issue 86 June 2007

Regional geochemical study paves way for national survey �

Regional geochemical study paves way for national surveyGeochemistry of near-surface regolith points to new resources

Megan Lech and Patrice de Caritat

TheThomsonRegionalGeochemicalSurveycoveredanareaofapproximately155000squarekilometresinnorthwesternNewSouthWales(figure1),encompassingpartsoftheThomsonOrogenandtheadjacentLachlanandDelamerianOrogens(Greenfieldetal2006a).

Theprojectaimedtoboostgreenfieldmineralexplorationintheregionbyprovidinganinternallyconsistent,backgroundgeochemicaldatasetforthesouthernThomsonOrogenregion.ItalsoaimedtorefinesamplingprotocolsdevelopedinotherregionstoensuretheirsuitabilitytoarangeofenvironmentsacrossAustralia.

Theregioniscoveredbysubstantialtransportedregolith,includingsedimentsfromtheCainozoicLakeEyre,Bulloo–BancanniaandMurray–DarlingriverbasinsandCainozoicsanddunesandsandplains.OutcroppingcrystallinebasementincludesvariousmineraloccurrencesintheBrokenHill(lead,zinc,tin),Tibooburra(gold)andCobar(lead,copper,gold)regions.

InterestinconductingageochemicalsurveyintheThomsonregionarosefromasimilarsurveyintheRiverinaregionofNewSouthWalesandVictoria(Caritatetal2004),andthepreliminaryinterpretationsthatresulted(Caritatetal2005).

TheRiverinasurveyidentifiedgeochemicalpatternsinthenear-surfaceregoliththatrelatetogeologicalandmineralisationfeaturesbelowthecover.Theseincludedelevatedregolithgoldconcentrationsincatchmentscontainingknowngolddepositsoroccurrences,dispersiontrainsofarsenicandantimonyfromtheVictoriangoldfields,andanomaloussilver,leadandzincintheeasternpartofthestudyareawhichbordersthebasemetal-richLachlanFoldBelt.

TheRiverinaandThomsonresultsshowhowregionalgeochemicalsurveyscanprovidefundamentaldatasetsformineralexplorationandresourcepotentialevaluation,environmentalmonitoringandpolicydevelopment,andgeomedicalstudiesintothehealthofhumans,animalsandplants.

TheThomsonRegionalGeochemicalSurveywasacollaborativeprojectbetweenGeoscienceAustralia,theCooperativeResearchCentreforLandscapeEnvironmentsandMineralExploration(CRCLEME),andtheNSWDepartmentofPrimaryIndustries(Greenfieldetal2006b).

Figure 1. Samplelocationsoverimageofthefirstverticalderivativeofthetotalmagneticintensity(courtesyofNSWDepartmentofPrimaryIndustries).

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issue 86 June 2007

Regional geochemical study paves way for national survey �

Floodplain sediments provide evidence

Themainsamplingmediumtargetedforthesurveywasoverbanksedimentsinfloodplains.Thisismorelikelythanothermediatorepresentanaveragecompositionofentirecatchments,enablinglow-costsamplingoflargeareas(Ottesenetal1989,Bølvikenetal2004).Targetsamplesiteswereselectedbyconductinghydrologicalanalysistodeterminethelowestpointineachcatchment.Sampleswerecollectedatbackgroundlocations,awayfromareasofobviousdisturbanceorcontamination.

Ateachsite,twointervalsapproximately10centimetresthickweresampled:oneat0to10centimetresbelowtherootlayer,ifpresent(topoutletsediment,TOS),andanotherusuallyatadepthof60to90centimetres(bottomoutletsediment,BOS).Atotalof99catchmentoutletsitesweresampledoverthearea,givinganaveragesampledensityof1per1540squarekilometres.Todate,geochemicalresultsareavailableforthefirst76sites.

Samplesweredriedandsievedat<75µmand<180µmfractionsbeforegeochemicalanalysis.Theconcentrationsofover60majorandtraceelementsweredeterminedbyX-rayfluorescenceandinductivelycoupledplasma–massspectrometry(ICPMS).SamplepreparationforICPMSinvolved4-acid‘near-total’digestionformulti-elementanalysisandmulti-aciddigestionforseleniumanalysis.Ionselectiveelectrodeanalysiswasundertakentomeasurefluorine,andgraphitefurnace–atomicadsorptionspectrometryforgold.

High concentrations at several sitesFigures2atocand3provideinformationontheconcentrationsofcopper,leadandantimonywiththeircorrespondinggeochemicaldistributions.

AcopperanomalyispresentintheBOS<75µmand<180µmfractions(32milligramsperkilogram(mg/kg)and24mg/kg,respectively)neartheBarrierRangesnorthofBrokenHilland50kilometressoutheastofBrewarrina(29mg/kgand27mg/kg).SeventykilometressoutheastofTibooburrathereisasamplewithanotablyhighcopperconcentration(25mg/kg)inthe<180µmfraction.Higherconcentrationsofcopperinthecentralwestareonlyafewkilometresawayfromarecentexplorationcompanydrillholethatintersectedbeddedpyritemineralisation(figure2a).

Elevatedleadconcentrations(21.7mg/kg)arevisibleinthesoutheast,presumablyreflectingproximitytotheBarrierRanges,andnortheastofCobarnearoutcropoftheLachlanFoldBelt.Bothregionshaveknownleadmineraloccurrences.Elevatedleadvalues(19.48mg/kgand19.22mg/kginthe<180µmfraction)alsooccurinthecentralwest,coincidentwithelevatedcoppervalues(figure2b).

Antimonyshowssimilargeochemicalpatternstoleadandcopper.AntimonyisnotablyhighernortheastofCobarandinthecentralwest(0.70mg/kg)inthe<180µmfraction.Valuesof0.86mg/kgand0.79mg/kgforthe<180µmfractionwererecordedinsamplescollected40kilometresnorthofCobaronYandaCreek(figure2c).

Anomalousconcentrationsoflead,copperandantimonymostlyoccurproximalto

“TheRiverinaandThomsonresultsshowhowregionalgeochemicalsurveyscanprovidefundamentaldatasetsformineralexplorationandresourcepotentialevaluation,environmentalmonitoringandpolicydevelopment,andgeomedicalstudiesintothehealthofhumans,animalsandplants.”

issue 86 June 2007

Regional geochemical study paves way for national survey �

outcroppingcrystallinebasement.Threemainareashavebeenidentified:

southwestregion,correspondingtotheCurnamonaCratonwhichhoststheBrokenHillorebody

centraltosouthwestregionatthejunctionoftheKoonenberry,ThomsonandLachlancrustalelements

easternandsouthernregion,wheretheeastandwestLachlancrustalelementsadjoinbeneaththeCobarBasin.

Lessons for explorationLowerelementconcentrationsgenerallycoincidewithareasofthickerGreatAustralianSuperBasincover.Thresholdsinthisareamayneedtobeloweredtocorrectlyassesstheprospectivityoftheseterrains.

Generally,catchmentswithknownmineraloccurrencesrelatingtolead,copperorantimonyhaveanomalousconcentrationsofthatelementintheoutletsample.Dispersionofelementsappearstobeatthecatchmentscale.

Severalanomalousconcentrationsoccurawayfromknownmineralisationandwarrantfurtherinvestigation.TheseincluderesultsfromrecentcompanydrillingnorthoftheKoonenberryBeltandnortheastofCobar.

ThesuccessoftheThomsonandotherpilotgeochemicalsurveysledtotherecentannouncementofthecontinent-wideNationalGeochemical

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Figure 2. (a)GeochemicalmapofcopperconcentrationsintheBOS<180µmfractionwithknowncopperoccurrences;(b)GeochemicalmapofleadconcentrationsintheBOS<180µmfractionwithknownleadoccurrences;(c)GeochemicalmapofantimonyconcentrationsintheBOS<180µmfractionwithknownmineraloccurrences

issue 86 June 2007

Regional geochemical study paves way for national survey �

SurveyofAustraliaproject(Baldwin2007).ThatprojectispartoftheAustralianGovernment’sOnshoreEnergySecurityInitiative(Johnson2006)andwillgathergeochemicaldatathatwillprincipallybeusedtosupportexplorationforAustralia’senergy-relatedresources.

ReferencesBaldwinB(ParliamentarySecretaryforIndustry,TourismandResources).2007.ZeroinginonAustralia’sonshoreenergyandminerals.Mediarelease,2March2007,DepartmentofIndustryTourismandResources.Availableat:minister.industry.gov.au/index.cfm?event=object.showContent&objectID=0F26ADF9-C2A0–4C89-FC1574CD3883BEAA

BølvikenB,BogenJ,JartunM,LangedalM,OttesenRT&VoldenT.2004.Overbanksediments:anaturalbedblendingsamplingmediumforlarge-scalegeochemicalmapping.ChemometricsandIntelligentLaboratorySystems74:183–199.

CaritatPde,JairethS,LechM&PykeJ.2004.Regionalgeochemicalsurveys:RiverinaPilotproject—methodologyandpreliminaryresults.CooperativeResearchCentreforLandscapeEnvironmentsandMineralExplorationOpenFileReport160.Availableat:www.crcleme.org.au/Pubs/OFR160/CRCLEME_OFR160.pdf

CaritatPde,LechM,JairethS,PykeJ&LambertI.2005.Riverinageochemicalsurvey—ANationalFirst.AusGeo News 78.Availableat:www.ga.gov.au/image_cache/GA6632.pdf

GreenfieldJ,ReidW,GilmoreP,CaritatdeP,LechM,HillS,HulmeK,WatkinsJ&WorrallL.2006a.TheThomsonOrogenProject—aworkinprogress.In:FitzpatrickRW&ShandP(eds),Regolith2006—Consolidationanddispersionofideas.ProceedingsoftheCRCLEMERegolithSymposium,Hahndorf,SouthAustralia,6–9November2006.Availableat:crcleme.org.au/Pubs/Monographs/regolith2006/Greenfield_J.pdf

GreenfieldJ,CaritatdeP,HillP&ReidW.2006b.TheNSWThomsonOrogenProject:newfrontiersinexploration,In:NSWDPIMinesandWinesConference,Cessnock,25–26May2006,Cessnock.Availableat:www.smedg.org.au/Greeab.pdf

JohnsonJ.2006.OnshoreEnergySecurityProgramunderway.Webpage:www.ga.gov.au/ausgeonews/ausgeonews200612/onshore.jsp(accessed29February2007).

OttesenRT,BogenJ,BølvikenB&VoldenT.1989.Overbanksediment:arepresentativesamplemediumforregionalgeochemicalsampling.JournalofGeochemicalExploration32:257–277.

Related websites/articlesAusGeo News 78:Riverinageochemicalsurveyanationalfirst

www.ga.gov.au/ausgeonews/ausgeonews200506/geochem.jsp

For more informationphone MeganLechon+61262499459email [email protected] PatricedeCaritaton+61262499378email [email protected]

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Figure 3. Boxplotsofcopper,leadandantimonyconcentrationsintheTOSandBOS<75and<180µmfractions.