Omega Mine (Omega Tunnel; Omega Extension), Monument Mountain, Helvetia, Helvetia-Rosemont District, Santa Rita Mts, Pima Co., Arizona, USA

‡Ref.: Schrader, F.C. & J.M. Hill (1915), Mineral deposits of the Santa Rita and Patagonia Mountains, Arizona, USGS Bull. 582: 92, 115-117.

Johnson, V.H. (1941) Geology of the Helvetia mining district, Pima County, Arizona: Tucson, University of Arizona, Ph.D. dissertation, 111 p.

Creasey, S.C. & G.L. Quick (1955), Copper deposits of part of Helvetia Mining district, Pima County, Arizona, USGS Bull. 1027-F: 315.

Arizona Bureau of Mines Field Notes (1971): 1(2).

Drewes, H.D. (1971) Geologic map of the Sahuarita quadrangle, southeast of Tucson, Pima County, Arizona: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-613, 1 sheet, scale 1:48,000.

Keith, Stanton B. (1974), Arizona Bureau of Geology & Mineral Technology, Geological Survey Branch Bull. 189, Index of Mining Properties in Pima County, Arizona: 127 (Table 4).

Anthony, J.W., et al (1995), Mineralogy of Arizona, 3rd. ed.: 123, 319, 374, 384.

USGS Sahuarita Quadrangle topo map.

Arizona Bureau of Mines file data.

MRDS database Dep. ID file #10103745, MRDS ID #M050179; and, Dep. ID #10283863, MAS ID #0040190339.

A former small surface and underground Cu-Ag-Pb-Zn-Au-Mn-W-Mo mine located in West-central sec. 24, T.18S., R.15E., on the steep southwest slope of Monument Mountain, nearly 1½ miles east of Helvetia and a ¼ mile south of the Isle Royal Mine, and a few miles south of the present-day town of Sahuarita, at about 5,100 feet of altitude. It adjoins the Isle Royal property on the SW and is comprised of the Omega and Omega Extension claims. Produced 1877-1920. Owned at times, or in part, by the Zeckendorfs (since the early 1880's); the Omega Copper Co.; and, McKay. The first Cu produced in any considerable quantity in Pima County came from this mine.

Mineralization is partly oxidized copper and minor lead-zinc minerals, largely soft and earthy, with manganese and iron oxides, and local massive chalcopyrite and pyrite, in replacement bodies with garnet-magnetite gangue along a fault zone between Paleozoic limestone and Precambrian granodiorite. Host rock units include the Martin Formation, Escabrosa Limestone, and the Horquilla Limestone. Ore control is the thrust fault zone. Alteration includes epidotization and altered limestone.

The limestone is partly mineralized, much epidotized, and otherwise altered. It is usually crystalline and near the contact diops 30ºNE. The granite for a distance of nearly 100 feet from the contact is sheared and altered and contains subordinate faults, dipping in all directions, especially to the north and to the south. At the joints occur much limonite, manganese oxide, and mica. Aplite dikes are also intruded along the contact between the granite and the limestone in places.

The ore is mostly a soft earthy mixture of copper and iron sulfides, with the manganese, copper, and iron-ore minerals of the oxidized zone. In the upper tunnel, a lens of almost pure massive pyrite and chalcopyrite, in places coated with limonite, occured. Additionally, a 2 foot wide vein of chalcopyrite and pyrite occurs in one tunnel with gouge on either wall. At the end of a drift, next to a very siliceous dike, probably an aplitic rock, a band of epidotized granite 10 feet wide occurs containing considerable pyrite and chalcopyrite.

Local structures include homoclinal, tilting and broad open folds in the south and extensive faulting in the north. WNW-striking thrust fault zone, permeable fracture zones.

Workings include 4 tunnels, a shaft, a winze, an open cut and minor openings, aggregating some 2,000 feet of workings distributed over a vertical distance of 300 feet (91.44 meters). One of the earliest mines, producing sporadically some 7,000 tons of ore averaging about 6% Cu, 1 oz. Ag/T and minor Pb from the 1880's to 1920. Produced 500,000 pounds Cu valued at $95,000 (period values)(1883).