Stable isotope (H, C, O, and S) systematics of the Pueblo Viejo acid sulfate system, the world's largest bulk mineable acid sulfate gold deposit, were investigated to determine the importance of magmatic fluids in hydrothermal systems of this type. Mineralization at Pueblo Viejo is hosted by a maar-diatreme system that cuts sedimentary rocks and spilites of the Late Cretaceous Los Ranchos Formation. The deposit is characterized by two stages of advanced argillic alteration: stage I fluids produced deep alunite + quartz and shallower kaolinite + quartz, both associated with disseminated pyrite; stage II fluids overprinted stage I and produced deep pyrophyllite + diaspore and an overlying silica cap. An increase in temperature from stage I ( approximately 250 degrees C) to stage II ( approximately 250 degrees -300 degrees C) is indicated by mineral stabilities and sulfur isotope thermometry of coexisting sulfide-sulfate pairs. Stage I quartz-alunite-kaolinite-pyrite alteration and associated disseminated Au-Ag mineralization are interpreted to have precipitated in equilibrium with magmatic discharge fluids (delta D = -35 to -15ppm; delta 18 O = 6.5 to 11ppm) that experienced only minor (< or =25%) dilution by meteoric water (delta D = -30ppm; delta 18 O = -5ppm). Stage II advanced argillic alteration resulted from a renewed influx of essentially pure magmatic discharge fluid. The shallow silica cap generated during stage II resulted from significant ground-water dilution of magmatic fluids.Circulation of conductively heated seawater adjacent to the acid sulfate system caused deposition of a carbonate-sulfate zone that sealed the acid sulfate mineralization from ground-water and seawater infiltration. It is possible that the regional greenschist facies alteration of Los Ranchos Formation rocks was also related to such seawater circulation.These results suggest that the dominant proportion of metals and fluids in acid sulfate systems are derived from bulk condensates of magmatic vapors. Relatively small (lessthanorequalto 25%) proportions of nonmagmatic fluids, that is, fluids from meteoric or seawater sources, may, however, have been important in causing precipitation of the ore components.