Abstract
The Taikeu ore cluster comprises four tantalum-bearing ore deposits: Taikeu, Ust-Mramorny, Longotyugan, and Neudachny. Their ores are related to microcline-albite and less frequent quartz-albite-mica (phengite) metasomatic rocks replacing granites. The major ore minerals belong to the fergusonite and pyrochlore groups (mainly plumbopyrochlore). The compositional evolution of these minerals is considered in detail. High-Yb fergusonite containing up to 22–27 wt % Yb2O3 was established in the ore cluster for the first time. Its formula is ({Yb0.30Er0.15Dy0.05Lu0.04Tm0.04…}0.61Y0.39)[Nb0.99Ta0.01]O3.96. The density of this mineral determined by microvolumetric method is 6.47 ± 0.06 g/cm3. Fergusonite evolves from high-Yb to low-Yb varieties, with relatively small variations in Er and Dy. It is suggested that this fractionation trend is caused by high solubility of Yb2+ complexes (primarily, fluoride complexes) in aqueous fluid during albitization. The chemical composition of the pyrochlore-group minerals evolves as follows: Na+Ca → U+Th → Pb. Plumbopyrochlore was formed at the late metasomatic stage of tantalum-niobium mineralization. The characteristic features of this stage are a high oxidation potential, on the one hand, and a low activity of sulfide sulfur, on the other hand.
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