Omega Mine, Monument Mountain, Helvetia, Helvetia-Rosemont Mining District, Pima County, Arizona, USAi

Regional Level Types
Omega Mine	Mine (Abandoned)
Monument Mountain	Mountain
Helvetia	Town (Former)
Helvetia-Rosemont Mining District	Mining District
Pima County	County
Arizona	State
USA	Country

A former small surface and underground Cu-Ag-Pb-Zn-Au-Mn-W-Mo mine located in West-central sec. 24, T18S, R15E, on the steep southwest slope of Monument Mountain, nearly 1.5 miles (2.41 km) east of Helvetia and 0.25 mile (ca. 402 m) south of the Isle Royal Mine, and a few miles south of the present-day town of Sahuarita, at about 5,100 feet (ca. 1,554 m) of altitude. It adjoins the Isle Royal property on the SW and comprises the Omega and Omega Extension claims. Produced 1877-1920. Owned at times, or in part, by the Zeckendorfs (since the early 1880s); 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 oxides 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 dips 30ºNE. The granite for a distance of nearly 100 feet (ca. 30 m) 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 oxides, 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 sulphides, 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, occurred. Additionally, a 2-foot (ca. 60 cm) 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 (ca. 3 m) 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 (ca. 610 m) 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 1880s to 1920. Produced 500,000 pounds (226.8 t) of Cu valued at $95,000 (period values) (1883).

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Link to USGS MRDS:	10103745

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References