Abstract

The Norbeau gold deposit comprises quartz veins within four differently oriented sets of shear zones which cut the Bourbeau sill, a metamorphosed layered mafic intrusion. Despite their differing orientations all shear zones display evidence of oblique-reverse shear which three-dimensional analysis shows to result from a nearly uniaxial downdip extension of the sill with a concurrent component of shortening along the strike of the sill. This guiding of strain axes can be attributed to the response of a stiff layer, the sill, in a matrix of softer rocks during a regional compressive deformation. This study demonstrates that veins in such rocks are strongly controlled by the geometry of the bodies and less by external stress conditions. Thus, applications of regional exploration models that rely on the prediction of vein patterns based solely on a consideration of paleostress axes will fail to provide an accurate assessment of the specific vein distribution in such layered rocks which host a number of important Archean lode gold deposits.

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