Rock-magnetic study of gold mineralization near a weakly deformed Archean syenite, Thunder Bay, Canada

Abstract

Gold mineralisation occurs near granitoid plutons in the Shebandowan granite-greenstone belt of the Superior Province in the Canadian Shield. Conventional structural and petrographic observations show that the Moss Lake Syenite pluton escaped penetrative, regional deformation. Nevertheless, anisotropy of magnetic susceptibility (AMS) reveals a well-developed, consistent, cryptic magnetic fabric, largely due to multidomain magnetite. The magnetic foliation is subparallel to the earlier regional schistosity and reveals the dissipating, regional stress field that was insufficient to produce conventional tectonic fabrics in the syenite. Palaeomagnetic studies reveal that the characteristic remanence carried by magnetite scatters around the known palaeofield for this region. The feeble strain that imposed the (AMS) magnetic fabric barely affected the palaeomagnetic signal. Gold mineralisation occurs where shear zones intersect the eastern margin of the pluton. Late haematisation concentrates in the auriferous sheared zones. Unlike the palaeomagnetic signature of magnetite, haematite-remanences disperse in a plane parallel to the regional schistosity due to tectonic effects rather than magnetic refraction. We infer that the shear-zones' haematite-remanences developed after the "post tectonic", cryptic magnetic fabric was imposed on the pluton. Since gold is found in the haematised shear zones, we infer that gold mineralisation may postdate the feeble, "regional" deformation of the syenite and is therefore clearly postmagmatic.