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).
The Bannack district, about 22 miles west-southwest of the town of Dillon, is best known for the rich placer deposits discovered along Grasshopper Creek in 1862, but lode deposits on both sides of Grasshopper Creek have produced significant amounts of gold, silver, lead, and copper.
Winchell (1914a, p. 19, 75) credited the placer deposits along Grasshopper Creek with a gold production prior to 1905 of about $2.5 million (120,950 ounces) to $3 million (145,140 ounces), most of which was mined during the 1860's, whereas Shenon (1931, p. 28, 43-44) reported the total placer production for about the same period to be about $8 million (387,034 ounces). Winchell (1914a, p. 75) reported that the Bannack district produced about $1.5 million (72,569 ounces) in gold bullion from its lode deposits, whereas Shenon (1931, p. 28) listed the gold production from lodes to be over $2 million (96,760 ounces).
Total gold production through 1959 was at least 240,400 ounces - 132,000 ounces from placers and 108,400 ounces from lodes. The district was virtually idle from 1950 through 1959.
The Bannack district is underlain by the Madison Limestone of Mississippian age, the Quadrant Quartzite of Carboniferous age, and red beds of Triassic(?) age. These rocks were intruded by several small masses of granodiorite. The eastern part of the district is covered by Tertiary volcanic rocks. The sedimentary rocks are folded and cut by faults, the most prominent of which is a thrust fault along which Madison Limestone overrides folded Triassic (?) red beds (Shenon, 1931, p. 14-26).
The ore deposits are chiefly irregular replacement bodies in the Madison Limestone near the granodiorite (Shenon, 1931, p. 39). They have yielded silver and gold and smaller amounts of lead, zinc, and copper. Much of the ore is oxidized. The most common ore minerals are native gold, tetrahedrite, chalcopyrite, galena, sphalerite, pyrite, specularite, and magnetite, and their oxidation products - malachite, azurite, chrysocolla, cerussite, anglesite, smithsonite, and manganese and iron oxides. The gangue minerals include calcite, quartz, siderite, garnet, epidote, and vesuvianite (Shenon, 1931, p. 39-40).
The Bryant (Hecla) district, in northeastern Beaverhead County about 12 miles west of Melrose, produced chiefly silver and lead and minor amounts of copper, gold, and zinc. Mining began with the first discovery of rich silver-lead ore in 1873 (Winchell, 1914a, p. 86) and continued through 1920, although production declined sharply after 1904 and was intermittent and small during the 1920's.
After the price of gold was raised in 1934, the district again was active through 1949, but since then it has been idle. From 1873 through 1912 ore valued at about $15,425,000, chiefly in silver and lead, was mined, including 11,744 ounces of gold valued at $242,800 (Winchell, 1914a, p. 86). Total gold production through 1959, all from lode deposits, was about 17,440 ounces.
Paleozoic and Cretaceous sedimentary rocks and a stock of quartz monzonite and basic dikes of Late Cretaceous or Tertiary age underlie the district. The rocks were warped into a dome and subjected to compressive forces that produced overturned folds, thrusting, and tear faults. Later, probably at the end of Tertiary time, the rocks were offset by normal faults (Karlstrom, 1948, p. 15-50).
Most of the ore deposits are replacement shoots and pockets in dolomitic limestone of Cambrian age near the quartz monzonite intrusive. Small deposits have also been found in the quartz monzonite and along dikes (Winchell, 1914a, p. 79-86). Most of the ore is oxidized, but sulfides are found in the lower levels of some mines.
The oxidized ores contain native silver, gold, cerargyrite, cerussite, malachite, azurite, chrysocolla, cuprite, smithsonite, and cala-mine; the sulfide ores contain galena, tetrahedrite, argentite, pyrite, sphalerite, chalcocite, and chalcopyrite. The gangue consists chiefly of calcite, dolomite, hematite, and quartz (Karlstrom, 1948, p. 50-51).