Redox-driven tellurium isotopic fractionation in magmatic-hydrothermal system: Implications from the Dashuigou Te-Bi deposit, SW China

Abstract

To date, research on Te isotopic behavior within ores from independent tellurium deposits has yet to be conducted. To address this shortcoming, we measured δ130/125TeNIST SRM 3156 values of tetradymite and tsumoite from the Dashuigou Te-Bi deposit, one of only two known independent tellurium deposits worldwide, to resolve the fractionation direction and degree of Te isotopes caused by geological processes with an additional investigation of Fe-Cu isotopes and XPS (X-ray Photoelectron Spectroscopy). The observed Te isotopic variations (tetradymite: 0.61‰ to 2.14‰; tsumoite: 2.38‰ to 4.05‰) are primarily attributed to isotopic fractionation driven by progressive reduction of tellurium during precipitation from oxidized aqueous species in the ore-forming fluid. This interpretation is supported by: (1) previous studies have demonstrated that redox transformations exert a primary control on Te isotopic fractionation, with the reduction of dissolved Te(IV) to Te(II) or Te(0) during mineral formation preferentially incorporating light isotopes into the reduced solid phases; (2) systematic correlations among Te-Fe-Cu isotopic compositions and Te-Bi concentrations for chalcopyrite and tsumoite, coupled with XPS valence state analyses, revealing a redox process involving the reduction of copper and tellurium and oxidation of iron; (3) progressively increasing δ130/125Te values from early-formed tetradymite to later-formed tsumoite, further supporting progressive reduction during ore formation via a Rayleigh distillation model. This model implies the possibility of significant tellurium at depth. Collectively, Te isotopes serve as a potential tracer of redox changes in magmatic-hydrothermal systems, and may develop into a useful tool for tracing metal sources, monitoring fluid evolution and ore-forming processes, and guiding ore exploration.