Influence of structural setting on sulphur isotopes in Archean orogenic gold deposits,Eastern Goldfields Province,Yilgarn, Western Australia |
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| Authors: | P F Hodkiewicz D I Groves G J Davidson R F Weinberg S G Hagemann |
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| Institution: | (1) Centre for Exploration Targeting, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia, 6009, Australia;(2) SRK Consulting, 10 Richardson Street, West Perth, Western Australia, 6005, Australia;(3) School of Earth Sciences, University of Tasmania, Private Bag 79, Hobart, Tasmania, 7001, Australia;(4) School of Geosciences, Monash University, Building 28, Clayton, Victoria, 3800, Australia |
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| Abstract: | The published mean δ34S values of ore-related pyrites from orogenic gold deposits of the Eastern Goldfields Province, Yilgarn Craton lie between
−4‰ and +4‰. As for orogenic gold deposits worldwide, most deposits have positive means and a restricted range of δ34S values, but some have negative means and wider ranges of δ34S values. Wall-rock carbonation and back-mixing of similar-source fluids with different fluid pathways can explain some of
the more negative δ34S signatures. However, structural setting appears to be the most important factor controlling ore-fluid oxidation state and
hence the distribution of δ34S 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 δ34S values are more common in gently dipping dilational structures, compared to more compressional 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 δ34S 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 δ34S values and large variations in δ34S 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.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. |
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| Keywords: | Archean Yilgarn Orogenic lode gold Sulphur isotopes |
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