Crystal-scale heterogeneity of Sn isotopes in Cassiterite: implications for reconstructing ore-forming processes in magmatic-hydrothermal systems

Cassiterite, as the primary ore mineral for critical metal tin, often forms in a variety of granite-related magmatic-hydrothermal systems. Sn isotopes in cassiterite provide unique insights into the mechanisms of tin precipitation and fluid evolution in magmatic-hydrothermal systems, but Sn isotope data based on traditional sample dissolution methods or in situ on cassiterite fragments have led to ambiguous interpretations. In this study, we first select a well-developed cassiterite crystal from the Weilasituo deposit in North China to perform in situ Sn isotope and trace element analyses, accompanied by scanning electron microscopy-cathodoluminescence (SEM-CL) imaging and electron backscatter diffraction (EBSD). The cassiterite sample reveals distinct primary sector zonations, including CL-bright and CL-dark zones. EBSD analyses indicate that the CL-bright zones are commonly developed within each individual crystallographic plane, while CL-dark zones are formed along the boundaries of different planes such as {001} and {201}. Moreover, the CL-dark zones are predominantly hosted in the {001}, with minor overlaps into {201} domains. Our results reveal significant enrichment of trace elements (W, Nb, and Ta) and heavier Sn isotopes in the CL-dark sectors relative to the CL-bright sectors. In contrast to traditional protosite model predictions, this enrichment is primarily attributed to structural complexities along the plane boundaries. Elevated lattice defects and dislocation in these structurally disturbed regions likely promote selective incorporation of trace elements and the retention of heavier Sn isotopes. In both dark and bright sectors, Sn isotopes decreased similarly from the center outwards. The decline of Sn isotope ratio and trace element concentrations across the oscillatory growth bands from the core to the rim, caused by element depletion in hydrothermal fluids through progressive mineral precipitation, is mainly attributed to Rayleigh fractionation. Challenging the conventional view, this study reveals that the heavier Sn isotopes in dark sectors do not result from the superposition of multi-stage fluids. This finding reshapes our understanding of cassiterite formation mechanisms and has important implications for reconstructing ore-forming processes.