Uranium at Carswell Canada

The outer limit of the Carswell impact structure in Sasketchewan (Canada; 58o27'N/109o30'W) is only vaguely defined, but the diameter of the exposed structure is generally estimated at 39 km, although Grieve and Masaitis (1994) suggested that the original diameter of the Carswell Structure could have been as large as 50-55 km. The structure comprises an annular, about 5 km wide trough around a 20-km-wide core terrane composed of metamorphic basement rocks (Fig. 6). Despite its relatively young age of 115 ± 10 Ma, the structure - similar to Vredefort- is deeply eroded to a level below the entire crater fill. Only dike breccias, as well as some other bodies of impact breccia and impact melt rock, remain in the exposed basement rocks.

- Major iBult t 1 CarEwII Formaîrcn - dcrlnmits kW^l Forrrmtron ■ sandstone, siltstonc 0__10

I WAsm RruEr Subgrojjj - sandstone, conglomerate um r.'.lArcteanbasement-grsnitotd, grteis^

- Major iBult t 1 CarEwII Formaîrcn - dcrlnmits kW^l Forrrmtron ■ sandstone, siltstonc 0__10

I WAsm RruEr Subgrojjj - sandstone, conglomerate um r.'.lArcteanbasement-grsnitotd, grteis^

Fig. 7. Geology and structure of the Carswell impact structure. After Grieve and Masaitis (1994).

The general geology of the uplifted basement core and the surrounding younger formations has been reviewed by Grieve and Masaitis (1994) and in various papers in Laine et al. (1985). The basement core comprises a mixture of Archean gneisses and granitoids that are surrounded by unmetamorphosed sedimentary lithologies including sandstones, siltstones, conglomerates, and carbonates of the Athabasca Group. Radial faulting has truncated and offset, in places, the contact between the basement rocks of the core and overlying Athabasca Group lithologies. Uranium mineralization is concentrated along the southern/southwestern contact between the uplifted core and the Williams River Subgroup of the Athabasca Group, and occurs in both these stratigraphic settings (Fig. 7). Harper (1983) estimated a reserve of 46,500 metric tons of uranium in the deposits known at that time. Ore formation is believed to be the result of regolith development due to lateritic weathering of the basement rocks under tropical climatic conditions, prior to the deposition of the Athabasca Group. The main mineralization apparently formed as a consequence of a hydrothermal event at approximately 1000 Ma ago that produced a uraninite-polymetallic sulfide assemblage. The ore evolution is complex and involves a series of remobilization events (further details are provided in Laine et al. 1985).

Due to the impact event and associated kilometer-scale uplift of the basement core, the pre-impact ores were uplifted by about 2 km, in the formation of the central uplift structure, and brought into their present position where they can be exploited. Impact effects on these ores include some brecciation, as well as minor post-impact remobilization that produced a coffinite-sulfide paragenesis.

The Carswell uranium deposits are currently exploited. Grieve (2003) estimated that a reserve in excess of 45 000 tons remains.

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