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Potassium metasomatism is a widespread alteration type in porphyry copper deposits and is often spatially associated with hypogene sulfide ore formation. At Ajo, potassic alteration composes the dominant alteration type and is spatially, and to some extent, temporally, associated with chalcopyrite and bornite mineralization. Physiochemical conditions prevailing during potassic alteration thus describe a significant portion of the hydrothermal ore-forming process. Studies of fractures, fluid inclusions in quartz, and structural and compositional characteristics of K-feldspar, all in the potassic alteration zone at Ajo, indicate that: fracturing was strongest during potassic alteration and sulfide deposition, pressure were at approximately 650 bars, temperatures started a a minimum of 580°C, continued through 470°C, log aK+/aH+ changed from 2.6 at 580°C to 3.2 at 470°C, log aNa/aH+ changed from 2.7 at 580°C to 3.8 at 470°C, and the ratio aK+/aNa+ radically decreased from 0.8 to 0.3 in this temperature range.

Copper from dump, heap, and in situ leaching accounts for about 15% of United States copper production. In situ leaching is the least understood and most difficult to evaluate of these leaching methods. Several types of supergene copper deposits are derived from porphyry copper systems depending on the geologic and climatic factors involved in the formation of a deposit. The geologic and climatic factors involved in the formation of a deposit. The geologic evaluation of a deposit should provide most of the basic information required to access a deposit for in situ leaching. Metallurgical testing can determine the leachability and acid consumption of the ore as well as the need for an oxidizing agent through the use of agitation and column leaching tests. The location of the deposit with respect to the water table determines the type of ground preparation required. Hydraulic fracturing can be used for deposits below the water table and blasting or caving can be used for deposits above the water table. Fluid flow through the deposit depends on the method of application, the permeability of the deposit, and the method of egress from the deposit. The leaching of a copper "oxide" deposit depends on the strength and volume of sulfuric acid which reaches the ore. The leaching of a copper sulfide deposit depends on an oxidant as well as sulfuric acid reaching the ore.