Zinc, cadmium and sulphur isotopic compositions reveal biological activity during formation of a volcanic-hosted massive sulphide deposit

Zinc and Cd isotopes fractionate strongly within a range of hydrothermal systems; consequently, metal stable isotopes have been used to understand the formation of hydrothermal ore deposits. We investigated the Zn-Cd-S isotopic compositions of sulphides from the Carboniferous-Permian Xiaobaliang Cu-Au deposit, Inner Mongolia, China. This ancient seafloor hydrothermal system is the only known Cyprus-type volcanic-hosted massive sulphide (VHMS) deposit in the Central Asian Orogenic Belt. The S isotopic compositions of sulphide minerals are highly variable with 834SCDT values ranging from -21.2%0 to 25.2%0. We infer that negative 834SCDT values reflect mixing of volcanogenic S and S derived from bacterial sulphate reduction (BSR), and that positive 834SCDT values reflect mixing of volcanogenic S with S derived from thermochemical sulphate reduction (TSR). Zinc isotopic compositions are relatively consistent at the deposit; 866Zn values range from 0.10%0 to 0.70%0. These values are similar to those reported for modern seafloor sulphides. We infer that consistency of 866Zn values relates to the Zn species ([ZnCl4]2-) present in the ore-forming fluids during sulphide precipitation. All of the sulphides are enriched in the light Cd isotopes; 8114/110Cd values range from -0.74%0 to -0.08%0. Zinc and Cd can be affected by biological activity, and we infer that biological activity caused the negative 8114/110Cd values at this deposit because: (1) biological processes (e.g., bacterial activity) enrich fluids in the light Cd isotopes, leading to the precipitation of sulphides with negative 8114/110Cd values; and (2) the Cd isotopic composition of ore-forming fluid is readily modified because the concentration of Cd is much lower than that of Zn in ore and basalt. Our results indicate that Cd isotopes might record ancient biological activity, and that organisms might play a more important role than previously thought in the formation of VHMS systems. (c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.