Multiple Au enrichment processes in the Early Cretaceous Heilongtan–Xiejiagou deposit, Tongbai Orogen, Central China

The Tongbai Orogen is an important Au province in central China, but the processes responsible for localized high-grade Au enrichment in its southern segment remain poorly constrained. Here, we investigate the Heilongtan–Xiejiagou Au deposit using ore geology, hand-specimen-scale element mapping, ore-stage geochronology, and in-situ trace-element and S–Pb isotope analyses of sulfides. The deposit records three stages of mineralization: Stage I quartz–sericite–pyrite alteration, Stage II quartz–pyrite vein, and Stage III quartz–polymetallic sulfide vein. Ore-stage geochronology based on Rb-Sr and U-Pb isotopic systems of Stage I sericite and monazite, and Stage III muscovite constrains the main Au–Ag mineralization to ca. 140–134 Ma. The S–Pb isotope data, together with the Early Cretaceous ages and spatial association with granitoid magmatism, indicate a major magmatic–hydrothermal contribution, with variable input from Neoproterozoic host rocks. Pyrite records a systematic evolution from As-rich invisible-Au-bearing Py-I, through porous and inclusion-rich Py-II associated with Au redistribution and polymetallic enrichment, to trace-element-poor Py-III associated with late polymetallic sulfides. These features indicate that Stage II was the main stage of Au remobilization and local reconcentration. During this stage, coupled dissolution–reprecipitation of earlier auriferous pyrite, together with additional Au input from the infiltrating fluid, generated porous reaction domains that promoted Au release, redistribution, and electrum precipitation. These results link the deposit to Early Cretaceous magmatic–hydrothermal activity and suggest that exploration in the southern Tongbai Orogen should focus on reactivated vein systems capable of localized high-grade upgrading. Overall, this study provides new insight into fluid-mediated remobilization and local reconcentration of pyrite-hosted Au in high-grade lode gold systems.