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PRINCIPAL PUBLICATION AND AUTHORS
Revealing gold speciation in hydrothermal fluids by in situ high energy resolution X-ray absorption spectroscopy, G.S. Pokrovski (a), E. Desmaele (b), C. Laskar (a), E.F. Bazarkina (c,d), D. Testemale (c), J.-L. Hazemann (c), R. Vuilleumier (b), A.P. Seitsonen (b), G. Ferlat (e), A.M. Saitta (e), Am. Min. 107, 369-376 (2022); https:/doi.org/10.2138/am-2022-8008 (a) Experimental Géosciences Team (GeoExp), Géosciences Environnement Toulouse (GET), CNRS, University of Toulouse (France) (b) PASTEUR, Département de Chimie, École Normale Supérieure, Paris (France) (c) Institut Néel and ESRF, Grenoble (France) (d) Institute of Resource Ecology Dresden (Germany) and ESRF (e) IMPMC, Sorbonne Université, Paris (France)
 G.S. Pokrovski et al., Geol. Soc. London Spec. Publ. 402, 9-70 (2014).  G.S. Pokrovski et al., Proc. Natl. Acad. Sci. USA 112, 13484-13489 (2015).  D. Testemale et al., Rev. Sci. Instrum. 76, 043905-043909 (2005).  O. Proux et al., J. Environ. Quality 46, 1146-1157 (2017).  Y. Joly, FDMNES User s Guide (2022); https:/ fdmnes.neel.cnrs.fr/  G.S. Pokrovski et al., Proc. Natl. Acad. Sci. USA 118, e2109768118 (2021).
However, distinguishing between these key sulfur ligands for Au is challenging because in all its sulfur complexes, AuI is always in linear coordination with the two ligands, S-Au-S, which are seen by traditional methods, whereas the presence of other more distant atoms, like H in HS/H2S or S-S in Sn types of ligands, is virtually undetectable.
High-energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) experiments were performed at beamline BM16 (FAME-UHD) using a unique hydrothermal reactor developed at the Néel Institute  and adapted to the geometry of the X-ray emission spectrometer with a crystal analyser for HERFD measurements  (Figure 6). The setup enables the simultaneous acquisition of high-quality X-ray absorption near-edge structure (XANES) spectra in HERFD mode and measurement of ppm-level Au concentrations in hydrothermal fluids under controlled laboratory conditions, similar to those that led to the formation of gold deposits in the Earth s crust (~350 °C, ~2 km depth). The HERFD method provides spectra with highly-resolved features that can be compared with those generated by quantum- chemistry calculations based on the FDMNES programme , and using different configurations for the Au-HS/ S3 types of complexes calculated by density functional theory (DFT, i.e. static) and first-principles molecular dynamics (FPMD, i.e. dynamic) approaches. Finally, in-situ measured aqueous Au concentrations were analysed using thermodynamic models of Au and S chemical speciation in the fluid phase [1,2].
The results revealed significant differences between the Au-HS and Au-S3 types of complexes (Figure 7). The di- hydrosulfide species, [Au(HS)2] , is dominant in reduced fluids at neutral-to-alkaline pH, whereas [Au(HS)S3] controls Au speciation in more oxidised and more acidic fluids. This suggests that gold can be transported by S-bearing hydrothermal fluids over a wide range of redox and acidity conditions in the Earth s crust. Such fluids are capable of dissolving hundreds to thousands of grammes
of gold per cubic metre of fluid up to a million times greater than average gold content in terrestrial rocks. These findings therefore significantly enlarge the potential geological settings where concentrations of gold and associated metals may be found. Furthermore, soluble complexes of gold and similar metals (e.g., platinoids) with the [S3 ]− ion may find applications in ore extraction technologies and hydrothermal synthesis of noble metal- based nanomaterials . This work opens perspectives for studying other metals and volatile elements in planetary fluids at depths inaccessible to direct observation.
Fig 7: Comparison between measured and calculated Au L3-edge HERFD-XANES spectra of aqueous Au species. Measured spectra (solid curves) are from the two experiments in acidic and slightly alkaline sulfidic solutions. Simulated XANES spectra are obtained by FDMNES calculations using the species structures shown in ball-and-stick style (Au = pink, S = yellow, S3 = blue, H = grey) from DFT (dotted curves) or FPMD (dashed curves) simulations. Grey arrows indicate differences in the experimental spectra, also apparent in the corresponding calculated spectra, supporting the change in Au speciation from [Au(HS)S3] in acidic to [Au(HS)2] in alkaline solution.