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Geologic samples are extremely diverse and share a tendency for both heterogeneity and complexity. This is especially true for ores, which commonly result from a complex interplay of processes in highly dynamic environments. In recent years, a number of tools allowing the chemical mapping of major (e.g., mineral liberation analysis, MLA), minor (e.g., electron microprobe, EPMA), and trace (e.g., laser ablation-inductively coupled plasma-mass spectrometry, LA-ICP-MS) elements in geologic samples at ~1- to 50-µm resolution and over mm2 areas have seen rapid development and have become readily available. To date, the application of synchrotron-based X-ray fluorescence (SXRF) mapping has been limited to addressing key questions because of low availability and high cost. This paper demonstrates how recent advances in X-ray fluorescence detector technology are bringing new possibilities to ore petrology. Millisecond dwell times allow collection of thin section size maps at resolutions of a few µm in hours, while improvements in data analysis software simplify the production of quantitative elemental maps. Based on the imaging of six samples representative of different commodities (Pt, U, Cu, Ge) and different geologic contexts (PGE deposit; sandstone-hosted U deposit; vein-type polymetallic hydrothermal deposit; iron oxide-copper-gold (IOCG) deposit), we demonstrate that megapixel SXRF (MSXRF) can efficiently provide the information necessary to understand metal speciation in the context of thin section-scale textural complexity. Image analysis revealed a number of new results for the studied deposits, for example, (1) the distribution of micrometer-sized Pt-rich grains and Ti mobility during the formation of schistosity at the Fifield Point prospect (New South Wales, Australia); (2) the presence of Ge contained in organic matter and of Hg minerals associated within quartzite clasts in the Lake Frome U ores (South Australia); (3) confirmation of the two-stage Ge enrichment in the Barrigão deposit, with demonstration of the presence of Ge in solid solution in the early chalcopyrite (Portuguese Iberian pyrite belt); and (4) the enrichment of U during late dissolution-reprecipitation reactions in the bornite ores of the Moonta and Wallaroo IOCG deposits (South Australia). These results illustrate that MSXRF is a powerful technique for locating nano- to microparticles of precious metals (Pt) and trace contaminants (e.g., Hg) that form distinct (micro) minerals. In addition, it is a powerful tool for understanding commodities with relatively low ore grades and complex distribution (100-1,000 ppm; e.g., U, Ge). [ABSTRACT FROM AUTHOR]

(Bio)geochemical processes mediate the dispersion and re-concentration of gold (Au) in Earth surface environments. The fundamental mechanisms underlying the mobilization of Au in surface environments and the chemical speciation of mobile Au remain largely unexplored. For more than half a century authors have argued that highly reactive manganese (Mn) oxides are involved in oxidizing, and hence mobilizing, Au in waters, soils and sediments. However, no systematic study has assessed the mechanisms of Au oxidation under conditions occurring in natural environments. Therefore, this paper explores the links between Mn-oxides and the oxidation/mobilization of Au under acidic to circum-neutral conditions. Using a newly developed high performance liquid chromatography–inductively coupled plasma-mass spectrometry (HPLC–ICP-MS) method, a range of Au(I/III)-complexes occurring in aqueous solution were directly quantified. We show that the oxidation of Au(I) to Au(III) with birnessite occurs in acidic environments, and that the rapid release of Au(III) sorbed (on Mn- or Fe-minerals) via influx of neutral waters transfers Au(III) into solution, making it mobile and transportable. This provides a viable pathway for explaining the occurrence of mobile Au(III)-complexes under neutral to alkaline conditions, as was reported from a Western Australian salt lake environment. In addition, we show that the [(bpy) 2 Mn III (μ-O) 2 Mn IV (bpy) 2 ] 3 − complex (bpy = 2,2′-bipyridyl), a useful model compound for the interaction of Au with bioorganic Mn in natural systems, mediates the formation of soluble Au(III)-complexes from metallic Au and from Au(I)-complexes under oxic and anoxic conditions. In conclusion, this study shows that reactive Mn-oxide minerals are important drivers of Au mobilization, and further suggests that natural organometallic compounds play an important role for the mobilization of Au in Earth surface environments. [ABSTRACT FROM AUTHOR]

Water activity is generally considered to affect ionic association in aqueous electrolyte solutions; however, it is usually ignored when the association reactions or constants are discussed. In this work, the effect of water activity on the association reaction $$ {\text{Ni}}_{{ ( {\text{aq}})}}^{2 + } + n{\text{Cl}}_{{ ( {\text{aq)}}}}^{ - } \rightleftharpoons {\text{NiCl}}_{{n{\text{ (aq)}}}}^{2 - n} $$ was investigated by EXAFS and UV-Vis spectroscopy measurements on NiCl aqueous solutions at room temperature with constant Cl/Ni ratio (~66) and various water activities; the latter were adjusted by adding MgCl and Mg(ClO). Both the EXAFS and the UV-Vis spectra measurements indicated that the extent of Ni-Cl association increases in decreasing water activity environments, independent on the Cl/Ni ratio. Thus, the effect of water activity on the ionic association should not be ignored and more emphasis should be paid on it, especially, when the water activity is very low. [ABSTRACT FROM AUTHOR]

Knowledge of the structure and speciation of aqueous Ni(II)-chloride complexes is important for understanding Ni behavior in hydrometallurgical extraction. The effect of concentration on the first-shell structure of Ni(II) in aqueous NiCl2 and NiCl2-MgCl2 solutions was investigated by Ni K edge X-ray absorption (XAS) and UV-Vis spectroscopy at ambient conditions. Both techniques show that no large structural change (e.g., transition from octahedral to tetrahedral-like configuration) occurs. Both methods confirm that the Ni(II) aqua ion (with six coordinated water molecules at RNi-O = 2.07(2) Å) is the dominant species over the whole NiCl2 concentration range. However, XANES, EXAFS and UV-Vis data show subtle changes at high salinity (> 2 mol∙kg-1 NiCl2), which are consistent with the formation of small amounts of the NiCl+ complex (up to 0.44(23) Cl at a Ni-Cl distance of 2.35(2) Å in 5.05 mol∙kg-1 NiCl2) in the pure NiCl2 solutions. At high Cl:Ni ratio in the NiCl2-MgCl2-H2O solutions, small amounts of [NiCl2]0 are also present. We developed a speciation-based mixed-solvent electrolyte (MSE) model to describe activity-composition relationships in NiCl2-MgCl2-H2O solutions, and at the same time predict Ni(II) speciation that is consistent with our XAS and UV-Vis data and with existing literature data up to the solubility limit, resolving a long-standing uncertainty about the role of chloride complexing in this system. [ABSTRACT FROM AUTHOR]

Sulfur and chlorine are the two most important ligands accounting for metal transport in the upper crust. In this study, four metal- and sulfur-saturated model fluids with varying salinities and redox states were simulated in the Fe-Cu-Pb-Zn-Au-S-C-H-O system, over a wide pressure-temperature (P-T) range (50–650 °C, 0.8–5.0 kbar), in order to compare the roles of chloride and bisulfide complexing for metal transport at the light of the latest available thermodynamic properties. The range in simulated Zn and Pb concentrations of the model fluids compares well with those of natural hydrothermal fluids, suggesting that the model can be used to evaluate hydrothermal ore-forming processes in Nature. The modeling reveals two different modes of Cu, Pb and Zn complexing in sulfur-saturated hydrothermal solutions. At lower temperature, chloride complexes are the predominant Cu, Pb and Zn species in sulfide-saturated systems, as expected from previous studies. However, hydrosulfide Cu, Pb and Zn complexes predominate at higher temperature. The predominance of bisulfide complexing for base metals at high temperature in sulfur-saturated systems is related to the prograde dissolution of pyrite and/or pyrrhotite, which results in a rapid increase in sulfur solubility. Metals transport as chloride or bisulfide complexes determines the modes of metal enrichment. In chloride-complexing dominated systems (e.g., Mississipi Valley Type deposits), low sulfide solubilities mean that the ore fluids cannot carry both reduced sulfur and metals, and ore precipitation is triggered when the ore fluid encounters reduced sulfur, e.g., via fluid mixing or via sulfate reduction. In contrast, in fluids where bisulfide complexing is predominant, cooling and desulfidation reactions are efficient mechanisms for base metal sulfide precipitation. Since both Au and base metals (Cu, Pb and Zn) are predicted to be transported as hydrosulfide complexes in high-temperature primary magmatic fluids in equilibrium with sulfide minerals, high-salinity is not a necessity for magmatic hydrothermal deposits such as porphyry- and skarn-style deposits. [ABSTRACT FROM AUTHOR]