In situ elemental and sulfur isotopic variations of Au-bearing iron–sulfides from the Alin Au deposit in the Shuixie Cu–Co Orefield, western Yunnan Province, SE Tibet: Insights into enrichment processes of invisible Au

The Shuixie Cu–Co ore field, located in the southwestern part of the Lanping Meso-Cenozoic Basin, SE Tibetan Plateau, is a well-known structure-controlling polymetallic ore concentration area. Recently, several independent and associated Au-rich orebodies have been discovered, most of which are hosted in iron–sulfides (arsenopyrite and pyrite) in the form of invisible Au. The mechanism of Au enrichment remains unclear. Based on typical orebodies in the Alin Au deposit, this study presents fine microtextural features and a dataset of in situ elemental and sulfur isotopic variations in iron–sulfides. Iron–sulfides are characterized by alternately precipitated pyrite and arsenopyrite with core- and rim-crystalline textures. Which can be divided into four generations of pyrite (PyI₁–PyI₄) and arsenopyrite (ApyI₁–ApyI₄), corresponding to precipitation pulses 1–4, respectively. The earliest crystallized iron–sulfides had low concentrations of Au, which considerably increased starting with pulse 2. The Au contents in pulses 2 and 3 were distinctly lower than those in pulse 4. Au was primarily incorporated into the iron sulfide lattice structure in the form of (Au⁺). The iron–sulfides have a narrow range and homogeneous δ³⁴S values, suggesting a magmatic origin. We propose the following key suggestions: the lower the deviation from the stoichiometric level, the higher the Au enrichment in iron–sulfides; high concentrations of invisible Au exist coeval with the precipitating iron–sulfides; and Au mineralization originates from episodic influxes of magmatic-hydrothermal fluids. These insights provide a novel perspective on Cu–Co and associated Au mineralization in the Shuixie Cu–Co ore field and similar regions.