Uranium and lead isotope ratios were measured on nine ore samples and four mineral separates from the Deftmann orebody of the Key Lake uranium-nickel deposit. The results indicate that major uranium mineralization occurred at Key Lake 1,350 _ 4 m.y. ago, i.e., at least 200 m.y. after deposition of the Manitou Falls Formation of the Athabasca Group. Uranium and lead were fractionated at 300 __+ 6 m.y. These ages are defined by a straight line on the concordia diagram determined by four of nine whole-rock ore analyses. The data from the four samples define a linear relationship with mean square weighted deviation of 0.24. The time of primary mineralization is the oldest reported for Proterozoic unconformityrelated uranium deposits in the Athabasca basin. Although there is ample evidence that rocks and minerals have been open to the loss and gain of lead and/or uranium at other times, no other specific times can be determined from the data. The two-stage model of uranium and lead evolution indicated by our discordia precludes the hypothesis that the 1,350-m.y. event was an in situ redistribution of uranium mineralization that was emplaced during the Hudsonian orogeny.Data obtained in this study show that lead loss (or uranium gain) has occurred in highgrade rocks and that lead gain (or uranium loss) has occurred in low-grade rocks. Lead loss rather than uranium gain in high-grade ore is suggested by the petrologic work of Cumming and Rimsaite (1979) and the presence of excess radiogenic lead in the overlying sandstone documented by Curtis and Gancarz (1980). Some of the results obtained in this study provide evidence of preferential movement of 23su intermediate daughters out of uranium minerals and into the matrix.

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