Magmatic-hydrothermal transition and lithium enrichment: Li–B isotope evidence of elbaite for fluid recirculation mechanisms

The magmatic-hydrothermal transition in rare-metal pegmatites is crucial for understanding mineralizing processes, particularly for strategic resources like lithium and beryllium. Tourmaline, a common mineral in pegmatites, serves as an excellent geochemical tracer for this transition. This study examines the Bailongshan Li pegmatite deposit in the western Kunlun Orogen, northwestern China, through detailed textural and geochemical analyses of elbaite (Elb). By integrating data for major and trace elements and Li–B isotope compositions of Elb, the present study delineates the magmatic-hydrothermal transition and its role in ore formation. Elbaite in the Bailongshan Li pegmatite deposit crystallized progressively from Elb-1 (core, magmatic origin, dark green), through Elb-2 (mantle, late magmatic origin, dark green), to Elb-3 (rim, magmatic-hydrothermal transition origin, light green). Li and B isotopes of Elb show different evolutionary trends from Elb-1 to Elb-3: δ 11 B increases from Elb-1 to Elb-2, whereas no statistically significant variation is observed from Elb-2 to Elb-3.This pattern is interpreted to reflect fractional crystallization (the resolvable core-to-mantle increase) followed by the re-involvement of an early-exsolved, weakly fractionated hydrothermal fluid that imparts no resolvable δ 11 B change to Elb-3. In contrast, δ 7 Li remains stable from Elb-1 to Elb-2 but shows a “V”-shaped variation from the late-crystallized range of Elb-2 to Elb-3. This “V”-shaped δ 7 Li trend likely records a combination of fractional crystallization, melt-fluid separation, and Li diffusion. Boron isotope modeling indicates ≥80% fractional crystallization during Elb-1 and Elb-2 deposition, while Elb-3 formed from exsolved hydrothermal fluids derived from early melt-fluid separation that are re-involved in the crystallization of lithium-bearing minerals (elbaite) during the magmatic-hydrothermal transition with ∼25% fractional crystallization. Elevated δ 7 Li values (avg. + 14.1 ‰ to + 14.6 ‰) and an increase in mean Li content of ∼1800 ppm between Elb-2 (avg. 5723 ppm) and Elb-3 (avg. 7513 ppm) might reflect Li isotope diffusion driven by concentration gradients. Reintroduced, weakly fractionated exsolved fluids govern Li mineralization (increasing concentrations of Be, Sn, Ta, and Nb) in elbaite during the magmatic-hydrothermal transition at the Bailongshan Li pegmatite deposit. This study identifies that early-exsolved hydrothermal fluids are re-injected into pegmatitic magmas during pegmatite evolution, a phenomenon conducive to rare metal mineralization. Our work enhances understanding of pegmatite evolution and provides a new mechanistic explanation for pegmatite-hosted rare metal mineralization.