High-sulfidation epithermal deposits are genetically associated with calc-alkaline volcanism in subduction zones, and although these ore deposits are excellent places to focus research, subduction zone stratovolcanoes provide important windows on magmatic-hydrothermal processes that are not available from study of the corresponding ore deposits. There is general agreement that the hydrothermal alteration accompanying the high-sulfidation epithermal ores is the product of volcanic degassing, however, there is considerably less agreement on the nature and origin of the ore fluid. Opinion is divided over whether the ore fluid is a vapor or a liquid, and whether it is entirely volcanic or of mixed volcanic-meteoric origin.The research presented here details a field-based investigation of the magmatic-hydrothermal environment of Kawah Ijen volcano, an active stratovolcano (mainly andesitic pyroclastics and lavas) located in the Ijen Caldera Complex in Java, Indonesia. The Kawah Ijen crater is approximately one kilometer in diameter and contains a hyperacidic lake (pH ~ -0.5) and a small and actively degassing solfatara, which is surrounded by a much larger area of acidsulfate alteration that was exposed during a phreato-magmatic eruption of the volcano in 1817; the eruption excavated the crater to a depth of 250 m. The research described in this thesis involved sampling and chemical analyses of the gases and their condensates (the surface equivalents of the ore-forming magmatichydrothermal fluids) collected from the solfatara and rock samples taken from the alteration center.Condensed fumarolic gases (pH ~ -0.5) released from the solfatara and sampled at temperatures between 330 and 495 °C contain up to 3 ppm Cu and 3.8 ppm As; the concentration of Ag is below detection. The alteration center is characterized by zones of residual silica, alunite-pyrite and kaolinite/dickite; based on alunite-pyrite geothermometry, the area formed at a temperature between 200 and 300 °C. High sulfidation epithermal mineralization occurs in this area in the form of massive and vein-hosted pyrite that contains up to 200 ppb iii Au, 16 ppm Ag, 6,800 ppm Cu, and 3,430 ppm As; these elements are invisible at the highest resolution of scanning electron microscopy, and thus occur either in the form of nano-particles or are in solid solution in the pyrite.The manuscript in Chapter 3 summarizes the observations detailed above to support a model in which highly acidic gases condensed ~ 250 m beneath the floor of the pre-1817 crater at Kawah Ijen volcano. In the area near the source of the vapors, the ratio of fluid to rock was extremely high and resulted in the leaching of elements from the andesitic host rock, leaving behind a residue of "vuggy silica." With increasing distance from the source, in an area of intermediate fluid/rock ratio, the condensed liquids replaced the primary minerals of the host with alunite and pyrite. The kaolinite/dickite zone formed in a rockbuffered environment (low fluid/rock ratio), in the zone furthest fr...