Interactions between Volatile Reduced Sulfur Compounds and Metals in the Seine Estuary (France)

Concentrations of volatile hydrogen sulfide (H₂S), carbonyl sulfide (OCS), methane thiol (MeSH), dimethyl sulfide (DMS), and dimethyl disulfide (DMDS) in the Seine estuary (France) were investigated in spring 2005 using samples collected from the first meter beneath the surface. Levels of dissolved metals (Ag, Cd, Cu, Zn, Ni, Co, Pb), suspended particulate matter, and particular organic carbon were also assessed. Maximum concentrations were 0.80 nM for H₂S, 0.64 nM for OCS, 3.06 nM for MeSH, 11.06 nM for DMS, and 1.18 nM for DMDS, and different features were observed for the five volatile reduced sulfur compounds (VRSCs). Experiments were conducted to determine silver–VRSC conditional stability constants. Major interactions were observed between H₂S and Ag, which may partially account for the absence of volatile H₂S at the lowest salinities. OCS, MeSH, and DMS did not interact with Ag, as revealed by their insignificant K′ values. Variations in concentrations along the salinity gradient may be explained by the influence of phytoplanktonic compounds.     

Fe-Ni-Co-O-S Phase Relations in Peridotite-Seawater Interactions

Serpentinization of abyssal peridotites is known to produceextremely reducing conditions as a result of dihydrogen (H₂,aq)release upon oxidation of ferrous iron in primary phases toferric iron in secondary minerals by H₂O. We have compiled andevaluated thermodynamic data for Fe–Ni–Co–O–Sphases and computed phase relations in fO₂,g–fS₂,g andaH₂,aq–aH₂S,aq diagrams for temperatures between 150 and400°C at 50 MPa. We use the relations and compositions ofFe–Ni–Co–O–S phases to trace changesin oxygen and sulfur fugacities during progressive serpentinizationand steatitization of peridotites from the Mid-Atlantic Ridgein the 15°20'N Fracture Zone area (Ocean Drilling ProgramLeg 209). Petrographic observations suggest a systematic changefrom awaruite–magnetite–pentlandite and heazlewoodite–magnetite–pentlanditeassemblages forming in the early stages of serpentinizationto millerite–pyrite–polydymite-dominated assemblagesin steatized rocks. Awaruite is observed in all brucite-bearingpartly serpentinized rocks. Apparently, buffering of silicaactivities to low values by the presence of brucite facilitatesthe formation of large amounts of hydrogen, which leads to theformation of awaruite. Associated with the prominent desulfurizationof pentlandite, sulfide is removed from the rock during theinitial stage of serpentinization. In contrast, steatitizationindicates increased silica activities and that high-sulfur-fugacitysulfides, such as polydymite and pyrite–vaesite solidsolution, form as the reducing capacity of the peridotite isexhausted and H₂ activities drop. Under these conditions, sulfideswill not desulfurize but precipitate and the sulfur contentof the rock increases. The co-evolution of fO₂,g–fS₂,gin the system follows an isopotential of H₂S,aq, indicatingthat H₂S in vent fluids is buffered. In contrast, H₂ in ventfluids is not buffered by Fe–Ni–Co–O–Sphases, which merely monitor the evolution of H₂ activitiesin the fluids in the course of progressive rock alteration.The co-occurrence of pentlandite–awaruite–magnetiteindicates H₂,aq activities in the interacting fluids near thestability limit of water. The presence of a hydrogen gas phasewould add to the catalyzing capacity of awaruite and would facilitatethe abiotic formation of organic compounds. KEY WORDS: serpentinization; ODP Expedition 209; sulfide; oxygen fugacity; sulfur fugacity; hydrothermal system; metasomatism; Mid-Atlantic Ridge     

Hydrothermal fluid evolution and structural control of the Guarim gold mineralisation, Tapajós Province, Amazonian Craton, Brazil

Fluid inclusion and structural studies were carried out at the Guarim gold deposit in the Palaeoproterozoic Tapajós province of the Amazonian craton. Guarim is a fault-hosted gold deposit cutting basement granitoids. It consists of a quartz vein, which is massive in its inner portions, grading laterally either to a massive or to cavity-bearing quartz vein associated with hydrothermal breccias. The wallrock alteration comprises chlorite, carbonate, white mica and sulphide minerals, with free gold occurring within quartz grains and spatially associated with sulphide mineral grains. Petrographic, microthermometric and Laser Raman investigations recognised CO₂-rich, mixed H₂O–CO₂, and H₂O fluid inclusions. The coexisting CO₂ and H₂O–CO₂ inclusions were interpreted as primary immiscible fluids that formed the gold-bearing vein. The H₂O inclusions were considered a product of later infiltration of fluids unrelated to the mineralising episode. The mineralising fluid has CO₂ ranging typically from 5–10 mol%, contains traces of N₂, has salinities of ∼5 wt% NaCl equiv., and densities varying between 0.85 and 0.95 g/cm³. The P–T estimations bracket gold deposition between 270–320 °C and 0.86–2.9 kb; ƒ_O2–ƒ_S2–pH estimates suggest a reduced, near-neutral character for the fluid. Variations in the physico-chemical properties, as demonstrated by the fluid inclusion study, resulted from a combination of fluid immiscibility and pressure fluctuation. This interpretation, combined with textural and structural evidence, suggests the emplacement of the mineralised vein in an active fault and at a rather shallow level (4–7 km). The geological and structural setting, deposit-scale textures and structures, wallrock alteration and physico-chemical fluid properties are compatible with those of epizonal to mesozonal orogenic lode gold deposits. Received: 3 March 2000 / Accepted: 21 October 2000     

Abiotic formation of hydrocarbons under hydrothermal conditions: Constraints from chemical and isotope data

To understand reaction pathways and isotope systematics during mineral-catalyzed abiotic synthesis of hydrocarbons under hydrothermal conditions, experiments involving magnetite and CO₂ and H₂-bearing aqueous fluids were conducted at 400 °C and 500 bars. A robust technique for sample storage and transfer from experimental apparatus to stable isotope mass spectrometer provides a methodology for integration of both carbon and hydrogen isotope characterization of reactants and products generated during abiogenic synthesis experiments. Experiments were performed with and without pretreatment of magnetite to remove background carbon associated with the mineral catalyst. Prior to experiments, the abundance and carbon isotope composition of all carbon-bearing components were determined. Time-series samples of the fluid from all experiments indicated significant concentrations of dissolved CO and C₁-C₃ hydrocarbons and relatively large changes in dissolved CO₂ and H₂ concentrations, consistent with formation of additional hydrocarbon components beyond C₃. The existence of relatively high dissolved alkanes in the experiment involving non-pretreated magnetite in particular, suggests a complex catalytic process, likely involving reinforcing effects of mineral-derived carbon with newly synthesized hydrocarbons at the magnetite surface. Similar reactions may be important mechanisms for carbon reduction in chemically complex natural hydrothermal systems. In spite of evidence supporting abiotic hydrocarbon formation in all experiments, an “isotopic reversal” trend was not observed for ¹³C values of dissolved alkanes with increasing carbon number. This may relate to the specific mechanism of carbon reduction and hydrocarbon chain growth under hydrothermal conditions at elevated temperatures and pressures. Over time, significant ¹³C depletion in CH₄ suggests either depolymerization reactions occurring in addition to synthesis, or reactions between the C₁-C₃ hydrocarbons and carbon species absorbed on mineral surfaces and in solution.     

RETRACTED ARTICLE: The hydrochemical framework of surface water basins in southern Ghana

Surface water resources play a crucial role in the domestic water delivery system in Ghana. In addition, sustainable food production is based on the quality and quantity of water resources available for irrigation purposes to supplement rain-fed agricultural activities in the country. The objective of this research was to determine the main controls on the hydrochemistry of surface water resources in the southern part of Ghana and assess the quality of water from these basins for irrigation activities in the area. R-mode factor and cluster analyses were applied to 625 data points from 6 river basins in southern Ghana after the data had been log transformed and standardized for homogeneity. This study finds that surface water chemistry in the south is controlled by the chemistry of silicate mineral weathering, chemistry of rainfall, fertilizers from agricultural activities in the area, as well as the weathering of carbonate minerals. A Gibb’s diagram plotted with total dissolved solids (TDS) on the vertical axis against (Na⁺ + K⁺)/(Ca²⁺ + K⁺ + Na⁺) on the horizontal axis indicates that rock weathering plays a significant role in the hydrochemistry. Activity diagrams for the CaO–Na₂O–Al₂O–SiO₂–H₂O and CaO–MgO–Al₂O₃–SiO₂–H₂O systems suggest that kaolinite is the most stable clay mineral phase in the system. In addition, an assessment of the irrigation quality of water from these basins suggests that the basins are largely low sodium—low to medium salinity basins, delivering water of acceptable quality for irrigation purposes.     

Melting of albite and dehydration of brucite in H2O–NaCl fluids to 9 kbars and 700–900°C: implications for partial melting and water activities during high pressure metamorphism

 The melting reaction: albite_(solid)+ H₂O_(fluid) =albite-H₂O_(melt) has been determined in the presence of H₂O–NaCl fluids at 5 and 9.2 kbar, and results compared with those obtained in presence of H₂O–CO₂ fluids. To a good approximation, albite melts congruently at 9 kbar, indicating that the melting temperature at constant pressure is principally determined by water activity. At 5 kbar, the temperature (T)- mole fraction (X _(H2O) ) melting relations in the two systems are almost coincident. By contrast, H₂O–NaCl mixing at 9 kbar is quite non-ideal; albite melts ∼70 ^°C higher in H₂O–NaCl brines than in H₂O–CO₂ fluids for X _(H2O) =0.8 and ∼100 ^°C higher for X _(H2O) =0.5. The melting temperature of albite in H₂O–NaCl fluids of X _(H2O)=0.8 is ∼100 ^°C higher than in pure water. The P–T curves for albite melting at constant H₂O–NaCl show a temperature minimum at about 5 kbar. Water activities in H₂O–NaCl fluids calculated from these results, from new experimental data on the dehydration of brucite in presence of H₂O–NaCl fluid at 9 kbar, and from previously published experimental data, indicate a large decrease with increasing fluid pressure at pressures up to 10 kbar. Aqueous brines with dissolved chloride salt contents comparable to those of real crustal fluids provide a mechanism for reducing water activities, buffering and limiting crustal melting, and generating anhydrous mineral assemblages during deep crustal metamorphism in the granulite facies and in subduction-related metamorphism. Low water activity in high pressure-temperature metamorphic mineral assemblages is not necessarily a criterion of fluid absence or melting, but may be due to the presence of low a _(H2O) brines. Received: 17 March 1995/Accepted: 9 April 1996     

CO<Subscript>2</Subscript>-rich fluid inclusions with chalcopyrite daughter mineral from the Fenghuangshan Cu–Fe–Au deposit,China: implications for metal transport in vapor

CO₂-rich fluid inclusions containing opaque mineral crystals were found in the Fenghuangshan skarn-porphyry Cu–Fe–Au deposit in Tongling, Anhui, China. These inclusions show variable CO₂ contents and are accompanied by aqueous inclusions, both occurring as secondary inclusions in quartz and being locally associated with chalcopyrite mineralization. Laser Raman microspectroscopic analyses confirm the predominance of CO₂ in the vapor and the presence of H₂S as high as 8 mol%, and identify the opaque mineral with yellow reflectance color in the inclusions as chalcopyrite. More than half of the CO₂-bearing inclusions contains chalcopyrite, whereas few of the associated aqueous inclusions do so. The chalcopyrite, occupying less than 1% (volume) of the inclusions, is interpreted to be a daughter mineral, and calculated Cu concentrations in the inclusions range from 0.1 to 3.4 wt%. Copper is inferred to have been transported in CO₂-dominated fluids as HS⁻ complexes. The occurrence of chalcopyrite daughter crystals in CO₂-rich fluid inclusions indicates that CO₂-rich vapor has the capacity of transporting large amounts of Cu, and possibly Au. This finding has significant implications for metal transport and mineralization in hydrothermal systems enriched in CO₂, such as orogenic-type and granitic intrusion-related gold deposits.     

Microbial Diversity in Hydrothermal Systems and Their Influence on Geological Environments

Abstract: Microorganisms found at terrestrial and seafloor hydrothermal systems are classified based on their optimum growth temperatures. Prokaryotes exhibiting optimum growth temperatures above 45C are defined as thermophiles, while those with optimum growth temperatures above 80C are called hyperthermophiles. Hyperthermophiles have chemically and thermodynamically stable cytoplasmic‐membrane consisting of ether–lipids. Hyperthermophiles observed at temperature up to 113C in seafloor hydrothermal areas should have thermostable proteins. Presumably, the thermostable proteins are replacing certain amino acid residue by proline instead of alanine in order to increase hydrogen bonds and salt bridges. Microorganisms inhabiting terrestrial and seafloor hydrothermal areas obtain energy mainly by use of chemical disequilibrium, so that their habitat may be restricted to the redox boundary between ambient water and reduced hydrothermal fluids. Among more than 200 species of cultivable thermophiles and hyperthermophiles, hyperthermophiles mainly obtain energy by sulfur reduction, because this reaction can produce energy efficiently at > 80C. Some hyperthermophilic sulfur reducers can reduce Cr(VI), Mn(IV), Fe(III), Co(III), As(V), Se(VI), Tc(VII), Au(III) and U(VI) through hydrogen or organic compounds that act as reducing agents for yielding energy. Some hyperther‐mophiles use W instead of Mo for activating their enzymes. Bacteria can pump out toxic metal ions from their cells, however toxic metal resistance systems of archaea, including hyperthermophiles, have not yet been studied. Methane producing hyperthermophiles can yield energy by the autotrophic reaction, when sufficient carbon dioxide and molecular hydrogen can be supplied and methane can be removed from hydrothermal systems. If nitrogen can be supplied into a hydrothermal system, hydrogen oxidization by nitrogen species (e.g. NO₃^‐, NO₂^‐, NO and N₂O) may be expected through the metabolism of some hyperthermophiles.     

Thermodynamic modelling of a cooling C–O–H fluid–graphite system: implications for hydrothermal graphite precipitation

Carbon-saturated crustal fluids in the C–O–H system comprise H₂O, CO₂ and CH₄ as the most important fluid species. Graphite precipitation from a cooling C–O–H is discussed for two different systems, namely for a fluid–rock system in which no transfer of atomic oxygen and hydrogen between the fluid and the rock is possible (closed fluid system), and for an open fluid system. Thermodynamic model calculations show that the graphite-forming reactions and the graphite precipitation potential are different for these two systems. Furthermore, the calculations demonstrate that for both systems, the following factors play a role in determining the graphite precipitation potential, i.e. (1) the redox state of the fluid, (2) the initial pressure and temperature conditions and (3) whether cooling is combined with decompression. Open and closed fluid system graphite precipitation can be distinguished from each other using fluid inclusion and stable carbon isotope studies. The results of this study provide insight in the formation of hydrothermal graphite deposits.     

Stable isotope geochemistry and diagenetic mineralization associated with the Tono sandstone-type uranium deposit in Japan

The Tono sandstone-type uranium mine area, middle Honsyu, Japan is composed of Miocene lacustrine sedimentary rocks in the lower part (18–22 Ma) and marine facies in the upper part (15–16 Ma). Calcite and pyrite occur as dominant diagenetic alteration products in these Neogene sedimentary rocks. The characteristics of calcite and pyrite differ significantly between lacustrine and marine facies. Abundant pyrite, calcite, organic matter, and small amounts of marcasite or pyrrhotite occur in the lacustrine facies, whereas small amounts of calcite and framboidal pyrite, organic matter and no marcasite or pyrrhotite are found within the marine units. The δ¹³C values of calcite in the lacustrine deposits are low (−19 to −6‰ PDB) but those in marine formation are high (−11 to +3‰). This implies that the contribution of marine carbonate is larger in upper marine sedimentary rocks, and carbon in calcite in the lower lacustrine formation was derived both from oxidation of organic matter and from dissolved marine inorganic carbon. The δ³⁴S values of framboidal pyrite in the upper marine formation are low (−14 to −8‰ CDT), indicating a small extent of bacterial seawater sulfate reduction, whereas those of euhedral-subhedral pyrite in the lower lignite-bearing arkose sandstone are high (+10 to +43‰), implying a large extent of closed-system bacterial seawater sulfate reduction. The δ³⁴S and δ¹³C data which deviate from a negative correlation line toward higher δ¹³C values suggest methanogenic CO₂ production. During diagenesis of the lacustrine unit, large amounts of euhedral-subhedral pyrite were formed, facilitated by extensive bacterial reduction of seawater sulfate with concomitant oxidation of organic matter, and by hydrolysis reactions of organic matter, producing CH₄ and CO₂. Uranium minerals (coffinite and uraninite) were also formed at this stage by the reduction of U⁶⁺ to U⁴⁺. The conditions of diagenetic alteration within the lacustrine deposits and uranium mineralization is characterized by low Eh in which nearly equal concentrations of CH₄ and HCO₃ ⁻ existed and reduced sulfur species (H₂S, HS⁻) are predominant among aqueous sulfur species, whereas diagenetic alteration of the marine formations was characterized by a predominance of SO₄ ²⁻ among dissolved sulfur species. Modern groundwater in the lacustrine formation has a low Eh value (−335 mV). Estimated and measured low Eh values of modern and ancient interstitial waters in lacustrine environments indicate that a reducing environment in which U⁴⁺ is stable has been maintained since precipitation of uranium minerals. Received: 9 February 1996 / Accepted: 11 April 1997     

300 Million years of episodic hydrothermal activity: stable isotope evidence from hydrothermal rocks of the Eastern Iberian Central System

The Eastern Iberian Central System has abundant ore showings hosted by a wide variety of hydrothermal rocks; they include Sn-W, Fe and Zn-(W) calcic and magnesian skarns, shear zone- and episyenite-hosted Cu-Zn-Sn-W orebodies, Cu-W-Sn greisens and W-(Sn), base metal and fluorite-barite veins. Systematic dating and fluid inclusion studies show that they can be grouped into several hydrothermal episodes related with the waning Variscan orogeny. The first event was at about 295 Ma followed by younger pulses associated with Early Alpine rifting and extension and dated near 277, 150 and 100 to 20 Ma, respectively (events II–IV). The δ¹⁸O-δD and δ³⁴S studies of hydrothermal rocks have elucidated the hydrological evolution of these systems. The event I fluids are of mixed origin. They are metamorphic fluids (H₂O-CO₂-CH₄-NaCl; δ¹⁸O=4.7 to 9.3‰; δD ab.−34‰) related to W-(Sn) veins and modified meteoric waters in the deep magnesian Sn-W skarns (H₂O-NaCl, 4.5–6.4 wt% NaCl eq.; δ¹⁸O=7.3–7.8‰; δD=−77 to −74‰) and epizonal shallow calcic Zn-(W) and Fe skarns (H₂O-NaCl, <8 wt% NaCl eq.; δ¹⁸O=−0.4 to 3.4‰; δD=−75 to −58‰). They were probably formed by local hydrothermal cells that were spatially and temporally related to the youngest Variscan granites, the metals precipitating by fluid unmixing and fluid-rock reactions. The minor influence of magmatic fluids confirms that the intrusion of these granites was essentially water-undersaturated, as most of the hydrothermal fluids were external to the igneous rocks. The fluids involved in the younger hydrothermal systems (events II–III) are very similar. The waters involved in the formation of episyenites, chlorite-rich greisens, retrograde skarns and phyllic and chlorite-rich alterations in the shear zones show no major chemical or isotopic differences. Interaction of the hydrothermal fluids with the host rocks was the main mechanism of ore formation. The composition (H₂O-NaCl fluids with original salinities below 6.2 wt% NaCl eq.) and the δ¹⁸O (−4.6 to 6.3‰) and δD (−51 to −40‰) values are consistent with a meteoric origin, with a δ¹⁸O-shift caused by the interaction with the, mostly igneous, host rocks. These fluids circulated within regional-scale convective cells and were then channelled along major crustal discontinuities. In these shear zones the more easily altered minerals such as feldspars, actinolite and chlorite had their δ¹⁸O signatures overprinted by low temperature younger events while the quartz inherited the original signature. In the shallower portions of the hydrothermal systems, basement-cover fluorite-barite-base metal veins formed by mixing of these deep fluids with downwards percolating brines. These brines are also interpreted as of meteoric origin (δ¹⁸O< ≈ −4‰; δD=−65 to −36‰) that leached the solutes (salinity >14 wt% NaCl eq.) from evaporites hosted in the post-Variscan sequence. The δD values are very similar to most of those recorded by Kelly and Rye in Panasqueira and confirm that the Upper Paleozoic meteoric waters in central Iberia had very negative δD values (≤−52‰) whereas those of Early Mesozoic age ranged between −65 and −36‰. Received: 9 June 1999 / Accepted: 19 January 2000     

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.     

Experimental investigation of single carbon compounds under hydrothermal conditions

The speciation of carbon in subseafloor hydrothermal systems has direct implications for the maintenance of life in present-day vent ecosystems and possibly the origin of life on early Earth. Carbon monoxide is of particular interest because it represents a key reactant during the abiotic synthesis of reduced carbon compounds via Fischer-Tropsch-type processes. Laboratory experiments were conducted to constrain reactions that regulate the speciation of aqueous single carbon species under hydrothermal conditions and determine kinetic parameters for the oxidation of CO according to the water water-gas shift reaction (CO₂ + H₂ = CO + H₂O). Aqueous fluids containing added CO₂, CO, HCOOH, NaHCO₃, NaHCOO, and H₂ were heated at 150, 200, and 300 °C and 350 bar in flexible-cell hydrothermal apparatus, and the abundances of carbon compounds was monitored as a function of time. Variations in fluid chemistry suggest that the reduction of CO₂ to CH₃OH under aqueous conditions occurs via a stepwise process that involves the formation of HCOOH, CO, and possibly CH₂O, as reaction intermediaries. Kinetic barriers that inhibit the reduction of CH₃OH to CH₄ allow the accumulation of reaction intermediaries in solution at high concentrations regulated by metastable thermodynamic equilibrium. Reaction of CO₂ to CO involves a two-step process in which CO₂ initially undergoes a reduction step to HCOOH which subsequently dehydrates to form CO. Both reactions proceed readily in either direction. A preexponential factor of 1.35 × 10⁶ s⁻¹ and an activation energy of 102 kJ/mol were retrieved from the experimental results for the oxidation of CO to CO₂. Reaction rates amongst single carbon compounds during the experiments suggest that ΣCO₂ (CO₂ + HCO₃⁻ + CO₃²⁻), CO, ΣHCOOH (HCOOH + HCOO⁻), and CH₃OH may reach states of redox-dependent metastable thermodynamic equilibrium in subseafloor and other hydrothermal systems. The abundance of CO under equilibrium conditions is strongly dependent on temperature, the total carbon content of the fluid, and host-rock lithology. If crustal residence times following the mixing of high-temperature hydrothermal fluids with cool seawater are sufficiently long, reequilibration of aqueous carbon can result in the generation of additional reduced carbon species such as HCOOH and CH₃OH, and the consumption of H₂. The present study suggests that abiotic reactions involving aqueous carbon compounds in hydrothermal systems are sufficiently rapid to influence metabolic pathways utilized by organisms that inhabit vent environments.     

Groundwater Evolution and Mineral Alteration Reactions in the Basaltic Rock Sequence of Mt. Wasserkuppe, Germany: A Case Study

Groundwater sampling was accomplished in the basaltic sequence of the Rh?n mountain range, Germany, in order to investigate hydrochemical groundwater evolution and to delineate mineral alteration reactions involved in natural weathering. The hydrochemical compositions of near-surface groundwaters indicate a Ca/Mg–HCO₃ type with near-neutral pH and evolve to a Na–HCO₃ type with high pH at greater depth. Column experiments were performed with basaltic and phonolitic rock samples to determine individual mineral alteration reactions. The basic reactions could be related to the alteration of olivine, Ca-pyroxene, plagioclase, pyrrhotite, and feldspathoids under formation of secondary clay minerals (smectites, illite) and goethite. The mineral alteration reactions deduced from the leaching experiments by inverse modelling were found to be consistent with the mineral reactions associated with the natural groundwaters. The reactions calculated for groundwater evolution involve the alteration of primary and secondary minerals to produce low-T mineral phase. The conversion of secondary Na-beidellite to illite occurs at a later stage of groundwater evolution, reducing the concentrations of K⁺ and Mg²⁺. Near-surface groundwaters do not indicate significant cation exchange. Initial cation exchange requires elevated pH values, with Mg²⁺ removed from solution preferred to Ca²⁺. Na-alkalisation of the groundwaters at greater depth suggests the exchange of Na⁺ for Mg²⁺ and Ca²⁺ on Na-beidellite, supported by cation exchange on coatings of iron hydroxides as alteration products. Among the mature high-pH groundwater at greater depth, the dissolution of anorthite and albite has significant effect on groundwater composition.     

Study on Composition of Inclusions in Minerals and Simulation Experiment on Hydrothermal Metasomatic Process of the Bayan Obo Iron Deposit

On the basis of the mechanism of formation of mineral inclusions, it may be assumed that a certain relation exists between the compositions of fluid inclusions in various minerals formed at the same stage of hydrothermal activity. In order to study the genetic relationships between different minerals in the Bayan Obo iron deposit, the compositions(K~+, Na~+, Ca~+, Mg~+, F~+, Cl~+, CO_2~(2-), ΣSO_4~(2-) and pH) of inclusions in fluorite(23), hematite(13), magnetite(3), sodium pyroxene(2) and dolomite(5) from the main mine and the eastern mine were determined by using the vacuum decrepitation and leaching methods, and cluster analyses of the data on the compostions were made. The Q-mode cluster analysis indicates that some iron oxide minerals in the deposit are related to dolomite of sedimentary origin, while others are related to fluorite and sodium pyroxene--products of hydrothermal activity. The R-mode cluster analysis shows that the components of the leaching solution may be divided into two groups: one includes CO_2~(2-), Mg~(2+) and H~+(pH), which are obviously associated with dolomite; the other comprises Na~+, K~+, Ca~+, F~+, Cl~+ and SO_4~(2-), which may possibly represent the composition of hydrothermal solutions.The reaction of the Na-F-Cl solution(pH 4.72) with hematite dolomite at 300℃ and 5 × 10~7 Pa and under alternately"static and dynamic" conditions produced large amounts of hematite and fluorite and small amounts of smectite and Na(Fe) silicates, and the hematite-fluorite assemblage accords with the actual geological conditions in the deposit. From a comparison between the compositions of"static" and"dynamic" solution samples, it may be known that the flow reaction facilitates the migration of Fe, F, Ca and other components as well as Na-metasomatism(Na and Si are fixed in a solid phase).The study of the compositions of mineral inclusions and simulation experiments on hydrothermal metasomatism have provided new evidence for the hypothesis of metamorphosed-sedimentary and hydrothermal-remoulding origin of the Bayan Obo deposit, and pointed out emphatically that hydrothermal metasomatism plays an important role in the formation of the mineral deposit, particularly in the main and the east mine.     

Sulfur isotope variations from orebody to hand-specimen scale at the Mežica lead–zinc deposit, Slovenia: a predominantly biogenic pattern

The Mississippi Valley-type (MVT) Pb–Zn ore district at Mežica is hosted by Middle to Upper Triassic platform carbonate rocks in the Northern Karavanke/Drau Range geotectonic units of the Eastern Alps, northeastern Slovenia. The mineralization at Mežica covers an area of 64 km² with more than 350 orebodies and numerous galena and sphalerite occurrences, which formed epigenetically, both conformable and discordant to bedding. While knowledge on the style of mineralization has grown considerably, the origin of discordant mineralization is still debated. Sulfur stable isotope analyses of 149 sulfide samples from the different types of orebodies provide new insights on the genesis of these mineralizations and their relationship. Over the whole mining district, sphalerite and galena have δ ³⁴ S values in the range of –24.7 to –1.5‰ VCDT (–13.5 ± 5.0‰) and –24.7 to –1.4‰ (–10.7 ± 5.9‰), respectively. These values are in the range of the main MVT deposits of the Drau Range. All sulfide δ ³⁴ S values are negative within a broad range, with δ ³⁴ S _pyrite <δ ³⁴ S _sphalerite <δ ³⁴ S _galena for both conformable and discordant orebodies, indicating isotopically heterogeneous H₂S in the ore-forming fluids and precipitation of the sulfides at thermodynamic disequilibrium. This clearly supports that the main sulfide sulfur originates from bacterially mediated reduction (BSR) of Middle to Upper Triassic seawater sulfate or evaporite sulfate. Thermochemical sulfate reduction (TSR) by organic compounds contributed a minor amount of ³⁴S-enriched H₂S to the ore fluid. The variations of δ ³⁴ S values of galena and coarse-grained sphalerite at orefield scale are generally larger than the differences observed in single hand specimens. The progressively more negative δ ³⁴ S values with time along the different sphalerite generations are consistent with mixing of different H₂S sources, with a decreasing contribution of H₂S from regional TSR, and an increase from a local H₂S reservoir produced by BSR (i.e., sedimentary biogenic pyrite, organo-sulfur compounds). Galena in discordant ore (–11.9 to –1.7‰; –7.0 ± 2.7‰, n = 12) tends to be depleted in ³⁴ S compared with conformable ore (–24.7 to –2.8‰, –11.7 ± 6.2‰, n = 39). A similar trend is observed from fine-crystalline sphalerite I to coarse open-space filling sphalerite II. Some variation of the sulfide δ ³⁴ S values is attributed to the inherent variability of bacterial sulfate reduction, including metabolic recycling in a locally partially closed system and contribution of H₂S from hydrolysis of biogenic pyrite and thermal cracking of organo-sulfur compounds. The results suggest that the conformable orebodies originated by mixing of hydrothermal saline metal-rich fluid with H₂S-rich pore waters during late burial diagenesis, while the discordant orebodies formed by mobilization of the earlier conformable mineralization.     

Effects of groundwater level fluctuation on its chemical composition in karst soils of Lithuania

Groundwater regime and mineralization process in moraine sandy loam and peat soils of the active sulphatic karst zone (karst processes develop in the Upper Devonian gypsum–dolomites) in Lithuania and the dependence of chemical compounds concentrations on water level fluctuations are reviewed. According to ion sum, groundwater mineralization in peat soil is 1.1–1.3 times higher than in loam soil. Based on this result, lower levels of groundwater predetermine a more intensive mineralization process. A stronger correlation was determined between groundwater levels and concentrations of chemical compounds (Ca²⁺, Mg²⁺, SO₄ ²⁻ and HCO₃ ⁻) enhancing groundwater mineralization. In mineral soil (sandy loam) nitrate (NO₃)⁻ concentration is highly influenced by changing stages of groundwater level as well as by nearby sinkholes.     

Hydrothermal processes in partially serpentinized peridotites from Costa Rica: evidence from native copper and complex sulfide assemblages

Native metals and metal alloys are common in serpentinized ultramafic rocks, generally representing the redox and sulfur conditions during serpentinization. Variably serpentinized peridotites from the Santa Elena Ophiolite in Costa Rica contain an unusual assemblage of Cu-bearing sulfides and native copper. The opaque mineral assemblage consists of pentlandite, magnetite, awaruite, pyrrhotite, heazlewoodite, violarite, smythite and copper-bearing sulfides (Cu-pentlandite, sugakiite [Cu(Fe,Ni)₈S₈], samaniite [Cu₂(Fe,Ni)₇S₈], chalcopyrite, chalcocite, bornite and cubanite), native copper and copper–iron–nickel alloys. Using detailed mineralogical examination, electron microprobe analyses, bulk rock major and trace element geochemistry, and thermodynamic calculations, we discuss two models to explain the formation of the Cu-bearing mineral assemblages: (1) they formed through desulfurization of primary sulfides due to highly reducing and sulfur-depleted conditions during serpentinization or (2) they formed through interaction with a Cu-bearing, higher temperature fluid (350–400 °C) postdating serpentinization, similar to processes in active high-temperature peridotite-hosted hydrothermal systems such as Rainbow and Logatchev. As mass balance calculations cannot entirely explain the extent of the native copper by desulfurization of primary sulfides, we propose that the native copper and Cu sulfides formed by local addition of a hydrothermal fluid that likely interacted with adjacent mafic sequences. We suggest that the peridotites today exposed on Santa Elena preserve the lower section of an ancient hydrothermal system, where conditions were highly reducing and water–rock ratios very low. Thus, the preserved mineral textures and assemblages give a unique insight into hydrothermal processes occurring at depth in peridotite-hosted hydrothermal systems.     

Evidence for a magmatic origin for Carlin-type gold deposits: isotopic composition of sulfur in the Betze-Post-Screamer Deposit, Nevada, USA

We report here new sulfur isotope analyses from the Betze-Post-Screamer deposit, the largest Carlin-type gold deposit in the world. Carlin-type deposits contain high concentrations of arsenic, antimony, mercury, tellurium and other elements of environmental interest, and are surrounded by large volumes of crust in which these elements are also enriched. Uncertainty about the source of sulfur and metals in and around Carlin-type deposits has hampered formulation of models for their origin, which are needed for improved mineral exploration and environmental assessment. Previous studies have concluded that most Carlin-type deposits formed from sulfide sulfur that is largely of sedimentary origin. Most of these studies are based on analyses of mineral separates consisting of pre-ore diagenetic pyrite with thin overgrowths of ore-related arsenian pyrite rather than pure, ore-related pyrite. Our SIMS spot analyses of ore-related pyrite overgrowths in the Screamer zone of the Betze-Post-Screamer deposit yield δ³⁴S values of about −1 to 4‰ with one value of about 7‰. Conventional analyses of realgar and orpiment separates from throughout the deposit yield δ³⁴S values of about 5–7‰ with one value of 10‰ in the Screamer zone. These results, along with results from an earlier SIMS study in the Post zone of the deposit and phase equilibrium constraints, indicate that early arsenian pyrite were formed from fluids of magmatic origin with variable contamination from sulfur in Paleozoic sedimentary rocks. Later arsenic sulfides were formed from solutions to which sulfur of sedimentary origin had been added. The presence of Paleozoic sedimentary sulfur in Carlin-type deposits does not require direct involvement of hydrothermal solutions of sedimentary origin. Instead, it could have been added by magmatic assimilation of Paleozoic sedimentary rocks or by hydrothermal leaching of sulfur from wall rocks to the deposit. Thus, the dominant process delivering sulfur, arsenic, gold and mineralizing fluids to Carlin-type systems and their surrounding country rocks was probably separation of fluids from a magmatic source. Editorial handling: G. Beaudoin     

Groundwater recharge and agricultural contamination

Agriculture has had direct and indirect effects on the rates and compositions of groundwater recharge and aquifer biogeochemistry. Direct effects include dissolution and transport of excess quantities of fertilizers and associated materials and hydrologic alterations related to irrigation and drainage. Some indirect effects include changes in water–rock reactions in soils and aquifers caused by increased concentrations of dissolved oxidants, protons, and major ions. Agricultural activities have directly or indirectly affected the concentrations of a large number of inorganic chemicals in groundwater, for example NO₃ ^–, N₂, Cl, SO₄ ^2–, H⁺, P, C, K, Mg, Ca, Sr, Ba, Ra, and As, as well as a wide variety of pesticides and other organic compounds. For reactive contaminants like NO₃ ^–, a combination of chemical, isotopic, and environmental-tracer analytical approaches might be required to resolve changing inputs from subsequent alterations as causes of concentration gradients in groundwater. Groundwater records derived from multi-component hydrostratigraphic data can be used to quantify recharge rates and residence times of water and dissolved contaminants, document past variations in recharging contaminant loads, and identify natural contaminant-remediation processes. These data indicate that many of the world's surficial aquifers contain transient records of changing agricultural contamination from the last half of the 20th century. The transient agricultural groundwater signal has important implications for long-term trends and spatial heterogeneity in discharge. Electronic Publication