Uranium deposits in the Athabasca Basin, northern Canada, are associated with graphitic metasediments and faults zones in the Archean basement and are often “blind”, covered by hundreds of meters of resistive Proterozoic sandstones. Hence airborne electromagnetic (EM) is commonly used in initial exploration over large areas. In recent years new airborne EM systems have become commercially available which have the capability to detect large graphitic zones in or near the basement at depths approaching 1000 m, examples of which are in this paper. Conductivity depth inversions and forward modeling of graphitic conductors in a conductive half-space are important tools in interpretation as they provide quantitative confirmation that deep conductors are responsible for the observed responses.
The Athabasca Basin straddles the Alberta-Saskatchewan border in Canada and occupies an area of about 100,000 sq km in northern Saskatchewan, accounting for approximately 30% of global primary uranium production. The Basin is filled with the Proterozoic Athabasca Group consisting of relatively undeformed and flat-lying sedimentary rocks. The high-grade uranium deposits are loosely associated with the unconformity between these sandstones and underlying Archaean-Palaeoproterozoic metamorphic and igneous basement rocks.
The deposits occupy a range of positions from wholly basement-hosted to wholly sediment-hosted, at structurally favorable sites in the interface between deeply weathered basement and overlying sandstones. In almost all cases the deposits appear to be structurally controlled by basement faults and fracture zones, which are localized in graphitic metapelitic gneisses that often flank structurally competent Archaean granitoid domes and may extend for up to 10 km or more. In some cases alteration zones associated with the fault zones and mineralization extend above the unconformity into the sandstone.
Geological cartoon for unconformity-style uranium deposits in the Athabasca Basin. The basement and overlying sandstones are generally very resistive, while the graphitic metapelite is relatively conductive. Alteration zones in the sandstone have variable resistivity, ranging from 50-20,000 ohm-m. The regolith at the unconformity is generally not a significant conductor.
Overburden and lake water may have relatively low resistivities, but are generally thin enough that they do not significantly hinder EM methods. Lake sediments have resistivities in the range 100-800 ohm-m and where thick may decrease the depth of penetration of EM systems. The depth to the unconformity between sandstone and basement varies from outcrop around the edges of the Basin to over 1500 m near the center. Initial exploration focused on the outer edges where the unconformity is shallow, but as the shallower deposits were discovered, it has moved increasingly into the deeper parts of the Basin.