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Late Eocene tectono-magmatic evolution and genesis of reduced porphyry copper-gold mineralization at… Smithson, David Mark 2004

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L A T E E O C E N E T E C T O N O - M A G M A T I C E V O L U T I O N A N D GENESIS OF R E D U C E D P O R P H Y R Y C O P P E R - G O L D M I N E R A L I Z A T I O N A T T H E N O R T H F O R K DEPOSIT, WEST C E N T R A L C A S C A D E R A N G E , W A S H I N G T O N , U.S .A. By DAVID M A R K SMITHSON B.Sc. (Hons), James Cook University, 2000 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE F A C U L T Y OF GRADUATE STUDIES (Department of Earth and Ocean Sciences) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA March, 2004 © David Mark Smithson, 2004 Library Authorization In presenting this thesis in partial fulfillment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Name of Author (please print) Date (dd/mm/yyyy) Department of E ^ v - ^ - u . <**~<A 0 L<-^ ^ Sc \-*^ The University of British Columbia Vancouver, BC Canada A B S T R A C T The North Fork porphyry Cu-Au deposit is located in the west central Cascades Mountain Range, Washington, U.S.A., and belongs to a belt of Eocene to Miocene porphyry Cu (Au) deposits that extend northward into the Coast Mountains of southern British Columbia. The deposit has a geological reserve of 80.4 million tonnes @ 0.44% Cu and 0.003 ounces (oz) Au (a 218,000 oz Au reserve) and is hosted in three main rock units. The oldest and most spatially extensive unit is the Mount Persis andesite (38.9 ±0.3 Ma). It is intruded by quartz monzodiorite (2 samples dated at 37.2 ±0.1 Ma and 37.0 ±0.2 Ma) and mafic latite porphyry (2 samples dated at 37.1 ±0.2 Ma and 36.8 ±0.2 Ma). Plutonic rocks are weakly to moderately peraluminous, calc-alkaline, I-type granitoids that have crystallized at uncommonly low oxygen fugacities (/C»2's) ranging from the quartz-fayalite-magnetite (QFM) oxygen buffer to one log unit above (QFM+1). The older andesites are even more reduced and have crystallized at /O2's approximating QFM-1. Field relationships, age constraints, mineralogy, oxidation state, and whole-rock trace-element data indicate that plutonic and volcanic rocks are consanguineous. It appears that the reduced I-type granitoids have intruded into their own volcanic pile during construction of a late Eocene volcanic arc. Hypogene Cu-Au mineralization is associated with three stages of vein formation, but primarily occurs with banded and crustiform Main-stage quartz-actinolite-albite-chlorite-sulfide veins and accompanying sodic-calcic (albite-actinolite) alteration. Main-stage veins contain abundant hypogene pyrrhotite and lack primary hematite and sulphate minerals indicating formation from relatively reduced hydrothermal fluids. Studies of quartz-hosted fluid inclusions in Main- and Early-stage veins reveal that the North Fork deposit has formed from a thermally prograding system with Cu-Au sulfide deposition occurring at pressures of ~ 400 to 690 bars and temperatures between 348° to 576°C. These pressures are hydrostatic and correspond to depths of ~ 4 to 7 km because fluids were undergoing immiscible ii phase separation (boiling) into a dense aqueous brine (up to 51 weight % NaCl equivalent) and coexisting low-density vapor (1.4 to 3.4 weight % NaCl equivalent) at the time of trapping and Main-stage vein formation. These physicochemical conditions of ore formation are typical of porphyry Cu-Au deposits worldwide, and together with a direct genetic association with reduced I-type magmas, classify the North Fork deposit as a "reduced porphyry copper-gold" deposit. Measurement of 671 brittle structures (fractures and faults) define three main structural trends that are consistent with the various stress fields operative during the Eocene. The most important of these structures are the NNW-striking (320-340°) fractures and faults that have focused the intrusion of ~ 37 Ma mafic latite porphyry, hypogene Cu-Au mineralization, and related hydrothermal alteration. Repeated use of these structural conduits has resulted in overprinting episodes of magmatism, hydrothermal alteration, and Cu-Au mineralization. Argon-argon dating of hydrothermal sericite from halos surrounding Late-stage quartz-sulfide veins yields an age of 35.5 ± 0.2 Ma, which is at least 900,000 m.y. younger than the emplacement age of the youngest mafic latite porphyry, but well within the time-frame that many porphyry Cu-Mo±Au deposits form. The 834S values of hypogene sulfide minerals from all stages of mineralization lie between 2.3 to 3.0 %o, a range typical of magmatic sulfur and further support of a magmatic origin for the North Fork fluids. The recognition of reduced porphyry Cu-Au mineralization and related arc magmatism at -37 Ma, highlights the prospectivity of the Mount Persis andesites and raises the possibility that Late Eocene porphyry Cu-Au mineralization may be more common in the west central Cascades than has been predicted previously from the localized exposures of quartz monzodiorite and mafic latite porphyry at North Fork. iii Table of Contents Abstract ii Table of Contents iv List of Figures ix List of Table x Foreword xii Acknowledgements xiii C H A P T E R I - General introduction 1 Methodology 3 Geological mapping 3 Petrographic and mineralogical studies 4 U-Pb and Ar-Ar geochronological studies 4 Lithogeochemical studies 5 Fluid inclusion studies 6 Sulfur isotope studies 6 Thesis presentation 7 References 9 CHAPTER II - Late Eocene magmatism and reduced porphyry copper-gold mineralization at the North Fork deposit, west central Cascade Mountain Range, Washington, U.S.A 11 Abstract 12 Introduction 14 Regional geologic setting 16 Local geology 17 Deposit geology 18 Mount Persis andesite 18 Quartz monzodiorite to granodiorite 20 iv Mafic latite porphyry 22 Other intrusive rocks 24 Geochemistry of igneous rocks 24 Major-elements 25 Trace-elements 26 Oxidation state of igneous rocks 27 Geochronology 29 Mount Persis andesite 30 Quartz monzodiorite 31 Mafic latite porphyry 31 Hypogene mineralization and alteration 32 Early biotite hornfels 32 Early-stage mineralization and alteration 33 Quartz-potassium feldspar-biotite-sulfide veins 33 Quartz-sulfide veins 34 Main-stage mineralization and alteration 35 Banded and crustiform quartz-actinolite-albite-chlorite-sulfide-veins 35 Late-stage mineralization and alteration 38 Quartz-sericite veins 38 Calcite-chlorite veins 39 Supergene mineralization 40 Structure of the North Fork deposit 41 Brittle elements 41 Tectono-magmatic evolution of the west central Cascades and porphyry copper-gold mineralization at the North Fork deposit 42 Chronological constraints on tectono-magmatic evolution model 43 Tectono-magmatic evolution 44 39-37 Ma: Slab roll-back and crustal extension 44 37 Ma: Riedel "P" shearing associated with dextral strike-siip faulting 44 V 30 Ma: Block rotation and north-south crustal extension 45 Discussion 46 Application of the reduced porphyry copper-gold model to the North Fork deposit 46 Relationship between mafic latite porphyry, NNW-striking structures, hypogene mineralization and associated alteration 48 Implications for Cu-Au metallogeny of the Mid-Tertiary Cascade magmatic arc and exploration in the North Fork area 50 Conclusions 52 References 55 C H A P T E R III - Genesis and fluid evolution of reduced porphyry Cu-Au mineralization at the North Fork deposit, W A , U.S.A 88 Abstract 89 Introduction 91 Regional geologic setting 92 Geology of the North Fork deposit 93 Host rocks 93 Geochemistry and redox state of igneous rocks 94 Structural geology : 96 Hypogene mineralization and primary alteration 96 Fluid inclusion microthermometry 98 Sample selection and methodology 98 Fluid inclusion petrography 99 vi Microthermometric results 102 Freezing data 102 Heating data 103 Early-stage veins 103 Main-stage veins 104 Interpretation of fluid inclusion data 105 Early-stage veins 105 Main-stage veins 106 Sulfur isotopes 108 Sample selection and methodology 108 Results: 834S data 109 Interpretation of the 534S data = 109 A r - A r geochronology 110 Sample selection and methodology I l l Results and interpretation 112 Discussion 113 Hydrothermal fluid evolution and genesis of the North Fork deposit 113 Implications for Cu-Au metallogeny and exploration in the Cascades Mountain Range 117 Conclusions 119 References 122 Chapter IV - General conclusions 144 vii Appendix A - Geochemical analyses 147 Appendix B - Fluid inclusion analyses 150 Appendix C - Structural data 153 Appendix D - Sulfur isotope data 168 Appendix E - Geochronological data 170 viii List of Figures C H A P T E R II Figure 2.1 Location map and geological domains comprising the North Cascade physiographic province 68 Figure 2.2 Simplified geological map of the North Fork deposit and surrounding mineral occurrences 68 Figure 2.3 Simplified geology map of the North Fork deposit 69 Figure 2.4 Schematic deposit-scale cross-sections through the North Fork deposit 70 Figure 2.5 Photomicrographs and hand specimen photographs of the major intrusive and extrusive rocks hosting the North Fork deposit 71 Figure 2.6 Mineral mode abundances of equigranular and porphyritic rocks plotted on the IUGS QAP ternary diagram 72 Figure 2.7 Major-, and trace-element geochemical plots of representative intrusive and extrusive rocks hosting the North Fork deposit 73 Figure 2.8 Electron microprobe compositions of co-existing biotite and ilmenite plotted in diagram of temperature versus oxygen fugacity 74 Figure 2.9 U-Pb concordia plots of intrusive and extrusive rocks from the North Fork deposit 75 Figure 2.10 Photomicrograph and hand specimen photographs of the styles and nature of veining and associated alteration deposit 76 Figure 2.11 Intensity of alteration zonation associated with Early-, Main-, and Late-stage veins 78 Figure 2.12 Compilation of mapped brittle structures presented on rose diagrams 79 Figure 2.13 Simplified map of the brittle structure mapped with the North Fork deposit area 79 Figure 2.14 Schematic tectono-magmatic evolution model for the formation of the North Fork copper-gold deposit 80 ix C H A P T E R III Figure 3.1 Simplified geology of the North Fork deposit and surrounding areas and deposit-scale geological map with brittle structures 133 Figure 3.2 Photomicrograph and hand specimen examples of Early-, and Main stage vein material used for fluid inclusion analysis 134 Figure 3.3 Photomicrographs of the types of fluid inclusions present in Early-, and Main-stage veins at the North Fork deposit 135 Figure 3.4 NaCl versus T h ( L i q u j d 0 r Halite) and T h (Halite) versus T h (Liquid) diagrams determined from micro thermometry 136 Figure 3.5 Estimated pressure-temperature conditions based on microthermometric behavior of Early-stage fluid inclusions 137 Figure 3.6 Estimated pressure-temperature conditions based on microthermometric behavior of Main-stage fluid inclusions 137 Figure 3.7 Age plateaus from Ar-Ar analysis of hydrothermal sericite grains 138 Figure 3.8 Simplified space-time evolution of hydrothermal fluid flow, alteration and mineralization at the North Fork deposit 139 x List of Tables C H A P T E R II Table 2.1 Mineral modes of the major intrusive and extrusive rocks hosting the North Fork deposit 81 Table 2.2 Representative major-, and trace-element geochemical compositions of the main intrusive and extrusive rocks hosting the North Fork deposit 82 Table 2.3 Representative electron microprobe analyses of biotite and ilmenite grains in andesite 83 Table 2.4 Representative electron microprobe analyses of biotite and ilmenite grains in quartz monzodiorite 84 Table 2.5 Character and styles of Early-, Main-, and Late-stage veins 85 Table 2.6 Generalized paragenetic sequence of alteration and mineralization 86 Table 2.7 Field criteria for characterization of brittle structure elements at the North Fork deposit 87 C H A P T E R III Table 3.1 Generalized paragenetic sequence of alteration and mineralization 140 Table 3.2 Hypogene 834S sulfur isotope values from the North Fork deposit and surrounding Low-sulphidation mines 141 Table 3.3 Summary of petrographic characteristics and microthermometric data from fluid inclusion assemblages in Early-, and Main-stage veins 142 xi Foreword This thesis represents the collaborative work of researchers from The University of British Columbia and the Weyerhaeuser Mineral Resources division. The findings of this research comprise two papers prepared for publication in peer-reviewed earth science journals. The purpose of this forward is to properly acknowledge the contributions of these collaborators. The paper that constitutes Chapter 2 has been prepared for submission to The Geological Society of America Bulletin and was co-authored with Dr. Stephen Rowins, Dr. James Mortensen (The University of British Columbia), and Mr. Grant Newport (The Weyerhaeuser Company). These co-authors contributed to the field work, geochronology, and were involved in the development of many of the concepts in this paper. Dr. Stephen Rowins also contributed editorially. The paper that comprises Chapter 3 was prepared for submission to Ore Geology Reviews, and was co-authored by Dr. Stephen Rowins. Dr. Rowins provided guidance and supervision in all aspects of this paper, and contributed both editorially, and in the development of ideas central to this paper. xii Acknowledgements Throughout the course of my research at The University of British Columbia, many have benevolently contributed their ideas, knowledge and support. I would like to thank my supervisor, Dr. Stephen Rowins, for his guidance and patience during this thesis and for allowing me the creative latitude to pursue the aspects of the project that interested me the most. Steve is especially thanked for sharing his uniquely creative style of research from which I have learned so much. Dr. Jim Mortensen, Dr. Greg Dipple and Dr. James Scoates are thanked for serving on my committee, and Grant Newport for introducing me to the geology of the North Fork-Snoqualmie area. Dr. Ken Hickey is thanked for his invaluable discussions in many aspects of structural geology and for sharing his expert knowledge of geographic information systems. Dr. Mati Raudsepp and Dr. Elisabetta Pani are thanked for their assistance in acquiring the vast majority of the analytical data in this thesis. Dr. Thomas Ullrich is especially thanked for his time and patience expert knowledge of Ar-Ar geochronology. Many thanks also to my fellow grad students Geoff Bradshaw, Michael Henrichsen, Steve Quane, Simon Haynes, Steve Israel, Laurence Winter and Scott Heffernan for your advice and friendship and for making my time in Canada unforgettable. Above all, I thank my family who continue to be the backbone of my every success. Their constant emotional support and encouragement during the course of this thesis has been unwavering. Finally, I would like to thank my wonderful friend and partner Julie for her understanding, constant emotional support, and friendship. Simple words could not ever convey my sincerest gratitude for all you have done. Above all your unique sense of humor, kindness, and friendship have kept me smiling even during the darkest days. xiii C H A P T E R I G E N E R A L INTRODUCTION The North Fork deposit is one of several porphyry Cu (Au) deposits of Eocene to Miocene age in the Cascades magmatic arc of northwestern Washington State. The deposit is located approximately 40 km east of Seattle on the western flank of the west central Cascade Range. The Weyerhaeuser Company originally discovered the deposit during a regional stream sampling program in the North Cascades in the mid-1960's (Bloom, 1965). Drilling in the 1960's and 1970's, and again in the 1990's lead to the definition of a geological reserve of 80.4 million tonnes (Mt) @ 0.44% Cu and 0.003 ounce (oz) per tonne Au (218,000 contained oz; Herdrick et al., 1995). There are no modern studies of the porphyry Cu-Mo±Au deposits in the Cascades Range. The few older studies that do exist reveal that the Cascadian porphyries have many features typical of porphyry copper deposits in the American southwest and the South American Cordillera, although Cascadian porphyries are typically smaller and contain an unusual abundance of hypogene pyrrhotite (e.g., Hollister, 1978). This later feature lead Hollister (1978) to refer to them as "pyrrhotite" porphyries and Rowins (2000a, 2000b) proposed that they represent a "reduced" porphyry Cu-Au (RPCG) district or province. Reduced porphyry Cu-Au deposits are similar in most respects to the classically oxidized porphyry Cu-Mo±Au deposits (e.g., Highland Valley, Bingham, Morenci, Chuquicamata) except that they have formed from relatively reduced ore fluids derived from intrinsically reduced I-type felsic magmas. A consequence of this lower fluid and magmatic redox state is that RPCG deposits generally contain lower grades and tonnages of Cu than the oxidized variants, but many contain similar 1 amounts of Au (Rowins 2000a; 2000b). In RPCG systems, these Au zones are commonly located distal to the relatively Cu-poor core of the deposit. This is due, in part, to the enhanced transport of Au and Cu in the magmatic vapor phase in reduced magmatic-hydrothermal systems (Heinrich et. al., 1992; 1999 Rowins 2002; 2003). Consequently, RPCG deposits may have unrecognized Au potential, which if realized, could significantly enhance the economic viability of a deposit. Assessment of the North Fork deposit for its Au potential was, thus, one major reason for this study. A second equally important reason to study the North Fork deposit revolves around the ~10 Ma K-Ar age of the deposit reported by Hollister and Baumann (1979). If true, this Miocene age makes the North Fork deposit the youngest porphyry Cu-Au deposit in the Cascades Range, significantly younger than the 18 to 25 Ma porphyry Cu (Au) deposits (e.g., Middle Fork, Quartz Creek, and Glacier Peak) related to the emplacement of the Snoqualmie Batholith and its satellitic plutons (e.g., Hollister, 1978; Lasmanis, 1995). A careful re-evaluation of the ages of the magmatism and porphyry Cu-Au mineralization at the North Fork deposit is therefore critical to understanding the evolution and Cu-Au metallogeny of the Tertiary Cascade magmatic arc. To address these issues, we undertook a comprehensive field and laboratory study of the North Fork deposit and surrounding area. The field studies consisted of detailed mapping at 1:4800 scale of the entire deposit (10.2 km ). The different lithologies, brittle structures, and types and styles of mineralization and associated alteration were mapped. These studies formed the basis for interpreting the whole-rock geochemistry, U-Pb geochronology, and mineralogical estimates of magmatic oxidation states. We were particularly interested in obtaining quantitative estimates of the oxygen fugacities (/(Vs) of the causative plutonic and volcanic rocks given the importance of this parameter in distinguishing oxidized from reduced porphyry Cu-Au deposits. This is the first comprehensive study of any porphyry Cu-Mo-Au deposit in the Tertiary 2 Cascades Range and, consequently, the geochemical, petrological, and geochemical data in this paper essentially constitutes the only published data for Cascadian porphyries. M E T H O D O L O G Y The multidisciplinary approach adopted by this study combines detailed geological mapping with modern laboratory techniques including; (1) whole-rock, trace-element and mineral chemistry; (2) petrography and mineral modes (3) U-Pb and Ar-Ar geochronology; (4) stable isotopes and (5) fluid inclusion microthermometry. In isolation, these techniques provide critical constraints on the age and petrology of the deposit as well as specific variables such as oxygen fugacity, fluid P-T-X conditions, and the sources of fluids responsible for mineralization. Together, these data represents a significant advancement in the understanding of the fundamental constraints on the genesis of magmatism, and the hydrothermal processes that lead to the formation of the North Fork deposit. Geological mapping Initial reconnaissance-scale mapping and sampling of the major intrusive and extrusive rocks hosting the North Fork deposit and surrounding Blackhawk and Lennox Creek epithermal mines was done by the author with the aid of Dr. Stephen Rowins (UBC) and Grant Newport from the Weyerhaeuser mineral resources division in the fall of 2001. More comprehensive deposit-scale mapping (1:4800) of rock type, alteration mineralogy, style and intensity of alteration and mineralization patterns, and structure in outcrop and drill core was done by the author with the assistance of Jason Pellet in the summer of 2002. Additional drill core 3 examination and sampling was conducted during the winters of 2002 and 2003 by the author with the aid of Dr. Stephen Rowins. Petrographic and mineralogical studies Seventy-one samples of the major intrusive and extrusive rock hosting the North Fork deposit were submitted for polished and doubly polished thin sections. A combination of optical mineralogy, scanning electron microscopy, and electron microprobe analysis were used to fully characterize (1) the mineralogy and textural nature of intrusive and extrusive rocks; (2) the mineralogy and paragenesis of veining and hydrothermal alteration associated with all stages of mineralization; and (3) quantitative and qualitative estimation of the oxidation state of the magmas that produced the igneous rocks hosting the North Fork deposit. The hematite component in ilmenite (hem%/ilm%) grains used in oxythermometry were calculated according to Buddington and Linsdsley (1965) using QUILF (Anderson et. al., 1993). Isopleths of (Fe/Fe+Mg) in biotite calculated according to Wones and Eugster (1965). The data derived from these studies were critical in identifying many of the geological processes that contributed to the genesis of the North Fork magmatic-hydrothermal system and subsequent production and localization of Cu-Au mineralization. U-Pb and A r - A r Geochronological studies Five samples of intrusive and extrusive rocks were collected for U-Pb (zircon) geochronology. The methodology for the process of zircon selection, abrasion-dissolution, chemical preparation, and mass spectrometry is described in Mortensen et al., (1995). U-Pb 4 geochronological studies of the intrusive and extrusive rocks were undertaken to assess whether rocks at the North Fork deposit are part of a magmatic continuum or whether they formed from unrelated magmatic events superimposed over time. The absolute ages of the plutonic rocks were also required to evaluate a K-Ar (biotite) age of 9.9 Ma reported for copper mineralization associated with secondary biotite (e.g., Hollister and Baumann, 1978). Rock samples containing hydrothermal sericite for Ar-Ar analysis were crushed, pulverized to a coarse powder, and sieved. Flakes of sericite were selected by handpicking under a binocular microscope. The resulting separates had a purity of >99 % sericite. Separates were subsequently washed in de-ionized water, rinsed, and then air-dried at room temperature for 48-hours. Sericite separates were then re-picked, wrapped in aluminum foil, and stacked with similar-aged samples and flux monitors in an irradiation capsule. Samples were irradiated at the McMaster Nuclear Reactor in Hamilton, Ontario, for 72 hours. The samples were then analyzed using the 4 0 Ar/ 3 9 Ar technique (Merrihue and Turner, 1966; Dallmeyer, 1975; Dalrymple et. al., 1981) at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia (Vancouver, British Columbia), and step-heated at incrementally higher powers using a 10W CO2 laser until fused. The gas evolved from each heating step was analyzed using a VG5400 mass spectrometer. An estimate for the error in the J value (i.e., a constant in the age equation related to irradiation; McDougall and Harrison, 1988) is ~ 0.5 %. Lithogeochemical studies The major and trace element abundances of twenty-one plutonic and volcanic rocks from the deposit area were determined by X-ray fluorescence, inductively coupled plasma-mass spectrometry (ICP-MS) and fire assay (for gold) at Chemex Labs Ltd, Vancouver, British 5 Columbia. These geochemical data were collected to permit comparison with thin section mineral mode estimations of the rocks and to investigate the possibility that plutonic and volcanic rocks are related to the same magma source at depth. Finally, the trace element data are used to interpret tectonic environments in which magmatism related to the North Fork deposit may have occurred. Fluid inclusion studies Microthermometric studies were completed by the author using a Fluid Inc®. modified USGS gas-flow heating-freezing stage at The University of British Columbia under the supervision of Dr. Stephen Rowins. Microthermometric data were obtained from one hundred and sixty three fluid inclusions in twenty-two fluid inclusion assemblages trapped in nine Early-, and Main-stage veins. Studies of fluid inclusion assemblages were done in order to obtain pressure-temperature-composition (P-T-X) data of metalliferous fluids responsible for Early-, and Main-stages of mineralization. In addition, these data were collected for information on processes responsible for hypogene metal transport and deposition (i.e., fluid pressure-temperature paths, fluid boiling). Sulfur isotope study Sulfides were physically separated from their host-rocks by hand picking under a binocular microscope and, by HCI dissolution. Isotope analyses were preformed at the G.G Hatch Isotope Laboratories at the University of Ottawa (Ontario). The sulphur isotopic composition of sulfides were determined by Continuous Flow (CF). Helium was used to carry 6 the SO2 gas produced by flash combustion with vanadium pentoxide at 1800°C on an Elemental Analyser, followed by separation (from other gases) by an SO2 gas chromatographic column, into a Finnigan MAT Deltaplus mass spectrometer. Reference gas was injected from the bellows of the dual inlet. The routine precision of the analyses is 0.20%o. Duplicate samples yielded an average analytical reproducibility of ±0.2 %o. Sulfur isotope studies of primary sulfide minerals associated with Cu-Au mineralization at the North Fork deposit were done to determine the source(s) of sulfur and, by inference, metals in the different stages of vein formation. The 5 3 4S values of primary sulfides from the Blackhawk and Lennox Creek epithermal Au-Ag mines located 4 to 5 km SSE of the North Fork deposit, were also measured to identify source(s) of sulfur and metals for comparison with North Fork sulfur isotope data. THESIS P R E S E N T A T I O N The results of the research on the North Fork deposit are presented as two separate papers. The first paper fully characterizes the genesis of the North Fork deposit through detailed field studies and evaluation of the age and petrology of the associated magmatism. The redox state of the causative plutonic and volcanic rocks are determined, and the effect that magmatic oxidation state had on fluids subsequently exsolved, was examined. The brittle structural elements mapped in the deposit are compared with changes in known paleostress fields operative in the west central Cascades between 40 and 30 Ma, thus permitting reconstruction of the kinematic history of the North Fork porphyry Cu-Au deposit. This paper presents the first detailed study of a large porphyry copper-gold deposit in the west central Cascades magmatic arc and it demonstrates that Cu-Au mineralization commenced much earlier that previously thought 7 in this very poorly understood metallogenic arc. This paper was prepared for submission to the Geological society of America bulletin. . . The second paper specifically employs mineralogical, fluid inclusion, sulfur isotope, and Ar-Ar geochronological data to investigate the source(s) of the mineralizing fluids and the physicochemical conditions (P-T-X) controlling sulphide deposition within the North Fork fluid system. Integration of these data with key structural, petrographic, and geochronological constraints provide useful insight into the magmatic-hydrothermal system that produced mineralization within the North Fork area. In addition, these data prove useful for the further evaluation of spatial relationships between the North Fork deposit and numerous proximal deposits and occurrences. These combined data are used to derive exploration models for the evaluation of the other RPCG porphyry deposits and associated epithermal gold deposits in this portion of the west central Cascades Range and elsewhere in the world. This paper has been prepared for submission to Ore Geology Reviews. Complete compilations of geochemical, mineralogical, fluid inclusion, geochronology and sulphur isotope data are included as appendices to this thesis. The presentation of this thesis as two complimentary research papers allows for the timely publication of results. Some minor repetition is unavoidable, however, it is hoped that any inconvenience to reviewers of this thesis is minimal and is justified by the benefits of rapid publication in peer-reviewed geoscience journals. 8 References Anderson, D.J., Lindsley, D.H., and Davidson, P.M., 1993, QUILF: A PASCAL program to assess equilibria among Fe-Mg-Mn-Ti oxides, pyroxenes, olivine and quartz: Computers and Geosciences, v. 19, 1333-1350. Bloom, H., 1965, Results of follow-up of reconnaissance geochemical survey in Washington and Oregon: Weyerhaeuser Company report, p. 9. Dallmeyer, R. D., 1975 , 4 0 Ar/ 3 9 Ar of biotite and hornblende from progressively re-metamorphosed basement terrane. Their bearing on interpretation of release spectra: Geochimica et Cosmochimica Acta, v. 39 p. 1655-1669. Dalrymple, G. B., Clague, D. A. Garcia, M. O. & Bright, S. W., 1981, Petrology and K-Ar ages of dredged samples from Laysan Island and Northampton Bank volcanoes, Hawaiian Ridge, and evolution of the Hawaiian-Emperor chain: Geological Society of America Bulletin v. 92, 884-933. Herdrick, M.A. Newport, G.R. Heinermeyer, G.R., 1992, Geology of the North Fork Snoqualmie Porphyry copper deposit, King county, Washington, in, Pierce, F.W., and Bolm, J.G., eds, Porphyry Copper Deposits of the American Cordillera, Arizona Geological Society Digest, v. 20, p. 224-250. Heinrich, C.A., Ryan, C.G., Mernagh, T.P, and Eadington, P.J., 1992, Segregation of ore metals between magmatic brine and vapor: A fluid inclusion study using PIXE microanalyses. Economic Geology, v. 87, p 1566-1583. Heinrich, C.A., Gunther, D., Audetat, A., Ulrich, T., and Frischknecht, R, 1999, Metal fractionation between magmatic brine and vapor, determined by microanalysis of fluid inclusions: Geology, 27, 755-758. Hollister, V.F., 1978, Geology of the porphyry copper deposits of the Western Hemisphere. New York, American Institute of Mining and Metallurgical Engineers, 219 p. Hollister, V.F. and Baumann, F.W., 1978, The North Fork porphyry copper deposit of the Washington Cascades: Mineralium Deposita, v. 13, p. 191-199. Lasmanis, R., 1995, Regional geological and tectonic setting of porphyry deposits in Washington State. In: Schroeter, T.G., (ed) Porphyry deposits of the Northwestern Cordillera of North America: Canadian Institute of Mining, Metallurgy, and Petroleum Special Volume, v. 46, p. 77-102. Merrihue, C , and Turner, G., 1966, Potassium-argon dating by activation with fast neutrons: Journal of Geophysical Research. 71, 2852-2857. McDougall, I., and Harrison, T.M., 1988, Geochronology and thermochronology by the 4 0 Ar/ 3 9 Ar method: Oxford Monographs on Geology and Geophysics v. 9, p. 212. 9 Mortensen, J.K., Ghosh, D., and Ferri, F., 1995, U-Pb geochronology of intrusions associated with Cu-Au porphyry deposits in the Canadian Cordillera, in, Schroeter, T. G. (ed) Canadian Institute of Mining and Metallurgy, Special Volume 46, p. 142-158. Rowins, S.M., 2000a, A model for the genesis of "reduced" porphyry copper-gold deposits: The Gangue, GAC-MDD Newsletter, v. 67, p. 1-7. Rowins, S.M., 2000b, Reduced porphyry copper-gold deposits: a new variation on an old theme. Geology, v. 28, p. 491-494. Rowins, S.M., Yeats, C.J., and Ryan, C.G., 2002, New PIXE evidence for magmatic vapour phase transport of copper in reduced porphyry copper-gold deposits. Eighth Biennial Pan-American Conference on Research on Fluid Inclusions (PACROFI), Halifax, Canada, v. 8, p. 88-90. 10 A B S T R A C T The North Fork porphyry Cu-Au deposit lies on the western flank of the west central Cascades Mountain Range, Washington, U.S.A. It belongs to a suite of Eocene to Miocene porphyry Cu (Au) deposits that extends northward into the Coast Mountains of southern British Columbia. The deposit has a geological reserve of 80.4 million tonnes (Mt) @ 0.44% Cu and 0.003 ounces (oz) Au per tonne (a 218,000 oz Au reserve). Detailed field mapping, petrographic analysis, whole-rock geochemistry, microprobe mineral analysis, and 5 new U-Pb (zircon) age determinations reveal that three main rock units host the deposit. The oldest and most spatially extensive unit is the Mount Persis andesite (38.9 ±0.3 Ma). It is intruded by quartz monzodiorite (2 samples dated at 37.2 ±0.1 Ma and 37.0 ±0.2 Ma) and mafic latite porphyry (2 samples dated at 37.1 ±0.2 Ma and 36.8 ±0.2 Ma). Amphibole and biotite are the main mafic silicate minerals in plutonic and volcanic rocks, and ilmenite greatly predominates over magnetite. Major-element whole-rock data indicate that the plutonic and volcanic rocks are calc-alkaline and weakly to moderately peraluminous. Chondrite-normalized rare earth element (REE) profiles of plutonic and volcanic rocks are highly fractionated (La/Ybcn = 4.9 to 9.2) and very similar. The felsic and plutonic rocks plot well within the volcanic-arc field on Nb-Y-Rb tectonic discrimination diagrams. Compositional data from primary ilmenite-biotite pairs in unaltered andesite indicate that these rocks crystallized at oxygen fiigacities (/02's) approximately one log unit below the quartz-fayalite-magnetite oxygen buffer (QFM-1). The /02's of unaltered quartz monzodiorite determined from primary ilmenite-biotite pairs are somewhat higher than those of the andesite and lie between QFM and QFM+1. The similarity of REE profiles, ages of formation, low redox state of the magmas, and inferred tectonic setting indicate that the plutonic rocks are reduced I-type granitoids that have intruded into their own volcanic pile 1 to 2 m.y. 12 after the start of andesitic volcanism and early arc construction at ~ 39 Ma. Hypogene Cu-Au mineralization is developed most strongly in mafic latite porphyry and immediately adjacent andesite. The quartz monzodiorite is unmineralized except where intruded by dikes of mafic latite porphyry. Three stages of vein formation are recognized, but banded and crustiform Main-stage quartz-actinolite-albite-chlorite-sulfide veins host most of the hypogene Cu-Au mineralization. Pyrite, pyrrhotite, and chalcopyrite are the dominant sulfides with rare bornite, arsenopyrite, and molybdenite. Sodic-calcic alteration (quartz-actinolite-albite-chlorite) surrounds the Main-stage veins, whereas potassic (quartz-biotite-K-feldspar), phyllic (quartz-sericite), and late propylitic (calcite-chlorite-epidote) alteration is associated with other stages of vein formation. The association of the North Fork deposit with reduced I-type magmas coupled with the abundance of hypogene pyrrhotite, but the absence of primary hematite and sulphate minerals, classify North Fork as a reduced porphyry Cu-Au deposit. Brittle structures (faults and fractures) define three main structural trends consistent with various stress fields operative during the Eocene. The most important of these structures, with respect to the genesis of the North Fork deposit, are the NNW-striking (320-340°) faults and fractures, which have focused the intrusion of mafic latite porphyry at ~ 37 Ma and localized overprinting zones of alteration and intense hypogene Cu-Au mineralization. This NNW-striking orientation should be viewed as important for targeting Cu and Au mineralization outside of the immediate deposit area. The -37 Ma age of the North Fork deposit demonstrates that Cu-Au mineralization in the Cascades arc commenced much earlier than the previously reported 10 Ma age for the deposit. The consanguineous relationship that exists between the volcanic and plutonic rocks highlights the prospectivity of regionally extensive Mount Persis andesite (100 km) for hosting other Eocene porphyry Cu-Au deposits in the Cascade arc of the Pacific Northwest. 13 INTRODUCTION The North Fork deposit is one of several porphyry Cu deposits of Eocene to Miocene age in the Cascades magmatic arc of northwestern Washington State (e.g., Hollister 1978; Lasmanis, 1995). It is located approximately 40 km east of Seattle on the western flank of the west central Cascade Range between the Tolt and Snoqualmie rivers (Fig. 2.1). The Weyerhaeuser Company discovered the deposit during a regional stream sampling program in the mid-1960's (Bloom, 1965). Between 1967 and 1972, a variety of geophysical surveys, metallurgical studies, and geological and geochemical data from 29 diamond drill holes were used to define the potential size of the porphyry Cu system (Herdrick et al., 1995). In 1991 an additional 243 short (3m in length), horizontal drill holes were completed with a percussion drill in order to produce Cu and, in particular, Au assay data. These new chemical data lead Herdrick et al. (1995) to calculate a reserve of 80.4 million tonnes (mt) @ 0.44% Cu and 0.003 ounces (oz) per tonne or 218,000 ounces of Au. Modern studies of the porphyry Cu deposits in the Cascades Range are absent. The few older studies that do exist reveal that the Cascadian porphyries have many features typical of porphyry Cu deposits in the American southwest and the South American Cordillera, although they are typically smaller and contain an unusual abundance of hypogene pyrrhotite (e.g., Hollister, 1978). This latter feature lead Hollister (1978) to refer to them as "pyrrhotite" porphyries and Rowins (2000a, 2000b) proposed that they represent a "reduced" porphyry Cu-Au (RPCG) district or province. Reduced porphyry Cu-Au deposits are similar in most respects to classically oxidized porphyry Cu deposits (e.g., Highland Valley, Bingham, Morenci, Chuquicamata), except that they formed from relatively reduced ore fluids derived from intrinsically reduced I-type felsic magmas. A consequence of this lower fluid and magmatic 14 redox state is that RPCG deposits generally contain lower grades and tonnages of Cu than the oxidized variants, but many contain similar amounts of Au (Rowins 2000a; 2000b). The Au zones in RPCG systems are located commonly distal to the relatively Cu-poor core of the deposit due, in part, to the enhanced transport of Au and Cu in the magmatic vapor phase in reduced magmatic-hydrothermal systems (Heinrich et al., 1992; 1999; Rowins 2002; 2003). Consequently, RPCG deposits may have unrecognized Au potential, which if realized, could significantly enhance the economic viability of a property. Assessment of the North Fork deposit for its Au potential was thus one major reason for this study. A second equally important reason to study the North Fork deposit revolves around the ~10 Ma K-Ar age of the deposit as reported by Hollister and Baumann (1979). If true, this Miocene age makes the North Fork deposit the youngest porphyry Cu deposit in the Cascades Range, significantly younger than the 18 to 25 Ma porphyry Cu deposits (e.g., Middle Fork, Quartz Creek, Glacier Peak) related to the emplacement of the Snoqualmie batholith and its satellitic plutons (Hollister, 1978; Lasmanis, 1995). A careful re-evaluation of the ages of magmatism and porphyry Cu-Au mineralization at the North Fork deposit is therefore critical to understanding the evolution and metallogeny of the Tertiary Cascade magmatic arc. To address these issues, we undertook a comprehensive field and laboratory study of the North Fork deposit and surrounding area in order to fully characterize its genesis. The field studies consisted of detailed mapping at 1:4800 scale of the entire deposit (10.2 km ). The different lithologies, brittle structures, and types and styles of mineralization and associated alteration were mapped. These studies formed the basis for interpreting the whole-rock geochemistry, U-Pb geochronology, and mineralogical estimates of magmatic oxidation states. We were particularly interested in obtaining quantitative estimates of the oxygen fugacities (/CVs) of the causative plutonic and volcanic rocks given the importance of this parameter in distinguishing oxidized from reduced porphyry Cu-Au deposits. This is the first comprehensive 15 study of a porphyry Cu-Mo±Au deposit in the Tertiary Cascades Range and, consequently, the geochemical, petrological, and geochemical data in this paper constitutes the only modern published data for Cascadian porphyries. R E G I O N A L G E O L O G I C SETTING The North Cascade physiographic province is the southerly extension of the Coast, Intermontane, and Omineca belts that pass from British Columbia into northern Washington state (Monger and Journeay, 1994; Haugerud et al., 1994). These tectonic domains offer natural boundaries within which Jurassic to Miocene porphyry Cu-Au-Mo deposits and associated calc-alkaline intrusions are grouped (Fig. 2.1). The North Cascades has been divided into four major geologic domains by Haugerud et al. (1994; Fig. 2.1). From west to east these are: (1) The Northwest Cascade system, comprising a stacked sequence of Paleozoic and Mesozoic eugeosynclinal strata and Early Cretaceous blueschist facies metamorphic rocks; (2) the Western and Eastern Melange belts, which are separated from the Northwest Cascade system by the Darrington-Devils Mountain fault zone (DDMF). The DDMF is characterized by a series of fault-bounded Mesozoic intrusive rocks and Mesozoic to Paleozoic marine metasedimentary rocks; (3) the Cascade Crystalline Core, which is divided by the Entiat fault into the Wenatchee and Chelan blocks. These blocks consist of plutons, gneisses, migmatites, and schistose rocks metamorphosed to lower greenschist and upper amphibolite grade; (4) the Methow block, which is separated from the Cascade Crystalline Core by the Ross Lake fault, and consists of metamorphosed late Mesozoic sedimentary rocks that are deformed by east-verging thrusts and associated folds. 16 L O C A L G E O L O G Y The North Fork deposit is located in the Jurassic to Early Cretaceous Western Melange belt (Fig. 2.1b), which is composed of argillite, phyllite, greywacke, and minor amounts of conglomerate, pelite, chert and marble (Fig. 2.2; Tabor et al., 1982; 1993). Mafic and ultramafic rocks are rare. This'melange sequence is interpreted to have formed in an accretionary wedge (Jett and Heller, 1986), with emplacement, tectonic disassemblement and significant strike-slip translocation completed by the early Tertiary (Beck et al., 1982; Frizzell et al., 1987). In the area of the North Fork deposit, rocks of the Western Melange are represented by the Callagan Formation, a sequence of intensely folded pelitic metasedimentary rocks (Tabor et al., 1993). The Callagan Formation is overlain unconformably by a series of shallowly dipping extrusive rocks that form part of the Eocene Mount Persis volcanic group (Tabor et al., 1993). This group is andesitic in composition and consists of flows, breccias, pyroclastic rocks, stocks, and dikes (e.g., Hollister and Baumann, 1978; Tabor et al., 1982; 1993). Oligocene granodiorite to tonalitic rocks of the Index batholith (~35 Ma; Ponzini and Tepper, 2003) intrude both the Callagan Formation and the Mount Persis Group. The Index batholith and smaller satellitic bodies (i.e., Sunday Creek pluton) crop out immediately north and south of the deposit area, respectively (Fig. 2.2). Similarly, rocks of the 18 to 25 Ma Snoqualmie batholith (Erikson, 1969; Tabor et al., 1982) intrude the Callagan Formation and the Mount Persis Group. This composite gabbroic to granitic batholith is located immediately east of the North Fork deposit, and occupies an area in excess of 700 km2. On the western flank of the batholith, numerous stocks and dikes of various composition and texture intrude the Western Melange and Mount Persis Group volcanic rocks (Livingston, 1971; Hollister and Baumann, 1978; Herdrick et al., 1992). 17 In addition to the North Fork deposit, other porphyry Cu (Au) deposits in the region include Middle Fork and Quartz Creek situated 10 and 15 km southeast of North Fork, respectively. The North Fork area has seen considerable historic small scale Au mining from numerous quartz-sulphide veins of the epithermal variety (Derkey et al, 1990). The North Fork deposit itself is surrounded by many such low-sulphidation epithermal veins, the most notable of which includes the Lennox, Beaverdale, Blackhawk, and Damon-Pythias mines. DEPOSIT G E O L O G Y Mount Persis andesite Volcanic rocks of the Mount Persis group are dominated by massive to porphyritic flows and subvolcanic intrusions of andesitic to dacitic composition. Volcaniclastic rocks of andesite to dacite composition with a wide variety of textures are less common, but significant in the mapped area. For simplicity, however, all these rocks are collectively referred to in this study as andesites. In the central and southwest portions of the deposit area, andesites display north-striking (±030°) beds associated with successive lava flows (Figs. 2.3 and 2.4). Andesites are typically massive and characterized by abundant (20-30 volume %) randomly oriented, euhedral, microphenocrysts (<1 mm in diameter) of quartz, plagioclase, and potassium-feldspar set in a very fine-grained grayish green groundmass of similar mineralogy. Microphenocrysts of acicular amphibole and euhedral flakes of biotite are uncommon (1-5 vol. %) and difficult to recognize in strongly altered and weathered rocks. In the northern parts of the map area, massive andesite may contain sharp-edged and rounded enclaves of unclassified mafic igneous rock. These eqiudimensional enclaves typically range from 5 to 15 cm in diameter with rare specimens 18 having diameters in excess of 25 cm. At other locations andesite exhibits columnar joints which localize weakly developed kaolinitic alteration due to surficial weathering of feldspars. In other areas of the deposit, matrix-supported, heterolithic, volcaniclastic rocks are interbedded with massive to porphyritic andesite flows and subvolcanic intrusions. Bedding relationships indicate that volcaniclastic rocks and massive to porphyritic flows and intrusion are broadly contemporaneous. Volcaniclastic rocks crop out throughout the property with the main exposures mapped in the southeast. Volcaniclastic rocks consist of very fine-grained, sub-angular to rounded rock fragments (5 mm to 50 cm in diameter) that are heterogeneously distributed within a fine-grained, grayish green groundmass. Abundant, typically small (<1 mm diameter), phenocrysts of feldspar (30 to 40 vol. %) and lesser amounts of amphibole and biotite (2 to 5 vol.%) constitute the groundmass. Rock fragments and matrix are andesitic or, less commonly, dacitic in composition. All andesites are altered by chlorite + epidote ± calcite to some degree in the mapped area. These minerals constitute apropylitic alteration assemblage, which defines the outermost alteration halo to the deposit and is the earliest formed alteration assemblage associated with intrusion of plutonic rocks. This style of hydrothermal alteration replaces a mineralogically similar pre-existing regional greenschist facies metamorphic mineral assemblage in andesites (e.g., Tabor et al., 1982). Recognition of the propylitic alteration associated with North Fork magmatism is therefore difficult, although it can be made most easily in areas away from intense hydrothermal alteration. Andesite adjacent to dikes of mafic latite porphyry is thermally metamorphosed to a very fine-grained, dark brown, highly fractured, biotite hornfels (Fig. 2.3). This hornfels commonly consists of >90 vol. % secondary biotite. Petrographic examination reveals that euhedral laths of plagioclase are the most common phenocryst in porphyritic andesite. Laths range from 0.2 to 0.7 mm in long dimension (Table 19 2.1). Less abundant microphenocrysts of euhedral to subhedral amphibole and euhedral biotite range from 0.3 to 1 mm and 0.5 to 1 mm, in the long dimension, respectively. The fine-grained (<1 mm) groundmass in both massive and porphyritic andesite is composed typically of euhedral plagioclase, subhedral to anhedral hornblende, euhedral biotite, anhedral quartz and subhedral to euhedral potassium-feldspar. Ilmenite is the dominant opaque mineral. Magnetite is minor (Figs. 2.5a, b). Quartz monzodiorite to granodiorite Equigranular intrusive rocks from the main southeast exposure of the North Fork deposit (Fig. 2.3) were slabbed, stained with sodium-cobalt nitrate, and point-counted on a 1 mm resolution grid transparency to determine the mineralogical modes. Subsequent classification using the International Union of the Geological Sciences (IUGS) Quartz-Alkali-feldspar-Plagioclase (QAP) ternary diagram (Fig. 2.6) reveals that the main intrusive mass in the North Fork map area ranges from quartz monzodiorite to granodiorite in composition. For simplicity, these rocks are collectively referred to as quartz monzodiorites in this study. Quartz monzodiorites occur as elongate, NNW-trending, largely fault-bounded intrusions (e.g., Fig. 2.4) exposed over 3.5 km2 in the southeast and western portions of the deposit area (Fig. 2.3). Sharp intrusive contact with Mount Persis andesite indicates that emplacement postdates the formation of the volcanic rocks. Quartz monzodiorite on the southern corner of the map area (Fig. 2.3) is relatively fresh, medium-grained, equigranular and grayish green in color. Primary minerals include hornblende, biotite, plagioclase, potassium-feldspar and quartz (Table 2.1). Systematic changes in quartz content were not recognized. The minor exposures on the western edge of the map area have suffered significant surficial weathering rendering the study of primary 20 mineralogy and texture difficult. The main intrusive body exhibits fine-grained chilled margins defined by abrupt decreases in grain size (<1 mm) as the contact with andesite is approached. A similar chill margin is developed where quartz monzodiorite cross-cut dikes of older mafic latite porphyry (see below). Petrographic examination reveals that quartz monzodiorite has a medium-grained hypidiomorphic-granular texture defined by plagioclase, potassium-feldspar, quartz, hornblende, biotite, ilmenite and rare magnetite (Figs. 2.5c, d). Euhedral to subhedral laths of plagioclase range from <1 to 8 mm in length and vary in modal abundance from 45 to 60 % (Table 2.1). Most plagioclase laths display concentric or oscillatory zonation with pericline, Carlsbad and albite twins. Less commonly, plagioclase grains exhibit brittle fracturing of their margins, along twin planes and, more rarely, in their cores. Such textures are indicative of crystal movement in semi-solidified magma. Hornblende grains are equant, euhedral, and range from <1 to 3 mm in diameter. Hornblende constitutes 5 to 15 modal % of the rock, but increases to 40 % where present as fine-grained patches ranging from 5 to 10 cm in long dimension (Table 2.1). Euhedral laths of biotite typically display foxy-red to yellow-brown pleochroism and range from <1 up to 4 mm in length. Biotite constitutes ~ 5 modal % of the rock with most grains free of secondary chloritization. Potassium-feldspar laths have euhedral habit and range in modal abundance from 15 to 20 %. Quartz consists of 5 to 25 vol. % of the rock with anhedral grains typically ranging between 0.1 and 1 mm in diameter. Accessory minerals such as ilmenite, magnetite and apatite occur in limited quantities (Table 2.1). 21 Mafic latite porphyry Classification of very fine-grained porphyry dikes can be made both chemically and mineralogically. Mineralogical classification was accomplished using the method of Stringham (1969) as recommended by Titley (1982) for porphyry Cu deposits. Specifically, where igneous rocks are too fine-grained for point counting techniques, the mineralogical mode of the phenocryst population is determined and classification made using the IUGS QAP ternary diagram for plutonic rocks (Fig. 2.6). The use of this technique indicates that these fine-grained porphyritic rocks are mineralogically equivalent to quartz monzodiorite and granodiorite. Geochemically, these rocks plot as mafic latites on the total alkali versus silica diagram of Le bas (1986) for volcanic rocks (Fig 7a). In this study, we use the chemical classification and refer to these rocks as mafic latite porphyry. Exposures of steeply-dipping, northwest-trending and fault-bounded mafic latite porphyry dikes and subvolcanic intrusions cover several square kilometers and crosscut andesites and the majority of other intrusive rock-types at the North Fork deposit (Figs. 2.3 and 2.4). Although the vast majority of contact relationships mapped in the field show that the mafic latite porphyry is younger than quartz monzodiorite, in rare cases, enclaves of mafic latite porphyry were present in quartz monzodiorite. This indicates temporal synchroneity, although, it can be concluded that the emplacement of mafic latite porphyry was generally later and outlasted emplacement of quartz monzodiorite which is consistent with the U-Pb ages obtained for these rock-types (see below). Mafic latite porphyry crops out throughout the deposit area as dikes and larger intrusions in the southeast and northeast areas, respectively. Dikes are typically tens of meters in thickness but continuity at this scale on surface is rare and most dikes cannot be 22 traced for more than a few meters. Mafic latite porphyry is characterized by phenocrysts of rounded, cracked, glassy gray quartz, white plagioclase, pinkish potassium-feldspar, dark green amphibole, and dark brown biotite (Table 2.1). These phenocrysts are set in a very fine-grained, medium to dark grey, equigranular groundmass. Phenocryst abundances vary greatly in this rock unit with some localities containing up to 40 vol. % feldspar phenocrysts. These zones of abundant feldspar phenocrysts, however, typically grade over several meters into zones of relatively sparse phenocryst populations (10 to 15 vol. %). Systematic variation in feldspar phenocryst populations, in terms of overall abundance and the plagioclase: potassium-feldspar ratios, was not recognized during mapping. Euhedral plagioclase laths and highly resorbed, rounded to subangular, quartz grains are the dominant phenocrysts in the mafic latite porphyry (Table 2.1). Less abundant phenocrysts include euhedral hornblende and euhedral to subhedral biotite (Figs. 2.5e, f). Plagioclase with oscillatory zonation commonly exhibit Carlsbad, pericline and albite twins. Plagioclase phenocrysts typically occur either as mineral aggregates or as individual crystals. Biotite laths display foxy-red to yellow-brown pleochroism and commonly have ragged crystal margins. The groundmass in mafic latite porphyry is generally too fine-grained for positive mineral identification, but in some cases, individual grains of plagioclase, potassium-feldspar, quartz and hornblende can be recognized. Accessory minerals include ilmenite, magnetite, apatite and zircon. 23 Other intrusive rocks Aplite dikes striking NNW are mapped in the southeast region of the study area. These narrow (20 to 40 cm in width) dikes are very rare, occur in isolation, and consist almost entirely of fine-grained plagioclase and quartz. Although field evidence constraining the age of the aplite dikes is absent, they appear related to the magmatic event which produced the igneous rocks associated with Cu-Au mineralization at North Fork. No direct relationship, however, has been found between aplites and zones of hydrothermal alteration and mineralization. Dark green, medium-grained, lamprophyre dikes (1 to 2 m in width) striking to the east occur within pre-existing east-striking fracture/fault zones that crosscut all rock-types and styles of alteration and mineralization at the North Fork deposit (Fig. 2.3). These contact relationships indicate that lamprophyre dike emplacement post-dates formation of the North Fork deposit. G E O C H E M I S T R Y O F IGNEOUS R O C K S The major and trace element abundances of twenty-one plutonic and volcanic rocks from the deposit area were determined by X-ray fluorescence, inductively coupled plasma-mass spectrometry (ICP-MS) and fire assay (for Au; Table 2.2). These geochemical data were collected to permit comparison with thin section mineral mode estimations of the rocks and investigate the possibility that plutonic and volcanic rocks are related to the same magma source at depth. Finally, the trace element data, in particular, are used to provide insights into the possible tectonic environments in which magmatism related to the North Fork deposit occurred. 24 Major elements The fine-grained porphyritic dikes (SMR01-113; NF-OC-230; NF-OC-266) tentatively classified as quartz monzodiorites (Fig. 2.6) based on their phenocryst proportions, plot in the mafic latite/andesite field on a total alkalies (Na20 + K2O) versus silica (SiC>2) diagram for volcanic rocks (Fig. 2.7a, the "TAS" diagram of Le Bas et al., 1986). The distinction between andesite and mafic latite is made based on the modal proportions of plagioclase and alkali feldspar phenocrysts, with higher proportions of alkali feldspar in the latter (e.g., Barker, 1983). Consequently, we term these fine-grained rocks mafic latite porphyry. The massive, weakly porphyritic volcanic rocks of the Mount Persis group also plot in the andesite/mafic latite field but are classified as andesites due to the preponderance of plagioclase over alkali feldspar phenocrysts (Fig. 2.7a). The equigranular medium-grained plutonic rocks that are classified as quartz monzodiorite and granodiorite based on their mineral modes, also fall in the quartz monzodiorite and granodiorite fields in addition to the diorite field on the TAS diagram (Fig. 2.7b) for plutonic rocks (Wilson, 1989). For simplicity, we have collectively referred to these rocks as quartz monzodiorites. The medium-grained plutonic rocks hosting the Blackhawk and Lennox Creek Au mines south of the North Fork deposit tend to be slightly more mafic than the plutonic rocks associated with the North Fork deposit and range from granodiorite to meladiorite in composition (Fig. 2.7b). Consideration of these comparative studies indicates that the mineralogical and chemical classification schemes for volcanic and plutonic rocks from the study area are largely consistent. Major element data also reveal that volcanic and plutonic rocks are calc-alkaline and moderately to weakly peraluminous, excluding several quartz monzodiorites and a sample of mafic latite porphyry which have tholeiitic compositions (Figs. 2.7c, d). 25 Trace-elements Rare earth element (REE) abundances for all North Fork rock types are very similar and have been normalized to the chondritic values of Nakamura (1974; Table 2.2). All patterns are highly fractionated with steep negative slopes and negligible to very small negative Eu anomalies (Fig. 2.7e). Lanthanum abundances range between 50 and 100 times chondrite, whereas those of the heavy rare earth elements (HREE) range between 6 and 14 times. The La /Ybcn ratios for andesite (6.9-9.1), quartz monzodiorite (5.8-8.2), and mafic latite porphyry (7.5-8.3) are similar and overlapping. The lack of systematic increase in the La/Yb c n ratio and the absolute abundances of light rare earth elements (LREE) from andesite through quartz monzodiorite to mafic latite porphyry discounts the possibility that these rock types are related by fractional crystallization of amphibole from a common parental magma body (e.g., Cullers and Graf, 1984; Tsususe et al., 1987; Rowins et al., 1993). Although a cogenetic relationship between these rock types through fractional crystallization and mineral accumulation is not supported by the REE data, the similarity of REE profiles is suggestive of a consanguineous relationship. Such a relationship is further supported by similar crystallization ages for quartz monzodiorite and mafic latite porphyry as discussed below. Tectonic discrimination diagrams based on Rb, Y , and Nb abundances in granitic rocks show that these rock types from the study area including the neighboring Lennox Creek and Blackhawk mines, have formed in a volcanic arc setting (Figs. 2.7f and g; Pearce et al., 1984; Christiansen and Keith, 1996). Conclusions drawn from these diagrams, however, must be accepted with caution because immobility of Rb, Y , and Nb is assumed during secondary 26 alteration processes, which is not necessarily true (e.g., Rollinson, 1993). The volcanic arc setting identified in this study, however, is entirely consistent with the documented arc magmatism in the Tertiary Cascades and the steep, negative R E E profiles of the North Fork rocks are very characteristic of intermediate to felsic arc magmas (Fig. 2.7e; G i l l , 1981; Wilson, 1989; Rollinson, 1993). Finally, a Zr versus G a / A l diagram, which discriminates between several types o f granite (Whalen et al., 1987), indicates that the North Fork rocks are not anorogenic (A-type) granites. The mineralogy and geochemistry of the North Fork rocks are consistent with classification as I-type granitoids, albeit of the reduced variety (e.g., Christiansen and Keith, 1996; see below) O x i d a t i o n s t a t e o f i g n e o u s r o c k s Qualitative estimates of the intrinsic oxidation state of the plutonic and volcanic rocks that host the North Fork deposit may be obtained from a variety of techniques including the relative abundances of magnetite and ilmenite (Ishihara, 1981), the pleochroic colors of primary biotite (Lalonde and Bernard, 1993), and the whole-rock Fe203/FeO ratios (Ishihara, 1981; Levil le et al., 1988; M c C o y et al., 1997). More rigorous quantitative estimates may be derived from electron microprobe compositions of co-existing primary biotite and ilmenite plotted in temperature (T) - oxygen fugacity (/O2) space (Tables 2.3 and 4; Fig. 2.8). The calculation of T-JO2 equilibria in plutonic rocks using magnetite-ilmenite oxythermometer of Buddington and Lindsley (1964) has been problematic because of subsolidus re-equilibration of magnetite in particular. More recently, Ague and Brimhall (1988), Candela, 1989 and M c C o y et al. (1997) have advocated the use of the ilmenite-biotite pair to estimate the 7D2-T conditions in reduced plutonic rocks. Unlike magnetite, ilmenite tends not to change its composition significantly upon cooling, and Wones and Eugster (1965) have demonstrated that the annite component in 27 biotite is a function of T and JO2. Consequently, the intersection of biotite and ilmenite isopleths in JO2-T uniquely defines these intensive parameters. In the case of the volcanic and plutonic rocks in the North Fork study area, all have quartz, biotite, potassium-feldspar, ilmenite, and magnetite as a stable mineral assemblage. Thus, all biotite and ilmenite compositions are correctly buffered for the experimental systems in which the hematite-ilmenite solid-solution (hem-ilmss) and annite stability curves were calibrated. Due to the small size and generally poor preservation of biotite grains in the samples of mafic latite porphyry that were studied in thin section, only ilmenite-biotite pairs in andesite and quartz monzodiorite were used for/O2-T estimates. Examination of Figure 2.8 reveals that the andesites crystallized at oxygen fugacities, approximately 1 log unit below the quartz-fayalite-magnetite (QFM) solid oxygen buffer curve. Quartz monzodiorites crystallized at lower temperatures and higher oxygen fugacities ranging between QFM and QFM + 1 or the nickel-nickel-oxide (NNO) buffer. The JO2S of andesites are at the lower end of the range for typical intermediate to mafic arc magmas (e.g., Carmichael, 1991) and theytVs of the quartz monzodiorites are much lower than those calculated for the magnetite-series or oxidized I-type felsic magmas associated with classic porphyry Cu-Au-Mo deposits (e.g., Burnham and Ohmoto, 1980; Burnham, 1981). The jOiS of these oxidized felsic magmas range from the NNO buffer up to the hematite-magnetite (HM) buffer. The magmas in the North Fork study area are, therefore, all reduced with the quartz monzodiorites and, by inference, the mafic latite porphyry classified as reduced I-type magmas (e.g., Blevin and chappell, 1992; Pollard et al., 1995). The Fe203/FeO ratio can be used as an indicator of oxidation state of magmas for volcanic, and in very fortuitous circumstances, plutonic rocks (e.g., Kress and Carmichael, 1988). These whole-rock ratios, however, are notoriously inaccurate due to the effects of 28 secondary oxidation and/or deuteric alteration inherent in cooling plutonic rocks (e.g., Clarke, 1996). This caveat, not withstanding, at North Fork all volcanic and plutonic rocks possess Fe2CVFeO ratios below the empirical cut-off value of 0.6, used by Leveille et al. (1988) and McCoy et al. (1997) to discriminate "reduced" from "oxidized" felsic plutonic rocks. The optical properties of igneous biotite are also strongly suggestive of crystallization under low magmatic./LVs. In andesites, quartz monzodiorites, and mafic latite porphyry, the biotites (in the sense of Deer et al., 1962) have Fe/Fe+Mg ratios ranging between 0.48 and 0.56. Examination of 34 thin sections of volcanic and plutonic rocks indicates that >90 % of all magmatic biotite displays foxy-red pleochroism. Work conducted by Lalonde (1992) and Lalonde and Bernard (1993) show that the oxidation state of iron exerts a strong control over the pleochroic color of biotite. Red biotites are enriched in total Fe and Fe2 +, a characteristic of reduced peraluminous granitoids; conversely, greenish-brown biotites are enriched in Mg and Fe3+, a characteristic of oxidized metaluminous granitoids (e.g., Rowins et al., 1991). Finally, primary ilmenite is much more abundant than magnetite in all igneous rock samples studied. Point counting revealed an ilmenite: magnetite ratio of 6:1, 10:1 and 2:1 in andesites, quartz monzodiorites and mafic latite porphyry, respectively. All of these qualitative results are consistent with the low redox state of the North Fork magmas determined from ilmenite-biotite oxythermometry. G E O C H R O N O L O G Y Geochronological studies of the volcanic and plutonic rocks associated with the North Fork deposit were done to help assess whether these rocks represent parts of a magmatic 29 continuum or unrelated magmatic events superimposed over time. This is necessary for the interpretation of the genesis of the North Fork deposit and for understanding the tectono-magmatic evolution of this part of the west central Cascades. The absolute ages of the plutonic rocks were also required to evaluate an old K-Ar (biotite) age of 9.9 Ma reported for Cu mineralization associated with secondary biotite by Hollister and Baumann (1978). If correct, this K-Ar age makes North Fork the youngest porphyry Cu deposit in the Cascade Range and not part of the 18-25 Ma magmatic event that produced the Snoqualmie batholith and its numerous satellite plutons associated with porphyry-Cu mineralization (e.g., Middle Fork; Quartz Creek; Glacier Peak; Lasmanis, 1995). To clarify these geochronological questions, one sample of fresh Mount Persis andesite and two samples each of quartz monzodiorite and mafic latite porphyry were collected for U-Pb (zircon) geochronology. The methodology for the process of zircon selection, abrasion-dissolution, chemical preparation, and mass spectrometry is described in Mortensen et al. (1995; appendix E). Errors on the ages are reported at the 2c level and displayed in concordia plots (Fig. 2.9). Mount Persis andesite A sample of the Mount Persis andesite (SMR01-110; Fig. 2.9) shows a single concordant U-Pb (zircon) analysis and yields an emplacement age of 38.9 ± 0.1 Ma. Due to the intermediate composition of the sample, however, only a small amount of zircon (19 micrograms) was recovered and, subsequently, only one zircon fraction was analyzed. Consequently, the possibility that these zircons are xenocrysts inherited from rocks of the Callagan Formation, part of the Western melange beneath the deposit, cannot be completely precluded although this seems impractical because these rocks are Cretaceous in age (Tabor et al., 1982). Moreover, under 30 plane-polarized light at high magnification, the zircons from sample SMR01-110 are equant, euhedral, clear and possess elongate grain morphologies. These features are inconsistent with magmatic resorption and transport abrasion, but are typical of zircons crystallized from a magma (e.g., Barrie, 2002). Finally, the U-Pb age of 38.9 ± 0.1 Ma obtained in this study is consistent with the K-Ar age of 38.1 Ma ± 3.3 Ma obtained by Tabor et al. (1982) for samples of Mount Persis andesite collected further to the north. Quartz monzodiorite Two samples of quartz monzodiorite (SMR01-108, SMR01-111; Fig. 2.9) collected from the large southeastern exposure yielded abundant equant euhedral zircon. These rocks yielded multiple concordant fractions and overlapping U-Pb ages of 37.2 ± 0.1 Ma and 37.0 ± 0.2 Ma. These emplacement ages are ~1 to 2 m.y. younger than the age of the andesites. This is consistent with field relationships that show quartz monzodiorite always intrudes the andesite. Mafic latite porphyry Multiple concordant fractions in two samples of mafic latite porphyry (SMR-106, SMR-113; Fig. 2.9) collected from the southeast portion of the mapped area yielded overlapping U-Pb ages of 37.1 ± 0.2 Ma and 36.8 ± 0.2 Ma, respectively. These ages are identical, within error, to those obtained for the quartz monzodiorites and consistent with field evidence for contemporaneity. Dikes of mafic latite porphyry are commonly observed crosscutting quartz monzodiorite, although rare reversals in the relative chronology have been mapped and enclaves of mafic latite porphyry in quartz monzodiorites have been found. The emplacement of mafic latite porphyry, however, appears to have outlasted that of quartz monzodiorite because the vast 31 majority of crosscutting relationships show that mafic latite porphyry is younger. The hydrothermal alteration and hypogene mineralization cuts all major rock types and closely follows the distribution of mafic latite porphyry suggesting a causual relationship (Fig. 2.11). This aspect is investigated below. H Y P O G E N E M I N E R A L I Z A T I O N AND A L T E R A T I O N The principal types of veins and associated styles of hydrothermal alteration responsible for hypogene Cu-Au mineralization at the North Fork deposit were identified and characterized by detailed field mapping and petrographic analysis. Veins were classified based on several criteria including: (1) vein growth textures; (2) vein mineralogy; (3) associated wallrock alteration and mineralogy; (4) and relative age (Tables 2.5 and 2.6). Individual vein-types commonly exhibit significant variation along strike in terms of mineral abundance and vein growth textures, although vein and wallrock mineralogy remains the same. The types of veins and their alteration assemblages directly correlate with the large-scale alteration features mapped at the North Fork deposit. In this study alteration is described as selective, pervasive, and selectively replacive following the terminology of Titley (1982). Early biotite hornfels The earliest stage of hydrothermal alteration associated with formation of the North Fork deposit is the development of biotite hornfels in andesite adjacent to mafic latite porphyry. In the field, this change is marked by the dark brown color of hornfels and the complete recrystallization of andesite to a fine-grained equigranular mixture of biotite, plagioclase, and 32 minor quartz. Although the development of biotite hornfels precedes all stages of vein formation, it is important to recognize as it signals the introduction of potassium into the wallrocks and the possible presence of a porphyry Cu system. The hornfels commonly is replaced by mineral assemblages representing all other styles of hydrothermal alteration at the North Fork deposit attesting to its early age (Table 2.6). Early-stage mineralization and alteration Quartz-potassium feldspar-biotite-sulfide veins These are the oldest veins at the North Fork deposit and they typically occur in isolation, although rare swarms of anastomosing veinlets have been recognized. These veins are characterized by massively textured, fine-grained, quartz and potassium feldspar with lesser biotite and sulfide (Fig. 2.10a-c). Accessory minerals including magnetite, sericite, and ilmenite constitute <5 mod. % of the vein. Pyrite, chalcopyrite, and molybdenite occur as fine- to coarse-grained disseminations and commonly constitute 8 to 10 mod. % of the vein. Locally, however, the disseminations are replaced by segments of massive pyrite and chalcopyrite and constitute up to 90 mod. % of the vein. In handsample, vein margins appear sharp and wavy, whereas in thin section they are diffuse and gradational with the wallrock. Veins range in width from 1 to 10 mm and have strike-lengths on the order millimeters to several meters. Alteration halos surrounding these veins consist of very fine-grained biotite with lesser potassium feldspar and quartz. This mineralogy is identical to that in the veins and is termed potassic alteration in this study. Potassic alteration is strongly developed (>30 mod. % replacement) immediately adjacent to the wallrock. Halos are diffuse, wavy, and range in 33 thickness from 1 mm to 3 cm. Pyrite, chalcopyrite, and lesser molybdenite constitute 5 to 10 mod. % of the vein selvage. Grains are euhedral to subhedral and occur as fine- to coarse-grained disseminations. The areas of strong potassic alteration mapped at the North Fork deposit correlate directly with areas of abundant quartz-potassium feldspar-biotite-sulfide veins, with the most intense zones of potassic alteration corresponding to areas of the most intense fracturing and quartz-potassium feldspar-biotite vein development (Fig. 2.1 la). In the field, potassic alteration is identified by the presence of dark brown "shreddy" hydrothermal biotite and lesser pale pink potassium feldspar. The intensity of alteration is defined according to modal abundances of potassium feldspar and biotite. Weak, moderate, and strong alteration intensities correspond to <5%, 5 to 10%, and 10 to 20% combined biotite and potassium feldspar, respectively. Potassium feldspar selectively replaces plagioclase phenocrysts and groundmass crystals along grain boundaries and cleavage planes (Fig. 2.10a). Biotite selectively replaces hornblende and some feldspar in the groundmass (Fig. 2.10a, b). Potassic alteration and early-stage quartz-potassium-feldspar-biotite-sulfide veins at the North Fork deposit is developed preferentially within and adjacent to the mafic latite porphyry implying that the potassium-rich fluids are related directly to these magmas. Quartz-sulfide veins Quartz-sulfide veins cut quartz-potassium feldspar-biotite-sulfide veins and are composed almost entirely of anhedral quartz with rare acicular actinolite, fibrous chlorite, and euhedral chalcopyrite, pyrite, and molybdenite (Fig. 2.1 Of). These veins are uncommon and 34 rarely mineralized. They typically occur in isolation, although they may occur as steeply dipping veins stockwork zones, where they constitute up to 10 mod. % of the rock. Quartz-sulfide veins range from 1 mm to 1 cm in thickness and are continuous on the scale of centimeters to meters. Vein margins are sharp with irregular to wavy outlines. Alteration halos, where present, are weakly developed (<15% replacement of wallrock) and consist of very fine-grained quartz, sericite, and minor biotite and potassium feldspar (Fig. 2.10d, e). Disseminated, euhedral chalcopyrite and pyrite account for up to 7 mod. % of the alteration halo. This alteration style is best described as a combination of potassic (biotite) and phyllic (quartz-sericite) styles of alteration. Main-stage mineralization and alteration Banded and crustiform quartz-actinolite-albite-chlorite-sulfide veins Steeply dipping quartz-actinolite-albite-chlorite-sulfide veins are the most common type of vein at the North Fork deposit (Fig. 2. lOi, 1). They cut Early-stage veins and display both banded and crustiform textures. Banded textures are characterized by wallrock-parallel bands (~1 to 10 mm in width) of acicular actinolite, fibrous chlorite and rare equant epidote and biotite that are separated from one another by bands composed of anhedral grains of quartz and feldspar, and plucked fragments (< 1 cm wide) of altered wallrock (Fig h, i). Veins vary in width from 1 to 5 cm and possess sharp, straight to wavy, edges that may be traced from centimeters to meters. The crustification textures are characterized by medium- to coarse-grained crystals of quartz and albite whose long axes are orientated perpendicular to vein margins indicating open-space growth towards the center of the vein (e.g., Ramsay and Hubert, 1987). Minor actinolite, tourmaline, and chlorite fill the interstices between quartz and feldspar crystals 35 (Fig. 2.10k). Chalcopyrite and pyrrhotite are the dominant sulfide minerals, with pyrite and molybdenite only locally abundant in some veins. Sulfide minerals constitute up to 10 mod. % of the vein and typically occur in either wallrock parallel bands or concentrated as anhedral to subhedral masses in the center of the veins. Magnetite and much less commonly, ilmenite, occur together with chalcopyrite and pyrrhotite. The alteration halos that surround the quartz-actinolite-albite-chlorite-sulfide veins consist of fine-grained quartz, albite, actinolite, chlorite and minor epidote. The minerals in this alteration assemblage are the same as those found in the veins and this assemblage is best defined as sodic-calcic alteration (Fig. 2.10g, i). Other minerals that occur in varying amounts include very fine-grained magnetite, tourmaline, biotite, and clinozoisite. Chalcopyrite, pyrrhotite, and pyrite account for 3 to 10 mod. % of the vein selvage and most commonly occur as disseminated, anhedral, blebs. An exception to this distribution is where clots of actinolite, chlorite and epidote are well-developed and associated with up to 10% sulfide minerals. Sulfide abundances diminish with increasing distance from vein margins. Alteration selvages range in thickness from 1 to 4 cm, and may be either weakly or strongly developed (i.e., 10 to 45 % replacement of wallrock) depending upon the number of stages of vein growth. For example, veins with mineralogical and textural evidence for six stages of open-space filling possess much more intensely developed alteration halos than those with evidence of only one stage of open-space filling. The intensity and distribution of sodic-calcic alteration mapped at the North Fork deposit relates directly to the formation of quartz-actinolite-albite-chlorite-sulfide veins (Fig. 2.1 lb). Zones of most intense sodic-calcic alteration correspond to areas of most abundant fracturing 36 and quartz-actinolite-albite-chlorite-sulfide veins. In the field, sodic-calcic alteration is recognized by the presence of fine- to medium-grained pale green actinolite and whitish-yellow albite which imparts a "bleached" appearance to the rock. The intensity of sodic-calcic alteration is mapped according to combined modal percentages of albite + actinolite with weak, moderate, and strong intensities corresponding to <5 %, 5-10 %, and <20 % albite + actinolite. A striking feature in the field that results from strong sodic-calcic alteration is the formation of "pseudobreccias". They consist of rounded to angular clots of actinolite + chlorite + albite greater than 15 cm in diameter in a matrix of kaolinitized feldspar and quartz (Fig. 2.10j). These pseudobreccias, however, are simply the result of intense surficial weathering where kaolinization of feldspar surrounding these clots has occurred in fractured andesite at the highest exposed elevations of the North Fork property. These features were previously mapped as "hydrothermal breccias" by Herdrick et al. (1995a) and their high Au content (~1 oz/t Au; Herdrick et al., 1995) is a result of their development in zones of intense sodic-calcic alteration and Main-stage quartz-actinolite-albite-chlorite-sulfide veins. Additional evidence that these pseudobreccias are the result of surficial weathering comes from their restriction to the top 30 m of drill core (e.g., SC-8; Herdrick et al., 1995a) and decreasing intensity of kaolinization of feldspar in fractures with distance away from the highest elevations on the property. The common observation in the field that sodic-calcic alteration increases in intensity with distance away from mafic latite porphyry is simply the result of destruction of the sodic-calcic alteration assemblage by later Late-stage quartz-sericite veins and associated phyllic alteration (see below) 37 Late-stage mineralization and alteration Quartz-sericite veins Quartz-sericite veins cut Early- and Main-stage and are mineralogically simple. They consist of inequigranular quartz, sericite, and the sulfide minerals pyrite, pyrrhotite, and chalcopyrite (Table 2.6; Fig. 2.10m-o). In some veins, however, the assemblage pyrite-bornite ± chalcopyrite replaces the pyrrhotite-pyrite-chalcopyrite (Table 2.6). Sulfide minerals may constitute of up to 10% of the vein and they occur as isolated 3 to 10 mm long clots. Veins are typically <1 cm wide and possess sharp, straight margins that are continuous on the scale of several meters. Alteration selvages adjacent to these veins are well developed and consist of very fine-grained quartz, sericite, and pyrite that replace up to 45 mod. % of the wallrock. The widths of vein selvages typically range from 1 to 2 cm. The alteration minerals in the selvages define a classic "phyllic" alteration assemblage in porphyry Cu deposits (e.g., Titley and Beane, 1982; Fig. 2.10m, n). The intensity of phyllic alteration was mapped and scaled according to modal percentages of quartz and sericite, with weak, moderate, and strong alteration corresponding to <5%, 5 to 20% and 20 to 45% quartz + sericite, respectively (Fig. 2.1 lc). Zones of strong phyllic alteration correlate with zones of intensely fractured rock and abundant quartz-sericite veins. In these zones, the replacement of primary minerals is pervasive and complete destruction of primary rock textures common. Where quartz-sulfide veins are absent, zones of less strongly developed phyllic alteration were mapped not only by the presence of quartz and sericite, but also on the development of rusty-brown gossans resulting from oxidative weathering of hypogene sulfide minerals. Zones of 38 weak phyllic alteration are recognized by a noticeable sheen in sunlight due to the replacement of feldspar by microscopic flakes of secondary sericite. Calcite-chlorite veins Calcite-chlorite veins are best recognized in fresh drill core due to their susceptibility to surficial weathering and resultant poor field preservation. Consequently, they are probably more abundant than the few occurrences mapped in the field. Calcite-chlorite veins cut Late-stage quartz-sulfide veins/veinlets and associated zones of phyllic alteration thereby identifying them as the youngest type of hypogene vein at the North Fork deposit. These veins consist of calcite, chlorite, sulfide and minor quartz (Fig. 2.10p-r). Sulfides in calcite-chlorite veins are rare and constitute <2% of the vein. These veins are characteristically narrow (<1 cm wide) with sharp, straight edges and strike-lengths on the order of centimeters to meters. Chalcopyrite and bornite occur as sparse, fine-grained disseminations. Medium- to coarse-grained disseminations of anhedral chalcopyrite and pyrite are common in wallrock where phenocrysts of felsic and mafic minerals are selectively replaced. Isolated grains of anhedral bornite display exsolution lamellae of chalcopyrite resulting in a characteristic "basket-weave" texture (e.g., Ixer, 1990). Alteration selvages that surround the calcite-chlorite veins possess the same mineralogy as the veins and are weakly developed (<15% replacement of wallrock) with sulfides consisting of <2% of the halo. These selvages are sharp, wavy, and typically range in thickness from approximately 1 to 10 mm. 39 S U P E R G E N E M I N E R A L I Z A T I O N The poor development and preservation of secondary zones of Cu and Au enrichment at the North Fork deposit can be attributed to low hypogene Cu grades and low abundances of pyrite, together with climatic factors. The formation of a supergene enrichment zone in a porphyry Cu-Au deposit requires movement of acidic solutions through permeable structures (Titley, 1982). To accomplish this, large volumes of pyrite are necessary for the production of sulfuric acid, which is required for the destruction of chalcopyrite and the liberation of Cu ions into solution (e.g., Blanchard, 1968). At the North Fork deposit, the areas of phyllic alteration contain the highest abundances of pyrite but even in these zones quartz-sericite veins constitute only a small percentage of the rock and abundances of pyrrhotite, chalcopyrite, and bornite rarely exceed several percent. Despite the lack of a well-defined supergene enrichment zone, the deposit has a variety of secondary Cu minerals exposed on surface and in shallow drill core. These include chrysocolla, malachite, neotocite, chalcocite, and covellite. The best zones of Cu oxide mineralization occur in association with Main-stage hypogene mineralization and sodic-calcic alteration (Fig. 2.1 lb), which implies that Cu-oxide mineralization results from surficial oxidation of exposed Main-stage hypogene mineralization. Similarly, the rare grains of bornite and chalcopyrite in calcite-chlorite veins commonly show replacement by indigo-blue supergene covellite along their margins. Subsequent supergene chalcocite replacement of this covellite is common. Although zones of economically significant secondary enrichment do not presently exist at the North Fork deposit, the previous development of such zones cannot be discounted. It is possible that Pleistocene glaciation within the North Fork area (e.g., Beg'et et al., 1997) has 40 removed a near-surface supergene Cu oxide zone. S T R U C T U R E O F T H E N O R T H F O R K DEPOSIT The structural elements of the North Fork study area were measured and their location mapped, in order to understand the distribution of lithologies and which structures, if any, controlled the location of hypogene and supergene Cu-Au mineralization and associated alteration. Brittle structural elements recognized include joints, fractures, normal dip-slip faults, and normal oblique strike-slip faults (Figs. 2.12 and 2.13). Lithological offsets were difficult to locate in the field due to discontinuous exposure and steep topography. Consequently, the sense of movement on many faults (i.e., dip-slip versus oblique strike-slip) commonly was impossible to determine with confidence. The unavailability of lithological contacts also made distinction between massive extrusive flows and subvolcanic mafic latite porphyry dikes especially problematic in the SW corner of the map area (Fig. 2.13). The structural elements at the North Fork deposit were categorized by their width, the nature of material in faults and fractures, continuity of strike, sense of movement, amount of displacement and strike orientations (Table 2.7; e.g., Ramsay and Hubert, 1987). Brittle elements Fractures are structures that lack any sense of offset and are <20 cm in width, although most range from 0.5 to 5 cm in width. Fractures commonly are filled with kaolinite, goethite, and hematite which are the products of surficial weathering of primary minerals in the rock. Fractures are planar features that are continuous on the scale of centimeters to meters (Table 2.7). 41 Small faults are structural zones that range between 20 cm and 1 m in width. These faults typically contain grayish-green gouge that has been moderately to intensely argillized. In the field, small faults are typically marked by abundant reddish-brown hematite and yellow goethite. Substantial fault offsets (i.e., >10m) are extremely rare and only recognized where mafic latite porphyry dikes are truncated. Shallowly dipping mineral elongation lineations and slickensides are common, however, and indicate some degree of oblique-slip movement. Small faults typically strike N N W and are continuous on the scale of hundreds of meters. As discussed below, these faults are of particular importance in the genesis of the North Fork deposit because they have focused the intrusion of dikes of mafic latite porphyry (Fig. 2.13; Table 2.7). Large faults are defined as structures greater that l m in width with significant mappable strike-lengths. Similar to the small faults, these structures commonly contain minor amounts of argillized fault gouge. Slip vectors are identified commonly by the presence of mineral elongation lineations and slickensides. These steeply pluging features consistently trend to either the east or the west. Large faults typically possess moderate to steep dips, and have a preferred EW strike. They cut all lithologies, types of mineralization, and types of alteration (Fig. 2.13; Table 2.7). TECTONO-MAGMATIC EVOLUTION OF THE WEST CENTRAL CASCADES AND PORPHYRY COPPER-GOLD MINERALIZATION AT THE NORTH FORK DEPOSIT The results of this study reveal that the structure of the North Fork area is dominated by a system of faults and fractures that reflect a history of Eocene oblique-slip faulting and brittle 42 deformation in an extensional regime. Paleostress data for the Eocene in the west central Cascades established by Atwater (1989) and Wells (1990) are used to interpret the brittle structures and reconstruct the kinematic evolution of the North Fork porphyry Cu-Au deposit (Fig. 2.14). Chronological constraints on tectono-magmatic evolution Detailed relative chronological data were derived by comparison of various structures mapped in different lithologies (Fig. 2.12; Table 2.7) with orientations consistent with the various stress fields active during the Eocene. This comparison of measured structures with documented paleostresses (Atwater, 1989; Wells, 1990; Lasmanis, 1995) reveals a clear temporal sequence of deformation events. Constraints on this paleo-reconstruction include the assumption that: (1) fractures at the North Fork deposit record the orientation and magnitude of paleostresses at the time of fracture generation; (2) variation in strike orientations within a single fracture set results from local stress perturbations in a common stress field (e.g., Tosdal and Richards, 2001); and (3) extensional structures (i.e., those opening without a shear component) have tensile stress orthogonal to the fractures and maximum principal compressive stress (ai) colinear with the strike of the fractures. Brittle structures with shear components form under triaxial compression where o"2 is parallel to the strike of the fractures and principal stress (di) forms at an angle less than 45° to the faults (e.g., Ramsay and Hubert, 1987). 43 Tectono-magmatic evolution 39-37 Ma: Slab roll-back and crustal extension The formation of andesites at ~ 39 Ma and earliest quartz monzodiorites at ~ 37 Ma is synchronous with steepening of the Juan de Fuca plate during the Eocene (Wells, 1990; Lasmanis, 1995). This increased angle of subduction would have promoted slab roll-back and accompanying crustal extension (e.g., Goldfarb et al., 1998). The earliest or first generation faults and fractures (350-010°) at the North Fork deposit strike in the expected orientation of extensional structures created by Eocene slab roll-back and crustal relaxation (Fig. 2.14a). Initial slab roll-back also may have caused the reactivation of pre-existing north-striking normal faults in the metasedimentary rocks of the Mesozoic Callaghan Formation, which underlies the North Fork deposit (Tabor et al., 1993). These preferential zones of crustal weakness would have been ideal conduits for the upward movement of magmas and fluids from the deep crust and upper mantle generated by anatexis accompanying slab roll-back. 37 Ma: Riedel "P" shearing associated with dextral strike-slip faulting Quartz monzodiorite and earliest mafic latite porphyry were emplaced into the Mount Persis andesite at ~ 37 Ma. Field relationships and geochronological data strongly suggest that emplacement of mafic latite porphyry continued after cessation of quartz monzodiorite activity. During this period of pluton emplacement, the Straight Creek fault (SCF) was active (Monger and Journeay, 1994) and associated deformation was dominantly dextral strike-slip faulting (Schiarizza et al., 1997). The second generation NNW-striking faults (320-340°) at North Fork 44 correspond to pressure or "P" shears synthetic to transcurrent faulting on the SCF in a Riedel-type shear model (e.g., Tchalenko, 1968; Hodgson, 1989). These NNW-striking faults appear to have been conduits focusing the intrusion of mafic latite porphyry, which also strikes in a similar orientation. ~30 Ma: Block rotation and north-south crustal extension NNE-striking (060-100°) normal faults correspond to a period of NS extension at ~ 30 Ma (Wells, 1990) that caused horst-and-graben style faulting in the North Fork area. These late-stage faults cut all lithologies, types of mineralization, and patterns of alteration. Based on the new U-Pb age data for igneous rocks hosting the North Fork deposit, the NS extension in the deposit area must be at least 36.6 Ma. This late-stage extensional event may represent Cenozoic extension associated with a bulk change to oblique subduction of oceanic plates to the west (Atwater, 1989). This resulted in basin-and-range geology in Oregon and California (Wells, 1990). A thermally elevated basin-and-range region would exert a ridge-push force against the southern part of the Cascade arc and cause it to rotate clockwise at a fixed point (the Olympic-Willamette Lineament; OWL) south of the North Fork deposit (Lasmanis, 1995). Rotation around the OWL and southerly movement of the rotating Willamette block would account for the EW extensional structures in the North Fork area. East-west-striking lamprophyre dikes mapped in the northern and southeast portions of the map area appear to have utilized these faults to ascend to shallow crustal levels (Figs. 2.13 and 2.14). 45 DISCUSSION Application of the reduced porphyry copper-gold model to the North Fork deposit. The mineralogy and geochemistry of the magmatic rocks in the North Fork study area are typical of intermediate to felsic igneous rocks formed in a volcanic arc environment. Rocks are calc-alkaline (with minor exceptions), weakly to moderately peraluminous, and have amphibole as the predominant mafic silicate. A volcanic arc setting is further supported by the relative abundances of trace-elements such as Rb, Y, Nb, Zr, and Ga (e.g., Pearce et al., 1984; Whalen et al., 1987; Christiansen and Keith, 1996), highly fractionated REE profiles, and La/Yb c n ratios ranging from 5.9 to 9.1 (e.g., Cullers and Graf, 1984; Wilson, 1989; Rollinson, 1993). Finally, the volcanic arc setting identified for the North Fork area in this study is entirely consistent with the documented arc magmatism in this section of the Tertiary Cascade Range (e.g., Tabor et al., 1993). The /TVs of the andesites, however, are at QFM-1, which is at the lower end of the range for intermediate to mafic arc magmas (Carmichael, 1991). The /Da's of the quartz monzodiorites are somewhat higher than those of the andesites and lie between QFM and QFM+1 (~NNO). This range is much lower than that of typical magnetite-series or oxidized I-type felsic magmas found in most volcanic arcs (e.g., Burnham and Ohmoto, 1980), but is consistent with the relatively low /TVs of the andesites with which they share a consanguineous relationship. Consanguinity is implied from the similarity of REE profiles, inferred tectonic setting, and late Eocene ages of formation. However the lack of systematic increase in both the La/Yb c n ratios and the absolute abundances of light LREE precludes the possibility that plutonic and volcanic rocks are part of a simple amphibole fractionation series from a common parental magma (e.g., Cullers and Graf, 1984; Tsususe et al., 1987; Rowins et al., 1993). It appears that the felsic to 46 intermediate plutonic rocks have intruded into their own volcanic pile 1 to 2 m.y. after the start of andesitic volcanism and early arc construction at ~39 Ma. The geological and geochemical characteristics of the magmas discussed above can have significant ramifications in terms of both ore-forming processes and the distribution of Cu and Au at the North Fork deposit. Porphyry Cu-Mo±Au deposits in magmatic arcs (e.g., Highland Valley, Chuquicamata, El Salvador) typically form from fluids exsolved from oxidized I-type or magnetite-series granitoids. These oxidized ore fluids precipitate hypogene sulfide minerals (e.g., pyrite, chalcopyrite, and bornite) in equilibrium with primary magnetite, hematite, and anhydrite. In contrast, the North Fork deposit has formed from ilmenite-series or reduced I-type granitoids, which are classified as "reduced" if they have magmatic /Ch's less than the NNO buffer (Blevin and Chappell, 1995; Pollard et al., 1995). Hypogene Cu-Au mineralization formed from fluids exsolved from these reduced granitoids is associated with abundant pyrrhotite, but no hematite or anhydrite. Rowins (2000a, b) termed these "Reduced Porphyry Cu-Au deposits" (RPCG) and the North Fork deposit has all the characteristics of this new porphyry subclass. Interestingly, a number of porphyry Cu (Au) deposits in close proximity to the North Fork deposit (i.e., Middle Fork and Quartz Creek) contain large amounts of hypogene pyrrhotite, and have all the necessary features (Lasmanis, 1995) to tentatively classify them as RPCG deposits as well. In fact, the abundance of pyrrhotite in many of these Cascadian porphyry deposits lead Hollister (1978) to refer to them as "pyrrhotite porphyries" and Rowins (2000a, b) proposed they very likely constitute a RPCG province or district. As explained in Rowins (2000a, b), RPCG deposits commonly have smaller tonnages and lower grades of Cu compared to the classically oxidized variants because of the decreased solubility of Cu in relatively reduced hydrothermal fluids at the P-T-X conditions of the porphyry Cu environment (e.g., Hemley et al., 1992). The solubility of Au, however, is relatively insensitive to the redox state of the porphyry fluid (Gammons and Williams-Jones, 1997) and RPCG systems may have similar quantities of Au. 47 Consequently, subeconomic RPCG deposits like North Fork may have unrecognized Au potential, especially in the distal regions of the deposit. Some RPCG deposits are surrounded by distal Au occurrences up to 5 or 6 km away which is consistent with analytical evidence that Cu, and by analogy Au, preferentially partition into the diffuse vapor phase during fluid boiling in reduced hydrothermal systems (Heinrich et al, 1992,1999; Rowins 2002). This enhanced vapor phase mobility of Cu and Au is interpreted to produce a low grade Cu core but distal zones of / Cu-Au enrichment (Rowins, 2000a, b). This scenario is particularly applicable to the North Fork deposit as it is surrounded by many low-sulfidation epithermal Au-Ag veins, some of which have been mined for Au (i.e., Lennox Creek, Blackhawk, Apex, Damon, and Cleopatra; Derky et al., 1990; Fig. 2.2). In addition, North Fork is a relatively Au enriched porphyry deposit with a 218,000 oz Au reserve (Herdrick et al., 1995). Relationship between mafic latite porphyry, NNW-striking structures, hypogene mineralization and associated alteration Hypogene Cu-Au mineralization and associated zones of hydrothermal alteration are developed most strongly within mafic latite porphyry and adjacent andesite. In contrast to the inference drawn from previous reconnaissance mapping by Herdrick et al. (1995a), the quartz monzodiorite is unmineralized except where intruded by mafic latite porphyry. The brittle structures within the deposit area have strikes that define three main structural trends (350°-010°; 320°-340°; 030°-060°). The hypogene Cu-Au mineralization, however, is spatially associated with mafic latite porphyry along the NNW-striking (320°-340°) brittle structures indicating that they have focussed both the emplacement of mafic latite dikes and ascending hydrothermal fluids. This relationship between mafic latite porphyry, NNW-striking structures, hypogene 48 mineralization and associated alteration explains why concentric zones of overprinting alteration and mineralization are not distributed around any one porphyry stock or porphyry cluster. No single intrusion has acted as a thermal anomaly or "point source" from which magmatic fluids emanate in a more-or-less concentric pattern (i.e., the fluid flowlines in the classic porphyry model of Lowell'and Guilbert, 1970). The lack of success experienced by Herdrick et al. (1995a) in attempting to map concentrically distributed outward zones of potassic, phyllic, and propylitic alteration from the "centre" of the North Fork deposit is now readily explained. The North Fork deposit has formed from numerous batches of mafic latite porphyry emplaced as mutually crosscutting dikes in the NNW-striking fault/fracture zones. Repeated use of these structural conduits has resulted in overprinting episodes of mineralization, alteration, and magmatism creating the deposit presently exposed. Similar use of a well-developed fracture system to focus consecutive episodes of magmatism and hydrothermal fluid flow has occurred at the Yerington porphyry Cu deposit (Dilles et al., 2000). Mapping at North Fork has revealed that the Main-stage banded and crustified quartz-albite-actinolite-chlorite-sulfide veins with sodic-calcic alteration (quartz-actinolite-albite-chlorite) selvages cut Early-stage quartz-potassium-feldspar-biotite veins and potassic (quartz-biotite-K-feldspar) alteration. If sodic-calcic alteration is derived from a magmatic fluid, then such an alteration sequence is suggestive of a thermally "prograding" system, because sodic-calcic alteration forms at higher temperatures than potassic alteration in the porphyry environment (Orville, 1963; Carten, 1986; Lang et al., 1995). Although it has been proposed that sodic-calcic alteration in the porphyry environment may be derived from an evaporitic source without a magmatic component (Barton and Johnson, 1996; Dilles et al., 1995), the restriction of sodic-calcic alteration to mafic latite porphyry and adjacent andesite, together with an absence of evaporatic rocks in the North Fork area (Tabor et al., 1993), suggest 49 that sodic-calcic alteration is related to high-temperature magmatic fluids. A similar conclusion has been drawn for numerous alkalic porphyry Cu-Au deposits in the Canadian Cordillera. Galore Creek, Mount Polley, and Cu Mountain (Lang et al., 1995) all possess multiple stages of high-temperature sodic-calcic alteration, which have a strong spatial and temporal correlation with intrusive centres of restricted composition. In summary, hydrothermal alteration and Cu-Au mineralization at the North Fork deposit is related to emplacement of successive batches of mafic latite porphyry magma having slightly different compositions and fluid saturation temperatures, rather than the effects of cooling of a single pulse of magmatic fluid or the involvement of formational fluids of evaporitic origin. Implications for Cu-Au metallogeny of the Mid-Tertiary Cascade magmatic arc and exploration in the North Fork area The -37 Ma North Fork deposit represents a newly recognized magmatic event intimately associated with RPCG mineralization. The previously published K-Ar age of-10 Ma (Hollister and Baumann, 1979) for the mineralization is either a reset age or in error due problems inherent with the K-Ar dating technique. This magmatic event has several important implications for Cu-Au metallogeny and mineral exploration in this part of the Cascade arc. The I-type magmas associated with the Cu-Au mineralization at North Fork are much more reduced than typical I-type arc magmas. The oxidation state of the North Fork magmas are similar to those of S-type granitoids associated with Sn-W deposits (e.g., Chappell and White, 1974), although they are not chemically or compositionally equivalent to "two-mica" S-type granites containing primary garnet, muscovite, and cordierite (Takamashi et al., 1980; Clarke, 1981). It is possible that the ultimate upper mantle source region for these magmas was anomalously 50 reduced (cf., Pollard et al., 1995). This would explain the widespread occurrence of RPCG systems in this part if the Cascade arc (i.e., Middle Fork, Quartz Creek, and Glacier Peak) and the recognition by Hollister (1978) more than 25 years ago that "pyrrhotite is unusually abundant in porphyry Cu-Mo±Au deposits from the Cascade Range". Structural analysis and kinematic reconstruction of the North Fork study area reveals that fracture/fault formation and hydrothermal activity is closely associated with the emplacement of mafic latite porphyry in space and time. All features are linked to the same mid-Tertiary tectono-magmatic event at 39-37 Ma. The NNW-striking (320°-340°) fractures and faults that focused ore fluids in the near-porphyry environment may have also facilitated their movement to distal areas up to 5 km away from the source magma. Interestingly, low-sulfidation epithermal Au veins (the Lennox Creek and Blackhawk mines), situated 3 to 4 km south-southeast from the North Fork deposit are on strike with the NNW-striking brittle structures (Fig. 2.2). Although Ar-Ar dating (next chapter) reveals that the Lennox Creek epithermal Au-Ag mineralization is Early Miocene in age, the NNW-striking structures have clearly focussed hydrothermal fluids and possibly magmas for many millions of years. Consequently, these NNW-striking structures should be viewed as important Cu-Au exploration targets in the west central Cascades Range. The ~37 Ma age of the North Fork deposit demonstrates that Cu-Au mineralization in the Cascade arc commenced much earlier than previously reported. Prior to this study, the oldest age of the porphyry Cu-Mo±Au deposits in the Cascades Range was 18-25 Ma. This Miocene magmatic event produced the Snoqualmie batholith and supposedly the porphyry Cu (Au) deposits at Glacier Peak, Quartz Creek and Middle Fork (Lasmanis, 1995). Perhaps most importantly, the ~37 Ma age of mafic latite porphyry and quartz monzodiorite magmatism is only 1-2 m.y. younger than the age of Mt. Persis andesite. This is significant from an exploration point of view because the Mt. Persis andesite has never been considered prospective because it was so much older than the porphyry Cu-Au mineralization. These new U-Pb ages together with 51 the geochemical and petrological data showing a consanguineous relationship exists between the plutonic and volcanic rocks makes the M t . Persis Group an attractive first-order exploration target for C u - A u mineralization. Its extensive exposure raises the possibility that Late Eocene porphyry C u - A u mineralization may be far more common in the west central Cascades than has been predicted from the amount of quartz monzodiorite and mafic latite porphyry exposed. CONCLUSIONS The geological and geochemical features that characterize the North Fork deposit reveal the importance of both plutonic and volcanic rocks in the genesis of reduced porphyry C u - A u mineralization. Detailed field mapping, petrographic mineral mode analysis, whole-rock geochemistry, and five new U-Pb (zircon) age determinations show that three main rock units host the deposit. The oldest and most spatially extensive unit is the Mount Persis andesite. It has been intruded by quartz monzodiorite and mafic latite porphyry. Amphibole is the predominant mafic silicate in plutonic units and ilmenite occurs in greater abundance than magnetite. Major-element whole-rock data indicate that the plutonic and volcanic rocks are calc-alkaline and weakly to moderately peraluminous. Chondrite-normalized R E E profiles for plutonic and volcanic rocks are highly fractionated and remarkably similar. On N b - Y and Rb-Y+Nb tectonic discrimination diagrams, the plutonic and volcanic rocks plot well within the volcanic-arc field. The similarity of geochemical profiles, inferred tectonic setting, and ages of formation all suggest that the plutonic rocks are I-type granitoids which have intruded into their own volcanic pile during construction of a late Eocene calc-alkaline volcanic arc. In contrast to magnetite-series or oxidized I-type felsic magmas associated with classic porphyry Cu-Mo±Au deposits, this study demonstrates that all magmatic rocks in the North Fork 52 study area are relatively reduced, with the quartz monzodiorites and, by inference, the mafic latite porphyry classified as "reduced" I-type magmas. The deposit also exhibits a number of key features consistent with classification as a reduced porphyry Cu-Au deposit. These features include the existence of substantial hypogene pyrrhotite in all stages of mineralization, an absence of primary hematite and sulphate minerals, and a close association with reduced I-type ilmenite-series granitoids. Hypogene Cu-Au mineralization is developed most strongly within mafic latite porphyry and adjacent andesite. Quartz monzodiorite is largely unmineralized except where intruded by dikes of mafic latite porphyry. Although three stages of vein formation are recognized, the banded and crustified Main-stage quartz-albite-actinolite-chlorite-sulfide veins host most of the hypogene mineralization. Sodic-calcic alteration (quartz-actinolite-albite-chlorite) surrounds these banded and crustified veins, with potassic (quartz-biotite-K-feldspar) and phyllic (quartz-sericite) types of alteration associated earlier and later stages of vein formation, respectively. They have only limited hypogene Cu-Au mineralization. Low abundances of primary sulfide minerals (especially pyrite) together with climatic factors prevented the production of the large volumes of sulfuric acid necessary for the development of zones of strong secondary supergene Cu and Au enrichment. Rare Cu oxide minerals, preferentially preserved in zones of Main-stage vein formation associated sodic-calcic alteration, result from the effects of surficial oxidation. The secondary oxidation of hypogene Cu sulfide minerals is very localized and does not form continuous zones of supergene Cu-Au enrichment. The brittle structures (faults and fractures) measured in the North Fork deposit area indicate three episodes of deformation. The oldest at ~ 39 Ma (350°-GT0°) and youngest ~ 30 Ma (030°-060°), however, are not directly associated with the formation of the North Fork deposit. Only the NNW-striking structures (320°-340°) at -37 Ma are closely associated with 53 emplacement of the mafic latite porphyry and development of overprinting zones of alteration and strong hypogene mineralization. These 39 to 37 Ma structures are part of a newly recognized magmatic event associated with reduced porphyry Cu-Au mineralization in the mid-Tertiary Cascade arc. The geological and geochronological data demonstrate that Cu-Au mineralization commenced much earlier than the Miocene magmatic event at 18-25 Ma, which produced the Snoqualmie batholith and several other porphyry Cu-Au deposits. The recognition that North Fork is a RPCG deposit, along with the identification of through going fluid channel ways on strike with low-sulfidation epithermal vein occurrences, raises the possibility that distal vein-hosted Au occurrences may represent a distal Au halo associated with the reduced porphyry Cu-Au mineralization. Furthermore, the NNW-striking structures may have remained important as conduits for millions of years focusing later Miocene reduced porphyry Cu-Au, and epithermal Au-Ag mineralization. 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Wilson, M., 1989, Igneous Petrogenesis'- A Global Tectonic Approach. Unwin Hyman, London, UK. Wones, D. R., and Eugster, H. P., 1965, Stability of biotite: experiment, theory and application. American Mineralogist, v. 50, p. 1228-1272. 61 Figure captions Figure 2.1. (a) The North Cascades physiographic province of northern Washington state incorporating portions of the Coast, Intermontane and Omenica belts that extend southward from British Columbia, (b) Geological domains of the North Cascades physiographic province. Cretaceous to Miocene porphyry Cu-Mo-Au deposits, represented by sold circles, are distributed across the Cascades volcanic arc. The North Fork deposit is located within the Western Melange belt, which hosts several other "pyrrhotite" porphyry copper-gold deposits with geological similarities to the North Fork deposit. The Darrington-Devils Mountain fault zone is abbreviated as D D M F . Geology adapted from Haugerud et al. (1994), Lasmanis (1995) and Monger and Journey (1994). Figure 2.2. Simplified geological map of the North Fork study area with the main geologic rock units. Solid black triangles indicate significant porphyry Cu and epithermal Au in the area. Open rectangle outlines the area of the North Fork deposit mapped in detail. Figure 2.3. Simplified geological map showing the distribution of major fractures and faults at the North Fork deposit. Figure 2.4. (a) Northeast cross-section A - A ' across the North Fork deposit as shown in figure 2.3. (b) Northwest cross-section B-B' across the North Fork deposit as shown in Figure 2.3. (c) Expanded portion of the cross-section B-B', shown as C - C through the part of the deposit with the highest copper grades and strongest sodic-calcic alteration. 62 Figure 2.5. (a) Photomicrograph of massive, fine-grained andesite with seriate texture defined by randomly oriented hornblende (Hbi), potassium-feldspar (Ksp), plagioclase (PI) and ilmenite (Iim). (b) Hand specimen photograph of porphyritic textured andesite defined by phenocrysts of plagioclase and hornblende in a massive very fine-grained groundmass. (c) Photomicrograph of seriate texture defined by biotite (Bt), plagioclase, quartz (Qtz), potassium-feldspar in quartz monzodiorite from the southeast exposure, (d) Hand sample photograph of quartz monzodiorite showing typical equigranular texture defined by interlocking grains of hornblende, plagioclase and quartz, (e) Photomicrographs of mafic latite porphyry showing typical porphyritic texture defined by plagioclase, hornblende and highly resorbed quartz in a fine-grained matrix of similar mineralogy, (f) Hand specimen photograph of mafic latite porphyry showing the highly rounded and resorbed nature of quartz phenocrysts and tabular nature of plagioclase phenocrysts. Mineral abbreviations after Kretz (1983). Figure 2.6. IUGS classification and nomenclature of plutonic rocks (Streckeisen, 1973) from the North Fork deposit based on modal percentages of primary alkali feldspar, plagioclase, and quartz. Samples 4 (SMR01-108) and 10 (SMR01-106) correspond Table 2. 1 - NF-OC-223; 2 - SMR01-11; 3 - NF-OC-064; 4 - SMR01-108; 5 - NF-OC-201; 6 - NF-OC-179; 7 - NF-OC-396; 8 - NF-OC-397; 9 - SMR01-113; 10 - SMR01-106; 11 - N F - O C -266; 12 - NF-OC-285; 13 - NF-OC-179; 14 - NF-OC-230. Figure 2.7. Whole-rock geochemical and tectonic discrimination diagrams for igneous rocks from the North Fork area. Legends for all diagrams are listed in figures 7a and 7b. (a) Total alkalis versus silica (TAS) diagram with superimposed alkaline and subalkaline fields after Irvine and Baragar, (1971). (b) TAS diagram for plutonic rocks after Wilson 6 3 (1989). (c) Subdivision of subalkaline igneous rocks into high-, medium-, and low-K series using K 2 0 versus S i0 2 fields after Lemaitre et al. (1989) and Rickwood (1989). (d) Metaluminous versus peraluminous classification of igneous rocks from the North Fork area (after Maniar and Piccoli, 1989 and Shand, 1927). (e) Chondrite-normalized rare earth element patterns for igneous rocks from the North Fork area. Rare earth element data have been normalized to the chondrite values of Nakamura (1974). (f) Nb versus Y tectonic discrimination diagram for igneous rocks from the North Fork area. Fields after Pearce et. al. (1984). (g) Rb versus Y+Nb tectonic discrimination diagram for igneous rocks from the North Fork area. Fields after Pearce et. al. (1984). (h) Ga/Al versus Zr granite discrimination diagram for igneous rocks from the North Fork area. Field after Whalen et al. (1987). Note: The andesite that plots in the "dacite" field (Figure 7c) and in the "peraluminous" field (Figure 7d) is likely altered. The quartz monzodiorite in the "medium K " field (Figure 7c) has also likely suffered some degree of silica alteration. Figure 2.8. Log/C>2 versus temperature with biotite-ilmenite pairs from unaltered andesite and quartz monzodiorite plotted at isopleth intersections. Isopleths of [Fe/(Fe+Mg)] * 100 (solid lines numbered 30-100) from biotite (annite component) are from the experimental data of Wones and Eugster (1965). Isopleths of the hematite component in ilmenite (dashed lines labeled "hem%/ilm%") are from the experimental data of Buddington and Lindsley (1974). Andesites from the North Fork area plot at/th's approximately QFM-1, whereas those of the quartz monzodiorites lie between Q F M and QFM+1 or the NNO buffer (see inset diagram). These oxygen fugacities are lower than those of andesite and oxidized I-type felsic magmas associated with classic porphyry Cu-Au deposits (e.g., Burham and Ohmoto, 1980) 64 Figure 2.9. U-Pb concordia diagrams for plutonic and volcanic rocks at the North Fork deposit. All samples excluding a sample of the Mount Persis andesite (SMR01-110) show multiple concordant fractions and, therefore, ages are therefore considered conclusive, (a) SMR-106; mafic latite porphyry, (b) SMR-108; quartz monzodiorite. (c) SMR-111; quartz monzodiorite. (d) SMR-113; mafic latite porphyry, (e) SRM-110; Mount Persis andesite. Figure 2.10. Photographs and photomicrographs of the different types of veins and resulting styles of alteration identified at the North Fork deposit, (a) Photomicrograph of secondary quartz (Qtz) and potassium feldspar (Ksp) replacing primary phenocrysts of plagioclase (PI) and hydrothermal biotite (Bt) replacing mafic minerals in the groundmass (b) Early-stage quartz-potassium feldspar-biotite-sulphide vein with weak alteration halo of biotite-potassium feldspar-quartz, (c) Outcrop photograph of Early-stage quartz-potassium feldspar-biotite-sulfide with visible alteration halo of secondary potassium feldspar-quartz-biotite. Magnet-pen for scale, (d) Photomicrograph of secondary quartz and potassium feldspar replacing primary phenocrysts of plagioclase. In the groundmass mafic and felsic minerals are replaced by an assemblage of biotite-quartz-sericite (Ser)-potassium feldspar, (e) Photomicrograph of cross-cutting Early-stage quartz-sulphide veins which define a vein stockwork zone, (f) Hand specimen photograph of cross-cutting Early-stage quartz-sulphide vein stockworking. (g) Photomicrograph of sodic-calcic alteration in porphyryritic andesite with the assemblage of quartz- albite (Ab)-actinolite (Act)-chlorite (Chl)-epidote (Ep) replacing a primary phenocryst of hornblende, (h) Photomicrograph of banded actinolite-quartz-albite-chlorite-sulfide vein with characteristic wallrock parallel growth bands of alternating sulphide and silicate minerals, (i) Hand specimen of Main-stage banded quartz-albite-actinolite-chlorite-sulfide vein. 65 Figure 2.10 cont. (j) Outcrop photograph of intense sodic-calcic alteration defined by large clots of secondary quartz-albite-actinolite-chlorite surrounded by weathered feldspar (i.e., kaolinite -Kin) and hydrothermal quartz. This intense style of sodic-calcic alteration, or "pseudobreccia", was mapped previously as "hydrothermal breccias" by Herdrick et. al. (1995a). (k) Photomicrograph of Main-stage crustified quartz-albite-actinolite-chlorite-sulfide vein with characteristic inward growth of quartz and feldspar grains. Actinolite, chlorite, tourmaline and sulfide minerals fill the interstices. (1) Photograph of Main-stage crustified quartz-albite-actinolite-chlorite-sulfide vein with intense alteration selvage of quartz-albite-actinolite-chlorite. (m) Photomicrograph of strong phyllic alteration defined by an assemblage of quartz-sericite-pyrite (Py). (n) Photomicrograph of Late-stage quartz-sericite grain with alteration selvage of quartz and sericite. (o) Hand specimen photograph of texturally destructive quartz-sericite altered rock, (p) Photomicrograph of secondary calcite-quartz-sericite replacing a primary phenocryst of plagioclase in mafic latite porphyry, (q) Photomicrograph of Late-stage calcite-chlorite vein with calcite-chlorite and sericite alteration halo, (r) Hand sample photograph of calcite-quartz veins and calcite veinlets in mafic latite porphyry. Mineral abbreviations after Kretz (1983). Figure 2.11. Distribution alteration types and relative intensities associated with Early-, Main-, and Late-stages of vein mineralization. Alteration intensity was determined in the field according to modal percentages of alteration minerals. Alteration maps clearly show a strong positive correlation between mafic latite porphyry, NNW-striking fractures and faults, and distribution of alteration, (a) Alteration intensity contour map potassic alteration. Zones of the strongest potassic alteration correlate with Early-stage quartz-potassium feldspar-biotite and quartz-sulfide veins, (b) Alteration intensity contour map 66 sodic-calcic alteration. Zones of the strongest sodic-calcic alteration correlate with Main-stage quartz-albite-actinolite-chlorite-sulfide veins, (c) Alteration intensity contour map phyllic alteration. Zones of the strongest phyllic alteration correlate with Late-stage quartz-sericite veins. Figure 2.12. A compilation of brittle structural elements presented on the rose diagrams. First generation fractures and faults (~39-37 Ma) correlate with extensional structural developing at deep crustal levels in response to slab roll-back. These structures are inferred be synchronous with the formation of the Mount Persis andesites. Second generation brittle structures are consistent with pressure shears associated with dextral strike-slip motion on the Straight Creek fault at ~37 Ma. The third generation brittle elements are associated with rotation of the down-going slab at ~ 30 Ma. These fractures and faults cut all lithologies, alteration and mineralization trends at the North Fork deposit. Figure 2.13. Simplified structural map of the North Fork deposit. Structural data have been divided into first, second, and third generation structures based field relationships and documented extensional and contractional stresses active during the late Eocene in Washington state (see text). Figure 2.14. (a-c) Schematic model of the tectono-magmatic evolution and formation of the North Fork copper-gold deposit. See text for details. 67 Figure 2.1. [Smithson et. al., 2004] Mineral deposit Figure 2.2. [Smithson et. al., 2004] 68 Figure 2.3. [Smithson et. al., 2004] 69 Figure 2.5. [Smithson et. al., 2004] 71 Figure 2.6. [Smithson et., al. 2004] 7 2 16 N so ox 12 10 o s + 6 o 4 z 2 0 & Mafic latite porphyry <^  Mount Persis andesite Alkaline R h y o l i t e (b) j 1 2 O 9 + 6 o a Z 3 • Blackhawk diorites • Lennox Creek granodiorite to meladiorite A North Fork granodiorites/quartz monzodiorites 35 40 45 50 55 60 65 70 75 Si0 2 (wt. %) Si0 2 (wt. %) O HighK (calc-alkaline series) s ' Medium K (Calc-alkaline series) • • • A 0 LowK A (tholeiite series) (d) o £ + < Metaluminous Peralkaline 60 70 Si0 2 (wt. %) 0.4 0 5 Peraluminous A _1 I i i _ Al/(Ca+Na+K) molar -a c o •gl 22 U 10 o I— UJ (f) L a C e P r N d P m S m E u G d T b D y H o E r T m Y b L u I M 1 111] 1 I I 11 l l [ 1 1/1 111 l l [ 1000 r j y n - C O L G / W P G E £ 100 3 /-^ \ / 1 Rb / 10 I V A G / O R G — i I f 1 1 Y+Nb (ppm) Ga / A l Figure 2.7. [Smithson et. al., 2004] 73 400 500 600 700 Temperature (°C) 800 930 goo %o IO'OO Figure 2.8. [Smithson et. al., 2004] 74 Figure 2.9. [Smithson et. al., 2004] 75 76 77 1 Early-stage alteration ^ fl -.. X " * X X X ; X / X X X W / \ y *• y Lithologic Key Mafic latite porphyry Quartz monzodiorite Andesite (undifferentiated) Normal fault Alteration Key a. Potassic alteration intensity Strong alteration ( 10-20 %Ksp-Bt) Moderate alteration (5-10%Ksp-Bt) Weak alteration ( < 5 % Ksp-Bt) Alteration Key b. Sodic-Calcic alteration intensity Strong alteration ( 10-20%Act-Ab) Moderate alteration (5-10%Act-Ab) Weak alteration (<5 %Act-Ab) Alteration Key c. Phyllic alteration intensity Strong alteration ( 20-45 % Qtz-Ser-Py ) Moderate alteration (5-20%Qtz-Ser-Py) Weak alteration ( < 5 % Qtz-Ser-Py) Figure 2.11. [Smithson et. al., 2004] 78 Fault data .. N=46 N=31 N=93 Small faults in Mount Persis andesite Fracture data ft T Small faults in quartz monzodiorite. Small faults in mafic latite porphyry. N=24 Mount Persis andesite. Quartz monzodiorite. Mafic latite porphyry Large faults in all intrusive and extrusive rocks Structural Elements 1st generation (-39-37 Ma). 350-010° striking faults and fractures. 2nd generation. (-37 Ma) 320-340° striking faults and fractures. 3rd generation. (~30Ma) 60-100" striking extensional fractures and faults. Figure 2.12. [Smithson et., al. 2004] Figure 2.13. [Smithson et., al. 2004] 79 a. -39-37 Ma Eruption of Mount Persis andesite at~ 39 Ma. may have resulted from extension associated with slab roll-back. Ascent of andesite to shallow crustal levels via extensional structures created during roll-back or pre-existing N-striking faults basement melange rocks. Steeply imbricated and rotated accretionary wedge metasedimentary rocks. Faulting associated with deep crustal extension or accretionary wedge imbrication of basement melange were possible conduits for monzodioritic rocks to intrude at -37 Ma to more shallow crustal levels Eocene steepening of the down-going slab causes slab roll-back and extension in the deep crust. Straight Creek fault (SCF) was active during the middle Eocene. c. -25 - 30 Ma ENE normal faulting associated with extension at deeper crustal levels produces horst-and-graben structures and minor block rotation. 20" clockwise rotation Cenozoic extension (in basin- and- range province) has contributed substantially to clockwise rotation of western Oregon and southeast Washington state causing local extension. Lamprophyre dykes ascend through the crust along normal faults that cross-cut all lithologies mineralization, and related alteration. The SCF is inactive by the end of the Eocene as indicated by stitching plutons along its length. Figure 2.14. [Smithson et. al., 2004] 80 TABLE 2.1. MODAL MINERALOGY OF PLUTONIC AND VOLCANIC ROCK-UNITS ROCK UNIT* MPA QMD MLP Primary minerals" Phenocryst Groundmass Phenocryst Groundmass Plagioclase 20-30 30-35 45-60 20-30 50-60 Quartz 15-25 15-25 5-25 7-10 15 Potassium feldspar -15 — 15-20 20 Hornblende 3-5 20-25 5-40 3-10 3-7 Biotite <l-3 10-15 5 3-5 <1 Ilmenite 1.5-3 <l-3 3-5 <2 Magnetite <1 <1 <1 <1 Accessory minerals' Titanite <1 <1 Zircon Tr Tr Apatite — <1 <1 Table 2.1. Modal ranges are based on 300 to 500 counts per thin section in conjunction with estimation techniques for coarse-grained samples. Tr = trace amounts. (*) Rock units are as follows: MPA, Mount Persis andesite; QMD, quartz monzodiorite; MLP, mafic latite porphyry. (§) Mineral abundances are in modal percent. Table 2.1. [Smithson et. al., 2004] 81 TABLE 2.2. REPRESENTATIVE MAJOR- AND TRACE-ELEMENT DATA FOR PLUTONIC AND VOLCANIC ROCKS, NORTH FORK, W.A. Sample* 125 123 122 118 117A 117B 120 116 115A 114 108 111 110 105 104 106 Unit 8 BHM MLP MLP LCMD LCMD LCMD LCMD QMD QMD QMD QMD QMD MPA QMD QMD QMD Oxides (wt. %) Si0 2 60.46 58.68 61.69 63.97 62.64 57.79 53.47 61.13 65.69 61.22 62.24 64.63 57.52 62.50 58.18 59.77 TiOj 0.70 0.63 0.65 0.27 0.64 0.71 1.46 0.83 0.72 0.84 62.24 0.73 0.83 0.80 0.92 0.89 AI 2O a 16.05 15.54 15.80 5.27 16.16 16.44 16.97 15.80 14.62 15.41 15.69 15.31 16.57 15.43 16.49 15.44 FeO 4.27 0.46 not/ss 4.25 • 3.99 4.80 7.02 3.81 2.63 3.54 2.81 2.68 4.70 3.14 3,15 3.79 Fe 20 3 6.37 5.29 5.28 6.30 5.74 6.92 10.12 6.38 5.07 6.52 5.57 5.47 7.13 6.58 6.74 7.28 MnO 0.11 0.09 0.09 0.39 0.07 0.10 0.24 0.26 0.16 0.11 0.07 0.08 0.12 0.08 0.07 0.15 MgO 3.06 1.79 2.20 2.97 2.51 3.68 3.96 3.10 1.45 3.16 2.76 1.99 3.10 2.41 2.95 2.65 Cr 2Oj <0.01 <0.01 <0.01 O.01 <0.01 O.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 CaO 6.12 5.01 4.43 7.56 5.34 6.80 6.93 6.31 3.70 6.09 5.56 4.36 6.23 4.79 4.43 5.62 Na 20 3.19 2.90 3.17 0.01 3.20 3.54 3.67 3.58 3.48 3.82 4.03 3.57 3.02 3.79 5.07 2.90 K 2 0 1.68 0.22 1.47 1.35 1.64 1.42 0.72 0.38 1.93 0.73 0.65 1.48 1.92 0.72 0.80 0.79 P205 0.14 0.10 0.13 0.05 0.13 0.15 0.27 0.13 0.15 0.14 0.14 0.13 0.17 0.15 0.18 0.20 BaO 0.04 O.01 0.03 <0.01 0.03 0.01 <0.01 <0.01 0.04 0.02 <0.01 0.04 0.01 <0.01 0.01 0.04 SrO 0.03 0.02 0.03 O.01 0.03 0.03 0.04 0.03 0.03 0.02 0.03 0.03 0.03 0.02 0.04 0.05 L O I f 1.10 8.67 4.14 10.57 0.92 1.22 0.51 1.11 1.69 0.92 1.26 1.22 2.24 1.69 2.97 3.48 Total 99.05 98.94 99.11 98.70 99.05 98.81 98.36 99.04 98.73 99.00 98.84 99.04 98.89 98.96 98.85 99.26 Trace elements (ppm) Ag <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 Au <1 2 <1 1 <1 <1 1 <1 <1 7 5 420 1 2 <1 1 Ba 444 113.5 359 180.5 359 285 230 252 459 266 167.5 422 238 218 251 424 Ce 33 34 34.5 9.5 30 33 28 36 54 42.5 49 46 39.5 45.5 46.5 40.5 Co 31.5 35.5 20 27.5 26 27 39 30.5 27.5 35.5 27.5 30 26.5 33 24.5 27 Cs 3.3 0.9 0.4 3.6 7 6.3 0.5 0.2 0.2 0.2 0.3 0.7 0.5 0.1 0.5 0.4 Cu 50 40 45 25 75 40 120 65 40 55 295 20 135 1035 765 105 Dy 3.7 3 2.8 1 2.9 4 4.6 4 4.9 4.1 4.3 4.3 3.2 4.2 3.4 3.1 Er 2.2 1.6 1.7 0.6 1.8 2.6 3 2.3 3.1 2.5 2.6 2.6 2 2.6 2.1 1.8 Eu 0.9 1 1 1.2 0.8 0.9 1.7 1.2 1.4 1.2 1.2 1.2 1.2 1.2 1.4 1.1 Ga 17 15 19 7 20 20 22 20 20 21 21 20 22 21 23 21 Gd 3.9 3.4 3.2 1.2 3.4 4.1 5.4 4.4 5.8 4.9 5 4.6 3.9 4.9 4.4 3.8 Hf 4 4 3 <1 4 3 4 6 8 6 5 6 4 6 5 4 Ho 0.8 0.6 0.6 0.2 0.7 0.9 1.1 0.9 1 0.8 0.9 0.9 0.7 0.9 0.8 0.7 La 15.5 18 19 5 14 14 11 17.5 26 20.5 22.5 22.5 20.5 21 22.5 20 Lu 0.3 0.2 0.3 <0.1 0.3 0.4 0.4 0.3 0.4 0.4 0.3 0.4 0.3 0.4 0.3 0.3 Nb 7 7 8 3 7 7 11 10 14 12 11 12 10 11 10 9 Nd 17.5 15 16 4.5 15 19 20 20 27.5 22 24 23 19 23.5 24 20.5 Ni 20 25 25 10 15 20 40 40 15 45 30 25 45 30 40 30 Pb 5 5 5 5 5 10 25 15 20 25 15 5 15 5 15 5 Pr 3.8 3.6 4 1.1 3.4 4.2 3.9 4.4 6.3 4.9 5.6 5.3 4.5 5.3 5.4 4.5 Rb 53.6 5.6 37.4 40 72.8 71.6 14.8 6.6 35.2 15.2 17.2 29.4 46.4 18.6 26.2 17.8 Sm 3.8 3 3.3 1.1 3.2 4.4 4.8 4.4 5.6 4.5 5 5 3.9 4.9 4.4 4.1 Sn <1 <1 <1 <1 1 7 <1 <1 4 <1 5 1 <1 2 <1 <1 Sr 254 200 286 64.2 266 279 359 299 249 248 261 249 288 229 294 422 Tb 0.6 0.5 0.5 0.1 0.6 0.7 0.9 0.7 0.9 0.7 0.8 0.8 0.6 0.8 0.6 0.6 Th 4 3 4 <1 5 4 <1 3 5 3 3 4 3 3 2 3 TI <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 Tm 0.4 0.3 0.3 <0.1 0.3 0.5 0.5 0.4 0.5 0.4 0.4 0.4 0.3 0.4 0.3 0.3 U 2 1 1.5 <0.5 2 1.5 <0.5 1 2 1 0.5 1.5 1 1 0.5 1 V 155 95 105 65 120 155 180 135 80 135 135 105 155 120 155 135 w 114 33 36 147 93 58 61 87 141 102 111 141 45 127 39 43 Y 20 15 17 6.5 17 23.5 26.5 22 28 22.5 23.5 24 18.5 24 19.5 17 Yb 2.1 1.6 1.7 0.6 1.8 2.7 2.7 2.4 3 2.3 2.4 2.5 2 2.5 2 1.7 Zn 60 60 50 25 55 65 175 420 170 75 35 40 75 50 70 105 Zr 160.5 137.5 126.5 54.5 142.5 115.5 138.5 234 277 229 174.5 218 165 227 176 175 Note: Analysis by Chemex Labs Ltd. in Vancouver, British Columbia. In house standards submitted with analyzed samples indicate 2o error for major element abundances is ± <0.3 % and for trace element abundances is ± <0.5 ppm with the exception of La, Ce and Nd which are ± <2 ppm. *Note that the prefix SMR01 precedes all sample numbers. § Rock-units as follows: MPA = Mount Persis andesite; QMD = Quartz monzodiorite; MLP = Mafic latite porphyry; LCMD = Lennox Creek monzodiorite; BHM = Black hawk monzodiorite. not/ss = not suitable sample, f LOI = wt. % loss on ignition at 1000°C. 82 T J T TJ T J X C> X X >0 T J U f l t i T J 0\ N - ' fN X> O 00 . . 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HH [SJ O ra oo X — a. »• S OH & < .ii 0- S S -5 OH £ 3 < + M H + K O oo ^ 2 S3 A « < •5 Q H c • C3 JD E ^ S o z 5 - u n 1 ^ Ji M . 2 ?3 | •s "2 H 1 ca o « m - O X o £ t n ~ c < a" « *3 ^H (rt •5 C 3 < 2 u <-> 8 g •S 1 * ra 3 • £ £ o ra 0 M S S S " ^ xi T ; 1 =? RA -o o X . a s »r ? o « • o n ^ ~ ai" S . 2 E w • JD oo - 3 ^ ra ra ra S.| 2 < -H " 5 E P J X J <u 0- _ , < > I 84 TABLE 2.5. TYPES OF VEINS AND ASSOCIATED ALTERATION AT THE NORTH FORK DEPOSIT, WA. USA Ape and structural Stvle S i l i c a t e a s s e m b l a g e Sulphide assemblage Alteration halo Vein type A S e a n d structuiai Myie a n ( ] t e x t u r e a n d texture 1. Quartz-K-s pa r-biotite-sulfide 2. Quartz-sulfide 3. Banded and crustiform quartz -albite-actinolite-chlorite-sulfide 4. Quartz-sericite 5. Calcite-chlorite Veins are sharp at hand-sample scale, however appear gradational and irregular with diffuse margins in thin section. They range in thickness from 1-10 mm. They are commonly discontinuous on a millimeter to centimeter scale, but are commonly continuous for centimeters to meters. At times, veins are sheeted. They are cross-cut by all other vein types. Vein boundaries are sharp and typically irregular and wavy. They are typically the principal vein type found within stock work zones and make up to 10 vol. % of the total rock volume in local-ized areas. Veins are approx-imately l-10mm wide and contin-uous on a centimeter to meter scale. Although veins are typically steep, the appear to be randomly orientated. Younger than quartz-sulphide veins, banded quartz-albite-actinolite are steeply dipping, antitaxial, composite and vary rarely show ataxial fracture propagation and growth. Widths usually range from l-5cm and have sharp regular boundaries. Veins strike continuously from centi-meters to a few meters. Veins represent multiple event crack-seal events. Vein margins are sharp and range regular to irregular. They vary in thickness from l-3cm. Veins display mineral growth inwards to the center of the vein perpendicular to the long axis of the vein. Veins cut Quartz-K-spar-biotite-sulphide veins and quartz-sulphide veins. Veins are commonly <1 cm wide with diffuse, regular boundaries. They are continuous on a meter scale. Veins cut act-qtz-alb veins, quanz-K-spar-biotite-sulphide veins and quartz-sulphide veins. Veins commonly display straight boundaries and are typically 5-15 mm wide. Calcite-chlorite veins cross-cut all other vein styles. They strike continuous from centi-meters to meters, however calcite-chlorite vein lets are commonly discontinuous on a centimeter scale. Veins are observed cutting quartz-sulphide veins and quartz-sericite alteration patterns. K-spar-quartz-biotite-sulphide and rare ilmenite and sericte. K-spar and quartz are fine-grained, subhedral to anhedral, inequi-granular-polygonal. Biotite and sericite are inequigranular, very fine- to fine-grained anhedral to acicular. Quartz-sulphide and very rare traces of chlorite-actinolite. Mineralization is typically absent, however chalcopyrite exists as rare isolated disseminations. Quartz ranges from serrated and polygonal to inequigranular and interlobate. Quartz-albite-actinolite and sulphide with rare K-spar, chlorite and biotite. Albite and quartz are typically inequigranular, and at times serrated to polygonal. Other minerals are concentrated in bands oriented parallel to the long axis of the vein and are fine-grained and inequigranular. Actinolite-quartz-albite with minor tourmaline and chlorite is typical, however veins commonly contain almost pure actinolite. Two populations of quartz are recog-nizable: Fine-grained inequigranular-interbolaie and coarse-grained crustiform quartz at die edges and core respectively. Subhedral sulphides and acicular to subhedral epidote and chlorite grow interstitially at the margins and the core of the vein. Quartz-sericite. Fine-grained inequigranular quartz, and very fine-grained equigranular euhedral to acicular sericite. Coarse euhedral sulphides are segmented in areas of the veins. Calcite-chlorite-sulphide-rutile and minor quaitz. Quartz is coarse to very fine-grained, hypodidi-morpic and inequigranular to poly-gonal. Calcite is coasre- to very fine-grained, euhedral to subhedral and inequigranular. Anhedral sulphides are distributed evenly throughout the veins. Euhedral to subhedral pyrite-chalcopyrite exist as fine- to coarse disseminations and are massive in some veins. Sulphides typically constitute 8-10 vol. % and 5-10 vol. % of material in wall rocks and veins respectively. In massive sulphide examples, sulphides make up 70-100 vol. % of vein material. Subhedral chalcopyrite-pyrite-magnetite is weakly disseminated Uiroughout the veins and con-stitutes approximately 3-7 vol. % of the total vein material. Disseminated and interstitial subhedral to euhedral chalcopyrite, magnetite and lesser pyrite within veins are distributed as vein parallel bands. Chalcopyrite, magnetite and occasional pyrite within alteration halos are weakly disseminated. Sulphides within veins and alteration halos constitute 3-5 and 3-!0 vol. % of total material respectively. Interstitial and finely disseminated euhedral to subhedral chalcopyrite and pyrrhotite inter-growths with minor pyrite is characteristic within veins. Sulphides are distributed as coarse disseminations within alteration halos. Within veins, sulphides are concentrated at the core with weak isolated coarse disseminations at vein margins. Sulphides constitute 3-7 and 10-15 volume percent of material in halos and veins respectively Euhedral coarse bornite with fine grained, disseminated pyrite. Sulphides make up 7-10 and 1-1.5 vol. % of vein and alteration halo material respectively. Occasional isolated anhedral fine- to medium-grained disseminations of chalcopyrite are common. Sulphides constitutes approximately < 2 vol. % of vein material. Chalcocite rims are due to oxidation. Intergrowth of bornite and chalcopyrite display "basket weave" textures with covelite rims. Alteration halos are defined by very fine-grained biotite with lesser k-spar and quaitz (potassic alteration). Halos are "diffuse" and irregular, and range in thickness from 1 mm to 3 cm. Alteration halos typically replace 40-45 vol. % of the wall rock. Lack of alteration halos is characteristic. Occasionally, irregular very fine-grained inter-lobate quartz grains define halos typically <1 mm wide. Quartz alteration typically replaces 15 vol. % of the wall rock. Alteration halos are commonly 1-3 mm wide. Halos of chlorite and lesser epidote are fine-grained euhedral to acicular and inequigranular (sodic-cacic alteration). Alteration minerals typically replace 40-45 vol. % of the wall rock. Alteration halos are approximately l-4cm wide. Halos of actinolite-quartz-albite with minor tourmaline, quartz, biotite, epidote with clinozoisite rims and chlorite are very fine-grained subhedral to euhedral and acicular. Alteration minerals commonly replace approximately 10-15 vol. % of the wall rock. Alteration halos are typically 3-4cm wide Very fine-grained quaitz-sericite-pyrite alteration halos (phyllic). Halos replace 40-45 vol. % of the wall rock and are typically 1-2 cm wide. Veins are typically regular and wavy. Calcite-chlorite-rutile-quartz-sulphide alteration halos are characteristic (late low temp-erature propylitic alteration). Alteration minerals range from very fine-grained in wall rock ground mass to coarse where phenocrysts are replaced. Typically, 7-15 vol. % of the wallrock is replaced. Halos are approximately 0.5-lem wide 85 Table 2.5. [Smithson et. al., 2004] TABLE 2.6. GENERALIZED PARAGENETIC SEQUENCE OF ALTERATION AND MINERALIZATION AT NORTH FORK, W.A Mineralization Hornfels Early stage Early stage Main stage Late stage Late stage Alteration assemblage Bt Qtz-kfs-Bt Qtz Qtz-Alb-Act-Chl Qtz-Ser-Py Cal-Chl-Ser Vein assemblage Qtz-Kfs Qtz Qtz-Act-Alb Qtz-Ser Cal-Chl Hypogene sulphides Pyrrhotite Chalcopyrite Pyrite Bornite* Molybdenite • - - -Hypogene sulphides Hypogene sulphides Hypogene sulphides Hypogene sulphides Hypogene sulphides Primary alteration minerals Chlorite Albite Biotite Quartz K-feldspar Sericite Actinolite Tourmaline Epidote Calcite Primary alteration minerals Primary alteration minerals Primary alteration minerals Primary alteration minerals Primary alteration minerals Primary alteration minerals Primary oxides Ilmenite Magnetite Supergene minerals Neotocite Crysocolla Malachite Chalcocite Covellite - - • • -Time • Table 2.6. Widths of solid lines indicate the degree of intensity of a particular mineral. Dashed lines indicate mineral occurrence is not always present. Act - actinolite, Ab - albite, Bt - biotite, Cal - calcite, Chi - chlorite, Kfs -Potassium-feldspar, Py - pyrite, Qtz - quartz, Ser - sericite (Kretz, 1985). Table 2.6. [Smithson et. al., 2004] 86 TABLE 2.7. FIELD CRITERIA FOR CHARACTERIZATION OF BRITTLE STRUCTURES Brittle element Size Mappable (width) length Internal features Lineation type and strike Lineation dip Preferred strike orientation contained orientation of *Host lithology on fault/fracture plane fracture/fault General Comments Large faults (dip-slip) >lm 10-100's meters Minor fault gouge. Intense kaolinization Steeply plunging Steeply plunging 60°-mineral elongations 90" Contained within and slickensides. 21% of mapped 060°-100" structures 060"-100° MLP, MPA, QMD Large faults cut all primary alteration-mineralization features. Lithological offset is common. Small faults (strike-slip) >20cm <lm 10's of meters Fault gouge variably kaolinized. Hematite and goethite also common. Shallowly plunging Shallowly plunging mineral elongations 03"-59". Contained and slickensides. within 16% of mapped ' structures 320"-340° 350"-010" MPA, QMD Structures have the same orientation as dykes of mafic latite porphyry. Offset is minor. Fractures >20cm (typically 0.5-5cm) Centimeters to meters Kaolinite, geothite, hematite alteration on fracture planes. nd nd 060°-100" 320"-340" 350°-010" MLP, MPA, QMD * MLP = Mafic latite porphyry, MPA= Mount Persis andesite QMD = Quartz monzodiorite. (nd) = no data Table 2.7. [Smithson at., al. 2004] 87 C H A P T E R III Genesis and fluid evolution of reduced porphyry Cu-Au mineralization at the North Fork deposit, west central Cascades, Washington, U.S.A. Smithson, David M. and Rowins, Stephen M. Dept. Earth and Ocean Sciences, The University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada Prepared for submission to Ore Geology Reviews, March 2004 88 A B S T R A C T The North Fork deposit is located on the western flank of the west central Cascades Mountain Range in the Pacific Northwest of Washington, U.S.A. It belongs to a belt of Eocene to Miocene porphyry Cu-Mo±Au deposits that extend northward into the Coast Mountains of southern British Columbia. The deposit has a geological reserve of 80.4 million tonnes @ 0.44% Cu and 0.003 ounces (oz) per tonne Au (a 218,000 oz Au reserve) and is hosted in three main rock units. The oldest and most spatially extensive unit is the -39 Ma Mount Persis andesite, which is intruded by -37 Ma quartz monzodiorite and mafic latite porphyry. The plutonic rocks are weakly to moderately peraluminous, calc-alkaline and of I-type affinity, although they have crystallized at uncommonly low oxygen fugacities (/O2's) ranging from the quartz-fayalite-magnetite (QFM) oxygen buffer to one log unit above (QFM+1). They are therefore "reduced" I-type granitoids. The older andesites are even more reduced and have crystallized at /O2's approximating QFM-1. Geochemical and geochronological data demonstrate that the plutonic and volcanic rocks are consanguineous, with mafic latite porphyry and quartz monzodiorite intruding into their own volcanic pile 1 to 2 m.y. after the start of andesitic volcanism and initial arc construction at -39 Ma. Hypogene Cu-Au mineralization at the North Fork deposit is associated with three stages of vein formation, but occurs primarily with banded and crustiform Main-stage quartz-actinolite-albite-chlorite-sulfide veins and accompanying sodic-calcic (albite-actinolite) alteration. Main-stage veins contain abundant hypogene pyrrhotite and lack primary hematite and sulphate minerals indicating formation from relatively reduced hydrothermal fluids. Microthermometric data were collected from quartz-hosted fluid inclusions in Early-stage quartz-sulfide veins 89 associated with potassic alteration (quartz-K-feldspar-biotite) and Main-stage quartz-albite-actinolite-chlorite-sulfide veins associated with sodic-calcic alteration (albite-actinolite). Studies of quartz-hosted fluid inclusions in Main- and Early-stage veins reveal that the North Fork deposit formed from a thermally prograding system with Cu-Au sulfide deposition occurring at pressures of ~ 400 to 690 bars and temperatures between 348° to 576°C. These pressures are hydrostatic and correspond to depths of ~ 4 to 7 km because fluids were undergoing immiscible phase separation (boiling) into a dense aqueous brine (up to 51 weight % NaCl equivalent) and coexisting low-density vapor (1.4 to 3.4 weight % NaCl equivalent) at the time of trapping and Main-stage vein formation. These physicochemical conditions of ore formation are typical of porphyry Cu-Au deposits worldwide, and together with a direct genetic association with reduced I-type magmas, classify the North Fork deposit as a "reduced porphyry Cu-Au" deposit. Argon-argon dating of hydrothermal sericite from haloes surrounding Late-stage quartz-sulfide veins yields an.age of 35.5 ± 0.2 Ma, which is at least 900,000 m.y. younger than the emplacement age of the youngest mafic latite porphyry, but well within the time-frame that many porphyry Cu-Mo±Au deposits form. The 8 3 4S values of hypogene sulfide minerals from all stages of mineralization lie between 2.3 to 3.0 %o, a range typical of magmatic sulfur and additional support of a magmatic origin for the North Fork fluids. The recognition of reduced porphyry Cu-Au mineralization and related arc magmatism at -37 Ma, highlights the prospectivity of the Mount Persis andesites and raises the possibility that Late Eocene porphyry Cu-Au mineralization may be far more common in the west central Cascades than has been predicted previously from the localized exposures of quartz monzodiorite and mafic latite porphyry at North Fork. 90 INTRODUCTION The Late Eocene North Fork porphyry Cu-Au deposit is located the west central Cascade Mountain Range of Washington state, USA. The deposit is one of numerous porphyry Cu-Mo±Au deposits (e.g., Quartz Creek; Middle Fork; Glacier Peak) and low-sulfidation epithermal Au-Ag mines (e.g., Blackhawk, Lennox Creek, Damon-Pythias, Beaverdale) hosted in the Mid-Tertiary Cascades magmatic arc (Fig. 3.1). A number of these porphyry deposits contain an unusual abundance of hypogene pyrrhotite, which lead Hollister (1978) to refer to them as "pyrrhotite porphyries". Rowins (2000a; 2000b) subsequently proposed that they likely formed from relatively reduced ore fluids and represent a suite of "reduced porphyry Cu-Au deposits" (RPCG) that define a RPCG district or province. Such deposits typically have lower grades and tonnages of Cu than classically oxidized porphyry Cu-Au systems and are generally smaller and/or subeconomic. Like the oxidized porphyry Cu-Au deposits, however, some RPCG deposits contain significant amounts of Au and thus represent unrecognized Au exploration targets. The North Fork deposit exhibits many of the features that characterize it as a RPCG deposit (Smithson et al., in prep.), which make it prospective for Au and an ideal candidate to undertake the first comprehensive geological, geochemical, and geochronological study of a Cascadian porphyry Cu-Au deposit. Smithson et al., (inprep.) showed that NNW-striking fractures and faults controlled both the emplacement of intrusive rocks and the exsolved magmatic-hydrothermal fluids responsible for hypogene Cu-Au mineralization at the North Fork deposit. The formation of the NNW-striking structures was largely synchronous with magmatism at ~37 Ma. This structural model also explained the regional distribution of porphyry Cu-Au and epithermal Au-Ag deposits and occurrences in this section of the Cascade 91 arc. In the present study, mineralogical, fluid inclusion, sulfur isotope, and Ar-Ar geochronological data are used to investigate the source(s) of the mineralizing fluids and the physicochemical conditions (P-T- X -fXj2-f&i) that attended hypogene Cu-Au mineralization at the North Fork deposit. Integration of these new isotopic and geochemical data with the key structural, petrological, and geochronological constraints discussed in Smithson et al., (inprep.) permits construction of a fluid evolution model for the North Fork deposit. These data also give insight into the relationship, if any, between the North Fork deposit and several surrounding low-sulfidation epithermal Au-Ag deposits. Finally, this study provides predictive exploration guidelines for the evaluation of North Fork and other RPCG porphyry deposits and associated epithermal Au-Ag deposits in the west central Cascades Range and elsewhere in the world. R E G I O N A L G E O L O G I C SETTING The North Fork porphyry Cu-Au deposit is located within the North Cascade physiographic province, which is the most southerly extension of the Coast, Intermontane, and Omineca belts that cross into British Columbia from central Washington state (Monger and Journeay, 1994; Haugerud et al., 1994; Lasmanis, 1995). The deposit is situated within the Jurassic to Early Cretaceous Western Melange tectonic domain (e.g. Tabor et al., 1982, 1993; Haugerud et al , 1994). Basement rocks within the deposit area consist of a sequence of Jurassic to Early Cretaceous pelitic metasedimentary rocks of the Callagan Formation (Tabor, 1982). The Callagan Formation is overlain unconformably by a series of shallowly dipping extrusive rocks that form part of the Eocene Mount Persis volcanic Group (chapter 2; Tabor et al., 1993). This group is predominately andesitic in composition, and consists of flows, breccias, pyroclastic rocks, stocks and dikes (e.g., Hollister and Baumann, 1978; Tabor et al., 1982, 1993). 92 Granodioritic to tonalitic rocks of the -35 Ma Index Batholith and its smaller satellitic bodies (e.g., Ponzini and Tepper, 2003; i.e., Sunday Creek) intrude the Callagan Formation and the Mount Persis Group and crop out immediately north and south of the deposit area (Tabor et al., 1982). The composite gabbro to granite Snoqualmie batholith (25-18 Ma; Erikson et al., 1969; Tabor et al., 1982) intrudes the Callagan Formation and the Mount Persis group immediately east of the North Fork deposit and occupies an area exceeding 700 km2. G E O L O G Y O F T H E N O R T H F O R K DEPOSIT Host rocks Late Eocene andesites (38.9 ± 0.3 Ma) of the Mount Persis Group are the primary host for the North Fork porphyry Cu-Au deposit. The andesites are typically massive flows characterized by abundant (up to 30 volume.%) whitish-yellow microphenocrysts (i.e., <lmm in long dimension) of quartz, plagioclase, and potassium-feldspar set against a very fine-grained, dark grayish green groundmass of the same mineralogy. Small microphenocrysts of acicular amphibole and books of euhedral biotite are consistently present but of minor abundance (1-5 vol. %). In the southwest part of the map area (Fig. 3.1b), massive to porphyritic andesites are interbedded with matrix-supported heterolithic volcaniclastic flow breccias. Quartz monzodiorite and mafic latite porphyry are the two other significant hosts for the hypogene Cu-Au mineralization. The quartz monzodiorite has been dated at 37.2 ± 0.1 Ma and 37.0 ± 0.2 Ma (Smithson et al., in prep.) and is exposed in the southeastern and westernern portions of the North Fork deposit area. Sharp intrusive contact relationships indicate that emplacement of quartz monzodiorite postdates the formation of the Mount Persis andesites. Quartz monzodiorite 93 is a medium-grained, equigranular grayish green rock consisting of hornblende, biotite, plagioclase, potassium-feldspar and quartz. It is only mineralized in narrow zones adjacent to crosscutting dikes of mafic latite porphyry, indicating an association between the hypogene Cu-Au mineralization and mafic latite porphyry, but not quartz monzodiorite. Mafic latite porphyry in the North Fork study area has been dated at 37.1±0.2 Ma and 36.8±0.2 Ma (Smithson et al., in prep.). It occurs as steeply-dipping, NNW-striking dikes and subvolcanic intrusions bounded by faults. The mafic latite porphyry is exposed over several square kilometers and crosscuts all other plutonic and volcanic rocks in the North Fork area. Distribution of alteration indicates that the mafic latite porphyry is intimately associated with the development of zones of alteration and strong hypogene mineralization. The mafic latite porphyry is characterized mainly by phenocrysts of rounded, cracked, glassy gray quartz (quartz "eyes") and white laths of plagioclase and lesser pink potassium-feldspar. Less common phenocrysts include dark green amphibole and dark brown biotite. A l l phenocrysts are set in a fine-grained, medium gray equigranular groundmass, imparting a very noticeable porphyryritic texture to the rock. Geochemistry and redox state of igneous rocks Whole rock major and trace element data, together with mineralogical constraints, reveal that the volcanic and plutonic rocks are calc-alkaline and weakly to moderately peraluminous I-Type granitoids (Smithson et al., in prep.). The similarity of rare-earth element (REE) profiles and ages of formation for quartz monzodiorite, mafic latite porphyry and andesite strongly suggests that these rocks are part of the same Late Eocene magmatic event (Smithson et al., in 94 prep.). Immobile trace element data plotted on tectonic discrimination diagrams for granitoids based upon Rb-Y-Nb variations (Pearce et al., 1984; Christian and Keith, 1996) indicate that the quartz monzodiorite and mafic latite porphyry have formed in a volcanic arc s'etting (Smithson et al., in prep.). More specifically, the plutonic rocks have intruded their own volcanic pile during construction of a late Eocene magmatic arc. Quantitative estimates of magmatic oxygen fugacity (/O2) for andesite and quartz monzodiorite were derived from the compositions of co-existing primary biotite and ilmenite (Smithson et al., in prep.). These data plotted in temperature (T) - JO2 space show that the /TVs of andesites are approximately 1 log unit below the quartz-fayalite-magnetite buffer (QFM-1) or at the lower end of the range for typical intermediate to mafic arc magmas (e.g., Carmichael, 1990). The /TVs of the quartz monzodiorites, however, lie between QFM and QFM+1. These /TVs are much lower than those of the magnetite-series or oxidized I-type felsic magmas associated with classic porphyry Cu-Mo±Au deposits, which characteristically have /TVs between the nickel-nickel oxide (NNO) and hematite-magnetite (HM) oxygen buffers (e.g., Burnham and Ohmoto, 1980; Burnham, 1981). These low magmatic JO2 estimates are supported by whole-rock Fe203/FeO ratios less than 0.6 (cf, McCoy et al., 1997), igneous biotite with foxy-red pleochroism (e.g., Lalonde, 1992; Lalonde and Bernard, 1993) and the predominance of primary ilmenite over magnetite in all rocks investigated (e.g., Ishihara, 1977; 1981). Consequently, the magmas that produced the plutonic and volcanic rocks hosting the North Fork deposit are classified as "reduced" I-type felsic magmas (e.g., Blevin and Chappel, 1995; Pollard et al, 1995; Christiansen and Keith, 1996). Structural geology The structure of the North Fork deposit area is dominated by a system of faults and fractures that reflect a history of Eocene oblique-slip normal faulting and brittle deformation in an extensional regime (Smithson et al., in prep.). Detailed relative chronological data were derived by comparison of various structures mapped in different lithologies with orientations consistent with various stress fields active during the Eocene (Smithson et al, in prep.). These data indicate three temporally distinct episodes of deformation at -39 Ma (350°-010°), -37 Ma (320°-340°), and at - 30 Ma (030°-060°). The NNW-striking faults (320°-340°), however, are the key structural elements in the North Fork system because they have focused the intrusion of mafic latite porphyries and associated hydrothermal fluids. These second generation faults correspond to "P" shears synthetic to Eocene transcurrent faulting on the Straight Creek fault in a Riedel-type shear model (e.g., Hodgson, 1988; Monger and Journey, 1994; Schiarizza et al, 1997). The ENE-striking normal oblique-slip faults formed at -30 Ma have created some vertical offset of lithologic units, but have not significantly dismembered the mineralizing system. H Y P O G E N E M I N E R A L I Z A T I O N AND P R I M A R Y A L T E R A T I O N Three stages of vein formation associated with deposit-scale alteration features and hypogene Cu-Au mineralization are recognized at the North Fork deposit (Smithson et al., in prep.). They are categorized into Early-, Main-, and Late-stage varieties (Table 3.1; Fig. 3.1). Hypogene Cu-Au mineralization occurs during all stages of vein formation, but is dominantly associated with the formation of banded and crustified Main-stage quartz-albite-actinolite-96 chlorite-sulfide veins. These veins are associated with sodic-calcic alteration. In addition to these veins, this study focuses on the Early-stage quartz-sulfide veins associated with potassic alteration. These two vein-types dominate the Early- and Main-stages of vein formation and Cu-Au mineralization at the North Fork deposit and are used in the fluid inclusion study to characterize the P-T-X conditions of mineralization. Potassic alteration (biotite-potassium-feldspar) at the North Fork deposit is related directly to the formation of quartz-K-feldspar-biotite and quartz-sulfide veins (Fig 2a, b; Table 3.1). The Early-stage quartz-sulfide veins range from 1 mm to 1 cm in thickness and are composed almost entirely of anhedral quartz and lesser potassium-feldspar with rare biotite, actinolite, and fibrous chlorite. Sulfides are mainly euhedral chalcopyrite, pyrite, and molybdenite. Alteration halos adjacent to quartz-sulfide veins consist mainly of very fine-grained quartz and sericite, with lesser fine-grained biotite (Fig. 3.2b). Where present, disseminated sulfides (chalcopyrite and pyrite) may constitute up to 7 % of the halo material. The Main-stage quartz-actinolite-albite-chlorite-sulfide veins associated with sodic-calcic alteration display both banded (Fig 2c, d) and crustiform textures indicative of open-space filling (Fig. 3.2e, f). Chalcopyrite, pyrrhotite, ± pyrite ± molybdenite constitute up to 10 % of the vein and commonly from multiple, millimeter-wide, bands oriented parallel to the long axis of the vein in banded veins. In crustiform veins, they fill the central seam as anhedral to subhedral masses. Sulfides also occur at vein/wallrock contacts as fine disseminations. These banded and crustified Main-stage veins vary in width from 1 to 5 cm and are surrounded by 1 to 2 cm wide alteration halos consisting of fine-grained quartz, albite, actinolite, chlorite and epidote. Fine-grained disseminated anhedral chalcopyrite is the most abundant sulfide mineral in vein halos. 97 Together with minor pyrite, these sulfides account for 3-10 % of the selvage material. FLUID INCLUSION M I C R O T H E R M O M E T R Y A study of quartz-hosted fluid inclusions in Early- and Main-stage veins was done to obtain pressure-temperature-compositional (P-T-X) data of the metalliferous fluids responsible for these stages of vein formation. In addition, these fluid inclusion data can provide information on the physicochemical processes (i.e., fluid immiscibility) responsible for metal transport and deposition, which is important for understanding the location and distribution of hypogene C u -A u mineralization at the North Fork deposit. The decision to study the Early- and Main-stage veins was based on the fact that they represent an opportunity to study the transition from early potassic alteration associated with limited C u - A u mineralization to later sodic-calcic alteration associated with widespread C u - A u mineralization. Late-stage quartz-sericite and calcite-chlorite veins were not suitable for fluid inclusion microthermometry. Very little hypogene mineralization was associated with these later stages of vein formation and the absence of these fluid inclusion data does not affect the conclusions concerning fluid evolution at the North Fork deposit made from the present study. Sample selection and methodology Samples of vein material for fluid inclusion studies were selected based on detailed field mapping and inspection of drill core from areas of strong alteration and mineralization associated with abundant Early- and Main-stage veins. Microthermometric data from fluid inclusions 98 hosted in grains of hydrothermal quartz were collected from nine veins in two drill holes (SC-72A and SC-72B) collared in the central zone of the deposit where Cu and Au grades are, highest, and potassic and sodic-calcic alteration is most strongly developed. Microthermometric measurements were made using a Fluid Inc®. modified USGS gas-flow heating-freezing stage at the University of British Columbia. The accuracy of the measurements was ensured by calibration against the triple point of pure CO2 (-56.6 °C), the freezing point of H 2 0 (0.0 °C), and the critical point of H2O (374.1 °C) using synthetic fluid inclusions supplied by Fluid Inc®. Precision of temperatures obtained was ± 0.2 °C for freezing experiments and ± 3.0 °C for heating experiments (40 to 500 °C). NaCl equivalent salinities were calculated by measuring the dissolution temperature of NaCl in halite-saturated aqueous inclusions, and using the equation of state of Bodnar and Vityk (1994). Salinities in halite-undersaturated aqueous inclusions were calculated using final melting temperature of ice and the equation-of-state of Bodnar (1993). Microthermometric data reduction was accomplished using the Macflincor software package of Brown and Hagemann (1995). Fluid inclusion petrography Doubly polished thin sections of Early- and Main-stage veins were examined petrographically in order to characterize the size, shape and types of fluid inclusions present, as well as their spatial and temporal distribution with respect to each other and the host quartz grains. Attempts were made to measure data from fluid inclusion assemblages (FIA's) as described in Goldstein and Reynolds (1994). Fluid inclusion assemblages are groups of co-genetic fluid inclusions that were all trapped at same time. For example, fluid inclusions trapped in a growth zone in quartz (primary fluid inclusions), or along a healed fracture (secondary fluid 99 inclusion), or in a three-dimensional cluster in areas of clear, unstrained, quartz (pseudosecondary fluid inclusions; Bodnar, 2003) . Consequently, a FIA represents a single "fluid event" in the history of an evolving hydrothermal system. Determination of simultaneous trapping of fluid inclusions was based initially on the distribution and phase proportions (liquid: vapor: solid) of individual fluid inclusions at room temperature. FIA's used in this study are hosted in grains of quartz that demonstrate good textural evidence for equilibrium crystallization with chalcopyrite. As noted above, individual fluid inclusions within a FIA may be classified as primary, pseudosecondary and secondary following the recommendations of Roedder (1984). Briefly, secondary fluid inclusions post-date the growth of quartz-hosting primary and pseudosecondary inclusions, and typically form from fluids unrelated to primary mineralization, although in some types of deposits, such as orogenic or mesothermal Au deposits, it is the late fluids that commonly produce the Au ores. Consequently, secondary fluid inclusions are actually the inclusions of interest in this example (e.g., Hagemann and Cassidy, 2000). At the North Fork deposit, the common superposition of multiple fluid inclusion populations in most quartz grains made the classification of fluid inclusions as primary, pseudosecondary, and secondary difficult, but not impossible. The sequence of fluid entrapment within Early- and Main-stage veins recorded by fluid inclusions was inferred from both careful petrography and relative vein paragenesis (Smithson et al., in prep.). In hydrothermal quartz from both Early- and Main-stage veins, two types of primary aqueous fluid inclusions with identical optical characteristics were recognized according to proportions of phases present at room temperature (Fig. 3.3). Type I fluid inclusions are aqueous and liquid-rich (>70 vol. % liquid), with halite and 100 commonly sylvite, calcite, and opaque daughter minerals. The opaque minerals are tentatively identified as chalcopyrite. Type I inclusions typically have elongate to equant negative crystal morphologies and range from 2 to 14 um in long dimension, although most inclusions analyzed in this study were <8 rim in length. Aqueous liquid fills >70% of the Type I inclusion volume with aqueous vapor typically filling <10% and daughter minerals filling the remaining volume. Type I inclusions are abundant in grains of hydrothermal quartz within Early- and Main-stage veins and occur in pseudosecondary clusters and, less commonly, primary growth zones (Fig. 3.3). Type II inclusions are two-phase (vapor + liquid) aqueous, vapor-rich inclusions (>85 vol. % vapor) with rounded to elliptical shapes or negative crystal morphologies. The liquid, which occupies up to 15 vol. % of the inclusion, is commonly invisible due to internal reflections off the inclusion walls (e.g., Bodnar, 1985). Type II inclusions generally range in size from 2 to 14 um in long dimension with most averaging ~ 6 um in length. Similar to Type I liquid-rich inclusions, vapor-rich inclusions are common in Early- and Main-stage veins and typically occur together with the Type I inclusions in pseudosecondary clusters. Accurate measurement of phase changes in the vapor-rich inclusions, however, could only be made where minor irregularities in fluid inclusion walls or sharp inclusion corners (i.e., inclusions with negative crystal morphologies) permitted the concentration of liquid. 101 M I C R O T H E R M O M E T R I C R E S U L T S Freezing data Microthermometric data collected from Early- and Main-stage veins are summarized in Table 3.1. The range of eutectic temperatures (Te) are similar for Type I and Type II inclusions (-69 to -22 °C) within both stages of vein formation. These T e data are suggestive of K + and divalent cations such as Mg 2 + , Fe2+, and Ca 2 + in solution in addition to Na+ (Shepherd et al., 1985). This observation is consistent with the presence of sylvite and carbonate daughter minerals, and fluid compositions that have produced sodic-calcic and potassic styles of alteration. The range in T e also may be attributed, in part, to the optical difficulties encountered during measurement of first melting temperatures in small inclusions (<7 urn in length) and to the small amounts of liquid available for observing the T e phase transition in vapor-rich Type II inclusions. Final ice melting temperatures (Tm) in Type II inclusions from Early-stage veins ranges from -3.1 to -7.2 °C corresponding to salinities of 5 to 10.7 wt.% NaCl equiv. (mean = 7.6). T m data for Type II inclusions in Main-stage veins range from -0.9 to -3.2 °C corresponding to salinities of 1.4 to 3.4 wt.% NaCl equiv. (mean = 3.1). Note that salinities for the halite-saturated Type I inclusions were calculated from the halite dissolution temperature during heating runs. 102 Heating data Early-stage veins Heating data for all FIA's from Early- and Main-stage veins are summarized in Table 3.2. The homogenization of the vapor phase into the liquid (Th i) is the first phase change observed during heating of Type I inclusions (n= 71) in Early-stage veins. The Th (D'S of Type I inclusions range from 195 to 367 °C (mean = 281 °C). The next phase change observed during heating was halite-dissolution (Tm Halite) which also represented the total or final homogenization of Type I inclusions in Early-stage veins. Salinities calculated from the halite dissolution temperatures range from 235 to 450 °C. These temperatures correspond to a wide range of salinities from to 33.7 to 53.3 wt. % NaCl equiv., although salinities do not vary more than ± 6 wt % NaCl equiv. in any single FIA. In detail, Six FIA's of Type I inclusions containing multiple daughter minerals (halite, sylvite, carbonate and opaque minerals) had Tm Halite's from 326 to 387 °C (40.2 to 46 wt. % NaCl equiv.; FIA 18), 387 to 421 (46 to 49.7 wt. % NaCl equiv.; FIA 34), 235 to 238 °C (33.7 to 33.9 wt. % NaCl equiv. FIA 37), 350 to 355 °C (42.2 to 42.8 wt. % NaCl equiv.; FIA 39), 312 to 315 °C (39 to 39.3 wt. % NaCl equiv.; FIA 48), and 340 to 356 °C (38.6 to 42.7 wt. % NaCl equiv.; FIA 52). Five FIA's of Type I inclusions containing a single halite daughter mineral had Tm Halite's from 359 to 378 °C (43.2 to 45.1 wt. % NaCl equiv.; FIA 38), 331 to 341 °C (40.6 to 41.6 wt.% NaCl equiv.; FIA 41), 401 to 451 °C (47.4 to 53.3 wt. % NaCl equiv.; FIA 50), 361 to 374 °C (43.4 to 44.7 wt. % NaCl equiv.; FIA 54), and 282 to 292 °C (36.8 to 37.5 wt. % NaCl equiv.; FIA 55). 103 Main-stage veins In contrast to Type I fluid inclusions in Early-stage veins, Type I fluid inclusions in Main-stage veins (n = 58) undergo final homogenization to a supercritical fluid by disappearance of the vapor phase into the liquid phase. The Tm Halite's measures prior to vapor phase homogenization yielded a wide range from 172 to 421 °C. Although these Tm H aiite correspond to a wide range of salinities from 30.0 to 51.0 wt. % NaCl equiv. (mean = 34.1), Tm Halite's within a single FIA are typically within ± 2 wt % NaCl equiv. Table 3.2. The final Homogenization temperatures (Th(L)) of Type I inclusions in Main-stage veins determined from the disappearance of the vapor into the liquid phase range from 348 to 575 °C (mean = 475 °C) and were measured by the FIA's 10a and 26a represent the immiscible liquid in a "boiling" FIA from a Main-stage vein (i.e., simultaneously trapped Type I liquid-rich and Type II vapor-rich inclusions) and have Th (L)'s from 441 to 479 °C (31.1 to 31.8 wt. % NaCl equiv.) and 547 to 554 °C (33.4 to 33.9 wt. % NaCl equiv.), respectively. Four FIA's with Type I inclusions containing multiple daughter minerals (halite, sylvite, carbonate and opaque minerals) have Th (D'S from 416 to 449°C (FIA 3), 423 to 450 °C (FIA 4), 371 to 378 °C (FIA 14), and 348 to 381 °C (FIA 15). Four FIA's with Type I inclusions containing a single halite daughter mineral have Th (L)'s ranging from 541 to 575 °C (FIA 1), 460 to 505 °C (FIA 8), 373 to 379 °C (FIA 11), and 525 to 530 °C (FIA 32). The existence of two distinct Type I fluid inclusion populations based on final homogenization temperatures and the mode of homogenization is easily recognized in Figure 4. Final homogenization temperatures of Type II vapor-rich inclusions (n = 4) in Early-stage veins were measured by the disappearance of the liquid phase into the vapor phase (Th (V)) . Th (v)'s could only be made for inclusions in one FIA (51) and they ranged from 362 to 367 °C. 104 Final homogenization temperatures for Type II inclusions in Main-stage veins (n= 12) were also measured by the disappearance of liquid into the vapor phase and they ranged from 401 to 549 °C. Note that Type II inclusions in Main-stage FIA's 10b and 26b represent the immiscible vapor phase in a boiling fluid inclusion assemblage. The conjugate immiscible liquids are represented by FIA's 10a and 26a. Final homogenization temperatures for the Type II vapor-rich inclusions in these FIA's ranged from 478 to 549 °C. I N T E R P R E T A T I O N O F FLUID INCLUSION D A T A Early-Stage veins The mode of final homogenization in Type I liquid-rich inclusions (halite dissolution to the liquid phase) precludes the liquid- and vapor-rich inclusions from being immiscible end members of a fluid undergoing phase separation (e.g., Bodnar, 1994; Bodnar and Vityk, 1994). Type I fluid inclusions must have been trapped at pressure and temperature conditions such that their inclusion isochores intersect the halite liquidii before cooling and decompressing to the 3-phase halite-liquid-vapor curve (Fig. 3.5). Consequently, the halite dissolution temperatures measured only provide minimum estimates of the trapping temperatures. To obtain true trapping temperatures requires knowledge of the pressures at the time of trapping. At the North Fork deposit, we can only estimate these pressures indirectly by stratigraphic reconstruction, which is difficult in this part of the Cascade Range. Therefore, we are only able to calculate a range of minimum trapping pressures and temperatures by constructing isochores from inclusions having the lowest to highest salinities using the method of Bodnar and Vityk (1994). A pressure-temperature diagram (Fig. 3.5) with isochores for three fluid inclusions (SC72A-296.5-3-1; 105 SC72A-269.0-2-9; SC72A-296.5-6-5) representative of the compositional and thermal range of Early-stage Type I inclusions are projected from the three-phase liquid + vapor + halite curve at the temperature of vapor phase disappearance. Upon heating of an inclusion and vapor disappearance, fluid inclusions leave the three-phase curve and follow an isochoric path until halite dissolution occurs at the halite liquidus (Fig. 3.5). The pressures at which Tm Halite occurs range from 460 to 520 bars (Fig. 3.5). These pressures accord with 4.0 to 5.3 km of overburden assuming a hydrostatic load (rock density =1.0 g/cm3) or 1.5 to 1.9 km of overburden assuming a lithostatic load (rock density = 2.7 g/cm3; see Shepherd, 1985 for discussion of rock density p. 162). Both depth estimates are feasible in the porphyry environment (e.g., Sillitoe, 1973; Hedenquist and Richards, 1998), but which is more likely cannot be determined from these Early-stage fluid inclusion data alone. Main-stage veins Fluid inclusion data collected from Main-stage veins satisfy the 3 main criteria used to recognize fluid immiscibility during inclusion entrapment in quartz and vein formation (Roedder, 1984). These criteria are: (1) the presence of co-existing FIA's with substantially variable liquid/vapor ratios; (2) final homogenization by disappearance of the vapor phase in liquid-rich inclusions and of the liquid phase in co-existing vapor-rich inclusions, and; (3) approximately equal Tn's for both inclusion populations. Supercritical fluids undergo immiscible phase separation along the two-phase solvus and therefore the Th's measured for fluid inclusions in the Main-stage veins are true trapping temperatures and pressures at the time of hypogene Cu-sulfide mineralization (e.g., Sourirajan and Kennedy, 1.962; Roedder, 1984; Fig. 3.6). The well-studied FkO-NaCl system in Figure 3.6 illustrates the three-dimensional relationship between pressure, 106 temperature and composition (salinity) through the temperature interval 250 to 700 °C and at pressures up to 1240 bars (Sourirajan and Kennedy, 1962; Bodnar et al., 1985; Pitzer and Pabalan, 1986). The topology of this system demonstrates that under the range of P-T-X conditions typical of the porphyry Cu-Au environment, a supercritical fluid may separate into a high-salinity liquid (i.e., Type I inclusions) and a low-salinity vapor (i.e., Type II inclusions). The common presence of co-existing hypersaline liquid- and vapor-rich inclusions, such as those in hydrothermal quartz from the Main-stage veins at North Fork (Fig. 3.3e), and in numerous porphyry Cu deposits including Bingham (Utah), El Salvador (Chile), Santa Rita (New Mexico) and Sierrita (Arizona) attest to the importance of immiscible phase separation in these huge hydrothermal systems (e.g., Roedder, 1984, Table 15-5). Immiscible phase separation of an originally homogeneous, moderately saline, supercritical magmatic fluid into a hypersaline liquid and a low-salinity vapor is usually ascribed to cooling and decompression - the hot buoyant magmatic fluid rises and intersects its solvus or isotherm as shown in Figure 3.6 (cf., Hedenquist and Richards, 1998). The temperature and salinity data measured in boiling FIA's 26a - 26b, and 10a - 10b (Table 3.2), are plotted on the P-T-X diagram for the H20-NaCl system (Fig. 3.6). The Th(L)'s of 26a (547 to 554 °C) and 26b (534 to 550 °C) are virtually identical, and when plotted on Figure 6 with the relevant salinity data, yield a narrow range of trapping pressures from 630 to 690 bars. Because these immiscible fluids were trapped on the 2-phase liquid-vapor curve or solvus, the pressures correspond to hydrostatic conditions (Hass, 1971) and depths of ~6 to 7 km. These pressure (depth) estimates suggest that a hot, relatively buoyant, supercritical magmatic fluid with a typical salinity of perhaps 10 wt% NaCl equiv. (e.g., Burnham, 1981), ascended to depths of 6 or 7 km before separating into a hypersaline liquid of ~ 34 wt% NaCl equiv. and a 107 low-salinity vapor of ~ 2 wt.% NaCl equiv. The immiscible liquid and vapor phases were trapped subsequently in quartz during the formation of Main-Stage quartz-albite-actinolite -chlorite-sulfide veins and sodic-calcic alteration. The other two FIA's recognized from fluid inclusion petrography and similar Th (L)'s as representing immiscible liquid and vapor are, respectively, 10a (441 to 479 °C) and 10b (478 to 485 °C). Plotted on Figure 6 with the relevant salinity data, these FIA's indicate trapping pressures between 400 and 550 bars. These pressures correspond to depths of 4 to 5.5 km assuming hydrostatic pressure conditions. These pressures are slightly lower than those calculated for FIA's 26a and 26b, but identical to the minimum pressures attained during formation of the Early-stage veins (460 to 520 bars, Fig. 3.5). SULFUR ISOTOPE STUDIES Sulfur isotope studies of primary sulfide minerals associated with Cu-Au mineralization at the North Fork deposit were done to determine the source(s) of sulfur and, by inference, metals in the different stages of vein formation. The 8 3 4S values of primary sulfides from the Blackhawk and Lennox Creek epithermal Au-Ag mines located 4 to 5 km south-southeast of the deposit (Fig. 3.1), were measured to identify source(s) of sulfur and metals for comparison with North Fork sulfur isotope data. Sample selection and methodology All sulfides isotopically analyzed were physically separated from their host-rocks by handpicking under a binocular microscope followed by HCI dissolution. Isotopic analyses were done at the G.G Hatch Isotope Laboratories at the University of Ottawa (Ontario). The sulfur 108 isotopic composition of sulphides was determined by Continuous Flow (CF). Helium is used to carry the SO2 gas produced by flash combustion with vanadium pentoxide at 1800°C on an Elemental Analyser, followed by separation (from other gases) by an SO2 gas chromatographic column, into a Finnigan M A T Delta p l u s mass spectrometer. A reference gas is injected from the bellows of the dual inlet. The routine precision of the analyses is 0.20 %o. Duplicate samples yielded an average analytical reproducibility of ±0.2 %o. Isotopic compositions of sulfur compounds are expressed as delta (8) values, defined as the per mil (%o) deviation between the ratio of heavy isotope ( 3 4S) to the light isotope ( 3 2S) in the sample relative to the standard for sulfur (i.e., Canon Diablo Troilite). In this study, sulfur isotope ratios were measured in 10 samples containing sulfides that include pyrite, chalcopyrite, bornite, pyrrhotite and arsenopyrite (Table 3.2; Appendix D). Results: 5 J 4S data Three 8 S values of sulfide samples from the Blackhawk mine range between 2.3 and 2.5 %o. The two sulfide samples from the Lennox Creek mine gave 8 3 4 S values between 2.9 and 3.0 %o. The five sulfide samples from the North Fork deposit, yielded 8 3 4S values between 2.3 and 2.9 %o. The total range of 8 3 4 S values measured in sulfides from the North Fork deposit and surrounding low-sulfidation epithermal Au-Ag veins is between 2.3 and 3.0%o. Interpretation of the 8 M S data The 8 3 4S values of hydrothermal sulfide minerals are primarily controlled by the sulfur isotope composition and S0 4 2 "/H 2 S ratio of the mineralizing fluid (Ohmoto and Rye, 1979). The 109 sulfide minerals at the North Fork deposit are consistent with formation from relatively reduced ore fluids given the abundance of hypogene pyrrhotite in all stages of vein formation (Table 3.1). These data, together with a lack of primary hematite and sulphate minerals indicate that sulfide formation occurred below the SO<427H2S boundary and that H2S is the dominant sulfur species in the ore fluids (e.g., Ohmoto and Rye, 1979). Under such reducing conditions, the 834S value of most sulfide minerals will be at most ~ l%o higher'relative to the 834S value of coexisting H2S at temperatures above 200 °C (Ohmoto and Rye, 1979). Therefore, the 834S values of sulfide minerals from the North Fork deposit and the surrounding low-sulfidation epithermal Au-Ag mines, which also have primary mineralogy indicative of relatively reduced conditions (i.e., abundant arsenopyrite), can be considered equivalent to the 834S value of the ore fluid assuming equilibrium was achieved. Isotopic studies of porphyry Cu-Mo±Au deposits reveal that the majority of hypogene sulfides have 834S values of 0±3%o, a range implying an igneous source of sulfur given the geological setting of these deposits (e.g., Ohmoto and Rye, 1979; Titley and Beane, 1981). The narrow range of 834S values of sulfides at the North Fork deposit therefore is consistent with sulfur derived from a homogeneous igneous source. A r - A r G E O C H R O N O L O G Y Ar-Ar geochronology on hydrothermal sericite associated with Late-stage quartz-sericite-sulfide veins (Table 3.1) and phyllic alteration in mafic latite porphyry was done for comparison with crystallization ages of mafic latite porphyry determined by U-Pb geochronology on zircon (Smithson et al., in prep.). Ar-Ar geochronology on sericite from intensely silicified and sericitized monzodiorite adjacent to quartz-adularia-sericite-sulfide veins from the Lennox Creek mine (Fig 1 a) was done for comparison with the ages of mineralization of the North Fork 110 deposit. Sample selection and methodology Sample selection was based on surface mapping of the major volcanic and plutonic rocks hosting the North Fork deposit and surrounding Blackhawk and Lennox Creek epithermal Au-Ag mines. The rock sample selected for Ar-Ar dating from the North Fork deposit (OC-42-1) came from a zone of pervasively sericitized andesite in the northwest portion of the deposit area. At this locality, part of the intense phyllic alteration zone associated with Late-stage quartz-sulfide vein formation, pervasive alteration of both potassium-feldspar formed from earlier alteration events and of primary ferromagnesian silicates, yields abundant, fine-grained, sericite, quartz and pyrite. The rock sample selected for Ar-Ar geochronology, from the Lennox Creek mine (SMR01-118) came from hydrothermally altered wallrock adjacent to a 3 cm-wide milky white quartz-adularia-sericite-chlorite vein with bladed quartz (after calcite) and well-developed crustification textures (e.g., comb quartz and colloform banding). Sericite in this altered wallrock occurred as medium- to coarse-grained clots of sericite (5 mm to 5 cm in long dimension) surrounded by fine-grained quartz and chlorite. Surficial weathering and secondary oxidation was negligible and removed from the sample before processing. Rock samples containing the hydrothermal sericite were crushed, pulverized to a coarse powder, and sieved. Flakes of sericite were selected by handpicking under a binocular microscope. The resulting separates had a purity of >99 % sericite. Seperates were subsequently washed in de-ionized water, rinsed, and then air-dried at room temperature for 48-hours. Sericite separates were then re-picked, wrapped in aluminum foil, and stacked with similar-aged samples 111 and flux monitors in an irradiation capsule. Samples were irradiated at the McMaster Nuclear Reactor in Hamilton, Ontario, for 72 hours. The samples were then analyzed using the 4 0 Ar/ 3 9 Ar technique (Merrihue and Turner, 1966; Dallmeyer, 1975; Dalrymple et al., 1981) at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia (Vancouver, British Columbia), and step-heated at incrementally higher powers using a 10W CO2 laser until fused. The gas evolved from each heating step was analyzed using a VG5400 mass spectrometer. An estimate for the error in the J value (i.e., a constant in the age equation related to irradiation; McDougall and Harrison, 1988) is ~ 0.5 %. Results and interpretation Although argon loss or gain is commonly observed in the initial heating steps (e.g., Dallmeyer, 1975; Dalrymple et al., 1981), an age spectrum is produced in the later heating steps, and a plateau of similar ages is subsequently produced comprising >50 % of the total gas released (Fig. 3.7). The plateau age of hydrothermal sericite grains adjacent to Late-stage quartz-sericite veins at North Fork (OC-42-1; Fig. 3.7a) yield an age of 35.5 ± 0.2 Ma. This age postdates formation of the mafic latite porphyry (37.1 ± 0.2 Ma and 36.8 ± 0.2 Ma) by at least 900 000 years, although only two U-Pb ages are available for the mafic latite porphyry and there certainly could be younger phases not yet dated. The formation of sericite at -35.5 Ma therefore is consistent with magmatic fluid-rock interaction accompanying intrusion of mafic latite porphyry and development of Late-stage quartz-sericite veins and associated phyllic alteration. The plateau age produced from sericite grains sampled from the Lennox Creek mine yield an age of 19.8 ± 0.1 Ma (Fig. 3.7b). The hydrothermal alteration at the Lennox Creek mine therefore postdates the intrusion of mafic latite by -17 m.y. and is very likely related to hydrothermal 112 activity associated with the 18-25 Ma magmatic event that produced the Snoqualmie batholith (Fig. 3.1) and its numerous satellite plutons with porphyry Cu (Au) mineralization (e.g., Glacier Peak; Middle Fork; Quartz Creek; Lasmanis, 1995). DISCUSSION Hydrothermal fluid evolution and genesis of the North Fork deposit The P-T-X characteristics and types of fluid inclusions in hydrothermal quartz from Early- and Main-stage veins from the North Fork deposit are very typical of those in porphyry Cu-Mo±Au deposits elsewhere in the world. Specifically, the occurrence of coexisting vapor-and hypersaline liquid-rich inclusions with similar high-temperatures of homogenization in primary growth zones or pseudosecondary clusters identifies the important and ubiquitous process of fluid immiscibility in the Main-stage veins (e.g., Nash, 1976; Roedder, 1984). Despite the many similarities, fluid evolution at the North Fork deposits has several features that are uncommon in calc-alkaline porphyry Cu-Au deposits, but may be explained by integration of vein paragenesis, types of alteration, and P-T-X data from the fluid inclusion studies. The principal differences between the North Fork deposit and most other porphyry Cu-Au deposits is the prograding hydrothermal system, from lower temperature Early-stage potassic alteration through to Main-stage sodic-calcic alteration. In concert with this thermally prograding path is the restriction of fluid immiscibility to the hotter Main-stages of vein formation and sulfide deposition, and not formation of the cooler Early-stage veins. The question may arise as to whether these temperature differences are real because the halite dissolution temperatures 1 1 3 measured in the Early-stage quartz-sulfide veins are minimum temperature estimates (i.e., the pressure of fluid entrapment is unconstrained). They are, however, very likely close approximations of the true trapping temperatures because of the steep positive slope of the isochores for halite-saturated Type I fluid inclusions in Figure 5. Even a doubling of the pressures estimated for Main-stage vein formation (-500 bars) to 1000 bars, only results in a temperature correction, or increase, of 20 to 40° C except for the most saline inclusions (50 wt.% NaCl equiv.) where an increase of -60° C is indicated. Consideration of the true trapping pressures calculated for Main-stage vein formation, however, reveals that pressures at North Fork did not exceed -700 bars. Consequently, the differences in fluid inclusion homogenization temperatures measured are real, and pressure increases of 100's of bars will not sufficiently raise the homogenization temperatures measured in halite-saturated Type 1 inclusions from Early-stage veins to the true trapping temperatures measured in Type 1 inclusions from the Main-stage veins. Thermally prograding porphyry Cu-Mo±Au systems are rare, but have been documented at Cerro Colorado in Chile (e.g., Bouzari and Clark, 2003) and Island Cu, Canada (Arancibia and Clark, 1996). Direct analogies may be made with precious metal-bearing geothermal systems in the Phillipines (Tongonan, and especially Tiwi) and New Zealand (Broadlands-Ohaki; Henley et al., 1984). The higher trapping temperatures measured in fluid inclusions from the Main-stage veins are entirely consistent with formation of sodic-calcic alteration because thermodynamic studies have shown that it is stable at higher temperatures than potassic alteration in the porphyry Cu environment (e.g., Orville, 1963; Carten, 1986; Lang et al., 1995). Due to its higher temperature of formation, sodic-calcic alteration, which is relatively rare in calc-alkaline porphyry Cu deposits but common in many alkalic porphyry Cu-Au deposits, normally precedes 114 the potassic and phyllic stages of alteration as pulses of magmatic fluid migrate away from parental intrusions and cool. This cooling of a single pulse of magmatic fluid, however, cannot apply to the North Fork system because the sodic-calcic alteration is later than the potassic alteration. Another way proposed to form sodic-calcic alteration in a porphyry Cu-Au deposit involves the circulation of heated formational brines enriched in Na and Ca due to leaching of evaporitic sequences (e.g., Dilles et al., 1995; Barton and Johnson, 1996). Such a process has been used to explain the widespread sodic-calcic alteration at the Yerington porphyry Cu (Mo) deposit in Nevada (Dilles et al., 1995; 2000). This explanation, however, is unlikely to apply to the North Fork deposit because evaporitic sequences are neither expected nor mapped in the Cascade magmatic arc (e.g., Tabor et al., 1982). The sodic-calcic alteration at North Fork is also spatially restricted to the occurrence of mafic latite porphyry and adjacent andesite. Incursion of cooler formational fluids enriched in Na and Ca into a hydrothermal convection cell surrounding a large intrusive body would create a pattern of widespread alteration (e.g., Yerington, Ann-Mason) that lacks any spatial association with a specific intrusive body (e.g., Cathles, 1981). The best explanation for prograde evolution and the change from potassic to sodic-calcic alteration would appear to be that later batches of mafic latite magma exsolved fluids (became fluid-saturated) at higher temperatures and were more enriched in Na and Ca than earlier batches of magma that produced the potassic alteration. The type of alteration from such hot Na-Ca enriched fluids would be dominantly sodic-calcic and pervasive replacement of the earlier potassic alteration would be expected as these fluids ascended up the same NNW-striking structures that focused earlier fluids and mafic latite porphyry dikes. The cause of renewed heating and prograde fluid evolution at the North Fork deposit could be explained by the incursion of new magma batches into an underlying magma chamber (cf, Bouzari and Clark, 2003). 115 The second main difference is the distribution of primary alteration and hypogene Cu-Au mineralization at the North Fork deposit. Instead of alteration being distributed around a single "point source", typically a cluster of nested porphyritic intrusions, it is distributed in relatively elongated zones of overprinting alteration types surrounding and replacing the mafic latite porphyry dikes, which are bounded by NNW-striking faults. This distribution of alteration and mineralization also may be partly a function of crustal depth. Fluid inclusion data suggest formation depths of 5 to 7 km, which classifies North Fork as a relatively deep-seated porphyry Cu-Au deposit such as the Ann-Mason mine in Nevada (Carten, 1986; Dilles and Einaudi, 1992). Other geological features suggestive of a relatively deep porphyry environment include a lack of hydrothermal breccias, subdued intensity of hydrothermal alteration in general, relatively weakly developed vein stockworks and, of course, the development of sodic calcic alteration. Finally, the present level of erosion reveals that exposed mafic latite porphyry and quartz monzodiorite intrudes the base of the Mt. Persis Group just above the unconformable contact with the Mesozoic Callaghan Formation. Although we have no direct evidence for the original thickness of the Mt. Persis Group, an accumulation of 5 or 6 km of andesite would not be unusual during construction of a calc-alkaline volcanic arc (e.g., Gill, 1981). Much of this volcanic pile probably would be present at the time of mafic latite porphyry and quartz monzodiorite emplacement, which occurred only 1 to 2 m.y. later according the 38.9 + 0.3 Ma age for andesite. Today, 35 to 40 m.y. later, much of the Mt. Persis Group has been eroded away leaving only the roots exposed. It is possible that the presently exposed North Fork deposit was part of a much bigger system that has been eroded away. Denudation rates in young magmatic arcs like the Cascade Range can be very high (meters per year). Given the topographic severity of the Cascade Range, it seems very likely that erosion rates have been high throughout the Tertiary. 116 The Ar-Ar (sericite) age (35.5 ± 0.2 Ma) of the Late-stage phyllic alteration is only -900,000 m.y. younger than the U-Pb (zircon) age of the mafic latite porphyry dikes (37.1 ± 0.2 Ma and 36.8 ± 0.2 Ma). These data, together with spatial and temporal coincidence of mafic latite porphyry dikes, zones of strong hydrothermal alteration and hypogene mineralization, and NNW-striking structures, strongly support a direct genetic linkage between hypogene Cu-Au mineralization and mafic latite porphyry. Mineralization in many porphyry Cu-Mo±Au systems evolves over several millions of years (e.g., Hedenquist and Richards, 1998; Sillitoe et al., 2001) making linkages between mafic latite porphyry and hypogene Cu-Au mineralization at the North Fork deposit entirely permissible from a strict chronological point of view. Implications for Cu-Au metallogeny and exploration in the Cascades Mountain Range. In contrast to traditional magnetite-series or oxidized I-type felsic magmas associated with classic porphyry Cu-Mo-Au deposits, the North Fork deposit exhibits several features consistent with formation under relatively reduced conditions (e.g., Smithson et al., in prep.). These features characterize North Fork as a reduced porphyry Cu-Au deposit. The reduced nature of the North Fork ore system, together with the identification of key physicochemical processes such as fluid immiscibility (boiling) attending Main-stage mineralization, are significant in terms both local Cu-Au mineralization and the development of a Au (Cu) halo up to several kilometers from the North Fork deposit. Metal solubility studies of classically oxidized porphyry Cu-Mo±Au deposits show that Cu and Au may partition into the saline aqueous brine during immiscible phase separation (e.g., 117 Williams et al, 1995). More recent analytical studies suggest that the opposite may occur under reducing hydrothermal conditions. Specifically, fluid inclusion studies of reduced porphyry Sn-W-Ag and Cu-Mo-Au deposits demonstrate that Au and (Cu), strongly partition into the low-density H2S rich vapor phase rather than the coexisting high-density chlorine-rich liquid phase during immiscible phase separation (e.g., Heinrich et al., 1992; Heinrich et al., 1999; Rowins et al., 2002). Although the exact partition mechanism remains unclear, the enhanced mobility of Au and Cu in the vapor phase directly affects how these metals may be zoned in a reduced porphyry Cu-Au system (Rowins, 2000a). Specifically, the vapor phase could potentially transport large quantities of Au and lesser Cu (as reduced sulfur complexes?) to distal sites far from any parental intrusion. Mineralization in this peripheral environment may take several forms including Au (Cu) mantos or carbonate-replacement deposits in calcareous rocks, and epithermal Au (Cu) deposits in more competent and chemically unreactive igneous rocks. At the North Fork deposit, it has been shown that fluid immiscibility accompanied the formation of Main-stage veins and widespread Cu-Au mineralization. This observation, together with the potential for preferential transport of Au and Cu in the immiscible vapor phase, raises the possibility that significant Au and Cu has been mobilized to distal sites away from the dikes and stocks of mafic latite porphyry. The prominent NNW-striking brittle structures associated with the emplacement of mafic latite porphyry and hypogene Cu-Au mineralization would have been ideal conduits for focussing metal-charged fluids many kilometers away from the centre of the North Fork system. Interestingly, the 19.8 Ma age for epithermal Au-Ag mineralization at the Lennox Creek mine south-southeast of the North Fork deposit, suggests that hydrothermal fluids have exploited the NNW-striking structures for at least 17 m.y. since formation of the North Fork deposit at -37 Ma. This new Late Eocene age for reduced porphyry Cu-Au 118 mineralization greatly enhances the Cu-Au potential of the Cascade Range and, in particular, the west central Cascades where the North Fork deposit is located. CONCLUSIONS Three stages of vein formation are recognized at the North Fork deposit, but banded and crustified Main-stage quartz-albite-actinolite-chlorite-sulfide veins host the majority of the hypogene Cu-Au mineralization. Hydrothermal fluid evolution differs from that documented in most porphyry Cu-Au deposits at North Fork as it is a thermally prograding system. Fluid inclusion studies reveal that the Early-stage quartz-sulfide veins and associated potassic alteration formed at significantly lower temperatures than the Main-stage veins associated with sodic-calcic alteration. In tandem with this thermally prograding path is the restriction of fluid immiscibility (boiling) to the hotter Main-stages of vein formation and sulfide deposition, and not formation of the cooler Early-stage veins. The higher trapping temperatures measured in fluid inclusions from Main-stage veins (348 to 576 °C) compared to those in Early-stage veins (238 to 451 °C) are entirely consistent with the formation of sodic-calcic alteration in place of potassic alteration because thermodynamic studies indicate that sodic-calcic assemblages are stable at higher temperatures in the porphyry Cu environment. It is suggested that this prograding fluid path and change from potassic to sodic-calcic alteration results from later batches of mafic latite magma that became fluid-saturated (i.e., exsolving fluids) at higher temperatures, and were more enriched in Na and Ca than the earlier batches. Fluid inclusion data suggest that the North Fork deposit has formed at depths of 5 to 7 km. Other geological features suggestive of a relatively deep porphyry environment include a lack of hydrothermal breccias, a generally subdued intensity of hydrothermal alteration, weakly 119 developed vein stockworks, and the development of sodic calcic alteration. The present level of erosion suggests that much of the volcanic pile, which was likely present at the time of mafic latite porphyry and quartz monzodiorite emplacement, has been eroded away leaving only the roots exposed. It is possible that the present-day North Fork deposit was part of a much larger system that has been eroded away. Denudation rates in young magmatic arcs like the Cascade Range can be very high (meters per year). Given the topographic severity of the Cascade Range, it seems very likely that erosion rates have been high throughout the Tertiary. Unlike traditional magnetite-series or oxidized I-type felsic magmas associated with classic porphyry Cu-Mo-Au deposits, the North Fork deposit exhibits several features consistent with formation under relatively reduced conditions. These features characterize North Fork as a reduced porphyry Cu-Au deposit. The reduced nature of the North Fork ore system, together with the identification of key physicochemical processes such as fluid immiscibility (boiling) attending Main-stage mineralization, are significant in terms of Cu and Au distribution in the North Fork deposit. The preferential transport of Au and Cu in the immiscible vapor phase in reduced porphyry Cu-Au systems like North Fork (e.g., San Anton and 17 Mile Hill; Rowins et al., 2002), raises the possibility that significant Au and Cu has been mobilized to distal sites. The prominent NNW-striking brittle structures associated with the emplacement of mafic latite porphyry and hypogene Cu-Au mineralization would have been ideal conduits for focussing metal-charged fluids many kilometers away from the centre of the North Fork system. Interestingly, the new 19.8 m.y. age for epithermal Au-Ag mineralization at the Lennox Creek mine south-southeast of the North Fork deposit, suggests that hydrothermal fluids have exploited the NNW-striking structures for at least 17 m.y. or since formation of the North Fork deposit at -37 Ma. This new Late Eocene age for reduced porphyry Cu-Au mineralization greatly enhances the Cu-Au potential of the Cascade Range and, in particular, the west central Cascades where the 120 North Fork deposit is located. References Arancibia, C.J., and Clarke, R., 1996, Calcic-sodic alteration at Island Copper Porphyry deposit, Vancouver Island, British Columbia: Economic Geology, v. 9, p. 672-695. 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Economic Geology. 75th Anniversary, v., 214-235. 126 Williams, T .J . , Candella, P.A., and Piccoli, P.M., 1995, The partitioning of copper between silicate melts and two-phase aqueous fluids: An experimental investigation at 1 kbar, 800°C, and 0.5 kbar, 850 °C: Contributions to Mineralogy and Petrology, v. 121, p. 388-399. 127 Figure Captions Figure 3.1. Simplified geology of the North Fork deposit and surrounding areas, a. Simplified regional geological map showing the location of the North Fork study area and the main rock units that crop out in the general vicinity of the deposit. Solid black triangles indicate other know deposits in close proximity to the North Fork deposit, b. Simplified deposit scale geological map showing the distribution of selected deposit-scale features (i.e., faults and fractures). Figure 3.2. Photomicrograph and hand specimen examples of Early- and Main stage vein material used for fluid inclusion analysis, a. Early-stage stock-worked quartz-sulfide veins, b. Photomicrograph of Early-stage quartz-sulphide veins exhibiting weak quartz-biotite ± potassium-feldspar alteration selvages, c. Hand specimen of a Main-stage banded quartz-albite-actinolite-chlorite vein. In hand specimen, banded veins exhibit as many as six crack-seal events associated successive hydrothermal events, d. Photomicrograph of Main-stage banded vein demonstrating at least four crack seal events. Polygonal quartz and albite grains have undergone minor recrystallization from successive high temperature hydrothermal fluid events, e. Hand sample of a crustified vein with strongly developed sodic-calcic (quartz-albite-actinolite-chlorite) alteration envelopes adjacent to vein margins, f. Photomicrograph of a Main-stage quartz-albite-actinolite-chlorite vein showing the inward crustified growth of bimodal grains of quartz and albite. Sulphides fill the interstices between quartz and albite grains. 128 All vein-types are hosted in porphyritic and massive varieties of the Mount Persis andesite. Figure 3.3. Photomicrographs of quartz-hosted fluid inclusions in Early- and Main-stage veins at the North Fork deposit, a. Aqueous brine (liquid-rich) inclusion containing a single halite daughter crystal coexisting with vapor-rich inclusions in an Early-stage vein with chalcopyrite. b. Multiphase brine inclusion coexisting with vapor-rich inclusions in an Early-stage quartz-sulphide vein. c. Multiphase brine inclusion with halite and carbonate daughter minerals in an Early-stage quartz-sulphide vein. d. Brine inclusions with single and multiple halite daughter minerals in a crustified Main-stage quartz-albite-actinolite-chlorite-sulfide vein. e. Multiphase brine inclusions co-existing with vapor-rich inclusions in a classic "boiling" assemblage, f. Multiphase brine inclusions densely packed with sylvite and halite in a banded Main-stage quartz-albite-actinolite-chlorite-sulfide vein. g. Quartz grain with growth zones containing primary inclusions and crosscutting trails hosting secondary inclusions from a crustified Main-stage quartz-albite-actinolite-chlorite-sulfide vein. Figure 3.4. a. Salinity vs. final homogenization temperature (by vapor disappearance or by halite dissolution) for all FIA's (numbered 1 - 55) at the North Fork deposit (see table 2 for details on individual FIA's). Stippled lines connect co-existing vapor-rich and liquid-rich inclusion populations in "boiling" FIA's. b. Final homogenization by halite dissolution versus final dissolution by disappearance 129 of the vapor phase for halite-saturated Type I inclusions in Early and Main-stage veins. Fluid inclusions plotting in the field above the line undergo final homogenization by halite dissolution, whereas inclusions in the field below the line homogenize to the liquid by vapor disappearance. Despite a similar appearance at room temperature, these differing modes of final homogenization for the halite-saturated Type I inclusions indicate that they have a fundamentally different genetic relationship to spatially associated Type II vapor rich inclusions. Type I inclusions that undergo final homogenization by halite dissolution cannot have formed by immiscible phase separation of a supercritical aqueous fluid (e.g., Bodnar, 1994). Figure 3.5. Pressure-temperature diagram illustrating trapping conditions for Early-stage fluid inclusions. Diagram adopted from Bodnar and Vityk (1994). Isochores for low-temperature "A" (SC72A-296.5 wafer 3-1), intermediate-temperature "B" (SC72A-269.0 wafer 2-9) and high-temperature " C " (SC72A-296.5 wafer 6-5) type I halite-saturated fluid inclusions from Early-stage quartz-sulphide veins at the North Fork deposit. Type I fluid inclusions from the Early-stage veins undergo final homogenization by dissolution of halite. Isochores calculated based on the equation of state H 2 0 - N a C l for by Bodnar and Vityk (1994). Halite liquidii from Bodnar (1994). Figure 3.6. The System H 2 0 - N a C l system after Sourirajan and Kennedy, (1962) with modifications from Bodnar et. al. (1985), and Pitzer and Pablan (1986). FIA's 130 from main-stage veins, representative of boiling assemblages, yield trapping pressures of 620 to 690 bars (26a and 26b) and 400 to 550 bars (10a and 10b). Figure 3.7. Age spectra from A r - A r analysis of hydrothermal sericites. a. Age spectra for hydrothermal sericite (OC-42-1) collected from a zone of quartz-sericite-sulfide alteration adjacent to quartz-sericite veins at the North Fork deposit. Sample includes 56.9 % of the 3 9 A r released, b. Age spectra for hydrothermal sericite (SMR-01-118) collected from quartz-sericite alteration adjacent to an epithermal quartz-adularia-sericite-chorite vein at the Lennox Creek mine. Sample 39 includes 53.7 % of the A r released. Plateau steps are filled, rejected steps are open. Error in plateau ages are reported at the 2o level and represented by height of plateau steps. Figure 3.8. Simplified space-time evolution of hydrothermal fluid flow, alteration and sulphide deposition at the North Fork deposit, a. Early dykes of mafic latite porphyry result in the development of quartz-k-feldspar-biotite and quartz-sulphide veins associated with zones of biotite-k-feldspar (potassic) alteration and Early-stage mineralization, b. Later, higher temperature magma batches produce dykes of mafic latite porphyry and quartz-albite-actinolite-chlorite veins associated with zones of sodic-calcic alteration and Main-stage mineralization. This stage represents a prograde heating up of the hydrothermal system, c. Late dykes of mafic latite porphyry, quartz-sericite-pyrite veins and chlorite-calcite-sericite veins associated with phyllic alteration are produced. The sequence of alteration and 131 mineralization that correspond to each vein-type is adapted from Smithson and others, in prep. 132 133 135 0 100 200 300 400 500 600 700 Total homogenization either by Th ( L ) or T m ( H a l i t e ) (°C) 0 100 200 300 400 500 600 700 T h ( L ) (°C) Figure 3.4. [Smithson and Rowins, 2004] 136 1500 I 1000 3 co CD 520 bars 480 bars^SQ. 460 bars 100 -3- o "3-o 200 2 / L H 300 400 500 600 700 800 Temperature (°C) Figure 3.5. [Smithson and Rowins, 2004] X 1000 900 800 700 600 0) Ul § 500 ii CU 400 300 200 100 10 50 Wt% NaCl Vajjor- I . iqu i l • Si ' ; / r | 10 wt. y =-/—1 \ o \° Boili ng asserr ^ blage 10b ^ assemb ACtCfC 0 + Liquid rich (brine) FIA % Vapor-rich FIA 100 Figure 3.6. [Smithson and Rowins, 2004] 137 70 60 ft) OC-42-1 Plateau age = 35.57 ±0.21 Ma 50 h 30 20 20 40 60 80 100 Cumulative A r % © SMR-0 Plateau 1-118 age= 19.88 ±0.15 Ma -'1 W W ' M ' ' 1 20 40 60 80 100 Cumulative 3 9 A r % Figure 3.7. [Smithson and Rowins, 2004] 138 139 T A B L E 3.1. G E N E R A L I Z E D PARAGENETIC SEQUENCE OF ALTERATION A N D MINERALIZATION AT NORTH FORK, W.A Mineralization Hornfels Early stage Early stage Main stage Late stage Late stage Alteration assemblage Bt Qtz-kfs-Bt Qtz Qtz-Alb-Act-Chl Qtz-Ser-Py Cal-Chl-Ser Vein assemblage Qtz-Kfs Qtz Qtz-Act-Alb Qtz-Ser Cal-Chl Hypogene sulphides Pyrrhotite Chalcopyrite Pyrite Bornite* Molybdenite • - - -Hypogene sulphides Hypogene sulphides Hypogene sulphides Hypogene sulphides Hypogene sulphides Primary alteration minerals Chlorite Albite Biotite Quartz K-feldspar Sericite Actinolite Tourmaline Epidote Calcite Primary alteration minerals Primary alteration minerals Primary alteration minerals Primary alteration minerals Primary alteration minerals Primary alteration minerals Primary oxides Ilmenite Magnetite Supergene minerals Neotocite Crysocolla Malachite Chalcocite Covellite Time • Table 1. Widths of solid lines indicate the degree of intensity of a particular mineral. Dashed lines indicate mineral occurrence is not always present. Act - actinolite, Ab - albite, Bt - biotite, Cal - calcite, Chi - chlorite, Kfs -Potassium-feldspar, Py - pyrite, Qtz - quartz, Ser - sericite (Kretz, 1985). Table 3.1. [Smithson and Rowins, 2004] 140 T A B L E 3.2. H Y P O G E N E S^S SULFUR ISOTPE V A L U E S , NORTH FORK A R E A , WA. USA Sample BH-1 BH-2 BH-3 BH-4 BH-5 LC-1 LC-2 LC-3 NF-1 NF-2 NF-3 NF-4 NF-5 Pyrite Py Py Py Py Asp Py Py Py Py Py Py Py Py Delta 3 4S 2.49 2.38 2.49 2.51 2.42 2.92 2.98 3.0 2.94 2.71 2.77 2.32 2.46 Note: B H = Blackhawk mine; L C = Lennox Creek mine; NF = North Fork deposit Table 3.2 [Smithson and Rowins, 2004] 141 Table 3.3. Summary of microthermometric data from fluid inclusion assemblages in Early- and Main-stage veins Early-stage quartz-sulfide veins FIA* Sample # and vein texture Petrographic description Inlcusion Type Vol % vapor Wt.% NaCl equiv. Tm l u„ u CC) orTm ,„(°C). Th CC) „.v, 18 SC72A- 464.8 stockwork qtz-Sx vein. Daughter minerals of halite and sylvite. Inclusion morphologies are rounded and range in size from 7 to 10 um. Inclusions are distributed as a psuedosecondary cluster. Type I multiphase assemblage 12 to 20 40.2 to 46.0 326 to 387 262 to 321 (L) 34 SC72A- 296.5 stockwork qtz-Sx vein. Inclusions contain up to four tightly packed halite, sylvite and carbonate daughter minerals. Inclusions are elongate and irregular ranging in size from 3 to 11 um in long dimension. Psuedosecondary inclusions are loosely clustered and cut by secondary liquid-rich inclusion trails. Type I multiphase assemblage 5 to 8 46.0 to 49.7 387 to 421 226 to 290 (L) 37 SC72A- 296.5 stockwork qtz-Sx vein. Daughter mineral of halite, sylvite and carbonate. Inclusions are elongate to round and equant ranging from 5 to 7um in long dimension. Psuedosecondary inclusions are tightly clustered with to vapor-rich inclusions. Type 1 multiphase assemblage 33.7 to 33.9 235 to 238 195 to 199 (L) 38 SC72A- 296.5 stockwork qtz-Sx vein. Single daughter minerals of halite. Inclusions posses round and equant morphologies and range from 5 to7 um in long dimension. Inclusions are distributed along primary growth trails. Type I assemblage 5 to 10 43.2 to 45.1 359 to 378 222 to 361 (L) 39 SC72A- 296.5 stockwork qtz-Sx vein. Daughter minerals of halite and sylvite. Inclusions posses round and equant morphologies and are 6 to 7 um in long dimension. Inclusions are distributed along primary growth trails. Type I multiphase assemblage 4 to 6 42.2 to 42.8 350 to 355 291 to 294 (L) 41 SC72A- 296.5 stockwork qtz-Sx vein. Single halite daughter mineral and rare sylvite and opaques. Inclusions are round and equant to elongate ranging in length from 4 to 8 um in long dimension. Inclusions are distributed as a tight psuedosecondary cluster. Type I assemblage 12 to 19 40.6 to 41.6 331 to 342 249 to 295 (L) 48 SC72A- 296.5 stockwork qtz-Sx vein. Single halite daughter with opaques minerals (<1 %). Inclusions are irregular, round to square shaped and range from 4 to 15 um in long dimension. Psuedosecondary inclusions are randomly distributed. Type 1 opaque assemblage 17 to 35 39.0 to 39.3 312 to 315 274 to 300 (L) 50 SC72A- 296.5 stock-work qtz-Sx vein. 1-2 halite daughters. Inclusions are elongate and posses negative crystal shapes and range from 5 to 10 um in long dimension. Pseudosecondary inclusions are randomly distributed in a tight cluster with vapor-rich inclusions. Type I assemblage 8 to 17 47.4 to 53,3 400 to 451 308 to 436 (L) 51 SC72A- 296.5 stockwork qtz-Sx vein. Vapor-rich inclusions. Inclusions posses negative and negative crystal shapes and are - 10 um in long dimension. Inclusions are distributed in a tight cluster with aqueous brines. Type II vapor-rich assemblage 90 5.0 to 10.7 -3.1 to-7.2 363 to 367 (V) 52 SC72A- 269.0 stockwork qtz-Sx vein. Single daughter mineral of halite with rare sylvite. Inclusions morphologies are elongate to equant ranging in length from 3-7um in long dimension. Inclusions are distributed as a single tight cluster. Type I multiphase assemblage 24 to 30 38,6 to 42,7 340 to 356 265 to 326 (L) 54 SC72A- 269.0 stockwork qtz-Sx vein. Single daughter mineral of halite. Inclusions are elongate and ~5 um in long dimension. Inclusions are psuedosecondary in nature. Type I assemblage 6 to 8 43.4 to 44.7 361 to 374 277 to 282 (L) 55 SC72A- 269.0 stockwork qtz-Sx vein. Single daughter mineral of halite. Inclusions are elongate and ~5 um long. Inclusions are distributed in a psuedosecondary cluster. Type I assemblage 10 36.8 to 37.5 282 to 292 215 to 286 (L) * Fluid inclusion assemblage Table 3.3. [Smithson and Rowins, 2004] 142 Table 3.3. Summary of microthermometric data from fluid inclusion assemblages in Early- and Main-stage veins Main-stage banded and crustiform veins Sample # and vein texture Petrographic description Inlcusion Type Vol % vapor W r . % N a C l equiv. T m H „ , u CC) or T m ,„ (°C). Th CC) „ SC72A- 296.5 banded qtz-act-alb-chl vein. SC72A- 296.5 banded qtz-act-alb-chl vein. SC72A- 296.5 banded qtz-act-alb-chl vein. Single halite daughter mineral. Inclusions posses rounded morphologies ranging from 5 to 12 um in long dimension. Inclusions occur on primary growth trails. Halite and carbonate daughter minerals. Inclusions are elongate and irregular in shape ranging from 8 to 11 um in long dimension. Inclusions occur on concentric primary growth trails cut by secondary trails. Dense packing of up to 4 daughter minerals of halite, sylvite and calcite. Inclusions are elongate and range in length from 6 to 2 um. Inclusions occur in a psuedosecondary cluster. SC72A- 296.5 Single daughter mineral of halite. Inclusions are banded qtz-act- round and equant and range in length from 6 to 8 um. alb-chl vein. Inclusions are in a psuedosecondary cluster. Type I brine assemblage Type I multiphase assemblage Type I assemblage Type I assemblage 27 to 35 38.3 to 51.0 Sto 10 50 47.3 to 49.7 15 to 28 30.5 to 33.2 35.0 to 38.1 302 to 432 399 to 421 172 to 226 255 to 299 541 to 575 (L) 416 to 449 (L) 423 to 450 (L) 460 to 505 (L) 10a 10b 14 15 26a 32 SC72A- 296.5 banded qtz-act-alb-chl vein. SC72A- 296.5 banded qtz-act-alb-chl vein. SC72A- 269.0 banded qtz-act-alb-chl vein. SC72A- 269.0 banded qtz-act-alb-chl vein. SC72A- 296.5 crustified qtz-act-alb-chl vein. 26b SC72A- 296.5 caistified qtz-act-alb-chl vein. Single halite and rare opaque (cpy; < 1 %) daughter minerals. Inclusions exhibit negative and rounded morphologies ranging from 5 to 7.5 um. Inclusions psuedosecondary cluster. Inclusion morphologies are elongate and irregular. Inclusions are in close proximity lo brine inclusions in a psuedosecondary cluster. Type I (boiling assemblage) Type II (boiling assemblage) 2 to 3 halite daughter minerals . Inclusions are Type I elongate, irregular and ~ 7um in long dimension. assemblage Psuedosecondary inclusions are clustered and cut by secondary linear vapor-rich inclusion trails. A single halite with variable proportions of sylvite and Type I carbonate daughter minerals . Inclusions possess multiphase negative and elongate shapes (6 to 8 um long) and assemblage occur in a randomly distributed psuedosecondary clusters cut by.secondary inclusion trails. Halite + sylvite daughter minerals. Inclusion are Type I elongate and range in length from 7 to 10 um. multiphase Inclusion are distributed as a tight psuedosecondary assemblage cluster. Single halite daughter with opaques of chlacopyrite (< Type I 1%). Inclusions are elongate to rounded and equant. (boiling They are - 5 um in long dimension.. Randomly assemblage) distributed in pseudosecondary cluster. Inclusions possess rounded and irregular Type II morphologies in a randomly distributed (boiling pseudosecondary cluster with co-existing brine assemblage) inclusions. Single halite daughter. Rounded and eqviant inclusion Type I morphologies 6 to 9 um in long dimension. assemblage Randomly distributed inpsuedosecondary cluster. 4 to 10 85 to 90 85 to 90 31.1 to 31.1 -3.4 31.3 to 31.4 33.4 to 33.8 22 to 35 38.2 to 39.0 12 to 18 33.4 to 33.9 1.9 to 2.1 25 to 33 36.8 to 39.0 185 to 202 -2.0 to -2.1 191 to 192 230 to 236 302 to 354 230 to 240 296 to 311 441 to 479 (L) 479 to 485 (V) 373 to 379 (L) 371 to 378 (L) 348 to 381 (L) 547 to 554 (L) 536 to 550 (V) 525 to 530 (L) * Fluid inclusion assemblage Table 3.3 cont. [Smithson and Rowins, 2004] 143 C H A P T E R IV G E N E R A L CONCLUSIONS Detailed geological mapping, lithogeochemistry, petrography, geochronology, stable isotope studies, and fluid inclusion microthermometry have been integrated in this thesis to produce a genetic model for the formation of the North Fork porphyry Cu-Au deposit. This multidisciplinary research approached has provided critical information on the age and petroiogical characteristics of the deposit, as well as insight into the physicochemical conditions attending mineralization and the source of metalliferous fluids. The field mapping, petrographic mineral mode analysis, U-Pb (zircon) geochronology, and whole-rock geochemistry conducted at the beginning of this study, identified several features that are key to understanding the relationship between magmatism and reduced porphyry Cu-Au mineralization at North Fork. Mapping at 1:4800 scale revealed that the Mount Persis andesite is the oldest and most spatially extensive unit in the North Fork area. Its is intruded by quartz monzodiorite and mafic latite porphyry, which share a consanguineous relationship with the Mount Persis andesite based on the similarity of rare-earth element profiles, inferred tectonic setting, and ages of formation. It is concluded that the mafic latite porphyry and quartz monzodiorite have intruded into their own volcanic pile during construction of a late Eocene calc-alkaline volcanic arc. The preponderance of primary ilmenite over magnetite, and electron microprobe studies of ilmenite-biotite pairs in unaltered andesite and quartz monzodiorite, also demonstrate that these rocks are relatively reduced. These features lead to the classification of quartz monzodiorites and by inference, the mafic latite porphyry, as reduced I-type magmas. The low redox state of the magmas together with substantial hypogene pyrrhotite 144 in all stages of mineralization, and an absence of primary hematite and sulphate minerals, classify North Fork as a "reduced" porphyry Cu-Au deposit. The brittle structures (faults and fractures) measured in the North Fork deposit area indicate three episodes of deformation, although only the NNW-striking structures (320°-340°) are associated with emplacement of mafic latite porphyry and development of overprinting zones of alteration and strong hypogene Cu-Au mineralization at -37 Ma. Three stages of vein formation are recognized at the North Fork deposit, but only the Main-stage quartz-albite-actinolite-chlorite-sulfide veins are associated with abundance hypogene mineralization. Sodic-calcic alteration (quartz-actinolite-albite-chlorite) surrounds these banded and crustified veins whereas potassic (quartz-biotite-K-feldspar), phyllic (quartz-sericite), and late calcite-chlorite types of alteration are associated with other stages of vein formation and limited hypogene Cu-Au mineralization. Fluid inclusion studies reveal that the Early-stage quartz-sulfide veins and associated potassic alteration formed at significantly lower temperatures than the Main-stage veins associated with sodic-calcic alteration and the bulk of hypogene mineralization. The higher trapping temperatures measured in fluid inclusions from Main-stage veins (348 to 576 °C) compared to those in Early-stage veins (238 to 451 °C) are consistent with the formation of sodic-calcic alteration in place of potassic alteration. In tandem with this prograding temperature path is the restriction of fluid immiscibility (boiling) to the hotter Main-stages of vein formation. These physicochemical data together with differences in temperatures and types of alteration suggest that overprinting generations of magmatic fluid have ascended repeatedly along the same NNW-striking structures that focused intrusion of mafic latite porphyry to depths of 4 to 7 km -the North Fork deposit is a relatively deep-seated porphyry Cu-Au deposit. The identification of fluid immiscibility (boiling) attending Main-stage mineralization is equally significant in terms of Cu and Au distribution in the North Fork deposit study area. The preferential transport of Cu 145 and, by analogy, Au in the immiscible vapor phase in reduced porphyry Cu-Au systems raises the possibility that significant Au and Cu have been mobilized to distal sites. The prominent NNW-striking brittle structures associated with the emplacement of mafic latite porphyry and hypogene Cu-Au mineralization would have been ideal conduits for focussing metalliferous fluids several kilometers away from the centre of the North Fork system. Interestingly, the new 19.8 m.y. age for epithermal Au-Ag mineralization at the Lennox Creek epithermal Au-Ag mine located ~ 5 km south-southeast of the North Fork deposit, suggests that hydrothermal fluids have exploited the NNW-striking structures for at least 17 m.y. since formation of the North Fork deposit at -37 Ma. The new Late Eocene age for reduced porphyry Cu-Au mineralization significantly enhances the potential for new discoveries of reduced porphyry Cu-Au mineralization within the North Fork area. Furthermore, the geochemical and petrological consanguinity of plutonic and volcanic rocks coupled with the extensive exposures (>100 km2) of Mount Persis andesites, raises the possibility that Late Eocene porphyry Cu-Au mineralization is far, more common in the west central Cascades than has been previously predicted based on the limited abundance of quartz monzodiorite and mafic latite porphyry exposed in the west central Cascades Range. 146 APPENDIX A G E O C H E M I C A L ANALYSES 147 x as S ai ai 3 S -Bi § S 2 ai S § -as 3 a! 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(WD D. = _ " E u 5 ° w » ° = .2 •S I S = • • : » » U U C3 S >> S •= 5 J= a. «j •o . « E S D j = ! 2 § g1 5^  2 s S ,E CA J P CJ wi ^ S b j= 3 — 152 APPENDIX C STRUCTURAL DATA 153 in u-i — — II < o o r - i n T t m © r - ^ i n 'T vo vo vo i - oo oo oo in o Tr fN — CN — CN — o o o o T t T t r - — ~ r N i n m o o c N o o r N m — — C N in — oo C N © Ch ON O — — fN - r s m r r © o o © rN fN (N CN O O — — — — in - m in fN o in — 0> m — »n — — n o \ D t o o o o o m « n o\ oo a> O N ^ m v o r - ^ O T t m — fN — fN fN r*1 — — m fN CO rn TJ- o oo CO oo in m in r-^  r-^  fNr fN" CN" o o o VO vc vo — vo — o OO fN rn in in co h — © © o vo vo oo © — — G\ G\ 0\ Q\ ©^ 0_ o" o" o" ©" —" —" —" oo oo oo oo oo oo oo CN CN CN CN fN fN fN^ m" >n 1/1 in m m m CfN t— o\ «n T J - oo —' rN m N oo Tt T vo m c- in — o o — — — — in in in © © o o < N r - \o r -m t~- in — fN r-' Tt Tt* ON O — © *o r-^  r-^  r-C N " C N " C N " fN" © o o o VO VO VD oo r- r— fN T t © <n — O N — © " OO CO fN fN m" in m m fN fN r*i fN fN fN fN © © © VO VO VO C v i u m 154 — tN to m CN fN O T j - o v i r N v o o f N a s ^ i n v o — m O N k n O r N r ^ t ^ - - — v o v D O t ^ - v o v o C N m m ro — CN —1 VO VO fN fN Tj-\t rN — in in - H ( N v O i r i O v t v O a CC O O x t v O ^ O O O v o t V O r n O O fN OO VO m - \t 1" h m m C N — — 1 r - r-- t-- r -o vo m vo oo Tt \t oo h h r t i r N i n O N T ' f i i n v Q O ' t ' t — r ^ u o r ^ T T m r ^ r - - - - t ^ v o o o m o o o o m m v - j v o v - i v o c N — o ^ . - — r ^ m r ^ r ^ o m ^ T t t ^ f N « c o i n m f N O N r - - - c N r N ^ - m oo in C N rN — o> C vO CJ m si •£ S N 155 m Tf Tf Tf fN CN vo r--Tt Tt fN fN OO Ch O Tt Tt m fN (N CN o o i n - i n o o i f i ' o - | n - t ^ o o ( N - iri in 1/1 oo - - v i co m vo o V D m >n in rn — m — i n O i n i n f N C h c N r - - c o r^-chor— — f N c N T t ^ t — V D m — rs — m — — C N C N - — o m o o o m o in Tt in Tt — oo o m - o o C N C N rn Tt Ch OO -— CO oo r- co r- oo r- fN CN Tt oo t— C C V D Ch C O ^ O Ch V O Ch C O C O Ch Ch Ch Ch in m C O rn r-i Ch m Tt Ch C N —' rN oo oo rN rN m C N Ch co Ch oo Ch Ch Ch Ch in *n in vo m m fN fN m — — m m vovo — — — — • — ' m — vo— ' in-—'VD — — — m — vovovo «—« r- r— Tt O Ch vo „ m vo in \ D m m — — ^ T t f N C h o o i n o o o o m m — i n T t f N — T t v o — o o T f o o c h r ^ f N — r ^ T t — r N ^ t r - f N i n — ' o o v o — — m f N — f N f N r N — f N f N fN fN — fN N (N - CN — r-. m — in — oo — o — oo oo rs (N in m — — C O od — vo Ch m C N 156 VO ON VO VO CN CN ro — r N — — — oo vo vo vo o ~ - ON m vo rn CN \ t o \ t t in vo oo CN oo T J -- h <n m t N vo VO Os ro — (N CN ro i n n \ t ^ ^ M i n o o ( N ( N | n f N c o r N > 0 \ t C N r> — i/i — r N C f s o — r o o O T t T t v s v o r o f N — ' C N — ' r o — C N f N ^ ro CN (N CN O vo 0"S — — fN OO oo rN cN vs" m" CN ro vo —' vo CO CN rN oo oo CN CN — f N rN Os oo oo CN CN 00 00 rN CN —- CN ro t^ <n vo vs vs vs vi vs vs fN fN CN CN CN ro OO OS o ITS V S vo CN CN CN — CN VO vo CN CN r— — — oor-vs — — — — — ,— v s o o v s v s o o v s v s o o — — — C h t v s o o s t o v i i n O S T t r - r - r o r o — vs CNCN — — —' — ro — r - ~ - r ~ - O s O s r o v s v o O O v o o O o o o o r - v s T t T t t - o > o s o o v o v o — CN CN VO O O CN O ro vd os os od ir~- ov OO ro vs Tt Os VD fN CN CN CN CN t— CN" r-T rN" CN" CN" CN" OO OO OO OO oo oo CN^  CN^  CN^  CN^  fN^ CN_ vs vs vs" vs vs vs O O O O O O r N O r o Tt Tt C s Os O O O O —1 Os O o o o o VS vo vo vo vo vo CN VO VS O vO -ro CN CN CN" CN" CN" oo oo OO ° i ^ r i vs vs vs o o o Os VO vs ro ro Tt VS VO —' Tt OS Os ro ro Tt^ o " O" O" O O O vo vo VD CN tN OO oo CN CN B vo « rO i » 5 a 6 g A ±-157 r - c o O N O f N r n T t o o _ o o o — — — — — m m m c n m r n m m m — CN CN CN rn m in o> O — C N m C N ( N m m m m rn II m r n m r n m m r n | | Tf CO Tf- -T t t t vo r~* t Tt co m *vt fN vo rs in m >n C N m m — - v o c N v o — i n T t m c o i n o T J in V O V O C N — o o t - - r N r n — i c N — rn rn *— — •—< r - — T t O s m m o o r N v o Tt r- — Tt — Ch VO (-» vo C N . . — c o o o c h i n t N i n c N — -in in in »n vo^  rn fN rN^  Oh C N " rN" C N " C N " C N" fs" rsf C N " — " o o o o o o o o o o o o o o o o o o CN fN fN CN CN CN fN fN fN in m" in" in" in m" m" in in rN m rn vo or-m rn co — Tt o Tt C N ri r~ ri — VO — — vo CO — co o^  Tt^ Tt^ Tt_|| OO CO CO oo CO oo (N^  fN^ CN_ fN fN^ fN^  in m" m" in in" in m || o oo Ch o o in O oo o O o o rn t~~ o Ch f-; o CO oo o o o o r-CN o CN o •n fN o rn CN vd Ch Tt o CN r-^  m r-^  rn r-r-_ en fN Ch CO VO vd r-rn 00 Tt r-o_ d VO rn oo rn fN Tt 599, Ch Ch m Ch" ON in Ch Ch in Oh Ch in Ch Ch in Ch Ch in Ch Ch in Ch" Ch m Ch Ch in Ch Ch m ON ON in r- r- rn Ch ON Ch Ch Ch ON m in m o o o o in o t vo t~- r - m m iri h d ^ Ch ON O ON ON O _ m in vo in fN m Tt OO CO CO fN fN fN — r-ON ON CN CN — fN rn O O O m m m _ _ „ _ l i n i r ) _ l n i n i n i n i n m O O O o o r — v o C h o o o o r ~ — V O C O V D O O — v o v o c N r n r - o i n o o c N o r ^ m ' — • —* — vo 'st vo C N vo r- — —< oo r- vo ON rn —• CN CN CN CN rN — CNCN Tt co t m vo rn r- Tt —' — r* — C N m —• »n m Tt o r~- vo — ON C N m rN m m oo cs fN rn fN (N CN OO CO OO fN^ CN, CN^  •n m «n CN CN oo oo CN CN in in m fN" fN~ fN" oo oo oo C N ^ rN^  rs_ in in" m" — m oo r- oo Tt ON 00* Tt — VO ON CN_ rn^ rn^ ON Ch ON ON ON ON m m in r s fN oo oo fN CN if > B vo 158 ON CN vs r - oo ON vs vo vo vo VD vo m m m m m m r- r- r-m m m TJ- vo r- oo r-- r- r- r-m m m m — — — r - r - vs — cs vs Tt — r - r--Tt o vs — Tt — vs oo cs r - Tt vs r - vo oo VS O VD • VS VS VS CN O vs vo CN CO VD OS VO ( VD VD Tt C N m C N * — — CN CN CN — VD VS OS O OS VS OS t— o Tt VO VO VS VS VO — CN CN — — CN vs — CN OO VO m T t t oo — o CN vo CN rN o m — ON — CN m cs — r- Tt vd — — m vs vd vd CN CN oo oo CN CN VS ON — | — vd od c-" — CN VS C S — t— VO vo vo CN (N OO oo CN CN cs — O m Tt ~ CN CN oo oo rN rN vo m — os VO vs vs m m CN CN CN oo oo oo CN^ CN^ CN, vs vs vs o o o r- Tt o OO — VO ci —• r-" i— i— vs vs vs vs o o r- r-VI Tf_ Tt_ CN" CN" CN" CN" oo oo oo oo CN. CN^  CN^  CN^  vs" vs vs vs O O O O fN Tt VD VD vq Tt Tt TT — — ON VS VO O O O tn vo_ vo; vo O O O O vo VO vo vo VS vo m m cs m m Tt vs r—• oo Tt Tj" Tt Tf Tt m m m m m — — vs — — i o o > — • — vs — v s v s v s c c — CN CO vs vs vs oo ON OS CO O O r~-— — m CN VS VO VS CN Tj- m —< — — m VS Tt Tf Tj" Tf m o oo Tt rN m rN — — O VS vs ON [-- ON (N m o o — c N T f — T t r -— CN CN m — vs ON ON od CN CN oo oo CN CN m ON ON m cs r- r-n r-- r-.^  r-^  CN rN CN CN" CN" CO CO oo oo oo <N^  CN^ CN^ CN_ CN^ vs" vs" vs" vs" vs" O O O O O CN <N oo oo rN CN — r— CN O o —' .5 s 159 vo I—• OO ON o o o o Tf Tf Tf Tf rs m Tt in vo Tt Tt Tt Tt Tf - i in co in — — Tt oo Tt r-oo r— m Tt — — rs — m r - o o ^ o i n i n r n m m Tt in Tf Tf — m Tf — i rs rs — r- — o vo r- m C O O Tf vo - in in C O E — VO o" o" o" m m m m vo oo rN — vo m m rs rs" rs" rs" m rs vo m vo vo ( N — C N" rs" r- vo m rs — i n rn Tf rs o oo r-vo^  Tf^ —^ o" o" o" o" m m m i n o o o o o rs o in o o o o vo vo vo vo — in in m rn ON — C O _ fN —" m vo t— *n vo °\ °\ r"~~ ON O N O N ON ON r— r— r— rs rs rs rs rS_ in" in" <n in" in" o o o o o r- o — rs r-r- rs oo — oo vo rn oo oo m —" — o" o" o" o o o o o V D VO V O VO VO o fN m Tt oo oo oo oo rn rn rn m V D [ — OO O — ON ON m m rN m T T in vo r-ON ON ON O N ON ON rn m m m rn m oo ON ON ON m m in in — — — in m r - m m m r-* oo o Tt o fN in in vo in m co oo in — vo ON o oo o — m vo rs Tt m r-» o vo Tt t-» — — rn —< rs m m oo rs rs rn m — rn vo — r- rs n rs^  rs^  rs" rs" rs" rs" C O C O C O oo fN^  fN^  fS_ rS^  in" in" u-T in" o o o o O N Tf Tf Tf o o o o vo vo vo VO m Tf m ON m O rs oo rs —i q o^  rs" rs" rN C O C O C O rs^  rs^  rs^  in in" m" rs rs ON O — — O O rs r- r- r-V D m rS ON ON ON m vo rs O N vo m —' vd r-* rn Tt vo. — rn t- O N m vo r-» t— c— oo o" o" o" o" o" o" oo oo oo oo oo oo rN. ° i <"si rs^  rs^  rs^  in" in" in in in in o o o o o o m Tf m o vo Tf oo O ON — O N — od rs •—• rs oo od rN ON r- rs — o r- vo vo vo \o_ vo^  fN" r-T rs" rs" rs" rs" o o o o o o VO VO VO vo vo vo rs vo — Tt vo m s ^ 'aj ~ *• B \0 gjj .Sf >, a «r .S a 160 h vo in \ t n t N - o a c o h v o i n ^ t m i S - o a c o r - vo m Tr m CN — o r- r- r— r-- r ^ r ^ r ^ t ^ v o v o v o v o v o v o v o v o v o v o i n «n in m m m in in in >n VO i/l 1/1 \D VO VO VD rN CN in in in O in oo m o r- oo co Tt m vo r- oo r- ON cN — in O in o O "n o vs co o h CN T t c N « n r M T t m rN — — CN — m m m m CN Tt ON m oo o o O T f r O N m m o o — Tt O oo CN vo o o o r~- m fN — —; av —; Tt CN — r-" — cd T t od ON od fN — ON rn' oo r~" — vo o O O vs CN T t m P-" — rsi in T t os t— T t c N c N O C N i n * — o v v o v o T t r n T t v o f N rN >n o v m o v m v o — Tt Ov ON CO CO O O C O C O M C O M C O h V O V O v D v D vo_ vq_ r-^  oc oq_ OO^  ON Ov_ 0_ —^  — O O O O O O O O 0 O C O O 0 00 00 0 O 0 O 0 O 00 00 00 O 0 O O 0 O O 0 O O O O OO OO OO OO CO CO t » CN CN (N fN N CS N CS M N CN N (N fN tN (N CN^  CN^  <N^  CN^  fN^  CN^  fN CN^  CN^  CN^  CN^  CN j O O O O o o o o o o o o o o o o o o o O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O — oo ON in r - r - . — oo — o — v o o N r - - T i - o v o m o o o o o r ^ r o T t c N i n O N oo od — m' O T t v o ' v o ' o o r o v d o o T t — i n ' o N v o o o o ro CN m ON t-^  od vd f l (N Oi VO — V O T t v o O C O f N O N i n ' T O O O O O r - - r N t~ -m ro ON vo ro fN CN fO <N CN — — o o o N o r N r n f N — — o o o o r - r - r — vo vo vo^  vs^  in in vs^  in O O O O o o o o o o o o o o o o o o o O O O O O O O O O VO VO VO VO V D V O V O V O V O V O V O V O V D V O V O V O V D V O V O VOVO VO VO VO VO VO VD VO o r - o o o N O — f N r o T f v s v o r - o o o N O — rN m Tt m vo r— o ON oo — — c N c N C N r N t N t N C N C N C N C N r o r o r o ro m m m m o o r - r -T t T t T t T t T t T f T f T f T t T t T t T t T t T f T t T t T t Tt Tt T f T f VO OO fN fN ON — CO OO Tf — co m vo oo rN ro —« oo oo co r— Tt rN (N r- oo Tt CN in T t r- o CN co o Tt m rN oo v o m r - vo vo m v i in m oo DO m — cN — m — CN ro r - m m T t O N T t o o o r - T t r - - o r - c o o o vs o in C N O O O ( ON — o — o" vd co co r~Tt"o'-— vdvoo'-^ m VD Tt" — m o o t r N T t T t r - ~ o o i n c o v o r - O N T t c N r n T t P - r - m ON o T t in m VD I m m m i n i n o o c o O N O N O N — — — c N C N m Tt^ TS*-_ in vs^  vs^  rN —; < ON" CN" ON" ON" ON" O " CN CN c N r N m m m m m m m m m m m CN CN ( i - , r - i > r - ( - . a ! X X c o c o c o o o o o o o o o o o co c o c o o o o o o o o o t C N r N r N C N C N C N C N C N ( N r N r N C N r N C N C N C N CN CN fN rN fN fN CN { vs vs vs vs vs vs v s v s m v s v s v s v s v s in O O O O O O O O O O O O O O O O o o o o o o o o i n T t T t r - v o r - O N O N m ON t~- — m — — ON — vo ON m o o o o o T t i n o r - m o N O N O N — TT in ON in CN ON O — co mi m — o o o c d r N r ~ - o d o v d c d i — r ^ o d i n r - " r - o r ~ - — in o ON in od d 6 611 m c o c o T t O T j - v o T t c o c o c - - m o o v s o o c o T t r - T t r^  vo r- oo vs II T t m r N — o v o m m vo vo oo os os fN ro r--^  ON P. "™V r i i f > i f Nill o" o" o" o' d " ON CN ON O N O N O N O N O N O O O O — — _ _ O O O O O O N O N O N ON ON ON ON ON O O O O O O O O O O O v o v o v o v o v o i n m i n i n i n i n i n i n v o v o v o vo vovo vo vo vo vo vo 161 U "a lm S l l n \ o T m ^ ^ M II o rj- o o o o o m "n O CN O O t— t— fN t CO m o w n o o o o c o i n o i n c o V O V O — O O C O C O O O T j - O O f N rn — r -o o o o m n in a •—1 CN rN o o o o o o in in m in a t - as CN m m oo — — — m — CNCNCN a 5 — — o r CN" CN" CN" OO OO CO CN^  cN_ rN^  in" m" vn" oo oo vo o o o vo vo vo CN rs OO CO (N rN VO O fN T f r-i K ( N m — Ch oo o_ o^ a\ ch_ rs" <N —" —" OO CO CO CO CN^  CN^  CN^  CN^  m" in" m" m" o o o o o o o o o (N in rn ori od Ch —i co »n -rf o o> Ch Ch —" o " o " o " o o o o VO VO vo VO C h o o r - v o m T j - r n c N - -o o c N C h T j - i n o o o o h \ c m - oo vo vo m — —• — m rN o r s — i n m o o o o o — Tr' v d —• m' —- d o ' o" oo rN in m in r- — Tr r-—^ rN^  fN^ rn_ rn rn TT^  TT^  TT^  CN" CN" rs" CN" of r i rN" rs" c i o o o o c o o o o o o o c o c o c o CN rN rN rN rN fN fN fN fN in m" m" m" in in in" in" m" o o o o o o o o o o o o o o o o o o C h r - r - O C N T j - O O O — m m r~-TT m vo in m m m in O CO O O O O O O O O O O d d d d o o o fN CN CN (N rN (N fN 00 00 00 OO CO CO CO CN^  CN CN^  CN^  CN^  fN^ fN^ in" in in in in" in" in" o o o o o o o o o o o o o o o o o o o o o ci <z> <zi <6 <~> <d- •d' O O T f O O O O O O r n ( N f N f N f N f N f N r N f N r N r N O O O O O O O O C O O O O O C O C O O O fN^  fN^  fN^  fN^ CN^  fN^  CN^  rN^  CN^  CN^  m" m" m" in in m <n in in in o o o o o o o o o o o o o o o o o o o o o o o o o o o o o v o d o o d d d d d d d c h o o o o VO VO vo vo a >A S sr S S 162 rS \o r> oo ^ VD ID Tf fN C N C N C N C N m — co •—• •—• •— —^  us oo t— vs vs vs vs vs vs co vs oo vi vo vo • O N © T t O O O O m T r O N — T f V O O f N O O v O — t <S -o ^ M r n ^ ^ l n ^ c o n ^ T ^ o v ^ l f l ^ o o r , ^ v o o ^ ^ f N fN fN fN fN — CN rn m — fN M — CN — VS TJ- m ov oo vo o o r- vo vs fN m — — cN — VS vs —; — m m Tt O O © vd rn O © © r~- — Tt C N fN O N rn, C N ^ C N ^ ON, C N " C N " C N " C N " — • " C O oo oo oo oo C N , ( N , ( N ^ C N , ( N , VS VS VS VS vs O O O O O © O O O © —* CNI O O O m *—" o © © tN © m rn m vo rn 0 \ ON_ VD^ © © O S ON ON © © ON O N ON vo vo vs vs vs v o v s T t m r N — c N r n — T t v s v o r - — - © o s v o v s T t m o o o o o o o o o o o o o o o N O N O O t - - - r - - r - - r - -fN fN fN fN — • VS VS r**~ co Tt t © vs vs © © vs vs © r - T t v o r - T t o v o T t v o —• — m rn fN — C N r- © — oo r- r- o vd t-^  —'> vd vs rn © - CO — h- vo ON I— vo r— *— vo vo t vs © © o © © o © o Tt r— Tt rn ™ © © o o o o" ©' ©' — rN Tt rN © a > © O N O N v o T t — oo vs vs m — c o r ^ c o r - - r - - r - - r ^ r - - v o v s f N C N C N — ^ ^ „ ^ „ _ _ ^ _ _ ^ _ ^ _ l _ ^ - , r s | r N f N f N c o o o o o o o c o c o o o o o o o o o o o o o o o o o o o c o c o c o o o o o °i rS. °i ^ rH. f N i , (N„ ^  rH, r " i ^ ^ r s l °i °i ""1 '"H. 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CN^  fN^ fN, fN^ in" m" m" m" m" in in in in in in o o o o o o o o o o o o O O o o o o o o o o o o o o o o o o o o p o o o O o o o o o o o o o o o o vo o © Tf o d fN O © ON ON © fN ON © VO CO d m oo © oo o Tf r-^  OIL' © oo VD, © m VO, ,500. ,440. ,380. ,350. d r-fN^  ,230. ,200. ,080. ,940. ,380. ,240 ,280 601, 601, 601, 600, 600, 600. 600. 600. o" o VO o o VO o" o vo o" O VD o o vo o o vo o o VO o o vo o" o vo o o VO o o VO o o VO ON ON *n o o vo o VO o VO .5 o B 8 165 OO C\ m m m m vo vo rn m _ H „ _ ^ _ ^ _ _ H _ - _ ^ _ H —, _ ^ _ »ri v> i—i co ^ —. so vo vo — —' — 1 >n o c o u n v o o o o o o N O m — CN •—i — m T t r N v o L n o r - r - T t v o r - - o o r - - o o c o c o r - - -v o r n t ^ r ^ c o u n o o u n v o u n r - - T r ^ O h i n r n o s T t i r i r n v o o O T j - o o — CN <N — cs CN r s — CN © i/i vo — oo un rs vp rn ON un i/i CN" CN OO OO rN rs rs rN oo oo rs rs rs rs oo oo cs cs CN m TT un vo ON un i/i i/i un un r-u n v o v o u n u n u n r ~ - u - ) u - i i / i v o u * i — u n v o -r- r- oo TT un r- os r— o o o o o o c N O v - T f o o m O N v o o v u n — — ON © cs m ON ^ ^ _ — rs rs — rs r - ( N u n r s r s v o o u n o — o o c s o o o o t ~ - r n c s r n r - - T f u n m r s — — cs — CN rN — rs m r s © O © © © ON d d © ' © ' © " oo Tf vo P - ON © oo rs rs rn m T f © DO oo oo oo oo oo rs^ rs^ rs^ rs^ cs_ cs_ u-f un un un un un © © O O O O O © © © © © © © © o o o © © © d d © I ON O Tf o vo vo r-^  oo_ oo^  o\ o" ©" ©" ©" ©" ©" © © © © o o vo vo vo vo vo vo C vo « rn S ST .5 fl B S 166 1 « r- i n oo —H i n i n ON VD — O Tt O vO t— ON o t— m fN vo — — r n n CN oo i n T r T t CN m © o n CN ON Tt vd i n — CN r- r-CN" CN oo oo CN fN r- vo ON OO — CN OO ON OO CO fN fN m Tt in vo ON ON ON ON fN fN fN fN m v o v D i n T t v o i n m m oo m m in — « o - T i C o ™ - i r i o o - « " ' H r - r - - H - vs rn vo r-o Tt o co O Tt r- r- v o O m T t v o v o — v o — - c o — 0 \ r- r-—i — fNCN — O m T f f N C N — O oo vo i n r n m co fN o i n Tt t i n CN m —' CN —* m m fN i n rN fN OO CO CN CN fN ON m ON vo, vo^  rsf rN CO CO fN CN CN US _g > C v i S -a 8 .SP ft 13 *« .5 a 6 S 2 S3 a •= e w 167 APPENDIX D SULPHUR ISOTOPE DATA 168 New Sample No. Old Sample No. Mineral percentages Comments Analysis Delta S (%o) BH-1 BH-2 BH-4 BH-4 BH-5 LEN-1 LEN-2 LEN-3 NF-1 NF-2 NF-3 NF-4 NF-5 50% Pyrite, 20% Cu-Au occurrence Blackhawk O.C 5 (1) Ccy proximal to 20% Asp, 10% Pyrr North Fork Blackhawk O.C 5 (la) 50% Pyrite, 20% duplicate Ccy 20% Asp, 10% Pyrr 60% Py, 10% Cu-Au occurrence Blackhawk O.C 5 (2) Ccy proximal to 10% Pyrr, 20% Asp North Fork Blackhawk O.C 5 (2a) 60% Py, 10% duplicate Ccy 10% Pyrr, 20% Asp 90% Asp, 10%o Cu-Au occurrence Blackhawk O.C 5 (3) Py proximal to N.F Lennox mine W. A 80% Py, 15% Ccy, 5% Gangue 75% Py, 15% Epithermal Cu-Au Ccy, vein occerence 5% Asp, 5% proximal to North Gangue Fork 75% Py, 15% Epithermal Cu-Au Ccy, vein occerence 5% Asp, 5% proximal to North Gangue 40% Ccy, 50% Py, S-isotope duplicate 2.49 2.38 S-isotope duplicate 2001 (1) Lennox mine W.A 2001 (2) Lennox mine W.A 2001 (3) DH 17. 84'2" S-isotope S-isotope S-isotope S-isotope Fork Diamond drill hole S-isotope DH 17. 18'0" SC-720A. 229'8" North Fork upper main road O.C-268 Diamond drill hole Diamond drill hole 5% Asp, 5% Gangue 60% Py, 40% Gangue 65% Py, 15% Ccy, 20% Gangue 30%, Py, 10% Mafic latite Ccy, porphyry 60% Gangue 60% Py, 20%, Mafic latite Ccy, porphyry 5% Asp, 5% Bn, 20% Gangue S-isotope S-isotope S-isotope S-isotope 2.49 2.51 2.42 2.92 2.98 3.0 2.94 2.71 2.77 2.32 2.46 169 A P P E N D I X E GEOCHRONOLOGICAL DATA 170 U-Pb geochronology of igneous rocks in the North Fork area J.K. Mortensen Introduction Uranium-lead zircon dating of five samples of intrusive and extrusive rock units from the North Fork area was undertaken using conventional ID-TIMS methods in order to establish precise crystallization ages for the main igneous events in the region. Methodology Zircon was separated from 5-15 kg samples using conventional crushing, grinding, Wilfley table, heavy liquids and Frantz magnetic separator techniques. U-Pb analyses were done at the Pacific Centre for Isotopic and Geochemical Research at the University of British Columbia. The methodology for zircon grain selection, abrasion, dissolution, geochemical preparation and mass spectrometry are described by Mortensen et al. (1995). Al l zircon fractions were air abraded (Krogh, 1982) prior to dissolution to minimize the effects of post-crystallization Pb-loss. Procedural blanks for Pb and U were 2 and 1 pg, respectively. U-Pb data are plotted on a conventional U-Pb concordia plot in Figure 2.9. Errors attached to individual analyses were calculated using the numerical error propagation method of Roddick (1987). Decay constants used are those recommended by Steiger and Jager (1975). Compositions for initial common Pb were taken from the model of Stacey and Kramer (1975). Al l errors are given at the 2 sigma level. I i 171 Results Sample SRM-113: Zircons recovered from this and all of the rest of the samples in the study were mainly colorless to pale brown, euhedral, stubby to elongate prisms with no visible growth zoning or cores. Many of the zircons in each of the five dated samples contained rare to moderately abundant, clear, bubble-shaped inclusions, which presumably contained much of the common Pb contained within the fractions and account for the relatively low measured 2 0 6Pb/ 2 0 4Pb ratios. Five fractions of the coarsest, least magnetic zircons from sample SRM-113 were analyzed after strong abrasion. Al l of the analyses are concordant, but yield a narrow range 90*' 9"?R of Pb/ U ages, which is. attributed to the effects of minor post-crystallization Pb-loss that affected the different fractions to varying degrees. Fraction A yields the oldest Pb/ U age of 37.2 ± 0.2 Ma, which is considered to be the best estimate for the crystallization age of the rock. Sample SMR-111: Five strongly abraded zircon fractions all yield concordant analyses with a narrow range of 2 0 6 Pb / 2 3 8 U ages. Sample SMR-108: Four strongly abraded fractions yield systematics similar to the previous sample). Fractions A and D yield overlapping analyses with, which is interpreted as the crystallization age of the sample. SampleSMR-106: Five strongly abraded zircon fractions analyzed. Four fractions display similar systematics to the previous two samples, and a crystallization age of 36.8 ± 0.2 Ma is 172 assigned based on the total range of Pb/ U ages for two overlapping analyses (A and D). 206 238 Fraction C was visually identical to the other fractions but gives a considerably older Pb/ U age (52.8 Ma), indicating that this fraction contained a "cryptic" older inherited zircon component, either as one or more xenocrystic zircons or as inherited cores that could not be distinguished visually. Sample SRM-110: A very small amount of fine grained zircon was recovered from this sample. Al l of the zircon from the sample was strongly abraded and analyzed as a single fraction, which gave a concordant, although imprecise analysis with a Pb/ U age of 38.9 ± 0.3 Ma. Since we cannot assess the amount of Pb-loss that these zircons may have experienced (if any), we interpret the Pb/ U age as a minimum age of crystallization for the sample. References Krogh, T.E., 1982, Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique: Geochimica et Cosmochimica Acta, v. 46, p. 637-649. Mortensen, J.K., Ghosh, D., and Ferri, F., 1995, U-Pb age constraints of intrusive rocks associated with Copper-Gold porphyry deposits in the Canadian Cordillera, in Schroeter, T.G., ed., Porphyry deposits of the northwestern Cordillera of North America: Canadian Institute of Mining and Metallurgy, Special Volume 46, pp. 142-158. Roddick, J.C., 1987, Generalized numerical error analysis with application to geochronology and thermodynamics: Geochimica et Cosmochimica Acta, v. 51, p. 2129-2135. Stacey, J.S. and Kramer, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two-stage model: Earth and Planetary Science Letters, v. 26, p. 207-221. Steiger, R.H. and Jager, E., 1977, Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, p. 359-362. 173 U-Pb analytical data for igneous rocks North Fork area. Sample Description Wt (mg) U (ppm) 2 Pb (PP m) 206 p b /204 p b (mcas.) total common Pb (Pg) % 208 p b 2 206 p b /238 u ' . ( ± % l o ) 207 p b / 235 l / ( ± % l a ) 207 p b / 206 p b 4 ( ± % l o ) 2 0 6 p b / 2 3 8 U a g c (Ma; ± % 2 a) 2 0 7 P b / 2 0 6 P b a (Ma; ± % 2o-Sample SRM-113 A: N2.+I04 0.195 448 0.7 233 34 14.0 0.00578(0.24) 0.03726(3.09) 0.04678(2.96)0. 37.1(0.2) 38.0(141.0) B: N2.+104 0.125 682 1.0 840 9 12.3 0.00572(0.19) 0.03695(1.24) 0.04686(1.18) 36.8(1.2) 42.3(56.2) C: N2.+104 0.212 624 0.8 556 20 11.3 0.00557(0.15) 0.03599(0.48) 0.04684(0.38) 35.8(0.1) 41.2(18.3) D:N2,+ I04 0.265 764 1.1 796 22 13.2 0.00565(0.17) 0.03654(1.43) 0.04692(1.38) 36.3(0.1) 45.0(66.1) E: N2.+ 104 0.135 843 1.2 1023 9 13.3 0.00572(0.14) 0.03676(0.39) 0.04659(0.31) 36.8(0.1) 28.0(14.8) Sample SRM-111 A: N 1.+ 104 0.129 310 0.5 541 6 16.1 0.00568(0.22) 0.03663(1.56) 0.04682(1.50) 36.5(0.2) 39.9(71.8) B: N1.+ 104 0.170 724 1.0 716 14 11.4 0.00578(0.14) 0.03724(0.42) 0.04672(0.34) 37.2(0.1) 35.1(16.3) C: N1.+ 104 0.144 393 0.6 827 6 16.6 0.00573(0.21) 0.03698(1.15) 0.04680(1.08) 368(0.2) 39.0(51.5) D:N1,+ 104 0.142 302 0.4 274 14 16.3 0.00571(0.26) 0.03658(0.90) 0.04643(0.75) 36.7(0.2) 19.7(36.2) E: N1 ,+104 0.172 78 0.5 579 8 16.5 0.00579(0.14) 0.03733(1.15) 0.04673(1.10) 37.2(0.1) 35.6(52.6) Sample SRM -108 A: N1.+104 0.147 177 1.1 316 31 19.2 0.00578(0.15) 0.03723(1.72) 0.04675(1.64) 37.1(0.1) 36.5(78.6) B: N1.+ 104 0.136 197 1.3 514 19 19.4 0.00570(0.14) 0.03673(0.48) 0.04672(0.39) 36.7(0.1) 34.9(18.7) D: N1.+104 0.157 188 1.2 452 24 19.8 0.00575(0.20) 0.03731(2.22) 0.04703(1.13) 37.0(0.1) 50.9(102.5) E: N1.+ I04 0.103 195 1.2 415 18 19.0 0.00574(0.12) 0.03719(0.54) 0.04702(0.46) 36.9(0.1) 50.3(22.0) Sample SRiV -106 A: N2.+74 0.092 114 0.7 347 11 12.5 0.00573(0.24) 0.03692(0.97) 0.04674(0.87) 36.8(0.2) 36.2(41.5) B: N2.+74 0.048 198 1.2 510 7 12.6 0.00566(0.16) 0.03658(0.75) 0.04691(0.68) 36.4(0.1) 55.7(32.6) C: N2.+74 0.053 121 1.0 901 4 13.1 0.00823(0.18) 0.05330(0.73) 0.04699(0.67) 52.8(0.2) 48.9(32.0) D: N2.+74 0.062 224 1.4 844 6 19.3 0.00571(0.19) 0.03694(0.57) 0.04689(0.52) 36.7(0.1) 43.6(25.1) E: N2.+74 0.034 161 0.9 498 4 14.5 0.00555(0.17) 0.03582(1.18) 0.04681(1.11) 35.7(0.1) 39.8(53.3) Sample SRM-110 A: N10.+44 0.019 153 1.0 166 7 18.3 0.00605(0.44) 0.03930(5.9) 0.04714(5.66) 38.9(0.3) 56.1(262.0) N1 ,N2 = non-magnetic at n degrees side slope on Frantz magnetic separator; grain size given in microns; n umber = number of grains in the analysis; t = tabular grains; s = stubby prisms; e = elongate irisms; u = abraded. radiogenic Pb; corrected for blank, initial common Pb, and spike corrected for spike and fractionation corrected for blank Pb and U, and common Pb 174 

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