Seawater Sulfate Research Articles

SULFIDE SULFUR ISOTOPE SOURCES OF THE NEOARCHEAN IRON ORE DEPOSITS IN THE KOSTOMUKSHA GREENSTONE BELT, KARELIA (RUSSIA)

In this paper we provide primary data on multi-isotope sulfur composition of the Neoarchean BIF sulfides of the Kostomuksha greenstone belt in Karelia (the Karelian Craton on Fennoscandian Shield). These sulfides formed at 2760–2740 Ma. Pyrite associated with fine-grained magnetite showed negative δ34S values (up to –7.8‰) and positive Δ33S values (up to +0.7‰). At the same time, pyrrhotite of vein-disseminated ores associated with shale demonstrated both negative δ34S (up to –6‰), and Δ33S values (up to –0.46‰). Based on the obtained data we assume that sulfur for the BIF sulfides was transported from three reservoirs: - atmospheric (photolytic), hydrospheric (seawater sulfate), and magmatogenic. Sulfur isotope ratios in sulfides preserved the evidence of interaction between abiological (atmospheric, hydrothermal) and biological processes (dissimilatory microbial reduction) that occurred during the formation of iron ore deposits of the Kostomuksha greenstone belt in Karelia.

Volcanogenic massive sulfide (VMS) deposits of the Dunnage Zone of the Newfoundland Appalachians: setting, styles, key advances, and future research

The Dunnage Zone of the Newfoundland Appalachians hosts diverse Cambrian–Ordovician volcanogenic massive sulfide (VMS) deposits. The peri-Laurentian Notre Dame Subzone contains Cu–Zn–Au mafic and bimodal mafic deposits in ∼501–485 Ma ophiolitic rocks and Zn–Pb–Cu–(Au–Ag) deposits in ∼471–465 Ma bimodal rifted continental arc sequences (e.g., Buchans). The peri-Gondwanan rocks of the Exploits Subzone host Zn–Pb–Cu–(Au–Ag) bimodal felsic, felsic siliciclastic, and Zn–Ag–Au hybrid bimodal felsic deposits in the ∼513–486 Ma Victoria Lake supergroup; Cu–Zn bimodal felsic to bimodal mafic deposits of the ∼486 Ma Wild Bight Group; and Cu–(Au) mafic siliciclastic deposits of the ∼466 Ma Great Burnt Lake/South Pond belt. Regardless of age or stratigraphic hosts, all VMS deposits are associated with specific magmatic assemblages and extensional tectonism (i.e., rifting). Gold-enriched deposits of the Rambler-Ming district are associated with felsic rocks that formed via slab melting and subsequent melt-mantle wedge interaction, which likely enhanced precious metal enrichment in these deposits. Whereas many deposits exhaled on the seafloor, some deposits formed via subseafloor replacement of host units or as re-sedimented sulfides generated in sediment-gravity flows. Metals in the deposits were derived from leaching of underlying footwall rocks; however, Au–Ag- and epithermal suite element-enriched deposits show evidence for metal contributions from magmatic hydrothermal fluids. Sulfur in deposits was derived predominantly from leaching of H2S from underlying footwall rocks and from thermochemical sulfate reduction of seawater sulfate, with lesser input from bacteria-derived H2S and magmatic-hydrothermal-derived H2S. Despite recent research advances and historic mining, numerous questions remain unresolved and provide opportunities for future study.

Uranium and molybdenum isotope evidence for globally extensive marine euxinia on continental margins and in epicontinental seas during the Devonian-Carboniferous Hangenberg Crisis

The end-Devonian Hangenberg Crisis was one of the biggest Phanerozoic mass extinctions. However, the mechanism(s) that triggered this event is still debated. In this study, multiple geochemical paleoredox proxies (redox-sensitive trace metals [e.g., Mo, U, Re, V] and isotope systems [Mo, U, S, C]) were applied to the Exshaw Formation black shales to infer ocean redox conditions during the Hangenberg Crisis. The measured δ13Corg values generally decrease upsection in the Exshaw Formation. The Exshaw black shales have increasing maturity levels from east to west in the Western Canada Sedimentary Basin. Large sulfur isotope fractionations (–15‰ to 65‰) between original seawater sulfate and pyrite is best explained by microbial sulfate reduction during deposition and early diagenesis rather than thermochemical sulfate reduction. Precise Re-Os ages previously reported for the Exshaw Formation suggest that metal geochemical signatures in these shales, including overmature shales, were minimally affected by hydrocarbon maturation and reflected depositional conditions. Both Sr/Ba and TS/TOC ratios (the latter only from immature shales affected minimally by hydrocarbon generation) suggest predominantly brackish and marine settings with significant water exchange between the basin and open ocean. The TOC contents, redox-sensitive trace metal concentrations (Mo, U, Re), and Fe speciation indicate local bottom waters ranged from suboxic (with sulfidic pore waters) to euxinic. The authigenic δ98Mo and δ238U values for the Exshaw shales range from 0.3‰ to 1.1‰, and from –0.23‰ to 0.39‰, respectively. The δ98Mo of the Exshaw Formation may have been offset to lower values than coeval seawater because of a local particulate Fe-Mn shuttle and/or local weakly euxinic bottom waters. Two different correlation patterns (positive correlation and no correlation) of δ238U and U enrichment factors (EF) were observed for Exshaw black shales deposited from locally euxinic bottom waters. The positive correlation group samples (−0.13‰ to 0.23‰) suggest U isotope fractionations (0.4‰ to 0.8‰) between sediments and seawater like those observed in modern euxinic basins. Higher δ238U values (>0.2‰) in the no correlation group suggest even larger U isotope offsets (0.8–1.2‰) from seawater, possibly associated with removal of U into organic floccule layers during deposition. Applying the above Mo and U isotope fractionations to the Exshaw shales, global seawater δ98Mo and δ238U at the Devonian-Carboniferous boundary (DCB) may have been in the range of 1.4‰ to 1.9‰ and −0.9‰ to −0.5‰, respectively. A Mo and U isotope mass balance model suggests a large areal extent of euxinic seafloor (6–10%) at the DCB, which could be pervasive along continental margins and in epicontinental seas during transgression. Therefore, our data support expanded ocean euxinia at the DCB as an important contributor to the Hangenberg Crisis.

Isotopic constraints on the sulfur cycle of the Bottom Convective Layer in the Black Sea

The Black Sea is the largest euxinic seawater basin with H2S at depths of below 90 m. Previously found that the presence of H2S in water was caused by microbial sulfate reduction. Herein, we report the sulfur isotopic composition of sulfate and sulfide in seawater at stations in the central part of the Black Sea, especially from the Bottom Convective Layer (BCL) located below a water depth of 1750 m. Our study shows that the sulfur isotopic composition of sulfate (δ34S(SO4)) in the surface layer of the oxic zone is +21.1‰ ± 0.1‰ relative to the Vienna Canyon Diablo troilite (VCDT), and does not differ from sulfate in the Mediterranean Sea. In the BCL, the average δ34S(SO4) value is +23.0‰ ± 0.2‰, which does not change vertically or laterally owing to effective convective mixing. The sulfur isotopic composition of H2S varies from −40.0‰ to −41.9‰ between 200 m water depth down to the seafloor with an average δ34S(H2S) value of −40.6‰ ± 0.4‰ for the BCL.Based on the water mass balance and sulfur isotopic composition of sulfate in the relevant water masses affecting the BCL, we have formulated a model to calculate the sulfate reduction rate in the BCL. After evaluating the sulfate and sulfide sources in the BCL, we assumed that the only apparent sulfide source is a bacterial sulfate reduction in the water column, and the main sulfate source is a mixture of water from the Black Sea and the lower Bosporus current with an initial sulfate isotopic composition of +21‰. The weak water exchange in the BCL leads to the largest observed isotopic difference ΔSR = δ34S(SO4) − δ34S(H2S) of 63.6‰ between sulfate and sulfide in the sulfidic water column of the Black Sea. To achieve a steady state with respect to the sulfate isotopic composition +23‰ in the BCL, an average sulfate reduction (SR) rate of 1.1 ± 0.1 nmol L−1 day−1 is required. A steady state relative to sulfur isotopic composition of sulfate in the BCL water was established 4200 years ago. The estimated residence time of H2S was 1.5 times shorter (946 years) than that of sulfate (1390 years). The latter is close to the water renewal time of the BCL (1432 years). Different residence times of sulfide and sulfate in the BCL are explained using their different fluxes from the BCL. Sulfate is mainly removed from the BCL via vertical advection, while a considerable part of the sulfide generated in the BCL (1/3 of total annual production) is probably spent on the sulfidation (FeS → FeS2) of muddy turbidites. The estimated annual H2S production in the BCL is 0.78 ± 0.07 Tg. The developed model predicts that the sulfide concentration in the BCL remains practically unchanged (380 ± 5 μM) over the last 1800 years (after 5700 years from the onset of anoxia).

U-Pb DATING OF APATITE FROM SILVERMINES DEPOSIT, IRELAND: A MODEL FOR HYDROTHERMAL ORE GENESIS

AbstractUranium-Pb dating of unusual coarse-grained apatite crystals from hydrothermal dolostone breccia in the barite-rich Magcobar zone at the Silvermines deposit, Ireland, indicates an age of 331 ± 5.6 Ma for hydrothermal alteration. This age is in agreement with an Re-Os age on pyrite-sphalerite but differs from previous estimates that were based on palemomagnetism and sphalerite Rb-Sr geochronology at Silvermines. The new U-Pb age indicates the deposit largely formed epigenetically rather than as a synsedimentary deposit. The trace element composition of the apatite provides additional constraints on the geochemical evolution of the Silvermines hydrothermal system indicating that it was initiated with the expulsion of oxidizing fluids responsible for early hematite-silica precipitation. This was followed by the formation of hydrothermal dolostone breccias containing barite and the apatite dated here. The infiltration of metal-bearing hydrothermal fluids, likely derived from the basement, overlapped and followed the development of hydrothermal dolostone breccias, interacting with Carboniferous seawater during dolomitization and barite precipitation. Near-sea-floor biogenic activity, possibly enhanced due to escape of hydrothermal fluids, resulted in the reduction of seawater sulfate and formation of the sulfide-bearing fluids that facilitated the development of the Silvermines Zn-Pb deposit.

Paleo-biodegradation and hydrocarbon mixing in a major hybrid petroleum reservoir

Some of the parental material for hydrocarbons produced from low-permeability reservoirs in Western Canada corresponds to thermal products from biodegraded oil. This has been proved by the occurrence of framboidal pyrite, which is often formed during microbial sulfate reduction (MSR). In addition, the identified pyrite framboids are associated with the presence of phosphorus (P). Phosphorus (as phosphate) is a key nutrient and energy carrier for sulfate-reducing bacteria. The pyrite-P assemblage occurs embedded in solid bitumen (thermal residue), which confirms that migrated hydrocarbons provided the environment for microbial growth. Molecular products of severe biodegradation such as 17-nortricyclic terpanes were also detected. Biodegradation effects have been masked not only by thermal degradation of biodegraded oil during maximum burial, but also due to hydrocarbon mixing with late gas-condensate charges. Suitable conditions for biodegradation (< 80 °C, basin uplift) occurred during the Early Cretaceous. The confirmation of paleo-biodegradation means that there was a significant hydrocarbon loss that we have not accounted for. Likewise, MSR and Early Cretaceous seawater sulfate might have played an important role in the generation of the hydrogen sulfide (H2S) detected today.

Geochemical features of HC- occurrence in carbonates of the Minyar formation of the Upper Riphean, Bashkir Meganticlinorium, Southern Urals

The results of a geochemical study of organic matter found in carbonate deposits of a typical section of the Minyar formation of the Upper Riphean, exposed within the Karatau structural complex in the western part of the Bashkir meganticlinorium, are presented.&#x0D; Analytical methods of gas chromatography and chromato-mass spectrometry in extracted hydrocarbons have been used to diagnose diamantanes, alkanes and specific biomarkers – tricyclic terpanes, gopanes and steranes. The thermal maturity corresponding to the upper part of the oil window is determined by the diamantanes. The quantitative ratios of other hydrocarbons strongly depend on the facies nature of carbonates – in thin-layered lagoon carbonates, where there are significantly more of them, they are recognized as autochthonous, in the stromatolite bioherm located next to them, light alkanes were lost, organic matter was probably subject to consedimentary biodegradation due to periodic subaeral exposure. Dibenzothiophene, an organosulfur biomarker, was also found in organic matter, which can be used to assess the content of sulfates in the sedimentation basin. The concentration of dibenzothiophene is significantly higher than the concentrations in the occurrence of hydrocarbons in Neoproterozoic sediments of other regions of the world, which indicates the existence of strong regional differences in the content of sulfate in Late Precambrian seawater.

S, Pb, and Fe isotope compositions of sulfides in middle and southern Okinawa Trough: Implying the complicated hydrothermal systems in back-arc spreading centers

Sulfur, lead, and iron isotopic systematics of sulfides from the Iheya North hydrothermal field in middle Okinawa Trough and the Yonaguni Knoll IV hydrothermal field in southern Okinawa Trough were investigated to understand the origin of ore-forming material and isotope fractionation during sulfide precipitation in back-arc spreading centers. The δ34S values (from 8.60‰ to 9.68‰) of sulfide minerals in Iheya North field suggest sulfur was derived from the leaching of basement rocks and the reduction of seawater sulfate. But an additional light sulfur source from the dissolution of biogenic sulfides in sediments is indispensable for the observed low δ34S values (between 4.78‰ and 4.97‰) of Yonaguni Knoll IV sulfides. Lead isotope compositions of all sulfides vary in a relatively wide range (206Pb/204Pb = 18.432 to 18.563, 207Pb/204Pb = 15.579 to 15.671, and 208Pb/204Pb = 38.539 to 38.979, respectively) which involves the mixing of mantle-derived and sediment-derived lead in varying proportions. Unusually high radiogenic lead isotope ratios of Yonaguni Knoll IV sulfides (207Pb/204Pb = 15.628 to 15.671, and 208Pb/204Pb = 38.854 to 38.979) imply the presence of old crustal fragments beneath southern OT. The hydrothermal fluids in OT are characterized by extremely low calculated δ56Fe values (−1.00‰ to −1.32‰) which is attributed to reducing sediments. The paired Fe-isotope compositions of pyrite and chalcopyrite revealed significantly different iron isotope compositions of hydrothermal fluids in study areas and extremely complicated ore-forming processes. Rapid precipitation of pyrite from FeS precursors accompanying iron isotopic disequilibrium moves from FeS-fluid equilibrium to pyrite-fluid equilibrium during pyrite recrystallization. The combination of sulfur, lead, and iron isotopic data suggests that hydrothermal systems in back-arc spreading centers related to plate subduction are generally affected by mantle, overlying sediments as well as potential old continental crust fragments, which results in various hydrothermal fluids and complicated mineralization processes.

Interlinked marine cycles of methane, manganese, and sulfate in the post-Marinoan Doushantuo cap dolostone

The strongly 13C-depleted calcite in the Doushantuo cap dolostones (∼635 Ma) of South China is considered to form via thermochemical or biological oxidation of methane by sulfate in an environment with sulfate concentrations close to that of modern seawater (∼28 mM). Here we demonstrate that the coeval seawater sulfate levels were indeed low (∼1 to 3 mM) because all bulk δ34S values of pyrites are close to and higher than the coeval seawater, with only a few showing a wide in situ δ34S range from 5 to 62‰. Such low sulfate concentrations were initially consumed by sulfate reducing microorganisms to extremely low levels (<∼0.2 mM), and thus Mn-driven anaerobic oxidation of methane (AOM) by sulfate occurred in pore water to produce H2S, which was in turn oxidized by Mn oxides back to sulfate. Consequently, with increasing Mn reduction and decreasing sulfate concentrations in pore waters, the produced pyrite shows an increase of in situ δ34S values, and is in association with calcite with more negative δ13C values, and an inherited positive Ce anomaly from Mn oxides. Finally, the pyrite with the highest δ34S value may have co-precipitated with the 13C-depelted calcite (with a nadir of −58‰), and thus show similarly high Mn concentrations (∼20000 μg/g). This is the first report of sulfate-dependent AOM driven by Mn- or Fe- oxides from the entire Precambrian record. Such a previously unrecognized biogeochemical cycling during Marinoan global deglaciation may serve as the mechanism for major methane sink during the Proterozoic and Archean with low sulfate concentrations in the oceans.

The Origin of the Caiyuanzi Pb–Zn Deposit in SE Yunnan Province, China: Constraints from In Situ S and Pb Isotopes

Located at the intersection of the Tethys and Pacific Rim metallogenic belts, the Laojunshan polymetallic metallogenic province in SE Yunnan Province hosts many large-scale W–Sn and Sn–Zn polymetallic deposits. The newly discovered Caiyuanzi medium-sized Pb–Zn deposit is located in the northern part of this province and has eight sulfide ore bodies. All the ore bodies occur in the siliceous rocks of the Lower Devonian Pojiao Formation (D1p). The ore bodies are conformable with stratigraphy and controlled by a lithologic horizon. The sulfide ores have banded or laminated structures. The ore minerals are mainly pyrite, chalcopyrite, sphalerite, and galena. In this study, in situ sulfur and lead isotopes were used to constrain the origin of the Caiyuanzi Pb–Zn deposit. The results show that the in situ δ34S values of pyrite, chalcopyrite, and sphalerite range from 0.1‰ to 6.0‰, with an average of 4.7‰. This δ34S signature reflects the mixing between magmatic-derived and reduced seawater sulfate sulfur. The in situ Pb isotopes characteristics of pyrite, galena, and sphalerite suggest that the sulfur and lead of ore minerals come from the upper crust. Integrating the data obtained from the studies including regional geology, ore geology, and S–Pb isotope geochemistry, we proposed that the Caiyuanzi Pb–Zn deposit is a hydrothermal deposit formed by sedimentary exhalative and magmatic hydrothermal superimposition.

Pyrite Sulfur Isotope Systematics Suggest Low Marine Sulfate Levels across the Ediacaran–Cambrian Transition

The Ediacaran–Cambrian (E–C) transition was a critical period in Earth’s history, and it was characterized by variable but potentially significant increases in atmospheric oxygen levels (pO2). Sulfate is an essential oxidant and plays a key role in regulating Earth’s surface redox conditions. However, there is uncertainty regarding seawater sulfate levels during the E–C transition. To address this issue, organic carbon (δ13Corg) and pyrite sulfur isotope (δ34Spy) analyses of the Heyu section (the Dengying Formation) in the Sichuan Basin, South China, were carried out. The δ13Corg varies between −36.4‰ and −27.4‰, and δ34Spy ranges from −6.6‰ to 27.3‰. The TOC and pyrite content ranges from 0.13 wt.% to 4.28 wt.% and from 0.01 wt.% to 0.94 wt.%, respectively. The marine sulfate concentration was modeled using a one-dimensional diffusion–advection–reaction model (1D-DAR). The modeling results indicate that the seawater sulfate level remained at a relatively low level of ~2–6 mM. The increased pyrite burial at extensive marine anoxia during the terminal Ediacaran period was likely a driver of such a low sulfate level.

The influence of submarine hydrothermal systems on seawater sulfate

Seawater sulfate is a major carrier of oxidizing capacity and influences the redox budget of the Earth’s surface on geologic timescales. Records of oxygen and sulfur isotopes in seawater sulfate are used to track changes in sulfate cycling through geologic time. Interpretations of these records are typically based on models that describe the seawater sulfate reservoir as a mass balance between microbial, riverine and sedimentary sulfate fluxes. Here, we investigate the influence of hydrothermal sulfate cycling, which remains an unconstrained but potentially significant additional flux in this mass balance. We find that anhydrite (CaSO4) from eight submarine hydrothermal vent fields is consistently offset from seawater sulfate in δ18O but not Δ’17O or δ34S. Experiments at hydrothermal pressure–temperature conditions indicate that this δ18O offset is driven by oxygen isotope exchange between sulfate and hydrothermal H2O at high temperature. An updated isotope mass balance model shows that a flux of hydrothermal sulfate into seawater, derived from retrograde dissolution of hydrothermal anhydrite, could cause high-temperature oxygen isotope exchange to buffer seawater sulfate δ18O and Δ’17O by as much as 25%. Hydrothermal sulfate preserved in the Troodos Ophiolite (Cretaceous) also records a δ18O and Δ’17O signal of high-temperature oxygen isotope exchange, supporting the conclusion that geologic records of seawater sulfate oxygen isotopes may include a hydrothermal component.

Digging Deeper: Bioturbation increases the preserved sulfur isotope fractionation

Bioturbation enhances mixing between the seafloor and overlying ocean due to changes the redox state of the sediment and influences the biogeochemical cycling of redox-sensitive elements such as sulfur. Before the widespread appearance of burrowing fauna over the Proterozoic-Phanerozoic transition, marine sediments were largely undisturbed and transport of material across the sediment-water interface was diffusion-dominated. Through both a microcosm experiment and numerical model, we show that the effect of bioturbation on marine sediments is to enhance the drawdown of sulfate from the water column into the sediment and thus “open-up” the sedimentary system. The key finding is that bioturbation increases the difference between the isotopic signature of seawater sulfate and pore water sulfide, the latter of which is preserved in sedimentary sulfide minerals. Our study empirically demonstrates a long-held assumption and helps identify the isotopic impact of bioturbation in the geological record and its environmental effects in modern marine systems.

The role of immiscible sulfides for sulfur isotope fractionation in arc magmas: Insights from the Talkeetna island arc crustal section, south-central Alaska

While sulfur isotopes have proven powerful tracers of sulfur (re)cycling within subduction zones, the origin of the 34S-enrichment seen in arc magmas remains a subject of debate, with competing hypotheses implicating both mantle and crustal processes. Herein, we investigate these competing models through the study of the Early–Middle Jurassic Talkeetna Arc section exposed in the Chugach Mountains, south-central Alaska, reporting the sulfur isotope systematics of rocks spanning a depth transect that extends from the upper mantle (∼0.9–1.2 GPa, ∼30–35 km) up into the mid-crust (∼0.2–0.5 GPa, ∼5–15 km). Marked by an increase in δ34S values from −1.28 to +5.61‰, these data reveal a significant isotope effect associated with melt differentiation, with 32S being preferentially sequestered from primitive basaltic melts into ultramafic and mafic cumulates, yielding progressively more evolved melts enriched in 34S. Leveraging a quantitative petrological model that constrains the liquid line of descent and sulfide saturation as a function of sulfur content, oxygen fugacity, pressure and temperature, we then demonstrate that, in the presence of dissolved oxidized sulfur species, the saturation of immiscible magmatic sulfides is capable of generating the observed sulfur isotope fractionation. While fluid saturation is unlikely to occur at lower crustal depths, ascending melts will almost certainly experience fluid saturation and degassing. As a result, sulfide immiscibility and magma degassing are not mutually exclusive and may, instead, represent complementary processes that combine to explain the observed range of positive δ34S compositions. In Talkeetna, crustal assimilation is unlikely to have played a role to increase δ34S values as there was little to no assimilation of pre-existing oceanic crust containing seawater sulfate during the formation of the Talkeetna Arc section. Furthermore, our model suggests that primitive melt in isotopic equilibrium with the most primitive ultramafic cumulates have mantle-like δ34S values between ∼0 and +1.1‰, which suggest limited input of 34S-enriched slab-derived sulfur into the sub-arc mantle. Collectively, these findings emphasize that mantle-like δ34S values for primitive arc melt may be common, while revealing the underappreciated importance of deep crustal crystallization of immiscible magmatic sulfides to generate positive δ34S compositions in erupted arc magmas.

Implications of multiple fluids in the deposition of Pb-Zn-Ba deposits in the Alvand Mountain, Golpayegan, Iran: Evidence from fluid inclusions and O, C, S isotopes

Middle Cretaceous sedimentary carbonates of Alvand Mountain host Pb-Zn-Ba veins. This mineralization occurs mainly as veins, breccia fillings, and to a lesser extent disseminated and host rock replacements. Ore mineralogy is simple and consists of sphalerite, galena, and barite, with minor pyrite, rare chalcopyrite, and tetrahedrite. Host rock dolomitization and hydrothermal silicification are typically associated with ore. Two types of primary liquid–vapor fluid inclusions were distinguished in sphalerite, quartz, and barite. Type I (salinity, 17–23 wt% NaCl; Th, 130 ± 30 °C) is dominant in sphalerite, whereas type II (salinity, 4.5–10 wt% NaCl, Th, 190 ± 40 °C) is dominant in quartz and barite; these fluids have characteristics of basinal brines and show a negative mixing trend in an evolving process of sulfide and gangue saturation. The δ18O and δ13C values for host rock and altered minerals range from 22.0 to 12.9 ‰ and 3.8 to −3.2 ‰, respectively, which suggest the influence of increased temperature fluids, presence of organic carbon, and fluid mixing. The δ34S values of sulfides and barite varies from 2 to 17 ‰ and 23 to 24 ‰, respectively, suggesting that reduced sulfur could be derived by thermochemical reduction of Paleogene seawater sulfate in the presence of organic matter; however, the wide ranges of δ34S sulfide values (15 ‰) exhibits that reduction of sulfur probably originated by different sulfur reduction processes. The data present in this study suggest that during increased tectonic activity in the late Cretaceous-Paleogene, mixing of basinal brines and water–rock interaction resulted in Pb-Zn-Ba deposition in carbonate host rocks. Therefore, it may represent low-temperature mineralization, possibly analogous to MVT depositional systems.

Mineral textures, mineral chemistry and S isotopes of sulphides from the Tianbaoshan Pb–Zn–Cu deposit in the Sichuan–Yunnan–Guizhou triangle: implications for mineralization process

AbstractThe carbonate-hosted Pb–Zn deposits in the Sichuan–Yunnan–Guizhou (SYG) triangle region are important Indosinian deposits in South China. The Tianbaoshan deposit is a typical large Pb–Zn deposit in the SYG area and occurs as pipe-like type, hosted by Sinian dolostone. It contains ∼26 Mt Zn–Pb ore (7.76–10.09 % Zn, 1.28–1.50 % Pb and 93.6 g t−1 Ag) and &gt;0.1 Mt Cu ore (2.55 % Cu). In this study, the detailed mineral textures, mineral chemical and sulphur isotopic compositions of the various sulphides have been analysed to constrain the abnormal enrichment mechanism and mineralization relationship. Four mineralization stages have been recognized: Stage 1, minor early pyrite (Py1) with relics and infill of intergranular dolomite or quartz grains; Stage 2, Cu mineralization with coarse-grained, elliptical crystal chalcopyrite (Cp1); (3) Stage 3, Zn mineralization with dark fine-grained sphalerite (Sph1) and light coarse-grained sphalerite (Sph2); and (4) Stage 4, as represented by a quartz–calcite assemblage with galena, minor pyrite (Py2) and chalcopyrite (Cp2). The petrography of the sulphide minerals (Py1, Cp1, Sph1 and Sph2) demonstrates a mutual inclusion relationship. The nature of this relationship from core to rim and their similar sulphur isotope values (5.5–8.3 ‰) indicates a single sulphur source, suggesting that the different mineralization types are the result of different stages of a continuous hydrothermal system. Sphalerite geothermometer study suggests that sphalerite in the Tianbaoshan deposit formed in a low-temperature (&lt;200 °C) hydrothermal system. The low concentrations of Mn and In, low In/Ge ratios and high Fe/Cd ratios in the sphalerite are consistent with those of Mississippi Valley-type (MVT) deposits, but different from those of magmatism-related deposits (e.g. epithermal, skarn and VMS deposits). The positive δ34S values for Py1 (5.1–7.9 ‰), Cp1 (5.1–7.2 ‰), Sph1 (4.7–7.4 ‰), Sph2 (3.9–8.7 ‰), Py2 (4.4–9.3 ‰) and Cp2 (5.0–6.8 ‰) indicate a sulphur source from thermochemical reduction of coeval seawater sulphate. Widely developed dissolved textures (caverns and breccias) with massive sulphide infillings and deformed host rock remnants suggest that replacement of host dolostones by ore fluids was volumetrically significant and the ore formed nearly simultaneously with the cavities. The Tianbaoshan deposit is a typical MVT deposit, which resulted from mixing of a H2S-rich fluid and a metal-rich fluid, with thermochemical sulphate reduction occurring before ore precipitation rather than during ore precipitation.

Sulfate and phosphate oxyanions alter B/Ca and δ11B in inorganic calcite at constant pH: Crystallographic controls outweigh normal kinetic effects

We report new results from inorganic experiments in which the effects of several oxyanions on the concentration and isotopic composition of boron (B/Ca and δ11B, respectively) in calcite were explored. The oxyanions examined here (sulfate, phosphate, nitrate, and nitrite) differ in their size, charge, and geometric configuration, but they are all common dissolved constituents in natural aqueous solutions including seawater. Of those, sulfate and phosphate which are known to inhibit calcite nucleation/precipitation had pronounced impacts on the B incorporation and isotope fractionation during calcite precipitation. Additions of up to 5 mmol sulfate and up to 2 μmol phosphate into experimental solutions of otherwise unaltered condition caused comparable declines in calcite precipitation rates R, increases in B/Ca, and decreases in calcite δ11B towards the calculated δ11B of B(OH)4−. The pattern of changes in B/Ca and δ11B as a function of R observed here is at odds with normal kinetic effects confirmed in previous studies, where B/Ca and δ11B were shown to increase and decrease, respectively, with an increase in degrees of calcite saturation in solutions and thus R. We argue that the paradoxical and apparently reversed kinetic trends in B/Ca and δ11B in the presence of sulfate and phosphate can be attributed to deformations of the calcite lattice structure due to their substitutions for lattice CO3, which in turn enhances the retainment and eventual incorporation of B(OH)4−. Our new findings could have important implications for paleo-reconstructions of ocean carbonate chemistry using B/Ca and δ11B, if the mechanism proposed above is similarly in effect during natural precipitation of biogenic calcite (e.g., foraminiferal tests) in seawater. Though this needs further testing, if true, the gradual but sizable increase in seawater sulfate over the Cenozoic could have biased foraminiferal B/Ca and δ11B. Our experimental data additionally indicate that Na+ has the capability of providing charge compensations for substitutions of CO3 by B(OH)4−. However, our data also demonstrate that changes in Na+ concentration do not control the degrees of B incorporation.

Genesis of the Qilinchang Pb–Zn deposit, southwestern China: Evidence from mineralogy, trace elements systematics and S–Pb isotopic characteristics of sphalerite

The Qilinchang (Pb–Zn)–Ag-Ge deposit of the Yangtze Platform, southwest China are hosted in the Early Carboniferous carbonate rocks of the Baizuo Formation. The deposit is located within the Sichuan-Yunnan-Guizhou (SYG) metallogenic province, which is western bounded by the Yangtze Platform in the southwestern part of China. Ore minerals are sphalerite, galena, pyrite, chalcopyrite, and marmatite, whereas gangue minerals are dolomite, calcite, and quartz. This study highlights the results of sphalerite chemistry analyzed by electron microprobe analyzer (EPMA), the laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and isotopic analysis. These techniques were used to investigate the major and trace concentrations in sphalerite from the Qilinchang deposit, in an attempt to provide noteworthy discernments into the ore-forming temperature and metal sources of the deposit. The calculated FeS (mole %) results obtained from Fe (wt. %) with the assumption that the sphalerite is stoichiometric in composition range from 0.02 to 13.48 mol % and mode at 3 mol %. This low FeS content falls within the bornite + pyrite + chalcopyrite field on the Fe–Zn–S system suggesting an increase in sulfur activity. The application of Ga/Ge ratios in sphalerite against the formation temperature was used to determine the homogenization temperature of this deposit. This expression is based on the existing Al/Si geothermometers as (Al/Si)total atomic ratio as a function of temperature calculated using available constants of hydrolysis reactions. The log10 (Ga/Ge) ratios calculated for this study range from −2.8 to 0.87, corresponding to formation temperatures of 100–227 °C, with an average of 180.3 °C. Likewise, correlation between PC1* and the homogenization temperature indicated that the calculated formation temperatures for this study ranged from 119.3 to 217.9 °C, which suggests that the deposit is sourced from a low-temperature environment. The δ34S of sphalerite is related to the thermochemical sulfate reduction (TSR) of Carboniferous seawater sulfate and are mainly derived from evaporites in the host strata producing H2S−rich fluid by TSR with restricted inducement from mantle-derived sulfur. The homogenous composition of Pb isotopic data of sphalerite indicates that the metals were predominantly originated from the basement rocks of the Mesoproterozoic-early Neoproterozoic rocks of the Kunyang Group. From these geochemical signatures, it can be inferred that the Qilinchang deposit was not linked to magmatic activities. It is proposed that the ore-forming fluids responsible for the formation of the deposit were generated from a low-temperature environment. Economic potential reveals the most important attributes of this deposit are high Zn and Pb grade (approximately 17.5% Zn and 6.6% Pb, 3.32 Mt of metal). Overall, the mineralization of the Qilinchang Pb–Zn deposit shows it a high-grade remarkable Pb–Zn deposit transiting between MVT and high-temperature carbonate-replacement Pb–Zn deposits. We suggest that the determination of the genetic type of this exceptional deposit is important to the research and exploration of similar Pb–Zn deposits in other metallogenic provinces of China.

Understanding the impact of main seawater ions and leaching on the chloride transport in alkali-activated slag and Portland cement

A growing body of evidence has suggested an aggravated chloride diffusion in concrete exposed to seawater than to pure NaCl solution of the same chloride concentration, indicating the potential impact of other constitutional ions in seawater, e.g., sulphate and magnesium. This study aims to systematically investigate the impact of sulphate, magnesium and leaching, and their interaction on the chloride transport in alkali-activated slag (AAS) and ordinary Portland cement (OPC). The results indicate that the aggravating effect of magnesium on chloride diffusion overwhelms the suppressing effect of sulphate, leading to increased chloride diffusion in both AAS and OPC. Despite without the pore filling effect by ettringite formation as in OPC, the sulphate in seawater also suppresses the chloride transport in AAS, which is likely linked to the electrical coupling effect during multi-ion transport and the pore wall thinning effect by the aggregation of the formed Na–SO4 clusters in the gel pore. The origin of the peaking behaviour is found to be contributed by not only the impact of leaching, but also the effect of sulphate and magnesium on releasing the bound chloride from the reacted phases at the exposure front.

Sulfur sources for ultramafic‐hosted sulfide mineralization in the Tianzuo hydrothermal field, 63.5° E, South‐west Indian Ridge: Insights from sulfur and carbon isotopes

To date, discovered ultramafic‐hosted seafloor massive sulfide (UM–SMS) deposits are rare, remain poorly understood in terms of their genesis, and typically form along slow‐ and ultraslow‐spreading mid‐oceanic ridges. The Tianzuo hydrothermal field (THF) is the only UM–SMS deposit identified so far along the ultraslow‐spreading South‐west Indian Ridge (SWIR). This study presents new detailed mineralogical and S–C isotopic data for serpentinized peridotite, gabbro, serpentinite, and sulfide ore samples from the THF and to provide new insights into the processes that formed the deposit. Chromium‐reduced sulfur (CRS) and sulfate–sulfur (SS) in samples of serpentinized peridotite have positive δ34S values (up to 10.22‰ and 20.67‰, respectively). The CRS formed through thermochemical reduction of seawater sulfate during reactions between seawater and ultramafic rocks, whereas the SS most likely occurred as dissolved sulfate phases in seawater occupying the pore space of serpentinized peridotite after exposure at the seafloor. The serpentinite samples contain SS with δ34S values of 19.21‰–20.9‰, indicating infiltration by seawater that led to the deposition of seawater sulfate. The sulfide ores have acid‐volatile sulfur (AVS), CRS, and SS δ34S values of 10.99‰–12.07‰, 2.34‰–10.98‰, and −4.13‰ to 2.76‰, respectively, indicating that the AVS and CRS within the samples are dominated by sulfur reduced from seawater sulfate (e.g., anhydrite) and leached from the surrounding wall rocks, although the re‐oxidization of microbial‐reduced sulfur most likely generated the sulfate with low and negative δ34S values. The total carbon and total organic carbon δ13C values of the THF samples are negative and range from −21.99‰ to −0.34‰ and −26.7‰ to −21.02‰, respectively, further suggesting that the samples record microbial activity. Gabbro samples have strongly negative AVS (−10.69‰) and CRS (−7.2‰) δ34S values, indicating that microbial‐reduced sulfur derived from shallow levels in the crust was incorporated into the hydrothermal system that circulated through the deeper‐seated gabbroic units. Our results suggest that the ultramafic and gabbroic rocks, seawater, and microbial activity all provided sulfur to the THF, with well‐developed fractures and local magma supply probably being the key factors controlling the formation of this field and similar UM–SMS deposits elsewhere.