Geology and genesis of major copper deposits and districts of the world : a tribute to Richard H. Sillitoe 9781934969465, 193496946X

Citation preview

Special Publication Number 16

Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe

Jeffrey W. Hedenquist, Michael Harris, and Francisco Camus, Editors

Special Publications of the Society of Economic Geologists

Special Publication Number 16 Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe Jeffrey W. Hedenquist, Michael Harris, and Francisco Camus, Editors

First Edition 2012 Printed by Cenveo Publisher Services 3575 Hempland Road Lancaster, PA 17601

Additional copies of this publication can be obtained from Society of Economic Geologists, Inc. 7811 Shaffer Parkway Littleton, CO 80127 www.segweb.org

ISBN 978-1-934969-46-5 ISSN 1938-4548

On the DVD label: Aerial view of the Bingham Canyon copper mine, Kennecott Utah Copper, September 2003. Photo courtesy of Rio Tinto plc.

SPONSOR The Society of Economic Geologists Publications Board thanks Rio Tinto Exploration for its generous financial support of this volume

In recognition of Richard H. Sillitoe’s contribution to the understanding of the world’s major copper deposits

SOCIETY OF ECONOMIC GEOLOGISTS, INC.

Special Publication Number 16

Contents Appendices for Borg et al. and Schuh et al. and archived SEG papers, 1971 to 2013, by Richard H. Sillitoe are included on this DVD. Papers by R. H. Sillitoe are listed following the Table of Contents

Foreword

Eric Finlayson

ix

Jeffrey W. Hedenquist, Michael Harris, Francisco Camus

xiii

Richard H. Sillitoe

1

Sergio L. Rivera, Hugo Alcota, John Proffett, Jaime Díaz, Gabriel Leiva, and Manuel Vergara

19

Miguel Hervé, Richard H. Sillitoe, Chilong Wong, Patricio Fernández, Francisco Crignola, Marco Ipinza, and Felipe Urzúa

55

Geologic Setting and Evolution of the Porphyry Copper-Molybdenum and CopperGold Deposits at Los Pelambres, Central Chile

José Perelló, Richard H. Sillitoe, Constantino Mpodozis, Humberto Brockway, and Héctor Posso

79

Protracted Magmatic-Hydrothermal History of the Río Blanco-Los Bronces District, Central Chile: Development of World’s Greatest Known Concentration of Copper

Juan Carlos Toro, Javier Ortúzar, Jorge Zamorano, Paticio Cuadra, Juan Hermosilla, and Cristian Spröhnle

105

John P. Porter, Kim Schroeder, and Gerry Austin

127

Carl Hehnke, Geoff Ballantyne, Hamish Martin, William Hart, Adam Schwarz, and Holly Stein

147

James R. Lang and Melissa J. Gregory

167

David Crane and Imants Kavalieris

187

Acknowledgments Introduction 1

Copper Provinces

Major Deposits 2

3

4

5

6 7

8

9

Update of the Geologic Setting and Porphyry Cu-Mo Deposits of the Chuquicamata District, Northern Chile Geologic Overview of the Escondida Porphyry Copper District, Northern Chile

Geology of the Bingham Canyon Porphyry Cu-Mo-Au Deposit, Utah Geology and Exploration Progress at the Resolution Porphyry Cu-Mo Deposit, Arizona Magmatic-Hydrothermal-Structural Evolution of the Giant Pebble Porphyry Cu-Au-Mo Deposit with Implications for Exploration in Southwest Alaska Geologic Overview of the Oyu Tolgoi Porphyry Cu-Au-Mo Deposits, Mongolia

v

10 11 12

13

Copper-Gold ± Molybdenum Deposits of the Ertsberg-Grasberg District, Papua, Indonesia

Clyde A. Leys, Mark Cloos, Brian T.E. New, and George D. MacDonald

215

Kathy Ehrig, Jocelyn McPhie, and Vadim Kamenetsky

237

Wolfram Schuh, Richard A. Leveille, Isabel Fay, and Robert North

269

Stephen E. Box, Boris Syusyura, Reimar Seltmann, Robert A. Creaser, Alla Dolgopolova, and Michael L. Zientek

303

Constantino Mpodozis and Paula Cornejo

329

Richard A. Leveille and Ralph J. Stegen

361

Alexander Yakubchuk, Kirill Degtyarev, Valery Maslennikov, Andrew Wurst, Alexander Stekhin, and Konstantin Lobanov

403

The Iron Oxide Copper-Gold Roberto Perez Xavier, Lena Virgínia Soares Monteiro, Systems of the Carajás Carolina Penteado N. Moreto, André Luiz Silva Pestilho, Mineral Province, Brazil Gustavo Henrique Coelho de Melo, Marco Antônio Delinardo da Silva, Benevides Aires, Cleive Ribeiro, and Flávio Henrique Freitas e Silva

433

Gregor Borg, Adam Piestrzynski, ´ Gerhard H. Bachmann, Wilhelm Püttmann, Sabine Walther, and Marco Fiedler

455

Murray W. Hitzman, David Broughton, David Selley, Jon Woodhead, David Wood, and Stuart Bull

487

D. R. Burrows and C. M. Lesher

515

Andreas Audétat and Adam C. Simon

553

Kalin Kouzmanov and Gleb S. Pokrovski

573

Geology and Mineralogical Zonation of the Olympic Dam Iron Oxide Cu-U-Au-Ag Deposit, South Australia Geology of the Tenke-Fungurume SedimentHosted Strata-Bound Copper-Cobalt District, Katanga, Democratic Republic of Congo Dzhezkazgan and Associated Sandstone Copper Deposits of the Chu-Sarysu Basin, Central Kazakhstan

Premier Provinces 14 15 16

17

18

19

Cenozoic Tectonics and Porphyry Copper Systems of the Chilean Andes The Southwestern North America Porphyry Copper Province Tectonomagmatic Settings, Architecture, and Metallogeny of the Central Asian Copper Province

An Overview of the European Kupferschiefer Deposits The Central African Copperbelt: Diverse Stratigraphic, Structural, and Temporal Settings in the World’s Largest Sedimentary Copper District

Genetic Themes 20

Copper-Rich Magmatic Ni-Cu-PGE Deposits

21

Magmatic Controls on Porphyry Copper Genesis

22

Hydrothermal Controls on Metal Distribution in Porphyry Cu (-Mo-Au) Systems

vi

Richard H. Sillitoe Papers (1971–2013) Included on the CD-ROM Sillitoe, R.H., and Sawkins, F.J., 1971, Geologic, mineralogic, and fluid inclusion studies relating to the origin of copper-bearing tourmaline breccia pipes, Chile: Economic Geology, v. 65, p. 1028–1041. Sillitoe, R.H., 1972, A plate tectonic model for the origin of porphyry copper deposits: Economic Geology, v. 67, p. 184–197. Sillitoe, R.H., 1973, Geology of the Los Pelambres porphyry copper deposit, Chile: Economic Geology, v. 68, p. 1–10. Sillitoe, R.H., 1973, The tops and bottoms of porphyry copper deposits: Economic Geology, v. 68, p. 799–815. Sillitoe, R.H., 1973, Environments of formation of volcanogenic massive sulfide deposits: Economic Geology, v. 68, p. 1321–1325. Sillitoe, R.H., Halls, C., and Grant, J.N., 1975, Porphyry tin deposits in Bolivia: Economic Geology, v. 70, p. 913–927. Sillitoe, R.H., 1975, Lead-silver, manganese, and native sulfur mineralization within a stratovolcano, El Queva, northwest Argentina: Economic Geology, v. 70, p. 1190–1201. Sillitoe, R.H., 1975, Subduction and porphyry copper deposits in southwestern North America – a reply to recent objections: Economic Geology, v. 70, p. 1474–1477. Sillitoe, R.H., 1977, Permo-Carboniferous, late Cretaceous and Miocene porphyry copper-type mineralization in the Argentinian Andes: Economic Geology, v. 72, p. 99–103. Sillitoe, R.H., Jaramillo, L., Damon, P.E., Shafiqullah, M., and Escovar, R., 1982, Setting, characterisitics, and age of the Andean porphyry copper belt in Colombia: Economic Geology, v. 77, p. 1837–1850. Silliitoe, R.H., 1983, Enargite-bearing massive sulfide deposits high in porphyry copper systems: Economic Geology, v. 78, p. 348–352. Sillitoe, R.H., Jaramillo, L., and Castro, H., 1984, Geologic exploration of a molybdenum-rich porphyry copper deposit at Mocoa, Colombia: Economic Geology, v. 79, p. 106–123. Sillitoe, R.H., Baker, E.M., and Brook, W.A., 1984, Gold deposits and hydrothermal eruption breccias associated with a maar volcano at Wau, Papua New Guinea: Economic Geology, v. 79, p. 638–655. Sillitoe, R.H., and Bonham, H.F., Jr., 1984, Volcanic landforms and ore deposits: Economic Geology, v. 79, p. 1286–1298. Sillitoe, R.H., 1985, Ore-related breccias in volcanoplutonic arcs: Economic Geology, v. 80, p. 1467–1514. Sillitoe, R.H., Grauberger, G.L., and Elliott, J.E., 1985, A diatreme-hosted gold deposit at Montana Tunnels, Montana: Economic Geology, v. 80, p. 1707–1721. Cunnean, R., and Sillitoe, R.H., 1989, Paleozoic hot spring sinter in the Drummond Basin, Queensland, Australia: Economic Geology, v. 84, p. 135–142.

Sillitoe, R.H., 1989, Gold deposits in western Pacific island arcs: The magmatic connection, in Keays, R.R., Ramsay, W.R.H., and Groves, D.I., eds., The geology of gold deposits: The perspective in 1988: Economic Geology Monograph 6, p. 274–291. Arnold, G.O., and Sillitoe, R.H., 1989, Mount Morgan goldcopper deposit, Queensland, Australia: Evidence for an intrusion-related replacement origin: Economic Geology, v. 84, p. 1805–1816. Sillitoe, R.H., 1991, Gold metallogeny of Chile – an introduction: Economic Geology, v. 86, p. 1187–1205. Vila, T., and Sillitoe, R.H., 1991, Gold-rich porphyry systems in the Maricunga belt, northern Chile: Economic Geology, v. 86, p. 1238–1260. Sillitoe, R.H., McKee, E.H., and Vila, T., 1991, Reconnaissance K-Ar geochronology of the Maricunga gold-silver belt, northern Chile: Economic Geology, v. 86, p. 1261–1270. Vila, T., Sillitoe, R.H., Betzhold, J., and Viteri, E., 1991, The porphyry gold deposit at Marte, northern Chile: Economic Geology, v. 86, p. 1271–1286. Sillitoe, R.H., 1992, Gold and copper metallogeny of the central Andes – Past, present, and future exploration objectives (SEG Distinguished Lecture): Economic Geology, v. 87, p. 2205–2216. Sillitoe, R.H., 1993, Giant and bonanza gold deposits in the epithermal environment: Assessment of potential genetic factors, in Whiting, B.H., Mason, R., and Hodgson, C.J., eds., Giant ore deposits: Society of Economic Geologists Special Publication 2, p. 125–156. Sillitoe, R.H., and Lorson, R.C., 1994, Epithermal gold-silvermercury deposits at Paradise Peak, Nevada: Ore controls, porphyry gold association, detachment faulting, and supergene oxidation: Economic Geology, v. 89, p. 1226–1246. Sillitoe, R.H., and McKee, E.H., 1996, Age of supergene oxidation and enrichment in the Chilean porphyry copper province: Economic Geology, v. 91, p. 164–179. Sillitoe, R.H., Hannington, M.D., and Thompson, J.F.H., 1996, High-sulfidation deposits in the volcanogenic massive sulfide environment: Economic Geology, v. 91, 204–212. Sillitoe, R.H., Marquardt, J.C., Ramírez, F., Becerra, H., and Gómez, M., 1996, Geology of the concealed MM porphyry copper deposit, Chuquicamata district, northern Chile, in Camus, F., Sillitoe, R.H., and Petersen, R., eds., Andean copper deposits: New discoveries, mineralization styles and metallogeny: Society of Economic Geologists, Special Publication 5, p. 59–69. Hannington, M.D., Poulsen, K.H., Thompson, J.F.H., and Sillitoe, R.H., 1999, Volcanogenic gold in the massive sulfide environment, in Barrie, C.T., and Hannington, M.D., eds., Volcanic-associated massive sulfide deposits: Processes and examples in modern and ancient settings: Reviews in Economic Geology, v. 8, p. 325–356. vii

McInnes, B.I.A., Farley, K.A., Sillitoe, R.H., and Kohn, B.P., 1999, Application of apatite (U-Th)/He thermochronometry to the determination of the sense and amount of vertical fault displacement at the Chuquicamata porphyry copper deposit, Chile: Economic Geology, v. 94, p. 937–947. Sillitoe, R.H., 2000, Geologic analogy: A vital field component of mineral exploration: Society of Economic Geologists Newsletter 42, p. 6–9. Gendall, I.R., Quevedo, L.A., Sillitoe, R.H., Spencer, R.M., Puente, C.O., León, J.P., and Povedo, R.R., 2000, Discovery of a Jurassic porphyry copper belt, Pangui area, southern Ecuador: Society of Economic Geologists Newsletter 43, p. 1, 8–15. Sillitoe, R.H., 2000, Gold-rich porphyry deposits: Descriptive and genetic models and their role in exploration and discovery, in Hagemann, S. G., and Brown, P. E., eds., Gold in 2000: Reviews in Economic Geology, v. 13, p. 315–345. Sillitoe, R.H., 2002, Rifting, bimodal volcanism, and bonanza gold veins: Society of Economic Geologists Newsletter 48, p. 24–26. Sillitoe, R.H., Cooper, C., Sale, M.J., Soechting, W., Echavarria, D., and Gallardo, J.L., 2002, Discovery and geology of the Esquel low-sulfidation epithermal gold deposit, Patagonia, Argentina, in Goldfarb, R.J., and Nielsen, R.L., eds, Integrated methods for discovery: Global exploration in the twenty-first century: Society of Economic Geologists Special Publication 9, p. 227–240. Sillitoe, R.H., and Burrows, D.R., 2002, New field evidence bearing on the origin of the El Laco magnetite deposit, northern Chile: Economic Geology, v. 97, p. 1101–1109. Sillitoe, R.H., and Hedenquist, J.W., 2003, Linkages between volcanotectonic settings, ore-fluid compositions, and epithermal precious metal deposits, in Simmons, S.F., and Graham, I.J., eds., Volcanic, geothermal, and ore-forming fluids: Rulers and witnesses of processes within the Earth: Society of Economic Geologists Special Publication 10, p. 315–343. Sillitoe, R.H., 2004, Musings on future exploration targets and strategies in the Andes, in Sillitoe, R.H., Perelló, J., and Vidal, C.E., Andean metallogeny: Mineralization styles, new discoveries, and deposit updates: Society of Economic Geologists Special Publication 11, p. 1–14. Sillitoe, R.H., 2004, Distal-disseminated and Carlin-type gold deposits: Are they fundamentally different?: Society of Economic Geologists Newsletter 59, p. 28–30. Sillitoe, R.H., 2005, Supergene oxidized and enriched porphyry copper and related deposits: Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., and Richards, J.P., eds., Economic Geology 100th Anniversary Volume, p. 723–768. Sillitoe, R.H., and Perelló, J., 2005, Andean copper province: Tectonomagmatic settings, deposit types, metallogeny, exploration, and discovery: Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., and Richards, J.P., eds., Economic Geology 100th Anniversary Volume, p. 845–890.

Sillitoe, R.H., and Thompson, J.F.H., 2006, Changes in mineral exploration practice: Consequences for discovery, in Doggett, M.D., and Parry, J.R., eds., Wealth creation in the minerals industry: Integrating science, business, and education: Society of Economic Geologists Special Publication 12, p. 193–219. Sillitoe, R.H., Hall, D.J., Redwood, S.D., and Waddell, A.H., 2006, Pueblo Viejo high-sulfidation epithermal gold-silver deposit, Dominican Republic: A new model of formation beneath barren limestone cover: Economic Geology, v. 101, p. 1427–1435. Sillitoe, R.H., 2007, Hypogene reinterpretation of supergene silver enrichment at Chañarcillo, northern Chile: Economic Geology, v. 102, p. 777–781. Sillitoe, R.H., 2008, Major gold deposits and belts in the North and South American Cordillera: Distribution, tectonomagmatic settings, and metallogenic considerations: Economic Geology, v. 103, p. 663–687. Sillitoe, R.H., 2009, Supergene silver enrichment reassessed, in Titley, S.R., ed., Supergene environments, processes, and products: Society of Economic Geologists Special Publication 14, p. 15–32. Sillitoe, R.H., 2010, Porphyry copper systems: Economic Geology, v. 105, p. 3–41. Sillitoe, R.H., 2010, The challenge of finding new mineral resources: An introduction, in Goldfarb, R.J., Marsh, E.E., and Monecke, T., eds., The challenge of finding new mineral resources: Global metallogeny, innovative exploration, and new discoveries: Society of Economic Geologists Special Publication 15, p. 1–4. Irarrazaval, V., Sillitoe, R.H., Wilson, A.J., Toro, J.C., Robles, W., and Lyall, G.D., 2010, Discovery history of a giant, high-grade, hypogene porphyry copper-molybdenum deposit at Los Sulfatos, Los Bronces-Río Blanco district, central Chile, in Goldfarb, R.J., Marsh, E.E., and Monecke, T., eds., The challenge of finding new mineral resources: Global metallogeny, innovative exploration, and new discoveries: Society of Economic Geologists Special Publication 15, p. 253–269. Sillitoe, R.H., and Mortensen, J.K., 2010, Longevity of porphyry copper formation at Quellaveco, Peru: Economic Geology, v. 105, p. 1157–1162. Sillitoe, R.H., 2010, Grassroots exploration: Between a major rock and a junior hard place: Society of Economic Geologists Newsletter 83, p. 11–13. Sillitoe, R.H., Perelló, J., and García, A., 2010, Sulfide-bearing veinlets throughout the stratiform mineralization of the Central African Copperbelt: Temporal and genetic implications: Economic Geology, v. 105, p. 1361–1368. Sillitoe, R.H., Tolman, J., and Van Kerkvoort, G., 2013, Geology of the Caspiche porphyry gold-copper deposit, Maricunga belt, northern Chile: Economic Geology, v. 108 (preprint).

viii

Foreword

A Tribute to Richard H. Sillitoe

ERIC FINLAYSON Head, Rio Tinto Exploration (2007–2011)

The decision by Rio Tinto Exploration to support a Society of Economic Geologists (SEG) volume dedicated to Dick Sillitoe is in recognition and appreciation of his contributions to the understanding of the world’s major copper districts on behalf of thousands of industry and academic geologists. It is fitting that this is done through the SEG, the Society that has been so closely associated with many of his best known publications. Dick has been widely honored throughout his career. His many awards in economic geology include the SEG Lindgren Award (1975), Wollaston Fund from the Geological Society (1977), SEG Thayer Lindsley Lecturer (1988), SEG Distinguished Lecturer (1992), Haddon Forrester King Medal from the Australian Academy of Science (1995), William Smith Medal from the Geological Society (1996), SEG Silver Medal (2002), Sir Julius Wernher Memorial Lecturer for the Institute of Materials, Minerals and Mining (2008), SEG International Exchange Lecturer (2012), and Herbert Thomas Prize from the Geological Society of Chile (2012). From a start with the UK Ministry of Overseas Development studying supergene enrichment of copper deposits in Chile, from 1965 to 1968, Dick then spent three years with the Instituto de Investigaciones Geológicas (Chilean Geological Survey) studying porphyry copper deposits before becoming an independent geologic consultant in 1971. Since 1968, Dick has published 120 technical publications in international journals, books, bulletins, and conference proceedings. His papers in Economic Geology and other publications of the SEG are reproduced on the CD-ROM included

with this volume. He has also written more than 800 unpublished technical reports while consulting for over 200 mining companies, seven governments, and four international agencies in 97 countries around the world. When not working in the field or writing up his observations, Dick has committed his time to education in economic geology. He has been an invited speaker at 95 international conferences to date, co-convened five major conferences, presented 40 public lectures at universities, geological surveys, and scientific meetings, given 16 short courses at universities, geological societies, and conferences, delivered 45 in-house courses for mining companies, run 17 field courses for mining companies and other organizations, acted as Associate Editor, Revista Geológica de Chile (1986−present), Member of the Editorial Board, Journal of Geochemical Exploration (1994−1999), and Honorary Editor, Resource Geology (2007−present); and was SEG President (1999−2000), the IAGOD Regional Councillor for Europe (1980−1984), and the UK correspondent for IGCP Project 342, “Age and Isotopes of South American Ores” (1993−1997). This prodigious body of work has made it virtually impossible for a geologist to work on a copper deposit today without benefiting from Dick’s field observations and genetic models. His ferocious work rate has led him to see more orebodies and prospects and to meet more economic geologists than possibly any other geologist to date. A highly commendable aspect of Dick’s work is the large number of people who have shared in his studies and publications. Dick is consistently generous in the recognition of ix

other people’s ideas and meticulous in referencing previous work. Another commendable aspect is his clarity of writing and oral presentation. He goes quickly to the issues of most importance to his audience and delivers conclusions and opinions that are valued. He is also never afraid to admit uncertainty when these circumstances arise. It seemed an almost impossible task to summarize Dick’s most significant accomplishments but an attempt has been made in the Table below. Reading through this list gives some appreciation of his contributions. However, to many of us who know Dick well, it will fall short in really describing the man. He is never condescending or self important, is always good

natured and humorous, and always treats people with the same courtesy and respect regardless of their age or position. Finally, while Dick’s name does not appear on the editorial list, I can confirm that his skill as an author and his commitment to this project have resulted in an outstanding publication. The eagerness of the most knowledgeable copper geologists to volunteer papers for the volume reflects enormous respect and admiration. It is a great pleasure for Rio Tinto Exploration to help with the production of this volume. It will stand as a cornerstone reference on copper deposits and as a fitting appreciation to Dick for his outstanding contributions.

x

R. H. Sillitoe Career Highlights 1970: First formal recognition of subduction control on porphyry Cu and related ore deposits.

1983: Remapped Wau low sulfidation epithermal Au-Ag deposit, Papua New Guinea (mining completed), leading to first formal recognition of maar-diatreme systems as controls of some epithermal ore deposits.

1970: Sited discovery hole at Los Pelambres porphyry Cu deposit, Chile (in production).

1984: Application of diatreme model to exploration of Montana Tunnels Au-Ag-Zn-Pb deposit, Montana, United States (mine now exhausted).

1971: Recognized control of ore by K-silicate alteration and used the relationship to plan the definition drilling program at Bajo de La Alumbrera porphyry Cu-Au deposit, Argentina (in production). Also drew attention for the first time to its high Au content.

1985: Initial sampling and recommendation of Choquelimpie high sulfidation epithermal Au-Ag deposit, Chile (mine now exhausted).

1971: First formal recognition of genetic relationship between porphyry Cu deposits and volcanoes.

1987 (and subsequently): Involvement in continuous extension of ore reserves at Lo Aguirre manto-type Cu and Sagasca exotic Cu deposits, Chile (Lo Aguirre exhausted, Sagasca in production).

1972: Predicted that volcanogenic massive sulfide (VMS) deposits would be found at oceanic spreading centers (confirmed in 1989).

1988−1990: Involvement in exploration and discovery of Au deposits in the Maricunga belt, Chile, including first recognition of porphyry Au deposits.

1973: Recognized potential and designed exploration program for VMS deposits in greenstone belts of Upper Volta (now Burkina Faso), which led to discovery of Perkoa Zn-Pb-Ag deposit (mine under construction).

1990: Recommendation of Cerro Vanguardia low sulfidation epithermal Au-Ag deposit, Argentina (in production).

1974: Conducted basic geologic work and supervised drilling at Saindak porphyry Cu-Au deposit, Pakistan (in production). Work led to first recognition of abundant hydrothermal magnetite as an indicator of Au-rich porphyry Cu deposits.

1990: Geologic reappraisal of Paradise Peak high sulfidation epithermal Au-Ag-Hg district, Nevada, United States (subsequently mined out). 1991: Successful prediction of enhanced Au contents at depth in Wafi porphyry Cu-Au deposit, Papua New Guinea (deeper drilling ongoing).

1976: First recognition of sediment-hosted stratiform ZnPb-Ag prospects and potential, Khuzdar district, Pakistan (in production).

1991: Preparation of first geologic model for MM (Ministro Hales) porphyry Cu deposit, Chuquicamata district, Chile (mine under construction).

1978: Recognition of concealed porphyry potential and discovery of Mocoa porphyry Cu-Mo deposit, Colombia. 1979: Discovery in South Korea of first Climax-type porphyry Mo deposit outside the United States on behalf of Amax.

1993: Prediction of existence of concealed Damiana exotic Cu deposit, El Salvador, Chile (mine now exhausted). 1994: Recognition of sediment-hosted Au mineralization, Sepon, Laos (3 Moz deposit in production).

1980: Discovery with J. Cabello of La Coipa high sulfidation epithermal Au-Ag deposit, Chile (in production).

1994: Recommendation of Zarsharan sediment-hosted Au prospect, Iran (3 Moz deposit defined).

1981: Recognition of bulk gold potential in Mount Leyshon breccia system, Queensland, Australia (mine now exhausted).

1994−2002: Participation in brownfields exploration of El Peñón low sulfidation epithermal Au-Ag district Chile, leading to discovery of several new bonanza-grade veins (in production).

1981: Recognition of concealed porphyry Cu-Au potential at Lepanto, Philippines, using diatreme clasts (Far Southeast deposit at definition drilling stage).

1995: First formal recognition of existence of high sulfidation VMS deposits.

1983: Recommended exploration for concealed manto-type Cu-Ag mineralization at Las Luces, Chile, leading to deposit discovery (mine now exhausted).

1996: Assistance with geologic modeling of Yanacocha high sulfidation epithermal Au deposits, Peru.

xi

1996: Recommended exploration of porphyry Cu prospects, Pangui district, southern Ecuador (four deposits defined to date, mine development planned).

2006: Assistance with geologic modeling of Resolution porphyry Cu-Mo deposit, Arizona, United States (at prefeasibility stage).

1997: Recommendation of the Opache porphyry Cu prospect, Chile (reserve defined).

2006: Recommended drill hole that discovered the Eureka West low sulfidation epithermal Au-Ag deposit, Cerro Negro district, Argentina (recently purchased by Goldcorp Inc. for US$3.2 billion; mine under construction).

1997 (through 2000): Geologic modeling of Gaby porphyry Cu deposit, Chile (in production). 2000: Predicted major subsurface extension of main Galadriel-Julia vein, Esquel low sulfidation epithermal Au deposit, Argentina, and recommended successful drill testing (mine construction halted by local community).

2007: Geologic modeling of breccia-hosted Cu-Au deposits in Gaoua district, Burkina Faso (infill drilling stage).

2000 (and 2001): Geologic modeling of the Boyongan porphyry Cu-Au deposit, Mindanao, Philippines (at feasibility stage).

2007: Assistance with geologic modeling of La Colosa porphyry Au deposit, Colombia (at prefeasibility stage).

2007: Recognition of porphyry Au system at Biely Vrch, Slovakia (prefeasibility stage).

2007: Assistance with geologic modeling of Navidad Ag-Pb deposit, Argentina (feasibility stage).

2002: Reinterpretation of origin of Pueblo Viejo high sulfidation epithermal Au-Ag deposit, Dominican Republic, beneath barren limestone cover.

2007−2011: Involvement in brownfields exploration of Escondida porphyry Cu district, northern Chile, which led to discovery of Pampa Escondida and Escondida Este deposits.

2004: Prediction of eastward blind extension of Pebble West porphyry Cu-Au deposit, Alaska, United States (higher grade Pebble East deposit subsequently discovered and now at prefeasibility stage).

2008−2010: Geologic modeling of Caspiche porphyry Au-Cu deposit, Chile (at feasibility stage).

2004−2005: Assistance with geologic modeling of Hugo Dummett porphyry Cu-Au deposit, Oyu Tolgoi, Mongolia (mine under construction).

2008−2010: Reinterpretation with J. Perelló of model for stratiform Cu-Co mineralization in Central African Copperbelt based on work in Zambia, DRC, Botswana, and Namibia.

2005: Recommended low sulfidation Au-Ag vein targets under cover and at depth, Cerro Bayo, Chile (two new veins discovered and exploited).

2009: Recommended search for low sulfidation epithermal Au deposits in Afar depression, Ethiopia and Djibouti, leading to discovery of several previously unknown auriferous veins (drilling underway).

2005−2008: Recommended exploration leading to discovery of the San Enrique-Monolito and Los Sulfatos porphyry Cu-Mo deposits, Los Bronces district, Chile (Anglo American team received 2011 Prospector of the Year Award from the Prospectors and Developers Association of Canada).

2010: Recommended drilling leading to discovery of Pompeya high sulfidation Au-Ag deposit, La Coipa district, Chile (definition drilling stage).

2006−2007: Assistance with geologic modeling of Fruta del Norte bonanza-grade intermediate sulfidation epithermal Au-Ag deposit, Ecuador (at feasibility stage).

2011: Recommended drilling leading to discovery of Ortaçam North high sulfidation Au deposit, Turkey (definition drilling stage).

xii

Acknowledgments We wish to acknowledge Rio Tinto Exploration (RTX) for proposing this tribute volume in honor of one of the world’s leading economic geologists, Richard (Dick) Sillitoe, and we thank RTX for generously financing the costs of the volume. We thank Mabel Peterson, Vivian Smallwood, and Alice Bouley for copyediting, layout, and print production, and we thank Stuart Simmons (Publications Board) and Brian Hoal (Executive Direc-

tor) who oversaw production of the volume on behalf of the SEG from its inception. The following reviewers helped maintain the quality expected of SEG publications, for which we are grateful. Finally, we thank the authors for their efforts in preparing the papers and for their patience in meeting the reviewers’ and editors’ comments. Without them, this tribute would not have been possible.

Reviewers of Papers in Special Publication 16 Bill Atkinson Geoff Ballantyne Sarah-Jane Barnes Derek Blundell Gregor Borg David Burrows Jacques Cailteux Phil Candela John Dilles Marco Einaudi David Groves Lew Gustafson Murray Hitzman Sue Kay Steve Kesler Ken Krahulec Duncan Large Ross Large Peter Laznicka Peter Lewis Lucas Marshall Scott Manske Larry Meinert

Philippe Muchez Jim Mungall Ruben Padilla Peter Pollard John Proffett Victor Ramos Patrick Redmond Jeremy Richards Steve Roberts Brian Rusk Eric Seedorff Richard Sillitoe George Steele John Thompson Spencer Titley Dick Tosdal David Vaughan James Webster Caroline Wilhem Alan Wilson Alexander Yakubchuk Marcos Zentilli Gustavo Zulliger Jeffrey W. Hedenquist, Ottawa, Michael Harris, London, and Francisco Camus, Santiago, Editors

xiii

© 2012 Society of Economic Geologists, Inc. Special Publication 16, pp. 1–18

Chapter 1 Copper Provinces RICHARD H. SILLITOE† 27 West Hill Park, Highgate Village, London N6 6ND, England

Abstract It has been recognized for the past century that copper deposits, in common with those of many other metals, are heterogeneously concentrated in Earth’s upper crust, resulting in areally restricted copper provinces that were generated during several discrete metallogenic epochs over time intervals of up to several hundred million years. Various segments of circum-Pacific magmatic arcs, for example, have total contained copper contents that differ by two orders of magnitude. Each metallogenic epoch introduced its own deposit type(s), of which porphyry copper (and related skarn), followed by sediment-hosted stratiform copper and then iron oxide copper-gold (IOCG), are globally preeminent. Nonetheless, genesis of the copper provinces remains somewhat enigmatic and a topic of ongoing debate. A variety of deposit-scale geometric and geologic features and factors strongly influence the size and/or grade of porphyry copper, sediment-hosted stratiform copper, and/or IOCG deposits. For example, development of major porphyry copper deposits/districts is favored by the presence of clustered alteration-mineralization centers, mafic or massive carbonate host rocks, voluminous magmatic-hydrothermal breccias, low sulfidation-state core zones conducive to copper deposition as bornite ± digenite, hypogene and supergene sulfide enrichment, and mineralized skarn formation, coupled with lack of serious dilution by late, low-grade porphyry intrusions and breccias. Furthermore, the copper endowment of all deposit types undoubtedly benefits from optimization of the ore-forming processes involved. Tectonic setting also plays a fundamental role in copper metallogeny. Contractional tectonomagmatic belts, created by flat-slab subduction or, less commonly, arc-continent collision and characterized by crustal thickening and high rates of uplift and exhumation, appear to host most large, high-grade hypogene porphyry copper deposits. Such mature arc crust also undergoes mafic magma input during porphyry copper formation. The premier sediment-hosted stratiform copper provinces were formed in cratonic or hinterland extensional sedimentary basins that subsequently underwent tectonic inversion. The IOCG deposits were generated in association with extension/transtension and felsic intrusions, the latter apparently triggered by deep-seated mafic magmas in either intracratonic or subduction settings. The radically different exhumation rates characteristic of these various tectonic settings account well for the secular distribution of copper deposit types, in particular the youthfulness of most porphyry relative to sediment-hosted stratiform and IOCG deposits. Notwithstanding the importance of these deposit-scale geologic, regional tectonic, and erosion-rate criteria for effective copper deposit formation and preservation, they seem inadequate to explain the localization of premier copper provinces, such as the central Andes, southwestern North America, and Central African Copperbelt, in which different deposit types were generated during several discrete epochs. By the same token, the paucity of copper mineralization in some apparently similar geologic settings elsewhere also remains unexplained. It is proposed here that major copper provinces occur where restricted segments of the lithosphere were predisposed to upper-crustal copper concentration throughout long intervals of Earth history. This predisposition was most likely gained during oxidation and copper introduction by subduction-derived fluids, containing metals and volatiles extracted from hydrated basalts and sediments in downgoing slabs. As a result, superjacent lithospheric mantle and lowermost crust were metasomatized as well as gaining cupriferous sulfide-bearing cumulates during magmatic differentiation—processes that rendered them fertile for tapping during subsequent subduction- or, uncommonly, intraplate extension-related magmatic events to generate porphyry copper and IOCG districts or belts. The fertile lithosphere beneath some accretionary orogens became incorporated during earlier collisional events, commonly during Precambrian times. Relatively oxidized crustal profiles—as opposed to those dominated by reduced, sedimentary material—are also required for effective formation of all major copper deposits. Large sedimentary basins underlain by or adjoining oxidized and potentially copper-anomalous crust and filled initially by immature redbed strata containing magmatic arc-derived detritus provide optimal sites for large-scale, sediment-hosted stratiform copper mineralization. Translithospheric fault zones, acting as giant plumbing systems, commonly played a key role in localizing all types of major copper deposits, districts, and belts. These proposals address the long-debated concept of metal inheritance in terms of the fundamental role played by subduction-metasomatized mantle lithosphere and lowermost crust in global copper metallogeny.

numbers of exceptionally endowed deposits, and copper is no exception (Singer, 1995; Laznicka, 1999). Indeed, approximately one-third of the world’s defined copper resources are contributed by just seven districts (Fig. 1), and approximately 2.5% of producing mines currently supply 25% of total copper

Introduction THE GLOBAL INVENTORY of metals is critically dependent on the inordinately large contributions made by relatively limited † E-mail:

[email protected]

1

2

RICHARD H. SILLITOE

FIG. 1. The world’s supergiant copper deposits and districts (defined as those containing ≥24 Mt [Singer, 1995] to ≥25 Mt [Laznicka, 1999] Cu in resources and past production) and preeminent provinces, keyed to deposit types. The newly discovered Kamoa deposit in the Central African Copperbelt (Broughton and Rogers, 2010) contains 22 Mt Cu, but is also considered as a supergiant because of the likelihood of further growth. Data compiled from numerous published and unpublished sources, including company press releases.

output (M. Harris, Rio Tinto, unpub. comp., 2012). Furthermore, large proportions of most major metals, particularly well exemplified by copper, are concentrated in areally restricted provinces (Fig. 1), which were typically assembled during several discrete metallogenic epochs. At least in the case of intrusion-related deposits, individual epochs commonly have durations of ≤10 m.y. (e.g., Sillitoe, 1988). This spatial and temporal confinement of copper and other metal deposits was appreciated by Lindgren (1909) and subsequent pioneers, as reviewed by Turneaure (1955), but has become much better defined over the ensuing century as a result of numerous discoveries and geologic advances, particularly direct isotopic dating of ore-related minerals. Although the fundamental reasons for the development of the world’s largest copper deposits and premier copper belts and provinces are not well understood, this introductory paper explores some of the more plausible possibilities. The principal contributors to the global copper inventory, namely porphyry and any associated skarn deposits (~70%), sediment-hosted stratiform deposits (~15%), and, a distant third, iron oxide copper-gold (IOCG) deposits, are emphasized both herein (Fig. 1) and throughout the rest of this volume. Other relatively minor copper sources, including magmatic nickel-copper, volcanogenic massive sulfide (VMS), nonporphyry-related skarn, vein, Chilean manto-type, and carbonatite-hosted deposits 0361-0128/98/000/000-00 $6.00

are not specifically discussed, although because of the importance of Noril’sk, Russia (Fig. 1), the first of these sources has a paper devoted to it (Burrows and Lesher, 2012). The copper endowment considered herein (≈2,500 million metric tons [Mt]; Figs. 1, 2) exceeds the global inventory of 1,900 Mt determined by Kesler and Wilkinson (2008), and is more than four times larger than some other recent estimates (e.g., ~570 Mt; U.S. Geological Survey, 2011, p. 48–49). Although only formal resources plus past production are taken into account, the greater copper tonnage may be attributed to major recent expansions of hypogene resources, particularly in the central Andes (e.g., Sillitoe, 2010a), and application of lower cutoff grades. If probabilistic methodologies employed by Cunningham et al. (2007) and Kesler and Wilkinson (2008) are followed, then at least twice the number of copper deposits exist (most at greater depths), albeit probably mainly confined to the currently defined belts and provinces. Deposit-Scale Contributions to Large Size and High Grade For the sake of brevity, only copper deposits and districts that attain supergiant status by containing ≥24 Mt (Singer, 1995) or ≥25 Mt (Laznicka, 1999) of copper metal are discussed and individually plotted in Figure 1; most exceed the 31.1-Mt Cu threshold used to define behemothian deposits 2

COPPER PROVINCES

3

FIG. 2. Total copper endowment (resources and past production) of different segments of Phanerozoic circum-Pacific magmatic arcs. Only Paleozoic arc terranes, potentially somewhat more deeply eroded because of their greater antiquity, are present in eastern Australia. Note the two orders of magnitude difference among the segments. Data compiled from numerous published and unpublished sources, including company press releases.

(Clark, 1993). These truly exceptional copper concentrations (Fig. 1) include 14 porphyry ± skarn districts (Pebble, Butte, Bingham, Resolution, Morenci, Chuquicamata, CollahuasiQuebrada Blanca, Escondida, Los Pelambres-El Pachón, Río Blanco-Los Bronces, El Teniente, Reko Diq, Oyu Tolgoi, and Grasberg), four and probably five sediment-hosted stratiform districts (Lubin-Polkowice, Konkola, Kolwezi, Udokan, and probably Kamoa), one IOCG deposit (Olympic Dam), and one magmatic nickel-copper district (Noril’sk). Six of these porphyry copper districts are the principal contributors to the central Andean copper province (Figs. 1, 2).

Nevertheless, as currently understood, Bingham and Resolution comprise single, zoned deposits rather than clusters or alignments (e.g., Hehnke et al., 2012; Porter et al., 2012). Such clusters and alignments are inferred to occur above cupolas on the roof zones of parental plutons located at depths of several kilometers (e.g., Emmons, 1927; Dilles and Proffett, 1995; Sillitoe, 2010b). The volume of porphyry intrusions within the confines of a deposit appears unrelated to its copper content, as emphasized by comparison of the largely porphyry-hosted Collahuasi deposits with the largely wall rock-hosted El Teniente deposit (e.g., Camus, 2003). Nonetheless, it is critically important that the earliest, generally best-mineralized porphyry phase and its immediate wall rocks remain as a physically coherent entity little diluted by subsequent pulses of intermineral and, especially, late mineral porphyry, which typically contain progressively lower copper contents as they become younger. Such low-grade porphyry phases are volumetrically restricted in most of the supergiant copper deposits (Fig. 3). It is well known that magmatic-hydrothermal breccias (Sillitoe, 1985) in porphyry copper deposits can give rise to substantially higher-grade hypogene (and supergene) mineralization because of the greater permeability and resultant fluid focusing that they provide (Fig. 3). The prime example of the key

Porphyry copper deposits Intrusion, host-rock, and alteration-mineralization features may all be discerned as controls on the large size and/or high grade of the world’s preeminent porphyry copper deposits and districts, as shown schematically in Figure 3. The presence of several closely spaced, mineralized porphyry stocks, which define clusters (e.g., Escondida and Reko Diq districts) or alignments (e.g., Collahuasi-Quebrada Blanca, Chuquicamata, Los Pelambres-El Pachón, Río Blanco-Los Bronces, and Oyu Tolgoi districts), is an important localizer of supergiant copper concentrations. At least 13 discrete deposits constitute the Reko Diq cluster (Perelló et al., 2008). 0361-0128/98/000/000-00 $6.00

3

4

RICHARD H. SILLITOE

FIG. 3. Anatomy of a non-eroded, telescoped porphyry copper system showing spatial interrelationships of a centrally located porphyry Cu ± Au ± Mo deposit in a porphyry stock and its immediate host rocks, including overlying high- and intermediate-sulfidation epithermal deposits in and alongside the lithocap environment. Not all depicted features are necessarily present in any single district. The legend explains the temporal sequence of rock types, with the porphyry stock predating maar-diatreme emplacement, which, in turn, overlaps lithocap development and related phreatic brecciation. Shallow alteration types generally overprint deeper ones. Circled letters highlight deposit-scale features that can enhance hypogene grade development: A. Minimal dilution caused by restricted volume of lower grade, intermineral and late-mineral porphyries; B. Large volume of well-mineralized magmatic-hydrothermal breccia; C. Late-stage, barren diatreme located beyond rather than within the ore zone; D. Mafic wall rocks induce effective copper precipitation; E. Massive, impermeable carbonate wall rocks inhibit dispersion of mineralizing fluids and favor internal copper precipitation and grade development; F. Presence of bornite and digenite in the deep, central parts of the potassic zone increases hypogene copper tenor; G. Hypogene enrichment by high sulfidation-state sulfide minerals in the roots of the sericitic zone; and H. Skarn development with copper tenors exceeding those in the adjacent porphyry stock. Modified from Sillitoe (2010b). 0361-0128/98/000/000-00 $6.00

4

COPPER PROVINCES

role played by magmatic-hydrothermal brecciation in the development of both deposit size and grade is at Río Blanco-Los Bronces, the world’s largest copper district (Fig. 1), where on the order of 50% of the ore may be breccia hosted (Serrano et al., 1996; Irarrazaval et al., 2010; Toro et al., 2012). However, magmatic-hydrothermal breccias make only relatively minor contributions to the other premier deposits, with Pebble, Bingham Canyon, and Chuquicamata being almost devoid of them (Ossandón et al., 2001; Lang and Gregory, 2012; Porter et al., 2012; Rivera et al., 2012). If present, late, lowgrade breccias, particularly diatremes, are best to be situated beyond deposits so as not to destroy ore (Fig. 3), although such destruction can be tolerated in the largest deposits (e.g., El Teniente; Howell and Molloy, 1960). Wall-rock composition and permeability seem to be important controls on large size and hypogene grades exceeding 1% Cu at several of the premier deposits (Sillitoe, 2010b; Fig. 3). Intensely biotitized, ferrous iron-rich wall rocks—a Mesoproterozoic diabase sill complex at Resolution (Hehnke et al., 2012; Leveille and Stegen, 2012), gabbro and dolerite sills and dikes at El Teniente (Skewes et al., 2002), and submarine tholeiitic basalt at Oyu Tolgoi (Kirwin et al., 2005; Crane and Kavalieris, 2012)—acted as highly effective copper precipitants from oxidized magmatic fluids. By the same token, quartzite at Bingham Canyon acted as a poor host (Porter et al., 2012). In contrast, the massively bedded limestone wall rocks at Grasberg may have created a relatively impermeable sleeve that inhibited magmatic fluid escape and promoted grade development within the porphyry stock (Sillitoe, 1997). A comparable role has also be ascribed to hornfelsed wall rocks, including those above both the East zone at Pebble (Lang and Gregory, 2012) and the world’s largest (17 Mt Cu) skarn copper deposit at Antamina, Peru (Love et al., 2004). The evolution of alteration, including the accompanying sulfide minerals, over the lifespans of porphyry copper deposits profoundly influences grade development and conservation (Einaudi et al., 2003). In deposits characterized by major copper introduction in the early, high-temperature, potassic stages (e.g., Bingham Canyon, Los Pelambres-El Pachón, El Teniente, Grasberg), the absence of appreciable overprinting by sericitic or chlorite-sericite alteration, which can cause partial or even total copper removal (e.g., Kouzmanov and Pokrovski, 2012), favors conservation of high copper tenors; these attain their maxima in the deep, central parts of systems where low-sulfidation conditions can lead to bornite ± digenite accompanying or even dominating chalcopyrite (Fig. 3; e.g., Bingham Canyon; Porter et al., 2012). Coalescence of several bornite-rich centers favors development of large orebodies, as at Los Pelambres and El Teniente (Vry et al., 2010; Perelló et al., 2012). Nonetheless, where copper deposition is relatively late, sericitic and even, in some cases, chlorite-sericite alteration appears to accompany grade development (e.g., Oyu Tolgoi; Crane and Kavalieris, 2012). The highest hypogene grades in some major porphyry deposits are present with sericite ± pyrophyllite ± dickite alteration and high sulfidation-state copper minerals, in the telescoped roots of advanced argillic lithocaps (Sillitoe, 2010b; Fig. 3), with Butte (Meyer et al., 1968), Chuquicamata (Ossandón et al., 2001; Rivera et al., 2012), and Resolution (Hehnke et al., 2012) and, to a lesser extent, Escondida 0361-0128/98/000/000-00 $6.00

5

(Hervé et al., 2012) and Pebble East zone (Lang and Gregory, 2012) being prominent examples. Both calcic and magnesian skarns, developed proximally with respect to porphyry copper stocks emplaced into shelfcarbonate sequences, as in the Grasberg and Bingham districts, can also give rise to high-grade, hypogene copper mineralization (Meinert et al., 1997; Leys et al., 2012; Porter et al., 2012). Particularly large tonnages can be developed where the receptive carbonate horizons either flank the apices of porphyry stocks (e.g., Antamina skarn; Love et al., 2004) or parallel the stock contacts (e.g., Ertsberg East skarn, Grasberg district; Gandler and Kyle, 2008; Leys et al., 2012). Supergene sulfide enrichment since ~40 Ma is well known to have been a key process in grade development, particularly in parts of the central Andes and southwestern North American porphyry copper provinces (Sillitoe, 2005, and references therein), with Morenci (Leveille and Stegen, 2012), Chuquicamata (Ossandón et al., 2001; Rivera et al., 2012), and Escondida (Hervé et al., 2012) being prime examples. However, particularly in the former region, exploration over the past decade or so has outlined far larger hypogene resources beneath the previously and/or currently mined parts of the Cenozoic supergene profiles (e.g., Escondida; Hervé et al., 2012). Sediment-hosted stratiform copper deposits The most obvious parameters controlling the size of sediment-hosted stratiform copper deposits, as exemplified by major examples in the Central African Copperbelt of Zambia and Democratic Republic of Congo (DRC), the Kupferschiefer province of Poland and Germany, Dzhezkazgan in Kazakhstan, and Udokan in Russia (Fig. 1), are geometric and depend on structurally uninterrupted stratigraphic continuity. The largest deposits have the greatest along-strike and downdip extents and/or thickest or greatest number of ore horizons, as exemplified by the sandstone-, carbonaceous shale-, and dolomite-hosted Lubin, Polkowice, and contiguous deposits of the Fore-Sudetic Monocline in Poland, which underlie an area of ~500 km2 and are mined to a depth of ~1,300 m (Oszczepalski, 1999; Borg et al., 2012). The subhorizontal, diamictite-hosted Kamoa deposit in DRC underlies an area of at least 80 km2 (Broughton and Rogers, 2010). The number of individual, ore-bearing sandstone beds approaches 30 over a 600-m stratigraphic interval at Dzhezkazgan, thereby accounting for its large size (Gablina, 1981; Box et al., 2012). In contrast, structural repetition of the main mineralized dolomitic shale horizons, in conjunction with their considerable strike and dip continuity, contributes to the large size of the Kolwezi and Tenke-Fungurumé district deposits, DRC (Hitzman et al., 2012; Schuh et al., 2012). The controls on ore grade in sediment-hosted stratiform copper deposits are less readily appreciated, although they appear to be regional in extent since province-wide average grades tend to be broadly similar, in marked contrast to the situation in porphyry copper and IOCG provinces. Consequently, availability of stratigraphic traps (e.g., pinchouts against basement highs, anticlinal crests) and original amounts and effectiveness of the copper-precipitating reductant in the mineralized horizons, be it in situ (as carbonaceous matter and/or diagenetic pyrite) or mobile and introduced 5

6

RICHARD H. SILLITOE

(e.g., Selley et al., 2005; Hitzman et al., 2012), are seemingly important factors. Nonetheless, the hypogene bornite- and chalcocite-bearing parts of the zoned deposits, especially where veining is well developed, consistently have the highest grade (e.g., Sillitoe et al., 2010; Schuh et al., 2012). Supergene chalcocite enrichment, suggested by some (e.g., Hitzman et al., 2005, 2012) to have enhanced copper grades in the shallow parts of the Central African Copperbelt deposits, is considered unimportant. Suppression of the enrichment process is caused by the abundance of carbonate-bearing host rocks and paucity of ore-related pyrite, and is reflected by the ubiquity of highgrade oxide copper mineralization throughout the supergene profiles (e.g., Tenke-Fungurumé; Fay and Barton, 2012).

Summary statement—deposit-scale contributions Optimization of all aspects of deposit-scale mineralization processes is a likely prerequisite for formation of large, highgrade orebodies (e.g., Richards, 2005, 2011); however, it is apparent in the case of porphyry copper deposits, and probably other ore types too, that no single geologic characteristic or set of characteristics seems able to adequately predict deposit size or grade (cf. Clark, 1993). Nonetheless, a number of geometric, host-rock, brecciation, and alteration-mineralization features may offer at least partial explanations for the preeminence of individual porphyry, sediment-hosted stratiform, and IOCG deposits and districts. Notwithstanding these proposals, it is difficult to see how any of these disparate, deposit-scale parameters or mechanisms can satisfactorily explain the origin of exceptionally endowed copper belts and provinces. In the case of the central Andean porphyry copper province, for example, the premier deposits are assigned quite different key geologic controls on grade and/or tonnage: for example, biotitized mafic wall rocks at El Teniente, exceptional development of magmatic-hydrothermal breccias at Río Blanco-Los Bronces, and a combination of hypogene and supergene copper enrichment at Chuquicamata. Furthermore, in the case of IOCG deposits, even orebodies within individual provinces display radically different geologic characteristics (e.g., Carajás; Xavier et al., 2012). Clearly, therefore, these deposit-scale parameters are secondary to more regionally relevant processes that are required to explain the localization of the world’s premier copper provinces.

IOCG deposits Notwithstanding the disparity of deposits assigned to the IOCG category and disaccord over their genesis (e.g., Sillitoe, 2003; Barton, 2009; Groves et al., 2010), the paucity of supergiant deposits limits assessment of deposit-scale controls on their size and grade. However, the overwhelming dominance of Olympic Dam within the IOCG grouping, both in terms of size and grade (90 Mt Cu; Fig. 1), must surely be related to the unusually large volume (~5 km3) of the host hematitic breccias (Ehrig et al., 2012), the products of highenergy hydrothermal fragmentation of granite (Reeve et al., 1990). Identical breccias occur at Carrapateena (Fairclough, 2005), 120 km southeast of Olympic Dam, but their far more restricted volume results in a commensurately smaller deposit (currently 4.4 Mt Cu). The world’s second-largest IOCG deposit, CandelariaPunta del Cobre (~11 Mt Cu) in northern Chile, contains relatively minor hydrothermal breccia, thereby showing that large size is by no means exclusively breccia controlled. Indeed, a combination of permeable volcaniclastic rocks, an anticlinal fold, faults, and an overlying limestone seal combined to localize Candelaria-Punta del Cobre (Marschik and Fontboté, 2001). The third largest IOCG deposit, Salobo (officially 7.8 Mt Cu) in the Carajás province (Fig. 1), is different again, being an elongate, steeply dipping, structurally controlled orebody associated with sheared and highly altered, granulite-facies, siliciclastic metasedimentary rocks (Réquia and Fontboté, 2000; Xavier et al., 2012; M.W. Hitzman, writ. commun., 2012). Fluorine-rich ore fluids were proposed as an influential control on size and grade at Olympic Dam (McPhie et al., 2011), but comparable fluorine enrichment at Carrapateena, Salobo, and elsewhere clearly did not generate the same size effect. The enhanced permeability provided by the breccia host seems to be a more likely explanation for the high copper grades, similar to the porphyry copper environment (see above). In common with sediment-hosted stratiform copper deposits, average grades are also clearly influenced by the copper contents of the hypogene sulfide species present, as underscored by the high grades at Olympic Dam where bornite and chalcocite are widespread (Reeve et al., 1990; Ehrig et al., 2012). However, in common with the sediment-hosted stratiform copper deposits, the overall deficiency of pyrite in IOCG orebodies militates against significant development of supergene chalcocite enrichment (e.g., central Andes IOCG deposits; Sillitoe, 2005). 0361-0128/98/000/000-00 $6.00

Tectonic Controls on Copper Belts and Provinces Porphyry copper deposits It has long been appreciated that porphyry copper deposits occur in accretionary orogens formed at sites of subduction of oceanic lithosphere (Sawkins, 1972; Sillitoe, 1972). However, copper endowment in magmatic arcs around the Pacific Ocean (Fig. 2), as well as elsewhere, is highly heterogeneous, ranging from nearly 1,000 Mt in the central Andes to 130 million metric tons (Mt) Cu in resources and past production. The mineralization occurs in various types of Eocene to early Oligocene porphyry Cu systems hosted by Paleozoic and Triassic volcanic and Triassic granodiorite rocks. Emplacement of the deposits occurred during E-W−directed contraction, crustal shortening, and uplift related to the Incaic orogeny between 38 and 31 Ma. The N-S−oriented West Fissure is the most important structural feature of the district, with significant postmineral, left-lateral displacement of 30 to 35 km estimated by regional mapping, although it remains to be confirmed by displaced alteration-mineralization features at the individual deposit scale. The West Fissure, across which both the geology and mineralization differ at any given point, divides the district into two domains. The Eastern Block comprises Paleozoic metamorphic and intrusive complexes overlain by Permian and Triassic volcanic and sedimentary rocks and intruded by Triassic granodiorite. These are overlain in the north by Cretaceous continental sedimentary and volcanic rocks and in the south by Eocene to Miocene sedimentary rocks of the Calama Basin, the lowermost units of which show evidence of syntectonic deposition. The Eastern Block contains the Chuquicamata and Radomiro Tomic deposits, both hosted by the Chuqui Porphyry Complex, a N-NE−oriented, 14- × 1.5-km megadike intruded into Triassic volcanic and intrusive rocks, and with a SHRIMP U-Pb age of ~36 Ma. Hypogene mineralization at Chuquicamata occurs mainly in the East porphyry, the dominant phase of the porphyry complex and does not show a close relationship to smaller, later porphyry bodies with SHRIMP U-Pb ages of ~34 Ma. Much of the Cu was introduced early (~34−35 Ma; 40Ar/39Ar) during potassic alteration, which comprises a large low-grade body containing biotitized hornblende. Within this large low-grade body higher Cu grades occur as bornite and other Cu-bearing sulfides, without pyrite, in intense potassic alteration halos (K-feldspar-sericite) along early fractures. This was followed by introduction of quartz-molybdenite veins, with an Re-Os age of ~35 Ma. Sericite-quartz-pyrite alteration, with advanced argillic alteration near veins, is later, as shown by crosscutting relationships, and returns 32 to 31 Ma 40Ar/39Ar ages. Veins with high sulfidation Cu-bearing sulfide-pyrite assemblages are part of this later stage, which overprinted and sulfidized the earlier Cu-bearing sulfides. Similar early and late mineralization occurs at Radomiro Tomic, but there the late-stage veins are of lesser importance. The Western Block comprises Paleozoic metamorphic complexes, Permian and Triassic volcanic and sedimentary rocks, and Triassic granodiorite in the south, overlain by Jurassic carbonates and continental sedimentary rocks. In the north, Cretaceous and Lower Tertiary volcanic and continental sedimentary rocks overlie the Jurassic unit. Jurassic to Eocene strata in the central to northern parts of the Western Block hosts the Eocene Los Picos and Fortuna batholiths. The Mina Ministro Hales deposit, at the eastern edge of the Western Block, is associated with Eocene porphyries (~39−35 Ma SHRIMP and LA-ICP-MS U-Pb ages) that intrude wall rocks similar to those at Chuquicamata. Mina Ministro Hales is characterized by mineralization similar to that at Chuquicamata, but late-stage sericitic and advanced argillic alteration (~ 32 Ma 40Ar/39Ar ages) are of greater importance, especially at the shallower levels. The Toki Cluster deposits, farther to the west, are associated with swarms of small porphyries thought to be late-stage phases of the ~38 Ma Fortuna granodiorite batholith. The mineralization consists of bornite and/or chalcopyrite, commonly with magnetite but little or no pyrite, in A-type veins accompanying potassic alteration. The strongest mineralization is in and near the earliest porphyries and is truncated by the next younger porphyries, which in turn are cut by similar but less intense † Corresponding author: e-mail, [email protected] *Present address: Coro Mining Corporation, Manuel Barros Borgoño 254, Providencia, Santiago, Chile. **Present address: Minería Activa S.A. Avda. Presidente Riesco 5335, Of. 2104 Las Condes, Santiago, Chile. ***Present address: Antofagasta Minerals, Avda. Apoquindo 4001 Piso 18, Las Condes, Santiago, Chile.

19

20

RIVERA ET AL.

mineralization and still younger, less-mineralized porphyries. Late pyrite-sericite alteration occurs mostly on the peripheries of these deposits and carries little Cu. A favorable combination of desert climate and morphotectonic evolution resulted in the formation and preservation of significant supergene enrichment and/or oxidation. The potassic zones at Chuquicamata, Radomiro Tomic, and the Toki Cluster underwent in situ oxidation. Supergene enrichment blankets formed in quartz-sericite-pyrite alteration at Chuquicamata and Mina Ministro Hales. Supergene processes also resulted in lateral migration of Cu and formation of the Mina Sur exotic deposit. From an exploration perspective, the history of the district demonstrates how geologic observations and interpretation have played a key role in development of the resource base. Only the Chuquicamata oxide Cu mineralization cropped out, and original open-pit development in 1912 was followed by exploration and evaluation of the giant, high-grade enrichment blanket from the 1930s onward. District-scale exploration resulted in discovery of Radomiro Tomic in the 1950s, Mina Sur in the 1960s, Mina Ministro Hales in the 1990s and the Toki Cluster at the beginning of this century, none of which were exposed at surface. The Chuquicamata district, now producing ~900,000 t Cu/yr, has been operating for 100 years and retains a substantial resource base that will enable it to continue for many years to come.

Introduction THE CHUQUICAMATA district is located in the Atacama Desert of northern Chile, 1,650 km north of the capital, Santiago, 200 km northeast of the port of Antofagasta, and 15 km north of Calama, at an elevation of 2,800 m above sea level (Fig. 1A). CODELCO Chile owns 100% of the mining complex which includes three open pits: Chuquicamata, Radomiro Tomic (formerly Pampa Norte), and Mina Sur (formerly Exotica), as well as beneficiation plants and a smelter. During 2011, the complex produced approximately 913,000 t Cu, representing 17% of worldwide production, together with approximately 10,700 t Mo. These figures confirm the Chuquicamata district’s place as one of the world’s greatest mining camps, with a historic production of more than 35.5 Mt Cu and with identified resources of >100 Mt of contained Cu. The future plans of CODELCO are to develop an underground mine to replace the Chuquicamata open pit, maintain production from Radomiro Tomic and Mina Sur and their extensions, and to develop the oxide deposits of the Toki Cluster. The Mina Ministro Hales (formerly Mansa Mina or MM) open-pit mine is also currently being developed. Figure 2 shows Cu production from 1915 through 2011. The district hosts a cluster of late Eocene to early Oligocene porphyry Cu-Mo deposits with a complex pre-, syn- and postmineral history. A favorable combination of desert climate and morphotectonic evolution resulted in the formation, and preservation, of well-developed deep oxidation and/or enrichment profiles in all deposits, as well as lateral transport of Cu in solution to produce exotic oxide mineralization such as that mined at Mina Sur. For example, the potassic zones at Chuquicamata, Radomiro Tomic, and the Toki Cluster underwent in situ oxidation, and supergene enrichment blankets formed in quartz-sericite-pyrite alteration at Chuquicamata and Mina Ministro Hales. The objective of this paper is to present information on district geology based on advances made in the past 10 years, resulting from systematic 1:5,000 mapping supported by modern technologies of digital mapping and capture of field observations (Fig. 1A). The paper also provides an update for the geologic framework of the main deposits, particularly the hypogene alteration and mineralization at Chuquicamata and Radomiro Tomic, as well as Mina Ministro Hales and the Toki Cluster. Because the main district deposits such as Chuquicamata and Radomiro Tomic have almost been exhausted of both oxides and supergene sulfides and although there are 0361-0128/98/000/000-00 $6.00

still supergene resources in the North Extension of Mina Sur, part of Mina Ministro Hales, and the Toki Cluster, the description and analysis of these enrichment processes is beyond the scope of this paper. More information about this can be found in several published papers (Jarrell, 1944; Newberg, 1965; Mortimer et al., 1978; Flores, 1985; Münchmeyer, 1996; Chavez, 2000; Cuadra and Rojas, 2001; Ossandón et al., 2001; Lorca et al., 2003; Faunes et al., 2005; Sillitoe, 2005; Rivera et al., 2009) This paper is based in large part on internal maps and company reports (N. Blanco, writ. commun., 2007; J. Dilles, writ. commun., 2008; Alcota et al., 2009; García et al., 2009; CODELCO, 2010; and reports and maps by consultants), which build on previous publications by Dilles et al. (1997), Tomlinson and Blanco (1997a, b, 2007), Cuadra and Rojas (2001), Tomlinson et al. (2001), Müller and Quiroga (2003), Rivera and Pardo (2004), Rivera et al. (2009), Dilles et al. (2011), Barra et al. (in press), and others as cited in the text. Chuquicamata: A Century of Copper Production, Exploration, and Discovery Archeological studies, especially those related to the Hombre de Cobre (fossilized Copper Man; Petersen, 2011) and remains of encampments, have shown that the earliest mining at Chuquicamata took place ~500 AD, which predates the Inca incursion by several hundred years (Dunbar, 1952; Sutulov, 1978; Núñez et al., 2003). The green Cu oxides were mined from shallow surface workings and smelted in various sites along the Rio Loa. The Inca caravan routes distributed the metal throughout the Andean region, and Cu from the deposit was instrumental in the development of the Inca Empire from 700 to 1,400AD (Núñez et al., 2003; Petersen, 2011). No references of relevant mining activity exist for the next four centuries, mostly because of the greater importance at that time of precious metals. The modern mining and exploration history is summarized in Table 1 and Cu production is illustrated in Figure 2. Some minor activity persisted at the end of the 19th century, and the Cu potential started to be recognized in reports by engineers commissioned by the Chilean government (Valdés, 1887; Gandarillas, 1915; Martínez, 1943; Millán, 2006). At least three factors contributed to the subsequent development of a true Cu rush in the area: the railroad construction from Antofagasta to Calama, the increase in Cu price, and the enactment of a new mining code. Several mining companies 20

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE

FIG. 1. Location map of the Chuquicamata district. A. Limits of the 1:5,000 scale geology map area shown in Figures 4 and 5 (Alcota et al., 2009). Note the location of main operations and prospects aligned on a northeast trend from Radomiro Tomic at the north toward the Toki Cluster to the south. Calama and Loa River are noted for reference. B. Sketch map of north-central Chile showing locations of Santiago, Antofagasta, and main porphyry Cu districts described in this publication, such as Escondida and Río Blanco-Los Bronces. 0361-0128/98/000/000-00 $6.00

21

21

22

RIVERA ET AL.

FIG. 2. History of mine production (data from CODELCO and COCHILCO). Chuquicamata is an excellent example of mining, metallurgical, and geologic improvement over nearly a century of continuous production, exploration, and development. Past production is estimated as 35.5 Mt Cu and remaining resources are >90 Mt Cu. A summary of mine and exploration milestones is presented in Table 1.

were created and the area became famous enough to attract the interest of mining promoters such as A.C. Burrage, who negotiated an option for most of the properties with the Guggenheim Group. The Chile Exploration Co. was thus created in January 1912, and in just 3 years Chuquicamata was explored and evaluated by means of churn drilling. Leaching and electrowinning recovery of Cu was tested as a new largescale metallurgical method, mine and facilities were constructed, and production began on May 19, 1915 (Yeatman, 1916, 1932; Parsons, 1933). The foundations of geologic understanding of the Chuquicamata deposit were established by the geologists and engineers working for Chile Exploration Co., especially Pope Yeatman and his assistants E.S. Berry and R. Marsh, Jr. (Yeatman, 1916, 1932) as well as by W. Lindgren who, aided by E. Bastin, generated the first geologic map of the deposit (Zentilli et al., 1994b). Details of the first drilling campaign and resource estimates are given in Table 1; Cu production for the first years is shown in Figure 2. The purchase by Anaconda Copper Co. in 1923 doubled production from ~40,000 t Cu/yr to 90,000 t. By means of successive expansions, production was increased to 250,000 t Cu/yr (Table 2; Fig. 2). As summarized by Perry (1991), geology played a key role in development of the Chuquicamata 0361-0128/98/000/000-00 $6.00

mine by Anaconda. During a visit in 1930, Reno Sales became interested in ore potential beneath the oxides and also in a zone to the west, close to the already defined West Fissure, which showed an attractive leached capping. Three drill holes were sufficient to demonstrate potential for >200 Mt at 2.75 wt % Cu in sulfides at depth and west of the oxide pit. Thanks to these results, Anaconda was able to survive the Great Depression according to Perry (1991). The company’s geologists generated the first structure-lithology-alteration-mineralization models that sustained the operation in the subsequent years (Taylor, 1935; López, 1939, 1942; Jarrell, 1944; Perry, 1952). While drilling related to metallurgical studies resulted in the discovery of Mina Sur in 1957, geology played a fundamental role in the exploration and definition of a new model for the mineralization based on the lateral migration of Cu in the supergene environment (Newberg, 1965; Roa, 1975; Perry, 1991). The placing in production of this new mine allowed for the substantial extension of the life of the leach plant. Between 1915 and 1971, oxides at Chuquicamata produced 6.5 Mt Cu (Fig. 2). Mina Sur has produced more than 2 Mt Cu and production now continues from the channel that links it to its source, the Chuquicamata deposit. Anaconda’s 22

Mine History event

Historic record of the first mining claims, small miners exploited very high grade veins; the first claimed mines where the Zaragoza and Lérida veins; 50 mining claims are registered covering part of the mineralized area

Several individuals and medium-size mining companies owned up to 288 mining claims; strong growth in the prospecting and mining activity is due to the increase in copper price (1886), the railroad extension up to Calama (1886) and a New Mining Code (1888); the deposit potential attracts investors such as Albert C. Burrage and the Guggenheim Group, who evaluated and negotiated options on the several properties in the best mineralized areas most of them owned by two Chilean and Chilean-British companies; Burrage and the Guggenheim group negotiate an agreement for exploration and project development

Creation of the Chile Exploration Company by Guggenheim Group and a minority participation of Albert C. Burrage on January 11, 1912; as stated by Pope Yeatman, after the option was signed the exploration drilling was initiated in April, 1912, with such encouraging results that the option was exercised in October, 1912, the Chile Copper Co was incorporated in January, 1913, construction work started and operation inaugurated in May 19, 1915

Anaconda Copper Co. purchased Chile Exploration Co., February 1923, and subsequently doubled production and expanded to mine the sulfide ore in 1958 production increased to more than 200,000 t Cu/yr

The Chilean State became the owner of the company by means of a first step of “Chilenization” (Sociedades Mixtas) aquiring 51% in 1966, followed by a change in the Constitution which allows “Nationalization” in 1971; Codelco takes control of the operations in 1976; Radomiro Tomic oxide mine and SX-EW operation starts in 1997 and Mina Ministro Hales in 2010; future projects include copper oxide production from Toki Cluster, and sulfides from both Radomiro Tomic and underground Chuquicamata mine

Year

1882-1885: Interest by small miners and first geologic appraisal

1886-1911: “Copper rush” times; explosive interest by Chilean and foreign companies and individuals

1912-1922: Exploration and development by the Chilean Exploration Co

0361-0128/98/000/000-00 $6.00

1923-1966: Exploitation and further development by the Anaconda Copper Co

23

1967-to date: Codelco’s period of growing to a world-class operation

Codelco doubled the 300,000 t production in 1966 to 600,000 in 1988 and to 900,000 t in year 2000; successfully managed the supergene to hypogene sulfide transition and expanded to open new operations; from 1966 to 2000 the total production, including Radomiro Tomic, reached about 27 Mt Cu

During Anaconda’s period the mine produced a total of 7.5 Mt Cu, by means of successive expansions, from 90,000 in 1923 to 200,000 in 1935, reaching close to 250,000 t in 1965

Chuquicamata produces 229,000 t of copper from 1915 to 1923. A mining town for 15,000 people was constructed close to the mine and a power station at the port of Tocopilla. This was the first of the newly recognized porphyry copper type deposits to use leaching and electro wining for treating ore.

In 1903 production reached 3,325 t of copper corresponding to 10% of total Chilean production. Data from 1910 and 1913 indicates that the annual production was on the order of 17,000 to 20,000 t ore averaging more than 15% Cu

In 1884 approximately 40 small miners or “pirquineros” produced 60,000 tons of mineral

Copper production

Codelco’s geologists define geologic models extending from Chuquicamata mine to Exótica (Mina Sur) and Radomiro Tomic; also performed successful district exploration leading to the discovery of MM (1991) and Toki Cluster deposits (19982007); geologic investigation is carried out and several studies including structural geology, petrology, and mineralization-alteration characterization and isotopic dating are completed and published in several thesis, congress presentations, and papers

After a visit by Reno Sales in 1930 a potential of more the 200 Mt, averaging 1.75% Cu was discovered below leach capping west from the main oxide orebody; subsequently Anaconda geologists produced the first geologic models, including rockstructure distribution as well as alteration and mineralization; Anaconda’s geologic work resulted in the exploration and discovery of Pampa Norte (1952) and Exótica (1957), and also drilling of some holes at the northernmost part of the MMH deposit

In April, 1912 start of the first drilling campaign, including 60 holes totaling about 12,000 m; the deposit is described as an outcrop of about 2,400 by 1,500 m; a supergene profile containing upper leached zone, oxide, mixed and sulfide zones is described, including details on mineralogy of copper oxide, sulfides, and alteration; the program discovered a deposit containing 150 Mt of oxidized ore averaging 2.8 % Cu; a paper by Pope Yeatmen published in 1916 gives details on geology and a total reserve of 303 Mt averaging 2.2 % Cu, including 202 Mt of oxide ore averaging 1.95% Cu; a year later, these figures changed to a total ore reserve of 700 Mt, averaging 2.12 % Cu, including 339 Mt, averaging 1.91% Cu of oxide ore

A technical report by Carlos Avalos is published in 1901, and describes an area of 3,500 m north-south by 1,600 m east-west with numerous veins, and a central core of roughly 1,600 m by 500 m containing fractures filled with copper oxide and estimated average grade of about 3 % Cu; called “llamperas,” these are easy to mine, high grade, and can be treated by known metallurgical methods; similar description of deposits emphasizing the huge size is given in a report by Doming Silva in 1908; rock types and alteration-mineralization mineralogy is given by Alfredo Sundt in 1910

First technical reconnaissance report by Samuel Valdes; the mineralized area described with outcropping oxides covers 4,200 m north-south by 1,400 m east-west; an increase in grade and size is noted from north to south; high-grade mineralization occurs as “llamperas” or fine copper oxide-rich material

Geologic milestones

Ambrus (1975), Alvarez et al. (1980), Ossandón and Zentilli (1997), Zentilli et al. (1994b), Ossandón et al. (2001), Faunes et al. (2003)

Dunbar (1952), Perry (1991), Zentilli et al. (1994b)

Gandarillas (1915), Yeatman (1916, 1932), Parsons (1933), Dunbar (1952), Sutulov (1978), Zentilli et al. (1994b), Domic (2001), Millán (2006)

Gandarillas (1915), Martinez (1943), Millán (2006)

Valdés (1887), Martínez (1943), Gutierrez-Viñuales (2008), Millán (2006)

Reference

TABLE 1. Summary of Main Historic and Geologic Milestones in the Discovery and Development During More than a Century of Mining in the Chuquicamata District

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE

23

24

RIVERA ET AL. TABLE 2. Summary of Stratified and Intrusive Units in the Western and Eastern Blocks on Either Side of the West Fissure (the Table complements the map in Figure 3)

Western Block

Eastern Block

References

Quaternary age alluvial gravels interbedded with 10 Ma ashes

Tomlinson and Blanco (2007), García et al. (2009), Alcota et al. (2009), Rivera et al. (2009)

Loa Group gravels, ignimbrites and fine lake sediments. Miocene

May et al. (2005), Blanco (2007), Tomlinson and Blanco (2009), García et al. (2009)

Cluster Toki and MMH porphyries (38–35 Ma)

El Abra and Chuquicamata-Radomiro Tomic porphyries (36–35 Ma)

Dilles et al. (1997), Dilles (2008), Tomlinson and Blanco (2007), García et al. (2009), Alcota et al. (2009)

Sichal Formation ( continental clastic sedimentary rocks), middle Eocene-Oligocene in age

Calama Formation (continental clastic sedimentary rocks with lavas interbedded at the base). Eocene-Oligocene in age

Blanco (2007), Tomlinson and Blanco (2009), García et al. (2009)

Los Picos Diorite and Fortuna Granodiorite; middle Eocene-Lower Oligocene in age

Pajonal Diorite and El Abra Granodiorite Middle Eocene-Lower Oligocene in age

Dilles et al. (1997), Dilles (2008), Tomlinson and Blanco (2007), García et al. (2009)

Paleocene intrusions

García et al. (2009)

Cinchado Formation (reds beds and pyroclastic rocks) and Icanche Formation (andesitic and dacitic volcanic rocks interbedded with continental sedimentary rocks), Paleocene to Eocene in age

Tomlinson and Blanco (2007), García et al. (2009)

Cretaceous diorites and monzodiorites

Dilles et al. (1997), Dilles (2008), Tomlinson and Blanco (2007), García et al. (2009)

Quebrada Mala Formation (volcanic rocks overlain by sedimentary rocks), widely distributed in the southwestern part; Upper Cretaceous to Paleocene in age

Tomlinson and Blanco (2007), García et al. (2009)

Tolar Formation (continental sediments and interbedded acid pyroclastic rocks) unconformably overlying Quehuita Formation Upper Member); Upper Cretaceous in age

Tomlinson and Blanco (2007), García et al. (2009)

Tolar Formation, unconformably overlying Collahuasi Formation

Lower Cretaceous volcanics and continental sedimentary rocks Quehuita Formation (lower member composed by marine sediments; upper member containing transitional to continental sediments); Lower to Upper Jurassic in age

Tomlinson and Blanco (2007), García et al. (2009) Quehuita Formation only Lower Member, unconformably overlying Paleozoic basement units

Tomlinson and Blanco (2007), García et al. (2009)

Elena Granodiorite and N-S- to NE-trending dacitic-rhyolitic hypabyssal porphyry and dike swarms; Middle-Upper Triassic intrusions

Tomlinson and Blanco (2007), García et al. (2009), Alcota et al. (2009)

Middle-Upper Triassic volcanic and sedimentary rocks (Estratos de Quetena and Estratos de Chuquicamata, ex Agua Dulce Formation) including some calcareous sediments interbedded and conglomerates and sandstones containing material eroded from Paleozoic basement rocks

Tomlinson and Blanco (2007), García et al. (2009), Alcota et al. (2009)

Collahuasi Formation, andesites, rhyolites minor sedimentary rocks, Permian-Carboniferous in age

Tomlinson and Blanco (2007), García et al. (2009), Tomlinson et al. (2012)

Cerros de Chuquicamata Igneous Complex (granites, diorites, and migmatitic metadiorites); Sierra Limón Verde Igneous Complex (granites and migmatized amphibolitic meta-plutonic rocks); Permian-Carboniferous in age Sierra de Moreno Metamorphic Complex (probably Precambrian schist and amphibolites, migmatized and intruded by Ordovician-Silurian granites; main metamorphic event of probably Cambrian-Ordovician in age); unconformably covered by epimetamorphic sedimentary rocks

Limon Verde Metamorphic Complex Tomlinson and Blanco (2007), Tomlinson et al. (muscovite schist and amphibolites intruded by (2012), Morandé et al. (2012) banded and partly migmatized granites) PermianCarboniferous in age, affected by high-pressure amphibolite grade metamorphism of Permian age; recently discovered Neoproterozoic Diamictites

geologists were also responsible for the exploration and discovery in 1952 of the Radomiro Tomic deposit; its subsequent development was carried out in various stages by Anaconda and CODELCO (Perry, 1991; Cuadra and Camus, 1998), and it finally entered production in 1996. To date Radomiro Tomic has produced approximately 3.5 Mt Cu from oxide ore (Fig. 2). 0361-0128/98/000/000-00 $6.00

Tomlinson and Blanco (2007), García et al. (2009), Tomlinson et al. (2012)

Since the nationalization of the Chilean Cu industry in 1971, it has fallen to CODELCO to continue with the development and growth of the Chuquicamata district, which through a series of expansions has now reached a production level of 900,000 t Cu/yr (Table 1; Fig. 2). The first challenge, aside from maintaining production levels, was to generate a 24

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE

reserve inventory at Chuquicamata itself, which in 1975 stood at 1,400 Mt at 1.20 wt % Cu down to an elevation of 2,300 m above sea level (Ambrus, 1975), and which was sufficient to maintain production from the sulfide enrichment blanket until 1999. The generation of the geologic model for Chuquicamata and Exotica, together with exploration in the district, was the responsibility of teams lead by Jozef Ambrus, and later by Orlando Alvarez, Juan Pallauta, Guillermo Ossandón, and Roberto Fréraut. The work of these teams has been published in several papers, including Newberg (1965), Ambrus and Soto (1974), Münchmeyer and Urqueta (1974), Ambrus (1978, 1979), Alvarez et al. (1980), Alvarez and Flores (1985), Zentilli et al. (1995), Münchmeyer (1996), Sillitoe et al. (1996), Fréraut et al. (1997), Cuadra and Camus (1998), Lindsay (1998), Cuadra and Rojas (2001), Ossandón et al. (2001), Camus (2003), Faunes et al. (2005), and in a recent workshop by CODELCO in 2010. More detailed studies on the structure, petrogenesis, and age of Chuquicamata also have been published (Reutter et al., 1996; Zentilli et al., 1994a, b, 1995; Lindsay et al., 1995; Dilles et al., 1997; Reynolds et al., 1998; Maksaev and Zentilli, 1999; McInnes et al., 1999; Ballard et al., 2001; Campbell et al., 2006), with more detailed work contained in several academic theses and Chilean geologic congress presentations. New geologic information was provided by regional mapping completed by the Chilean Geological Survey (SERNAGEOMIN; Tomlinson and Blanco, 2007; Tomlinson et al., 2010), and by detailed mapping at 1:25,000 and 1:5,000 scales by CODELCO in the last 10 years (Alcota et al., 2009; García et al., 2009). In addition, over the past several decades, CODELCO’s teams have achieved a number of important milestones (Table 1; Fig. 2) that include managing the transition from supergene to hypogene ores, implementation of computerized resource estimation and grade control, overcoming geotechnical challenges, particularly close to the West Fissure, development and placing into production of Radomiro Tomic, discovery and development of Mina Ministro Hales in 1991 (Alvarez and Miranda, 1991; Alvarez, 1993) and, more recently, discovery of the Toki Cluster from 1998 to 2007 (Rivera and Pardo, 2004; Rivera et al., 2009). The mine geologists have continued to maintain a good quality geologic database and have completed studies necessary to meet the demands of production and medium- to long-term development. However, the greatest knowledge and understanding of the deposits has come from the logging of millions of meters of core and reverse circulation chips, from bench mapping, and from interpretation of maps and sections, and it is these activities that have produced the growth in resources that will carry the district forward. Few mines in the world have archived the great majority of their diamond drill core as has been done at Chuquicamata, an invaluable resource to develop deposit models and for modern research.

are hosted by pre-Cenozoic rocks, mainly Permo-Carboniferous and Triassic intrusions and volcanic or volcano-sedimentary units. This is the first advance in understanding the geologic framework of the district, which in recent years has brought into focus the role that these units and the basement architecture have played in the emplacement of the great Chuquicamata cluster of deposits (Tomlinson and Blanco, 2007; García et al., 2009; Tomlinson and Cornejo, 2012). Secondly, structural analysis has provided new insights into the geologic development of the district, especially the evolution of the main Incaic stage of deformation, which is coincident with formation of the porphyry Cu systems, but also postmineral deformation, which reached its maximum expression with development of the famous West Fissure (Lindsay et al., 1995; Dilles et al., 1997; Tomlinson and Blanco, 1997a, b, 2007; Lindsay, 1998). Thirdly, the relevance of syntectonic Tertiary sediments of the Calama Basin, and of sedimentary rocks deformed during the Incaic uplift further north, has now been recognized. These sedimentary deposits, with only rare associated volcanism, are evidence for the tremendous uplift that accompanied the synchronous emplacement of the porphyry Cu-Mo deposits (e.g., Maksaev and Zentilli, 1999). Fourthly, the evolution of the Cretaceous and Tertiary intrusions, which were emplaced as precursor complexes to the introduction of the porphyry intrusions, has been documented through a large number of isotopic ages, verifying the role that these intrusions played in the subsequent development of mineralization (Dilles et al., 1997, 2011; Barra et al., in press; Proffett et al., in prep.). The following synthesis of the geologic framework is summarized from recent 1:50,000 scale maps published by SERNAGEOMIN (Tomlinson and Blanco, 2007; Tomlinson et al., 2010) and by CODELCO in its 1:25,000 scale mapping (García et al., 2009; summarized in Fig. 3, Table 2). Due to its structural significance and to facilitate description, the area has been divided into Eastern and Western Blocks, on either side of the West Fissure (Fig. 3, Table 2). The basement, as defined by geologic mapping and isotopic ages, is exposed in several nuclei, the oldest of which corresponds to a metamorphic complex comprising a variety of schists and amphibolites in the southern part of the Sierra de Moreno, 30 km northwest of Chuquicamata (Fig. 3). These rocks are affected by migmatization and intruded by granites of probable Ordovician and Silurian age. Based on this and on isotopic ages (García et al., 2009), the protolith has been assigned a Precambrian age and the metamorphic event ascribed to the Cambrian to Ordovician (Tomlinson and Blanco, 2007). In Sierra Limón Verde, about 30 km south of Chuquicamata, a metamorphic complex dominated by muscovite schists and amphibolites is exposed (Fig. 3; Table 2), including the recently discovered matrix-supported conglomerates interpreted as Neoproterozoic diamictites (Morandé et al., 2012). The complex lacks migmatites but has undergone high-pressure amphibolites-grade metamorphism of Permian age (Tomlinson et al., 2012) and is intruded by banded and partly migmatized Permo-Carboniferous granites. Schists similar to those of Sierra Limón Verde are exposed in Sierra San Lorenzo, west of the Toki Cluster, suggesting the presence of this basement in the Western Block (Fig. 3). In the Sierra de Moreno complex, epimetamorphic sedimentary

Geologic Setting The age of mineralization of the porphyry Cu-Mo deposits in the district is Eocene to Oligocene, and the postore development and subsequent preservation of supergene enrichment, and of the systems themselves, have been key to their economic feasibility. All the mineral-related porphyry intrusions 0361-0128/98/000/000-00 $6.00

25

25

26

RIVERA ET AL.

FIG. 3. Chuquicamata district geologic setting (simplified from García et al., 2009). Legend and summary of main lithologic units are explained in Table 2. The West Fissure divides the Western and Eastern Blocks; south from Loa River this major structural zone is currently being interpreted to border Sierra Limón Verde. The El Abra porphyry Cu district and Atahualpa breccias and porphyry Cu prospect are also shown. 0361-0128/98/000/000-00 $6.00

26

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE

rocks unconformably overlie the lower Paleozoic units. These rocks continue to the south in the Atahualpa area and equivalent rocks have been recognized as some of the host rocks in the western part of the Toki Cluster. Plutonic and metaplutonic intrusive complexes, which form part of the basement in the Eastern Block, are exposed in the Cerros de Chuquicamata, east of the deposit. These comprise the Mesa granite, diorites, and abundant migmatitic metadiorites (Fig. 3; Table 2). Comparable rocks crop out in Sierra Limón Verde (Fig. 3), including granites similar to the Mesa granite, as well as migmatized, amphibolitic metaplutonic rocks (Tomlinson and Blanco, 2007; Tomlinson et al., 2012). The Permo-Carboniferous radiometric ages are similar, although the greater abundance of metaplutonic rocks and more intense foliation suggest a deeper level of exhumation in the Cerros de Chuquicamata. Volcanic rocks of the Collahuasi Formation (Tomlinson and Blanco, 2007; García et al., 2009; Tomlinson et al., 2012), which consist of volcanic and volcaniclastic rocks with PermoCarboniferous ages similar to those of the plutonic complexes, border these complexes in the Cerros de Chuquicamata and Sierra Limón Verde. These volcanic rocks overlie the Sierra San Lorenzo schists and constitute the host to the Toki Cluster porphyries. They also occur in Mina Ministro Hales and along the west side of Sierra Limón Verde, in the Western Block, and northeast of Radomiro Tomic in the Eastern Block (Fig. 3; Table 2). Middle to Upper Triassic sedimentary, volcanic, and intrusive rocks are widespread in both the Eastern and Western Blocks (Fig. 3). Conglomerates near the base contain material eroded from the Paleozoic complexes. Middle to Upper Triassic intrusions are mostly equigranular granodiorite plutons, which are intruded by Triassic dikes (Tomlinson and Blanco, 2007; García et al., 2009). The Jurassic is represented by the Quehuita Formation (Tomlinson and Blanco, 2007; García et al., 2009), composed of a calcareous marine lower member transitional to a continental sedimentary upper member. These rocks are exposed mainly in the south and west of the district. They overlie Triassic sedimentary rocks or granites in Sierra Limón Verde, and elsewhere they overlie various basement units in both blocks. They are absent near Mina Ministro Hales and in the northern parts of the Eastern Block, where the Jurassic was apparently eroded and Cretaceous sedimentary rocks directly overlie Triassic or Upper Paleozoic rocks (Fig. 3; Table 2). The Early Cretaceous, represented by lavas and continental sedimentary rocks, is found only in the central-western parts of the area. The Late Cretaceous is represented by continental sedimentary rocks, with interbedded felsic pyroclastic units of the Tolar Formation, which, north of Chuquicamata, unconformably overlies the Collahuasi Formation in the Eastern Block and the Quehuita Formation in the Western Block (Fig. 3). An Upper Cretaceous sequence of andesitic volcanic and sedimentary rocks correlated with the Quebrada Mala Formation is exposed in the southern and central part of the Western Block. Although some outcrops described northeast of Radomiro Tomic in the Eastern Block appear similar to the Quebrada Mala, Garcia et al. (2009) assigned the sequence to the Tolar Formation (Fig. 3; Table 2). 0361-0128/98/000/000-00 $6.00

27

In the Western Block (Fig. 3; Table 2), Lower Tertiary stratified units comprise red beds and pyroclastic rocks of the Cinchado Formation in the south and andesitic to dacitic volcanic and continental sedimentary rocks of the Icanche Formation in the north (Tomlinson and Blanco, 2007). These rocks conformably overlie the Tolar Formation and have Paleocene to Eocene isotopic ages. The mid-Eocene to Oligocene continental clastic sedimentary sequences of the Sichal Formation are considered to be syntectonic deposits related to the Incaic uplift and are commonly folded and faulted (Blanco and Tomlinson, 2006). In the Calama Basin, the south-central part of the Eastern Block, the lower part of the Calama Formation (May et al., 2005; N. Blanco, writ. commun., 2007; Tomlinson and Blanco, 2007) consists of conglomerates with lava interbeds near the base, similar in age to the Cinchado and Icanche Formations, ranging from 53 to 43 Ma. The middle and upper parts of the Calama Formation, of Eocene to Oligocene age, comprise syntectonic gravels of similar age to the Sichal Formation. These gravels are interpreted to have been generated during the Incaic uplift, and their age (Blanco et al, 2003; Tomlinson and Blanco, 2007) overlaps with emplacement of the porphyry Cu systems. The upper part of the Tertiary section, of Miocene age, is the El Loa Group, comprising gravels and fine-grained lake sediments (N. Blanco, writ. commun., 2007; Tomlinson and Blanco, 2007). Late Cenozoic alluvial gravels, containing ash beds dated at 10 Ma, are the youngest sediments in the district. Eocene intrusions are important in the economic geology of the district, as well as to the north at El Abra, both as precursor intrusions that preceded the porphyry Cu systems and as ore-related porphyries. In the Eastern Block at Chuquicamata and Radomiro Tomic, the Chuqui Porphyry Complex is the main intrusion of this age; it cuts Triassic or Permo-Triassic volcanic rocks and Triassic granodiorite. In the eastern part of the Western Block, similar porphyry intrudes similar host rocks at Mina Ministro Hales. Farther west, the Eocene Los Picos Diorite and Fortuna Granodiorite batholiths intrude Jurassic through Eocene sedimentary and volcanic rocks in the central and northern parts of the area. Associated latestage porphyries of the Toki Cluster, south of the main Fortuna batholith, intrude Paleozoic Collahuasi Formation volcanic rocks. Recently, mineralized porphyries that intruded into older schists, similar to those in the Sierra San Lorenzo, have been found south of the Opache deposit (Fig. 4). The most important structure in the district is the West Fissure, but important regional-scale reverse faults that control the uplift of Paleozoic and Triassic metamorphic and plutonic complexes are also present. These structures include N- to NNE−striking faults in the Sierra de Moreno and Sierra Limón Verde in the Western Block, and E-striking faults at Pampa Cere and Pampa Moctezuma in the Eastern Block (Fig. 3). Because the faults do not cross the West Fissure they are thought to be older. They place older rocks in faulted contact with Calama Formation gravels and may have formed during the Incaic uplift. Displacement on the West Fissure postdates deposition of the Calama and Icanche Formations but mostly predates deposition of the El Loa Group (Tomlinson and Blanco, 1997a, b). Field evidence, petrogenetic studies, and isotopic ages, 27

FIG. 4. Chuquicamata district bedrock interpreted geology, displaying main lithologic and structural units. A. Distribution of exposed bedrock areas (modified from Alcota et al., 2009) Lines of sections in Figure 5B-E are shown in the geologic map (B); age of units is the same as in Figure 3 and Table 3. UTM coordinate datum is Provisional South American 1956.

28

0361-0128/98/000/000-00 $6.00

RIVERA ET AL.

28

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE

discussed below, suggest left-lateral displacement on the West Fissure of as much as 35 km (R.C. Baker, writ. commun., 1969; Dilles et al., 1997; Tomlinson and Blanco, 1997a, b). While evidence for major strike-slip displacement is compelling, this interpretation has to date not been validated by discovery of the offset portion of the Chuquicamata deposit that should be in the Western Block, ~35 km to the south, and the offset portion of Mina Ministro Hales that should be in the Eastern Block, ~35 km to the north.

2003; Tomlinson and Blanco, 2007; Fig. 5D), are assigned to the Collahuasi and Agua Dulce Formations, rather than to a previously assigned Cretaceous unit. In the southwest of the area, in the Sierra Quetena (Fig. 4), a 2- to 20-m-thick unit of polymict conglomerate, containing clasts of Paleozoic muscovite schist and vein quartz is exposed. This unit had previously been interpreted as fill or gouge related to a thrust fault, but detailed mapping demonstrated that it is a basal conglomerate showing a transition upward into Triassic volcanic breccia and into the Quehuita Formation marine sedimentary rocks (this study; Tomlinson et al., 2010). The conglomerate and overlying volcanic breccia are named Quetena Strata (Tomlinson and Blanco, 2007) and based on their stratigraphic position and the isotopic age of the volcanic material (Alcota et al., 2009); the conglomerate has been assigned to the Permo-Triassic. It provides evidence for uplift and erosion of the Paleozoic basement prior to the mid-Triassic. Middle to Upper Triassic volcanic and sedimentary rocks previously assigned to the Agua Dulce Formation consist principally of andesitic flows and breccias, with interbeds of conglomerate, and sandstone, and with uncommon dacitic pyroclastic interbeds. Immediately east of Chuquicamata and Radomiro Tomic, calcareous strata and shale appear to be interbedded with the volcanic rocks that are assigned to the Triassic Cerros de Chuquicamata Strata by Tomlinson and Blanco (2007). Some Permian volcanic rocks may locally be included in this unit. Jurassic marine sedimentary rocks of the lower Quehuita Formation (Tomlinson and Blanco, 2007; Alcota et al., 2009) are exposed in the Sierra Quetena and around the Toki Cluster, and consist of limestones, siltstones, and shales, partially affected by contact metamorphism (Figs. 4, 5E). The upper member of the Quehuita Formation, comprising sandstones and siltstones, crops out in the central-north part of the area, where it is in erosional contact with the overlying Tolar Formation. The Tolar Formation, consisting of red beds, conglomerates (with Paleozoic clasts), and dacitic tuffs, is distributed throughout both the Western Block, where it overlies the upper member of the Quehuita Formation, and the Eastern Block, overlying Collahuasi Formation volcanic rocks; it is best exposed in the San Lorenzo anticline (Figs. 3, 4), and has been assigned a Late Cretaceous age based on its stratigraphic position and the isotopic ages of the tuffs (Tomlinson and Blanco, 2007). In the Cerro Negro area, west of Mina Ministro Hales, Collahuasi Formation volcanic rocks are unconformably overlain by sedimentary breccias, tuffs, conglomerates, and sandstones. These rocks are overlain by andesitic lavas and tuffs, with dacitic pyroclastic interbeds, which are contact metamorphosed by an adjacent Eocene pluton (Alcota et al., 2009), and have returned a 72.5 ± 3.1 Ma zircon age (Table 3; Tomlinson et al., 2010). These volcanic rocks and the underlying sedimentary rocks are correlated with the Quebrada Mala Formation. In the northern part of the Western Block, the Tolar Formation is conformably overlain by volcanic rocks assigned to the Icanche Formation, which comprise >500 m of andesitic lavas and pyroclastic rocks, with minor sandstone interbeds that are metamorphosed to quartzites in proximity to the

District Geology Earlier geologic understanding of the district was based on mapping at scales of 1:250,000 (Thomas, 1969; Marinovic and Lahsen, 1984) and 1:100,000 (Chong and Pardo, 1994). Subsequently, these maps were updated and reinterpreted with information from various sources and in the context of new ideas about the region’s geologic evolution; the first subsurface map of the district was completed by R. Pardo and S. Rivera (writ. commun., 1999). Between 2004 and 2009, CODELCO completed a multiscale mapping program whose objective was to further update the geology of the Chuquicamata district in order to assist the Corporation’s exploration and development activities. A large area was mapped at 1:50,000 scale in cooperation with SERNAGEOMIN (Tomlinson and Blanco, 2007; Tomlinson et al., 2010), and some of the area was mapped and compiled in more detail at 1:25,000 scale (García et al., 2009; Fig. 3). In addition, a 36-km north-south by 15-km east-west area around the mines was mapped at 1:5,000 scale (Alcota et al., 2009; Figs. 4, 5), which forms the basis for most of the following discussion. The more detailed mapping by CODELCO personnel used Tablet PCs and GVMapper © software, which allowed the generation of lithology, alteration, and mineralization maps. This program included contributions from consultants John Proffett, on topics related to geology of the ore deposits and their surroundings, and John Dilles, who undertook a study of the Fortuna and Los Picos batholiths. A semisystematic lithogeochemical investigation was also carried out. Approximately 60% of the area is covered by recent gravels, the so-called pampas, but with information provided by exploration drilling it has been possible to produce surface geologic maps as well as partially inferred subsurface maps (Fig. 4), and a set of 19 cross sections, with the final objective of producing three-dimensional models (Fig. 5). Stratified units The oldest stratified units in the vicinity of the ore deposits in the Western Block are volcanic rocks of the Permo-Carboniferous Collahuasi Formation. In the Toki deposit, these comprise more than 500 m of andesitic flows and dacitic to rhyolitic pyroclastic rocks, with intercalations of sedimentary breccias and sandstones, and with lutite interbeds toward the top (Fig. 4). In the deepest drilling at the Genoveva deposit (Figs. 4, 5A, E), a higher metamorphic grade and pronounced development of foliation are observed in these rocks. Upper Carboniferous to Permian ages of 304 to 274 Ma (Alcota et al., 2009) were determined for the Collahuasi Formation (Table 3; Tomlinson and Blanco, 2007; Alcota et al., 2009). Volcanic rocks at Mina Ministro Hales, which occur as structural slices and roof pendants in Triassic granodiorite and have isotopic ages of 300 to 230 Ma (Müller and Quiroga, 0361-0128/98/000/000-00 $6.00

29

29

FIG. 5. Chuquicamata district bedrock interpreted mineralization. A. Geologic and mineralization sections interpreted from surface and drill hole data. B. through E. Modified from Alcota et al. (2009). Rock units, colors, and symbols same as in Figure 4. Compare the host-rock and structural controls of mineralization along the West Fissure, especially at Radomiro Tomic, Chuquicamata, and Mina Ministro Hales. Note the clustered nature of porphyry Cu deposits in the Toki area west of Mina Ministro Hales. In the Toki Cluster, oxide Cu mineralization occurs in proximity to the top of bedrock. Most zones denominated “sparse, partly oxidized sulfides and minor leached capping” contain limonite and/or Cu oxide minerals after only trace sulfides, and those shown as “pyrite dominant” contain only trace pyrite; however, both include smaller areas with more abundant pyrite and/or limonite after pyrite.

30

0361-0128/98/000/000-00 $6.00

RIVERA ET AL.

30

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE

Eocene plutons. The Icanche Formation, dated at 51 ± 2.4 Ma by K-Ar on hornblende (Tomlinson et al., 2001), is cut by numerous andesite porphyry dikes and locally affected by advanced argillic alteration. This combination of diking and alteration, together with the volcanic facies, is suggestive of a partly preserved lower to middle Eocene volcanic edifice, whose equivalent-age intrusions are partly exhumed farther west. The intrusions predate the major uplift event and deposition of the syntectonic gravels coincident with emplacement of the porphyry Cu systems (Blanco and Tomlinson, 2006). An important objective of the 1:5,000 scale mapping was to understand the geology of the Calama Basin. The study was carried out by N. Blanco (writ. commun., 2007), with some of the results reported by Tomlinson and Blanco (2007). The Calama Formation is the most relevant unit for understanding the link between district stratigraphy and porphyry emplacement (May et al., 1999, 2005; Blanco et al., 2003; Tomlinson and Blanco, 2007; N. Blanco, writ. commun., 2007). The formation is exposed in several small hills in the Eastern Block and in holes drilled in the eastern part of the Mina Ministro Hales deposit but has not been found in the Western Block within the area studied. It consists of a lower unit (Topater Member) of conglomerate with interbedded andesitic lavas and laharic breccias and an upper unit of conglomerates (Chorrillos Member). The base of the unit on Cerro Milagro, ~15 km southeast of Chuquicamata, is marked by conglomerate containing clasts of locally derived Mesa Granite, but in general, most fragments are of volcanic and intrusive rocks similar to the Eocene plutons and volcanic units exposed in the Western Block. Isotopic ages of lavas interbedded with the Topater Member range from 51 to 47 Ma (Tomlinson and Blanco, 2007; García et al., 2009). An angular clast with advanced argillic alteration from conglomerate at Loma Negra (3 km SE of Mina Ministro Hales, Fig. 4) returned a hypogene alunite Ar-Ar age of 31.5 ± 1.1 Ma (Table 3; Alcota et al., 2009; Proffett et al., in prep.). At Mina Ministro Hales, more than 500 m of conglomerates correlated with the Calama Formation are in fault contact with mineralized rocks to the west across the West Fissure (Alvarez and Miranda, 1991; S. Rivera, unpub. data, 1995; Sillitoe et al., 1996; Müller and Quiroga, 2003). Deposition of the Calama Formation spans the time of porphyry Cu mineralization, and although deposition was not necessarily continuous, the conglomerates and the lack of volcanic units in all but the lower part are strongly suggestive of active tectonic uplift during a time of little or no contemporaneous volcanism. It can be concluded that there is a direct temporal and spatial relationship between uplift, erosion, and deposition of these syntectonic sediments and emplacement of the porphyry Cu systems of the Chuquicamata district. Similar relationships have been noted by Perelló et al. (2011) at Telégrafo and other deposits of the Sierra Gorda district, Hervé et al. (2013) in the Escondida district, and Sillitoe and Perelló (2005) for other Eocene to Oligocene porphyry Cu deposits associated with the Domeyko fault system, of which the West Fissure is a part. Gravels and fine-grained sedimentary rocks, including lacustrine limestones of the El Loa Group, overlie the Calama Formation in the Eastern Block and Mesozoic and Paleozoic rocks in the Western Block. The Group consists of two units: 0361-0128/98/000/000-00 $6.00

31

the lower Jalquinche Formation, comprising mudstones, sandstones, and gypsum beds, with intercalated tuffs at the base and top, dated at 16.3 to 9.6 Ma (Tomlinson and Blanco, 2007; Alcota et al., 2009); and the upper Opache Formation, comprising limestones and lacustrine sediments, with intercalated tuffs dated at 8 to 3 Ma. These units represent the final stages of deposition in the Calama Basin and mostly postdate the last movements on the West Fissure. Intrusive complexes The oldest intrusive rocks in the area are those of the Cerros de Chuquicamata plutonic and metaplutonic complex (Figs. 3, 4; Alcota et al., 2009), which is made up of NNE−oriented belts that decrease in age from east to west. The oldest member of the complex is the Mesa granite suite, comprising foliated granites, syenogabbros, and monzogranites dated at 305 to 295 Ma (Tomlinson and Blanco, 2007; Tomlinson et al., 2010). These are intruded by diorites, quartz diorites, and metadiorites, also with marked foliation. The youngest phase is the Elena (or Este) granodiorite, dated at 238 to 229 Ma (Tomlinson and Blanco, 2007; Tomlinson et al., 2010; Proffett et al., in prep.), which occurs as a northeast elongate body. It intrudes the diorites on its eastern side and is in contact with Triassic limestone and volcanic rocks (Cerros de Chuquicamata Strata, previously assigned to the Agua Dulce Formation; Tomlinson and Blanco, 2007) to the west across the Mesabi fault. A second body of Elena granodiorite intrudes the western side of the Triassic volcanic rocks west of the Mesabi fault and occurs as wall rock on both the eastern and western sides of the Chuqui Porphyry Complex. It appears to be the southwestern end of a large body of Triassic granodiorite, which occurs northeast of Radomiro Tomic. These granodiorites are themselves cut by Triassic dacitic to rhyolitic hypabyssal bodies, mainly dikes that strike N-NE to nearly E-W. The Elena granodiorite in the Eastern Block is truncated to the southwest by the West Fissure, but similar intrusions have been recognized farther south as wall rock in Mina Ministro Hales and eastern parts of the Toki Cluster in the Western Block (Figs. 4, 5E). In Mina Ministro Hales, the Elena intrusion was dated at 234 to 211 Ma and is crosscut by dacitic dikes similar to those mentioned above, dated at 238 to 222 Ma (Müller and Quiroga, 2003). A similar granodiorite at the Miranda deposit was dated at 229 ±2 Ma (Table 3; Proffett et al., in prep.). Petrography, radiometric ages, and the relationship with hypabyssal intrusions all support correlation of the Elena granodiorite with the granodiorites to the south across the West Fissure (Table 3). Recent petrogenetic studies of these granodiorites indicate adakitic affinities, similar to, but less pronounced than, those of the Eocene to Oligocene porphyries (J. Proffett, unpub. data, 2007; Wilson et al., 2011). Triassic granodiorites differ from Chuqui Porphyry Complex rocks in lacking significant amounts of sphene (J. Proffett, unpub. data, 2005). In the Western Block, from the Cretaceous onward, a succession of plutons was emplaced that decrease in age from west to east (Figs. 3, 5D, E). The oldest of these are monzonites and granodiorites, dated at 69 to 63 Ma (Ballard et al., 2001; Campbell et al., 2006), which cut sedimentary rocks of the upper member of the Quehuita Formation, producing contact metamorphism and metasomatism. These intrusions 31

32

RIVERA ET AL. TABLE 3. Isotopic Ages for Samples from the Chuquicamata District (area same as map of Fig. 4),

UTM North

UTM East

Place

Lithology

7536250 7536750 7537300 7536400 7536500 7536200 7536750 7536400 7538500 7535900 7535750 7535616 7535787 7536133 7535787 7535892 7536741 7536741 7534326 7534326 7535750 7535750 7535750 7535750 7535750 7535750 7536314 7535080 7535080 7535080 7533750 7533750 7533750 7537995 7535175 7535175 7534636 7534821 7521700

511000 510900 510900 511200 511650 510500 510900 511200 509900 506900 510600 509543 509868 510035 509868 509707 509660 509660 509770 509770 510600 510600 510600 510600 510600 510600 510367 509917 509917 509917 507680 507680 507680 512155 510197 510197 510119 510231 503200

Chuquicamata mine Chuquicamata mine Chuquicamata mine Chuquicamata mine Chuquicamata mine Chuquicamata mine Chuquicamata mine Chuquicamata mine Northwest from Chuquicamata Sierra San Lorenzo Chuquicamata Chuquicamata Chuquicamata Chuquicamata Chuquicamata Chuquicamata Chuquicamata, bench C2 (1993) Chuquicamata, bench C2 (1993) Chuquicamata, bench I1 (1993) Chuquicamata, bench I1 (1993) Chuquicamata, DDH1843-157,30-157,50m Chuquicamata, DDH1843-157,30-157,50m Chuquicamata, DDH1843-37,03-37,10m Chuquicamata, DDH2234 and DDH2242 Chuquicamata, DDH2234-219,84m Chuquicamata, DDH2242-149,60m Chuquicamata, DDH2967-576,98m Chuquicamata, DDH3472-224,33m Chuquicamata, DDH3472-602,60m Chuquicamata, hole DDH3472-322,20m North from Chuquicamata town North from Chuquicamata town North from Chuquicamata town Cerros de Chuquicamata Chuquicamata mine, CHDD 7202-1, 443,3 m Chuquicamata mine, CHDD 7202-2, 886.5 m Chuquicamata mine, CHDD 7251, 540,5 m Chuquicamata mine, CHDD 7522, 728.3 m Genoveva, AD-966, 246 m

Granodiorite porphyry Sericite from vein halo Potassic altered granodiorite porphyry Potassic altered granodiorite porphyry Granodiorite Quartz sericite rock Sericite from vein halo Granodiorite porphyry Granodiorite Granodiorite Sericitic altered granodiorite porphyry Aplite porphyry Granodiorite Granodiorite Granodiorite Granodiorite Granodiorite Granodiorite Granodiorite Granodiorite Sericitic superimposed to potassic altered granodiorite porphyry Sericitic superimposed to potassic altered granodiorite porphyry Sericitic superimposed to potassic altered granodiorite porphyry Sericitic superimposed to potassic altered granodiorite porphyry Sericitic superimposed to potassic altered granodiorite porphyry Sericitic superimposed to potassic altered granodiorite porphyry Sericitic altered granodiorite porphyry Sericitic altered granodiorite porphyry Sericitic altered granodiorite porphyry Potassic altered granodiorite porphyry Granodiorite Granodiorite Granodiorite Granodiorite Quartz-molybdenum vein Quartz-molybdenum vein Quartz-molybdenum vein Quartz-molybdenum vein Quartz-molybdenum-chalcopyrite vein

7520699 7520900 7520478 7517100 7517500 7517507 7521799

506670 506800 507079 503524 503408 503652 504099

Miranda, DDH 2565, 393,5 m Miranda, DDH 2569, 782.7 m Miranda, DDH2550, 636.5 m Opache, DDH 396, 441.0 m Opache, DDH 430, 333.0 m Opache, DDH 432, 363,0 m Quetena project, AD-1185, 277.3 m

Quartz-moybdenite vein Quartz-molybdenum-chalcopyrite vein Molybdenite-chalcopyrite vein Moybdenite-chalcopyrite breccia Moybdenite vein Moybdenite-chalcopyrite breccia Molybenite vein in tonalite

7522000 7525570 7525570 7525570 7525570 7525570 7525570 7525570 7525570 7525570 7517301 7517100 7517300 7521795 7520599 7521405 7520994 7520401 7521002 7521256

504400 509060 509060 509060 509060 509060 509060 509060 509060 509060 503660 503604 503059 504809 505529 505287 505779 505899 505490 505506

Quetena, DDH 1163, 499.5 m MMH MMH MMH MMH MMH MMH MMH MMH MMH Opache-DDH AD310, 305 m depth Opache-DDH AD396, 294 m depth Opache-DDH AD427, 202 m depth Quetena-DDH AD1707, 314 m depth Toki-DDH AD1132, 597 m depth Toki-DDH AD1174, 544 m depth Toki-DDH AD1191, 297 m depth Toki-DDH AD945, 7 m depth Toki-DDH DDH870, 378 m depth Toki-DDH DDH872, 273 m depth

Tonalite porphyry Granodiorite Granodiorite Dacite Dacite Porphyry Quartz eye porphyry Porphyry Porphyry Advanced argillic altered rock Porphyry Igneous breccia Andesite Sericitized dacite Chalcopyrite-bornite mineralized porphyry Sericitized dacite Sericitized dacite Ash tuff Sericite altered tonalite Potassic altered porphyry

0361-0128/98/000/000-00 $6.00

32

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE Ordered Alphabetically by Reference (UTM datum is Provisional South American 1956) Geological Unit

Method

Material dated

Age (Ma) and error

Reference

Banco porphyry East Porphyry West porphyry East Porphyry Elena granodiorite East Porphyry East Porphyry East Porphyry Fiesta granodiorite Antena Granodiorite Chuquicamata Porphyry Complex Tetera porphyry Fiesta granodiorite Fiesta granodiorite Fiesta granodiorite Fiesta granodiorite Fiesta granodiorite Fiesta granodiorite Fiesta granodiorite Fiesta granodiorite Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Fiesta granodiorite Fiesta granodiorite Fiesta granodiorite Not described East Porphyry East Porphyry East Porphyry East Porphyry Metandesite with moderate chloritediorite alteration Tonalite porphyry Tonalite porphyry Tonalite porphyry Igneous breccia Tonalite porphyry Igneous breccia Fortuna Granodiorite with minor biotite-chlorite alteration Tonalite porphyry MM Granodiorite MM Granodiorite Dacite dykes Dacite dykes MM Porphyry MM Porphyry MM Porphyry MM Porphyry Not described San Lorenzo Porphyry San Lorenzo related igneous breccias Estratos de Quetena Collahuasi, rhyolite-dacite facies San Lorenzo Porphyry Collahuasi, rhyolite-dacite facies Collahuasi, rhyolite-dacite facies Artola ignimbrite Fiesta granodiorite San Lorenzo Porphyry

K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar U-Pb ELA-ICP-MS Re-Os Re-Os Re-Os Re-Os

Biotite Sericite Biotite Biotite Biotite Sericite Sericite Biotite Biotite Biotite Sericite K-feldspar K-feldspar K-feldspar Hornblende/biotite Hornblende K-feldspar Biotite Biotite Hornblende K-feldspar Biotite K-feldspar K-feldspar Biotite Biotite Sericite Sericite Sericite Sericite K-feldspar Biotite Hornblende Zircon Molybdenite Molybdenite Molybdenite Molybdenite

33.8 ± 1.3 31.0 ± 1.2 32.6 ± 0.9 34.4 ± 0.9 125 ± 3.8 28.0 ± 1.1 31.2 ± 1.3 35.7 ± 1.9 38.9 ± 3.0 36.2 ± 1.4 31.7 ± 0.7 35.5 ± 0.4 35.5 ± 0.5 35.8 ± 0.5 36.0 ± 0.3 37.2 ± 0.5 35.0 ± 0.4 36.9 ± 0.5 36.4 ± 0.6 37.9 ± 2.0 31.5 ± 0.6 31.7 ± 0.4 31.8 ± 0.3 31.1 ± 0.7 31.3 ± 0.3 31.1 ± 0.2 31.2 ± 0.2 31.1 ± 0.3 31.1 ± 0.3 30.9 ± 0.3 35.5 ± 0.4 36.4 ± 0.4 37.2 ± 1.1 37.7 ± 1.0 32.0 ± 0.2 32.0 ± 0.2 32.0 ± 0.2 32.0 ± 0.2

Alvarez et al. (1980) Ambrus (1979) Ambrus (1979) Ambrus (1979) Ambrus (1979) Ambrus (1979) Ambrus (1979) Ambrus (1979) Ambrus (1979) Ambrus (1979) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Arnott (2003) Ballard et al. (2001) Barra et al. (in press) Barra et al. (in press) Barra et al. (in press) Barra et al. (in press)

Re-Os Re-Os Re-Os Re-Os Re-Os Re-Os Re-Os

Molybdenite Molybdenite Molybdenite Molybdenite Molybdenite Molybdenite Molybdenite

38.1 ± 0.2 36.0 ± 0.2 36.5 ± 0.2 36.2 ± 0.2 37.6 ± 0.2 36.8 ± 0.2 36.4 ± 0.2

Barra et al. (in press) Barra et al. (in press) Barra et al. (in press) Barra et al. (in press) Barra et al. (in press) Barra et al. (in press) Barra et al. (in press)

Re-Os U-Pb U-Pb U-Pb U-Pb U-Pb U-Pb U-Pb 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar K-Ar K-Ar K-Ar U-Pb ELA-ICP-MS Re-Os U-Pb ELA-ICP-MS U-Pb ELA-ICP-MS K-Ar K-Ar K-Ar

Molybdenite Zircon Zircon Zircon Zircon Zircon Zircon Zircon Secondary biotite K-feldespar Primary alunite Biotite Whole rock Whole rock Zircon Molybdenite Zircon Zircon Biotite Whole rock Biotite

38.4 ± 0.2 38.6 ± 0.7 233.5 ± 1.5 211.0 ± 6,6 237.6 ± 0,4 222.0 ± 4.9 38.9 ±0.4 35.53 ±0.8 34.33 ± 0.2 32.08 ± 0.15 31.41± 0.15 36.9 ± 1.0 38.0 ± 1.5 37.9 ± 1.2 273.5 ± 11.0 48.2 ± 0.2 278.5 ± 9.0 279.0 ± 9.0 9.6 ± 0.4 37.5 ± 1.7 37.3 ± 1.3

Barra et al. (in press) Barra et al. (in press) Bertens (in Boric, pers. commun., 2005) Bertens (in Boric, pers. commun., 2005) Bertens (in Boric, pers. commun., 2005) Bertens (in Boric, pers. commun., 2005) Bertens (in Boric, pers. commun., 2005) Bertens (in Boric, pers. commun., 2005) Bertens (in Boric, pers. commun., 2005) Bertens (in Boric, pers. commun., 2005) Bertens (in Boric, pers. commun., 2005) Codelco (unpub. information) Codelco (unpub. information) Codelco (unpub. information) Codelco (unpub. information) Codelco (unpub. information) Codelco (unpub. information) Codelco (unpub. information) Codelco (unpub. information) Codelco (unpub. information) Codelco (unpub. information)

0361-0128/98/000/000-00 $6.00

33

33

34

RIVERA ET AL. TABLE 3.

UTM North

UTM East

Place

7521256 7521256 7521301

505506 505506 503571

Toki-DDH DDH872, 290 m depth Toki- DDH872, 327 m depth Genoveva-DDH AD1012, 286 m depth

Andesite Sericite altered porphyry Sericitized dacite

7521701

503324

Genoveva-DDH AD966, 213 m depth

Meta sandstone

7521996

504346

Quetena-DDH AD1159, 491 m depth

Potassic altered porphyry

7521996

504136

Quetena-DDH AD1163, 424 m

Sericitic and potassic altered granodiorite porphyry

7521797

503958

Quetena-DDH AD1185, 239 m depth

Sericite altered tonalite

7522196

504856

Quetena-DDH AD1705, 294 m depth

Altered porphyry

7520586

505624

Toki-DDH AD1005A, 614 m depth

Chalcopyrite-pyrite mineralized porphyry

7520196

506045

Toki-DDH AD1123, 435 m depth

Chalcopyrite mineralized porphyry

7519803

505859

Toki-DDH AD1157A, 832 m depth

Chalcopyrite-bornite mineralized porphyry

7521794

505541

Toki-DDH AD1162, 590 m depth

Hydrothermal breccia

7537840 7535730 7530880 7538500 7536560 7538200 7536237 7536237 7529060 7528616 7528616 7537312 7536237 7530484

508790 506850 510880 513000 511850 512150 510732 510732 503450 501945 501945 512592 510732 515018

Granodiorite Granodiorite Ash Granodiorite Ash tuff Granodiorite Potassic altered granodiorite porphyry Potassic altered granodiorite porphyry Foliated granodiorite Granodiorite Granodiorite Granodiorite Granodiorite porphyry Diorite

7529060 7532398 7533935 7535318 7535750 7535750 7535750 7535750 7520288 7536646 7536646 7536625 7536625 7536666 7536710 7536710 7536369 7536459 7536722 7536722 7536829 7536917 7536634 7525570 7525570 7525570 7525570 7525570 7525570 7525570 7525570

503450 502432 502575 503720 510600 510600 510600 510600 523729 510455 510455 510694 510694 510336 510790 510790 510235 510240 510954 510954 509979 509305 511649 509060 509060 509060 509060 509060 509060 509060 509060

North west from Chuquicamata Sierra San Lorenzo Mina Sur Cerros de Chuquicamata Chuquicamata mine Chuquicamata mine Chuquicamata mine Chuquicamata mine SW de Chuquicamata SW de Chuquicamata SW de Chuquicamata 1 km east from Chuquicamata (3150m) Chuquicamata (2580m) Cobrizo hill south east from Chuquicamata (2600m) West from Chuquicamata (2750m) West from Chuquicamata (2970m) West from Chuquicamata (3080m) West from Chuquicamata (3270m) Chuquicamata Chuquicamata Chuquicamata Chuquicamata South from Salar de Talabre- hole SE4, 28m depth Chuquicamata (1996 m) Chuquicamata (1996 m) Chuquicamata (2190 m) Chuquicamata (2190 m) Chuquicamata (2210 m) Chuquicamata (2372 m) Chuquicamata (2372 m) Chuquicamata (2415 m) Chuquicamata (2580 m) Chuquicamata (2610 m) Chuquicamata (2610 m) Chuquicamata (2770 m) Chuquicamata (2980 m) Chuquicamata (3010 m) MMH MMH MMH MMH MMH MMH MMH MMH

0361-0128/98/000/000-00 $6.00

Lithology

34

Granodiorite Granodiorite Granodiorite Biotite-hornblende granodiorite porphyry Sericitic altered granodiorite porphyry Molybdenite vein Molybdenite vein Potassic altered granodiorite porphyry Ash tuff Granodiorite porphyry Granodiorite porphyry Granodiorite porphyry Granodiorite porphyry Granodiorite Granodiorite porphyry Granodiorite porphyry Granodiorite Granodiorite Granodiorite porphyry Granodiorite porphyry Granodiorite Granodiorite Granodiorite porphyry MMH Porphyry Upper high sulfidation zone Porphyry Porphyry MMH Porphyry Upper high sulfidation zone Porphyry Porphyry

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE

35

(Cont.) Method

Collahuasi formation andesite facies San Lorenzo Porphyry Collahuasi Formation

K-Ar K-Ar Re-Os

Whole rock Whole rock Molybdenite

35.0 ± 1.2 37. 6 ± 1.5 38.0 ± 0.2

Collahuasi Formation

Re-Os

Molybdenite

38.2 ± 0.2

San Lorenzo Porphyry

Re-Os

Molybdenite

38.3 ± 0.2

San Lorenzo Porphyry

U-Pb ELA-ICP-MS

Zircon

34.6 ± 1.5

San Lorenzo Porphyry

Re-Os

Molybdenite

38.4 ± 0.2

San Lorenzo Porphyry

Re-Os

Molybdenite

38.0 ± 0.2

San Lorenzo Porphyry

Re-Os

Molybdenite

37.7 ± 0.2

San Lorenzo Porphyry

Re-Os

Molybdenite

37.9 ± 0.2

San Lorenzo Porphyry

Re-Os

Molybdenite

37.9 ± 0.2

San Lorenzo related igneous breccias

Re-Os

Molybdenite

38.1 ± 0.2

Fiesta granodiorite Antena Granodiorite Sifon ignimbrite Elena granodiorite Mio-Pliocene gravels Elena granodiorite East Porphyry East Porphyry Granodiorite Antena Antena Granodiorite Antena Granodiorite Elena granodiorite East porphyry Cerros de Chuquicamata metaplutonic complex Antena Granodiorite Los Picos Diorite Los Picos Diorite San Lorenzo Porphyry Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Opache Formation Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Fiesta granodiorite Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Fiesta granodiorite Fiesta granodiorite Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Fiesta granodiorite Fiesta granodiorite Elena granodiorite MMH deposit MMH deposit MMH deposit MMH deposit MMH deposit MMH deposit MMH deposit MMH deposit

U-Pb TIMS U-Pb TIMS K-Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar K-Ar Fission track Fission track

Zircon Zircon Biotite Hornblende Amphibole Organic material K-feldspar Biotite Biotite Biotite Biotite Apatite Apatite

37.6 ± 0.7 39.3 ± 0.4 8.6 ± 0.4 204 ± 3 2.8 ± 0.7 30.8 ± 1.3 31.4 ± 0.2 31.7 ± 0.4 36.3 ± 0.6 37.1 ± 0.5 39.0 ± 1.2 32.7 ± 4.4 30.2 ± 4.4

Codelco (unpub.information) Codelco (unpub. information) Codelco (unpub. information); Barra et al. (in press) Codelco (unpub. information); Barra et al. (in press) Codelco (unpub. information); Barra et al. (in press) Codelco (unpub. information); Barra et al. (in press) Codelco (unpub. information); Barra et al. (in press) Codelco (unpub. information); Barra et al. (in press) Codelco (unpub. information); Barra et al. (in press) Codelco (unpub. information); Barra et al. (in press) Codelco (unpub. information); Barra et al. (in press) Codelco (unpub. information); Barra et al. (in press) Dilles et al. (1997); Tomlinson et al. (2001) Dilles et al. (1997); Tomlinson et al. (2001) Hunt ( in Münchmeyer and Urqueta.1974) Lindsay (1998) Lindsay (1998) Lindsay (1998) Maksaev (1990) Maksaev (1990) Maksaev (1990) Maksaev (1990) Maksaev (1990); Boric et al. (1990) Maksaev and Zentilli (1999) Maksaev and Zentilli (1999)

Fission track Fission track Fission track Fission track Fission track Re-Os Re-Os Re-Os Re-Os 40Ar/39Ar (U-Th)/He Fission track (U-Th)/He Fission track (U-Th)/He Fission track (U-Th)/He (U-Th)/He (U-Th)/He (U-Th)/He Fission track (U-Th)/He (U-Th)/He (U-Th)/He 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar

Apatite Apatite Apatite Apatite Apatite Pyrite Molybdenite Molybdenite Chalcopyrite Biotite Apatite Apatite Apatite Apatite Apatite Apatite Apatite Apatite Apatite Apatite Apatite Apatite Apatite Apatite Alunite Alunite Sericite Biotite Alunite Alunite Sericite Biotite

35.4 ± 5.2 30.3 ± 3.6 32.0 ± 3.8 30.8 ± 3.6 37.1 ± 3.6 31 ± 2 31.7 ± 0.2 32.2 ± 0.2 ca. 33 7.82 ± 0.10 26.8 ± 2.68 28.0 ± 5.4 30.6 ± 3.08 33.3 ± 4.8 17.1 ± 1.88 29.7 ± 4.4 35.27 ± 3.6 19.2 ± 1.8 30.1 ± 3.8 30.8 ± 3.08 32.0 ± 5.6 33.6 ± 4.6 30.1 ± 5.6 31.0 ± 1.4 31.4 31.8 32.5 34.3 31.4 31.8 32.5 34.3

Maksaev and Zentilli (1999) Maksaev and Zentilli (1999) Maksaev and Zentilli (1999) Maksaev and Zentilli (1999) Maksaev and Zentilli (1999) Mathur et al. (2000) Mathur et al. (2000) Mathur et al. (2000) Mathur et al. (2000) May et al. (1999); May et al. (2005) McInnes et al. (1999) McInnes et al. (1999) McInnes et al. (1999) McInnes et al. (1999) McInnes et al. (1999) McInnes et al. (1999) McInnes et al. (1999) McInnes et al. (1999) McInnes et al. (1999) McInnes et al. (1999) McInnes et al. (1999) McInnes et al. (1999) McInnes et al. (1999) McInnes et al. (1999) Müller and Quiroga (2003) Müller and Quiroga (2003) Müller and Quiroga (2003) Müller and Quiroga (2003) Müller and Quiroga (2003) Müller and Quiroga (2003) Müller and Quiroga (2003) Müller and Quiroga (2003)

0361-0128/98/000/000-00 $6.00

Material dated

Age (Ma) and error

Geological Unit

35

Reference

36

RIVERA ET AL. TABLE 3.

UTM North

UTM East

Place

7525570 7537000 7538202 7522398

509060 511000 512093 504500

MMH Chuquicamata Carmen mine northeast from Chuquicamata Quetena Prospect

Granodiorite Blue molybdenite vein Granodiorite porphyry Tonalite

7521109

506986

Miranda

Granodiorite

7538670 7538580 7535776 7534572 7536186 7535030 7536314 7536158 7536158 7536642 7536578 7536642 7536184

512498 512250 510559 509896 510642 510161 510367 510978 510978 511011 511203 511011 511298

Chuquicamata, Carmen sector Chuquicamata, Carmen sector Chuquicamata Chuquicamata Chuquicamata Chuquicamata Chuquicamata Chuquicamata Chuquicamata Chuquicamata Chuquicamata Chuquicamata Chuquicamata

7536184

511298

Chuquicamata

7536184

511298

Chuquicamata

7538301 7538301 7538301 7538012 7538012 7521405 7521405 7537282 7537282 7537282 7537282 7534546 7536852 7525568 7525568 7525570 7525570 7529969 7516199 7515721 7517442 7520380 7520303 7520304 7537560 7519438 7520390 7521802 7521394 7522248 7518024 7521260 7521780 7521780 7526057 7538980 7521200

512391 512391 512391 512304 512304 505287 505287 510675 510675 510675 510675 510301 510331 509037 509037 509060 509060 500800 510520 510750 510975 501205 501398 501370 515330 503412 502890 503459 503579 501743 504810 516960 518001 518001 509503 505278 501860

North east from Chuquicamata pit North east from Chuquicamata pit North east from Chuquicamata pit North east from Chuquicamata pit North east from Chuquicamata pit Toki Toki Chuquicamata, bench D-4 Chuquicamata, bench D-4 Chuquicamata, bench D-4 Chuquicamata, bench D-4 Chuquicamata, bench G-2 Chuquicamata, bench G-3 MMH, exploration shaft (55 m depth) MMH, exploration shaft (93 m depth) MMH MMH Andacollo mine Calama hill Calama hill Calama hill Cerro Quetena eastern flank Cerro Quetena eastern flank Cerro Quetena eastern flank Cerros de Chuquicamata East from Cerro Quetena Genoveva Genoveva-DDH AD1209, 840,19 m depth Genoveva-DDH AD1736, 600,10 m depth Genoveva hill La Cruz hill Milagro hill Milagro hill Milagro hill MMH-DDH DD-5549-157,9 m depth Northeast from Cerro Aralar Northeast from Cerro Quetena

Granodiorite Granodiorite Sericitic superimposed to potassic altered granodiorite porphyry Sericite halo vein Quartz-sericite altered rock Sericitic superimposed to potassic altered granodiorite porphyry Quartz-sericite altered rock Sericitic superimposed to potassic altered granodiorite porphyry Sericitic superimposed to potassic altered granodiorite porphyry Potassic altered granodiorite porphyry Potassic altered granodiorite porphyry Potassic altered granodiorite porphyry Chlorite-sericite superimposed to potassic altered granodiorite porphyry Chlorite-sericite superimposed to potassic altered granodiorite porphyry Chlorite-sericite superimposed to potassic altered granodiorite porphyry Mylonite in granodiorite Mylonite in granodiorite Mylonite in granodiorite Potassic altered granodiorite porphyry Potassic altered granodiorite porphyry Tonalite Sericite halo of D vein Top of supergene blanket in altered porphyry Leach capping Sericitized and antlerite mineralized porphyry Sericitized and antlerite mineralized porphyry Leach capping Leach capping Leach capping Leach capping Alunite related to enargite mineralization Alunite related to enargite mineralization Monzodiorite Andesite clast Andesite Dacitic ignimbrite clast Volcaniclastic andesite Sedimentary breccia Potassic altered andesite Microcline granite Andesitic porphyry Rhyolite tuff Amphibolite Amphibolite Monzonite Rhyolite Andesite clast Andesite Andesite Quartz diorite clast Monzodiorite Andesitic porphyry

7521070 7529090

502000 507261

North east from Cerro Quetena North from Cerro Negro

Potassic altered porphyry Meta andesite in mylonitic zone

7518072 7518090

500687 500411

North from Rio San Salvador North from Rio San Salvador

Monzodiorite Aplite porphyry

0361-0128/98/000/000-00 $6.00

Lithology

36

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE (Cont.) Method

MMH granodiorite Chuquicamata Porphyry Complex East porphyry Toki Tonalite (also previously mapped as Antena Granodiorite) Granodiorite of Miranda (equivalent to Elena Granodiorite) Elena granodiorite Elena granodiorite East porphyry East porphyry East porphyry East porphyry East porphyry East porphyry East porphyry East porphyry Banco porphyry East porphyry

U-Pb Re-Os U-Pb SHRIMP

Zircon Molybdenite Zircon

223.6 ± 1.2 34.9 ± 0.17 36.2 ± 0.4

Müller and Quiroga (2003) Ossandón et al. (2001) Proffett and Dilles (unpub. information)

U-Pb SHRIMP

Zircon

44.6 ± 0.6

Proffett and Dilles (unpub. information)

U-Pb SHRIMP U-Pb SHRIMP U-Pb SHRIMP 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar

Zircon Zircon Zircon Biotite Sericite Sericite K-feldspar Sericite Biotite K-feldspar Biotite K-feldspar K-feldspar

229.4 ± 2.6 231.4 ± 2.0 233.1 ± 2.2 31.2 ± 0.2 31.4 ± 0.4 31.4 ± 0.4 31.6 ± 0.2 31.8 ± 0.4 31.9 ± 0.2 32.1 ± 0.2 32.8 ± 0.2 33.1 ± 0.2 33.6 ± 0.2

Proffett and Dilles (unpub. information) Proffett and Dilles (unpub. information) Proffett and Dilles (unpub. information) Reynolds et al. (1998) Reynolds et al. (1998) Reynolds et al. (1998) Reynolds et al. (1998) Reynolds et al. (1998) Reynolds et al. (1998) Reynolds et al. (1998) Reynolds et al. (1998) Reynolds et al. (1998) Reynolds et al. (1998)

East porphyry

40Ar/39Ar

K-feldspar

33.7 ± 0.2

Reynolds et al. (1998)

East porphyry

40Ar/39Ar

Biotite

34.0 ± 0.4

Reynolds et al. (1998)

East porphyry East Porphyry, Mesabi fault zone East Porphyry, Mesabi fault zone East Porphyry, Mesabi fault zone East porphyry East porphyry Tonalite host rock Tonalite porphyry Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex Chuquicamata Porphyry Complex West Fault gouge MMH deposit MMH deposit MMH deposit MMH deposit Aralar quartz monzonite Calama Formation, Topater member Calama Formation, Topater member Calama Formation, Chorrillos member Estratos de Quetena Estratos de Quetena Estratos de Quetena Mesa granite Permian andesite porphyry Collahuasi, rhyolite-dacite facies Genoveva metamorphic rocks Genoveva metamorphic rocks Genoveva quartz monzonite Collahuasi, rhyolite-dacite facies Calama Formation, Topater member Calama Formation, Topater member Calama Formation, Topater member “Red Gravels” from MMH Aralar quartz monzonite Antena Granodiorite related andesite porphyry Andesite porphyry Quebrada Mala Formation upper member Antena Granodiorite Tetera Aplite

40Ar/39Ar

Biotite K-feldspar- sericite K-feldspar K-feldspar K-feldspar Biotite Zircon Sericite Alunite Alunite Alunite Alunite Alunite Alunite Alunite Alunite Alunite Alunite Biotite Hornblende Whole rock Biotite Zircon Zircon Biotite Zircon Ground mass Zircon Hornblende Hornblende Zircon Zircon Hornblende Hornblende Plagioclase Biotite Biotite

35.2 ± 0.2 31.5 ± 0.2 32.8 ± 0.2 33.0 ± 0.6 33.4 ± 0.4 33.9 ± 0.3 38.8 ± 0.5 34.5 ± 0.2 15.2 ± 1.0 16.5 ± 1.0 17.6 ± 1.2 18.1 ± 1.4 16.3 ± 1.2 16.8 ± 2.4 20.4 ± 1.2 20.8 ± 1.2 32.9 ± 1.2 33.8 ± 1.0 40.8 ± 0.5 51.8 ± 0.5 51.9 ± 1.7 60 ± 2 234.5 ± 3.2 235.4 ± 1.6 38.98 ± 0.16 296.9 ± 2.1 46.0 ± 1.8 300.3 ± 1.8 236.2 ± 1.4 86.0 ± 0.9 43.75 ± 0.01 303.9 ± 0.1 46.9 ± 0.5 51.0 ± 0.6 52.1 ± 0.8 44.9 ± 0.8 40.8 ± 0.4

Reynolds et al. (1998) Reynolds et al. (1998) Reynolds et al. (1998) Reynolds et al. (1998) Reynolds et al. (1998) Reynolds et al. (1998) Rivera and Pardo (2004) Rivera and Pardo (2004) Sillitoe and McKee (1996) Sillitoe and McKee (1996) Sillitoe and McKee (1996) Sillitoe and McKee (1996) Sillitoe and McKee (1996) Sillitoe and McKee (1996) Sillitoe and McKee (1996) Sillitoe and McKee (1996) Sillitoe et al. (1996) Sillitoe et al. (1996) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007)

Hornblende Biotite

39.9 ± 0.5 39.18 ± 0.18

Tomlinson and Blanco (2007) Tomlinson and Blanco (2007)

Hornblende Biotite Biotite

44.7 ± 0.6 39.8 ± 0.6 40.4 ± 0.2

Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007)

0361-0128/98/000/000-00 $6.00

40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar U-Pb 40Ar/39Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar 40Ar/39Ar 40Ar/39Ar K-Ar K-Ar U-Pb SHRIMP U-Pb SHRIMP 40Ar/39Ar U-Pb TIMS K-Ar U-Pb SHRIMP 40Ar/39Ar 40Ar/39Ar U-Pb CA-TIMS U-Pb CA-TIMS 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar

Material dated

Age (Ma) and error

Geological Unit

37

Reference

37

38

RIVERA ET AL. TABLE 3.

UTM North

UTM East

Place

Lithology

7518130 7521235 7521176 7521176 7517410 7517970 7522258 7523290 7530373 7520065 7520287 7535850 7538570 7532070 7538450 7524450 7536660 7537900 7538700 7537840 7520240 7520400 7538520 7536010 7539030 7518100 7536237 7535750 7536184 7536500 7536158 7535750 7536500 7536500 7536158 7536500

500362 507057 507024 507024 501352 501927 501755 501980 504362 503978 503977 507150 515430 515890 514900 502920 511650 505760 508600 508790 501100 501600 507030 506630 505350 500760 510732 510600 511298 511670 510978 510600 511670 511670 510978 511670

North from Rio San Salvador North west from Calama North west from Calama North west from Calama Rio San Salvador Rio San Salvador South east from Cerro Genoveva South east from Cerro Genoveva South from road to Tocopilla West from Toki camp West from Toki camp Antena hill west from Chuquicamata Cerros de Chuquicamata Cerros de Chuquicamata Cerros de Chuquicamata East from Genoveva hill Eastern upper border from Chuquicamata pit Mine 8 km WNW from Chuquicamata North west from Chuquicamata North west from Chuquicamata Quetena Mine Quetena Mine San Manuel mine, 4 km WNW from Chuquicamata Sierra San Lorenzo Sierra San Lorenzo South from Cerro Quetena Chuquicamata mine Chuquicamata mine Chuquicamata mine Chuquicamata mine Chuquicamata mine Chuquicamata mine Chuquicamata mine Chuquicamata mine Chuquicamata mine Chuquicamata mine

Granitic porphyry Welded tuff Ash tuff Ash tuff Aplite Granitic porphyry Monzonite Microdiorite porphyry Granodiorite in mylonitic zone Monzodiorite Monzodiorite Monzodiorite Foliated granite Meta quartz diorite Diorite Granodiorite porphyry Granodiorite Quartz-sulfide-tourmaline vein with sericite halo Granodiorite Granodiorite Ash tuff in colluvium Schist clast Chalcopyrite-pyrite vein with sericite halo Granodiorite porphyry Monzodiorite Monzodiorite Granodiorite porphyry Sericite altered granodiorite porphyry Sericite - chlorite altered porphyry Dike Sericite over potassic background altered porphyry Granodiorite porphyry Dike Dike Sericite over potassic background altered porphyry Dike

Coordinates corresponds to UTM Datum Provisional South America 1956

are succeeded by the Los Picos diorite, dated at 43 to 42 Ma (Dilles et al., 1997, 2011; Campbell et al., 2006; Tomlinson et al., 2010), composed of diorites, monzonites, and monzodiorites distributed in a north-south belt about 30 km long by 7 to 8 km wide, intruding the Collahuasi, Agua Dulce, Quehuita, Quebrada Mala, and Icanche Formations. The Fortuna granodiorite complex crops out in a belt subparallel to and east of the Los Picos diorite (Figs. 4, 5B-E; Dilles et al., 1997, 2011). The oldest intrusion is the Antena (or Gray) granodiorite, a medium-grained, hornblende-biotite−bearing phase, commonly with a N-NE foliation, that crops out in the center-west of the complex. It forms a belt 3 km west of Chuquicamata that is traceable for ~30 km northsouth and 5 to 6 km east-west. Dated at 39 to 38 Ma, it intruded Los Picos diorite, Cretaceous intrusions, and volcanic units of the Icanche Formation. Antena is intruded by Fiesta granodiorite, a rock with large hornblende and feldspar crystals, locally porphyritic, which exhibits local cupolas with weak potassic alteration (Dilles et al., 1997, 2011). The youngest intrusions of the Fortuna Complex are the Tetera and San Lorenzo porphyries, dated at ~38 Ma (Table 3; Dilles et al., 1997, 2011; Barra et al., 2006, in prep.). The Tetera porphyries are granitic in composition and most common in the northern 0361-0128/98/000/000-00 $6.00

part of the complex. The younger San Lorenzo porphyries, which are prevalent in the central and southern parts, are granodiorites and quartz monzodiorites, with associated Cu mineralization (Rivera and Pardo, 2004; Dilles et al., 2011). Based on detailed mapping, petrography, and lithogeochemistry, J. Dilles (writ. commun., 2008) and Dilles et al. (2011) concluded that the Fortuna Complex evolved from Antena granodiorite through to the San Lorenzo porphyries, with roots of the latter, together with parts of the Fiesta granodiorite, representing cupola zones that are altered and mineralized. The well-mineralized Toki porphyries, which are similar in composition and age to the San Lorenzo porphyries (Barra et al., 2006), intrude the Collahuasi Formation volcanic rocks, ~45 Ma tonalites, and Elena-type granodiorites with ~229 Ma ages (Table 3; Proffett et al., in prep.). The Toki porphyries occur as syn- to postmineral, NE-striking, subvertical to steeply E-dipping dikes, 20 to 50 m wide, with strike lengths of a few hundred meters, which have been recognized to depths of approximately 1,000 m below surface. They are crosscut and displaced by conspicuous systems of NW-oriented, D-type veins with sericitic alteration. Because of their location near the Fortuna Complex, J. Dilles (writ. commun., 2008) postulated that Fortuna-type cupolas should occur at 38

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE

39

(Cont.) Geological Unit

Method

Tetera porphyry Artola ignimbrite Artola ignimbrite Jalquinche Formation Tetera Aplite Tetera Porphyry Genoveva quartz monzonite Cretaceous andesite porphyry Antena Granodiorite Genoveva quartz monzonite Genoveva quartz monzonite Antena Granodiorite Mesa granite Cerros de Chuquicamata Diorites Cerros de Chuquicamata Diorites San Lorenzo Porphyry Elena granodiorite San Lorenzo Porphyry Fiesta granodiorite Fiesta granodiorite Miocene-Pliocene colluvium deposits Estratos de Quetena Fiesta Granodiorite San Lorenzo Porphyry Aralar quartz monzonite Antena Granodiorite East Porphyry East Porphyry East Porphyry Dike in Elena Granodiorite East Porphyry East Porphyry Dike in Elena Granodiorite Dike in Elena Granodiorite East Porphyry Dike in Elena Granodiorite

K-Ar K-Ar 40Ar/39Ar K-Ar 40Ar/39Ar K-Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar 40Ar/39Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar U-Pb TIMS K-Ar K-Ar U-Pb TIMS K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar U-Pb TIMS K-Ar 40Ar/39Ar 40Ar/39Ar K-Ar Rb-Sr K-Ar K-Ar K-Ar 40Ar/39Ar

Material dated Sericite/alunite Biotite Biotite Biotite Biotite Sericite Biotite Hornblende Biotite Biotite Biotite (chloritized) Biotite Whole rock Biotite Hornblende Biotite (chloritized) Zircon Sericite Biotite Titanite Biotite Muscovite Sericite Biotite Biotite Biotite Zircon Sericite K-feldspar K-feldspar K-feldspar Total rock Biotite K-feldspar Biotite Biotite

depth beneath the Toki porphyries, although these have not been found to date. The Fortuna Complex and Toki porphyries are good examples of porphyry-style mineralization in cupolas of well-differentiated batholiths. The middle parts of a system subcrop at Toki, with a well-preserved mineralized column extending to the deepest parts nearest the roof of the batholith, such that the roots of the system may be observed. Some of the Mina Ministro Hales porphyries have been dated at 39 Ma, comparable to those at Toki, although dates of 36 Ma, closer to those of the Chuqui Porphyry Complex, have also been reported (Müller and Quiroga, 2003). In contrast to the Toki porphyry dikes, those at Mina Ministro Hales have strike lengths of several kilometers (Figs. 4, 5A, D). The Chuqui Porphyry Complex (Ambrus, 1975, 1978; Ossandón et al., 2001; Faunes et al., 2005), located entirely in the Eastern Block, comprises a N-NE−oriented megadike ~14 km long and ~1.5 km wide, extending from Chuquicamata to north of Radomiro Tomic (Figs. 3, 4, 5B, C). Hosted by the Triassic Elena granodiorite and Triassic volcano-sedimentary rocks, and with a vertical to steep west dip and commonly foliated contacts, it is dominated by the East porphyry. Where nonfaulted, nonfoliated contacts are mapped, they strike N-NW, N-S, and 0361-0128/98/000/000-00 $6.00

Age (Ma) and error 40.5 ± 0.8 10.0 ± 0.5 10.00 ± 0.18 10.6 ± 0.3 37.4 ± 1.6 38.2 ± 1.1 43.7 ± 0.6 68.1 ± 0.5 36.6 ± 0.5 44.9 ± 0.3 46.3 ± 1.2 36.3 ± 1.1 196 ± 6 267 ± 6 273 ± 9 38.5 ± 1.1 227 ± 2 38.3 ± 1.3 37.3 ± 1.2 37.5 ± 0.03 7.3 ± 1.6 43.8 ± 1.4 37.8 ± 1.2 37.1 ± 0.9 45.1 ± 1.2 39.6 ± 1.2 32.3 ± 5.0 31.4 ± 0.8 34.0 ±1 34.2 ± 0.68 34.9 ± 0.9 35.0 ± 2.0 36.6 ± 1.0 37.7 ± 1.0 38.7 ± 1.0 43.8 ± 0.88

Reference Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson and Blanco (2007) Tomlinson et al. (2001a) Tomlinson et al. (2001a) Tomlinson et al. (2001a) Tomlinson et al. (2001a) Tomlinson et al. (2001a) Tomlinson et al. (2001a) Tomlinson et al. (2001a) Tomlinson et al. (2001a) Tomlinson et al. (2001a) Tomlinson et al. (2001a) Tomlinson et al. (2001a) Tomlinson et al. (2001a) Tomlinson et al. (2001a) Tomlinson et al. (2001a) Tomlinson et al. (2001a) Zentilli et al. (1994) Zentilli et al. (1995) Zentilli et al. (1995) Zentilli et al. (1995) Zentilli et al. (1995) Zentilli et al. (1995) Zentilli et al. (1995) Zentilli et al. (1995) Zentilli et al. (1995) Zentilli et al. (1995)

locally N-NE, and the overall N-NE orientation appears in large part to be due to dextral offsets on later NE-striking faults (J. Proffett, unpub. mapping, 2006). The East porphyry is granodioritic, with plagioclase, biotite, hornblende, euhedral sphene, and deformed quartz phenocrysts, in a fine- to coarsegrained groundmass of quartz and K-feldspar (Ossandón et al., 2001). It has been dated beyond the zone of pervasisve potassic alteration by the SHRIMP U-Pb method on zircon at 36.2 ± 0.4 Ma (Table 3; J. Proffett et al., in prep.). The East porphyry is cut by small bodies of West porphyry in the centralnorth part of the Chuquicamata mine, and of Banco and Fino porphyry in the central and northeastern part (Ossandón et al., 2001, fig. 2). The West porphyry is mineralogically and chemically similar to the East porphyry, but with a finer grained, aplitic groundmass. Like the East porphyry, it is locally foliated and contains K-feldspar megacrysts, and near contacts between the two porphyries, gradations from fine-grained aplitic to coarser groundmass are observed (J. Hunt, unpub. report, 1962). The Banco porphyry is a monzogranite, finer grained and more porphyritic than the East porphyry (Ossandón et al., 2001). SHRIMP U-Pb ages on zircon of 34.1 ± 0.3 and 34.0 ± 0.3 Ma were obtained from the Banco and West porphyries, respectively (Table 3; Ballard et al., 2001). 39

40

RIVERA ET AL.

At Radomiro Tomic, at least two types of porphyry intruding the East porphyry can be distinguished. These include the Fino porphyry, which has abundant quartz eyes in a finegrained groundmass, and occurs as sinuous and irregularly shaped bodies in the central parts of the deposit, and the West porphyry, which occurs as NE-oriented, straight-sided dikes (Parra et al., 2010). At Radomiro Tomic, U-Pb zircon ages of 35.0 ± 0.3 Ma by the SHRIMP method and 34.3 ± 0.3 by the LA-ICP-MS method were reported on a sample of the so-called major porphyry, described as similar to the East and West porphyries (Campbell et al., 2006), and an LA-ICP-MS U-Pb zircon age of 33.9 ± 0.4 Ma was reported by the same authors for a finer grained, darker, minor porphyry, presumably the Fino intrusion. Similar ages are reported by Barra et al. (in review) and are listed in Table 3. The porphyry complex at Chuquicamata has now been recognized to extend to depths of 2,000 m below surface and therefore represents one of the largest porphyry systems in the world. The bulk of the East porphyry has a background Cu content of 0.1 to 0.2 wt % Cu, and within it, higher grade mineralization is hosted by zones of structural weakness that have been accessed both by the later porphyries and by vein and stockwork systems.

2003). Intrusive contacts of the East porphyry generally strike north to north-northwest, subparallel to much of the foliation. The Mesabi fault, which trends north-northeastward from the southeastern part of the Chuquicamata mine and dips west, places locally foliated Triassic volcanic and sedimentary rocks on its west side against Triassic Elena granodiorite on its eastern side (Fig. 4). Dextral shear fabrics in the Mesabi fault gouge have been reported by G. Chong and R. Pardo (writ. commun., 1993) and Lindsay et al. (1995) and Chong and Pardo also mention evidence for sinistral displacement. More recent mapping (J. Proffett, unpub. mapping, 2005) confirms dextral fabrics in a N-NW−striking branch of the Mesabi fault but also indicates that the part of the fault zone that forms the main contact between volcano-sedimentary rocks and granodiorite is a chlorite-healed gouge with a reverse-sense shear fabric. The chloritic gouge cuts the East porphyry in the southeastern part of the mine. The Estanques Blancos fault and other steep NE-striking dextral faults displace the Mesabi fault, Chuqui Porphyry Complex, and all older rocks by a few to a few hundred meters (G. Chong and R. Pardo, writ. commun., 1993; Lindsay et al., 1995). Where faults of this set cut rocks of the Chuqui Porphyry Complex in the Chuquicamata and Radomiro Tomic mines, they tend to split into multiple strands, many of which are mineralized. Many strands bend from southwest to southsouthwest to south near the West Fissure, apparently due to sinistral drag, and are apparently ultimately truncated by the West Fissure. A second set of faults, which strike northwest, dip steeply, and have small sinistral displacements, also occurs locally in the Chuquicamata mine (e.g., Perry, 1952; Lindsay et al., 1995; Fig. 5A). Lindsay et al. (1995) documented early potassic-related Cu sulfide mineralization, quartz-molybdenite veins, late pyrite-sericite mineralization, and postmineral gouge along the dextral northeast set and late pyrite-sericite mineralization and postmineral gouge along the sinistral northwest set. Recent underground observations and pit mapping have documented early potassic-related Cu sulfide mineralization and quartz-molybdenite veins along the northwest set as well (J. Proffett, unpub. data, 2003, 2006). It is evident that the Chuqui Porphyry Complex and its wall rocks have been subjected to a variety of deformation events, which have acted either continuously or discontinuously over an extended period of time. Some of the resulting structures may have facilitated emplacement of the porphyry complex and influenced an original north-northwest orientation of the porphyry megadike, later modified to a north-northeast orientation due to displacements on NE-striking faults of the Estanques Blancos set. At least some of the ductile fabrics closest in age to porphyry emplacement and Cu mineralization indicate approximate east-west shortening and crustal thickening, and mineralized brittle fault sets also indicate east-west shortening, but with north-south extension, during and after the period of mineralization. The significance of early ductile dextral fabrics is uncertain. Some dextral displacement appears to have taken place at the site of the Chuqui Porphyry Complex and/or Mesabi fault (e.g., Lindsay, 1998) but the presence of nearly identical Triassic granodiorite on both sides of the Chuqui Porphyry Complex and on both sides of the Mesabi fault would seem to preclude major dextral displacement on the scale of the later West Fissure.

Structure The Chuquicamata district has suffered the effects of successive deformations since the Paleozoic, culminating with the Incaic event, which produced uplift as well as basins, resulting in deposition of the Calama Formation syntectonic sediments. In a broad sense, the district is within the area of influence of the Domeyko fault system, interpreted to have exerted a direct regional control on emplacement of the porphyry Cu systems of northern Chile (Maksaev and Zentilli, 1988, 1999; Camus, 2003; Sillitoe and Perelló, 2005). Eastern Block: Intrusions of the Cerros de Chuquicamata igneous complex show a marked foliation, usually oriented NE (Tomlinson et al., 2010). Field relationships show evidence for several ages of deformation east and northeast of the Chuquicamata mine. Intense N-NW−striking, NE-dipping mylonitic foliation, with E-plunging lineations, in the eastcentral part of the mine, are truncated by the Triassic Elena granodiorite, whereas foliation with a similar orientation overprints the Elena intrusion and locally forms its contact with Triassic volcanic rocks northeast of the mine (Lindsay, 1998; J. Proffett and H. Alcota, unpub. mapping, 2005). Some of this foliation in the Elena intrusion has east-side-up shear sense indicators, whereas some of the similarly oriented foliation in the Triassic volcanic rocks has northeast-side-down indicators (J. Proffett, unpub. data, 2005, 2006). Northeast of the mine, the East porphyry truncates foliation in the Elena intrusion but is itself overprinted locally by a similar foliation. Northeast-side-up sense of shear of some of the ductile foliation that overprints the East porphyry was documented by Lindsay (1998), and dextral shear on steep N-NE−striking ductile shear zones in East porphyry was reported by Lindsay et al. (1995). Foliation that overprints the East porphyry also overprints the smaller porphyry bodies that intrude it as well as potassic alteration (Ossandón et al., 2001), but this foliation is older than late-stage sericitic alteration and associated pyrite Cu sulfide mineralization (J. Proffett, unpub. data, 0361-0128/98/000/000-00 $6.00

40

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE

Western Block: In the area of the Toki Cluster, the Paleozoic volcanic rocks of the Collahuasi Formation and Jurassic marine sedimentary rocks were folded and faulted. The Late Cretaceous Quebrada Mala Formation volcanic and sedimentary rocks were folded into a north-south syncline in the Cerro Negro area. Farther north, Cretaceous units of the Tolar Formation were deformed to produce the SE-trending San Lorenzo anticline (Fig. 3). Roof pendants and wall-rock exposures with steep NNE−striking mylonite zones in volcanic rocks occur along the length of the N-NE−trending Fortuna batholith (Dilles et al., 2011). The mylonitic foliation is cut by all phases of the Fortuna, but foliation of similar orientation overprints some of the intrusive rocks, indicating deformation approximately coincident with emplacement. These observations are interpreted in terms of a ductile fault zone along which the Fortuna batholith was intruded (Dilles et al., 2011). The results of drilling of covered areas and mapping indicates the likely presence of an important NW-striking fault, the Toki North fault, beneath gravels north of the Toki Cluster (Fig. 4). On the northeastern side of this proposed fault, Upper Cretaceous volcanic and clastic sedimentary rocks lie directly upon a basement of Collahuasi Formation, Triassic volcanic rocks and Triassic granodiorite, and are intruded to the west by the Fortuna granodiorite. On the southwestern side of the proposed fault, basement of Collahuasi Formation and Triassic conglomerate, volcanic rocks, and granodiorites are overlain by a thick section of Jurassic carbonates and other sedimentary rocks, and are intruded by Los Picos-type quartz monzodiorites and porphyries of the Toki Cluster. Northwest of the covered area, where the Toki North fault should be exposed, an intrusive contact between Fortuna granodiorite to the north and Los Picos-type quartz monzodiorites to the south is displaced by minor, W- to W-NW−striking faults, but a large fault is not present. It is thought that the main movement on the Toki North fault may have predated the Eocene plutons, and that minor postintrusion reactivation took place later. Other NW- to N-NW−striking faults, mostly with small sinistral displacements, are common in parts of the Western Block (Figs. 3, 4; Tomlinson et al., 2010; Dilles et al., 2011, Fig 4B), including in the Mina Ministro Hales deposit; these may be related to the NW-striking set described above in the Chuquicamata mine. West Fissure: The postore West Fissure, described in many previous publications, is the most prominent structure in the district. Its fault plane is clearly mappable between Radomiro Tomic and Mina Ministro Hales and is less well constrained to both the north and south. At Chuquicamata, it juxtaposes mineralized Chuquicamata porphyries and Elena granodiorite on the east against barren, petrographically distinct Fiesta granodiorite on the west, and at Mina Ministro Hales it places the mineralized system on the west against Calama Formation gravels on the east (Figs. 3, 5D). Sinistral displacement of 30 to 35 km along the West Fissure since the Eocene has been proposed based on correlation of intrusions of the Fortuna complex and their host rocks with intrusions of similar ages, compositions, and host rocks in the El Abra intrusive complex to the north (R.C. Baker, unpub. mapping, 1969; Dilles et al., 1997, 2011; Tomlinson 0361-0128/98/000/000-00 $6.00

41

and Blanco, 1997a, b). Results of the recent multiscale mapping program are consistent with this interpretation. Triassic granodiorite, such as the Elena intrusion, is more widespread than Eocene intrusions on both sides of the West Fissure, and its distribution is consistent with anywhere from about 13 to >22 km of sinistral displacement. The probable location of source rocks for the predominant clasts found in the Calama Formation conglomerates suggests a displacement of 7 to 11 km (N. Blanco, writ. commun., 2007). If it is assumed that mineralization at Mina Ministro Hales is displaced from Chuquicamata, 7 to 9 km of sinistral displacement would be indicated, but alternatively these two orebodies may not have originally been part of the same deposit (e.g., Alvarez, 1993; Sillitoe et al., 1996). If they were originally parts of different deposits, however, the offset portions of neither have been discovered. The great thickness of Calama Formation conglomerates in fault contact with Mina Ministro Hales mineralization across the West Fissure (Alvarez and Miranda, 1991; S. Rivera, unpub. data, 1995; Sillitoe et al., 1996; Müller and Quiroga, 2003) indicates that the latest displacement on the fault took place after the Calama Formation was deposited, and after the final alteration-mineralization events at ~31.5 Ma. This displacement occurred before Jalquinche Formation sedimentation at ~16 Ma, which took place in both the Eastern and Western Blocks. Tomlinson and Blanco (2007) interpreted various episodes of fault displacement during this time interval. Update of the Geology of the Porphyry Cu-Mo Deposits This section provides a brief update on the hypogene alteration and mineralization of the principal deposits of the district based on presentations at a CODELCO workshop in 2010 and unpublished work by the authors. For more detailed deposit descriptions, the reader is referred to publications by Ossandón et al. (2001) and Faunes et al. (2005) for Chuquicamata; Cuadra and Camus (1998), Cuadra and Rojas (2001), and Lorca et al. (2003) for Radomiro Tomic; Alvarez and Miranda (1991), Alvarez (1993), Sillitoe et al. (1996), Müller and Quiroga (2003), and Boric et al. (2009) for Mina Ministro Hales; Münchmeyer and Urquieta (1974), Mortimer et al. (1978), and Münchmeyer (1996) for Exótica (Mina Sur); and Rivera and Pardo (2004) and Rivera et al. (2009) for the Toki Cluster and Marquardt et al. (2009) for Miranda. The locations of these deposits are shown in Figure 4, the districtwide distribution of mineralization in Figure 5, and maps and sections of the deposits in Figures 6 to 9. A compilation of all isotopic ages for the district is presented in Table 3. Chuquicamata New underground ramp exposures and deeper drilling at Chuquicamata (Córdova et al., 2010) has provided improved understanding of the distribution of hypogene alteration and mineralization (Fig. 6A, B). The mineralization is hosted within the Chuqui Porphyry Complex, which as described above, is a megadike up to 1.5 km wide that extends well to the north and south of the deposit (Fig. 4). The orebody consists of a large volume of background potassic alteration within which zones of more intense potassic alteration occur and which is overprinted by younger sericite-pyrite alteration. 41

42

RIVERA ET AL.

FIG. 6. Chuquicamata update: Plan and section showing (A) alteration and (B) mineralization on the 1,900-m level (left) and section 3550 N. Taken from Córdova et al. (2010). The coordinates used are the Chuquicamata mine grid as shown in figure 2 of Ossandón et al. (2001); north on this coordinate system is about 10° east of true north. 0361-0128/98/000/000-00 $6.00

42

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE

43

(A)

Fine Porphyry

Background Potassic Intense Potassic

West Porphyry

Quartz-sericite

East Porphyry

Strong supergene

(B)

FIG. 7. Radomiro Tomic update. A. Rock types and mineralization plan. B. Alterationmineralization section. Taken from Parra et al. (2010). The coordinates shown are the Chuquicamata mine grid. 0361-0128/98/000/000-00 $6.00

43

44

RIVERA ET AL.

(A)

(B)

FIG. 8. Mina Ministro Hales update. A. Plan. B. Section, showing rock types and mineralization distribution. Veins and breccias in (A) are simplified as straight lines for representation purposes. Modified from Díaz (2010). Coordinates are UTM, Provisional South America 1956, with first two digits removed, i.e., 8500E = 508500E, etc., and 24500N = 7524500N, etc. 0361-0128/98/000/000-00 $6.00

44

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE

45

FIG. 9. Toki Cluster sections. A. Integrated mineralization. B. Rock type and structure. Miranda deposit is 200 m due east. Note that the porphyry intrusions widen and merge downward, possibly toward a subjacent cupola, with the accompanying mineralization tightly zoned around the porphyry swarm. Also note the development of an in situ oxidation profile. Modified from Rivera et al. (2009).

As described above ductile foliation commonly overprints potassic alteration assemblages but is older than late sericitepyrite alteration (Fig. 10E). Background potassic alteration is characterized by alteration of hornblende to hydrothermal biotite, sphene to rutile, and destruction of most magmatic magnetite. Feldspars may be nearly fresh or weakly altered to hydrothermal K-feldspar or sodic plagioclase, but plagioclase is commonly partly sericitized or argillized due to overprinting by later alteration types. Copper grades in the background potassic alteration are low (generally 0.2−0.5 wt % Cu) and occur as disseminated and fracture-controlled chalcopyrite and/or bornite, generally without pyrite (although pyrite may be present with later sericitic alteration overprint). The outer limits of the background potassic zone, marked by the presence of nonbiotitized hornblende, is commonly located in the outer parts of the porphyry intrusion, suggesting that the megadike is an older wall rock rather than being directly related to the mineralizing events. Within the background potassic zone, intense potassic alteration, with abundant disseminated Cu sulfides, occurs in 0361-0128/98/000/000-00 $6.00

alteration halos a few millimeters to a few centimeters wide along early fractures and veinlets (Fig. 10A, B; Proffett, 2009). In these halos, feldspars are altered to sericite and hydrothermal K-feldspar, with the latter mineral commonly occurring between clusters of the sericite and either magmatic or hydrothermal quartz (Fig. 10C, D). Mafic minerals may be recognizable as clusters of rutile grains intergrown with sericite and/or K-feldspar. At Chuquicamata, there are two main types of these early potassic alteration halos, termed early sericite and K-sil. In the early sericite (Fig. 10B, C), most feldspars are altered to sericite, and only minor K-feldspar is present between the sericite clusters and quartz grains. In the K-sil alteration (Fig. 10D, E; equivalent in part to the quartzK feldspar alteration of Ossandón et al., 2001), K-feldspar is abundant, and sericite occurs only as small relict clusters in the cores of K-feldspar grains. It appears that both types are the products of reaction between sericite-quartz on the one hand and K-feldspar on the other. In the early sericite type, the reaction progressed toward the quartz-sericite side whereas in the K-sil, reaction proceeded toward the K-feldspar side (with consumption of some quartz as well as sericite). 45

46

0361-0128/98/000/000-00 $6.00

RIVERA ET AL.

46

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE

Both types of intense potassic alteration generally contain abundant anhydrite and Cu sulfides and lack magnetite. Where overprinting by late sericite-pyrite alteration is absent, the sulfides are dominated by bornite (without pyrite), but where there is overprinting by late sericitic alteration, chalcopyrite or other Cu sulfide assemblages that include pyrite may be present. Copper sulfides may be sufficiently abundant for early sericite or K-sil halos to contain as much as 5 to 10 wt % Cu. Lower grade background potassic alteration occurs between the halos, but where the halos are wide and/or closely spaced, broad zones of 0.6 to 2 wt % Cu can occur (potásico intenso or PI zones). Locally, breccias that cut through PI zones can be even higher grade (e.g., Ossandón et al, 2001; Fig. 3H), possibly due to remobilization and concentration of Cu sulfides in the breccia. Potassic alteration is clearly younger than the East porphyry host, and much is also younger than most of the small, later porphyries (West and Banco porphyries, discussed above and by Ossandón et al., 2001) that intrude the East porphyry (J. Proffett, unpub. data, 2003). Although the higher grade PI zones as well as these small porphyry intrusions occur within the same large volume of background potassic-altered East porphyry, there is not generally a close spatial relationship between high Cu grades and individual porphyry intrusions. Early sericite halos are distinguished from the later sericitepyrite alteration because they occur along structurally early fractures and veinlets, as determined by crosscutting relationships (Fig. 10B). Compared to later sericite, most of the early sericite is darker gray or gray-green in color, associated with more abundant disseminated Cu sulfides, and results in more texture destruction. Both A- and B-type quartz veins (cf. Gustafson and Hunt, 1975), with or without Cu sulfides, occur at Chuquicamata, but other than the quartz-molybdenite veins described below, comprise only about 1 to 5 vol % of the rock in most areas. The A-type veins with barren K-feldspar selvages are abundant only locally, as in the center-north part of the deposit, where they occur in a small stockwork in West Porphyry (J. Hunt, unpub. report, 1962); A- and B- type veins do not normally control significant zones of higher Cu grade.

Banded quartz-molybdenite veins, several centimeters to 1 m or more wide, are common throughout most of the deposit. Except for their greater width, these are similar to some varieties of B-type veins but are called blue veins at Chuquicamata (Ossandón et al., 2001) due to the presence of molybdenite. Many occur along minor NE-striking, dextral or NW-striking, sinistral faults, but some have other orientations. Blue veins may occur without alteration halos; some have potassic or sericitic halos, but these may have formed as earlier or later alteration guided by the same structures. In some examples, where later pyritic veins associated with sericitic alteration cut the blue veins, molybdenite was removed from a halo a few millimeters or centimeters wide along the pyritic veins (J. Proffett, unpub. data, 2002). Near such examples, molybdenite may occur as aggregates in pyrite-rich veins and may have been redeposited by the fluids that removed the molybdenite from the blue veins. Chloritic alteration of biotite, or of nonbiotitized hornblende, occurs around the periphery of the background potassic zone and may overprint its outer edges (Alvarez and Aracena, 1985). Relict magmatic magnetite may also occur in this zone. Veins or breccia zones containing hematite are also common. Minor pyrite and, in some places, minor chalcopyrite may also be present. Sericite-quartz-pyrite alteration occurs as halos along pyritic veins (D-type veins; Gustafson and Hunt, 1975) that cut the earlier veins and alteration discussed above (Fig. 10A, E). The veins commonly contain various Cu sulfides together with pyrite, and below the top of sulfates, also anhydrite. The veins occur in a variety of orientations, but many larger examples, up to 1 m or so wide, follow minor NE-striking, dextral or NW-striking, sinistral faults, and may occupy some of the same structures as the earlier blue veins. Where pyritic veins are closely spaced and their alteration halos wide enough to overlap, pervasive sericite-quartz-pyrite alteration is present, and inner halos of advanced argillic alteration (generally with alunite; Ossandón et al., 2001) or silicification may be present along some of the veins. Pervasive alteration of this type occurs mainly within a zone immediately east of the West Fissure, where it cuts and overprints

FIG. 10. Mineralization in the Chuquicamata deposit. A. D-type veins (horizontal) with pyrite and chalcopyrite cut early sericitic alteration halos (subvertical, on left) in East porphyry of the Chuqui Porphyry Complex. Yellow is weathered clay in plagioclase sites. Chuquicamata pit, 4100N, 3423E, 2,259 masl. B. Part of an early sericitic halo (left) with abundant Cu sulfides (black grains) in East porphyry, drill hole 6604, 9.3 m. Edge of halo (black arrows) is displaced by a thin A-type quartz veinlet (red arrow), which, in turn, is cut by a later quartz veinlet (orange arrow). C. Polished thin section (cross-polarized light) of early sericitic halo, showing a thin rim of hydrothermal K-feldspar between sericite and quartz. Drill hole 6602, 479.3 m. D. Polished thin section (cross-polarized light) of K-sil alteration, with a relict grain of quartz surrounded by hydrothermal K-feldspar (shades of gray). Relict clusters of sericite in upper left are partly surrounded by K-feldspar, and to right, traces of relict sericite (bright laths) remain in K-feldspar grains. Drill hole 6602, 475 m. E. K-sil alteration (upper and lower parts of photo) containing abundant bornite (black grains) in East porphyry, cut by a D-type vein (subhorizontal, center). Bornite is truncated at the edge of the D-type vein sericite-quartz halo, which is almost devoid of Cu sulfides except for chalcopyrite along centerline. Cu grade in D-type vein and its halo is about half that of the K-sil alteration, indicating that Cu was removed during D-type vein formation; Fe and S were introduced. Foliation in K-sil alteration trends lower left toward upper right but does not affect the D-type vein. Drill hole 3522, 124.8 m, on 3100N section. F. PIR, (relict intense potassic alteration) drill hole 7250, 132 m. Dark disseminated grains are chalcocite and lesser pyrite formed by sulfidation of original pyrite-free bornite in an early potassic halo. The pyritic vein and its inner halo contain the same chalcocite-pyrite assemblage but with a much lower ratio of chalcocite to pyrite; Cu was removed. G. Polished thin section in plane-polarized reflected light of covellite (blue) and pyrite (nearly white) formed by sulfidation of bornite in a relict early potassic halo during late sericitic overprint (PIR), Chuquicamata underground, 4200N crosscut at 3368E, ~1,881-m elevation. H. Same as (G) in cross-polarized transmitted light, showing alteration minerals. Abbreviations: b = bornite, K = K-feldspar, q = quartz, s = sericite. Coordinates, where given, are Chuquicamata coordinates (see fig. 2 of Ossandón et al, 2001 for location of coordinates in pit). 0361-0128/98/000/000-00 $6.00

47

47

48

RIVERA ET AL.

the central part of the potassic zone. Lower sulfidation-state bornite and/or chalcopyrite assemblages without pyrite, including those in the K-sil and early sericite halos, are altered to high sulfidation assemblages containing pyrite, such as bornite-pyrite, chalcocite-pyrite, and covellite-pyrite, with this overprinting (Fig. 10F-H). Although Cu is remobilized on a local scale in these sulfidized zones, in most places the newly sulfidized Cu sulfides have remained within the overprinted early halos, a process that resulted in relict zones of high Cu grades within the sericite-quartz-pyrite alteration (potásico intenso relicto or PIR zones, Fig. 10B, F, G). In addition to these relict high-grade zones, high Cu grades may also occur within the late D veins that guide the sericitequartz-pyrite halos. D-type veins and sericite-quartz-pyrite alteration, as well as earlier alteration and mineralization, are locally cut by veinlets of anhydrite, hematite, and Cu-bearing sulfides, generally without alteration halos of their own. These veinlets usually contain little or no pyrite; the Cu sulfides can consist of a combination of chalcopyrite, bornite, digenite, and/or covellite. Chalcopyrite occurs with pyrite, pyrrhotite, magnetite, and calc-silicate minerals in carbonate rocks east of Chuquicamata, and with magnetite replacements of carbonate rocks along the Mesabi fault. A SHRIMP U-Pb age reported for zircon from the East porphyry within the potassic zone is 35.2 ± 0.4 Ma (Ballard et al., 2001), about 1 m.y. younger than the SHRIMP age reported above for the East porphyry beyond this zone. SHRIMP U-Pb ages for West and Banco porphyries of ~34 Ma were cited above, and 34 to 35 Ma 40Ar/39Ar ages were reported for biotite and K-feldspar from the potassic alteration (Zentilli et al., 1995). An Re/Os age of 34.9 ± 0.17 Ma was reported from molybdenite from a blue vein (J. Ruiz in Ossandón et al., 2001). Zentilli et al. (1995) also reported Ar/Ar ages of 31 to 32 Ma for sericite from late pyritic veins, and Mathur et al. (2000) provided an Re-Os isochron age of 31 ± 2Ma for pyrite associated with the late sericitic alteration (Table 3).

zone (Fig. 7A, B), and limited mining of the sulfide ore has commenced, enabling comparisons with Chuquicamata (Parra et al., 2010). Host rocks are similar to those at Chuquicamata, as described above. The orebody consists of a large body of background potassic alteration, similar to that at Chuquicamata, within which intense potassic alteration occurs as alteration halos a few millimeters to centimeters wide along early fractures and veinlets (Fig. 11A; Proffett, 2009). In this intense potassic alteration, feldspar and mafic sites are replaced by sericite and K-feldspar, with the Kfeldspar occurring between sericite clusters and quartz. In some halos, andalusite occurs with the sericite (Fig. 11B). Where overprinting by late sericite-pyrite alteration is weak or absent, hydrothermal biotite is also present in feldspar as well as mafic sites, and abundant bornite (without pyrite) is the main sulfide. These are referred to as EDM halos because of their resemblance to the early dark micaceous (EDM) halos at Butte, Montana (Meyer, 1965). Where late sericite-pyrite alteration has overprinted the early halos, biotite is altered to chlorite or sericite and bornite is altered to chalcopyrite, and this alteration, referred to as SGV (sericita gris-verde), resembles the early sericite at Chuquicamata. Both types lack magnetite. Background potassic alteration occurs between the early halos, but where the halos are sufficiently wide and closely spaced, they define zones of higher Cu grade. Although these higher grade zones are in the same general area as the small bodies of Fino and West porphyry (see above and Fig. 7), there does not seem to be a close relationship in detail between the high-grade zones and individual porphyry intrusions. As at Chuquicamata, A- and B-type quartz veins, some containing Cu sulfides, are present but not abundant, and only locally define zones of higher Cu grades. Wide, banded quartzmolybdenite blue veins are common, and like at Chuquicamata, many occupy steep, NE- and NW-striking structures. Sericite-quartz-pyrite alteration occurs as halos along pyritic veins, some of which contain Cu sulfides. In contrast to Chuquicamata, this type of alteration seems to be restricted to relatively narrow halos, and large zones of pervasive sericite-quartz-pyrite alteration are not present. This fact may be due to a deeper level of exposure at Radomiro Tomic. Many of these pyritic veins are steep and strike northeasterly (Fig. 7A, B).

Radomiro Tomic Mining at Radomiro Tomic has been mainly for Cu oxide ores, but recent drilling has explored the underlying hypogene

FIG. 11. Mineralization in other deposits of the Chuquicamata district. A. Early potassic halos of the EDM type (brownish), Radomiro Tomic drill hole 4886, 160.9 m. Small black grains in early halos are bornite. Curvature of halos is due to cylindrical drill core surface. B. Polished thin section of early halo of (A), cross-polarized light. Andalusite is surrounded by sericite, which, in turn, is surrounded and separated from quartz by hydrothermal K-feldspar. Biotite and bornite (without pyrite) also occur in the halo. C. EDM halo along early quartz-bornite-molybdenite vein in granodiorite from deep part of Mina Ministro Hales (drill hole 7509, 1,386 m). The halo contains abundant disseminated Cu sulfides but no pyrite. D. Polished thin section of EDM halo, Mina Ministro Hales drill hole 4903, 1,077.1 m, showing sericite surrounded by rim of hydrothermal K-feldspar (stained yellow) between the sericite and quartz. Plane-polarized light. Opaques (black) are Cubearing sulfides. E. Early potassic halo overprinted by later sericite-quartz-pyrite alteration, Mina Ministro Hales hole 7509, 1,117 m. Fine dark grains are Cu-bearing sulfides with pyrite in the relict early halo that formed by sulfidation of original pyrite-free bornite. Thin, late pyrite veins (D-type veins) cross left part of image, with Cu-bearing sulfides leached from their inner halos. F. EDM halo, Miranda deposit (drill hole 2558, 740 m). Fine-grained biotite partly replaces plagioclase. The halo contains abundant disseminated bornite (difficult to see in photo). G. A-type quartz vein (upper part of image), with abundant disseminated bornite and minor chalcopyrite as well as hydrothermal magnetite and K-feldspar along margin. East part of Toki, drill hole 2558, 747.2 m. H. Contact (arrows) between two porphyries at Toki, hole 1718, 547.4 m. A-type veins in the earlier porphyry (on left) are truncated at the contact with the later porphyry. Other A-type veins cut both porphyries. Abbreviations as in Figure 10 and a = andalusite, m = molybdenite. 0361-0128/98/000/000-00 $6.00

48

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE

0361-0128/98/000/000-00 $6.00

49

49

50

RIVERA ET AL.

Mina Ministro Alejandro Hales

Miranda

The host rock of the Mina Ministro Hales deposit, which lies beneath late Cenozoic gravels, is Triassic granodiorite cut by dacitic dikes of similar age (Boric et al., 2009; Díaz, 2010), which extend along the western side of the West Fissure for >10 km. To the west, the Triassic granodiorite intrudes late Paleozoic volcanic rocks, which farther west still are unconformably overlain by W-dipping, Late Cretaceous conglomerate and andesitic rocks. Steep, relatively narrow, N-trending, dike-like bodies of late Eocene porphyry cut the host rocks and seem to widen slightly with depth. The Mina Ministro Hales mineralization is present in a broad zone within and surrounding the Eocene porphyries (Fig. 8A, B). Most rocks in the upper part of the deposit are altered to sericitic and advanced argillic assemblages, but relict textures indicate that much of this alteration overprinted earlier potassic alteration. At depth, intense sericite-pyrite alteration is restricted to halos of pyritic veins or veinlet stockworks, and deeper drill holes intersect a zone of background potassic alteration, similar to that described above at Chuquicamata. This zone occurs in the porphyry and a broad zone in the adjacent Triassic granodiorite and appears to dip steeply westward (Boric et al., 2009). Within the background potassic zone, intense potassic alteration occurs as alteration halos along early veins and fractures (Proffett, 2009). Where not overprinted by later sericite, these alteration halos (Fig. 11C) are similar to the EDM halos at Butte, Montana (Meyer, 1965), with feldspars and mafic minerals altered to sericite and hydrothermal biotite and with hydrothermal K-feldspar occurring as rims between the sericite-biotite clusters and quartz grains (Fig. 11D). Abundant bornite and/or chalcopyrite (without pyrite) are disseminated in these halos, and where they are closely spaced, they define zones of high Cu grade. Where these EDM halos are overprinted by later sericite-pyrite alteration, biotite and some of the hydrothermal K-feldspar are altered to sericite, bornite may be altered to chalcopyrite with or without pyrite, and the halos resemble the early sericite halos of Chuquicamata and Radomiro Tomic. These early sericite halos are abundant in the upper part of the deposit, and where closely spaced, may define zones of high Cu grade (PIR zones; Fig. 11E). Much of the ore-grade material in the zone of sericitic alteration is attributable to Cu-bearing sulfides plus pyrite that occur in veins, many with inner halos of advanced argillic alteration, including alunite (Fig. 8A, B). Some of the veins are up to a few meters wide, and much of the vein fill is brecciated. Many of the veins are oriented north-northwest, with a steep dip, and appear to occupy faults with minor sinistral displacement. A few of the veins trend northeast. The sulfides define high sulfidation-state assemblages, such as pyrite-bornite, pyrite-digenite, pyrite-covellite, and pyrite-enargite. The mineralized system is limited to the east by the West Fissure, but one or more internal faults subparallel to the West Fissure occur in the orebody (e.g., Sillitoe et al., 1996). Isotopic dates for the mineralization include 40Ar/39Ar ages of 34.3 ± 0.2 Ma for hydrothermal biotite and 32.5 Ma (errors not provided) for later stage sericite (Table 3; Müller and Quiroga, 2003; Boric, pers. commun., 2005).

The Miranda deposit is located about 5 km south-southwest of Mina Ministro Hales, at the eastern end of the Toki Cluster, beneath late Cenozoic gravels (Marquardt et al., 2009). The host rocks include volcanic rocks of late Paleozoic and possibly Triassic age, granodiorite cut by dacitic dikes, both of Triassic age, small intrusions of Eocene tonalite and a few narrow porphyry dikes of probable Eocene age. The mineralization occurs within a large body of background potassic alteration, which lies below rock with nonbiotitized, but locally chloritized hornblende. At depth within this background potassic zone, intense potassic alteration of EDM type, as described above, occurs as halos along early veins, and contains high Cu grades (Fig. 11F). Zones of pyritic D-type veins with sericite-quartz-pyrite halos overprint parts of the background potassic zone and adjacent rock. At depth in the western part of the deposit, A-type quartz veins containing disseminated bornite, typical of mineralization in the adjacent Toki deposit, cut the Miranda EDM veins. Re-Os ages of 36.5 to 36.0 Ma were returned for molybdenite from quartz-molybdenite veins (Table 3; F. Barra in Marquardt et al., 2009).

0361-0128/98/000/000-00 $6.00

Toki Cluster The Toki Cluster consists of several centers of porphyry Cu mineralization, including Toki, Quetena, Genoveva, and Opache (Fig. 4), concealed beneath late Cenozoic gravels (Rivera and Pardo, 2004). The deposits are related to dike swarms and stocks of Eocene porphyry that appear to be of the San Lorenzo type associated with the Fortuna batholith (Fig. 9). The porphyries intrude late Paleozoic volcanic rocks as well as quartz monzodiorite to quartz diorite precursor plutons that are a few million years older than the porphyries. The porphyry intrusions are of limited extent at the higher levels, but drilling shows them to widen markedly downward (Fig. 9A, B), which suggests that they may merge into cupolas in the roofs of source plutons at a relatively shallow depth (cf. Proffett, 1979, 2009; Setyandhaka et al. 2008; Dilles et al., 2011). Multiple intrusions of petrographically similar porphyries occur in clusters in each porphyry center, and zones of background potassic alteration are centered within and around these centers. The background potassic alteration contains low Cu grades and differs from the background potassic at Chuquicamata and other deposits discussed above, mainly in that magmatic magnetite has commonly survived the potassic alteration. High Cu grades occur in A-type quartz veins in the form of disseminated bornite without pyrite, commonly accompanied by hydrothermal magnetite (Fig. 11G). Where late sericitic overprinting has taken place the Atype veins contain disseminated chalcopyrite, with or without pyrite. Some A-type veins lack alteration halos and some have irregular halos in which plagioclase is replaced by hydrothermal K-feldspar. Zones of high Cu grades occur where the Atype veins are abundant, which is generally within and immediately surrounding the earliest porphyry intrusion in a given deposit. This high-grade mineralization is truncated by the next younger porphyry, which, in turn, is cut by similar, but less intense mineralization (Fig. 11H). The latest porphyries truncate most or all such mineralization. Igneous breccias 50

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE

with potassic alteration and Cu mineralization in fragments, and in some cases also as cement, are locally present. Crosscutting relationships indicate that the Cu mineralization is similar in age to the porphyries (Fig. 11H), from which an LA-ICP-MS U-Pb zircon age of 38.6 ± 0.7 Ma was reported, consistent with ~38 Ma Re-Os ages for molybdenite (Table 3; Barra et al., 2006). Pyritic D-type veins, sericitic alteration, and tourmaline-cemented breccias overprint rocks beyond and above the potassic zone, and sodic-calcic alteration is present locally at depth (Rivera and Pardo, 2004; Dilles et al., 2011). In the Opache deposit, farther south (Fig. 4), igneous and early hydrothermal breccias are common and contain clasts of A-type veins and porphyry cut by A-type veins. There is a greater development of sericite-quartz-pyrite alteration, which has overprinted earlier hydrothermal biotite with chalcopyrite. The deposit has the best development of supergene enrichment in the Toki Cluster, in contrast to the others that show mostly in situ oxidation of the hypogene mineralization (Rivera et al., 2003, 2006, 2009; Rivera and Pardo, 2004).

host rocks that are slightly (Chuquicamata, Radomiro Tomic) to significantly (Mina Ministro Hales, Miranda) older than the late Eocene mineralization event. Although porphyries close in age to the mineralization are present, they are relatively narrow bodies that show only limited widening with depth. This observation suggests that the magmatic source of these porphyries was located well below the present lower limits of drilling. High Cu grades do not generally show a close space-time relationship to individual porphyry intrusions, other than that both occur within the same large background potassic zones. In contrast, in deposits of the Toki Cluster (other than the eastern part of Miranda), most zones of high Cu grades occur as low sulfidation-state Cu sulfide mineral assemblages disseminated in A-type quartz veins. Zones of abundant A-type veins and high Cu grades are most intensely developed in and near the earliest porphyries and are truncated by the next younger porphyries of the sequences, which, in turn, are cut by similar, though less intense mineralization, and this, in turn, by yet younger porphyries. These high Cu grades thus show an intimate time-space relationship with porphyry emplacement. The Toki Cluster porphyries show significant widening with depth, suggesting that magmatic source cupolas for porphyry magmas and mineralizing fluids may be only short distances below the deepest levels drilled (Fig. 9). There is little direct evidence for the paleodepth of porphyry emplacement in the district. However, the magmatic sources of porphyries and fluids appear to be at shallower depths below the deposits in which the highest Cu grades occur in A-type veins than in those in which the higher grades occur in early potassic halos, as discussed above. This situation is similar to that observed in the Yerington district, Nevada (Proffett, 2009). In most of the deposits in which most high Cu grades occur in the early potassic halos, high grades also occur in late, pyritic veins and stockworks associated with sericitic and advanced argillic alteration, which are located within and overprint part of the potassic zone. This is similar to the pattern observed at Butte, Montana (Meyer et al., 1968; Proffett, 1973, 1979), as also is the abundance of early potassic alteration halos. In the deposits in which most high Cu grades are present in A-type veins, sericite-pyrite mineralization mostly occurs outside the main potassic zone. This situation may have been caused by the presence of a still-cooling, central porphyry cluster in these deposits, keeping temperatures too high for sericite-pyrite development. High Cu grades in late sericitepyrite veins are much less important in these deposits (at exposed levels), possibly because of this restriction of sericitepyrite to parts of the system in which the early-formed Cu mineralization, that could be remobilized into the late veins, was low grade or absent. The Fortuna Complex, west of Chuquicamata, was cited by Sillitoe (1973) as an example of the bottom of a porphyry Cu system. Although later work has shown that the Fortuna Complex is not the upfaulted roots of Chuquicamata, nevertheless some of the features of the Fortuna Complex recognized by Sillitoe (1973), such as a large phaneritic granitic pluton containing pegmatitic patches, small Cu-bearing stockworks, and limonite associated with weakly developed

Conclusions: Overview of Porphyry-Related Mineralization in the Chuquicamata District Porphyry Cu deposits of the Chuquicamata district occur in two belts: a western belt (Toki Cluster, possibly including El Abra; Dilles et al., 1997) associated with porphyries related to the N-NE−trending Fortuna batholith, and an eastern belt associated with porphyries that are ~2 to 3 m.y. younger (Radomiro Tomic, Chuquicamata, Mina Ministro Hales). Miranda, at least its eastern part, located on the eastern side of the Toki Cluster, seems more similar in characteristics and age to the eastern belt deposits. The porphyries that are similar in age to mineralization in the eastern belt are small at exposed levels, but the large sizes of the deposits suggest the existence of larger plutonic source bodies at depth. Although voluminous volcanism is known to have preceded deposit formation throughout the district, the stratigraphic record, reviewed above, shows little evidence of volcanism during the few million year time period synchronous with mineralization. Abundant mafic xenoliths or mafic intrusions contemporaneous with the ore-related porphyries are lacking, consistent with the interpretation that mafic injection, which can act as a trigger for volcanism (e.g., Pallister et al., 1996), was not an important process during the mineralizing period. Stratigraphic units that span the mineralization interval include voluminous, coarse, locally derived conglomerates suggestive of uplift during the Incaic orogeny. Structural features within and adjacent to deposits, as reviewed above, suggest approximately east-west contraction and structural shortening, consistent with crustal thickening, uplift, and structural limiting of eruptive pathways. The lack of volcanism during an extended period of intermediate to silicic magmatic activity, due to contractional tectonics and lack of injection of mafic magma into the system, may have contributed to the abundance and large size of deposits in the Chuquicamata district (e.g., Camus, 2003). At Radomiro Tomic, Chuquicamata, Mina Ministro Hales, and the eastern part of Miranda, most high Cu grades in potassic alteration occur as part of early potassic alteration halos (Proffett, 2009). The mineralization occurs mainly in 0361-0128/98/000/000-00 $6.00

51

51

52

RIVERA ET AL.

Camus, Jeffrey Hedenquist, and Richard Sillitoe were helpful in producing this final version.

hydrothermal biotite, are known to be characteristic of porphyry Cu root zones. For example, such features, in addition to albitization or sodic-calcic alteration, which is also now recognized in the Fortuna Complex (Dilles et al., 1997, 2011), characterize exposed root zones of tilted porphyry Cu systems (C. Meyer, unpub. report, 1966; Proffett, 1979; Carten, 1986; Dilles and Einaudi, 1992). Thus, the concept that parts of the Fortuna Complex can be interpreted as bottoms of porphyry Cu systems (though not of Chuquicamata) is supported by current work.

REFERENCES Alcota, H., Anguita, P., Vargas, X., Ahumada, C., Blanco., N., Canales, A., Cepeda, A., Fajardo, J., Lucero, C., Münster, H., Piquer, J., Urrutia, C., Wettke, E., and Baeza, L., 2009, Marco geológico escala 1:5.000, geoquímica, geofísica y potencial de exploración del área Distrito Chuquicamata, Distrito Codelco Norte, Región de Antofagasta: CODELCO, unpublished report, 144 p. Alvarez, O., 1993, MM, the other side of Chuquicamata?: Society for Mining, Metallurgy and Exploration, Annual Meeting, Reno, Preprint 93-139, 3 p. Alvarez, O., and Aracena, I., 1985, Algunas consideraciones de la petrología y alteración del complejo plutónico de Chuquicamata, Chile: Congreso Geológico Chileno, 4th, Antofagasta, 1985, Actas v. 4, p. 1−30. Alvarez, O., and Flores, R., 1985, Alteración y mineralización hipógena en el yacimiento de Chuquicamata, Chile: Congreso Geológico Chileno, 4th, Antofagasta, Actas, v. 2, p. 78−100. Alvarez, O., and Miranda, J., 1991, Primeras consideraciones geológicas del yacimiento Mansa Mina [ext. abs.]: Congreso Geológico Chileno, 6th, Viña del Mar, 1991, Extended Abstract, 6 p. Alvarez, O., Miranda, J., and Guzmán, P., 1980, Geología del complejo Chuquicamata, in Minería de cobres porfídicos: Anales del Congreso Cincuentenario, Instituto de Ingenieros de Minas de Chile, Santiago, Actas, v. 2, p. 314−363. Ambrus, J., 1975, Compañía de Cobre Chuquicamata. El Yacimiento, in Sutulov, A., ed., El Cobre Chileno, Corporación Nacional del Cobre de Chile: Santiago, Editorial Universitaria, p. 225−231. ——1978, Chuquicamata deposit, in Sutulov, A., ed., International molybdenum encyclopedia, 1778−1978: Santiago, Chile, Internet Publications, v. I, p. 87−93. ——1979, Emplazamiento y mineralización de los pórfidos cupríferos de Chile: Unpublished Ph.D. thesis, Salamanca, Spain, Universidad de Salamanca, 308 p. Ambrus, J., and Soto, H., 1974, Estudio geológico del molibdeno en Chuquicamata: Studia Geológica [Salamanca, Spain], v. 8, p. 45−48. Arnott, A.M., 2003, Evolution of the hydrothermal alteration at the Chuquicamata porphyry copper system, northern Chile: Unpublished Ph.D. thesis, Halifax, Canada, Dalhousie University, 450 p. Ballard, J.R., Palin, J.M., Williams, I.S., Campbell, I.H., and Faunes, A., 2001, Two ages of porphyry intrusion resolved for the super-giant Chuquicamata copper deposit of northern Chile by ELA-ICP-MS and SHRIMP: Geology, v. 29, p. 383−386. Barra, F., Hidalgo, E., Valencia, V., Pardo, R., and Ruiz, J., 2006, Geology and geochronology of the Toki Cluster: Chuquicamata district, northern Chile [abs.]: Geological Society of America Abstracts with Programs, v. 38, no. 7, p. 372. Barra, F., Alcota, H., Rivera, S., Valencia, B., Munizaga, F., and Maksaev, V. (in press), Timing and formation of porphyry Cu-Mo mineralization in the Chuquicamata district, northern, Chile: New constraints from the Toki Cluster: Mineralium Deposita. Blanco, N., and Tomlinson, A., 2006, Formación Sichal: sedimentación aluvial (Eoceno-Oligoceno) sintectónica al evento orogénico incaico. Región de Antofagasta, Chile: Congreso Geológico Chileno, 11th, Antofagasta, 2006, Actas, v. 2, p. 29−32. Blanco, N., Tomlinson, A.J., Mpodozis, C., Pérez de Arce, C., and Matthews, S., 2003, Formación Calama, Eoceno, II Región de Antofagasta (Chile): estratigrafía e implicancias tectónicas: Congreso Geológico Chileno, 10th, Concepción, 2003, Actas, CD-ROM, 10 p. Boric, R., Diaz, J., Becerra, H., and Zentilli, M., 2009, Geology of the Ministro Hales mine (MMH), Chuquicamata district, Chile: Congreso Geológico Chileno, 10th, Santiago, 2009, Actas, Pendrive, S11-055, p. 1−4. Campbell, I.H., Ballard, J.R., Palin, J.M., Allen, Ch., and Faunes, A., 2006, U-Pb zircon geochronology of granitic rocks from the Chuquicamata-El Abra porphyry copper belt of northern Chile: Excimer laser ablation ICPMS analysis: Economic Geology, v. 101, p. 1327−1344. Camus, F., 2003, Geología de los sistemas porfíricos en los Andes de Chile: Santiago, Chile, Servicio Nacional de Geología y Minería, 267 p. ——2005, La minería y la evolución de la exploración en Chile, in Lagos, G., ed., Minería y desarrollo. Foro en economía de minerales, v. 3: Santiago, Chile, Ediciones Universidad Católica de Chile, p. 229−270. Carten, R.B., 1986, Sodium-calcium metasomatism: Chemical, temporal, and spatial relationships at the Yerington, Nevada, porphyry copper deposit: Economic Geology, v. 81, p. 1495−1519.

Acknowledgments Many geologists have contributed to the discovery and development of the deposits of the supergiant Chuquicamata Cu district. Many of them have been named in the text, although many more can only be recognized through their anonymous and dedicated contributions to core logging and geologic mapping of surface exposures and pit benches, and their valuable observations which added to the geologic knowledge of the district. In particular, the great contributions of the late Jorge Miranda, Pedro Guzman, Felipe Rosas, Leonardo Mardones, and Rodolfo Hein must be recognized. The first author thanks the team of geologists who supported him during the exploration of the district between 1999 and 2011, especially Ruben Pardo, a valuable partner in the discoveries of the Toki Cluster, and Marcelo Garcia and Juan Carlos Castelli who helped lead the 1:25,000 scale mapping. This work was completed as the first author’s Master’s thesis in Economic Geology at Universidad Católica del Norte, Antofagasta. He would like to thank Mario Pereira for his kind cooperation. The program of 1:5,000 and 1:25,000 scale mapping was managed by Juan Carlos Marquardt, who provided guidance and valuable discussion throughout the work. Marcelo García, John Dilles, and others contributed to the mapping, and field discussions with them and with Rubén Pardo, Juan Rojo, John Hunt, Guillermo Rochefort, Rodrigo Stefoni, Eduardo Guerra, and Carlos Ahumada provided valuable input. The section on the Chuquicamata deposit benefited from revision of the geologic model, the subsequent underground ramp program, and discussions with Armando Siña, Mario Vivanco, John Hunt, Hector Veliz, René Albornoz, Darryl Lindsay, and the late Felipe Rosas, among others. Sections on the other deposits benefited from discussions with Ricardo Boric, Eduardo Guerra, Juan Jose Latorre, and others. Some of the SHRIMP U-Pb ages reported here were determined under the direction of Joe Wooden and a 40Ar/39Ar age on alunite was done by John Huard. The invitation by Richard Sillitoe and Francisco Camus to provide this contribution and their patience in awaiting its completion are appreciated. The writers wish to thank Carlos Huete of Codelco’s Gerencia de Exploraciones for permission to publish this paper and to Exploraciones Mineras Andinas S.A. for providing support. Alan Stephens is thanked for his support in preparing the text and translation of the first version of the manuscript, as well as Roberto Oyarzún for review and comment on several versions of the manuscript. Special thanks are due to Carolina Fontecilla for preparing the figures and Table 3. Reviews by John Dilles and Marcos Zentilli and editorial advice from Francisco 0361-0128/98/000/000-00 $6.00

52

GEOLOGIC SETTING & PORPHYRY Cu-Mo DEPOSITS, CHUQUICAMATA DISTRICT, N. CHILE

Lindsay, D., Zentilli, M., and Rojas de la Rivera, J., 1995, Evolution of an active ductile to brittle shear system controlling mineralization at Chuquicamata porphyry copper deposit, northern Chile: International Geology Review, v. 37, p. 945−958. López, V.M., 1939, The primary mineralization at Chuquicamata, Chile, S.A.: Economic Geology, v. 34, p. 674−711. ——1942, Chuquicamata, Chile, in Newhouse, W.H., ed., Ore deposits as related to structural features: Princeton, NJ, Princeton University Press, p. 126–128. Lorca, G., Rojas, G., and Valdés, R., 2003, Oxide mineralization in the Radomiro Tomic-Chuqui Norte orebody [abs.]: Congreso Geológico Chileno, 10th, Concepción, 2003, Actas, CD-ROM, 1 p. Maksaev, V., 1990, Metallogeny, geological evolution, and thermochronology of the Chilean Andes between latitudes 21° and 26° south, and the origin of major porphyry copper deposits: Unpublished Ph.D. thesis, Halifax, Nova Scotia, Dalhousie University, 554 p. Makasev, V., and Zentilli, M., 1988, Metallogenic framework of the large porphyry copper deposits of the Andes of northern Chile: Congreso Geológico Chileno, 5th, Santiago, 1988, Actas, v.1, p. B181−B212. ——1999, Fission track thermochronology of the Domeyko Cordillera, northern Chile: Implications for Andean tectonics and porphyry metallogenesis: Exploration and Mining Geology, v. 8, p. 65−89. Marinovic, N., and Lahsen, A., 1984, Hoja Calama, Región de Antofagasta. Servicio Nacional de Geologia y Mineria, Carta Geológica de Chile, N° 58, escala 1:250.000, 140 p. Marquardt, J., Rojo, J., Rivera, S., and Pizarro, J., 2009, Geología del pórfido cuprífero Miranda: nuevo descubrimiento en el clúster Toki, distrito de Chuquicamata, Chile: Congreso Geológico Chileno, 12th, Santiago, 2009, Actas, Pendrive, 4 p. Martínez, G., 1943, Orígenes y desarrollo de Chuquicamata bajo la Chile Exploration Company: Archivo Fotográfico y Digital, Biblioteca Nacional, Chile: www.memoriachilena.cl/temas/documento_detalle.esp?id=MC0037067 Mathur, R., Ruiz, J., and Munizaga, F., 2000, Relationship between copper tonnage of Chilean base-metal porphyry deposits and Os isotope ratios: Geology, v. 28, p. 555−558. May, G., Hartley, A.J., Stuart, F.M., and Chong, G., 1999, Tectonic signatures in arid continental basins: An example from the upper Miocene-Pleistocene Calama Basin, Andean forearc, northern Chile: Palaeo, v. 151, p. 55−77. May, G., Hartley, A.J., Chong, G., Stuart, F., Turner, P., and Kape, S.J., 2005, Eocene to Pleistocene lithostratigraphy, chronostratigraphy and tectonosedimentary evolution of the Calama Basin, northern Chile: Revista Geológica de Chile, v. 32, no. 1, p. 33−58. McInnes, B.I.A., Farley, K.A., Sillitoe, R.H., and Kohn, B.P., 1999, Application of apatite (U-Th)/He thermochronometry to the determination of the sense and amount of vertical displacement at the Chuquicamata porphyry copper deposit, Chile: Economic Geology, v. 94, p. 937−947. Meyer, C., 1965, An early potassic type of wall-rock alteration at Butte, Montana: American Mineralogist, v. 50, p. 1717−1722. Meyer, C., Shea E.P., and Goddard, C.C., 1968, Ore deposits at Butte, MT, in Ridge, J.D., ed., Ore deposits of the United States (1933–1967), GratonSales Volume, v. 2: New York, American Institute of Mining, Metallurgical, and Petroleum Engineers, p. 1373−1416. Millán, A., 2006, La minería metálica en Chile en el Siglo XX: Santiago, Chile, Editorial Universitaria, 179 p. Morandé, J., Mpodozis, C., Valencia, V., Arriagada, C., and Marquardt, C., 2012, Las diamictitas de Sierra Limón Verde, Antofagasta: Evidencias de glaciación Neoproterozoica en el norte de Chile?: Congreso Geológico Chileno, 12th, Antofagasta, 2012, Actas, Pendrive, p. 271−273. Mortimer, C., Münchmeyer, C., and Urqueta, I., 1978, Emplazamiento del yacimiento Exótica, Chile: Revista Geológica de Chile, no. 6, p. 41−51. Müller, G., and Quiroga, J., 2003, Geology of the MM copper deposit, Chuquicamata district—an update [abs.]: Congreso Geológico Chileno, 10th, Concepción, 2003, Actas, CD-ROM, 1 p. Münchmeyer, C., 1996, Exotic deposits—products of lateral migration of supergene solutions from porphyry copper deposits: Society of Economic Geologist Special Publication 5, p. 43−58. Münchmeyer, C., and Urqueta, I., 1974, Geología del yacimiento Exótica, in Klohn, E., ed., Coloquio sobre fenómenos de alteración y metamorfismo en rocas volcánicas e intrusivas: Santiago, Departamento de Geología, Universidad de Chile, p. 213−253. Newberg, D.W., 1965, Geochemical implications of chrysocholla-bearing alluvial gravels: Economic Geology, v. 62, p. 932−956.

Chavez, W.X., 2000, Supergene oxidation of copper deposits: Zoning and distribution of copper oxide minerals: Society of Economic Geologists Newsletter 41, p. 1 and 10−21. Chong, G., and Pardo, R., 1994, Geología del Distrito Chuquicamata. Segunda Región de Antofagasta: Mapa Escala 1:100.000, CODELCO unpublished map. CODELCO, 2010, Jornadas de Geociencias, Codelco Norte, Primera Reunión Bi-Anual Geociencias 2010, Subgerencia de Geología y Geotecnia, Gerencia de Recursos Mineros y Desarrollo, Calama, 2010, 398 p. Córdova, S., Demané, E., Fortt, L., Ordenes, T., and Ramírez, F., 2010, Alteración y mineralización hipógena del yacimiento de Chuquicamata: Jornadas de Geociencias, Codelco Norte, Primera Reunión Bi-Anual Geociencias 2010, Subgerencia de Geología y Geotecnia, Gerencia de Recursos Mineros y Desarrollo, Calama, 2010, p. 11−21. Cuadra, P., and Camus, F., 1998, The Radomiro Tomic porphyry copper deposit, northern Chile, in Porter, T.M., ed., Porphyry and hydrothermal copper and gold deposits. A global perspective: Adelaide, Australian Mineral Foundation, p. 99−109. Cuadra, P., and Rojas, H., 2001, Oxide mineralization at the Radomiro Tomic porphyry copper deposit, northern Chile: Economic Geology, v. 96, p. 387−400. Cuadra, P., Grez, E., and Gröpper, H., 1997, Geología del yacimiento Radomiro Tomic: Congreso Geológico Chileno, 7th, Antofagasta, 1997, Actas, v. 3, p. 1918−1922. Díaz, J., 2010, Actualización de la geología del yacimiento Mina Ministro Hales (MMH): Jornadas de Geociencias, Codelco Norte, Primera Reunión Bi-Anual Geociencias 2010, Subgerencia de Geología y Geotecnia, Gerencia de Recursos Mineros y Desarrollo, Calama, 2010, p. 109−120. Dilles, J.H., and Einaudi, M.T., 1992, Wall-rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposit, Nevada—a 6km vertical reconstruction: Economic Geology, v. 87, p. 1963−2001. Dilles, J.H., Tomlinson, A.J., Martin, M.W., and Blanco, N., 1997, El Abra and Fortuna Complexes: A porphyry copper batholith sinistrally displaced by the Falla Oeste: VIII Congreso Geológico Chileno, 8th, Antofagasta, 1997, Actas, v. 3, p. 1883−1887. Dilles, J.H., Tomlinson, A. J., García, M., and Alcota, H., 2011, The geology of the Fortuna Granodiorite Complex, Chuquicamata district, northern Chile: Relation to porphyry copper deposits: Biennial Meeting of the Society of Geology Applied to Mineral Deposits (SGA), 11th, Antofagasta, 2011, Proceedings Volume, p. 399−401. Domic, E., 2001, Hidrometalurgia: Fundamentos, procesos y aplicaciones: Santiago, Chile, Andros Impresiones, 947 p. Dunbar, D.M., 1952, History of Chuquicamata copper: Mining Engineering, December 1952, v. 4, p. 1164−1165. Faunes, A., Hintze, F., Siña, A., Veliz, H., and Vivanco, M., 2005, Chuquicamata, core of a planetary scale Cu-Mo anomaly, in Porter, T.M., ed., Super porphyry copper and gold deposits—a global perspective: Adelaide, PGC Publishing, v. 1, p. 151−174. Flores, R., 1985, Control del enriquecimiento supérgeno en el yacimiento Chuquicamata, Chile: Congreso Geológico Chileno, 4th, Antofagasta, 1985, Actas, v. 3, p. 228−249. Fréraut, R., Ossandón, G., and Gustafson, L.W., 1997, Modelo geológico de Chuquicamata: Congreso Geológico Chileno, 7th, Antofagasta, 1997, Actas, v. 3, p. 1898−1902. Gandarillas, J., 1915, Bosquejo del estado actual de la industria minera del cobre en el extranjero i en Chile: Santiago, Chile, Sociedad Nacional de Minería, 130 p. García, M., Dilles, J.H., Fuentealba, J., Daroch, G., Gómez, I., Pérez, F., Münster, H., Castelli, J.C., and Piquer, J., 2009, Informe de resultados del programa de mapeo escala 1:25.000 (2004−2009): CODELCO, unpublished report, 64 p. Gustafson, L.B., and Hunt, J.P., 1975, The porphyry copper deposit at El Salvador, Chile: Economic Geology, v. 70, p. 857−912. Gutierrez-Viñuales, A., 2008, Chuquicamata: patrimonio industrial de la minería del cobre en Chile: Apuntes, [Bogotá; Colombia], v. 21, p. 75−91. Hervé, M., Sillitoe, R.H., Wong, C., Fernández, P., Crignola, F., Ipinza, M., and Urzúa, F., 2013, Geologic overview of the Escondida porphyry copper district, northern Chile: Society of Economic Geologists Special Publication 16, p. 55–78. Jarrel, G.W., 1944, Oxidation at Chuquicamata, Chile: Economic Geology, v. 39, p. 251−286. Lindsay, D.D., 1998, Structural control and anisotropy of mineralization in the Chuquicamata porphyry deposit, Chile: Unpublished Ph.D thesis, Halifax, Nova Scotia, Dalhousie University, 381 p. 0361-0128/98/000/000-00 $6.00

53

53

54

RIVERA ET AL.

Núñez, L., Agüero, C., Cases, B., and De Souze, P., 2003, El campamento minero Chuquicamata-2 y la explotación cuprífera prehispánica en el Desierto de Atacama: Estudios Atacameños [San Pedro de Atacama], n° 25, p. 7−34. Ossandón, G., and Zentilli, M., 1997, El Distrito de Chuquicamata: una concentración de cobre de clase mundial: Congreso Geológico Chileno, 7th, Antofagasta, 1997, Actas, v. 3, p. 1888−1892. Ossandón, G., Fréraut, R., Gustafson, L.B., Lindsay, D.D., and Zentilli, M., 2001, Geology of the Chuquicamata mine: A progress report: Economic Geology, v. 96, p. 249−270. Pallister, J.S., Hoblitt, R.P. Meeker, G.P., Knight, R.J., and Siems, D.F., 1996, Magma mixing at Mount Pinatubo: Petrographic and chemical evidence from the 1991 deposits, in Newhall, C.G., and Punongbayan, R.S., eds., Fire and mud, eruptions and lahars of Mount Pinatubo, Philippines: Seattle, University of Washington Press, p. 687−731. Parra, A., Zamarreño, J., Herrera, P., Carrasco, M., and Cabrera, J., 2010, Actualización de la geología del yacimiento Radomiro Tomic y su relación con Chuquicamata: Jornadas de Geociencias, Codelco Norte, Primera Reunión Bi-Anual Geociencias 2010, Subgerencia de Geología y Geotecnia, Gerencia de Recursos Mineros y Desarrollo, Calama, 2010, p. 23−37. Parsons, A.B., 1933, The porphyry coppers: New York, American Institute of Mining, Metallurgy, and Petroleum Engineers, 581 p. Perelló, J., Mühr, R., Mora, R., Martínez, E., Brockway, H., Swaneck, T., Artal, J., Mpodozis, C., Münchmeyer, C., Clifford, J., Acuña, E., Valenzuela, D., and Argandoña, R., 2011, Wealth creation through exploration in a mature terrain: the case history of the Centinela district, northern Chile porphyry copper belt: Society of Economic Geologists Special Publication 15, p. 229−252. Perry, V., 1952, Geology of the Chuquicamata orebody: Mining Engineer, December, v. 4, p. 1166−1168. ——1991, A half century as mining and exploration geologist with the Anaconda Company: An oral history conducted by Eleanor Swent: Regional Oral History Office, Bancroft Library, University of California, Berkeley. Petersen, G., 2011, Minería y metalurgia en el Antiguo Perú: Lima, Peru, Visadeva, 118 p. Proffett, J.M., 1973, Structure of the Butte district, Montana: Montana Bureau of Mines and Geology Publication O1, p. G-1−G 12. ——1979, Ore deposits of the western United States: A summary: Nevada Bureau of Mines and Geology Report 33, p. 13−32. ——2009, High Cu grades in porphyry Cu deposits and their relationship to emplacement depth of magmatic sources: Geology, v. 37, p. 675−678. Reutter, K.J., Scheuber, E., and Chong, G., 1996, The Precordilleran fault system of Chuquicamata, northern Chile: Evidence for reversals along arcparallel strike-slip faults: Tectonophysics, v. 259, p. 231−228. Reynolds, P., Ravenhurst, C., Zentilli, M., and Lindsay, D., 1998, High-precision 40Ar/39Ar dating of two consecutive hydrothermal events in the Chuquicamata porphyry copper system, Chile: Chemical Geology, v. 148, p. 50−60. Rivera, S., Kovacic, P., and Pardo, R., 2003, Descubrimiento del pórfido de cobre Quetena y actualización del conocimiento geológico del Clúster Toki: Convención Anual del Instituto de Ingenieros de Minas, 54th, Antofagasta, 2003, CD-ROM, 21 p. Rivera, S.L., and Pardo, R., 2004, Discovery and geology of the Toki porphyry copper deposit, Chuquicamata district, northern Chile: Society of Economic Geologists Special Publication 11, p. 199−211. Rivera, S.L., Pardo, R., Alcota, H., Fontecilla, C., Kovacic, P., Pizarro, J. and Rojo, J., 2006, Reseña de la exploración y geología del clúster de pórfidos de cobre Toki, distrito Chuquicamata: Congreso Geológico Chileno, 11th, Antofagasta, 2006, Actas, v. 2, CD-ROM, p. 343−346. Rivera, S., Alcota, H., Fontecilla, C., and Kovacic, P., 2009, Supergene modification of porphyry columns and the application to exploration with special reference to the southern part of the Chuquicamata district, Chile: Society of Economic Geologists Special Publication 14, p. 1−14. Roa, R., 1975, Compañía Minera Exótica, in CODELCO, El Cobre Chileno: Santiago, Chile, Editorial Universitaria, p. 294−301. Setyandhaka, D., Arif, J., and Proffett, J., 2008, Characteristics of the roots of a classic Cu-Au porphyry system: the Batu Hijau porphyry deposit, Indonesia [ext. abs.]: PACRIM Congress 2008, Australia, Extended Abstracts, p. 445−450. Sillitoe, R.H., 1973, The tops and bottoms of porphyry copper deposits: Economic Geology, v. 68, p. 799−815. ——2005, Supergene oxidized and enriched porphyry copper and related deposits: Economic Geology 100th Anniversary Volume, p. 723−768. 0361-0128/98/000/000-00 $6.00

Sillitoe, R.H., and McKee, E.H., 1996, Age of supergene oxidation and enrichment in the Chilean porphyry copper province: Economic Geology, v. 91, p. 164−179. Sillitoe; R.H. and Perelló, J., 2005, Andean copper province: Tectonomagmatic settings, deposit types, metallogeny, exploration, and discovery: Economic Geology 100th Anniversary Volume, p. 845−890. Sillitoe, R.H., Marquardt, J.C, Ramírez, F., Becerra, H., and Gómez, M., 1996, Geology of the concealed MM porphyry copper deposit, Chuquicamata district, northern Chile: Society of Economic Geologists Special Publication 5, p. 59−69. Sutulov, A., 1978, Chilean mining: Santiago, Chile, Centro de Investigaciones Mineras y Metalúrgicas, 241 p. Taylor, A.V., Jr., 1935, Ore deposits of Chuquicamata, Chile: International Geological Congress, 16th, Washington, DC, Proceedings, v. 2, p. 473−484. Thomas, A., 1969, Resumen sobre la geología de las Hojas Chuquicamata y Soledad, Provincia de Antofagasta: Santiago, Chile, Instituto de Investigaciones Geológicas, Unpublished report, 18 p, 1 map. Tomlinson, A.J., and Blanco, N., 1997a, Structural evolution and displacement history of the West Fault system, Precordillera, Chile: Part I. Synmineral history: Congreso Geológico Chileno, 7th, Antofagasta, 1997, Actas, v. 3, p. 1873−1877. ——1997b, Structural evolution and displacement history of the West Fault system, Precordillera, Chile: Part II. Postmineral history: Congreso Geológico Chileno, 7th, Antofagasta, 1997, Actas, v. 3, p. 1878−1882. ——2007, Geología de la franja El Abra-Chuquicamata, Región II (21°45'−22°30'S): Servicio Nacional de Geología y Minería [Chile], Informe Registrado IR-07, 194 p. Tomlinson, A.J., and Cornejo, P., 2012, Regional distribution of middle Eocene-early Oligocene porphyry copper centers in northern Chile: Second-order patterns and possible causes: Congreso Geológico Chileno, 12th, Antofagasta, 2012, Actas, Pendrive, p. 40−42. Tomlinson, A.J., Blanco, N., Maksaev, V., Dilles, J.H., Grunder, A., and Ladino, M., 2001, Geología de la Precordillera andina de Quebrada BlancaChuquicamata, Regiones I y II (20°30'−22°30'S): Servicio Nacional de Geología y Minería [Chile], Informe Registrado IR01-20, 444 p. Tomlinson, A.J., Blanco, N., and Dilles, J.H., 2010, Carta Calama, Región de Antofagasta: Carta Geólogica de Chile, Serie Preliminar, No. 8, Servicio Nacional de Geología y Minería [Chile], Escala 1:50,000. Tomlinson, A.J., Blanco, N., García, M., Baeza, L., Alcota, H., Ladino, M., Perez de Arce, C., Fanning, M., and Martin, M., 2012, Permian exhumation of metamorphic complexes in the Calama area: Evidence for flat-slab subduction in northern Chile during the San Rafael tectonic phase and origin of the Central Andean Gravity High: Congreso Geológico Chileno, 12th, Antofagasta, 2012, Actas, Pendrive, p. 209−211. Valdés, S., 1887, Informe sobre el Estudio Minero i Agrícola de la región comprendida entre el paralelo 23 i la Laguna de Ascotán presentado al Ministerio de lo Interior: Santiago, Chile, Imprenta Nacional, 100 p. Wilson, J., Zentilli, M., Boric, R., Díaz, J., and Maksaev, V., 2011, Geochemistry of the Triassic and Eocene igneous host rocks of the Mina Ministro Hales porphyry copper deposit, Chuquicamata district, Chile: Biennial Meeting, Society of Geology Applied to Mineral Deposits (SGA), 11th, Antofagasta, 2012, Proceedings Volume, p. 426−428. Yeatman, P., 1916, Mine of Chile Exploration Co., Chuquicamata, Chile: Engineering and Mining Journal, v. 101, p. 307−314. ——1932, Problems at the Chuquicamata and Braden Copper properties, in Yeatman, P., and Bain, F., eds., Choice of methods in mining and metallurgy: AIME Series, v. 1, p. 1−34. Zentilli, M., Krogh, T.E., Maksaev, V., and Alpers, C.N., 1994a, Uranium-lead dating of zircons from the Chuquicamata and La Escondida porphyry copper deposits, Chile: Inherited zircon cores of Paleozoic age with Tertiary overgrowths: Comunicaciones [Departamento de Geología, Universidad de Chile, Santiago], v. 45, p. 101−110. Zentilli, M., Leiva, G., Rojas, J., and Graves, M.C., 1994b, The Chuquicamata porphyry copper system revisited: Congreso Geológico Chileno, 7th, Concepción, 1994, Actas, v. 2, p. 1647−1651. Zentilli, M., Graves., Lindsay, D., Ossandón, G., and Camus, F., 1995, Recurrent mineralization in the Chuquicamata porphyry copper system: Restrictions on genesis from mineralogical, geochronological and isotopic studies, in Clark, A.H., ed., Proceeding, Giant Ore Deposits II Workshop, Kingston, ON, Queens University, p. 86−100 (second corrected printing, p. 90−113).

54

© 2012 Society of Economic Geologists, Inc. Special Publication 16, pp. 55–78

Chapter 3 Geologic Overview of the Escondida Porphyry Copper District, Northern Chile MIGUEL HERVÉ1,* RICHARD H. SILLITOE,2 CHILONG WONG,1 PATRICIO FERNÁNDEZ,1,** FRANCISCO CRIGNOLA,1,*** MARCO IPINZA,1 AND FELIPE URZÚA3 1 Minera

Escondida Limitada, Avenida de la Minería 501, Antofagasta, Chile

2 27 3 BHP

West Hill Park, Highgate Village, London N6 6ND, England

Billiton, 10 Marina Boulevard 50-01, Marina Bay Financial Centre Tower 2, Singapore 018983

Abstract The giant Escondida district in northern Chile, discovered in 1981, includes the major porphyry copper deposits at Escondida-Escondida Este, Escondida Norte-Zaldívar, Pampa Escondida, and two small deposits (the Escondida cluster), besides the Chimborazo deposit. The district contains at least 144 million metric tons (Mt) of copper. The Escondida district is part of the middle Eocene to early Oligocene porphyry copper belt, which follows the trench-parallel Domeyko fault system, a product of the Incaic transpressional tectonic phase. At the district scale, the major N-striking Portezuelo-Panadero oblique-reverse fault juxtaposes latest Carboniferous to Early Permian igneous basement with an andesitic volcanic sequence of late Paleocene to early Eocene age, both of which host the porphyry copper mineralization. Immediately before and during porphyry copper formation, a thick siliciclastic sequence with andesitic volcanic products intercalated toward the top (San Carlos strata) filled a deep basin, generated by clockwise rigid-block rotation, within the confines of the Escondida cluster. The presence of these volcanic rocks suggests that an eruptive center was still active within the confines of the Escondida cluster when deposit formation began. The deposits are all centered on multiphase biotite granodiorite porphyry stocks, which were predated by dioritic to monzodioritic precursors and closely associated with volumetrically minor, but commonly highgrade, magmatic-hydrothermal breccias. The earliest porphyry phases consistently host the highest grade mineralization. Alteration-mineralization zoning is well developed: potassic and overprinted gray sericite assemblages containing chalcopyrite and bornite at depth; more pyritic chlorite-sericite and sericitic zones at intermediate levels; and shallow advanced argillic developments, the remnants of former lithocaps that could have attained 200 km2 in total extent. The latter are associated with high-sulfidation, copper-bearing sulfide mineralization, much of it in enargite-rich, massive sulfide veins. The Escondida and Escondida Norte-Zaldívar deposits, formed at ~38 to 36 Ma, are profoundly telescoped, whereas the earlier (~41 Ma) Chimborazo and later (~36−34 Ma) Escondida Este and Pampa Escondida deposits display only minor telescoping, suggesting that maximal Incaic uplift and erosion took place from 38 to 36 Ma. The Portezuelo-Panadero and subsidiary longitudinal faults in the district—inverted normal structures that formerly delimited the eastern side of a Mesozoic backarc basin—were subjected to sinistral transpression prior to deposit formation (pre-41 Ma), which gave rise to the clockwise block rotation responsible for generation and initial synorogenic filling of the San Carlos depocenter. The Escondida district was then subjected to transient dextral transpression during emplacement of the NNE- to NE-oriented porphyry copper intrusions and associated alteration and mineralization (~38−34.5 Ma). This dextral regime had waned by the time that a N-trending, late mineral rhyolite porphyry was emplaced at Escondida Este and was replaced by transient sinistral transpression during end-stage formation of NW-striking, high and intermediate sulfidation, massive sulfide veins and phreatic breccia dikes. Since 41 Ma, the faults in the district have undergone no appreciable displacement because none of the porphyry copper deposits shows significant lateral or vertical offset. Renewed uplift and denudation characterized the late Oligocene to early Miocene, during which the extensive former lithocap was largely stripped and incorporated as detritus in a thick piedmont gravel sequence. Development of hematitic leached capping and attendant chalcocite enrichment zones, along with subsidiary oxide copper ore, was active beneath the topographic prominences at Escondida, Escondida Norte-Zaldívar, and, to a lesser degree, Chimborazo from ~18 to 14 Ma, but supergene activity was much less important at the topographically lower, gravel-covered Pampa Escondida deposit. After ~14 Ma, supergene processes were soon curtailed by the onset of hyperaridity throughout much of northern Chile.

† Corresponding author: e-mail, [email protected] Present addresses: *Antofagasta Minerals S.A., Avenida Apoquindo 4001, piso 18, Las Condes, Santiago, Chile. **BHP Billiton, Avenida Américo Vespucio 100, piso 8, Las Condes, Santiago, Chile. ***BHP Billiton, 10 Marina Boulevard 50-01, Marina Bay Financial Centre Tower 2, Singapore 018983.

55

56

HERVÉ ET AL.

Introduction THE SUPERGIANT Escondida porphyry copper district is located ~170 km southeast of the port city of Antofagasta, the capital of Antofagasta Region, northern Chile (Fig. 1). The district comprises the Escondida cluster and, 15 km northwest, the Chimborazo deposit. The Escondida cluster is made up of the Escondida (including Escondida Este), Escondida Norte-Zaldívar, Pampa Escondida, and small Baker and Pinta Verde deposits. Escondida Norte and Zaldívar are the eastern and western parts of a single deposit, separated by a property boundary. The district is located in the Atacama Desert, where it lies beneath rounded, talus-mantled hills and broad, alluviated plains of the Domeyko Cordillera (Fig. 2), part of the Precordillera morphostructural province, at elevations of 2,600 to 3,500 m above sea level. The giant status of the Escondida district is underscored by its current resources plus past production of 144 million metric tons (Mt) of Cu, as detailed in Table 1. The resources also contain locally elevated molybdenum and gold contents. The Escondida and Escondida Norte-Zaldívar deposits are mined from two open pits. Escondida and Escondida Norte, owned and operated by Minera Escondida Limitada (BHP Billiton 57.5%, Rio Tinto 30%, JECO Corp. 10%, and JECO 2 Ltd. 2.5%), produce enriched sulfide ore, treated by conventional flotation, and oxide and low-grade enriched sulfide ores, subjected to heap leaching. The much smaller Zaldívar oxide and

enriched sulfide leach operation (Table 1) is owned and operated by Barrick Gold Corporation. The Escondida-Escondida Norte operation is the world’s largest copper producer, and since 2004 has shipped >1 Mt of copper per year for a 20-year total of 17.7 Mt (Comisión Chilena del Cobre, 2011). The Pampa Escondida, Chimborazo, Baker, and Pinta Verde deposits, also owned by Mineral Escondida Limitada, remain under study. This overview of the Escondida district provides geologic and alteration-mineralization descriptions of the Chimborazo, Escondida (including Escondida Este), Escondida Norte-Zaldívar, and Pampa Escondida deposits, the last three over 1,500 to 2,000 vertical meters. The deposit descriptions are prefaced by summaries of the exploration history and geologic setting and followed by a synthesis of the district chronology and general discussion. The present work is based on district-scale geologic remapping at 1:25,000 scale and logging and interpretation of ~1,000,000 m of diamond drill core, over half from 430 holes drilled to depths of >1,000 m, during a six-year brownfields exploration program led by the first author; however, the results reported here must still be considered as work in progress. Exploration History The discovery history of the Escondida deposit by a joint venture between Utah International and Getty Minerals was exhaustively described by Ortíz et al. (1985, 1986), Lowell (1990, 1991), and Ortíz (1995), and briefly summarized by Sillitoe (1995). In 1979, the joint venturers began a search for chalcocite enrichment zones concealed beneath postmineral cover in northern Chile. Initial fieldwork included a regional drainage geochemical survey, which defined a copper-molybdenum anomaly centered on a well-known alteration zone at Cerro Colorado, now the Escondida deposit. Before the drainage geochemical results became available, the joint-venture’s landman drew attention to the prominent color anomaly at Cerro Colorado (Fig. 2a), and two claims were filed immediately. A field inspection confirmed porphyry copper potential, leading to a campaign of grid rock-chip geochemical sampling. Zoned alteration and geochemical patterns, presence of peripheral veins, high molybdenum geochemistry, and partially hematitic leached capping led in early 1981 to the decision to drill five rotary holes at 1-km intervals. The holes were collared in the gravel-filled depression between what are now the Escondida and Escondida Norte-Zaldívar deposits, but copper values encountered were 1,000 km long and coincides with the Domeyko Cordillera and its northern and southern extensions between latitudes 21° and 31° S (Sillitoe, 1988; Camus, 2003; Sillitoe and Perelló, 2005; Fig. 1). At the latitude of Escondida, the Domeyko Cordillera is a 35-km-wide, up to 4,500-m-high, internally broken, N-trending range that separates the Central Depression, to the west, from large endorheic sedimentary basins to the east (Atacama and Punta Negra salars; Fig. 1). Most of the Domeyko Cordillera is formed by Late Carboniferous to Triassic volcanic rocks (La Tabla Formation in 0361-0128/98/000/000-00 $6.00

58

ESCONDIDA PORPHYRY COPPER DISTRICT, N. CHILE

the Late Cretaceous and Incaic events (Amilibia and Skarmeta, 2003; Amilibia et al., 2008). Richards et al. (2001) proposed that the main porphyry copper clusters in the Domeyko Cordillera were located at intersections between the fault system and NW-trending, trans-Andean lineaments (Salfity, 1985)—possible translithospheric fault zones—that are well marked farther east in Argentina as linear Miocene to Recent volcanic complexes. Nonetheless, no field evidence for major NW-trending faults has been recognized to date in the Domeyko Cordillera (Maksaev et al., 1991; Mpodozis et al., 1993b; Gardeweg et al., 1994; Urzúa, 2009).

59

Escondida Este, Escondida Norte-Zaldívar, and Pampa Escondida deposits is well established by 20 U-Pb zircon ages determined for both the andesitic and rhyolitic volcanic rocks and coeval monzogranite, monzogranite porphyry, rhyolite porphyry, tonalite, quartz monzodiorite porphyry, and quartz diorite porphyry intrusions, which range in age from ~300 to 282 Ma (Richards et al., 1999; Jara et al., 2009; Urzúa, 2009; this study). The youngest Early Permian rhyolitic and andesitic strata may postdate the intrusive suite (Table 2). Comparable latest Carboniferous to Early Permian ages, for both igneous rocks and associated alteration, were obtained immediately north of the Escondida Norte-Zaldívar deposit (Cornejo et al., 2006). The Augusta Victoria Formation in the vicinity of the Escondida cluster is dominated by calc-alkaline andesitic flows, also shallowly dipping and calc-alkaline in composition, although more felsic units occur farther west. The volcanic rocks were dated by the U-Pb zircon method at ~58 to 53 Ma (Urzúa, 2009; Table 2), confirming a Paleocene age, in accord with previous, more regional work (Marinovic et al., 1995). Hand-sample distinction between hydrothermally altered La Tabla and Augusta Victoria andesitic volcanic rocks in and around the deposits is impossible (cf. Jara et al., 2009); however, rare earth element patterns for the two units are distinctive (Richards et al., 2001) and are used routinely to separate them for mapping purposes. The oldest post-Paleozoic intrusive rocks in the Escondida district are alkaline gabbro and associated diorite, monzodiorite, monzonite, and granite of Late Cretaceous age (~77–72

Escondida district geology Figure 3 is a geologic map of the Escondida district grossly simplified from Urzúa (2009). The district is dominated by two units: La Tabla Formation basement rocks in the east and the Mesozoic backarc sedimentary succession (El Profeta and Santa Ana Formations) and unconformably overlying Augusta Victoria Formation in the west. The Escondida cluster spans the faulted contact between these two environments, whereas Chimborazo is hosted exclusively by the Augusta Victoria strata. La Tabla Formation comprises generally shallowly dipping, andesitic and rhyolitic volcanic rocks, the former principally flows and the latter dominated by welded ignimbrite, as proposed by Richards et al. (2001). Both the volcanic and coeval intrusive rocks are calc-alkaline in composition (Richards et al., 2001; Urzúa, 2009). The latest Carboniferous to Early Permian age of La Tabla Formation host rocks within the

TABLE 2. U-Pb Zircon Ages for Late Paleozoic Intrusive Rocks and La Tabla and Augusta Victoria Formations, Escondida District Sample

Location

Age (Ma)

Andesite Andesitic tuff Andesite Rhyolitic tuff Andesite Biotite granodiorite porphyry Andesite Andesite Rhyolite

La Tabla Formation volcanic rocks Escondida Este 282.0 ± 2.0 Baker-Escondida Norte 284.0 ± 4.0 Escondida 286.0 ± 2.0 Escondida Este 287.0 ± 3.0 Pampa Escondida 288.0 ± 2.4 Zaldívar 287.1 ± 4.4 Pampa Escondida 288.8 ± 2.4 Zaldívar 294.4 ± 4.6 Zaldívar 298.2 +5.5/-4.9

Quartz monzodiorite porphyry Monzogranite Rhyolite porphyry Monzogranite Monzogranite porphyry Quartz diorite porphyry Rhyolite porphyry Monzodiorite porphyry Monzogranite porphyry Monzogranite Tonalite

Pampa Escondida Zaldívar Zaldívar Zaldívar Escondida Norte Pampa Escondida Escondida Norte Escondida Este Escondida Este Escondida Norte Escondida Este

Andesitic flow Andesitic flow Andesitic flow Andesitic flow Andesitic flow

Escondida cluster Escondida cluster Escondida cluster Escondida cluster Escondida cluster

Late Paleozoic intrusive rocks 287.6 ± 3.3 289.9 ± 3.5 290.0 ± 4.0 291.1 ± 2.3 293.0 ± 6.0 293.0 ± 4.3 294.2 ± 2.4 296.8 ± 3.2 296.6 ±4.4/-3.8 298.8 ± 2.6 300.1 ± 3.5

Reference

Urzúa (2009) Urzúa (2009) Urzúa (2009) Urzúa (2009) This study Jara et al. (2009) This study Jara et al. (2009) Jara et al. (2009)

This study Morales (2009) Richards et al. (1999) Morales (2009) This study This study P.J. Pollard and R.G. Taylor, unpub. rept., 2002 This study This study P.J. Pollard and R.G. Taylor, unpub. rept., 2002 This study

Augusta Victoria Formation volcanic rocks

0361-0128/98/000/000-00 $6.00

56.8 ± 0.5 55.8 ± 0.8 54.9 ± 0.8 53.9 ± 0.8 57.0 ± 0.6

59

Urzúa (2009) Urzúa (2009) Urzúa (2009) Urzúa (2009) Urzúa (2009)

60

HERVÉ ET AL.

Recent alluvium (Holocene) Salar (Miocene-Holocene) Pampa de Mulas Formation: Piedmont gravel (late Oligocene-middle Miocene) Granodiorite, dacite, andesite, + rhyolite porphyries (middle-late Eocene, ~38-34 Ma) San Carlos strata: Siliciclastic + volcaniclastic rocks Eocene) Diorite, quartz diorite, manzodiorite, + quartz monzonite intrusions (middle Eocene, ~43-41 Ma) Augusta Victoria Formation: Andesitic + dacite volcanic rocks (late Paleocene-early Oligocene) Gabbro, diorite, + monzodiorite intrusions (Late Cretaceous, ~77-72 Ma) Santa Ana Formation: Siliciclastic rocks (Late Jurassic-Neocomian) El Profeta Formation: Limestone + gypsum (Late Triassic-Kimmeridgian) Diorite to granite intrusions (late Paleozoic-Late Triassic) La Tabla Formation: Rhyolitic + andesitic volcanic rocks (Late Carboniferous-Early Permian) Subsurface limit of San Carlos strata Porphyry copper deposit (>0.3 % Cu) Observed fault Inferred fault High-angle reverse fault Syncline FIG. 3. Geology of the Escondida district, showing the locations of the Chimborazo, Escondida-Escondida Este, Escondida Norte-Zaldívar, Pampa Escondida, Baker, and Pinta Verde porphyry copper deposits. Note the subsurface extent of the San Carlos strata immediately east of Escondida Este and Escondida Norte and the NE-striking reverse fault that bounds them to the southeast. Grossly simplified and slightly modified from Urzúa (2009). 0361-0128/98/000/000-00 $6.00

60

ESCONDIDA PORPHYRY COPPER DISTRICT, N. CHILE

Ma; U-Pb zircon) in the southwestern quadrant of Figure 2, which are part of a N-trending, 15-km2 pluton (Urzúa, 2009). Their geotectonic significance remains uncertain, although a backarc setting is suspected (Richards et al., 2001; Urzúa, 2009) by comparison with alkaline gabbro intrusions of similar age farther north in the Domeyko Cordillera (Mpodozis et al., 2005). Two additional gabbro-to-granite complexes of Late Cretaceous to early Paleocene age and backarc affiliation are present along the western side of the Escondida district (Urzúa, 2009). The next intrusive activity in the district gave rise to the epizonal complexes associated with the porphyry copper deposits. These commence with fine-grained hornblende diorite and monzodiorite, which constitute a series of stocks, occupying an area of 45 km2, in the northwestern part of the district (Fig. 3) as well as occurring as small bodies within the Chimborazo and Pampa Escondida deposits and near Escondida Este and Baker. Based on U-Pb zircon ages, most of these rocks were emplaced from ~43 to 41 Ma (Urzúa, 2009; cf. ~38–36 Ma Ar/Ar ages of Richards et al., 2001), although those at Baker and Pampa Escondida returned ages of 38.2 ± 0.5 and 37.6 ± 0.5 Ma, respectively (Table 3; see below). These diorites and monzodiorites (including those at Baker and Pampa Escondida) are clearly precursors to the ore-related intrusions (Richards et al., 2001). The ore-related intrusions in the Escondida cluster are multiphase biotite granodiorite porphyries, which report U-Pb zircon ages between ~38 and 34.5 Ma; however, the undated porphyry at Chimborazo is older, probably ~41 Ma (Table 3; see below). The last sizable intrusion in the district is the rhyolite porphyry at Escondida Este, dated at ~34 Ma (Table 3), which is compositionally the most evolved and plots nearer the quartz apex in a Streckeisen diagram. Recent drilling has defined a remarkably thick sequence of siliciclastic sedimentary and andesitic volcanic rocks, immediately east of Escondida Este and Escondida Norte, which accumulated both before and during porphyry copper deposit formation (Fig. 3). These rocks crop out in the immediate footwall of the northwest-vergent Hamburgo reverse fault (Fig. 3), where they were designated as San Carlos strata by Urzúa (2009). The strata attain maximum thicknesses of >1,200 m and comprise gray-green and red sandstone and conglomerate, which in their upper parts are interbedded with a cumulative thickness of up to 500 m of andesitic laharic breccia, ignimbrite, and subsidiary flows, which reported two U-Pb zircon ages of 38.0 ± 2.1 and 37.7 ± 0.6 Ma (Urzúa, 2009). Urzúa (2009) compared the San Carlos strata with similar siliciclastic units farther north, which include the upper Calama (Blanco et al., 2003; May et al., 2005) and Loma Amarilla (Mpodozis et al., 2005) Formations in the Calama and Salar de Atacama basins, respectively (Fig. 1). These sedimentary units are the erosional products resulting from Domeyko Cordillera uplift (Jordan et al., 2007; Wotzlaw et al., 2011). The final stratigraphic unit in the district is the Pampa de Mulas Formation, a widespread, flat-lying, crudely stratified, poorly consolidated, piedmont gravel sequence of mass-flow origin, which is up to 240 m thick. In proximity to the porphyry copper deposits, the sequence contains abundant clasts of altered rocks, especially advanced argillic lithocap. 0361-0128/98/000/000-00 $6.00

61

The formation is assigned to the Oligocene to mid-Miocene interval by Marinovic et al. (1995) and Urzúa (2009), which accords well with ages of 8.7 ± 0.4 to 4.2 ± 0.2 Ma for overlying felsic air-fall tuff horizons at Escondida and Zaldívar (Alpers and Brimhall, 1988; Morales, 2009). Escondida district structure The principal faults and associated fold axes in the Escondida district are orogen parallel and strike N to NNE (Mpodozis et al., 1993b; Marinovic et al., 1995; Richards et al., 2001; Urzúa, 2009; Fig. 3). The faults constitute the eastern side of a giant cymoid loop, ~180 km long and up to 20 km wide (Mpodozis et al., 1993a, b; Fig. 1). In the Escondida district, the pre-eminent Portezuelo-Panadero fault is a 65° E-dipping, reverse structure which places La Tabla over Augusta Victoria units (Navarro et al., 2009; Urzúa, 2009; Fig. 3). Its total vertical and transcurrent displacement remains uncertain, but the movement history is complex, as discussed below. The NE-striking, 70° SE-dipping Hamburgo reverse fault (Fig. 3) is the southernmost of several such structures along the eastern side of the giant shear lens (Fig. 1) that were produced by clockwise rigid-block rotations about vertical axes coeval with a component of sinistral motion on the N-striking faults (Mpodozis et al., 1993a, b; Arriagada et al., 2000). The accommodation space generated incrementally during this rotation was filled by the San Carlos strata (see above). Chimborazo Porphyry Copper Deposit Deposit summary Chimborazo is a large porphyry copper system, within which the only resource defined to date is a supergene enrichment zone (Table 1). The enrichment is situated in the northern part of the system, north of Cerro Chimborazo, which is formed by an advanced argillic lithocap (Figs. 2c, 4). The entire Chimborazo system to a depth of ~1,000 m is mainly hosted by dacitic tuffs and lesser andesitic breccias and flows assigned to the Augusta Victoria Formation (Petersen et al., 1996). North of the lithocap, the Chimborazo system is concealed beneath an extensive, northward-thickening wedge of Pampa de Mulas gravels, which are omitted from Figure 4. Precursor, phaneritic hornblende diorite and monzodiorite intrusions, dated at ~42 Ma (Table 3), cut the volcanic succession, both south (Fig. 4) and north of the alteration zone as well as at depth (Fig. 5a). At ~1,000 m below the surface, drill holes have intersected the uppermost parts of an early biotite granodiorite porphyry intrusion, the isotopic age of which is pending. Minor bodies and dikes of late mineral hornblende diorite, dated at 38.1 ± 0.3 Ma by the Ar/Ar method (Richards et al., 1999), cut the lithocap immediately south of the supergene copper deposit and appear to be closely related to the inter- to late mineral Chimborazo intrusive complex that flanks the deposit to the northwest (Figs. 4, 5a). This intrusive complex, dated at ~39 to 37 Ma (Table 3), comprises fine-grained, inequigranular diorite to granodiorite phases, which are controlled and cut by NE-striking faults, some displaying ductile synintrusion deformation textures at depth. 61

62

HERVÉ ET AL. TABLE 3. U-Pb, Re-Os, Ar/Ar, and K-Ar Ages for Porphyry Copper Deposits in the Escondida District

Sample

Location

Method, Material

Age (Ma)

Reference

Precursor diorite Precursor monzodiorite Precursor monzodiorite Early granodiorite porphyry Early granodiorite porphyry Early granodiorite porphyry Rhyolite dome Early granodiorite porphyry Early granodiorite porphyry

Pampa Escondida Chimborazo Baker Escondida Escondida Escondida Escondida Zaldívar Escondida Norte

U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon

37.6 ± 0.5 42.0 ± 0.9 38.2 ± 0.5 37.9 ± 1.1 37.7 ± 0.8 37.2 ± 0.8 37.5 ± 0.6 38.0 ± 0.5 37.5 ± 0.5

Early granodiorite porphyry Early granodiorite porphyry Early granodiorite porphyry Early granodiorite porphyry Early granodiorite porphyry Early granodiorite porphyry Intermineral granodiorite porphyry Late-mineral rhyolite porphyry Late-mineral dacite porphyry Late-mineral dacite porphyry Late-mineral dacite porphyry Intermineral granodioritie porphyry Intermineral granodioritie porphyry Hornblende dacite late mineral porphyry Inter- to late mineral granodiorite porphyry Inter- to late mineral monzodiorite Inter- to late mineral monzodiorite Inter- to late mineral quartz diorite Inter- to late mineral quartz monzodiorite Molybdenite mineralization Molybdenite mineralization Molybdenite mineralization Molybdenite mineralization Molybdenite mineralization Molybdenite mineralization Molybdenite mineralization Molybdenite mineralization Early granodiorite porphyry Early granodiorite porphyry Biotitic alteration Biotitic alteration Biotitic alteration Biotite from granodiorite porphyry Biotite from granodiorite porphyry Biotite from granodiorite porphyry Biotite from granodiorite porphyry Biotite from granodiorite porphyry Biotite from granodiorite porphyry Biotite from granodiorite porphyry Advanced argillic alteration Advanced argillic alteration Advanced argillic alteration Supergene alunite Supergene alunite Supergene alunite Supergene alunite Supergene alunite Supergene alunite Supergene alunite Supergene alunite

Escondida Este Escondida Este Pampa Escondida Pampa Escondida Pampa Escondida Baker Escondida Escondida Este Escondida Norte Zaldívar Zaldívar Pampa Escondida Pampa Escondida Pampa Escondida Chimborazo Chimborazo Chimborazo Chimborazo Chimborazo Escondida Escondida Escondida Este Escondida Norte Escondida Norte Escondida Norte Chimborazo Chimborazo Escondida Escondida Escondida Escondida Escondida Zaldívar Zaldívar Zaldívar Zaldívar Zaldívar Zaldívar Zaldívar Escondida Escondida Escondida Zaldívar Zaldívar Escondida Escondida Escondida Escondida Chimborazo Chimborazo

U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon U-Pb, zircon Re-Os, molybdenite Re-Os, molybdenite Re-Os, molybdenite Re-Os, molybdenite Re-Os, molybdenite Re-Os, molybdenite Re-Os, molybdenite Re-Os, molybdenite Ar/Ar, igneous biotite Ar/Ar, igneous biotite Ar/Ar, hydrothermal biotite Ar/Ar, hydrothermal biotite Ar/Ar, hydrothermal biotite Ar/Ar, igneous biotite Ar/Ar, igneous biotite Ar/Ar, igneous biotite Ar/Ar, igneous biotite Ar/Ar, igneous biotite Ar/Ar, igneous biotite Ar/Ar, biotite Ar/Ar, alunite Ar/Ar, alunite Ar/Ar, alunite K-Ar, alunite K-Ar, alunite K-Ar, alunite K-Ar, alunite K-Ar, alunite K-Ar, alunite K-Ar, alunite K-Ar, alunite

34.5 ± 0.6 34.5 ± 0.5 35.0 ± 0.6 36.1 ± 0.7 36.0 ± 1.2 37.1 ± 0.7 35.4 ± 0.7 34.2 ± 0.7 35.7 ± 0.7 36.0 ± 0.8 35.5 ± 0.8 35.0 ± 0.8 34.5 ± 0.4 35.2 ± 0.8 38.5 ± 0.5 37.5 ± 0.5 37.4 ± 0.5 37.9 ± 0.7 38.9 ± 0.8 36.1 ± 0.2 35.2 ± 0.2 33.7 ± 0.3 37.8 ± 0.2 37.6 ± 0.2 36.6 ± 0.2 41.9 ± 0.4 36.6 ± 0.4 35.8 ± 0.2 35.9 ± 0.3 37.5 ± 0.6 36.0 ± 0.4 37.5 ± 0.6 37.4 ± 0.2 37.7 ± 0.4 37.1 ± 0.5 36.6 ± 0.9 36.5 ± 0.5 36.0 ± 0.3 35.6 ± 0.7 35.7 ± 0.3 35.9 ± 0.3 35.2 ± 0.2 14.7 ± 0.7 16.8 ± 0.6 17.7 ± 0.7 14.7 ± 0.6 16.4 ± 0.7 18.0 ± 0.7 17.6 ± 1.1 15.9 ± 1.0

This study Urzua (2009) This study Richards et al. (2009) Padilla-Garza et al. (2004) Padilla-Garza et al. (2004) This study Jara et al. (2009) P.J. Pollard and R.G. Taylor, unpub. rept., 2002 This study This study This study This study This study This study This study This study This study Jara et al. (2009) Jara et al. (2009) This study This study This study This study This study This study This study This study Romero et al. (2010) Romero et al. (2010) Padilla-Garza et al. (2004) Romero et al. (2010) Romero et al. (2010) Romero et al. (2010) This study This study Padilla-Garza et al. (2004) Padilla-Garza et al. (2004) Padilla-Garza et al. (2004) Padilla-Garza et al. (2004) Padilla-Garza et al. (2004) Campos et al. (2009) Campos et al. (2009) Campos et al. (2009) Campos et al. (2009) Campos et al. (2009) Campos et al. (2009) Campos et al. (2009) Padilla-Garza et al. (2004) Padilla-Garza et al. (2004) Véliz (2004) Morales (2009) Morales (2009) Alpers and Brimhall (1988) Alpers and Brimhall (1988) Alpers and Brimhall (1988) Alpers and Brimhall (1988) This study This study

The volcanic rocks and monzodiorite are cut by a series of pipe-like, magmatic-hydrothermal breccias (Petersen et al., 1996), which are now known to be developed over a 1,000-m vertical interval (Fig. 5a). The breccias are cemented by anhydrite with chalcopyrite and molybdenite at 0361-0128/98/000/000-00 $6.00

depth, an assemblage that grades upward through quartz, schorlitic tourmaline, sericite, pyrite, and gypsum (after anhydrite) to quartz, alunite, pyrite, enargite, native sulfur, and abundant open space (produced by gypsum dissolution) in the advanced argillic lithocap, in conformity with 62

ESCONDIDA PORPHYRY COPPER DISTRICT, N. CHILE

63

FIG. 4. Geology of the Chimborazo porphyry copper deposit, at sub-Pampa de Mulas Formation bedrock surface. Outer limit of advanced argillic lithocap is also shown.

the system-scale alteration-mineralization zoning pattern (Fig. 5b, c).

Farther from the center of the deposit, illite dominates over sericite (fine-grained muscovite). The chlorite-sericite alteration is gradational downward to and overprints a deep potassic zone, comprising pervasive biotitization in the volcanic rocks and orthoclase, minor quartz, and varied amounts of anhydrite in the granodiorite porphyry intrusion (Fig. 5b). A localized zone of gray sericite development, associated with quartz and andalusite, overprints the biotitized zone (Fig. 5b). The potassic-altered rocks contain minor amounts of chalcopyrite, pyrite, and subordinate magnetite, whereas the overprinted gray sericite zone is characterized by chalcopyrite and bornite (Fig. 5c, d) plus elevated gold values. The chlorite-sericite zone is dominantly pyritic (Fig. 5c), shows clear evidence for hypogene leaching of preexisting chalcopyrite, and in its external parts contains a well-defined zinc halo. The advanced argillic alteration is associated with early pyrite and enargite ± tennantite as disseminations, veinlets, and veins, and late chalcopyrite-sphalerite ± pyrite ± galena veins, which tend to occur peripherally. The veins consistently strike NE and follow minor faults and fractures (Petersen et al., 1996; Fig. 4).

Hypogene alteration and mineralization The topographically higher parts of the lithocap, beneath the upper slopes of Cerro Chimborazo (Figs. 2c, 4, 5b), are dominated by fine-grained quartz-alunite, which grades downward nearer its exposed base to quartz-alunite-kaolinite plus minor dickite. Crystalline hypogene alunite from an outlying lithocap exposure, 6 km east, was dated at 42.4 ± 2.0 Ma (Table 3). The lithocap includes a series of downward-penetrating prongs, some attaining a depth of ~1,000 m below the surface (Fig. 5b). These prongs grade downward from quartz-alunitedickite to quartz-pyrophyllite-dickite, the latter assemblage containing minor diaspore, topaz, and andalusite. Green tourmaline coexisting with lesser dumortierite is present throughout the advanced argillic prongs, the upper parts of which also contain native sulfur. The advanced argillic alteration passes downward and outward through sericitic alteration to an earlier chlorite-sericite assemblage, containing minor schorlitic tourmaline (Fig. 5b). 0361-0128/98/000/000-00 $6.00

63

64

HERVÉ ET AL.

FIG. 5. Chimborazo sections. a. Geology. b. Alteration. c. Mineralization zones. d. Copper distribution. Section line shown in Figure 4. In this and all following sections, only selected drill holes are shown to avoid obscuring geologic detail.

Supergene mineralization The Chimborazo supergene enrichment zone is low grade (0.4–0.7% Cu), laterally continuous, ~2 × 1 km in areal extent, and averages ~100 m thick (Figs. 4, 5c, d); it is chalcocite dominated at shallow levels but contains more covellite at depth. The zone is strongly controlled by the NE-striking faults, containing the high-sulfidation pyrite-enargite veins, along which the enrichment extends considerably deeper (Petersen et al., 1996; Fig. 5c). The enrichment zone is overlain by hematitic leached capping, up to 150 m thick, but only minor oxide copper (brochantite-dominated) mineralization (Fig. 5c). Supergene alunite veins at and near Chimborazo yielded ages of ~18 to 16 Ma (Table 3).

Este center is developed within andesitic volcanic rocks of La Tabla Formation and coeval intrusions. The latter comprise a large stock of quartz monzodiorite porphyry, dated at 296.8 ± 3.2 Ma, monzogranite porphyry dikes, dated at 296.6 + 4.4/3.8 Ma, and, at a depth of 2,000 m, a foliated phaneritic tonalite, dated at 300.1 ± 3.5 Ma (Fig. 7a; Table 2). The Augusta Victoria volcanic sequence at Escondida is cut by a composite biotite granodiorite porphyry stock, within which the NE-trending, early phases, traditionally referred to as Colorado Grande and Escondida porphyries (Ojeda, 1986, 1990; Padilla-Garza et al., 2001, 2004; Quiroz, 2003a), are dated at 37.9 ± 1.1, 37.7 ± 0.8, and 37.2 ± 0.8 Ma (Richards et al., 1999; Padilla-Garza et al., 2004; Table 3). At depths of >500 m and westward, the early granodiorite porphyries are cut by a large stock of late intermineral biotite granodiorite porphyry (Figs. 6, 7a), which explains the low-grade of the hypogene mineralization reported beneath the high-grade supergene enrichment zone by previous workers (e.g., Ojeda, 1986; Fig. 7d). This intrusion was dated at 35.4 ± 0.7 Ma (Table 3). The granodiorite porphyry stock and copper mineralization are cut northward by a biotite rhyolite dome (Figs. 2a, 6), which is notably richer (>10 vol %) in quartz phenocrysts (Ojeda, 1986, 1990; Padilla-Garza et al., 2001); it is dated at 37.5 ± 0.6 Ma (Table 3). Numerous small magmatic-hydrothermal breccia bodies constitute roughly 5% of the Escondida deposit and invariably

Escondida Porphyry Copper Deposit Geologic summary Recent exploration at Escondida shows that it comprises two discrete porphyry copper centers. The western center developed the major chalcocite enrichment blanket exploited to date, whereas the eastern center, informally named Escondida Este, lies deeper and east of the Panadero fault (Figs. 6, 7a). The two centers formed at different times, as documented below. The Escondida center is hosted, at least at shallow levels, by andesitic flows and subordinate breccias of the Augusta Victoria Formation (Ojeda, 1986), whereas the Escondida 0361-0128/98/000/000-00 $6.00

64

ESCONDIDA PORPHYRY COPPER DISTRICT, N. CHILE

65

FIG. 6. Geology of the Escondida-Escondida Este porphyry copper deposit, compiled from various sources: bench mapping supplied by the mine geologists and relogging of core from exploration drill holes within pit limits; top of bedrock geology from historic drill holes and surface exposures beyond pit limits.

host the highest grade hypogene and supergene copper mineralization (Ojeda, 1986, 1990; Véliz, 2004). The breccia clasts, commonly polymict in nature, are surrounded by varied proportions of quartz-sulfide cement and rock-flour matrix (Ojeda, 1986, 1990; Véliz, 2004). There are also late, poorly mineralized pebble dikes of phreatic origin (cf. Sillitoe, 1985), most of them striking NW (Ojeda, 1986, 1990). The Escondida deposit is bounded eastward by a late mineral biotite rhyolite porphyry, measuring 3 × 1.5 km at surface, which follows the trace of the N-striking PortezueloPanadero fault (Figs. 6, 7a). Recent deep drilling east of the fault encountered a new biotite granodiorite porphyry center at a depth of ~800 m beneath the rhyolite porphyry. The 0361-0128/98/000/000-00 $6.00

existence of this deep Escondida Este porphyry copper center was presciently predicted by Padilla-Garza et al. (2004) on the basis of shallow high-sulfidation mineralization affecting the rhyolite porphyry. The Escondida Este center, comprising early porphyry, dated at 34.5 ± 0.6 and 34.5 ± 0.5 Ma (Table 3), and at least two intermineral phases (Fig. 7a), is therefore ~2 m.y. younger than the Escondida center. The upward-flared rhyolite porphyry (Ojeda, 1986, 1990; Padilla-Garza et al., 2001), displaying local flow foliation and partly broken quartz phenocrysts, was emplaced at 34.2 ± 0.7 Ma (Table 3), an age preferred to that (34.7 ± 1.7 Ma) reported by Richards et al. (1999) because of the smaller error. The N-trending, dike-like form of the rhyolite porphyry 65

66

HERVÉ ET AL.

FIG. 7. Escondida-Escondida Este sections. a. Geology. b. Alteration. c. Mineralization zones. d. Copper distribution. Section line shown in Figure 6. 0361-0128/98/000/000-00 $6.00

66

ESCONDIDA PORPHYRY COPPER DISTRICT, N. CHILE

suggests control by the Portezuelo-Panadero fault, which borders it westward (Figs. 6, 7a); however, postmineral fault displacement is relatively minor because rhyolite porphyry dike offshoots occur both within and immediately west of the broad zone of fractured rock that defines the fault zone. A series of NW-striking rhyodacite porphyry dikes cut all the late Eocene porphyry phases at Escondida and are especially abundant in the rhyolite porphyry (Ojeda, 1986, 1990; Padilla-Garza et al., 2001, 2004; Quiroz, 2003a; Véliz and Camacho, 2003; Véliz, 2004). Small magmatic-hydrothermal breccias are widespread at Escondida Este, with most of them being clast supported, monomict, and cemented by high-sulfidation sulfide assemblages and anhydrite; hence, they are late-stage additions to the system. Even later, barren, NW-striking pebble dikes are also present.

chlorite-sericite and sericitic zones (Fig. 7c). Molybdenum values average ~70 ppm (Romero, 2008). Padilla-Garza et al. (2001) recognized that copper, as chalcopyrite, was added at the chlorite-sericite stage. High sulfidation-state assemblages occur in the advanced argillic zone. In the underlying late intermineral porphyry intrusion, pyrite dominates over chalcopyrite and copper grades are 0.05 to 0.25%, decreasing downward (Fig. 7c, d). The deep, potassic-altered rocks in the Escondida Este center contain chalcopyrite-pyrite mineralization, whereas the gray sericite zone is bornite rich at depth, but contains more chalcopyrite in its upper parts (Fig. 7c); both sulfide minerals occur mainly as disseminated grains in the veinlet halos. The gray sericite zone gives rise to the highest copper and molybdenum grades (Fig. 7d), the latter in preexisting Btype quartz veinlets and later pyrite veinlets. The overlying advanced argillic zone at Escondida Este contains disseminated, high-sulfidation-state assemblages (Fig. 7c), which are higher grade than the surrounding mineralization (hypogene enrichment). A series of NW-trending, banded, massive sulfide veins, up to 3 m wide, cut the quartz porphyry as well as occurring in faults and fractures farther east and west. The veins contain a variety of high-sulfidation assemblages, which include abundant pyrite accompanied by one or more of enargite, tennantite, chalcopyrite, bornite, covellite, and chalcocite; they also include molybdenite and minor amounts of gold. Sphalerite is especially abundant in the youngest vein fillings (Padilla-Garza et al., 2001, 2004; Quiroz, 2003a, b). Vein halos, up to 1 m wide, include pyrophyllite, dickite, kaolinite, and alunite, with subordinate diaspore, andalusite, and corundum (Padilla-Garza et al., 2001; Quiroz, 2003a, b; Véliz, 2004), which are transitional to sericitic alteration at depth.

Hypogene alteration and mineralization Much of the early granodiorite porphyry at Escondida displays sericitic alteration, although an advanced argillic zone is also widely developed at the shallowest levels (Cerro Colorado Grande and Chico; Fig. 2a) and at much greater depths along fault zones (Fig. 7b). Quartz, pyrophyllite, and subordinate alunite, diaspore, and svanbergite are reported (Brimhall et al., 1985; Alpers and Brimhall, 1988), with some of the quartz-pyrophyllite displaying a distinctive patchy texture (Ojeda, 1986; J. Perelló, pers. commun., 2009), comparable to that reported from Yanacocha, Peru (Gustafson et al., 2004). At depth and as remnants in the sericitic zone, there are patches of chlorite-sericite alteration, which give way downward to biotite in andesitic volcanic rocks and Kfeldspar>biotite in the early porphyries (cf. Padilla-Garza et al., 2001; Fig. 7b). The potassic and overprinted sericitic alteration contain abundant A- and B-type quartz veinlets (cf. Gustafson and Hunt, 1975). The late intermineral porphyry underwent weak potassic alteration and veining, but typically displays a pervasive chloritic overprint within which remanent hydrothermal K-feldspar is prominent (Fig. 7b). The Escondida Este center is characterized at depths of >~1,800 m by K-feldspar and biotite alteration, which is cut by a variety of veinlet types, including early biotite and sparse early dark micaceous (EDM) veinlets (cf. Meyer, 1965). The potassic zone is overprinted at shallower levels (~1,000–1,800 m below surface) by gray sericite (±andalusite), which is developed as centimetric halos to discontinuous quartz-anhydrite veinlets that partly coalesce resulting in pervasive sericitization (Fig. 7b). Similar alteration is reported at Chuquicamata, Chile (Ossandón et al., 2001). Outward, these zones are transitional to chlorite-sericite alteration. The rhyolite porphyry underwent white sericitic and advanced argillic alteration, which completely dominate the shallow parts of the Escondida Este center (Fig. 7b). The early gray sericite is richer in magnesium and iron (phengite) than the shallower, lower temperature white variety (muscovite) based on systematic portable SWIR spectrometer readings (cf. Pontual et al., 1997) and confirmed by QEMSCAN analysis. The hypogene sulfide assemblage at Escondida is largely obliterated by the effects of the supergene enrichment. Nonetheless, chalcopyrite and bornite are reported from potassic remnants, and chalcopyrite and pyrite from the overprinted 0361-0128/98/000/000-00 $6.00

67

Supergene mineralization The Escondida deposit is dominated by a mature, kaoliniterich supergene profile, whereas Escondida Este displays very limited supergene effects (Fig. 7c). The Escondida profile comprises a hematitic leached capping, averaging ~200 m thick but double this figure in places, underlain by a NWtrending enrichment zone, with an areal extent of 4.5 × 1 km and maximum thickness of ~400 m (Ojeda, 1986, 1990; Fig. 7c). The NW-striking faults, fractures, and veins combined with the highest hypogene copper contents appear to have been the main controls on both the form and depth of the enrichment zone (Ojeda, 1986, 1990; Padilla-Garza et al., 2001; Fig. 6). The zone is dominated by chalcocite-group minerals in its upper, highest grade parts, with lower grade covellite and remanent hypogene sulfides becoming dominant downward (Alpers and Brimhall, 1989). The initial mineable reserve comprised 662 Mt at 2.12% Cu, parts of which averaged >3% (Ojeda, 1990). Supergene alunite from the leached capping and underlying enrichment zone ranges in age from ~18 to 14 Ma (Alpers and Brimhall, 1988; Table 3). The relatively small volumes of oxide copper mineralization at Escondida (Table 1) tend to be concentrated in biotite- and chlorite-sericite-altered andesitic volcanic rocks, in which brochantite and antlerite are the principal minerals, along with minor amounts of chrysocolla, atacamite, several copper phosphate minerals, cuprite, and native copper, the last two 67

68

HERVÉ ET AL.

concentrated at the top of the enrichment zone (Ojeda, 1986; Véliz and Camacho, 2003).

small Pinta Verde satellite deposit, at the southwestern extremity of Escondida Norte-Zaldívar, is associated with a markedly different, porphyritic biotite tonalite intrusion (Fig. 8), which is yet to be dated. Small bodies of polymict magmatic-hydrothermal breccia are associated with the early and intermineral porphyries (Figs. 8, 9a). These breccias display sericitic or chloritesericite alteration and are cemented by quartz, pyrite, and varied amounts of chalcopyrite at shallow depths but by pegmatoidal quartz-biotite-anhydrite ± K-feldspar ± magnetite along with chalcopyrite and bornite at depth (e.g., Monroy, 2000; Navarro et al., 2009). Hydrothermal breccia and porphyry bodies appear to be controlled by the PortezueloPanadero fault (Navarro et al., 2009). The breccias are typically higher grade than surrounding rocks.

Escondida Norte-Zaldívar Porphyry Copper Deposit Geologic summary Escondida Norte and the eastern part of the contiguous Zaldívar property are hosted by volcanic rocks of La Tabla Formation and coeval intrusive phases. In the east and at depth, La Tabla Formation comprises andesitic rocks, dated at 294.4 ± 4.6 Ma (Jara et al., 2009; Table 2), which are overlain westward by a rhyolitic sequence, principally welded ignimbrite, dated at 290.0 ± 4.0, 294.2 ± 2.4, and 298.2 + 5.5/−4.9 Ma (Richards et al., 1999; Jara et al., 2009; Table 2). The intrusions comprise coarse-grained monzogranite (298.8 ± 2.6, 293.0 ± 6.0, 291.1 ± 2.3, 289.9 ± 3.5 Ma; Morales, 2009; Table 2), granodiorite porphyry (287.1 ± 4.4 Ma; Jara et al.; 2009; Table 2), and diorite (Figs. 8, 9a). Additionally, in the southeastern part of the deposit, the Early Permian quartz diorite and quartz monzodiorite porphyry defined and dated at Pampa Escondida are also present (see below). The western part of the Zaldívar property, west of the demonstrably west-vergent Portezuelo-Panadero reverse fault, is underlain by andesitic volcanic rocks, which are assigned to La Tabla Formation at depth and the Augusta Victoria Formation at shallow levels (Navarro et al., 2009; Fig. 8). The above rocks are intruded by a series of NE-striking dikes and larger bodies of biotite granodiorite porphyry, which include early, inter-, and late mineral phases (Figs. 8, 9a). The early and intermineral phases, not differentiated in Figures 8 and 9a, yielded ages of 38.0 ± 0.5 and 37.5 ± 0.5 Ma, whereas the late mineral phase gave ages of 36.0 ± 0.8, 35.7 ± 0.7, and 35.5 ± 0.8 Ma (Jara et al., 2009; Table 3). The

Hypogene alteration and mineralization Potassic alteration is developed at depth throughout the deposit (Fig. 9b). A biotite-K-feldspar assemblage is best developed in the felsic rocks, whereas biotite and minor magnetite predominate in the andesitic volcanic rocks and diorite. The potassic alteration contains early biotite and magnetite veinlets and abundant K-feldspar and quartz-K-feldspar veinlets, the latter of A type. Gray sericite veinlets overprint the potassic zone but do not constitute coherent, mappable zones like that at Escondida Este (see above). At shallower levels, widespread chlorite-sericite alteration, characterized by chlorite-sulfide veinlets, overprints and largely destroys the potassic assemblage (Fig. 9b). This, in turn, is overlain by a sericitic zone, which is overprinted locally by quartz-pyrophyllite ± alunite alteration, closely associated with NW-striking, high-sulfidation veinlet zones, some of them sheeted.

FIG. 8. Geology of the Escondida Norte-Zaldívar porphyry copper deposit, 2,750-m elevation. Escondida Norte geology from logging of exploration drill holes, Zaldívar geology from Monroy (2009). 0361-0128/98/000/000-00 $6.00

68

ESCONDIDA PORPHYRY COPPER DISTRICT, N. CHILE

69

FIG. 9. Escondida Norte sections. a. Geology. b. Alteration. c. Mineralization zones. d. Copper distribution. Section line shown in Figure 8.

Most of the hypogene sulfide mineralization at Escondida Norte-Zaldívar consists of chalcopyrite and pyrite (Fig. 9c), with development of only localized centers of chalcopyrite-bornite ± chalcocite mineralization in the potassic zone, mainly recognized in the Zaldívar part of the deposit. There is a tendency for copper tenors to drop at depth in 0361-0128/98/000/000-00 $6.00

the potassic zone (Fig. 9c, d), implying that appreciable copper was introduced at the chlorite-sericite stage (cf. Escondida, see above). Molybdenum values average ~100 ppm (Romero, 2008). Pyrite, enargite, tennantite, and sphalerite are observed in the high-sulfidation veinlet zones (Williams, 2003). 69

70

HERVÉ ET AL.

Supergene mineralization A well-developed supergene profile is present at Escondida Norte-Zaldívar (Fig. 9c), comprising a hematitic leached capping, averaging 100 to 200 m (up to 350 m) thick, underlain by an enrichment zone from 20 to 250 m thick, which constitutes the main orebody (Fig. 9c, d). The enrichment zone is 2 × 1.5 km in areal extent, trends northeast, and roughly parallels the present topography (Monroy, 2000; Williams 2003; Fig. 2b); it is divided into a high-grade, chalcocite-dominated, upper zone and a lower grade, covellitebearing, basal part in which pyrite is little replaced. Supergene kaolinite is ubiquitous, and associated supergene alunite returned ages of ~17 to 14 Ma (Morales, 2009; Table 3). South of Escondida Norte, a 5-km-long paleochannel contains low-grade, exotic copper mineralization, hosted by both the basal Pampa de Mulas gravels and immediately subjacent bedrock. Oxide copper mineralization is irregularly developed above the enrichment zone, principally antlerite and brochantite in the central, highest grade parts (Maturana and Saric, 1991; Monroy, 2000; Williams, 2003), but chrysocolla and atacamite peripherally (Fig. 9c). In contrast, the oxide copper mineralization developed at the expense of low-grade chalcopyrite mineralization in andesitic volcanic rocks at Pinta Verde is dominated by chrysocolla, atacamite, black oxide phases, and copper-bearing clay (cf. Monroy, 2000).

FIG. 10. Geology of the Pampa Escondida porphyry copper deposit, 2,000-m elevation.

Pampa Escondida Porphyry Copper Deposit Geologic summary The large Pampa Escondida deposit (Table 1), completely concealed beneath 10 to 130 m of Pampa de Mulas piedmont gravel (Fig. 2d), is hosted by La Tabla andesitic tuffs and flows, dated at 288.0 ± 2.4 Ma, which are intruded at depth by dark-colored quartz diorite and quartz monzodiorite porphyries, dated at 293.0 ± 4.3 and 287.6 ± 3.3 Ma, respectively (Figs. 10, 11a; Table 2). Shallowly dipping lenses of calcareous sedimentary rock, dacitic tuff, and andesitic lava are interbedded with the upper parts of La Tabla succession (Fig. 11a). A multiphase porphyry stock is present at Pampa Escondida, most of it at depths of >600 to 700 m below the surface (Fig. 11a). The first phase, the precursor hornblende diorite dated at 37.6 ± 0.5 Ma (Table 3), occurs as dikes cutting La Tabla Formation. The copper mineralization spans several phases of biotite granodiorite porphyry, which intrude all the Early Permian units. The earliest porphyry, dated at 36.1 ± 0.7, 36.0 ± 1.2, and 35.0 ± 0.6 Ma, constitutes a NE-oriented, 2- × 0.5-km stock, which is cut centrally, at depths of >1,000 m, by an intermineral porphyry, dated at 35.0 ± 0.8 and 34.5 ± 0.4 Ma (Figs. 10, 11a; Table 3). These, in turn, are cut in the northeast of the deposit by a late intermineral hornblende dacite porphyry dike, 20 to 30 m wide, dated at 35.2 ± 0.8 Ma (Figs. 10, 11a; Table 3). In the northeast of the deposit there are also several minor monomict magmatic-hydrothermal breccias as well as NW-striking dikes of barren, polymict breccia cutting the granodiorite porphyry stock (Figs. 10, 11a). The latter breccias have rock-flour cement, postdate all alteration and mineralization, and are considered phreatic in origin (cf. Sillitoe, 1985). 0361-0128/98/000/000-00 $6.00

Hypogene alteration and mineralization The alteration and mineralization at Pampa Escondida have a classic dome-shaped form centered on the early granodiorite porphyry stock (Fig. 11b). The core of the deposit displays potassic alteration, composed of K-feldspar > biotite in the early porphyry and, locally, also in the intermineral phase. Outward, biotite > K-feldspar alteration affects the Early Permian porphyries, whereas intense biotitization characterizes the Early Permian andesitic volcanic rocks. Local garnet-diopside skarn is developed in the calcareous lenses within the upper part of the andesitic sequence. Beyond the potassic core zone, chlorite-sericite alteration is widely developed in the andesitic sequence as well as affecting the late mineral dacite porphyry dike (Fig. 11b). Sericitic alteration is not abundant but is mapped at the shallowest levels of the deposit (Fig. 11b). The inner potassic zone with K-feldspar > biotite contains the highest grade mineralization, ranging between 0.5 and 1.0% Cu, as chalcopyrite and bornite (2/1) along with lesser amounts of hypogene digenite, chalcocite, covellite, and molybdenite (Fig. 11c, d); however, maximum molybdenite concentrations constitute an annular zone, partly overlapping and external to the copper-rich core, in common with many gold-rich porphyry copper deposits (e.g., Sillitoe, 1979; Perelló et al., 2004). The sulfides occur in early K-feldspar and later A- and B-type quartz veinlets as well as in disseminated form. Minor gray sericite veinlets are also present. The chalcopyrite-bornite mineralization grades outward to chalcopyritepyrite, averaging 0.2 to 0.5% Cu, which in turn is transitional to a pyrite halo, containing 2 to 3 vol % pyrite and 1.2 m.y. are excluded from consideration. b. U-Pb zircon, Re-Os molybdenite, and Ar/Ar biotite ages for intrusion, alteration, and mineralization events in the Escondida and Escondida Este deposits. Note the ~2-m.y. interval between early porphyry emplacement at Escondida and Escondida Este. Data sources detailed in Table 3. 0361-0128/98/000/000-00 $6.00

73

73

74

HERVÉ ET AL.

Tectonic setting Most early interpretations of the tectonic setting of middle Eocene to early Oligocene porphyry copper emplacement in northern Chile assumed that the N-striking, controlling faults of the Domeyko system underwent transcurrent motion (e.g., Reutter et al., 1991, 1996; Richards et al., 2001). It is now appreciated, however, that the faults are transpressional structures that underwent kilometer-scale reverse displacement as well as varied degrees of transcurrent motion (McInnes et al., 1999; Amilibia and Skarmeta, 2003; Amilibia et al., 2008; Niemeyer and Urrutia, 2009). Indeed, fission-track thermochronologic results suggest that 4 to 5 km of uplift and consequent unroofing took place in the Domeyko Cordillera during the Incaic orogeny (Maksaev and Zentilli, 1999). During the Incaic orogeny in the Escondida district, sinistral transpression—to generate the clockwise block rotation implicated in the initial opening and synorogenic filling of the San Carlos depocenter (Mpodozis et al., 1993a, b; Arriagada et al., 2000; see above)—prevailed for an indeterminate time prior to formation of the Chimborazo deposit at ~41 Ma (Fig. 13a). Comparisons with the Salar de Atacama basin suggest that the initial synorogenic sedimentation could have commenced at ~45 Ma or even earlier (Hammerschmidt et al., 1992; Mpodozis et al., 2005). Most of the San Carlos strata were clearly in place at the time of porphyry copper formation because their upper, partly volcanic parts are dated at ~38 Ma (Urzúa, 2009) and cut by polymetallic veins peripheral to the porphyry copper deposits (see above). A dextral transcurrent stress regime then seems to have been imposed during porphyry copper formation, between ~41 and 34.5 Ma, in order to explain the systematic NNE to NE orientation of the early, inter-, and late mineral porphyry intrusions at Chimborazo, Escondida, Escondida NorteZaldívar, Baker, and Pampa Escondida as well as the latestage Chimborazo veins (Fig. 13b). However, this dextral transpression was only incipiently developed in the Escondida district because there was no appreciable offset of ~38 to 34.5 Ma porphyry copper elements across the PortezueloPanadero and subsidiary N-striking faults (Figs. 3, 6, 8). The Escondida Este rhyolite porphyry was intruded as a Ntrending body, apparently controlled by the PortezueloPanadero fault (Figs. 6, 7a), at ~34 Ma, implying that by then the dextral stress regime no longer prevailed. Immediately after ~34 Ma, when the NW-striking, high-sulfidation veins and phreatic breccia dikes at Escondida-Escondida Este and Pampa Escondida (Fig. 10), were formed, the stress regime had once again become sinistral transpressive, although again the amount of fault displacement within the district was minimal (Fig. 13c). Therefore, the porphyry copper deposits of the Escondida district developed immediately after the main phase of Incaic sinistral transpression, in a transient dextral transpressive regime that was transitional to transient sinistral transpression before the end-stage, high-sulfidation vein emplacement and phreatic brecciation. These systematic district-scale and timeprogressive changes in the orientation of porphyry copper elements, from NE through N to NW in perhaps as little as 0.5 m.y., preclude deposit formation in a static stress regime. Such rapid tectonic switchovers are best explained by the 0361-0128/98/000/000-00 $6.00

FIG. 13. Schematic structural evolution of the Escondida district. a. Pre41 Ma: Large-scale sinistral transpression, giving rise to clockwise rigid-block rotations and opening of accommodation space progressively filled by the San Carlos strata. b. ~41–34.5 Ma: Cessation of sinistral transpression, imposition of a transient dextral transpressive regime, localized andesitic volcanism, and porphyry copper emplacement. c. ~34 Ma: Switchover to a transient sinistral transpressive regime and formation of end-stage high-sulfidation massive sulfide veins and phreatic breccia dikes. d. Late Oligocene-Miocene: Reimposition of transient sinistral transpression, continued uplift and unroofing of porphyry copper deposits, accumulation of Pampa de Mulas piedmont gravels, and supergene oxidation and enrichment (until ~14 Ma). See text for further details. Inspired by Mpodozis et al. (1993a, b).

progressive bending of the central Andean orogen to produce the Bolivian orocline (Arriagada et al., 2008), rather than to changes in plate vectors at the Andean margin (e.g., Reutter et al., 1991; Richards et al., 2001). This tectonic evolution implies that the Escondida porphyry copper cluster was not emplaced during either sinistral transcurrent faulting (Padilla-Garza et al., 2001) or change from regional-scale dextral to sinistral transcurrent motion (Richards et al., 2001). Nor were the deposits sinistrally offset by postmineral transcurrent faulting as appears to have occurred farther north in the Chuquicamata porphyry copper district (e.g., Lindsay et al., 1995; Tomlinson and Blanco, 1997; Astudillo et al., 2008; Fig. 1), although appreciable Oligo-Miocene offset may have taken place on parallel structures well beyond the immediate Escondida district (e.g., Niemeyer and Urrutia, 2009). 74

ESCONDIDA PORPHYRY COPPER DISTRICT, N. CHILE

75

Alteration-mineralization telescoping The profoundly telescoped Escondida, Escondida NorteZaldívar, and Baker deposits, in which the advanced argillic alteration and attendant high-sulfidation copper mineralization affected quartz-veinlet stockworked early porphyry intrusions (Fig. 7b), formed from ~38 to 36 Ma (Fig. 12a, b); a situation suggesting that deposit generation coincided temporally with the maximal rates of Incaic uplift and unroofing. The younger Escondida Este and Pampa Escondida deposits underwent little telescoping, perhaps implying that exhumation rates were in decline by the time they formed at ~36 to 34 Ma (Fig. 12a, b). Furthermore, by analogy, the little-telescoped Chimborazo deposit, emplaced at ~41 Ma (Fig. 12a), may have immediately preceded onset of the main phase of tectonic uplift and related denudation. These data suggest that maximal exhumation in the Escondida district commenced sometime between ~41 and 38 Ma and was already waning by ~36 Ma. Synmineral volcanism The laharic breccias, ignimbrite flows, and subsidiary lava flows, all andesitic in composition, that are a major component of the upper parts of the San Carlos strata within the Escondida cluster seem likely to be distal, ring plain products of a stratovolcano or large dome complex (cf. Fisher and Schmincke, 1994). The most likely former position for the source volcano is within the confines of the Escondida cluster, the only known nearby magmatic center of appropriate age. Certainly, there is no evidence for an eruptive source within the San Carlos depocenter itself. However, no in situ volcanic remnants have yet been recognized within the cluster, although it is possible that they could have been confused with andesitic rocks of the late Paleocene to early Eocene Augusta Victoria Formation. Furthermore, if the volcano overlay all or part of the Escondida cluster, the latter must have remained concealed because of the notable absence of altered and mineralized clasts in the San Carlos strata. The contemporaneity of the subaerial volcanism and initial formation of the Escondida cluster at ~38 Ma opens the possibility of more widespread volcanic activity during the Incaic orogeny and associated porphyry copper emplacement than has been documented to date in the Domeyko Cordillera (e.g., Mpodozis and Ramos, 1990; Mpodozis and Perelló, 2003).

FIG. 14. Inferred former extents of lithocaps in the Escondida district based on exposed advanced argillic alteration. Isotopic ages of hypogene alunite (Table 3) and locations of porphyry copper deposits are also shown.

component at Escondida Este where the late mineral quartz porphyry displays extensive but currently undated advanced argillic alteration. Additional isotopic dating may well document even more complex temporal histories. Supergene history Supergene enrichment is spectacularly developed at Escondida and Escondida Norte-Zaldívar, although oxide copper ore is also present, particularly peripherally in potassic-altered andesitic host rocks characterized by lower neutralization potentials (Figs. 7c, 9c). Enrichment is also more modestly and thinly developed at Chimborazo (Fig. 5c). In contrast, Escondida Este and Pampa Escondida possess little enrichment (Figs. 7c, 11c). There are three main reasons for the high-grade enrichment zones at Escondida and Escondida Norte-Zaldívar: The zones are hosted by advanced argillic and sericitic alteration with low neutralization potentials, high pyrite contents, and copper-bearing, high sulfidation-state sulfide assemblages, partly occurring in steep veins (cf. Véliz and Camacho, 2003; Padilla-Garza et al., 2004). The zones underlie topographic highs (Fig. 2a, b) that were undergoing erosion and paleoground-water table depression as a result of the renewed tectonic uplift that gave rise to accumulation in flanking depressions of the Pampa de Mulas piedmont gravels (Fig. 13d), which contain abundant limonitic clasts of advanced argillic lithocap. Lateral transport of copper to form exotic copper

Advanced argillic lithocaps An appreciable volume of advanced argillic lithocap is preserved at Chimborazo (Fig. 2c) and more restricted remnants occur at Escondida-Escondida Este (Fig. 2a), Escondida Norte (Fig. 2b), and Baker, as described above. If these partially mineralized remnants are combined with other, apparently barren lithocap exposures, two large lithocaps could have existed at ~34 Ma, when porphyry copper formation ended (Fig. 14). In fact, the two lithocaps could easily have coalesced to form a single advanced argillic zone, covering ~200 km2. On the basis of the available isotopic age data (Fig. 14; Table 3), the northern lithocap may have formed at ~42 Ma, in association with the Chimborazo deposit, whereas the southern one is composite and includes 37 to 36 Ma parts at Escondida, Escondida Norte, and Baker and a post-34 Ma 0361-0128/98/000/000-00 $6.00

75

76

HERVÉ ET AL.

deposits, a common process in northern Chile (Münchmeyer, 1996), was apparently only incipiently developed in the district. Enrichment was inhibited at Pampa Escondida because the rocks subjected to oxidation were pyrite poor and included calcareous horizons with a high acid-neutralizing capacity; and the deposit lies beneath a gravel-filled, topographic low (Figs. 2d, 3) and, judging by the limited oxidation depth (Fig. 11c), had a relatively near-surface paleogroundwater table. Enrichment was poorly developed at Escondida Este because of a deficiency of near-surface copper and is relatively low grade and thin at Chimborazo for the same reason, except in the pyrite-enargite veins, and because most of it underlies low, gravel-covered terrain. Oxidation and enrichment were active in the Escondida district between ~18 and 14 Ma based on ages of supergene alunite, a proxy for pyrite oxidation (Alpers and Brimhall, 1988; Morales, 2009; this study; Table 3), in keeping with the supergene chronology throughout the middle Eocene to early Oligocene porphyry copper belt of northern Chile (Sillitoe and McKee, 1996; Arancibia et al., 2006). Contemporaneously, the Pampa de Mulas Formation was accumulating in nearby depressions (Marinovic et al., 1995). Since about 14 Ma, aridity intensified, erosion rates decreased, and groundwater recharge and supergene processes ceased, as reflected by the absence of younger supergene alunite development and near-surface preservation of the tuff horizons as old as 8.7 Ma (Alpers and Brimhall, 1988; Sillitoe and McKee, 1996; Arancibia et al., 2006).

peak of the Incaic uplift underwent extreme telescoping, with advanced argillic alteration overprinting originally deeply emplaced, early porphyry intrusions, potassic alteration, and chalcopyrite-bornite mineralization. The earlier Chimborazo and later Escondida Este and Pampa Escondida deposits display much lesser degrees of telescoping. Synchronously, the uppermost synorogenic San Carlos strata were accumulating, including andesitic volcanic rocks dated at ~38 Ma and therefore coeval with early porphyry intrusion at Escondida and Escondida Norte-Zaldívar; this situation implies that volcanism, probably associated with construction of a stratovolcano or major dome complex, was active nearby in the magmatic arc, possibly above the Escondida cluster. The porphyry copper deposits were likely still overlain at ~34 Ma by extensive, composite advanced argillic lithocaps, which, judging by the localized high-sulfidation epithermal gold mineralization preserved in the remnants at Escondida and Chimborazo may have been auriferous. The main stripping of the lithocaps took place during renewed uplift and erosion in the late Oligocene to middle Miocene, as charted by the abundance of advanced argillic-altered clasts in the piedmont gravels of the coeval Pampa de Mulas Formation. Once high-sulfidation copper mineralization in the basal parts of the lithocaps became exposed to weathering, the thick leached cappings and their underlying chalcocite enrichment blankets began to form. Their recorded age spans the ~18 to 14 Ma interval, contemporaneous with gravel accumulation. The topographically prominent Escondida and Escondida Norte-Zaldívar deposits developed high-grade enrichment zones, whereas the topographically recessive, gravel-covered Pampa Escondida deposit underwent only shallow oxidation and incipient enrichment.

Conclusions The Escondida district is the largest of several giant porphyry copper clusters that characterize the middle Eocene to early Oligocene magmatic arc of northern Chile (Fig. 1). Deposits of the Escondida cluster are assumed to have formed above a single parental magma chamber, from which ascent of the first magma batches and related fluid involved in deposit generation took place in two discrete pulses separated by ~1 to 2 m.y. (Fig. 12a). These initial magma batches, represented by the early porphyries, were followed by additional discrete aliquots of magma and fluid, giving rise to the interand late mineral porphyry sequences and their associated, but progressively declining intensities of alteration and mineralization. Nonetheless, there was temporal overlap between the two porphyry copper events. The deeply concealed Chimborazo porphyry initiated intrusion and copper introduction at the district scale, but it remains uncertain if the magma and fluid were supplied by the same parental chamber as that responsible for the Escondida cluster. Chimborazo was formed prior to the main phase of Incaic uplift and exhumation, possibly toward the end of an episode of sinistral transpressional faulting, related clockwise rigidblock rotation, and the consequent opening and initial filling of the San Carlos sedimentary depocenter. Formation of Escondida, Escondida Norte-Zaldívar, and the small Baker center coincided with maximal uplift and erosion rates in a transient dextral transpressive setting, whereas Escondida Este and Pampa Escondida emplacement accompanied waning uplift immediately prior to imposition of a weak, sinistral transpressive regime. The entire Escondida cluster postdated all major fault displacement. The deposits generated at the 0361-0128/98/000/000-00 $6.00

Acknowledgments The writers wish to acknowledge the contributions to geologic understanding of the Escondida district by many past and present members of the Escondida brownfields team. Javier Urrutia of Minera Escondida Limitada, Ricardo Muhr of Antofagasta Minerals, and Geoff McKinley, Geoff Woad, and Angus Campbell of BHP Billiton MinEx facilitated the collective preparation of the first draft of the manuscript in Santiago, Chile. Constantino Mpodozis provided valuable instruction on the regional geologic setting of the Escondida district. Reviews of the first manuscript draft were provided by Leyla Vaccia and Walter Véliz of Minera Escondida Limitada and Jean des Rivières and Rick Preece of BHP Billiton Base Metals. Geoff Ballantyne and Rubén Padilla carried out the formal manuscript reviews. Edgar Basto, President of Minera Escondida Limitada, and BHP Billiton authorized publication. REFERENCES Alpers, C.N., and Brimhall, GH., 1988, Middle Miocene climatic change in the Atacama Desert, northern Chile: Evidence from supergene mineralization at La Escondida: Geological Society of America Bulletin, v. 100, p. 1640–1656. ——1989, Paleohydrologic evolution and geochemical dynamics of cumulative supergene metal enrichment at La Escondida, Atacama Desert, northern Chile: Economic Geology, v. 84, p. 229–255. Amilibia, A., and Skarmeta, J., 2003, La inversión tectónica de la Cordillera de Domeyko en el norte de Chile y su relación con la intrusión de sistemas porfídicos de Cu-Mo: Congreso Geológico Chileno, 10th, Concepción, 2003, Actas, CD-ROM, 7 p.

76

ESCONDIDA PORPHYRY COPPER DISTRICT, N. CHILE

Chuquicamata porphyry copper deposit, northern Chile: International Geology Review, v. 37, p. 945–958. Lowell, J.D., 1990, The La Escondida exploration project: Pacific Rim Congress 90, Gold Coast, Queensland, 1990, Proceedings, v. 3, p. 263–274. ——1991, The discovery of the La Escondida orebody: Economic Geology Monograph 8, p. 300–313. Maksaev, V., and Zentilli, M., 1988, Marco metalogénico regional de los megadepósitos de tipo pórfido cuprífero del Norte Grande de Chile: Congreso Geológico Chileno, 5th, Santiago, 1988, Actas, v. 1, p. B181–B212. ——1999, Fission track thermochronology of the Domeyko Cordillera, northern Chile: Implications for Andean tectonics and porphyry copper metallogenesis: Exploration and Mining Geology, v. 8, p. 65–89. Maksaev, V., Marinovic, N., Smoje, I., and Mpodozis, C, 1991, Mapa geológico de la Hoja Augusta Victoria, Región de Antofagasta. Escala 1:100,000: Servicio Nacional de Geología y Minería [Chile], Documentos de Trabajo 1. Marinovic, N., Smoje, I., Maksaev, V., Hervé, M., and Mpodozis, C., 1995, Hoja Aguas Blancas, Región de Antofagasta. Escala 1:250,000: Servicio Nacional de Geología y Minería, Carta Geológica de Chile 70, 150 p. Maturana, M., and Saric, N., 1991, Geología y mineralización del yacimiento tipo pórfido cuprífero Zaldívar, en los Andes del norte de Chile: Revista Geológica de Chile, v. 18, p. 109–120. May, G., Hartley, A.J., Chong, G., Stewart, F., Turner, P., and Kape, S.J., 2005, Eocene to Pleistocene lithostratigraphy, chronostratigraphy and tectono-sedimentary evolution of the Calama basin, northern Chile: Revista Geológica de Chile, v. 32, p. 33–58. McInnes, B.I.A., Farley, K.A., Sillitoe, R.H., and Kohn, B.P., 1999, Application of apatite (U-Th)/He thermochronometry to the determination of the sense and amount of vertical fault displacement at the Chuquicamata porphyry copper deposit, Chile: Economic Geology, v. 94, p. 937–946. Meyer, C., 1965, An early potassic type of wall rock alteration at Butte, Montana: American Mineralogist, v. 50, p. 1717–1722. Monroy, C., 2000, Nuevos antecedentes geológicos del pórfido cuprífero Zaldívar, II Región, Chile: Congreso Geológico Chileno, 9th, Puerto Varas, 2000, Actas, v. 1, p. 293–297. ——2009, Análisis del tamaño de bloques económico optimo de producción, para el yacimiento tipo pórfido cuprífero Zaldívar, Región de Antofagasta, Chile: Unpublished Memoria de Título, Antofagasta, Universidad Católica del Norte, 114 p. Morales, P., 2009, Geología y edad de la zone hipógena del yacimiento Zaldívar, II Región, Chile: Unpublished Memoria de Título, Antofagasta, Universidad Católica del Norte, 136 p. Mpodozis, C., and Perelló, J., 2003, Porphyry copper metallogeny of the middle Eocene-early Oligocene arc of western South America: Relationships with volcanism and arc segmentation [abs.]: Congreso Geológico Chileno, 10th, Concepción, 2003, CD-ROM, 1 p. Mpodozis, C., and Ramos, V., 1990, The Andes of Chile and Argentina: Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, v. 11, p. 59–90. Mpodozis, C., Marinovic, N., and Smoje, I., 1993a, Eocene left lateral strike slip faulting and clockwise block rotations in the Cordillera de Domeyko, west of the Salar de Atacama, northern Chile: International Symposium on Andean Geodynamics, 2nd, Oxford, U.K., 1993, Proceedings, p. 225–228. Mpodozis, C., Marinovic, N., Smoje, I., and Cuitiño, L., 1993b, Estudio geológico-estructural de la Cordillera de Domeyko entre Cerro Limón Verde y Sierra Mariposas, Región de Antofagasta: Servicio Nacional de Geología y Minería [Chile], Informe Registrado IR-93-04, 282 p. Mpodozis, C., Arriagada, C., Basso, M., Roperch, P., Cobbold, P., and Reich, M., 2005, Late Mesozoic to Paleogene stratigraphy of the Salar de Atacama basin, Antofagasta, northern Chile: Implications for the tectonic evolution of the Central Andes: Tectonophysics, v. 399, p. 125–154. Münchmeyer, C., 1996, Exotic deposits—products of lateral migration of supergene solutions from porphyry copper deposits: Society of Economic Geologists Special Publication 5, p. 43–58. Nalpas, T., Hérail, G., Mpodozis, C., Riquelme, R., Clavero, J., and Dabard, M.-P., 2005, Thermochronological data and denudation history along a transect between Chañaral and Pedernales (~26°S), North Chilean Andes: Orogenic implications [ext. abs.]: International Symposium on Andean Geodynamics, 6th, Barcelona, 2005, Extended Abstracts, p. 548–551. Navarro, M., Monroy, C., Rubio, M., Bustamante, V., Morales, P., Ramírez, C., Osorio, K., Machulás, K., Maldonado, M., Vera, C., Solís, S., and Merino, R., 2009, Actualización de la geología del yacimiento Zaldívar: Congreso Geológico Chileno, 12th, Santiago, 2009, Actas, Pendrive, 4 p.

Amilibia, A., Sàbat, F., McClay, K.R., Muñoz, J.A., Roca, A., and Chong, G., 2008, The role of inherited tectono-sedimentary architecture in the development of the central Andean mountain belt: Insights from the Cordillera de Domeyko: Journal of Structural Geology, v. 30, p. 1520–1539. Arancibia, G., Matthews, S.J., and Pérez de Arce, J., 2006, K-Ar and 40Ar/39Ar geochronology of supergene processes in the Atacama Desert, northern Chile: Tectonic and climatic relations: Journal of the Geological Society, London, v. 163, p. 107–118. Ardill, J., Flint, S., Chong, G., and Wilke, H., 1998, Sequence stratigraphy of the Mesozoic Domeyko basin, northern Chile: Journal of the Geological Society, London, v. 155, p. 71–88. Arriagada, C., Roperch, P., and Mpodozis, C., 2000, Clockwise block rotations along the eastern border of the Cordillera de Domeyko, northern Chile (22°45’–23°30’ S): Tectonophysics, v. 326, p. 153–171. Arriagada, C., Roperch, P., Mpodozis, C., and Cobbold, P.R., 2008, Paleogene building of the Bolivian orocline: Tectonic restoration of the central Andes in 2-D map view: Tectonics, v. 27, TC6014, 14 p. Astudillo, N., Roperch, P., Townley, B., Arriagada, C., and Maksaev, V., 2008, Importance of small-block rotations in damage zones along transcurrent faults: Evidence from the Chuquicamata open pit, northern Chile: Tectonophysics, v. 450, p. 1–20. Bahlburg, H., and Hervé, F., 1997, Geodynamic evolution and tectonostratigraphic terranes of northwestern Argentina and northern Chile: Geological Society of America Bulletin, v. 109, p. 869–884. Blanco, N., Tomlinson, A.J., Mpodozis, C., Pérez de Arce, C., and Matthews, S., 2003, Formación Calama, Eoceno, II Región de Antofagasta, Chile: Estratigrafía y implicancias tectónicas: Congreso Geológico Chileno, 10th, Concepción, 2003, Actas, CD-ROM, 10 p. Brimhall, G.H., Alpers, C.N., and Cunningham, A.B., 1985, Analysis of supergene ore-forming processes and ground-water solute transport using mass balance principles: Economic Geology, v. 80, p. 1227–1256. Campos, E., Wijbrans, J., and Andriessen, P.A.M., 2009, New thermochronologic constraints on the evolution of the Zaldívar porphyry copper deposit, northern Chile: Mineralium Deposita, v. 44, p. 329–342. Camus, F., 2003, Geología de los sistemas porfíricos en los Andes de Chile: Santiago, Chile, Servicio Nacional de Geología y Minería, 267 p. Chong, G., 1977, Contribution to the knowledge of the Domeyko Range in the Andes of northern Chile: Geologische Rundschau, v. 66, p. 374–404. Comisión Chilena del Cobre, 2011, Estadísticas históricas: www.cochilco.cl. Cornejo, P., and Mpodozis, C., 1996, Geología de la región de Sierra Exploradora (25°−26° S): Servicio Nacional de Geología y Minería [Chile], Informe Registrado IR-96-09, 330 p. Cornejo, P., Matthews, S., Marinovic, N., Pérez de Arce, C., Basso, M., Alfaro, J., and Navarro, M., 2006, Alteración hidrotermal y mineralización recurrente de Cu y Cu-Mo durante el Pérmico y el Triásico en la Cordillera de Domeyko (zona de Zaldívar-Salar de los Morros): Antecedentes geocronológicos U-Pb, 40Ar/39Ar y Re-Os: Congreso Geológico Chileno, 11th, Antofagasta, 2006, Actas, v. 2, p. 219–222. Fisher, R.V., and Schmincke, H.-U., 1994, Volcaniclastic sediment transport and deposition, in Pye, K., ed., Sediment transport and depositional processes: Oxford, Blackwell Scientific Publications, p. 351–388. Gardeweg, M., Pino, H., Ramírez, C.F., and Davidson, J., 1994, Mapa geológico del área de Imilac y Sierra Almeida, Región de Antofagasta. Escala 1:100,000: Servicio Nacional de Geología y Minería [Chile], Documentos de Trabajo 7. Gustafson, L.B., and Hunt, J.P., 1975, The porphyry copper deposit at El Salvador, Chile: Economic Geology, v. 70, p. 857–912. Gustafson, L.B., Vidal, C.E., Pinto, R., and Noble, D.C., 2004, Porphyry-epithermal transition, Cajamarca region, northern Peru: Society of Economic Geologists Special Publication 11, p. 279–299. Hammerschmidt, K., Döbel, R., and Friedrichsen, H., 1992, Implication of 40Ar/39Ar dating of Early Tertiary volcanic rocks from the north-Chilean Precordillera: Tectonophysics, v. 202, p. 55–81. Jara, C., Rabbia, O., and Valencia, V., 2009, Petrología y dataciones U-Pb del depósito tipo pórfido Cu Zaldívar, II Región de Antofagasta, Chile: Congreso Geológico Chileno, 12th, Santiago, 2009, Actas, Pendrive, 4 p. Jordan, T.E., Mpodozis, C., Muñoz, N., Blanco, N., Pananont, P., and Gardeweg, M., 2007, Cenozoic subsurface stratigraphy and structure of the Salar de Atacama basin, northern Chile: Journal of South American Earth Sciences, v. 23, p. 122–146. Lindsay, D.D., Zentilli, M., and Rojas de la Rivera, J., 1995, Evolution of an active ductile to brittle shear system controlling the mineralization at the 0361-0128/98/000/000-00 $6.00

77

77

78

HERVÉ ET AL. Romero, B.P., 2008, Caracterización y distribución del molibdeno en los yacimientos Escondida y Escondida Norte, Segunda Región de Antofagasta, Chile: Unpublished Memoria de Título, Antofagasta, Universidad Católica del Norte, 87 p. Romero, B., Kojima, S., Wong, C., Barra, F., Véliz, W., and Ruiz, J., 2010, Molybdenite mineralization and Re-Os geochronology of the Escondida and Escondida Norte porphyry deposits, northern Chile: Resource Geology, v. 61, p. 91–100. Salfity, J.A., 1985, Lineamientos transversales al rumbo andino en el noroeste argentino: Congreso Geológico Chileno, 4th, Antofagasta, 1985, Actas, v. 1, p. 2/119–2/137. Samoza, R., and Zaffarana, C.B., 2008, Mid-Cretaceous polar standstill of South America, motion of the Atlantic hotspots and the birth of the Andean cordillera: Earth and Planetary Science Letters, v. 271, p. 267–277. Sillitoe, R.H., 1979, Some thoughts on gold-rich porphyry copper deposits: Mineralium Deposita, v. 14, p. 161–174. ——1985, Ore-related breccias in volcanoplutonic arcs: Economic Geology, v. 80, p. 1467–1514. ——1988, Epochs of intrusion-related copper mineralization in the Andes: Journal of South American Earth Sciences, v. 1, p. 89–108. ——1994, Erosion and collapse of volcanoes: Causes of telescoping in intrusion-centered ore deposits: Geology, v. 22, p. 945–948. ——1995, Exploration and discovery of base- and precious-metal deposits in the circum-Pacific region during the last 25 years: Resource Geology Special Issue 19, 119 p. ——2010, Exploration and discovery of base- and precious-metal deposits in the circum-Pacific region—a 2010 perspective: Resource Geology Special Issue 22, 139 p. Sillitoe, R.H., and McKee, E.H., 1996, Age of supergene oxidation and enrichment in the Chilean porphyry copper province: Economic Geology, v. 91, p. 164–179. Sillitoe, R.H., and Perelló, J., 2005: Andean copper province: Tectonomagmatic settings, deposit types, metallogeny, exploration, and discovery: Economic Geology 100th Anniversary Volume, p. 845–890. Tomlinson, A.J., and Blanco, N., 1997, Structural evolution and displacement history of the West fault system, Precordillera, Chile: Part 2. Postmineral history: Congreso Geológico Chileno, 8th, Antofagasta, 1997, Actas, v. 3, p. 1878–1882. Urzúa, F., 2009, Geology, geochronology and structural evolution of La Escondida copper district, northern Chile: Unpublished Ph.D. thesis, Hobart, Australia, University of Tasmania, 486 p. Uyeda, S., and Kanamori, H., 1979, Back-arc opening and the mode of subduction: Journal of Geophysical Research, v. 84, B3, p. 1049–1061. Véliz, W.O., 2004, Relación espacio-temporal del sistema pórfido cuprífero y epitermal en el yacimiento Escondida, Provincia de Antofagasta, Segunda Región, Chile: Unpublished Masters thesis, Antofagasta, Universidad Católica del Norte, 139 p. Véliz, W., and Camacho, J., 2003, Antecedentes geológicos del yacimiento La Escondida: Congreso Geológico Chileno, 10th, Concepción, 2003, Actas, CD-ROM, 10 p. Williams, M.J., 2003, Geology and resources of the Escondida Norte deposit, Region II, Chile [abs.]: Congreso Geológico Chileno, 10th, Concepción, 2003, Actas, CD-ROM, 1 p. Wotzlaw, J.F., Decou, A., von Eynatten, H., Wörner, G., and Frei, D., 2011, Jurassic to Palaeogene tectono-magmatic evolution of northern Chile and adjacent Bolivia from detrital zircon U-Pb geochronology and heavy mineral provenance: Terra Nova, v. 23, p. 399–406.

Niemeyer, H., and Urrutia, C., 2009, Transcurrencia a lo largo de la Falla Sierra de Varas (Sistema de fallas de la Cordillera de Domeyko), norte de Chile: Andean Geology, v. 36, p. 37–49. Ojeda, J.M., 1986, The Escondida porphyry copper deposit, II Region, Chile: Exploration drilling and current geological interpretation, in Mining Latin America: London, Institution of Mining and Metallurgy, p. 299–318. ——1990, Geology of the Escondida porphyry copper deposit, II Region, Chile: Pacific Rim Congress 90, Gold Coast, Queensland, 1990, Proceedings, v. 2, p. 473–483. Ortíz, F.J., 1995, Discovery of the Escondida porphyry copper deposit in the Antofagasta region, northern Chile: Arizona Geological Society Digest, v. 20, p. 613–624. Ortíz, F.J., Lowell, J.D., Rojas, N.D., and Burns, P.J., 1985, Reseña técnica del descubrimiento del yacimiento de pórfido cuprífero Escondida, II Región, Chile: Congreso Geológico Chileno, 4th, Antofagasta, 1985, Actas, v. 4, p. 3.703–3.729. Ortíz, F.J., Lowell, J.D., Bratt, J.A., Rojas, N.D., and Burns, P.J., 1986, Escondida porphyry copper deposit, II Región, Chile: History of the discovery, in Mining Latin America: London, Institution of Mining and Metallurgy, p. 319–331. Ossandón, G., Fréraut, R., Gustafson, L.B., Lindsay, D.D., and Zentilli, M., 2001, Geology of the Chuquicamata mine: A progress report: Economic Geology, v. 96, p. 249–270. Padilla-Garza, R.A., Titley, S.R., and Pimentel, F., 2001, Geology of the Escondida porphyry copper deposit, Antofagasta Region, Chile: Economic Geology, v. 96, p. 307–324. Padilla-Garza, R.A., Titley, S.R., and Eastoe, C.J., 2004, Hypogene evolution of the Escondida porphyry copper deposit, Chile: Society of Economic Geologists Special Publication 11, p. 141–165. Pananont, P., Mpodozis, C., Blanco, N., Jordan, T.E., and Brown, L.D., 2004, Cenozoic evolution of the northwestern Salar de Atacama basin, northern Chile: Tectonics, v. 23, TC6004, 20 p. Perelló, J., Brockway, H., and Martini, R., 2004, Discovery and geology of the Esperanza porphyry copper-gold deposit, Antofagasta Region, northern Chile: Society of Economic Geologists Special Publication 11, p. 167–186. Petersen, C.R., Rivera, S., and Peri, A., 1996, Chimborazo copper deposit, Region II, Chile: Exploration and geology: Society of Economic Geologists Special Publication 5, p. 71–80. Pontual, S., Merry, N., and Ganson, P., 1997, G-MEX Volume 1. Spectral interpretation field manual: Queenstown, New Zealand, AusSpec International Pty. Ltd., 169 p. Quiroz, F., 2003a, Geology and hypogene alteration and mineralization at Escondida, northern Chile: Porphyry and high-sulfidation events: Unpublished M.Sc thesis, Tucson, University of Arizona, 82 p. ——2003b, Evolution of hypogene alteration and mineralization at Escondida, northern Chile: Porphyry copper and high-sulfidation events [abs.]: Congreso Geológico Chileno, 10th, Concepción, 2003, Actas, CD-ROM, 1 p. Reutter, K.-J., Scheuber, E., and Helmcke, D., 1991, Structural evidence of orogen-parallel strike slip displacements in the Precordillera of northern Chile: Geologische Rundschau, v. 80, p. 135–153. Reutter, K.-J., Scheuber, E., and Chong, G., 1996, The Precordillera fault system of Chuquicamata, northern Chile: Evidence for reversals along arcparallel strike-slip faults: Tectonophysics, v. 259, p. 213–228. Richards, J.P., Noble, S.R., and Pringle, M.S., 1999, A revised late Eocene age for porphyry Cu magmatism in the Escondida area, northern Chile: Economic Geology, v. 94, p. 1231–1248. Richards, J.P., Boyce, A.J., and Pringle, M.S., 2001, Geologic evolution of the Escondida area, northern Chile: A model for spatial and temporal localization of porphyry copper mineralization: Economic Geology, v. 96, p. 271–305.

0361-0128/98/000/000-00 $6.00

78

© 2012 Society of Economic Geologists, Inc. Special Publication 16, pp. 79–104

Chapter 4 Geologic Setting and Evolution of the Porphyry Copper-Molybdenum and Copper-Gold Deposits at Los Pelambres, Central Chile JOSÉ PERELLÓ,1,† RICHARD H. SILLITOE,2 CONSTANTINO MPODOZIS,1 HUMBERTO BROCKWAY,1 AND HÉCTOR POSSO3 1 Antofagasta 2 27 3 Anaconda

Minerals S.A., Apoquindo 4001, piso 18, Las Condes, Santiago, Chile West Hill Park, Highgate Village, London N6 6ND, England

Perú, Avenida Paseo de la República 3245, piso 3, San Isidro, Lima, Peru

Abstract The porphyry copper mineralization at Los Pelambres is contained in two contiguous deposits, Los Pelambres (Cu-Mo) and Frontera (Cu-Au), which together constitute the third largest copper concentration (~36 million metric tons (Mt) Cu) in the Miocene to early Pliocene belt of central Chile. Los Pelambres is centered on a composite, N-oriented, ~4.5- × 2.5-km precursor quartz diorite stock emplaced within the regional, NNW-striking, E-vergent Los Pelambres reverse fault. The fault places intensely deformed Late Cretaceous volcanic and late Oligocene to early Miocene volcanic and volcanosedimentary rocks of the Los Pelambres Formation over gently folded early Miocene volcanic rocks of the Pachón Formation. Copper-gold mineralization at Frontera is hosted mainly by andesite of the Pachón Formation. Hydrothermal alteration at Los Pelambres-Frontera conforms to the classic zonal pattern in which a potassic center grades laterally to an annular sericitic zone surrounded by a propylitic halo. The bulk of the hypogene metal resource is hosted by multiple veinlet generations within potassic alteration, of which type 4 (quartz ± K-feldspar ± biotite ± sericite ± phengite ± andalusite ± corundum), A, and B types are volumetrically and economically the most important. The type 4 veinlets are regularly distributed throughout Los Pelambres and Frontera, whereas highest intensities of A and B veinlets display a spatial correlation with at least 20 small (~200-m diam), SE-plunging magmatic-hydrothermal centers. These centers comprise one or more intermineral porphyry intrusions of dacitic (porphyry B) and andesitic (porphyry A) compositions along with igneous and hydrothermal breccias, the apical parts of which contain aplite and pegmatite pods. These centers acted as a series of miniature porphyry copper deposits whose coalescence generated the Los Pelambres-Frontera orebody. This coalescence also led to deposit-scale sulfide zoning, from internal chalcopyrite-bornite through chalcopyrite-pyrite to external pyrite. Abundant hydrothermal magnetite accompanies the gold-bearing copper mineralization in biotitized andesite at Frontera. The sericitic alteration is largely pyritic, but a NE-striking, SE-dipping corridor of D-type veinlets that overprints the potassic alteration in the northwestern quadrant of Los Pelambres contains copper sulfosalts. The internal portions of this corridor are characterized by advanced argillic assemblages, defining the roots of a once more extensive lithocap. On the basis of detailed U-Pb zircon dating, the intrusive magmatism at Los Pelambres-Frontera lasted ~3.8 m.y., from emplacement of the precursor Los Pelambres stock between ~14 and 12.5 Ma, through generation of numerous porphyry B and A phases and associated magmatic-hydrothermal centers between ~12.3 and 10.5 Ma, to intrusion of late mineral porphyry at Frontera at ~10.2 Ma. Similarly, the copper, molybdenum, and gold mineralization was introduced during a protracted interval of ~1.7 m.y., between 11.8 and 10.1 Ma, as constrained by Re-Os molybdenite geochronology. The entire system cooled to nearly ambient temperatures by ~8 Ma, as supported by temporally overlapping K-Ar, Ar/Ar, and (U-Th)/He ages, and was exposed to the effects of supergene oxidation and immature enrichment by ~5 Ma. Plio-Pleistocene glaciation partially eroded a former, more widespread supergene chalcocite blanket, the remnants of which accounted for the bulk of the ore mined during the first 10 years of the Los Pelambres open-pit operation. The southeast-inclined geometry of the entire Los Pelambres-Frontera system, including the porphyry centers and northeast structural corridor defined by sericitic and advanced argillic alteration, are ascribed to synmineral tilting. The tilting accompanied regional tectonic uplift during crustal shortening and thickening, which were controlled by thick-skinned reverse faults active ~60 km farther east in Argentina.

at elevations between 3,200 and 3,600 m above sea level (Fig. 2a). Los Pelambres and Frontera along with the copper-molybdenum deposit at El Pachón, 5 km southeast across the international frontier in Argentina, constitute the Los Pelambres-El Pachón porphyry copper cluster (Fig. 1). The supergiant status of the Los Pelambres deposits is defined by their current resource of 6,165 million metric tons (Mt) at 0.56% Cu and 0.011% Mo, using a cutoff of 0.35% Cu (Perelló et al., 2011).

Introduction LOS PELAMBRES, the northernmost and third largest copper concentration in the Miocene to early Pliocene belt of central Chile (Fig. 1), comprises two contiguous deposits, Los Pelambres copper-molybdenum and Frontera copper-gold. The deposits underlie the vegetation-free talus slopes of a Ushaped glacial valley in the Principal Cordillera of the Andes, † Corresponding

author: e-mail, [email protected]

79

80 74º 30º

Chile-Peru trenc h

PERELLÓ ET AL.

La Serena

64º Wadati-Benioff zone contours (km)

LOS PELAMBRES-EL PACHON

Ju

100 km

175 200 69º 120

z an Fernánde

Ridge

80 100

Flat-slab segment

Mendoza 40 1 Santiago

RIO BLANCO-LOS BRONCE S Principal deposits Other deposits and prospects

EL TENIENTE

Southern Volcanic Zone

FIG. 1. Location of the Los Pelambres-El Pachón porphyry copper cluster in the Miocene to early Pliocene porphyry copper belt of central Chile (diagonal shading). The principal deposits are named. The position of the belt with respect to the transition between amagmatic flat-slab subduction and the Southern Volcanic Zone of the Andes is defined by depth contours on the present-day Wadati-Benioff zone (after Cahill and Isacks, 1992; Anderson et al., 2007).

The deposits at Los Pelambres are owned by Minera Los Pelambres (Antofagasta Minerals S.A. 60%, Japanese consortium 40%). The copper-molybdenum deposit, mined in an open pit (Fig. 2b) at a current ore throughput of 176,000 t/d averaging 0.74% Cu and 0.019% Mo, produced 411,800 t of copper, 9,900 t of molybdenum, and 39,800 oz of gold, and 1,774,300 oz of silver in 2011. The ore is processed by conventional flotation and the resulting copper concentrate is transported ~120 km by slurry pipeline to the company’s port for shipment to overseas smelters. Los Pelambres and Frontera are currently the subject of another major infill drilling campaign, which is likely to further increase resources for a planned future mine expansion. This contribution summarizes the historic and recent exploration history of the district, describes the regional geologic

setting and the geology and alteration-mineralization features of Los Pelambres and Frontera, documents the lifespan of the hydrothermal system, and discusses the geologic evolution of the porphyry mineralization within a regional tectonomagmatic framework. The paper is based on more than three years of fieldwork by the authors during a brownfields exploration program that included 1:50,000-scale regional, 1:10,000scale district, and 1:2,000-scale pit mapping as well as 1:100scale logging of 160,000 m of preexisting and newly obtained drill core. Previous published studies by Sillitoe (1973), Skewes (1985), Skewes and Atkinson (1985), Atkinson et al. (1996), Bertens et al. (2003, 2006), Perelló et al. (2007, 2009, 2011), and Mpodozis et al. (2009) as well as extensive unpublished in-house data provide the basis for this synthesis. The porphyry and veinlet nomenclature of Skewes and Atkinson

a

b

FIG. 2. Views of Los Pelambres porphyry copper deposit, looking north. a. In 1970 prior to mining. Note the jarositic leached capping (yellowish-brown) developed over the pyrite-rich sericitic halo. The ore-bearing potassic zone underlies the U-shaped glacial valley. b. The open pit in 2007 after seven years of large-scale mining. 0361-0128/98/000/000-00 $6.00

80

PORPHYRY Cu-Mo & Cu-Au DEPOSITS, LOS PELAMBRES, CENTRAL CHILE

(1985) and Atkinson et al. (1996) is followed throughout, but the relative timing of porphyry phases, the economic relevance of certain veinlet generations, and the volumetric importance of hydrothermal breccias are considered to be different.

marine sedimentation in a backarc setting during the JurassicEarly Cretaceous, and subaerial, subduction-related, calc-alkaline volcanism and associated plutonism during the Cretaceous through Cenozoic (Mpodozis and Ramos, 1990). Central Chile and contiguous parts of Argentina, including the porphyry copper belt, underwent contractional tectonism from the early Miocene through early Pliocene in response to subduction zone shallowing (Jordan et al., 1983). This triggered crustal shortening and thickening through hybrid thinand thick-skinned thrusting to generate the Aconcagua foldthrust belt (Ramos et al., 1996; see below). The slab shallowing is generally ascribed to the diachronous oblique subduction of the buoyant Juan Fernández ridge on the Nazca plate (Fig. 1; Pilger, 1981; Yáñez et al., 2001). The copper mineralization in the belt took place between 12 and 4 Ma and accompanied multikilometer, regional-scale uplift and concomitant exhumation (Skewes and Holmgren, 1993; Kurtz et al., 1997). The porphyry copper stocks have rare earth element signatures interpreted to reflect the thickening of the crust (Kay et al., 1999; Kay and Mpodozis, 2002).

Exploration History In 1967, the Instituto de Investigaciones Geológicas prepared the first geologic report for Los Pelambres, which formally identified the porphyry copper affiliation of the prospect (Thomas, 1967). Exploration was resumed in 1969 under a joint program conducted by the United Nations and Empresa Nacional de Minería (ENAMI), the state mining agency. Drilling defined a mineral inventory of 430 Mt at 0.80% Cu and 0.035% Mo (Sillitoe, 1995). No further exploration took place until 1979 when Anaconda South America purchased the property from the local owners and undertook detailed exploration, culminating in 1983 with completion of a feasibility study for a 60,000 t/d operation based on a resource of 3,300 Mt at 0.63% Cu and 0.016% Mo (Sillitoe, 1995; Atkinson et al., 1996). At the copper prices prevailing at the time, such a large-scale project was considered uneconomic and all work was discontinued. In 1985, Antofagasta Holdings acquired Anaconda’s interests in Chile and a wholly owned subsidiary, Compañía Minera Los Pelambres, developed a 5,000-t/d sublevel caving and flotation operation based on high-grade, breccia-hosted ore (see below). Through 1999, Compañía Minera Los Pelambres mined approximately 12 Mt averaging 1.5% Cu (Perelló et al., 2011). Simultaneously, a feasibility study for an 80,000-t/d plant was carried out, with completion in 1996 and, the following year, a Japanese consortium acquired a 40% interest in Compañía Minera Los Pelambres and committed the same proportion of development funding. Project construction began in early 1998 and in January 2000 the plant attained its rated capacity. Successive expansions through 2004 increased ore throughput to 130,000 t/d, with the most recent expansion completed in 2010 increasing capacity to ~175,000 t/d. Since the completion of Anaconda’s feasibility study in 1983, no further exploration was carried out at Los Pelambres until late 2005 when the brownfields program was initiated by Antofagasta Minerals S.A. on behalf of the operating company. The program resulted in discovery of the Frontera copper-gold deposit (700 Mt at 0.52% Cu and 0.1 g/t Au, using a 0.4% Cu cutoff) and definition of additional resources at Los Pelambres, which together constitute the current global resource cited above (Perelló et al., 2011).

Tectonics and stratigraphy of the greater Los Pelambres region The greater Los Pelambres region, spanning the Chile-Argentina frontier between latitudes 31°35' and 32°03'S, comprises three main tectonic domains bounded by high-angle, E- or W-vergent, N- to NNW-striking, reverse faults, herein named the Los Pelambres, Totoral, and González faults (Fig. 3). These structural elements form the northern termination of the larger Aconcagua and smaller (e.g., La Ramada) foldthrust belts (Cegarra and Ramos, 1996; Cristallini and Ramos, 2000). The eastern domain, east of the E-vergent Los Pelambres reverse fault, contains the large basement block of the Cordillera de Santa Cruz (Fig. 3), composed of late Paleozoic rhyolite and felsic tuff along with comagmatic granitoids, collectively assigned to the Choiyoi Group (Alvarez, 1996; Cristallini and Ramos, 2000). Beyond the limits of Figure 3, this basement block is thrust eastward over synorogenic, continental, siliciclastic deposits of Miocene age (Jordan et al., 1996; Pérez, 2001). To the west of the Cordillera de Santa Cruz block, the Paleozoic basement is overlain by Triassic continental volcanic and sedimentary strata and Jurassic to Early Cretaceous marine and continental sedimentary rocks (Alvarez, 1996), which together represent the northernmost exposures of the sedimentary fill to the Neuquén backarc basin, amply developed farther south (Mpodozis and Ramos, 1990; Cristallini and Ramos, 2000). These bedded units are unconformably overlain by continental volcaniclastic conglomerate and breccia, rhyolitic tuff, and pyroxene- and hornblendebearing andesite and dacite, herein informally grouped as the Mondaca Strata (Fig. 3), which yield U-Pb zircon ages of 22.1 ± 0.4 and 21.6 ± 0.4 Ma (Table 1). On the Chilean side of the frontier, at Laguna del Pelado (Fig. 3), these strata are unconformably overlain by >400 m of subhorizontal, hornblendebearing andesitic lava flows, which provide U-Pb zircon ages between 21.3 +0.4/-0.3 and 18.3 ± 0.4 Ma (Table 1). Immediately to the north, these rocks are tectonically overlain by Paleozoic basement and Mesozoic strata along the E-vergent Mondaca reverse fault (Fig. 3).

Tectonomagmatic Setting Central Chile porphyry copper belt The Miocene to early Pliocene porphyry copper belt of central Chile and contiguous Argentina extends for ~400 km between latitudes 31° and 35° S and contains an exceptional copper endowment (~360 Mt) contained in a series of supergiant and smaller sized deposits (Fig. 1). The belt was constructed within the Chilenia terrane, a microcontinental block accreted to the Gondwana margin in the Devonian and underpinned by basement rocks of Proterozoic age (Ramos, 2009). Following terrane accretion, the belt was the site of extension-related, bimodal magmatism during the Permo-Triassic, 0361-0128/98/000/000-00 $6.00

81

81

82

PERELLÓ ET AL. 370,000

345,000

R CO

DIL

A LE R

ga Río Chalin

DE

Rí

oC arn icer ía

A S A NT

da Piu quene s

6,500,000

N

Los Pelambres

C R UZ

Quebra

Frontera El Pachón

Totoral fault

ón

ch

Pa Río

Cros

s sec tion

Rí o M

on

au

Ar

ral

a tin e

to To

ge n Ch il

Mo nd ac af

6,475,000

lt

da ca

Yunque

lt

fau ez

ío

Gon

zá

l

Pocuro fault

Río Choapa

R

Laguna del Pelado

s bre Los Pelam

ral R í o Toto

lt fau

González fau lt

5 km Quebrada Piuquenes

Chile Argentina Los Pelambres El Pachón

Río Santa Cruz Río Pachón

Meters above sea level 4,000 2,000

Mondaca fault

Chalinga intrusive complex Totoral fault

a

STRATIFIED ROCKS Laguna del Pelado volcanic rocks (20-18 Ma) Mondaca Strata (22 Ma) a: Basal conglomerate Pachón and Abanico Formations (28-21 Ma) Los Pelambres Formation (33-18 Ma)

Cretaceous volcanic rocks (Salamanca Formation) Early Cretaceous redbeds Jurassic continental and marine sedimentary sequences Triassic volcanosedimentary rocks

Paleocene volcanic rocks Late Cretaceous (75-70 Ma) volcanic and sedimentary rocks a a: Intensely deformed sequences of the central domain

Late Paleozoic-Triassic basement (Choiyoi Group)

0

INTRUSIVE ROCKS Middle-late Miocene intrusive rocks (14-10 Ma) Totoral pluton (18 Ma) Chalinga intrusive complex a b c: Phase 3 (16-15 Ma) b: Phase 2 (18 Ma) c a: Phase 1 (23-22 Ma) Late Cretaceous-Eocene intrusive rocks Reverse fault (teeth on upper plate) Normal fault Undifferentiated fault

FIG. 3. Regional geologic map of the greater Los Pelambres region, based on Alvarez (1996), Mpodozis et al. (2009), and more recent mapping by the authors. UTM datum: Prov. S. Am 56, Zone 19 South. 0361-0128/98/000/000-00 $6.00

82

83

PORPHYRY Cu-Mo & Cu-Au DEPOSITS, LOS PELAMBRES, CENTRAL CHILE TABLE 1. U-Pb Zircon Ages of Selected Geologic Units from Los Pelambres Area and Greater Los Pelambres Region Sample no.

Age (Ma ± 2σ)

UTM E

UTM N

General location

Geologic unit

Comments

PEL 761 PEL 681

22.1 ± 0.4 21.6 ± 0.4

376,152 378,011

6,490,687 6,480,767

Mondaca Strata Mondaca Strata

Quartz-sanidine rhyolitic tuff Welded rhyolitic tuff

PEL 211

21.3 (+0.4/-0.3)

378,020

6,458,681

Lower Río Carnicería North of El Yunque prospect Laguna del Pelado

PEL 161

18. 3 ± 0.4

379,343

6,461,318

Laguna del Pelado

FRONT 42

22.7 ± 0.2

360,775

6,487,750

Los Pelambres

Laguna del Pelado Sequence Laguna del Pelado Sequence Pachón Formation

PELLAG 012 DDH 71962 PEL 11

21.69 ± 0.26 21.36 ± 0.80 33.4 ± 0.5

358,375 350,000 357,720

6,494,200 6,491,440 6,485,634

Río Carnicería Los Pelambres Río Pelambres

Pachón Formation Pachón Formation Los Pelambres Formation

PEL 411 PEL 471 PEL 511

18.5 ± 0.4 24.9 ± 0.5 22.2 ± 0.4

364,693 367,718 364,854

6,464,923 6,447,831 6,447,799

Río Totoral Río Chicharra Río Chicharra

Los Pelambres Formation Abanico Formation Abanico Formation

PEL 461 Sec1-12

18.5 ± 0.4 23.32 ± 0.23

359,196 355,053

6,460,543 6,486,484

Río Totoral Río Pelambres

PEL2603

21.62 ± 0.67

352,151

6,485,829

Quebrada del Perro

PEL2473

18.59 ± 0.43

351,281

6,494,431

PEL2463

18.11 ± 0.52

353,490

6,494,503

PEL 2014

16.5 (+0.3/-0.2)

351,270

6,509,363

Upper Quebrada Piuquenes Upper Quebrada Piuquenes Río Tres Quebradas

PEL 1031

15.1 (+0.6/-0.6)

350,890

6,504,830

PEL 651

15.4 ± 0.4

373,153

6,483,804

Los Helados, Río Chalinga Río Pachón

PEL 661

15.0 ± 0.3

378,397

6,475,054

El Yunque prospect

PEL 1021

70.1 ± 1.5

358,755

6,488,350

Los Pelambres

Totoral pluton Chalinga intrusive complex (Phase 1) Chalinga intrusive complex (Phase1) Chalinga intrusive complex (Phase 2) Chalinga intrusive complex (Phase 2) Chalinga intrusive complex (Phase 3) Chalinga intrusive complex (Phase 3) Northwest-trending intrusive belt Northwest-trending intrusive belt Country rock at Los Pelambres

Daciandesitic, pumice-rich, lithic tuff Hornblende-bearing porphyritic andesite Fine-grained porphyritic andesite Aphanitic andesitic lava Quartz-eye dacitic sill Fine-grained andesitic volcanic breccia Recrystallized daciandesite Andesitic lithic tuff Fine-grained porphyritic andesite Biotite (pyroxene) monzogranite Pyroxene granodiorite Pyroxene-hornblende-biotite granodiorite Pyroxene-biotite diorite Hornblende-biotite granodiorite Pyroxene-biotite quartz monzodiorite Pyroxene-biotite quartz monzodiorite Hornblende-biotite dacitic porphyry Hornblende-biotite dacitic porphyry Quartz-eye rhyolitic tuff

Notes: UTM datum for all samples is Prov. S. Am 56, Zone 19 South 1 Dated at the University of Arizona, Tucson, Arizona 2 Dated at Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW, Australia 3 Dated at University of Tasmania, Tasmania, Australia 4 Dated at Washington State University, Washington

A well-defined belt of pyroxene ± olivine-bearing basaltic to andesitic lava flows and minor felsic tuff interbeds extends continuously throughout the greater Los Pelambres region for ~60 km and constitutes the Pachón Formation (Fernández et al., 1974; Lencinas and Tonel, 1993). Near Los Pelambres, as well as east of the international frontier, this unit yields U-Pb zircon ages between 21.69 ± 0.26 and 22.7 ± 0.2 Ma and, in the Los Pelambres open pit, it is intruded by a quartz-eye dacite sill with a U-Pb zircon age of 21.36 ± 0.80 Ma (Perelló et al., 2009; Table 1). The Pachón Formation is the westernmost unit of the eastern domain, and throughout the area is delimited westward by the Los Pelambres fault (Fig. 3). The central domain straddles the international frontier and corresponds to a N- to NW-striking, ~5-km-wide, faultbounded zone of strongly deformed andesitic to basaltic lava flows and tuffs, fluviatile epiclastic strata, and local lacustrine limestone of the Los Pelambres Formation (Rivano and Sepúlveda, 1991) as well as tectonic slivers of Cretaceous volcanic rocks (Fig. 3). The domain, defined and bounded by the E-vergent Los Pelambres and Totoral and W-vergent 0361-0128/98/000/000-00 $6.00

González faults is intensely deformed, as indicated by vertical and overturned strata, anastomosing, thrust-bounded tectonic lenses, and widespread mesoscale, subisoclinal folds. Newly obtained U-Pb zircon ages for the Los Pelambres Formation, ranging from 33.4 ± 0.5 to 18.0 ± 0.4 Ma (Table 1), confirm its early Oligocene to early Miocene age (Mpodozis et al., 2009; Perelló et al., 2009), contrary to previous Early Cretaceous age assignments (Rivano and Sepúlveda, 1991; Bertens et al., 2006). West of the Totoral and González faults, the western domain comprises a >2-km-thick, gently E-dipping sequence of continental volcanic, volcanosedimentary, and sedimentary rocks of Cretaceous age, including the Salamanca Formation (Rivano and Sepúlveda, 1991) and other undifferentiated units (Fig. 3). The steeply dipping Pocuro fault, first defined farther south in central Chile by Carter and Aguirre (1965), constitutes the tectonic contact between the Salamanca Formation to the west and the other volcanic units farther east, which become more intensely deformed on approach to the central domain. The tectonic wedge of Cretaceous volcanic rocks between the Totoral and González faults is an example of 83

84

PERELLÓ ET AL.

particularly intense deformation (Fig. 3). The volcanic rocks exposed east of the Pocuro fault yield U-Pb zircon ages between 75 and 70 Ma, similar to the 70.10 ± 1.50 Ma age (Table 1) obtained from the rhyolitic tuff at Los Pelambres (see below). The Cretaceous volcanic rocks of the western domain are unconformably overlain northwest of Los Pelambres by andesitic volcanic sequences of Paleocene age and, at Río Totoral, by similar rocks of latest Oligocene to early Miocene age (Fig. 3). Immediately south of the area covered by Figure 3, the latter rocks provided U-Pb zircon ages of 24.9 ± 0.5 and 22.2 ± 0.4 Ma (Table 1) and are correlated with the northernmost expressions of the Abanico Formation, widely distributed farther south (Charrier et al., 2002).

basaltic andesite, and basalt, the great majority with SiO2 contents between 61 and 53%. They display FeO/MgO versus SiO2 ratios transitional between the tholeiitic and calc-alkaline fields and possess flat rare earth element patterns, with La/Yb between 5 and 19 and La/Sm between 3 and 6 (Fig. 4). Intrusive and volcanic rocks with ages between 18 and 15 Ma display more felsic compositions (67–62% SiO2), stronger arclike signatures, and higher La/Yb (6–34) but similar La/Sm (3–7) ratios. The Los Pelambres stock and related porphyry copper centers (see below) have low Nb/Ta ratios (15 Ma intrusive rocks compared to 600

DAM-18

3,450

3,200

400 × 80 × >700

DAM-01

150 × 140 × 450

3,150

180 × 90 × >700

Portezuelo Este

MAM-03

3,500

170 × 130 × 210

Mina Portezuelo

3,150

3,400

170 × 70 × 300

RAM-05

160 × 70 × >300

3,600

160 × 120 × 250

Center

Mina Victoria

Elevation of highest exposure (m asl)

Dimensions (m) length × width × vertical

Moderate to high (10–30/m2)

Low to moderate (3–10/m2)

Low (30/m2)

Low to moderate (3–10/m2)

Moderate to high (10–20/m2)

Low (100 m continued to exploit massive to semimassive chalcopyrite (>23% Cu) during the Austral summer seasons. At this time, the complex ownership of the mining district (ten different producing companies; R.H. Sales, unpub. report for Andes Exploration Company, 1916) led to operational inefficiencies and boundary disputes. When Los Bronces was eventually 0361-0128/98/000/000-00 $6.00

consolidated in 1916, the frequent legal disputes over mining rights that had plagued it since discovery resulted in the new company being aptly named Compañía Minera Disputada de Las Condes (Disputed mining company of Las Condes). At the beginning of the 20th century, there were a number of mines in production at Los Bronces, among them Disputada, which sent its high-grade ore to the Perez Caldera smelter, but there were only minor mine workings at Río Blanco. In 1920, the engineer Felix Corona established the first mining company (Compañía Minera Aconcagua) at the Río Blanco deposit. In 1922, Corona sold the property to North American Nichols Copper Corporation, which consolidated the district mining property rights in 1965 as Compañía Minera Andina, a wholly owned subsidiary of the Cerro de Pasco Corporation.

107

108

TORO ET AL.

In 1952, the French company Societé Miniere et Metallurgique de Peñarroya acquired Disputada (W. Swayne, unpub. report for Anaconda Copper Mining Company, 1958). During this period, the process of government involvement in the Chilean copper mining industry (“chileanization”) was advancing, with the creation of the Corporación del Cobre in 1966. In 1967, Law 16.624 was issued by the Chilean government and the “chileanization” process began. As a consequence, the Sociedad Minera Andina was created from the assets of the Compañía Minera Andina, in which the Chilean state controlled 30% and Cerro de Pasco the balance (Correa, 1975). The subsequent democratic election of the socialist government of Salvador Allende heralded the nationalization of the entire Chilean copper mining industry. In 1972, during the Allende government, almost 99% of Compañía Minera Disputada de Las Condes (also including the El Soldado mine and the Los Chagres smelter) was purchased by the state mining agency Empresa Nacional de Mineria (ENAMI) for US$5M. Up until the 1973 coup d’état led by General Augusto Pinochet, the nationalized Chilean mines were kept under state control. In 1976, the military government established CODELCO to operate Chile’s large copper mines and the Division Andina was created from Sociedad Minera Andina and became one of the operational divisions of CODELCO. Following six years of operation, Disputada was considered a noncore asset by the Chilean government, and the company was privatized once again via an international tender, eventually being purchased in 1978 by an affiliate of Exxon Mobil Corporation for US$97 million (Warnaars et al., 1985). Based on the positive results of an extensive drilling program, Exxon initiated a major expansion in 1989, and this was commissioned in 1993. In mid-2001, Anglo American conducted due diligence studies of Disputada after Exxon Mobile determined that Disputada was a noncore asset. Anglo American’s geologic review team concluded that not only did the existing Los Bronces orebody have the potential for resources to support substantial future expansions, but also that the La Paloma sector had characteristics consistent with potential for 350 to 500 Mt of additional mineralized material at grades between 0.5 to 1.0% copper. The review team speculated that Los Sulfatos was a contiguous extension of the overall La PalomaLos Sulfatos system, implying significant further upside potential (V. Irarrazaval and J.C. Toro, unpub. report for Empresa Minera de Mantos Blancos S.A., 2001). In November 2002, Exxon Mobil sold Disputada to Anglo American for US$1.300M. Anglo American completed a major expansion of Los Bronces in 2011, investing US$2.200M. Concurrently, the brownfield exploration team discovered two additional major centers of mineralization, La Paloma-Los Sulfatos (now referred to as the Los Sulfatos deposit) and San Enrique-Monolito, which together have added 65 Mt of contained copper to the mineral resource base of the district (Irarrazaval, 2010; Table 1). Geologic Setting The late Miocene to early Pliocene porphyry copper belt that hosts Río Blanco-Los Bronces lies near the southern end of a Neogene metallogenic belt that extends for ~6,000 km along the Andes from west-central Colombia to central Chile 0361-0128/98/000/000-00 $6.00

and west-central Argentina (Fig. 1). From late Eocene to early Miocene (~36−20 Ma), continental tholeiitic to calc-alkaline volcanism (Los Pelambres, Abanico, and Coya Machalí Formations) accumulated in a N-S-elongated, extensional intra-arc basin. Between latitudes 31° and 35° S, the Miocene-Pliocene magmatic evolution of the southern portion of the belt in central Chile and west-central Argentina may be assigned to two main stages (Sillitoe and Perelló, 2005; Perelló et al., 2009, and references therein): (1) early to late Miocene (~18 to −