NEW AIRBORNE GEOPHYSICAL DATA FOR THE LAKE SUPERIOR REGION OF NORTHWESTERN ONTARIO: A NEW TOOL FOR THE IDENTIFICATION OF NEOARCHEAN TO MESOPROTEROZOIC STRUCTURES AND ASSOCIATED MAFIC-ULTRAMAFIC INTRUSIONS

New high-resolution (200m line-spacing) airborne magnetic and radiometric surveys released in 2015 (Ontario Geological Survey - Geophysical Datasets 1077, 1078, 1246) cover a large portion of the northwest Lake Superior region of Ontario. The surveys cover areas underlain by Midcontinent Rift-related rocks, Paleoproterozoic and Mesoproterozoic sedimentary rocks of the Animikie and Sibley Groups, as well as parts of the Archean Quetico, Wawa and Wabigoon subprovinces.

In the Lake Superior region, Ni-Cu-PGE mineralized mafic-to-ultramafic rocks were emplaced during several tectonic events and in a variety of settings, over a time period extending from the Mesoarchean to the Mesoproterozoic. As a result, these intrusions display a wide range of geochemical affinities and morphologies. In spite of these geochemical and physical differences, most of these mafic-to-ultramafic intrusions have a close spatial association to major crustal-scale structures and many intrusions can be recognized by their distinctive magnetic signatures (i.e., positive or negative anomalies) regardless of age - of which more than 60 examples are illustrated.

The new airborne magnetic data highlight several rift-parallel structures that could have controlled the emplacement of mineralized MCR-related intrusions into Archean country rocks along the northwest margins of the Midcontinent rift north of Thunder Bay. One such structure, marked by several magnetic discontinuities and anomalies, can be traced for at least 145 km along an east-northeast (063⁰) trending line from the southeast end of Northern Light Lake (near the Ontario-Minnesota border), through to Greenwich Lake (50 km northeast of Thunder Bay). Ni-Cu-PGE mineralized mafic-ultramafic rocks of the Sunday Lake, Steepledge Lake and Current Lake intrusive complexes occur in a linear array that closely follows this Northern Light-Greenwich Lakes structure (NLGLS), suggesting that it may have played a role in their emplacement. The NLGLS is also located in close proximity to Neoarchean gold (Tower Mountain) and komatiite-hosted Ni-Cu-PGE (Bateman Lake) mineralization in Conmee Township, suggesting that the NLGLS may have been tectonically active both during the accretion of the Superior Craton, and during the Midcontinent Rift event.