Étude métallographique des différentes associations minérales et caractérisation chimique des minéraux uranifères du gisement de Cigar Lake (Saskatchewan, Canada)

Abstract

The Cigar Lake deposit, discovered in 1981, represents one of the richest uranium occurrences known in the province of Saskatchewan. The eastern margin of the Athabasca Basin can be considered as one of the richest uraniferous provinces in the Western World.Being located at the unconformity between the Aphebian metasediments and the Helikian sandstone cover, this deposit is classified in the group of deposits spatially related to the Proterozoic unconformity. The study of this deposit, exceptional in its dimensions, grade, and tonnage, was made possible through a series of 170 boreholes drilled to delineate the whole orebody to a depth of 440 m.The metallographic study of the mineralizations in the main orebody, hosted in the basement, and the so-called "perched" mineralization in the cover rocks, led to the definition of three major mineralizing stages, which contributed in varying degrees to ore deposition.A major stage is characterized by the association of U–Ni–Co–As–S ± Mo–Bi–Cu–Zn–Pb and reflects the polyphased nature of this hydrothermal stage. This early deposition of the uraniferous phase (massive, botryoidal, and euhedral uraninite) was followed, on one hand, by a phase precipitating nickel–cobalt and Feron arsenides and sulfoarsenides, and on the other hand, by a phase exclusively of sulphides. The age of emplacement of this stage is 1341 Ma, and it corresponds to the very important metallogenic period in the Athabasca during which the main economic ore occurrences "related to unconformities" were emplaced.The second stage appears only in the metamorphic units of the Aphebian basement. It is represented by an uraninite–sulphide paragenesis, the uraninite being characterized principally by the presence of thorium and, as well, a lead content lower than that in the uranous oxides stage.The third mineralizing stage affects the basement rocks, the main orebody, and the cover rocks with their perched mineralization. It is a simple pitchblende – iron or iron–nickel sulphide and (or) hematite paragenesis with an age of emplacement of 323 Ma. This type of mineralization is identical to that shown in a certain number of occurrences in the Athabasca Plateau and it is considered to be related to the Hercynian orogeny.The first and third of these three mineralizing stages fit perfectly in the metallogenic model for this part of the Canadian Shield. A comparison of these stages with the main mineralizing stages defined in the Carswell Dome structure and with some occurrences in eastern Athabasca was carried out in order to define a model of the metallogenic evolution of northern Saskatchewan. [Journal Translation]