Radioactivity recorded by clay minerals in the Shea Creek area, Athabasca Basin (Canada): Implications for uranium transfers and exploration

摘要

The understanding of uranium mobility in the geosphere is a prerequisite for the modelling of high-level nuclear waste repositories and economic uranium deposit genesis. To complement more classical geochemical and mineralogical approaches, this understanding can be improved by measuring the record of past cumulative radioactivity as stable radiation-induced defects in clay mineral structure. This study focuses on world-class unconformity-related uranium deposits of the Athabasca Basin (Canada) for which the source, timing, and paths of the uranium-bearing fluids are still matters of debate. A set of 46 samples collected from the Athabasca Group sandstones in the Shea Creek area of the western Athabasca Basin, up to 634m above either the unconformity (barren drill hole) or uranium mineralization, was selected in order to locate the paleo-occurrences of radioelements. A relevant three-dimensional view is shown by plotting (i) the concentrations of radiation-induced defects (RID's) in clay minerals, (ii) the present dose rate, and (iii) the distance to the mineralization or unconformity. The results clearly reveal different cases, such as geochemical background, equilibrated dose rate, late accumulations of radioelements, and/or records of their past temporary occurrence. Noticeable paleo-occurrences, now leached away, are revealed within 100m of the structures hosting present-day mineralized bodies, which is in line with a recent model of long range lateral paleofluid flow in a basinal permeable formation, and may be useful for exploration, albeit within a proximal range. Such results rely on the detection of RID's in clay minerals, as chemical analysis or gamma counting alone detect only the present concentration of radioelements and are unable to distinguish between accumulations, equilibration of transfers, or temporary occurrences of uranium. This study represents a first step toward spatial 3D quantitative reconstruction of U transfers, which will require time constraints and artificial dosimetry to improve models of genesis of high-grade unconformity-related deposits and to identify paleo-pathways of U migration.