The Fond-du-Lac uranium deposit

CIM Bulletin, Vol. 75, No. 841, 1982

LA. HOMENIUK, R.J. McH. CLARK and R. BONNAR, Eldorado Nuclear Limited, Ottawa, Ontario

The Fond-du-Lac uranium deposit is located on the northern rim of the Athabasca Basin. The uranium mineralization is, for the most part, confined to the Athabasca Group, which is underlain by pre-Helikian metamorphic rocks of the Western Craton.The deposit is characterized by a remarkable train of mineralized glacial erratics which led to the original discovery. The mineralization is present as a "high-grade" core and a diffuse "low-grade" aureole. The "high-grade" core occurs as a stockwork of steeply dipping pitchblende veins which strike N 50°E. Hematite, siderite and secondary silicate are also abundant. The "low-grade" aureole contains uraniferous goethite, argillaceous material and rare carbon buttons. Recent alteration to coffinite has taken place along the margins of preexisting ore minerals.The "high-grade" core is 390 m long and 10 to 40 m wide, and is surrounded by an irregularly shaped zone of "low-grade" mineralization with a similar length and a width of up to 350 m.Comparing the Fond-du-Lac deposit to other unconformity-type deposits, several important differences can be noted: Ninety per cent of the ore is above the unconformity, in the Athabasca sandstone. The deposit is not associated with underlying graphitic rocks or a major structure. The clay-rich alteration zone is not extensive around the Fond-du-Lac deposit. Although there are differences with other Athabasca unconformity-type deposits, the high-grade portion below the unconformity suggests a common origin.The ore deposit is postulated to have been formed from uranium-bearing solutions which entered the stockwork of fractures. Following the reduction in part of these solutions by pre-existing carbonaceous matter and pyrite and/or chemical reaction with pre-existing hematite, deposition continued as a result of decreasing temperature and pressure. Subsequently, minor supergene redistribution of the uranium has taken place.
Keywords: Geology, Uranium deposits, Fond-du-Lac deposit, Athabasca Basin, Boulder trains, Pitchblende, Alteration, Ore genesis, Exploration, Mineralogy.