Extreme hydrothermal pressure transitions and fluid evolution in a felsic magmatic-hydrothermal system: a case study of Wondong Fe-W deposit, Korea

• Two distinct brine fluids evolved during the transition from lithostatic to hydrostatic pressure conditions. • The Wondong magmatic–hydrothermal system is associated with a small and isolated magma chamber. • Limited skarn development and rapid magma fractionation at Wondong constrained extensive W mineralization. We investigated magmatic-hydrothermal evolution associated with Paleogene quartz–feldspar porphyry intruded into Cambrian carbonates in the Wondong area, Korea. These “Wondong porphyry” intrusions are spatially associated with skarn mineralization containing magnetite, subeconomic scheelite, and Zn–Pb carbonate-replacement ores. 40 Ar/ 39 Ar dating of phlogopite from magnetite-bearing skarn yielded an age of 50.0 ± 0.03 Ma. The porphyries were subsequently crosscut by numerous thin, subparallel quartz veins. Microthermometry of pseudosecondary fluid inclusions hosted in quartz veins and phenocrysts in the porphyry revealed two distinct brine inclusion types based on their homogenization characteristics: (1) inclusions that homogenize by bubble disappearance at temperatures of ∼300–560 °C, interpreted to record boiling of ascending magmatic-derived fluids at pressures of ∼400 bar, and (2) inclusions that homogenize by halite dissolution at ∼300–550 °C, indicating direct exsolution of brine from lithostatically pressured magma at ∼1200 bar. These observations indicate a significant transition from lithostatic to hydrostatic pressure conditions at approximately 4 km below the paleosurface. Fluid inclusion compositions further show pronounced variations in Rb/Sr and Cs/Na ratios in the Wondong porphyry system, with Rb/Sr ratios varying by up to three orders of magnitude. This contrasts sharply with the relatively invariant ratios observed in the adjacent supergiant Sangdong W–Mo deposit. These geochemical features suggest substantial plagioclase fractionation within a small and/or physically isolated magma chamber, possibly separated from the much larger batholithic systems typically required for the formation of high-grade, high-tonnage W deposits.