Structual Geology and Resources 2012

43 l ke

ynot

e

Structural Controls to, and exploration for, epithermal Au-Ag Deposits

GReG CoRBett

Structural Controls to, and Exploration for, Epithermal Au-Ag Deposits

GREG CORBETT

Introduction Structural controls to the formation of epithermal Au-Ag deposits within magmatic arc (extending into back arc) environments are apparent as:• Localisationbymajorstructures,• Provisionoffluidflowpathstoenablefluidevolution,• Dilatantsettingsofenhancedfluidflowandmineral

deposition,• Sitesofmineraldepositionbyfluidmixing,locallyat

structural intersections.Structural controls to epithermal Au-Ag mineralisation must be considered as an interaction with other factors

suchasmineralisationstyle,hostrockcharacteristicsandmechanisms of precious metal deposition. The path to discoverydelineatedhereinreliesontheuseofevolvinggeologicalmodels,builtupfromconsiderationofmanyexplorationexamples(Corbett,2009),butwhichshouldneverbetreatedtoorigidly(Lowell,2012).

Classification of Epithermal Deposits Epithermal Au-Ag deposits are classified (Corbett and Leach,1998;Corbett,2009andreferencestherein)asformedabovetheporphyryenvironmentatcrustallevelswithinabout1kmofthesurfaceandsubdividedonthebasisoforemineralogy,hydrothermalalterationandfluidflowpaths(figure1).

Figure 1. Conceptual model for the styles of Au-Ag-Cu mineralization developed within magmatic arcs (from Corbett, 2009).

Structual Geology and Resources 2012

44 l

High sulphidation epithermal Au+Ag+Cu deposits arecharacterisedbythereactionwithwallrocksofhotacidicfluidstoformhydrothermalalterationwhichiszonedoutwardsasmineralassemblagesdominatedbyvughysilica,silica-alunite,pyrophyllite-diaspore,andmoremarginaldickite-kaolin.AuintheSWPacific,withadditionalAginLatinAmerica,andCuatdeepercrustallevels,areassociatedwithenargite-pyrite-barite-alunite,partlyoverprintingalteration.Hydrothermalfluidflowiscontrolledinitiallybystructurethenpermeablelithologiesaswell as phreatomagmatic (diatreme) breccia pipes. LowsulphidationepithermalAu-Agdepositsarecharacterisedbyneutralwallrockalterationassemblagesdominatedbyilliticclays,althoughacidicwaterscollapsingfrom near surfical acid sulphate caps deposit kaolin within someoreassemblages(below).Magmaticallyderivedhydrothermalfluidsdepositquartz-sulphideAu+Cuveinsearliestandcommonly(butnotalways)atdeepercrustallevels,andthendivergeintotwofluidflowtrends(figure1).Typicallyinextensionalsettings,suchasbackarcenvironments,polymetallicAg-AudepositsdominateinLatinAmerica(SierraMadreofNorthernMexico,Peru,Patagonia),andareverticallyzonedpassingfromAu-Cudominant at depth to higher level Ag with associated sphalerite-galenaandsulphosaltsandquartz-carbonategangue.Thesedepositspassupwardstobandedchalcedony-ginguroepithermalAu-Agorescharacterisedbybandsofchalcedony,depositedfromdominantlymetroricwaters,andginguro(pyrite-electrum-sulphosalts)derivedfromthe magmatic source. These deposits are well represented throughoutthecircumPacificrimmagmaticarcs(Hishikari,Japan;Kupol,Siberia;Waihi,NewZealand;VeraNancy-Pajingo,Queensland;MidasUS;Ares,Peru;CerroMoroandCerroVanguadia,Patagonia).Epithermaloresdepositedfrommainlyintrusion-relatedfluidsarerecognisedinSWPacificrimmagmaticarcsgradingfromquartz-sulphideAu+Cutocarbonate-basemetalAu,andatthehighestcrustallevel,epithermalquartzAu-Agstylemineralisation(CorbettandLeach,1998;Corbett,2009;figure1).Augradesincreaseintheevolvinghydrothermalfluidinitiallylocalisedwithinsulphideswhichvaryintimeandspacetowardshighercrustallevels,fromearlypyriteandlesserchalcopyriteatdepth,tosphalerite>galenaandsulphosalts,andthenganguepoorhighfinenessfreeAuathighestcrustallevels.Dilatantstructuresarerequiredtobleedorefluidsfromtheintrusionsource rocks at depth to cooler elevated settings where mineral deposition takes place and also to facilitate the mixingofrisingorefluidswithoxidisingnearsurficalwatersto promote Au deposition. Carlin(sedimenthostedreplacementAu)depositsformbythereactionofquartz-sulphidefluidswithimpurelimestonehost rocks at elevated crustal settings and are not confined to thewesternUS.Thetransitionbetweenporphyryandepithermaloresisapparentaslowsulphidationquartz-sulphideAu+

Cumineralisationwithindilatantsheetedveinsystems,commonlyreferredtoaswallrockporphyries,locallyextendingsomedistanceawayfromthesourceintrusion,butwithtypicallylowmetalgrades(Cadia,NSW;MaricungaBelt,Chile;Gaby,Ecuador;WhitewashMo,Queensland).

Major Structures Majorcrustal-scalestructureswhichlocalizemagmaticsource rocks for low and high sulphidation epithermal Au-Agdepositsaredivided(Corbett,1994;CorbettandLeach,1998)into3classesas:• Arcparallelstructures(DomeykoFault,Chile;Gilmore

Suture,NSW;KalimantanSuture,Borneo),whichcommonlydevelopedasterrainboundarieswithdominantlyreversesensesofmovement,oftenundergotransient changes to strike-slip deformation to localise oresystemsindilatantstructuralsites.Arcparallelstructuresmayalsoparallelthestructuralgraininextensionalsettings(Carlin,GoldstrikeTrends,Nevada;northernMexico)wheretheymayactasregionalfeederstructuresorhostveins,locallywithinlistricfaultssuchas in the Sierra Madre (below).

• ArcnormaltransferstructuresmayextendthroughmagmaticarcsintotheunderlyingbasementtotapmantlederivedmeltsassourcesofAumineralization,orfocusoverprintingmagmatismandlocallydelineatechangesinthediporrateofsubductioninadjacentplates(Porgera,WafiandGrasbergTransferstructures,NewGuinea;LachlanTransferZone,NSW).

• Conjugatefracturesarerecognizedascontrolstomagma emplacement in some orthogonal magmatic arcs (Andes,NorthSulawesiandBandaarc),commonlyatthe intersections with arc-parallel terrain boundaries. Strike-slipmovementonthesefracturesystemsduringorthogonal compression or transient episodes of relaxation aids the creation of dilatant structural sites for ore formation.

Hydrothermal Fluid EvolutionEpithermaldepositsdevelopbyorefluidevolutionduringstructurallycontrolledflow.Highsulphidationepithermalorefluidsdisplaycharacteristic evolution and vertical separation between the magmatic source and epithermal environment of ore deposition. The structural control therefore represents anessentialelementtofacilitatetheflowofrapidlyrisingvolatile(SO2)richmagmaticfluidswhichbecomedepressurisedandexsolveSO2,whichthenoxidises(toH2SO4)toformhotacidicfluids.Atepithermallevelsthesefluidsareprogressivelycooledandneutralisedbyreactionwithwallrockstoprovidethecharacteristichydrothermalalteration described above. Sulphide mineralisation is depositedbytheliquid-richphaseoftheorefluid,mostlylater.Thisfluidevolutionprovidesalowerlimittohighsulphidation deposits which are therefore rootless.

Structural Controls to, and Exploration for, Epithermal Au-Ag Deposits

GREG CORBETT

Structual Geology and Resources 2012

45 l

Lowsulphidationveinsystemscommonlycomprisebandedveins in which different bands are deposited within a hoststructurefromvariablefluidsources(Corbett,2008).Dilatantstructuresmayhostshallowcirculatingcellsofmeteoric-dominated waters which deposit barren gangue mineralssuchaschalcedony,carbonateandadularia.SomedeepercirculatingfluidsentrainamagmaticcomponentanddepositquartzwithdisseminatedsulphidesandlowgradeAu.Structuresmayoccasionallytapdeepmagmaticorefluidstodepositmostpreciousmetalsinthinsulphidebands.Ofinteresttoexplorationists,somerecentdiscoveriesfeaturesurficialbarrenquartzunderlainbymineralisedsulphide-bearingveins(HuevosVerdeandCerroMoro,ArgentinePatagonia),asonlysomeelementsofveinshostmineralisation.

Controls to Epithermal OresManyepithermalAudepositshostmostorewithinoreshoots(clavosinSpanish),developedaswiderandhigherpreciousmetalgradeveinportions,mosteasilydiscernibleongramxplots(Corbett,2007).Epithermalveinsystems(mostlylowsulphidation)arecontrolledbythe interaction of:• Competenthostrocksfracturetoaidveinformation.

Volcanicsequencesofinterlayeredcompetent(andesite)andincompetent(tuffs)providestackedflatplungingoreshoots.Thismaybeaccentuatedasmoderatelydippingfaultspreferentiallyhostveinswheretheyrefracttosteeperanglesinbandsofcompetenthostrock(figure2).Thereareanumberofexplorationexamplesofdiscoveryof epithermal veins within competent host rocks obscuredbyoverlyingincompetenttuffs.

• Permeablehostrockscontrolmostlyhighsulphidationepithermalfluidflowwithoreshootsattheintersectionwithfeederstructures,althoughhostpermeabilityislesscommonlyrecognisedinlowsulphidationores(LihirPapuaNewGuinea;RoundMountain,Nevada).

• ThestyleofepithermalmineralisationdescribedaboveinfluencesAugrade,distributionofAgandCuandmetallurgicalcharacteristics,asthestructuralenvironmentinfluencesfluidevolution.Oresmayoccurasveins,brecciamatrixordisseminations.

• Fieldstudiesusingmodelsderivedfromanalysesofmagmatic arc (as different to back-arc) geothermal systems(CorbettandLeach,1998;LeachandCorbett,2008)demonstratebonanzaAugradesmostlyresultfromthemixingofrisingorefluidswithcollapsingnearsurficialwatersorrapidcoolingofrisingorefluids.Boilingdepositsbarrengangueminerals(adularia,quartzpseudomorphingplatycalcite)andmostlylowgradeAu-Ag.Mixingenvironmentsarestronglystructurallycontrolled,typicallyatstructuralintersections(below).

• Structuresalsoinfluencetoformoforeshootsdevelopedatdilatantstructuralsettingsofenhancedfluidflow.

Structural Control to Epithermal OresStructural environments provide the dominant control to the form(typicallyplunge)oforeshootsdevelopedasregionsofelevatedfluidflowunderdifferentconditions(figure2)as:• Orthogonalextension(SierraMadrenorthernMexico,

southernPeru,ArgentinePatagonia;Gosowong,Indonesia;HiddenValley,PNG)generallyoccursonlistricfaultswhichhostepithermalveinswithinflatplunging ore shoots confined to the steeper dipping faultportions.Refractionofdippingfaultsthroughcompetent host rocks will provide steep dipping portions asveinhosts(ElPenon,Chile;PalmarejoMexico;Kupol,Siberia).Variationsofjustafewdegreesdipcontrolchangesfromoreshootstowaste.AtLihir,PapuaNewGuinea,similartoMtStHelens,USA,sectorcollapseof a volcanic edifice has occurred on listric faults which thenhostoreshootsinsteeperportions(MinifieZoneatLihir).CalderacollapseprovidedflatoreshootswithinreactivatedbeddingplanesofadjacentlavasatEmperorGoldMine,Fiji.

• Transpressionalenvironmentscharacterisedbystrike-slip structures host steep plunging ore shoots formed in dilatantstructuralsettingssuchasfaultjogscontainingtensionveinlinksbetweentwostructures,flexuresasbendsinthroughgoingstructures,splaysorhorsetails(inolderliterature).Normal(growth)faultswhichparticipate in pull-apart basin formation host tension veinsatdepthandsplaysatdeepestlevels,withinverticallystackednegativeflowerstructures(CorbettandLeach,1998).Twotranspressionalendmembersareapparentasflexureswithinthroughgoing(champion)veinswithinterveningpoorlymineralisedveinportions(VeraNancy,Queensland),ortensionveinarraysconstrainedbetweenbarrenstrike-slipstructures(Waihi,NewZealand).Cautionisurgedasmineralizedtensionveins formed at high angles to the structural grain of thedistrict(controllingstructures),prospectedastheelongationofsoilgeochemicalanomalies,maythenbesub-parallel to the drilling direction and provide erratic results.Geologicalmappingandnotgridsshouldlocalizedrill holes and veins at low angles will suggest the structuralmodelrequiredfurtherwork.

• Orthogonalcompressionisanuncommonoresettingdespite the overall compressional nature of magmatic arcs.Flatplungingoreshootsdevelopintheflatterdipping portions of moderate dipping reverse faults (Kencana,Gosowong,Indonesia;Hamata,MorobeGoldfield,PapuaNewGuinea)andmaybeblindatthesurface(Kencana).Curiously,eachofthesetwoexamplesis oriented normal to outcropping mineralized veins withinlistricfaults(GosowongveinandHiddenValleyoresystemsabove).

• Plungingoreshoots(ElInido,Chile)developbytheinteractionofstrike-slipandmostlynormalfaultmovement,althoughareversecomponentispossible.

Structural Controls to, and Exploration for, Epithermal Au-Ag Deposits

GREG CORBETT

Structual Geology and Resources 2012

46 l

Repeatedactivationofthekinematiccontrolstoveinformation promotes the development of banded veins with polyphasaleventsofAu–Agdepositionandassociatedelevated precious metal grades.

Fluid Mixing and Ore ShootsLowsulphidationepithermalveinsystemsmayhostbonanza Au grades derived from the mixing of rising pregnantorefluidswithcollapsingoxidizingnearsurficialwaters.Here,oreshootsofvariableplungearelocalizedatthe intersection of the separate structures which transport thedifferentfluidsandsocommonlydisplayrod-likeforms.Therisinghydrothermalfluidssetupcirculatinghydrothermalcellswhichpromotethecollapseofthenearsurficialwatersdowncrossstructures.LowpHwaterscollapsing from near surficial acid sulphate caps to settings of low sulphidation vein formation provide the best mixing environments,evidencedbyhypogenekaolininthesulphideoreassemblage.Whileaninterestingprospectingmodel,oreshootsmaybottomatthelimitofthedownwardcollapseofthelowpHwaters.Goldisalsodepositedbythemixingofrisingorefluidswithoxidizinggroundwatersevidencedbyhypogenehematiteintheoreassemblage,ormixingwith bicarbonate waters recognized in carbonate-base metalAudeposits(CorbettandLeach,1998;Leachand

Figure 2. Epithermal vein ore shoots in different structural settings including the development of bonanza Au at a site of fluid mixing (modified from Corbett, 2007).

Corbett,2008).Incompetentacidsulphatecapsinhibitveinformationandsomayobscureblindbonanzagradeoresystems(GuadalupeatPalmarejo,Mexico).

ConclusionsMineral exploration benefits from an understanding of the controls to epithermal mineralization as:• Majorstructureslocaliseoresystemsandlocallydisplay

components of movement which control dilational settingsoforeformation.Epithermalfluidsevolveduringfluidflowwithinstructurestoprovideviableoresystems.Thecompositefluidtypesresponsibleforbandedlowsulphidationveinformationmayprovidebarren veins at the surface which are mineralised at depth,anddifferentportionsofveinsmaydisplaydramaticallydifferentpreciousmetalgrades.

• HighsulphidationepithermalAudepositsarecharacterizedbyatwostage(earlyvolatileandlaterliquid)fluiddevelopedbyevolutionduringtherisefroma deep magmatic source to higher crustal level epithermal sitesofmineraldeposition.ManyhighsulphidationepithermalAusystemshostoreshootsattheintersectionoffeederstructuresandpermeablehostrocks,typicallyalteredtovughysilica,anddisplaydistinctbottoms.

Structural Controls to, and Exploration for, Epithermal Au-Ag Deposits

GREG CORBETT

Structual Geology and Resources 2012

47 l

• Incompetentandcommonlyclayalteredtuffsmayobscureveinswithinunderlyingcompetentandesiteorfelsicsills(Arcata,Peru;ElPenon,Chile).AcidsulphatecapsmayobscurebonanzaAu-Aggradesdevelopedbyfluidmixing(GuadalupeatPalmarejo,Mexico).Rod-likelinear ore shoots develop as a result of Au deposition byfluidmixingatstructuralintersections(COSE,ArgentinePatagonia,Kupol,Siberia).

• Kinematicenvironmentscontroltheformoforeshootswhichhostmostmineralizationinmanyveinsystems.Steepplungingflexuresinstrike-slipstructuresmaybeidentifiedbymagnetitedestructioninandesiteterrains,orusingintegratedresistivityandchargeabilitydatainmorepermeablerocktypes.Onlysteepdippingportionsoflistricfaultshostoreasflatplungingoreshoots,whilesimilarplungingoreshootsinmoderatelydippingreversefaultsarecommonlyblindatthesurface.Interaction of extensional and strike-slip controls are commonandmayprovideplungingoreshoots.

References CitedCorbett,G.J.,2007,ControlstolowsulphidationepithermalAu-Ag:Talk

presentedatameetingoftheSydneyMineralExplorationDiscussionGroup(SMEDG)withpowerpointandtextonSMEDGwebsitewww.smedg.org.au

Corbett,G.J.,2008,Influenceofmagmaticarcgeothermalsystemsonporphyry-epithermalAu-Cu-Agexplorationmodels:TerryLeachSymposium,AustralianInstituteofGeoscientists,Bulletin48,p.25-43.

Corbett,G.J.,2009,Anatomyofporphyry-relatedAu-Cu-Ag-Momineralisedsystems:Someexplorationimplications:NorthernQueenslandExplorationandMining2009,ExtendedAbstracts,p.33-46.

Corbett,G.J.,andLeach,T.M.,1998,SouthwestPacificgold-coppersystems:Structure,alterationandmineralization:SpecialPublication6,SocietyofEconomicGeologists,238p.

Leach,T.M.andCorbett,G.J.,2008,Fluidmixingasamechanismforbonanzagradeepithermalgoldformation:TerryLeachSymposium,AustralianInstituteofGeoscientists,Bulletin48,p.83-92.

Lowell,D.2012,LowellandRebagliatiurgefreethinking:Editorial,NorthernMiner,12-18March,2012,Toronto.

Structural Controls to, and Exploration for, Epithermal Au-Ag Deposits

GREG CORBETT