Progression in sulfur isotopic compositions from coal to fly ash: Examples from single-source combustion in Indiana

Sulfur occurs in multiple mineral forms in coals, and its fate in coal combustion is still not well understood. The sulfur isotopic composition of coal from two coal mines in Indiana and fly ash from two power plants that use these coals were studied using geological and geochemical methods. The two coal beds are Middle Pennsylvanian in age; one seam is the low-sulfur (< 1%) Danville Coal Member of the Dugger Formation and the other is the high-sulfur (> 5%) Springfield Coal Member of the Petersburg Formation. Both seams have ash contents of approximately 11%. Fly-ash samples were collected at various points in the ash-collection system in the two plants. The results show notable difference in δ³⁴S for sulfur species within and between the low-sulfur and high-sulfur coal. The δ³⁴S values for all sulfur species are exclusively positive in the low-sulfur Danville coal, whereas the δ³⁴S values for sulfate, pyritic, and organic sulfur are both positive and negative in the high-sulfur Springfield coal. Each coal exhibits a distinct pattern of stratigraphic variation in sulfur isotopic composition. Overall, the δ³⁴S for sulfur species values increase up the section in the low-sulfur Danville coal, whereas they show a decrease up the vertical section in the high-sulfur Springfield coal. Based on the evolution of δ³⁴S for sulfur species, it is suggested that there was influence of seawater on peat swamp, with two marine incursions occurring during peat accumulation of the high-sulfur Springfield coal. Therefore, bacterial sulfate reduction played a key role in converting sulfate into hydrogen sulfide, sulfide minerals, and elemental sulfur. The differences in δ³⁴S between sulfate sulfur and pyritic sulfur is very small between individual benches of both coals, implying that some oxidation occurred during deposition or postdeposition.The δ³⁴S values for fly ash from the high-sulfur Springfield coal (averaging 9.7‰) are greatly enriched in ³⁴S relative to those in the parent coal (averaging 2.2‰). This indicates a fractionation of sulfur isotopes during high-sulfur coal combustion. By contrast, the δ³⁴S values for fly-ash samples from the low-sulfur Danville coal average 10.2‰, only slightly enriched in ³⁴S relative to those from the parent coal (average 7.5‰). The δ³⁴S values for bulk S determined directly from the fly-ash samples show close correspondence with the δ³⁴S values for SO₄^− 2 leached from the fly ash in the low-sulfur coal, suggesting that the transition from pyrite to sulfate occurred via high-temperature oxidation during coal combustion.     

Graphite–sulfide deposits in Ronda and Beni Bousera peridotites (Spain and Morocco) and the origin of carbon in mantle-derived rocks

This paper describes unusual graphite–sulfide deposits in ultramafic rocks from the Serranía de Ronda (Spain) and Beni Bousera (Morocco). These deposits occur as veins, stockworks and irregular masses, ranging in size from some centimeters to a few meters in thickness. The primary mineral assemblage mainly consists of Fe–Ni–Cu sulfides (pyrrhotite, pentlandite, chalcopyrite and cubanite), graphite and chromite. Weathering occurs in some sulfide-poor deposits that consist of graphite (up to 90%), chromite and goethite. Texturally, graphite may occur as flakes or clusters of flakes and as rounded, nodule-like aggregates. Graphite is highly crystalline and shows light carbon isotopic signatures (δ¹³C≈− 15‰ to − 21‰). Occasionally, some nodule-like graphite aggregates display large isotopic zoning with heavier cubic forms (probably graphite pseudomorphs after diamond with δ¹³C up to − 3.3‰) coated by progressively lighter flakes outwards (δ¹³C up to − 15.2‰).Asthenospheric-derived melts originated the partial melting (and melt–rock reactions) of peridotites and pyroxenites generating residual melts from which the graphite–sulfide deposits were formed. These residual melts concentrated volatile components (mainly CO₂ and H₂O), as well as S, As, and chalcophile elements. Carbon was incorporated into the melts from the melt–rock reactions of graphite-bearing (formerly diamonds) garnet pyroxenites with infiltrated asthenospheric melts. Graphite-rich garnet pyroxenites formed through the UHP transformation of subducted kerogen-rich crustal material into the mantle. Thus, graphite in most of the studied occurrences has light (biogenic) carbon signatures. Locally, reaction of the light carbon in the melts with relicts of ¹³C-enriched graphitized diamonds (probably generated from hydrothermal calcite veins in the subducting oceanic crust) reacted with the partial melts to form isotopically zoned nodule-like graphite aggregates.     

Sulfur isotopic compositions of the Huize super-large Pb–Zn deposit, Yunnan Province, China: Implications for the source of sulfur in the ore-forming fluids

The Huize Pb–Zn deposit of Yunnan Province, China, is located in the center of the Sichuan–Yunnan–Guizhou Pb–Zn–Ag district. Four primary orebodies (orebody No. 1, No. 6, No. 8 and No. 10), with Pb + Zn reserves from 0.5 Mt to 1 Mt, have been found at depth in this deposit. This paper provides new data on the sulfur isotopic compositions of the four orebodies. The data show that the principal sulfide minerals (galena, sphalerite and pyrite) in the four orebodies are enriched in heavy sulfur, the δ³⁴S values between 10.9‰ and 17.7‰ and where δ³⁴S_pyrite > δ³⁴S_sphalerite > δ³⁴S_galena. The δ³⁴S values of sulfide are close to that of the sulfates from the carbonate strata within the region. The similarity in sulfur isotope composition between sulfides and sulfates indicates the sulfur in the ore-forming fluids was likely derived by thermochemical sulfate reduction of sulfates contained within carbonate units.     

The diagenesis of carbohydrates by hydrogen sulfide

Carbohydrates react with hydrogen sulfide under low temperature (100° to 200°C) yielding a variety of organosulfur compounds including thiophenes, thiols, sulfides and sulfones. A polymer is also produced, whose elemental composition is within the range of natural coals. When reductive dehydration is carried out in the presence of hydrocarbon, organosulfur compounds are formed in the carbon number range of the hydrocarbon used. In these processes, an active hydrogen transfer catalyst is produced which facilitates the passage of hydrogen between normal paraffins and saccharide units, distributing sulfur between these two families primarily in the form of thiophene rings. The simplicity of these systems—H₂S, carbohydrates, H₂O, hydrocarbon—and the facility of the chemistry would suggest that the carbohydrates and hydrogen sulfide may be important agents in the diagenetic processes leading to petroleum and coal. Carbohydrate reduction by hydrogen sulfide may constitute an important route through which certain organosulfur compounds found in petroleum and coal entered these materials in early diagenesis.     

Simultaneous determination of sulfide, polysulfides and thiosulfate as an aid to ore exploration

The interaction of water and sulfide minerals yields dissolved species which can be utilized to trace back the presence of sulfide minerals and associated minerals. Computer modeling and laboratory and field results show that the most characteristic dissolved species are hydrogen sulfide (H₂S, HS⁻), polysulfide ions (S_n²⁻) and thiosulfate (S₂O₃²⁻), derived from the hydrolysis of sulfide minerals. Typical concentration ranges are: 10⁻⁵ – 10⁻⁷ mole/l for hydrogen sulfide, 10⁻⁶ – 10⁻⁹ mole/l for polysulfides and 10⁻⁵ – 10⁻⁸ mole/l for thiosulfate. The chemical reactivity of these species at contact with air makes them difficult to assess unless determined immediately after sampling.These sulfur species can be determined rapidly and accurately in field conditions by simultaneous titration with mercuric chloride employing an Ag/Ag₂S electrode for the determination of the end points.The application to ore exploration is exemplified by the results of the research on roll-type uranium deposits in the southwest of France.     

Reduced sulfur and iron species in anoxic water column of meromictic crater Lake Pavin (Massif Central, France)

The vertical distribution of reduced sulfur species (RSS including H₂S/HS⁻, S⁰, electroactive FeS) and dissolved Fe(II) was studied in the anoxic water column of meromictic Lake Pavin. Sulfide concentrations were determined by two different analytical techniques, i.e. spectophotometry (methylene blue technique) and voltammetry (HMDE electrode). Total sulfide concentrations determined with methylene blue method (∑H₂S_MBRS) were in the range from 0.6 µM to 16.7 µM and were substantially higher than total reduced sulfur species (RSS_V) concentrations determined by voltammetry, which ranged from 0.1 to 5.6 μM. The observed difference in the sulfide concentrations between the two methods can be assigned to the presence of FeS colloidal species.Dissolved Fe was high (> 1000 µM), whereas dissolved Mn was only 25 µM, in the anoxic water column. This indicates that Fe is the dominant metal involved in sulfur redox cycling and precipitation. Consequently, in the anoxic deep layer of Lake Pavin, “free” sulfide, H₂S/HS⁻, was low; and about 80% of total sulfide detected was in the electroactive FeS colloidal form. IAP calculations showed that the Lake Pavin water column is saturated with respect to FeS_am phase. The upper part of monimolimnion layer is characterized by higher concentrations of S(0) (up to 3.4 µM) in comparison to the bottom of the lake. This behavior is probably influenced by sulfide oxidation with Fe(III) oxyhydroxide species.     

Hydrogen from coal gasification: An economical pathway to a sustainable energy future

Although hydrogen is the most abundant element in the universe, it does not occur naturally in large quantities or high concentrations on Earth. Hydrogen must be produced from other compounds such as fossil fuels, biomass, or water and is therefore considered an energy carrier like electricity. Gasification of carbonaceous, hydrogen-containing fuels is an effective method of thermal hydrogen production and is considered to be a key technology in the transition to a hydrogen economy. However, for gasification to play a major role during the transition period, capital and operating cost must be reduced and reliability and performance must be improved.Analyses show that hydrogen produced from coal-based gasification can be competitive with production from natural gas provided the cost of natural gas remains above $4/10⁶ Btu and the high reliability of gasification-based processes can be demonstrated. But for coal to be considered in a carbon-constrained environment, the cost of natural gas would have to be greater than $5.50/10⁶ Btu. The development of advanced technologies, however, offers the potential for significant reductions in capital costs, improved thermal efficiencies, and increased reliability. If these advanced technologies are capable of achieving their goals, the cost of producing hydrogen from coal could be reduced by 25–50%, even with the capture and sequestration of CO₂. With these reductions, the cost of natural gas would have to be less than $2.50/10⁶ Btu to compete, a scenario that is very unlikely to occur in the future. This potential cost reduction provides considerable impetus for continuing research and development in the production of hydrogen from coal.     

Iodine in Chinese coals and its geochemistry during coalification

To determine the I distribution in Chinese coals, a nationwide survey was undertaken based on the distribution, periods of formation, rank and production yields of various coal deposits. A total of 305 coal samples were collected and their I contents were determined by catalytic spectrophotometry with pyrohydrolysis. The geochemistry of I during coalification (including both peat diagenesis and coal metamorphism) was assessed. It was found that the I contents of Chinese coals range from 0.04 mg kg^–1 to 39.5 mg kg^–1 and exhibit a lognormal distribution, with a geometric mean of 1.27 mg kg^–1. Statistical correlation analysis and the observation that I contents increase with coal rank indicate that coal I is chalcophile in nature, and not generally organically bound. When peat developed into lignite through diagenesis, 95–99.9% of the original I was lost. The composition and structure of clay minerals present in the coal were controlled by the original depositional environment. The higher the I content of coals, the more likely the original sediments were affected by a marine environment. Iodine contents increased from lignite through sub-bituminous and bituminous coals to anthracite. This indicates that coal absorbed excess I from hydrothermal fluids during metamorphism (including geothermal metamorphism and telemagmatic metamorphism). The telemagmatic metamorphism was caused by magmatic activities that depended on the specific geological structure of the region. In China, most high-rank coals were formed by telemagmatic metamorphism.     

Carbon isotopes of Middle–Lower Jurassic coal-derived alkane gases from the major basins of northwestern China

Coal-derived hydrocarbons from Middle–Lower Jurassic coal-bearing strata in northwestern China are distributed in the Tarim, Junggar, Qaidam, and Turpan-Harmi basins. The former three basins are dominated by coal-derived gas fields, distributed in Cretaceous and Tertiary strata. Turpan-Harmi basin is characterized by coal-derived oil fields which occur in the coal measures. Based on analysis of gas components and carbon isotopic compositions from these basins, three conclusions are drawn in this contribution: 1) Alkane gases with reservoirs of coal measures have no carbon isotopic reversal, whereas alkane gases with reservoirs not of coal measures the extent of carbon isotopic reversal increases with increasing maturity; 2) Coal-derived alkane gases with high δ¹³C values are found in the Tarim and Qaidam basins (δ¹³C₁: − 19.0 to − 29.9‰; δ¹³C₂: − 18.8 to − 27.1‰), and those with lowest δ¹³C values occur in the Turpan-Harmi and Junggar basins (δ¹³C₁: − 40.1 to − 44.0‰; δ¹³C₂: − 24.7 to − 27.9‰); and 3) Individual specific carbon isotopic compositions of light hydrocarbons (C_5–8) in the coal-derived gases are lower than those in the oil-associated gases. The discovered carbon isotopic reversal of coal-derived gases is caused by isotopic fractionation during migration and secondary alteration. The high and low carbon isotopic values of coal-derived gases in China may have some significance on global natural gas research, especially the low carbon isotope value of methane may provide some information for early thermogenic gases. Coal-derived methane typically has much heavier δ¹³C than that of oil-associated methane, and this can be used for gas–source rock correlation. The heavy carbon isotope of coal-derived ethane is a common phenomenon in China and it shed lights on the discrimination of gas origin. Since most giant gas fields are of coal-derived origin, comparative studies on coal-derived and oil-associated gases have great significance on future natural gas exploration in the world.     

Sulfide petrology of basal chilled margins in layered sills of the Archean Deer Lake Complex,Minnesota

Sulfide minerals in amounts up to 3 vol% are found in basal, chilled marginal zones of two layered peridotite-pyroxenite-gabbro sills in the Early Precambrian Deer Lake Complex, northcentral Minnesota. The sulfides occur interstitially to silicate minerals, and consist of pyrrhotite with minor exsolved cobaltian pentlandite, chalcopyrite, gersdorffite, and marcasite±pyrite as an alteration product of pyrrhotite.The basal chilled units (3–6 m) of the sills are divisable into three zones based primarily on textures. The lowermost zone is an equigranular basalt, whereas the overlying zone is characterized by skeletal, spinifex-like actinolite after clinopyroxene. The upper zone of the basal margins contains elongate and swallow tail plagioclase, and is barren of sulfide minerals.Electron microprobe analyses of sulfide minerals and modal data suggest that sulfide bulk compositions at 1,100–1,000 ° C represent a pyrrhotite solid solution and a coexisting Cu-rich sulfide liquid. Cooling of the Cu-rich liquid and low temperature transformations are thought to have produced chalcopyrite or chalcopyrite plus pyrrhotite. The sulfide minerals have reequlibrated to temperatures near 300 ° C or less.Analyses of sulfur content and ³⁴S values suggest that assimilation of sulfur from adjacent country rocks was the principal mechanism responsible for anomalous concentrations of sulfides in the basal chilled margins. The distribution of sulfides in the peridotite-pyroxenite-gabbro portions of the sills, and calculations of settling rate preclude an origin involving gravitational settling of immiscible droplets through the magma body.     

Simultaneous removal and measurement of sulfide on the basis of turn-on fluorimetry

Hydrogen sulfide is a flammable and poisonous gas pollutant often emitted to air as a by-product of water supply, chemical, petroleum and coal industries. It can be transferred into sulfur dioxide in the air under some meteorologic conditions. Herein, a novel functional copper complex is reported which can selectively absorb hydrogen sulfide and subsequently release a highly fluorescent molecule. The copper complex is functionalized with a benzoxadiazole moiety as a fluorescence reporter, and the copper center serves as the recognition and binding site for sulfide. The application of the copper complex has been demonstrated for sensitive and selective measurement of hydrogen sulfide on the basis of “turn-on” fluorescence method, and the limit of detection is determined to be 0.1 μM. Other relevant anionic ions such as bisulfite, sulfate and mercapto compounds showed no interference for the detection of sulfide. These results suggest that the compound and method can be potentially applied for on-site measurement and effective removal of sulfide from environment.     

Serpentinization of abyssal peridotites from the MARK area, Mid-Atlantic Ridge: sulfur geochemistry and reaction modeling

The opaque mineralogy and the contents and isotope compositions of sulfur in serpentinized peridotites from the MARK (Mid-Atlantic Ridge, Kane Fracture Zone) area were examined to understand the conditions of serpentinization and evaluate this process as a sink for seawater sulfur. The serpentinites contain a sulfur-rich secondary mineral assemblage and have high sulfur contents (up to 1 wt.%) and elevated δ³⁴S_sulfide (3.7 to 12.7‰). Geochemical reaction modeling indicates that seawater-peridotite interaction at 300 to 400°C alone cannot account for both the high sulfur contents and high δ³⁴S_sulfide. These require a multistage reaction with leaching of sulfide from subjacent gabbro during higher temperature (∼400°C) reactions with seawater and subsequent deposition of sulfide during serpentinization of peridotite at ∼300°C. Serpentinization produces highly reducing conditions and significant amounts of H₂ and results in the partial reduction of seawater carbonate to methane. The latter is documented by formation of carbonate veins enriched in ¹³C (up to 4.5‰) at temperatures above 250°C. Although different processes produce variable sulfur isotope effects in other oceanic serpentinites, sulfur is consistently added to abyssal peridotites during serpentinization. Data for serpentinites drilled and dredged from oceanic crust and from ophiolites indicate that oceanic peridotites are a sink for up to 0.4 to 6.0 × 10¹² g seawater S yr⁻¹. This is comparable to sulfur exchange that occurs in hydrothermal systems in mafic oceanic crust at midocean ridges and on ridge flanks and amounts to 2 to 30% of the riverine sulfate source and sedimentary sulfide sink in the oceans. The high concentrations and modified isotope compositions of sulfur in serpentinites could be important for mantle metasomatism during subduction of crust generated at slow spreading rates.     

Mechanisms of sulfur introduction chemically controlled: δS imprint

Organic sulfur in marine sediment is ³⁴S enriched relative to the co-existing pyrite. This phenomenon is still enigmatic. Timing of the sulfur incorporation, immobilization and different sulfur species involved are part of the explanations. The reduced sulfur species incorporation into organic matter (OM) is generally assumed to have negligible δ³⁴S fractionation. This assumption has never been confirmed by laboratory experimental data. The present study measures the δ³⁴S changes resulting from reduced sulfur species (sulfides and polysulfide anions) incorporation into organic model compounds in an aquatic and low temperature (25 °C) system that simulates diagenetic marine environment. In addition, we also investigate the δ³⁴S fractionation and the isotope chemical mixing in the formation of polysulfide anions produced from elemental sulfur and sulfide anions. The results showed total isotope mixing between the two species in the formation of polysulfides. Acidification of the polysulfides solution caused δ³⁴S fractionation between the released elemental sulfur and H₂S. The incorporation of polysulfides and sulfides into carbonyl groups, caused ³⁴S enrichment relative to the starting polysulfides and sulfide of 4–5‰. The ³⁴S enrichment of the sulfurized carbonyl groups showed a minimal effect by temperature (0–70 °C) and is not affected by salinity, polysulfides composition, reaction time or solubility in water. The incorporation of polysulfides and sulfides into brominated organic compounds was negligibly ³⁴S enriched. The chemical mechanisms controlling the polysulfides incorporation into OM depend mostly on the functional groups and determine the ³⁴S enrichment of the sulfurized OM. The results presented in this study can explain part of the difference between pyrite δ³⁴S and sulfurized OM δ³⁴S in natural marine sediments.     

Sources of environmental sulfur in the groundwater system,southern New Zealand

Sulfide minerals commonly occur in sediments and basement rocks in southern New Zealand, as authigenic precipitates from groundwater below the oxygenated surface zone. There are two principal potential sources for sulfur in the groundwater system: weathering of sulfide minerals in the metamorphic basement and rainwater-derived marine aerosols. We present data for these two key sulfur sources: metamorphic sulfide and associated hydrothermal Au-bearing veins within the Otago Schist (average δ³⁴S = −1.8 ± 2.4‰), and an inland saline lake (S derived entirely from rainwater, δ³⁴S = 21.4 ± 0.8‰). We use these two end member δ³⁴S values to estimate the contributions of these sources of sulfur in authigenic groundwater sulfide minerals and in waters derived from oxidation of these sulfide minerals, across a range of environments. We show that authigenic groundwater pyrite along joints in the Otago schist is derived primarily from metamorphic basement sulfur. In contrast, authigenic groundwater pyrite cementing Miocene-Recent aquifers shows a substantial marine aerosol component, and represents a distinct hydrogeological system. We suggest that marine aerosols represent a significant flux to the terrestrial sulfur cycle that has been present through the groundwater system in Otago over the past 20 million years.     

Sulfur cycling in a stratified euxinic lake with moderately high sulfate: Constraints from quadruple S isotopes

We present a 3-year study of concentrations and sulfur isotope values (δ³⁴S, Δ³³S, and Δ³⁶S) of sulfur compounds in the water column of Fayetteville Green Lake (NY, USA), a stratified (meromictic) euxinic lake with moderately high sulfate concentrations (12-16 mM). We utilize our results along with numerical models (including transport within the lake) to identify and quantify the major biological and abiotic processes contributing to sulfur cycling in the system. The isotope values of sulfide and zero-valent sulfur across the redox-interface (chemocline) change seasonally in response to changes in sulfide oxidation processes. In the fall, sulfide oxidation occurs primarily via abiotic reaction with oxygen, as reflected by an increase in sulfide δ³⁴S at the redox interface. Interestingly, S isotope values for zero-valent sulfur sampled at this time still reflect production and recycling by phototrophic S-oxidation. In the spring, sulfide S isotope values suggest an increased input from phototrophic oxidation, consistent with a more pronounced phototroph population at the chemocline. This trend is associated with smaller fractionations between sulfide and zero-valent sulfur, suggesting a metabolic rate control on fractionation similar to that for sulfate reduction. Comparison of our data with previous studies indicates that the S isotope values of sulfate and sulfide in the deep waters are remarkably stable over long periods of time, with consistently large fractionations of up to 58‰ in δ³⁴S. Models of the δ³⁴S and Δ³³S trends in the deep waters (considering mass transport via diffusion and advection along with biological processes) require that these fractionations are a consequence of sulfur compound disproportionation at and below the redox interface in addition to large fractionations during sulfate reduction. The large fractionations during sulfate reduction appear to be a consequence of the high sulfate concentrations and the distribution of organic matter in the water column. The occurrence of disproportionation in the lake is supported by profiles of intermediate sulfur compounds and by lake microbiology, but is not evident from the δ³⁴S trends alone. These results illustrate the utility of including minor S isotopes in sulfur isotope studies to unravel complex sulfur cycling in natural systems.     

Time dependence of organic matter decay and mixing processes in Framvaren, a permanently anoxic fjord in South Norway

Three different layers have been identified in Framvaren, which has a maximum water depth of 184 m. One oxic layer above the redoxcline at 18–20 m. One anoxic layer from 20 to 100 m which is occasionally ventilated by a flow over the sill (which has a depth of 2.5 m), and finally a stagnant layer below 100 m. Using the release rate of silica from the bottom and measurements of the concentration of HTO it is possible to make some calculations on the annual volume of interleaving in the layers 25–50 m, 50–75 m, and 75–100 m together with the advective flows. Reliable values of the sulfide concentration were obtained by precipitating and weighing HgS together with careful protection of all anoxic water samples with argon. The light yellow color of the precipitate in the depth range 25 to 80 m indicates that the occasional ventilation will cause such reactions as 0.50₂ + H₂S S(colloidal) + H₂O. The elemental sulfur, being stabilized with HS^–, is set free upon the precipitation of HgS. The new data for the concentration of sulfide give an acceptable stoichiometry for the decay reaction of organic matter. This is not the case with the data of Yao and Millero. The mean values for the concentrations of ammonium and phosphate agree with the new data of Yao and Millero. The mol/mol C/N ratio of 10.1 found in trapped material by Naess and coworkers (1988) agrees with the stoichiometry of the dissolved constituents, i.e. C/N = 9.92 ± 0.45. A denitrification reaction is suggested to explain the high values of C/N. The vertical diffusion coefficient at 100 m calculated from the depth profile of silica was 0.92 × 10^–6 m² s^–1 which lies in the range of values given by Fröyland. Finally, the ¹⁴C age of the total dissolved inorganic carbon (Ct) in the water below 90 m was about 1600 years indicating a bioproduction in the period 8000 years B.P. to A.D. 1853 when a channel was opened between the fjord outside (Helvikfjord) and Framvaren.     

A new view on gold speciation in sulfur-bearing hydrothermal fluids from in situ X-ray absorption spectroscopy and quantum-chemical modeling

Despite the common belief that Au^I complexes with hydrogen sulfide ligands (H₂S/HS⁻) are the major carriers of gold in natural hydrothermal fluids, their identity, structure and stability are still subjects of debate. Here we present the first in situ measurement, using X-ray absorption fine structure (XAFS) spectroscopy, of the stability and structure of aqueous Au^I–S complexes at temperatures and pressures (T–P) typical of natural sulfur-rich ore-forming fluids. The solubility of native gold and the local atomic structure around the dissolved metal in S–NaOH–Na₂SO₄–H₂SO₄ aqueous solutions were characterized at temperatures 200–450 °C and pressures 300–600 bar using an X-ray cell that allows simultaneous measurement of the absolute concentration of the absorbing atom (Au) and its local atomic environment in the fluid phase. Structural and solubility data obtained from XAFS spectra, combined with quantum-chemical calculations of species geometries, show that gold bis(hydrogensulfide) Au(HS)₂⁻ is the dominant Au species in neutral-to-basic solutions (5.5  pH  8.5; H₂O–S–NaOH) over a wide range of sulfur concentrations (0.2 < ΣS < 3.6 mol/kg), in agreement with previous solubility studies. Our results provide the first direct determination of this species structure, in which two sulfur atoms are in a linear geometry around Au^I at an average distance of 2.29 ± 0.01 Å. At acidic conditions (1.5  pH  5.0; H₂O–S–Na₂SO₄–H₂SO₄), the Au atomic environment determined by XAFS is similar to that in neutral solutions. These findings, together with measured high Au solubilities, are inconsistent with the predominance of the gold hydrogensulfide Au(HS)⁰ complex suggested by recent solubility studies. Our spectroscopic data and quantum-chemical calculations imply the formation of species composed of linear S–Au–S moieties, like the neutral [H₂S–Au–SH] complex. This species may account for the elevated Au solubilities in acidic fluids and vapors with H₂S concentrations higher than 0.1–0.2 mol/kg. However, because of the complex sulfur speciation in acidic solutions that involves sulfite, thiosulfate and polysulfide species, the formation of Au^I complexes with these ligands (e.g., AuHS(SO₂)⁰, Au(HS₂O₃)₂⁻, Au(HS_n)₂⁻) cannot be ruled out. The existence of such species may significantly enhance Au transport by high T–P acidic ore-forming fluids and vapors, responsible for the formation of a major part of the gold resources on Earth.     

The stable isotope geochemistry of Australian coals

D/H, ¹³C/¹²C, ¹⁸O/¹⁶O and ³⁴S/³²S ratios in the organic matrix and organic solvent extracts of Australian coals, and in the fluids and minerals associated with these coals, are reported and reviewed against similar isotopic data for coals from other regions.Where coals are immature, original isotopic differences between macrolithotypes, and between solvent extracts (lipid concentrates) and insoluble residues, are largely preserved. However, with increasing maturity these characteristic differences, particularly those between macrolithotypes, are rapidly erased. Conversely, where, as indicated by low total sulfur contents, coals of Cretaceous to Permian age were deposited under essentially freshwater conditions, δ³⁴S values^* for the organically-bound sulfur remain remarkably constant at +4 ± 3‰ relative to meteoritic sulfur. In similar, younger Tertiary coals, the organic sulfur is markedly enriched in ³⁴S.Five distinctive isotopic patterns, which may be interpreted in terms of the environment of sulfate reduction, can be recognized from ³⁴S/³²S ratio measurements on the various forms of sulfur in Australian coals.Isotopic studies of seam gas hydrocarbons collected in situ show these to be unexpectedly strongly depleted in the heavier isotopes of hydrogen and carbon relative to natural gases from proposed humic sources. Furthermore, no pronounced increase in the ¹³C content in methane with increase in rank of the parent coal was observed. In addition, several sources of associated carbon dioxide have been delineated, including normal maturation processes, invasion of the seams by magnetic carbon dioxide, and interaction of the coal with intrusive magma.Isotopic exchange between free seam gases is not accepted as an explanation for some unusual isotopic fractionations seen, rather the data suggest that these gases may be formed in a state approaching isotopic equilibrium. This argument also satisfactorily explains the isotopic compositions of primary siderite and secondary calcite associated with bituminous coal seams. However, where seams are invaded and permeated with externally derived carbon dioxide, usually of magnetic origin, carbonates are frequently absent, presumably as a result of the action of carbonic acid.     

Electron accepting capacity of dissolved organic matter as determined by reaction with metallic zinc

Information about the chemical electron accepting capacity (EAC) of dissolved organic matter (DOM) is scarce owing to a lack of applicable methods. We quantified the electron transfer from metallic Zn to natural DOM in batch experiments at DOC concentrations of 10–100 mg-C L^− 1 and related it to spectroscopic information obtained from UV-, synchronous fluorescence, and FTIR- spectroscopy. The electron donating capacity of DOM and pre-reduced DOM was investigated using Fe(CN)₆³⁻ as electron acceptor. Presence of DOM resulted in release of dissolved Zn, consumption of protons, and slower release of hydrogen compared to reaction of metallic Zn with water at pH 6.5. Comparison with reaction stoichiometry confirmed that DOM accepted electrons from metallic Zn. The release of dissolved Zn was dependent on pH, DOC concentration, ionic strength, and organic matter properties. The reaction appeared to be completed within about 24 h and was characterized by pseudo first order kinetics with rate constants of 0.5 to 0.8 h^− 1. EAC per mass unit of carbon ranged from 0.22 mmol g^− 1 C to 12.6 mmol g^− 1 C. Depending on the DOM, a calculated 28–127% of the electrons transferred from metallic Zn to DOM could be subsequently donated to Fe(CN)₆³⁻. EAC decreased with DOC concentration, and increased with aromaticity, carboxyl, and phenolic content of the DOM. The results indicate that an operationally defined EAC of natural DOM can be quantified by reaction with metallic Zn and that DOM properties control the electron transfer. Shortcomings of the method are the coagulation and precipitation of DOM during the experiment and the production of hydrogen and dissolved Zn by reaction of metallic Zn with water, which may influence the determined EAC.     

Hydrothermal venting activities in the Early Cambrian,South China: Petrological,geochronological and stable isotopic constraints

Multi-episodic (three episodes at least) hydrothermal silica chimneys are identified in the black chert successions (up to ～ 100 m thick locally), overlain by thick black shales (up to a few 100 m thick), in the Ediacaran–Cambrian boundary successions along the southern marginal zone of the Yangtze Platform, South China. Their occurrences are constrained within the lowermost Cambrian by the SHRIMP U–Pb zircon dating and stable isotopic chemostratigaphic data, and are stratigraphically coincident with the negative isotopic excursions of organic carbon (～ 10‰ VPDB in magnitude) and sulfide sulfur (between 0 and 10‰ CDT), i.e., in bases of Nemakit–Daldynian and Tommotian stages in the Lower Cambrian. In this case, a causal link could have existed between which the hydrothermal venting could have released huge amounts of reduced silica-rich hydrothermal fluids with abundant ¹³C-depleted greenhouse gases (methane) and volcanic-originated H₂S into the ocean and/or atmosphere, resulting in an extreme warm climate and oceanic anoxia, oceanic chemical perturbation, and subsequent massive precipitation of silica (chert), and large-magnitude negative isotopic excursions of organic carbon and sulfide sulfur. This scenario provides a unique chance to explore the causes of apparent tectono-depositional, oceanic and geochemical perturbations during this critical interval in South China and elsewhere.