The separation mechanism of W and Sn in the Fei’eshan deposit, Southeast China: Insights from trace element and Sn isotope compositions of cassiterite

• The Fei’eshan deposit exhibits greisen-type Sn and quartz vein-type W orebodies. • Cassiterite from different orebodies show variable trace element and Sn isotope compositions. • Different fluids are responsible for the Sn and W mineralization. The spatial separation of W and Sn mineralization represents a fundamental characteristic of W-Sn deposits. While mechanisms responsible for regional-scale separation have been widely investigated, processes controlling deposit-scale zonation remain poorly constrained. This study investigates the Fei’eshan W-Sn deposit in eastern Guangdong Province, South China, which exhibits a well-defined spatial zoning pattern: (1) proximal greisen-type Sn orebodies and (2) distal quartz vein-type W orebodies. To unravel the factors controlling this zonation, we conducted an integrated analysis of U-Pb ages, trace element and Sn isotopic compositions of cassiterite from three ore types: (1) greisen (Cst-G), (2) quartz-wolframite veins (Cst-QWV), and (3) quartz-sulfide veins (Cst-QSV). LA-ICP-MS U-Pb dating of cassiterite yields concordant ages for all ore types: Cst-G (135.9 ± 2.8 Ma), Cst-QWV (136.5 ± 2.2 Ma), and Cst-QSV (134.5 ± 3.7 Ma), demonstrating synchronous W-Sn mineralization during the Early Cretaceous. Cathodoluminescence (CL) and electron backscatter diffraction (EBSD) imaging reveal that Fe, W, and U contents in cassiterite display crystallographic orientation-dependent variations, whereas other trace elements (Zr, Hf, Nb, Ta, Ti, Sc, and V) are less affected by crystallographic orientations, making them reliable tracers for fluid migration pathways. CL-dark domains correlate with Fe-W enrichment through quenching effects, while Zr-Hf-Ti enrichment generates CL-bright domains as activators. Progressive depletion of Zr-Hf-Nb-Ta from Cst-G to Cst-QWV and Cst-QSV reflects temperature gradients in the ore-forming fluids. Cst-QWV exhibits significant Sc, V, and Ti enrichments, indicating elevated oxygen fugacity and fluid evolution. In situ Sn isotope analyses reveal core-to-rim decreases in δ 122/118 Sn values within individual cassiterite crystals, attributed to Rayleigh fractionation. The preferential incorporation of heavy Sn isotopes into cassiterite progressively lightens the residual fluid. Systematic decreasing δ 122/118 Sn values from Cst-G ( δ 122/118 Sn mean = − 0.13 ‰) to Cst-QSV ( δ 122/118 Sn mean = − 0.20 ‰) is consistent with the evolutionary trend. However, Cst-QWV exhibits significantly elevated δ 122/118 Sn values ( δ 122/118 Sn mean = +0.18 ‰), implying derivation from a distinct source. Integrated with whole-rock Sn isotope data, this study proposes that W-Sn separation at Fei’eshan was controlled by two ore-forming fluids with contrasting physicochemical properties: (1) a reduced, Sn-rich fluid generating greisen and quartz-sulfide-type ores, and (2) a related oxidized, W-rich fluid precipitating quartz vein-type wolframite ores.