Hillacre and two co-authors reported in early 2021 that recent significant discoveries of uranium mineralization in the southwestern Athabasca basin, northern Saskatchewan, Canada, have been associated with a series of geophysical conductors along a NE- to SW-trending structural zone, termed the Patterson Lake corridor. The Arrow deposit (indicated mineral resource: 256.6 Mlb U3O8; grade 4.03% U3O8) is along this trend, hosted exclusively in basement orthogneisses of the Taltson domain, and is the largest undeveloped uranium deposit in the basin.
This study is the first detailed analysis of a deposit along this corridor and examines the relationships between the ductile framework and brittle reactivation of structures, mineral paragenesis, and uranium mineralization. Paragenetic information from hundreds of drill core samples and thin sections was integrated with structural analysis utilizing over 18,000 measurements of various structural features. The structural system at Arrow is interpreted as a partitioned, strike-slip–dominated, brittle-ductile fault system of complex Riedel-style geometry. The system developed along sub-vertical, NE- to SW trending dextral high-strain zones formed syn- to post-D3 deformation, which were the focus of extensive pre-mineralization metasomatism (quartz flooding, sericitization, chloritization), within the limb domain of a regional-scale fold structure. These zones evolved through post-Athabasca dextral and sinistral reactivation events, creating brittle fault linkages and dilation zones, allowing for hydrothermal fluid migration and resulting uraninite precipitation and associated alteration (white mica, chlorite, kaolinite, hematite, quartz veins).
This study of the structural context of Arrow is important as it emphasizes that protracted reactivation of deep-seated structures and their subsidiaries was a fundamental control on uranium mineralization in the southwestern Athabasca basin. The authors concluded that the geologic relationships and structural history of the Patterson Lake corridor is exceedingly complex, spanning at minimum from the Paleoproterozoic (crystallization age of ca. 2.46 Ga; Card et al., 2014) to the Mesoproterozoic (U mineralization at ca. 1.45 Ga; Hillacre et al., 2018).
The Patterson Lake corridor displays evidence of episodic structural reactivation and exhumation at progressively shallower crustal levels, related to the protracted tectonic evolution of the Precambrian shield in this region, and accompanied by various episodes of hydrothermal fluid flow with associated alteration and uranium mineralization, recrystallization, and remobilization. Arrow is a structurally controlled uranium deposit, hosted within a NE-SW–striking brittle-ductile deformation zone approximately 250 m wide, over 1 km in strike length, and more than 1 km in depth extent, which cuts Paleoproterozoic granulite facies rocks of the Taltson domain. Foliations within the Arrow deformation zone have been deflected and transposed, and the resulting anastomosing transposition foliation (S1-S3) accommodated protracted deformation along the zone, allowing for uranium mineralization to penetrate the reactivated shears. Plastic deformation within the major high-strain zones (A1-A5) provides evidence for early shearing under a ductile to brittle-ductile regime, whereas formation of Riedel shears and incohesive fault rocks indicates ongoing deformation through the brittle-ductile transition
into a brittle regime. The prominence of these brittle rocks indicates prolonged reuse and reactivation of the preexisting A1 to A5 shear zones pre-, post-, and syn-mineralization, overprinting ductile deformational fabrics and incorporating angular clasts of ductile-deformed wall rock into their structure.
Furthermore, highly altered and evolved fault gouge structures demonstrate that the portion of the Patterson Lake corridor hosting the Arrow deposit likely acted as a conduit for the movement of a significant volume of fluid over a substantial period of geologic time. Overall, there are aspects of the alteration stages identified through the paragenetic sequence at Arrow that are similar to other basement-hosted deposits in the eastern Athabasca basin. These include the presence of pre-mineralization clinochlore and white mica, syn-mineralization muscovite, and syn- to post-mineralization sudoite and dravite. In addition, the primary uraninite has been altered, and secondary uranium minerals precipitated in subsidiary structures. However, we suggest the processes active within the portion of the Patterson Lake corridor hosting the Arrow deposit, such as softening of shear zones and metasomatism and/or hydrothermal alteration of host rocks, were a critical factor in preparing the ground for the formation of world=class, high-grade uranium deposits, such as Arrow.
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