Tantalum Oxide Minerals from Complex Granitic Pegmatites Inthe Moldanubicum, Czech Republic: Primarvversussecondary Compositional Trends

6s9 The CanadianM ineraLo g is t Vol.36, pp. 6s9-672(1998)

NIOBIUM-TANTALUM OXIDE MINERALS FROM COMPLEX GRANITIC PEGMATITES INTHE MOLDANUBICUM, CZECH REPUBLIC: PRIMARVVERSUSSECONDARY COMPOSITIONAL TRENDS

MILANNOVAKI

Departrnent of Mineralogy and Petrography, Moravian Museum" Zeln! trh 6, 659 37 Bnto, Czech Republic

PETREERNfI

Depamnent of Geological Sciences, University of Manitoba, Wnnipeg, Manitoba R3T 2N2

ABSTRACT

Compositional trends of M,Ta-oxide minerals in four complex granitic pegmatites (lepidolire and petalite subtypes) in the Moldanubicum, Czech Republic, covering primary (magmatic) and secondary (hydrothermal replacement) stages of crystallization, were investigated. The primary stage is characterized by ferrocolumbite to manganocolumbite, followed after Fe- and Mn-depletion by Ca- or Sb-rich minerals: microlite (Nov6 Ves), rynersonite (Chvalovice) and stibiotantalite (Dobr6 Voda, La5tovidky). Compositional paths in columbite are similar to those of lepidolite pegmatites;the other primary Nb, Ta-oxide minerals exhibit Ta/(Ta+M) values significantly higher relative to columbite. Decreasing Nb/Ta and the cation sequenceFe - Mn - (Sb,Ca) are rypical of the primary stage.The secondarystage displays a broad spectrum of mainly fracture- filling secondary phases, such as diverse microlite-group minerals, cesstibtantite,manganotantalite, ferrotantalite and ferrotapiolite, which replace stibiotantalite, microlite and rynersonite; columbite in outer pegmatite units remains unaffected. The Tal(Ta + Nb) values of the secondary phasesare comparable to those of their precursors. The general sequenceof major A-site cations in hydrothermal Nb,Ta-oxide minerals, (Sb) - Ca - Mn - Fe, is reversed relative to that of the primary stage,and it may representa universal pattern ofhydrothermal replacementofprimary Nb,Ta-oxide minerals in comparablegranitic pegmatites. The composition of microlite-type minerals and textural relations indicate that the secondary hydrothermal stage includesabroadrangeofP-T-Xconditionsfromearlysubsolidusreplacementat-500-350oC,-2.5-2.Okbar(Nov6Ves, Dobr6 Voda) to near-surfaceweathering at <100oC (Chvalovice). The likely source of the Sb, Ca, Mn and part of the Fe seems to be the fluid phase exsolved from the residual melt or the replaced primary mineral; the host rock may be a source ofFe (Chvalovice).

Keywords: niobium, tantalum, columbite, microlite, cesstibtantite, rynersonite, stibiotantalite, tantalite, fenotapiolite, Moldanubicum, Czech Republic.

Solvernr

Nous avons 6tudi6 les variations en composition des oxydes de M et Ta provenant de quatre massifs de pegmatite granitique complexe (sous-typesd lepidolite et petalite) dans le Moldanubicum, en Rdpublique Tchbque, tant d'origine primaire (magmatique) que secondaire (remplacement hydrothermal). Le stade primaire a donn6 la s6rie ferrocolumbite I manganocolumbite, qui a par la suite donn6 lieu l des min6raux appauvris en Fe et Mn, et riches en Ca et Sb: microlite (Novd Ves), rynersonite (Chvalovice) et stibiotantalite @obrd Voda, LaStovidky). Le trac6 des compositions de columbite ressemble d celui des pegmatites d l6pidolite; les aures oxydes de M et Ta du stade primaire ont une valeur Ta./(Ta+ Nb) plus 6lev6e que dans la columbite. Au cours du stadeprimaire, une diminution en Nb/Ia et la s6quenced'enrichissement Fe - Mn - (Sb,Ca) sont typiques. Au stade secondaire,nous documentonsle remplissagede fissures par plusieurs min6raux secondaires,par exemple les min6raux du groupe du microlite, cesstibr,ntite, manganotantalite,ferrotantalite et ferrotapiolite, en remplacement de stibiotantalite, microlite et rynersonite; la columbite des zones externes de la pegmatite resle inchang6e.Le rappon Tal(Ta + Nb) des phases secondaites est comparable tr celui des pr6curseurs. En g6n6ral, la s6quencedes cations occupant la position A dans les min6raux d Nb et Ta d'origine hydrothermale, (Sb) - Ca - Mn - Fe, est renvers6epar rapport aux min6raux primaires, et pourrait t6moigner d'un comportement g6n6ral suite au r6-dquilibrage hydrothermal dans des situations comparables rencontr6es dans les pegmatites $anitiques. D'aprbs la composition des min6raux du groupe du microlite et les relations texturales, le stade hydrothermal repr6sente un large intervalle de conditions P-T-X, en partant du remplacement

I E-mnil addresses:[email protected], [email protected]

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subsolidusprdcocea-500-350'C,-2.5-2.0kbar(NovdVes,Dobr6Voda),jusqu'dlam6t6orisationprdsdelasurfacedune temp6ratureinf6rieure ou 6gale A 100'C (Chvalovice). ks 6l6ments Sb, Ca, Mn et une partie du Fe seraient ajout6s vla la phase fluide exsolv6e du magma r6siduel ou viendraient du min6ral pr6curseur.A Chvalovice, la roche-h6te a peut-6tre 6t6 A I'origine du Fe. (Traduit par la R6daction)

Mots-clds: niobium, tantale, columbite, microlite, cesstibtantite, rynersonite, stibiotantalite, tantalite, ferrotapiolite, Moldanubicum, R6publique Tchbque.

Irrnopucuolt articles quoted above, are largely restricted to a mere identification. We presenthere the resultsof a systematic Compositional trends of primary Nb,Ta-oxide study of the secondary minerals, remarkable in the minerals in complex pegmatitesare a useful tool for uniformity of their evolution in four widely separated providing information about the degree of melt pegmatites; in some respects, we significantly depart fractionation. Distinct compositional trends in terms of from the traditional views of the alteration processes. the Nb-Ta and Fe-Mn fractionation are recognized in columbite-type minerals from individual subtypesof PEcMATITEBoums ExaulxBo granitic pegmatites(e.g., Cen! & Ercit 1985, 1989, Spilde & Shearer 7992,Ercit 1994, Nov6k & Povondra Complex pegmatitesin the Moldanubicum occur in 1995"Mulja et al. 1996). They result from differences several spatially isolated regions in southern Bohemia in the composition of parent melts, and particularly in and western Moravia (Fig. 1). Pegmatites of the the activity of F, P and their complexes (Keppler 1993, lepidolite subtype predominateover those of the elbaite Wolf & London 1993, Wolf et aI. 1994, Linnen & subtype; several pegmatite dikes of the lepidolite Keppler 1996). Typical cations in the primary Nb, subtype carry variable amounts of petalite and can be Ta-oxide minerals include Fe and Mn; however, locally designated as transitional to the petalite subtype. Na, Ca, Sb, Bi, Y Sn andAl may alsoplay a significant Four bodies of pegmatite are examined here: Nov6 role, particularly in diverse minerals of the microlite Ves and Chvalovice (eeskf Krunilov region, southern subgroup, simpsonite and in the stibiotantalite group, Bohemia), and Dobr6 Voda and La5tovidky (Velkd all of which are typical of highly fractionated granitic MeziYidi region, western Moravia) (Fie. l). The Nov6 pegmatites (Ercit 1986, Voloshin & Pakhomovskyi Ves pegmatite belongs to the transitional lepidolite- 1988). petalite category, if not to the petalite subtype proper Secondary (hydrothermal) Nb,Ta-oxide minerals (Standk 1992).The other three pegmatites are of the represent a broad spectrum of mineral species, lepidolite subtype, locally with abundantelbaite and commonly carrying exotic cations: Li, Na, K, Cs, Fe, amblygonite (Nov6k et al. 1992). Mn, Sr, Ba, Pb, Al, Sb, Bi, Sn and U, in addition to All four pegmatitestypically form symmetrically abundant and widespread Ca. However, compositional zoned dikes (Fig. 2), consisting of the following trends evolving during the hydrothermal stage are not textural and paragenetic units, from the contacts understood at a level comparable with those of the inward: (1) fine-grained granitic unit, (2) graphic unit, primary stage. Diverse views were expressedabout (3) coarse-grainedalbite - qtJafiz- tourmaline - mica the behavior of the ratio Nb/Ta (Cf Voloshin 1983,Ercit unit, (4) blocky K-feldspar, and (5) lepidolite unit in the 1986), and attempts at establishing a general sequence center.Amblygonite - montebrasiteis commonly found of cations are rather controversial (Voloshin & in the lepidolite unit (5) or locally in the coarse-grained Pakhomovskyi 1988). albite - quzrtz - tourmaline - mica unit (3). A blocky The complex pegmatites in the Bohemian and petalite unit (6) is developed in the center of the Nov6 Moravian part of the Moldanubicum typically contain Ves pegmatite (Fig.2), rare petalite or pseudomorphs Nb,Ta-oxide minerals, predominantly manganocolum- of spodumene+ quaftz after petalite are located in the bite. Other primary minerals include ferrocolumbite, internal zones of the Chvalovice and Dobrd Voda dikes manganotantalite,"wolframoixiolite", ixiolite, wodginite, (StanEk 1965, 1992). Neither petalite nor amblygonite pyrochlore, microlite, stibiotantalite and rynersonite, - montebrasite were found at La3tovidky (Stan6k but most of them are rare (Nov6k et al. 1992, dernf & I 973). Additional information concerning thickness, Ndmec 1995,Nov6k & Divi5 1996,unpubl. data of the host rock and mineral assemblagesof the individual authors). The last three minerals were hydrothermally pegmatite dikes is given in Table 1. altered at various stagesofthe subsolidusreconstitution The pegmatites examined are very similar in their of the pegmatites,producing a variety of secondary internal structure (Fig. 2) and mineral paragenesis. mineralssuch as microlite, stibiomicrolite, cesstibtantite, Petalite is more abundantin localities from southern manganotantalite, ferrotantalite and ferrotapiolite Bohemia, pollucite was found only at Nov6 Ves (StanEk1963,1973,Ercit et al. 1993,Nov6k & Srein (Teertstra et aI. 1995). The Dobr6 Voda and La5tovidky 1998). However, the available data, scatteredin the pegmatitescontain small vugs lined with crystals of

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r 30km ' t

+-+ + +

+ +++ LASTOVICKY {:i:ffi+++ + +++ Y+ + + + + + + + + +l+ + ''Rl

,(_ CHUALOVICEF F ,9. \ NourvEsc.zuDEJOvrCE 6ermony W

I rf\_

Ftc. 1. Locations of complex pegmatitesexamined (solid dots) in the Moldanubicum. Granitic (crosses)and syenitic (x) intrusions are emplaced into a dominantly gneissic terrane (open).

Ol2lm Irtl t DOBRAVODA sw

Ftc. 2. Sections across the Novd Ves and Dobr6 Voda pegmatites. Ctosses: fine- to medium-grained granitic unit; check marks: graphic unit, open stipple: coarse-grained albite - quartz - tourmaline - mica unit, cross-hatched:blocky K-feldspar, white: blocky petalite, densestipple pattem: albite-rich unit, black: lepidolite unit, subdivided into an outer and inner oart at Dobr6 Voda.

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TABLB 1. MINERAL ASSH\,IBIAGES,EOST N@K AND TgIGNBSS TextunaL aNo PeRacnuertc RELATIoNSoF TIIE M,Ta-BnanrNG PHASES

U$€adrg mlffil. lF[,ldollh lata Primary minerals petallE l+1- spoduEe* o.+- elbdto aoaa Columbite typically forms black bladelike crystals ooa- up to 5 mm in size enclosedin the coarse-grainedalbile Nb,Taddenincis - quartz - tourmaline - mica unit (3). It is commonly prbnot ftq@bEffie associatedwith zoned tourmaline (black schorl to green nflg4natunHF or pink elbaite), colorless to purple Iithian muscovite to niqolie lepidolite, and locally with amblygonite.Columbite is rytrmits sdblobrralfre exceptionally found in blocky K-feldspar (Nov6 Ves), in blocky petalite completely converted to quartz + wndol spodumene (Dobr6 Voda), EisotiF +- and in the medium-grained casdbbDdle .? lepidolite unit (5) (Nov6 Ves, Dobr6 Voda) (Standk siblodmlle 1963, 1973, 1992, Cech & Standk 1960). Columbite in ffigmahrnffi Eegarot@talitB the outer units of the pegmatitesremains unaffected by ftmtanta[to ?- any secondary replacement processes. Beside columbite, stibiotantalite is found at Dobr6 Voda and La5tovidky as a primary phase; primary microlite occurs at Nov6 Ves, and at Synbols: I mjor, r 6@q O Eiu, + EE,. vqy EB, ? prcbldic, - sbs€dn, rynersonite 's@Ddry. Thic,trsinffi Chvalovice (Table 1). The primary rynersonite deserves specialcomment, as it is generallyknown as a secondary mineral. At Chvalovice, however, it occurs in the columbite-free blocky pegmatite unit (4)" the medium- to coarse-grainedlepidolite unit (5), and inside the columbite-bearing coarse-grainedunit (3). It forms subhedral inclusions in amblygonite, feldspars and quartz, and in most casesis extensively replaced by quartz, albite, elbaite and lepidolite. The smaller dikes secondaryminerals. Extensive depletion of Fe and Mn at Chvalovice and La5tovidky are very simitar to their could have led to the stabilization of an anhydrous larger counterparts at Nov6 Ves and Dobr6 Voda, Ca,Ta,M-oxide mineral as a primary phase. respectively,but their internal structure and mineral paragenesisare relatively simple. Secondary assemblages

ExpERIMENTATMetHoos Late assemblages of secondary minerals form fracture-filling, generally very fine-grained and locally Electron-microprobe analyses were performed porous aggregates within the replaced primary Nb, in the wavelength-dispersion mode on a Cameca Ta-oxide minerals. As some differences were found Camebax SX-50 instrument at the Department of among individual localities, the conditions of the Geological Sciences,University of Manitoba, Winnipeg. secondary phases are described separately for each A beam diameter of 1-2 pm, accelerating potential of pegmatite. 15 kV, a sample current of 20 mA and a counting time At Novti Ves, primary euhedral to subhedral of 20 s were used for Na, Ca, Nb, Ta and F" and 40 mA microlite in a medium-grained "cleavelandite" - and 50 s for K, Cs, Sr, Ba, Fe, Mn, Pb, Y, Sb, Bi, Sn, lepidolite subunit of the lepidolite unit (5) (dech & Ti, U, and W. The following standardswere used: Stan6k 1960) exhibits two distinct manifestations manganotantalrte('IaMo'), MnNbrOu (MnKq, NbLc[), of hydrothermal alteration. The first one, more FeNbrOu @eKo), microlite (NaKcl, FKcr,), CaNbrOu widespread, is representedby secondary microlite (CaKa), SnO, (Snl,c), rutile (TiKcr), BiTaOo (Bllt4d.), penetrating along fractures of the primary phase; such m i meti te (PbM a), sribi orantalite (Sbtcr), NaS cSirOu alteration was described in detail from complex (ScKct),UO, (UMo), YAG (Ytcr), Ba2NaNbrO,,@aLa), pegmatites by Lumpkin & Ewing (1992). The second SrBaNboO,o (SrZc), pollucite (Cslcr), orthoclase style is extremely rire: manganocolumbiteseems to (KKct) and metallic W (WMc[). Data were reduced cement fractured primary microlite. However, the using the PAProutine (Pouchou& Pichoir 1985). textural.relationshipsare complicated in thesecases, Identification of some of the minerals examined was and alternative interpretations may be possible. verified by X-ray powder diffraction, using standard At Chvalovice, primary rynersonite commonly is procedureson the Philips PW1710 and SiemensD5000 enclosed in amblygonite. It is almost completely instruments. replaced by a mixture of secondaryminerals: microlite,

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manganotantaliteand ferrotantalite. They locally form fracrure-filling aggregates;microlite seemsto be the earliest to crystallize, followed by Fe-poor manganotantalite. Onion-skin replacementtextures consisting of almost alternating monomineralic zones of microlite and manganotantalitealso were found (Fig. 3a). Based on EDs-study, poorly polished dark areasin Figure 3a consist largely of manganotantalite. The X-ray powder-diffraction study yielded peaks only of microlite, but none of manganotantalite(F. Veselovskf, pers. commun . 1995). A very fine-grained mixture of manganotantalite and ferrotantalite with a random distribution of both minerals also was identified, in which both rynersonite and secondary microlite were completely replaced. At Dobrd Voda, stibiotantalite forms irregular grains or euhedral dipyramidal crystals up to 3 cm in size, enclosedin medium-to coarse-grainedalbite - "rubellite" subunit of the lepidolite unit (5) (StanEk 1963). It is replacedalong fracturesby an assemblageof secondary hydrothermal minerals (Fig. 3b). The sequence stibiomicrolite - microlite - manganotantalite is much easier to recognize here than in the Chvalovice pegmatite. However, the last two minerals locally form extremely fine-grained mixtures, situated mainly in the center of veinlets (Fig. 3c). Poorly polished dark areas (Fig. 3c) correspond to Fe-enriched manganotantalite, or perhaps ferrotapiolite. Its presence as a secondary phase is very likely, becausea tapiolite-like mineral was detected by X-ray powder diffraction in several samples.Very rare niobian cesstibtantitealso was found in the assemblage of secondary minerals (Ercit et al. 1993,Nov6k & Srein 1998). At InStovidl{y, small fragments of compositionally heterogeneouscrystals of stibiotantalite,up to 8 mm in size, occur mainly as loose grains in pocket clay, and only exceptionally are enclosed in lepidolite, "rubellite" or quartz. Stibiotantalite is replaced along fractures by very fine-grained ferrotapiolite. Stibiomicrolite and cesstibtantitealso were recognized as extremely rare and very small grains in fenotapiolite; both minerals seemto have formed before ferrotapiolite.

Crm,vrcar CovrposmoN

Primary minerals

Frc. 3. Back-scattered elechon image of secondary Ferro- to manganocolumbitefrom the outer pegmatite M,Ta-oxide minerals. A. Onion-skin texture of microlire units have a very similar composition at all localities (bright) and manganotantalite (dark) from Chvalovice. examined (Table 2). The composition is characterized The scale bar is 100 trrm long. B. Veins of secondary by variable Fe contents, but relatively stable and high microlite (gray) (pale and manganotantalite gray) in the M/Ta values (Fig. ), decreasing in the late stagesof center of veins penetrating primary stibiotantalite primary crystallization. The relative Mn enrichment is from Dobr6 Voda. The bottom edge of the photograph is locally extreme, with Mn/(Mn + Fe) very close to 1 I 100 pm long. C. Fine-grained intergrowths of microlite except pegmatite (TabIe y (gray) and manganotanrrlite (bright) in rhe center of veins for the LaltoviEky 2, Fig. 4 shown in B. Dark poorly polished areas correspond to Tlpical minor cations include particularly W [up to Fe-rich manganotantalite to ferrotantalite. The scale bar is 2.88 wt.VoWO3, 0.09 atoms per formula unit (apfu)l 80 pm long. and Ti (up to 0.65 wt.VoTiO2,0.06 Ti apfu); they are

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TABLE 2. REPRESENTATTVE COMPOSITIONS to 0.95 (Fig. a). Except the stibiotantalite from OF PRIMARY MANGAI.IOCOLIJMBITE La5tovidky, they are fairly uniform on a single-grain 345678 scale, but locally variable among different grains wo! 0.93 0.9 L3E O.9 2.25 0.48 0.@ 1.44 l.l5 Nbro, 68.60 6E.70 59.70 49.30 65.70 59.10 37.tO 63.m 59.E0 (Fie.4). Tqot 9.60 9.b. t7.70 30.20 ll.l0 m.l0 44.& t4.20 17.t0 Although not completely developed at all four lio, 0.14 0.17 0.n 0.00 0.51 0.04 0.@ 0.43 0.t6 localities, a general sequenceof the A-site cations in s!q 0.@ 0.06 0.@ 0.04 o.tz 0.09 0.88 0.07 0.00 UO, 0.@ 0.06 0.m 0.@ 0.@ 0.00 0.00 0.00 0.00 primary M,Ta-oxide minerals can be defined as ScrO3 0.@ 0.@ 0.@ 0.05 0.02 0.92 0.05 0.02 0.t2 Fe-Mn-(Ca,Sb). Lrp 0.@ 0.00 0.@ 0.00 0.@ 0.@ 0.@ 0.00 0.01 CaO o.u2 0.02 0.01 0.03 0.01 0.01 0.03 0.00 0.01 IvItrO 17.E0 17.@ 14.20 18.60 11.90 18.@ 16.30 9.15 I Lto Secondary minerals 0.16 7.79 l.16 0.00 t0.20 7.41 Total 9.12 9.26 98L6 937 9.4 99.@ 99.16 9E.71 98.15 Chemical compositions of secondary minerals are cmicorem highly variable and complicated compared to the we 0.028 0.030 0.u4 0.033 0.069 0.015 0.000 0.045 0.037 primary minerals, and considerable differences were Nbe 3.655 3.655 3.33t 2.874 3.52,6 3.298 2.3t3 3.455 3.342 found individual localities (Fig. 5). Secondary Ta$ 0.30E 0.295 0.594 1.059 0.358 0.67i 1.680 0.4d? 0.598 among 0.012 0.015 0.02s 0.0@ 0.046 0.@4 0.@ 0.039 0.01s minerals locally form extremely hne-grained masses; sne 0.000 o.@3 0.0@ 0.o2 0.005 0.004 0.048 0.003 0.0@ consequently, some microprobe data may represent u* 0.000 0.002 0.000 0.0m 0.000 0.000 0.000 0.0@ 0.@ phases,either microlite with manga- s.f 0.000 0.000 0.w) o.m6 0.002 0.@2 0.006 o.trD 0.092 mixtures of two l41 0.@ 0.000 0.@0 o.o0o o.c{n o.@0 0.000 0.000 0.@2 notantalite or manganotantalite with ferrotantalite c€F 0.@3 0.@3 0.@l 0.@4 0.@l 0.@t 0.@4 o.@ 0.001 (ferrotapiolite) in a variable ratio. Mf t.m t.754 t.M 2.031 1.197 1.8E2 t.w 0.937 1.235 was Ff 0.200 0.2,10 0.516 0.017 0.7R 0.120 0.@0 1.031 0.766 AtNovd Ves,secondary microlite formed during t cd. 5.9t5 5.96 5.998 6.02J 5.981 6.@1 5.958 5.9E3 6.000 early and late hydrothermal alteration, as described by Lumpkin & Ewing (1992); Na- and F-depletion is It - NovdVos, FnTIe Vc6; 3,4- Cteslqeirs,sql€ MMCI6;5-7 - DoH Vod4 5 - r@pleMMC39, 6 - BanpleCDVs, 7 - r@pleMMglT; 89 - Idoviaky, typical of early alteration; however, significant Ca-, Na- 8qleVC3. Cdi@qdrmDq@liz€dto12 cl€@; 0.m-belrydd.di@liEil and F-depletions combined with a considerable AAcYSb @dBi beld ddeci@l#t Cqositi@ rcpq&di! st% 6id6 increasein U contents(up to 7. 11 wt.VoUO2, 0. 14 U apfu) were found in late secondarymicrolite (Fig. 5, Table 3). In both types of alteration, the MlTa values remain virtually constant.Late manganocolumbiteis most abundant in the Dobr6 Voda columbite and verv Fe-free, locally with minor amount of Ca and R but it low in its Nov6 Ves counterpart. exhibits a significantly depressedTa.i(Ta + M), similar Primary microlite from Nov6 Ves is homogeneous, to that of the primary manganocolumbite (Fig. 4). The with Na = Ca; negligible vacanciesin the A site and overlap with associated microlite may explain high F contents are characteristic (Table 3). The ratio increased Ca and F contents in some compositions. Ta/(Ta + Nb) in two grains varies from 0.73 to This manganocolumbitemay be the only secondary 0.90 (Fig. 4), but each individual grain is almost mineral examined to display a changein M/Ta relative homogeneous. to its primary precursor. Rynersonitefrom Chvalovice is homogeneous,with At Chvalovice, highly variable secondary minerals Ta/(Ta + Nb) in the range 0.68 to 0.71; it displays a involve mainly Ca-, Na-, F-poor microlite with a high relatively simple chemical composition, with W Pb, proportion of vacanciesin the A site; H2O may be a Sn, Fe and particularly Mn the typical minor to trace dominantA-site constituent (Ercit et al. 1994). A wide elementsdetected. Low totals of oxides (about97 wt.Vo) spectrum of minor to trace elements W, U, Sn, Bi, Ba, and a small excessof R?+cations were observed (Table and particularly Pb and K (Table 4) is typical. Pb and K 4), but cannotbe readily explained. may locally represent dominant A-site cations Stibiotantalite from Dobr6 Voda is fairly homoge- (plumbomicrolite and "kalimicrolite"). Secondary neous,with Ta./(Ia + M) between 0.7 | to 0.76 (Fig. 4). manganotantalite associated with microlite exhibits Except moderate M-contents and minor to trace variable but mostly low Fe- and Ca-contents.Highly amounts of W Sn and Bi, it is close to its end-member heterogeneousferrotantalite is associatedwith Fe-rich composition; moderateY contents (up to l.l3 wt.Vo manganotantalite (Figs. 4, 5, Table 4). Minor to trace Y2O3;Table 5) are remarkable. The composition of amounts of W, Si, Sn, Bi, Ba, Pb, and Ca are typical in heterogeneousstibiotantalite in the Laltovidky pegmatite both minerals; except Sn and W, the increase in these is similar to that from the above locality, except for the cations is concomitant with increasing FeAvIn. Tal(Ta + Nb) values, which range from 0.68 to 0.95 Ferrotantalite displays low oxide totals, high excessof (Fig. 4, Table 6). Relatively rare Nb-enriched areasare trP*(plus Rk) cations, far above the limits of the ABrOu randomly distributed within the stibiotantalite grains. stoichiometry, and appreciableSi (Table 4). Most of the Primary microlite, rynersonite and stibiotantalite are Fe is probably in a trivalent form; however, admixture more evolved relative to the primary columbite. Their ofFe-rich hydroxide minerals and quartz in the very Tal(Ta + M) values vary in the broad range from 0.68 fine-grained porous mass is possible.

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t NOVAVES CHVALOVICE

ooo 8\ o + o z + g

' .. . ortfol rc

OOBRAVODA LASTOVICKY

xx x T

zo + .0 F

a a

t ta .t P

-o I Mn/(Mn+ h I Mn/(Mn+F!l

FIc.4. Compositions ofprimary and secondary Nb,Ta-oxide minerals in the columbite quadrilateral. Horizontal dashesalong the manganoan side of the diagram mark only the Ta./(Ta+ M) value, and are not related to the Mn/(Mn + Fe) scale. Symbols: O primary columbite-tantalite, O secondary manganotantalite to ferrotantalite, x secondary manganocolumbite - manganotantalite (secondary ferrotapiolite at LaStovidky), + secondary manganotantalite, F primary stibiotantalite -l (primary rnicrolite at Nov6 Ves, primary rynersonite at Chvalovice), secondary microlite (stibiomicrolite at La5tovidky).

At Dobrd Voda, secondary minerals are highly Manganotantalite is dominantly Fe-enriched, with low variable in their chemical composition (Fig. 5, Table 5). oxide totals, but stoichiometry close to the A8206 Microlite-type minerals vary from stibiomicrolite formula, and low contents of minor elements(Table 5). through Sb- and Sr-enriched microlite varieties to At la{tovidlq, ferrotapiolite is the significantly microlite proper. Most of them are Na-, Ca-, and dominant secondary mineral; cesstibtantite (not F-poor, with significant vacancies in the A site (Table analyzed quantitatively becauseof its small grain-size) 5, Fig. 6); H2O may be a dominant constituentin the and stibiomicrolite (only one spot analyzed, Table 6) A site (Ercit et al. 1994). The minor to trace elements are very rare. Two distinct varieties of ferrotapiolite include Sn, Bi, Mn, Fe, Sr, Ba, Na and K. were recognized, Sb-free fenotapiolite with moderate

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TABIE 3. REPRESIENTATIVECOMPOSITIONS OFMIROLTTE MANGAI{OCOILnAAFB AlrlD ITRANMIROLIIE The Nb,Ta-oxide minerals from all localities FROMNOVAVES examined exhibit a similar sequenceof A-site cations, (Sb) - Ca - Mn - Fe, but with significant quantitative rgor 0.16 0.00 0.28 0.2t 0.04 0.53 0.06 variations conholled by the precursors.The secondary Nbro, 14.10 14.90 49.63 5.30 4.93 5.O2 s.Vz phases the localities with Ca-rich primary minerals T%Ot il.q $.60 30.45 74.n 75.4 n.n 76.70 at Tlo, 0.00 0.@ 0.m 0.00 0.@ 0.00 0.@ (microlite, rynersonite) are characteized by gradual StrO, 0.05. 0.ll' 0.0r 0.101 0.n 0.08. 0.2t decrease in Ca and increase in Mn, followed at uo, l.l9 3.05 0.@ 0.00 0.00 4.42 7.ll (Fig. sbro. 0.36 0.16 0.@ 0.16 0.14 0.@ 0.00 Chvalovice by significant late enrichment in Fe 5). Bto, 0.09 0.@ 0.@ 0.@ 0.07 0.00 0.0E The two localities with stibiotantalite exhibit somewhat CeO 10.60 9.79 0.05 10.30 10.20 1.35 0.7t different evolution. Primary stibiotantalite is initially lvttrO 0.00 0.00 15.35 0.01 0.01 0.09 0.73 FeO 0.00 0.@ 0.00 0.@ 0.01 0.36 0.00 replacedby stibiomicrolite or Sb-rich microlite, but the SrO 0.@ 0.@ 0.00 0.@ 0.@ 0.@ 0.47 subsequentstages of replacementare very similar to B8O 0.00 0.@ 0.00 0.@ 0.@ 025 0.210 P'bo 0.10 0.14 0.00 0.0t 0.13 0.55 0.zA Nqo 5.09 2.n 0.00 6.04 6.08 0.ll 0.00 KrO 0.00 0.oo 0.06 0.01 0.01 0.10 0.r5 F 3.01 2.57 0.m 3.18 3.t1 o.lt 0.@ O=F -l-n -1.0t 0-00 -t 34 -l 33 4 05 0.00 TABLE4. REPRESENTATTVECOMFOSITIONSOF 9t.38 95.16 95.95 98.95 98.9t 90.A2 91.t9 RYNERIIO}qTq MICROLIT4 MAI{GANOTAT.{TALITE aloaicffi AND FERROTAI{TAIXTE FROM CIryAIOVICB w6 0.003 0.@ 0.010 0.@5 0.@l 0.012 o.ml Nb$ 0.530 0.561 2.9t3 0.210 0.196 0.t92 0.1t5 Ta$ t.67 t.439 t.lol 1.785 1.t03 t.7 l.tl3 wor 2.t0 0.45 0.54 0.53 1.23 0.s8 fi& 0.0m 0.000 0.002 0.m 0.@0 0.000 0.@ Nb2o' 17.@ 12.tO 10.40 8.43 11.00 t6.14 Sno 0.(o2+ 0.@4+ 0.004 0.004+ 0.005+ 0.@3+ 0.@t+ TarO5 9.90 54.60 58.00 56.70 4.@ s9.42 [r 0.t2. 0.056 0.000 0.000 0.000 0.083 0.138 11O, 0.00 0.(p o.02 0.00 0.00 0.10 sb4 0.012 0.005 0.@0 0.006 0.@5 0.@l 0.@ SnO" 0.48 1.11r l.l0* 1.30* t.t1 0.85 Bi3 0.@2 0.@ 0.@ 0.000 0.@2 0.000 0.@2 SiO, trd nd- rd- nd nd. O.79 co} 0.944 0.873 0.@7 0.96i1 0.961 0.14 o.f73 uo, 0.00 2.9 2.57 1.90 0.@ 0.00 I4q& 0.@ 0.@0 r.728 0.@l 0.@1 0.006 0.054 sblq 0.03 0.00 0.00 0.00 0.06 0.12 Ff 0.@ 0.0@ 0.000 0.@0 0.@l 0.0126 0.@ Birq o.oo 0.@ 0.10 0.41 0.@ 0.33 sfl 0.0@ 0.@ 0.000 0.000 0.0@ 0.000 o.ou CsO 12.30 3.20 1.95 1.56 0.67 0.47 BE 0.@ 0.@ 0.0@ 0.@0 0.000 0.@t 0.014 Ivtro 0.35 1.90 0.95 0.66 I 1.40 10.87 Pb& 0.@2 0.@3 0.000 o.w2 0.003 0.013 0.011 Fgo 0.20 l.6t 1.35 o.v3 0.r2 3.0E Na' 0.820 0.471 0.000 t.t26 l.m7 0.018 0.000 slo 0.00 o.27 0.24 0.10 0.00 0.u K" 0.@ 0.@ 0.010 0.@l 0.001 0.0u 0.017 B@ 0.00 2.16 t.1't 1.08 0.08 0.00 F 0.791 0.676 0.@0 o.tEl 0.tt2 0.t29 0.000 Pbo 0.41 0.01 0.04 4.54 2.98 0.37 or 6.031 5 903 12000 6c62 6Ml 5 ?s6 5479 NqO 0.@ 0.00 0.@ 0.@ 0.00 0.@ ca. 3.t04 3.4t4 5.850 4.@7 4.015 2.292 2.34 f,, KrO 0.oo o.72 t.22 0.30 0.00 0.01 In 6.42. 6.579 l2cxm 6 7 6945 5386 5479 CaO 0.m 0.m o.tE 0.16 0.10 0.00 F 0.00 0.00 l, 4 @d 5 - prindy Ei@lit€;2,6 @d ? - s@dey Ei@ho to 0.@ 0.09 0.16 0.@ lll@imlie; 3 -'wdr/ nfrg''ql@titE Cdio ot* nomalized to Ts+NbrTi+Wd2 (Eirclilo #1, 2, 4 Torat n.19 E0.E2 80.46 78.60 94.2t $.4 5, 6, 7) @d 12 q!€@ ('llfig''l'@l'fihito #3). 0.00 - b€los d6€di@ liEir8; 6mc6@E 'SD I StrO, +Sq,. Cs belos det6{i@ liEit Co@o8ni@ rcpoftd in wt% qida we 0.082 0.011 0.014 o.ol4 0.054 0.t2A Nbs 1.165 0.535 0.456 0,394 0.349 t.til Tsl 2.675 1.4s3 1.530 l.ss2 3.m 2.6ts Ils 0.0@ 0.000 0.000 0.000 0.0@ o.ol2

0.I28 0.055 o.04 0.000 0.@2 inferredFe3+ content (up to 0.25apfu), anda rare S- 3.ffi 3.$l 0.0@ 0.@0 0.004 0.008 Sb-enrichedferrotapiolite (up to 9.76 wt.Va5b2O3,0.67 Bis o.oo0 o.ooo 0.003 0.@l 0.@ 0.014 apfu;rabre6,Fig. s). o.2c3 0.t73 0.121 0.081 ffi. lil l:i3f 0.078 0.058 l.g1 0.4t7 Fsr o.oE 0.132 0.110 0.080 0.131 0.00-/ General observations sP o.ooo o.olj 0.01I 0.006 0.000 1.490 Bsf o.@ o.os3 0.067 o.u4 0.005 0.0m Almost all secondary phases Pbe o.ol7 0.@ 0.001 o.t26 0.t37 0.016 from the localities Na* 0.0@ 0.000 0.000 0.000 0.000 0.0@ examined are characterizedby a fairly stable Nb/Ta K' 0.0@ 0.090 0.t51 0.040 0.000 0.@2 value during all stages of hydrothermal alteration Cs- 0.000 0.@0 0.07 0.@7 0.@7 0.@l (Fig. a). The only exception is very rare "secondary" F 0.019 0.028 0.049 0.0m 0.m0 0.0@ e n.9Er 5.903 5.741 5.741 12.000 12.0@ manganocolumbite associatedwith primary microlite L cd. 6.V17 2.919 2.7U 2.ffi 6.04 6.052 from Nov6 Ves (Fig. 3). However, textural relations I m 12 5 951 s74a 564 l2 t2 of both minerals are complicated, and this mangano- I - pdnry rynqBoniq 2a secodry nimliq 5 - sscmdEy columbite may in fact be a primary mineral replacedby nngnoffilife; 6 s€coillgy hngnoffitite. microlite. Such interpretation also is supported by the Cdim cm noma.lizd to 12 o46w (rynm@ns #l), ed very similar T8+Nb}T!+W=2 composition of primary manganocolumbite (Eimlits #24). 0.@ = b€low de(sctid limi! +SnI Sm, +sd' at this locality. Compositim rcpored iu wtolooxids.

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TABLE 5. REPRESENTATTVECOMP]OSMONS OF STIBIOTANTALTTq grain- STIBIOMCROLITE MICROUTEAND MANGA}IOTAI{TNITE Poor polish in some samples, extremely fine FROMDOBRAVODA size and high porosity ofsome secondary aggregates, possible presenceof very fine-grained admixture of wq 0.46 0.08 0.00 o.2t 0.60 r.23 0.35 Fe-hydroxides and quartz are typical features of the Nbro3 9.t4 ll.l0 t2.@ 13.90 13.@ 16.20 ll.@ latest sr4geof the hydrothermal alteration. They render Taror 46.80 52.4 62.4 63.20 62.70 63.50 70.90 StrO, 0.67 1.00* 0.211 0.59* 0.3t. 0.85 0.96 the study quite difncult, and may obscuresome aspects YrO3 l.l3 0.47 o.tz 0.00 0.t2 0.00 0.@ of the mineral assemblagesexamined. However, the sbzq 40.60 20.10 7.55 4.41 2.73 0.ll O.O0 generally stable values sequence Biro, o.u 1.08 l.@ 0.17 0.06 0.@ 0.05 M/Ta and the of c@ 0.00 0.99 6.08 6.45 9.59 0.0s 0.13 A-site cations, (Sb) - Ca - Mn - Fe, characterizethe MaO 0.03 0.16 0.52 0.31 0.530 1t.50 8.45 products of hydrothermal alteration of primary F€O 0.01 0.74 0.n 0.14 0.68 4.10 6.42 sro 0.10 3.30 t.yt 4.@ 223 0.00 0.00 M,Ta-oxide minerals at the localities examined beyond B8O 0.09 O.72 o.37 0.55 0.38 0.00 0.00 any doubt (Fie. 5). PK' 0.07 0.04 0.09 0.@ 0.00 0.00 0.03 N&O 0.00 o.sz 1.66 0.28 0.90 0.00 0.@ K?O 0.00 o.lE o.lE 0.10 0.03 0.01 0.01 CqO 0.00 0.00 0.10 0.00 0.14 0.00 0.00 F 0.00 0.31 o.47 0.38 0.30 0.00 0.00 O=F 0-m 411 4m Jll6 42t ono 00O To&l [email protected] 93.06 95.34 94.53 .a6 n.sl n.9t TABLE 6. REPRESENTATIVECOMPOSMONS OF AtoEir@@ STIBIOTAI{TAI.ITE STIBIOMICROI.,ITEAI\D cP o.on o.@2 0.000 0.@s 0.013 0.050 0.0t5 FERROTAPIOLITEFROM LASTOVI*Y Nbs l.ol4 o.i2o o.ifit 0.534 o.sn t.l5l 0.E05 TsI z.nt L478 L4n l.,t6l 1.460 2.it3 3.t20

sb$ 3.E15 0.8s9 0.n5 0.155 0.096 0.007 o.@ vo, 0.45 0.31 0.55 1.33 r.r2 0A4 0.34 Bi$ 0.t20 0.v29 0.023 0.004 0.@l 0.000 0.002 Nqo5 t2.E0 8.56 3.90 13.05 11.30 9.20 9.41 c8! 0.000 0.llo 0.575 o.sts o.rEo o.@8 o.aj T&rO, 40.90 48.20 53.70 63.89 70.N 70.80 67.75 Md! 0.006 o.ol4 0.039 o.olt 0.03E 1.530 l.160 sio" 0.00 0.@ 0.00 nd. 0.00 0.10 0.04 Fe} 0.m2 0.06lt o.om 0.010 0.049 0.s39 0.869 Tio, 0.02 0.01 0.02 0.30 0.04 0.02 0.02 sF o.ol3 0.r9E 0.r0r o.l9? o.lll 0.000 0.0@ SnO, 0.53 0.52 0.67 0.00 0.39 0.7r 0.r0 Btr 0.@r o.c29 0.013 0.018 o.ol3 0.@0 0.000 pb* 0.004 0.001 0.002 0.000 0.000 o.@0 0.@l uo, 0.0E 0.m 0.00 0.@ 0.00 0.@ 0.@ Na" 0.000 0.105 0.2t4 0.046 0.149 0.000 0.000 sbro, 43.@ 4t.40 N.@ 12.82 0.54 4.rr 9.t7 K 0.000 o.ta 0.o2! 0.011 0.003 0.@2 0.002 Biro. o.0o 0.07 0.37 0.00 0.00 0.0o 0.06 Cs" 0.0@ 0.m0 0.004 0.000 0.@5 0.@0 0.@0 c@ 0.00 0.@ 0.@ 4.2E 0.04 0.M 0.@ F 0.029 0.201 0.131 0.102 0.135 0.m0 0.0@ IUnO 0.01 0.00 0.00 0.30 0.35 0.?a 0.17 o& 15.971 6.895 6.310 6.092 6.287 12.000 t2.@o FeO 0.00 0.06 0.00 0.25 13.75 t2.49 10.51 t.00E 3.506 3.369 3.tt3 3.362 6.051 6.037 t,cd 0.23 f @ 16 6.950 6.432 6.177 6.4M 12 l2 F%Or 0.00 0.00 0.00 0.00 t.06 0.00 BaO 0.@ 0.00 0.19 nd o.lt 0.00 0.@ I - priEEy stibio@liq 2 - 8@ndry Simimliq 15 - ssudEy Pbo 0.00 0.16 0.00 nd. 0.00 0.@ 0.05 - nimlite, 6 od 7 wdry |@geot8@lile. NaO O0O 000 000 I l0 0.00 0_00 0_00 Cdi@ c.'a€@ rcmsliad ro 16 ei6 (@iaEmlib #l), TarNbrf i+W = 2 (Ei@lib# 2 !o 5),4d 12oq/gEns (|@g@ot@fte{6,7). Total n.79 99.29 1.ffi.00 n32 9.17 9.25 98.9E 0.00- b€lowdd€qtio limiq .Sn o SnO, + Sf. n ed U belw deb(.i@ linit slonic cmtfils Co4ositi@ rEporbdin wtolc w* o.an 0.019 0.034 0.v29 0.046 0.019 0.015 Nbs t.32S 0.900 0.420 0.495 o.El4 0.677 0.701 Ta: 2.553 3.u7 3.475 1.4s7 3.051 3.t34 3.037 si* 0.0@ 0.000 0.000 0.000 0.016 0.@7 those given above for Ca-rich primary phases(compare Ti4 0.003 0.002 0.004 0.019 0.005 0.002 0.002 in Fig. 5). No other substantialA-site cationsparticipate in the process at the localities examined. u* 0.004 0.000 0.000 0.@0 0.000 0.000 0.001 Evident differences exist in the participation of sb$ 4.06.8 3.967 3.913 0.443 0.035 0.327 0.67r Bi+ 0.000 0.004 0.023 0.0m 0.000 0.000 0.003 the individual A-site cations among the four localities cf 0.@0 0.@0 0.0fl) 0.3Es 0.@7 0.007 0.0@ (Fig. 5). In the Nov6 Ves pegmarire, the early It{f O.W2 0.000 0.@0 0.021 0.M7 0.039 o.ou subsolidusphase (secondarymicrolite) is preserved, F€fl 0.@0 o.O1,2 0.0@ 0.01E 1.E32 l.'tCE 1.449 indicating high activity of Ca relative to Mn, which Fgk 0.000 0.0@ 0.000 0.000 0.127 0.02t 0.000 is present only in the very rare manganocolumbite Bf 0.000 0.@0 0.01E 0.011 0.000 0.000 0.w2 of uncertain affiliation and negligible volume. The Pbl 0.000 0.010 0.000 0.000 0.000 Na* 0.000 0.000 0.000 0.179 0.000 0.000 0.0@ La5tovidky pegmatite is characterizedby high activity e t6.@o 16.0@ 16.0@ 6.rt3 12.00012.000 12.@0 of Fe, but very low activities of Ca and Mn, generating abundantferrotapiolite. Both the Chvalovice and Dobr6 t @. 16 16 16 6.1t3 12 12 12 Voda pegmatites exhibit a virtually complete replacement sequence,including early Ca(Sb)-rich phases such as 1-3- primarystibiotmlits; 4- secondarysuaiomicrolito; - stibiomicrolite microlite, 5-7 smndry ftrroqiolito. and and late Fe-enrichment cdion conteffi normalizedto 16 otygos (stibiotant4lib#1-3), characterizedby presenceof Fe-rich manganotantalite Ta+NFTi+W = 2 (stibiomicrolite+i4); 12 ox)€Eos @d 6 cdions (both localities), ferrotantalite (Chvalovice) and (ferrotapiolite#5-?). n

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9t lmc s€cmic Mn col

Sbtn Sbmic Sbt

t \ I \ 1\ i i .'tco -.t' - Mn ryn s€cmic Mntn Mtnt[ Fefn Sbtn Sbmic fttp

Frc. 5. Variations in the A-site cations Ca, Mn and Fe in primary and secondary M,Ta-oxide minerals. A1l data normalized to IrIb + Ta + Ti +W = 2 atoms. Vertical bars: ranges of individual cations. Symbols: NV Nov6 Ves, CH Chvalovice, DV Dobr6 Voda, LA LaStovidky, prmic: primary microlite, secmic: secondary microlite, Mncol: "secondary" manganocolumbite, Mntnl: secondary manganotantalite associated with secondary microlite, Mntntr: Fe-rich secondary manganotantalite associated with ferrotantalite, Sbtn: primary stibiotantalite, Sbmic: secondary stibiomicrolite, Mntn: secondary manganotantalite, Fetp: secondary ferrotapiolite, Fetn: secondary ferrotantalite.

DISCUSSIoN of rynersonite instead of the chemically similar and more abundant microlite may be influenced by low Primaryminerals activity of F in the parent medium. The general sequenceof enrichment of the A-site Compositional trends in columbite and other cations in primary minerals, Fe - Mn - Ca - (Sb), is primaryM,Ta-oxide minerals(microlite, rynersonite, quite common in the Nb,Ta-oxide minerals of stibiotantalite)exhibit geochemicalsignatures typically moderately to considerably fractionated rare-element observed in granitic pegmatitesof the lepidolite pegmatites (as summarized by Cernf & Ercit 1985, subtype:in columbite-typeminerals, the buildup in Mn 1989). Greater complexity is, however, routinely over Fe precedesthe enrichmentin Ta over Nb, and the encounteredin exfemely evolved pegmatites,in which Mn/(Mn + Fe) values are commonly close to 1; additional cations (such as Li, Na, K, Cs, Pb, Bi, Al, microliteand stibiotantalite represent tle mostevolved U and Sn) are encountered(Ercit 1986, Voloshin & Nb,Ta-oxide-minerals(cf Foord 1976,eernf & Ercit Pakhomovskyi 1988). 1985, 1989,Cern! 1989).Rynersonite from Chvalovice is very likely the first occurrenceofttris speciesas a Secondnry minerals primarymineral in graniticpegmatites. It is generally known as a secondaryphase replacing stibiohnhlite, Minerals crystallized during the secondary bismutotantalite,microlite or otherminerals (Foord & (hydrothermal) stage in the pegmatites examined, Mrose1978, von Knorring& Fadipe1981, dernf & including stibiomicrolite, microlite, cesstibtantite, Ercit 1985,1989, Baldwin 1989), or asa.late phase in manganotantalite, ferrotantalite and ferrotapiolite, are pegmatitepockets (Nov6k et al. 1994).The formation commonly describedfrom similar mineral assemblages

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Sequenceof A.site cations in the secondary minerals

The complete sequence of the A-site cations (Sb) - Ca - Mn - Fe, as recognized at Chvalovice and Dobr6 Voda, has only rarely been recorded in complex granitic pegmatites (Voloshin 1983). However, diverse segmentsof the sequence,such as Sb - Ca - Mn, Sb - Ca, Ca- Mn and Mn - Fe, are commonly observed at many localities (e.g.,iernf & Ercit 1985, 1989,Ercit 1986,Voloshin & Pakhomovskyi1988, Baldwin 19891. Increased activities of Mn and Fe relative to Ca are also recorded in late varieties of microlite (Wise & dernf 1990, Lumpkin & Ewing 1992). This indicates that the overall sequencedocumented here, (Sb) - Ca - Mn - Fe (Fig. 6), may representa universal pattern of hydrothermal replacement of primary M,Ta-oxide minerals such as microlite, stibiotantalite, simpsonite and bismutotantalite. The variable abundance of the individual A-site cations may result from the compositionsof residual fluids, and from the compositionof primary M,Ta-oxide minerals.The anay of the A-site cations is consequently subject to broader variations at individual Iocalities (e.g., Na Li, K, Cs, Ba, Al, Bi, Sn; Voloshin 1983,Ercit 1986). However, the sequenceof (Al) - K, Li - Cs, Na - Ca, derived by Voloshin & Pakhomovskyi (1988), cannot be accepted as a valid generalization. The have a Ftc. 6. Generalized trends in the evolution ofA-site cation composition of metamorphic fluids may also abundancesat the localities examined, complemented by significant role in the latest stagesof the hydrothermal published observations (see the text for references,,. crystallization (as discussed below), and meteoric A. The Sb - Ca * Mn - Fe sequence.B. The Ca - Mn - Fe waters may be involved under conditions of near-surface, sequencein Sb-free assemblages. lowtemperature weathering-inducedreplacement (e. 9., cesstibtantite and kimrobinsonite: Nickel & Robinson 198s). The suggestedsequence of the A-site cations in secondary replacement, (Sb) - Ca - Mn - Fe, is reversed from that characteristic of the primary crystal- consideredto be metasomatic,formed at theexpense of Iization at the localities examined. However, the primary phasesanalogous to those describedhere. mechanism producing such a reversed sequenceis not However,they alsoreplace simpsonite, stibiotantalite, understood at present. bismutotantalite.holtite and other less common minerals (e.g., (em! & Ercit 1985, Ercit 1986, Sources ofA-site cations in the secondary phases Voloshin& Pakhomovskyi1988, Baldwin 1989). The majorA-site cations in the M,Ta-oxide minerals Therarto NblTa in the secondaryminerals of the secondary stage at the localities studied include Sb, Ca, Mn and Fe (Fig. 5). The likely sourceof Sb and A universaldecrease in NblTa in not only primary Ca seemsto be the highly evolved fluids exsolved from but alsosecondary minerals was suggested by Voloshin the residual pegmatite-forming melt, or the replaced (1983).However, Cernf & Ercit (1985),Ercit (1986) primary mineral, or both; elevated Ca-contents were and Lumpkin & Ewing (1992) found this ratio in found in the residual melt in the experiments on the hydrothermallyformed late minerals rather stable, and Macusani glass (London et al. 1989).Infiltration of Ca comparableto that oftheir precursors.Our dataare in vra metamorphic fluids from the host rocks is another good agreementwith the resultsof the lafter groupof possible process, particularly in granitic pegmatites authors.Evidently, the mobility of Nb andTa is about hosted by Ca-rich rocks (London 1990). As Ca equal during the replacementprocesses at most participates in very eaily stages of the hydrothermal localities,and the two metalsare incorporated into the replacementof primary M,Ta-oxide minerals, possibly metasomaticproducts in propOrtionsmore or less Lreforemixing of residual and metamorphic fluids, and identicalto thoseestablished in the primaryphases. as in most casestle host rocks are not Ca-rich (Table

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l), this processdoes not seemto be probable.Release ferrotapiolite-tantalitegap (eem! et at. 1992).However, of Ca during hydrothermal alteration of blocky a total lack of experimental data does not allow realistic K-feldspar is not likely because of virtual absence of estimates of the P-T-X conditions of all stages of this processat most localities. Highly evolved fluids secondary hydrothermal processes,particularly at the also are the source of Mn, becausehigh activity of Mn low-temperature end. in pore fluids in the host rocks (Table l) can be ruled out. CoNcLusrous High activity of Fe in late stagesof the replacement process may have several possible explanations. Iron (1) A general sequence of A-site cations was may have the same residual source as the other A-site established for primary M,Ta-oxide minerals in four cations Sb, Ca and Mn; such an idea is supportedby complex pegmatitesin the Bohemian and Moravian late hydrothermal crystallization of Fe-rich tourmaline Moldanubicum, Fe - Mn - Ca - (Sb) in columbite, (foitite) from residual pegmatite-generated fluids microlite, rynersonite and stibiotantalite, with many uncontaminated by metamorphic fluids, suggestedfor analogs in similar granitic pegmatites worldwide. the Dobr6 Voda pegmatite by Nov6k & Taylor (in (2) A reversed sequence(Sb) - Ca - Mn - Fe prep.). The array of minor A-site cations such as Sb, Bi, characterizesthe products of subsolidusreplacement of Sr, Ba, Pb and U recorded at all localities, general lack the primary minerals of Ca and Sb, also with numerous of columbite alteration in outer pegmatite units, and analogies at other localities. It may represent a virtual absenceof Mg in most compositions, also point universal pattern of secondary replacement at the to residual fluids as the main source of all A-sire expenseof the above primary minerals in moderatelyto cations. On the other hand, some phenomena, such as considerably fractionated rare-element pegmatites. onion-skin textures, nonstoichiometriccompositions (3) The qualitative and quantitative representation of Si-enriched ferrotantalite with a high excess of Fe of the A-site cations is controlled by the relationships (Fig. 5), and the chemical composition of most of among the primary phases and by the composition of the secondary microlite (Table 4; cl also Lumpkin & subsolidus fluids. Further complications are encoun- Ewing 1992), recognized particularly at Chvalovice, tered in highly fractionated pegmatites,with secondary suggest low-temperature formation, possibly under minerals containingLi, Na, K, Cs" Pb, Al, U or Sn. conditions of near-surfaceweathering. Consequently,a (4) participation of mixed pegmatite and host-rock-derived The A-site cations in the secondary minerals pegmatite-derived fluids in the secondary process at the Chvalovice originate mainly from the residual parent locality is fairly probable, and the source of Fe may be fluids and ttre minerals. Early involvement of the host rock. country-rock-derived solutions is unlikely, but possible locally to explain the pattern of Fe in late stages. P-T-X conditions of secondary replacements (5) The ratio M/Ta decreasesduring primary crystallization, but the secondary minerals inherit the ratio The early stage of hydrothermal replacement of the of their respective precursors. primary M,Ta-oxide minerals at the localities studied (6) The P-T conditions of the secondaryreplacement is representedby the reaction of primary minerals with processesvary from early subsolidus at -500-350'C, highly evolved pegmatite-derived fluids soon after 2.5-2.0 kbar to near-surfaceweatherine. crystallization, i.e., atabout 500 to 350"C, 2.5-2.0kbar (typical of low-P, high-T petalite-bearing pegmatites; AcKNowrslcEMENTS London 1990). Such fluid has a relatively high pH, high activity of Ca, and low activity of Na, is com- This study was supported by the NSERC Research monly accompanied and followed by elevated activities Grants and Major Installation Grant to P.C., and by of Mn and Fe. Participation of metamorphic fluids NSERC Equipment and Infrastructure Grants to releasedfrom the host rock is unlikely at this stage.The F.C. Hawthorne. A Faculty of Science, University of formation of Fe-rich secondary phases (Fe-rich Manitoba Postdoctoral Stipend supported M.N. Our manganotantalite, ferrotantalite and ferrotapiolite), thanks to to F. Veselovskf, P. Korbel and N. Ball for latest to crystallize in most of the mineral assemblages help in X-ray-diffraction work, and to Ron Chapman studied, seems to be more complicated. Veins of for assistanceduring the electron microprobe study. ferrotapiolite in stibiotantalite from La5tovidky formed A.U. Falster and W.B. Simmons, Jr. provided thorough at relatively early stages.However, onion-skin textures reviews which substantially improved the presentation. and chemical composition of microlite from Chvalovice and in part also from Nov6 Ves suggest a REFERENCES low temperature of

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