Multiple Sulfur Isotope Analyses Research Articles

Oceanic and Sedimentary Microbial Sulfur Cycling Controlled by Local Organic Matter Flux During the Ediacaran Shuram Excursion in the Three Gorges Area, South China.

The increased difference in the sulfur isotopic compositions of sedimentary sulfate (carbonate-associated sulfate: CAS) and sulfide (chromium-reducible sulfur: CRS) during the Ediacaran Shuram excursion is attributed to increased oceanic sulfate concentration in association with the oxidation of the global ocean and atmosphere. However, recent studies on the isotopic composition of pyrites have revealed that CRS in sediments has diverse origins of pyrites. These pyrites are formed either in the water column/shallow sediments, where the system is open with respect to sulfate, or in deep sediments, where the system is closed with respect to sulfate. The δ34S value of sulfate in the open system is equal to that of seawater; on the contrary, the δ34S value of sulfate in the closed system is higher than that of seawater. Therefore, obtaining the isotopic composition of pyrites formed in an open system, which most likely retain microbial sulfur isotope fractionation, is essential to reconstruct the paleo-oceanic sulfur cycle. In this study, we carried out multiple sulfur isotope analyses of CRS and mechanically separated pyrite grains (>100 μm) using a fluorination method, in addition to secondary ion mass spectrometry (SIMS) analyses of insitu δ34S values of pyrite grains in drill core samples of Member 3 of the Ediacaran Doushantuo Formation in the Three Gorges area, South China. The isotope fractionation of microbial sulfate reduction (MSR) in the limestone layers of the upper part of Member 3 was calculated to be 34ε = 55.7‰ and 33λ = 0.5129 from the δ34S and Δ33S' values of medium-sized pyrite grains ranging from 100 to 300 μm and the average δ34S and Δ33S' values of CAS. Model calculations revealed that the influence of sulfur disproportionation on the δ34S values of these medium-sized pyrite grains was insignificant. In contrast, within the dolostone layers of the middle part of Member 3, isotope fractionation was determined to be 34ε = 47.5‰. The 34ε value in the middle part of Member 3 was calculated from the average δ34S values of the rim of medium-sized pyrite grains and the average δ34S values of CAS. This observation revealed an increase in microbial sulfur isotope fractionation during the Shuram excursion at the drill core site. Furthermore, our investigation revealed correlations between δ34SCRS values and CRS concentrations and between CRS and TOC concentrations, implying that organic matter load to sediments controlled the δ34SCRS values rather than oceanic sulfate concentrations. However, these CRS and TOC concentrations are local parameters that can change only at the kilometer scale with local redox conditions and the intensity of primary production. Therefore, the decreasing δ34SCRS values likely resulted from local redox conditions and not from a global increase in the oceanic sulfate concentration.

Atmospheric ozone destruction and the end-Permian crisis: Evidence from multiple sulfur isotopes

The end-Permian mass extinction (EPME) is considered the largest biotic crisis of the Phanerozoic. To explain the worldwide destruction and mutation of land plants, previous work has emphasized the role of enhanced UV irradiation linked to volcanism-induced disruption of the ozone shield. However, direct evidence for a link between volcanic SO2 emission and stratospheric ozone deterioration is missing. Previous bulk analysis (i.e., SF6 method) found no Mass-Independent Fractionation of sulfur isotopes (MIF-S, noted Δ33S). However, possible grain scale sulfur isotope anomaly can be easily overlooked during bulk analysis. To detect possible sulfur isotope anomaly in grain scale, we applied in-situ multiple sulfur isotope analysis of the sulfide using secondary ion mass spectrometry (SIMS). We found a marked positive shift (higher than +0.30‰ and up to +0.94‰) just before the end-Permian extinction in the Meishan section. Modelling shows that these positive Δ33S anomalies cannot be explained by Mass Dependent Fractionation (MDF) processes alone, rather by UV-induced photolysis of volcanic SO2. The formation and preservation of MIF-S anomalies in the atmosphere and subsequent transport to the oceans require intense and prolonged irradiation of the Earth's surface with solar UV, thus fingerprinting severe disruption of the atmospheric ozone balance at the onset of Earth's largest mass extinction.

Sulfur in Archean komatiite implies early subduction of oceanic lithosphere

Komatiite is a magnesium-rich volcanic rock that erupted almost exclusively during the Archean era, requiring high partial melting of Earth's upper mantle to form. Archean komatiites, therefore, provide the most reliable information on the mantle geochemistry of the early Earth. However, it remains unclear whether komatiite compositions reflect only remnants of an early deep mantle or may provide evidence of recycling of subducted components. Sulfur mass-independent fractionation (S-MIF) could be useful to distinguish the two scenarios because S-MIF is produced only by photochemistry in anoxic atmospheres. We report here multiple sulfur isotope analyses of the 2.7 Ga Belingwe komatiites and related volcanics. Some Belingwe komatiites show clear S-MIF signatures (Δ33S from −0.20‰ to −0.12‰). Petrology, trace element chemistry, and Sr-Nd-Pb isotope compositions demonstrate that the samples contain Ni-poor pristine igneous sulfides and crustal contamination is minimal. The observed S and initial Nd isotope covariation can be reconciled if the komatiite source involved an early depleted shallow mantle which acquired S-MIF from seawater (Δ33S < 0‰) via seafloor alteration. We suggest that the Belingwe komatiites were derived at least partly from a past subducted oceanic lithosphere that stagnated in the mantle from the Archean era to the present. Therefore, the sulfur isotopes in Archean komatiite provide new constraints on the history of subduction and the recycling of crustal material.

Genesis of the Island Gold Deposit, Ontario, Canada: Implications for Gold Mineralization in the Wawa Subprovince of the Superior Province

Abstract The Island Gold deposit represents an uncommon example of known economic mineralization in the Wawa subprovince, which has been tectonically correlated with the neighboring Abitibi subprovince that hosts worldclass orogenic gold deposits. The Island Gold deposit is hosted in dacite, gabbro, and tonalite-trondhjemite, and accompanied D2 deformation. The main ore zone dips steeply toward the south and consists of early shear-hosted laminated quartz veins and late extensional veinlets; both vein sets host gold mineralization. A set of shallowly dipping extensional quartz veins in the Goudreau zone located north of the main ore zones also host economic gold mineralization. Multiple sulfur isotope analysis of pyrite associated with gold-bearing alteration envelopes in the main ore zone indicates no involvement of sulfur affected by mass-independent fractionation, which rules out sulfur (± gold) sourced from nearby banded iron formation or metasedimentary material. However, a single analysis of pyrite from an auriferous Goudreau zone vein indicates the involvement of sulfur that underwent mass-independent fractionation, which suggests a different fluid source. Zircon U-Pb geochronology of pre- and postmineralization rock samples at Island Gold restricts the timing of mineralization to between ca. 2724 and 2672 Ma. Including previous results of detrital zircon geochronology, gold mineralization occurred between ca. 2680 and 2672 Ma. This age range is similar to the timing of gold mineralization at the Hemlo deposit in the Wawa subprovince but is slightly older than the bulk of orogenic gold mineralization in the neighboring southern part of the Abitibi subprovince. Multiple sulfur isotopes indicate that gold mineralization at Island Gold results from fluids of igneous affinity associated with second-generation transpressional deformation. The ore-controlling structures were developed in bends in regional shear zones focused around relatively competent premineralization plutons.

Atmospheric and hydrothermal sulfur isotope signatures recorded in Neoarchean deep marine sedimentary pyrites from the Yilgarn Craton, Western Australia

We report in-situ multiple sulfur isotope analyses for pyrites from deep marine sediments that are interbedded with ∼ 2.7 Ga greenstone lava flows in the Kalgoorlie-Kambalda area of the Eastern Goldfields Superterrane, Western Australia. Two endmember sediment types are recognised: shale and chert, with transitional chert as an intermediate. Petrologic studies are consistent with both the pyrite and pyrrhotite having a syngenetic/diagenetic origin. Pyrites from the shales and transitional cherts have positive Δ33S, whereas those from the cherts have Δ33S ∼ 0. We suggest that the principal sources of S are atmospheric photolytic S8, with Δ33S > 0, and nanoparticulate sulfides from hydrothermal seafloor vents, with neutral Δ33S. In an anoxic Archean ocean, nanoparticles of pyrite and/or pyrrhotite, issuing from black and white smokers, were dispersed through the ocean by currents. S8, together with nanoparticulate sulfides and fine detrital particles, rained down slowly and accumulated on the sea floor to form the shales. During diagenesis, pyrrhotite reacted with available S to form pyrite until all the S was consumed, with unreacted pyrrhotite remaining in the shale. Variations in Δ33S in the sedimentary pyrites are therefore attributed to variations in the relative proportions of pyrite derived directly from black and white smokers, and pyrite formed by the diagenetic reaction between nanoparticulate pyrrhotite and photolytic S8. The cherts are interpreted to have formed close to hydrothermal vents where rapid accumulation of amorphous silica and pyrite from white smokers negated the influence of slow S8 rain.The Δ33S isotopic trend across individual sedimentary layers, can be explained by variations in the hydrothermal flux as local volcanic activity waxed and waned. The marked global increase in Δ33S in sedimentary pyrites at ca. 2,650 Ma is attributed to the emergence of several cratons above sea level at that time, associated with a marked increase in sub-aerial felsic volcanism.

Theoretical estimates of equilibrium sulfur isotope effects among aqueous polysulfur and associated compounds with applications to authigenic pyrite formation and hydrothermal disproportionation reactions

Inorganic polysulfur compounds (polysulfides, Sx2-; polysulfur radical ions, Sx·-; thiosulfate, S2O32-; polythionate, SxO62-; elemental sulfur, e.g. S8) participate in numerous geochemical processes related to the sulfur cycle. These include authigenic pyrite formation in sediments undergoing early stages of diagenesis, reactions associated with magmatic-hydrothermal processes, and numerous other aquatic sulfur redox processes (e.g., pyrite and sulfide oxidation). Sulfur isotope fractionations among many of these and associated compounds (e.g., H2S, HSO4-) are either unknown or unconstrained over wide ranges of temperatures. We present theoretical estimates of equilibrium sulfur isotope fractionation factors among aqueous polysulfur compounds (including select polysulfides, polysulfur radical anions, and polythionates) and select aqueous sulfide and sulfate compounds that correspond to all three stable isotope ratios of sulfur (33S/32S, 34S/32S, 36S/32S). Our estimates are based on electronic structure calculations performed at the B3LYP/6–31+G(d,p) level of theory and basis set implemented in concert with an explicit solvation model whereby molecules are encapsulated in water clusters of varying size (30–52 H2O) to simulate the aqueous solvation environment. These calculations yield relatively small magnitude fractionation factors between aqueous polysulfides, polysulfur radicals, and reduced sulfur moieties in polythionates relative to the aqueous sulfide compounds but reveal numerous crossovers that result in non-intuitive temperature dependencies. Our predictions of 34S/32S-based fractionation factors among aqueous sulfur compounds generally agree with previous experimental constraints where available within estimated uncertainties (e.g., HSO4-/H2S(aq), H2S(aq)/HS-, HSO4-/S0, H2S(aq)/S0). We use our calculations to explore equilibrium isotope fractionations among polysulfur and sulfide compounds that are precursors to authigenic pyrite in the framework of established mechanisms (e.g., the polysulfide mechanism). We examine possible explanations for why pyrite formation may be associated with relatively small isotope fractionation with respect to precursor aqueous sulfur compounds. We additionally use our theoretical calculations to constrain multiple sulfur isotope (33S/32S, 34S/32S, 36S/32S) mass balance models associated with the abiotic hydrolytic disproportionation of intermediate sulfur compounds (SO2, S8, S3·-) relevant to hydrothermal-magmatic-volcanic systems in order to illustrate the potential for subtle but potentially resolvable effects expressed in values of Δ33S and Δ36S associated with these processes. We apply a SO2 disproportionation mass balance model based on previous work but newly constrained by our theoretical calculations to (hyper-) acid crater lakes associated with active volcanoes, and newly highlight the potential for the utility of multiple sulfur isotope analyses in volcanic gas monitoring and constraining sulfur cycling processes in such systems.

Integration of multiple sulfur isotopes with structural analysis unveils the evolution of ore fluids and source of sulfur at the Kanowna Belle Archean orogenic gold deposit, Yilgarn Craton, Western Australia

The Kanowna Belle deposit is a world-class Archean orogenic gold system that witnessed multiple fluid episodes over a protracted deformation history. The hydrothermal fluid circulation episodes at the Kanowna Belle deposit initiated with the precipitation of early gold-bearing carbonate-famatinite-pyrite-telluride-electrum veins (V1a). These early veins were subsequently folded during a NE-SW shortening event (D1bKB) that led to the development of sericite-chlorite-pyrite stringers (V1b) and foliation (S1KB) dated at c. 2658 ± 9 Ma (U-Pb, xenotime). D1bKB structures are overprinted by quartz-carbonate-sericite-pyrite-gold veins (V2) controlled by the reverse faulting formed as a result of N-S shortening during D2KB. A subsequent deformation event (D3aKB) is related to sinistral shearing produced under ENE-WSW shortening and associated with the development of the Troy lodes and deposition of quartz-pyrite-sericite-gold veins (V3a) dated at c. 2628 ± 9 Ma (U-Pb, xenotime). The application of multiple sulfur isotope analyses of sulfides related to the different mineralization events resolves the hydrothermal fluid isotopic evolution through time. Despite the ore mineralogy differences of the V1, V2, and V3 vein sets, their associated sulfides yield small positive ∆33S (+ 0.1 to + 0.4‰; n = 231) values with two outliers (∆33S = + 0.5‰ and + 0.6‰) across all lithology types. The constant value of MIF-S through the three temporally different gold mineralization episodes implies that sulfur was derived from a single homogenized source of sulfur distal from the deposition site, irrespective of the Au endowment. The consistent small positive ∆33S sulfur isotope signature may support that the Archean orogenic gold system sourced sulfur and possibly hydrothermal fluids from a mantle/magmatic dominated source that homogenized with crustal sulfur at depth prior to gold deposition.

A significant seawater sulfate reservoir at 2.0 Ga determined from multiple sulfur isotope analyses of the Paleoproterozoic Degrussa Cu-Au volcanogenic massive sulfide deposit, Western Australia

The Proterozoic rock record displays secular change from ferruginous to an oxic hydrosphere over the course of 2 billion years; however, debate continues on the periodicity, rate of change and steps in following atmospheric oxygenation that ultimately led to an oxygenated ocean. This is partly due to poor preservation of the Paleoproterozoic marine sedimentary record in the few hundred million years after the Great Oxidation Event. Whereas the 2.0 Ga rock record preserves only rare chemical sediments, it contains significant mafic igneous provinces, which are known to locally host volcanogenic massive sulfide (VMS) deposits. These hydrothermal environments fossilize the ancient interaction at the seafloor interface between volcanic rocks and seawater. In this context, the 2.01 Ga Degrussa VMS deposit of the Paleoproterozoic Capricorn Orogen, Western Australia offers an opportunity to probe the ancient ocean composition. The Degrussa VMS deposit preserves massive sulfide mineralisation (pyrite – chalcopyrite – pyrrhotite ± sphalerite ± galena) hosted in turbiditic sedimentary rocks interlayered with basaltic flows and cut by numerous gabbroic sills. Exhalite layers are composed of hematite and jasper associated with magnetite. This study documents the multiple sulfur isotope composition of the Degrussa VMS deposit through an integrated analytical approach, which comprises bulk fluorination gas chromatography isotope ratio mass spectrometry and in situ secondary ion mass spectrometry. By comparing the ultra-high precision bulk measurements (n = 21) with in situ measurements of variably-textured grains of pyrite, chalcopyrite and pyrrhotite (n = 252), we determine that VMS mineralisation yields δ34S between +2‰ and +5‰ with a peak at ∼+2.9‰, and negative Δ33S signal ranging from −0.08 to 0.00‰. A two component δ34S-Δ33S mixing model indicates 11% of H2S derived from thermochemically reduced seawater sulfate mixed with magmatic H2S. The most negative Δ33S values must be explained by interaction with sulfate in the near-surface, undergoing complex dissolution-reprecipitation reactions, necessitating a minimum seawater sulfate reservoir of ∼2 mmol/L, or 7% modern seawater at 2.01 Ga.

Textural and geochemical investigation of pyrite in Jacobina Basin, São Francisco Craton, Brazil: Implications for paleoenvironmental conditions and formation of pre-GOE metaconglomerate-hosted Au-(U) deposits

The Jacobina Basin has a well-preserved sedimentary record, including continental and marine deposits, and hosts Au-(U)-Py mineralization in metaconglomerate beds similar to the Witwatersrand gold province. Based on petrographic observations, in situ trace-elements, and multiple sulfur isotope analyses (32S, 33S, 34S, and 36S) on pyrite from alluvial and marine facies, several types of pyrite were recognized. The pyrite grains identified in the alluvial metaconglomerates resemble those found in several pre-GOE gold-bearing metaconglomerates, including detrital and epigenetic varieties. Detrital inclusion-bearing pyrite is enriched in several redox-sensitive trace-metals as well as Au, which indicate an association with carbonaceous shales. On the other hand, the sources of detrital massive pyrite are more variable and include igneous, metamorphic, and hydrothermal sources from the Paleoarchean hinterland of Jacobina Basin. Epigenetic pyrite in metaconglomerates formed during metamorphism by the recrystallization of detrital pyrite with negligible contributions from external hydrothermal fluids to the basin. Diagenetic and epigenetic pyrite are found in marine lithologies. In a metapelite sample, the pyrite grains formed near the sediment-water column interface, with S sourced from the photolytic sulfate (SO42−, Δ33S < 0) dissolved in the water column. A quartzite sample, in turn, has detrital pyrite grains that were likely reworked from continental deposits, as well as pyrite formed by the assimilation of elemental sulfur (S8, Δ33S > 0) that accumulated in sediment pore water during diagenesis. Significantly, the pyrite associated with terrestrial metasediments shows a wide range in δ34S values but quite restricted ranges in Δ33S and Δ36S values, whereas pyrite associated with the marine metasedimentary rocks exhibits limited δ34S values but has a wide range in Δ33S values and correlated Δ36S values close to the Archean array. These data suggest distinct preservation routes for MIF-S from atmospheric SO42− and S8 in terrestrial and marine environments. Conditions on the terrestrial surface resulted in re-equilibration of distinct S sources, diminishing the amplitude of the Archean atmospheric signal. In contrast, SO42− dissolved in shallow marine settings while S8 settled to the floor, favoring the preservation of MIF-S isotopic signatures. Such processes may also explain the apparent differences in interpretations of atmospheric conditions derived from uncharacterized pyrites from Archean sources. Our data suggest that the Earth’s atmosphere remained anoxic, and terrestrial conditions were such as to allow the syngenetic accumulation of gold, as recently proposed for the Witwatersrand gold deposits.

Tracking Water‐Rock Interaction at the Atlantis Massif (MAR, 30°N) Using Sulfur Geochemistry

AbstractHydrothermal alteration and serpentinization of ultramafic rocks at oceanic core complexes involve extensive element exchange between fluid and rock and a wide range of biogeochemical processes. These processes influence the global sulfur cycle due to both biogenic and abiogenic removal of seawater sulfate. Hence, ocean floor serpentinization connects the hydrosphere, biosphere, and lithosphere. This work presents a study of the sulfur geochemistry of highly altered mafic and ultramafic samples from the Atlantis Massif located at 30°N along the Mid‐Atlantic Ridge. The analyzed samples were drilled during International Ocean Discovery Program Expedition 357 and collected during Alvin dives in 2000, 2003, and 2005. Multiple sulfur isotope analyses of sulfide and sulfate phases indicate that several processes took place during progressive hydrothermal alteration: (1) incorporation of seawater sulfate, (2) thermochemical sulfate reduction during interaction with high‐temperature (high‐T; 350–400 °C), low‐pH fluids and input of H2S derived from leaching gabbroic intrusions, (3) microbial sulfate reduction, and (4) oxidation of sulfides at high water‐rock ratios. Petrological examinations show that high‐T fluids mostly postdated the bulk serpentinization and that these fluid pulses were relatively localized (&lt;1 dm scale) resulting in a highly heterogeneous mineralogy. Locally, high‐T fluid influx took place subsequent to microbial sulfate reduction and oxidation as indicated by geochemical modeling. Overall, this study documents the complex interplay of magmatic processes, fluid‐rock interaction, and microbial activity that take place during the formation of oceanic core complexes and where mantle rocks are exposed to seawater.

Multiple sulfur isotopes (δ34S, Δ33S) of organic sulfur and pyrite from Late Cretaceous to Early Eocene oil shales in Jordan

We present the first multiple sulfur isotope study (32S, 33S, 34S, 36S) of bulk kerogen sulfur (KS) and disulfides (‘chromium-reducible sulfur’, CRS) from the oil shale of the Umm Rijam Chert-Limestone and Muwaqqar Chalk Marl Formation, Jordan (Late Cretaceous to Early Eocene, appr. 50–70 Ma).Analysis of the sulfur isotopic composition of KS (δ34SKS) shows values ranging from 0.3 to 17.9‰, which are 34S-enriched compared to the δ34SCRS ranging from −23.5 to −3.7‰. Values for CRS and KS are significantly 34S-depleted compared to seawater sulfate sulfur which suggests a major input of early-diagenetic, microbially-generated sulfide. A minor contribution of assimilated seawater sulfate to KS is assumed. The 34S-enrichment of KS compared to CRS can be partly explained by (1) sulfide oxidation to intermediate sulfur species prior to its incorporation into organic material, by (2) fractionations during organic sulfur generation, as well as by (3) a post-depositional timing of formation.Additionally, we hypothesize, based on parallel depth trends of maturity parameters (e.g., vitrinite reflectance) and δ34SCRS and δ34SKS values, that the sulfur isotopic compositions were influenced by thermal maturation (catagenesis). We suggest that the CRS pool comprises a contribution of sulfide released during the thermal decomposition of KS.Overall, our study highlights the importance of organic sulfur in sulfur isotopic studies and the potential of multiple sulfur isotope analyses in maturated sedimentary successions.

Sulfur isotope and metal variations in sulfides in the BIF-hosted orogenic Cuiabá gold deposit, Brazil: Implications for the hydrothermal fluid evolution

Textural, chemical and multiple sulfur isotope analyses of sulfides from the Archean world-class Cuiabá orogenic Au deposit, Quadrilátero Ferrífero district, Brazil, were conducted to track distinct trace element signatures from syngenetic to epigenetic-hydrothermal pyrite types in different host units, and to deduce the nature and source of the mineralizing fluids. Gold mineralization in Cuiabá is hosted in greenschist facies volcanic and sedimentary host rocks belonging to the Archean Rio das Velhas Supergroup, especially in association with quartz veins and sulfide minerals. The BIF-hosted Fonte Grande Sul orebody is controlled by quartz veins and disseminated gold-bearing pyrite, pyrrhotite and arsenopyrite in the wall rocks. Five pyrite types are classified based on textural relations as spongy, syngenetic (Py1), porous early-(Py2), smooth main-(Py3) and smooth isolated and overgrown late-stage (Py4 and Py5) types variably present in metamorphosed carbonaceous pelites, BIF and andesite. Besides pyrite, pyrrhotite and arsenopyrite are also defined according to textural and paragenetic differences, and they display significant trace element contents. Trace element abundance maps and LA-ICP-MS analyses display trace element incorporations in Py1 (present only in carbonaceous pelite), Py2 and Py3 (in all host rocks), and Py4 and Py5 (only in BIF). Py1 yields high As, Co, Ni, Pb and Ag concentrations, whereas Py2 (formed by agglomeration of Py1) maintains high trace element concentrations with slightly less of those elements. Trace element incorporation in Py3 is similar to Py1 in carbonaceous pelite. Early-stage Py2 and main-stage Py3 in BIF and andesite have increased Co and Ni, but have less Au and As. Late-stage Py4 is characterized by lower trace element concentrations, whereas Py5 is further enriched in As, Bi, Co, Ni and Pb. The hydrothermal alteration is divided into early-, main- and late-stage, which correlate to the sulfide hydrothermal evolution. The study shows that carbonaceous pelite is pre-enriched in Co, Ni and Pb, whereas certain elements like Ag, Au, Bi and As are only hydrothermally concentrated either during the early, main or late stage in BIF and andesite. This geochemical pattern supports a syngenetic versus hydrothermal origin of distinct elements, including some trace elements now in the ore zones. Multiple sulfur isotope data suggest that mineralizing fluids at the Cuiabá gold deposit reflect a complex mixing of sulfur evolved from three possible sources: seawater, mantle, and reduced elemental sulfur. Spongy, syngenetic Py1 yields Δ33S values ranging from −2.28 to −0.25‰, separated into two ranges, the first from −2.28 to −1.97‰, and the second from −0.96 to −0.25‰. The first range suggests Py1 in the carbonaceous pelite was deposited in a seawater environment. These pyrites probably mixed with later mantle sulfur carrying a near-zero Δ33S signature, which is indicated by the second Δ33S range, or could have mixed with fluids sourced from sedimentary rocks at depth carrying a positive Δ33S signature. The second scenario is more common in sedimentary and diagenetic pyrites. An unequivocal contrast is shown by early-, main- and late-stage pyrite types (Py2, Py3, Py4 and Py5) in carbonaceous pelite, BIF and andesite, which present a continuous process of crustal assimilation values towards increasingly positive δ34S and Δ33S values. This confirms evidence for a sedimentary-derived sulfur source as also indicated by fluid inclusion data of the andesite-hosted orebody at the Cuiabá deposit.

A simple and reliable method reducing sulfate to sulfide for multiple sulfur isotope analysis.

RationalePrecise analysis of four sulfur isotopes of sulfate in geological and environmental samples provides the means to extract unique information in wide geological contexts. Reduction of sulfate to sulfide is the first step to access such information. The conventional reduction method suffers from a cumbersome distillation system, long reaction time and large volume of the reducing solution. We present a new and simple method enabling the process of multiple samples at one time with a much reduced volume of reducing solution.MethodsOne mL of reducing solution made of HI and NaH2PO2 was added to a septum glass tube with dry sulfate. The tube was heated at 124°C and the produced H2S was purged with inert gas (He or N2) through gas‐washing tubes and then collected by NaOH solution. The collected H2S was converted into Ag2S by adding AgNO3 solution and the co‐precipitated Ag2O was removed by adding a few drops of concentrated HNO3.ResultsWithin 2–3 h, a 100% yield was observed for samples with 0.2–2.5 μmol Na2SO4. The reduction rate was much slower for BaSO4 and a complete reduction was not observed. International sulfur reference materials, NBS‐127, SO‐5 and SO‐6, were processed with this method, and the measured against accepted δ34S values yielded a linear regression line which had a slope of 0.99 ± 0.01 and a R 2 value of 0.998.ConclusionsThe new methodology is easy to handle and allows us to process multiple samples at a time. It has also demonstrated good reproducibility in terms of H2S yield and for further isotope analysis. It is thus a good alternative to the conventional manual method, especially when processing samples with limited amount of sulfate available.

A new negative and positive gas ion source for isotope ratio mass spectrometry.

The goal of this research was to construct an effective gas ion source for isotope ratio mass spectrometry, which generates sufficiently strong and stable ion beams for multiple isotope ratio analysis of sulfur and oxygen from readily available gases such as SO2 , CO2 , CO, O2 and chlorine from CH3 Cl gas. By means of a 60° sector mass spectrometer equipped with a new type of sensitive ion source and with a dual-inlet system and triple collector assembly, we have measured 34 S/32 S and challenging 36 S/32 S isotope ratios using positive sulfur ions generated from SO2 gas. The new ion source differs from the Nier-type ones by the electron beam being replaced with a tungsten filament. The obtained isotope signals of 32 S+ , 34 S+ and 36 S+ were from 1 to 5V (depending on the pressure of admitted gas) on high-ohm resistors of 200MΩ, 5 GΩ and 500 GΩ, respectively. The variable 33 S peak was not recorded as useful due to interference from H32 S+ ions. Although the ionizing electrons in the new ion source have significant energy dispersion the obtained peak profiles are flat-topped and they lead to good resolving power for the mass spectrometer (M/∆M≈100). The achieved precision of δ34 S and δ36 S determination was better than ±0.01 and ±0.1‰, respectively. The determination of 37 Cl/35 Cl in a volatile organic compound CH2 Cl2 was easily achieved on Cl- negative ions with a precision of ±0.005‰. The new ion source has great potential in the quantitative analysis of gases on positive ion mass spectra as well as, in the case of gases with high electron affinity such as CH3 Cl, CH3 Br, SF6 , SiF4 , on negative ones. It usefulness was demonstrated on an example of multiple sulfur isotope analysis and 37 Cl/35 Cl ratio determination with great precision. A negative ion source of this type will probably be useful in a more complicated instruments used in SIMS or AMS. Copyright © 2017 John Wiley & Sons, Ltd.

In situ multiple sulfur isotope analysis by SIMS of pyrite, chalcopyrite, pyrrhotite, and pentlandite to refine magmatic ore genetic models

With growing interest in the application of in situ multiple sulfur isotope analysis to a variety of mineral systems, we report here the development of a suite of sulfur isotope standards for distribution relevant to magmatic, magmatic-hydrothermal, and hydrothermal ore systems. These materials include Sierra pyrite (FeS2), Nifty-b chalcopyrite (CuFeS2), Alexo pyrrhotite (Fe(1−x)S), and VMSO pentlandite ((Fe,Ni)9S8) that have been chemically characterized by electron microprobe analysis, isotopically characterized for δ33S, δ34S, and δ36S by fluorination gas-source mass spectrometry, and tested for homogeneity at the micro-scale by secondary ion mass spectrometry. Beam-sample interaction as a function of crystallographic orientation is determined to have no effect on δ34S and Δ33S isotopic measurements of pentlandite. These new findings provided the basis for a case study on the genesis of the Long-Victor nickel-sulfide deposit located in the world class Kambalda nickel camp in the southern Kalgoorlie Terrane of Western Australia. Results demonstrate that precise multiple sulfur isotope analyses from magmatic pentlandite, pyrrhotite and chalcopyrite can better constrain genetic models related to ore-forming processes. Data indicate that pentlandite, pyrrhotite and chalcopyrite are in isotopic equilibrium and display similar Δ33S values +0.2‰. This isotopic equilibrium unequivocally fingerprints the isotopic signature of the magmatic assemblage. The three sulfide phases show slightly variable δ34S values (δ34Schalcopyrite=2.9±0.3‰, δ34Spentlandite=3.1±0.2‰, and δ34Spyrrhotite=3.9±0.5‰), which are indicative of natural fractionation. Careful in situ multiple sulfur isotope analysis of multiple sulfide phases is able to capture the subtle isotopic variability of the magmatic sulfide assemblage, which may help resolve the nature of the ore-forming process. Hence, this SIMS-based approach discriminates the magmatic sulfur isotope signature from that recorded in metamorphic- and alteration-related sulfides, which may not be resolved during bulk rock fluorination analysis. The results indicate that, unlike the giant dunite-hosted komatiite systems that thermo-mechanically assimilated volcanogenic massive sulfides proximal to vents and display negative Δ33S values, the Kambalda ores formed in relatively distal environments assimilating abyssal sulfidic shales.

Multiple sulfur isotopes (δ34S, Δ33S) and trace elements (Mo, U, V) reveal changing palaeoenvironments in the mid-Carboniferous Chokier Formation, Belgium

In this study, multiple sulfur isotopes are combined with widely-used redox-sensitive trace element concentrations (Mo, V, and U) in order to further validate the use of δ34S and Δ33S as a paleoredox proxy. The approach is applied to the mid-Carboniferous Chokier Formation, for which a detailed microfacies study identified a marked change in the environmental conditions during sediment deposition.Sediments from the upper part of the Chokier Formation exhibit negative δ34S (mean −24.6‰) and positive Δ33S (mean +0.040‰) values that are attributed to microbial sulfate reduction (MSR) and/or microbial sulfur disproportionation (MSD). Samples from the lower part of the Chokier Formation display less negative δ34S (mean −9.7‰) and negative Δ33S (mean −0.029‰) values. Paired δ34S-Δ33S values are located on a mixing curve between two endmembers: (1) sulfur generated by MSR and/or MSD, and (2) seawater sulfate. This latter observation is attributed to a restricted seawater exchange that limited the sulfate supply, thus, promoting closed-system conditions. Consequently, sulfur isotope values approached the isotopic composition of contemporaneous seawater.Trace element abundances exhibit a stronger enrichment in the upper Chokier Formation (EFMo=9–77, EFV=1–4; EF=Enrichment Factor) than in the lower Chokier Formation (EFMo=16–21, EFV ≈1). However, a hydrographic rather than a redox control is inferred for both sections based on Mo/TOC≤15 and Mo/U ratios well below seawater values. For the upper Chokier Formation, Mo/TOC≈15 indicates a moderately restricted basin, whereas the lower part is more strongly restricted (Mo/TOC≪15).Multiple sulfur isotopes and trace element abundances/ratios reveal a consistent picture of restricted basinal conditions during deposition of the lower Chokier Formation and more open conditions for the upper Chokier Formation. Hence, it can be concluded that results support the application of multiple sulfur isotope analyses for revealing paleoenvironmental conditions.

Improved analysis of micro- and nanomole-scale sulfur multi-isotope compositions by gas source isotope ratio mass spectrometry.

Multiple sulfur isotope compositions are usually measured on relatively large samples (in the range of micromoles); however, sometimes only small amounts are available and thus it is necessary to analyze small (sub-micromole) samples. We report an improved method to measure multiple sulfur isotope compositions: δ(33) S, δ(34) S and δ(36) S values on the SF6 molecule (m/z 127, 128, 129, 131) for quantities down to 0.1 micromole, and δ(33) S and δ(34) S values for quantities down to 20 nanomoles. Multiple sulfur isotope analyses including fluorination and purification of two international Ag2 S standards, IAEA-S1 and IAEA-S3, were carried out at various low concentrations on a dual-inlet isotope ratio mass spectrometer using a microvolume and modified resistor capacities. The analyses yielded a narrow range of δ(34) S values vs CDT (the international standard), with an overall standard deviation of ±0.2 ‰, which was within the range of certified values. This demonstrates the feasibility of determining both Δ(33) S and Δ(36) S values on the sub-micromole scale, and Δ(33) S values on the nanomole scale with similar accuracy to conventional dual-inlet analyses. The analysis of the three S-isotope ratios on the SF6 molecule using the so-called conventional fluorination method and dual-inlet ion ratio mass spectrometry is reliable for sample sizes down to ~20 nanomoles. Despite being close to the theoretical limits for maintaining the viscous flow regime of gas in the capillary, errors were not limited by counting statistics, but probably relate to sample gas purification. Copyright © 2016 John Wiley & Sons, Ltd.

Multiple S isotopes, zircon Hf isotopes, whole-rock Sr-Nd isotopes, and spatial variations of PGE tenors in the Jinchuan Ni-Cu-PGE deposit, NW China

Previous geochemical data for the Jinchuan Ni-Cu-(platinum-group elements, PGE) deposit, the single largest magmatic sulfide deposit in the world, are derived primarily from the upper parts of the deposit. This paper reports new PGE and S-Hf-Sr-Nd isotope data for the lower parts of the deposit that have become accessible for sampling by ongoing underground mining activity. New PGE data from this study, together with previous results, indicate that PGE tenors in the bulk sulfide ores of the deposit increase eastward, except for two fault-offset ore zones which occur together within the western part of the deposit. Generally, these two ore zones show depletions in IPGE (Ir, Ru, Rh) but not in PPGE (Pt, Pd) and Cu, and more fractionated olivine and Cr-spinel compositions than the rest of the deposit. These differences can be explained by a more evolved parental magma for the IPGE-depleted ore zones. The eastward increase of PGE tenors in the rest of the deposit can be explained by upgrading of preexisting sulfide liquid in a subhorizontal conduit by a new surge of magma moving through the conduit from west to east, which took place before the formation of the IPGE-depleted ore zones. The Jinchuan ultramafic rocks are characterized by elevated initial 87Sr/86Sr ratios from 0.7077 to 0.7093, negative e Nd values from −9.2 to −10.5, and zircon e Hf values from −4 to −7. These data indicate up to 20 % of crustal contamination in the Jinchuan magma. Four of nine multiple sulfur isotope analyses for the Jinchuan deposit show anomalous ∆33S values varying from 0.12 to 2.67 ‰. These results, together with elevated δ34S values (>2 ‰) for some of the samples analyzed previously by other researchers, indicate the involvement of external sulfur from Archean and Proterozoic sedimentary rocks. Modeling results based on our olivine data and magma compositions estimated previously by other researchers indicate that fractional crystallization did not play a major role in triggering sulfide saturation in the Jinchuan magma and that the parental magma experienced previous sulfide segregation during ascent or in staging chambers.

Multiple Sulfur Isotope Analyses Support a Magmatic Model for the Volcanogenic Massive Sulfide Deposits of the Teutonic Bore Volcanic Complex, Yilgarn Craton, Western Australia

We report sensitive high mass resolution ion microprobe, stable isotopes (SHRIMP SI) multiple sulfur isotope analyses (32S, 33S, 34S) to constrain the sources of sulfur in three Archean VMS deposits—Teutonic Bore, Bentley, and Jaguar—from the Teutonic Bore volcanic complex of the Yilgarn Craton, Western Australia, together with sedimentary pyrites from associated black shales and interpillow pyrites. The pyrites from VMS mineralization are dominated by mantle sulfur but include a small amount of slightly negative mass-independent fractionation (MIF) anomalies, whereas sulfur from the pyrites in the sedimentary rocks has pronounced positive MIF, with ∆33S values that lie between 0.19 and 6.20‰ (with one outlier at −1.62‰). The wall rocks to the mineralization include sedimentary rocks that have contributed no detectable positive MIF sulfur to the VMS deposits, which is difficult to reconcile with the leaching model for the formation of these deposits. The sulfur isotope data are best explained by mixing between sulfur derived from a magmatic-hydrothermal fluid and seawater sulfur as represented by the interpillow pyrites. The massive sulfide lens pyrites have a weighted mean ∆33S value of −0.27 ± 0.05‰ (MSWD = 1.6) nearly identical with −0.31 ± 0.08‰ (MSWD = 2.4) for pyrites from the stringer zone, which requires mixing to have occurred below the sea floor. We employed a two-component mixing model to estimate the contribution of seawater sulfur to the total sulfur budget of the two Teutonic Bore volcanic complex VMS deposits. The results are 15 to 18% for both Teutonic Bore and Bentley, much higher than the 3% obtained by Jamieson et al. (2013) for the giant Kidd Creek deposit. Similar calculations, carried out for other Neoarchean VMS deposits give value between 2% and 30%, which are similar to modern hydrothermal VMS deposits. We suggest that multiple sulfur isotope analyses may be used to predict the size of Archean VMS deposits and to provide a vector to ore deposit but further studies are needed to test these suggestions.

Mass-independently fractionated sulfur in Archean paleosols: A large reservoir of negative Δ33S anomaly on the early Earth

Results of multiple sulfur isotope analyses from five Archean paleosols show the widespread presence of mass-independently fractionated sulfur in the regolith developed on the pre-2.5Ga Earth. Analysis of sulfur from a small set of diamictite samples gave similar results. All values of Δ33S are negative, indicating that the Archean surface environments preferentially retained atmospheric S from the SO42− pathway, which carried a negative Δ33S signal, whereas a portion of the S from the S8 pathway, with a positive Δ33S, was transferred to the oceans. The soil SO42− was then converted to sulfide by bacterial sulfate reduction with terrestrial organic matter in the weathering horizon acting as a reductant. Some S from the S8 pathway also included, which resulted in a net soil Δ33S anomaly from atmospheric S in the range −0.3 to −0.6‰, similar to values for pyrites from floodplain sediments. Excess S8-derived S was carried to the oceans, resulting in the negative/positive dichotomy between terrestrial and marine systems. Pyrite that formed in paleosols and pyrite grains that formed in flood-plain deposits, both carrying the terrestrial Δ33S signal, were then recycled into detrital pyrite grains now found in sandstones and conglomerates deposited before the rise of atmospheric oxygen. Therefore the Earth's early regolith constituted a reservoir of S with negative Δ33S values that could be important for balancing the predominantly positive Δ33S signature found in marine sediments.