Mission pit, Mission complex, San Xavier, Pima Mining District, Sierrita Mountains, Pima County, Arizona, USAi

Regional Level Types
Mission pit	Pit
Mission complex	Complex
San Xavier	- not defined -
Pima Mining District	Mining District
Sierrita Mountains	Mountain Range
Pima County	County
Arizona	State
USA	Country

A large surface producer Cu-Ag-Zn-Mo-Pb-Au-Sn-W-Fluorspar-Cd-Sn-W-PGE mine located in East-central sec. 36, T16S, R12E & West-central sec. 31, T16S, R13E, G&SRM, 17 miles S of Tucson, on private land. Discovered by geologists of ASARCO in 1953. Owned and operated by the American Smelting & Refining Corporation (ASARCO).

ASARCO geologists discovered this deposit by drilling through about 200 feet of valley gravels in 1953. By 1961, a major new open-pit mine was opened, and a mill was constructed.

Mineralization is a skarn-related copper porphyry deposit with seams and disseminations of copper carbonates and sulfides with minor zinc, lead and molybdenum minerals in a pyrometasomatic deposit in altered, complexly folded, and faulted Paleozoic limestone and Triassic sedimentary and volcanic formations intruded by Laramide and Tertiary intrusives. There is a thin secondary enrichment zone. The host rock unit is the Scherrer Formation. The ore zone is 1,609.3 meters long, 804.65 meters wide, depth to top of 60.96 meters, depth to bottom of 213.36 meters, at 152.4 meters thick, and is flat-lying. Ore control was tactite and hornfels sedimentary units, particularly limy units, less in argillite; contact of the Papago Formation with underlying Paleozoic sedimentary rocks; low-angle thrust faults. Ore concentration was hydrothermal-metasomatic fluids migrating through altered porphyry, tactite, and hornfels. Alteration includes tactite, hornfels, leaching and enrichment confined to a thin layer.

There was a 40 foot thick zone of supergene enrichment clays, malachite, native copper; oxidation occurred to about 200 feet along some faults; there was also a calcite zone 20 feet thick. Late fluorite in calcite-scheelite-galena-pyrite. PGM recovered as a by-product at the smelter. The bulk of the replacement ore occurred in garnet and pyrozene tactites. There was a 2 mile wide alteration halo.

Local structures include thrust and normal faults; an NW-trending anticline.

Discovery of this deposit was by scout drilling. Initial production rate was 15,000-20,000 tons Cu ore/day after investment of $34 million; expansion: $13.1 million. ASARCO approved spending of $94 million in February of 1989 to increase capacity at Mission and to refurbish the adjacent Pima mill and concentrator that it bought for approximately $6 million. Mill capacity: 41,000 short tons/day.

Workings are a large open pit operation overall 2,225.04 meters long, 2,194.56 meters wide and 301.75 meters deep. The above figures represent the merged product of the Mission, Pima, and Eisenhower Mines into a single, large open pit. Produced over 77,600,000 tons of ore from 1961 through 1972. This ore averages about 0.7% Cu, 0.13 oz. Ag/T and considerable by-product zinc, molybdenum and lead. Very little gold.

Reserve-Resource 1978 estimate excludes contribution of 31.5 million tons to the Eisenhower Mining Co. in September, 1985. ASARCO acquired the adjacent Pima property with 39 million tons of ore. It obtained another 14 million tons of reserves in April, 1987, when it bought the Eisenhower operation. In late 1987 it acquired an additional 14 million tons of reserves with the undeveloped Mineral Hill property. These purchases boosted Mission's reserves by 56%. Purchase of the Helvetica property, 15 miles east of the Mission complex, added 280 million tons of sulfide ore grading 0.6% Cu and 23 million tons at 0.8%.

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Link to USGS MRDS:	10111429

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