Yandal greenstone belt, Western Australia: 12 million ounces of gold in the 1990s

Abstract

After a century of virtual neglect, exploration in the Yandal greenstone belt of the Yilgarn Craton of Western Australia has yielded resources of 12 Moz Au during the 1990s. Success has come from a combination of conceptual geological models, surface prospecting, understanding the weathering environment, and systematic drilling. The Archaean Yandal greenstone belt comprises a lowermost banded iron formation, extensive basalt and dolerite sills, ultramafic rocks, intermediate to felsic volcanic rocks, and variable clastic sedimentary rocks. Early shear zones trend NNW and form the greenstone belt margins, or trend N–S within the belt. Later brittle cross-faults are critical in gold localization. Gold resources and past production at major deposits include Bronzewing (4 Moz Au), Jundee (5 Moz) Mt.␣McClure (1 Moz) and Darlot (3␣Moz, some of which was produced before 1990). All major deposits are hosted by Fe-rich mafic rocks, and mineralization displays a combination of different orientations and morphologies. Quartz veins are surrounded by broad carbonate alteration with proximal K-mica and Fe-sulphides. The recognition of a critical role for the late brittle structures in localizing gold implicates mid-crustal processes within the greenstone belt for fluid generation, and with the host rock control, supports the model in which fluid was derived by metamorphic devolatilization.