Combined Lithological and Structural Controls on Unconformity Related Uranium Deposits (Eastern Athabasca Basin, Canada)

The Proterozoic Athabasca Basin (Canada) hosts the highest-grade uranium deposits known. They are unconformity-related uranium deposits, meaning that uranium high grades are localized at the interface between the basin and its underlying crystalline basement. Reactivated fault zones are considered to be a first-order control on the "plumbing system" that triggered the circulation of mineralizing fluids. The geometrical relation between alteration, mineralization and fault/fractured zones is however still unclear in massive uranium deposits (e.g. McArthur River, Cigar Lake) due to massive replacement by clays and massive pitchblende of primary lithological and structural markers. Recent discoveries of small to medium size uranium mineralization in Waterfound and McClean South prospects (eastern Athabasca Basin) allow to transects of mineralized systems with less degree of alteration in the basal part of the Athabasca sandstone and related basement. First results suggest that the clay-replacement halo and the uranium oxides emplacement, is controlled by the combination of a specific structural network and by lithological variations in the basal MFb formation which forms the bottom basin. In such context, mineralizing fluids were strongly channelized along steep/vertical pathways (fracture network) and along sub-horizontal pathways formed by conglomeratic layers, leading to alteration and mineralization zones characteristic of this type of uranium deposits.