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

Abstract

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 δ7Li 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 δ7Li 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 Li-rich pegmatites during the late stage of magmatic differentiation. The lower average δ7Li 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.