Extreme variation and apparent reversal of Nb-Ta fractionation in columbite-group minerals from the Scheibengraben beryl-columbite granitic pegmatite, Marsikov, Czech Republic

Compositional variation was studied in columbite-group minerals from the beryl-columbite pegmatite at Scheibengraben, Marsikov, Northern Moravia, Czech Republic. The pegmatite consists of five textural-paragenetic units, from the least to the most evolved: volumetrically dominant coarse-grained unit, subordinate graphic and blocky units and a minor cleavelandite unit; a saccharoidal albite unit is rather randomly distributed within the dike. It replaces and/or crosscuts all other units except the cleavelandite unit. Columbite-group minerals are the dominant Nb,Ta-oxide phases in all units. They are associated with other Nb,Ta-oxide minerals: minerals of the pyrochlore subgroup and fersmite in the coarse-grained unit, and minerals of the microlite subgroup, ferrotapiolite and rynersonite in the cleavelandite unit. The extreme Nb-Ta [Ta/(Ta+Nb) = 0.06 to 0.97 (microlite 0.99)] and appreciable Fe-Mn [Mn/(Mn+Fe) = (ferrotapiolite 0.22) 0.35 to 0.90] fractionations in columbite-group minerals differ from those observed in beryl pegmatites examined to date, but they are comparable with those in some highly fractionated, complex, Li-rich pegmatites. High activity of F (facilitated by low contents of B, P and Li in the pegmatite melt) very likely maximized such an extensive Nb-Ta fractionation, over and above differential solubilities of columbite and tantalite in pegmatite melt. The apparent reversal of Nb-Ta and Fe-Mn fractionations found in columbite from the saccharoidal albite unit seems to be an artefact from early units (particularly the coarse-grained one), which were extensively replaced by saccharoidal albite.