To support the Bureau's field in situ copper leach project at the Santa Cruz deposit near Casa Grande, Arizona, laboratory experiments are being done to study the reactions occurring during leaching (5-pct sulfuric acid) of oxide copper ores (atacamite and chrysocolla) hosted by biotite-poor granite and biotite-rich granite porphyry. Evaluation of the various reactions responsible for the observed fluid chemistry is determined by detailed pre- and postleach petrographic examination of the rock samples combined with the leach solution chemistry resulting from core-leaching experiments. Biotite dissolution (apparently nonstoichiometric, as it is progressively altered to vermiculite and finally to amorphous silica) also occurs, contributing k, mg, fe, and al to the leach solution. The texture of leached chrysocolla remains relatively intact, as copper removal from the chrysocolla leaves an amorphous silica residue with a texture similar to that of unreacted chrysocolla. Biotite also is pseudomorphed by its reaction products, but results in a volume expansion of up to 30 pct. This expansion may have the potential to significantly affect the structure and permeability of biotite-rich ores. Atacamite dissolves completely, with no noticeable collapse of the resulting voids observed in these experiments. The amorphous silica resulting from complete reaction of biotite and chrysocolla is completely devoid of cu, but some cu remains in the biotite that is altered to vermiculite. However, this cu is subsequently released by further alteration of vermiculite to amorphous silica.