Pétrographie et minéralogie des halos d'altération autour du gisement de Cigar Lake et leurs relations avec les minéralisations

Abstract

Around the Cigar Lake orebody, the present zoneography of alteration halos reflects several alteration episodes, some of which are anterior to and others coeval with the mineralizing events and have a regional extension. The basement retromorphism is characterized by crystallization of muscovite with a low iron and magnesium content and of trioctahedral ferromagnesian chlorites. The later regolith alteration, more obvious at the top of the basement, is marked by iron-bearing 1T kaolinite, by hematite, and by local crandallite–florencite and diaspore. In the Athabasca sandstones far from any mineralization, the diagenetic quartzification was followed by crystallization of aluminous 2M hydromuscovite, dickite, and crandallite–goyazite.In the main pod, the uraninite mineralization was dated 1341 ± 12 Ma. In the sandstones, it is surrounded by ferromagnesian chlorites with a variable sudoitic character. This proximal alteration halo grades into a more distal envelope, visible in the sandstone and in the basement, that is composed of magnesian sudoite and 3T hydromuscovite. During this mineralizing event, dravite crystallized in the form of urchin-like clusters in the basement and xenotime overgrowths, around altered zircon, and apatite formed in the sandstones.Around the main pod and in some perched orebodies, an alteration zone of vanadium-bearing ferrikaolinite and iron-bearing 3T hydromuscovite, crosscut by a later siderite, surrounds the pitchblende dated 323 ± 4 Ma. Coffinite and an aluminous hydromuscovite crystallized during a later fracture event. The aluminous hydromuscovite also appears, with a silica–carbon–uranium complex, in perched mineralizations. Kaolinization and iron-sulfide oxidation into iron hydroxides occurred in perched orebodies that were more exposed to meteoric alteration.