Mineralogy and geochemistry of pegmatite in the Gaoligong High-Grade metamorphic belt, southwestern China: Implications for rare metal mineralization

Pegmatite rare metal deposits are vital sources of rare metals. However, their metallogenic mechanism remains controversial. Numerous pegmatites have been discovered in the Gaoligong High-Grade Metamorphic Belt, southeastern margin of the Tibetan Plateau, southwestern China. These pegmatites can be categorized into rare metal-bearing pegmatites, such as albite pegmatite and perthite–albite pegmatite, and non-rare metal-bearing pegmatites, including orthoclase–albite pegmatite and biotite pegmatite. The rare metal minerals columbite, tantalite, cassiterite, and beryl intergrown with hydrothermal muscovite, topaz, and apatite are observed in the rare metal-bearing pegmatites. In the rare metal-bearing pegmatites, magmatic muscovite that is enriched in rare metals exhibits significantly higher Rb2O and Cs2O levels, lower MgO and FeO concentrations, and lower K/Rb ratios than magmatic muscovite found in non-rare metal-bearing pegmatites, which show higher MgO, FeO, and K/Rb values but lower Rb2O and Cs2O contents. Hydrothermal muscovite in rare metal-bearing pegmatites has distinctly higher F, Li2O, Rb2O, and Cs2O values than its magmatic counterpart. Additionally, orthoclase and albite from rare metal-bearing pegmatites have relatively high levels of P2O5. The decreasing K/Rb values from non-rare metal-bearing to rare metal-bearing pegmatites indicate an increase in the degree of pegmatite evolution. The increase in F, Li2O, Rb2O, and Cs2O concentrations in muscovite from the magmatic to hydrothermal stages in rare metal-bearing pegmatites suggests the involvement of F-, Li-, Rb-, and Cs-rich fluids in rare metal enrichment. Cassiterite U–Pb dating of rare metal-bearing albite pegmatite and perthite–albite pegmatite yields ages of approximately 14.79 ± 0.76 Ma and 16.81 ± 0.36 Ma, respectively, indicating that rare metal mineralization occurred during the Miocene. The rare metal mineralization in the Gaoligong High-Grade Metamorphic Belt (GHMB) is related to movements along strike-slip faults following the collision between the Indian and Eurasian plates. The differentiation of these magmas and the associated hydrothermal activity played a crucial role in rare metal mineralization, and hydrothermal processes were particularly instrumental in the GHMB.