YANKEE FORK DISTRICT
The Yankee Fork district is between lat 44Â°20' and 44Â°30' N. and long 114Â°40' and 114Â°50' W., in northwestern Custer County.
Gold was discovered in the gravels of Jordan Creek in the mid-1870's, but these yielded only about $50,000 in gold (Umpleby, 1913a, p. 89). Ores from silver-gold lode deposits, the first of which was discovered in 1875, proved to be extremely rich. The General Custer mine alone produced $8 million before 1900 (Anderson, 1949, p. 14). However, these high-grade deposits proved to be shallow, and the district began to decline in the 1890's, and its mill closed in 1905. There were sporadic attempts to revive some properties, but no significant activity occurred until the reopening of the Lucky Boy mine in 1939. Placer mining along the Yankee Fork was also renewed about that time. World War II curtailed activities, but a few properties were reopened in 1946 and 1947. Production in the late 1940's was almost entirely by a dredge that operated along the Yankee Fork, although some small-scale production from lode deposits continued through 1957 (T. H. Kiilsgaard, written commun., 1962). The most productive placers in the district were along the Yankee Fork of the Salmon River, from the mouth of Jordan Creek almost to the mouth of the Yankee Fork.
Anderson (1949, p. 14) credited the district with a total production of gold and silver valued at $13 million to about 1948. Of this, $12 million was mined before 1910. Umpleby (1913a, p. 78) estimated that about 40 percent of this was in gold (about 252,400 ounces). From 1948 through 1959 the district produced 14,253 ounces; most of it was from dredging operations. Total gold production through 1959 was about 266,600 ounces.
Bedrock in the Yankee Fork district, according to Anderson (1949, p. 8-11), consists of contorted Paleozoic and Mesozoic sedimentary and volcanic rocks that are intruded by quartz monzonite and granodiorite of Mesozoic age. The Paleozoic rocks are the Wood River Formation of Pennsylvanian age and the Casto Volcanics of Permian (?) age. These are cut in the northwest part of the district by quartz monzonite of the Idaho batholith. The Paleozoic rocks were subjected to two periods of deformation - one at the close of the Jurassic and one at the close of the Cretaceous. During Oligocene time the Challis volcanic flows covered most of the older rocks, and these were intruded in Miocene time by relatively small masses of dacite and rhyolite porphyry (Anderson, 1949, p. 8-10). The Challis Volcanics were gently warped and fractured, and these fractures were filled by epithermal silver-gold deposits. Most of the lodes are simple fissure fillings, but where the rock was complexly fractured, the ore minerals are disseminated and the deposits resemble stockworks (Anderson, 1949, p. 15). Typical veinfilling is quartz which may be fine grained, coarse comb, or drusy. Veins characteristically contain pyrite, chalcopyrite, sphalerite, tetrahedrite, arsenopyrite, enargite, galena, steph-anite, miargyrite, pyrargyrite, argentite, aguilarite, gold, and electrum; some calcite may be present. In the weathered zones, native silver, argentite, cerargyrite, azurite, malachite, chalcocite, and co-vellite are present in variable amounts (Anderson, 1949, p. 16-17).
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