STRUCTURAL CONTROL OF MINERALIZATION IN SHEAR-ZONE-HOSTED GOLD DEPOSITS

Several examples of shear-zone-hosted gold deposits can be found in the Archean Superior Province of the Canadian Shield in northwestern Ontario. Field investigation and microstructural analysis show that strain incompatibility due to competence contrast creates significant control on gold mineralization in these deposits. This control is evident at various scales. On the microscopic scale, gold is commonly found in fractures within and in strain shadows adjacent to rigid porphyroclasts. For example, at Musselwhite Mine gold is associated with rigid garnet porphyroblasts in metamorphosed and mylonitized banded iron formation. At Hammond Reef gold is associated with euhedral, subhedral and brecciated porphyroblasts of pyrite. On the macroscopic scale, gold is commonly associated with competence contrast along lithologic contacts as well as near margins of competent lithons between anastomosing shear zones. At the regional scale, relatively low strain areas adjacent to granitic plutons within broad regional shear zones appear favourable for large economic deposits.

On the microscopic and macroscopic scales, strain incompatibility during ductile deformation creates sparse local porosity through loss of cohesion along boundaries between ductile and rigid materials, and by brittle deformation of the more competent material. At mid to lower crustal depths this porosity is temporary, eliminated by mineral precipitation and ductile deformation. The difference in rates for fluid flow versus crystal-plastic flow allows fluid to exploit this porosity, creating favourable conditions for gold mineralization.

Large volumes of fluid interaction are required to produce economic gold deposits. In order for sparse fluid moving through temporary porosity to concentrate large deposits, deformation on the hosting shear zone must be long-lived. Crustal-scale shear zones with the longest history of deformation are most prospective for gold. In the Superior Province and elsewhere, steeply dipping shear zones near plate or terrane boundaries with large magnitudes of transcurrent displacement are most likely to host giant gold deposits.