CopperMission pit, Mission complex, San Xavier, Pima District, Sierrita Mts, Pima Co., Arizona, USA
Photo: Michael Cline Latitude: 31°59'31"N
Longitude: 111°3'48"W
A large surface producer Cu-Ag-Zn-Mo-Pb-Au-Sn-W-Fluorspar-Cd-Sn-W-PGE mine located in East-central sec. 36, T.16S., R.12E. & West-central sec. 31, T.16S., R.13E., 17 miles S of Tucson, on private land. Discovered by geologists of ASARCO in 1953. Owned and operated by the American Smelting & Refining Corp. (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 meetrs 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%.
References
Richard, K.E. & Courtright, J.H. (1959), Some geologic features of the Mission copper deposit, in Southern Arizona Guidebook II, Arizona Geological Society Digest: 2: 201-204.
Cooper, J.R. (1960) Some geologic features of the Pima mining district, Pima County, Arizona: U.S. Geological Survey Bulletin 1112-C, p. 63-103, 1 sheet, scale 1:31,680.
Skillings Mine Review (1961, 1962).
Argall, George D., Jr. (1962) ASARCO’s Mission Copper. Mining World: 24(1): 19-42.
Engineering & Mining Journal (1962).
Gale, R.E. (1965) Geology of the Mission copper mine, Pima mining district, Arizona: Stanford, Stanford University, Ph.D. dissertation, 176 p.
Kinnison, J.E. (1966), The Mission copper deposit, Arizona, in S.R. Titley and C.L. Hicks (editors), Geology of the porphyry copper deposits, southwestern North America, University of Arizona Press, Tucson: 281-287.
World Mining (1972) Pima Mining Five Year Ore production and Copper Sales, Mining World: January 1972: 57.
Keith, Stanton B. (1974), Arizona Bureau of Geology & Mining Technology, Geological Survey Branch Bull. 189, Index of Mining Properties in Pima County, Arizona: 136 (Table 4).
Niemuth, N.J. (1987), Arizona Mineral Development 1984-1986, Arizona Department of Mines & Mineral Resources Directory 29, 46 pp.
Dayton, S.H. (1988) Mission Today, Trim, Efficient, and in Third-Stage Growth. Engineering and Mining Journal: 189(9): 35-41.
Walenga, Karen (1989) Mission, Ray Expansion to Assure Copper Feed: Overall project will reduce need for outside sources, cut costs. Southwestern Paydirt, March, 1989: 4A-8A.
Carter, Russell A. (1992) Expansion Almost Complete at Arizona’s Mission and Morenci Mines. Engineering and Mining Journal, 2/92: C14-C16.
Anthony, J.W., et al (1995), Mineralogy of Arizona, 3rd. ed.: 111, 146, 166, 185, 205, 207, 224, 242, 402, 421.
Singer, D.A., Berger, V.I., and Moring, B.C. (2005): Porphyry Copper Deposits of the World: Database, Map, and Grade and Tonnage Models. USGS Open-File Report 05-1060.
USGS Twin Buttes Quadrangle topo map.
Arizona Bureau of Mines file data.
MRDS database Dep. ID file #10111429, MRDS ID #M050387; and, Dep. ID #10234710, MAS ID #0040190317.
External Links
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