Palaeochannel-controlled depositional features of the cretaceous lower Mahadek uranium Mineralisation in Umthongkut area, West Khasi Hills, Meghalaya

Abstract

Sedimentological studies along Horin nala section of Umthongkut area in the lower Mahadek sediments have enabled the characterization of channel and resulting sandstone body. Nine lithofacies repeat to generate multistoried sandstone bodies deposited in channel belts. Lithofacies types identified were grouped into facies assemblages and into three different architectural elements. Based on the architectural elements the depositional palaeo-environment, resemble multichannel, low sinuosity braided system, considered as the product of the complex evolution of a fluvial fan segment. Palaeo-current data analysis indicates a low sinuosity river system with consistent current directions pointing toward south. The provenance for the sandstone type uranium mineralisation is primarily related to the basement crystalline rocks like granite and granite-gneisses, which apparently supplied the uranium. As the channel fills tend to have more porosity due to difference in the sizes of grains and clasts and with the tendency of concentration of more organic matter along these loci, the interaction of mineralizing fluids along with the reductants caused the precipitation of the uranium.