Low δ34S Values Research Articles

Quantitative characterization on multistage formation of sedimentary pyrite driven by H2S derived from biogenic process in the northeastern Ordos Basin, China

Negative δ34S values of sedimentary pyrite associated with organic matter are routinely assumed to be the result of biogenic process. However, the distribution and evolution of isotopic values mediated by bacteria across sedimentary strata remains poorly understood. Abundant pyrite-rich nodules (PyRNs) are distributed in the bottom of the sandstone unconformably overlying the coal in middle Jurassic strata in the northeastern Ordos Basin, providing a good opportunity to quantitatively characterize the distribution, morphology and compositions of sulfur isotope and trace element of pyrite in the sandstone influenced by organic matter in the coal. A total of 1007 PyRNs occurs within a range of 4 m from the coal. From bottom to top in the sandstone, the shape of the nodule changes from oval to round on the vertical section, and the length and number gradually decrease at the rate of ~45 mm and ~ 228 for every 1 m increasing in distance, respectively. Microscopically, pyrite occurs as euhedral crystals, and trace element mapping reveals multistage growth (up to a dozen times) and a marked compositional zoning with respect to Co, Ni, As, Se and Mo. The positive correlation between Co and Ni, with ratios of Co/Ni ranging from 0.06 to 0.45, indicates that Fe and those trace elements are sourced from diagenetic fluid. The organic sulfur in coal, serving as a sulfur source, is reduced by bacteria to generate H232S at slow reduction rate. The H232S migrates upwards and reacts with Fe to form pyrite in the sandstone, resulted in extremely low δ34S values (from −53.9 to −43.1 ‰), which gradual decrease both from core to margin in individual grain and at the rate of 2.2 to 8.8 ‰ for every 1 m increasing in distance away from the coal. This study highlights the significant variability of mineralogical (e.g., number, size, morphology) and geochemical (trace elements, sulfur isotope) characteristics of sedimentary pyrite. Results allow the relation of multistage growth of pyrite to biogenic fractionation, and provide fresh insights into biogenically derived sulfur from coals to be fixed in sandstones, which can be applied to quantitative characterization of formation processes of sedimentary minerals controlled by organic matter in sedimentary environments worldwide.

Seasonal variability in the feeding ecology of an oceanic predator

Complementary approaches (stomach contents, DNA barcoding, and stable isotopes) were used to examine seasonal shifts in the feeding ecology of an oceanic predator, yellowfin tuna (Thunnus albacares, n = 577), in the northern Gulf of Mexico. DNA barcoding greatly enhanced dietary resolution and seasonally distinct prey assemblages were observed for both sub-adults and adults. In general, diet was characterized by ommastrephid squids and exocoetids in spring, juvenile fishes (i.e., carangids and scombrids) in summer, migratory coastal fishes during fall, and an increased consumption of planktonic prey (e.g., amphipods) in winter. Seasonal variability in bulk stable isotope values (δ13C, δ15N, and δ34S) was also observed, with low δ15N values and high δ34S values during late summer/early fall and high δ15N values (low δ34S) during late winter/early spring. Bayesian stable isotope mixing models corroborated seasonal diet shifts, highlighting the importance of oceanic nekton in spring/summer, coastal nekton during fall, and oceanic plankton during winter. Seasonal shifts in diet appeared to be influenced by prey reproductive cycles, habitat associations, and environmental conditions. Findings highlight the complex food web dynamics supporting an opportunistic oceanic predator and the importance of seasonal cycles in prey availability to predator resource utilization in open-ocean ecosystems.

Geology and in-situ S-Pb isotopes of the Luoboshan carbonate-hosted Pb-Zn deposit in Yunnan Province, SW China

The Luoboshan Pb-Zn deposit (0.2 Mt Pb + Zn metal reserves, @ 0.94 % Pb and 5.33 % Zn) is located in the Lanping-Simao Basin in the southern part of the Sanjiang Tethys metallogenic domain in Yunnan Province, SW China. The deposit comprises three ore bodies hosted in the lower Permian bioclastic and micritic limestone. Ore body I is stratiform (mineralization is small in scale and ores have been mined out), with average grades of 0.02–1.44 % Pb and 1.5–14.88 % Zn. Ore bodies II and III are lens-like, the average grades of them are 0.10–5.07 % Pb and 1.68–12.39 % Zn, and 0.10–1.71 % Pb and 1.14–11.79 % Zn, respectively. Two hydrothermal stages were identified in the Luoboshan deposit: Pyrite-I-sphalerite-galena-calcite (stage I) and Pyrite-II-calcite (stage II). The in-situ δ34S values of pyrite and sphalerite reveals a medium range of −8.80 ‰ to +10.1 ‰. Furthermore, Py-I and sphalerite exhibit low δ34S values from −8.80 ‰ to −3.04 ‰, but Py-II displays high δ34S values (+2.37 ‰ to +10.0 ‰). These results suggest distinct formation mechanism for reduced sulfur during different stages of Pb-Zn mineralization. A reasonable explanation is that a transition from bacterial sulfate reduction (BSR) in the early stage to thermochemical sulfate reduction (TSR) in the late stage as the likely mechanism for the formation of reduced sulfur. In addition, the in-situ Pb isotopes exhibit a narrow range (206Pb/204Pb = 18.660–18.672, 207Pb/204Pb = 15.665–15.677 and 208Pb/204Pb = 38.940–38.980), indicating that the metals originated from either a single source or a uniform mixture of different sources. Comparison of geological charateristics and isotopic geochemical data (S and Pb) indicates that the basin sedimentary strata (wall rocks) is the only and most probable source. Hence, we propose that the formation of the Luoboshan deposit involved biogenic in-situ acid generation of marine sulfates, which subsequently mixed with Pb-and-Zn-rich hydrothermal fluids, initiating TSR and resulting in Pb-Zn mineralization.

Formation of the giant Maoping Mississippi Valley-type Pb–Zn (Ge) deposit via fluid mixing: Evidence from trace element and sulfur isotope geochemistry of pyrite

The SYG (Sichuan–Yunnan–Guizhou) area is one of the most economically significant Mississippi Valley Type (MVT) ore provinces, providing approximately 27 % of the Pb–Zn resources in China. The Pb-Zn deposits in this region are renowned for their high grade, and the Maoping deposit with 20.3 % Pb-Zn grade is one prime example. However, the mechanism of such high-grade mineralization remains unclear, and pyrite may record valuable ore-forming information before and during Pb-Zn mineralization. Based on field geology and petrography, three hydrothermal stages were identified in the Maoping deposit: dolomite–pyrite vein (stage I), dolomite–sphalerite–galena vein (stage II), and calcite vein (stage III). Three types of pyrite are recognized: Py1 and Py2 occur in stage I, and Py3 is closely associated with sphalerite and galena in stage II. Py1 exhibits the highest concentrations of S, Pb, Sb, Cu, Co, Ni, V, Ag, Mn, Se, and Mo, and Py3 shows the highest Fe and As contents but the lowest levels of S, Pb, Sb, Cr, Ti, Co, Ni, Mn, and Mo. The element composition of Py2 shows transitional characteristics between those of Py1 and Py3. The Co and Ni contents gradually decrease from Py1 through Py2 to Py3, while their ratios remain within the range of 0.1–1.0. Compared with Py1 and Py2, Py3 exhibits suddenly elevated As levels and apparent acicular structures, indicating that a rapidly precipitating environment was likely triggered by an abrupt temperature decrease. Py1 exhibits δ34S values of 19.7–21.5 ‰, followed by Py2 between 18.6 and 21.1 ‰; both indicate that the sulfur was sourced from sulfates by thermochemical sulfate reduction (TSR). In contrast, Py3 exhibits lower δ34S values ranging from 7.5 to 11.0 ‰, potentially attributed to bacterial sulfate reduction (BSR). The differences in mineral structure, element composition, and sulfur isotopes among the three pyrite types indicate the involvement of two distinct fluids: metal-bearing basin brine and fluid containing reduced sulfur. The former was derived from basin brine that extracted ore metals (Pb2+ and Zn2+) from host sedimentary piles. The latter originated from carbonate strata in the Maoping area containing reduced sulfur formed by BSR. We propose that the ore-bearing basin brine reacted with organic matter in the wallrock through TSR to generate Py1 and Py2 in stage I and then mixed with the positioned fluid containing reduced sulfur via BSR to precipitate Py3, sphalerite, and galena in stage II. Hence, fluid mixing is the primary ore-forming mechanism and effectively accounts for the high-grade Pb–Zn ores in the Maoping deposit.

Using sulfur isotopes to constrain the sources of sulfate in PM2.5 during the winter in Jiaozuo City

Sulfate (SO42−) is one of the main driving forces behind the explosive growth of atmospheric fine particulate matter (PM2.5), and it scatters sunlight, contributing to atmospheric cooling. Investigating the sources and transformation of SO42− is crucial for understanding PM2.5 pollution and its implications for climate change. Despite a continuous decline in PM2.5 concentrations in China since 2015, it remains unclear whether there have been changes in the sources and transformation processes of SO42−. Furthermore, previous studies that utilized isotope techniques for source identification have often overlooked the impact of isotope fractionation. This study focuses on atmospheric PM2.5 in Jiaozuo based on samples collected from December 2017 to February 2018. The analysis encompassed both the water-soluble ions and sulfur isotope compositions of SO42− (δ34S–SO42−). By employing a Bayesian isotope mixing model, quantitative analysis of SO42− sources in winter atmospheric PM2.5 in the study area was conducted. The findings revealed that the δ34S–SO42− values of winter PM2.5 ranged from −0.1‰ to 9.8‰, with an arithmetic mean of 4.9 ± 1.5‰ (n = 74). After considering sulfur isotope fractionation effects and the impact of primary SO42−, the calculated results suggested that on average, the contributions of primary SO42−, coal combustion, vehicle exhaust, biomass burning, and oil combustion to the SO42− in winter PM2.5 were 24.9 ± 15.7%, 41.2 ± 2.6%, 27.9 ± 5.3%, 4.8 ± 3.7%, and 1.2 ± 0.7%, respectively. Compared with the clean period, the contribution of the primary SO42− decreased significantly during haze episodes, while the contribution of the secondary SO42− increased. The contribution of coal combustion to the secondary SO42− increased notably by 21.9%, while the contributions from other secondary sulfate remained relatively stable. The lower δ34S values of the atmospheric PM2.5 in Jiaozuo affirm the significant impact of the anthropogenic SO2 input on the sulfur isotope composition of the atmospheric sulfate. Analysis confirmed that the decrease in the contribution of coal combustion to atmospheric SO42− in the study area, indicating the positive effects of regional air pollution control measures. Nevertheless, continued attention should be paid to the influences of coal combustion and vehicle exhaust on regional air pollution.

Relationship between Pb-Zn and Au mineralization in the Laoyachao deposit, South China: Constraints from geology and S-Pb-He-Ar isotopes

Both Pb-Zn and Au mineralization can simultaneously occur at the deposit, camp or regional scale, but their spatial-temporal and genetic relationships have received less attention so far. Here we present detailed field investigations and S-Pb-He-Ar isotope data for the Laoyachao deposit, South China, to interpret the intrinsic associations between Pb-Zn and Au mineralization. Both Pb-Zn and Au ores in the studied area are distributed independently but spatially associated with the same granodiorite. Field and microscopic observations, together with previous dating, show that the Au mineralization postdated the Pb-Zn mineralization. The relatively restricted range and lower δ34S values for the Au ores (−0.72 to +0.94 ‰) and the Pb-Zn ores (−1.98 to +2.36 ‰) indicate that both metal mineralization types are attributed to a homogeneous magmatic sulfur source. Lead isotope compositions for the ores and the feldspars from the granodiorites define a well-defined linear array, with the Au ores displaying less radiogenic Pb. In addition, the Pb-Zn ores exhibit lower 3He/4He and 40Ar/36Ar ratios (0.01–0.26 Ra, 298–313) than the Au ores (0.16–2.93 Ra, 341–382). All evidence demonstrated that, besides the granodiorite end-member, an end-member with high-radiogenic Pb and lower 3He/4He values participated in the early Pb-Zn mineralization, in contrast, the other end-member with less radiogenic Pb and higher 3He/4He values was involved in the late Au mineralization. Considering that both Pb-Zn and Au mineralization in the studied deposit are spatially, temporally and genetically related to the same granodiorite, the Pb-Zn mineralization associated with granodiorite can provide an indicator for gold exploration.

Sulfur isotopic systematics of hydrothermal precipitates in the Okinawa Trough: Implication for the effect of sediments and magmatic volatiles

The Okinawa Trough is a sediment-covered intra-continental back-arc basin with active hydrothermal activities. The sulfur isotopic systematics of the hydrothermal systems in the Okinawa Trough remain unclear owing to the complex sulfur reservoirs. To address these uncertainties, we conducted in situ S isotopic combined with whole-rock geochemical, Pb isotopic, and barite Sr isotopic analyses of typical hydrothermal precipitates from three hydrothermal fields with varying sediments thickness (Iheya North Knoll (INK) < Yonaguni Knoll IV (YK) < Noho) in the Okinawa Trough. The δ34S values of barite from INK (+16.4‰ to +19.6‰, median + 18.5‰) and YK (+16.2‰ to +22.2‰, median + 19.5‰) mostly indicate lower values than those obtained for seawater (~ +21‰), combined with the observation of high S and O fugacity (fO2–fS2) mineral assemblages (e.g., low-Fe sphalerite ± enargite ± tennantite ± chalcopyrite), and high Cu and Au contents of hydrothermal precipitates, consistent with the addition of magmatic volatile to these two fields. In contrast, the in situ δ34S values of sulfide minerals from INK (+5.6‰ to +10.4‰, median + 9.0‰) and YK (+3.5‰ to +5.5‰, median + 4.7‰) fields were all positive, inconsistent with the S isotopic characteristics of H2S formed from SO2 disproportionation. We conclude that a large proportion of thermochemical SO42− reduction-derived heavy sulfur likely obscure the characteristics of H2S sourced from disproportionation. This was likely resulting from (1) sediment controlled long-scale lateral hydrothermal circulation, (2) the existence of early precipitated anhydrite, and (3) high concentrations of H2 in the high-temperature reaction zone. The Noho field have low δ34S values (−0.2‰ to +5.3‰, median + 0.9‰) of sulfide minerals, combined with high sediment contributions reflected by high Pb isotopic ratios and low-fS2–fO2 mineral assemblages (e.g., high-Fe sphalerite ± pyrrhotite ± isocubanite), which indicates that the low δ34S value sulfur was derived from leaching of biogenic pyrite in thick sedimentary strata by hydrothermal fluid. Near surface, the high-temperature discharging hydrothermal fluids interacted with different thicknesses of sedimentary strata and leached different proportions of biogenetic pyrite, resulting in a negative correlation of sedimentary strata thickness and sulfide δ34S values of the Okinawa Trough hydrothermal fields.

Trace elements and sulfur isotopic compositions of sulfides in the giant Dahongshan Fe-Cu-(Au-Co) deposit, SW China: Implications for fluid evolution and Co enrichment in IOCG systems

Iron-oxide-copper–gold (IOCG) deposits are characterized by Cu-Au polymetallic mineralization and may contain large amounts of critical metals (e.g. Co, REE, and U) as by-products. Sources of ore-forming fluids and metals of the IOCG deposits remain controversial, either a magmatic or basinal source has been advocated. The ∼1660 Ma Dahongshan Fe-Cu-(Au-Co) deposit in the Kangdian region in southwestern China has spatially separated Fe and Fe-Cu orebodies, in which Co-rich pyrite has been identified in different types of ores and host rocks. Here, we integrate in-situ sulfur isotopic compositions and trace element geochemistry of sulfides to characterize the nature of ore fluids and the Co enrichment processes.Paragenetic sequences of sulfides in the Dahongshan deposit include: Pyrite (Py I) associated with magnetite mineralization in Fe ores, which has rims (Py II-1) intergrown with hematite-chalcopyrite assemblage; Pyrite and chalcopyrite (Sulfide II) in the Fe-Cu ores with protolith of arenaceous dolostone (Sulfide II-2) and interbedded argillaceous siltstone and dolostone (Sulfide II-3); and Sulfide III in post-ore reworking veins formed during regional metamorphism. Py I shows a magmatic sulfur isotopic signature (−2.2 to 1.2‰) and relatively high Au-Cu-Zn contents, indicating a predominance of magmatic-hydrothermal component during magnetite formation. By contrast, the combined sulfur isotopes and trace element compositions of Sulfides II in Fe-Cu ores indicate the mixing of two distinct fluid endmembers: Sulfides precipitated from one endmember show magmatic affinity with low δ34S values (as low as −2.1‰), high Se/S ratios (∼11.8 × 10-4), high Co contents (∼10600 ppm) and Co/Ni ratios (∼110), likely from a magmatic-hydrothermal source of mafic magmas affinity; whereas sulfides from the other endmember have overall higher δ34S values (up to 15.8‰), relatively lower Se/S ratios (∼0.09 × 10-4), lower Co contents (∼900 ppm) and Co/Ni ratios (∼13). Together with the published B-O isotopic data, the high-δ34S fluid is attributed to externally-derived basinal brine at Dahongshan. The clear negative correlations between δ34S values and Se/S ratios, Co contents, as well as Co/Ni ratios of Py II highlight that the incursion of exchanged basinal brines may be vital for triggering economic sulfide mineralization in Dahongshan. Furthermore, when assuming a similar predominance of magmatic fluids, Py I associated with magnetite ores is less enriched in Co (with lower Co/Ni ratios) than Py II from Fe-Cu ores, which can be tentatively ascribed to the decreasing Co solubility limits and formation of cattierite (CoS2) solid solution in pyrite as the fluid evolved. Finally, sulfide III in reworking veins show comparable δ34S values and Se/S ratios, but higher Co contents (up to 3.1%) than sulfide II, implying that Co can be further enriched during later remobilization events. This study exemplifies the utility of sulfide as a robust monitor of ore-forming process in IOCG system.

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.

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.

Inorganic sulfur cycles in sediments of the Pearl River Estuary: Processes, mechanisms, and isotopic indicators

The burial of inorganic sulfur is one of the important components in the global sulfur cycle. In this study, the cycling of inorganic sulfide species and the underlying control mechanisms were examined in sediments found in distinct sub-regions of the Pearl River Estuary (PRE). Specifically, the content and isotopic composition of SO42−, acid-volatile sulfides (AVS), and pyrite were determined, along with sedimentary sulfate reduction rates (SRR). The results show that the formation of iron sulfides (such as AVS and pyrite) was mainly controlled by the content of available reactive iron in the sediments of the PRE, and therefore formed mainly in the shallow sediments, with little variation in the deeper parts of the sediments. The δ34S values of AVS and SO42− increased synchronously with depth in the middle and lower part of the sediment, indicating a closed diagenetic environment. In contrast, the δ34S value of pyrite only increased slowly with depth in the case of a small portion of AVS converted to pyrite, which was controlled by the burial time. There are obvious differences in the formation and burial of pyrite in sediments of different sub-regions of the PRE. The surface layer of stations QA and HQ in the upper estuary and brackish coast was an open environment due to sedimentary reworking, and the contents of AVS and pyrite in the sediment was extremely low due to re-oxidation, while the oxidation of 32S-rich H2S also resulted in lower δ34S values of sulfate in the sediment pore water than in the overlying seawater. Station GS in the estuarine mouth had a much higher percentage of AVS converted to pyrite in the sediments than that at stations QA and HQ due to the relatively long burial time. In addition, near the sulfate methane transition zone (SMT), only the estuarine mouth (station GS) produced iron sulfides (AVS) synchronously with the anaerobic oxidation of methane (AOM), and then converted to pyrite gradually; in contrast, the effect of AOM on the formation of iron sulfides in the sediments of the upper estuary and the brackish coast was almost negligible. The results show that the contribution of AOM processes to the final burial of inorganic sulfur is generally small in PRE.

Metallogenic mechanism of the Houge’zhuang gold deposit, Jiaodong, China: Evidence from fluid inclusion, in situ trace element, and sulfur isotope compositions

The Houge’zhuang gold deposit, located in the Penglai–Qixia gold belt of the Jiaodong peninsula, is a representative auriferous quartz vein-style deposit. Pyrite is the most common and main gold-bearing mineral and shows complex textures in the Houge’zhuang gold deposit. Study of ore-related pyrite is of great significance for understanding the metallogenesis of this deposit, especially the gold precipitation mechanism. The present study applied systematic microscopic observation, fluid inclusion analysis, electron microprobes,in situ LA-ICP-MS trace-element analysis, and in situ sulfur isotope analyses. Three types of fluid inclusions were identified, among which the ore-forming fluids exhibited medium–low salinity and temperature, with the fluid inclusions mainly comprising H2O and CO2. Three types of pyrites were identified: 1) Py0, characterized by low concentrations of As and Au and low δ34S values (5.51–6.86‰). 2) Py1, found in the gold-quartz-pyrite veins and homogeneous in chemical composition with no obvious zonal growth but with notably more gold and chalcopyrite inclusions. Py1 contained medium and uniform concentrations of As and was Au-rich, with δ34S values ranging from 7.13 to 7.89‰ (mean 7.44‰). 3) Py2 contained arsenic-bearing pyrite and was found in quartz-polymetallic sulfide veins, with distinct As enrichment and As-rich rims of pyrite and growth zoning. Consequently, the primary ore-forming fluids passed through some arsenic and δ34S-rich sedimentary rocks, such as the Jingshan, Fenzishan, and Penglai groups. With the occurrence of stable water-rock interaction, the extracted fluids were enriched for As and δ34S. Furthermore, Au was closely associated with As, visible gold grains tended to occur in association with Py1 at stage II, and invisible gold was related to the Au-As-rich Py2 of stage III. The As-bearing pyrites shared a close spatiotemporal relationship with gold, playing an important role in the formation and exploration of high-grade gold deposits.

Trace-element distribution and ore-forming processes in Au–Ag-rich hydrothermal chimneys and mounds in the TA25 West vent field of the Tonga Arc

We report detailed mineralogy and geochemistry of hydrothermal mounds and chimneys in the TA25 West vent field (TA25 WVF), a newly discovered magmatic–hydrothermal system in the Tonga (Tofua) arc. Chimney samples are classified as sulfate- or sulfide-rich, based on major sulfide, sulfosalt, and sulfate minerals. The former type represents a simple mineral assemblage of predominance of anhydrite/gypsum + barite + pyrite, whereas the sulfide-rich chimneys show three different stages of mineralization with decreasing fluid temperature: sphalerite–pyrite dominated stage I, sphalerite–sulfosalts dominated stage II, and stage III is dominated by seawater alteration. Mound samples are characterized by sulfide assemblages and paragenesis similar to those of sulfide-rich samples, but abundant chalcopyrite indicates a relatively high-temperature mineralization. The chimney and mound samples are enriched in Au (average 9.2 ppm), Ag (297 ppm), As (1897 ppm), Sb (689 ppm), Hg (157 ppm), and Se (34.6 ppm). LA–ICP–MS and FE–TEM studies indicate that most of these elements occur in sulfides or sulfosalts in solid solution, although some occur as nanoparticles. This is mainly controlled by the combined effects of fluid conditions (temperature and redox state) and influx of ambient seawater. Petrography and trace-element compositions of sulfides and/or sulfosalts suggest that most concentrations of Au and Ag in the TA25 WVF result from the precipitation and/or adsorption of Au–Ag-bearing nanoparticles on rapidly crystallized sulfides, the substitution of Au and Ag in sulfide and/or sulfosalt minerals, and the saturation of Ag in hydrothermal fluids during late, relatively low-temperature mineralization (< 150 °C). The maximum measured temperature (242 °C) of venting fluids and calculated formation temperatures of sphalerite (229–267 ℃) are below the boiling temperature of seawater at the depths (966–1096 m) of the TA25 WVF, suggesting fluid boiling had little effect on Au–Ag-rich mineralization in the TA25 WVF. The presence of enargite–tetrahedrite–tennantite assemblages, high concentrations of magma-derived elements (e.g., Au, Ag, As, Sb, Hg, and Se), low δ34S values (2.1 to 4.3‰) of sulfide minerals relative to the host rocks, and the distribution of CO2-rich hydrothermal plumes (500 to 1000 ppm) suggest that the TA25 WVF is a submarine hydrothermal system influenced by a magmatic contribution in an arc setting. Our results indicate that the magmatic contribution is most likely to play an important role in supplying various metals, including Au and Ag, to the TA25 WVF. Subsequently, the rapid crystallization of sulfides induced by abundant fluid-seawater mixing significantly contributes to the precipitation of Au–Ag-rich mineralization.

Novel sulfur isotope analyses constrain sulfurized porewater fluxes as a minor component of marine dissolved organic matter

Marine dissolved organic matter (DOM) is a major reservoir that links global carbon, nitrogen, and phosphorus. DOM is also important for marine sulfur biogeochemistry as the largest water column reservoir of organic sulfur. Dissolved organic sulfur (DOS) can originate from phytoplankton-derived biomolecules in the surface ocean or from abiotically "sulfurized" organic matter diffusing from sulfidic sediments. These sources differ in 34S/32S isotope ratios (δ34S values), with phytoplankton-produced DOS tracking marine sulfate (21‰) and sulfurized DOS mirroring sedimentary porewater sulfide (∼0 to -10‰). We measured the δ34S values of solid-phase extracted (SPE) DOM from marine water columns and porewater from sulfidic sediments. Marine DOMSPE δ34S values ranged from 14.9‰ to 19.9‰ and C:S ratios from 153 to 303, with lower δ34S values corresponding to higher C:S ratios. Marine DOMSPE samples showed consistent trends with depth: δ34S values decreased, C:S ratios increased, and δ13C values were constant. Porewater DOMSPE was 34S-depleted (∼-0.6‰) and sulfur-rich (C:S ∼37) compared with water column samples. We interpret these trends as reflecting at most 20% (and on average ∼8%) contribution of abiotic sulfurized sources to marine DOSSPE and conclude that sulfurized porewater is not a main component of oceanic DOS and DOM. We hypothesize that heterogeneity in δ34S values and C:S ratios reflects the combination of sulfurized porewater inputs and preferential microbial scavenging of sulfur relative to carbon without isotope fractionation. Our findings strengthen links between oceanic sulfur and carbon cycling, supporting a realization that organic sulfur, not just sulfate, is important to marine biogeochemistry.

Organoclastic sulfate reduction in deep-buried sediments: Evidence from authigenic carbonates of the Gulf of Mexico

The fate of organic matter in deep-buried marine sediments determines the formation of hydrocarbon reservoirs, shapes the deep biosphere, and affects the global carbon cycle. Several geochemical processes such as silicate weathering, iron and manganese (hydr)oxide reduction, and sulfate reduction influence the remineralization of organic matter in buried sediments. However, little information on deep-seated organoclastic sulfate reduction is available and the involved geochemical processes are largely unknown. Here, authigenic carbonates recovered from Ship Shoal Block 296 (SS296) of the Gulf of Mexico, all dominated by low-magnesium calcite, were investigated to constrain organoclastic sulfate reduction in deep-buried sediments. The SS296 carbonates had been previously suggested to be of a deep origin and transported upward by salt diapirism. The presence of cone-in-cone textures in samples 3110R1 and 3110R2 as well as vitrinite reflectance values of 0.48% to 0.58% indicate carbonate formation at a relatively deep burial. Likewise, clumped isotope thermometry yielded formation temperatures from 45 °C to 53 °C for sample 3110R1, 60 °C for sample 3110R2, and 11 °C to 37 °C for sample 3110R3. In spite of formation at greater depth, the temperatures of samples 3110R1 and 3110R2 are still within the temperature range of microbial sulfate reduction, reflecting formation depths between 1 km and 2.7 km. The presence of framboidal pyrite and its relatively low δ34S values (−27.8‰ to +11.5‰) agree with the occurrence of microbial sulfate reduction during carbonate precipitation. The carbon isotopic composition of the carbonates (δ13C: −16.6‰ to −8.9‰) and enclosed organic matter (δ13Corg: −26.8‰ to −20.5‰) suggests that organic matter is the dominant carbon source of carbonate minerals. This study confirms that microbial sulfate reduction occurs in deep-buried sediments, driving the degradation of organic matter. Our findings suggest that organoclastic sulfate reduction might be common in deep-seated sedimentary environments, controlling the fate of organic matter in deep subsurface environments and affecting the global carbon cycle.

Geochemical Evidence for a Topographically Driven Regional Mineralizing Fluid in the Polaris Zn District, Arctic Canada

AbstractThe Polaris district in Canada’s Arctic Archipelago contains numerous carbonate rock-hosted Zn + Pb showings and rare, anomalous Cu showings in a 450- × 130-km area. As in many metallogenic districts, a genetic relationship between the mined deposit and surrounding showings has been assumed but not tested. This study uses an in situ, multianalytical approach combining optical and scanning electron microscopy petrography, fluid inclusion microthermometry, evaporate mound analysis, trace element analysis, and in situ stable isotope analysis on sphalerite and carbonate gangue to characterize the fluid histories of individual showings and the district as a whole. Results indicate that a regional, marine-derived fluid dissolved subsurface evaporite minerals, interacted with their connate brines, and transported metals and sulfate to sites of mineralization. Initial fluid mixing with local reduced sulfur accumulations resulted in precipitation of sulfides with lower δ34S values; after exhaustion of the local reduced sulfur pool, thermochemical sulfate reduction (TSR) of transported sulfate became dominant, resulting in higher δ34S. Differences in main-stage δ34S values among different showings indicate a variable extent of TSR among sites. The mineralized volume of each showing is predominantly a function of local fluid flux and availability of a local reductant. The nature and consistency of geochemical characteristics throughout the district confirm the genetic relationship between the large deposit (Polaris) and surrounding showings and indicate that a uniform mineralizing fluid, topographically mobilized during the mid-Paleozoic Ellesmerian orogeny, was responsible for the main, district-wide mineralization, after initially mixing at a smaller scale with local, on-site fluids.

Genesis of the Danping bauxite deposit in northern Guizhou, Southwest China: Constraints from in-situ elemental and sulfur isotope analyses in pyrite

The super large bauxite deposit known as the Danping deposit is hosted in the Dazhuyuan formation, Lower Permian, and is located in northern Guizhou (Southwest China). The geochemical signatures of five generations of pyrite from Danping have been identified, and their genesis has been constrained by analyzing their geochemical signatures. Oolitic pyrite (Py-O), which has two sub-generations, occurs in the upper part of the oolitic bauxite. Typically, Py-O2 has a core and rim structure, where the core may contain altered primary Py-O1. While Py-M appears as occurs in the form of interstitial material in the lower part of the oolitic bauxite, Py-C is cubic in shape and is reported in the earthy bauxite. Moreover, Py-Q is found in the Qixia formation. Py-M has a Co/Ni ratio of ∼ 1 and a δ34SV-CDT value of ∼ 0 ‰, thus suggesting a magmatic hydrothermal origin. The primary Py-O1 has a uniform geochemical composition. The reformed Py-O1 and the core of Py-O2 have considerably lower δ34S values of −19.15 ‰ to −4.72 ‰, indicating that a brief pulse of seawater injection can result in the replacement of Py-O1 and the formation of Py-O2. Py-M, Py-O and Py-Q all have positive Ce anomalies, while Py-C has negative Ce anomalies. These contrasting characteristics demonstrate that the original aluminous rock series was largely deposited in an oxidic environment, while the earthy bauxite was formed in a reducing environment. Lithium, Sc and Ga are positively correlated with one another, reflecting their similar geochemical behavior during sedimentary–weathering processes. Py-M has higher contents of Li, Sc and Ga, implying that magmatic fluids contributed to the presence of critical metals even in the weathering conditions.

Episodic ore-forming fluid evolution processes in the Jiudian gold deposit, Jiaodong Peninsula: Constrains from texture, trace element and S isotope composition of pyrite

Jiudian is a representative quartz vein-type gold deposit in the Jiaodong world-class gold province, southeast of the North China Craton. Four stages and ten pyrite generations can be identified based on the paragenetic relationships: (1) pyrite (Py1)-milky quartz, (2) pyrite (Py2-1 and Py2-2)-smoky grey quartz, (3) siderite-pyrrhotite-pyrite (Py3, Py3-1 and Py3-2)-quartz, (4) quartz-polysulfide-siderite-marcasite (Py4-mc)-pyrite (Py4-1, Py4-2 and Py4-3), suggesting the complex and multistage ore-forming processes.The course-grained euhedral Py1 from the pre-ore stage 1 quartz veins is enriched in lattice-bound Co, Ni, and Se with relatively low δ34S values (+4.7 to + 5.5‰), precipitated from the equilibrated fluid environment with higher temperature (ca. 400 ℃). The dissolution-reprecipitation textures are characterized by the pristine As-rich Py2-1 as parent phase and porous Py2-2 as product phase hosting abundant microscale galena enriched in low-melting chalcophile elements (LMCE, e.g., Pb, Bi, Ag, Te, and Sb) and minor electrum. Both of Au-HS and Ag-Cl complexes destabilized by decompression would result in the precipitation of electrum with low gold fineness (672 to 713). Py3 is a poor host for majority of trace elements coexisting with pyrrhotite, precipitated from the late second pulse fluid.Py3 was overprinted by the third ore fluid pulse and replaced by siderite into sieve-shaped Py3-1 and anhedral aggregates Py3-2. Subsequently, fluid oxidation caused by pyrite replacement and meteoric water influx would result in the precipitation of marcasite (Py4-mc). Furthermore, euhedral pyrite aggregates were directly deposited from this ore-forming fluid, characterized by the Co-rich core (Py4-1) with abundant inclusions of LMCE-dominant sulfosalt (lillianite homologues) and galena, overgrown by pristine As-rich rim (Py4-2). The decoupled As and Co in Py4-1 and Py4-2 were probably attributed to abrupt temperature drop due to meteoric water influx. Nevertheless, the precipitation of visible gold with higher gold fineness in siderite or intergrown with Py4-mc was primarily resulted from fluid oxidation which has significant effect on the stabilities of Au-HS complexes but little on those of Ag-Cl complexes. The last generation Py4-3 was the post-oxidation product, evidenced by the extremely negative δ34S values (-34.3 to −28.4‰).Hence, the textural and geochemistry characteristics of different pyrite generations of Jiudian have revealed the presence of three fluid pulses as well as the distinctive evolution processes and changes of physicochemical conditions leading to the characteristic mineral assemblages and multiple-stage gold mineralization, which could provide implications for the genesis of siderite-rich gold deposits in the Jiaodong district.

Testing the global significance of the sulfur isotope record of the ca. 2.0 Ga Zaonega Formation: A micro-scale S isotope investigation

The Paleoproterozoic metasedimentary rocks of the Zaonega Formation (Onega Basin, NW Russia) are important archives of inferred global environmental change following the initial oxygenation of the Earth’s atmosphere and oceans. However, the geochemical signals preserved in these exceptionally organic-and pyrite-rich metasedimentary rocks and their environmental meaning remain contested. In particular, the Zaonega Formation’s unusually high pyrite sulfur isotope ratios (δ34S) have been explained by either global or local forcings acting on sulfur cycling processes. We tested former interpretations of the Zaonega Formation’s sedimentary pyrite record by integrating bulk and micro-scale δ34S analysis to discriminate the isotopic signatures of different generations of pyrite and determine the underlying mechanisms contributing to δ34S variability. We show that the prolonged genesis of pyrite occurred via multiple stages and included precipitation from early diagenetic fluids, organic matter pyritization, and late-stage alteration fluids. Our results demonstrate that early-stage pyrite typically carries more variable and lower δ34S values than late-stage pyrite. Although the early pyrite captures pore water S isotope signatures least evolved from the seawater, their contribution to the bulk δ34S results can be dwarfed by the greater volume of late-stage coarse pyrite. Consequently, determining the sequence of pyrite precipitation and δ34S characteristics of individual generations in any given sample are fundamental to interpreting bulk δ34S records. Our micro-scale results suggest that previous estimates based on bulk pyrite data (ca. 6 to 18‰) should not be related to the original seawater sulfate’s isotopic composition. These results demonstrate that a thorough understanding of the geological context and mechanisms associated with S-cycling, and pyrite formation is necessary to interpret bulk δ34S records accurately.

Elemental and isotopic compositions of New Zealand regional soils identifies human and climate-induced effects

A regional soil study of 61 sites from the Wairau and Awatere Valleys, Marlborough region, New Zealand characterised the geochemical baselines of different land-use types. The study used three stable isotopes (δ13C, δ15N, δ34S) and 65 elements to describe the influence of parent-rock geology, human and climate-induced effects and natural transport processes on the soil quality and soil health of this region. The temperate Marlborough region produces high quality wine and the elemental and isotopic composition of a subset of vineyard soils are examined in more detail to identify soil characteristics specific to this agriculturally productive region. Higher levels of Cd, K, N, P, S and Zn were found in vineyard soils compared to forested and rural soils, indicative of anthropogenic agricultural inputs. Stable isotopes in vineyard soils had higher δ13C and δ15N values and lower δ34S values than forested and rural soils, typical of more highly cultivated soils. The natural geogenic inputs from the ultra-mafic mountains enclosing the Wairau and Awatere Valleys were geochemically distinct from localised anthropogenic contaminants. Comparisons with a further 240 New Zealand urban (Dunedin) and rural (Southland-Otago region) soils suggests the elemental contaminants found in the Marlborough vineyard soils may be specific to high-yielding agricultural zones across New Zealand.