HYDRODYNAMIC CONTROL ON LOCALIZATION OF URANIUM DEPOSITS IN SEDIMENTARY BASINS

Uranium deposits can be found in various geologic environments, among which sedimentary basins are economically most important. Uranium deposits within a basin are commonly concentrated in certain parts or stratigraphic intervals of the basin, which may be related to 1) the distribution of potential uranium source rocks, 2) the distribution of reducing agents required for uranium deposition, and 3) zones or stratigraphic intervals with enhanced permeabilities. Formation of uranium deposits requires a favorable combination of these factors, linked through a hydrodynamic framework.

Fluid flow in sedimentary basins may be driven by fluid overpressure, topographic relief, fluid density variation due to temperature or salinity change, and tectonic deformation. The fluid pressure regime (hydrostatic, underpressured or overpressured) plays an important role in basinal fluid flow. Generally overpressure drives upward fluid flow, and topographic relief drives downward fluid flow; development of strong fluid overpressure is unfavorable for fluid convection induced by temperature or salinity changes. If a sedimentary basin is strongly overpressured, due to rapid sedimentation, abundance of low-permeability sediments and /or generation of oil and gas, fluid flow is dominantly upward and uranium mineralization is likely limited in shallow depths, such as in the Gulf of Mexico basin. If a sedimentary basin is moderately overpressured, upward moving, reducing agents-carrying fluids may meet topography-driven, downward moving, oxidizing, uranium-bearing fluids in the middle of the basin, forming uranium deposits at moderate depths, such as in the Ordos and Chu-Sarysu basins. If a basin is nearly in a hydrostatic regime, either due to slow sedimentation or dominance of high-permeability lithologies, such as the Athabasca basin, minor topographic disturbance or density variation may drive surface-derived, oxidizing fluids to the bottom of the basin, leaching uranium either from the basin or the basement, forming unconformity-type uranium deposits. Localization of uranium deposits should be analyzed based on basin-scale hydrodynamic framework and geologic conditions specific to individual basins.