Mica geochemistry as an indicator of magmatic-hydrothermal processes in the Ta-Nb-Sn-W mineralization of the Limu deposit, South China

Abstract

Limu deposit is one of the representative deposits with coexisting Ta-Nb and Sn-W mineralization in the Nanling Range, South China. It consists of three stages of granites, with the Ta-Nb and Sn-W mineralization mainly restricting in the third-stage granite. However, the details of the polymetallic mineralization process in this deposit are still obscure. This study utilizes mica geochemistry to trace the ore-related magmatic and hydrothermal processes. The primary micas evolved from Li-phengite in the first- and second-stage granites to zinnwaldite-lepidolite in the third-stage granite. The micas are increasingly enriched in Li, F, Rb and Cs elements with decreasing K/Rb and K/Cs ratios, suggesting a dominant fractional crystallization of feldspars, particularly plagioclase, with minor mica fractionation during magma evolution. The Li-phengites from the first- to the second-stage granites increase in Ta (average value: 38 to 61 ppm) and Nb concentrations (average value: 160 to 180 ppm) but decrease in Nb/Ta ratios (average value: 4.4 to 3.5), consistent with the bulk-rock Ta-Nb enrichment trends. However, the zinnwaldite in the third-stage granite has significantly lower Nb (110 ppm in average) but higher Ta concentration (170 ppm in average), and evolves to lepidolite toward decreasing Nb and Ta concentration (51 and 63 ppm in average, respectively) and increasing Nb/Ta ratios (average value: 0.7 to 1.0). These chemical features of the micas are controlled by co-crystallization of columbite group minerals (CGM) during the evolution of third-stage granitic magma, which was likely Nb-rich at the early stage and evolved toward Ta enrichment. Thus, magmatic processes played a critical role in the Ta-Nb mineralization. Secondary hydrothermal micas in the third-stage granite distinctly depleted in Li, F, Rb and Cs recrystallized from the primary zinnwaldite-lepidolite via replacing reaction, which are closely associated with cassiterite, wolframite and pyrite in the granite. The consistent δ34S values of the disseminated pyrite (0.71‰) and vein-type pyrite (1.12‰) in the granite indicate that the hydrothermal system was dominated by magmatic fluids without notable involvement of external fluids. The lower Ta and Nb concentrations (31 and 35 ppm in average, respectively) of the hydrothermal micas compared to primary ones suggest that at least small amounts of Ta and Nb were transferred by the fluids. Indicated by the mica concentrations, Sn and W were enriched with magma evolution and likely concentrated in the volatile and flux components of the third-stage magma. Furthermore, cassiterite and wolframite in the granite precipitated by fluid-rock interaction, and a portion of W and Sn migrated away with exsolved fluids to form the quartz vein W-Sn mineralization. This study demonstrates that mica geochemistry can effectively indicate magmatic-hydrothermal processes in granite-related Ta-Nb and Sn-W mineralization.