Cobalt mineralization in an evolving skarn system: Insights from co-bearing minerals in the Cihai Fe-Co deposit, NW China

Cobalt is a critical metal and is usually a by-product in many skarn Fe deposits. Understanding Co-bearing minerals and origin of Co enrichments in such deposits is essential for both efficient resource utilization and mineral exploration. The Cihai skarn Fe deposit in northwestern China is well known to contain considerable amounts of Co and is taken as an example to decipher this issue. In this deposit, Fe orebodies are associated with diabase-gabbro complexes and skarn alteration. Discrete Co minerals include cobaltite, siegenite, ferroskutterudite, and clinosafflorite, with siegenite being the most widespread. Cobalt is mainly enriched in pyrite, reaching concentrations of up to 1.4 wt%, and is slightly enriched in pyrrhotite (45–67 ppm) and is negligible in chalcopyrite. High-resolution atom probe tomography analysis identifies Co nanoclusters within Co-rich pyrite, formed through the heterogeneous substitution of Fe by Co. The enrichment of Co in the Cihai deposit is attributed to magmatic-hydrothermal processes. The Co-rich fluids are likely generated through leaching of mafic rocks by fluids. Moreover, the extensive crystallization of magnetite during hydrothermal evolution further contributes to Co enrichment. Temperature and oxygen fugacity play minor roles in this process. A three-stage Co mineralization model is proposed. In the prograde skarn stage, Co is incorporated into garnet and pyroxene. In the retrograde skarn stage, Co is enriched in fluids due to magnetite crystallization. In the sulfide stage, Co precipitates in pyrite and Co minerals. This study underscores the significance of fluid composition and evolution in controlling Co mineralization in the skarn system.