The effects of basement faults on thermal convection and implications for the formation of unconformity‐related uranium deposits in the Athabasca Basin, Canada

Abstract

The formation of the world‐class, high‐grade unconformity‐related uranium deposits in the Athabasca Basin (Canada) requires circulation of large amounts of fluids, the mechanisms for which are still not well understood. Recent studies advocate thermal convection as a possible driving force for the fluid flow related to uranium mineralization; however, little is known regarding how basement faults, which are spatially associated with most unconformity‐related uranium deposits, influence fluid convection and how this may affect the localization of mineralization. This study addresses these questions through simulations of thermal convection with various configurations of basement faults using the FLAC3D software. Modelling results indicate that the location, spacing, orientation and thermal conductivities of basement faults influence the size and location of thermally driven fluid convection. In a model with a single isolated fault, the fault coincides with an upwelling plume and the dip angle of the fault does not affect the fluid flow pattern; when the fault is moved laterally, the upwelling plume shifts accordingly. In the case of two vertical faults, the faults may either coincide with upwelling flow between two convection cells or be located below individual convection cells, depending on fault spacing. In the latter case, fluid may flow into and out of individual fault zones. Similar results were also obtained for models with two nonvertical (i.e. dipping) faults. Convective flow can penetrate the uppermost basement when the permeability is less than two orders of magnitude lower than that of the overlying sandstone. In this case, the basement faults not only can control the location of ascending flow, but also can passively act as fluid conduits of either flow from the basin into the basement (ingress), or flow from the basement into the basin (egress), depending on their thermal conductivities and relative locations in the models.

Uranium-rich diagenetic fluids provide the key to unconformity-related uranium mineralization in the Athabasca Basin

“…The third mechanism requires a fluid convection system in which basinal brines infiltrated into the upper part of the basement and circulated back into the basin 27 . Such a fluid convection system is theoretically possible based on numerical modeling 41,42 , and may have been further facilitated by elevated geothermal gradients as implied in the shallow-burial model 36 discussed above. However, the efficiency of this mechanism depends on the permeability of the upper part of the basement, which remains poorly constrained.…”

“…1540 Ma), the sediments probably remained poorly consolidated due to shallow burial 36 , which further facilitated brine migration and fluid-rock reaction. Finally, the high geothermal gradients implicated by the high fluid temperatures of ~200 °C 12,19,26,37 and shallow burial (~3 km) environments as constrained by regional geochronological and stratigraphic data 36 , may have enhanced fluid convection 41,42 , which further facilitated fluid-rock reactions and extraction of U from source rocks including the uppermost part of the basement.…”

Precambrian deformation belts in compressive tectonic regimes: A numerical perspective

“…The origin of the vertical structures within the WMTZ and the Hearne Craton was examined in the framework of this thermo-mechanical study. Recent fluid-thermal simulations (e.g., Li et al, 2016Li et al, , 2018 suggest that the reactivation of these inherited structures have played an important role in the formation of this metallogenic province.…”

Hydrodynamic Links between Shallow and Deep Mineralization Systems and Implications for Deep Mineral Exploration

“…5a, scenario C) is controversial too, e.g., in the North Sea Basin (Haszeldine et al, 1984;Bjørlykke et al, 1988). Fluid convection has been proposed to have played an important role in the formation of some mineral deposits in sedimentary basins, e.g., U deposits in the Athabasca Basin (Raffensperger and Garven, 1995;Li et al, 2016), and Zn-Pb-Ag mineralization in the Mt Isa Basin (Yang et al, 2006), but these studies are generally based on numerical modeling and do not have independent evidence. However, a recent petrographic study of quartz cementation and dissolution pattern combined with reactive transport modeling provided strong evidence supporting that fluid convection indeed took place in the Athabasca Basin and played an important role in U mineralization (Wang Y et al, 2021).…”

“…5a, scenario E) and then recycled back into the basin (Russell, 1988;Koziy et al, 2009;Boiron et al, 2010;Richard et al, 2010; Xue et al, 2006Xue et al, ). al., 2012bMercadier et al, 2012;Bons et al, 2014;Li et al, 2016Pek and Malkovsky, 2016;Rabiei et al, 2021). The ingress (from basin into basement) and egress (from basement into basin) fluid flow may be driven by fluid density variation or deformation (Figs.…”