By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
In 1866, prospectors from the Elk Creek area found gold on Napias Creek in the unexplored north-central part of Lemhi County. The town of Leesburg was soon founded and became a base from which discoveries were made in the northern part of the county at Gibbonsville, Moose Creek, Bohannon Bar, and Yellow Jacket (Umpleby, 1913b, p. 21). Gold placers and then lode deposits were discovered at Yellow Jacket in 1868. Nearly all the gold discoveries in Lemhi County were made before 1900, and most of the production, which was largely from placers, occurred in the early days. In recent years byproduct gold from copper-cobalt ores in the Blackbird district caused a significant increase in gold output.
Umpleby (1913b, p. 21) estimated that the total gold production of Lemhi County to 1911 was worth $13,702,256 (about 665,060 ounces), of which $6 million represented the output of placers before 1881.
Staley (1946, p. 22) listed annual gold production data for Lemhi County from 1874 through 1942. Total output for this period was 514,430 ounces. From 1942 through 1958, the county was credited with 56,295 ounces of gold. Total production through 1959 was 570,725 ounces, according to Staley's data, or 720,000 ounces, according to Umpleby's estimate.
The oldest rock is Archean granitic gneiss, exposed around Shoup, in the northwestern part of Lemhi County. This is unconformably overlain by argillites, phyllites, and quartzites of the Belt Series of Precambrian age. In the southern part, the Precambrian rocks are overlain by Cambrian quartzite and Ordovician, Silurian, Devonian, and Mississippian limestones and dolomites (Umpleby, 1913b, p. 30-35). Farther north in the Leesburg quadrangle, Ordovician quartzite lies unconformably on the Belt Series (Shockey, 1957, p. 10). Lake beds of Miocene age, occupying an area 8 miles wide and 90 miles long, occur in the valley of the Lemhi River near the east boundary of the county. At several localities in the northern part, granitic rocks probably related to the Idaho batholith cut the pre-Tertiary rocks (Umpleby, 1913b, p. 42-43). Numerous dikes, ranging from granite porphyry to lamprophyre, are associated with the larger intrusives. The dominant igneous rocks of Tertiary age are lava flows and welded tuffs that cover much of the central part of the county. These are known regionally as the Challis Volcanics of Oligocene age (Shockey, 1957, p. 12).
The gold lodes are fissure veins and replacement deposits along shear zones. There are two recognized periods of mineralization: Late Cretaceous or early Eocene, and late Miocene or early Pliocene (Umpleby, 1913b, p. 49).
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