Geological Background of Sandstone-Type Uranium Deposits in Ordos Basin, Northwest China 9811960275, 9789811960277

Citation preview

Springer Geology

Ruoshi Jin · Ren’an Yu · Peisen Miao et al.

Geological Background of Sandstone-Type Uranium Deposits in Ordos Basin, Northwest China

Springer Geology Series Editors Yuri Litvin, Institute of Experimental Mineralogy, Moscow, Russia Abigail Jiménez-Franco, Barcelona, Spain Tatiana Chaplina, Institute of Problems in Mechanics, Russian Academy of Sciences, Moscow, Russia

The book series Springer Geology comprises a broad portfolio of scientific books, aiming at researchers, students, and everyone interested in geology. The series includes peer-reviewed monographs, edited volumes, textbooks, and conference proceedings. It covers the entire research area of geology including, but not limited to, economic geology, mineral resources, historical geology, quantitative geology, structural geology, geomorphology, paleontology, and sedimentology.

Ruoshi Jin · Ren’an Yu · Peisen Miao

Geological Background of Sandstone-Type Uranium Deposits in Ordos Basin, Northwest China

Ruoshi Jin China Geological Survey Tianjin Center Tianjin, China

Ren’an Yu China Geological Survey Tianjin Center Tianjin, China

Peisen Miao China Geological Survey Tianjin Center Tianjin, China

ISSN 2197-9545 ISSN 2197-9553 (electronic) Springer Geology ISBN 978-981-19-6027-7 ISBN 978-981-19-6028-4 (eBook) https://doi.org/10.1007/978-981-19-6028-4 Jointly published with Science Press The print edition is not for sale in China (Mainland). Customers from China (Mainland) please order the print book from: Science Press. Translation from the Chinese Simplified language edition: “Eerduosi pen di shay anxing you kuang cheng kuang di zhi bei jing” by Ruoshi Jin et al., © Science Press 2019. Published by Science Press. All Rights Reserved. © Science Press 2023 This work is subject to copyright. All rights are reserved by the Publishers, whether the whole or part of the material is concerned, specifically the rights of reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publishers, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publishers nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publishers remain neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Foreword

Uranium is an important strategic key mineral resource in China. With the rapid development of China’s economy and the need for national ecologically sound construction, the development of nuclear power can help to ensure the safety of China energy supply while providing environmental protection. However, the proven reserves of uranium resources in China are far from meeting the long-term development needs of a national nuclear power program. Currently, sandstone-type uranium deposits have been the most important type of uranium deposits worldwide and the most important economically recoverable type inland. Therefore, strengthening the investigation and exploration of sandstone-type uranium resources and enhancing the support capacity of uranium resources serve as an important basis for the sustainable development of nuclear power. The Ordos Basin is one of the most important uranium-producing basins in China. Since the beginning of the twenty-first century, the Nuclear Geological System and the Central Geological Exploration Fund Management Center, affiliated with the former Ministry of Land and Resources, and other geological exploration units have made a series of important uranium prospecting achievements in this basin, laying a foundation for the construction of a uranium resource base. In 2010, the Tianjin Center of the China Geological Survey, as a national basic, public welfare, strategic investigation, and research institute, discovered that the “secondary development” of coalfield data is crucial to sandstone-type uranium ore prospecting in the Ordos Basin. The work team has undertaken a number of geological survey projects led by the China Geological Survey, the Key Project of Chinese National Programs for Fundamental Research and Development (973 Program), and the National Key Research and Development Project, the latter two of which are funded by the Ministry of Science and Technology of the People’s Republic of China. All the above projects have comprehensively opened a new round of sandstone-type uranium prospecting and scientific research in the main Mesozoic basins in northern China, and a series of theoretical innovations and prospecting achievements have been obtained and refined. Proceeding from basic geological studies, and taking the sedimentary basin as a unit, this book investigates the basic geology, geophysics, geochemistry, and remote v

vi

Foreword

sensing image characteristics of the Ordos Basin. The favorable uranium geological background brought by the changes of sedimentary environment conditions has been recovered and is well recognized. On the basis of comparative studies regarding numerous practical data extracted from uranium, coal, and oil and gas exploration boreholes, rocks exhibiting vertical color zonation were found indicate an original depositional environment. The vertical zoning signature differs from that of horizontal color zoning, as has been proposed for the “interlayer oxidation zone” sandstone-type uranium deposits. The sedimentary environment and orecontrolling sequences of the Ordos Basin have been preliminarily determined. The redox conditions reflecting the sedimentary environment and the dry and humid lithologic sequence index have been essentially established. The established metallogenic model along the northern margin of the Ordos Basin has not only laid the foundation of geological facts but also played an important role in guiding and promoting the determination of prospecting direction and target optimization, thereby opening up new uranium prospecting potential. By utilizing of big data drilling and radioactive anomaly parameter characteristics to screen coal and oil borehole data, numerous metallogenic prospective areas and prospecting target areas were quickly delineated, and specific deployment direction for uranium prospecting was defined. The systematic “232” prospecting method was proposed, providing ideas for overall deployment of prospecting work. The four stages of the working procedure—selection, type research, engineering validation, and exploration demonstration—represent the successful experience summarized from the project implementation process. Relying on the public welfare platform of the China Geological Survey, Mr. Jin Ruo-Shi and his scientific research team devised the technical idea of secondary development of coalfield and oilfield data and realized data sharing and cooperation among personnel in the geological and mineral, nuclear, and coal and oil industries. By making full use of tens of thousands of boreholes in coal and oil industrial systems, logging data, and rich uranium prospecting experience in nuclear industrial systems, in combination with multi-means, multi-mineral, and multi-field prospecting information from geological and mineral industrial systems, a systematic scientific prospecting cross-industry system was truly realized. The above creative work not only contributes to open industrial barriers, revitalizes decades-old accumulated data in various industries, and mobilizes the enthusiasm of scientific research and exploration teams in various industries but also generates active participation in this work with less investment. Many mineral resources have been discovered and proved by collaborative research and prospecting exploration. This innovative work mode is worth learning from. This book reflects the scientific achievements of the use of secondary development based on existing coalfield and oilfield data systems. The new achievements pertaining to and understandings of sandstone-type uranium deposit investigation and prospecting in the Ordos Basin led by the team from the Tianjin Center afford a good start to a new round of comprehensive research and uranium prospecting breakthroughs and further reveal that the Ordos Basin and other northern basins still have great uranium potential. I firmly believe that, with the strong support of the

Foreword

vii

Ministry of Natural Resources and the China Geological Survey, and the unremitting efforts and struggle of geological workers, uranium metallogenic theory and prospecting technology will continue to advance and engender creative ideas, which will not only promote uranium prospecting in China but also boost a greater vigorous development of uranium geology. August 2019

Mao Jin-Wen Chinese Academy of Engineering Academician Beijing, China

Preface

Despite the history of sandstone-type uranium deposits, most of the development and utilization of such mineral resources has only taken place in recent decades. In 1850, Czechoslovakia first exploited uranium ores as mineral products. In 1880, the USA discovered sandstone-type uranium deposits on the Colorado Plateau. On July 16, 1945, the USA detonated the first atomic bomb in the world. The Wuqikuduk sandstone-type uranium deposit was discovered by the Soviet Union in 1952. In 1954, the first nuclear power plant was built in Obuninsk in the Soviet Union. In 1967, an in situ leaching experiment on a uranium deposit in Bujinayi in the Soviet Union was successful, demonstrating that such sandstone-type uranium deposits can provide a non-fossil-fuel-based clean energy source with a large scale and a low cost of in situ leaching while having minimal environmental impact. The 26th International Geological Congress held in Paris, France, in 1980 put forward the plan of “Geological environment of sandstone-type uranium deposits.” This paper systematically put forward the definition of sandstone-type uranium deposits and studied the distribution scope and tectonic setting of the world’s important sandstonetype uranium deposits (based on IAEA, 1985). Research work on sandstone-type uranium deposits has been conducted by numerous scholars. An American team led by Shawe (Shawe et al., 1959) put forward the notion of “roll-type” uranium mineralization, and the zoning metallogenic model of “roll” was proposed by Granger and Warren (1969). Russian scientists proposed “phreatic infiltration-type” and “interlayer permeability-type” uranium mineralization. Chinese scholars proposed “the interlayer oxidation zone” metallogenic model, which transformed from the “interlayer permeability” type. It has been more than ten years since the discovery of sandstone-type uranium deposits in the Ordos Basin in China. In 2000, the Zaohuohao sandstone-type uranium deposit was discovered in the Ordos Basin by Brigade No. 208 of the China Nuclear Industry; this was followed by the discovery of the Nalinggou uranium deposit in 2010. The Daying uranium deposit, the largest sandstone-type uranium deposit in China up to now, was discovered by the Central Geological Exploration Fund Management Center of the former Ministry of Land and Resources. Since 2012, the Tianjin Center of the China Geological Survey and other units have followed the ix

x

Preface

guidance of “coal and uranium co-exploration” and “oil and uranium co-exploration” on the basis of previous work, promoting quick discovery of Tarangaole, Ningdong, Huangling, and Jinchuan uranium deposits. A three-dimensional visualization model of the northeastern margin of the Ordos Basin and a database of uranium exploration boreholes have been established. New understandings of the metallogenic geological background and role of the basin have been put forward. This book consists of two volumes: Volume 1 describes the geological background of sandstone-type uranium deposits in the Ordos Basin and Volume 2 is related to sandstone-type uranium mineralization in the Ordos Basin. The two volumes contain highly refined achievements from the “Comprehensive Investigation and Evaluation of Coal, Uranium, and Other Minerals in Major Basins of China” (2013–2014), the “Sandstone-type Uranium Survey Project” (2015–2021), the National Basic Research Program of China (973 Program) entitled “Sedimentary Environment and Large-scale Mineralization in Continental Basins of Giant Sandstone-type Uranium Metallogenic Belt in Northern China” (2015–2019), and the National Key Research and Development Program entitled “Demonstration of Deep Exploration Technology for Energy and Mineral Base of Northern Sandstone-type Uranium Deposits” (2018– 2021). This book aims to discuss the favorable metallogenic geological background of sandstone-type uranium deposits in the Ordos Basin, including the closure of the Paleo-Asian ocean at the end of the Paleozoic in northern China, and the formation of sedimentary materials and the spatio-temporal evolution of sedimentary materials in the basin during the formation of continental mountain basins after the closure of the Paleo-Asian ocean. The basic geology, geophysics, geochemistry, and remote sensing image characteristics of the basin are systematically studied to restore the favorable geological background of uranium generation brought by the change of sedimentary environment. Relying on the function orientation and platform advantages of the Central Public Welfare Geological Survey, through the working principle of “five insistences” and “five unification” quality management, we have devised a new mechanism for uranium prospecting work, which is a combination of multi-industry and multi-department cooperation and a combination of production, teaching, and research, giving full play to the advantages of departments and units such as coal, oil, and nuclear industries, as well as colleges and universities. With the Central Public Welfare Geological Survey team as the leader, comprehensive and scientific prospecting has been implemented. Without the secondary development and application of massive coalfield and oil and gas field drilling data, it would be impossible to perform detailed research on sedimentary facies, horizontal zoning of rock alteration, and vertical zoning of redox environments. Without the new cognition of vertical zoning of the redox environment, it would be impossible to expand the exploration space, and advances in sandstone-type uranium ore prospecting in northern China would be limited. Relying on the platform of the Uranium 973 Program and the Uranium Survey Project, team members published 177 papers, including 36 and 37 papers included in the Science Citation Index and Engineering Index, respectively. A uranium survey

Preface

xi

and scientific research team with nearly 70 people has been initially established at the Tianjin Center. The uranium mining team has been rated as the model collective of Tianjin in 2015 and an advanced collective of the National Land and Resources Management System. Two team members were selected as Tianjin labor models. Two were included in the “Top Ten Outstanding Youth.” One was designated as an “Excellent Youth.” Four were deemed “Excellent Geological Talents” by the China Geological Survey. Through comprehensive uranium prospecting, numerous coal, oil and gas, geological, and mineral experts have become uranium experts. The discovery of a series of uranium ore-producing areas also provides opportunities and platforms for different geological exploration units to adjust their industrial structure and enhance their business carrying capacity. Based on the analysis of the geophysical characteristics of the basin and its surrounding areas, a new understanding of the tectonic framework of the whole Ordos Basin has been achieved. Combined with the distribution characteristics of sandstone-type uranium ore concentration areas, the favorable structural positions of sandstone-type uranium deposits have been analyzed. Based on the analysis of geochemical characteristics around the basin, the material origin of ore-forming materials has been inferred. The airborne energy spectrum of the basin exhibits the characteristics of low values in the north and high values in the south, which is mainly controlled by the present landform and Quaternary sedimentary distribution after the formation of the basin topography. The remote sensing information revealed that there are many linear and annular structures in the basin, and the ore-controlling effect of these structures is worthy of attention. Through a comparative study of the sedimentary environment of the boreholes in both the whole basin and the metallogenic concentration area, it is found that the rock color zoning representing the oxidation–reduction formation conditions during diagenetic formation is vertical, with most color variations of red–yellow–green– gray–black proceeding downward. Sandstone-type uranium deposits are mostly found in the upper part of the coal-bearing rock series or the upper part of the oil-bearing rock series, (i.e., generally in the grayish rock above the coal seam or oil layer). This kind of zonation is common in the data of tens of thousands of boreholes in the basin. This phenomenon is quite different from the horizontal zoning of rock color in the metallogenic area, which is characterized by red and yellow color in the oxidation zone along the upper part of slope zone, green and gray color in the oxidation–reduction transitional metallogenic belt in the middle part, and gray and black color in the reduction zone in the lower part. Moreover, in the ore-forming area, the core along the slope zone was systematically scanned by short-wave infrared spectroscopy, and it was found that the altered minerals were zoned horizontally along the slope zone. This book has established detailed rules and unified investigation standards for the secondary development of coalfield and oilfield exploration data. Four working steps are defined: (1) screening and delineation of prospecting targets by abnormal boreholes, (2) discovery of ore-producing areas by drilling verification, (3) determination of prospecting methods through geological surveys of ore-producing areas, and (4) expansion of resources by mineral surveys. A combination of prospecting methods

xii

Preface

is established by utilizing comprehensive information, ore-controlling elements, and the uranium “232” survey method. This book puts forward theoretical viewpoints on the background conditions of sandstone-type uranium deposits, such as a prospecting model sequence of sandstone-type uranium deposits in coal-bearing basins, restriction of coupling occurrence of red–black rock series on the metallogenic environment of sandstone-type uranium deposits, the structural style of uranium-bearing rock series, and large-scale ore formed under large basins and large sandbodies. Guided by these theoretical understandings and using a series of new prospecting methods, a number of sandstone-type uranium deposits have been found in this basin. The book consists of six chapters. Chapter 1 is the Introduction. Chapter 2 was written by Sun Li-Xin, Li Jian-Guo, Zhao Xin-Xin, Zhang Guo-Li, Zhang Su-Rong, Teng Fei, Cheng Yin-Hang, Zhang Yuan-Qing, Zheng Guo-Qing, and Wang Wei. Chapter 3 was written by Ruoshi Jin, Ren’an Yu, Zhang Tian-Fu, Sun Li-Xin, Yang Jun, and Xiao Peng. Peisen Miao, Li Xiu-Hua, Peng Sheng-Long, Cao Hui-Feng, Zhu Qiang, Si Qing-Hong, Tang Chao, Liu Xiao-Xue, Xiao Peng, and Zhao LiJun prepared Chap. 4. Chapter 5 was prepared by Ruoshi Jin, Ren’an Yu, Wang Shan-Bo, and Zhang Yuan-Qing. The final chapter was written by Ruoshi Jin, Sima Xian-Zhang, Ren’an Yu, Wang Shan-Bo, Tang Chao, Xiao Peng, and Wen Si-Bo. The final version of the book was confirmed by Ruoshi Jin and Ren’an Yu. Yang Jun, Liu Xiao-Xue, Zhao Li-Jun, and Wang Ya-Fei have been involved in the compilation of the numerous maps in the book. Mr. Gao Ping, the director general of the Science and Technology Development Department of the Ministry of Natural Resources, and Mr. Hou Zeng-Qian, an academician from the Chinese Academy of Science, implemented the project. Mr. Mao Jing-Wen, an academician from the Chinese Academy of Engineering, and leaders from resource evaluation departments of the China Geological Survey gave careful guidance and assistance to the operation of the 973 Progam. Mr. Zheng Da-Yu, the former chief engineer of the China Nuclear Industry Geological Bureau, guided the uranium survey and research work throughout the project. In addition, Mr. Hou Hui-Qun, the former president of the China Nuclear Science and Technology Information and Economy Research Institute, Mr. Chen Bing from Brigade No. 224 of the China Shaanxi Nuclear Industry Group, Mr. Guo Jian-Yu from the Ningxia Nuclear Industry Exploration Institute, and Mr. Zhang Yu-Long and other comrades from the Gansu Nuclear Industry Geological Bureau gave strong support to improve the investigation technology and methods of the project and ensure the work quality. With the long-term support of the China Geological Survey and the Ministry of Science and Technology, the work was organized and led by the Tianjin Center, in cooperation with the Coalfield Geological Bureau of the Inner Mongolia Autonomous Region, the Brigade No. 208 of the China Nuclear Industry Geological Bureau, the Geological Bureau and Nuclear Industry Exploration Institute of the Ningxia Hui Autonomous Region, the China Coalfield Geological Bureau, and the Inner Mongolia Geological Survey Institute; the Shaanxi Nuclear Industry Geological Bureau and other subordinate teams jointly undertook the completion of the project. We would like to express our heartfelt thanks to the colleagues who have worked hard and

Preface

xiii

contributed their wisdom to this endeavor and all leaders and experts who care and support this project! Tianjin, China November 2018

Ruoshi Jin

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Analysis of Uranium Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Research on Sandstone-Type Uranium Deposits in Ordos Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Breakthroughs and Main Achievements Related to Sandstone-Type Uranium Resources . . . . . . . . . . . . . . . . . . . . . . . .

1 1

2 Background of the Ordos Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Regional Geological Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Regional Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Regional Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3 Magmatic Activity and Metamorphism . . . . . . . . . . . . . . . . . . 2.1.4 Tectonic–Sedimentary Evolution of the Ordos Basin . . . . . . 2.1.5 Tectono–Thermal History of the Basin Since the Jurassic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Characteristics of the Regional Geophysical Field . . . . . . . . . . . . . . . 2.2.1 Physical Properties of Rock Strata . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Characteristics of the Regional Gravitational Field . . . . . . . . 2.2.3 Characteristics of the Regional Magnetic Field . . . . . . . . . . . 2.2.4 Radiological Anomalies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5 Comprehensive Interpretation of Regional Geophysical Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Geology Inferred from Remote Sensing . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Overall Image Characteristics of the Ordos Basin and Its Periphery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Main Linear Structural Features . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Remotely Sensed Geological Characteristics of Typical Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Geochemical Characteristics of the Basin Periphery . . . . . . . . . . . . . 2.4.1 Regional Distribution of Elements . . . . . . . . . . . . . . . . . . . . . .

7 7 9 29 37 41

2 4

45 48 48 58 58 64 67 69 69 70 81 85 86

xv

xvi

Contents

2.4.2 Spatial Variation Characteristics of Elements and Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3 Association of Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.4 Enrichment and Depletion Characteristics of the Basement and Caprock in the Basin . . . . . . . . . . . . . . . 2.4.5 Elemental Distribution Characteristics of the Main Rock Masses Around the Basin . . . . . . . . . . . . . . . . . . . . . . . . 2.4.6 Geochemical Background of Uranium Mineralization Along the Periphery of the Basin . . . . . . . . . . . . . . . . . . . . . . . 2.4.7 Delineation of Uranium Source Rocks Along the Periphery of the Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Hydrogeological Characteristics of the Basin . . . . . . . . . . . . . . . . . . . 2.5.1 Hydrogeological Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.2 Types and Distribution of Groundwater . . . . . . . . . . . . . . . . . . 2.5.3 Groundwater Replenishment, Diameter, and Discharge Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.4 Hydrochemical Characteristics of Groundwater . . . . . . . . . . . 2.5.5 Characteristics of the Fissure Pore Aquifer in Clastic Rock of the Zhiluo Formation in the Middle Jurassic . . . . . . 2.6 Internal Structural Units of the Basin . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.1 Western Margin Thrust Belt . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.2 Northern Uplift Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.3 Yulin Uplift Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.4 Central Uplift Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.5 Southeastern Uplift Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.6 Central Subsidence Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Geological Characteristics of Uranium-Bearing Rock Series in Key Metallogenic Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Classification of Uranium-Bearing Rock Series . . . . . . . . . . . . . . . . . 3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock Series in Key Metallogenic Prospective Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Northeastern Margin of the Ordos Basin . . . . . . . . . . . . . . . . . 3.2.2 Southeastern Margin of the Ordos Basin . . . . . . . . . . . . . . . . . 3.2.3 Western Margin of the Ordos Basin . . . . . . . . . . . . . . . . . . . . . 3.2.4 Southwestern Margin of the Ordos Basin . . . . . . . . . . . . . . . . 3.2.5 Central Part of the Ordos Basin . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Comparison of Logging Parameters of Uranium-Bearing Rock Series in Key Metallogenic Prospective Areas . . . . . . . . . . . . . 3.3.1 Logging Response of the Zhiluo Formation . . . . . . . . . . . . . . 3.3.2 Logging Identification of Different Lithologies in the Zhiluo Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

88 89 94 94 95 100 102 102 103 106 108 110 111 113 115 116 116 116 117 117 119 121 121

122 122 145 149 159 166 170 178 182

Contents

3.4 Sedimentary Environment of Uranium-Bearing Rock Series in the Ordos Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Restriction of Color Zoning of Uranium-Bearing Rock Series on the Paleo-sedimentary Environment . . . . . . . 3.4.2 Indicative Significance of Biological Fossils to the Paleoclimate of Uranium-Bearing Rock Series . . . . . . 3.4.3 Geochemical Characteristics and Provenance Indication Significance of the Zhiluo Formation of Uranium-Bearing Rock Series in the Ordos Basin . . . . . . 3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Geological Characteristics of Typical Deposits and Newly Discovered Ore-Producing Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Geological Characteristics of the Uranium Concentration Area Along the Northeastern Margin . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Tarangaole Mining Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 Zaohuohao Uranium Deposit . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3 Nalinggou Uranium Deposit . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.4 Daying Uranium Deposit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.5 Wulanxili Mining Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Geological Characteristics of Uranium Ore Concentration Areas Along the Western Margin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Yangchangwan Mining Area . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Shicaocun Mining Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Ciyaobao Uranium Deposit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Geological Features of the Uranium Concentration Area Along the Southeastern Margin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Huangling Uranium Deposit . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Shuanglong Uranium Deposit . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Geological Characteristics of the Uranium Ore Belt Along the Southwestern Margin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Huanxian Ore Spot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Pengyang–Jingchuan Deposit . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Geological Characteristics of the Uranium Mineralization Belt in the Jinding Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1 Metallogenic Geological Background . . . . . . . . . . . . . . . . . . . 4.5.2 Geological Characteristics of Uranium Mineralization . . . . . 4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xvii

184 184 210

218 238 240 245 247 247 258 262 265 271 275 275 283 289 292 292 301 304 304 308 318 318 320 322 323

5 Basin Uranium Mineralization Law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325 5.1 Overview of Uranium Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325 5.1.1 Tarangaole–Dongsheng Uranium Belt Along the Northeastern Margin of the Ordos Basin . . . . . . . . . . . . . 325

xviii

Contents

5.1.2 Ningdong Uranium Belt Along the Western Margin of the Ordos Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.3 The Huating–Jingchuan Uranium Deposit Belt Along the Southwestern Margin of the Ordos Basin . . . . . . . . . . . . . 5.1.4 The Huangling Uranium Deposit Belt Along the Southeastern Margin of the Ordos Basin . . . . . . . . . . . . . 5.2 Ore-Controlling Elements and Prospecting Signs . . . . . . . . . . . . . . . . 5.2.1 Ore-Controlling Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Prospecting Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Regularity of Metallogeny . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Time Distribution Law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Laws of Spatial Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Prospecting Methods, Prediction Models, and Metallogenic Prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Technical Prospecting Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1 Strategic Constituency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.2 Drilling Verification and Effect . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.3 Determination of the Combination of Prospecting Technology and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.4 Demonstration of Selective Exploration . . . . . . . . . . . . . . . . . 6.1.5 Secondary Development Technology System for Coal and Oilfield Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Prospecting Forecast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Comprehensive Assessment of Ore-Controlling Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Prospecting Prediction Model . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Mineralization Prediction and Analysis . . . . . . . . . . . . . . . . . . . . . . . . 6.3.1 Principles of Delineation of Scenic Spots . . . . . . . . . . . . . . . . 6.3.2 Features of the Scenic Spot . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

326 327 327 328 328 347 349 350 350 355 356 357 357 359 364 365 367 367 369 369 369 375 375 378 397 398

Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399

Chapter 1

Introduction

1.1 Analysis of Uranium Resources Uranium is a trace element in the earth’s crust. Because of its low content and uneven distribution, the abundance of uranium in the earth’s crust varies greatly. The general value is on the order of parts per million (10−6 ), while the highest value can reach 10−4 . Although it may seem that the content of uranium is minor, In fact, it is higher than that of tungsten, mercury, silver, and other familiar metals, being even thousands of times higher than that of gold! Uranium primarily exists in various complexes in the valence states of +4 and +6. The simplest and basic chemical combination is UO2 and UO3 , which exist in nature in the form of more complicated complexes. Although the existence of uranium is complex and diverse, its regularity is also relatively strong. In various solutions, U6+ and UO 22+ can easily dissolve in water, whereas U4+ and UO 2 generally exist in precipitation states. At present, >170 kinds of uranium-bearing minerals have been found. Among them, the minerals with industrial value are pitchblende, uraninite, autunite, torbernite, carnotite, vanadate, brannerite, coffinite, and tyuyamunite. Uranium resources provide an important guarantee of national defense and energy security, factors that are crucial for adjusting and optimizing China’s energy structure and improving the ecological environment. The development of nuclear power needs a large and stable long-term supply of uranium resources. Given the development trend of nuclear power, proven uranium resources in China will be unable to meet the needs of China’s medium- and long-term development of nuclear energy. Only by relying on domestic resources can China ensure the supply and price stability of uranium resources. Therefore, expanding the amount of uranium resources in China ensures a long-term stable supply and a strategic reserve of uranium resources. According to the International Atomic Energy Agency (IAEA), uranium deposits have been found in 75 countries around the world. The top five countries are Australia, Kazakhstan, Niger, Canada, and Namibia. According to their economic importance, the IAEA divides uranium deposits into 15 types, the most important of which is sandstone-type uranium deposits. The international community’s © Science Press 2023 R. Jin et al., Geological Background of Sandstone-Type Uranium Deposits in Ordos Basin, Northwest China, Springer Geology, https://doi.org/10.1007/978-981-19-6028-4_1

1

2

1 Introduction

Fig. 1.1 (Left) Proportion of world uranium resources and (right) uranium production in 2015 (IAEA 2016)

growing panic about nuclear power plants and the continuous decline of global uranium prices have seriously restricted global uranium exploration and mining work. Sandstone-type uranium deposits, with the advantages of their large scale, low-cost in situ leaching mining, and relatively environmentally benign mining process, are becoming increasing popular as a supply of uranium resources. At present, sandstonetype uranium deposits rank first in terms of economic value both in the world and in China. According to the IAEA red book (2016), the proven resources of sandstonetype uranium deposits accounted for 27% of the global total (Fig. 1.1a). However, the actual resource supply in 2015 accounted for 49% of the total global supply of uranium resources in that year (90,000 m of verification boreholes, combined with the research results of the 973 Program, traditional metallogenic theory has been enhanced, and the theoretical and practical exploration space has been expanded to account for basin sedimentary evolution and ore-forming redox

1.3 Breakthroughs and Main Achievements Related to Sandstone-Type …

5

environment evolution. From the traditional single-ore-controlling model of horizontal zoning to vertical oxidation–reduction zoning, the three-dimensional orecontrolling model of horizontal ore-forming fluid zoning has evolved into a fourdimensional space–time constraint mechanism for the ore-forming redox environment. The vertical zoning of the redox environment has been found to have a broader metallogenic space and ore-forming physicochemical conditions. The red–black rock series plays an important role in controlling the uranium-bearing rock series. From the Yan’an Formation to the Anding Formation, the vertical stratigraphic structure of black–gray–green–red is generally developed in the whole basin. This vertical zoning formed by the transformation of the ancient sedimentary environment from reduction to oxidation comprises the geological background conditions for large-scale mineralization, which provides favorable conditions for uranium pre-concentration. Moreover, the horizontal and vertical zoning characteristics of orebodies, their geochemistry and altered minerals, and their evolution have been identified. These provide the basis for the dissolution, migration, and precipitation of uranium minerals by supergene fluids in the process of uranium mineralization.

Chapter 2

Background of the Ordos Basin

2.1 Regional Geological Background The Ordos Basin is located in the central part of North China, with its main body located in Shanxi Province, Inner Mongolia Autonomous Region, and its western part spanning from Ningxia Province to Gansu Province. The land forms of the basin are distinctive. According to the genesis and morphological types, the Ordos Basin can be divided into desert plateau in the north, loess plateau in the south, and the mountainous and rifted depression surrounding the Ordos Basin (Fig. 2.1). The terrain is generally high in the south and low in the north, with the middle axis high in the north–south direction and low in the eastern and western flanks. Local areas are undulating, with the Dongsheng and Yanchiliang regions in the north of the basin, at an altitude of ~1500 m, and the Baiyu Mountains in the center and the Ziwu Ridge in the southeast, at an altitude of 1500–1800 m, forming the surface watershed within the basin and presenting two distinct landscapes to the north and south. The desert plateau is dotted with the Kubuqi Desert, the Maowusu Sands, and denuded hills, with an altitude of 1100–1500 m; the topography is relatively undulating, with a relative height difference from 30 to 80 m. The Loess Plateau area is crisscrossed by ravines, which are strongly cut and fragmented, with an altitude of 1000–1700 m and a thickness of 100–300 m of loess cover. The coarse sand area covers ~144,500 km2 , which is the largest unit of the basin. The northern part of the Huanxian and Wuqi regions is characterized by loess and mountains, while the southern part is dominated by loess or residual loess. The geomorphology of the Ordos Basin is mainly controlled by neotectonic movements. Since the Quaternary, the Ordos Basin has experienced integral large-scale uplifting, and the uplift rate is higher in the north than in the south. In the north, except for several lakes formed in the relative depression area, most of the area is under a denudation or denudation–accumulation stage, presenting a denuded plateau and a wide landscape of wind and sand accumulation. The southern part received a thick layer of loess. During the Middle and Late Pleistocene, the erosion of rivers

© Science Press 2023 R. Jin et al., Geological Background of Sandstone-Type Uranium Deposits in Ordos Basin, Northwest China, Springer Geology, https://doi.org/10.1007/978-981-19-6028-4_2

7

8

2 Background of the Ordos Basin

Fig. 2.1 Topography of the Ordos Basin

and gullies intensified, and a loess plateau landscape with a combination of loess plateau, alpine plateau, and gullies was formed. Based on thesis of the continental dynamics system, the Ordos massif is part of the North China block and is located in the western part of the North China massif. It has an irregular rectangular shape and a nearly north–south long axis. It is bounded on the east by the central tectonic belt of the North China land block, on the west by

2.1 Regional Geological Background

9

the Alashan land block and the western margin of the alluvial belt, on the north by the Baiyun’erbo–Chifeng Fault and the Xingmeng orogenic belt (central Central Asian orogenic belt), and on the south by the Luonan–Luanchuan Fault and the Qinqikun orogenic belt (central and western orogenic belt). The Ordos massif is one of the rare tectonically stable massifs on the North China land mass. Its main tectonic features are the Late Paleozoic kratonization, the formation of a set of argillic trough- and rift-trough-type deposits in the Mesozoic and Cenozoic, the formation of thick and stable land surface sea deposits in the Early Paleozoic, a set of sea-lake basin deposits in the Late Paleozoic, and the development of thick terrestrial basin clastic rocks and volcanic deposits in the Mesozoic. The Ordos region was a huge stable sedimentary basin during the Palaeozoic, being part of the wider epicontinental sea of North China. The real evolution of the Ordos Basin as an independent sedimentary basin took place mainly during the intraplate dynamics of the Mesozoic and Cenozoic. The Erdos Basin in the Mesozoic encompassed some of the surrounding small basins, while the Cenozoic evolution of these peripheral small basins and the main body of the Erdos Basin were distinctly different. As the peripheral basins have their own independent spatial and developmental evolutionary processes, the Ordos Basin in a narrow sense generally no longer includes the small and medium basins of the Cenozoic around the Ordos Basin. The Ordos Basin is one of the typical large inland sedimentary basins of the Mesozoic era in China. It is tectonically an asymmetrical oblique basin with a north– south orientation, gentle in the east and steep in the west, and surrounded by a system of folded mountains, starting from the Yinshan Mountains and Daqing Mountains in the north, reaching the Qinling Mountains in the south, the Helan Mountains and Liupan Mountains in the west, and the Luliang Mountains and Taihang Mountains in the east, with a total area of >250,000 km2 . The basin is rich in oil, natural gas, coal, uranium, salts, oil shale, and many other valuable energy deposits and nonmetallic minerals, making it one of the most important energy and mineral bases in China.

2.1.1 Regional Stratigraphy The Ordos Basin and its peripheral areas have strata of various geological ages; these have different depositional types and construction characteristics. Among them, the Mesozoic strata are widely distributed; the Tauric, Proterozoic, and Palaeozoic strata are mostly exposed in the mountainous areas of the periphery and are scattered; the Cenozoic strata are well developed in the peripheral fault basin; and the desert, low mountain hills, and loess plateau areas in the basin are distributed in areas of Quaternary loose sediments (Fig. 2.2). According to the North China regional stratigraphic scheme, the stratigraphic zoning of the Ordos Basin belongs to the North China Stratigraphic Region of the Ordos Stratigraphic Division (Chen and Wu 1997), and the stratigraphic type belongs to the typical North China–type sedimentary stratigraphy. The stratigraphy

10

2 Background of the Ordos Basin

Fig. 2.2 Geological map of the Ordos Basin (revised based on Zhang 2016). 1. Quaternary–Paleogene. 2. Lower Cretaceous. 3. Jurassic. 4. Triassic. 5. Palaeozoic. 6. Lower Paleozoic. 7. Yuanguyu. 8. Taiguyu. 9. Yanshanian granite

of the Ordos Basin from old to new consists of the Taikyu–Palaeoproterozoic gneiss system; Middle Neoproterozoic clastic rocks and carbonates with a few volcanic rocks; Cambrian–Ordovician clastic rocks and carbonates; Carboniferous–Permian interfacial clastic rocks, limestones, and coal-bearing rocks; Triassic coal-bearing and oil-bearing terrestrial clastic rocks; Jurassic coal-bearing and uranium-bearing terrestrial clastic rocks and bainite rocks; terrestrial clastic facies; Cretaceous clastic rocks; and Cenozoic loose accumulations (Table 2.1). Because of the large differences

2.1 Regional Geological Background

11

between the new and past series named during the stratigraphic clean-up process in the 1990s, the coalfield and oil and gas borehole data are all based on the traditional rock group as the main presentation system. During the preparation of this book, to conform to the habits of most readers, a combination of the old and new system was used for the stratigraphic series; that is, the units below the rock group are based on the traditional local descriptions, and the units above the rock group are based on the new stratigraphic system.

2.1.1.1

Basement

The Ordos massif has gone through a long history of development and evolution. It can be divided into three major evolutionary stages, corresponding to the formation of three major tectonic layers. The first stage is from the Middle Archean to the end of the Paleoproterozoic, which is the initial stage of crustal (plate) formation, and to the end of the Paleozoic, when the regional kraton was formed and a set of metamorphic crystalline rock system was formed. The second stage is from the Mesoproterozoic to the Palaeozoic, which is the stage of development of the North China Basin, forming a set of thick and relatively stable sedimentary strata, including Early Palaeozoic marine sediments and Late Palaeozoic continental–oceanic interaction sediments, with some areas having substable Meso–Neoproterozoic sedimentation. The third stage comprises the Mesozoic and Cenozoic, being characterized by the deposition of terrestrial clastic rocks and volcanic rocks of different grain sizes. For the Mesozoic to Cenozoic basins, therefore, the basin basement has a distinct two-layer structure: a metamorphic crystalline basement layer and a Mesoproterozoic–Paleozoic sedimentary basement. Alternatively, the direct basement of the Mesozoic and Cenozoic basins is a Mesozoic–Palaeozoic sedimentary layer, and the indirect basement is a Mesoproterozoic–Palaeozoic crystalline rock series. The crystalline basement is mainly composed of metamorphic and mixed granites of the Tauric (Ar) sepiolitic and hornblende phases and metamorphic rocks of the Palaeogene (Pt1 ) hornblende phase, diorite hornblende, and shallowly metamorphosed green schist phase. The spatial distribution of the crystalline basement is characterized by an old north–south trend, with a correspondingly deep north–south trend in metamorphism. The crystalline basement is exposed to varying degrees in the alteration source areas around the basin, providing not only a source of sediment for the formation of the indirect basement of the basin but also a rich source of sediment and uranium for the deposition of the Middle and Cenozoic. The direct basement includes the Middle Palaeogene, Neogene, and Lower and Upper Palaeozoic, but the Middle Neogene was deposited to a very limited extent and thickness, with the main body being the Lower Palaeozoic deposits. The Upper Palaeozoic material was deposited extensively, but generally not in great thickness. The Lower Palaeozoic material was mainly deposited in the coastal, shallow marine and lagoonal phases, with carbonates and mud shales dominating; the Upper Palaeozoic material consisted of mainly clastic rocks that intersected with sea and land. The stratigraphic characteristics of each unit are described in the following.

12

2 Background of the Ordos Basin

Table 2.1 Lithostratigraphy, tectonic movement, and basin evolution in the Ordos Basin Age

Lithostratigrap hy

Unifie d

Quate rnary Cen oz oic

Main rock

Tectonic movement

Basin evolution

Himalayan movement

Ordos fault block uplift and the formation of Cenozoic fault depression basin around it

Fluvial lacustrine facies and eolian clastic deposits, 0–2500 m thick

Neog ene

Red fluvial lacustrine clastic rock, 0– 10,000 m thick

Paleo gene Upper seri es

Yanshan movement III Jingchua n Formation

Aeolian fluvial lacustrine clastic rock

Luohand ong Formation

Fluvial pebbly sandstone

Huanhe Formation

Fluvial lacustrine clastic rock

Luohe Formation

Aeolian sandstone

Yijun Formation

Debris flow conglomerate

All-out uplift

S Creta ceous (K) Lowe r seri es

Upper seri es es Meso zoi c

Jurass ic (J)

Middl e unif icati on Lowe r seri es Upper seri es

Triass ic (t)

Middl e unif icati on Lowe r seri es

Upper seri es

Permi an (p)

Middl e unif icati on

e c u ri t y g r o u p

Xiangxianghe Formation Formation Diazepam Group Zhiluo Formation Yan'an Formation Fuxian Formation Extension Group

Yanshan movement II

Reddish brown alluvial mediumcoarse conglomerate, 100–1000 m thick 100 m

Formation , development, and extinction of the Ordos Basin

Fluvial lacustrine clastic rock, 150– 270 m thick Fluvial lacustrine clastic rock, 200– 280 m thick

Yanshan movement I

Coal-bearing terrigenous clasts in Hehupo Delta, 200–450 m thick

Fluvial lacustrine clastic rock, locally coal-bearing, 1100–3500 m thick

Large area uplift

Tongchuan Formation Paper room Group Shangshanggo u Formation

Fluvial lacustrine clastic rocks; along the western margin of the Ordos Basin, the lithology is generally coarsened, with clastic flow and debris flow conglomerate, or transformed into clastic flow boulders, 960–1630 m thick

Indosinian movement

Foreland basin Slow settlement

Liujiagou Formation

Shiqianfeng Formation

Shihezi Formation

Fluvial lacustrine clastic rock, with several layers of tuff and tuffaceous sandstone in the middle and upper parts of the Helan Mountain area, with a thickness of 340–720 m

Residual basin

(continued)

2.1 Regional Geological Background

13

Table 2.1 (continued) Upper series Carbo niferous (C)

Taiyuan Formation Benxi Formation

Platform carbonate rock, clastic argillaceous tidal flat coal-bearing deposit, 120–1250 m thick

Lower series

Devo nian (D) P ale oz oic

Caledonian movement

Siluri an (s)

Sedimenta ry loss

Upper series

Ordo vician (o)

Middle unif icati on

Lower series

Upper series

Camb rian (c)

Majiagou Formation

Liangjiashan Formation Yeli Formation Fried rice shop Group Gushan Formation Zhang xiazu

Middle unif icati on Lower series

Neopr oterozoic rot ero zoi c

Large area uplift

Meso proterozoic Paleo proterozoic

Archean universe

Steamed bread Group Changping (Zhushadong) Formation

Very thick limestone, bioclastic limestone with dolomite

huaiyuan movement

Chert block dolomite and dolomitic limestone Limestone, gravelly limestone Bamboo leaf limestone, banded limestone, and glauconite sandstone Epicontin ental basin

Banded limestone and shale

Slow settlement

Oolitic limestone, bioclastic limestone, and intercalated shale Red clastic rock intercalated with argillaceous dolomite

Phosphorous limestone, dolomite, and dolomitic limestone

Quartz sandstone formation: mainly composed of quartzite, metamorphic sandy conglomerate, slate and shale; carbonate formation: mainly dolomite and cryptoalgal dolomite with limestone; 1400–10,000 m thick

Shallow metamorphic rock series, 2000–15000 m thick

Deep metamorphic gneiss and tTG rock series, 3400–16000 m thick

Jinning movement Caprock formation

Luliang movement

Fuping movement

Extended consolidation of continental crust Land nucleus formation

1. Mesoarchean The Mesoarchean metamorphic rock is the oldest rock in the Ordos Basin and the surrounding ancient land mass, with outcrops mainly in the Huoxian, Zanhuang, Jining, Blossom and Jiehekou regions in the Shanxi. it is estimated that it is also distributed in the deep part of the basin. The rocks are mainly sepiolitic and high hornblende phase rocks, and the main lithologies are sepiolite, hornblende black cloud

14

2 Background of the Ordos Basin

metagranite, plagioclase-bearing plagioclase, plagioclase hornblende and magnetite quartzite. 2. Neoarchean The Neoarchean metamorphic system is part of the metamorphic basement in the deep part of the basin and its periphery, being exposed in Wutai, along the Longhua River and in Jiangxian, Shanxi. It is mainly a high and low hornblende phase metamorphic system, with the main lithologies being black cloud metagranite, plagioclase, hornblende metagranite, garnet quartzite, mica schist, and magnetite quartzite. 3. Paleoproterozoic The Palaeoproterozoic metamorphic system is generally distributed in the deep part of the basin and its periphery and is a low hornblende phase and high green schist phase rock, with the main lithologies being slate, metamorphic sandstone, metamorphic andesite, metamorphic basalt, dacite, quartzite, and micrite. 4. Mesoproterozoic The Changcheng system and Jixian system of the Middle Palaeogene are sporadically exposed in Pingliang, Gansu, Yongshou, and Shaanxi and along the Weihe River. The Great Wall system is a combination of sand shale and volcanic rocks intersected by sea and land and is unconformable to the former Great Wall system. The Great Wall series of the Helan Mountains is composed of the maritime quartz sandstone and quartzitic sandstone of the Huangqikou Group with a few slates and sand slates. The Jixian series is dispersed along the northern and southwestern margins. There is a small distribution in the Helan Mountains on the northern margin. It is a grayishwhite medium-thick laminated siliceous band or siliceous agglomerate dolomite that is 726 m thick. It is exposed along the southwestern margin in the western part of Weibei, the Qianyang Jingfu Mountains in Shaanxi, in the southern mountains of Pingliang in Gansu, and in the Yunwu Mountains and Qinglong Mountains in Ningxia. Drilling work reveals that the Jixian series is dispersed as far east as the Yoshen 1 well in Liulin Town, Tongchuan, Shaanxi, and as far north as the Ren 3 well in Inner Mongolia. The lithology is mainly dark gray and gray-white mediumto-thick laminated siliceous bands or siliceous masses of dolomite, with occasional conglomerate lenses in the lower part. The thickness of this layer is >2000 m in the Qishan area of Shaanxi and 500–700 m in the Qiyang area of Shaanxi and Longxian area of Gansu and is dominated by thin to medium-thick gray-white laminated siliceous bands containing gray dolomite in the Qinglong Mountain area of Ningxia, with a thickness of >708 m. The Jixian system is in angular unconformity with the underlying former Great Wall system and the overlying Cambrian system. 5. Neoproterozoic The Neoproterozoic system is mainly composed of the Qingbaikou series and the Aurora series. The Qingbaikou series is generally found in the Tableshan, Qinglongshan, and Guyuan areas along the western margin and around Liulin and Wubao along the eastern margin and is a set of quartz sandstones, quartzites, siltstones, and

2.1 Regional Geological Background

15

dolomites with siliceous bands and nodules, interspersed with a few slates, in the shallow coastal marine phase. The Aurignacian system consists of gray and light gray thickly bedded flintstriped dolomitic limestone and dolomite, rose-colored medium to thickly bedded siliceous shale, yellow calcareous shale and marl in the Longxian and Qianyang area in the south, and 1350-m-thick gray massive siliceous conglomerate and maroon siltstone with a few fine sandstones in the Qinglongshan area of Ningxia. It is in unconformable contact with the underlying Middle Palaeogene system and is 35.8 m thick. 6. Lower Paleozoic The Lower Palaeozoic system is the most important and stable stratigraphic unit in the basin, covering the whole basin and the surrounding areas, with outcrops on the western edge of the basin from Table Mountain, Black Mountain, Sun Mountain, and Yunwu Mountain in the north to Jingfu Mountain, Tiehuatian Mountain, and Jinli Mountain on the southern edge, and Jikwang Mountain, Hangaoshan Mountain, and Yiyuanguan Mountain on the eastern edge, while the central part of the basin is buried deep underground. The Lower Palaeozoic system is mainly developed in the Lower, Middle, and Upper Cambrian and the Lower and Middle Ordovician. Sea erosion developed from south to north and soon covered the whole basin. The Cambrian system, from the old to the new in order, is the Vermilion Sand Cave Group, the Steamed Bun Group, the Maozhuang Group, the Xuzhuang Group, the Zhangxia Group, the Mountains Group, the Changshan Group, and the Fengshan Group. The Ordovician System, in order, is the Yeli Group, the Liangjiashan Group, and the Majiagou Group, which are widely distributed throughout the basin and the surrounding areas. The Zhushadong Formation is exposed in Qinglong Mountain, Ningxia, and Guyuan Laoyan Mountain. It is a set of grayish-white and dark gray mediumthick-bedded dolomite and dolomitic tuff, occasionally interbedded with thin- to medium-thick-bedded tuff (of 13.1–47 m in thickness). The Mantou Formation is distributed in a ringlike pattern in Table Mountain, Qianli Mountain, and Gandel Mountain. It is overlain by the Qingbaikou series in a parallel unconformity and is in integrated contact with the Middle Cambrian Zhangxia Formation. The lithology is purple-brown sandy dolomite with gray-white quartz sandstone, sand conglomerate, shale, and limestone (125.5–263.37 m thick); purple-red, gray-violet, and dark gray calcareous shale, mudstone, mud siltstone mainly interbedded with marl, oolitic limestone, and dolomitic limestone in Wanrong and Fuping (103–535 m thick); yellow-gray and brown–red marl, dolomite, oolitic limestone in Hejin and Hancheng; and purple-red shale, oolitic tuff, thin-slabbed tuff and light gray limestone, interbedded with sand conglomerate and lenticular quartz sandstone (166–230 m thick), and with a general trend of gradual thinning from south to north and from west to east. The lithology of the Mantou Formation, which is exposed in the upper reaches of the Youguan River in the Jin–Shaan–Meng area on the eastern edge of the basin,

16

2 Background of the Ordos Basin

is brownish-red sand conglomerate, brick-red sandy shale with fine sandstone, graypurple shale and a small amount of dolomite and limestone in the lower part, and purple-red and yellowish-brown shale and dolomitic tuff in the upper part. In the upper part of the southern area, oolitic tuffs, limestones, and muddy striped tuffs are seen. These are 49.5–108.5 m thick, being thin in the north and thick in the south. The Maozhuang Formation (∈2 m) is mainly exposed in the southern part of the basin. It is integrated over the Bantou Formation and comprises mainly purple-red shale interbedded with thinly bedded siltstone, containing trilobite fossils. The Xuzhuang Formation (∈2x) is integrated over the Maozhuang Formation. Its depositional range basically covers the whole basin and the surrounding area. The lithology is mainly dark gray medium-thick-bedded silt-bearing marl tuffs, producing trilobite fossils. The Zhangxia Formation (∈2z) lies along the inner and northwestern, southern, and eastern margins of the basin, while the southwestern margin formations are known as the Taosigou and Hulustai formations. Its integration overlies the Xuzhuang Formation. It is dominated by gray medium- to thick-bedded oolitic tuffs and mudbearing streaked oolitic tuffs, interbedded with thin-bedded tuffs, dolomitic clastic tuffs, and bamboo-leaf tuffs. The base is often underlain by a layer of thin yellow-gray slaty mud-banded tuffs interbedded with yellow-green calcareous shale. In Gansu, the lower part is purple shale and siltstone sandstone interbedded with thinly bedded tuff, and the upper part is dark gray thinly bedded oolitic tuff interbedded with dark purple and purple gray shale (160 m in thickness) and calcareous shale, bamboo leafy tuff, and bioclastic tuffs. It is 46.0–111.3 m thick, with a pattern of variation from thick in the south to thin in the north, and from thick in the east to thin in the west. The oolitic tuffs, dolomitic tuffs, and bamboo-leaved tuffs, interbedded with muddy tuffs, are 77–110 m thick in the Jin–Shaan–Meng area on the eastern margin, with the dolomite content increasing from south to north and the thickness increasing. The phase changes to muddy banded tuff and shale interbedded with thinbedded tuff around Table Mountain in Inner Mongolia. The lithology of the Taosigou Formation is grayish-white and grayish-yellow thin- to medium-bedded fine-grained quartz sandstone, dolomite, limestone, and shale, with a thickness of 109.5. It is in integrated contact with the underlying Zhusha Cave Formation. The lithology of the Hulustai Formation consists of gray-green and purplish-red shales interbedded with thin- to medium-thick-bedded tuffs and muddy striped tuffs of unequal thickness, interbedded with oolitic tuffs and bamboo leaf tuffs. The Gushan Formation (∈3 g) is integrated overlying the Zhangxia Formation. It mainly consists of greenish-gray bamboo-leaved tuff interbedded with purple calcareous shale. The Changshan Formation (⧠3 c) is consolidated over the Mesa Formation. It mainly consists of purple bamboo leafy tuff with dolomitic crystalline tuff. The Fengshan Formation (⧠3 f ) is integrated overlying the Changshan Formation. It mainly consists of light gray thin-bedded dolomitic tuffs, marls, and dolomites. The Yeli Formation (O1 y) is integrated over the Cambrian Fengshan Formation. It mainly consists of yellow-gray thinly and thickly bedded crystalline dolomite.

2.1 Regional Geological Background

17

The Liangjiashan Formation (O1 l) is integrated over the Yeli Formation, It mainly consists of light gray thin- and thick-bedded dolomitic tuffs with a few dolomites, containing flint nodules, and yellow-green shales at the base, containing trilobites and cephalopod fossils. The Majiagou Formation (O2 m) is integrated overlying the Liangashan Formation. It mainly consists of dark gray and gray-brown thinly and massively laminated tuffs and leopard skin tuffs. The middle and upper parts are light yellow dolomitic tuffs and the bottom is fine-grained quartz sandstone. In the central part of the Ordos Basin, multiple layers of thicker saltstone layers and gypsum layers are developed. 7. Upper Paleozoic After the deposition of the Ordovician Majiagou Formation, the Ordos area, as a part of the North China epicontinental basin, began to uplift as a whole and suffered weathering and denudation. In the Late Carboniferous, it sank again and this continued into the Permian. A set of relatively stable in thickness marine continental interactive, continental clastic rocks, and coal-bearing rock series were developed in the Ordos area, and these are rich in coal, iron ore, and bauxite. The Upper Carboniferous depositions are divided into the Benxi Formation and the Taiyuan Formation, and the Permian depositions are divided into the Shanxi, Lower Shihezi, Upper Shihezi, and Shiqianfeng formations. There are small sedimentary discontinuities between groups, the main bodies of the upper and lower groups are in integrated contact, and there is local parallel unconformity or angular unconformity. The Upper Carboniferous rocks are sporadically exposed in the west, south, and east surrounding areas of the basin and can be seen in varying degrees in the boreholes in the basin. The Permian rocks are mainly exposed in the gullies east of the basin and in the table mountain area in the northwest, followed by intermittent sporadic exposure in the west and south. The whole basin is buried under Mesozoic and Cenozoic sedimentary layers in a large area. The characteristics of each group are as described in the following. The Benxi Formation (C2 b) is a parallel unconformity covering the Lower Paleozoic strata. At the bottom lies paleoweathed crustal-type clayey material, mainly mudstone, quartz sandstone, siltstone, limestone, and ferroaluminous rock, with brachiopods and coral fossils. This group is rich in iron ore, bauxite, and other minerals. The Taiyuan Formation (C2 t) is integrated and covered over the Benxi Formation. The upper and lower parts are coal measure strata, the middle part is quartz sandstone, and the bottom is gray-white sandy conglomerate. This formation is rich in coal. The Shanxi Formation (P1 s) is the integration covering the Taiyuan formation. It consists of gray to gray-white quartz sandstone, dark gray siltstone, argillaceous rock, and bioclastic limestone with thin coal seams. The Lower Shihezi Formation (P1 x) is the main integration covering the Shanxi Formation, and some areas are similar to integration or small-angle unconformity. It is mainly yellowish brown and grayish green clayey siltstone and shale mixed with purple medium-grained sandstone; the bottom is grayish white gravelly coarse sandstone.

18

2 Background of the Ordos Basin

The Upper Shihezi Formation (P2 s) is the overall integration covering the Lower Shihezi Formation. It is mainly brownish yellow medium-fine-grained siltstone and silty mudstone, dark purple mudstone mixed with gravelly coarse sandstone, and gray-white gravelly coarse sandstone at the bottom. The Shiqianfeng Formation (P2 sh) is the overall integration covering the Upper Shihezi Formation. It is mainly brownish red sandy mudstone, gray-white and graygreen medium-fine-grained siltstone, and gray-white conglomerate at the bottom.

2.1.1.2

Mesozoic–Cenozoic

The caprock of the Ordos Basin refers to Mesozoic and Cenozoic sedimentary and volcanic rocks. Before the Mesozoic, the development and evolution of the Ordos area occurred mainly as a part of the North China landmass and North China epicontinental sea, being different from the surrounding geological units but generally exhibiting consistency. From the Mesozoic to the Cenozoic, although there was still some unity, more particularity is evident. This is mainly determined by the tectonic background of the Ordos area. After the Mesozoic, the Pacific plate subducted under the Eurasian plate, and the Tethys–Himalayan tectonic domain plate drifted northward. Tectonic activity such as pushing, subduction, and collision affected the area, resulting in large-scale faulting and series of basin deposits. The sedimentary characteristics and tectonic evolution are different in each basin. Table 2.2 presents the sedimentary strata in the Ordos Basin and surrounding basins. Compared with the Weihe and Hetao basins, which developed on the Ordos block, the sediments in the Ordos Basin were mainly deposited during the Mesozoic, whereas those in the former basins were mainly deposited during the Cenozoic. The strata developed in the Ordos Basin include Triassic (T), Jurassic (J), Lower Cretaceous (K) of the Mesozoic and Paleogene (E), and Neogene (N) and Quaternary (Q) of the Cenozoic (Fig. 2.3). The development of different strata varies greatly in horizontal and vertical directions, among which the Triassic, Jurassic, and Lower Cretaceous sediments are the main ones in the basin. The Jurassic strata are important coal-bearing strata in the eastern part of the basin, being exposed in a north–south belt and dipping westward and southwestward. They are also the main rock series for uranium exploration. The Lower Cretaceous sediments are widely distributed in the northern part of the basin, and they also one of the main rock series for uranium prospecting in the basin. Paleogene and Neogene sediments are sporadically exposed in the Ordos Basin, being mainly developed in peripheral basins. Quaternary loose sediments are widely distributed in the central and southern parts of the basin. 1. Triassic The Triassic rocks form a set of inland river, lake, and swamp facies clastic rock sedimentary formations, and the main body is continuously transitional with the Permian. From bottom to top, the strata can be divided into the Liujiagou Formation

2.1 Regional Geological Background

19

Table 2.2 Correlation of Mesozoic and Cenozoic strata in the Ordos Basin and its periphery Stratum

Liupanshan Basin Bayanhot Basin

Overlying strata

Pal eogene (E2+3)

Pal eogene (E2+3)

Helan Mountain area

Pal eogene (E2+3)

Ordos Basin

Qingshuiying Formation (E3q)

U pper Cretac eous

Pale ogene (E)

Ningwu Basin

Neogen e (N)

Zhu ma auxiliary Group

Ma dongsha n Formati on

Cretaceous

Liw axia Formati on

Jiyuan Basin

Yima Basin

Yinshan area

Pal eogene (E2+3)

Ne ogene (N)

Paleo gene (E2+3)

Do ngmeng cun Formati on

Do ngmeng cun Formati on

D ongshe ng Format ion

Nai jiahe Formati on

L ower Cretac eous

Datong Basin

Jingchuan Formation

Ba yanhot Group

Guya ng Formation

Zuo yun Formatio n

Lisan gou Formation

Mia oshan Lake group

Mo nk auxiliary Group

Luohandong Formation

San qiao Formati on

Huachi– Huanhe Formation Luohe Formation

Zho ngzhuang auxiliary Group

Mesozoic

Yijun Formation Whit e female sheep plate Group Xi angxian ghe Formati on

U pper jurassi c

Xia ngxiang he Formati on Dia zepam Group

Jurassic

M iddle Jurass ic

Zhi luo Formati on Shi yanzi Formati on

Diazepam Group

Zhiluo Formation

Yun gang Formatio n

Yungan g Formation

Ma ao Formati on

Ya n'an Formati on

Yan'an Formation

Dato ng Formatio n

Datong Formation

Ya ngshuzh uang Formati on

Fu xian Formati on

Triassic

Wa yaobao Formati on

Ha nzhuang Formati on

Tian hechi Formatio n

Zhi luo Formati on

L ower Jurass ic

U pper Triass ic

Qia nqiuzhe n Formati on

Xi angxian ghe Formati on

Ext ension Group

Extension group

Yon gdingzhu ang Formatio n

Exte nsion group

Do ngmeng cun Formati on

Yi ma Formati on

Sad dle waist group

Tan zhuang Formati on

Daqi ngshan Formation

Chan ghangou Formation

Zhao gou Formation

Wud anggou Formation

Tan zhuang Formati on

(continued)

20

2 Background of the Ordos Basin

Table 2.2 (continued) Yo ngping Formati on Huj iacun Formati on Ton gchuan Formatio n

Ton gchuan Formati on M iddle Triass ic

L ower Triass ic

Overlying strata

T ongchu an Format ion

Ch unshuya o Formati on

Ch unshuya o Formati on

Yo ufangzh uang Formati on

Yo ufangzh uang Formati on

Zhi fang Group

Zhi fang Group

Ermaying Formation

Erm aying Formatio n

Ermayi ng Formation

Er maying Formati on

Er maying Formati on

Sha ngshang gou Formati on

Sha ngshang gou Formati on

Shangshanggo u Formation

Shan gshanggo u Formatio n

Shangs hanggou Formation

Sha ngshang gou Formati on

Sha ngshang gou Formati on

Liu jiagou Formati on

Liu jiagou Formati on

Liujiagou Formation

Liuji agou Formatio n

Liujiag ou Formation

Liu jiagou Formati on

Liu jiagou Formati on

Shi qianfeng Formati on

Shiqianfeng Formation

Shiq ianfeng Formatio n

Shiqian feng Formation

Shi qianfeng Formati on

Shi qianfeng Formati on

Shi qianfeng Formati on

Ya nghugo u Formati on

Lao wopu Formation

Sulc us Group

(T1 l), the Heshanggou Formation (T2 h), the Ermaying Formation (T2 e), and the Yanchang Formation (T3 y). The Liujiagou Formation (T1 l) is widely distributed and exposed along the eastern and southern margins of the basin. It is basically continuous with the underlying Permian Shiqianfeng Formation. It is a set of variegated medium and fine sandstone dominated by fluvial facies, mixed with a small amount of mudstone and conglomerate. The Heshanggou Formation (T2 h) is distributed throughout the whole region. Its integration covers the Liujiagou Formation. It is a set of sedimentary formations dominated by lacustrine facies. The lithology is mainly purplish red mudstone, mixed with a small amount of purplish red sandstone and glutenite. The Ermaying Formation (T2 z) is widely distributed and exposed in the eastern and southeastern parts of the basin, contacting conformably with the Heshanggou Formation. It is a set of fluvial, lacustrine, and delta facies sedimentary formations with frequent facies changes. The lithology is mainly composed of purplish red silty mudstone and sandstone interbedded. The Yanchang Formation (T3 y) is widely developed, widely exposed in the eastern denudation area, and integrated and covering the Ermaying Formation. It is a set of sedimentary formations dominated by fluvial facies. The lithology is mainly grayish green gravelly sandstone, with argillaceous siltstone and sandy mudstone in the middle, and occasionally extremely unstable thin coal seams.

2.1 Regional Geological Background

21

Fig. 2.3 Comprehensive histogram of strata in the Ordos Basin (modified from Zhao et al. 2012)

22

2 Background of the Ordos Basin

2. Jurassic rocks The Jurassic rocks comprise a set of fluvial and lacustrine clastic rocks intercalated with coal seams, and their surface is mainly exposed in the east. Drilling data confirm that the whole basin is developed and is parallel unconformable above the Triassic strata, with a thickness of >2000 m. From bottom to top, they are the Fuxian Formation of the lower series, the Yan’an Formation, the Zhiluo Formation, the Anding Formation of the middle series, and the Fenfanghe Formation of the upper series. The lower series is mudstone, mixed with sandstone and a small amount of marl and sandstone, conglomerate, and mudstone, and oil shale, mixed with thin coal seam fluvial–lacustrine facies deposits. The middle series consists of sandstone, gravelly sandstone, and sandstone, shale, and mudstone with different thicknesses, interbedded with coal seams or coal lines, and the top is a fluvial–lacustrine sedimentary combination of oil shale, shale, calcareous siltstone, and marl. The upper series is a glutenite accumulation of piedmont facies, which is only sporadically exposed in the eastern piedmont and southwest of the table along the western margin. The Jurassic strata comprise an important ore-bearing bed for oil, coal, and uranium resources in this area. The Jurassic coalfield formed in the northern part of the basin is one of the most important coalfields in China. Unstable burnt rocks with a thickness of 5–15 m and a maximum thickness of 50 m are formed in shallow or exposed areas of local coal seams. The Zhiluo Formation in Ningdong, Huangling, and Dongsheng areas in the basin is an important uranium-resource-bearing bed. The Fuxian Formation (J2 f ) is the earliest Jurassic sedimentary strata in the Ordos Basin; it developed on the uneven denudation surface caused by the Indosinian movement. The sediments are characterized by filling and are in parallel unconformity contact with the underlying Triassic Yanchang Formation. The formation is mainly distributed in the east and southeast of the basin, with a sedimentary discontinuity with the Triassic. It comprises a set of sedimentary formations dominated by fluvial–lacustrine facies. The lithology is mainly yellow-green sandstone and mudstone, interbedded with a small amount of marl and sandstone, conglomerate, and mudstone, and oil shale mixed with thin coal seams. The Yan’an Formation (J2 y) is widely developed in the northern part of the basin and exposed in a large area in Ordos City and Yulin in the eastern part. Its integration covers the Fuxian Formation. It comprises a set of sedimentary formations dominated by fluvial–lacustrine facies. The lithology is mainly gray mudstone and siltstone, and coal seams are developed. This formation is one of the main coal-bearing and oilbearing horizons in the basin, and it is also one of the key prospecting target horizons of sandstone-type uranium deposits in the Ordos Basin. The Zhiluo Formation (J2 z) is distributed in the whole basin and its surrounding areas. It is exposed in the eastern part of the basin in a north–south belt and also exposed in the Ciyaobao section in the west, being basically parallel and unconformable above the Yan’an Formation. It comprises a set of sedimentary formations dominated by fluvial–lacustrine facies. The lithology of the lower part is gray, gray-green sandstone, and glutenite, and that of the upper part is gray, gray-green

2.1 Regional Geological Background

23

mudstone, and sandstone. This formation is an important prospecting target layer for sandstone-type uranium deposits in the Ordos Basin. The Anding Formation (J2 a) is well distributed in the basin. It comprises a set of sedimentary formations dominated by lacustrine facies. The lithology is mainly purplish red and brown and variegated sandy mudstone interbedded with maroon red, grayish green, and grayish white sandstone and siltstone. The Fenfanghe Formation (J3 f ) comprises a set of sedimentary formations dominated by piedmont diluvial facies. It is only exposed in the western part of the basin along the eastern margin of the table and inthe western part of the southern margin of the basin. It comprises a set of piedmont facies and piedmont alluvial facies deposits, with a stratum thickness of 100–1200 m. The lithology is mainly brown and extremely thick layered, much like massive conglomerate, conglomerate mixed with fine conglomerate, and glutenite. It is in parallel unconformity contact with the underlying Anding Formation and in angular unconformity contact with the overlying Cretaceous rocks. 3. Cretaceous rocks At the beginning of Early Cretaceous, the eastern margin of the basin rose to a mountain, and the southern and western margins also rose again, forming a closed and unified basin with uplifting all around. The Zhidan Group (Baoan Group) continental clastic rock deposits with a thickness of >1300 m were deposited during the Lower Cretaceous. It is a set of purplish red and variegated continental clastic rock formations, which are angularly unconformable on the Jurassic strata. There is only the lower series (Zhidan Group) in the Cretaceous in the Ordos Basin; it is a set of purplish red and variegated continental clastic rocks, and its angle is unconformable above the Jurassic strata. From bottom to top, it is divided into the Yijun, Luohe, Huachi–Huanhe, Luohandong, and Jingchuan formations. The Yijun Formation (K1 y) is also known as the Yijun Conglomerate. It is mainly exposed in the Qianyang, Binxian, and Xunyi areas in the southern part of the basin, is sporadically exposed in the Ansai, Yijun, Ganquan, Huangling, and Yaozhou areas in the eastern margin, but is missing in the northern part of the basin. Because of its limited distribution and continuous transition with the Luohe Formation in most areas, it is collectively referred to as Luohe Formation later. The Yijun Formation is a set of near-source piedmont flood alluvial fan deposits with a thickness of 0–302 m in the basin. Its lithology is mainly (variegated) purple conglomerate, sandstone lens with gravel, and a small amount of a mudstone thin layer. Its shape is mostly fan-shaped, wedge-shaped, mound-shaped, and lenticular, which rapidly thins, pinches out, or transits into the Luohe Formation sandstone from the edge to the basin. In addition, it is distributed in a large area near Lingwu in the western margin thrust zone, with a thickness of >887 m. The Yijun Formation and the underlying Jurassic Anding Formation, Zhiluo Formation, and Fangfanghe Formation are mostly in micro-angle unconformity or parallel unconformity contact. Along the southern and southwestern margins of the basin, high-angle unconformity lies on different horizons of Jurassic strata. The thrust belt along the western margin

24

2 Background of the Ordos Basin

of the basin, Lingwu gravel well, and the Houjiahe area in Ningxia is directly unconformable above the Triassic strata. The gravel composition is mainly limestone, granite, and siliceous rock, with a small amount of sandstone, and the sorting is poor. The gravel diameter is 2–30 cm and, generally, 5–6 cm. The gravel is well rounded and subangular–subround (mainly subround), with a slightly directional arrangement of individual gravels and silica–calcareous cementation. The Luohe Formation (K1 l), also known as the Luohe Sandstone, is mainly exposed in a wide range from Qianyang, Binxian, Xunyi, and Yijun lines in the south of the basin to Huangling, Zhidan, Yulin, and Erdos cities along the eastern margin of the basin. The underground distribution is stable and can be seen in boreholes. The Luohe and Yijun formations in the southern basin exhibit a continuous sedimentary integration transition, while other places are parallel and unconformable on the Anding Formation or Jurassic Zhiluo Formation, and the northeastern part of the basin also overlaps on different horizons of Triassic and Permian strat. The thickness varies greatly under the influence of sedimentary paleotopography and geomorphology differences and later denudation, but it is generally characterized by being thin in the east and thick in the west. The thickness generally varies from 250 to 350 m, and the maximum thickness occurs roughly along the towns of Mahuangshan, Hongdecheng, and Sancha along the western margin of the basin. The thickness is generally >400 m. The maximum thicknesses exposed by drilling are 844.5 m (Well A256) and 855 m (Well A242), respectively. The residual thickness near Narinxili on the northeast side of the basin is also relatively great, being 529 m (Well C181) and 472 m (Well A1111). The thickness at Well B10 near Ningxian County in the south is 481 m. The thickness of the formation in Bin County on the edge of the basin is 241.8 m, that at Qianyang Caobigou is 130 m, and that at other places are only tens of meters to nearly 100 m. The Luohe Formation comprises a set of near-source alluvial fan-braided river– desert facies deposits. Desert facies deposits are the main underground deposits in the Yijinhuoluo Banner and at Wushenzhao in Inner Mongolia; Yanchi in Ningxia; Hongdecheng, Huanxian, and Jingchuan in Gansu; and Changwu in Shaanxi. The deposits along northern, western, and southern margins of the basin; in the vicinity of Yijun and Zhiluo towns along the southeastern margin; in Chahe and Erlintu towns along the northeastern margin; and in the southern area of the Ordos City are mainly fluvial deposits. The desert facies sedimentary area of the Luohe Formation accounts for about two-thirds of the sediments in the same period, with a thickness of 200–300 m, and the thickness of its sedimentary center is generally >450 m (around Wangpanshan in Dingbian and Tiebiancheng in Wuqi). They are mainly composed of aeolian dune sandstone and fine siltstone and argillaceous rock between dunes. Dune sandstone is the main sedimentary body of the Luohe Formation desert, and its lithology is mainly brick-red, brown–red, and purplish red massive medium- and fine-grained feldspathic Shi Ying sandstone and feldspathic sandstone, with a small amount of gravelly sandstone, coarse sandstone, and silty fine sandstone. It is characterized by the development of giant cross-bedding and plate bedding. The maturity of the rock structure and composition is high, the structure is loose, the pores are developed,

2.1 Regional Geological Background

25

and the connectivity is good. The fine siltstone and argillaceous rock between hills are limited in distribution, small in thickness, and discontinuous. In terms of the combination of deserts, it has become the most important aquifer in the basin owing to its few mudstone interlayers, high proportion of sandstone (>90%), stable extension, huge scale, and loose structure. The fluvial facies of the Luohe Formation are mainly alluvial fan and braided river deposits. At the bottom of the northern part of the basin (Wells Bl, B14, B16, D27, D31, etc.), there are alluvial fan deposits composed of variegated conglomerate, glutenite, gravelly sandstone, and sandstone. Its thickness is 64–170 m, and the gravels are mixed in size. Among them, there are many boulders larger than 50 cm; these are poorly rounded, with muddy and sandy filling and exhibit basement cementation. The alluvial fan experienced a transition to a braided fan in the vertical and longitudinal direction. Braided river deposits are widely distributed along the northern and western margins of the basin, being mainly lithic feldspathic sandstone, feldspathic sandstone, feldspathic Shi Ying sandstone, and gravelly sandstone, mixed with silty mudstone, argillaceous siltstone, and thin layers of mudstone, and partially containing gypsum. The sandstone mostly has unequal grain structure, which is moderately to poorly sorted and rounded, and is produced in lenses with different thicknesses. In this work, it is newly discovered that this group is a uranium-bearing target layer along the southwestern margin of the basin. The Huachi–Huanhe Formation (K1 hc + h) includes the original Huachi Formation and the Huanhe Formation, which are in integrated contact with the Luohe Formation. It is exposed in pieces in Inner Mongolia in the north and distributed in the bottom of valleys in the southeast. The exposed range shrinks westward compared with the Luohe Formation, and the eastern boundary is west of the Yijinhuoluo Banner, Jingbian County, and Zhidan County. The strat thickness is generally 200– 600 m, with a maximum thickness still located in the core of the Tianhuan syncline (800–900 m), and the thickness of the eastern edge is 0–100 m. Surface outcrops and underground boreholes reveal that there are obvious differences in lithology and lithofacies between the northern and southern parts of the Huanhe Formation, which are roughly bounded by Yanchi and Jingbian north of Baiyushan, and fluvial deposits are the main ones in the north, with the delta and lacustrine deposits and a small amount of the fluvial deposits in the south in delta and lake regions. Alluvial fan facies, gray and gray-green conglomerate and gravel-bearing medium-coarse sandstone can be seen at the bottom of the Huanhe Formation, with coarse gravel diameter and poor sorting at the northern and western margins of the basin. The gravel composition is mainly quartzite and gneissic granite. Wells B1, B6, and B3 reveal thicknesses of 39, 58 and 61.34, respectively. However, braided river and meandering river deposits are the main deposits in the northern part of the basin, and the lithology is mainly purple-gray, brown–red, and cyan-gray lithic feldspathic sandstone, feldspathic sandstone, glutenite, and gravelly sandstone, mixed with brown–red mudstone and argillaceous siltstone. Calcareous and argillaceous contact-pore cementation is the main form, and several coarse–fine sedimentary cycles are developed. On the plane, the grain size in the north of the basin tends to change from coarse to fine from west to east and from north to south.

26

2 Background of the Ordos Basin

The southern part of the basin is dominated by lacustrine deposits, and its lithology is obviously different from that of the northern part. These deposits are mainly composed of bluish gray, gray medium-fine sandstone, siltstone, and mudstone, with a small amount of fine-grained materials in a gypsum-salt layer. The water-bearing medium is mainly delta water and an underwater distributary channel sandbody. The thickness of the single sandbody is thick in the north (263 m at Well C93) and thin in the south (32 m at Well D10 and 23 m at Well C165), thick in the west (28 m at Well B9 and 23 m at Well ZX1), and thin in the east (10 m at Well C315 and 5 m at Well ZX2). Statistics show that, from north to south, from the river deposition to the lake deposition center, the cumulative thickness of sand layer gradually decreases, from 70% of the thickness of land layer in the north to 43% in the southeast. The vertical sandbody is characterized by multicycle superposition, reflecting its progradation and retrogradation evolution. On the plane, it often extends into the lake in irregular strips and networks, reflecting its wandering characteristics. The surface of the Luohandong Formation (K1 lh) is exposed in an inverted “L” shape at the edge of the basin, north of the Ordos –Etuoke in the northern part of the basin and west of Dingbian–Huanxian–Qingyang–Changwu in the west. In the Yimeng uplift, the Zhuozhuoshan area, and the north–south “paleo-ridge” of the western margin of the basin, they are unconformably overlaid on the Ordovician, Triassic, and Jurassic Zhiluo Formation and Anding Formation, respectively, and integrated under the overlying Jingchuan Formation. Other places are integrated or eroded with the underlying Huanhe Formation. The outcropping thickness of the Luohandong Formation varies greatly, with basin thickness in the Yikewusu area in the north reaching >350 m, that in the Etuoke Banner and Zhenyuan–Jingchuan area in the west exceeding 250 m, and that in other areas being generally 0–150 m. The Luohandong Formation is a clastic rock deposit dominated by fluvial and desert facies. After deposition of the fluvial and lacustrine sediments of the Huanhe Formation, the Ordos Basin was uplifted again and again, and the climate gradually turned to drought conditions. Some volcanic overflow facies basalt interbeds formed as a result of volcanism. The Luohandong Formation along the northern edge of the basin is mainly an alluvial fan facies–braided river facies sedimentary framework, and its lithology is a set of brown–red, purple-red, orange, and ginger sandstone, gravelly sandstone, conglomerate mixed with lenticular mudstone, and sandy mudstone. The distribution area of the Luohandong Formation near the western edge of the southern basin is mainly a braided river (alluvial fan) facies–desert dune subfacies sedimentary framework, and the lithology is mainly brown–red, orange-red, purple-red unequal grain, medium- and medium-fine grain lithic feldspathic sandstone, calcareous fine sandstone, feldspathic quartz sandstone mixed with purple-red mudstone, and silty fine sandstone thin layers. In the area of Heishitougou, Talagou Township, and Hangjinqi, this group is grayyellow, turmeric, yellow-green, gray-green medium-coarse sandstone and gravelly coarse sandstone mixed with conglomerate; the middle and upper parts are mixed with ~10 m of black porous and almond-shaped Yidinite basalt.

2.1 Regional Geological Background

27

The Jingchuan Formation (K1 j) is mainly distributed in the basin margin area north of the Yikewusu–Hangjinqi line in the northern basin and west of the Bulongmiao– Yanchi–Huan County–Jingchuan line in the west. It is intermittently exposed in a north–south strip and continuously deposited with the Luohandong Formation. The maximum thickness in the north is >300 m, while that in the Jingchuan–Zhenyuan area in the southwest is ~200 m. These form two sedimentary centers in the north and south, and the thickness in other areas is tens of meters to >100 m. The lithology is obviously different between the north and the south, with coarse particles with bright colors in the north and fine particles with dim colors in the south. In the northern part of Yikewusu–Hangjinqi, the lower part of this group comprises typical flood alluvial facies and braided river facies, and the lithology is yellow-green and gray-green conglomerate mixed with gray-white, brown–red, and gray-yellow gray sandstone. The upper part is soil red, yellow-green medium-fine sandstone, gravelly coarse sandstone, and interbedded conglomerate, which is rich in gray nodules. Most of the northern area is covered by Cenozoic rocks, and the surface outcrop is only seen sporadically. According to the drilling data, the stratum thickens from south to north, and the exposed thickness at Well A892 is 251 m. Deposits in Bulongmiao in the Etuoke Banner, Damiao in the west of Etuoke Qianqi, and Beidachi along the western margin of the northern basin are lacustrine deposits. The lithology is blue-gray, gray-green, brown-gray, and brick-red medium-thin-layered mudstone, mixed with gray-green and yellow-gray calcareous fine sandstone and marl, locally mixed with thin-layered pseudooolitic limestone lenses, with a residual thickness of 120.87 m (Wujiamiao). From this, the Haba lacustrine facies in the south to Yanchi County changed into interbedded deposits of iron–calcareous cemented mudstone, silty fine sandstone, medium-grained feldspathic sandstone, and lithic feldspathic sandstone, with a thickness of 42.47 m. The Longdong area is mainly composed of freshwater lacustrine facies and meandering river facies deposits. The lithology is dark purple and light gray sandy mudstone and interbedded argillaceous siltstone, with light gray marl, dolomitic mudstone, and light gray and light yellow sandstone in the middle. The stratum thickness is 142–446 m (Qianyang Caobigou–Chongxin Xiangfanggou). Plant fossils are also found. The stratum is a continuous deposit with the underlying Luohandong Formation, which overlaps and unconformity with the Jurassic and the Sinian strata to the west. 4. Paleogene–Neogene rocks The western margin of the Ordos Basin lacks Paleocene strata, and the Shikou Formation of Eocene is only exposed in the Liupanshan sedimentary area. The Qingshuiying Formation of the Oligocene and the Hongliugou and Ganhegou formations of the Miocene are generally developed in the area. The Sikouzi Formation is a set of Eocene coarse- to medium-grained clastic rocks composed of brick-red and brown–red conglomerate, glutenite, pebbly sandstone, and sandstone. It is not integrated above the Mesozoic. The Qingshuiying Formation is a set of Oligocene maroon and brick-red mudstone, siltstone, mixed with gray-green sandstone, mudstone, and a gypsum

28

2 Background of the Ordos Basin

layer. It was continuously deposited with the Lower Fuzikou Formation and lies in parallel unconformity contact with the Hongliugou Formation. The Hongliugou Formation is a set of Middle Neocene orange-red and orangeyellow clayey sandy soil, clay mixed with gray-white feldspathic quartz sandstone and gritstone lens, and upward clayey sandy soil with clay gradually increasing, occasionally mixed with light gray marl. Its upper and Ganhegou Formation are continuous sediments. The Ganhegou Formation is a set of Middle Neocene clastic rocks and clayey rocks composed of gray glutenite, gray-white quartzite, earth-yellow and earth-red siltstone, and sandy mudstone, occasionally containing gypsum or marl interlayers. Its top is covered by Quaternary rocks. 5. Quaternary rocks Quaternary deposits are widely developed in the basin. They are mainly composed of loess and aeolian sand. They can be divided into a diluvial layer, a lacustrine layer, and an aeolian layer according to genetic type. The loess deposit comprises an alluvial–lacustrine deposit, alluvium, an alluvial–proluvial deposit, and an aeolian sand deposit. Both Lower Pleistocene and Middle Pleistocene and Holocene alluvial layers are developed. They are mainly distributed on the foothills or terraces of the Yinshan alluvial plain, Helan Mountain, and Liupanshan Mountain, with some material also being distributed in large and small gullies and alluvial–proluvial fans in front of the mountains. The lithology is gray or variegated conglomerate, glutenite, and gravelly pebble, mixed with sand, gravel, and clayey sand or lens. 5–130 Mo thick The aeolian loess layer includes the Lower Pleistocene Wucheng Formation, the Middle Pleistocene Li Shi Formation, and the Upper Pleistocene Malan Formation, exhibiting the typical geomorphological features. It is mainly distributed in the west, south, and east of the basin while being scattered in other areas. The lower part of the Wucheng Formation is light flesh-red soil-like subclay (stone loess) with several to dozens of layers of light brown–red paleosoil; the upper part is a light flesh-red stone loess layer with 10–20 layers of calcareous nodules and is 2–84 m thick. The Lishi Formation is grayish yellow and light yellowish brown silty loess, with several layers of maroon paleosoil on the top and a gray-white calcareous nodule layer on the bottom, with columnar joints developed. The 2- to 235-m-thick Malan Formation is light yellow, brown-yellow silty loess and calcareous nodules, with vertical joint development. The aeolian lacustrine deposits include lacustrine deposits of the Upper Pleistocene Salawusu Formation and the Holocene. The Salawusu Formation is mainly distributed in the middle eastern and southern parts of the basin, and it is composed of river–lake facies and aeolian facies. The lithology bottom is 1-to 2-m-thick blackgray peat and a muddy sand layer; the middle is light brown yellow fine silty sand, silt, sandy clay, and a medium-coarse sand interbed; the upper part is light a gray clayey calcareous silt layer, which are the main aquifers and have thickness of 5–90 m. Holocene alluvial–lacustrine strata are mainly distributed along the two banks of the Yellow River Basin, Yinwu Basin, Weining Basin, Qingshui River Valley, Kushui

2.1 Regional Geological Background

29

River Valley, and other large tributaries. They belong to the transitional deposition of the fluvial facies of the lake marsh river and are composed of gray-yellow and gray-black fine sand, silt, clay, and silt or are produced in interbeds with thickness of 1–30 m. The alluvial–proluvial layer is mainly found in the fan-shaped plain, alluvial– proluvial fan, and major rivers and first and second terraces. It is earth-yellow pebblebearing sand gravel and gravel-bearing medium-coarse sand, mixed with a thin layer of clay sand, exhibiting horizontal and cross-bedding and being up to 5 m thick. It belongs to the main water-bearing horizon. Alluvium is distributed over major gullies, floodplains, and pedestal terraces at all levels. It includes gray-yellow and gray-green secondary loess with horizontal bedding and other alluvial sandy soils with gravel lens bodies and being calcareous at the bottom, loess blocks, and mud balls. The alluvium is generally 1–5 m thick. The aeolian sand layer is mainly distributed in the north and at the edge of the basin, forming the Kubuqi, Maowusu, and Wulanbu deserts, and distributed sporadically in other sections. It is mainly light yellow fine sand, followed by medium and silty sand, with a thickness of 0–15 m. Various types of sand dunes are formed, with relative height differences of 5–80 m, to form the main aquifer.

2.1.2 Regional Structure 2.1.2.1

Geotectonic Setting

The Ordos Basin is located west of the North China continental block. It has an irregular rectanglular shape with a long axis running nearly in the north–south direction. The east side is the central tectonic belt of the North China continental block, the west side is the Alashan block and the western margin thrust belt, the north side is bounded by the Baiyun Obo–Chifeng Fault and the Central Asian orogenic belt, and the south side is bounded by the Luonan–Luanchuan Fault and adjacent to the Qin–Qi–Kun orogenic belt (Fig. 2.4). Some studies have shown that at least the Siberian plate in the north and north of the North China continental block in the Ordos Basin were divided by the Paleo-Asian Ocean, and the oceanic basin between the two plates closed during the Late Permian to form the Paleo-Asian continent (Wang 1987). The Qinling–Qilian Paleozoic oceanic basin between the North China plate and the Yangtze plate on the south side of the Paleo-Asian continent was closed in the early Middle-Oceanic period at the end of the Permian, forming a collocation of the North China and Yangtze plates and the basic outline of the Chinese mainland (Ren et al. 2000). The Ordos block is the main geological unit of the longitudinal middle axis of the Middle Mongolia continent (Ma and Zheng 1981), which was recognized as the dominant part of the Qilv tectonic unit by Li et al. (1990). The Ordos block (meso-axis) is the most stable geological unit in China. The main basin trends, folds and faults, bounded by the Ordos Block, are NE and NNE trending in the eastern

30

2 Background of the Ordos Basin

Fig. 2.4 Regional geotectonic map of the Ordos Basin (modified from a project of the China’s Mineral Resources Assessment)

part and NW trending in the western part. These features further demonstrate that the Meso–Cenozoic tectonic evolution of the eastern Ordos Basin was controlled by the subduction of the (Paleo) Pacific plate, and the western part was related to the Tethys tectonic system (Alpine–Himalayan orogeny). The results of tectonic activity in the east and west are the uplift and denudation of the peripheral mountain range, which makes the Meso–Cenozoic sedimentary near-source source material extremely abundant in the basin, and the sandstone, glutenite, and sandy mudstone deposits are very thick. There are not only abundant uranium sources but also vast storage spaces. The Ordos block and Ordos Basin are important geotectonic units of the North China block, which is famous for its long-term stable development and evolutionary history and richness in energy resources and minerals. The Ordos massif, Ordos Meso–Cenozoic basin, and Ordos area Meso–Cenozoic sedimentary range (the socalled Great Ordos Basin) have a high degree of spatial consistency, but they are also not completely consistent, especially in the northern and southern ends. The east– west boundary of the Ordos Basin marks the boundary of the Ordos block. On the north side, it is a Cenozoic basin with a long axis in nearly the east–west direction; on the south side, it is a Cenozoic basin with a long axis in the NEE direction. The Ordos block contains the main parts of the Ordos, Hetao, and Weihe basins, with the Ordos Basin occupying the main part of the Ordos block. The Ordos Basin is an important geological tectonic unit of the North China continental block, with a similar metamorphic basement and sedimentary cover as

2.1 Regional Geological Background

31

the North China continental block. During its geological evolution, the Ordos Basin in the Middle and Neoproterozoic Palaeozoic became part of the North China plate, belonging to the giant North China Craton sedimentary basin, especially during the Paleozoic period, which has a strong consistency with the surface sea to the general uplifting denudation and then to the land–sea intersedimentation. The independent Ordos Basin generally refers to Mesozoic basin sedimentation. The sedimentary range of the Mesozoic Ordos area also includes some small and medium basins around the basin. After the Cenozoic, the Ordos Basin was generally uplifted and the sedimentary thickness of the peripheral basin expanded greatly, already belonging to different tectonic units. In terms of its geological structure, the Ordos Basin is a large syncline-type sedimentary basin composed of Paleozoic rocks in the axial direction from the north and south, with a length of 640 km in the north and 400 km from the east to west. The syncline axis aligns westward, and the east and west wings are extremely asymmetric: The east wing is a gently dipping monocline with a width of >300 km; the west wing is composed of several fault–fold belts extending in the north–south direction to the east, with a width of 6000 m, Palaeozoic– Mesozoic clastic rocks, and all kinds of new gneisses are deposited in the basin. The basement of Precambrian crystalline metamorphic rock in the Ordos Basin is of asymmetric skip type. The carbonate rocks in the basin are mainly Cambrian and Ordovician; these are exposed only in the eastern and southern parts of the basin. They are exposed locally along the western margin of the basin by thrust faults and buried deeply in the north of the basin as a result of faulting. The top surface of Ordovician limestone has been denuded for a long time and fluctuates greatly. The buried depth in the center of the basin can reach >4000 m. The carbonate rocks in the area are dominated by limestone with dolomite. The total sedimentary thickness is >2000 m, and there are rich mineral resources such as natural gas and rock salt. The bottom surface of Carboniferous–Jurassic clastic rock (the top surface of Lower Paleozoic carbonate rock) is an unconformity denudation surface, and the basin shape is not symmetrical. This layer is mainly gently exposed in the eastern and southern parts of the basin, mostly in as strip-shaped outcrops steeply exposed in the west and deeply buried in the northern part of the basin as a result of fault subsidence. The buried depth at the bottom of the middle of the basin is >4000 m, This layer is mainly composed of sandstone and mudstone interbeds, with a total thickness of >3000 m, and contains rich mineral resources such as coal, oil, natural gas, coalbed methane, uranium, and bauxite. Cretaceous clastic rocks generally refer to the Baoan Group (formerly known as the Zhidan Group) located in the middle and western parts of the Ordos Basin,

Fig. 2.5 Section map of the main geological units in the Ordos Basin (according to data from the Ministry of Geology and Mineral Resources in Sanpu, 1979). a North–south section map, b East–west profiles. c plane schematic diagram of profile position

32 2 Background of the Ordos Basin

2.1 Regional Geological Background

33

forming a rectangular distribution area with a length of 600 km from north to south, a width of 300 km from east to west, and an area of 134,200 km2 . Lying east of the Cretaceous basin is a broad and gentle anticline; in the west, there is a steeply inclined wing damaged by a series of thrust faults. In the south, the basin inclines upward, and, in the north, the fault strike downward. The maximum buried depth of the Baoan Group is 1200–1500 m in the middle and western parts of the basin. The lithology is mainly thick sandstone (including gravel) and interbedded sand and mudstone, with poor mineral resources. The discontinuous deposits of the Cenozoic in the Ordos Basin are located on the top surface of the moderately undulating Middle Paleozoic strata and are dominated by Quaternary strata, and the Paleogene and Neogene strata are locally developed. The Quaternary system is dominated by aeolian sand (mainly distributed in the north) and loess (mainly distributed in the south). Roughly bounded by the Great Wall, the northwest surface is mostly covered by an aeolian sand layer with varying thickness and an alluvial lacustrine layer with a thickness of 40–120 m; the southeast surface is mostly covered with loess with a thickness of tens of meters to 200 m, and the Neogene Pliocene mudstone with a thickness of 10 m to tens of meters is usually developed under the loess layer. According to the seismic profile, the underlying strata of the Cenozoic of the Fen–Wei graben are characterized as old and both ends and young in the middle: The basement of the Cenozoic of the Weihe, Xinding, Datong, and Yanhuai basins is of Archaeozoic or Proterozoic–Paleozoic age, the basement of the Yuncheng basin is of Carboniferous–Permian age, and the basement of the Linfen and Taiyuan basins is of Triassic age (Wang 1995; Xing et al. 2005). Seismic exploration and drilling data indicate that the bottom of the Cenozoic Hetao fault basin in front of Mount Daqing is composed Lower Cretaceous strata, which are widely distributed. The basin is developed from Baotou to Hohhot, and its underlying stratigraphic age is unknown. The subsidence range of the new boundary of the Yinchuan graben reaches 7 km. At present, only Well Yincan 2 located in the Wuzhong area reveals that the Cenozoic strata lie directly over the Ordovician strata, and no other well has drilled through the Cenozoic strata. Well Yinshen 2 is located in the east–west trending tectonic unit at a latitude of 38°N. Therefore, the drilling results in the fault tectonic belt do not reflect the basement stratigraphic age of the Yinchuan graben (Zhao et al. 2010).

2.1.2.2

Main Fault Structure

The main structural aspects of the Ordos Basin include faults, wide and gentle folds, monoclines, and ring structures (to be introduced in the remote sensing part). Faults, in particular have large development scales and long activity times, exerting obvious control over the regional structural framework. According to the scale and spatial position of fault activity, faults can be divided into boundary faults and internal faults. According to their strike, they can be divided into three groups of main faults:

34

2 Background of the Ordos Basin

east–west, northeast, and north–south. The main fault features are described in the following. 1. East–west-trending faults The Zhengyiguan–Pianguan Fault starts from Zongbieli in Inner Mongolia in the west, passes through Zhengyiguan in the east, near Tiekesu Temple, proceeds south of the Etuoke and Yijinhuoluo banners, and then extends to Pianguan in the northeast direction, exhibiting an anti-“S” shape, which is generally distributed in the east– west direction. This fault is exposed near Tiekesumiao, which is roughly the southern boundary of the Yimeng uplift, with the Archean Helanshan Group on the north side and Lower Paleozoic limestone and Upper Paleozoic coal-bearing measures on the south side. The fault generally dips northward, with a dip angle of 70°–80°, and undergoes right-lateral translation. It may have been formed in the Neoarchean, and there was still some activity in the Mesozoic and Cenozoic to the west of Tiekesu Temple. The Weining–Lishi fault zone lies basically along a latitude of 38°N. Its distribution starts from Huianbao in the west, passes through Zhongning, Jingbian, Zizhou, and Suide, and still extends eastward to north of Lishi, with a length of ~300 km in the basin. The fault zone is located in (Zhongwei)–(Zhongning) Beishan, exhibiting a Paleozoic fold–thrust zone, and its main axis is composed of open and gentle arches or nose-shaped uplifts and associated low-order faults. The main arches are Wuzhen, Gaozhen–Zizhou, Xinjiagou, and Anbian–Dingbian. The two sides of the arch are basically symmetrical with an inclination angle of 60°–80°. Largescale faults include the Huangwan Fault, Gaoyuanshan Fault, Zhuanmiao–Tianjiacha Dault, Luopangyuan–Wucangbao Dault, and the Dingbian–Wuqi concealed fault. Faults are mostly tensile, while a few are compressive and compressive–shear. In the Cuiyaoping area, a compression fracture zone with a width of 50 m can be seen; it has fault distance of 30–40 m and a cross section dipping northward with an inclination angle of 78° (Zhang 1994). This tectonic belt is caused by a series of large-scale basement faults, which may have been formed in the Archean (Wang 1996), with weak activity in the late stage. The Guyuan–Hancheng Fault starts from Guyuan in the west, passes through Qingyang, Zhengning, Yijun, and Hancheng in the east to the west of Linfen, spreads nearly ENE, and may continue to extend eastward, but the magnetic field characteristics are not very clear. The fault is mainly based on a magnetic anomaly, and it is speculated that the later activity is weak. The Linyou–Tongguan fault zone starts from Qianyang in the west, passes through Linyou and Yongshou to Kouzhen and Tongguan in the east, and continues to extend eastward. In the Yongshou–Sanyuan section, the fault is directly exposed at the surface and inclines southward from several faults with an inclination angle of 55°– 70°. The compressive faults are formed by overlapping thrusts from south to north, which cut the Paleozoic strata, while the Mesozoic strata basically did not dislocate. The portion east of Kouzhen is hidden underground, and a linear structure can be seen on satellite images. The fault may have been formed in the Caledonian, and it was still active in the Mesozoic and Cenozoic.

2.1 Regional Geological Background

35

2. Northeast-trending faults The Datong–Huanxian Fault starts from southwest of Huanxian County and extends to the northeast of Datong via Jingbian, Hengshan, Yulin, and Shenmu. The fault extends in the northeastern direction and is basically hidden under the Phanerozoic caprock. It is clearly reflected on aeromagnetic isoline and Bouguer gravity anomaly maps, being most obvious on the aeromagnetic map, in which a large-scale aeromagnetic gradient zone with a maximum gradient of 50 nT/km can be observed. The southeast side of the fault exhibits a high positive anomaly of regional northeasttrending magnetic force, while the northwest side exhibits a low negative anomaly of regional northeast-trending magnetic force, which quickly transitions into a regional east–west-trending magnetic anomaly. This fault is the longest extending fault in the basin. After the aeromagnetic extension of 45 km, the characteristics are still very clear. Geological data show that this fault is one of the faults controlling basement lithology, with Mesozoic–Neoarchaic spreading in the east–west direction to the north of the fault and Paleoproterozoic spreading in the northeast direction to the south of the fault. Formed in the Neoarchean–Paleoproterozoic, it may be the northern boundary of the Proterozoic Shanxi–Shaanxi depression trough (Ma et al. 1979; Ren 1990). The fault exhibited weak activity in the Phanerozoic. The Qingyang–Jiaxian Fault starts from the warm water in Longxian County and extends northeast through Qingyang, Zhidan, Mizhi, Jiaxian, and other places to Zuoyun in northern Shanxi Province, extending 650 km in the area and striking 35°–40°, running parallel to the Fuxian–Lishi fault. The fault is basically covered by Phanerozoic caprock, and linear structures or linear anomalies can be seen on satellite images and aeromagnetic anomaly maps. It may have been mainly active in the Neoarchean and Paleoproterozoic. The Fuxian–Lishi Fault also extends in the northeast direction, starting from Xinxian County in Shanxi Province in the north and passing through Lishi and Fuxian County to Yongshou in the south. The fault zone extends 45 km. The fault formed in the Paleoproterozoic, and its activity was weak after that era. It may be the southeast boundary of the Proterozoic Shanxi–Shaanxi depression, which is the same as the Qingyang–Jiaxian Fault. The southwestwern part of the Lower Station–Fenxi Fault intersects with the Linyou–Tongguan Fault from the Lower Station, and the northeast reaches Fenxi from Heyang and Yumenkou. Geomorphologically, the fault zone is the boundary between the Loess Plateau and the Weihe Plain. It is exposed on the surface near Heyang with an inclination angle of 65°–80°. The fault distance is ~80–200 m. The aeromagnetic anomaly and gravity anomaly maps show a northeast-trending cascade zone. It may have been formed in the Proterozoic, was a thrust fault in the Mesozoic, and reversed into a normal fault in the Cenozoic. 3. North–south-trending faults The Yellow River and Qingtongxia–Guyuan faults are the western boundaries of the basin. The western margin of the Ordos Basin is the area with the strongest tectonic changes in the later period. After flattening, the western Ordos Basin continued to be

36

2 Background of the Ordos Basin

compressed and formed a foreland basin in the Early Cretaceous. At the end of the Paleogene, under the action of nearly east–west pressure, fault block mountains such as Liupan Mountain and Helan Mountain were formed. Fault depression occurred in the southern and northern sides of the basin, forming the Hetao and Weihe basins. In the Early Neogene, primarily under the action of north–south compressive stress, fault depressions occurred on the eastern and western sides of the Ordos Basin, forming the Fenhe and Yinchuan grabens. Regional geological data indicate that the region west of the Qingtongxia–Guyuan Fault belongs to the Liupanshan arc structural belt, which is obviously different from the Ordos Basin in terms of stratigraphic and tectonic evolution characteristics. The western boundary of the basin is composed of the Yellow River Fault and deep Qingtongxia–Guyuan Fault along the northern boundary of the Yinchuan Basin. The “western margin thrust belt” of the western tectonic unit of the basin as a whole exhibits a continuous residual gravity high anomaly zone and a low negative magnetic anomaly zone. If we take this as the boundary, the gravity anomaly field in Helan Mountain, Yinchuan Mountain, and Liupanshan Mountain on the west side changes strongly and the zoning is complex, and the magnetic anomaly field exhibits a large area with a relatively high magnetic anomaly area distributed regionally. In contrast, the gravity and magnetic anomaly fields in the Ordos Basin on the east side are relatively calm, with simple anomaly structures, and the main secondary anomaly zones are distributed regularly in the northeast direction. The Lishi Fault lies at the eastern margin of the Ordos Basin. This region is a complex tectonic–geomorphic boundary zone formed during the Yanshan movement. The Yanshan movement uplifted Luliang Mountain and pushed it westward, and, under the influence of basement faults, a north–south torsion fold belt in western Shanxi was formed. The eastern margin of the basin is adjacent to the Shanxi Fault uplift with the Lishi Fault as the boundary, which is the boundary fault between the Shanxi Fault uplift and the Ordos block in the central structural belt of the North China block. The strata cut off by this fault are from the former Great Wall system to the Triassic system.

2.1.2.3

Gravity and Magnetic Interpretation of Internal Faults in the Basin

Not only is the Ordos Basin an irregular rectangular fault block surrounded by faults, but numerous faults of different scales also exist in the basin. Consequently, observing and identifying these faults can be difficult because of insufficient surface exposure conditions. However, they have obvious gravity and magnetic characteristics. According to the comprehensive analysis and interpretation of aeromagnetic and gravity data, supplemented by other information, 130 (concealed) faults of different scales were interpreted in and around the basin (Fig. 2.6). Among them, in the northern part of the basin, the east–west faults are the principal faults, while in the central and southern parts of the basin, large-scale northeast-trending faults and their accompanying northwest-trending faults are developed. The north–south faults are

2.1 Regional Geological Background

37

located on the eastern and western sides of the basin and control the east–west boundary of the basin. Most of these hidden faults have no obvious geological display in the basin caprock. The aeromagnetic field data indicate that the northeast aeromagnetic anomaly in the Ordos Basin is the strongest, followed by the east–west aeromagnetic anomaly. The northeast-trending structure is controlled and influenced by the northwest-trending structure, which causes local migration and dislocation, exhibiting the characteristics of northeast belt formation and northwest block formation. The northwest-trending fault obviously formed later than the east– west-trending fault, which cuts the east–west-trending fault in the magnetic field and obviously blocks the continuity of the northeast-trending fault. It can be seen that the occurrence and development sequence of the main structures in the basin are east–west, and north-east–north-west.

2.1.3 Magmatic Activity and Metamorphism The sedimentary layers in the Ordos Basin are relatively stable and thick, covering essentiall the entire the surface. Magmatic rocks and metamorphic rocks are only distributed sporadically in local areas. The orogenic belt around the basin developed rock masses and metamorphic rocks of different ages. The outcrops and some boreholes around the basin indicate that there are a very small number of sporadically exposed volcanic rocks and volcanic sedimentary rocks of different ages in the Mesozoic and Cenozoic.

2.1.3.1

Magmatic Activity

The magmatic activity in the Ordos Basin and its surrounding area is characterized by multistage evolution. Archean, Proterozoic, Early Paleozoic, Late Paleozoic, and Mesozoic rock masses are developed. The magmatic activity of Luliang, Jinning, and the West Seas is the most intense, and Meso–Cenozoic magmatic activity is relatively weak, mainly comprising the Yanshanian intrusion. Some of the boreholes produced Yanshanian and Himalayan volcanic rocks, and it is inferred that there may be magmatic intrusions below the Mesozoic and Cenozoic caprocks in the basin. The magmatic rocks exposed in and around the Ordos Basin can be divided into southern and northern zones. The Yinshan distribution area in the north belt is dominated by Hercynian granite, followed by Luliang, Jinning, and Yanshanian granites. The Early Yanshanian granites are mainly exposed in the Qinling area of the southern belt, followed by Luliang, Jinning, Caledonian, and Haixi granites. In addition, the Archean and Proterozoic granites are sporadically exposed in the distribution areas of Luliang Mountain and Wutai Mountain in the eastern Ordos Basin, and the Early Paleozoic and Proterozoic granites are sporadically exposed in Helan Mountain and Qilian Mountain in the western and southwestern regions of the basin (Fig. 2.7).

38

2 Background of the Ordos Basin

Fig. 2.6 Gravity and magnetic comprehensive inference fault structure and basin boundary map

2.1 Regional Geological Background

39

Fig. 2.7 Distribution map of magmatic rocks in the Ordos Basin and its surrounding areas (revised based on the 1:1 million geological and mineral map of the North China Platform)

40

2 Background of the Ordos Basin

Magmatic activity cannot be easily observed in the basin because of the thick overburden. With strong ascending and descending motions, the local fault depression exhibits evidence of a small amount of continental volcanic eruptions. The first magmatic activity since the Mesozoic is reflected in the tholeiitic basalt of Rujigou in Helanshan in the north of the western margin of the basin. The basalt is stratiform in the Upper Triassic Yanchang Formation and Middle Jurassic Yan’an Formation. The total rock K–Ar dating isotopic age is 229 ± 15 Ma, placing is formation at roughly the end of the Indosinian (Zhao and Liu 2003). The tuff of the Fuxian Formation in the Lower Jurassic is the second oldest, with zircon U–Pb ages concentrated at 168.4–175.2 Ma. In addition, zircon U–Pb ages of 130.7 and 133.9 Ma were obtained in the Zijinshan rock mass in the middle of the eastern margin of the basin, and an age of 108 Ma was recorded in the Tongcheng rock mass along the southwestern margin of the basin. The regional tectonic environment of the basin indicates that the north–south margin was a strong collisional orogenic belt at the end of the Paleozoic to the beginning of the Mesozoic, while the Late Paleozoic–Triassic deposits in the basin are both relatively stable continental-platform-type deposits, which indicates that the magmatic activity in the basin and the margin have different properties and intensities. The magmatic rocks at the margin are generally formed by thrusting and nappe migration from the orogenic belt into the basin.

2.1.3.2

Metamorphism

Metamorphic rock series are developed around the Ordos Basin. The metamorphism mainly occurs in the peripheral orogenic belt and the old basement of the basin. By the end of the Paleoproterozoic, the cratonic age of the basement rocks in the basin represented the most extensive metamorphic activity in the basin. The oldest rocks around the basin are Mesoarchean metamorphic rocks, and the outcrops are mainly distributed in Huoxian, Zanhuang, Jining, Qianshui, and Jiehekou of Shanxi Province. The main lithology is granulite, hornblende black cloud granulite, pyroxene-bearing plagioclase amphibolite, plagioclase amphibolite, and magnetite quartzite, with metamorphism degrees up to granulite facies and high amphibolite facies. The outcrop distribution of the metamorphic rock series in the Neoproterozoic is consistent with that in the Middle Archean. The lithology mainly consists of biotite metamorphic granulite, plagioclase amphibolite, amphibolite granulite, garnet quartzite, mica schist, and magnetite quartzite. The Paleoproterozoic metamorphic rocks are composed mainly of low amphibolite facies, high greenschist facies slate, metamorphic sandstone, metamorphic andesite, variable basalt, marble, quartzite, and phyllite. The metamorphism changes from granulite facies in the northeast to high amphibolite facies, low amphibolite facies, and green schist facies.

2.1 Regional Geological Background

41

2.1.4 Tectonic–Sedimentary Evolution of the Ordos Basin The Ordos Basin is a typical Mesozoic basin developed on the North China continental block. Throughout the whole evolutionary history of the basin, the basement exhibited a double-layer structure. The lower part is cratonized crystalline basement, and the upper part is Palaeozoic-dominated stable surface–sea sedimentation, and the distribution of bilayer substrates has wide consistency. The range and basement of Mesozoic sedimentary basins differ greatly, mainly being Meso–Cenozoic continental strata. Such basin structures are often referred to as superimposed composite basins. However, sedimentary and uranium mineralization in the Ordos Basin occurred during the Mesozoic era, which has its own typical characteristics. Therefore, some experts contend that the Ordos Basin is a Mesozoic basin, and the basement level has no substantive significance. There are many Meso–Cenozoic basins in North China and in the area of the Tarim Basin. As a part of the North China continental block, the basement of the Ordos Basin has the general characteristics of the North China continental block and is also influenced by the formation and evolution of the paleocean basin around the basin, which has its own uniqueness and complexity. The tectonic–sedimentary evolution of the basin generally underwent multiperiod tectonic movements such as the Luliang, Jinning, Caledonian, Haixi, Indosinian, Yanshan, and Himalayan and developed multiple sets of sedimentary strata and fault systems, forming multiple tectonic layers. The lithostratigraphic characteristics, structural styles, and sedimentary construction types of each tectonic stratum are different. The basin development and evolution underwent six main stages: (1) an Archean–Paleoproterozoic basin crystalline basement formation stage; (2) a MesoNeoproterozoic rift stage, mainly developing sediments of aulacogens; (3) the Early Paleozoic continental surface sea deposition stage, mainly developing marine platform carbonate rock and clastic rock deposition; (4) a Late Paleozoic sea–land interaction basin stage, mainly developing sea–land cross-deposition; (5) a Mesozoic intracontinental basin stage, mainly developing fluvial facies, delta facies, and lacustrine facies deposition; and (6) a Cenozoic intracontinental fault depression stage, mainly developing fluvial facies, lacustrine facies deposition, and aeolian loess and desert. The pre-Permian evolution stage is the basement evolution stage of the Mesozoic basin, which primarily lays down the “internal static and external movement” structural pattern of the Ordos block and also forms the uranium-rich construction around the Ordos block. The evolution stage since the Triassic is the intracontinental basin evolution stage, which mainly controls the formation of uranium-bearing construction and the uranium mineralization process.

42

2.1.4.1

2 Background of the Ordos Basin

Formation of the Crystalline Basement in the Archean–Paleoproterozoic Basin

Archean–Paleoproterozoic strata form the basement of the North China Craton Basin. Because of the Qianxi, Fuping, Wutai, and Luliang tectonic movements, the basement rock system of the basin underwent intense metamorphism, mixed rock formation, and folding and thus formed a complex metamorphic rock system composed of granulite facies and green schist facies rocks. The North China continental mass eventually formed a stable craton after the Luliang movement (at ~1.8 billion years ago).

2.1.4.2

Sedimentary Stage of the Middle–Late Proterozoic Fracture Trough–Aural Trough

After the cratonization, the North China continental block remained very fragile, and the rift trough and the depression trough were widely developed. The southern margin of the basin developed three major aural grooves (Helan, Shanxi–Shaanxi, and Shanxi–Henan–Shaanxi), which deposited the Great Wall system littoral facies clastic rocks, Jixian system flint-bearing zonal algal dolomite, Qingbaikou system carbonate rocks, and marine clastic rocks. The northern margin of the basin mainly developed the Baiyun Obo rift system and Yanliao depression, with a depositional thickness of ~2000–10,000 m.

2.1.4.3

Early Paleozoic Continental and Marine Sedimentary Stage

The Middle Ordovician period of the Cambrian is the passive continental margin sedimentary stage. As a part of North China continental block, the basin is characterized by stable marine clastic rock and carbonate sedimentary interaction. The east–west Yinshan uplift was formed in the north of the basin, the north–south Jingbian saddle uplift was formed in the middle and west, and the Luliang uplift was formed in the east of the basin. The Early Ordovician period was an active continental margin stage. The Qinqi Ocean on the south side of the basin subducted to the north and the Xingmeng Ocean on the north side subducte to the south, and the north–south extrusion further intensified. Along with this, the two sides of the basin were transformed into an active continental margin, a trench–arc period basin system developed, the North China continental block was uplifted as a whole, seawater was withdrawn from the whole region, and some areas in the middle of the basin became extremely arid to form massive salt rock beds. With the strengthening of orogeny on both sides of the north and south, the basin entered the whole denudation stage.

2.1 Regional Geological Background

2.1.4.4

43

Late Paleozoic Sea–Land Interaction Basin Stage

After long-term weathering and denudation, the north–south orogeny also entered a relatively calm period, and the North China continental block entered the development stage of the sea–land interaction basin. The weathering surface residual deposits were transformed to neritic deposits. The Ordos massif underwent regional subsidence and began to accept Carboniferous–Permian deposition. Paleogeomorphism is high in the north and low in the south, with the Asian continent in the north and the Tethys Ocean and marginal basins in the south and the transgression direction running from south to north.

2.1.4.5

Mesozoic Intracontinental Basin Stage

At the end of the Paleozoic, the main body of the Chinese mainland was formed, and the Ordos Basin was basically located in the interior of the continental block, dominated by intracontinental basin deposition. Only the southwest direction is close to the Tethys Ocean (Qinqi Ocean), which is also a strong active region of Meso–Cenozoic plate subduction and collision orogeny. The long-range effect of the southwest subduction collision caused the continuous uplift of the mountains around the basin, and the Ordos Basin accepted a large amount of near-source clastic deposits. This sedimentation of the basin was coupled with the uplift and denudation of the mountains around the basin, resulting in the deposition of large, thick, and continuous Mesozoic strata. The Early Mesozoic sedimentary range covers the Ordos Basin and the surrounding area. Although the basin range gradually shrinks after the Yanshan movement, it basically covers the main part of the basin. The tectonic movement of the basin was dominated by horizontal ascending and descending movements, and tectonic and magmatic activity was relatively weak. At present, the crustal thickness is still relatively thick and the crust is stable. To the east of the Ordos Basin, the crustal thinning of the North China Craton during the Yanshanian period was stronger than that of the Ordos Basin. The Early and Middle Triassic Ordos region and the western part of North China exhibit some structural differentiation, which is basically the result of inherited Permian structural patterns and sedimentary characteristics, and the region has a stable continental basin sedimentary environment. From the view of stratum thickness and lithology, the sedimentary environment of the Middle Triassic strata varies greatly, and the local deposition thickens and a conglomerate interlayer appears. It is speculated that the north–south thrust belt began to form at this time. The Upper Triassic Yanchang Formation formed the Qianyuan depression with a thickness of ~3000 m and mainly conglomerate around the Helanshan mountains (Xiangchizi conglomerate) and Shigouyi (gravel). The western margin of the basin was restricted by the thrust fault at that time. The west wing of the depression is steep and the east wing is shallow. Its provenance mainly comes from the west side, and it rapidly tapers to the inside of the basin. The basin is characterized by a foreland basin. At the end of the Middle Triassic during the Indo-Chinese movement, the whole area was

44

2 Background of the Ordos Basin

rapidly and extensively uplifted and denudated, forming an unconformity surface in the shape of valleys and hills without complete leveling. At this time, the thrust belt of the western margin of the basin was in its initial form. The lower part of the Fuxian Formation of the Lower Jurassic and the Yan’an Formation of the Middle Jurassic were deposited under this background, and most of the Ordos Basin is river– lake facies and river facies. This process continued into the Middle Jurassic, and the basin kept settling, and the fluvial and lacustrine deposits filled the whole basin. The Early Triassic strata were deposted in a sedimentary environment with a dry, hot climate and undeveloped vegetation and are mainly composed of red fine clastic rocks of river–lake facies, which are mainly composed of sandstone and mudstone. In the Middle Triassic, red conglomerate and sand mudstone were deposited along the eastern margin of the basin, and gray-green mudstone was deposited in the middle, with local coal seams interbedded, and plants were flourishing. The second stage of the Indosinian movement occurred at the end of the Middle Triassic, resulting in the discontinuity of Middle and Late Triassic strata. The northern part of the basin was uplifted and the Late Triassic strata are missing, while the depression on the western margin continued to fall. In the Late Triassic, except in the north, gray-green mudstone was deposited in other areas, with local coal seams. The third stage of the Indosinian movement occurred at the end of the Late Triassic, and the basin was once again uplifted, resulting in denudation of some Upper Triassic strata. The Early and Middle Jurassic Ordos Basin strata comprise a set of continental sediments. In the Middle and Late Jurassic, only a set of >100-m-thick variegated continental detritus was deposited in the Yifuxian Formation south of the Zhungeer Banner. In the Middle Jurassic, the basin experienced a warm and humid subtropical climate environment with developed vegetation and a set of coal-bearing structures gradually thinning from west to east were deposited. The Late Jurassic climate was dry and hot with undeveloped vegetation. The sediments mainly comprise red fine clastic rocks of river and lake facies, which were mainly composed of sandstone and mudstone. The Ii curtain of the Yanshan movement occurred at the end of the Late Jurassic, which led to folding and fracturing of the Early and Middle Jurassic strata and brought about a denudation area. The fourth stage of the Yanshanian movement (155–140 Ma) is an important tectonic event affecting the eastern part of China. It is manifested in the strong tectonic action around the Ordos Basin, the obvious uplift of the Helanshan and Luliangshan mountains, and the north–south thrust and overthrust fault zones of nearly 600 km long formed along the western margin of the basin. This resulted in the unconformity of the angle between the Lower Cretaceous and Triassic strata and the Middle–Lower Jurassic strata along the western margin of the basin and the slight angle unconformity and pseudconformity between the Lower Cretaceous and the Middle Jurassic strata in the direction of the basin. During Early Cretaceous, the Ordos Basin started to shrink. Early Cretaceous sediments were accepted in most areas of the region. These early sediments were red clastic and aeolian sediments of river and lake facies, and the late sediments were lacustrine sand and argillaceous sediments. The sedimentary center was on the Linhe line in the north of the basin, and it was a skip basin extending in the north–south direction. The eastern part of the basin had retreated to the Dongsheng area. The basin began to shrink in the middle

2.1 Regional Geological Background

45

of the Early Cretaceous, and the sedimentary Dongsheng Formation was constructed by red coarse clastic deposition. In the late Early Cretaceous, the Ordos Basin was uplifted and the lake water withdrew. By the Late Cretaceous, the basin had become denuded. In the Mesozoic, the Ordos Basin was mainly controlled by the Tethys tectonic system, and the remote impact of collision orogeny caused uplifting of the mountains around the basin and eventually led to the revival and development of the Luliang uplift, Liupanshan thrust belt, and Yinshan tectonic belt. The Mesozoic Ordos Basin developed and evolved as an independent sedimentary basin, exhibiting obvious stages and cycles.

2.1.4.6

Cenozoic Basin Intracontinental Fault Depression Stage

During the Paleogene period, the basin was mainly constructed with gypsum–red sand argillaceous debris in river and lake facies. The sediments of the western Oligocene basin are widely distributed, mainly composed of a set of red gypsumbearing sediments. Neogene strata are not very developed in the basin. The Cenozoic is the period of violent change in the tectonic pattern in the west and the southwest and the Yunnan–Tibet blocks and Indian plates continue to drift northward and collide. The remote orogenic influence causes the mountains around the Ordos Basin and the basin itself to rise together. The formation of the Himalayan and Kunlun mountains in the southwest blocks the humid air flow in the southwest direction, and the climate of the Ordos Basin developed toward arid conditions. There are sandstone and gypsum deposits of different thickness in the basin. During the uplift of the Ordos Basin, a series of Cenozoic basins were formed on the outer edge of the Ordos Basin; these include the Weihe, Hexi, Yinchuan, and Fenhe fault systems on the southern, northern, western, and eastern sides, respectively.

2.1.5 Tectono–Thermal History of the Basin Since the Jurassic The Mesozoic tectonic evolutionary history of the basin is key to uranium formation and mineralization. Based on eight apatite fission trail samples from the Middle Triassic Ermaying Formation (T2e) to the Early Cretaceous Luohe Formation (K1l) in the northwest-trending section of the Yimeng uplift (Table 2.3), the results suggest that the Ordos Basin has experienced four tectonic uplift events since the Late Jurassic, during 150–125, 110–100, 100–80, and 50–23 Ma (Fig. 2.8).

4.139 (771)

2.341 (438)

35

30

35

T3y-2

T2e-1

J2y-1

2.835 (813)

3.345 (936)

11.945 (2049)

35

35

29

J3a-1

K3l-1

K1d-1

26.135 (4483)

9.126 (2554)

6.082 (1744)

10.577 (3380)

6.955 (1301)

10.028 (1868)

10.038 (3208)

6.17 (2095)

ρi (105 /cm2 ) (N i )

12.703 (7124)

13.541 (7124)

14.378 (7124)

15.215 (7124)

16.053 (7124)

16.681 (7124)

12.285 (7124)

13.122 (7124)

ρd (105 /cm2 ) (N)

0.4

93.2

5.7

12.0

83.5

6.7

15.4

79.2

P(χ2 ) (%)

110 ± 8 114 ± 6 136 ± 8 101 ± 6 118 ± 6

110 ± 8 114 ± 7 137 ± 8 101 ± 6 117 ± 7

124 ± 7 140 ± 9

125 ± 7 142 ± 10

127 ± 8

Pool age (Ma) (±1σ)

127 ± 8

Central age (Ma) (±1σ)

12.8 ± 1.4 (137)

12.5 ± 1.2 (110)

12.6 ± 2.1 (96)

13.0 ± 1.9 (116)

13.2 ± 1.6 (104)

12.7 ± 2.5 (107)

12.6 ± 2.6 (104)

12.6 ± 2.2 (105)

L (μm) (N)

Notes N s = number of spontaneous apatite fission tracks; ρS = spontaneous apatite fission track density; N i = number of induced apatite fission tracks; ρi = induced apatite fission track density; P( χ2) = χ2 inspection probability; age ± 1 σ = apatite fission track age ± standard deviation; L ± σ = mean apatite fission track length ± standard deviation; N—number of closed apatite fission tracks

3.915 (1251)

34

34

J2y2

J2z-1

4.985 (1593)

2.948 (1001)

Number of ecogniz (n)

Sample number

ρs (105 /cm2 ) (N s )

Table 2.3 Apatite fission track analysis results

46 2 Background of the Ordos Basin

2.1 Regional Geological Background

47

Fig. 2.8 Thermal history simulation diagram of apatite fission tracks in the Ordos Basin. PAZ: Apatite annealing band (70°–120°). K-S: Fitting value of simulated track length and actual track length of apatite. GOF: Simulated age of apatite. Green thermal history simulation curve: Acceptable thermal history simulation curve. Purple-red thermal history simulation curve: Better thermal history simulation curve. Black thermal history simulation curve: Best thermal history simulation curve. Light green columnar part: First stage uplift (150–125 Ma). Blue column: Second stage uplift (110– 100 Ma). Orange column: Third stage uplift (100–80 Ma). Yellow column: Fourth-order segmental uplift (50–23 Ma)

48

2.1.5.1

2 Background of the Ordos Basin

Time Frame for Uplift of the Western Margin of the Ordos Basin

The age of apatite fission traces in the retrogradational thrust tectonic zone of the Helan Mountains at the western margin of the Yimeng uplift has four major peaks [(116 ± 6)–(107.2 ± 5.8), (89 ± 7)–(71.7 ± 3.6), (58.3 ± 5)–(30.9 ± 3.5), and (11.4 ± 0.9)–(10.0 ± 1.4 Ma)] as ascertained by simulating apatite fission traces in the Ordos Basin in the east central part of the Yimeng uplift. The four cooling events obtained from fission trail simulations are 150–125, 110–100, 100–80, and 50–24 Ma. The Yimeng uplift and Helan Mountains together record three tectonic cooling events from 24 to 110 Ma. During this period, the Imeng uplift and the Helan Mountains retrogradational thrust belt were a unified sedimentary basin, and the Helan Mountains were the western margin of the paleo-Imeng uplift. As the western margin of the paleo-Imeng uplift, the Helan Mountains rebound thrust tectonic formation has three phases: 150–125, 110–76, and 50–24 Ma.

2.1.5.2

Time Frame for Uplift of the Eastern Edge of the Ordos Basin

The age of the apatite fission traces in the Luliang Mountains has four peaks: 138– 110, 90–70, 60–30, and 25 Ma. The Luliang Mountains and the Ordos Basin were deposited and uplifted synchronously during the Early Cretaceous and Cenozoic, and the uplift cooling accelerated since the Pliocene, separating the Luliang Mountains from the Ordos Basin. The northern and middle sections of the Luliang Mountains are older, and the age of the fission trails in the middle section is gradually decreasing. The northern and middle sections of the Luliang Mountains are presumed to have been influenced by the southwestward subduction of the Indian Ocean plate in stages, which is consistent with the southwestward iterative subduction of multiple blocks.

2.2 Characteristics of the Regional Geophysical Field 2.2.1 Physical Properties of Rock Strata The Ordos Basin is composed of various rocks and strata, which have different physical properties. Various anomalies can be formed, which reflect the spatial distribution and combination of various geological bodies. Physical parameters are the basis for the interpretation and analysis of these various anomalies.

2.2 Characteristics of the Regional Geophysical Field

2.2.1.1

49

Density and Magnetic Characteristics

1. Density and Magnetism of Sedimentary Rocks The density and magnetic parameters of sedimentary rocks in the Ordos Basin are listed in Tables 2.4 and 2.5, respectively. Cambrian and Ordovician These periods are dominated by coastal-phase carbonate rocks, represented by marl and dolomitic limestone. Their density is (2.7–2.72) × 103 kg/m3 , their magnetic susceptibility is (0–43) × 10−5 SI, and their residual magnetization is (0–127) × 10−3 A/m. Therefore, these rocks have a high density and are weakly magnetic. Carboniferous–Permian system This system lies at the intersection between sea and land and it is mainly composed of various clastic rocks. The Upper Carboniferous Taiyuan Formation and the Shanxi Formation are important coal strata in the basin. Mesozoic Jurassic and Cretaceous The Mesozoic Jurassic and Cretaceous rocks comprise a set of river and lake facies and continental clastic sedimentary rocks. They are represented by sandstone, glutenite, and tuff. The density of sandstone and glutenite is (1.89–2.33) × 103 kg/m3 , their magnetic susceptibility is (13–23) × 10−5 SI, and their residual magnetization is (9–23) × 10−3 A/m. These are low-density, weakly magnetic rocks. The local volcanic rock–sedimentary series tuff has a density of (2.5–2.74) × 103 kg/m3 , a magnetic susceptibility of 660 × 10−5 SI, and a residual magnetization of 2210 × 10−3 A/m, identifying it as a low-density, strongly magnetic rock. Cenozoic Paleogene and Neogene Widely distributed in plains and mountain basins, Cenozoic Paleogene and Neogene rocks were deposited by rivers and lakes. They are represented by sandstone, conglomerate, claystone, and loess, with densities of 2.53 × 103 kg/m3 , magnetic susceptibilities of (3–100) × 10−5 SI, and residual magnetizations values of (1– 30) × 10−3 A/m. For low-density, weakly magnetic rocks, Neogene clay and loess generally have weak magnetic properties. The above-mentioned sedimentary rocks are nonmagnetic or weakly magnetic rocks, and they all exhibit a stable negative magnetic field or an extremely insignificant positive magnetic field on aeromagnetic maps. 2. Magmatic rock density and magnetism A. Basic intrusive rocks The main lithologies of basic rocks are gabbro, pyroxene amphibolite, gabbro porphyrite, and amphibolite. The standard rock, gabbro, has a density of 2.91 × 103 kg/m3 , a magnetic susceptibility of (1992–4390) × 10−5 SI, and a residual magnetization of (290–4300) × 10−3 A/m. It is a high-density, strongly magnetic

1.91

2.37

2.519

2.33

2.48

2.58

2.725

2.71

2.7

2.53

394

1

18

6

361

197

1642

434

254

1217

221

1275

101

5

5

Loess

Calcareous subclay

Calcareous clay

Claystone

Conglomerate

Glutenite

Sandstone

Shale

Marl

Dolomite

Dolomitic limestone

Limestone

Mudstone

Quartzite

Slate

2.017

1.773

2.079

2.21

2.017

1.8

2.039

2.006

Minimum value

2.931

2.757

3.07

2.98

2.931

2.8

2.5

2.696

Maximum value

2

171

24

18

170

24

149

Number of samples

11.01

0.3

1.26

2.48

2.92

21.13

1.7

Minimum

25.99

7.47

77.9

18.7

22.92

31.9

12.59

Maximum

18.5

3.75

0

43.29

11.21

13.02

23.63

2.51

Average

0.74

0.38

0.2

0.28

0.2

0.54

0.39

Minimum

1.28

1.05

246.98

2.09

26.6

74.88

2.43

Maximum

1.01

0.9

0

127.59

1.65

9.46

23.13

0.88

Average

Magnetic susceptibility κ (10−5 SI) Remanence Jγ (10−3 A/m)

Magnetic properties

Note Statistical data are from “Report on Application Results of Gravity Data for Mineral Resources Potential Evaluation in North China” (provided by G. Zhao)

2.58

2.588

2.52

2.47

1.89

Average

Density (103 kg/m3 )

Number of samples

Rocks

Table 2.4 Statistical table of physical parameters of sedimentary rocks in the Ordos Basin

50 2 Background of the Ordos Basin

MEsozoic

52

Gray siltstone, basic volcanic rock

45

Volcanic clastic rock

Sandstone, mudstone

55

Gray-green sandstone

Jurassic

Triassic

77

Brown sandy shale glutenite and fine sandstone

2.56

2.58

2.45

2.33

Gray-purple 46 mudstone, brick-red conglomerate 47

2.45

55

Gray-green sand shale

2.25

43

2.23

2.11

2.05

Gray sandstone

Cretaceous

Paleogene

46

Brown–red clay

Neogene

50

Loess alluvium

Quaternary

NEozoic

2.59

2.41

2.2

2.05

2.3–2.50

2.0–2.35

2.12–2.32

2.11–2.19

1.95–2.17

Number Average Density Density of density stratification range 3 3 3 3 (10 kg/m ) (10 kg/m ) (103 kg/m3 ) density specimen blocks

Main rock lithology

Ground layer

Table 2.5 Magnetic parameters of the sedimentary strata in the Ordos Basin

40

1091

333

757

40

14

1210

160

3

60

120

Number Magnetic of susceptibility magnetic (10−5 SI) specimen blocks

Magnetic susceptibility Layered (10−5 SI)

12 (continued)

2750

540

1

6

60

Magnetic Residual susceptibility magnetization −5 Range (10 Jr (10−3 A/m) SI)

2.2 Characteristics of the Regional Geophysical Field 51

54

Cambrian

Limestone,

80 63

68

Limestone

Ordovician

Marl, clastic rock

Carboniferous

67

2.77

2.72

2.7

2.61

2.61

2.68

2.59–2.84

2.59–2.77

2.66–2.77

2.29–2.71

2.33–2.61

Number Average Density Density of density stratification range 3 3 3 3 density (10 kg/m ) (10 kg/m ) (103 kg/m3 ) specimen blocks

Cambrian-Ordovician Limestone, dolomite

Medium fine sandstone, black shale

Permian

PPaleozoic Upper Paleozoic

Lower Paleozoic

Main rock lithology

Ground layer

Table 2.5 (continued)

180

190

284

657

400

12.9

45

3

9

16

Number Magnetic of susceptibility magnetic (10−5 SI) specimen blocks

12.9

Magnetic susceptibility Layered (10−5 SI)

2–35

5

20

1

7

12

Magnetic Residual susceptibility magnetization −5 Range (10 Jr (10−3 A/m) SI)

52 2 Background of the Ordos Basin

2.2 Characteristics of the Regional Geophysical Field

53

rock. Mesozoic rocks have a large distribution range, with many basic veins, which often lead to areas of high-gravity and strong magnetic anomalies. B. Neutral intrusive rocks Neutral rocks are widely distributed around the Ordos Basin, both in the exposed area of bedrock and in the shallow cover area. The main lithology is diorite, pyroxene diorite, diorite porphyrite, and granodiorite. The standard rock, diorite, has a density of (2.62–2.72) × 103 kg/m3 , a magnetic susceptibility of (1067–2456) × 10−5 SI, and a residual magnetization of (372–1049) × 10−3 A/m. It is a medium-density, strongly magnetic rock. Its relatively large distribution range, high density, and strong magnetism often lead to large-scale local gravity and magnetic anomalies with high gravity and strong magnetism. C. Acidic intrusive rocks Acidic rocks around the Ordos Basin are widely distributed and have a large area. The main lithology is granite, granite porphyry, monogranite, monogranite porphyry, and Shi Ying monozonite. The granite and monogranite are the standards, and their density is (2.50–2.65) × 103 kg/m3 , their magnetic susceptibility is (183–633) × 10−5 SI, and their residual magnetization is (33–660) × 10−3 A/m. These rocks have relatively low-density and are weakly magnetic. Acidic rocks invaded the highdensity strata of the pre-Paleozoic are often distributed as rock bases, causing a large range of low-gravity anomalies and stable low-value positive and negative magnetic anomalies. From the above analysis, it can be seen that the density of intrusive rocks gradually decreases and their magnetism gradually decreases from ultrabasic to basic to acidic (alkali) rocks. This is primarily due to the gradual decrease in the content of ferromagnesian minerals and the gradual increase in the content of quartz and feldspar minerals. D. Volcanic rocks Volcanic rocks are widely distributed around the Ordos Basin and in the basin; they range from acidic to basic and from Proterozoic to Cenozoic in age. The main lithology is basalt, andesite, rhyolite, trachyte, tuff, and pyroclastic rock, with densities of (2.28–2.91) × 103 kg/m3 , magnetic susceptibilities of (200–3409) × 10−5 SI, and residual magnetization values of (302–4540) × 10−3 A/m. These are mediumdensity, strongly magnetic rocks with a wide range of density and magnetism. Most of them are distributed in volcanic basins. Volcanic–sedimentary rock series distributed in Mesozoic and Cenozoic volcanic basins often cause low-gravity and chaotic magnetic anomalies. Volcanic basins dominated by andesite and basalt can form abnormal areas with high gravity and strong growth. 3. Density and magnetism of metamorphic rocks The old metamorphic rock series of the Ordos block is widely developed, and the periphery of the basin is also exposed. It can be divided into three sets of rock series: the Middle Archean, the New Archean, and the Palaeoproterozoic metamorphic rock

54

2 Background of the Ordos Basin

series. These rock series are relatively difficult to sample because they are buried deep inside the basin, but they a certain influence on the regional background field.

2.2.1.2

Radioactivity and Electrical Characteristics

1. Radioactive characteristics of rocks From the statistical results of γ spectroscopy measurement data of samples in the Ordos Basin and adjacent areas (Table 2.6), it can be seen that the contents of radionuclides such as granite and diorite are generally high and obvious high-value fields are often formed; the radionuclide content of medium-based rocks is generally low, generally exhibiting obvious low-value fields; the radionuclide content of sedimentary rocks is mostly medium to low. 2. Electrical characteristics of rocks The electrical characteristics of rocks can be obtained from regional peripheral tests, and the data are presented in Table 2.7. Metal minerals generally have low resistivity and can exhibit different degrees of electromagnetic response to electrical measurement. The resistivity of medium-based intrusive rocks is low, especially that of medium-based volcanic rocks, such as andesite and altered andesite. Granite and Table 2.6 γ spectrum parameters of rock Lithology

Main road (Ur) Uranium (10−6 ) Thorium (10−6 ) Potassium (%)

Granite

38.23

4.15

25.08

4.90

Diorite

27.20

4.22

16.57

3.12

Gabbro

12.10

0.66

7.63

1.60

Ultrabasic rock

12.78

1.94

6.56

1.70

Tuff

37.43

1.60

17.06

6.18

Shi Ying schist

15.03

0.69

9.86

1.96

Marble

15.50

2.68

7.11

1.96

Limestone

10.45

1.27

4.85

1.39

Sandstone

13.73

3.29

5.02

1.65

Conglomerate

14.58

1.55

5.50

2.06

Mudstone

24.63

0.26

13.35

3.82

Quaternary undiagenetic 20.43 sediments

0.02

12.11

3.15

Note Data are from actual measurements by the China Aero Geophysical Survey and Remote Sensing Center for Land and Resources from 1993 to 2004 (provided by AGRS) * In the measurement of the total amount of uranium in geological exploration, the combined amount can be represented by Ur, where 1 Ur is the content of radioactive material when the count rate is equal to that of 1 μg/g of equivalent equilibrium uranium under the specified measurement conditions (China Nuclear Industry Standard EJ/T 584-2014)

2.2 Characteristics of the Regional Geophysical Field Table 2.7 Electrical parameters of rock

Name of rock

55 Resistivity ρ (Ω· m) Range of change

Common value

Copper nickel cobalt ore

1–320

25

Alteration mineralization zone

150–250

175

Peridotite

110–890

310

Diorite

91–169

130

Granite

149–189

167

Granite porphyry

104–192

148

Rhitic porphyry

96–149

118

Andesite

76–95

85

Altered andesite

54–83

63

Tuff

82–918

219

Breccia

57–110

84

Volcanic breccia

131–221

191

Shi Ying vein

153–211

170

Phyllite

313–440

377

Note Data are from the Geological Survey Institute of Inner Mongolia

most sedimentary rocks have high resistivity, which makes it difficult to obtain an obvious electromagnetic response. In actual measurement, the surface or shallow distribution of the saline-alkali gypsum layer and high-salinity water have very low resistivity. They generally produce a very obvious electromagnetic response; these interference factors often have a greater impact on the extraction of electromagnetic anomalies, but these anomalies are generally easier to identify.

2.2.1.3

Characteristics of the Physical Parameters of the Cenozoic Strata in the Borehole in the Basin

The strata exposed by boreholes in the Ordos Basin are mainly Mesozoic to Cenozoic in age. Through the statistics of logging parameters of 113 boreholes in the basin, the γ, resistivity, density, and other parameter characteristics of different horizons and different lithologies in the Ordos Basin can be summarized. 1. Formation radioactivity characteristics From the horizon point of view, the γ background value of the lower segment of the Zhiluo group is the highest, with an average of 4.1 nC/(kg·h), followed by the Yan’an Group, with an average γ background value of 3.5 nC/(kg·h). The average γ background value of the upper segment of the Zhiluo Group is 3.32 nC/(kg·h); the

56

2 Background of the Ordos Basin

average γ background value of the Anding Group is 3.28 nC/(kg·h). In contrast, the γ background value of the Luohe Group is low, with an average of 2.68 nC/(kg·h), indicating that the lower section of the Zhiro Group has a greater capability of providing secondary uranium sources. In addition, the logging γ background values of different lithologies vary greatly. The γ background values of coarse sandstone range from 0.1 to 0.85 nC/(kg·h), with an average of 2.618 nC/(kg·h); those of medium sandstone range from 0.3 to 12.42 nC/(kg·h), with an average of 3.09 nC/(kg·h), while those of fine sandstone ranges from 0.3 to 10.38 nC/(kg·h), with an average of 3.23 nC/(kg·h). The γ background value of siltstone ranges from 0.3 to 8.64 nC/(kg·h), with an average of 3.32 nC/(kg·h). Mudstone has the highest γ background, with values ranging from 0.3 to 16.71 nC/(kg·h), with an average of 3.58 nC/(kg·h). It can be seen that, the finer the particle size, the higher the γ background value (Fig. 2.9), indicating that the background value of uranium in the formation is chiefly related to the adsorption of fine-grained rocks. 2. Formation resistivity characteristics On the whole, the resistivity still exhibits a decreasing trend with the decrease of particle size (Fig. 2.10, left). There are large differences in logging resistivity values among different groups (Fig. 2.10, right), which is primarily affected by the difference in lithology within different groups. Among them, the Lower Cretaceous Luohe Formation is mainly composed of medium and coarse sandstone with large overall grain size and high resistivity. The Middle Jurassic Anding Formation is mainly composed of mudstone and has relatively low resistivity. 3. Formation density characteristics

Fig. 2.9 (Left) Comparison of γ background values of each segment in the Ordos Basin and (right) comparison of γ background values of different lithologies

2.2 Characteristics of the Regional Geophysical Field

57

Fig. 2.10 (Left) Resistivity comparison of different lithologies in the Ordos Basin and (right) resistivity comparison of each group

The overall rock density has a negative correlation with rock grain size, which increases with the decrease of grain size (Fig. 2.11, left). However, in the same area, under the condition of different horizons and the same lithology, the density parameter does not change much, indicating that the compaction effect is not obvious and that the density is related only to the rock composition (Fig. 2.11, right).

Fig. 2.11 (Left) Comparison of different lithological densities in the Ordos Basin and (right) comparison of stratigraphic density in each group (right)

58

2 Background of the Ordos Basin

Conglomerate is characterized by high resistivity and high density. Because most of it exists in the Lower Cretaceous, mud, sand, and gravel are mixed and the conglomerate exhibits a large range of property values, making it impossible to gather parameter statistics.

2.2.2 Characteristics of the Regional Gravitational Field A low-gravity anomaly in the Ordos Basin lies along the western Daxing’anling– Taihang Mountain gravity gradient zone. The general tendency of the Bouguer anomaly is to increase from west to east, reflecting the sedimentary characteristics of caprock in the Ordos Basin. There is a negative correlation between crustal attenuation and gravity anomalies. The gravity anomaly lineations fall into three groups of banded distributions in the east–west, south–north, and northeast directions (Fig. 2.12). The gravity anomalies in the middle and southwest are distributed near the north– south direction and are related to the corresponding geological structure or sedimentary thickness. The gravity anomaly in the middle of the basin changes gently, which reflects the relatively stable tectonic movement in the basin after the formation of the crystalline basement. In the southeastern part of the basin, there are many gravity anomaly highvalue traps, most of which are caused by tilling and tilted fault blocks in the late reconstruction. There are gravity anomaly traps in the northwest (Yinchuan–Shizuishan area) with high and low values, corresponding to the uneven lifting caused by the Yinchuan graben and later reconstruction. The gravity anomaly field in the northeast is a dustpan distribution opening to the northeast, corresponding to the northeasterly structure in this area and reflecting the deep inherited structure in the northeasterly direction.

2.2.3 Characteristics of the Regional Magnetic Field The aeromagnetic anomaly reduced to the pole in the Ordos Basin (Fig. 2.13) shows that the Ordos Basin is bounded by three regional magnetic anomaly strips dominated by positive anomalies. To the north is a belt dominated by positive anomalies along the northern margin of the North China landmass. To the southwest of the Qinling– Qilian–Kunlun orogenic belt is a belt dominated by positive anomalies. The central tectonic belt of the North China landmass is also dominated by positive anomalies in the east. The interior of the basin with three limits is dominated by a negative anomaly. Therefore, the structural framework reflected by the aeromagnetic anomaly is consistent with the gravity anomaly. The aeromagnetic anomaly field in the basin is also clear. There is an east–west-trending negative magnetic anomaly zone to the

2.2 Characteristics of the Regional Geophysical Field

59

Fig. 2.12 Bouguer gravity anomaly map of the Ordos Basin (according to Changqing Oilfield data)

east and west in the northern part. The scope is basically the same as the east–westtrending Yimeng uplift. The south-central part is the high-intensity band anomaly trending northeast. From northwest to southeast, there are four northeast abnormal zones, alternating from negative to positive. The anomaly distribution is characterized by alternating positive and negative anomalies. Both scale and intensity of the NEtrending positive anomaly zone in the central part are relatively large, exceeding the basin boundary in the NE direction and extending to the Shuozhou, Datong, and Zhangjiakou areas. Compared with the gravity anomaly, the annular anomaly on the edge of the basin in the aeromagnetic anomaly map reduced to the pole also shows to a certain

60

2 Background of the Ordos Basin

Fig. 2.13 Aeromagnetic anomaly map reduced to pole in the Ordos Basin (according to the Regional Geophysical Survey Results Integration and Methodology Research Project Internal Data Compilation of Tianjin Geological Survey Center (2012–2015)

degree, but it is not very obvious. The regional structural unit features exhibited in the vertical first-derivative aeromagnetic anomaly reduced-to-pole map (Fig. 2.14) are more obvious. The zonal features of the northern margin of the North China landmass, the Qinqi–Kun orogenic belt, and the central tectonic belt of the North China landmass are more obvious. Like the gravity anomaly, the vertical first-derivative anomaly also exhibits an obvious annular anomaly around the edge of the basin. Compared with the anomaly reduced to the pole, the first-derivative anomaly in the northern direction in the south-middle of the Ordos Basin and the east–west-trending anomaly in the north are also more obvious. In the south-central basin and the east– west anomaly in the north basin, the first-derivative anomaly in the vertical direction is more recognizable. The high consistency of the gravity and magnetic information indicates that the origin of the anomaly has a considerable degree of homology. Except for volcanic rocks, the magnetic susceptibility of sedimentary rocks in the

2.2 Characteristics of the Regional Geophysical Field

61

Ordos Basin is generally very low, and so can be regarded as weakly magnetic and nonmagnetic. Therefore, the regional magnetic anomaly field in the Ordos Basin primarily reflects the spatial distribution characteristics of the crystalline basement of the Ordos block. In other words, the aeromagnetic anomaly in the Ordos Basin is a relatively low (negative) annular anomaly in the positive anomalous zone in the northern margin of the North China landmass, the Qinqi–Kun normal anomalous zone, and the central North China landmass. The internal local anomaly is a north–south banding anomaly in the west, the northern anomaly is an east–west anomaly, and the south-central one is a northeast anomaly. The distribution of internal local anomalies is relatively consistent with the gravity anomaly. From these observations, we infer that the old metamorphic rock series outcrop along the northern margin of the Ordos Basin is consistent with the deep metamorphic

Fig. 2.14 Vertical first-derivative map of aeromagnetic anomalies in the Ordos Basin (according to the Regional Geophysical Survey Results Integration and Methodology Research Project Internal Data Compilation of Tianjin Geological Survey Center (2012–2015)

62

2 Background of the Ordos Basin

Fig. 2.15 Energy spectrum anomaly diagram of the airborne radioactivity measurement main channel for the Ordos Basin

rock series. Therefore, the study of their magnetic field characteristics is the key to investigating the old basement stripe extending in the northeast direction of the basin. Meanwhile, the northern part of the basin is also a uranium concentration area, and their sources are directly related to the old basement. Two distinctive east–west anomalous bands appear in pairs in the aeromagnetic anomaly map at the northern

2.2 Characteristics of the Regional Geophysical Field

63

Fig. 2.16 Aeroradiometric U content spectrum anomaly and zoning diagram for the Ordos Basin

margin. The northern side is the east–west strong magnetic anomaly zone and the southern side is the east–west low negative anomaly zone. The northern strong magnetic anomaly zone is located in the Etoke Banner, Yijin Horo Banner, Qingshui River, and Linxian County in the Kelan area. The magnetic field exhibits a wide normal anomaly band with an abnormal strength of 10–600 nT. After reduction to the pole and upward continuation to different elevations for

64

2 Background of the Ordos Basin

the aeromagnetic anomaly field, this positive magnetic anomaly zone can still be observed. According to regional geological data, the basement of the North China landmass is composed of the Middle–Upper Archeozoic and Paleoproterozoic crystalline rock series. This set of strata is exposed along the Wula Mountains in northern basin and the Liangshan region in the east. In the Wula Mountain region, it is known as the Wula Mountain Group and, in the Luliang Mountain region, it is known as the Hutuo Group. The Wulashan Group is mainly composed of granulite, gneiss, amphibolite, and amphibolite gneiss sandwiched with magnetite quartzite. It can be seen that this set of strata is hypometamorphic and has abundant dark-colored minerals. It is ferromagnetic, containing magnetite, and can often cause strong normal magnetic anomaly lineations (zones). The Wula Mountain Group distributed in Baotou and the Wulat Qianqi area corresponds well to the normal anomaly zone. The intensity, shape, and trend of the anomaly zones can be compared completely. Therefore, it is generally regarded that the wide and gently rising normal anomaly zones in the basin should reflect the ferromagnetic bedrock of the Taigu Yu Wulashan Group. In the southern low negative magnetic anomaly zone, the Dalat Banner along the northern margin of the basin and Yinchuan area in the west of the basin, the Wushen Banner area in the middle and east of the basin, and the Shenmu, and Fugu areas in the northeast are characterized by gently changing negative anomaly areas. The strength varies from −100 to 0 nT in general, being up to −200 nT in some areas. After reduction to the pole and upward continuation to different elevations for the aeromagnetic anomaly field, the wide and gentle negative magnetic anomaly zone can still be observed. Along the western margin of the basin, Helan Mountain and Zhuozi Mountain form the Archean Qianlishan Group, whose lithology is mainly composed of gneiss, biotite plagioclase gneiss, granulite, and migmatite. This formation is weakly magnetic. The Seltenshan Group in the Paleoproterozoic (whose lithology is mainly migmatitic gneiss, migmatite, and schist) is also a weakly magnetic stratum with a susceptibility on the order of only 10−3 SI. Therefore, the negative anomaly region of gentle change is a reflection of the Archean–Paleoproterozoic Qianlishan Group and Selten Mountain Group. This formation constitutes the weak magnetic basement of the region. The formation is thick, exhibits weak magnetism, and has a wide distribution range. The basement of the Ordos Basin is composed of a Mesoto Neoarchean and Paleoproterozoic crystalline complex, which exhibits different magnetic field characteristics because of the different lithologies and lithofacies.

2.2.4 Radiological Anomalies 2.2.4.1

Abnormal Energy Spectrum Characteristics of Regional Aerial Radioactivity Surveys

In the anomalous energy spectrum diagram from the airborne radioactivity survey in the Ordos area (unpublished report provided by G. Zhang), the energy spectrum

2.2 Characteristics of the Regional Geophysical Field

65

anomaly of the general road in this area exhibits an anomaly pattern of being high in the south and low in the north. Along the line of Yanchi, Yulin, and Dian Tower, it can be divided into two north– south regions: the high anomaly area of the energy spectrum of the Xifeng–Tongzhen Road in the south (I) and the low anomaly area of the energy spectrum of the Etuoke Banner–Dongsheng Road in the north (II). The high anomaly area can be divided into three zones: the Xia Maguan–Zhenyuan low value zone (I1), the Wuqi–Huachi high value zone (I2), and the Suide–Tongzhen low value zone (I3), along two nearly south–north lines (i.e., the Xutuan Town–Huan County–Xifeng line and the Shiwan Town–Yanhewan Town-Fu County line). From west to east, the form is two lows and one high, in which the abnormal trend is generally northwest with a small number of northeast anomalies. Corresponding closely with the Mesozoic trend and spatial location exposed on the surface, there are many higher peaks in Xutuan Town, Anbian Town, Wuqi County, Huachi County, etc. Among them, for Wuqi–Huachi (I2), the background value of the total channel energy spectrum count rate in the abnormal area is ~2360 counts per second (cps), and the lower limit of the abnormality is generally >2460 cps. In Huachi, Wuqi, and Huan counties and other places, the peak value is >2700 cps, and the anomaly is more continuous, forming strips with short axes, and the strike is generally northwest. The abnormalities in the other two abnormal areas I1 and I3 are relatively scattered and unevenly distributed. Their abnormal values are lower than those in abnormal area I2. Along the east–west lines of Etuoke Banner, Yijinhuoluo Banner, Hazhen, and Dengkou County and north of Dongsheng, the low anomaly area can be divided into two communities (II1 and II2). The energy spectrum anomaly in the north is higher than that in the south. The energy spectrum anomaly of the total road in the northern Wuhai–Dongsheng (II1) area is dominated by low and gentle anomalies, with a background value of ~2000–2100 cps, and the anomaly trend is mostly northwestward, but a small area near the north–south direction appears around the Wuhai area in the west. The east–west energy spectrum anomaly appears near the Dongsheng area in the east, and the position of the anomaly corresponds to the outcrop of the Zhiluo Formation on the surface. This indicates that the Zhiluo Formation has a high radioactive background, and there are uranium deposits such as Zaohuohao and Abuhai around it. The Zhiluo Formation may provide a certain uranium source for the enrichment of uranium deposits in these areas. By combining modern geomorphic features and remote sensing information, the total energy spectrum anomalies can be divided into two categories: a regional background (trend) and a local anomaly. The regional background is high in the south and low in the north. The anomalous northwest zoning is highly consistent with the modern surface dune distribution area and sand dune strike zone. The trend of the high-background area and anomalous belts on the south side is highly correlated with the Mesozoic strata outcropping area and the distribution of mountains and valleys. The real significance of metallogenic research should be the local anomalies that are not closely related to the trend. In the northwestern and northeastern parts of the basin, the anomaly is relatively strong and may provide an important uranium

66

2 Background of the Ordos Basin

source for the enrichment of regional uranium deposits, which is related to the high southern background value.

2.2.4.2

Abnormal Characteristics of Uranium Content in Regional Aerial Radioactivity Surveys

The uranium content anomaly measured by aerial radioactivity surveys in the Ordos Basin indicates an anomaly that is high in the south and a low in the north as a whole, which is similar to the overall shape of the total channel energy spectrum anomaly map. The two areas are the Xifeng–Tongzhen high-uranium-content anomaly area (I) in the south and the Etuoke Banner–Dongsheng low-uranium-content anomaly area (II) in the north. The North Etuoke Banner–Dongsheng low-uranium-content anomaly area can also be divided into two communities along the east–west line of the Etuoke Banner, Yijinhuoluo Banner, Hazhen, Dengkou County, and the Dongsheng Region. In the Hangjinqi–Dongsheng Region (II2), the uranium content value is mainly high and there is a medium anomaly. The background value is 1.8 ppm. The anomalous strikes are mostly northwest, which is consistent with the shape of the Cretaceous outcrop. The lithology of the Cretaceous outcrop is purple-red, gray-white mudstone, sandstone, glutenite, and marl. The strong adsorption of mud and sandstone facilitates uranium enrichment, which provides the original uranium source for the enrichment of uranium deposits in this area. Compared with the obvious correspondence between the background characteristics of the main energy spectrum and modern landforms and sand dunes, the distribution range and strip direction of uranium anomalies in the northern low-background area are also clearly consistent with the distribution and direction of modern sand dunes. In the southern high-background area, the distribution range is equivalent to the outcropping area of Mesozoic sandstone. However, the direction of the anomaly is not inconsistent with the distribution of mountains and valleys. It is filtered out, so the local uranium anomaly is relative to the general channel anomaly, which has a stronger indicative significance for uranium prospecting. The South Xifeng–Tongzhen high-uranium-content anomaly area is similar to the total road energy spectrum anomaly zone, and it can be divided into three zones. The western zone boundary is also divided along Xutuan Town, Huanxian, and Xifeng. The eastern division boundary shifts slightly to the east. It is divided along the nearly north–south line of Tongzhen, Mizhi County, and Laojundian Town. It is in the form of two lows and one high from west to east. The abnormal trend is generally northwestward. There are some northeast and north–south anomalies, which are closely related to the Jurassic and Cretaceous strikes and spatial locations exposed on the surface. The background value of uranium content in the central Wuqi County–Huachi County area is relatively high, ~2.6 ppm, and the lower limit of the anomaly is generally >2.8 ppm. In the Xifeng District, Xutuan Town, Anbian Town, Zhouwan Town, and other places, the peak values are all >4.1 ppm. In this work, we use an

2.2 Characteristics of the Regional Geophysical Field

67

anomalous value of >3 cps as the lower limit to delineate the distribution range of anomalies. The relatively high value anomaly tends to be generally northwestward, in a beaded shape, but the distribution pattern is not very obvious. The distribution pattern of the overall and modern river channels is relatively consistent.

2.2.5 Comprehensive Interpretation of Regional Geophysical Fields 2.2.5.1

Inference and Interpretation of the Ordos Basin Fault and Basin Boundary

We delimit the current basin boundary according to the deep fault development shown by the gravity data. The northern boundary fault is the Dengkou–Tokto fault zone, the western boundary comprises the Yellow River Fault and the Qingtongxia–Guyuan deep fault, the southern boundary is the Weihe Basin northern boundary fault, and the eastern boundary is the Lishi Fault. Some Cenozoic faulted basins are developed between the Ordos Basin and the peripheral orogenic belts. The Hetao Basin lies at the northern margin, the Yinchuan graben lies along the northern segment of the western margin, the Weihe graben lies along the southern margin, and the Shanxi graben lies along the eastern margin. Orogeny plays an important role in restricting basin evolution.

2.2.5.2

Local Structure Inferred by Combining Geophysical Anomalies

The Mesozoic uplift in the central part of the Ordos Basin is generally northeastern trending, the southern and northern uplift is all east–west trending, and the western part is generally north–south trending, which is controlled by the basin boundary fault (Fig. 2.17). According to the analysis of physical property data and the characteristics of sedimentary formation scale, the Bouguer gravity anomaly in the Ordos Basin mainly reflects the density difference between the Lower Paleozoic tectonic layer and its overlying sedimentary cover. The aeromagnetic anomaly mainly reflects the distribution characteristics of the magnetic basement and volcanic rocks (Li and Gao 2010). Based on the regional geological background and the available gravity and magnetic data, the distribution of the Mesozoic uplift in the region can be deduced and interpreted comprehensively. Mesozoic uplift in the northeast direction in the central basin is parallel and causes a series of high-gravity anomaly belts, which provide important clues for searching for uranium deposits in the basin. For example, previous seismic-data-based research has suggested that the fold in the Dongsheng uplift has a short-axis nose and fornix structure (Teng 2008). The faults are more developed and the northeast normal faults are the main ones. If the velocity contour line of 5.6 km/s is roughly regarded as the top surface of the ancient

68

2 Background of the Ordos Basin

Fig. 2.17 Mesozoic micro-uplift distribution map inferred from gravity and magnet data (according to the Regional Geophysical Survey Results Integration and Methodology Research Project Internal Data Compilation of Tianjin Geological Survey Center (2012–2015)

2.3 Geology Inferred from Remote Sensing

69

Fig. 2.18 Seismic Pg wave tomography of the upper crustal fault in the Yinshan orogenic belt and Ordos Basin (revised based on Teng et al. 2008). The solid line is the undulating surface of the ancient crystalline basement

crystalline basement (Fig. 2.18), the buried depth of the top surface of the basement in the Ordos block is basically distributed within a depth range of 4–6 km. The Dongsheng uplift is relatively shallow (i.e., the southern part is close to 5 km), and depth of the northern edge is uplifted to ~4 km. The velocity structure of the upper crust of the Ordos block, the Yinshan orogenic belt, and the Inner Mongolia fold zone are obviously different. The upper crust of the northern Ordos block is noticeably affected by the Yinshan orogenic activity, and the influence gradually weakens from the northern margin to the south. The compression in the orogenic process leads to uplift of the northern block basement, reactivation of faults, and tectonic deformation of the sedimentary formation.

2.3 Geology Inferred from Remote Sensing 2.3.1 Overall Image Characteristics of the Ordos Basin and Its Periphery Vegetation is not developed in most areas of the Ordos Basin, and remote sensing images can clearly reveal the distribution law of large-scale geological bodies. The Ordos Basin has an obvious rectangular block image, surrounded by the Yellow River on the east, west, and north and by the Fenwei rift zone on the south. Except for the northwest and southeast corners of the basin, the overall tone and texture are uniform. Fine interpretation reveals that the whole block can be divided into northern and southern blocks, which are bounded by a northeast-trending fault. Surface desertification in the north is serious, and desert textures such as sand chains and sand

70

2 Background of the Ordos Basin

dunes are developed. The water system is undeveloped, consisting primarily of a long, straight northwest water system, with residual lakes distributed in a northwest string and well-developed northeast and northwest structures. In the south, except for the southeast corner, the surface vegetation is not developed, but the overall vegetation is denser than that in the north, the texture is rough, and small branched water systems are widely developed. The northern block of the Ordos Basin is adjacent to the Hetao Plain and Yinshan block from west to east. There is a northwestern sand belt in the middle. To the north of the sand belt is the Yellow River. The numerous water systems and their inflection points distributed in parallel from the northeast indicate that there are at least two parallel fault structures on its northern boundary. There are two obvious double-Vshaped structural blocks in the northeast, and the central axis of the structural block is crossed by a northeast-trending fault. The formation of these two blocks should be closely related to the northeast compression of the Ordos block. In fact, the larger scale remote sensing images clearly show that, because of the compression stress from northeast and southeast, the whole North China continent exhibits signs of escaping eastward, which is clearly shown in the western section of the Xilamulun River forest.

2.3.2 Main Linear Structural Features The linear structure in the remote sensing image of the Ordos Basin is very obvious. According to the spatial location of the structure, it can be roughly divided into a basin edge fault, an intrabasin fault, and a ring structure.

2.3.2.1

Basin Margin Fault Structure

There are a series of faults on the edge of the ring of the Ordos Basin. According to the spatial combination characteristics of its fault structures, these ring Ordos faults include the western edge of the western Ordos, the southern edge of the northern Hetao fault basin, the eastern Lishi fault zone and the southern Ordos fault zone In addition, the Daqingshan Fault in the north and the Datong Shuozhou Fault in the northeast are also present. They form an important fault structure in the Ordos Basin (Fig. 2.19). 1. Southern margin fault of the Hetao Basin (F5 ) The southern boundary fault between the Hetao and Ordos basins starts from the riverside area in the west, passes through Hetao Town to Lamawan, and strikes northwest. There are great differences in the arcs and waves of the linear image and in the hue and shadow pattern between the two sides of the structural line. The north side is green, and the geomorphic unit is the Hetao Plain. The south side is brownish

2.3 Geology Inferred from Remote Sensing

71

Fig. 2.19 Structural interpretation map of the Ordos Basin and its periphery (Thematic Mapper image) F5 : Fault along the southern margin of the Hetao Basin. F6 : Lishi fault zone. F8 : Qingtongxia– Luoshan–Haodian fault zone. F12 : Northern edge fault of the Weihe Fault Depression. F13 : Tianshui– Xi’an–Sanmenxia Fault. F14 : North Qilian fault zone (Youfanggou–Huanghe Fault zone) platform fracture. F33 : North foot of the Xiangshan and Miboshan–Taoshan–ShiXiakou east fault zone. F34 : Lijun–Nokou–Jingyuan (Liupanshan) Fault. F36 : Goose feather mouth fracture

yellow and brownish red, and the geomorphic unit is a flat plateau. According to the interpretation marks, it is a fault structure. According to the landform, geology, and image characteristics, the fault is inclined to the north, and the Hetao Plain on the north side is a descending wall, which is a normal fault. 2. Lishi fault zone (F6 ) The Lishi fault zone is located in the eastern portion of the basin, which is consistent with the Lishi fault zone on the geological map and the eastern boundary of the Loess Plateau. The fracture is in southern Baode County, Shanxi Province, with clear image signs, but in northern Baode County, the image signs are not obvious. The fault runs through the west of Shanxi in a north–south direction. The >400-km-long fault starts from Liujiata in Hequ at the boundary with Inner Mongolia in the north and passes through Jiaoloushen in Xingxian County, the Chengjiata turbulent head in Linxian County, east of Liulinzhai, Shilou Jiebangou, and the west side of the Zijing in Xi County to Jinjiachuan in Linfen. On the remote sensing image, the linear trace of the fault is very obvious, and there are obvious differences in hue, shadow patterns, and

72

2 Background of the Ordos Basin

landforms on both sides. The west side of the fault is a Quaternary loess landform, and the east side is scarp formed by typical Cambrian strata. 3. Northern margin fault zone of the Weihe Fault Depression (F12 ) The northern margin fault zone is the boundary fault between the southern margin of the Ordos Basin and the Weihe Fault Depression. It is distributed in the front of the mountain on the North Bank of the Weihe River, ends at the Lijun Nikou Jingyuan (Liupanshan) Fault on the west, and enters the Linfen Basin on the east, with a total length of 570 km. The fault generally forms an arc protruding to the southeast, and some parts are often broken line or arc linear images. The fault image is clear and the marks are obvious, and the image, geomorphic, and geological marks are relatively consistent. 4. Western Ordos fault zone (F9 ) The fault zone along the western margin of the Ordos consists of several faults: the Helan western margin fault, the Dengkou Wuda Fault, the table eastern margin fault in Inner Mongolia, and the Majiatan Tianshuibao and Niushoushan–Guyuan faults in Ningxia. This fault zone is called the Helan Mountain meridional structural system in the geological records of the Ningxia Hui Autonomous Region (1990). It is composed of some nearly north–south and NNE branch faults. It starts from the eastern edge of Inner Mongolia in the north and passes through the Yinchuan Wuzhong Basin, Majiatan, Qinglongshan, and Pengyang in the south to the south of Pingliang in Gansu, with a length of 500 km from north to south and a width of ~40–100 km from east to west. It constitutes the boundary between the Mesozoic Ordos Basin and the Alxa block. It is also the second boundary of different structures, strata, and geomorphic landscapes from east to west in China (the first boundary being the fault at the eastern foot of Taihang), and it is also an important modern seismic activity zone. 5. Knob structure Three northwest faults—the Lijun–Nokou–Jingyuan (Liupanshan) Fault, the Xiangshan–north foot of Miboshan–Taoshan–east of Shi Xiakou fault, and the Qingtongxia–Luoshan–Haodian fault—constitute an important structural type of knob structure in the southwestern Ordos Basin. From the macroscopic image, the three faults form a group of structures that open radially in the northwest and converge in the southeast. After convergence, they are tangent to the circular structure of the Loess Plateau in Northern Shaanxi in Guyuan and Huating, and they end at the Weihe Fault (F12 ) in the south. The circular structure of the Loess Plateau in Northern Shaanxi in remote sensing images may indicate the existence of a more stable giant continental core in the southern Ordos block. With the collision between the Indian and Eurasian plates, under the continuous action of tectonic stress from the southwest, the three faults can only cut in at the western edge of the Ordos giant block and gradually converge close to the western edge of the block to form a knob structure. According to the structural stress analysis, the knob structure has the characteristics of leftward walking and sliding.

2.3 Geology Inferred from Remote Sensing

73

• Lijun Nokou Jingyuan (Liupanshan) Fault (F34 ) The structural direction of the Liupanshan Fault is nearly north–south-northwest. It is a fault on the southwestern edge of the Ordos Basin that is distributed from Baoji to the north through Jingyuan County and Gulang Town. The southern part ends at F13 , strikes northwest–NNW, and tends to southwest or northeast. The northwestern section is a normal fault, and the middle and southern sections are reverse faults. The landforms and geology on both sides are vastly different. The Qilian Mountains southwest of the fault are intermittently distributed with Cretaceous, Mesoproterozoic, Silurian, and Carboniferous Permian strata, and the Tengger Desert in the northeast is mostly Paleogene, Neogene, and Quaternary strata. Geological data indicate a left-handed slip property, and the fault is still active in the Quaternary period. The linear image has clear marks, and the hue and shadow patterns vary greatly between the two sides of the structure, indicating different geomorphic forms. • North foot of Xiangshan and Miboshan Taoshan Shi Xiakou eastern fault zone (F33 ) The fault is clear, extending northwest from Guyuan through Wufengtai, basically forming the southwestern boundary of Tengger Desert. The amount of sandy land southwest of the fault is greatly reduced, and the amount of the green land is increased, and the bedrock is gradually exposed. The fault strikes northwest, with a linear image, different colors and shadow lines on both sides, and great lithological differences. According to the geological data, the Cambrian Xujiaquan Formation is exposed southwest of the fault in Weizhou Town. The lithology is gray conglomerate, breccia, sandstone splint, and microcrystalline limestone. The northeast side consists Paleogene, Neogene, and Quaternary accumulation and piedmont alluvial fan accumulation landform. The fault dips southwest with a left-handed strike-slip. • Qingtongxia–Luoshan–Haodian fault zone (F8 ) The fault strikes northwest, extends northward from Guyuan to the northwest through Wanglejing, breaks the Helan Mountain, forms a northwest gap, crosses the desert, and pierces the southern part of the Ejina Banner in the Gobi Desert. The fault runs linearly, with the different landforms on the sides being connected, and bifurcates the Tengger Desert. The desert landform on the southwestern side is obvious, with more sand and large accumulation thickness. Although the northeastern side is desert landform, mountains and lakes are exposed. Jilantai Town is located in a lake depression. The fault controls the desert landform and Quaternary accumulation. The fault signs in the desert-covered area indicate that the fault is still active in the Quaternary period.

74

2 Background of the Ordos Basin

2.3 Geology Inferred from Remote Sensing

2.3.2.2

75

Fault Structure in the Basin

The main directions of the structural line are northwest, NNE, WNW, and northeast. The structural lines are dense, with many in the north, very few in the middle, and generally few in the south (Fig. 2.20). The northwestern fault is mainly distributed north of Jingbian, Yan’an, in the central southern region and Linyou County in the southwest. A series of northwesttrending structures are developed north of Jingbian, especially in the Mu Us Desert. The northwestern structures are not far away from each other and are approximately equally spaced, being 18–38 km apart. The strike is basically consistent with the fault direction of the southern margin of the Hetao Basin, which is obviously affected by the northern regional tectonic belt. The most important and significant faults are the southern margin fault (F5 ), the Pojianghaizi Fault (F21 ), and the Erlin rabbit fault (F10 ) of the Hetao Basin. There are five groups of northwestern structures in the Yan’an area, with a minimum separation of 17 km and a maximum separation of 45 km. Linyou County in southwestern China is also a WNW structural development zone. The structure is affected by the fault on the northern edge of the Weihe River, and the structural direction is basically the same. However, there are few WNW structures here, and the scale is small. The NNE fault is mainly distributed in the eastern and western sides of the basin. There are also NNE-trending structures in the northern Maowusu Desert. The main faults are the Etokeqi Fault (F7 ) and the Luoyukou Yanshuiguan Fault (F11 ). The former is located in the western part of the basin and the latter is located in the eastern part of the basin. They have the same strike and similar characteristics with basin controlling faults. The WNW fault is mainly distributed in the Maowusu sag in the middle of the basin and at the edge of the Weibei uplift in the south. The northwest-trending faults in the Maowusu sag are equally spaced, intersect with the NNE and northwest faults,

⯇Fig. 2.20 Structural interpretation of the Ordos Basin: F1-1 : Western edge fault of the Langshan Fault block. F1-2 : Eastern edge fault of the Langshan Fault block. F2 : Narincili Fault. F3 : Chengcaigou–Dazhaogou structure. F4 : Daqingshan–Yudao Mouth fracture. F5 : Fault along the southern margin of the Hetao Basin. F6 : Lishi fault zone. F7 : Otok Banner Fault. F8 : Qingtongxia–Luoshan– Haodian fault zone. F9-1 : Deep fault along the western margin of the Helanshan Fault block. F9-2 : Deep fault along the eastern edge of the Helanshan Fault block. F10 : Erlin rabbit fracture. F11 : Luoyukou–Yanshuiguan Fault. F12 : Northern edge fault of the Weihe Fault Depression valley. F13 : Tianshui–Xi’an–Sanmenxia Fault. F14 : North Qilian fault zone (Youfanggou–Huangtai Fault). F15 : Southern margin fault of the North China block. F16 : Early Paleozoic suture zone. F17 : Shangnan– Shangcheng fault zone (Late Paleozoic suture zone). F18 : Cooperative Dangchang–Liangdang deep fault. F19 : Jiudianliang Fault. F20 : Fengzhen–Shanyang Fault. F21 : Pojiang–Haizi Fault. F22 : Yulin Fault. F23 : Jingbian Fault. F24 : Yan’an Fault. F25 : Linzhen Fault. F26 : Yichuan Fault. F27 : Yuele Fault. F28 : Laocheng Fault. F29 : Xunyi Fault. F30: Ankou fracture. F31 : Huanghaocha Fault. F32 : Jingyuan Fault. F33 : North foot of the Xiangshan and Miboshan–Taoshan–ShiXiakou east fault zone. F34-1 : Lijun–Nokou–Jingyuan (Liupanshan) Fault. F34-2 : Nanhua Mountain–Xihua Mountain north foot fault. F35 : Yuerhong–Baiquanmen plate junction zone. F36 : Shuozhou Fault. F37 : Ulan Mulun River Fault. F38 : Shijinqi Fault. F39 : Wushenqi Fault. F40 : Red pier boundary fault. F41 : Balasu fracture. F42 : Chahe Fault. F43 : Gaojiapu Fault

76

2 Background of the Ordos Basin

and form a lattice structure pattern and fault block structures of different sizes. The northwest fault is usually the edge fault of the fault block in the south, which controls the distribution of the Weibei uplift. The northeastern fault is generally undeveloped, being mainly distributed in Yulin, Yan’an, and Linyou counties. The more important structures are the Yulin Fault (F22 ) and the Jingbian Fault (F23 ).

2.3.2.3

Ring Structure

There are 65 ring structures interpreted in the whole region (Fig. 2.21). According to the origin of the ring structure, they can be preliminarily divided into intrusive rock ring structures, color anomaly ring structures, and concealed structure ring structures. There are 62 concealed structure ring structures, 2 color anomaly ring structures, and only 1 intrusive rock ring structure. The ring structures are mostly elliptical circular in shape and usually circular. They combine as single rings, multiple rings, composite rings, and other forms or exist alone, intersect, or are tangential. Their size varies, ranging from a few kilometers to >100 km in diameter. The smallest annular structure recognizable in regional remote sensing images is only 2 km in diameter, and the largest annular structure is 380 km in diameter. The structural scale is mostly medium and large. The ring structures are mainly distributed around the edge of the Ordos Basin, and their direction is very focused. There are three ring structure belts and two giant ring structures in the Ordos Basin, and two ring structure belts and one ring structure group are developed outside the basin. The circular structures in the basin include the Etuoqi, Lishi, Dongsheng, and Tongchuan circular structure belts and the giant circular structure in the Loess Plateau of Northern Shaanxi. The Jilantai ring structure belt, Huashan complex ring structure belt, and Tianshui complex ring structure group are developed along the outer edge of the Ordos Basin. Because sandstone-type uranium deposits are primarily produced in the basin, the main ring structural features in the basin are briefly introduced. 1. Etuoke Banner ring structure belt The Etuoke Banner ring structure belt is distributed in the Wuhai Yuwangzhen area in the western Ordos Basin. The tectonic environment entails the southern end of Tianhuan sag crossing the Yuerhong Baiquanmen plate suture zone and entering the edge of Qilian Mountain orogenic belt. The circular structure of the Etuoke Banner is mainly composed of seven circular structures (H1 –H7 ) connected from head to tail, and their formation is principally related to the depression structure. 2. Lishi ring structure zone Located in the east of the Ordos Basin, the Lishi ring structure zone is distributed along the eastern edge of the basin and strikes NNE. The zone is controlled by the Lishi Fault and distributed along the Lishi fault zone from north to south. There are concealed ring structures and Mesozoic intrusive rock ring structures. The ring

2.3 Geology Inferred from Remote Sensing

77

Fig. 2.21 Interpretation of the circular structure in the Ordos Basin (1:100,000 TM742 false color composite film). 1: Intrusive rock ring structure. 2: Color anomaly ring structure. 3: Concealed ring structure

78

2 Background of the Ordos Basin

structure is dominated by Precambrian metamorphic rocks and surrounded by Paleozoic strata. Some ring structures coincide with folds and monoclinic structures, and most of them are folds. For example, the H24 ring structure is an anticline structure. Concentric circular shadow lines or east–west transverse lines with ring structure development characteristics are more conspicuous. 3. Tongchuan ring structure belt The Tongchuan ring structure belt is located in the Tongchuan area in the southeast of the Ordos Basin and at the southeastern end of the giant annular structure in the Loess Plateau of Northern Shaanxi. It strikes northeast and is consistent with the fault structural line at the northern edge of the Weihe Fault Depression. The northeastern end of the Tongchuan annular structural belt is connected with the southern end of the Lishi annular structural belt, which is composed of six concealed structural annular structures (H16 –H20 and H33 ). The tectonic environment is the Yishan Slope. It is inferred that the development of the formation may be related to tectonic uplift. 4. Dongsheng ring structure belt A number of ring structures are arranged in turn to form a northwest-trending ring structure belt. The Dongsheng annular structural belt is located at the junction of the Yimeng uplift and the Northern Shaanxi Slope. The southeastern end of the structural belt meets the Lishi annular structural belt. The H23 annular structure at the intersection is the intersection of the Dongsheng annular structural belt and the Lishi annular structural belt. The annular structural belt is cut by many northwesttrending faults such as F21 . It is controlled by the Yimeng uplift, Northern Shaanxi Slope, and Western Shanxi flexure fold belt. The Dongsheng ring structure belt is an important structure for uranium mineralization. The Yijinholoqi ring structure (H13 ) and Jungar ring structure (H14 ) are related to uranium deposits. A. Yijinholo Banner ring structure The ring structure (H13 ) of the Yijinholo Banner is composed of concentric circular water systems and has fingerprint texture and clear image marks. As shown in Fig. 2.22, the ring structure is elliptical, the axis is NNE, and the axis is ~50 km long. The ring structure is cut by a northwest fault, and the southeastern end is incomplete. However, there is no doubt about the ring structure form. B. Ring structure of Xinjie Town The Xinjie Town ring structure is located at the junction of the Yimeng uplift and the Northern Shaanxi Slope belt. It is composed of gray powder, brown-yellow-green, and gray-pink rocks forming the ring center and brown-yellow-green rocks forming the outer edge of the ring, as shown in Fig. 2.23. Its water system is well developed. According to the image, the annular structure is low-lying in the ring and uplifted at the ring edge. It looks like a shallow dish annular structure and has the geomorphic characteristics of the basin.

2.3 Geology Inferred from Remote Sensing

79

Fig. 2.22 Ring structure of the Yijinholo Banner

Fig. 2.23 Ring structure of Xinjie Town

5. Giant ring structure of the Loess Plateau in Northern Shaanxi The giant ring structure (H79 ) of the Loess Plateau in Northern Shaanxi presents a very clear giant ring structure in the TM satellite image of the southern Ordos Basin. The annular image at the edge of the annular structure shows an arc ridge, an arc water system, and an arc fault. The structure is nearly circular and has a diameter of

80

2 Background of the Ordos Basin

380 km. The northern, western, southern, and eastern boundaries of the ring structure are clear, but the northeast boundary is fuzzy. The northern and western boundaries of the ring edge of the ring structure have perfect arc images. The northern boundary lies just at the junction of the Mu Us Desert and the Loess Plateau, and the boundary is a northward protruding arc. The ring is pink gray and brown gray and locally green, and the color tone is uneven, indicating the heterogeneous nature of the block. The principal circular structure of the Loess Plateau in Northern Shaanxi is the landform of the Loess Plateau, and a primarily dendritic water system is developed. The bedrock is exposed at the valley bottom and mainly consists of Triassic, Jurassic, and Cretaceous sedimentary strata. At the southeastern end, there is a protruding green patch, which is trapezoidal in shape, which is perhaps a structural uplift (the Weibei uplift). The light green depression in the southwest is suspected to be a depression structure (Fig. 2.24) (the Jingchuan depression). The circular structure of the Loess Plateau in Northern Shaanxi has long been described in the geoscience literature and has been given different names. According to the morphological characteristics of remote sensing images, the Chinese Academy of Geological Sciences (1981) proposed naming it the Shaanxi Gansu Ningxia ring,

Fig. 2.24 Giant ring structure of the Loess Plateau in Northern Shaanxi (tm742)

2.3 Geology Inferred from Remote Sensing

81

which Zhu et al. (1994) called the central ring structure of the Loess Plateau. Meanwhile, Li and Shen (2001) called it the south Ordos ring structure and discussed the movement characteristics of the arc fault zone in the west and the morphology of the ring structure. The earth heat flow values in Huachi, Chenghao, and Liuwan within the ring structure are 82–91 mW/m2 (Zhang et al. 1987). Ma (1986) concluded that the annular structure corresponds to the uplift of the upper mantle. Through a comparative study of the interpretations and geophysical exploration results, we consider the annular structure to be not only an uplift structure but also the result of regional northeast compressive stress. At the same time, it may also reflect the existence of a giant block in the deep part of the southern Ordos Basin.

2.3.3 Remotely Sensed Geological Characteristics of Typical Deposits The Zaohuohao, Daying, and Ningdong uranium deposits in the west and Huangling uranium deposit in the southeast found in the Yimeng uplift in the northern Ordos Basin were selected as the research objects to study the remotely sensed characteristics of typical deposits and to analyze and construct remote sensing prospecting indicators.

2.3.3.1

Zaohuohao Uranium Mine

Located in the southern Dongsheng District, the Zaohuohao deposit occurs at the junction of the southern edge of the Yimeng uplift and the Northern Shaanxi Slope. The boundary fault of the two structural units passes through the south side of the deposit, which is a typical ring line intersection ore control. According to TM image analysis, the deposit has a circular structure denoted by purple spots. There are obvious tonal differences in the image. The inner ring is formed by multiple green color blocks surrounding the ore occurrence. The color of the outer ring is purple gray and dark, forming a dark color outer ring, as shown in Fig. 2.25. The annular structure is nearly circular, with a diameter of 150 km. Faults are developed in the mining area, and the main structural directions are northwest and northeast. The image mark of the northwest fault is a long and straight linear image with the characteristics of a compressive structural plane. The ring structure is controlled by the Luoyukou Yanshuiguan Fault (F11 ), the southern edge fault of the Hetao Fault Depression basin (F5 ), the Erlin rabbit fault (F10 ), and F2 . The Luoyukou Yanshuiguan Fault (F11 ) controls the eastern space of the ring structure, the southern margin fault (F5 ) of the Hetao Basin controls the northern space of the ring structure, and the Erlin rabbit fault (F10 ) controls the southern space of the ring structure. The Pojiang Haizi Fault (F21 ) crosses the middle of the annular structure and intersects with the Ulan Mulun River Fault (F37 ). The uranium deposit occurs

82

2 Background of the Ordos Basin

Fig. 2.25 Remote sensing images of (left) the Zaohuohao uranium deposit with (right) characteristics of linear ring structure highlighted. 1: Main interpreted normal faults and their numbers. 2. Main faults with unknown interpretation nature and number. 3. Annular structure. 4. Ore occurrence

at the intersection of F37 and F21 , which is closely related to the ring structure and fault structure. Moreover, in addition to line and ring structures, the occurrence of orebodies seems to be closely related to ancient rivers.

2.3.3.2

Daying Uranium Mine

The Daying deposit is located on the Yimeng uplift of the Ordos block. The TM image (Fig. 2.26) reveals a triangular circular ring structure in the mining area. The ring structure is composed of inner and outer rings with obvious color differences. The outer ring is composed of an arc water system, and the inner ring is light purple gray with a disordered and rough texture. The two known ore spots in the mining area are present in the color inner ring. According to the image, in addition to the ring structure, the fault structure is also the principal structural type in the mining area. The fault structure in the mining area is developed, and the structural directions are NWW, NNE, and NNW. The main faults in three directions are the southern edge fault of the Hetao Fault depression basin (F5 ), the Etokeqi Fault (F7 ), and the Pojianghaizi Fault (F21 ) and the parallel fault F2 , respectively. The southern margin fault (F5 ) of the Hetao Basin is the boundary fault between the Hetao Basin and the Ordos block, which controls the northern space of the annular structure. The Etuokeqi Fault (F7 ) is the boundary fault between the Yimeng uplift and the Tianhuan depression of the Ordos block, which controls the western space of the annular structure. The Pojiang Haizi Fault (F21 ) is the boundary fault between the Yimeng uplift and the Northern Shaanxi Slope, and fault F2 is the eastern boundary. According to the image, the

2.3 Geology Inferred from Remote Sensing

83

Fig. 2.26 Images of the Daying uranium deposit. 1: Main interpreted normal faults and their numbers. 2: Interpretation of normal fault and its number. 3: Faults with unknown interpretation properties. 4: Annular structure. 5: Ore occurrence

fault cuts off the southern edge of the inner ring. It is inferred that the Pojianghaizi fault (F21 ) controls the southern space of the annular structure. The occurrence of the deposit is also closely related to the line and ring. The tone of the inner ring related to mineralization is obviously shallow. Whether it is fading alteration caused by oil and gas remains to be further verified by other geological work. Similar to the Dongsheng uranium deposit, the occurrence of the orebody seems to be controlled by the ancient river channel in addition to the line and ring structure. The orebody generally occurs in the sandbodies of braided rivers. The plane projection of the orebody indicates that it is closely related to the distribution of modern river channels (Fig. 2.27). Under the weak tectonic background, there may be an obvious inheritance relationship between the modern river channel and the Zhiluo river channel.

2.3.3.3

Ningdong Uranium Deposit

The Ningdong uranium deposit is located in Majiatan Town, Lingwu County, Ningxia. The structural unit is the Tianhuan subsag of the Ordos block in the North China block. The structural form of the Ningdong uranium deposit is complex, and four blocks of different sizes cut by linear structures or faults are distributed from north to south. These blocks are elliptical in shape and have obvious ring structure characteristics.

84

2 Background of the Ordos Basin

Fig. 2.27 Uranium deposits and Quaternary channels along the northeastern margin of the Ordos Basin

The Ningdong uranium deposit is located in the ring structure formed by the southernmost block (Fig. 2.28), which is clamped between NNE and NNW faults. The annular structure is nearly circular, with a diameter of 8.5 km, making it a medium-sized composite annular structure. Three secondary elliptical ring structures are developed in the ring, whose axes are aligned orth–south, northeast, and northwest, and their scales are very similar. The Ningdong uranium deposit is closely related to and controlled by the northwest-trending structure. The two ore occurrences are located on the northwesttrending fault zone. The image of the ore control fault is clear, striking northwest. The image shows a brown linear structure, which is wide and stable and is cut off from the annular structure.

2.3.3.4

Huangling Uranium Deposit

The Huangling uranium deposit is located in Diantou Town, Huangling County, Shaanxi Province. The structural unit is the Weibei uplift of the Ordos block in the North China continental block. The Huangling uranium deposit is located in the middle of the Weibei ring structure in the inner ring of the Loess Plateau ring structure in Northern Shaanxi. The structure is the Weibei uplift, and the periphery of the Weibei uplift is also controlled by faults. Faults are developed in the Weibei uplift, and the structural direction is divided into three groups: northwest, northwest, and northeast. The principal ring structure of the Loess Plateau in Northern Shaanxi is the landform of the Loess

2.4 Geochemical Characteristics of the Basin Periphery

85

Fig. 2.28 Remotely sensed geological interpretation of the Ningdong uranium deposit. 1: Main interpretation of normal faults. 2: Faults of unknown general nature. 3: Annular structure. 4: Ore occurrence

Plateau, and the bedrock is exposed at the valley bottom. It comprises mainly Triassic, Jurassic, and Cretaceous sedimentary strata. The image of the mining area is green and circular and the area is surrounded by faults, resembling a tadpole with a wide head. The Huangling uranium mine lies at the head of the tadpole and at the intersection of the faults (Fig. 2.29).

2.4 Geochemical Characteristics of the Basin Periphery So far, no systematic geochemical survey has ever been conducted in the Ordos Basin, reflecting a significant blank area of geochemical investigation. We have collected data from the 1:200,000 stream sediment survey and some 1:250,000 multipurpose regional geochemical survey data of soil media in the peripheral area of the basin. Based on these data, the distribution characteristics of geochemical elements along the periphery of the Ordos Basin were studied.

86

2 Background of the Ordos Basin

Fig. 2.29 Remotely sensed geological interpretation of the Huangling uranium deposit. 1: Large fault. 2: Medium fault. 3: Small fault. 4: Annular structure. 5: Ore occurrence

2.4.1 Regional Distribution of Elements The statistical results from the geochemical data collected in the study area have revealed the regional geochemical distribution of elements in the area with the parameters given in Table 2.8. Compared with the distribution of elements in arid desert areas of China, except for the relative depletion of CaO, K2 O, Sr, and Na2 O, other elements all exhibit enrichment to some extent. High-temperature and lowtemperature ore-forming elements, such as Hg, Sb, Cd, Au, As, Bi, W, and Sn, generally have high contents. Rare earth and scattered elements such as B, Li, Nb, Zr, Th, La, Be, U, Y, and Ba are relatively enriched. The degree of element enrichment and dispersion is expressed by the concentration Clark value (K1 ). When calculating the concentration Clark value, the abundance data of the stream sediments in the arid desert area of China from Ren et al. (1998) were used as the reference standard. One can see that the enrichment of Hg is the highest in the region, being a factor of 372 greater than the abundance value of stream sediments in the arid desert region of China. CaO, K2 O, Sr, and Na2 O are the most depleted, being ~70% of the abundance values of stream sediments in the arid desert area of China. The elements in this group demonstrate strong depletion along the southern margin of the Ordos Basin owing to the difference of climatic conditions between the north and the south but exhibit high

2.4 Geochemical Characteristics of the Basin Periphery

87

Table 2.8 Geochemical distribution parameters of elements in the Ordos region Element

Mean [% (major elements), × 10−6 (minor elements)]

50.84 × 10−9

Standard deviation

Coefficient of variation

Concentration Clark value (K1 )

Abundance of stream sediments in arid desert areas of China (Ren 1996)

669.15

13.16

3.72

Sb

1.34

14.30

10.70

2.73

0.490

B

50.05

29.71

0.59

1.98

25.290

Cd

197.04

602.29

3.06

1.86

106.000

Li

33.58

17.04

0.51

1.81

18.600

14.16

6.68

1.72

1.230

Hg

Au

2.12 × 10−9

13.670

Pb

24.29

47.55

1.96

1.69

14.380

Ni

28.48

23.67

0.83

1.68

16.970

W

1.87

14.03

7.51

1.64

1.140 39.620

Cr

64.45

209.22

3.25

1.63

Nb

15.43

8.31

0.54

1.59

9.680

Bi

0.34

0.74

2.15

1.57

0.220

As

11.32

14.73

1.30

1.55

7.290

Zr

230.25

92.39

0.40

1.51

152.800

Th

11.67

6.11

0.52

1.48

7.860

Sn

2.68

1.85

0.69

1.42

1.890

La

36.26

26.12

0.72

1.41

25.740

F

546.87

439.02

0.80

1.39

394.660

Ag

78.42

178.60

2.28

1.35

58.000

Ti

3948.24

1717.84

0.44

1.35

2931.460

Zn

69.49

75.94

1.09

1.34

51.780

Co

11.93

5.42

0.45

1.33

9.000

V

83.59

46.87

0.56

1.30

64.110

Be

2.07

0.85

0.41

1.29

1.610

U

2.33

16.89

7.26

1.26

1.850

Mo

1.08

5.62

5.18

1.20

0.900

Cu

24.48

27.07

1.11

1.18

20.670

4.32

1.58

0.37

1.16

3.730

24.36

6.90

0.28

1.14

21.350

Fe2 O3 Y Al2 O3 P MgO

12.39

2.55

0.21

1.11

11.160

639.81

443.71

0.69

1.08

594.710

1.81

1.10

0.61

1.07

1.690 (continued)

88

2 Background of the Ordos Basin

Table 2.8 (continued) Element

Mean [% (major elements), × 10−6 (minor elements)]

Standard deviation

Coefficient of variation

Concentration Clark value (K1 )

Abundance of stream sediments in arid desert areas of China (Ren 1996)

Ba

596.03

506.92

0.85

1.05

568.580

Mn

655.97

353.53

0.54

1.05

627.060

SiO2

63.71

10.63

0.17

1.03

61.920

CaO

3.99

3.59

0.90

0.89

4.490

K2 O

2.43

0.82

0.34

0.83

2.940

185.45

139.84

0.75

0.69

270.110

1.61

0.89

0.55

0.62

2.570

Sr Na2 O

Source Modified after geochemical data of national mineral resources assessment

background values along the northern margin of the basin. According to the statistics of the distribution of the whole region and the abundance values in arid desert areas of China, the abundance of elements in the region are depleted. The abundances of P, MgO, Ba, Mn, and SiO2 are close to those of elements in arid desert areas of China. The distribution of elements (e. g., Hg, Sb, Au, Pb, As, Co, Cr, Ni, Ag, and Cd) are mainly related to the types of rock formations, and their covered areas are mainly concentrated in Paleozoic, Jurassic, and Cretaceous strata and in acidic and intermediate-acidic rock masses of the Permian and Triassic.

2.4.2 Spatial Variation Characteristics of Elements and Compounds The heterogeneity of the regional distribution of elements is chiefly manifested in the distribution variation of elements both in time and space, which also reflects the complexity of the regional distribution patterns of different elements. The heterogeneity and degree of differentiation of the regional distribution of elements are indicators of metallogenic elements or potential metallogenic elements to a certain degree. The coefficient of variation (Cv) of a parameter reflects the unevenness of the elemental distribution and is an important indicator for determining the regional metallogenic potential. A small coefficient of variation indicates that the distribution of elements in the region or different geological units tends to be even, and the regional distribution variation is very weak. A large coefficient of variation indicates that the uneven distribution of elements in the region or different geological units may lead to enrichment in local areas. Ore deposits can be formed when the local anomaly

2.4 Geochemical Characteristics of the Basin Periphery

89

area is strongly enriched to a certain extent. Therefore, the variation characteristics of the elemental distribution in a region are crucial for exploring ore-forming elements or potential ore-forming elements. According to the regional geochemical map of each element and the statistical results of the coefficient of variation of the elemental distribution, the differentiation characteristics of the elemental distribution in the study area are summarized in Table 2.9. Intensively differentiated and extremely differentiated elements include Zn, Cu, As, Pb, Bi, Ag, Cd, Cr, Mo, Au, U, W, Sb, and Hg. Most elements of this type are polymetallic mineralization indicators or metallogenic elements. The study of geochemical maps combined with geological maps reveals that the high-background area of these elements is mainly outcropped in the Qinling area along the southern margin of the basin. Some exposed rock masses are closely related to the spatial distribution of the ancient strata and locally present high anomalies, which are coincident with the polymetallic deposits discovered in the region. There are four obviously differentiated elements and compounds, namely F, Ni, Ba, and CaO. The high-background area of this group is related to the distribution of rock formations in the area. For example, the high-background area of CaO in the southwest of the southern margin of the basin is closely related to the spatial distribution of limestone. The elements and compounds exhibiting uneven distribution include Li, Th, Nb, Mn, Na2 O, V, B, MgO, Sn, P, La, and Sr, which are mostly rock-forming elements and compounds and rare and rare earth elements (REEs). The high-background areas of Sr and Na2 O are mainly distributed along the northern margin of the Ordos Basin. Evenly and quasi-evenly distributed elements and compounds are mainly petrogenic and siderophile, including SiO2 , Al2 O3 , Y, K2 O, Fe2 O3 , Zr, Be, Ti, and Co.

2.4.3 Association of Elements The original geochemical data were normalized to calculate the correlation coefficient matrix R for 39 element variables. On the basis of the symmetric matrix R, a principal component analysis was conducted to find the principal factor solution by using the factor analysis model. The typical association of each factor was calculated by the orthogonal rotation of the maximum variance of the principal factor solution (Table 2.10). Variables such as Fe2 O3 , Mn, Co, Ti, V, Al2 O3 , Y, P, SiO2 , MgO, Zn, Li, Cu, Nb, Ni, K2 O, and Na2 O have high loadings on factor F1. The association is related to rock-forming elements and compounds and basic and ultrabasic rock bodies in the study area. Variables such as CaO, SiO2 , MgO, Sr, Na2 O, Al2 O3 , and P have high loadings on factor F2. The association is related to limestone or alkaline rocks. Variables such

Y, K2 O, Fe2 O3 ,Zr, Li, Th, Nb, Mn, F, Ni, Ba, CaO Be, Ti, Co Na2 O, V, B, MgO, Sn, P, La, Sr

SiO2 , Al2 O3

Unevenness of regional distribution increasing →

Distribution feature

0.75 ≤ Cv < 1.0 Obviously differentiated

Element or compound

0.45 ≤ Cv < 0.75 Uneven

0.25 ≤ Cv < 0.45

Medium even

Cv < 0.25

Cv value

Even

Table 2.9 Cv values and fractionation characteristics of 65 elements and compounds 1.0 ≤ Cv < 1.5

Zn, Cu, As

Intensively differentiated

Cv ≥ 1.5

Pb, Bi, Ag, Cd, Cr, Mo, Au, U, W, Sb, Hg

Extremely differentiated Cv = standard deviation/mean

Remarks

90 2 Background of the Ordos Basin

−0.046

−0.938

0.809

−0.048

−0.088

−0.448

0.704

CaO

SiO2

0.793 −0.01

−0.003

0.021

W

−0.017

−0.011

−0.269

0.017

−0.02

Sb

Sr

−0.053

−0.549

0.387

MgO

0.007

−0.025

0.008

0.015

0.003 0.044

0 −0.005

0.002

−0.002

0.009

0.018

0.032

U

−0.059

−0.041

0.107

Cd

0

0.171

0.005

0.015

0.009

0.038

−0.013

−0.009

0.015

0.05

−0.002

0.011

−0.007

0.003

F4

Au

0.003 −0.008

0.003

−0.015

0.042

0.057

−0.004

−0.005

Pb

0.01

Hg

−0.039 −0.011

−0.035

−0.001

−0.007

0.464

Cr

0.301

0.038

Ni

Mo

V

−0.093

0.161

0.109

0.582

0.068

Al2 O3

−0.018

−0.028

0.759

0.03

Ti

Ba

0.002 −0.055

−0.031

−0.069

0.316

0.316

Nb

0.788

F3

0.188

F2

C

F1

−0.108

Variable

Na2 O

Table 2.10 Loading factors after varimax rotation F5

0.009

0.01

0.004

0.041

0.006

0.004

0.004

0.003

0.026

0.002

0.021

0.029

0.057

−0.078

0.013

0.059

0.062

0.046

0.01

0.362

−0.026

F7

F8

−0.013

0.04

−0.013

0.034

−0.011

−0.006

−0.926

−0.001

0.069

0.01

0.101

0.003

0.001

0.042

0.014

−0.032

−0.042 0.003

0.002

−0.002

0.014

0.114

0.054

−0.011

−0.077

−0.101

−0.276

0.037

−0.034 0.147

0

−0.005

0

−0.008

0.001

0

−0.002

0.979

0.001

0

0.001

0.226

−0.007

−0.019

0.007

0.014

−0.324

0.027

0.004

0.037 −0.119

−0.007

0.001

−0.011

−0.009

0.017

0.085

−0.064

−0.11

−0.132

0.031

−0.026

0.031

F6

F9

−0.023

0.039

−0.011

−0.018

−0.01

−0.001

−0.021

0.001

−0.041

−0.002

−0.008

−0.007

−0.012

0.018

0.008

−0.042

−0.024

−0.035

−0.138

−0.078

−0.027

F10

0.004

−0.02

0.994

0.006

0

0

0.01

0

0.004

0.052

0.005

0.006

0.016

−0.017

0

0.011

0.011

0.005

0.008

0.006

−0.008

F11

0.001

0.003

0

0

0

0.999

0.006

0

0

0.01

0.002

0.006

0.004

−0.005

0

0.002

0

0

0.005

0.04

−0.003

F12

0.998

−0.003

0.004

0.004

0.01

0.002

0.009

0

0.011

0.002

0.005

0.001

−0.001

−0.006

0

−0.003

−0.001

−0.002

0.001

−0.001

−0.011

(continued)

−0.005

0.019

−0.003

0.019

−0.006

−0.005

−0.005

0.001

−0.045

0

0

−0.001

−0.006

0.037

0.032

−0.116

0.006

0.019

−0.026

−0.073

0.002

F13

2.4 Geochemical Characteristics of the Basin Periphery 91

0.019

0.007

0.085

0.174

Be

0.003

Source Same as Table 2.8

32.15

5.62 36.90

4.75

6.82

19.71

19.71

Variance contribution

Cumulative percentage

26.53

0.041

−0.039

0

0.119

Sn

0.003 0.073

0.028 −0.005

0.065

−0.007

0.289

0.02

Zr

0.013

0.125

Bi

0.012

−0.233

−0.023

−0.019

0.132

0.363

0.002

−0.008

0.009

B

0.135

La

0.004

0.047 −0.034

0.086

0.007

0.972

0.019

−0.075

Zn

0.85

0.798

Fe2 O3

Co

0.014

−0.001

0.082

0.004

Th

0.009

−0.015

−0.002

−0.074

0.049

0.136

Ag

F

−0.075

−0.07

0.022

0.322

Li

−0.01

0.045 −0.09

0.127

0.802

0.026

0.021

0.019

F4

−0.08

−0.149

K2 O

0.139

Mn

−0.162

−0.044

−0.144

0.545

0.454

Y

−0.052

P

F3

−0.05

0.09

F2

F1

As

Variable

Table 2.10 (continued)

41.26

4.36

0.133

0.021

0.018

0.074

0.094

0.053

0.886

0.052

0.067

0.127

0.87

0.008

0.099

0.13

0.008

0.122

0.17

0.024

F5

44.91

3.65

−0.019

−0.011

48.25

3.34

−0.035

0.002

0.079

−0.072

−0.02 0.023

−0.075

0.026

−0.013

0.055

0.048

−0.067

0.007

0.01

−0.063

0.08

−0.932

−0.044

−0.154

0.008

F7

−0.454

−0.038

−0.002

0.007

0.026

−0.004

−0.016

−0.051

−0.049

−0.087

0.032

−0.062

−0.078

−0.044

F6

0.02

51.45

3.19

0.003

0

0.006

−0.002

0.005

−0.009

0.008

54.44

3.00

−0.222

−0.958

0.05

−0.017

0.06

−0.017

−0.005

0.027

−0.001 0.035

−0.034

−0.012

−0.073

−0.051

−0.023

0.004

−0.003

−0.06

−0.006

F9

−0.002

−0.005

0.001

0.022

0.05

−0.014

0.016

0.033

−0.002

F8

57.29

2.84

0.011

59.89

2.61

0.005

0.001

−0.002

−0.006 0.011

0.004

0.004

0

0

0.001

0.002

0.004

0.001

0.002

0.014

0.006

−0.001

0.002

0

0.002

F11

0.001

0.017

0.004

−0.002

0.003

0.001

0.001

0.006

0.007

0.027

0.003

−0.012

0.014

0.012

0.083

F10

62.43

2.54

0.017

0.025

0.011

0.022

0.036

0.026

−0.005

0.009

0.01

0.006

0.017

0.008

0.011

0.016

0.003

0.006

0.005

0.012

F12

64.95

2.51

−0.054

−0.034

−0.109

−0.113

−0.005

0.013

−0.08

−0.029

−0.04

−0.962

−0.041

−0.018

−0.09

−0.01

−0.062

−0.025

−0.118

−0.01

F13

92 2 Background of the Ordos Basin

2.4 Geochemical Characteristics of the Basin Periphery

93

as Ag, Pb, and Zn have high loadings on factor F4, which are metallogenic and indicator elements of typical medium- and low-temperature hydrothermal polymetallic deposits. Variables such as La, F, Nb, Y, Sn, Mn, Th, and P have high loadings on factor F5. This group of elements is an element association related to mineralization of rare elements and REEs. Variable U has high loadings on factor F11, which indicates that the correlation between U and other elements is weak from a regional perspective. On the basis of factor analysis, the original geochemical data were normalized, and an R-type clustering analysis of elements was performed using the correlation coefficient method in the study area. The cluster dendrogram is shown in Fig. 2.30. The figure shows that the geochemical paragenetic association of elements in the clustering analysis is consistent with the results of the factor analysis. The mutual verification of the two multivariate geochemical statistical analysis methods further confirms the reliability of the paragenetic association between elements.

Fig. 2.30 Cluster dendrogram (based on the data in Table 2.8)

94

2 Background of the Ordos Basin

2.4.4 Enrichment and Depletion Characteristics of the Basement and Caprock in the Basin The basement of the Ordos Basin consists of a metamorphic crystalline basement and Meso–Neoproterozoic and Paleozoic erathem. To understand the enrichment and depletion characteristics in different geological units, the enrichment coefficient C1 (the ratio between the mean of elements in stream sediments in each geological unit and the background value in the whole Ordos Basin) is introduced to describe the distribution variation and enrichment and depletion characteristics of elements in each geological unit and their corresponding relationship with geological bodies, thereby enabling study of the metallogenic pattern and prospecting potential of elements. To depict the degree of element enrichment and fractionation, C1 is divided into five categories: C1 > 1.2 for obvious enrichment, 1.0 < C1 ≤ 1.2 for enrichment, 0.8 < C1 ≤ 1.0 for nonobvious enrichment and depletion, 0.6 < C1 ≤ 0.8 for depletion, and C1 ≤ 0.6 for obvious depletion. The enrichment and depletion characteristics of the basement are listed in Table 2.11. One can see that U, La, Li, Zr, Th, Be, Y, and other rare elements and REEs exhibit strong spatial variation and obvious enrichment and are closely related to U mineralization. Carboniferous strata can be used as an important indicator for finding source horizons of U mineralization. The enrichment and depletion characteristics of elements in the caprock in the northern part of the basin are listed in Table 2.12. Both the Fuxian and Yan’an formations are obviously enriched in U, and the Fuxian and Yan’an formations are also ore-hosting strata of sandstone-type uranium deposits along the northern margin of the Ordos Basin.

2.4.5 Elemental Distribution Characteristics of the Main Rock Masses Around the Basin There are few exposed rock bodies in the Ordos Basin. The main rock bodies along the periphery consist of Archean, Proterozoic, Ordovician, Silurian, Carboniferous, Permian, Triassic, Jurassic, and Cretaceous massifs ranging from old to young. Their depletion and enrichment characteristics are listed in Table 2.13. From the enrichment and depletion of elements in the residues of the above-mentioned rock bodies, it can be seen that the U-enriched residues are mainly from Jurassic and Cretaceous rock bodies and are slightly enriched in the Triassic rock body. Where Jurassic and Cretaceous rock bodies are exposed can be used as the target horizon or the source horizon of sandstone-type uranium ore for finding hard-rock-type uranium.

2.4 Geochemical Characteristics of the Basin Periphery

95

Table 2.11 Enrichment and depletion characteristics of elements and compounds in the basement around the Ordos Basin Geological unit

Obvious Enrichment Depletion Obvious enrichment (1.0 < C1 ≤ 1.2) (0.6 < C1 ≤ 0.8) depletion (C1 > 1.2) (C1 ≤ 0.6)

Upper Paleozoic Permian

La

Carboniferous B, Hg, La, Li, Mo, U, Zr

Lower Paleozoic Ordovician Cambrian

B, CaO, MgO

Ba, Cd, Zr, Al2 O3 , CaO

Au, Cu, Hg, Li, Mo, P, U, W, Zn, MgO

As, Bi, Sb, Sn, Na2 O

As, Be, Cd, Co, Cr, F, Nb, Ni, Sn, Th, Ti, V, W, Y, Al2 O3 , Fe2 O3 , CaO

Au, Ba, Sr, MgO

Na2 O

Cr, F, Li, Nb, Ni, Ti, V, W, Zr

Au, Ba, Bi, Sr, Na2 O

B, Zr, MgO Cr, F, La, Li, Mn, Nb, Ni, P, Sn, Th, Ti, V, W, Y, CaO

Au, Ba, Bi, Hg, Mo, Sr, Na2 O

Mesoproterozoic Shinagan Group

Ba, Sr, Na2 O, MgO

Au, F, Li, Mo, Ni, P, Sn, Th, Ti, Y, Zn, Zr

Paleoproterozoic Erdaoao Group

Ag, Au, Be, La, Nb, Ni, Ba, Co, Cr, P, Sr, Y, Al2 O3 , Cu, Mn, Ti, CaO V, Fe2 O3 , Na2 O, MgO

Archean

Ag, Co, Mn, SiO2 , K2 O

As, B, Bi, Cd, Hg, Sb, U, W

Cd, Hg, Li, Mo, As, B, Sb Pb, Sn, U

Seertengshan Group

Ba, Co, Cr, Ag, Cu, Mn, Pb, Sn, Th, Y, Sr, Na2 O Nb, Ni, V, SiO2 , CaO Al2 O3 , Fe2 O3 , MgO

Jining Group

Ba, Fe2 O3 , Co, Cr, La, Th, K2 O Ti, V, Y, Zr, SiO2 , Al2 O3

As, B, Bi, Cd, Hg, Li, Sb, U, W

Au, Cd, Li, Mo, As, B, Bi, Pb, Sn, U, CaO Cd, Hg, Li, Sb, U, W

Note Elements not indicated in the table are elements with no obvious depletion (i.e., 0.8 < C1 ≤ 1.0) Source Same as Table 2.8

2.4.6 Geochemical Background of Uranium Mineralization Along the Periphery of the Basin The Qinling Mountains stretch from east to west in Central China. They act as a windshield to prevent cold air from moving southward in the winter and stop northward movement of the southeast monsoon in the summer. The Qinling Mountains form a natural dividing line between China’s subtropical and warm-temperate zones

Mesozoic

Cretaceous

Lower Series

Zhidan Group

As, B, Cu, F, La, Li, Mn, Nb, Ni, P, Pb, Th, Ti, U, V, W, Zn, Zr, MgO

Bi, Sb, CaO

Huachi–Huanhe Formation

Au, Ba, Cd

Cd, Co, Cu, F, Li, Mn, Nb, Ni, V, Zn

Bi, F, Li, P, Pb, Sb, Au, Mo Sr, W, MgO Sb, Sr, Zr, SiO2 , CaO

As, CaO

Luohandong Formation

Jingchuan Formation

Be, Cu, F, La, Ni, U, V, Zr, K2 O

Au, Hg, Sr

Oligocene

Paleogene

Ag, B, Ba, Cd, Co, Cr, Mn, Mo, Nb, P, Pb, Ti, W, Y, Zn, Fe2 O3 , CaO, MgO

As, B, Co, Li, Be, Bi, Cr, Mo, Ni, Au, Cd, Hg, P, Pb, Mn, Nb, U, V, W, Sb, Sn, Th, Ti, Y, Sr Zr Zn, SiO2 , Al2 O3 , Fe2 O3

Pliocene

Depletion (0.6 < C1 ≤ 0.8)

Neogene

As, B, Bi, Cd, Cu, F, Hg, Li, Ni, Sb, Sn, Th, U, W, MgO

Enrichment (1.0 < C1 ≤ 1.2)

CaO

Obvious enrichment (C1 > 1.2)

Quaternary

Group or formation

Cenozoic

Series

System

Erathem

Table 2.12 The enrichment and depletion characteristics of elements in the caprock in the northern part of the Ordos basin

(continued)

Hg, Mo

Au, Hg,

Cd, Hg

Na2 O

Na2 O

Obvious depletion (C1 ≤ 0.6)

96 2 Background of the Ordos Basin

Jurassic

Mesozoic

Lower Series

Middle Series

Series

Middle Series

Jurassic

System

Series

System

Erathem

Erathem

Table 2.12 (continued)

Fuxian Formation

Yan’an Formation

Zhiluo Formation

Group or formation

Be, Co, La, Ni, Ti, U, V, Zn, Al2 O3 , Fe2 O3

Depletion (0.6 < C1 ≤ 0.8)

SiO2

Ba, Mo, SiO2 , K2 O

Ba, Cr, Cu, Hg, Li, Pb, Th, Y,

Cd, Mo

Obvious depletion (C1 ≤ 0.6)

As, Au, B, Bi, Mn, Sb, MgO

Obvious depletion (C1 ≤ 0.6)

Bi, Sr, W, CaO, MgO

(continued)

As, Au, B, Sb, Na2 O

As, B, Bi, Sb, Sr, W, Zr, Au, Na2 O MgO

Cd, Co, Cr, Cu, F, Hg, Li, Nb, Ni, P, Sn, Sr, Th, Ti, V, W, Y, Zn, Zr, Fe2 O3 , Na2 O, CaO

Depletion (0.6 < C1 ≤ 0.8)

Ag, As, B, Be, Bi, Au, Cu, F, Hg, Co, Cr, La, Li, Mn, Nb, P, Sb, W Mo, Ni, Pb, Sn, Sr, Th, Ti, U, V, Y, Zn, Al2 O3 , Na2 O, CaO, MgO

Ag, As, Be, Co, F, Au, Ba, Hg La, Li, Mn, Nb, Ni, Sb, Sn, Th, Ti, U, W, Zr, Fe2 O3 , MgO

Enrichment (1.0 < C1 ≤ 1.2)

Enrichment (1.0 < C1 ≤ 1.2)

B, Bi, Cu, P, Pb, V, Zn, CaO

Obvious enrichment (C1 > 1.2)

Cd, Co, Mo, U, Fe2 O3 , Ba, Be, Hg, La, Mn, V, CaO Y, Zn, Al2 O3

Obvious enrichment (C1 > 1.2)

Anding Formation

Luohe Formation

Group or formation

2.4 Geochemical Characteristics of the Basin Periphery 97

Ermaying Formation

Heshanggou Formation

Middle Series

Lower Series

Liujiagou Formation

Yanchang Formation

Upper Series

Triassic

Group or formation

Series

System

Zr, CaO

Obvious enrichment (C1 > 1.2)

Depletion (0.6 < C1 ≤ 0.8)

Bi, Mo

Ag, B, La, Nb, Ti, Y, Zr, CaO

Au, Bi, Mo, Sr, Na2 O

B, Cr, La, Nb, Ni, Ti, V, Bi, Mo, Na2 O W, MgO

Ba, La, Zr, CaO

As, B, Co, Cr, Cu, La, Hg, Mo Li, Mn, Ni, P, Sb, Ti, U, V, Y, Zn, Zr, Al2 O3 , Fe2 O3 , CaO, MgO

Enrichment (1.0 < C1 ≤ 1.2)

Note The elements not indicated in the table are elements that are not obviously enriched of depleted (i.e., 0.8 < C1 ≤ 10) Source Same as Table 18

Erathem

Table 2.12 (continued)

Hg

Hg

Hg

Obvious depletion (C1 ≤ 0.6)

98 2 Background of the Ordos Basin

2.4 Geochemical Characteristics of the Basin Periphery

99

Table 2.13 Element enrichment and dilution characteristics of main rock masses around the Ordos Basin Geological unit

Obvious enrichment (C1 > 1.2)

Cretaceous massif

W, Th, U, Nb, Bi, Cd, Au, Sr, Hg, Be, Mo, Zr, Zn, Ag, Cu, K2 O Pb, La, Sn, P, F, Y, Ti, Fe2 O3 , Na2 O, Mn, Li, Co, V, Al2 O3

Jurassic massif Bi, Mo, U, Nb, Na2 O, W, Be, P, Mn, Pb, Th, Ag, Sr, V

Enrichment (1.0 < C1 ≤ 1.2)

Depletion (0.6 < C1 ≤ 0.8)

Obvious depletion (C1 ≤ 0.6)

Sb, B, As

CaO

Ti, Y, F, Zn, K2 O, Hg, B Ba, Al2 O3 , Co, Fe2 O3 , Li, La, Zr, Sn, Ni, Cu, Sb

CaO

Triassic massif Nb, Bi, Be, Th, W, K2 O, Y, Li, Ti, F, Na2 O, Pb, Zn, La, Al2 O3 , Zr Mn, P Fe2 O3 , Ag, Co, U, Sn, V, SiO2 , Ba, Cu, Cr, Sr

Fe2 O3

CaO

Permian massif

Na2 O, Sr, Ba, K2 O

Al2 O3 , SiO2

Pb, Mo, Sn, La, Th, Y, Fe2 O3 , Bi, Mn, P, Co, F, CaO, Nb, Cr, Cu

V, Zn, Cd, Zr, Au, As, Ti, Li, W, U, MgO, Ni, B, Hg, Sb

Carboniferous massif

Na2 O, Bi, K2 O, Be, Th

Pb, Sn, SiO2 , Al2 O3 , Sr, Ba

VF, Nb, U, Zn, V, Sb, Hg Mn, P, Fe2 O3 , Co, W, Ti, MgO, Cu, B, Zr, Au, Cr, As, Mo, Cd, Ni, CaO

Silurian massif Nb, F, Sb, P, Zn, Th, Pb, W, Y, Zr, Mn, La, V, Ni, Na2 O, Be, Cu, Ti, Co, Bi

Li, U, Al2 O3 , Fe2 O3 , Cr, MgO, K2 O, Sn, Ag, Sr, Mo

B, Hg, CaO

Ordovician massif

Na2 O, Sr, Cu, P, Nb, Mn, Be, Co, V, Al2 O3

MgO, Ti, Bi, Fe2 O3 , Ba, Zn, SiO2

Sn, CaO, W, Sb, As, Cd, Hg

B, Au

Proterozoic massif

Na2 O, Sr, Mo, Ba, Be, Co, Al2 O3 , Cr K2 O, Nb, Fe2 O3 , P, SiO2 , Mn, La, V

Li, Bi, CaO, Cd, W, U, As

Hg, Sb

Archean massif

Hg, Na2 O, Sr, Pb, Fe2 O3 , Ti, Cr, Ag, Au, Ba, Al2 O3 , V, Cu, Co K2 O, Zr, Ni, Mn, Y, SiO2 , Zn, La

Bi, Li, CaO, U, Sb B, As

Note The elements that are not indicated in the table are elements that are not obviously enriched or depleted (i.e., 0.8 < C1 < 10)

100

2 Background of the Ordos Basin

in eastern Asia. There are obvious differences in climate, hydrology, and the vertical natural spectrum between south and north across the line. The climate in the northern mid-latitude region is characterized by greatly fluctuating daily temperatures, less rainfall, and underdeveloped vegetation. Typical uranium-rich rocks such as acidic magmatic rock formations are extensively developed in the region. Because of the large daily temperature differentials, severe thermal expansion and cold contraction of rocks occur under most landscape conditions in the north, and surface rocks are weathered and disintegrated severely. Thick weathered oxidation zones developed as a result of long-term supergene geological processes. Uranium is a chemically active element. The standard electrode potential in +3 and +4 uranium ion valence states is lower than the standard electrode potential of hydrogen, and both can react strongly with water, reducing H+ and oxidizing itself to U+4 or U+6 . Uranium is also very easily oxidized by oxygen dissolved in water. Under natural superficial conditions, it is an element easily oxidized and dissolved in oxygen-rich water. At the same time, the dry and hot climate in the north tends to produce evaporation, resulting in a variety of secondary uranium minerals, such as schroeckingerite, uranyl vanadate, and silicate minerals. The geochemical properties of uranium and the differences in climate between northern and southern regions make the uranium produce obvious spatial differences in the supergene geochemical process. The Ordos Basin and its surrounding area north of the Qinling Mountains are low-value anomaly areas of uranium. In contrast, the area south of the Qinling Mountains is a high-background area of uranium (Fig. 2.31).

2.4.7 Delineation of Uranium Source Rocks Along the Periphery of the Basin There are many factors that affect the mineralization and enrichment of sandstonetype uranium deposits, with the uranium source condition being one of important conditions necessary for mineralization. The uranium source mainly depends on the lithology of uranium source rocks and the physical and chemical properties of uranium content. The prerequisite condition for uranium source rocks is that the uranium content is abundant and uranium can easily precipitate. Uranium occurs in the form of uranium minerals, dispersed adsorption, and isomorphic replacement. Uranium in the uranium-bearing source rock is activated and migrated into the basin through weathering, leaching, tectonic hydrothermal activity, and other geological processes. Under the dissolution and transportation of oxygen-containing surface water and groundwater, active uranium in the erosion source area can continuously migrate to the mineralization area, providing a uranium source for mineralization. Geochemical element factor analyses of stream sediments along the periphery of the Ordos Basin indicate that La, Nb, Y, Th, Mn, and other REEs are strongly correlated. These elements are characterized by having ionic radii and electronegativity

2.4 Geochemical Characteristics of the Basin Periphery

101

Fig. 2.31 Distribution of high-background areas of rare elements and REEs and uranium along the periphery of the Ordos Basin. (The data source is the same as that of Table 2.8.)

values similar to those of uranium. Uranium can be isomorphically replaced with them and enter into their mineral lattices to form minerals such as thorium, monazite, allanite, and other uranium-containing minerals. In addition, the content of uranium in the rock is generally positively related to the content of K2 O. A linear transformation using a normalized standard deviation was performed on La, Nb, Y, Th, Mn, and K2 O contents, so that the result values fall between 0 and 1 to eliminate the dimensional influence among parameters. The normalized data were accumulated to study the spatial distribution characteristics of the geochemical fields of La, Nb, Y, Th, Mn, and K2 O in the peripheral area of the basin, and the high-background areas of uranium were superimposed and analyzed. Combined with the geological background, 10 high-background zones of REEs and uranium around the basin are delineated. Zone I is located in the northwestern area of the basin periphery. This highbackground area well coincides with the Cretaceous Suhongtu Formation (K1 s). The lithology of the Suhongtu Formation is composed mainly of basalt, basaltic trachyandesite, andesitic basalt, sandstone, and silty mudstone. Zone II is located west of Baotou City. This high-background area occurs in Proterozoic diorite and granite dikes trending in the northeast direction. Zone III is located in the northeastern periphery of the basin. This high-background area occurs mainly in the Carboniferous Taiyuan Formation (C2 t). Its lithology consists of sandstone, siltstone, shale, and bauxite intercalated with limestone and coal seam. Zone IV is located south of

102

2 Background of the Ordos Basin

Xichang City on the western margin of the basin. This high-background area occurs in the outcropped Ordovician plagiogranite, tonalite, adamellite, Silurian adamellite, and syenite rock masses. Zone V is located southwest of Lanzhou City, where Ordovician adamellite, quartz diorite, diorite, and Jurassic adamellite are exposed. Both Zones VI and VII are located along the southwestern periphery of the basin. The strata exposed in Zone VI are mainly of the Sinian Crystal Formation (Z2 sh), Xianglongka Formation (Zx), Cambrian Taiyangding Formation (mt), and Silurian Bailongjiang Group (Sbl). The Sinian Crystal Formation (Z2 sh) is composed of dolomite, siliceous dolomite, and marble; the Xianglongka Formation (Zx) consists of conglomerate, sandstone, graywacke, and silty slate; the Cambrian Taiyangding Formation (mt) consists of grayish-black siliceous rock and siliceous slate intercalated with stone coal; and the Silurian Bailongjiang Group (Sbl) consists of quartz sandstone, slate, siliceous rock, and crystalline limestone. The strata exposed in Zone VII include the Silurian Diebu Formation (S1 d), Zhouqu Formation (S2 z), Zhuowukuo Formation (S3 zw), and Devonian Tonggou Formation (D1 p). The Silurian Diebu Formation (S1d) is composed of carbonaceous siliceous slate, siliceous rock, meta-sandstone, and phyllite. The upper part of the Zhouqu Formation (S2 z) consists of carbonaceous slate and meta siltstone intercalated with limestone. The middle and lower parts consist of gray to dark gray slate and limestone. Zone VIII is located along the southern periphery of the basin. Geographically, it is located e southwest of Baoji City. The exposed rocks include Triassic adamellite and syenogranite. Zone X is located north of Nanyang City. This high-background area is relatively large. Rare element and REEs and uranium are all high in content. Rocks related to uranium mineralization include primarily Cretaceous rocks, Carboniferous granites, and Paleoproterozoic migmatized granites. In summary, the distribution of rocks in the high uranium background area suggests that they may theoretically contribute to uranium sources in the Mesozoic sediments in the Ordos Basin. However, it is also understandable that some high-content areas seem to be inconsistent with the current distribution of uranium deposits. Although the current spatial distribution of geological units does not represent the spatial distribution of geological bodies at that time, the transportation and migration of uranium entail very complicated mechanisms. Physical transport and chemical transport are related to physical and chemical changes in the process of weathering and denudation. Understanding this process requires further study.

2.5 Hydrogeological Characteristics of the Basin 2.5.1 Hydrogeological Units The Ordos Basin is located on the second step of the three major terrain steps in China. The surrounding area of the basin is mountainous, and the main body of the Yellow River surrounding the basin is composed of desert plateau in the north and the

2.5 Hydrogeological Characteristics of the Basin

103

Loess Plateau in the south. If Baiyu Mountain in the middle of the basin is taken as the boundary, the basin can be divided into two relatively independent regional hydrogeological areas (Fig. 2.32): the southern Loess Plateau (I) and the northern Ordos Plateau (II). The hydrogeological conditions of the basin are recognizably different. The northern hydrogeological region can be further divided into two parts: the western part of the Ordos Plateau (II2 ) and the eastern part of the Ordos Plateau (II1 ) by the Dongshengliang–Shililiang–Yanchi watershed in the middle. The southern region can be divided into two hydrogeological subregions: the Loess Plateau in eastern Gansu Province (I2 ) and the hydrogeological subregion of the Loess Plateau in northern Shaanxi Province (I1 ) by Ziwuling, which runs through the boundary of Shaanxi and Gansu provinces. In different hydrogeological regions, topography and geomorphology play important roles in controlling the spatial distribution, storage, and circulation of groundwater in the region.

2.5.2 Types and Distribution of Groundwater The Ordos Basin is a large artesian basin with Middle Jurassic and Lower Cretaceous water-bearing formations as the main body. The formation, distribution, and movement of groundwater are controlled by geological structures, lithology, and stratigraphic structures. According to the occurrence conditions of groundwater and the structure of water-bearing rocks in the north of the basin, combined with the sedimentary characteristics of the basin, all kinds of rock series before the Triassic are regarded as bedrock. Therefore, the groundwater in the basin can be divided into three main types: bedrock fissure water, clastic rock fissure pore water, and loose rock pore water. The distribution and characteristics of the three types of groundwater are described in the following.

2.5.2.1

Bedrock Fissure Water

Bedrock fissure water is mainly distributed in the bedrock exposed area of the structural uplift zone around the basin. The water-bearing rock is mainly schist, gneiss, shale, siliceous limestone, and sandstone. The fissure development is uneven. The water yield and burial conditions are also different. Because of the dry climate, the atmospheric water supply is limited, the water content is poor, seasonal variation is substantial, and the burial depth of the general groundwater level is 200 m. The eastern sandbody is relatively thin, usually 40–60 m thick. One can see from the contour map of sandbody thickness and sand content that the sandbody obviously has high value bands spreading from northwest to southeast, which are the specific manifestation of the main channel. The north is dominated by large channel sandbodies of greater size and thickness, while the east is dominated by branched channel sandbodies of lesser size and thickness (Fig. 3.18).

140

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.18 Contour maps of sandbody thickness (upper panel) and sand content (lower panel) in the lower member of the Zhiluo Formation on the northeastern edge of the basin

2. From the sandbody comparison profile (Fig. 3.19), one can see that, compared with the sandbody in upper part of the Zhiluo Formation, that in the lower part has a large scale and good transverse connectivity. In the lower part of the Zhiluo Formation, fine clastic rock deposits are few, there is basically no water-proof layer in the main sandbody, sand and mud interbed in the upper part, and the proportion of mudstone increases obviously. At the bottom of the lower member of the Zhiluo Formation, the coal seam area of the Yan’an Formation is stable, forming good water-proof and reduction layers. 3.2.1.6

Characteristics of Sedimentary Facies of the Uranium-Bearing Rock Series

Based on the research of sedimentary structure, sedimentary facies sequence, geochemistry, and paleontology of outcrop sections, combined with a comparative analysis of drilling and seismic data, one can conclude that the Yan’an Formation developed a fluvial delta sedimentary system, from the early fresh fluvial facies deposition to the triangle continental sedimentary evolution, and that the Zhiluo Formation is a fluvial sedimentary system, which evolved from braided fluvial facies in the early stage to meandering fluvial facies in the late stage. The sedimentary environment is continental fluvial–lacustrine delta. The Anding Formation is mainly

3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock …

141

Fig. 3.19 Comparison of the Yan’an and Zhiluo formation sandbodies in the Ailaiwukugou area on the northeastern edge of the Ordos Basin

lacustrine sedimentary environment, while the Luohe Formation is a channel and eolian desert sedimentary environment. 1. Competing river facies Polemic fluvial facies are mainly developed in the lower part of the Yan’an and Zhiluo formations and can be further divided into channel deposits, Xintan deposits, and floodplain subfacies (Fig. 3.20). The braided river deposits of the early Yan’an Formation and the early Zhiluo Formation appeared in the deep gully development zone above the ancient weathering crust at the top of the Yanchang Formation. The sandbodies were deposited quickly in a geological sense and were close to the provenance, and their grain size was coarse. The sandbodies had good continuity in the transverse direction. A. Channel deposits A channel sandbody is a skeleton sandbody of fluvial facies, with a common erosion scour surface at the bottom. It is lenticular in the transverse section and is composed of multilayer or single-layer medium-coarse sandstone, reflecting strong channel migration. Trough, tabular cross-bedding (Fig. 3.20b, d), modal cross-bedding, and sometimes graded bedding at the bottom, containing gravel, plant debris, and charcoal (Fig. 3.20e, f), are developed. Compared with the gravelly braided channel at the bottom of the Yan’an Formation, the range of floodplain in the sandy channel in the Zhiluo Formation was enlarged, peat bog deposition occurred, and local minable coal seams were formed. B. Heart beach deposit Channel bar sedimentary consists mainly of thick sandbodies. Lateral accretion is dominant, and the vertical direction is composed of several rhythmites from

142

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.20 Typical sedimentary structures of the Yan’an and Zhiluo formations along the northeastern margin of the Ordos Basin. a Parallel bedding and climbing sand bedding (wavy bedding). b Small plate cross-bedding. c Large porphyritic cross-bedding. d Trough interleaving bedding. e Retained deposits such as the scouring surface of the river bottom and mud gravel. f Large amount of carbon debris developed on the scouring surface of the river bottom in the lower part of the Zhiluo Formation

coarse grained to fine grained, with tabular cross-bedding and trough cross-bedding developed. C. Floodplain deposits Located at the top of the polemical fluvial facies, the floodplain deposits are mainly composed of siltstone and mudstone. Because of the cutting and transformation between the channel sandbodies, the fine-grained deposits are mostly removed and pinched out in a lenticular shape on both sides. In the transition part from the upper meandering river facies or delta facies, with the weakening of the hydrodynamic force, the scope of the floodplain increases, peat bog deposits may occur, and locally mineable coal seams may be formed. 2. Meandering river facies Meandering river facies are mainly developed in the middle and upper Zhiluo Formation as lower braided river deposits. Because of the amount of space and sediment recharge, the sedimentary facies changed from polemical river to meandering river and to mudstone deposition in the floodplain. The size of the sandbody became smaller and the amount of mudstone increased continuously. Meandering river facies can be divided into channel sedimentary facies and floodplain subfacies with a typical binary structure. Channel deposits Channel deposits form the skeleton part of the system. They include the riverbed beach deposition, the riverbed deposition in the lower part of the meandering

3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock …

143

river deposition, and large-scale cross-bedding in the development of a set of grit, containing the elder brother of grit, granularity and upward development erosion at the bottom of the scour surface, including gravel etc. lag deposit, on the transverse section on a flat convex lens shape. The bank (point bar and meandering bar) deposits are primarily formed by lateral accretion, which consists of inclined lateral accretion sandstone beds with low-angle cross-bedding. Floodplain deposits Formation in floods during flood overflowing, sediment mainly for suspended load of water, such as powder sandstone, mudstone fine particle deposition is given priority to, bedding is generally not development, sometimes visible horizontal bedding, possess higher proportion in the meandering river system, because the climate is dry, early general is given priority to with exposed amaranthine oxidation (Fig. 3.20a), plant development, organic matter content is more Low. Through outcropping observation, the crevasse fan sandbody can be identified. It has poor stability, and is distributed in sheet and lenticular shape in the fine-grained sediments of the flooding plain. The upper and lower interfaces are relatively smooth, consisting of mainly fine sandstone, and small oblique bedding, climbing liver bedding and horizontal bedding are developed. 3. Delta facies The lacustrine delta facies comprises a terrigenous clastic sedimentary system formed by fluvial progradation to lacustrine deposits, which form the main part of the Yan’an Formation. It developed on the lower competing river deposits of the formation, forming the middle and upper layers of the Yan’an Formation. It can be divided into delta plain, delta front, and foredelta subfacies from top to bottom, forming a typical delta facies sequence structure of fine to coarse to fine. A. Delta plain The delta plain is the overland portion of the delta that extends horizontally from the large forks of the river to the shoreline of the lake. In the section, distributary channel sandbodies, natural levees, and peat bogs are deposited. The distributary channel sandbodies are mainly medium-coarse grained with developed cross-bedding. The natural dikes are located on both sides of the channel sandbody and are mainly interbedded with fine sandstone, siltstone, and mudstone, in which the climbing sand bedding is developed (Fig. 3.20d). Peat bogs are distributed at the upper and top of the delta sequence and are composed of mudstone, siltstone, and coal seams. In the lower part, numerous vertically growing plant root fossils are developed, namely the root soil layer, and coal seams or coal lines are developed above the root soil layer (Fig. 3.20). B. Delta front The delta front is the main part of the underwater delta. Overall, it is tilted toward the lake, thinning to a wedge. The delta is the most active sedimentary center. Its profile

144

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

can be identified on the underwater distributary channel and distributary mouth bar, estuarine dam sand sheet, and far sand dam (through, e.g., sedimentary microfacies) to outflow river mouth dam with alternating layers composed of underwater distributary channel sandbody as the main body, in vertical upward. The thickened sequence, that is, the transition from the distal part of the leading edge to the proximal part, constitutes a complete prograding sequence. In profile, the size and grain size of delta front (distributary mouth bar and underwater distributary channel sandbody) development change significantly, reflecting the change of water depth. The middle and lower part of the Yan’an Formation is dominated by medium-fine-grained sandstone, and the channel sandbodies migrate and superposition in the lateral direction, reflecting the shallow water body. Siltstone–argillaceous siltstone is dominant in the middle and upper part of the dam. The fine-grained deposits in the far sand bar increase notably, indicating the deepening of the water body. The upper horizon is between the two, and the overall development process from lake basin to lake basin atrophies. Anterior delta The foredelta is located in front of the delta front. It is mainly composed of dark gray mudstone and silty mudstone, and sequence bedding and horizontal bedding developed, marking a transition from the deposition of the far bar in the lower part of the delta front. Because of different water depths, the development degree is inconsistent, and its thickness reaches its maximum in the middle SQ3 sequence. Numerous bivalve fossils are developed in the mudstone of the foredelta of this horizon in the upper siltstone (Ferganoconcaibica), a large number of well-preserved were found (Cladophebis cf. Asiataca How et Yeh; Coniopterishymenophylloides Brongn; Cekanouskiarigida Heer et al.), It shows a shallow half-deep lake environment. 4. Lacustrine facies Shallow lake and lacustrine sediments developed in the middle of the Anding Formation, the upper part of the Zhiluo Formation, and the middle part of the Yan’an Formation. These are distributed in the central part of the basin, west of Yan’an, Wuqi, Jingbian, and Qingyang and north of the Huangling extensive area. The lithology is mainly oil shale at the bottom, marl and mudstone in the middle and lower parts, and marl and siltstone in the upper part, and horizontal bedding and grain sequence bedding are developed. The basin has a typical lacustrine sedimentary sequence, and fossils such as andiformis, ostracods, and palynus are found. 5. Desert facies Desert facies deposits are typical deposits of the Luohe Formation of the Baijian system in the Ordos Basin (Xie et al. 2005). They are characterized by giant crossbedding with a wide distribution area in the basin and relatively stable deposition thickness and are often connected with an early-valley glutenite continuous sedimentary sequence.

3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock …

145

Fig. 3.21 a Radioactive anomaly intensity map and b anomaly thickness map in the Tarangaole area on the northeastern edge of the Ordos Basin

3.2.1.7

Radioactive Anomaly Characteristics of Uranium-Bearing Rock Series

A total of 229 abnormal radioactive boreholes were identified by screening the borehole logging data of the coalfield in the Tarangaole area along the northeastern margin. Their natural γ intensity values are high. The natural γ value of most boreholes is > 700 API units, with a maximum value of > 6000 API units; there are > 100 with an abnormal thickness of > 2 m. The radioactive abnormal drilling hole has obvious characteristics of continuous zonation and has a large scale (Fig. 3.21). The anomaly in the central part is generally distributed in the north–south direction, and the anomaly in the eastern Nalinggou area is distributed in the northeast and east–west directions, which is relatively consistent with the orebody shape of the Nalinggou deposit. Because the uranium drilling data from the Nalinggou deposit were not collected, whether the abnormal thickness truly reflects the thickness characteristics of the orebody of the Nalinggou deposit cannot be determined. The scale of the anomaly indicates that the area has a very good uranium prospecting potential.

3.2.2 Southeastern Margin of the Ordos Basin 3.2.2.1

Regional Geological Background

The southeastern margin of the Ordos Basin is located in the southern slope of the Yishan Slope and the northern slope of the northern uplift. The current tectonic landform of the northern uplift is nearly east–west, slightly southeast of the arc protruding, and has the geomorphological characteristics of relative uplift. The uplift began in the Late Mesozoic, and, after the Cenozoic Weihe Fault depression sank, the North Weihe uplift further uplifted. The Mesozoic strata were incomplete after multiple tectonic movements and intense denudation. The Mesozoic strata along the southern margin exhibit great differences horizontally and vertically. The loess covers a wide area, with poor bedrock exposed. Only on both sides of the main water

146

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

system and its tributaries, the Jurassic (J) and Cretaceous (K) strata are exposed, with a gentle dip angle of, generally, 3°–4°, trending northwest to north. From the bottom up, strata was composed by fine clastic rocks of the Upper Triassic Yanchang Formation (T3 y), coal-bearing clastic rocks of the Middle Jurassic Yan’an Formation, and clastic and fluvial clastic rocks of the Zhiluo Formation. The Zhiluo Formation was unconformably overlain by the Lower Cretaceous Zhidan Group., The Yan’an Formation is an important coal-bearing system, while the Zhiluo Formation is the main uranium ore-bearing horizon.

3.2.2.2

Stratigraphic Characteristics of Uraniferous Rock Series

The Zhiluo Formation of the Middle Jurassic can be divided into upper members (1 and 2) and a lower member (J2' ) according to the stratigraphic structure, lithology and the lithofacies characteristics. There characteristics can be described as follows: 1. Lower Member of the Zhiluo Formation (J2 Z1 ): This is an ore-bearing horizon, commonly known as Qilizhen sandstone, with a stable distribution in the whole area. It is a fluvial deposit with a thickness of 37–94 m under warm and humid paleoclimate conditions. It can be further divided into a lower submember (J2 z2 ) and an upper submember (1, 2) of the lower member of Zhiluo Formation according to lithologic, lithofacies, and rhythmic characteristics (Fig. 3.22). 2. Lower submember of the Zhiluo Formation of the Middle Jurassic (J2 Z1−1 ): The lower submember of the Zhiluo Formation was in contact with the Yan’an Formation, which was a sandy shafted river sedimentary system formed in a humid climate at the early stage of deposition, with a thickness of 32–81 m. The lithology of this section is mainly gray and gray-white medium-coarsegrained sandstone, partially sandwiched with a lenticular conglomerate thin layer. It has a medium-coarse-grained structure and consists of mainly quartz, feldspar, secondary debris, mica, and accessory minerals. The sorting of debris is poor and the roundness is a subnuclear angle. Argillaceous cementation is dominant, with siliceous and ferric cementation being secondary. The degree of rock consolidation is dense. The rock has weak water permeability and is rich in clastic, fine line, strip, coal block and other charred plant fragments. Pyrite is developed, mostly in the form of particles scattered in sandstone but also in the form of lumps and carbon debris, mud, and gravel symbiosis. Oil sand can be seen in local areas. The sandbody is a large compound channel deposit composed of one to rhythmic layers, with large massive bedding, trough inclined bedding, and plate inclined bedding. 3. Lower member and upper submember of the Zhiluo Formation of the Middle Jurassic (J2 Z1−2 ): The upper submember is a meander river depositional system formed in a humid climate in the middle sedimentary period, with a thickness of 11–39 m. The main lithology is brown, light yellow, gray-green, green-gray medium-fine sandstone, sandwiched between the top siltstone and mudstone. The rock has good sorting ability, the roundness of grinding is subround, mud

3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock …

147

Fig. 3.22 Comprehensive stratigraphic histogram of the southeastern margin of the basin (in the Huangling area)

148

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

cementation is dominant, calcareous cementation is secondary, and there is no organic matter. Evident is horizontal bedding, block bedding, and some crossbedding. 4. Upper member of the Zhiluo Formation of the Middle Jurassic (J2 Z2 ): The upper member of the Zhiluo Formation is a dry early tidal mooring and meandering river depositional system formed in the late sedimentary period. It was a multicolored deposition formed under an arid climate, with a thickness of 50–105 m. The middle and upper parts are interbedded with purplish red mudstone, brownish red mudstone, silty mudstone, and siltstone, interbedded with reddish brown medium-fine feldspar quartz sandstone lens, mostly containing thin layers of gypsum interbedded and gray and gray-green mudstone, and siltstone can be seen at the bottom. Sandstone, siltstone, and mudstone are composed of multiple sandstone–mudstone dual structures. Meandering river features are obvious. The upper mudstone is thickened and mostly purplish red and is a flood deposition. The formation is dominated by an oxidation environment and no uranium mineralization has been found. 3.2.2.3

Roof and Floor Structure of Uraniferous Rock Series

The lower member of the Zhiluo Formation along the southern margin of China has a great variation in stratigraphic undulation, which is higher in the southeast, lower in the northwest, and highest in the southeast. The roof of the ore-bearing aquifer is composed of sand and mud deposits in the lower member and sand–sand interbedding in the upper member of the Zhiluo Formation, exhibiting good regional stability and water isolation in general. The roof elevation is 800–2200 m. The eastern Jiaoping–Jianzhuang area is the uplift area, the northern and western areas are low-lying, and the axial extension of the uplift zone is in the northeast direction. The floor of the ore aquifer is the Middle Jurassic Yan’an Formation, consisting of fluvial facies deposition of coal-bearing fine broken shoulder rocks. The elevation of the floor is 600–140 m, and the structural morphology is similar to that of the roof. The uplift range of the eastern Jiaoping area is relatively reduced. At present, uranium deposits and ore spots found in the region are mainly distributed west of the Weibei uplift belt, indicating that the distribution of orebodies is closely related to the slope belt of positive structure.

3.2.2.4

Spatial Distribution Characteristics of the Ore-Bearing Aquifer

The main prospecting target layer along the southeastern margin is the Zhiluo Formation, which is mainly a fluvial facies deposition, and the lithology of the lower part is mainly green and gray sand and sandy conglomerate intercalated mudstone. The upper part is interbedded with purplish red mudstone and gray-green sandstone, indicating floodplain and meandering river deposits. The lower member of the Zhiluo Formation is the main ore-bearing target layer, and the braided river sandbody is the

3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock …

149

important ore-bearing sandbody in the area. The shape of the sandbody is stratified, and the distribution in the study area is relatively stable. The thickness of the sandbody in the study area is generally 5–90 m, with an average thickness of ~ 45 m. It is generally thick in the northeast and thin in the southwest (Fig. 3.23d).

3.2.2.5

Abnormal Characteristics of Radioactivity in the Zhiluo Formation

The radioactive anomaly horizon observed along the southeastern margin is primarily in the lower member of the Zhiluo Formation, and the anomaly distribution trends northeast–southwest, which is relatively consistent with the extension direction of the northern uplift zone. The anomaly in the Huangling–Shuanglong area in northeast China extends nearly east–west, and the area with an anomaly intensity of > 1000 Y is nearly 100 km. Because of the high-background value of natural γ-ray background of boreholes in this area, 100 Y cannot be used as the discrimination index of potential uranium mineralization. The radioactive anomalies in the Jianzhuang–Miowan area extend northeastward and are mainly distributed west of the Jianzhuang–Miowan uplift. The radioactive anomalies in the Binxian–Beibei area exhibit an elliptical distribution, mainly distributed between Xinminzhen and Tingkou (Fig. 3.23).

3.2.3 Western Margin of the Ordos Basin 3.2.3.1

Western Margin Thrust Belt

The western margin thrust belt is one of the most important structural units in the Ordos Basin. In the Mesozoic, it was a depression. Since the Cenozoic, it has been thrust from west to east. It is generally distributed in the north–south direction, is 50–200 km wide from east to west and 600 km long from north to south, covering an area of ~ 6000 km2 . The Ningdong uranium mining area is located in the Majiatan Tianshuibao section in the middle of the western thrust belt. The structure in the area is relatively complex, being composed of a series of wide and gentle folds trending NNW or near north–south and associated faults, reflecting the characteristics of multistage tectonic movement in the area. The north–south structural characteristics of the belt are also different. The north is dominated by folds and a few faults. The south fault is generally developed, which destroys the integrity of the fold (Fig. 3.24). Folds control the distribution and morphological characteristics of ore-bearing aquifers, and fault structures affect the integrity and hydrodynamic characteristics of ore-bearing aquifers. Fold and fault structures control the formation and development of the interlayer oxidation zone, as well as the enrichment and formation of the uranium mineralization state distribution (Fig. 3.25).

150

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.23 Map of the southeastern edge of the Ordos Basin. a Geological sketch of the southeastern edge of the basin, 1. Quaternary Paleogene. 2. Lower Cretaceous. 3. Zhiluo Formation of the Middle Jurassic. 4. Middle Jurassic Yan’an Formation. 5. Middle and Upper Triassic. 6. Lower Triassic. 7. Paleozoic. 8. Fault. 9. Sandstone-type uranium deposit (mineralization) point. 10. Scope of the Huangling research area. b Contour map of the lower section of the Zhiluo Formation. c Bottom plate contour map of the lower Zhiluo Formation. d Equal thickness diagram of the sandbody in the lower section of the Zhiluo Formation. e Intensity map of the natural γ anomaly in the lower section of the Zhiluo Formation

3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock …

151

Fig. 3.24 Structural outline of the western margin of the basin

Six anticlines and four synclines composed of Triassic and Jurassic strata are developed. From west to east, these are the Yandunshan syncline, Shenjiazhuang Yangzhuang anticline, Yezhuangzi Xiaoshawanzi syncline, Jijiajing Tianshuibao anticline, Haizihu Hejiayao syncline, Zhoujiagou Yujialiang anticline, Jianerzhuangzi anticline, Changliangshan Majiatan syncline, and Yuanyanghu Fengjigou anticline. A total of 18 main faults are developed, of which the Wandunshan, Shangtaizi, and Maliu faults are first-order reverse faults and are the main faults in this area. The middle zone of Sandunshan and Maliu faults is the Ciyaobao Dashuikeng fault depression, which is the primary metallogenic belt in this area.

152

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.25 Comprehensive histogram of strata along the western margin

3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock …

153

The crystalline basement on the western edge of the basin is a pre-Paleoproterozoic metamorphic rock series, which is composed of granulite facies, amphibolite facies metamorphic rocks, and granite. Most of them are covered with several kilometers of overlying material. They are only exposed on the surface in the middle and northern sections of Helan Mountain in the western part of the basin and Table Mountain in Inner Mongolia. The Neoproterozoic and Paleozoic basements of the basin are exposed to varying degrees in the western part of the basin and can directly provide rich sediment and uranium sources for the deposition of the basin caprock. The caprock is mainly composed of Triassic, Jurassic, Cretaceous, Paleogene, Neogene and Quaternary strata. The Triassic layer is an important oil and gas reservoir along the western margin of the basin, the Jurassic layer is the main coal seam and uranium-bearing layer, with the Cretaceous layer also being of some value.

3.2.3.2

Stratigraphic Characteristics of Uraniferous Rock Series

Jurassic rocks are widely distributed in this area; from bottom to top, there are middle and upper series rocks and Yan’an, Zhiluo, and Anding formation rocks. The Yan’an Formation is mainly composed of coal-bearing shoulder rock, which is the main coal-bearing horizon and also one of the principal uranium-bearing target horizons. The Zhiluo Formation is located above the Yan’an Formation at a moderate burial depth and mainly represents fluvial facies deposition, which is the main prospecting target layer in this area (Fig. 3.26). 1. Middle Jurassic Yan’an Formation (Jy) The Middle Jurassic Yan’an Formation is the principal coal-bearing rock series in this area and also one of the targets of uranium exploration. The minimum, maximum, and average thicknesses revealed by drilling are 250.23, 429.09, and 36.61 m, respectively. It is in angular unconformity contact with the underlying Triassic Ueda Formation. The Yan’an Formation is a fluvial–lacustrine delta–inland lacustrine sedimentary system. The lithology is mainly composed of gray and gray-white feldspar quartz sandstone, gray and gray-black siltstone, mudstone, carbonaceous mudstone, and coal. The bottom comprises a set of pale white, white, and local yellow rocks with erythematous coarse sandstone and pebbly coarse sandstone. 2. Middle Jurassic Zhiluo Formation (J2z) The Zhiluo Formation has minimum, maximum, and average thicknesses of 336, 495, and ~ 431.1 m, respectively. It is in conformity contact with the lower Fuyan’an Formation. The sedimentary environment evolved from warm and wet to dry, with fluvial facies in the early stage and lacustrine facies in the late stage. The lower member of the Zhiluo Formation can be further divided into an upper submember (J2'' ) and a lower submember (J2'' ) according to the sedimentary characteristics and lithologic structures of different stages in the sedimentary process. Multilayer

154

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.26 Comprehensive histogram of ore-bearing rock series along the western margin thrust belt

3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock …

155

uranium mineralization has been identified in the Zhiluo Formation, and the lower member of the Zhiluo Formation sandstone uranium mineralization is the most developed and is the main prospecting target layer. The Zhiluo Formation lithology in the upper class of yellow, green, green with purple, purple with green swan, amaranthine, reddish brown powder sandstone, fine sandstone, clip a thin layer of feldspar quartz sandstone, mudstone, is in dry and semidry early ancient gas deposits and entertainment environment, belong to high curvature meander sedimentary system, the floodplain deposits, lithology combination for sand mixing layer structure, meandering river sedimentary characteristics of dual structure. The sandbody is mainly fine sandstone with a thickness of ~ 10 m. The upper submember is a low-curvature meandering river depositional system with large phase transformation and limited distribution. It also has the characteristics of floodplain depositional development, obvious dual structure, and frequent occurrence of sand and mud interlayers. Uranium mineralization in some areas indicates that it is a subtarget layer for prospecting. The siltstone and fine gray-green and graygreen sandstone with purple bands are mainly mixed with thin sandstone. The lower submember is a sandy braided river sedimentary system formed in a humid environment, which shows that the sandbody mostly appears in the deep valley and has the sedimentary characteristics of filling. In the vertical direction, it is composed of multiple rhythmites overlapping from coarse to fine sand, silty sand, and mudstone, forming a widely connected network. It is the main uranium mineralization layer in the area. For light gray, gray, gray green. At the bottom there is 30,160 m thick pebbly feldspar stone sandstone.

3.2.3.3

Roof and Floor Structure of the Axis-Bearing Rock Series

The western margin of the Ordos Basin in the east Ningxia region, group under straight section of the strata of roof, floor mark near the overall reflect the north west or north and south to the rolling skirt knit group of tectonic characteristics, because of the influence and Cenozoic thrust fault zone in skirt wrinkled, porcelain top secret fort and south Zhuang Zi in northern region of roof and floor elevation of 1100–1300 m, central ma beach roof and floor elevation for 900–1200 m, indicating that the roof and floor of the lower member of the Zhiluo Formation in Majiatan area in central China are low, the Majiatan area exhibits the saddle characteristics of the north–south anticline. It can be seen that the uranium deposits and ore spots in Jimibao, Maiduoshan, Majiatan, and Yezhuangzi are mainly distributed on the axis and on two wings of the north–south skirt belt, indicating that the skirt controls the distribution and morphological characteristics of uranium orebodies in this area (Fig. 3.27).

156

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.27 a Roof elevation contour map and b floor elevation contour map of the lower subsection of the lower Zhiluo Formation in the Ningdong area

3.2.3.4

Spatial Distribution Characteristics of Ore-Bearing Aquifers Along the Drawing Edge

Based on the statistics of sandbody thickness and sand-to-ground ratio, the sedimentary facies maps and sandbody distribution maps of each member of the Zhiluo and Anding formations were compiled by using the dominant facies mapping method. These are described as follows: 1. Sedimentation, facies, and environment of the lower submember of the Zhiluo Formation The lower submember of the Zhiluo Formation developed fresh river deposits, including channel filling, floodplain, and heart bank. Three channels of the Shaoxing River were identified in the survey area. Three channels of the braided River were identified in the survey area, with the main channel developed along the direction from Shigouyi towards Yeerzhuang. The main channel bifurcates in the Majitan zone

3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock …

157

and continues to extend eastward along the Zhangjiajuan. The channel width is large, the bend is low, the sand ratio is mostly > 80%, in the center of the channel sand ratio is > 90%, even up to 100%. In addition, two subchannels were developed in the northern part of the survey area. The width of the channel was small, the ratio of sand to soil was > 70%, and the channel extended from west to the Gujia circle. The two subriver channels converge in Zhangjiatu, widening the width of the channel and continuing to extend to the east and southeast (Fig. 3.28). Sedimentary facies zone controls the development degree of sandbody. Located in the main channel, the sandbody is very well developed, and the sandbody is continuous slab. The thickest place is near well Shui13-1, the sand thickness can reach 208.6 m, the average thickness of sandbody is 110.7 m. The sand thickness in the main channel is > 100 m. The thickness of the sandbody in the two subchannels in the north is slightly less than that in the main channel, and its value is mostly distributed at ~ 80 m, while the thickness of the sandbody in the floodplain facies near the two sides of the main channel decreases obviously. The whole sandbody deposition area accounts for ~ 70% of the area.

Fig. 3.28 a Sedimentary facies map and b sandbody isopach map of the lower submember of the lower Zhiluo Formation in the Ningdong area along the western margin

158

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

2. Sedimentary facies and environment of lower and upper submembers of the Zhiluo Formation Meandering river deposits were developed in the upper and lower members of the Zhiluo Formation, including channel filling, flood bank, and edge bank. There is a main meandering river channel in this area. The river extends to the east and southeast along the Cimibao–Maiza Mountain–Jigmo Wan–Ye’er Zhuang–Tan Sheep Farm area. The river presents many bends and the curvature is larger. The width of the channel is up to 30 m, and the sand ratio is > 30% at the development of the channel, > 50% at the center of the channel, and > 95% in the eastern part of Yeerzhuang. Floodplain deposits are developed in the northeast and southwest of the region (Fig. 3.29). The sandbodies of the lower and upper submembers of the Zhiluo Formation are distributed along the area of Cimibao–Maiduo Mountain–Pimenwan–Ye’er Zhuang– Tan Yangchang. Compared with the lower submember of the Zhiluo Formation, the sandbody is obviously thinner and smaller in size. The thickness of the sandbody corresponding to the main channel is generally > 30 m, and the thickness of the sandbody at the center of the channel is generally > 50 m, and the maximum thickness can reach 94.8 m. The sandbody has suffered a small range of denudation in majiatan area. The whole sandbody deposition area accounts for ~ 60% of the area.

3.2.3.5

Abnormal Characteristics of Radioactivity in Uranium-Bearing Rock Series

The West east Ningxia region through the screening of coalfield drilling logging data, found that the intensity of natural γ-ray value is higher, the region at fort radioactive anomaly pattern from the porcelain—the wheat-rick mountain—ma beach—when Chuang Tzu is nearly ns-trending distribution, abnormal shape, zonal characteristics significantly, γ anomaly strength os > 14 PA/kg (Fig. 3.30) larger area, one of the northern and central intensity and anomaly thick. The degree is larger than that of the southern area, indicating that there is more prospecting space in the north and central area. This understanding has been confirmed by the uranium survey in recent years. However, there are few boreholes collected in the coalfield in the southern Hui’anpu area, and the abnormal morphology does not show the form of the Hui’anpu deposit discovered by predecessors. On the whole, the distribution of natural γ radioactivity anomaly in eastern Ningning is consistent with that of the north–south anticline zone, which further indicates that uranium enrichment in this area is closely related to the enrichment of uranium deposits.

3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock …

159

Fig. 3.29 a Sedimentary facies map and b sandbody isopach map of the upper submember of the lower member of the Zhiluo Formation in the Ningdong area along the western margin

3.2.4 Southwestern Margin of the Ordos Basin 3.2.4.1

Regional Geological Background

The area covers some administrative areas of Huating County, Chongxin County, Jingchuan County, Pingliang City in Lingtai County, Zhengning County, Ning County, and Qingyang City in Huan County. The structural division involves secondary structural units such as the Weibei uplift, western margin thrust belt, and Tianhuan syncline of the Ordos Basin (Fig. 3.31). The continental Mesozoic–Cenozoic sedimentary strata along the southwestern margin of the basin are well developed, with a total thickness of > 500 m and mainly fluvial and lacustrine deposits. The developed caprocks include Triassic (T), Jurassic (J), Lower Baimo (K) of the Mesozoic, and Paleogene (I), Neogene (N), and Quaternary (Q) of the Cenozoic. Among them, the Triassic, Jurassic and Lower Cretaceous deposits are the main strata in the basin, while the Jurassic and Lower Baimo deposits are the main prospecting targets in this area. The main features are as follows:

160

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.30 Isoline maps of a radioactive anomaly intensity b abnormal thickness in the lower section of the Zhiluo Formation in the Ningdong area on the western edge

1. Yanchang Formation The Yanchang Formation is mainly a set of red clastic rock formations that outcrop to the south outside the survey area. They include the Lower Triassic Liujiagou and Shougou formations, the Middle Triassic Zhifang Formation. And the Upper Triassic Yanchang Formation. 2. Jurassic rocks The Jurassic rocks comprise mainly the terrigenous clastic formation of fluvial lacustrine facies, including the Middle Jurassic Yan’an Formation, Zhiluo Formation, Upper Jurassic Anding Formation, and Xianghe Formation. Most areas of the Fuxian Formation of the Lower Jurassic in the area are missing. The Fuxian Formation (J1f) is the product of leveling after the Indian branch Yungong, and its distribution is very sporadic, being scattered throughout Ningxian, Chongxin, and Huanxian counties. The coalfield has been exposed in the north of the

3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock …

161

Fig. 3.31 Structural outline of the southwestern margin

Huanxian working area and is in parallel unconformity contact with the underlying Triassic Wayaobao Formation. The Yan’an Formation (J2y) is widely distributed in the Huating Chongxin area, Huanxian county, and Tianshuibao coal producing areas 60 km north of the survey area along the western margin and almost throughout the basin. The lithology is mainly gray and gray-black siltstone, mudstone, and light gray and gray-white sandstone, intercalated with carbonaceous mudstone and coal seams. Thick coal seams or extra thick coal seams often exist at the bottom, with oil shale in some parts. The formation is rich in plant fossils and sporopollen fossils such as equisetite lateralis Phillps, with a small amount of bivalves and fish fossils. The formation in the Chongxin area is generally 95–296 m thick, and that in the Shajingzi area in Huanxian

162

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

county is 103–285 m thick. The Yan’an Formation is in parallel unconformable contact with the underlying Fuxian Formation. The lithology of the Zhiluo Formation (J2z) is variegated siltstone, argillaceous siltstone, and sandy grayish green, grayish, and purplish red mudstone, intercalated with fine sandstone, and the bottom is grayish white gravelly coarse sandstone, containing plant and sporopollen fossils. The western edge is 150–669 m thick, being thin in the south and thick in the north. The formation is thin in the survey area, with an average thickness of ~ 50 m. It is in parallel unconformity contact with the underlying Yan’an Formation. Compared with the distribution of the Yan’an Formation, the distribution scope of the Anding Formation (J2a) is slightly narrowed. The lithology of the western edge is grayish brown, purple, purplish red, grayish yellow, and other variegated mudstone (mostly in mass, called pimple mudstone) and silty mudstone mixed with sandstone, and the upper part is mixed with dolomitic marl, marl, and limestone. The residual thickness is 56–493 m, being thin in the south and thick in the north, The Anding Formation contains ostracods and sporopollen fossils, which are in integrated contact with the underlying Zhiluo Formation. The Xiangxianghe Formation is mainly distributed in the foreland depression on the western edge of the basin. It is mainly a set of proluvial and deluvial brownish red and purple gray massive conglomerate and boulder mixed with a small amount of brownish red sandstone and argillaceous siltstone. It is poorly sorted and rounded. This set of sediments is thick in the west and thin in the east, with a thickness of 100–200 m, and it gradually pinches out to the east. 3. Cretaceous (k) rocks The Yanshan movement at the end of Jurassic raised the periphery of the Ordos Basin, and the Lower Cretaceous rocks were deposited along the western margin of the Ordos Basin, which was mainly alluvial fan fluvial lacustrine facies under the condition of an arid climate. It is called Zhidan Group in the basin and the Liupanshan Group in the Liupanshan area, which corresponds to each group of the Zhidan Group. Most of the strata in this area belong to the Liupanshan Group and a small part of the Lower Cretaceous strata belongs to the Zhidan Group. The first group (K1 L1 ) of the Liupanshan Group (K1 L) is the Sanqiao Formation, which is equivalent to the eastern Luohan Formation and is sporadically exposed. The lower part is dark purple conglomerate mixed with purple coarse sandstone, and the upper part is yellow-green conglomerate mixed with coarse sandstone and light green and purple sandy mudstone. The local stratum is purple sandy mudstone and light green sandstone mixed with grayish green conglomerate. The average thickness of the stratum is > 250 m, which is continuously deposited with the overlying group II. The second group (K1 L2 ) of the Liupanshan Group is the Shangshangpu Formation, which is equivalent to the eastern Luohan Formation, Jingchuan Formation, and the lower part of the Jingchuan Formation, and it is exposed in some areas of the whole region. The lower part is green-gray sandy mudstone and siltstone mixed

3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock …

163

with green-gray coarse sandstone and purplish red sandy mudstone, the middle part is mainly purplish red sandy mudstone mixed with yellow-green coarse sandstone, and the upper part is light green and yellow-green sandy mudstone and argillaceous mixed with siltstone of the same color, with an average thickness of ~ 200 m, in angular unconformable contact with the overlying Neogene and Quaternary strata. 4. Neogene (n) rocks The Ganhegou Formation (N2 g) is in angular unconformity contact with the underlying stratum and unconformity contact with the overlying Quaternary system. The bottom is gray and grayish red thick layered conglomerate, the lower part is graywhite and grayish yellow sandy conglomerate, and the upper part is orange and orange-red sandy mudstone and mudstone, intercalated with fine conglomerate and pebbly sandstone lens. 5. Quaternary (q) rocks Quaternary rocks are mainly alluvial, proluvial, deluvial, and eluvial sediments, in unconformity on all old strata.

3.2.4.2

Stratigraphic Characteristics of Uraniferous Rock Series

Mesozoic and Cenozoic continental sedimentary strata are well developed along the southwestern margin of the basin, with a total thickness of > 5000 m and primarily fluvial and lacustrine deposits, including Triassic (T), Jurassic (J), Lower Baimo (K), Paleogene (E), Neogene (N), and the fourth series (Q) (Fig. 3.32). The Triassic, Jurassic, and Lower Cretaceous are the main sedimentary bodies of the basin, similar to other parts of the basin, and the Lower Cretaceous Luohe Formation is the main uranium-bearing horizon discovered in this new study.

3.2.4.3

Characteristics of the Sedimentary System

The alluvial fan, fresh channel sand (SQ), and shallow lacustrine sand bar (SO) in the low system region of the lower member of the Luohe Formation were further identified as the principal ore-bearing horizon newly discovered in the Early Cretaceous of the Naerdos Basin. There are 2 regional (second-order) sedimentary cycles or structural sequences in the Lower Cretaceous in the southwestern Ordos Basin, which can be divided into 8 third-order sequences, 17 third-order sequences, and dozens of parasequences (groups) (Fig. 3.33). Under the established sequence stratigraphic framework, four sets of sedimentary systems can be identified: alluvial fan, braided river–meander river, dry early shallow lake, and desert deposits (Fig. 3.34). The Luohe Formation is composed of alluvial fan (marginal facies) and fluvial coarse broken shoulder deposits (5Q-502) at the bottom and desert facies eolian deposits (SQ-5q.) at the top, and intermittent

164

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.32 Comprehensive histogram of strata in the southwestern edge of the Ordos Basin

3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock …

165

Fig. 3.33 Sequence stratigraphic division of Lower Cretaceous rocks in the Zhenyuan area

desert developed during the late stage shallow tidal facies deposition (Fig. 3.34). The Huachi–Huanhe Formation is composed of dry early shallow tidal fine clastic rock deposits (50-Sq.), interspersed with eolian deposits (sQ, -5q.) in the early stage and a large number of gypsum intercalations (SQ-50) in the middle stage. The early (50, -Sq) and late (Squ) Luohandong Formation are both desert eolian deposits, and the middle is desert shallow lacustrine deposits (SQN-5q.) formed under a dry early background. The Jingchuan Formation as a whole was transformed into a set of fluvial deposits under a high system tract. The above sequence stratigraphic structure generally reflects the depositional cycle of the Tianhuan depression from early to late from water inflow (Luohe Formation–Huachi–Huanhe Formation) to water regression (Luohandong Formation–Jingchuan Formation). During the deposition of the Huachi–Huanhe Formation, the tidal level reached its peak. The alluvial fan in the lower member of the Luohe Formation, the shafted channel sandbody (SQ1 ),

166

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.34 Sedimentary system division of the Lower Cretaceous in the Jingchuan Zhenyuan area. 1. Mudstone. 2. Argillaceous siltstone. 3. Fine sandstone. 4. Medium sandstone. 5. Coarse sandstone. 6. Conglomerate. 7. Oil immersion. 8. Gypsum. 9. Uranium orebody; 10. Uranium mineralized body. 11. Pyrite

and the shallow-phase sand bar (SQ5 ) in the tidal extension system of the Huachi– Huanhe Formation are the primary and secondary ore-bearing horizons in this areas respectively.

3.2.5 Central Part of the Ordos Basin The central part of the basin (the Zhidan Dingbian area) is located in the Yishan Slope of the secondary structural unit of the Ordos Basin. Loess is widely covered in the central area of the basin, and the exposure of bedrock is poor. Basement fluctuation in

3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock …

167

the area is very minor, and the dip angle of sedimentary caprock is gentle. The central part is an uplift area from the Late Neoproterozoic to Early Paleozoic. Denudation occurred and did not accept sedimentation. Marine strata with a total thickness of 500–1000 m were deposited in the middle Late Cambrian and Early Ordovician. The present structural feature is a gentle monocline inclined to the west, with an average slope of 10 m/km and an inclination of < 1°. Nose structures are developed in some areas.

3.2.5.1

Regional Geological Background

Jurassic (J) and Cretaceous (k) strata are exposed on both sides of the main water system and its tributaries. The Middle Jurassic stratum is the main prospecting target layer in the working area; it can be divided into the Yan’an Formation (J2y), the Zhiluo Formation (J2z), and the Anding Formation (J2a) from bottom to top. The Lower Cretaceous rocks are mainly the Lower Cretaceous Luohe Formation (K1l) and the Huachi–Huanhe Formation (K1hc+h) (Fig. 3.35), and the Yijun Formation (K1y) is less exposed. The Yan’an Formation (J2y) is widely distributed in the area and exposed in a large area in the eastern Shenmu and Hengshan areas. It is fluvial facies consisting of 5–7 layers of coal. Vertically, the whole stratum has a coarse to fine to coarse stratigraphic structure. The rock stratum is generally ~ 160 m thick, with a maximum thickness of 450 m. This formation is the primary coal-bearing stratum in the basin. The Zhiluo Formation (J2z) is widely developed in the area. It is exposed in an arc in the area of Hantaichuan, Daliuta, and Hengshan in the eastern part of the basin. The edge is in micro-angle unconformity contact with the Yan’an Formation, and it gradually transits to parallel unconformity contact into the basin. The Zhiluo Formation consists mainly of fluvial facies, with a thickness of 52–300 m. It is the main target layer for uranium deposit research. The Diazepam group (J2a) east of the area is adjacent to the west side of the Zhiluo Formation and is exposed in a north–south strip. It is commonly drilled in Northern Shaanxi, Longdong, Gansu, Dongsheng, Inner Mongolia, and Ningxia. The Anding Formation is obviously divided into three parts in the region. The lower part is black shale and grayish black oil shale and a small amount of calcareous dolomite and clayey dolomite. The middle part is yellow-green and dark peach shale, grayish green mudstone, and calcareous siltstone. The upper part is interbedded with gray dolomite, yellow marl, and calcareous shale. The Anding Formation has stable lithology and only a minor variations in thickness, being generally tens of meters to > 100 m thick but 243 m thick in Qujiawan, Qianyang. A relatively complete Anding Formation is exposed in the north mountain of Wangyao west of Zhao’an Town to the west of Yan’an (Fig. 3.36). The bottom is grayish black silty mudstone intercalated with grayish black oil shale (8–12 m thick) with horizontal bedding. The lower part is gray and gray-black thick calcareous marl mixed with yellow silty shale and limestone in mainly a horizontal bedding. The middle part is gray and gray-white thick marlstone intercalated with argillaceous siltstone. In the upper part, there are alternately layers

168

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.35 Comprehensive histogram of Mesozoic and Cenozoic strata in central Ordos Basin

3.2 Geological Characteristics and Correlation of Uranium-Bearing Rock …

169

of gray-black, egg-green and gray-red marls, and chert strips and siliceous nodules are mostly seen in the rock stratum. The top is gray-purple mud shale and silty shale, with rhythmitic bedding developed. Numerous paleontological fossils are found in this group, including fish, ostracods, sporopollen, and other fossils. The middle and upper parts of the Anding Formation comprise mainly gray-purple marl, intercalated with purple-gray mudstone, and the bottom is light gray sandstone, which is in parallel unconformity contact with the underlying Zhiluo Formation. The marl section of this group is one of the major marker beds divided by Mesozoic strata, with a stable distribution and thickness of > 10 m to tens of meters. The logging curve

Fig. 3.36 Measured geological profile of the Anding Formation

170

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

is characterized by high resistivity, which contrasts starkly with the low resistivity of upper and lower adjacent strata. The γ anomalies found are mainly distributed in the Anding Group.

3.2.5.2

Stratigraphic Characteristics of Uranium-Bearing Rock Series in Central China

The Anding Formation in Zhidan and Dingbian areas in the central Ordos Basin is thick in the southeast and thin in the northwest. The tectonic movement after the deposition of the Anding Formation put it in unconformable contact with the Cretaceous strata in most parts of the basin. For the Anding Formation, which experienced multistage reconstruction and deep burial compaction in the Early Cretaceous, the thickness recovery method is limited and the reliability of the results is low. However, the variation regularity of residual stratum thickness and buried depth of the roof and floor is obvious. Therefore, it is still a significant reference point for exploring the spatial distribution of the Anding Formation for the study of uranium mineralization. The thickness of the Anding Formation is shown in Fig. 3.37. The thickness in the southeast is ~ 60 m and transits to the northwest of the study area. The stratum thickness is ~ 110 m. The thickness exhibits a linear increasing trend from the edge of the basin to the sedimentary center. During the sedimentary deposition period of the Anding Formation, the height difference of paleogeomorphology inside and outside the basin became greatly reduced and the fluctuation tended to be gentle. A stable provenance supply and a tectonic environment endow the strata of the Anding Formation with a good metallogenic potential. The burial depths of the roof and floor of the Anding Formation (Figs. 3.38 and 3.39) also have similar regularity. Generally, the formation is a complex anticline with an anti-Y shape and dipping to the northwest, being high in the southeast and low in the northwest, with a high wave peak and low two wings. The stable formation tendency well controls the later fluid transport process in the uranium reservoir, creating favorable conditions for uranium enrichment.

3.3 Comparison of Logging Parameters of Uranium-Bearing Rock Series in Key Metallogenic Prospective Areas Changes in the characteristics of different logging parameter curves can reflect the sedimentary strata and paleo-sedimentary environment in different periods. Previous researchers have focused on analyzing and testing the lithology and physical parameters of the exposed geological bodies around the Ordos Basin, and magnetic susceptibility and density data of the whole sedimentary strata have been obtained. The statistics of formation radioactivity and physical parameters were determined by

3.3 Comparison of Logging Parameters of Uranium-Bearing Rock Series …

171

Fig. 3.37 Stratigraphic thickness of the Anding Formation in the Zhidan Dingbian area of the central Ordos Basin

using some boreholes in the northeastern margin area (Zhang 2010). However, a systematic statistical comparative analysis has not been performed on the logging geophysical parameter data of the main uranium-bearing rock series in different areas around the basin. Based on the statistics of natural γ, resistivity, spontaneous potential, and other logging data of nearly 100 uranium deposits in four uranium concentration areas of the basin, in this section we analyze the differences of physical properties of uranium-bearing rock series in the Zhiluo Formation and the various lithologies in different areas. The purpose of this section is to provide guidance and reference for the rapid identification of lithology and the search for in situ leaching sandstone-type uranium deposits in the basin.

172

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.38 Top burial depth of the Anding Formation in the Zhidan Dingbian area of the central Ordos Basin

At present, the nine curve series used in conventional logging refer to three lithologic curves (natural γ, natural potential, and well diameter), three resistivity curves (shallow, medium, and deep), and three porosity curves (density, sound waves, and neutrons). Natural γ and natural potential logging can reflect the lithology and sedimentary environment of the reservoir, but the logging response is easily affected by the salinity of the drilling fluid and radioactive minerals, making it difficult to identify effectively (Chu et al. 2007). Resistivity logging can indirectly reflect the pore structure of the target layer. Density, acoustic moveout, and neutron porosity logging can directly reveal the physical properties of the target layer. Therefore, only through comprehensive analysis of multiple parameters can we accurately classify lithology, identify strata, and interpret ore beds. The three logging curves of natural

3.3 Comparison of Logging Parameters of Uranium-Bearing Rock Series …

173

Fig. 3.39 Bottom burial depth of the Anding Formation in the Zhidan Dingbian area of the central Ordos Basin

potential, three lateral resistivity curves, and density logging have been widely used to interpret lithology and sedimentary facies and to analyze permeability in coalfields and in oil and gas exploration areas. In addition, the quantitative γ logging tool is calibrated at the radioactive exploration and metering station of the nuclear industry. The logging data are consistent, ensuring the comparability of radioactivity data in different regions and for different lithologies Therefore, in this book, the above four logging parameters with different lithologies in the upper and lower segments of the Zhiluo Formation of four uranium-concentrated areas in the Ordos Basin are selected for classification and statistics (Table 3.1). At the same time, combined with the logging curve shape of the borehole bar chart, the formation logging responses in different areas are compared and analyzed. Finally, the characteristics of logging parameters of the various lithologies are summarized.

Western margin

Upper member of the Zhiluo Formation

Northeastern margin

Upper member of the Zhiluo Formation

Lower member of the Zhiluo Formation

Stratum

Area

3.23 3.57

0.6–6 1.3–5.6 2.4–5 2.1–6.3

Medium sandstone

Fine sandstone

Siltstone

Mudstone

0.10–7.78 0.52–6.28 0.34–7.37

Medium sandstone

Fine sandstone

2.71

1.5–4.2 1.6–7.7

Siltstone

Mudstone

Coarse sandstone

3.00

1.5–5

Fine sandstone

3.27

2.97

2.54

3.16

3.06

1.2–5.3

Medium sandstone

3.32

1.7–5.2

Coarse sandstone

3.51

3.31

1.5–4

2.64

1.11–2.69

1.48–2.67

1.47–2.71

1.47–2.51

2.13–2.48

2–2.46

1.87–2.45

1.89–2.32

2.04–2.52

2.06–2.63

1.68–2.49

1.96–2.43

1.9–2.41

Density Range of change

Average value

Quantitative γ Range of change

Coarse sandstone

Lithology

2.25

2.29

2.36

2.25

2.31

2.23

2.14

2.08

2.30

2.31

2.22

2.21

2.18

Average value

3.05–102.37

5.78–73.57

4.97–67.6

7–23

7–16

8–76

8–37

8–55

6–35

7–36

7–50

7–114

7–140

23.51

22.56

18.83

10.93

11.88

12.29

13.19

16.69

9.40

11.03

11.86

12.92

15.16

Range of change Average value

Resistivity

Table 3.1 Statistics of stratigraphic parameters of the Zhiluo Formation in different areas of the Ordos Basin Spontaneous potential

− 94.13 − 90.21 − 79.10

− 255–395 − 255.38–374.83 − 256.85–393.05

(continued)

28.50

95.12

− 310–488

99.82

89.62

− 310–490

− 386–494

90.45

− 310–490

− 315–490

101.43 101.80

103.10

− 316–493 − 310–490

107.03

− 310–495

− 310–494

107.27

− 311–493

Range of change Average value

174 3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Southwestern margin

Area

Upper member of the Zhiluo Formation

Lower member of the Zhiluo Formation

Stratum

Table 3.1 (continued)

/

Fine sandstone

0.54–8.69

Mudstone

2.76–4.75

0.52–8.64

Siltstone

Medium sandstone

0.31–10.38

Fine sandstone

2.76–4.55

0.50–12.42

Medium sandstone

Coarse sandstone

0.32–10.85

Coarse sandstone

0.31–7.98

Mudstone

/

3.77

3.66

3.98

3.86

3.62

3.76

4.68

3.76

3.58

/

2.26–2.85

2.36–2.75

1.2–2.68

1.06–2.93

1.68–2.70

1.39–2.70

1.37–2.72

1.24–2.77

1.49–2.69

Range of change

0.43–8.11

Density

Range of change

Average value

Quantitative γ

Siltstone

Lithology

/

2.57

2.56

2.29

2.27

2.37

2.37

2.26

2.16

2.24

Average value

/

11–29

13–37

5.37–77.3

4.21–100

6.02–114

6.75–106

4.61–92.31

4.04–95.01

3.87–65.6

/

20.40

25.00

24.42

31.56

34.93

30.41

30.97

22.59

21.25

Range of change Average value

Resistivity

− 86.68 − 85.66 − 85.09 − 83.90 − 84.50 − 15.12 − 15.19

− 244.42–402.28 − 249.68–395.41 − 256.85–398.27 − 258.41–398.52 − 252.15–398.09 − 8.68 to − 21.55 − 8.28 to − 29.54

(continued)

/

− 83.29

− 246.80–392.79

/

− 80.06

− 254.77–392.86

Range of change Average value

Spontaneous potential

3.3 Comparison of Logging Parameters of Uranium-Bearing Rock Series … 175

Southeastern margin

Area

Upper member of the Zhiluo Formation

Lower member of the Zhiluo Formation

Stratum

Table 3.1 (continued)

2.08–4.04 2.16–4.26

Fine sandstone

Siltstone

1.34–6.84

Mudstone

1.79–3.86

1.89–2.92

Siltstone

Medium sandstone

1.59–5.22

Fine sandstone

1.23–3.18

1.29–1.92

Medium sandstone

Coarse sandstone

1.49–1.72

Coarse sandstone

1.14–6.84

Mudstone

3.17

3.03

2.50

2.21

3.83

2.33

2.62

1.61

1.61

2.81

1.98

2.15–2.87

2.35–2.93

2.48–2.95

/

2.29–2.89

2.59–2.99

2.21–2.94

2.21–2.94

2.51–2.94

2.34–2.94

2.24–2.96

Range of change

1.18–3.26

Density

Range of change

Average value

Quantitative γ

Siltstone

Lithology

2.48

2.68

2.73

/

2.56

2.79

2.59

2.63

2.73

2.73

2.57

Average value

27.45–45.45

38.46–85.35

55.69–82.45

106.45–201.45

12–44

14–29

15–34

16–38

19–39

11–32

14–35

34.60

58.08

69.52

153.95

22.25

21.00

24.00

27.13

29.00

21.80

23.50

Range of change Average value

Resistivity

− 36.46

− 19.19 to − 41.27

23.29 − 125.52 − 81.35

− 85.15–310 − 865.45–450 − 340.45–227.43

(continued)

271.70

− 20.77

− 8.69 to − 32.85 265.14–278.25

− 19.02

− 7.69 to − 30.85

− 9.3 to − 25.80 − 19.72

− 9.5 to − 25.80 − 18.17

− 9.1 to − 25.67 − 17.39

− 27.15

− 24.29 to − 34.87

Range of change Average value

Spontaneous potential

176 3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Lower member of the Zhiluo Formation

Stratum

3.00 /

2.07–4.87 / 3.845.29

Fine sandstone

Siltstone

Mudstone

4.40

2.96

1.87–4.56

Medium sandstone

2.82

1.45–4.91

4.32

2.21–3.02

/

2.11–2.95

2.28–2.90

2.22–2.98

2.39–3.05

Range of change

2.18–5.97

Density

Range of change

Average value

Quantitative γ

Coarse sandstone

Mudstone

Lithology

2.73

/

2.62

2.59

2.67

2.70

Average value

14.45–29.55

/

51.25–100.72

69.62–265.45

86.56–945.21

11.56–27.45

21.64

/

73.56

140.41

273.07

17.68

Range of change Average value

Resistivity

/

− 157.71

− 523.45–375.47

− 59.09

− 214.18

− 612.31–367.44

/

− 171.37

− 665.45–376.42

− 356.14–372.25

− 60.31

− 356.14–354.45

Range of change Average value

Spontaneous potential

Note The γ value is the range of normal values. It does not include the quantitative γ logging value of the ore section. A slash (/) means that the number of samples is small and does not participate in statistics

Area

Table 3.1 (continued)

3.3 Comparison of Logging Parameters of Uranium-Bearing Rock Series … 177

178

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

3.3.1 Logging Response of the Zhiluo Formation 3.3.1.1

Comparison of the Characteristics of Radioactive Parameters

The statistical analysis results of γ logging parameters of the Zhiluo Formation in the whole basin reveal that there is little difference in γ exposure rate of the Zhiluo Formation in different ore concentration areas. The average value varies in the range of 1.98–4.68 nC/(kg h). The average γ exposure rate of the Zhiluo Formation along the western margin is 4.6 nC/(kg h), which is slightly higher than that in other areas. This high value may be related to the overall high shale content of the Zhiluo Formation in this area. There is a certain adsorption and pre-enrichment of uranium during deposition. Vertically, the γ background value of the upper member of the Zhiluo Formation is lower than that of the lower member of the Zhiluo Formation, indicating that the latter may have a relatively strong ability to provide secondary uranium sources.

3.3.1.2

Correlation of Physical Parameters of Strata

By taking the average statistics of different formation logging parameters in Table 3.2, the following conclusions can be made: 1. The resistivity of the Zhiluo Formation along the northeastern margin is the lowest, with an average of 9.4–16.6 Ω m. The resistivity curve zigzags between medium and low resistivity. The average rock density is 2.08–2.3 g/cm3 . The resistivity of the lower member of the Zhiluo Formation is relatively high. Its natural potential is mainly box-shaped, which is related to the braided river sedimentary system of this section. The spontaneous potential and natural γ curve in the upper segment of the Zhiluo Formation exhibit a toothlike negative abnormality, reflecting intermittent deposition (Fig. 3.40). Logging parameters Table 3.2 Statistics of average parameters of upper and lower strata of the Zhiluo Formation in different areas Area

Stratum

Quantitative γ [nC/(kg h)]

Density (g cm−3 )

Resistivity (Ω m)

Spontaneous potential (mV)

Northeastern margin

J2z2

3.252

2.244

12.074

104.126

J2z1

3.05

2.202

12.996

80.702

3.224

2.26

21.748

− 85.358

J2z1

3.98

2.312

30.458

− 85.166

Southwestern margin

J2z2

3.055

2.6075

22.675

− 23.48

J2z1

2.4

2.66

24.676

− 19.014

Southeastern margin

J2z2

3.046

2.6475

66.766

5.562

J2z1

3.295

2.6525

127.17

− 150.588

Western margin J2z2

3.3 Comparison of Logging Parameters of Uranium-Bearing Rock Series …

179

and field observation reveal that the sandstone of the Zhiluo Formation along the northeastern margin is characterized by loose lithology, low cementation, and high water cut. 2. The resistivity logging value of the Zhiluo Formation along the southeastern margin area is obviously on the high side as a whole, with an average value for the upper member of the Zhiluo Formation of 66.76 Ω m. The average resistivity of the lower member of the Zhiluo Formation is 127.17 Ω m, which is much higher than that in other parts of the basin. The average density of the Zhiluo Formation is 1.73–2.56 g/cm3 . According to the core condition of mud logging in this area, the degree of siliceous cementation in the lower part of the Zhiluo Formation is high and the rock is relatively dense. The ore lithology is mainly gray medium-coarse feldspar quartz sandstone. The mass fraction of the major chemical constituent SiO2 is between 60.42 and 86.65%, with an average content of 53% (based on 21 samples). However, the SiO2 content of the primary sandstone in the lower member of the Zhiluo Formation of the Daying and Nalinggou deposits along the

Fig. 3.40 Logging curves and lithologic correlation of the Zhiluo Formation in different ore concentration areas of the Ordos Basin. a Northeastern margin area. b Southwestern margin area. c Western edge area. d Southeastern margin area. SP = spontaneous potential. DEN = density. GR = natural γbackground. RD = resistivity

180

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

northeastern margin is 66.5–73.87%, with an average content of 70.44% (Yi et al. 2015). The SiO2 content of sandstone of the Zhiluo Formation in the southern margin area is generally higher than that in the northern margin area. 3. The resistivity values along the western and southwestern margins are relatively close to each other, averaging 21.75–30.46 Ω m. The resistivity of the lower section of the straight group is higher than that of the upper section. However, the density in the southwestern margin area is relatively high, being 2.59–2.73 g/cm3 . The higher resistivity of the lower member of the Zhiluo Formation compared to that of the upper member of the Zhiluo Formation in the basin is related to the braided river sedimentary system of this section. Because the channel sedimentation is developed and the sand-to-mud ratio is relatively large, the density parameters of the same lithology in the upper and lower members of the Zhiluo Formation change little, so compaction is not obvious. The density is only related to the rock composition. The variations in resistivity and density in the region are generally high in the south and low in the north, indicating that the cementation degree of the Zhiluo Formation along the northern margin is lower and that the permeability is greater than that along the southern margin.

3.3.1.3

Logging Parameters of Different Lithologies

According to national standards of Classification and nomenclature scheme of sedimentary rocks (GB/T17412.2-1998) and petrological characteristics of the Zhiluo Formation in the Ordos Basin (Qi et al. 2007), five lithologic grades can be established: coarse sandstone, medium sandstone, fine sandstone, siltstone, and mudstone. Conglomerate is generally distributed in the Lower Cretaceous strata with some appearing in the thin layer of glutenite at the bottom of the Zhiluo Formation. The logging curve does not reflect this clearly. Therefore, there are no statistical parameters for conglomerate this time. The corresponding relationship between lithology and logging parameters is found by summarizing the parameters of the five grain size lithologies in different areas. The logging results of rocks with different grain sizes are shown in Fig. 3.41. The coarse sandstone is mainly distributed in the braided river in the lower member of the Zhiluo Formation, and the γ curve exhibits a low value response. The average γ exposure rate ranges roughly from 2.5 to 4 nC/(kg h). It is slightly higher than that of medium sandstone and lower than that of other lithologies. The undercutting of the channel at the bottom of the lower member of the Zhiluo Formation results in the sandbody containing a reduction medium (e.g., carbon debris, coal lines, and pyrite). This forms enrichment by reducing and adsorbing uranium. Finally, the γ value of the local area is on the high side. The resistivity and density are slightly higher than those of other small grain sizes. The logging parameters of medium sandstone and fine sandstone are similar. The values of the two parameters lie chiefly between those of coarse sandstone and mudstone. γ logging of medium and fine sandstone exhibits low value responses.

3.3 Comparison of Logging Parameters of Uranium-Bearing Rock Series …

181

Fig. 3.41 Relationship between different lithology and logging parameters of the Zhiluo Formation in the Ordos Basin

182

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

The variation range of γ exposure rate is 2.6–3.2 nC/(kg h), and the density is 2.2– 2.50 g/cm3 . There is a high resistivity response on the three lateral resistivity curves. The range of resistivity is 15–58 Ω m. The γ exposure rate of mudstone is higher than that of other lithologies, with values ranging from 3.1 to 4.32 nC/(kg h). One can see that, the finer the particle size, the higher the γ background value. This indicates that the high-background value of uranium in strata may be closely related to the adsorption of fine-grained rocks. The density range is 2.16–2.70 g/cm3 . Because of the close relationship between density and rock composition, it is not obvious that there is a negative correlation between density and rock grain size in the study area. The resistivity of mudstone is the lowest, with the range being 9.4–24.4 Ω m. With the increase of shale content and the decrease of rock particle size, the apparent resistivity decreases accordingly. Therefore, the characteristics of different lithologic logging parameters from coarse sandstone to mudstone reflect a distinct trend. With the thinning of rock particles and the increase of argillaceous content, the adsorption of uranium by rocks increases gradually. The background value of γ increases also gradually. With the increase of specific surface area, the content of bound water on the surface of rock particles increases. The abnormal amplitude of resistivity and spontaneous potential changes from large to small.

3.3.2 Logging Identification of Different Lithologies in the Zhiluo Formation Because of the differences in sedimentary environment, rock grain size, miscellaneous base content, and pore structure of different lithologies, the corresponding logging parameters are also different. There are some limitations in lithology identification with single logging parameters. Therefore, a lithology identification model can be established through the logging parameter characteristics of different lithologies. At present, there are four kinds of lithologic logging identification methods (Zhao et al. 2015). This book relies on the crossplot method to effectively identify the various different lithologies of the Zhiluo Formation. Because there are many boreholes along the northern margin of the Ordos Basin, the statistical logging parameters are more representative. It is difficult to establish a recognition model for different lithologies in different areas, so the borehole logging parameters in the northeastern margin area were selected for lithology identification. Through a comparative analysis of the identification results of different logging parameters in the northeastern margin area, one can see that the resistivity and density intersection chart enables relatively good recognition (Fig. 3.42). From mudstone to coarse sandstone, the grain size of the rock increases gradually, while the density decreases gradually and the resistivity increases gradually. Therefore, the chart of

3.3 Comparison of Logging Parameters of Uranium-Bearing Rock Series …

183

Fig. 3.42 Rendezvous chart of different logging parameters in the northeastern basin

resistivity and density can be used to better identify mudstone, medium-fine sandstone, and coarse sandstone of the Zhiluo Formation. There is a partial overlap between mudstone and siltstone in the plate, and it is possible that the siltstone in this area is argillaceous cementation. If the argillaceous content is high, it is difficult to effectively identify the two logging parameters. In summary, the following conclusions can be obtained: (1) The characteristics of logging parameters of the Zhiluo Formation in four uranium ore concentrated areas in the basin are different. There is little difference in the value of the γ exposure rate. The resistivity in the southeastern margin area is obviously higher than that in other areas. The density is high in the south and low in the north. The γ background value and resistivity of the upper member of the vertical formation are lower than those of the lower member of the vertical formation. However, the density value is relatively consistent, indicating that compaction is not evident. The γ background value increases gradually from coarse sandstone to mudstone in the Zhiluo Formation in the region. With the relative increase of density, the abnormal amplitude of resistivity and natural potential changes from large to small. (2) By optimizing the boreholes in the northeastern margin area, the lithology recognition model of resistivity and density intersection chart can be established. However, the recognition accuracy needs to be further improved by implementing other interpretation methods.

184

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

3.4 Sedimentary Environment of Uranium-Bearing Rock Series in the Ordos Basin 3.4.1 Restriction of Color Zoning of Uranium-Bearing Rock Series on the Paleo-sedimentary Environment The color of sedimentary rocks is one of the important macroscopic characteristics used for identifying rocks, dividing and comparing strata, and analyzing and judging paleoclimate and paleoenvironment. Color indicators are also used to reconstruct the paleoclimate environment or to explore pre-Quaternary climate transition events (Song et al. 2005; Yan et al. 2017). In a sense, color represents the material source of rock formation (Huang et al. 2016). Sandstone-type uranium deposits are formed by epigenetic fluid with sandstone as the ore-bearing carrier. The formation of uranium deposits is restricted by many factors, such as uranium source, paleoclimate, and epigenetic fluid transformation. Among them, paleoclimate and paleo-sedimentary environment are the most important conditions for the mineralization of sandstone-type uranium deposits (Chen 1994). Considerable research work has been conducted on the stratigraphic color zoning of sandstone-type uranium deposits. Most of these studies focus on the anatomical scale of typical ore deposits. The research is mainly based on the interlayer infiltration metallogenic theory of Soviet and American scholars. Earlier work also established an ideal infiltration model for ore-forming fluid (Fig. 3.43) and a uranium metallogenic model of the Ordos Basin (Fig. 3.44). According to the horizontal direction, the interlayer sandbody can be divided into three zones (Adams and Smith 1981): the oxidized zone (reddish brown), the secondary oxidized zone (brownish yellow), and the primary zone (gray). Chinese scholars have additionally introduced four zones (Cai et al. 2006), but three zonations are proposed by some scholars for the Ordos Basin (Yang et al. 2009). A (brownish yellow, light gray green) transverse oxidation–(light gray) redox transition–(gray) reduction zone is developed in the ore-bearing sandbodies along the northeastern margin of the Ordos Basin from north to south. For this book we analyzed big data borehole from sandstone-type uranium deposits in the Ordos Basin in the past five years, and we summarized the results of exploration and verification. Based on the statistics of rock colors of nearly 20,000 coalfields and oilfields in the Ordos Basin, a drilling database was established. Big data analysis and mapping of the ore concentration area and basin were performed, respectively. It was found that whether on the ore concentration area scale or on the basin scale, the color of the rocks exhibits the characteristics of vertical zoning. The geochemical data for different color layers also show obvious vertical variation characteristics. However, the difference of horizontal change is minor. The basic characteristics and laws of the formation of controlled sandstone-type uranium deposits discovered using big data book are quite different from those found in earlier studies. Therefore, an objective understanding of the color occurrence law of uranium-bearing rock series is crucial

3.4 Sedimentary Environment of Uranium-Bearing Rock Series …

185

Fig. 3.43 Formation model of sandstone-type uranium deposits in the interlayer oxidation zone (revised based on Guo 2006). 1. Strongly oxidized sandstone. 2. Weakly oxidized sandstone. 3. Sandstone-type uranium orebody in the redox zone. 4. Original sandstone. 5. Mudstone. 6. Siltstone. 7. Conglomerate. 8. Uranium-bearing geological bodies (represented by granite). 9. Uranium-bearing basement rock. 10. Direction of groundwater movement

for guiding the prospecting and exploration of sandstone-type uranium deposits in the Ordos Basin and for studying the metallogenic theory of sandstone-type uranium deposits in continental basins.

3.4.1.1

Vertical Color Zoning of Uranium-Bearing Rock Series in the Basin

1. Color zoning characteristics of uranium-bearing rock series in typical uranium deposits in the ore concentration area Taking the ore concentration area as a unit, we study the characteristics of rock changes caused by mineralization. The mineralization is examined by dissecting typical ore deposits and analyzing the spatio-temporal evolution characteristics of deposit mineralization. The known typical uranium deposits, ore spots, and mineralization points in the Ordos Basin are distributed mainly along the margin of the basin (Fig. 3.45). Uranium mineralized bodies have also been found in some oilfields in the basin. There are four uranium concentration areas in the basin. According to the different ore-controlling

186

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.44 Superimposed metallogenic model of sandstone-type uranium deposits in the northeastern Ordos Basin (according to Li et al. 2009). a Pre-enrichment stage. b Oxidation stage of paleophreatic water. c Paleo-interlayer oxidation stage. d Reduction and heating transformation of oil and gas

3.4 Sedimentary Environment of Uranium-Bearing Rock Series …

187

Fig. 3.45 Geology and distribution of ore deposits in the Ordos Basin (revised based on Sun et al. 2017)

structural units, they can be divided into the Dongsheng uplift ore concentration area, the Ningdong fault–fold ore concentration area, the Jingchuan fault uplift ore concentration area, and the Weibei uplift ore concentration area. These are discussed in turn. A. Dongsheng uplift ore concentration area The Dongsheng uplift ore concentration area is located along the northeastern margin of the basin and is an important ore concentration area in the Ordos Basin. There are some large uranium deposits such as Daying, Nalinggou, and Zaohuo trenches and some medium-sized uranium deposits such as the Chaideng trench and Nongshengxin. Recently, the newly submitted mineral sites are Tarangaole and Ulan Xili. On the plane, uranium mineralization is distributed in a semi-annular shape on the western side of the Dongsheng uplift belt, similarly to the extension direction of the outcrop line of the stratum. (i) Daying uranium deposit The vertical color zoning of uranium-bearing rock series of the Zhiluo Formation in the Daying uranium deposit is obvious. The red layer is chiefly located in the middle

188

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

and upper parts of the upper Zhiluo Formation. On the section, the buried depth of the bottom of the red layer gradually decreases from northeast to southwest (from 550 to 450 m) (Fig. 3.46). The lithology is characterized by purplish red and reddish brown sand and mudstone mixed with green medium-coarse-grained sandstone, exhibiting a remarkable binary structure. The green layer is mainly located at the boundary between the bottom of the upper member and the top of the lower member of the Zhiluo Formation. Its thickness is 50–90 m, and the layer thickness is relatively stable. The lithology is grayish green, light green medium-fine grained sandstone with silty mudstone. The connectivity of the overall sandbody is relatively poor. Sandbodies are mostly distributed in the shape of lenses. The gray layer is distributed in the middle and lower parts of the lower member of the Zhiluo Formation. The thickness on the section varies from 30 to 100 m. The lithology is gray medium-coarse-grained sandstone. Residual gravel and mud and coal chips are found in the bottom of sandbody. Calcareous sandstone intercalation is common. Vertically, the sandbody is composed of multiple positive rhythmites stacked on top of each other. This is also the main ore-bearing horizon in this area. The Daying uranium orebody is distributed along the northeast–southwest–southeast direction, and the morphology is U shaped with an opening to the northeast. The ore belt is ~ 20 km long and 0.4–2 km wide. The orebodies are distributed in plate shape in the lower and middle parts of the lower member of the Zhiluo Formation. (ii) Nalinggou uranium deposit The red layer of the Zhiluo Formation in the Nalinggou uranium deposit is located in the upper part of the upper member of the Zhiluo Formation and the top part of the lower member of the Zhiluo Formation in the Middle Jurassic. The sedimentary burial depth from northwest to southeast is 280–320 m. The thickness varies from 40 to 100 m (Fig. 3.46). The lithology of the upper part of the Zhiluo Formation is mainly interbedded with siltstone and mudstone. The mudstone and siltstone are pink, purplish red, and grayish purple, containing blue and blue-green sandy masses or nest sand. The sandstone is purple, gray green, grayish white, and so on. Some of the coarse clastic rocks in the upper member of the Zhiluo Formation in the Nalinggou and Daying areas are subjected to denudation. In most boreholes, the conglomerate beds at the bottom of the Lower Cretaceous directly overlay a set of motley mudstone deposits at the top of the Zhiluo Formation. The green layer is widely developed in this area. It is primarily distributed in the middle and upper parts of the lower member and the bottom of the upper member of the Zhiluo Formation. The thickness is relatively stable and varies from north to south from 100 to 140 m. The lithology is mainly grayish green medium-coarse sandstone. At the bottom of the upper part of the middle section, there are gray-green siltstone, argillaceous siltstone, and other floodplain sediments. The gray layer is the ore-bearing parent rock in the ore area. It is primarily distributed in the lower part of the Zhiluo Formation. Its thickness is 20–80 m, and the thickness increases from north to south. The lithology is dark gray and light

3.4 Sedimentary Environment of Uranium-Bearing Rock Series …

189

Fig. 3.46 Profile of boreholes and wells in typical uranium deposits in the Ordos Basin. a Profile of 1–1' borehole connecting well in the Daying uranium deposit. b Profile of 2–2' borehole connecting well in the Nalinggou uranium deposit. c Profile of 3–3' borehole connecting well in the Zaohuohao uranium deposit. d Profile of 4–4' borehole connecting well in the Ningdong uranium deposit area. e Profile of 5–5' borehole connecting well in the Huangling uranium mine area

190

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

gray lithic feldspar sandstone. The degree of rock diagenesis is not high and the boron structure is loose. It consists of multiple upward tapering positive cycles. The argillaceous strata are thin or rarely developed, indicating that the channel is unstable and has the characteristics of multistage channel sandbody deposition. The orebodies of the Nalinggou deposit are distributed in the northeast direction of southwest direction on the plane. On the profile, the uranium orebody mainly occurs in the gray sandbody under the green layer in the form of plates and layers. (iii) Zaohuohao uranium deposit The red bed of the Zhiluo Formation in the Zaohuohao uranium deposit is primarily distributed in the middle and upper parts of the upper member. The sedimentary thickness varies from 10 to 80 m from east to west (Fig. 3.46c). The lithology is mainly composed of purple-red and red-brown mudstone and argillaceous siltstone, mixed with green sandstone. The buried depth of the bottom decreases gradually from west to east. The Upper Cretaceous stratum is less preserved. The eastern part is affected by tectonic uplift, and the oxidation of the upper member of the Zhiluo Formation is relatively strong. The color is more complex. The green layer in the middle and lower parts is partially oxidized in the later stage. The green layer is primarily composed of the upper member of the Zhiluo Formation and the upper part of the lower member of the Zhiluo Formation. The thickness is relatively stable, ranging from 30 to 50 m. The lithology of the upper member is grayish green medium-fine-grained sandstone intercalated with silty mudstone. Generally, two or three layers of sandbodies can be seen. It has relatively obvious characteristics of a dual structure. The lithology of the upper part of the lower member is mainly gray-green medium- to coarse-grained sandstone. This layer is well developed in the central and western parts of the mining area. The gray layer is primarily distributed in the middle and lower parts of the lower member of the Zhiluo Formation, with a thickness of 10–30 m. The lithology is dark gray and light gray lithic feldspathic sandstone. The rock structure is loose. The deposit consists of a number of upward thinning positive cycles. The argillaceous strata are thin or rarely developed. The bottom is rich in carbonized plant debris and organic matter. The Zaohuo trench uranium orebody is distributed in an intermittent belt in the east–west direction on the plane. The profile is mainly platelike and layer-like. The orebodies are primarily distributed at the interface between gray and green layers, where they are rich in organic matter or shale bands. B. Ningdong fault–fold ore concentration area The Ningdong fault–fold ore concentration area is located along the western margin of the basin. At present, there are small and medium-sized uranium deposits such as Ciyaobao and Huianbao, as well as newly discovered large and medium-sized ore deposits such as Shicaocun, Jinjiaqu, Maiduoshan, and Yangchangwan. The spatial occurrence of the orebody is controlled by the north–south-trending fault–fold fault zone. It has the characteristics of multistage, multilayer wing mineralization.

3.4 Sedimentary Environment of Uranium-Bearing Rock Series …

191

In addition to the red layer, green layer, and gray layer, there is also a yellow lenticular sand layer in eastern Ningxia (Fig. 3.46d). The north–south thrust nappe structure is well developed in this area. The upper stratum is denuded seriously, and the Cretaceous stratum is basically unpreserved. The red layer is mainly located in the upper part of the Zhiluo Formation, with a thickness of 50–120 m. The buried depth of the bottom of the red layer is 280–70 m from north to south. Therefore, the stratigraphic structure is more complex. The lithology is mainly soil yellow, green, purplish red, reddish brown siltstone and fine sandstone, intercalated with sandstone and mudstone in thin layers. The interlayer structure of sand and mud is obvious. The green layer is widely developed in the area. It is mainly distributed in the upper member of the Zhiluo Formation and the middle and upper parts of the lower member of the Zhiluo Formation. The lithology is mainly gray-green siltstone and fine-grained sandstone, intercalated with thin gray medium-grained sandstone. The thickness varies greatly from 130 to 350 m. The cumulative thickness of the green layer from the SCZK5-1 borehole in the Shicaocun area is 350 m. There are two sets of yellow lenticular sand layers with a thickness of 3–5 m. The yellow sandstone is located at the bottom of the upper member of the Zhiluo Formation and in the middle of the Zhiluo Formation. The gray layer is mainly located in the middle and lower parts of the lower member of the Zhiluo Formation, with a thickness of 50–180 m. The lithology is primarily composed of gray and gray-white medium-coarse sandstone, which is the main orebearing horizon. A layer of yellow sandwiched red sandbody with a thickness of 2–5 m can be seen on the upper contact surface of the orebody. The sandbody developed stably in the Shicaocun area and can be used as a landmark layer for ore prospecting in this area. Controlled by the structure of the north–south thrust fault–fold belt, the Ningdong uranium orebody is generally distributed in the north–south direction. The orebodies mainly occur in the sandbodies of the lower member of the Middle Jurassic Zhiluo Formation and the Yan’an Formation. The strike of the orebody is consistent with the axial direction of the anticline. Vertically, the multilayered orebodies generally occur in the gray-white coarse sandstone at the bottom of the Zhiluo Formation. Their shapes are layer-like and platelike, and a few of them are lenticular. C. Weibei uplift uranium deposit concentration area The ore concentration area of the Weibei uplift is located along the southeastern margin of the basin. The area includes small and medium-sized uranium deposits such as Shuanglong and Diantou, mineralization points such as Jiaoping and Miaowan, and the newly discovered Huangling ore deposit. The horizontal mineralization is distributed in the northern part of the Weibei uplift and the structural slope belt of the Tongchuan uplift. The Zhiluo Formation in the Huangling area is dominated by red and gray layers. The green layer is relatively thin but stable (Fig. 3.46e). The buried depth gradually becomes shallower from northeast to southwest. The red layer is mainly distributed at the top of the upper member of the Zhiluo Formation and at the top of the lower member of the Zhiluo Formation. The thickness of the deposit is relatively stable, ranging from 80 to 100 m. The lithology of the upper member

192

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

of the Zhiluo Formation is mainly purple-red mudstone and argillaceous siltstone, intercalated with thin gray-green and reddish brown sandstone. One can see that there is the development of a thin layer of gypsum. Most of the strata in the lower part of the Zhiluo Formation of the R66 borehole near the Weibei uplift area have been oxidized. The green layer is primarily distributed in the upper part of the lower member of the Zhiluo Formation. The lithology is grayish green mudstone and argillaceous siltstone, mixed with purplish red sandstone. This layer is thinner but relatively stable, with a thickness of 10–30 m. The gray layer is distributed in the middle and lower parts of the lower member of the Zhiluo Formation, with a thickness of 30–50 m. The lithology is mainly light gray and grayish white medium-fine sandstone. The degree of siliceous cementation is high. The sandstone is rich in organic matter layers, and oil spots and oil traces can be seen locally. There is an unconformable contact between the gray layer and the underlying strata. The Huangling uranium orebody exhibits a continuous zonal distribution from northeast to southwest on the plane. On the profile, the orebodies are mainly platelike and layer-like, and a few are lenticular. There are two major layers in the mineralized layer. The first layer of uranium mineralization is located in the green layer in the middle of the lower member of the Zhiluo Formation, and the lower orebody is located in the gray bed channel sandbody. 2. Color variation characteristics of rocks in the basin The purpose of taking the basin as a unit to study the sedimentary environment is to compare the similarities and differences between the ore-forming sedimentary environment and the nonmetallogenic sedimentary environment, thereby enabling the ore-forming background and the remains left by mineralization to be distinguished. In this work, we selected two large north–south sections with complete data and good representativeness that cut through the northeastern margin of the ore concentration area (6–6' and 7–7' ) (Fig. 3.47). The northern part of section 6–6' begins in the northern part of the Daying uranium deposit in the Yimeng uplift belt, and the southern part finally approaches the Wushen Banner coalfield exploration area in the Yishan Slope belt in the central part of the basin. The north–south extension length of the section is 280 km (Fig. 3.45). The north of section 7–7' starts from Gaojialiang in the north of the Zaohuo trench and ends in the exploration area of the Balasu coalfield in Yulin in the south. The north–south extension is nearly 200 km. Combined with the macroscopic characteristics of the field core, the comprehensive fine anatomy can be determined from the basin scale section. Furthermore, the variation characteristics of rock color of the uranium-bearing rock series in the basin can be analyzed. A. Characteristics of vertical color change in section 6–6' In the section from north to south, the red layer dominated by miscellaneous colors is mainly distributed in the upper member of the Zhiluo Formation. The thickness gradually decreases and varies from 300 to 50 m. The lithology gradually changes from medium-fine-grained sandstone to fine sandstone and argillaceous siltstone.

Fig. 3.47 Profiles of a 6–6' and b 7–7' borehole connecting wells in the northeastern Ordos Basin

3.4 Sedimentary Environment of Uranium-Bearing Rock Series … 193

194

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

The green layer in the north is mainly located in the lower part of the upper member and the upper part of the lower member of the Zhiluo Formation with a thickness of 50–90 m. The lithology is mainly medium-fine grained sandstone, partially mixed with purplish red mudstone. Because of the lateral deposition of the channel, the finer deposits are superimposed on the thicker deposits in turn. A polycyclic binary structure sedimentary sequence from coarse to fine has been formed. The thickness of the green layer increases gradually to the south of the section, with a thickness of 150–300 m, being generally distributed in the middle and lower parts of the lower member of the Zhiluo Formation. The gray layer is located in the lower part of the lower member of the Zhiluo Formation, and its thickness varies greatly. The thickness of gray layer in the Daying area in the north is 20–100 m, and the medium-coarse-grained sandstone (gravel) is well developed. There is an obvious positive correlation between the number of sandstone layers and the thickness of the lower member. The stratum is thinner and there is less sandstone. The sand content is high and the braided river sedimentary system is well developed. Because the Wushenqi area in the south of the section is relatively far away from the provenance area, the lithology of the gray layer in the lower part of the Zhiluo Formation is mainly medium-fine sandstone and local siltstone. B. Characteristics of vertical color change in section 7–7' The red layer of this section is primarily located in the upper member of the Zhiluo Formation. Its thickness gradually decreases from the soap trench in the north to the Zhiluo Formation in the south. The stratigraphic structure of the northern Gaojialiang and Zaohuogou mining areas is similar to that of the Daying area. Most of the Cretaceous strata were denuded as a result of tectonic uplift in this area. The buried depth of the Zhiluo Formation is shallow, with a thickness of 30–80 m. There is no red layer in the upper member of the Zhiluo Formation in the southern Yulin area. The spatial distribution location and thickness of the green layer vary greatly from north to south, with a thickness range of 20–180 m. The northern green layer is mainly located in the lower part of the Zhiluo Formation. Because of the late oxidation of the green layer in this area, it is relatively thin. In the southern section, there are green layers in both the upper and lower members of the Zhiluo Formation. Thick green layers can be seen even in the Zhidan Group. The depositional system of the lower member of the Zhiluo Formation in the southern Chahasu area is delta plain. The lower member of the Zhiluo Formation is basically a green layer with a thickness of 30–100 m. The lithology is mainly fine-grained sandstone and siltstone. The lithology of the upper and lower members of the Zhiluo Formation in the Yulin area at the southern end of the section is dominated by medium- and fine-grained sandstone. The sedimentary facies are all part of a meandering river sedimentary system. The color is basically green, and a thin red interlayer can be seen locally. The northern gray layer is principally located in the middle and lower parts of the lower member of the Zhiluo Formation. The thickness of the gray layer in the

3.4 Sedimentary Environment of Uranium-Bearing Rock Series …

195

Zaohuohao area is greater than that in other areas. The southern part is mainly distributed in the thin sand and gravel strata at the bottom of the lower member of the Zhiluo Formation.

3.4.1.2

Sedimentary Environment Reflected by Geochemical Characteristics

The geochemical characteristics of sedimentary rocks are a good indicator of the state the of paleo-environment and paleoclimate during the sedimentary period. By studying the characteristics of major and trace elements in sedimentary rocks or sediments, we can trace the ancient sedimentary environment and understand the sedimentary characteristics at that time. The solubility of some trace elements is controlled by the redox state of the sedimentary environment. These elements (e.g., U, Th, V, Cr, Fe, Co, Cu, Zn, Mo, and other trace elements) tend to migrate to reductive water bodies and sediments, where they accumulate spontaneously or precipitate in the form of sulfides (Francois 1988; Calvert and Padersen 1993; Russell and Morford 2001; Algeo and Maynard 2004). The high state ions of Cr, U, and V can be reduced and enriched in an anoxic denitric acid environment, whereas Ni, Cu, Co, Zn, Cd, and Mo are enriched in environments where sulfate reduction occurs. Because the geochemical properties of Th and U are similar under the reduction condition, the difference is particularly great under the oxidation state. Iron is sensitive to redox because of the presence of + 2 and + 3 valence states. The presence of boron changes with different Eh and pH. Therefore, the content or ratio of these elements is often regarded as an important index parameter to distinguish the redox environment. The criteria for judgment are as follows: The ratios of V/Cr, Ni/Co, and U/Th are > 4.25, > 7, and > 1.25, respectively, while ratios of < 2, < 5, and < 0.75 correspond to an oxidation environment, respectively (Jones and Manning 1994). The ratio of V/(V + Ni) is usually used to judge the stratification of bottom water during sediment deposition (Hatch and Leventhal 1994). Stratification is strong when this ratio is > 0.84. The appearance of H2 S in the water body indicates an anaerobic environment. A V/(V + Ni) ratio of 0.6 to 0.84 indicates that the stratification of the bottom water is moderate, while that of 0.4 to 0.6 indicates that the stratification is weak. The ratio of Cu/Zn varies with the rise and fall of oxygen fugacity in the medium. High ratios reflect more reduction, whereas low ratios reflect oxidation (Dypvik 1984). It is generally believed that Fe2+ /Fe3+ >> 1 is a reduction environment, Fe2+ /Fe3+ > 1 is a weak reduction environment, Fe2+ /Fe3+ = 1 is a neutral environment, Fe2+ /Fe3+ < 1 is a weak oxidation environment, and Fe2+ /Fe3+ 100 m developed along the northeastern margin of the Ordos Basin (Zhang et al. 2016). Fossils in the Anding Formation in the Ordos Basin are scarce, and the sporopollen fossil data have not been reported yet. However, the content of Classopollis in the equivalent strata of the Late Jurassic in other areas is generally > 50%. For example, the Classopollis content is > 90% in the Tuchengzi Formation in western Liaoning, > 90% in the Penglaizhen Formation in western Hubei, and 81.7% in the Karazza Formation in the Turpan–Hami Basin, Xinjiang. All these results indicate an arid climate in the Late Jurassic (Table 3.6). Therefore, a comparative analysis of the contents of Classopollis, Cyathidites, and Deltoidospora in the sporopollen assemblage of the Yan’an and Zhiluo formations along the northeastern and western margins of the basin reveals that the content of this assemblage in the Yan’an Formation is only about half of that of the Zhiluo Formation. The climatic conditions reflected are generally the transformation from subthermal-temperate humid and warm climatic conditions to dry and hot climatic conditions.

3.4.2.2

Paleoplant Fossil Assemblage Indications

The macroscopic characteristics of rocks in continental basins and paleoplant fossils are of qualitative significance to paleoclimate. The lithology of the Yan’an Formation is gray-gray sandstone, gray-black siltstone, and mudstone, being rich in pyrite nodules and coal seams. The fossils of ancient plants are mainly composed of true ferns, Ginkgo biloba, and gymnosperms, with the development of Czekanowskia rigida Heer and Coniopteris hymenophylloides Brongn. Cladophlebis cf. asiataca and others are all Coniopteris–Cladophlebis combinatorial molecules. They are similar to the flora of the Jurassic Yaopo Formation in Xishan, Beijing. The presence of coal seam is an indicator of a humid climate, reflecting the warm and humid paleoclimate of the Yan’an Formation. The lower part of the Zhiluo Formation is interbedded by gray, yellowish green sandstone and purplish red argillaceous siltstone. The sandstone contains considerable amounts of coal debris and pyrite debris, being similar to that of the Yan’an Formation, but the ancient plants are mainly pteridophytes and cycads, with a certain amount of cycads and Ginkgo biloba. This indicates that the basin was in a transitional climatic environment of humidity and drought at that time. The common genera and species of cycads are Cycas sp. and Cycadeoidea sp. The common genera and species of Ginkgo

8.33

0.36

13.17

35.23

1.64

1.64

4.10

33.61

3.26

1.09

6.52

16.30

0.52

1.05

7.33

25.92

8.64

54.97

10.00

2.17

1.57

3.14

0.26

Neoraistrickia

0.71

6.25

Verrucosisporites

2.08 0.71

Converrucosisporites

8.33

Apiculatisporis

Planisporites

2.17

1.09

0.52

0.79

0.79

0.26

0.26

1.05 0.82

6.25

4.17

0.26

1.07

0.71

0.26

0.26

0.52

Lophotriletes

8.33

8.33

0.82

2.08

O. wellmanii

O. parvus

Osmundacidites

10.00

0.36

Granulatisporites

Cyclogranisporites

0.71

2.14

Toroisporis

Punctatisporites

2.08

2.08

14.58

6.25

8.33

1.09

33.70

0.26

38.10

8.33

62.50

Cotiumspora 10.00

8.33

12.30

56.56

0.26

16.67

10.32

67.97

Calamospora

7.69

7.69

C. australis

Concavisporites

7.69

C. minor

Cyathidites

52.38

90.48

(continued)

0.74

0.74

1.47

1.47

2.94

1.47

0.74

4.41

20.59

7.35

16.18

59.56

20.00

7.69

Deltoidospora

58.33

60.00

30.77

Number of spores in pteridophytes (%)

16.67

BF-220 BF-339.5 BF-353 BF-366 BF-380 BF-403.5 BF-406 BF-418 BF-429 BF-473 BF-506

Sample number

Table 3.6 Chemical alteration index (CIA) values of core YCZK2-1 and paleoclimate information for the Middle and Late Jurassic reflected by sporopollen fossils in eastern Ningxia

3.4 Sedimentary Environment of Uranium-Bearing Rock Series … 215

7.69

31.52

Q. anellaeformis

3.56

0.82

3.28

3.56

Quadraeculina

Q. minor

0.82

1.64 0.36

Protoconiferus

Piceites

Pseudopinus

0.36

1.09

8.70

1.09

0.52

1.57

0.52

0.26

1.31

P. rotundiformis

3.26

2.17

1.83 2.08

4.17

1.42

0.82

0.71

Pseudopicea

1.31

0.26

16.75

Protopinus

4.17

8.33

0.26

0.82

30.33

1.31

45.03

0.26

0.52

Keteleeriaepollenites

1.07

8.54

3.26

16.67

1.09

33.33

3.26

66.30

Podocarpidites

40.00

2.08

37.50

Alisporites

Abietineaepollenites/Pinuspollenites

0.82

8.33

1.78

7.69

1.78

9.52

Callialasporites

41.67

Cerebropollenites

83.33

43.44

40.00

32.03

69.23

Number of gymnosperms pollen (%)

0.82

0.82

0.36

0.71

Laevigatosporites

Densosporites 10.00

8.33

Lycopodiumsporites

Asseretospora

8.33

(continued)

1.47

2.94

3.68

3.68

14.71

1.47

0.74

40.44

0.74

BF-220 BF-339.5 BF-353 BF-366 BF-380 BF-403.5 BF-406 BF-418 BF-429 BF-473 BF-506

Klukisporites

Sample number

Table 3.6 (continued)

216 3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

10

50.00

16.67 9.52

14.58

21

281

12

Number of fossils identified per sample (grains) 13 6

0.36

Perinopollenites

122

48

92

7.61

382

2.36

0.26

7.69

C. annulatus

2.08

8.90

38.46

Classopollis 2.46

1.05

Cycadopites 5.69

7.69

C. hians 1.09

1.42

C. minor

0.79

2.88

1.83 0.79 1.09

1.09 1.07

0.82

0.82

Chasmatosporites

0.36

136

8.82

1.47

1.47

BF-220 BF-339.5 BF-353 BF-366 BF-380 BF-403.5 BF-406 BF-418 BF-429 BF-473 BF-506

Psophosphaera

Araucariacites

Concentrisporites

Q. limbata

Sample number

Table 3.6 (continued)

3.4 Sedimentary Environment of Uranium-Bearing Rock Series … 217

218

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

biloba are Phoenicopsis and Czekanowskia. The sporopollen assemblage in the lower part of the Zhiluo Formation is similar to that of the Yan’an Formation. The upper strata of the Zhiluo Formation are mainly red with a few fossils and sporopollen. Dry and hot conditions are not conducive to plant growth. In summary, there is a paleoclimatic evolutionary cycle from humid to arid from the Yan’an Formation to the Zhiluo Formation in the Ordos Basin. The humid period of the cycle is conducive to the formation of coal seams and primary reductive strata, while the arid period is conducive to the formation of primary oxidizing red strata. The paleoclimate changing from temperature and humid to drought conditions is beneficial not only to the formation of the redox zone but also to the migration and accumulation of uranium. This creates conditions for the formation of uranium deposits. The Zhiluo Formation of the Middle Jurassic formed just in this period, creating beneficial metallogenic conditions for uranium deposits.

3.4.3 Geochemical Characteristics and Provenance Indication Significance of the Zhiluo Formation of Uranium-Bearing Rock Series in the Ordos Basin At present, there are different understandings of the complex transport system and zoning characteristics of the north–south provenance in the whole basin. There remains a dispute about the influence scope and source of different provenances and the tectonic evolution and development of the provenance area, especially in the south. As a result, the analysis of the spatial distribution characteristics of sandbodies in the Zhiluo Formation and the expansion of uranium prospecting space are affected. Provenance analysis is an important basis for reproducing sedimentary basin evolution and analyzing the paleoenvironment in the basin (Xu et al. 2007). Through the study of major elements, trace elements, and REEs of sandstone in different tectonic environments, the composition characteristics of sandstone from sedimentary basins formed at different types of plate boundaries and in their interior can be summarized, and the provenance and sedimentary tectonic background of the basin can be ascertained (Bhatia 1983; McLennan and Taylor 1991). According to the geochemical test results of major elements, trace elements, and REEs in the sandstone of the lower member of the Zhiluo Formation, as well as the regional sedimentological background established by numerous coalfield and uranium drilling data, and combined with previous research data, in this book we further expound upon the structural background and provenance attributes of the sandstone of the Zhiluo Formation of uranium-bearing rock series in the Ordos Basin.

3.4 Sedimentary Environment of Uranium-Bearing Rock Series …

3.4.3.1

219

Geochemical Characteristics of Major Elements

The sandstone of the Zhiluo Formation along the northern margin of the middle eastern part in the three areas has the lowest SiO2 content, with an average content of 64.83%. The content of SiO2 along the southeastern margin is the highest, with an average of 71.63%. It is generally believed that the content of unstable components such as Al2 O3 , TiO2 , and Fe2 O3 decreases with the increase of SiO2 content. These results show that the maturity of sandstone increases gradually. The relative contents of Al2 O3 and Fe2 O3 in the samples are relatively high. These samples are rich in clastic heavy minerals, such as rutile, ilmenite, and ilmenite (Table 3.7). The samples were placed on the geochemical classification map of sandstone; rock types represented by different areas are considered to be different. The Huangling area in the southeastern margin is mainly feldspar sandstone. The northeastern margin area is mainly lithic sandstone. The eastern Ningxia area along the western margin is composed of mixed sandstone and lithic sandstone (Fig. 3.52). The type of sandstone basically reflects the characteristics of near provenance.

3.4.3.2

Geochemical Characteristics of Trace Elements

The average contents of Co, Ni, Cr, V, and other mafic elements in the continental upper crust are similar to those in the continental upper crust (Rudnik and Gao, 2003), exhibiting a trend of intermediate acidity (see Table 3.8). In the mid-ocean ridge basalt standardized trace element spider web diagram (Fig. 3.53), the rocks are relatively enriched in large ion lithophile elements (e.g., K and Rb) and high field strength elements (e.g., Zr and Hf) and depleted in typical inactive elements (e.g., Nb, Ta, P, and Ti). Contents of Sr, Y, and Yb are low. The contents and characteristic parameters of REEs in sandstone are presented in Tables 3.9 and 3.10. Figures 3.54a–c show the REE distribution models of sandstone obtained based on the normalization by Sun and Mcdonough (1989) chondrite. The ∑ total amount of REEs∑ ( REE) in sandstone varies greatly in these three areas of the Ordos Basin. The REE values for the northeastern margin are (36.49–154.71) ∑ −6 × 10−6 , with ∑an average of 95 × 10 . The value of the ratio of light REEs (LREEs) to heavy REEs (HREEs) ranges from 7.5 to 12.6, with an average of 10.82. The value of (La/Yb)N ranges from 6.5 to 17.9, with an average of ∑ 12.3. No obvious Ce abnormality was found in most of the samples. However, the REE and ∑ ∑ LREE/ HREE values of the western and southeastern margins are relatively low, being 61 and 63, respectively, with corresponding average (La/Yb)N values of 7.0 and 5.5 respectively. There are no Eu anomalies or weak Eu negative anomalies, and only a few of them are weakly positive anomalies. Although the absolute content of REEs varies greatly in the three regions, the standardized distribution patterns of chondrite are basically the same. They all exhibit the characteristics of LREE enrichment, a flat HREE content, and a moderate negative Eu anomaly. This is similar to the distribution pattern of REEs in the upper continental crust.

Ningdong

Al2 O3

62.77

60.06

SCZK23-2-4

SCZK23-2-5

12.36

17.56

7.34

11.17

77.26

86.5

SCZK15-1-8

SCZK15-1-12

10.76

58.88

7.51 17.9

SCZK15-1-4

81.01

62.17

skys21-05

6.26

7.7

SCZK15-1-2

87.42

skys21-04

10.55

68.33

84.33

skys32-7

skys21-02

14.9

67.45

skys32-5

8.55 19.61

79.88

56.25

skys-03-17

8.05

8.49

skys32-1

70.92

skys-03-16

6.03

88.19

76.2

skys-03-8

skys-03-10

14.67

69.66

skys-03-7

15.82 7.5

68.7

78.89

skys-03-5

10.92

19.68

13.98

skys-03-6

59.72

74.83

skys-03-2

skys-03-3

SiO2

65.78

Sample number

skys-03-1

Area

Huangling

10.35

2.8

0.16

0.95

1.14

3.49

0.72

0.16

0.33

2.88

1.85

2.04

2.11

3.45

2.8

0.24

1.52

1.08

2

3.28

6.54

6.36

Fe2 O3

1.45

3.78

0.39

0.84

1.38

2.92

1.07

0.21

0.69

3.1

1.53

4.83

0.66

1.17

0.62

0.38

2.21

0.4

1.77

0.48

0.85

0.96

FeO

2.05

0.72

0.54

1.25

11.17

0.62

1.79

0.59

0.44

0.33

1.35

0.43

0.76

4.41

2.19

0.13

0.41

3.5

0.31

0.87

0.21

0.98

CaO

0.79

1.72

0.15

0.84

0.89

2.14

0.4

0.2

0.31

1.08

1.33

1.9

0.34

0.46

0.62

0.23

1.17

0.48

1.24

0.74

1.55

1.6

MgO

2.41

2.7

2.5

2.76

2.26

2.61

2.75

2.54

3.03

3.57

4.15

4.01

3.01

2.78

2.89

2.21

4.34

2.7

4.55

3.46

5.68

3.6

K2 O

2.33

1.56

1.32

2.38

2.34

1.6

1.42

1.36

1.27

1.07

1.1

0.72

1.67

1.58

1.24

1.26

0.55

1.08

0.56

1.56

0.56

1.38

Na2 O

0.5

0.92

0.1

0.31

0.52

0.91

0.18

0.2

0.29

0.57

0.72

0.89

0.47

0.17

0.37

0.28

0.74

0.43

0.78

0.55

0.84

0.92

TiO2

Table 3.7 Major element analysis of sandstone of the Zhiluo Formation in different areas of the Ordos Basin (in wt%) P2 O5

0.074

0.075

0.021

0.047

0.066

0.036

0.045

0.035

0.042

0.11

0.095

0.21

0.059

0.04

0.066

0.049

0.1

0.036

0.075

0.06

0.06

0.12

MnO

0.047

0.048

0.01

0.029

0.51

0.064

0.18

0.013

0.04

0.31

0.042

0.11

0.033

0.13

0.05

0.01

0.044

0.048

0.023

0.015

0.012

0.05

Burning loss

(continued)

7.41

4.94

0.92

2.07

9.92

5.22

2.8

0.99

1.46

7.76

5.31

8.47

2.38

6.71

4.42

0.95

4.36

3.84

3.97

3.19

4.22

4.16

220 3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

12.2

11.93 17.11

72.3

UZK4-6

13.51

12.89 8.41

73.86

81.95

66.6

ACM

PCM

UCC

0.84

0.98

1.99

1.68

6.66

1.51

1.09

1.48

5.1

4.44

0.45

4.6

4.57

2.21

1.05

Fe2 O3

5.04

1.76

1.58

3.05

5.52

0.68

1.02

1.51

2.22

1.66

1.29

1.4

0.27

0.93

1.21

2.38

1.38

7.63

0.24

0.44

FeO

3.59

1.89

2.48

2.68

5.83

3.08

1.5

1.03

0.64

1.38

0.92

1.11

16.6

5.62

0.94

1.81

0.75

1.66

2.3

1.07

CaO

2.48

1.39

1.23

1.97

3.65

0.97

1

1.31

2.29

0.92

1.09

1.12

0.78

1.74

1.59

1.45

1.77

1.74

0.54

0.33

MgO

2.8

1.71

2.9

1.89

1.6

3.32

3.65

3.16

3.31

2.98

3.53

3.38

2.3

2.85

3.48

3.32

3.38

2.43

3

3.2

K2 O

3.27

1.07

2.77

3.21

4.1

1.78

2.12

3.54

2.08

1.85

2.15

1.97

1.73

1.78

1.92

1.58

1.95

0.56

1.45

1.83

Na2 O

0.64

0.49

0.46

0.64

1.06

0.34

0.31

0.63

0.82

1

0.56

0.4

0.34

0.45

0.56

0.58

0.67

0.8

0.45

0.31

TiO2

0.15

0.12

0.09

0.16

0.26

0.08

0.075

0.13

0.15

0.13

0.093

0.08

0.092

0.09

0.11

0.096

0.13

0.15

0.029

0.044

P2 O5

0.1

0.05

0.1

0.1

0.15

0.068

0.046

0.056

0.048

0.075

0.042

0.041

0.34

0.21

0.098

0.065

0.093

0.38

0.031

0.016

MnO

4.56

2.7

3.29

3.17

8

2.27

2.39

13.92

7.32

5.14

4.04

4.12

10.46

4.39

2.05

Burning loss

Note OIA = average chemical composition of oceanic island sandstone. CIA = average chemical composition of continental island arc sandstone. ACM = average chemical composition of active continental margin sandstone. PCM = average chemical composition of passive continental margin sandstone. (These data are derived from Bhatia (1983).) UCC = average chemical composition of the continental upper crust. (These data are derived from Rudnick and Gao (2003).)

15.4

14.04

58.83

70.69

OIA

12.07

69.68

74.42

14.22

UZK27-2

69.1

UZK27-1

11.31

12.98

12.23

9.02

UZK27-4

73.42

63.86

UZK16-3

UZK16-4

74.65

UZK16-2

12.55

61.25

53.09

UZK4-8

UZK4-10

13.81

66.58

UZK4-3

12.42

66.72

14.29

16.48

71.45

73.13

52.29

SCZK00-3-2

SCZK00-3-4

10.65

Al2 O3

UZK4-1

78.97

SCZK23-2-7

UZK4-2

SiO2

Sample number

CIA

Tarangaole

Area

Table 3.7 (continued)

3.4 Sedimentary Environment of Uranium-Bearing Rock Series … 221

222

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.52 Identification of clastic rock types of the Zhiluo Formation in the Ordos Basin

In addition, compared with the standardized REE chondrite distribution patterns of metamorphic rocks and magmatic rocks in the surrounding source areas (e.g., Yinshan in the north, Luliang in the east, Alashan in the west, and Qinling in the south), the Archean and Paleoproterozoic metamorphic rocks (granite gneiss, diorite gneiss, hornblende plagioclase gneiss, etc.) in the Yinshan area of the northern margin exhibit enrichment of LREEs. Except for migmatite, other rocks are relatively depleted in HREEs (Fig. 3.54d). The basement rocks in the Alashan area are characterized by enrichment in LREEs and positive, negative, or no abnormalities of europium (Eu). The partition curve is obviously right-leaning (Fig. 3.54e). The distribution curve of metamorphic rocks in the Qinling area dips to the right with slight negative abnormal pin, but the HREE curve is relatively flat. However, the distribution curve of intrusive rocks exhibits an obvious right inclination. The comprehensive results reveal that the Zhiluo Formation in the northern and western parts of the basin is related to the Archean granitic gneiss, diorite gneiss, and monzonitic granite in the Yinshan area. The REE distribution curve of Jurassic rocks along the western margin of the basin is also a right-dipping type of LREE enrichment (Fig. 3.54f), indicating that the provenance may come from the Alashan area on the west side of the basin. The distribution curve of metamorphic rocks in the Qinling area is similar to that in the Huangling area, but the distribution curve of intrusive rocks is quite

Sr

Ba

Ningdong

5.74 15

skys-03-17

15.4 100

2.72 216 280

5.16 9.85 6.5

4.27 225

1.71 17.1 3.04 11.5 5.09 4.42 7.87 79.3 1.48 165 662

skys21-02

skys21-04

skys21-05

3.19 120 390

2.36 238 382

SCZK23-2-4

24.2 17.5 85.1 57

2.04 143 173

2.94 5.26 74.4 1.35 134 549

25.5 13.8 39.4 122

SCZK15-1-12 0.86 11.1 6.82 5.67 3.9

13.4 24.4 18.1 7.37 5.81 12.1 81.7 1.36 225 573

10.4 13.2 42.2 32.5 16.4 11.6 16.3 69.5 2.52 222 566

4.8

SCZK15-1-4

SCZK15-1-8

4.25 149 268

26.6 20

SCZK15-1-2

99.6 58.5 25.9 15.9 40.3 118

2.11 14.9 5.46 4.02 4.33 81.9 2.03 123 478

25.4 6.25 3.62 10.3 105

30.3 36.9 14.4 8.91 25.1 110

10.4 17

skys32-7

11.9 165 345 10.7 156 292

32.9 15.7 86.6 75.4 31.5 15.6 87.9 161

19.2 13.9 54.1 63.8 31.6 12.4 38.8 174

skys32-1

skys32-5

7.19 16.4 5.99 4.94 7.59 85.3 1.89 160 574

10.8 6.37 10.4 82.6 1.95 153 382

3.17 13.4 31.4 15

skys-03-16

Sc

Nb

Ta

Zr

0.7

246 695

76.6 3.63 14.7 1.08 236

30.8 4.58 9.2

79.9 2.71 15.8 1.1

0.78 246

3.75 18.2 1.29 178

44.7 4.45 11

132

99.6 7.49 17.2 1.24 587

V

3.8

7.68 0.54 208

8.81 16.1 1.04 155

2.11 5.02 0.36 61.8 8.14 15.8 1.05 226 6.05 0.39 87.2 91.5 5.36 15.3 1.02 261

11.9 2.75 3.11 0.23 55.9

29.9 5.6

60.4 8.68 8.52 0.56 164

110

16.7 4.39 3.81 0.26 83

140

24.7 3.49 6.75 0.51 234

59.6 4.27 10.5 0.75 506

72.3 3.97 15.3 1.06 257

117

21.3 3.68 7.15 0.52 236

42.8 4.26 4.09 0.22 67.4

126

22.6 7.71 5.17 15.9 75.4 2.25 122 1140 18.9 3.22 5.99 0.46 306

14.1 12.3 179

4.58 24.4 9.32 30.4 12.1 6.6

skys-03-8

skys-03-10

15.1 10.6 23.8 63.6 26.2 12.1 44.1 190

skys-03-7

9.66 148 142

22.2 8.06 36.8 66.8 33.4 10.5 46.6 205

35.8 11.7 13.8 43.6 8.81 4.58 12.2 97.6 3.75 127 330

124 70.1

skys-03-5

27

skys-03–6

33.6 10.5 64.8 292

9.14 200 244

Cs

11.9 116 132

Rb

45.8 161

Li

8.25 128 445

Co

39.8 15

Ni

11.5 12.2 26.7 42.2 14.7 6.51 19.5 133

Cr

skys-03-3

Zn

18.4 16.2 66.4 107

Pb

17.7 18.2 70.4 90

Cu

skys-03-2

Sample number

Huangling skys-03-1

Area

Ga

U

Th

2.02 16.4 2.89 13.2

Be

4.42 0.72 7.92 1.36 2.42

8.43 7.58 2.03

7.42 12.8 4.26 3.75 0.89 7.83 1.68 1.53

1.6

12.1 2.28 5.84

7.08

(continued)

7.81 1.94 19.1 1.27 3.36

1.77 0.65 5.89 0.63 0.87

2.65 1.16 9.93 1.55 2

4.66 1.2

6.93 2.04 20.9 2.2

2.55 0.58 6.44 0.63 2.05

1.95 0.66 5.53 9.32 1.4

6.6

13.9

7.78 1.92 17.6 2.8

4.78 2.55 23.4 2.71 9.59

6.6

2.03 0.8

6.14 1.18 8.36 56

8.48 0.49 4.67 1.41 0.72

15.9 1.24 2.14

0.72 7.68 4.17 9.91

7.07 2.3

19.1

7.43 2.26 16.6 1.18 1.84

7.44 1.02 10.4 1.63 5.43

5.67 4.38 21.1 0.74 1.45

16.6

Hf

Table 3.8 Analysis results of trace elements in sandstone of the Zhiluo Formation in the study area (mass values are in micrograms)

3.4 Sedimentary Environment of Uranium-Bearing Rock Series … 223

Sr

Ba

22.5 10

30.5 171

6.61 135 188

2.74 148 449

13.1 43

11.8 10.9 33.7 49.1 16.9 9.7

4.77 11.6 14.3 22.2 8.4

6.63 13.4 37.5 29.3 9.63 4.58 10.6 85.6 1.48 302 978

UZK27-2

UZK27-4

UZK4-6

97.1 1.44 317 775

UZK27-1

14 1.76 219 357

5.16 10.3 90.1 1.23 292 832

15.9 94.9 1.37 213 659

52.2 78.4 20.2 11.3 35.4 102

23.7 11.7 11.9 82.3 1.56 318 686

41.9 11.6 6.9

20.9 43.5 56

9.42 19

16

UZK16-3

40

277 801

86.3 1.18 336 533

9.16 58.1 1.4

UZK16-4

25.3 28.8 10.2 7.4

6.63 11

8.49 59.4 21.1 29.7 9.41 5.21 11

UZK4-10

UZK16-2

8.78 26.2 99.3 2.55 241 922

14.4 13.9 39.8 68.7 17

12.4 14.9 37.5 54.4 17.1 8.22 21.6 79.2 2.16 227 742

UZK4-3

Sc

Nb

Ta

Zr

6.49 13.9 0.9

171

148

10.7 0.69 187 8.29 0.54 197

183

6.04 14.2 0.84 340

5.89 19.1 1.05 495

34.6 7.4

6.54 0.45 104

31.4 6.07 6.29 0.39 103

64.4 8.33 11.1 0.68 323

189

95

48.7 6.33 9.86 0.6

71.3 6.68 7.35 0.43 123

36.1 7.46 5.82 0.38 106

64.1 10

82.4 8.92 11.6 0.76 222

6.2

74.3 9.91 12.8 0.88 275

110

48.1 5.73 8.65 0.5

33.7 4.03 5.71 0.35 120

29.8 4.75 8.49 0.51 144

V

11.5 5.88 17.9 89.1 2.11 295 1040 145

UZK4-8

7.95 15.7 32.1 58

UZK4-2

16.4 13.3 60.5 69.7 17.8 9.01 27.3 98.1 2.78 242 941

45.8 62

13.4 10.8 17.4 104

23.5 15

Cs

SCZK00-3-4

Rb

9.77 15.8 18.5 30

Li

SCZK00-3-2

Co

27.3 24.7 22.5 17.6 77.6 1.84 195 475

Ni

16.3 43.1 23

Cr

4.66 13.2 25.3 19.6 5.95 5.35 9.71 88.5 1.65 175 693

Zn

SCZK23-2-5

Pb

SCZK23-2-7

Cu

Sample number

Tarangaole UZK4-1

Area

Table 3.8 (continued) Be

Ga

U

17.1 17.2 1.89

2.83 0.64

3.3

Th

6.08

11.5 0.79 2.25

17.1 4.66 3.24

1.48 12.5 24.4 7.63 1.6

3.12 1.06 12.5 14.6 3.63

3.02 1.14 11.4 18.7 1.78

9.24 1.34 13.9 1.29 3.52

9.3

13.3

5.24 1.38 13.1 24.8 2.83

3.58 1.2

2.97 1.16 10.6 3.98 3.45

5.48 1.59 16.8 8.51 6.49

6.08 1.94 16.5 29

5.38 1.34 13.4 1.97 6.24

7.75 2.05 17.2 14.4 6.66

5.09 2.38 18.8 1.68 11.3

4.62 1.4

3.46 0.94 10

4.16 1.34 11.1 1.6

Hf

224 3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

3.4 Sedimentary Environment of Uranium-Bearing Rock Series …

225

Fig. 3.53 Mid-ocean ridge basalt standardized cobweb map of trace elements in sandstone of the Zhiluo Formation in the Ordos Basin (based on standardized data from Pearce et al. 1984)

different from that of the Zhiluo Formation along the southeastern margin, indicating that their contribution as a provenance area is minor.

3.4.3.3

Provenance Analysis

By extracting the sedimentological, petromineralogical, and geochemical information from the sedimentary strata, we can comprehensively analyze and connect the original sedimentary relationship between the basin and the source area and identify the main provenance supply direction of the primitive basin, the main rock types of the source area, and the geotectonic environment of the source area (Zhao et al. 2010). 1. Sedimentary geochemical analysis In the discriminant map of the provenance area (Fig. 3.55), the sandstone samples of the Zhiluo Formation in three areas of the Ordos Basin primarily fall into the felsic and neutral igneous provenance areas. The former comprises mature continental margin arcs and continental transformation margin pull-apart basins, while the latter involves pyroclast in the sandstone that is mainly andesite, indicating mature magmatic arcs and immature continental margin magmatic arcs. Some samples from the eastern Ningxia area along the western margin are located in the mafic igneous provenance area, having the property of immature marine island arcs. In addition, some samples from the Tarangaole area along the northeastern margin are located in the quartzite sedimentary provenance area, the sedimentary basin within the craton, and the recycled orogenic belt, indicating a mature continental source area. The

Ningdong

Ce

7.63

SCZK23-2-4

19.2

9.15

12

7.65

2.92

SCZK15-1-8

SCZK15-1-12

69.2

24.6

9.12

64

SCZK15-1-4

4.71

22.5

skys21-05

4.36

8.33

SCZK15-1-2

6.25

skys21-04

38

13.2

5.25

skys32-7

skys21-02

35.7

19.1

skys32-5

10.5

81.9

6.41

28.6

skys-03-17

43.1

24

skys32-1

6.33

skys-03-16

3.88

1.57

7.78

skys-03-8

skys-03-10

16.8

4.99

skys-03-7

12.8

58

3.44

22

skys-03-5

30.2

7.79

61.4

skys-03-6

1.77

14.3

skys-03-2

skys-03-3

La

25.8

Sample number

skys-03-1

Area

Huangling

2.55

0.75

1.93

5.42

5.57

1.22

1.45

1.41

2.94

5.79

6.71

1.51

2.91

2.19

0.32

1.3

5.1

0.89

3.58

0.5

6.8

Pr

10.2

2.98

7.36

19.2

21.2

4.74

5.43

5.51

10.5

23

24.9

5.7

13.3

8.86

1.14

5.19

18.9

3.5

13.8

2

26.2

Nd

2.13

0.53

1.39

3.1

3.99

1.04

0.89

1.08

1.81

4.56

4.53

1.03

2.96

2

0.23

1.11

3.51

0.68

2.5

0.44

4.98

Sm

0.48

0.33

0.62

0.9

0.88

0.58

0.41

0.4

0.49

0.92

0.99

0.46

0.82

0.52

0.57

0.27

0.73

0.17

0.63

0.1

0.89

Eu

1.82

0.48

1.37

3.12

3.61

1.05

0.83

0.93

1.84

3.83

4.19

0.96

2.4

1.6

0.23

1.05

3.44

0.67

2.23

0.4

4.36

Gd

0.32

0.064

0.21

0.4

0.57

0.18

0.11

0.15

0.26

0.61

0.65

0.15

0.37

0.26

0.039

0.2

0.58

0.12

0.35

0.084

0.68

Tb

1.95

0.33

1.15

2.04

3.13

1.14

0.55

0.95

1.55

3.61

3.55

0.89

1.87

1.54

0.25

1.37

3.58

0.83

2.04

0.65

3.96

Dy

0.4

0.065

0.23

0.4

0.61

0.23

0.11

0.2

0.33

0.73

0.69

0.19

0.34

0.32

0.057

0.31

0.74

0.19

0.43

0.16

0.82

Ho

1.15

0.19

0.64

1.22

1.75

0.66

0.34

0.6

1.03

2.08

1.95

0.56

0.94

0.96

0.17

0.92

2.12

0.59

1.27

0.52

2.44

Er

Table 3.9 Analysis results of REEs in sandstone of the Zhiluo Formation in the study area (masses are in mircograms) Tm

0.19

0.031

0.1

0.2

0.28

0.1

0.057

0.1

0.17

0.33

0.31

0.095

0.15

0.16

0.03

0.15

0.35

0.1

0.21

0.094

0.4

Yb

1.41

0.24

0.74

1.34

1.95

0.71

0.42

0.78

1.27

2.31

2.06

0.72

1.04

1.22

0.22

1.08

2.46

0.77

1.51

0.7

2.89

Lu

0.22

Y

8.75

1.81

5.83

10.8

14.6

6.36

3.38

5.6

8.66

18.7

17.6

5.36

8.9

8.94

1.48

6.61

20.4

3.94

10.8

2.82

21.1

(continued)

0.038

0.12

0.22

0.3

0.11

0.07

0.13

0.21

0.36

0.31

0.12

0.16

0.2

0.036

0.17

0.41

0.13

0.25

0.12

0.48

226 3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Tarangaole

Area

14.8

11.1

6.61

21

UZK27-1

UZK27-4

UZK4-6

24

UZK16-4

UZK27-2

8.51

7.83

UZK16-2

30.3

UZK4-10

UZK16-3

29.6

42.2

UZK4-3

UZK4-8

35.1

UZK4-2

19.4

34.4

SCZK00-3-4

UZK4-1

1.64

3.59

SCZK23-2-7

12.1

SCZK23-2-5

SCZK00-3-2

La

Sample number

Table 3.9 (continued)

32.8

14.1

25.6

41.2

67.1

15.1

16.6

47

55.6

37.8

57

48.9

39.2

10.2

6.26

28.9

Ce

4.76

1.83

3.34

3.13

5.83

2.16

2.31

6.02

8.55

6.38

7.34

8.04

5.7

1

0.4

2.95

Pr

17.3

7.39

13.4

11.4

22.1

8.25

8.97

21.9

30.4

21.8

25.9

29

22.5

4.13

1.54

11.5

Nd

2.82

1.48

2.55

1.81

3.81

1.52

1.66

3.62

4.94

3.15

4.12

4.63

4.35

0.96

0.32

2.07

Sm

0.88

0.79

0.77

0.16

0.99

0.73

0.81

1.09

1.29

0.79

1.11

1.01

0.87

0.45

0.39

0.68

Eu

2.28

1.24

2.09

1.8

3.55

1.34

1.42

3.27

3.99

2.63

3.37

3.45

3.59

1.1

0.31

1.91

Gd

0.32

0.2

0.31

0.23

0.52

0.19

0.2

0.46

0.56

0.36

0.49

0.49

0.56

0.19

0.05

0.29

Tb

1.6

1.13

1.55

1.14

2.88

1.13

1.14

1.41

2.88

1.86

2.6

2.49

3.18

1.27

0.33

1.62

Dy

0.31

0.22

0.3

0.22

0.57

0.23

0.22

0.48

0.56

0.38

0.5

0.48

0.63

0.27

0.068

0.32

Ho

0.86

0.62

0.91

0.72

1.66

0.64

0.63

1.3

1.63

1.1

1.44

1.4

1.77

0.78

0.2

0.93

Er

0.13

0.1

0.15

0.12

0.28

0.1

0.1

0.18

0.25

0.18

0.22

0.22

0.28

0.13

0.035

0.15

Tm

0.87

0.68

1.03

0.9

1.9

0.75

0.72

1.14

1.61

1.18

1.52

1.52

1.91

0.95

0.27

1.07

Yb

0.14

0.1

0.17

0.14

0.3

0.12

0.12

0.19

0.25

0.18

0.24

0.25

0.29

0.16

0.044

0.17

Lu

7.77

5.53

6.98

5.6

14.6

5.33

5.11

14.4

14

9.45

12.4

11.6

15.6

7.11

1.85

7.83

Y

3.4 Sedimentary Environment of Uranium-Bearing Rock Series … 227

228

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Table 3.10 REE parameters of sandstone in the study area Tectonic background

ΣREE

La/Yb

Huangling area

61.83

8.02

Ningdong area

60.86

Tarangaole area

95.13

9.68 18.3

(La/Yb)N 5.4

ΣLREE/ΣHREE 7.32

(Gd/Yb)N 1.5

6.53

8.28

1.75

12.34

10.89

2.37

Fig. 3.54 Comparison of standardized REE distribution patterns in sandstone of the Zhiluo Formation and surrounding provenance area in the Ordos Basin (according to Sun and Mcdonough 1989). a Standardization of chondrite in the Tarangaole area of the northeastern margin. b Standardization of chondrite in the Ningdong area of the western margin. c Standardization of chondrite in the Huangling area of the southeastern margin. d Archean crystalline basement samples from the Yinshan and Luliang mountains (Chen et al. 2012). e Metamorphic rock and granite samples from the Alashan area. f Metamorphic rock samples from the Qinling Group in the Qinling orogenic belt (Shi et al. 2009) and intrusive rock samples from various rock areas (Zhou et al. 1999)

source area belongs to a deeply weathered granite–gneiss geological body or ancient sedimentary body. The sandstone K2 O content of the Zhiluo Formation in the three areas is relatively high. The average mass ratios of w (K2 O)/w (Na2 O) along the northeastern, western,

3.4 Sedimentary Environment of Uranium-Bearing Rock Series …

229

Fig. 3.55 F1–F2 discriminant map of sandstone provenance of the Zhiluo Formation in the Ordos Basin

and southeastern margins are 1.5, 1.5, and 3, respectively. These values are close to or even higher than that of passive continental margin sandstone (~ 1.60), reflecting the addition of numerous mature components. Microscopic observation reveals that there are few illite clay minerals in the sandstone. Therefore, one can infer that the high potassium content of sandstone mainly comes from clastic particles rather than epigenetic minerals. This indirectly reflects the high potassium nature of the source region. Especially in the Huangling area of the southeastern margin, the sedimentation was influenced by the provenance of the passive continental margin for a long time in the Early and Middle Permian, and the sandstone has the characteristics of high SiO2 and low Na2 O. The Archean–Proterozoic Taihua, Qinling, and Kuanping groups are characterized by high SiO2 content and K2 O/Na2 O > 1, which is consistent with what is found in the Huangling area. Floyd et al. studied the geochemical characteristics of Early Proterozoic metamorphic sedimentary rocks in northwestern Scotland. He proposed to use a Hf–La/Th discriminant diagram to distinguish sedimentary provenance in different tectonic environments. In the Hf–La/Th diagram (Fig. 3.56), most of the samples from the Ordos Basin fall into the mixed zone of feldspathic and basic rocks, reflecting that they originate from the source of the mixture of volcanic arc material and felsic material in the continental upper crust. This indicates that the original material should come from the upper crust. This material is dominated by feldspathic rocks and mixed with neutral magmatic rocks with a high feldspar content.

230

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.56 Environmental identification map of the La/Th–Hf source area of sandstone in the Zhiluo Formation in the Ordos Basin (according to Bhatia 1983; McLennan et al.; Taylor; Floyd)

In addition, McLennan and Taylor (1991) found that the mixed sandstone of post-Archean age is characterized by a flat HREE distribution pattern of w (Gd)N /w (Yb)N < 2. The ratio of Eu/Eu* varies roughly between 0.65 and 1.0. The Archean complex sandstone is distributed in the range of < 1.0 and > 2.0 in the form of w (Gd)N /w (Yb)N . Eu/Eu* values are roughly > 0.85. One can see from Table 3.8 that the provenance is dominated by material of post-Archean age and also contains a small portion of Archean detritus. 2. Regional sedimentological analysis The analysis of the spatial distribution of the sandbody of the target layer can indicate the internal relationships among the deposits. The alluvial fan deposits represent the coarsest and worst sorted near-source units in the onshore sedimentary system of the basin. They usually evolve into a fine-grained fluvial depositional system with a lower slope in the downdip direction. Then this system transitions to a delta or lacustrine system and finally evolves into a lacustrine sedimentary system, constituting the configuration pattern of sedimentary facies zones in continental basins. In general, places with large superimposed layers of sandstone should be areas where rivers often flow. Therefore, the distribution and variation of thick sandstone also reflect the provenance direction of sediment and the distribution characteristics of the sedimentary system. By using data from numerous coalfield and uranium boreholes, thickness maps of sandbodies in the lower part of the Zhiluo Formation in the three uranium ore concentrated areas in Ordo Basin have been compiled. These

3.4 Sedimentary Environment of Uranium-Bearing Rock Series …

231

maps more accurately reflect the characteristics of the paleo-sedimentary system. As a result, the trend of ancient running water can be discussed and the direction of provenance can beidentified. Along the western margin of the basin, three braided rivers are developed in the lower part of the Zhiluo Formation (Fig. 3.57a). The sandbody in the main channel of Shigouyi–Yeerzhuang exhibits continuous plate development, with an average sandbody thickness of 110.7 m. Two rivers are developed in the Yechawan– Lijiamiaoziliang area, and the thickness of the sandbody there is ~ 80 m. The river extends from the exposed area of the Yan’an Formation in the north to Maiduoshan in the southeast. The parts near the highlands on both sides of the main river belong to the floodplain facies, and the sandbody is clearly thinner. Previous studies have shown that the direction of the paleocurrent in the Ningdong area is southeast (108°– 176°) (Guo 2010). This is consistent with the paleocurrent direction reflected by the thickness distribution pattern of the sandbody in this area. Along the southeastern margin of the basin, in general, the sandbody in the Shuanglong area of the northeast is thicker while, in the Beibei–Miaowan area of the

Fig. 3.57 Isopach map of sandbodies in the lower member of the Zhiluo Formation in different areas of the Ordos Basin

232

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

southwest, it is thinner (Fig. 3.57b). The braided river sandbody system is developed in the lower member of the Zhiluo Formation in the Shuanglong–Huangling area in the east. The distribution of sandbodies is stable. The sandbody thickness is generally 30–90 m, with an average of ~ 45 m. Because the western Huangling– Shuanglong area is close to the center of the basin, there is a lack of borehole data. Therefore, analysis of the paleocurrent in this area was combined with the results of previous studies. Based on detrital zircon and paleocurrent analysis, some scholars proposed that the provenance is mainly from the Alxa block (Lei et al. 2017). Sedimentary facies analysis shows that the paleocurrent is northwest–southeast trending and concentrated along 120°–140° (Jia 2005; Zhao et al. 2010). Based on the study of paleo-groundwater dynamic conditions, Zhang et al. (2017) concluded that the paleocurrent direction of the Zhiluo Formation is a south–north confluence into the basin. Based on the analysis of the thickness characteristics of sandbodies and the comparison of the geochemical characteristics of the Alashan area, we conclude that the paleo-running water strike should be from northwest to southeast or from west to east, rather than from southeast to northwest or from south to north. Along the northeastern margin of the basin, the most prominent distribution feature of the sandbody in the lower member of the Zhiluo Formation in the Tarangaole area is that it is thickest near the north–south direction. This is along the main river, which bifurcates continuously to the south, west, and southeast and evolves into a series of smaller branch channels. The main river is located in the Tarangaole–Nalinggou area, with a length of ~ 20 km and a width of 5–10 km (Fig. 3.57c). The maximum thickness of the sandbody can be up to 260 m, and the sand content is as high as 85%. A large area of floodplain is developed around the braided river. Crevasse fan deposits are sporadic and their distribution range is small. The distribution of sandbodies in this area generally reflects the flow characteristics of paleocurrent from north to south. Based on the above analysis, we can conclude that the sandstone in the west and northeast of the basin is thicker than that in the southeastern margin of the basin. This indicates that the thickness of sandstone decreases gradually from west to east and from northwest to southeast and reveals that the western and northeastern margins of the basin are the basin’s main provenance areas. The Qinling trough closed to form the uplift area of the Qinling orogenic belt in the Early and Middle Jurassic. Combined with the previous paleocurrent measurements in the southeastern margin, this indicates that a few paleocurrents flowed northward (Zhao et al. 2010) and that the Qinling orogenic belt in the southern basin may also provide provenance for the basin during this period. 3. Analysis of heavy minerals The statistical results of heavy minerals are shown in Fig. 3.58. Zircon, apatite, sphene, epidote, garnet, and ilmenite are the major heavy minerals in the sandstone of the Zhiluo Formation in the Daying, Nalinggou, and Zaohuogou areas in the northern part of the basin (Zhang et al. 2016). The main stable heavy minerals in the Huangling area are zircon, apatite, garnet, and leucoxene, as well as the unstable heavy mineral pyrite. The heavy minerals along the western margin are mainly zircon, apatite, garnet, and perovskite. The relatively high content of garnet indicates that

3.4 Sedimentary Environment of Uranium-Bearing Rock Series …

233

Fig. 3.58 Heavy mineral composition of sandstone in the lower member of the Zhiluo Formation in the Ordos Basin

some metamorphic detritus has been mixed in. In addition, the content of pyroxene in the samples is very low, and pyroxene is not detected in the western and southeastern margin areas, indicating that the basic rocks are not the primary source of sandstone of the Zhiluo Formation. 4. Age analysis of clastic zircon The clastic zircon U–Pb ages from the sandstones in the Middle Jurassic Zhiluo Formation of the Daying–Nalinggou area in the northeastern Ordos Basin exhibit four peak ages of 270–280, 320–340, 1800–2000, and 2300–2500 Ma (Zhang et al. 2016). Compared with the isotopic chronology of the surrounding source bodies, the main body of zircon comes from metamorphic rocks and magmatic bodies in the Yinshan–Wulashan–Langshan area of the northern margin (Fig. 3.59a). The sandstone clastic zircons of the Middle Jurassic Zhiluo Formation along the western margin area are mainly formed in three age groups: 175–525, 1550–2050, and 2100–2450 Ma (Guo 2010). Hercynian clastic zircons account for the vast majority (Fig. 3.59b). An isotopic chronological comparison with the geological bodies in the source area shows that the sandstone source area of the Zhiluo Formation along the western margin is mainly the Alashan block. The sandstone clastic zircon ages of the Middle Jurassic Zhiluo Formation in the Huangling area of the southeastern basin are mainly 169–438, 1435–2083, and 2300–2767 Ma in age (Lei et al. 2017). In the first group, the clastic zircons from the Middle Mesozoic to the late Early Paleozoic (the peak being 274 Ma) account for the vast majority of the content. The main source of the Zhiluo Formation in the Huangling area comes from the Alashan block in the northwest, and there is also a contribution from the southern Qinling orogenic belt (Fig. 3.59c).

234

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.59 Comparison between zircon U–Pb age spectrum of sandstone of the Zhiluo Formation in the Ordos Basin and zircon age spectrum of the adjacent area (where N is the statistical age) (according to (a) Zhang et al. (2016); (b) Guo et al.; (c) Lei et al. (2017)

3.4 Sedimentary Environment of Uranium-Bearing Rock Series …

3.4.3.4

235

Tectonic Background

In the mountain margin with strong tectonic uplift, the rocks are rapidly denudated, transported, and deposited in a short time. Weathering and transportation processes have little effect on the composition and geochemical composition of source rocks (Spalletti et al. 2008). Therefore, the geochemical characteristics of clastic rocks can provide important information about source rocks, inland tectonic evolution, and the tectonic setting of sedimentary basins (Colombo 1994; Zimmermann and Bahlburg 2003). For clastic rocks, especially recycled sedimentary rocks and sedimentary rocks originating from the source area of the mixture, the discrimination diagram of trace element geotectonic environment is difficult to correlate to the specific geotectonic environment. However, at least it can reflect the difference between the source area and the source rock. 1. Major quantitative elements and tectonic background Because Fe and Ti elements in sediments cannot easily be lost, they can better reflect their provenance properties. Although Mg is not as good as Fe and Ti, it can basically represent the original content of the parent source. The Al2 O3 /SiO2 ratio can be used to judge the enrichment degree of quartz in sandstone. The K2 O/Na2 O ratio can reflect the content of potash feldspar, mica, and plagioclase in the rock. Al2 O3 /(CaO + Na2 O) is a parameter reflecting the relationship between the most stable elements and the most unstable elements. Therefore, the contents of these element oxides in sandstone can be used as research parameters to better reflect the properties of the source area and its tectonic background. The average values of w (Al2 O3 )/w (SiO2 ) along the northeastern, western, and southeastern margins are 0.19, 0.22, and 0.17 s respectively, which are similar to those of continental margin arc (0.15–0.22). The TiO2 contents are 0.52%, 0.62% and 0.53%, respectively, which is basically consistent with the range of active continental margin arc sandstone (0.5–0.7%). According to the discriminant diagram (Fig. 3.60), the sandstone samples of the Zhiluo Formation along the northeastern and western margins of the Ordos Basin mainly fall on the active continental margin. Some sandstone samples of the Zhiluo Formation along the southeastern margin fall on the passive continental margin, and a few of the samples are on the continental island arc and the active continental margin. 2. Trace elements, REEs, and tectonic setting Bhatia (1983) summarized the REE eigenvalues of sandstones from provenance ∑ areas ∑ under ∑ different tectonic settings (Table 3.11). He concluded that the REE, LREE/ HREE, and La/Yb values from the substable passive continental margin to the unstable continental island arc all clearly decrease. A comparison of the REE parameters of sandstones of the Zhiluo Formation in the three areas of the Ordos Basin with those of sandstones under various tectonic settings shows that the petrochemical composition is similar to that of active continental margin and continental island arc, indicating that the tectonic background of the provenance area is active continental margin and continental island arc.

236

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Fig. 3.60 Identification of major elements in the geotectonic background of the sandstone source area (according to Bhatia 1983) Table 3.11 REE parameters of sandstone under various tectonic settings Tectonic Provenance ΣREE background area type 58 ± 10

La/Yb

(La/Yb)N

4.2 ± 1.3 2.8 + 0.9

ΣLREE/ΣHREE Eu/Eu* 3.8 ± 0.9

1.04 ± 0.11

7.5 ± 2.5

7.7 ± 1.7

0.79 ± 0.13

12.5

8.5

9.1

0.6

210

15.9

10.8

8.5

0.56

Huangling area

61.83

8.02

5.4

7.32

0.1

Ningdong area

60.86

9.68

6.53

8.28

0.1

Tarangaole area

95.13

18.3

12.34

10.89

0.08

Oceanic island arc*

Uncut magmatic arc

Continental island arc*

Cut magma 146 ± 20 11 ± 3.6 arc

Active continental margin*

Basement uplift

186

Passive continental margin*

Tectonic highland within craton

Note Data are from Bhatia (1983)

3.4 Sedimentary Environment of Uranium-Bearing Rock Series …

237

In addition, Bhatiam and Crook established a series of discriminant diagrams by studying the trace element geochemical characteristics of the Paleozoic turbidite in eastern Australia. The elements Co, Sc, and Zr are considered to have good stability, and Sc and Co are compatible elements and have a good correlation. They represent an immature tectonic background. Zirconium indicates the degree of sediment sorting. However, the large ion lithophile Th is relatively active, which represents a mature tectonic background. In the Th–Sc–Zr/10 and La–Th–Sc diagrams, the sandstone samples of the Zhiluo Formation in the three areas are distributed in a continental island arc area. Some samples in the Huangling area along the southeastern margin are distributed in the passive continental margin (Fig. 3.61). This feature indicates that the formation of clastic rocks of the Zhiluo Formation in the Ordos Basin is closely related to the island arc but may be mixed with some active continental margin detritus. This is consistent with the geotectonic background revealed by the main quantitative elements. It indicates that the structural nature of the provenance area of the Zhiluo Formation along the northeastern and western margins has been in the active continental margin and continental island arc for a long time. However, the tectonic nature of the provenance area of the Zhiluo Formation in the southeastern margin area has been dominated by passive continental margin and continental island arc for a long time, followed by active continental margin. The tectonic environment may be the collision orogeny between the active continental margin and the passive continental margin with a trench–arc–basin system. In summary, the main results are as follows:

Fig. 3.61 Identification Th–Sc/10 and La–Th–Sc diagrams of source tectonic setting of the Zhiluo Formation sandstone

238

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

(1) The sandstone in the lower member of the Zhiluo Formation along the northeastern margin of the Ordos Basin is mainly lithic sandstone. The Ningdong area is composed of mixed sandstone and lithic sandstone. The Huangling area along the southeastern margin is mainly feldspar sandstone. They all represent the properties of near sources. Compared with the average content of elements in the upper crust, K, Rb, Zr, and Hf are enriched, whereas Nb, Ta, P, and Ti are depleted. The total amount of REEs is low. The sandstone is characterized by enrichment of LREEs, no obvious differentiation of HREEs, and no Eu anomaly or weak positive and negative anomalies. (2) La–Th–Sc and Th–Sc–Zr/10 diagrams show that the source rock tectonic settings of the Zhiluo Formation in the three areas are slightly different. The source rocks in the northern and western parts of the basin were formed in the active continental margin–continental margin arc environment. The source rocks in the south are mainly formed in the passive continental margin and continental island arc environment. The La/Th–Hf and F1–F2 provenance discrimination results of the Zhiluo Formation sandstone reveal that the original material should come from the upper crust, which is mainly composed of feldspathic rocks and mixed with neutral magmatic rocks with high feldspar content. (3) Combined with the regional sedimentary characteristics of the lower member of the Zhiluo Formation in the three areas and the previous research results, the following results are obtained: The provenance of the Zhiluo Formation in the northern Ordos Basin mainly comes from metamorphic rocks and magmatic rocks in the northern orogenic belt, and the paleocurrent runs from north to south. The provenance of the Zhiluo Formation in the western basin mainly comes from the metamorphic rocks and magmatic rocks of the Alashan block, and the paleocurrent runs from northwest to southeast. The provenance of the Huangling area along the southeastern margin mainly comes from metamorphic rocks in western mountain areas, magmatic rocks, and some metamorphic rocks in North Qinling Mountains, and the paleocurrent runs generally from northwest to southeast.

3.5 Summary 1. The main uranium-bearing strata and their characteristics in different areas of the Ordos Basin have been preliminarily identified. The uranium-bearing rock series in the Ordos Basin is mainly the Jurassic Zhiluo Formation, with secondary contributions from the Jurassic Yan’an Formation and the Cretaceous Luohe Formation. The strata of the Zhiluo Formation are widely distributed. Fluvial sandbodies are developed. The formation is the main ore-bearing target horizon along the northeastern, western, and southeastern margins. Uranium deposits primarily occur in the channel sandbodies of the lower member of the Zhiluo Formation. The lithology of

3.5 Summary

239

the lower submember is gray-white and gray-green medium- to coarse-grained sandstone intercalated with a thin layer of gray silty mudstone. The upper submember is interbedded with gray medium-thick sandstone and grayish purple, purplish red, and red mudstone. The Yan’an Formation is the main coal-bearing strata. The rocks are mainly composed of gray and gray-green mudstone and sandstone. The formation can be divided into five stages according to the characteristics of rock assemblage, the evolution of the sedimentary system, and the periodicity of coal accumulation. The first member of the Yan’an Formation is a river system. The second to fourth segments are composed of several lake delta system units. The sediments are mainly fine detritus, and exploitable coal seams are developed. Fluvial sandbodies are developed in the fifth member. As a secondary ore-bearing layer along the western margin of the basin, uranium orebodies are mainly distributed in the Yanyi Formation. The Luohe Formation is a newly discovered uranium-bearing layer along the southwestern margin of the basin. In this study we have further defined the characteristics of the sedimentary system of the Luohe Formation. The Luohe Formation consists of alluvial fan in the lower part, coarse clastic deposits in fluvial facies, and eolian deposits in the upper desert facies. Intermittent desert shallow lacustrine deposits were developed in the late stage. The main ore-bearing strata in the lower part of the Luohe Formation along the southwestern margin of the basin are alluvial fan bodies and eolian sandbodies in lowstand system tracts. 2. The lithologic logging response characteristics of the Zhiluo Formation has been preliminarily analyzed. By means of classified statistics and morphological analysis of logging curves, the parameters of quantitative γ, spontaneous potential, and three lateral resistivity and density parameters of the Jurassic Zhiluo Formation in four main uranium ore concentrated areas in the Ordos Basin have been comprehensively analyzed. The results reveal that there is little difference in γ exposure rate in different regions. The resistivity along the southeastern margin is notably higher than that in other areas. The change of density is high in the south and low in the north. Vertically, the γ background and resistivity of the upper section of the straight group are lower than those of the lower section, but the density changes are minor. Regionally, the γ background value from coarse sandstone to mudstone in the Zhiluo Formation increases gradually, and the density increases slightly. The abnormal amplitude of resistivity and spontaneous potential changes from large to small. Based the differences of logging parameters and facies series of different types of rocks, the lithology identification model of resistivity and density intersection chart along the northeastern margin of the basin has been established. When interpreting the lithology of coalfield borehole logging, the grain size of the sandstone section with a radioactive anomaly should be increased by one or two grades. 3. The restriction of color zoning, geochemical characteristics, and biological fossils on the paleo-sedimentary environment of the Zhiluo Formation have been discussed.

240

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

From the uranium mining area on the basin margin to the non-mining area in the basin, the red–green–gray color zoning is seen vertically in the Zhiluo Formation. It is vertical zoning and cannot be used as mineralization stage zoning. Vertically, U, Th, V, Cr, Fe, Co, Cu, Zn, Mo, and other elements indicating the oxidation–reduction environment exhibit similar geochemical characteristics and they are relatively stable. The formation’s geochemical characteristics basically reflect the gradual transition from a paleo-sedimentary environment of oxidation to one of weak reduction and then one of reduction. Horizontally, it does not have a wide range of color zoning. The results of this study contrast with our understanding of transverse color zoning in the metallogenic theory of the interlayer oxidation zone. In the process of uranium prospecting, color is only one of the prospecting indicators. Primary color and postcolor should be distinguished. The primary color is related to the metallogenic environment but is unrelated to mineralization. This understanding can be used as a reference for the transformation of the uranium prospecting idea that the redox front line is delimited through the color change of the prospecting target layer in the early stage and for study on the lateral sedimentary environment for uranium-bearing rock series. 4. The provenance and structural background of the Zhiluo Formation in different ore concentration areas of the basin have been preliminarily analyzed. Through the analysis of the geochemical characteristics of the Zhiluo Formation in different ore concentration areas, combined with the regional sedimentary characteristics and previous research results, the following understanding has been obtained: The provenance of the Zhiluo Formation along the northern margin of the Ordos Basin mainly comes from metamorphic rocks and magmatic rocks in the northern orogenic belt, while that along the western margin mainly comes from the metamorphic rocks and magmatic rocks of the Alashan landmass. The provenance of the Huangling area along the southeastern margin mainly comes from metamorphic rocks and magmatic rocks in the western mountain area and some metamorphic rocks in the northern Qinling Mountains in the south. The source rocks in the northern and western parts of the basin were formed in an active continental margin–continental margin arc environment. The source rocks in the southern part of the basin are primarily formed in a passive continental margin and continental island arc environment. The discriminant diagrams of La/Th–Hf and provenance F1–F2 of the Zhiluo Formation sandstone indicate that the original material in the source area should come from the upper crust, which is mainly composed of feldspathic rocks and mixed with neutral magmatic rocks with high feldspar content.

References Adams SS, Smith RB (1981) Geology and recognition criteria for sandstone uranium deposits in mixed fluvial-shallow marine sedimentary sequences South Texas. U. S. Department of Energy Grand Junction Office, Colorado

References

241

Algeo TJ, Maynard JB (2004) Trace-element behavior and redox facies in core shales of Upper Pennsylvanian Kansas—type cyclothems. Chem Geol 206:289–318 Bhatia MR (1983) Plate tectonics and geochemical composition of sandstones. J Geol 91(6):611– 628 Cai G, Huang Z, Li S (2006) Alteration mineral assemblages in interlayer oxidation zone of the Shihongtan in-situ leachable sandstone-type uranium deposit. Acta Geol Sin (01):119–125 (in Chinese with English abstract) Calvert SE, Pedersen TD (1993) Geochemistry of recent oxic and anoxic marine sediments: implications for the geological record. Mar Geol 113:67–88 Chen D (1994) Discussion on the present research status and developmental direction for the sandstone type uranium deposit in Meso-cenozoic Basins in China. Uranium Ore Geol 10(4):203– 206 (in Chinese with English abstract) Chen Y, Dai D, Yang C (1981) Lithofacies palaeogeography: a case study of Ordos Basin. Geological Publishing House, Beijing, pp 96–115 (in Chinese with English abstract) Chen Q, Li W, Hu X, Li L, Pang J, Guo Y (2012) Tectonic setting and provenance analysis of the Late Paleozoic sedimentary rocks in the Ordos Basin. Chin J Geol 86(7):1150–1162 (in Chinese with English abstract) Chu Z, Gao J, Huang L et al (2007) Geophysical logging methods and principles: part II. Petroleum Industry Press, Beijing (in Chinese with English abstract) Colombo F (1994) Normal and reverse unroofing sequences in syntectonic conglomerates as evidence of progressive basinward deformation. Geology 22(3):235–238 Dypvik H (1984) Geochemical compositions and depositional conditions of Upper Jurassic and Lower Cretaceous Yorkshire clays, England. Geol Mag 121(5):489–504 Francois R (1988) The study on the regulation of the concentrations of some trace metals (Rb, Sr, Zn, Cu, V, Cr, Ni, Mn, and Mo) in Saanich inlet sediments, British Columbia, Canada. Mar Geol 83:285–308 Gao R, Zhao C, Qiao X et al (1999) Cretaceous petroleum stratigraphy in Songliao Basin. Geological Publishing House, Beijing, pp 1–373 (in Chinese with English abstract) Guo Z (2006) Meso-neozoic intracontinental orogenesis and sand-type uranium mineralization. Geological Publishing House, Beijing (in Chinese with English abstract) Hatch JR, Leventhal JS (1994) Relationship between inferred redox potiential of the depositonal environment and geochemistry of the Upper Pennaylvanian (Missourian) Stark Shale Member of the Dennis Limestone, Wabaunsee County, Kansas, USA. Chem Geol 99(1–3):65–82 Hu L (2010) The geological-geochemical characteristics and genesis of the main epigenetic alteration in the Dongsheng uranium area of the Ordos Basin. Master dissertation of Northwest University (in Chinese with English abstract) Huang C, Zhang J, Ding W, Lei H, Fan H (2016) Stugy on quantitative analysis of rock color based on Munsell color system. Sci Technol Eng 16(25):1671–1815 (in Chinese with English abstract) Jia L (2005) Study on evolvement in Mesozoic group and geological information about sandstonetype uranium deposits in south of Ordos Basin. Master thesis of China University of Geosciences, Beijing (in Chinese with English abstract) Jiao Y, Chen A, Wang M, Wu L, Yuan H, Yang Q, Zhang C, Xu Z (2005) Geneyic of the bottom sandstone of Zhiluo Formation, northeastern Ordos Basin: predictive base of spatial orientation of sandstone-type uranium deposits. Sediment J (03):371–379 (in Chinese with English abstract) Jones B, Manning DAC (1994) Comparion of geochemical indices used for the interpretation of palaeoredox conditions in ancient mudstones. Chem Geol 111(1–4):111–129 Lei K, Liu C, Zhang L, Wu B, Wang J, Cun X, Sun L (2017) Detrital zircon U-Pb dating of MiddleLate Mesozoic strata in northern Ordos Basin: implications for tracing sediment sources. Geol J 91(7):1522–1541 (in Chinese with English abstract) Li Z, Fang X, Chen A, Ou G, Sun Y, Zhang K, Xia Y, Zhou W, Chen F, Li M, Liu Z, Jiao Y (2009) Superposition metallogenic model of sandstone-type uranium deposit in the Northeastern Ordos basin. Uranium Geol 25(2):65–70 (in Chinese with English abstract)

242

3 Geological Characteristics of Uranium-Bearing Rock Series in Key …

Li Z, Feng S, Yuan X, Qu H (2014) Chronology and its significance of the Lower Jurassic tuff in Ordos Basin and its periphery. Oil Gas Geol 35(5):729–741 (in Chinese with English abstract) Li X, Yi C, Gao H, Chen X, Zhang K, Wang M (2016) Study on formation mechanism of epigenetic altered zone in Zhiluo Formation, northeastern Ordos Basin,north China. Mod Geol 30(04):739– 747 (in Chinese with English abstract) Mclennan SM, Taylor SR (1991) Sedimentary rocks and crustal evolution: tectonic setting and secular trends. J Geol 99(1):1–21 Pearce JA, Harris NBW, Tindle AG (1984) Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J Petrol 25:956–983 Qi F, Qin M, Liu W, Xiao S, Wang Z, Zou S, Huang J (2007) Time space configuration of uranium mineralization sedimentary facies and oil & gas induced alteration zone in Zhiuo Formation, Ordos Basin. Uranium Geol 23(2):65–70 (in Chinese with English abstract) Rudnick RL, Gao S (2003) Composition of the continental crust. In: Holland HD, Turekian KK (eds) The crust: treatise on geochemistry. Elsevier-Pergamon, Oxford, pp 1–64 Russell AD, Morford JL (2001) The behavior of redox-sensitive metals across a laminated-massivelaminated transition in Saanich Inlet, British Columbia. Mar Geol 174:341–354 Shi Y, Yu J, Xu X, Qiu J, Chen L (2009) Geochronology and geochemistry of the Qinling group in the eastern Qinling Orogen. J Petrol 25(10):2651–2670 (in Chinese with English abstract) Song C, Bai J, Zhao Y, Jin H, Meng Q (2005) The color of lacustrine sediments recorded climate changes from 13 to 4.4 Ma in Linxia Basin. J Sedimentation 23(3):507–513 (in Chinese with English abstract) Spalletti LA, Queralt I, Matheos SD et al (2008) Sedimentary petrology and geochemistry of siliciclastic rocks from the upper Jurassic Tordillo Formation (Neuquén Basin, western Argentina): implications for provenance and tectonic setting. J S Am Earth Sci 25(4):440–463 Sun SS, Mcdonough WF (1989) Chemical and isotopic system atics of oceanic Basalts: implication for the Mantle composition and process. In: Saunder AD, Norry MJ (eds) Magmatism in the ocean basins. Geological Society of London Special Publication, pp 313–345 Sun L, Zhang Y, Zhang T, Cheng Y, Li Y, Ma H, Yang C, Guo J, Lu C, Zhou X (2017) Jurassic sporopollen of Yan’an Formation and Zhiluo Formation in northern-eastern Ordos Basin, Inner Mongolia, and its paleoclimatic significance. Earth Sci Front 24(1):32–51 (in Chinese with English abstract) Wang S (1996) Coal accumulation regularity and evaluation of coal resources in Ordos Basin. Coal Industry Publishing House, Beijing (in Chinese with English abstract) Xie Y, Wang J, Jiang X, Li M, Xie Z, Luo J, Hou G, Liu F, Wang Y, Zhang M, Zhu H, Wang D, Sun Y, Cao J (2005) Sedimentary characteristics of the cretaceous desert facies in Ordos Basin and their hydrogeological significance. J Sedimentation 23(1):73–84 (in Chinese with English abstract) Xu Y, Zhang W (1980) Neojurassic spore pollen. See: Compiled by Institute of geology, Chinese Academy of Geological Sciences. Mesozoic stratigraphic paleontology in Shaanxi Gansu Ningxia Basin. Geological Publishing House, Beijing, pp 144–186 (in Chinese with English abstract) Xu Y, Du Y, Yang J (2007) Prospects of sediment provenance analysis. Geol Sci Technol Inf 26(3):26–32 (in Chinese with English abstract) Yan Y, An C, Miao Y, Song Y, Yang S, Cai X (2017) Relationship between color index of modern surface sediment and climate parameters in the region of Xinjiang and Qinghai. Arid Geogr 40(2):355–364 (in Chinese with English abstract) Yang X, Ling M, Lai X (2009) Metallogenic model of the Dongsheng in-situ leaching sandstonetype uranium deposit in the Ordos Basin. Geol Front 16(02):239–249 (in Chinese with English abstract) Yi C, Gao H, Li X, Zhang K, Chen X, Li J (2015) Study on indicative significance of major elements for sandstone-type uranium deposit in Zhiluo Formation in northeastern Ordos Basin. Mineral Geol 34(4):801–813 (in Chinese with English abstract) Zhang Q (2010) Analysis of well logging parameters and well logging facies in Hoosiliang area. J Henan Univ Technol (Nat Sci Ed) 29(suppl):41–47 (in Chinese with English abstract)

References

243

Zhang L, Wu B, Liu C, Lei J, Hou H, Sun L, Cun X, Wang J (2016) Provenance analysis of the Zhiluo Formation in the sandstone-hosted uranium deposits of the northern Ordos Basin and implications for uranium mineralization. Geol J 90(12):3441–3453 (in Chinese with English abstract) Zhang Z, Fan H, He F, Liu H, Jia L, Yi L, Yang M (2017) Features of hydrogeochemistry and uranium metallogenesis in Binxian County area on the southern margin of Ordos Basin. Geol Bull China 36(4):503–510 (in Chinese with English abstract) Zhao X, Zhao C, Guan X et al (1992) The application of spore-pollen data in a quantitative explanation of tertiary palaeoclimate in our country. Petrol J 13(2):215–225 (in Chinese with English abstract) Zhao J, Liu C, Liang J, Wang X, Yu L, Huang L, Liu Y (2010) Restoration of the original sedimentary boundary of the middle jurassic Zhiluo Formation–Anding Formation in the Ordos Basin. J Geol 84(4):553–568 (in Chinese with English abstract) Zhao X, Wang G, Zhou Z, Wang D, Ran Y, Sun Y, Zhang X, Li M (2015) A review of lithology interpretation methods using geophysical well logs. Prog Geophys 30(3):1278–1287 (in Chinese with English abstract) Zhou W, Song M, Zhang B, Zhao Z, Xie H (1999) Chemical characteristics of syn-and postcollisional plutons in Qinling orogen. Geol Geochem 27(1):27–32 (in Chinese with English abstract) Zimmermann U, Bahlburg H (2003) Provenance analysis and tectonic setting of the Ordovician clastic deposits in the southern Puna Basin, NW Argentina. Sedimentology 50(6):1079–1104

Chapter 4

Geological Characteristics of Typical Deposits and Newly Discovered Ore-Producing Areas

The uranium geological survey has led to important prospecting breakthroughs along the northeastern, western, and southeastern margins of the Ordos Basin, and new discoveries have been made in the southwestern and central areas. A total of 8 newly discovered mineral areas, 6 ore spots, and 12 mineralization spots have been noted. Among them, there are two large-scale mineral deposits (Tarangaole and Yangchangwan) and three medium-sized mineral deposits (Huangling, Shicao village, and Jinjiaqu) (Table 4.1). 1. Ore concentration area along the northeastern margin The newly discovered uranium deposits include the Tarangaole, Wuding Bragg, and Chaidengnan deposits and the Kujigou, Nalinsili, Hongqingliang, Naimadai, Selianerhao, Gao Jia Liang, and Zhongji occurrences. Typical deposits include Daying, Nalinggou, Zaohuohao, and Abuhai. 2. Ore concentration area along the western margin The newly discovered mineral deposits and occurrences include the Yangchangwan, Jinjiaqu, Shicaochun, and Maiduoshan deposits and the Ye Zhuangzi, Zaoquan, and Qingshuiying occurrences. Typical deposits include the Ciyaobao and Huianbao deposits. 3. Ore concentration area along the southwestern margin The new discoveries include the Jingchuan ore spot, the Huanxian miniaturized spot, and the Chongxin minimized spot. Typical deposits include the Guoguowan deposit. 4. Ore concentration area along the southeastern margin The new discoveries include the Huangling mineral field and the Binchang, Xinbaozi, and Dafosi miniaturized spots. Typical deposits include the Shuanglong deposit. 5. Central metallogenic prospect area The new discovery is the Jinding miniaturized spot. According to different metallogenic geological conditions, some typical deposits, mineral fields, and ore occurrences were selected for description. We mainly © Science Press 2023 R. Jin et al., Geological Background of Sandstone-Type Uranium Deposits in Ordos Basin, Northwest China, Springer Geology, https://doi.org/10.1007/978-981-19-6028-4_4

245

246

4 Geological Characteristics of Typical Deposits and Newly Discovered …

Table 4.1 Statistics of typical uranium deposits and occurrences in the basin Region

Typical uranium deposit

Ore-bearing Newly Ore-bearing Newly horizon discovered horizon discovered ore-producing mineral and areas in this mineralized round points in this round

Ore-bearing horizon

Northeastern margin

Camp

J2 z

J2 z

Tarangaole

J2 z

Kujigou ore occurrence

Naling ditch

Wuding Prague

Narinsili mine

Soap fire trench

Ulansili

Narinsili mine

Abuhai Southeastern margin

Shuanglong J2 z

Huangling

J2 z

Binchang J2 z mineralization point Xinbaozi mineralization point Dafosi mineralization point

Western margin

Porcelain kiln castle

J2 z

Porcelain kiln castle

Yangchang Bay

K1 md

Yezhuangzi mine

Jinjiaqu

Huanxian mineralization point

Shicao Village

Chongxin mineralization point

Mai Duo mountain Southwestern National margin Bay

J2 z+J2 y

J2 z

J2 z Jingchuan mine

K1 l

Chongxin J2 z mineralization point Huanxian J2 z mineralization point Central section

Jinding J2 a mineralization point

4.1 Geological Characteristics of the Uranium Concentration Area Along …

247

focus on describing the structure, stratum, radioactive anomaly characteristics, orebody, and ore characteristics of the mining area.

4.1 Geological Characteristics of the Uranium Concentration Area Along the Northeastern Margin 4.1.1 Tarangaole Mining Area 4.1.1.1

Metallogenic Geological Background

1. Structure of the mining area The structural unit of the mining area is located in the north central part of the Yimeng uplift on the northern margin of the Ordos Basin (Fig. 4.1a). From the contour map of floor elevation and burial depth of the Zhiluo Formation in this area (Fig. 4.2), one can see that its structural form is basically consistent with that of regional coalbearing strata, with contour lines essentially equidistant from northwest to southeast. It is generally a monoclinic structure inclined to the southwest, tending to southeast at 220°–250°. Generally, it is higher in the northeast and lower in the southwest. The elevation of the Tanggongliang–Husiliang area in the north of Tarangaole is >1300 m, which is the uplift region in this area. The elevation of the Xinsheng area in the south of Tarangaole is generally 800–900 m, which is the relative depression region in this area. The elevation difference between the uplift and depression regions is 400–500 m, the dip angle of the stratum is 1°–5°, and the occurrence of the stratum changes along the strike, with wide and gentle undulation. This indicates that the paleotopography of the Zhiluo Formation is relatively gentle, which creates favorable structural conditions for the stable development of a fluvial sedimentary system. There is no obvious fault and fold structure in the area. There are two typical large-scale sandstone-type uranium deposits to the east and west of the Tarangaole area (Daying and Nalinggou, respectively), both of which are located on the northern uplift slope. Not only does this provide a good channel for the migration of uranium ore-forming fluids, but unloading sedimentation of minerals also provides favorable space. 2. Mining area strata The mining area is located at the northern edge of the Dongsheng Coalfield. The Cenozoic geological process was relatively strong. The upper strata has been eroded and destroyed by branch-shaped gullies. The developed strata in the area from old to new are the Middle–Lower Jurassic Yan’an Formation (J1-2 y), Middle Jurassic Zhiluo Formation (J2 z), and Lower Cretaceous and Quaternary (Q) strata. The exposed strata are mainly Lower Cretaceous and Quaternary (Q) (Fig. 4.1b).

248

4 Geological Characteristics of Typical Deposits and Newly Discovered …

Fig. 4.1 Geological map of Tarangaole and surrounding areas (according to Xiaoxue et al. 2016). a Location of the Tarangaole and surrounding geotectonic structures. b Geological map of the Tarangaole and surrounding areas. 1. Quaternary. 2. Neogene. 3. Lower Cretaceous Dongsheng Formation. 4. Lower Cretaceous Yijinhuoluo Formation. 5. Middle Jurassic Anding Formation. 6. Middle Jurassic Zhiluo Formation. 7. Middle Jurassic Yan’an Formation. 8. Triassic. 9. Sandstonetype uranium deposit. 10. Surface radioactive anomaly point. 11 Tarangaole range

A Lower part of the Zhiluo Formation (J2 z1 ) The main lithology of the lower section of the Zhiluo Formation is gray, light gray, and green sandstone with mudstone interposed with thin coal seams (Fig. 4.2). The sandstone is dominated by clastics with an average content of 90%. The main component of the clastics is quartz, with feldspar as a secondary component. Charcoal chips, coal chips, and pyrite are more common in gray sandstone, and most of the pyrite is in the form of agglomerates and fine crystals. Sandstone grains are mostly subangular, with a low degree of consolidation, and dominated by argillaceous cementation. Among them, the lower submember is in contact with the Yan’an Formation, which is a sandy braided river sedimentary system formed in a humid climate during the early depositional stage. The lithology is gray, light gray, and green sandstone with mudstone, with thin coal seams in some areas. The upper subsection comprises a set of meandering river sedimentary systems developed in a humid climate. The main lithology is gray, gray-green, and greengray medium- and fine-grained sandstone, with siltstone and mudstone on the top.

4.1 Geological Characteristics of the Uranium Concentration Area Along …

249

Fig. 4.2 Contour map of the elevation and depth of the roof and floor of the lower section of the Zhiluo Formation in the Tarangaole area. Contour maps of a the top plate elevation of the lower section of the Zhiluo Formation, b the buried depth of the top plate of the lower section of the Zhiluo Formation, and c the bottom plate elevation of the lower section of the Zhiluo Formation

The rock sortability is good, and the rocks are rounded. The degree of consolidation is subcircular, mainly muddy cementation, with some calcareous cementation. The upper subsection does not contain organic matter. It has horizontal bedding and small cross-bedding. The debris is wrapped, and numerous spherical chlorite aggregates are developed in the gaps of the particles. B Upper part of the Zhiluo Formation (J2 z2 ) This rock section in the upper part of the Zhiluo Formation is sedimentary strata formed under arid paleoclimatic conditions and a high-curvature meandering river sedimentary system. The lithology is mainly sandstone, siltstone, and mudstone interbedded. The mudstone and siltstone are pink, purple, and grayish purple, and the sandstone is purple, gray green, and gray white. Limonite mineralization is generally developed, and it is porphyritic or banded along the fissures (Fig. 4.3). The sandstone is dominated by detritus, with a content of ~80%. The detrital composition is mainly quartz, with feldspar being secondary and a small amount of mica. The sandstone grain size is generally fine, mainly fine-grained, medium-fine grained, and sorted. It has a subangular shape and is dominated by argillaceous cementation. The degree of consolidation is loose and the diagenesis is relatively low. This section is dominated by an oxidizing environment, and no uranium mineralization has been found.

250

4 Geological Characteristics of Typical Deposits and Newly Discovered …

Fig. 4.3 Features of the Zhiluo Formation sandbody in the Tarangaole area. a Thick-layered graygreen sandstone in the lower member of the Zhiluo Formation. b Gray-green sandstone section of the lower member of the Zhiluo Formation. c Interbedded gray-green fine sandstone and mudstone in the upper submember of the lower member of the Zhiluo Formation. d Gray-green silty mudstone in the upper submember of the lower member of the Zhiluo Formation. e Purple-red fine sandstone of the upper member of the Zhiluo Formation. f Purple-red fine sandstone section of the upper member of the Zhiluo Formation

3. Distribution Characteristics of Sandbodies in the Target Layer The thickness of the sandbody in the lower part of the Zhiluo Formation in the Tarangaole area exhibits a nearly south–north distribution. This is a specific manifestation of the main river channel, and it continuously diverges to the south, west, and southeast, evolving into a series of smaller scales. The main channel is located between the Nalinggou and Tarangaole areas. The width of the sandbody is ~10 km, its thickness is >150 m, and the thickest point can reach 260 m (Fig. 4.4a). The lower member of the Zhiluo Formation is a braided river–meandering river sedimentary system formed in a humid climate during the early depositional period. The bottom is a gravel braided river sedimentary system, and the upper part transitions to a sandy braided river sedimentary system, which appears as the sandbody, mostly occurring in deep-cut valleys, and has the characteristics of filling and filling. In the vertical direction, it is composed of multiple rhythmitic layers ranging from

4.1 Geological Characteristics of the Uranium Concentration Area Along …

251

Fig. 4.4 Contour maps of a sandbody thickness and b sand content of the lower part of the Zhiluo Formation in the Tarangaole area

coarse sandstone to fine sandstone (or siltstone and mudstone). The whole is a thick connected body. On the plane, the sand content map shows the development of a braided branch channel in the Tarangaole and Nalinggou areas. The sandbody is relatively thick, and the sand content is as high as 85%. The entire braided channel is ~20 km long and 5–10 km wide. (Fig. 4.4b). A large area of floodplain is developed around the braided channel, and there are sporadic fracture fan deposits on it. The distribution range is small. The wide development of braided channel sandbodies in the area indicates the migration of oxygen-containing uranium water. They provide an effective channel and a huge storage space for sandstone-type uranium ore.

4.1.1.2

Deposit Features

1. Orebody characteristics Uranium orebodies mainly occur in braided river sandbodies in the lower section of the Middle Jurassic Zhiluo Formation. The uranium orebodies are generally distributed in a northeast–southwest or nearly north–south direction on the plane. The average buried depth of the orebodies is >500 m, which is deeply affected by topography and stratigraphic occurrence, but overall the buried depth gradually increases from east to west and from north to south. On the profile, the orebodies are developed in the lower middle and lower parts of the Zhiluo Formation (Fig. 4.5). Affected by the stratum and the distribution direction of the channel sandbodies, the occurrence of the orebodies is consistent with that of the target sandbodies. The mineralized bodies are mainly distributed along the periphery of the industrial orebodies. The vertical distribution of the orebodies is similar to that of the Nalinggou uranium deposit, and most of them occur in the gray sandbodies of the lower submember of the lower member of the Middle Jurassic Zhiluo Formation. Two layers of orebodies are developed in individual boreholes. There are six industrial orebodies delineated in this area. Their thickness varies greatly from 1.30 to 7.50 m, with an average value of 4.33 m. The average orebody grade is 0.0342%. The distribution of the orebody uranium mineralization is relatively

252

4 Geological Characteristics of Typical Deposits and Newly Discovered …

Fig. 4.5 (Top) North–south and (bottom) east–west connecting well profiles of the Tarangaole deposit

uniform. The uranium content of the orebodies varies from 1.76 to 11.20 kg/m2 , with an average value of 3.49 kg/m2 . 2. Features of mineralization alteration Through optical microscopy and electron probe analysis, seven main types of mineral alteration were identified, namely, limonite mineralization, pyrite mineralization, selenium iron mineralization, carbonation, sulfation, claying, and uranium mineralization (Fig. 4.6). According to the characteristics of element changes, one can infer that different sandbodies of the uranium-bearing rock series have a unified provenance, sedimentary environment, and structural background. Under the action of fluids, the migration and enrichment of REEs occurred during the later period. Limonite mineralization is mostly present in sandstone lenses or as sandy clumps (spots) in gray-green sandstone. The composition is dominated by goethite. Microscopic observation reveals that the clastic particles and cements are stained brown or brown as a whole. Only the edges of the debris particles are impregnated (Fig. 4.7). From the perspective of the formation stage and the relationship with uranium mineralization, pyrite can be divided into three stages. The first stage is diagenetic pyrite, including berry spherical pyrite and granular pyrite. The second phase is fluid transformation of pyrite, which consists mostly of colloidal pyrite or euhedral pyrite between the clastic particles. The third phase is altered pyrite. Its main feature is

4.1 Geological Characteristics of the Uranium Concentration Area Along …

253

Fig. 4.6 a Gray carbon-bearing chips and b pyrite sandstone core photographs of the lower submember of the lower member of the Zhiluo Formation

Fig. 4.7 Microphotographs of limonite in sandstone of the Zhiluo Formation. a Clastic particles in the red sandstone disseminated as a whole in red, taken in cross-polarized light, 10 × 10. b Immersed edges of the clastic particles in the red sandstone, taken in cross-polarized light, 10 × 10. c Inhomogeneous limonite mineralization inside the pyrite, taken in reflected light, 10 × 20. d Internal limonite mineralization of ilmenite, taken in reflected light, 10 × 20

symbiosis with biotite. The main source of Fe is the alteration and precipitation of biotite. This also includes two forms: euhedral–semiautomorphic and colloidal (Fig. 4.8). Selenite is mostly filled in grain pores or fissures, and it coexists with pyrite and other selenium-containing minerals. Previous research results have led to the discovery of only a very small number of sandstone-type uranium deposits. There are small amounts of independent selenium minerals such as selenium lead ore and selenium iron ore. The discovery of selenium iron ore in this study area may indicate that this area has experienced the transformation of medium- and low-temperature

254

4 Geological Characteristics of Typical Deposits and Newly Discovered …

Fig. 4.8 Micrographs of pyrite in the Zhiluo Formation sandstone. a Berry globular pyrite. b Starpointed pyrite. c Rhomboidal and hexagonal pyrite. d Colloidal pyrite between detrital particles. e Colloidal pyrite in the mica cleavage seam. f Semiautomorphic pyrite in mica cleavage cracks

hydrothermal action. Ilmenite mineralization is also relatively common in this area, with the cogeneration of ilmenite and pyrite or the cogeneration of titanium oxide and ilmenite occurring (Fig. 4.9). In the core, one can see that the red calcareous sandstone develops carbonate fine veins, and the plant charcoal in the gray fine sandstone develops grid-like calcite fine veins (Fig. 4.10). These characteristics all indicatge the later fluids. The transformation of the role of carbonation can be roughly divided into three stages. In the first stage, micrite calcite is formed, and the calcite grain diameter is only a few micrometers. In the second stage, coarse-crystalline calcite is formed, and the calcite grains are relatively coarse, with a diameter of ≥0.5–2 mm, poor brightness, and

Fig. 4.9 Micrographs of low-temperature hydrothermal minerals in the Zhiluo Formation sandstone. a Selenite replacing pyrite. b Pyrite being replaced by ilmenite. c Ilmenite being replaced by titanium oxide

4.1 Geological Characteristics of the Uranium Concentration Area Along …

255

Fig. 4.10 Micrographs of carbonate in sandstone of the Zhiluo Formation. a Grid-like calcite veins developed inside the plant charcoal chips. b Micrite calcite, taken in orthogonally polarized light, 10 × 10. c Coarse-crystal calcite, taken in orthogonally polarized light, 10 × 10. d Coarse-crystal calcite at the edge of feldspar grains, taken in orthogonally polarized light, 10 × 10. e Bright crystal calcite, taken in orthogonally polarized light, 10 × 20. f Calcite cutting through the quartz particles, showing strong carbonation

obvious traces of recrystallization. Calcite often metasomatizes heterogeneous bases and partially metasomatizes detrital particles. The third stage of carbonation is the most advanced carbonation in the region. Calcite veins or microvessels are produced, and two or three groups of extremely complete cleavage can be seen, and there are more metasomatic clastic particles. Agglomerated or bedding gypsum veins can be seen in the sandstone body of the mining area. The alteration spectrum scan revealed that the gypsum cementation phenomenon is also widespread in the sandbody. The gypsum may have recrystallized under the action of the younger fluid, coarsening the particles. Clarification of the uranium-bearing rock series in the mining area includes chloritization, kaolinization, and hydromicaization. Sandstone kaolinization includes\mica kaolinization, potash feldspar kaolinization, and heterogeneous high ridge petrification (Fig. 4.11). Hydromicaization in sandstone is mainly manifested by the hydromicaization of plagioclase, followed by the hydromicaization of heterogeneous bases. Chloritization includes the chloritization and detrital particles of biotite. There are two types of membrane-like chlorites. Part of the biotite is eroded into chlorophyllite. The chlorites are leaf-shaped and light green. The surface of the debris particles is coated with very fine needle-like green mud. Stone aggregates cannot be identified under an optical microscope. Only extremely thin green borders are distributed on the edges of the debris particles. Under a scanning electron microscope, they are identified as fine needle-like chlorite aggregates (Fig. 4.11).

256

4 Geological Characteristics of Typical Deposits and Newly Discovered …

Fig. 4.11 Photomicrographs of kaolinization in the Zhiluo Formation sandstone. a Biotite undergoing kaolinization, taken in cross-polarized light, 10 × 10. b Feldspar undergoing kaolinization, taken in cross-polarized light, 10 × 10. c Feldspar undergoing hydromicaization, taken in crosspolarized light, 10 × 10. d Cuttings that are hydromicaized, taken in cross-polarized light, 10 × 10. e Biotite being eroded into chlorophyllite, taken in single-polarized light, 10 × 10. f Chlorite bordering on the edge of clastic particles, taken in single-polarized light, 10 × 20

3. Ore quality The content of detritus is relatively high, ranging from 79 to 98%. The composition is mainly quartz, followed by feldspar. The detritus contains a certain amount of mica, rock debris, organic matter, and a few heavy minerals (Fig. 4.12). The interstitial content of the sandstone is 8% to 33%, which is mainly composed of miscellaneous bases and cements. The miscellaneous bases include mainly illite, kaolinite, and hydromica. The content of interstitials in calcareous sandstone is relatively high (~18%), and the cement is primarily calcite and pyrite, with very small amounts of goethite and limonite. The grain size of the detritus is chiefly medium grain, followed by fine grain and coarse grain. The proportions of sandstone of various grain grades are different. Medium-grain sandstone is the most common, accounting for 55.22%; coarse- and fine-grain sandstones account for 15.30% and 26.51%, respectively, while silt and argillaceous matter account for only 2.22%. The ore-bearing sandstone has a relatively coarse grain size, and the contents of argillaceous and siltstone are relatively small. The sandstone in the area has good permeability as a whole. The clastic materials are primarily contact and pore cementation, accounting for 80.8%, and basal cementation is less, accounting for only 19.2%. Some sandstones with carbonate content of 10–20% (or calcareous sandstone) is basal cementation. The roundness of detritus particles is generally low, being mainly angular and subangular, accounting for 72.3%, followed by subangular and subcircular with medium roundness, accounting for 25.7%. Good roundness accounts for 0.70%,

4.1 Geological Characteristics of the Uranium Concentration Area Along …

257

Fig. 4.12 Features of rock and micrographs in the Zhiluo Formation sandstone. a Gray medium sandstone. b Light gray medium-coarse sandstone, with a grain-level sequence. c Micrographs of medium- and fine-grained sandstones, taken in orthogonally polarized light. d Micrographs of calcareous medium-sized sandstone, taken in orthogonally polarized ligh. e and f Micrographs of medium-coarse-grained sandstone, taken in cross-polarized light

and there is a certain range of variation in the percentage of debris with different roundness values. The chemical compositions of the ore are basically similar, all of which being mainly SiO2 , Al2 O3 , and TFe2 O3 . These three compounds account for 63.40–90.17% of the total. The average contents of SiO2 , Al2 O3 , TFe2 O3 , and FeO and are 68.33%, 12.94%, 3%–9.8%, and 1%–6.1%, respectively. The average content of the harmful component P2 O5 is low (only 0.10%), and the average content of CaO is 1.80%. The chemical compositions of the ore, surrounding rock, and calcareous ore are basically the same. The content of CaO in the calcareous ore is increased, and the content of SiO2 is correspondingly reduced. The average loss on ignition of the ore is 5.58%, indicating that the content of organic matter in the ore is relatively high and that its own adsorption capacity and reduction ability is strong. The ore particle size is mainly medium sand and fine sand, accounting for 60.29% and 27.36%, respectively, followed by coarse sand, accounting for 10.97%. Silt and

258

4 Geological Characteristics of Typical Deposits and Newly Discovered …

Fig. 4.13 Uranium minerals (adsorption form) in the Tarangaole survey area

clay are rare. The contents of medium-coarse sand, medium sand, and fine sand in the ore is as high as 98.62%, and permeable ore is absolutely dominant. The Clark values of most of the co-associated elements of uranium (e.g., Au, Ag, Cu, Pb, Zn, Mo, Cs, V, Se, and Re) are greater than the crustal Clark value, reflecting the existence of certain associated elements above the region. Although these elements are enriched, none of them has reached the value of industrial use. Uranium minerals occur mainly in the form of adsorption, and some also exist in the form of independent minerals and uranium-containing minerals. Adsorbed uranium minerals generally exist in the form of clay minerals. They are adsorbed by other minerals such as coal and rock debris or exist on the surface of cements and mineral debris (Fig. 4.13). Common uranium-containing minerals include crystalline uranium ore, uranium thorium, cristobalite, and secondary uranium minerals. Most uranium-containing minerals are attached with pyrite, ilmenite, sphene, organic matter, and apatite at their edges. Independent uranium minerals are found mainly in uranium stone, and they appear as a single crystal or mineral spherulitic aggregate under an electron microscope. They are mostly in symbiosis with calcite and chlorite or attached to quartz or carbonaceous cuttings (Fig. 4.14). Compared with the adsorbed uranium minerals, the contents of their main components MgO and SiO2 have been reduced, while the content of the harmful component P2 O5 has increased.

4.1.2 Zaohuohao Uranium Deposit 4.1.2.1

Geological Background of Mineralization

The Zaohuohao uranium deposit is located along the southern margin of the Yimeng uplift in the northeastern part of the Ordos Basin. Its development has laid the foundation for the development of the later oxidation zone.

4.1 Geological Characteristics of the Uranium Concentration Area Along …

259

Fig. 4.14 Uranium minerals in the form of independent minerals (Left) Elemental map scanning (Right) Backscattered image

The ore-bearing strata comprise the lower section of the Middle Jurassic Zhiluo Formation, which mainly occurs in a channel extending from northwest to southeast. The ore-bearing lithology is mainly gray, light gray green, and gray green The principal medium- and coarse-grained sandstones are intercalated with fine sandstones; exhibit poor sortability, subangular clastics, and well-developed cross-bedding; and are rich in organic matter and calcified wood and pyrite. The gravel in the sandbody has a complex composition, large gravel diameter, poor sorting, and large thickness. The lower section of the Middle Jurassic Zhiluo Formation is mainly composed of braided river and braided river delta facies. Uranium mineralization occurs primarily in sandy braided river sandbodies. The sandbodies are mainly composed of medium and coarse sandbodies of granular sandstone. The bottom often develops retained gravel, mud, and charcoal chips. The sandbodies have a low degree of consolidation and are relatively loose. The sandy braided channel is generally distributed in the northwest–southeast direction, and the length is ~150 km. It is affected by the swing of the channel. A pan-connected grand sandbody is developed in the mining area, with a width of 20–30 km. In Sunjialiang, Shashagetai, and Xinmiaohao, interchannel sediments with relatively thin sandbodies are developed.

4.1.2.2

Deposit Features

1. Orebody characteristics From east to west, the Zaohuohao uranium deposit is composed of five ore sections: Wulansetai, Sunjialiang, Shashagetai, Zaohuohao, and Xinmiaohao. The ore belt is generally distributed in a nearly east–west direction (Fig. 4.15). The ore belt is nearly 40 km long from east to west and ~5 km wide from north to south. The continuity of the orebody is good in the east and slightly worse in the west. The tendency of ore sandbodies is basically the same, and the orebody morphology of each section is also different. The orebody distribution in the Sunjialiang section is

260

4 Geological Characteristics of Typical Deposits and Newly Discovered …

relatively concentrated, and on the plane it is shaped like a large pie. The orebody in the Shashage platform section consists of two nearly parallel belts spreading in a nearly east–west direction. The orebodies in the Zaohuohao section are relatively scattered. The orebodies and mineralized bodies are in a belt-like shape, with a U or lens shape protruding southeast. The orebodies in the Xinmiaohao section extend northwest–southeast. The shapes of the orebodies on the profile are mainly platelike and layer-like, and a few are lenticular (Fig. 4.16). The lower orebody is thin and long, with good continuity and a long extension distance. The upper layer is also thin and long, and the orebody is lenticular and produced near the roof, being thin with poor continuity. The orebody generally changes from thin to thick from the east to the west and tilts toward the south. The middle part is concave; that is, the eastern portion of the orebody is output close to the bottom of the sandbody. The buried depth of the roof of the orebody is 67.05–209.55 m, and the buried depth of the floor of the orebody is 74.20–219.25 m. The orebody is generally inclined to the southwest, which is basically consistent with the stratigraphic tendency. The elevation of the orebody gradually decreases from northeast to southwest. The body depth is still primarily controlled by topography. The orebody grade (from a single borehole) varies from 0.0177 to 0.3623%, with an average value of 0.0641% and a coefficient of variation of 145.60%. Some orebodies with grades of >0.1% are seen in some boreholes. The average grade of the orebody on exploration line A3 is the highest, and the average grade on both sides of the orebody gradually decreases. The uranium amount per square meter (single project) varies from 1.01 to 48.47 kg/m2 , the average is 7.63 kg/m2 , and the coefficient of variation is 124.51%.

Fig. 4.15 Schematic diagram of the plane distribution of the Zaohuohao uranium deposit (modified based on Jindai et al. 2015). 1. Industrial uranium orebodies. 2. Industrial uranium holes. 3. Exploration lines and numbers. 4. Uranium mineralization points. 5. Place names

4.1 Geological Characteristics of the Uranium Concentration Area Along …

261

Fig. 4.16 Schematic diagram of the section of exploration line A3 in the Sunjialiang section of the Zaohuohao deposit (based on Li and Chen). 1. Gray sandstone. 2. Green sandstone. 3. Mudstone. 4. Front line of the interlayer oxidation zone. 5. Industrial orebody (mineralized body). 6. Drill holes and numbering

2. Ore quality The ore-bearing lithology is lithic feldspar sandstone of the lower submember of the lower member of the Zhiluo Formation. The color of the rock is mainly dark gray and light gray, and the light part or near the surface is mostly grayish green. The surface is grayish yellow and light yellowish brown as the result of strong oxidation. The diagenetic degree of the rock is not high, and the structure is loose, exhibiting coarse and fine rhythmic changes and cross-bedding, and locally it contains more mud gravel (mostly at the bottom of the grain sequence layer). There is much rock debris, and the content of mica debris is also high. This reflects the characteristics of near source; that is, the mining area is located at the edge of the basin and not far from the erosion source area. From the perspective of the composition of the debris, the erosion source area mainly exposes rocks. It is composed of granite and metamorphic rock, with a small amount of volcanic rock. The associated elements in the ore include primarily Mo, V, Se, and Sc. The concentration of these elements is greater than the Clarke value of the crust, which reflects the enrichment of associated elements above the study area. None of the associated elements in the belt has reached the value of industrial use, but Se and Mo have potential use value. Uranium exists in two forms in the ore: in an adsorption state and as uranium minerals. Adsorbed uranium is closely related to clay minerals, powdered pyrite, and carbonaceous debris in the ore. Uranium minerals are mainly bituminous uranium and uranium. Bituminous uranium and pyrite coexist closely and are mainly produced on the surface of pyrite in sandstone cements or filled on the edge of pyrite between chlorite layers and nearby. There is also a small amount of filling on the surface of pyrite. Among the biotite planes, uranium is mainly distributed around feldspar clastics or around pyrite in a colloidal state. Uranium minerals are round and elliptical single crystals and aggregates. The size is mostly 7–9 μm. They are distributed along the growth line of pyrite crystals. Pyrite exhibits obvious stages. According to its shape, reflectivity, and composition, it can be divided into early berry globular pyrite and colloidal pyrite in the diagenesis stage and irregular pyrite and euhedral pentagonal pyrite in the middle diagenesis

262

4 Geological Characteristics of Typical Deposits and Newly Discovered …

stage. Uranium minerals only exist in the pores of irregular pyrite. In the inner cracks or microcracks, coffinite can also be seen distributed in the illite–montmorillonite mixed layer and the dissolved pores of potassium feldspar particles (Yiqun et al. 2006). Xia et al. used the U–Pb isochron method to obtain the uranium ore ages from the pentagrams of 149 ± 16, 120 ± 11, 85 ± 2, 20 ± 2, and 8 ± 1 Ma in the mining area. The mineralization has the characteristics of multiple stages.

4.1.3 Nalinggou Uranium Deposit 4.1.3.1

Geological Background of Mineralization

The Nalinggou uranium deposit is located in the north central area of the Yimeng uplift in the northeastern part of the Ordos Basin. The surface fault structure is not developed. The ore-bearing horizon of the Nalinggou uranium deposit is the lower submember of the lower member of the Middle Jurassic Zhiluo Formation. There is no stable water barrier between the upper and lower sublayers. It belongs to the same orebearing sandbody and is thick. However, there are local water barriers above and below the orebody. The lithology is mainly composed of green and gray mediumand coarse-grained sandstone, with thin layers of mudstone and siltstone, and the structure is loose. The overall distribution is in the northwest–southeast direction, gradually thinning from the center of the channel sandbody to both sides, with an average thickness of 124.1 m and a maximum thickness of >160 m and only minor variations in thickness change. The stability is good. The lower subsection can be further divided into two sections: The upper part is dominated by green and red sandstone deposited by sandy braided channels, and the lower part is dominated by gray conglomerates and sandy conglomerates deposited by gravel braided channels. Uranium deposits are widely distributed and are pan-connected. They are the main stratum where uranium mineralization occurs. Sandstone has a relatively coarse grain size and contains many fine gravels. Carbon chips, coal dust, and pyrite are more common in gray sandstone. The upper submember of the lower member of the Zhiluo Formation is dominated by green, light green, and dark green sandstone. Gray sandstone can be seen in the middle and lower parts of individual boreholes. In the southern part of the deposit, industrial uranium mineralization has been discovered in this layer. Sandstone is common with muddy interlayers. The sedimentary environments are all braided river sedimentary environments.

4.1 Geological Characteristics of the Uranium Concentration Area Along …

4.1.3.2

263

Deposit Features

1. Orebody characteristics On the plane, the orebodies are distributed in the northeast–southwest direction, with stable development along the strike and good continuity. The width of the orebodies varies greatly on each exploration line (Fig. 4.17). The horizontal projection of the main orebody (No. 1) is in the shape of an irregular belt with narrow ends, being wide in the middle, with multiple small skylights appearing inside the orebody. Other orebodies are produced on both sides of the main orebody and are distributed in discontinuous blocks. On the profile, the orebody is platelike and layered in the gray sandstone at the transition between the green sandstone and gray sandstone far away from the top and floor (Fig. 4.18). The orebody is gently inclined from northeast to southwest, with an inclination angle of 1°. The elevation of the top boundary of the orebody is 912.20–1106.85 m, and the average elevation is 1065.14 m. The burial depth of the top boundary of the orebody is 315.00–630.00 m, and the average burial depth

Fig. 4.17 Schematic diagram of the plane distribution of the Nalinggou uranium deposit (according to Jindai et al. 2015). 1. Exploration lines and numbers. 2. Industrial uranium holes. 3. Uranium mineralization holes. 4. Unusual uranium holes. 5. No uranium holes. 6. Industrial uranium orebodies

264

4 Geological Characteristics of Typical Deposits and Newly Discovered …

Fig. 4.18 Sectional view of exploration line N8 of the Nalinggou uranium deposit (modified based on Gui et al. 2017). 1. Lower Cretaceous. 2. Upper Zhiluo Formation. 3. Upper submember of the upper Zhiluo Formation. 4. Lower submember of the lower Zhiluo Formation. 5. Yan’an Formation. 6. Geological boundary. 7. Mudstone and conglomerate. 8. Mudstone intercalation and calcareous sandstone intercalation. 9. Green sandstone. 10. Gray sandstone. 11. Industrial uranium orebody and uranium mineralized body. 12. Drilling location, number, and depth

is 410.000 m. The influence of stratigraphic occurrence is relatively obvious, but, in general, the burial depth of the top boundary of the orebody gradually increases from east to west and from north to south. The ore grade in the Nalingou uranium mine bed sheet project is 0.0193%– 0.3014%, the average grade is 0.0641%, and the coefficient of variation is 66%. The amount of uranium per square meter in a single project is 1000–48.81 kg/m2 , the average is 4.23 kg/m2 , and the coefficient of variation is 113%. The orebody is relatively thin and the grade coefficient of variation of the Nalinggou uranium deposit is relatively small, but the uranium content per square meter has a high coefficient of variation, which indicates that the thickness and grade of the orebody are relatively stable, and the amount of uranium per square meter varies greatly. The sandbodies of the lower part of the Zhiluo Formation in the Nalinggou uranium deposit are macroscopically pan-connected, which is the result of multistage vertical stacking of channel sandbodies and lateral connections. The ore-bearing lithology is concentrated in the middle. The sandstone is primarily grained, mediumcoarse, and coarse-grained. On the plane, the sandbodies are distributed in a northwest–southeast direction, and the southwest side is nearly north–south. The thickness of the sandbodies varies little, mostly between 120 and 140 m. The sandbody of exploration line N16 on the northeast side is slightly thicker, being mostly 130–140 m. The thickness of the sandbody on the southwest side is generally 120–130 m. The sedimentary area with a thickness of 3.5 PA/kg have been found in the Zhiluo Formation. The uranium anomaly zone is located at the turning point of the secondary fold on the east wing of the Yuanyanghu anticline and distributed along the turning end. The anomaly in the eastern part is 6.25 km along the north–south direction, while that in the southwest part is 4.60 km along the NNE direction. The depth of the radioactive anomaly layer is 37–430 m, and the thickness is generally 0.30–1.72 m, with an average of 0.65 m. The radioactivity intensity is generally 7.20–163.48 PA/kg, with an average of 28.93 PA/kg and a maximum intensity of 191.12 PA/kg. Four coalfield boreholes with radioactive intensity of >3.5 PA/kg are found in the Yan’an Formation; these are also distributed in the east wing of the Yuanyanghu anticline. The depth of the radioactive anomaly layer is 57–165 m, and its thickness is 0.35–0.65 m, with an average of 0.48 m, which is thinner than that of the Zhiluo Formation. The radiation intensity is generally 6.42–23.37 PA/kg, with an average of 15.27 PA/kg, and the abnormal radiation intensity is significantly lower than that of the Zhiluo Formation.

4.2.1.2

Deposit characteristics

1. Orebody characteristics There are eight uranium orebodies delineated in the Yangchangwan mining area. Orebody No. I occurs in the sandbody of the upper part of the lower member of the Zhiluo Formation, orebody No. VIII occurs in the sandbody of the upper Yan’an Formation, and other orebodies occur in the sandbody of the lower part of the lower member of the Zhiluo Formation (Fig. 4.29). The overall orebody is characterized

4.2 Geological Characteristics of Uranium Ore Concentration Areas Along …

281

Fig. 4.29 Distribution of uranium orebodies in the Yangchangwan area

by a single large orebody with length varying in the range of 0.72–2.2 km and width in the range of 200–650 m. The ore (mineralized) body is in the form of layer (plate). Uranium mineralization is closely related to clay, organic matter, pyrite, and carbonate. Orebody No. I is located at the turning end of the Heigeda anticline and the Yuangeda syncline. It occurs in the sandbody of the upper part of the lower member of the Zhiluo Formation. The strike of the orebody is 320°–340° and tends to the northeast with a length of 795 m. The roof elevation of the orebody is 1081.16– 1083.56 m, its buried depth is 300.2–302.6 m, its thickness is 2.40 m, and the grade of the orebody is 0.0352%. Orebody No. II is located in the east wing of the Suishijing anticline. The orebody strikes northeast, with a length of ~2200 m and a width of ~200–650 m. The roof elevation of the orebody is 1097–1207 m, its buried depth is 154–285 m, its thickness is 2.10 m, and the grade of the orebody is 0.1686%. Orebody No. III is ~1600 m long and ~200 m wide. The roof elevation of the orebody is 972.64 m, its buried depth is 424.61 m, its thickness is 2.45 m, and the grade of the orebody is 0.0522%. Orebody No. IV strikes 320°–340° and trends northeast with a length of 798 m. The roof elevation of the orebody is 966.2–978.18 m, and its buried depth is 401.78– 413.76 m. The thickness of the orebody is 1.57 m and its ore grade is 0.0326%.

282

4 Geological Characteristics of Typical Deposits and Newly Discovered …

Orebody No. V is located in the west wing of the Heigeda anticline, near the turning point of the Heigeda anticline and the Shangeda syncline. The orebody strikes NNW, with a length of ~1562 m and a width of ~288 m. The roof elevation of the orebody is 940.81–972.64 m, and its buried depth is 410.8–455.23 m. The thickness of the orebody is 2.7 m and the grade of the orebody is 0.0556%. The length of orebody No. VI is ~1600 m. The orebody has an elevation of 942.09 m, a buried depth of 452.5 m, a thickness of 4.2 m, and a grade of 0.1164%. Orebody No. VII is ~725 m long and ~ 200 m wide. It strikes NNW and tends to the northeast. The roof elevation of the orebody is 997.02 m, its buried depth is 409.61 m, its thickness is 7.62 m, and the grade of the orebody is 0.0100%. Orebody No. VIII occurs in the upper sandbody of the Yan’an Formation. The orebody is ~1580 m long and ~200 m wide. The orebody strikes NNW and dips to the southwest. The elevation of the roof of the orebody is 936.19 m. The orebody has a buried depth of 458.4 m, a thickness of 2.10 m, and a grade of 0.1361%. 2. Ore quality The ore texture is mainly granular and layered with a massive structure, especially characterized by a thick layered structure. The sandstone detritus comprises primarily quartz, feldspar, and lithic sandstone serving as the main sandstone clasts, followed by muscovite, biotite, and chlorite, a small amount of carbon and heavy minerals, and a small amount of authigenic minerals (pyrite and clay minerals). The content of metallic minerals is 1%–5%, including irregular granular pyrite, goethite, ilmenite, and limonite. The sandstone texture of the lower Zhiluo Formation in the mining area is loose to relatively loose, placing it as an aluminosilicate type. Aluminosilicate accounts for ~85% and varies greatly, while other elements exhibit little variation. The organic carbon in the ore ranges from 0.09 to 5.60%, with a great variation, mostly being >0.4%, with an average of 0.81%. It reflects the positive correlation between uranium and organic carbon. The sulfur component is 0.018–1.89%, which varies greatly, mostly being >0.4%, with an average of 0.39%, reflecting the positive correlation between uranium and sulfur. There is abundant organic matter such as carbon chips and deep oil and gas in the Ningdong area. The principal alteration phenomena of sandstone include carbonation, pyritization, chloritization, kaolinization, hematization, and limonitization. Analysis of associated elements V, Ni, Mo, Cu, and Se in the lower member of the Zhiluo Formation reveals that most of the contents of V, Ni, Mo, and Cu elements are higher than their crustal abundance values, but no obvious enrichment is found. Individual samples of Se reach industrial grade, but the correlation is not obvious. Uranium mainly occurs in the form of adsorption in sandstone cements, with a small amount in the form of uranium minerals. Uranium gets adsorbed by Fe–Ti oxides and pyrite and by small amounts of mica and clay. The primary uranium mineral is pitchblende with a small amount of uraninite in the form of kidney-like and grape-like aggregates (Fig. 4.30). The U–Pb isotopic age of pitchblende is 11.83 ± 0.53 Ma, which indicates that the metallogenic age is Miocene.

4.2 Geological Characteristics of Uranium Ore Concentration Areas Along …

283

Fig. 4.30 Distribution of uranium minerals in the ore. In the image on the left, the bright to dark minerals are uranium minerals, pyrite, apatite, potassium feldspar, albite, quartz, and clay minerals. In the image on the right, the bright to dark minerals are uranium minerals, pyrite, potassium feldspar, albite, quartz, and clay minerals

4.2.2 Shicaocun Mining Area 4.2.2.1

Geological Setting of Metallogeny

1. Structural characteristics The Shicaocun mining area is located in the east wing of the Yuanyanghu anticline. The Yuanyanghu anticline is an important fold structure, belonging to a regional fold in terms of the large scale. The fault structure, which is roughly parallel to the anticline axis, develops on both wings. The main fold in the mining area is the Yuanyanghu anticline; secondary folds include the Lijiajuan syncline and Lijiajuan anticline. The Yuanyanghu anticline strikes nearly north–south and inclines to the south along the strike direction. The two wings are asymmetric. The dip angle of the west wing is 30°–38°, which is slightly greater than that of the east wing. The dip angle of the stratum at the east wing varies, generally between 10° and 30°. The east wing of the anticline, especially the southeastern part, is affected by the Zhangjiamiao and Lijiaquan anticlines and the fault structure, and locally it forms a NNW wavelike undulation. The Lijiajuan syncline is located in the southeastern part of the east wing of the Yuanyanghu anticline; its axial direction is ~25° northwest and it dips northward, extending for 3400 m. Because of the influence of the Lijiajuan reverse fault, the two wings of the syncline are asymmetric, with an inclination of 13°–18° in the west and 13°–25° in the east. The maximum amplitude of the fold is 130 m. The Lijiajuan anticline is ~450 m away from the east of the Lijiajuan syncline axis. Their structural axes are roughly parallel. The strike of the anticline axis is ~23° northwest and it inclines northward. The extension length is ~3040 m, and the two wings are asymmetric because of the effect of the Lijiajuan Fault. The dip angle of

284

4 Geological Characteristics of Typical Deposits and Newly Discovered …

the west wing is ~13°–25°, while the dip angle of the east wing is ~10°–23°, with the maximum amplitude of 100 m. Two groups of faults are well developed: the northwest-trending reverse fault and the dominant northeast-trending normal fault. The normal faults in the mining area tend to be northeast striking with a southeast inclination. Most of the reverse faults strike northwest, with both groups inclining toward the east and west. 2. Strata of the mining area The strata in the area include the Upper Triassic Shangtian Formation (T3 S), Middle Jurassic Yan’an Formation (J2 y) and Zhiluo Formation (J2 z), Upper Jurassic Anding Formation (J3 a), Oligocene Qingshuiying Formation (Eq), and Quaternary (Q) strata. 3. Characteristics of ore-bearing sandbodies Uranium mineralization occurs mainly in the braided river sedimentary coarse sandstone at the lower part of the lower member of the Middle Jurassic Zhiluo Formation and the meandering river medium and fine sandstone at the bottom of the upper member of the Middle Jurassic Zhiluo Formation. The ore-bearing sandbody at the bottom of the upper member of the Lower Zhiluo Formation is mainly purplish red and grayish white, medium-coarse and mediumfine sandstone, with loose cementation, good water permeability, and poor continuity. The thickness of the sandbody is 15.60 m, with a complete mudstone and siltstone water-resisting roof and floor. The ore-bearing sandbody at the bottom of the lower member of the Middle Jurassic Zhiluo Formation is purplish red, grayish white, grayish green, and gray coarse sandstone and coarse sandstone with fine gravel, with loose cementation and good water permeability. The thickness is 3.00–130.34 m. The buried depth gradually becomes shallower from north to south, and the thickness from the anticline wing to the axis becomes thinner. The southern part is larger than the northern part, and the eastern part is larger than the western part. The thickness of the southwestern part is >60 m, while that of the center is >100 m (Figs. 4.31 and 4.32). The thickness of sandbodies gradually thins toward the north and west. The main channel was gradually replaced by a branch channel. The varying thickness of the sandbody reflects the variation of groundwater dynamic conditions. The moderate sandbody thickness and the good mud–sand–mud stratum structure both favor the formation of sandstone-type uranium deposits. 4. Characteristics of the roof and floor of the ore-bearing aquifer The ore-bearing aquifer is layered. Its roof and floor strata have poor water permeability, with a thickness of 10–50 m, and is an aquifuge. The roof consists of brown, grayish green, and gray silty mudstone sandwiched with a thin layer of siltstone, composed primarily of argillaceous material and a small amount of silt, with argillaceous texture, massive structure, and poor water permeability. Along the strike and dip direction, the variation is small, and the continuity and stability of the strata are good. A gradual contact relationship between the strata and the aquifer is observed.

4.2 Geological Characteristics of Uranium Ore Concentration Areas Along …

285

Fig. 4.31 Contour map of sandbody thickness in the lower section of the Zhiluo Formation in the Shicao Village area

Fig. 4.32 Three-dimensional diagram of braided river sandbody thickness at the bottom of the lower submember of the lower member of the Zhiluo Formation (in units of meters)

286

4 Geological Characteristics of Typical Deposits and Newly Discovered …

The floor consists of gray and dark gray mudstone and siltstone, with carbon chips, and exhibits argillaceous or silty texture. The rocks are extremely hard and stable with undeveloped joints. There is good continuity along the strike and dip direction of the stratum and stability. However, the varying thickness varies along the strike and dip direction indicates a gradual contact relationship between the floor and the aquifer. The three-dimensional map of the sandbody roof buried depth and floor elevation of the lower member of the Zhiluo Formation (Fig. 4.33) reveals that the sandbody floor in the western and southern Shicaochun mining area is higher than that in the northeast, with an elevation difference of >700 m. The stratum is gently inclined from the west and south to the northeast. The groundwater containing uranium and oxygen is supplied by runoff along the sand layer from the west (the axis of the Yuanyanghu anticline) to the northeast. Because of the change of hydrodynamic conditions, uranium is also precipitated and enriched in the part where the sandbody is gradually buried. 5. Characteristics of the radioactive anomaly A total of 106 coalfield boreholes were checked. The radioactive abnormal horizons were located primarily in the lower part of the lower member of the Zhiluo Formation and the Yan’an Formation. In total, 21 potential uranium holes and 6 potential mineralization holes were found in the lower part of the lower member of the Zhiluo Formation. The lithology is mainly coarse sandstone. The γ intensity of radioactive abnormal holes is 4.70–72.70 PA/kg, the thickness is 0.30––24.62 m, and the buried depth is 75.12–558.78 m. There are seven potential uranium ore holes and two potential mineralized holes in terms of the Yan’an Formation. The lithology is medium-coarse sandstone. The γ intensity of radioactive abnormal holes is 5.02– 16.13 PA/kg, the thickness is 0.43–11.94 m, and the buried depth is 100.02–603.50 m. The anomaly is generally distributed in the north–south direction and is obviously controlled by the north–south anticline structure.

Fig. 4.33 Three-dimensional map of the bottom plate elevation of the braided river sandbody at the bottom of the lower submember of the lower Zhiluo Formation (in units of meters)

4.2 Geological Characteristics of Uranium Ore Concentration Areas Along …

4.2.2.2

287

Deposit Characteristics

1. Characteristics of orebodies There are five delineated uranium orebodies in the Shicaochun mining area, and the ore (mineralized) body is in the form of a layer (plate) (Figs. 4.34 and 4.35). Among them, the four orebodies SI, SII, SIII, and SIV are located in the lower part of the lower member of the Zhiluo Formation. Orebody SV is located in the Yan’an Formation. All the orebodies exhibit a large scale, with lengths of 0.8–3.75 km and widths of 200–540 m. Orebody SI is ~3.75 km in length and 200–540 m in width. The roof elevation of the orebody is 858.53–1027.18 m. Its buried depth is 355.90–514.60 m. The average thickness of the orebody is 4.23 m, and its ore grade is 0.0297%. Orebody SII strikes north–south, with a length of ~800 m and a width of ~200 m. The roof elevation of the orebody is 1108.17 m. Its buried depth is 242.61 m. The thickness of the orebody is 1.12 m, and its grade is 0.0465%. Orebody SIII strikes north–south and partially transfers from NNE to NNW, with an eastward inclination. The length and width of the orebody are ~1620 and ~200– 400 m, respectively. The roof elevation of the orebody is 969.45–1050.43 m. Its buried depth is 304.30–417.02 m. The thickness of the orebody is 5.33 m, and its grade is 0.0191%. Orebody SIV strikes nearly north–south and inclines to the east. Its length and width are 800 and ~175–200 m, respectively. The roof elevation of the orebody is 958.14 m. Its buried depth is 406.0 m. The thickness of the orebody is 3.25 m, and its grade is 0.0338%. Orebody SV strikes northeast with a length of 1210 m and a width of 200 m. The roof elevation of the orebody is 1103.14 m. Its buried depth is 262.23 m. The thickness of the orebody is 11.94 m, and its grade is 0.0150%.

Fig. 4.34 Profile of the orebody in the Shicao Village mining area (north–south borehole connected well profile)

288

4 Geological Characteristics of Typical Deposits and Newly Discovered …

Fig. 4.35 Distribution of uranium orebodies in the Zhiluo Formation in the Shicao Village area

2. Ore quality The ore generally exhibits granular texture, with the clastic particles being closely arranged and obvious extrusion deformation of the plastic shale debris and mica. Some of the shale debris and mica even gets squeezed into the intergranular pores to form a pseudo matrix. The cementation types include calcite cementation and kaolinite cementation. The ore displays both bedded and massive structure. The thick-bedded structure serves as the dominant structure. The primary sandstone clasts of the ore include quartz, feldspar, and lithic sandstone, followed by muscovite, biotite, and chlorite, a small amount of carbon and heavy minerals, and a small amount of authigenic minerals (pyrite and clay minerals). The content of metallic minerals (including pyrite, goethite, ilmenite, and limonite) is 1–5%; they exhibit irregular granular texture. Based on chemical composition, the ore belongs to the aluminosilicate type, with aluminosilicate accounting for ~85%. The aluminosilicate content varies greatly, while other elements exhibit little variation. The organic carbon in the ore ranges from 0.19 to 1.70%, mostly being >0.4%, with an average of 0.71%, reflecting a positive correlation between uranium and organic carbon. The sulfur content ranges

4.2 Geological Characteristics of Uranium Ore Concentration Areas Along …

289

from 0.06 to 2.21%, mostly being >0.4%, with an average of 0.30%, also reflecting a positive correlation between uranium and sulfur. Regarding mineralization alteration characteristics, spotted ferritization can be observed in the sandbody of the oxidation zone, which is widely developed in clastic materials and cements in rocks, along pores, in fissuresm and even along bedding planes. In the intercalated calcareous sandstone layer, there are many rose red patches and bands, with some rose red calcified mud and gravel. Feldspar and rock debris exhibit varied degrees of clayization, with strong limonitization and hematite. The rocks are reddish brown and yellowish brown. The grayish white sandbody is well developed in reducing media such as carbon and pyrite. Uranium mineralization occurs mainly on the side of the grayish white sandbody. Electron microprobe analysis shows that the uranium minerals in the ores of the Lower Zhiluo Formation are primarily pitchblende and uranite, which are kidneylike and grape-like aggregates. Uranium is mainly adsorbed by Fe–Ti oxides and pyrite and by small amounts of mica and clay.

4.2.3 Ciyaobao Uranium Deposit 4.2.3.1

Geological Background of Metallogeny

The Ciyaobao uranium deposit is located in the structural unit of the Majiatan submember in the north of the southern segment of the western margin thrust belt, with relatively weak structural deformation. The Middle Jurassic Zhiluo Formation and Yan’an Formation are both well preserved. In the northern Ciyaobao area, asymmetric folds (steep in the west and gentle in the east) are developed, while the faults are not well developed. The gentle slope east of the folds is favorable for uranium enrichment. The ore-bearing target layer is mainly the Middle Jurassic Lower Zhiluo Formation, which is a set of braided river sedimentary systems. The lithology is mainly grayish white and light gray medium-coarse sandstone, with a small amount of grayish black mudstone lens in some parts. The sandbody is stable, with a thickness of 60–100 m. The profile generally has one to three semirhythms with the upper part being coarse grained and lower part being fine grained, forming multiple mud–sand– mud lithological associations. In each association, the thickness of a single sandbody is 20–40 m. The secondary target layer is the upper Yan’an Formation, which is a set of braided river sedimentary systems. It is composed of grayish white feldspar quartz sandstone, gray and grayish black siltstone, mudstone, and coal seam, with abundant organic matter, carbon chips, pyrite, and other reducing media. The Middle Jurassic Lower Zhiluo Formation and the Upper Yan’an Formation are the main ore-bearing horizons in this area. The Lower Zhiluo Formation is composed primarily of braided river channel subfacies and floodplain subfacies. On the plane, it has a nearly east–west distribution, with the length of the controlled channel in the range of 3.5–25 km. On the profile, it is a composite channel sandbody superimposed

290

4 Geological Characteristics of Typical Deposits and Newly Discovered …

by a multistage braided channel, which is characterized by one to three channel sandbodies. The Upper Yan’an Formation is principally a fluvial sedimentary system, with well-developed channel sandbodies and rich reducing media. In summary, the litho-facies lithologic conditions are favorable for uranium reduction and enrichment.

4.2.3.2

Deposit Characteristics

1. Orebody characteristics The Ciyaobao uranium orebody is irregular in the plane, being generally related to the heterogeneity in the sandbody. The uranium orebodies are distributed on both sides of the Yuanyanghu–Fengjigou anticline. Much of the uranium mineralization body along the western margin of the Ordos Basin is layered. Uranium mineralization mainly occurs in members 1–4 of the Yan’an Formation and the Lower Zhiluo Formation. The ore-bearing host rocks are gray and grayish white coarse sandstone and light yellow medium-fine sandstone. Spatially, uranium mineralization is distributed primarily in the Sujiajing–Dashuikeng area, and it is more developed in the wing of the fold–thrust anticline (Qingyin et al. 2010). The ore-bearing sandbody exhibit certain changes in characteristics in going from north to south, with the trend of increasing thickness and decreasing number of layers. The lower Zhiluo Formation exhibits one to four layers of sandbody in the Fengjigou area in the north, with a total thickness of 64–146 m, whereas the area of Jinjiaqu in the south displays only one or two layers of sandbody, with a total thickness of 116–174 m. The thickness of the Yan’an Formation is 10–30 m (Fig. 4.36). The uranium mineralization thickness of the Zhiluo Formation ranges from 0.20 to 22.30 m, with grade varying from 0.0105 to 0.1328% and uranium content in the range of 0.07–7.41 kg/m2 . The uranium grade of the Yan’an Formation is 0.0118– 0.0596%, with thickness in the range of 0.1–3.80 m, and the uranium content is in the range of 0.28–4.98 kg/m2 . 2. Ore quality The mineral composition of the ore can be described, respectively, according to the ore-bearing horizon of the Zhiluo and Yan’an formations (Qingyin 2010). The ore-bearing host rocks of the Zhiluo Formation are gray and dark gray sandstone. The rocks are poorly sorted and rounded, mainly composed of gravelly coarse sandstone and medium-coarse sandstone, with a small amount of medium-fine sandstone. Based on the composition, the sandstone can be classified as lithic arkose, with quartz content of 20–50%, feldspar content of 10–45%, lithic material content of 9–49%, and low contents of biotite, muscovite, and chlorite. The rock debris includes mainly altered debris, volcanic felsic rock debris, and metamorphic quartz rock debris. The clastic grains are generally subangular but a few are angular. The main cementation type is contact pore cementation, followed by basement cementation, with porosity of 1–10%. The cement consists primarily of clay minerals and

4.2 Geological Characteristics of Uranium Ore Concentration Areas Along …

291

Fig. 4.36 Uranium orebody exploration line No. 57 in the Ciyaopu area in the lower section of the Zhiluo Formation (according to the 208 nuclear industry team). 1. Middle Jurassic Yan’an Formation. 2. Middle Jurassic Zhiluo Formation. 3. Sandstone. 4. Siltstone. 5. Mudstone. 6. Stratum and lithology boundary. 7. Stratum parallel unconformity contact boundary. 8. Uranium orebody

a small amount of carbonate (mainly calcite and a small amount of dolomite) and pyrite. The ore-bearing host rocks of the Yan’an Formation are grayish white and gray lithic arkose. The contents of quartz and K-feldspar + plagioclase are in the ranges of 15%–55% and 10%–45% respectively, with subordinated 15%–35% debris and a small amount of mica (