Pa-231 and Th-230 relations in several ground water saturated sandstones containing U ore indicate that much of the U has been accumulating in very recent times. Samples from the Hauber mine, Crook County, Wyoming, were selected to illustrate the concept of recent accumulation and the methods of calculation of the estimated minimum and maximum dates of the start of the U accumulation. The radiochemical results of 8 samples from this mine show extremely consistent radioactive daughter product distribution, and a close correlation between the estimated dates of the start of U accumulation and the U content of the ore. The results for mill pulp samples, representing large tonnages of ore, indicate that the major part of U deposition started between 40,000 and 130,000 years ago and the rate of deposition has increased approaching the present time.

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