Genesis of the Ganfang Li-Rb-Cs-Be-Ta-Sn rare metal deposit: Evidence from geochemistry, geochronology, and Nd isotopes

Abstract

Rare metal deposits are generally associated with highly fractionated granites with unique geochemical signatures and the mineralization is controlled by composition of magmas, degree of magma fractionation, and magmatic-hydrothermal processes. The Ganfang rare metal deposit, located in Jiangxi Province, south China, is hosted within a strongly peraluminous, P and F-rich S-type rare-metal granite pluton (the Ganfang composite granite pluton) of Early Cretaceous age in the Jiuling Neoproterozoic granitic batholith. The pluton comprises two-mica monzogranite, topaz-bearing muscovite-albite granite, and felsite (locally aplitic) dikes. The topaz-bearing muscovite-albite granite is highly enriched in Li, Ta, Sn, Nb, Be, Rb, and Cs, while the felsite displays ultra-high enrichment of P, Li, Cs, Rb, Be, W, Sn, Nb, and Ta. The cassiterite U–Pb (137–140 Ma), monazite U–Pb (∼140 Ma), and muscovite Ar–Ar (140–142 Ma) ages indicate a coeval magmatic association for the pluton. Nd isotopic compositions indicate magmatic origination in the Late Paleoproterozoic metasedimentary basement. However, the significant differences in contents of SiO₂, Al₂O₃, Li₂O, P₂O₅, Rb₂O, Cs₂O, and F among these three components suggest unlikely a single parental magma source. The columbite minerals in the topaz-bearing muscovite-albite granite show complex replacement textures and unique chemical composition, suggesting involvement of locally Ta-saturated magma and melt-fluid metasomatism during the magma evolution process. The felsite shows high F (up to 1 wt%), P₂O₅ (up to 1.2 wt%), and Li₂O (up to 1.5 wt%), indicating that the magma sequestered a large quantity of incompatible elements and ascended rapidly through the melt column to its emplacement level. In summary, enrichment of source magmas with rare metals, extreme fractionation, and melt-fluid metasomatism are the key factors in controlling genesis of the Ganfang Li-Rb-Cs-Be-Ta-Sn deposit.