McDevitt Mining District, Lemhi County, Idaho, USAi

Regional Level Types
McDevitt Mining District	Mining District
Lemhi County	County
Idaho	State
USA	Country

According to Umpleby (1913), the McDevitt District is located about 20 miles southeast of Salmon. It extends from Haynes Creek on the northwest to Reese Creek on the southeast and from the crest of the Lemhi Range to the Continental Divide.

Most of the mining activity in the district has been concentrated in the Lemhi Pass area along the Continental Divide. Early prospecting was for precious and base metals. In 1883, the Copper Queen deposit was located and production began around 1893. Mineralization consists of copper-bearing quartz veins, sulfide veins, and replacement bodies which occur in a fault breccia zone trending N 40-70 E and dipping 40-70 NW in quartzite. The mine eventually produced a large amount of copper and significant amounts of gold and silver.

Thorium and rare-earth oxides were discovered in the Lemhi Pass area by Vhay (1951) during 1949, and by Trites and Tooker (1953) during 1950. Their discovery resulted in a large surge in exploration activity, claim staking, and numerous studies.

According to Staatz (1979a), thorium, rare-earth oxides, and copper occur in steeply dipping veins in Precambrian sedimentary rocks, generally quartzite and siltite. The veins range in length from a few feet to as much as 4,350 feet and in thickness from thin seams to as much as 40 feet. Of approximately 250 veins mapped, thorium veins constituted 87 percent while copper and barren quartz veins each made up 6 percent of the total.

Thorite is the predominant thorium mineral in the veins; monazite, a thorium and rare-earth mineral, is the next most common. The ratio of total rare-earth oxides to thoria in 25 veins was four to three. Many of the larger veins contain more than 0.4 percent ThCh (Staatz, 1979a).

Indicated thorium resources in the Lemhi Pass area are 176,500 tons ThCb and inferred resources are 128,900 tons ThC>2. Indicated rare-earth oxide resources are 186,000 tons and inferred resources are 122,000 tons (Staatz, 1979a, p. Al).