SusChEM: Solubility of Iron Oxides and Gold in Acidic Hydrothermal Brines: Applications to the Origin of Iron Oxide Copper-gold Deposits

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As the world's population continues to expand and become increasingly industrialized, there will be a steady if not increasing need to discover new mineral deposits to provide the raw commodities that humans have come to depend on. In the case of gold, several decades of aggressive exploration by the mining industry in the 1970s and 1980s led to a global boom in the discovery of economically mineable gold deposits. However, the rate of discovery of new gold deposits has greatly declined in the past 10 years, partly because of a downturn in the industry, and partly because most of the deposits that are near surface and that fit conventional exploration models have already been discovered. This project will investigate the causative origins of a newly recognized class of hydrothermal gold deposits, referred to as 'iron oxide copper gold' (IOCG) deposits, that has the potential to generate a new cycle of exploration and discovery in the metal mining industry. Despite the fact that the IOCG class includes one of the largest metal deposits in the world - Olympic Dam, Australia - there is no consensus as to how - in a geologic, geochemical, petrologic, or plate tectonic sense - these deposits form. Part of the reason for this lack of consensus is a gap in our understanding at a fundamental, thermodynamic level of how metals such as iron, gold, and copper dissolve and precipitate in high temperature hydrothermal fluids. This project will help fill in this knowledge gap by performing several sets of hydrothermal experiments under controlled laboratory conditions. This type of study will have direct benefits to the mining industry and to society in general, which demands a steady supply of these mineral commodities. As for any scientific study that generates new, high-quality thermodynamic data, the results will also be useful to scientists in other disciplines to further understand the processes that have shaped the ancient and present-day Earth. Most researchers agree that the fluids that form IOCG deposits were hot, saline brines that were unusually oxidized: beyond this the ore deposit models diverge. This project will examine the solubility of iron oxide and gold in acidic, oxidized, saline brines at temperatures of 100 to 350 degrees C, and at oxidation states that are buffered near the aqueous ferric (Fe3+)/ferrous (Fe2+) boundary. The project will generate new thermodynamic data on the stability of aqueous ferric-, ferrous-, and gold-chloride complexes. The new data will be used to develop more accurate geochemical models for how IOCG deposits (and other types of Au-Cu-Fe deposits) form.