Beaverhead County Montana Gold Production |
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BANNACK DISTRICT
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).
BRYANT DISTRICT
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).
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