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Chalcopyrite (CuFeS2) and bornite (Cu5FeS4) are the most abundant Cu-bearing minerals in hydrothermal Cu deposits, forming under a wide range of conditions from moderate-temperature sedimentary exhalative deposits to high-temperature porphyry Cu and skarn deposits. We report the hydrothermal synthesis of both chalcopyrite and bornite at 200-300 °C under hydrothermal conditions. Both minerals formed via the sulfidation of hematite in solutions containing Cu(I) (as a chloride complex) and hydrosulfide, at pH near the pKa of H2S(aq) over the whole temperature range. Polycrystalline chalcopyrite formed first, followed by bornite. Assuming that Fe behaves conservatively, the transformation of hematite to chalcopyrite involves a large increase in volume (~290%). The reaction proceeds both via direct replacement of the existing hematite and via overgrowth around the grain. Chemical exchanges between bulk solution and hematite are enabled by a network of micrometer-size pores. However, in some cases the chalcopyrite overgrowth develops large grain sizes with few apparent pores and in these cases fluid transport may have been via a network of fractures. Similarly to the replacement of hematite by chalcopyrite, bornite forms via the replacement of chalcopyrite. The reaction has a large positive volume (~230%), and proceeds both via chalcopyrite replacement and via overgrowth. This study shows that replacement reactions can proceed via coupled dissolution-reprecipitation even where there is a large volume increase between parent and product mineral. This study also provides further evidence about the controls of reaction pathways onto the final mineral assemblage. In this case, the host initial fluid was undersaturated with respect to Fe-bearing minerals. Upon slow release of Fe at the surface of hematite, a mineral assemblage of chalcocite, bornite, and finally chalcopyrite is expected. However, in practice chalcocite did not nucleate on the surface of hematite. Rather relatively slow nucleation of bornite enabled high concentrations of Fe to build up near the dissolving hematite, so that chalcopyrite (high-sulfidation experiments) or chalcopyrite+pyrite (low sulfidation) crystallized first. [ABSTRACT FROM AUTHOR]

Calaverite, krennerite, and sylvanite are Au-Ag-tellurides with close compositions and related crystal structures. Previous experimental studies show that both calaverite and sylvanite transform to porous "mustard" gold under hydrothermal conditions; however the transformation of sylvanite follows a complex reaction path with several intermediary products, contrasting with the simple replacement of calaverite by gold. Here we report results of an experimental study of the transformation of krennerite, a phase with Ag contents intermediate between those of calaverite and sylvanite. Krennerite was replaced by Au-Ag alloy under all experimental conditions explored (160 to 220 °C; pHT ~ 3 and 9; varying availability of oxygen). No reaction was observed at the same temperature under dry conditions. The replacement was pseudomorphic and the resulting Au-Ag alloy was porous, consisting of worm-like aggregates with diameters ranging from 200 nm to 1 µm. The replacement of krennerite proceeds via an interface coupled dissolution-(re)precipitation (ICDR) reaction mechanism. Tellurium is lost to the bulk solution as Te(IV) complexes, and may precipitate away from the dissolution site. In contrast, Au-Ag alloy precipitates locally near the krennerite dissolution site. Overall, the hydrothermal alteration of krennerite is very similar to that of calaverite, but differs from the alteration of sylvanite, for which multi-step reaction paths led to complex products and textures under similar conditions. These striking differences are driven by the competition between solid-state reactions and ICDR reaction in sylvanite. This reflects the fact that a metastable, Ag-rich calaverite nucleates on sylvanite during the early steps of its dissolution, as a result of the close relationship between the structures of these two minerals and the enrichment in Au, Ag, and Te in solution at the reaction front. In contrast, for calaverite and krennerite, no such phase precipitates, and both minerals are transformed in a pseudomorphic manner into Au-Ag alloy. [ABSTRACT FROM AUTHOR]

Abstract: In spite of the similar chemical properties of gold (Au) and platinum (Pt), Au is highly mobile in surface environments, whereas Pt appears to be far less so. In this study we assess if geomicrobial processes are likely to cause these differences, as the mobility of Pt and Au should differ little based on thermodynamic solubility alone. To achieve an accurate comparison it is important that both metals occur in the same environment. Mineral- and groundwater samples were obtained from the Fifield Pt–Au field in New South Wales, Australia, where Pt and Au nuggets occur in a series of Tertiary eluvial and alluvial paleo-placers. In particular, we studied the μm-scale dispersion of Au and Pt within an extraordinary 10mm-sized fragment of ferruginous paleochannel material, which contained abundant native Au- and isoferroplatinum grains. Gold grains displayed complex secondary morphologies indicative of biogeochemical transformations, whereas isoferroplatinum grains appeared smooth and well-rounded and showed no signs of supergene transformation. Gold grains were surrounded by a dusting of highly pure metallic Au particles (<10nm to >10μm in diameter), whereas no metallic Pt particles were detected. A search for ionic Pt was also unsuccessful. A series of biotic and abiotic incubation experiments was conducted to investigate the hypothesis that these differences in mobility are driven by interactions with microbiota. Biofilms consisting of metallophilic bacteria formed on ultraflat Au surfaces and caused significant surface transformations. In contrast, only subtle changes were observed on Pt surfaces incubated under similar conditions. Minimal inhibitory concentrations for Au complexes are more than an order of magnitude lower than those measured for Pt complexes in Cupriavidus metallidurans cells. This higher cell-toxicity of mobile Au- compared to Pt-complexes can lead to toxic levels of mobile Au in the vicinity of Au grain surfaces. The elevated toxicity drives the formation of Au-detoxifying biofilms that catalyze the biomineralization of spheroidal nano-particulate and bacteriomorphic Au. In contrast, this does not occur on isoferroplatinum grains due to the lower toxicity of Pt. In conclusion, these results are consistent with microbial adaptation to element toxicity driving the cycling of precious metals in surface environments. [Copyright &y& Elsevier]

The crystal structure of pseudojohannite from White Canyon, Utah, was solved by charge-flipping from single-crystal X-ray diffraction data and refined to an Robs = 0.0347, based on 2664 observed reflections. Pseudojohannite from White Canyon is triclinic, P1, with a = 8.6744(4), b = 8.8692(4), c = 10.0090(5) Å, α = 72.105(4)°, β = 70.544(4)°, γ = 76.035(4)°, and V = 682.61(5) ų, with Z = 1 and chemical formula Cu3(OH)2[(UO2)4O4(SO4)2](H2O)12. The crystal structure of pseudojohannite is built up from sheets of zippeite topology that do not contain any OH groups; these sheets are identical to those found in zippeites containing Mg2+, Co2+, and Zn2+. The two Cu2+ sites in pseudojohannite are [5]- and [6]-coordinated by H2O molecules and OH groups. The crystal structure of the pseudojohannite holotype specimen from Jáchymov was refined using Rietveld refinement of high-resolution powder diffraction data. Results indicate that the crystal structures of pseudojohannite from White Canyon and Jáchymov are identical. [ABSTRACT FROM AUTHOR]

Although the crustal abundance of tellurium (Te) is about half of that of gold (Au), several classes of Au deposits are highly enriched in Te. Our understanding of the nature of this Au-Te association is hampered by the lack of experimental studies of Te geochemistry at elevated temperature. We characterized the structure of polytelluride solutions from room temperature to 599 °C at 800 bar using in situ X-ray absorption spectroscopy. Both ab-initio XANES and EXAFS fits show that polytellurides are stable up to the highest temperature, with planar structures (four- or threefold coordination of Te) giving way to linear chains (e.g., Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed) at temperatures above ~200 °C. This is the first experimental confirmation of the thermal stability of polytelluride species. The data show that polytellurides play an important role in Te transport in reduced S-rich or CO2-rich solutions and vapors. [ABSTRACT FROM AUTHOR]

A large-volume single-pass flow-through cell for in situ neutron diffraction investigation of hydrothermal crystallization processes is reported. The cell is much more versatile than previous designs owing to the ability to control independently and precisely temperature (up to 673 K), pressure (up to 46 MPa), flow rate (0.01-10 ml min−1) and reaction-fluid volume (≥65 ml). Such versatility is realized by an innovative design consisting of a room-temperature and ambient-pressure external fluid supply module, a high-pressure reaction module which includes a high-temperature sample compartment enclosed in a vacuum furnace, and a room-temperature and high-pressure backpressure regulation module for pressure control. The cell provides a new avenue for studying various parameters of hydrothermal crystallizations independently, in situ and in real time at extreme hydrothermal conditions ( e.g. supercritical). The cell was successfully commissioned on the high-intensity powder diffractometer beamline, Wombat, at the Australian Nuclear Science and Technology Organisation by investigating the effect of pressure on the hydrothermal pseudomorphic conversion from SrSO4 (celestine) to SrCO3 (strontianite) at a constant temperature of 473 K and flow rate of 5 ml min−1. The results show that the increase of pressure exerts a nonlinear effect on the conversion rate, which first increases with increasing pressure from 14 to 20 MPa, and then decreases when pressure further increases to 24 MPa. [ABSTRACT FROM AUTHOR]

In situ spatially resolved X-ray absorption near edge structure (m-XANES) spectra are obtained for natural Cu-In-bearing sphalerite. Copper K-edge spectral data show that, in Cu-In-bearing sphalerite, in which an excellent correlation between the Cu and In contents is noted, Cu is present in the Cu+ state. This offers indirect proof for the coupled substitution 2 Zn2+ ↔ Cu+ + In3+, which allows indium to enter the sphalerite structure. The study clearly demonstrates the utility of synchrotron radiation to accurately determine oxidation state in small volumes of mineral in which the concentration of the element of interest is low or very low. The study also demonstrates that good quality m-XANES spectra can be collected on TEM foils prepared in situ at a chosen position on the surface of a polished sample using the focused ion beam-scanning electron microscope method. [ABSTRACT FROM AUTHOR]

Argandite, Mn7(VO4)2(OH)8, is a new mineral from the metamorphosed synsedimentary exhalative Mn deposit located underneath the Pipji glacier (Pipjigletscher) in the Turtmann valley, Central Alps, Switzerland. The mineral is dedicated to the Swiss geologist Émile Argand (1879--1940). Argandite occurs in manganosite-rich ores in association with the V-minerals pyrobelonite, reppiaite, and an unknown silico-vanadate with chemical formula (Mn,Mg)24(V,As,Si)4Si2O27H38; these minerals result from the remobilization of ore components during the Tertiary Alpine metamorphism under upper greenschist facies conditions (~450 °C, 4--6 kbar). Argandite forms isolated anhedral grains up to 60 µm in diameter. The mineral is transparent, orange in color with a pale orange streak and vitreous luster, Mohs hardness ~3.5--4, Dcalc 3.67(1), and Dmeas 3.71(5) g/cm³. It is brittle with one distinct cleavage, probably parallel to {001}. Argandite is biaxial negative, with α ~1.74, β = 1.762(4), γ ~1.77 (white light), 2V (calc) = ~62°. It shows a distinct pleochroism under polarized light, orange-yellow to orange. The empirical chemical formula is (Mn6.54Mg0.38Ni0.04Ca0.02Zn0.01Sr0.01)∑=7.00(V1.46,As0.54)∑=2.00O8(OH)8.00. Argandite is monoclinic, space group P21/n, a = 5.5038(2), b = 12.2665(5), c = 10.1055(5) Å, β = 95.559(4)°; V = 679.04(5) ų; Z = 2. The six strongest lines measured in the X-ray powder diffraction pattern are [d in Å (I)(hkl)]: 3.074 (100)(131), 2.687 (70)(140;113), 3.395 (60)(112), 3.708 (50) (112̅121), 2.945 (50)(041;113̅), and 2.522 (50)(004;202̅). The crystal structure was solved with direct methods on the basis of 1661 unique reflections with I > 4σF and refined to R1 = 3.40%. Argandite is isostructural with allactite, Mn7(AsO4)2(OH)8, and raadeite, Mn7(PO4)2(OH)8. [ABSTRACT FROM AUTHOR]

The transformation of pyrrhotite to Fe disulfide (pyrite and/or marcasite) under hydrothermal conditions was studied experimentally by probing the effects of temperature (up to 220 °C, vapor-saturated pressures), ∑S(-II) concentrations, pH, and availability of oxygen on reaction progress and on the resulting textures. The pyrrhotite to Fe disulfide reaction proceeded by a dissolution-reprecipitation mechanism under all conditions. Marcasite and pyrite formed under both oxic and anaerobic conditions, which is inconsistent with the traditionally assumed polysulfide route for FeS2 formation (oxidants required for polysulfide formation). The nature of the products was controlled by the level of supersaturation of the solution with respect to Fe disulfide minerals. Marcasite formed preferentially at low pH or S(-II)-deficient solutions (saturation index << 1000), while pyrite was the main product at saturation indices >1000. Different textures were obtained for the replacement of pyrrhotite by either pyrite or marcasite. Pyrite formation proceeded by direct replacement of pyrrhotite and, simultaneously, by overgrowth from solution. The pyrite crystals were >10 µm in size and randomly oriented. In comparison, marcasite crystals were <1 µm in size, and no significant overgrowth was observed. At pH21°C <3, the marcasite nanocrystals showed the well-known crystallographic relationship with pyrrhotite, but at pH21°C 3.96, the marcasite crystallites were randomly oriented. These experimental results confirm that the preservation of the crystallographic orientation is not a distinguishing feature between dissolution-reprecipitation and solid-state reactions. The different textures among pyrite and marcasite reflect the dominance of crystal growth (pyrite) vs. nucleation (marcasite) as a precipitation mechanism. [ABSTRACT FROM AUTHOR]

Paulscherrerite, UO2(OH)2, occurs as an abundant dehydration product of metaschoepite at the Number 2 Workings at Radium Ridge, Northern Flinders Ranges, South Australia. The mineral name honors the contribution of Swiss physicist Paul Scherrer (1890-1969) to mineralogy and nuclear physics. Individual paulscherrerite crystals are tabular, reaching a maximum of 500 nm in length. Paulscherrerite has a canary yellow color and displays no fluorescence under UV light. Chemically, paulscherrerite is a pure uranyl hydroxide/hydrate, containing only traces of other metals (<1 wt% in total). Bulk (mg) samples always contain admixtures of metaschoepite (purest samples have ~80 wt% paulscherrerite). A thermogravimetric analysis corrected for the presence of metaschoepite contamination leads to the empirical formula UO3⋅1.02H2O, and the simplified structural formula UO2(OH)2. Powder diffraction shows that the crystal structure of paulscherrerite is closely related to that of synthetic orthorhombic α-UO2(OH)2. However, splitting of some X-ray diffraction lines suggests a monoclinic symmetry for type paulscherrerite, with a = 4.288(2), b = 10.270(6), c = 6.885(5) Å, β = 90.39(4)°, V = 303.2(2) ų, Z = 4, and possible space groups P2, P21, P2/m, or P21/m. Paulscherrerite-like material was synthesized using various methods, including heating metaschoe- pite in water at 150 °C and slow hydration of UO3(am) in air; material synthesized using hydrothermal techniques displayed peak splitting indicative of monoclinic symmetry. Paulscherrerite has been reported under the name "dehydrated schoepite" as an early weathering product of uraninite/pitchblende in several deposits, such as Shinkolobwe, Zaire; Nopal I deposit, Mexico; and the granitic pegmatites of New Hampshire, U.S.A. [ABSTRACT FROM AUTHOR]

Abstract: We present the results of an experimental study into the sulfidation of magnetite to form pyrite/marcasite under hydrothermal conditions (90–300°C, vapor saturated pressures), a process associated with gold deposition in a number of ore deposits. The formation of pyrite/marcasite was studied as a function of reaction time, temperature, pH, sulfide concentration, solid-weight-to-fluid-volume ratio, and geometric surface area of magnetite in polytetrafluoroethylene-lined autoclaves (PTFE) and a titanium and stainless steel flow-through cell. Marcasite was formed only at pH21°C <4 and was the dominant Fe disulfide at pH21°C 1.11, while pyrite predominated at pH21°C >2 and formed even under basic conditions (up to pH21°C 12–13). Marcasite formation was favored at higher temperatures. Fine-grained pyrrhotite formed in the initial stage of the reaction together with pyrite in some experiments with large surface area of magnetite (grain size <125μm). This pyrrhotite eventually gave way to pyrite. The transformation rate of magnetite to Fe disulfide increased with decreasing pH (at 120°C; pH120°C 0.96–4.42), and that rate of the transformation increased from 120 to 190°C. Scanning electron microscope (SEM) imaging revealed that micro-pores (0.1–5μm scale) existed at the reaction front between the parent magnetite and the product pyrite, and that the pyrite and/or marcasite were euhedral at pH21°C <4 and anhedral at higher pH. The newly formed pyrite was micro-porous (0.1–5μm); this micro-porosity facilitates fluid transport to the reaction interface between magnetite and pyrite, thus promoting the replacement reaction. The pyrite precipitated onto the parent magnetite was polycrystalline and did not preserve the crystallographic orientation of the magnetite. The pyrite precipitation was also observed on the PTFE liner, which is consistent with pyrite crystallizing from solution. The mechanism of the reaction is that of a dissolution–reprecipitation reaction with the precipitation of pyrite being the rate-limiting step relative to magnetite dissolution under mildly acidic conditions (e.g., pH155°C 4.42). The experimental results are in good agreement with sulfide phase assemblage and textures reported from sulfidized Banded Iron Formations: pyrite, marcasite and pyrrhotite have been found to exist or co-exist in different sulfidized Banded Iron Formations, and the microtextures show no evidence of sub-μm-scale pseudomorphism of magnetite by pyrite. [Copyright &y& Elsevier]

Mineralogically relevant chalcogenide glasses with the composition Cu xBi5Ag5 (As0.33S0.335Se0.335)90− x ( x=2, 6, and 10, in mol%) that approximate the chemistry of some unusual sulfide mineral assemblages were synthesized. The differential thermal analyses revealed that the glass transition temperature ( Tg), onset crystallization temperature ( Tc), and peak temperature of crystallization ( Tp) first increase significantly with an increasing Cu content from 2 to 6 mol%, and then remain almost constant when the Cu content further increases to 10 mol%; the glass thermal stability also increases with an increasing Cu content. Both dry annealing and hydrothermal treatments were carried out in the temperature range of 207°–331°C for up to 96 h. X-ray diffraction analyses on annealed samples showed that dry annealing and hydrothermal treatments produced different phase assemblages. Dry annealing produced crystallized phases AgBiSe2 (bohdanowiczite), Cu2Se (berzelianite), Ag3AsS3 (xanconthite), Cu3AsS4 (luzonite), AgBi3S5, and As4CuS9, while hydrothermal treatments produced phases AgBiSe2 (bohdanowiczite), AgBi3S5, and AgAsSe2. This case study presents a new way for studying the formation and alteration of minerals in nature. [ABSTRACT FROM AUTHOR]

Abstract: The understanding of the physico-chemical processes leading to the formation and weathering of ore deposits plays an increasingly important role in mineral exploration. Synchrotron, neutron, and nuclear radiation are contributing to this endeavour in many ways, including (i) support the modelling of ore transport and deposition, by providing molecular-level understanding of solvent–solute interaction and thermodynamic properties for the important metal complexes in brines, vapours, and supercritical fluids over the range of conditions relevant for the formation of ore deposits (i.e., temperature 25–600°C; pressure 1–109 Pa; and fluid compositions varying from hypersaline (e.g., >50wt% NaCl) to volatile-rich (e.g., CO2, CH4, and H2S)); (ii) track the fluids that travelled through rocks and predict their ore-forming potential by analysing hydrothermal minerals and remnants of those fluids trapped in these minerals as ‘fluid inclusions’; (iii) characterize the biochemical controls on metal mobility in soils to predict the geochemical footprint of a buried mineral deposit. X-ray fluorescence (XRF), particle-induced X-ray emission (PIXE), and X-ray absorption spectroscopy (XAS) are the most common techniques used in support of mineral exploration. Analytical challenges are related to (i) the complexity of heterogeneous natural samples, which often contain only low concentrations of the elements of interest; (ii) beam sensitivity, especially for redox-sensitive elements in aqueous fluids or biological samples; (iii) extreme sample environments, e.g., in-situ study of fluids at high pressure and temperature. Thus, critical improvements need to be made on a number of fronts to: (i) develop more efficient detectors, able to map large areas in heterogeneous samples (e.g., 106–108 pixels per map), and also to collect a maximum number of photons to limit sample exposure and beam damage; (ii) integrate techniques (e.g., XRF, XAS, and X-ray diffraction (XRD)) on a single beamline, and promote synergy between neutron-, synchrotron-, and nuclear microprobe-based methods; (iii) advance the theory (e.g., quantitative XANES interpretation; X-ray extended range technique (XERT) measurements) to gain maximum information from the hard-won datasets. [Copyright &y& Elsevier]

Abstract: X-ray absorption spectroscopy (XAS) measurements were used to determine the coordination structure and to derive the speciation of aqueous ferrous chloride complexes in acidic chloride brines over a wide range of conditions (25–450 °C, 500 bar, 0.5–12 m chloride molality), covering the range from sedimentary brines to magmatic hydrothermal fluids. EXAFS analysis coupled with ab initio free potential XANES calculations confirmed the octahedral geometry of the different Fe chlorocomplexes at low temperature (<200 °C) and low (<1 m) chloride concentration ([FeCl x (H2O)6− x ]2− x , x =0–2), and attest the stability of a high-order tetrahedral Fe(II)-chloride complex at high-temperature (>300 °C) and high (>2 m) chloride molality ([FeCl y ]2− y ; y =4 or y =3; Fe–Cl distance=2.31±0.01 Å). These spectroscopic results contrast with the interpretation of most recent high-temperature studies of Fe(II) speciation in brines, which assumed that [FeCl2]0 is the predominant species in brines at high temperature. A reinterpretation of the experimental Fe solubilities measured by Fein et al. [Fein, J.B., Hemley, J.J., D''Angelo, W.M., Komninou, A., Sverjensky, D.A., 1992. Experimental study of iron-chloride complexing in hydrothermal fluids. Geochim. Cosmochim. Acta 56, 3179–3190.] for the magnetite–pyrite–pyrrhotite–quartz–muscovite–K-feldspar assemblage in KCl solutions at 300 °C/500 bar and 400 °C/500 bar shows that these solubility data can be explained using the high-order [FeCl4]2− complex. This study illustrates the complementarity between solubility and spectroscopic studies, and provides further evidence of the importance of high-order chlorocomplexes for the transport of transition metals (e.g., Zn, Ni) in high-temperature and/or supercritical fluids. [Copyright &y& Elsevier]

Abstract: Although pseudomorphic mineral replacement reactions are common in all geological environments, and have long been considered important to many geological processes such as metamorphism, metasomatism, diagenesis, and chemical weathering, their mechanisms are still not well known. We present a combined textural and kinetic study of the replacement of pentlandite, (Fe,Ni)9S8, by violarite (NiFe)3S4, and describe the mechanisms and kinetic behavior of this reaction by considering the role of the fluid phase, the causes of coupling between pentlandite dissolution and violarite precipitation, the rate-limiting steps controlling the kinetic behavior, and the origin of the length scale of the features preserved during pseudomorphism. The experiments were conducted under mild hydrothermal conditions (80–210°C, vapor saturated pressures). Reaction kinetics shows a complex behavior depending on various physical and chemical parameters including temperature, pH, concentrations of various reaction species, solid-weight-to-fluid-volume-ratio and specific surface area. The rate of replacement (i) increases with temperature from 80 to 125°C, then decreases as temperature further increases to 210°C, (ii) first increases then decreases with decreasing pH from pH 6 to 1, (iii) increases with increasing concentration of oxidants such as O2(aq), H2O2, and KMnO4, but decreases with increasing concentration of Ni2+ and Fe3+, and with increasing solid-weight-to-fluid-volume ratio, (iv) increases linearly with the specific surface area. This kinetic behavior as well as the resulting textures revealed a coupled dissolution–reprecipitation reaction mechanism. Nanometer-scale pseudomorphic replacement, through which the crystallographic orientation of pentlandite is inherited by violarite, occurs only between 1

Abstract: We used µ-XANES spectroscopy to measure the oxidation state of europium in-situ at near-µm resolution in hydrothermal scheelite from the giant Archean gold deposits of Kalgoorlie, Western Australia. By combining these measurements with µ-XRF imaging, it is possible to distinguish inhomogeneities in Eu2+/Eu3+ ratios that developed during mineral precipitation from the effects of subsequent hydrothermal alteration and weathering. Thermodynamic analysis reveals that under the conditions typical of the formation of many Au deposits, the Eu2+/Eu3+ ratio in the hydrothermal fluid is highly sensitive to pH. The range in pH calculated from the Eu2+/Eu3+ ratios in the analyzed scheelite corresponds to a maximum in Au solubility under the ore-forming conditions, and suggests buffering of pH by the CO2(aq)-rich fluid. The primary heterogeneity of the Karlgoorlie scheelite most likely results from pH oscillating between fluid- and rock-buffered conditions, and reflects the dynamics of the hydrothermal system. [Copyright &y& Elsevier]

Abstract: The solubility of nantokite (CuCl(s)) and the structure of the predominant copper species in supercritical water (290–400bar at 420°C; 350–450°C at 290bar; 500°C at 350bar; density=0.14–0.65g/cm3) were investigated concurrently using synchrotron X-ray absorption spectroscopy (XAS) techniques. These conditions were chosen as they represent single phase solutions near the critical isochore, where the fluid density is intermediate of typical values for vapour and brine and is highly sensitive to even small changes in pressure. X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption spectroscopy (EXAFS) analyses show that aqueous copper occurs in a slightly distorted linear coordination in the solutions studied, with an average of 1.35(±0.3) Cl and 0.65(±0.3) O neighbours. The solubility of CuCl(s) decreases exponentially with decreasing water density (i.e., decreasing pressure at constant temperature), in a manner similar to the solubility behaviour of salts such as NaCl in water vapour. Based on this similarity, an apparent equilibrium constant for the dissolution reaction of 0.5±0.4 was calculated from a regression of the data at 420°C, and it was determined that each Cu atom is solvated by approximately three water molecules. This indicates that under these conditions, copper solubility is controlled mainly by the structure of the second-shell hydration, which is essentially invisible to the XAS techniques used in this study. These results demonstrate that for a supercritical fluid near the critical isochore, decreasing pressure may initiate precipitation of copper even before boiling or phase separation. Such a process could be responsible for near-surface ore deposition in seafloor hydrothermal systems, where supercritical fluids experience rapid pressure changes during the transition between lithostatic and hydrostatic domains. [Copyright &y& Elsevier]

The development of numerical modeling of reactive transport relies on the availability of thermodynamic properties for the solid, surface, aqueous, and vapor species stable at the conditions of interest. The lack of experimental studies and comprehensive activity-composition models severely limits the predictive capabilities of these models in systems involving highly saline fluids or low-density volatile-rich fluids. X-ray absorption near-edge structure spectroscopy (XANES) is a powerful technique to study the speciation of transition metals in aqueous fluids: it is element specific; sensitive to the oxidation state of the metal, the ligand field and the coordination geometry of the complex; and it is suitable for measuring trace amounts of metals (>1 wt%) in solutions over a wide pressure and temperature range. Formation constants for the aqueous complexes of transition metals can be determined from a series of XANES spectra obtained on solutions containing a constant amount of the metal of interest and variable concentrations of a ligand. The method relies on a non-linear, least-squares-fitting approach, with full distribution of species calculations based on a complete thermodynamic model for the experimental system under consideration. The technique is particularly suitable for following octahedral to tetrahedral transitions among weak chloride complexes of transition metals. The log K for the reaction Zn(octahedral)2++ 4Cl- = ZnCl4 2- (tetrahedral) at 25°C is retrieved to be 0.1(6), within error of the accepted literature value. The same method applied to Fe2+-chloride complexes shows that the log K for the reaction Fe(octahedral)2+ + 4Cl- = FeCl4(tetrahedral) 2- increases from -6.2(6) at 25 °C to -2.9(3) at 150 °C. This study confirms that tetrahedral chloride complexes play an important role in Fe transport in hypersaline brines especially at elevated temperatures, and shows that XANES is well suited to study systems that may be difficult to study with other techniques. [ABSTRACT FROM AUTHOR]

Abstract: The modelling of the speciation and mobility of metals under surface and hydrothermal conditions relies on the availability of accurate thermodynamic properties for all relevant minerals, aqueous species, gases and surface species. Spectroscopic techniques obeying the Beer–Lambert law can be used to obtain thermodynamic properties for reactions among aqueous species (e.g., ligand substitution; protonation). BeerOz is a set of Matlab routines designed to perform both qualitative and quantitative analysis of spectroscopic data following the Beer–Lambert law. BeerOz is modular and can be customised for particular experimental strategies or for simultaneous refinement of several datasets obtained using different techniques. Distribution of species calculations are performed using an implementation of the EQBRM code, which allows for customised activity coefficient calculations. BeerOz also contains routines to study the n-dimensional solution space, in order to provide realistic estimates of errors and test for the existence of multiple local minima and correlation between the different refined variables. The paper reviews the physical principles underlying the qualitative and quantitative analysis of spectroscopic data collected on aqueous speciation, in particular for studying successive ligand replacement reactions, and presents the non-linear least-squares algorithm implemented in BeerOz. The discussion is illustrated using UV–Vis spectra collected on acidic Fe(III) solutions containing varying LiCl concentrations, and showing the change from the hexaaquo Fe(H2O)6 3+ complex to the tetrahedral FeCl4 − complex. [Copyright &y& Elsevier]

Accessory gangue scheelite (CaWO4) from the Archaean Mt. Charlotte lode Au deposit can be divided into two types with different rare earth element (REE) signatures. In some scheelite grains, specific REE signatures are reflected by different cathodoluminescence colours, which can be used to map their often complex oscillatory intergrowths. Domains with specific REE contents from two grains were sampled for Sm/Nd, Rb/Sr and Pb isotopic analyses using a micro-drilling technique.Type I scheelite is strongly enriched in middle REE (MREE) and Eu anomalies are either absent or slightly positive. Four fragments collected from Type I regions of two crystals have initial 87Sr/86Sr and &epsiv;Nd values ranging from 0.70141 to 0.70163 and +2.5 to +3.5, respectively, and Pb isotope ratios reflecting the composition of greenstone sequence. This may indicate that Nd and Pb have their source, either locally or regionally, in the greenstones. Basic greenstone lithologies have 87Sr/86Sr<0.7015, and the radiogenic Sr signatures indicate that part of the Sr originated from felsic lithologies located either within or beneath the host greenstone pile. Alternatively, the Sr signature may have evolved from preferential leaching of a Rb-rich mineral during hydrothermal alteration of the greenstone.The REE patterns of Type II scheelite are either flat or MREE-depleted and have strong positive Eu anomalies. Three fragments collected from Type II regions of the same two crystals have initial 87Sr/86Sr ratios and &epsiv;Nd values between 0.70130 and 0.70146, and +1.1 to +2.6, respectively, and Pb isotope signatures that are once again similar to that of the greenstone. This implies that 87Sr/86Sr ratios in Type II fluids were closer to those of the host dolerite (0.7008–0.7013), due to more extensive fluid interaction with the dolerite.A positive correlation between Na and REE suggests that REE3+ are accommodated by the coupled substitution REE3++Na+=2 Ca2+ into both Type I and Type II scheelite. This is consistent with a fractional crystallisation model to explain the change in REE patterns from Type I to Type II, but not with a model involving different coupled substitutions and fluids from different origins. We propose that the complex REE and isotopic signatures of scheelite at Mt. Charlotte are related to small (

The article discusses application of In-situ experiments and molecular dynamics (MD) simulation techniques to study mobility of metals in hydrothermal ore fluids. Topics discussed include investigation of metal speciation and solubility at high temperature and pressure using Ultraviolet?visible spectroscopy, estimation of chemical speciation and thermodynamic properties at extreme temperatures by MD and numerical modelling of metal transport in ore fluids.

Sulphur is the third most abundant volatile element in deep Earth systems. Analytical methods for accurately and efficiently determining the sulphur content and oxidation state in natural minerals are still lacking. Natural apatite is widely distributed in the Earth and incorporates a large amount of sulphur. Therefore, apatite is an ideal mineral for performing sulphur measurements. Here, we used spectroscopic, Raman, X-ray diffraction, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), electron microprobe (EMPA) and micro-X-ray fluorescence spectrometry (micro-XRF) analysis techniques and developed a new analytical approach (i.e., micro-X-ray absorption near-edge structure (micro-XANES) analysis of the sulphur K-edge) to investigate the chemical characteristics of natural apatite. These multiple methods were developed to measure in situ sulphur concentration and S oxidation states and to assess a potential natural apatite reference material. Apatite contains chemically homogeneous sulphur, with micro-XANES located at the peak energies corresponding to S6+ (sulphate; ~2482 eV), S4+ (sulfite; ~2478 eV), and S2− (sulphide; ~2467, 2470 and 2474 eV). The Durango apatite contains total S presented as SO3 at amount of 0.332 ± 0.012 wt.% (1σ), with a large amount of S6+ and a small contribution of S4+. The Kovdor apatite contains 44–100 ppm of S and is dominated by S6+. These results indicate that the Durango apatite crystallised under relative oxidising conditions, and the Kovdor apatite has a higher oxygen fugacity than Durango. In addition, this study indicates the potential use of the natural apatite reference material with its S composition and S oxidation state. [ABSTRACT FROM AUTHOR]

In Earth's near-surface environments, gold biogeochemical cycling involves gold dissolution and precipitation processes, which are partly attributed to bacteria. These biogeochemical processes as well as abrasion (via physical transport) are known to act upon gold particles, thereby resulting in particle transformation including the development of pure secondary gold and altered morphology, respectively. While previous studies have inferred gold biogeochemical cycling from gold particles obtained from natural environments, little is known about how metal contamination in an environment could impact this cycle. Therefore, this study aims to infer how potentially toxic metal contaminants could affect the structure and chemistry of gold particles and therefore the biogeochemical cycling of gold. In doing so, river sediments and gold particles from the De Kaap Valley, South Africa, were analysed using both microanalytical and molecular techniques. Of the metal contaminants detected in the sediment, mercury can chemically interact with gold particles thereby directly altering particle morphology and "erasing" textural evidence indicative of particle transformation. Other metal contaminants (including mercury) indirectly affect gold cycling by exerting a selective pressure on bacteria living on the surface of gold particles. Particles harbouring gold-tolerant bacteria with diverse metal resistant genes, such as Arthrobacter sp. and Pseudomonas sp., contained nearly two times more secondary gold relative to particles harbouring bacteria with less gold-tolerance. In conclusion, metal contaminants can have a direct or indirect effect on gold biogeochemical cycling in natural environments impacted by anthropogenic activity. Gold biogeochemical cycling in a metal contaminated environment. Unlabelled Image • Metal contamination affects gold biogeochemical cycling in natural environments. • Gold particles contain (structural, chemical, molecular) evidence of gold cycling. • African gold particles and their resident bacterial communities were analysed. • Chemical interactions between particles and mercury formed spherical gold amalgam. • Metal (including mercury) resistant bacteria resided on gold-enriched particles. [ABSTRACT FROM AUTHOR]