By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Lincoln County is in the northwest corner of Montana along the Idaho border. Its most valuable mineral deposits are vermiculite deposits near Libby. Lead-silver ores are also important; gold is a minor commodity. Although both placers and lodes were worked before 1901, there are no production data on this early activity. From 1901 through 1957 a minimum of about 29,000 ounces of lode gold and 4,318 ounces of placer gold was mined.
Most of the lode production came from the Libby and Sylvanite districts and the placer production came chiefly from the Libby district.
The Libby (Snowshoe) district, in southwest Lincoln County south of the town of Libby in the Libby Creek drainage basin, was the largest producer of both lode and placer gold in the county. Placer gold was discovered as early as 1867, but it was not until the early 1880's that the first mining was done, on Libby Creek (Gibson, 1948, p. 67). After 1904 these placers contributed at least 75 percent of the placer gold of Lincoln County (Lyden, 1948, p. 76-78). There was a small but steady placer production from 1931 to July 1947, when operations were suspended.
Lodes of lead-silver ore were discovered about 1887, but gold-quartz veins were not discovered until the 1890's (Gibson, 1948, p. 67), and the most productive mines were developed in the early 1900's. Production from these deposits was sporadic and small, and no lode gold production was recorded from 1945 through 1959.
From 1901 through 1937 the Libby district produced a total of about 12,400 ounces of gold (Gibson, 1948, p. 70) including about 650 ounces of placer gold. The total gold production of the district from 1901 through 1959 was about 16,300 ounces from lodes and 3,225 ounces from placers.
The following notes on geology and ore deposits of the Libby district were abstracted from the report by Gibson (1948).
The area is underlain by Precambrian sedimentary rocks of the Belt Series, consisting chiefly of quartzite, slate, and calcareous and magnesian argillite and having a total thickness of about 40,000 feet. They have been folded and faulted and intruded by metadiorite sills and dikes of Precambrian age and by stocks and dikes of granodiorite and quartz monzonite of probable Late Cretaceous or early Tertiary age.
The lodes in the Libby district are gold-quartz veins, silver-lead-zinc veins that carry some gold, and a few scattered and commercially unimportant copper veins. The lodes occupy faults and shear zones in Precambrian sedimentary rocks of the Belt Series and in metadiorite dikes and sills that intrude the Belt Series rocks. Some of the veins, especially the gold-bearing veins, have formed along bedding planes. Most of the gold has been recovered as a byproduct from the silver-lead-zinc veins, in which galena, pyrite, and sphalerite are accompanied by quartz and calcite.
The gold-bearing veins are chiefly in a small area about 20 miles south of Libby. They are quartz veins with small amounts of native gold, pyrite, galena, sphalerite, and pyrrhotite. The less common minerals in these veins are chalcopyrite, arsenopyrite, tetrahedrite, magnetite, and scheelite (Gibson, 1948, p. 71-81).
The Sylvanite (Yaak) district is a little-known district in northwest Lincoln County. As early as 1905 Sylvanite was a ghost town; the mines were idle, presumably having been abandoned when the oxidized ore was mined out (Gibson, 1948, p. 69). From 1901 to 1930 the district produced only about 75 ounces of gold (Gibson, 1948, p. 70), but from 1932, when the mines were reactivated, through 1940 it produced about 10,850 ounces of lode gold. Since then only desultory work has been done in the district.
In the Sylvanite district gold-bearing quartz-pyrite veins are in sandstones that were intruded by mafic dikes (Emmons, 1937, p. 140).
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