科研之友
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摘要
Previous studies have suggested that buoyancy-driven convection could only have developed in thick sandstone aquifers within Proterozoic intracratonic sedimentary basins that host unconformity-related uranium deposits. On the other hand, oxygen and hydrogen isotopic and fluid inclusion studies of quartz and carbonate veins have shown that basinal brines have interacted with basement rocks and basement-derived fluids pervasively. Finite element modeling was conducted to investigate the potential mechanisms driving fluid interaction across a basement/cover unconformity. We firstly constructed a simplified conceptual model by integrating the known features shared by the Athabasca, Thelon and Kombolgie basins and their Phanerozoic counterparts. Based on this conceptual model, various numerical scenarios were designed to examine buoyancy-driven (heat and solute transport are coupled) fluid flow patterns and the corresponding solute transport. The results show that thermohaline convection may have penetrated into the basement for up to 1 to 2 km below the unconformity, when typical hydrological parameters for these Proterozoic hydrogeological units were used. Fluid flow velocities in the sandstone sequence were several orders of magnitude larger than those in the basement. If a uranium source (a pore fluid with 500 mg/l uranium) was assumed to be located in the center of the basin below the unconformity, uranium was able to gradually spread into the sandstone aquifer through thermohaline convection without considering any contribution from fluid-rock interaction. The uranium concentration of basinal fluids above the uranium source approached 15 and 24 mg/l after 1 and 5 m.y. of modeling time, respectively. If the uranium source was initially located at the center of the aquifer, a uranium plume developed and percolated down to 2 km below the unconformity at 5 m.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, led to a wider uranium plume than if it was 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 Proterozoic basins where unconformity-related uranium deposits have developed, and that enough uranium could have been leached from the uranium-rich basement to form large, high-grade unconformity-related uranium deposits.
- 出版日期2012-11
- 单位CSIRO
