On the genesis of Rabbit Lake and other unconformity-type uranium deposits in northern Saskatchewan, Canada

A description is given of the geology, mineralogy, and geochemistry of the Rabbit Lake deposit, located in the immediate vicinity of the unconformity between an Aphebian/Archean metamorphosed basement and overlying unmetamorphosed sandstones of the Helikian Athabasca Formation. In the deposit several stages of alteration, mineralization, remobilization, and redeposition, separated by intermittent brecciation, are recognized. The alterations, predominantly chloritization with some tourmalinization, silicification, and dolomitization, are accompanied by introduction of H 2 O, MgO, probably also B, and by leaching of SiO 2 , Na 2 O, K 2 O, CaO, and Sigma Fe 2 O 3 . The deposit is of complex mineralogy and typified by a broad suite of elements including U, Ni, Co, Cu, Pb, Zn, Fe, Mn, As, S, and Se.In the history of the deposit conditions have repeatedly oscillated between oxidizing and reducing, and the temporal relationships between oxidizing, reducing, and mineralizing episodes indicate the importance of redox reactions in shaping the deposit.After comparison with other unconformity-type deposits in the Athabasca Basin, a diagenetic-hydrothermal metallogenetic model is presented. The model invokes percolation of oxidizing diagenetic solutions in the Athabasca Formation, which, being heated under the influence of the geothermal gradient, eventually attained temperatures of around 200 degrees C at the floor of the basin. Reaction of the heated diagenetic solutions with graphitic basement rocks generated methane, which acted as a mobile reductant. Mineralization resulted from interaction of flows of the methane-bearing reducing solutions and the oxidizing diagenetic solutions. The latter have carried the ore constituents to the site of reduction and, hence, of deposition.The ore constituents may have been derived from the Athabasca Formation by leaching during a process of intrastratal weathering and oxidation. Thus, in a broad sense, processes governing the formation of the Saskatchewan unconformity-type deposits may have been similar to those that give rise to sandstone-type roll-front deposits.