Joel Simon Mbai^1,, Akumbom Vishiti¹, Bravo Martin Mbang Bonda¹, Gilbert François NGON NGON², Elie Constantin Bayiga², Martine Gérard³, Jacques Etamè^{1, 2}

The lolodorf syenite axis is known for its radiometric indices. In the Ngombas area the migration and retention of uranium in the regolith developed on syenite is studied using a combination of mineralogy and geochemistry in a bid to understand the processes involved in the dissolution and redistribution of uranium in the secondary environment. A trench dug in the area shows three horizons from the bottom to the top. They include the saprolite, B and Ah horizons. Petrographic and XRD investigations of the syenites reveals minerals such as plagioclase, potassic feldspar, amphibole, pyroxene, biotite, quartz, hematite, zircon coupled with uraninite, U-monazite. The regolith developed on the syenite shows relics of plagioclase, amphibole, quartz, hematite, goethite, chlorite, vermiculite, kaolinite. This is associated with U-bearing minerals such as uranothorite, U-monazite, U-zircon, U-florencite and U-rhadophane. The chemical alteration index (CIA), and gain and loss diagrams indicate that the horizons are more weathered from the top to the bottom. The presence of uranium bearing phases such as uranothorite indicates that U-minerals were dissolved, migrated and sorbed on thorite. The occurrences of U-florencite and U-rhabdophane in the weathering blanket indicate that uranyl is stabilized by phosphate minerals. Under oxidizing conditions the stability of hexavalent uranium is favored by the presence of clay minerals and Fe/Mn-oxyhydroxides. Thus, the migration of uranium in Ngombas is sequestrated by clay blended on Fe-oxides through the process of sorption. The U-bearing phases in the regolith that survived weathering include monazite and zircon. The presence of accessories minerals (U-zircon, U-monazite), sorption of uranium by phosphates, by Fe/Mn-oxyhydroxides, and clays minerals play important roles to reduce the U migration in environmental impact of Ngombas region.

uranium, migration, weathering, uraniferous syenite, tropics, Cameroon