Abstract

Uranium-lead isotopic ages of the Colorado Plateau primary-uranium ores have been measured by the isotope-dilution method on discrete mineral phases. The ages obtained from Jurassic rocks range from 65 to 175 m.y. (million years) and from Triassic rocks 22–220 m.y.; it is unlikely that uranium was introduced into the Plateau during a single period.

The U238-Pb206 ages from one mine vary up to a factor of three. The U238-Pb206 and U235-Pb207 ages on the same sample commonly differ by more than the experimental error. Two hypotheses proposed to explain these problems are based on (1) subsequent loss of radiogenic lead and (2) incorporation of old radiogenic lead at the time of mineral formation. The new data present serious questions for the original radiogenic-lead hypothesis.

The isotopic ages obtained from the three ratios U238/pb206, U235/Pb207, and Pb207/Pb206 on Colorado Plateau samples are usually not concordant, and no direct relationship to the absolute geologic age can be obtained. Under the lead-loss hypothesis, however, the U235-Pb207 ages are considered the most reliable, providing at all times a minimum age.

The theory of origin of these deposits that is most probable and also consistent with the isotopic data involves deposition of uranium by ground water in H2S-rich locations. This generally occurs shortly after sedimentation but may also occur later, if circulatory conditions are favorable.

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