MEGA FALLBACK BRECCIA AND IMPACT MELT BODIES DISCOVERED AT THE DEEPLY ERODED CARSWELL IMPACT STRUCTURE, NORTHWESTERN SASKATCHEWAN, CANADA

Voluminous impact melt rocks, mega fallback breccia, and extensive impact-induced melting of host granitoid gneisses were discovered during recent uranium exploration diamond drilling by Alpha Exploration Inc. on its Middle Lake property within the central gneissic basement uplift of the Carswell Impact Structure, located in the western part of the Proterozoic Athabasca Basin in NW Saskatchewan, Canada. The impact structure’s age is Ordovician, based on previous studies in the 1970s and 80s. The mega fallback breccia occupies a wedge-shaped mass 160 m wide, at least 200 m long, and up to 100 m thick along the downthrown side of a major impact-related fault that crosses the basement uplift. The breccia comprises a heterolithic suite of intensely fractured clasts up to 8 m across within a clast-rich impact breccia matrix accompanied by impact melt veins and dykes. Irregular curvilinear contacts with embayments indicate a semi-plastic state of the boulder-size clasts. In addition to drill core intersections, pink, olive brown and green, vesicular, amygdaloidal, clast-bearing, volcanic-like impact melt bodies were previously mapped on surface, and are spatially associated with the mega fallback breccia. Impact melt bodies occur in dyke to sill-like intrusions up to 30 m wide and several hundred metres long, most commonly along faults. They have a microcrystalline texture with vesicles and amygdules typically lined with chlorite and or quartz. Many amygdules have a white clay core. Quartz is the primary clast type and shows various stages of assimilation into the melt and recrystallization to very fine quartz. Flow banding is common in the impact melt rocks. The intense heat generated by the impact produced in situ melting of granitoid gneisses in this part of the structure, and resulted in the progressive destruction (melting) of the mafic mineral component and metamorphic textures, which in turn produced darker colored rocks. Quartz and feldspars also underwent progressive melting/assimilation, and are commonly present as ragged, relict grains in the black melt, with quartz showing various stages of recrystallization. Some black melt material has recrystallized to very fine, randomly oriented biotite and finer opaque grains. Decorated planar deformation features in quartz occur in all these impact-related rocks.