Geochemical fingerprints of ore-forming fluids in skarn copper systems: Implications for evaluating mineralization mechanisms and potential

• Fluids from all paragenetic stages show magmatic signatures. • Fluid mixing serves as the principal factor for copper precipitation. • Fluid-rock reaction may exert a secondary influence on copper deposition. • Fluids of mineralized porphyry-skarn copper system show high Cu/Na and medium Cs/Na ratios. The Tongshan skarn copper deposit is situated in the North Wuyi area of South China. Three distinct paragenetic stages have been identified: the pre-mineralization prograde stage, the syn -mineralization retrograde stage, and the post-mineralization carbonate stage. This study systematically conducted fluid inclusion petrography on gangue minerals from different stages, identifying three typical types of fluid inclusions: brine inclusions, vapor-rich inclusions, and liquid-rich inclusions. These inclusions were subsequently subjected to detailed microthermometry and LA-ICP-MS composition analysis. The analytical results show that in the pre-mineralization stage, garnet-hosted brine inclusions and vapor-rich inclusions exhibit homogenization temperatures of 455–529 °C and 470–535 °C, with salinities of 30.5–35.2 wt% NaCl equiv. and 1.74–3.06 wt% NaCl equiv., respectively. Liquid-rich inclusions in syn -mineralization stage quartz have homogenization temperatures of 290–400 °C and salinities of 0.88–9.21 wt% NaCl equiv. In the post-mineralization stage, calcite-hosted liquid-rich inclusions show homogenization temperatures of 122–221 °C and salinities of 0.70–4.96 wt% NaCl equiv. The marked decrease in fluid inclusion homogenization temperature and salinity values from the pre-mineralization to syn -mineralization stage suggests the involvement of meteoric water. Additionally, the depletion of major elements such as Rb, Na, and K in the syn -mineralization fluids correlates with the declining temperature, further supporting the notion of meteoric water dilution. These observations indicate that the mixing of magmatic fluids with meteoric water was instrumental in facilitating metal precipitation. Moreover, the elevated Ca/K ratios observed in mineralizing fluids associated with quartz imply substantial interaction between fluids and calcareous rock. In line with this, retrograde alteration assemblages (chlorite ± epidote), formed through fluid–rock interaction, are closely spatially associated with sulfide mineralization. This relationship underscores fluid–rock interaction as another critical mechanism driving metal precipitation. Comparative analysis of the fluid geochemical characteristics in global porphyry-skarn copper systems reveals that mineralized systems have higher Cu/Na ratios and moderate Cs/Na ratios, whereas barren systems exhibit lower Cu/Na ratios and higher Cs/Na ratios. The mineralization potential of these systems may primarily depend on the copper content in the early exsolved fluids from the granitic parental magma. The level of parental magma fractionation appears not to enhance the initial copper endowment in these systems.