Many new isotopic age determinations by the uranium-lead method have been made on most of the important North American localities from which primary uranium minerals have been reported. With the aid of other published isotopic U-Pb age measurements the writers attempted to interpret the most probable age for each locality and to assign a reasonable uncertainty to that age. The recognition of differential lead loss, the relation of alteration to mineral type, the importance of radon leakage for young samples, and the potential analytical errors make it timely to conduct: such a re-evaluation. Among the well-established ages are: Spruce Pine district, North Carolina, 360 ± 20 m.y.; Portland, Connecticut, 265 ± 10 m.y.; Front Range, Colorado, 59 ± 5 m.y.; Black Hills, South Dakota, 1620 ± 20 m.y.; southeast Manitoba, 2650 ± 100 m.y.; Lake Athabasca, Saskatchewan, 1900 ± 50 m.y.; and the Wilber-force area, 1050 ± 20 m.y. Other probable ages are Bedford, New York, 350 m.y.; Colorado Plateau, 60 m.y.; Coeur d'Alene, 1200 m.y.; and Great Bear Lake, Northwest Territories, 1500 m.y. Although there may have been several regional metamorphic events in the Appalachian Province in the Precambrian, the post-Cambrian effects appear at only two periods centering on 260 m.y. and 360 m.y. The Grenville metamorphism appears limited to a narrow time interval about 1050 m.y. ago. The Front Range and Colorado Plateau uranium mineralization may have been one event occurring about 60 m.y. ago.

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