Occurrence and Enrichment Mechanism of Cobalt in Cu-Ni-Fe-Ti Paragenetic Deposits: A Case Study of the Xiangshanxi Deposit, East Tianshan

The study of the occurrence state and enrichment mechanism of cobalt in Cu-Ni-Fe-Ti paragenetic deposits is critical for precise ore blending and comprehensive utilization of associated cobalt resources. This work investigates the Xiangshanxi Cu-Ni-Fe-Ti deposit (eastern Tianshan) using petrogeochemistry, petrographic mineralogy and element-partitioning calculations to decipher cobalt’s temporal–spatial distribution and partitioning behavior at the rock–mineral scale. The Xiangshanxi intrusion comprises peridotite, Ti-Fe gabbro and gabbro rock types. Cobalt peaks in peridotite rock type (113–146 ppm) followed by Ti-Fe gabbro rock type (67–115 ppm). Mineralogical analyses reveal the highest Co contents in independent minerals (cobaltite, siegenite, gersdorffite) and then isomorphous cobalt-rich pentlandite. Pyrite and pyrrhotite contain <1 wt.% Co, whereas silicate and Fe-Ti oxide minerals have low Co content. Cobalt occurs as independent minerals, isomorphous substitution (replacing Fe, Ni in sulfide lattices) and micro-inclusions. Pentlandite preferentially crystallizes from monosulfide solid solution (MSS). Cobaltite–siegenite–gersdorffite formed in late-magmatic–hydrothermal overprinting. Sulfide melt liquation–exsolution drives initial Co pre-enrichment, and later hydrothermal overprinting triggers Co mineral precipitation. Combined processes control Co enrichment; an evolutionary model for Co enrichment is proposed for the Xiangshanxi deposit.