Geological setting and genesis of Archean gold deposits of Australia

Most Archaean gold production from a Australia has come from Greenstone Belts in the Western Australian Shield. On the regional scale, large deposits occur predominantly in younger rift-phase greenstones, whereas younger and older platform-phase greenstones are progressively less well mineralized. Mineralization occurs mainly within mafic volcanic or intrusive rocks, although BIF is also a significant host rock. Ores commonly occur in structural sites, typified by extensive wallrock alteration involving carbonation, K-metasomatism, and pyritization with synchronous Au deposition. Large deposits commonly have great depth extension but lack significant zoning. Sub-amphibolite facies settings are dominant. A metamorphic-replacement model explains the majority of characteristics of many of the deposits. The ore fluid and ore components were derived by devolatilization of dominantly volcanic sequence during high-grade metamorphism at the base of the greenstone pile. Gold was transported as a reduced sulfur complex in reduced, near-neutral, H/sub 2/O-CO/sub 2/ fluids of low salinity. Faults and shear zones provided fluid access to depositional sites where gold deposition occurred at 300-400C and 1-2 kb, largely in response to fluid-wallrock interaction. Pyritization of Fe-rich host rocks resulted in development of several of the larger gold deposits and other less-specific reactions produced the variety of other generally smaller deposits.