An XPS study on the valence states of arsenic in arsenian pyrite: Implications for Au deposition mechanism of the Yang-shan Carlin-type gold deposit,western Qinling belt

The enrichment of gold in arsenian pyrite is usually associated closely with the enrichment of arsenic in the mineral, generally known as As¹⁻-pyrite [Fe(As, S)₂]. Direct analyses of the valence state of Au in pyrite are, however, difficult due to generally low (∼ppm level) Au concentrations. By means of X-ray photoelectron spectroscopy (XPS), this study obtained reliable valence states of As in pyrite from the Yang-shan gold deposit, a giant “Carlin-type” Au deposit in the western Qinling orogen, central China. The arsenian pyrite specimens were sputtered with Ar⁺ beam in the vacuum chamber of an XPS to obtain pristine surfaces and to avoid As oxidation during sample preparation. Analyses before and after sputtering show that the As³⁺ peak are only present on surface that was once exposed to the air. In contrast, the peak of As⁻¹ was essentially unchanged during continuous sputtering. The results indicated that As⁻ is the predominant state on the pristine surface of arsenian pyrite; the peak of As³⁺ previously reported for Au-bearing arsenian pyrite was probably due to oxidation when exposed to air during sample preparation. It is unlikely that the coupled substitution of (Au⁺ + As³⁺) for 2Fe²⁺ takes place in the pyrite lattice. The so-called As³⁺-pyrite proposed by previous studies may occur in some special (oxidizing) geologic settings, but it is not observed in the Yang-shan gold deposit, and is unlikely to be important in typical orogenic or Carlin-type gold deposits, in which arsenian pyrite is a dominant Au carrier. Combining previous studies on Carlin-type Au deposits with our XPS experimental results, we suggest that the most likely state of Au in the Yang-shan Au deposit is lattice-bounded Au with or without nanoparticles (Au⁰).     

Mineralogy and geochemistry of gold-bearing arsenian pyrite from the Shuiyindong Carlin-type gold deposit, Guizhou, China: implications for gold depositional processes

Arsenian pyrite in the Shuiyindong Carlin-type gold deposit in Guizhou, China, is the major host for gold with 300 to 4,000 ppm Au and 0.65 to 14.1 wt.% As. Electron miroprobe data show a negative correlation of As and S in arsenian pyrite, which is consistent with the substitution of As for S in the pyrite structure. The relatively homogeneous distribution of gold in arsenian pyrite and a positive correlation of As and Au, with Au/As ratios below the solubility limit of gold in arsenian pyrite, suggest that invisible gold is likely present as Au¹⁺ in a structurally bound Au complex in arsenian pyrite. Geochemical modeling using the laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of fluid inclusions for the major ore forming stage shows that the dominant Au species were Au(HS)₂⁻ (77%) and AuHS_(aq)⁰ (23%). Gold-hydroxyl and Gold-chloride complexes were negligible. The ore fluid was undersaturated with respect to native Au, with a saturation index of −3.8. The predominant As species was H₃AsO₃⁰ _(aq). Pyrite in the Shuiyindong deposit shows chemical zonation with rims richer in As and Au than cores, reflecting the chemical evolution of the ore-bearing fluids. The early ore fluids had relatively high activities of As and Au, to deposit unzoned and zoned arsenian pyrite that host most gold in the deposit. The ore fluids then became depleted in Au and As and formed As-poor pyrite overgrowth rims on gold-bearing arsenian pyrite. Arsenopyrite overgrowth aggregates on arsenian pyrite indicate a late fluid with relatively high activity of As. The lack of evidence of boiling and the low iron content of fluid inclusions in quartz, suggest that iron in arsenian pyrite was most likely derived from dissolution of ferroan minerals in the host rocks, with sulfidation of the dissolved iron by H₂S-rich ore fluids being the most important mechanism of gold deposition in the Shuiyindong Carlin-type deposit.     

First-principles calculations of the thermodynamic mixing properties of arsenic incorporation into pyrite and marcasite

The thermodynamic mixing properties of As into pyrite and marcasite have been investigated using first-principles and Monte Carlo calculations in order to understand the incorporation of this important metalloid into solid solution. Using quantum-mechanical methods to account for spin and electron transfer processes typical of sulfide minerals, the total energies of different As–S configurations were calculated at the atomic scale, and the resulting As–S interactions were incorporated into Monte Carlo simulations. Enthalpies, configurational entropies and Gibbs free energies of mixing show that two-phase mixtures of FeS₂ (pyrite or marcasite) and FeAsS (arsenopyrite) are energetically more favorable than the solid solution Fe(S,As)₂ (arsenian pyrite or marcasite) for a wide range of geologically relevant temperatures. Although miscibility gaps dominate both solid solution series, the solubility of As is favored for X_As < 0.05 in iron disulfides. Consequently, pyrite and marcasite can host up to ∼6 wt.% of As in solid solution before unmixing into (pyrite or marcasite) + arsenopyrite. This finding is in agreement with previously published HRTEM observations of As-rich pyrites (> 6 wt.% As) that document the presence of randomly distributed domains of pyrite + arsenopyrite at the nanoscale. According to the calculations, stable and metastable varieties of arsenian pyrite and marcasite are predicted to occur at low (X_As < 0.05) and high (X_As > 0.05) As bulk compositions, respectively.     

Experimental analysis of arsenic precipitation during microbial sulfate and iron reduction in model aquifer sediment reactors

Microbial SO₄²⁻ reduction limits accumulation of aqueous As in reducing aquifers where the sulfide that is produced forms minerals that sequester As. We examined the potential for As partitioning into As- and Fe-sulfide minerals in anaerobic, semi-continuous flow bioreactors inoculated with 0.5% (g mL⁻¹) fine-grained alluvial aquifer sediment. A fluid residence time of three weeks was maintained over a ca. 300-d incubation period by replacing one-third of the aqueous phase volume of the reactors with fresh medium every seven days. The medium had a composition comparable to natural As-contaminated groundwater with slightly basic pH (7.3) and 7.5 μM aqueous As(V) and also contained 0.8 mM acetate to stimulate microbial activity. Medium was delivered to a reactor system with and without 10 mmol L⁻¹ synthetic goethite (α-FeOOH). In both reactors, influent As(V) was almost completely reduced to As(III). Pure As-sulfide minerals did not form in the Fe-limited reactor. Realgar (As₄S₄) and As₂S₃(am) were undersaturated throughout the experiment. Orpiment (As₂S₃) was saturated while sulfide content was low (∼50 to 150 μM), but precipitation was likely limited by slow kinetics. Reaction-path modeling suggests that, even if these minerals had formed, the dissolved As content of the reactor would have remained at hazardous levels. Mackinawite (Fe_1 + xS; x ? 0.07) formed readily in the Fe-bearing reactor and held dissolved sulfide at levels below saturation for orpiment and realgar. The mackinawite sequestered little As (<0.1 wt.%), however, and aqueous As accumulated to levels above the influent concentration as microbial Fe(III) reduction consumed goethite and mobilized adsorbed As. A relatively small amount of pyrite (FeS₂) and greigite (Fe₃S₄) formed in the Fe-bearing reactor when we injected a polysulfide solution (Na₂S₄) to a final concentration of 0.5 mM after 216, 230, 279, and 286 days. The pyrite, and to a lesser extent the greigite, that formed did sequester As from solution, containing 0.84 and 0.23 wt.% As on average, respectively. Our results suggest that As precipitation during Fe-sulfide formation in nature occurs mainly in conjunction with pyrite formation. Our findings imply that the effectiveness of stimulating microbial SO₄²⁻ reduction to remediate As contamination may be limited by the rate and extent of pyrite formation and the solubility of As-sulfides.     

Dissolved and particulate metals and arsenic species mobility along a stream affected by Acid Mine Drainage in the Iberian Pyrite Belt (SW Spain)

Intensive mining has taken place in the Iberian Pyrite Belt since 3000 B.C. generating Acid Mine Drainage (AMD) and releasing high amounts of SO₄, acidity, metals and metalloids into surface water. Concentrations of elements in AMD-impacted waters are regulated by the precipitation of Fe-rich materials and particulate matter can influence the mobility and the bioavailability of metals. In this paper a study on the dissolved As species concentration along a polluted stream has been performed. Two sampling campaigns were conducted during the dry and the rainy seasons. Concentrations of dissolved elements are higher during the dry season and increase progressively along the water course in both seasons. Concentrations up to 80 μg L⁻¹ of As³⁺ and 5 mg L⁻¹ of As⁵⁺ were determined. The concentration of As species increases and the As³⁺/As⁵⁺ ratio decreases downstream. The Fe²⁺/Fe³⁺ and As³⁺/As⁵⁺ ratios show the same pattern with respect to pH for all the examined samples, except those taken from Au cyanidation wastes. The particulate phase is mainly composed of Fe, As and Pb, with As being associated with the Fe minerals while Pb seems to be associated with the clay colloids.     

Distribution of arsenic and its mobility in shallow aquifer sediments from Ambikanagar, West Bengal, India

Sediments from a core retrieved during installation of a shallow drinking water well in Ambikanagar (West Bengal, India) were analyzed for various physical and chemical parameters. The geochemical analyses included: (1) a 4-step sequential extraction scheme to determine the distribution of As between different fractions, (2) As speciation (As³⁺ vs. As⁵⁺), and (3) C, N and S isotopes. The sediments have a low percentage of organic C and N (0.10-0.56% and 0.01-0.05%, respectively). Arsenic concentration is between 2 and 7 mg kg⁻¹, and it is mainly associated with the residual fraction, less susceptible to chemical weathering. The proportion of As³⁺ in these sediments is high and ranges from 24% to 74%. Arsenic in the second fraction (reducible) correlates well with Mn, and in the residual fraction As correlates well with several transition elements. The stable isotope results indicate microbial oxidation of organic matter involving SO₄ reduction. Oxidation of primary sulfide minerals and release of As from reduction of Fe-(oxy)hydroxides do not seem important mechanisms in As mobilization. Instead, the dominance of As³⁺ and presence of As⁵⁺ reducing microorganisms in this shallow aquifer imply As remobilization involving microbial processes that needs further investigations.     

Progressive oxidation of pyrite in five bituminous coal samples: An As XANES and Fe Mössbauer spectroscopic study

Naturally occurring pyrite commonly contains minor substituted metals and metalloids (As, Se, Hg, Cu, Ni, etc.) that can be released to the environment as a result of its weathering. Arsenic, often the most abundant minor constituent in pyrite, is a sensitive monitor of progressive pyrite oxidation in coal. To test the effect of pyrite composition and environmental parameters on the rate and extent of pyrite oxidation in coal, splits of five bituminous coal samples having differing amounts of pyrite and extents of As substitution in the pyrite, were exposed to a range of simulated weathering conditions over a period of 17 months. Samples investigated include a Springfield coal from Indiana (whole coal pyritic S = 2.13 wt.%; As in pyrite = detection limit (d.l.) to 0.06 wt.%), two Pittsburgh coal samples from West Virginia (pyritic S = 1.32–1.58 wt.%; As in pyrite = d.l. to 0.34 wt.%), and two samples from the Warrior Basin, Alabama (pyritic S = 0.26–0.27 wt.%; As in pyrite = d.l. to 2.72 wt.%). Samples were collected from active mine faces, and expected differences in the concentration of As in pyrite were confirmed by electron microprobe analysis. Experimental weathering conditions in test chambers were maintained as follows: (1) dry Ar atmosphere; (2) dry O₂ atmosphere; (3) room atmosphere (relative humidity ∼20–60%); and (4) room atmosphere with samples wetted periodically with double-distilled water. Sample splits were removed after one month, nine months, and 17 months to monitor the extent of As and Fe oxidation using As X-ray absorption near-edge structure (XANES) spectroscopy and ⁵⁷Fe Mössbauer spectroscopy, respectively. Arsenic XANES spectroscopy shows progressive oxidation of pyritic As to arsenate, with wetted samples showing the most rapid oxidation. ⁵⁷Fe Mössbauer spectroscopy also shows a much greater proportion of Fe³⁺ forms (jarosite, Fe³⁺ sulfate, FeOOH) for samples stored under wet conditions, but much less difference among samples stored under dry conditions in different atmospheres. The air-wet experiments show evidence of pyrite re-precipitation from soluble ferric sulfates, with As retention in the jarosite phase. Extents of As and Fe oxidation were similar for samples having differing As substitution in pyrite, suggesting that environmental conditions outweigh the composition and amount of pyrite as factors influencing the oxidation rate of Fe sulfides in coal.     

Gold deposition on pyrite and the common sulfide minerals: An STM/STS and SR-XPS study of surface reactions and Au nanoparticles

Gold species spontaneously deposited on pyrite and chalcopyrite, pyrrhotite, galena, sphalerite from HAuCl₄ solutions at room temperature, as well as the state of the reacted mineral surfaces have been characterized using synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS), scanning tunneling microscopy and tunneling spectroscopy (STM/STS). The deposition of silver from 10⁻⁴ M AgNO₃ has been examined for comparison. Gold precipitates as metallic nanoparticles (NPs) from about 3 nm to 30 nm in diameter, which tends to aggregate forming larger particles, especially on pyrite. The Au 4f binding energies increase up to 1 eV with decreasing size of individual Au⁰ NPs, probably due to the temporal charging in the final state. Concurrently, a positive correlation between the tunneling current and the particle size was found in STS. Both these size effects were observed for unusually large, up to 20 nm Au particles. In contrast, silver deposited on the minerals as nanoparticles of semiconducting sulfide showed no shifts of photoelectron lines and different tunneling spectra.The quantity of gold deposited on pyrite and other minerals increased with time; it was lower for fracture surfaces and it grew if minerals were moderately pre-oxidized, while the preliminary leaching in Fe(III)-bearing media inhibited the following Au deposition. After the contact of polished minerals with 10⁻⁴ M solution (pH 1.5) for 10 min, the gold uptake changed in the order CuFeS₂ > ZnS > PbS > FeAsS > FeS₂ > Fe₇S₈. It was noticed that the open circuit (mixed) potentials of the minerals varied in approximately the same order, excepting chalcopyrite. We concluded that the potentials of minerals were largely determined by Fe(II)/Fe(III) couple, whereas the reduction of gold complexes had a minor effect. As a result, the deposition of gold, although it proceeded via the electrochemical mechanism, increased with decreasing potential. This suggests, in particular, that the accumulation of “invisible” gold in arsenian pyrites and arsenopyrite under hydrothermal conditions may be explained by the low electrochemical potentials but not structural relationships between As and Au in solids.     

Solubility of gold in arsenian pyrite

Although Au and As can be enriched up to the weight percent level in arsenian pyrite, there is little knowledge of their limiting concentrations and nature of incorporation. This study reports SIMS and EMPA analyses showing that As and Au contents of arsenian pyrites plot in a wedge-shaped zone with an upper compositional limit defined by the line

CAu=0.02⋅CAs+4×10⁻⁵     

Role of Fe(II) and phosphate in arsenic uptake by coprecipitation

Natural attenuation of arsenic by simple adsorption on oxyhydroxides may be limited due to competing oxyanions, but uptake by coprecipitation may locally sequester arsenic. We have systematically investigated the mechanism and mode (adsorption versus coprecipitation) of arsenic uptake in the presence of carbonate and phosphate, from solutions of inorganic composition similar to many groundwaters. Efficient arsenic removal, >95% As(V) and ∼55% in initial As(III) systems, occurred over 24 h at pHs 5.5-6.5 when Fe(II) and hydroxylapatite (Ca₅(PO₄)₃OH, HAP) “seed” crystals were added to solutions that had been previously reacted with HAP, atmospheric CO_2(g) and O_2(g). Arsenic adsorption was insignificant (<10%) on HAP without Fe(II). Greater uptake in the As(III) system in the presence of Fe(II) was interpreted as due to faster As(III) to As(V) oxidation by molecular oxygen in a putative pathway involving Fe(IV) and As(IV) intermediate species. HAP acts as a pH buffer that allows faster Fe(II) oxidation. Solution analyses coupled with high-resolution transmission electron microscopy (HRTEM), X-ray Energy-Dispersive Spectroscopy (EDS), and X-Ray Absorption Spectroscopy (XAS) indicated the precipitation of sub-spherical particles of an amorphous, chemically-mixed, nanophase, Fe^III[(OH)₃(PO₄)(As^VO₄)]·nH₂O or Fe^III[(OH)₃( PO₄)(As^VO₄)(As^IIIO₃)_minor]·nH₂O, where As^IIIO₃ is a minor component.The mode of As uptake was further investigated in binary coprecipitation (Fe(II) + As(III) or P), and ternary coprecipitation and adsorption experiments (Fe(II) + As(III) + P) at variable As/Fe, P/Fe and As/P/Fe ratios. Foil-like, poorly crystalline, nanoparticles of Fe^III(OH)₃ and sub-spherical, amorphous, chemically-mixed, metastable nanoparticles of Fe^III[(OH)₃, PO₄]·nH₂O coexisted at lower P/Fe ratios than predicted by bulk solubilities of strengite (FePO₄·2H₂O) and goethite (FeOOH). Uptake of As and P in these systems decreased as binary coprecipitation > ternary coprecipitation > ternary adsorption.Significantly, the chemically-mixed, ferric oxyhydroxide-phosphate-arsenate nanophases found here are very similar to those found in the natural environment at slightly acidic to circum-neutral pHs in sub-oxic to oxic systems, such phases may naturally attenuate As mobility in the environment, but it is important to recognize that our system and the natural environment are kinetically evolving, and the ultimate environmental fate of As will depend on the long-term stability and potential phase transformations of these mixed nanophases. Our results also underscore the importance of using sufficiently complex, yet systematically designed, model systems to accurately represent the natural environment.     

The formation, structure, and ageing of As-rich hydrous ferric oxide at the abandoned Sb deposit Pezinok (Slovakia)

The abandoned Sb deposit Pezinok in Slovakia is a significant source of As and Sb pollution that can be traced in the upper horizons of soils kilometers downstream. The source of the metalloids are two tailing impoundments which hold ∼380,000 m³ of mining waste. The tailings and the discharged water have circumneutral pH values (7.0 ± 0.6) because the acidity generated by the decomposition of the primary sulfides (pyrite, FeS₂; arsenopyrite, FeAsS; berthierite, FeSb₂S₄) is rapidly neutralized by the abundant carbonates. The weathering rims on the primary sulfides are iron oxides which act as very efficient scavengers of As and Sb (with up to 19.2 wt% As and 23.7 wt% Sb). In-situ μ-XANES experiments indicate that As in the weathering rims is fully oxidized (As⁵⁺). The pore solutions in the impoundment body contain up to 81 ppm As and 2.5 ppm Sb. Once these solutions are discharged from the impoundments, they precipitate or deposit masses of As-rich hydrous ferric oxide (As-HFO) with up to 28.3 wt% As₂O₅ and 2.7 wt% Sb. All As-HFO samples are amorphous to X-rays. They contain Fe and As in their highest oxidation state and in octahedral and tetrahedral coordination, respectively, as suggested by XANES and EXAFS studies on Fe K and As K edges. The iron octahedra in the As-HFO share edges to form short single chains and the chains polymerize by sharing edges or corners with the adjacent units. The arsenate ions attach to the chains in a bidentate-binuclear and monodentate fashion. In addition, hydrogen-bonded complexes may exist to satisfy the bonding requirements of all oxygen atoms in the first coordination sphere of As⁵⁺. Structural changes in the As-HFO samples were traced by chemical analyses and Fe EXAFS spectroscopy during an ageing experiment. As the samples age, As becomes more easily leachable. EXAFS spectra show a discernible trend of increasing number of Fe-Fe pairs at a distance of 3.3-3.5 Å, that is, increasing polymerization of the iron octahedra to form larger units with fewer adsorption sites. Therefore, although ferrihydrite is an excellent material for capturing arsenic, its use as a medium for a long-term storage of As has to be considered with a great caution because it will tend to release arsenic as it ages.     

The occurrence and geochemistry of arsenic in groundwaters of the Newark basin of Pennsylvania

Elevated As concentrations in groundwater in the eastern United States have been recognized predominantly in the accretionary geologic terranes of northern New England. A retrospective examination of more than 18,000 existing groundwater samples from the Pennsylvania Department of Environmental Protection (PA DEP) Drinking Water and Sampling Information System database indicates that elevated groundwater As concentrations occur throughout the northern half of the Piedmont Province of Pennsylvania. Chemical analyses of 53 samples collected in 2005 from drinking water wells in this area all had detectable As, and 23% of these samples contained elevated (>133 nmol/L or >10 μg/L) concentrations of As. Elevated concentrations of As in the groundwater samples were most common in the Mesozoic sedimentary strata composed of sandstone and red mudstone with interbedded gray shale, and gray to black siltstone and shale. Arsenic was typically not elevated in groundwater of diabase intrusions of the Newark Basin or in crystalline and calcareous aquifers to the north of the Newark Basin. Geochemical parameters such as pH and oxidation–reduction potential can indicate mobility mechanisms of As in some regions. In this area, measured groundwater conditions were predominantly oxidizing (Eh > +50 mV), and more than 85% of samples contained arsenate as the dominant As species. Variations in pH were strongly correlated to the As concentration, with highest As concentrations observed at pH values greater than 6.4. The original source of As is most likely the black and gray shales that contain some arsenian pyrite with groundwater concentrations likely to be controlled by adsorption/desorption reactions with Fe oxides in the red mudstone aquifer materials.     

Invisible gold in ore and mineral concentrates from the Hillgrove gold-antimony deposits, NSW, Australia

Vein-hosted mesothermal stibnite-gold mineralisation at the Hillgrove Au-Sb mine in northeastern New South Wales has a halo of veinlet and disseminated auriferous arsenopyrite and arsenian pyrite in metasedimentary and granitic host rocks. About 50–55% of the gold produced at Hillgrove occurs invisibly in arsenopyrite and pyrite. Gold losses of ∼20% into tailings are due to this mineral chemical factor. From PIXE probe analyses, it has been found that arsenopyrite contains 255–1500 ppm Au and pyrite 24–223 ppm Au, with Au contents of each mineral correlating moderately with As content. Arsenopyrite and pyrite also contain anomalous values of Cu, Ag and Sb, whereas paragenetically later stibnite contains little invisible gold, but minor Fe, As, Ag, Cu and Pb. The precipitation of invisible gold in arsenopyrite and pyrite by a possible (Fe, Au)³⁺= (As-S)³⁻ substitution mechanism may have been facilitated by rapid, non-equilibrium conditions involving pressure decreases and wall rock reaction (sulphidation, carbonatisation), as a prelude to the main stage of stibnite and gold deposition. Received: 15 January 1999 / Accepted: 12 October 1999     

The role of carbonate ions in pyrite oxidation in aqueous systems

The mechanism of pyrite oxidation in carbonate-containing alkaline solutions at 80 °C was investigated with the help of rate experiments, thermodynamic modeling and diffuse reflectance infrared spectroscopy (DRIFTS). Pyrite oxidation rate increased with pH and was enhanced by addition of bicarbonate/carbonate ions. The carbonate effect was found to be limited to moderately alkaline conditions (pH 8-11). Metastable Eh-pH diagrams, at 25 °C, indicate that soluble iron-carbonate complexes (FeHCO₃⁻, FeCO₃⁰, Fe(CO₃)(OH)⁻ and FeCO₃²⁻) may coexist with pyrite in the pH range of 6-12.5. Above pH 11 and 13, the Fe(II) and Fe(III) hydroxocomplexes, respectively, become stable, even in the presence of carbonate/bicarbonate ions. Surface-bound carbonate complexes on iron were also identified with DRIFTS as products of pyrite oxidation in addition to iron oxyhydroxides and soluble sulfate species. The conditions under which thermodynamic and DRIFTS analyses indicate the presence of carbonate compounds also correspond to those in which the fastest rate of pyrite oxidation in carbonate solutions was observed. Following the Singer-Stumm model for pyrite oxidation in acidic solutions, it is assumed that Fe(III) is the preferred pyrite oxidant under alkaline conditions. We propose that carbonate ions facilitate the electron transfer from soluble iron(II)-carbonate to O₂, increase the iron solubility, and provide buffered, favorable alkaline conditions at the reaction front, which in turn favors the overall kinetics of pyrite oxidation. Therefore, the electron transfer from sulfur atoms to O₂ is facilitated by the formation of the cycle of Fe(II)-pyrite/Fe(III)-carbonate redox couple at the pyrite surface.     

Visible gold in arsenian pyrite at the Shuiyindong Carlin-type gold deposit, Guizhou, China: Implications for the environment and processes of ore formation

Carlin-type gold deposits are best known for the scarcity of visible gold in their ores. It has long been recognized that the majority of gold is “invisible”, such that it cannot be resolved by conventional microscopy, and resides in arsenian pyrite. Shuiyindong differs in that sub-μm to μm-sized native gold is present in arsenian pyrite veinlets and disseminations. It is also the largest (55 tonnes) and highest grade (7 to 18 ppm), stratabound, Carlin-type gold deposit in Guizhou, China and has produced 5 tonnes of gold from sulfide refractory ores extracted by underground mining methods. In this study, an electron microprobe analyzer (EMPA) was used to map the spatial distribution of “invisible” gold and sub-μm to μm-size visible gold particles in arsenian pyrite in high-grade ore samples from the Shuiyindong. The samples studied are hosted in Permian bioclastic ferroan limestone of the Longtan Formation and exhibit evidence of decarbonation, silicification and sulfidation. Arsenian pyrite with detectable Au (> 400 to 3800 ppm) is disseminated in altered limestone and was deposited in two stages separated by an episode of corrosion in a veinlet.The results show that there are two populations of native gold in arsenian pyrite. One is comprised of sub-μm size gold particles (0.1 to 0.2 μm) that are occasionally present in the gold-bearing arsenian pyrite disseminated in the host rocks. This arsenian pyrite is interpreted to have been formed by sulfidation of ferroan calcite and dolomite. Another is comprised of coarser (1 to 6 μm) native gold grains present in the arsenian pyrite veinlet, either on the first stage where it has been corroded or on the second stage. The lack of fluid inclusion or other evidence of boiling and the low iron content of fluid inclusions in quartz, suggest the veinlet formed by sulfidation of another fluid containing Fe. The Fe-bearing fluid may be a depleted ore fluid that gained Fe by dissolution of ferroan limestone after H₂S had been consumed. The association of the largest visible gold grains with an episode of corrosion suggests that fluids episodically became undersaturated with arsenian pyrite while remaining saturated with gold (e.g., pH decrease or an increase in the oxidation state). This may have resulted from incursion of relatively acidic or oxidized fluids that were able to dissolve arsenian pyrite and remain saturated with gold. In this case, sulfidation of iron from the host rock, was the most important depositional mechanism for Au-bearing arsenian pyrite with, or without, grains of native gold.     

甘肃阳山金矿田载金矿物特征及金赋存状态研究

采用电子探针分析,详细研究了甘肃阳山类卡林型金矿田原生矿石中不同成矿阶段载金矿物的Au、As、S、Fe等元素含量及其分布规律,确定含砷黄铁矿和毒砂是最重要的载金矿物,发现不同成矿阶段的黄铁矿具有不同的成分特点;沉积成岩期黄铁矿为草莓状、胶状,砷和金含量最低,分别为0.10%和0.08%;热液成矿期早阶段黄铁矿粒度较粗(0.40～1.00mm),是较高温度(270～300℃)下缓慢结晶的产物,其砷和金含量较低,分别为0.27%和0.09%;热液成矿期主阶段(包括M1,M2和M3亚阶段)黄铁矿粒度微细(0.05～0.20mm),是210～270℃条件下快速结晶的产物,砷和金含量最高,M1亚阶段分别为3.45%As和0.11%Au,M2亚阶段分别为3.88%As和0.14%Au.在含砷黄铁矿中,金可能有自然金和离子金两种存在方式.沉积成岩期和热液成矿期早阶段低砷黄铁矿中金主要以纳米级自然金(Au~0)颗粒形式分布,而在热液成矿期主阶段含砷黄铁矿中金主要以Au+的形式存在.当热液中As活度高时,含砷黄铁矿在快速生长条件下,其生长面的空穴和缺陷较多,有利于热液中Au(HS)~0络合物通过吸附反应直接进入含砷黄铁矿生长表面.此外,主阶段流体的硫化和沸腾作用均可导致H_2S的减少,有利于形成砷黄铁矿和Au沉淀富集.     

Insights for size-dependent reactivity of hematite nanomineral surfaces through Cu sorption

Macroscopic sorption edges for Cu²⁺ were measured on hematite nanoparticles with average diameters of 7 nm, 25 nm, and 88 nm in 0.1 M NaNO₃. The pH edges for the 7 nm hematite were shifted approximately 0.6 pH units lower than that for the 25 nm and 88 nm samples, demonstrating an affinity sequence of 7 nm > 25 nm = 88 nm. Although, zeta potential data suggest increased proton accumulation at the 7 nm hematite surfaces, changes in surface structure are most likely responsible for the preference of Cu²⁺ for the smallest particles. As Cu²⁺ preferentially binds to sites which accommodate the Jahn-Teller distortion of its coordination to oxygen, this indicates the relative importance of distorted binding environments on the 7 nm hematite relative to the 25 nm and 88 nm particles. This work highlights the uniqueness of surface reactivity for crystalline iron oxide particles with decreasing nanoparticle diameter.     

Mobilization of arsenic from mine tailings through reductive dissolution of goethite influenced by organic cover

Mine tailings at the former Delnite gold mine in northern Ontario were characterized to assess the impact of a biosolids cover on the stability of As species and evaluate options for long-term management of the tailings. Arsenic concentrations in the tailings range from 0.15 to 0.36 wt% distributed among goethite, pyrite and arsenopyrite. Pyrite and arsenopyrite occur as small and liberated particles that are enveloped by goethite in the uncovered tailings and the deeper portions of the biosolids-covered tailings. Sulfide particles in the shallower portions of the biosolids-covered tailings are free of goethite rims. Arsenic occurs predominantly as As⁵⁺ with minor amount of As¹⁻ in the uncovered tailings. Coinciding with the disappearence of goethite rims on sulfide particles, the biosolids-covered tailings have As³⁺ species gradually increasing in proportion towards the cover. Leaching tests indicated that the As concentrations in the leachate gradually increase from less than 0.085 to 13 mg/L and Fe from 28 to 179 mg/L towards the biosolids cover. These are in sheer contrast to the leachate concentrations of less than 0.085 mg/L As and 24–64 mg/L Fe obtained from the uncovered tailings confirming the role of biosolids-influenced reduction and mobilization of As in the form of As³⁺ species. The evidence suggests that reductive dissolution of goethite influenced by the biosolids-cover caused the mobilization of As as As³⁺ species.     

The arsenic source term for an in-pit uranium mine tailings facility and its long-term impact on the regional groundwater

Detailed field sampling and analyses and laboratory-based diffusion-cell experiments were used in conjunction with 3-D reactive transport modeling (MODFLOW and MT3D99) to quantify the fate and long-term (10 ka) transport of As in the Rabbit Lake In-pit Tailings Management Facility (RLITMF), northern Saskatchewan, Canada. The RLITMF (300 m × 425 m × 90 m thick) was engineered to ensure solute transport within the RLITMF is dominated by diffusion. Concentrations of As in the tailings pore fluids ranged from 0.24 to 140 mg/L (n = 43). Arsenic speciation analyses indicate 90% of this arsenic exists as As⁵⁺. This observation is supported by pH–Eh measurements of pore fluids (n = 135). Geochemical analyses yielded a strong inverse correlation between the Fe/As molar ratio in the tailings solids and the corresponding concentration of dissolved As, which is attributed to the adsorption of As to secondary 2-line ferrihydrite present in the tailings. Diffusion-cell testing yielded values for the effective diffusion coefficient, sorption coefficient, and effective porosity of As in the tailings of 4.5 × 10⁻¹⁰ m²/s, 2–4 cm³/g and 0.36, respectively. Reactive transport simulations using the field and laboratory data show adsorption of As to the tailings and diffusive transport of dissolved As in the tailings should reduce the source term concentration of As to between 40% and 70% of the initial concentrations over the 10 ka simulation period. Based on these simulations, the As concentrations in the regional groundwater, 50 m down gradient of the tailings facility, should be maintained at background concentrations of 0.001 mg/L over the 10 ka period. These findings suggest the engineered in-pit disposal of U mine tailings can provide long-term protection for the local groundwater regime from As contamination.     

Geochemistry and mineralogy of arsenic in (natural) anaerobic groundwaters

Here new data from field bioremediation experiments and geochemical modeling are reported to illustrate the principal geochemical behavior of As in anaerobic groundwaters. In the field bioremediation experiments, groundwater in Holocene alluvial aquifers in Bangladesh was amended with labile water-soluble organic C (molasses) and MgSO₄ to stimulate metabolism of indigenous SO₄-reducing bacteria (SRB). In the USA, the groundwater was contaminated by Zn, Cd and SO₄, and contained <10 μg/L As under oxidized conditions, and a mixture of sucrose and methanol were injected to stimulate SRB metabolism. In Bangladesh, groundwater was under moderately reducing conditions and contained ∼10 mg/L Fe and ∼100 μg/L As. In the USA experiment, groundwater rapidly became anaerobic, and dissolved Fe and As increased dramatically (As > 1000 μg/L) under geochemical conditions consistent with bacterial Fe-reducing conditions. With time, groundwater became more reducing and biogenic SO₄ reduction began, and Cd and Zn were virtually completely removed due to precipitation of sphalerite (ZnS) and other metal sulfide mineral(s). Following precipitation of chalcophile elements Zn and Cd, the concentrations of Fe and As both began to decrease in groundwater, presumably due to formation of As-bearing FeS/FeS₂. By the end of the six-month experiment, dissolved As had returned to below background levels. In the initial Bangladesh experiment, As decreased to virtually zero once biogenic SO₄ reduction commenced but increased to pre-experiment level once SO₄ reduction ended. In the ongoing experiment, both SO₄ and Fe(II) were amended to groundwater to evaluate if FeS/FeS₂ formation causes longer-lived As removal. Because As-bearing pyrite is the common product of SRB metabolism in Holocene alluvial aquifers in both the USA and Southeast Asia, it was endeavored to derive thermodynamic data for arsenian pyrite to better predict geochemical processes in naturally reducing groundwaters. Including the new data for arsenian pyrite into Geochemist’s Workbench, its stability field completely dominates in reducing Eh–pH space and “displaces” other As-sulfides (orpiment, realgar) that have been implied to be important in previous modeling exercises and reported in rare field conditions.