Abstract
Reconnaissance geochemical investigations of plutonic rocks in the western United States were undertaken for the purpose of determining geochemical guides and favorable areas for vein-type uranlum deposits. Gamma ray spectrometric analyses for uranium, thorium, and potassium and semiquantitative emission spectrographic analyses were obtained on approxiraately 500 samples collected from throughout the western U. S. Quantitative majorelement analyses were obtained on selected samples. The regional variations of uranium, thorium, and potassium concentrations in plutonic rocks were investigated on the basis of average values for one-degree latitude-longitude quadrilaterals and average vslues and frequency distributions for geologic subdivisions. The results in both cases indicate a general decrease in the concentrations of the three elements from the continental interior to the continental margin. Geologic subdivisions with the highest average uranium content are (a) the Northern Rocky Mountain batholiths of eastern Washington and northern Idaho, (b) the Idaho- Boulder batholiths, (c) the Southern Rocky Mountains of Colorado, and (d) the Mexican Highland of southeastern Arizona and southwestern New Mexico. Geologic subdivisions with the lowest average uranium values are the Columbia Plateau and the Colorado Plateau. Subdivisions containing major known vein-type uranium deposits are not necessarily characterized by high average uranium content. Possible geochemical guides formore » vein-type uranium deposits were determined from comparisons of chemical data for samples from plutonic bodies associated with known deposits and all samples from the western U. S. or selected groups of samples. The results indicate that plutonic bodies associated with known vein deposits are characterized by very high silicon, high aluminum, potassium, rubidium and Nockolds-Allen differentiation index, a limited range of values of the ferric to total iron ratio, low sodium, calcium, titanium, manganese, and phosphorus, very low iron and vanadium, and a high range of values of the uranium- potassium ratio. Uranium and thorium concentrations and the uranium-thorium ratio are commonly but not always higher in the plutonic bodies associated with vein deposits. Of the various chemical elements and indices considered the best possible indicators are the Nockolds-Allen index, potassium, silicon, the ferric- total iron ratio, and the uranium-potassium ratio. Uranium content is not useful as an indicator on a continental scale but it may be useful on a local or regional (geologic province) scale. Results indicate that plutonic bodies associated with known vein-type uranium deposits are characterized by unique geochemical abundances and relationships and that the determination of geochemical guides and favorable areas for vein-type uranium deposits is possible. (auth)« less
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