Madison County Montana Gold Production
The Renova district, which includes Bone Basin, is located in northern Madison County in the north end of the Tobacco Root Mountains. The district produced chiefly gold and some silver, most of which came from the Mayflower mine. The date of discovery of ore in the district is not known, but there was little mining activity until 1896 when the Mayflower ore body was discovered, after which the region became prominent (Winchell, 1914a, p. 97). The Mayflower mine closed in 1905, and other mines in the district operated only intermittently and on a small scale through 1912. The district again became active during the early 1930's; the Mayflower mine was reopened in July 1935 and again became the chief producer. The district reached a peak production of 21,539 ounces of gold in 1940, but activity declined sharply after 1942 and no produc¬tion was recorded from 1953 through 1959.
Production prior to 1896 was probably of little significance. From 1896 through 1912 the district produced 60,023 ounces of gold valued at $1,282,052; most of this was from the Mayflower mine and was produced before 1905 (Winchell, 1914a, p. 101). From 1932 through 1953 production was 102,036 ounces, or a total of about 162,000 ounces.
The geology and ore deposits of the Renova district were briefly described by Winchell (1914a, p. 99-101). The oldest rocks are arkosic sandstone, sandy shale, and slate of the Belt Series of Precambrian age. These are overlain by rocks of Cambrian age and possibly younger rocks, consisting, in ascending order, of a basal conglomerate, quartzite, and shale, and limestone. Dikes of andesite and quartz porphyry cut the sedimentary rock.
The Mayflower ore, consisting chiefly of telluride minerals, is along a bedding fault in limestone. The ore above the 300-foot level was oxidized.
The ore in the other mines of the district is in veins that cut rock of the Belt Series. This ore is oxidized and contains free gold in iron oxide and quartz. The unoxidized ore consists of pyrite, chalcopyrite, and galena in a gangue of calcite, dolomite, and siderite.
SHERIDAN DISTRICT
Located in the western part of Madison County 10 to 12 miles northeast of Virginia City, the Sheridan district, which includes Ramshorn, is important chiefly for gold, but silver, copper, and lead have also been recovered. Small amounts of gold have been mined from placers. Quartz veins were discovered in 1864 (Winchell, 1914a, p. 133), and mills were erected as early as 1865. The district was a steady producer through about 1952, although output dropped sharply after 1948. No activity was reported in 1959.
Production data for 1864-1904 are not available (Winchell, 1914a, p. 139); however, several mines are known to have produced more during that period than after 1904. The total gold production from 1905 through 1952 was about 33,500 ounces, of which about 2,100 ounces was from placers.
The Sheridan district is underlain by Precambrian schist and gneiss interbedded with quartzites and limestones. These rocks are intruded by small stocks of quartz monzonite and dikes and sills of porphyry of Late Cretaceous or Tertiary age (Winchell, 1914a, p. 133-139; Hart, in Tansley and others, 1933, p. 40-45).
The ore deposits are veins and limestone replacement deposits in the Precambrian rocks. The chief sulfide minerals are pyrite, arsenopyrite, chalcopyrite, galena, and tetrahedrite in a gangue of quartz and small amounts of siderite. The gold occurs with the pyrite. Tellurides are not common but have been reported in the Indian Creek area (Hart, in Tansley and others, 1933, p. 41). Much of the ore was oxidized and consisted of gold in hematite with some copper oxides and carbonate.