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And Ta-Bearing Oxide Minerals in the Greer Lake Pegmatitic Granite and Its Pegmatiteaureole, Southeasternmanitoba

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And Ta-Bearing Oxide Minerals in the Greer Lake Pegmatitic Granite and Its Pegmatiteaureole, Southeasternmanitoba American Mineralogist, Volume 71, pages 501-517, 1986 Fractionation trends of the Nb- and Ta-bearing oxide minerals in the Greer Lake pegmatitic granite and its pegmatiteaureole, southeasternManitoba Pnrn ennmi, Bnucn E. Golo,r FuNx C. H,lwruronNr, RoN CrHplr,lN Department of Earth Sciences,University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada Arsrucr The Greer Lake pegmatitic granite and related exterior rare-element pegmatites of the beryl-columbite type intrude metabasalt and tonalite gneiss in the Archean English River Subprovince of southeasternManitoba. Columbite-tantalite is the predominant Nb- and Ta-bearingmineral, associatedwith subordinateixiolite, microlite, niobian-tantalian rutile, and rare tantalian cassiterite, wodginite, and ilmenite. In coexisting mineral pairs, Tal(Ta + Nb) of microlite exceedsthat of columbite-tantalite, ixiolite, rutile, and cassiterite;in cassiteriteand rutile, Tal(Ta + Nb) is higher and Mn/(Mn * Fe) lower than in columbite- tantalite and ixiolite. In Li-, Rb-, Cs-, and F-poor environments, limited Mn enrichment accompaniesthe fractionation of Ta, which culminates in ixiolite and subordinate microlite. In Li-, Rb-, Cs-, and F-rich parageneses,extensive Mn enrichment precedesthe main Ta fractionation, which subsequentlygenerates near-end-member manganotantalite, wodginite, and mi- crolite. The Greer Lake and other fractionation trends indicate that a late-stageF-rich environment promotes extreme Fe/Mn fractionation prior to the main stage of Nb-Ta separation. The abundancesof Sn, Ti, and Sc are not related to fluorine or rare-alkali enrichment, but increasefrom pegmatitic granite to pegnatites. The relative accumulation of Ti and Sc in the pegmatite aureole seemsto be due to internal fractionation rather than assimilation. Columbite-tantalite in the pegmatitic granite shows an intermediate to near-ordered structural state, but is highly disordered in the pegmatite veins. Crystallization in the disorderedstate is suggested,with subsequentordering in slowly cooling granitebut thermal quenchingin the exterior pegmatites.However, the increasein Ti, Sn, Sc, and Fe3*in the Nb- and Ta-bearing minerals of the pegmatite aureole may also have retarded ordering. INrnooucrroN pegmatitesand restrictedgroups ofcogenetic veins (eern!' Parageneticand geochemicalrelationships among Nb- and Turnock, I 97 I ; Grice et al., | 972; Foord, | 976; l-ahri, and Ta-bearingminerals from cogeneticgranite-pegmatite 1984;v. Knorring and Condliffe,1984; Lumpkin et al., suites are the most poorly documented and least under- 1986), but even here the available information is often stood aspectsof this mineral family. Despite recent work sketchy. on the compositional and structural variability of these The presentpaper is the first in a seriesofcomprehen- species(Foord, 1982; eerni and Ercit, 1985),the geo- sive studies, conducted at our department, dealing with chemical featuresof their evolution in differentiating re- the parageneticand geochemicalevolution of Nb- and sidual granitic magmas are obscure. Early work lacks spe- Ta-bearing minerals in geologicallydefined and petrolog- cific data and tends to be oversimplified (Ginsburg, 1956; ically well-characterizedpegmatite sequences. The general Beus,1960; Solodov, l97l; Kuzmenko, 1978).More re- researchprogram is basedon the development of reliable cent studieson the regionalmineralogy ofpegmatite fields standards and procedures for electron-microprobe anal- and provincesgenerally lack information on the behavior ysis (Ercit, 1986) and on extensivefield work (Anderson, of the Nb- and Ta-bearing phasesin different pegmatite 1984;Ferreira,1984; Goad, 1984;Paul, 1984;Wise and typesand on their individual fractionation paths (Sahama, Grn9, 1984a;Cernj'et al., 1985b;Ercit, 1986;Cernj'and 1980;Wang et al., l98l; v. Knorring and Fadipe, l98l). Ercit, 1985; M. A. Wise, unpub. ms.). The presentstudy The only exceptionsare studies dealing with individual utilizes some of the early results of Cernf and Turnock (1971), but it is based mainly on Goad's (1984) thesis derived from field and laboratory studies terminated in ' Presentaddress: I I l5l KingsbridgeDrive, Richmond,Brit- 1979and on extensiveadditional samplingand laboratory ish ColumbiaV7A 4Tl, Canada. work conductedby the other authors in 1979-1984. 0003-004v86/03044501$02.00 50r 502 egnvt ET AL.: FRACTIoNATIoN TRENDSoF Nb- AND Ta-BEARINGoxIDEs - \ o oo o MAP AREA ENGLISH BLOCK innipeg o-'r+ 5km= Fig. l. Location of the study area and its geology:Bird River GreenstoneBelt (open) between the Winnipeg River Batholitic Belt (WRBB) and the Manigotagan-Ear Falls Gneiss Belt (MEGB); Maskwa Lake (MWL) and Marijane lake (MJL) tonalitic diapirs, eastern end of the complex Lac du Bonnet batholith (LdB). Most pegmatite groups (each marked by ditrerent spot symbol) are associatedwith pegmatitic granites(black; GL, ENL, AX, OL, and TL). Modified from Cern! (1982). Gnnnn L.lxn cnc,NrrE AND ITs pEGMATTTE AUREoLE Cernf, l98l; Iongstaffeet al., l98l; Cern!'et al., l98la). It is Iocation and regional geology hosted largely by metabasalt. At the eastern and western extrem- ities ofthe intrusion and at its southwesternoffshoot, the contacts Greer Lake is located 135 km east-northeastof Winnipeg, at trend narth at high angle to the layering and foliation of the lat 50"20'39'N, long 95'19'W (Fig. 1).The pegmatiticgraniteand metabasalts. These contacts are sharp, truncating both the lay- its pegmatite aureole are part of the Cat Lake-Winnipeg River ering and foliation with no evident deformation. Host-rock in- pegmatite field situated in the westernmost English River Sub- clusions are rare in the pegmatitic granite, except for angular province ofthe SuperiorProvince (Cernj et al., I 98 la). The Bird blocks ofmetabasalt in the southwesternofshoot indicative of River GreenstoneBelt, host of the pegmatitefield, consistsof six stoping. A fault, striking west-northwest and transecting the Greer formations of metavolcanic and largely related metasedimentary Lake intrusion, displays subvertical movement and separatesthe rocks of the Rice Lake Group, which form a broad and complex granite into two segments (Fig. 2). It seems likely that the peg- synclinorium (Trueman, 1980). Two major episodesof folding matitic granite is also truncated by a fault marking the boundary afected the greenstone belt; the second correlates with the dia- between the Bird River Greenstone Belt and the Winnipeg River piric intrusion of the Maskwa Lake and Marijane lake batholiths BatholithicBelt (Figs. l,2). and with the peak of the regional metamorphism. Metamorphism The pegmatitic granite is compositionally and texturally het- attained the greenschist-facies level over most of the map area erogeneous:(l) pegmatitic leucograniteis dominant, consisting of Figure I but reached amphibolite grade in its eastern and of megacrysticmicrocline-perthite (5-100 cm in size)intergrown northeasternparts. Rb-Sr isochron dating of regional metamor- with graphic quzrtz and embedded in a coarse-grained matrix of phism and igneousactivity placesthe regional evolution in the albite, quartz, and muscovite, with or without garnet; (2) fine- Kenoranorogeny (2.7-2.55 Ga; eernf et al., 198I a). grained, massive,and homogeneousleucogtanite is an equigran- The dominant tonalite and trondhjemite components of the ular facies consisting ofmicrocline perthite, quartz, albitic pla- Maskwa Lake and Marijane I-ake diapirs are rimmed and pen- gioclase,and muscovite, with accessorygarnet and zircon; (3) etrated by largely undeformed biotite granites. The lac du Bonnet sodic aplite typically consists of albite, quartz, muscovite, and batholith consists predominantly of an analogous late biotite garnet, with accessory apatite, gahnite, monazite, and opaque granite, but an early and strained leucogranite constitutes its east- minerals;it is commonly layeredwith prominent bandsofgarnet; ern extremity (Fig. l) and irregular areas along its margin to the (4) potassicpegmatite layers altemate with aplite or residein the west. The leucogranite phase is alaskitic and highly fractionated; pegmatitic leucogranite;they consistofgraphic to blocky micro- geochemical characteristics indicate that it is compositionally cline-perthiteand quartz, commonly in concentricallyzoned pat- related to the late pegmatitic granites to the east and northeast. terns, locally with cleavelanditeand muscovite, accessoryberyl, garnet, cordierite, columbite-tantalite, and extremely rare apatite. Greer Lake pegrnatitic granite All four phasesofthe pegmatitic granite are highly fractionated: The Greer Lake intrusion is the southwestern one among the average data for five samples of the fine-grained leucogranite five pegmatitic granites of the Winnipeg River district that were show WRb 87, K/Cs 5100, K/Ba 767,BalSr 7.4, Rb/Sr 32.1, emplaced along subvertical faults and adjacent d€collement struc- il'/:gLi 4.7, AUGa 1526,andZr/Hf 48.3. The potassicpegmatite tures late in the history of regional events (Fig. 1; Goad and is the most fractionated phase with K/Rb 28, Rb/Sr 56.5, Mg/ ennm? ET AL.: FRACTIoNATIoN TRENDSoF Nb- AND Ta-BEARINGoxIDES 503 WINNIPEG RIVER ') - "r--'-7'?ry"-') ?7ze " / ,/t--1' 'i'-':.--",....ii-/"i'" x ' Fig.2. The Greer lake pegmatitic granite (open, heavy outline) in metabasalt (ruled) and the GL pegmatites (heavy bars in the gneissicterrane (X) south ofeast-trending fault. An east-southeast-trendingfault separatesthe southwestern segnent ofthe pegnatitic granite from its main body (GRS and GRN, respectively). Locality
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