Post-diagenetic hydrothermal events control Co enrichment in sediment-hosted Cu-Co deposits: Evidence from the Shizishan Cu-Co deposit, southwestern China

The role of post-diagenetic hydrothermal processes in cobalt (Co) enrichment in sediment-hosted stratiform Cu-Co (SSC) deposits remains controversial, particularly regarding whether high-grade Co mineralization is primarily diagenetic or requires later tectonic overprinting. The Shizishan deposit, SW China, exhibits a well-defined vertical Co zoning and coexisting disseminated and veinlet sulfide assemblages, making it an ideal example to address this issue. Here we integrate in-situ sulfide trace element geochemistry, Fe–S isotopes, and rutile U–Pb dating to distinguish two mineralization events. Early disseminated chalcopyrite, cobaltite, and pyrite formed during diagenesis, with Co distribution controlled by organic matter in the host rocks. The later post-diagenetic stage formed veinlet chalcopyrite, bornite, cobaltite and cobaltian pyrite cross-cutting early-stage mineralization. Veinlet chalcopyrite shows systematic depletion of Co, Ni and As relative to disseminated chalcopyrite, while cobaltite and pyrite show opposite trends, indicating remobilization and recrystallization of pre-existing sulfides. Chalcopyrite from Co-rich layers exhibits lower δ 5 ⁶Fe values (−0.53‰ to −0.17‰) than Co-depleted layers (−0.31‰ to −0.02‰), suggesting more reduced fluids. Hydrothermal rutile associated with veinlet ores yields a U–Pb lower intercept age of 1449 ± 33 Ma, defining a post-diagenetic tectonic-hydrothermal event. This overprinting event significantly upgraded Co grades, particularly in dolostone enriched in organic matter. Our results demonstrate that while the diagenetic stage provided primary metals and traps, it is the later orogenic-related hydrothermal modification that was essential for the formation of economic Co concentrations, and a reduced setting is a key factor controlling Co precipitation.