Study of pyrrhotite-bearing volcanic units such as the El Chichon, St. Helens, and Fish Canyon tuffs demonstrates that the activity of sulfurous gases in calc-alkaline systems is often quite high. Oxidizing systems have especially high activities of SO 2 in early volatile phases.Upon cooling, oxidized compositions react to form sulfate-rich solutions through the reaction 4SO 2 + 4H 2 O = 3(H 2 SO 4 ) + H 2 S whereas reduced compositions form hydrogen sulfide-rich solutions by the reaction SO 2 + 3H 2 = H 2 S + 2H 2 O.In both cases, the quenched magmatic component is high in total sulfur content and posseses high oxygen and sulfur fugacities. These conditions fall near the sulfide-sulfate-dominant boundary which defines a maximum in sulfur fugacity for solutions in the oxygen-sulfur system. Such solutions fall within the stability fields of native sulfur, pyrite, pyrite + bornite, iron-free sphalerite, galena, alunite, and other highly oxidized and sulfidized minerals. Upon mixing with lower sulfur content meteoric waters these solutions will trace a path through f (sub O 2 ) - f (sub S 2 ) space which duplicates much of the mineral zoning seen with depth in porphyry systems. In addition, this magmatic component can account for much of the chemical and mineral data from fumaroles associated with recent calderas. The high sulfate content, either inherited directly from the reaction of magmatic gases or derived from the oxidation of magmatic H 2 S by meteoric waters, may be an important component for the transport of certain ore-forming elements. In addition, the changing conditions during mixing of these sulfur-rich magmatic waters with meteoric systems may cause deposition of ore minerals.The conditions derived from these calculations duplicate those determined from natural magmatic-dominated systems such as Summitville, Colorado.