Trans-crustal geophysical responses beneath the supergiant
Timmins-Porcupine orogenic gold camp, Canada
Abstract
A new 80-site magnetotelluric (MT) survey, integrated with reprocessed
seismic reflection profiles, across the supergiant Timmins-Porcupine
gold camp of the Abitibi greenstone belt (AGB) was conducted to
investigate the architecture of crustal-scale structures. Resistivity
sections derived from 3-D MT inversions reveal a major 40 km by 20 km
sub-horizontal 500-1000 S conductor north of the Porcupine Destor fault
zone at 5–10 km depth. A horizontal component of this conductor,
attributed to a deeply buried >2687 Ma graphitic
argillaceous unit at the base of Porcupine assemblage, strikes east-west
parallel to the Pipestone fault zone. A second steeply-dipping component
strikes northwest-southeast parallel to the Buskegau River fault. This
conductor correlates spatially with lateral breaks in seismic reflectors
and velocity models in the upper, middle, and lower crust, and provides
evidence of a crustal-scale suture which also resulted in imbrication of
<2698 Ma metasedimentary rocks onto the southern AGB. Enhanced
conductivity and spatially complex electrical structure of the crust to
the north of the Porcupine Destor fault zone reflects the asymmetric
distribution of metasedimentary packages, second- and third-order
bounding structures, and gold mineralization. The MT resistivity models
also resolve an upper crustal conductor located 10 km south of the
surface trace of the Porcupine Destor fault zone, providing support for
a south-dipping crustal fault. Breaks in seismic reflectors underlying
this conductor provide additional evidence of sub-vertical structures
extending through the middle crust, postdating post-tectonic collapse or
orogen-parallel ductile flow at ∼2660-2590 Ma, and consistent with late
strike-slip deformation.