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The Northwest orebody, located in Pima County, Arizona, is a large tonnage skarn containing copper, silver, and molybdenum mineralization. The orebody is separated from the main orebodies of the Twin Buttes mine by the Twin Buttes fault. Sulfide mineralization and silicate alteration are systematically zoned along a westerly trend away from the fault and from a central intrusive complex. Original host rock chemistry was the major control on the silicate and sulfide mineralogy of the skarns. The highest grade ore occurs in a garnet-diopside alteration zone within beds of altered limestone and dolomite. Lower grade ores formed in cal-silicated siltstones, quartzites, and granitic rocks. The skarns exhibit a complex history of sequential veining and replacement. The alteration can be subdivided into an early thermal event, followed by three stages of hydrothermal alteration. The first stage of hydrothermal alteration is characterized by the formation of anhydrous Ca-Mg-Fe-Al silicates, predominantly garnet and diopside. Stage II is a period of sulfide mineralization. The copper sulfides were deposited when copper in the hydrothermal solutions reacted with Fe⁺⁺⁺₋ rich garnet to form chalcopyrite. Mineralization continued during Stage III as increasingly pyritic sulfide mineralization was deposited with hydrous silicate minerals, predominantly epidote and actinolite. Several points of evidence show that the mineralization in the Northwest orebody is related to the intrusive activity at the center of the Twin Buttes mine area and not to either the pre-mineral Ruby Star Granodiorite or to a buried intrusive at depth.

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