Positive δ34S Research Articles

Trace elements and in situ sulfur isotopes of pyrite from the Baiyunpu Au deposit in central Hunan, South China: Implications for gold mineralization

The Central Hunan metallogenic province, located in the southeastern part of the Yangtze Block, South China, is one of the most important Sb–Au metallogenic belts in China. The Baiyunpu gold deposit is one of the important Carlin‐type gold deposits occurring in Devonian sedimentary strata in Central Hunan. Optical microscopy and in situ laser ablation (multi‐collector) inductively coupled plasma mass spectrometry (LA‐[MC]‐ICP‐MS) are used to determine the texture, trace element compositions and sulfur isotope for pyrites from the pre‐ore and syn‐ore stages to constrain the sources of reduced sulfur and metals and the mechanism of gold precipitation. Four generations of pyrite were identified by petrography and pyrite compositions. Pre‐ore Py1 is characterized by relatively low concentrations of As, Au, Sb and Cu and high concentrations of Co, Ni, Bi and Pb. Py1 has positive δ34S values of 6.5–10.1‰, which we ascribe to a bacterial reduction of marine sulfate in closed marine systems, or from weak Rayleigh fractionation by bacterial sulfate reduction (BSR) or thermochemical sulfate reduction (TSR) during sedimentary‐diagenetic processes. Pre‐ore Py2 presents as rims of Py1 and contains relatively high As (69,530 ppm), Sb (123.2 ppm) and low Au (4.89 ppm). Compared with Py2, Py3 has relatively high Au (194.4 ppm) and Cu (24.6 ppm) and low As (40,801 ppm). We interpret that Py2 and Py3 could have precipitated from an Au‐free but As‐rich early hydrothermal fluids or from low‐Au and As‐rich fluids at the beginning of the ore stage. The δ34S values of Py2 and Py3 in the Baiyunpu deposit are concentrated in a narrow range of −1.2 to 1.2‰, suggesting the involvement of magmatic sulfur. Syn‐ore pyrites (Opy) are enriched in Au (517.8 ppm), As (39,719 ppm), Cu (261.9 ppm) and Sb (171.4 ppm). Irregular boundaries between Py1 and Opy suggest that the replacement may have been important for the formation of pyrites at the syn‐ore stage. The variation of As concentration in Opy1, Opy2 and Opy3 appear to have recorded fluctuations in the chemical composition of ore fluids. The δ34S values of Opy (0.1–5.9‰) are slightly higher than Py2 and Py3 due to mixing with sedimentary components with elevated δ34S values in late‐stage hydrothermal fluids. Profile analyses for Baiyunpu auriferous pyrite grains suggest that Au was positively correlated with Cu, but not with As. The correlation between Au, Cu and As in Opy may be dominated by their concentrations of ambient evolving fluids.

Temporal and spatial separation mechanisms of the Cu and Mo mineralization in the Dabate porphyry deposit, Western Tianshan, Xinjiang, China

Dabate deposit is a typical porphyry Cu-Mo deposit in the Western Tianshan, Xinjiang, China. Both the Cu and Mo mineralization are mainly associated with intrusion of rhyolite porphyry. Distinguishing from typical porphyry deposits, the Cu mineralization and Mo mineralization are separated from each other on both the temporal and spatial scales. The Cu orebodies are mainly hosted in the contact zone between the rhyolite porphyry and Tuosiku’ertawu Formation and predominantly occur as fluorite-arsenopyrite-Cu-sulfides ± K-feldspar ± quartz veins (Cu-1 stage), whereas the Mo orebodies are mostly hosted in the inner part of the rhyolite porphyry and are typically presented as Mo-2 stage quartz-molybdenite veins (external Cu and internal Mo). Total homogenization temperatures (Th, L-V) for boiling fluid inclusion assemblage of the Mo-2 stage occurred at 302 to 358 °C, with corresponding salinities of 1.4 to 38.1 wt% NaCleqv. H-O isotope data indicate that the Mo mineralization fluids were mainly derived from magmatic water. The non-boiling fluids of the Cu-1 stage originated from magmatic fluids and meteoric water, which were characterized by high temperature (323 to 379 °C) and low salinity (2.6 to 5.9 wt% NaCleqv) as well as Cu, As, and volatile enrichment. These features are different from the initial magmatic fluids of this deposit, indicating that the Cu mineralization may occur before the Mo mineralization (early Cu and late Mo) and the magmatic fluid components of the Cu-1 stage should be derived from the condensation of the low-salinity vapor generated by boiling of the hydrothermal fluids (351 to 405 °C; 1.8 to 40.1 wt% NaCleqv) for the earliest Mo-1 stage barren quartz veins. The positive δ34S values (5.5 to 9.8‰) of sulfides indicate that the sulfur in the ore-forming fluids was derived from magma and country rocks. The subtle change in fluid conditions from the high temperature (peak range 340 to 360 °C), alkaline, and slightly more oxidized of the Cu-1 stage to high temperature (peak range = 320 to 340 °C), more reduced, and acidic of the Mo-2 stage gave rise to the temporal separation of Cu and Mo mineralization. The Cu, As, and volatiles (e.g., CO2, HF), preferentially partitioned into the vapor phase, were highly enriched in Cu ore-forming fluids, whereas Mo mineralization fluids contained the majority of Mo with strong preference for the brine phase. Therefore, the spatial separation of the Cu and Mo mineralization could be attributed to selectively fractionating between the vapor phase and hypersaline liquid of the Cu and Mo metals during the process of fluid boiling.

Multiple Sulfur Isotope Compositions in Mesoarchean Sulfide Deposits of the Karelian Craton: Implications for Determining the Sulfur Source, Biogeochemical Processes, and Deposit Genesis

Abstract —In the present paper we demonstrate that most sulfides of the studied deposits of the Archean Sumozero–Kenozero greenstone belt within the Karelian Craton on the Fennoscandian Shield have nonzero Δ33S values. This indicates that proportions of seawater sulfate and elemental sulfur in Mesoarchean, included into the ores and resulting from UV photolysis, are different. Our results show that systematics of sulfur isotopes of sulfides generally reflects the mixing of mass-independently fractionated sulfur reservoirs with positive and negative Δ33S values. Pyrite is depleted in 34S isotope, which was interpreted as evidence for microbial sulfate reduction. Variations in the positive Δ33S anomalies of the Leksa deposit and the general tendency for Δ33S sulfide content to increase with stratigraphic levels in certain boreholes most likely reflect the change in temperature and the fluid mixing throughout the life of the hydrothermal system. The presence of sulfides with strongly negative Δ33S anomalies suggests that atmospheric sulfur and seawater sulfate, rather than volcanic sulfur, were the prevailing source for mineral systems of the studied deposits. The presented data require the Mesoarchean seawater to contain sulfates at least locally.

Transition of seawater conditions favorable for development of microbial hydrocarbon source – Reservoir assemblage system in the Precambrian

Several commercial Precambrian petroleum systems have been discovered worldwide. The microorganisms found in the Precambrian successions play a significant role in the development of high-quality hydrocarbon source rocks and microbialites with a high reservoir porosity. Yet, it remains unclear regarding issue of environments in which microbial activities can lead to the development of organic-rich hydrocarbon source rocks or reservoirs, and how the microbial source rock and reservoir constitute an effective hydrocarbon source-reservoir assemblage system. During the Edicaran Doushantuo Formation period, high-quality black shale source rocks were widely developed, and the organic matter in the source rocks was mainly derived from microbes including bacteria and algae. The thickness of the source rocks is up to 200 m, and the total organic carbon (TOC) value is ∼ 17.9%. During the Dengying Formation period, thick-layered microbialites, including microbial stromatolite, thrombolite, and botryoidal dolomite were developed extensively, with the abundant primary framework and secondary dissolution pores. During the Precambrian evolution, the transition of the seawater environment controlled the development of microbial source rocks or microbialite reservoirs. In a deep, euxinic reducing seawater condition, microbial organic matter was well preserved and accumulated in fine-grain sediments, thus forming the high-quality microbial source rocks. Meanwhile, under such a reducing condition, a large amount of layered, nodular, granular, and strawberry pyrites were precipitated due to the effect of BSR reaction with extremely positive δ34S values (up to 39.0‰). Meanwhile, in a shallow turbulent high-energy oxidized seawater condition, especially in the intermittent “Dolomite Sea” environment, microbial dolomitization promoted the massive development of microbial dolomite mound/shoal frameworks, constituting the matrix for the development of large-scale microbialite reservoirs. During the later burial evolution processes, the oil and gas generated from the Doushantuo Formation source rocks migrated and accumulated into the overlying Dengying Formation microbialite reservoirs, which constituted an effective hydrocarbon source-reservoir assemblage system. Overall, the areas surrounding the Mianyang, west side of the Chengkou, and northwest side of the western Hubei troughs are considered to be the potentially favorable areas with microbial source rock-reservoir assemblage system. The microbial source-reservoir assemblages in these Precambrian strata worldwide are worth further exploring.

Mineralogy and geochemistry of clastic sulfide ores from the Talgan VHMS deposit, South Urals, Russia: Signatures of diagenetic alteration

Diagenetically altered thin-layered sulfide ores from the Talgan volcanic-hosted massive sulfide deposit (South Urals) represent an intercalation of thin sulfide and hyaloclastite layers up to 3 and 0.5 cm thick. Their mineralogy and trace element, including rare earth elements (REE), and S, C and O isotopic geochemistry are considered in this work. They occur at the slopes of ore bodies and pinch outside them within hyaloclastites and their thickness increases towards the flanks of the ore bodies. The sulfide layers are mostly composed of authigenic pyrite, which forms zoned nodules, anhedral aggregates, euhedral crystals and pseudomorphs after pyrrhotite crystals. The pyrite nodules and anhedral aggregates host inclusions of authigenic chalcopyrite, sphalerite, galena, barite, cotunnite, native gold, stromeyerite, tellurobismutite and hessite. The trace element content of sulfide layers widely varies and depends on the amount of non-sulfide minerals (chlorite, illite, calcite, quartz, rutile, scheelite), which formed as a result of alteration of dacitic hyaloclastite components mixed with sulfide clasts. The sulfide layers are enriched in REEs (up to 561 ppm) due to the presence of REE-bearing minerals (bastnaesite, parisite, synchysite, galgenbergite, REE-bearing xenotime), which do not occur in massive sulfide ores. A narrow range of δ34S values of pyrite (1.41–3.27 ‰) from sulfide layers is explained by equilibrium or kinetic dissolution–precipitation processes of redeposition of sulfide ore clasts during diagenetic processes. The δ13C values of bulk samples of sulfide layers (–5.29 to –17.89 ‰) indicate that the authigenic carbonates formed from pore fluids circulated in former organic-rich hyaloclastite layers. The δ18O values of the same samples (+4.00 to + 8.36 ‰) are higher compared to seawater δ18O values due to diagenetic isotopic exchange between the hyaloclastitic components and pore fluids at elevated temperatures of burial process. The positive δ34S–Mn, δ13C–Mn, δ13C–As, δ13C–Sb, δ18O–Mn and δ18O–U correlations reflect organic matter consumption during the evolution of authigenic minerals. Data presented in this study helps to define the origin of diagenetically altered layered sulfide ores in volcanic-hosted massive sulfide deposits and are important for understanding diagenetic REE accumulations in sulfide-rich sediments of these deposits.

Diagenetic barite-calcite-pyrite nodules in the Silurian Longmaxi Formation of the Yangtze Block, South China: A plausible record of sulfate-methane transition zone movements in ancient marine sediments

The sulfate-methane transition zone (SMTZ) serves as an important geochemical transition especially during early diagenesis. Organoclastic sulfate reduction (OSR), sulfate-driven anaerobic oxidation of methane (SD-AOM), fermentation, and methanogenesis, the dominant diagenetic processes near the SMTZ, play key roles in marine sulfur and carbon cycling. However, the paleo-SMTZ has rarely been identified in the geologic record because of equivocal evidence. The presence of barite-calcite-pyrite assemblages (abundant nodules and pyritic laminae) concentrated over a 1-m thick interval of the Silurian Longmaxi Formation, Yangtze Block, South China, provide a unique opportunity to investigate possible vertical movements of the paleo-SMTZ in ancient sediments. The inferred paleo-SMTZ comprises nodules made up of barite (BaSO4), calcium carbonate (CaCO3), and pyrite (FeS2), and pyritic laminae. Strong 34S enrichment of barite (50 to 62.1‰ VCDT) and modestly 13C-depleted calcite (−11.4‰ to −5.3 VPDB) of the nodules associated with positive δ34S values of pyritic laminae (4 to 6.2‰ VCDT) suggest precipitation of these minerals sourced by mixed carbon and sulfur sources close to and/or within the paleo-SMTZ. Nodules from center to edge display mineral zonation that may reflect the effects of depth fluctuations of the paleo-SMTZ. Initial (stage 1) barite mineralization in nodule centers appears to have occurred within the base of the sulfate reduction zone (SRZ). By the time if stage 2 mineralization, the stratigraphic locus of precipitation had migrated closer to highly alkaline SMTZ resulting in barite dissolution and calcium carbonate precipitation. Higher proportions of calcium carbonate exhibiting some enrichment of 12C associated with cubic pyrite crystals in nodule centers are consistent with the occurrence of SD-AOM within the SMTZ. Stage 3 acicular barite mineralization along nodule edges likely took place near at the base of SRZ immediately above the SMTZ, whereas the replacement of barite by calcite during stage 4 mineralization occurred again within the SMTZ. Vertical movement of paleo-SMTZs suggested by the occurrences of barite-calcite-pyrite assemblages in the Longmaxi Formation across Yangtze Block, are likely the consequence of the complex interplay of varying sedimentation rate, organic matter quality, methane flux, and perhaps seawater sulfate concentration.

Atmospheric and hydrothermal sulfur isotope signatures recorded in Neoarchean deep marine sedimentary pyrites from the Yilgarn Craton, Western Australia

We report in-situ multiple sulfur isotope analyses for pyrites from deep marine sediments that are interbedded with ∼ 2.7 Ga greenstone lava flows in the Kalgoorlie-Kambalda area of the Eastern Goldfields Superterrane, Western Australia. Two endmember sediment types are recognised: shale and chert, with transitional chert as an intermediate. Petrologic studies are consistent with both the pyrite and pyrrhotite having a syngenetic/diagenetic origin. Pyrites from the shales and transitional cherts have positive Δ33S, whereas those from the cherts have Δ33S ∼ 0. We suggest that the principal sources of S are atmospheric photolytic S8, with Δ33S > 0, and nanoparticulate sulfides from hydrothermal seafloor vents, with neutral Δ33S. In an anoxic Archean ocean, nanoparticles of pyrite and/or pyrrhotite, issuing from black and white smokers, were dispersed through the ocean by currents. S8, together with nanoparticulate sulfides and fine detrital particles, rained down slowly and accumulated on the sea floor to form the shales. During diagenesis, pyrrhotite reacted with available S to form pyrite until all the S was consumed, with unreacted pyrrhotite remaining in the shale. Variations in Δ33S in the sedimentary pyrites are therefore attributed to variations in the relative proportions of pyrite derived directly from black and white smokers, and pyrite formed by the diagenetic reaction between nanoparticulate pyrrhotite and photolytic S8. The cherts are interpreted to have formed close to hydrothermal vents where rapid accumulation of amorphous silica and pyrite from white smokers negated the influence of slow S8 rain.The Δ33S isotopic trend across individual sedimentary layers, can be explained by variations in the hydrothermal flux as local volcanic activity waxed and waned. The marked global increase in Δ33S in sedimentary pyrites at ca. 2,650 Ma is attributed to the emergence of several cratons above sea level at that time, associated with a marked increase in sub-aerial felsic volcanism.

A metamorphic devolatilization model for the genesis of the Baiyun gold deposit in the North China Craton: A novel Fe-S isotopes perspective

The Mesozoic gold deposits in the North China Craton (NCC) were hosted by the Precambrian basement and Mesozoic intrusions. Thus, most researchers consider that these gold deposits were genetically linked to the Mesozoic intrusions. However, we suggest that a metamorphic devolatilization model provides an alternative based on a combined Fe and in-situ S isotopes study on auriferous pyrites from the Baiyun gold deposit in the NCC. The Triassic Baiyun gold deposit contains the quartz vein and altered rock ores that were developed in the Paleoproterozoic metavolcanic-sedimentary rocks (the Liaohe Group). Our in-situ S isotopic analyses show that pyrites from the quartz vein ores are characterized by negative δ34S values (-10.7 ∼ -5.5‰), while those from the altered rock ores have two distinct groups of δ34S values, one being positive (+13.5 ∼ +16.2‰) and the other negative (-10.6 ∼ -3.0‰). We suggest that pyrite grains with positive δ34S values should be relicts from the host rocks, because they show comparable δ34S values with those from the host rocks schists (+3.3 ∼ +16.1‰). Thus, only the negative δ34S values of pyrites in ores (-10.7 ∼ -3.0‰) and the Fe isotopes of the quartz vein ores (δ56Fe = +0.30 ∼ +0.48‰) can represent the isotopic characteristics of ore-forming fluids at Baiyun. Our study shows that the sulfur were probably from the pyritic volcanic-sedimentary sequences of the Liaohe Group, rather than from magmas. The calculated δ56Fe values of the ore-forming fluids (-0.78 ∼ -0.37‰; pyrite-fluid isotope fractionation) could be modelled in a metamorphic devolatilization model with Fe-species (pyrite&magnetite) of the Liaohe Group as sources. Therefore, our combined S- and Fe- isotope data indicate that the metamorphic devolatilization of the Liaohe Group could account for the genesis of the Baiyun gold deposit.

Silver enrichment and trace element deportment in hydrothermal replacement reactions: Perspective from the Nageng Ag-polymetallic deposit, East Kunlun Orogen, NW China

Mineral replacement reactions (dissolution-reprecipitation and solid-state diffusion) have been reported in various types of hydrothermal deposits. However, the metal transfer and partitioning between the parent and daughter minerals and the role of different replacement reactions in silver enrichment remains poorly understood. Here we present the petrographic textures, major and trace elements, and sulfur isotopes of different generations of pyrite and marcasite associated with replacement reactions from the Nageng epithermal Ag-polymetallic deposit, East Kunlun Orogen (EKO, NW China). The mineralization includes the pyrite-quartz (I), quartz-siderite-sulfides (II), quartz-fluorite-rhodochrosite-sulfides-sulfosalts (III), and quartz-calcite (IV) stage. This deposit is characterized by a metal zonation from deep-level Cu-Pb-Zn mineralization to shallow-level Ag-Pb-Zn mineralization, which correspond to main ore stages II and III, respectively. Stage II ores contain the colloform pyrite (Py1) and porous pyrite aggregate (Py2), both of which have higher Co, Ni, and W contents than stage III pyrite (Py3 and Py4) and marcasite (Mc1 and Mc2). In contrast, stage III pyrite/marcasite are enriched in Ag and other ore metals (e.g., Pb, Zn, Sb, Mn, and Sn). The sharp contact between Py3 and Py4/Mc1, together with the porosity in Py4/Mc1, suggests that Py3 and Py4/Mc1 represent the parent and daughter phase of dissolution and reprecipitation. The crystallographically-oriented and metallic inclusion-rich Mc1 has higher Ag concentrations (median 246 ppm) than Py3 (median 180 ppm), and commonly occurs in breccia-type ores related to hydraulic fracturing. This suggests that (i) the dissolution-reprecipitation reaction may have remobilized and concentrated silver and associated ore metals, and (ii) the fluid oxidation related to hydraulic fracturing may contribute to dissolution-reprecipitation reaction. However, the randomly-oriented Mc2 is porous and has homogeneous compositions. This, together with the prevailing occurrence with rhodochrosite in vein-type ores, indicates that Mc2 was transformed from Mc1 via Mn-rich fluid-driven solid-state diffusion under relatively mild physicochemical changes. Mc2 has the most homogeneous and highest Ag concentrations (avg. 1142 ppm) and other ore meals among all the studied Fe disulfides, suggesting that the marcasite has higher trace element compatibility than pyrite, and that the solid-state diffusion reactions promoted Ag enrichment more efficiently than dissolution-reprecipitation reactions. The mostly positive δ34S values (−1.09 to 8.50‰) of the Nageng pyrite and marcasite are consistent with a magmatic origin. The δ34S variations among different pyrite and marcasite generations are attributed to the fluctuations between episodic replenishment of magmatic fluids and subsequent oxidation in respond to hydraulic fracturing and mixing with meteoric water. Our findings reveal higher Ag compatibility of marcasite over pyrite, and that replacement reactions (especially solid-state diffusion) greatly promote ore metal enrichment.

Genesis of the sediment-hosted Haerdaban Zn-Pb deposit, Western Tianshan, NW China: Constraints from textural, compositional and sulfur isotope variations of sulfides

The sediment-hosted Haerdaban Zn-Pb deposit is a newly discovered Zn-Pb deposit in the Sailimu terrane, Western Tianshan (NW China), containing ca. 0.64 Mt of ore at 6.19 wt% Zn and 1.09 wt% Pb. The mineralization consists mainly of veins, breccias and semi-massive sulfide ores within the dolomitic limestone and calcareous slate. Using LA-ICP-MS, this study investigates trace element and sulfur isotope composition of sulfide to constrain the genesis of the deposit. Four pyrite types were identified by variations in texture and composition. Porous pyrite (Py1) shows a significant enrichment in Mn, Ag, Sb, Tl, Ba, Pb, Cu, and Zn, whereas the pyrite overgrowth (Py2) and euhedral pyrite (Py3) are enriched in Co and As, but depleted in all other trace elements. This compositional trend is attributed to the release of trace elements from pyrite during recrystallization from a porous to massive texture. Pyrite (Py4) associated with sphalerite is inclusion-rich, but exhibits a depletion in most trace elements, except Ni and Sb. The underlying carbonaceous sediments are interpreted as a source of Ni, Sb and other metals for the ore-forming fluids. Sphalerite displays a wide variety of colors from dark brown to pale yellow, primarily related to variations in Fe contents. The overall Fe contents (0.25 to 6.12 wt%), Cd contents (524 to 1875 ppm) and Zn/Cd ratios (320 to 1258) in sphalerite are compatible with those of SEDEX Zn-Pb deposit. The contents of other elements in sphalerite are generally low. Calculated temperatures using trace element contents in sphalerite (GGIMFis geothermometer) range from 163 °C to 309 °C, indicating progressive cooling of ore-forming fluid. Pyrite, sphalerite and galena display a wide range of δ34S values from +3.9 to +18.3‰. The association of lower positive δ34S values (+3.9 to +8.8‰) and epithermal suite elements (Ni, As, Sb, and Tl) enrichment in porous pyrite indicate that sulfur in early sulfides was derived from thermochemical sulfate reduction (TSR) with a possible magmatic input. The dominant population of heavier δ34S values (+13.8 to +18.3‰) of massive sulfide support TSR as the main source of sulfur. The intermediate δ34S values (+7.0 to +13.0‰) of vein sulfide are likely a mixing of TSR-derived sulfur with light sulfur leached from diagenetic pyrite. The current data support the classification of Haerdaban as a metamorphosed SEDEX Zn-Pb deposit, where the main mineralization was the product of remobilization and upgrading of early exhalative ores during hydrothermal events.

Mechanisms for invisible gold enrichment in the Liaodong Peninsula, NE China: In situ evidence from the Xiaotongjiapuzi deposit

Mechanisms for invisible gold enrichment hosted within Paleoproterozoic Fe-rich metamorphic rocks (marble, biotite schist, and quartz-feldspar schist) in the Liaodong Peninsula, NE China are still unclear. In this study, we choose the Xiaotongjiapuzi deposit as a case study, and present a detailed ore deposit geology and dataset of in situ chemical compositions of Fe-sulfides to address this issue. Fe-sulfides have the following paragenesis: metamorphic pyrite (Py0), first-ore stage pyrite (Py1, Py2), and a second-ore stage comprising arsenopyrite (Apy1, Apy2) and two phases of marcasite (Mas(a-b)). Early metamorphic pyrite (Py0) has relatively low concentrations of Au (<0.06–0.17 ppm). The significantly higher concentrations of Au in Py2 (1.55–423 ppm) and Apy2 (105–341 ppm) than those of Py1 (0.068–0.123 ppm) and Apy1 (2.90–7.70 ppm) suggest that gold prefers to incorporate into non-stoichiometric hydrothermal Fe-sulfides. Substitution of As1- and As3+ in Fe-sulfides may account for the coupled geochemical behavior between As and Au, Ag, Pb, Zn, Cu. Replacement led to fractured Py1 becoming enriched in Au, As, Ag, Cu, Sb and Pb, and Py0 being replaced by Au, As, Ag, Cu, Sb and Pb-rich marcasite. In situ sulfur analysis shows that Py0 has highly positive δ34S values (+13.62 to +18.55‰), suggesting it is associated with metasediments (+14 to +20‰). Py1, Py2, Apy1, and Apy2 are isotopically homogeneous and have δ34S values of +3.67 to +7.50‰, indicative of a magmatic phase. Both generations of marcasite, which replaced Py0, have yielded mixed sulfur isotopic signatures (δ34S values of +8.66 to +13.18‰). The gold mineralization in the Xiaotongjiapuzi gold deposit was the result of magmatic-hydrothermal processes. We propose two key factors: i) abundant defects in non-stoichiometric Fe-sulfides, and ii) replacement processes can cause invisible gold enrichment in Fe-sulfides in the Liaodong Peninsula. This provides a new perspective for invisible gold mineralization in other similar regions worldwide.

Multiple sulfur isotopes discriminate organoclastic and methane-based sulfate reduction by sub-seafloor pyrite formation

The marine sulfate inventory represents the largest standing pool of electron acceptors, which, via microbial sulfate reduction, is responsible for roughly half of the organic matter mineralization globally in marine sediments. In addition to the oxidation of buried organic matter, sulfate reduction can be coupled to the oxidation of methane migrating upward. Multiple sulfur isotope ratios were measured for porewater sulfate, sulfide, elemental sulfur and pyrite from core samples collected from continental shelf (19–96 m water depth) in the Baltic Sea and slope (1098 m water depth) in the Andaman Sea, in order to test if their isotope systematics can be used to trace the two different sulfate reduction processes. For shallow shelf sediments (<43 cm below seafloor), both δ34S and Δ33S values of porewater sulfate increased with increasing depth as a result of organoclastic sulfate reduction (OSR). However, the Δ33S values of both porewater sulfate and sulfide decreased as the δ34S values increased downcore at 43–73 cm depth where sulfate reduction was coupled to anaerobic oxidation of methane (AOM). Pyrite in shelf sediments yielded positive Δ33S values, reflecting Δ33S of porewater sulfide produced by OSR. In contrast, pyrite in slope sediments yielded negative Δ33S values as low as −0.15‰, as a result of mixing of sulfides derived from OSR and AOM. This is consistent with the slowly migrating sulfate–methane transition (SMT) in slope sediments inferred from sulfur and iron speciation, carbon isotope ratios of carbonates, and magnetic susceptibility data. A stable SMT, focusing AOM at roughly the same depth over the past few millennia, further provides favorable conditions for prolonged pyrite formation with the development of cubic crystals, while the sulfur isotope difference between bulk pyrite and macroscopic pyrite reflects the significance of OSR throughout the sediment column. We conclude that minor sulfur isotopes provide a unique proxy that can be applied to differentiate OSR and AOM and the origin of pyrite in diverse sediments and sedimentary rocks. This contribution highlights strong local diagenetic controls on pyrite formation and its multiple sulfur isotopic composition in various marine settings.

Kinetic factors control trace element and isotope zoning in Archean pyrite corona nodules

Pyrite corona nodules from the ∼2.7 Ga Kapai Slate, a thin, sulfidic carbonaceous shale horizon interbedded with basaltic lava flows in the Yilgarn Craton, Western Australia, have concentric compositional and isotopic zoning with distinctive textural differences between cores and mantles. The sieved-textured cores are enriched in highly compatible trace elements, incl. Ni, As, Ag, Te, Sb, Bi and Pb, and depleted in incompatible Mo and Tl, whereas the radiating-textured mantles are strongly depleted in compatible elements and enriched in incompatible elements, relative to the cores. A striking feature of the data is that both the compatible and incompatible elements are linearly correlated, with correlation coefficients as high as 0.99. A marked drop in the concentration of compatible elements and an increase in incompatible elements at the core-mantle boundary is attributed to a sudden change in the rate of growth of the nodules produced by eruption of the voluminous overlying Paringa Basalt. The weight of the basalt produced sudden compaction of the unconsolidated clays below resulting in upward advection of pore fluid, which thinned the boundary layer around the growing nodules, leading to a marked increase in the rate of pyrite growth. Rapid pyrite growth led to a dramatic depletion in highly compatible elements, and to a build-up in incompatible elements, in the boundary layers around the growing nodule mantles, which resulted in extreme depletion of compatible elements, and enrichment in incompatible elements in the nodule mantles, relative to the cores. The corona nodules are also isotopically zoned with cores that have higher δ34S, with small positive Δ33S values, and mantles that have lower δ34S and higher Δ33S. The increase in Δ33S towards the rims is attributed to S8 being advected to the growing mantles by upward fluid movement during sudden compaction, and the decrease in δ34S to the lighter S isotope, with its higher reactivity and diffusivity, being preferentially incorporated into the fast growing pyrite mantle.The extreme changes in the growth rates of the Kapai Slate corona pyrite nodules provide a new constraint on the partition coefficients of the trace elements between Archean ocean water and sedimentary pyrite. The compatibility of the analysed trace elements decreases in the order Bi > Te > Sb > Ag > Cu > Pb > Ni ≈ As > (Co, Zn, Se, Cd, Mn, W) > Tl > Mo, which is consistent with the order obtained from modern sedimentary pyrites by Large et al. (2014), except for the redox-sensitive elements Mn, Tl and Mo. These differences are attributed to the lower oxygen content of the Archean atmosphere and oceans.

Sulfur isotopic and trace‐element compositions of pyrite from the Zankan iron deposit, West Kunlun Orogenic Belt, China: Possible Early Cambrian banded iron formations

Abundant iron deposits characterized by stratified orebodies have recently been discovered in the Taxkorgan Terrane, West Kunlun Orogenic Belt, China. Of these deposits, the Zankan iron deposit was the first to be discovered and has the largest estimated tonnage. However, the iron mineralization types for this deposit and others in this area are uncertain, mainly because of the apparent conflict between their typical banded architecture and Cambrian ages. In this study, in situ sulfur isotopic and trace‐element compositions of pyrite, including that in orebodies coexisting with magnetite (Py(o)), pyrite veinlets in strata (Py(v)), and rhyolite and magnetite veins cross‐cutting rhyolite (Py(r)), were conducted to ascertain the genesis of the Zankan deposit. Our results reveal that the pyrite in Zankan deposit has marked positive δ34S values (17.20‰–29.86‰). In addition, Py(o) has low Co/Ni ratios, which are distinct to Py(r) and Py(v), meaning that they belong to typical marine chemical precipitates and magmatic–hydrothermal pyrite, respectively. The coexistence of pyrite and magnetite suggests that the Zankan iron deposit was precipitated in a euxinic basin. Together, our results suggest that seawater sulfate concentrations were low, and the scale and sulfur isotopic compositions of this euxinic basin were most likely dominated by changes in sea level eustacy, which controlled the proportions of seawater and riverine inputs. Comparison with typical Precambrian banded iron formations (BIFs) suggests that the Zankan iron deposit belongs to a unique class of Cambrian BIFs that were controlled predominantly by basins in a shelf environment with locally anoxic conditions.

Fluid origin and evolution of the Ruanjiawan W-Cu-(Mo) deposit from the Edong District in the Middle-Lower Yangtze River metallogenic belt of China: Constraints from fluid inclusions and H-O-C-S isotopes

Ruanjiawan W-Cu-(Mo) deposit, located in the Edong District of the Middle-Lower Yangtze River metallogenic belt, is one of the largest W deposits (40348 tons WO3 at an average grade of 0.32 wt%) in this ore district. The orebodies mainly occur in the contact zone between the granitoid and Ordovician carbonate strata. A detailed field geology and petrographic observation indicates four stages of ore formation: (1) prograde skarn stage, (2) retrograde skarn stage, (3) quartz-sulfide stage, and (4) quartz-carbonate stage. The δ18O values of the ore fluids vary from 3.0‰ to 11.2‰, and the δD values vary from −88‰ to −105‰, suggesting a dominant magmatic water origin, with a small amount of meteoric water mixing during the late stages. The δ13C values of fluid inclusions vary from −15.0‰ to −7.1‰, also indicating a major magmatic carbon source but with some mixing of organic carbon component in the sedimentary strata that probably leached by circulated meteoric water. The sulfides show a large δ34S variation from −4.8‰ to +16.3‰, and the sulfur may have two different sources, with a majority magmatic source (with δ34S around 0), and some marine sulphate source (with high positive δ34S value) in sedimentary strata in the region leached by meteoric water. In the prograde skarn stage, garnet shows a fluid inclusion homogenization temperature above 517 °C. The estimated salinity of the vapour-liquid two-phase inclusions ranges from 15.7 to 21.4 wt% NaCl equiv., and the salinity of the daughter mineral-bearing inclusions ranges from 39.8 to 57.1 wt% NaCl equiv. In the retrograde skarn stage, the fluid inclusions in epidote show a homogenization temperature of 385 °C to 463 °C, and the salinity estimated from the vapour-liquid two-phase inclusions ranges from 9.7 to 16.4 wt% NaCl equiv. The fluid inclusions in coarse-grained garnet from the retrograde skarn stage show similar homogenization temperature of 339 °C to 398 °C, and similar salinity of vapour-liquid two-phase inclusions of 7.4 to 15.3 wt% NaCl equiv., however, the salinity estimated from the daughter mineral-bearing inclusions ranges from 35.2 to 42.0 wt% NaCl equiv. In the quartz-sulfide stage, fluid inclusions in quartz show a homogenization temperature of 274 °C to 344 °C. The salinity varies from 6.5 to 14.0 wt% NaCl equiv. for the vapour-liquid two-phase inclusions, from 5.1 to 8.0 wt% NaCl equiv. for the CO2-bearing three phase inclusions, and from 35.3 to 39.8 wt% NaCl equiv. for the daughter-mineral-bearing inclusions. In the quartz-carbonate stage, the fluid inclusions have homogenization temperatures from 137 °C to 261 °C, and the salinity for the vapour-liquid two-phase inclusions shows a range from 3.9 to 1.0 wt% NaCl equiv. Fluid boiling and water-rock reaction in the retrograde skarn stage lead to the first precipitation of scheelite. The precipitation of scheelite, chalcopyrite and molybdenite in the quartz-sulfide stage was mostly triggered by the mixing of meteoric water and magmatic fluids, and fluid immiscibility also plays an important role. According to petrographic evidence and all available geological and geochemical data, we conclude that the Ruanjiawan deposit is an oxidized skarn-type W-Cu-(Mo) deposit.

Sulphur isotopes (δ34S and Δ33S) in sulphides from cratonic mantle eclogites: A glimpse of volatile cycling in ancient subduction zones

Multiple sulphur isotopic compositions of sulphides from Kaapvaal craton mantle eclogites allow to elucidate the recycling of sulphur into the deep Earth and to differentiate between recycled crust and mantle origins of eclogite-hosted sulphides, including the precious metals that they capture. We present multiple sulphur isotope ratio measurements by secondary ion mass spectrometry for sulphides from a collection of mantle-derived eclogite xenoliths from Proterozoic and Mesozoic kimberlite occurrences in South Africa (Premier, Roberts Victor, Jagersfontein). Previous work established that the host eclogites have elemental and oxygen isotopic compositions in support of seawater-altered oceanic lithosphere protoliths, and for many of these xenolith suites Archean ages have been suggested.The eclogite-hosted sulphides have δ34S values from −5.7 to +29‰, with the upper end of this wide range representing the highest-ever recorded δ34S composition of material derived from the Earth's mantle. The Δ33S values range from −0.29 to +0.18‰ and do not record significant mass-independent sulphur isotope fractionation, i.e., there is no compelling S-MIF signature. Most of the sulphide grains have δ34S values that fall within a range between −6 and +4‰, and they probably retain an isotopic record of sulphides that formed originally within altered oceanic crust. In contrast, the highly positive δ34S values from +13 to +29‰ detected in sulphide grains from a single eclogite xenolith are similar to those of marine sulphates, which were probably a minor sulphur component of the oceanic crustal protolith. The lack of a significant S-MIF signature in the eclogitic sulphides that show δ34S evidence for a recycled crust origin implies that this sulphur component stems from a <2.4Ga post-Archean surficial reservoir. This finding suggests that the cratonic mantle eclogites may have formed from post-Archean oceanic crust (e.g., Paleoproterozoic eclogite protoliths), or – as is preferred here – the ‘surficial’ sulphur was introduced into the cratonic root during relatively young metasomatic events and is thus unrelated to eclogite petrogenesis and Archean continent formation.

The Paleoarchean Northern Mundo Novo Greenstone Belt, São Francisco Craton: Geochemistry, U–Pb–Hf–O in zircon and pyrite δ34S-Δ33S-Δ36S signatures

Greenstone belts contain several clues about the evolutionary history of primitive Earth. Here, we describe the volcano-sedimentary rock association exposed along the eastern margin of the Gavião Block, named the Northern Mundo Novo Greenstone Belt (N-MNGB), and present data collected with different techniques, including U–Pb–Hf–O isotopes of zircon and multiple sulfur isotopes (32S, 33S, 34S, and 36S) of pyrite from this supracrustal sequence. A pillowed metabasalt situated in the upper section of the N-MNGB is 3337 ± 25 Ma old and has zircon with εHf(t) = −2.47 to −1.40, Hf model ages between 3.75 Ga and 3.82 Ga, and δ18O = +3.6‰ to +7.3‰. These isotopic data, together with compiled whole-rock trace element data, suggest that the mafic metavolcanic rocks formed in a subduction-related setting, likely a back-arc basin juxtaposed to a continental arc. In this context, the magma interacted with older Eoarchean crustal components from the Gavião Block. Detrital zircons from the overlying quartzites of the Jacobina Group are sourced from Paleoarchean rocks, in accordance with previous studies, yielding a maximum depositional age of 3353 ± 22 Ma. These detrital zircons have εHf(t) = −5.40 to −0.84, Hf model ages between 3.66 Ga and 4.30 Ga, and δ18O = +4.8‰ to +6.4‰. The pyrite multiple sulfur isotope investigation of the 3.3 Ga supracrustal rocks from the N-MNGB enabled a further understanding of Paleoarchean sulfur cycling. The samples have diverse isotopic compositions that indicate sulfur sourced from distinct reservoirs. Significantly, they preserve the signal of the anoxic Archean atmosphere, expressed by MIF-S signatures (Δ33S between −1.3‰ to +1.4‰) and a Δ36S/Δ33S slope of −0.81 that is indistinguishable from the so-called Archean array. A BIF sample has a magmatic origin of sulfur, as indicated by the limited δ34S range (0 to +2‰), Δ33S ~ 0‰, and Δ36S ~ 0‰. A carbonaceous schist shows positive δ34S (2.1‰–3.5‰) and elevated Δ33S (1.2‰–1.4‰) values, with corresponding negative Δ36S between −1.2‰ to −0.2‰, which resemble the isotopic composition of Archean black shales and suggest a source from the photolytic reduction of elemental sulfur. The pillowed metabasalt displays heterogeneous δ34S, Δ33S, and Δ36S signatures that reflect assimilation of both magmatic sulfur and photolytic sulfate during hydrothermal seafloor alteration. Lastly, pyrite in a massive sulfide lens is isotopically similar to barite of several Paleoarchean deposits worldwide, which might indicate mass dependent sulfur processing from a global and well-mixed sulfate reservoir at this time.

Extreme sulfur isotope fractionation of hydrothermal auriferous pyrites from the SW fringe of the Taupo Volcanic Zone, New Zealand: Implications for epithermal gold exploration

The Taupo Volcanic Zone and its environs, New Zealand, represent an ideal place for epithermal gold exploration. We have documented structurally-bound gold in low temperature hydrothermal pyrite veins from the Erua Basin area on the outside fringe of the Taupo Volcanic Zone. Extreme 34S/32S fractionation of 122.2‰ from auriferous pyrite is documented in hydrothermal veins hosted in alluvial deposits from the Erua Basin. We document three generations of pyrite based on petrographic and Nano-SIMS microstructural analysis. First generation pyrite (Py assemblage -1) has extremely high δ34S values of +70.3 to +100.0‰ probably resulting from sequential-cyclic bacterial reduction and Rayleigh fractionation of limited sulfate. Second generation pyrite (Py assemblage -2) has relatively lower δ34S values of -39.8 to -3.3‰, interpreted to reflect the influx of magmatic-related oxidized fluids mixed with remobilized, reduced sedimentary sulfides that then formed the host calcite-pyrite veins. Third generation pyrite (Py assemblage -3) incorporated much heavier 34S resulting in positive δ34S values ranging from +6.8 to +72.2‰, indicating greater reduction of sulfate to H2S in an anoxic (reducing) environment. One single composite ~ 10 um pyrite grain encompassing all three types of subgrains has a large δ34S range from -22.2‰ along its rim and +100.0‰ from its core (122.2‰ for a single grain, and 139.8‰ for the system). This is the largest known variation in δ34S ever reported in a single pyrite grain from any system on Earth, but is comparable to δ34S ranges in pyrite systems of up to 186‰ reported from microbial sulfate reduction in deep bedrock fracture systems in Sweden. The growth stages from Py-2 to Py-3 indicate a transformation of the environment from an open to a closed (highly reducing) system, and it is these late-stage vein pyrites that are slightly auriferous. This process is interpreted to be induced by the precipitation of sulfides perhaps near the margin of the hydrothermal system or closed growth of pyrite in hydrothermal veins, which both restrict circulation and convection of sulfate within the veins. Our results suggest that the Erua Basin may sit on the fringe of a low-temperature epithermal gold system; other “low-temperature” areas around the TVZ may also preserve similar, potentially enriched zones on the edge of an epithermal Au belt.

Sources of auriferous fluids associated with a Neoarchean BIF-hosted orogenic gold deposit revealed by the multiple sulfur isotopic compositions of zoned pyrites

Internal textures, multiple sulfur isotopic compositions, and contents of gold, selenium, and molybdenum of ore-related pyrites from a Neoarchean carbonate-facies BIF-hosted gold deposit in Quadrilatero Ferrifero were investigated to elucidate the source(s) of sulfur. Sodium hypochlorite etching and BSE imaging revealed pervasive zoning in pyrites. Five different growth zones (Py1a, Py1b, Py2, Py3, and Py4) and six types of zoning (Type A–Type F) were identified. Two pyrite generations were distinguished in ores: G1 (Py1a and Py1b) and G2 (Py2, Py3, Py4). Both G1 and G2 have positive Δ33S, but the magnitudes of G1 are higher. The G1-relevant fluids can be shallow-sourced, whereas the G2-related auriferous fluids are most likely deep-sourced metamorphic fluids derived from devolatilization of the lower succession of the Nova Lima Group (metavolcanics and metasedimentary rocks) during metamorphism, with fluid–rock interactions during fluid ascent and at the depositional site. The negative δ34S, higher selenium contents, and carbonates inclusions of Py1a contrast with the positive δ34S, lower selenium contents, and carbonaceous material inclusions of Py2, Py3, and Py4. The Lamego system possibly started with shallow-sourced oxidized and low-gold fluids (Py1a), closely followed by mixing in of deep-seated reduced auriferous fluids (Py2, Py3, Py4) initiated by tectonic activities. The oscillatory zoning of Py2 with the highest gold contents consists of alternating gold-rich and gold-poor laminae, indicating that fault-valve activity is a trigger of gold deposition. The sulfidation of siderite in BIF (desulfidation of auriferous fluids) related to the formation of ore-related pyrites also contributed to gold precipitation.

Sulfur and lead isotopic compositions of ore sulfides and mining economic potential of the High Atlas Mississippi Valley-type ore province, Morocco

The Moroccan High Atlas metallogenic province hosts numerous PbZn (Ba-Sr-Fe) ore deposits hosted in the Jurassic carbonates. The high Atlas basin was formed during the Triassic-Jurassic rifting and underwent a Cenozoic tectonic inversion during the Alpine orogeny. This work presents a study of Pb and S isotopes of ore sulfides (galena, sphalerite, pyrite) from the Central and Oriental High Atlas of Morocco (CHA, OHA). The investigated ore deposits are Mississippi Valley-type (MVT) ore deposits with a simple paragenesis consisting of galena and sphalerite with subordinate barite, celestine, and pyrite. These ore deposits belong to three geographic zones; the northern border zone (NZ), the southern border (SZ), the central zone also called the diapiric zone (CZ), and the western zone (WZ) in the High Atlas. Based on the sulfur isotopic data, we suggest the Triassic-Jurassic sulfates and/or coeval pore water sulfates as the source of sulfur. Sulfur isotope compositions (δ34S = −18.6 to 22.2‰) are regionally zoned with isotopically heavy δ34S (14.3‰) in the center of the basin, negative δ34S values at the northern border (−16.9‰) and western border (−9.1‰), and intermediate δ34S values (4.4‰) at the southern border. This δ34S zonation suggests different sulfate reduction mechanisms; a low-temperature bacterial sulfate reduction (BSR) that induced the negative δ34S values and a high-temperature thermochemical sulfate reduction (TSR) that yielded positive δ34S values. The intermediate sulfur isotopic composition reflects the mixing of the TSR- derived sulfur and the bacteriogenic sulfur. The western part of the High Atlas near the Precambrian-Paleozoic paleohigh is characteristically more radiogenic (206Pb/204Pb = 18.709 207Pb/204Pb = 15.658 and 208Pb/204Pb = 38.808) than the rest of the basin (206Pb/204Pb = 18.151 to 18.369, 207Pb/204Pb = 15.631 to 15.668 and 208Pb/204Pb = 38.433 to 38.555). The Paleozoic rocks are the primary source of lead and by inference other metals. The metallogenic zone CZ with heavy sulfur isotopic composition and abundant Triassic evaporites (diapiric zone) constitutes a potential zone for future ore exploration. The SZ is also a promising target for ore exploration since it hosts ore sulfides that contain the thermally reduced sulfur (TSR).