From metasedimentary rocks to evolved pegmatitic melts: Tourmaline geochemistry and the origin of Nb-Ta mineralization in the Mufushan area, South China

The Mufushan region in the South China Jiangnan Orogenic Belt hosts significant rare-metal mineralization, yet the sources of fluids and metals for its pegmatite-hosted Nb-Ta deposit have remained unclear. This study employs tourmaline, an accessory mineral throughout the deposit, to trace Nb-Ta variations in the melt or fluid. Integrated textural and geochemical analyses of tourmaline from slate, schist, granite, ore-bearing pegmatite, and ore-free pegmatite reveal that all tourmalines belong to the alkali group (predominantly schorlitic, hydroxy species), with substitutions governed by (Fe 2+ Fe 3+ )(MgAl) −1 and AlO[R(OH)] −1 vectors, defining a foitite to magnesio-foitite trend. Tourmalines from metasedimentary rocks are Mg-rich, whereas those from granite and pegmatites are Fe-rich, with high Fe/(Mg+Fe) ratios (0.61–0.74) in the latter. Metasedimentary-hosted tourmalines show elevated Sr, Sc, Ni, and V, indicating host-rock control. Ore-bearing pegmatite tourmalines record melt evolution with progressive Nb, Li, and Cs enrichment, distinct from Be-enriched ore-free pegmatite. Boron isotope values (−15.30‰ to −9.54‰, excluding one anomalous slate-hosted outlier at −4.19‰) point to mixed granite and metasedimentary boron reservoirs. We conclude that the Nb-Ta mineralization originated from an evolved magmatic-hydrothermal melt/fluid enriched in Sr, Sc, V, and Mn, derived from the Mufushan granite batholith, which itself was sourced from the Lengjiaxi metasedimentary Group. Among the granitoids, the most evolved phase, muscovite monzogranite associated with rare-metal pegmatites, is the most promising exploration target.