Geology of the uranium deposits related to the Sub-Athabasca Unconformity, Saskatchewan

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Abstract

The Athabasca Basin is a large oval, dish shaped structure, 425 km by 225 km (80 000 km2 ) containing about 1500 m of mainly flat lying quartz-rich sandstone of the Athabasca Croup. The sandstone has a white clay cement and is chiefly white but locally is grey to black or multicoloured. The basin lies with marked angular unconformity a cross a Hudsonian basement of deformed and metam9rphosed Archean and Apheman sedimentary, volcanic and plutonic rocks trending north to northeast beneath the basin. The basement rocks in the southeast half of the basin are mainly Aphebian whereas those in the northwest appear to be predominantly Archean. In the Carswell Circular Structure in the central western half of the basin basement rocks are brought to surf ace through 1200 m of sandstone. The rocks of the basin are less than one per cent exposed. Overburden locally reaches 90 m thick. Uranium deposits have been found near the southeast edge of the basin, within the Carswell Circular Structure, and along the northern rim of the basin. They are (1) at the unconformity as high grade masses elongated in and parallel to major faults, hosted mainly in highly white clay-altered feldspar-rich basement rocks and associated with graphitic metasediments and cale-silicate rocks; (2) within the first 40 m above the unconformity in grey to black and multicoloured Athabasca sandstones and shales as a coating on quartz grains, as disseminations in the clay matrix and as veins; and (3) within 100 m below the unconformity as fracture fillings and disseminations in basement rocks. Pitchblende and coffinite are the main uranium minerals. Many of the deposits have a multielement association and some have appreciable amounts of nickel, cobalt, gold, arsenic and/or other elements in addition to uranium. The richest zones are at the intersections of faults or where faults change direction. Several stages of uranium concentration were required for the formation of the se deposits. Some of the stages took place before deposition of the Athabasca rocks, that is, during the deposition, anatexis and subsequent weathering of the basement rocks. The significant deposits formed after de position of the Athabasca rocks by remobilization of the uranium from the basement rocks and the regolith in hot circulating groundwater, and by its precipitation at the unconformity and spatially related traps in highly white clay-altered basement rocks and/or grey to black and multicoloured sandstone. Uranium is being mined (1981) at Rabbit Lake and Cluff Lake (D orebody).

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