Lithium isotope fractionation during fluid exsolution: Implications for Li mineralization of the Bailongshan pegmatites in the West Kunlun, NW Tibet

Abstract

Granitic pegmatites are considered significant host rocks for the rare metal deposits. However, the enrichment mechanisms for the rare metal elements (e.g., Li, Be, Nb and Ta) remain debated. Here we report the whole-rock and mineral (e.g., muscovite and spodumene) major, trace elements and Li isotopic compositions of the Bailongshan granitic pegmatites in the West Kunlun, NW Tibet, to address this issue. The Bailongshan pegmatites (hosting a super-large Li–Rb polymetallic deposit) are composed of Li-poor and Li-rich pegmatites. The Li-poor pegmatites display high δ7Li values in the whole rock (2.30–4.94‰) and muscovite (2.22–7.55‰). In contrast, the Li-rich pegmatites show low δ7Li values in the whole rock (−1.89 to 0.35‰), muscovite (−3.39 to 4.49‰) and spodumene (−2.83 to 1.87‰). These results, together with the variations of the elements, suggest that the Li-poor and Li-rich pegmatites were produced by the melt-fluid separation during the late stage of granitic magma evolution. This means that the Li-poor pegmatites were formed in a H2O-poor silicate-rich melt system, while the Li-rich pegmatites were generated in a H2O-rich silicate-poor melt (supercritical fluid) system. The melt-fluid separation can lead to significant Li isotope fractionation, with 7Li enriched in the strongly-bonded residual silicate melt, and 6Li preferring to weaker hydrated bonds in the fluid. Alternatively, the Li-rich pegmatites enriched in 6Li could have been induced by preferential accumulation of the faster-diffusing 6Li in the supercritical or near critical fluid. The Li-rich pegmatites experienced extensive crystal-fluid interaction in a relatively closed magmatic-hydrothermal system, which resulted in the leaching and reprecipitation of ore-forming elements, but produced restricted Li isotope fractionation. The exsolved supercritical fluid is more enriched in alkali-, fluxing components (Li, Rb, Cs, P and F) by the continuous interaction with the residual silicate melt. Therefore, the supercritical fluid exsolution plays an important role in the Li isotope fractionation and rare metal mineralization.