The slightly oxidizing environment of the Mrak orebody, Wyoming, has led to U-234 deficiencies of 40 to 60 percent in ore and slight excess below the ore. In contrast, the very small amount of fractionation at the Burro mine, Colo., is attributed to the strongly reducing environment below the zone of oxidation. On the assumption that only authigenic U-234 (generated in situ from disintegration of Th-234 and Pa-234) can be preferentially leached, equilibrium relationships indicate a minimum duration of 280,000 yrs for the residence time of uranium.

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