Jurassic microbial communities in hydrothermal manganese crust of the Rifian Calcareous Chain,Northern Morocco

The stratigraphic record of the Middle and Upper Jurassic in the Western Tethys is characterized by successive eustatic and tectonic events recorded as stratigraphic unconformities, which are revealed by hardgrounds, palaeokarsts, palaeosoils, and by the deposition of Fe–Mn crusts. The study of a Mn crust from the Middle-Upper Jurassic discontinuity in the Jbel Moussa Group (Rifian Cordillera), from stratigraphic, geomicrobiologic, mineralogical, and geochemical standpoints allows us to establish its hydrothermal origin. The manganese crust is composed by Ca-birnessite, cryptomelane and coronadite. Major- and trace-elements analyses of the whole crust show high contents in MnO (> 70 wt.%), a negative Ce anomaly and a positive Eu anomaly. Analysis of the microstructures under scanning electron microscopy reveals crystalline and microbial laminations, probably owing to fungal mycelium. Mineralogical and geochemical composition, together with microbial structures, suggest that this Mn crust formed as a result of venting hydrothermal fluids through synsedimentary faults. Chemosynthetic microbes were probably involved in the precipitation of Mn.     

Further evaluation of the Re-Os geochronometer in organic-rich sedimentary rocks: a test of hydrocarbon maturation effects in the Exshaw Formation, Western Canada Sedimentary Basin

Re-Os isotopic analyses of a single organic-rich sedimentary rock unit (ORS) of known depositional age, and at three levels of regional hydrocarbon maturity, show that hydrocarbon maturation does not affect the ability the ¹⁸⁷Re-¹⁸⁷Os chronometer to yield a depositional age for such rocks. We present Re-Os isotope analyses from the Late Devonian Exshaw Formation in the subsurface of the Western Canada Sedimentary Basin, Alberta, and obtain a Re-Os isochron age of 358 ± 10 Ma (2σ, Model 3, λ = 1.666 × 10⁻¹¹.a⁻¹) for samples ranging from hydrocarbon immature to overmature. This age is within uncertainty of the established absolute age for the Exshaw Formation. Hydrocarbon immature, and mature plus overmature samples show no significant age differences if regressed individually, indicating that hydrocarbon maturation did not greatly disturb the Re-Os isotope system in the Exshaw Formation. As such, we propose that the Re-Os geochronometer may be used as a reliable tool for measuring the depositional ages of ORS regardless of their level of hydrocarbon maturity. We find that minimizing natural variation in the initial ¹⁸⁷Os/¹⁸⁸Os ratio is more important than avoiding hydrocarbon maturation in obtaining precise Re-Os ages. In particular, the Exshaw Formation appears to contain a nonhydrogenous component of unradiogenic Os, in addition to the hydrogenous Os load. A subset of Exshaw Formation samples with >5% total organic carbon (TOC), which should best reflect the hydrogenous Os load alone, yields a very well-fitted isochron having a depositional age of 358 ± 9 Ma (2σ, λ = 1.666 × 10⁻¹¹.a⁻¹) with an initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.59 ± 0.05 (Model 3, Mean Square of Weighted Deviates (MSWD) = 1.8). The initial ¹⁸⁷Os/¹⁸⁸Os ratio of this regression may provide an estimate of the Os isotopic composition of local seawater at the time of deposition.     

Identification of last interglacial deposits in eastern Beringia: a cautionary note from the Palisades,interior Alaska

Last interglacial sediments in unglaciated Alaska and Yukon (eastern Beringia) are commonly identified by palaeoecological indicators and stratigraphic position ～2–5 m above the regionally prominent Old Crow tephra (124 ± 10 ka). We demonstrate that this approach can yield erroneous age assignments using data from a new exposure at the Palisades, a site in interior Alaska with numerous exposures of last interglacial sediments. Tephrochronology, stratigraphy, plant macrofossils, pollen and fossil insects from a prominent wood‐rich organic silt unit are all consistent with a last interglacial age assignment. However, six ¹⁴C dates on plant and insect macrofossils from the organic silt range from non‐finite to 4.0 ¹⁴C ka BP, indicating that the organic silt instead represents a Holocene deposit with a mixed‐age assemblage of organic material. In contrast, wood samples from presumed last interglacial organic‐rich sediments elsewhere at the Palisades, in a similar stratigraphic position with respect to Old Crow tephra, yield non‐finite ¹⁴C ages. Given that local permafrost thaw since the last interglaciation may facilitate reworking of older sediments into new stratigraphic positions, minimum constraining ages based on ¹⁴C dating or other methods should supplement age assignments for last interglacial sediments in eastern Beringia that are based on palaeoecology and stratigraphic association with Old Crow tephra. Copyright © 2011 John Wiley & Sons, Ltd.     

Sources of osmium to the modern oceans: new evidence from the Pt-Os system

High precision Os isotope analysis of young marine manganese nodules indicate that whereas the composition of modern seawater is radiogenic with respect to ¹⁸⁷Os/¹⁸⁸Os, it has ¹⁸⁶Os/¹⁸⁸Os that is within uncertainty of the chondritic value. Marine Mn nodule compositions thus indicate that the average continental source of Os to modern seawater had long-term high Re/Os compared to Pt/Os. Analyses of loess and freshwater Mn nodules support existing evidence that average upper continental crust (UCC) has resolvably suprachondritic ¹⁸⁶Os/¹⁸⁸Os, as well as radiogenic ¹⁸⁷Os/¹⁸⁸Os. Modeling the composition of seawater as a two-component mixture of oceanic/cosmic Os with chondritic Os compositions and continentally-derived Os demonstrates that, insofar as estimates for the composition of average UCC are accurate, congruently weathered average UCC cannot be the sole continental source of Os to seawater. Our analysis of four Cambrian black shales confirm that organic-rich sediments can have ¹⁸⁷Os/¹⁸⁸Os ratios that are much higher than average UCC, but ¹⁸⁶Os/¹⁸⁸Os compositions that are generally between those of chondrites and average-UCC. Preferential weathering of black shales can result in dissolved Os discharged to the ocean basins that has a much lower ¹⁸⁶Os/¹⁸⁸Os than does average upper crust. Modeling the available data demonstrates that augmentation of estimated average UCC compositions with less than 0.1% additional black shale and 1.4% additional ultramafic rock can produce a continental end-member Os isotopic composition that satisfies the requirements imposed by the marine Mn nodule data. The interplay of these two sources provides a mechanism by which the ¹⁸⁷Os/¹⁸⁸Os of seawater can change as sources and weathering conditions change, yet seawater ¹⁸⁶Os/¹⁸⁸Os varies only minimally.     

Extreme positive Ce-anomalies in a 3.0 Ga submarine volcanic sequence,Murchison Province: Oxygenated marine bottom waters

Systematic positive anomalies of Ce, where Ce/Ce* spans 2.1 to 11.4, are present in basalts and rhyolites of a 2.96 Ga submarine volcanic sequence of the Murchison Province, Western Australia. This volcanic sequence is host to a stratabound Cu–Zn deposit that formed on the seafloor from a seawater hydrothermal system. These are true Ce anomalies as Pr/Pr* < 1. In modern oxygenated marine water Ce is sequestered by Mn-oxides and hydroxides, which coprecipitate with Fe-oxides and hydroxides as nodules and crusts on the ocean floor, as well as Fe–Mn chemical sediments from hydrothermal systems at ocean spreading centers. Fe–Mn sediments have positive Ce anomalies and marine water complementary negative anomalies. Such Ce anomalies have not formerly been reported for Archean hydrothermally altered volcanic rocks. These extreme anomalies are attributed to Mn-transport in shallow-circulating oxygenated marine bottom waters peripheral to the deeper, hotter, hydrothermal system from which the Cu–Zn deposit formed, and record an oxygenated marine environment ~ 500 Ma before the so-called great oxidation event at ~ 2.4 Ga. Results for positive Ce anomalies in the Golden Grove volcanic sequence are complementary to negative anomalies in Archean BIF, collectively stemming from particulate scavenging of Ce^+ 3 in an oxic water column.     

Rare elements and Nd and Sr isotopic composition in micronodules from the Brazil Basin,Atlantic Ocean

The Sr isotope stratigraphy of the biogenic apatite was used to determine the age of pelagic sediments in the Brazil Basin (Station 1541) that contain ferromanganese micronodules, nodules, and coatings on the weathered volcanic rocks. The age of sediments at horizons 0–5 and 86–90 cm was estimated at 24.1 ± 0.2 Ma and 24.8 ± 0.2 Ma, respectively. The average sedimentation rate in the Late Oligocene was about 13 mm/ka. The hydrogenous Fe–Mn nodule on the sediment surface with the Mn/Fe value of 1.05–1.95 was formed at a rate of 1.2–2.4 mm/Ma, which is 1000 times lower than the growth rate of buried nodule (Mn/Fe 0.4) at depth of 83 cm. Diagenesis provoked changes in the mineral composition of the buried nodule (asbolane-buserite partially replaced by goethite), leading to the loss of a part of Mn, Ni, Li, and Tl but accumulation of trace elements linked with iron oxyhydroxides (Ce, Th, Be, As, and V) were retained. The composition of manganese micronodules at two studied depths in sediments evolved in the course of two stages of ore formation: related to the oxic and suboxic diagenesis. The Sr isotopic composition in manganese micronodules from both horizons do not differ from that of dissolved Sr in the ocean water. The ¹⁴³Nd/¹⁴⁴Nd ratio, which reflects the Nd isotopic composition in the paleocean during the micronodule formation, varies in manganese micronodules from different horizons and is constant in different size fractions.     

大西洋洋中脊TAG热液区中块状硫化物的Os同位素研究

新测得TAG热液区中5件海底块状硫化物样品的锇含量及其同位素组成,187Os/186Os比值在2.305～7.879之间,均值为5.986,介于现代海水和上部洋壳岩石的锇同位素组成之间,表明该区海底块状硫化物中锇是海水和上部洋壳来源锇混合的产物.在海底热液循环过程中,海水的混入对该区热液流体的Os浓度及其同位素组成产生了明显的影响。     

Re–Os pseudo-isochron of disseminated ore from the Kalatongke Cu–Ni sulfide deposit,Xinjiang, Northwest China: Implications for Re–Os dating of magmatic Cu–Ni sulfide deposits

Re–Os dating of disseminated ore from the Kalatongke Cu–Ni sulfide mineral deposit, Xinjiang, Northwest (NW) China, yields an apparent isochron age of 433 ± 31 Ma with an apparent initial ¹⁸⁷Os/¹⁸⁸Os (433 Ma) ratio of 0.197 ± 0.027. This apparent age is older than not only the zircon U–Pb age of the host intrusion (287 ± 5 Ma, Han et al., 2004) but also the stratigraphic age of the intruded country rock. Thus, the regression line is a pseudo-isochron. However, previous Re–Os dating of massive ores of the same deposit yielded an age that is consistent, within analytical uncertainty, with the zircon U–Pb age (Zhang et al., 2008). This relationship is similar to that observed in the Jinchuan deposit, NW China. Therefore, we suggested that the same mechanism, post-segregation diffusion of Os (Yang et al., 2008), is applicable to the Kalatongke deposit.Re–Os isotopic studies of Kalatongke, Jinchuan and representative magmatic Cu–Ni sulfide deposits suggest that the massive ores of mafic–ultramafic-rock-associated Cu–Ni sulfide deposits would yield geologically meaningful Re–Os age, whereas a pseudo-isochron would be obtained for the disseminated ores. Therefore, to obtain a geologically meaningful Re–Os age, the type of the deposit, the type of the ore and the ore-forming process should be taken into account.     

Continuous growth of hydrogenetic ferromanganese crusts since 17 Myr ago on Takuyo-Daigo Seamount,NW Pacific,at water depths of 800–5500 m

Ferromanganese crusts cover all outcrops on Takuyo-Daigo seamount traversed during remotely operated underwater vehicle (ROV) dives, except in places covered by foraminifera sand. Takuyo-Daigo is a Cretaceous seamount located in the northwest Pacific Ocean. Geological and bathymetric mapping provide the framework for this study. Chemical and mineralogical analyses of the hydrogenetic ferromanganese crusts show temporal and spatial variations typical of those found in previous studies. Outcrops from 800 to 5500 m water depths are covered with ferromanganese crusts up to 105 mm thick. Beryllium isotope dating shows that the crusts have apparently been growing continuously at all water depths, even through the modern oxygen minimum zone (OMZ), contrary to some earlier models for deposition. Growth rates vary from 2.3 to 3.5 mm/Myr, with Fe or Mn fluxes of 0.07–0.11 g/cm²/Myr since the early-middle Miocene. Co/Mn ratios decrease with water depth while Fe/Mn and other metallic elements increase or show no change, based on the analysis of the uppermost crust surface. This is probably because Co is the most abundant redox-sensitive element derived from seawater that occurs in crusts.     

Re–Os isotopic constraints on the source of platinum-group minerals (PGMs) from the Vestřev pyrope-rich garnet placer deposit,Bohemian Massif

Mineralogical studies of the heavy fraction from a Holocene pyrope-rich garnet placer deposit at Vestřev (Krkonoše Piedmont Basin, Bohemian Massif) have identified the presence of very rare grains of platinum group minerals (PGM). Pt–Fe alloy grains are accompanied by Os–Ir–Ru minerals (native osmium, iridium, and ruthenium) with inclusions of Pt–Fe alloy and hongshiite (PtCu). This mineral assemblage is typical for several mantle settings including ophiolites. The chemistry of the Os–Ir–Ru minerals shows an enrichment of the PGM in Ru, which is typical of ophiolites. The grain morphology of PGM and pyrope-rich garnet (mostly rounded with numerous euhedral/subhedral grains) does not exclude a common source. In-situ laser-ablation MC-ICP-MS was used to measure the Re–Os isotopic compositions of single Os-rich grains, which show heterogeneous subchondritic Os isotopic compositions (¹⁸⁷Os/¹⁸⁸Os = 0.12082–0.12505 ± 0.00003). This precludes their low-temperature origin and indicates derivation of platinum-group elements (PGEs) essentially from mantle-derived rocks without a significant contribution of crustal Os. The mantle model age (T_MA) and Re-depletion model age (T_RD) model ages range from ~ 0.4 to ~ 1.0 Ga and most likely reflect a long history of melt depletion that affected the mantle sources of PGM.     

High‐Precision Measurement of 187Os/188Os Isotope Ratios of Nanogram to Picogram Amounts of Os in Geological Samples by N‐TIMS using Faraday Cups Equipped with 1013 Ω Amplifiers

N(¹⁸⁷Os)/N(¹⁸⁸Os) ratios of six geological reference materials were measured using static Faraday cups (FCs) with 10¹³ Ω amplifiers by N‐TIMS. Our results show that the repeatability precision was 2–3‰ (2 RSD, n = 3), when taking ~ 1 g of BHVO‐2 with 76 pg g^?1 of Os mass fraction and ~ 2 g of BCR‐2 with 21 pg g^?1 of Os mass fraction for each sample, whether measured by FCs or by secondary electron multiplier. The repeatability precision measured by FCs was 1–0.2‰ (2 RSD, n = 3) when taking ~ 1 g of BIR‐2 with 350 pg g^?1 of Os mass fraction, ~ 1 g of WGB‐1 with 493 pg g^?1 of Os mass fraction or ~ 0.5 g of WPR‐1 with 13.3 ng g^?1 of Os mass fraction for each sample, which is much better than those measured by secondary electron multiplier. Instead, when taking ~ 2 g of AGV‐2 with 4 pg g^?1 Os mass fraction, the repeatability precision measured by secondary electron multiplier is 3–4‰ (RSD, n = 3), which is better than those measured by FCs. Of the six reference materials analysed, WPR‐1 and BIR‐1a are the most homogeneous with regard to Os isotopic composition (2 RSD of 0.08% and 0.23%, respectively) when test portion masses are 0.5–1 g.     

Re–Os dating of sulphides from the Yushui Cu-polymetallic deposit in eastern Guangdong Province,South China

The Yushui Cu-polymetallic deposit, which is associated with Ag, Pb, and Zn, is located in the middle part of the Yongan–Meixian Late Paleozoic Hercynian depression. It was discovered in eastern Guangdong Province in the late 1980s and is one of the richest copper deposits in China with high-grade copper averaging 3.25% and locally reaching 50–60%. The main ore body is located along the unconformity between the Upper Carboniferous Hutian Group limestone and the Lower Carboniferous Zhongxin Formation quartz sandstone with a bedded and lenticular morphology. The ores exhibit massive textures dominated by chalcopyrite, bornite, chalcocite, pyrite, sphalerite, galena, and a trace amount of argentite. Although researchers began studying the Yushui deposit in the early 1990s, the ore genesis remains controversial because of the lack of precise mineralisation age constraints. In this study, direct Re–Os dating of Cu sulphides aided in facilitating a better understanding of the timing of formation of the Yushui deposit. This study is the first attempt to use the Re–Os isotopic system for directly dating chalcopyrite and bornite ores for the Yushui deposit. The contents of Re, common Os, ¹⁸⁷Re and ¹⁸⁷Os in nine sulphides are 1.68–219.35 ppb, 0.003–0.427 ppb, 1.05–137.31 ppb, and 0.045–0.734 ppb, respectively. The isotope data yielded an isochron age of 308 ± 15 Ma (mean square weighted deviates = 2.4) using the ⁸⁷Re/¹⁸⁸Os–¹⁸⁷Os/¹⁸⁸Os plot, which is interpreted to represent the age of formation for these sulphides, suggesting that the mineralisation age of the Yushui deposit is close to the age of the host rocks. The ¹⁸⁷Os/¹⁸⁸Os initial value obtained from the Re–Os isochron is 1.81 ± 0.34, which corresponds to the γOs value of + 1349. This value indicates that the ore-forming materials were derived from the crust without mixing with materials from the mantle, and that the Yushui massive sulphide deposit may be of sedimentary exhalative origin.     

Geochronology and geochemical properties of the large-scale graphite mineralization associated with the Huangyangshan alkaline pluton,Eastern Junggar,Xinjiang, NW China

The Huangyangshan alkaline pluton is located within the southern part of the Eastern Junggar orogenic belt in Xinjiang Province, and forms part of the Kalamaili alkaline granite belt. The pluton hosts the Huangyangshan super-large graphite deposit, which develops unique spherical structure and coexists with metal sulfides. This study examines the genetic relationship between the alkaline magmatism that formed the pluton and the graphite mineralization using zircon LA–ICP–MS U–Pb dating, geochemical analysis for representative rock types in the Huangyangshan pluton, and new Re–Os isotope dating for the graphite in the Huangyangshan graphite deposit. Zircons from medium-grained arfvedsonite granite, medium–fine-grained amphibole granite, medium-grained biotite granite, and fine-grained biotite granite phases of the Huangyangshan pluton yield weighted mean U–Pb ages of 322.7 ± 4.5, 318.3 ± 4.0, 303.9 ± 2.1, and 301.1 ± 3.6 Ma, respectively, indicating that all of the granite phases were emplaced during the Late Carboniferous over a period of around 20 Myr. Six graphite samples from the deposit yield a Re–Os isochron age of 332 ± 53 Ma. Combining these ages with the genetic relationship between the graphite mineralization and magmatism in the study area and the relatively large uncertainty on the Re–Os isochron age for the graphite suggests that the mineralization formed at ca. 320 Ma. The graphite samples yield an initial ¹⁸⁷Os/¹⁸⁸Os value of 0.38 ± 0.2, indicative of carbon derived from a mixture of organic and mantle-derived sources. The different granite phases in the Huangyangshan pluton are geochemically similar with relatively high SiO₂ (75.6–78.2 wt%) and Na₂O + K₂O (8.01–9.04 wt%) and relatively low CaO (0.18–0.7 wt%), MgO (0.06%–0.13 wt%) and Fe₂O₃ (T_Fe2O3 = 1.08–2.06 wt%) contents. The granites are enriched in light rare earth elements (LREE), large-ion lithophile elements (LILEs) (e.g. Rb, Th, and K), and high field strength elements (HFSEs) (e.g. Zr and Hf), depleted in heavy rare earth elements (HREEs), and have negative Ba, Sr, P, Ti, and Eu anomalies. These geochemical characteristics are indicative of derivation from juvenile basaltic oceanic crustal materials in the lower crust. This suggests that the Huangyangshan pluton formed from magmas generated by partial melting caused by mantle-derived magma underplating, with the magmas then undergoing mixing, separation, and significant fractional crystallization. Diorite enclaves within the granites have weaker trace element anomalies that are indicative of magma mixing. In addition, the mantle-derived intermediate–basic end-member involved in the magma mixing is likely one of the important carriers of carbon and metal. In summary, the Late Carboniferous Huangyangshan pluton and its associated graphite mineralization formed in a post-collision extensional tectonic setting in the Kalamaili area.     

Geochemistry and U-series dating of Holocene and fossil marine hydrothermal manganese deposits from the Izu-Ogasawara arc

This study investigated Holocene and fossil hydrothermal manganese deposits in the Izu-Ogasawara arc. Mineralogically, these deposits comprise 10 Å and 7 Å manganate minerals, and the fossil samples showed higher 10 Å stabilities. Chemical compositions of the Holocene samples are typical of other hydrothermal manganese deposits, including low Fe/Mn ratios, low trace metals, and low rare earth elements. Although the fossil samples generally have similar chemical characteristics, they exhibit significant enrichment in Ni, Cu, Zn, Cd, Ba, REE, Tl, and Pb contents. Furthermore, the chondrite-normalized REE patterns showed more light REE enrichment trends. These chemical characteristics suggest post-depositional uptake of these metals from seawater. U-Th dating of a Holocene hydrothermal manganese deposit from the Kaikata Seamount indicated 8.8 ± 0.94 ka for the uppermost layer and downward growth beneath the seafloor with a growth rate of ca. 2 mm/kyr. This is approximately three orders of magnitude faster than that of hydrogenetic ferromanganese crusts. U-Pb age of a fossil hydrothermal manganese deposit from the Nishi-Jokyo Seamount showed 4.4 ± 1.6 Ma, which was contemporary with basaltic volcanism (5.8 ± 0.3 Ma). Hydrothermal manganese deposits contain high concentrations of high value Mn, but only small amounts of valuable minor metals; their ages constrain the periods of past hydrothermal activity and provide a vector to explore for polymetallic sulfide deposits.     

Secondary Ion Mass Spectrometry Bias on Isotope Ratios in Dolomite–Ankerite,Part II: δ13C Matrix Effects

This study is Part II of a series that documents the development of a suite of calibration reference materials for in situ SIMS analysis of stable isotope ratios in Ca‐Mg‐Fe carbonates. Part I explored the effects of Fe²⁺ substitution on SIMS δ¹⁸O bias measured from the dolomite–ankerite solid solution series [CaMg(CO₃)₂–CaFe(CO₃)₂], whereas this complementary work explores the compositional dependence of SIMS δ¹³C bias (calibrated range: Fe# = 0.004–0.789, where Fe# = molar Fe/(Mg+Fe)). Under routine operating conditions for carbonate δ¹³C analysis at WiscSIMS (CAMECA IMS 1280), the magnitude of instrumental bias increased exponentially by 2.5–5.5‰ (session‐specific) with increasing Fe‐content in the dolomite structure, but appeared insensitive to minor Mn substitution [< 2.6 mole % Mn/(Ca+Mg+Fe+Mn)]. The compositional dependence of bias (i.e., the matrix effect) was expressed using the Hill equation, yielding calibration residual values ≤ 0.3‰ relative to CRM NBS‐19 for eleven carbonate reference materials (6‐μm‐diameter spot size measurements). Based on the spot‐to‐spot repeatability of a drift monitor material that ‘bracketed’ each set of ten sample‐spot analyses, the analytical precision was ± 0.6–1.2‰ (2s, standard deviations). The analytical uncertainty for individual sample analyses was approximated by combining the precision and calibration residual values (propagated in quadrature), suggesting an uncertainty of ± 1.0–1.5‰ (2s).     

First Reliable Re–Os Ages of Pyrite and Stable Isotope Compositions of Fe(‐Cu) Deposits in the Hami Region,Eastern Tianshan Orogenic Belt,NW China

The Eastern Tianshan Orogenic Belt (ETOB) in NW China is composed of the Dananhu–Tousuquan arc belt, the Kanggurtag belt, the Aqishan–Yamansu belt and the Central Tianshan belt from north to south. These tectonic belts have formed through arc–continent or arc–arc collisions during the Paleozoic. A number of Fe(‐Cu) deposits in the Aqishan–Yamansu belt, including the Heifengshan, Shuangfengshan and Shaquanzi Fe(‐Cu) deposits, are associated with Carboniferous–Early Permian volcanic rocks and are composed of vein‐type magnetite ores. Metallic minerals are dominated by magnetite and pyrite, with minor chalcopyrite. Calcite, chlorite, and epidote are the dominant gangue minerals. Pyrite separates of ores from those three deposits have relatively high and variable Re contents ranging from 3.7 to 184 ppb. All pyrite separates have very low common Os, allowing us calculation of single mineral model ages for each sample. Pyrite separates from the Heifengshan Fe deposit have an ¹⁸⁷Re–¹⁸⁷Os isochron age of 310 ± 23 Ma (MSWD = 0.04) and a weighted mean model age of 302 ± 5 Ma (MSWD = 0.17). Those from the Shuangfengshan Fe deposit have an isochron age of 295 ± 7 Ma (MSWD = 0.28) and a weighted mean model age of 292 ± 5 Ma (MSWD = 0.33). The Shaquanzi Fe‐Cu deposit has pyrite with an isochron age of 295 ± 7 Ma (MSWD = 0.26) and a weighted mean model age of 295 ± 6 Ma (MSWD = 0.23). Pyrite separates from these Fe(‐Cu) deposits have δ³⁴S_CDT ranging from ?0.41‰ to 4.7‰ except for two outliers. Calcite from the Heifengshan Fe deposit and Shaquanzi Fe‐Cu deposit have similar C and O isotope compositions with δ¹³C_PDB and δ¹⁸O_SMOW ranging from ?5.5‰ to ?1.0‰ and from 10‰ to 12.7‰, respectively. These stable isotopic data suggest that S, C, and O are magmatic‐hydrothermal in origin. The association of low‐Ti magnetite and Fe/Cu‐sulfides resembles those of Iron–Oxide–Copper–Gold (IOCG) deposits elsewhere. Our reliable Re–Os ages of pyrite suggest that the Fe(‐Cu) deposits in the Aqishan–Yamansu belt formed at ～296 Ma, probably in a back‐arc extensional environment.     

A massif‐type (~1.86 Ga) anorthosite complex in the Yeongnam Massif,Korea: late‐orogenic emplacement associated with the mantle delamination in the North China Craton

To unravel the petrogenesis of a massif‐type anorthosite in terms of the crust‐mantle geodynamics, we dated zircons separated from six samples from the Sancheong‐Hadong (SH) complex, Korea, using a sensitive high‐resolution ion microprobe. The weighted mean ²⁰⁷Pb/²⁰⁶Pb age of two anorthosites is 1862 ± 2 Ma, whereas the ages of the hornblende gabbro and granitic gneiss are 1873 ± 4 Ma and 1875 ± 5 Ma respectively. Zircon rims from mafic granulite and migmatitic gneiss yielded ages of 1860 ± 5 Ma and 1858 ± 4 Ma, respectively, implying that the granulite facies metamorphism and anatexis are associated with anorthosite emplacement. Our results, together with available Re–Os data, are compatible with the ~1.9–1.86 Ga collisional orogeny prevalent in the North China Craton and the Korean Peninsula, and suggest that orogenesis was accompanied by mantle delamination beneath the craton. It is thus likely that the SH anorthositic rocks are a product of late‐orogenic magmatism during the post‐collisional extension‐dominated phase of orogeny.     

Rhenium–Osmium Isotope Measurements in Marine Shale Reference Material SBC‐1: Implications for Method Validation and Quality Control

In this study, the USGS black shale reference material SBC‐1 was investigated as a matrix‐matched reference material for both intra‐laboratory calibration and inter‐laboratory comparison of high‐precision Re‐Os dating for organic‐rich sedimentary rocks. This reference material was analysed for Re‐Os isotopic composition by three digestion protocols – inverse aqua regia, CrO₃‐H₂SO₄ and H₂O₂‐HNO₃. The results for SBC‐1 obtained by inverse aqua regia digestion yielded similar Re mass fractions but slightly (~ 5%) higher Os mass fractions and lower ¹⁸⁷Os/¹⁸⁸Os values than the CrO₃‐H₂SO₄ and H₂O₂‐HNO₃ digestions. The data set of inverse aqua regia digestion exhibited strong correlations in plots of ¹⁸⁷Os/¹⁸⁸Os vs. 1/¹⁹²Os and ¹⁸⁷Os/¹⁸⁸Os vs. ¹⁸⁷Re/¹⁸⁸Os, which may signify the incorporation of detrital Re and Os into organic matter in the Re‐Os system. Similar correlations were also observed for the CrO₃‐H₂SO₄ digestion data set, but not for that of H₂O₂‐HNO₃. The data indicate that there is an amount of non‐hydrogenous Os in SBC‐1 and that CrO₃‐H₂SO₄ and H₂O₂‐HNO₃ digestions would minimise liberation of the non‐hydrogenous Os component. We propose that SBC‐1 may be a more suitable reference material to monitor the influence of detrital Re and Os on Re‐Os isochron age data, especially for samples with less organic matter and more siliceous detritus.     

Pyrite Re-Os isotope systematics at the Zijinshan deposit of SW Fujian,China: Constraints on the timing and source of Cu-Au mineralization

Zijinshan is the largest high-sulfidation epithermal Cu–Au deposit on mainland China. The primary mineralization at the deposit is characterized by pyrite, digenite and covellite. Although some RbSr isochron ages of the fluid inclusions in quartz and the apparent K-Ar age of the Cu-bearing alunite alteration zone have been previously interpreted as the ore-forming age, the Rb-Sr and K-Ar dating systems are usually readily reset due to their closure temperature, and thus document the latest thermal event. In order to precisely determine the age of the Cu mineralization, eight-pyrite separates were Re-Os dated, and seven yielded an isochron age of 103 ± 4 Ma with an initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.45 ± 0.14. This date is interpreted as the age of Cu mineralization, which is in agreement with geochronological data from molybdenite and the porphyritic granodiorite that hosts Cu-Mo mineralization at Luoboling (located 2 km NE of Zijinshan), suggesting that these two deposits were formed during the same metallogenic event. The relatively low initial ¹⁸⁷Os/¹⁸⁸Os ratio indicates that the source of the Cu at Zijinshan is predominantly the crust with input from the mantle.     

Petrography,mineral chemistry,fluid inclusion microthermometry and Re–Os geochronology of the Küre volcanogenic massive sulfide deposit (Central Pontides,Northern Turkey)

The Re–Os isotopic system is applied for the first time to the sulfide ores and the overlying black-shales at the Küre volcanogenic massive sulfide deposit of the Central Pontides, Northern Turkey. The ore samples collected include predominantly pyrite, accompanied by chalcopyrite, sphalerite and other species. Massive ore is almost free of gangues, whereas the stockwork ore includes quartz and calcite gangue. The composition of sphalerite is similar to ancient and modern massive sulfide mineralizations globally. Microthermometric studies from quartz from the stockwork ore has shown two populations of two-phase fluid inclusions with vapor/liquid ratios between 4 to 28%, low to intermediate T_h (161.5–317.0 °C) and low salinities (0.9–5 wt.% NaCl equiv.) which are mostly in good agreement with the ranges for volcanogenic massive sulfide mineralizations. These studies also suggest a H₂O–CaCl₂–KCl–MgCl₂ ore-forming fluid system in a shallower subsurface near the seafloor vents. The Re–Os dating of the LLHR sulfides yield a nominal depositional age of upper Toarcian for the massive sulfide mineralization. Two largely different model ages obtained are attributed to other pyrite crystallization events prior to and postdating the main sulfide deposition. Some lower homogenization temperatures (< 200 °C) from the quartz of the stockwork may also similarly be related to the post-VMS events. It is concluded that a submarine volcanic extrusion episode has continued until upper Toarcian in the Küre Basin, when it has entered a stagnation period that allowed the discharge of hydrothermally circulated sulfide-laden fluids from the seafloor vents. This age data promotes the palaeotectonic models interpreting the Küre Basin as a Permian–early Jurassic marginal back-arc basin of the Devonian-Triassic Karakaya Ocean. The Re–Os data from the overlying black-shale provide a glimpse to the initial Os isotope ratio of the water column at the time of the sedimentation (0.45–0.46 for 180 Ma). The lack of common Os from the sulfides does not let us to infer a source of Os and initial ¹⁸⁷Os/¹⁸⁸Os ratios from the black shale are not statistically robust to make a significant deduction. A further detailed study on the isotopic composition of the black shale strata may help us to make an apporach to the Os source(s) in the deposition environment of the Küre VMS deposit.