Geochemical characteristics and boron isotopes of tourmaline from the Baishaziling tin deposit, Nanling Range: Constraints on magmatic-hydrothermal processes

Abstract

Greisenization is an important stage of Sn-W mineralization, its mechanism of Sn enrichment and deposition remains debated (e.g., boiling, cooling, fluid-rock interaction, and fluid mixing). In this study, we performed detailed petrography and geochemical studies of different types of tourmalines in the Baishaziling greisen-type tin deposit, aiming to investigate the magmatic-fluid evolution and ore-forming processes. Major elements show that almost all tourmalines have high Fe/(Fe + Mg) ratios with affinity of alkali group and schorl compositions. Disseminated tourmalines (Tur-G1) occurring in granite are primary magmatic origin with high Li, Zn, Sc, V, and Ga contents. In the late magmatic-hydrothermal transition stage, tourmalines occur as tourmaline-quartz nodules (Tur-G2) in the granitic body, which have similar geochemical characteristics to Tur-G1. The lower Sn content of Tur-G3 in early quartz-tourmaline veins indicates that the dissolution of fluid has not lead to the initial Sn enrichment in the Baishaziling deposit. In the greisen-type orebodies, Tur-O1 with highest content of Sn (up to > 1000 ppm) and HREE, result of the interaction between magmatic-hydrothermal fluid and host granite due to the silicate minerals alteration, especially biotite. On the other hand, both syn-ore Tur-O2 and Tur-O3 exhibit low Sn, Li, Ca, HREE contents, which are suggested to be related to the co-crystallization of cassiterite and accessory mineral. The inverse Fe–Al correlation, NaR(X□Al)-1 and NaR2(OH)(X□Al2O)-1 substitution vectors of tourmalines in ore bodies (Tur-O) imply the relatively oxide environment and high salinity during the precipitation of cassiterite. Based on calculation, the δ11B value of primary magma, initial hydrothermal fluids and ore-formation fluids are ranging from −15.5‰ to −14.5‰, −12.63‰ to −12.22‰, −11.90‰ to −10.01‰, respectively, indicating that they have the same boron source from magma.