Deep Geological Controls on Formation of the Highest‐Grade Uranium Deposits in the World: Magnetotelluric Imaging of Unconformity‐Related Systems From the Athabasca Basin, Canada

Abstract

The Athabasca Basin in Canada (Figure 1) is the premier exploration region for uranium deposits, hosting the highest grade deposits in the world (Jefferson et al., 2007). Traditionally, uranium deposits in the eastern Athabasca Basin region occur within 400 m of the basin-basement unconformity surface and are associated with graphite-rich faults rooted in metapelitic gneisses (Jefferson et al., 2007 and references therein). The Patterson Lake corridor (PLC) in the western Athabasca Basin hosts the high-grade Triple R and Arrow uranium deposits and several prospects such as Spitfire, Cannon, Arrow South and Harpoon. However, the known PLC deposits do not occur in geological settings typical of unconformity-related uranium (URU) deposits (Jefferson et al., 2007); they are hosted in altered, metamorphosed granite, granodiorite and ultramafic to mafic basement rocks up to 900 m below the unconformity contact, and in some cases, outside the present-day basin margins. The fluid-conduits for PLC deposits are reactivated fault zones rich in hydrothermal graphite and sulfide minerals. Recent geochemical and thermochronology studies of the PLC district suggest a deep, fertile heat source enhanced hydrothermal fluid flow and mineralogical, fluid inclusion, isotopic and structural studies document incursion of basinal brines to ∼1 km depth along inherited ductile fault zones reactivated under a brittle regime (

Role of Hydrothermal Circulation along and above Inherited Basement Structures Relating to Unconformity-Related Uranium Mineralization

“…Moreover, field data, such as the presence of carbonatites within the associated deep-seated shear zones of the Patterson Lake corridor, indicates that these deep-seated structures reach mantle depths [20]. Similar crustal-scale low-resistivity corridor networks are also well documented in the Gawler craton, Australia, and refer these high-strain zones for potential fluid flow [22,[102][103][104]. Subsequently, these tectonic structures are assumed to be strongly modified through long-standing chemical and physical processes.…”

“…At a regional scale, magnetotelluric profiles have been conducted to characterize the structure of the lithosphere and structures' association to mineral deposits (e.g., [5,6,101]). Magnetotelluric profiles conducted towards southern Trans-Hudson Orogeny by Jones et al [5] and in the Taltson orogeny towards the SW Athabasca Basin show lowresistivity anomalies that have a close spatial relationship with high-strain tectonic domains and to major mineral occurrences [6,21,22]. These observed anomalies extend over strike for hundreds of kilometres, (e.g., North American Central Plans, [5]).…”

“…As revealed by exploration drill holes and its magnetic geophysical signature below the Athabasca Basin, this tectonic structure is composed of steeply dipping anastomosed shear zones (Figure 1(b), [16][17][18]). At its western extent, recent discoveries of basement hosted world class uranium deposits along the Patterson Lake conductive trend [19] and extensive geophysical and geological surveys [6,[20][21][22] reveal the presence of crustal to lithospheric scale deeply rooted structures. These structural features can be considered as critical elements for the location of major hydrothermal systems and generation of giant uranium deposits.…”