By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Fergus County in north-central Montana is east of the main mining area of the State. Gold accounts for about 99 percent of the value of metals produced in the county. From 1886 through 1950 the total gold production of Fergus County was about 653,000 ounces; all but 500 to 1,000 ounces was from lodes. The periods of greatest activity were 1901-22 and 1936-42. The bulk of the gold production from Fergus County has come from the Warm Springs district in the Judith Mountains and from the North Moccasin district near Kendall in the North Moccasin Mountains.
NORTH MOCCASIN DISTRICT
Located in the North Moccasin Mountains in west-central Fergus County about 15 to 18 miles north-northwest of Lewistown, the North Moccasin (Kendall) district produced about 425,000 to 450,000 ounces of gold through 1959, about two-thirds of the county production. In 1903 and 1904, when Fergus County was the leading gold producer in Montana, nearly all the gold came from this district.
These deposits were discovered in 1893, about 13 years after the discovery of gold ore in the Judith Mountains, but they were not developed until about 1900, when a cyanidation plant was built to recover the gold from the refractory ores which were not amenable to amalgamation. Mining thereafter was almost continuous through 1922, at which time the North Moccasin mine, the most productive in the district since 1911, closed (U.S. Bureau of Mines, 1924, pt. 1, p. 350).
The district was practically idle until the middle 1930's. In 1936 the old Barnes-King property was reopened, a 50-ton cyanide plant was built, and there was substantial production from the district through 1942. Only small-scale operations were carried on after World War II, and the district was idle from 1948 through 1959.
The North Moccasin Mountains, which lie west of the Judith Mountains, are a single, major, dome-shaped uplift formed by the laccolithic intrusion of syenite porphyry into Paleozoic and Mesozoic sedimentary rocks. Dikes and sills are also present (Blixt, 1933, p. 5).
The gold deposits are irregularly distributed in bituminous and argillaceous layers in limestone near the top of the Madison Formation, but some ore has also been found in dikes and in sandstone and shale. Mining has centered on the eastern slope of the mountains near Kendall. The ore is oxidized and consists of finely disseminated native gold, quartz, fluorite, and limonite. Unoxidized ore in prospects contains chalcopyrite, argentiferous galena, sphalerite, and pyrite (Blixt, 1933, p. 21).
WARM SPRINGS DISTRICT
The Warm Springs (Maiden-Gilt Edge) district is near the southwest end of the Judith Mountains in the central part of the county about 10 miles northeast of Lewistown. The first ore discoveries in Fergus County, which were placer gravels in the Judith Mountains, were made in either 1879 or 1880 (Lyden, 1948, p. 28). Soon afterward gold-silver lodes were found. The placers were worked on a small scale but their output is not known.
The lode mines of the district yielded gold ores worth $939,230 (about 45,400 ounces) before 1900 (Robertson, 1950, table 4, p. 10). Lode mining continued after 1900; the most prosperous period probably was 1901-21. From 1932 through 1954 activity was desultory, and the district was idle from 1954 through 1959. Total gold production through 1959 was about 200,000 ounces.
The Judith Mountains are a group of dome-shaped uplifts formed by laccolithic intrusions of Late Cretaceous or early Tertiary age into sedimentary rocks that range from Cambrian to Cretaceous in age (Weed and Pirsson, 1898, p. 457). Associated with the laccoliths are sills and dikes. The intrusive rocks are chiefly acidic porphyries. The major ore deposits are found in limestone near porphyry contacts. The ore minerals are gold and sylvanite accompanied by pyrite, galena, argentite, sphalerite, and chalcopyrite. The gangue minerals are calcite, chalcedonic quartz, and fluorite (Corry, 1933, p. 39-40).