Študentská vedecká konferencia, 22. apríl 2009, Bratislava. Zborník recenzovaných príspevkov.

Ferrocolumbite-ferrotantalite and ferrotapiolite in the leucogranites from the Považský Inovec Mts., Western Carpathians, Slovakia: chemical composition and evolution

Peter Chudík1, Pavel Uher1, Milan Kohút2

1Comenius University in Bratislava, Department of Deposits, Mlynská dolina, 842 15 Bratislava, Slovakia; [email protected] 2Dionýz Štúr State Geological Institute, Mlynská dolina 1, 817 04 Bratislava, Slovakia

1. Introduction Accessory Nb-Ta oxide belong to the uncommon but relatively the most characteristic accessory phases in evolved granitic rocks. Studies of these disseminated accessory minerals of granites reveal important informations about the degree of melt fractionation and are useful tools for interpretation of P-T-X conditions of their host rocks. The Nb-Ta oxide minerals usually associate with highly evolved, specialized granites, commonly rich in volatile elements (F, B, P), and their greisenized cupolas, connected with cassiterite, , topaz, Li-silicates and other characteristic minerals (e.g [3, 6, 7, 8]). However, our knowledge of accessory Nb-Ta phases in non-specialized, “barren“ granites without Sn±W±Li mineralization is still very scarce in comparison to the specialized granites or rare-element granitic pegmatites. In this article we describe accessory Nb-Ta oxide minerals in such non-specialized S- type pegmatitic leucogranite near Duchonka, the Považský Inovec Mountains, Western Carpathians, Slovakia as an example of unusual Nb-Ta fractionation in highly peraluminous environment.

2. Geological setting Studied granite is a medium to coarse-grained leucocratic rock, which consists of quartz, K-feldspar, plagioclase (An04–28), biotite and muscovite, with accessory garnet (almandine – ) and fibrolitic sillimanite, rarely zircon, apatite, monazite and identified Nb-Ta minerals. They can be geochemically characterized as relatively highly fractionated granites with S-type characteristics and they are comagmatic with other

1074 Študentská vedecká konferencia, 22. apríl 2009, Bratislava. Zborník recenzovaných príspevkov. lowercarboniferous granites to granodiorites of the Bojná complex of the Považský Inovec Mts.

3. Analytical methods Electron-microprobe analyses (EMPA) were carried out in the wavelength-dispersion mode using the Cameca SX-100 electron-microprobe at the State Geological Survey of Slovak Republic, Bratislava (Nb-Ta and silicate minerals) and the same instrument at the Institute of Geological Sciences, Masaryk University, Brno. For the Nb-Ta minerals, the following analytical conditions were used: accelerating voltage of 15 kV, a sample current of 20 nA, beam diameter of 1-3 μm and a counting time of

20 to 40 s. The following standards were applied: metallic W (W Lα), LiNbO3 (Nb Lα),

LiTaO3 (Ta Lα), TiO2 (Ti Kα), SnO2 (Sn Lα), ZrSiO4 (Zr Lβ), ThO2 (Th Mα), UO2 (U Mβ),

ScPO4 (Sc Kα), YPO4 (Y Lα), Sb (Sb Lα), fayalite (Fe Kα), (Mn Kα), MgO (Mg Kα), wollastonite (Ca Kα), ZnS (Zn Kα), and PbS (Pb Mα). Data were reduced using the PAP routine.

4. Results -(Fe) to -(Fe) forms discrete tabular crystals 30-350 m in size, in association with quartz, alkali-feldspar, muscovite and sillimanite. The mineral commonly shows prograde zoning with central parts enriched in Nb (columbite) and rims enriched in Ta (tantalite). However, irregular convoluted zoning of border parts, as a result of the late- magmatic to subsolidus dissolution-reprecipitation processes are also present, or a reversal trend of zoning with decrease of Ta towards to border parts of the crystals was detected. The composition of columbite-(Fe) to tantalite-(Fe) shows a relatively constant Mn/(Mn+Fe) = 0.20-0.27 (locally 0.35-0.40), but an extreme range of Nb-Ta fractionation: Ta/(Ta+Nb) = 0.18-0.72. Moreover, group of analyses in the rim zones of the tantalite crystals attain Ta/(Ta+Nb) ratio over 0.63 value and pass to the field of the miscibility gap between tantalite and tapiolite (Fig. 1). Contents of Ti, W, Sn, Mg and other elements in columbite-group minerals are low. Titanium attains mostly about 0.5 wt.% TiO2 and a slightly decrease of Ti with increasing values of Ta/(Ta+Nb) was recorded. Locally slightly elevated Zr and Pb occur

(≤0.6 wt.% ZrO2 and ≤1.2 wt.% PbO) (Tab. 1).

1075 Študentská vedecká konferencia, 22. apríl 2009, Bratislava. Zborník recenzovaných príspevkov.

Table 1. Representative compositions of columbite-(Fe) (Col-Fe), tantalite-(Fe) (Tan-Fe), tantalite-(Fe) in the miscibility gap field (Tan-Fe X) and tapiolite-(Fe) (Tap-Fe) from Duchonka pegmatitic leucogranite (in wt. %) Col-Fe Tan-Fe Tan-Fe Tan-Fe X Tan-Fe X Tan-Fe X Tap-Fe Tap-Fe

WO3 0,25 0,10 0,00 0,15 0,05 0,00 0,12 0,17 Nb2O5 41,66 25,36 20,83 21,79 17,94 15,88 6,14 1,58 Ta2O5 39,81 58,90 63,45 62,44 66,56 68,18 78,91 83,22 TiO2 0,07 0,09 0,04 0,46 0,16 0,44 1,28 0,83 SnO2 0,07 0,00 0,18 0,17 0,00 0,13 0,46 0,45 ZrO2 0,25 0,06 0,24 0,24 0,05 0,24 0,21 0,12 ThO2 0,00 0,00 0,00 0,02 0,00 0,00 0,04 0,00 UO2 0,00 0,00 0,00 0,00 0,00 0,00 0,01 0,00 Sc2O3 0,00 0,03 0,00 0,00 0,01 0,00 0,00 0,00 Y2O3 0,00 0,00 0,04 0,02 0,04 0,00 0,00 0,00 Sb2O3 0,06 0,10 0,00 0,03 0,00 0,04 0,10 0,05 Fe2O3 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 FeO 11,73 10,57 9,22 10,65 9,89 10,14 12,31 11,90 MnO 4,22 3,81 5,22 3,05 3,66 2,90 0,48 0,41 MgO 0,16 0,19 0,02 0,28 0,23 0,26 0,00 0,00 CaO 0,02 0,02 0,02 0,01 0,04 0,00 0,01 0,00 ZnO 0,01 0,00 0,00 0,02 0,00 0,01 0,00 0,00 PbO 0,58 0,21 0,23 0,43 0,36 0,27 0,00 0,02 Total 98,31 99,22 99,26 99,31 98,63 98,21 100,07 98,73

Formulae based on 6 oxygen atoms and valence calculation

W 0,004 0,002 0,000 0,003 0,001 0,000 0,002 0,004 Nb 1,275 0,844 0,712 0,735 0,626 0,561 0,225 0,061 Ta 0,733 1,179 1,304 1,268 1,397 1,450 1,740 1,925 Ti 0,004 0,005 0,002 0,026 0,009 0,026 0,078 0,053 Sn 0,002 0,000 0,005 0,005 0,000 0,004 0,015 0,015 Sum B 2,019 2,030 2,023 2,037 2,033 2,041 2,061 2,058

Zr 0,008 0,002 0,009 0,009 0,002 0,009 0,008 0,005 Th 0,000 0,000 0,000 0,000 0,000 0,000 0,001 0,000 U 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 Sc 0,000 0,002 0,000 0,000 0,001 0,000 0,000 0,000 Y 0,000 0,000 0,002 0,001 0,002 0,000 0,000 0,000 Sb 0,002 0,003 0,000 0,001 0,000 0,001 0,003 0,002 Fe3+ 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 Fe2+ 0,664 0,651 0,583 0,665 0,638 0,663 0,835 0,846 Mn 0,242 0,237 0,334 0,193 0,239 0,192 0,033 0,029 Mg 0,016 0,021 0,002 0,031 0,026 0,031 0,000 0,000 Ca 0,002 0,002 0,002 0,001 0,004 0,000 0,001 0,000 Zn 0,001 0,000 0,000 0,001 0,000 0,000 0,000 0,000 Pb 0,011 0,004 0,005 0,009 0,007 0,006 0,000 0,000 Sum A 0,945 0,922 0,936 0,911 0,919 0,903 0,881 0,882

Mn/(Mn+Fe) 0,267 0,267 0,364 0,225 0,272 0,225 0,038 0,033 Ta/(Ta+Nb) 0,365 0,583 0,647 0,633 0,691 0,721 0,885 0,969

Tapiolite-(Fe) forms discrete irregular or shortly prismatic crystals, ca. 15-75 m in size, in association with quartz and sillimanite. The tapiolite-(Fe) displays slightly irregular

1076 Študentská vedecká konferencia, 22. apríl 2009, Bratislava. Zborník recenzovaných príspevkov. compositional zonality in BSE with Mn/(Mn+Fe) = 0.03-0.04 and Ta/(Ta+Nb) = 0.88-0.97 (Tab. 1), Ti and Sn contents are slightly higher than in columbite group minerals; ≤0.8 wt.%

SnO2 and ≤1.3 wt.% TiO2.

FeTa O MnTa O 1 2 6 2 6 Tapiolite-(Fe)

0,8 Miscibility gap columbite-tantalite tapiolite

) Tantalite-(Mn) b 0,6 N Tantalite-(Fe) + a T ( / 0,4 a T

0,2 Columbite-(Mn) Columbite-(Fe)

0 0 0,2 0,4 0,6 0,8 1 FeNb2O6 MnNb2O6 Mn/(Mn+Fe)

Fig. 1. Quadrilateral diagram of columbite-tantalite and tapiolite from the Duchonka pegmatitic leucogranite. The boundaries of the tantalite-tapiolite miscibility gap are taken for single-phase, non-paired compositions of tantalite and tapiolite (grey solid line, after [4]) and coexisting tantalite-tapiolite pairs

(black dashed line, after [5]).

Consequently, due to the low contents of W, Ti, Sn, Zr, Mg, Pb and other elements, only single monovalent TaNb-1 and MnFe-1 substitutions are essential for the both columbite- tantalite-(Fe) and tapiolite-(Fe).

5. Discussion and conclusion The Duchonka pegmatitic leucogranite revealed a presence of accessory columbite- tantalite and tapiolite with unusually wide and up to extremely high Ta/Nb fractionation level. Tantalite-(Fe) compositions with Ta/(Ta+Nb) = 0.63-0.72 and Mn/(Mn+Fe) = 0.20-0.27, which penetrate the empirically defined natural tapiolite-tapiolite miscibility gap [5], belong to striking feature of the Duchonka leucogranite. Despite lack of XRD determination, the

1077 Študentská vedecká konferencia, 22. apríl 2009, Bratislava. Zborník recenzovaných príspevkov. phase indoubtedly belongs to tantalite, and not tapiolite. These anomalous compositions form external parts of columbite-tantalite crystals and they represent a part of the same columbite to tantalite evolutional trend with rapid Ta/Nb increasing and stable Mn/Fe ratio. On the contrary, tapiolite-(Fe) occurs as separate grains with different textural pattern and compositional field (Fig. 1). Such tantalite or tapiolite compositions are exceptional and they are considered as metastable [5]. However, some “hyper-tantalite-(Fe)” compositions with Ta/(Ta+Nb) between 0.62 and 0.97 were described from some rare-element granitic pegmatites in Africa [10, 2] and Separation Rapids pegmatite group, Ontario, Canada [9]. Moreover, W,Ti-rich tantalite-(Fe) from the Suzhou and Yichun specialized granites, China, shows Ta/(Ta+Nb) ratio from 0.50 to 0.73, and from 0.14 to 0.95, respectively [6, 11]. All these mentioned tantalite-(Fe) compositions lie within the field of the tantalite-tapiolite miscibility gap, or even in the tapiolite composition field (tantalite-(Fe) from the Rubicon Mine, Namibia; Baldwin 1989). However, such data, based commonly only by EMPA and without precise XRD determination of tantalite or tapiolite phase, must be taken with a large caution. On the other hand, an empirically based field of tantalite-tapiolite miscibility gap [5] is based only on known data from granitic pegmatites and some natural (stable or metastable) compositions could be penetrated them as the consequence of specific P-T-X conditions. Namely, variations of Ta/Nb ratio in Nb-Ta oxide minerals strongly depends on pH of the host pegmatitic melt; the decreasing of pH (increasing of acidity) caused the increasing of the Ta/Nb ratio of the solution and precipitated Nb-Ta phases [1]. The Duchonka pegmatitic granite represents such example, where possible combination effect of relatively higher solidus temperature in comparison to common granitic rare-element pegmatites, together with increased fractionation level and strongly peraluminous composition caused the precipitation of tapiolite-(Fe) and exceptionally Ta-rich tantalite-(Fe).

Acknowledgements The authors thank R. Škoda and I. Holický for the electron-microprobe assistance. This work was supported by the Slovak Research and Development Agency under the contract No. APVV-0557-06.

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1078 Študentská vedecká konferencia, 22. apríl 2009, Bratislava. Zborník recenzovaných príspevkov.

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