RE-EXAMINATION OF ISOTOPIC RELATIONSHIPS IN COLORADO FRONT RANGE URANIUM ORES

Some new isotopic analyses of Front Range pitchblende samples and comparisons with published data indicate a complex history of isotopic disturbance in these ores. Relationships in the U²³⁸-Pb²⁰⁶ and U²³⁵-Pb²⁰⁷ systems indicate extensive and protracted mobilizations of parents and daughters, and particularly intermediate daughters, in both isotopic series. These relationships preclude a simple, much less unique interpretation of the absolute age of the uranium mineralization in the Front Range. Considering the lack of close definition in the stratigraphic age assignment for this mineralization, it is suggested that the common use of the inferred age in the construction of geologic time scales is not justified.