Lemhi Pass drilling project, Montana

Abstract

The Lemhi Pass district has the largest thorium resources of any vein district in the United States (Staatz and others, 1979, p. 10). Most of the resources occur in veins that have an average grade of between 0.1 and 2.0 percent Th0 2 . More than 200 thorium-bearing veins have been mapped in this district (Staatz, 1979, p. A34). The Lemhi Pass thorium district lies astride the Continental Divide in the east-central part of the Beaverhead Mountains along the Idaho-Montana State line. Although the area in which thorium veins are found covers more than 150 square miles, the drilling project was confined to three veins in the northwestern part of this district on the Montana side of the Continental Divide (fig. 1). An unimproved dirt road that crosses the Continental Divide at Lemhi Pass furnishes access to the drilling area. This road connects Idaho State Highway 28 at Tendoy, Idaho, 13 miles west of Lemhi Pass, with Interstate Highway 15 at the Clark Canyon reservoir/dam, 31.5 miles east of Lemhi Pass. Salmon, Idaho, the principal town to the west, is 19 miles northwest of Tendoy; and Dillon, Mont., the principal town to the east, is 18 miles northeast of Clark Canyon Reservoir. The drilling project was proposed by the Department of Energy headquarters personnel in Washington, D.C., during the spring of 1978. The purpose of the project was to demonstrate downward extension of the veins. Drilling was proposed to indicate whether several veins had a depth of 500 feet or more. Little exploration has been carried out on thorium veins. The lack of a ready market for thorium has resulted in little economic incentive for exploration. Most veins in this district are covered by 50 feet of soil.