Seismic reflection images of the major ore-controlling structures in the Eastern Goldfields province, Western Australia

Abstract

Gold mineralisation in the Eastern Goldfields Province of the Yilgarn Block, Western Australia, is associated with shear zones within the greenstone supracrustal succession. Regional shear zones are imaged in seismic reflection sections as bands of strong reflections. Although individual wavelets within the bands of reflections can only be correlated over small distances, the bands of reflections can be correlated over tens of kilometres. The Bardoc Shear, adjacent to which considerable mineralisation has been found, dips west and links with the Ida Fault, which forms the boundary between the Eastern Goldfields Province and the Southern Cross Province farther west. The Ida Fault dips east at about 40°, and can be traced from the surface to about 27 km depth. Bands of reflections within the upper and middle crust have a similar seismic signature to the Ida Fault, Bardoc Shear Zone and the basal detachment of the greenstones, and are therefore interpreted as shear zones. Interpreted shear zones in the upper crust under the greenstones mostly dip west. Shear zones in the lower crust dip east. The upper crust east of the Ida Fault and below the Bardoc Shear is an exception. There, east-dipping shear zones, including the Ida Fault, are interpreted to extend from the lower crust into the upper crust, thereby providing a simple plumbing system for mineralising fluids migrating from the lower crust, into the Bardoc Shear, and then to high levels in the greenstones.