METAMORPHIC-HYDROTHERMAL REE MINERALS IN THE …

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The Canadian MineralogistVol.48,pp.81-94(2010)DOI:10.3749/canmin.48.1.81

METAMORPHIC-HYDROTHERMAL REE MINERALS IN THE BACÚCH MAGNETITE DEPOSIT, WESTERN CARPATHIANS, SLOVAKIA: (Sr,S)-RICH MONAZITE-(Ce) AND Nd-DOMINANT HINGGANITE

JaroslavPrŠEK§

Department of Mineralogy and Petrology, Faculty of Natural Sciences, Comenius University, Mlynská dolina G, 842 15, Bratislava, Slovakia and AGH University of Science and Technology, Department of Economic and Mining Geology, Al.

Mickiewicza 30, 30-059 Kraków, Poland

MartinonDrEJKaanDPEtErBaČÍK

Department of Mineralogy and Petrology, Faculty of Natural Sciences, Comenius University, Mlynská dolina G, 842 15, Bratislava, Slovakia

BartoszBUDzYŃ

Institute of Geological Sciences, Polish Academy of Sciences, Kraków Research Centre, Senacka 1, 31-002 Kraków, Poland

PavElUHEr

Department of Mineral Deposits, Faculty of Natural Sciences, Comenius University, Mlynská dolina G, 842 15, Bratislava, Slovakia

aBstract

AccessoryREEmineralsoccurinasmallmetamorphicmagnetiteoredepositatBacúch,VeporicSuperunit,centralSlovakia.Wedistinguish twopopulations ofmonazite.Monazite I forms subhedral to euhedral crystals associatedwithmagnetite. Itcontains≤12wt.%ThO2,≤2.7%UO2,≤0.85%SO3,withlowCaandSrcontents.Comparedtothecommonmonazite-(Ce)I,monazite-(Nd)I(≤26.1%Nd2O3)occasionallyoccurswithanatomicratioNd:Ceupto1.17.MonaziteIIispresentasirregularaggregateswithhingganiteinyoungerhydrothermalquartz–albite–chloriteveinlets,orasrimzonesonmonaziteI.MonaziteIIisdepletedinThandUandhasanunusuallyhighcontentofS(≤11.3%SO3,0.31apfuS)andSr(≤8.7%SrO,0.18apfuSr).Thiscompositionindicatesa(Ca,Sr)S(REE,Y)–1P–1substitutionasadominantmechanismofSrandSentryintothemonazitestructure.SomemonaziteIIcrystalsdisplayanelevatedEucontent(≤1.2%Eu2O3).Xenotime-(Y)formssubhedralcrystals,inassociationwithmonazite-(Ce)I,magnetite,pyritetransformedtogoethite(?)andquartz.Gadolinite-groupmineralsatBacúcharerepresentedbyhingganitewithanatomicvalueofX□/X(□+Fe)intherange0.51–0.72.NeodymiumislocallythemostabundantREE(17.8–18.7%Nd,~0.56apfu),andanNd-dominantmemberofthegadolinitegroupwasidentified.Thecomposi-tionofhingganite-(Y)wasalsodetermined.TheprincipalmechanismofsubstitutioninhingganiteisFe2+O2□–1(OH)–2.PrimarymonaziteIandxenotimearemostlikelyproductsofregionalmetamorphism,togetherwithmagnetitemineralization.Onthecontrary,Sr-andS-richmonaziteIIandhingganiteoriginatedduringayounger(Alpine)metamorphic-hydrothermaloverprintinafluid-richregime.

Keywords:REEminerals,Sr-andS-richmonazite-(Ce),monazite-(Nd),xenotime-(Y),hingganite-(Y),Nd-dominanthingganite,metamorphicmineralization,WesternCarpathians,Slovakia.

soMMairE

Noussignalons laprésencedeminérauxaccessoiresde terres raresdansunpetitgisementmétamorphiquedemagnétiteàBacúch, superunité deVeporic, enSlovaquie centrale.Nousdistinguonsdeuxpopulations demonazite.Lamonazite I seprésenteencristauxsubidiomorphesàidiomorphesassociésàlamagnétite.Ellecontient≤12%ThO2,≤2.7%UO2,≤0.85%SO3(poids),avecdefaiblesteneursenCaetenSr.Comparéeàlamonazite-(Ce)Icommune,lamonazite-(Nd)I(≤26.1%Nd2O3)est localementdéveloppée,avecunrapportatomiqueNd:Cejusqu’à1.17.LamonaziteIIseprésenteenagrégats irréguliersaveclahingganitedansdesveinuleshydrothermalestardivesdequartz–albite–chlorite,oubienensurcroissancesexternes

§ E-mail address:[email protected]

82 tHEcanaDianMinEralogist

demonaziteI.LamonaziteIIestpauvreenThetU,etpossèdeuneteneuranormalementélevéeenS(≤11.3%SO3,0.31apfuS)etSr(≤8.7%SrO,0.18apfuSr).Seloncettecomposition,leschémadesubstitution(Ca,Sr)S(REE,Y)–1P–1seraitdominantpourexpliquerl’incorporationdeSretdeSdanslastructuredelamonazite.CertainsdescristauxdemonaziteIIfontpreuvedeteneursélevéesd’europium(≤1.2%Eu2O3).Lexénotime-(Y)formedescristauxsubidiomorphes,enassociationaveclamona-zite-(Ce)I,magnétite,pyritetransforméeengoethite(?)etquartz.LesminérauxdugroupedelagadolinitesontreprésentésàBacúchparlahingganiteayantunevaleuratomiquedeX□/X(□+Fe)entre0.51et0.72.Lenéodymeestlocalementlaterrerarelaplusabondante(17.8–18.7%Nd,~0.56apfu),etnousidentifionsunmembredugroupedelagadoliniteàdominancedeNd.Nousavonsaussiétablilacompositiondelahingganite-(Y).LemécanismeprincipaldesubstitutiondanslahingganiteseraitFe2+O2□–1(OH)–2.LamonaziteIprimaireetlexénotime,ainsiquelaminéralisationenmagnétite,seraientlesproduitsprobablesd’unmétamorphismerégional.Enrevanche,lamonaziteIIricheenSretenSetlahingganiteauraientuneoriginependantunévénementdecirculationd’unfluidemétamorphiqueethydrothermalplusjeune(alpin).

(TraduitparlaRédaction)

Mots-clés:minérauxdeterresrares,monazite-(Ce)richeenSretenS,monazite-(Nd),xénotime-(Y),hingganite-(Y),hingganiteàdominancedeNd,minéralisationmétamorphique,Carpatesoccidentales,Slovaquie.

of substitution and possible scenario for their originandevolution.

gEologicalsEtting

Mineralization in the Bacúch area,NízkeTatryMountains, in central Slovakia, is hosted bymeta-morphosedPaleozoic rocks of theVeporicSuperunit(Veporicum) close to the Čertovica fault line.TheVeporicum consistsmainly of Paleozoic crystallinerocks(Fig.1)dominatedbymicaschists,varioustypesof granitic rocks and gneisses,withminor amountsofmetavolcanosedimentary andmetacarbonate rocks(Bielyet al.1992).

ThemineralizationoccursatBacúch–BielaSkala,closetoBacúchvillage,ontheeasternslopesofBielaSkala Peak (1250m a.s.l.).The belt ofmagnetite-bearingmicaschistsrunsfromBeňušvillageandBielaSkalatotheJavorinkaPeak;themostimportantminedumpsoccurbetween750mand900ma.s.l.(Fig.1).Theoresampleswerecollectedfromthedumpsofthisore-lenssystemat750ma.s.l.Lisý (1957)describedquartz, chlorite andmagnetite. Petro (1973) reportedthe presence of pyrrhotite, chalcopyrite, sphalerite,hematiteandK-feldspar.Bothauthorsnotedamassive,banded(quartzandmagnetitebands)anddisseminatedmagnetitemineralizationinthemicaschists.Chovanet al.(2000)describedmonazite,xenotime,zircon,biotiteand tourmaline of the dravite–schorl series. J. Pršek(unpublisheddata)alsoidentifiedrarecassiterite,nativebismuthandbismuthsulfosalts.

Chovanet al.(2000)proposedasedimentaryoriginfor the Feminerals, affected, however, by youngermetamorphism.The Permian age of themagnetitemineralizationasapartoftheJánovGrúňvolcanosedi-mentary formationwas documented byU–Pb zircondatingof the adjacentmetarhyodacites (255±2Ma:Kotov et al. 1996).The JánovGrúňFormationwaslater overprinted byAlpine (Cretaceous) low-grademetamorphism, indicated by theAr–Armuscovitedatingoftheadjacentmetavolcanicrocks(Dallmeyeret

introDUction

Monazite, a phosphatemineral of the light rare-earthelements (LREE), ispresent inawide rangeofigneous,metamorphicandsedimentaryrocks.Althoughmonaziteisusuallyanaccessorymineral,itmaycontroltheREEbulk-rockcompositionbecauseitincorporateslarge amounts of REEwithin its structure. Ceriumrepresentsthemostcommoncation,andmonazite-(Ce)isthemostwidespreadmemberofthemonazitegroup.Rarely,occurrencesofmonazite-(Nd),monazite-(La),andmonazite-(Sm)havebeendescribed(e.g.,Graeser&Schwander1987,Anthonyet al.2000,Masauet al.2002).InadditiontocommonCa,ThandSiadmixtures,rareSr-andS-richcompositionsofmonazitehavealsobeen reported in somemagmatic andmetamorphicrocks(Kukharenkoet al.1961,1965,Chakhmouradian&Mitchell1998,1999,Krenn&Finger2004,Ondrejkaet al.2007,Krennet al.2008).

Thegadolinitegroupofmineralsarerelativelyrare(Y,REE),Be-bearingsilicatephasesoccurringinsomemagmaticandmetamorphicassemblages.Inadditiontothemostcommongadolinite-(Y)andgadolinite-(Ce),octahedral-site-vacanthingganite-(Y),hingganite-(Yb)and hingganite-(Ce) have been described from somegranitic–pegmatiticoccurrences(e.g.,Dinget al.1981,Pezzottaet al.1999,Miyawakiet al.1987,2007).Somegadolinite–hingganite compositions also showhigherNdcontents(upto8.4wt.%Nd2O3,0.26apfu:Pezzottaet al.1999);however,theNd-dominantmemberhasnotyetbeendescribed.

Inthispaper,wereporttheoccurrenceofmonazite-(Ce) unusually enriched in S and Sr, togetherwithS,Sr-poormonazite-(Ce) andmonazite-(Nd), and thefirst natural occurrence ofNd-dominant hingganiteinmetamorphic, banded and disseminatedmagnetitemineralization in quartz lenses occurring in themicaschists near Bacúch in theWesternCarpathians ofcentral Slovakia.Our detailed electron-microprobeinvestigationof themonaziteandhingganiterevealedtheir compositional variations, principalmechanisms

rEEMinEralsintHEBacúcHMagnEtitEDEPosit,slovaKia 83

Fig.1. Simplifiedgeological sketch-mapofSlovakia (A) andBacúch surroundings (B) (modified afterBielyet al. 1992).Symbols:1–3Paleozoicmetamorphicrocks(Hroncomplex,JánovGrúňformation):phyllitesmicaschists,metarhyolites,4Kráľovahoľacomplex:graniticrocks,5Ľubietovácomplex:paragneisses,6TatricUnit:granitoids,gneisses,quartzites,limestones,7VeľkýBokunit:quartzites,limestones,dolomites,8Quaternary,9alluvium,10nappeslines,aAlpine,bHer-cynian,11aupthrusts,bfaults,12minesanddumps.


al.1993).TheP–TconditionsofthisAlpineoverprintwas estimated to be 600–700MPa and 330–350°C,accordingtophengitegeobarometryandmineralasso-ciation in theadjacentmetavolcanosedimentary rocks(Korikovskyet al.1997).

The newest geochronological data onmonazitesupportthisPermianandalsoAlpineevents(J.Pršek,unpubl.data).

ExPEriMEntalMEtHoDs

The REE-bearing minerals were analyzed inpolished thin sections usingCameca SX–100 elec-tronmicroprobesat theDionýzŠtúrStateGeologicalInstitute (GeologicalSurveyof theSlovakRepublic)inBratislavaandatMasarykUniversity,Brno,CzechRepublic.The instruments, operated inwavelength-dispersion (WD)mode, are equippedwith fourWDspectrometers.ThesampleswereanalyzedwithLLIF,LPETand standardPETcrystalsunder the following

conditions:acceleratingvoltage15kV,samplecurrent40nAandabeamdiameterof1–3mm.Thestandardsutilized, spectral lines, detection limits and standarddeviationsfortheREEelementsarepresentedinTable1.Thematrix effectswere corrected using the PAPprocedure.Moreover,specialcarewastakentoensurethat lineoverlapswereproperlycorrectedandalsotoavoidbackgroundinterference.Empiricallydeterminedcorrectionfactorsappliedtothefollowinglineoverlapswereused:Th ! U,Dy ! Eu,Gd ! Ho,La ! Gd,Ce ! Gd,Eu ! Er,Gd ! Er,Sm ! Tm,Dy ! Lu,Ho ! Lu,Yb ! LuandDy ! As(Konečnýet al.2004).Analyticaltimeswere15to40secondsduringmeasure-mentdependingon theexpectedconcentrationof theelement in themineral phase.Major elementsweremeasured using shorter times,whereas longer timeswereappliedforelementswithlowconcentrations.

Thechemicalformulaofmonazitewascalculatedonthebasisoffouroxygenatomsperformulaunit(apfu).The chemical formula of hingganitewas calculated


on thebasis of two (S6++P5++As5++Si4++Al3+)cationsperformulaunit.TheBe2+contentwasfixedat2apfu.Thecontentof(OH)–wascalculatedfromthedeviationbetweentheidealanionchargesandthesumofthecationcharges.

rEsUlts

The REE-bearing minerals [monazite-(Ce) I,monazite-(Nd) I, xenotime-(Y), and hingganite-(Y)]are present as individual small crystals or aggregatesdisseminatedinthequartz–magnetiteoresinthemassivemagnetite,ortheyformapartofyoungerhydrothermalveinlets[monazite-(Ce)II,hingganite-(Nd)]withalbite,chlorite and quartz crossing themagnetite-bearingrocks.The parent rocks consistmainly ofmagnetiteandquartz(Fig.2A),withminoramountsofchlorite,albite,hematite,cassiterite,ilmenite,tourmaline,pyrite(usuallyalteredtogoethiteor“limonite”),K-feldspar,muscoviteandREEminerals.Thesemineralsusuallyformdisseminated,bandedtextures,whereasmagnetiteformingmassive ore is less common.Hydrothermalveinletsoccasionallyconsistofquartz,pyrite,hematite,sphalerite,chalcopyriteandBi sulfosaltscross-cutbymagnetiteores.

Monazite and xenotime

Monazite-(Ce)isthemostabundantaccessoryphaseandthemajorLREEcarrierinthesamplesstudied.TwopopulationsofmonaziteweredistinguishedonthebasisofBSEimagesandchemicalcomposition.MonaziteIforms subhedral to euhedral, <200mmmatrix grainsthatareassociatedwithFe–Tioxides,mainlymagnetite(Figs. 2B,C,D), and rarely alsowith xenotime-(Y).MonaziteIshowsahighlyvariableThcontent(0.0–12.4wt.%ThO2,0.00–0.12apfuTh)andslightlyelevatedScontent(<0.85%SO3,<0.03apfuS)(Table2).TheSi andCa contents (<4.1%SiO2, <0.18apfu Si and1.3%CaO,0.06apfuCa)indicatethatThisprimarilyincorporated into themonazite I structure accordingto both huttonite [ThSi(REE)–1P–1] and cheralite[CaTh(REE)–2]mechanisms of substitution (Fig. 3).Somegrains showslightlyelevatedconcentrationsofU (<2.7%UO2,<0.02apfuU)without analogousSicontent.

MonaziteIIispresentasirregularaggregates<100mm in size associatedwith hingganite in youngerhydrothermalquartz–albite–chloriteveinlets(Fig.2F),ora<10mmthickrimonmonaziteI(Figs.2B,C).ThesecondpopulationissignificantlyenrichedinS(<11.3%SO3,0.31apfuS),Sr (<8.7%SrO,0.18apfuSr)anddepletedinTh(<0.04%ThO2)(Table2,Fig.4).Mona-zite II compositions show that theelevatedScontentcorrelateswithCa andSr enrichment (up to 0.37Ca+Srapfu),suggestingadominant(Sr,Ca)S(REE,Y)–1P–1mechanismof substitution at the expense of the

cheralite CaTh(REE)–2 substitution (Figs. 3 to 5).TheconcentrationofUismainlybelowthedetectionlimit.Very lowcontents ofTh+U are accompaniedby systematically elevated amounts of Pb (up to 0.2wt.%PbO).

Monazite I andmonazite II populations differ intheirREEandYdistribution.Bothpopulationsexhibita predominance of the LREE,withCe usually thedominantREEinmonaziteI(0.32–0.51apfuCe)andmonazite II (0.33–0.48apfuCe).TheNd content ofmonaziteI,0.14–0.37apfu,ishigherthaninmonaziteII(0.10–0.25apfuNd).Moreover,thecompositionofmonazite-(Nd) I,with an atomic ratioNd/Ceof 1.17(26.1%Nd2O3)was occasionally detected (Table 2).Monazite I usually shows higher concentrations ofSm,Eu,GdandY,andcommonly lowerLacontentscomparedtomonaziteII(Table2).Chondrite-normal-ized values of elements such asSm,Eu,Gd andTbinmonazite I arehigher than inmonazite II (Fig.6).ThegreatestdifferenceswereobservedinEucontent.MonaziteIcontains<1.9%Eu2O3,whereasmonaziteIIhasalowercontent,<0.4%Eu2O3(Table2).

The arsenic concentration in bothmonazite popu-lations is similar.The atomicAs:P ratio is very low,withamaximumvalueof0.02,correspondingto0.5%As2O5,which indicates insignificant solid-solutiontowardsgasparite.

Xenotime-(Y)formssubhedralcrystals,30to90mminsize,inassociationwithmonazite-(Ce)I,magnetite,pyritetransformedtogoethite(?),andquartz(Fig.2E).XenotimeshowsacommoncompositionwithstronglydominantY,HREE>>LREE,andverylowS,As,Si,Th,U,ZrandCacontents.

Hingganite

Mineralsofthegadolinite–datolitegroupinBacúchareexclusivelyrepresentedbyhingganite,(Y,REE)2(□)Be2Si2O8(OH)2.They formanhedral grains 5–30mminsizeandgrainaggregatesassociatedwithmonazite,albite,chloriteandmuscovite(Fig.2F).TheX□/X(□+Fe)valuerangesbetween0.51and0.72,andtheFecontentreaches0.49apfu (Table3,Fig.7).Thehing-ganite is generallyCa-poor, (≤0.01apfu), although aslightly higher content, 0.09–0.13apfuCa,was alsoencountered (Table3,Fig.6).Amajorityof analysesreveal a hingganite-(Y) compositionwith a predomi-nance ofY over otherREE.However, some grainsdisplayNdpredominanceoverY(1.27<Nd/Y<1.41),suggesting the presence of anNd-dominantmemberof the gadolinite group, “hingganite-(Nd)”.TheNdconcentration attains 17.8–18.7%Nd2O3, 0.56–0.57apfuNd (Table 3, Fig. 8).Concentrations ofYvarybetween 0.40 and 0.49 apfu in “hingganite-(Nd)“and between 0.53 and 1.10apfu in hingganite-(Y),respectively.Contents of the otherREE are signifi-cantlylowerinbothhingganitemembers(Table3).A


Fig.2. Microphotoandback-scatteredelectron(BSE)imagesofREEaccessorymineralsfromtheBacúchmagnetitemineral-ization.A.Microphotoofquartz(Qtz)–magnetite(Mgt)ore.B.Primarymonazite-(Ce),typeI(MnzI)withwell-developed(S,Sr)-richrimsofmonazite-(Ce),typeII(MnzII)scatteredinquartzmatrix(Qtz).C.Primarymonazite-(Ce),typeI(MnzI)with(S,Sr)-richrimsofmonazite-(Ce),typeII(MnzII)inassociationwithmagnetite(Mgt)scatteredinthequartzmatrix(Qtz).D.Aggregateofprimarymonazite-(Ce),typeI(MnzI)inassociationwithmagnetite(Mgt)scatteredinthequartzmatrix (Qtz).E.Xenotime-(Y) (Xnt) in associationwithmagnetite (Mgt) andpyrite altered to goethite (gt) or limonitescatteredinthequartzmatrix(Qtz).F.Irregularaggregateofnewlyformed(S,Sr)-richmonazite-(Ce),typeII(MnzII)inassociationwithhingganite-(Y)and“hingganite-(Nd)”(Hin),chlorite(Chl),muscovite(Ms)andalbite(Ab)enclosed inhydrothermalquartz–albite–chloriteveinlets.


Fig. 3. Compositionofmonazite-(Ce)fromtheBacúchmagnetitemineralization in plot ofCa +Sr + S versus REE +Y + (P+As) normalized to 2 apfu,with ideal cheralite Ca(Th,U)REE–2(dashedline)and(Ca,Sr)S(REE,Y)–1(P,As)–1substitutionvectors(solidline).


normalizedplotoftheREEgenerallyshowsanalmostflatpattern,withincreasefromLatothemiddleREEand a subsequent slight decrease toLu (Fig. 6).Thecontentofotherelements, suchasU,Ti,Al,MnandMg, is very lowor below the detection limit of theelectronmicroprobe.The composition of theBacúchhingganiteismainlycontrolledbythegadolinite-typesubstitution [Fe2+O2□–1(OH)–2],whereas a datolite-

typesubstitution,alongtheCaB(Y,REE)–1Be–1vector,isvery restrictedowing to a lowCacontent inhing-ganite(<0.13apfu,Table3,Fig.7).

DiscUssion

Monazite-(Ce) represents the dominant LREEphosphatephaseof theBacúchmagnetite ore assem-

Fig.4. BSEimageandcorrespondingX-raycompositionalmapsofCa,S,P,SrandCeformonaziteI(mnzI)andmonaziteII(mnzII).MapsshowrelativeenrichmentofS,Sr,CaanddepletionofP,CeinmonaziteII.TheinversetrendisobservedformonaziteI.


blage (Fig. 2). Primary accessorymonazite-(Ce) I tomonazite-(Nd) I forms large subhedral to euhedralcrystalswithachemicalcompositionclosetotheidealLREEPO4endmember, with a highTh content insomeareasdefiningamonazite–cheralite–huttonitesolidsolution(e.g.,Bea1996,Förster1998).Asecond

generation usually forms anhedral and fine-grainedirregularmonazite-(Ce)IIaggregatesorrecrystallizedrimsontheprimarymonaziteI.Ananalogousformofsecondarymonazite of hydrothermal originwas alsodocumented bySchandl&Gorton (2004),Krenn&Finger(2007),andZych-Habelet al.(2007).

The variation inmonazite composition is drivenby isomorphous chemical substitutions.The simplestone involves the replacement of oneREE3+ (orY3+)cation by anotherREE3+ (orY3+) cation.The larger,nine-fold-coordinatedLREEcationsaremorefavorablyincorporatedintothemonoclinicstructureofmonazitethan in the smaller, eight-fold-coordinated sitewellsuitedfortheheavyrare-earthelementsandY(HREE+Y),whicharedominantinthetetragonalstructureofxenotime(Niet al.1995).TheenrichmentinNdorLawasexplainedinrelationshiptotheformationofmona-ziteinastronglyoxidizingenvironmentandhighpH,causingthehydrolysisofCe4+andsubsequentremovalofmost of theCe from the solution (Demartinet al.1991).Consequently,thepresenceofNd-richmonazite-(Ce)Itomonazite-(Nd)IinassociationwithmagnetiteatBacúch indicatesahigh fugacityofoxygenduringitsprecipitation.Inaddition,theelevatedEucontentinmonazite I (<1.9%Eu2O3)and the lackofanegativeEuanomaly inchondrite-normalizedpatterns (Fig.5)mayreflect thepresenceofEu3+,whichcanenter thestructuremoreeasilythanthelargerEu2+.Thispossiblypoints tomonazitecrystallizationunderhighfugacityofoxygen(Krennet al.2008).

ThemonaziteIIfromBacúchrevealsthehighestSandSrcontentsreportedinnature(<11.3%SO3,<0.31

Fig. 5. Composition ofmonazite-(Ce) from theBacúchmagnetitemineralization inCa+Sr versus S correlationdiagramnormalizedto2apfu.Thestraightlinerepresentstheideal1:1ratioin(Ca,Sr)SO4.

Fig.6. Averagechondrite-normalizedREEpatternsofmonazite-(Ce)fromtheBacúchmagnetitemineralization.ChondritevaluesfromTaylor&McLennan(1985).


Fig.7. Classificationplotofthegadolinite-groupmineralsX□/ X(□+Fe)versusW(Y+REE)/ W(Y+REE+Ca).

Fig.8. TriangularY–Ce–NdplotshowingtheoccupantsattheWsiteinhingganite.


apfuS;Sr<8.7%SrO,0.18apfuSr).Sulfurshowsapositive correlationwithCa+ Sr (Fig. 5), but doesnotcorrelatewithSicontent.There isalsonegligibleTh content (Table 1, Fig. 4).Therefore, the substitu-tion (Ca,Sr)S(REE,Y)–1P–1 or (Ca,Sr)2++ (SO4)2– $ (REE,Y)3+ + (PO4)3– is interpreted as the dominantmechanism (Fig. 3).The synthesis of amonoclinic,monazite-typeCaSO4 polymorph (“clinoanhydrite”)at2 to28GPaandroomtemperature(Borg&Smith1975,Crichtonet al.2005,Bradbury&Williams2009)indicatesthepossibilityofincorporatingCaSO4withinthemonazitestructureinsignificantamounts.TheroleofSrTh(REE,Y)–2(Chakhmouradian&Mitchell1998)orSiSP–2(Cresseyet al.1999,Jercinovic&Williams2005)substitutionsisnegligibleowingtoverylowSiandThcontentsinthemonaziteIIatBacúch.ElevatedcontentsofSandSr innaturalmonazite(<8.5%SO3andup to<8.3%SrO)and the (Ca,Sr)S(REE,Y)–1P–1mechanismofsubstitutionhaveonlybeenreportedfromafewlocalities,includingcarbonatiteoccurrencesintheKolaPeninsula,Russia(Kukharenkoet al.1961,1965,Chakhmouradian&Mitchell1998),calcitekimberlitefromInternatsional’naya,Yakutia(Chakhmouradian&Mitchell 1999), garnet-rich high-pressure rocks fromtheBohemianMassif (Krenn&Finger2004),A-typerhyolitefromTisovec–Rejkovo,Slovakia(Ondrejkaet al.2007),andore-bearingmyloniticmicaschistsandapliticmaterialfromtheSchellgadenminingdistrictinAustria(Krennet al.2008).TheprecipitationofsuchanomalouslyS-andCa(Sr)-richmonaziteattheabove-mentionedsitesisinterpretedasaproductofsecondaryandmainly hydrothermal alteration of the primarymagmaticrocks,oritsformationasaresultofplagio-clasebreakdownduringhigh-pressuremetamorphism.ThetexturalappearanceoftheBacúchsulfatian–stron-tianmonaziteIIalsosupportsitssecondaryoriginaftertheformationofprimary,S,Sr-poormetamorphicmona-ziteI.Thispresumablyoccurredduringayounger,mostlikelyAlpinetectonicandhydrothermaloverprintoftheparentalrock.Theyoungerhydrothermalveinletswithsulfidemineralizationweremost likely formed fromH2O–NaCl–KCl-typefluidsoveratemperatureintervalof100–250°Candasalinityof9–24wt.%equiv.NaCl,aswaspreviouslydeterminedinsimilarhydrothermalmineralization in the surrounding areas (Pršek&Chovan2001).ThismineralizationisproposedtobeasourceofSformonaziteII formation.TheextremelylowThcontentinthemonaziteIIalsosuggestsgrowthunder hydrothermal conditions (Schandl&Gorton2004).Moreover, the enrichment in S coupledwiththerelativedepletioninREE,mayindicatealowpH,atleastduringthegrowthofmonaziteII(Fulignatiet al.1999).

ThehighSrcontentcorrelatedwiththeenrichmentofEu2O3,upto2.7%,wasinterpretedasthebreakdownof plagioclase under greenschist-facies conditions(Krennet al.2008).However,monaziteIIisSr-enrichedanddepleted inEu.The elevatedSr content together

withCaenrichmentmaybeconsideredtobetheresultofhydrothermalleachingofSrandCafromCa-bearingplagioclase,withsimultaneouscrystallizationofalbiteduringtheAlpineoverprint.

The chemical compositionof the gadolinite–dato-lite-groupmineralscanberepresentedinthequadrilat-eralsystembygadolinite(R3+Fe),hingganite(R3+□),homilite (Ca2+Fe) and datolite (Ca2+□) components(Fig. 7; Pezzotta et al. 1999).Hingganite can formby substituting in gadolinite theFe vacancy through□(OH)2Fe–1O–2 (Burt 1989).This substitution trendisthemostinfluentialonthechemicalcompositionoftheBacúchhingganite.Here,thehingganitehasaverylowCacontent(mostly<0.01apfu),indicatingaverysmallamountofthedatolitecomponentinthemineral.Hingganite-(Y)andgadolinite-(Y)mostlypreferREEwithionicradiiclosertoY(Sm–Tb),whicharemainlyDyandGd(Demartinet al.1993).However,ourhing-ganite compositions showadistinctive enrichment inNd(0.42to0.57apfu)and,insomecases,Nd-dominanthingganitewasdetected(Table3,Fig.7).Unfortunately,thesmallsizeandtheintimateintergrowthswithassoci-atedmineralsprecludestructuralanalysisanddefinitionasanewmineral.Consequently,theNdcontentsoftheBacúch hingganite are the highest so far reported asnaturally occurring, and they are significantly higherwhencomparedtootheroccurrencesofhingganiteorgadolinite (e.g.,Demartinet al. 1993,Pezzottaet al.1999,Miyawakiet al.2007).

Gadolinite-groupminerals andmonazite from theveins and veinlets have been described in variousmetamorphic rocks in theAlps, and the surroundingrockswere interpreted tobeasourceofBeandREE(Demartinet al.1991,1993).Texturalaswellascompo-sitional evidence indicates at least twomain evolu-tionarystagesofREEmineralizationatBacúch.(1)TheoriginofmonaziteIandxenotimeisrelatedtoanolder,probablyHercynianmetamorphicevent,togetherwiththeformationofmagnetite.(2)Thepartialdissolution–re-precipitationofmonaziteIandxenotimeandtheformationofsecondaryS-andSr-richmonaziteIIandNd-richhingganiteoccurredduringtheyounger,Alpine(Cretaceous) tectonometamorphic and hydrothermaloverprintoftheBacúchmagnetitemineralization.

acKnowlEDgEMEnts

The authors thankDanielOzdín,PatrikKonečný,IvanHolický,RadekŠkodaandPetrGadasforelectron-microprobeassistance,andRayMarshallforareviewoftheEnglishcontent.ReviewsbyMichaelL.WilliamsandDanielE.Harlovgreatlyimprovedthispaper.ThisworkwassupportedbytheSlovakResearchandDevel-opmentAgency under contractNo.APVV–0557–06andbyresearchandequipmentgrantsfromtheSlovakagencyVEGA,grantnumber1/1048/07.B.BudzyńisafellowoftheFoundationforPolishScience(STARTprogrammein2008–2009).


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Received December 15, 2008, revised manuscript accepted January 5, 2010.

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