URANIUM-LEAD METHOD OF AGE DETERMINATION

Abstract

A suite of samples of pitchblende, clausthalite, and galena from various mines in the Lake Athabasca uranium province was subjected to isotopic and chemical analyses as well as radon-leakage measurements in order to obtain apparent ages from the isotopic ratios Pb/sup 2-6//U/sup 238/, /pb/sup 206, Pb/ sup 210/pb/sup 206/ measurements were made to check equilibrium in the decay series. The apparent isotopic ages range from 220 to 1860 m.y. Aiteration effects which might explain these anomalies are considered. Uranium leaching and radon leakage are inadequate to explain these anomalies. The diverse ages can be calculated on the hypothesis of a single period of pitchblende deposition at 1.90 plus or minus 0.04 b.y. ago followed by two periods of exsolution of lead at about 1.2 and 0.15 b.y. This is consistent with the 207/206 ratio of the radiogenic lead in the clausthalite and galena which apparently formed at the times of recrystallization. However, the existing data do not preclude more than two periods of lead exsolution. An alternative hypothesis involving successive epochs of pitchblende deposition is more complex geologically and, without contemporaneous lead exsolution, cannot account for the observed isotopic relations. The recrystallization presumably requires only the elevation of the regionalmore » temperature to 150 to 300 deg C in the presence of water. The variable fractions of lead removed during the periods of recrystallization can be ascribed to variations in temperature and water content throughout the district. it is quite certain that (1) the initial uranium deposition occurred about 1.9 b.y. ago, (2) there have been at least two periods of lead exsolution, (3) one period of lead exsolution must have occurred subsequent to 0.22 b.y. ago; (4) another period of lead exsolution probably occurred about 1 ago. (5) alteration of subsurface pitchblende has been negligible during the last million years. These conclusions resulting from an intensive study of an important uranium province suggest some general principles which may be applied to other areas. If samples are chosen to avoid recent weathering effects and if the radon leakage is measured, the remaining anomalies in the isotopic ages are probably due to partial lead removal at some time or times during the geologic history of the mineral. In all cases the 207/206 age will be closest to the true age but will be minimal. The true age can be approximated if a large number of samples from a given locality are studied with their associated common leads. Uranium minerals that give accordant isotopic ages have a known age and indicate the absence of significant regional thermal metamorphism following formation.« less