Omaha - Lone Jack Mines

Site Name

Primary: Omaha - Lone Jack Mines

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Commodity

Primary: Gold
Secondary: Silver

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Location

State: California
County: Nevada
District: Grass Valley

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Land Status

Land ownership: Private
Note: the land ownership field only identifies whether the area the mine is in is generally on public lands like Forest Service or BLM land, or if it is in an area that is generally private property. It does not definitively identify property status, nor does it indicate claim status or whether an area is open to prospecting. Always respect private property.
Administrative Organization: Nevada County Planning Department

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Holdings

Not available

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Workings

Not available

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Ownership

Not available

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Production

Not available

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Deposit

Record Type: Site
Operation Category: Past Producer
Deposit Type: Hydrothermal vein
Operation Type: Underground
Discovery Year: 1855
Years of Production:
Organization:
Significant: Y

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Physiography

Not available

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Mineral Deposit Model

Model Name: Low-sulfide Au-quartz vein

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Orebody

Form: Tabular

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Structure

Type: R
Description: Wolf Creek Fault Zone, Gillis Hill Fault, Melones Fault Zone

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Alterations

Alteration Type: L
Alteration Text: Ankeritic, sericitic, and pyritic replacement of wall rocks adjacent to veins

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Rocks

Name: Diabase
Role: Host
Age Type: Host Rock
Age Young: Mesozoic
Age Old: Paleozoic

Name: Granodiorite
Role: Host
Age Type: Host Rock
Age Young: Early Cretaceous

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Analytical Data

Not available

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Materials

Ore: Gold
Ore: Pyrite
Ore: Galena
Gangue: Quartz
Gangue: Calcite
Gangue: Chalcopyrite
Gangue: Sphalerite

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Comments

Comment (Location): The location point selected 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

Comment (Geology): An important structural feature in the district is a group of "crossing" vertical or steeply dipping fractures that strike northeast, about normal to the long axis of the granodiorite body. In places they are simple fractures; elsewhere they form sheeted fracture zones several feet wide. Some are tight, some are open and form watercourses, and few contain any quartz. Two main stages of primary or hypogene mineralization are recognized - 1) a hypothermal stage represented by one vein and one mineralized crossing, in which magnetite, pyrrhotite, pyrite, and specularite were deposited, and 2) a mesothermal stage, in which the gold quartz veins were formed. The mesothermal stage is further divided into two sub-stages - an older one, in which quartz is the principal gangue mineral, and a younger one, marked by the deposition of carbonates. Pyrite and arsenopyrite, deposited in the quartz stage, are the earliest sulfides of the gold-quartz veins. Sphalerite, chalcopyrite, and galena are somewhat later. No secondary or supergene minerals have been noted except limonite, calcite, and gypsum, which are being deposited in the oxidized zone. The distribution of gold in the ore shoots is extremely erratic and assays of adjacent vein samples commonly differ widely. Some ore shoots have a pitch length of several thousand feet, but most are much smaller. Adjacent to veins and crossing fractures, the wall rocks are generally highly altered. Ankerite, sericite, and pyrite have replaced the original rock-forming minerals. Lesser amounts of chlorite and epidote have been found. The wall rock has not been replaced by quartz. LOCAL GEOLOGY 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/ton. The ratio of silver to gold in the sulfides is great being over 2 oz. of silver to 1 oz. gold.

Comment (Workings): A map of the underground workings of the Omaha-Lone Jack (and Homeward Bound Mine) can be found in Johnston (1940, fig. 56).

Comment (Geology): REGIONAL GEOLOGY The Omaha-Lone Jack mines are within the Grass Valley District, home to California's two largest underground gold mines, the Empire and the Idaho-Maryland. The district is located in the northern portion of the Sierra Nevada Foothills Gold Belt. This belt averages 50 miles wide and extends for about 150 miles in a north-northwest orientation along the western slope of the Sierra Nevada range. The Foothills Gold Belt roughly coincides with the Foothills Metamorphic Belt, which can be subdivided into four major lithotectonic belts: Western Belt, Central Metamorphic Belt, Feather River Peridotite Belt, and Eastern Belt. The Grass Valley District lies within the Central Belt, where in the Grass Valley area it is marked by an 8-mile-wide north-trending assemblage of two accreted terranes that range from Late Triassic to Late Jurassic in age. The Central Belt is bounded on the east and west by regional-scale tectonic suture zones; the Wolf Creek Fault Zone on the west and the Gills Hill Fault/Melones Fault Zone on the east. The oldest rocks in the area are those of the Carboniferous-Triassic metasedimentary Calaveras Complex. Originally clastics, these rocks were converted to schistose or slaty rocks during the Late Paleozoic orogeny and locally into a contact-metamorphic biotite gneiss by intruded granodiorite during Late Mesozoic time. The slates of the Jurassic Mariposa Formation, which outcrop in a small part of the area, are relatively unaltered. Igneous rocks in the district include granodiorite, diabase, porphyrite, amphibolite schist, serpentinite, gabbro, diorite, quartz porphyry, and various dike rocks (Johnston, 1940). The veins of the Grass Valley and neighboring Nevada City districts are not connected with or continuations of the famous Mother Lode vein system to the south. The last veins of the Mother Lode end about 20 miles to the south. Also, the Grass Valley veins differ in general character from those of the Mother Lode. Generally, the Grass Valley veins are narrower and produce a higher-grade ore than those of the Mother Lode. The veins trend in two primary directions. One set trends N-S (dipping E or W), and the other trends E-W (dipping N or S). The major feature of the Grass Valley District is a body of Lower Cretaceous granodiorite and diabase five miles long from north to south and half a mile to two miles wide (probably the apex of a larger batholitic mass). It which is intruded into older sedimentary and igneous rocks, including diabase of the Mesozoic-Paleozoic Lake Combie Complex, and is itself cut by various dike rocks. Gold-quartz veins cut the granodiorite and diabase (and in some cases, serpentinite) throughout the district. Most of the veins strike generally north, parallel to the intrusive body, and display gentle dips averaging 35?. Others strike northwest, parallel to a diabase contact with the granodiorite. The veins fill minor thrust faults that occur within fracture zones of various width and degree of fracturing. The maximum measured reverse displacement is 20 feet (Johnston, 1940). In all veins, quartz is the principal vein material and occurs in four textural types: 1) Comb quartz that forms crustifications and lines vugs, 2) massive milky quartz with a granular texture that displays many sharp crystal faces and has not undergone deformation, 3) sheared quartz developed with little or no dilation of the vein fracture and commonly showing ribbon or shear-banding structures, and 4) brecciated quartz formed where vein movement dilated the interwall space (Johnston, 1940). Gold occurs in quartz and in sulfides, principally pyrite. Although specimen ore has been found, most ore from the district occurs as fine and coarse free-milling gold in ores averaging between 0.25 to 0.5 ounces per ton.

Comment (Development): 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 33? W. 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. The chief work on these mines was begun in 1875, and during the 1880s and 1890s, 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 for a total production of more then $2,000,000. By 1896, the Omaha shaft had attained a depth of 1500 feet on the incline and the Lone Jack, located 700 feet farther south, attained a depth of 1600 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).

Comment (Economic Factors): The Omaha-Lone Jack mines produced well in excess of $2,000,000.

Comment (Commodity): Ore Materials: Free-milling coarse and fine gold in quartz (850 fine). Auriferous pyrite and galena.

Comment (Commodity): Gangue Materials: Quartz, calcite, chalcopyrite, sphalerite

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References

Reference (Deposit): Lindgren, W., 1896a, Geologic atlas of the United States - Nevada City Special Folio: U.S. Geological Survey Folio 29.

Reference (Deposit): Lindgren, W., 1896b, Gold-quartz veins of Nevada City and Grass Valley: Seventeenth Annual Report of the U.S. Geological Survey, Part 2, p. 1-262

Reference (Deposit): MacBoyle, E.M., 1919, Mines and mineral resources of Nevada County: Sixteenth Annual Report of the State Mineralogist, California State Mining Bureau, p. 1-270.

Reference (Deposit): Clark, W.B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 191, p. 53.

Reference (Deposit): Johnston, W.G., Jr., 1940, The gold quartz veins of Grass Valley, California: U.S. Geological Survey Professional Paper 194, 101 p.

Reference (Deposit): Koschmann, A.H., and Bergendahl, M.H., 1968, Gold-producing districts of the United States: U.S. Geological Survey Professional Paper 610, 283 p.

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