Study on the Occurrence State and Enrichment Process of Cobalt in Jinchuan Giant Magmatic Ni−Cu Sulfide Deposit

As the largest Ni–Co (PGE) magmatic sulfide deposit in China, the Jinchuan is located in the eastern segment of Longshoushan terrane. The most important sulfide assemblages are pyrrhotite, pentlandite, and chalcopyrite. A small account of independent cobalt minerals, such as cobaltite occurrences mainly in the disseminated ores. The whole rock analytical results show a good positive relationship between Co and S, Ni, but have no correlation with As. The Co/Ni ratio decrease with the increase of sulfide content. Based on the EMPA data of sulfide minerals. The cobalt content of pentlandite arranges from 0.32 to 1.93% and the average value is 0.81%, which is much bigger than the cobalt content in pyrrhotite (0.02%～0.11%) and chalcopyrite (0.01%～0.08%). The EPMA map of chalcophile elements also shows that the area of high cobalt content is completely consistent with the pentlandite, indicating that cobalt mainly occurs in the pentlandite. The average Co/Ni ratio of the Jinchuan deposit is 0.042, which is almost equal to the typical peridotite mantle (0.055), implying that the primitive magma of Jinchuan derived from the peridotite mantle. Compared to the typical basalt, the Co/Ni ratio of the Jinchuan deposit is much lower, and litter lower than the picrite, which indicates that a high degree of partial melting occurred in the mantle source. During the sulfide segregation, the cobalt mainly migrates to the sulfide melt, but the cobalt content is less than nickel content, causing the negative correlation between the S and Co/Ni. The fractional crystallization of sulfide melt further promotes the enrichment of cobalt into pentlandite.