Diagenetic and geochemical studies of sandstones from drill core DV10-001 in the Athabasca basin, Canada, and implications for uranium mineralization

Abstract

The Proterozoic Athabasca basin, located in northern Saskatchewan and Alberta, Canada, is known for its richness in uranium resources. The lithologies in the basin are dominated by sandstones, with minor amounts of mudstones and conglomerates. It has been the subject of debate whether the uranium in the deposits, located near the sub-Athabasca unconformity, was derived from the sedimentary rocks in the basin or from the igneous and metamorphic rocks in the basement. A diagenetic and geochemical study of sedimentary rocks from unmineralized areas in the basin may provide information about the geochemistry of the background basinal fluids, thus helping to evaluate whether or not the sedimentary rocks in the basin may have contributed uranium to the mineralizing systems. In this paper, we present the study results of sandstone samples from drill core DV10-001, which was drilled in the central part of the Athabasca basin, and includes petrographic studies of 80 polished thin-sections, point-counting data of 30 polished thin-sections, density measurements of 80 samples, and major and trace element analyses of 135 samples.The strata in the basin are generally characterized by red beds due to the presence of iron oxides and/or hydroxides (IOHs), indicating overall oxidizing conditions during sedimentation and diagenesis, but a significant portion of them have been bleached to a variety of relatively light colours. Petrographic studies indicate that precipitation of IOH and kaolinite started in early diagenesis, and may have lasted throughout much of the diagenetic history, except during bleaching events, when IOHs were dissolved. Some bleaching events may have occurred in early diagenesis, but the majority of bleaching events likely took place after a significant burial of the rocks, as indicated by the sharp contacts between the red beds and bleached parts that appear to have been controlled by fractures. Quartz overgrowths were formed after the early phase of IOH, and may have been interrupted by quartz dissolution events. Illite was formed relatively late in diagenesis, and both kaolinite and illite, as well as quartz overgrowths, are relatively enriched in the bleached rocks.Bleaching is interpreted to have been caused by introduction of relatively reducing fluids, which lowered the oxygen fugacity and increased the pH of the basinal fluids. These changes could have caused precipitation of uraninite if the basinal fluids initially contained tens of ppm of uranium, as demonstrated by some previous studies assuming that uranium was extracted from the sedimentary rocks and carried by the basinal fluids. No systematic gain or loss of uranium during bleaching has been found in our study, suggesting low concentrations of uranium in the basinal fluids at the time of bleaching. However, this interpretation does not exclude the possibility that high concentrations of uranium may have been developed in basinal fluids in early diagenesis, especially during the reddening processes. Although this study can neither confirm nor discard the hypothesis that uranium found in the unconformity-related uranium deposits was extracted from the sedimentary rocks in the basin, it puts a constraint on the timing of such uranium extraction, limiting it to early diagenesis, which is consistent with the first mineralization ages.