Geologically, the district is composed of fine grained gneisses and mica and hornblende schists of Archean age. Sills and dikes of aplitic granite have been intruded roughly parallel to the banding in the metamorphic rocks which in general trend northeast and dip 15-40 degrees to the northwest. Stocks of diorite and andesite flows are also present in the area east of Rochester. Basalt flows occur to the west. The main body of the Boulder batholith is exposed a few miles to the north and the igneous bodies of Rochester may be upward reaching fingers of the main bodies which may underlie the whole region (Winchell 1914; Sahinen 1935).
The ore deposits are in well defined veins which usually strike north or northeast and dip steeply to the west. The veins are associated with the granite dikes. They are commonly narrow but locally very rich. A few veins are wide and of low grade. Most veins are valued chiefly for their gold content but some are silver-lead veins. The ore minerals include: native gold, argentiferous galena, cerussite, malachite, chrysocolla, pyrite in a gangue of quartz. Vanadinite and exdemite have also been reported (Winchell 1914; Sahinen 1935).
Although statistics are not available for the early years, it has been estimated that the district produced $2 million from 1868 to 1903. From 1904 to 1912 the district produced 10,314 tons of ore that was reduced to 4,893.1 ounces of gold worth $101,149; 24,923 ounces of silver worth $15,496; 1,913 pounds of copper worth $321; and 382,086 pounds of lead worth $17,685. Total value of the ores during this period was $134,651. Prior to 1932 the district was credited with a total production of 2.5 million (Winchell 1914; McClernan 1981).
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