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.
YELLOW PINE DISTRICT
The Yellow Pine district is between lat 44°50' N. and long 115°00' and 115°30' W., near the town of Stibnite.
In about 1900, during the rush to the Thunder Mountain district, deposits of quicksilver, antimony, and gold were found in the Yellow Pine district. No work of any consequence was done, however, until 1917, when the demand for quicksilver encouraged development of several properties, notably the Fern and Hermes mines (Cooper, 1951, p. 152). Gold-antimony deposits were developed in 1929 at the Meadow Creek mine, but this property was closed in 1938.
The Yellow Pine deposit, the major producer of the district, was discovered in the early 1900's but was not mined until 1937. At first only gold and antimony were recovered, but in 1941 scheelite was found. Activity accelerated, and during World War II the Yellow Pine mine became the largest tungsten producer in the United States. The tungsten ore was exhausted by 1945; nevertheless, large-scale mining of the antimony-gold ore continued (Cooper, 1951, p. 174-175).
At the end of 1952, the Yellow Pine mine was shut down and virtually no gold was produced from the district afterward. The nearby town of Stibnite was almost deserted by 1958. The Hermes mine remained active, however, and was still producing mercury in 1959.
The gold production of the Yellow Pine and Meadow Creek mines through 1945 was 101,437 ounces (Cooper, 1951, p. 155). Little if any gold was produced from any of the other properties. Total district gold production through 1959 was 309,734 ounces.
The oldest rocks in the Yellow Pine district are quartzite, quartzitic conglomerate, mica schist, altered limestone, dolomite, and tactite, all probably Ordovician in age (Cooper, 1951, p. 156). These rocks were folded and faulted, then intruded by a mass of quartz monzonite related to the Idaho batholith of middle Cretaceous age. There was also some postintrusion faulting that dislocated the igneous rocks (Cooper, 1951, p. 162-163). Dikes ranging from basalt to rhyolite in composition cut the quartz monzonite and fill many of the faults and shear zones.
Ore deposits, according to Cooper (1951, p. 164), are of two types: deposits of low-grade disseminated gold ore containing local concentrations of antimony, silver, and tungsten; and deposits of quicksilver. Currier (1935, p. 16-17), on the other hand, classified three types: arsenical gold ores, antimony-gold-silver ores, and mercury ores.
The gold-bearing deposits are characterized by auriferous pyrite and arsenopyrite, scheelite, and stibnite. Cooper (1951, p. 165) noted a zoning of the deposits. East of a north-south line, 1 mile east of Stibnite, only mercury deposits are found; west of the line the important tungsten-antimony-silver-gold deposits are found.
All deposits are localized along faults; the zoning is probably due to depth below the land surface at the time of mineralization (Cooper, 1951, p. 165). Most of the deposits consist of a network of small mineralized fractures and disseminations in the adjacent wallrock.