Objective Qixin intrusion is the second largest mafic−ultramafic intrusion in Beishan block, which is located in the southern of the Central Asian orogenic belts, the exposed surface area is about 150 km². It mainly comprises lherzolite, olivine gabbronorite, olivine gabbro and gabbro. The cobalt−rich lithofacies is olivine gabbronorite. The grade of Co in the mottled ore is 0.02%~0.09%, with an average of 0.03%, and the grade of Ni is 0.23%~0.54%.

Methods This study employs microscopic observations, TIMA scans, in−situ composition analysis and in−situ sulfur isotope of typical ore samples to delineate the occurrence and enrichment process of Co.

Results The ore contains pyrrhotite, pentlandite, and chalcopyrite, but lacks independent cobalt minerals. Sulfides exhibit a decreasing cobalt content in the following order: pentlandite, pyrrhotite and chalcopyrite, the Co content of the first generation pentlandite (Pn¹) (4.45%~10.31%) is higher than that of the second generation pentlandite (Pn²) (2.26%~3.89%). Co mainly exists in pentlandite, substituting for Ni²⁺ and Fe²⁺ through isomorphism. The significant enrichment of Co in pentlandite may be mainly controlled by the magma source and the separation and crystallization of sulfides. The in−situ sulfur isotope δ³⁴S values of pyrrhotite and pentlandite are between 0.9‰ and 2.2‰, with an average of 1.6‰, indicating that there is no significant addition of shell−derived S during the mineralization. The assimilation and contamination of silicon−rich components or the separation and crystallization of magnesian minerals play a key role in the melting of sulfides.

Conclusions Owing to its considerable size, notable geochemical anomalies in Ni, Co, and Cr, significant mineralization in olivine gabbronorite, the Qixin intrusion exhibits substantial prospecting potential.