Abstract

Isotopic analyses for U 234 were made on 30 samples from 5 vertical suites of ore-grade uranium-bearing sandstone from the Ambrosia Lake district, McKinley County, New Mexico. U 238 , U 235 , and U 234 were measured by mass spectrometry, and U 234 /U 238 ratios were verified by alpha spectrometry. No U 238 /U 235 isotopic variations were found above the 1/2 percent level of accuracy of these measurements. Uranium-234 variations were as great as 58 percent deficient and as much as 36 percent enriched as compared to the radioactive equilibrium value attained in a reference sample of Republic of Congo pitchblende. The pattern of U 234 fractionation found for both primary ore (classified as prefault) and redistributed ore (post-fault) samples generally shows an enrichment just above the ore and a large relative deficiency near the richest uranium ore. Samples taken above the water table show another enrichment in U 234 at the bottom boundary of the ore but samples taken below the water table do not. This pattern of U 234 fractionation can be explained by assuming that the uranium was redistributed and U 234 was gradually enriched at the boundaries of mineralization except where the lower boundary is below the water table. Samples from two vertical suites believed to be redistributed ore show the greatest deficiency in U 234 with respect to the reference standard.Radiochemical analyses of Th 230 and Pa 231 indicate a complicated history of uranium migration in the deposit, significant redistribution of uranium having occurred during the last 200,000 years. The distribution of Th 230 and Pa 231 indicates that interphase isotopic exchange of uranium was the predominant mechanism producing U 234 enrichment, although deposition of U 234 enriched uranium from solution can be a contributing factor. Leaching of uranium apparently prevails more commonly where U 234 is relatively deficient, but uranium deposition is indicated in a few samples that have a deficiency of U 234 .

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