Bacterial Sulfate Reduction Processes Research Articles

Pb-Zn-Ba deposits in the Nigerian Benue Trough: A synthesis on deposits classification and genetic model

The Benue Trough stretching about 800–1000 km with a width ranging between 100 and 150 km, is a megarift structure whose origin was related to the opening of the south Atlantic during the early Cretaceous. This mega rift structure contains economically significant Pb-Zn-Ba-F deposits that form well-defined metallogenic belt about 600 km long in Nigeria. In the Southern Benue Trough (SBT), the Pb-Zn-Ba-F deposits are localized in four districts namely Abakaliki, Ishiagu, Wanikande and Gabu-Oshina and are hosted by carbonaceous shale; in the Central Benue Trough (CBT) the deposit is hosted in the silicified limestone and arkosic sandstone localized in Arufu, Akwana and Azara districts; while in the Northern Benue Trough (NBT) ores deposits occur in the arenaceous sediments of Yolo, Gombe and Gulani districts. Previously published sulfur (S) isotopic data of sulfides suggest that the ore-forming S was sourced from marine sulfates and the reduction of sulfate mainly by thermochemical sulfate reduction process in the CBT and NBT. Whereas in the SBT, bacterial sulfate reduction process is dominant. The compiled C-O isotopic compositions suggest that limestone and slightly metamorphosed sedimentary rocks supplied ore-forming materials to the hydrothermal systems through water/rock interaction. The available bulk Pb isotopic compositions for the ore minerals, the host sedimentary and crystalline rocks comprising the basement rocks as well as the basaltic rocks from the Trough suggest that the arkosic sandstone which is composed of detrital feldspars that was eroded from the Precambrian basement rocks could be the likely source of ore-forming metals. Our model proposed that the ore-forming fluids were sourced from the hot connate-brine that was expelled through compaction during the development of the deep sedimentary basins. Mesozoic magmatic events distributed throughout the basin likely provided the initial thermal flow and elevated the geothermal gradient. The hot brines could have leached the metals from the syn-rift continental formations. The tensional fractures serve as the conduit through which the metals migrated, leached abundant sulphate and precipitated their contents in the transcurrent fault system as sulfides, barite and associated polymetallic ores. The geological, mineralogical, and geochemical characteristics of the Nigerian Benue Trough Pb-Zn deposits are consistent with Mississippi Valley-type (MVT) Pb-Zn deposits in North Africa and the typical MVT ore deposits worldwide.

ГЕОХИМИЯ РАННЕГО ДИАГЕНЕЗА ЛИМНОГЛЯЦИАЛЬНЫХ ОТЛОЖЕНИЙ НА ПРИМЕРЕ ОЗЕР НОРИЛО-ПЯСИНСКОЙ ВОДНОЙ СИСТЕМЫ (РОССИЙСКАЯ АРКТИКА)

For the first time, some features of the early diagenesis of bottom sediments of glacial lakes in the Arctic zone of Russia (Pyasino and Melkoe) were studied using long drilling cores. For the first time, the chemical composition of pore waters for the entire thickness of bottom sediments was determined for the lakes of the Russian Arctic. The enrichment of pore waters with biogenic compounds NO3 –, PO4 3– and dissolved organic carbon (DOC) is shown to be a consequence of microbiological destruction of organic matter (OM), as a result of which the labilest components of OM pass into the pore water, and reducing environment arises in the sediments. An increase in Fe, Mn, Al, As, B, Ba, Co, Mo, Ni, Si, Sr, V, Y concentrations in pore waters was found in comparison with supra-bottom water due to the diagenesis of bottom sediments. The role of OM in changes concerned with mineral composition of sediments and formation of authigenic minerals (pyrite, calcite) was shown. Sediments poor in OM are characterized by low concentrations of reduced forms of S (sulphur in the composition of Fe sulphides, H2S, thiosulphfates, etc.), which indicates a low intensity of bacterial sulfate reduction processes.

The elemental sulfur ore deposit of Salmerón: Las Minas de Hellín basin (Late Miocene, SE Spain)

Strata-bound elemental sulfur deposits occur in different circum-Mediterranean Miocene sedimentary successions containing evaporites and high amounts of organic matter. It is widely known that bacterial sulfate reduction processes are the triggering mechanisms involved in the origin of hydrogen sulfide and the subsequent native sulfur. However, in most of these sedimentary successions, there is controversy over whether elemental sulfur formed in the basin floor, as the same time as the sediments (biosyngenetic), or later, during diagenesis (bioepigenetic). Las Minas de Hellín basin, in the SE Spain, contains one of the largest elemental sulfur deposits in Europe. Based on data recovered from mining company (1903 -1960) and a borehole campaign performed by MINERSA between 1987 and 1988, two native sulfur ore bodies are recognized. The upper sulfur body is hosted by carbonates and diatom-rich levels, whereas the lower sulfur body, only exploited in underground mines now closed, is hosted by gypsum. This work shows a petrological and geochemical study of core samples from the upper sulfur body in the Salmerón area (Murcia), 500 m west of the widely exploited area of Las Minas de Hellín (Albacete). In Salmerón, elemental sulfur occurs as pseudomorphs after primary sedimentary gypsum as well as filling fractures and bed joints. The elemental sulfur replacement is also related to calcification and silicification of the sedimentary biomediated dolomite. The mineralization is considered bioepigenetic formed during early diagenesis. Contribution from hydrothermal waters circulating through adjacent faults are not ruled out.

Desulphurization of FGD wastewater through bacterial sulfate reduction process with agricultural incineration bottom ash as an amendment

Coal-fired power plants produce a large amount of sulfate-laden flue gas desulphurization (FGD) wastewater, while a sustainable amount of incineration ash was produced as solid waste in biomass power plants. Both FGD wastewater and incineration ash led to serious environmental problems without proper treatment. Sulfate reduction was an efficient technology for FGD wastewater treatment, however, the carbon source was insufficient for reducing sulfate in FGD wastewater which needed to be replenished. In this study, pyrolyzed incineration bottom ash (PIA) was introduced to sulfate reducers for FGD wastewater treatment. Sulfate-reducing activity increased (up to 2.9-fold) with PIA300 (incineration ash pyrolyzed under 300 °C) addition. Remarkably increasing abundance of Clostridium (12-fold) and Paraclostridium sp. (79-fold) bacteria were observed in sulfate-reducing sludge with PIA300 addition. Pyrolyzed incineration ash not only acted as a carrier for sulfate reducers, but also provided additional carbon, alkalinity and trace elements (Fe, K, Ca, etc.) for sulfate reduction. Incineration ash was partially degraded by sulfate-reducing bacteria (up to 39.4%), meanwhile, heavy metals (Mg, Al, Cr, Co, Ni, Cu, Zn, etc.) leached from ash and existed in FGD wastewater were efficiently co-removed (95–100%). Addition of PIA to sulfate reducers led to increased formation of extracellular polymeric substances, which might enhance the biotransformation and degradation of organic matters in both ash and FGD wastewater. Thus, a promising FGD wastewater treatment technology was established in this study, to achieve simultaneous FGD wastewater reclaim and solid waste utilization.

Textural and geochemical analysis of celestine and sulfides constrain Sr-(Pb-Zn) mineralization in the Shizilishan deposit, eastern China

Breccia-hosted Sr-(Pb-Zn) mineralization from the Shizilishan deposit (Edong district, eastern China) is located at the contact between a Lower Cretaceous (∼139 Ma) quartz diorite porphyry stock and Triassic evaporite-bearing limestone/dolomite sequences. Backscatter electron imaging, electron probe microanalysis, and laser ablation (multicollector-) inductively coupled-plasma mass spectrometry constrain ore genesis based on the textural evolution and trace element and isotope geochemistry in multiple stages of celestine and sulfides (pyrite and marcasite). Complex growth and/or dissolution and/or recrystallization textures of celestine and superposition of pyrite and marcasite are suggestive of evolving fluid compositions and physicochemical conditions during which pH first decreased (favorable for marcasite precipitation) and then increased, facilitating precipitation of Ba-rich celestine and As-rich pyrite. Sphalerite is characterized by enrichment in structurally-bound Sb, Cu, Ag, As, and Tl. Celestine displays Sr and S isotope signatures comparable to Middle Triassic seawater, indicating derivation of both elements from evaporite sequences. Decreasing δ34S in sulfides through the paragenetic sequence may reflect multiple sulfur sources produced by thermochemical reduction of sulfate and/or bacterial sulfate reduction processes. This research highlights that low to moderate salinity fluids, expected to carry significant concentrations of Sr, are able to form large celestine deposits through extensive water–rock interaction when fluid flow is either protracted, or there are superimposed episodes of fluid introduction with high Sr/Ba ratios and moderate dissolved sulfate. The Shizilishan deposit shares some alteration features and geochemical signatures with low-temperature Carlin-type Au deposits (e.g., relative enrichment in As and Tl in ore-stage pyrite) and Fe-poor MVT-type Pb-Zn ores (e.g., high content of Ga and Ge but low Fe, Mn, and In in sphalerite).

Desulphurization of Fgd Wastewater Through Bacterial Sulfate Reduction Process with Agricultural Incineration Bottom Ash as an Amendment

Desulphurization of Fgd Wastewater Through Bacterial Sulfate Reduction Process with Agricultural Incineration Bottom Ash as an Amendment

Paleo‐thermal constraints on the origin of native diagenetic sulfur in the Messinian evaporites: The Northern Apennines foreland basin case study (Italy)

AbstractRecent studies on the genesis of sedimentary native sulfur deposits indicate diagenetic mid‐low temperature Bacterial Sulfate Reduction (BSR) as the main process, involving organic compounds (kerogen/hydrocarbons), bacterial colonies and gypsiferous rocks. In the peri‐Mediterranean area (Southern Spain, Sicily, Northern Apennines, Israel), the main sulfur accumulations are always associated with late Miocene sulfates and organic‐rich successions encompassing the Messinian salinity crisis (MSC). In particular, the Messinian successions of the Apennine‐Adriatic foreland basin system, due to a large amount of high‐resolution stratigraphic data, represent a perfect case study for understanding the diagenetic conditions controlling the development of the BSR process during sedimentary basin evolution. In this work, thermal models performed in three sub‐basins in a sector of the Northern Apennines comprised of the Sillaro and Marecchia rivers (Italy), calibrated by means of organic and inorganic geothermometers, indicate a general thermal immaturity of the studied successions attained as a result of a constant heat flow similar to the present day one (ca. 40 mW/m2) since Late Tortonian and lithostatic loads between 615 and 1,710 m depending on different sub‐basins. These results suggest that the MSC deposits experienced maximum temperatures between about 39°C and 65°C. Temperatures derived from thermal models have been used to constraint occurrence of the diagenetic BSR associated with evaporitic deposits providing thermal constraints in sulfur genesis as well as new useful thermal‐constraints for basin analysis studies.

Genesis, controls and risk prediction of H2S in coal mine gas

Abnormal H2S concentration in coal mine gas is a serious threat to normal mining activities, which has caused serious loss of life and property in many coal mines. This study explores the genesis and influencing factors of abnormal H2S concentration in coal mine gas, taking the Xishan coal mine in the Fukang mining area as a case study. The H2S formation by bacterial sulfate reduction (BSR) is simulated with a bacterial culture experiment and that by thermochemical sulfate reduction (TSR) is simulated with a thermal reduction experiment. The potential for a magmatic genesis is assessed using data regarding the tectonic evolution and history of magma intrusion in the study area. The factors influencing H2S formation and enrichment are then analyzed by a comprehensive consideration of the characteristics of coal, the gas composition, the coal seam groundwater geochemistry and other geological factors in the study area. The results show that the study area meets the necessary conditions for the BSR process to operate and that there is widespread BSR derived H2S. TSR genesis H2S mainly forms in coal fire areas and their vicinity, while there is little contribution from magmatically formed H2S. The concentration of H2S is negatively correlated with the buried depth of the coal seam, the concentrations of CH4, N2 and CO2, and the ash yield; and it is positively correlated with the volatiles yield and total sulfur content. In addition, in areas with abnormally high H2S concentration, the concentration of SO42− is obviously lower, HCO3− + CO32− concentration is higher, and the HCO3−/SO42− value is larger than that in non-anomalous areas. Geologically, H2S enrichment is found to be controlled by lithology, tectonism, and hydrogeological conditions. Moreover, the results of predictive modeling show that areas prone to abnormal H2S concentration are generally spatially correlated with coal fire areas. In this study, the genetic types of H2S and the factors controlling their formation and retention are discussed, producing research results that have guiding significance for the prediction and prevention of the coal mine disasters that arises from abnormal H2S concentration.

Insight into the mechanisms of denitrification and sulfate reduction coexistence in cascade reservoirs of the Jialing River: Evidence from a multi-isotope approach

The coexistence of denitrification and bacterial sulfate reduction (BSR) processes is commonly observed in natural water systems. However, its formation mechanism remains unclear at a basin scale due to the difficulty of precise identification of these processes. To address this issue, we investigated the spatial-temporal variations in water chemistry and isotopic compositions (e.g., δ13CDIC, δ15NNO3, δ18ONO3, δ34SSO4, and δ18OSO4) in cascade reservoirs (artificial dam lakes) of the Jialing River, SW China in 2016. The results showed that the denitrification and BSR processes coexisted in the studied reservoirs, which was supported by the positive correlation between δ15NNO3 and δ18ONO3 and between δ34SSO4 and δ18OSO4, and by the decreasing concentrations of NO3− and SO42−. Moreover, covariation of Δ13CDIC, Δ15NNO3, and Δ34SSO4 indicated the dominance of heterotrophic denitrification (HD) in the reservoir waters along with the occurrence of bacterial sulfide oxidation (BSO). In addition to SO42− and NO3−, the coexistence of HD and BSR processes were also controlled by the dissolved organic carbon (DOC) in winter and dissolved oxygen (DO) contents in other seasons. Overall, the cumulative effect of cascade reservoirs caused δ15NNO3 and δ34SSO4 to display an upward trend from upstream to downstream in the Jialing River, while δ13CDIC showed an opposite downward trend, which implying that cascade reservoirs may be in favor of the coexistence of the HD and BSR processes. This study therefore concludes that the multi-isotope approach could be a useful technique to ascertain the coexistence mechanism of HD and BSR processes in reservoir water systems.

New insights into the multi-layer metallogenesis of carbonated-hosted epigenetic Pb-Zn deposits: A case study of the Maoping Pb-Zn deposit, South China

The Maoping Devonian and Carboniferous carbonate-hosted epigenetic Pb-Zn deposit, a typical representative of multi-layer (two or more than two ore-bearing strata) Pb-Zn metallogenesis, is located in the central Upper Yangtze Pb-Zn metallogenic province, which consists of >400 carbonate-hosted Pb-Zn deposits (spots) accounting for at least a quarter of total Pb and Zn reserves in China. The Maoping deposit is the second largest one in the province and contains ~30 Mt ores with grades of Pb + Zn ranging from 15 wt% to 35 wt%. A total of three ore bodies (groups) as layers, lens or veinlets are distributed in the NW-trending inverted wing of the Maomaoshan anticline and its secondary NE-trending bedding fracture. Nos. I, II and III ore bodies (groups) are hosted by dolostone of the late Devonian Zaige Fm., early Carboniferous Baizuo Fm., and late Carboniferous Weining Fm., respectively. Sphalerite, galena and pyrite are the most abundant sulfide minerals, while dolomite, calcite, quartz and barite are gangue minerals. These minerals have a massive, disseminated or veined structure, and a granular, metasomatic, co-edge, colloidal, aggregate or fragmentation texture, suggesting an epigenetic origin. NanoSIMS in-situ sulfur isotope analysis shows that the δ34S values of sulfides ranging from −20.4‰ to +25.7‰, of which the pyrite framboid and colloidal sphalerite have δ34S values ranging from −20.4‰ to −8.7‰, and the δ34S values of euhedral granular pyrite and xenomorphic granular sphalerite vary from +22.1‰ to +25.7‰. Such sulfur isotope signatures indicate that the formation of S2- in ore-forming fluids has undergone both marine bacterial sulfate reduction (BSR) and thermochemical sulfate reduction (TSR) processes. The processes of BSR and TSR are chiefly temperature depended, so the reduction of sulfate in the Maoping deposit is most likely to occur at the local and may first undergo a relatively low temperature’s BSR process, then goes through a relatively high temperature’s TSR process. LA-MC-ICPMS in-situ Pb isotope analysis shows that the Pb isotopic ratios of galena are relatively uniform and 206Pb/204Pb = 18.707–18.773, 207Pb/204Pb = 15.780–15.802 and 208Pb/204Pb = 39.375–39.530. Such Pb isotope signatures indicate that the source of Pb is single or well-mixed. After comparing with all the potential metal source rocks, we implied that the metals in the Maoping deposit were mainly derived from wall rocks with a certain contribution from basements and basalts. In addition, the in-situ Pb isotopic ratios increase as altitude increases (from 720 to 760 m), which suggests that the migration of metal-bearing fluids is most likely to go upward, and with the evolution of hydrothermal system, the contribution of radiogenic Pb-depleted basements and basalts was declining. This paper proposed that (i) Pb-Zn ores do not select strata, but are selective for lithology, (ii) various processes are responsible for the ore formation at Maoping, including BSR and TSR, fluids mixing and sulfide rapid precipitation, and (iii) the multi-layer metallogenesis of the Maoping carbonated-hosted epigenetic Pb-Zn deposit is a coupling mineralization of ore-forming fluids, trapped structure systems and favorable lithological combinations, which is different from that of typical Mississippi Valley-type (MVT) deposits.

Selective crystallization and precipitation of authigenic pyrite during diagenesis in uranium reservoir sandbodies in Ordos Basin

The occurrence characteristics of authigenic pyrite during diagenesis in uranium reservoir sandbodies of Zhiluo Formation in Ordos Basin are investigated. Based on outcrop area investigations and drilled cores observations, the pyrite is closely related to carbonaceous debris. Microscopically, the host components for pyrite consist of carbonaceous debris, clay minerals, biotite, the earlier pyrite, and ilmenite by emphasizing on δ34S values via LA-ICP-MS, as well as observing through optical microscope and scanning electron microscope equipped with energy dispersive spectroscope. Three kinds of distribution pattern of pyrite are classified as: (i) periphery; (ii) infilling; (iii) the combined form of the above two. In-situ micrometer-scale sulfur isotope analyses of pyrite demonstrated that broad-scale isotopes are heterogeneous (from −52.5‰ to +35.4‰ of δ34S) and sulfur is derived from more than a single source. Significant differences of host components effects on authigenic pyrite formation are discussed. The carbonaceous debris can supply the energy for the bacterial sulfate reduction process, and the biotite can provide the source of iron for pyrite precipitation. The ferrous ion can be adsorbed on the surface of the clay minerals. Meanwhile, the authigenic pyrite is distributed in or around the earlier pyrite and ilmenite by the indirect adsorption of ferrous ion, and the ilmenite may also provide the source of iron. Futhermore, the uranium minerals are mainly associated with the authigenic pyrite relevant to carbonaceous debris and clay minerals, which indicates there exists constraints of authigenic pyrite during diagenesis on the uranium mineralization.

Genesis and evolution of framboidal pyrite and its implications for the ore-forming process of Carlin-style gold deposits, southwestern China

Southwestern Guizhou, an indispensable component of the “golden triangle” area of southwestern China, is famous for its numerous Carlin-style gold deposits. Unlike other sulfides in the deposits that have been well studied, the genesis of framboidal pyrite is still controversial because of its tiny size. Here, we first determined the chemical and in situ sulfur isotopic compositions of framboidal pyrites from three typical Carlin-style gold deposits (namely Shuiyindong, Taipingdong and Nibao) using secondary electron image (SEI), back scattered electron (BSE), electron probe micro analysis (EPMA), and nanometer-scale secondary ion mass spectroscopy (Nano-SIMS) methods. Our EPMA data show that the framboidal pyrites contain a minor amount of trace elements (Ni < 0.344 wt%, Co = 0.017–0.295 wt%, Cr = 0.009–0.292 wt% and As < 1.479 wt%). Incorporation of the trace elements into the framboidal pyrites resulted in a slight decrease in S and Fe concentrations compared to the theoretical values. The in situ δ34S values of the framboidal pyrites, ranging from −53.2‰ to −30.9‰, are in good agreement with the sulfur isotopic fractionation triggered by the bacterial sulfate reduction (BSR) process in an open-SO42− ocean condition. Combined with SEI, BSE, and Nano-SIMS high-resolution multiple elemental mapping, four consecutive processes responsible for their genesis and evolution are summarized as follows: (1) initial microcrystal nucleation via the BSR process, (2) abundant microcrystal aggregation to form individual framboid and multiple-framboids during sedimentation or diagenesis, (3) partial replacement and recrystallization of preexisting framboids by As-rich ore fluids, and (4) eventual framboidal pyrite transformation to larger euhedral grains via continuous recrystallization. These processes not only present compelling mechanisms responsible for the variations in morphology and chemical composition of the framboidal pyrites, from the initial framboid with a low 75As32S signal (Py1) to the framboid with a recrystallization origin (Py3), but also indicate that their genesis and evolution may have an intimate correlation with the formation of Carlin-style gold deposits.

In situ geochemical characterization of pyrite crystals in burial dolomites of St. George Group carbonates

Secondary ion mass spectrometry (SIMS) was used to measure the δ34S of pyrite disseminated in burial dolomite matrix of Boat Harbour formation at Main Brook and Daniel’s Harbour (about 130 km apart). At Main Brook, δ34S values for the pyrite grains show wide variation (–15 to +20‰ (n = 20)), but are mostly negative. Combined with a paucity of two-phase fluid inclusions in the host burial dolomite, the depleted δ34S values suggest that the pyrite is a direct product of bacterial sulfate reduction (BSR). Predominantly negative δ34S values were also obtained for sampled pyrite at Daniel’s Harbour and Port au Choix; however, relatively high homogenization temperatures (&gt;100 °C) of two-phase fluid inclusions in the host dolomite is incompatible with a BSR process for pyrite formation. More so, Daniel’s Harbour is a site for main stage sulfide mineralization (Mississippi Valley-type (MVT) system), hosted in similarly burial dolomite and that has been previously constrained to be associated with thermochemical sulfate reduction (TSR). A relative proximity of the currently studied pyrite samples to this MVT system deposit is thus inconsistent with an in-situ BSR for these pyrites. The analyzed pyrite grains are commonly encased in bitumen, and they postdate their host burial dolomite and predate deep burial saddle dolomite. Taken together, the depleted δ34S signature in the pyrite was likely inherited from migrated hydrocarbons in the reservoir. Incursion of an initial pulse of hot sulfate-rich brine into the formation can cause thermal cracking of hydrocarbons, thereby releasing its low δ34S. Thus the inherited low δ34S signature was likely a product of an earlier BSR that occurred in the kerogen or source organic materials in source rock. Subsequently, the main stage sulfide mineralization (MVT deposit) occurred via in-situ TSR. These findings have an implication for the paragenetic history of sulfide minerals precipitated during MVT mineralization episode.

Tracing sulfate recycling in the hypersaline Pétrola Lake (SE Spain): A combined isotopic and microbiological approach

Sulfur (S) plays a significant role in saline environments, and sulfate (SO42−) is an important component of the biogeochemical S-cycle since it acts as the main electron acceptor in anoxic sediments. The purpose of this paper is to evaluate the fate of S, its origin, and processes affecting sulfate outcome in the hypersaline Pétrola Lake in the Castilla-La Mancha region (High Segura Basin, SE Spain). The lake is the terminal discharge zone of an endorheic basin with considerable anthropogenic pressures. Anthropogenic activities (mainly agricultural inputs and wastewater discharge), together with bedrock leaching of sulfate and sulfide-rich sediments, increase dissolved SO42− in surface and groundwater up to 123,000mg/L. The source and fate of sulfate in this environment was investigated coupling hydrochemistry, including hydrogen sulfide (H2S) microprofiles, isotopic analyses (δ34S, δ18OSO4, δ2HH2O, δ18OH2O, and tritium), mineralogical determinations, and molecular biology tools (16S rDNA amplification and sequencing). The origin of dissolved SO42− in water is related to pyrite oxidation from Lower Cretaceous sediments, and secondary gypsum dissolution. Under the lake, dissolved SO42− decreases with depth, controlled by three main processes: (1) seasonal evaporation cycles, (2) hydrodynamic instability caused by the different density-driven groundwater flow, and (3) sulfate-reduction processes, i.e. dissimilatory bacterial sulfate reduction (BSR). These processes control the continuous recycling of sulfur in the system. Lake water and groundwater are in hydraulic connection, and a density-driven flow (DDF) is able to transport reactive organic matter and dissolved SO42− towards the underlying aquifer. Hydrochemical evolution in depth, H2S production (up to 0.024nmol/cm3·s) and the presence of sulfate-reducing bacteria suggest the existence of BSR processes. However, isotope techniques are insufficient to elucidate BSR processes since their isotopic effect is masked by low isotope fractionation and high SO42− concentrations. The pattern here described may be found in other saline basins worldwide.

Geological and geochemical constraints on the origin of the Early Cambrian Kalaizi Fe–Ba deposit in Western Kunlun, NW China

The Kalaizi deposit in the Bulunkuole Group of Taxkorgan area, Western Kunlun (NW China) is a unique regional Fe–Ba deposit accompanied with sporadic Fe–Pb–Zn sulfide mineralization within a series of iron deposits. All the ore bodies are stratabound and hosted by the Early Cambrian metamorphosed argillaceous clastic rocks. Here, we analyzed C–S–O–Sr isotopes and trace elements in individual minerals (e.g., magnetite, barite, ankerite) from the Kalaizi deposit to investigate the origin of this deposit for the first time. The S isotope fractionation (average of 21‰) between sulfate and sulfide minerals provided clear evidence that the deposit was formed in a semi-closed marine depositional environment with moderate replenishment of seawater sulfate. The distinct Post-Archean Australian Shale (PAAS)-normalized positive Eu anomalies (2.02–11.03) and lower Y/Ho ratios (27.2–33.4) relative to modern seawater in all individual magnetite minerals separated from magnetite–barite ores and the δ18O isotope signatures (10.0–13.0‰) in individual sulfate minerals suggested that hydrothermal activity played an important role in the formation of the deposit. Barium was likely derived from the dissolution of marine barite with a contribution from more radiogenic terrigenous sediments considering the slight deviations (within ±0.0013) of 87Sr/86Sr ratios of Kalaizi barites from the Early Cambrian seawater. The sulfate and sulfide minerals in the ore deposit should evolve from coeval seawater containing residual SO42− and resultant H2S by means of bacterial sulfate reduction process because (i) δ34S values (37.8–42.8‰) of anhydrite and barite minerals were slightly higher than those of penecontemporaneous seawater, (ii) pyrite and galena minerals owned δ34S values (16.3–23.5‰) were lower than coeval seawater, and (iii) 13C isotope values (−10.0 to −12.1‰) of single ankerite minerals were depleted. To summarize, the Kalaizi bedded Fe–Ba deposit is interpreted as a cold seep barite deposit overprinted by hydrothermal fluids.

Sulfur Recycling Processes in a Eutrophic Hypersaline System: Pétrola Lake (SE, Spain)

Agricultural pollution and wastewater effluents from the village of Pétrola cause a high pressure over Pétrola Lake-aquifer system (Central Spain). The lake is rich in sulfate (SO42−), with concentrations up to 1,280 mmol/L. Pétrola lake is considered one of the most symbolic saline wetlands in Castilla-La Mancha Region, due to its geomorphology, hydrology and salinity (up to 109.4 mS/cm). At the water-sediment interface anaerobic conditions and high dissolved organic carbon (up to 21.5 mmol/L) and sulfide production (up to 5 nmol/L) suggest the development of bacterial sulfate reduction processes (BSR). Hydrochemical and isotopic results confirm a mixing trend between high density brine water from the lake and freshwater from the regional groundwater flow. The sulfur origin in the system seems to be linked to sulfide oxidation of Lower Cretaceous sediments (Utrillas Facies) that are rich in sulfide and organic matter. Sulfur and oxygen isotope data show the sulfate-bearing mineral dissolution as the main source of SO42− in surface water. The study concludes that the regulation of the amount of sulfur depends on the relationship between the kinetics of BSR and the hydrodynamic instability driven by the different density of groundwater flow.

Sulfur Isotopes as Indicators of Amended Bacterial Sulfate Reduction Processes Influencing Field Scale Uranium Bioremediation

Aqueous uranium (U(VI)) concentrations in a contaminated aquifer in Rifle Colorado have been successfully lowered through electron donor amended bioreduction. Samples collected during the acetate amendment experiment were analyzed for aqueous concentrations of Fe(ll), sulfate, sulfide, acetate, U(VI), and delta(34)S of sulfate and sulfide to explore the utility of sulfur isotopes as indicators of in situ acetate amended sulfate and uranium bioreduction processes. Enrichment of up to 7% per hundred in delta(34)S of sulfate in down-gradient monitoring wells indicates a transition to elevated bacterial sulfate reduction. A depletion in Fe(II), sulfate, and sulfide concentrations atthe height of sulfate reduction, along with an increase in the delta(34)S of sulfide to levels approaching the delta(34)S values of sulfate, indicates sulfate limited conditions concurrent with a rebound in U(VI) concentrations. Upon cessation of acetate amendment, sulfate and sulfide concentrations increased, while delta(34)S values of sulfide returned to less than -20% per hundred and sulfate delta(34)S decreased to near-background values, indicating lower levels of sulfate reduction accompanied by a corresponding drop in U(VI). Results indicate a transition between electron donor and sulfate-limited conditions at the height of sulfate reduction and suggest stability of biogenic FeS precipitates following the end of acetate amendment.

A model for oxygen and sulfur isotope fractionation in sulfate during bacterial sulfate reduction processes

We present a model of bacterial sulfate reduction that includes equations describing the fractionation relationship between the sulfur and the oxygen isotope composition of residual sulfate (δ 34S SO4_residual, δ 18O SO4_residual) and the amount of residual sulfate. The model is based exclusively on oxygen isotope exchange between cell-internal sulfur compounds and ambient water as the dominating mechanism controlling oxygen isotope fractionation processes. We show that our model explains δ 34S SO4_residual vs. δ 18O SO4_residual patterns observed from natural environments and from laboratory experiments, whereas other models, favoring kinetic isotope fractionation processes as dominant process, fail to explain many (but not all) observed δ 34S SO4_residual vs. δ 18O SO4_residual patterns. Moreover, we show that a “typical” δ 34S SO4_residual vs. δ 18O SO4_residual slope does not exist. We postulate that measurements of δ 34S SO4_residual and δ 18O SO4_residual can be used as a tool to determine cell-specific sulfate reduction rates, oxygen isotope exchange rates, and equilibrium oxygen isotope exchange factors. Data from culture experiments are used to determine the range of sulfur isotope fractionation factors in which a simplified set of equations can be used. Numerical examples demonstrate the application of the equations. We postulate that, during denitrification, the oxygen isotope effects in residual nitrate are also the result of oxygen isotope exchange with ambient water. Consequently, the equations for the relationship between δ 34S SO4_residual, δ 18O SO4_residual, and the amount of residual sulfate could be modified and used to calculate the fractionation-relationship between δ 15N NO3_residual, δ 18O NO3_residual, and the amount of residual nitrate during denitrification.

Microbial fabrics from Neogene cold seep carbonates, Northern Apennine, Italy

A number of hydrocarbon seep-carbonate masses are hosted within Miocene and Pliocene siliciclastic units along the Apennine Range and Sicily, and are mainly composed of micritic and microsparitic textures. Geomicrobiological analysis of some of these deposits shows that preserved microbial processes are few and the contribution of microbe-derived textures to their construction is volumetrically very limited. In the carbonate groundmass of two northern Apennine Miocene and Pliocene seep-carbonate deposits uncommon and well-preserved features suggest direct and indirect evidence of microbial activity. Some of these features are interpreted as the mineralization products of microbial textures, and include complex alveolar textures and fossilized biofilms that line and fill microconduits, pores and other voids, similar to living bacterial aggregations recovered in modern seep-carbonates. Other mineral structures, such as spheroids and rhombs, with concentric fabrics composed of calcite–dolomite alternations, abound as cement components and suggest a nucleation via bacterial sulfate reduction processes. A similar origin is also assumed for the acicular aragonite bundles that in the Miocene seep-carbonate mass enclose dense clusters of millimeter-size, filamentous morphologies of biological origin.

In situ metal precipitation in a zinc-contaminated, aerobic sandy aquifer by means of biological sulfate reduction.

The applicability of in situ metal precipitation (ISMP) based on bacterial sulfate reduction (BSR) with molasses as carbon source was tested for the immobilization of a zinc plume in an aquifer with highly unsuitable initial conditions (high Eh, low pH, low organic matter content, and low sulfate concentrations), using deep wells for substrate injection. Batch experiments revealed an optimal molasses concentration range of 1-5 g/L and demonstrated the necessity of adding a specific growth medium to the groundwater. Without this growth medium, even sulfate, nitrogen, phosphorus, and potassium addition combined with pH optimization could not trigger biological sulfate reduction. In column experiments, precipitation of ZnS(s) was induced biologically as well as chemically (by adding Na2S). In both systems, zinc concentrations dropped from about 30 mg/L to below 0.02 mg/L. After termination of substrate addition the biological system showed continuation of BSR for at least 2 months, suggesting the insensitivity of the sulfate reducing system for short stagnations of nutrient supply, whereas in the chemical system an immediate increase of Zn concentrations was observed. A pilot experiment conducted in situ at the zinc-contaminated site showed a reduction of zinc concentrations from around 40 mg/L to below 0.01 mg/L. Termination of substrate supply did not result in an immediate stagnation of the BSR process, but continuation of BSR was observed for at least 5 weeks.