Structural geology, tectonostratigraphy, and unconformity-related uranium mineralization of the Aberdeen Lake area, northeast Thelon Basin, Nunavut, Canada

Abstract

The Aberdeen Lake supracrustal belt is located alongside the northeast Thelon Basin in southcentral Nunavut. The belt hosts recently-discovered unconformity-related uranium occurrences, including the Tatiggaq, Qavvik and Ayra prospects. New geological mapping, structural analysis, whole rock lithogeochemistry, U-Pb zircon geochronology, and fluid inclusion analysis were conducted to interpret the stratigraphy, tectonic setting, and structural geology of the belt, and its uranium metallogeny. The revised lithostratigraphy of the Aberdeen Lake supracrustal belt consists of: 1) a ca. 2750 Ma Lower Sequence of komatiite, TTG (tonalite-trondhjemitegranodiorite) intrusions, and mafic gneiss; 2) a ca. <2687 Ma Middle Sequence of ca. psammopelitic gneiss, iron formation and a ca. 2680 Ma felsic gneiss; and 3) a ca. < 2650 Ma Upper Sequence of pelitic to psammopelitic gneiss with minor iron formation and arkosic gneiss. Transecting these Neoarchean supracrustal rocks are the ENE-trending Thelon and Judge Sissons faults, which record long-lived, dextral transcurrent movement with complex reactivation and fluid flow histories. A ca. 1830-1760 Ma initial faulting event produced damage zones consisting of multiple fracture sets, quartz veins and hydrothermal breccias, surrounding core zones of mosaic to chaotic breccias and cataclasites with dextral slip. A ca. 1760-1750 Ma epithermal faulting event comprises cross-cutting comb, crustiform-cockade, and lattice bladed quartz veins associated with Pitz Formation volcanism. Late reactivation events at ca. 1600-1300 Ma are expressed as irregular, fractures, non-cohesive crackle to mosaic breccias and gouges, which were the primary pathways for uranium-bearing hydrothermal fluids. The Tatiggaq, Qavvik and Ayra prospects are located between or along ENE- to NE-trending secondary faults associated with the Thelon and Judge Sissons fault zones. Steep-dipping fracture and non-cohesive breccia zones along these faults host most of the uranium mineralization within medial to proximal alteration halos characterized by strong clay, sooty sulphide and hematite. Fluid inclusion work suggests that a ≥300ºC, reduced, sulphur-bearing basement fluid acted as the primary reductant for uranium mineralization. This fluid was transported by fault-valve seismically-driven fluid injections into a large interconnected and reactivated fault system, which initially formed during dextral transcurrent movement along the Thelon and Judge Sissons faults.