Abstract

Unconformity-type uranium deposits from the Athabasca Basin area are considered to be the result of mixing between oxidized basinal brines and basement-derived reduced fluids/gases, and/or reduced basement rocks. Graphite and its breakdown products are suggested to be responsible for uranium mineralization by acting as a reductant that could trigger deposition of uranium. In addition, the presence of well-connected graphite and sulfides within graphitic pelitic rocks is considered to be potentially indicative of basement structures, as the minerals are often concentrated along the structures, which hence become electromagnetic (EM) conductors. Thus, exploration for uranium deposits is often focused on the search for EM conductors.

Variably graphitic pelitic schists with a steeply dipping fabric underlie the sedimentary rocks of the basin in the Dufferin Lake zone, south-central Athabasca Basin (Saskatchewan, Canada). Up dip, and just below the unconformity with the Athabasca Group, the pelitic schists are replaced by texturally similar rocks within “graphite-depleted zones”. These zones consist of chlorite and hematite alteration (Red/Green Zone; RGZ) and a bleached zone immediately adjacent to the unconformity. Both zones are characterized by a lower concentration of carbon and sulfur, with the bleached zone showing higher concentrations of uranium and boron, the latter corresponding to a high dravite content. The major elements composition of the graphite-bearing pelitic schists and altered equivalents (RGZ) are similar. Raman analyses indicate that well-ordered carbon species (graphite to semi-graphite) are present in the pelitic schists, with both types more common within shear zones. In contrast, only rare low-ordered carbon species (carbonaceous matter) were detected in the graphite-depleted samples within the RGZ. This variation is interpreted to be the result of graphite consumption by oxidizing fluids migrating downward from the Athabasca Group. This graphite consumption may have resulted in the production of a mobile reductant (gas or fluid), which may have played a subsequent role in the deposition of uranium mineralization.

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