Rhombohedral Carbonates from Carbonatites of the Khibina Massie

453

The Cannlian Mineralo gist Vol. 34, pp. 453468 (1996)

RHOMBOHEDRALCARBONATES FROMCARBONATITES OF THE KHIBINA MASSIE KOLAPENINSULA, RUSSIA

ANATOLY N. ZAITSEV Departmentof Minerdogy,University of St.Petersburg, St. Petersbarg 199034, Russin

ABSTR,ACT

The easternpart of the I(hibina alkaline massif, Kola peninsul4 Russia is made up of ijolite, foyaite, olivine mela:re- phelinite,melanephelinite, phonolite, alkali ftachyte,and rocks ofthe carbonatiteseries. The latter canbe divided into (1) biotite - aegirine- apatiterocks containingless than 507ocarbonate, (2) early and (3) late carbonatitesand (4) carbonats-zeoliteveins. Theserocks are distinguishedby their relative agesand by their major and accessoryminerals. The rock-forming minerals include rhombohedralcarbonates of Ca, Mn, Fe and Mg: calcite, manganoancalcite, dolomite, ankerite,manganoan arkerite, ferroan kutnohorite, ferroan rhodochrosite, manganoan siderite and siderite. Petrographic and cathodoluminescence observationsreveal two textural types of carbonates:primary and secondary(measomatic). The Sr contentis high in pdrnaql minerals,but low in tlose of netasomaticorigin. Values of isotoperatios 613Cin the range-:7.8 to -2,8%o(PDB) and 6180in the range+5.9 to +14.3%o(SMOW) for the rhombohedralcarbonates from tle Khibina carbonatitesand carbonate-zeoliteveins are similar to those from carbonatiteselsewhere. For most sarnples,there is a positive correlation between613C and 6180, which can be explainedby Rayleigh and temperaturefractionation. However, in one type of the late carbonatitesand the carbonate-zeoliteveins, cmbonatesshow wide variatious in theL 6l3C and 6180values; this has been interpretedas loss of heavy C to a gas phase.Field relations, petrographicand cathodoluminescenceobservations suggest that formation of the late carbonatitesand carbonate-zeoliteveins is more likely to have been from a carbohydrothermal(fluid) systemthan from a melt.

Keywords: carbonatite,calcite, dolomite, ankerite,kutnohorite, rhodochrosite, siderite, isotope composition,Khibina massif, Russia.

SoM:ra,qlRg

L,a partie orientale du massif alcalin de Khibina dansla pdninsulede Kola, en Russie,contient une associationd'ijolite, de foyalte, de m6lan6ph6linitei olivine, de m6lan6phdlinite,de phonolite, de fachyte alcaline et de rochescarbonatitiques. Cette dernidre suite se subdivise en (1) roches i biotite + aegyrine + apatite contenantmoins de 507o de carbonate, (2) carbonatitepr6coce, (3) carbonatitetardive, et (4) veines i carbonate+ z6olites.Ces rochesse distinguentpar leurs iges relatifs et leurs mindrauxnajeurs et accessoires.Parmi les min6rauxmajeurs, on trouve les carbonatesrhomboddriques de C4 Mn, Fe et Mg: calcite, calcite manganifdre,dolomite, ark6rite, ark6rits manganifbre,kutnohorite feneuse, rhodocbrosite feneuse,sid6rite manganifbre et sid6rite.Les observationsp€trographiques et par cathodoluminescencer6vblent deux types de textures,indicatives de cristallisationprimaire et de remplacement,respectivement. La teneuren Sr est6levde dans les min6raux primaires,mais faible dansles carbonatessecondaires. Irs rapportsisotopiques de C (613Centre -7,8 et -2.8%oapar rappoft e l'6talonPDB) et de O (6180entre +5.9 et +14,3%oparrapport e l'6talonSMOW) descarbonates rhomboddriques des carbonatites de Khibina et des veines i carbonate-z6ottesressemblent aux valeurs d6termin6e$pour les carbonatitesailleurs. Dans la plupart des 6chantillons,il y a une corr6lation positive enfie 613Cet 6180, qui r6sulteraitd'un fractionnementde Rayleigh et de sa ddpendancesur la temp6ran[e.Toutefois, dans un facidsde carbonatitetardive et dansle casdes veines e carbonale+ zdolites,les carbonatesvarient beaucoupdans leurs rapports 613Cet 6180, variations qui ont 6t6 interpr6t6esen invoquant une perte de C lowd par d6gazage.D'aprbs les relations de terrain, l'6vidence pdtrographiqueet les observationspar cathodoluminescence,les carbonatitestardives et les veines d carbonates+ z6olites se seraientform6es i partir d'une phase fluide riche en gaz carboniqueet en eauplut6t qu'i partir d'un maema. Clraduit par la R6daction)

Mots-cl.6s:carbonatite, calcite, dolomite, ank6rite, kutnohorite, rhodocbrosite,sid6rite, composition isotopique, massif de I(hibina" Russie.

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Ilrmooucuor.r Gmmnar Gsolocv

The Kola alkaline province of Russia contains The geological relationshipsin the eastempafi of twenty-four alkaline complexesthat rangein agefrom the Khibina massif are complex (Dudkin et al. 1984, 380 to 360 Ma (Kramm et al. 1.993).The Khibina Zutsev 1992). Strong tectonism has producedwide- massif,364.5 * 4.0 Ma (Kogarkoet al. L98|),located spread brecciation and faulting. Reactions between in the central part of the Kola peninsula (67'48'N, silicate wallrocks and aqueousCO2-bearing fluid have 33"48'E), is one of the best known and most resultedin the formation of variousmetasomatic rocks interesting alkaline-carbonatitecomplexes. It is a (fenites), consisting mainly of albite, biotite and composite pluton @ig. 1A), elliptical in shape and calcite. The unalteredsilicate rocks are representedby concentricallyzoned, consist'ng of variousultrabasic ijolite, foyaite, olivine melanephelinite,melane- and alkaline silicate rocks and carbonatites.On the phelinite, phonolite and alkali frachyte (from oldest to basis of field relationships, petrography and youngest). mineralogy,four principal groupsof silicaterock-types are distinguished(Kostyleva-Labuntsova et al. 1978): Silicate rocla ultrama.fic rocks (peridotite, pyroxenite), nepheline syenites (khibinite, foyaite, rischolite), foidolites The ijolite is a fine-grained porphyritic rock (urtite, ijolite, melteigite),and apatite-nephelinerocks. occurring as xenoliths.(5 cm - 20 m) in foyaite. It Numerous dykes of essexite, theralite, teschenite, consists of aegirine-augiteand nepheline with trace olivine melanephelinite,melanephelinite, nephelinite, amountsof tignite, microcline, cancriniteand apatite. camptonite, phonolite and alkali trachyte cut tle The mineralogical and chemical compositionsof complex (Arzamastsevet al. L988). The massif is ijolite, as well as the chemical composition surroundedby a meta$omaticaureole of fenite and of nepheline,are similar to thosein other examplesof hornfels. ijolite at Khibina. The clinopyroxenefrom ijolite from The carbonatitesof the Khibina were discoveredin the easternpart of the massif containsmore Mn, and 1969during explorationdrilling within the easternpart less Ti and Al than that in ijolite elsewherein the of the massif (Fig. 1A), where they are covered by complex (Kostyleva-Labuntsovaet al. 1978, Zutsev alluvium 20 to 90 m thick The carbonatiteshave been et al. L990. describedby Minakov & Dudkin (1974), Minakov Foyaite is the dominant silicate rock in the area et al. (198L) and Dudkin et aI. (1984). studied. It is medium gained and has a massive or In this study, I present new geological, mineral- trachytic texture, and consists of alkali feldspar, ogical and C and O isotopic data concerning nepheline,arfvedsonite and aegirine, with a variable carbonatitesand associatedsilicate rocks.The samples amountof biotite. Accessoryminerals include titanite, investigatedwere collected from drill core;the location apatite, iLaenite, eudialite and astrophyllite. of the drill holes is shownin Fisure 1B. Mineralogical and whole-rock chemical data for

66. 1. $imFlified geologicalmap of the Khibina alkaline massif (A) and drill-hole localities in the areastudied (B). Symbols in A: I foyaite, 2 nepheline syenite, 3 khibinite, 4 urtite, ijolite, melteigite and rischorrite, 5 apatite-nephelinerocks, 6 location of carbonatites;in B: L numberand location of drill holes,2 axeaof greatestdevelopment of carbonatite.

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foyaite from the easternpart of the massifare similar to Both the biotite - aegirine - apatite rocks @AA thosefor foyaite from elsewherein the Khibina massif. rocks) and calcite - biotite - aegirine - apatite Olivine melanephelinite,melanephelinite, phonolite carbonatite(C I) occur mainly at a depth of 1700 m, and alkali trachyte occur as dykes from 10 cm to andthey representapproximately 57o by volume of the 10m wide.A stock-likebody ofphonolite, 150-200 m carbonatiteseries. BAA rocks form veins in silicate across,has also beendocumented in this area@udkin rocks, with thicknessesfrom 0.5 to 2.0 m, but more et al. L984). The geochemistry and mineralogy of often occur as relics (1 cm to 30 cm in size) in the silicate dykes of the Khibina massifwere discussedby carbonatites.The BAA rocks are fine grained and Arzamastsevet al. (1988). homogeneouswhere they contain less than 57o carbonate,and are medium to coarse grained and The carbonatitesertes heterogeneouswhere more that 5-l0%o carbonateis present.Petrographic observations show the presence Ijolite, foyaite and all silicate dyke-rocksare cut by of at least two parageneticmineral associations.The various carbonate-bearingrocls, which have been first associationincludes apatite, aegirine, pyrrhotite assignedto a carbonatiteseries. According to Bulakh & and pyrochlore,and seemsto be primary. The second Ivanikov (1984),rocks ofthe carbonatiteseries include associationis representedby biotite and pyrite, which phoscorite, carbonatite, late quartz-+arbonateveins replace aegirine and pyrrhotite, respectively. In and various alkali-silicate metasomatic rocks comparisonto the field relationshipsand mineralogy of surrounding carbonatite. Rocks belonging to the other carbonatitecomplexes within the Kola peninsul4 carbonatite series were formed at approximately the BAA rocks are interpreted to be belong to the the sametime, have similar mineralassociations. and phoscorites (Za;tsev 1992), as suggestedby their invariably occur in the sameorder of emplacement. associationwith garbonatites,their relative age and At least eight minslalogical types of carbonate- mineral chemis!ry. In contrast to typical phoscorite bearingrocks have been identified at Khibin4 on the (e.g., from Kovdor, Vuonjarvi), the BAA rocks, like basis of relative ages and their major and accessory the carbonatitesfrom Khibina"do not containolivine or minerals (Zaitsev & Pavlov 1988,Zaitsev 1992).T\e magnetite.Olivine-poor but aegirine- and biotite-rich rocks are classified into biotite - aegirine - apatite phoscoritevei1fr minql amountsof magnetitehas been rocks containing less than 50Vo carbonate,various describedfrom the Seblyaw massif,Kola peninsulaby carbonatitesand carbonate-zeoliteveins. Relative ages Lapin (1979), and Bulakh & Ivanikov (1984) have of these rocks, establishedby cross-cuttingrelation- shown that the mineral composition of phoscoriteis ships,are given in Table 1. Rocls of stageI and tr are correlatedwith the mineralogyof the principal silicate regardedas magmatic, and rocks of stageItr and tV rocks in the ultrabasic-alkalinemassif. Olivine-poor, are considered of carbohvdrothermal-metasomatic pyroxene-amphibole-and mica-rich varieties of origin. phoscoriteoccur in alkaline massifscontaining mainly foidotte, and olivine-rich phoscoriteoccurs in alkaline massifscontaining dunite. Calcite - biotite - aegirine - apatite carbonatites TABLE I. SEQUENCEOF FORMATIONOF ROCKS (C f) occur as veins 0.1 to 4.0 m wide. They are OF T}IE KHIBINA CARBONATITESERIFS' medium to coarsegrained, heterogeneous and contain minor amountsof pyrrhotite, pyrite with traceamounts Tlpe of @k of ilnenite, magnetite,pyrochlore and chalcopyrite. Bandingparallel to the walls of the carbonatiteveins is I ph6&!it! bic'titFa€giriE-apali!9 rarely observed. Locally, the carbonatites contain xenoliths of the wallrocks: foyaite, foyaite pegmatite, Iledyerbonafites @fcibbiq!'tea€giii@apatile CI melanepheliniteand alkali trachyte. These xenoliths seem unaltered, angular, and have sha4l contacts. eloite sith butantite C trl with the surrounding carbonatite.Contacts between silicate wallrocks are sharp. In mhb@b@dits mrBMalksile€lcite carbonatitesand the witlsyrchlsib-{Ca) CII-2 contrast,there is a gradual transition from the BAA rocks to the C I carbonatites.This transition is &m ftodocbroiteaaagM @kqite withsyrclrpio-{Ce) ctr-3 expressedas increasing contents of apatite, aegirine aud biotite in the carbonatites.Field relations, petro-

q$1"e" sidsito-@lg@ @l€riletalolite CZ m-l graphic observationsand mineral chemistry show that IV qboalqalite all non-carbonateminerals were incorporatedinto the mnghr eidtrito{ordluadditeetrolit& @ln-2 reia carbonatites from the BAA rocks (Zutsev 1992). Datulite{@ga'@sirkrnb<'flmile Czfra Relative age, minslalegy and geochemisty of the C I carbonatites(abundances ofmajor and trace elements) . aff{ Zaitsev& Pavlw 0988) ed Aibry (1992) indicate that they belong to the category of "early

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Ef l zF-l,sF;&q m rffilzlWl rKlqn FIc. 2. Relationsbetween the late carbonatitesand the carbonate- zenhtevein. A. 'Undisturbed" relics at tle vein contact between late calcit€ and manganoananlerite carbonatiteswith synchysite-(Ce);1 calcitq,2 yellow synchysite-(Ce)+ strontianite+barite+calcite,3redsynchysite-(Ce)+strontianite+barite+manganoanankerite+relicsofcalcite, 4 manganoanankerite + relics of calcite; red color of synchysite-(Ce)is due to of small inclusionsof Fe phase.B. Changes in mineral composition of the carbonate-zeolitevein cutting late carbonatite; I manganoanankerite, 2 orthoclase, 3 manganoansiderite + magnetite(black), 4 natrolite + relics of ordroclase.

carbonatites" (Kapustin 1980) or to siivite (calcio- wirhin manganoanankerite carbonatite.This feature carbonatite)(Woolley & Kempe 1989). gives evidence of the metasomaticcharacter of the Carbonatites containing Ba-Sr-REE-bearing latter(Krasnova 1988). minerals (C II) make up about 90Voby volume of The field relations and mineralogy of the the carbonatite series in the Khibina massif. Three carbonatitescontaining Ba-Sr-ltEE minerals(C tr) are different types of the C tr carbonatiteswere selected interpretedto suggestthat they are "late carbonatites" for study on the basis of cross-cuttingrelations and (Kapustin1980) or REE carbonatites@ecora 1956). mineralogy Cfable 1). They occur as veins I cm to According to the chemical classification of 6 m wide, and are medium to coarse grained and carbonatitesproposed by Woolley & Kempe (1989), heterogeneous.The contactsbetween C tr carbonatite the carbonatitesof this grcup can be subdividedinto and wallrocks may be gradational or sharp. Where calciocarbonatiteand ferrocarbonatite(A.N. Zaitsev gndational, the transition to the wallroctrs is charac- & M.J. Le Bas, unpubl. data). Some samplesof teized by a high content of extremely altered ferrocarbonatiie,however, contain more Mn than Fe (metasomatized)relics of ijolite, foyaite, melane- (e.g., 11.37wL.Vo MnO versus8.O9 wt.7o FeOr6 and phelinite and alkali trachyie, into which carbonatehas 3J2 wt.VoMgO,24.59 wt.VoCaO), and they can be beenintroduced along fracturesand grain boundaries. classified as manganocarbonatitewith CaO/(CaO + The carbonatiteveins that have sharpcontacts with the MgO + FeO + Fe2O,+ MnO) lessthan 0.8, MgO < wallrocks are characterized,by crustification (vein- (FeO + FqO, + MnO) and MnO > (FeO + FqO:). filling) and contain druses.In the case of wallrocks Carbonate-zeolite veins (CZ m), the youngest surreundingt}re carbonatites with Ba-Sr-REE mineral- carbonaterocks in the Khibina massif (Zaitsev & 1?anon,a high contentof carbonate,albite, biotite and Pavlov 1988),are rarely more than 20 cm across,and fluorite is typical. Field and microscopeobservations are medium to coarse g&ined and heterogeneous. in someinstances show 'lrndisturbed" relics at the vein Three types of carbonate-zeoliteveins were distin- contactsbetween late carbonatites.Figure 2A shows guished accordingto their cross-cuttingrelations and manganoan ankerite carbonatite that cuts calcite mineralogy (Iable 1). The veins are discordant to carbonatite. Petrographic observations reveal that silicale wallrocks and are charasteizedby crustifica- calcite is replaced by manganoanankerite, and tion (vein-filling) and commonly contain numerous synchysite-(Ce)- strontianite - barite intergrowths druses, Where tle carbonate-zeolite veins cut from calcite carbonatiteare preservednear the contact carbonatite,they may show significantchanges in their

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mineral composition,so as to reflect the surrounding C for calcite, sideriteand rhodochrosite, and two atoms carbonatite(Frg. 2B). This feature indicates metaso- of C for dolomite,ankerite and kutnohorite per formula matic formation of such carbonate-zeoliteveins unit. (Krasnova1988). Calcite Mnmnerocv oF RHoIvBoHDRALC.qRBoNarFs Calcite, the dominant carbonate in the early Analyticalmethods carbonatites(C I) and two types of late carbonatite (C tr-l and C tr-2), occurs as white subhedralor Identification of the carbonates \ilas based on anhedral grains, 0.5 mm to 12 mm in size. Calcite staining, optical examination, infrared spectroscopy crystals are usually twinned, and they have nearly (UR-20 Carl Zeiss spectrophotometer), X-ray straight crystal boundaries and almost 120' riple diffraction (RKU camera, 57.3 mm in diameter junctions. Calcite shows an orange-redcathodolumi- and FeKcrr radiation, and 114.6 mm in diameter and nescence(CL) activated by Mn2+ (Marshall 1988). CrKcrl radiation), scanningelectron microscopy inves- Rarely, in early carbonatites(C I) and late carbonatites tigations (SEM, Hitachi 5-430), cathodoluminescence with burbankite(C tr-l), internal zonesof calcite can (Iechnosyn cold cathodeLuminescence, model 8200 be observed(Frg. 3). This zoning is similar to that Mark tr), and electron-microprobe analysis. Both observedin calcite phenocrystsfrom the Kerimasi and energy- and wavelength-dispersion analyses of Kaiserstuhl carbonatites(Mariano & Roeder 1983, carbonateswere made using a JEOL JXA-8600S Keller 1989). electron microprobe (Leicester University), a Electron-microprobeanalyses show that the calcite Cambridge Microscan (MK 5) electron microprobe of the Khibina carbonatitesis characterizedby wide (CarletonUniversity), and a CamecaMS-46 electron variations in its trace-elementcontent. Selected microprobe (Geological Institute, Apatity, Russia). compositions are given in Table 2, and results are Acceleration voltages of 15 or 20 kV and beam plotted on Figures4 and 5. Calcite in the C I and C tr currents of 20 or 25 nA were used for analyses.The carbonatiteshas a low Mg content.The differencein its following standards were used: natural dolomite chemical compositionin various types of carbonatite (Ca, Mg), diopside (Ca), wollastonite (Ca), pyrope is mainly in the contentof Mn, Fe and Sr. Early calcite (Mg), siderite @e), ilmenite (Fe), rhodochrosite(Mn), in the C I carbonatiteshas a low Mn and Fe contento celestite(Sr) and syntheticphases MgO (Mg), Fe (Fe), but it is rich in Sr. Late calcite in the C II carbonatites SrF2 (Sr), LaF3 (La) and CeF3 (Ce). CO2 was is characterizedby similm contentof Fe and much calculated by stoichiometry based on one atom of more Mn than the early calcite. Sr is enrichedin the

Flc. 3. Internal zonation of calcite crystal under cathodoluminescence.Late calcite carbonatitewith burbankite (black), C tr-l. Widttr of image is 2 mm. Drawn from color slide.

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TABLE 2. RESIJLTSOF SELECTEDANALYSES OF CALCITE AND MANOANOANCALCITE

Pock synbot ct cr cr cu-l c[-l ctr-l cll-2 crl-2 cn-2 o Satrplsf 6328 6328 6328 608 W 607 603 @3 633A 1984.01963.0 1847.0 97.0 451.0 281.5 36.3 165.5253.0 tr s CaO vlo/o 52.74 53.29 51.57 52.42 5t.54 47.@ 53.34 54.90 53.70 u qf Mgo 0.35 0.,18 0.25 0.21 o.2l 0.25 0.06 0.04 bd B 2.9 1.31 l.5E n3 -tt MnO 0.45 0.3E 0.61 194 3.37 5.69 .tr FuO" t.t2 0.60 0.96 0.64 1.07 r.75 0.55 0.19 0.40 Enn sro t.49 1.51 r.7r r.56 l.ll 0.92 0.13 0.47 bd tr la2G3 nd 0.06 nd nd 0.15 rd 0.06 nd !d 2 atr u CeUO: nd 0.12 trd !d o.o7 d bd nd !d 43.39 43.68 42.81 43.63 M.0l 42.6 44.20 M.27 43.3'l tr coz a Tdal 99.54 100.12 98.41 100.40r0r.53 98.91 101.33lol.tE 99.05

Feo' - total Feelcddcd I Feo; nd - not d€lsmine4 bd - belw detctim rrrrr lrl

0.5 1 1.5 SrO wtTo

late calcite from carbonatitetype C tr-l; the range of r cI tr cil.l . cllL.z Sr contentsoverlaps with datafor the early calcite,but the level of Sr is very low in carbonatitetype C tr-l. Flc. 5. Compositionof the calciteand manganoan calcite in termsof SrOversu.r MnO. Symbols: see Fig. 4. Manganoancalcite

Manganoancalcite, with more than 4.0 wt.VoMnO, occurs in carbonatiteas pink subhedralor anhedral grains that range in size from 0.2 to 3.0 mm. Two distinct textural types of the mineral were distinguished. The first is found only in the late C tr-l carbonatite. Here, manganoan calcite occurs as subhedral crystals, 1,-3 mm wide. Cathodo- luminescenceobservations reveal internal zonation of CaCO3 the crystals,which are similar to those of calcite. The second textural type forms vein-like, fine-grained aggregates that occur along fractures and calcite boundariesin the early C I carbonatitesand late C II-1 and C tr-2 carbonatites.Here, calcite and manganoan calcite are easily distinguishedone from anotherby cathodoluminescencecolor (orange-redfor calcite and deep red for manganoancalcite). CL observations show that calcite is replaced by manganoancalcite. This featureis evidencefor the late secondaryorigin of the secondfype of manganoancalcite. Manganoancalcites of the fust and secondtextural typesare also distinguished by their Sr content.High Sr is typical ofthe first type ofmanganoancalcite (range 0.86-1.19 wt.7o SrO), whereaslow Sr contentwas textural type (range FeC03 MnCO: determined for the second 0.354.49 wt.7o SrO). Selectedcompositions of the first type are given in Table 2 and shown on Figures4 rcI trcJJ-l .clt-z and5. Dolomite FIc.4. Composition(moJ.Zo) of calcite andmanganoan calcite in the system CaCO3 - FeCO3 - MnCq. Symbols: Dolomite was found in only one vein of late C tr-3 C I early carbonatite,C tr-l late calcite carbonatitewith carbonatite. This vein contains ferroan kutnohorite" burbankite, C n-2 |o'te manganoanankerite - calcite manganoan ankerite and ferroan rhodochrosite as carbonatitewith synchysite-(Ce). major carbonatephases. Dolomite occursas xenoliths

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TABI.E 3. RESULTSOF SELECTEDANALYSES OF DOLOMITE,ANKERITE! repor0edSr concentrationsin dolomite-ankeritefrom MANOANOANANKERITE AND FERROANKUTNOHORITE Lueshe, Newania, Kangankundeand Chipman Lake complexesranging from 0.38 wt.Voto 0.80wt.7o SrO; Rckgrabol Ctr-3 CII-3 BAA CII-I CII-2 CII-2 Ctr-3 Ctr-3 CII-3 carbonatites, Suple# 6S4 604 6328 604 603 603 60E @4 @4 dolomite from the Meech Lake Quebec 413.0 413.0 1953.0 113.2 225.2 36.3 2t3.2 413.5 413.0 contains0.9 to 1.1'ut.Vo SrO @. Hogarth,pers. comm., CR 1993).Compositions of dolomiteare given in Table3, and the results are plotted in terms of CaMg(CO.), - CaO $.o/o n34 21.EE 28.41 28;74 28.05 2'1.65 26.51 24.64 27.01 CaFe(CO3)2- CaMn(CO3)2 in Figure6. Mso 20.66 20.55 11.89 5.87 4.19 4.39 2.58 2.6A 0.97 MnO l.l7 0.82 1.85 8.4t 8.10 13.10 tI.n 20.to 21.52 FoOi 0.39 0.39 11.99 t2.99 t'1.86 11.69 ll.3E 10.28 E.ll Ankerite sro 3.88 2.32 0.52 0.41 0.06 0.24 nd 0.09 0.12 IaZOS M0.l5Mdndbd trd 0.14 0.10 cqot MMMnd!d0.12 !d bd bd Ankerite was found only in a thin (5 mm) vein co2 46.63 46.t2 44.W 4227 42.67 4l.U 4t.44 4t.t0 40.6 Total 9.93 98.19 98.66 98.6910l.ll 98.85 99.38 98.9 98.49 cutting the BAA rock. It occurs as white or grey subhedral grains, 0.5-1.5 mm in diameter. A

Feoltobl Fe @l&red I FeO; ad-Bot dedoeired; bd{slN deteli$t Cs€, R{im composition is given in Table 3, and the results are presentedin Figure 6. The CaFe(COr), component rangesfrom 31,to 46 mo'l,.Vo,with only small amounts of Mn (1.03-2.05 wt.VoMnO). The Sr content is 0.424.82 wt.7oSrO.

(or relics?)that rangein diameterfrom 5 1s lJ mm, and Manganoanankerite consistof white subhedralg&ins (0.5-2.0 mm). Electron-microprobe analyses reveal that the Manganoan ankerite occurs variously in major, dolomite is zoned, with Sr content desleasing,from accessoryor trace amounts in all types of the late coreto rim" from 3.88 wt.Voto 2.32 vrt.VoSrO. These carbonatites (C tr) and carbonate-zeoliteveins valuesare unusually high. lusklsy & Woolley (1990) GZ nD, and forms white or grey, subhedral or

CaMn(COj2

Ftc. 6. Composition (mol.7o) of dolomite - ankerite - manganoan ankerite - ferroan kutnohorite in the system CaMn(CO:)z - CaMg(CO3)2 - CaFe (COr)r. Symbols:C ank ankerite carbonatite, C II-1 calcite carbonatite with burbankite, C 'lI-2 manganoan ankeriG - calcite carbonatite with synchysite-(Ce),C II-3 ferroan rhodocbrosite - manganoanankerite carbonatite with synchysite- (Ce). Symbols of the 50 D+S fields: D: dolomite solid- CaMg(CO3)2 CaFe(CQj2 solution, C: Ca-rich solid-solution,S: Ca-poor solid-solution.T = 450oC. P.oy in the range2,1-2.6 .cank trcII-t .cll-2 +cII-3 kbars @osenberg1968).

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'l \ l\ i\ \\ i\ '.n tu

:iD l\.. r ^ ^ -..1r ,'4'. \ 'E C\ a ii + A "'.ol .r\ .. d.-

CaMg(CQ)2 CaFe(CO3)2

^ czill-l o czlil-z

Flc. 7. Composition (mol.7o) of manganoanankerite in the system CaMn(COt2 - CaMg(CO)2 - CaFe(CO)2. Symbols: CZ fr-| manganoansiderite - manganoan ankerite - natrolite veins, CZ m-2 manganoansiderite - nordstrandite- natrolite veins. Svmbolsofthe fields are definedin Fis.6.

anhedral grains, 0.05-5.0 mm in diameter. Selectedcompositions of manganoanalkerite from Petographic observationsreveal two distinct textural both late C tr carbonatiteardCZ III carbonate-zeotte types.In the first the mineral forms subhedralcrystals veins are given in Table 3 and shown on Figures 6 (1-5 mm) coexisting variously with either calcite or and 7. The mineral from the late carbonatitesdefines ferroan rhodochrosite or manganoan siderite; this a continuous series from low-manganeseankerite textural q/pe was found in the late carbonatites(C ID (calcite carbonatite with burbankite), through and carbonate-zeolite veins (CZ m-1). Contacts manganoanankerite [manganoanankerite - calcite between tle manganoan ankerite and associated carbonatite with synchysite-(Ce)l to ferroan carbonatesare sharp,with no evidenceofreplacement kutnohorite,which occursin someveins of manganoan and the manganoanankerite seemsto be a primary ankerite - ferroan rhodochrosite carbonatite with phase.In the secondtextural fype, which was found in synchysite-(Ce)(Frg. 6).The chemicalcomposition of the late carbonatites (C II=2 and C II-3) and the manganoanankerite from the CZ mA and CZ carbonate-zeolite veins (CZ Itr-l and CZ m-2), Itr-2 carbonate-zeoliteveins is similar to that of the manganoanankerite is representedby veinJike aggre- manganoanankerite from the late C tr-2 carbonatite. gates of anhedral gains, 0.05-1 mm in diameter, Sr is abundantin the manganoana:rkerite of the fint developedalong grain boundariesof calcite, ferroan textural type from both the late carbonatite and rhodocbrosite or rnanganoansiderite, and cleavage carbonate-zeoliteveins (average0.37 vtt.VoStO, range crackswithin theseminerals. The typical featureof the 0.18-0.41wt.Vo),but Sr is low in the secondtextural oobelow manganoaoankerite of the secondtextural type is a type (average0.07 wt,cloSrO, range detection" fan-shapedor wavy extinction. These observations 4.I2 *.Vo). Manganoanankerite of the second show that (he manganoanankerite of the second textural type also has a high content of CaFe(CO3)t textural type is a late-stagesecondary mineral. relative to that of the fust textural type.

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Fenoankutnafutrite MgC03 n Ferroankutnohorite is the major carbonatephase in / \ someveins of late C tr-3 j ^L \ manganoanankerite ferroan t rhodochrosite carbonatite with synchysite-(Ce). It 'I forms white or grey aggregalesof fine needlesoro less ^ - commonly, banded aggregateswith fan-shaped or wavy extinction. Petrographicobservations show that ferroan kutnohoritereplaces both manganoanankerite a o+ and ferroanrhodochrosite" and is secondarv. a r ^^ l+^f+ Selected electron-microprobe data are given in Table 3, and the results are plotted in Figure 6. The maximum content of CaMn(COr), component is FeC03 MnC03 69 mol.Vo. The average amount of Sr in ferroan kutnohoriteis 0.10wt.7o SrO. + cII.3 ^ Czfit-t o CZm-2 o CZilI-3 Ferroan rhodachrositeand manganoansilerite FIc. 8. Composition (mol.7o) of ferroan rhodochrosite - Ferroan rhodochrositeand manganoan manganoansiderite - siderite in the syst€m MgCO: - siderite - are the dominant carbonatesin one late FeCO3 MnCO3.Symbols: C tr-3 ferroan rhodocbrosite carbonatite - manganoan (C tr-3) and in all carbonate-zeolite (CZ ankerite carbonatitewifl synchysite-(Ce), veins m\ CZ m-l manganoan siderite - manganoan respectively. ankerite Thesecarbonates take the form of brown - natrolite veins, CZ fII-2 manganoan siderite - or grey anhedralor subhedralgrains ranging in size nordstrandite - natrolite veins. CZ ltr-3 natrolite - from 0.1 to L0 mm. Two distinct textural typesmay be mnqganoansiderite - dawsoniteveins. distinguished.In the fusg ferroan rhodocbrosite(in the C tr-3 carbonatites)and manganoan siderile (in the CZ m-I carbonate-zeolite veins) form subhedral crystals(up to 10 mm in size) and have sharpcontacts with manganoan ankerite, without evidence of 0.19 tvt.VoSrO, range 0.L24.29 wt.7o) than those replacementbetween the two. In the secondtextural of the second textural type (secondary) (average type, manganoansiderite occnrs as anhedral, small 0.05 wt.VoSrO, range "below detection'n4,O7 wtVo). grains (0.1-2.0 mm) with wavy extinction. The analytical data (Table 4) show that there is a Si.d.erite large compositionalrange of rhodochrosite- siderite solid solution. The FeCO3content gradually insreases Siderite was found in tle natrolite - manganoan from 37.5 mol.Vo tn late carbonatite to 68 mol.Vo siderite - dawsonite veins (CZ Itr-3) only and is in carbonate-zeolite veins (Fig. 8). The ferroan presgntin minor to accessoryamounts. It forms either rhodocbrosite and manganoansiderite of the first as white or grey subhedralgrains (0.5-2.0 mm) or as textural type (primary) contain more Sr (average rhombohedralcrystals in vein cavities. The composition (Table 4, Fig. 8) shows limited substitutionof Mn for Fe.

CensoNAND OxycEN Isoropn Corvrposrnon TABLE4. RESULTSOF SELECTEDANALYSFSOF FERROAN RHODOCHROSITE.MANGANOANSIDERITE A}ID SIDERITE Isotopic analysesof C and O .were made at the Institute of Geochemisty and Geophysics,Minsk. Ro*idq Ctr-3 C II-3 c tr-3 czm-l ufr:3 Measurementswere performedon a MI-1201 B mass Sanple @. @ @3 @- @ *100-K" 4i3.0 417.O 23t.0 75.0 2l.?.0 spectrometer with sample (Kiev) as the laboratory standard. The "l0G-K" standard was (613C= CaO Et% 1.39 1.08 o24 t.4l 0.44 calibrated to the PDB and SMOW scales MgO 1;14 l.E3 3.58 2.4 0.15 4.9Voo PDB and 616Q = +20.3VooSMOW). The MtrO 34.00 ?2.07 2t.ol 1E.41 t.29 uncertaintyis consideredto be *0.243%o. reo" 2397 24.69 28.80 37.59 6A2l The carbon sro 0.06 022 !d 0.06 0.02 and oxygen isotope data are shown in Figure 9 and Ls2q 0.10 d qd bd !d Table 5. CeZOI bd !d !d bd !d CO2 38.n 37.95 39.12 3E.50 3E22 In general,the isotoperatios of C (613Cin the range Total 98.63 q7.u 9.75 98.6r [email protected] -7.8 to -2.8VooPDB)and O (61EOin the range+5.9 to +l4.3VooSMOW) for the rhombohedral carbonates feor - trcal Feolrulated as FsO;nd - !.rt de$smi!€d:bd - belm &b.tim from the l(hibina carbonatitesand caxbonate-zeolite

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ii -6 F{

9-I o -8 -9 4 6 8 10 t2 t4 16 deltal8Oo/ooSMOW

rCI nCII-1 .Cil-2 +CII-3 LCZIII-7

Flc. 9. Carbon and oxygen isotopic composition of the carbonatesin the Khibina carbonatites.Symbols: C I early calcite carbonatite,C tr-l late calcitecarbonatite with burbankite,C II-2 late manganoanankerite - calcite carbonatitewith synchysite-(Ce), C tr-3 late ferroan rhodochrosite- manganoanankerite carbonatite with synchysite- (Ce), CZ [I-1 manganoansiderite - manganoanankerite - natrolite veins. Field of "carbonatitebox" is after Taylor et al. (1967),Sheppard & Dawson(1973) and Hoefs (1987).Dotted tines join coexistingcarbonates.

ttglrzg 180/160 veins are similar to carbonatitesfrom many other where carbonateshave hrgfier and localities (Deines 1989). Data for fifteen carbonate valuesthan the calcitefrom the carbonatitesof group 1. samples1613C in the range-7.8 to -4.07ooPDB and Manganoanankerite and calcite have similar carbon 6tEOin the ranger5.9 to +L0.2%oSMOW) plot within isotopiccompositions (613C in therange -3.9 to 45%o or near the oocarbonatitebox'o of primary, high- PDBj, but show a very wide rangein 6180 1+8.0to temperaturecarbonatite according to Taylor et al, +I4.3VaoSMOW). There is a positive correlation (1967),Sheppard & Dawson(1973) and Hoefs (1987). bet'ween 613C and 6180 values for the calcite, Data for five carbonate samples show slight manganoan calcite and manganoan ankerite from enrichmentin 13C(613C in the range4.2 to -23%o carbonatiteof groups I and2. PDB) and 18O(6180 in the range +12.0 to +l4.3Vao Group 3 includes the ferroan rhodochrosite- SMOW), as shownin Figure 9. manganoanankerite carbonatites with synchysite-(Ce) On the basisofisotope ratios ofC andO in various (C [-3) andmanganoan siderite - manganoanaxkerite carbonates, the early C I carbonatites, late C II - natrolite veins (CZ [I-1), and these carbonates carbonatitesard CZ III carbonate-zeoliteveins are exhibit a wide variation in isotopic compositionof C divided into three groups.Group 1 includes early C I (613Cin the range-7.8 to -2.8%oPDB)and o (61EOin carbonatites and late C II-l carbonatites with the range +:7.9 to +l4.Moo SMOW). In this group, the burbankite.Calcite from thesef)?es of carbonatiteshas isotopiccomposition of carbon(6t3C) in manganoan similarcarbon 16t:C in the range-6.8 to -5.5%oPDB) ankerite,ferroan rhodochrosite and manganoansiderite and oxygen (6t8O in the range +5.9 to +I0.2%o correlateswith depth of sample location in the drill SMOW) isotopic composition. This indicates a hole. In the upper part of a drill hole, carbonateshave commonsource for carbonand oxygenin eachcase. a lower 6l3C values than those obtained from the Group 2 is representedby manganoanankerite - deeperparts of the drill hole (Fig. 10).ft i5 imFortantto calcite carbonatitev/ith synchysite-(Ce)(C II-2), note, however,that the carbon and oxygen data from

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TABLE5.6I3C.AI.ID 6I8GVALUESFORCARBONATES IN THE KHIBINA DrscussrouAND CoNcLUsroNs CARBONATITES The compositions of carbonates from most pDB * Holc Dcpth (n) MireEl 6l3co/@ 6t8ool@ SMOW carbonatite complexes :re well representedin the system CaCO3- MgCO3 - FeCO3.Carbonates from Early elcib e$onatitc C I plot 6328 1934.0 eloite -62 16.t early varieties of carbonatiteusually within the 6328 1960.0 @lcite "5.5 19.0 subsystemCaCO, - CaMg(CO3)2- CaFe(CO3)2,and 632 1984.0 €lcite -5.7 +9.1 those from late varieties of carbonatites are well Iae elcite qbontits witl bubaDliteC IIl describedin the subsystemCaMg(CO3)2 - CaFe(CO)2 603 158.0 €lcite -5.6 +8.4 - - 603 191..9 €lci6 -5.8 +10.2 FeCO3 MgCO3. The evolution of carbonatesin ffi1 2ElJ oalgumelcite -5.9 19.1 carbonatiteshas beendiscussed in detail by Samoylov 633 471.7 €lcite {.E +5.9 (1984), Sokolov (1985) and Buckley & Woolley fale o@gM arkqitFqlcite carto@lit$ wilh synchFilo{Ca)C tr-2 (1990). According to their data, one common scheme 603 122.1 elcite 4.0 fI0.5 603 165.5 dcnc -3.5 +t4.3 for the mineralogicalevolution of carbonatites,based l0 603 ns,z @g@ aikerits 4.2 +12.0 on agerelationships and chemicalcomposition, can be lt 6334 414.1 mig@ a*qile -3.9 +12.8 6M 454.0 malg@artFilo 4.3 +E.0 describedby the following succession(from early to late): calcite + calcite + dolomite + calcite + lue fe!@ drcdohmitemlg@ addito @rbolditd rith sy!ob8it6-(Ce) C tr-2 l3 @3 34.5 n@Ba,@atkerite -7.4 +10.2 dolomite + ferroan dolomite -) ferroan dolomite 14 &3 34.5 ftr@ ftodocJruite -5.2 +8.4 + alkerite -? ankerite+ siderite+ magnesite. 15 603 89.0 ndqgu@anksite 4.4 +7.1 16 603 89.0 fell@ rhod@hmite -5.4 +9.5 Someof the carbonatesdescribed above are unusual 11 @3 238.0 rmgman&qite -3.8 +13.0 for carbonatitecomplexes. For example, at Kovdor, 18 603 238.0 ftr'@ rbodchrcle 4.4 +E.4 Vuorijarvi, Sallanlawaand other carbonatiticmassifs, Maie@ sidqilH@rg@a*qits-rdolite reias CZ fin only Ca-Mg-Fe phases (calcite, dolomite-ankerite, 19 603 49.8 m'ne@sidqitr -6.9 +:1.9 20 603 184.0 mngql@ sid*ile -3.5 +11.3 magnesite-siderite)have been observed as major 21 608 235.0 nogm sidrite -2.8 +14.0 minerals (Kapustin 1980). However, carbonateswith a high Mn content are not unique to the carbonatites from Khibina. Other carbonatite complexescontain manganoancalcite @olshetaginskii), kutnohorite(?) @olshetaginskii),rhodochrosite (Kovdor, Vuorijarvi, Sallanlagva,Mbeya Amba Dongar), and manganoan- magnesiansiderite (Kanga::kunde),but in all of these manganoan ankerite, ferroan rhodochrosite and complexes, these carbonatesoccur as accessory manganoansiderite from deeperparts of drill hole are minerals, and few descriptions are available in the in agreementwith carbon-oxygen relations in the literature. carbonatesof groups L and 2. These data plot in the The field relations of carbonatites and 613C-and 618o-enriched field on the diaeram. carbonate-zeoliteveins and mineral chemical data show tlat there is a mineralogical evolution of carbonatesin the Khibina carbonatites.The earliest carbonate phase to crystallize was calcite in tle C I carbonatites.In the late C tr carbonatites.calcite evolvedto manganoancalcite, which was followed by an associationof calcite and manganoanankerite or -2 manganoanankerite only. The late carbonatephases to m-3 crystallize in the C II carbonatitesare representedby the association of manganoanankerite and ferroan FA + rhodochrosite.and the formation of ferroankutnohorite was followed by the same coexisting carbonates.In €-s +'+ u-6 CZ II carbonate-zeolite veins, the sequence ca manganoan ankerite + manganoan siderite (or manganoanankerite alone) followed by manganoan 'd siderite and sideriteis observed. !0 -8 This sequenceof carbonateevolution is supported -9 by data from mineral chemistry. The rhombohedral 0 50 100 150 200 250 carbonatesshow gradationalchanges in their content Depth(m) of major and trace elements from early to late carbonatites.High-Sr, low-Mn calcite in the early Frc. 10. Variations h 6l3C in the carbonatesin a vertical C I carbonatiteschanged to high-Sr,high-Mn calcitein sectionofthe drill hole. Symbols:see Fig. 9. the late C tr-1 carbonatites,and to low-Sr, high-Mn

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calcite in the C tr-2 carbonatites(Frg. 5). Such a equilibrium are probably close to their temperatureof disribution of Sr and Mn in early- and late-stage formation. calcite (high-Sr - low-Mn early calcite and low-Sr - Minerals of the dolomite group from the late C tr high-Mn late calcite) has also been documentedfrom carbonatitesand CZ III carbonate-zeoliteveins are - other carbonatites@ouliot 1970,Hogarth et al. L985, well representedin the system CaMg(CO)2 Sokolov 1985, Platt & Woolley 1990, Clarke et aI. CaFe(COr), - CaMn(CO3)r.This dolomite composi- 1992) and seemsto characterizemost early- and late- tional spaceis not as well studied experimentallyas stagecalcite. Compositionalevolution is also observed other carbonatesystems. The subsolidusrelationships for ankerite-kutnohorite and rhodochrosite-siderite in this system are indicated on Figures 6 and 7 solid solutions.The CaMn(CO3)2component increases (Rosenberg1968). At a T of 450"C and a P,oo1of from low-Mn ankerite(C tr-l carbonatites)to ferroan 2.1-2.6 kbars, the dolomite compositional space kutnohorite (C tr-3 carbonatites)(Fig. 6), and ferroan contains one single-phase,one three-phaseand three rhodochrositein the C tr-3 carbonatitesis followed by two-phaseareas. The three-phasefield of coexisting manganoansiderite (CZ m carbonate-zeoliteveins), dolomite solid-solution and Ca-rich and Ca-poor with pure siderite being found in the latest CZ m-3 solid-solutions indicates a limited solubifity of the [email protected]). CaFe(CO3)2component in dolomite, up to approxi- There are no clear criteria to indicate the relative mately 74 mo1'%o,and this field extends from the position of dolomite and 'opure" ankerite in this join CaMg(CO)z - CaFe(CO:)z to approximately schemeobut the high Sr contentin theseminerals may 40 mol.Voof CaMn(CO3)r.fr" experimentaldata of indicate formation in the early stages,possibly after Rosenberg(1968), as well as data from the mineral calcite and before manganoanankerite. chemistryof natural samples@ssene 1983), show that Petrographicand cathodoluminesenceobservations there is complete solid-solution between reveal two different textural types of carbonate dolomite-ankerite and kutnohorite, and there is no minerals, In the first type, carbonatesoccur as solid solutionbetween kutnohorite and the CaFe(COr)t subhedral crystals, in cases with internal zonation, component. which show an equilibrium relation to coexisting The resultsof carbonateanalyses from the Khibina mineralsand seemto be primary phases.In the second carbonatitesand carbonate-zeoliteveins plot in four textural type, carbonates occur as vein-like fine- fields: 1) one-phasedolomite solid-solutionfield, gmined aggregatescontaining anhedralgrains, which 2) two-phase dolomite solid-solution + Ca-poor are developed along grain boundariesand cleavage solid-solution (magnesite- siderite - rhodochrosite cracls of crystalsand envelopgrains of otherminerals. solid-solutions) field, 3) two-phase Ca-poor solid- Carbonates showing such textural relations are solution + Ca-rich solid-solution (calcite) field, and interpretedto be secondaryand metasomaticin origin. 4) three-phasedolomite solid-solution+ Ca-poorsolid- Thesepetrographic observations are also supportedby solution + Ca-rich solid-solution field (Figs. 6, 7). dataon Sr distributionbetween minerals from different Equilibrium assemblagesof carbonatesin the Khibina textural types. The Sr content is relatively high in carbonatites are calcite + manganoan ankerite, minerals of the first type, but is relatively low in the manganoan ankerite + ferroan rhodochrosite and sameminerals of the secondtextural type. manganoanankerite + manganoansiderite, and the The rhombohedral carbonatesof the Khibina presenceof thesemineral pairs is generallyconsisient carbonatitescan be describedin terms of the CaCO3- with experimental data. MgCO3 - FeCO3 - MnCO3tetrahedron. Coexisting Data from somesamples of the manganoanankerite calcite and low-Mn ankeritefrom the late C tr-l and [with high content of CaFe(CO)2] and ferroan C tr-2 carbonatitesbelong to the CaCO3- MgCO3 - kutnohorite plot outside their stability fields, and can FoCO3system. Subsolidus relationships in this system probably be related to the difference in the P-T were determinedby Goldsmi0l Rosenbergand other conditions of experimentsand those of carbonatite investigators(Goldsmith 1983). The calcite-dolomite formation. Limited experimental data in the geothermometerof Talantsev(1981), which includesa CaMg(CO.), - CaFe(CO)2 and CaFe(COJz - correctionfor Mn, suggestsequilibration temperatures CaMn(CO3)2systems show displacement of phase of 260-330'C for carbonatesfrom the late C tr-l and boundarieswith changing of temperature.However, C Il-2 carbonatites.Coeristing pairs of calcite - with a fall in temperature,the stability fields of ankerite manganoanankerite yield equilibration temperatures and kutnohorite shrink, and this explanation is of. 260', 2i70" and 300'C (all *15'C) for calcite regardedas less likely. carbonatite with burbankite (C tr-l) and 285' and An alternativeexplanation of this phenomenonmay 330"C (all tl5'C) for manganoanankerite - calcite lie in the metastable compositions of manganoan carbonatite with synchysite-(Ce)(C II-2) (A.S. ankerite [with high content of CaFe(CO)z] and Talantsev,pers. comm., 1989,7,aitsev 1992). The lack ferroan kutnohorite. The tendencyfor compositional of subsolidusalteration, e.9., exsolution or recrystal- metastabilityat low temperatureof high-Fecarbonates lization, shows that temperatures of carbonate from the dolomite group has beennoled by Goldsmith

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(1983).As was describedabove, high-Fe manganoan respectively.These featuresshow that the coexisting ankerite and ferroan kutnohorite from the Khibina carbonateswere not in isotopic equilibrium during carbonatitesshow evidenceof a metasomaticorigin, their formation (Deines 1989). Petrographicand CL and their temperatureof formation can be 260'C and observationsdo not revealany evidenceof alterationor below. other secondaryprocesses in these samples of the Carbonatesbelonging to the siderite-rhodochrosite carbonatitesor the carbonate-zeoliteveins. solid solution from the late C tr-3 carbonatitesand The depletion in heavy carbon in carbonatesfrom CZ m carbonate-zeoliteveins lie close to the the upper part of drill holes may be explained as FeCOr-MnCOljoin (Fig. 8). The carbonarecomposi- partitioning of heavy C into CO2 gas phase, which tions are consistent with experimental data of escaped during carbonatite emplacement. The Rosenberg(1963) and with data on the mineral possibility of sucha mechanismhas beenproposed by chemistry of natural samples @ssene1983), which Suwaet al. (1975)and Knudsen & Buchardt(1991) to show complele solid-solution between FeCO3 and explainlow 6l3Cvalues in carbonatitesof the Oldoinyo MnCO3, and limited incorporation of MgCO, (up to Lengai and Qaqarssuk,and by Clarke et al. (1992) for 3Omol.Vo) in this system. carbonatitetuffs at Kruidfontein. The evolution of carbonatites,as well as of Field relations, petrographicand CL observations carbonatesin carbonatites,is also revealed by their suggest that the late C tr carbonatitesand CZ U. C and O isotope compositions. Relations between carbonate--zeoliteveins aremore likely to haveformed carbonand oxygenisotope compositions usually show from a fluid phase rather than from a melt. The a trend of enrichment in 613C and 6180 during evidencefor this hypothesisincludes: field relationsof carbonatiteevolution @eines1989). Interpretation of a carbonatitesand wallrocks (gradational contacts, positive correlation between 61lC and 6180 values presenceof extremelymetasomatized relics of silicate in carbonatitesincludes: 1) a Rayleigh fractionation wallrocls in carbonatite, intensive fluoritization of model and 2) a temperature-controlledfractionation wallrocks), relationships between various types of model. carbonatitesand carbonate-zeoliteveins (metasomatic Rayleighfractionation of C and O isotopeshas been characterofcontacts between carbonatites, changing of demonstratedby Pineau et al. (L973) and Deines mineral composition of carbonate-zeolite veins in (1989).According to Pineauet al. (L973),theC andO conjunction with those of surrounding carbonatite), isotopic composition of carbonatitesdepends on the textural and structural features of carbonatites[two molar ratio H2OlCO2in the CO2-H2Ofluid phasethat distinct textural types of carbonates,primary and accompaniescarbonatite magma. The temperature- secondary,crustification (vein-filling), drusesl. The controlled fractionation model was presentedby best explanationof theseobservations is precipitation Plyusnin et al. (L980), Samoylov & Plyusnin (1982) of carbonatitesfrom gas-richfluid phase. and Samoylov (1984). According to this model, The geochemistryoflate carbonatitesalso supports carbonaiephases precipitating in the systemCaCO3 - this suggestion;these rocls are extremelyenriched in COz - CO32-- HzO will be characterizedby an rare-earthelements, with La + Ce + Nd contentsin the indease in bt3c -d 6tto values as the temperature rangebetween 12200 alord 74350 ppm (A.N. Zaitsev & decreases,and this is in accordwith the.observedfield M.J. t€ Bas, rrnFubl. data). Experimentaldata show relations,which show low 6l3C and 6180values in that enrichmentia REE in CO2-richgas phaserelative high+emperature,early-stage carbonatites, a4d high to carbonatitemelt increaseswith decreasingT and P 6l3C and 6180 valuesin lo\il-temperature,late-stage fWendlandt& Hanison 1979). carbonatites(Samoylov & Plyusnin 1982, Sarnoylov Experimental data suggest that the primary 1984). carbonatiticfluid necessaryfor the fonnation of late Isotope data describingthe Khibina carbonatesare carbonatitesand carbonate-zeoliteveins must have consistentwith both models; it is possible that both been an alkaline (Na K) carbohydrothermalsystem Rayleigh and temperature-inducedfractionation took containinga Ca(CO)o(OH)ff or HCq - CO?-- CO2 place during formation of the Khibina carbonatites. - OH- aqueoussolution and CO2-H'O vapor (Malinin However, these models cannot explain the wide & Dernov-Pegarev1974, Dernov-Pegarev& Malinin variation in carbon and oxygen compositionsof the 1976). Thermodynamiccalculations for a range of carbonatesfrom C tr-3 carbonatitesand CZ m-L temperatures(35M00"C) and P = 1 kbar show the carbonate-zeoliteveins (group 3). The tendencyfor existence of such fluids with X(CO) in the range changing613C in carbonatesfrom this groupwith depth 0.m.6. This fluid is homogeneousat temperatures o1 sample location has already been noted. An other higher than 25V270"C, and heterogeneous(HrO-rich important observation is the difference in isotopic solution and CO2-richvapor) at temperaturesless than composition between coexisting manganoanankerite 25U270'C. The processof fluid transformationfrom and ferroan rhodochrosile.Values of Al3C***o_ homogeneousto heterogeneouscan be accompaniedby atqite - fmm rhodrchrorl6 :ttrd AluOruemom mkerite - im* explosive events @ulakh & Ivanikov 1984). More rhod@hrcsiteare -2,6; +1,0; +0.6 and-+1.8; -2.4; +4.6, detailed information about thermophysicalproperties

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of CO2-H2O mixtures, namely P-V-T-X University), L.I. Polezhaeva(Geological Instifute, measurements,equations of state and experimentally Apatity) and P. Jones (Carleton University) for determinedphase-equilibria is availablefrom the work assistancewith electron-microprobeanalyses, and of Miider(1991). I. Kolosov (Instifute of Geochemistry,Minsk) for Temperaturesof equilibration of 260-330oC for assistancewith isotope analyses.R.F. Martin (McGill coexistingcalcite - manganoanankerite in the C tr-l University), R.H. Mitchell (Lakehead University), and C tr-2 carbonatitesowhich are probably similar to F. Watl (Natural History Museum,London) and B.C. their temperaturesof fornation, suggestthe formation Jago Qnco Exploration and Technical Services) are of C II-2 and C II-3 carbonatites and CZ In thanked for extensivereviews and help in the final carbonate-zeoliteveins at a temperatureof 260'C or preparation of the manuscript. K. Bell (Carleton less. At such temperatures,a late-stagecarbonatitic University), D. Hogarth (University of Ottawa), M.J. systemwill be heterogeneous(solution + vapor), and Le Bas (,eicester University), T. Frish (Geological volatiles can be lost through explosive events Survey of Canada) and A. Simonetti (Carleton accompanyingtransformation of the fluid. Loss of a University) provided critical reviews of an eadier vapor phasefrom carbonatiteis also supportedby data version of the manuscript. G. Ivanuk (Geological on He, Ne andAr isotopic compositionsof gasesin the Instifute, Apatiry) kindly drafted the figures. Most Khibina carbonatites.The isotopic compositionof inert sincereand specialthanks go to my wife Helen for her gasesshows that the carbonatiteswere intruded close support. This work was partly funded by the Royal to the surfacewith intensivedegassing (Tolstihin er a/. Sociefy (project 638U 2.P699). 1985).There is no sign of oxidation accompanyingthe degassing. Rnrannrcss This study shows that the carbonate-bearingrocks from the Khibina massif belong to the rocks of ARzAMAsrsEv,A.A., KavrnNa, V.A. & PoL;eaIAEvA,LJ. carbonatiteseries and that they define a crystallization (1988):Dykes of theKhibhn Massif and ix Surroundings. of Sciences, sequence from biotite - aegirine - apatite rocks Kola ScienceCentre of theUSSR Academy Apatity,Russia (in Russ.). through various carbonatites to carbonate-zeolite veins. The study of the rhombohedral catbonates, Benrr, D.K. (1993):Carbonate magmas. J. GeoI.Soc. calcite, manganoan calcite, dolomite, ankerite, Inndon150. 637-651. manganoan ankerite, ferroan kutnohorite, ferroan rhodocbrosite,mangaooan siderite and siderite,reveals BucKr,By.H.A. & Woou:ev,A.R. (1990):Carbonates of the a mineralogical and chemical evolution of these magresite-sideriteseries from four carbonatite complexes. minerals during carbonatiteemplacement. That most Mineral.Mag. 54, 4134L8. carbonatitesare of magmaticorigtn has beenaccepted (1978):Origin of in the Westernscientific literaturefor many years(e.9., BULAKH,A.G. & Isroz-Dor.nu.la,I.P. carbonatitesin the Centralnyymassif, T\riy peninsula. Bailey 1993).At the sametime, Russianinvestigators Int. Geol.Rev. 20, 822-828. proposeda hydrothermal-metasomaticorigin for some carbonatites(Kukharenko et aI. 1965,Bulakh & Iskoz- & IvANtr(ov,V.V. (1984):Problems of Mineralogy Dolinina 1978,Kapustin 1980).This study showsthat and Petrol.ogyof Carborwrttes.Lenin$ad State Univ.' the Khibina carbonatitesare polygenetic in origin. lrningrad, Russia.(in Russ.). Early C I carbonatites are magmatic, late C tr carbonatites are interpreted as precipitated from CLARKB,L.B., Ls Bas, M.J. & Srno, B. (L992)tRare earth, volatile-rich fluid (carbohydrothermalsystem), derived trace element and stable isotope fractionation of /n hoc. from early-crystallizing magmatic carbonatite, and carbonatitesat Kruidfonteiq Transvaal,S. Africa- Conf. 1. Kimberlite, Related Rocks and CZfr. carbonatp-zeoliteveins arehydrotherrnal veins. 5th Kinrberlite Mantle Xenoliths. CPRM, Brasilia Bruzil (236-251). The mineral assemblages,mineral chemistry, and C and O isotopic data suggestthat the evolution of the DstrrFs,P. (1989):Stable isotope variations in carbonatites.ln Khibina carbonatitesmay be interpretedas produced Carbonatites:Genesis aad Evolution (K. Bell, ed.).Unwin by crystallization-inducedfractionation. Hyman,London, U.K. (301-359).

Acm{owLEDGBuEr.ITs DmNov-PEGAREv,V.F. & MALn{D,I,S.D. (1976):Solubility of calcite in high temperatureaqueous solutions of alkali This paperresulted from a Ph.D. thesis,carried out carbonatesand the problem of formation of carbonatites. Int. l3(3), 1'-13. in the Department of Mineralogy, St. Petersburg Geochetn, University, under the supervisionof A.G. Bulakh. The DrDK[.], O.8., MINAKov, F.V., KRAVcIDNKo,M.P., author thanks V.P. Pavlov (Murmansk Geological KRAVCHBNKo,E.V., Krnarov, A.N., Pouananva, L.I., Expedition, Apatity) for help with field work, Yu.P. Pnpactu

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coLDsMrH, J.R. (1983): Phase relations of rhombohedral Maruaro, A.N. & RoDR, P.L. (1983):Kerimasi: a neglected carbonates.1z Carbonates:Mineralogy and Chemistry carbonatitevolcano. J. Geol. 91.449455. @.J.Reeder, ed,.). Rev. Mineral. 11,49-76. Maxsuau,, D.J. (1988): Cathodoluminescenceof Geological Homs, J. (1987): Stable Isotope Geochemisrry.Springer- Materials. Unwin Hymaa, Boston,Massachusetts. Verlag, Berlin, Germany. MD{AKov,F.V. & Dunml, O.B. (1974):Possible presence of HoGARIH,D.D., Hanrns, R., Ioop, S. & Sor,sERc,T.N. carbonatitein the Khibiny alkalig plulea. f okl. Acad. Sci. (1985): Rare-earthelement minerals in four carbonatites USSR(Earth Sci.Sect.) 215,109-11,2. near Gatineau,Quebec. Am. Mineral T0, 1135-1.1.42. & KaMENsv,Ys.A. (1981):The Khibiny KApusrIN, Yu.L. (1980): Mineralogy of Carbonatites. carbonatitecomplex. Dokl. Acad. Sci. USSR(Eanh Sci. Amerind Publishing,New Delhi, India- Sect.)259,58-60.

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KRAMM, U., Kocamo, L.N., KoNoNovA, V.A. &. RosEI.IBERG,P.E. (1963): Synthetic solid solutions in the Vannanrlw,H. (1993):The Kola alkalineprovince of the systems MgCO3-FeCO3 and MnCO3-FeCO3.Am. CIS and Finland: preciseRb-Sr agesdefine 380-360 Ma Mineral. 48, 1396-1,400. ageraage for all magmatism.Lithos 30,3344. (1968):Subsolidus relations on the dolomitejoin Knasrova, N.I. (1988): Diagnosticfeatures of metasomatism. CaMg(COf 2-CaFe(COf2-CaMn(CO)2. Am. Mineral. Int. Geol. Rev.30, 1070-1083. s3. 880-889.

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