32'7

The Canadian M ine ralo gist Vol. 32, pp. 327-332 (1994)

GEIKIELITEFROM THE NAATANIEMI SERPENTINITE MASSIF, KUHMOGREENSTONE BELT, FINLAND

ruSSI P. LIPO, JOUNI I. WOLLO, VESA M. NYL{NEN ANDTALINO A. PIIRAINEN Departmentof Geology,University of Oulu,SF-90570 Oulu 57, Finland

Arsrtecr

Geikielite has only occasionallybeen reponed as an accessoryphase in metamorphosed,hydrothermally altered mafic and ultramafic rocks. This paperrepons a new occurence,in serpentiniteof the Niin$iniemimassif, in the Kuhmo greenstonebelt, easternFinland. Electron-microprobeanalyses show that its compositionranges from IlmroHem2GeiusPyhtrto Ilm2aGeir6Pyhto.The geikielitein theserocks is richerin Mn thanthat previously reported in serpentinizedultramafic rocks.

Keywords:geikielite, serpentinite,Niiiitiiniemi, Kuhmo greenstonebelt, Finland.

SoMMARS

Il est assezrare de rencontrerla geikielite commephase accessoire dans les rochesmafiques et ultramafiquesqui ont subi des effets mdtamorphiqueset m6tasomatiques.Nous en d&rivons un exemple,d'une serpentinitedu massif de Niiiitiiniemi, dansla ceinturede rochesvertes de Kuhmo, dansla partie orientalede la Finlande.l,es analysesi la microsondedlectronique montrentque sa compositionvarie de llm2sFlem2Gei66Pyh,si Ilm2aGei56Pyh26. La geikielite dans ces roches est enrichieen Mn comparativementaux compositionsdocumentdes antdrieurement dans les rochesultramafiques serpentinis6es. (Traduit par la R6daction)

Mots-cl6s: geikielite, serpentinite, Ndiitiiniemi, ceinture de roches vertes de Kuhmo, Finlande

INTRoDUCTIoN Geikielite hasonly occasionallybeen reported as an accessoryphase in metamorphosed,hydrothermally The end membersthat comprisethe ilmenite series altered mafic and ultramafic rocks. This brief report in igneousand metamorphicrocks are ilmenite providesadditional information on its chemistryand (FeTiO3),hematite (Fe2O3), geikielite (MgTiO3), mode of occurrencein theserocks. In the courseof pyrophanite(MnTiO), and ecandrewsite(ZnTiOr) our researchon chromianspinel (Liipo et al. in prep.), (Waychunas1991). Significant solid-solution minor amountsof geikielite were discoveredin existsamong the following end-members:1) between Archeanmetadunitic serpentinite from the Niiiitiiniemi ilmenite and hematite (Ilm-Hem..), 2) benveenilme- massif.This is the first reportedoccurrence of nite and geikielite (Ilm-Gei,,), 3) betweenilmenite geikielite in Finland. and pyrophanite (Ilm-Pyh""), and 4) between ilmenite and ecandrewsite(Ilm-Eca.,) (Haggerty Locanon ANDGEoLocICAL SETTING 1976,Waychunas 1991). Geikieliteoccurs predominantly in kimberlites,as The Kuhmo greenstonebelt in easternFinland xenocrystsderived by the crystallizationof melts in (Taipaleet al. 1980,Piirainen 1988) is a typical the mantle (Haggerty& Thompkins 1984) and, less Archeangreenstone belt surroundedby granitic rocks. commonly,in carbonatites(Mitchell 1978),granulite- It forms a compositenorth-trending belt approxima- grade ferrous metapyroxenitelenses in gneisses tely 20 km wide and200 km long. The belt preserves (Scharbert1979), n lensesof retrogradedeclogite in upper-greenschist-faciesassemblages and can be gneisses(Pinet & Smith 1985,Smith & Pinet 1985), divided into three parts, namely the southern in serpentinizedultramaJic rocks (Dietrich et al.1986, (Tipasjiirvi), middle (between Kuhmo and Dymek et al. 1.988,Windley et al. 1989), and in Suomussalmi),and northern(Suomussalmi) areas marblein contactaureoles (Gier6 1987). (Fig. 1).It hasbeen dated at between2.99 (Vaasjoki&

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/32/2/327/3446667/327.pdf by guest on 03 October 2021 Ultramaficand mafic volcanic rocks

Felsicvolcanic rocks

I Serpentinite

Micaschist

Gneissand tl metasedimentary rocks

Frc. 1. Geologicalskerch map of theKuhmo greenstone belt. Modified afterTaipale (1983) and Hanski (1984).

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/32/2/327/3446667/327.pdf by guest on 03 October 2021 GEIKIELITE. KTJI{MOGREENSTONE BELT, FINLAND 329

Sakko1991) and2.75 Ga (Vaasjokiet al. 1989)and rs presentas an inclusion in magnetite,and as composite composedprincipally of volcanicrocks and, subordi grains with magnetite.Geikielite is grey under nately,of volcano-sedimentaryand sedimentary rocks, reflectedlight, and resemblesilmenite. On the basisof which havebeen divided into threelithological units. texturesand occurrence,chromite is interpretedto be Eachunit is characterizedby its own volcanicseries: the only primary mineral in this sample. l) andesite(Luoma Broup), 2) serpentinite- tholeiite- komatiite - basaltickomatiite (Kelloj:irvi group), and ANATYilCALPnocsPunss 3) rhyolite - rhyodacite- Fe-rich tholeiite - komatiite - basaltickomatiite (Ontojiirvi group) (Taipaleet al. The mineral analyseswere performedwith a 1980). wavelength-dispersionJEOL JCXA 733 electron- The ultramaficrocks have a multistagehistory, microprobeequipped with LINK AN10 automationat affectedby at leastthree phasesof deformationand the Institute of Electron Optics,University of Oulu- two phasesof intrusion of granitic magma,and are The accelerationvoltage was l5 kV, the sample composedpredominantly of serpentinite,having been current, 15 nA, and the spot size,2 'pm.A rangeof deformedand metamorphosedunder conditionsof the natural and synthetic standardswere used (Alapieti & greenschistor upperamphibolite facies (Piquet 1982, Sivonen1983). Results were corrected with an on-line Taipale1983, Blais & Auvray 1990,Tuisku & ZAF program.Ferrous and ferric iron were distributed Sivonen1984). on the basisof stoichiometryby the methodof Carmichael(1967). PsrnocRAPHY Owing to small grain-sizeof most of the geikielite grains, only analysesfrom grains I and 2 could be In the samplescollected from the Niiiitiiniemi ser- consideredsatisfactory, i.e., not affectedby adjacent pentinite massif,one sampleof serpentinitewas found minerals. to containseveral small (G15 pm) grainsof geikielite. This sampleis dunitic in composition,and character- Mnener CmursrnY ized by a metamorphicassemblage of lizardite and antigorite,and minor olivine (Fig. 2). Serpentinemin- Geikielite occursas small grainsin or nearthe mag- eralswere formedat the expenseof olivine, which is netite of the Archeanserpentinite (Fig. 2). The two extensivelyserpentinized, though minor amountsof largestgrains of geikielitefound in this investigation olivine are still present.Accessory minerals include are about 15 pm in size,and homogeneous(Figs. 2, magnetite,Fe-rich chromite and geikielite. Magnetite 3). Electron-microprobeanalyses of geikielite (Fig. a) occurs as anhedralgrains, and chromite, as subhedral revealhigh contentsof Mg and Mn, and lead to the grains,within serpentineminerals. Geikielite is compositionsIlm2qHe2Gei66Pyhts for grain I and Ilm2rHe2Gei5aPyhr2forgrain 2. (Table 1). On the Uasii of O aio-s 5f oxygen,Mg, Mn2* and Fe2+ amountto slightly more than 2 cations,and Ti is slightly lessthan 2 cations.Mg and Mn2+seem to t"ptu." Fe2*in the octahedralsite. No Fe3+-bearing phases,such as magnetiteand hematite,are present,

FIc. 2. Back-scatteredelecuon image of geikielite (grain 1) and other small grains of geikielite, and their textural relationship with associatedminerals. Symbols: GEI FIc. 3. Back-scatteredelectron image of geikielite (graln 2) geikielite, MAG magnetite,OL olivine, and SRP serpen- as an inclusion in magnetite.Symbols: GEI geikielite' tine.Scale bar: 50 pm. MAG magnetite,SRP serpentine. Scale bar: 10 pm.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/32/2/327/3446667/327.pdf by guest on 03 October 2021 330 TIIE CANADIAN MINERAIOGIST

TABLE !. REPRESET$ATTIIE@MPOTilIIONS OF GEIKIEUTE GRATNS, core rim MATANEMT SRPENm.{IE MAssF to is correlatedwith decreasein concentration of Mg, Mn, and Ji. An interestingfeature of the 1a x456 chemistryof the GEitr 1 Gnin 2 chromitein the samesamples as the geikielite is the high Mn contentfound in their core(up to 6.13wtTo MnO). ra2 57J6 Xfr s9.14 58.9 5&A fi,67 FeCJS 3I)1 2.77 0.m 0m 792 L79 The magnetiteenclosing the geikielite showsno FeO 1113 l^gl 13.09 1244 113 1039 differencecompared grains MrO 1007 rLyl 7037 1060 98 995 to other of magnetite l'lSO 1860 16J4 1667 16J0 18,9 1825 occuring in tfie samesample; it containsa relatively gzt Tdd tm36 99A9 98a1 9891 98.45 high level of Mg (Table2). Nmbq of iq bced o 6 at@s of qtg6 The main serpentiniteminerals are lizardite and 11.. 1.X87 1.ffi6 1.989t LXn L9431 Ls426 antigorite, the former conespondingto the lizardite tr Fdl 0J998 0.091:1 o(m 0.m 0&r describedbvBrais.ff:::t' F*+ 0.4tm o.4sn 0.487 0,67 0.42t0 Mn 03161 0.4545 0398 0.&5 03494 lvIS ^ 1.45 L1055 7.tt12 t.121 12115 M&/Rz+ 0.6m6 05493 05574 |Xy 0.61a lffi Molg Ipt€trt @d B@ber Geikieliteshould be expectedonly in extremely Ilaaite 79.92 X.98 u.x 8.4t m.n ts magnesianassemblages, in which Mgi(Mg + Fez+) HeBatite 2'42 Z,Z, 0.m otr 158 (Windley Caitielito 5938 53.70 55J4 5636 @]6 ;X [Xrr] in spinelis at least0.90 et al.1989) or 4trophuite 1816 n.@ 19.69 m2t n.a 18.6s olivine is more magnesianthan Fon, (Frost 1991).In the samplestudied, the Xrn value of the chromite !@R2+ - vd(vg+uotrl*) rangesfrom 0.26 in the coreto 0.08 in the outerrim, andolivine compositions range from Foq to Foe6. Becauseof extensivemetamorphism, which has obliteratedall primary textures,it is unclearif eitheras inclusionsor as exsolutionlenses in geikielite representsthe product of recrystallizationof geikielite. Calculationsof stoichiometrvindicate small an ilmenite poorer in Mg or a primary igneous amountsof Fe3*in thegeikietite. ilmenite solid-solution.Its occurrenceas an inclusion Olivine rangesin compositionfrom Fo9, to Fo96, in magnetiteand as compositegrains with magnetite, and is extensivelyaltered to lizardite.The relatively and its chemical composition,suggest that in these high value for MgO and MnO (Table 2) indicatesthat rocks, the geikielite is a result of metamorphicand the olivineis metamorphicin origin (e.g.,Piquet 1982, hydrothermalprocesses, as the Mn and Mg contents Blais& Auvray 1990). of primary ilmenite from mafic rocks are usually low, The chromitegrains are compositionallyzoned 0.1-2.3 wt%oMgO,and 0.1 - 1.4tvt%oMnO (Frost& from core to rim. The decreasein level of Cr from Lindsley1991).

TABIA Z REPR,ESENTATIVE@MP(NrIIq{ OFMAGIIBTITE.CHROMITE AND 'I|IIE OLIVINB GURRING IN SAMESAMPLE AS tr{E CMjTE TABIT 3. REPRSSENTAITSECQMPOSITIONS OF 56 GEIKIELITE REPORTEDFROM Magoclite Magoeiito Oltui@ METAMORPHOSEDULTRAMAFIC ROCKS ,iltvo siq TtOZ r.13 0,2, t63 o,ioi Y Tlo2 ciA tn ltl t7I5 1L44 3A x 5735 6LL @.ti F%o1 6635 61.X ,75 '/18 65.15 FeO 4.4 ?a92 $.14 16'01 FeO aAt 8@ 199J 2S9o ?9.3l 3.,t8 MrO 0.&l 232, L05 5fr MnO 05) fib 6.13 z4 0.4i 0.s MSO 11.48 11.42 2LF9 1894 Mso rJ9 tj/g 3& 2n L49 9Ja Totd 1mO5 1m3r 7m,21 [email protected] ZaO 0.15 0.m 2.9 056 0.m Total rm90 rcm4 1m.61 7ma 99.83 tols2 Nmber of iw bacd otr 6 at@ of 6!E@

Nmber of ioo barod oo 3a (circnir ad nqgndne) Ti 2-tM lWA zN/3 7.W2 &d 4 at@ (olivire) of clig@ F;z+ 1.1369 1.U70 054'9 05893 si Mn 0B2A 0.096 0.m& o.r&4 ogv Ms o25y 004yJ 03566 o.rw 00517 0.7890 0.786r L40n t2424 Ct 0.41tr/ 0.4979 8-!83 2.5,912 0.7D8 ufuPz+ o,ga 039f3 0Jw 0.6156 Fr 749t4 152;/a 652m 12308 74.m5 rJ* 7.3478 7.t5?9 4t596 6.& 7m ooeas Molc IEr@t ond mmber Mo 0.tx9 0.0{4 1J125 osa2 011(B 0t)196 MS LTWZ 0.7515 15886 l-(x}75 06n4 19185 Ilmelite 58.05 Xg2 tt.42 4.79 7^ oBn o.flm 0J03 o.7a o.dm Hma&e 0.m 0.m 0.m 0& XMg 0.0880 0.@1 02,,6/ 0,t352 0.mt 0.%55 @Keue 40,9 9.42 70.65 6rJ6 Plrophmire 1.65 4J5 r.92 9A

Propeni6 of FP+ dd FJ+ ee alhsed @ding to 6toichiomeryi! ths @ of - spirel-group phss XMg M&/(Mg+F*+) r - 3 : Witrdloy€L aL (1989),4 | Dietrich €t aL (1986).

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/32/2/327/3446667/327.pdf by guest on 03 October 2021 GEIKIELITE. KI,'HMO GREENSTONEBELT, FINLAND 331 Mgo Mn0

o Thisstudy ^ . Dietrichet al.(1986) v Windleyet al.(1989)

Tio, FeO Ftc. 4. Chemical compositions of geikielite in the Niiiitiimiemi serpentinite massif. The compositions of Dietrich et al. (1986) and Windley et al. (1989) are plotted for com- parison.

Comparedwith otherexamples of geikielite(Fig. 4, DrETRrcH.H., KoLLER,F., RtcIffER,W. & Ktrsl, W. (1986): Table 3) reportedfrom metamorphosedultramafic Petrologieund Geochemie des Rodingitvorkommens vom (Dorfertal, Tauern).Schweiz. Mineral. rocks (Dietrich et al. 7986,Windley et al. 1989), the Islitzfall Hohe Petrogr. Mitt. 66, 163-192. geikielite reportedin this paper has a higher Mn to increasewith Mg at the content,which seems DvMEK,R.F., BRorHERs, S.C. & Scur,'rurs,C.M. (1988): expenseof Fe in the octahedralsite. The relatively Pelrogenesisof ultramaficmetamorphic rocks from the high componentof pyrophanitein geikieliteindicates 3800 Ma Isua supracrustalbelt, West Greenland.J. that there is extensivesolid-solution between these Petrol 29, 1353-1397. phases. Fnosr, R.B. (1991):Stability of oxide mineralsin metamor- ACKNOWLEDGEMENTS phic rocks.1n Oxide Minerals:Petrologic and Magnetic Significance(D.H. Lindsley,ed.). Rev. Mineral.2.5' 469' The authorsthank ResearchProject of Archean 487. Areas for providing the samples,and Mr. Malcolm D.H. (1991):Occurrence of iron- Hicks, for correctingt}le English of the manuscript. & Lnoslev, titanium oxides in igneousrocks. 1z Oxide Minerals: provided to Liipo by the Financial supportwas J.P. Petrologicand Magnetic Significance(D.H. Lindsley' Foundationfor Researchin Natural Resourcesin ed.).Rav. M ineral.?5, 433-468. Finland.An earlierversion of the manuscriptwas con- structively reviewedby D.L. Whitney, R.A. Mason GIERE,R. (1987):Titanian clinohumite and geikielitein and one anonymousreferee. marblesfrom the Bergell contactaureole. Contrib. M ineral. Pet rol 96, 496-502. RSFEREI.{cEs HAGoERry,S.E. (1976):Opaque mineral oxides in terrestial Aleprrrr, T. & SrvoNsN,S. (1983):Use of the electron igneousrocks. In OxideMinerals (D. RumbleIII, ed.r. microprobein the investigationof the Early hoterozoic Rev.Mineral. 3, Hgl01-300. Koillismaalayered igneous complex, NE Finland.Geol. Surv.Finlanl, Rep.Invest.6l. & Tuouprrxs. L.A. (1984):Subsolidus reactions in kimberlitic ilmenites: exsolution,reduction and the Blars, S. & Auvnav, B. (1990):Serpentinization in the redox state of the mantle. 1z Kimberlites. I. Kimberlites Archeankomatiitic rocks of the Kuhmo greenstonebelt, and RelatedRocks (J. Kornprobst,ed.). Elsevier, easternFinland. Can. Mineral. X.55-66. Amsterdam,The Netherlands(335-357).

CanvrcuanqI.S.E. (1967): The iron-titaniumoxides of salic fhNsKI, E. (1984):The Gabbro-WehrliteAssociation in the volcanicrocks and their associatedferromagnesian sili- Eastem Part of the Baltic Shield. Phil. Lic. thesis,Univ. cates.Contrib. Mineral. Petrol. 14, 36-64. of Oulu,Oulu, Finland.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/32/2/327/3446667/327.pdf by guest on 03 October 2021 332 THE CANADIAN MINERALOGIST

MncHE[, R.H. (1978):Manganoan magnesian ilmenite and Tuorco, I. & Pmawnq, I. (1980):A brief intro- titanian clinohumitefrom the Jacupirancacarbonatire, duction in the geology and geochemistryof the Kuhmo 56oPaulo, Brazil. Am. Mineral, 63. 544-547. greenstonebelt, easternFinlancL. lz Nickel Sulfides in Ultramafic and Mafic Rocks (F[. Papunen,ed.). IGCP PIIRAnTEN,T. (1988):The geologyof the Archaeangreen- Project 16l, Field ExcursionGuilebook, 37-73. stone-granitoidterrain in Kuhmo, easternFinland. GaoL Surv.Finlan[ Spec.Pap.4,39-51. TuIsKU,P. & SrvoNrN,S. (1984):Paine- ja lAmpdrilaolo- suhteetKuhmon vihredkivivyiih5'kkeenmeamorfoosissa (Geothermometry,geobarometry PnrEr,M. & Surnr, D.C. (1985):Petrochemistry of opaque and metamorphiccondi- in greenstone mineralsin eclogitesfrom the WestemGneiss region, tions the Kuhmo belt). Univ. of Oulu, Oulu, (19 Norway. l. Petrologyof the oxide micro-assemblages. Finland p.). Che m. Geol. 50.225 -249. VAAsJoKr,M. & SAKKo,M. (1991):Isotopic constraints on the age and genesisof the Finnish greenstonebelts and PIeuEr,D. (1982):Mdcanisntes de recristallisations m4fti- their ore deposits.& The Types,Evolutionary Epochs morphiquesdans les ultrabasites: exempledes roches and Metallogenyof GreenstoneBeltst Soviet- Finnish veftes arch4ennesde Finlande (Ceinture orientale de Symp.(Kostomuksha), Abstr., 15-16. Suomusalmi- Kuhmo). Thbsede 3bme cycle, Universit6 de Rennes,Rennes. France. WARD,P. & LalrranreN, S. (1989):U-Pb zircon agesfrom the late ArchaeanHaftu schist belt, eastem ScHanaenr,H.G. (1979):Mg-reicher Ilmenit in einem Finland.Geol. Surv. Finland, Spec. Pap.10,33-36. Granatwebsteritim Granulitkcirpervon St. Leonard, Mittleres Kamptal, Niederiisterreich.Anzeig. iister. Akad. WAvcHUNAS,G.A. (1991):Crystal chemistry of oxides der Wissen.116. 161-165. and oxyhydroxides.In Oxide Minerals: Petrologicand MagneticSignificance (D.H. Lindsley, ed.). Rev. SMnH,D.C. & PrNEr,M. (1985):Petrochemistry of opaque Mineral.25.11-68. mineralsin eclogitesfrom the Westem Gneissregion, Norway. 2. Chemistry of the ilmenite mineral group. WrNolrv, B.F.,HERD, R.K. & ACKERMAND,D. (1989): Chem.Geol. 50. 25 l-266. Geikielite and ilmenite in Archeanmeta-ultramafic rocks, Fiskenaesset,West Greenland.Eur. J. Mineral. 1.427- Terram, K. (1983): The geologyand geochemistryof the 437. ArchaeanKuhmo greenstone-graniteterrain in the Tipasjiirvi area,eastern Finland. Acta Univ. Oulu A 151, ReceivedOctober 20, 1992, revisedmanuscript accepted Geol.5. July 11,1993.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/32/2/327/3446667/327.pdf by guest on 03 October 2021