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AN ABSTRACT of the DISSERTATION of Robert G. Lee

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AN ABSTRACT of the DISSERTATION of Robert G. Lee AN ABSTRACT OF THE DISSERTATION OF Robert G. Lee for the degree of Doctor of Philosophy in Geology presented on September 29, 2008 . Title: GENESIS OF THE EL SALVADOR PORPHYRY COPPER DEPOSIT, CHILE AND DISTRIBUTION OF EPITHERMAL ALTERATION AT LASSEN PEAK, CALIFORNIA. Abstract approved: ______________________________________________________________________ John H. Dilles The El Salvador porphyry copper deposit in the Indio Muerto district of northern Chile has been geologically investigated for more than 60 years and provides one of the best bases for understanding similar environments of ore formation elsewhere in the world. Fourteen new zircon U/Pb isotopic ages obtained via in situ SHRIMP-RG analysis are here coupled with previous geological studies to allow refinement of the timing of Eocene porphyry magma emplacement responsible for copper and molybdenum mineralization that occurs in several ore bodies within the district. The earliest intrusions are rhyolites that crop out throughout the district, but are more abundant in the north. In contrast, the later granodiorite porphyries were emplaced only in the central and southern parts of the district. Two age periods of mineralization have been documented using zircon U/Pb geochronology. The low grade and small copper deposit at Old Camp in the northern district is associated with a quartz porphyry intrusion that yielded an age of 43.6 ± 0.5 Ma, whereas the main copper molybdenum deposit at Turquoise Gulch is associated with emplacement of the granodioritic L porphyry plug that yielded an age of 42.0 ± 0.5 Ma. The final intrusion is a series of latite porphyry dikes, which post-date ores and yielded a U/Pb zircon age of 41.6 ± 0.5 Ma. Inherited Eocene zircons with ages from ~45 Ma to ~47 Ma are found within younger porphyry intrusions and likely formed via magmatic recycling of older intrusions. Therefore, the zircon U/Pb ages suggest magmatism spanned approximately 5 million years from 47 to 42 Ma, with hydrothermal copper-molybdenum ores dominantly forming during the final stages of porphyry emplacement. Geochemical analyses by XRF, ICP-MS, electron microprobe and laser-ablation ICP-MS define a wide range of major, minor and trace element contents for the Eocene porphyry intrusions within the district. The early rhyolite and quartz porphyry intrusions have rare earth contents with strong negative europium anomalies and relatively low Sr/Y and Sm/Yb ratios consistent formation via fractional crystallization of plagioclase-rich mineral assemblages from more mafic parental melts. The granodiorite porphyries have no europium anomalies and a wide range of Sr/Y and Sm/Yb ratios that support an origin via fractional crystallization of garnet, hornblende ± titanite, and minor plagioclase from an andesitic parental melt. The granodiorite intrusions at M Gulch – Copper Hill are ~1 m.y. older and have less evolved trace element ratios than the younger granodiorite intrusions associated with the main mineralization event. The evolving Eocene intrusions are the result of lower to mid crust melts ascending to mix with silica-rich differentiated melts derived from fractional crystallization of older andesitic magmas. Progressive decrease of Eu/Eu* ratios in the zircons with decreasing age gives direct evidence in support of the hypothesis that the main ore mineralization is directly related to the evolution of the upper crustal magma reservoir to progressively more oxidized conditions. A second goal of this study was to document the mineralogy and zonation of altered wall rock at Lassen National Volcanic Park in northern California, in order to understand the pressure, temperature, fluid composition, and epithermal processes along the southern flank of Lassen Peak. Extensive epithermal wall rock alteration occurs along the southern flank of the Cascadia volcano and includes both active and fossil geothermal systems. Geologic mapping coupled with mineral identification using a portable infrared spectrometer and X-ray diffraction outline several hydrothermal systems within the park. Currently active, steam-heated acid sulfate alteration is characterized by kaolinite, alunite, opal, and cristobalite with accessory iron sulfates. The active hydrothermal zones are proximal to thermal pools and fumaroles at Sulphur Works, Pilot Pinnacle, Little Hot Springs Valley, and Bumpass Hell. Three fossil systems occur within andesite lavas and flow breccias of the eroded Pleistocene Brokeoff Volcano. Intermediate argillic alteration occurs at higher elevations on the flanks of the eroded volcano and is characterized by mixed layer illite-smectite, quartz, pyrite, and albite. Propylitic alteration occurs within the eroded lower elevations of Little Hot Springs Valley and is characterized by chlorite, calcite, quartz, pyrite, illite, and rare epidote. Also present at a lesser extent is an advance argillic alteration defined by pyrophyllite, dickite, alunite, kaolinite, and quartz formed at Pilot Pinnacle. ©Copyright by Robert G. Lee September 29, 2008 All Rights Reserved Genesis of the El Salvador Porphyry Copper Deposit, Chile and Distribution of Epithermal Alteration at Lassen Peak, California by Robert G. Lee A DISSERTATION Submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Presented September 29, 2008 Commencement June 2009 Doctor of Philosophy dissertation of Robert G. Lee presented September 29, 2008 . APPROVED: Major Professor, representing Geology Chair of the Department of Geosciences Dean of the Graduate School I understand that my dissertation will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my dissertation to any reader upon request. Robert G. Lee, Author ACKNOWLDEGEMENTS I would like to thank my advisor John H. Dilles for his guidance throughout my tenure at Oregon State University. My committee members Adam J.R. Kent, Frank J. Tepley III, Anita L. Grunder, and my graduate representative Mary Flahive have been valuable assets during the progression of my research. I am indebted to my co-authors Richard M. Tosdal, Joe L. Wooden, Frank K. Mazdab, David A. John, and Tanya L. Abela. Exploraciones Mineras of the Corporacion Nacional del Cobre de Chile provided funding for the zircon geochronology. Analytical and miscellaneous costs were covered by a 2007 Society of Economic Geology (SEG) student research grant and a generous donation by Bill Williams of Freeport McMoRan Copper & Gold, Inc. I would also like to thank Lew Gustafson and Enrique Tidy for providing samples for this study and initiating this project. Walter Orquera of the Exploraciones Mineras at the El Salvador mine provided invaluable assistance in the collection and processing of samples while I was working in Chile along with Ricard Santelices, Christian Rojas, and Eduardo Gonzalez. The Mineral Resource External Research Program (MRERP) of the US Geological Survey provided funding for the Lassen Peak project. Patrick Muffler, George Breit, and Jim Crowley of the USGS all provided valuable assistance during my fieldwork at Lassen Volcanic National Park. My fellow colleagues at Oregon State University have been a great help during the course of my studies and include Michael Rowe, Allison Weinsteiger, Mark T. Ford, Barry A. Walker, Denise Giles, Isabelle Chamberfort, Tony Longo, and Abigail Stephens. Finally I would like to thank my wonderful wife Katie and my two children Robert and Audrey for sticking with me through this long process and their loving support. CONTRIBTUTION OF AUTHORS Dr. John Dilles assisted with the study design, interpretations of fieldwork, analytical data and provided oversight for all manuscripts presented herein. Dr. Richard M. Tosdal contributed samples and assisted with analytical collection, along with the writing of Chapter Two. Dr. Joe L. Wooden and Dr. Frank K. Mazdab of the U.S. Geological Survey provided analytical training and assisted with the analytical work at the Stanford USGS Micro Analysis Center for zircon trace element analysis. Dr. Frank J. Tepley III provided training and assistance with electron microprobe analysis. Dr. Adam J.R. Kent assisted with collection of trace element data by laser-ablation ICP-MS of mineral separates. Dr. David A. John (USGS-Department of minerals) and Tanya L. Abela provided assistance with fieldwork at Lassen Park, CA and with the writing of Chapter Four. TABLE OF CONTENTS Page CHAPTER ONE General introduction to porphyry and epithermal deposits …………. 1 Scope of this study …………………………………………………… 5 The El Salvador porphyry copper deposit, Chile …………………… 6 Zircon geochronology and trace element composition …………….. 11 Previous geochronlogic work at El Salvador ………………………. 12 Geochemical analysis at El Salvador ………………………………. 13 Lassen Volcanic National Park, California ……………….………… 14 CHAPTER TWO TRACE ELEMENTS AND U/PB AGES OF ZIRCON FROM GRANODIORITE PORPHYRY: TEMPORAL, THERMAL, AND GEOCHEMICAL EVOLUTION OF PORPHYRY COPPER MAGMAS AT EL SALVADOR, CHILE ……………………………………………… 16 Abstract …………………………………………………………….. 17 Introduction ………………………………………………………… 18 Tectonic setting of the Andean precordillera, northern Chile ……… 22 Porphyry intrusions of the Indio Muerto district …………………… 24 Methods ……………………………………………………………. 30 El Salvador porphyry samples ……………………………... 30 Zircon separation procedure ……………………………….. 32 SHRIMP-RG analyses ……………………………………... 34 TABLE OF CONTENTS (Continued) Page Results ……………………………………………………………… 41 Zircon U/Pb
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