Structural Setting of the Gold Mineralization at Stawell, Victoria, Australia
R. B. Watchorn, C. J. L. Wilson, 1989. "Structural Setting of the Gold Mineralization at Stawell, Victoria, Australia", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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The Stawell gold field, on the western edge of the Lachlan fold belt, is dominated by regional northwest-trending tectonostratigraphic units upon which are imposed shear zones that display repeated reactivation within weak and highly strained zones. Gold mineralization was preceded by regional deformations D, to D3 and low-grade regional metamorphism. Two distinct types of shear zone-hosted mineralization associated with deformations D4 to D6 exist in the area: vein deposits superimposed on the regional metamorphic sequence in the Magdala mine, and hydrothermal alteration along a fracture system within the contact metamorphic aureole of the Stawell granite in the Wonga open pit.
In the Magdala mine, gold mineralization can be related to late-stage D4 and D5 movements in a west-dipping fault system with initial reverse and subsequent oblique strike-slip movements. Three types of mineralization associated with arsenopyrite and pyrite are found in the Magdala mine: (1) laminated and massive quartz veins occurring as flat veins in dilational openings within shear zones or in deformed west-dipping quartz sheets within shear zones, (2) gash and stockwork veins superimposed on the D4 shear zones, and (3) sulfide lodes where shear zones are superimposed on syngenetic pyrrhotite concentrations. All gold mineralization occurs within or above a footwall volcanic sequence. Mineralization and metasomatic alteration are retrogressive with respect to, and postdate, the peak of regional metamorphism and are controlled by fluid ingress during shearing.
In the Wonga open pit, an east-dipping reverse shear system is contemporaneous with initial gold deposition and is synchronous with the very late stages of contact metamorphism produced by the emplacement of the Stawell granite. Movement in the shear system changes to a major sinistral strike-slip system which becomes the locus for sericite-carbonate metasomatic alteration. The Wonga quartz-carbonate vein deposits are spatially and temporally associated with fluids accompanying the felsic plutonism.
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When the price of gold rose from about $200 (U.S.) an ounce in 1979 to nearly $700 an ounce by the end of the same year, the gold rush of the 1980s was under way. Gold production in the western world rose dramatically; from 1981 to 1986 production increased by 300 to 1,282 metric tons per year. Annual production may reach 1,500 to 1,600 metric tons by 1990 (Woodall, 1988). The major contributors to the increased stream of gold have been Australia, Canada, Brazil, and the United States together with other circum-Pacific countries. The increased price of gold and new methods of extraction have allowed many older deposits to be reopened, but the most important factor has been the high success level of exploration. This success has resulted in large part from the application of new genetic models and from the development of new exploration techniques.
There are hundreds of thousands of reported gold occurrences around the world. The majority are alluvial placers, but large numbers of bedrock occurrences have also been discovered. Most of these occurrences prove to be very small and are relatively unimportant in the overall world production level. Most mined gold has come from a small number of giant deposits, which were found by prospectors. It is becoming increasingly clear, however, that the discovery of giant deposits in the future will involve more than the sharp eyes and persistence of the old prospector. The use of sound geologic principles, and exploration programs based on those principles, is what the future holds. An example can be seen in the successful search for gold deposits in the South Pacific. There, exploration models have been based on principles developed in the study of modern geothermal systems. Giant deposits such as Lihir and Porgera have been the reward. Another example is the giant copper-gold-uranium deposit at Olympic Dam, South Australia, discovered beneath 300 m of cover using an exploration program based on models developed by Western Mining Corporation geologists for Zambian copper belt-type deposits.
Gold deposits are widely dispersed throughout many geologic settings and in virtually all kinds of rocks, but they do not seem to have formed at a uniform rate throughout geologic history. On the contrary, two very distinct metallogenic periods have been defined. The first is the Archean era, when most of the great deposits in greenstone belts were formed and the vast Witwatersrand basin deposits in