SOCIETY OF ECONOMIC GEOLOGISTS, INC.

Special Publication, No. 7

Geology and Ore Deposits of the Central Andes

EditorB.J. Skinner

Special Publications of the Society of Economic Geologists

Special Publication, No. 7

Geology and Ore Deposits of the Central AndesB.J. Skinner, Editor

First Edition, 1999

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Preface Nowhere else in the world can the genetic connection between tectonics, magmatism, and widespread

mineralization be made more clearly or more definitively than in the Andes. The central Andes of Peru, Bolivia, and northern Chile, in particular, are the site of a remarkable range of

deposit types. Details of the association between tectonic and magmatic activities arising from eastward sub­duction of the Nazca plate beneath South America and events in the upper crust leading to the formation of ore deposits are not always straightforward. However, close temporal connections between tectonics and min­eralization indicate that a fundamental tectono-magmatic control of mineralization must exist. A full under­standing of those controls is slowly emerging and this forms an underlying theme that serves as the framework for this volume.

This volume is dedicated to Professor Ulrich Petersen of Harvard University, for his contributions to the wealth of information about mineralizing processes that has come from geological studies in the central Andes. A two-day conference was held at the Department of Earth and Planetary Sciences, Harvard University, May 10 and 11, 1996. The occasion marked Ulrich Petersen's retirement from teaching-but not from research. Ulrich was reared in Peru and received much of his education there. He spent time working in the mining industry and eventually became a foremost scientific investigator of mineralization in the Peruvian Andes.

No attempt was made to seek conference speakers who could address all of the topics that bear on issues of mineralization in the central Andes. Rather, leading researchers in some of the currently most important topics were sought. As might be expected, some of the speakers had been students of Professor Petersen, but many were simply professional colleagues. The present volume contains papers given at the conference and papers on additional topics specially prepared to give balance to the volume.

David James and Selwyn Sacks have provided a geophysical perspective of the formation of the central Andes during the Cenozoic era. Their perspective covers the entire lithosphere. By contrast, a very detailed paper by Vic­tor Benavides addresses the organic evolution of the Peruvian Andes throughout the Andean cycle that com­menced in the late Triassic and continues to the present. Dr. Benavides' paper �is ·c0n:Gerned mainly with the crust. Building on the background fabric of Mesozoic and Cenozoic geophysics and 'ocogenesis, Ulrich Petersen's paper considers the magmatic and metallogenic evolution of the central Andes from the onset of subduction beneath South America in the Cambrian up to the present. Suzanne Kay, Constantino Mpodozis, and Beatriz Coira report on their detailed studies of Neogene magmatism, tectonism, and mineralization. Although the geographic span covered by Kay and her coworkers. ranges only from 22° to 33° S, the span contains many of the greatest deposits of northern Chile. One of the most challenging but also most interesting questions concerns the sources of the metals deposited, which is addressed by Andrew Macfarlane in an interesting paper on ore metal resources using isotopic signatures. His results indicate that both crustal and mantle sources are involved.

Several papers deal specifically with the timing and/ or details of mineralization. Don Noble and Edwin McKee have compiled a very detailed account of mineral deposition in the Miocene metallogenic belt of cen­tral and northern Peru. Raymond Jannas, Ulrich Petersen, and Richard Beane report on a long-awaited study of the high-sulfidation mineralization in the El Indio district of Chile; and Richard Vargas, Lew Gustafson, Mon­ica Vucasovic, Enrique Tidy, and Alexandra Skewes discuss the intriguing ore breccias of the Rio Blanco-Los Bronces prophyry deposits of Chile. Andrew Macfarlane, Richard Tosdal, Cesar Vidal, and Jorge Paredes pre­sent their work on the isotopic constraints of the age and origin of gold-quartz veins in the Parcoy district of Peru. In many respects, the most unusual and least well understood deposits of the region are the magnetite deposits of El Laco, in northern Chile. Long considered a type-example of magmatic differentiation involving an immiscible oxide melt, the El Laco deposits have many enigmatic features. Amy Larson Rhodes and Naomi Oreskes provide the first detailed field, petrographic, and rare earth element study of the deposits, together with an intensive study of the oxygen isotope geochemistry. They have demonstrated that the deposits cannot be exclusively magmatic, and that the El Laco event involved a major hydrothermal component-mineraliza-tion may even have been an early hydrothermal process.

·

Much remains to be done in the central Andes and new deposits are being discovered with an encourag­ing frequency, so we can anticipate much interesting work to be reported in the future. The work of the authors of the papers in this volume will, I am sure, play an important role in continuing exploration for new ore deposits and in many further studies of Andean mineralization.

Brian J Skinner, Editor

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SOCIETY OF ECONOMIC GEOLOGISTS, INC.

Special Publication Number 7

TABLE OF CONTENTS

PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

CHAPTER 1: CEN OZOIC FORMATION OF THE CENTRAL ANDES: A GEOPHYSICAL P ERSPECTIVE

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Tectonic Framework for the Central Andes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Review of Geophysical Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Seismic Structure, Gravity Anomalies, and the Geoid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Temporal and Spatial Pattern of Magmatic Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 Four Stages of Andean Subduction . . . . .

. · . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Formation of the Modern Central Andes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

CHAPTER 2: NEOGENE MAGM ATISM, TECTONISM, AND MINERAL DEPOSITS OF THE CENTRALANDES (22° TO 33° S LATITUDE)

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Special Features of the Central Andes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 A Model for the Neogene Magmatic and Tectonic Evolution of the Central Andes . . . . . . . . . . . . . . 30 Changes in the Mantle and Crust Deduced from Mafic and Silicic Lava Compositions . . . . . . . . . . . 41 Neogene Tectonic and Magmatic Controls on Giant Andean Au and Cu Deposits . . . . . . . . . . . . . . 45 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Appendix: Analytical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

CHAPTER 3: OROGENIC EVOLUTION OF THE P ERUVIAN ANDES: THE ANDEAN CYCLE

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 The Andean Cycle . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Epilogue . . . . . . . . . . · . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . . . . 100

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CHAPTER 4: MAGMATIC AND METALLOGENIC Ev OLUTION OF THE CENTRAL ANDES

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 09 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 The Evidence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . · . . . . . . . 1 1 0 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . 14 7

CHAPTER 5: THE MIOCENE METALLOGENIC BELT OF CENTRAL AND NORTHERN PERU

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 156 Geologic Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7 Igneous Rocks .................................................................. 161 Mineral Deposits ................................................................ 166 Exploration Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 Space-Time Distribution of Hydrothermal Activity and Mineralization . . . . . . . . . . . . . . . . . . . . . . 176 Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

CHAPTER 6: ISOTOPIC STUDIES OF NORTHERN ANDEAN CRUSTAL EvOLUTION AND ORE METAL SOURCES

Abstract ........................................................................ 195 Introduction .................................................................... 195 Previous Research ................................................................ 197 Geologic Setting of the Hualgayoc Area .............................................. 198 Analytical Methods ............................................................... 200 Results ........................................................................ 200 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214

CHAPTER 7: HIGH-SULFIDATION DEPOSIT TYPES IN THE EL INDIO DISTRICT, CHILE

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . 219 Introduction .................................................................... 219 Geologic Framework . . . . . . . . . . . . . . . . . . . . . .. . . . · . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 Ore Deposits of the El Indio District . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 El Indio Deposits .................................. ; ............................. 225 Tambo Deposits . .. . . . . . . . . . . . . . . . . . . . . . . . · . . . . . . . . . . . . . . . . . . . . . . . . . · .............. 232 Stable Isotope Compositions of Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 How are the El Indio and Tambo Deposits Related? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 Chemical Equilibria at El Indio and Tambo Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248 Precipitation of Gold and Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 Model for the Origin of the Tambo and El Indio Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 Appendix: Reactions and Equilibrium Constants ....................................... 266

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CHAPTER 8: GEOLOGIC AND ISOTOPIC CONSTRAINTS ON THE AGE AND ORIGIN OF AURIF EROUS QUARTZ VEINS IN THE PARCOY MINING DISTRICT, PATAZ, PERU

.

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 Introduction . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 Geologic Framework . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268 Overview of Mineralization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

CHAPTER 9: ORE BRECCIAS IN THE RIO BLANCO -LOS BRONCES P ORPHYRY COPPER DEPOSIT, CHILE

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 Textural Variations . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 Temporal Relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 Zonal Variations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289 P

.orphyry Dike Associations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291

Geochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 Fluid Inclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . · . . . . 292 Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 Implications for Exploration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296

CHAPTER 10: GEOLOGY AND RARE EARTH ELEMENT GEOCHEMISTRY OF MAGNETITE DEPOSITS AT ELLACO , CHILE

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300 Geologic Description of El Laco . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 Interpretation of Textural Features in Magnetite ....................................... 314 Rare Earth Element Analyses .................... : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323 Summary ...................................................................... 329

CHAPTER 11: OXYGEN ISOTOPE COMPOSITION OF MAGNETITE DEPOSITS AT EL LACO , CHILE: EviDENCE OF FORMATION FROM ISOTOPICALLY HEAVY FLUIDS

Abstract ........................................................................ 333 Introduction .................................................................... 333 Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . 334 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 Discussion of Isotopic Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342 Summary . . . . . . . . . . . . . . . . . . _. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348

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