Phase 2 geoscientific preliminary assessment: acquisition, processing and interpretation of high-resolution airborne geophysical data, township of Hornepayne 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 Hornepayne area to safely host a deep geological repository (Geofirma, 2017). This study followed the successful completion of a Phase 1 Geoscientific Desktop Preliminary Assessment (Geofirma, 2013). The desktop Phase 1 study identified potentially suitable areas warranting further studies such as high-resolution surveys and geological mapping. High-resolution surveying was completed over two potentially suitable areas; one located in the Black-Pic batholith south of the Township of Hornepayne and one located in the metasedimentary rocks of the Quetico Subprovince north of the Township. 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 Hornepayne area. Both magnetic and gravimetric data were acquired during the surveys 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 pluton in 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 gravity and magnetic signatures of the Black-Pic batholith and internal plutons and greenstone belt units, as well as the metasedimentary rocks of the Quetico Subprovince and the dominant signature of the Quetico-Wawa subprovince boundary. The Black-Pic batholith corresponds to a fairly gentle gravity gradient decreasing to the south away from the subprovince boundary. Although previously mapped as homogeneous gneissic tonalite, the Black-Pic batholith has internal variations in magnetic character. The magnetic data along the subprovince boundary shear zone in the northern portion of the Black-Pic batholith is interpreted as a complex and heterogeneous mixture of lithologies which has undergone deformation and metamorphism, and may also include slivers of mafic to ultramafic metavolcanic rocks. In the central portion of the Black-Pic batholith is a narrow band of a strong magnetic fabric trending east-northeast that may contain some amount of mafic metavolcanic bedrock. The weakly magnetized southern portion of the Black-Pic batholith that is lacking in fabric is interpreted to represent the core of a large-scale fold through the Black-Pic batholith where gneissic layering is suggested to be either shallowly dipping or non-existent. Intense deformation and metamorphism apparent in the magnetic and gravity data in the Quetico Subprovince is limited within a 14 km wide zone north of the mapped subprovince boundary with tight folding apparent farther north. The Quetico Subprovince becomes generally non-magnetic farther away from the deformation zone. The Black-Pic batholith and the Quetico metasedimentary rocks are intruded by numerous dyke swarms (Matachewan, Abitibi, Biscotasing, Sudbury and unclassified dykes). Preliminary forward modelling was completed on two profile lines covering the principle features of the Black-Pic batholith and Quetico metasedimentary rocks and surrounding intrusions and greenstone belts within the study area. The Black-Pic batholith has been modelled with depths ranging from 4 km to 7 km with density and magnetic susceptibility variability and intensity increasing towards the subprovince boundary. Rocks of the Quetico Subprovince have been modelled locally as extending to depths typically greater than 6 km. (author)