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A study of the Pb2O6 /[J and Pb207/^235 ages and the Pb2O7/Pb2°6 ratios of /the Colorado Plateau uranium ores has been undertaken by the Geological Survey on behalf of the U. S. Atomic Energy Commission because of its direct bearing on the origin of these ores. A preliminary treatment of the data for 4l ore samples containing more than 0.1 percent uranium gives a mean Pb206/U age of approximately 71 million years, a mean Pb207/^235 age of approximately 82 million years, and a mean Pb2O7/Pb2°6 ratio equivalent to an age of 425 million years . At least part of the large discrepancy between i ^^f"1*7 / >?f"ifi the mean lead-uranium ages and the much less reliable Pb /Pb ages is due to small systematic mass spectrometric errors. The extreme sensitivity of 007 , iPofi the Pb /Pb ratio to small mass speetrometric errors invalidates not only 5*0 7 i ^Ofi the Pb /Pb ages but also the correction? for the presence of old radiogenic lead and the selective loss of radon -which must be based on precise determinations of the Pb2o7/Fb2OS ratio. Errors in the chemical analyses for lead and uranium introduce uncertainties in the mean Pb2°6 /[J age of approximately jh ^ million years. The small systematic mass spectrometric errors should not increase the mean Pb2O6/Q age by more than ^ million years . Better corrections for common lead and additional corrections for selective loss of uranium and the presence of old radiogenic lead should lower the mean Pb2O6/U age by approximately 10 million years. If the entire Fb207/^)206 ratio anomaly is assumed to be due to the selective loss of radon, an improbable assumption, the mean Pb2O6/U age would increase approximately 10 million years. Our data suggest that the calculated ages are close to the true age of the ores. From these calculated ages it is reasonable to assume that the uranium "was introduced into the sediments not later than the end of the Cretaceous or the beginning of the Tertiary (55 "to 75 million years ago). This assumption differs markedly from the assumption that the present uranium deposits -were formed in the late Triassic and late Jurassic sediments of the Colorado Plateau (152 and 127 million years ago, respectively) during or soon after deposition of the sediments Careful study is continuing in order to reduce the uncertainties in interpretation of both field and laboratory data so that a satisfactory hypothesis of origin of these ores may be securely established. To place this laboratory study on a firmer basis, ages are being determined by the lead-uranium methods on many additional carefully selected samples of uraninite and other primary ore minerals.

A preliminary determination of the age of some Colorado Plateau uranium ores by the lead-uranium method