Pegmatitic Nb-Ta oxide minerals in alluvial placers from Limbach, Bratislava Massif, Western Carpathians, Slovakia: compositional variations and evolutionary trend

Abstract

*Corresponding author Niobium-tantalum mineral assemblage of granitic pegmatite origin was found in alluvial placers near Limbach, the Bratislava granitic massif, Male Karpaty Mountains, Central Western Carpathians, southwest Slovakia. The most widespread mineral is ferrocolumbite to ferrotantalite, rarely manganocolumbite to manganotantalite I [at. Mn/(Mn + Fe) = 0.17–0.52 and Ta/(Ta + Nb) = 0.19–0.70] shows primary magmatic, fine to coarse, regular oscillatory zoning with an evolutionary trend indicating a moderate degree of fractionation, analogous to beryl–columbite subtype of rare-element granitic pegmatites. On the contrary, irregular oscillatory and patchy zoning of ferrocolumbite to manganotantalite II [Mn/(Mn + Fe) = 0.17–0.66, Ta/(Ta + Nb) = 0.16–0.81] together with precipitation of anhedral Ta-rich intergrowths and inclusions of ferrotapiolite [Mn/(Mn + Fe = 0.05–0.07, Ta/(Ta + Nb) = 0.87–0.93], Ta>Nb-rich rutile [Mn/(Mn + Fe) = 0.02–0.03, Ta/(Ta + Nb) = 0.80–0.86], Sn-rich ixiolite [Mn/(Mn + Ta) = 0.40–0.41, Ta/(Ta + Nb) = 0.81] and uraninite + uranmicrolite [Ta/(Ta + Nb) = 0.81–0.94] indicates their subsolidus origin.