Gawler Project breaks cover �

issue 83 Sept 2006

GawlerProject breaks cover

New datasets aid area selection and targetingRoger Skirrow, Patrick Lyons, Anthony Budd, Evgeniy Bastrakov

Overthepastfiveyears,theGawlerMineralPromotionProjecthasshedconsiderablelightonthe1590Maironoxide–copper–gold(IOCG)mineralsystemsoftheeasternGawlerCratonandthecoevallode-goldsystemsofthecentralGawlerCraton.Theproject,undertheNationalGeoscienceAccord,hasbeenajoint-effortofGeoscienceAustraliaandPrimaryIndustriesandResourcesSouthAustralia.

AcquisitionandinterpretationofdeepcrustalseismicreflectiondataintheregionofthegiantOlympicDamCu–Au–Umine(seeAusGeo News 76)deliveredhugegainsinourunderstandingofthecrustalarchitectureandtectonicevolutionoftheeasternandcentralGawlerCraton.WenowrecognisethattheIOCGmineralisation,whichincludestheOlympicDamdeposit,occurredinboardofaconvergentmargin,withfirst-ordercontrolsonfluidpathwaysbeingnorthwest-trendingthrustfaults.

Thethree-dimensionalpictureaffordedbytheseismicresultsconstrainsourinterpretationofpotential-fielddataandenablesclearcomparisonswiththeMesozoic–CainozoicIOCGsystemsoftheAndeanmarginofSouthAmerica.

Apartfromgeophysics,otheradvancesintheprojecthavecomefromgranitestudiesandgeochemistry.WehavealsorefinedandimprovedthegeochronologicalframeworkoftheeasternandcentralGawlerCraton.

TheresultisacoherentmodelofmineralisationoftheeasternandcentralGawlerCratonthatlinkstheIOCGswiththeircoevallode-golddepositsforthefirsttime.Thisknowledgegivesexplorersvaluablespatialguidestowardsmineralisationinthesealmostcompletelyhiddenterranes.

Potential-field dataInareasoflittleornooutcrop,potential-fielddataarecriticaltounderstandingthegeologicalmake-upofthebasement.BecausetheGawlerCratonisparticularlydeficientinoutcrop—withthecrystallinebasementoftheeasternGawlerCratonalmostcompletelycoveredbyyoungerrocks—potential-fielddataarevitalforgainingnecessaryregionalknowledgeoftheprospectivebasement.

Interpretationofpotential-fielddataisnormallyconstrained

bytheextentofavailablepetrophysicalandspatialdata,butthesedatamaybedifficulttoobtain.Fortunately,theuniquedensityandmagnetisationoftheironoxidealterationofIOCGmineralisationallowsustomapthelikelylocationsoforedepositionusing3Dinversionofthepotential-fielddata(figure1),evenwhereconstraintsarelacking(seeAusGeo News 74).

Figure 1. Three-dimensionalinversionofmagneticandgravitydatapredictsthedistributionofhaematiteandmagnetitealteration,representedhereasisosurfaces(haematiteinbrown,magnetiteinblue).MappingthedistributionofironoxidesassociatedwithIOCGmineralisationalsogivesinformationaboutpossiblestructurescontrollingmineralisation,suchasthefault-planeshowningreen,allowingexplorerstodefinedrilltargets.Althoughthisexamplecoversanareaofabout30kmby30km,theinversionmethodisindependentofscale.

Gawler Project breaks cover �

issue 83 Sept 2006

New subdivision of Hiltaba granitesAnewsubdivisionoftheHiltabaAssociationgranites(whichsupersedestheHiltabaSuite)andtheircomagmaticGawlerRangevolcanicscomprisesfoursupersuites.TheRoxbyandVenusSupersuitesshowA-typecharacteristics,whiletheVenusandMalboomaSupersuitesareI-type.

Significantly,theA-typegraniteswerehotterandrequiredmoreelevatedgeothermsthantheI-types.TheOlympiccopper–goldprovinceintheeasternGawlerCratonischaracterisedbythepresenceofA-typesupersuites,indicatingthattheelevatedcrustaltemperaturesoftheeasternGawlerCratonat1590MawereakeyingredientforgeneratingIOCGmineralisation.

ThelowertemperatureI-typesupersuitesshowmanyofthecharacteristicsofgranitesassociatedwithintrusion-relatedgolddeposits.ThesegranitesaremoreabundantinthecentralGawlergoldprovince.

ThereisnodemonstrateddirectgeneticlinkbetweenthegranitesupersuitesandthebroadlycoevalIOCGandlode-goldmineralisation.Itmaybethatthegranitesaresymptomaticofcertaincrustalgeothermsthatdeterminethestyleofmineralisation,withthehottercrustproducingsomeoftheconditionsrequiredforIOCGmineralisationandcoolercrustleadingtolode-goldmineralisation.

Chemical modellingChemicalmodellingpredictsthatreactionofalow-grademagnetiteprotore(containing0.1%Cu,inchalcopyrite,and0.1g/tAu)withanoxidisedfluidmayresultinamineralassemblagewithappreciableupgradingofcopperandgold(upto~3%Cuasbornite,chalcociteandchalcopyrite;upto1g/tAu)(figure2).Mineralisationwillbecontainedwithinthehaematitealterationzoneadjacenttothemagnetite–haematiteoxidationfront.Ourmodellingagreeswithobservationsfromseveralknownoccurrencesofcopper–goldmineralisationintheGawlerCratonandshowsthatfavourablesitesofmineralisationarelesslikelywithinmagnetitealteration,butmayoccuratthejunctureofoffsethaematiteandmagnetitealterationzones.Thetwo-stageupgradingmodelcomplementspreviouslypublishedmodelsofcopperandgoldprecipitationduetosimultaneousmixingofreducedandoxidisedfluidsinaccountingfortherangeofstylesofcopper–goldmineralisationintheeasternGawlerCraton.

Mineral potential mapA1:500000scalemapreleasedinFebruarythisyear—Iron oxide Cu–Au (–U) potential of the Gawler Craton, South Australia (figure3)—summariseskeyresultsofworkonIOCG

Figure 2. Computermodellingshowsthatflushingofmagnetitealteration,containingsub-economiccopperandgold,byoxidisedfluidsproduceshaematitealterationandupgradescopperandgold.ThecharacteristicsuiteofsulfidesassociatedwithIOCGmineralisationispredicted.Aconsequenceisthatmaximaofmagneticanomaliesarenotoptimaldrilltargetsforcopper–gold.Onemustcombinemineralogicaldatawithmagneticandgravitydatatolocatehaematitealterationadjacenttomagnetitealteration.

Gawler Project breaks cover �

issue 83 Sept 2006

systemsandshowsthedistributionofseveral‘essentialingredients’ofIOCGore-formingsystems,including:

supersuitesoftheHiltabagranites

faultsandshearzoneswithinterpretedageofyoungestsignificantmovement

coppergeochemistry(>200ppm)fromdrillholesintersectingcrystallinebasement

hydrothermalalterationassemblagesandzones,basedondrillholelogging

distributionofIOCGalterationbasedoninversionmodellingofpotential-fielddata

hostsequenceunitsconsideredimportantinlocalisingIOCGalterationandmineralisation

neodymiumisotopicdataandthemineralisotopicagesoflatePalaeoproterozoictoearlyMesoproterozoicmagmatismandhydrothermalminerals

prioritisedareasofmaximumpotentialforIOCGmineralisation.

ThemapmaybedownloadedfromtheGawlerProjectwebsite.

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Figure 3. IronoxideCu–Au(–U)potentialoftheGawlerCraton,SouthAustraliamapproducedbytheGawlerMineralPromotionProjectisasummaryof‘essentialingredients’forIOCGmineralisationintheGawlerCraton.This map can be downloaded from the Gawler Project website.

Project lessonsTheGawlerMineralPromotionProjectdemonstratedhowtocometogripswiththegeologyandmineralisationofaregiongenerallyobscuredbyhundredstothousandsofmetresofcover.Themostimportantlessonisthatamultidisciplinaryandcollaborativeapproachisessential.

Geophysics,geochemistry,petrology,drillholedata,andgeochronologycombinedtoprovideguidestooreandtohelpusunderstandthetectonicenvironmentoftheGawlerCraton’sIOCGmineralsystemsandcoevallode-goldsystems.

Gawler Project breaks cover �

issue 83 Sept 2006

WearenowabletotakewhatwehavelearnedaboutworkinginsuchadifficultterranetootherfrontierpartsoftheAustralianProterozoic.

Related links

AusGeo News 76 Gawlerseismicstudy.

AusGeo News 74 Thickcovernoobstacletofieldinversion,IronoxideCu–Au(–U)potentialoftheGawlerCraton,SouthAustralia1:500000scalemap.

www.ga.gov.au/minerals/research/regional/gawler/gawler.jspGawlerMineralPromotionProject

More information

phone RogerSkirrowon+61262499442email roger.skirrow@ga.gov.auweb www.ga.gov.au/minerals/research/regional/gawler/gawler.jsp link

https://www.seegrid.csiro.au

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