The differentiation mechanism of Sn isotopes in cassiterite at crystal scale and its implications for studying the Sn mineralization

Cassiterite, as a primary ore mineral for critical metal tin, is often widely formed in various kinds of granite-related magmatic-hydrothermal systems. The Sn isotopic composition of cassiterite can provide a unique view for studying the cassiterite precipitation mechanism and the fluid evolution process within magmatic-hydrothermal systems. However, Sn isotopic data, obtained by using the analysis of traditional sample dissolution methods or the in situ analysis of cassiterite fragments, have been ambiguously interpreted because the internal crystal structural heterogeneity of cassiterite is commonly overlooked. A recent study of a well-developed cassiterite crystal from the Weilasituo deposit in the North China Craton has been undertaken by a research team of Nanjing University using the integrated approach combining with the SEM-cathodoluminescence (CL), electron backscattered diffraction (EBSD), in situ Sn isotope analysis, and trace-element analysis. The results demonstrate that the cassiterite crystal is characterized by primary sector zonations, expressed as CL-bright and CL-dark sectors, which have obvious differences in trace-element concentrations and Sn isotopic compositions. Especially, the CL-bright sector corresponds to each individual crystallographic plane, while the CL-dark zones, which are mainly concentrated in boundaries of the {001} and {201} crystallographic planes, are enriched in W, Nb, Ta, and heavier Sn isotopes. These features of differences in trace element contents and Sn isotopic compositions in CL-bright and CL-dark sectors of cassiterite cannot be attributed to the superposition of multiple hydrothermal fluid pulses but can be interpreted to result from the influence of structural complexities along the plane boundaries. In addition, the gradual decrease tendency of Sn isotopic ratios and trace element concentrations across the oscillatory growth bands from the core to the rim of cassiterite reflects the Rayleigh fractionation of Sn isotopes during the progressive mineral precipitation in the closed hydrothermal system. This study refined current understandings of the cassiterite precipitation and Sn isotopic fractionation mechanism and is of important enlightment significance for reconstructing the granite-related hydrothermal ore-forming process.