Core-rim zoning of pyrite as a record of redox-controlled gold enrichment in South Qinling deposits, China

Unraveling the enrichment mechanism of gold (Au) within arsenian pyrite rims is critical for understanding the genesis of refractory gold deposits; however, the isotopic signatures of the underlying physicochemical processes remain poorly constrained. In this study, two representative gold deposits from the Qinling metallogenic belt were investigated using in situ laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) trace-element mapping and micro-scale LA-multicollector-ICP-MS S-Fe isotope analyses. The results reveal that Au is preferentially concentrated within the arsenian pyrite rims, which spatially coincide with pronounced negative excursions in both δ34S and δ56Fe values relative to the pyrite cores. Paragenetic relationships indicate that arsenopyrite crystallized during the transitional stage between core and rim formation. A comparison of the S-Fe isotopic compositions of this transitional arsenopyrite with those of the core-rim zoned pyrite reveals that the negative shift in δ34S likely records an evolution in the sulfur source of the ore-forming fluids, whereas the synchronous decrease in δ56Fe provides key isotopic evidence for an increase in oxygen fugacity (fO2). Based on these observations, we propose a genetic model in which Au precipitation from initially reducing hydrothermal fluids was triggered by a localized oxidation event, highlighting the critical role of fO2 fluctuations in controlling invisible gold enrichment and high-grade mineralization in arsenian pyrite-hosted gold deposits. Overall, this study demonstrates that pairing Fe-S isotopes at the micro-scale provides a diagnostic tool to decode the role of redox fluctuations in controlling high-grade mineralization in sediment-hosted gold systems.