Thermodynamic modelling of ore transport and deposition: The good, the bad, and the ugly

Brugger, Joël1; Etschmann, Barbara1; Gonzalez, Christropher1; Liu, Weihua2; Yuan, Mei2; Guan, Qiushi1; Raiteri, Paolo3; Testemale, Denis4; and Xing, Yanlu5

1Monash University, Clayton, Australia, 2 CSIRO Mineral Resources Flagship, Clayton, Australia, 3Curtin University, Perth, Australia, 4Université Grenoble Alpes, Grenoble, France, 5University of Minnesota, Minneapolis, USA

Ore deposit formation and the associated fluid-induced alteration require effective advective mass transfer of fluids, volatiles, and metals over length scales of meters to hundreds of kilometres. Over the past 20 years, our understanding of the geochemical aspects of ore transport and deposition has seen major advances driven by revolutions in theoretical, experimental, and characterization capabilities. This has improved our ability to predict metal behaviour at scales ranging from the ore system to the hand specimen, but this new knowledge raises also important new challenges.

  1. Thermodynamic modelling enables prediction of the metal-carrying capacity of geological fluids, and mapping the distribution and efficiency of metal-precipitating processes through time and space. In the past 20 years a large amount of in situ spectroscopic data complemented by increasingly accurate first principle molecular dynamic simulations have dramatically improved our understanding of the molecular-level nature of the hydrothermal reactions that are responsible for metal transport in the mid-to upper-crust. This underpins the development of more accurate models of reactive transport over wide ranges in pressure, temperature, fluid composition, and physical states.
  2. Supercritical aqueous fluids link subducting plates and the return of water, carbon, sulfur, and metals to the Earth’s surface. Innovative theoretical thermodynamic extrapolations have extended our capacity to model the role of aqueous fluids in the deep Earth. These new predictions suggest a rather more profound role for deep fluids than originally thought: for example, dissolved organic carbon species stable at high pressure recycle large amounts of carbon out of the subduction zone and into the atmosphere; and polysulfide species, stable at high pressure, may form Au deposits rather than the reduced sulfur species stable at lower pressure. With regard to metal complexes, these extrapolations are ultimately based on a large body of experimental studies; the vast majority were conducted at near ambiant temperature and pressure; a reasonable number investigated solutions to ~300 ˚C, P ≤ 300 bar; but few experimental data are available at pressures above 1 kbar, and hardly any reliable data is available beyond 10 kbar for any metal complex. Molecular dynamics can be used to address this fundamental limitation. The data produced by MD provide realistic properties in PT space that underpin accurate simulations of element transfer by fluids expelled from subducting slabs and their contributions to the deep Earth’s volatile, redox and metal budgets.
  3. Finally, the role of fluids in controlling both the kinetics and pathways of mineral replacement reactions is now firmly established. The positive feedback between these reactions and porosity creation is one of the key mechanisms that explains the pervasive nature of many alteration reactions. On-going experiments demonstrate the important role of trace elments in controlling the fate of fluid-driven reactions. For example, we discovered that the presence of trace amounts of dissolved cerium (Ce) increases the porosity of hematite (Fe2O3) formed via fluid-induced replacement of magnetite (Fe3O4), thereby increasing the efficiency of coupled magnetite replacement, fluid flow, and element mass transfer.

Biography

Joël Brugger obtained his PhD in Basel, Switzerland. Following 12 years  in a joint role at the South Australian Museum and the University of Adelaide, he took up the chair in Synchrotron Geosciences at Monash University. Joël uses state-of-the-art experimental techniques to study the transport and deposition of metals.

About the GSA

The Geological Society of Australia was established as a non-profit organisation in 1952 to promote, advance and support Earth sciences in Australia.

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