Uranium series disequilibrium in a young surficial uranium deposit, northeastern Washington, U.S.A.

A recently discovered ore-grade accumulation of U in organic-rich sediments of late Quaternary age provides an opportunity for studying the early association of U, U-daughters, and organic matter in a natural setting. The U occurs in valley-fill sediments of peat, peaty clay, silt, and sand along the north fork of Flodelle Creek, Stevens County, Washington. Radiometric techniques (delayed neutron, high-resolution gamma-ray spectrometry, thin-source alpha spectrometry) were employed to determine the abundance and distribution of U-series nuclides, the extent of secular equilibrium within the U decay series, and the apparent U-series ages of U incorporation.

Sixteen lithologically distinct intervals were sampled from a 292 cm core. Uranium contents range from 140 to 2790 ppm and are positively correlated with organic contents. Measured alpha activity ratios of ²³⁴U/²³⁸U (1.31–1.38) are very similar to those reported in coexisting waters, suggesting a rather constant isotopic composition of introduced U. Much lower Th contents of <10–40 ppm are controlled by the type and abundance of silicate detritus. The youth of the host sediments (<15 000 a) and the paucity of associated radioactivity suggested large excesses of U relative to radioactive daughters and such excesses were observed, particularly in the shallowest intervals. Apparent ages of U emplacement determined by the (alpha) activity ratio of ²³⁰Th daughter to ²³⁴U parent show a general increase with depth and fair agreement with estimated depositional ages. This observation suggests dominantly syndepositional or early post depositional emplacement of U followed by decay-generated buildup of ²³⁰Th daughter with time. However, interval by interval comparisons of the relative abundances of other daughters, particularly ²²⁶Ra and ²¹⁰Pb, indicate variability caused by processes other than closed-system growth and decay, probably because chemically diverse daughters that are decay-generated in situ have differing mobilities and because upwelling ground water continuously adds more U and minor amounts of daughters. If ²³⁰Th is considered the least susceptible to these modifications, the data suggest some addition of ²³⁴U in the deepest intervals and some loss of ²²⁶Ra and/or gain of ²²²Rn throughout the studied core.