Drift Prospecting for Porphyry Copper‐Gold, Volcanogenic … · 2019-04-22 · Geoscience BC...

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GeoscienceBCReport2013‐151

DriftProspectingforPorphyryCopper‐Gold,VolcanogenicMassiveSulphideMineralizationandPreciousandBaseMetalVeinswithinthe

QUESTProjectArea,CentralBritishColumbia(NTS093J)

BrentC.Ward,MatthewI.Leybourne,DavidA.Sacco,RaymondE.Lett,LambertusC.Struik

INTRODUCTION.............................................................................................................................................3

Locationandphysiography.......................................................................................................................5

Bedrockgeology............................................................................................................................................5MineralOccurrences...............................................................................................................................................8RegionalQuaternaryGeology............................................................................................................................10

Methods.........................................................................................................................................................12Geochemistry...........................................................................................................................................................12HeavyMinerals........................................................................................................................................................13

QUALITYCONTROL...................................................................................................................................13

Results...........................................................................................................................................................14Au,As,AgandHginTill........................................................................................................................................17Cu,MoandSbinTill..............................................................................................................................................39Pb,Bi,ZnandCdinTill.........................................................................................................................................50RareEarthElements,U,Th,K,Ca,Mg,Na,Cr,Hf,Co,MnandNiinTill...............................................51HeavyMineralConcentrates:visiblegold,pyriteandcinnabar...........................................................51

TillGeochemicalExploration................................................................................................................52Preciousandbasemetalveins..........................................................................................................................52PorphyryCu‐Au.......................................................................................................................................................52VolcanogenicMassiveSulphideDeposits......................................................................................................53Mercury......................................................................................................................................................................54

HEAVYMINERALCONCENTRATEGEOCHEMISTRY........................................................................54

Conclusions..................................................................................................................................................54

Acknowledgments.....................................................................................................................................55

REFERENCES................................................................................................................................................56

DriftProspectingforPorphyryCopper‐Gold,VolcanogenicMassiveSulphideMineralizationandPreciousandBaseMetalVeinswithintheQUESTProjectArea,

CentralBritishColumbia(NTS093J)

BrentC.Ward1,MatthewI.Leybourne2,DavidA.Sacco1,RaymondE.Lett3,LambertusC.Struik4

1DepartmentofEarthSciences,SimonFraserUniversity,Burnaby,BCV5A1S6;bcward@sfu.ca2ALSGeochemistry,2103DollartonHwy,NorthVancouver,BCV7H0A73BCGeologicalSurveyEmeritus3956AshfordRd,Victoria,BCV8P3S5

4GeologicalSurveyofCanada,NaturalResourcesCanada,1500‐605RobsonStreet,Vancouver,BCV6B5J3

INTRODUCTION ThisreportsummarizestheresultsofregionaltillgeochemicalandmineralogicalsurveysandQuaternarystudiesconductedintheheartoftheQUESTProjectarea(GeoscienceBC’sprogramtoattractnewexplorationinanunder‐exploredportionofcentralBritishColumbia,initiatedin2007;http://www.geosciencebc.com/s/Quest.asp)(Figure1).TheQUESTProjectareahasgoodpotentialforCu‐Auporphyryandvolcanogenicmassivesulphide(VMS)mineralization,butmineralexplorationactivityhasbeenhinderedinsomeareasduetothethickcoverofsurficialdeposits.Regional‐scaletillsamplingfollowedbydetailedsurveysaroundsampleswithelevatedoranomalousvalueswascarriedouttoassessmineralpotentialofthisarea.Tillisthepreferredsamplingmediumforgeochemicalandmineralogicalsurveysinglaciatedterrainbecauseitiscommonlyconsideredafirstderivativeofbedrock(Dreimanis,1989;Levson,2001).Thismethodologyhasbeenshowntoaidinidentifyingpotentiallymineralizedbedrockunitsinareascoveredwiththickglacialdeposits(Levson,2001;McClenaghanetal.,2002;McClenaghan,2005).TheQuaternarygeology,specificallythedistributionofsurficialdepositsandtheice‐flowhistory,includingthedominanttransportdirection,wereusedtoplanthesurveysanddeterminethetransporthistoryoftillgeochemicalandmineralogicaldata.Theprimarygoalofthisstudyistouseregional‐scalemajor‐,minor‐andtrace‐elementtillgeochemicaldata,goldgraincountsandheavymineralseparationandidentificationtoidentifymineralizedbedrock.Thegeochemicalandmineralogicaldatapresentedherehighlightnewexplorationtargetsand,incombinationwiththearea’sglacialhistory,provideasurficialgeologicalcontextforcompaniestointerprettheirowngeochemicalandbedrockgeologydatasets.

LOCATIONANDPHYSIOGRAPHYThestudyarea,locatedinwest‐centralBritishColumbia,comprisessix,1:50000‐scalemapsheets(NTS093J/05,06,11,12,13,and14;Figure1).Highway97,whichlinksPrinceGeorgetoFortSt.John,runsalongsidetheeastsideofthestudyareaandHighway16,whichlinksPrinceGeorgetoFortSt.James,runstothesouth.AmoderatelydensenetworkofForestServiceroadsprovidesaccesstomostpartsofthestudyareaforfieldworkbasedoutofPrinceGeorge,FortSt.James,McLeodLakeandMackenzie.ThemajorityofthestudyarealiesintherelativelylowreliefoftheInteriorPlateau(Holland,1976;Mathews,1986),includingitssubdivisions,theNechakoPlainandtheFraserBasin.Theseareasarecharacterizedbydrumlinizedtill,glaciofluvialoutwashandeskerdepositswithglaciolacustrinedepositsoccurringinthesouthernregions.ThenorthwesternpartofthestudyareaoccurswithintheNechakoPlateauandischaracterizedbytillmantlesandexposedbedrock.

BEDROCK GEOLOGY ThestudyareastraddlesfouroftheterranesthatmakeuptheCanadianCordillera(CacheCreek,SlideMountain,Quesnel,Kootenay)whilethemostnortheasterncornerofitextendsintotheRockyMountainAssemblage(Figure2).AcomplexassemblageofintrusiveandextrusiverocksoftheSlideMountainterraneoccursintheeast.TheCacheCreekterraneisrepresentedbyPennsylvanianandPermianlimestoneinthesouthwesternportionofthestudyarea,withbasaltsoccurringjusttothesouth.TheRockyMountainassemblageinthenortheasterncornerofthestudyareacomprisesSiluriantoDevoniansandstoneandquartzite.TheQuesnelterranedominatesthestudyareaandiscomposedprimarilyofLateTriassictoEarlyJurassicarcvolcanicrocksoftheWitchLakesuccessionandvolcaniclasticrocksoftheCottonwoodRiversuccession,bothpartoftheNicolaGroup(Loganetal.,2010).TheNicolaGroupwaspreviouslyreferredtoastheTaklaGroup(Struik,1994),followingfirstusage.ItwascorrelatedwithTaklaGrouprockstothewestwithintheStikineterrane.TheNicolaGroupcomprises:a)mainlybasaltictodaciticvolcaniclasticrocksandsubordinatecoherentvolcanicrocks,eachwithaugite‐porphyrytextures(particularlycharacteristicoftheQuesnelterrane),whichformaneasternfaciesofalkalinetosubalkalineaugite‐phyricbasalticandesite;b)coevalandpartlycomagmaticplutonsrangingfromcalcalkaline(inthewest)toalkaline(intheeast);andc)sedimentaryrocks,includingshale,limestoneandepiclasticdeposits.StratigraphicallyoverlyingtheseterranesareaseriesofoverlapassemblagesrangingfromUpperCretaceoustoMiocenesedimentaryrocksandCretaceoustoPliocenevolcanicrocks.ThelatterincludesthedominantlyMioceneChilcotinbasaltsandEocenefelsicvolcanicrocks.Intrusiverocks,paragneissandmetasedimentaryrocksoftheWolverinemetamorphiccomplexwereexposedduringEoceneextension.ThemetamorphismandplutonismoccurredinthelateCretaceousandPaleogene,andtheprotolithfortheparagneissand

metasedimentaryrocksarelikelyPrecambrianandEarlyPaleozoic(Struik,1994).RecentcompilationhasassignedtheserockstotheKootenayterrane(Loganetal.,2010).

Mineral Occurrences

WithinthestudyareaaretwelveCushowings,sixAushowings,oneplatinumgroupelements(PGE)showing,twoHgshowingsandtwopast‐producingAuandPtdeposits(MINFILE;BCGeologicalSurvey,2010;Figure2).ThetwopastproducersaretheMcDougallRiverandMcLeodRiverplacerdeposits(MINFILE093J007and012;BCGeologicalSurvey,2010).Bothdepositsoccurinthenortheasternpartofthestudyarea,underlainprimarilybytheMississippianSlideMountainGroup.CaribooNorthernDevelopmentCo.Ltd.andNorthernReefGoldMinesLtd.workedtheMcDougallRiverplacergolddepositfromaround1931to1935,withtotalreportedproductionofapproximately1750gAu(62oz).From1981topresent,theareahasreceivedrenewedinterest,includingheavymineral,soil,siltandrocksampling;geologicalmapping;airborneverylowfrequency(VLF)andmagnetometersurveys;andgroundVLFandmagnetometersurveysbyavarietyofcompanies.AtMcDougallRiver,AuandPtwereextractedfromshallowgraveldepositsonbothbanksoftheriver,withadditionalclastsretrievedfromcracksandcrevicesinthebedrock.LocallyshearedrocksandquartzveinsmaybethesourceoftheplacerAuandPGE.HeavymineralsampleshaveyieldedhighAuandAgcontents,andmanyoftheplacerGoldgrainsrecoveredareangulartowiry,consistentwithminimaltransportfromalocalbedrocksource.Thecoincidentelectromagnetic(EM)andmagneticanomaliescouldrepresentthelocalsourceforAu.ThetwoHgshowings(MountPrinceSoutheastandNorthwest,MINFILE093J010,093J011)inthesouthwesternpartofthestudyareaareassociatedwiththePinchifault.Bothshowingsarecharacterizedbysmallvolumesofcinnabarhostedbycarbonate‐alteredandshearedNicolaGroupmaficvolcanicrocks,commonlyassociatedwithquartzstringers.Mostoftheothermineralshowingsinthestudyareaaresmallwithminimalassociatedexplorationactivity.MountMilligan(MINFILE093N194)isaCu‐AuporphyrydevelopedprospecttothenorthwestofthestudyareainQuesnelterrane.Inthisarea,TriassictoLowerJurassicvolcanicandsubordinatesedimentaryrocksofNicolaGroupareinterpretedtobetheextrusivephaseoftheHogemintrusivesuite.ManyCu‐AumineralshowingsareassociatedwiththeHogembatholithandsmallercoevalintrusions(LeFortetal.,2011).TheNicolaGroupintheMountMilliganareaisinformallysubdividedintoalower,predominantlysedimentaryInzanaLakesuccession,andanupper,predominantlyvolcaniclastic,WitchLakesuccession.TheWitchLakesuccessionhoststheMountMilligandepositandischaracterizedbyaugite‐phyricvolcaniclasticandcoherentbasalticandesite,withsubordinateepiclasticbeds.RegionalmappingandpetrographicstudiesintheMountMilliganareaindicatethattheWitchLakebasalticandesiteandassociatedsedimentaryrockshavebeensubjectedtostrongpotassicalterationdetectableforupto4kmfromthedeposit.WitchLakesuccessionvolcanicrockswereintrudedbysyn‐andpost‐depositionalgabbro,diorite,granodiorite,monzoniteandsyenite(Loganetal.,2010).RecentworkhasshownthatmineralizationatMountMilligan

isdominatedbyanearlyCu‐richporphyrystage,withlatermineralizationcharacterizedbyenrichmentsinAu,PGE,As,Sb,Bi,TeandB(LeFortetal.,2011).

Regional Quaternary Geology

TheCordilleranIceSheethasrepeatedlycoveredBritishColumbiaandportionsofYukon,AlaskaandWashingtonoverthelasttwomillionyears(Armstrongetal.,1965;Clague,1989).Atitsmaximumextent,theCordilleranIceSheetwasupto900kmwideandupto2000–3000mthickovertheInteriorPlateau,closelyresemblingthepresent‐dayGreenlandIceSheet(Clague,1989).AmorecomprehensivehistoryoftheCordilleranIceSheetcanbefoundinJacksonandClague(1991)andClagueandWard(2011).ThemajorsourcesofregionalicethatcoveredcentralBritishColumbiaadvancedfromaccumulationcentresintheCoast,Skeena,OmenicaandCariboomountains(Tipper,1971a,b;LevsonandGiles,1997;Plouffe,1997,2000,Wardetal,2009).Thestudyareaoccursneartheconvergenceofthesethreeadvancingicefronts,makingitdifficulttodeterminewhichicecentre(s)hadthemostinfluenceoniceflowdirectionanddispersalintheearlypartsoftheLateWisconsinan.Previouslyreportedice‐flowindicators(Tipper,1971a;PaulenandBobrowsky,2003),incombinationwithdatafromthisstudy,suggestthatitwasmainlyicefromtheCoastMountainstothewestandtheCoastandCariboomountainstothesouththataffectedthearea.Absolutechronologicalinformationonthemovementand/orconfluenceoficefrontsthroughthestudyareaislimited.AlthoughitisknownthaticewasadvancingoutoftheCoastMountainsby28.8ka(GSC‐95,Clague,1989),itisnotclearwhenthisadvancereachedthecentralInteriorPlateau.Ice,possiblysourcedfromtheCaribooMountains(PaulenandBobrowsky,2003),coveredtheBowronValleysometimeafter19.9ka(AA44045,Wardetal.,2008).AccordingtoBobrowskyandRutter(1992),iceadvancingfromtheOminecaMountainsintowhatisnowthenortharmofWillistonLakeoccurredsometimeafter15180±100BP(TO‐708).Thedominanticeflowinthestudyareaisrelativelyeasytodemonstrateusingtheorientationsofthenumerousmacro‐forms.Themacro‐formswereinitiallycompiledfromexistingmaps(Tipper1971)andtheresolutionwasincreasedwithobservationsmadeduringmappingforthisproject(Figure3).Thedominanticeflowindicatorsgenerallyconsistofdrumlins,flutings,cragandtails,andstreamlinedbedrock(Figure3).Theinteractionofthetwosourcesresultedinageneralnortheasterntransportdirectionwithminorvariationacrossthe6sheets;thelargestreamlinedformsareapproximatelyENEinthesouthwestportionofthestudyareaandcurvenorthward,beingNNEinthenorthofthestudyarea.Small‐scaleiceflowindicatorsweremeasuredinthefieldsuchasgrooves,striationsandrat‐tails.Thesemicro‐flowindicatorsweremeasuredatatotalof22sites(Figure3).Atsomeofthesesites,multipleiceflowdirectionswererecorded,atothersonlyonedominantdirectionwasobserved.Findingmicro‐flowindicators(e.g.,striations,rat‐tails)waschallengingduetothelackofbedrockexposuresinpartsofthefieldarea,andtheweatherednatureofsomeoftheoutcropspresent.Inmostcases,exceptforsomefreshroadcuts,micro‐flowindicatorswereonlyfoundaftersediment,usuallytill,wasscraped,brushedorwashedoffbedrocksurfaces.

Theorientationofelongateclastsintillweremeasuredat12sites.Thesetillfabricscanbeusedtointerpretthegenesisofatillunitandthedirectionoficeflowthatdepositedit.FabricsandstriationswereusedtoadddetailandarelativechronologytotheiceflowhistorythatisdescribedinSaccoetal.(2012).

METHODS

Geochemistry

Basaltillsampleswerecollectedatapproximately760siteswithinthestudyarea,duringthesummersof2008,2009,and2010.Thereareslightlydifferentnumbersofsamplesforthedifferentsizefractionsanalyzedowingtoasmallnumberoflostormislabeledsamples(seeAppendices).Basaltillinthestudyareaistypicallyadense,darkgrey,sandytoclayeysiltmatrixsupporteddiamictoncontaining25‐40%gravelsizedmaterial(clasts).Theaveragesampledensityisapproximately1sampleper8km2,buttherearesomezoneswithnosamplesandsomezoneswithhigherdensity.Insomeareassamplingwasnotpossiblebecauseofaccessproblems,roaddeactivationandlackofroads,orlackofsuitablesamplemedia,suchasareasofeolian,glaciofluvialandglaciolacustrinedeposits.Inaddition,nosamplingoccurredinCarpLakeProvincialPark.Ateachsamplesitethreeseparate~900gsampleswerecollectedfor:1)analysisoftheclay‐sizefraction(<0.002mm)byinductivelycoupledplasmamassspectrometry(ICP‐MS),followinganaquaregiadigestion,atAcmeAnalyticalLaboratoriesLtd.(Vancouver,BC);2)analysisofthesilt‐plusclay‐sizedfraction(<0.063mm)byinstrumentalneutronactivationanalysis(INAA)atActivationLaboratoriesInc.(Ancaster,ON);and3)forarchiveattheGeologicalSurveyofCanada.Thesearchivesampleswillbeavailableforfutureanalysisforeitherimproveddetectionlimitsordifferentelements.Clay‐sizedseparationswereproducedbycentrifugeatAcmeAnalyticalLaboratoriesLtd.(Vancouver,BC).Typically,between0.5–0.8kgoftillwereprocessed,whichonaverageyieldedapproximately5gofclay.TheclaysplitswereanalyzedbyICP‐MSfor36elements(analyticalpackage1DX)followingleachinginahot(95°C)aqua‐regiadigestion(detectionlimitsarelistedintheappendixeswiththedata).Upto5gofclayisanalyzedtoovercomepotentialnuggeteffectsforAu.Thesiltplusclay‐sizedfraction(<0.063mm)oftillsamples(onaverage,24gofmaterialwasused)wereanalyzedfor35elementsbyINAA(analyticalpackage1D,enhancedoption)(detectionlimitsarelistedintheappendixeswiththedata).Hoffman(1992)describestheanalyticalprocedureasfollows.Analiquotandaninternalstandard(oneforeveryelevensamples)areirradiatedwithfluxwiresatathermalneutronfluxof7x1012·n·cm‐2·s‐1.Afteraseven‐daydecay,thesamplesarecountedonahighpurityGedetector.Usingthefluxwires,thedecay‐correctedactivitiesarecomparedtoastandardcalibrationcurve.Thestandardincludedisonlyacheckonaccuracyandisnotusedforcalibrationpurposes.From10–30%ofthesamplesarerecheckedbyre‐measurement.

Heavy Minerals

Bulktillsamples(>10kg)werecollectedatevery4–5sitessampledforgeochemicalanalysis.Intotal,152sampleswerecollected.Heavymineralconcentrates(HMCs)wereseparatedatOverburdenDrillingManagementLimited(Nepean,ON)andwerepannedforgoldgrains,platinumgroupmetals(PGM)anduraninite.Bulksamplesweredisaggregated,followedbyseparationofthe>2mmand<2mmfractions.The<2mmfractionwasthenpreconcentratedonashakingtable,withthefinest,heaviestfractionbeingpanned.Gold,uraninite,andplatinumgroupelements(PGEs)werethenexaminedunderopticalmicroscopetoprovidegraincountsaswellasgrainmorphology.MoredetaileddescriptionsofthemethodsareprovidedinAverill(2001).Sulfideandcinnabargraincountswerealsomade,althoughwheren>20,thesecountsareestimates.Thetableconcentratewasthensievedandthe<0.25mmfractionsubsequentlyseparatedusingheavyliquidat3.2g/cm3.This<0.25mmfractionwasthenanalyzedbyINAAatBecquerelLaboratories(Mississauga,ON)usingtheirBQ‐NAA‐1package(withtheadditionofHgfor2009and2010samples).Theconcentrateisplacedinvials,whicharestackedintoone‐foot(30cm)longbundlesforirradiationattheMcMasterNuclearReactor,whichhasfluxof8x1012·n·cm‐2·s‐1.Afteratypicaldecayperiodofsixdays,theirradiatedsamplesareloadedontoahighresolution,coaxialgermaniumdetectorthatconstructsaspectrumofgamma‐rayenergiesversusintensities.Thecountingtimeistwentytothirtyminutespersample.Quantitativeelementalcontentsarederivedbycomparisonofpeakpositionsandareawithlibrarystandards.Forthe2009and2010samples,severalelements,suchasHg,Ni,Zr,Rb,Au,hadvariableandhigherthanusualdetectionlimitsbecauseofelevatedCr,REEandThcontents(SteveSimpson,pers.comm.,2011).Forexample,Auusuallyhasadetectionlimitof2ppbbuthereitrangesfrom5to42ppbdependingonthesample.However,thesamplestakenin2008didnothavethisissue.

QUALITY CONTROL Discriminatinggeochemicaltrendscausedbygeologicalfeaturesfromvariationduetospurioussamplingoranalyticalerroriscriticalforassessingthereliabilityofregionalgeochemicalsurveydata.Acombinationoffieldduplicates,analyticalduplicatesandreferencestandardsareusedtoestimatetheaccuracyandprecisionoftheseanalyticaldata.Every20samplegroupsubmittedforcommercialanalysiscontainafieldduplicate,ananalyticalduplicate(splitandinsertedintothesamplesequenceatthelabafterpreparation),andareferencestandard(eitheranin‐houseBCGeologicalSurveystandardoracertifiedCanadaCentreforMineralandEnergyTechnology[CANMET]standard).Nofieldoranalyticalduplicatesweredonefortheheavymineralsamples,owingtothelargesamplesizesrequired.Scatter‐plotsoftheanalyticalandduplicatepairsweregeneratedandaselectionisshowninFigure4and5,withtherestoftheanalysisinAppendix1.Precisionfortheaquaregia(clayfraction)ICP‐MSanalysesistypicallymuchbetterthanfortheINAA(silt+clay)analyses.For

theICP‐MSanalyses,correlationcoefficientsforthefieldduplicatesaretypicallyhigherthanthosefortheanalyticalduplicatesandaregenerallyabove+0.9.Morethan80percentofthetotalvariationamongtheduplicatesisaccountedforbytheFieldDuplicate1–FieldDuplicate2correlation.CorrelationcoefficientsfortheINAAduplicatesaregenerallymuchlower,withbetterprecisionforCo,Cr,ThandtheREEthantheotheranalytes.ThehigherprecisionfortheICP‐MSdataisinpartrelatedtolowerdetectionlimitsandlessvariationingrainsize,butalsolikelyreflectsbetterinstrumentcapabilities.FourCANMETtillstandards(TILL1,2,3and4),BCGeologicalSurveysediment(RD29)andtillstandards(SM,Till99),andtwoquartzblanksampleswereanalysedwiththesurveysamplestomeasureanalyticalaccuracyandprecision.AllofthestandardsdataiscompiledinAppendix1.TheINAAquartzblanksampleresultsrevealamountsofAs,Br,Co,Cr,La,Ce,ThandScinthe1to10ppmrangeandlessthan0.2%Fe.Traceamounts(<than1ppm)ofCu,Pb,Co,Mn,SrandBaintheblanksaredetectablebytheaquaregiadigestion‐ICP‐ESanalysis.Elementconcentrationsdetectedintheblanksamplesmayreflecttracesinthequartzandnotnecessarilysamplecontaminationduringtheanalysis.Precisionisbelow+/‐8%RSDforAs,Cd,Co,Cr,Cu,Hg,Mn,Mo,Ni,Pb,Sb,VandZninstandardsanalysedbyaquaregiaICP‐MSandbelow+/‐8%RSDforAs,Br,Co,Cr,Fe,La,Ce,ThandScinGSBTill99.Accuracycanbeaccessedfromthenear‐totalINAAanalysesandmeasuredamountsformostelementsarewithin5%oftherecommendedvalue.PrecisionandaccuracyforelementsatdifferentconcentrationisbestshownbytheCANMETstandardsdata(Appendix1).TheINAAanalysesofCANMET.Goldshowsconsiderablevariationalthoughthemeanandrecommendedvaluesareverysimilar.ThelargemeasuredBadifferencescomparedtotherecommendedvaluearedifficulttoexplain.Table2listsprecisionandCANMETrecommendedvalues(hotaquaregiaanalyses)forCANMETTill1,2,3and4.Goldprecision(%RSD)forthreeofthestandardsisbelow+/‐30%andmostotherelementshaveaprecisionbelow+/‐8%RSD.Lowerprecision(i.e.above8%)commonlyreflectsalowelementconcentrationsmeasuredinthestandard.

RESULTS Elementswiththemostsignificancetopotentialeconomicmineralizationintheprojectareaarediscussedbelow.Notethatweanalyzeddifferentsizefractionsbydifferentanalyticalmethods.Insomecases,commodityelementsarenotanalyzedbybothmethods(e.g.,CuwasnotdeterminedbyINAA).Furthermore,dependingonwhereanelementissequesteredinasample,comparisonsbetweensizefractionsmayhavegreaterorlesserutility,asdiscussedbelow.Notealsothataquaregiadigestionisnottotal,silicateandresistantoxidesarenotdigestedtoanygreatextent.Bycontrast,INAAisatotalmethod.Thus,thedifferencesbetweenresultsfortheclay‐sizeversusthesiltplusclay‐sizefractionsarenotonlyafunctionofanalyticalmethod(partialversustotal),butalsoafunctionofwhereananalyteresideswithina

sample.Forexample,Asshowsgoodcorrelationsforbothfieldandlaboratoryduplicatesforbothsizefractions,becauseAsisprimarilyassociatedwithadsorptiontoclay‐sizemineralsurfaces(Figure4).Bycontrast,Auismorepronetothenuggeteffectinthesiltplusclay‐sizefractionthanAuintheclayfraction(Figure5).Tillgeochemicaldata(ICP‐MSandINAAanalysis)isinappendix2.Heavymineraldata(Graincounts,INAAanalysisontheheavymineralfraction)isinAppendix3.Averagevaluesarepresentedbelowalongwithonestandarddeviationandsamplecount.Samplecountsarevariableasresultsbelowdetectionarenotincludedinthestatisticalcalculations.Thresholdvalues(percentile)aresetbasedoninflexionpointsonthecumulativefrequencyplots(Figure6);wehavenotdefinedasetbackgroundvalueowingtothevariabilityinthegeochemicallandscape.Wherecorrelationsarestated(rvalues),theserepresentPearsonProductMomentcorrelations,statisticallysignificantatleastatthe95%confidenceinterval.Insomecases,wealsoemployedR‐modefactoranalysis,astatisticalmethodtoinvestigateinter‐relationshipsbetweenanalytesinacorrelationmatrix.

Au, As, Ag and Hg in Till

Goldcontentsintheclayfractionrangefromlessthandetection(0.5ppb)to294ppb(average=5.1±11ppb,n=704).Goldcontentsintheclayfractionshowhighlyanomalousvaluesaroundthe98thpercentile(10ppb),althoughthereisalsoasubtlechangeinslopearoundthe90thpercentile,or8ppb(Figure6a).Inthesiltplusclay‐sizefraction,Aucontentsrangefrombelowdetection(2ppb)upto635ppb.Forthesiltplusclay‐sizefraction,anomalousAucontentsoccurabovethe80thpercentile(~8ppb;Figure6a);mostsamplesbelowthisthresholdwerebelowthedetectionlimitbythismethod(2ppb).AnomalousAucontentsoccurinthenortheasternandnorthwesternpartsofthemapareaforbothsizefractions(Figures7a,b),largelycoincidentwithknownAushowings.TherearealsoanomalousAucontents,inparticularinthesiltplusclay‐sizefraction,tothesouth,andtoalesserextent,totheeastofCarpLake.TherearenoknownAushowingshere.FortheclayfractionAushowsthebestcorrelationwithCu(r=0.410).ThehighestAucontentbybothmethodsoccursinthesamesample(Figures7aand7b)indicatingthatthisanomalousvalueisnottheresultofthenuggeteffect,consistentwiththisalsobeingthesamplewiththehighestAscontentsbybothmethods(Figures7cand7d).Alltillsamplesprocessedforheavyminerals(n=152)containvisiblegold(Figure8a).Notsurprisingly,thedistributionofAubyINAAontheheavymineralconcentratesmimicsthegoldgraindistribution(Figure8b)alongwiththesilt+clayfractionanalyzedbyINAA(Figure7b).FortheINAAanalysesoftheHMCs,goldrangesfrombelowdetection(5to42ppbdependingonthesample)to2,630ppb.Goldcontentsshowclearlyanomalousvaluesaroundthe95thpercentile(~750ppb),althoughthereisalsoasubtlechangeinslopearoundthe80thpercentile(~400ppb).

ArsenicistypicallyconsideredapathfinderelementforAu.Inthisstudy,thresholdAscontentsintillare32and26ppm,atthe95thand98thpercentilesfortheclayandsiltplusclay‐sizefractions,respectively(Figure6a).Arseniccontentsareanomalousinboththenortheasternandnorthwesternsectionsofthestudyarea(Figures7c,d),largelycoincidentwithAuanomalies.However,AscontentsdonotappeartobeanomaloussouthofCarpLake;incontrast,thereareanomalousAscontentsinthewest‐centralpartofthestudyarea,primarilyinthesiltplusclay‐sizefraction.AuandAsshowamoderatepositivecorrelation(r=0.372)fortheclayfractionbasedontheR‐modefactoranalysis,statisticallysignificantatthe99.9%confidenceinterval.Bycontrast,thestatisticalcorrelationbetweenAsandAuforthesiltplusclay‐sizefractionispoor(r=0.112),despitetheevidentspatialassociation(Figures7b,d).Figure7(Next18pages):Proportionaldotmapsofselectedelementsfromtillgeochemicalanalyses,centralBritishColumbia:a)Aucontents(clay‐sizedfraction)byinductivelycoupledplasma–massspectrometry(ICP‐MS);b)Aucontents(clayplussilt–sizedfraction)byinstrumentalneutronactivationanalysis(INAA);c)Ascontents(clay‐sizedfraction)byICP‐MS;d)Ascontents(clayplussilt–sizedfraction)byINAA;e)Agcontents(clay‐sizedfraction)byICP‐MS;f)Hgcontents(clay‐sizedfraction)byICP‐MS;g)Cucontents(clay‐sizedfraction)byICP‐MS;h)Mocontents(clay‐sizedfraction)byICP‐MS;i)Sbcontents(clay‐sizedfraction)byICP‐MS;j)Pbcontents(clay‐sizedfraction)byICP‐MS;k)Bicontents(clay‐sizedfraction)byICP‐MS;l)Zncontents(clay‐sizedfraction)byICP‐MS;m)Cdcontents(clay‐sizedfraction)byICP‐MS;n)Lacontents(clayplussilt–sizedfraction)byINAA;o)Lacontents(clay‐sizedfraction)byICP‐MS;p)Crcontents(clay‐sizedfraction)byICP‐MS;q)Sbcontents(silt+claysizedfraction)byINAA;r)Thcontents(silt+claysizedfraction)byINAA.SizeofdotsareproportionaltothecontentwithAu‐ICPandAu‐INAAbeingrepresentedbylogplots.DataareoverlaidonthebedrockgeologymappresentedinFigure2.

Arseniccontentsintheheavymineralfractionrangefrom9to414ppmandshowclearlyanomalousvaluesaroundthe95thpercentile(60ppb)(Figure6a).ThespatialdistributionofAscontentsmimicthoseofgoldandmostanomalousvaluesareinthenortheastpartofthestudyareawithslightlylowervaluesinthenorthwest(Figure8c).Silvercontentsfortheclayfractionrangefromlessthandetection(0.1ppm)to1.1ppm(average=0.21±0.14ppm,n=520),withanomalousvalues>0.5ppm(~95thpercentile;Figure6a).Silvershowsamoderatelypositive,statisticallysignificantcorrelation(r=0.313)withAu,withanomalousvaluesinthenortheasternandnorthwesternpartsofthestudyarea(Figure7e),beingcoincidentwiththeAuanomalies.Silverforthesiltplusclay‐sizefractionandtheheavymineralfractionwasbelowdetectionlimitforallsamples.Mercurywasonlydetectedintheclayfraction,althoughheavymineralconcentratedata(seebelow)indicatesmanysampleshavesignificantquantitiesofcinnabargrains.Intheclayfraction,Hgrangesfrom0.02to1.0ppm(average=0.29±0.13ppm).Inthecumulativefrequencyplottherearenomajorbreaksinslope,consistentwithaclosetonormaldistribution(Figure6a).Settingthethresholdatthe95thpercentile(0.51ppm),anomalousHgcontentsoccurinthewest‐centralportionofthestudyarea(Figure7f),northofthetwoknownHgshowings.SeveralanomalousHgcontentsoccurinthenortheasternandnorthwesternpartsofthestudyarea,coincidentwithAu,AsandAganomalies.Mercuryvaluesdonot,however,statisticallycorrelatewellwithAuvalues(r=0.083),butdostatisticallycorrelatemoderatelywithAsvalues(r=0.360).Cinnabarcountsrangefrom0to400grains.Anomalouscinnabargraincounts(>60grains)occurinthewesternpartofthestudyarea,andmarkthestartofatrendofdecreasingvaluestothesoutheast(Fig.8e).Becauseofthelargenumberofcinnabargrainsidentified,the2009and2010sampleswerealsoanalyzedforHgbyINAA;unfortunately,becauseofdetectionlimitissuesduetointerferencewithotherelements,only14sampleshavevaluesabovedetection.Thesesamplesareonthewestandeastsideofthestudyareaanddomimichighsinthecinnabargraincounts(Fig.8f).

Cu, Mo and Sb in Till

Copperwasanalyzedonlyfortheclayfractionsamplesandrangesfrom33to408ppm(average=125±38ppm).Basedonthecumulativefrequencyplot,Cucontentsintheclayfractionareanomalousatthe90thpercentile(165ppm;Figure6a).AnomalousCucontentsoccurinthenorthwesterncornerofthestudyarea,withsmalleranomaliesinthenortheasternFigure8(next10pages):Proportionaldotmapsofselectedelementsandmineralsfor<0.25mmfractionoftillheavymineralseparatesforINAAgeochemicalanalysesalongwithselectedgraincounts.a)Goldgraincounts,b)Au,c)As,d)Sb,e)cinnabar,f)Hg,g)Ce,h)Th,i)Cr,andj)pyrite.SizeofdotsareproportionaltothecontentwithAu,Cinnabar,andpyritegrains,andAu,As,andThbeingrepresentedbylogplots.DataareoverlaidonthebedrockgeologymappresentedinFigure2.

corner(Figure7g).ThereisapositiveconcentrationcorrelationbetweenCuandanumberofotheranalytessuchasFe(r=0.712),Sc(r=0.654),V(r=0.656),As(r=0.538),Au(r=0.410),Co(r=0.341)andMo(r=0.313).TheseelementassociationsindicatethatCuintheclayfractionofthetillinthenorthwesternandnortheasternpartsofthestudyareaisassociatedwithCu‐Aumineralization.Molybdenumwasanalyzedforbothsizefractions.AllclaysamplesreturnedMocontentsabovethedetectionlimit,rangingfrom0.3to12ppm(average=1.74±1.12ppm).Bycontrast,thesiltplusclay‐sizefractionhadonly137samplesabovedetectionlimit(1ppm),rangingfrom3to28ppm.Fortheclayfraction,theanomalousthresholdisaroundthe97thpercentile(3.5ppm;Figure6a),whereasforthesiltplusclay‐sizefraction,allsampleswithdetectableMocanbeconsideredanomalousatthe85thpercentile(≥3ppm;Figure6a).Thetwosizefractionsshowdifferentspatialrelationships.Fortheclayfraction,anomalousMocontentsoccurmainlyinthenortheasternsectionofthestudyarea;Mocontentsarenotanomalousinthenorthwesternsection(Figure7h).ThehighestMocontentisforasampleinthewest‐centralpartofthestudyarea.Bycontrast,thesiltplusclay‐sizefractionhasanomalousMocontentsscatteredovermuchofthestudyarea,withthemostconsistentlyelevatedcontentsintheeast‐centralandsouthernareas.Notably,Mocontentsinthesiltplusclay‐sizefractionarebelowdetectionforthenorthwesternareawherehighCu,AuandAsvaluesoccur.Antimonywasmeasuredforbothsizefractions,withclaycontentsrangingfrom0.1to5.6ppm(average=0.80±0.48ppm),andclay+siltcontentsrangingfrom0.5to13.1ppm(average=1.84±0.87ppm).Thresholdvaluesarearoundthe95thpercentileforbothfractions,at1.5and2.7ppmfortheclayandsiltplusclay‐sizefractions,respectively(Figure6a).Spatially,thetwosizefractionsshowsimilardistributions,withthemostanomalouscontentsoccurringinthenortheasternsectionofthestudyarea(Figure7i).AntimonyisalsoacommonpathfinderelementofporphyryCu‐AuandVMSmineralization.Antimonyintheheavymineralfractionrangesfrom1.2to26.3ppm,andisanomalousatthe90thpercentile(~7ppm)(Figure6a).ThetwohighestvaluesintheheavymineralfractionaretothesoutheastandnortheastofCarpLake,withintermediatevaluesinthenorthwest,thezoneofpotentialporphyryCu‐Austylemineralization.

Pb, Bi, Zn and Cd in Till

Leadwasonlyanalyzedintheclayfraction,andrangesfrom6.6to64ppm(average=15.0±5.21ppm).Leadshowsanearnormaldistribution,althoughthereisasubtleinflectioninthecumulativefrequencyplotnearthe85thpercentile(Figure6a);settingthethresholdvalueatthe95thpercentilegivesanomalousPbat>24ppm.ThestrongestcorrelationswithPbareshownbyK(r=0.591),La(r=0.501),Bi(r=0.668),Th(r=0.720)andU(r=0.621).ThespatialdistributionofanomalousPbconcentrationsisdistinctfromtheothercommodity‐relatedelements,withthehighestPbcontentsinthenorth‐centralpartofthemaparea,betweenthenortheasternandnorthwesternareasthatareanomalousinAu,CuandAs(Figure7j).Bismuth(Figure7k)showsasimilarspatialdistributiontoPb,alongwithU,Th

andtherareearthelements(REE).Bismuthrangesfromlessthandetection(0.1ppm)to2.7ppm(average=0.33±0.26ppm),withathresholdaroundthe95thpercentile(0.8ppm;Figure6b).ZncontentswereanalyzedinbothsizefractionsandCdwasonlyanalyzedintheclayfraction.Zinccontentsinclayrangefrom83to531ppm(average=187±41ppm)comparedto60to400ppmintheclay+silt(average=167±49ppm,witharound280samplesbelowdetection).CadmiumshowssimilarspatialdistributiontoZn,andrangesfrom0.1to4.0ppm(average=0.75±0.48ppm).Bothmetalsshowthelargestanomalies(thresholdatthe95thpercentile=245ppmZnforclay,250ppmZnforclay+siltand1.6ppmCdforclay;Figure6b)alongtheeasternsideofthemaparea(Figure7l,m),althoughCdalsoshowsseveralanomalousvaluesinthewest‐centralportionofthestudyarea.BothZnandCdhavestrongpositivecorrelationswithMo(r=0.743and0.586,respectively),As(r=0.421and0.364,respectively)andSb(r=0.464and0.334,respectively),aswellaswitheachother(r=0.719).AlthoughZncorrelatespoorlywithmajorelements,CdisstronglycorrelatedwithCa(r=0.595).

Rare Earth Elements, U, Th, K, Ca, Mg, Na, Cr, Hf, Co, Mn and Ni in Till

BoththebasaltillgeochemicalresultsandtheINAAdeterminationsontheheavymineralfractionshowthatvaluesforrareearthelements(REE),U,Th,K,Ca,Mg,Na,Ni,andCr(onlyLa,ThandCrshowninFigure7asexamples)havespatialrelationshipsthatareconsistentwithchangesinthedominantunderlyingbedrocklithology(e.g.,Figures7n,o,p).Thus,incompatibleelementsthatareenrichedinfelsicrocks(i.e.,theREE,U,Th,KandHf)areelevatedinthenorthernpartofthestudyareacoincidentwiththeWolverinemetamorphiccomplex,whichcontainsfelsicrockssuchasgraniticpegmatite,granite,granodioriteandrhyolite(Struik,1994).InthelargeHMCsamplesfromthisareaofthemap,>33%oftheclaststhatare>2mmaregranitoid,indicatingagreaterprevalenceofalkalicvolcanicrocksandassociatedlatestageintrusiverocks.NickelandCrcontentsarehighestinthesouth‐southwestportionofthestudyareawheremaficandvolcaniclasticrocksoftheQuesnelTerraneoccur(Struik,1994).Conversely,whereasCoshowsastrongcorrelationwithMnintheclayfraction,CrandNi,whicharestronglyadsorbedbyMnoxidesandoxyhydroxides(NicholsonandEley,1997;Leybourneetal.,2003),showdistributionpatternsthatfollowthemajormaficvolcanicbedrockunitsinthesouthernpartofthestudyarea(Figure7p).HighTa,CeandThcontentsinthenorthtonortheastsuggestmorefelsicrocks,likelythegraniticpegmatite,granite,granodioriteandrhyoliteand/ormoreenrichedmaficrocks;whereashighCrcontentsinthesouthindicatesmoreprimitivemaficrocks(Figures7g,h,i,j).

Heavy Mineral Concentrates: visible gold, pyrite and cinnabar

Alltillsamplesprocessedforheavyminerals(n=152)containvisiblegoldgrains.Thenumberofgoldgrainsper~10kgofsamplerangesfrom1to91(Figure8a)andthecalculatedAucontentsrangefrom1to23,491ppb.Intotal,1584goldgrainswereclassifiedonthebasisofsizeandmorphology.Goldgrainmorphologiesaresubdividedintothreegroups:pristine,modifiedandreshaped,basedontheclassificationschemeofDilabio(1990).Themajorityof

goldgrainsinthisstudyareclassifiedasreshaped(82.5%),withlesscommonmodifiedgrains(14%)andrarepristinegrains(3.5%).Thethresholdvalueforthetotalnumberofgoldgrainsisaround12–15(80–85thpercentile),basedonchangesinslopeofaprobabilitydistribution.Althoughtheyareonlyestimates,graincountsofpyriteandcinnabarareuseful.Pyritecountsrangefromzerotoahighof~10000grains.Mostofthetillsampleswithelevatedpyritegraincounts(whereanomalousvaluesareapproximately>50grains)occurintheeasternandsouthernpartsofthestudyarea(Figure8b),distinctlysouthoftheareawithanomalousmetalvalues(northeasterncornerofthestudyarea;Figures7a–i,k–m).Bycontrast,cinnabarcountsrangefrom0to400,withanomalouscinnabargraincounts(approximately>60grains)inthewesternpartofthestudyarea,withatrendofdecreasingvaluestothesoutheast(Figure8c).

TILL GEOCHEMICAL EXPLORATION

Precious and base metal veins

Inthenortheasternpartofthestudyarea,thereareanumberofAuandCu‐Aushowings,aswellastwosmallpast‐producingplacerdeposits(discussedpreviously).Historicgoldrecoveredfromtheplacerdepositswasdescribedaswirytoangular(MINFILE093J007),suggestingthattheplacergoldhadnotbeentransportedfarfromsource.Samplesoftheclayfractionwereanalyzedbyaqua‐regiadigestionfollowedbyICP‐MS,whereasthesiltplusclay‐sizefractionwasanalyzedbyINAA,thustheICP‐MSresultswillbelessbiasedbythenuggeteffect.Goldintheclayfractionoccurseitherasclay‐sizedgoldgrains,mostlikelyaresultofglacialcomminutionand/orsmall‐scalehydromorphicgolddispersionandadsorptiontoclayandoxyhydroxidemineralsurfacesintheclayfraction.Otherthanasmallnumber(~3)ofhighlyanomalousAuvaluesintheICP‐MSresults,thereisarelativelystrongcorrelation(r=0.410)betweenCuandAu;thissuggeststhatmuchoftheAuisassociatedwithCu‐sulphideminerals.Thepathfinderelementalassociationspresentedhere(i.e.,Sb,As,Se,Tl,Cd,Zn)areconsistentwiththisstyleofmineralization(Taylor,2007).Thisassociationiscoherentwithdescriptionsofmanyoftheshowingsinthenortheasternsectionofthestudyarea;showingsofquartzveinswithAu,Cu±Agand/orPGE(MINFILE093J007,093J012,093J027,093J037),likelyofepigeneticorigin.Themainclusteroftillsampleswithanomalousvaluesisessentiallyspatiallycoincidentwithmanyoftheshowings.Thedominanticeflowtowardsthenortheastcanbeusedasavectortoguidefurtherprospecting.

Porphyry Cu‐Au

ThereispotentialforporphyryCu‐Au–stylemineralizationinthestudyareabasedonthepresenceoftheMountMilliganporphyryCu‐Audevelopedprospectincorrelativerockstothenorthwestofthestudyarea.ThetillgeochemicaldatashowselevatedvaluesofCuandAuandanumberofpathfinderelements(e.g.,As,Hg,Sb)inthenorthwesternpartofthestudyarea(Figures7a,b,c,d,f,g,i).Theseanomaloustillsamplesstronglyindicatesourcesofmineralizationup‐ice,towardsthesouthwest.ThereareanumberofCuandCu‐Aushowings

coincidentandup‐iceofthisareaofelevatedgeochemicalvalues(Figure2).Forexample,attheTsilshowing(MINFILE094C180),inthenorthwesterncornerofthestudyarea,therearereportsofoutcropsofintermediatehornblendeandfeldsparporphyriticrocksexhibitingquartz‐carbonatealterationwithpyrite,pyrrhotiteandrarechalcopyriteveins.ThemainclusterofCuandAuanomaliesinthenorthwesternpartofthestudyareadirectlyoverliethemainclusterofCuandAushowingsinthisarea(Figure2).ThehighestheavymineralcontentscorrespondtotheareaidentifiedashavingpotentialporphyryCu‐Austylemineralization(cf.Wardetal.,2011).

Volcanogenic Massive Sulphide Deposits

Volcanogenicmassivesulfideshowingsoccurtothesoutheastandtothenorthwestofthestudyareaalongthetrendofthemajorbedrockunits.Giventhepresenceextensivevolcanicrocks,thereshouldbesignificantpotentialforVMSmineralizationinthestudyarea,eventhoughtherearenoVMSshowingsordepositslistedinMINFILE.However,thisstudyindicatesthereisrelativelylittlespatialcorrelationbetweenthecommodityelementsassociatedwithVMSmineralization.LeadanomaliesareclearlydistinctfrombothCuandZn.TherelativelylowPbcontentsinthetillinthestudyarea,comparedtootherareasofVMSdeposits(cf.,Halletal.,2003;ParkhillandDoiron,2003),couldberelatedtothreefactors: GiventhepreponderanceofmaficvolcanicrocksinthispartofBC,VMSmineralization,if

present,wouldlikelybelead‐poorgiventhegenerallyjuvenilenatureofthesourceofthevolcanicrocks(Smithetal.,1995;PatchettandGehrels,1998;Dostaletal.,1999;Erdmeretal.,2002;Rossetal.,2005).Furthermore,VMSdepositsassociatedwithoceanfloorandoceanicarcsettingsarelead‐poorcomparedtothoseassociatedwithcontinentalmargins(Franklinetal.,1981;Galleyetal.,2007).

OnlytheclayfractionwasanalyzedforPb,byaqua‐regiadigestionfollowedbyICP‐MS,anditispossiblethatPbispresentinalesslabileformorinacoarsersizefraction.

ItispossiblethatthethicktillunitsofthestudyareahavedilutedthegeochemicalsignatureofunderlyingbedrocklithologiesresultinginsubduedanomaliesforPb.

DespiteanapparentlackofgeochemicalresponseforPbintillsamplesthereisstillpotentialforVMS‐stylemineralizationinthestudyarea.ThegenerallackofspatialcorrelationbetweenanomalousCuandZnmaysimplybeafunctionofVMS‐relatedCuanomaliesbeingmaskedbyanomaliesassociatedwithporphyryCu‐Auandpreciousandbasemetalveinmineralizationorbydilutionduetothicktill.ZincshowspoorcorrelationswithNi,CrandMgindicatingthatanomalousZncontentsinthetillarenotsimplyafunctionofweatheringofmaficvolcanicrocks.InadditiontoanomalousZnalongtheeasternportionofthestudyarea(Figure7l),therearecoincidentanomaliesforCd,BiandTl,suggestingthepotentialforconcealed,presentlyunrecognized,Znmineralization.TheHMCsampleswiththehighestpyritegraincountsarealsofromtheeast‐centralpartofthemaparea(Figure8b),furthersuggestingthepresenceofVMS‐stylemineralizationinthisarea.MoredetailedworkfollowingupthesourceofanomalouspyritegraincountsandZn,Cd,BiandTlcontentsinthetilliswarranted.

Mercury

PinchiLakemercurymine(MINFILE093K049)islocatedonthePinchifaultapproximately45kmtothenorthwestofthetwoHgoccurrencesinthesouthwesternportionofthestudyarea(Figure2).ThePinchiLakemineoperatedfrom1940to1944and1968to1975,andwasoneofonlytwomercury‐producingminesinCanada(Plouffe,1998).ThetwoHgshowingswithinthestudyareaareassociatedwiththeextensionofthePinchifault,butanomalouscinnabarcountsandHgcontentsintheclayfractionarenotspatiallyassociatedwiththeseshowings(Figures7f,5c).Elevatedcinnabargraincountsoccurtothenorthoftheshowings,suggestingadditionalsourcesoffault‐associatedHgmineralizationupicefromthecinnabargrains.ModeratelyelevatedHgintheclayfractionalsooccursintheareaofhighcinnabargraincounts.Follow‐upworkthatincludesananalysisofthesiltplusclay‐sizefractionusingananalyticalmethodwithlowerdetectionlimitsforHgiswarranted.

HEAVY MINERAL CONCENTRATE GEOCHEMISTRY TheINAAdeterminationsonheavymineralconcentratespresentedhereaddtothepreviouslypublisheddataandbegintobuildacoherentstoryonthepotentialformetallicmineralizationinthestudyarea.ThespatialdistributionofAu,AsandSbcontentsconfirmthepotentialforpreciousandbasemetalveinmineralizationandtoalesserextentporphyryCu‐Auinthenortheastandnorthwestareasofthestudyarea,respectively.Withgreaterthan30sampleshavingAucontentsgreaterthan400ppb,thereisthepotentialformineralizedbedrocktooccurthere.GoldanomaliescommonlycoincidewithanomaliesinotherpathfinderelementssuchasAsandSb.Althoughonlyalimitednumberofvaluesareabovedetectionlimit,thespatialdistributionsofHgvaluesintheheavymineralconcentratesdomimicthecinnabargraincounts.AreaswithelevatedINAAvaluesandgraincountsmaybeassociatedwithfaultssimilartothePinchiLakefault.ElevatedlightREEsandThinthenorthernpartofthestudyareaarecoincidentwith,andlocateddown‐icefrom,granitepegmatite,granite,andgranodioritesuggestingthepossibilityofREEmineralizationbeingassociatedwithfelsicporphyrybodiesinthearea.Ourpreviousstudiesontheclay+siltandclayfractionsofstudyareatillsandthepresenceoflargenumbersofpyrite/marcasitegrainsinsomeoftheHMCsamplessuggestpossibleVMSmineralization(Wardetal.,2011).However,theinherentlimitationsoftheINAAmethod(e.g.,lackofCuandPbdeterminationsandhighdetectionlimitsforZn,Cd,andAg)meanthatINAAdeterminationspresentedhereonheavymineralconcentratesdonotaddanyinsightintothepotentialforVMSstylemineralizationwithinthestudyarea.

CONCLUSIONS InpartoftheQUESTProjectarea,centralBC,approximately760tillsampleshavebeencollectedwherethickglacialdepositscoverbedrock,hinderingbothbedrockmappingandmineralexplorationprograms.ThestudyareaoccurswithintheQuesnelterrane,andisdominatedbymiddletoupperTriassicmaficvolcanicrocksandvolcaniclasticsedimentary

rocksoftheNicolaGroup.TheMountMilliganCu‐Auporphyrydepositoccursjusttothenorthwestofthestudyareaincorrelativerocks,partofanearlinear,northwest‐trendingseriesofCu±Modepositsthatoccurwithinthisterrane.Tillgeochemicaldata(clayandclay+silt)andheavymineralgraincountdataandmetalcontentsofthe<0.25mmfraction,highlightfourareasthatwarrantfurtherwork:1) Inthenorthwesternpartofthestudyarea,thereisalargenumberoftillsampleswith

significantlyanomalousCuandAucontents(andcoincidentbutlesssignificantAsandAganomalies).TheunderlyingrocksarecorrelativewiththosethathosttheMountMilliganCu‐Auporphyrydeposit.ConsistentwiththepotentialforporphyryCu‐Austylemineralization,thereareanumberofshowingsassociatedwithalkalicvolcanicandporphyriticrocks.ThisareaalsohastillsampleselevatedinHf,REE,Th,Ti,FeandV,reflectingFe‐richalkalicigneousrocksintheunderlyingandup‐icebedrock.

2) Inthenortheasternpartofthestudyarea,thereareAu,Cu,As,Ag,SbandCdanomaliesintilloccurnearseveralpreciousandbasemetalveinshowingsandtwosmall‐scalepast‐producingAu(andPt)placermines.

3) Intheeast‐centralportionofthestudyarea,tillsampleshaveelevatedZn,CdandBicontents,aswellashighpyritegraincounts(upto10000grainsina10kgsample).TherearenoknownshowingsormineralizationinthispartofthestudyareabutthetillgeochemicalresultssuggeststhereispotentialforconcealedVMS‐typemineralization.

4) Inthewest‐centralportionandintothecentralportionofthestudyarea,Hgvaluesandelevatedcinnabargraincountssuggestthereisfault‐associatedHgmineralizationup‐ice(i.e.,tothesouthwest),perhapssimilartothePinchiLakemercuryminelocatedtothewestofthestudyarea.

Inthesefourareasincreasedtillsampledensitycouldprovidesomeinsightintocoveredbedrocklithologiesandthepotentialformetallicmineralization.Tillsamplingcanbecomemorechallenging,however,assampledensityincreasesappropriatesamplematerialcanbedifficulttofindandaccesstogoodsamplesitescanbelimited.Insuchcases,prospecting(includinganexaminationofclastsindrift)andtrenchingcouldbecarriedouttofurthertesttheseareas.

ACKNOWLEDGMENTS GeoscienceBCprovidedthemajorityoffundingforthisprojectandtheauthorsextendmanythankstoC.AnglinandC.Sluggettforhelpingtomakethisprojecthappen.TheMountainPineBeetle(MPB)ProgramunderthedirectionofC.Hutton(GSC,NaturalResourcesCanada)facilitatedbyAlainPLouffeprovidedfundingforaportionofthegeochemicalanalysesforsamplestakenin2008.AlainPlouffealsoarrangedforthearchivingofsamplesattheGSC.M.Casola,M.Dinsdale,K.Kennedy,V.Levson,J.McDonald,C.Pennimpede,S.Reichheld,I.SellersandD.Visprovidedableassistanceinthefield.AthoroughreviewbyT.Ferbeyhelpedtogreatlyimprovethemanuscript.

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