Three-dimensional structure of greenstone belts in Western Australia: implications for gold exploration

The three-dimensional geometry of greenstone belts in Africa and Canada is reasonably well constrained by gravity, geoelectric and seismic reflection data. A comparison of greenstone belts throughout the world shows that their overall geometries are everywhere similar. They are typically about 5 km thick and have relatively flat bases. The magnitude and direction of the dip of their contacts with surrounding granitoid rocks is variable, probably owing to local differences in their tectonic histories. The greenstones are sometimes intruded by sheet-like granitoid bodies. Thicker granitoid intrusions which form basement ridges are also recognised. Fewer geophysical studies are available from Western Australian greenstone belts. However, gravity data show that in some cases the granitoid?greenstone contact dips at a shallow angle away from the greenstone belt. There is also evidence that some of the granitoids are sheet-like. These observations carry the implication that mineralised greenstones may be present in the sub-surface below areas of granitoid outcrop. Evidence from the Murchison Greenstone Belt in South Africa, and from the Eastern Goldfields, suggests that a spatial correlation exists between mineralisation and concealed granitoid bodies. As the latter granitoids may be revealed as gravity 'lows', regional gravity surveys may be a useful first-order exploration tool within greenstone belts.