Mesothermal gold-bearing quartz veins along the Juneau gold belt formed in the early Tertiary from H2O-CO2(±N2,H2S,CH4)-rich, low-salinity fluids at temperatures of 200 to 325 °C and pressures in excess of 1 - 1.5 kbar. Isotopically heavy ore-forming fluids, with δ18O values between +8‰ and +12‰ and δD values of -20‰ to -30‰ , indicate a deep-seated fluid source. The data are compatible with a model of ore-fluid generation via metamorphic devolatilization within material subducted below the southern Alaska continental margin. Prograde metamorphic fluids infiltrated up zones of mechanically enhanced permeability associated with the Coast Range megalineament and formed the gold-bearing veins within brittle units at higher, retrograding crustal levels. Deposition of gold was accomplished by several mechanisms, including boiling, fluid-wall rock reactions, and decreases in pressure and temperature of the hydrothermal fluids.