Differential behavior of components of the 238U-206Pb and 235U-207Pb isotopic systems in polymineralic U ores

U-Pb systems were examined in samples (ranging from 4 to 10 cm(3) in volume) of ore material taken from along a 3.5-m profile across a zone of U mineralization exposed in an underground mine at the Strel'tsovskoe U deposit in eastern Transbaikalia. The behaviors of two isotopic U-Pb systems (U-238-Pb-206 and U-235-Pb-207) are principally different in all samples from our profile. While the individual samples are characterized by a vast scatter of their T(Pb-206/U-238) age values (from 112 to 717 Ma), the corresponding T(Pb-207/U-235) values vary much less significantly (from 127 to 142 Ma) and are generally close to the true age of the U mineralization. The main reason for the distortion of the U-Pb system is the long-lasting (for tens of million years) migration of intermediate decay products in the U-238-Pb-206((RDU)-U-238) in the samples. This process resulted in the loss of (RDU)-U-238 from domains with high U concentrations and the subsequent accommodation of (RDU)-U-238 at sites with low U concentrations. The long-term effect of these opposite processes resulted in a deficit or excess of Pb-206 as the final product of U-238 decay. The loss or migration of (RDU)-U-238 are explained by the occurrence of pitchblende in association with U oxides that have higher Si and OH concentrations than those in the pitchblende and a higher U+6/U+4 ratio. The finely dispersed character of the mineralization and the loose or metamict texture of the material are the principal prerequisites for (RDU)-U-238 loss and an excess of Pb-206 in adjacent domains with low U concentrations. Domains with low U contents in the zone with U mineralization serve as geochemical barriers (because of sulfides contained in them) at which long-lived (RDU)-U-238(Ra-226, Po-210, Bi-210, and Pb-210) were accommodated and subsequently caused an excess of Pb-206. The U-235-Pb-207 system remained closed because of the much briefer lifetime of the U-235 decay products. This may account for the significant discrepancies between the T(Pb-206/U-238) and T(Pb-207/U-235) age values. (RDU)-U-238 was most probably lost via the migration of radioisotopes at the middle part and end of the U-238 family (starting with Ra-226). The heavy Th, Pa, and U radioisotopes (Th-234, Pa-234, U-234, and Th-230) that occur closer to the beginning of U-238 decay, before Ra-226, only relatively insignificantly participated in the process. Our results show that the loss and migration of (RDU)-U-238 are, under certain conditions, the main (or even the only) process responsible for the distortion of the U-Pb system.