Omilak

Mine, Inactive

Commodities and mineralogy

Main commodities Ag; Pb; Sb
Other commodities Au; Cu
Ore minerals cerussite; chalcopyrite; galena (argentiferous); stibnite
Gangue minerals arsenopyrite; limonite; pyrite; pyrrhotite

Geographic location

Quadrangle map, 1:250,000-scale BN
Quadrangle map, 1:63,360-scale A-2
Latitude 65.043
Longitude -162.66
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Omilak mine is on the west end of the ridge between the North and South Forks of Mosquito Creek at an elevation of 1,450 feet. Mosquito Creek flows westward from headwaters in the northern Darby Mountains to Telephone Creek, a tributary to Fish River in McCarthy's Marsh. The confluence of the North and South Forks is at the range front where the drainages leave the mountains and flow onto the lowlands of McCarthy's Marsh. This is locality 14 of Cobb (1972; MF 417; 1975; OFR 75-429).

Geologic setting

Geologic description

The Omilak mine is one of the oldest in Alaska (Cobb, 1975). It was discovered before 1880, claims were staked in 1881, ore shipments took place between 1881 and 1890, and the claims were patented in 1884 (Brooks and others, 1901; Mendenhall, 1901; Smith and Eakin, 1911). Forty- one tons of galena were shipped in 1890; they contained 74.7% Pb, 142.2 ounces/ton Ag, and 0.1 ounces/ton Au (Smith and Eakin, 1911). Nine assays of ore shipped in the 1880s ranged from 60.7 to 173.0 ounces/ton Ag and 10.27 to 82.0% Pb and some contained up to 0.4 ounces/ton Au (Smith and Eakin, 1911). The mineral deposits consist of discontinuous lenses or veins of argentiferous galena and cerussite in fractured marble. Samples from the shaft dump contain limonite after pyrite or pyrrhotite (and possibly chalcopyrite) and marble cut by veinlets of calc-silicate minerals with disseminated pyrrhotite and pyrite(?). Typical specimens of high grade ore from the shaft dump contained 55.2% Pb, 12.1% Fe, 1.0% Sb, 0.15% Cu, 0.2% Sn, 0.1% Zn, 0.13 ounces/ton Au, and 88.01 ounces/ton Ag (Mulligan, 1962). Thin stibnite veinlets and disseminations are scattered through marble float on the dump of a caved prospect shaft 350 feet southeast of the main shaft (Herreid, 1965; Briskey, 1983). A specimen from this dump contained 0.06% Pb, 3.4% Fe, 31.7% Sb, 0.05 ounces/ton Au, and 0.19 ounces/ton Ag (Mulligan, 1962). The host rock is slightly recrystallized, partly dolomitic marble intercalated with schist in the core of a small anticline that plunges northwest and is overturned to the northeast (Herreid, 1965). The Omilak mine had a 180 foot main shaft, two working levels, and a 500 foot adit. Smith and Eakin (1911) estimated about 600 feet of workings in the main mine area and Herreid (1965) suggested that the adit may have not reached the ore zone due to an inferred plunge of the mineralization. This is a polymetallic vein and replacement in high grade metasedimentary rocks of probable Lower Paleozoic or Precambrian age (Till and others, 1986).
Geologic map unit (-162.662615824325, 65.0422929412488)
Mineral deposit model Polymetallic vein or replacement in marble (Cox and Singer, 1986; model 19a? or 22c?).
Mineral deposit model number 19a? or 22c?
Age of mineralization Cretaceous ?; Epigenetic mineralization in metamorphic rocks of Seward Peninsula is primarily of Cretaceous age.
Alteration of deposit The deposits are variably oxidized with secondary iron oxides and lead carbonates developed.

Production and reserves

Workings or exploration The Omilak mine had a 180 foot main shaft, two working levels, and a 500 foot adit. Smith and Eakin (1911) estimated about 600 feet of workings in the main mine area and Herreid (1965) suggested that the adit may have not reached the ore zone due to an inferred plunge of the mineralization. Some stopes are now caved to the surface. An inclined prospect shaft (now caved) is 350 feet southeast of the main shaft.
Indication of production Yes; small
Production notes Smith and Eakin (1911) estimate that a total of 300 to 400 tons of high grade ore was shipped from the Omilak mine. Forty-one tons of galena shipped in 1890 contained 74.7 % Pb, 142.2 ounces/ton Ag, and 0.1 ounces/ton Au (Smith and Eakin, 1911). Nine assays of ore shipped in the 1880s ranged from 60.7 to 173.0 ounces/ton Ag and 10.27 to 82.0 % Pb and some contained up to 0.4 ounces/ton Au (Smith and Eakin, 1911).

References

MRDS Number A012745

References

Briskey, J.A., 1983, Summary of field observations on Seward Peninsula mineral deposits: U.S. Geological Survey, unpublished administrative report, 34 p.
Brooks, A.H., Richardson, G.B., Collier, A.J., and W.C. Mendenhall, 1901, A reconnaissance in the Cape Nome and adjacent gold fields of Seward Peninsula, Alaska, in 1900: U.S. Geological Survey Special Publication, p. 1-185, maps.
Mendenhall, W.C., 1901, A reconnaissance in the Norton Bay region, Alaska, in 1900, in Brooks, A.H., Richardson, G.B., Collier, A.J., and Mendenhall, W.C., Reconnaissance in the Cape Nome and Norton Bay regions, Alaska, in 1900: U.S. Geological Survey Special Publication, p. 187-218.
Reporters Travis L. Hudson (Applied Geology)
Last report date 3/15/1999