Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Milan Novák, Petr Černý

Distinctive compositional trends in columbite-tantalite from two segments of the lepidolite pegmatite at Rožná Western Moravia, Czech Republic

Journal of the Czech Geological Society, volume 46 (2001), issue 1-2, 1 - 8

The Fe-Mn and Nb-Ta fractionation trends were studied in columbite-group minerals from two spatially separated segments of the zoned lepidolite pegmatite dike at Rožná. In both segments, Hradisko and Borovina, columbite-tantalite occurs exclusively in the lepidolite subunit and in the adjacent quartz core. Manganocolumbite from Hradisko is largely homogeneous and displays extreme values of Mn/ (Mn+Fe)_at. = 0.99-1.00, but commonly low Ta/(Ta+Nb)_at. = 0.06-0.10, rarely up to 0.44. Manganocolumbite to manganotantalite from Borovina is heterogeneous and exhibits moderate to high Mn/(Mn+Fe) = 0.48-0.97, as well as Ta/(Ta+Nb) = 0.43-0.89. Columbite-tantalite contains low concentrations of W and Ti in both segments, slightly higher in Hradisko. Rare, late, oscillatorilly zoned columbite veinlets in manganocolumbite from quartz core at Hradisko display highly variable Mn/(Mn+Fe) of 0.08-0.98, but Ta/(Ta+Nb) = 0.10-0.23 which is slightly higher than that of the primary host manganocolumbite. Columbite-tantalite is largely highly-ordered, but intermediate disorder was found in some samples from Hradisko. Different compositional trends in columbite-tantalite from both segments were compared with parent mineral assemblages and with composition of associated lepidolite and elbaite. Both lepidolite and elbaite are Fe-enriched in Borovina relative to the Hradisko segment. The advanced Fe-Mn fractionation in Hradisko is very likely related to high activity of F and alkali fluorides inferred from the abundance of lepidolite and topaz; whereas the Nb-Ta fractionation is probably controlled by a different factor. Compositional homogeneity of manganocolumbite from Hradisko suggests equilibrium crystallization; however, high heterogeneity in columbite-tantalite from Borovina indicates disequilibrium conditions.