Hydrothermal gold deposits of the Trabong district in Vietnam occur as single-stage quartz $pm$ calcite veins (0.3-1.2 m thick) which fill fault fractures in graphite-bearing gneiss and schist of the Chulai Complex and Kham Duc Formation of the Proterozoic age. Ore grades are 1.3 to 92.4 g/ton Au. Ore mineralogy is very simple, consisting mainly of pyrite with minor amounts of base-metal sulfides and electrum. Gold grains occur in two assemblages as follows: (1) early, Fe-rich (7.2-10.4 mole % FeS) sphalerite + electrum (50.4-64.3 atom % Au) assemblage occurring as inclusions in pyrite; (2) late, Fe-poor «4.7 mole % FeS) sphalerite + galena + electrum (47.6-81.7 atom % Au) assemblage occurring along fractures of pyrites. Based on fluid inclusion data and thermochemical considerations of ore mineral assemblages, ore minerals were formed at high temperatures (about $230^{circ}C$ to $420^{circ}C$) from $H_{2}O-CO_{2}(-CH_{4})$-NaCI fluids with the sulfur fugacity of about $10^{-6}$ to $10^{-10}$ atm. Fluid inclusion data also indicate that ore mineralization occurred mainly as a result of fluid unmixing accompanying $CO_2$ effervescence. Calculated oxygen and measured hydrogen isotope compositions of mineralizing waters (${delta}^{18}O_{V-SMOW}$ values = 5.3 to 8.6$ extperthousand$, ${delta}D_{V-SMOW}$ values = - 60 to - 52$ extperthousand$), along with the sulfur isotope compositions of vein sulfides (${delta}^{34}S_{CDR}$ values = - 1.2 to 2.8$ extperthousand$) and carbon isotope compositions of inclusion $CO_2$ (${delta}^{13}C_{PDB}$ values = - 4.7 to - 2.0$ extperthousand$) indicate that the high temperature (mesohypothermal) gold mineralization formed from a magmatic fluid.