Crustal-scale shear zones and their significance to Archaean gold mineralization in Western Australia

Many large Archaean epigenetic gold deposits show a broad spatial relationship to regional lineaments in greenstone belts, although in detail they are sited in subsidiary brittle-ductile fault structures. Fluids, originating from a deep source, follow a complex path and re-equilibrate with different lithologies and with metamorphic fluid during migration to higher crustal levels. Temperature and pressure conditions at or below the amphibolite/greenschist metamorphic boundary, where most gold deposits are located, favour the establishment of brittle-ductile and brittle subsidiary structures, the preferred structural setting of gold deposits. Physical gradients between the regional ductile structures and more brittle subsidiary structures ensure transient, strongly localized, fluid flow into the latter, where lower temperatures and suitable host rocks with high Fe/(Fe + Mg) ratios favour gold-deposition. The multi-source origin and continuous re-equilibration of the fluid with crustal rock, which includes granitoid and greenstone-belt lithologies of different ages, is reflected in the diverse isotopic and geochemical signature of the gold deposits.