Host Black Shale Research Articles

Origin and preservation mechanisms of organic matter in carbonate concretions from Lower Cambrian black shales in South China

Carbonate concretions are widely used in paleoclimate, paleoenvironmental, and paleontological studies, and even in the study of potential life on Mars. These petrological, elemental, isotopic, and lipid biomarker signals in Meso-Cenozoic carbonate concretions (relatively low thermal maturity) can effectively preserve details of seawater conditions and benthic ecosystems during the deposition of their host sediments/rocks. However, such research on Precambrian-Cambrian carbonate concretions under highly mature conditions remains scarce, and the ability of these ancient carbonate concretions to retain their original biogenic information remains uncertain. To achieve that, this study examines two Cambrian carbonate concretions, using samples from the center, transition, and rim of each, and their adjacent host black shales from the Lower Cambrian Qiongzhusi Formation in the Yangtze Block, South China. Organic and inorganic geochemical analyses were combined to elucidate the origin and preservation process of organic matter (OM) in these ancient Cambrian carbonate concretions. The results show that the thermal maturity of OM within these concretions (1.8 % EqVRo) is relatively low compared to their adjacent host shales (2.9 % EqVRo). Hopanes and steranes are detectable in both free and calcite-occluded hydrocarbons within these concretions, with concentrations of individual compounds ranging from 0.001 to 0.800 μg/g TOC, whereas kerogen-bound hydrocarbons lack detectable biomarkers. The results indicate that the two Cambrian carbonate concretions were formed mainly within the iron reduction and bacterial sulfate reduction zones, extending to depths of 10 to 38 m below the sediment-water interface. The OM within these concretions mainly inherited the initial unaltered signature of OM from the Qiongzhusi host shale. The carbonate concretions protected the internal OM from further thermal and secondary (e.g., biodegradation) alteration processes and might also prevent the formation of the conventional macromolecular skeletal kerogen manifested by the absence of bound biomarkers. The biomarkers, in both free and occluded forms, in the Cambrian carbonate concretions still retained their original source information, providing valuable insights into ancient biogeochemical processes during sediment burial and ancient seawater chemistry during the early Cambrian.

Diagenetic effect on barium isotope compositions of barites in the lower Cambrian successions

The pelagic barite precipitation in the seawater column is tightly associated with the biological pump, and thus the barite Ba isotope composition (δ138/134Ba) has been advocated as a novel paleo-productivity proxy. However, pelagic barites in sediments are susceptible to diagenetic processes that may modify primary δ138/134Ba signals and hamper the application. To constrain the diagenetic effect, we report δ138/134Ba data of diagenetic barites from the black shales of the lower Cambrian Yurtus Formation in the Aksu area of the Tarim Basin. The δ138/134Ba values vary within a large range from −0.15‰ to +0.49‰ and show a generally positive correlation with the 87Sr/86Sr ratios, indicating the mixing of two end-members. The samples with 87Sr/86Sr ratios close to the coeval seawater value record relatively low δ138/134Ba values from −0.15‰ to +0.14‰, which are inferred to inherit from the primary barites; whereas the samples with 87Sr/86Sr ratios exceeding the coeval seawater value record higher δ138/134Ba values from −0.05‰ to +0.49‰, which may have resulted from the alteration by the host black shales during early diageneses. Based on these investigations, we propose that pore waters below the sulfate-methane transition zone may provide an additional source of Ba (enriched in heavy isotopes) for the formation of diagenetic barites. We suggest to combine barite 87Sr/86Sr ratio with δ138/134Ba value to evaluate diagenetic influence on primary Ba isotope signatures especially for an over-determined system, which is important for applying Ba isotope compositions of barites to understand paleo-oceanic evolution.

Zinc enrichment and isotopic fractionation in a marine habitat of the c. 2.1 Ga Francevillian Group: A signature of zinc utilization by eukaryotes?

Constraining the timing of eukaryogenesis and the divergence of eukaryotic clades is a major challenge in evolutionary biology. Here, we present trace metal concentration and zinc isotope data for c. 2.1 billion-year-old Francevillian Group pyritized structures, previously described as putative remnants of the first colonial multicellular organisms, and their host black shales. Relative to the host rocks, pyritized structures are strongly enriched in zinc, cobalt and nickel, by at least one order of magnitude, with markedly lighter zinc isotope compositions. A metabolic demand for high concentrations of aqueous zinc, cobalt, and nickel combined with preferential uptake of lighter zinc isotopes may indicate metalloenzyme utilization by eukaryotes in marine habitats c. 2.1 billion years ago. Once confirmed, this would provide a critical calibration point for eukaryogenesis, suggesting that this major evolutionary innovation may have happened contemporaneously with elevated atmospheric oxygen levels during the latter part of the Great Oxidation Event, some 400 million years earlier than is currently widely accepted.

Rare earth elemental and Sr isotopic evidence for seawater intrusion event of the Songliao Basin 91 million years ago

Petrogenesis of lacustrine dolostone is closely related with paleo-lake water conditions. Here we report the high spatial-resolution petrographic and geochemical results of a lacustrine dolomite nodule from the Qingshankou Formation, the Songliao Basin. Sedimentary and elemental signatures confirm the protogenetic origin of this nodule and its effectiveness in recording geochemical characteristics of paleo-lake water during dolomitization. The low Y/Ho ratios, middle rare earth element (MREE) enrichment and subtle positive Eu anomalies within the nodule indicate a fresh water source. However, the Sr isotope values in the core of the nodule (0.7076–0.7080) are close to contemporaneous seawater (0.7074), yet different from the modern river (0.7120) and the host black shale (0.7100). On the premise of excluding the influence of hydrothermal fluids, the significantly low strontium isotope values of the lacustrine dolomite might be caused by seawater interference during dolomitization. Our findings demonstrate that lacustrine dolomite within black shales is not only a faithful tracer of diagenetic water environment, but also a novel and easily identified mineralogical evidence for episodic seawater intrusion event (91 Ma) in the Songliao Basin, which supplements other paleontological and geochemical evidence.

Multielement Imaging Reveals the Diagenetic Features and Varied Water Redox Conditions of a Lacustrine Dolomite Nodule

The genesis of dolostone has long been puzzling for more than two centuries. Although much work has been done on investigating the process of dolomitization, little emphasis has been put on examining the diagenetic water redox condition with the wealthy geochemical information preserved in primary dolomite, which is believed to archive the aqueous environment as well as biotic and/or abiotic effects during formation. In situ interpretation with high resolution is a prerequisite in refined research of dolomite. Here, we reported the multielement imaging results of a lacustrine dolomite nodule with the host black shale from the Songliao Basin, northeast of China. Micro X-ray fluorescence (μ-XRF) with a spatial resolution down to 10 μm was used for in situ scanning. Two key parameter settings of the μ-XRF, including single-point exposure time and spatial resolution, were optimized to achieve a better result in a reasonable scanning time scale. The final imaging data graphically revealed dynamic variation of elemental distributions, including elements enriched in dolomite (e.g., Ca, Mg, Fe, and Mn), clastic quartz (Si), and clay minerals (e.g., Al and K) and redox-sensitive trace elements (e.g., Cr, Mo, V, and U). The well-preserved laminated structures inside the nodule and the features with a magnesium-rich core wrapped with an iron-concentric outer layer and a manganese-concentric shell together indicated its primary form as dolomite and a gradual transformation into ankerite as well as manganese-ankerite. The elemental variation indicates a varied bottom water redox condition, which involved from sulfidic to ferruginous and manganous zones. Here, we propose that the intermittent supplies of sulfate and Fe-/Mn-oxidized minerals interrupting the black shale deposition while favoring dolomitization might be brought by the oxidized and salted seawater. And this lacustrine dolomite is expected to be a potential fingerprint mineral in tracking the seawater intrusions to the Songliao Basin which happened 91 million years ago.

Trace element and C-S-Fe geochemistry of Early Cambrian black shales and associated polymetallic Ni-Mo sulfide and vanadium mineralization, South China: Implications for paleoceanic redox variation

Early Cambrian black shales are widely exposed along the southeastern margin of the Yangtze Platform in South China for a length of over 1600 km. The basal part of this sequence host a large spectrum of mineralization, including polymetallic Ni-Mo sulfide, vanadium, phosphorite, barite, as well as combustible shale (stone coal). In particular, the polymetallic Ni-Mo sulfide ore layer is stratigraphically equivalent but geographically separated from the V mineralization. In the Zhangjiajie region in Hunan province, the tenor of the polymetallic Ni-Mo sulfide mineralization in the northeastern segment decreases gradually and grades into vanadium mineralization in the southwestern segment. Based on the stratigraphic correlation, we compared the trace element composition and carbon–sulfur-iron system of the polymetallic Ni-Mo sulfide ore, vanadium ore, and their host black shales.The polymetallic Ni-Mo sulfide and V ores display similar PAAS-normalized REE distribution patterns with negative Ce and positive Y anomalies, similar to present-day seawater. The polymetallic sulfide ores show elevated seawater-like Y/Ho ratios (mean: 52.7 ± 6.8) and the V ores with an average Y/Ho ratio of 40.2 ± 3.8 reflect the influence of terrigenous material. Although both the polymetallic Ni-Mo sulfide ore and the V ore formed under anoxic conditions, the degree of oxygen deficiency was variable, i.e., euxinic condition for the polymetallic sulfide ore but euxinic and suboxic conditions for the vanadium ore. This model is supported by the more elevated redox-sensitive elements (e.g., U and Co) in the polymetallic Ni-Mo sulfide ores compared to the V ores. Excess sulfur of the polymetallic Ni-Mo sulfide ores with S/Fe ratios>1.15 suggests that they formed under iron-limited euxinic conditions. In contrast, the V ores formed under euxinic to suboxic conditions with S/Fe ratios close to or lower than 1.15. The host black shales display positive correlations for TOC and redox-sensitive elements, suggesting that they formed under suboxic conditions. We infer that the polymetallic Ni-Mo sulfide ores formed in a sediment-starved semi-restricted euxinic marine basin. In contrast, the V ores formed under both euxinic and intermittently suboxic to euxinic conditions at the slope of the basin with terrestrial input from enhanced continental weathering. Repeated transgressive–regressive episodes led to the formation of V ores interlayered with suboxic black shale with more subdued oxygen deficiency. The results of this study suggest that exploration for V mineralization should focus on the southeastward lateral stratigraphic equivalents of the polymetallic Ni-Mo sulfide mineralization.

Gold in the Farallones Block of the Shale-Hosted, Clastic-Dominated Castellanos Zinc-Lead Deposit (Northwest Cuba)

The Zn-Pb ores of the Castellanos shale-hosted, clastic-dominated deposit in northwest Cuba average nearly 1 g/t Au, with local maximum concentrations up to 34 g/t Au. This deposit is stratiform with respect to the bedding in the host black shales and shows a bottom to top zoning of ore assemblages made up of a stockwork underlying the main orebody, a basal pyrite-rich zone and a disseminated to massive Zn-Pb ore zone capped by a discontinuous, thin barite-rich zone. Petrographic data and textural relations allow distinguishing five textural types of pyrite (framboidal Py I, colloform Py IIa, euhedral Py IIb, massive Py IIc and banded colloform Py III) successively formed during ore deposition. The main Zn-Pb ore formed after the crystallization of disseminated, sedimentary framboidal pyrite (Py I) in black shales by the superimposition of several crystallization events. The crystallization sequence of the main ore-forming stage evolved from the precipitation of colloform sphalerite and pyrite (Py IIa) with skeletal galena and interstitial dolomite-ankerite to similar ore assemblages but showing subhedral to euhedral crystal habits (Py IIb) and interstitial calcite-rich carbonates. This stage ended with the development of massive pyrite (Py IIc), mainly occurring at the base of the stratiform orebody. A late fracturing stage gave way to the development of a new generation of colloform banded pyrite (Py III) just preceding the crystallization of early barite. Au is mainly concentrated in pyrite showing variable contents in the different textural types of pyrite and a bottom to top enrichment trend. Minimum contents occur in massive pyrite (Py IIc) from the basal pyrite-rich zone (0.18 ppm Au average), increasing in pyrite IIa (from 0.29 to 2.86 ppm Au average) and in euhedral pyrite (Py IIb) (from 0.82 to 9.02 ppm Au average), reaching maxima in colloform banded pyrite (Py III) formed just before the crystallization of early barite at the top of the orebody. Au enrichment in pyrite correlates with that of Sb (0.08–4420 ppm), As (0.7–35,000 ppm), Ag (0.03–1560 ppm) and to a lesser extent Cu (3–25,000 ppm), Ni (0.02–1600 ppm) and Mn (0.6–5030 ppm). Au deposition should have taken place by oxidation and, probably cooling, of reduced (H2S-dominated) fluids buffered by organic matter-rich black shales of the host sedimentary sequence. The input of such reduced fluids in the ore-forming environment most probably occurred alternating with that of the main oxidized fluids which leached Zn and Pb from the large volume of sandstones and siltstones making up the enclosing sequence, thus being responsible for the precipitation of the majority Zn-Pb ore. Supply of Au-carrying reduced fluids might progressively increase over the course of ore formation, reaching a maximum at the beginning of the late fracturing stage. This evolution of Au supply is consistent with the early crystallization of barite since Ba can also only be transported at low temperature by highly reduced fluids. These results highlight the potential of medium-sized, shale-hosted, clastic-dominated deposits to contain economic (by product) Au amounts and show that ore-forming fluids can change from oxidized (SO42+ dominated) to reduced (H2S-dominated), and vice versa, throughout the evolutionary history of a single deposit.

From source to sink: Glacially eroded, Late Devonian algal “cysts” (Tasmanites) delivered to the Gulf of Mexico during the Last Glacial Maximum

Abstract The source of reworked Devonian algal “cysts” in last glacial maximum (LGM) sediment in the Gulf of Mexico is traced to their host black shales, which ring the southwestern Great Lakes. The source-to-sink pathway includes intermediate storage in fine-grained LGM glacial lacustrine sediment and till. The “cysts” are pelagic chlorophyllous algae (Tasmanites and Leiosphaeridia), collectively referred to herein as tasmanitids. Radiocarbon dates of syndepositional Gulf of Mexico foraminifera, derived from accelerator mass spectrometry, bracket the Gulf of Mexico sediment age with common tasmanitids from 28.5 ± 0.6–17.8 ± 0.2 cal kyr B.P. Approximately 1400 km north of the Gulf of Mexico, tasmanitids are abundant in Upper Devonian black shales (New Albany, Antrim, and Ohio Shales) that ring the Michigan, Illinois, and Appalachian intracratonic basins. Tasmanitids were eroded from bedrock and incorporated in glacial sediment dating from ca. 28.0–17.6 cal kyr B.P. by the Lake Michigan, and Huron-Erie lobes of the Laurentide Ice Sheet. The physical characteristics of tasmanitids are ideal for long-distance transport as suspended sediment (density: 1.1–1.3 g/cc, size ranging from 63 µm to 300 µm), and these sand-sized tasmanitids traveled with the silt-clay fraction. Thus, the source-to-sink journey of tasmanitids was initiated by subglacial erosion by water or friction, sequestering in till or glaciolacustrine sediment, re-entrainment and suspension in meltwater, and final delivery in meltwater plumes to the Gulf of Mexico. River routes included the Mississippi, Illinois, Ohio, Wabash, Kaskaskia, and many of their tributaries. Reworked Devonian tasmanitids are a previously unrecognized link between their occurrence in deep-water deposits of the Gulf of Mexico and the late Wisconsin glacial history of the Upper Mississippi Valley. We propose that tracking occurrences of tasmanitid concentrations from the source area to sink, along with adjunct proxies such as clay minerals, will facilitate a more refined analysis of the timing and duration of megafloods. This study also demonstrates that isotopically dead carbon, from reworked Devonian tasmanitid “cysts,” can contaminate radiocarbon dating of LGM bulk sediment samples toward older ages.

Origin of chert nodules in the Ediacaran Doushantuo Formation black shales from Yangtze Block, South China

Abstract Chert nodules are widely developed in Ediacaran deposits of South China, yet the mechanism responsible for their formation remains disputed. Petrological and geochemical studies of chert nodules and host shale of the Ediacaran Doushantuo Formation of the Yangtze Block, South China, were carried out to more fully assess silicification of these deposits. Doushantuo chert nodules display a concentrically layered internal structure dominated by quartz and lesser concentrations of calcite, carbonate fluorapatite, organic matter, pyrite, and sphalerite. Individual layers become thinner from nodule centers to edges. Our results suggest that (1) Doushantuo host black shale accumulated under persistent anoxic bottom water conditions and (2) chert nodules formed as a consequence of the anaerobic oxidation of methane focused along sulfate-methane transition zones at shallow burial depth. Nodules likely originated from porous proto-nodules that consisted of 13C-depleted authigenic calcite produced by the anaerobic oxidation of methane. Decaying of organic matter within nodules helped to create porosity that was filled by authigenic calcite and also released phosphate and zinc ions to pore water, thus favoring precipitation of carbonate fluorapatite and sphalerite. Silica supplied principally from the diagenesis of clay minerals of the host shale cemented the nodules and replaced early formed calcite. The progressive infilling of residual porosity by 18O-depleted pore fluids associated with increasing burial depth is reflected in the strongly negative δ18Ocarb values documented from the nodules. Thus, the formation of Doushantuo chert nodules reflects a protracted diagenetic history initiated by bacterial sulfate reduction and anaerobic oxidation of methane close to and within the sulfate-methane transition zone and continued in association with the diagenetic release of silica from host shale as well hydrocarbon generation with increasing burial depth.

The Koudia El Hamra Ag–Pb–Zn deposit, Jebilet, Morocco: Mineralogy and ore fluid characterization

The Koudia El Hamra silver - base metal (Pb-Zn) deposit is located in the Upper Visean - Namurian Sarhlef schist, Jebilet Hercynian Massif (Morocco). Ore bodies consist of N-S to NE-SW quartz-chlorite veins hosted by Hercynian black shale. Mineralogical and paleo-fluid investigations (microthermometry, Raman spectroscopy) and stable isotope data (O, C, S) have shown that high salinity brines (30 to 35 wt% NaCl+CaCl2 equiv.), likely from sedimentary origin deposited base metal (Zn) - carbonate assemblage at minimal temperature around 220°C. Then brines deposited silver ore - galena assemblage at lower salinity (9 to 15 wt% NaCl equiv.) and similar average minimal temperature (around 220°C). The main driving mechanism for silver ore deposition is the dilution of ore-bearing brines by low-salinity fluids probably combined with reduction of brines in the host black shale. Ore brines likely have come from the Permian-Triassic Atlasic basins related to the extension coeval with the Central Atlantic opening. Those brines probably have percolated into the basement through Hercynian faults (N-S to NE-SW) reworked during the Permian-Triassic to Lower Jurassic extensional events. The source of silver seems to be in black shale and/or in abundant mafic to intermediate magmatic rocks (i.e. microdiorite dykes) consistently observed close to the ore bodies. Silver ores are superimposed on the same lineament to preexisting Fe-As-(Sn) assemblages related to Hercynian events in the central Jebilet and mainly due to the circulation of magmatic and/or metamorphic fluids. However, silver ores have no genetic relationship with magmatic-metamorphic events as suggested by former models. The metallogenic model proposed for the Koudia El Hamra deposit is similar to those proposed for several other silver - base metal (Pb-Zn) deposits from Morocco like the Roc Blanc silver deposit (Jebilet massif) and major silver deposits from the Anti-Atlas belt (Imiter, Zgounder and Bou Azzer) and from Western Europe.

Mineral and chemostratigraphy of a Toarcian black shale hosting Mn-carbonate microbialites (Úrkút, Hungary)

Toarcian black shale that hosts Mn-carbonate microbialites at Úrkút, Hungary was investigated by mineralogical, inorganic, and organic geochemical methods for characterization and comparison with other European black shales representative of the Toarcian Oceanic Anoxic Event. Based on the authigenic mineral composition, calculations were made to estimate environmental conditions during sediment accumulation and early diagenesis. Geochemical and petrographic results of organic, carbonate, and REE multiple-proxy analyses revealed a strong congruence between the host black shale and the Mn-carbonate ore beds. The Úrkút black shale is really a gray shale with moderate to low TOC contents that accumulated in a starved basin. The organic matter and anoxic characteristics resulted from rapid accumulation of organic matter from microbial booms, accompanied by a geothermally generated hydrothermal circulation system, and a high rate of authigenic mineral formation (clay minerals and proto-ore minerals). The inferred enzymatic Mn and Fe oxidation blocked carbonate formation by decreasing the pH. The system remained suboxic via syngenetic mineral accumulation (Fe-rich biomats), and became anoxic during diagenesis in conjunction with pyrite generation. The separation of black shale beds and Mn-ore beds is not distinct through the section. Instead, a distal hydrothermally induced clay-rich authigenic assemblage (marlstone) best describes the black shale, in which Mn-oxide proto-ore beds (Mn-rich laminae) formed from the beginning of black shale deposition, when the oxygen supply in the sedimentary basin was insufficient for enzymatic Mn(II) oxidation. Mn-oxide proto-ore was transformed to Mn-carbonate ore during microbially mediated processes during early diagenesis. The drivers for Mn-bearing organic matter-rich marlstones were most probably a combination of regional and local processes, with generation of a tectonic rift system that promoted geothermally generated hydrothermal fluids, which initiated microbial blooms. Black shale mineralogy, geochemistry, and organic matter at Úrkút differ from those of the epicontinental shelf black shales of the Tethyan Ocean.

MINERALOGICAL AND GEOCHEMICAL EVIDENCE FOR MULTI-STAGE FORMATION OF THE CHERTOVO KORYTO DEPOSIT

Introduction . The Lena gold province is one of the largest known gold resources in the world. The history of its exploration is long, but the genesis of gold mineralization hosted in black shales in the Bodaibo synclinorium still remains unclear. The studies face the challenge of discovering sources for the useful component and mechanisms of its redistribution and concentration. This study aims to clarify the time sequence of the ore mineralization in the Chertovo Koryto deposit on the basis of detailed mineralogical and geochemical characteristics of the ore, wallrock metasomatites and the Early Proterozoic host black shales, and to assess the applicability of the Sukhoi Log model for clarifying the Chertovo Koryto origin. Geological setting . The Lena gold province is located in the junction area of the Siberian platform and the Baikal mountain region (Fig. 1). The main element of its geological structure is the Chuya-Tonoda-Nechera anticline. Its axial segment is marked by horsts composed of the Early Proterozoic rocks with abundant granitoid massifs. The Chertovo Koryto deposit is located within the Kevakta ore complex at the Tonoda uplift, the largest tectonically disturbed block between the Kevakta and Amandrak granitoids massifs. The 150 m thick and 1.5 km long ore zone of the Chertovo Koryto deposit is confined to the hanging wall of the fold-fault zone feathering the Amandrak deep fault (Fig. 2). Composition . In the ore zone, rocks of the Mikhailovsk Formation include carbonaceous shales of the feldspar-chlorite-sericite-quartz composition with nest-shaped ore accumulations of the pyrite-quartz composition and quartz veinlets. In our study, we distinguish five mineral associations resulting from heterochronous processes that sequentially replaced each other: - The earliest association related with the quartz-muscovite-sericite metasomatism and the removal of REE and other elements from the rocks and their partial redeposition; - Metamorphic sulphidization presented by scattered impregnations of pyrrhotite, as evidenced by small lenses of pyrrhotite, which are considerably elongated (axes up to 0.7 cm long) along the foliation planes (Figs 3, a , b ); - Ore mineralization represented by a superimposed hydrothermal gold association with arsenopyrite (Fig. 3, d ); - Late chalcophilic mineralization formed at the final stage of hydrothermal-metasomatic process (Figs 3, e , f ); - Post-ore silification. Geochemical characteristics . The geochemical study of rocks and ores from the Chertovo Koryto deposit show that the rocks of the Mikhailovsk Formation are characterized by higher contents of rock-forming elements, such as of Al 2 O 3 , Fe 2 O 3total , MgO, K 2 O, and P 2 O 5 , in comparison to the PAAS standards [ Condie, 1993 ] and the black shale standard composition (SChS-1) [ Petrov et al., 2004 ]. A characteristic feature of the ore zone is that the contents of practically all the oxides, except SiO 2, tend to decrease (Table 1). The distribution of rare elements repeats the pattern established for major elements. The least metamorphosed rocks of the Mikhailovsk Formation have higher contents (up to three times) of Cu, Mo, Ba, W, As, Pb relative to the values in the PAAS and SChS-1 standards. In the ore zone, the contents of almost all rare elements are considerably reduced (Table 2). The contents of elements in the siderophile group (Co, Ni) are clearly correlated with the ore processes and increased more than twice in the area of metamorphic changes. Samples with gold-ore grade contents show the highest concentrations of Co and Ni. Conclusion. In our opinion, the Chertovo Koryto deposit was formed in five stages, the first two of which were pre-ore, with ore preparation, and probably considerably distant in time from the main ore-generating event. The staged formation of the Chertovo Koryto deposit correlates with the basic stages in the tectono-metamorphic history of the study region and is consistent with the model showing the formation of Sukhoi Log-type deposits [ Nemerov, 1989; Buryak, Khmelevskaya, 1997; Large et al., 2007 ].

Pyritization induced by anaerobic oxidation of methane (AOM) – An example from the upper devonian shale succession, western New York, USA

The diagenetic history of the Upper Devonian Rhinestreet Shale of the western New York region of the Appalachian basin appears to reflect the upward diffusion of methane through these deposits shortly after their accumulation. The most obvious diagenetic signature of this process is numerous horizons of authigenic calcium carbonate concretions interpreted to reflect mineralization induced by the anaerobic oxidation of methane (AOM) focused along the sulfate-methane transition (SMT) zone. Additionally, a 2-m-thick section of the lower Rhinestreet preserves a diagenetic record consistent with AOM in the form of two pyritiferous stratigraphic horizons. The stratigraphically lowest horizon is a 2-cm-thick layer containing abundant 34S-enriched (11.8 and 18.4‰ V-CDT) coarsely crystalline pyrite. The thinness of this interval suggests that AOM-related diagenesis remained fixed at this stratigraphic position for a brief period of time before these deposits were buried below the SMT or the SMT shifted position within the sediment column. A second paleo-SMT appears to have stabilized 60 to 80 cm above the 2-cm-thick pyritiferous horizon, meter-scale carbonate concretions being the most obvious evidence of this episode of AOM. More than this, though, the apparently Fe-limited nature of the host black shale induced the downward diffusion of 34S-enriched hydrogen sulfide produced by AOM resulting in the precipitation of 34S-enriched (23.1 to 27.3‰ V-CDT) pyrite below the SMT. The maximum depth of diffusion of the sulfidization front was limited by the amount of hydrogen sulfide produced within and near the SMT as well as by the availability of reactive iron in underlying deposits. The focusing of AOM-related diagenesis at the two stratigraphic positions was likely caused by non-steady-state sedimentation. However, it is impossible to decipher relative timing of formation of the two paleo-SMTs.

The chemical composition of specular hematite from Tilkerode, Harz, Germany: implications for the genesis of hydrothermal hematite and comparison with the Quadrilátero Ferrífero of Minas Gerais, Brazil

The black shale-hosted selenide vein-type deposit at Tilkerode, eastern Harz, Germany, has specular hematite enclosed in clausthalite (PbSe). The specular hematite has Ti and V in amounts of up to ∼1 wt.% TiO2 and ∼3 wt.% V2O5, and subordinate, but important, contents of Mo (22–372 ppm) and B (up to 68 ppm). The Tilkerode hematite serves as a reference for hydrothermal hematite formed at relatively low temperatures (<150 °C). The composition of the Tilkerode hematite is compared with that of two generations of specular hematite from itabirite-hosted iron-ore deposits in the Quadrilatero Ferrifero of Minas Gerais, Brazil. The first generation of specular hematite represents an early tectonic hematitisation of dolomitic itabirite at Aguas Claras and occurs as fine-grained crystals. Reconnaissance data indicate that the Aguas Claras hematite is poorer in Ti and V, relative to the Tilkerode hematite, but has ∼5–10 ppm B and ∼7–11 ppm Li. The second generation of specular hematite defines the pervasive tectonic foliation of the Gongo Soco iron ore. This hematite has Ti contents of up to ∼2 wt.% TiO2 and subordinate amounts of V (62–367 ppm); its B and Li concentrations are mostly below <2 ppm B and <1 ppm Li. The presence of Ti and B in the Tilkerode hematite can be explained by highly saline, B-bearing fluids that were capable of mobilising otherwise immobile Ti. The Mo signature of the Tilkerode hematite suggests that Mo was derived from the host black shale. In Minas Gerais, B and Li were incorporated into the early tectonic hematite from saline fluids at relatively low temperatures (Aguas Claras) and then released during metamorphic hematite growth at higher temperatures, as suggested by the foliation-defining hematite without B–Li signature (Gongo Soco).

Seawater contribution to polymetallic Ni–Mo–PGE–Au mineralization in Early Cambrian black shales of South China: Evidence from Mo isotope, PGE, trace element, and REE geochemistry

The Early Cambrian black shale sequence of the Niutitang Formation in South China hosts a synsedimentary, organic carbon-rich, polymetallic sulfide layer with extreme metal concentrations, locally mined as polymetallic Ni–Mo–PGE–Au ore. In combination with previously reported data, we present Mo isotope, platinum-group element (PGE), and trace and rare-earth element (REE) data for the polymetallic sulfide ores and host black shales from four mine sites (Dazhuliushui and Maluhe in Guizhou Province, and Sancha and Cili in Hunan Province, respectively), several hundred kilometers apart. The polymetallic sulfide ores have consistently heavy δ98/95Mo values of 0.94 to 1.38‰ (avg. 1.13±0.14‰, 1σ, n=11), and the host black shale and phosphorite have slightly more variable δ98/95Mo values of 0.81‰ to 1.70‰ (n=14). This latter variation is due to variable paleoenvironmental conditions from suboxic to euxinic, and partly closed-system fractionation in isolated marine sedimentary basins. Both the polymetallic sulfides and host black shales show PGE distribution patterns similar to that of present-day seawater, but different from those of ancient submarine-hydrothermal deposits and modern submarine hydrothermal fluids. The polymetallic sulfide bed has a generally consistent metal enrichment by a factor of 107 compared to present-day seawater. PAAS-normalized REE+Y patterns of the polymetallic sulfide bed are characterized by a remarkably positive Y anomaly, consistent with an origin of the REE predominantly from seawater. Small positive Eu anomalies in some of the sulfide ores could reflect minor hydrothermal components involved. The Mo isotope, PGE, and trace and rare-earth element geochemical data suggest that metals in the polymetallic Ni–Mo–PGE–Au sulfide ore layer were scavenged mostly from Early Cambrian seawater, by both in-situ precipitation and local re-deposition of sulfide clasts.

Selenium speciation in Lower Cambrian Se-enriched strata in South China and its geological implications

To understand the impact of Selenium (Se) into the biogeochemical cycle and implications for palaeo-redox environment, a sequential extraction method was utilized for samples including black shales, cherts, a Ni–Mo–Se sulfide layer, K-bentonite and phosphorite from Lower Cambrian Se-enriched strata in southern China. Seven species (water-soluble, phosphate exchangeable, base-soluble, acetic acid-soluble, sulfide/selenide associated, residual Se) and different oxidation states (selenate Se(VI), selenite Se(IV), organic Se, Se (0) and mineral Se(-II)) were determinated in this study. We found that the Ni–Mo–Se sulfide layer contained a significantly greater amount of Se(-II) associated with sulfides/selenides than those in host black shales and cherts. Furthermore, a positive correlation between the degree of sulfidation of iron (DOS) and the percentage of the sulfide/selenide-associated Se(-II) was observed for samples, which suggests the proportion of sulfide/selenide-associated Se(-II) could serve as a proxy for palaeo-redox conditions. In addition, the higher percentage of Se(IV) in K-bentonite and phosphorite was found and possibly attributed to the adsorption of Se by clay minerals, iron hydroxide surfaces and organic particles. Based on the negative correlations between the percentage of Se(IV) and that of Se(-II) in samples, we propose that the K-bentonite has been altered under the acid oxic conditions, and the most of black shale (and cherts) and the Ni–Mo–Se sulfide layer formed under the anoxic and euxinic environments, respectively. Concerning Se accumulation in the Ni–Mo–Se sulfide layer, the major mechanism can be described by (1) biotic and abiotic adsorption and further dissimilatory reduction from oxidized Se(VI) and Se(IV) to Se(-II), through elemental Se, (2) contribution of hydrothermal fluid with mineral Se(-II).

Emerald deposits and occurrences: A review

Emerald, the green gem variety of beryl, is the third most valuable gemstone (after diamond and ruby). Although it is difficult to obtain accurate statistics, Colombia supplies most (an estimated 60%, worth more than $500,000,000 per year) of the world's emeralds. However there is speculation that the emerald mines in Colombia are becoming depleted. Brazil currently accounts for approximately 10% of world emerald production. Emeralds have also been mined in Afghanistan, Australia, Austria, Bulgaria, China, India, Madagascar, Namibia, Nigeria, Pakistan, South Africa, Spain, Tanzania, the United States, and Zimbabwe. Because it is difficult to obtain accurate analyses of beryllium, most published analyses of beryl are renormalized on the basis of 18 oxygen and 3 Be atoms per formula unit. The color of emerald is due to trace amounts of chromium and/or vanadium replacing aluminum at the Y site; in most cases the Cr content is much greater than that of V. To achieve charge balance, the substitution of divalent cations at the Y site is coupled with the substitution of a monovalent cation for a vacancy at a channel site. Beryl is relatively rare because there is very little Be in the upper continental crust. Unusual geologic and geochemical conditions are required for Be and Cr and/or V to meet. In the classic model, Be-bearing pegmatites interact with Cr-bearing ultramafic or mafic rocks. However in the Colombian deposits there is no evidence of magmatic activity and it has been demonstrated that circulation processes within the host black shales were sufficient to form emerald. In addition, researchers are recognizing that regional metamorphism and tectonometamorphic processes such as shear zone formation may play a significant role in certain emerald deposits. A number of genetic classification schemes have been proposed for emerald deposits. Most are ambiguous when it comes to understanding the mechanisms and conditions that lead to the formation of an emerald deposit. Studies of individual emerald deposits show that in most cases a combination of mechanisms (magmatic, hydrothermal, and metamorphic) were needed to bring Be into contact with the chromophores. This suggests the need for a more flexible classification scheme based on mode of formation. Stable isotopes can be used to estimate the contribution of each mechanism in the formation of a particular deposit. Such estimates could perhaps be more precisely defined using trace element data, which should reflect the mode of formation. Emerald may be identified in the field by color, hardness, and form. It will tend to show up in stream sediment samples but because its specific gravity is relatively low, it will not concentrate in the heavy mineral fraction. In Colombia, structural geology, the sodium content of stream sediment samples, and the lithium, sodium, and lead contents of soil samples have all been used to find emerald occurrences. Exploration for gem beryl could result in the discovery of new occurrences of non-gem beryl or other Be minerals that could become new sources of Be and Be oxide. Future efforts should go towards creating a comprehensive data base of emerald compositions (including trace elements), determination of the role of metamorphism in the formation of some emerald deposits, improved classification schemes, and more effective exploration guidelines.

Highly metalliferous carbonaceous shale and Early Cambrian seawater: COMMENT and REPLY: COMMENT

[Lehmann et al. (2007)][1] reported trace, rare earth element (REE), and Mo-isotope data from the Ni-Mo sulfide layer and host black shale of the Lower Cambrian Niutitang Formation in southern China. They argue that the metals in the sulfidic rocks originated purely from Early Cambrian seawater.

Trace- and rare-earth element geochemistry and Pb–Pb dating of black shales and intercalated Ni–Mo–PGE–Au sulfide ores in Lower Cambrian strata, Yangtze Platform, South China

The Lower Cambrian black shale sequence of the Niutitang Formation in the Yangtze Platform, South China, hosts an extreme metal-enriched sulfide ore bed that shows >10,000 times enrichment in Mo, Ni, Se, Re, Os, As, Hg, and Sb and >1,000 times enrichment in Ag, Au, Pt, and Pd, when compared to average upper continental crust. We report in this paper trace- and rare-earth-element concentrations and Pb–Pb isotope dating for the Ni–Mo–PGE–Au sulfide ores and their host black shales. Both the sulfide ores and their host black shales show similar trace-element distribution patterns with pronounced depletion in Th, Nb, Hf, Zr, and Ti, and extreme enrichment in U, Ni, Mo, and V compared to average upper crust. The high-field-strength elements, such as Zr, Hf, Nb, Ta, Sc, Th, rare-earth elements, Rb, and Ga, show significant inter-element correlations and may have been derived mainly from terrigenous sources. The redox sensitive elements, such as V, Ni, Mo, U, and Mn; base metals, such as Cu, Zn, and Pb; and Sr and Ba may have been derived from mixing of seawater and venting hydrothermal sources. The chondrite-normalized REE patterns, positive Eu and Y anomalies, and high Y/Ho ratios for the Ni–Mo–PGE–Au sulfide ores are also suggestive for their submarine hydrothermal-exhalative origin. A stepwise acid-leaching Pb–Pb isotope analytical technique has been employed for the Niutitang black shales and the Ni–Mo–PGE–Au sulfide ores, and two Pb–Pb isochron ages have been obtained for the black shales (531±24 Ma) and for the Ni–Mo–PGE–Au sulfide ores (521±54 Ma), respectively, which are identical and overlap within uncertainty, and are in good agreement with previously obtained ages for presumed age-equivalent strata.

Sulfate Reduction by Organic Matter in Colombian EmeraldDeposits: Chemical and Stable Isotope (C, O, H) Evidence

Colombian emeralds occur within carbonate-silicate-pyrite veins and breccias in black shale-limestone– hosted deposits. They are contained within two zones, the eastern and western emerald zones, in the Eastern Cordillera. Mineralizing fluids were derived from the interaction of low-salinity fluids with primary halite and anhydrite. The source of beryllium is sedimentary and of sulfide sulfur is evaporitic. The presence of organic matter, as bitumen, in both the black shales and the emerald-bearing hydrothermal veins invites discussion of the role played by organic matter in thermochemical sulfate reduction, the mechanism proposed for H 2 S production. Organic matter is found in the orebodies in two forms: altered and unaltered. It is also found in primary fluid inclusions of emeralds. Rock-Eval analyses (hydrogen index <1; 19 < oxygen index < 61) as well the elemental H/C ratio (hydrogen index <1) confirmed the intense cracking suffered by the original organic matter. In hydrothermal bitumen, infrared microscopy revealed the absence of aliphatic and aromatic bands and the disappearance of oxygen functional groups. The eastern emerald zone contains 13 C-depleted carbonates (mean δ 13 C = –11.3 ± 1.3‰, n = 14) with an homogeneous oxygen isotope composition range ( δ 18 O = 17.8 ± 0.3‰). The western emerald zone contains carbonates with relatively heavy δ 13 C (mean δ 13 C = –5.1 ± 0.5‰, n = 25) and notable δ 18 O variation (17–23.8‰). In both emerald zones, 13 C-depleted calcite points to thermochemical sulfate reduction by oxidation of organic matter, but most δ 13 C values are heavier than predicted by thermochemical sulfate reduction. A model that mixes heavy CO 2 evolved from dissolution of local limestone and light CO 2 evolved by oxidizing organic matter explains the observed δ 13 C range of carbonates. Variations in δ 18 O of carbonate also reflect the local heterogeneity of the host rocks. Redox reactions produced HCO 3 – and H 2 S; these compounds then reacted with the predominant cations (Fe 2+ and Ca 2+ extracted from the host black shale and limestones by the hydrothermal fluid) and induced calcite and pyrite precipitation. The oxygen and hydrogen isotope compositions of fluid inclusions and structural waters of the gangue minerals and emeralds constrain the origin of the mineralizing fluids. The composition of these fluids was compared with the isotope composition of fluids in the Zipaquira halite-bearing salt intrusions, which are near the emerald deposits. The calculated fluid δ 18 O in equilibrium with calcite and dolomite (eastern emerald zone: 7.7 δ 18 O Fluid < 12.8‰; western emerald zone: 8.2 δ 18 O Fluid < 17.8‰) and emerald (eastern emerald zone: 15.5 δ 18 O Fluid < 17.1‰; western emerald zone: 17.5 δ 18 O Fluid < 23.6‰) are typical of metamorphic waters or highly evolved sedimentary formation waters (temperature of formation = 300°C). The high δ 18 O H 2 O values of the minerals suggest that the fluid may have been continuously enriched in δ 18 O owing to exchange with limestones and black shales. In both emerald zones, the δ D values of carbonates and quartz parental waters (–64 δ D < –40.7‰) are similar to those found for halite (–66.4 δ D < –41.2‰), whereas δ D values of emerald, muscovite and albite are –51.4 to –7.1 per mil. The increase in δ D is interpreted as a consequence of mixing of fluids of different isotope composition during the mineralizing event. Two observations can be made about the O and H isotope ratios of fluid inclusions and structural waters from salt, muscovite, albite, and emerald. In the eastern emerald zone, they indicate that the fluids from halite and aluminosilicates are the same and are the result of mixing of a meteoric water ( δ 18 O = –12.7‰, δ D = –91.6‰) with a fluid representing the previous mixture of a water with the Early Cretaceous evaporite brines. In the western zone, the isotope ratios do not show the covariation seen in the eastern zone. The 18 O H 2 O shift found in emerald and muscovite is interpreted to be the result of oxygen isotope exchange reactions with sedimentary formations during the precipitation of the silicate phase.