Metallogenesis of intrusive rocks of Victoria �
issue 83 Sept 2006
Metallogenesisof intrusive rocks of VictoriaNew datasets target gold and base-metal mineralisationDavid Champion
GeoscienceAustraliahasbeencompilingandsynthesisingdatasetsofvariousmetallogenicparametersforintrusiveandcountryrockunitsoftheTasmanides(orTasmanOrogenicZone—theyoungereasternpartofAustralia,whichisjoinedtothecontinent’sProterozoicandoldercorealongtheTasmanLine)ofeasternAustralia.
TheworkispartoftheFelsicIgneousRocksofAustraliaprojecttoassisttheexplorationindustryinthesearchforintrusion-relatedmineralisationsystems(see,forexample,Champion&Blevin2005).AsreportedinAusGeo News 74and79,thisprojectisbeingundertakenasregionalmodules.DatasetsfornorthQueenslandarecurrentlybeingpreparedforrelease,anddatasetsforTasmaniaarenowcompletedandreleasedasajointMineralResourcesTasmania–GeoscienceAustraliaproduct.
IncollaborationwithGeoscienceVictoria,theprojectisnowextendingintotheTasmanFoldBeltofVictoriaandiscurrentlysynthesisingmetallogenicdataforthatstate.Productsofthisstudy,whichwillincludedownloadabledatatablesforVictorianrockunits
(linkedtoGeoscienceAustralia’sNationalMapdigitalgeology)willbereleasedinearly2007.
Preliminary implications for intrusion-related mineralisationGranitescompriseasignificantpartoftheLachlanFoldBeltinVictoria(upto20%bysurfacearea;figure1).TheyaredominantlySiluriantoDevonian,rangingfromabout430Mato350Ma.Mineralisationassociatedwiththesegranitesincludesarangeofcommodities,includingtungsten,tin,molybdenum,copperandgold.
Oneoftheproject’sprimaryaimsisthecompilationandinterpretationofchemical-basedmetallogenicparametersforthegranites,usingthecharacteristicsidentifiedbyBlevinandotherworkers(forexample,Blevin2004;seesummaryinChampion&Blevin2005).Thesimplestfirst-passregionalapproachhasbeentousethegranitedatatocalculateparametersofoxidationstateanddegreeofevolution.TheseparameterscanthenbeusedasdefinedbyBlevin(2004)
Figure 1. Simplifiedpre-PermiangeologyofVictoria.GeologymapcourtesyofGeoscienceVictoria.
Metallogenesis of intrusive rocks of Victoria �
issue 83 Sept 2006
tohighlightmineralpotential(figure2).
Useofthistechniqueisrelevantforregionalapplicationtonewexplorationmodels,andparticularlyforintrusion-relatedgoldsystems(Thompsonetal1999,Blevin2005),giventhatBierlein&McKnight(2005)haverecentlydocumentedseveralexamplesofintrusion-relatedgoldmineralisationinwesternVictoria,andraisedthepossibilityofadditional,asyetundiscovered,mineralisationofsimilarstyleselsewhereinthestate.
GeoscienceAustraliahasaccesstopublicandconfidentialgeochemicaldataforover200Victoriangraniteunits.Apreliminaryclassificationofthisgeochemicaldata,plotted(contoured)ontheoxidation–fractionationdiagramofBlevin(2004;figure2),illustratesanumberofpoints.
First,itisevidentthatthegranitescollectivelyspanarangeofcompositions,encompassingvariousmetallogenicassociations,consistentwithknownmineraloccurrencesforthestate.Second,itisapparentthatonlyafewofthegranites(forwhichwehavedata)havecharacteristicsconsideredprospectiveforthegranite-relatedporphyrycopper–goldclassofdeposits(suchasthoseassociatedwithOrdovicianmagmatisminNewSouthWales,forexampleCadia).Third,itisclearthatthemajorityofVictorianI-typegraniteshave
10310210110010-110-210-3
Blevin (2004)
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10 0
10 1
Rb/Sr ratio
Blevin et al. (1996)
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Fe2O3FeOratio
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IncreasingFractionation
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Figure 2. A. Relationshipbetweentheoxidationstate(calculatedusingtotalrockFe2O3/FeOratio),andthedegreeofcompositionalevolution(calculatedusingtotalrockRb/Srratio)ofgranites,andrelatedmetallogenicassociations,asdocumentedbyBlevinetal(1996)andBlevin(2004).
Rb/Sr ratio
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Victorian Granites
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Figure 2. B. Oxidation–compositionalevolutionplot,contouredusingavailablegeochemicaldataforVictoriangranites.ThebulkofthegeochemicaldatafortheI-typegranites(greyfield)stronglyoverlapswiththesuggestedfield(Blevin2004)forintrusion-relatedgold.
Metallogenesis of intrusive rocks of Victoria �
issue 83 Sept 2006
chemicalcharacteristicssimilartothoseofgranitesknowntobeassociatedwithintrusion-relatedgoldmineralisation(figure2).Thisdoesnotnecessarilyindicatethesegraniteswillhaveassociatedgoldmineralisation,butdoesshowthatmuchofVictoriahasthepotentialforsuchmineralisation,consistentwiththesuggestionsofBierlein&McKnight(2005).
Generalmodelsforintrusion-relatedgoldsystemsalsohighlighttheimportanceofcontinentalsedimentaryassemblagesashostrocks,especiallythosewithcarbonaceousorcarbonate-bearingunits(seethecompilationtableinAusGeo News 79).Theseandotherparametersarecurrentlybeingcompiledforallpre-MesozoicVictoriancountryrockunits.AllsynthesiseddatawillbelinkedtoGeoscienceAustralia’sNationalMapdigitalgeology.Thislinkagewillallowintegrateduseofthecountryrockandgranitedatainaspatialenvironment,andwillhelptorefineareaselectionforintrusion-relatedgoldandothercommoditiesinVictoria.
More information
phone DaveChampionon+61262499215email [email protected]
References
BierleinFP&McKnightS.2005.Possibleintrusion-relatedgoldsystemsinthewesternLachlanOrogen,southeastAustralia.EconomicGeology100:385–398.BlevinPL.2004.RedoxandcompositionalparametersforinterpretingthegranitoidmetallogenyofeasternAustralia:Implicationsforgold-richoresystems.ResourceGeology54:241–252.BlevinPL.2005.IntrusionRelatedGoldDeposits.ExplorationmodelandPDFfile,www.ga.gov.au/about/corporate/ga_authors/Expl_models/PL_Blevin_IRGD.jsp.BlevinPL,ChappellBW&AllenCM.1996.IntrusivemetallogenicprovincesineasternAustraliabasedongranitesourceandcomposition.TransactionsoftheRoyalSocietyofEdinburgh:EarthSciences87:281–290.ChampionD&BlevinP.2005.Newinsightsintointrusion-relatedgold–coppersystemsintheTasmanides.PresentationgivenatMining2005,Brisbane,26October.www.ga.gov.au/minerals/research/pubs/presentations/irg_mining2005.jspThompsonJFH,SillitoeRH,BakerT,LangJR&MortensenJK.1999.Intrusion-relatedgolddepositsassociatedwithtungsten–tinprovinces.MineraliumDeposita34:323–334.
Related websites/articles
AusGeo News 74 Granitehostsfocusofnewmineralsprojectlink
AusGeo News 79 ProspectslookgoodforgoldinnorthQueensland
link
GeoscienceAustralia’sFelsicIgneousRocksofAustraliaproject
link
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