RECENT AEROMAGNETIC SURVEYS IN THE SOUTHERN CANADIAN CORDILLERA: CONTRIBUTIONS TO GEOLOGICAL MAPPING AND MINERAL EXPLORATION

Several government-funded airborne geophysical surveys flown in the last two decades have provided a new perspective on the geology along an approximately 400 km long corridor stretching roughly from Prince George to Kamloops, and falling mainly on the prospective Quesnel Terrane. These surveys were invariably combined magnetic-radiometric surveys flown at 200-500 m line-spacing and 125 m mean terrain clearance, distances providing significant detail in images derived from the measured parameters. In a region where extensive glacial drift and Tertiary volcanic cover present major challenges for geological mapping and mineral exploration, the capability of magnetic data to “see through” such cover is a critical means for appraising and enhancing existing geological maps. Magnetic fabrics in images of the 1st and 2nd vertical derivatives of the magnetic field reveal internal structures and characterize phases and lithological units, which are otherwise unrecognized.

Changes in patterns of the fabrics also help delineate contacts between first order geological units. Magnetic patterns indicate revisions to the location of contacts between the Jurassic Thuya Batholith and Miocene Chilcotin Group and Eocene Kamloops Group basaltic volcanic rocks. Patterns and profile data strongly suggest that an area mapped as Chilcotin Group south of Bonaparte Lake is underlain by the Kamloops Group. A linear magnetic fabric in the central part of the Thuya batholith similar to fabric in the heterogeneous, more mafic, more altered and more prospective eastern lobe of the batholith may likewise be associated with prospective rocks. Farther north, magnetic images reveal an internal complexity within the Cretaceous Naver pluton. A strong positive magnetic signature characterizing the northwestern lobe terminates abruptly along a N-S line, undoubtedly reflecting a major fault. This fault has significant expression in radiometric potassium and thorium/potassium ratio images, demonstrating the value of integrating magnetic and radiometric data. Patterns in images of vertical derivatives of the magnetic field are suggestive of other major faults, internal phases, and possible roof pendants.