Thorium-rich veins occur in an area of about a half square mile about 3 miles east of the border station of Porthill, Idaho. Some of these veins are of particular interest because they have more thorium, coarser grain size, and less iron oxides than other known thorium veins in the United States. The veins on Hall Mountain cut Precambrian quartzite and quartz diorite. The quartz diorite is one of several Purcell sills that intrude Belt rocks. Most of the veins occur adjacent to the upper contact of the thickest sill on Hall Mountain. The veins strike from N. 42 degrees W. to N. 30 degrees E. and dip steeply. They range in known length from 6 to 700 feet and in thickness from thin seams to 13 feet. The thorium content of 23 samples from 11 veins ranged from 0.0095 to 21.0 percent. Twelve samples had a thorium content greater than 1 percent. The thorium and the mineral content of the veins are erratic, varying both among the veins and within an individual vein. Quartz and calcite are the principal gangue minerals and thorite is the principal thorium mineral. Chlorite, magnetite, limonite, pyrite, and biotite occur in many samples of these veins. In addition the following minerals in general order of abundance were found in a few of the samples: goethite, sphene, zircon, apatite, plagioclase, allanite, chalcopyrite, rutile, actinolite, pyrrhotite, muscovite, hematite, epidote, ilmenite, malachite, chrysocolla, monazite, cenosite, jarosite, dolomite, fluorite, and an unknown brown thorium mineral. Mineralogically these veins differ considerably from most thorium veins. A comparison of the veins of Hall Mountain with those at Lemhi Pass on the Idaho-Montana border indicates the following differences: (1) at Hall Mountain veins contain only sparse limonite or hematite, but at Lemhi Pass these minerals are both widespread and abundant; (2) at Hall Mountain plagioclase was the only feldspar identified and it is rather sparse, whereas at Lemhi Pass microcline is one of the common gangue minerals, and plagioclase was not noted; (3) at Hall Mountain allanite is the second most common thorium and rare-earth mineral, and monazite and cenosite were found in one sample each, whereas at Lemhi Pass monazite is the second most common thorium and rare-earth mineral, brockite is common, and allanite is rare; (4) at Hall Mountain sphene, zircon, and ilmenite are present, but are absent from Lemhi Pass; and (5) barite is absent from Hall Mountain but common at Lemhi Pass.

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