Sulfur Isotopic Composition Of Pyrite Research Articles

Sulfur isotope geochemistry of the submarine hydrothermalcoastal vents of Punta Mita, Mexico

Coastal hydrothermal submarine vents occur near Punta Mita (Mexico) through a seafloor fissure hosted by basaltic rocks and partially covered by recent sediments. Hydrothermal venting produces calcareous tufa mounds, with minor sulfides, barite and apatite. Pyrite is the most abundant sulfide mineral that precipitates around the submarine hot springs. It occurs as framboids, euhedral crystals, thin botryoidal coatings interbedded with calcite, and as replacement of detrital magnetite grains. Vent fluids have temperature below 100 °C, and methane is abundant. Sulfur isotopic composition of pyrite predominantly has δ 34S values from − 10.7 to +4.9 suggesting that the source of sulfur is microbially reduced sulfate seawater.

Coupled primary production, benthic foraminiferal assemblage, and sulfur diagenesis in organic-rich sediments of the Benguela upwelling system

Abstract Episodically deposited, dark, organic-rich Pleistocene and Late Pliocene sediments from the lower continental slope off southwest Africa reveal complex interactions between changes in primary production, benthic foraminiferal assemblage, and anaerobic microbial processes. The organic-rich layers contain diatom assemblages characteristic of intense seasonal coastal upwelling whereas stratigraphically adjacent sediments reflect pelagic primary production. Coastal upwelling-dominated depositional intervals coincide with periods of enhanced carbon flux to the seafloor. Enhanced organic carbon export during dark layer deposition was accompanied by decreases in the diversity of benthic foraminifera to few opportunistic species adapted to high phytodetritus accumulation rates and low O 2 conditions. In all sediments the sulfur isotopic composition of pyrite indicates redox cycling of sulfide close to the sediment/water interface. The sulfur isotopic evidence and the permanent presence of abundant low O 2 -adapted benthic foraminifera throughout the organic-rich layers suggest an oxygenated benthic environment. Efficient oxidation of sulfide and removal of sulfide by sulfidization of organic matter inhibited buildup of toxic hydrogen sulfide from bacterial sulfate reduction at the sediment/water interface. These data imply that in continental slope sediments underneath productive surface waters benthic dysoxic conditions are maintained by the lateral advection of dissolved oxygen to support a small, but well-adapted benthic community.

Stable Sulfur Isotopic Evidence for Historical Changes of Sulfur Cycling in Estuarine Sediments from Northern Florida

Data on abundance and isotopic composition of porewater and sedimentary sulfur species are reported for relatively uncontaminated and highly contaminated fine-grained anoxic sediments of St. Andrew Bay, Florida. A strong contrast in amount and composition of sedimentary organic matter at the two sites allows a comparative study of the historical effects of increased organic loading on sulfur cycling and sulfur isotopic fractionation. In the contaminated sediments, an increase in organic loading caused increased sedimentary carbon/sulfur ratios and resulted in higher rates of bacterial sulfate reduction, but a lower efficiency of sulfide oxidation. These differences are well reflected in the isotopic composition of dissolved sulfate, sulfide, and sedimentary pyrite. Concentration and isotopic profiles of dissolved sulfate, organic carbon, and total sulfur suggest that the anaerobic decomposition of organic matter is most active in the upper 8cm but proceeds at very slow rates below this depth. The rapid formation of more than 90% of pyrite in the uppermost 2 cm which corresponds to about 3 years of sediment deposition allows the use of pyrite isotopic composition for tracing changing diagenetic conditions. Sediment profiles of the sulfur isotopic composition of pyrite reflect present-day higher rates of bacterial sulfate reduction and lower rates of sulfide oxidation, and record a profound change in the diagenetic cycling of sulfur in the contaminated sediments coincident with urban and industrial development of the St. Andrew Bay area.

Origins of pyrites in the ∼2.5 Ga Mt. McRae Shale, the Hamersley District, Western Australia

The Mt. McRae Shale is the footwall rock unit of the Brockman Iron Formation (∼2.5 Ga in age) in the Hamersley Basin, Australia. It is characterized by a high concentration of organic carbon (∼2–8 wt%), an abundance of disseminated pyrite (∼1 to ∼10 wt% S in the bulk rocks), and abundant pyrite nodules (∼1–10 cm radius). We have examined microscale (∼200 μm to 1 cm) variations in sulfur isotopic compositions of pyrite and S and C contents in six rock samples from a ∼27 m drill core section of the base of the Mt. McRae Shale at the Whaleback Mine. The microanalyses of sulfur isotope compositions were performed in situ on single or aggregates of pyrite crystals (eighty-four analyses) using a Nd-YAG laser microprobe. The carbon contents of thirty powdered samples, drilled from different parts of the six rock samples, are similar (∼2 to ∼8 wt% C) with the exception of one sample (0.5 wt%). However, the occurrence, morphology, abundance, and δ 34S value of pyrite reveal distinct differences between the two samples from the upper part and the four samples from the lower part of the drill core section. Pyrite crystals in the upper part occur mostly as disseminated fine grains (∼5 μm). The pyrite S contents are uniform within each sample, but the δ 34S values vary from −6.3 to +7.1‰. These data suggest that pyrite crystals in the upper section were formed by bacterial reduction of seawater sulfate. Pyrite crystals of the lower section occur in the form of veinlets, nodules, or laminae of coarse grains (∼200 μm): the pyrite contents are highly variable within each specimen (0 to >10 wt%), and the δ 34S values vary from +2.2 to +11.8‰. The formation process of pyrites in the lower section appears to have been complicated: pyrite laminae were first formed by bacterial sulfate reduction during early diagenesis, and then some of the early pyrites were dissolved and reprecipitated to form pyrite nodules by later diagenetic or hydrothermal solutions in a closed-system. The sulfur isotope data obtained in this study can be best explained by a model postulating that the seawater about 2.5 Ga ago in the Hamersley Basin had the δ 34S value of +10 to +15‰ and that the kinetic isotope effects accompanying bacterial sulfate reduction were 8–13‰ and 16–21‰, respectively, during the deposition of the lower and upper sections of the McRae Shale. The variable δ 34S values of microscale area and the magnitudes of the kinetic isotope effects suggest that: (1) the sulfate concentration of the 2.5 Ga seawater was already more than one-third of the present seawater value, and (2) the activity of sulfate-reducing bacteria in the 2.5 Ga ocean was generally higher than that in the modern ocean. Suggestion 1 further implies that, by 2.5 Ga, (3) the atmosphere became oxic, and (4) the chemical and isotopic characteristics of sulfur in the earth’s near surface reservoirs (oceans and sediments) were controlled by the Phanerozoic-style biogeochemical cycles, rather than by the mantle-buffer (i.e., magmatic) mechanism. Suggestion 2 may imply that (5) the Archean oceans were generally warmer compared to the modern oceans, and (6) they produced a higher abundance of organic matter that were easily metabolized by sulfate-reducing bacteria.

Pyrite formation by reactions of iron monosulfides with dissolved inorganic and organic sulfur species

Pyrite formation has been investigated at 70°C and pH 6–8 by aging precipitated, disordered mackinawite, Fe9S8, and greigite, Fe3S4, in solutions containing aqueous H2S, HS−, Sx2−, S2O32−, SO32−, colloidal elemental sulfur, and the organic sulfur species thiol, disulfide, and sulfonate. Pyrite formed in all experiments where unoxidized iron monosulfides were aged with species containing zero-valent sulfur, i.e., polysulfides and colloidal elemental sulfur, but not with hydrogen sulfide (or bisulfide), the sulfoxy anions, or the organic sulfur species. Pyrite formation also occurred in experiments where the starting monosulfides were air-exposed prior to aging in sulfide solutions, or when air was bubbled through a reaction vessel containing iron monosulfides suspended in a sulfid sulfide solution. The experiments indicate the rate of conversion from iron monosulfides to pyrite is not only a function of solution chemistry (i.e., pH and aqueous speciation), but also depends on the surface oxidation state of the precursor iron monosulfides.Measurements of δ34S of reactants and products from pyrite-forming experiments suggest that the conversion from iron monosulfides to pyrite may proceed via loss of ferrous iron from, rather than via addition of zero-valent sulfur to, the precursor monosulfides. The sulfur isotopic composition of pyrite in sedimentary environments should reflect the sulfur isotopic composition of the precursor iron monosulfide plus sulfur sources incorporated during surface-controlled growth processes.Pyrite forms produced in this study ranged from poorly developed octahedral grains, in experiments where initial pyritization rates were the slowest, to framboidal aggregates in experiments where initial pyritization rates were the fastest. Although greigite formation occurred in experiments that produced framboids, not all experiments that produced greigite led to framboid formation. The formation of pyrite with framboidal texture is apparently favored when iron monosulfides rapidly convert to pyrite.

The genesis of Lincang germanium deposit — A preliminary investigation

The mechanism of formation of the Lincang germanium deposit is discussed in the light of the spatial distribution of Ge-rich coal and siliceous rocks, the sulfur isotopic composition of pyrite in the Ge-rich coal, the variation of Ge abundance in the coal seams and the geochemical characteristics of the siliceous rocks. The results show that the siliceous rocks intercalated with the coal seams were deposited from a hydrothermal medium through which germanium was enriched in the coal beds. The primary source of germanium is thought to be the Ge-rich granite in the basement of the sedimentary basin.

Hydrocarbon-water interactions during brine migration: Evidence from hydrocarbon inclusions in calcite cements from Danish North Sea oil fields

Crude oils in primary and secondary fluid inclusions in calcite from fractures in seven offshore oil fields associated with diapiric salt structures in the Danish sector of the North Sea were analyzed by capillary column gas chromatography and compared with crude oils produced from the same reservoirs. Oils from fluid inclusions in all fields show evidence of biodegradation (decreased n- C 17/pristane and n- C 18/phytane ratios and loss of n-C 7, 2-methyl hexane, and 3-methyl hexane relative to methyl cyclohexane) and water washing (absence of benzene and depletion of toluene). Some oils in inclusions are extremely enriched in C 6 and C 7 cyclic alkanes suggesting that these samples contain hydrocarbons exsolved from ascending, hotter formation waters. Compared to inclusion oils the produced oils are less biodegraded, but are water washed, indicating that both types of oil interacted with large volumes of formation water. The carbon isotopic composition of the calcite host of the fluid inclusions in the Dagmar and Skjold fields is as light as −16.5%. PDB and the sulfur isotopic composition of pyrite in and adjacent to the calcite veins in the Skjold field is as light as −39.6%. CDT, indicating that biodegradation of the oils was a source of some of the carbon in the calcite and sulfate reduction was the source of sulfur for the pyrite. The evidence for microbial degradation of petroleum is consistent with present-day reservoir temperatures (65°–96°C) but is not consistent with previous estimates of the temperatures of calcite vein filling (95°–130°C) which are much higher than the temperatures of known occurrences of biodegraded oil.

Stable isotope compositions of some vein minerals from the Oe mine, southwestern Hokkaido, Japan.

Sulfur isotopic compositions of pyrite, sphalerite, galena and barite, and also carbon and oxygen isotope ratios of rhodochrosite and calcite were measured so as to estimate the origin of ore-forming sulfur and carbon responsible for mineralizations found in the Oe deposit within the so-called “Green Tuff” Region, Japan. The δ34S values of sulfides are in a comparatively narrow range of 5.0 to 10.0‰, which are similar to those of sulfide minerals from most Kuroko deposits in Japan. On the other hand, the δ13C and δ18O values of rhodochrosite measured are -6.4 to -9.4‰ and 2.9 to 9.9‰, respectively. Because formation temperature at the main stage is assumed to be near 240°C on the basis of sphalerite-galena and pyrite-sphalerite thermometers, the bulk isotope compositions of sulfur and carbon in the fluids can be considered on the log fO2-pH diagram at 240°C. Consequently, it is proved that the isotope ratios of total sulfur and total carbon in the fluids can be regarded to be approximately 22‰ and -10‰, respectively. The former value corresponds to that for marine sulfate in the Neogene Tertiary age, so sea water sulfate is thought to have been related considerably to the sulfide mineralization. The latter value and oxygen isotope data, however, suggest a possibility that the ore-forming carbon responsible for the carbonate mineralization is meteoric in origin. Therefore, such a mechanism as leaching of “fossil” sea water sulfate by meteoric water should be taken into account for the Mn-Pb-Zn type mineralization observed in southwestern Hokkaido.

In situ microanalysis for 34S/ 32S ratios using the ion microprobe SHRIMP

The SHRIMP ion microprobe has been used to determine the relative sulfur isotopic compositions of pyrite, pyrrhotite, chalcopyrite, galena, sphalerite, and barite, employing a 10 kV, 3nA, negative oxygen primary ion beam and measuring positive secondary ions accelerated to 11 kV. Tests showed the minerals to be stable emitters of secondary ions, and intensities of the 32S + secondary ion beams ranged from 0.3 to 0.07 MHz in the order: sphalerite > pyrite > chalcopyrite > pyrrhotite > galena > barite, enabling measurement of 34S/ 32S ratios to a precision and accuracy near ±2 permil (2σ). Materials of diverse 34S concentration and/or matrix were repeatedly analyzed without detectable memory effect. Isotopic fractionation during analysis was mineral species-dependent and linear in terms of 32S, 33S, and 34S over a range in delta values of 60 permil, producing easily corrigible ratios and eliminating the need for standards isotopically similar to the samples. Fractionation varied from 1.5 to 6% u −1 in the order: galena < chalcopyrite < pyrrhotite = pyrite < barite and follows the relative bond strengths of the minerals. This apparently simple fractionation behavior suggests that, ultimately, it may be possible to reference all analyses to a single standard. Some irregularity in the fractionation of ratios from sphalerite, possibly related to matrix effects, has delayed its incorporation into this scheme.

A sulfur isotope study on pyrite deposits of Southern Tuscany, Italy

The S-isotope composition (δ34SCDT) of 213 samples of sulfides, sulfates and native sulfur from the pyrite mineralizations of southern Tuscany and associated country rocks were determined. The sulfur isotopic composition of pyrite is quite homogeneous and similar for all studied ore bodies, with an average δ34S value near +9,5‰. Pyrite disseminated within the Filladi di Boccheggiano formation, and thought to be authigenic, shows a much larger range of δ34S values (-13.1 to +14.5‰). The isotopic compositions of other sulfides associated with pyrite in the deposits show that isotopic equilibrium among sulfides was approached on a regional scale, but seldom fully attained. Isotopic data suggest that sedimentary marine sulfate was the ultimate source of sulfur in ores.