Omaha Consolidated Mine (Omaha Cons.; Lone Jack Mine; Omaha vein), Empire West Mine (Lone Ridge), Empire Star group (Empire Star Mines Company holdings), Grass Valley, Grass Valley Mining District, Nevada County, California, USAi

Regional Level Types
Omaha Consolidated Mine (Omaha Cons.; Lone Jack Mine; Omaha vein)	Mine
Empire West Mine (Lone Ridge)	Mine
Empire Star group (Empire Star Mines Company holdings)	- not defined -
Grass Valley	Valley
Grass Valley Mining District	Mining District
Nevada County	County
California	State
USA	Country

A former lode Au-Pb-Ag occurrence/mine located in sec. 3, T15N, R8E, MDM, about 2.6 km (about 1.5 miles) S of Grass Valley proper on private (patented) land. Discovered in 1855. The property comprises 79.87 acres. MRDS database stated accuracy for this location is 100 meters. The location point selected by USGS for latitude and longitude represents the Omaha Mine symbol as shown on Lindgren's 1896 1:14,400-scale Grass Valley Special Map (contained in Lindgren's 1896 Nevada City Special Folio) and transcribed onto the USGS Grass Valley 7.5-minute quadrangle.

The Omaha and Lone Jack mines, as well as the neighboring Homeward Bound Mine, all followed the northerly striking Omaha vein, which dips an average 33W. This vein is one of several that comprise the Omaha-Wisconsin-Hartery vein group, a system of northerly striking, westerly dipping veins lying wholly within granodiorite. Other veins in this group include the Lone Jack, Homeward Bound, Wisconsin, Hartery, Surprise, Allison Ranch, Mary Ann-Phoenix, and several others. All mines in the vein system (except for the Phoenix) were closed and their shafts flooded by 1940 (Johnston, 1940).

The Omaha and Lone Jack mines were worked by the same company and were heavy producers during the early days of the Grass Valley District. The Lone Jack was located in 1855 and by 1867 had a shaft 600 feet deep on the incline. By this time, the Lone Jack was reported to have produced $500,000 (period values). The chief work on these mines was begun in 1875, and during the 1880's and 1890's, were steady producers. In 1885, the Omaha, Lone Jack, and Homeward Bound mines were joined under the name Omaha Consolidated Mining Co. and from 1890 to 1899, the Omaha Consolidated produced 54,966 tons of ore valued at $883,970 (pweriod values) for a total production of more then $2,000,000 (period values). By 1896, the Omaha shaft had attained a depth of 1,500 feet on the incline and the Lone Jack, located 700 feet farther south, attained a depth of 1,600 feet. The two shafts were connected by drifts on most levels. The company also operated a 28-stamp mill on the property. In 1903, the Empire West Mines acquired the group. In 1906, the mines were shut down and never reopened (Johnston, 1940).

Mineralization is a vein deposit (Mineral occurrence model information: Model code: 273; USGS model code: 36a; Deposit model name: Low-sulfide Au-quartz vein; Mark3 model number: 27) hosted in Early Cretaceous granodiorite and Mesozoic-Paleozoic diabase, and slate. The ore body is tabular, strikes N, dips 32W at a thickness of 0.91 meters and lies near a syenite contact. The vein averaged 12 inches (30 cm) thick and carried an average of 4% sulfides. Controls for ore emplacement involved mineralization occuring as erratic shoots within mesothermal, gold-bearing quartz deposited within fracture zones. Local alteration included ankeritic, sericitic, and pyritic replacement of wall rocks adjacent to veins. Associated rocks include syenite. Local rocks include Mesozoic volcanic rocks, unit 2 (Western Sierra Foothills and Western Klamath Mountains).

Beginning at the Omaha Mine and extending down the west side of Wolf Creek for over a mile are a series of parallel veins having many common features. The Omaha vein is the most prominent in this series and is traceable for 4500 feet. The most northerly outcrops appear on the east side of Wolf Creek a short distance north of the Omaha Mine. The Omaha vein, which has a consistent strike at the surface, develops noticeable curves in strike at depth and dips on average of 33OW.

The Omaha vein averages 1 foot wide, which is narrower than most veins in the district. Hanging and foot walls are sharply defined in granodiorite, though sheeting parallel to the walls is present in some places. North of the Omaha shaft, all drifts soon run from granodiorite into diabase. Although wall rocks contain pyrite, they are less altered by the introduction of carbonate and sericite than is general in the district. Quartz is confined to the ore shoots and is principally massive, with little ribboning or shearing. This character, with the narrowness of the vein, indicates little or no post-quartz movement occurred and that there were fewer pulses of quartz deposition than is common in the district.

The principal pay shoot dips to the south, beginning at the upper part of the Omaha Shaft and extending toward the bottom of the Lone Jack. A strong crossing of barren fissures traverses the vein along the principal pay shoot with a steep dip to the south. On the twelfth level the vein is faulted about 1 foot and downthrown to the south.

Outside of pay shoots, the vein closes to a seam, so virtually all quartz was good ore. The country rock next to the vein is impregnated with pyrite, but is unusually fresh and hard. Calcite occurs to only a limited extent in the wall or vein.

Ore is high-grade and ranges from $20-$30 per ton with abundant free gold of 825-845 fineness.

Pyrite and galena are the principle sulfides with little chalcopyrite and sphalerite. Assays of sulfides, which make up to 4% of the ore, range from $60 up to $460 (period values)/ton. The ratio of silver to gold in the sulfides is great being over 2 ounces of Ag to 1 ounce Au.

Regional geologic structures include the Wolf Creek Fault Zone, Gillis Hill Fault, and the Melones Fault Zone.

Workings include underground openings with a length of 1,524 meters and an overall depth of 426.72 meters and were comprised of 2 inclines and extensive drifting on 17 levels. A map of the underground workings of the Omaha-Lone Jack (and Homeward Bound Mine) can be found in Johnston (1940, fig. 56).

Ore materials include free-milling coarse and fine gold in quartz (850 fine), auriferous pyrite and galena. Gangue materials include quartz, calcite, chalcopyrite, and sphalerite.

Production data are found in: Johnston (1940).

The Omaha-Lone Jack mines produced well in excess of $2,000,000 (period values). Total production opn the Omaha vein was $3,500,000 (period values) (per MRDS file #10101214).

Analytical data results: Averaged $20 to $30 (period values) Au/ton, sulfides averaged $80 to $130 (period values) Au/ton (concentrate).

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Link to USGS MRDS:	10101214

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References