Redox-controlled antimony isotope fractionation in the epithermal system: New insights from a multiple metal stable isotopic combination study of the Zhaxikang Sb–Pb–Zn–Ag deposit in Southern Tibet

Abstract

Redox reactions, biological processes, evaporation and precipitation processes, adsorption, and Rayleigh distillation mechanisms have been proposed to account for the fractionation of antimony (Sb) isotopes in natural materials. However, only one paper was published to support and characterize the Rayleigh distillation induced Sb isotopic fractionation in a hydrothermal system. Therefore, the Sb isotopic fractionation mechanism research in the epithermal system is still negligible, which severely restricts future application. The Zhaxikang Sb–Pb–Zn–Ag deposit is the only super-large deposit within the North Himalayan Metallogenic Belt (NHMB), which comprises two episodes of mineralization (First episode: Pb–Zn mineralization, Stages 1 and 2; Second episode: Sb mineralization, Stages 3 to 6). Herein, we measured the δ123SbNIST 3102A values of stage 4 sulfosalt minerals and stage 5 stibnite from the Zhaxikang Sb–Pb–Zn–Ag deposit, to resolve the fractionation direction and relative magnitude of Sb isotopes caused by these aforesaid geologically prominent mechanisms in the epithermal systems with an auxiliary investigation of Fe–Cu isotopes. The second episode of Sb-containing hydrothermal fluid has leached metals (e.g., Pb, Ag, Fe, Cu) from pre-existing Pb–Zn orebodies, to initiate the formation of stage 4 sulfosalt minerals. According to XPS (X-ray Photoelectron Spectroscopy) analyses, the Cu isotopic variation of stage 4 bournonite (0.08‰ – 0.89‰) must be controlled primarily by the reduction reaction (Cu2+ + e− → Cu+) during leaching and precipitation. Keeping in mind a good linear correlation between δ123Sb and δ65Cu values (R2 = 0.99) for bournonite, the same mechanism was used to decipher the variation in δ123Sb values (0.10‰ – 0.48‰) of bournonite (Sb5+ + 2e− → Sb3+). Abundant native sulfur within the orefield allowed for the overall reaction equation of CuFeS2 + FeS2 + PbS + Sb5+(aq) = CuPbSbS3 + Fe2+(aq) + 2S to describe the bournonite formation. Accordingly, the Sb isotopic variations (−0.27‰ to 0.48‰) of Sb-bearing minerals (boulangerite, bournonite and stibnite) were empirically attributed to redox reactions. The Rayleigh distillation model indicated that the observed fractionation was most likely induced by redox reactions, while the related theoretical calculations revealed a great antimony exploration potential in deeper parts of the orefield. Overall, the Sb isotopes could be used to identify redox changes in the epithermal system, and exhibit great potential in tracing metal sources, monitoring fluid evolution and ore formation, and providing insights into mineral exploration.