Changes in the H-O-Ar isotope composition of clays during retrograde alteration

The K-Ar ages of illite samples from the unconformity-type uranium deposits of the Athabasca basin, Saskatchewan, show a wide range. Correlations between these ages and measured delta D, delta 18O values and water contents suggest that some of the illites associated with the original deposition of the ore have undergone varying degrees of retrograde alteration by meteoric waters. The same effect is shown in some of the chlorite and kaolinite samples from the deposits. Two processes are invoked to explain the degree of alteration of the illite. Initially, the interlayer region is hydrated by meteoric water and concomitant Ar release lowers the apparent K-Ar ages. Continued interaction results in hydrogen and oxygen isotope exchange between the interlamellar water and the mineral. Chemical composition and XRD analysis of the altered illite show no detectable smectite component, but the isotopic composition suggest this may be an early precursor to a mixed-layer illite/smectite. (author)