Cobalt mineralization in the Northeastern Hunan Province of South China: New evidence from the Jintang hydrothermal Co polymetallic ore district

The hydrothermal vein-hosted Co deposits as one of the significant deposit types, have economic importance in the world. The Jiangnan Orogen of South China hosts abundant hydrothermal Co deposits, of which several Co (-polymetallic) deposits cluster together along the Changsha-Pingjiang fault zone in the northeastern Hunan Province. However, the variations of cobaltiferous mineral assemblages exhibit along the Changsha-Pingjiang fault zone. The Jintang ore district as the new prospecting target could provide an insight into the understanding of Co mineralization in the northeastern Hunan Province. The TIMA and EPMA results identify pyrite, arsenopyrite and minor cobaltite and glaucodot as the main cobaltiferous sulfide phases in the Jintang district, of which Co and Ni occur via substituting Fe in pyrite and arsenopyrite. In contrast with the barren pyrite (PyI) associated with marcasite in the early stage, the PyII of the middle stage is featured by the successive precipitation of Co-rich PyII-a (Avg. 2.93 wt%, up to 5.05 wt% Co), Ni-rich PyII-b (Avg. 1.56 wt%, up to 3.84 wt% Ni) and As-rich PyII-c (Avg. 1.36 wt%, up to 4.10 wt% As). The sharp chemical gradients across the different pyrite species as well as irregular reaction fronts indicate fluid-mediated dissolution and reprecipitation are principal mechanism for the incorporation of Co, Ni and As into sulfides. The fractionation of Co and Ni in pyrite was possibly caused by the decreasing salinity which favors high Ni/Co ratios in pyrite, rather than different fluid origins. Successively, the As-rich PyII-c together with arsenopyrite and glaucodot were precipitated, indicating a progressive increase of As concentrations in fluids and decreasing fO2 condition. The sulfur isotopic analyses yield the delta 34SV-CDT values of -8.0 to -4.9 parts per thousand for the Jintang sulfides, suggesting both magma and wall rock as potential sulfur reservoirs. In combination with S and Pb isotopic results in this area, the Co mineralization in the northeastern Hunan Province was most likely formed via fluid-rock interactions along cataclastic fault zones with metal leached from the crystalline basement. The migration of ore-forming fluids along the Changsha-Pingjiang fault zone potentially contributed to the development of contrasting cobaltiferous mineral assemblages in the region, but the changes in local physicochemical conditions generated the partitioning of elements within a single deposit.