Interplay between thermal convection and compressional fault reactivation in the formation of unconformity-related uranium deposits.

A number of large and high-grade unconformity-related uranium deposits formed in intracratonic Proterozoic basins, including the Athabasca Basin in Canada and the McArthur Basin in Australia. These deposits occur close to the unconformities between the Paleo- to Mesoproterozoic basinal sequences and their Archean to Paleoproterozoic metasedimentary basement, and are spatially associated with reactivated basement faults showing reverse offsets due to contractional deformation. Previous studies have suggested that fluid flow responsible for uranium mineralization in such basins may be related to thermal convection and tectonic deformation; however, the relative roles of these two driving forces, as well as their interrelationships, have not been explored in detail. In this study, two-dimensional numerical modeling of fluid flow in relation to coupled tectonic compression and heat transport was carried out to evaluate the relationships between deformation-driven flow and fluid convection, and their relative importance for uranium mineralization. The results indicate that tectonic compression at a relatively high strain rate (6.66 × 10−11 s−1) may instantaneously destroy pre-existing thermal convection developed in the sedimentary rocks in the basin, due to the rapid development of overpressure in the system. Convection may then reappear over time as deformation progresses and the overpressure wanes. By contrast, tectonic compression at a relatively low strain rate (6.66 × 10−13 s−1) does not affect pre-existing thermal convection, and deformation-driven fluid flow in the basement coexists with thermal convection in the basin. It is proposed that during an overall tectonically active period associated with a far-field tectonic event, compressional reactivation of basement faults creates permeability and causes fluid to flow toward dilatant zones accompanying individual seismic events, whereas during seismically quiet times, fluid flow driven by thermal convection dominates. The formation of the unconformity-related uranium deposits likely involves alternating relatively short-lived, deformation-driven fluid flow and relatively prolonged fluid convection events. [ABSTRACT FROM AUTHOR]