Unconformity-associated uranium deposits of the Athabasca Basin, Saskatchewan and Alberta

Abstract

This review of the geology, geophysics, and origin of the unconformity-associated uranium deposit type is focused on the Athabasca Basin. Pods, veins, and semimassive replacements of uraninite (var. pitchblende) are located close to unconformities between late Paleo-to Mesoproterozoic conglomeratic sandstone basins and metamorphosed basement rocks. The thin, overall flat-lying, and apparently unmetamorphosed but pervasively altered, mainly fluvial strata include red to pale tan quartzose conglomerate, sandstone, and mudstone. Beneath the basal unconformity, red hematitic and bleached clay-altered regolith grades down through chloritic altered to fresh basement gneiss. The highly metamorphosed interleaved Archean to Paleoproterozoic granitoid and supracrustal basement gneiss includes graphitic metapelitic that preferentially hosts reactivated shear zones and many deposits. A broad variety of deposit shapes, sizes and compositions ranges from monometallic and generally basement-hosted veins to polymetallic lenses located just above or straddling the unconformity, with variable Ni, Co, As, Pb and traces of Au, Pt, Cu, REEs, and Fe. Résumé Cet examen de la géologie, de la géophysique et de l'origine des gîtes d'uranium associés à des discordances est focalisé sur le bassin d'Athabasca. Les minéralisations d'uraninite (variété pechblende) qui prennent la forme de lentilles, de filons ou de corps semi-massifs de remplacement se situent près de la discordance entre des grès conglomératiques de bassin du Paléoprotérozoïque tardif au Mésoprotérozoïque et les roches du socle métamorphisées. La succession sédimentaire de bassin est mince, repose dans l'ensemble à plat, est apparemment non métamorphisée, mais profondément altérée, et se compose principalement d'unités fluviatiles constituées de conglomérats quartzeux, de grès et de mudstones de couleur rouge à chamois pâle. Sous la discordance marquant la base de la succession sédimentaire, un régolite hématitique rouge, décoloré par endroits par une altération argileuse, passe progressivement vers les profondeurs du socle à des gneiss chloritisés puis à des gneiss non altérés. Les roches très métamorphisées du socle, formées d'une intercalation de gneiss granitoïdes et de gneiss supracrustaux de l'Archéen au Paléoprotérozoïque, incluent des métapélites graphitiques qui renferment de manière préférentielle des zones de cisaillement réactivées et un grand nombre de gîtes. Les gîtes sont de formes, de dimensions et de compositions très variées, passant de minéralisations monométalliques, généralement sous forme de filons encaissés dans le socle, à des lentilles polymétalliques présentant des concentrations variables de Ni, Co, As, Pb et des traces de Au, Pt, Cu, de terres rares et de Fe, qui se situent à cheval sur la discordance ou à peu de distance au-dessus. Definition Unconformity-associated uranium deposits are pods, veins, and semimassive replacements consisting of mainly uraninite dated mostly 1600 to 1350 Ma, and located close to basal unconformities between Proterozoi...

Tectono-metamorphic evolution of the pre-Athabasca basement within the Wollaston–Mudjatik Transition Zone, Saskatchewan

“…1). A common feature of unconformity-type uranium deposits located in the eastern portion of the Athabasca Basin is their close spatial association with the WMTZ and associated reactivated basement faults intersecting the unconformity surface (Jefferson et al 2007a). However, significant gaps remain in our understanding of the role of the basement in the formation of the Athabasca unconformity-type uranium deposits.…”

“…The rocks underlying the eastern part of the Athabasca Basin of northern Saskatchewan (Canada) comprise a complexly deformed and strongly metamorphosed crystalline basement, part of the Canadian Shield basement (Jefferson et al 2007a). The western Canadian Shield of North America is underlain by Archean cratonic provinces, including the Churchill, Sask, and Superior provinces, separated by Paleoproterozoic orogens and major crustal shear zones (e.g., Hoffman 1988).…”

“…1.8 Ga continent-continent collision of the Trans-Hudson Orogen (THO) before the deposition of detrital sediments into the post-THO Athabasca Basin that began ca. 1.72-1.71 Ga (Cumming and Krstic 1992) and the subsequent formation and remobilization of uranium mineralization located at the unconformity, perched in the Athabasca sandstones, and (or) within the basement, in the 1.6-1.3 Ga period Quirt 1984, 1987;Jefferson et al 2007aJefferson et al , 2007b; and references therein). The close spatial relationship observed between Athabasca unconformity-type uranium mineralization and (i) the occurrence within the WMTZ of numerous uraniumenriched intrusive rocks (granitic pegmatites and granites), (ii) the presence of graphite-bearing pelitic to psammopelitic gneisses (Thomas 1983;Annesley et al 2005;Yeo and Delaney 2007;among others), and (iii) brittle faults that reactivated Trans-Hudsonian ductile high-strain zones (Hoeve et al 1980) suggests that preAthabasca deformation, metamorphism, and partial melting may constitute a major first stage of uranium remobilization and elevation of uranium content that could be a critical factor necessary to obtain the world-class uranium deposits at or near the subAthabasca unconformity.…”

Magnetotellurics and Airborne Electromagnetics - a combined method for assessing basin structure and exploring for unconformity-related uranium

“…The Cariewerloo Basin, South Australia is prospective for unconformity-related uranium, in a style that is analogous to some of the world's largest deposits in the Athabasca Basin, Canada (Jefferson et al, 2007, Wilson et al, 2010. Such similarities include Palaeo-Mesoproterozoic unmetamorphosed sedimentary rocks which unconformably overlie deformed and metamorphosed Palaeoproterozoic and Archean basement; reactivated basement faults; and elevated uranium concentration in basement rocks (Cowley 1991, Jefferson et al 2007).…”

Preliminary Survey for Uranium in Cretaceous Sandstones of Bonako-Sole Areas, Ngondo Complex, Southwestern Cameroon

“…An example includes the Palaeoproterozoic unconformity-type uranium deposits in the Athabasca basin, Canada which is known to produce about one-third of the global uranium [21,22]. These deposits usually occur as veins or massive pods or scattered either in the sandstone of the Athabasca Formation overlying the basement rocks or in the basement rocks themselves [2,19,1].…”