Anastassia Y. Borisova

Gold speciation and transport in geological fluids: insights from experiments and physical-chemical modelling

Abstract This contribution provides an overview of available experimental, thermodynamic, and molecular data on Au aqueous speciation, solubility, and partitioning in major types of geological fluids in the Earths crust, from low-temperature aqueous solution to supercritical hydrothermal-magmatic fluids, vapours, and silicate melts. Critical revisions of these data allow generation of a set of thermodynamic properties of the AuOH, AuCl−2, AuHS, and Au(HS)−2 complexes dominant in aqueous hydrothermal solutions; however, other complexes involving different sulphur forms, chloride, and alkali metals may operate in high-temperature sulphur-rich fluids, vapours, and melts. The large affinity of Au for reduced sulphur is responsible for Au enrichment in S-rich vapours and sulphide melts, which are important gold sources for hydrothermal deposits. Thermodynamic, speciation, and partitioning data, and their comparison with Au and S contents in natural fluid inclusions from magmatic-hydrothermal gold deposits, provide new constraints on the major physical-chemical parameters (temperature, pressure, salinity, acidity, redox) and ubiquitous fluid components (sulphur, carbon dioxide, arsenic) affecting Au concentration, transport, precipitation, and fractionation from other metals in the crust. The availability and speciation of sulphur and their changes with the fluid and melt evolution are the key factors controlling gold behaviour in most geological situations.

Amorphous Materials: Properties, Structure, and Durability{dagger} Arsenic enrichment in hydrous peraluminous melts: Insights from femtosecond laser ablation-inductively coupled plasma-quadrupole mass spectrometry, and in situ X-ray absorption fine structure spectroscopy

Abstract Despite the ubiquity of arsenic in hydrothermal-magmatic environments, its abundance, distribution, and chemical and structural status in natural silicate melts and glasses remain poorly known. Here we report the first in situ measurements of the redox state and molecular structure of As, using X-ray absorption fine structure (XAFS) spectroscopy, in a rhyolitic peraluminous glass from Macusani (SE Peru) that is representative of anatectic melts derived from metasedimentary crustal protoliths. Arsenic abundances as well as the concentrations of other trace elements were measured in the glass using a femtosecond laser ablation-inductively coupled plasma-quadrupole mass spectrometry (LA-ICP-QMS). The glass shows enrichments, by factors of 10 to 100, in comparison with the mean continental crust values, for As and other incompatible trace elements (e.g., Be, B, Rb, Sn, Sb, and Ta), and by factors of 100 to 200 for Li, Cd, and Cs. Arsenic is present in the peraluminous glass in trivalent state in the form of oxy-hydroxide complexes like AsO(OH)2- and/or As(OH)3, similar to those dominant in the aqueous fluid vapor phases at hydrothermal-magmatic conditions. The similar As chemical speciation between the fluid and the melt is consistent with As fluid/melt partitioning coefficients close to one, as observed in experiments on rhyolite-water systems. The depolymerized melt structure caused by elevated H2O, F, and P contents is likely to allow accomodation of high concentrations of metalloid hydroxide/hydrated complexes. Consequently, hydrous silicate melts may be important transporting media in shallow magmatic-hydrothermal settings for As and similar elements like B and Sb due to their high affinity to water and hydroxide ligands

Commentary: Is the Neoproterozoic oxygen burst a supercontinent legacy?

The oxygen content of the atmosphere is thought to have increased in two steps: the Paleoproterozoic Great Oxidation Event and the Neoproterozoic Oxidation Event. The younger event is still a matter of debate (Och and Shields-Zhou, 2012). Macouin et al. (2015) suggest that it may have been triggered by volcanic degassing from unusually oxidized magmas occurring in a subduction ring surrounding the Rodinia supercontinent. Evidence for subduction-related oxidized magmas is supported by the study of 780 Ma biotite and pink granites of high K-calc-alkaline affinities and associated gabbros from Socotra Island (Denèle et al., 2012). Magnetite and subordinate hematite are revealed by magnetic properties, reflected-light microscopy, and scanning electron microscopy. Macouin et al. (2015) suggest that hematite is a primary phase witnessing a very high oxygen fugacity at magmatic conditions. In igneous rocks, Ti-rich titanohematite (ilmenite) is a primary phase, whereas stoichiometric hematite represents a common secondary mineral due to post-magmatic alteration processes. Indeed, hematite was uniquely found to be a liquidus phase in an experimental peralkaline residua Na 2 O-Al 2 O 3-Fe 2 O 3-SiO 2 system (Edgar, 1974), but the investigated system had no ferrous (Fe 2+) iron, hence it has no terrestrial equivalent in common tectonic settings. The microphotographs of iron oxides presented as evidences of primary hematite by Macouin et al. (2015) in their Figures 6, 7 show subhedral magnetite crystals with typical subsolidus exsolution features such as fine trellis-lamellae of ilmenite and minor hematite parallel to (111) planes, or larger (sandwich) ilmenite lamellae in only one (111) plane. The petrographic nature (biotite granite, pink granite, or gabbro) of the relevant samples is not provided. Macouin et al. (2015) infer that some primary magnetite grains were fractured and subsequently filled by ilmenite (see caption of their Figure 7). We argue that sandwich ilmenite lamellae in Fe-Ti oxide grains do not arise from a reaction with biotite, unlike the small ilmenite needles at the contact between magnetite and biotite (Figure 7d of Macouin et al., 2015). Indeed, primary igneous magnetite is generally titanomagnetite containing a percentage of ulvöspinel (Fe 2 TiO 4) in solution. During slow cooling, subsolidus oxidative exsolution takes place from ∼650 • C to 450 • C yielding ilmenite + magnetite (Figure 1A). Temperature and oxidation conditions control the exsolved phase proportions, hence the width of ilmenite lamellae. This is the common interpretation of Fe-Ti oxide intergrowths supported by experimental evidence (Buddington and …