Seeing the forest for the trees: Regional and national-scale magnetotelluric models and their application to mineral systems exploration

Abstract:

As linkages between deep-Earth structure and near-surface mineral systems are increasingly recognized, the quest to map the lithosphere over large areas has turned a spotlight on national-scale geophysical arrays. Within North America, magnetotelluric (MT) data from the EarthScope and LithoProbe programs, together with public archives of legacy MT data, have led to a suite of 3D electrical resistivity models at unprecedented scale. Such models, together with geologic, seismic, potential field and geochronologic data are being used to refine the lithospheric architecture of North America and its evolution through time. This understanding in turn provides the tectonic context for identifying a series of deep-seated conductive zones related to paleosubduction zones, extensional provinces, and past continental margins. These inherited structures reflect changes in composition, mechanical strength and permeability which may act as preferential pathways for heat, fluids and metals during subsequent tectonic events. The 3D models also reveal the geometry of ancient sedimentary basins which can serve as both a physical/chemical trap during mineralization and a potential sulfur source for a variety of magmatic and hydrothermal deposits. Archean cratonic nuclei and their internal subdivisions are further imaged, the boundaries of which are often associated with steps in lithospheric thickness and are often associated with world-class mineral occurrences.

We present a preliminary ‘lithospheric prospectivity’ map of the conterminous United States based largely upon the 3D resistivity models. Several examples will be discussed, including paleosubduction zones of the Trans-Hudson Orogen and their relation to the Black Hills gold province and the Thompson Nickel Belt, extensional domains beneath the Missouri Iron province and the Proterozoic Animikie and Belt Basins.