Dean R. Cutting

Field mapping (1:5000) and study of joints in an area of 7.3 square kilometres centered around the Arntfield Gold Mines, during the summer of 1979, forms the basis for this study.

East-striking, north-dipping rocks of the Archean Age Blake River Group, consisting predominantly of metamorphosed intermediate (andesitic) and felsic (rhyolitic) volcaniclastics and flows, occur in the south limb of a regional syncline. They are intruded by small bodies of diorite, quartz-feldspar porphyry and syenite. Meta-greywackes and conglomerates of the Proterozoic Cobalt Group unconformably overlie all the above rocks.

Now-abandoned and inaccessible gold mines lie within the Francoeur-Arntfield shear zone, an east-trending, sinuous, wide (over 100 m), regional feature that traverses the area. Field relationships seem to indicate that the formation of this major fault was later than the deposition of the volcanic pile but earlier (or synchronous with ?) than the intrusions. Metamorphism to greenschist facies has affected all rocks.

Analysis of over 400 joint sets in different structural domains in the area and comparison with experimental data for rock deformation suggest that the major shear zone evolved as a “Riedel” type sinistral (left lateral) strike slip shear. The joint patterns are distinct in the proximity of the shear zone, even in the absence of outcrops of sheared rock. Systematic studies of this kind may provide a practical tool for locating the shear zone in the field and thus may provide a useful guide to the controls of gold-telluride-quartz-carbonate mineralization locally associated with the shear zone.

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Pages: 135
Supervisor:  Marcos Zentilli