Jurassic (166 + or - 2 Ma) gold vein deposits of the Samhwanghak mine, Youngdong area, are examples of mesothermal gold deposits in the Republic of Korea. The deposits are composed of gold-bearing but relatively sulfide-poor massive quartz veins which occupy shear zones in graphite-bearing paragneiss of Precambrian age.Vein quartz was deposited mostly prior to deposition of sulfides and gold, and contains three main types of fluid inclusions: (I) CO 2 -CH 4 fluids, (II) low-salinity (<9 wt % NaCl equiv) H 2 O-CO 2 -CH 4 fluids with variable CO 2 + CH 4 contents, (III) dominantly aqueous (0-14 wt % NaCl equiv) fluids with small amounts of CO 2 . Type II inclusions are largely primary in origin and represent early, immiscible fluids formed by extensive fluid unmixing (CO 2 -CH 4 effervescence) at temperatures from 480 degrees to 220 degrees C and pressures up to >2 kbars. Initial hydrothermal fluids were near their critical states at temperatures of 410 degrees to 480 degrees C and were homogeneous compositionally with X (sub CO 2 ) +CH 4 of 0.45 (X (sub CH 4 ) = 0.14) and salinity of [asymp] 9 wt percent NaCl. Type III inclusions appear to be secondary and represent later fluids which formed through extensive fluid unmixing of type II fluids with progressive decreases in temperature (below 370 degrees C) and pressure (down to [asymp] 1 kbar), but may also include deep circulating meteoric waters.Thermochemical calculations indicate the following evolution of hydrothermal fluids: fugacity of sulfur decreased progressively with decreasing temperature near the pyrite-pyrrhotite sulfidation curve; fugacity of oxygen was controlled largely through graphite-fluid interaction and evolved near the QFM buffer; pH was maintained near neutrality. Temperature versus f (sub O 2 ) relationships indicate that hydrothermal fluids were derived originally from a nearby S-type magma, the Middle Jurassic Kimcheon Granite. Gold occurs as gold-rich (avg 72 at. % Au) electrum intimately associated with sulfide minerals, and precipitated from later aqueous (type III) fluids at temperatures of 340 degrees to 240 degrees C, as a combined result of cooling and decrease of sulfur activity (by sulfide precipitation and/or H 2 S loss accompanying fluid unmixing).Calculated sulfur isotope compositions of hydrothermal fluids (delta 34 S (sub Sigma S) ) near 2 per mil indicate that ore sulfur was derived from an ilmenite series (S-type) granitic melt. Measured and calculated oxygen and hydrogen isotope compositions of ore fluids (delta 18 O water = 6.1-8.4ppm; delta D water = -62 to -72ppm) are consistent with derivation from and interaction with magmatic rocks. Mesothermal gold vein deposits of the Youngdong area were formed by extensive fracturing and veining during the Jurassic Daebo orogeny and developed from fluids that may have included an important component of magmatic fluids. Mineralogy and ore fluid chemistry of the deposits of the Youngdong area are similar to those of Archcan mesothermal gold vein deposits, possibly validating a more general application of a magmatic hydrothermal model for the formation of mesothermal gold vein deposits.