By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
The gold production of Gem County is virtually equivalent to that of the Westview (Pearl-Horseshoe Bend) district, which sprawls across the Boise-Gem County line, about 18 miles north-northwest of the city of Boise.
Gold was mined chiefly from lodes in the Westview district. According to Anderson (1934, p. 17-18), the first development of note was at the Red Warrior in 1870, although greatest activity occurred between 1900 and 1907. Thereafter, interest waned as the easily milled oxidized ores were depleted. Lindgren (1898, p. 708) estimated that the district produced $80,000 in gold (about 4,000 ounces) to 1896, but Anderson (1934, p. 18) listed an estimate (by R. N. Bell) of ores worth more than $1 million.
Including Lindgren's estimate for the early production, the minimum total gold production for this district was about 20,000 ounces.
Country rock in the Westview district consists of a batholithic mass of quartz diorite and granodiorite (Anderson, 1934, p. 5-12). An elongate mass of diorite cuts the granodiorite and a large number of porphyry dikes cut both the granodiorite and diorite. These dikes, which are in a belt that trends east-northeast, are composed of dacite porphyry, granite porphyry, syenite porphyry, and rhyolite porphyry; some are moderately mafic in composition.
The ore deposits are mineralized fissures in the dike zone, and they may be in granodiorite, diorite, or in or along the dike contacts (Anderson, 1934, p. 18). The deposits are stringers of arsenopyrite and pyrite and contain subordinate sphalerite and galena and small amounts of chalcopyrite, tetrahedrite, boulangerite, and stibnite.
Small amounts of quartz, dolomite, and calcite gangue accompany the ore minerals, but the chief gangue component is broken and altered wallrock. Gold accompanies the sulfides and is extremely fine grained (Anderson, 1934, p.19).