Variable enrichment of Co by deformation and hydrothermal processes in the Huangshandong Ni–Cu sulfide deposit, Eastern Tianshan

• The Huangshandong deposit underwent a complex post-magmatic history that modified the orebodies. • Three stages of Co enrichment are identified – 1) primary magmatic, 2) metamorphic with related deformation, and 3) hydrothermal remobilization. • The Co content of olivine and pyroxene can be effectively concentrated into base-metal sulfides and individual Co minerals through reworking processes, improving the economics of magmatic sulfide deposits. Magmatic sulfide deposits are the main global resource of Ni, Cu, and platinum-group elements (PGEs), with significant Co concentrations that enhance their economic importance for the low-carbon transition. Nickel, Cu, and PGEs are enriched via magmatic processes, while Co enrichment typically occurs due to post-magmatic activities. Despite the importance of these post-magmatic processes to the evolution of mineralized systems, their significance to the enrichment of Co remains ambiguous. The Huangshandong (HSD) Ni–Cu sulfide deposit is located in the Huangshan–Jing’erquan belt, Eastern Tianshan, and was modified by regional ductile shearing and later hydrothermal activity. Therefore, this deposit serves as an excellent case study to assess the role of reworking processes on the enrichment of Co in magmatic sulfide systems. Ores within the HSD deposit are classified into three mineralogically and texturally distinct varieties — primary magmatic, metamorphosed-deformed, and hydrothermally altered. Although Co is primarily hosted by pentlandite (18,315–24,776 ppm) and pyrrhotite (44–203 ppm) in all of the ore types, cobaltite–gersdorffite (9.27–26.54 wt% Co) is restricted to the hydrothermally altered ores where it occurs as disseminations (Cob–Gdf-I) and in calcite–chlorite veins (Cob–Gdf-II). The main contributors of Co in the primary magmatic and metamorphosed-deformed ores are pentlandite followed by pyrrhotite. Despite its low abundance, cobaltite–gersdorffite is the major Co contributor in hydrothermally altered ores. Three stages of Co enrichment are identified at HSD based on variations in silicate mineralogy and texture, and trace-element characteristics of base-metal sulfides (BMS) — primary enrichment of Co ( Stage 1 ), mobilization of Co by metamorphism–deformation ( Stage 2 ), and remobilization of Co by hydrothermal fluids ( Stage 3 ). Stage 1 is characterized by elevated concentrations of Co in magmatic pyrrhotite and pentlandite, as well as pyroxene and olivine. Stage 2 is characterized by the replacement of Co-bearing pyroxene and olivine by muskoxite, chlorite, and serpentine, which mobilized Co. Additionally, given that BMS were ductily deformed, it is possible that some Co was mobilized via deformation processes. Based on the FeAsS–CoAsS–NiAsS components of Cob–Gdf-I and Cob–Gdf-II, both formed at similar temperatures of ∼450–600 °C. The restriction of cobaltite–gersdorffite in Stage 3 mineralization implies that Co–Ni–As were remobilized. Cobalt and Ni were likely remobilized from Stage 2 and/or Stage 1 mineralization by hydrothermal fluids given the systematically lower Co and Ni contents of BMS in Stage 3 compared to Stage 2 . Arsenic was likely added to the system from an external source by the hydrothermal fluids given the typically low As contents of sulfide liquids. This contribution demonstrates that Co can be mobilized via deformation, metamorphism, and hydrothermal alteration processes, and highlights the importance of these processes to the overall enrichment of Co in magmatic sulfide systems, which are otherwise valued for their Ni, Cu, and PGE contents. Considering that cobaltite–gersdorffite may significantly contribute to the Co budget of a sulfide deposit, even at low abundances, attention should focus on assessing its occurrence in magmatic sulfide deposits, particularly those that have been reworked by secondary processes.