Lithium isotopic systematics and numerical simulation for highly-fractionated granite-pegmatite system: Implications for the pegmatite-type rare-metal mineralization

Abstract

Lithium (Li) isotope behavior during fractionation of highly-evolved granitic pegmatites and associated rare metal mineralization remains unclear, especially concerning the relative importance of magmatic versus metasomatic processes. In this study, we investigated the geochemistry and Li isotopes of the giant Renli-Chuanziyuan lithium-niobium-tantalum (Li-Nb-Ta) deposit in the Mufushan batholith (South China). We found that muscovite granite in the Mufushan complex can be divided into two groups (high-Li and low-Li). δ7Li value of the high-Li and low-Li group increases and decreases, respectively, with increasing Li content, which is likely resulted from fractional crystallization and/or magmatic volatile phase (MVP) exsolution. The MVP exsolution began with the solidification of muscovite granite, as evidenced by the zoned micas. The Li isotope fractionation, modeled by Rayleigh distillation during MVP exsolution, is consistent with the δ7Li value of Li-poor pegmatite, indicating that MVP exsolution is related to the pegmatite formation.The Mufushan Li-poor and Li-rich pegmatites have very high and low δ7Li value, respectively. Based on the Δ7Li(bulk-mica) value and numerical simulation, we suggest that the pegmatite formation at Mufushan could be linked to extensive MVP accumulation, while the different Li elemental and isotopic compositions of the two types of pegmatite are probably caused by MVP migration. In fact, published granite-pegmatite data worldwide also show negative Li vs. δ7Li correlation, indicating that such MVP exsolution, accumulation and migration are probably a common phenomenon for evolution of granitic pegmatite. Therefore, the different Li isotopes and Li-Rb-Cs proportion of Li-poor and Li-rich pegmatites indicate that MVP exsolution, migration and accumulation are key to rare metals enrichment and distinguish various types of rare-metal mineralization (Li and Nb-Ta).