Rare-earth elements in thorium-bearing veins in the Lemhi Pass quadrangle are almost as abundant as thorium, and are currently of greater potential economic importance. Total rare-earth oxide content of 31 samples from 21 veins in the area ranges from 0.073 to 2.24 percent. Rare earths and thorium commonly occur together in the same minerals, though in different proportions. The three most common of these minerals are thorite, monazite, and brockite. Some thorium is contained in goethite. Although the rare earths and thorium occur in the same minerals, the amount of rare earth found in the veins has little relation to the amount of thorium. Total rare-earth-oxide to thoria ratios in the 31 analyses range from 0.05 to 9.2.The relative amounts of the individual lanthanides commonly varies in the different veins. The most abundant lanthanides in six of the 31 samples is cerium, in 17 is neodymium, in three is gadolinium, in two is dysprosium, and in three samples two lanthanides are equally abundant. Mineral content, except where brockite predominated, could not be directly correlated to the lanthanide distribution in the veins. Gadolinium is the predominant lanthanide in the three brockite-rich samples. The yttrium content of these three samples is also relatively large. The veins in this area are unusual in that most of them have neodymium as the most abundant rare-earth element, and in that europium is also concentrated in many of them more than it is in the earth's crust. The concentration of neodymium and europium in the veins for the most part is caused by fractionation of the rare earths close to, but not at the present site of the vein. The rare-earth content of a mineral depends not only on the crystal structure of the mineral but also on which of the rare earths are available in the vein fluids. The anomalous neodymium concentration in many of the veins may be due either to the removal of much of the generally more abundant cerium or to the oxidation of the cerium in the fluids before they reached the vein site. Cerium that is oxidized from Ce (super 3+) to Ce (super 4+) will commonly be excluded from structural sites available to the Ce (super 3+) form.

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