Structural Analysis, Paragenesis, and Geochronology of the Arrow Uranium Deposit, Athabasca Basin, Saskatchewan: Implications for Controls on Mineralization

Posted on 2018-03-19 in Events
Mar 23, 2018

Please join us for a graduate student seminar this Friday March 23 at 3:30 pm in rm 155 Geology presented by Sean Hillacre, MSc Candidate:

Structural Analysis, Paragenesis, and Geochronology of the Arrow Uranium Deposit, Athabasca Basin, Saskatchewan: Implications for Controls on Mineralization

The Athabasca Basin in northern Saskatchewan hosts the world’s highest-grade uranium deposits. These deposits are commonly spatially associated with structural zones that have undergone multiple episodes of brittle reactivation. This study examines the relationship between the ductile framework and brittle reactivation and reuse of structures, mineral paragenesis, and uranium mineralization at the Arrow Deposit, which is the largest undeveloped high-grade uranium resource in the world. Hand sample examination, thin section microscopy, and electron microprobe analysis has been used to generate a detailed paragenesis of the Arrow Deposit. Semi-massive to massive uraninite is associated with hematite-limonite and chlorite alteration, and crosscut by carbonate and quartz veins, whereas disseminated uraninite is present in clay/chlorite/muscovite-rich hydraulic breccia zones. Sulphides and graphite are pre-, syn-, and post-mineralization, whereas dravitic structures are dominantly post-mineralization. Paragenesis information has been integrated with structural analysis utilizing over 18,000 measurements of individual brittle and ductile structural features. Through this study, the structural system at Arrow has been interpreted as a strike-slip dominated fault system of complex Riedel-style geometry. The Arrow system originally developed along sub-vertical, northeast-southwest trending chloritic-graphitic L-S phyllonites (named the A1 through A5 shears) along the limb of a possible regional-scale fold, and further evolved through episodic reactivation events creating various small-scale brittle fault linkages oblique to and connecting the main fault zone. Detailed analysis of the major structural trends suggests a predominantly sinistral Riedel-type system, where multiple brittle reactivations of primary shear fractures have occurred, allowing for migration of hydrothermal fluids, alteration of host rocks, and precipitation of uranium mineralization. Structural, paragenetic, and EDS information were used to select uraninite samples that were categorized based on cross-cutting relationships, textures, and chemical composition. Uranium mineralization at Arrow occurs as botryoidal, cubic, vein, semi-massive, and massive uraninite (UO2), as well as younger alteration phases including uranium-silicate (coffinite) and uranyl oxy-hydroxide minerals (uranophane). Uraniferous phases have been characterized into two broad groups based on their chemical composition and textural relationships, and a suite of U/Pb chemical ages have been calculated from electron microprobe data. Regression of the concentrations of substituting elements including Fe, Si, and Ca give a chemical age of initial uraninite crystallization of approximately 1450 Ma. In-situ SIMS U-Pb ages obtained in this study (~700, ~1200, and ~1300 Ma) are comparable with those obtained from the Shea Creek area and correspond to large scale thermo-tectonic events. The relative timing and spatial association of structural events/re-activations, alteration, and mineralization has been constrained, which is critical to understanding the mineralized system, and provides the framework for integration of the ages of mineralizing events. Overall, the Arrow Deposit is interpreted as a structurally-controlled uranium deposit entirely hosted within crystalline basement rocks below a thin veneer of Athabasca Group sandstones, and continued studies on these controls will aid in developing a structural template for exploration of new target areas within the recently established southwestern Athabasca Basin uranium camp.