Polymetallic veins—a possible bridge between porphyry and intermediate-sulfidation deposits: A case study from Xiongcun ore district, Tibet, China

Understanding the poorly constrained transition between porphyry and intermediate-sulfidation epithermal mineralizations is critical for deciphering the genetic link and fluid evolution in porphyry systems. In this study, we investigate late-stage, post-porphyry polymetallic (Zn-Ag-Au-Cu ± Pb) veins from the No. 1 porphyry Cu-Au deposit in the Xiongcun ore district, Gangdese belt, Tibet, China, which represents “transitional” mineralization bridging porphyry and intermediate-sulfidation epithermal systems. Here we combine detailed petrography, in situ trace element geochemistry of sulfides and quartz, and in situ secondary ion mass spectrometry (SIMS) oxygen isotope analysis of vein quartz to constrain the origin of the polymetallic veins. The veins are paragenetically divided in to three stages: the first stage (stage A) comprises pyrite + sericite, the second stage (stage B) is characterized by sphalerite + pyrrhotite + chalcopyrite + galena + quartz, while the third stage (stage C) consists of actinolite + epidote + chlorite + magnetite + quartz. The precipitation of Zn, Ag, Au, Cu, and Pb mainly occurred during stage B. Silver is mainly incorporated in chalcopyrite but also subordinately in acanthite-argentite, pyrargyrite, or hessite, while electrum inclusions, the only Au-bearing phase, are commonly hosted by chalcopyrite and sphalerite. Mineral geochemical compositions and associated mineral assemblages suggest that the ore-forming fluid displays decreasing temperatures (∼317 °C) as well as decreasing oxygen and sulfur fugacities (fO2 and fS2) from stage A to B, thereby triggering the precipitation of sulfides rich in Zn, Ag, Au, Cu, and Pb. The late-stage fluid underwent a further temperature and fS2 decrease and a pH increase during the subsequent stage C. In situ trace element compositions of vein ore minerals and δ18O values (−4.2‰ to +4.8‰) of the ore-forming fluid, calculated by in situ analysis of quartz from the polymetallic veins, suggest that the decrease in temperature, log fO2, and log fS2 and the increase in pH of the ore-forming fluid are due to the mixing of metal-rich magmatic fluid separated from the magma and relatively reduced alkaline groundwater. This mixing process is also thought to be responsible for the precipitation of sulfides rich in Cu, Au, and Ag along fractures and filling open space. The metal-rich magmatic fluid may have lost some metals (e.g., Cu) during the deep-seated interaction with alkaline groundwater prior to reaching an epithermal environment, thereby prohibiting the formation of high-grade Cu intermediate-sulfidation epithermal deposits. This study provides new insight into the transitional evolution of porphyry and polymetallic mineralizations, thereby improving our understanding of the genetic relationship of porphyry and epithermal mineralizations in porphyry systems and providing a guide for mineral exploration.