The role of magmatic volatile phase exsolution in lithium enrichment in pegmatites: Tourmaline evidence from the Mufushan complex, South China

Over one-third of the global lithium supply is sourced from pegmatites, yet their complex evolution obscures Li mineralization mechanisms. This study investigates the Mufushan granite-pegmatite system (South China) using tourmaline geochemistry and boron isotopic compositions to identify the key processes controlling lithium enrichment. Tightly clustered δ11B values in tourmalines from both granite and pegmatite confirm a common magmatic source. Compared with muscovite granite tourmalines, those from northern Mufushan pegmatites exhibit lower YAl/(YAl + Fe) ratios at the Y site and reduced concentrations of incompatible elements (e.g., Li, Sn), suggesting a less evolved character and derivation from a more primitive melt. Rayleigh modeling indicates that fractional crystallization generally leads to the enrichment of incompatible elements in the melt, yet in the northern Mufushan pegmatites, Li contents increased only to ∼720 ppm in late-stage tourmaline, still far below ore-formation levels, consistent with the absence of Li mineralization in the north. In contrast, tourmalines from southern pegmatites record a decrease in Li contents from 814 ppm in early stages to 42 ppm in late stages, a pattern that cannot be explained by fractional crystallization and is likely related to the exsolution of a magmatic volatile phase (MVP). Coupled Rayleigh modeling of Li and δ11B indicates that after ∼70% fractional crystallization, at least 36% of the melt-derived MVP must have exsolved in the southern pegmatites. The exsolved MVP transported large amounts of Li and other volatile elements, ultimately controlled Li mineralization, and created the observed spatial contrast between Li-rich southern pegmatites and barren northern pegmatites.