Enrichment of cobalt at Baijian skarn Fe-Co deposit in the Handan-Xingtai region, North China Craton: Insights from mineral trace elements and pyrite sulfur isotopes

Cobalt (Co) in skarn deposits generally occurs as discrete Co minerals, or dispersed in sulfides. Recent studies have shown that magnetite from Baijian Fe-Co deposit (North China Craton) contains significant amounts of Co, but the mechanism for its enrichment was unclear. In this study, we present in situ mineral trace element and pyrite sulfur isotopic data from Baijian deposit (reserves of 112 Mt Fe at an average grade of 48.5 %): our results show that Co is mainly hosted in cobaltite, magnetite, and pyrite. Magnetite contains 47.6 ppm Co on average, accounting for most of the Co resource in the deposit. Elevated Co concentrations correspond to elevated levels of both Ti + V and Al + Mn, indicating the influence of elevated temperature. Pyrite grains from different lithologies in the deposit have variable Co contents, with those from the related diorite intrusion having the highest (averaging 9264 ppm Co). Sulfur isotopes of pyrite from the magnetite ores range from 16.6 to 19.4 ‰, similar to those from the Cixian Formation limestone and dolemite, which suggests that the sulfur was mainly derived from evaporites in those strata. Pyrite in magnetite-bearing skarn exhibits core-rim zoning, with the core having higher δ³⁴S_V-CDT values than the rim, which has values similar to those of pyrite in the diorite. A comparison of our results with other skarn Fe deposits shows that magnetite tends to have a higher Co concentration with increasing δ³⁴S_V-CDT values, suggesting that oxygen fugacity has an important influence on Co enrichment in magnetite.