Published lead-uranium ages of 11 pitchblende ores from deposits of classes 2 and 3, as defined by Sims and Sheridan, in the central Front Range, vary from a low of 35 m.y. to a high of 113 m.y. These age determinations are re-evaluated in the light of the following real or potential sources of error: (1) precision of the lead analyses; (2) precision of the lead isotope determinations; (3) uncertainties in the common lead correction arising from inappropriate choice of isotopic composition, excessive amounts of common lead, and postdepositional uranogenic enrichment; (4) leaching of uranium relative to lead; (5) radon loss; (6) sample size; and (7) loss of lead relative to uranium. It is concluded that the largest Sample to sample age differences stem from factors 1, 4, and 6 and that the largest discordancies in the measured Pb206/U238 and Pb207/U235 ages within single samples can be related to factors 2 and 3.
The ages previously published were recalculated using improved common lead corrections and newer decay constants and were ranked in accordance with criteria 1 through 7. In 4 of the 11 samples dated, the use of the improved common lead correction reversed the age sequence Pb206/U238 < Pb207/U235 < Pb207/Pb206 commonly attributed to radon loss. Samples ranked the highest for dating purposes on the basis of criteria 1 through 7 yielded concordant Pb206/U238-Pb207/U235 ages. Two such samples from class 2 deposits yielded Pb206/U238 ages of 56.6 and 58.9 m.y.; the rounded average, 58 m.y., is considered to be a best estimate of the true age of the class 2 deposits. One sample from class 3 deposits was judged to be markedly superior to the other three on the basis of criteria 1 through 7, and, in addition, yielded the most concordant Pb-U ages of any samples from the Front Range. Its Pb206/U238 age, 72.5 m.y., is considered to be a best estimate of the true age of the class 3 deposits. The preferred ages of the class 2 and class 3 deposits are in harmony with the published K-Ar ages obtained on separate members of the Cretaceous-Tertiary intrusive sequences to which, it is inferred, class 2 deposits are directly and class 3 deposits are indirectly related.