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New Cornelia Mine – Ajo
New Cornelia Mine – Ajo

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A former large-output surface Cu-Ag-Au-Mo-Zn-Ti-Nb-Rb-Gemstones mine located in the SE¼ sec. 22, SW¼ sec. 23, NW¼ sec. 26 & the NE¼ sec. 27, T12S, R6W, G&SRM, about ½ mile (0.8 km) south of Ajo. Discovered by the Spaniards in 1750. First produced in modern times in 1917. Owned at times, or in part, by the Arizona Copper Mining & Trading Co.; St. Louis Copper Co.; the Rescue Copper Co.; Cornelia Copper Co.; New Cornelia Copper Co. (1912-1916); Calumet & Arizona Copper Co.; and the New Cornelia Division, Phelps Dodge Corp.

Mineralization is an elliptical ore body 2,011.63 meters long, 1,206.98 meters wide, 487.68 meters deep, and 304.8 meters thick, with veinlets and disseminations of chalcopyrite, bornite, and minor pyrite, oxidized in the upper part to carbonates, silicates, and minor chalcocite, deposited in fractured and faulted, altered Laramide quartz monzonite and bordering quartz diorite facies, and often in close association with irregular pegmatite zones in the intrusive. Mineralization extends into bordering intruded and altered Cretaceous volcanics. Minor spotty molybdenite and sphalerite. Ore control was the fracturing of the porphyry. Ore concentration was governed by the permeability of rocks to mineralizing fluids; some secondary copper on fractures. Alteration included sericitic, clay, chloritization, kaolinization, silicification with oxidation down to the water table. The host rock unit is the Chico Shuni (Cornelia). An associated rock unit is the Cornelia pluton.

Contact of oxidized and sulfide ore is sharp. There is no leached overburden. Gold is more closely associated with copper sulfides than with pyrite. Units present include quartz monzonite, quartz diorite, granodiorite, equigranular quartz monzonite, porphyritic quartz monzonite, and porphyritic micro-quartz monzonite.

Local structures include NNW fracture zones and a number of major faults with large displacements.

The working is a huge open pit operation (about 3,000 feet long and nearly 2,000 feet wide) at 750 feet deep. Haulage was by internal railroad installed in the pit. Total production from 1917 through 1972 amounts to some 350,000,000 tons of ore averaging about 0.8% Cu, 0.05 ounce Ag/ton and 0.004 ounce Au/ton. Higher grade ore was mined in the early years and lower grade ore in more recent years.

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