By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Beaverhead County, in the southwest corner of the State, had a recorded gold production of about 370,000 ounces through 1959, but early production records are incomplete and total output may be considerably larger. Before 1900 production from placers probably was considerably larger than from lodes; however, from 1904 through 1958 the county produced about 116,350 ounces of lode gold and only about 14,800 ounces of placer gold.
Rich gold placer deposits were discovered at Bannack along Grasshopper Creek in 1862. These deposits were the first significant ore discoveries in Montana and started the first rush of prospectors to Montana. The first lode mine in Montana, also in the Bannack district, was located in 1862, soon after discovery of placer gold (Shenon, 1931, p. 27).
The placer deposits along Grasshopper Creek have been the most productive in the county and probably are the only placers that have produced more than 10,000 ounces of gold. Gold has been recovered from placers along many of the other streams in the county (Lyden, 1948, p. 8-12), but operations have been intermittent and, according to available records, none of these placers yielded more than 3,000 ounces.
The lode deposits are chiefly in the north half of the county where the Mount Torrey granitic stock and several smaller satellite bodies (Corry, 1933, fig. 6) intrude Precambrian, Paleozoic, and Mesozoic sedimentary rocks. Most of the deposits are in or near the granitic intrusives and are valuable chiefly for silver and lead, but significant amounts of gold and copper have also been recovered. Gold production from lode deposits has come mainly from the Bannack, Argenta, and Bryant (Hecla) districts.
The Argenta district, about 12 miles west of Dillon, has produced gold, silver, and lead, and smaller amounts of copper and zinc, mostly from lode deposits. After the discoveries of placer deposits in the Bannack district in 1862, some placer deposits were discovered near Argenta in the early 1860's, but apparently (Winchell, 1914a, p. 66) these were successfully mined on a small scale for only a short time in the 1870's. No figures are available as to the amount of placer gold produced. Lode deposits in the district were discovered in the spring of 1865 (Shenon, 1931, p. 57), and the early lode discoveries presumably consisted of rich argentiferous lead ore which carried only small amounts of gold. According to fragmentary early mining records, eight furnaces were built at Argenta in the first few years. The ore bodies were soon depleted, and by 1875 the district was almost deserted. The next 50 years was a period of dormancy, broken by brief periods of activity (Winchell, 1914a, p. 66). In 1926 the Ermont deposits were discovered and the bulk of the gold production of the district has come from them (W. B. Myers, written commun., 1947). The amount of gold produced prior to 1904 has not been ascertained but it probably was very small. From 1904 through 1957 the district produced about 65,350 ounces of gold.
The geology and ore deposits of the Argenta district were briefly described by Shenon (1931, p. 44-77) and by Winchell (1914a, p. 65-69).
The Argenta district is underlain by folded and complexly faulted sedimentary rocks of Precambrian and Paleozoic age that are intruded by a stock of quartz monzonite and by sills and dikes of ande-site and dacite of Late Cretaceous or Tertiary age.
The ore occurs chiefly in contact metamorphic deposits in limestone, in pipelike bodies in limestone, in tabular shoots along bedding planes in limestone, and in veins in both the sedimentary rocks and in the quartz monzonite. The deposit at the Ermont mine, the most productive in the district, is in andesite along the Ermont fault and in shale beneath an andesite sill. The ore is oxidized and contains gold and limonite which is pseudomorphic after pyrite (W. B. Myers, written commun., 1947).
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