Abstract

The uranium-lead ages for two massive uraninite samples from two deposits, one each in the Gas Hills and Shirley Basin districts, are both 22 + or - 3 m.y. as determined by 206 Pb/ 238 U and 207 Pb/ 235 U. These are sufficiently concordant to confirm an early Miocene time of mineralization for both deposits. Uranium-lead ages for several ore-grade samples from an upper sandstone layer of the Shirley Basin indicate 24 + or - 5 m.y. The ore samples show less concordancy in age than the massive uraninite samples and are believed to give a less reliable age because of a large common lead correction.Radioactive secular equilibrium between 238 U and 234 U was determined by alpha spectrometry to be > 99 percent for the massive uraninite samples; equilibrium between 238 U and 230 Th (ionium) was established by a direct method of isotopic mass ratios. A method of indexing the common lead correction using 208 Pb is used with these young samples which contain negligible 232 Th.Both the Gas Hills and Shirley Basin uranium deposits, located about 85 miles (137 km) apart, are in the Wind River Formation. The radioactive equilibrium found in the massive uraninite samples supports the reliability of the age found for these two samples. The loss of equilibrium in the ore-grade samples, along with the less reliable lead age, indicates that some local migration of elements occurred surrounding the roll at the Shirley Basin. Apparently this has not affected the lead-uranium system sufficiently to invalidate a date of 22 m.y. for the emplacement of both deposits and the conclusion that ore deposition stopped at about that time.

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