Influence of structural setting on sulphur isotopes in Archean orogenic gold deposits, Eastern Goldfields Province, Yilgarn, Western Australia

摘要

The published mean delta~(34)S values of ore-related pyrites from orogenic gold deposits of the Eastern Gold-fields Province, Yilgarn Craton lie between -4 per thousand and +4 per thousand As for orogenic gold deposits worldwide, most deposits have positive means and a restricted range of delta~(34)S values, but some have negative means and wider ranges of delta~(34)S values. Wall-rock carbonation and back-mixing of similar-source fluids with different fluid pathways can explain some of the more negative delta~(34)S signatures. However, structural setting appears to be the most important factor controlling ore-fluid oxidation state and hence the distribution of delta~(34)S values in gold-related pyrites. Shear-hosted deposits appear to have experienced fluid-dominated processes such as phase separation, whereas stockwork, vein-hosted or disseminated deposits formed under conditions of greater rock buffering. At Victory-Defiance, in particular, negative delta~(34)S values are more common in gently dipping dilational structures, compared to more compres-sional steeply dipping structures. It appears most likely that fluid-pressure fluctuations during fault-valve cycles establish different fluid-flow regimes in structures with different orientations. Rapid fluid-pressure fluctuations in dilational structures during seismic activity can cause partitioning of reduced gas phases from the ore fluid during extreme phase separation and hence are an effective method of ore-fluid oxidation, leading to large, local fluctuations in oxidation state. It is thus not necessary to invoke mixing with oxidised magmatic fluids to explain delta~(34)S signatures indicative of oxidation. In any case, available, robust geochronology in the Eastern Goldfields Province does not support the direct involvement of oxidised magmatic fluids from adjacent granitic intrusions in orogenic gold genesis. Thus, negative mean delta~(34)S values and large variations in delta~(34)S values of ore-related pyrites in world-class orogenic gold deposits are interpreted to result from multiple mechanisms of gold precipitation from a single, ubiquitous ore fluid in varying structural settings, rather than from the involvement of oxidised ore fluids from a different source. Such signatures are indicative, but not diagnostic, of anomalously large orogenic gold systems.