Occurrence and enrichment of cobalt and nickel in the Yindongshan ultramafic-mafic intrusion-hosted iron deposit, western Hubei Province, China

The Yindongshan iron deposit in western Hubei Province of China is hosted within Ordovician ultramafic-mafic intrusions. Recent field geological surveys have detected cobalt (Co) and nickel (Ni) enrichment within the Yindongshan clinopyroxenite, suggesting good potential for subeconomic Co-Ni mineralization. This study investigated the occurrence modes and enrichment processes of Co and Ni in the Yindongshan deposit through field geology and in situ analysis of the rock texture, geochronology, and geochemistry of sulfides, Fe-Ti oxides, and silicate minerals. The Yindongshan clinopyroxenites were emplaced at 440.2 +/- 7.2 Ma via titanite LA-ICPMS U-Pb dating, and underwent extensive post-magmatic metamorphism at 401.2 +/- 8.6 Ma via apatite U-Pb dating. This metamorphism led to the widespread formation of amphibole via the replacement of clinopyroxene. Subsequently, hydrothermal epidote-albite veins, calcite-pyrite veins, and later actinolite veins developed. Pyrites from both clinopyroxenites and calcite veins show limited delta 34S variations from -5.4 %o to -1.2 %o, suggesting a magmatic sulfur source. Late-vein actinolites display geochemical characteristics distinct from those of amphiboles, indicating possible involvement of external fluids. LA-ICP-MS trace element results reveal a general increase in Co and Ni contents from silicate minerals and Fe-Ti oxides to pyrite and from earlier-crystallized coarse-grained clinopyroxenite to later medium- to fine-grained clinopyroxenite. The highest Co and Ni contents observed in pyrite from coarse-grained clinopyroxenite suggest their preferential incorporation into early sulfide minerals and then progressively depleted through magmatic evolution such as fractional crystallization. The positive correlation between Co and Ni with Fe in pyrite, along with the consistent parallel time-resolved LA-ICP-MS depth profiles among different mineral phases, indicates that isomorphic substitution occurred in pyrite. In silicate minerals, the Co and Ni contents increase from pyroxene, amphibole, to actinolite, with almost no Co or Ni present in epidote. The elevated Co and Ni contents in metamorphic amphiboles are attributed to their remobilization from magmatic pyrite to newly formed amphiboles. During the late hydrothermal phase, external fluids might transport additional Co and Ni, contributing to higher concentrations in late actinolite veins. In general, Co and Ni in the Yindongshan iron deposit are primarily hosted in sulfide minerals and can be remobilized during regional metamorphism and metasomatism. Thus, magmatic iron or iron-titanium oxide deposits hosted by mafic-ultramafic intrusions with sulfide mineralization may serve as promising targets for further Co-Ni exploration.