Nano scale study of cobaltite in pyrite: insights and significance

Abstract The cobalt in pyrite from skarn deposits often exhibits strong heterogeneous distribution at the micrometer scale, but its genetic mechanism is still unclear. In this study, we investigate pyrite form the Longqiao cobalt bearing skarn iron deposit in the Middle-Lower Yangtze River Metallogenic Belt in eastern China. By comprehensively utilizing multi-scale analysis methods such as LA-ICP-MS, EPMA, EBSD, FIB-SEM, and STEM, the distribution characteristics and occurrence states of cobalt in pyrite were systematically revealed. The results indicate that Co and As are enriched in a concentric ring shape in the core of pyrite, essentially in the form of nanoscale cobaltite, which has the same crystal band axis [2-1-1] as the pyrite matrix. EBSD analysis reveals the absence of significant plastic deformation in pyrite, indicating that cobalt migration is dominated by diffusion rather than dislocation pipe diffusion. The fluid-induced solid-state diffusion may drive Co and As redistribution toward the pyrite-fluid interface, producing local supersaturation with respect to CoAsS and thereby promoting the nanoscale nucleation and growth of cobaltite. The formation of these nano phases consumes cobalt from adjacent pyrite and fluids, leaving domains with locally lower- Co abundances and resulting in the observed micrometer-scale heterogeneous distribution of cobalt. This study emphasizes that whether cobalt enrichment occurs in pyrite depends not only on the composition of the initial magmatic-hydrothermal fluid, but also on the potential for diffusion-controlled reorganization of cobalt within pyrite itself.