Pima District (Olive District; Mineral Hill District; Twin Buttes District), Sierrita Mts, Pima Co., Arizona, USA

‡Ref.: Guild, F.N. (1910), The mineralogy of Arizona, The Chemical Publishing Co., Easton, PA.

Ransome, F.L. (1922) Ore deposits of the Sierrita Mountains, Pima County, Arizona, in Contributions to Economic Geology (Short Papers and Preliminary Reports), 1921 - Part I.--Metals and Nonmetals except Fuels: U.S. Geological Survey Bulletin 725, p. 407-428.

Webber, B.N. (1929), Marcasite in the contact metamorphic ore deposits of the Twin Buttes District, Pima Co., AZ, Econ.Geol.: 24: 304-310.

Guild, F.N. (1934), Microscopic relations of magnetite, hematite, pyrite and chalcopyrite, Economic Geology: 29: 107-120.

Mayuga, M.N. (1942) The Geology and Ore Deposits of the Helmet Peak Area, Pima County, Arizona, PhD thesis, University of Arizona.

Wilson, E.D., et al (1950), Arizona zinc and lead deposits, part I, Arizona Bureau of Mines Bull. 156: 39-51.

Galbraith, F.W. & D.J. Brennan (1959), Minerals of Arizona: 39, 60, 61, 81, 90, 92, 100.

Irvin, G.W. (1959), Pyrometasomatic deposits at San Xavier mine, in Southern Arizona guidebook II, Arizona Geological Society Digest: 2: 195-197.

Lacy, W.C. (1959) Structure and ore deposits of the east Sierrita area, in Heindl, L.A., ed., Southern Arizona Guidebook II, combined with the 2nd annual Arizona Geological Society Digest: Arizona Geological Society, p. 184-192.

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.

Keith, Stanton B. (1974), Arizona Bureau of Geology & Mineral Technology, Geological Survey Branch Bull. 189, Index of Mining Properties in Pima County, Arizona: 133 (Table 4).

Anthony, J.W., et al (1995), Mineralogy of Arizona, 3rd. ed.: 101, 118, 126, 158, 174, 184, 205, 207, 215, 216, 229, 245, 248, 250, 284, 292, 341, 346, 374, 377, 386.

Arizona Bureau of Mines file data.

A Cu-Pb-Zn-Ag-Au-Mo-W mining area located in T.16-18S., R.11-13E., on the eastern margin of the Sierrita Mountains, from 18 to 30 road miles SSW of Tucson. As generally considered, it comprises the Mineral Hll, San Xavier, Olive Camp, and Twin Buttes areas, although the US. Minerals Yearbooks include with it the Papago and other districts of the Sierrita Mountains.

A pediment slopes gently eastward towards the Santa Cruz Valley from the foot of the Sierrita Mountains. This pediment ranges in altitude from approximately 4,500 feet on the west to 3,300 feet on the east. Rising prominently above it are scattered hills, such as the Twin Buttes, Helmet Peak, and Mineral Hill. Numerous eastwrd-flowing arroyos carry the runoff towards the Santa Cruz River. Topography is shown on the Twin Buttes quadrangle sheet.

Because of faulting, igneous intrusion, and alluvial cover, few if any of the sedimentary rock units are exposed in their original entire thickness. Over much of the area structural relations are obscure, and many features of the igneous bodies are not revealed.

The ore deposits in this district include contact, replacement, and vein types. These types are somewhat related to one another. Available evidence suggests that the ore solutions ascended through fissures of northeast to eastward strike. The principal mines are less than a mile from the granite outcrop.

The contact zone, nearest the granitic intrusive, is characterized by garnet and other ferric iron minerals; next outward is a zone containing ferrous iron, marked by hedenbergite; and farther outward, sulpides occur associated with little or no contact minerals (Mayuga).

Mineralization is varied: (1) Small, shallow-seated, low temperature fissure veins with spotty, partly oxidized base metal sulfides and relatively rich gold and silver values, in faults and fractures in Cretaceous sedimentary formations and andesite intrusives within the San Xavier thrust sheet; (2) Pyrometasomatic deposits with copper and/or lead-zinc mineralization in silicified Paleozoic limestone, often associated with fault contacts and bordering Laramide intrusions; and, (3) Large, irregular disseminated copper orebodies with minor or considerable molybdenite and locally minor sphalerite and galena in pyrometamorphosed sedimentary and volcanic formations or in complex intrusives.

The contact deposits are limestone replacements which are characterized by association with garnet, epidote, wollastonite and other silicates. These minerals occur somewhat erratically distributed in masses along fissures of various trends. In places they appear to be genetically connected with fissures of NE to eastward strike.

Fissure deposits containing Pb-Ag-Zn occur in arkosic or volcanic rocks of the Olive Camp area. Some gold-pyrite veins are found n quartzite breccia of the Alpha group area. Quartz-scheelite veins occur in arkosic beds of the Senator Morgan mine area. In general the veins trend northeasterly to N.8ºW. and dip steeply.

Workings include numerous small to large underground mines and prospects, and large open pit operations. Total estimated and reported production up through 1972 (now greatly outdated), would be some 369,707,400 tons of ore containing 2,100,000 tons of copper, 42,000 tons of lead, 116,000 tons of zinc, 31,000,000 oz. of silver, 53,700 oz. of gold and 33,000 tons of molybdenum.