Reviews in Economic Geology, Vol. 2
Transcription
Reviews in Economic Geology, Vol. 2
Reviews in Economic Geology Volume 2 GEOLOGY AND GEOCHEMISTRY OF EPITHERMAL SYSTEMS CONTENTS The Geothermal Framework for Epithermal Deposits R.W. Henley A Practical Guide to the Thermodynamics of Geothermal Fluids Hydrothermal Ore Deposits R.W. Henley and K.L. Brown The Behavior of Silica in Hydrothermal Solutions R.O. Fournier Carbonate Transport and Deposition in the Epithermal Environment R.O. Fournier Fluid Inclusion Systematics in Epithermal Systems R.J. Bodnar, T.J. Reynolds, and C.A. Kuehn Light Stable-Isotope Systematics in the Epithermal Environment Geologic, Mineralogic, and Geochemical Characteristics of Volcanic-Hosted Epithermal Precious-Metal Deposits C.W. Field and R.H. Fifarek D.O. Hayba, P.M. Bethke, P. Heald, and N.K. Foley Geologic Characteristics of Sediment-Hosted, Disseminated Precious-Metal Deposits in the Western United States W.C. Bagby and B.R. Berger Relationship of Trace-Element Patterns to Alteration and Morphology in Epithermal Precious-Metal Deposits M.L. Silberman and B.R. Berger Relationship of Trace-Element Patterns to Geology in Hot-Spring Type Precious-Metal Deposits B.R. Berger and M.L. Silberman Boiling, Cooling, and Oxidation in Epithermal Systems: A Numerical Modeling Approach M.H. Reed and N. Spycher Using Geological Information to Develop Exploration Strategies for Epithermal Deposits S.S. Adams Editors B.R. Berger and P.M. Bethke SOCIETY OF ECONOMIC GEOLOGISTS, INC. Society of Economic Geologists, Inc. Reviews in Economic Geology, Vol. 2 Geology and Geochemistry of Epithermal Systems B.R. Berger and P.M. Bethke, Editors J.M. Robertson, Series Editor Additional copies of this publication can be obtained from Society of Economic Geologists, Inc. 7811 Shaffer Parkway Littleton, CO 80127 www.segweb.org ISBN: 978-1-629495-60-6 The Authors: Robert 0. Fournier Samuel S. Adams Branch of Igneous and Geothermal Processes 3030 Third Street Boulder, CO 80302 U.S Geological Survey MS 910 William C. Bagby Branch of Western Mineral Resources 345 Middlefield Road Menlo Park, CA 94025 U.S. Geological Survey Daniel 0. Hayba MS 901 345 Middlefield Road Branch of Resource Analysis Menlo Park, CA 94025 U.S. Geological Survey MS 959, National Center B. R. Berger Branch of Exploration Geochemistry Reston, VA 22092 U.S. Geological Survey Pamela Heald Branch of Resource Analysis . U.S. Geological Survey MS 959, National Center Reston, VA 22092 MS 973 Box 25046, Federal Center Denver, CO 80225-0046 Philip M. Bethke Branch of Resource Analysis R. W. Henley U.S. Geological Survey Chemistry Divsion MS 959, National Center D.S.I.R., Private Bag Taupo Reston, VA 22092 New Zealand R. J. Bodnar Department of Geological Sciences Virginia Polytechnic Institute and State University C. A. Kuehn Blacksburg, VA 20461 The Pennsylvania State University Department of Geosciences University Park, PA 16802 K. L. Brown Chemistry Division Mark H. Reed D.S.I.R., Private Bag Department of Geology Taupo University of Oregon New Zealand Eugene, OR 97403 Cyrus W. Field T. J. Reynolds FLUID, Inc. Department of Geology Oregon State University P.O. Box 6873 Corvallis, OR 97331-5506 Denver, CO 80206 Richard H. Fifarek M. L. Silberman Department of Geology Branch of Exploration Geochemistry U.S. Geological Survey MS 912 Southern Illinois University Carbondale, IL 62901 Box 25046, Federal Center Denver, CO 80225-0046 N. K. Foley Branch of Resource Analysis N. Spycher U.S. Geological Survey Department of Geology MS 959, National Center University of Oregon Eugene, OR 97403 Reston, VA 22092 ll GEOLOGY & GEOCHEMISTRY OF EPITHERMAL SYSTEMS CONTENTS FOREWORD X • PREFACE xvi B I OGRAPHIES CHAPTER 1 THE GEOTHERMAL FRAMEWORK OF EPITHERMAL DEPOSITS R. W. Henley INTRODUCT ION HYDROTHERMAL SYSTEMS IN GENERAL Co l l i s ion-Related Amagma t ic Hydro thermal S y s tems Terre s trial Magma-Rel a t e d Hydro thermal Sys tems 4 TERRESTRIAL MAGMATIC-HYDROTHERMAL SYSTEMS Large Scale S t ructure Na tura l D i s charge s Hydrothermal Erup t i on Vents He a t and Ma s s Fl ow in Geothermal Sys tems 11 CHEMISTRY OF GEOTHERMAL D I S CHARGES EPITHERMAL ORE-FORM ING SY STEMS • 12 • Re quirememts for Ore Dep o s i t ion Chemis try o f Sys tems Re s pons ible for Ore Fo rmation Chemical and Phy s i ca l Proce s s e s in Ore Fo rmation Ho s t-Ro ck Relat ions SUMMARY 19 EP ILOGUE 21 ACKNOWLEDGMENTS 21 REFERENCE S • • 21 • CHAPTER 2 A PRACTICAL GUIDE TO THE THERMODYNAMICS OF GEOTHERMAL FLUIDS AND HYDROTHERMAL ORE DEPOSITS R. w. Henley and K. L. fuoo'/Jn INTRODUCTION • • • • • • 25 • GEOLOG I CAL CHARACTERI S T I C S OF THE BROADLANDS GEOTHERMAL SYSTEM • 25 26 FLU ID CHEMISTRY iii FLUID-MINERAL EQUILIBRIA : ALTERATION MINERALOGY . . . . . . . FLUID-MINERAL EQUILIBRIA : TRACE-METAL CONTENTS Lead Gol d �r Me t a l s : . . . . . . . . . . . . 28 32 Copper, Sil ver, and Ars e nic . . . . . . . . . . . . . . MINERAL DEPO S I T ION . . . . 36 . . . . . 41 . . . Silica caiCfte Met a l S u l f id e s and Gold . . . . . ACKNOWLEDGMENTS . . . . . . . . . . . . . REV IEW QUE S TIONS . . . . . . . . . . 41 REFERENCE S APPENDI X 41 43 • CHAPTER 3 TBR BEHAVIOR OF SILICA IN HYDROTHERMAL SOLUTIONS R. 0. Fournie1' INTRODUCTION 45 SOLUBILITIES OF S IL I CA MINERALS 45 THE BEHAV IOR OF D I S SOLVED S IL I CA IN HOT- SPRING SYSTEMS . . . . . . . . . . . . . . . 46 ALKALINE WATERS 48 ACID WATERS 50 REACTION WITH GLA S S 51 . . . . . . . . . . . . . . . . . AMORPHOUS S I L I C A- CHALCEDONY RELATIONS 51 51 SPECULATIONS REGARDING SOME TEXTURES OF QUARTZ Ja speroid and Mas s ive Replacement of Lime s t one by Sil ica quart z Solub i l i t y a t High Tempera tures C ONCLU S IONS • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • ACKNOWLEDGMENTS REFERENCE S APPENDIX • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv 55 56 56 60 CHAPTER 4 CARBONATE TRANSPORT AND DEPOSITION IN THE EPITHERMAL ENVIRONMENT R. 0. Four>nie1' INTRODUCTION CO z • • • • • 63 • 63 DI S SOLVED IN AQUEOU S SOLUTIONS THE SOLUBILITY OF CALCITE IN AQUEOU S SOLUTIONS 67 SUMMARY 71 REFERENCE S 71 CHAPTER 5 FLUID-INCLUSION SYSTEMATICS IN EPITHERMAL SYSTEMS R. J. Bodna1', T. J. ReynoLds, and C. A. Kuehn INTRODUCTION • • • • • • • • • • • • • • • • • • • • 73 • INFORMATION AVAILABLE FROM FLUID-INCLU SION PETROG RAPHY 73 IDENTIFICATION OF FLUID INCLU SIONS TRAPPED FROM BOILING SOLUTIONS 79 IDENTIFICATION OF GAS E S IN FLUID INCLU SIONS FROM THE EPITHERMAL ENVIRONMENT • • • • • • • • • • • • • • • • • • • • • • • 83 • 93 INTERPRETATION OF FLUID INCLU SIONS FROM THE EPITHERMAL ENVIRONMENT APPLICATION OF FLUID INCLU SIONS IN EXPLORATION FOR EPITHERMAL PRECIOU S-METAL DEPOSITS • • • • • • • • • • 94 • 95 SUGGESTIONS FOR FUTURE FLUID-INCLU SION RE SEARCH REFERENCE S • • • • • 96 • CHAPTER 6 LIGHT STABLE-ISOTOPE SYSTEMATICS IN THE EPITHERMAL ENVIRONMENT C. W. FieLd and R. H. Fifa1'ek INTRODUCTION 99 CONVENTIONS , SYSTEMATI C S , AND RATIONALE 99 Fra c t iona t i on Equ i l ibrium Rea c t ion App l icat ions GEOLOGIC DI STRIBUTIONS • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 110 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 113 Hydrogen and Oxygen Carbon Sul fur EPITHERMAL DEPOSITS • • Carbon Sul fur v Hyd rogen and Oxyg en SUMMARY 124 RE FERENCE S 125 CHAPTER 7 GEOLOGIC, MINERALOGIC, AND GEOCHEMICAL CHARACTERISTICS OF VOLCANIQ-HOSTED EPITHERMAL PRECIOUs-METAL DEPOSITS D. 0. Hayba, P. M. Bethke, P. Heatd, and N. K. Fotey INTRODU CTION 129 • 129 SUMMARY OF THE CHARACTERISTI C S OF VOLCANI C-HOSTED EPITHERMAL ORE DEPOSITS Charac t e r i s t i c s o f Adu laria- Se r i c i t e-Typ e Dep o s i t s Chara c t e r i s t i c s o f Ac i d-Sul fate-Type Depo s i t s Summary o f Cha rac t e r i s t ics • • • • • • • • • • • • • • • • • 136 • • • • • • • • • • • • • • • • • 151 GEOTHERMAL INTERPRETATION OF VOLCANI C-HO STED EPITHERMAL nEPOSITS • • • • • • • • • • • • • 158 THE ADULARIA- S ERICITE ENVIRONMENT : CREEDE AS AN EXAMPLE Creede as an Exemp lar Summ a ry o f Importan t Stud i e s Geolog i c and Mineralogic Charac t e r i s t i c s Geochemical Env i ronment Hydrologic Env i ronment Boil ing and Mixing in the Ore Zone Summary o f Creede Mineral i z a t i on THE ACID- SULFATE ENVIRONMENT : SUMMITVILLE AS AN EXAMPLE Geolog ic and Mine ralogic Charac t e r i s t i c s Geochemical Env i ronment Summ a ry of Summi tville Minera l i z a t i on Adu laria- Se r i c i t e Depo s i t s Ac id-Su l f a t e Dep o s i t s ME CHANISMS O F ACID- SULFATE ALTERATION 159 ACKNOWLEDGMENTS 162 REFERENCE S • • 162 • CHAPTER 8 GEOLOGIC CHARACTERISTICS OF SEDIMENT-HOSTED, DISSEMINATED PRECIOUs-METAL DEPOSITS IN THE WESTERN UNITED STATES W. C. Bagby and B. R. Berger INTRODUCTION • 169 • 169 CLAS SIFICATION REGIONAL GEOLOGI C CHARACTERISTI C S OF DEPOSITS IN MINERAL TRENDS AND ISOLATED DEPO SITS • • • • • • • • • • • • • The Getche l l Trend The Carlin Trend vi • • • • • • • • • • • • • • • • • • • • 172 The Co r t e z Trend Is o l a t e d Depos i t s GEOLOGIC CHARACTERI STIC S O F THREE END-MEMBER , SEDIMENT-HOSTED , DI SSEMINATED PRECIOU S-METAL DEPOSITS • • • • • • • • • • • • • • • • • • • • • • • • • • 183 Carlin Taylor Preble GENERAL ASPECTS OF TRACE ELEMENT AND STABLE-I SOTOPE GEOCHEMISTRY 189 • 192 SUMMARY OF GEOLOGIC CHARACTERI S TIC S Re g ional and D i s t r i c t Sc ale Depos i t Sc ale ENVIRONMENT OF FORMATION 195 EXPLORATION APPLICATION 195 INFLUENCE OF GEOLOGIC CHARACTERI STIC S ON MINING 196 Grade and Tonnage Mine ab i l i ty REFERENCE S • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 1 99 CHAPTER 9 :RELATIONSHIP OF TRACE-ELEMENT PATTERNS TO ALTERATION AND MORPHOLOGY IN EPITHERMAL PRECIOUs-METAL DEPOSITS M. L. Silberman and B. R. Berger INTRODUCTION • • 203 • GEOTHERMAL SY STEMS 204 • Morphology and Charac t e r i s t i cs Al t e ra t i on Pa t t e rns Geo chemical Zone s EPITHERMAL ORE DEPOSITS • • • • • • • • • • • • • • • • • • • • • • • • • • • 208 Morphology and Charac t e r i s t ic s Al terat ion Pat t e rns NATURE OF FLUID S INVOLVED IN GEOTHERMAL SYSTEMS AND EPITHERMAL ORE DEPOSITS TIMING • • • 21 3 214 • GEOCHEMICAL ZONING IN EPITHERMAL DEP O SITS 214 BODIE MINING DISTRICT 21 5 Large- scale Vertical Zoning at Bod i e Bluf f--The Big Pic ture De t a i le d Lat e ra l Zoning PARAMOUNT MINING DISTRICT- -VERTICAL ZONING 224 SUMMARY 227 vii 228 A CKNOWLEDGMENTS REFERENCES • • 230 • CHAPTER 10 RELATIONSHIPS OF TRACE-ELEMENT PATTERNS TO GEOLOGY IN HOT-SPRING-TYPE PRECIOUs-METAL DEPOSITS B. R. Berger and M. L. Silberman INTRODU CTION • • • • • • • • • • 233 • CONTROL S ON TRACE-ELEMENT PATTERNS 233 TRACE-ELEMENT PATTERN S IN STUDIED DEPOSITS 235 Ha sbrouck Mount ain , Ne vada Round Mount ain , Ne vada DI S CU S SION 245 REFERENCES 246 CHAPTER 1 1 BOILING, COOLING, AND OXIDATION IN EPITHERMAL SYSTEMS: A NUMERICAL MODELING APPROACH M. H. Reed and N. F. Spyaher INTRODUCTION 249 BOILING 249 BOILING RESULTS 252 DISCU S SION OF BOILING AND COOLING 252 Sul f i de and Carbona t e M inera l Precipi t a t ion Pre c i p i t a t i on o f S i l icates Bo i l i ng Wi thout Frac t i onat i on and Cool ing Onl y SUPER- AND SU B-I SOENTHALPIC BOILING 258 BOILING AND GOLD PRE CIPITATION 261 THE HOT-SPRING ENVIRONMENT • 262 • Conden s a t ion of the Bo iled Gas Oxida t i on o f Ga s e s t o Produce Ac i d- S u l f a t e Wat e r s Re act ion o f Ga s e s with Me teoric G r ound Wat e r G o l d P r e c i p i t a t ion f rom M ixing o f Ac id- Su l f a t e Wa t e r w i t h Bo i l e d Aqueous Phas e Go ld P r e c i p i t a t ion f rom Mixing of Oxyg enated Ground Wa t e r wi th Bo iled Aqueous Phas e SUMMARY 269 ACKNOWLEDGMENTS 270 REFERENCES • • 270 • viii OIDW�Rl2 USING GEOLOGICAL INFORMATION TO DEVELOP EXPLORATION STRA�GIES FOR EPITHERMAL DEPOSITS S. S. Adams INTRODUCTION • • • • • • • • • • • • • • • • • • • • • • S OME CONSIDERATIONS IN THE USE OF GEOLOGICAL INFORMATION IN EXPLORATION 273 273 • 274 STRATEGIC FACTORS Organ i z a t i onal Obje c t ives Commod i ty Prices Financ ial Re s ourc e s Explora t ion Organi zat ion Regu l a t i ons and Land Avai l abi l i t y Compet i t o r Act ivity Previous Exp lorat ion Geolog i c Inf o rma t ion Explora t i on Me thod s Opportun i t i e s Ri sk HUMAN FACTORS • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 279 • • • • • • • • • • • • • • • • • • • • • • • • 282 • • • • • • • • • • • • • • • • • • • • • • • • 286 Personal Objec t ives Educat i on and Training Problem Solv ing Intu i t ion and Crea t i v i t y Uncertainty Avers ion t o Lo s s DEVELOPMENT OF MINERAL-DEPO SIT MODELS Organ i z a t ion of Geologic Informat i on Model Te rminology Level o f Mod e l Developmen t DATA- P RO CE S&- C RITERIA MODEL De f in i t ion of a Mine ra l-Depo s it Type Comp i l a t i on of Analog Depo s i t s Se l e c t ion o f G e o l o g i c Dat a Dat a-Proce s s Linking Ident i f i c a t ion o f Format ion Proce s s e s Evaluat ion o f Dat a-Proc e s s Links Se l e c t ion of Diagno s t ic Cri t e r i a Evalua t i on o f Data-Proc e s s - Cr i t eria Mode l App l ica t i on o f Data-Proce s s-Criteria Model t o Explorat i on Summary of Da t a-Proc e s s-Gr i t e r i a Model 296 CONCLU SIONS REFERENCE S 297 • TABLE OF CONVERSION FACTORS • • • • • • • • . • • • • • • • . • • • • • • • • . • In s i de Back Cover FOREWORD Exploration Geochem istry, U.S. G eological Survey, Denver, Colorado. These ladies, M arilyn A. Billone, Candace A. Vassalluzzo, and especially Pamela S. Detra and Dorothy B. Wesson, accom plished the long, arduous, and often frustrating j ob of assem bling, editing, and form attin g the book with a uniformly high level of professionalism and good che er. Their efforts are gratefully acknowledged. Carol Hjellming of the New Mexico Bureau of Mines and Mineral Resources (NMBM MR) editing staff checke d, balanced, and helped interpret the chemical equations; Lynne McNeil (N MBMMR) form at ted the cutlines. Lastly, I wish to express my continuing appreciation to the New Mexico Bureau of Mines and Min eral Resources and its Director, Frank Kot tlowski, who provide the Series Editor with time, space, and encouragement. Geology and Geochem istry of Epithermal Systems--Volume 2 of Reviews in Economic Geology--was created to accompany a Society of E conomic Geologist s (SE G) short course of the same nam e that was given in October, 1 9 85, prior to the annual m eetings of the Geological Society of Am erica and Associate d Societies in Orlando, Florida. As was the case with Volum e 1, the final published version of Volum e 2 unfo rtunately postdates the short course by som e m onths. Geology and Geochemistry of Epithermal Systems presents a synthesis of the current under standing of the processes responsible for the concentration of m etals (especially gold and silver) in near-surface environments, provides an overview of the system atics of the m ost important appr oaches to the study of epithermal ores and processes, and summ arizes the geology of both sediment-hoste d and volcanic-hoste d epitherm al precious-metal deposits. After the volum e edi tors, the m ost signifi cant contributors to the production of this volum e were the mem bers of the Editorial Suppo rt Group, Branch of James M. Robertson Series Editor Socorro, N M M arch, 1 986 X PREFACE In a speech on May 1 0, 1 9 1 1 , before the Geological Society of Washington, Waldemar Lindgren described his system atic classification of all types of mineral deposits. One of his categories included deposits related to intrusive and eruptive igneous rocks that form veins at shallow depths that contain open cavity filling textures and that have been a primary source of "bonanza" grades of gold and silver--the epitherm al deposits. Historically, most of the ores in epithermal systems have been mined from quartz veins, breccias, or disseminations that are as.sociated with non-marine volcanic rocks. Open-space filling textures and structures are common--com b structure, crustification, symm etrical banding, and crystal-lined vugs. Ore minerals include native gold, native silver, electr um , argentite, sulfosalts, tellurides, and selenides and often the com mon sulfides sphalerite, galena, and chalcopyrite. Common gangue minerals are quartz, adularia, calcite, barite, rhodochrosite, and fluorite. Alteration is commonly widespread in epi thermal systems, particularly in the upper portions of the vein systems; among the alteration phases are quartz, adularia, illite, chlorite, alunite, and kaolinite. Lindgren ( 1 928) recognized the dif ficulty of developing a rigid subsidiary classification scheme for epithermal deposits; he separated them into six categories: 1. 2. 3. 4. 5. 6. Gold deposits Argentite-gold deposits Argentite deposits Gold selenide deposits Gold telluride deposits Gold telluride deposits with alunite Nolan ( 1 933) and Fer guson ( 1 929) felt that few of these six characteristics were restricted enough to be diagnostic and proposed only two classes of epithermal systems based on the weight ratio of gold to silver, silver-gold, and gold-silver. Based on his experience with deposits in Nevada, Ferguson ( 1 929) found that there is a bim odal distribution of gold-silver ratios, and Nolan ( 1 933) felt that the bim odality was due to genetic processes. For the silver-gold deposits, Nolan ( 1 9 33) noticed that through-going fault fissures control the ore and felt that this implies a deep origin for the source of the m etals. Nolan ( 1 933) also n oted that the precious metal ores are ver y com monly sharply limited above and below by approxim ately parallel surfaces refer red to as the ore "horizon." He suggested that these limits are related to tem perature. Base metals tend to increase at and below the base of the lower surface of the precious-metal ore. Figure 1 is a longitudinal, vertical proj ection of the Last Chance-Confidence silver-gold vein in the Mogollon mining district, New C o nfi d e n c e La s t C h a n c e 70o-FT LEVEL 500 0 I 000 FEET 900-FT LEVEL Figure P.l. Vertical, lorigitudinal projection of the Confidence-last Chance vein in the Mogollon m1n1ng district, New Mexico (Ferguson, 19 2 7). Banded quartz vein is continuous along strike with ore grade material occurring in specific masses {stippled areas ) in the vein. The tops and bottoms of the silver-ridh ore bodies describe near parallel surfaces referred to as the "ore hor i zon." xi I Mexico (Ferguson, 1 927) il lustrating the ore horizons, the shape of ore bodies, and the typical distribution of ore grades within a continuous banded quartz-adularia sericite vein. Burbank ( 1 9 3 3) reported that base m etals appear to be m ore abundant in silver-gold deposits in regions of sedimentary rocks with overlying volcanic rocks and in thick, volcanic sequences with a long history of volcanic activity. In contrast to the silver-gold deposits, Nolan ( 1 933) noted that gold silver deposits are com monly within or close to small, shallow intrusive bodies and that the ore-controlling fracture systems are frequently more discontinuous than those associated with silver-gold deposits. The gold-silver ores are also m ore irregular in distribution than the silver-gold ores. Nolan felt that this irregularity may be related to the com plex thermal regimes in these types of systems due to the shallow intrusive activity. Figure 2a shows a series of plan views of the January mine, Goldfield mining distrct, Nevada and a cross section through the January shaft ( Ransom e, 1 909) showing the relationships of ore to quartz-alunite-kaolinite replaced wallrock ("ledge m atter" ) and the host rocks. Figure 2b shows two cross sections from Ransome ( 1 909, p. 1 54) of the Com bination m ine in Goldfield illustrating the irregular vertical distribution of bonanza-grade ore masses within the "ledge matter." Also, the ore bodies were not persistent along strike. Although Waldemar Lindgren ( 1 928) recognized the correlation between epithermal systems and active geothermal systems, it was Donald E. White ( 1 955, 1 98 1 ) who cham pioned the detailed study of active systems and the application of the results and concepts derived from these studies to epither mal ore deposits. The im pact of White's leadership in the study of hydrothermal systems, in general, and epithermal systems, in particular, was recognized by the Society of Economic Geologists when it held a sym posium in w Ore a JANUARY SHAFT I 09-FT LEVEL I 09-FT LEVEL 51-FT LEVEL 81-FT LEVEL 232-FT LEVEL CROSS SECTION 0 I 60-FT LEVEL 232-FT LEVEL 50 I 00 1 50 FEET 283-FT LEVEL Figure P. 2 . a ) . Plan views of the January m ine at selected mining levels and a cross sec tion through the January shaft Goldfield mining d i s t r i ct , Nevada ( Ransome, 1 909) . Bonanza-grade gold ores occur in replaced dacite referred to as "ledge matter." The ore bodies are not persistent either down dip or along strike, and occur both on the hang ing wal l s ide of the ledge and on the foot wall s ide. b ) . Two cross sections from Ransome (1909, p. 154) of the Combina t ion m ine in the Gold field distr i ct. Bo nanza-grade gold ores occur in i rregular , discontinuous masses within the ledge. The ledge fol lows a l i thologic contact and flattens with depth. 8Q-Ft Lev•l I 30-Ft Level I 8Q-Ft Level 23Q-Ft Level 28o-Ft Level xii his hon or in February, 1 9 84 entitled: Geothermal Syste rns and Ore Deposi ts. It clearly em phasized the val ue of using active ge othermal areas as models of fossil , ore-forming hydrothermal systems. Thus, the evolution of understanding of the geology and genesis of epithermal precious-metal deposits has fol lowe d a pathway from the early, vividly descriptive studies of mining districts such as the Com stock Lode, Nevada (Becker, 1 882), Cripple Creek, Colorado (Lindgren and Ransome, 1 906), and Waihi, New Zealand (Bell and Fraser, 1 9 1 2) to the later, topical studies on st ructure (Wisser , 1 960), alteration (Hemley and Jones, 1 964), stable isotopes (Taylor, 197 3), and fluid chemistry (Barton et al., 1 977). The most recent research on epithermal deposits has built on these past studies and has em phasized the thermal and com positional roles of volcanic rock terranes; the genesis, significance , and pattern of alteration miner alogies; the sources of the geothermal fluids and the paleohydrology of the systems; and, the chemical con ditions surrounding the deposition of the ore minerals. The present volume is an attem pt to provide a synthesis of the current state of geological and geochemical knowledge of epithermal precious-metal systems. It follows on, and should be used in conj unction with, the first volume in this series: Mineral-Fluid Equilibria in H ydrothermal Systems by Henley et al. ( 1 984). In the present volume we have attempted to provide a framework for understanding the system atics of controls on fluid com positions and of metal and gangue transpor t and deposition. The structure, dynamics, and transport properties of active geothermal systems are use d as a starting point. With active systems as a reference, the evolution of fluid com positions and the constraints on m etal and gangue transport and deposition in the epithermal environment are explored. The systematics of fluid inclusion and light stable-isotope applications is developed because these two approaches have been so useful in the development of our understanding of epitherm al processes. The im portance of boiling, cooling, and oxidation in transport properties of epitherm al systems is evaluated through a numerical m odelling approach. With the foregoing as background, the observational base and its inter pretation for epitherm al ore deposits in continental volcanic and sedimentary ter ranes is explore d through sum m aries of the geologic, mineral ogical, and geochemical characteristis of, and trace element distributions in, some well-studied epithermal ore deposits. The final chapter is devoted to the use of our understanding of epithermal systems in the development of exploration strategies. This volume does not attem pt to be the final word on epitherm al ore deposits, nor does it claim comprehensive treatment. The absence of a chapter on the hydrology of epithermal systems documents the fact that our current understanding of this aspect is woefully inadequate. It does not reflect a lack of recognition of the im portance of hydrologic controls. Sim ilarly, this volum e focuses on volcanic- and sediment-hosted epithermal deposits in the cordil lera of western North Am erica, particularly the United States. It does not treat aspects of alkaline- or basaltic-rock related deposits such as Cripple Creek, Colorado, and Vatacoula, Fiji, nor does it treat the relationship of epithermal systems to deeper hydro thermal systems responsible for the formation of porphyry-type deposits. A gain our reason is the lack of an adequate observational base. Our primary purpose in organizing this volume and the related Sho rt Course has been to stim ulate critical studies to improve our cur rent understanding of epit hermal deposits and processes rather than to document it. Perhaps our omissions will serve this purpose equally as well as o ur inclusions. REFERENCES Barton, P . B., Jr., Bethke, P. M., Roedder , E., 1977, Environm ent of ore deposition in the Creede mining district, San J uan Mountains, Colorado: III. Progress toward interpretation of the chemistry of the ore-forming fluid for the OH vein: Economic Geology, v. 72, p. 1-25. Becker, G . F., 1 882, Geology of the Com stock lode and the W ashoe district: U.S. Geological Survey Monograph 3, 4 42 p. Bell, J. M . , and Fraser, C., 1 9 1 2, The great Waihi gold mine: New Zealand Geological Survey, B ulletin 1 5. Burbank, W . S., 1 9 33, Epithermal base-metal deposits in Ore deposits of the Western States: Am erican Institute of M ining Metallurgical Engineers, New York, Part VI, p. 64 1 -652. Ferguson, H. G., 1 927, Geology and ore deposits of the Mogollon mining district, New Mexico: U.S. Geological Survey Bul letin 787, 100 p. Ferguson, H . G., 1 929, The mining districts of Nevada: Economic Geology, v. 24, p. 1 3 1 - 1 4 1 . Hemley, J. J . , and Jones, W. R . , 1 964, Chemical aspects of hydrothermal alteration with em phasis on hydrogen m etasom atism : Economic Geology, v. 59, p. 538-569. Henley, R. W., Truesdell, A. H., and Barton, P. B., Jr., 1 9 84, Fluid-mineral equilibria in hydrothermal systems: Society of Economic Geologists , Review in Economic Geology, v. 1, p. 267. Lindgren, W ., 1 928, M ineral Deposits: Third Edition, M cGraw Hill, New York, 1 049 p. Lindgren, W ., and Ransome, F. L., 1 906, Geology and gold deposits of the Cripple Creek district, Colorado: U.S. Geological Survey, Professional Paper 54, 5 1 6 p. Nolan, T. B., 1 9 33, Epithermal precious-m etal deposits in Ore deposits of the Western States: A m erican Institute of M ining Metal lurgical Engineers, New York, Part VI, p. 623-640. Ransome, F. L., 1 909, The geology and ore deposits of Goldfield, Nevada: U.S. Geological Survey Professional Paper 66, 258 p. Taylor, H . P . , Jr., 1 973, 1 8 /o 1 6/ 0 evidence for meteoric-hydrothermal alteration and ore deposition in the Tonopah, Com stock Lode, and Goldfield mining districts , Nevada: Economic Geology, v. 68, p. 747-7 64. xiii Wisser, E., 1 960, Rei a tion of ore deposition to doming in the North American Cordil lera:. Geological Society of Am erica, Memoir 77. White, D. E., 1 95 5, Thermal springs and epithermal ore deposit s: Economic Geology, Fiftieth Anniversary Volume, p. 99- 1 54. White, D . E., 1 98 1 , Active geothermal systems and hydrothermal o re deposits: Economic Geology, Sevent y-fifth A nniversary Volume, p. 392-42 3. xiv ACKNOWLEDGMENTS As is tr ue for any effort of the scope of this vol ume, m any people in addition to the editors played key roles along the r oad to final publication. T he time and effort expended by each author is greatly appreciated as are the co ntributions of the large cadre of individual reviewers who have of fered insights and alternative perspectives to the authors. Technical support to the e ditors including manuscript pre paration and revision, final format ting for publication, and badgering of both editors and authors was provided by the Editorial Support Group, Branch of E xploration Geochemistr y, U .S. Geological Survey. Within this group we would especially like to thank Pamela Detra, Dorothy Wesson, Marilyn Billone, and Candy Vassalluzzo. An e arlier version of this text was assembled for use at the Society of Economic Geologists Short Course by the B ranch of Exploration Geochem istry Cler ical Support Group. Finally, we would like to express appreciation for the patience of Jamie Robertson, Series Editor, Reviews in Econom ic Geology, and the support of the Society of Economic Geologists. Byron R. Ber ger Philip M. Bethke XV BIOGRAPHIES B YR ON R. BERGER received a B.A. degree in EconomicSTGeology from Occidental College in 1 966 and a M .S. in Geology from the University of California, Los Angeles in 1 97 5. He worked as a petroleum exploration geologist for Standard Oil Company of California from 1 968- 1 9 70 and a minerals exploration geologist and research scientist for Continental Oil Compan y from 1 9 7 1 - 1 977. He j oined the U.S. Geological S urvey in 1 977, and has been involved in research on epitherm al precious-m etal deposits and the relationship of m agma genesis to ore genesis. He is currently the Chief of the Branch of Exploration Geochemistr y. He is an adj unct assistant professor of geology in the Department of Geologi cal Sciences at the University of Colorado, Boulder, where he has taught courses on the geology and geochem istry of epithermal ore deposits and exploration geo chem istr y. He is a mem ber of several professional societies including the G eological Society of America and the Am erican Geophysical Union. government agencies in the areas of mineral deposits, exploration, and resource assessm ent. His principal research interest is the representation of data and concepts for all types of m ineral deposits in coherent and predictive m odels for exploration and resource studies. He is currently a Councillor of the Society of Economic Geologists and the Geological Society of America. WILLIA M� BABGY received a Ph.D. degree in Earth Science from the University of California, Santa Cruz, in 1 979 based on petrogenetic resear ch of Tertiar y volcanic rocks in the Sier ra Madre Occidental, Mexico. His industry experience includes geologic eval uation of volcanic-hosted uranium in the McDermitt caldera com plex, Nevada, and the bulk mineability potential of the am ythest silver vein system at Creede, Colorado. Industry research included development of an occur rence model for hot spring-related gold deposition based on the McLaughlin gold deposit in California. Present research interests are focused on the genetic aspects of sediment-hosted precious-m etal deposits. PHILIP M. BET HKE received a B.A. degree in Geology from Am herst College in 1 952 and a Ph.D. in Geology {specialization in Mineralogy and Ore Deposits) from Colum bia University in 1 957. He was Assistant Professor of Geology at the Missouri School of Mines and Metallurgy {now the University of Missouri-Rolla) from 1 95 5 to 1 959. He joined the U.S. Geological Survey as a W AE research geologist in 1 957 and transfer red to full time in 1 959. His research has com bined field and laboratory approaches to the study of hydrother mal ore deposits, particularly to epithermal vein systems. He has held several adm inistrative positions with the U.S.G.S., most recently, Chief of the Branch of Experimental Geochemistr y and Mineralogy. He is a mem ber of several professional societies and is cur rently a Councillor of the Society of Economic Geologists. He has been active in the establishment of the SEG Short Course Series, and is currently Chairman of the Short Course Com m ittee. ROBERT 2:. BODNAR received an M.S. degree from the University of Arizona and a Ph.D. degree from The Pennsylvania State University and has been involved in various aspects of fluid-inclusion research for the past 10 years. He worked for 1 year as a resear ch geochem ist in the Ore Deposits Group of Chevron Oil Field Research Com pany and is currently an assistant professor in the Department of Geological Sciences at Virginia Polytechnic Institute and State University. KEVIN BROWN received an M.S. degree in Chem istry in 1 969 and a Ph.D. degree in Chemical Crystallography in 1 9 72 from the University of Auckland, New Zealand. Except for a two-year soj ourn at the E.T.H. in Zurich, he has worked at the Departm ent of Scientifi c and Indust rial Resear ch, New Zealand. Initial ly in Wellington, his research interest centered around the crystal str uctures of organic reaction intermediates, but he gradual ly cam e down to earth with the crystal str uctures of some new epithermal minerals. In 1 98 1 , he shifted to the Geothermal Section at Wairakei, where his present research is concerned with experimental studies of mineral deposition from geotherm al fl uids. SA MUEL S. ADAMS received B.A. and M.A. degrees from Dar t mouth College in 1 959 and 1 96 1 , and a Ph.D. degree from Harvard University in 1 967. From 1 964 to 1 977 he served as mine geologist, exploration geologist, exploration manager, and exploration vice president, em ployed by International Minerals and Chemical Corporation and then the Anaconda Company. During this period, his work em phasized sedim ent-hosted mineral deposits, particularly potash and uranium. Since 1977 he has served as a lecturer and consultant to industry, research o rganizations, and C YRUS W. FIEL D received a B.A. degree in Geology from Dartmouth College in 1 95 6 and M.S. and Ph.D. degrees in Economic Geology, Geochem istry, and Petrology from Yale University in 1 957 and 1 96 1 , xvi respectively. He worked as an exploration geologist during the sum m ers of 1 955, 1 956, and 1957 for the Oliver Iron Mining Company and Quebec Cartier Mining Com pany subsidiaries of the U.S. Steel Corporation, and served as a research geologist from 1 960 to 1 963 with the Bear Creek Mining Com pany division of Kennecott Copper Corporation. In 1 963, he j oined the faculty of Oregon State University where he is cur rently Professor of Geology. His research interests are largely concerned with the geology and geochemistry of hydr othermal mineral deposits; particularly the application of stable isotope and maj or-minor-trace element investigations to their genesis. He is a mem ber of several professional societies and was Vice President of the Society of Economic Geologists in 1 9 8 1 . sili ca species in water and saline solutions. He has also been a leader in the development of several chemical geothermom eters and mixing models that are now widely used in the exploration for geothermal resources. His present research focuses mainly on internally consistent chemical, isotopic, and hydrologic m odels of presently active hydrothermal systems. He has served on NATO com mittees to review geothermal energy development programs in Iceland, France, Greece, Port ugal, and Turkey, and other com m ittees to review geothermal exploration programs in Argentina and Thailand. He was Chairman of the Organizing Committee for the 1975 United Nations International Symposium on Geothermal Energy, and Chairman of the Technical Program Com mittee for the 1 9 85 GRC International Symposium on Geothermal Energy. He now serves on panels to oversee geothermal developments in Costa Rica and Panam a, and several U.S. Col'ltinental Scientific Drilling Committees. He is a member of several societies and has served on the Board of Directors of the Geochemical Society and the Geothermal Resources Council. RICHARD H . FIFAREK received a B.S. degree in Geology from the University of Washington in 1 974, and M.S. and Ph.D. degrees in Geology (specialization in Economic Geology) from Oregon State University in 1982 and 1985, respectively. From 1 9 7 4 to 1 9 84, he worked periodically as an exploration geologist (4 yrs.) for several mir.ing com panies, as a research assistant/ scientist (1 yr.) at the facilities of the Branch of Isotope Geology (Denver), U.S. Geological Survey, and as an instr ucto r for Oregon State University. Presently, he is an assistant professor in the Department of Geology at Southern Illinois University where he teaches and conducts research in economic geology and isotope geochemistry. His research interests include integrated geologic (field) and geochemical investigation of massive sulfide and epithermal Au-Ag deposits, and m odeling the isotopic evolution of fluids and rocks in hydrothermal systems. DANIEL 0. HAYBA received a B.A. degree in Geology from the College of Wooster in 1976 and an M.S. degree in Geochemistry and Mineralogy from the Pennsylvania State University in 1979 following a study of the Salton Sea geothermal system. From 1 978 to 1 9 80, he worked for Exxon Production Research Com pany on com puter m odeling of ore deposits. Since that time, he has been a research geologist with the U.S. Geological Survey where his research has been directed towards understanding the ore-forming processes in epithermal systems. NORA .!S..:. FOLE Y received a B.S. degree in Geology and Min eralogy from the University of Michigan in 1 978 and an M.S. degree in Geological Sciences from Virginia Polytechnic Institute and State University in 1 980. She is cur rently working towards a doctoral degree in Geology through Virginia Polytechnic Institute and State University. Since 1 980, she has been a research geologist at the U.S. Geological Survey in Reston, Virginia. Her research has included fluid-inclusion and isotopic studies of different types of ore deposits, including Ag- and base-metal-bearing, epithermal deposits, sediment-hosted, stratabound, Pb-Zn deposits, and K uroko-type massive sulfides. PA MELA HEALD received a B.A. degree in Geology in 1971 from Vassar Col lege and an M.S. degree in Geology from George Washington University in 1 977. She has been a research geologist at the U.S. Geological Survey since 1 9 72. Her research has included spectral reflectance and structural studies in Nevada, with a focus on ore deposits, and mineralogical and geochemical studies to evaluate ore forming processes in epithermal precious- and base metal deposits. RICHARD W. HENLEY received a B.S. degree in Geology in 1 968 from the University of London and a Ph.D. degree in Geochemistry from The University of Manchester in 1 9 7 1 following experimental studies of gold transport in hydrothermal solutions and the genesis of som e Precambrian gold deposits. He was Lecturer in Economic Geology Memorial University of Otago, New Zealand, from 1 9 7 1 to 1 975, and at Memorial University, Newfoundland, until 1 9 77. Research interests have focused on the mode of origin of a number of dif ferent types of ore deposits including post-metamorphic gold-tungsten veins, porphyry copper, massive sulfide, and placer gold deposits. He is cur rently with the Geothermal Chemistry Section of the Departm ent of Scientific and Industrial Research at Wairakei, New Zealand, and a visiting lecturer at the Auckland Geother mal ROBERT 0. FOURNIER received an A.B. degree in Geology in 1 9 5 4 from Harvard College and a Ph.D. in Geology (specializing in Economic Geology, in general, and the Ely porphyry copper deposit, in particular) from the University of California at Berkeley in 1 958. Since then, he has been a research geologist with the U.S. Geological Survey. His research interests have ranged from laborato ry studies of mineral-water interactions at hydrothermal conditions appropriate for shallow levels in the crust, to field studies of presently active hydrothermal systems, including Yellowstone National Park, Coso and Long Valley, California, and Zuni!, G uatemala. Experimental studies have em phasized solubilities of xvii Institute. Through 1 9 8 3-84, he was a Fulbright Fellow and G uest Investigato r at the U.S. Geological Survey and during that tim e pr oduced Volume 1 of this Review series. His present research includes a number of isotope and chemical studies relating to the exploration and development of geothermal systems and geothermal im plications for the origin of ore deposits. C. A. KUEHN received an M.S. degree from the Pennsylvania State University and has 7 years of experience in exploration for sedim ent-hosted gold deposits. He is cur rently an NSF Resea�ch Assistant and Ph.D. candidate at the Pennsylvania State University and part-time employee of the U.S. Geo ! ogical S urvey working on the Carlin gold deposit. MARK H. REED received a B.A. degree in Chemistry and in Geology from Carleton College in 197 1 and M.A. and Ph.D. degrees in Geology at the University of California, Berkeley, in 1 977. His Ph.D. research was on the geology and geochem istry of the m assive sulfide deposits of the West Shasta District, California. From 1 9 7 7 through 1979, he worked for the Anaconda Minerals Company at Butte, Montana. Since that time, he has taught and conducted research at the University of Oregon, where he is currently Associate Professor of Geology. His research has focuse d on alteration and metal zoning in the porphyry copper and large vein deposits at Butte and the geochem istry of hydrothermal alteration, metal transport, and ore deposition in massive sulfide and epithermal systems. T. J. REYNOLDS received an M.S. degree from the Uni versity of Arizona and has been an exploration xviii geologist specializing in the application of fluid inclusions to mineral e xploration for the past 5 years. MILES b. SILBER MAN received a B.S. degree from the City University of New York and M .S . and Ph.D. degrees from the University of Rochester, New York. He is a m ember of the B ranch of Exploration Geo chem istr y of the U.S. Geological Survey, with current assignments to the Redding, California (CUS MAP) project, and to the study of the geochemistry of volcanic and metamorphic-hosted gold deposits in the western U.S. and northern Mexico. Previous work for the U.S;G.S. included geochronological, geochemical, and regional geological studies of precious- and base m etal deposits in the Great Basin and Alaska, and tectonic syntheses with particular focus on the relationships of hydr othermal precious-metal deposits to magmatic and metamorphic evolution. Between tours at the U.S.G.S., he designed and supervised exploration programs for precious-metal deposits in the Great Basin for the Anaconda Minerals Company. NICOLAS F. SPYCHER received a B.S. degree in Earth Sciences in 1 979 and a Dip!. es Sc. in Exploration Geophysics in 1 980 from the University of Geneva, Switzerland. He is now a Ph.D. candidate and research assistant at the University of Oregon. His present research includes studies of the transport of arsenic and antimony in hydrotherm al solutions, the m ixing properties of geothermal gases, and the geochemical modeling of hot spring syste ms.