Summary:
The Phoenix deposit is classified as an Athabasca Basin unconformity-associated (also unconformity-related and -type) uranium deposit. Phoenix straddles the unconformity contact between the Athabasca sandstone and underlying basement, signifying the unconformity as a major fluid pathway for uranium mineralization.
The prominent structural feature at the Phoenix deposit is the northeast-southwest trending (052º azimuth) WS Shear, which dips 58° to the southeast and lies within or at the base of the graphitic pelitic gneiss unit along the east edge (hangingwall) of the quartzite ridge, which appears to have acted as a buttress for thrusting and reverse faulting (Kerr, 2010; Kerr et al., 2011). Deformation along the WS Shear has occurred partly by ductile shearing but mainly by fracturing. A progressive fracturing sequence is evident by variations in the strike and dip of slickensides. The principal stress directions responsible for early deformation were northwest southeast. A change in the principal stress to an east-west direction led to later strike-slip movement along the WS Shear. The later extension is indicated by northwest-striking normal faults, which dip steeply to the southwest.
Reverse fault displacements on the western edge of the quartzite ridge occurred primarily within the highly resistant quartzite unit. Within the Wheeler River area, vertical offset on the footwall of the quartzite unit can be as much as 60 m; however, at the Phoenix deposit, known vertical displacements in the hangingwall sequence are always less than 10 m.
Mineralization hosted in the lower 15 m of the Athabasca sandstone appears to have some relationship to the extensions of the WS Shear and its various hangingwall splays; hence, movement on these faults must have continued after the deposition of rocks of the Read Formation and probably the MFd of the Manitou Falls Formation. The WS Shear and its various interpreted hangingwall splays may have been the main conduit for the mineralizing fluids. Thus, determining favourable locations along the WS Shear, where zones of long-lived permeability are present, is critical. A series of east-west oriented cross faults or tear faults are also observed at Phoenix. These features are not well documented in drill core as most structures have been replaced by high-grade mineralization. They are inferred by changes in geologic strike or flexures in the geology underlying the deposit. These cross faults are believed to have enhanced the permeability of select portions of the deposit during deposition, subsequently allowing for the formation of thicker and high-grade uranium mineralization.
Uranium mineralization at Phoenix generally occurs at the Athabasca Basin unconformity at depths ranging from 390 m to 420 m. It is interpreted to be structurally controlled by the northeast-southwest trending (052° azimuth) WS Shear, which dips about -58° to the southeast on the east side of the quartzite ridge. Mineralization is separated into three zones (Zone A, B C and D), with Zone A hosting most of the mineralization. No mineral resources have been declared for Zones C and D to date.
The quartzite ridge forms an interpreted structural barrier and mineralizing fluid trap to the footwall of Phoenix and dominates the basement geology in the area. The basement units exhibit variable dips from -36° to -59° to the southeast, averaging -50°, with undulating azimuth between 028° to 048°. Immediately overlying the quartzite (QZIT) is a garnetiferous pelitic gneiss (GTPL), which varies from 7 to 60 m in thickness. This generally competent and unmineralized unit contains distinctive porphyroblastic garnets and acts as a marker horizon. Overlying the garnetiferous pelitic gneiss is a graphitic pelitic gneiss (GFPL) in which the graphite content varies from 1% to 40%. The graphitic pelitic gneiss is approximately 5 m wide in the southwest, increases to approximately 70 m at Zone A and is 50 m wide at the northeast extremity. Overlying the graphitic pelitic gneiss is a massive, non-graphitic pelitic gneiss unit (PELT) and semi-pelitic gneiss in the northeast (SMPL).
The intensity of the uranium mineralization, along with the intense alteration associated with the mineralizing event, often makes it incredibly difficult to identify the unconformity in the drill core.
The unconformity surface has been interpreted primarily using field logs. However, immobile element ratios (specifically ZrO2 / (TiO2)2) were used in some cases where it was difficult to distinguish the original lithologies due to intense mineralization and alteration. A property-wide review of geochemical and core logging data found that ratios were the most accurate in picking the location of the unconformity in both altered and unaltered drill core.
The mineralization within the Phoenix deposit is dominated by massive to semi-massive uraninite associated with an alteration assemblage comprising hematite, dravitic tourmaline, illite and chlorite. Secondary uranium minerals, including uranophane and sulphides, are trace in quantity. Average nickel, cobalt, and arsenic concentrations are at the low end of the range found in other uranium deposits in the Athabasca basin.
Phoenix Zones A and B exhibit elevated concentrations of certain rare earth elements (REEs). While there is a strong correlation between the REEs and uranium mineralization, the correlation between heavy rare earth elements (HREEs) and the high-grade uranium domains is comparatively stronger than the correlation between high-grade uranium mineralization and light rare earth elements (LREEs).