Abstract

The thorium mineralization in the Lemhi Pass area of southeastern Lemhi County, Idaho, is directed along simple to complex shear and fracture zones, and reopened copper and gold-quartz veins and lodes, in impure quartzitic and phyllitic rocks of the Precambrian Belt series. Some of the shear and fracture zones are more than 40 feet across and comprise broad zones of irregularly mineralized rock reaching distances to 2,000 feet in length. These zones contain notable concentrations of thorium and rare-earth elements along with considerable amounts of barium, alkali metals, calcium, iron, phosphorus, sulfur, and silicon and meager amounts of columbium, uranium, and perhaps other related elements. Minerals identified so far include thorite, allanite, monazite, xenotime(?), euxenite(?), apatite, specularite, barite, alkali feldspar, calcite, biotite, phlogopite, sericite, chalcedony, and quartz. The most characteristic minerals are the thorite, specularite, barite, and quartz. The specularite and thorium-bearing minerals are intimately associated and were introduced into the deposits after the micas and in advance of the barite, feldspar, calcite, and quartz. Except for the specularite and quartz and in part the barite, feldspar, calcite, and thorite, the minerals are not distinguishable without the microscope.Some of the deposits contain several percent thoria and comparable amounts of rare-earth oxides, but the average is generally under 1 percent. The area has an appreciable reserve of lode thorium. Some of the deposits are in or are being brought into production.

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