Geochemical characterization of tourmaline in rare-metal pegmatites in the Mufushan area of central China and its metallogenic significance

Abstract

Pegmatites, as significant sources of rare metals, have long attracted considerable scientific attention due to their complex magmatic–hydrothermal evolutionary histories. However, the processes governing pegmatite evolution and rare-metal mineralization remain controversial. The Mufushan area in central China is a significant concentration area of rare-metal deposits, where multiple phases of magmatic activity and multistage mineralization have given rise to extensive rare-metal pegmatite deposits. Previous studies have primarily focused on magmatic-stage ore forming processes and secondary rare-metal enrichment during the hydrothermal stage. However, systematic investigations into the complete magmatic-hydrothermal evolution and associated rare-metal mineralization remain debate. Tourmaline is widely developed in various types of pegmatites throughout the Mufushan area, recording the geochemical characteristics of magmatic evolution at different stages within the region. This study conducted in-situ major and trace element analyses of different tourmaline types to investigate their geochemical characteristics and implications for magmatic-hydrothermal evolution and rare-metal mineralization. The results indicate that black tourmaline is schorl, formed during the late magmatic stage; dark-green tourmaline is elbaite, formed during the magmatic-hydrothermal transition; and pink tourmaline is rossmanite, formed during the hydrothermal stage. The evolutionary sequence of tourmaline colors (black → dark green → pink) reflects the continuous differentiation process of volatile-rich (e.g., F-enriched) pegmatitic magma. This evolution corresponds to the increasing intensity of rare-metal mineralization. As magmatic evolution progresses, the Li, Be, Nb, and Ta contents in tourmaline continue to increase. Elemental composition variations in tourmaline are governed by the LiAlFe₋₂ and Fe³⁺Al_-1 substitution mechanism, which reflect a transition of the magmatic system from Fe-rich to Li- and Al-rich compositions. Fractional crystallization is the primary mechanism for rare-metal enrichment in the residual melt. Hydrothermal metasomatism during the fluid-rich stage further enhances rare-metal mineralization. Moreover, an F-enriched magmatic environment facilitates rare-metal concentration and mineralization. Compared to tourmaline from other pegmatite-hosted rare-metal deposits, the ones in Mufushan pegmatites, characterized by higher concentrations of rare-metal elements and lower Nb/Ta ratios, demonstrate a higher degree of magmatic evolution and greater metallogenic potential of regional pegmatites. These findings offer new insights for utilizing the geochemical tourmaline composition (e.g., Li + Be + Nb + Ta, Li/Sr, Nb/Ta) as an indicator of rare-metal mineralized pegmatites.