The purpose of this trip is to demonstrate the manner in which Laramide and middle Tertiary faults controlled the original emplacement of, and subsequent disruption of porphyry copper mineralization. Two copper skarn deposits are present in the district. The larger of these, the Rosemont deposit of ASARCO Inc. contains approximately 400 million tonnes of ore with grades of 0.53% to 0.61% copper (Anzalone, 1995). The smaller Peach-Elgin deposit contains about 42 million tonnes of ore with grades of 0.58% copper (Anzalone, 1995). The field trip will examine both Laramide faults that controlled the original emplacement of ore-related intrusions, as well as middle Tertiary faults that subsequently tilted, truncated and reorganized the original mineral system.