875 Thz Caradian M fuc ralo g is t Vol.35, pp. 875-885(1997) FIRSTTERRESTRIAL OCCURRENCE OF TITANIUM.RICH PYRRHOTITE, MARCASITEAND PVRITEIN A FENITIZEDXENOLITH FROMTHE KHIBINA ALKALINE COMPLEX. RUSSIA ANDREI Y. BARKOVT nxp KAUKO V.O. LAAJOKI Irxtiruteof Geosciencesand Astrorcmy, University of Oula FIN-90570OuIW FinLand YT]RIP.MEN'SHIKOV Geological Instirute, Kola Science Cente, Russian Acadenry of Scimces, 14 Fersman Street, Apatity 1M200 , Russia TUOMO T. AI.APIETI Instituteof Geoscierrcesand Astrotnnry, University of Ouh+FN-90570 OuIu Finlnnl SEPPOJ. SryONEN Instiarcof ElcctronOptics, University of OuhaFN-90570 Ouh+ Finlnnd ABSTRACT The first terrestrial titanium-rich sulfides, the fust natural niobium-rich sulfide FeMgSe, fluorine-rich (ca. 6 wt.Vo D end-memberphlogopite (S().04 wLTo FeO) and ferroan alabandite occurlocally in aheterogeneous xenolit\ enclosed within nepheline syenite in the l(hifiaa alkalins gs6plex, Kola Peninsul4 northwestem Russia- This assemblage is exclusively associated with an alkali-feldspar-rich rock, probably fenite. Associat€d ninerals include corundum, Nb-Zr-bearing rutile and monazite. The maximum Ti content reaches 3.9 w.7o in pynhotite and 2 wt.7o in marcasite and pyrite, which represent prducts of replacement of the pynhotite. Titanium is distributed rather homogeneously within single grains of the pyrrhotite; however, a strong grain-to-gain variation is observed. The Tl-rich sulfides invariably contrin an elevated level ofvanadium (0.2-0 .4 wr.Vo).T\e results indicate that both Ti and V enter into solid solution in the sulfides. Presumably, there is an environmental similarity between the occurrences ofTl-bearing sulfides in trftibina and in meteorites (enstatite chondrites), where Tl-bearing hoilite occurs. As in the meteorites, strongly reducing conditions probably existed locally at the time of crystelliTation. Concentration of Mg wa a fluid phase accounts for the formation ofthe Mg-rich phlogopite, associated with the relatively Mg-poor rock. Keyvords:Tr-beainn s"lfi6es, V-bearing sulfides, M-rich sulfide, end-member phlogopite, alabandite, fenite, xenolitb, I(hibina alkalins se6plgA Russi4 FennoscandianShield. Souuane Nousprdsenlons une description du premierexemple terrestre de sulfuresenrichis en titaneet du premierexemple naturel d'un sulfure cont€nant du niobium, FeMgSe, en association avec une phlogopite fluor6e (environ 6% de E en poids) et I faible teneuren fer (<0,MVoFeO) + alabandite ferreuse. Cette association est d6velopp6e dans une enchys.lans une sy6nite n6ph6linique dans le complexe alcalin de Khibin4 p6ninsule de KoIa secteur nord-ouest de Ia Russie; l'enclave serait une f6nite emichie en feldspath alcalin. Pami les min6raux associdssontcorindon, rutileenrichi en Nb etZ, et mon:zite.lspyilstitecontientjusqu'i 3.9Vo &'Ti, etla marcasite et la pyrite, qui remplacent la pyrrhotile, en contiennentenvtrot2%o,I*Ti y est r6parti de fagon assez homogBne rlgns gfuaquecristal, mais montre une grande variabilit6 parmi les grains. Les sulfifes riches en Ti sont aussi enrichis en vanadium (0.24.4Vo). D'aprds nos r6suhats, Ti et V sont incorpor6s dsns la structure des sulfures de cette enclave. tr y a proba- blement une ressemblance dans le milieu de formation de ces sulfures riches en Ti I l(hibina et dans les m6t6orites (chondrites i enstatite), dans lesquelles on peut trouver une trofite riche en Ti. Dans les deux cas, les conditions semblent avoir 6t6 fortement r6ductrices. [a concentration du Mg dgns une phase fluide pourrait expliquer Ia formation de pblogopite trBs magn6sienne dans un milisg lslatiyement pauvr,e en Mg. (Ibaduit par la R6daction) Mo*-clds: sulfure.s riches en Tl, sulfures riches en V, sulfure de Nb, phlogopite, mica riche en R alabandite, f6nite, x6nolithe, complexe alcalin de Khibin4 Russie, bouclier fennoscandien. I E-mail address: [email protected] 876 TI{E CANADIAN MINERALOGIST IxrnouucrroN the typical assemblagesof minerals.The xenoliths are presentasnunreous isolated bodies having wide variations In the courseofa detailedmineralogical study ofone in theirsize(from d m up to approximately0.5 km across) of the contactmetamorphicrockxenolitbs in the Khibina andmineralasse, blages. The mineralory of somexenolitbs alkaline complex, northwesternRussia, a unique is exceptionallydiverse. occurrenceof titanium-rich pyrrhotite, marcasiteand The assemblagedescribed here was identified in a pyrite,aniobium-rich sulfide and end-memberpblogopite relativelysmall xenolith (<()2kminthe longestdimension) wasrecorded. It is well known thatpyrrhotite, pyrite and surroundedby nephelinesyenite (foyaite). The xenolith marcasiteare abundantin a wide variety of geological is extremelyheterogeneous texfrrrally and mineralogically, environments(e.g., Deer et al. 1992).These minerals andseems to becomposed of vruiouscontact metamorphic have been the subject of numerousstudies; however, reks, accide,ntallyincorporated into foyaitic magma"The Ti-e,nrichedteneshialexampleswerenotpreviously known rcks te,ndtobefinegrained: however, medium-to coarse- The Iftibina pynhotite, reportedin this paper,contains grained rocks and pegmatitic facies locally also are up to 3.9 wt.VoT1 being considerablyricher in Ti than commonin theoccurrence. It is notewo,rthytbat, in general, troilite from enstatitechondrites (up to 1.0wtTa Tt), the the rocks in the xenolith show an essentialenricbment only otherTi-bearingsulfide sofarlnown in nature(Keil in titanium, and an abundanceof Ti-(Fe) oxide occurs &Andersen1965, Keil 1968).Inaddition, thebase-metal at the contactwith the host nephelinesyenite. snlfide mineftrlsare reported here to containremarkable Alkali feldspar,nepheline, sodalite, analcirne, mic4 concenhationsof vanadium (up to 0.4 wt.VoY rn the conmdum,nrile, danite,ilmenite and topaz occur in places pyrrhotite).The prese,nce of theNb-rich sulfide and a nealy asthe chief or essentialminerals in the xenolith. It is of iron-free,fluorine-rich phlogopite, occurring in therather interestto note1tru1m1s mineral speciesfreudenbergte, Mg-poor environment,is also of particularinterest. representingtle first Fennoscandianoccurrence, and membn of thecrichto'nite - loveringite- landauitesolid- Occunnrncp solutionseries were found to occurin a closeassociation with otheroxides in thexenolith @arkov et al.,nprep.). The uniquemineral assemblage occurs in the tftibina Zircon andminute grainsof uraninite(?) (310 pm) also '43'N; alkalineso6plsx (Kola PeninsulaNW Russia:67 wereobserved. Base-metal sulfide mineralization is rather 33'47'E), oneof thelargest (about 1330 km2) and most abundantin places. well-knownalkaline complexes in the world.As noted Within the xenolith, the unusual assemblageis taKogarkoet aL (195), only theGuIi nlkalineulfabasic intimately associatedwith amedium- to coarse-grained complex in Siberia may be somewhatlarger (1500- rock consistingpredominantly of alkali feldspar (bulk 16@km2). The Khibina complex is lcated at thecontract compositionAb ss.g-sgsOrn o-+r.oAn r.r-a.z), the modal betweenArchean granite gneissesand Proterozoic abundanceofwhich is not uniform, but seemsto exceed volcanic-sedimentarycomplexes. Ikamm et al. (1993) 75-3OVoon the whole. This rock may be regardedas an haverecently reported a very narrowrange ofRb-Sr ages, atkali-fetdspar-richvariety of fenite. Platy pblogopite, from 361.7tn 377.3Ma (i.e.,late Middle Devonianto subhedralto euhedralcorundum, pyrrhotite (up to Up'perDevonian),forigneous activity in theKola-Finnish ca.34 vol.7a)and rutile arepresent in varying propor- alkalineprovince, including that of theKhibina complex. tions. Monaziteis a wide,spreadaccessory mineral. It is This complexis composedof variousalkaline 16skso noteworthy that a graphite-like mineral (idenffied by predominantly(from theperiphery to thecenter) alkaline X-ray powderdiftaction) occursin thexenolitb, relatively syenite,nepheline syenite, khibinite, rischorrite,ijolite, closeto theTi-rich sulfideoccurrenceo as small spherules urtite,apatite-*phelinerocks, melteigite, nepheline syenite of lessthan 0.5 mm in diameter.This fenite xenolithmay andcarbonatite; it is surroundedby abundantfenites and be an exampleof "contactfenite", which may consistof homfelses(a9., Ge,rasimovskya aI l% 4, hlakhov l% 5, almostmonomineralic feldsparrock Examples of thelater Kostyleva-Iahmaovaet aI. Lg78).T\egeolory,pehography areknowninthefenitizedxenoliths in theOldoinyo kngai and mineralogy of the Khibina complex have been carbonatiticlavas (Morogan & Martin 1985). describedin numerouspublications (a9., Kuplesky 1928, The occurrenceof titanium-rich sulfides seems Eltseeve t al. 1939,Zak et al. l9T 2, Gerasimov sky et aI. to be a local featureof the trftibina xenolith. Electron- 1974,Galakhov 1975, Kostyleva-Iabuntsova et aL L978, microprobeanalyses ofpyrrhotite andchalcopyrite from Shlyukova 1986).However, in spite of a large number a related fine-grainednepheline-rich rock in the same of publications,only Kupletsky(1932) and Men'sbikov xenolithic body do not show detectableconcentrations (1978)have addressed the geologicaland mineralogical ofTi andV. featuresofheterogeneous xenolithic homfelsand fenite bodies,which are lcated within alkalinerocks in thecentral Et-ucrRoN-MrcRoPRoBEANALYSES part of the complex. It is important to emphasizethat xenoliths are widespreadover a large areain Khibina. Mineral compositionswere determinedusing Men'shikov(198) reporteda zoneof theconmdum-bearing both wavelength-dispersiontechnique (WDS) and xenolithic bodiesof over 20 km in length anddescribed energy-dispersiontechnique