How the solution lies in hydrothermal fluids
Over the last 15 years the Mineralogy Department of the South Australian (SA) Museum has become one of the powerhouses in fundamental research on the formation of ore minerals not only in Australia, but also across the world. Our research group explores the physics and chemistry of ore deposit formation, applying state-of-the-art experimental techniques to study the transport and deposition of metals and mineral-microbe-fluid interaction in geological environments.
Under our leadership, the minerals, microbes and solutions (MMS) research group at the South Australian Museum grew to over 20 research fellows and PhD students, and received over $10 million in research funding, mainly from the Australian Research Council, but also from the mining industry. We adopted a team approach, with Joël focusing on understanding the properties of hydrothermal solutions, computer modelling, and applications of synchrotron radiation, and Allan on the mechanisms of mineral formation reactions and developing reactions cells to study mineral formation reactions in real time at high temperatures and pressures. The third strand of the group, the study of bacterial mobilisation of gold and other precious metals, was developed by Dr Frank Reith, who joined the group as a post-doctoral researcher and is currently a senior research fellow at the University of Adelaide. Research groups like this are successful because they collaborate widely; the SA Museum group hosted research students from all three universities in Adelaide, and developed extensive collaborations both locally and overseas.
Ore deposits in a test tube
Most hydrothermal ore deposits form within the Earth crust at elevated pressure and temperature. Much of our current knowledge of ore-forming processes has come from direct observation about the structure of deposits, chemical analyses of rocks, minerals and fluids. Modelling has been very useful, but the problem is that much of the base data is extrapolated from measurements made at low temperatures in the laboratory.
Our approach has been to build equipment so that we can measure fluid properties and reactions at temperatures and pressures much closer to those that operate in the Earth’s crust, as deposits are formed. Taking advantage of new materials and of the new generation of synchrotron and research reactors with much higher x-ray and neutron fluxes, we have been able to conduct experiments on mineralising hydrothermal fluids under crustal conditions and to observe directly the structure and composition of the metal-carrying fluids and the fluid-rock interactions leading to ore deposition (Figure 1).
Aqueous hydrothermal fluids are ubiquitous in the crust and are the most important medium for ore deposit formation or enrichment. These fluids evolve over wide ranges in pressures (ambient to many kbar) and temperatures (up to 600 ˚C or more) and show a wide range in compositions (eg high salinity; varying amounts of volatiles). The understanding of dissolution, transport, and deposition of metals in hydrothermal fluids is central to understanding hydrothermal ore deposits genesis, and is key to successful hydrometallurgical and in-situ recovery.
Over the past 10 years, we have used in-situ spectroscopy of metals in hydrothermal fluids to develop a molecular-level understanding of how changes in P, T and solution composition affect the transport properties of the fluids over wide ranges of conditions (Figure 2).
Mineral replacement reactions
One of our major recent research projects is dedicated to understanding the formation of the mineral assemblage at IOGC deposits like Olympic Dam. Replacement processes are central to the formation of these deposits, be it the hematite replacement of the granite during brecciation, the replacement of early magnetite by hematite, or the deposition of the copper sulphide and uranium minerals, which are key to the economics of these deposits. These minerals are generally formed by hydrothermal replacement reactions. The nature of fluid-mediated replacement reactions can vary over different length and time scales. In general the dissolution takes place at the reaction interface and the products at the interface (an interface coupled dissolution reprecipitation reaction (ICDR)) or alternatively precipitates some distance away from the reaction front, or even remain in solution and are expelled from the system.
In Figure 3 we show a typical image from our experiments that illustrate how chalcopyrite and bornite can form by the replacement of hematite under hydrothermal conditions. One of the key features in these mineral replacement reactions is the generation of porosity in the product minerals (chalcopyrite and bornite) that is a fundamental feature of the molecular-level reaction mechanism. This reaction-generated porosity enables fluid transport from the bulk solution to the reaction front.
Recent advances in experimental hydrothermal geochemistry, computational geochemistry, and mineralogy are transforming our understanding of the physics and chemistry of ore deposits. We have not only to apply this knowledge to improve outcomes in mineral exploration and metallogenesis, but also to geometallurgy and in-situ recovery. For example, mineral replacement reactions are key features in in-situ leaching pads, and many opportunities exist to exploit deep water flow for recovering metals as well as energy.
This work is funded by the Australian Research Council Project (DP0878903; DP0772299; DP1095069) and South Australian Museum. We thank our former students Drs Jing Zhao, Yuan Mie and Tian Yuan for the images used in this article, which are taken from their thesis work.
Feature image – Ore from Olympic Dam: massive chalcopyrite in strongly hematised breccia. Used under the Creative Commons License. Photo courtesy Geomartin.