High δ34S Values Research Articles

Geochemistry Insights on the Genesis of the Subduction-Related Heishan Magmatic Ni-Cu-(PGE) Deposit, Gansu, Northwestern China, at the Southern Margin of the Central Asian Orogenic Belt

The Devonian subduction-related Heishan mafic-ultramafic intrusion hosting a magmatic Ni-Cu-(PGE) deposit is situated at the southern margin of the Central Asian orogenic belt. The outcrop of the intrusion is ~800 m long and ~470 m wide and dips to the southwest to depths up to 1,300 m. The Heishan intrusion is emplaced in Neoproterozoic dolomitic marble and siliceous slate and dominantly comprises of harzburgite and lherzolite. The stratigraphic reversals of Fo and Ni contents of olivine within the lerzolite and harzburgite indicate injections of several pulses of magmas. The two orebodies (Nos. 1 and 4) within the harzburgite and lherzolite at the lower part of the intrusion host ~35 million metric tons (Mt) of disseminated sulfide mineralization with average grades of 0.6 wt % Ni and 0.3 wt % Cu. The sulfides of the No. 4 orebody are lower in PGE contents on the basis of 100% sulfide (580–1,860 ppb Pt, 720–1,450 ppb Pd, 50–120 ppb Ir) and higher in Cu/Pd ratios (48,000–75,000) relative to the sulfides of the No. 1 orebody and mineralized harzburgites, which have 2,350 to 4,110 ppb Pt, 3,460 to 5,840 ppb Pd, 130 to 160 ppb Ir and Cu/Pd ratios of 16,000 to 26,000. Additionally, the olivines in the No. 1 orebody and mineralized harzburgites have higher forsterite (Fo) and Ni contents than those in the No. 4 orebody. We propose that the sulfide segregation of the No. 4 orebody was associated with extensive introduction of crustal sulfur, which is indicated by high δ 34 S values (1.9–6.1‰) and low Se/S ratios (230–390 × 10 −6 ). Reworking of early fractionated massive or net-textured sulfides by the new wave of magma containing unfractionated sulfide droplets resulted in the positive correlation between Pd/Ir and Ni/Ir for the disseminated sulfides of the No. 4 orebody. In contrast, low δ 34 S values (0.43–1.01‰) and restricted range of Pd/Ir ratios indicate that the high PGE contents of the sulfides in the No. 1 orebody and mineralized harzburgites resulted from reaction between the sulfides and new pulses of S-undersaturated magmas. The low PGE grades of the Heishan sulfides indicate that the sulfides were segregated from PGE-depleted parental magmas.

Stable isotopes reveal rail-associated behavior in a threatened carnivore

Human–wildlife conflict is a leading cause of adult mortality for large carnivores worldwide. Train collision is the primary cause of mortality for threatened grizzly bears (Ursus arctos) in Banff National Park. We investigated the use of stable isotope analysis as a tool for identifying bears that use the railway in Banff. Rail-associated bears had higher δ15N and δ34S values than bears sampled away from the rail, but similar δ13C values. Because elevated δ15N values are indicative of higher animal protein consumption, rail-associated bears likely preyed on ungulates that foraged along the rail or scavenged on train-killed animals. The higher δ34S values in bear hair could have resulted from bears consuming sulfur pellets spilled on the rail or through the uptake of sulfur in the plants bears or animals consumed. Similar δ13C values suggest that the two types of bears had generally similar plant-based diets. Results from this study suggest that stable isotopes analysis could be used as a non-invasive, affordable, and efficient technique to identify and monitor bears that forage on the railway in Banff and potentially other transportation corridors worldwide.

Geochemistry of Early Frasnian (Late Devonian) pyrite-ammonoid level in the Kostomłoty Basin, Poland, and a new proxy parameter for assessing the relative amount of syngenetic and diagenetic pyrite

Pyrite geochemistry (isotope and trace element composition, degree of pyritization, S/Corg ratio) was used in context of selected lithogeochemical parameters (major and trace elements, including sulphur, organic carbon, and δ13C of carbonate carbon) to constrain fluctuations in depositional conditions during the Early to Middle Frasnian carbon isotopic perturbation (punctata Event) in the Kostomłoty Basin, Poland. Based on the ratio between the sum of oxyanionic elements and transition metals in pyrite, a new proxy parameter (index of syngenetic pyrite, ISYP) is proposed for assessing the relative amount of syngenetic pyrite in a sample. The distribution of the ISYP along the Kostomłoty – Małe Górki section (upper Szydłówek to the basal Kostomłoty beds) is in concert with conclusions inferred from paleoecologic data and other geochemical parameters (degree of pyritization, S/Corg, δ34Spyrite). According to these, the lower segment of the Szydłówek Beds was deposited in a normally oxygenated environment, but undergoing increasing primary productivity in surface water, as indicated by an increase in δ13Ccarb and in Cu/Zr ratio in bulk rock, which triggered the periodic deposition of sediments slightly enriched in organic matter, notably within the pyrite-ammonoid level (=Goniatite Level). Fluctuating, but in general high S/Corg ratios, DOPR values and ISYP values suggest that during this time – against the background of a generally dysoxic environment – shorter or longer lasting episodes of more restricted (anoxic and possibly even euxinic) bottom water conditions developed. Low sedimentation rates enabled a continuous and practically unlimited supply of sulphate during bacterial sulphate reduction (BSR), which in turn led to a strong depletion of pyrite sulphur in 34S in this interval (constantly around –29‰). In contrast, below and above the Goniatite Level, higher δ34S values (up to +3‰), are compatible with closed system conditions and higher sedimentation rates. The Styliolina Horizon is distinguished by the overall highest portion of syngenetic pyrite, which suggests that in this marker layer most of the pyrite precipitated from euxinic bottom waters.

Mineralisation of amethyst-bearing geodes in Ametista do Sul (Brazil) from low-temperature sedimentary brines: evidence from monophase liquid inclusions and stable isotopes

Fluid inclusion studies in combination with hydrogen, oxygen and sulphur isotope data provide novel insights into the genesis of giant amethyst-bearing geodes in Early Cretaceous Parana continental flood basalts at Amestita do Sul, Brazil. Monophase liquid inclusions in colourless quartz, amethyst, calcite, barite and gypsum were analysed by microthermometry after stimulating bubble nucleation using single femtosecond laser pulses. The salinity of the fluid inclusions was determined from ice-melting temperatures and a combination of prograde and retrograde homogenisation temperatures via the density maximum of the aqueous solutions. Four mineralisation stages are distinguished. In stage I, celadonite, chalcedony and pyrite formed under reducing conditions in a thermally stable environment. Low δ34SV-CDT values of pyrite (−25 to −32 ‰) suggest biogenic sulphate reduction by organotrophic bacteria. During the subsequent stages II (amethyst, goethite and anhydrite), III (early subhedral calcite) and IV (barite, late subhedral calcite and gypsum), the oxidation state of the fluid changed towards more oxidising conditions and microbial sulphate reduction ceased. Three distinct modes of fluid salinities around 5.3, 3.4 and 0.3 wt% NaCl-equivalent characterise the mineralisation stages II, III and IV, respectively. The salinity of the stage I fluid is unknown due to lack of fluid inclusions. Variation in homogenisation temperatures and in δ18O values of amethyst show evidence of repeated pulses of ascending hydrothermal fluids of up to 80–90 °C infiltrating a basaltic host rock of less than 45 °C. Colourless quartz and amethyst formed at temperatures between 40 and 80 °C, while the different calcite generations and late gypsum precipitated at temperatures below 45 °C. Calculated oxygen isotope composition of the amethyst-precipitating fluid in combination with δD values of amethyst-hosted fluid inclusions (−59 to −51 ‰) show a significant 18O-shift from the meteoric water line. This 18O-shift, high salinities of the fluid inclusions with chloride-sulphate composition, and high δ34S values of anhydrite and barite (7.5 to 9.9 ‰) suggest that sedimentary brines from deeper parts of the Guarani aquifer system must have been responsible for the amethyst mineralisation.

Barite-rich massive sulfides from the Semenov-1 hydrothermal field (Mid-Atlantic Ridge, 13°30.87′ N): Evidence for phase separation and magmatic input

The ultramafic-hosted Semenov-1 hydrothermal field is a part of the Semenov group of sulfide deposits located at 13°30′ N Mid-Atlantic Ridge. The hydrothermal deposits comprise fine-crystalline, colloform, nodular, and banded barite–marcasite–pyrite assemblages, which are a result of moderately- to low-temperature venting of (Fe, Ba)-rich hydrothermal fluids. The precipitates contain low concentrations of Cu (0.02–0.55 wt.%) and Zn (0.01–0.08 wt.%), and variable Au content (0.35–4.76 ppm). Fe-disulfides are relatively enriched in Au and typical low-temperature trace elements (As, Ag, Pb, Mn, Tl). The concentrations of Au and of most trace elements are higher in the earliest, fine-grained Fe-disulfides and decrease in the late coarse-crystalline pyrite. Based on fluid inclusion data, barite precipitated from moderately- to low-temperature (244–83°С), low-salinity (0.6–3.8 wt.% NaCl eq.), carbonate–sulfate–aqueous fluids, which incorporated a low-density component produced by fluid phase separation. The presence of CO2 and SO2 in fluid inclusions in barite, as revealed by Raman spectroscopy, may indicate a magmatic volatile contribution to the hydrothermal fluid. The main amount of marcasite and pyrite was formed at temperatures below 240°С, after the precipitation of barite. The occurrence of isocubanite, chalcopyrite and pyrrhotite in precipitates from station 292 indicates a later high-temperature (~ 300°С) overprint. The sulfur isotopic composition of barite (+ 21.0 and + 21.3‰) closely matches that of seawater. The isotopic composition of sulfur in sulfides from station 186 (− 3.26 to − 0.08‰) suggests a possible contribution of light reduced sulfur derived from disproportionation of magmatic SO2 or from leaching of basalt that has degassed SO2. The higher δ34S values in sulfides from station 292 (− 0.08 to + 1.53‰) may reflect a heavy sulfur contribution from reduced seawater sulfate. Although the Semenov-1 field is considered to be associated with serpentinized ultramafic rocks (Beltenev et al., 2007), the mineralogical composition of the hydrothermal precipitates, particularly, their high barite and pyrite contents, is more typical of EMORB-hosted seafloor hydrothermal deposits (e.g., Lucky Strike or Menez Gwen fields). Thermodynamic modeling of fluid–rock hydrothermal systems using a flow reactor model demonstrates that basalt–seawater interaction generates fluids that can produce only minor amounts of barite, irrespective of the Ba content of the basalt. Addition of a magmatic gas to the system produces highly acidic hydrothermal fluids capable to extract larger amounts of Ba and Fe, and form mineral assemblages similar to the studied ones. In contrast, interaction between ultramafic rock (peridotite) and seawater, with or without magmatic gas, results in precipitation of barite-free mineral assemblages. The above results do not support an ultramafic signature for the Semenov-1 hydrothermal system and suggest a mafic control on the hydrothermal fluid composition.

東北地方太平洋沖地震による岩手，宮城，福島県沿岸域における津波堆積物の供給源

Geochemical analyses (total nitrogen: TN, total organic carbon: TOC, total sulfur: TS, and stable sulfur isotope ratio: δ34S) were performed on tsunami deposits by the 2011 off the Pacific coast of Tohoku Earthquake obtained from Iwate, Minamisanriku, Kesen-numa, and Sendai Bay areas for a total of 133 samples. Although TOC/TN ratios ranged widely (0.7–90), most of the samples (more than 80% of samples) ranged from 10–30, which were similar results to those for marine surface sediment in the Pacific coast area. Low δ34S values (from -29.6‰ to -11.3‰) from Iwate, Minami Sanriku, and Sendai Bay tsunami deposits were observed. Generally, such low δ34S values result from large isotope fractionation caused by the activity of sulfate-reducing bacteria in marine sediment under a sulfate (SO42-) rich environment. These results suggest that the tsunami deposits were mainly derived from marine sediment. High correlations between TS and As contents (r=0.958) were observed in tsunami deposits from the Kesen-numa area. Furthermore, the high δ34S values (from -3.1‰ to -0.7‰) in the tsunami deposits from the Kesen-numa area correspond well with those of hydrothermal ore samples from the Kesen-numa area. These results indicate that sulfur and arsenic in tsunami deposits from the Kesen-numa area originated in slag from the mines.

The sulfur isotope signatures of Marinoan deglaciation captured in Neoproterozoic shallow-to-deep cap carbonate from South China

We present high-resolution sulfur and carbon isotope records from the cap carbonate of the Doushantuo Formation, South China, a unique suite of depositional facies spanning across a pronounced paleo-oceanic depth gradient. The δ34S of carbonate-associated sulfate (CAS) decreases across a shelf-to-slope depth transect. We propose that the deeper-water slope profile represents syn-glacial sulfur isotope compositions of the seawater sulfates with impact of 34S-depleted hydrothermal sulfur inputs, while the δ34S of the shallow inner-shelf were dominantly controlled by the 34S-enriched terrigenous input. We suggest that the terrigenous sulfur inputs were mainly from weathering and erosion of the pre-Marinoan Datangpo Formation—glacially ground and then freshly exposed during rapid deglaciation. The widespread Datangpo Formation, deposited between the Sturtian and Marinoan glacial intervals, is noted for the unusually high δ34S values of its pyrite.The low sulfate conditions in the ocean allowed for strong local controls and consequent heterogeneities in seawater chemistry. The post-Marinoan ocean was stratified in the immediate wake of glaciation, with anoxic deep waters overlain by an oxic, melt-water layer. During the transgression, the shallow water was disturbed by upwelling. The oxidation of dissolved organic carbon (DOC) from deep water would lead to transiently enhanced 13C depletions but 34S enrichments in the shallow waters.

Discovery of an abnormally high-δ34S barite deposit and a new understanding of global sulfur isotope variation during geological history

The evolution of the global sulfur isotope curve was plotted based on the δ34S values of evaporates resultant from oceanic evaporation. In the long period of geological history the δ34S values showed obvious peaks for three times during the process of ancient oceans’ sulfur isotope evolution, namely the Early Cambrian (+30‰), the Late Devonian (+25‰) and the Permian-Triassic transition interval (+17‰), but the causes of the abnormal rise of sulfur isotopic values during the geological period are still in question. In this paper, 18 samples collected from a large Devonian barite deposit from Zhenning County were analyzed to determine their δ34S values, revealing that the 18 samples have very high δ34S values (δ34S=41.88‰–+68.39‰), with an average close to 56.30‰, which are higher than the isotopic values of contemporary sulfates (+17‰–+25‰). A comparative analysis was conducted of the emerging of high δ34S barite deposits (from Cambrian and Devonian) and the δ34S variation curves of the ancient oceans. The results indicate that the time when the obvious peaks of δ34S values appeared and the time of massive sedimentation of high δ34S barite deposits are very close to each other, which, in our opinion, is not a coincidence. There may exist some correlations between the sulfur isotope evolution of ancient oceans during the diverse periods of geological history and the massive sedimentation of high δ34S barite deposits. Therefore, it is inferred that perhaps it was the massive sedimentation of high δ34S barites that caused the sharp rise of δ34S values in a short period of time.

Microscale sulfur isotopic compositions of sulfide minerals from the Jinding Zn–Pb deposit, Yunnan Province, Southwest China

The Jinding Zn–Pb deposit, located in the Lanping basin in Northwest Yunnan Province, is the largest Zn–Pb deposit in China, and also probably the youngest sediment-hosted super giant Zn–Pb deposit in the world. Its genesis differs from the well-known major types of sediment-hosted Zn–Pb deposits. Based on mineral paragenesis and textures, there are two stages of mineralization: stage 1 that is typically characterized by fine-grained sulfide minerals (galena, sphalerite, pyrite and marcasite) disseminated in sandstones of the Lower Cretaceous Jingxing Formation (K1j), and massive sulfides in limestone breccias of the Paleocene Yunlong Formation (E1y); and stage 2 which mainly occurs as coarse-grained galena veins crosscutting stage 1 sulfides, and minor amounts of colloform sphalerite intergrown with galena. In situ sulfur isotopic analyses of galena, sphalerite and pyrite were determined by secondary ion mass spectrometry (SIMS), and showed highly variable δ34S values (−42.1‰–7.7‰) of different ore types. Stage 1 mineralization has δ34S values from −42.1‰ to −10.2‰ with the majority ranging from −26‰ to −14‰. Stage 2 mineralization has higher δ34S values (−8.3‰–7.7‰). Combined with the geological settings and mineral paragenesis, the sulfur isotopic data presented here suggest multiple sulfur sources (biogenic sulfur+evaporites) and formation mechanisms for reduced sulfur (H2S). H2S responsible for stage 1 sulfide precipitation was associated with bacterial sulfate reduction (BSR). However, H2S of stage 2 was likely derived from thermochemical sulfate reduction (TSR). The most reasonable scenario for the stage 1 mineralization is a metal-bearing brine mixing with an H2S-rich fluid, thereby causing rapid sulfide precipitation. Till the stage 2, the ore-forming fluid shifted to the meteoric water that infiltrated and reacted with evaporitic rocks, leached metals and transported them as sulfate- or sulfite-complexes to the Jinding dome where the oxidized sulfur was reduced by organic matters to H2S, leading to precipitation of metal sulfides. In contrast to other sulfide deposits in the Lanping basin, biogenic sulfur might have played a key role in the mineralization process, especially during the early stage of formation of the Jinding Zn–Pb deposit.

Paleoproterozoic Mississippi Valley Type Pb–Zn mineralization in the Ramah Group, Northern Labrador: Stable isotope, fluid inclusion and quantitative fluid inclusion gas analyses

Mississippi Valley Type (MVT) Pb–Zn sulfide mineralization is common in Phanerozoic rocks but relatively few MVT deposits have been reported from Paleoproterozoic rocks. This study investigates the genesis of numerous Pb–Zn showings in the ca.2.0Ga Ramah Group, northern Labrador. Pb–Zn mineralization is hosted within the Reddick Bight Dolomite Member (RBDM), Ramah Group. The RBDM is characterized by a well developed secondary porosity associated with solution dissolution of tepee structures in the dolomite. Mineralization occurs as open fill in this secondary porosity, with dark-brown sphalerite, galena and pyrite with gangue quartz, dolomite, calcite and feldspar. The presence of blocks of mineralized RBDM in thrust faults indicates that mineralization predates propagation of these faults during the Torngat Orogeny from 1.85 to 1.87Ma.Fluid inclusion analysis of sphalerite has identified H2O–NaCl–CaCl2+CO2±N2±CH4 mineralizing fluids with high salinities (up to 19.4eq.wt.% NaCl+CaCl2) and homogenization temperatures of 104 to 177°C. The presence of significant volatiles in the mineralizing fluids has been confirmed through quantitative fluid inclusion gas analysis of sphalerite, which has recorded 5.9mol% CO2. Fluid inclusion data from quartz suggests high temperature (135 to 231°C), high salinity (up to 20.5eq.wt.% NaCl+CaCl2) fluids with significant volatile contents (quantitative fluid inclusion gas analysis up to 21.5mol% CO2).Carbon isotope data show a decrease in δ13C from host dolomite (−1.3±0.9‰) through pre-ore dolomite (−4.7±1.5‰) to post-ore calcite (−5±1.7‰), indicating an influx of isotopically light hydrocarbon-bearing fluids. The δ34S isotopic ratios range from 8.3 to 11.1‰ for early pyrite mineralization, 23.3 to 31.8‰ for late stage pyrite, 16.7 to 32.9‰ for galena and 23.2 to 33.8‰ for sphalerite. The relatively high δ34S values associated with Pb–Zn mineralization, high fluid temperatures (>175°C) and presence of CO2 fluids indicate ore deposition was associated with thermochemical sulfate reduction. Pb isotope data from galena separates suggests that the Pb was derived from a relatively non-radiogenic source consistent with the Archean basement rocks of the Nain Province.The Pb–Zn mineralization in the RBDM shares many features with classic MVT deposits including geological setting, mineralogy, fluid characteristics and crustal sources for both metals and sulfur. However, MVT mineralization in the RBDM is unusual when compared with Phanerozoic MVT deposits, with elevated fluid temperatures, high CO2 contents of mineralizing fluids and minimum trapping pressures for fluid inclusions corresponding to mineralization depths of >4.5km. Similar characteristics have been described from other Paleoproterozoic MVT deposits (e.g. Pering Zn–Pb deposit, South Africa, Kamarga Pb–Zn deposit, Australia).

Sulfur biogeochemistry of cold seeps in the Green Canyon region of the Gulf of Mexico

Cold seeps in the Gulf of Mexico provide a natural laboratory to study biogeochemical cycling of sulfur, carbon, and oxygen at hydrate- and hydrocarbon-rich deep marine settings with obvious additional relevance to studies of diverse modern and ancient seeps. Of particular interest are the sulfur isotope signatures of microbial sulfate reduction coupled to anaerobic oxidation of methane and other non-methane liquid and gaseous hydrocarbons.Whereas most of the published sulfur isotope data from cold seep systems pertain to pore-water species, our study integrates both solid and dissolved sulfur: acid-volatile sulfides (SAVS), pyrite (Spy), elemental sulfur (S°), dissolved sulfate and ΣH2S. Modeled and 35SO42− reduction rates and δ13C and δ18O data for authigenic carbonates are integrated within this sulfur framework. Our results indicate extreme variability over narrow spatial and temporal scales within short distances (meters) from active seeps. High rates of microbial sulfate reduction can lead to complete consumption of the sulfate within the upper few centimeters of burial, while meters away the sulfate profile shows little depletion. Such small-scale variability must reflect the structure and temporal dynamics of hydrocarbon migration in the presence of low amounts of background organic matter. Our past work demonstrated that electron donors other than methane drive significant levels of microbial activity at these seeps, and very recent work has demonstrated that oxidation of higher chain volatile hydrocarbons can contribute to the high levels of microbial activity. These findings are consistent with our new results.Elevated concentrations of pyrite and diagenetic carbonate relative to background sediments are diagnostic of active seepage, yet the S isotopes tell more complex stories. Low levels of the transient, ‘instantaneous’ products of S cycling—AVS and S°—show high δ34S values that increase with depth. Most of the pyrite formation, however, seems to be very early as limited by the availability of reactive Fe phases. As such, δ34S values for pyrite at ancient seeps can show consistently low δ34S values that undersell the full intensity of microbial sulfate reduction. Low sedimentation rates, and the resulting low detrital iron fluxes, may in fact limit our ability to recognize seeps in the geologic record using only δ34S compositions for pyrite.

Re[sbnd]Os and S isotopic constraints on the origins of two mineralization events at the Tangdan sedimentary rock-hosted stratiform Cu deposit, SW China

The Tangdan stratiform Cu deposit in the Kangdian region of South China is hosted in sedimentary rocks of the late Paleoproterozoic to early Mesoproterozoic Dongchuan Group. Orebodies mainly occur in dolostone and black shale above a thick sequence of hematitic sandstone and siltstone. Mineralization consists mainly of bedding-parallel stratiform ores, with minor amounts of discordant vein-type ores. Both stratiform and vein-type ores are composed of bornite and chalcopyrite with minor chalcocite and digenite, associated with gangue minerals including ankerite, dolomite, calcite, and quartz.Cu-sulfides from vein-type ores contain ~122–293ppb Re, much higher than those of stratiform ores (~0.6–14ppb Re). They have highly radiogenic Os with ReOs model ages ranging from 1413 to 1457Ma and a weighted mean ReOs model age of 1432±19Ma (2σ, MSWD=0.30), which is within uncertainty in agreement with isochron ages of 1401±30Ma (2σ, MSWD=0.67) and 1397±71Ma (2σ, MSWD=0.22) using 187Re/188Os versus 187Os/188Os and 187Re versus 187Os regression, respectively. In contrast, sulfides from stratiform ores did not yield meaningful model ages due to the extremely low concentrations of Re and radiogenic Os. However, early studies have shown that the stratiform ores were likely formed at ~1700Ma during the diagenesis of the host strata.Sulfides from the vein-type ores have δ34S values ranging from −15‰ to −8‰, significantly lower than those of stratiform ores (−3‰ to +10‰), suggesting that organic matter may have been involved in the formation of the vein-type ores. Contrasting ReOs and S isotopes suggest that the two types of ores at the Tangdan deposit were likely formed from different ore-forming fluids. We propose that stratiform ores were precipitated from basinal fluids during the early evolution of the rift basin, whereas the vein-type ores were formed due to a much younger thermal event during which hydrothermal fluids interacted with overlying black shale to account for the high Re and negative δ34S values. This study demonstrates that ReOs isotopes of sulfides can be used to study the origin of hydrothermal system.

Contribution of isotopologue self‐shielding to sulfur mass‐independent fractionation during sulfur dioxide photolysis

Signatures of sulfur mass‐independent fractionation (S‐MIF) are observed for sulfur minerals in Archean rocks, and for modern stratospheric sulfate aerosols (SSA) deposited in polar ice. Ultraviolet light photolysis of SO2 is thought to be the most likely source for these S‐MIF signatures, although several hypotheses have been proposed for the underlying mechanism(s) of S‐MIF production. Laboratory SO2 photolysis experiments are carried out with a flow‐through photochemical reactor with a broadband (Xe arc lamp) light source at 0.1 to 5 mbar SO2 in 0.25 to 1 bar N2 bath gas, in order to test the effect of SO2 pressure on the production of S‐MIF. Elemental sulfur products yield high δ34S values up to 140 ‰, with δ33S/δ34S of 0.59 ± 0.04 and Δ36S/Δ33S ratios of −4.6 ± 1.3 with respect to initial SO2. The magnitude of the isotope effect strongly depends on SO2 partial pressure, with larger fractionations at higher SO2 pressures, but saturates at an SO2 column density of 1018 molecules cm−2. The observed pressure dependence and δ33S/δ34S and Δ36S/Δ33S ratios are consistent with model calculations based on synthesized SO2 isotopologue cross sections, suggesting a significant contribution of isotopologue self‐shielding to S‐MIF for high SO2 pressure (>0.1 mbar) experiments. Results of dual‐cell experiments further support this conclusion. The measured isotopic patterns, in particular the Δ36S/Δ33S relationships, closely match those measured for modern SSA from explosive volcanic eruptions. These isotope systematics could be used to trace the chemistry of SSA after large Plinian volcanic eruptions.

Microstructure and geochemistry studies on Messinian gypsum deposits from the Northern Coast of Egypt

Messinian gypsum deposits from Dir El-Baraqan area, Northern Coast of Egypt, were investigated by stable sulfur isotope method, X-ray diffraction, infrared spectroscopy, optical microscopy, and scanning electron microscopy to differentiate features formed under substantial microbial influences as indicator of paleoenvironments. Petrographically, gypsum deposits were classified into three types: biolaminated gypsum, disordered selenite, and swallow-tail selenitic crystals. Biolaminated gypsum is characterized by regular alternating dark and light laminae, which were formed due to the seasonal environmental changes in Dir El-Baraqan area. Stable sulfur isotope data show that the gypsum deposits are characterized by δ34S values ranging from +18.1 to +28.1 ‰. In swallow-tail gypsum, the δ34S values are characterized by a narrow range (from +20.0 to +20.2 ‰) which is considered as the primary phase. In biolaminated gypsum, the δ34S values ranged from +22.8 to +28.1 ‰ which is considered as the secondary phase. However, the white laminae are characterized by δ34S values ranging from +22.8 to +24.1 ‰, while dark laminae are characterized by δ34S values ranging from +27.2 to +28.1 ‰. The high δ34S values of dark laminae revealed the increasing activity of sulfate-reducing bacteria.

Geology, mineralogy, and isotope (Sr, S) geochemistry of the Likak celestite deposit, SW Iran

Celestite mineralization in the Likak deposit is hosted by Early to Middle Miocene Gachsaran evaporitic formation. The celestite-bearing layers are composed of medium- to coarse-grained celestite crystals with different morphologies in a carbonate groundmass. The presence of carbonate and gypsum inclusions within celestite crystals as well as the pseudomorphic habits of some crystals suggests that celestite replaced preexisting carbonate and gypsum. The lithofacies features and faunal assemblage of the host rocks indicate subtidal–supratidal environments of deposition for country rocks. Strontium isotope analyses indicate that Sr originated from Middle Miocene seawater. Sulfur isotopic ratios indicate higher δ34S values in analyzed samples than those of Middle Miocene seawater. These higher values were probably resulted by bacterial reduction of sulfate. Based on petrographic evidences and Sr-isotope ages, it is revealed that mineralization occurred as a late-diagenetic process during deposition of Gachsaran Formation. Taking into account the available data, it can be suggested that precursors of the mineralizing fluids were probably produced by evaporation of seawater in a coastal sabkha setting. As these brines entered into underlying sediments, they leached considerable amounts of Sr from host sediments. Once these Sr-enriched fluids discharged back up into overlying beds containing gypsum and carbonate materials, celestite precipitation took place by replacement of the preexisting minerals as well as by mixing of the mineralizing fluids with sulfate-enriched brines entrapped within the beds.

The genetic relationship between hydrocarbon systems and Mississippi Valley-type Zn–Pb deposits along the SW margin of Sichuan Basin, China

The Chipu Mississippi Valley-type (MVT) deposit is located on the southwest (SW) margin of the Sichuan Basin. Occurrence of plentiful organic matter (bitumen) at this deposit and abundant hydrocarbon reservoirs in the SW Sichuan Basin implies a link between lead–zinc mineralization and hydrocarbon systems in this area. The high δ34S values of most metal sulphides from the different ore stages suggest that H2S-bearing gases and/or thermochemical sulphate reduction (TSR) by organic matter could have been the source of reduced sulphur involved in ore formation. Sulphides with small positive to negative δ34S values can be attributed to organically bound sulphur at the Chipu deposit. Carbon and oxygen isotopic compositions from sparry carbonates suggest mixing of organic carbon with seawater-derived carbon in the mineralization process. From the early to the later ore stages, δ13CPDB values of ore-hosting carbonates are increasingly more negative, which indicates strengthening of the TSR role during mineralization. Hydrogen and oxygen isotopes in fluid inclusions in the quartz gangue indicate the provenance of the ore-forming fluids in the hydrocarbons. Moreover, some extremely low hydrogen isotope values suggest the addition of hydrogen from the same source. The low H/C ratios and high non-hydrocarbon component of the bitumen (Zhang et al. 2010) also suggest that the organic matter may have been involved in TSR and subjected to a strong oxidation by ore-bearing fluids. This study attempts to explain the lead–zinc mineralization process and the role of organic matter in it. As there is a demonstrable relationship between the evolution of the hydrocarbons and regional lead–zinc mineralization along the SW edge of the Sichuan Basin, we propose a possible model in which the MVT mineralization coincided with the degradation of hydrocarbon reservoirs due to the large-scale migration of basinal fluids, most likely driven by the late Indosinian orogeny in response to the closure of the Palaeo-Tethys Ocean.

External sulphur in IOCG mineralization: Implications on definition and classification of the IOCG clan

Although the sources of the ore metals remain problematic in most Iron-oxide Cu and Au (IOCG) deposits, external sulphur, either from surficial basinal brines and seawater (e.g., Central Andean and Carajás deposits) or from formation water and metamorphic fluids (e.g., the Cloncurry deposits), or introduced by magmatic assimilation of metasedimentary units (e.g., Phalaborwa), has been documented in many major Cu-rich IOCG centres. However, only the evaporite-sourced fluids yield diagnostically high δ34S values (i.e., >10‰), while sedimentary formation water or metamorphic fluids commonly have lower values and are less clearly distinguishable from magmatic fluids, as in the Cloncurry deposits in which the involvement of external fluids is revealed by other evidence, such as noble gas isotopes. On the basis of these arguments, IOCG deposits could be redefined as a clan of Cu (AuAgU) deposits containing abundant hypogene iron oxide (magnetite and/or hematite), in which externally-derived sulphur probably plays an important role for the Cu (AuAgU) mineralization. In this definition, all “Kiruna-type” magnetite deposits, hydrothermal iron deposits (e.g., skarn Fe deposits) and magnetite-rich porphyry CuAu and skarn CuAu deposits are excluded. Two subtypes of IOCG deposits are recognized on the basis of the predominant iron oxide directly associated with the Cu (Au) mineralization, whether magnetite or hematite. Neither magnetite- nor hematite-rich IOCG deposits show any preference for specific host rocks, and both range in age from Neoarchean to Pleistocene, within a broad tectonic environment.

Implications of sulfur isotope fractionation in fracture-filling sulfides in crystalline bedrock, Olkiluoto, Finland

A detailed investigation of 34S/32S ratios in fracture-bound pyrite and marcasite was conducted at Olkiluoto, western Finland, in order to gain information on paleohydrogeochemical conditions in a deep groundwater (0–400m b.s.l.) environment. The bedrock at Olkiluoto is being considered for a final repository of spent nuclear fuel. The 34S/32S ratios were measured in situ on individual sulfide grains, using secondary ion mass spectrometry (SIMS) supplemented by analyses from bulk sulfide material. The δ34S values of pyrite and marcasite show considerable variations, from −50‰ to +82‰ VCDT. Using published records and new data from fracture filling minerals, the pyrite fillings were classified into groups representing distinct episodes of fracture mineral formation. The δ34S values of these groups record a transition from hydrothermal activity to a low temperature groundwater environment. The δ34S values of hydrothermal pyrite vary between −18‰ and +34‰. However, hydrothermal pyrite that definitively precipitated at 128–245°C, evidenced by fluid inclusion data from co-genetic calcite, is characterized by a more limited variation in δ34S from −8.6‰ to +3.9‰. The high variability of δ34S within the hydrothermal pyrite precipitates indicates that biogenic fractionation effects cannot be completely excluded. Hydrothermal pyrite precipitation is likely related to hydrothermal systems produced by Mesoproterozoic rapakivi granite and diabase intrusions. Subsequent pyrite generations show a wider spread of δ34S values. Pyrite types representing formation temperatures from ca. 50 to 90°C have δ34S values ranging from −40‰ to +82‰, and the latest low temperature (<80°C) pyrite precipitates are characterized by δ34S from −50‰ to +78‰. The wide range of δ34S values is attributed to bacterial SO42- reduction, affected by Rayleigh fractionation under restricted SO42- input. It can be calculated that the highest δ34S values in pyrite represent consumption of ca. 90% of the original SO42- in the deep groundwater in a fracture section. Furthermore, in situ variations of the δ34S values in pyrite grains indicate that pulses of fresh SO42- with relatively non modified S-isotopic composition have periodically entered the fractures. The latest low temperature pyrite formation event most likely occurred during the Holocene period when brackish, SO42--rich waters of the Litorina Sea infiltrated the fracture network at 8000–4000 BP, providing substrate for SO42--reducing bacteria. Due to the complex Quaternary history of the site and low temperature conditions existing for millions of years, latest fracture filling calcite and pyrite may represent a compilation of several low temperature precipitation events. The records of δ13C in calcite and δ34S in coexisting pyrite indicate that paleogroundwaters have been influenced by the migration of a redox front between methanic and sulfidic environments. These findings can be used to consider the possible geochemical changes that may affect the materials used to contain spent nuclear fuel.

Excitation band dependence of sulfur isotope mass-independent fractionation during photochemistry of sulfur dioxide using broadband light sources

Ultraviolet photolysis of sulfur dioxide (SO2) is hypothesized to be the source of the sulfur isotope mass-independent fractionation (S-MIF) observed in Archean sulfate and sulfide minerals and modern stratospheric sulfate aerosols. A series of photochemical experiments were performed to examine the excitation band dependence of S-MIF during the photochemistry of SO2 under broadband light sources (a xenon arc lamp and a deuterium arc lamp). Optical filters (200±35nm bandpass and 250nm longpass filters) were used to separately access two different excitation bands of SO2 in the 190–220nm and the 250–330nm absorption regions, respectively.UV irradiation of SO2 in the 190–220nm and 250–330nm regions both produced elemental sulfur (S0) and sulfur trioxide (SO3) as end products but yielded very different sulfur isotope signatures. The elemental sulfur products from direct photolysis in the 190–220nm region were characterized by high δ34S values (154.7–212.0‰), modest Δ33S anomalies of 21±3‰, and relatively constant 33λ (=ln(δ33S+1)/ln(δ34S+1)) values of 0.64±0.3, all with respect to the initial SO2. Photoexcitation in the 250–330nm region produced elemental sulfur with δ34S values of 7.7–29.1‰ and Δ33S values of 15.0±1.6‰. In both excitation regions, the SO3 products were mass dependently fractionated relative to the SO2 reservoir. The two different absorption regions produced contrasting Δ36S/Δ33S signatures in the elemental sulfur products, with Δ36S/Δ33S=−1.9±0.3 and 0.64±0.3 for the 190–220nm and 250–330nm bands, respectively.Our results provide several critical constraints on the origin of the S-MIF signatures observed in modern stratospheric aerosols and in the Archean geological record. A lack of S-MIF in the sulfate product and positive Δ36S/Δ33S ratios for the elemental sulfur from SO2 photo-oxidation demonstrate that photoexcitation in the 250–330nm region is not a likely source for the S-MIF observed in modern stratospheric aerosols. Large δ34S fractionation, 33λ values, and Δ36S/Δ33S ratios observed for the 190–220nm band are qualitatively consistent with predictions from synthetic isotopologue-specific cross sections. These isotope patterns, however, are not compatible with the Archean rock record. We explore the possibility that S-MIF from both the 190 to 220nm and the 250 to 330nm absorption bands could have contributed to the Archean S-MIF signatures.

Silica- and sulfate-bearing rock coatings in smelter areas: Part II. Forensic tools for atmospheric metal(loid)- and sulfur-isotope compositions

Black silica- and sulfate-bearing rock coatings in the Greater Sudbury area, Canada provide a record of atmospheric processes and emitted particulate matter associated with historical smelting operations in this area. Coating samples collected over the Greater Sudbury region are characterized with scanning electron microscopy, electron microprobe analysis, laser ablation inductively coupled mass-spectrometry, S-stable isotope measurements and micro X-ray fluorescence. On the micrometer scale, Cu, Pb, As, Se and S occur in close association within metal–sulfate rich layers composed of Fe- and Cu-sulfates. The concentrations of these and other elements do not represent their chemical proportions in the smelter plumes due to dissolution–precipitation processes, element substitutions and the stability of various phases involved in the coating formation. On the regional scale, the atomic ratios of Pb:Ni, As:Ni and Se:Ni decrease in the coatings with increasing distance from the smelting centers. This observation is consistent with higher wet deposition rates of small diameter Pb, As and Se-bearing primary sulfate aerosols (<2.5μm), compared to larger diameter (>2.5μm) Ni-bearing particulate matter. The mixing of primary (higher δ34S values) and secondary (lower δ34S values) sulfates explains the δ34S values of sulfates within the coatings close to smelting centers and the decrease in these values is attributed to the decrease in the ratio of primary to secondary sulfates with distance from the smelting centers. The information preserved in mineral surface-coatings together with an understanding of stoichiometry, geochemical processes and former environmental conditions provide a valuable record of atmospheric compositions, mixing, scavenging, deposition rates and oxidation processes, and the nature and source of anthropogenic releases to the atmosphere.