STRUCTURAL ANALYSIS, PARAGENESIS, AND GEOCHRONOLOGY OF THE ARROW URANIUM DEPOSIT, WESTERN ATHABASCA BASIN, SASKATCHEWAN, CANADA: IMPLICATIONS FOR THE DEVELOPMENT OF THE PATTERSON LAKE CORRIDOR

The Athabasca Basin in northern Saskatchewan hosts the world’s highest-grade uranium deposits, which are commonly spatially associated with structural zones that have undergone episodes of brittle reactivation, alteration, and polyphase fluid movement. The most recent significant discoveries of uranium mineralization in the Athabasca Basin have been associated with a series of geophysical conductors along a NE-SW-trending structural zone, termed the Patterson Lake corridor, in the southwestern portion of the Basin. The Arrow Deposit, which is along this trend and hosted exclusively in the basement rocks below the Athabasca Supergroup sandstones, has an indicated mineral resource of 179.5 Mlbs U3O8 at a grade of 6.88% U3O8, and is the largest undeveloped uranium resource in the Basin. The present study examines the relationships between the ductile framework and brittle reactivation of deep-seated structures, mineral paragenesis, and radiogenic and stable isotope analyses of uranium mineralization at the Arrow Deposit. Hand sample examination, structural analysis from oriented drill core, thin section microscopy, and electron microprobe analysis has been used to generate a detailed paragenesis of the Arrow Deposit, which was used to select mineralized samples for isotopic analysis that were categorized based on cross-cutting relationships, textures, and chemical composition. Paragenetic information was integrated with structural analysis utilizing over 18,000 measurements of foliation, fractures, veins, shears, mylonites, breccias, cataclasites, fault gouges, and plunge and trend of slickenstriae and ductile lineations. Through this study, the structural system at Arrow has been interpreted as a partitioned, sinistral strike-slip dominated, brittle-ductile fault system of complex Riedel-style geometry. The Arrow system developed along sub-vertical, NE-SW-trending heterogeneous high strain zones (named the A1 through A5 shears) along the limb of a regional-scale fold, and further evolved through episodic reactivation events creating small-scale brittle fault linkages oblique to, and connecting the main fault zone, allowing for migration of fluids, alteration of host rocks, and precipitation of uranium. Uranium mineralization at Arrow occurs as botryoidal, cubic, vein, semi-massive, and massive uraninite (UO2), as well as younger alteration phases including uranium-silicates (e.g. coffinite) and uranyl oxy-hydroxide minerals (e.g. uranophane). Regression of the concentrations of substituting elements including Fe, Si, and Ca give an average chemical age of initial uraninite crystallization of approximately 1,425 Ma. In-situ secondary ionization mass spectrometry (SIMS) U-Pb ages obtained in this study (~700, ~1,200, and ~1,300 Ma) are comparable with those obtained from the Shea Creek area and reveal numerous episodes of uranium mineralization, remobilization, and alteration associated with multi-stage deformation during the Proterozoic. The geochronological data on uranium mineralization and post-mineralization alteration and resetting events broadly correspond to major orogenic events that have affected the North American shield. The oldest uraninites (~1,300 to 1,425 Ma) are botryoidal, cubic, and semi-massive occurrences commonly replacing clay minerals and micas. Younger (~1,200 and ~700 Ma) uraninites occur as cubic crystals, semi-massive and massive lenses, and form the matrix of breccias. The youngest uraniferous minerals are the products of alteration and/or remobilization of uraninite through subsequent fluid-flow events. This study demonstrates that the U–Pb isotope systematics of uranium-rich minerals from the Arrow Deposit have been affected by paleo-fluid-flow events that were controlled by regional and global-scale tectonic events. The precision and high spatial resolution of the SIMS method allowed for measurement of δ18O values from ~10 μm spots on uraninites from the Arrow Deposit. The range of δ18O values (-34.5 to -15.2 ‰) are low, and comparable to those obtained from other unconformity-type deposits in the Basin such as Cigar Lake and Shea Creek. The low δ18O values indicate that the uraninite likely underwent recrystallization via interaction with late, relatively low temperature Athabasca fluids with δ18O values in the range of -20 to -16 ‰. The other discoveries along the Patterson Lake corridor (Triple R, Cannon, Bow, Harpoon, Spitfire) have not been studied in detail, and so this study of the structural context of the Arrow Deposit is important as it emphasizes that protracted reactivation of deep-seated NE-SW-trending structures and their subsidiaries was a fundamental control on uranium mineralization in the SW Athabasca Basin. Continued studies integrating mineral paragenesis, geochemistry, and structural geology with geochronological context will aid in understanding the evolution of uranium deposits within the recently established southwestern Athabasca Basin uranium camp.

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