Distribution and enrichment mechanism of rhenium in sediment-hosted stratiform copper deposits: A case study from the Tangdan deposit, SW China

Sediment-hosted stratiform copper (SSC) deposits are globally significant copper resources and contain critical metals such as Co, Re, and Ge. The Tangdan deposit, located in the Kangdian metallogenic belt within the western Yangtze block, is a prime example of SSC deposits in China. It comprises two types of ores: bedded ores with parallel bedding, and minor discordant vein-type high-grade ores. The sulfides in both types include chalcopyrite, bornite, chalcocite, tetrahedrite, digenite, and traces of galena and pyrite. Sulfides from vein-type ores show Re contents 10 to 100 times higher (0.01–1.67 ppm) than those in the bedded ores (0.06–0.27 ppm). However, the mechanism driving this disparity in Re content between ore types remains unclear. In the bedded ores, Re is preferentially partitioned into secondary digenite over primary sulfides such as chalcopyrite, bornite, chalcocite, and tetrahedrite. The relative Re enrichment in digenite is likely due to oxidization alteration, as Re is highly sensitive to oxygen fugacity. In contrast, Re in the vein-type ores predominantly associates with chalcocite, likely because of its high Mo content. Additionally, sulfides in vein-type ores typically have higher Zn, Mo, Sb, Re, and Pb concentrations than bedded ores. The δ³⁴S values of Cu-sulfides in bedded ores range from +1.7 ‰ to +11.5 ‰, whereas those in vein-type ores span from −13.3 ‰ to −4.8 ‰. The sulfur isotope geothermometer for coprecipitated chalcopyrite and bornite indicates that the average formation temperature for vein-type ores is 364 °C. Based on trace elements, sulfur isotope, and formation temperature of sulfides, we propose that bedded and vein-type ores have different formation mechanisms. Copper and other ore-forming elements were leached from the source bed by low-temperature (171 °C), Re-poor basin brine, while marine sulfate provided reducing sulfur, triggering redox reactions that formed bedded ores. In contrast, vein-type ores accumulated Re through the interaction between high-temperature hydrothermal fluids (364 °C) and Re-rich carbonaceous slate. The continuous dissolution of dolostone in high-temperature fluids increases pH and promotes the precipitation of sulfides, ultimately leading to the formation of Re-rich vein-type ores. This model for Re migration and enrichment at the Tangdan deposit could have broader applications for SSC deposits globally and serves as an exploration guide for critical metals in SCC deposits.