Isotopic Composition Of Pyrite Research Articles

Multiple sulfur isotope systematics of pyrite for tracing sulfate-driven anaerobic oxidation of methane

Sulfate-driven anaerobic oxidation of methane (SD-AOM) is ubiquitous in marine sedimentary environments, governing the global methane budget and the redox evolution of Earth's surface. Tracing SD-AOM and organoclastic sulfate reduction (OSR) from the pyrite archive is key to the reconstruction of SD-AOM activity and paleoenvironmental interpretation. However, discriminating the origins of pyrite – basically SD-AOM and OSR – is commonly challenging due to frequent overlap of δ34Spy values. Multiple sulfur isotopes of pyrite are expected to be an effective tool to distinguish between OSR and SD-AOM, yet variable uncertainties and unknowns remain. Here we investigated the δ34S and Δ33S values of pore-water sulfate and authigenic pyrite from a piston core taken on the continental slope of the South China Sea. A positive Δ33S - δ34S correlation of pore-water sulfate is observed in the upper OSR-dominated zone, resulting in Δ33S and δ34S values of sulfate diffusing into the sulfate-methane transition zone (SMTZ) >0.1‰ and >30‰ larger than the corresponding values of seawater sulfate. A negative Δ33S - δ34S trajectory of pore-water sulfate and pyrite is observed for the SMTZ, agreeing with low sulfur isotope fractionation characteristic for SD-AOM and a diagnostic large Δ33S - δ34S field of SD-AOM-derived pyrite. These findings elucidate that the multiple sulfur isotope systematics of pyrite in methane-bearing sediment depends on (1) mass transport effects of dissolved sulfate and sulfide, (2) the relative contribution of OSR and SD-AOM to the pore-water sulfide and pyrite pools, and (3) the sulfur isotope fractionation during microbial sulfate reduction. Our study highlights the importance of mass transport dynamics on the isotopic composition of pyrite, a factor that needs to be considered in any attempt to reconstruct the origin of early diagenetic pyrite and the paleoenvironmental setting with multiple sulfur isotopes.

Trace element and S-Pb isotopic compositions of pyrite from the Precambrian metamorphic rocks and their derivative pegmatites in the Xiaoqinling district, southern North China Craton: Implications for possible gold source of the Early Cretaceous gold deposits

The North China Craton is well endowed with numerous Early Cretaceous gold deposits that are mainly hosted in Archean to early Paleoproterozoic metamorphic rocks and late Mesozoic granitoid intrusions intruding the former. Whether or not gold was sourced from those metamorphic rocks remains hotly debated. Here we present trace element and S-Pb isotope data of pyrite from the Neoarchean to early Paleoproterozoic metamorphic rocks in the Xiaoqinling district of southern North China Craton and late Paleoproterozoic pegmatites derived from the former rocks, in an attempt to provide new insights into the gold sources for Early Cretaceous gold lodes genesis in this district. Pyrite from the metamorphic rocks typically occurs as inclusions in rock-forming minerals or disseminated grains roughly distributed along the foliation of the rocks, whereas pyrite from the pegmatites usually occurs as disseminations closely associated with quartz and magnetite. The textural data suggest that pyrite in the metamorphic rocks and pegmatites are of metamorphogenic and magmatogenic origins, respectively. LA-ICP-MS spot analyses show that pyrite both from the metamorphic rocks and pegmatites has Au contents mostly below the detection limit, with minor analyses revealing Au marginally higher than the detection limit. Considering that Au is moderately incompatible and highly siderophile, the very low gold contents in pyrite allow us to infer that the metamorphic rocks contain negligible gold. Pyrite grains from the metamorphic rocks and pegmatites have broadly consistent δ34S values ranging from −2.7 to 31.5 (mean = 17.6) per mil and 6.3 to 17.9 (mean = 14.3) per mil, respectively. Similarly, they have comparable, highly radiogenic lead isotopes, which are 16.859–24.889 for 206Pb/204Pb, 15.355–15.694 for 207Pb/204Pb, and 37.480–43.848 for 208Pb/204Pb. The sulfur and lead isotopes of pyrites from the metamorphic rocks and pegmatites are distinct from those of Early Cretaceous gold deposits hosted by these rocks. Taken together, the pyrite trace element and S-Pb isotope data presented here preclude the Precambrian metamorphic basement rocks as an important gold source for the extensive Early Cretaceous gold mineralization in the Xiaoqinling district and, by inference, other major Early Cretaceous gold provinces across the North China Craton.

Deep ore‑forming fluid characteristics of the Jiaodong gold province: Evidence from the Qianchen gold deposit in the Jiaojia gold belt

The Jiaodong gold province hosts world-class gold deposits with giant resources of >5000 t, where the Jiaojia-Xincheng fault belt and the Sanshandao-Cangshang fault belt occupy more than two-thirds of the gold deposits. Recent developments of exploration have advanced in tracking deep mineralization. The nature of the ore-forming fluid, source, and the extreme enrichment mechanism of the gold in this region remain debated. Here, we investigate the deep ore body of the Qianchen gold deposit at the southern part of the Jiaojia gold belt based on mineralogy, fluid inclusion, elemental and isotopic composition of pyrite, and the primary halo geochemistry. Natural gold is the important form of Au in this deposit with a relatively large (up to 200 μm) grain size, suggesting the prospect of mineralization at deeper domains. The accompanying PGE minerals (Hexaferrum) hint at contributions from mantle material. The ore-forming fluids of the Qianchen gold deposit are characterized by medium–low temperatures and salinities and have a general H2O–NaCl–CO2 ± CH4 composition, which indicates that the Jiaodong gold province has a unified ore-forming fluid system from shallow to deep. However, the trace elements and S isotopes of pyrite at different elevations show slight differences, suggesting the composition and properties of the fluid changed during the process of ascending, affected by ore-forming conditions such as temperature, pressure, and oxygen fugacity. Furthermore, the evidence of a decrease of salinity with depth and temperature and fluid immiscibility indicates that fluids from different sources continuously mixed during the upward migration of fluid, with the mineralization triggered possibly by phase separation. The primary halo model suggests promising prospecting potential. We formulate a comprehensive metallogenic model for the mineralization process in the Jiaodong gold province. The Au-bearing hydrous magma derived from the partial melting of the metasomatized SCLM, underwent further magmatic-hydrothermal differentiation and fluid exsolution, and thus the Au was enriched and partitioned into exsolved fluids that evolved to auriferous fluids. During the upward migration of ore-forming fluids (magmatic-hydrothermal), many factors affected mineralization, mainly including fluid-rock interaction and mixing of the metamorphic-hydrothermal and meteoric fluids. The fluid activities, multiple sources of Au in the mantle and ancient basement, together with the large fault structures resulted in the formation of the giant Jiaodong gold province.

The Passa Três Granite Intrusion-Related/Hosted Neoproterozoic Gold Deposit (Paraná State, Brazil): Mineralogical, Geochemical, Fluid Inclusion and Sulphur Isotope Constraints

The Passa Três granite is a 5 km2 intrusion in southern Brazil with an NNE–SSW-elongated shape, hosting gold-bearing quartz veins with fluorite, carbonates, sulphides (pyrite, chalcopyrite, aikinite, molybdenite) and native gold. Orebodies are hosted by the pluton roof zone, which is marked by various textures indicating magmatic–hydrothermal transition processes. Mineralisation formed between 613 and 608 Ma in extensional pull-apart structures controlled by two conjugated N–S and E–W fault systems. We report results from petrography, quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), scanning electron microscopy (SEM), electron probe microanalyses (EPMA), X-ray fluorescence (XRF), trace element analyses of pyrite by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), sulphur isotope (δ34S) analyses, and fluid inclusion microthermometry. Hydrothermal alteration is dominantly developed as phengite–quartz–carbonate and sericite–carbonate–chlorite assemblages along mineralised structures. Fluid inclusion study indicates mineralising fluids with H2O-CO2-NaCl composition, low to moderate salinity (0.2–12.84 wt % NaCl eq.), and temperatures from 400 to 150 °C. The sulphur isotopic composition of pyrite (−0.1 to 1.1‰) suggests magmatic origin. These data, in conjunction with structurally controlled mineralisation enriched in Au-Bi minerals shortly post-dating the granite emplacement, appoint towards similarities between the Passa Três deposit and intrusion-related gold systems. The specific location of the mineralisation in the core (and in the roof zone, regarding its vertical position) of the Passa Três granitic intrusion defines it as “granite-hosted” thus, it is representative of a specific model that can be used for exploration of other intrusion-related/hosted gold deposits near the studied area or in other locations.

Texture, in-situ geochemical, and S isotopic analyses of pyrite and arsenopyrite from the Longshan Sb-Au deposit, southern China: Implications for the genesis of intrusion-related Sb-Au deposit

Longshan is an important Sb-Au ore deposit (3.7 Mt @4.5 wt% Sb and 4.6 g/t Au) in the Xiangzhong metallogenic province (XZMP), southern China. The Sb-Au mineralization is characterized by vein ores comprising quartz, stibnite, native gold, auriferous pyrite, and arsenopyrite within the Neoproterozoic Jiangkou Formation slate. The paragenesis of the Longshan deposit can be divided into three stages, i.e., (1) stage 1 (pre-ore) composed of euhedral coarse to nodular sedimentary pyrite and deformed irregular quartz veins, (2) stage 2 marked by NE-striking and NW-dipping quartz, stibnite, pyrite, arsenopyrite, and minor native gold veins, and (3) stage 3 characterized by NW-striking and NE-dipping quartz, stibnite, pyrite, arsenopyrite, scheelite, and minor native gold veins. Seven subtypes of pyrite and four subtypes of arsenopyrite from the three stages are recognized based on mineral assemblages, textures, and geochemistry. Visible gold shows the complexity with three distinct categories, including Au1 from stage 2, Au2 and Au3 from stage 3. In this study, we integrated laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) analyses to document textural, compositional, and S isotopic variation among texturally complex pyrite and arsenopyrite assemblages, and constrain the genesis of the Longshan Sb-Au deposit.Py1a and Py1b of sedimentary origin from stage 1 are enriched in δ34S (from 13.3 to 16.9‰, average 15.3‰, n = 8) and have low Au concentrations (from 0.43 to 4.10 ppm, n = 37). Two subtypes of pyrite (Py2a–Py2b) and arsenopyrite (Apy1a–Apy1b) from stage 2 are characterized by oscillatory zoning textures in backscattered electron (BSE) images and are obvious by their relatively high content of invisible Au (up to 351 and 1357 ppm for pyrite and arsenopyrite, respectively) and restricted range of δ34S values (from 2.1 to 4.7‰, average 3.7‰, n = 19). Three subtypes of pyrite (Py3a–Py3c) and two subtypes of arsenopyrite (Apy2a–Apy2b) from stage 3 have relatively low Au concentrations (up to 95.1 and 64.9 ppm for pyrite and arsenopyrite, respectively), and show a narrow variation in δ34S values (from –2.5 to 3.7‰, average 0.8‰, n = 22). These data suggest that magmatic-hydrothermal ore-forming fluids contribute to most of the Au and S budget in the Longshan Sb-Au deposit.The major differences between mineralization in terms of texture, trace elements, and S isotopic compositions of pyrite and arsenopyrite can reflect contrasting mechanisms of Au precipitation and provide implications for the genesis of this intrusion-related Sb-Au deposit. The new data underpin the importance of performing complementary in-situ mineralogical analyses to elucidate the source and evolution of ore-forming fluids and enable correct interpretation of the hydrothermal Sb-Au system architecture.

Spatial distribution and speciation of sulfur in Ediacaran limestones with μ-XRF imaging and XANES spectroscopy: Implications for diagenetic mobilization of sulfur species

The Ediacaran period was characterized by numerous events, including the emergence of large multi-cellular metazoans and surface environmental perturbations. This period was also characterized by an increase in atmospheric oxygen concentrations that was critical for the evolution of life. Oceanic sulfate concentrations varied in association with atmospheric oxygen concentrations, which have been constrained by sulfur isotopic compositions of sedimentary sulfates and pyrite, and sulfur concentrations of pyrite and carbonate associated sulfate (CAS). However, other parameters such as sedimentation rate or iron availability have a strong impact on the abundance and isotopic composition of pyrite. In addition, recent studies have demonstrated that some Phanerozoic limestones include a mix of various carbonate minerals, the compositions and diagenetic histories of which differ at the μm scale. Thus, the μm scale description of sulfur species is necessary to accurately extract information preserved in carbonate rocks. In this study, we investigated the speciation and concentrations of sulfur in the matrix of the Ediacaran Doushantuo and Dengying limestones exposed in South China using μ-X-ray fluorescence (XRF) mapping and S K-edge X-ray absorption near edge structure (XANES) analyses. In addition to pyrite, the XANES spectra of the Doushantuo limestone indicate that sulfur occurs as CAS, while the Dengying limestone contains CAS and abundant organic sulfur. Pyrite oxidation and re-mineralization of organic sulfur had little influence on CAS content in the samples, whereas sparitization produced decreases in CAS and organic sulfur concentrations. The CAS content of the Dengying sparite was lower than that of the Dengying micrite, indicating that the CAS content decreased even during marine diagenesis. Thus, the micrite is more appropriate for extracting paleo-oceanic information. On the other hand, variations in the CAS concentrations of the limestone matrix in the Dengying Formation were larger than those in the Doushantuo Formation, regardless of grain size. The large variations in the Dengying limestone resulted from local alkalinity fluctuations caused by temporal changes in microbial activity within microbial mats. Existence of abundant organic sulfur in the Dengying limestones has another implication to ancient sedimentary environment. The low pyrite content of the Dengying limestone is likely due to a deficient supply of reactive iron to the sediment–water interface, because the supply of organic matter was likely sufficient.

Disseminated Gold–Sulfide Mineralization in Metasomatites of the Khangalas Deposit, Yana–Kolyma Metallogenic Belt (Northeast Russia): Analysis of the Texture, Geochemistry, and S Isotopic Composition of Pyrite and Arsenopyrite

At the orogenic gold deposits of the Yana–Kolyma metallogenic belt (northeast Russia) both Au–quartz-sulfide mineralization with native gold and disseminated sulfide mineralization with invisible Au developed. The textural and mineralogical-geochemical features, isotope-geochemical characteristics of gold-bearing sulfides from proximal metasomatites, and possible forms of Au occurrence in pyrite and arsenopyrite have been studied using electron microprobe, atomic absorption, LA-ICP-MS trace element, isotope analysis, and computed microtomography. Four generations of pyrite (Py1, diagenetic; Py2, metamorphic; Py3, metasomatic; Py4, veined) and two generations of arsenopyrite (Apy1, metasomatic; Apy2, veined) have been identified at the Khangalas deposit. In the proximal metasomatites, the most common are Py3 and Apy1. Studying their chemical composition makes it possible to identify the features of the distribution patterns of typochemical trace elements in pyrite and arsenopyrite, and to establish the nature of the relationship between Au and these elements. In Py3 and Apy1, structurally bound (solid solution) Au+ prevails, isomorphically entering the crystal lattice or its defects. Isotope characteristics of hydrothermal sulfides (δ34S = −2.0 to −0.6‰) indicate that mantle/magmatic sulfur was involved in the formation of the deposit, though the participation of sulfur from the host rocks of the Verkhoyansk clastic complex cannot be ruled out. The Khangalas deposit has much in common with other gold deposits of the Yana–Kolyma metallogenic belt, and from this point of view, the results obtained will help to better reveal their gold potential and understand their origin.

Spatially and temporally variable sulfur cycling in shallow-sea hydrothermal vents, Milos, Greece

Shallow-sea hydrothermal systems are ideal for studying the relative contributions to sedimentary sulfur archives from ambient sulfur-utilizing microbes and from fluxes of hydrothermally derived sulfur. Here we present data from a vent field in Palaeochori Bay, Milos, Greece using a suite of biogeochemical analytical tools that captured both spatial and temporal variability in biotic and abiotic sulfur cycling. Samples were collected along a transect from a seagrass meadow to an area of active venting. The abundance and isotopic composition of sulfide captured in situ, together with geochemistry from sedimentary porewaters and the overlying water column and solid phase sulfide minerals, record evidence of ephemeral activity of microbial sulfate reduction as well as sulfide oxidation. The sulfur and oxygen isotope composition of porewater sulfates indicate active sulfate reduction within the transition zone between the vents and seagrass, rapid recycling of biologically produced sulfide within non-vent sediments, and reoxidation of abiotic sulfide within the vent field. A phylogenetic survey of sediments also indicates the pervasive presence of a suite of putative sulfur-metabolizing bacteria, including sulfate reducers and sulfide oxidizers, many of which can utilize intermediate valence sulfur compounds. The isotopic composition of pyrite in these sediments consistently records a microbially influenced signature (δ34Spy of −4.4 to −10.8‰) relative to the hydrothermal endmember (δ34S ~ + 2.5‰), independent of distance from the vent source. The narrow range of pyrite δ34S across sediments with a highly variable hydrothermal influence suggests that physical mixing (e.g., by storm events) homogenizes the distribution of biogenic and hydrothermal Fe-sulfides throughout the region, overprinting the spatially and temporally variable interplay between biological and hydrothermal sulfur cycling in these environments.

Genesis of the Permian karstic Pingguo bauxite deposit, western Guangxi, China

More than 0.5 billion tons of late Permian bauxite overlies the karstic topography of the Maokou Formation of western Guangxi in China. Here, we provide new mineralogical, geochemical, Sr–Nd–Pb isotopic, and pyrite S isotope and trace element compositional data for the Pingguo bauxite deposit, aiming to further our understanding of the genesis of Permian bauxite. The Pingguo bauxite contains three distinct layers: a lower layer dominated by ferric clay or weathered iron ore, a middle layer of cryptocrystalline and oolitic bauxite ore, and an upper layer dominated by argillaceous bauxite. The bauxite ore is mainly diaspore, pyrite, chamosite, and anatase, whereas the argillaceous bauxite contains diaspore, kaolinite, pyrophyllite, pyrite, and anatase. Two types of pyrite have been identified within the bauxite: fine-grained and framboidal pyrite (Py1) occurring in aggregates and coarse-grained and euhedral pyrite (Py2). Py1 is enriched in trace elements and is thought to have a diagenetic origin, whereas Py2 is deficient in trace elements and is considered to have formed by later recrystallization. The S isotopic composition of pyrite (−34.11 to −18.91‰) and visible ovoid microorganisms within the bauxite provide evidences of microbial activity during bauxite formation. The Sr–Nd–Pb isotopic composition of the bauxite indicates that these ores were generated by the weathering of basalts belonging to the Emeishan Large Igneous Province (LIP) and limestones of the Maokou Formation. Microorganisms were likely to have enhanced the dissolution and weathering of the parent rock and facilitated the precipitation of diaspore under near-surface conditions.

Constraints on the Genesis of the Archean Oxidized, Intrusion-Related Canadian Malartic Gold Deposit, Quebec, Canada

With a current resource of 13.4 Moz Au, plus past production of 5.1 Moz Au, the Canadian Malartic deposit represents the first bulk tonnage (measured and indicated resources of 372.9 Mt at 1.02 g/t Au) mine in the Superior province. Canadian Malartic is thus an important example of a large-tonnage, low-grade Archean gold deposit in which the mineralization is disseminated (or in fine veinlets) and hosted partly by felsic to intermediate intrusions. The deposit is located in the Abitibi greenstone belt, Quebec, within and immediately south of the Cadillac-Larder Lake tectonic zone, and occurs in porphyritic monzodiorite intrusions as well as clastic metasedimentary rocks of the Pontiac Group and mafic-ultramafic volcanics of the Piche Group. These rocks have undergone pervasive potassic alteration, carbonatization, pyritization, and local silicification. The main ore minerals are native gold and subordinate gold tellurides, accompanied by pyrite and minor chalcopyrite, galena, sphalerite, hematite, molybdenite, and Ag-Pb-Bi-bearing tellurides. Gold is concentrated in two generations of thin, discontinuous veins, and as finely disseminated grains in alteration envelopes around the main ore-stage veinlets. The main-stage veinlets consist of quartz-carbonate ± biotite ± albite surrounded by alteration haloes of K-feldspar-biotite ± pyrite ± calcite, whereas later veins are dominated by quartz-pyrite-calcite ± muscovite ± biotite ± chlorite. Alteration and gold mineralization were accompanied by large mass gains in K and S, extremely large mass gains in Ag, Te, and Au, and significant mass gains in Sb, W, Bi, and Pb; Cu underwent significant mass loss. The oxygen and hydrogen isotope composition of the mineralizing fluid determined from the corresponding compositions of quartz, biotite, and hematite ( δ 18Ofluid of 5.2 to 9.8‰, δ Dfluid of −52.0 to −45.0‰) is consistent with a predominantly magmatic source. A magmatic fluid source is supported by the composition of fluid inclusion leachates from quartz. Based on the isotopic composition of pyrite, sulfur was dominantly of magmatic origin but included a small contribution from a sedimentary source ( ![Formula][1] from −4.5 to +3.3‰, with small, positive Δ33S values). The deposit is interpreted to have formed at a temperature of ~475°C, based on oxygen and sulfur isotope geothermometry. Using this temperature in conjunction with the titanium-in-quartz geothermobarometer, the deposit is interpreted to have been emplaced at a pressure of ~3 kbar or a depth of ~10 km. Consistent with the constraints noted above, we propose a genetic model for the Canadian Malartic deposit in which felsic to intermediate, borderline alkaline to subalkaline magmas, emplaced at midcrustal levels, exsolved relatively oxidized ![Formula][2] , CO2- and sulfur-rich (∑ a S ≥ 0.1) auriferous fluids. These fluids rose to higher levels (~10 km; ~3 kbars), where they interacted with associated porphyritic monzodiorite intrusions, clastic Pontiac metasedimentary and Piche Group mafic to ultramafic rocks. The porphyries and metasedimentary rocks buffered the fluids to near-neutral pH, whereas the mafic-ultramafic rocks buffered the fluids to higher pH and lower ![Formula][3] . Ore deposition resulted from pyritization of the host rocks and oxidation of the mineralizing fluid, which reduced ![Formula][4] and caused destabilization of gold bisulfide species, leading to precipitation of native gold. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif [3]: /embed/mml-math-3.gif [4]: /embed/mml-math-4.gif

Origin, mobility, and temporal evolution of arsenic from a low-contamination catchment in Alpine crystalline rocks

The reduction to 10μg/l of the limit for arsenic in drinking water led many resource managers to deal with expensive treatments. In the very common case of arsenic levels close to the recommended maximum concentration, knowing the origin and temporal evolution of As has become of great importance. Here we present a case study from an alpine basin. Arsenic speciation, isotopic compositions of pyrite, sulfate and water, and concentrations of major and trace elements demonstrate a geogenic source for arsenic linked to the dissolution of pyrite. We provide new tools to further study As at low concentrations where many processes may be masked. The observed negative correlation between δ34SSO4 and [As] is interpreted as a Rayleigh-type sulfur-isotope fractionation during increasing pyrite dissolution. The observed positive correlation between δ18OSO4 and AsV/AsIII could help to retrieve initial redox conditions. A 3-year long monitoring at high-resolution demonstrated that drought conditions enhance pyrite dissolution whose degradation products are scavenged by recharge water. An increase in As in groundwater may result from droughts due to enhanced oxygen entry in the unsaturated zone. The 2003 European heatwave had a major effect.

Major oceanic redox condition change correlated with the rebound of marine animal diversity during the Late Ordovician

AbstractTo understand probable influences of environmental changes on the decrease and rebound of biodiversity during the Late Ordovician, a detailed analysis of total organic carbon (TOC), pyrite, V/(V + Ni) ratios, and S isotopic compositions of pyrite in the Late Ordovician sediments from Poland was performed. The significant enrichments of TOC and pyrite as well as high V/(V + Ni) ratios (>0.6) for the Jeleniów Formation of the latest Darriwilian to middle Katian Stages provide evidence for predominant anoxic bottom-water conditions. In contrast, low TOC and pyrite as well as low V/(V + Ni) ratios for the Wólka Formation of late Katian age indicate oxic depositional conditions. The S isotopic data are consistent with the interpretations of oceanic redox conditions based on multiple proxies for paleoredox. The results from the Baltica area reveal a major oceanic redox condition change during the late Katian and suggest that oceanic oxygenation may have contributed to the rebound of marine animal diversity during the Late Ordovician.

Relationships between development of organic-rich shallow shelf facies and variation in isotopic composition of pyrite (Middle Triassic, Spitsbergen)

Relationships between development of organic-rich shallow shelf facies and variation in isotopic composition of pyrite (Middle Triassic, Spitsbergen) 51 samples from the Middle Triassic black shales (organic carbon-rich silt-stones; up to 4.9% TOC - Total Organic Carbon) from the stratotype section of the Bravaisberget Formation (western Spitsbergen) were analyzed with respect to isotopic composition of pyritic sulphur (Δ34S) and TOC. Isotopic composition of syngenetic pyrite-bound sulphur shows wide (Δ34S from -26‰ to +8‰ VCDT) and narrow (Δ34S from -4‰ to +17‰ VCDT) variation of the Δ34S in upper and lower part of the section, respectively. Range of the variation is associated with abrupt changes in dominant lithology. Wide Δ34S variation is found in lithological intervals characterized by alternation of black shales and phosphorite-bearing sandstones. The narrow Δ34S variation is associated with the lithological interval dominated by black shales only. Wide and narrow variation of the Δ34S values suggests interplay of various factors in sedimentary environment. These factors include oxygen concentration, clastic sedimentation rate, bottom currents and burrowing activity. Biological productivity and rate of dissimilatory sulphate reduction had important impact on the Δ34S variation as well. Wide variation of the Δ34S values in the studied section resulted from high biological productivity and high rate of dissimilatory sulphate reduction. Variable degree of clastic sedimentation rate and burrowing activity as well as the activity of poorly oxygenated bottom currents could also cause a co-occurrence of isotopically light and heavy pyrite in differentiated diagenetic micro-environments. Occurrence of organic matter depleted in hydrogen could also result in a wide variation of the Δ34S values. Narrow variation of the Δ34S values was due to a decrease of biological productivity and low rate of dissimilatory sulphate reduction. Low organic matter supply, low oxygen concentration and bottom currents and burrowing activity were also responsible for narrow variation of the Δ34S. The narrow range of the Δ34S variation was also due to occurrence of hydrogen-rich organic matter. In the studied section the major change in range of the Δ34S variation from wide to narrow appears to be abrupt and clearly associated with change in lithology. The change of lithology and isotopic values may suggest evolution of the sedimentary environment from high- to low-energy and also facies succession from shallow to deeper shelf. The evolution should be linked with the Late Anisian regional transgressive pulse in the Boreal Ocean.

Stable isotope analysis of the Cretaceous sulfur cycle

We report the sulfur and oxygen isotope composition of sulfate (δ 34S SO4 and δ 18O SO4, respectively) in coexisting barite and carbonate-associated sulfate (CAS), which we use to explore temporal variability in the marine sulfur cycle through the middle Cretaceous. The δ 34S SO4 of marine barite tracks previously reported sulfur isotope data from the tropical Pacific. The δ 18O SO4 of marine barite exhibits more rapid and larger isotopic excursions than the δ 34S SO4 of marine barite; these excursions temporally coincide with Ocean Anoxic Events (OAEs). Neither the δ 34S SO4 nor the δ 18O SO4 measured in marine barite resembles the δ 34S SO4 or the δ 18O SO4 measured in coexisting CAS. Culling our data set for elemental parameters suggestive of carbonate recrystallization (low [Sr] and high Mn/Sr) improves our record of δ 18O SO4 in CAS in the Cretaceous. This suggests that the CAS proxy can be impacted by carbonate recrystallization in some marine sediments. A box model is used to explore the response of the δ 34S SO4 and δ 18O SO4 to different perturbations in the marine biogeochemical sulfur cycle. We conclude that the δ 34S SO4 in the middle Cretaceous is likely responding to a change in the isotopic composition of pyrite being buried, coupled possibly with a change in riverine input. On the other hand, the δ 18O SO4 is likely responding to rapid changes in the reoxidation pathway of sulfide, which we suggest may be due to anoxic versus euxinic conditions during different OAEs.

Fluid evolution and large-scale gold metallogeny during Mesozoic tectonic transition in the Jiaodong Peninsula, eastern China

AbstractThe Jiaodong Peninsula or eastern Shandong Province, the most important gold producing in region China, is located in the southeastern margin of the North China Craton. The gold deposits in the Jiaodong Peninsula are divided into three gold belts, from west to east, the Zhaoyuan–Laizhou, Penglai–Qixia and Muping–Rushan belts. The deposits occur as gold-bearing quartz veins and disseminated- and stockwork-style ores adjacent to fault zones. Most of the gold deposits can be classified in four stages: stage I, quartz–(minor) pyrite; stage II, pyrite–quartz–gold; stage III, quartz–base metal sulphide minerals; stage IV, quartz–carbonate. Ar–Ar ages, Rb–Sr isochrons, and hydrothermal zircon sensitive high-resolution ion microprobe U–Pb ages obtained from these deposits suggest a gold mineralization time of 120±10 Ma. The Sr–Nd isotopic compositions of pyrites and the associated rocks suggest that the ore-forming materials were probably derived from a mixed source. Fluid inclusion studies show that ore-forming fluids of gold deposits are consistent throughout the Jiaodong Peninsula, with similar mineralizing temperature and pressure conditions. Ore-forming fluids are characterized by H2O–CO2–NaCl±CH4. The optimal mineralizing temperature and pressure ranges are 170–335 °C and 0.7–2.5 kbar. Oxygen and hydrogen isotope data show that ore fluids are of magmatic origin. Gold deposits in the Jiaodong Peninsula formed in the same mineralizing–geodynamic conditions, and are related to the Mesozoic tectonic transition in the eastern North China Craton. Gold metallogeny is only one expression of the Mesozoic tectonic transition.

The Sulfur-Isotopic Composition of Cenozoic Seawater Sulfate: Implications for Pyrite Burial and Atmospheric Oxygen

The availability of high-temporal-resolution C- and S-isotope curves for the Cenozoic permit for the first time modeling of the influence of the C and S cycles on the partial pressure of atmospheric O2 on comparable time scales. A simple isotope mass-balance model was used to calculate atmospheric O2 levels from the burial rates of organic C and pyrite S. Burial rates were derived from the C- and S-isotope records of seawater-dissolved inorganic C and sulfate. Results indicate that in the early Cenozoic atmospheric O2 levels were about 16% higher than current levels. Extension of the model to Phanerozoic time scales yields atmospheric O2 levels that are inconsistent with geological evidence that suggests that the mass of atmospheric O2 has not changed by more than a factor of two from the present atmospheric level since the Cambrian (Berkner and Marshall, 1974; Watson et al., 1978; Jones and Chaloner, 1991). These results indicate that either our knowledge of the parameters controlling atmospheric O2 is incomplete, or that the assumptions used in such models inadequately represent the complexity of the natural systems. Here we critically examine the assumptions inherent in isotope mass-balance models to determine whether they may be the source of the model-data discord. A major problem with these models is the extreme sensitivity of the mass of atmospheric O2 to very small changes in the much larger masses of oxidized and reduced C and S reservoirs. For example, small variations in continental weathering fluxes and the associated isotope ratios of river input have profound effects on calculated O2 levels and need to be accounted for. Similarly, variations in the isotopic composition of pyrite and organic C buried in sediments, which are strongly influenced by changes in isotopic fractionation, dramatically influence calculated O2 concentrations. Thus, constant fractionation factors should not be applied in such models. In addition, the assumption that the isotopic composition of dissolved inorganic C is controlled only by the relative amounts of reduced and oxidized C buried in sediments and their respective isotope ratios is questionable when relatively short time scales are considered. Isotope mass-balance models do not adequately encompass and simulate the actual processes being modeled because of the simplifications and assumptions made. More “realistic” models are required to achieve stabilization of atmospheric O2 over geological time.

Organic matter and trace element rich sapropels and black shales: a geochemical comparison

A distinct Pliocene eastern Mediterranean sapropel (i-282), recovered from three Ocean Drilling Program (ODP) Leg 160 Sites, has been investigated for its organic and inorganic composition. This sapropel is characterized by high organic carbon (C org) and trace element contents, and the presence of isorenieratene derivatives. The latter suggests that the base of the photic zone was sulphidic during formation of the sapropel. Combined with evidence of bottom water anoxia (preservation of laminae, high redox-sensitive trace element contents, and the abundance and isotopic composition of pyrite) this leads to the tentative conclusion that almost the entire water column may have been anoxic. This anoxia resulted from high productivity and not from stagnation, because an approximation of the trace element budget during sapropel formation shows that water exchange with the western Mediterranean is needed. Entire water column anoxia has been suggested earlier for several black shales. With regard to the depositional environment and the C org content, however, only the Cenomanian/Turonian Boundary Event (CTBE) black shales appear to be comparable to this sapropel. The proposed trace element removal mechanism of scavenging and (co-)precipitation in an anoxic water column, is thought to be similar for both types of deposits. The ultimate trace element source for the sapropel, however, is seawater, whereas it is hydrothermal and fluvial input for CTBE black shales (because they have a larger temporal and spatial distribution). Nonetheless, the C org-rich eastern Mediterranean Pliocene sapropel discussed here may be considered to be a younger analogue of CTBE black shales.

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

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

The genesis of Lincang germanium deposit — A preliminary investigation

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

Geochemistry of inorganic and organic sulphur in organic-rich sediments from the Peru Margin

We have determined (1) the abundance and isotopic composition of pyrite, monosulphide, elemental sulphur, organically bound sulphur, and dissolved sulphide; (2) the partition of ferric and ferrous iron; (3) the organic carbon contents of sediments recovered at two sites drilled on the Peru Margin during Leg 112 of the Ocean Drilling Program. Sediments at both sites are characterised by high levels of organically bound sulphur (OBS). OBS comprises up to 50% of total sedimentary sulphur and up to 1% of bulk sediment. The weight ratio of S to C in organic matter varies from 0.03 to 0.15 (mean = 0.10). Such ratios are like those measured in lithologically similar, but more deeply buried petroleum source rocks of the Monterey and Sisquoc formations in California. The sulphur content of organic matter is not limited by the availability of porewater sulphide. Isotopic data suggest that sulphur is incorporated into organic matter within a metre of the sediment surface, at least partly by reaction with polysulphides.Most inorganic Sulphur occurs as pyrite. Pyrite formation occurred within surface sediments and was limited by the availability of reactive iron. But despite highly reducing sulphidic conditions, only 35–65% of the total iron was converted to sulphide; 10–30% of the total iron still occurs as Fe (III). In surface sediments, the isotopic composition of pyrite is similar to that of both iron monosulphide and dissolved sulphide. Either pyrite, like monosulphide, formed by direct reaction between dissolved sulphide and detrital iron, and/or the Sulphur species responsible for converting FeS to FeS2 is isotopically similar to dissolved sulphide. Likely stoichiometries for the reaction between ferric iron and excess sulphide imply a maximum resulting FeS2 :FeS ratio of 1:1. Where pyrite dominates the pool of iron sulphides, at least some pyrite must have formed by reaction between monosulphide and elemental sulphur and/or polysulphide. Elemental sulphur (S0) is most abundant in surface sediments and probably formed by oxidation of sulphide diffusing across the sediment-water interface. In surface sediments, S0 is isotopically heavier than dissolved sulphide, FeS and FeS2 and is unlikely to have been involved in the conversion of FeS to FeS2. Polysulphides are thus implicated as the link between FeS and FeS2.