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EcoaomicGeologyVol.92, 1997,pp 807-826BriefDurationof HydrothermalActivityat ronologyCHRISTOPHERD. HENRY, NevadaBureauof MinesandGeology,Universityof Nevada,Reno,Reno,Nevada89557HALLET B. Nevada89045WILLIAM C. MCINTOSH, MATTHEW T. HEIZLER,NewMexicoBureauof MinesandMineralResources,Socorro,NewMexico87801AND STEPHEN B. CASTORNevadaBureauof MinesandGeology,Universityof Nevada,Reno,Reno,Nevada89557AbstractThe RoundMountaingold-silverdepositis one of the ;reservesplusproductiontotalat least16 millionoz (500,000kg)Au.The depositoccursin ash-flowtuff thatis pondedwithinits Theseareinterpretedeitherasa temporalsequencefromearlypropyliticto latesilicicandargillicor asa spatialprogressionfroma potassiccoreoutwardto a ProPYlitichalo. øAr/a9Aragesdeterminedbysinglecrystalfusionof aindicatea mostlikelydurationof hydrothermalactivityof 0.1 m.y.,possiblyno morethan0.05m.y.,anda maximumof 0.5 m.y.Eruptionof hostash-flowtuff andcalderacollapseoccurredat 26.5Ma; alateor postalterationtuff thatrestsuponmineralizedrockis 26.0Ma. Hydrothermalcirculationis unlikelyto ,sodurationwasprobablyconsiderablylessthan0.5 m.y.Eightadulariasamplesgiveagesbetween25.94 0.09and26.090.05Ma, andoneis 26.20 0.05Ma; the mean of all nine is 26.03 0.08 Ma. The maximum and minimum valuesand their uncertaintiesallowsa durationof asmuchas0.4 m.y.,butfor .The absoluteagesof sevensamplesthatcanbe placedin thepossibletemporalalterationsequencedo etemporalseq uencebutdoesindicatethatalterationoccurredin a ,whichthemselvesargueagainsta dulariarevealdiffusiondomainswithAr closuretemperaturesbetween 230 øand 390øC.Adulariathat crystallizedand remainedat temperaturesof 275øC,the maximumtemperatureinferredfrom fluidinclusiondata,wouldyieldagespectrawith agegradientsspanningthe lifetimeof the system.No spectraareconsistentwitha thermalhistorythatincorporatessustained( 0.1 m.y.)temperaturesat or above275øC.Episodicactivityto butisinconsistentwithmostdata.Therefore,the hydrothermalsystemprobablywasactivefor lessthanthe uncertaintyin the ars.We concludethathydrothermalactivityat RoundMountainwasa briefeventat 26.0 Ma andwascloselylinkedto nitialash-floweruptionby 0.5 m.y.andmayhavebeendrivenby a icsignificanceoftheRoundMountaindeposithasled to intensestudythatincludesgeologicmappingof RoundMountainandadjacentareas;analysisof tion;geochemicalstudiesof magmatismandalteration;andK-ArandTHE ROUNDMOUNTAINgold-silverdepositin Nye County,Nevada,hasproducedmorethan4 millionounces(Moz;124,000kg)of reservesat thebeginningof 909;proximately10.2Moz (509 Mt at 0.02 oz/t). Becauseonly 4øAr/39Ar1921;FergusonandCathcart,1924;Mills, 1984;approximately65 percentof the minedgoldwasrecovered Ferguson,1985;TingleyandBerger,1985;Bergeret MozAu. Harrington,1986; Boden, 1986, 1992; Shawe, 1986, 1995; Mills et al., Sander,1988;Sanderand Einaudi,1990;Henry etReno, Nevada 89511.mineralizationand calderamagmatismand structure.The* kesideDrive, al., 1996). These studiesdocumenta 07
808HENRY ETAL.RoundMountaindepositis locatedalongthe easternring The Round Mountain calderafractureof a esof malfluids.Circulationmayhavebeendrivenby a et al., 1986;Boden,1992;Shawe,1995;Henryet al., alderacycle.Henryet al. (1996)describein detailthe in generalmod- tionof the Round Mountain caldera Ma'or strati ra hic units.ß ' 40j g. pßelsof epithermalmineraldeposits.It is one of the world'stheirages,mostlydeterminedby Ar/ Ar datingof ilverdeposits. anddine, are listed in Table 1.It is citedasa typeexampleof a hot-springpreciousmetaldepositandusedto emphasizethe connectionbetweenepi- Pre-Round Mountain caldera rocksthermal mineralizationand active dimentaryrocks,Cre(Berger,1985;Bergerand Silberman,1985;Henley,1985;granite,andEoceneto TingleyandBerger,1985).The taceousrockshotspringconnectionis notuniversallyaccepted.For exam- nic rocks(Figs.2 and3; Table1). The velytotheple, Sander(1988)and ximumhydro- eastin theToquimaCambrianto uresat RoundMountainwere approxi- sinseveralmately270øC,whichtheyinterpretto (Shaweetal.,1986).of about400 m for the mainorebody.In contrast,Henryetswarmof 34 to 37 Ma, northeast-striking,al ng An extensiveandesitic,andmonzoniticdikesanda smallbodyofto arguethat mineralizationwasindeedshallow,with some rhyolitic,granodioritecuttheCretaceousgraniteof ShoshoneMounoreforminglessthana fewtensofmetersbelowthepaleosurtain immediatelyeastof RoundMountain(Figs.2 and3;face.similardikescutPaleoRoundMountainis alsoan idealsiteto investigatethe Shaweet al., kesareprobablytiminganddurationof hydrothermalactivity.Thethorough- zoicrocksnessof previousstudieson geologyandalterationconstitute also34 to 37 Ma but are highlyalteredandhavenot beendated.a comprehensiveframeworkin whichto examinetiming.Theof Roundproposedalterationsequenceof SanderandEinaudi(1990), The tuff of Dry Canyoncropsout northeastandappearsto fill themostlyburiedDry Canyonalthoughcontroversial,providesa relativetemporalframe- Mountainworkwithwhichto comparethe geochronologicdata.Adu- caldera(Figs.2 and3; Boden,1986;Henryet al., 1996).Drillholes thatlaria,whichiswellsuitedto precisedatingbythe4øAr/39Arbottom in the tuff indicate a minimum thicknessof RoundMountain.The tuff is denselymethod,is abundantin the depositandformedthroughout of 288 m northeastwelded,containsclastsof Paleozoicsedimentaryrocksandthe historyof alteration.Finally,the importanceof Mountainbotheconomicallyandasa typeexampleof epirocksinthermalmineraldepositsmeansthat the resultshavebroad 1988),andpinchesout his studyexaminesthe geologicsettingand outcropto the eastand in the / Ardateshydrothermalalterationat thatthetuffofDry Canyontensivenewdataon the precisetimingof hydrothermalacdatesat 32.2Ma,nearly6 m.y.olderthanthevolcanicrockstivity.of the calderaash-flowtuffs cropout in the ToquimaGeologicSettingcalderacomplexapproximately5 km to the northeast(Fig.Regionalsetting1; Boden,1992),outsidethe areashownin Figure2.The RoundMountaindepositoverliesthe easternmargin Tuffof RoundMountainofa mostlyburiedcalderaatthewesternedgeoftheToquimaThe RoundMountaincalderaformedat 26.5 Ma, duringRangein the GreatBasinof centralNevada(Fig. 1; Henryet al., 1996).The ToquimaRangeis one of manynorth- eruptionof thetuff of RoundMountain(Henryet al., dvalleysgen- The tuff is dividedinto two informal members,a lower memeratedby late Cenozoicextension.CentralNevadawasan ber that is morethan600 m thick,andan uppermemberareaof intensemid-Cenozoicash-flowmagmatismthatcre- (originaltuff of RoundMountainof Tingleyand Berger,ated numerouscalderas(Boden, 1986; Shawe et al., 1986; 1985)that is asmuchas390 m thick(Figs.2 and3). BothBestet al.,1989;andreferencesin Fig.1).At least13calderas that rangein agebetweenabout32 and22 Ma havebeen wall of the caldera.The lower member, which is encounteredto moderatelyweldedandconmappedorinterpretedin theToquimaRange,in theToiyabe onlyin drillholes,is denselydebrislensesand ewest,andintheMoni- rainsnumeroustor Rangeto the east(Fig. 1). Severalof thesecalderasare blocks of Paleozoic rock as much as 60 m thick. The thercal- dmapping wellasbrecciashedfromthe calderawall.Thetuff ismostlya tein the regionprobablywill identifystillmorecalderas.
4øAl:Ua9ARGEOCHRONOLOGY,ROUNDMTN., NV809Caldera withI10 miF]c. 1. Regionalgeologicsettingof the RoundMountaingolddeposit.RoundMountainliesat thewesternedgeofthe ToquimaRange,oneof numerousnorth-northeast-trendingrangesin the centralGreatBasinof Nevada.At alley.Data from McKee(1974,1976),Stewartand Carlson(1978),Bremand Snyder(1983),Snyderand rger(1985),Boden(1986,1992),Shaweet al. (1986),ShaweandSnyder(1988),Bestet al. (1989),Bremet al. yet al. (1996).breaksduringeruption,particularlyin the upperpart.Thetuff wedgesout tainandthickensmarkedlyto thewestasthe top of the Paleozoicrocksdropssteeply(sectionA-A';Fig. 3). The steep,southwest-dippingcontactof the lowermemberwithpre-Cenozoicrocksin Figure3 oregentlecontactto the northeastalongsectionA-A' dipstooshallowlyto beTABLE1. Stratigraphyat cktypeTuff of l air-falland(m)23-30Age(Ma)26.07 0.05225.93 0.15.291ash-flow tuffTuff of Round MountainUppermember Denselyweldedash-flowtuffNonwelded ash-flow tuffLower memberTuff of Dry lowtuffRhyoliticto andesiticdikesGranite of Shoshone MountainPaleozoicSedimentaryRocksAgesin boldare4øAr/aOArdeterminationsfromHenryet al.(1996);otheragesarefromShaweet al.(1986) Originaltuffof hronages,respectively,ofa singlesample22050-9.10 60026.50 0.0626.54 0.0726.48 0.089.6.52 0.1526.53 m 0.0732.18 0.1334 to 37CretaceousCambrian-Ordovician
810HENRYETAL.117 ø 05','.' ' TrudD1 9"."."- . Ott ' D175A.' .'.' ß .' . .'.:;: :;: : : : : : :;Round.:.:.:,:,:.:-:.:.:3[::Mountain,o. .' ./".'. .'.::::::::::::::::::::::::::::::::::Vru'::: ?. :. :. : ;::.::::::::::::::::::::::::::::::::QTrud.' .'Q9" Qg:.Kg.,D26;Trod: :[::'/y//TrunKe0:NI/ ./Q½ Trunp0500200010001500 m4000ftFIG.2. gextrapolationtosurfaceofsamplesfromdrillholesor pit).SeeFigure3 forexplanation.a calderaboundaryfaultsoit is mostlikelya scarperoded at least390 m about1,300m southwestof RoundMountain,alongthe fault.Thesecharacteristicsindicatethatthe lower thelower,nonweldedpart,whichis98 m thickbelowRoundduringitseruption,andthatpartof thetuffwasdepositedMountainandmorethan210m thenthinssouthwestThe uppermemberof the tuff of RoundMountainforms ta singlecoolingunitseparatedfromthelowermemberbya of the nonwelded tuff in drill hole D26 at the southwesterndistinctcoolingbreak(Figs.2 and3). Mostof the upper endof sectionNE-SW(Fig.3) is no morethan50 m lying,andis poorlyto selyweldedtuffnearthe wasdepositedalongthe erodedcalderawall,shortlyaftertopof RoundMountain(Figs.2 and3; TingleyandBerger, eraplacementof the lowermember,whilethe wall stillhad1985;Sander,1988);it hasbeenremovedby miningbutis ehadpreservedonthedownthrownsideof a systemof northeast-occurred.The coolingbreakbetweenthe two membersisstrikingBasinand Rangefaultswestof RoundMountain mostdistinctat thecalderawall,whichwasprobablya heat(sectionB-B'; Fig. 3). Coarse,massivedebrislensescon- tainingelastsof pre-Cenozoierocksup to severalmetersin diameterarecommonin thelower,nonweldedpartandare ermember.exposedin the pit.Theuppermemberwedgesouteastwardagainsta moder- Bothmemberscontainabundantphenocrystsof ierocksand ,Fethetuff of Dry Canyon(sectionA-A';Fig.3). Theupper Ti oxides,and zircon. The lower member contains30 to 40memberthickensgraduallysouthwestwardintothe caldera. percentphenocrysts.Phenocrystabundancein the upperIt is about 290 m thick at Round Mountain and thickens to memberincreasesupwardfromlessthan10to morethan40
4øAlVaOARGEOCHRONOLOGY,ROUND MTN., NVA811AITs1994 pitD26I --D8271 .: :::': P.--"' .'.' ::::::::"Trud . :.:.:: :i:i:i:i:i ::iiiiiii i;iiiiiiiiiii:i:!: ::. D--- --- ---::"- ':- :i : ::i::ii::i . . .TrudQIT 1' 'Approximatelocation,ringfracture, - Approximatelocation,ringfracture,Dry Canyon calderaRound Mountain stuffofBigSmokyValleyStebbinsHillsequenceB!tuff of Round Mountainupper memberi poorlyweldedTrun f¾ densely weldednonweldedlower member tuffofDryCanyon//34-37Madikes Cretaceousgranite Paleozoicrocksß 1 Sampleor drilllocationholeD26KgBasinand Rangefault systemI I50010001500m2O0040 30fiNo verticalexaggerationFIG.3. Crosssectionsacrossthe RoundMountaindeposit;seeFigure2 for lysampleddrillholesareshown.SectionA-A':thetuffof RoundMountain,includingtheuppermember(originaltuff of ,is pondedwithinandrestsuponpreCenozoicrocksin dpre-Cenozoicrocks.Ringfractureof DryCanyoncalderaisprobablyjustnortheastof steepcontactof tuffof Dry theast-strikingBasinandRangefaultsdropthetuffof RoundMountain,StebbinsHill sequence,andtuffof BigSmokyValleydownto the northwest.withinpercent,reflectinga combinationofcompactionandcomposi- zoning,withthe tuff becomingslightlylesssilicicup- theclosedcalderabasinfollowingtheendof majoreruption.ward (Boden, 1986).The considerablethicknessof the StebbinsHill pper demonstratesa significanthiatusbetweenemplacementofmembersof the tuff of RoundMountainrangefrom26.48 the tuff of RoundMountainandthe overlyingtuff of Big 0.08 to 26.54 0.07 Ma with a mean of 26.51 0.03 y.dif(Henryet al., 1996;Table1). The agesof the twomembers ferencein ages(Table1).are indistinguishable;this is consistentwith the field dataThe StebbinsHill thetwomemberseruptedcloselyin time einscutfrom the Round Mountain caldera.the sedimentaryrocks,andmuchof the sequenceis of senceIntracalderasediments:StebbinsHill sequenceandsignificanceofsinterintheStebbinsHill sequence,whichTheStebbinsHill sequenceof volcaniclasticrocksandmi- uffofRound that ain.TheserocksarepreservedonlybeneathlateTer- fled,brecciated,andrecementedwith ositson the 1985)andsideof the BasinandRangefaultswheretheyare asmuch B.R. Berger(pers.commun.,1995)reportclastsof thinlyas91 m thick(Fig.3). However,about8 m of thelowermost laminatedchalcedonicsinterin whattheyinterpretto bepartof theStebbinsHill sequencecappedRoundMountain hydrothermalexplosionbrecciaformedno yto coarsecon- arguethat anysinterin the StebbinsHill lylaminated,planar- havebeenseveralhundredmetersbelowthe palcosurface
812HENRY ET AL.IITil]Highlevelsilicic Potassicß 00% SL Datedsample Ftc. 4. Distributionof hydrothermalalterationoftheuppermemberofthetuffof RoundMountainfromSanderandEinaudi(1990;theirfig.4) superimposedonpartof geologicmapof Figure2. licicalterationtohaveoccurredsequentiallyin bydegreeof replacementof sanidinephenocrystsby whicharemarkedbylinearbandsof antherangeof ysesshowthatalterationprodandthereforenot relatedto the ore-formingsystem.Silica- tic.Petrographicopal,anderistobalite.The sigrichlensesin drillcoreexaminedbyusareprobablysilieified uctsincludemontmorillonite,tuff and sedimentratherthan subaeriallyor subaqueouslynificance of this alteration is uncertain. It could be a latephaseof alterationrelatedto he late,high-levelsilieiealterationof SanderandEiLateor postalterationtuffof BigSmokyValleynaudi(1990).Alternatively,it couldrepresenta tat RoundMountain,therhyolitictuffof eventthathasno rpretation,of the goldsystem.sideof the BasinandRangefaults.The tuff hasa densely thanat the endof alterationweldedbasalvitrophyrethat passesupwardinto denselyof theRoundMountaincalderawelded, devitrifiedrock. It is as much as 30 m thick, but ippeditoffrom muchof the area(Fig. 4). The rockis sparselyand tion of an arcuatering fracturesystemovera distancethedeposit(Figs.2 and3; Henryet al.,finelyporphyritic,with5 to 10 percentphenocrysts,mostly about6 tite,quartz,Fe- 1996). However, the continuationof the calderawestwardTi oxides,and deradoesMeanandisochronagesof sanidinefromthe tuff of Big not extendintothe easternpartof the ToiyabeRange(Fig.rockscrop out thereSmokyValley(sampleD22-837)are26.07 0.05and25.93 1), ).Thislimitsthecalderadiame bleof thegiventhe combineduncertainties,andthebestestimateof ter to about11 kin. The loweranduppermembersthetuffs ageis probably26.0Ma, the averageof the mean tuff of RoundMountainare eitherfiat lyingor dip gentlysouthwestwardwhereexposedor encounteredin drilling.andisochronages.or resurThe sourceof thetuffof BigSmokyValleyanditsrelation Therefore,eitherthe calderawasnot resurgent,either to the Round Mountain caldera or to other calderas genceaffectedonlya smallareain themiddleof thecaldera,in the regionare uncertain(Henryet al., 1996).The tuff which has not been examined.couldbe a lateeruptionfromthe RoundMountaincalderaAlterationassemblagesat RoundMountainor a distaldepositfroma distantcaldera.of thegeometryandhistoryof alterationMostofthetuffof BigSmokyValleyisunaltered.However, Anunderstandingnonwelded tuff at the base of the unit is bleached and limoniat RoundMountainis criticalto evaluatingthe durationof
4øAPPEARGEOCHRONOLOGY,ROUND MTN., NV813T BI. .2. AlterationSequenceandTemperaturesof SanderandEinaudi(1990)LateEarlyAlterationT argillicPotassic- 180- 180250to n)PropyliticMainoredeposition250to 270OvergrowthveinsAdularizedsanidine4øAr/a ArsamplesSHHGV,SL,TypeII 1//3 StringerD8 continuousprocessinterruptedof is onlyby the suddeninfluxof cold groundwaterwhichdebated.In a comprehensivestudyof the alterationof the quenchedthesystemanddepositedgold.A possibleimplicauppermemberof thetuffof RoundMountain,Sander(1988) tion of their modelis ed,whereasthe restof the ateargillicassemblagesis likelyto pretedthesetohave rmedsequentiallyin thatorder.In contrast,Tingleyand quence(Table2). rrentminegeologists,andothers(H. F. m- ticblagestobea spatialprogressionfromapotassiccoreoutward to to a propylitichalo,all of whichformedapproximatelyat grainedadulariaprecipitatedin veinsandin cavitiesin ationof and tz-adulariaveinsformedduringlatesilieifieationin rkasa upperpartsof RoundMountain.frameworkin whichto rationischaracterizedbythemineralassemblage tile Samplesepidote.Accordingto sanidinephenocrystsduringpropylitic 4øAr/a9Ardatingwasdoneonadulariacollectedto encomalteration.Theydefinedzonesof differentintensityonthe passthe proposedalterationsequenceandthroughoutthebasisof degreeof replacement(Fig.4); the mostintense depositto testspatialvariations(Table2; Fig.4; sandjointsthat Theseincludetwo samplesof rmalfluids.Propyliticalterationreplacedsanidine(D8 27! andH95-51A),a fluidwithmaximumovergrowthvein (//3 en250øand270øC(seealsoBerger,1985). alteration(HGV,TypeII, andSL),andonesampleof a late,Althoughtherelationshipof earlypropyliticalterationto ore coxcombvein (SH). All sevensamplesare hostedby themineralizationis unclearin manypreciousmetaldistricts, uppermemberof the tuff of ludedthatpropyliticalter- edationat RoundMountainwasan earlymanifestationof the uponmatchingtheirtexturaltypesanddistributionof assemore-forminghydrothermalsystemandessentialin developingblagesand,fortwosamples(D8 271and//3Stringer),specificbulk mineable from bly litictopotassicalteration,whenhydrothermaltempera- D175A,fromtheuppermember,liesnorthwestof the Basinturesdroppedfromabout250øto 150øCatconstantpH and andRangefaults,outsidethe areaexaminedby ransitionto Einaudi(1990),and ofcoldgroundwaterresultingfromfractur- Sample421-796is fromPaleozoicrocksthatunderliea thiningduringregionalextension.Overgrowthveins,in which wedgeof the lowermember.The inephenocrysts,formedin haveundergonepropyliticalteration,but the s.CrosscuttingrelaOtherwise,potassicalterationis bestdevelopedin poorly ichweldedtuff (Fig.4).ispartof thereasoncompanygeologistsareskepticalof hichare dfromfurthercoolingwithtem- e;peratureslessthan180øC.Theseformedonlyat highlevels coarse,open-space-fillinggrains;and fine replacementofin theRoundMountainsystem(Fig.4). Minoralterationthat groundmass(Fig. 5). Adulariareplacingsanidineformsaaffectedthetuffof BigSmokyValleycouldbe a partof the coarsemosaicthatmimicsthe Einaudi(1990)emphasizethat onlybedistinguishedfromunalteredsanidinein handspeci-
814HENRY ET AL.ationof pumiceandrangesup to about7 mm in diameter(Fig. 5B). Coarseadulariais commonlymilkywhite fromß massorpumice,especiallyin nonweldedash-flowtuffoftheuppermember,andoccursin grainsrangingfromaboutI to 200 m in diameter(Fig.5C). In somesamples(e.g.,SL), fineadulariaoccursasa memberK feldspar(Table3). However,adularizedsanidinetypicallycontains1.5 wt percentNa O( Ors7 Abxa),possiblyretainedfromthe sanidinephenocrysts,whichoriginallycontainedabout4 wt percentNa O(Henryet al., 1996).B10 n dinephenocrystswerecrushedandsievedto 20 to 40 mesh,thenrun rehandpickedandleachedwithdiluteHF to ndpickedfromcavitiesorselvages.The grainswereleachedwithdiluteHF andseparatelywithnitricacidto removecommonironoxyhydroxidecoatings.The resultingadulariasamplesweresievedto lessthan10meshandconsistmostlyof individualcrystals.Fineadulariain samplesSL andHGV wasconcentratedbycrushingtherockandsievingto 80to 100mesh.Magneticmaterial,mostlygroundmassand itha magneticseparator.AfterleachingwithdiluteHF, sampleswereseparatedintofiveto sixdensityfractionsbyprogressivesettlingin dilutedbromoform. Grain mounts of these fractions were examinedwith a petrographicmicroscopeßThe edbymagneticanddensity separation.PetrographicandSEM examinationsindicatethatall adularia containsfluid and mineralinclusions,mostof which arealsoproductsof nquartz,albite,andprobablesmectiteandchlorite (Fig.5A). Adularizedsanidinein tainssparsequartzinclusions,andfineadulariais intergrownwith quartz;however,quartzconstitutesnomorethanabouti percentof the tedin twobatchesat the Universityof Michigan(10h) or TexasA&M University(6 h) andanaFIG.5. A. c mottledextinction(sampleH95-51A).Noteinclusionsof calcite,albite,FishCanyonsanidine(27.84Ma, relativeto an ageandsmectite.Datedseparateconsistsof formerindividualphenocrysts.B. sources.Coarseadulariarhombsupto 5 mmacross(coresample421-796,a veinin of 520.4Ma s)ßDate
Round Mountain Gold Corporation, Round Mountain, Nevada 89045 WILLIAM C. MCINTOSH, MATTHEW T. HEIZLER, New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico 87801 AND STEPHEN B. CASTOR Nevada Bureau of Mines and Geology, University of Nevada, Reno, Reno, Nevada 89557 . ore forming less than a few tens of meters below the .
