Formation of Ni-Co-polymetallic sulfarsenide-sulfide mineralization in the Longhua Ni-Co hydrothermal vein deposit, Dayaoshan area, South China: Constraints from in-situ apatite U-Pb geochronology, sulfide S-Pb isotopes and trace element compositions

The Longhua Ni-Co deposit, situated in the Dayaoshan area of South China, is characterized by Ni-Co arsenide and Ni-Co-polymetallic sulfarsenide-sulfide mineralization primarily hosted within quartz veins. While previous studies have focused on the origin for the Ni-Co arsenide mineralization, the Ni-Co-polymetallic sulfarsenide-sulfide mineralization remains less understood. In this study, microscopic observation and scanning electron microscopy (SEM) revealed the Ni-Co-polymetallic sulfarsenide-sulfide mineralization stage that postdates the earlier Ni-Co arsenide mineralization stage. In-situ U-Pb dating of apatite coexisting with Ni-Co sulfarsenide and Ni-Co-polymetallic sulfide from this later stage yielded an lower intercept age of 436.2 ± 6.9 Ma, contemporaneous with the Caledonian granites in the Dayaoshan area. Sphalerite geothermometer indicate a crystallization temperature ranging from 379 °C to 388 °C, suggesting a moderate-high temperature ore-forming fluid. In-situ S-Pb isotope analysis of sulfide from this stage revealed δ³⁴S values of −0.5 ‰ to + 6.5 ‰ (mean + 1.4 ‰) for pyrite and 2.7 ‰ to 3.0 ‰ (mean 2.9 ‰) for sphalerite, with pyrite showing similar Pb isotopic ratios to those for Caledonian granites in the region, indicating a predominantly magmatic source characteristic. These new findings, combined with evidences from previous studies, suggest a multi-stage evolutionary process for Ni-Co arsenide and Ni-Co-polymetallic sulfarsenide-sulfide mineralization, with magmatic hydrothermal fluids playing a crucial role during the Ni-Co-polymetallic sulfarsenide-sulfide mineralization stage. The Ni-Co-polymetallic sulfarsenide-sulfide mineralization represents a subsequent overprinting process superimposed on the earlier Ni-Co arsenide mineralization. This study provides new insights into the Ni-Co enrichment mechanism in hydrothermal Ni-Co vein deposits and offers valuable guidance for future prospecting in the Dayaoshan area.