Textural and compositional evolution of quartz and cobalt-bearing minerals from the Wubaoshan Deposit, Jiangxi Province, South China: Implications for cobalt mineralization

Cobalt resources are primarily derived from hydrothermal-related deposits; however, our understanding of the mechanisms governing cobalt transportation and deposition, particularly for cobalt arsenides and sulfarsenides within hydrothermal fluids, remains limited. In this study, we present a comprehensive investigation into the Wubaoshan cobalt deposit located in the Qin-Hang ore Belt, South China. This deposit exhibits mineralization associated with multiple hydrothermal fluids, resulting in complex paragenesis and the formation of cobalt minerals. By employing a combination of petrographic and geochemical observations, we explore the conditions of the ore-forming fluid and the hydrothermal mechanisms underlying cobalt mineralization. Our analysis reveals three distinct hydrothermal stages of mineralization: Stage 1 is characterized by quartz-arsenopyrite-pyrite assemblages formed at relatively high temperatures (352-390 degrees C), neutral pH, and high fS2 (log fS2 = -6.9 -8.8); Stage 2, the pivotal stage for cobalt mineralization, displays various sulfides-sulfarsenides formed at lower temperatures (229 +/- 6.0 degrees C), under acidic conditions, and low fS2 (log fS2 = -12.0 - -14.0); and Stage 3 comprises minor sulfides and carbonates formed at relatively lower temperature and pH. Sulfur isotope analysis of pyrite from Stage 1 (+13.49 to +14.30 %o) and sphalerite from Stage 2 (+5.53 to +9.21 %o) suggests a mixed sulfur source derived from both Permian seawater sulfate and related magmatic rocks. Additionally, our results reveal that a transformation from cobalt arsenide to sulfarsenide during Stage 2, likely influenced by the change of As/S ratio in ore fluids, which is affected by the influx of external sulfur from magmatic hydrothermal fluids. Our findings, together with the previous thermodynamic modelling results, indicate that the variations of the ore fluid condition, particularly temperature and pH, govern the precipitation and dissolution of mineral during cobalt mineralization. These findings contribute to our understanding of the formation processes of the Wubaoshan deposit and provide valuable insights into the mechanisms of cobalt enrichment.