Structural evolution of auriferous deformation zones at the Campbell mine, Red Lake greenstone belt, Superior Province of Canada

Abstract

The Campbell mine, the largest gold mine in the Red Lake camp, lies within the regional (first-order) NW-trending foliation-parallel Cochenour-Gullrock Lake deformation zone. The auriferous veins at the mine are hosted by subvertical second-order deformation zones within the Cochenour-Gullrock Lake deformation zone. These second-order deformation zones strike NW, NNW and E-W. Across the foliation-parallel NW-trending ore zones (A, F, F2 and NL ore zones), the rock and magnetic foliations show progressive increase in strain intensity, but their orientations do not change. This, together with the absence of other shear-sense indicators within the foliated rocks, strongly suggests that the foliation parallel NW-trending ore zones were developed during NE-SW subhorizontal shortening. Foliation-oblique ore zones strike NNW (G and P zones) and E-W (part of A and L zones). They display shear-sense indicators such as asymmetrical folding of thin gold-bearing quartz-carbonate veins and progressive rotation of petro and magnetic fabrics. These shear-sense indicators indicate that NNW- and E-W-trending ore zones are dextral and sinistral strike-slip shear zones, respectively. The intersection of these second-order shear zones is approximately parallel to the subvertical boudin axis of foliation-parallel veins in the NW-trending deformation zones, and the NW-trending deformation zones bisect the NNW- and E-W-trending shear zones. These suggest that the NNW- and E-W-trending shear zones are conjugate shear zones developed in association with the NE-SW subhorizontal shortening, which is normal to the NW-trending deformation zones. Cross-cutting relationships, the deformation recorded in post-ore lamprophyre dykes, and stress tensors inverted from the fault-slip data suggest that the NE-SW compression deformation spanned a prolonged period. Auriferous veining under the compression regime is followed by lamprophyre dyke intrusion, then by brittle-ductile faulting under NE-SW compression. The mode of deformation may characterize other parts of Cochenour-Gullrock Lake deformation zone and is of significant importance in gold exploration in the eastern part of the Red Lake greenstone belt. Field data from the Cochenour-Gullrock Lake deformation zone and the margin of the Trout Lake batholith suggest that the NE-SW shortening is likely related to the batholith emplacement and that the Cochenour-Gullrock Lake deformation zone corresponds to a highly attenuated limb of a NW-trending regional fold which overprints earlier NE-trending folds in the area.