Abstract

Gold in the Abitibi greenstone belt in the Superior craton, the most prolific gold-producing greenstone terrane in the world, comes largely from complex orogenic mineralizing systems related to deep crustal deformation zones. In order to get a better understanding of these systems, we therefore combined new magnetic, gravity, seismic, and magnetotelluric data with stratigraphic and structural observations along a transect in the Matheson area of the Abitibi greenstone belt to constrain large-scale geologic models of the Archean crust. A high-resolution seismic transect reveals that the well-known Porcupine Destor fault dips shallowly to the south, whereas the Pipestone fault dips steeply to the north. Facing directions and gravity models indicate that these faults are thrust faults where older mafic volcanic rocks overlie a younger sedimentary basin. The depth of the basin reaches ~2 to 2.5 km between these two faults, where it is interpreted to overlie mafic-dominated volcanic substrata. Regional seismic and magnetotelluric surveys image the full crust down to 36-km depth to reveal a heterogeneous architecture. Three crustal-scale layers include a resistive (104–105 Ωm) upper crust of granite-greenstone rocks, a low-resistivity (~10–50 Ωm) middle crust dominated by granitic plutons for which low resistivity is attributed to the presence of graphite, and a low to moderately resistive (50–1,000 Ωm) and seismically homogeneous lower crust interpreted as granulite gneisses. The significant resistivity transition between upper and middle crust is interpreted to be the result of interconnected micrographite grain coating, precipitated from carbon-bearing crustal fluids emplaced during Neoarchean craton stabilization. A major subvertical, seismically transparent, and extremely low resistive (<10 Ωm) corridor connects the lower and middle crust with the upper crust. The geometry of this low-resistivity feature supports its interpretation as a deep-rooted extensional fault system where the corridor acted as a regional-scale conduit for gold-bearing hydrothermal fluids from a ductile source region in the lower crust to the depositional site in the brittle upper crust. We propose that this newly discovered whole crustal corridor focused the hydrothermal fluids into the Porcupine Destor fault in the Matheson region.

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