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).
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