Genesis of the Xianghualing tin-polymetallic deposit in southern Hunan, South China: Constraints from chemical and boron isotopic compositions of tourmaline

Abstract

The Xianghualing skarn Sn polymetallic deposit (Nanling Range), with ca. 0.25 Mt Sn metal, 0.85 Mt Pb + Zn metal, and 22 Kt (Ta, Nb)2O5, is a Late Jurassic biotite granite-related Sn-Pb-Zn-Nb-Ta deposit. However, our understanding of how magmatic-hydrothermal processes contributed to its mineralization remains poor. Tourmaline has the capacity to retain primary elemental and isotopic information throughout the tin-related ore-formation process, allowing us to use the in-situ major, trace elemental, and boron isotopic compositions of tourmaline to research the origin of tin-bearing fluids and to trace the magmatic-hydrothermal evolution associated with tin mineralization. Five types of tourmaline are recognized based on petrography: i) late-magmatic tourmaline (Tur-I) disseminated in the biotite granites; ii) hydrothermal tourmaline (Tur-II) in massive schorl-fluorite association from the endoskarn zone; iii) hydrothermal tourmaline (Tur-III) with long columnar crystals in massive schorl-epidote association from the exoskarn zone, is related to Sn-Pb-Zn mineralization; iv) hydrothermal tourmaline (Tur-IV) from tourmaline-fluorite veins in exoskarn within the marble; and v) hydrothermal tourmaline (Tur-V) from cassiterite-sulfide-tourmaline-fluorite-topaz veins hosted by quartz sandstone, and is directly related to Sn-Pb-Zn mineralization.Tourmaline from the Xianghualing tin-polymetallic deposit dominates the alkali group and the schorl-dravite solid-solution series. Distinct Fe/(Fe + Mg) and Na/(Na + Ca) ratios of Tur-II to Tur-IV reveal different degrees of B-rich fluid-rock (granite or carbonate) interaction. Trace element variations in tourmaline are principally governed by local fluid-rock interactions and melt/fluid compositions in a low fO2 environment. Late-magmatic tourmaline (Tur-I) and hydrothermal tourmaline (Tur-V) related to vein-type Sn-Pb-Zn mineralization have similar Ca, Na, Al, Mg, Fe, and REE + Y contents, indicating that magmatic-hydrothermal exsolution controls Sn-Pb-Zn enrichment. This process occurs in a reducing environment without significant water–rock reactions, demonstrating that high fO2 or acid-consuming reactions are unnecessary for cassiterite precipitation. Variations in water–rock reactions between magmatic fluids and the carbonates cause differences in tourmaline elemental concentrations (e.g., Ca, Al, and Pb) between the magmatic and skarn phases. There are two main paths of tin mineralization in the Xianghualing deposit: (1) fluorine- and boron-rich fluid exsolution at the magmatic-hydrothermal transition stage is crucial for releasing tin and forming cassiterite during the direct cooling process; (2) fluid-rock interaction between tin-bearing magmatic fluids and carbonates facilitates the precipitation of cassiterite. The small variation in δ11B values for all types of tourmaline (-14.8 to −11.6 ‰) suggests that B-rich fluids are mainly derived from magmatic differentiation and fluid exsolution, with insignificant B isotopic fractionation. Magmatic-hydrothermal evolution at the Xianghualing Sn-polymetallic deposit can be unraveled by using the element and B-isotope fingerprints of tourmaline, which might be used as a prospecting aid for Sn-polymetallic deposits.