Diagenetic Pyrite Research Articles

Controls on the Distribution of Invisible and Visible Gold in the Orogenic Gold Deposits of the Yangshan Gold Belt, West Qinling Orogen, China

Six orogenic gold deposits constitute the Yangshan gold belt in the West Qinling Orogen. Gold is mostly invisible in solid solution or in the sulfide lattice, with minor visible gold associated with stibnite and in quartz-calcite veins. Detailed textural and trace-element analysis of sulfides in terms of a newly-erected paragenetic sequence for these deposits, together with previously published data, demonstrate that early magmatic-hydrothermal pyrite in granitic dike host-rocks has much higher Au contents than diagenetic pyrite in metasedimentary host rocks, but lower contents of As, Au, and Cu than ore-stage pyrite. Combined with sulfur isotope data, replacement textures in the gold ores indicate that the auriferous ore-fluids post-dated the granitic dikes and were not magmatic-hydrothermal in origin. There is a strong correlation between the relative activities of S and As and their total abundances in the ore fluid and the siting of gold in the Yangshan gold ores. Mass balance calculations indicate that there is no necessity to invoke remobilization processes to explain the occurrence of gold in the ores. The only exception is the Py1-2 replacement of Py1m, where fluid-mediated coupled dissolution-reprecipitation reactions may have occurred to exchange Au between the two pyrite phases.

Extending the paleogeographic range and our understanding of the Neoarchean Monteville impact spherule layer (Transvaal Supergroup, South Africa)

AbstractThe Monteville spherule layer (MSL) was deposited in the Griqualand West Basin (GWB) on the Kaapvaal Craton approximately 2.63 Ga. The spherules were generated by a large impact and reworked by impact‐generated waves and/or currents. The MSL has been intersected in three previously undescribed cores. Two of the cores, GKF‐1 and GKP‐1, were drilled ~30 km west of the southernmost outcrop of the MSL. The third core, BH‐47, was drilled ~250 km south and east of the GWB. The MSL contains medium to coarse sand‐size spherules like those described previously in all three cores but each one was emplaced in a different way. In GKF‐1, the MSL is 90 cm thick and contains large rip‐up clasts of basinal carbonate and early diagenetic pyrite. In GKP‐1, the MSL is only 1.5 cm thick and consists largely of fine carbonate sand, yet it contains pyrite intraclasts up to ~1 cm long. In BH‐47, the MSL consists of a lower coarse sandy zone ~37 cm thick rich in spherules, carbonate peloids/ooids, pyrite intraclasts, and quartzose sand and an upper, finer sandy zone ~46 cm thick; neither zone contains any large intraclasts. The new occurrences triple the known extent of the MSL from ~15,000 to ~46,000 km2, support the oceanic impact model for the formation of the MSL, demonstrate that it is a persistent regional time‐stratigraphic marker, place new constraints on the Kaapvaal paleoshoreline at the time of impact, and support the existence of Vaalbara.

Frequent euxinia in southern Neo-Tethys Ocean prior to the end-Permian biocrisis: Evidence from the Spiti region, India

In this study, we reconstruct water-column oxygen availability during the deposition of Late Permian shales (Wuchiapingian to approximately early Changhsingian) using the geochemistry and morphology of sedimentary pyrite (FeS2). Deposition of the shales occurred prior to the end-Permian mass extinction within the southern Neo-Tethys Ocean (Spiti region, Himachal Pradesh, India). We found that the variability of pyritic sulfur to organic carbon (Spyr/Corg) and the pyritic sulfur isotopic composition (δ34Spyr) did not record changes in oxygen availability at Spiti sections. This was due to the removal of original pyritic sulfur by modern oxidative weathering and diagenetic pyrite formation which overwhelmed the original δ34Spyr values. However, pyrite framboid size distributions was a reliable recorder of paleoredox conditions. The degree of oxygen limitation varied with multiple transient euxinic events interrupting a predominantly oxic-dysoxic trend near the top of the Gungri Formation upper member (approximately late Wuchiapingian to early Changhsingian). These transient euxinic intervals provide important insight and regional evidence of environmental stress and instability in the southern Neo-Tethys well prior to the end-Permian mass extinction.

Ore genesis of the Bake gold deposit, southeastern Guizhou province, China: Constraints from mineralogy, in-situ trace element and sulfur isotope analysis of pyrite

The Bake gold deposit is exceptional for its high native gold abundance relative to the other orogenic gold deposits in the southeastern Guizhou district. The grades of gold can be up to 2 to 3 wt% in bonanza ore, and native gold preferentially occurs within voids and fractures in pyrite, arsenopyrite and quartz. Gold content can be up to 6.9 ppm in hydrothermal pyrite, however, the majority of the pyrite analyses have Au contents less than 1 ppm (mean = 0.32 ppm) based on LA-ICPMS analyses. Meanwhile, EMPA reveals that arsenopyrite has a maximal Au content of up to 0.14 wt%, but most analyses are below detection limit (0.01 wt%). Moreover, native gold grains in cataclastic pyrite and arsenopyrite are commonly high-purity with fineness ranging from 949 to 977. Given that paragenetically early hydrothermal pyrite and arsenopyrite contain variable but relatively high gold concentrations than later generations of sulfide, it is suggested that early invisible gold may have been remobilized to form at least part of the microscopic visible gold. Post-mineralization shear zones, which cut the vein-bearing anticline, may have led to the deformation of the lode quartz vein system on the macro level, and microscopically, sulfide fragmentation and elemental remobilization. Although diagenetic pyrite from unaltered host rocks is enriched in invisible gold with mean values at 0.83 ppm, evidence supporting the release of Au from them is weak. Corresponding evidence comes from the distinct sulfur isotope signatures in different pyrite generations. Diagenetic pyrite has δ34S values in the narrow range of +9.27‰ to +10.75‰, whereas hydrothermal pyrite shows a broad range (+6.81‰ to +17.42‰), based on LA-MC-ICPMS analyses. The Mesoproterozoic Lengjiaxi Group underlying the Xiajiang Group is most likely the source region, on the basis of geological background and assuming the metamorphic devolatilization model.

Liberation of selenium from alteration of the Bowland Shale Formation: evidence from the Mam Tor landslide

The Bowland Shale Formation is anomalously rich in selenium (Se) at levels an order of magnitude greater than other black shales. The Mam Tor landslide, Derbyshire, England, offers an opportunity to measure whether the Se anomaly is conferred to the alteration products formed by oxidative water flow through the shale. Selenium in the shale is concentrated in diagenetic pyrite. Alteration of the shale causes decomposition of the pyrite to iron oxyhydroxide, which is carried in colloidal form (ochre) by springs draining the landslide. The iron oxyhydroxide contains anomalously high Se, and anomalously high Se was measured in water ponded where the ochre precipitated, although not in flowing groundwater. Other trace elements including cadmium and thallium also occur at concentrations higher than in other ochres. Given the widespread nature of the Se anomaly in the Bowland Shale Formation and equivalents across Britain and Ireland, any alteration products derived from workings through the shale should be disposed of with care.

Oceanic redox condition during the late Ediacaran (551–541 Ma), South China

The late Ediacaran (551–541 Ma) oceanic environment could represent a significant preparation for the enrichment of redox sensitive elements during the early Cambrian and even for the evolution of life. However, the marine environment during the late Ediacaran has not been thoroughly characterized to date. To better understand late Ediacaran oceanic redox conditions, we investigated the Fe and S isotopic compositions of cherts from the slope (Yinjiang) and basin (Silikou) sections of the Nanhua basin in South China. The slope (Yinjiang) section is characterized by a wide range of δ56FeT values (−0.12‰ to 1.06‰) and by continuously decreasing δ34SPy values from 40.4‰ to 4.1‰. The variable δ56FeT values in the Yinjiang section primarily reflect the various oxidation degrees of dissolved Fe2+ in ferruginous seawater. The homogeneous δ56FeT signatures (0.02–0.40‰) along the Silikou section and the decreasing trend of δ56FeHR values from 0.86‰ to near zero primarily depend on the mixing of Fe3+ hydroxide/oxide precipitates in a ferruginous water column, possibly followed by partial reduction, the authigenic framboidal pyrite trapped within an intermittent euxinic condition and detrital silicate Fe minerals. A 20–30‰ discrepancy in δ34SPy values between the Yinjiang and Silikou sections may be ascribed to the formation of diagenetic pyrites and framboidal pyrites, respectively. Therefore, the continuously decreasing δ34SPy values in the Yinjiang and Silikou sections may reflect a significant increase of sulfate diffusion into sediments at the slope location and the enlarged marine sulfate reservoir at the basin location. These new results indicate that the deep water in the Nanhua basin is primarily characterized by a ferruginous condition with intermittent euxinic conditions in the basin location during the late Ediacaran.

Multistage pyrite in the Getang sediment-hosted disseminated gold deposit, southwestern Guizhou Province, China: Insights from textures and in situ chemical and sulfur isotopic analyses

Getang is a representative sediment-hosted disseminated gold (SHDG) deposit in the southwestern Guizhou Province, China. The majority of gold is present as structurally bound Au+ in pyrite, with a minor amount occurring as nanoparticles (Au0). Four generations of pyrite (Py0, Py1, Py2, and Py3) were distinguished based on morphology and paragenesis. Electron microprobe (EMP), laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) analyses were conducted to determine the chemical and sulfur isotopic composition of the pyrite.The pre-ore diagenetic pyrite (Py0) has a framboidal texture and is characterized by relatively high concentrations of As, Co, Ni, Cu, Zn, Pb, and Mo. It has negative δ34S values of −38.4 to −4.0‰, indicating the derivation of sulfur by bacterial reduction of marine sulfate during sedimentation and diagenesis. The early ore stage pyrite (Py1) coexists with arsenopyrite as subhedral-anhedral grains, and contains high concentrations of As, Ni, Cu, and Zn, with large variation in Co and Pb content. It has positive δ34S values of +3.8 to +17.7‰, which are interpreted to be complete thermochemical reduction of marine sulfate. The intermediate ore stage pyrite (Py2) is disseminated in the host rocks, and occurs as overgrowths on Py0 and Py1. It is enriched in Au and Ag but depleted in As, Co, Ni, Cu, and Bi. The late ore stage pyrite (Py3) occurs as acicular to columnar aggregates, and is intergrown with calcite. It contains high concentrations of As and low concentrations of Pb, Mo, and Bi. Py2 and Py3 display a comparable narrow range of δ34S values from +1.9 to +8.4‰, which are interpreted to result from a magmatic source.All the stable isotope data together with the gravity and magnetic evidences suggests that sulfur, gold, and the mineralizing fluids in the Getang deposit were probably originated from deep magmatic hydrothermal systems. The absence of igneous intrusions in southwestern Guizhou may be attributed to the relatively shallow denudation depth in this area.

In-situ sulfur isotope analysis of pyrite from the Pangjiahe gold deposit: Implications for variable sulfur sources in the north and south gold belt of the South Qinling orogen

The sedimentary- and granite porphyry-hosted Pangjiahe gold deposit is located in the north belt of the South Qinling Orogen, with a proven reserve of 38 t Au at 6.3 g/t. A total of 26 samples, from various rock types and locations from the Panjiahe deposit, were taken in order to determine the paragenesis, sulfur source and absolute age of mineralization. Magmatic activities of ca. 240 Ma occurred around the whole study area, which is common in the northwest part of the West Qinling orogen. Field observations confirm that the emplacement of granite porphyry dykes is earlier than the gold mineralization, because they were both altered and mineralized. Moreover, post ore stage diabase dykes, which cut cross the ore bodies, display an age of ca. 220 Ma. Zircons from strongly mineralized granite porphyry samples show typical hydrothermal trace element characteristics. Theses zircons return a concordia age at ca. 230 Ma. Therefore, we infer that the ore-forming age of the Pangjiahe deposit is at ca. 230 Ma.Optical microscopy, EMPA and LA-(MC)-ICPMS have been used to determine the physical and chemical features of pyrite. Five stages of pyrite have been identified in mineralized phyllite and granite porphyry rocks. Diagenetic pyrite in phyllite, Py1-P, displays a wide range of δ34S (−1.5% to +9.4%). The magmatic pyrite in the granite porphyry, Py1-G, display relatively narrow range of δ34S (+2% to +5%) consistent with magmatic sulfur. The main ore stage pyrite in both rocks, Py2 (Py2-P and Py2-G), is the most common sulfide in the deposit. Py2 grains predominantly show oscillatory zoning and either replace or overgrown the Py1-P and Py1-G, or occur as individual fine grains. Arsenopyrite is typically associated with Py2. Pyrite grains in late stage quartz (Qz) veins that cross cut phyllite and granite, Py-Qz, display weak oscillatory zoning. Native gold is commonly observed as inclusions in the Py-Qz grains or within/proximal to quartz veins. Py3 pyrite (Py3-P and Py3-G), which also belong to the later ore stage and can be distinguished by the native gold inclusions in them, all occur as the outer rims that overgrown or replace on Py2. The ore stage Py2, Apy, Py-Qz and Py3 have a similar narrow range in sulfur isotopic composition, from +8% to +10%. This range is slightly lower than the age equivalent SEDEX Pb-Zn deposits situated in graben basins to the East, but totally different from the earlier Py1-P and Py1-G. EMPA element mapping and spot analyses, and in-situ sulfur isotope analyses indicate that the ore stage pyrites in both phyllite and granite rocks have similar major element chemistry and sulfur isotopic composition, indicating that they are likely sourced from the same ore fluids. Therefore, we propose that prograde metamorphism of the underlying Devonian sedimentary sequences during the seafloor exhalation, which are thought to be enriched in Au, As, Sb, B, F and S, may have released H2S that forming these auriferous sulfides.Gold mineralization in the south belt of the South Qinling orogen shows the same mineralization style as observed in the North belt. However, mineralization is younger and the δ34S values of the hydrothermal ore stage sulfide are consistent with magmatic sulfur, which is consistent with intense magmatic activities within the South Qinling orogen. The δ34S values in the South Belt are distinctly different from those of gold deposits in the north belt. We propose that the source of the sulfur and gold in north belt came from the Devonian graben basins, and in the south belt came from a deep magmatic source. More focused studies of the gold and Devonian graben basin hosted SEDEX Pb-Zn deposits are required to further prove or develop this hypothesis.

Sulfur isotope and metal variations in sulfides in the BIF-hosted orogenic Cuiabá gold deposit, Brazil: Implications for the hydrothermal fluid evolution

Textural, chemical and multiple sulfur isotope analyses of sulfides from the Archean world-class Cuiabá orogenic Au deposit, Quadrilátero Ferrífero district, Brazil, were conducted to track distinct trace element signatures from syngenetic to epigenetic-hydrothermal pyrite types in different host units, and to deduce the nature and source of the mineralizing fluids. Gold mineralization in Cuiabá is hosted in greenschist facies volcanic and sedimentary host rocks belonging to the Archean Rio das Velhas Supergroup, especially in association with quartz veins and sulfide minerals. The BIF-hosted Fonte Grande Sul orebody is controlled by quartz veins and disseminated gold-bearing pyrite, pyrrhotite and arsenopyrite in the wall rocks. Five pyrite types are classified based on textural relations as spongy, syngenetic (Py1), porous early-(Py2), smooth main-(Py3) and smooth isolated and overgrown late-stage (Py4 and Py5) types variably present in metamorphosed carbonaceous pelites, BIF and andesite. Besides pyrite, pyrrhotite and arsenopyrite are also defined according to textural and paragenetic differences, and they display significant trace element contents. Trace element abundance maps and LA-ICP-MS analyses display trace element incorporations in Py1 (present only in carbonaceous pelite), Py2 and Py3 (in all host rocks), and Py4 and Py5 (only in BIF). Py1 yields high As, Co, Ni, Pb and Ag concentrations, whereas Py2 (formed by agglomeration of Py1) maintains high trace element concentrations with slightly less of those elements. Trace element incorporation in Py3 is similar to Py1 in carbonaceous pelite. Early-stage Py2 and main-stage Py3 in BIF and andesite have increased Co and Ni, but have less Au and As. Late-stage Py4 is characterized by lower trace element concentrations, whereas Py5 is further enriched in As, Bi, Co, Ni and Pb. The hydrothermal alteration is divided into early-, main- and late-stage, which correlate to the sulfide hydrothermal evolution. The study shows that carbonaceous pelite is pre-enriched in Co, Ni and Pb, whereas certain elements like Ag, Au, Bi and As are only hydrothermally concentrated either during the early, main or late stage in BIF and andesite. This geochemical pattern supports a syngenetic versus hydrothermal origin of distinct elements, including some trace elements now in the ore zones. Multiple sulfur isotope data suggest that mineralizing fluids at the Cuiabá gold deposit reflect a complex mixing of sulfur evolved from three possible sources: seawater, mantle, and reduced elemental sulfur. Spongy, syngenetic Py1 yields Δ33S values ranging from −2.28 to −0.25‰, separated into two ranges, the first from −2.28 to −1.97‰, and the second from −0.96 to −0.25‰. The first range suggests Py1 in the carbonaceous pelite was deposited in a seawater environment. These pyrites probably mixed with later mantle sulfur carrying a near-zero Δ33S signature, which is indicated by the second Δ33S range, or could have mixed with fluids sourced from sedimentary rocks at depth carrying a positive Δ33S signature. The second scenario is more common in sedimentary and diagenetic pyrites. An unequivocal contrast is shown by early-, main- and late-stage pyrite types (Py2, Py3, Py4 and Py5) in carbonaceous pelite, BIF and andesite, which present a continuous process of crustal assimilation values towards increasingly positive δ34S and Δ33S values. This confirms evidence for a sedimentary-derived sulfur source as also indicated by fluid inclusion data of the andesite-hosted orebody at the Cuiabá deposit.

Links between seawater paleoredox and the formation of sediment-hosted massive sulphide (SHMS) deposits — Fe speciation and Mo isotope constraints from Late Devonian mudstones

Many models of sediment hosted massive sulphide (SHMS) deposit formation invoke basin restriction events that resulted in long-term stagnation and anoxia, in which sulphidic (euxinic) conditions ultimately prevailed. Euxinic conditions are then thought to provide a chemical trap for hydrothermally exhaled base metals. Here, we present Fe speciation and Mo isotope data for organic-rich mudstones from two drill-holes intersecting Upper Devonian strata, deposited along the passive margin of ancestral North America. One drill-hole intersects a 35 m thick sequence of SHMS mineralisation, while the other intersects correlative, un-mineralised strata. All samples have high FeHR/FeT values (>0.38), indicating water-column anoxia. For the majority of samples in the un-mineralised drill-hole, the levels of pyritisation fall below the threshold typically used to define euxinic conditions (FePY/FeHR ≤ 0.70). In contrast, higher levels of pyritisation in the mineralised drill-hole (median FePY/FeHR = 0.86) likely developed via diagenetic pyrite enrichment. Whereas Pb and Zn are negatively correlated with Mo, Mo-U co-variation is consistent with Fe (oxyhydr)oxide particulate shuttling in the water-column. In addition, a weak correlation between TOC/P and Mo provides further evidence that Mo was sourced via authigenic, rather than hydrothermal, processes. The δ98Mo values (+0.66 to +1.02‰) are uniform between both drill-holes, and substantially lower than constraints for Late Devonian seawater (+1.5 to +2.0‰), consistent with Mo adsorption to Fe (oxyhydr)oxides. Collectively, the data provide evidence that local seawater was dominantly ferruginous (anoxic, non-sulphidic) at Macmillan Pass. Regional variability in the extent of ferruginous (low TOC/P) and euxinic (high TOC/P) conditions likely contributed to a balance between P regeneration and P enrichment that maintained nutrient availability and productivity in the Selwyn Basin during the Late Devonian. We argue that high primary productivity and enhanced organic carbon burial are key variables for promoting sulphate reduction in the sub-surface. Moreover, how such conditions are maintained over long periods of basin evolution is more important for producing effective metal traps in SHMS systems, rather than a specific, localised redox condition of seawater (i.e. euxinia).

Gold mineralization associated with Emeishan basaltic rocks: Mineralogical, geochemical, and isotopic evidences from the Lianhuashan ore field, southwestern Guizhou Province, China

A number of gold deposits are newly discovered in the Lianhuashan ore field, southwestern Guizhou Province, China. The gold orebodies are restricted to the Emeishan basaltic rocks and the interlayer fractures between the Emeishan basaltic rocks and the limestone of the Maokou Formation. Three generations of pyrite (Py0, Py1, and Py2) have been recognized in these deposits based on morphology and paragenetic relationships. Electron microprobe (EMP), laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) analyses were carried out to characterize their chemical and sulfur isotopic compositions.The framboidal Py0 is enriched in As but depleted in most of the other trace elements. It has negative δ34S values of −14.5–−1.9‰, which are typical of diagenetic pyrites resulted from bacterial reduction of marine sulfate. Py1 and arsenopyrite exhibit high concentrations of Au and As, making them the most dominant auriferous minerals. They yield comparable δ34S values of −2.8– + 2.1‰, consistent with the derivation of sulfur from the Emeishan basaltic rocks. Py2 is characterized by well developed oscillatory-zoned textures, that are interpreted in terms of internal self-organization processes. It contains low concentrations of Au and highly variable concentrations of As, Sb, Co, and Ni. The Py2 grains show slightly positive δ34S values of + 0.4– + 7.0‰, indicating that the sulfur was mainly sourced from the Emeishan basaltic rocks with variable amounts of contamination from reduced marine sulfate.The Emeishan basaltic rocks may have provided not only ore-hosting space but also Au, S, and other associated elements for gold mineralization in the Lianhuashan ore field. Similar gold deposits are expected to be discovered in the areas of Emeishan basaltic rocks with high Au backgrounds and small thicknesses.

Organic remains in late Palaeoproterozoic granular iron formations and implications for the origin of granules

Toward the end of the Palaeoproterozoic era, over 109 billion tonnes of banded (BIF) and granular (GIF) iron formations were deposited on continental platforms. Granules in iron formations are typically sub-spherical structures 0.2 to 10 mm in size, whereas concretions are >10 mm. Both types of spheroids are preserved throughout the sedimentological record. Their formation has typically been interpreted to originate from reworked Fe-rich sediments in high-energy, wave-agitated, shallow-marine environments. New evidence from six different late Palaeoproterozoic granular iron formations (GIF), however, suggests that some granules are the result of diagenetic reactions, in addition to other features driven by microbial processes and mechanical movements. Characteristic coarse grain interiors and septarian-type cracks inside granules, akin to those features in decimetre- to meter-size concretions, are interpreted as desiccation features from hydrated diagenetic environments where sulphate and/ or ferric iron were reduced while organic matter (OM) was oxidised inside granules. Those granules derived from sulphate reduction preserve diagenetic pyrite rims, whereas those formed via ferric iron reduction preserve diagenetic magnetite along their rims. Other diagenetic minerals including apatite mixed with OM, and various carbonate phases are commonly preserved within granules. Combined with systematically 13C-depleted carbonate, these diagenetic mineral assemblages point to the oxidative decay of OM as a major process involved in the formation of granules. Spheroidal equidistant haematite laminations surround some granules and contain apatite associated with carbonate, OM, and ferric-ferrous silicates, and oxides that further suggest these structures were not shaped by wave-action along sediment-water interfaces, but rather by chemical wave fronts and biomineralisation. Our results demonstrate that the formation mechanisms of GIF also involve microbial activity and chemically-oscillating reactions. As such, granules have excellent potential to be considered as promising biosignatures for studying Precambrian biogeochemistry, as well as astrobiology.

Microenvironmental constraint on δ13C depletion: Garudamangalam Sandstone, Cauvery Basin, India

Within the hydrocarbon-producing Cretaceous marine Uttatur Group, Cauvery Basin, India, the Garudamangalam Sandstone Formation is at top of a TST-HST transit. The δ13C value is expectedly depleted within the calcareous Garudamangalam Sandstone, which is the top most unit of the Uttatur Group, overlying the Karai Shale. The calcareous sandstone was deposited in a coastal setting around a shore-parallel river mouth bar. Instances of excessive depletion of δ13C up to −44.5‰ in the carbonate cement is suggestive of methane generation and its subsequent sequestration. The common occurrence of early diagenetic pyrite in these rocks testifies to the proliferation of sulfate-reducing bacteria and is suggestive of methane generation beneath the sulfate reduction zone. Upward-moving diffusive methane was possibly consumed by methanotrophs at the base of the sulfate-reduction zone. Abundant fabric-selective carbonate cement corroborates microbially-controlled anaerobic oxidation of methane. The presumed high rate of nutrient supply, abundance of vegetative material and moderately high organic carbon content in sediments (av. 1.6%), support this contention. All the samples which have the greatest δ13C depletion are characterized by enriched organic carbon and are derived from a tidal inlet-mouth facies, and selectively from mud drapes on cross-bedding in tidal strata. Calcarenite at the base of the same cross-strata are invariably much less depleted in δC13. This range of relationships indicates the transport of methanotrophs that settled on foreset beds mostly as tides slackened under the broader control of neap-spring cycles.

Isotopic evidence for oxygenated Mesoarchaean shallow oceans

Mass-independent fractionation of sulfur isotopes (MIF-S) in Archaean sediments results from photochemical processing of atmospheric sulfur species in an oxygen-depleted atmosphere. Geological preservation of MIF-S provides evidence for microbial sulfate reduction (MSR) in low-sulfate Paleoarchaean (3.8–3.2 billion years ago (Ga)) and Neoarchaean (2.8–2.5 Ga) oceans, but the significance of MSR in Mesoarchaean (3.2–2.8 Ga) oceans is less clear. Here we present multiple sulfur and iron isotope data of early diagenetic pyrites from 2.97-Gyr-old stromatolitic dolomites deposited in a tidal flat environment of the Nsuze Group, Pongola Supergroup, South Africa. We identified consistently negative Δ33S values in pyrite, which indicates photochemical reactions under anoxic atmospheric conditions, but large mass-dependent sulfur isotope fractionations of ~30‰ in δ34S, identifying active MSR. Negative pyrite δ56Fe values (−1.31 to −0.88‰) record Fe oxidation in oxygen-bearing shallow oceans coupled with biogenic Fe reduction during diagenesis, consistent with the onset of local Fe cycling in oxygen oases ~3.0 Ga. We therefore suggest the presence of oxygenated near-shore shallow-marine environments with ≥5 μM sulfate at this time, in spite of the clear presence of an overall reduced Mesoarchaean atmosphere.

Basic-to-acid and acid-to-basic zonation in the sulfur isotope composition of diagenetic sulfide nodules

The typical for marine sediments decrease in the amount of sulfate ion in interstitial solutions with increasing depth owing to sulfate reduction and attenuation of the diffusion of near-bottom waters is manifested as the basic-to-acid zonation in the sulfur isotope composition of the diagenetic pyrite nodules (increase of δ34S values from the center to edge). In sandstones of the Early Carboniferous Emyaksin Formation, which is developed at middle reaches of the Vilyui River (Sakha-Yakutia), we detected a peculiar sulfide nodule. In contrast to common nodules, this nodule is marked by the δ34S value decreasing from 7.5…8.6‰ at the center to –30…–31.4‰ at the edge. Such acid-to-basic zonation is attributed to the origin of nodule in a freshened basin (delta) with a sulfate-depleted environment, which gave way to a sulfate-enriched environment owing to the expulsion brines from the underlying gypsiferous sequences.

NanoSIMS element mapping and sulfur isotope analysis of Au-bearing pyrite from Lannigou Carlin-type Au deposit in SW China: New insights into the origin and evolution of Au-bearing fluids

Sulfur isotope signatures of Au-bearing pyrite from Lannigou Au deposit, a typical Carlin-type Au deposit in SW China, provide valuable information about the origin of the ore-forming minerals. Analysis by NanoSIMS was used to determine S isotope compositions of Au-bearing pyrite and to map the grain-scale distributions of Au, Cu, As and S in pyrite from the deposit. Based on different textural pattern of pyrites revealed by back-scattered electron (BSE) images, they are divided into three types: Py-1 diagenetic pyrite without core-rim structure, Py-2 pyrite with an Au-free core and a rhythmically-zoned Au-bearing rim, Py-3 Au-bearing pyrite with rhythmic zoning across the entire grain. The element distributions and S isotope compositions of four paragenetic stages are recognized on the basis of textural observation. Py-1 grains and the Au-free homogeneous cores of zoned crystals were formed in Stage 1 while the Au-bearing rims of the zoned crystals with rhythmic zonation of As and Cu, and to a lesser degree Au, were formed in two superimposed stages: stage 2 formed the inner zone that is enriched in As alone; and stage 3 formed the outer zone that is enriched in both Au and As. Other sulfides such as realgar, cinnabar and stibnite are formed in the last stage. The relationship between Au and As distributions in pyrite rim is complicated, changing from coupled to decoupled at the nanoscale. Such complexity is interpreted to reflect fluctuation of fluid composition and temperature with time, which in turn affect the modes of occurrence of As and Au. It is inferred that As mainly occurs in the crystal lattice replacing S whereas Au is mainly present as nanoparticles that were trapped in pyrite during crystal growth. The Au-bearing rims of the zoned pyrite crystals are characterized by highly variable δ34S values from 1.1 to 18.1‰, which exceed the values of the Triassic calcareous host rocks (10–14‰). In contrast, the δ34S values of the Au-free cores of zoned pyrite crystals vary over a narrower interval and are mainly between 6 and 12‰, close to the values of pyrite crystals in the sedimentary country rocks. Our new analyses also reveals that the δ34S values of the Au-bearing fluids generally increase during the formation of the deposit. The observed S isotope variations are consistent with mixing between a magmatic-related fluid with mantle-like δ34S value (∼0‰) and a sedimentary or deep basin brine fluid with elevated δ34S value (>18‰), with an increasing contribution from the latter with time. The notably varied values of δ34S and the disseminations of Au and other trace elements such as As and Cu in pyrite crystals indicate that the process responsible for Au precipitation in this deposit occurred in an open hydrothermal system.

The Role of Seawater on the Trace Element Geochemistry of Some UK Coals and a Tribute to Goldschmidt

The Pennsylvanian coals in the UK were deposited in a low-lying delta plain which was periodically inundated by the sea. This resulted in high S contents, mainly due to early diagenetic pyrite. Based on a statistical analysis of the geochemical data, pyrite is the major host for As, Se, Hg, Tl and Pb and an important host for Mo, Cd, Ni, Sb and Cu. All of these elements are enriched in the coals compared with non-marine mudrocks in the sequence, as is Ge and Be, although theirs is an organic association. The pattern of enrichment in the coals is similar to that in marine shales and coals overlain by marine shales, demonstrating the seawater influence. Goldschmidt was a pioneer in the geochemical study of coals and this paper will demonstrate that many of his ideas have stood the test of time.

Evidence of local sourcing of sulfur and gold in an Archaean sediment-hosted gold deposit

Determining the source of sulfur in an ore deposit is key to understanding the nature of the ore forming processes. The Neoarchaean Paulsens sediment-hosted gold deposit (∼1Moz @ 7.6g/t) located in the Pilbara Craton of Western Australia exhibits many of the characteristics of Phanerozoic shale hosted gold deposits (e.g. Huijiabao Trend, Northern Carlin Trend and Sukhoi Log), in that 1) black shales are the dominant host rock, 2) gold is hosted in pyrite as both free gold and dissolved gold in the lattice of the pyrite, and 3) multiple generations of pyrite have formed due to a variety of geological processes. In this contribution we utilised Secondary Ion Mass Spectrometer (SIMS) to measure the in-situ quadrupole (32S, 33S, 34S and 36S) sulfur isotope compositions of the different generations of pyrite. Our results indicate that the both diagenetic and hydrothermal pyrite generations display similar and anomalous Δ33S signatures (up to +0.4‰). Further, the Δ33S-Δ36S arrays in the hydrothermal pyrite generations lie on a slope which is similar to that of the diagenetic pyrite. These data support the hypothesis that the sulfur in the ore zones came from the host Hardey Formation black shales.We also performed trace element analyses of syn-sedimentary and early diagenetic pyrite from the Hardey Formaiton using Nano Secondary Ion Mass Spectrometry (NanoSIMS), Electron Probe Microanalysis (EPMA) and Laser Ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS), all of which show that the syn-sedimentary and early diagenetic pyrite contain high concentrations of many trace elements (As, Ni, Co, Cu, Ag, Se, Te, Bi), including up to 1.5ppm Au. These metals contents are also abundant in the several generations of mineralised hydrothermal pyrite and form clear patterns of growth associated with couple dissolution reprecipitation reactions. These findings clearly indicate that the fluid that transported the Au must have also been enriched in the base and precious metals that are contained in the early, syn-diagenesis pyrite.Data from this study clearly support the hypothesis that in some sediment-hosted gold systems, all the sulfur and gold required to form the deposit are sourced from the local sedimentary package. By using the presence of anomalous mass independent sulfur isotope signatures as chemically conservative and indelible tracers, it is possible to fingerprint the source of sulfur in a wide range of mineral systems, thus enhancing predictive exploration strategies at the regional to camp scales.

Ore genesis of the Xiyi Pb‐Zn deposit, western Yunnan province, China: Geologic, fluid inclusion, and isotopic (C‐H‐O‐S‐Pb) evidence

The Xiyi Pb‐Zn deposit, located in the Baoshan Block of the Sanjiang region in western Yunnan, China, contains 1.2 Mt of Pb‐Zn resources. The orebodies are hosted by Devonian‐Carboniferous limestone, structurally controlled by the NE‐trending fractures, and occur in stratiform, vein, and lenticular forms. The mineral assemblage of the ores includes sphalerite, galena, pyrite, arsenopyrite, chalcopyrite, and marcasite. Calcite, barite, and quartz occur as gangue minerals. Three stages of hydrothermal mineralization are recognized based on the ore textures, crosscutting relationship, and mineral assemblages: an early stage of sphalerite + galena + calcite (Stage 1), a middle stage of galena + arsenopyrite + calcite ± sphalerite (Stage 2), and a late stage of calcite + pyrite ± barite ± quartz (Stage 3). Two types of fluid inclusions are associated with the different ore‐forming hydrothermal stages: aqueous inclusions and hydrocarbon‐H2O inclusions. The ore‐forming fluids were characterized by low temperatures (100 ~ 200 °C), medium salinities (8 ~ 20 wt% NaCleq), and medium densities (1.0 ~ 1.1 g/cm3) and contained minor amounts of CH4. The δDH2O and calculated δ18OH2O values of the Stage 1 calcite range from −89‰ to −103‰ and from 1.5‰ to 5.5‰, respectively, indicating that the H2O in the hydrothermal fluids was derived from seawater that had reacted with organic matter. The δ13CPDB and δ18OSMOW values of the calcite associated with Stages 1 and 3 range from −5.2‰ to 3.2‰ and from 12.5‰ to 20.0‰, respectively. These data plot in the field between marine carbonate and granites and are close to the marine carbonate field, suggesting that the CO2 in the ore‐forming fluids was derived from the dissolution of marine carbonate in the sedimentary rocks. The galena, sphalerite, and pyrite in Stages 1 and 3 have a narrow range of δ34S values (0–3‰), which is lower than the δ34S values of early Carboniferous to Middle‐Late Devonian seawater sulfate (+18‰ to +23‰). Therefore, because the mineralization is unrelated to magmatic activity, the sulfur in the hydrothermal sulfides was derived from regional marine sulfates primarily via organic reduction at temperatures of 100 to 200 °C. The lead isotopic compositions in the galena, sphalerite, and pyrite in Stages 1 and 3 are very similar to those in the diagenetic pyrite in the host rocks, plotting in the field of the upper crust Pb evolution curve. Thus, the lead source was likely the Carboniferous and Devonian strata. The geologic, fluid inclusion, and isotopic (C‐H‐O‐S‐Pb) evidence in this study suggests that the Xiyi Pb‐Zn deposit is a Mississippi Valley‐type deposit.

Multiple sulfur isotope constraints on sulfate-driven anaerobic oxidation of methane: Evidence from authigenic pyrite in seepage areas of the South China Sea

Multiple sulfur isotope signatures and secondary ion mass spectroscopy (SIMS) sulfur isotope compositions of pyrite from two seafloor sites (DH-CL11 and HD109) in seepage areas of the South China Sea were measured in order to study isotope effects of sulfate-driven anaerobic oxidation of methane (SO4-AOM). The multiple sulfur isotopes of pyrite reveal variable ranges for both sites (δ34S: between −44.1‰ and −2.9‰ for DH-CL11 and between −43.8‰ and −1.6‰ for HD109; Δ33S: between 0.02‰ and 0.17‰ for DH-CL11 and between −0.03‰ and 0.14‰ for HD109). SIMS analysis reveals an extreme variability of δ34S values (between −50.3‰ and −2.7‰ in DH-CL11; between −50.1 and 52.4‰ in HD109) for three types of pyrite: (1) framboids, (2) zoned aggregates with radial overgrowth surrounding a framboidal core, and (3) euhedral pyrite crystals. The synchronous changes of geochemical proxies (sulfate and methane concentrations, δ34Ssulfate and δ18Osulfate, δ34Spyrite, and pyrite content) at the sulfate-methane transition zone (SMTZ) at site DH-CL11 are interpreted to be induced by SO4-AOM under steady state conditions. In contrast, pyrite content and δ34S value fluctuations throughout core HD109 suggest that the sediment at this site was affected by multiple pyritization events during diagenesis. Multiple sulfur isotope signatures of early diagenetic pyrite (i.e., with low and high δ34S values, the latter above 315 cmbsf in DH-CL11; above 70 cmbsf in HD109) in the upper sediment column suggest that organoclastic sulfate reduction (OSR) and sulfur disproportionation generated the observed isotopic signatures. In contrast to the early diagenetic 34S depleted framboids, the higher SIMS δ34S values of overgrowth and euhedral crystals suggest a late diagenetic 34S enriched pool of dissolved sulfide derived from SO4-AOM at the current and paleo-SMTZs. Interestingly, pyrite resulting from SO4-AOM in the SMTZ at site DH-CL11 reveals a distinct pattern with higher Δ33S values, different from pyrite resulting from OSR and sulfur disproportionation. Therefore, paired δ34S and Δ33S values may allow to differentiate OSR and SO4-AOM, although a full understanding of the isotope effects associated with SO4-AOM is hampered by uncertainties on the actual electron transfer mechanism in the syntrophic SO4-AOM consortium.