Formation of the Zashikhinsky Rare-Metal Deposit, Russia, by the Filtration Differentiation of Alkali Granite Melt

A model proposed for the formation of the Zashikhinsky Nb–Ta, Zr, and REE deposit is based on the results of geological, petrographic, and mineralogical–geochemical studies of rocks of the massif and ores hosted in it. Data are presented on the zonal structure of the massif and the distribution of ore minerals in the vertical section of the massif and between distinct zones identified in it. The behavior of ore components in the ore-forming process is elucidated. According to the proposed model, the deposit was formed by the interplay of two processes: (1) the in-chamber filtration differentiation of leucogranite melt, which was initially enriched in F, other volatile components (H2O, P2O5, CO2, and S), and trace elements: Nb–Ta, Zr, (Y + REE), U, Th, Pb, and Sn [Nb > 300 ppm, Zr > 300 ppm, (Y + REE) > 110]; and (2) autometasomatic alteration of the rocks under the effect of residual intergranular melt–aqueous solution without any significant input of material from the outside. This process operated during all evolutionary stages of the melt and led to the formation of multicomponent ores containing zircon, REE salts, and columbite of variable composition. The final phase in the formation of the ores was late hydrothermal and weathering processes. The high cryolite content in the rocks, and hence, the high fluorine concentrations in the solutions, caused secondary alterations of the rocks without significant changes in their bulk chemical composition.