The occurrence of cobaltite nanoparticles in pyrite from the De’erni deposit, NW China

• Cobaltite was discovered within pyrite at the De’erni deposit for the first time. • The lattice defects provided sites for the nucleation of cobaltite within pyrite. • Cobaltite NPs contributed to the uneven distribution of Co in pyrite. Cobalt (Co) is a critical metal that occurs in many types of deposits, Co minerals and sulfide hosts are the main forms of Co occurrence. Pyrite is the most important cobalt-bearing mineral in the De’erni Cu-Zn-Co ultramafic-hosted volcanogenic massive sulfide deposits. However, the occurrence and enrichment of Co in pyrite remain unclear. In this study, a combination of LA-ICP-MS and STEM techniques was employed to conduct a detailed mineralogical investigation of pyrite in the De’erni deposit. The results revealed significant variations in cobalt content among pyrite samples from different mineral assemblages. Pyrite associated with magnetite (Mag), pyrrhotite (Po), chalcopyrite (Ccp), arsenopyrite (Apy), and bornite (Bn) (Py-Mag-Po-Ccp-Apy-Bn suite of mineral assemblages) exhibited the highest cobalt content, which ranged from 672.6 ppm to 2007 ppm. Cobalt occurs in two forms in the pyrite from the De’erni deposit: as cobaltite nanoparticles (NPs) and as a substitute for iron (Fe) in the pyrite lattice. The enrichment mechanism of cobalt in pyrite was explored at the deposit and mineral scales. The results indicate that a decrease in ore-forming fluid temperature and an increase in cobalt content may be significant factors contributing to cobalt enrichment at the deposit scale. Lattice defects may play a crucial role in cobalt enrichment within the pyrite lattice. Furthermore, the discovery of cobaltite NPs in pyrite could provide new insights for explaining the complex zonation of the cobalt element in pyrite.