Cobalt distribution and enrichment in skarn iron deposits: A case study of the Zhuchong skarn iron deposit, Eastern China

Global Cobalt (Co) is mainly mined from sediment-hosted, magmatic sulfide, or laterite deposits, but rarely in skarns. The Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB), eastern China, is an important skarn iron ore producer, where skarn iron deposits are often associated with Co. The Zhuchong iron deposit is a typical high-grade skarn iron deposit with Co enrichment in this belt. This deposit contains 50 Mt Fe ore @50.1 %, 4.3 Mt Cu ore @1.15 %, and similar to 10000 t Co metal with an average grade of 0.019 %. In this study, the distribution and enrichment of Co at Zhuchong were investigated by detailed petrographic observations, mineral geochemistry, and whole-rock major and trace element data. Our results indicate that Co contents in the ore and gangue minerals at Zhuchong follow these trend: pyrite (median = 2138 ppm) > pyrrhotite (705 ppm) > sphalerite (352 ppm) > magnetite (72.1 ppm) > diopside (42.5 ppm) > chalcopyrite (3.11 ppm) > garnet (1.23 ppm). The negative correlation between Co + Ni and Fe in pyrite suggests that the incorporation of Co into pyrite can be achieved by the substitution mechanism of 2Fe(2+) <-> Co2+ + Ni2+. Depth profile analysis reveals that the Co-enriched pyrite is mainly developed in the contact zone between the diorite pluton and the skarn iron ore body. Pyrite far from the pluton has lowered Co contents ranging from several to hundreds ppm. This study highlights that the enrichment of Co in pyrite at Zhuchong is probably temperature-dependent. Co/Ni ratios of pyrite range from 0.01 to 10000, and where pyrrhotite occurs the ratios increase. Additionally, Our results establish pyrite as the primary carrier of Co at the Zhuchong deposit, contributing approximately 5000 tons of Co. Pyrrhotite and magnetite account for about a quarter proportion of total Co reserves, respectively. Our study emphasizes that finding out the Co-rich location and its occurrence nature of the deposit is of great significance for the recovery and utilization of Co in skarn iron deposits.