Geological, Petrographic and Geochemical Characterization of the Roughrider West Zone Unconformity-Type Uranium Deposit, Athabasca Basin, Saskatchewan

Abstract

The Roughrider unconformity-type uranium deposit is located in northeastern Athabasca Basin, Saskatchewan, and currently comprises 3 mineralized zones: the West Zone, East Zone, and Far East Zone. This study, focused on the West Zone (RWZ), comprises petrographic, paragenetic, and geochemical studies, and aims to document both the mineralization and alteration, and to construct a model for the genesis of the deposit, which may assist in future exploration in the Athabasca Basin. The RWZ is dominantly hosted in the deformed basement rocks (Wollaston Group paragneisses and Archean orthogneisses) underneath the Paleoproterozoic Athabasca Group siliciclastics. Despite being predominantly basement-hosted, it exhibits some signatures, such as extensive clay alteration and the presence of base metals that are akin to sandstone-hosted unconformitytype uranium deposits. Spatial analysis of the clay alteration pattern indicates an illite-dominant pervasive alteration halo that extends vertically to surface, laterally for up to 50 metres surrounding the deposit, and reaches 200 metres into the basement below the unconformity. Conversely, strong chlorite alteration occurs only proximal to mineralization. Detailed petrographic analysis of approximately 100 polished thin sections in reflected and transmitted light, and selective electron microscopy and electron microprobe studies were used to document host rock, ore, and alteration mineralogy, and unravel the paragenetic history. Results indicate that RWZ comprises at least six phases of uranium mineralization (U1 to U6) with local enrichment of base metals including Cu, Ni, Co and Zn. Two episodes of uraninite mineralization (U1 & U2) are interpreted to represent the primary mineralization event, whereas the other four uraninite phases (U3 – U6) represent remobilization of the primary ore. U1 predates most base metalbearing sulphides, although some copper sulphides may predate primary uranium mineralization. Fluid-rock interactions between oxidizing basinal fluids and basement rocks (especially graphite and ferrous iron-bearing minerals) before and throughout uranium mineralization were mainly responsible for precipitation and remobilization of uraninite. U/Pb isotopic geochronology yields an upper intercept age of 1,188±52 Ma for U1 uraninite, and 262±24 Ma for U4 and U5 uraninite, whereas the chemical ages of U1 to U5 range from ca. 1,482 to ca. 35 Ma. The young ages reflect lead loss and the ability of the system to mobilize and re-precipitate uraninite after the primary mineralization over a period of 900 Ma. The conceptual fluid flow model suggests that basinal fluids infiltrated the basement to the west of RWZ, flowed eastward and upward in the basement, channelled along networks of fractures, and expelled back into the basin. This overall upward fluid flow model explains the extensive development of alteration in the sandstones above the sites of mineralization, and it does not exclude the possibility that downward flow took place after the main phase of mineralization, causing uranium remobilization.