Lithium isotope behavior during magmatic differentiation and fluid exsolution in the Jiajika granite-pegmatite deposit, Sichuan, China

The Jiajika lithium (Li) deposit in Sichuan Province is the largest pegmatite-type Li deposit in China. The petrogenesis and metallogenesis of the Jiajika granitic pegmatites remain debatable. This study presents the Li isotopic compositions of two-mica granites, Li-rich and Li-poor pegmatites, and muscovites in Li-poor and Li-rich pegmatites to unravel the Li enrichment mechanism in the Jiajika deposit. The two-mica granites have a lower average delta 7Li values than those of the Li-rich and Li-poor pegmatites. The Li contents of the two-mica granites are almost the same as those of the Li-poor pegmatites but are much lower than those of the Li-rich pegmatites. Muscovites in the Li-rich pegmatites have higher average Li contents and delta 7Li values than those of the Li-poor pegmatites. All these results indicate the Jiajika granitic pegmatites were the final products of the extreme fractional crystallization of two-mica granitic magmas rather than direct anatectic melts, and that the Li-poor pegmatites derived from early differentiation of the two-mica granitic magmas before they evolved into Lirich pegmatites during the late stage of magmatic differentiation. The lower average delta 7Li values of the more evolved Li-rich pegmatites compared with the Li-poor pegmatites may have been caused by fluid exsolution and kinetic diffusive fractionation during melt-fluid separation. We discovered that fluid exsolution during melt-fluid separation can cause significant Li isotopic fractionation, with 7Li enriched in a H2O-poor silicate-rich melt system. Considering the crystallization ages of the two-mica granites and granitic pegmatites and other geochemical evidence (e.g., major- and trace-element compositions), the magmatic differentiation and fluid exsolution in the late stage of the granitic magma evolution together with the Li-rich surrounding strata led to the multistage enrichments in lithium, thus contributing to the formation of the Jiajika large Li deposit.

Automated summary

The Jiajika lithium deposit in Sichuan Province is the largest of its kind in China, and the petrogenesis and metallogenesis of the granitic pegmatites in the deposit have been debated. This study reveals that the Li-rich and Li-poor pegmatites in the deposit were the result of extreme fractional crystallization of two-mica granitic magmas, rather than direct anatectic melts. Additionally, fluid exsolution during melt-fluid separation played a significant role in causing Li isotopic fractionation in the late stage of granitic magma evolution, contributing to the formation of the large lithium deposit.