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Genesis of the El Salvador porphyry copper deposit, Chile and distribution of epithermal alteration at Lassen Peak, California

Dissertation
Author: Robert G. Lee
Abstract:
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, albite 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.

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 data …………………………………………… 41

Zircon U/Pb age calculations….……………………………. 42

Trace element geochemistry of zircons ……………………. 49

Thermal history of granodiorite porphyries and latite dikes ………. 57

Zircon saturation …………………………………………… 57

Titanium in zircon …………………………………………. 60

Discussion …………………………………………………………. 65

Geologic evolution of the El Salvador magmatic system … 65

Cerro Pelado – Old Camp …………………………………. 68

M Gulch – Copper Hill ……………………………………. 69

Turquoise Gulch porphyry Cu mineralization …………….. 72

Magma recycling and porphyry formation ………………………... 72

Model for the Cu-Mo ore formation of the El Salvador deposit .….. 75

Conclusions ………………………………………………………... 79

Acknowledgements ………………………………………………... 81

References ………………………………………………………….. 81

CHAPTER THREE

THE GEOCHEMISTRY OF PORPHYRY INTRUSIONS FROM THE INDIO MUERTO DISTRICT, EL SALVADOR, CHILE: INSIGHTS INTO MAGMATIC PROCESSES THAT PRODUCE PORPHYRY COPPER DEPOSITS………………………………………..……………… 87

Abstract …………………………………………………………….. 88

TABLE OF CONTENTS (Continued) Page

Introduction ………………………………………………………… 90

Geologic setting ……………………………………………………. 93

Porphyry intrusions ………………………………………………… 96

Quartz rhyolite porphyry …………………………………… 96

Quartz porphyry and late quartz porphyry …………………. 96

X porphyry …………………………………………………. 97

K porphyry …………………………………………………. 98

L porphyry and associated A & R porphyries ……………... 98

Latite porphyry dike ………………………………………... 100

Methods ………………………………………………………….…. 100

Sample preparation …………………………………..……... 100

Whole rock chemical analysis ……………………..……...… 101

Electron microprobe ………………………………………... 102

Quadrupole LA-ICP-MS ………………..………………..… 103

Whole rock geochemistry ……………………..…………………….. 104

Major elements …………………….………………………… 104

Trace elements ………………………………………………. 105

Mineral composition ………………………………………………… 112

Amphibole …………………………………………………… 112

Apatite ……………………………………………………….. 121

Biotite ………………………………………………………... 125

TABLE OF CONTENTS (Continued) Page

Plagioclase …………………………..……………………… 132

Titanite …………………………..………………………….. 138

Discussion ………………………………..……………………….… 142

Amphibole geothermobarometry ...…………………………. 142

Trace element mass balance ..………………………………. 150

Geochemical modeling ……………………………………... 152

Plagioclase-melt equilibrium ……………………………….. 159

Formation of porphyries and copper mineralization ……………….. 163

Acknowledgements …………………………………………..…….. 169

References ……………………………………………………..….... 170

CHAPTER FOUR

THE HYDROTHERMAL ALTERATION ASSEMBLAGES AROUND ACTIVE GEOTHERMAL SYSTEMS IN LASSEN VOLCANIC NATIONAL PARK, NORTHERN CALIFORNIA ……………………….. 178

Abstract …………………………………………………………….. 179

Introduction …………………………………………………………. 181

Geological and volcanological setting ……………………………… 184

Geology of Brokeoff Volcano ………………………………. 186

Methods and analytical techniques ……………………………….... 189

Hydrothermal alteration mineralogy ……………………………….. 194

Bumpass Hell …………………………………………….… 195

Little Hot Springs Valley …………………………………... 201

TABLE OF CONTENTS (Continued) Page

Pilot Pinnacle ……………………………………………..… 205

Sulphur Works …………………………………………..….. 206

Structural Features ………………. ………………………………… 207

Landslides ……………………………………………………….….. 207

Geochemistry ……………………………………………………..… 208

Hydrogen isotopes ……………………………………….…. 210

Discussion …………………………………………………….….…. 210

Conclusions ……………………………………………………….… 216

Acknowledgements ……………………………………………….… 218

References ……………………………………………………….….. 218

CHAPTER FIVE

Conclusions ……………………………………………………….… 221

Bibliography ………………………………………………………………... 226

Appendices ………………………………………………………………..… 241

Appendix A: El Salvador samples ..……………………………….… 241

Appendix B: SHRIMP-RG analytical procedures and results …….… 252

Appendix C: Electron microprobe analytical procedures and results .. 269

Appendix D: LA-ICP-MS analytical procedures and results ………... 285

Appendix E: Lassen Peak ……………………………………………. 308

LIST OF FIGURES Figure Page

1.1 General distribution of porphyry Cu and epithermal Au-Ag deposits that occur throughout North and South America …………………… 3

1.2 Regional geologic map of the Indio Muerto district ………………... 8

1.3 Geologic map of the Indio Muerto district and El Salvador porphyry Cu(Mo) deposit, northern Chile ……………………………………. 10

1.4 Location map outlining major volcanic centers in the Cascade Mountains …………………………………………………………… 15

2.1 Tectonic map of northern Chile outlining major fault zones, Cordilleras and porphyry copper deposits ………………………….. 23

2.2 Geologic map of the 2600 m level of the Indio Muerto district, northern Chile ……………………………………………………..... 25

2.3 Photomicrographs of main porphyry types analyzed ………………. 27

2.4 Cathodoluminescence images of selected zircon grains ………….... 33

2.5 Terra-Wasserburg Concordia diagrams showing U/Pb geochronologic data with interpreted weighted mean age for selected samples …….. 47

2.6 Rare earth element (REE) plots for selected samples analyzed by SHRIMP-RG ………………………………………………………... 53

2.7 Trace element plots from individual zircon grains …………………. 57

2.8 Zircon Hf variation diagrams ………………………………………. 59

2.9 Whole rock SiO 2 wt. % variation diagram ………………………….. 62

2.10 Cathodoluminescence images of El Salvador zircon grains with corrected temperatures based on Ti content (± 2ºC) ……………….. 64

2.11 Trace element content vs. Ti temperature of zircon illustrating core, rim, and sector zones from the K porphyry, A porphyry, and latite dike 66

2.12 Probability density plot of all U/Pb zircon spot ages analyzed from the fourteen samples but excluding spots with inherited (Mesozoic), discordance, and probable Pb loss …………………………………. 67

LIST OF FIGURES (Continued) Figure Page

2.13 Summary of El Salvador chronology comparing relative ages vs. robust U/Pb zircon ages from this study …………………………… 71

2.14 Enlarged Th/U vs. Yb/Gd ratio plot for El Salvador zircons ………. 76

2.15 Enlarged Hf ppm vs. Eu/Eu* plot for Eocene-age zircons …………. 77

2.16 Conceptual north-south cross-sectional model for the formation of the El Salvador porphyry-Cu district ……………………………….. 80

3.1 Geologic map and ore distribution of the El Salvador porphyry copper deposit, northern Chile ……………………………………… 94

3.2 Major element oxide versus SiO 2 concentrations normalized to volatile free for El Salvador porphyry intrusions ………………….... 109

3.3 Trace element variation diagrams for El Salvador porphyry intrusions …………………………………………………………..… 110

3.4 Amphibole Al 2 O 3 vs. TiO 2 (in wt. %) diagram from L porphyry and latite dike samples from Turquoise Gulch ….………………..... 115

3.5 Classification diagrams for calcic-amphiboles after Leake et al. (1997) 116

3.6 Photomicrographs of L porphyry amphiboles from Turquoise Gulch 117

3.7 Backscattered electron image (BSE) of L porphyry amphibole analyzed by electron microprobe ……………………………………. 118

3.8 Photomicrographs of amphiboles and sieved plagioclases from the El Salvador latite dike samples …………………………………….. 119

3.9 REE diagram for amphiboles from latite dike sample ES-12792 normalized to chondrite ……………………………………………. 120

3.10 Photomicrographs of apatite phenocrysts from El Salvador latite porphyry dike ………………………………………………………. 122

3.11 Reflected, BSE, and X-ray images of latite dike apatite grain ES-12792ap-7 ……………………………………………………… 123

3.12 Chondrite normalized REE diagram for apatite from the El Salvador district …………………………………………………………….… 126

LIST OF FIGURES (Continued) Figure Page

3.13 Plane-polarized photomicrograph of biotite from the K porphyry of Turquoise Gulch ……………………………………………………. 127

3.14 Cross-polarized photomicrograph of L porphyry from Turquoise Gulch ……………………………………………………………….. 128

3.15 Variation diagrams for biotites from selected El Salvador porphyries. 130

3.16 Photomicrograph of plagioclase phenocryst from sample ES-12800 L porphyry from M Gulch ………………………………………….. 135

3.17 Photomicrograph of sieved plagioclase from sample ES-12792 latite dike from Turquoise Gulch …………………………………………. 136

3.18 Ba (ppm) vs. Sr (ppm) plot of plagioclase grains from the El Salvador district ……………………………………………………………….. 137

3.19 Reflected light images of titanites from sample ES-12792 latite dike from Turquoise Gulch ……………………………………………….. 139

3.20 Variations of cations plotted as a function of molar Ti content for titanites from the K porphyry (ES-12785a), L porphyry (ES-12787), and latite dike (ES-12792)……………………………………….…. 141

3.21 Titanite REE diagrams ……………………………………………... 143

3.22 Temperature vs. pressure diagram outlining amphibole crystallization fields………………………………………………………………….. 147

3.23 Mass balance REE diagram for whole rock and mineral phenocrysts from the Turquoise Gulch porphyries ……………………………….. 151

3.24 Y (ppm) vs. Sr/Y for whole rock and mineral phases from the El Salvador porphyry suite …………………………………………. 155

3.25 Sm/Yb vs. La/Sm plots for El Salvador whole rock and mineral Analyses ……………………………………………………………. 158

3.26 Calculated melt equilibrium concentrations in equilibrium with measured plagioclase Ba and Sr compositions.……………………... 162

3.27 Geologic evolution of the El Salvador porphyry copper deposit …… 168

LIST OF FIGURES (Continued)

Figure Page

4.1 Geologic and tectonic setting of Lassen volcanic region …………... 183

4.2 Geologic map of the Brokeoff Volcano region showing rock sequences, structure, active fumaroles and intrusive dikes and plugs 188

4.3 Infrared spectra analysis of dickite and kaolinite …………………… 191

4.4 Representative infrared spectra “stack plots” from Bumpass Hell, Pilot Pinnacle, and samples from northern and southern Little Hot Springs Valley………………………………………………………. 192

4.5 Sample maps denoting major mineral locations defined by PIMA for the Brokeoff volcano area …………………………………………... 196

4.6 Representative XRD spectra for selected Brokeoff Volcano samples 197

4.7 Scanning electron microscope images of hydrothermal alteration textures and minerals at Brokeoff Volcano ………………………… 198

4.8 Photographs outlining alteration and hydrothermal features within Lassen Volcanic National Park……………………………………… 203

4.9 Geochemical plots of major and trace elements vs. titanium (Wt. %). 209

4.10 Distribution of hydrogen isotopic values and oxygen isotope contours along the south flank of Lassen Peak………………………………. 212

4.11 Simplified sketch from southwest to the northeast outlining the geothermal systems on the south flank of Lassen Peak……………. 215

4.12 Map of the hydrothermal alteration assemblages in the Brokeoff Volcano region …………………………………………………...… 217

LIST OF TABLES Table Page

2.1 U/Pb geochronologic data for zircos from the El Salvador Porphyries …………………………………………………………… 36

2.2 Summary of interpreted zircon 206 Pb/ 238 U ages for El Salvador porphyry samples …………………………………………………... 43

2.3 Composition of zircon grains from the El Salvador district ……..… 50

2.4 Zircon saturation temperatures defined from whole rock major element concentrations …………………………………………….. 61

2.5 Trace element composition for Latitie, K porphyry, and A porphyry zircons analyzed by SHRIMP-RG for entire trace element suite ….. 63

3.1 Whole-rock geochemical results for selected El Salvador intrusions . 106

3.2 Composition of selected amphiboles by electron microprobe analysis 114

3.3 Composition of selected apatites by electron microprobe analysis …. 124

3.4 Average composition of biotites from electron microprobe analysis 129

3.5 Composition of selected plagioclase by electron microprobe analysis 134

3.6 Composition of selected titanites by electron microprobe analysis … 140

3.7 Calculated temperatures and pressures for amphiboles from ES-12792 Latite dike ……………………………………………….. 148

3.8 Calculated melt compositions derived from inherited Mesozoic age zircons for La, Sm, and Yb …………………………………………. 160

4.1 Hydrothermal alteration assemblages at Brokeoff Volcano, California 200

4.2 Hydrogen isotopic composition of whole rock and <15 mm size fractions from Brokeoff Volcano, California ………………….…… 211

LIST OF APPENDIX TABLES Table Page

A1 El Salvador whole rock X-ray fluorescence analyses ……………… 246 A2 El Salvador whole rock ICP-MS analyses ………………………..... 250 B1 SHRIMP-RG analytical data from El Salvador zircon separates …... 255 C1 Quantification settings for mineral analyses by electron microprobe 270 C2 El Salvador amphibole composition by electron microprobe analysis 273 C3 El Salvador apatite composition by electron microprobe analysis …. 275 C4 El Salvador biotite composition by electron microprobe analysis ….. 278 C5 El Salvador plagioclase composition by electron microprobe analysis 280 C6 El Salvador titanite composition by electron microprobe analysis … 282 D1 El Salvador mineral compositions by Quadrapole LA-ICP-MS analysis 286 E1 PIMA and XRD Mineral Identifications from Lassen Peak samples 310 E2 Whole Rock Geochemical Analyses for Lassen samples ………….. 314

LIST OF CD APPENDICES CD Appendices

CD Appendix I: Zircon images CD Appendix II: Mineral calculations CD Appendix III: PIMA files CD Appendix IV: Lassen GIS files

This Dissertation is dedicated to the memory of my mother and father; Sheila A. Box-Lee and Robert P. Lee.

GENESIS OF THE EL SALVADOR PORPHYRY COPPER DEPOSIT, CHILE AND DISTRIBUTION OF EPITHERMAL ALTERATION AT LASSEN PEAK, CALIFORNIA

CHAPTER ONE

General introduction to porphyry and epithermal deposits Magmatic-hydrothermal systems include economic porphyry deposits and associated epithermal deposits and are important contributors of gold, silver, copper, molybdenum, tungsten, manganese, tin, lead, and zinc that annually contribute billions of dollars to the industrial world. These deposits result when hydrothermal fluids are released from shallow granitoid magma chambers of intermediate to silicic composition and react with overlying rock to form hydrothermal ore minerals, veins, and wall-rock alteration (Gustafson and Hunt, 1975; Arribas, 1995; Sillitoe, 1997; Lang and Titley, 1998; Seedorff et al., 2005). Magma chambers that form these deposits typically reside at depths of 3 to 10 km and sequentially intruded upward in small volumes to produce porphyry dikes and porphyry type (Cu-Mo-Au) deposits mainly at depths of one to four km but sometimes as much as six to eight km. Magma-derived aqueous fluids accompany the porphyry dikes and stocks and commonly un-mix at low pressure (<1.4 kb) to form a vapor and a brine. The volumetrically dominantly vapor may rise to the surface where it interacts with meteoric waters to form high-sulfidation epithermal gold-silver deposits characterized by advanced argillic alteration and pyrite-enargite-luzonite-covellite mineralization. While there is geologic evidence that high-sulfidation epithermal deposits and porphyry deposits are related (Hedenquist et al., 2000; Gustafson et al., 2004), the details of the timing and evolution of these porphyry-epithermal systems remains unclear.

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Ore-related magmatic-hydrothermal systems typically display long periods of ore- forming magmatism, generally 3-10 m.y. with multiple intrusive centers occurring within the same region, e.g. Butte, Montana (~4 m.y.); Chuquicamata, Chile (~4 m.y.); El Salvador, Chile (~5 m.y.); and Yanacocha, Peru (~6 m.y) (cf., Longo, 2005). These deposits may consist of multiple intrusive events (Rohrlach and Loucks, 2005; Seedorff et al., 2005) with ore deposition occurring late during the lifespan of the magmatic systems (cf., Gustafson and Hunt, 1975). However, this is not always the case and the nature between barren and economic intrusions and the associated hydrothermal fluids remains unknown. The source of components in the upper crustal magma chambers from which ore-bearing fluids are derived is still under debate. Hypotheses include those that propose derivation via fractionation of a hydrous mantle-derived basaltic melt in the lower or middle crust (Kay and Mpodozis, 2001; Rohrlach and Loucks, 2005) and those that propose derivation by assimilation, fractional crystallization, mafic recharge with open-system volatile loss within a upper crustal magma chamber (cf., Field et al., 2005; Chambefort and Dilles, 2006). Recent studies on fluid inclusions (Ulrich et al., 1999; Rusk et al., 2004; Rusk, 2007) indicate a dominantly magmatic source for the formation of the hydrothermal fluids, while magmatic vapor supplies the metal ligands for ore- transport and eventual deposition. Porphyry copper and epithermal gold-silver deposits occur within Cenozoic- Mesozoic arc terrains along the western edges of the Americas and within Paleozoic rocks of the Appalachians in northeastern United States (Figure 1.1). These types of economic deposits are commonly associated with magmatic arc segments where subduction-related magmas intrude crustally shortened terrains. Porphyry magmas

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Figure 1.1. General distribution of porphyry Cu and epithermal Au-Ag deposits that occur throughout North and South America. Figure is modified from compilations by John H. Dilles.

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are characterized by phenocrysts of plagioclase, quartz, orthoclase, biotite, and hornblende within a fine grained aplitic ground mass (Seedorff et al., 2005). Accessory minerals may vary within each deposit and consist of apatite, titanite, zircon, magnetite, and Fe-Ti oxides. Veins and sulfide deposition are associated with hydrothermal potassium silicate and sericitic alteration that are dominated by magmatic water (Sheppard et al., 1969; Sheppard and Gustafson, 1976; Bowman et al., 1995; Hedenquist et al., 2000). Hydro-fractures form in host-rocks as fluids are released from magmas allowing porphyry magmas to rise and be emplaced concurrently with hydrothermal fluid ascent. This rapid upward emplacement produces the porphyry aplitic texture due to aqueous fluid loss and “pressure-quenching” of the intruding magmas (Burnham, 1979). Fluids that ascend to the surface mix with meteoric water producing low-pH advanced argillic alteration directly above ascending fluids and near-neutral pH intermediate argillic alteration along the periphery of surface deposits (Arribas, 1995).

Determining when the magmatic and hydrothermal liquids that scavenge and deposit ore metals form in these systems as well as the source of the magma(s) that host and form these deposits is essential to forming a model of magmatic-hydrothermal system formation and ore deposition. The timing of magma/ore emplacement, the possibility of multiple magma recharge and magma generation events, as well as the oxidation state of the magma are key points to a comprehensive model of formation. Such models can be developed by studying the age and compositions of accessory minerals that form in magmatic-hydrothermal ore deposits.

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Scope of this Study This dissertation comprises three manuscripts that present detailed geochronologic and geochemical analyses from the El Salvador porphyry copper deposit, Chile (Chapters two and three), and the distribution and petrography of hydrothermal altered rocks at Lassen Volcanic National Park, California (Chapter four). This study expands the current geochronology of the El Salvador district and is the first to detail the geochemical compositions from pheonocrysts and accessory minerals from the deposit. Detailed geochemical investigation provides a model for the formation of the porphyry intrusions that host ore-mineralization in the deposit. Hydrothermal activity at Lassen Peak has formed multiple alteration events including one of the largest active geothermal systems in the Cascade Arc. Alteration is related to the mixing of meteoric water with magmatic water to produce advanced argillic and intermediate argillic alteration (Ingebritson and Sorey, 1985; 1987), similar to epithermal deposits that are closely associated with porphyry deposits at depth (Hedenquist et al., 2000; Sillitoe and Hedenquist, 2003). The Cascade Range however lacks the large deposits of Cu-Au-Ag deposits seen in other epithermal deposits around the world. This study provides a detailed alteration map along the southern flank of Lassen Peak and is part of the U.S. Geological Survey Cascades project in order to understand the epithermal processes that form in the Cascades compared with other epithermal deposits. Analyzing the alteration at Lassen Volcanic National Park provides key interpretations in understanding the formation multiple stages of alteration with time.

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The El Salvador Porphyry-Copper Deposit, Chile The El Salvador (Cu + Mo and trace Au) deposit is one of the southernmost late Eocene porphyry copper centers in northern Chile, and represents a unique deposit for determining the formation of porphyry copper deposits. The El Salvador porphyry copper deposit lies in the Indio Muerto district of the Third Region, northern Chile along the Pre-Cordillera part of the Atacama Desert at ~3000 m elevation immediately west of the Andean crest marking the Chile-Argentina border. The ore body is one of the best- documented porphyry Cu deposits on Earth. Geological mapping, petrology and geochemical analyses of hydrothermal alteration and mineralization, and age-dating have been collected over the last 40 years mainly by geologists from the Anaconda Company, the Chilean geological survey (Sernageomin), and Corporacion Nacional del Cobre de Chile (Codelco) (Gustafson and Hunt, 1975; Cornejo et al., 1997; Cornejo et al., 1999; Gustafson et al., 2001). Previous work has established the size and relative age of intrusions, the vein sequence and sulfides in the orebodies, and the zoning patterns of hydrothermal ore minerals, silicate alteration minerals and veins. The late Paleozoic Sierra Batholith to the east and Mesozoic volcanic and sedimentary rocks of the Sierra Fraga Formation and Mantos Gruesos sequence in the west form the basement rocks of the Indio Muerto district, and are overlain by Upper Triassic to Cretaceous age sedimentary rocks (Figure 1.2). These include sedimentary rocks and andesites of the Llanta Formation to the west and marine carbonates with interbedded volcanics of the Quebrada del Salitre, Montandón and Asientos Formations, and Quebrada Vicuñita sequence to the east (Cornejo et al., 1997).

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Magmatism at this latitude (26.3°S) has migrated eastward with time, with Jurassic ages along the Chilean coast, Cretaceous ages in the Central Valley, Paleogene and Eocene ages in the Pre-Cordillera, and Miocene and Quaternary ages along the Argentina frontier. The Pre-Cordillera lies along the Domeyko fault system, a north- south network of late Eocene reverse and strike-slip faults that parallel the continental margin (Figure 1.2, inset). Paleocene volcanism within the Indio Muerto district includes large volumes of trachybasalts, trachyandesites, and rhyolite lavas, domes, and tuffs (Cornejo et al., 1994, 1997). The Los Amarillos-Kilómetro Catorce volcanic sequence lies to the west of El Salvador and ranges in age from 62 to 60 Ma (Cornejo et al., 1997). The largest volume of volcanic and plutonic rocks during this time comprises diorites, ignimbrites, monzonites, and rhyolitic rocks of the El Salvador Caldera and Indio Muerto Domes. The El Salvador Caldera is defined by densely welded rhyolite ignimbrites that are cut by normal and “scissor” faults to the south of the El Salvador deposit (Figure 1.2). Cornejo et al. (1997) suggest that due to the fault structure and volcanic facies the caldera complex is a trap-door caldera. Whole rock and biotite K-Ar ages indicate a period of two million years between 61 and 63 Ma for the time of formation for the caldera (Cornejo and Mpdozis, 1996; Cornejo et al., 1999). Shortening occurred during the late Creataceous to the Paleocene with the activation of the Sierra Miranda thrust and the Mantos Gruesos fault as well as other faults which cut and deform Paleocene and older rocks. Syn- to post-tectonic Eocene intrusions include a series of porphyry and granitic intrusions that range from ~44 to ~41 Ma associated with Cu-Mo ores in the Indio Muerto district and principally from ~41 to

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Figure 1.2. Regional geologic map of the Indio Muerto district. Chile and Argentina map denotes tectonics and major porphyry copper deposits within northern Chile. Black square in geologic map denotes location of Figure 1.3 and roughly outlines the location of the El Salvador porphyry copper deposit.

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33 Ma associated with Au and Cu ores in the Potrerillos district (Marsh et al., 1997). Following mineralization, slow uplift accompanied by erosion and a drying climate led to the oxidation of the upper parts of the sulfide orebodies, and development of supergene Cu sulfide and exotic oxide ores during the period between ~38 and 15 Ma (Mote et al., 2001; Bissig and Riquelme, 2007). Hypogene and later supergene acidic fluids have altered many of the rocks above 3000 m elevation at the El Salvador deposit (Watanabe and Hedenquist, 2001). Since 15 Ma, hyper-arid conditions have prevailed in the Atacama Desert. The El Salvador ore deposit is centered on the largest granodiorite porphyry intrusive complex at Turquoise Gulch where the initial discovery was made (Perry, 1960). Other centers of porphyry intrusions occur in a 5 by 10 km area and are located at O-nose, Granite Gulch, M Gulch-Copper Hill, Red Hill, and Cerro Pelado. O-nose, M Gulch-Copper Hill and Old Camp centers have low-grade copper mineralization and have been mined where supergene enriched (Gustafson et al., 2001). Cerro Pelado contains porphyry Mo mineralization associated with quartz rhyolite porphyries that are about one m.y. older than the other porphyry intrusions (Gustafson and Hunt, 1975; Gustafson et al., 2001). The porphyries include, in order of decreasing relative age established by cross- cutting field relations, rhyolite porphyry, Quartz porphyry, X porphyry, K porphyry, L porphyry, and latite porphyry dikes (Figure 1.3). The Cu-Mo mineralization and hydrothermal alteration apparently began with emplacement of the X porphyry, reached a peak associated with the K porphyry, and declined with emplacement of the L porphyry (Gustafson and Hunt, 1975). Rhyolite porphyry predates all hypogene sulfide

Full document contains 345 pages
Abstract: 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, albite 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.