Unveil the Redox Evolution of Ore‐forming Fluids using Sulfur Isotope: A Case Study of the Zhengguang Intermediate Sulfidation Epithermal Au‐Zn Deposit, NE China

Oxygen fugacity (fO2) is a key intensity variable during the entire magmatic‐hydrothermal mineralization courses. The redox state and its variations between different stages of the ore‐forming fluids of intermediate sulfidation epithermal deposits are rarely deciphered due to the lack of appropriate approaches to determine fO2 of the fluids. Here, we reported the δ34S of the sulfides from three different stages (stage I, II, III) of Zhengguang, an Early Ordovician Au‐rich intermediate sulfidation (IS) epithermal deposit, to decipher the redox evolution of the ore‐forming fluids. The increasing δ34S values from stage I pyrite (py1, average –2.6‰) through py2 (average –1.9‰) to py3 (average –0.2‰) indicates a decrease of the oxygen fugacity of the ore‐forming fluids. A compilation of δ34S values of sulfides from two subtypes of IS deposits (Au‐rich and Ag‐rich) from NE China shows that the δ34S values of sulfides from Au‐rich IS deposits are systematically lighter than those of Ag‐rich IS Ag‐Pb‐Zn deposit, indicating the ore‐forming fluids of the former are more oxidized than the latter. We highlight that sulfur isotopic composition of hypogene sulfides is an efficacious proxy to fingerprint the oxygen fugacity fluctuations of epithermal deposits and could potentially be used to distinguish the subtypes of IS deposits.