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

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 and²³⁰Th, the latter produced by the decay of dissolved²³⁴U, 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 of²³⁰Th is estimated at about ninefold relative to²³⁸U. The adsorption of radionuclides chemically analogous to Th, such as Pu (IV) and Np, and²³⁷Np 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.