Micro-textural and chemical fingerprints of hydrothermal cobalt enrichment in the Jingchong Co-Cu polymetallic deposit, South China

Abstract

The Jiangnan Orogen, located at the margin of the southeastern Yangtze Block, has Co reserve more than 20,000 t, in which Co occurs mainly along with Cu deposits, with minor as independent Co deposits. As for Co enrichment in the Jiangnan Orogen, it remains unclear whether Co was enriched by the same mineralizing episodes as that precipitating Cu ores or from other processes. The Jingchong Co-Cu polymetallic deposit as a representative was chose to illuminate this issue, based on the integrated TIMA, EBSD, EPMA, LA-ICPMS and LA-MC-ICPMS analyses. The results show that cobaltiferous sulfides have complex textural and compositional zoning patterns. Four generations of pyrite (i.e., PyI, PyII, PyIII and PyIV) corresponding to four different ore stages (i.e., barren stage I, Cu-rich stage II, Co-rich stage III, and Pb-Zn-rich stage IV) were identified. The PyIII which consists of PyIII-1 (0.02–0.79 wt% Co and 0.16–3.08 wt% As) rimmed by PyIII-2 (0.19–13.66 wt% Co and 0.20–7.42 wt% As), arsenopyrite consisting of Apy-1 (up to 0.90 wt% Co and 39.70–42.06 wt% As) and Apy-1 (up to 0.60 wt% Co and 42.42–45.61 wt% As), and cobaltite-alloclasite (18.84–34.98 wt% Co) of stage III dominate the Co occurrence. These inhomogeneous features were explained by the fluid-mediated coupled dissolution-reprecipitation reactions (CDR), and the super-enrichment of the Co metal budget at the interface of PyIII-2 with PyIII-1 was caused by the non-equilibrium process at the advancing reaction front. In combination with the high Co and As contents and Co/Ni ratios (>1) at stage III, as well as the δ34S ranges (−4.47‰ to 3.57‰) similar to that of magmatic sulfur, Co enrichment in the Jingchong deposit could be ascribed to the interaction of a Co– and As-rich magmatic-hydrothermal fluid pulse with wall rock (including the early ores), later than Cu mineralization. The decreasing temperature together with increasing sulfur and oxygen fugacity serve as the critical factors controlling Co precipitation from solutions. This study provides a detailed mineralogical insight into Co enriching process in magmatic-hydrothermal Cu deposits. Finally, a significant mineral indicator (i.e., the coexistence of fine-grained pyrite and arsenopyrite) for Co ore exploration and one more suggestion (i.e., crushing sulfides ores into fine pieces, smaller than 15–20 μm) for efficient cobalt recovery were proposed.