537

TheC anadian M ine rala g h t Vol.33, pp. 537-545 (1995)

ZONEDZN.RICH CHROMITE FROM THE NAATANEMI SERPENTINITE MASSE, KUHMOGREENSTONE BELI,

ruSSI P. LIIPO, JOUNI I. YUOLLO, VESA M. NYI(ANEN N{D TAT]NO A. PIIRAINEN Departnwntof Geolngy,University of Oulw FN-90570 Finland

ABsrRAcr

Chromitefrom the ArcheanN2iilnniemi serpentinitemassif, Kuhmo greenstonebelq Finland, exhibits high concentrations of Zn, up to 4.4 wtVoZnO. The chromiteis zond optically and chemically;the best-preservedchromite corescan be divided into two groupson the basisof their composition.Type-I chromiteis interpret€das a relict core,with Cr(Cr + Al) valuesfrom 'ferrircbromit''; 0.58 to 0.76 and Mg(Mg + Fd+) valuesfrom 0.53 to 0.60; the core of type-tr cbromite has beBnalt€red to M/(Mg + Fd+) valuesrange from 0 to 0.37. In the core of type-I and -tr chromite,Zn showsno correlationwith the other divalent cations (Mg, Mn and Fe2*). On the basis of their Zn content,type-I and type-tr cbromite resemblethose examples reported from metamorphosedvolcanic-type iron-nickel sulfide ores from Western Austaliq though the suite of type-I chromitefrom the N66tltniemiserpentinite massif displays higher Mg (12.1 w17oMgO) and A1 (20.9 wt-VoN2O) contents.

Keyvords: cbromite,zoning, serpentinite,Archean, Kubmo greenstonebell Finland.

Somr{ens

la chroniitq du massifde serpentinitede Ndiiriiniemi,d'0ge archden, de la ceintue de rochesvertes de Kuhmo, en Finlande, contientjusqu') 4.47o(en poids) de ZnO. La chromiteest zon6e,optiquement aussi bien que chimiquemenl Certainsdes coeurs les mieux pr6serv6sont une compositioninterpr6tde comme relique magmatique(type D, avec desrapports Cr(Cr + Al) enfo 0.58 et 0.76, et Mg/(Mg + Fe2) entre 0.53 et 0.60. D'autres(dits de typ€ IIl ont une compositionhansform6e, conespndant maintenantI la 'Tenirchromit", et le rapportMg/(Mg + Fef va de 0 i 0.37.Dans le coeurdas deux types de chromite,la teneur en Zn ne montre aucunecorrdlation avec celle desaures cationsbivalents (Mg, Mn et Fez+).Les teneursen Zn destypes I et tr rappellela chromite desminerais I Fe-Ni mdtamorphis6set d'origine volcaniqueen Aushalie occidentale,quoique la chromite de type I de NAftiiniemi possbdeune teneurplus 6lev6een ME 02.lTo de MgO par poids) et en Al (20,9VoAJ2Q),

(Traduit par la R6daction)

Mots-cl6s:chomit€, zonation,serpentinite, arch6en, ceinture de rochesvertres, Kuhmo, Finlande.

INrRoDUcnoN stone belt of Archean age, in easternFinland. The chromite occurs as optically and chemically zoned Chromite rich in Zn is consideredunusual enough cumulategrains in Archean serpentinitesand exhibit to be reported in the literature wheneverit is found. high concentationsof Zn. Z*ich spinel has been reported from different geo- logical settings (Thayer et al. 1964, Weiser 1967, Locmou ANDGEoLoGIcAL Se'ruDlo Groves et al. 1977, 1983, Moore 1977, Bevan & Mallinson 1980,Wylie et al, L987,Bjerget aI. 1993, The Kuhmo belt in eastemFinland (lao:palaet al. HalkoahoL994),andaZncontentin excess of 0.5wt% 1980,Piirainen 1988) is a typical Archean greenstone ZnO is consideredsignificant (Groves et al.1983).ln belt surroundedby granitoid rocks. It forms a narrow most cases,hlgh Zn contentin cbromite is causedby composite north-trending belt approximately 20 km metasomatism;only in the example describedfrom wide and 200 km long. The belt, which is one of sequencesof ultamafic flows in Westem Australia the most intensively investigatedArchean belts in the (Groveset al. 1977,1983)is the Zn consideredto be Fennoscandianshield Cfaipale et aL 1980, Taipale inheritedfrom tle magmaticstage. 1983,Piquet 1982, Piirainen 1988, Luuktonen 1992), This paper discussesaccessory chromite from the can be divided into three parts, namely the southem Ntiiitiiniemi serpentinilemassf, in the Kuhmo green- Qipasjiirvi), middle (between Kuhmo and Suomus- FENNOSCANDIANSHIELD

uomussalmi /*

- Middleto lateProterozoic X cover g TransscandinaManGran- l-,1 ip-P6pfiyryBsll @l@ Earlysqr'' Proterozoic' rocks I StudY U area ll Archaeandomain N

Ultramaficand fi mafic volcanic rocks T Felsicvolcanic rocks N6ifittiniemi ffi

Serpentinite

Mica schist

Gneissand tt* metasedimentary rocks

Fto. 1. Geologicalskerch map of the Kuhmo greenstonebelt, showingthe locationsof Niiiitiiniemi se4rentinitemassif. Modified afterHanski (1984). ZNCIAN CHROMITE, KLIHMO GREENSTONEBELT 539 salmi),and northern() areas (Fig. l). It CnEItrcALCowosrnoN oF TI{ECHRoMTE has been dated at between 2.99 (Vaasjoki & Sakko 1991)and 2.75 Ga (Vaasjokiet al. 1989)and is com- Electron-microprobeanalyses were performed on posed principally of volcanic rocks and subordinate compositionally zoned grains of chromite from the volcano-sedimentaryand sedimentary rocks, which Niiiitiiniemi serpentinite massif. A representative have been divided into three lithological units. Each samplingof fhe 232 analysesis presentedin Table 1. unit is characterizedby its own volcanic series: The chromite has been extensively atalyzed to cover 1) andesite(Luoma group),2) serpentinite- tholeiite - the fuIl range of variations from aluminous chromite komatiite - basaltic komatiite (Kellojiirvi Group), through"ferritchromit" to chromianmagnetite. Table I and 3) rhyolite - rhyodacite - Fe-rich tholeiite - srrntnutrizesrelevant characteristicsof the chromite komatiite - basaltic komatiite (Ontojiirvi Group) $ains. (Taipaleet al. 1980).The Kellojiirvi group is cross-cut The mineral analyseswere performedwith a wave- by a dyke of Fe-rich tholeiite dated at 2.79 Ga length-dispersionJEOL JCXA 733 electron micro- (Luukkonen1988). probe equippedwith LINK AN10 automationat the The ulnamafic rocks have a multistage history, Institute of Electron Optics, University of Oulu. The affected by at least tbree phasesof defonnation and accelerationvoltage was 15 kV, the sample current, fwo phasesof intrusion of granitic magma and are 15 nA, and the spot size, 2 pm. The following composedpredominantly of serpentinite,having been standardswere used: syntheticrutile for Ti, synthetic deformedand metamorphosedunder conditionsof the corundum for Al, hematite for Fe, periclasefor Mg, greenschistor lower amphibolite facies @iquet 1982, andpure metallic standardsfor the remainingelements. Taipale1983, Blais & Auway 1990,Tuisku & Sivonen The practical limit of detection in these routine 1984). analysesvaries from 0.01 to 0.05 wtvo. Resultswere The serpentinitesin the ArcheanKubmo greenstone correctedwith an on-line ZAF program. Ferrous and belt have been consideredto be metamorphosed ferric iron were distributed on the basis of stoichio- cumulatesof the komatiitic series@lais et al. 1978), metry by the methodof Carmichael(L967). but on the basisof the high valuesof the (LalSm)ryand (Sm/Yb)N ratios and high TiO, contents, the Compositionalvariations in the core Niiiitiiniemi serpentinitesdiffer from typical ultramafic rocks of komatiitic origin (Hanski 1982, 1984). An The core of each preserved grain is iregularly ophiolitic nature of this serpentinitemassif has been shapedand grey in reflectedlight. It is optically homo- suggestedby Piirainen(1991). geneous,free ofinclusions andhas a sha4lcontact with a lighter grey rim, whereasin thosegrains that aremore Pr"rnocnepnv intensely altered,the grey core has disappeared;such grains can be classified as chromian magnetite or The samples used in this investigation were magnetite. selected from those of the ResearchProject on In terms of composition, the best-preservedcores Archean Areas (Piirainen 1991), and from outcrops may be divided into two groups.Type-I chromitehas a of the Niiiitiiniemi serpentinitemassif. The Niintiiniemi Xr,a"Mg/(Mg + Fez*)l value gteaterthan 0.4 andaXg, massif is located in the middle sub-belt and consists lCr(Cr + Al)l value between0.58 and 0.76. In type-tr of Archean serpentinites cross-cut by an early chromite,the core hasundergone more profound alter- Proterozoicdifferentiated dyke datedat2.2 Ga (Hanski ation, and is depletedin Mg and A1, and enrichedin 1984). Mn, Fe2+and Fe3+(Xoan from 0 to 0.37) comparedto 'feritchromif' The serpentinite is dunitic in composition; the type-I cbromite.It can be classifiedas primary mineralsother than chromitehave disappeared (fable 2). From an exploration viewpoint, the most completely.Olivine @oe6)and clinopyroxeneof meta- significant compositionaloddity aboutthe core zoneis 3.74wtVo morphic origin occur as relict grains in rare samples. its consistentlyhigh Zn content,from 2.18to 'n The dominant minerals of the serpentinite,antigorite in type-I chromite and from L.47 to 4.36 vttVo and lizardite, formed at the expense of olivine. type-tr chromite. Accessory minerals include magnetite and geikielite Compositional variations in spinel have generally (l,id.poet al. 1994). beenrepresented on bivariateplots ofXyn versusXlyot Chromite is ubiquitous as an acces$orycumulus XF"[Fe3*/(Cr + Al + Fe3*)](rvine 1965,1967,Dick & phase in all the samples analyzed. Most of the Bullen 1984). On such diagrnms, chromite from cbromite occursin the form of disseminatedeuhedral komatiites and stratiform intrusions cannot be dis- to subhedralgrains 20 to 400 pn in diameter.These criminated,though cbromitefrom komatiitesplots in a are zoned, with a homogeneouscore surrounded different field from cbromite in ophiolitic complexes. by an inner and an outer rim Gig. 2). This zonation Type-I cbromite plots in the overlapping field of is defined by optical and compositional differ- ophiolites and stratiform intrusions, and differs from ences. chromiteof komatiitic origin, whereastype-tr chrcmite 540 TIIE CANADIAN MINERALOGIST

""s j;/

Ftc. 2. Back-scatteredelectron images of the zoned grains of chromite from the Niintiiniemi serpentinitemassif. A) type-I chomite, and B) type-tr chromits. Lines show the locationsof the step-scanprofiles. Scalebar 10 pm in A and 100 pm in B. plots mostly outsidethose fields (Frg. 3). Fe3+,Fe2+, Mn, and Ti, and depletedin Mg, Cr, Al and Za. T\e outer rim has a higher reflectivity than the Intragrain compositionalvariations inner rim and consists of chromian magnetite. The outer rim is enrichedin Fe3+and Fe2+.Both inner and Euhedral to subhedral crystals of cbromite are outer rims contain small inclusions of secondary characterizedby an inner rim, which propagatesalong silicate. fractures,reflected in optical and compositionaldiffer- Step-scanprofiles from type-I chromite (Fig. 4) and ences.The inner rim showsa higher reflectivity than type-tr chromite (Frg. 5) revealthat the chemicalcom- the core and is light grey in reflected light. position of the core zonesshows only slight variations, Compositionally, it is classified as 'Territchromit". mainly restricted to its external border; important Comparedwith the core, the inner rim is enrichedin chemical changes occur within the inner rim. Zn ZNCIAN CHROMITE, KUHMO GREENSTONE BELT 541

TABI.EI. SUIIMARY OF GARAEfERATICS OF CEROI'ITBCRATNS DISCUSSION FROMTAB NAATI{NMfl SERPENTNiTBMASSIF, KTJEMOGRg{fiANB BB.T. FINI.AND Chromite from serpentinitescan exhibit a wide G@no Cholte l@rh @(do Tlpel ..tlpotr range in composition, which reflects differences in metamorphicconditioris & Frost 1975). The l|leal ce@fs cb@llg @gletlg osg!€dtg @vans Col@ IhcLgFy &rf gfty cFy rrghgly compositionof type-I chrcmiteplots in the overlapping Sapo Ainodnl AlMal Anbdnt .Aons&al fields Islud@ No No srn, roelt C@E@ly of statiform andophiolitic chromites,with inner rims ('Terrircbromit'), outerrims (chromianmagnetite) nO2 Vl o35 (169 0'56 02' AlrO3 m& 13.06 (I3E O@ and type-tr chromite following a regular path toward (}A !+t63 4,9 ?38 9.&t the magnetite and ferroan chromite corner 3). F%93 taf 7t2 ng fl.Q @ig. F€O 1455 n:n 2521 AXt This is in accordancewith an alteration trend estab- MnO o79 2-t4 l-55 0.i18 Mgo n.a) ls4 2,59 L.4t lishedby many @liss & Maclran 1975,Evans & Frost ?ao 2& 1J5 0i8 oxl 1975, Hoffman & Walker 1978, Kimbdl 1990). Hoftuan & Walker (1978) observedthat the thickness lts c@eocil@ Eeqted @ @ yrlB dt6 @lyK nE pqqdm of and composition of the chromian magnetite and Fo" @d Fe" @ a[!t&d saqdiug b dddffi.{rt. magnetite rims are correlated inversely with Fe3+ content of the core chromite and positively with the extent of serpentinizationof the ultamafic rocls. In type-I chromite,Fe3+ content ranges from 0.13 to 0.73, and in type-tr chromite, it rangesfrom 0.28 to 3.68, reachesits maximumvalue at the edgeof the core and though the thicknessand compositionof the rims A decreasestoward the contactwith outer rim, wherethe andB in type-Iand -tr chromitesare simil6l (Pigs.2,4 level of all the cationswith the exceptionof Fe2+,Fe3* and5). and Cr approachzero. On the basesof reasonablyhigh Xrurnand Xq, and The Zn conlentof the coresshows only slight varia- low Fe3+values, type-I cbromitemay be"interpretedas tions, andthe differencebetween the maximumcontenr a relict magmaticcore, only slightly modified by meta- in the edge of the core and the averagevalues in the morphism. In compmison with magmatic spinel in coreis small,below 1 wtVo.Inthe coresof type-I and komatiitesreported earlier from the Kuhmo greenstone type-tr chromite, Zn shows no correlation with the belt, Finlan4 which have beensuggested to be closest other divalent cations(Mg, Mn and Fef. Decreaseof to a possible primary composition (Blais & Auway Mg conelateswith increasein Mn and Fe2+. 1990), and with other occwrencesfrom serpentinized Archean ultramafic cumulates (Amdt 1977, Amdt et aI. 1977,Nisbet et al. 1977,Groves et al. 1977, 1983,Donaldson & Bromley 1981),type-I chromite massif TABII Z BBPBBSBNTATIVBNESULTS OF BACTRO,I-MI@OPROBB A!{ALYSBS from the Niiiitiiniemi serpentinite hashigher Mg @ moullts FRou ISBMATAIWI mTBtTNIIB MAssIF. (L2.1 wt%oMgO) and N (20.9 wt%oAl2O) contents, KTJUMO@EMIT(}TB BSLT.TNIAT{D resemblingthose from ophiolitic cumulates.However, on fhe basis of their Zn content, type-I and type-tr srrylo 9A 9A __L_ 23 1362 _L chromites resemble those reported from meta- morphosedvolcanic.type iron-nickel sulfide oresfrom d% western Australia (Groves et al. 1977" 1983. 110, o3i 0.51 0J0 0.|2 (L09 |I72 2.8 t-lt 7J/2 oJr, Al2or zL98 2039 19.90 (I4r 0.01 13.6 ().& 1006 (In qll Donaldson& Bromley 1981). azQ i16.15 46.15 4715 40.o s,s 432, &35 48.61 3&75 565 F.rq 25 3J4 2J0 28.rAt A.A &69 9.12 6tu A8 6L4' BO L4:t2 L424 L4$:l 22.* /r35 n6 A.g AAr 2r:r' {.76 Zinc contentof chrornite lrlo &89 oet o74 L4, (L28 zSL Zrl Un 2.s 0ll9 lrp 1132 1139 12.10 4.48 2n2 ztu 7XL 2.6 ott 0.05 !.(o o24 0.10 0.$ o.4 ll:' 0.15 0.:7 0.X2 0.4 (Is6 zfi J"48 3A L6 1.08 0.11 223 4;t6 3d$ 2J1 0.m The chromite in the Niiiitiiniemi serpentinitemassif T@l iln7, (n11 946 9069 99.4!1l(n{' 1m.1:' 1m39 1fl160 9.67 contains anomalouslybrgb Zn contents.Similar high tftobd of tod8€lslrled o 32 (O) bsle Zn contents occur in "ferrocbromit" associatedwith o10&) 0.@62 oJE60 0.0918 otDr5 ct& 0it9 0.2t18 oat 0gt6s nickel deposits in Western Australia (Groves et al. AI &L622 6i2k 5,Ct63 01466 0.OD5 44176 3n4B 3J@4 o,trt OJIi4l C. 9J861 9.1458 9.gn0 9.$d' 1r83r 93914 9J856 rO?14 9.142 LsTn t977, L983,Donaldson & Bromley 1981),where the r€r" UUn Os7tt 03962 6A, 14489 7:rW ts2e l.@2 6.@89 14.,1t4 &,2r 3.0dr6 2.98tr 2S!B 352L6 6.W 62Kt &${' 59138 6ttX2 79tn high Zn content is interpreted as primary magmatic ld! otg,l o17F oltl 03dt8 ut7zl 0.5855 05634 lltut 07163 ut3E }[8 4,J16 4liEX 4.5206 1Jtt3 09169 0&66 07616 1.166 ttStt (uIZn feature. l{ 004€6 afn9t o(zrl 01rD8 02157 0BA (urm lt.@n 0.u54 0.89t Step-scanprofiles madeon the chromitegrains from h 0.t{06 (l.60m a$n 0437 o@l 0.4617 0gD9 (L67n o4647 ofim the Niiiitiiniemi serpentinitemassif show that the Zn Xu" otltr t}5919 G6(5t orrft o.11dt 0116J oro78 (r1t5 urKl 0.fl8 )(G' 0.5961 0.dr9 oll79 0 L4 0U63 06gP 07316 0.7645 0.9nD 09!86 content of the core is constantand decreasesin inner xe c@83 (}.00ar 0&51 03816 0.99t1 0.u52 (LtS4 0.0910 0.3$5 09120 and outerrims. ConstantZn contentof the coremay be a result of contaminationby S-rich sedimentaryrocks, 4eE!4gf6dg+ Fot),Xc- Cd(Cr+AD,Xe.F€$(tts +G+ Al). ft !s'!tu cd Fd. ed Fs& @ alfrd oocddhg b dddi@t. ftm /qr h!6 dB d! B: @ rh. recrystallizationduring metamorphismor inheritance 542 T1IE CANADIAN MINERALOGIST

1.0 1.0 cf) o IL + + O Y 0.8 -|. 0.8 O o (.) o tL 0.6

ophiorites.>S

0.0 1.0 0.0 1.0 0.0 Mg/(Mg+Fe') Mg/(Mg+Fe')

Ftc. 3. Compositionof chromiteplotted on projectionsof the spinelprism of Stevens(1944). The fields for the various6pes of complexesshown in A (Xg, ver.rrrrXl,4r) and B (Xu"versus Xyln) are basedon Irvine (1967),Bfud & Clark (1976),Dick & Butlen (1984) and Zhou & Kerrich (1992).Arrows indicatebehavior expected as a result of alterationinvolving a decrease in Mg first and Al afterward.

from the magmatic stage, as noted by Groves et al. 3) At other localities noted for their Zn-enrichment (1977, 1983).The following facts supporta magmatic in chromite, metasomatismis clearly indicated and origin of the hrghzn contentof the chromite,although related to 7n-beaing mineralization (6ly7ie et al. crustal contaminationof magmais an equally possible 1987) or ores (Thayer et al. l9&, Weiser 19671, explanation, as pointed out by Irvine (1965) and whereasat Niiiitiiniemi, no significant sourcesfor 7n Groveset aI. (1917). are known to occur. This could be an indication of the l) In many cases,an increasein the level of Zn possible existence of mineralization in the area, correlateswith a decreasein Mg content(Y,lyhe et al. however. 1987, Bjerg et al. L993), whereasthe cores of type-I chromiteshow no correlationbetween concentration of CoNcr,usIoNs Zn and of other divalent cations.Type-I chromite also has a moderateto high contentof Mg and Al, together The optical and chemical zonation of the zincian with a high Zn content. chromite from the Archean Niiiitiiniemi serpentinite 2) Grains of type-tr chromite were affected more massif is interpreted to result from intense meta- intenselyby the serpentinizationprocess, having a core morphism and several phases of serpentinization. composedof 'ferrirchromit". T\e Zn content of the Despite the serpentinizanonothe core of type-I type-tr cbromite reachesup to 4.36 wt7o,which is not chromite has retained a primary magmatic signature, significantly higher than that in fype-I chromite. This as demonstratedby its X", and Xr* values.The high suggeststhat there is no enrichmentof Zn during the 7n content of the chromile ls inhefited from the alterationprocess. magmaticstage, which is demonsEatedby the gradient ZNCIAN CHROMIE. KT]HMO GREENSTONEBELT 543

:A Core s * 3 3 o o p "lt oX ox

il 14 x 3 3 o o c p 12 x ox o 10

' MgO eho

Distance in micrometers Distance in micrometers

Flc. 4. Compositionalvariations from outer rim to outer rim Flc. 5. Compositionalvariations from the centerofthe core in one grain of type-I cbromite from the Ndiitiiniemi to outer rim in one grain of type-tr chromite from serpentinitemassif. A: inner rim, and B: outer rim. the Niiiitilniemi serpentinitemassif. A: inner rim, and B: ouler rim.

in Zn contentin the chromite,with a unifonnly brghzn Mr. M. Hicks for correcting the English manuscript. content in the core. and a decreasetoward the outer Financial support was provided to J.P. Liipo by rim. the Foundation for Researchin Natural Resources in Finland. An earlier version of the manuscript Acnqowtpcmcnns was contructively rewiewed by D.J.M. Burkhard and S.J. Bames, and the presentversionn by S.J. The authorsthank the Researchhoject on Archaean Bames and one anonymousreferee. We thank them Areas, University of Oulu, for providing the samples, for their useful comments, which improved the the Institute of Electron Optics, University of Oulu manuscript.Comments by R.F. Martin are gratefirlly for providing the electron-microprobeanalyses, and acknowledged. 544 TI{E CANADIAN MINERALOGIST

Rmmmqcrs HAIroAso, T. (1994): Thz Sompujdni andAla-Penikl

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