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Multi-scale distribution of Potassium, Thorium, and Uranium in Paleoproterozoic granites from eastern Senegal

Potassium (K), thorium (Th) and uranium (U) are good tracers of magmatic or alteration processes and the surface concentrations of these elements may be mapped at the regional or local scale by airborne radiometric (gamma) surveys. Geological mapping and mineral exploration are therefore often supported by airborne radiometric surveys, especially in areas of scarce outcrops (e.g., tropical regions). However, a quantitative use of these data poses a problem of scale to the petrologist. The comparison between rock sample analyses and airborne values, corresponding to average concentrations over hundreds of square meters, requires knowledge of the distribution of K-Th-U concentrations at different scales. For this purpose, we undertook a multi-scale analysis of K-Th-U concentrations on four Paleoproterozoic granitoid massifs in eastern Senegal (Balangouma, Badon, Moussala and Saraya). Our method combines geochemical bulk rock analyses, four high-resolution grids (150 m*150 m, 5 m spacing) using a Field Portable Gamma Ray Spectrometer (FPGRS), and an airborne regional survey of eastern Senegal (?30 000 km2, 250 m line spacing). We find a large scatter of K-Th-U concentrations at the small scale, with the existence of right-skewed frequency distributions approaching a lognormal behavior. This implies that airborne concentrations may systematically differ from the most frequent values measured on the ground at the FPGRS or sample scale. The scale-dependence of K-Th-U distributions is considered to be, in part, a consequence of the central limit theorem applied on radiometric data.

Pages : 30-51
Année de publication : 2018
Revue : Journal of African Earth Sciences
N° de volume : 148
Type : Article
Mise en ligne par : NDIAYE Papa Moussa