Redistribution of natural 129I among mineral phases and ground water in the Koongarra uranium deposit, N.T., Australia

Redistribution of fission-product 129I between ore and ground water is being evaluated in field studies at four uranium ore deposits. Most attention has been given to the Koongarra deposit. Weathered primary ore samples have atomic 129I/U ratios ranging from 5 to 22% of values predicted for equilibrium, indicating preferential loss of 129I relative to U during weathering. Samples from zones of relatively recent U deposition under slightly oxidizing conditions show considerable enrichment in 129I relative to U, which is attributed to sorption of 129I onto Fe oxide or clay surfaces. Water samples from the four deposits also indicate strong partitioning of 129I into ground water during weathering of the ore. Maximum 129I concentrations and 129I/I ratios occur in boreholes nearest the primary ore bodies; these values are generally 1 to 2 orders of magnitude higher than those for boreholes up-gradient of the deposits. 129I contents in the water drop sharply with distance down-gradient of the Koongarra deposit, which is consistent with the significant sorption indicated by drill-core analyses. (author)