Large-scale resistivity structure of the Superior Craton, Canada

Abstract

The Superior Craton, located within the Canadian Precambrian Shield, hosts significant mineral wealth largely concentrated within its southeastern portion. However, little is known about a) the deep geophysical signatures, and b) the processes involved in the metal emplacement in the Archean greenstone belts of the Superior province. Furthermore, despite several decades of research involving a variety of geophysical methods, the electrical resistivity structure of the Superior Craton remains enigmatic. Via the analysis, inversion, and interpretation of new and legacy magnetotelluric data, this study aims to reveal the large-scale resistivity structure of the Superior Craton, and thus gain insights into the geodynamic processes which were active during its complex history as well as the underlying mechanisms responsible for the disparate levels of shallow metal endowment across the Superior.

New 3D inverse modelling results of broadband MT stations from southeastern Superior, including the well-endowed Abitibi and Pontiac subprovinces, reveals the details of a ‘whole-of-crust’ magmatic-hydrothermal system. East-west low resistivity structures broadly underlie the surface traces of the major deformation zones that are host to significant gold endowment, while upper and mid-crustal cross trends suggest mineralized fluids flowed along narrow pathways within and/or oblique to the fault planes. These resistivity features delineate relict mantle source/transit domains and crustal pathways enriched by the flow of magmas or metamorphic fluids which may be genetically related to a late-stage pulse of ore-bearing magmatism, possibly as a result of slab break-off or delamination.

Similar lower-crustal structures are imaged within the less endowed western Superior, where inverse modelling results reveal curvilinear conductors within the lower crust trending sub-parallel to major terrane boundaries and their associated fault systems. Similar to the southeastern Superior, these conductive features are interpreted as the geo-electric signatures of syn- to post- orogenic magmatism during crustal delamination. At depths of 100–200 km, the model is dominated by north-trending bands of alternating low and high resistivity, interpreted to be artefacts of electrical anisotropy. Subsequent anisotropic modelling of the data image an electrically anisotropic layer within the lithosphere, interpreted to represent channelized metasomatism as a result of mid-Proterozoic tectono-magmatic activity.