Ta–Nb Oxides in Rare Metal Pegmatites of the Vishnyakovskoye Deposit (Eastern Siberia): Chemical Composition, Zonation Patterns, and Evolution

The chemical composition, zonation, and crystallization sequence of minerals of the columbite and wodginite groups in quartz-albite-potassium feldspar-muscovite pegmatites, albitites, quartz-muscovite rocks and muscovite-lepidolite glimmerites of the Vishnyakovskoye rare metal pegmatite deposit were studied. The Ta/(Ta + Nb) (Ta*) = 0.39‒0.50 and Mn/(Mn + Fe) (Mn*) >0.80 ratios in the first crystallized columbite-(Mn) show that the pegmatite melt underwent deep differentiation before its emplacement and was enriched in fluorine. The evolution of the composition of columbite-group minerals is reflected in the increase of Ta* values from 0.80 to 0.91 with an increase in Mn* to 1.00. Wodginite is represented by a Sn- and Mn-rich variety with Sn/ΣB = 0.59‒0.82, Ti/ΣB < 0.20, and Mn/ΣA = 0.88‒1.00. The shape of the grains, their zonation, and the evolution of the chemical composition indicate a predominantly magmatic genesis of the columbite-group minerals and wodginite in quartz-albite-potassium feldspar-muscovite pegmatites. In the albitization and greisenization zones, the columbite-group minerals (CGM) are largely inherited from the pegmatites; in the quartz-muscovite zone, partial dissolution of tantalite-(Mn) and removal of Ta and Nb occur, and in the muscovite-lepidolite glimmerites, tantalite-(Mn) and wodginite are again formed. Wodginite-group minerals (WGM) from different zones of pegmatite veins have highly variable compositions, indicating an important role of post-magmatic processes in their genesis. In muscovite-lepidolite glimmerites, Ta- and Mn-rich wodginite with Ta/ΣB = 0.50‒0.70 and Mn/ΣA = 0.90‒1.00 (tantalowodginite) is formed, after which wodginite itself with Sn/ΣB = 0.50‒0.82 crystallizes. In albitites, wodginite is replaced by Ti- and Fe-rich wodginite and ferrotitanowodginite with Ti/ΣB = 0.52‒0.76 and Mn/ΣA = 0.29‒0.49. The albitization and acid leaching processes manifested in pegmatites do not significantly affect the distribution of ore components, but anomalous accumulation of Ta, Sn, and Nb is noted in locally developed muscovite-lepidolite glimmerites.