The Carswell structure in the western Athabasca Basin, northern Saskatchewan (Canada), has previously been interpreted as an eroded impact structure with a minimum diameter of ~36 km, the outer margin of which is broadly defined by an outer ring of sediments composed of the only algal reefs observed in the Athabasca Group, the Carswell Formation. This ring surrounds an 18-km-wide uplifted basement core composed of gneiss units of Archean to Paleoproterozoic age that display shatter cones, planar deformation features (PDFs), pseudotachylyte veins, and impact melts and breccias (Cluff melt sheet, Cluff breccias) indicating that pressures and temperatures locally exceeded 60 GPa and 1500 °C. Detailed analysis of the basement–Athabasca Group contact from field and drill core samples indicates that shock features are not present in the Athabasca Group sediments in direct contact with the highly shocked basement gneisses. This pattern is inconsistent with a post–Athabasca Group age for the impact. Moreover, our study has revealed PDF-bearing quartz grains in basal units of the Athabasca Group 14 km south of the southern edge of the basement core (outside the estimated outer ring). The new proposed model suggests that the impact event is of pre-Athabasca (Proterozoic) age and that it produced a multiring structure that controlled the paleogeography of the Athabasca Group units in the western part of the basin. The model is well supported by basin analysis and gravity data. The Carswell Formation is the result of algal reefs building on peak-ring–related seamounts at the end of Athabasca Group deposition. The overturned bedding observed locally adjacent to the basement core is interpreted as the result of gravity-driven readjustment of the central uplift.

Abstract The Wheeler River property, host to the Phoenix deposit, is located in the Athabasca basin, northern Saskatchewan, 35 km southwest of the McArthur River uranium mine complex. Depths to the unconformity between the overlying Paleoproterozoic to Mesoproterozoic undeformed Athabasca Group sandstones and the underlying Paleoproterozoic and Archean crystalline basement range from 170 to 600 m, which is shallow by Athabasca basin standards. The Wheeler and McArthur River properties were explored simultaneously, with fairly similar budgets, continuously from 1975 to 1988, until the McArthur River discovery in 1988. Subsequent to that, exploration on the Wheeler River property continued at minimal budget levels until Denison became the operator in 2004. Fifty-eight holes, each about 450 m, were drilled during the next four years prior to the Phoenix discovery hole, WR-249, which intersected 1.06 percent U 3 O 8 over 2.35 m in the summer of 2008. The Phoenix uranium deposit is a blind deposit (i.e., there is no indication of uranium at surface) located at a depth of 400 m. However, there were favorable regional features including a quartzite unit within the crystalline basement and extensive dravite-altered sandstones overlying the quartzite. Discovery drill hole WR-249 tested a direct current (DC)-resistivity anomaly adjacent to the quartzite unit, targeting an interpreted alteration chimney in the sandstone. Alteration chimneys represent clay alteration and structural disruption above unconformity or basement-hosted mineralization. Drill hole WR-251 tested the same resistivity low 600 m along strike to the southwest and also intersected favorable alteration, structures, and mineralization. Sandstone alteration is similar to that described from other unconformity-associated Athabasca basin uranium deposits and consists of silicification, desilicification, rotated bedding, drusy quartz, hydrothermal hematite, and a gray, pyritic zone. Alteration of the sandstone visually reaches a maximum height of 200 m above the unconformity. Basement alteration consists of irregular drusy quartz, often with clay gouge in the pelitic gneiss. The basement in the northeast part of the Phoenix deposit is much more bleached and clay altered than that to the southwest. To date (April 2010), the Phoenix deposit has been drill tested for >1 km of strike at generally 50-m spacings and remains open along strike to the northeast and southwest. The mineralization is primarily sandstone-hosted monomineralic uraninite, as pitchblende. Most mineralization occurs at or above the unconformity and is associated with a steeply easterly dipping, graphitic pelite basement unit, which has a maximum thickness of 75 m. Mineralization is generally located along the eastern margin of the quartzite ridge, within 25 m of the lithologic contacts of this unit. The mineralization is also directly associated with a thin, generally <1-m-wide, graphitic structure termed the WS shear. This is the only structural control recognized to date. The most enriched intersection is from drill hole WR-273, which returned 62.6 percent U 3 O 8 over 6.0 m. This represents the highest grade thickness reported worldwide from any uranium exploration property in the past decade. The mineralization drilled to date is estimated to contain from 45 to 95 million pounds (Mlb) U 3 O 8 , making Phoenix, even at the lower figure, the sixth largest known individual deposit in the Athabasca basin.