Mineralization in the Grants mineral belt, except for minor occurrences of secondary minerals, is commonly attributed to several distinct periods: (a) early epigenetic (Late Jurassic), (b) early redistributed (middle Cretaceous), or (c) late redistributed (Laramide). Previous geochronologic studies have supported the geologic data favoring these periods of mineralization. More recent geologic studies indicate post-Laramide mineralization, although the source for the uranium may well have been from an older, destroyed deposit as opposed to an entirely new supply of uraniferous solutions. Work on clay minerals formed penecontemporaneously with uranium mineralization at the Silver Spur Mine (host rocks, Dakota) and the Doris Mine (host rocks, Morrison) yield Rb-Sr isochron a es of 41 ± 9 m.y.B.P. with initial Sr(87/86) = 0.715 ± 0.001 and 44 ± 7 m.y.B.P. with initial Sr(87/86) = 0.724 ± 0.001. These clay minerals formed in the presence of fairly radiogenic ⁸⁷Sr as Sr(87/86) from pre-ore, host-rock calcites yields 0.709 ± 0.001; thus simple, in-situ rehomogenization of Sr isotopes did not occur. Further, at least two other suspected occurrences of Tertiary mineralization in the Churchrock district do not yield isochrons at all, but the preceding Rb-Sr data plot references 135, 120, and 90 m.y.B.P. isochrons with initial Sr(87/86) = 0.710. These data suggest incomplete rehomogenization of Sr isotopes during the Tertiary. At present the data suggest, but do not prove, a period of Tertiary mineralization in the Grants mineral elt at about 35 to 50 m.y.B.P.

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