Rubidium isotope compositions of biotite in granites record magmatic-hydrothermal process and rare metal enrichments

Magmatic-hydrothermal processes associated with granitic rocks are crucial for magma evolution and rare metal enrichment. This study presents Rb isotope compositions (δ87Rb) for whole rocks and Rb-enriched minerals (biotite and K-feldspar) from the Qitianling granite pluton in South China, which hosts one of the world's largest Sn-ore deposits. This pluton comprises three stages of granitic evolution, including less evolved initial (the first stage) granites to highly differentiated Sn-rich granites in subsequent stages (the second and third stages). The δ87Rb values of whole rocks and K-feldspar vary slightly from −0.34 ‰ to −0.03 ‰ and −0.43 ‰ to −0.09 ‰, respectively. In contrast, biotites show a systematic variation from the first stage (−0.55 ‰ to −0.28 ‰) to the second (0.21 ‰ to 0.39 ‰) and third stage (−0.20 ‰ to −0.05 ‰), which is influenced by magmatic fluids. Significantly, biotites from the second stage exhibit extremely high Rb concentrations exceeding 7600 μg/g, more than 4 times higher than the other two stages, indicating additional enrichment processes beyond fractional crystallization. Furthermore, δ87Rb values of biotites correlate positively with concentrations of various rare metal elements (Rb, Sn, Li, Cs, Nb, and W), suggesting the involvement of magmatic fluids enriched in heavy Rb isotope (87Rb) and fluid-mobile rare metals. Biotite, capable of accommodating these metals within the interlayer or octahedral sites, thus serves as a reliable recorder for the migration and enrichment of rare metals through magmatic-hydrothermal processes. Collectively, our findings demonstrate the potential utility of Rb isotopes of biotite as a robust tracer for magmatic-hydrothermal processes and rare metal mineralization in granitic systems.