Numerical Modeling Of Fluid Flow And Solute Transport:Implications For The Formation Of Unconformity-related Uranium Deposits

Although the unconformity-related uranium deposits hosted by Paleoproterozoic sedimentary basins in Canada and Australia currently supply more than30percent of global uranium, their genetic models are still uncertain. A series of numerical experiments based on the finite element and finite difference modeling have been carried to investigate ore-forming fluid system related to uranium mineralization. We constructed conceptual models by integrating important hydrogeological features shared by the Athabasca, Thelon and Kombolgie basins. Based on these conceptual models, various numerical scenarios were designed to investigate the interaction among fluid flow, heat transport, topographic relief and tectonic deformation. Equations governing these processes were solved by FEFLOW and FLAC.The modeling suggests that buoyancy-driven thermohaline convection develops in the thick sandstone sequence at any geothermal gradient of25-35℃/km during periods of tectonic quiescence. Thermohaline convection may penetrate into the basement for up to1-2km below the basal unconformity when typical hydrological parameters for these Proterozoic hydrogeological units are used. Fluid flow velocities in the sandstone sequence are several orders of magnitude larger than those in the basement. If a uranium source (a pore fluid with500mg/1U) is assumed to be located in the center of the basin below the unconformity, uranium is able to gradually spreads into the sandstone aquifer through thermohaline convection. If the uranium source is initially located at the centre of the aquifer, a uranium plume develops and percolates down to2km below the unconformity at5m.y.. The location of the uranium source also affects the solute transport efficiency. A uranium source located around the sloping basal unconformity, either in the basin fill or basement, close to the basin margin, leads to a wider uranium plume than if it is located near the center of the basin. Given appropriate hydrological conditions, thermohaline convection could have caused widespread interaction of basinal brines with basement rocks or basement-derived fluids in uranium-bearing Proterozoic basins, and that enough uranium could have been leached from the uranium-rich basement to form large, high-grade unconformity-related uranium deposits.Reactivating of preexisting basement structures and the generating of new faults suppress free convection and led to deformation-dominated fluid flow or mixed convection, depending on strain rates. During compressive deformation, reduced brines in the basement may be forced out along fractured zones and encounter uranium-bearing fluids in the clastic sequence to form sandstone-hosted deposits. By contrast, basement-hosted deposits are likely to form during extension, when oxidized basinal brines flow into faulted structures to interact with reduced minerals or fluids in the basement. The rate of pressure accumulation and dissipation is different in various geological units depending on their hydrogeological properties and strain rates, and this different cause fluid migration across adjacent sequences. Thus, the combined effect of thermohaline convection and tectonic deformation leads to the development of unconformity-related uranium deposits at interactions of the basal unconformity with faults or sheared zones.