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1.
The Paleoproterozoic McArthur Basin (McArthur Group) of northern Australia hosts world-class sedimentary ‘exhalative’ (SEDEX) McArthur type Zn–Pb deposits, which are largely hosted within a sequence of 1.64 Ga pyritic carbonaceous shales deposited in an extensional rift setting. A well-known example of these is McArthur River (or Here's Your Chance [HYC] Zn–Pb–Ag deposit). The ~ 1.78 Ga McDermott and ~ 1.73 Ga Wollogorang formations (Tawallah Group) both contain carbonaceous shales deposited in similar environments. Our observations suggest the carbonaceous facies of the Wollogorang Formation were deposited under mostly euxinic conditions, with periodically-high concentrations of sedimentary pyrite deposition. The carbonaceous shales in the older McDermott Formation contain considerably less early pyrite, reflecting a mostly sulfide-poor, anoxic depositional environment. Localized fault-bound sub-basins likely facilitated lateral facies variations, which is evident from synsedimentary breccias.The presence of evaporitic oxidized facies within the McDermott and Wollogorang formations, alongside evidence for synsedimentary brecciation in reduced shales are favourable criteria for SEDEX-style base metal deposition. Both formations overlie volcanic units, which could have been sources of base metals. Detailed X-ray petrography, new geochemical data and sulfur isotope data from historical drill cores indicate multiple horizons of stratiform and sediment breccia-hosted base metal sulfide within carbonaceous shale units, with high-grade Zn concentrations. A close association between sphalerite and ferromanganean dolomite alteration draws comparisons with younger SEDEX mineralization at HYC. Additionally, SEDEX alteration indices, used demonstrably as a vector to the younger orebodies, indicate the sedimentary rocks analyzed in this study are marginally below the ore window when compared to the overlying mineralized stratigraphy.Our data imply that localized active circulation of metalliferous brines occurred in the Tawallah Group basin. High-grade sulfide deposition in reduced facies alteration may represent distal expressions of larger SEDEX-style deposits. Furthermore, abundant pyrite and high molybdenum in the Wollogorang Formation suggest the global oceanic sulfate concentration was sufficient by ~ 1.73 Ga to engender intermittent but strong bottom-water euxinia during shale deposition, thus providing a robust chemical trap for base metal sulfide mineralization.  相似文献   

2.
Geochemical evidence reported from Paleoproterozoic sediments has long been used to evaluate the transition from the anoxic Archean atmosphere to an oxygenated atmosphere. Sulfur isotopes (32S, 33S, 34S and 36S) in sedimentary sulfides and sulfates are an especially sensitive means to monitor this transition, such that the timing of the Paleoproterozoic “Great Oxidation Event” can be investigated using mass-independently fractionated (MIF) sulfur isotope systematics expressed as Δ33S. Here we report data from 83 individual analyses of pyrite, pyrrhotite and chalcopyrite on a new suite of 30 different samples from Finland, South Africa, Wyoming and Ontario that span ∼600 My and follow one or several “Snowball Earth” events in the Paleoproterozoic. The samples were measured using a high-resolution secondary ion mass spectrometry technique in multicollection mode that investigates multiple sulfur isotopes in microdomains (<30 μm) within individual sulfide grains while preserving petrographic context. We focused on sediments deposited in the aftermath of the Paleoproterozoic glaciations (between 1.9 and 2.2 Ga) to trace fluctuations in atmospheric O2 concentrations that were likely affected by an interplay of O2 sinks in the atmosphere and the upper ocean and continental crust, and by the emergence and diversification of aerobic organisms. Our results demonstrate that MIF sulfur isotopes are absent in sediments deposited after the period of protracted global cooling in the Paleoproterozoic and independently confirm observations that MIF ceased during this time. We interpret our results by integrating Δ33S and δ34S data in sulfides, δ13C data in carbonates and the estimated timing of glaciation events in the Paleoproterozoic. Data strongly hint at the presence of microbial sulfate reduction and fluctuations in the concentration of dissolved seawater sulfate and/or in δ34Ssulfate in the aftermath of glaciations and likely were affected by changing erosion rates and nutrient delivery to the oceans. These changes modulated the population of primary producers, especially oxygenic photosynthesizers, and led to fluctuations in the abundance of atmospheric O2, CO2 and CH4. Our results support the interpretation that the world-wide δ13Ccarb excursion observed between ∼2.25 and 2.05 Ga (Karhu and Holland, 1996) was a period of significant accumulation of O2 in the atmosphere.  相似文献   

3.
The source of sulfur in giant Norilsk-type sulfide deposits is discussed. A review of the state of the problem and a critical analysis of existing hypotheses are made. The distribution of δ34S in sulfides of ore occurrences and small and large deposits and in normal sedimentary, metamorphogenic, and hypogene sulfates is considered. A large number of new δ34S data for sulfides and sulfates in various deposits, volcanic and terrigenous rocks, coals, graphites, and metasomatites are presented. The main attention is focused on the objects of the Norilsk and Kureika ore districts. The δ34S value varies from -14 to + 22.5‰ in sulfides of rocks and ores and from 15.3 to 33‰ in anhydrites. In sulfide-sulfate intergrowths and assemblages, δ34S is within 4.2-14.6‰ in sulfides and within 15.3-21.3‰ in anhydrites. The most isotopically heavy sulfur was found in pyrrhotite veins in basalts (δ34S = 21.6‰), in sulfate veins cutting dolomites (δ34S = 33‰), and in subsidence caldera sulfates in basalts (δ34S = 23.2-25.2‰). Sulfide ores of the Tsentral’naya Shilki intrusion have a heavy sulfur isotope composition (δ34S = + 17.7‰ (n = 15)). Thermobarogeochemical studies of anhydrites have revealed inclusions of different types with homogenization temperatures ranging from 685 °C to 80 °C. Metamorphogenic and hypogene anhydrites are associated with a carbonaceous substance, and hypogene anhydrites have inclusions of chloride-containing salt melts. We assume that sulfur in the trap sulfide deposits was introduced with sulfates of sedimentary rocks (δ34S = 22-24‰). No assimilation of sulfates by basaltic melt took place. The sedimentary anhydrites were “steamed” by hydrocarbons, which led to sulfate reduction and δ34S fractionation. As a result, isotopically light sulfur accumulated in sulfides and hydrogen sulfide, isotopically heavy sulfur was removed by aqueous calcium sulfate solution, and “residual” metamorphogenic anhydrite acquired a lighter sulfur isotope composition as compared with the sedimentary one. The wide variations in δ34S in sulfides and sulfates are due to changes in the physicochemical parameters of the ore-forming system (first of all, temperature and Pch4) during the sulfate reduction. The regional hydrocarbon resources were sufficient for large-scale ore formation.  相似文献   

4.
Sulfide sulfur in mid-oceanic ridge hydrothermal vents is derived from leaching of basaltic-sulfide and seawater-derived sulfate that is reduced during high temperature water rock interaction. Conventional sulfur isotope studies, however, are inconclusive about the mass-balance between the two sources because 34S/32S ratios of vent fluid H2S and chimney sulfide minerals may reflect not only the mixing ratio but also isotope exchange between sulfate and sulfide. Here, we show that high-precision analysis of S-33 can provide a unique constraint because isotope mixing and isotope exchange result in different Δ33S (≡δ33S-0.515 δ34S) values of up to 0.04‰ even if δ34S values are identical. Detection of such small Δ33S differences is technically feasible by using the SF6 dual-inlet mass-spectrometry protocol that has been improved to achieve a precision as good as 0.006‰ (2σ).Sulfide minerals (marcasite, pyrite, chalcopyrite, and sphalerite) and vent H2S collected from four active seafloor hydrothermal vent sites, East Pacific Rise (EPR) 9-10°N, 13°N, and 21°S and Mid-Atlantic Ridge (MAR) 37°N yield Δ33S values ranging from −0.002 to 0.033 and δ34S from −0.5‰ to 5.3‰. The combined δ34S and Δ33S systematics reveal that 73 to 89% of vent sulfides are derived from leaching from basaltic sulfide and only 11 to 27% from seawater-derived sulfate. Pyrite from EPR 13°N and marcasite from MAR 37°N are in isotope disequilibrium not only in δ34S but also in Δ33S with respect to associated sphalerite and chalcopyrite, suggesting non-equilibrium sulfur isotope exchange between seawater sulfate and sulfide during pyrite precipitation. Seafloor hydrothermal vent sulfides are characterized by low Δ33S values compared with biogenic sulfides, suggesting little or no contribution of sulfide from microbial sulfate reduction into hydrothermal sulfides at sediment-free mid-oceanic ridge systems. We conclude that 33S is an effective new tracer for interplay among seawater, oceanic crust and microbes in subseafloor hydrothermal sulfur cycles.  相似文献   

5.
Syngenetic iron sulfides in sediments are formed from dissolved sulfide resulting from sulfate reduction and catabolism of organic matter by anaerobic bacteria. It has been shown that in recent marine sediments deposited below oxygenated waters there is a constant relationship between reduced sulfur and organic carbon which is generally independent of the environment of deposition. Reexamination of data from recent sediments from euxinic marine environments (e.g., the Black Sea) also shows a linear relationship between carbon and sulfur, but the slope is variable and the line intercepts the S axis at a value between 1 and 2 percent S. It is proposed that the positive S intercept is due to watercolumn microbial reduction of sulfate using metabolizable small organic molecules and the sulfide formed is precipitated and accumulates at the sediment-water interface. The variation in slope and intercept of the C to S plots for several cores and for different stratigraphic zones for the Black Sea can be interpreted in relation to thickness of the aqueous sulfide layer or thinness of the oxygen containing layer and to deposition rate, but also may be influenced by availability of iron, and perhaps the type of organic matter (Leventhal, 1979).  相似文献   

6.
Multiple sulfur isotope ratios （^34S/^33S/^32S） of Archean bedded sulfides deposits were measured in the Yanlingguan Formation of the Taishan Group in Xintai, Shandong Province, East of China; 633S = -0.7%o to 3.8‰,δ^34S = 0.1‰-8.8‰, △^33S = -2.3‰ to -0.7‰. The sulfur isotope compositions show obvious mass-independent fractionation （MIF） signatures. The presence of MIF of sulfur isotope in Archean sulfides indicates that the sulfur was from products of photochemical reactions of volcanic SO2 induced by solar UV radiation, implying that the ozone shield was not formed in atmosphere at that time, and the oxygen level was less than 10-5 PAL （the present atmosphere level）. The sulfate produced by photolysis of SO2 with negative △^33S precipitated near the volcanic activity center; and the product of element S with positive △^33S precipitated far away from the volcanic activity center. The lower △^33S values of sulfide （-2.30‰ to --0.25‰） show that Shihezhuang was near the volcanic center, and sulfur was mostly from sulfate produced by photolysis. The higher △^33S values （-0.5‰ to -‰） indicate that Yanlingguan was far away from the volcanic center and that some of sulfur were from sulfate, another from element S produced by photolysis. The data points of sulfur isotope from Yanlingguan are in a line parallel to MFL （mass dependent fractionation line） on the plot of δ^34S--δ^33S, showing that the volcanic sulfur species went through the atmospheric cycle into the ocean, and then mass dependent fractionation occurred during deposition of sulfide. The data points of sulfur isotope from Shihezhuang represent a mix of different sulfur source.  相似文献   

7.
The biogeochemistry of sedimentary sulfur was investigated on the continental shelf off central Chile at water depths between 24 and 88 m under partial influence of an oxygen minimum zone. Dissolved and solid iron and sulfur species, including the sulfur intermediates sulfite, thiosulfate, and elemental sulfur, were analyzed at high resolution in the top 20 cm. All stations were characterized by high rates of sulfate reduction, but only the sediments within the Bay of Concepción contained dissolved sulfide. Due to advection and/or in-situ reoxidation of sulfide, dissolved sulfate was close to bottom water values. Whereas the concentrations of sulfite and thiosulfate were mostly in the submicromolar range, elemental sulfur was by far the dominant sulfur intermediate. Although the large nitrate- and sulfur-storing bacteria Thioploca were abundant, the major part of S0 was located extracellularly. The distribution of sulfur species and dissolved iron suggests the reaction of sulfide with FeOOH as an important pathway for sulfide oxidation and sulfur intermediate formation. This is in agreement with the sulfur isotope composition of co-existing elemental sulfur and iron monosulfides. In the Bay of Concepción, sulfur isotope data suggest that pyrite formation proceeds via the reaction of FeS with polysulfides or H2S. At the shelf stations, on the other hand, pyrite was significantly depleted in 34S relative to its potential precursors FeS and S0. Isotope mass balance considerations suggest further that pyritization at depth includes light sulfide, potentially originating from bacterial sulfur disproportionation. The δ34S-values of pyrite down to −38‰ vs. V-CDT are among the lightest found in organic-rich marine sediments. Seasonal variations in the sulfur isotope composition of dissolved sulfate indicated a dynamic non-steady-state sulfur cycle in the surface sediments. The 18O content of porewater sulfate increased with depth at all sites compared to the bottom water composition due to intracellular isotope exchange reactions during microbial sulfur transformations.  相似文献   

8.
Previous efforts to constrain the timing of Paleoproterozoic atmospheric oxygenation have documented the disappearance of large, mass-independent sulfur isotope fractionation and an increase in mass-dependent sulfur isotope fractionation associated with multiple glaciations. At least one of these glacial events is preserved in diamictites of the ∼2.4 Ga Meteorite Bore Member of the Kungarra Formation, Turee Creek Group, Western Australia. Outcrop exposures of this unit show the transition from the Boolgeeda Iron Formation of the upper Hamersley Group into clastic, glaciomarine sedimentary rocks of the Turee Creek Group. Here we report in situ multiple sulfur isotope and elemental abundance measurements of sedimentary pyrite at high spatial resolution, as well as the occurrence of detrital pyrite in the Meteorite Bore Member. The 15.3‰ range of Δ33S in one sample containing detrital pyrite (−3.6‰ to 11.7‰) is larger than previously reported worldwide, and there is evidence for mass-independent sulfur isotope fractionation in authigenic pyrite throughout the section (Δ33S from −0.8‰ to 1.0‰). The 90‰ range in δ34S observed (−45.5‰ to 46.4‰) strongly suggests microbial sulfate reduction under non-sulfate limiting conditions, indicating significant oxidative weathering of sulfides on the continents. Multiple generations of pyrite are preserved, typically represented by primary cores with low δ34S (<−20‰) overgrown by euhedral rims with higher δ34S (4-7‰) and enrichments in As, Ni, and Co. The preservation of extremely sharp sulfur isotope gradients (30‰/<4 μm) implies limited sulfur diffusion and provides time and temperature constraints on the metamorphic history of the Meteorite Bore Member. Together, these results suggest that the Meteorite Bore Member was deposited during the final stages of the “Great Oxidation Event,” when pO2 first became sufficiently high to permit pervasive oxidative weathering of continental sulfides, yet remained low enough to permit the production and preservation of mass-independent sulfur isotope fractionation.  相似文献   

9.
The sulfur cycle of Mariager Fjord was studied by following the pool of sulfide in the anoxic water and its isotopic composition during a period of 3 yr. Though most of the sulfide accumulating in the fjord was formed in the sediment, the isotopic composition of sulfide in the water was different from the isotopic composition of sulfide diffusing into the water from the sediment. The mean isotopic composition of the water column sulfide (δ34S) varied during the year between −13‰ and −21‰ with the most negative values reached during winter/early spring, while the sulfide diffusing into the water from the sediment had a mean isotope composition of −11.3‰. This annual pattern suggested that processes in the oxidative part of the sulfur cycle were responsible for the excess fractionation, and mass-balance considerations indicated that the excess fractionation of the sulfur isotopes could be accounted for by disproportionation of S0 or S2O32− in the water column, but not by water column sulfate reduction or sulfide oxidation alone. MPN counts demonstrated that a population of more than 3 × 104 cells mL−1 capable of growing by disproportionation of these two substrates was present in all depths of the fjord. The results presented in this communication demonstrate that the isotopic depletion of sulfide in anoxic systems may vary between periods of net sulfate reduction versus periods of net sulfide oxidation and indicate that disproportionation of sulfur compounds may be an important step in the sulfur cycle of euxinic basins.  相似文献   

10.
We present a 3-year study of concentrations and sulfur isotope values (δ34S, Δ33S, and Δ36S) of sulfur compounds in the water column of Fayetteville Green Lake (NY, USA), a stratified (meromictic) euxinic lake with moderately high sulfate concentrations (12-16 mM). We utilize our results along with numerical models (including transport within the lake) to identify and quantify the major biological and abiotic processes contributing to sulfur cycling in the system. The isotope values of sulfide and zero-valent sulfur across the redox-interface (chemocline) change seasonally in response to changes in sulfide oxidation processes. In the fall, sulfide oxidation occurs primarily via abiotic reaction with oxygen, as reflected by an increase in sulfide δ34S at the redox interface. Interestingly, S isotope values for zero-valent sulfur sampled at this time still reflect production and recycling by phototrophic S-oxidation. In the spring, sulfide S isotope values suggest an increased input from phototrophic oxidation, consistent with a more pronounced phototroph population at the chemocline. This trend is associated with smaller fractionations between sulfide and zero-valent sulfur, suggesting a metabolic rate control on fractionation similar to that for sulfate reduction. Comparison of our data with previous studies indicates that the S isotope values of sulfate and sulfide in the deep waters are remarkably stable over long periods of time, with consistently large fractionations of up to 58‰ in δ34S. Models of the δ34S and Δ33S trends in the deep waters (considering mass transport via diffusion and advection along with biological processes) require that these fractionations are a consequence of sulfur compound disproportionation at and below the redox interface in addition to large fractionations during sulfate reduction. The large fractionations during sulfate reduction appear to be a consequence of the high sulfate concentrations and the distribution of organic matter in the water column. The occurrence of disproportionation in the lake is supported by profiles of intermediate sulfur compounds and by lake microbiology, but is not evident from the δ34S trends alone. These results illustrate the utility of including minor S isotopes in sulfur isotope studies to unravel complex sulfur cycling in natural systems.  相似文献   

11.
Pyritization in late Pleistocene sediments of the Black Sea is driven by sulfide formed during anaerobic methane oxidation. A sulfidization front is formed by the opposing gradients of sulfide and dissolved iron. The sulfidization processes are controlled by the diffusion flux of sulfide from above and by the solid reactive iron content. Two processes of diffusion-limited pyrite formation were identified. The first process includes pyrite precipitation with the accumulation of iron sulfide precursors with the average chemical composition of FeSn (n = 1.10-1.29), including greigite. Elemental sulfur and polysulfides, formed from H2S by a reductive dissolution of Fe(III)-containing minerals, serve as intermediates to convert iron sulfides into pyrite. In the second process, a “direct” pyrite precipitation occurs through prolonged exposure of iron-containing minerals to dissolved sulfide. Methane-driven sulfate reduction at depth causes a progressive formation of pyrite with a δ34S of up to +15.0‰. The S-isotopic composition of FeS2 evolves due to contributions of different sulfur pools formed at different times. Steady-state model calculations for the advancement of the sulfidization front showed that the process started at the Pleistocene/Holocene transition between 6360 and 11 600 yr BP. Our study highlights the importance of anaerobic methane oxidation in generating and maintaining S-enriched layers in marine sediments and has paleoenvironmental implications.  相似文献   

12.
Concentrations of oceanic and atmospheric oxygen have varied over geologic time as a function of sulfur and carbon cycling at or near the Earth’s surface. This balance is expressed in the sulfur isotope composition of seawater sulfate. Given the near absence of gypsum in pre-Phanerozoic sediments, trace amounts of carbonate-associated sulfate (CAS) within limestones or dolostones provide the best available constraints on the isotopic composition of sulfate in Precambrian seawater. Although absolute CAS concentrations, which range from those below detection to ∼120 ppm sulfate in this study, may be compromised by diagenesis, the sulfur isotope compositions can be buffered sufficiently to retain primary values.Stratigraphically controlled δ34S measurements for CAS from three mid-Proterozoic carbonate successions (∼1.2 Ga Mescal Limestone, Apache Group, Arizona, USA; ∼1.45-1.47 Ga Helena and Newland formations, Belt Supergroup, Montana, USA; and ∼1.65 Ga Paradise Creek Formation, McNamara Group, NW Queensland, Australia) show large isotopic variability (+9.1‰ to +18.9‰, −1.1‰ to +27.3‰, and +14.1‰ to +37.3‰, respectively) over stratigraphic intervals of ∼50 to 450 m. This rapid variability, ranging from scattered to highly systematic, and overall low CAS abundances can be linked to sulfate concentrations in the mid-Proterozoic ocean that were substantially lower than those of the Phanerozoic but higher than values inferred for the Archean. Results from the Belt Supergroup specifically corroborate previous arguments for seawater contributions to the basin. Limited sulfate availability that tracks the oxygenation history of the early atmosphere is also consistent with the possibility of extensive deep-ocean sulfate reduction, the scarcity of bedded gypsum, and the stratigraphic δ34S trends and 34S enrichments commonly observed for iron sulfides of mid-Proterozoic age.  相似文献   

13.
Integrated studies of seven Proterozoic sediment-hosted, Pb-Zn-Ag sulfide deposits of Brazil, permit the estimation of the age of the hosting sequence and the mineralization, the nature of the sulfur and metal sources, the temperature range of sulfide formation and the environment of deposition. These deposits can be classified into three groups, according to their ages. (a) Archean to Paleoproterozoic: the Boquira deposit, in Bahia state, consists of stratiform massive and disseminated sulfides hosted by parametamorphic sequences of grunnerite-cummingtonite+magnetite that represent a silicate facies of the Boquira Formation (BF). Lead isotope data of galena samples indicate a time span between 2.7 and 2.5 Ga for ore formation, in agreement with the stratigraphic position of the BF. The relatively heavy sulfur isotope compositions for the disseminated and stratiform sulfides (+8.3 to +12.8 ‰ CDT)suggest a sedimentary source for the sulfur. (b) Paleo to Mesoproterozoic: stratiform and stratabound sulfides in association with growth faults are present in the Canoas mine (Ribeira, in Paraná state) and in the Caboclo mineralization (Bahia state). They are hosted by calcsilicates and amphibolites in the Canoas deposit, whereas in the Caboclo area the mineralization is associated with hydrothermally altered dolarenites at the base of the 1.2 Ga Caboclo Formation. The interpreted Pb-Pb age of the Canoas mineralization is coeval with the 1.7 Ga host rocks. Sulfur isotopic data for Canoas sulfides (+1.2 to +16 ‰ CDT) suggest a sea water source for the sulfur. The range between −21.1 and +8.8 ‰ CDT for the Caboclo sulfides could suggest the action of bacterial reduction of seawater sulfates, but this interpretation is not conclusive. (c) Neoproterozoic: stratiform and stratabound sulfide deposits formed during the complex diagenetic history of the host carbonate rocks from the Morro Agudo (Bambui Group), Irecê and Nova Redenção (Una Group), yield heavy sulfur isotope values (+18.9 to +39.4 ‰ CDT). The uniform heavy isotope composition of the barites from these deposits (+25.1 to +40.9 ‰) reflect their origin from Neoproterozoic seawater sulfates. The late-stage, and most important, metallic concentrations represent sulfur scavenged from pre-existing sulfides or from direct reduction of evaporitic sulfate minerals. Lead isotope data from the Bambui Group suggest focused fluid circulation from diverse Proterozoic sediment sources, that probably was responsible for metal transport to the site of sulfide precipitation. (d) Late Proterozoic to Early Paleozoic: lead-zinc sulfides (+pyrite and chalcopyrite) of Santa Maria deposits, in Rio Grande do Sul, form the matrix of arkosic sandstones and conglomerates, and are closely associated with regional faults forming graben structures. Intermediate volcanic rocks are intercalated with the basal siliciclastic members. Lead isotope age of the mineralization (0.59 Ga) is coeval with the host rocks. Sulfur isotopic values between −3.6 and +4.1 are compatible with a deep source for the sulfur.Geological, petrographic and isotopic data of the deposits studied suggest that they were formed during periods of extensional tectonics. Growth faults or reactivated basement structures probably were responsible for localized circulation of metal-bearing fluids within the sedimentary sequences. Sulfides were formed by the reduction of sedimentary sulfates in most cases. Linear structures are important controls for sulfide concentration in these Proterozoic basins.  相似文献   

14.
The Irece Basin is a relatively small late Proterozoic basin within the São Francisco craton of northeast-central Brazil. Stratabound Zn-Pb-Ag sulfide and phosphate-rich units occur within a 50-m-thick tidal-flat sequence of dolomitic limestone and cherty dolostone within the lower Salitre Formation of the Una Group. Phosphate is concentrated in stromatolitic structures; metallic mineralization is represented by stratiform masses and disseminations of sulfide minerals in a tidal-flat sequence that has relict evaporitic structures.

Zn-Pb-Ag sulfide concentrations in the Irece area are associated with parallel laminated, silty dolostone with shallow-water sedimentary structures, including laminar stromatolites, mud cracks, tepee structures, collapse breccias, length-slow quartz nodules, and pseudomorphs after evaporitic sulfates. Sulfide minerals include pyrite, sphalerite, galena, marcasite, jordanite, tetrahedrite, and covellite, in order of decreasing abundance. Nodular and bladed sulfide forms are pseudomorphous after evaporitic sulfate minerals. Replacement textures, such as corroded grain boundaries between sulfide minerals, are common. Sulfur-isotope analysis indicates relatively uniform heavy δ34S values. Barite has a δ34S range from +25 to +31‰, CDT. Pyrite, sphalerite, galena, and jordanite-representative of a variety of textural types-have a δ34S range of +19 to + 23‰; δ34S values for Irece barite probably represent original late Proterozoic sea-water sulfate values. The consistently heavy δ34S values of the Irece sulfides likely are the result of thermochemical reduction of a limited evaporitic sulfate source by organic matter that is abundant in the Salitre carbonates.

Major Zn-Pb-Ag concentrations appear to be an overprint on the earlier iron sulfide-rich zone. Limited fluid-inclusion data suggest precipitation in the range of 140 to 200°C from formation waters with salinities ranging from 3 to 12 wt%. Deep fluid circulation may have been promoted by reactivation of basement faults originally related to the Irece Basin's development. Petrographic and fluid-inclusion data suggest that sulfide mineralization may have occurred within a mixing zone of metal-bearing, high-temperature basinal brines with meteoric water. Metallic mineralization scavenged sulfur from pre-existing sulfides or from direct reduction of evaporitic sulfate minerals.

The metal and phosphate concentrations in the Irece Basin possess some similiar geologic characteristics that result from a common host-rock depositional environment. However, existing information indicates that the two deposit types are not directly related genetically, although the origin of both is related to the organic-rich nature of the host sequence.  相似文献   

15.
Based on evaporite sequences of the Irkutsk amphitheater, it is shown that sulfur in the Vendian-Lower Cambrian sedimentary sulfates displays a very wide scatter of sulfur isotope ratios and enrichment in heavy sulfur up to average values of δ34S ≈ +(27–30)‰. These features are related to sulfate reduction, which is distinctly expressed like other secondary alterations in the studied sections. Average δ34S values reflect the dynamics of isotopic effects rather than the initial sulfur composition of the oceanic water. The Irkutsk amphitheater can be considered natural model of sulfur isotopic variations in ancient evaporites. Existing concepts of the sulfur isotopic composition of Cambrian oceans need to be revised.  相似文献   

16.
The isotopic composition of sulfur has been studied in plants representative of various regions of the U.S.S.R., two oceanic islands, and atmospheric precipitations on land and in marine areas. In soils, the isotopic composition of sulfur in the atmospheric water varies as a result of sulfate reduction (increase of δ34S of the soil sulfate) and sulfate regeneration from hydrogen sulfide. The sulfur in plants from the oceanic islands has characteristically higher values of δ34S than the sulfur in the plants and in the atmospheric water of the continents. Compared to sea water, the sulfur from the island plants that were studied contains a considerably lesser proportion of the 34S isotope. This can be explained by the significant role in such plants of the sulfur of the atmospheric air masses coming from the continents.  相似文献   

17.
与基性-超基性侵入体有关的Ni-Cu-PGE硫化物矿床是镍-铜-铂族元素矿床的最重要类型。传统观点认为,Ni-Cu-PGE硫化物矿床是由成矿岩浆分异演化、熔离形成的,与围岩性质关系不大。实际上,大部分基性-超基性岩浆是硫化物不饱和的,在岩浆自身演化过程中难以聚集大量硫化物而形成有经济价值的大型高品位NiCu-PGE硫化物矿床。因此,壳源硫的加入是基性-超基性岩浆中硫化物浓度达到过饱和,熔离形成Ni-Cu-PGE硫化物矿床的关键。膏盐层是富含石膏等硫酸盐(SO24-)的蒸发沉积建造,除SO24-外,还富含Cl-、CO23-、Na+、K+等盐类物质,在自然界分布广、面积大,是地壳中重要的硫源层和氧化障。但膏盐层在Ni-Cu-PGE硫化物矿床中的作用长期被忽视,制约了Ni-Cu-PGE硫化物矿床成矿找矿理论的发展。文章以世界最大的俄罗斯诺里尔斯克Ni-CuPGE硫化物矿床为例,介绍了膏盐层与矿床分布的空间关系、石膏等硫酸盐矿物在矿床和蚀变围岩中的分布、成矿元素和硫同位素组成特征及变化规律,阐明了膏盐层在成矿中的作用和控矿机理。膏盐(SO24-)的加入,可以大幅度提高成矿系统的氧逸度,将成矿岩浆中Fe2+氧化成Fe3+,形成铁氧化物,SO24-自身被还原,向成矿系统提供还原硫S2-,与Cu2+、Ni2+等结合,形成铜镍硫化物等,使基性-超基性成矿岩浆由硫化物不饱和变为过饱和,形成硫化物小液滴,在岩浆房经聚集-熔离-富集,形成岩浆型Ni-Cu-PGE硫化物矿床。除膏盐层外,富含硫化物的地层也是形成Ni-Cu-PGE硫化物矿床的重要硫源层。  相似文献   

18.
The Palinuro volcanic complex and the Panarea hydrothermal field, both located in the Tyrrhenian Sea (Italy), are associated with island arc magmatism and characterized by polymetallic sulfide mineralization. Dissolved sulfide concentrations, pH, and Eh measured in porewaters at both sites reveal a variable hydrothermal influence on porewater chemistry.Multiple sulfur isotopic measurements for disseminated sulfides (CRS: chromium reducible sulfur) extracted from sediments at Palinuro yielded a broad range in δ34S range between ?29.8 and + 10.2‰ and Δ33S values between + 0.015 and + 0.134‰. In contrast, sediments at Panarea exhibit a much smaller range in δ34SCRS with less negative values between ?11.3 and ?1.8‰. The sulfur isotope signatures are interpreted to reflect a mixture between hydrothermal and biogenic sulfide, with a more substantial biogenic contribution at Panarea.Multiple sulfur isotope measurements were performed on sulfides and elemental sulfur from drill core material from the Palinuro massive sulfide complex. δ34S and Δ33S values for pyrite between ?32.8 and ?1.1‰ and between ?0.012 to + 0.042‰, respectively, as well as for elemental sulfur with δ34S and Δ33S values between ?26.7 and ?2.1‰ and between + 0.035 and + 0.109‰, respectively, point to a microbial origin for much of the sulfide and elemental sulfur studied. Moreover, data suggest a coupling of bacterial sulfate reduction, sulfide oxidation and sulfur disproportionation. In addition, δ34S values for barite between + 25.0 and + 63.6‰ are also in agreement with high microbial turnover of sulfate at Palinuro.Although a magmatic SO2 contribution towards the formation of the Palinuro massive sulfide complex is very likely, the activity of different sulfur utilizing microorganisms played a fundamental role during its formation. Thus, porewater and multiple sulfur isotope data reveal differences in the hydrothermal activity at Palinuro and Panarea drill sites and underline the importance of microbial communities for the origin of massive sulfide mineralizations in the hydrothermal subsurface.  相似文献   

19.
Sedimentary pyrite formation: An update   总被引：1，自引：0，他引：1  
Sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time. The amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter, dissolved sulfate, and reactive detrital iron minerals. Organic matter appears to be the major control on pyrite formation in normal (non-euxinic) terrigenous marine sediments where dissolved sulfate and iron minerals are abundant. By contrast, pyrite formation in non-marine, freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic-rich fresh water shales from marine shales. Under marine euxinic conditions sufficient H2S is produced that the dominant control on pyrite formation is the availability of reactive iron minerals.Calculations, based on a sulfur isotope model, indicate that over Phanerozoic time the worldwide average organic carbon-to-pyrite sulfur ratio of sedimentary rocks has varied considerably. High CS ratios during Permo-Carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time. Low CS ratios, compared to today, during the early Paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean. The latter could have been due to lower oceanic oxygen levels and/or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time.  相似文献   

20.
Five temperate seagrasses (Amphibolis antartica, Halophila ovalis, Posidonia australis, Posidonia sinuosa and Zostera nigricaulis) were surveyed along the south-west coast of Western Australia. These morphological different seagrasses grow in contrasting sediments with large variations in sedimentary organic matter, carbonate and iron contents. We tested if sulfur composition in the plants responded to sulfur dynamics in the sediments and if plant morphology affected the sulfur composition of the plants. The sediments were characterized by low sulfate reduction rates (<9 mmol m?2day?1), low concentrations of dissolved sulfides in the pore waters (<74 μM) and low burial of sulfides (total reducible sulfur <0.8 mol m?2) in the sediments. However, all seagrasses showed high intrusion in the below-ground parts with up to 84 % of the sulfur derived from sedimentary sulfides. There were no direct links between sulfur in the plants and sulfur dynamics in the sediments, probably due to low iron contents in the sediments limiting the buffering capacity of the sediments and exposing the plants to sulfides despite low rates of production and low pools of sulfides. The intrusion was linked between plant compartments (roots, rhizomes and leaves) for the two small species (H. ovalis and Z. nigricaulis), whereas the intrusion into the leaves was limited for the larger species (P. australis and P. sinuosa) and for A. antarctica, where extensive rhizomes and roots and the long stem for A. antarctica separate the leaves from the sediment compartment. Elevated intrusion was observed at two study locations, where natural deposition of organic matter or nutrient enrichment may be contributing factors to enhanced sulfide pressure.  相似文献   

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