Seismic images from surface to Moho depth at the McArthur River mine district; Saskatchewan, Canada

Abstract

Two subparallel, regional seismic reflection profiles of the multidisciplinary and multi-institutional EXTECH-IV project outline fundamental relationship between the local structures of the McArthur River mine of the Athabasca Basin and the underlying deep crustal tectonic framework. This deep reflection sounding investigation crosses over the P2 ore body, which is located in the region of a magnetic low. Tomographic inversion of the first-break (FB) arrival times indicated lateral changes in P-wave velocities of the sandstone along the survey lines. Distinct high velocity zones mark alterations of the sandstone lithology over known mineralized locations of the P2 ore body.The migrated time sections are characterized by robust reflection signals from near surface to past-Moho depths. The sandstonebasement unconformity is clearly imaged, however, its correlation is more difficult along the central and southern part of the profiles due to influences of major structural disturbances. The differing patterns of reflections observed along the profiles will potentially allow distinction of the Wollaston/Mudjatik Transition Zone from the Wollaston Domain. South dipping reflections, traceable to 4 km depth in the crust, are correlated with the P2 shear/fault that is well recognized below the main ore body of the McArthur mine site. This structure is over 2.5 km thick and its seismic signature suggests multi-phase deformation. Strong, arcuate, and in places subparallel, zones of reflectivity dominate the middle and lower crust. The origin of these prominent seismic events appears to be located at the southern margin of the seismic investigation.Highly prominent, bright and sub-horizontal reflections, at 2.3 s two-way time (TWT), are the most distinct seismic signatures on the sections. These events are directly comparable to reflections observed along the northern half of the 1994 LITHOPROBE regional line S2B. The origin of this enigmatic zone of reflectivity is still under considerable discussion; Mandler and Clowes (1997) associate the 1994 data with the post-Hudsonian Mackenzie magmatic activity (1.265 Ga). The Moho is well defined at about 10.5 s TWT and is characterized by a number of laterally recognizable gently south dipping reflections. Below the Moho, a zone of strong reflectivity delineates the upper mantle, revealing considerable tectonic involvement of the lithospheric mantle within the depth range of 35-45 km. A gentle southeastern dip in the tectonic development of the region symbolizes the overall structural attitude of this highly reflective zone.