The contribution of two-stage immiscibility for the formation of lithium-rich pegmatite: Insights from tourmaline in the Ke’eryin region

• Pure fractional crystallization alone cannot explain extreme Li enrichment. • Low- f O 2 , H 2 O-rich crustal magmas are required to generate Li-ore potential. • Fe 3+ -rich tourmaline is an effective exploration indicator for Li pegmatites. • A two-stage melt-melt then melt-fluid immiscibility drives Li enrichment. LCT-type granitic pegmatites represent a major global source of lithium, although their magmatic evolution and rare-metal enrichment mechanisms remain contentious. Owing to its structural flexibility and capacity to incorporate diverse trace elements, tourmaline effectively records magma-fluid evolution through its major-trace element and boron isotopic compositions, and thus serves as an excellent tracer for rare-metal migration and enrichment. Based on detailed petrographic observations, we present integrated major- and trace-element concentrations and boron isotope data for tourmaline from granites and associated pegmatites in the Ke’eryin district. Tourmaline from the Ke’eryin granite-pegmatite system defines a continuous evolutionary trend from schorl to elbaite: most grains are magmatic schorl, whereas magmatic-hydrothermal Li-rich tourmaline occurs exclusively in Li-rich pegmatites. Substitution mechanisms, combined with tourmaline δ 11 B signatures, indicate a predominantly continental crustal source and a pegmatite-forming environment that evolved from relatively reduced, low-salinity conditions to more oxidizing, saline regimes. Magmatic tourmaline exhibits limited intra-crystal δ 11 B variation (Δ 11 B < 0.8 ‰), whereas magmatic–hydrothermal Li-rich tourmaline shows significantly greater isotopic shifts (Δ 11 B > 2.0 ‰) that partially overlap the magmatic range, reflecting overprinting of tourmaline boron isotope compositions by late fluid exsolution. Rayleigh fractionation modeling indicates that transfer of ∼20 % of the total boron budget into an exsolved fluid phase optimally promotes strong lithium enrichment and ore formation in the residual melt-fluid system. Integrated mineralogical, geochemical, and boron isotopic evidence demonstrates that both early melt-melt immiscibility in the granitic magma and later melt–fluid immiscibility in pegmatitic melts collectively controlled the efficient concentration and mineralization of Li and other rare metals in the Ke’eryin LCT-type pegmatites.