Phase 2 geoscientific preliminary assessment: acquisition, processing and interpretation of high-resolution airborne geophysical data: town of Blind River, city of Elliot Lake and area, Ontario

This technical report documents the results of the acquisition, processing and interpretation of high resolution airborne geophysical data conducted as part of the Phase 2 Geoscientific Preliminary Assessment, to further assess the suitability of the Elliot Lake and Blind River area to safely host a deep geological repository (Golder, 2017). This study followed the successful completion of a Phase 1 Geoscientific Desktop Preliminary Assessment (Golder, 2014). The desktop study identified 3 potentially suitable areas warranting further studies such as high-resolution surveys and geological mapping one of which was covered by the survey. The purpose of the Phase 2 acquisition, processing and interpretation of geophysical data was to provide an updated interpretation of the geological characteristics of the potentially suitable bedrock unit identified in Phase 1 and to provide additional information to further assess the geology of the Elliot Lake and Blind River area. Both magnetic and gravimetric data were acquired during the survey in order to provide data to interpret the geometry and thickness of the potentially suitable bedrock units; the nature of geological contacts; bedrock lithologies; the degree of geological heterogeneities and the nature of intrusive phases within the area; as well as the nature of structural features such as faults, shears zones, and alteration zones. The grids of the acquired magnetic and gravimetric data and associated processed grids (first, second and horizontal derivatives, total gradient amplitude, trend analysis solutions and tilt angle) were analyzed and interpreted together with the mapped bedrock geology and other available geological information (e.g. magnetic susceptibility and rock density). The survey allowed for a characterization of the local gravity and magnetic signature of the granite granodiorite suite of the Ramsey-Algoma granitoid complex and the transition to the gneissic tonalite suite to the north. A gravity low at the southern end of the survey either represents the deepest portion of a uniformly dense granite-granodiorite suite or a local alternate phase of emplacement with a slightly lower density than the rest of the granite-granodiorite suite. A gravity high at the northern end represents a dipping transition from the granite-granodiorite suite to a denser gneissic tonalite suite. Low amplitude long wavelength variations in magnetic anomalies highlight some degree of lithological heterogeneity within the granite-granodiorite suite. The gravity and magnetic signature of the Benny Lake greenstone belt shows internal compartmentalization with variations in magnetic properties and a possible extension of the greenstone belt farther to the west of the mapped area. Preliminary forward modelling was completed on two profile lines covering the local features of the Ramsey-Algoma granitoid complex and intrusions and greenstone belts within the study area. Multiple model options looking at either uniformly or variably dense granite-granodiorite rocks were investigated. The base of the Ramsey-Algoma granitoid complex has been modelled no shallower than 5 km below Mean Sea Level (MSL) in all scenarios. (author)