By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
The major gold-producing districts of Custer County are in its western part.
From 1881 through 1942, the county produced 252,879 ounces of gold (Staley, 1946, p. 18) ; total production through 1959 was 329,586 ounces. The Yankee Fork district was the most productive, although the Loon Creek district also produced considerable gold. Small amounts of gold were produced as a byproduct of the copper ores of the Alder Creek district and the lead, zinc, and copper mines in the Bayhorse district. Placers were worked at several localities along the Yankee Fork of the Salmon River.
ALDER CREEK DISTRICT
The Alder Creek district is in southeastern Custer County near Mackay and includes Tps. 6 and 7 N., Rs. 23 and 24 E.
Ores rich in copper were discovered in this district in 1884, after the rich lead-silver discoveries at Nicholia to the northeast at the site of the district's chief mine, the Empire (Ross, 1930a, p. 7). After many failures to produce copper and after the expenditure of about $3 million, success was finally achieved in 1905 (Umpleby, 1917, p. 93), and the mine remained active through 1929. Sporadic production was also reported from 1940 through 1951.. Mining resumed in 1957 and was continuing in 1959.
From 1884 to 1913 the Empire produced about $100,000 (about 5,000 ounces) in byproduct gold (Umpleby, 1917, p. 94). The Empire and Horseshoe mines produced 24,710 ounces from 1912 through 1928 (Ross, 1930a, p. 8-9), and the district produced 3,770 ounces from 1939 through 1959. Total gold production for the district through 1959 was about 33,500 ounces.
The district is underlain by folded Paleozoic sedimentary rocks, intrusive granitic and monzonitic rocks, and volcanic rocks of the Challis Volcanics. The Paleozoic rocks are mostly thick-bedded dolomitic limestone containing Mississippian fossils (Ross, 1930a, p. 13). In the Empire mine area the limestone is intruded by a large mass of granitic rocks and by a swarm of porphyritic dikes that follow a broad zone of regional faulting.
Ore deposits in the district are largely of the contact-metamorphic type and are along the limestone-granite contact. Some ore bodies of the Empire deposit are in large blocks of limestone isolated well within the granite (Umpleby, 1917, p. 97). The primary ores contain intergrowths of garnet and chalcopyrite and subordinate amounts of pyroxene, pyrite, and pyrrhotite. Oxidized ores, which were highly productive, contain a mixture of chrysocolla, azurite, malachite, and cuprite. Secondary copper sulfides are rare (Umpleby, 1917, p. 98-99).
LOON CREEK DISTRICT
The Loon Creek district is between lat 44°32' and 44°38' N. and long 114°45' and 114°52' W., in the Loon Creek drainage area.
The first mineral discoveries in this area were in 1869 when gold placers were found along Loon Creek near the abandoned town of Casto (Ross, 1934, p. 117). During the next 10 years the placers were worked out, having yielded between $1.5 and $2 million in gold.
Lode mining became significant in the district about 1902 when the Lost Packer gold-copper prospect was developed. Despite transportation difficulties, the mine remained operative through 1917, but was mostly inactive after that time. Ross (1934, p. 118) reported a production of $600,000 worth of ore for this mine, but he did not mention the amount of gold.
Considering the more conservative estimates of early placer production and assuming that at least half of the lode output of the Lost Packer mine was in gold, the total production of the district through 1959 was about 40,000 ounces. This, however, is a conservative estimate, and the production may have been several times that amount.
The geology in the vicinity of the Lost Packer mine was described by Ross (1934, p. 120, pi. 8). The ore body is in contorted Precambrian schist that was intruded by dikes and irregular bodies of aplite, lamprophyre, dacite porphyry, granophyre, and quartz monzonite. Elsewhere in the district, large areas are covered by flows and tuffs of the Casto Volcanics of Permian (?) age and the Challis Volcanics of Oligocene(?) age.
Chalcopyrite is the principal ore mineral in the lodes, which occupy steplike fissures in the schist. Also present are small amounts of tetrahedrite, pyrite, and pyrrhotite, and near the surface is bornite, native copper, and oxidized iron and copper minerals (Ross, 1934, p. 121-123). Gold is associated with the chalcopyrite and in some veins, with quartz.
YANKEE FORK DISTRICT
The Yankee Fork district is between lat 44°20' and 44°30' N. and long 114°40' and 114°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).