By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Valley County, in west-central Idaho, was formed in 1917 and is one of the newest counties in the State. The early gold production of the area now known as Valley County was reported under Idaho or Boise Counties. Staley (1946, p. 28) credited Valley County with 96,578 ounces of gold from 1917 through 1942. Total gold production from 1917 through 1958 was 324,460 ounces, most of which was mined from lode deposits of antimony-gold ore in the late 1940's. The gold districts are in the northeastern part of the county in a triangular-shaped area with Edwardsburg at the apex and the Yellow Pine and Thunder Mountain districts at the southwest and southeast corners respectively. Each side of the triangle is about 15 miles long. Edwardsburg, however, had only minor gold production and will not be discussed here.
THUNDER MOUNTAIN DISTRICT
The Thunder Mountain district is in T. 19 N., R. 11 E., on Monumental Creek, in northeastern Valley County.
Discovered in 1896, the Thunder Mountain district is a typical example of the effect of rumor on the gold-fever-ravaged minds of that day. Accounts of wondrously rich gold ore attracted several thousand people to the district in 1902, and the towns of Belleco and Roosevelt sprang up; Roosevelt was the principal business center (Shenon and Ross, 1936, p. 18). The boom lasted until 1907 when the principal producer, the Dewey mine, closed regular operations. In 1909 a landslide destroyed the town of Roosevelt, and this disaster stifled the enthusiasm of those still remaining in the district (Shenon and Ross, 1936, p. 19). After 1909 there were only intermittent operations in the district, chiefly at the Dewey and Sunnyside mines.
The total value of production of the district to about 1940 was $400,000, most of which came from the Dewey and Sunnyside mines (Ross, 1941, p. 96). Although most of this was in gold, silver was also important as evidenced by the Dewey mine production record of 14,342 ounces of gold and 8,484 ounces of silver from 1902 to 1919 (Shenon and Ross, 1936, p. 38). Total gold production of the district through 1959 was probably about 17,500 ounces.
Bedrock in the Thunder Mountain district consists of tuffaceous and rhyolitic rocks interbedded with sandstone, shales, and breccias, all considered part of the Challis Formation of late Oligocene or early Miocene age (Shenon and Ross; 1936, p. 10). Locally, patches of basalt cap the higher areas.
The ore bodies are in altered rock, usually in the more permeable beds; at the Dewey mine, for example, the ore is in altered rhyolitic tuff, sandstone, rhyolitic lava, and breccia, in a steeply dipping shear zone. Pyrite and pyrargyrite were the only recognized ore minerals, and the gold was associated with pyrite irregularly concentrated in the rocks. The host rocks are highly silicified (Shenon and Ross, 1936, p. 39). At the Sunnyside mine the ore occurs in flow breccia overlain by interbedded sandstones, shales, and conglomerate; a mudflow overlies much of the area. At some places where the mudflow is close to the breccia, blanketlike ore bodies are formed. Apparently the mud acted as an impermeable barrier to the upward-moving ore solutions. The ore is highly oxidized although patches of pyrite can be found locally.
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