Seawater Sulfate Research Articles

Origin of the gypsum veins associated with the diagenetic dolomite nodules from the Neogene Marls of South-East Spain

The late Neogene marls from the Lorca, Murcia and Vera basins in S-E. Spain contain abundant dolomite nodules that were formed due to intense methane-rich fluid migration. The pathways for these fluids are evidenced by dense networks of fractures that are crossing the sedimentary layers and eventually the dolomite nodules. The fractures are generally filled by secondary fibrous gypsum that form veins of a few cm thick.The oxygen and sulfur isotopic compositions of the gypsum veins exhibit wide ranges (−2.2 < δ18O‰ VSMOW <6.7; −22.3 < δ34S‰ VCDT <10.5) that are far away from the average δ values of Tortonian-Messinian gypsum from the basins of Lorca, Fortuna and Sorbas (δ18O = 13.8 ± 2.3‰; δ34S = 21.3 ± 1.0‰) precipitated from the late Neogene seawater sulfate. These low δ values of the gypsum veins clearly indicate their diagenetic origin that would have resulted from sulfide oxidation with 18O-depleted water. The isotopic compositions of the diagenetic dolomite nodules close to the gypsum veins (−0.7 < δ18O‰ VPDB <3.7; −11.9 < δ13C‰ VPDB <7.3) are indicative of their formation in methane-rich fluids. In this context, the formation of sulfide is related to the bacterial sulfate reduction that is associated with the anaerobic oxidation of methane. During this process, pyrite coprecipitated with dolomite, either within the dolomite framework or within conduits where the fluids were seeping. The pyrite oxidation occurred later during diagenesis when more or less oxygenated groundwaters circulated within the fractures during the sealevel drawdown of the Messinian Salinity Crisis and after the regional uplift of the sedimentary deposits in relation with the Betic cordillera tectonics.

The Ab-Bagh Late Jurassic-Early Cretaceous sediment-hosted Zn-Pb deposit, Sanandaj-Sirjan zone of Iran: Ore geology, fluid inclusions and (S–Sr) isotopes

The Ab-Bagh clastic- and carbonate-hosted stratabound and stratiform Zn-Pb deposit in the Sanandaj-Sirjan zone, in the southeastern corner of the Malayer-Esfahan metallogenic belt of Iran, is typical of several sedimentary–exhalative (SEDEX) deposits in this metallogenic belt. The deposit is hosted in a late Jurassic-early Cretaceous sedimentary sequence. Based on position, there are two Zn-Pb-bearing stratigraphic ore horizons. Ore horizon 1 is hosted by late Jurassic-early Cretaceous black shale and siltstone. The wedge-shaped ore body is located close to a synsedimentary normal fault. Petrographic studies indicates that mineralization comprises three sulfide ore facies: stockwork, bedded and massive ore facies. Ore horizon 2 occurs in early Cretaceous carbonates and comprises a massive-replacement ore facies that is concordant with host rock layering; it is also underlain by a stockwork facies. Textures include framboidal, laminated, breccia, replacement, massive and vein-veinlet. Hypogene sulfide minerals are predominantly sphalerite, galena, pyrite and chalcopyrite. Calcite and quartz are major gangue minerals. Silicification and carbonatization are the main wallrock alteration styles; alteration intensity increases towards the feeder zone. Based on relationships between ore minerals and rock-forming minerals, sulfide mineralization in the Ab-Bagh deposit formed during two main stages: fine-grained sulfide bands (stage I) are intricately interlayered with organic matter-rich beds of black shale and siltstone. Sulfide bands exhibit classic sedimentary textures, such as laminations and bedding, indicative of a synsedimentary origin. Coarser-grained stage II base metal sulfides show breccia and vein-veinlet textures, and are considered to have formed by replacement during sub-seafloor fluid flow. Fluid inclusion microthermometry was carried out on calcite associated with sulfides of the stockwork ore facies. Homogenization temperatures are in the range of 180–301 °C, and the average salinity is 17.93 wt% NaCl eq. The size distribution of pyrite framboids of the bedded ore, geochemistry of host rock and presence of organic matter, collectively suggest anoxic to locally suboxic ambient conditions for the host basin. The δ34S composition of pyrite, sphalerite, and galena ranges from −5.08 to +4.51‰. The highest δ34S values are from massive and stockwork ore facies (+2.66 to + 4.51‰) and the lowest from bedded ore (−5.08 to −4.72‰). In the bedded ore facies, the main sulfur source was bacterially reduced seawater sulfate. Whereas in massive and stockwork ore facies, thermochemically reduced seawater sulfate was the main source of sulfur. The 87Sr/86Sr ratios of calcite indicate that origin of ore forming fluids is Paleozoic radiogenic basement. Collectively, the isotopic, sedimentological, textural, mineralogical, fluid inclusion, and geochemical data for the Ab-Bagh deposit support a vent-proximal sub-seafloor replacement SEDEX ore deposit.

Genesis of the Tuokesai Zn-Pb deposit in the Sayram Massif, Xinjiang, NW China: Constraints from geology, jasperite geochemistry and Si-S-Pb isotopes

The Zn-Pb potential of the Sayram Massif has drawn significant attention since the discovery of the large Tekeli SEDEX Zn-Pb deposit, which is located 15 km from the Kazakhstan and Chinese border in Xinjiang. The Tuokesai Zn-Pb deposit was discovered in 2009 in the northern margin of the Sayram Massif, a major Precambrian terrane of the Kazakhstan-Yili Block. The geological characteristics of the ore-bearing strata at Tuokesai are similar to those of the nearby large Tekeli SEDEX mineral system. Tuokesai’s lens-shaped Zn-Pb orebodies are stratiform and conformable with marble of the Neoproterozoic Wenquan Group. The ore-bearing marbles are commonly interbedded with jasperite that is mainly composed of fine-microcrystalline quartz. The ore minerals include sphalerite, galena and pyrite with minor pyrrhotite, and they are mainly fine-grained with a laminate texture. The gangue minerals are mainly calcite and quartz. Both the jasperite’s relatively high δ30SiV-NBS-28 values (1.3 ~ 2.2‰) and it’s REE patterns show a seawater signature, whereas both its Y/Ho ratios (concentrated at 30.48 ~ 48.57) and its weakly positive Eu anomalies imply a minor hydrothermal influx. The jasperite’s Fe-Al-Ti compositions and La/Ce ratios, combined with the lithology of the host Wenquan Group, suggest that the ore-bearing strata were formed in a passive margin basin. The S isotopic compositions of Tuokesai’s Zn-Pb ores (8.1 ~ 20.4‰) suggest that the sulfur was derived from contemporaneous seawater sulfate. The Pb isotopic compositions of the metallic ore minerals (206Pb/204Pb = 17.227 ~ 17.549, 207Pb/204Pb = 15.447 ~ 15.547, 208Pb/204Pb = 36.912 ~ 37.097) indicate that they came from a mixed crustal-mantle source. In summary, Tuokesai is a SEDEX-type Zn-Pb deposit that was formed in a passive margin basin during Neoproterozoic. The Tuokesai deposit’s discovery suggests that the Sayram Massif may represent the eastern extension of the Tekeli metallogenic belt and has a high potential for successful Zn-Pb prospecting.

Spatiotemporal evolution and causes of marine euxinia in the early Cambrian Nanhua Basin (South China)

Marine redox conditions and their dynamic variations in the early Cambrian are thought to have strongly influenced the evolution of early animals. However, the patterns and mechanisms of contemporaneous marine redox variation remain poorly understood. The Nanhua Basin (South China) was a failed rift basin located between the Yangtze Block (NW margin) and Cathaysia Block (SE margin) in the early Cambrian. In this study, we conducted a comprehensive study of marine redox conditions, sulfur cycling, and chemical weathering intensity of source areas for the lower Cambrian shales of the Qingxi Formation in the basinal Shimensuidao section, which was geographically close to the Cathaysia Block. We integrated these results with previously published data from the nearby Silikou section as well as the coeval inner-shelf Xiaotan, upper-slope Daotuo, lower-slope Longbizui, and basinal Zhalagou sections, where were closer to the Yangtze Block. At Shimensuidao, iron species and redox-sensitive trace element data suggest dominantly euxinic conditions for the organic-rich black shale interval (lower member; LM) and variable redox conditions (oxic-ferruginous-euxinic) for the overlying organic-lean shales (upper member; UM) of the Qingxi Formation. Integrated redox results show a similar lessening of euxinic conditions from LM to UM across the entire basin and that euxinia developed along both the Yangtze and Cathaysia basin margins, but it was mainly in upper-slope areas (e.g., Daotuo) on the Yangtze margin and reached basinal areas (e.g., Silikou and Shimensuidao) on the Cathaysia margin. A decline of δ34Spy from LM to UM was observed at Shimensuidao and other sections, which can be explained by decreasing microbial sulfate reduction rates due to decreasing organic availability and increasing riverine sulfate flux at study sections. Lower δ34Spy values in shallow-marine relative to deep-marine areas suggest increased terrestrial sulfate availability to the shallower sites. Chemical index of alteration (CIA) values at Shimensuidao are uniformly high for both the LM (CIAcorr1: 82.5 ± 0.4, CIAcorr2: 88.6 ± 0.5) and UM (CIAcorr1: 80.2 ± 2.3, CIAcorr2: 86.2 ± 2.5) compared with the low-to moderate values at Daotuo (CIAcorr2: 61.9 ± 3.9 for LM, 71.3 ± 4.8 for UM). This pattern is consistent with intense chemical weathering of source areas on the Cathaysia Block and weaker chemical weathering of source areas on the Yangtze Block. Based on these observations, we propose that strong chemical weathering of the Cathaysia Block led to regionally high terrigenous sulfate fluxes, resulting in expansion of mid-depth euxinia into deep basinal areas on the Cathaysia margin of the Nanhua Basin. In contrast, weak chemical weathering of the Yangtze Block yielded lower terrigenous sulfate fluxes, limiting euxinia to upper slope areas on the Yangtze margin of the basin. Weaker euxinia during UM deposition may have been due to its decreasing organic availability and/or small seawater sulfate reservoir caused by persistent euxinic consumption during LM deposition. Our results highlight the complex controls and the key role of continental weathering and resulting terrigenous sulfate fluxes in marine euxinic variations in early Cambrian marginal seas, providing new insights into the co-evolution of continents, oceans, and early animals.

Major and trace element and multiple sulfur isotope composition of sulfides from the Paleoproterozoic Surda copper deposit, Singhbhum shear Zone, India: Implications for the mineralization processes

The present study combines major and trace element composition, and sulfur (S) isotope data of pyrite and chalcopyrite from the Surda copper sulfide deposit in the Singhbhum Shear Zone, the most important copper and uranium producing belt (Singhbhum Cu-U Belt) in India. Three textural and compositional types of both pyrite and chalcopyrite were distinguished; unzoned to partially zoned Pyrite IA with high Co (up to 54900 ppm) and low Ni content is earliest, followed by oscillatory zoned Pyrite IB with high As (up to 25600 ppm) and Co (up to 46800 ppm), both occurring in pyrite I + chalcopyrite I + pyrrhotite + magnetite + apatite vein; Cobaltite-type substitution (Fe1−xCox)(S1−xAsx)2 is suggested for Pyrite IB. Gold occurs in Pyrite I as minor “invisible” gold and as electrum inclusions. It also occurs along with Cu, Mn, Ni, Hg, Ag, Pb, Sb, Zn, Ce, Y, U, and Th in micro-fractures that transgress the primary zoning pattern defined by As, Co, and Ni in Pyrite IB. The early inclusion-rich chalcopyrite generation (Chalcopyrite I) contains high concentration of Zn and Se, and minor to trace amounts of Co, Ni, Hg, Pb, Sb, Te, and Bi, appeared in between Pyrite I and II. Low Co, high Ni (up to 37700 ppm) content Pyrite II, and inclusion-free Chalcopyrite II enriched in Co, Ni, Hg, Bi, Mn, Ag, Sb, V, and Pb are cogenetic, and occurring in pyrite II + pyrrhotite + pentlandite + chalcopyrite II ± violarite vein. Low Co and Ni containing Pyrite III + Chalcopyrite III occur mainly as disseminated grains. The relative timing of formation of Pyrite II + Chalcopyrite II with Pyrite III + Chalcopyrite III remains uncertain.Pyrite + chalcopyrite textures indicate that all pyrite + chalcopyrite formed at some time prior to the end of deformation and metamorphism.Both in situ and mineral separates of all pyrite types and associated chalcopyrite yield a narrow range of positive δ34S values (between +3.8 and +6.9‰) suggesting sulfur being derived from a similar source. Consistent positive δ34S values and other circumstantial evidence indicate that most sulfur was derived from seawater sulfate or modified seawater (brine/evaporite). Δ33S values revealed mass dependent fractionation (MDF) signature. It is proposed that incorporation of MDF sulfur of the mineralization event in Paleoproterozoic Singhbhum Cu-U Belt took place after the great oxidation event. The high Se concentrations (260–400 ppm) and ∑Se/∑S ratios for both Pyrite I and II from Surda deposit (4.4–5.7 × 10−4) suggest a low temperature of the Cu–rich ores (250°–350 °C), and precipitation from a metalliferous fluid with a high ∑Se/∑S ratio (10−4 to 10−3) consistent with igneous input of these elements.

Organic sulfur: A spatially variable and understudied component of marine organic matter

AbstractSulfur (S) is a major heteroatom in organic matter. This project evaluated spatial variability in the concentration and molecular‐level composition of organic sulfur along gradients of depth and latitude. We measured the concentration of total organic sulfur (TOS) directly from whole seawater. Our data reveal high variability in organic sulfur, relative to established variability in total organic carbon or nitrogen. The deep ocean contained significant amounts of organic sulfur, and the concentration of TOS in North Atlantic Deep Water (NADW) decreased with increasing age while total organic carbon remained stable. Analysis of dissolved organic matter extracts by ultrahigh resolution mass spectrometry revealed that 6% of elemental formulas contained sulfur. The sulfur‐containing compounds were structurally diverse, and showed higher numbers of sulfur‐containing elemental formulas as NADW moved southward. These measurements of organic sulfur in seawater provide the foundation needed to define the factors controlling organic sulfur in the global ocean.

Sub-seafloor sulfur cycling in a low-temperature barite field: A multi-proxy study from the Arctic Loki’s Castle vent field

The Loki's Castle vent field at the ultraslow-spreading Arctic Mid-Ocean Ridge (AMOR) hosts a low-temperature venting area, which is characterized by microbial mats and numerous up to 1 m tall barite chimneys. An actively-venting barite chimney yielded δ18OSO4 and δ34SSO4 values heavier than ambient seawater and sulfide-oxidizing bacteria in microbial mats identified microbial sulfate reduction and sulfide oxidation as the main processes. In order to investigate the chemical and microbial structure below the barite field, we obtained two gravity cores and present chemical composition (e.g., H2S, SO42−, NH4+, DIC) and stable isotope data for the pore fluids (δ34SSO4, δ13CDIC) together with stable isotope (δ13Corg, δ34S, ∆33S) and lipid biomarker data on bulk sediments. The gravity core more distant to the high-temperature vents shows seawater-like pore fluid profiles with only minor vent fluid contribution (<1%), whereas the bulk sediments yield negative δ34S and positive ∆33S values indicative of sulfate reduction. In contrast, the pore fluid data in close proximity to the high-temperature vents (5–9% vent fluid contribution) record distinct horizons showing sulfate depletion, which coincide with δ34SSO4 values that are higher than those for ambient seawater sulfate. The sediments in these horizons record negative δ34S and positive ∆33S values, indicating that both the pore fluids and the sediments are influenced by active sulfate reduction. We also detected a greater abundance of archaeal mono- and dialkyl tetraether lipids (GMGTs, GDGTs) and bacterial fatty acids in the sediments at actively venting sites, pointing to a more diverse microbial community. Moreover, a positive correlation observed between GMGT abundance and sulfur concentration in the sediments indicates that the availability of sulfur is crucial for the presence of GMGT-producing archaea. Our multi-proxy approach suggests that sulfate reduction in the sub-seafloor sediments of the Loki's Castle barite field is largely driven by microbial processes.

Hydrothermal influence on barite precipitates in the basal Ediacaran Sete Lagoas cap dolostone, São Francisco Craton, central Brazil

Most Precambrian sediment-hosted barites have been interpreted to be diagenetic, hydrothermal exhalation, or methane-seepage in origin. Seafloor barite precipitates and void-filling barite cements in basal Ediacaran cap dolostones have been interpreted as sedimentary and early diagenetic in origin, and they have been used to infer ocean geochemistry in the aftermath of the terminal Cryogenian (or Marinoan) Snowball Earth glaciation. Barite crystals precipitated syndepositionally from seawater or authigenically from marine porewaters can potentially offer insights into Neoproterozoic ocean geochemistry. In this study, we analyzed void-filling barite cements from the basal Ediacaran Sete Lagoas cap dolostone to determine whether the barite records seawater geochemical signatures and whether it experienced post-depositional hydrothermal alteration. Sete Lagoas barite occurs as veins and major void-filling cement in multiple horizons, sometimes interbedded with carbonate fans. Barite crystals, commonly forming rosettes, grew upwards and downwards as radiating bladed crystals, isolated crystals in the matrix, and inclusions within carbonate fan crystals. Sulfur isotope compositions of coexisting barite and carbonate-associated sulfate (CAS) show similar values, suggesting a similar sulfate source, potentially seawater sulfate from a shallow marine environment. Assuming that CAS was sourced from seawater sulfate, we infer that the Sete Lagoas barite was precipitated from marine porewaters with seawater as a source of the sulfate. Carbonaceous material (CM) and fluid inclusions in barite crystals share a similar thermal history with a peak temperature in the range of 206–257 °C. CM was not detected in the host rock, possibly due to pervasive dolomitization, thermodegradation of CM or the fine-grained nature of the host rock. Estimated temperatures and salinity based on fluid inclusions are both higher than seawater, suggesting that hydrothermal activity, perhaps those related to Mississippi-Valley type mineralization in the São Francisco Craton, may have influenced the Sete Lagoas barite. Thus, although the Sete Lagoas barite may have originally precipitated from seawater or marine porewaters in communication with seawater, it may have been subsequently influenced by hydrothermal activity and therefore a more thorough diagenetic assessment must be done before the Sete Lagoas barites can be used to infer seawater geochemistry and environment in the aftermath of Marinoan Snowball Earth.

Secular variation in the elemental composition of marine shales since 840 Ma: Tectonic and seawater influences

Marine shales, the most common lithology in the stratigraphic record, represent an archive of long-term changes in sediment production, weathering intensity, and seawater chemistry through Earth history whose potential has been underexploited to date. In this study, we compiled compositional data for 14,531 samples from 268 shale and mud units ranging in age from the early Cryogenian (840 Ma) to the Recent. We examined secular variation in the Al-normalized concentrations of ten elements (Mg, Na, K, Ba, Sr, Y, Th, Rb, Nb, and Zr) selected for (1) their presence in typical X-ray fluorescence (XRF) datasets (thus providing sufficient data), and (2) their lack of redox sensitivity (thus excluding elements whose primary control is aqueous redox conditions). Relationships among the study elements were quantified using principal component analysis (PCA) and cluster analysis (CA), revealing several significant groupings: (1) a subset of “detrital elements” (K, Rb, Th, Nb, Y, and Zr) whose concentrations are primarily controlled by subaerial weathering processes; (2) a subset of “hydrogenous elements” (Mg, Na, and Sr) whose concentrations are significantly influenced by seawater chemistry, and (3) Ba, which is subject to multiple controls. The dominant influence on the detrital elements appears to have been not chemical weathering intensity per se but the ratio of Al-poor silt to Al-rich clay in the particulate weathering flux. Although possibly modulated by factors such as climate and land plant evolution, silt:clay ratios appear most closely linked to tectonism, with higher values associated with major orogenic events. The concentrations of the hydrogenous elements in marine shales were likely related to their seawater inventories, which were modulated at long time scales by rates of submarine hydrothermal alteration. Ba in marine shales may have been significantly influenced by atmospheric pO2 and seawater sulfate levels via their control of barite saturation in seawater. In summary, our findings document significant relationships of marine shale composition to first-order geological controls, suggesting that long-term secular changes in sediment and seawater chemistry can be reconstructed from fine-grained siliciclastic sediments.

The pyrite multiple sulfur isotope record of the 1.98 Ga Zaonega Formation: Evidence for biogeochemical sulfur cycling in a semi-restricted basin

The pyrite sulfur isotope record of the 1.98 Ga Zaonega Formation in the Onega Basin, NW Russia, has played a central role in understanding ocean-atmosphere composition and inferring worldwide fluctuations of the seawater sulfate reservoir during the pivotal times of the Paleoproterozoic Era. That, in turn, has led to a concept that Earth's atmospheric oxygen levels underwent global-scale changes. Here we present a steady-state isotope mass-balance model to gain insight into the mechanisms governing the sulfur cycle and sulfate reservoir during deposition of the organic-rich Zaonega Formation. We demonstrate that coupling between high microbial sulfate reduction rates and effective sulfate removal by pyrite precipitation can lead to Rayleigh distillation of the basinal sulfate reservoir and development of high amplitude positive δ34S excursions. This modelling approach illustrates that secular changes in sedimentary pyrite isotope trends can be explained by processes that reflect local (basin-scale) fluctuations in sulfur cycling rather than global mechanisms.

Geochronology, isotopic chemistry, and gold mineralization of the black slate-hosted Haoyaoerhudong gold deposit, northern North China Craton

The Haoyaoerhudong gold deposit is the largest gold deposit in the north margin of the North China Craton gold province, and contains over 7 Moz of gold at an average grade of 0.62 g/t. The deposit is hosted in the carbonaceous and pyritic slate, phyllite, and schist and is controlled by a tight syncline and shear zones. The high-grade orebodies contains abundant pyrite veins and pyrite-quartz veins. Three stages of pyrite have been identified, including the diagenetic disseminated pyrite, pyrite veins caused by peak matamorphism, and pyrite-quartz veins forming during post-peak metamorphism. Native gold has been observed in pyrite veins and pyrite-quartz veins.The 40Ar/39Ar plateau age of a biotite separate from a tails of boudinaged pyrite-quartz veins is 260.1 ± 2.9 Ma. Combined with previously published 40Ar/39Ar mica age data and low closure temperature of mica, these results suggest that pyrite veins and pyrite-quartz veins were formed during the peak, to post-peak metamorphism during 285–260 Ma, respectively. The lower limit of the formation age of sedimentary disseminated pyrites has constrained to 1670–1560 Ma by the intruded mafic–ultramafic dikes.Disseminated pyrite separates have δ34S values ranging from −39.40‰ to + 17.85‰, 206Pb/204Pb of 19.144–21.892, 207Pb/204Pb of 15.681–15.864, and 208Pb/204Pb of 37.502–38.925, suggesting they were formed from seawater sulfate and has experienced strong sulfur isotopic fractionation. In contrast, hydrothermal pyrites from pyrite veins and pyrite-quartz veins have δ34S values ranging from +6.8‰ to +16.47‰, 206Pb/204Pb of 18.566–18.922, 207Pb/204Pb of 15.645–15.684, and 208Pb/204Pb of 38.924–38.983, which may reflect dissolution-reprecipitation of disseminated sulfides from the pre-existing organic-rich sediments. The mineral paragenetic, geometric, and cross-cutting relationships of pyrite veins and pyrite-quartz veins at Haoyaoerhudong suggest that gold was most likely introduced into pyrite, accompanying sedimentation of the organic-rich shales, and then became enriched during diagenesis. Subsequently, the hydrothermal fluids following metamorphism and shear zone activity make dissolution of the gold in the diagenetic pyrite and precipitated in the intersection of shear zone and tight syncline.

Sulfur isotope ratios in co-occurring barite and carbonate from Eocene sediments: A comparison study

In order to investigate the utility of bulk carbonate as a recorder of seawater sulfate sulfur isotope ratios (δ34Ssw), co-existing pelagic barite and bulk carbonate in Eocene sediments from the Equatorial Pacific Ocean (IODP Exp. 320/321 Sites U1331 to U1333) were analyzed for their sulfur isotope ratios (δ34S). The δ34S from both minerals showed parallel fluctuation throughout the Eocene with carbonate associated sulfate (CAS) values about 0.8‰ heavier than those of barite. A similar offset was observed in CAS δ34S obtained using species-specific cleaned planktonic foraminifers in a previous study. The consistent results from two distinct minerals suggest that the original δ34Ssw can be derived from bulk CAS analysis, if post deposition carbonate recrystallization is minimal.The combined data set showed a ~5‰ increase of δ34Ssw during the Eocene (from 53 Ma to 36 Ma) with the majority of the shift within 7 myr between 53 and 46 Ma. This change is more gradual than previously reported. The timing of this δ34Ssw shift coincides with extensive pyrite burial in the Arctic Ocean, supporting the hypothesis that the 5‰ increase in Eocene δ34Ssw has been caused by 34S-enriched water outflow from the Arctic Ocean.

Geology, mineralogy, S and Sr isotope geochemistry, and fluid inclusion analysis of barite associated with the Lemarchant Zn–Pb–Cu–Ag–Au-rich volcanogenic massive sulphide deposit, Newfoundland, Canada

Barite in the approximately 513 Ma Lemarchant volcanogenic massive sulphide (VMS) deposit, Newfoundland, consists of granular and bladed barite intimately associated with mineralization. Regardless of type, the composition of barite is homogeneous at bulk rock and mineral scale containing predominantly Ba, S, and Sr, with minor Ca and Na. The barite has homogeneous sulphur isotope compositions (δ34Smean = 27‰), similar to Cambrian seawater sulphate (25–35‰) and Sr isotope compositions (87Sr/86Sr = 0.706905 to 0.707485). These results are consistent with barite having formed from fluid–fluid mixing between Cambrian seawater and VMS-related hydrothermal fluids. The 87Sr/86Sr values in the barite are lower than mid-Cambrian seawater, which suggests that some of the Sr was derived from the underlying Neoproterozoic basement. Fluid inclusions in bladed barite are low-salinity, CO2-rich inclusions with homogenization temperatures between 245°–250 °C, and average salinity of 1.2 wt.% NaCl equivalent. Estimated minimum trapping pressures of between 1.7 to 2.0 kbars were calculated from aqueous–carbonic fluid inclusion assemblages. The fluid inclusion results reflect regional metamorphic reequilibration during younger Silurian regional metamorphism, rather than primary fluid signatures, despite the preservation of primary barite and fluid inclusion textures. These results illustrate that barite in VMS deposits records the physicochemical processes associated with VMS formation and the sources of fluids in ancient VMS deposits, as well as seawater sulphate and basement isotopic compositions. The results herein are not only relevant for the Lemarchant deposit but also for other barite-rich VMS deposits globally.

Geological and geochemical constraints on the Farahabad vent-proximal sub-seafloor replacement SEDEX-type deposit, Southern Yazd basin, Iran

Farahabad is a Lower Cretaceous-age sandy dolomite, sandstone and silty limestone hosted zinc-lead deposit in the southern Yazd basin, Iran. This deposit occurs in the lower part of Taft Formation. The Farahabad deposit is a stratiform and stratabound accumulation of hydrothermal sulfides. The mineralized zone at Farahabad is lens shaped and has a relatively flat top. It also ranges greatly in thickness, from a few meters to >30 m. Three styles of ore facies have been differentiated at the deposit. Feeder zone, bedded ore facies, whereas the bulk of ore is contained within sphalerite and galena breccias that record a prolonged and texturally complex history of ore replacement (massive-replacement ore facies). The shape and size of the mineralized body suggest that dolomites replaced with sulfide minerals in a submarine channel. Dolomitization and sulfide mineralization's are the major hydrothermal products in the Farahabad deposit, and they occur in all host rocks, but only in the immediate vicinity of the syn-sedimentary normal fault that served as the main conduit for the mineralizing fluids. Based on microscopic studies, sulfide mineralization in the Farahabad deposit emplaced during two stages: fine-grained sulfide bands (stage I) are intricately interlayered with organic matter-rich beds and thin turbidite beds. They exhibit classic sedimentary textures, such as laminae, cross bedding, graded bedding and bedding, indicative of a synsedimentary origin. Main-stage (stage II) mineralization involved the progressive replacement of preexisting sulfides and the dolomite breccias, initially by replacement of the breccia matrix and ultimately by replacement of clasts.Dolomite minerals from the feeder zone and massive-replacement ore facies have 115–234 °C trapping temperature with salinities ranging from 0.99 to 16.70 eq. wt% NaCl eq. δ34S values of sulfide minerals range from −19.47 to +3.9‰, suggesting that sulfur in the hydrothermal fluids was derived predominantly from reduction of seawater sulfate by bacteriogenic sulfate reduction. The δ13CPDB and δ18OSMOW values of host limestone's and hydrothermal dolomites plot the near marine carbonate rocks field in a plot of δ13CPDB vs. δ18OSMOW diagram. It suggests that CO2 in the hydrothermal fluids was mainly originated from marine carbonate rocks. Abrupt lateral changes in facies and thickness, along with the existence of synsedimentary breccia's and debris flows within the ore sequence, suggest the proximity of synsedimentary faults and tectonic activity contemporaneous with the sedimentation in the Lower Cretaceous, favorable to the formation of deposit. So this deposit formed by combination of sub-seafloor replacement and on the seafloor processes. Characteristics of the Farahabad deposit are compatible with a vent-proximal sub-seafloor replacement SEDEX-type classification.

Trace element geochemistry of magnetite and pyrite and sulfur isotope geochemistry of pyrite and barite from the Thanewasna Cu-(Au) deposit, western Bastar Craton, central India: Implication for ore genesis

The Thanewasna copper belt in Bastar Craton of central India is well-known for Cu ± Au mineralization. In this study, we use major and trace element chemistry of magnetite, hematite, and pyrite, as well as the sulfur isotope composition of chalcopyrite, pyrite and barite from the deposit to characterize the mineralization and to constrain its petrogenesis. The mineralization is hosted in quartz-chlorite veins associated with 2.5 Ga and 1.6 Ga granitoids. The mineralization is focused along the NW-SE trending Thanewasna brittle-ductile shear zone and comprises disseminated and vein-hosted Cu-sulfides, mainly chalcopyrite and pyrite intergrown with minor magnetite, hematite, and sulfide-free barite. The associated hydrothermal alteration is characterized by pervasive potassic-sodic alteration and chloritization. Calcite and sulfide-bearing barite veins postdate the Fe-oxide and Cu-sulfide ores. At least two generations of magnetite, hematite, and pyrite were identified based on their textural and trace element characteristics.Texturally early magnetite (Mag-I) displays oxy-exsolution of ilmenite and is of magmatic origin, representing the pre-mineralization igneous assemblage of the host rocks. Texturally later magnetite (Mag-II) is of hydrothermal origin and replaces Mag-I. It has high Co and Mg concentrations and shows dissolution and re-precipitation texture as well as evidence for high-temperature annealing. Martitisation of Mag-I and II associated with chloritization and Cu-Au mineralization produced hematite.The δ34SVCDT of syn-mineralization pyrite and chalcopyrite (−0.77‰ to −4.28‰) is suggestive of magmatic sources for the sulfur. Post-mineralization barite veins have δ34SVCDT between +10.31‰ and +17.55‰ which indicate that the sulfur was derived from seawater sulfate without going through an intermediate stage of reduction. Variation in the sulfur isotopic compositions of sulfide and sulfate was the result of dilution and cooling of the metalliferous fluid after interaction with meteoric fluids, which triggered the deposition of Cu ± Au along structural weak planes.The widespread presence of iron-rich breccias, association with crustal-scale shear zone, pervasive K-Na alteration, chloritic and advanced argillic alteration, the involvement of magmatic and non-magmatic fluid and Cu-Fe-Au-Ag-Ba-REE metal association are all suggestive of IOCG-style mineralization at Thanewasna. This supported by the Ti, Al + Mn, Ti + V concentrations and Ti/V, Ni/Cr, Ni/(Cr + Mn) ratios of Mag-II and hematite. The Co/Ni ratios of pyrite further supports a hydrothermal origin of the deposit, unrelated to skarns or copper porphyry, having similarity with known IOCG deposits. The identification of IOCG-type deposits in the Thanewasna region gives a boost to deeper sub-surface exploration in this belt as well as in similar geological setting elsewhere.

Spatiotemporal redox heterogeneity and transient marine shelf oxygenation in the Mesoproterozoic ocean

The Mesoproterozoic Era (1.6–1.0 Ga), long regarded as an interval of sluggish biotic evolution and persistently low atmospheric-oceanic oxygen levels, has become the subject of recent controversy regarding putative large-scale oxygenation events. In this study, we conducted a comprehensive investigation of redox, productivity, seawater sulfate concentrations, and hydrographic conditions for the ∼1.4–1.32-Ga Xiamaling Formation in the shallow Hougou and mid-depth Huangtugui sections in the Yanshan Basin (North China). Integrated Fe-trace metal data suggest that bottom-water redox conditions mainly underwent a three-stage secular evolution: (1) suboxic (-oxic), (2) predominantly anoxic, and (3) oxic. Integrated productivity (TOC, P/Al, and Cu-Zn-Ni enrichment factors) and depositional system (major and trace elemental enrichment patterns) data suggest varying productivity levels during deposition of the Xiamaling Formation, with high productivity fueled mainly by intense open-marine upwelling. In combination with published data from the deep Xiahuayuan and Chicheng sections, a refined stratigraphic correlation framework was developed for the Xiamaling Formation based on the basinal upwelling event and existing lithostratigraphic framework. Within this new framework, sulfur concentration and pyrite δ34S (δ34Spy) data suggest a rise in marine sulfate concentrations at ∼1.36 Ga. Furthermore, strong spatiotemporal redox heterogeneity was observed within the ∼1.4–1.32-Ga Yanshan Basin, i.e., strong bottom-water anoxia (or euxinia) in deep sections and less anoxic (even oxic) conditions in shallow sections, although all sections experienced high productivity owing to oceanic upwelling. We propose a productivity-driven shelf oxygenation model to explain this spatiotemporal redox heterogeneity within the context of eustatic changes, an upwelling event, and the rise of oceanic sulfate concentrations at ∼1.36 Ga. In this model, from initially low atmospheric oxygen levels, increasing surface productivity in shelf areas due to elevated nutrient supplies from upwelling not only intensified bottom-water anoxia through organic matter export but also stimulated expansion of the ocean-surface oxic layer through photosynthetic O2 release, contributing to the development of contrasting bottom-water redox conditions between shallow and deep sections during the upwelling event identified in this study. Our model helps reconcile conflicting observations concerning Mesoproterozoic marine redox conditions, and provides new insights into putative Mesoproterozoic oceanic oxygenation events and their relationship to the evolution of early eukaryotes.

Influence of Combined Expansive Agent and Supplementary Cementitious Materials Against Chloride Effects in Offshore Concrete

The performance of hardened concrete may be degraded by the chemical action of chlorine ions (Cl-) or sulfate (SO42-) in sea water. Thus, supplementary cementitious materials (SCMs) are used in replacement of OPC for the purpose of improving the durability of concrete, but show weaknesses such as drying shrinkage. Judging that the sole use of SCMs has limitations in designing concrete durability to cope with the environment of salt damage, this study suggests the use of aluminum-based inorganic salts (AIS) in the form of chemical agents. This study aims to select material of the highest efficiency in enhancing the durability of offshore concrete, by performing the durability evaluation of typical four SCMs and four AISs that are intermixed as single materials. There has been almost no domestic or overseas research on Al-based organic salts intermixed in the form of chemical agents for the purpose of improving resistance to salt damage and offsetting drying shrinkage. It was confirmed that mortar intermixed with Al-based chemical admixtures showed great performance improvement in the rates of length change. The mixture proportion intermixed with sulfate ion (SO42-) as binding anion tended to show unfavorable reaction in terms of resistance to sulfate erosion and drying shrinkage. On the other hand, the mixture proportion set with nitrite ion (NO3-) as cation showed excellent performance improvement in all evaluation items, compared with non-intermixture. It is deemed that it will be necessary to highlight further and make use of the expansion properties and chloride binding performance of Al-based organic salts through the design of offshore concrete mixture proportions.

Dynamic carbon and sulfur cycling in the aftermath of the Lomagundi-Jatuli Event: Evidence from the Paleoproterozoic Hutuo Supergroup, North China Craton

The unprecedented positive δ13C excursion in carbonates deposited between 2.2 and 2.0 Ga, known as the Lomagundi-Jatuli Event (LJE), has been documented globally and linked to the rise of atmospheric oxygen. Increasing oxidation inevitably changed the atmosphere-hydrosphere system, but few chemostratigraphic or quantitative constraints for the aftermath of this event exists. Here, we describe a ~200 m-thick carbonate succession in the Huaiyincun Formation, Hutuo Supergroup, ~2.0–1.9 Ga, from the North China Craton. There is a lithological transition from pink-purple dolostones to grey dolostones at ca. 91.6 m above the base of the Huaiyincun Formation. The former are more enriched in hematite and detrital minerals, whereas the latter contain more organic matter but almost no detrital phases. Meanwhile, the frequent occurrence of tempesite structures, along with the abrupt decline of stromatolites in the upper Huaiyincun Formation, suggest a storm-dominated environment. These distinct features within the Huaiyincun Formation reveal increased water depth during a transgression event. Two types of Raman spectra of organic matter were found in the lower and upper Huaiyincun Formation, respectively, which is proposed to be the result of variable oxidation.At 65.6 m, 26 m below the lithological transition (~91.6 m), remarkable decreases in both δ13Ccarb and δ34SCAS are observed. This discordance between C-S isotopic excursions and sedimentological and mineralogical variations argues against a seawater depth gradient effect of the δ13Ccarb and δ34SCAS curves. Instead, the decline of δ13Ccarb rather correlates with the negative δ13Ccarb excursions in ca. 2.0 Ga carbonates from Gabon and Russia, known as the Shunga-Francevillian Event (SFE). The result of the quantitatively constrained paleo-seawater [SO42−]sw suggests a crash of the seawater sulfate reservoir compared with that during the preceding LJE. However, the decreased δ34SCAS and increased CAS concentration towards the top of the study unit represent the recovery of seawater sulfate reservoir. The coordinated decline in δ34SCAS and δ13Ccarb values is likely related to enhanced oxidation of continental pyrite and organic matter in the aftermath of the LJE. The Huaiyincun Formation therefore represents a critical interval that recorded dynamic carbon and sulfur cycles after the LJE.

A revised seawater sulfate S-isotope curve for the Eocene

The sulfur isotope ratio (δ34S) recorded in marine barite is commonly used to estimate secular changes in the sulfur isotopic composition of seawater sulfate (δ34SSO4) throughout Earth's history. The Cenozoic record demonstrates a drastic 5‰ increase in seawater δ34SSO4 during the Early Eocene. However, the gradient of this excursion is based on only a few data points. Taking advantage of a much improved biostratigraphic framework, we redefine the Eocene δ34SSO4 data with a new high-resolution barite based δ34S record between 60 and 30 Ma. Our results show that the rise of δ34SSO4 starts about 3 million years later (~53 Ma) and lasts about 9 million years longer (until 38 Ma) than previously depicted. As such, the gradient of the δ34S curve is ~0.4‰/Myr. The radiogenic strontium isotopic ratio in barite suggests that some of the samples used for the original barite record are affected by diagenetic alteration.

Heterogeneity in the Ediacaran–Cambrian coastal oceans: a sulphur isotope perspective

AbstractMultiple sulphur isotope compositions of sedimentary pyrites across the Ediacaran–Cambrian (Ed–C) transition and into the early Cambrian from the Xiaotan section, Yunnan, South China, are presented to explore the evolution of the sulphur cycle. The values of δ34Spyrange from 13.5 ‰ to 35.8 ‰, and the values of Δ33Spyrange from −0.044 ‰ to 0.063 ‰. The first-order observation of highly positive δ34Spyis consistent with sulphur isotope records from other sedimentary successions (with various degrees of enrichment in34S), reflecting a common feature in cycling of sulphur among ocean basins. The positive values suggest that pyrite was formed in a depositional setting with limiting availability of sulphate that suppressed the expression of microbial fractionations. The first-order observation of a 10-million-year period of negative Δ33Spybeginning around the Ed–C boundary likely reflects changes in isotopic compositions of sulphur influx to the oceans. Such changes are suggested to be linked to a pulse of preferred weathering of sulphides (with negative Δ33S) relative to sulphate, which may reflect enhanced exposure of pyrites in continental margins due to reorganization of continents at this time. Both δ34Spyand Δ33Spydata imply low seawater sulphate levels, and possibly heterogeneity in sulphate concentrations in the world’s coastal oceans. The predictions about sulphur isotope signatures of evolved seawater (with highly positive δ34S and negative Δ33S) at the Xiaotan section are testable with future measurements of carbonate-associated sulphate (CAS), a proxy of ancient oceanic sulphate that carries information about the operation of sulphur cycling on a global scale.