Smith 2009 - Sources and evolution of mineralising fluids in IOCG systems

Thesourcesandevolutionofmineralisingfluidsiniron

oxide–copper–goldsystems,Norrbotten,Sweden:Constraints

fromBr/ClratiosandstableClisotopesoffluidinclusion

leachates

S.A.Gleesona,*,M.P.Smithb

aDepartmentofEarth&AtmosphericSciences,UniversityofAlberta,Edmonton,Alta.,CanadaT6G2E3bSchooloftheEnvironment,UniversityofBrighton,CockcroftBuilding,LewesRoad,BrightonBN24GJ,UK

Received25February2009;acceptedinrevisedform11June2009;availableonline16June2009

Abstract

Wehaveanalysedthehalogenconcentrationsandchlorinestableisotopecompositionoffluidinclusionleachatesfrom

threespatiallyassociatedFe-oxide±Cu±AumineralisingsystemsinNorrbotten,Sweden.Fluidinclusionsinlate-stage

veinsinFe-oxide–apatitedepositscontainsalinebrinesandhaveawiderangeofBr/Clmolarratios,from0.2to

1.1Â10À3andd37ClvaluesfromÀ3.1&toÀ1.0&.LeachatesfromsalinefluidinclusionsfromtheGreenstoneandPorphyry

hostedCu–AuprospectshaveBr/Clratiosthatrangefrom0.2to0.5Â10À3andd37ClvaluesfromÀ5.6&toÀ1.3&.Finally,

theCu–AudepositshostedbytheNautanenDeformationZone(NDZ)haveBr/Clmolarratiosfrom0.4to1.1Â10À3and

d37ClvaluesthatrangefromÀ2.4&to+0.5&,althoughthebulkofthedatafallwithin0&±0.5&.

TheBr/Clratiosofleachatesareconsistentwiththederivationofsalinityfrommagmaticsourcesorfromthedissolution

ofhalite.MostoftheisotopicdatafromtheFe-oxide–apatiteandGreenstonedepositsareconsistentwithamantlederived

sourceofthechlorine,withtheexceptionofthefoursampleswiththemostnegativevalues.Theoriginofthelowd37Clvalues

inthesesamplesisunknownbutwesuggestthattheremayhavebeensomemodificationoftheCl-isotopesignaturedueto

fractionationbetweenthemineralisingfluidsandCl-richsilicateassemblagesfoundinthealterationhaloesaroundthedepos-

its.Ifsuchaprocesshasoccurredthenamodifiedcrustalsourceofthechlorineforallthesamplescannotberuledout

althoughtheamountoffractionationnecessarytogeneratethelowd37Clvalueswouldbesignificantlylarger.

ThesourceofClintheNDZdepositshasacrustalsignature,whichsuggeststheClinthissystemmaybederivedfrom

(meta-)evaporitesorfrominputfromcrustalmeltssuchasgraniticpegmatitesoftheLinaSuite.

Ó2009ElsevierLtd.Allrightsreserved.

1.INTRODUCTION

Theironoxide–copper–gold(IOCG)depositclasshas

attractedmuchattentioninrecentyears,bothintermsof

academicresearchandexplorationactivity.Thedeposits

arecharacterisedbyCu-sulphides±Auhydrothermalores

withabundantmagnetiteorhematiteandoccurin

rocksrangingfromLateArchaentotheCenozoicinage(Williamsetal.,2005).IOCGdepositsdonothaveaclear

spatialassociationwithigneousrocks,havedisputedtec-

tonicsettingsandvariablegeologicalcharacteristics.Also,

thesourcesofthemajorcomponentsinmanyofthedepos-

itsareunknown.However,allthedeposittypesarecom-

monlyfoundinsequenceswhichhaveundergonelarge

scalesodicalterationandcontainCl-richsilicateminerals

suchasscapolite,biotiteandamphiboles.

TherelationshipbetweenFe-oxide–apatite(e.g.Kiruna-

type)andtheIOCGdepositshasalsobeenacontentious

pointintheliterature.Thecommonspatialrelationship

0016-7037/$-seefrontmatterÓ2009ElsevierLtd.Allrightsreserved.doi:

10.1016/j.gca.2009.06.005*Correspondingauthor.E-mailaddress:sgleeson@ualberta.ca(S.A.Gleeson).

www.elsevier.com/locate/gcaAvailable online at www.sciencedirect.com

GeochimicaetCosmochimicaActa73(2009)

5658–5672ofthesedeposittypes(andinsomecasestheirdirectsuper-

position)hasleadtosuggestionsthattheyarepartofthe

samedepositclassorthattherearegeneticlinksbetween

thetwo(Hitzmanetal.,1992).Thisissupportedbyearly

stagemagnetitealterationinmanyIOCGdeposits(e.g.

Smithetal.,2007),thelate-stageoccurrenceofpyrite,chal-

copyriteandgoldinandnearmassivemagnetitedeposits,

andthecommonfeaturesinalterationassociatedwithboth

deposittypes(Sillitoe,2003).However,recentworkhas

highlightedasignificantdifferenceintimingofthetwode-

posittypesinsomeareas(Hitzman,2000)indicatingthata

directgeneticlinkbetweenthetwomaynotexist.

Oneofmaindifficultiesinconstructingageneticmodel

forthesedepositsisdeterminingthefluidsourcesatdiffer-

entstagesofmineralisation,inpartduetothemodification

offluidstableisotopecharacteristicsbywater–rockinterac-

tioninthedepositclassasawhole(Haynes,2000).Anum-

berofdifferentgeneticmodelshavebeensuggestedforthe

depositclassincluding(1)amagmaticsourcefortheminer-

alisingfluids(e.g.Pollard,2000,2006);(2)amagmatic

sourcethathasbeenmodifiedbylargescalefluidcircula-

tionformingtheregionalsodicalterationandaddingmet-

alstothemineralisingfluid(e.g.Oliveretal.,2004);(3)a

non-magmatic,evaporiteornear-surfacecontinentalbrine

derivedoriginforthefluids(BartonandJohnson,1996,

2000;Xavieretal.,2008);(4)ametamorphicsourcefor

thefluids(FisherandKendrick,2008);(5)amixedmag-

matic–basinalbrineorigin(Chiaradiaetal.,2006;Baker

etal.,2008;Kendricketal.,2008).Metamorphismofevap-

oriteshasalsobeeninvokedtoexplaintheregionaldistri-

butionofsodicalterationintheKirunadistrict,

Norrbotten,SwedenalthoughnoFennoscandianevaporitic

sedimentsarepreserved(Frietschetal.,1997).However,it

hasbeensuggestedthatsomeoccurrencesofapparently

stratigraphicallyrestrictedscapoliteintheGreenstone

grouphavearerelatedtoformerevaporitebeds(Martins-

son,1997).Itisnowrecognisedthattheclassrepresentsa

diversegroupwiththepotentialforawiderangeofpoten-

tialfluidsources(Williamsetal.,2005).

InNorrbottenbothdepositshostedbytheGreenstone

andPorphyryGroupmetavolcanicrocks,anddeposits

hostedregionallysignificantdeformationzonessuchas

theNautanenDeformationZone(NDZ),havebeenpro-

posedtobelongtotheIOCGclass.Theyarelinkedby

theubiquitouspresenceofmagnetiteasanalterationphase,

andthecommonoccurrenceofscapoliteinalteration

assemblages,despiteothervariations.Itis,therefore,perti-

nenttoexaminethedepositstogetherinordertoinvestigate

therangeofbrinesourcesoperatingovertimeinthearea,

andinparticulartoinvestigateifcommonsourcesperiodi-

callysuppliedfluidaccountingforthegeochemicalsimilar-

itiesinthedeposittypes.Inthisstudy,weexaminetheCl

andBrconcentrationsand,usinganon-linemassspectro-

metrictechnique,thestablechlorineisotopiccomposition

ofdiluteleachates(>20ppmtotalchloride)derivedfrom

microthermometricallywellcharacterisedveinquartzsam-

ples.WecomparethesourceofClinKiruna-typeFe-

oxide–apatiteandIOGCdepositsoftheNorrbottendis-

trict,Swedenandprovidenewconstraintsonthefluid

sourceandwater–rockinteractionhistoryindepositsandprospectsinthisimportantmetallogenicprovince.Bulk

leachatesarecurrentlytheonlywaytoexaminethechlorine

stableisotopechemistryofinclusionfluids.

2.BACKGROUNDGEOLOGY

ThemajorironoreprovinceofnorthernSwedenislo-

catedinNorrbottenCountyandismainlyhostedbyPalae-

oproterozoicrocks(seereviewsbyCarlon(2000)and

Bergmanetal.(2001)).ThesedepositsaremainlyKarelian

(2.5–2.0Ga)andSvecofennian(1.9–1.88Ga)inage(Fig.1)

andarepreservedindeformedandmetamorphosedbelts,

intrudedbyarangeofgranitoidplutons.Metamorphic

conditionspeakedatuppergreenschistorloweramphibo-

litefaciesduringtheSvecofennianOrogenyfrom1.9to

1.8Ga(Skio¨ld,1987).Adetailedlithostratigraphyofthese

rockshasbeenproposedbyMartinsson(1997).TheGreen-

stoneGroup(>1.9Ga),consistingofmainlytholeiitic(Ek-

dahl,1993)tokomatiitic(Martinsson,1997)volcanicrocks

overliesArchaeanbasement.Theseareoverlainfirstbythe

MiddleSedimentGroup(Witschard,1984),andthenbythe

PorphyryGroup,whichconsistsofvolcanicandsub-volca-

nicrocks,subdividedintheKirunaareaintothedomi-

nantlyandesiticPorphyriteGroup,andthesyeniticand

quartz-syeniticKirunaPorphyrieswhichhosttheKiruna-

vaaramagnetite–apatitedeposit.Inviewoftheirproximity

totheKirunaavaaradepositstheKirunaporphyriesmay

haveacquiredtheirsyeniticcharacterviaalkalimetamor-

phism,anditislikelythattheoriginalvolcanicrockswere

calc-alkalineincharacter.TheHaparandaandPerthite–

monzonitecalc-alkalineandalkali-calcicgranitesuitesin-

trudedtheserocksbetween1.9and1.8Ga(Skio¨ld,1987)

followedbytheLinaSuitegranitoidsataround1.79Ga

(Skio¨ld,1987;Bergmanetal.,2001).Theyoungestplutonic

rocksintheareaareTIB2granitoids,ataround1.71Gain

age,exposedattheSwedish–Norwegianborder(Romer

etal.,1994).Theareaiscross-cutbyaseriesoflargescale

shearsystemsincludingtheNDZ,whichisnotableforits

associationwithmineralisation.ItisaNNWtrendingtec-

tonicstructurewherestronglyschistoseormyloniticrocks

occurinseveralhighstrainbranchesinazoneupto3km

wide(MartinssonandWanhainen,2004).

ThePalaeoproterozoicrocksofNorrbottenareaareaf-

fectedbyscapoliteandalbitealterationatboththeregional

anddepositscale,wheretheyareassociatedwithbothiron

oxideandCu–(Au)mineralisation(Frietschetal.,1997).

Samplesforthisstudyweretakenfromthreegroupsof

mineralisingsystems;late-stagequartzveinscuttingFe-

oxide–apatitedepositsandassociatedalteration;minerali-

sation-relatedquartzveinsfromCu–(Au)prospectshosted

bytheGreenstoneandPorphyryGroups,andmineralisa-

tion-relatedquartzveinsfromtheheavilydeformedmeta-

volcanicrocksoftheNDZ.

Dataonthetimingofthescapolizationoftheareaare

limited,butSmithetal.(inpress)reportaU–Pbtitanite

ageof1903±8Mafortitaniteinascapolitealtereddiorite

atNunasvaara.TheFe-oxide–apatitedepositatKiirunava-

arahasbeendatedasformingbetween1884±6and

1875±9Ma(Romeretal.,1994),whilstStoreyetal.

(2007)showedthattitanitefromLuossavaarahaddistinctChlorineisotopesinironoxide–copper–golddepositsSweden5659