Formation mechanisms of unconformity-related uranium deposits: insights from numerical modeling

摘要

A series of numerical experiments based on the finite element and finite difference modelings have been carried out to investigate ore-forming fluid systems related to uranium mineralization. Conceptual models were constructed 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 thick sandstone sequences at any geothermal gradient of 25to35 C/km during periods of tectonic quiescence. Thermohaline convection may penetrate into the basement for up to1-2 km 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 is assumed to be located in the center of the basin below the unconformity, uranium is able to gradually spread into the sandstone through thermohaline convection. The location of the uranium source also affects the solute transport efficiency. 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. Reactivation of preexisting basement structures and the generation of new faults suppress free convection and lead 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. Thus, the combined effect of thermohaline convection and tectonic deformation leads to the development of unconformity-related uranium deposits at intersections of the basal unconformity with faults or shear zones.