The U.S. Bureau of Mines, using a variety of techniques, has made significant advances in tracking and modeling the flow of leach solutions in rock formations. However, the influence that leach solution composition has on the hydrology of these systems is poorly understood, and geochemically controlled changes in the hydrologic characteristic of ore deposits during in situ leach mining of porphyry-hosted, copper oxide ores have not been previously studied. The Cyprus Casa Grande in situ leach mining operation mines fracture-hosted and disseminated copper oxides (predominantly chrysocolla) hosted by granodiorite porphyry using a pattern of underground wells. The injection solution consists of sulfuric acid raffinate from a solvent extraction-electrowinning plant. Chemical analysis of representative injection and recovery solutions reveals that gangue cation concentrations in these solutions are high, and geochemical modeling shows that the solutions are saturated or nearly saturated with respect to gypsum, jarosite, and various amorphous oxides. Precipitation of these solids is evoked by calcium-exchange reactions, rising ph, and perhaps locally rising redox potential, as the leach solution reacts with the ore minerals, host rocks, and the atmosphere. Mass and volume balance calculations show that approximately 25 cm3 of precipitates are formed per gram of copper recovered at Cyprus Casa Grande. Chemical precipitation causes significant decreases in porosity and permeability during in situ leach mining of porphyry-hosted, copper oxide ore.