The Mujeong au-Ag hydrothermal vein type deposits occur within the Teriary igneous rocks of the Janggi basin. Ore minerals consist of pyrite, pyrrhotite, sphalertite, chalcopyrite, galena, cosalite, lillianite, argentite and electrum, and associated with epidotization, sericitization and pyritization. Fluid inclusion studies reveal that ore fluids were low saline with a simple NaCl-$H_{2}O$ system. Fluid inclusion data indicate that homogenization temperatures and salinities of fluid are 150 to $340^{circ}C$ and 1.0 to 6.5wt.% NaCl equivalent, respectively. Sulfur isotope compositions of sulfied minerals ( ${delta}^{34}S$=6.2 to 9.6$ extperthousand$) indicate that the ${delta}^{34}S_{H2S}$ value of ore fluids was about 10.4$ extperthousand$. This ${delta}^{34}S_{H2S}$ value is likely consistent with and hydrothermal sulfur, whereas the fluids were highly influenced by mixing with meteoric water. Measured and calculated oxygen and hydrogen isotope values (${delta}^{18}O_{H2O}$=-2.7 to 3.4 $ extperthousand$, ${delta}D_{H2O}$ = -83.6 to -52.7 $ extperthousand$) of ore forming fluids suggest mixing with hydrothermal and meteoric water. Equilibrium thermodynamic interpretation by mineral assemblages and chemistry indicates that sulfur fugacities (-log $fs_2$) ore forming fluids range from 9.0 to 12.6 atm stage II.