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The Marcona - Mina Justa District, South-Central

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The Marcona - Mina Justa District, South-Central THE MARCONA - MINA JUSTA DISTRICT, SOUTH-CENTRAL PERÚ: IMPLICATIONS FOR THE GENESIS AND DEFINITION OF THE IRON OXIDE-COPPER (-GOLD) ORE DEPOSIT CLAN by Huayong Chen A thesis submitted to the Department of Geological Sciences and Geological Engineering In conformity with the requirements for the degree of Doctor of Philosophy Queen’s University Kingston, Ontario, Canada May, 2008 Copyright © Huayong Chen, 2008 “I am looking for IOCG deposits” Frontispiece: The mysterious Nazca Lines (a hummingbird) in the Cañete basin, 70 km north of Marcona ii ABSTRACT The Marcona district of littoral south-central Perú represents the largest concentration of iron oxide-copper-gold deposits in the Central Andes. Hydrothermal activity occurred episodically from 177 to 95 Ma and was controlled by NE-striking faults. At Marcona, emplacement of massive magnetite orebodies with subordinate, overprinted magnetite-sulphide assemblages coincided with a 156-162 Ma episode of eruption of andesitic magma in the Jurassic arc, but mineralization is hosted largely by underlying, Lower Paleozoic metaclastic rocks. The magnetite orebodies exhibit smoothly curving, abrupt contacts, dike-like to tubular apophyses and intricate, amoeboid interfingering with dacite porphyry intrusions, interpreted as evidence for the commingling of hydrous Fe oxidic and silicic melts. An evolution from magnetite - biotite - calcic amphibole ± phlogopite assemblages, which are inferred to have crystallized from an Fe-oxide melt, to magnetite - phlogopite - calcic amphibole - sulphide assemblages coincided with quenching from above 700°C to below 450°C and with the exsolution of aqueous fluids with magmatic stable isotopic compositions. Subsequent, subeconomic chalcopyrite - pyrite - calcite ± pyrrhotite ± sphalerite assemblages were deposited from cooler fluids with similar δ34S, δ18O and δ13C values, but higher δD, which may record the involvement of both seawater and meteoric water. The much younger (95-110 Ma), entirely hydrothermal, Mina Justa Cu (-Ag) deposit is hosted by Middle Jurassic andesites intruded, on a district scale, by small dioritic stocks at the faulted SW margin of an Aptian-Albian shallow-marine volcano-sedimentary basin. Intense albite-actinolite alteration (ca. 157 Ma) and K-Fe metasomatism (ca. 142 Ma) long preceded the deposition of magnetite-pyrite assemblages from 500-600°C fluids with a magmatic isotopic signature. In contrast, ensuing chalcopyrite - bornite - digenite - chalcocite - hematite - calcite mineralization was entirely the product of non - magmatic, probably evaporite-sourced, brines. iii Marcona and Mina Justa therefore represent contrasted ore deposit types and may bear minimal genetic relationships. The former shares similarities with other Kiruna-type magnetite (-apatite) deposits. In contrast, the latter is a hydrothermal system recording the incursion of fluids plausibly expelled from the adjacent Cañete basin. Non-magmatic fluids are inferred to be a prerequisite for economic Cu mineralization in the Cu-rich IOCG deposits in the Central Andes and elsewhere. iv ACKNOWLEDGEMENTS This thesis research would not have been completed without the support of many people. First I present my esteem and appreciation to my co-supervisor, Dr. Alan Clark, who originally gave me the chance to come to Canada to continue my graduate study and his fatherly caring for my studies during these years in Kingston, and more important, for his extremely strict but definitely reasonable guidance in my Ph.D. project. Finally, although not completely, he opens my “hard granite brain” using his sharp axe, to make me more like an economic geologist, who should know everything. I also greatly thank my co-supervisor, Dr. Kurt Kyser, for his great patience in allowing an “over proud” but mindless Ph.D. student to grow up slowly. I will remember his endless understanding of my difficult situation and more important, his guidance in my research, concise but extremely important. I also thank my supervisors for their funding for both my research and living, dominantly from their grants funded by Natural Sciences and Engineering Research Council of Canada (NSERC), and partly from the scholarships offered by Queen’s University. Shougang Hierro Perú SA, Chariot Resources and Rio Tinto are thanked for their cooperation in my field work, particularly for comfortable lodging in San Juan. Mr. Yuming Chen, former chief geologist of Marcona Mine, is especially thanked for his great help at the very beginning of this project. I thank Mr. Nicholas Hawkes and Timothy Moody for their understanding and permission to sample Mina Justa drill cores, and also the protection afforded by their local mine team, who ensured my safe return to Canada, without falling into ancient mining workings. At Queen’s, I benefited greatly from the advice and help of Kerry Klassen for isotope analysis, April Vuletich for Laser-TOF-ICP-MS analysis, and Alan Grant for X-ray studies. Dr. Gema Olivo is thanked for her help and permission to use the fluid inclusion equipment and digital-camera microscope at Queen’s. Roger Innes and Jerzy Advent prepared thin and polished v sections. I also thank Thomas Ullrich and Peter Johns for their help and advice in Ar-Ar and microprobe analysis at UBC and Carleton University, respectively. I especially give my thanks to Joan Charbonneau, who was the bridge between Alan and me during my thesis correction, and other Geology staff members, Dianne Hyde, Linda Brown and Ellen Mulder for their kind help. Thanks to many grad students and postdocs for their useful discussions and help. Special thanks are due to Al Montgomery, Greg Lester, Jorge Benavides, Chan Quang, Amelia Rainbow, Mike Cooley, Dave Love, Farhad Bouzari, Rui Zhang, Jingyang Zhao and Luis Cerpa. Last, but not least, I would like thank my dear wife here in Kingston for her support, encouragement and sacrifices, especially when she has to work hard on her own Ph.D. project. Also I thank all my Chinese friends in Kingston, especially those from the Kingston Chinese Alliance Church: my life will be difficult without their help. vi STATEMENT OF ORIGINALITY I hereby certify that all of the work described within this thesis is the original work of the author. Any published (or unpublished) ideas and/or techniques from the work of others are fully acknowledged in accordance with the standard referencing practices. Huayong Chen May, 2008 vii TABLE OF CONTENTS Abstract……………………………………………………………………………………………iii Acknowledgments………………………………………………………………………………...v Statement of Originality…………………………………………………………………………vii Table of Contents………………………………………………………………………………...viii List of Figures…………………………………………………………………………….....…….x List of Tables…………………………………………………………………………………….xiv Chapter 1. Introduction The “Iron Oxide-Copper-Gold” Ore Deposit Clan: Problematic Definition and Genesis………………………………………………………………………………………1 The Scientific Contributions of This Study…………………………………...............……18 Thesis Organization………………………………………………………………………...21 Chapter 2. The Longlived, Marcona-Mina Justa Iron-Copper District, Perú: Implications for the Origin of Cu-poor and Cu-rich IOCG Mineralization in the Central Andes 2.1 Abstract…………………………………………………………………………………22 2.2 Introduction……………………………………………………………………………..24 2.3 Regional and District Geologic Setting…………………………………………………31 2.4 The Marcona Magnetite Deposit………………………………………………………..40 2.5 Paragenetic Relationships of the Marcona Orebodies…………………………….…….53 2.6 The Mina Justa Cu (-Ag-Au) Deposit……………………………………..............……69 2.7 Stable Isotope Geothermometry………………………………………………………...84 2.8 40Ar/39Ar Geochronology………………………………………………………….……88 2.9 Discussion 2.9.1 An Oxide Melt Origin for the Main Marcona Magnetite Orebodies?............……97 2.9.2 Evolution of the Marcona-Mina Justa District…………………………………..103 2.10 Conclusions…………………………………………………………………………..115 Chapter 3. Contrasted Fluids and Reservoirs in the Contiguous Marcona and Mina Justa Iron-Oxide Cu (-Au-Ag) Deposits, South-Central Perú 3.1 Abstract………………………………………………………………………………..119 3.2 Introduction……………………………………………………………………………120 viii 3.3 Ore Deposit Geology………………………………………………………..........……123 3.4 Alteration and Mineralization 3.4.1 Marcona Magnetite Deposit……………………………………………………..126 3.4.2 Mina Justa Cu Deposit…………………………………………………………..129 3.5 Sampling and Analytic Techniques……………………………………………………130 3.6 Results 3.6.1 Fluid Inclusions…………………………………………………………….……133 3.6.2 Stable Isotope Geochemistry…………………………………………………….150 3.7 Discussion 3.7.1 Fluid Evolution in the Marcona Deposit………………………………………...160 3.7.2 Fluid Evolution in the Mina Justa Deposit………………………………………165 3.7.3 Implications for Cu-mineralizing Fluids in IOCG Deposits……………….……169 3.8 Conclusions……………………………………………………………………………173 Chapter 4. Conclusions 4.1 Marcona – A Unique “Kiruna-type”, Magnetite Deposit in the Middle Jurassic Metallogenetic Sub-province of the Central Andes…………………………………...174 4.2 Mina Justa Cu (-Ag) Deposit – A Major Cu-rich IOCG Deposit in the Cretaceous Central Andes…………………………………………………………………….……………177 4.3 The Protracted History of Alteration and Mineralization in the Marcona-Mina Justa District and Other IOCG Centres……………………………………………………...183 4.4 Implications for the Genesis of IOCG Deposits: A Redefinition and Reclassification of the IOCG Clan…………………………………………………………………………185 References………………………………………………………………………………………194 Appendix A: Analytical Techniques………………………………………………….....……230 Appendix B: Summerized 40Ar/39Ar Analytical Data for Hydrothermal Minerals from Marcona
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