Economic Geology Vol. 92, 1997,pp. 578-600 Secondary PreciousMetal Enrichmentby Steam-Heated Fluidsin the Crofoot-Lewis Hot SpringGold-Silver DepositandRelationto Palcoclimate SHANEW. EBERTk* KeyCentrefor Strategic MineralDeposits, Department of Geology andGeophysics, University of YVestern Australia, Nedlands, Western Australia 6907, Australia AND ROBERT O. RYE U.S.Geological Survey,P.O. Box25046,Mail Stop963,DenverFederalCenter,Denver,Colorado80225 Abstract The Crofoot-Lewis depositis an adularia-sericite-type (low-sulfidation) epithermal Au-Agdeposit, whose well-preserved palcosurface includes abundant opalinesinters, widespread andintense silicification, bedded hydrothermal eruption breccias, anda largezoneof acidsulfatealteration. Radiogenic isotope agesindicate thatthe system wasrelatively long-lived, withhydrothermal activitystarting around4 Ma andextending, at leastintermittently, for the next3 m.y. Field evidence indicates that the surficial zone of acid sulfate alteration formed in a steam-heated environ- mentwithinan activegeothermal system. A dropin thewatertableenableddescending acidsulfatewaters to leachAu andAgfromzonesof low-grade disseminated mineralization, resulting in theredistribution and concentration of Au andAgintoore-grade concentrations. Thesezonesof secondary Au-Agenrichment are associated withopal+ alunite+ kaolinitc+ montmorillonite _ hematiteandweredeposited in openspace fractures at, andwithina fewtensof metersbelow,the palcowater table. The stableisotopesystematics of aluniteandkaolinitcin the steam-heated environment are relatively complex, duetovariations in theresidence timeof aqueous SO4thatformedfromtheoxidation of H2Sprior to precipitation of alunite,andthesusceptibility of fine-grained kaolinires to hydrogen isotope exchange with laterwaters.Mostof thealunites areenriched in 34Srelative to earlysulfide minerals, reflecting partialS isotopeexchange betweenaqueous SO4and H2S.Abouthalf of the alunitesgivereasonable calculated z•SOso4_ou temperatures fora steam-heated environment indicating O isotope equilibrium between aqueous SO4andwater.The6DH2o values of thehydrothermal fluidsvariedby almost60 perrailoverthelifeof the meteoric water-dominated system, suggesting significant climatechanges. Mineralization isbelieved tohaveresulted fromlarge-scale convection ofmeteoric watercontrolled largely bybasinandrangefractures anda highgeothermal gradient withH2SforAucomplexing derived fromorganic matterin basinsediments. A wet climateresultedin the formation of a largeinlandlakewhichprovided abundant recharge waterfor the hydrothermal system. A fluctuating watertablecontrolled by changing climaticconditions enabledsteam-heated acidsulfatefluidsto overprint lowergrademineralization resulting in ore-grade precious metalenrichment. Introduction amajorrolein thegenesis ofvarious metal,industrial mineral, and strategic fuel deposits (Cronin et al., 1983), but only THE Crofoot-Lewismine is an adularia-sericite(Heald et al., recently hasa linkbeensuggested between palcoclimate and 1987)or low-sulfidation (Hedenquist, 1987)typeepithermal epithermal systems (BergerandHenley,1989).The Great Au-Agdeposit,locatedin northwestern Nevada,nearthe Basinof thewesternUnitedStatesissusceptible to theaccuabandoned townof Sulphur(Fig.1). Badiometric agesshow mulation of largevolumes of surface waterduringperiods of that the hydrothermal system wasyoung(4.0-0.7 Ma) and abundant precipitation, andlargeinlandlakeshaveperiodiconsequently manyfeatures of thepalcosurface remain,in- callyformedwithinthe GreatBasinat leastasfar backas cludingsinter,beddedhydrothermal eruption breccias, and late Tertiarytimes(Forester,1991).Theseinlandlakesexsurficial acidsulfatealteration. CurrentAu-Ag(Hg) mining pandedandcontracted according to palcoclimatic conditions by HycroftResources andDevelopment Incorporated com- (Smith,1984;Forester,1991),reflectingmajorchanges in mencedin 1987,with provenandprobablereserves, asof the regionalwatertable.Changes in the levelof the water October1994,of 57.8milliontons(Mr) grading 0.65g/t Au tablecanhavea majoreffectonthe surficial environment of and containing 31,185kg Au. Miningis by openpit and activeepithermal systems, resulting in the overprinting of cyanide heap-leach methods. Currentannualproduction is differentalteration assemblages (Simmons, 1991). approximately 2,835kg (100,000oz) Au, 8,500kg (300,000 In thispaperwedescribe indetailtheacidsulfate alteration oz) Ag,and13,600kg Hg. andusefieldrelations, combined with lightstableisotope It haslongbeenrecognized thatpalcoclimate hasplayed data,K-Ardating,palcoclimate records, andgeologic constraints, topropose thatfluctuations in thewatertableduring theformation of theCrofoot-Lewis deposit enabled acidsul• Corresponding author: email,sebert@compuserve.com to overprint weaklymineralized near-surface * Presentaddress: Hot SpringGoldCorporation, 4650SierraMadre#813, fate alteration Reno, Nevada 89502. silicification, resulting in theleaching, redistribution, andsec0361-0128/97/1936/578-2356.00 578 CROFOOT-LEWIS HOTSPRING Au-Ag DEPOSIT, NV 119'30' 579 Tertiaryrocksare abundantin the surrounding ranges, 118'15' and includevolcanic,voleanielastie,intrusive,and fresh-water Sleeper 41'15' -tog Ranch >z Sulphur (abandoned) o ø e.> HUMBOLDT COUNTY ß-r' PERSHING COUNTY ... I I ;rofoot/Lewis Mine the basin 12 km north of the Crofoot-Lewismine (Garside et al., 1988). The late Cenozoic (Miocene-Holocene) basin 5•' Rosebud Gerlach Irnlay Florida Canyon 40'30' 5•'Standard'- Wind 5•' Mountain o 5 Io 15 sedimentary rocks(Willden,1964).Igneousrocksare commonandrangein composition fromolivinebasaltto rhyolite. Sedimentary rocksareabundant in the downdropped basins that formedduringthe development of basin-and-range structure (Stewart,1980).The sedimentary rockswithinthe BlackRockDesertbasinadjacent to theoredeposit arenot exposed. A general description oftheserocksisbasedondrill cuttings froma 2,417-m-deep oil exploration well drilledin 5•' Trinity F,• Rochester Nevada sediments andsedimentary rocksin the BlackRockDesert basincontaintracesof oil andgas(Garsideet al., 1988)and areinterpretedto be 2,160m thick.From 1,600to 1,775m, therocksarepredominantly poorlyconsolidated tounconsolidatedsiltstone. From1,775to 2,160m, thecuttings aretuffs, clays,shale,sandstone, conglomerate, marl,andlimestone, locallycontaining tracesof pyrite.From2,160m to thebottom of the hole (2,417 m) volcanicrockswere encountered. Unconsolidated sedimentary materialof Plioceneand youngerage(<6 Ma) formswidespread alluvialfan,stream terrace,floodplain,valleyflat,andplayadeposits thatgrade from coarsegrainednearthe rangefrontsto fine silt and clayin thevalleys. Largeinlandlakesoccupied manyof the present-day valleysof Nevadaduringthe Pleistocene, and manyof the presentlyexposed playaandshorelinedeposits originated in theselakes(Morrison,1964). Deposit geology A seriesof subparallel basin-and-range-type normalfaults offsetrocktypesandcontrolmuchof thenear-surface mineralization andalteration in thedistrict(Figs.3, 4). Thesefaults north(N 15ø-30øE) anddip55øto 85ø FIG. 1. Locationmapof the Crofoot-Lewis mine,HumboldtCounty, strikeapproximately Nevada, showing adjacent precious metaldeposits. to the west.Northwest-andwest-striking transferor relay faultslocallycause nilnoroffsets in orezonesandalongnorthondary enrichment of Au andAg.In addition we propose a strikingfaults. Theoldestrockunitin theSulphur district istheMesozoic geneticmodelfor the formation of younghot springgold deposits in theGreatBasin, whichhasimportant implicationsAuldLangSyncGroup,whichis presentbelowthe deposit for exploration, andto our knowledge, hasnot beenpre- (Fig.4) andcropsout5 km eastof the minesite.Here the unit consists of laminatedslateand siltstone. In places,the viouslyproposed. unitishighlycontorted andfolded,andgraphiteis abundant Geology alongbedding-foliation planes. overlyingthe Auld LangSyncGroupis A moredetailed description ofthegeology, mineralization, Unconformably group(informally namedby andsecondary Au enrichment at the Crofoot-Lewis mineis the TertiaryKammaMountains of predominantly eastgivenin Ebertet al. (1996),andonlya brief summary is Wallace,1987).Thisgroupconsists dippingfelsicvolcanic, volcaniclastic, andpyroelastic rocks. presentedhere. Rhyolitelavaflows(locallydomes), withwell-developed flow Regional geology banding,arecommon in the sequence. Minordaciticto anThe regional geology surrounding the Sulphur districtis desiticrocks,andrhyoliticflowbreccias andtuffaceous units dominated by late Cenozoicbasin-and-range extensionaloccurlocally. faulting.The majorrocktypesin the regionhavebeen The KammaMountains groupis overlainby the Sulphur mapped anddescribed byWillden(1964)andJohnson (1977) group(informally namedby Wallace,1987),whichconsists andaresummarized in Figure2. Triassic to Jurassic rocksof of Pliocene to Pleistocene sediments andsedimentary rocks. theAuldLangSyncGroup(BurkeandSilberling, 1974)are The Sulphurgroupis dividedintothe LowerSulphurgroup, widespread through theregion. TheAuldLangSyncGroup theCamelconglomerate, theCrofootbreccia, andsinterdeisa thicksequence ofvariably metamorphosed metasedimen-posits. TheLowerSulphur groupconsists offine-grained silttary rocks,consisting of phyllite,slate,metasiltstone, fine- stones,tuffs,sandstones, and minorpebbleconglomerates. grained quartzite, andlocalhornfels andispartof theJungo TheCamelconglomerate isa poorlysortedmatrix-supported Mesozoic allochthonous terrane(e.g.,Oldow,1984). conglomerate with clastsup to 25 cmin size,averaging 1.5 580 EBERTAND RYE Qal Qal Hot Spdng Black Rock Desert t Qal Crofoot/Lewis Valley Mine Oal .:• 40ø45 ' Qal Qal Oa, 119000 ' 118030 ' Quaternary • Undivided alluvium includes gravel, alluvial fan, landslide, playa, dune and stream deposits Quaternary- Tertiary? [-• Pliocene-Pleistocene basalt and Tertiary _-• Sedimentary felsic volcanics and •[.:• Rhyolite porpydtic rhyolite rocks • Undivided and sedimentary rocks'"• Basalt andesite Cretaceous ['• Granodiodte TriassicJurassic? Jungo :/..:• Phyllite, slate, fine grained quartzite, homfels (Auld Lang Syne Group?) •- Terrane Permian- Triassic? N []•Undivided volcanic and sedimentary rocks tBlack Rock "[-•Intermediated 10 km tobasic volcanic rocks (Happy Creek Group)Terrane I 8miles ' , Permian y Interpreted position ofMesozoic Terraneboundary (thrust fault) FIG.2. Generalized regional geology of south-central Humboldt andnorth-central Pershing Counties. Geology from Willden(1964)andJohnson (1977). cm.Theelasts in thisunitarederivedfromtheAuldLang ehaleedonie silica interbedded withsilieified CamelconglomSyneGrouportheKammaMountains group,andlithifieationerate.Sinterbedsrangefrom30 cm to severalmetersin isa productofhydrothermal alteration. Crofootbreeeiarefers thickness and occur interbedded with Crofoot breeeia and tohydrothermal eruption deposits thatarepervasively altered Camelconglomerate, havinga maximum cumulative thickandabundant nearthesurface attheCrofoot-Lewis deposit.nessof about45 m. Individual sinterbedsarefiatlyingand Thesinterdeposits consist of bedsof massive opaline and extendforhundreds of meters. Thesinters typically contain CROFOOT-LEYVIS HOTSPRING Au-AgDEPOSIT, NV 581 t Oiatamaceous Bay area / ....... , ..",• Lewis Pit i' '_T_L--..'-- i'•-•_.. "1/1^ ^ • Qa \ / • Pit / i ^ • ^ Kamma Mountains / / GroupVolcanics ii i x 1: ,4 I i ^ iI Gap Proposed SouthCentral Qal I ^ ,• A I__- I Qal Brimstone Pit • I• • A' • Location ofsections • f Normal fault ^ i - dashed where inferred < i • Alluvium • I:::::::::1 Conglomerate/Croloot Bmccia • ;.?...;• Basal acid leach altered Croloot Breccia i I • i--.r--.rl OpaI-K-teldspar altered Camel Qal ^ • '• ! I++1 Conglomerate • 4 ,• • / Illlte-smecltte altered Camel g ,• < 4 •,'• • \ \ *`*`\ .... ,,% x\ / / •11ver uamel xxX ,•,\ •. Hill -,x,•.. \ •. '• ,,.. - ,, , a .......... oa• -'.<.%.; / limitofknown Approximate surlace and subsurface alteration •--.• ... ^ .• •. Conglomerate (partially overprinted by blanket leach locally) • Mordenite-type alteration (overprinting sinter and opal-K-feldspar altered Camel Conglomerate + Crotoot Bmccia \. /^/ ^ ,• ,. r=l alteration I• I Mordenite (altering talus/alluvium) •, a < 4 • Weakly silicifled and argillized Camel x• I-'- I Conglomerate +Crofoot Bmccia I(2•1Ore zone/open pit \ ,,• \^ ;--;--.-•...... , 900m , ' ooo. " •... FIG. 3. Simplified mapof the Sulphurdistrictshowing structure, hydrothermal alteration, ore zones,andlimitof hydrothermal alteration. abundant opalizedaquaticreedcaststhat are oftenin an HydrothermalAlteration uprightorgrowth position. Thepresence ofopalized aquatic Introduction plantmaterialandthefiat-lying andextensive natureofthese deposits indicate thattheyareshallow-water lacustrine depos- Figures3 and4 summarize the distribution of hydrotherits,probably analogous to the subaqueous "poolsinter"de- malalteration in theSulphur district. Hydrothermally altered scribedat BuckskinMountain,Nevada(Vikre, 1985). rocksareextensive in the district,covering an areagreater 582 A A A' West East East Albert Oal Fault /f-16oom Boneyard OalFault 5000 ft- Oal 4000 ft- Central Fault,/ Fault * * •- > <' oOoO • • •' oø • • • • v• •' • • • • -1300 m 3500 ft_ roo -• Alluvium ,•-'• Kamma Mountains Group •'• Auld Lung Sync Group B A A' West EastEast Bone' ard Albert Q•I Fault m •" .• -16oo Y Fault Central Fault O,a, 4L ....•,'• • ....' "::::: i:i:i, i:!:i:i:iii:i:. !! •<-1450 m .+ I-- -///-\4--//T-?'17 t---I f .' '. I V*11 .... :F \'''''' i '++v^••ed• -1300m . Y• + • i ! > ' •<Grøu• p•c• ^ I 3000 ft- F[o.4. Cross section through theCrofoot-Lewis deposit showing (A)rocktypesand(B)hydrothermal alteration. than3 by7 kmatornearthesurface andextending todepths propylitic alteration ispoorly defined. Themineralogy ofthe greater than600m,thelimitof deepdrilling. Therearefive propylitic zoneis quartz+ K feldspar + chlorite+ calcite maintypesofhydrothermal alteration in theSulphur district: + illitc + marcasite + pyrite+ leucoxene _+siderite. (1) propylitic,(2) interstratified illite-smectite andillitc, (3) opal-Kfeldspar, (4) montmorillonite + mordenite (zeolite), Opal-Kfeldsparalteration alteration covers a largenear-surface area and(5) acidsulfate. Briefdescriptions of propylitic, opal-K Opal-Kfeldspar feldspar,illite-smectite, and montmorillonite + mordenite (Fig.4), rangingfromabout60 to 213m thick.Thisalteration alteration, followed bya detailed description of acidsulfate ischaracterized byintense silicification consisting ofopaland alteration, aregivenbelow. chalcedony, withbetween 15and50percent K feldspar and minor amounts of marcasite + pyrite _ stibnite _ leucoxene Propylitic alteration and/or rutfie.Potassium feldspar inthiszonereplaces volcanic Propyliticalterationoccursin the KammaMountains clasts andfineparticles in the matrixandis 4xtremel¾ finegroup,mainlylateralto, andlocallywithin,zonesof illite- grained, withindividual crystals lessthan0.1/.tinih size. smectite or illitc alteration. The extent and distribution of Opal-K feldspar alteration contains pervasive low-grade Au- CROFOOT-LEWIS HOTSPRING Au-Ag DEPOSIT, NV 583 Agvalues.Minorveinsof bandedchalcedony _+calciteoccur termed"blanketacidsulfatealteration," andassteeplydipwithinopal-Kfeldspar alteration. pingveinsandzonesthatextendbeneaththe blanketzone A blackto darkbrown,opaque(somethinedgesarered- intotheunderlying opal-Kfeldspar alteration, termed"acid dish-brown to dark brown) bitumen, with low reflectance sulfate veins"(Figs.5 and6B). (lessthanquartzor calcite)andbluefluorescence occursin opal-Kfeldspar alteration andquartzandcalciteveins.The Blanketacidsulfatealteration Blanketacidsulfatealteration is verticallyzoned,with an term"bitumen"is hereusedto describe solidor liquidorganiematerial thathasmigrated fromitssoume to formsolid upperzoneof intensely leachedmaterialconsisting of residhydrocarbon (Parnell,1993).Bitumen isgenerally notvisible ualquartzandcristobalite, termed"blanketacidleach,"and maeroseopieally, although a fewerustiform-eolloform banded a thinlowerzoneof intenseopalization, termed"basalacid chalcedony veinscontaindarkbitumen-rich bandsthat are leach"(Fig. 5). Blanketacidleachvariesin thickness from dearlyvisiblein handsamples. Veinsamples fromnearthe zeroto 100m andconsists mainlyofa barren,bleached white, present surface to over425m in depthcontain traceamounts skeletal residueof whitepowderyquartz+ cristobalite, with to over10percentbitumen,averaging between 2 and4 per- lesseramounts of opal-A,alunite,natroalunite, kaolinitc, nacentbitumen.Bitumentypically occursasdisseminations in tive sulfur,gypsum, cinnabar,andjarosite(Fig. 6C). This chalcedony, opal,quartz,orcalcite, asvugfillings, andlocally zoneispoorlyconsolidated andhasa highporosity; however, as discrete bitumen-rich bands in eolloform or erustiform despitethe intenseleaching, originalrocktextures arewell bandedveins.A sampleof coarse-grained, darkbrownto preserved in manyareas.The intensity of acidsulfatealterblackcalcite contains anestimated 20percent bitumen along ationgenerally increases towardmajorfaultsandextends to cleavage surfaces anddisseminated withincalciterhombs and greaterdepthsalongsomeof thesestructures. Scattered remmostlikelygetsits colorfromthe abundance of bitumen. nantsof unleached opal-Kfeldsparalterationoccurwithin blanketacidleach(Fig.5), indicating thatblanketacidleach Illite~smectite alteration alteration hasoverprinted opal-Kfeldsparalteration. An inBelowthenear-surface opal-Kfeldspar alteration, theSul- tervalofhematite-stained alluvium, ranging froma fewcentifur groupsediments areintensely argillized to interlayeredmeters to 7 m in thickness,is common at the interface of illite-smeetite + quartz+ pyrite_+ehlorite_+illitc_+calcite acidleachalteration andoverlying alluvium. +_kaolinitc _+pyrrhotite (Fig4).Thisillite-smeetite alteration Withtheexception ofcinnabar, sulfides donotoccurwithin ispervasive throughout thesediments andextends morethan blanket acid sulfate alteration. Cinnabar coats native sulfur 600 m belowthe surface.An envelopeof illitc alteration crystals or occurs disseminated in intenseacid-leached mate(<5% interstratified smeetite) + quartz+ pyrite_+ehlorite rial.Nativesulfuroccurs in beds,lenses, andfracture fillings + calciteis developed around steeply dippingsilieified fault within the blanket acid leach zone, and in someareas,native zones.This envelopegradesoutwardawayfrom the fault sulfurfillsopenspacefractures alongfaultzonesbelowthis zoneinto illite-smeetite alteration with increasing smeetite zone.Smallhorizontal bedsandoval-shaped elongate pods composition. Calciteveinsandveinletslocallycut the illite- of fine-grained alunite+ kaolinitc+ silica_+montmorillonite smeetitealteration,andsmall(30-50/•m) rhombsof calcite occurlocallyin theupperpartof theblanketacidleachzone aredisseminated in theillite-smeetite + quartzmatrix. (Fig.5). Theseappearto be remnantmudpoolswhichare Montmorillonite + moralehire alteration common at the surface in areas of steam-heated acid sulfate springs in geothermal systems (e.g.,Waiotapu, NewZealand; Montmorillonite + mordenite alteration occurs mainlyin Hedenquist andBrowne,1989).Remnantmudpoolsare esthenorthwest Sulphur district (Fig.3),whereit partially over- peciallyabundant in the upperportionof the Boneyard pit printsopal-Kfeldspar alteration andsinterdeposits, andalso wheretheyoccurashorizontal bedsandlenses offine-grained altersunconsolidated or poorlyconsolidated alluvium. This alunite+ kaolinitc+ opal-C+ opal-CT_+montmorillonite alteration isrestricted tothenearsurface, typically extendingup to 0.9 m thickand15 m in length(Fig.6D). Thesebeds nodeeperthan50 m belowthebaseof alluvium. Thisassem- aretypically surrounded above,below,andlaterallyby opablageistypically oxidized andconsists mainlyof montmoril- lizedor kaolinized hydrothermal eruptionbreeeia. lonite, the zeolite mordenite(Na2,K.•,Ca)[A12Si100•4] ß7H•O, Basal acid leach alteration and chalcedony or quartz.Lesser,but locallysignificant, Basal acid leach alteration occurs as a subhorizontal unit amounts of interlayered kaolinite-smeetite andkaolinitcare present.Minoramounts of jarositeandaluniteoccurwithin atthebaseofblanketacidleachalteration (Fig.5). Basalacid narrowfractures cuttingmordenite-type alteration, arevery leachalteration rangesfromlessthani m to over12 m thick finegrained, andappeartobelate.Wheremontmorillonite + andcommonly contains tracesto ore-grade Au (averaging up mordenite alteration hasoverprinted earlieropal-Kfeldspar to 0.6 g/t) andrelatively littleAg.Basalacidleachalteration alteration, mareasite andpyritehavebeenoxidized. hasoverprinted earlieropal-Kfeldsparalteration,as evidencedby remnantpatchesof opal-Kfeldsparalteration Acidsulfatealteration within this zone. Basal acid leach alteration is characterized Acidsulfate alteration isa typeofintense advaneed-argillie by denseopalization, normally whiteor grayin color,but alteration containing alunite+ kaolinitc + quartz(Hemley maybe a varietyof colorsincluding redwherehematiteis andJones,1964).Acidsulfatealteration is extensive andwell abundant in the matrix(Fig.6G).Typically, bothmatrixand preserved at the Crofoot-Lewis mine,andoccursbothasa elasts arealteredcompletely to opal-A,opal-CT,or chalcedwidespread blanket thatcapsa largeportionof thedeposit, ony,withthe originalbrecciaor conglomerate textures pre- 584 EBERTAND RYE Remnant podofopal-K-feldspar alteration withmoderateto low Nativesulfurin mounds AuandAg(0.3g/tAu,4.2g/tAg) Remnant mudpools andalong fractures CentralFault noAuorAg Blanketacidleach variable AuandlowAg • (0.3 g/t Au, <0.3 g/t Ag) ß ,,....• Basalacidleach Acid-sulfateveins/zones high AuandAg(1.4g/tAu, 28.2g/tAg) Pods orfragments ofopal- K-feldspar altered rock which arecoated by,orcompletely altered to,white opal moderateto low AuandAg(0.3 g/tAu,4.2g/tAg) Blanketacid-sulfatealteration [• Acid-sulfate alteration• Blanket acid-leach: residual quarlz +cristobalite + alunite, natroalunite, kaolinite, opal, gypsum cinnabar, hematite, andjarosite Basal acid-leach: massive opal +alunite, kaolinire, hematite Acid-sulfateveins/zones E• Acid-sulfate vein: alunite +kaolinite +opal + natroalunite, hematite, goethite, jarosite __ • Opal-K-feldspar altertion: Opal +chalcedony + K-feldspar + pyrite + marcasite +stibnite I 10rn I FIC.5. Schematic vertical section through theupper alteration zones attheCrofoot-Lewis mine, illustrating thevertical zonation andprecious metaldistribution withinacidsulfate alteration. Average Au-Ag grades in parenthesis. served.Locally,alunite+ kaolinite+ hematitearedominant m, andall of theveinsnarrow withdepth.Mostacidsulfate in the matrix.Basalacidleachalteration, although wide- veins do not extend more than 100 m below the base of the spread, isnotalways present andistypically poorly developedblanketacidleachzone,although a fewveinsextendmore or absentin the highlyfractured rocksoverlying zonesof than200mbelowthiszone.These veinsshow a crudespatial mineralized acidsulfate veins(Fig.5). andtemporal zoningfromearlyopal(commonly opal-CT)to Acidsulfateveins alunite + kaolinitc to late montmorillonite. Veins with white opalselvages andalunite+ kaolinire interiors occurlocally zonesof acidsulfatealteration, white At thebaseof theblanketacidsulfate alteration, steeply (Fig.5). In pervasive dippingveinsandzonesof opal+ alunite+ kaolinitc+ opalcommonly coatsfragments thataresurrounded byalumontmorillonite ___ natroalunite _+hematite_+goethite_+ nite + kaolinire + montmorillonite, andlateveinsof pink jarosite ___ antimony oxides _+pyrite(atdepth)fillopen-spacemontmorillonite locally cutthesezones. In places, acidsulfate fractures andbrecciated zones in theunderlying opal-Kfeld- veinsfill thecenterof earlierformedquartzor chalcedony sparalteration.There is a continuous transitionfrom the blanket acid leach zone (and basalacid leach zone where veins. At deeperlevels(>70 m depth)someof the acidsulfate developed) to the underlying acidsulfateveins,sincethe veinscontain fine-grained pyriteandlocally traces of marcaveins extend from the base of the blanket acid leach zone site.The pyriteisveryfinegrained (<0.1 mm)or occurs as but do notcutit (Fig.5). Acidsulfateveinsasthickas2.4 small cubes that are disseminated or form bands within or m exist,but mostrangefroma fewmillimeters to about0.5 adjacent toalunite, natroalunite, opal,and/orquartz, andka- CROFOOT-LE•VIS HOTSPRING Au-Ag DEPOSIT, NV 585 olinitc.Severalof thesepyriteshaveanomalous As,Ni, and overprinted, andoxidized, by near-surface montmorillonitemordenite alteration. Oxidation of thisorestyleis indepenlocally,Co. Somesteeplydippingacidsulfateveinscontainsections dentof structure, andlocallyis beddingcontrolled andis withpreserved horizontal bedsandlaminations of veryfine referredto as"horizontally controlled." Oxidizedorein this grainedmixturesof opal + alunite+ kaolinitc(Fig. 6E). zonecontains coinparable gradesasthe underlying unoxiSome of these horizontal laminations contain structures indicdizedmineralized opal-Kfeldspar alteration, andthereareno of precious metalremobilization or enrichment. ativeof softsediment deformation, aswall-rockfragmentsindications havefalleninto veinsand deformedthe laminations (Fig. 6F). Thesetextures indicatethattheveinsformedasopen- Mineralizedacidsulfatealteration spacefillingsandwerefilledfrointhebottomup. Thissug- The majority of oreminedto dateconsists of steeply dipgests thatmuchoftheopal+ alunite+ kaolinitc +_montmo- pingacidsulfate veinsandzones thatfillopen-space fractures rillonitein theseveinsprecipitated fromsolution. However, belowtheblanketacidleachzone(Figs.5 and7).TheLewis, pitsareexamples of this alteration of preexisting minerals suchasfeldspars to alunite Gap,SouthCentral,andBrimstone Areasofmineralized acidsulfate alteror kaolinitc is apparent in theveinselvages andin partially typeofmineralization. overprinted podsof opal-Kfeldsparalterationthat occur ationhaveformedwithinopal-Kfeldspar alteration thathas within blanket acid leach alteration, basal acid leach alter- beenintensely fractured, predominantly in the hanging wall ation, and mineralized acid sulfateveins or zones. of the Centralfaultwithina well-defined grabenstructure, Alunite froin the blanket acid leach zone and the acid whereoreis continuous for at least3 kin. Zonesof pervasive consistmainlyof white opaline sulfate veinsistypically whiteandgenerally veryfinegrained; acidsulfatemineralization in someof the deeperacidsulfateveins,alunitehasa light (+ chalcedony) fragments thataresurrounded by alunite+ pinkcolor.Alunitecrystals arelathor rhombshaped, or less kaolinitc+ opal-chalcedony + montmorillonite. The white commonly, cubicor anhedral. Mostalunitesamples contain fragments aretypically opalcoated and/oropalized fragments grainsthatrangein sizefrom< 1 to about20/•m; however, of earlieropal-Kfeldsparalteration. Hematiteandgoethite themajority ofgrains areapproximately thesamesize.Within +_jarositegivethezoneitsredand/oryellowhues.Pervasive the blanketacidleachzone,alunitelathsandrhombsrange acidsulfatemineralization givesway at depthto narrow, fromi to 18/•min length,averaging 5/•m. In theacidsulfate steeplydipping,mineralized acidsulfateveins. veins,aluniterangesfroinlessthan i to 15 /•m in length, Figure7 showsa wall frointhe southfaceof the South andthe deepest veinsampleis alsothe coarsest. AssociatedCentralpit andis anexample of themineralized acidsulfate kaolinRe crystals are typicallyveryfine grained,predomi- alteration currently beingminedat the deposit. In thisarea nantlyi/•m or less.Scanning electronmicroscope analysesthebulkoftheorebody consists ofnumerous, steeply dipping, revealthatseveral purealunites (K endmember)andnatroa- mineralized acidsulfateveinswhichare widelyspaced but lunites(Na end member)occur.However,manyalunites haverelatively highgrades, making thembulkmineable. Indicontainsignificant amounts of Na, andmanynatroalunitesvidualacidsulfateveinshavevaluesup to 63 g/tAu,withan containsignificant amounts of K. Mostalunitesandnatroa- average grade(20veinsamples) of6.4g/tAu.TheAgcontents lunitescontain nodetectable Fe or Ca,although, in a fewof of theseveinsreachvaluesover157 g/t andare typically' the deeperacidsulfateveins,aluniteor natroalunite is en- severaltimeshigherthanthe Au content.Adjacentunoxirichedin Ca at theexpense of K or Na. dizedopal-Kfeldspar alteration averages only0.27g/tAuand 3.43g/tAgthrough thissection, indicating thattheacidsulMineralization fateveinsaresubstantially enriched in Au andAg relativeto Introduction the surrounding opal-Kfeldspar alteredrock.The zoneasa Several zones of economic mineralization have been dewholehasanaverage gradeof 0.89g/tAu.In individual acid finedat the Crofoot-Lewis mine,including the Bayarea, sulfateveins,the Au-Agcontentis highestnearthe topand withdepth. Lewispit, SouthExtension pit, Gappit, SouthCentralpit, decreases Scanning electronmicroscope analyses showsthat Au in Boneyard pit, andBrimstone pit (Fig. 3). Mineralization is asmicron-size electruingrains, withinand hosteddominantly by Cainelconglomerate, Crofootbreccia, thesezonesoccurs Most and KammaMountains groupvolcanic rocks.Unoxidized,adjacentto opal,alunite,kaolinitc,or montmorillonite. of the electruin associated with acid sulfate mineralization mineralized, opal-Kfeldsparalteration extends overa large 31 percentAg. areaandconstitutes a hugeAu-Agresource(>116,000kg hasbetween27 and35 percentAg,averaging (AgC1)andlesscommonly Au; Ebert,1995),but is uneconomic dueto its low-grade Silveralsooccursascerargyrite (AgI)associated withalunite,kaolinite, orjarosite. (0.27g/t Au and4.15g/t Ag) andrefractory nature.Higher iodargyrite grades occurwithinthisunit,andthe Bayareacontains a Mineralized basal acid leach refractory resource of about7 Mt at a gradeof 1.20g/tAu. A thickened portionof mineralized basaladd leachhosts Economic near-surface Au-Agmineralization occurs in four pit.Thisoreischaracterized byCrofoot styles (Ebertetal.,1996);horizontally controlled near-surfaceorein theBoneyard ore, mineralizedacidsulfatealteration,mineralizedbasalacid breeeia in which both elasts and matrix have been altered to whiteopal+ alunite+ kaolinitc___ hematite.Texturesand remnants of mineralized opal-Kfeldsparalteration indicate Horizontally controlled near-surface ore mineralized basalacidleachhasoverprinted, and altered, Themajority of orein theBayareacamefrommineralized mineralized opal-Kfeldspar alteration. mineralized basalacid opal-Kfeldsparalterationandinterbedded sinterthat was leachaccounts for a smallportionof the oreat the Crofoot- leach,andlatequartz-chalcedony veins. 586 E•ERT,4iVD RYE lm i . G .• 3 i 5 6 1 8 i Di CROFOOT-LEWIS HOTSPRING Au-Ag DEPOSIT, •/ 587 B West C/L32 {O,IMa) Remnantmudpools C/,L 7 .• • CIL 9(2.1 Ma) / C/L ,1{2.4 Ma} 1400 m : 1300m C/L 12 (1.2 Ms)C,L15 C/L16 C/L 28 C/L 10 • ..• (1.$ , [• • Central Fault ! East Blanket add-leach Opal-K-feldspar alteration\• .[• ß Mineralized acid-sulfate [• Illite-srnectite alteration '•'• veins/zones • x sample location (2.4 Ma)K-Ar age iC/L 7 Sample number • 6{2.0 Ma) HorizontaJ laminations 100 m F•c. 7. Simplified east-west cross section throughthe SouthCentralpit showing alteration, sample locations, andKAr ages.An enlargement of anacidsnlfate veincontaining horizontal laminations is alsoshown. Rocktypes(notshown) consist dominantly of conglomerates andbreccias. 1994)andweredifficult to reconcile withthe Lewismine,consisting of 2,250,000t grading0.55 g/t Au, pers.commun., withrelatively lowAgvalues. geology of thedeposit andthetimingof acidsulfate alteraLatequartz-chalcedony veins Mineralizedquartz_ chalcedony veinsandveinletscut tion.Potassium-Ar and4øAr-a"Ar agedataaresummarized in Table 1. Examination ofopal-Kfeldspar alteredbreccia byscanning andinfiltrateblanketacidleachalteration, opal-Kfeldspar electron microscope revealed thatK feldspar in thebreccia alteration, andfilite+ quartz+ pyritealteration in theBrim- matrixisstoichiometric whereas K feldspar replacing thevolstonearea.Theseveinlets createlocalized zonesof ore-grade canicbreccia clasts contains significant amounts of CaOand mineralizationwithin the blanket acid leach zone, a feature Na20 whichincreasetowardthe centerof the dasts.Presum- uniquetotheBrimstone area.Mineralization withintheblan- ably,the presence of CaOandNa20in K feldspar within ket acidleachzonecouldaccount for up to halfof the eco- thevolcanic clastsis a resultof incomplete replacement of nomicmineralization withinthe Brimstone pit, whichcon- magmatic feldspar byhydrothermal K feldspar, andthisintainsabout39 Mt at a gradeof 0.58g/t Au.A mineralizedcomplete replacement mayaccount for the olderthanexstockwork containingsimilarveinletsoccursbeneaththe pectedK-Aragesobtained fromthebreccia samples (Ebert, Brimstone pit;however, its distribution andextentis poorly 1995). defined. To resolve the dilemma, laserprobe4øhr/3•hr datingof hydrothermal K feldspar withinthematrixandwithinclasts Ageof Mineralization ofopal-Kfeldspar altered breccias wasperformed. K feldspar Ageof opal-Kfeldsparandillite-smectite alteration inthebreccia matrix yielded 4øhr/3•Ar ages of3.8+_0.09and C/L77andC/L 78),whereas K feldUntilrecently theageofopal-Kfeldspar andillite-smectite3.9 _ 0.3 Ma (samples clasts yieldedagesof 10.6+_0.17and alteration at theCrofoot-Lewis minewasenigmatic. Unpub- sparin the volcanic C/L 79 andC/L 81).TheagesoblishedK-Aragesof K feldspar obtained fromopal-Kfeldspar 14.9_+1.6 Ma (samples to represent the age alteredbrecciagavedatesranging fromEarlyto LateMio- tainedfromthe matrixareinterpreted alteration whereas theolderagesobtained cene(samples C/L 74, 75, and76; D. JohnandT. McKee, of opal-Kfeldspar F•c. 6. A. Viewof the Crofoot-Lewis depositfromthe valleyfloor,facingeasttowardthe pedimentandrange.B. Viewof the southwallof the SouthCentralpit facingsouthwest. Blanketacidleachalteration (lightcolor)occurs likea blanketabovetheorezone(darkercolored ) thatextends belowthebaseof thepit.Thebenches are6 m high.TheSilver Camelhill,a partially exposed sfiicified faultzoneformsthe ridgein the background. C. Blanketacidleachalteration, southeast wallof southern mostextension of the SouthCentralpit. Benches are6 m high.D. Closeup of blanketacid leachalteration containing horizontal podscomposed of fine-grained alunite+ silica+ kaolinitc, interpreted asfossilized mudpools.E. Horizontal bedsandlaminations composed of alunite+ sfiica+ kaolinitc froma largeverticalacidsulfate vein.SouthCentralpit.F. Closeupofhorizontal laminations withina near-vertical acidsulfate vein.Notethesoftsedimenttypedeformation createdby thetwowall-rock fragments (rightcenter)thatappearto havefallenintothevein.G. Basal acidleach-altered breccia. Thematrixanddastsaredominantly massive opalwithminoraluniteandkaolinitc. The dasts appeardarkerdueto thepresence of hematite. H. Opal-Kfeldspar alteredhydrothermal eruption breccia. 588 EBERT AND RYE TABLE1. K-Arand4øAr-39Ar AgeDatafor theCrofoot-Lewis Deposit Sample no. Mineral C/L C/L C/L C/L C/L C/L 1 Alunite 2 Alunite 6 Alunite 9 Alunite 12 Alunite 15 Ainuitc Description Blanket acid leach zone Acid sulfatevein Blanket acid leach zone Blanket acid leach zone Acid sulfhtevein Acid sulfate vein C/L 32 Jarosite Fracture fillingin blanket acid K-Arage (Ma) 2.4 0.9 2.0 2.1 1.2 1.5 +_ 0.1 • +_0.6j _+ 0.1 j _+0.1• +_0.13• +_ 0.12• 0.7 _+0.22 leach zone C/L 71 Illitc Illitc+ quartz+ pyrite'altered 4.0_+0.2) volcanic rock 4øAr•9Ar age(Ma) hydrothermal, andmagmatic steam(Ryeet al., 1992).In the supergene enviromnent acidsulfate'alteration resultsfrom theacidwaters thatformbytheweathering ofsulfide-bearing rocksthatare exposed abovethe watertable(e.g.,Guilbert and Park, 1986). In the steam-heatedenviromnent,acid solu- tionsformby the oxidation of H.2Sto sulfatein thevadose zone;in thisenvironment H.2Sis derivedfromboilingliquid at depth(e.g.,Schoenet al., 1974).In somecasesthisH.2S maybe associated withburstsof magmatic steam(e.g.,the Cactus,CA, deposit, Ryeet al., 1992).The endproducts of supergene andsteam-heated acidsulfatealteration zonescan be similar,but the distinctionbetweenthe two can be made by isotope studies in the contextof time-space frameworks (Ryeet al.,1992).Magmatic-hydrothermal acidsulfate 'alterationisassociated withhigh-sulfidation-type oredeposits and breccia isformedin hydrothermal systems withacidmagmatic comC/L 77 K {'eldsparBreccia matrix 3.8+_0.092 ponents in theirfluids(Ryeet al., 1992).In thisenviromnent C/L 78 K t'eldsparBreccia matrix 3.9+_0.32 extremely acidconditions areformedby thedisproportionaC/L 79 K {bidsparFelsicvolem•ic breccia clast 10.6_+0.172 SOsandthe presence of HC1andother C/L 81 K l'eldsparFelsicvolcanic breccia clast 14.9_+1.62 tion of magmatic acids,subsequent to magmatic vaporsbeingabsorbed by • D. JohnandE. McKee,U.S.Geological Survey, MenloPark,Cali•bmia (unpub- groundwater (e.g.,Holland,1965;Henleyand McNabb, lished data) 1978;Stoffregen, 1987;Hedenquist et al., 1994).The mag2I. KigaiandM. Karpenko, Institute of Geology andOreDeposits, Petrography, matic steam environment is characterized by the formation Mineraloe,andGeochemistry, Bussian Academy of Sciences, Moscow (unpublished of veinsof almostpurecoarse-banded alunitein extensional data) C/L 74 K feldsparOxidized opal-K{'eldspar breccia 9.3 +_0.3• C/L 75 K feldsparOxidized opal-Kfeldspar breccia23• C/L 76 K feldsparUnoxidized opal-Kfeldspar 10.3+_0.31 faults. The alunites are believed to form froin the oxidation of the SOs-rich magmatic fluidsby the lossof hydrogen or of atmospheric oxygen. The dominance of fromthe volcanic clastsare thoughtto be somewhere be- the entrainment to resultfromthedecompression tweenthe ageof hydrothermal alteration andthe ageof the SOsin thefluidsisbelieved fluids(Rye,1993). volcanicelasts.Temperatures in thisnear-surface environ- of magmatic Field evidence indicates that the blanket acid leach zone mentappearto havebeeninsufficient to resetolderK felddeposit formedin a steam-heated envispar.An illitc sample(C/L 71) separated froman intensely at theCrofoot-Lewis rotancut. Here acid sulfate 'alteration occurs dominantly asa illitc + quartz+ PYrite altered zone ave a K-At age of 4.0 4 g horizontal blanket that caps the system, generally lacks sulwhichisconcordant withthe øAr/aOAr ages oftheK feldspar matrix. fides,and is zonedvertically,from a leachedresidualcap (blanket acidleach)at thetop,to a narrowintervalofintense Ageof acidsulfatealteration opalization (basalacidleach).Thelowerzoneof acidsulfate is inferredto coincide with the palcowater table AluniteK-Aragesvarybetween2.4 and0.9 Ma (Table1); alteration et al., 1974).The deepest extentof acid'alterthelocation of several alunites andthejarosite fromtheblan- (e.g.,Schoen of underlying veinsof opal-chalcedony + aluket acidleachzoneandunderlying acidsulfateveins'along ationconsists (Fig.5). Thiszonation is with available K-Ar agesareshownin Figure7. In Figure7 nite+ kaolinitc_+montmorillonite alunitesfromthe blanketzonegiveolderagesthanalunites common in steam-heated acid sulfate 'alteration zones in acsystems suchasSteamboat Springs, Nevada fromthe underlying veins.Fieldrelations (Fig.7) showthat tivegeothermal et al., 1974),Roosevelt Hot Springs, Utah(Parryet theveinsdonotcrosscut theoverlying blanketzonebutsim- (Schoen basinatYellowstone Park, plyextenddownward fromitsbase.Thisrelation,combined al.,1980),andin theNorrisGeyser (Whiteet al., 1988).Withintheblanketacidleach withstableisotope datadiscussed below,andprecious metal Wyoming of nativesulfur,remnantmudpoolssimidistribution, suggests thattheblanketacidleachzoneislikely zone,thepresence systems, andtheassociato be the sameageastheunderlying veinsandnotolder,as lartothosethatoccurin geothermal the K-Aragesindicate.Giventheuncertainty of theanalysestion with cinnabar indicate that this alteration formed in a steam-heated environment. In numerous localities at the Crothesesamples couldall be aboutthe sameage. Coarse-grained (upto4 ram)jarosite(sample C/L 32)from foot-Lewis mine bedded mounds and lenses of native sulfur vertical,elongate vesicles, a texturethatindicates the narrowfractures cuttingtheblanketacidbachzoneyielded contain a K-Arageof 0.7 + 0.2 Ma. Thiscoarse-grained samplehas sulfurwasmolten,andtherefore,hada minimumtemperaisotopic compositions characteristic of a steam-heated origin ture around 113ø to 120øC(White et al., 1988). Combined, precludesupergene proandrepresents theyoungest documented episode of geother- thesetexturesand temperatures cesses. mal activity. The presence of magmatie hydrothermal, acidsulfatealterField Evidencefor a Steam-Heated Origin ation can be ruled out based on the geometry of the add for Acid Sulfate Alteration sulfatealteration, the generalabsence of sulfideminerals in Acidsulfate alteration commonly formsin fourgeologicallythis zone, and as discussedbelow, the lack of a detectable distinctenvironments: supergene, steam-heated, magmatic magmatie component (e.g.,Byeet al., 1992).Stableisotope CROFOOT-LEWIS HOTSPRING Au-AgDEPOSIT, NV 589 data,presented below, support a steam-heated originforacid beingthatthe acidfluidsweregenerated bytheoxidation of sulfate alteration attheCrofoot-Lewis deposit; however, mi- H2Sgasandnot sulfideminerals. The mechanism by which noramounts of supergene aluniteprobably occurin thedis- Au andAgaretransported in theseacid,relatively oxidizing, trict. low-temperature fluidsis uncertain. The abilityof the acid sulfatefluidsto leachevenrelatively insoluble elements is Evidence for Fluctuations in the Water Table displayed by the largeareascontaining 90 to 100percent The levelof the watertablehasa majorcontrolon the residual silicathathavehadessentially all othermajorand distribution of epithermal mineralization andalteration as- minorelements removed(Ebert,1995).We speculate that semblages in thenear-surface environment (e.g.,Foleyet al., thiosulfate andC1arelikelyligands in thisenvironment (e.g., 1989;Simmons, 1991).At the Crofoot-Lewis depositalter- Listovaet al., 1968; Mann, 1984; Webster and Mann, 1984; ationassemblages andsinterbedshavebeenusedto constrain Stoffregen, 1986;Webster, 1986,1987);however, supporting thelevelofthewatertableduring theformation ofthedeposit experimental dataareneeded.The physical-chemical envi(Ebert,1995).Thepresence ofremnant opal-K feldspar alter- ronmentin the steam-heated environment is highlyvariable ation (an assemblage that formsbelowthe water table; anddeposition of Au andAgminerals fromsuchfluidscould Browne,1978;Henneberger andBrowne,1988)withinblan- be triggered by changing fo2,temperature, evaporation, or ket acidsulfatealteration (anassemblage thatformsabove mixing. the water table; Schoenet al., 1974) demonstrates that the StableIsotopeStudy watertabledropped between theperiodofearlyopal-Kfeldsparalteration andlateracidsulfate alteration. OverprintingSampledescriptions andanalytical results ofopal-Kfeldspar alteration byblanket acidsulfate alteration Briefsample descriptions andstable isotope dataaresumindicates thatthewatertabledropped a minimum of 18 m marizedin Table2. Samplelocations for several of the sampies are shown in Figure 7. The seven kaolinitc samples listed Mineralized stockwork quartzveining hasbeenexposed in in Table 2 were separated from mixtures of fine-grained alurecentminingexcavations andencountered in twocoreholes nite + kaolinitc + silica and are assumed to have formed in theproposed Brimstone pit. Thisstockwork cutsblanket with the alunite.Sixsamples of pyriteacid sulfate alteration in the Brimstone area. The distribution eontemporaneously mamasite were separated from samples of opal-K feldspar andextentof thisstockwork ispoorlyconstrained; however, alteration, and one sample of pyrite was separated froma thecoresamples demonstrate thatlatecrosscutting quartz sample of illite-smeetite alteration. In addition, two pyrite veiningextendsa minimumof 50 m abovethe baseof the samples (samples C/L 13 and C/L 17) were separated from blanket acidsulfate zone(Ebert,1995).Thisrequires atleast narrow alunite-kaolinite-pyrite veins that occur relatively a 50-mrisein the levelof the watertablefollowing acid deepin the system. A whole-rock •34Svaluewasobtained sulfate alteration. duringacidsulfatealteration. from an unaltered felsie volcanic rock from the Kamma Shallow Enrichment Processes for Precious Metals Mountains group(sample C/L 58)anda •34Svalueofpyrite was obtained from interlaminated slate and metasiltstone Figure5 illustrates the relationship betweenrelatively theAuldLangSyncGroup(sample C/L 43). high-grade oxidized acidsulfate mineralization, relatively low- from Three samples of present-day surface and subsurface acidic gradeunoxidized opal-Kfeldspar mineralization, andbarren waters have been analyzed for aqueous sulfur. The subsurface blanketacidleachalteration. The presence of remnant in anareawhichcontains nooxidized patches of weaklymineralized opal-Kfeldspar alterationacidwaterwassampled by oxidation (at shallow within barren blanket acid leach alteration is evidence that rockand mostlikelyoriginated depths) of HsS gas that is presently exsolving from a reservoir blanket acidleachalteration overprinted weakly mineralized of warm water at depth. Exploration drill holes have encounopal-K feldspar alteration andremoved AuandAg.Themintered warm (up to 40øC), locally acidic (pH to 3.9) water and eralizedacidsulfateveinsandzoneslocallycontain several H2S gas. Standard stable isotope techniques were used and timesmoreAu andAgthanadjacent opal-Kfeldspar alter- are summarizedin Ebert (1995). ation,andsimplemass-balance calculations demonstrate that leaching ofAufromtheoverlying blanket acid-leached mateDiscussion of IsotopicData rial can account for most of the observed Au increase in the Oxygen and hydrogen isotope datafor alunite, jarosite, underlying acidsulfateore(Ebert,1995;Ebertet al., 1996). and kaolinitc We propose thata dropping watertableenabled acidsulfate waters to overprint weaklymineralized opal-Kfeldspar alter- Figure8 shows thefieldsfor thepossible compositions of ation,resulting in theleaching of Au andAgasthesefluids supergene aluniteandkaolinitc (Ryeet al.,1992).Theshaded percolated downward towardthewatertable.Theseacidsul- regionlabeledSASFis the fieldfor •D-•18Oso 4 valuesfor fatefluidsprecipitated AuandAg(dominantly aselectrum,supergenealunitein the absenceof bacterialreductionof AgC1, andAgI),associated withopal+ alunite+ kaolinitc + aqueous sulfateor exchange of aqueous sulfate withlowpH montmorillonite, in open-space fractures at, and mainly waters. The area between the dotted lines labeled SAOZ within a few tens of meters below, the water table. This represents thepossible rangeof 6D-61sOon values in superprocess resulted in leaching of precious metals froma large geneequilibrium withmeteoricwaterbetween20øand80øC. areaandconcentration intorelatively confined zonesbelow Thesefieldsareforreference onlyandmanynonsupergene thewatertable;theyprobably formedin a similarmanner to alunites haveisotopic compositions withinbothofthesefields classical supergene enrichment zones,the maindifference (Ryeet al.,1992).Mostof thealunites in Figure8 cluster in 590 EBERT AND RYE CIlOFOOT-LEIVIS HOTSPIllNGAu-AgDEPOSIT, NV 591 592 EBEItT AND RYE acid sulfate veins. Four alunites from the blanket acid leach zonegivetemperatures between40øand90øC.Suchtemperaturesare reasonable for alunitesthat originatein the zone abovethewatertable,wheretemperatures •villgenerally not exceed100øC(Ryeet al., 1992).Fouralunitesamples from add sulfateveinshavetemperatures that rangefrom60øto 180øC.Suchhighertemperatures for alunitesin the add sulfateveinsare consistent with their greaterdepthin the geothermal system. Isotopicdataof sulfateandsulfides Figure 10A isaglot showing the6•SOso 4vs.6'•4S values of alunite,andthe 6 4Srangesfor sixsamples of pyriteand marcasite from opal-K feldspar alteration, and sulfatefrom -20• threesamples ofpresent-day acidwater.The634S values for -30 -20 -10 0 10 20 30 earlysulfides rangefrom -10.8 to -15.2 per mil andthese values approximate the634S values of H2Sin thegeothermal 680 fluid,because fraedonation between aqueous H2Sandpyrite lowtemperatures (< 180øC)likelyto FIC,.8. The6D,6•SOso4, 6•8OoH values of alunites andjarosite, 6D,and issmallat therelatively 6]sOofassociated kaolinires, andcalculated 6D.2o and6•SO._,o offluids for havebeenattained in thisalteration zone(Ohmoto andRye, alunitesandjarositesampleC/L 32. Fluid compositions calculated from 1979;Ryeet al.,1992).Allbuttwoofthealunites areenriched equations of Stoffregen et al. (1994)andRyeandStoffregen (1995).Lines in 34S relative to earlysulfides. andfieldsareMWL: meteoric •vaterlineofCraig(1961);PMW= primary The 634S value of pyritesample C/L 42 (-14.5%o)sepamagmatic waterfieldof Taylor(1979);kaolinitc line of SavinandEpstein illite-smeetite alteration iswithin (1970);SASF(shaded) = supergene aluniteSO4field;andSAOZ= super- ratedfromthewidespread genealuniteOH zoneasdescribed in Byeet al. (1992), theinterpreted 634S rangeof initialH.2S. Thesource of the S for pyritein the illite-Slnectite alterationzonewasmost likelythe H2Sthatwasexsolved fromboilingsolutions which a groupwith6D values around-120 per mil and6•SOo]•flowedalongpermeable zonesandcondensed (alongwith and6D-6•SOso4 values between about0 and10permil.Two CO2)into marginal,relatively stagnant, groundwater.This alunitesfall outsidethisgroup,with 6D valuesnear -160 sourceagreeswithmodelsforthe formation of interstratified per mil. The 6D valuesfor jarositesare muchlowerthan illite-smeetite alteration by Bartonet al. (1977)at Creede, thosefor alunite,with a rangefrom -183 to -145 per ]nil. andHedenquist (1990)attheBroadlands geothermal system, Supergene kaolinires orhalloysites typically have6Dand6•sO New Zealand. values closetothekaolinitc line(Lawrence andTaylor,1971; Figure10Bisa plotof alunite634S vs.A•SOso•_on values. Marumoet al.,1982).Thekaolinires allplotto theleftof the All of the alunites from the blanket add leach zone have supergene kaolinitc linein a tightgroupwithanaverage 6D partially exchanged 634S values, andthese values overlap with, of about-160 values anda rangeof 6•sOvalues of about0 but havea widerrangethan,thoseof the blanketalunites. to 7 per mil (Fig. 8). Mostof thealunites withexchanged 634S values (leastnegative) give reasonable A•SOso4_on temperatures. ThisindiAluniteA•SOso•_oH temperatures catesthat the residence time of aqueous sulfatein solution The6•sOoi• values ofalunite arecommonly in equilibriumforsomeof thesamples wassufficient forequilibrium oxygen with the parentwater,andif the aqueous SO4alsoreached isotope andsignificant sulfurisotope exchange betweenaqueoxygen isotope equilibrium withtheparentwater,theisotopic dataofalunitecanbeusedasa single-mineral isotopic geothermometer(Ryeet al., 1992;Stoffregen et al., 1994).The Vein Blanket average uncertainty of thesetemperatures isprobably about 2O _+30øC. Eightof the alunitesfromthe Crofoot-Lewis mine yield 18 o o A Oso4-oH temperatures thatrangefrom40 to 180C, con- E 40 • 6o v sistent witha steam-heated origin. TheA•SOso4_oi• values of theotheralunites yieldunrealistic temperatures forthisshallowenvironment andareinconsistent withthoseof thepresent mineralassemblages. Similarvalueshavebeenobtained foralunites fromthesteam-heated zones in activegeothermal systems (Ryeet al., 1992)andaretypicalof thoseof steamheatedalunites in whichoxygen isotope equilibrium wasnot obtainedfor the aqueous sulfate. Figure9 isa plotofdepthvs.calculated A•SOso4_oi• tem- lOO 0 .... 5•) .... 11•0 ' ' '1•0 .... 200 Temperature øC peratures foralunites yielding reasonable formation tempera- FIG.9. Depthvs.calculated alunite A]SOs%_oH temperature forblanket andacidsulfate vein(solidtriangles) samples. turesfromthe blanketacidleachzoneandthe underlying addleach(opensquares) CROFOOT-LEWIS HOTSPRING Au-Ag DEPOSIT, NV ism.Fluidinclusions arerareandtypically small(2-5 andidentification andmeasurements arehampered bystrong doublerefraction. Onlyonereliable homogenization temperatureof87øCwasobtained. Thesample hasanextremely low didvalueof -183 permilduetothelargeD-H fractionation between jarositeandwater(RyeandStoffregen, 1995).The calculated didvaluefor the parentfluidis about-120 per mil, consistent withthe rangein composition of the steamheatedalunitefluids.Thissamplehasa K-Ar ageof 0.7 _ 0.2Ma andrepresents theyoungest knownepisode of steam- ß ß ß ß G/l_2 I Pyrite/marcasite I heated alteration. I AcidwaterSO4 A I -17 b34S 40øC Twosamples of cryptocrystalline jarosite(C/L 33 andC/L 34) consist of lightbrownfracturecoatings that occurin oxidizedand unoxidized opal-Kfeldsparalterationzones whichhavenot beenacidleached.Bothof thesesamples haveunexchanged di34S values (- 11,- 13%,)andhavevery smallor negative A1SOso4_o, values. Thesejarosites could besupergene. It isquitelikelythattheoxidation of anyshallowsulfides wouldoccursimultaneously withthatof H2Sin a shallow geothermal environment. [] 180øC Sourceof SulfurandHydrocarbons ß [] ß [] [] Sulfides relatedtoprimary mineralization andalteration at theCrofoot-Lewis deposit havelowdis4S values, ranging from ß [] ß -10 Pyrite/ marcasiteI -•5 593 -10.8 to -15.2 perrail.Thesevalues areverydifferentfrom thoseof thelocalTertiaryvolcanic rocksor theunderlying metasedimentary AuldLangSyncGroup.Onewhole-rock dis4S analysis fromunaltered Tertiary volcanic rocksadjacent to theCrofoot-Lewis minehasa valueof 10permil.A single sample ofpyritefromtheAuldLangSyncGrouphasa dis4S valueof -1.9 per mil. Assuming a relatively homogeneous dis4S, the AuldLangSyncGroupis not a likelysource of (•=Blanket) B Vein -io •534S -• sulfur for mineralization. FIG.10. A. Alunite6•8Os04 vs.634S. Therangeof 634S values forearly Hydrocarbons orotherorganic material aretypically incorporated into hydrothermal solutions as they pass through sedA•8Oso4_o.vs. 634S values ofblanket (opensquares) andveinalunite (sold rocks(Hanor,1980).Dataondi34S values fromthe triangles) samples. Dashed linescorrespond to calculated A•SOso4_oH tem- imentary basinsediments adjacent to theCrofoot-Lewis peratures of 40øand180øC. The634S rangeof earlypyrite-marcasite isalso lateCenozoic shown. deposit arenotavailable. However, RiceandTuttle(1989) investigated thedis4S values oflatePleistocene toearlyHoloceneagesediments fromWalkerLake,in west-central Neous sulfate and the fluid. The fact that some of the vein vada.Sulfides andorganic sulfurin thesesediments contain alunitesamples showa greaterdegreeof S isotope exchangeisotopically lightsulfur, withdis4S values of sulfides ranging thansamples fromthe blanketzoneis consistent with the from -41.5to+24.4 •errail, averaging -15.6 per rail(total highercalculated aluniteA1SOso4_on temperature rangein of56samples), anddisSvalues oforganic sulfur ranging from theveinsamples. Figure10Bemphasizes thatindividual sam- -26.5 to +8.1 perrailandaveraging -4.4 perrail(totalof plesof steam-heated alunitein whichisotopicequilibrium 12 samples). RiceandTuttle's(1989)results showthatlow wasnotattainedmaynotbe distinguishable fromsupergene disaS values arecommon in organic materandsulfide minerals aluniteonthebasisof isotopic dataalone(Ryeet al., 1992). withinlateCenozoic basinsediment. HaS,likehydrocarbons, canbe derivedfromthe thermaldecomposition of organic Jarositeisotopic data materandhydrocarbons in sedimentary basins(Le Tran et organicmatter Darkbrown,relatively coarsely crystalline jarosite(upto al., 1974).Givensufficientheat,degrading pyrite-marcasite and for acid mine water is also shown.B. The 4 mm) occursin a veinlet that cuts the blanket acid leach within the basin sedimentscould contribute CO2, H.2S,and zone(sample C/L32).Because thereisnosulfide orevidence volatile hydrocarbons tothegasphase (Peabody andEinaudi, of weathered sulfides above,adjacent to, or immediately be- 1992). Alternatively organicmatter and/orhydrocarbons lowthissample, anoriginbysupergene oxidation of sulfides couldbe incorporated into the rechargefluidsand subseisunlikely. Thisjarosite hasanexchanged di34S valueof -6.4 quentlyhydrothermally decomposed (matured)alongthe permil andyieldsa A18Oso4_oH temperature of 70øC(Rye flowpath. Therearethreedominant sedimentary unitsin thedistrict, andStoffregen, 1995),suggesting a steam-heated origin.In thinsection, thejarositecrystals arezonedandaregenerally theAuldLangSyncGroup,bedswithinthe BlackRocktertransparent withlightyellow-green to deepbrownpleochro- rane,and the late Cenozoicbasinfill. Abundantgraphite 594 EBERT AND RYE withinthe Auld LungSyncGroupindicates that, at least quartzandcalcitesamples) aswellasa largerangein surface fromalunitesamples). Thisrangeofvalues locally,priorto ore deposition theserocksweresubjectto water(calculated of processes including variations temperatures abovethe oil stability windowandare an un- couldreflectcombinations likelysourcefor bitumen.The BlackRockterranecontains in theisotopic composition of thewatersovertime,modificawaters,boiling,and igneous andsedimentary rocks,including cherts,mudstones,tionsduringmixingof deepandshallow sandstones,courseelasticrocks, and minor lacustrinelime- water-rock interaction. Two of the calculated chloride waters stones (Oldow,1984;Russell, 1984).The degreeof maturity (fromquartz)havedidvaluesnear-118 perrail.Thisvalue waterandis the best of hydrocarbons in thisunit is unknown. The 2,160-m-thick fallswithinthe rangeof earlysurface valuefor earlychloridewater.Two sequence oflateCenozoic basinsediments thatfillstheBlack estimateof an average RockDesertbasinis considered to be the mostlikelysource calciteandonequartzsamplehave6DH2ovaluesof fluids of bitumenin the region.As previously mentioned, several rangingfrom -137 to -153 per rail.The paragenetic relaofthelateCenozoic basins throughNevadacontain showingstionsof thesetwosamples aredifficultto demonstrate; howof oil andgas,andanoil exploration welldrilledin theBlack ever,oneof the veinscutsopal-Kfeldsparalteration and RockDesertbasinencountered tracesofhydrocarbons above onecutsillite-smectite alteration, therefore bothsamples are a 2,150-mdepth(Garsideet el., 1988).Thus,available evi- relativelylate. The 19 to 35 per rail differencein the dencestrongly indicates that the late Cenozoic basinsedi- 6DH•,O valuebetweenthewaterfor thesesamples andearly mentsare the sourceof bothbitumenandisotopically light chloridewater(-118%•) cannotbe accounted forbyboiling (Truesdellet el., 1977). These low valuesresultedfrom mixsulfurat the Crofoot-Lewis deposit. ing of chloridewaterwith a low didvaluesurface water,or Characteristics of Fluids alternatively, the 6D valueof the chloridewatersdecreased Isotopic composition of chloride waters significantly duringthe laterstages of the system. A sample of quartz (+ chalcedony) fromthe mineralized The term"chloride waters"refersto the deephydrotherquartz stockwork that cuts the blanket acid leachzonein the malfluidandis analogous to the neutralpH chloridewaters areahasa relatively low6•sOvalueof4.1perrail in activegeothermal systems (White,1969;HenleyandEllis, Brimstone C/L 44). Thissampleformedverynearthe present 1983).The isotopic compositions of ohoride waters(Fig.11) (sample surface, at anestimated temperature between100øand120øC havebeencalculated from6•sovalues of quartzandcalcite (estimated from the hydrostatic boiling pointversusdepth usingfluid inclusion homogenization temperatures (Ebert, Thecalculated rangeof 618OH20 values forthiswater 1995)and6DH.,O valuesdetermined directlyfrominclusion curve). to low 6DHzo fluids.The quartzsamples havenumerous, but verysmall are -16.8 to -14.4 per rail, corresponding did = (mostly<2/.tin) fluidinclusions. The scarcity of sufficientlyvaluesof --158to --131perrailfromthe relationship 86•so + 10 (Craig, 1961). largefluidinclusions makesdetermination of accurate filling temperatures problematic andis a possible sourceof error Oxygen isotope shiftin deepthermalwaters in the calculated 6•so,,ovaluesof the quartzfluids.The Theamount ofoxygen isotope shiftduetowater-rock interstableisotope dataonfluidsin Figure11recorda largecomaction at depth in the deeply circulating geothermal fluid positional rangein chloridewater (calculated from vein -80 •.•t/ Ran ofearly .•'/ surface and '7 early chlodde fluids -3-• !.... -lOO • • •q- / • / -12o '-/--- q-' / +1•'- ' -t• • 3•. ...... ,-x--f- • ..... ,, / [] -14o , ,' , -16o ß A,•/ / / __ ........ I ' i I (chloride waters)canbe estimated fromthe 6•SOH•o-6DH.•O valueofwaterthatwasresponsible forveinquartzdeposition. Thewatersthatdeposited quartzfallalonga lineparallelto 4 Calculated alunitethe meteoricwaterline,but areenrichedabout4 per railin fluids (surface water) isattributed to [] Calculated quartz •sO(Fig.11).This4 perrail•sOenrichment oxygen isotope shift due to water-rock interaction and is simifwl•itdtr )(choride 0 Calculated calcite lar to that observedin moderngeothermalareassuchas fluids (exchanged Yellowstone (Craiget el.,1956;Craig,1963;Truesdell et el., chloride water or • ' a-- mixture) 1977). fluids (100øC) The calcitefluidsare enrichedabout10 to 11 per rail relativeto unexchanged meteoric water.CalcitesampleC/L Calculated koalinite ß Calculatedkoalinite • f,u•ds (soøc) 68 is from a calcite vein that occurs within illite-smectite alteration. Fluid inclusions fromthis samplehavevariable liquidtovaporratiossuggestive ofboiling(e.g.,Bodnar et el., 1985).Someinclusions in thissamplehavepositivemelting temperatures (upto2.6øC) whichmayresultfromtheformationof clathrate duringfreezing, indicating thepresence of AA • .• ,_, ,, ,' - b- • ' E•matedrangeof .'-'. I latesurfaceand late chlondefluids ' I CO2 or the formationof metestableice (Roedder,1984). This ' samplemostlikelyformedfroma COs-rich, steam-heated ground waterora mixtureofground waterandchloride water 8180 (e.g.,Hedenquist, 1990).CalcitesampleC/L 69 is froma darkbrownto black,coarsely bandedcalcite+_chalcedony F]c..11. Fluid•DH20and•18OH20 values fordeepandshallow fluids vein. Fluid inclusions from this sampleare alsodominantly calculated fromalunite 6D and•SO values and/¾SOso4_oH temperatures, liquidto vaporratios.The increase andquartz andcalcite •SOvalues, fluidinclusion •Dn2ovalues, andhomoge- twophasewithvariable in •SOshiftin thesecalcitefluidsoverthatfor the quartz nizationtemperatures. -30 -20 -10 0 10 CROFOOT-LEWIS HOTSPRING Au-Ag DEPOSIT, NV 595 exchange witha late,isotopically lighthydrofluidsis possibly dueto isotopic enrichment duringboiling bleto H isotope (Truesdellet al., 1977)or duringinteraction of CO2-rieh thermalfluid (O'NeilandKharaka,1976).Figure12 shows from the veinsare slightly steam-heated waterswith the countryrock,as described at that the 8D valuesof kaolinites in D relativeto kaolinites fromthe blanketzone. the Broadlands-Ohaaki geothermal systems, New Zealand depleted (Simmonsand Christenson,1994). Oneinterpretation of thistrendisthatkaolinites in theveins reached a greater degree ofisotopic equilibrium withanoverIsotopic composition of acidsulfatewaters printing fluidthankaolinites in theblanket zone.Thisdifferenee would be consistent with higher elevation oftheblanked Isotopic compositions of watersthatdeposited alunitecan zone which most likely resulted in lower temperatures or a becalculated fromtemperature information andthefraetionshorter duration of contact with an overprinting hydrothermal ationfraetors of Stoffregen et al. (1994).Figure11shows the fluid. calculated isotopic compositions ofwaterin equilibrium with that H isotopeexchange in kaolinitcrethosealunites thatyieldreasonable /X]SOso4_OH tempera- The observation quires the influx of fluids having a substantially lower tures.The8D and8]SOvalues of alunite waterplotnearthe 8Dmo value than that of the primary fluids, and along with meteoricwater line, and mosthave 6D valuesbetween -100 the low 8D value of alunite C/L 1, suggests low 8Dn2o v'alues and -123 per mil, whichis doseto the valueof chloride system. waterandis consistent withthe average value(-120%o)in- for fluidslatein the life of the geothermal Relation of Mineralization to Palcoclimate terpretedto havebeentypicalof meteoric waterin thisarea backto the late Tertiary(Friedmanet al., 1993).Alunite The Crofoot-Lewis depositrecordslargefluctuations in waterswhichplottotherightofthemeteoric waterlinecould both the isotopic composition of fluids and in the level of the beduetoevaporation in thesteam-heated environment (Ellis paleowater tableovertime.Thesechanges canbecorrelated and Mahon, 1977; Truesdell et al., 1977). with Plioeene to Pleistocene palcoclimate recordsfor the The fluidfor onealunitehasa 8D valueof -158 per mil western United States and are therefore interpreted to reflect andalongwiththecorresponding 8•SOH,2o valueplotsnear major elimate changes over the life of the system. the meteoric waterline (Fig.11).Thisalunitewascollected Studies of marineoxygen isotope, foraminiferal, anddiafroma remnantmudpoolnearthe top of the blanketacid tom records,combinedwith terrestri'altaxaand other indicaleachzoneand, basedon its positionin the system,it is 1991),suggest thatfromapproximately 4.8 youngerthanthe otheralunitesamples(although its K-Ar tors(Thompson, to 2.4 Ma relatively high rainfall and mild temperatures exageindicates thatit is slightlyolder).The low 6D}•2o value isted in the western United States. This wetter climate crefor thisalunitesamplecannotbe accounted for by temperalargelakesthroughout westernNorthAmerica ture-related fractionation or evaporation, andit wasalmost atedseveral (Forester, 1991). At about 2.4 Ma, the firstlargecontinental certainlydeposited froman isotopically lightsurface water. glaeiation events occurred in the northern hemisphere, and This isotopically light surfacewater is consistent with the thesecoolertemperatures andgreaterariditylastedfrom2.4 composition of the lateehoridewaterspresented above. to about2.0 Ma (Thompson, 1991).From2.0 to about1.8 Ma, warmer and moister conditions prevailed throughwestPostdepositional D exchange in kaolinites and•D em NorthAmerica(Thompson, 1991). of late-stage fluids Detailedstudies of late Cenozoic perennial lakedeposits Withoneexception (aluniteC/L 1), allkaolinite •D values fromSeades Lake,California, bySmith(1984)demonstrated are41 to 50 per mil lowerthanthoseof associated alunites sixdistinct paleohydrologie conditions in theGreatBasinover (Fig. 8). Basedon the fraetionation factorsof alunite-H.20 the past3.2 m.y.:3.2 to 2.5 Ma (wet);2.5 to 2.0 Ma (dry); (Stoffregen et al.,1994)andkaolinite-H•20 (LiuandEpstein, 1984), there is little differencebetween•D valuesof alunite andkaolinitewhichformedfromthe samefluidat tempera- turesabove150øC.Since/X•SOso4_on temperatures calculatedfor aluniteindicatethatseveral of the samples formed at temperatures between 40øand180øC, andthealuniteand kaolinite appearto havebeendeposited eontemporaneously, thegreaterthan40 per mil difference in •D valuesbetween coexisting aluniteandkaolinitccannotbe explained by temperature-related fraetionation. We suggest that•D valuesof thefine-grained kaolinites havebeenresetby exchange with late,isotopically lightfluids.Experimental studies showthat kaolinitcis susceptible to low-temperature H exchange in the absence of significant O isotopeexchange (O'Neil and Kharaka,1976;Kyserand Kerrieh,1991).The mechanism involves exchange with OH siteandis distinctfromthe DH changes thataccompany reerystallization or neoformation, bothwhichareassociated withchanges in •]SOvalues. 50 150 • KaoJinite Blanket ) ß 200 -170 Kaolinite Vein '-1•0 '-1•0 '-140 6D The kaolinitesamples analyzed in thisstudyareveryfine FIC. 12. Depthvs.•SDfor kaolinitc fromthe blanketacidleachzone grained, typically lessthan1/am,andarethereforesuseepti- and acid sulfate veins. 596 EBERTAND RYE 2.0 to 1.3 Ma (intermediate);1.3 to 1.0 Ma (wet), 1.0 to 0.6 ationto overprint opal-Kfeldspa• alteration, resulting in secMa (intermediate); 0.6to 0.3Ma (dry). ondaryAu-Ag-enriched zonesthatare economically mineThe variousepisodes of alterationat the Crofoot-Lewisable.Duringthelaterpartofthisperiod,6D values of fluids depositbetween4 to 0.7 Ma andthe •D valuesof meteoric decreased fromabout-120 to -158 per mil. watercorrelate wellwiththispalcoclimate record. A possible 3. 2.0 to 1.3 Ma: warmer, moister intermediate conditions history of the system basedonthe combined palcoclimateresulted in a risein thelakelevelandwatertableallowing studies of Thompson (1991)andSmith(1984)ispresentedthegeothermal system to overprint thenear-surface blanket belowandisillustrated schematically in Figure13: acidleach alteration (Fig.13C).Thehighwatertable,possibly related toglacial meltwater,enabled a lateepisode ofminer1. 4.8 to 2.5or 2.4 Ma:wetconditions andmildtempera- alizedquartzstockwork veiningto reachthe near-surface, tures created a lake in the Black Rock Desert basin. Geother- locally overprinting andmineralizing blanket acidleachaltermalactivity beganduringthisinterval, andfluiddischargeationin the Brimstone areaandresetting kaolinitc to lower •D values. nearthemargin of thelake,focused alongrange-bounding normalfaults,resulted in widespread near-surface opal-K 4. 1.3 to 1.0 Ma: relatively wet conditions mayhaverefeldspar alteration andsinterdeposition (Fig.13A). sultedin a furtherrisein the water-table(at least50 m above 2. 2.5 or 2.4 to 2.0 Ma: lowertemperatures and more thelowest watertablelevelduringthesecond episode). aridconditions associated withglaciation at highelevations 5. 1.0 to 0.6 Ma: intermediate anddry conditions again resulted in a reduced size of the lake in the Black Rock resulted in an increase in •D values of meteoric waters Desertbasin(Fig.13B).Theresulting drop(18m minimum) (-120%o)anda possible dropin thelakelevelandthewater in the water table enabled steam-heated acid sulfate alter- table(Fig.13D).Late(0.7Ma)steam-heated jarosite formed 4.0 to 2.5 Ma wet mildclimate A 2.5 to 2.0 Ma coolermore arid climate aD~-120 to-100 'Poolsinter' interbedded Build upofnear- Talus B High water table withsilicified conglomeratessurfaceSilicification debris Widespread ac•-surrate I alteration,/ •• .... •,•'I,• ' High lake level Drop in lake level . (lake) late change inaD from ~.o-120 to -158lowwatertable .. . Relatrvely .. (lake) •.... ,'.-' !• ß !''; : • • ""-• ..... :;".i: o½/' .. •,,. :.: C Rise in lake Pervasive day 0 -0 alteration 1.0 to 0.6 Ma warm moistclimate 2.0 to6D 1.0 warm moist climatecuts Late quartz stockwork D ~Ma -158 to -137 allotheralteration/ High Water table zones [ ••. level • Drop in lake level • steam-heated alteration Low w•ter table aD ~Minor -137 to -120 or greater End of lake? I (lake)• .... :' ii•i•' "':'" (lake?) . i': ....... ;;.• ;- '; .":•;. / I Opal-K-feldspar alteration containing low-grade Au- Ag r• Illite-smectite alteration -'• Blanket acid-leach alteration I 1km I Mineralized acid-sulfate veins Hydrothermal eruption breccia Stockwork quartz veining Boiling conditions Massive opaline sinter deposits Dominant fluid flow pattern FIG.13. Schematic cross section through theCrofoot-Lewis deposit showing formation through timeandinterpreted changes in 6D values (seetext). CROFOOT-LEWIS HOTSPRING AuoAg DEPOSIT, NV 597 section through theSulphur region thatisconsistent along fractures above thewatertableintheblanket acidleach rivecross with all of the data available from the area. zone. Mostactive geothermal systems andepithermal oredepos6. 0.6to 0.3 Ma:dryconditions caused a furtherdropin in timeandspace withsubaerial volcanism, thewatertable.Geothermal activityprobably ceased during itsareassociated heatsource isoftendemonstrated orinferred this time. Currentwarmwatersand H•S gasin the area andanigneous (Henley,1985).Figure14, however, is drawnwithoutan probably reflectconductive heating. Factors other than climatic conditions could account for igneous heatsource, although the modeldoesnotrequire theabsence of sucha heatsource. TheCrofoot-Lewis deposit the observed fluctuations in thepaleDwater table.However, andseveral geothermal systems innorthern Nevada aresomeastheknownvariations in paleDclimate canexplain boththe whatanomalous in thattheylackobvious coevalvolcanism fluctuationsin the water table and the variable 6D values, (Crewdson, 1976;Blackwell, 1983).Manyauthors consider changes in paleDclimate are considered to be the bestex- thehighgeothermal gradient in northern Nevada tobesuffiplaination ofthe6D-timedataat theCrofoot-Lewis deposit. cientto account formostof thepresentgeothermal systems intheregion (HoseandTaylor,1974;Sass etal.,1979;Black- Fluid Circulation Model well, 1983; Buchanan,1990). The regionsurrounding the Sulphur districtfallswithin Thissection proposes a simpleconvection modelfor the the Battle Mountain high, an area of anomalous heatflowin Crofoot-Lewis system in whichmeteoric waterflowedalong Basin andRange province thathasa geothermal extensional fractures in the basinandwasheatedmainlyby thenorthern ofabout50øC/km (Duffield etal.,1994).Thevolumithehighthermalgradient in theregion.Thismodelisconsis- gradient of silicasinterattheCrofoot-Lewis mineinditentwiththeSisotope dataandtheapparent longevity ofthe nousdeposits byfluidsfroma reservoir witha temperature Crofoot-Lewis system, but is admittedly speculative, mainly catedeposition than210øC (Fournier, 1985).Sincethedepthtothe because of a lackofgeophysical dataonthebasin,thepaucity greater transformation nearcentralPershing County anduncertainty of the radiometric agedataontheCrofoot- brittle-ductile 1992),meteoric waters Lewisdeposit, andthelackof supporting hydrologic model- isbetween12 and18km (Catchings, should be able to circulate easily to deeper than 4 km and ing studies.Nevertheless, it is importantto presentthis gradient to reach model,because, if correct,the potentialareathat canbe be heatedonlyby the highgeothermal above210øCasshown in Figure14. targeted forexploration of younghotspringgolddeposits in temperatures theGreatBasinincreases significantly. Figure14isa specula- Seismic reflection and refraction data from northwest NeNW CrofootJLewis Black Rock Range Mine Black Rock Desert • • SE Kamma Mountains King Lear Federal 1-17 • ,vwvv• ,•vvvvvvvv .................... • .............................................................. vv .• "?•"•-'•-•__ ,.,....... -_ ,,/• -•-,,-,•,•y v v, •L ..... ............................................................... • ß Geothermal Depth km • 0 gradient50øC/km -- 0oc -- 100oc -- 30OOC •Z;E; :::::::::::::::::::::"•7•;... '•;;;2;;;;;.' ......... ;•.......'•7;;;.';;Z ..........7.';.';.';;;.';.'.'; ;;;;.';;;Z;;:. ======================= ........................................................... v ....................... .':';7:2:..':;::.'E::2 '•.CC;:' 2.C';.'::.;:C. ,'.'.':.'.':.'.'.;'.';.'.;•C C'CCCC ,';:::!v ,••. ...' v Paleozoic clastlc sedimentary, '• •' ? carbonate and volcanic rocks ? 2 -- 8 Master fault -- ......... u•il•'tr a•si ti•'n r•xi•ate•y 500 - 700øC (e.g. Catchings,1992) B•tl•'-d (."?) •'pp •.::• Late Cenozoic basin sediments Triassic toJurassic Auld Lang Syne Group, fine-grainedmetasedimentaryrocks .• ß Talus-fanglomerate deposits Mesozoic (+Paleozoic) Black Rock terrane • Tertiary Kamma Mountains Group, felsic volcanic and volcaniclastic rocks rocks, and fine to course clastic rocks. includingintermediate volcanicand plutonic 10 --500øC --12 --600øC 0 2 I I krn Drawn to scale Fluid flow path Fla. 14. Simplified interpretive cross section through theBlackRockDesert andadjacent ranges, showing interpreted geology andhydrothermal fluidconvection. Anigneous heatsource isnotshown butcouldbepresent. 598 EBERT AND RYE vadaindicatethat the majorrange-bounding faultsflatten Thepaleoclimate wasanimportant factorin theformation with depthandthat the intensityof faultingandfracturing of the Crofoot-Lewis depositby controlling the levelof the ismuchgreaterwithinbasins thanin adjacent ranges (Catch- palcowater tableand the availability of meteoricrecharge ings,1992).Seismic reflection, gravity, andgeologic datafrom water.Duringperiodsof highsurface wateravailability, the the Dixie valleyarea,Nevada,alsoshowthe rangesto be highlyfractured areasbeneaththe basins areinterpreted to composed of a fairlysolidblock,whereas theadjacent basins be favorable sites for meteoric water convection. Meteoric are highlyfaultedandfracturedto depthsof 15 to 20 km watersrecharged alongfaultsthrough thelateCenozoic basin (Okaya,1985).The simplified structure presented in Figure sediments, acquiring Sandhydrocarbons. Thesedeeplycircu14 illustrates the highdensityof faultingandfracturing in- latinggeothermal fluidswere eventually focusedand disferred to occur below the Black Rock Desert basin. chargedalongbasin-and-range faultsnearthe marginof a The highdensityof faultingwithinthe basinsprovides lake.An igneous heatsourceis possible butnotapparent at pathways forthedownward migration of meteoric water(e.g., thesystem, andit issuggested thatthehighregional geotherHoseandTaylor,1974).Fluidconvection is favoredin frac- malgradient wassufficient to formtheoredeposit, withfluids turedrockswithhighpermeability thatareexposed to a geo- circulating to minimumdepthsof 4 km. thermalgradient(CrissandHofmeister, 1991).Convective Acknowledgments flowcanbecaused bytheinverse density gradient thatresults Discussions with DavidGroveshelpeddevelopthe confromthe thermalexpansion of water,suchthathot fluidis in thispaper,andhiseditingandreviewis displaced upwardby the inwardflowof cooler,denserfluid ceptsproposed DavidJohnandTed McKeefromthe (Crisset al., 1991;Bj0rlykke,1993).In Figure14 recharge greatlyappreciated. Geological Survey performed K-Aranalyses on forthesystem isshown to occurwithinthebasinwithupflow UnitedStates samples andoneillitcsample fromtheCrofoot-Lewis alongthe range-bounding faultsat thebasinmargins aspro- alumitc available for stableisotope posedbyMaseandSaGs (1980)fromtheirstudyof theBlack depositandmadethesesamples Rock Desert area. analyses; theirsharing ofdataandideasisgreatly appreciated. with BradWigglesworth, KenJones, AlainCotFluiddischarge, andthelocalization of geothermal activity Discussions contributed to andepithermal mineralization in northernNevada,is com- noir,andFrederickFelderhavesignificantly in thispaper.We acknowledge reviews monlyrestricted to therange-bounding faultsalongthepedi- the ideaspresented mentat the basinmargins. Geophysical studiesshowthat by ByronBerger,Phil Bethke,Jeff Doebrich,A1 Hofstra, and Economic manybasinsin northernNevadaare highlyasymmetrical,DavidJohn,Ed Mikucki,StuartSimmons, refereesthat havecontributed significantly to imwiththedeepersideadjacent to thedominant or masterfault Geology thismanuscript. Several of thegasextractions, isotoandthe othersidemarkedby valleyward tilt anda complex proving andcountless othertasks weredonein thelaboseriesof antithetic faults(Stewart,1971).We predictthatthe picanalyses, at the U.S.G.S. moresignificant anddeeperpenetrating masterfaultshavea ratoryby CarolGent,andher contributions in Denverare greatlyappreciated. significant role in focusing deepfluidupflow(andperhaps stableisotopelaboratory andDevelopment Inc. andGranges Inc. heat-flow fromdepth),andthereforeexerta majorcontrol HycroftResources financial support fortheproject. Additional support onthe distribution of epithermal andgeothermal systems in provided wasprovided bytheKeyCentreforStrategic MineralDeposthe basin-and-range setting. itsandanOverseas Postgraduate Research Scholarship at the Summary University of WesternAustralia. Radiogenic isotopeagessuggest that the Crofoot-LewisNovember 6, 1995;June26, 1997 system waslong-lived, with at leastintermittent activityfor REFERENCES over3 m.y.Field evidence indicates that the blanketacid leachzoneat the Crofoot-Lewis depositformedin a steam- Barton, P.B., Bethke, P.M., and Roedder, E., 1977, Environment of ore in the Creedeminingdistrict,SanJuanMountains, Colorado: heatedenvironment in a geothermal system. Stableisotope deposition PartIII. Progress towardinterpretation of thechemistry of theore-formdataconfirma steam-heated originfor thiszone. ingfluidfor the OH vein:ECONOMIC GEOLOGY, v. 72,p. 1-24. 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