Special Publication, No. 7
Geology and Ore Deposits
of the Central Andes
Editor
B.J. Skinner
SOCIETY OF ECONOMIC GEOLOGISTS, INC.
Special Publications of the Society of Economic Geologists
Special Publication, No. 7
Geology and Ore Deposits of the Central Andes
B.J. Skinner, Editor
First Edition, 1999
Graphic design and layout by
Johnson Printing
Boulder, CO
Printed by
Sheridan Books, Inc.
613 E. Industrial Drive
Chelsea, MI 48118
Additional copies of this publication can be obtained from
Society of Economic Geologists, Inc.
7811 Shaffer Parkway
Littleton, CO 80127
www.segweb.org
ISBN: 978-1-629496-25-2
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 ty pes. Details of the association between tectonic and magmatic activities arising from eastward sub­
duction 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 min­
eralization indicate that a fundamental tectono-magmatic control of mineralization must exist. A full under­
standing 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 Vic­
tor Benavides addresses the organic evolution of the Peruvian Andes throughout the Andean cycle that com­
menced 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 cen­
tral 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, Mon­
ica 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 pre­
sent 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-mineralization 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 encourag­
ing 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
iii
SOCIETY OF ECONOMIC GEOLOGISTS, INC.
Special Publication Number 7
TABLE OF CONTENTS
PREFACE
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CHAPTER 1: CEN OZOIC FOR MATION OF THE CENTRALANDES:
A GEOPHYSICAL P ERSPECTIVE
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Review of Geophysical Obser vations . . . . . . . . . . . . . . . .
Seismic Structure, Gravity Anomalies, and the Geoid . . .
Temporal and Spatial Pattern o f Magmatic Activity
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Introduction
Tectonic Framework for the Central Andes
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Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Formation of the Modern Central Andes
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NEOGENE MAGM ATISM, TECTONISM, AND MINERAL DEPOSITS
OF THE CENTRALANDES (22° TO 33° S LATITUDE)
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Special Features of the Central Andes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A Model for the Neogene Magmatic and Tectonic Evolution of the Central Andes . .
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Changes in the Mantle and Crust Deduced from Mafic and Silicic Lava Compositions
Neogene Tectonic and Magmatic Controls o n Giant Andean A u and C u Deposits
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Appendix: Analytical Methods
CHAPTER
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. 61
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1 00
THEANDEAN CYCLE
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Introduction . . . . . . . . . .
The Andean Cycle . . . . .
Epilogue
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OROGENIC EVOLUTION OF THE P ERUVIAN ANDES:
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CHAPTER 4: MAGM ATIC AND METALLOGENIC Ev OLUTION OF THE
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The Evidence
Discussion
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CHAPTER 5: THE MIOCENE METALLOGENIC BELT OF CENTRAL AND
NORTHERN PERU
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Geologic Setting ................................................................
Igneous Rocks ..................................................................
Mineral Deposits ................................................................
Exploration Considerations .......................................................
Space-Time Distribution of Hydrothermal Activity and Mineralization ......................
Future Directions ...............................................................
Introduction
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CHAPTER 6: ISOTOPIC STUDIES OF NORTHERNANDEAN CRUSTA L
EvOLUTION AND ORE META L SOURCES
Abstract ........................................................................
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Previous Research ................................................................ 197
Geologic Setting of the Hualgayoc Area .............................................. 198
Analytical Methods ............................................................... 200
Results ........................................................................ 200
Discussion
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Conclusions
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Introduction
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HIGH-SULFIDATION DEPOSIT TYPES IN THE EL INDI O
DISTRICT, CHILE
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Geologic Framework
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Ore Deposits o f the E l Indio District
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El Indio Deposits .................................. ; ............................. 225
Tambo Deposits ..
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Stable Isotope Compositions of Minerals ............................................ 242
How are the El Indio and Tambo Deposits Related?
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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 IS OTOPIC CONSTRAINTS ON THEAGE AND
QUARTZ VEINS IN. THE PARCOY
ORIGIN OFAURIF EROU S
MINING DISTRICT, PATAZ, PERU
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Geologic Framework
Overview of Mineralization
Results and Discussion
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CHAPTER 9: ORE BRECCIAS IN THE RIO BLANCO -LOS BRONCES
P ORPHYRY COPPER DEPOSIT, CHILE
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Introduction
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Implications for Exploration
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CHAPTER 10: GEOLOGY AND RARE EARTH ELEMENT GEOCHEMISTRY OF
MAGNETITE DEPOSITS AT ELLACO , CHILE
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Geologic Description of El Laco .................................................... 301
Interpretation of Textural Features in Magnetite ....................................... 314
Rare Earth Element Analyses .................... ................................. 323
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Summary ...................................................................... 329
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Introduction
CHAPTER 11: OXYGEN ISOTOPE COMPOSITION OF MAGNETITE DEPOSITS
AT ELLACO , CHILE: EviDENCE OF FOR MATION FROM
ISOTOPICALLY HEAVY FLUIDS
Abstract ........................................................................ 333
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Geology ....................................................................... 334
Methods ...................................................................... 335
Results ....................................................................... 337
Discussion of Isotopic Data ........................................................ 342
Summary
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Introduction
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