Uranium-series nuclides in the Golden fault, Colorado, U.S.A.: dating latest fault displacement and measuring recent uptake of radionuclides by fault-zone materials

Abstract Concentrations and isotopic ratios of U, Th and Ra were measured in a fault zone near Golden, Colorado where major displacement occurred between about 190 and 615 ka. Faulting created new surfaces for leaching and provided the pathways for U-rich ground water. Uranium and 230 Th, the latter produced by the decay of dissolved 234 U, are adsorbed by fault gouge, hematite-stained sand and brecciated sand- and claystones. The observed U enrichment is as much as six times baseline value and the simultaneous enrichment of 230 Th is estimated at about ninefold relative to 238 U. The adsorption of radionuclides chemically analogous to Th, such as Pu (IV) and Np, and 237 Np decay products, on fault-zone materials would contribute to the immobilization of high-level radioactive waste in the vicinity of a repository in the event of leakage from engineered barriers into fractured rock-mass.