Boise County Idaho Gold Production
The lode deposits, which are mainly in the Centerville and Gambrinus area, are of early Tertiary (?) age. They consist of fissure fillings in fracture zones in the quartz monzonite. The fissures were formed by reverse faults, in contrast with the fissures formed by horizontal movement, which characterize the lodes of Miocene age in the Pioneerville and Quartzburg districts (Anderson, 1947, p. 181).
The vein mineralogy is simple, consisting of quartz and small amounts of pyrite, arsenopyrite, sphalerite, tetrahedrite, chalcopyrite, galena, and stibnite. Gold occurs with quartz or with the sul-fides (Anderson, 1947, p. 183).
Pioneerville District
The Pioneerville (Summit Flat, Grimes Pass) district is at about lat 44°00' N. and long 115°50' W., near the settlements of Grimes Pass and Pioneerville, in the northern part of the Boise Basin. This has been predominantly a lode-mining district; its mining history is closely associated with that of Boise Basin. The district was most active before 1920. The Golden Age mine produced ore worth $200,000 between 1895 and 1920, and the Mammoth mine, $472,000 in the early days (Ballard, 1924, p. 75-76, 95). Most of this was in gold, although it included considerable silver and some lead. Only 3,340 ounces of gold was produced from 1939 through 1959. Total production for the district from 1895 through 1959 was about 25,000 ounces.
The Pioneerville district is at the north end of the "porphyry belt" discussed by Ross (1933a, p. 330-333) and Anderson (1947, p. 191). The country rock is quartz monzonite of the Idaho batholith which is cut by a zone of northeast-trending dikes of dacite porphyry, rhyolite, granophyre, granite porphyry, and diabase. These were intruded along preexisting shear zones in the quartz monzonite. The ore deposits are closely associated with the porphyry dikes and were emplaced in fissures that resulted from later movements along the old shears. The principal metallic minerals of the veins are pyrite, tetrahedrite, chalcopyrite, arsenopyrite, galena, and sphalerite. These occur in a gangue of sericitized dike rock and quartz monzonite, quartz lenses, and some calcite. Native gold occurs most abundantly in quartz or in or near the bismuth minerals galenobismutite, bismuthinite, and tetradymite. Some deposits, characterized by abundant miargyrite and pyrargyrite, are mined for silver alone. Electrum, containing about equal amounts of silver and gold by weight, is the chief ore mineral at the Comeback mine (Anderson, 1947, p. 195-203).