Sulfur Isotopic Composition Research Articles

Multi-isotopic analysis reveals the early stem turtle Odontochelys as a nearshore herbivorous forager

IntroductionAfter decades of debate on the origin of turtles, it is now widely accepted that they are diapsid reptiles originating in the Permian from a terrestrial ancestor. It seems that the initial development of the structures that will later form the unique turtle bony shell took place as a response to a fossorial lifestyle. However, the earliest stem turtle with a fully complete plastron, Odontochelys semitestacea from the Late Triassic (lower Carnian) of China, is somewhat controversially interpreted as an aquatic or even a marine form, raising the question of the environment in which the completion of the plastron happened.MethodsHere, we analyzed the stable carbon, oxygen and sulfur isotope compositions (δ13C, δ18O and δ34S) of bones from two specimens of Odontochelys along with bones and teeth of two associated specimens of the marine ichthyosaur Guizhouichthyosaurus tangae.Results and discussionWe first show that δ18O values of Odontochelys are incompatible with a terrestrial lifestyle and imply a semi-aquatic to aquatic lifestyle. Isotopic results also demonstrate that the aquatic environment of Odontochelys was submitted to a strong marine influence, therefore excluding the possibility of a strict freshwater aquatic environment. Additionally, an unusual carbon isotope composition shows that O. semitestacea was herbivorous, probably consuming macrophytic algae in coastal zones like the extant green sea turtle (Chelonia mydas) or the marine iguana (Amblyrhynchus cristatus) do.

Multiple Sulfur Isotopes of Iron Sulfides From Thick Greigite‐Bearing Sediments Indicate Anaerobic Oxidation and Possible Leakages of Coastal Methane

AbstractMagnetic greigite may be a valuable indicator for methane emissions in the geological past, if its formation pathway and diagenetic environment can be unambiguously defined. Here, we investigate sulfur isotopic compositions of iron sulfides and ferrous iron concentrations of thick greigite‐bearing sediments (TGBSs) in the South Yellow Sea, a shallow marginal sea with strong methane emissions. For the first time, isotopically heavy iron sulfides (up to 28.7‰ in δ34S and 0.19‰ in Δ33S) and enrichments of ferrous iron in the TGBSs are observed. We interpret the data as indicating synchronic occurrences of anaerobic oxidation of methane coupled to sulfate and iron reductions, which occur in coastal methanic zones with limited sulfate availability and therefore probably imply leakages of methane. Consequently, we suggest that greigite is a promising geological indicator for tracing methane liberated from coastal sediments, accounting for ∼60% of the currently rising global marine methane budget.

The Relationship between Bacterial Sulfur Cycling and Ca/Mg Carbonate Precipitation—Old Tales and New Insights from Lagoa Vermelha and Brejo do Espinho, Brazil

Over the few past decades, the concept of microbial sulfur cycling catalyzing the precipitation of CaMg (CO3)2 at low temperatures (<40 °C) has been studied intensely. In this respect, two hypersaline lagoons, Lagoa Vermelha and Brejo do Espinho, in Brazil, have been the subject of numerous studies investigating sedimentary Ca/Mg carbonate formation. Here, we present the sulfur and oxygen isotopic compositions of dissolved sulfate from surface water, as well as sulfate and sulfide from pore-water (δ34SSO4, δ18OSO4, and δ34SH2S), the sulfur isotopic composition of sedimentary pyrite (δ34SCRS), and sulfur and oxygen isotopic compositions of carbonate-associated sulfate (CAS, δ34SCAS and δ18OCAS). The pore-water profiles at Lagoa Vermelha indicate ongoing bacterial sulfate reduction by increasing δ34SSO4, δ18OSO4 and δ34SCRS values downcore. At Brejo do Espinho, the pore-water profiles displayed no depth-dependent isotope trends; the Ca/Mg ratio was, on average, lower, and the δ18OSO4 values in both surface and pore-water were strongly enriched in 18O. There was an overall mismatch between δ34SSO4 and the significantly higher δ34SCAS values. A negative correlation was observed between the Ca/Mg ratio and higher δ34SCAS values. The results show that the size difference between the two lagoons induces differences in the intensity of evaporation, which leads to the increased secretion of extrapolymeric substances (EPSs) by microbes in the smaller Brejo do Espinho. EPS provides the microenvironment where Ca/Mg carbonate can nucleate and preserve increased δ34SCAS values. Apart from EPS, increased sulfur oxidation is proposed to be a second factor causing relative enrichment of Ca/Mg carbonates at Brejo do Espinho. Our results emphasize the role of evaporative processes on Ca/Mg carbonate formation, and indicate that the respective δ34SCAS values reflect microenvironments rather than preserving an open marine δ34SSO4 signature.

Geochronology, pyrite trace elements, and sulfur isotope geochemical characteristics of the Saibagou gold deposit in the eastern part of the northern Qaidam Basin

Introduction: The Saibagou gold deposit, located in the eastern part of the tectonic belt of the northern Qaidam Basin in western China, has its gold ore bodies strictly controlled by the regional fault system. Despite this understanding, there remain controversies surrounding the deposit’s metallogenic epochs, sources of ore-forming materials, and properties of ore-forming fluids. To address these controversy, the metallogenic process of the Saibagou gold deposit can be further determined by analyzing the U-Pb ages of hydrothermal zircons in the gold-bearing quartz veins and investigating the trace element and sulfur isotope compositions of pyrite in the gold deposit.Methods: This study does not focus on the mineral characteristics susceptible to interference by the metallogenic condition in gold ore bodies. Instead, it offers a detailed discussion on stable associated minerals and their indicative markers formed in the process of gold mineralization.Results: The results of this study showed that the metallogenic process of the gold deposit can be categorized into the following stages: 1) quartz–pyrite veins, 2) milky-white quartz—pyrite—native gold veins, 3) hoary quartz—native gold—polymetallic sulfide veins. As indicated by the U-Pb ages of hydrothermal zircons in the gold-bearing quartz veins, the Saibagou gold deposit has two metallogenic ages, namely, 423.91 ± 4.5 Ma (the Silurian) and 470.18 ± 4.92 Ma (the Ordovician).Discussion: The Silurian metallogenic age, predominates and nearly aligns with the expansion of the regional NWW-trending brittle-ductile shear zone, followed by the Ordovician metallogenic age. Data on the trace elements and sulfur isotopes of pyrite, show that the Saibagou gold deposit has similar pyrite compositions in the three metallogenic stages. Gold in the deposit primarily occurs as native gold or minor petzite inclusions and has a very low lattice gold concentration. As indicated by the concentrations of elements such as Co, Ni, and as in the pyrite, as well as the variation range of δ34S values, the ore-forming fluids were derived from low-temperature arsenic-bearing acidic magmas. In addition, the primary ore-forming materials appear to encompass mantle-derived materials from the deep earth.

THE FIRST INFORMATION ABOUT THE GOLD-COPPER MINERALIZATION OF THE KONTORSKOYE ORE OPENING (ARIADNE INTRUSION OF ULTRABASITES, PRIMORYE)

Within the Sikhote-Alin orogenic belt, a new promising type of mineral resources has been identified – complex ores and placers spatially and genetically related to Late Mesozoic ultramafic intrusions. An example of this is the Ariadne massif of ultramafic rocks, in the center of which there is a manifestation of gold-titanium mineralization, and in the exocontact – gold-antimony. The main features of the mineralogy and geochemistry of gold-copper formations in its near-intrusive zone are determined. To reconstruct the conditions for the formation of these ores, the isotopic composition of sulfur in sulfide minerals was studied. The obtained materials testify to the significant metal-bearing potential of the ultramafic rocks of the Sikhote-Alin.

Physicochemical evolution and mechanism of a skarn system: Insights from the world-class Mazraeh Cu deposit, NW Iran

The Mazraeh Cu skarn deposit, NW Iran, is situated at the lithological contact between an Oligo−Miocene granodiorite and Cretaceous limestones. The causative granodiorite has I-type medium- to high-K calc-alkaline composition that formed in a subduction arc setting. Metasomatism generated exoskarn and endoskarn characterized by a temporal and spatial mineral-chemistry zonation with massive garnet and garnet-pyroxene proximal and epidote-amphibole skarn at the distal zones of the exoskarn. Our data reveal a multi-stage evolution of the hydrothermal system. Accordingly, the prograde stage was formed by barren, oxidized, high-temperature and hypersaline magmatic fluids, which suggests that the parental magma was emplaced at low lithostatic pressure conditions (∼1.5 km depth). By contrast, during the retrograde stage the regime changed from lithostatic to hydrostatic conditions, resulting in a significant decrease in pressure and oxygen fugacity, boiling, cooling, and mixing, respectively. Dilution by meteoric fluids led to additional cooling and lower salinity of the ore fluids. Sulfur isotope compositions (δ34S = −2.8‰−1.3‰) of sulfides indicate a unique magmatic source of the sulfur. The δ18Ofluid and δDfluid values of the prograde stage fluid also indicate a magmatic origin, while their gradual depletions from the prograde to retrograde stage imply increased mixing with meteoric water. We propose that the fluid mixing scenario continued over the entire life cycle of the mineral system in an open hydrostatic regime as reflected in the oxygen isotope values. It is suggested that boiling and fluid mixing played a significant role in the formation of this giant Cu skarn deposit.

Origin of dissolved organic sulfur in marine waters and the impact of abiotic sulfurization on its composition and fate

Despite being the largest reservoir of organic sulfur in the ocean, the major source and reactivity of non-volatile dissolved organic sulfur (DOS) pool in the water column remain uncertain. Three experiments were conducted to fill these gaps. In the first one, sulfur isotopic composition (δ34S) of marine and estuarine organic matter, extracted with various methods, indicated that biotic assimilatory sulfate reduction (ASR) is the main formation pathway of DOS in marine waters and of particulate organic sulfur (POS) in oxic sediment. The abiotic sulfurization by sulfides and polysulfides (e.g., in sulfidic sediment) appeared a negligeable contributor to surface DOS but can nevertheless account for up to 39% of deep oceanic DOS. In contrast, abiotic sulfurization appears to be the dominant source of POS in anoxic sediments. In the two other experiments, the possible contribution of sedimentary porewater DOS to the overlaying water column and the stability of DOS in oxic water, before and after abiotic sulfurization, were assessed. Results indicated that the main sources of water-column and sedimentary DOS are different. Laboratory abiotic sulfurization decreased the stability of the dissolved organic matter (DOM) in oxic conditions but seems to increase the proportion of carboxyl-rich alicyclic molecules (CRAM). In addition, the S/C ratio of the DOM changed during degradation: DOS was immobilized in S-poor non‑sulfurized DOM, but DOS was selectively degraded in S-rich sulfurized DOM. The high degradation rate of porewater DOS could explain why it is not the main contributor to deepwater DOS. Biotic ASR appears as the principal source of both labile and non-labile DOS in marine waters. The preservation of DOS may not be related to sulfur speciation or origin but may be controlled by alterations targeting other elements and leaving behind more recalcitrant molecules containing sulfur atoms.

A metasedimentary origin for gold deposits in the Dian-Qian-Gui “Golden Triangle” of Southwest China

The Dian-Qian-Gui “Golden Triangle” area of Southwest China hosts a large number of Au deposits. However, the sources of the ore-forming fluids are still controversial, hampering the formulation of a sound genetic model and effective ore prospecting. Here we investigate the sources of ore-forming fluids by an integrated study of in-situ sulfur isotope analyses and available trace element (e.g., Co and Ni) data for the pyrite in the “Golden Triangle” area. We find that the ore stage pyrites are featured by low Co/Ni ratios (mostly < 2), which is distinct from pyrite formed in magmatic-hydrothermal systems but resemble typical sedimentary pyrite. In addition, the significant variation (from −7.5 to +20.0‰) in sulfur isotopes (δ34S) of the ore stage pyrite in these deposits also support a sedimentary origin of the ore-forming fluids. Considering the difference in sulfur isotopic compositions of ore-stage pyrite and the sedimentary host rocks, we suggest the metasedimentary basement rocks (>500 Ma) in the area may have supplied most of the sulfur and gold for the ore-forming fluids during the Triassic orogenesis in the Dian-Qian-Gui “Golden Triangle” area of Southwest China.

Genesis of the Xiaobeigou gold deposit in the Jiapigou gold district, Northeast China: Constraints from fluid inclusions, quartz H–O isotopes, and pyrite in situ sulfur isotope and trace element compositions

The Xiaobeigou gold deposit (>20 t @ 17.36 g/t) is located in the Jiapigou gold district (JGD), Northeast China, representing a large-sized quartz vein-type gold deposit hosted by Neoarchean basement gneiss and is related to Mesozoic intrusions in space and time. Four hydrothermal stages were identified from early to late, namely, (1) barren quartz stage, (2) quartz–pyrite stage, (3) gold–quartz–polymetallic sulfide stage, and (4) quartz–carbonate stage. Three main types of fluid inclusions (FIs) have been identified: water-rich (H2O–NaCl; W-type), mixed aqueous-carbonic (CO2–H2O–NaCl; C-type) and pure carbonic (CO2; PC-type) FIs. The early-stage quartz contains W- and C-type FIs that yield moderate homogenization temperatures (270–379 °C) and moderate-low salinities (4.82–17.87 wt% NaCl equivalent). The main-stage quartz grains contain all three types of FIs that yield medium–low temperature (179–263 °C) and moderate-low salinity (2.82–9.86 wt% NaCl equivalent). Only W-type FIs are recognized in late-stage quartz grains, which have low temperatures (133–183 °C) and low salinities (1.91–7.73 wt% NaCl equivalent). The initial ore-forming fluid of the Xiaobeigou gold deposit was a moderate homogenization temperature and moderate-low salinity CO2–H2O–NaCl ± CH4 ± N2 hydrothermal fluid system. Fluid immiscibility is the main mechanism leading to the precipitation of gold and other metal sulfides. The H–O isotopic values (δ18OH2O = −6.87 to 5.90‰; δD = −102 to −78‰) and composition of the quartz suggest that the initial ore-forming fluids consisted mainly of magmatic water mixed with meteoric water during the late stage of the mineralization process. The S isotope data (δ34S = 3.2 to 4.4‰, avg. 3.6‰) indicate that the ore-forming materials are magmatic sources. EPMA and LA–ICP–MS data show that a large amount of gold mineralization mainly occurs in the main ore-forming stage. The high Co/Ni ratio indicates that the pyrite may be a magmatic–hydrothermal source. Taking into account all available data and regional geological history, we suggest that a magmatic–hydrothermal source for the Xiaobeigou gold deposit in the JGD and the ore formation is related to the subduction of the Paleo-Pacific Plate beneath Eurasia.

Extensive sea-floor oxygenation during the early Mesoproterozoic

The Mesoproterozoic ocean was previously proposed to be characterized by low oxygen concentrations that may have resulted in a stasis in eukaryotic evolution. Recent studies reveal several pluses of oxygenation during this period that may have facilitated eukaryote evolution. However, few quantitative constraints on the global oxygenation events have hindered the comprehensive understanding of interactions between eukaryotes and environments. Here we present new molybdenum isotope (δ98/95Mo, simplified as δ98Mo) data coupled with sulfur and carbon isotope compositions and trace element contents from the carbonates and mudstones of the Gaoyuzhuang Formation (ca. 1.60–1.54 Ga) in two-well preserved sections in the North China Craton. The δ98Mo data from the two sections show similar trends in stratigraphic variation, with a prominent positive excursion in the Gaoyuzhuang Member III and peak values reaching up to 2.03‰ and 1.92‰, respectively. We developed a revised isotope mass balance model with high manganese (Mn) (hydr)oxide burial rate due to higher manganese concentration in the Mesoproterozoic oceans. Model calculation suggests that at least more than half of global seafloor (>54%) was covered with oxygenated seawater during the deposition of the Gaoyuzhuang Member III. Our estimation points to a significant oxygenation event in the early Mesoproterozoic with magnitude and extent much higher than previously thought. Further, a simple mass-balance calculation suggests that the atmospheric oxygen level was at least > 4% PAL (present atmospheric level) and likely up to 30% PAL at ca. 1.57 Ga. This oxygenation event, together with other environmental and/or developmental factors, may have facilitated a short and rapid evolution of eukaryotes in the early Mesoproterozoic.

The Sulfur Isotopic Characteristics of Evaporites in the Yarkand Basin of Xinjiang Province in the Paleocene and Its Paleoenvironmental Evolution

The Yarkand Basin, located in the southwest of the Tarim Basin, is a northeastern part of the eastern Paratethys ocean in the Paleocene, and a significant amount of evaporites, with gypsum, anhydrite, and halite as the main types, were developed in this area. These evaporites record the sedimentary environment at that time. A study was conducted on the sulfur isotopic composition of gypsum in the Paleocene of the Yarkand Basin to explore the origin of the evaporites and interpret the sedimentary environment. The experimentally measured sulfur isotope δ34SCDT values of 187 gypsum samples ranged from 6.69‰ to 25.92‰ with an average value of 18.64‰. The overall trend of the Paleocene gypsum sulfur isotopic curve is consistent with the global seawater sulfur isotopic curve, which shows a decreasing trend. In the early and middle Paleocene, the curve shows four stages of sulfur isotope increase, indicating that the sedimentary environment during that time was mainly influenced by bacterial reduction and a relatively open sedimentary environment, while the late period shows a decreasing trend, suggesting that the late period may be primarily influenced by terrigenous freshwater. In addition, the sulfur isotope value has the characteristics of decreasing from northwest to southeast of the basin, which may indicate that the sedimentary environment of Paleocene evaporites in the Yarkand Basin may also be related to paleotopography and distance from the estuary, resulting in differences in sedimentary environments. The mainly original sulfur isotope values of the Paleocene evaporites in the Yarkand Basin should be in the range of 18‰–20‰, which is a supplement to the Paleocene global paleoseawater and is of great significance for the reconstruction of the marine transgressive–regressive cycle and sulfur isotopic composition of the eastern Paratethys ocean during this period.

Ore-forming processes of the Baisong carbonate-hosted Pb-Zn deposit, SW China: Constraints from in-situ sphalerite trace element and sulfide S-isotopic compositions

The western Hunan-eastern Guizhou (WHEG) Pb-Zn ore belt in the southwestern Yangtze block is a main Pb-Zn province in China. The metallogenic characteristics of this ore belt were widely reported, but the ore-forming process is still poorly understood. The lower Cambrian carbonate-hosted Baisong deposit is a representative Pb-Zn deposit in the southern WHEG belt. Sphalerite trace elements and sulfur isotopic compositions from the Baisong Pb-Zn deposit could reveal the ore-forming fluid source and metallogenic processes. Four types of sphalerites have been distinguished (black colloidal sphalerite (Sp) I, light-colored sector Sp II, red zoned Sp III and brown zoned Sp IV, indicating four sulfide mineralization stages from the early ore phase. Sp I (66-117℃, δ34S: +24.8 to + 27.8‰) has the highest Cd-Ga-Ag-Cu contents among the four sphalerite types, and commonly overgrow with marcasite. This suggests that Sp I was formed from acidic fluids under high precipitation rate. The trace elements and sulfur isotopic compositions of Sp II (80-128℃, δ34S: +26.5 to + 28‰) are similar to Sp I. Sp II was likely formed by a decrease in oversaturation and precipitation rate with fluid pH increase. Sp III (127-164℃, δ34S: +31 to + 32.5‰), commonly overgrow on pyrite (Py I), has higher Mn-Fe-Ge but lower Cd-Cu-Ga-Ag contents and heavier sulfur isotopes than Sp II. Changes in precipitation environment and fluid compositions are likely major factors in forming Sp III (temperature increase, pH increase and more reduced). The forming of Sp III and Py I likely consumed the Fe and Ge in the fluid, depriving those elements from the later-formed Sp IV (97-141℃, δ34S: +25.7 to + 29.8‰). The sulfide δ34S value (+22.5 to + 46.3‰) implies that the S2- at Baisong was sourced from seawater sulfates via thermochemical sulfate reduction (TSR). Sphalerite trace element features reflect that the Baisong is an MVT deposit.

Gold–Sulfide Mineralization in the Manitanyrd Region, Polar Urals, Russia

This article describes the characteristics of gold–sulfide–quartz and gold–sulfide (gold–arsenic) ore occurrences in the Manitanyrd region of the Polar Urals. Ore occurrences are confined to NE-trending shear zones and have the common features of a geological structure. The host rocks are metamorphosed volcanic and volcanic–sedimentary rocks. We analyzed the mineral and chemical composition of the ore mineralization in all studied ore occurrences, showing that they belong to the same mineral type—pyrite–arsenopyrite, with a variable ratio of the main minerals. Arsenic pyrite is present in all ore occurrences. Two stages of ore formation were distinguished: early gold–pyrite–arsenopyrite with finely dispersed gold and late gold–galena–chalcopyrite–sphalerite with coarse gold, fahlore, and sulfosalts Pb, Cu, Bi, Sb. Native gold of the first generation, finely dispersed in arsenopyrite and pyrite, had an average to high fineness (800‰–1000‰) with a relatively low dispersion. Native gold of the second generation was larger, and its fineness in ore occurrences varied; in one of them, it varied from 300‰ to 950‰, while in others, it varied from 800‰ to 950‰. The isotope composition of sulfur in sulfides (δ34S) ranged between −0.2‰ and −8.0‰. δ34S values of sulfides in the range of −0.2‰ to −3.5‰ were similar to meteorite, indicating the participation of a single deep magmatic source of sulfur in the ore formation. According to the study of fluid inclusions, the formation of ore quartz veins occurs in the temperature range of 467–109 °C. The similarity of the geological–structural, mineralogical–geochemical, and isotope–geochemical features of the gold–sulfide–quartz and gold–sulfide occurrences in the area suggest their formation in a single hydrothermal system.

In situ characterization of pyrite from multiple microenvironments in middle Jurassic strata, northeastern Ordos Basin, China: Evaluation of synsedimentary/diagenetic fluid evolution

The sulfur isotopic composition and trace element content recorded in sedimentary pyrite have been widely used as a proxy to establish the geochemistry of pyrite-forming fluids. However, a continuous and complete evolution process of these fluids from the sedimentary period to diagenetic period remains not well constrained. The middle Jurassic strata (the Yan'an Formation containing coal seams and its overlying Zhiluo Formation hosting U deposits in the sandstone) in the northeastern Ordos Basin contain four kinds of microenvironments favorable for pyrite formation, namely coal, carbonaceous debris in barren gray sandstone, barren gray sandstone without carbonaceous debris and U mineralized gray sandstone from bottom to top, providing a good opportunity to decode pyrite formation in response to synsedimentary/diagenetic fluid evolution. In these microenvironments, pyrite named as PC, PD, PB and PM respectively, displays a variety of micro morphologies (framboidal, euhedral, infilling, irregular and cement) formed during sedimentary and diagenetic stages. PC has δ34S values ranging from −36.1 to +26.5 ‰ and low contents of total trace elements (from 6.7 to 58 ppm of Co + Ni + As + Se + Mo). PD and PB present δ34S signatures from −33.6 to +14.4 ‰ and from +9.7 to +22.2 ‰, respectively, with moderate trace element contents (from 21 to 111 ppm and from 23 to 80 ppm, respectively). PM resulting from the oxidation of PD and PB shows negative δ34S values (from −16.9 to −11.1 ‰ for framboidal pyrite and from −56.7 to −34.8 ‰ for euhedral pyrite + pyrite cement) and high trace element contents varying from 350 to 931 ppm. Detailed sulfur isotope and trace element microanalyses indicate that S involved in the formation of pyrite is derived from the fractionation process of organic sulfur in the coal, while Fe and trace elements originate from the surface-derived meteoric water. A paragenetic sequence from early PC, to PD and PB, and finally PM implies an evolution trend of δ34S values increasing first and then decreasing (bacterial sulfate reduction for framboids and Ostwald ripening for euhedral pyrite and pyrite cement of PM) and gradual enrichment of trace elements from synsedimentary to diagenetic fluids. The quantitative results can contribute to a better understanding of synsedimentary/diagenetic fluid evolution both locally and regionally for sedimentary strata in the low-temperature system.

Occurrence and precipitation mechanism of silver in pyrite from chimney fragments in the Edmond hydrothermal field, Central Indian Ridge

The enrichment of precious elements including Au and Ag in submarine hydrothermal sulfide deposits attracts more and more attention. Previous studies indicate that Au and Ag combine distinctly different ligands under the physico-chemical conditions of submarine hydrothermal systems, therefore, the factors controlling their precipitation may be different. In general, silver mineralization has been much less studied than gold mineralization in submarine hydrothermal sulfide deposits. The Ag mineralization process and precipitation mechanism of submarine hydrothermal sulfide deposits formed at Mid-ocean Ridges are still poorly constrained. In this study, we studied the occurrence and precipitation mechanism of Ag in sulfide deposits of Edmond hydrothermal field, located on the intermediate-spreading Central Indian Ridge (CIR).Three ore-forming stages were identified in chimney fragments collected in the Edmond hydrothermal field. The corresponding minerals are (stage I) anhydrite + barite + colloidal/porous pyrite (Py1) + fine-grained sphalerite; (stage II) marcasite; (stage III) euhedral pyrite (Py2) + coarse-grained sphalerite + chalcopyrite + isocubanite. Py1, characterized by colloidal, porous, and micro-sized anhedral morphology, was usually overgrown by marcasite, which is, in turn, surrounded by euhedral-subhedral Py2, usually coexisting with coarse-grained sphalerite, chalcopyrite, and isocubanite. Compared to marcasite and Py2, only Py1 contains abundant native silver nanoparticles. LA-ICP-MS analyses suggest that Py1 contains higher Pb, Cu, Ag, Mn, Tl, Mo, Au, and lower Co as compared to Py2. In-situ LA-ICP-MS and nanoSIMS analyses indicate that Py1 with higher Ag contents (101–586 ppm) has larger variation in sulfur isotopic compositions (0.8 to 6.6‰) than those of Py2 with lower Ag contents (0.22–15.5 ppm; δ34S values: 2.1 to 4.8‰ for Fe-rich samples and 0 to 2.8‰ for Zn-rich samples).Texture, mineral assemblage, pyrite trace element and sulfur isotopic compositions indicate a progressive decrease on the degree of fluid-seawater mixing as the temperature gradually increases during the growth of the chimneys in Edmond hydrothermal field. Phase diagram analysis indicates that the increase in pH and in particular cooling, due to the mixing of the hot hydrothermal fluid with cold seawater, can significantly decrease the solubility of Ag and be the effective mechanisms of silver precipitation. Our results suggest that mixing of hydrothermal fluid with seawater is the main Ag precipitation mechanism for submarine hydrothermal sulfide deposits formed at Mid-ocean Ridges.

Geochemistry and Sr, S, and O stable isotopes of Miocene Abu Dhabi evaporites, United Arab Emirates

This study investigates for the first time the subsurface Miocene evaporite facies (Gachsaran Formation) in Abu Dhabi, United Arab Emirates. Forty-five evaporite rock samples were selected for petrographic, mineralogical, and geochemical investigations and stable isotope analyses to decipher their origin and constrain their age. Secondary gypsum with anhydrite relics dominates the investigated evaporitic rocks, with minor amounts of clays, dolomicrite, Fe/Ti oxides, and celestite. These samples are characterized by their excellent purity and low variability in geochemical composition. The distribution of trace element concentrations is significantly influenced by continental detrital intake. The main focus of the study is to determine the strontium, sulfur, and oxygen stable isotope compositions. The measured 87Sr/86Sr values of 0.708411–0.708739 are consistent with Miocene marine sulfates and indicate ∼21.12–15.91 Ma (Late Aquitanian-Burdigalian). The δ34S and δ18O values are 17.10‰–21.59‰ and 11.89‰–19.16‰, respectively. These values are comparable to those of Tertiary marine evaporites. The relatively low values of δ34S suggest that non-marine water possesses little influence on S distribution. The geochemical composition and Sr, S, and O isotope distributions of the Abu Dhabi gypsum facies from the Gachsaran Formation reveals that their source brines were marine (coastal saline/sabkha) with subordinate continental input.

Sulfide Trace Element Signatures and S- and Pb-Isotope Geochemistry of Porphyry Copper and Epithermal Gold-Base Metal Mineralization in the Elatsite–Chelopech Ore Field (Bulgaria)

The Elatsite–Chelopech ore field in the northern part of the Panagyurishte district in Central Bulgaria comprises numerous spatially associated porphyry copper and epithermal gold deposits and prospects. In addition to the mineralization and alteration features, trace elements, lead and sulfur isotope signatures of sulfide minerals from porphyry copper, base metal and gold-base metal deposits/prospects have been studied. LA-ICP-MS analyses of pyrite, arsenopyrite and sulfosalt minerals validate them as major carriers for Au, Ag, Sb, Se and Co. Pyrite from the three types of mineralization has specific geochemical characteristics. Pyrite from the porphyry copper deposits/prospects has generally lower total trace element content compared to pyrite from the epithermal prospects, except for Se, Co and Ni. Pyrite from the base metal and gold-base metal veins is enriched in As, Au, Ag, Sb and Pb. In pyrite from the base metal deposits, Co and Ni have contents comparable to the pyrite from the porphyry copper deposits, while pyrite from the gold-base metal veins shows lower Co and Ni. Arsenopyrite from these deposits shows similar features. Similarly, sphalerite from the gold-base metal veins also has lower Co content compared to sphalerite from the base metal veins but higher In and Cu contents. In addition to the close spatial relationships between the Elatsite and Gorna Kamenitsa porphyry Cu deposits and Negarstitsa-West and Dolna Kamenitsa base metal prospects, as well as similarities in the mineralization and alteration styles, the lead isotopic (206Pb/204Pb = 18.61–18.68, 207Pb/204Pb = 15.64–15.65 for porphyry and 206Pb/204Pb = 18.55–18.67, 207Pb/204Pb = 15.64–15.68 for base metal) and sulfur isotopic (δ34S values of −3 to +1‰ for porphyry and δ34S values of −1.7 to +3.5‰ for base metal) signatures of sulfides support the idea of a genetic link between these two types of deposits. The porphyry and base-metal mineralization result from a common major ore-forming event during the Late Cretaceous, corresponding to deep/higher-temperature and shallower/distal/lower-temperature environments, respectively. In particular, more radiogenic lead (206Pb/204Pb = 18.41–18.47, 207Pb/204Pb = 15.67–15.76) and slightly different sulfur isotopic compositions (δ34S values of +3.5 to +10.6‰) of sulfides from the distal gold-base metal veins of Kordunsko Dere, Svishti Plaz and Shipkite might be a consequence of the interaction of the ore-forming fluids with an external older crustal and isotopically positive S source. Alternatively, a different fluid source/event for the formation of these gold-base metal veins may be suggested.

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).

Metallogeny of the Xiaotongjiapuzi gold deposit, Liaodong Peninsula (Eastern China): Perspective from sulfide trace element geochemistry and sulfur isotopes

Most gold ores have variable sulfur isotopic and trace elemental compositions at micro-/nano-scale, which makes the understanding of the gold formation process more challenging. The advancement of in-situ analytical techniques offer opportunity to gain insight into gold mineralization processes and ore-material source. The Xiaotongjiapuzi is a representative intrusion-related gold deposit in the Qingchengzi orefield of the Liaodong Peninsula, Eastern China. Alteration and gold mineralization comprise four stages: (I) quartz-pyrite-Au, (II) quartz-pyrite-biotite-Au, (III) Au-barren quartz-pyrite, and (IV) post-ore quartz-calcite. Detailed scanning electron microscope (SEM) backscattered electron (BSE) imaging, electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) elemental and LA–multi-collector (MC)–ICP–MS sulfur isotope analyses were conducted to reveal the ore-material source and mineralization process. Three texturally distinct auriferous pyrite generations were identified: Py1 and Py2 (in both stage I and II) and Py3 (in stage II and III). Py1 is porous and weakly fragmented, poorly crystallized, and overgrown by Py2. Py2 is commonly intergrown with arsenopyrite and marcasite, whilst Py3 overgrew around Py2 or replace it.Gold exists mainly as solid solution (Au+) in the pyrite lattice and nanoparticles (Au0), as well as (rare) micron-scale inclusions along the Py2-Py3 interface. Py2 has the highest gold content (mean 47 ppm, max 344 ppm), especially when intergrown with marcasite (Mrc1). The δ34S values of pyrite (5.71–15.31‰, n = 18) and marcasite (-19.59 to 7.45‰. n = 6) are the highest and lowest, respectively, and the values increase steadily from Py1 to Py3 (Py1: 5.71–7.54‰ → Py2: 7.44–9.55‰ → Py2 + Mrc1: 12.88–13.92‰ → Py3: 15.17–15.31‰). The Paleoproterozoic Dashiqiao and Gaixian formation host rocks contain large amount of sulfate minerals (e.g., barite and gypsum), and the coeval global sulfate has heavy sulfur isotope enrichment, suggesting that sulfate reduction may have provided H2S-rich fluids [Au(HS)2- and/or Au(HS)0] for the Mesozoic mineralization. For the multistage gold enrichment at Xiaotongjiapuzi, we suggest that the Paleoproterozoic sedimentary and metamorphic rocks may have formed the initial sulfur reservoir, which was then reactivated by Mesozoic magmatic-hydrothermal events.

Evolution, Magmatic Source and Metallogenesis of A-Type Granites in the Fanchang Volcanic Basin, Middle and Lower Yangtze Metallogenic Belt: A Review

The Fanchang volcanic basin (FVB) is located in the Middle and Lower Yangtze Metallogenic Belt (MLYMB) between the ore districts of Ningwu and Tongling. The existing ore deposits in the FVB are relatively small in scale and related to late Mesozoic A-type granites. In this paper, the crystallization age, major and trace element composition, and Sr-Nd and Hf isotope compositions of the A-type granites are summarized from the literature; in addition, the magnetite composition, H and O isotopes of fluid inclusions, and sulfur isotope composition of metal sulfides in some typical ore deposits in the FVB are also summarized to give insights into the petrogenesis and mineralization of the A-type granites intruding into the FVB. The results show that: (1) Orthopyroxene, plagioclase, K-feldspar, and biotite are the main fractionating minerals controlling the evolution of the magmas of A-type granites in the FVB and other areas in the MLYMB. (2) The whole-rock Sr-Nd and zircon Hf isotopic characteristics show that the source of A-type granite magma is complex and includes the enriched mantle, lower crust, and upper crust, probably with stronger participation of Archaean–Paleoproterozoic crustal materials in the FVB granites than in other regions of the MLYMB. (3) The ores in the FVB are dominated by skarn and hydrothermal deposits. H and O isotopes of fluid inclusions indicate that ore-forming fluids have been derived from mixtures of magmatic hydrothermal fluid, meteoric waters, and deep brine related to gypsum layers. S isotopes of metal sulfides indicate that the sulfur may be a mixture of magmatically derived sulfur and sulfur originating from the Triassic gypsum-bearing layers. The deposit and ore characteristics of the main deposits in the FVB are also illustrated, and the evaluation of metal resources indicates that the skarn and hydrothermal iron–zinc ores in the FVB also have potential as sources of Cd, Ga, and Se. In addition, in terms of the oxygen fugacity, rock type, and geochemical characteristics of magmatic rocks, the metallogenic characteristics and potential of the A-type granites in the FVB are evaluated. It is considered that in addition to the dominant constituents of iron and zinc and the minor constituents listed above, the FVB could have the potential for providing copper, gold, molybdenum, uranium, and other metals as well.