6s9T he Canadian M ine raLo g is tVol. 36, pp. 6s9-672(1998)

NIOBIUM -TANTALUM OXIDE MINERALS FROM COMPLEX GRANITIC PEGMATITESIN THE MOLDANUBICUM, CZECH REPUBLIC:

PRIMARV VERSUSSECONDARY COMPOSITIONAL TRENDS

MILANNOVAKI

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

PETR EERNfI

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 theMoldanubicum, Czech Republic, covering primary (magmatic) and secondary (hydrothermal replacement) stages ofcrystallization, were investigated. The primary stage is characterized by ferrocolumbite to manganocolumbite, followed afterFe- and Mn-depletion by Ca- or Sb-rich minerals: microlite (Nov6 Ves), rynersonite (Chvalovice) and stibiotantalite (Dobr6Voda, 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 cationsequence Fe - Mn - (Sb,Ca) are rypical of the primary stage. The secondary stage displays a broad spectrum of mainly fracture-filling secondary phases, such as diverse microlite-group minerals, cesstibtantite, manganotantalite, ferrotantalite andferrotapiolite, which replace stibiotantalite, microlite and rynersonite; columbite in outer pegmatite units remains unaffected.The Tal(Ta + Nb) values of the secondary phases are comparable to those of their precursors. The general sequence of majorA-site cations in hydrothermal Nb,Ta-oxide minerals, (Sb) - Ca - Mn - Fe, is reversed relative to that of the primary stage, andit may represent a universal pattern ofhydrothermal replacement ofprimary Nb,Ta-oxide minerals in comparable graniticpegmatites. The composition of microlite-type minerals and textural relations indicate that the secondary hydrothermal stageincludesabroadrangeofP-T-Xcondi t ionsfromear lysubsol idusreplacementat-500-350oC, -2.5-2.Okbar(Nov6Ves,Dobr6 Voda) to near-surface weathering at <100oC (Chvalovice). The likely source of the Sb, Ca, Mn and part of the Fe seemsto 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 granitiquecomplexe (sous-types d 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 Imanganocolumbite, qui a par la suite donn6 lieu l des min6raux appauvris en Fe et Mn, et riches en Ca et Sb: microlite (NovdVes), rynersonite (Chvalovice) et stibiotantalite @obrd Voda, LaStovidky). Le trac6 des compositions de columbite ressembled celui des pegmatites d l6pidolite; les aures oxydes de M et Ta du stade primaire ont une valeur Ta./(Ta + Nb) plus 6lev6eque dans la columbite. Au cours du stade primaire, une diminution en Nb/Ia et la s6quence d'enrichissement Fe - Mn - (Sb,Ca)sont typiques. Au stade secondaire, nous documentons le remplissage de fissures par plusieurs min6raux secondaires, parexemple les min6raux du groupe du microlite, cesstibr,ntite, manganotantalite, ferrotantalite et ferrotapiolite, en remplacementde stibiotantalite, microlite et rynersonite; la columbite des zones externes de la pegmatite resle inchang6e. Le rapponTal(Ta + Nb) des phases secondaites est comparable tr celui des pr6curseurs. En g6n6ral, la s6quence des cations occupant laposition A dans les min6raux d Nb et Ta d'origine hydrothermale, (Sb) - Ca - Mn - Fe, est renvers6e par rapport aux min6rauxprimaires, et pourrait t6moigner d'un comportement g6n6ral suite au r6-dquilibrage hydrothermal dans des situationscomparables rencontr6es dans les pegmatites $anitiques. D'aprbs la composition des min6raux du groupe du microlite et lesrelations texturales, le stade hydrothermal repr6sente un large intervalle de conditions P-T-X, en partant du remplacement

I E-mnil addresses: mineral@mzm.anet.cz, cemyp@ms.umanitoba.ca

660 THE CANADIAN MINERALOGIST

subsol idusprdcocea-500-350'C, -2.5-2.0kbar(NovdVes,Dobr6Voda), jusqu'd lam6t6or isat ionprdsdelasurfacedune

temp6rature inf6rieure ou 6gale A 100'C (Chvalovice). ks 6l6ments Sb, Ca, Mn et une partie du Fe seraient ajout6s vla la phasefluide exsolv6e du magma r6siduel ou viendraient du min6ral pr6curseur. A Chvalovice, la roche-h6te a peut-6tre 6t6 A I'originedu Fe.

(Traduit par la R6daction)

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

Irrnopucuolt

Compositional trends of primary Nb,Ta-oxideminerals in complex pegmatites are a useful tool forproviding information about the degree of meltfractionation. Distinct compositional trends in terms ofthe Nb-Ta and Fe-Mn fractionation are recognized incolumbite-type minerals from individual subtypes ofgranitic pegmatites (e.g., Cen! & Ercit 1985, 1989,Spilde & Shearer 7992,Ercit 1994, Nov6k & Povondra1995" Mulja et al. 1996). They result from differencesin the composition of parent melts, and particularly inthe activity of F, P and their complexes (Keppler 1993,Wolf & London 1993, Wolf et aI. 1994, Linnen &Keppler 1996). Typical cations in the primary Nb,Ta-oxide minerals include Fe and Mn; however, locallyNa, Ca, Sb, Bi, Y Sn and Al may also play a significantrole, particularly in diverse minerals of the microlitesubgroup, simpsonite and in the stibiotantalite group,all of which are typical of highly fractionated graniticpegmatites (Ercit 1986, Voloshin & Pakhomovskyi1988) .

Secondary (hydrothermal) Nb,Ta-oxide mineralsrepresent a broad spectrum of mineral species,commonly carrying exotic cations: Li, Na, K, Cs, Fe,Mn, Sr, Ba, Pb, Al, Sb, Bi, Sn and U, in addition toabundant and widespread Ca. However, compositionaltrends evolving during the hydrothermal stage are notunderstood at a level comparable with those of theprimary stage. Diverse views were expressed aboutthe behavior of the ratio Nb/Ta (Cf Voloshin 1983, Ercit1986), and attempts at establishing a general sequenceof cations are rather controversial (Voloshin &Pakhomovskyi 1988).

The complex pegmatites in the Bohemian andMoravian part of the Moldanubicum typically containNb,Ta-oxide minerals, predominantly manganocolum-bite. Other primary minerals include ferrocolumbite,manganotantalite, "wolframoixiolite", ixiolite, wodginite,pyrochlore, microlite, stibiotantal ite and rynersonite,but most of them are rare (Nov6k et al. 1992, dernf &Ndmec 1995, Nov6k & Divi5 1996, unpubl. data of theauthors). The last three minerals were hydrothermallyaltered at various stages ofthe subsolidus reconstitutionof the pegmatites, producing a variety of secondaryminerals such as microlite, stibiomicrolite, cesstibtantite,manganotantalite, ferrotantalite and ferrotapiolite(StanEk 1963,1973,Ercit et al. 1993, Nov6k & Srein1998). However, the available data, scattered in the

articles quoted above, are largely restricted to a mereidentification. We present here the results of a systematicstudy of the secondary minerals, remarkable in theuniformity of their evolution in four widely separatedpegmatites; in some respects, we significantly departfrom the traditional views of the alteration processes.

PEcMATITE Boums ExaulxBo

Complex pegmatites in the Moldanubicum occur inseveral spatially isolated regions in southern Bohemiaand western Moravia (Fig. 1). Pegmatites of thelepidolite subtype predominate over those of the elbaitesubtype; several pegmatite dikes of the lepidolitesubtype carry variable amounts of petalite and can bedesignated as transitional to the petalite subtype.

Four bodies of pegmatite are examined here: Nov6Ves and Chvalovice (eeskf Krunilov region, southernBohemia), and Dobr6 Voda and La5tovidky (VelkdMeziYidi region, western Moravia) (Fie. l). The Nov6Ves pegmatite belongs to the transitional lepidolite-petalite category, if not to the petalite subtype proper(Standk 1992).The other three pegmatites are of thelepidolite subtype, locally with abundant elbaite andamblygonite (Nov6k et al. 1992).

All four pegmatites typically form symmetricallyzoned dikes (Fig. 2), consisting of the followingtextural and paragenetic units, from the contactsinward: (1) fine-grained granitic unit, (2) graphic unit,(3) coarse-grained albite - qtJafiz - tourmaline - micaunit, (4) blocky K-feldspar, and (5) lepidolite unit in thecenter. Amblygonite - montebrasite is commonly foundin the lepidolite unit (5) or locally in the coarse-grainedalbite - quzrtz - tourmaline - mica unit (3). A blockypetalite unit (6) is developed in the center of the Nov6Ves pegmatite (Fig.2), rare petalite or pseudomorphsof spodumene + quaftz after petalite are located in theinternal zones of the Chvalovice and Dobrd Voda dikes(StanEk 1965, 1992). Neither petalite nor amblygonite- montebrasite were found at La3tovidky (Stan6kI 973). Additional information concernin g thickness,host rock and mineral assemblages of the individualpegmatite dikes is given in Table 1.

The pegmatites examined are very similar in theirinternal structure (Fig. 2) and mineral paragenesis.Petalite is more abundant in localities from southernBohemia, pollucite was found only at Nov6 Ves(Teertstra et aI. 1995). The Dobr6 Voda and La5tovidkypegmatites contain small vugs lined with crystals of

M-Ta OXIDE MINERAI-S. MOLDANUBICUM PEGMATITES 661

r 3 0 k m '

t

' 'Rl,(_ CHUALOVICEF F

\ NourvEsW

I rf\_

,9.c.zuDEJOvrCE

6ermony

+ - + + +

{:i:ffi+ + + ++ + +

+ + + +Y + + + + + ++ + + + l + +

LASTOVICKY

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

O l 2 l mI r t l

t

DOBRA VODAsw

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

662 THE CANADIAN MINERALOGIST

TABLB 1. MINERAL ASSH\,IBIAGES, EOST N@K AND TgIGNBSS

U$€adrg mlffil.

lF[,ldollh

petallEspoduEe*elbdto

Nb,Taddenincisprbnot

ftq@bEffienflg4natunHF

niqolie

rytrmits

sdblobrralfre

wndol

EisotiF

casdbbDdle

siblodmlleffigmahrnffi

Eegarot@talitB

ftmtanta[to

Synbols: I mjor, r 6@q O Eiu, + EE,. vqy EB, ? prcbldic, - sbs€dn,'s@Ddry. Thic,trsinffi

quartz, albite, elbaite and lepidolite. The smaller dikesat Chvalovice and La5tovidky are very simitar to theirlarger counterparts at Nov6 Ves and Dobr6 Voda,respectively, but their internal structure and mineralparagenesis are relatively simple.

ExpERIMENTAT MetHoos

Electron-microprobe analyses were performedin the wavelength-dispersion mode on a CamecaCamebax SX-50 instrument at the Department ofGeological Sciences, University of Manitoba, Winnipeg.A beam diameter of 1-2 pm, accelerating potential of15 kV, a sample current of 20 mA and a counting timeof 20 s were used for Na, Ca, Nb, Ta and F" and 40 mAand 50 s for K, Cs, Sr, Ba, Fe, Mn, Pb, Y, Sb, Bi, Sn,Ti, U, and W. The following standards were used:manganotantalrte ('IaMo'), MnNbrOu (MnKq, NbLc[),FeNbrOu @eKo), microlite (NaKcl, FKcr,), CaNbrOu(CaKa), SnO, (Snl,c), rutile (TiKcr), BiTaOo (Bllt4d.),m i meti te (Pb M a), sribi orantalite (Sbtcr), NaS cS irOu(ScKct), UO, (UMo), YAG (Ytcr), Ba2NaNbrO,, @aLa),SrBaNboO,o (SrZc), pollucite (Cslcr), orthoclase(KKct) and metallic W (WMc[). Data were reducedusing the PAProutine (Pouchou & Pichoir 1985).

Identification of some of the minerals examined wasverified by X-ray powder diffraction, using standardprocedures on the Philips PW1710 and Siemens D5000instruments.

TextunaL aNo PeRacnuertc RELATIoNS oF TIIEM,Ta-BnanrNG PHASES

Primary minerals

Columbite typically forms black bladelike crystalsup to 5 mm in size enclosed in the coarse-grained albile- quartz - tourmaline - mica unit (3). It is commonlyassociated with zoned tourmaline (black schorl to greenor pink elbaite), colorless to purple Iithian muscovite tolepidolite, and locally with amblygonite. Columbite isexceptionally found in blocky K-feldspar (Nov6 Ves),in blocky petalite completely converted to quartz +spodumene (Dobr6 Voda), and in the medium-grainedlepidolite unit (5) (Nov6 Ves, Dobr6 Voda) (Standk1963, 1973, 1992, Cech & Standk 1960). Columbite inthe outer units of the pegmatites remains unaffected byany secondary replacement processes.

Beside columbite, stibiotantalite is found at Dobr6Voda and La5tovidky as a primary phase; primarymicrolite occurs at Nov6 Ves, and rynersonite atChvalovice (Table 1). The primary rynersonite deservesspecial comment, as it is generally known as a secondarymineral. At Chvalovice, however, it occurs in thecolumbite-free blocky pegmatite unit (4)" the medium-to coarse-grained lepidolite unit (5), and inside thecolumbite-bearing coarse-grained unit (3). It formssubhedral inclusions in amblygonite, feldspars andquartz, and in most cases is extensively replaced bysecondary minerals. Extensive depletion of Fe and Mncould have led to the stabilization of an anhydrousCa,Ta,M-oxide mineral as a primary phase.

Secondary assemblages

Late assemblages of secondary minerals formfracture-filling, generally very fine-grained and locallyporous aggregates within the replaced primary Nb,Ta-oxide minerals. As some differences were foundamong individual localities, the conditions of thesecondary phases are described separately for eachpegmatite.

At Novti Ves, primary euhedral to subhedralmicrolite in a medium-grained "cleavelandite" -lepidolite subunit of the lepidolite unit (5) (dech &Stan6k 1960) exhibits two distinct manifestationsof hydrothermal alteration. The first one, morewidespread, is represented by secondary microlitepenetrating along fractures of the primary phase; suchalteration was described in detail from complexpegmatites by Lumpkin & Ewing (1992). The secondstyle is extremely rire: manganocolumbite seems tocement fractured primary microlite. However, thetextural.relationships are complicated in these cases,and alternative interpretations may be possible.

At Chvalovice, primary rynersonite commonly isenclosed in amblygonite. It is almost completelyreplaced by a mixture of secondary minerals: microlite,

l a t al + 1 -o . + -a o a ao o a -

+ -. ?

? -

Nb-Ta OXIDE MINERALS. MOLDANIJBICTIM PEGMATTTES 663

Frc. 3. Back-scattered elechon image of secondaryM,Ta-oxide minerals. A. Onion-skin texture of microlire(bright) and manganotantalite (dark) from Chvalovice.The scale bar is 100 trrm long. B. Veins of secondarymicrolite (gray) and manganotantalite (pale gray) in thecenter of veins penetrating primary stibiotantalitefrom Dobr6 Voda. The bottom edge of the photograph isI 100 pm long. C. Fine-grained intergrowths of microlite(gray) and manganotanrrlite (bright) in rhe center of veinsshown in B. Dark poorly polished areas correspond toFe-rich manganotantalite to ferrotantalite. The scale bar is80 pm long.

manganotantalite and ferrotantalite. They locally formfracrure-filling aggregates; microlite seems to be theearliest to crystallize, followed by Fe-poor mangano-tantalite. Onion-skin replacement textures consisting ofalmost alternating monomineralic zones of microliteand manganotantalite also were found (Fig. 3a). Basedon EDs-study, poorly polished dark areas in Figure3a consist largely of manganotantalite . The X-raypowder-diffraction study yielded peaks only ofmicrolite, but none of manganotantalite (F. Veselovskf,pers. commun . 1995). A very fine-grained mixture ofmanganotantalite and ferrotantalite with a randomdistribution of both minerals also was identified, inwhich both rynersonite and secondary microlite werecompletely replaced.

At Dobrd Voda, stibiotantalite forms irregular grainsor euhedral dipyramidal crystals up to 3 cm in size,enclosed in medium- to coarse-grained albite - "rubellite"subunit of the lepidolite unit (5) (StanEk 1963). It isreplaced along fractures by an assemblage of secondaryhydrothermal minerals (Fig. 3b). The sequencestibiomicrolite - microlite - manganotantalite is mucheasier to recognize here than in the Chvalovicepegmatite. However, the last two minerals locally formextremely fine-grained mixtures, situated mainly in thecenter of veinlets (Fig. 3c). Poorly polished dark areas(Fig. 3c) correspond to Fe-enriched manganotantalite,or perhaps ferrotapiolite. Its presence as a secondaryphase is very likely, because a tapiolite-like mineralwas detected by X-ray powder diffraction in severalsamples. Very rare niobian cesstibtantite also was foundin the assemblage of secondary minerals (Ercit et al.1993, Nov6k & Srein 1998).

At InStovidl{y, small fragments of compositionallyheterogeneous crystals of stibiotantalite, up to 8 mmin size, occur mainly as loose grains in pocket clay,and only exceptionally are enclosed in lepidolite,"rubellite" or quartz. Stibiotantalite is replacedalong fractures by very fine-grained ferrotapiolite.Stibiomicrolite and cesstibtantite also were recognizedas extremely rare and very small grains in fenotapiolite;both minerals seem to have formed before ferrotapiolite.

Crm,vrcar CovrposmoN

Primary minerals

Ferro- to manganocolumbite from the outer pegmatiteunits have a very similar composition at all localitiesexamined (Table 2). The composition is characterizedby variable Fe contents, but relatively stable and highM/Ta values (Fig. ), decreasing in the late stages ofprimary crystallization. The relative Mn enrichment islocally extreme, with Mn/(Mn + Fe) very close to 1except for the LaltoviEky pegmatite (TabIe 2, Fig. 4 yTlpical minor cations include particularly W [up to2.88 wt.Vo WO3, 0.09 atoms per formula unit (apfu)land Ti (up to 0.65 wt.Vo TiO2,0.06 Ti apfu); they are

664

wo!Nbro,Tqotlio,s!qUO,ScrO3LrpCaOIvItrO

TABLE 2. REPRESENTATTVE COMPOSITIONSOF PRIMARY MANGAI.IOCOLIJMBITE

3 4 5 6 7 82.25 0.48 0.@ 1.44

65.70 59.10 37.tO 63.mll. l0 m.l0 44.& t4.200.51 0.04 0.@ 0.43o.tz 0.09 0.88 0.070.@ 0.00 0.00 0.000.02 0.92 0.05 0.020.@ 0.@ 0.@ 0.000.01 0.01 0.03 0.0011.90 18.@ 16.30 9.157.79 l.16 0.00 t0.20

Total 9.12 9.26 98L6 937 9.4 99.@ 99.16 9E.71 98.15

cmicoremwe 0.028 0.030 0.u4 0.033 0.069 0.015 0.000 0.045 0.037Nbe 3.655 3.655 3.33t 2.874 3.52,6 3.298 2.3t3 3.455 3.342Ta$ 0.30E 0.295 0.594 1.059 0.358 0.67i 1.680 0.4d? 0.598

0.012 0.015 0.02s 0.0@ 0.046 0.@4 0.@ 0.039 0.01ssne 0.000 o.@3 0.0@ 0.o2 0.005 0.004 0.048 0.003 0.0@u* 0.000 0.002 0.000 0.0m 0.000 0.000 0.000 0.0@ 0.@s.f 0.000 0.000 0.w) o.m6 0.002 0.@2 0.006 o.trD 0.092l41 0.@ 0.000 0.@0 o.o0o o.c{n o.@0 0.000 0.000 0.@2c€F 0.@3 0.@3 0.@l 0.@4 0.@l 0.@t 0.@4 o.@ 0.001Mf t.m t.754 t.M 2.031 1.197 1.8E2 t.w 0.937 1.235Ff 0.200 0.2,10 0.516 0.017 0.7R 0.120 0.@0 1.031 0.766t cd. 5.9t5 5.96 5.998 6.02J 5.981 6.@1 5.958 5.9E3 6.000

It - Novd Vos, FnTIe Vc6; 3,4 - Cteslqeirs, sql€ MMCI6; 5-7 - DoH Vod45 - r@ple MMC39, 6 - Banple CDVs, 7 - r@ple MMglT; 89 - Idoviaky,8qleVC3. Cdi@qdrmDq@liz€dto 12 cl€@; 0.m-belrydd.di@liEilAAcYSb @d Bi beld ddeci@ l#t Cqositi@ rcpq&d i! st% 6id6

most abundant in the Dobr6 Voda columbite and vervlow in its Nov6 Ves counterpart.

Primary microlite from Nov6 Ves is homogeneous,with Na = Ca; negligible vacancies in the A site andhigh F contents are characteristic (Table 3). The ratioTa/(Ta + Nb) in two grains varies from 0.73 to0.90 (Fig. 4), but each individual grain is almosthomogeneous.

Rynersonite from Chvalovice is homogeneous, withTa/(Ta + Nb) in the range 0.68 to 0.71; it displays arelatively simple chemical composition, with W Pb,Sn, Fe and particularly Mn the typical minor to traceelements detected. Low totals of oxides (about97 wt.Vo)and a small excess of R?+ cations were observed (Table4), but cannot be readily explained.

Stibiotantalite from Dobr6 Voda is fairly homoge-neous, with Ta./(Ia + M) between 0.7 | to 0.7 6 (Fig. 4).Except moderate M-contents and minor to traceamounts of W Sn and Bi, it is close to its end-membercomposition; moderate Y contents (up to l.l3 wt.VoY2O3; Table 5) are remarkable. The composition ofheterogeneous stibiotantalite in the Laltovidky pegmatiteis similar to that from the above locality, except for theTal(Ta + Nb) values, which range from 0.68 to 0.95(Fig. 4, Table 6). Relatively rare Nb-enriched areas arerandomly distributed within the stibiotantalite grains.

Primary microlite, rynersonite and stibiotantalite aremore evolved relative to the primary columbite. TheirTal(Ta + M) values vary in the broad range from 0.68

to 0.95 (Fig. a). Except the stibiotantalite fromLa5tovidky, they are fairly uniform on a single-grainscale, but locally variable among different grains(Fie.4).

Although not completely developed at all fourlocalities, a general sequence of the A-site cations inprimary M,Ta-oxide minerals can be defined asFe-Mn- (Ca,Sb) .

Secondary minerals

Chemical compositions of secondary minerals arehighly variable and complicated compared to theprimary minerals, and considerable differences werefound among individual localities (Fig. 5). Secondaryminerals locally form extremely hne-grained masses;consequently, some microprobe data may representmixtures of two phases, either microlite with manga-notantalite or manganotantalite with ferrotantalite(ferrotapiolite) in a variable ratio.

AtNovd Ves, secondary microlite was formed duringearly and late hydrothermal alteration, as described byLumpkin & Ewing (1992); Na- and F-depletion istypical of early alteration; however, significant Ca-, Na-and F-depletions combined with a considerableincrease in U contents (up to 7. 1 1 wt.Vo UO2, 0. 14 Uapfu) were found in late secondary microlite (Fig. 5,Table 3). In both types of alteration, the MlTa valuesremain virtually constant. Late manganocolumbite isFe-free, locally with minor amount of Ca and R but itexhibits a significantly depressed Ta.i(Ta + M), similarto that of the primary manganocolumbite (Fig. 4). Theoverlap with associated microlite may explainincreased Ca and F contents in some compositions.This manganocolumbite may be the only secondarymineral examined to display a change in M/Ta relativeto its primary precursor.

At Chvalovice, highly variable secondary mineralsinvolve mainly Ca-, Na-, F-poor microlite with a highproportion of vacancies in the A site; H2O may be adominantA-site constituent (Ercit et al. 1994). A widespectrum of minor to trace elements W, U, Sn, Bi, Ba,and particularly Pb and K (Table 4) is typical. Pb and Kmay locally represent dominant A-site cations(plumbomicrolite and "kalimicrolite"). Secondarymanganotantalite associated with microlite exhibitsvariable but mostly low Fe- and Ca-contents. Highlyheterogeneous ferrotantalite is associated with Fe-richmanganotantalite (Figs. 4, 5, Table 4). Minor to traceamounts of W, Si, Sn, Bi, Ba, Pb, and Ca are typical inboth minerals; except Sn and W, the increase in thesecations is concomitant with increasing FeAvIn.Ferrotantalite displays low oxide totals, high excess oftrP* (plus Rk) cations, far above the limits of the ABrOustoichiometry, and appreciable Si (Table 4). Most of theFe is probably in a trivalent form; however, admixtureofFe-rich hydroxide minerals and quartz in the veryfine-grained porous mass is possible.

0.93 0.9 L3E O.968.60 6E.70 59.70 49.309.60 9.b. t7.70 30.200.14 0.17 0.n 0.000.@ 0.06 0.@ 0.040.@ 0.06 0.m 0.@0.@ 0.@ 0.@ 0.050.@ 0.00 0.@ 0.00o.u2 0.02 0.01 0.0317.E0 17.@ 14.20 18.60

l . l 559.E017.t00 . t60.000.000.t20.010.01

I Lto7.410.16

oz+g

.o

oz+.0F

F

Nb-Ta OXIDE MINERAI.S, MOLDANUBICUM PEGMATTTES

t

NOVA VES

a

. . . ' o r t fo l

OOBRA VODA

x xx

T

aa

t t a

. t P

At Dobrd Voda, secondary minerals are highlyvariable in their chemical composition (Fig. 5, Table 5).Microlite-type minerals vary from stibiomicrolitethrough Sb- and Sr-enriched microlite varieties tomicrolite proper. Most of them are Na-, Ca-, andF-poor, with significant vacancies in the A site (Table5, Fig. 6); H2O may be a dominant constituent in theA site (Ercit et al. 1994). The minor to trace elementsinclude Sn, Bi, Mn, Fe, Sr, Ba, Na and K.

665

LASTOVICKY

Manganotantalite is dominantly Fe-enriched, with lowoxide totals, but stoichiometry close to the A8206formula, and low contents of minor elements (Table 5).

At la{tovidlq, ferrotapiolite is the significantlydominant secondary mineral; cesstibtantite (notanalyzed quantitatively because of its small grain-size)and stibiomicrolite (only one spot analyzed, Table 6)are very rare. Two distinct varieties of ferrotapiolitewere recognized, Sb-free fenotapiolite with moderate

rc

I

Mn/ (Mn + h I M n / ( M n + F ! l

FIc.4. Compositions ofprimary and secondary Nb,Ta-oxide minerals in the columbite quadrilateral. Horizontal dashes alongthe 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(primary rnicrolite at Nov6 Ves, primary rynersonite at Chvalovice), -l secondary microlite (stibiomicrolite at La5tovidky).

-o

CHVALOVICE

8 \o

+o o o

666 TTIE CANADIAN MINERALOGIST

rgor 0.16 0.00Nbro, 14.10 14.90T%Ot il.q $.60Tlo, 0.00 0.@StrO, 0.05. 0.l l 'uo, l. l9 3.05sbro. 0.36 0.16Bto, 0.09 0.@CeO 10.60 9.79lvttrO 0.00 0.00FeO 0.00 0.@SrO 0.@ 0.@B8O 0.00 0.@P'bo 0.10 0.14Nqo 5.09 2.nKrO 0.00 0.ooF 3.01 2.57O=F - l -n -1 .0 t

0.28 0.2t 0.0449.63 5.30 4.9330.45 74.n 75.40.m 0.00 0.@0.0r 0.101 0.n0.@ 0.00 0.000.@ 0.16 0.140.@ 0.@ 0.070.05 10.30 10.2015.35 0.01 0.010.00 0.@ 0.010.00 0.@ 0.@0.00 0.@ 0.@0.00 0.0t 0.130.00 6.04 6.080.06 0.01 0.010.m 3 .18 3 . t10-00 -t 34 -l 33

0.53 0.065.O2 s.Vzn.n 76.700.00 0.@0.08. 0.2t4.42 7.l l0.@ 0.000.00 0.0E1.35 0.7t0.09 0.730.36 0.000.@ 0.47025 0.2100.55 0.zA0.ll 0.000.10 0 . r5o . l t 0 .@4 05 0.00

TABIE 3. REPRESIENTATIVE COMPOSITIONS OFMIROLTTEMANGAI{OCOILnAAFB AlrlD ITRANMIROLIIE

FROMNOVAVESThe Nb,Ta-oxide minerals from all localities

examined exhibit a similar sequence of A-site cations,(Sb) - Ca - Mn - Fe, but with significant quantitativevariations conholled by the precursors. The secondaryphases at the localities with Ca-rich primary minerals(microlite, rynersonite) are characteized by gradualdecrease in Ca and increase in Mn, followed atChvalovice by significant late enrichment in Fe (Fig. 5).The two localities with stibiotantalite exhibit somewhatdifferent evolution. Primary stibiotantalite is initiallyreplaced by stibiomicrolite or Sb-rich microlite, but thesubsequent stages of replacement are very similar to

TABLE4. REPRESENTATTVE COMFOSITIONSOFRYNERIIO}qTq MICROLIT4 MAI{GANOTAT.{TALITE

AND FERROTAI{TAIXTE FROM CIryAIOVICB9t.38 95.16 95.95 98.95 98.9t 90.A2

aloaicffi91.t9

w6

Nb$

Ta$fi&

Sno

[rsb4Bi3co}I4q&FfsflBEPb&Na'K"For

0.003 0.@ 0.0100.530 0.561 2.9t3t.67 t.439 t. lol0.0m 0.000 0.0020.(o2+ 0.@4+ 0.0040.t2. 0.056 0.0000.012 0.005 0.@00.@2 0.@ 0.@0.944 0.873 0.@70.@ 0.@0 r.7280.@ 0.0@ 0.0000.0@ 0.@ 0.0000.@ 0.@ 0.0@0.@2 0.@3 0.0000.820 0.471 0.0000.@ 0.@ 0.0100.791 0.676 0.@06.031 5 903 12 000

0.@5 0.@l0.210 0.1961.785 1.t030.m 0.@00.004+ 0.005+0.000 0.0000.006 0.@50.000 0.@20.96i1 0.9610.@l 0.@10.@0 0.@l0.000 0.0@0.@0 0.000o.w2 0.003t.t26 l.m70.@l 0.001o.tEl 0.tt26c62 6Ml

0.012 o.ml0.t92 0.1t5t.7 l.t l30.000 0.@0.@3+ 0.@t+0.083 0.1380.@l 0.@0.000 0.@20.14 o.f730.006 0.0540.0126 0.@0.000 o.ou0.@t 0.0140.013 0.0110.018 0.0000.0u 0.0170.t29 0.0005 ?s6 5479

wor 2.t0Nb2o' 17.@TarO5 9.9011O, 0.00SnO" 0.48SiO, trduo, 0.00sblq 0.03Birq o.ooCsO 12.30Ivtro 0.35Fgo 0.20slo 0.00B@ 0.00Pbo 0.41NqO 0.@KrO 0.ooCaO 0.mF 0.@

0.45 0.5412.tO 10.4054.60 58.000.(p o.021.11r l . l0 *nd- rd-2.9 2.570.00 0.000.@ 0.103.20 1.951.90 0.95l .6 t 1 .35o.27 0.242.16 t.1't0.01 0.040.00 0.@o.72 t.220.m o.tE0.09 0.16

0.53 1.23 0.s88.43 11.00 t6.14

56.70 4.@ s9.420.00 0.00 0.101.30* t.t1 0.85nd nd. O.791.90 0.@ 0.000.00 0.06 0.120.41 0.@ 0.331.56 0.67 0.470.66 I 1.40 10.87o.v3 0.r2 3.0E0.10 0.00 0.u1.08 0.08 0.004.54 2.98 0.370.@ 0.00 0.@0.30 0.00 0.010.16 0.10 0.000.@ 0.00 0.00

f,, ca.I n

3.t04 3.4t46.42. 6.579

5.850 4.@7 4.015 2.292 2.34l2cxm 6 7 6945 5386 5479

inferred Fe3+ content (up to 0.25 apfu), and a rare S- 3.ffi 3.$lSb-enriched ferrotapiolite (up to 9.76 wt.Va 5b2O3,0.67 Bis o.oo0 o.oooapfu;rabre 6, Fig. s). ffi. lil l:i3f

Fsr o.oE 0.132sP o.ooo o.oljBsf o.@ o.os3Pbe o.ol7 0.@Na* 0.0@ 0.000K' 0.0@ 0.090Cs- 0.000 0.@0F 0.019 0.028e n.9Er 5.903

l, 4 @d 5 - prindy Ei@lit€;2,6 @d ? - s@dey Ei@ho tolll@imlie; 3 -'wdr/ nfrg''ql@titECdio ot* nomalized to Ts+NbrTi+Wd2 (Eirclilo #1, 2, 45, 6, 7) @d 12 q!€@ ('llfig''l'@l'fihito #3). 0.00 - b€los d6€di@ liEir8;'SD I StrO, +Sq,. Cs belos det6{i@ liEit Co@o8ni@ rcpoftd in wt% qida

General observations

Almost all secondary phases from the localitiesexamined are characterizedby a fairly stable Nb/Tavalue during all stages of hydrothermal alteration(Fig. a). The only exception is very rare "secondary"manganocolumbite associated with primary microlitefrom Nov6 Ves (Fig. 3). However, textural relationsof both minerals are complicated, and this mangano-columbite may in fact be a primary mineral replaced bymicrolite. Such interpretation also is supported by thevery similar composition of primary manganocolumbiteat this locality.

Torat n.19 E0.E2 80.46 78.60 94.2t $.46mc6@E

we 0.082 0.011 0.014 o.ol4 0.054 0.t2ANbs 1.165 0.535 0.456 0,394 0.349 t.tilTsl 2.675 1.4s3 1.530 l.ss2 3.m 2.6tsIls 0.0@ 0.000 0.000 0.000 0.0@ o.ol2

0.0550.0@0.003o.2c30.0780 . 1 1 00.01 I0.0670.0010.0000. t510.070.049

0. I28o.04 0.000 0.@20.@0 0.004 0.0080.@l 0.@ 0.0140.t73 0.121 0.0810.058 l.g1 0.4t70.080 0.131 0.00-/0.006 0.000 1.490o.u4 0.005 0.0mo.t26 0.t37 0.0160.000 0.000 0.0@0.040 0.000 0.@20.@7 0.@7 0.@l0.0m 0.m0 0.0@

5.741 5.741 12.000 12.0@

L cd. 6.V17 2.919 2.7UI m 12 5 951 s74a

2.ffi5 6 4

6.04l 2

6.052t 2

I - pdnry rynqBoniq 2a secodry nimliq 5 - sscmdEynngnoffilife; 6 s€coillgy hngnoffitite.Cdim cm noma.lizd to 12 o46w (rynm@ns #l), edT8+Nb}T!+W=2(Eimlits #24). 0.@ = b€low de(sctid limi! +Sn I Sm, +sd'Compositim rcpored iu wtolo oxids.

Nb-Ta OXIDE MINERAI-S. MOLDANIJBICUM PEGMATITES 667

TABLE 5. REPRESENTATTVE COMP]OSMONS OF STIBIOTANTALTTqSTIBIOMCROLITE MICROUTE AND MANGA}IOTAI{TNITE

FROMDOBRAVODA

Poor polish in some samples, extremely fine grain-size and high porosity ofsome secondary aggregates,possible presence of very fine-grained admixture ofFe-hydroxides and quartz are typical features of thelatest sr4ge of the hydrothermal alteration. They renderthe study quite difncult, and may obscure some aspectsof the mineral assemblages examined. However, thegenerally stable M/Ta values and the sequence ofA-site cations, (Sb) - Ca - Mn - Fe, characterize theproducts of hydrothermal alteration of primaryM,Ta-oxide minerals at the localities examined beyondany doubt (Fie. 5).

TABLE 6. REPRESENTATIVE COMPOSMONS OFSTIBIOTAI{TAI.ITE STIBIOMICROI.,ITE AI\D

FERROTAPIOLITE FROM LASTOVI*Y

wq 0.46 0.08 0.00 o.2t 0.60 r.23 0.35Nbro3 9.t4 ll.l0 t2.@ 13.90 13.@ 16.20 ll.@Taror 46.80 52.4 62.4 63.20 62.70 63.50 70.90StrO, 0.67 1.00* 0.211 0.59* 0.3t. 0.85 0.96YrO3 l.l3 0.47 o.tz 0.00 0.t2 0.00 0.@sbzq 40.60 20.10 7.55 4.41 2.73 0.ll O.O0Biro, o.u 1.08 l.@ 0.17 0.06 0.@ 0.05c@ 0.00 0.99 6.08 6.45 9.59 0.0s 0.13MaO 0.03 0.16 0.52 0.31 0.530 1t.50 8.45F€O 0.01 0.74 0.n 0.14 0.68 4.10 6.42sro 0.10 3.30 t.yt 4.@ 223 0.00 0.00B8O 0.09 O.72 o.37 0.55 0.38 0.00 0.00PK' 0.07 0.04 0.09 0.@ 0.00 0.00 0.03N&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 0.01CqO 0.00 0.00 0.10 0.00 0.14 0.00 0.00F 0.00 0.31 o.47 0.38 0.30 0.00 0.00O = F 0 - m 4 1 1 4 m J l l 6 4 2 t o n o 0 0 OTo&l l@.14 93.06 95.34 94.53 .a6 n.sl n.9t

AtoEir@@cP o.on o.@2 0.000 0.@s 0.013 0.050 0.0t5Nbs l.ol4 o.i2o o.ifit 0.534 o.sn t.l5l 0.E05TsI 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.@Bi$ 0.t20 0.v29 0.023 0.004 0.@l 0.000 0.002c8! 0.000 0.llo 0.575 o.sts o.rEo o.@8 o.ajMd! 0.006 o.ol4 0.039 o.olt 0.03E 1.530 l.160Fe} 0.m2 0.06lt o.om 0.010 0.049 0.s39 0.869sF o.ol3 0.r9E 0.r0r o.l9? o.l l l 0.000 0.0@Btr 0.@r o.c29 0.013 0.018 o.ol3 0.@0 0.000pb* 0.004 0.001 0.002 0.000 0.000 o.@0 0.@lNa" 0.000 0.105 0.2t4 0.046 0.149 0.000 0.000K 0.000 o.ta 0.o2! 0.011 0.003 0.@2 0.002Cs" 0.0@ 0.m0 0.004 0.000 0.@5 0.@0 0.@0F 0.029 0.201 0.131 0.102 0.135 0.m0 0.0@o& 15.971 6.895 6.310 6.092 6.287 12.000 t2.@o

t,cd t.00E 3.506 3.369 3.tt3 3.362 6.051 6.037f @ 16 6.950 6.432 6.177 6.4M 12 l2

I - priEEy stibio@liq 2 - 8@ndry Simimliq 15 - ssudEynimlite, 6 od 7 - wdry |@geot8@lile.Cdi@ c.'a€@ rcmsliad ro 16 ei6 (@iaEmlib #l),TarNbrf i+W = 2 (Ei@lib# 2 !o 5),4d 12 oq/gEns (|@g@ot@fte{6,7).0.00 - b€low dd€qtio limiq .Sn o SnO, + Sf. n ed U belw deb(.i@ linitCo4ositi@ rEporbd in wtolc

those given above for Ca-rich primary phases (comparein Fig. 5). No other substantial A-site cations participatein the process at the localities examined.

Evident differences exist in the participation ofthe individual A-site cations among the four localities(Fig. 5). In the Nov6 Ves pegmarire, the earlysubsolidus phase (secondary microlite) is preserved,indicating high activity of Ca relative to Mn, whichis present only in the very rare manganocolumbiteof uncertain affiliation and negligible volume. TheLa5tovidky pegmatite is characterized by high activityof Fe, but very low activities of Ca and Mn, generatingabundant ferrotapiolite. Both the Chvalovice and Dobr6Voda pegmatites exhibit a virtually complete replacementsequence, including early Ca(Sb)-rich phases such asstibiomicrolite and microlite, and late Fe-enrichmentcharacterized by presence of Fe-rich manganotantalite(both localities), ferrotantalite (Chvalovice) andprobable ferrotapiolite (Dobr6 Voda) (see Fig. 5).

Total n.79 99.29 1.ffi.00 n32 9.17 9.25 98.9Eslonic cmtfils

w* o.an 0.019 0.034 0.v29 0.046 0.019 0.015Nbs t.32S 0.900 0.420 0.495 o.El4 0.677 0.701Ta: 2.553 3.u7 3.475 1.4s7 3.051 3.t34 3.037si* 0.0@ 0.000 0.000 0.000 0.016 0.@7Ti4 0.003 0.002 0.004 0.019 0.005 0.002 0.002

u* 0.004 0.000 0.000 0.@0 0.000 0.000 0.001sb$ 4.06.8 3.967 3.913 0.443 0.035 0.327 0.67rBi+ 0.000 0.004 0.023 0.0m 0.000 0.000 0.003cf 0.@0 0.@0 0.0fl) 0.3Es 0.@7 0.007 0.0@It{f O.W2 0.000 0.@0 0.021 0.M7 0.039 o.ouF€fl 0.@0 o.O1,2 0.0@ 0.01E 1.E32 l.'tCE 1.449Fgk 0.000 0.0@ 0.000 0.000 0.127 0.02t 0.000

vo, 0.45Nqo5 t2.E0T&rO, 40.90sio" 0.00Tio, 0.02SnO, 0.53uo, 0.0Esbro, 43.@Biro. o.0oc@ 0.00IUnO 0.01FeO 0.00F%Or 0.00BaO 0.@Pbo 0.00NaO O0O

0.31 0.558.56 3.90

48.20 53.700.@ 0.000.01 0.020.52 0.670.m 0.00

4t.40 N.@0.07 0.370.@ 0.@0.00 0.000.06 0.000.00 0.000.00 0.190.16 0.00000 000

1.33 r.r2 0A4 0.3413.05 11.30 9.20 9.4163.89 70.N 70.80 67.75nd. 0.00 0.10 0.040.30 0.04 0.02 0.020.00 0.39 0.7r 0.r00.@ 0.00 0.@ 0.@12.82 0.54 4.rr 9.t70.00 0.00 0.0o 0.064.2E 0.04 0.M 0.@0.30 0.35 0.?a 0.170.25 13.75 t2.49 10.510.00 t.06 0.23 0.00nd o.lt 0.00 0.@nd. 0.00 0.@ 0.05I l0 0.00 0_00 0_00

Bf 0.000 0.@0 0.01EPbl 0.000 0.010 0.000

0.011 0.000 0.0000.000 0.000 0.w2

Na* 0.000 0.000 0.000 0.179 0.000 0.000 0.0@e t6.@o 16.0@ 16.0@ 6.rt3 12.000 12.000 12.@0

t @. 16 16 16 6.1t3 12 12 12

1-3- primary stibiotmlits; 4- secondary suaiomicrolito;5-7 - smndry ftrroqiolito.cdion conteffi normalized to 16 otygos (stibiotant4lib #1-3),Ta+NFTi+W = 2 (stibiomicrolite +i4); 12 ox)€Eos @d 6 cdions(ferrotapiolite #5-?). n<L - not decermined. K Cs ad F belowdetsction limit Composfions ropored in wt% oxides.

668 TIIE CANADIAN MINERALOGIST

9t lmc s€cmic Mn col

DISCUSSIoN

Primary minerals

Compositional trends in columbite and otherprimary M,Ta-oxide minerals (microlite, rynersonite,stibiotantalite) exhibit geochemical signatures typicallyobserved in granitic pegmatites of the lepidolitesubtype: in columbite-type minerals, the buildup in Mnover Fe precedes the enrichment in Ta over Nb, and theMn/(Mn + Fe) values are commonly close to 1;microlite and stibiotantalite represent tle most evolvedNb,Ta-oxide-minerals (cf Foord 1976, eernf & Ercit1 985, 1989, Cern! 1989). Rynersonite from Chvaloviceis very likely the first occurrence ofttris species as aprimary mineral in granitic pegmatites. It is generallyknown as a secondary phase replacing stibiohnhlite,bismutotantalite, microlite or other minerals (Foord &Mrose 1978, von Knorring & Fadipe 1981, dernf &Ercit 1985, 1989, Baldwin 1989), or as a.late phase inpegmatite pockets (Nov6k et al. 1994). The formation

of rynersonite instead of the chemically similar andmore abundant microlite may be influenced by lowactivity of F in the parent medium.

The general sequence of enrichment of the A-sitecations in primary minerals, Fe - Mn - Ca - (Sb), isquite common in the Nb,Ta-oxide minerals ofmoderately to considerably fractionated rare-elementpegmatites (as summarized by Cernf & Ercit 1985,1989). Greater complexity is, however, routinelyencountered in exfemely evolved pegmatites, in whichadditional cations (such as Li, Na, K, Cs, Pb, Bi, Al,U and Sn) are encountered (Ercit 1986, Voloshin &Pakhomovskyi 1988).

Secondnry minerals

Minerals crystallized during the secondary(hydrothermal) stage in the pegmatites examined,including stibiomicrolite, microlite, cesstibtantite,manganotantalite, ferrotantalite and ferrotapiolite, arecommonly described from similar mineral assemblages

Sbtn Sbmic

Sbt

i

t

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 toIrIb + Ta + Ti +W = 2 atoms. Vertical bars: ranges of individual cations. Symbols: NV Nov6 Ves, CH Chvalovice, DV Dobr6Voda, LA LaStovidky, prmic: primary microlite, secmic: secondary microlite, Mncol: "secondary" manganocolumbite,Mntnl: secondary manganotantalite associated with secondary microlite, Mntntr: Fe-rich secondary manganotantaliteassociated with ferrotantalite, Sbtn: primary stibiotantalite, Sbmic: secondary stibiomicrolite, Mntn: secondarymanganotantalite, Fetp: secondary ferrotapiolite, Fetn: secondary ferrotantalite.

Nb-Ta OXIDE MINERAI-S. MOLDANUBICUM PEGMATTTES 669

Ftc. 6. Generalized trends in the evolution ofA-site cationabundances at the localities examined, complemented bypublished observations (see the text for references,,.A. The Sb - Ca * Mn - Fe sequence. B. The Ca - Mn - Fesequence in Sb-free assemblages.

considered to be metasomatic, formed at the expense ofprimary phases analogous to those described here.However, they also replace simpsonite, stibiotantalite,bismutotantalite. holtite and other less commonminerals (e.g., (em! & Ercit 1985, Ercit 1986,Voloshin & Pakhomovskyi 1988, Baldwin 1989).

The rarto NblTa in the secondary minerals

A universal decrease in NblTa in not only primarybut also secondary minerals was suggested by Voloshin(1983). However, Cernf & Ercit (1985), Ercit (1986)and Lumpkin & Ewing (1992) found this ratio inhydrothermally formed late minerals rather stable, andcomparable to that oftheir precursors. Our data are ingood agreement with the results of the lafter group ofauthors. Evidently, the mobility of Nb and Ta is aboutequal during the replacement processes at mostlocalities, and the two metals are incorporated into themetasomatic products in propOrtions more or lessidentical to those established in the primary phases.

Sequence of A.site cations in the secondary minerals

The complete sequence of the A-site cations(Sb) - Ca - Mn - Fe, as recognized at Chvalovice andDobr6 Voda, has only rarely been recorded in complexgranitic pegmatites (Voloshin 1983). However, diversesegments of the sequence, such as Sb - Ca - Mn, Sb -

Ca, Ca- Mn and Mn - Fe, are commonly observed atmany localities (e.g., iernf & Ercit 1985, 1989, Ercit1986, Voloshin & Pakhomovskyi 1988, Baldwin 19891.Increased activities of Mn and Fe relative to Ca are alsorecorded in late varieties of microlite (Wise & dernf1990, Lumpkin & Ewing 1992). This indicates that theoverall sequence documented here, (Sb) - Ca - Mn - Fe(Fig. 6), may represent a universal pattern of hydro-thermal replacement of primary M,Ta-oxide mineralssuch as microlite, stibiotantalite, simpsonite andbismutotantalite.

The variable abundance of the individual A-sitecations may result from the compositions of residualfluids, and from the composition of primary M,Ta-oxideminerals. The anay of the A-site cations is consequentlysubject to broader variations at individual Iocalities(e.g., Na Li, K, Cs, Ba, Al, Bi, Sn; Voloshin 1983, Ercit1986). However, the sequence of (Al) - K, Li - Cs, Na- Ca, derived by Voloshin & Pakhomovskyi (1988),cannot be accepted as a valid generalization. Thecomposition of metamorphic fluids may also have asignificant role in the latest stages of the hydrothermalcrystallization (as discussed below), and meteoricwaters may be involved under conditions of near-surface,lowtemperature weathering-induced replacement (e. 9.,cesstibtantite and kimrobinsonite: Nickel & Robinson198s).

The suggested sequence of the A-site cations insecondary replacement, (Sb) - Ca - Mn - Fe, isreversed from that characteristic of the primary crystal-Iization at the localities examined. However, themechanism producing such a reversed sequence is notunderstood at present.

Sources ofA-site cations in the secondary phases

The majorA-site cations in the M,Ta-oxide mineralsof the secondary stage at the localities studied includeSb, Ca, Mn and Fe (Fig. 5). The likely source of Sb andCa seems to be the highly evolved fluids exsolved fromthe residual pegmatite-forming melt, or the replacedprimary mineral, or both; elevated Ca-contents werefound in the residual melt in the experiments on theMacusani glass (London et al. 1989).Infiltration of Cavra metamorphic fluids from the host rocks is anotherpossible process, particularly in granitic pegmatiteshosted by Ca-rich rocks (London 1990). As Caparticipates in very eaily stages of the hydrothermalreplacement of primary M,Ta-oxide minerals, possiblyLrefore mixing of residual and metamorphic fluids, andas in most cases tle host rocks are not Ca-rich (Table

670 TTIE CANADIAN MINERALOGIST

l), this process does not seem to be probable. Releaseof Ca during hydrothermal alteration of blockyK-feldspar is not likely because of virtual absence ofthis process at most localities. Highly evolved fluidsalso are the source of Mn, because high activity of Mnin pore fluids in the host rocks (Table l) can be ruledout.

High activity of Fe in late stages of the replacementprocess may have several possible explanations. Ironmay have the same residual source as the other A-sitecations Sb, Ca and Mn; such an idea is supported bylate hydrothermal crystallization of Fe-rich tourmaline(foitite) from residual pegmatite-generated fluidsuncontaminated by metamorphic fluids, suggested forthe Dobr6 Voda pegmatite by Nov6k & Taylor (inprep.). The array of minor A-site cations such as Sb, Bi,Sr, Ba, Pb and U recorded at all localities, general lackof columbite alteration in outer pegmatite units, andvirtual absence of Mg in most compositions, also pointto residual fluids as the main source of all A-sirecations. On the other hand, some phenomena, such asonion-skin textures, nonstoichiometric compositionsof Si-enriched ferrotantalite with a high excess of Fe(Fig. 5), and the chemical composition of most ofthe secondary microlite (Table 4; cl also Lumpkin &Ewing 1992), recognized particularly at Chvalovice,suggest low-temperature formation, possibly underconditions of near-surface weathering. Consequently, aparticipation of mixed pegmatite and host-rock-derivedfluids in the secondary process at the Chvalovicelocality is fairly probable, and the source of Fe may bethe host rock.

P-T-X conditions of secondary replacements

The early stage of hydrothermal replacement of theprimary M,Ta-oxide minerals at the localities studiedis represented by the reaction of primary minerals withhighly evolved pegmatite-derived fluids soon aftercrystallization, i.e., atabout 500 to 350"C, 2.5-2.0kbar(typical of low-P, high-T petalite-bearing pegmatites;London 1990). Such fluid has a relatively high pH,high activity of Ca, and low activity of Na, is com-monly accompanied and followed by elevated activitiesof Mn and Fe. Participation of metamorphic fluidsreleased from the host rock is unlikely at this stage. Theformation of Fe-rich secondary phases (Fe-richmanganotantalite, ferrotantalite and ferrotapiolite),latest to crystallize in most of the mineral assemblagesstudied, seems to be more complicated. Veins offerrotapiolite in stibiotantalite from La5tovidky formedat relatively early stages. However, onion-skin texturesand chemical composition of microlite fromChvalovice and in part also from Nov6 Ves suggest alow temperature of <l00oC, close to conditions ofnear-surface weathering (Lumpkin & Ewing 1992).Such conditions would explain the high Fe! content ofsecondary tantalite at Chvalovice, and violations of the

ferrotapiolite-tantalite gap (eem! et at. 1992). However,a total lack of experimental data does not allow realisticestimates of the P-T-X conditions of all stages ofsecondary hydrothermal processes, particularly at thelow-temperature end.

CoNcLusrous

(1) A general sequence of A-site cations wasestablished for primary M,Ta-oxide minerals in fourcomplex pegmatites in the Bohemian and MoravianMoldanubicum, Fe - Mn - Ca - (Sb) in columbite,microlite, rynersonite and stibiotantalite, with manyanalogs in similar granitic pegmatites worldwide.

(2) A reversed sequence (Sb) - Ca - Mn - Fecharacterizes the products of subsolidus replacement ofthe primary minerals of Ca and Sb, also with numerousanalogies at other localities. It may represent auniversal pattern of secondary replacement at theexpense of the above primary minerals in moderately toconsiderably fractionated rare-element pegmatites.

(3) The qualitative and quantitative representationof the A-site cations is controlled by the relationshipsamong the primary phases and by the composition ofsubsolidus fluids. Further complications are encoun-tered in highly fractionated pegmatites, with secondaryminerals containing Li, Na, K, Cs" Pb, Al, U or Sn.

(4) The A-site cations in the secondary mineralsoriginate mainly from the residual pegmatite-derivedfluids and ttre parent minerals. Early involvement ofcountry-rock-derived solutions is unlikely, but possiblelocally to explain the pattern of Fe in late stages.

(5) The ratio M/Ta decreases during primary crys-tallization, but the secondary minerals inherit the ratioof their respective precursors.

(6) The P-T conditions of the secondary replacementprocesses vary from early subsolidus at -500-350'C,2.5-2.0 kbar to near-surface weatherine.

AcKNowrslcEMENTS

This study was supported by the NSERC ResearchGrants and Major Installation Grant to P.C., and byNSERC Equipment and Infrastructure Grants toF.C. Hawthorne. A Faculty of Science, University ofManitoba Postdoctoral Stipend supported M.N. Ourthanks to to F. Veselovskf, P. Korbel and N. Ball forhelp in X-ray-diffraction work, and to Ron Chapmanfor assistance during the electron microprobe study.A.U. Falster and W.B. Simmons, Jr. provided thoroughreviews which substantially improved the presentation.

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Received September 15, 1996, revised manuscript acceptedFebruarv4,1998.