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Hydrothermal gold mineralization occurs in banded iron formation and amphibolite host rocks, respectively, in the Main Hill and Zakanaka deposits in the northern sector of the Pilgangoora greenstone belt of the Pilbara Craton. The supracrustal sequences of the greenstone belt have been folded into NS trending, megascale isoclinal F2A folds, which have been refolded into open to tight F2B folds. D2 to D3 strike-slip shear zones developed preferentially along the limbs of F2A isoclinal folds and along the granitoid-greenstone belt contact. Gold mineralization is controlled by F2B folds and shear zones were interpreted to have been activated or reactivated during D2B deformation in both deposits. Gold associated hydrothermal alteration comprises zoned quartz-calc silicate veins and pyrrhotite-pyrite at the Zakanaka deposit and quartz-amphibole-biotite±diopside and pyrrhotite-lollingite-arsenopyrite at the Main Hill deposit. Thermobarometric and textural studies indicate that gold mineralization occurred synchronous with the peak of amphibolite-facies metamorphism in both deposits. Lead isotope studies on gold-associated hydrothermal microcline, titanite and pyrrhotite in the Zakanaka deposit yield a PbPb isochron with an age of 2888.2±6.1 Ma for the alteration and, hence, gold mineralization. This age is confirmed by a concordant PbPb age using SHRIMP on hydrothermal titanite from the Zakanaka deposit of 2904 ± 70 Ma. The initial isotopic ratio in the ore system, estimated using the isotopic composition of hydrothermal microcline, is much more primitive than 2888 Ma Pb from accepted growth curves. Lead-isotope modeling using the possible ages of rock formation and metamorphic events in the eastern Pilbara Craton shows that the primitive Pb isotope signature is consistent with the 2888 Ma age of alteration, if the Pb was derived from old crust which underwent an early high-grade depletion event. The age of hydrothermal, gold-associated alteration at Zakanaka indirectly constrains the age of the F2B folding event and the peak of amphibolite-facies metamorphism in the Pilgangoora greenstone belt to ca. 2.89 Ga. The special position of the Pilgangoora greenstone belt in the central Pilbara, permits the interpretation of the age of gold mineralization in the light of the tectonic evolution in both parts of the craton, and indicates that mineralization and the associated structural and metamorphic events post-dated the main magmatic evolution in both the east and the west Pilbara Craton, but coincided with late deformational events along strike-slip shear zones in the west Pilbara and late syn- to post-tectonic granitoids and associated metamorphism in the east Pilbara.

Petrophysical properties (magnetic susceptibility, density) and gravity data are presented from greenstone belts of the Yilgarn Craton which have been subjected to greenschist and amphibolite facie...

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.