By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
From 1904, when systematic compilation of production data was begun, to 1938, Ferry County was the leading gold producer in Washington. After 1938, the productive Holden mine boosted Chelan County to the lead, but in the late 1950's accelerated activity in the Republic district of Ferry County probably restored this county to its former status.
From 1896 to 1958 a minimum of 839,000 ounces of gold was mined in Ferry County. More than 99 percent of it was from the Republic district, and the remainder - a recorded aggregate of 6,000 ounces - was from the Danville district and the Columbia River placers.
The Republic (Eureka) district is in western Ferry County near the town of Republic, about 25 miles south of the international boundary.
The Republic district has had the most consistent record of large gold production of any district in the State. Prospecting began in this area when the northern part of the Colville Indian Reservation was opened in 1896. Deposits assaying high gold content were soon found, and many shipments were made until 1901 when the district closed. By 1903 railroads linked the district with large smelters on the Pacific coast, and small-scale activity was resumed.
In 1909 the discovery of substantial amounts of high-grade ore at the Republic mine and of new ore bodies on other properties resulted in a revival of the district; later large-scale production from the Lone Pine and Knob Hill mines sustained activity through 1928. Mining fluctuated from 1928 until after World War II, when the Knob Hill mine emerged as the largest and most consistent producer. In 1960 it was the third most productive lode-gold mine in the United States.
The estimated minimum total production of gold from the Republic district from 1896 through 1959 was 836,393 ounces; all production was from lode mines except for a few hundred ounces from placers.
The dominant geologic feature of the area is the Republic graben, which is 4 to 10 miles wide and at least 52 miles long. It is bounded on the northwest by the Scatter Creek and Bacon Creek fault zones and on the southeast by the Sherman and other faults (Calkins and others, 1959; Muessig and Quinlan, 1959; Staatz, 1960, p. B304). Within the graben, mostly near the margins, are narrow belts of greenstone and other rocks of Permian and Triassic ages.
Most of the graben is filled with Tertiary lavas and bedded rocks consisting of, from oldest to youngest, the O'Brien Creek Formation, Sanpoil Volcanics, and the Klondike Mountain Formation (Muessig and Quinlan, 1959). Granodiorite of Cretaceous age underlies the area northwest of the Scatter Creek and Bacon Creek fault zones, and quartz monzonite of Tertiary and Cretaceous age is exposed on the southeast side of the Sherman fault.
Other intrusive rocks, of Tertiary age, exposed in the Republic graben consist mostly of plugs, dikes, and sills of diorite, rhyodacite, quartz latite porphyry, trachyandesite, andesite porphyry, rhyolite, and basalt and stocks of quartz monzonite and granodiorite (R. L. Parker, written commun., 1960).
The Republic graben was formed in early Tertiary time, and later faulting, possibly as late as early Miocene, disrupted rocks in it. The ore deposits are along the Republic fault, a high-angle fault that trends north within the graben. Throughout most of the Republic district, ore occurs as fissure fillings along one to four veins; most veins are in altered flows of the Sanpoil Volcanics. At the Republic mine, the lower part of the Klondike Mountain Formation forms the hanging wall of the
vein. The vein minerals consist of fine-grained to chalcedonic quartz with stringers of very fine grained pyrite, chalcopyrite, stephanite, naumannite, native silver, and electrum. Adularia is common, and laumontite, marcasite, and fluorite are present in small amounts (S. J. Muessig, written commun., 1960).