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The geologic study of the Lemhi Pass area in the Beaverhead Mountains, Lemhi County, Idaho, and Beaverhe.ad County, Mont., was undertaken to evaluate the area as a potential source of thorium. To accomplish this objective some of the geologic relations within the area were determined and the vein deposits were mapped and sampled. The Beaverhead Mountains in the vicinity of Lemhi Pass are underlain principally by dark-gray to greenish-gray micaceous quartzite and argillite of the Belt series of Precambrian age. Certain quartz veins in these rocks are known to contain thorite as an important constituent. Tertiary volcanic rocks that range in composition from olivine basalt to rhyolite porphyry overlie the Precambrian rocks on remnants of a well-developed erosional surface. Diorite dikes are the only intrusive rocks in the area. Faults are numerous in the Lemhi Pass area ; two major systems bound the district on the north and west. Quartz veins in the area are grouped into four types : ( 1) quartz-hematitethorite veins, ( 2) quartz-copper-bearing sulfide-thorite veins, ( 3) quartz-copperbearing sulfide veins, and (4) quartz-hematite veins. The veins range in size from a few inches wide and 100 feet long to 30 feet wide and several hundred feet long. The Th02 content of the veins ranges upward to 2.0 percent in local concentrations. A brief examination of most of the deposits shows that the deposits are small except for three localities. The three major deposits-the Last Chance, the 'Vonder Lode, and the Trapper-Lucky Strike claims-have been explored to some extent and may become economic sources of thorium.

Geology and thorium-bearing deposits of the Lemhi Pass area, Lemhi County, Idaho, and Beaverhead County, Montana