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 of the beryl-columbite type intrude metabasalt and tonalite gneiss in the Archean English River Subprovince of southeasternManitoba. Columbite- is the predominant Nb- and Ta-bearingmineral, associatedwith subordinateixiolite, , niobian-tantalian , and rare tantalian , , 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 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 symbols mark the occurrences of Nb- and Ta-bearing minerals in the pegmatitic granite and the pegmatiteveins (italics; cf. Table l). Compiled from Grnf and Tumock (1971) and Goad (1984).

Li 2.8, Zr/Hf 5.0, and AVGa 778. The southwestemsegnent of south of the southern fault (Fig. 2). Thus it seemsprobable that the intrusion is more fractionated than the main part north of the geochemically more primitive Greer Lake pegrnatites in the the fault. It also hosts three highly fractionated pegrnatite pods, south, and the main body of the pegmatitic granite in the north, the Annie Claim no. 2 and no. 3 and the SilverleafClaim offshoot are uplifted relative to the intervening greenstone wedge that (Fie. 2). hosts the more evolved granite segrnent. Annie Claim no. 2 is an irregular pod of blocky microcline- Greer Lake pegmatite group perthite and quartz with abundant cleavelandite, several gener- ations of Li-enriched rnicas, beryl, wodginite, and manganotan- The Greer Lake pegmatites are dominantly concordant intru- talite. The Annie Claim no. 3 is a subellipsoidal pod, highly sions, pinching and swelling within the foliation of the hosting enriched in Li, Rb, Cs, Mn, Be, Nb, Ta, and F. K- is tonalitic and granitoid gneisses,both along strike and downdip' extremely rare; lithian muscovite and lepidolite are the dominant Typical dimensions of the pegmatite outcrops are well under potassic phases and are associated with quartz, albite, cesian 100 x 10 m; the largestpegmatite, GLF, attains 400 x 15 m. beryl, and rare tourmaline, manganotantalite, microcline, apatite Most of the pegmatites show a regular pattern of concentric and cassiterite. Both AC2 and AC3 evolve from the surrounding zoning.The thin but continuousborder zoneconsists of medium- pegmatitic leucogranite through a narrow transitional zone. In grained oligoclase with microcline-perthite and quartz. Iarge contrast, the Silverleaf offshoot consists largely of garnetiferous crystals of graphic microcline-perthite + quartz intergrowths, aplite u/ith a large rounded pod of Li-, Rb-, Mn-, Nb-, Ta-, Be-, embedded in coarse-grained albite + quartz + biotite + mus- Sn- and F-rich pegmatite. The pod consists of microcline- covite, constitute an extensive wall zone. The core consists of perthite, quartz, albite, lithian muscovite, lepidolite, spessartine, blocky microcline-perthite and quartz, occasionally segregated cesian beryl, petalite, spodumene, topaz, cassiterite, manga.no- into a blocky core-margin of microcline-perthite surrounding a columbite-manganotantalite, triphylite-lithiophilite, and lenticular quartz core; cleavelandite and muscovite are wide- amblygonite-montebrasite, with rare apatite, tourmaline, and spread but subordinate. Iayers and pods of fine- to medium- sphalerite. Fractionation levels of all three pods are much more grained albite with quartz, muscovite, and garnet are located at advanced than those of the potassic pegmatite units: blocky the outer margins ofthe blocky core (or within and around the K-feldspar reachesK./Rb I 0, K/Cs I 39, and Rb/Cs I 4, and some segregatedmicrocline-perthite core margin), extensively replac- of the lithian micas have K/Rb 3, K./Cs 60, Rb/Cs 4.2, and Mg/ ing microcline-perthite and quartz. Most of the rare-element- Li 0.01. bearing and other accessory minerals are concentrated in this These pods emphasize the overall highly fractionated nature unit. Garnet, beryl, and cordierite are associatedwith subordinate of the southwestern segment of the pegmatitic granite, located in but widespread Nb-, Ta-, Ti-, and Sn-bearing oxide minerals, the wedge-shapedblock between two prominent faults. In con- local monazite, gahnite, zircon, xenotime, and extremely rare trast, the potassic pegmatite phase ofthe less fractionated north- apatite, pyrite, and chalcopyrite. ern part of the pegmatitic granite closely resembles the predom- Despite the rather simple mineral assemblage and uniform inant type of Greer Lake pegmatites hosted by the grreissicterrane internal structure, the Greer Iake pegmatites are conspicuously 504 ernuf ET AL.: FRACTIoNATIoN TRENDSoF Nb- AND Ta-BEARINGoxIDES

11 25 26 30 38 HT 22 L 2r4N ACl 22A36891

GAHNITE a CORDIERITE A. -i. GARNET A AAA ++ ++ o*-F BERYL T A + + +++ -14tcAs ++ Li +A a of T0URl,,lALlNE SPODUMENE PETALITE TOPAZ zlRc0N aa

APATITE MONAZITE t.. XENOTIME a A14BLYGONITE TRIPHYLITE

COLUI4EITE-TANTALITEaaaaao IXIOLITE I,{ODGINITE MICROLITE CASSITERITE NIOBIANRUTILE a TANTALIANRUTILE ILMENITE

URANINITE{?) EUXENITE(?) a Nb,Ta,Ti,Ca,Fe,Th,U a Ta,Nb, Ti , Pb,Th, U a

SPHALERITE PYRITE

Fig. 3. Assemblagesof Nb- and Ta-bearing oxide minerals and other subordinate and accesoryphases at Greer [ake. Microcline- perthite, albite, quartz, and muscovite present at all locations. GRN : northern segnent and GRS : southern segment of the Greer Lake granite; GL : Greer Lake pegmatites (see text for further details). O : subordinate rock-forming constituent; r : abundant accessoryphase; + : subordinateaccessory phase; o: rare mineral.

diversified. They rangefrom bodies with predominant REE, Ti, Dnscnrvrrvn MTNERALocY Fe > Mn and Nb > Ta in their accessoryminerals (GL6, GL8, GL9) to veins poor in Ti and REE but with Nb < Ta, Fe = Mn Experimental methods and increasedLi and Cs in beryl (GL2, GL2A, GLI I, GLl2A). All chemical analyses were done with a nec 5 electron micro- Variation in the accessorymineral assemblagescoincides with a probe operating predominantly in wavelength-dispersive (WD) broad rangeoftrace-element concentrationsshown by the rock- and partly in energy-dispersive (ED) modes. Standards used were forming minerals. For example, blocky microcline-perthite varies cassiterite (SnZa), chromite (FeKa), titanite (TiKd), mangano- from 96 to 13 in K,/Rb and from l0 to 1790 ppm of Cs; core- tantalite (MnKa,TaLa, Ma), stibiotantalite (Sb,Bi), pyrope (Al), margin muscovite and late lithian micas range from 21.8 to 3.6 Sc metal, BarNaNbrO,,, and CaNbrOu (NbIa). Conditions of in WRb, from 1027to 321 in K/Cs, and from 0.07 to l.l0 wto/o analysis were acceleratingpotentials of 20 and 15 kV, sample of Li. currents of 0 nA (WD) and 5 nA (ED) measured on brass (WD) The internal structure, mineralogy, and geochemistry of the and ZnS (ED), count times of l0 s (WD) and 200 s (live time; Greer I^ake pegmatites indicate that they can be classified as ED). Wavelength-dispersive spectral data were reduced by a lo- partly albitized blocky microcline + muscovite dikes of the beryl- cally modified version of the program EMIADRvrr (Rucklidge and columbite type (Cern!, 1982). Gasparrini, 1969). Energy-dispersive spectra were collected with a KEVExModel 7000 ED spectrometer and were reduced with Nb- and Ta-bearing parageneses xrvex software utilizing the program MAGrcv (Colby, 1980). The Nb- and Ta-bearing accessoryminerals occur principally Peak-overlapproblems encounteredduring analysisofED spec- in four environments: (1) GRN-the potassic pegmatite phase tra were resolved by stripping techniques involving library spec- of the main, northern, geochemicallyprimitive part of the Greer tra. Compositions of ixiolite and niobian-tantalian rutile calcu- I^akepegmatitic granite; (2) GRS-the potassicpegmatite phase lated with all as Fe2*showed cations in excessofstructurally of the southeastern, more fractionated segnent of this granite, available sites. Recalculationofunit-cell contents was adjusted within the fault-bound greenstonewedge; (3) AC2, AC3, SF- by converting as much Fe to Fer* as required to eliminate the the highly fractionated pegmatite pods within, and adjacent to cation surplus.The crystallochemicalassumptions ofErcit (1986) GRS: the Annie Claim no. 2 and no. 3 and the Silverleafoffshoot; were applied to the calculation of unit-cell contentsof both AC2 and (4) Gl-the Greer Lake pegmatitegoup south of the south- wodgifiles. ern fault. Figure 3 lists the associationsof accessoryminerals Electron-microprobeanalyses of microlite wereobtained under from all locations where Nb- and Ta-bearing minerals have been the same ED instrumental conditions as for columbite-tantalite, found. and with the same standards for Ti, Sn, Fe, Mn, and Nb. Other eenNi ET AL.: FRACTIoNATIoN TRENDSoF Nb- AND Ta-BEARINGoxIDEs 505

Table l. Chemical composition of columbite-tantalitesfrom Table 2. Chemical composition of columbite-tantalites Greer I-ake pegmatitic granite from Greer Iake pegmatites

sanple Ta^o- Nb^o- Ti 0^ sno^ sc^o^ t{no Fe0 total :iffil: ra205Nb205 Ti02 sno2 A120?sb201 Mno Feo Total cL2A-18 64.0 17.2 2.3 0-7 - 11.0 4.0 - 992 GRHT 15 5 63.0 0.4 0.05 0.1 7.8 12.2 0.06 99 ll ot GR3oA] 20.7 592 0.8 0.03 - 2316.6 0.05 99.68 GL6-t 17.7 56 4 3 I 2 4 1.4 8.1 0.06 98 36 - GLSC-31-626,6 47.1 4.6 - i.5 6.0 0.3 96.0* cR2t4NA40.8 4t7 03 0.4 7.8 l0.l t0l I - GR22A2 5l 0 32.0 0.3 03 - 5.4 ll.5 1005 clS}l-l 27.8 48.3 3 8 1.3 0.7 6.8 9.9 98.0 - GL9-18-3 39.5 37.7 4.7 0 6 1.3 8.2 8.8 - 100.8 sF22 37.9 408 0.2 4.6 15.4 I .5 100.4 - s FTr'l 62 9 204 0.02 0.7 - I 5.6 0.05 0.3 99.97 GL9-19 51 0 25.2 5.2 2 5 0.8 7.8 7.1 99.6 GLt2-t 34.6 43.5 3.6 0 I 0.6 9.5 8.0 - I 00.6 ACIAL 57 2 23.9 0.9 0.5 0.1 4.7 11.2 0.04 98 54 - - AC31D 67.6 15.9 02 0.,] - 13.1 I 9 0.04 98.94* GLt2A-l 72.2 ll 4 1.2 1.6 13.4 12 101.0 AC32B 77.5 7.8 0.1 0.4 - 14.2 0.2 100.2 4* Ta5* Nb5* Ti sn4+ s"3* MnZ+ FeZ* catt Totai o* J* Ta5* llb5* Ti snot Al sbJ* Mhz+ fez* caZr GL2A-18 5.25 2.34 05,] 008 2.81 l0l I 2.00 GRHT 1.02 6 88 0.07 0.005 - 0.01 1.60 2.47 0.02 12.075 GL5-1 l.l7 6.]9 0.57 0.23 0.30 1 67 1.87 0 02 12.02 - 1I GR3OAI I 38 6.55 0.15 0 0l 0.01 - 0.48 3.40 0.01 |99 GLSC-31-61.85 5.45 0 89 0.33 1.30 2.04 0.08 99** GR2T,INA2.94 4 99 0.06 0.04 - 1 75 2.24 lz 02 GLSl,l-I 1.91 5.5i 0.72 0.13 0.15 I 45 2.09 il .96 GR22A2 3.88 4.05 0.06 0.03 - t.2a 2.69 11.99 GL9--I 8- 3 2.77 4.40 0.9t'I 0.06 0.29 1.79 1.90 12.12 sF22 2.74 4.91 0.04 0.49 - 3.47 0 33 ll 98 GL9-19 3.83 3.18 .08 0.28 0.19 r.83 I 64 I 2.03 SFTM 5 13 2.77 0.0050.08 - 3.97 0.01 0.]0 12.065 GLl2-t 2.40 ,].605.01 0.69 0.08 0 l3 1.73 2.02 12.06 AC]AL 4.60 3 20 0.20 0.06 0 0t i.18 2.77 0 0l I 2.03 GLI2A-t 5.09 0 28 0.20 3.52 0.31 12.00 AC3ID 5.72 2.24 0.05 0.01 - 3.45 0.49 0 0t 12 .02** AC32B 6 82 Lt4 0.02 0.05 - 3.93 0.05 l2 0l a, A b, A c, A o 1,8 a, I v, E" GL2A-I 8 r4.247(ro) 5.739(3) 5.147(7) 420.8(4) GL6-I 14.r86(s) 5 721(3) 5.'t28(2\ 416.2(2) GR30A] 14.230(10) 5.725(4) 5.08r(5) 413.e(7) GLSC-31-6 r4.r73(6) s.7oe(3) s.r08(2) 413.3(3) GR2lvtNA t4-27s(6) 5.728(2) 5.08s(2) 4r5.8(5) GL9-I 9 r4.r77(3) 5.716(l) 5 il4(2) 414.4(r) GR22A2 14.248(8) 5 721(3) 5 0e2(3) 4rs.r(6) GLt2-t 14.217(6) s.722(3) 5.144(2\ 418.5(3) sF22 14.277(4) 5.748(2) s.r50(3) 4227(2) GLl2A-I 14.247(3\ 5.741(21 5 rsr(2) 4213(2) AC32B 14 402(3) s.751(l) 5.1020) 4?26(2\

Heated GL24-I8 't4.2se(4\r4.34e(4) 5.742(2) 5.083(2) 4t8 8(2) GR3OA] r4.280(9) 5 053(4) 413.e(5) cLSC-3r-6 5.i32(r) 5.064(2\ 415. r (2) GR2IlNA r4.33e (5) 5.738(2) 5.058(3) 4162(3) GL9-19 14.271(6) 5.724(2) 5.063(2) 413.5(3) r4.4i 7( 4) 5 75612) s.084(2) 42t 9(2) cLt2-1 r4.320(5) 5.732(2) 5.074(3) AC32B l4.4lr(2) 5.755(r) 5.085(r) 421.711) GLI2A-I r4.380(4) 5.746(r) s.084(2) 420.1(2)

Unit cell contents and unit cell dimensions calculated for the ordered unit cell contents and unit cell dimensions calculated for the ordered cell (24 orygens); - not detected; * includes 0.1 wt.% 1,490;** includes cell (24 oxygens); - not detected; * includes 0-2 wL- U0ti ** includes 0 05 Mg llunerals in brackets represent +lo in terms of the last 0.01 U. Nunerals in brackets represent +lo in terms of the last deciml place decimal place

standards for microlite were uranium @, microlite (Na, Ca, Ta), Columbite-tantalite ThO, (Th), and PbTe (Pb). Peak-stripping techniques were used columbite-tantalite group are the most to resolve overlap problems, using library spectra derived from Minerals of the phasesin spectra of microprobe standards with matrices similar to the abundant and widespreadNb- and Ta-bearing sample.Noniterative stripping was usedin casesof minor over- this granite pegmatitesuite. Small lath-shapedcrystals are lap. More severeoverlap problems (e.g., Nb-Pb and U-Sn-Ca) typical of the GRN and GRS occulrences, whereas large were dealt with by deconvoluting the region ofinterest prior to anhedral gains (to 2 cm) and subhedralcrystals (to 8 cm) energy stripping, using simple iterative methods and the same are found in AC and SF. Lath-shapedcrystals occur locally library spectra used in the noniterative approach. Unit-cell con- in the GL pegmatites, but microgranular aggregatesinti- : tents were calculatedassuming 2B-site 4, and all Sn as Sn'?* mately associatedwith other Nb-, Ta-, Ti-, and Sn-bearing unless)A-site exceeded4. In such a caseboth 2A and 2B were minerals are much more common. adjusted to 4 by converting an adequate part of Sn2t to Sn4*, Representativecompositions of columbite-tantalite in- giving a minimum theoretical estimate of Sn4*/Sn2*. AC, and SF minerals are poor Unit-cell dimensions were calculatedfrom X-ray powder-dif- dicate that the GRN, GRS, (Tables fraction data obtained with Philips X-ray diffractometers using in Ti and, with the exception of SF, also in Sn I Ni-filtered or graphite-monochromated CuKa radiation O : and 2, Fig. 4). In contrast, many of the GL columbite- 1.5418A) and scanningspeeds of 0.25'to 0.5'2dlmin. A min- tantalites are distinctly enriched in both Sn and Ti (Fig. imum of 8 and a maximum of 22 reflections measured between 5, Table 2), and they grade into the compositions typical 10" and 70 20 wereused in least-squaresrefinement ofthe data. of ixiolite. AnnealedCaF, (a: 5.4620 A)was usedas an internal calibration The structural state of columbite-tantalitescan be rep- standard. A modified version of the program cernnr (Appleman resented onan a-c diagram shown in Figqfe 6 (eern9 and and Evans, 1973) was used for unit-cell refinement. Turnock, 197l; cf. Wise et al., I 985, and Cerni and Ercit, In the heating experiments, separatefragments 0.5 to 2 mm 1985,for end-memberdata). The cell dimensionsare sen- in size were heated in air at 1000'C for 16 h. Heating relatively Fe/Mn substitution (primarily affecting coarse grained material minimizes oxidation of Fe'z* to such a sitive to both the degree that unit-cell dimensions of columbite-tantalites heated a) and the order-disorder (affectitg a and c alike). The in air and in a controlled atmosphere (CO/CO, : 2.5/l) arc order-disorder vector is parallel to the lines connecting statistically identical (unpub. data of P. Cernf and A. C. Tur- equal end-member compositions. nock). The columbite-tantalites show widely variable deglees 506 eenmi ET AL.: FRACTIoNATIoN TRENDSoF Nb- AND Ta-BEARINGoxIDEs

B t 2li G -t tl ti t it tlt-r \ t+ till ?. irz4 i*J I I I

,tt/ 2A

I

GRN o.o+ 00 02 04 0.6 08 10 Mn/(Mn+Fe) Mn/(Mn+Fe)

Fig. 4. Compositions of the Greer Lake Fe-, Mn-, Nb-, and Ta-bearing oxide minerals in the columbite-tantalite- quadrilateral. (A) Occurrences in the pegrnatitic granite; (B) minerals from the pegmatite veins. Atomic ratios urith all Fe as Fe2*. oforder-disorder, but they clusterinto well-definedgroups On heating, the columbite-tantalites develop ordered according to their provenance;(l) both GRN and GRS structuresthat plot closeto the joins of fully ordered syn- minerals show intermediate structural states;(2) the AC thetic end members of the a-c diagram. These data are and SF samplesshow moderate and extensivevariation, not included in Figure 6 to avoid cluttering, but examples respectively; the AC data indicate intermediate to con- of the GL data are shown in eernf and Turnock (197l). siderably ordered structures, the SF minerals have nearly disordered to considerably ordered structures; (3) the GL Ixiolite columbite-tantalites are highly to totally disordered,cor- Ixiolite is restricted to the GL pegmatitesand always respondingto "pseudo-ixiolite"ofNickel et al. (1963). occurs in fine-grained (<0.5 mm) aggegatesof diverse

(Fe,Mn)(Nb,Ta)2 (Fe,Mn)(Nb,Ta)2

Sn,Ti Fe,Mn Sn,Ti Fe,Mn

Fig. 5. Composition of the Greer lake Nb- and Ta-bearing oxide minerals in the (Sn,Ti)-(Nb,Ta)-(Fe,Mn) diagram. (A) Occur- rences in the pegmatitic granite; @) minerals from the pegrnatite veins. Black field in A is populated by 32 columbite-tantalites. Atomic ratios with all Fe as Fe2*. enmni ET AL.: FRACTIoNATIoN TRENDSoF Nb- AND Ta-BEARINGoxIDEs 507

(Mn,Ta)oOa c

EI

vL o taa ' --ri;-t

ra tL -(Fe,Nb)00, 'to--- A

4T^Bo24 \L MnONbrOrO

Fe4Nb8O24

14.10

Fig. 6. T|1e a-c diagram of structural order-disorder for the Greer Lake columbites-tantalites. Ixiolites (triangles) from the GL pegmatites are shown for comparison. All data for a relate to the ordered unit-cell dimension, irrespective of the actual degree of disorder. Seetext for sourcesof the end-member values.

Nb-, Ta-, Ti-, and Sn-bearingoxide minerals. It is com- and Ercit, 1985).Scandium is another elementthat locally monly associatedwith niobian and tantalian rutile, il- enters ixiolite in substantialamounts (v. Knorring et al., menite, and cassiterite;however, it is rare in Nb-, Ta-, 1969;Borisenko et al., 1969)but doesnot accumulatein Ti-, and Sn-bearingnodules containing Sn- and Ti-poor columbite-tantalite. Surprisingly,the subordinateSc con- columbite-tantalite, and the two minerals do not occur in tents in the orthorhombic GL phasesare, on average,high mutual contact. The ixiolites have distinctly hieher Tal(Ta + Nb) ratios Table 3. Chemicalcomposition of ixiolites from Greer than columbite-tantalitesofcorresponding Mn/(Mn + Fe) Iake pegmatites ratio, and they typically plot within the two-phase tan- talite + tapiolite region ofthe quadrilateral (Table 3, Figs. Sampl e Ta205 Nb205 Ti02 Sn02 Sc203Fe203 Feo Mno Total 48, 58; cf. Fig. I of eernf and Ercit, 1985).In Figure 58, nunber GL3-15-2 66.5 9.7 3.1 4.6 - 2.5 5.9 5.4 98.8 the GL ixiolites generally plot along the (Sn,Ti)- GL3-15-8 66.6 9.3 3.5 5.2 - 0.7 9.1 3.9 98 3 GL8C-31-3 20.9 39.7 4.0 19.5 l.t 0.2 8.4 5.0 98I (Fe,Mn)(Nb,Ta), join, extendingfrom the columbite-tan- GLSC-32-t3 66.5 9.9 3.4 4.3 - 3.3 8.1 3.9 99.3 GLSC-32-24 65.3 9.3 3.0 4.7 - 4.1 6.6 4.6 97.6 talite data. This indicates the dominance of the substi- cL-18-5 59.0 12.5 8.0 4.5 0.5 4.5 6.9 3.7 99 7 (Fe,Mn) + which reaches GL9-18-2 41-9 32.4 5.0 1.8 0 8 4.2 10.7 2.5 100.3 tution + 2(Nb,Ta) 3(Sn,Ti), GL9-18 36 I 35.8 8.6 3.2 0.6 0.3 7.3 7.8 100.4 much higher levels here than in stannian-titanian colum- Gl',f2-t-s 57.0 12.8 7.1 49 0.1 7.3 6.6 2.8 98.5 GLi2-1-4 68.5 6.4 2.8 6.6 - 3.1 5.8 5 6 98.8 bite-tantalite. Most of the deviations plot between the (Sn,Ti)-(Fe,Mn)(Nb,Ta), and (Sn,Ti)-(Fe,Mn)(Nb,Ta) Ti Sn Fe Total GL3-15-2 5.60 1.36 0.72 0.57 - 0.58 r.77 l.4l 12.00 joins, totals normalized to 24 oxygens - and their cation GL3,l5-8 5.65 1.3'l 0.82 0.65 0.16 '1.802.37 1.03 12.00 presence GLSC-3]-3 1.45 4.60 0.77 1.99 0.25 0.05 1.09 12.00 exceed 12.00. Both thesefeatures suggest the of GLSC-32-t3 5.52 I 37 0.78 0.52 - 0 75 2.06 l.0l 12.00 qLEC-32-24 - Fe3*,which seemsto be typical of ixiolite in general(e.g., 5.52 'l1.3t 0.70 0.58 0.95 1.13 1.21 ,]2.0012.00 cL9-18-5 4.55 .60 L 71 0.5'l 0.12 0.97 1.64 0.89 Borisenkoet al.,1969). Calculatedvalues ofFe3* are shown GL9-18-2 2.96 3.80 r.r7 0.19 0.18 0 82 2.33 0.55 12.00 GL9-18 2.53 4.r0 r.64 0.32 0.13 0 07 1.54 1.67 12.00 in Table 3. Ixiolite GL8C31-3, the most enriched in Ra* GLI 2-I -5 4.43 r .65 r .53 0.56 0 03 1.56 1.57 0.68 12.00 - 0.74 1.52 1 49 12.00 (25.1 at.o/oSn + Ti), has cation ratios close to those of Gllz-i- 5.85 0.91 0.66 0.83 ideal wodginite (R?+RX+RiOrr).This representsthe max- Natura l U,X v, Xr cL3-r5-2 ',|4.208(3)14.206(3) 5.729(1) 5.r4r(r) 418.4(r) imum level attained by the above substitution in colum- GLSC-31-13 5.722(1) s.n8(r) 416r(r) ginite-ixiolite GL9-18-2 14.172(8) s.7r9(3) 5.127(3) 4rs.5(4) bite-wod structures. GLr2-r-4 14.13602) 5.737(3) s.r57(r2) 418.2(7) It is noteworthy that most of the GL ixiolites are Ti- Unit cell contents (based on 8 oxygens) tripled for ease of conparison rich with only subordinate Sn. Stannian ixiolites are gen- with co'lumbite-tantalites in Tables I and 2; - not detected. Nuffials in brackets represent in terms of the last deciml place. erally much more common (Khvostova et al., 1983;Cerni !o 508 ennx'i ET AL.: FRACTIoNATIoN TRENDSoF Nb- AND Ta-BEARTNGoxIDES

Table 4. Chemical composition of wodginites, niobian and and cooling procedures.In any case, Komkov's (1970) tantalian rutiles, cassiterites,and ilmenite from Greer Iake resultsand the continuous gmdation from the rather pure pegmatitic granite and pegmatites GL columbite-tantalitesinto typical Ra*-richixiolites cast doubt on the existenceofa sharp,quantitatively definable Sample boundary between columbite-tantalite and ixiolite. r4inerat ;;fl;;; TaZ05 Nb205 Ti02 SnoZ FeZ03 FeO t4n0 Totat l,{ Ac2-81-tv 70-9 1.2 - 14.4 0.2 0.0 11.2 97-9. N Acz-79 64.6 6.6 0.2 15.8 l.t 0.0 10.7 gg.z' R GL3-15-9 34.8 8.3 43.2 2 2 8.2 4.2 _ 100.9 Wodginite R GLSC-32-127 6 4 9 82.4 1.8 4.1 0.7 _ 101.5 R GLS|/-28 18.4 I 5.9 52.4 2.5 8.0 3.7 _ 100.9. Wodginite occurs only at the AC2 locality, where two R GL9-18 44.9 6.0 34.2 t.5 8.4 5.0 - too.z" c sFl4 4.1 944 - 0.7 0.2 100.0 large subhedralgrains (2 and 4 cm across)associated with c sFt 5 4.9 94.0 1.0 0.05 99.95 c GLSC-31-4 3 3 13 - 93.0 0.9 - 985 cleavelanditewere found along the contact of blocky mi- IL GL8I,{-28 O B 0.2 5t 0 35.7 il.o 98.7 crocline-perthite with quartz. The sample AC2-79 has a Ta5* Nb5* Ti4* sn4* Fe3* F"2* r'4n2* To;ur composition closeto that of ideal wodginite (Table 4) but I,J AC2-81-tV 8.72 0.25 - 260 0.07 0.00 4.29 15.932 with some vacanciesand extensiveTa substitution for Sn l,l AC2-79 7 -52 1.28 0 06 2 70 0.35 0.00 3.88 I 5.88- R 613-15-9 0.34 0.13 I t6 0.03 0 22 0.13 - 2.O0 at the C site. Cell dimensionsare appreciablymonoclinic, R GLSC-32-t2 0.06 0.06 1.75 0 02 0.09 0.02 - 2.00 R GL8l,{-2B 0.16 0.23 1 28 0 03 0.20 0 t0 - 2.O0A enhancedby heatingto a B angleclose to its highestknown R GL9-18 0-47 0.l0 0.99 0.0? 0-24 0.t6 - 2.00- value for natural (Ercit c sFl4 0 07 190 003 0 0l 2.01 and,/or heated wodginilss et al., c sFl5 0.07 189 0.04 0.002 2 002 I 984). In contrast, the AC2-8 I -IV specimenshows broad c GLSC-31-4 0.05 0.03 - 1 B9 004 - 2.01 IL GL8l4l-28 0.03 0.02 5.91 461 1 44 12.01 diffraction peakscorresponding to tantalite. The Mn con- Natural b, A v,83 tent of AC2-8 I -IV exceedsthat allowed by wodginilg slei- 9 s23(3) il.476(3) 5.142(2) e0.78(3) s6l.e(2) chiometry. The sample may representa submicroscopic R GL8l,{-28 4.634(r) - 3.001(2) - 64.4(r) - intergrowth of manganotantalite wodginite, R GL9-18 4.65r(r ) 3.004(r) - 6s.0(t) and and it c sFt5 4 734(1) - 3.r84(r) - 71.4(l) deservesdetailed examination by electron microscopy and Heated diffraction. AC2-79 9.527(2) |.471(3) 5.119(l) 91.15(2) R GLsH-28 4.632(2\ - 2.999i4i 64.3(2) R GL9-r8 4.645(r) - 3.001(i) 64 7(1) Niobian and tantalian rutile c sFls 4.735(1) - 3.179(l) 7r.3(r) These varieties ljnit cell contents based on 32 oxygens for wodginite (|l,l), on 4 oxygens of rutile occur exclusively in the GL for rutile (R) and cdssiterite (C), and o1 18 oxygensfor itmeni!;"(IL); pegmatites,mostly as microscopic grains associatedwith not detected; rtotal inclgdes 0.2 Cao; ztotal ineludes ;Jtotal 0.09 Cdz+; includes 0.2 Sc203;4total includes 0.0'l Sc3+. Nurerals rn other Nb-, Ta-, Ti-, and Sn-bearingminerals. They are brackets represent +lo in terms of the last decirol place. particularly common in close association and intimate intergrowth with ixiolite and ilmenite. in columbite-tantalitesrelative to the ixiolites. In contrast, At other localities, niobian and tantalian rutiles com- Sc is not detectablein the GRN, GRS, AC, and SF co- monly form primary subhedral to euhedral crystals (the lumbite-tantalites. -rich varieties usually bearing exsolved titanian The ixiolites produce X-ray diffraction patterns typical ixiolite), or they occur as exsolution blebsin primary crys- of disordered columbite-tantalite. Table 3 and Figure 6 tals of titanian ixiolite. In the GL samples,both niobian show the unit-cell dimensions of ixiolite with d tripled, and tantalian rutile occur as anhedral grains coexisting for easy comparison with columbite-tantalites in Tables with other Nb- and Ta-bearingphases, with no indication I and 2 and in Figure 6. On heating, the ixiolites adopt of exsolution or metasomatic relationship. the wodginite structure (cf. Cern! and Ercit, 1985). Cell Rutile compositions vary widely in Ti/(R,* * R5*) and dimensions of the heating products of GL ixiolites could Tal(Ta + Nb) but always show extremely high Fe/(Fe + not be adequatelyrefined becauseof peak overlap with Mn) (Table 4, Fig. 5B). Cation totals calculatedwith 2Fe admixed phases.Nevertheless, the diffractions typical of as Fe2*exceed the theoretical maximum of 2.00 per unit wodginite were unequivocally identified in the powder cell, suggestingthe presenceofFe3*. This is also indicated patterns in all sampleslisted in Table 3. by the location of most ofthe rutile compositionsbetween The heating products of some of the "columbite-tan- the (sn,Ti)-(Fe,Mn)(Nb,Ta), and (Sn,Ti)-(Fe,Mn)(Nb,Ta) joins talites" containing 8-l I at.o/oZRa* (Fig. 5B) could not be of Figure 58. Table 4 shows unit-cell contents cal- : unambiguously identified, becauseofthe presenceofad- culated for a Fe2*/Fe3*ratio yielding )cat. 2. All these ditional phasesand the uncertain nature of some of their characteristicsare in agreementwith the generalbehavior prominent diffractions. Thus it is possible that some of of niobian-tantalian rutiles from other localities (eernf et these samplesconvert to (weakly monoclinic) wodginite, al., l98lb; Foord, 1982;eeny and Ercit, 1985). completely or in part. Komkov (1970) statedthat a wodg- inite component develops,along with an ordered colum- Cassiterite bite-tantalite phase,in amounts proportional to the (Sn,Ti) Cassiteriteis not common at Greer Lake. It occurs in content of the natural disorderedminerals. We could not significant quantities only in the most fractionated peg- observesuch a relationship in our current studies(unpub. matite pods (AC3 and SF) of the southwesternsegment data of P. eernf and M. A. Wise); however, this discrep- of the pegmatitic granite and as very rare microscopic ancy may be due only to delicatedifferences in the heating grains in some of the GL pegmatites.It is rather poor in CBMTr ET AL.: FRACTIONATIONTRENDS OF Nb- AND TA-BEARINGOXIDES 509

Table 5. Chemicalcomposition of microlitesfrom GreerLake pegmatiticgranite and pegmatites

Sanple Ta205 NbZ05 Ti02 Cao Na20 SnoZ Sno Uo2 Fe0 Tota l

0.3 0.4 9.5 2.1 1t 0.2 0.05 92.45 ACI-B 78.6 ,]0.3 - GL3-15-4 64.9 2.7 12.6 3.5 2.0 0.4 96.4 52.2 14 7 7.6 8.3 I .6 - 67 2.2 - 93.3 GLSC-32-7 - GL9-18 74.4 2.4 1.5 12.3 4 3 02 0.7 0.'] 0.8 96.6 0 7 0.1 05 0.4 98 4 GL12-1-3 74-0 .l.02.8 1.8 12.8 4 3 1.0 GL12A-2-4 78.3 0 4 10.8 4.0 0.I 0.3 0.3 0 05 95.25

-5n F"2 Total Eff.0

AC]-B 3.92 0.03 0.06 1.87 0.75 0.t0 - 003 0.0'l 2.78 12.36 cL3-15-4 2.90 0.77 0.33 2.22 1.12 0.l5 - 0.06 3.55 12.81 ,].00 22 0.28 2.31 I 1.87 GLSC-32-7 2.14 0.86 I . 34 0.47,l48 a GL9-15 3.60 0 19 0.20 2.34 00'l 005 0 005 012 3 985 13.16 GLt2-t-3 3.48 0.22 0.23 2.37 144 007 0.05 0.005 0 07 006 4 135 13l3 GL12A-?-4 3 86 0.08 0.05 2,10 1.41 0.01 0.01 0.05 0.01 3.5B 12.86 = - Contents of 0.25 unit cell normalized to:B cations (Ta+Nb+Ti) 4 00; n9t detected Totals of 0.25 unit cell contents qive rA cations (Ca+Na+Sn2++Uc*+Fe+l4n+Sbrr)' * i ncludes 0.1 Sb,0" **includes0.01 S53+

the "tapiolite" component, but it shows the same strong vary between 74 arrd 85 wt0/0.Two compositions are en- Ta > Nb and Fe > Mn preferencesas rutile (Table 4). countered most frequently. One of them has substantial Nbroj ? TarO, (XO wtolo),subordinate ThO, > UO, (2- Microlite 4 wto/o)and TiO, > (CaO,FeO) (7-10 and 24 wto/o,te- Microlite is absentfrom the GRN localities but occurs spectively).The otherphasehas TarO, > Nbros (XO wto/o) in minor quantities in ACI and AC3 locations of GRS. and PbO > ThO, > UO, (l-8 wto/oof each).A euxenite- It is also found as microscopic grains in fine-grainedag- like phaserich in Y is rather rare. gregatesof Nb-, Ta-, Ti-, and Sn-bearingminerals in all pARTITIoNING AMoNG Nb-,lNl GL pegmatites.Microlite forms rounded grainscoexisting Er,nvmxr Ta-nnlnrNc PHASES with columbite-tantalite or ixiolite, or veins and metaso- matic patchespenetrating these minerals. The distribution of elementsamong coexisting phases Microlite compositions are highly variable in all re- is indicative of their individual crystallochemical pref- spects.Table 5 shows representativeexamples of com- erencesand/or stabilities under given P-Z conditions in positions that gaverealistic totals (with allowancefor HrO the different bulk compositions of their parent system-In and Fr) and acceptableformulae when normalized to >B- the mineral associationsexamined, the spectrum of ele- site : 4 (cf. Experimental Methods and Ercit et al., 1985). ments involves some or all of Na, Ca, Fe2*,Mn, Sn, Ti, Many microlite grains are altered, being inhomogeneous Nb, Ta, O, and F. Other subordinatecations, such as Fe3+ in reflectedlight and giving electron-microprobetotals of and Sc,do not stabilizeany ofthe mineral speciespresent' 78 to 85 wto/0.This is particularly typical of but U, Th, and nonradiogenicPb may have some control enriched in U (to 8.7 wto/oUOr) and Bi(to27 wto/oBi'Or). over the crystallization of the unknown phases.Thus it is None of the Bi-bearing microlites conforms to a realistic desirable to review the compositional relationships among microlite stoichiometry. the coexistingphases. However, it should be emphasized that these are not necessarilyequilibrium relationships. Ilmenite This is particularly relevant to the fine-grainedaggregates Ilmenite occursin severalof the GL pegmatitesas rare from the GL pegmatites:their complex mineralogy, fine microscopic grains or skeletal aggregatesintimately in- grain size,and the variable composition of different grains tergrown with niobian-tantalian rutile or associatedwith of the same speciesin a single aryegate all indicate rapid ixiolite. Only a single electron-microprobeanalysis gave nucleation and growth, triggered by a sudden change in acceptablestoichiometry, with very low (Nb,Ta) but sur- conditions ofthe parent fluid, and a lack oflater re-equil- prisingly high Mn (Table 4); beam overlap with adjacent ibration. phaseswas the most probable cause of appreciable de- vs. viations in other data. The slight deviation from the Ixiolite and columbite-tantalite rutile A686018formula in favor of the A sites (i.e., Fe,Mn) sug- niobian-tantalian gestsa limited quantity of hematite in solid solution. With one prominent exception,columbite-tantalite and ixiolite are enriched in Nb and Mn with respect to co- Unidentified phases existing niobian-tantalian rutile (Fig. 7A), in agreement Several unidentified phaseswere repeatedly analyzed, with previous data (Cerni et al., l98lb). However, non- but their nature could not be determined. Theseminerals equilibrium is indicated by the coexistenceof a single are invariably somewhat heterogeneouson the scale of orthorhombic phase with two different rutile composi- the optical microscope; they are U and Th bearing and tions, and by an ixiolite + tantalian rutile pair with re- probably metamict as the totals of their chemical analyses versedNb/Ta partitioning. Compositions of niobian-tan- 510 ernm? ET AL.: FRACTToNATIoN TRENDSoF Nb- AND Ta-BEARINGoxIDES

. COLUMBITE-TANTALITE A IXIOLITE . RUTILE (Fe,Mn)(Nb.Ta)2 -CASSITERITE z + (Fe,Mn)(Nb,Ta) F

o0 o0 02 04 06 08 1 0O Sn.Ti Fe,Mn Mn/(Mn+Fe) Fig. 7. Compositions of coexisting minerals (except microlite) in the columbite-tantalite-tapiolite quadrilateral, and in the (Sn,Ti)-(Nb,Ta)-(Fe,Mn)diagram. Atomic ratios with all Fe as Fe2*. talian rutiles and coexisting orthorhombic phases sug- for coexistingpairs. The GL8W-28 ilmenite (Table 4) has gest a markedly asymmetric shape of the TiOr- negligible)(Ta,Nb) relative to rutile, with a possiblepref- (Fe,Mn)(Nb,Ta)rOusolvus, with a steep slope on the co- erence of ilmenite for Ta relative to Nb. Ilmenite does lumbite-tantalite side (Fig. 7B). This is in agreementwith concentrate Mn relative to the highly ferroan rutile com- o-ther mineral pairs that may be closer to equilibrium position. Another rutile * ilmenite pair from the GL8C- (eernj' et al., l98lb; eernf and Ercit, 1985). 32 specimen shows the same Fe/Mn partitioning but about equal Nb/Ta in both minerals. Ixiolite and columbite'tantalite vs. cassiterite The few data available indicate a strong preferenceof Ixiolite, columbite-tantalite,and niobian-tantalian cassiterite for Ta and Fe relative to Nb and Mn, in agree- rutile vs. microlite ment with the data of Wise and eern! (1984a)and -ernf Microlite is always conspicuously et al. (1985b). enriched in Ta rela- tive to Nb when compared with coexisting (or replaced) Niobian-tantalian rutile vs. cassiterite columbite-tantalites and related phases (e.g., Ginsburg, 1956).This is confirmed by all the AC and GL specimens Both rutile and cassiteriteconcentrate Fe and Ta rel- (Fie. 8). Nonequilibrium is indicated by irregular varia- ative to Mn and Nb in the coexisting orthorhombic Fe-, tions in tie-line slopes for identical speciespairs, irre- Mn-, Nb-, and Ta-bearingminerals. The principal reasons spective of the cocrystallization, overgrowth, or replace- are evidently thosesuggested by Weitzel (1976)to account ment relationship. for the differencein structure and symmetry betweenco- Compositional data on rutile coexistingwith microlite lumbite-tantalite and tapiolite, namely the interrelation- are very scarce.Figure 8 showsmicrolite to have high Tal ship between the difference in polarizing effectsof Tas* (Ta + Nb) relative to the rutile phase.In contrast, stibio- and Nbs* on one hand and between the ionic radii of Fer* betafitefrom its type locality and associatedniobian rutile and Mn2* on the other. In contrast,the partitioning of Fe/ have near-identicalTa,z(Ta + Nb); however,stibiobetafite Mn and Nb/Ta between rutile and cassiterite is virtually crystallized after and partly at the expenseof the niobian unknown, as this assemblageis extremely rare. A single rutile (Cernf et al., 1979). coexisting rutile * cassiteritepair (GL8C-31) shows Tal (Ta + Nb) higher in cassiteritethan in rutile, a relationship suggestedby the general compositional ranges of these FucnoN,r,uoN, CoNTAMINATToN, AND minerals in separatepegmatite occurrences (Fig. 4 in eer- ORDER.DISORDER TRENDS ni and Ercit, 1985) and observed for coexisting cassi- The data from Greer L,ake provide ample material for terite * rutile from Tanco (T. S.Ercit, 1985,pers. comm.). defining the fractionation paths followed by the Nb- and Ta-bearingoxide minerals in the granite and its pegmatite Niobian-tantalian rutile vs. ilmenite aureole, for considering the possible role of contamina- This mineral pair should not be particularly scarcein tion, and for interpreting the pattern of order-disorder granitic pegmatites,but there is little compositional data relationships. CNMN? ET AL.: FRACTIONATION TRENDS OF Nb- AND TA-BEARING OXIDES 5II

1.OO- atures. This could be the principal factor affecting Nb/Ta I fractionation in residualgranitic and pegmatitic melts. On F and individual crystals, 0.90 - a local scale of mineral aggregates the effects of this large-scalemechanism may be aug- F mented by a mass-relateddiference in diffirsion rates, o - z o.80 analogousto that proposed for Zt and Hf (Butler and Thompson, 1965). -uJ 0.70- The fractionation of Fe/Mn is not well understood.On o -=l! the scaleofgranitic magma chambers, preferential volatile x transfer of Mn was consideredby Shaw (1974) and Hil- 0.60- -oJ uJ (1979, l98l) the only viable mechanismof Fe/Mn E dreth F (1968) (1977) proposed J separation.Shawe and Bailey 0.50- preferential complexing of Mn with F to explain its sep- F z -o aration from Fe and late precipitation. z - - Fractionation paths of the Greer Lake columbite-tan- I 0.40 -+ uJ G talites (including the minor ixiolites and wodginites; Fig. ! 9A) consist of (1) vertical to subvertical segmentsof pro- dl - _o 0.30 F nounced Nb/Ta fractionation at almost constant Mn/ (Mn + Fe) and (2) subhorizontalsegments marking prom- o + 0.20 - inent Fe/Mn fractionation at almost constant Ta/(Ta Nb). z The Nb/Ta-dominated fractionation is characteristicof + o.1o- o internal evolution of the relatively primitive potassicpeg- F matite phase of the pegmatitic granite and of the less 6 o.oo- fractionated GL pegmatites (both at Mn/(Mn + Fe) < F the Li- and Fig. 8. The atomic TaJ(Ta + Nb) ratios of columbite-tan- 0.5). It also marks the internal evolution of talites, niobian and tantalian rutiles, and ixiolites related to those F-enriched Silverleaf offshoot, the AC2 pod of GRS, and ofcoexisting microlites. Pointed arrowheadon a tie line indicates the GLI2A pegmatite (at Mn/(Mn + Fe) > 0.8). In con- replacement by microlite; rounded arrowhead marks overgrowh trast, the fractionation segments(or gaps)with dominant by microlite. Unmarked tie lines indicate cocrysrallization of shift in Mn/(Mn + Fe) are parallel to major enrichment neighboring grains. in Li and F (Figs. 4, 9A): (l) A separateMn-enriched fractionation trend within the GL group is defined by the Nb/Ta and FelMn fractionation 2,2A, and I I pegmatitesthat carry moderateamounts of As shown by Wang et al. (1982), F-based and other late Li- and F-enrichedmuscovite. (2) Another, evenmore complexes of Nb and Ta have different thermal stabilities, Mn-enriched branch of the GL fractionation seemsto be the Ta-bearing ones generally persisting to lower temper- separatedfrom the main two-pronged trend; the GLI2A

z z + z + a F

6 F o F

Mn/(Mn+Fo)

Fig. 9. Fractionation trends of columbite-tantalites in granitic pegnatites, pegmatite groups, and fields, in atomic ratios. Inter- rupted arrows schematically indicate fractionation gaps between columbite-tantalite proper and ixiolite (IX) or tapiolite (T); W : wodginite. (A) Greer Lake pegmatitic granite and its pegmatite aureole condensed from Figure 4; (B) Black Hills, South Dakota pegmatite field (BH); Mounrain (TM) and Peerless(PRL) pegnatites, Black Hills, South Dakota; Plex pegrnatite, Baftn Island, N.W.T. (PX); Himalaya pegnatite districr, California (HM; Foord 1976); (C) Yellowknife pegmatite field, N.W.T. (YKF) and its Moosepegmatite (M. A. Wise, unpub. ns.); Southernpegmatite series, Cross t akefield, Mani,toba (CLS;Anderson, 1984);Mongolian Altai field(MGA; Wang et al., 1981).Nonreferenced trends from unpublished data of P. eern9. 512 ennmf ET AL.: FRACTToNATIoNTRENDS oF Nb- AND Ta-BEARINGoxIDES pegmatitecontains a lenticular unit of Li- and F-enriched lowed by extreme Nb/Ta fractionation, from its most muscovite in its core. (3) A much more enhancedFelMn primitive biotite-peristerite members to spodumene-rich fractionation is observedat the ACI locality and partic- pegmatites(Fig. 9C); these latter are highly fractionated ularly in the immediate vicinity of the nearby AC3 pod, in terms of all rare-alkali metals, but their mineralogy which is extremely enriched in Li and F. At this location, does not indicate prominent activity of F at any stageof the Mn/(Mn + Fe) ratio increasesfrom 0.3-{.5 in the their consolidation. v. Knorring and Fadipe (1981) also common GRS potassicpegmatite phase to 0.85-1.00 in indicated manganocolumbite as typical of Li-rich peg- the AC3 pod, over a distance of only 5 m. (4) The SF matites, with no referenceto a particular role of F. In aplite-cum-pegmatite offshoot also carries considerable contrast, the pegmatite field of Mongolian Altai (Wang et quantities of Li- and F-rich lithian muscovite and lepid- al., l98l) does evolve into lepidolite-rich, pollucite- and olite, and its columbite-tantalites show an isolated frac- manganotantalite-bearingpegmatites, but the composi- tionation trend at Mn/(Mn + Fe) > 0.84, despite the tions of columbite from even the most primitive of its intermediate 0.364.74 rangein Ta/(Ta + Nb). A similar members show Mn/(Mn + Fe) > 0.50 (Fig. 9C). Such an relationship is observedin AC2. examplesuggests the possibility of Mn-enriched protoliths The above relationshipsshow that Nb/Ta fractionation from which the parent granitic magmas were mobilized, can proceed with equal easein both the primitive, rela- or lithologios through which they passedduring their em- tively Li- and F-poor magmasand the highly fractionated, placement(e.9., the masutomilite-bearingTanakamiyama Li- and F-saturatedmelts, reachingthe highestlevels pre- pegmatitesin Japaq Harada et al., 1976). dominantly (but not exclusively)in the latter. In contrast, To summarize, Fe/Mn fractionation in columbite-tan- FelMn fractionation is greatly enhancedby high activity talite seemsto be greatly enhancedby the high activity of of F at late stagesof granite and/or pegmatite evolution, F in late stagesof pegmatite consolidation. This may be supporting the idea of F-basedcomplexing of Mn. not the only factor promoting Fe/Mn fractionation, but Additional evidencefor the link between FelMn frac- is evidently the main one responsiblefor the fractionation tionation and high late-stagea.- is provided by the co- pattern in the Greer Lake granite and pegmatite group. lumbite-tantalite compositions from other pegmatite lo- It should be noted here that high activity of F also calities (Fig. 9B). The Peerlesspegmatite (eern!'et al., promotes extreme fractionation of Nb and Ta. Rare-ele- 1985b) and the Tin Mountain deposit of the southern ment pegmatitesof the lepidolite type carry Ta-rich mi- Black Hills, South Dakota are complex pegmatitesrich in crolite as their most abundant Nb- and Ta-bearingphase, Li and F, with sizeableconcentrations oflithian muscovite manganocolumbitebeing of only minor importance (e.g., in their central parts. ConspicuousMn-enrichment ofNb- the Quartz Creek district, Colorado; Staatz and Trites, and Ta-bearing oxide minerals occurs at both localities. 1955). However, widespreadas microlite is in the Greer In the overall fractionation trend of the southern Black Lake area,it is only a minor phaserelative to columbite- Hills field, the Mn-rich side is populated by mangano- tantalite and ixiolite. columbites and manganotantalitesfrom Li- and F-en- riched pegmatites. Variation in Sn, Ti, and Sc Of the other localities shown in Figure 98, the Hima- In the pegmatitic granite,Sn is very low in the primitive laya, California, pegmatites(Foord, 1976) and the Brown potassic pegmatite lensesand the AC pods, but it is en- Derby swarm, Colorado, belong to the lepidolite type of riched in the SF ofshoot. In the GL pegmatites, Sn is the rare-elementpegmatite class;extreme enrichment in relatively abundant in the Nb- and Ta-bearingoxide min- Mn precedesmost of the Nb/Ta fractionation in their erals of all parageneticand geochemicaltypes, and cas- columbite-tantalites.Even the rather primitive, beryl-co- siterite is found in some of the more primitive veins (Ta- lumbite-type Plex pegmatite(Baffin Island, N.W.T.) shows bles l-3, Figs. l, l0). Thus Sn accumulatedin the residual a prominent Mn-enrichment from the coarseferrocolum- fluid phase within the pegmatitic granite and migrated bite crystals of its quartz core to the finer-grained man- into the pegmatite aureole where it precipitated at pre- ganocolumbite in late pods of Li- and F-enriched mus- sumably lower temperatures.No correlation is apparent covrte. between Sn distribution and the rare-alkali or fluorine In contrast to the above example, the Yellowknife peg- enrichment of the host assemblages. matite field of the N.W.T. showsan extensive,and locally Both Ti and Scare conspicuouslylow in the orthorhom- extreme, Nb/Ta fractionation but restricted Mn/(Mn + bic Fe-, Mn-, Nb-, and Ta-bearing oxide minerals of the Fe) range(0.05-0.45, Fig. 9C; Wise and Cernj,, 1984b; pegmatitic granite assemblagesbut more abundant in all M. A. Wise, unpub. ms.). This field consists mainly of types of the GL pegmatites (Tables l-3, Fig. l0). Two pegmatitesof the beryl-columbite and complex typesthat interpretationsare possible:(l) Ti and Sccould have been lack significantamounts of Li- and F-enrichedmicas (e.g., complexedand thermally stablein the granite magmaand the Moose pegmatite, Fig. 9C). precipitated only at lower temperaturesin the pegmatite In other cases,the Nb/Ta and Fe/Mn relationships do aureole (as in the thortveitite-bearing, uncontaminated not lend themselvesto unambiguousinterpretation. The pegmatiteshosted by their parent granitesat Kobe; Sak- Southern Seriesof the Cross Lake field, central Manitoba urai et al., 1962), or (2) they could have been "absent" (Anderson, 1984), shows an early enrichment in Mn fol- from the pristine pegmatitic granite magma and absorbed ennmi ET AL.: FRACTIoNATIoN TRENDSoF Nb- AND Ta-BEARINGoxIDES 513

.?ef'1l 1.O I

0.6

SnWt% .l o.4 I

a a o.2 a a a tt' a .l llllllll 1.O 1.2 1.4 1.6 Ti wt% Fig. 10. The Sn and Ti contents of the Greer Iake columbite-tantalites.The GRN, GRS, and AC compositions are represented only by their field boundariesbased on 8, 9, and 10 analyses,respectively. Ixiolite data extend the GL field to much higher Sn and Ti values outside this diagram.

by the pegmatite melt only during its passagethrough and garnet metabasalticcountry rocks (Norwegian and Madagascar Fractionation in columbite-tantalites districts as interpreted by Goldschmidt, 1958, and Neu- Garnet is the most widespreadaccessory mineral in the mann, 196l). It seemslikely that the first mechanismwas GRN, GRS, ACl, AC3 and GL parageneses;it seemsto operative at Greer Lake. However, detailed regional stud- be missing only in the AC2 pod (Fie. l). Thus it is of ies of Sc distribution in Nb- and Ta-bearing oxide min- interest to compare the FelMn behavior in columbite- erals are virtually nonexistent, and additional data are tantalitewith that in garnet(Goad and eern9, l98 1leern! neededfor comparison with pegmatite goups located in et al., l98la; Goad, 1984).Garnet commonly occursin diverse host rocks. several generationsin the GL parageneses,and even the

F U z U tr z

o o r f f

4

20 40 60 80 Mn/(Mn+Fe), COLUMBITE-TANTALITE TAl(TA+N b), COL U [,4 B IT E-T ANTAL ITE Fig. I l. Correlation of fractionation trends in columbite-tantalite and associated garnet. Data codes correspond to individual locations as shown in Figures 2 and3. 514 CENMr ET AL.: FRACTIONATIONTRENDS OF Nb- AND TA-BEARINGOXIDES

+t2s

ar tE CA o- o- o u L rE Y -4 o' CE

Y Y

Mn/(Mn+Fe),COLUMBITE-TANTALTTE Tal(Taf Nb). COLUMBITE_TANTALITE Fig. 12. Correlation of fractionation trends in columbite-tantalite with the WRb ratio of blocky, core-margin K-feldspar of parent pegmatites. Data codes correspond to the individual location as shown in Figures 2 and3 garnet most closely associatedwith columbite-tantalite is 1963) are more common than largely to completely or- not found in mutual contact with it. Thus the trend shown dered phases.Two different interpretations are available in Figure I lA indicates only a correlation ofconcurrent for the origin of different degreesof ordering in columbite- fractionation paths rather than partition coemcients. tantalite. Graham and Thornber (1974) proposed a szl Considering the lack of equilibrium partitioning, the generis metamictization, leading to a breakdown of orig- correlation shown in Figure I lA is surprisinglygood. The inally homogeneous,compositionally complex phasesinto initial rate of Mn enrichment is higher in garnet,whereas submicroscopicintergrowths of several phasesof simple columbite-tantalite reaches predominantly manganoan stoichiometries;the resulting mixture generatesX-ray dif- compositions only in the most fractionated pegmatites. fraction patterns of the disordered columbite-tantalite type Figure I I showsthat the correlation betweenMn-enrich- (cf. Ewing, 1975, and,Graham and Thornber, 1975, for ment in garnet and Ta-enrichment in columbite-tantalite discussion).In contrast, experimental evidenceindicates is rather poor. This is to be expectedas Fe/Mn and Nb/ that synthetic columbite-tantalites of even the most sim- Ta trends behavelargely independentlywithin the colum- ple end-member compositions grow initially in the dis- bite-tantalites themselves(Fig. a). ordered state, attaining partial to complete cation order under extendedsubsolidus heating. The disorderedstruc- Fractionation of columbite.tantalite and K-feldspar ture is formed initially as a metastable high-energy ar- Figure 12 shows the Fe/Mn and Nb/Ta fractionation rangement within the stability field of the ordered struc- in columbite-tantalite compared to one of the principal ture, which subsequentlyevolves from the disordered state. fractionation characteristics of granitic pegmatites, the The disordered structure is more persistentat low ?nand K./Rb ratio of the core-margin blocky K-feldspar (Goad P, whereasat higherP- ?"conditions the transition is rapid. and eernj', l98l;eernj, et al., l98la; Goad, 1984).Both There is a striking relationship between the degreeof correlations are rather good: in conjunction with Figure order in the Greer Lake columbite-tantalitesand the dis- I l, they indicate a concurrent and continual evolution of tance from the pegmatitic granite intrusion (Figs. 3, 6). the FelMn, Nb/Ta, and K,/Rb fractionations in the peg- Within the pegmatitic granite, the GRN, GRS, and AC matitic granite and its pegmatite derivatives. Neverthe- columbite-tantalites show intermediate to considerably less,Anderson (1984) showed that this is not always the ordered structures;the nearby SF offshoot shows a range case and that the degree of correlation may vary rather ofdisordered to considerablyordered phases,and the GL extensivelyamong different pegmatitegroups of the same pegmatitescarry highly disordered columbite-tantalites. field (cf. Cernf et al., 1985a). Such a distribution of structural states may have been controlled by differences in postcrystallization cooling Variation in structural state of columbite-tantalite rates, in accord with the experimental evidence quoted The data presentedby eern! et al. (1984) and eernf above. Assuming initial crystallization of all columbite- and Ercit (1985) show that partly to largely disordered tantalites in the disordered state, conditions ofrelatively columbite-tantalites ("pseudo-ixiolites" of Nickel et al., fast cooling in the small and dispersed GL pegmatites CenrVi ET AL.: FRACTIONATIONTRENDS OF Nb- AND Ta-BEARINGOXIDES 515 would lead to metastablepreservation of disorder, where- studies of natural assemblages.The experimental ap- as longer periods of higher I in the slow-cooling large proach should address(1) Fe, Mn, Nb, Ta, Sn, Ti, and Sc mass of the pegmatitic granite would promote ordering. speciationand mobility in pegrnatitemelts and fluids; (2) The SF columbite-tantalites would represent an inter- the role of F in FelMn fractionation; (3) ixiolite stability, mediate case.An analogy to the distribution pattern of as opposedto that ofwodginite, tapiolite + cassiterite+ order-disorder in columbite-tantalite is shown by the columbite-tantalite, tapiolite * rutile + columbite-tan- wodginite-ixiolite pair: relatively ordered wodginite oc- talite, and other relevant assemblages;and (4) order-dis- curs within the pegmatitic granite, but the disordered order in synthetic columbite-tantalite and wodginite- ixiolites occur only in the aureole of pegmatite veins. ixiolite, under conditions close to those of granitic The above interpretation may be oversimplified, and it pegrnatite crystallization. is not necessarilyapplicable to other localities. For ex- Thorough studiesare required ofthe paragenesis,com- ample, manganotantalitestend to be either highly ordered position, structural stateand mutual relationshipsofcom- or highly disordered (Cernf and Ercit, 1985); thus the plete assemblagesof Nb- and Ta-bearing oxide minerals structural state of the SF manganocolumbites-mangano- from individual pegrnatites and from parental granitoids tantalites may be subject to a specific crystallochemical plus their pegmatite aureoles.Particular attention should control. Electrostaticenergy calculations of Giese ( I 975) be paid to selectinggeologically well-characterizedpeg- suggest that manganocolumbites of different structural matites and pegmatite groups characteristic of different states may originate with about equal ease,because of classesand types.In order to fully exploit recentadvances minimal energeticdifferences between the different cation in crystal chemistry of the Nb-, Ta-, Ti-, and Sn-bearing arrangements.Consequently, the structural statemay eas- oxide minerals in a geologiccontext, complete structural, ily be affectedby other factors.The most obvious is com- chemical, and parageneticinformation on the oxide and positional variation disturbing the stoichiometry. It was coexisting silicate minerals is required; such essentialin- noted earlier that the GL columbite-tantalites contain formation is currently scarce. Sn, Ti, and Sc distinctly above their concentration in the AcrNowr.BocMENTS GRN, GRS, and AC and partly in the SF specimens.Thus it is possible that the presenceof R3* and Ro* cations in The authorsare indebted to T. S. Ercit andto E. E. Foordfor the structure may have been an important factor in pre- providing someof the electron-microprobeand structuraldata venting the GL columbite-tantalites from even incipient usedin this studyand for criticalreviews ofan earlydraft ofthe ordering. manuscript.Karen J. Ferreira and I-en Chackowskyprovided extensivehelp in the X-ray diffractionstudies. T' S.Ercit made and developedsome of Coxcr,uorNc R-EMARxs someof his unpublisheddata available the methodsof formulacalculation. D. L. Trueman'Reijo Al- Fractionation of FelMn and Nb/Ta in the Nb- and Ta- viola, andLance Barber were ofvaluable assistance in the field. bearing oxide minerals of the Greer Lake granite-peg- This work was supportedby the NSERCof CanadaOperating matite sequenceevolves with the general fractionation Grantsto Grnf and Hawthorne,by a University ResearchFel- characteristicsof these rocks, as exemplified by the Mn lowshipto Hawthorne,by the DEMR ResearchAgreements with enrichment in garnet and the increase in rare alkali metals Cernf; by the Canada-ManitobaSubsidiary Agreement on Min- lo- in K-feldspar. However, the Nb/Ta fractionation proceeds eral Explorationsubcontracted in part to Cernf; and by the grantsfrom the Mining at similar rates in all environments whereasMn enrich- gistic support and research CorporationofCanada, Ltd. to Cernf. ment is particularly enhanced by high activity of F. Pref- erential concentration of Sn occurs in the pegmatite au- RrrnnnNcns reole, but the mechanism of this is not understood. A.J. (1984)Geochemistry, mineralogy and petrology Ta-bearingoxide Anderson, Enrichment of Ti and Sc in the Nb- and of theCross Lake pegmatite field, Manitoba. M.Sc. thesis, Uni- minerals of the pegmatites seemsto be due to primary versityof Manitoba,WinniPeg. fractionation of these elements into the highly evolved Appleman,D. E.,and Evans, H.T., Jr. (1973)Job92l4'.Indexing pegmatite melts, rather than to assimilation from meta- attd least-sqoa.esrefinement of powderdiftaction data. U.S. SurveyComputer Contributions 20; NTIS Docu- country rocks. The order-disorder relationships Geological basaltic mentPB2-16188. in columbite-tantalite, wodginite, and ixiolite together with Bailey,J.C. (1977) Fluorine in graniticrocks and melts: A review' their distribution in the different units suggestscrystalli- zation ofall these phasesin the disordered state; subse- quent partial ordering proceedsin the solid state,at rates inverse to those of the cooling of the parent rock. How- ever,the degreeofdisorder alsoshows positive correlation with Ti, Sn, Fe3*,and Sc,and the possibility of a crystallo- chemical control of the structural state cannot be dis- 189,148-151. andThompson, A.J. (1965): ra- missed. Butler,J.R., tio in someigneous rocks. Geochimica et CosmochimicaActa, The paragenesisand geochemicalevolution of Nb- and 29,167-175. Ta-bearing oxide minerals in granitic pegmatitesshould eernf, Petr.(198]) Anatomyand classificationofgranitic peg- be examined by experimenlal modeling and painstaking matites.In P. Cern!, Ed. Granitic pegmatitesin scienceand 516 CNNN'r ET AL.: FRACTIONATION TRENDS OF Nb- AND TA.BEARING OXIDES

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