Impact of sulfur on the fate of economically critical metals in geological fluids – SOUMET

Advances of in-situ experimental and analytical tools for studies of high T-P fluids, coupled with recent major improvements of laser and synchrotron X-ray beam sources, and progress in thermodynamic and molecular modeling approaches for fluid and vapor phases enable now the determination of the solubility and chemical speciation of metals and sulfur in the complex multi-phase systems ‘vapor-brine-minerals-silicate melt’ ubiquitous in the Earth crust and the establishment of predictive models of element transfer in the wide range of natural conditions. In this program we will combine a) measurements of solubility and partitioning for Au, Pt and associated metals (Cu, Ag, Zn, Fe, Sn, Mo, Pd) under carefully controlled acidity, redox and sulfur fugacity conditions and using cutting-edge experimental and analytical techniques, b) in situ spectroscopic studies of the atomic environment of metals and sulfur in fluids and vapors in model H2O-CO2-salt-sulfur-bearing systems matching the composition of natural ore-forming fluids, c) thermodynamic and molecular modeling of hydrothermal solvents, solute-solvent interactions and mineral solubilities, d) analytical and methodological developments for characterizing natural S-rich fluids in natural fluid inclusions from key gold ore deposits.

At this stage it is only possible to speak about results that may be expected:
1) Acquisition of new fundamental experimental and theoretical data on sulfur and metals; 2) development of new analytical and experimental tools; 3) quantification of the fluid capacities to mobilize and transport metals and of processes leading to economic ore formation.

This project puts together, for the 1st time, approaches and methods from very different domains, which will allow creation of a core of French teams working at the interface “geochemistry-ore deposit geology-chemistry-physics”.
In addition, some applications of this project may extend far beyond ore deposit aspects such as:
- global geochemical cycles of metals and sulfur
- isotope fractionation
- CO2 geological storage
- ore mineral treatment
-hydrothermal synthesis of new materials

Kouzmanov K. and Pokrovski G.S. (2012) Hydrothermal controls on metal distribution in porphyry systems. Society of Economic Geologists Special Publication (accepted pending moderate revisions).

Pokrovski G.S., Roux J., Jonchiere R. Ferlat G., Seitsonen A., Vuilleumier R., Hazemann JL. (2012) Silver in saline hydrothermal fluids: insights from in-situ X-ray absorption spectroscopy and first-principles molecular dynamics (submitted to Geochimica et Cosmochimica Acta 06/2012).

The aim of this project is to develop new experimental, analytical and computational approaches for quantifying the geological role of sulfur in the mobilization, transport and precipitation of metals of high economic and technological value in geological fluids, at temperatures and pressures from the magmatic stage to porphyry and epithermal ore deposits of gold and associated metals (e.g. Cu, Ag, Pt, Zn, Fe, Mo, Sn), which host the major part of these metal resources on Earth. Although sulfur is a major constituent of crustal fluids, its impact on the metal extraction-transport-deposition process has not been resolved for most natural magmatic-hydrothermal settings. This is primarily due to i) the lack of reliable data on the chemical speciation of sulfur itself and the identity and stability of metal complexes with different sulfur ligands in the fluid and vapor phase, ii) the imperfection of existing physical-chemical models for predicting metal solubility and partitioning in vapor-fluid-melt systems, and iii) the paucity of analytical data on the amounts and distribution of sulfur in its different redox forms in ore-forming fluids over a wide temperature-pressure (T-P) range. Until present, this lack greatly hampered our understanding of the key processes responsible for ore-deposition of gold and related metals and identifying the metal sources and fluid reservoirs involved in the genesis of economic deposits.
Advances of in-situ experimental and analytical tools for studies of high T-P fluids, coupled with recent major improvements of laser and synchrotron X-ray beam sources, and progress in thermodynamic and molecular modeling approaches for fluid and vapor phases enable now the determination of the solubility and chemical speciation of metals and sulfur in the complex multi-phase systems ‘vapor-brine-minerals-silicate melt’ ubiquitous in the Earth crust and the establishment of predictive models of element transfer in the wide range of natural conditions. Elucidating the role of sulfur amongst other possible factors in these high T-P multiphase natural systems represents a great scientific challenge, necessitating a multidisciplinary approach that integrates a range of novel experimental, theoretical and analytical tools. In this program we will combine a) measurements of solubility and partitioning for Au, Pt and associated metals (Cu, Ag, Zn, Fe, Sn, Mo, Pd) under carefully controlled acidity, redox and sulfur fugacity conditions and using cutting-edge experimental and analytical techniques, b) in situ spectroscopic studies of the atomic environment of metals and sulfur in fluids and vapors in model H2O-CO2-salt-sulfur-bearing systems matching the composition of natural ore-forming fluids, c) thermodynamic and molecular modeling of hydrothermal solvents, solute-solvent interactions and mineral solubilities, d) analytical and methodological developments for characterizing natural S-rich fluids in natural fluid inclusions from key gold ore deposits. The results will enable quantification, for the first time, the metal-sulfur relationships in magmatic-hydrothermal systems from the molecular to ore-deposit scale. They will provide the necessary base for robust geological modeling in near-future programs integrating the new fundamental results and analytical developments with geological data on ore-deposits contexts.
The proposed research involves 4 French teams - leaders in experimental geochemistry, synchrotron-based methods, thermodynamic and molecular modeling of fluids, gases and silicate melts, in situ microanalysis, and economic geology. This consortium will create a core of French specialists working at the interface “geochemistry-ore deposit geology-chemistry-physics”, and contribute to the renaissance of Metallogeny in France, thus facing up the world crisis of mineral resources.