Le gisement d'uranium de Cigar Lake: decouverte et caracteristiques générales

CIM Bulletin, Vol. 79, No. 886, 1986

J.P. FOUQUES, M. FOWLER, H.D. KNIPPING et K. SCHIMANN, Coqema Canada Limitée Montréal, Québec

The Cigar Lake uranium deposit, in the Athabasca area of North Saskatachewan, was discovered in May 1981 by Cogema Canada Ltd., as operator of the Waterbury Lake Joint Venture. The deposit, which is devoid of direct surface expression, is located at the unconformity between the Lower Proterozoic (Aphebian) Wollaston Group metasediments and the Middle Proterozoic (Helikian) Athabasca sandstone, at a depth of 410 m to 450 m below surface. At the end of 1984, the deposit had been defined by 105 mineralized holes over a length of 1800 m and a width of between 25 m and 105 m. It has the shape of a flat-lying lens 5 m to 30 m thick and shows a remarkable longitudinal and lateral continuity from the information obtained to date. A well defined bulge of the unconformity underlies the mineralized zone over its entire length. The deposit appears to be controlled by an east-west-trending structure within which a particular rock fades ("augen gneiss") has developed in the graphitic metapelites of the Wollaston Group. It is surrounded by a strong alteration halo affecting both sandstone and basement, characterized by extensive development of Mg-Al rich clay minerals (illite-chlorite). The mineralization is hosted principally by the Athabasca sandstone, and consists mainly of uraninite and sulfo-arsenides of nickel and cobalt. Numerous geochemical and geophysical surveys have been carried out in the Cigar Lake area, and no specific geochemical or geophysical anomaly can be directly associated with the mineralized body, although there may be locally a good coincidence. However, a combination of several methods proved extremely useful in interpreting the regional geology and defining the drilling targets within the favourable areas. Although the Cigar Lake deposit shows many similarities with several classical deposits of the Athabasca region, it is characterized by the intensity of its alteration processes and the presence of massive, extremely high-grade mineralization. At the end of 1984, the global geological reserves estimated by geostatistics on the main mineralized "pod" of the deposit amounted to 110 000 metric tonnes of uranium at an average grade of 12.2% U, with a precision of 25% at the 95% confidence level.

Mots Clés: Mineral exploration, Uranium deposils, Unconformity-lype deposils, Geophysics, Saskatchewan