Abstract: Uranium-Lead Isotope Apparent Ages of Pitchblendes, Shirley Basin, Wyoming

K. R. Ludwig

Uranium-lead isotope apparent ages of several pitchblende samples from roll-type uranium deposits of the Shirley basin district, Wyoming, are all moderately to strongly discordant. The discordance and variability of apparent ages from clearly cogenetic pitchblendes show that isolated U-Pb isotope analyses of roll-type pitchblendes can be very misleading, even when applied to massive and apparently unoxidized samples. For example, analyses of three samples from the same pea-sized pitchblende lump differed in ²⁰⁷Pb/²³⁵U apparent ages by as much as 27 percent, and in ²⁰⁷Pb/²⁰⁶Pb ratios by as much as 11 percent. Analyses of five samples from the same hand specimen differed in ²⁰⁷Pb/²³⁵U apparent ages by as much as 12 percent and in ²⁰⁷Pb/²⁰⁶Pb ratios by as much as 18 percent.

The discordance and large variations in apparent ages evidently reflect varied degrees of both radiogenic lead loss and preferential ²³⁸U intermediate daughter leakage. Incorporation of older radiogenic lead by the pitchblende samples has not significantly affected the apparent ages. Overall ²³⁸U intermediate daughter leakage rates, integrated over the total history of the pitchblende samples, range from 12 percent (massive pitchblende) to 67 percent (equivalent to 100µ grains disseminated in the sandstone host).

Apparent ages of the pitchblende samples ranged from 6.1 to 29.6 m.y. for ²⁰⁶Pb/²³⁸U from 18.1 to 34.4 m.y. for ²⁰⁷Pb/²³⁵U, and from 295 to 2,220 m.y. for ²⁰⁷Pb/²⁰⁶Pb. The oldest and least discordant apparent ages were obtained from the purest and most massive sample. These apparent ages indicate that at least some of the uranium mineralization in the Shirley basin district must have occurred before 34 m.y. ago. No more precise information on the actual age of the pitchblende can be obtained from these data.

AAPG Search and Discovery Article #90979©1975 AAPG – SEPM Rocky Mountain Sections Annual Meeting, Albuquerque, New Mexico