Anomalous cobalt enrichment in pyrite from an altered mafic dike in the Linglong gold deposit, Jiaodong Peninsula, China: Mechanisms and implications

Traditionally, gold deposits in the Jiaodong gold province of China have been regarded as gold-only deposits lacking economic base metals. However, recent studies found some cobalt (Co) enrichment in those deposits. The mechanisms of this Co enrichment remain poorly understood. In this study, we identified a type of pyrite highly enriched in Co (concentrations up to 32800 ppm) at the Linglong gold deposit, Jiaodong gold province. The pyrite was collected from a hydrothermally altered mafic dike contacting with gold ore veins. To decipher the enrichment mechanisms of Co, in-situ textural, elemental and sulfur isotopic analyses were conducted on the pyrite. The pyrite typically exhibits core-mantle-rim textures. The cores (Py-m1) have high Co concentrations (up to 5950 ppm) with homogeneous texture and elemental distribution, which could be the primary pyrite formed by pyritization of the mafic dike. The mantles (Py-m2) have the highest Co concentrations (up to 32800 ppm) and contain abundant mineral inclusions (chalcopyrite, galena, and sphalerite) and pores, showing the formation through coupled dissolution-reprecipitation (CDR) reactions. The rims (Py-m3) have low concentrations of Co (0.14–559 ppm) and show cubic shapes, forming directly from hydrothermal fluids. The δ34S values increase from the cores (4.2 ‰ to 9.1 ‰, mean of 8.0 ‰) to the mantles (8.9 ‰ to 11.9 ‰), but vary between the above two in the rims (4.2 ‰ to 12.2 ‰, mean of 9.0 ‰). The sulfur isotopic compositions of the Py-m are similar to those of pyrite (4.8 ‰ to 9.5 ‰) from the neighboring auriferous quartz-sulfide veins at Linglong in previous study. Combined with the geological and textural features, it is inferred that hydrothermal fluids during gold mineralization extracted abundant Co from the mafic dikes, leading to the primary enrichment of Co in the Py-m1. The Py-m1 was dissolved and re-precipitated during subsequent hydrothermal activities, forming Py-m2 with enhanced re-enrichment of Co. Basically, the Co-rich mafic dike provided the initial enrichment of Co while the gold-bearing hydrothermal fluids contributed to concentrate Co further. Considering the widespread mafic rocks and extensive hydrothermal activities during gold mineralization throughout the Jiaodong gold province, there might be a considerable Co resource formed through the above processes.