Integrated mineralogical and thermodynamic constraints on cobalt enrichment in the Qibaoshan deposit, Jiangxi Province, South China: Implications for ore-forming processes

Cobalt (Co) is a critical strategic metal with increasing economic importance. However, compared to other base and precious metals, the mechanisms of Co enrichment and the paragenetic evolution of Co-bearing minerals during ore formation have received limited attention despite their key role in controlling metal distribution and zoning. This study addresses these gaps through an integrated mineralogical, geochemical, and thermodynamic investigation of the Qibaoshan Co-polymetallic deposit in the northern Jiangnan orogen, South China. By integrating detailed mineralogical observations, in situ microanalysis (electron probe microanalysis and laser ablation–inductively coupled plasma–mass spectrometry), and thermodynamic modeling, we reconstruct the paragenetic sequence, fluid evolution, and specific ore-forming processes that drove Co mineralization. Our findings reveal a five-stage hydrothermal history: (1) a diagenetic quartz-feldspar-carbonate assemblage with minor pyrite; (2) an early high-temperature arsenopyrite-pyrite stage (360–400 °C; log fS2 = −8.2 to −6.3); (3) the main Co mineralization stage (190–220 °C; log fS2 = −12.3 to −10.4), characterized by zoned Co-Ni arsenide and sulfoarsenide aggregates (e.g., CoAs3 → CoAs2 → CoAsS → NiAsS); (4) a late low-temperature sulfide stage (120–160 °C; log fS2 = −18.3 to −14.0) dominated by pyrite, sphalerite, and galena; and (5) a final carbonate vein-filling stage. The mineralogical transition from As-rich to As-poor phases reflects declining sulfur fugacity (fS2) and As/S ratios. Thermodynamic constraints indicate that Co and Ni were transported in Cl-rich fluids of mafic-ultramafic magmatic origin, with host rocks buffering the system and triggering cobalt precipitation. The distinct concentric zoning within Co-Ni minerals records this continuous fluid evolution, which is marked by cooling and progressive arsenic depletion. Mass-balance calculations, integrated with existing isotopic data, indicate a hybrid metal source dominated by magmatic input with a minor sedimentary contribution. Based on these findings, we propose a three-stage metallogenic model involving sedimentary pre-enrichment, subsequent magmatic-hydrothermal input and remobilization, and finally, structurally controlled precipitation of Co-Ni arsenides overprinted by later sulfides. Our results highlight how fluid evolution and fluid–rock interaction are key controls on Co mineralization and provide a robust genetic model to guide future exploration for similar deposits worldwide.