• 标题：Lithium isotope fractionation during flfluid exsolution: Implications for Li mineralization of the Bailongshan pegmatites in the West Kunlun, NW Tibet
• 关键词：Granitic pegmatite,Lithium isotope fractionation,Fluid exsolution,Crystal-fluid interaction,West Kunlun,Tibet
• 作者：Jing-Jing Fan, Gong-Jian Tan, Gang-Jian Wei, He Wang, Yi-Gang Xu, Qiang Wang, Jin-Sheng Zhou, Zhuo-Ying Zhang, Tong-Yu Huang, Zi-Long Wang

Granitic pegmatites are considered signifificant 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 LieRb polymetallic deposit) are composed of Lipoor and Li-rich pegmatites. The Li-poor pegmatites display high d7 Li values in the whole rock (2.30 e4.94‰) and muscovite (2.22e7.55‰). In contrast, the Li-rich pegmatites show low d7 Li 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-flfluid 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 flfluid) system. The melt-flfluid separation can lead to signifificant Li isotope fractionation, with 7 Li enriched in the strongly-bonded residual silicate melt, and 6 Li preferring to weaker hydrated bonds in the flfluid. Alternatively, the Li-rich pegmatites enriched in 6 Li could have been induced by preferential accumulation of the fasterdiffusing 6 Li in the supercritical or near critical flfluid. The Li-rich pegmatites experienced extensive crystal-flfluid 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 flfluid is more enriched in alkali-, flfluxing components (Li, Rb, Cs, P and F) by the continuous interaction with the residual silicate melt. Therefore, the supercritical flfluid exsolution plays an important role in the Li isotope fractionation and rare metal mineralization.

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