U-Pb dates from the Grants mineral belt of New Mexico are discordant and yield minimum dates of approximately 87 to 113 m.y. for the age of uranium mineralization. Analysis of ore stage and pre-ore clay minerals by the Rb-Sr isochron method is more reliable than other methods of age determination and indicates the minimum age of sedimentation of the Morrison Formation to be 139 to 157 m.y. (Late Jurassic). The age of uranium mineralization in the Ambrosia Lake and Smith Lake districts is interpreted to be about 139 ± 10 m.y., and that of uranium mineralization in the Laguna district is approximately 110 to 115 ± 10 m.y. Mixed provenance is indicated; volcanogenic detritus may represent contemporaneous Late Jurassic source material, but part of the plutonic detritus probably has been derived from a 1.3-b.y. old terrane.

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