Temperature dependent phase relationships and irreversible reactions between fluids and minerals in the systems K 2 O-Al 2 O 3 -SiO 2 -SO 3 -H 2 O and FeO-Cu 2 O-H 2 S-SO 3 -As 2 O 3 -H 2 O have been computed using thermodynamic data taken from the literature. Pyrophyllite, rather than kaolinite, forms at temperatures below 200 degrees C if the fluid phase is supersaturated with respect to quartz. Pyrophyllite formed under these conditions is expected to occur with a metastable silica phase. Decreasing temperature at constant ion activity favors precipitation of alunite over silicate minerals. Formation of alunite requires acid conditions and high sulfate activity. In volcanic rocks which are more reduced than the hematite-magnetite oxygen buffer, the presence of alunite indicates high H 2 S activity if the fluid contained greater than 0.001 molal total sulfur when the temperature of the fluid was 300 degrees C. This fact greatly enhances the likelihood that alunite-producing fluids also produced high total sulfur mineralization at depth. Decreasing temperature favors enargite (luzonite) and tennantite over copper sulfide minerals and favors copper sulfide minerals over iron minerals. If the volcanic rocks above a porphyry pluton are more oxidized than the pluton, the fluid will oxidize as it flows upward. This oxidation favors formation of alunite, quartz, enargite (luzonite), and tennantite over other minerals in the systems studied. These facts suggest that the vertical vein mineral zonation in porphyry copper plutons overlain by more oxidized volcanic rocks is from pyrite-chalcopyrite-potassium feldspar in the pluton upward to enargite (luzonite)-tennantite-alunite in the volcanic rocks.Irreversible oxidation of a hydrothermal fluid in equilibrium with potassium feldspar, quartz, biotite, pyrite, and chalcopyrite decreases the stability of all minerals with respect to the solution, except quartz, alunite, and in some instances, kaolinite or pyrophyllite. Irreversible reaction between an andesite and a solution in equilibrium with alunite and quartz produces the following zonations, alunite: kaolinite: montmorillonite: muscovite: potassium feldspar if the solution maintains quartz equilibrium or alunite: kaolinite: pyrophyllite: montmorillonite: potassium feldspar if the solution supersaturates with respect to quartz.