Hydrothermal fluids are one of the key media for transporting economically important elements in the crust. I will demonstrate some of the chemical principles by which a variety of trace elements are concentrated into ore deposits. Studies of metal solubilities have shown systematic changes as a function of redox. For example, Te solubility increases during reduction at oxygen fugacities below the hematite-magnetite boundary in hydrothermal fluids. Conversely reduction decreases Sb solubility. The solubility of many other elements are redox controlled, including all of the transition metals. The response of As to changing oxygen fugacity is important in gold deposits due to its strong association with economic Au grades. In particular Au is closely related to arsenian pyrite and arsenopyrite, which are known to precipitate due to reduction. As the behaviour of trace elements is affected by changing oxygen and sulfur fugacities, these elements can be used to track changes in hydrothermal fluids. In a range of geological settings pyrite is observed, hence sulfur isotopes can be used to track subtle changes in redox within individual pyrites across the entire history of ore formation and or granite development.
Wallaby, Argo and a collection of S and I type granites were selected to test if trace elements and sulfur isotopes systematically change across an independently determined redox gradient. Given that changes in pH or temperature are minimal, all changes in sulfur isotope and trace element concentrations can be attributed to changes in oxygen fugacity. The outcomes of this study will not only document the evolution of the samples but could also define wider geological processes involved in ore formation. This study also includes results from a range of pyrite synthesis experiments that span 3 different buffered conditions and discusses the implications for gold deposition.