This review of the geology and geochemical-geophysical attributes of the unconformity-associated uranium (U) deposit type is based mainly on deposits in the Athabasca Basin. Pods, veins, and semi-massive replacements of uraninite (var. pitchblende) are located close to unconformities between late Paleo- to Mesoproterozoic conglomeratic sandstone and metamorphosed basement. The Athabasca and Thelon basin strata are up to 1.5 km thick, overall flat-lying, apparently unmetamorphosed but pervasively altered, mainly fluvial, red to pale tan quartzose sandstone, lesser conglomerate and mudstone. The underlying red hematitic clay-altered regolith grades down through chloritic-illitic and chloritic altered rock to fresh basement gneiss. The highly metamorphosed interleaved Archean to Paleoproterozoic granitoid and supracrustal basement gneiss includes graphitic metapelite that preferentially hosts reactivated shear zones and deposits. A broad variety of deposit shapes, sizes and compositions ranges from polymetallic lenses at and above the unconformity, with variable Ni, Co, As, Pb and traces of Au, Pt, Cu, REEs and Fe, to near-monometallic and generally basement-hosted veins that in some cases are now recognized as ‘blind’ with few direct clues for exploration. Key geochemical/mineralogical anomalies for exploration include: illite, sudoïte, dravite, silicification, quartz dissolution, and >3 ppm U, as well as pathfinders such as B, Pb and Pb isotopes, Ni, Co, Cu and As in sandstone and till. Geophysical anomalies include low or high resistivity in sandstone related to clay alteration or silicification, respectively, high conductivity in basement graphitic metapelite gneiss, gravity lows related to quartz dissolution, and seismically detected offsets of the basal unconformity. Geological targets, based on integrated geophysics, multiparameter drill hole logging and lineament analysis of multiple media, include intersecting faults with specific geometries, favourable basement lithology and marked facies changes in the basal Athabasca Group.




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