The Gagok Zn-Pb skarn deposit, which is hosted in Cambrian calcareous rocks, is the result of late-stage hydrothermal activity associated with a Mesozoic granite porphyry in the eastern part of South Korea. The zoning of calc-silicates relative to igneous contacts is either lacking or not distinctive. Most calc-silicates are enriched in Mn and Fe3+, containing FeS 20–18 mol.% sphalerite. Two types of primary fluid inclusions (e.g., liquid-rich Type I and vapor-rich Type II) trapped in syn-ore fluorite are spatially associated. The Type I inclusions contain calcite daughter crystals, and all microthermometric analyses were performed in the presence of calcite. The Type I inclusions displayed systematic low first melting temperatures (e.g., eutectic event) close to −37 ± 2°C, vapor bubble disappearance temperatures from 197 to 344°C, and low salinities from 7.4 to 1.2 wt.% NaCl equiv. The Type II inclusions appear to have an extremely small concentration of a CO2 gaseous phase, no CO2 phase was detected during microthermometry. The presence of co-existing Type I and II inclusions, the variable range of vapor volume ratios, low density vapor, and low homogenization temperatures clearly indicate that the fluid boiled and fashed. The insolubility of calcite during the microthermometry analysis may be explained by the acid neutralization of the fluids mainly due to boiling at the time of trapping as fluid inclusions. Fluid inclusions displaying systematic low first melting temperatures and calcite-saturated nature indicate a seawater signature, producing a metasomatizing fluid that resulted from the mixing of different fluids, one of possible magmatic origin, and the other possibly derived from oxidized basinal or meteoric fluids. Pressure calculation of vapor pressures for H2O–NaCl–CO2, combined with mineral chemistry, suggests that the metasomatism and ore-formation occurred under a shallow environment at 450–600 bars. The boiling and fluid mixing played important roles in the destruction of the early formed skarn minerals and later deposition of economic ore-bearing skarn at the Gagok Zn-Pb skarn deposit.