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El Tiro Mine (El Tiro pit; El Tiro open pit; Red Rock group), Silver Bell Mine, Mineral Mountain, Silver Bell District, Silver Bell Mts, Pima Co., Arizona, USA

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Latitude & Longitude (WGS84): 32° 24' 57'' North , 111° 32' 12'' West
Latitude & Longitude (decimal): 32.41583,-111.53667
Other regions containing this locality:Sonoran Desert, North America
Köppen climate type:BSh : Hot semi-arid (steppe) climate


‡Ref.: Stewart, C.A. (1912), The geology and ore deposits of the Silverbell mining district, Arizona: AIME Bull. 65: 455-505 (AIME Transactions: 43: 240-290.

Weed, W.H. (1918) The Mines Handbook, Vol. XIII.

Tenney, J.B. (1928), Mineral Industries of Arizona, Arizona Bureau of Mines Bull. 125: 74-75.

Galbraith, F.W. (1947), Minerals of Arizona, Arizona Bureau of Mines Bull. 153: 9.

Kerr, P.F. (1951), Alteration features at Silverbell, Arizona, Geological Society of America Bull.: 62: 451-480.

Richard, K.E. & Courtright, J.H. (1954), Structure & Mineralization at Silver Bell, Arizona, Mining Engineering: 6(11): 1095-1099 (AIME Transactions, 1954 - Vol. 199 (1955)).

Galbraith, F.W. & D.J. Brennan (1959), Minerals of Arizona: 50, 61, 85, 108, 110.

Hardwick, W.R. (1963) Open-pit copper mining and concentrating methods and costs, Silver Bell unit, American Smelting and Refining Co., Pima County, Ariz.: U.S. Bureau of Mines Information Circular I.C. 8153, 72 p.

Richard, K.E. & Courtright, J.H. (1966), Structure & Mineralization at Silver Bell, Arizona, in Titley, S.R. and Hicks, C.L., eds., Geology of the porphyry copper deposits, southwestern North America: 157-163 (University of Arizona Press).

Keith, Stanton B. (1974), Arizona Bureau of Geology & Mineral Technology, Geological Survey Branch Bull. 189, Index of Mining Properties in Pima County, Arizona: 142-143 (Table 4).

Galey (1979) Society of Economic Geologists, Porphyry Copper Field Conference, 18p.

Mining World, Vol. 36: 1104-1107, 1147-1150.

Anthony, J.W., et al (1995), Mineralogy of Arizona, 3rd. ed.: 147, 158, 163, 166, 174, 186, 216, 266, 280, 292, 399.

Arizona Bureau of Mines file data.

MRDS database Dep. ID file #10039684, MRDS ID #M050650; and, Dep. ID #10259035, MAS ID #0040190205.

A former large production underground and surface Cu-Ag-Pb-Au-Mo open pit operation and is the continuance of the former El Tiro underground mine located in the SW ¼ sec. 33 T.11S., R.8E., & the NW ¼ sec. 4, T.12S. R.8E. First produced 1954. Owned/operated at times, or in part, by the Cleveland Arizona Mining Co.; the El Tiro Copper Co.; El Tiro Leasing Co.; Williams; Western American Mines Co.; Chemical Process Copper Co.; and the American Smelting & Refining Co. (ASARCO).

Mineralization is irregular, pyrometasomatic, partly oxidized, copper orebodies in a garnetized and sheared Paleozoic limestone block in contact with Laramide alaskite, hydrothermally altered dacite porphyry, and monzonite. It is comprised of supergene-enriched deposits of chalcocite with minor copper carbonates and silicates derived from sheeted, closely spaced, veinlets and seams of quartz, pyrite, chalcopyrite, and minor molybdenite. Alteration included argillic; partly oxidized, garnetized limestone. The ore zone is 670.56 meters long, 396.24 meters wide, 30.48 meters depth to top, and 91.44 meters thick. The chalcocite blanket was formed by 2-3 fold enrichment of the copper contained in the proto-ore. Ore is in the thin quartz sulfide veinlets that form a disseminated deposit in aggregate. Copper does not occur in dacite porphyry where shears aren't parallel.

Local structures include sheared limestone, parallel fissures in dacite porphyry.

Prospected and worked since the late 1800's and developed from the Daisy and Kurtz shafts. Underground mine operated until 1931, producing some 203,500 tons of ore averaging about 5% Cu, 0.2 oz. Ag/T and minor Au and Pb. Operations prior to 1931 were small. The open pit El Tiro operation is included under the Silver Bell Mine.


Mineral List


19 valid minerals.

Regional Geology

This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org

Ypresian - Campanian
47.8 - 83.6 Ma



ID: 2993483
Early Tertiary to Late Cretaceous granitic rocks

Age: Phanerozoic (47.8 - 83.6 Ma)

Description: Porphyritic to equigranular granite to diorite emplaced during the Laramide orogeny. Larger plutons are characteristically medium-grained, biotite +/- hornblende granodiorite to granite. Smaller, shallow-level intrusions are typically porphyritic. Most of the large copper deposits in Arizona are associated with porphyritic granitic rocks of this unit, and are thus named 'porphyry copper deposits'. (50-82 Ma)

Comments: Laramide metaluminous; associated with porphyry Cu deposits

Lithology: Major:{granite,granodiorite,diorite}

Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. [133]

Data and map coding provided by Macrostrat.org, used under Creative Commons Attribution 4.0 License



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