Lower Cambrian Black Shales Research Articles

Late-stage sulfides and sulfarsenides in Lower Cambrian black shale (stone coal) from the Huangjiawan mine, Guizhou Province, People’s Republic of China

The Ni-Mo Huangjiawan mine, Guizhou Province, People’s Republic of China, occurs in Lower Cambrian black shale (stone coal) in an area where other mines have recently extracted ore from the same horizon. Detailed electron microprobe (EMPA) and scanning electron microscope (SEM) analyses of representative thin sections have revealed a complex assemblage of sulfides and sulfarsenides. Early sulfidic and phosphatic nodules and host matrix have been lithified, somewhat fractured, and then mineralized with later-stage sulfides and sulfarsenides. Gersdorffite, millerite, polydymite, pyrite, sphalerite, chalcopyrite, galena, and clausthalite have been recognized. EMPA data are given for the major phases. Pyrite trace-element distributions and coeval Ni-, As-sulfides indicate that in the main ore layer, the last sulfide deposition was Ni-As-Co-rich. Mo and V deposition were early in the petrogenesis of these rocks. The assemblages gersdorffite-millerite-polydymite (pyrite) and millerite-gersdorffite (pyrite) and the composition of gersdorffite indicate a formation temperature of between 200° and 300 °C suggesting that the last solutions to infiltrate and mineralize the samples were related to hydrothermal processes. Environmentally sensitive elements such as As, Cd, and Se are hosted by sulfides and sulfarsenides and are the main source of these elements to residual soil. Crops grown on them are enriched in these elements, and they may be hazardous for animal and human consumption.

Trace and rare earth element geochemistry of phosphate nodules from the lower Cambrian black shale sequence in the Mufu Mountain of Nanjing, Jiangsu province, China

Phosphate nodules are abundant in the lowermost section of the Lower Cambrian black shale sequence along a NE-trend belt in the Yangtze Platform, south China. In this study, we examined the Neoproterozoic-Cambrian Mufu Mountain section near Nanjing of Jiangsu province, and conducted a detailed trace and rare earth element (REE) study on the phosphate nodules. We employed a step-leaching acid dissolution protocol of 1 M HAc and 1 M HCl, respectively, to separate carbonate from phosphate mineral contributions in the nodules. Both the HAc-leached and HCl-leached fractions of the phosphate nodules show similar symmetrical variations in trace and REE concentrations and ratios. The total REEs and a number of redox-sensitive trace metals such as V, Cr, Mo, Ni abundances show a systematic increase from the core to rim, whereas the U and Mn abundances decrease from the core to rim in most of the nodules. Similar trends are also observed for the Rb/Sr, U/Th, Co/Ni, and V/Ni ratios, but these trends in the HAc-leached fractions are less pronounced than the HCl-leached fractions. All of the HCl-leached fractions for the nodule cores show seawater-like shale-normalized REE patterns, but the rims of the nodules display slightly MREE-enriched and HREE-depleted patterns. We suggest that the phosphate nodules may have formed in a basinal setting beneath a stratified water column during Early Cambrian, and the compositional variations of REEs and redox-sensitive trace elements from the core to rim may record a changing redox condition and fluid mixing during different stages of nodule growth. The cores of the nodules preserve more pronounced negative Ce anomalies and progressively HREE-enriched patterns, which may retain primary Early Cambrian seawater chemistry, whereas the nodule rims record chemistry consistent with changed redox conditions and/or influences from pore fluids such as generated from degradation of organic matter.

Extreme enrichment of polymetallic Ni–Mo–PGE–Au in Lower Cambrian black shales of South China: An Os isotope and PGE geochemical investigation

Extreme metal enrichments occur in the Lower Cambrian Niutitang Formation, a thick black shale sequence deposited on the Yangtze Platform of South China, and form a regionally distributed conformable polymetallic Ni–Mo–PGE–Au sulfide horizon. The origin for the extreme enrichment of metals in these black shales is discussed based on a comprehensive geochemical investigation of Re–Os isotopes and platinum-group elements. The black shales of the Lower Cambrian Niutitang Formation in Guizhou province yielded an Re–Os isochron age of 535 ± 11 Ma with an initial 187Os/ 188Os ratio of 0.80 ± 0.04, which is similar to the initial 187Os/ 188Os ratio (0.78 ± 0.19) of the Ni–Mo sulfide ores, but slightly lower than the initial 187Os/ 188Os ratio (1.18 ± 0.02) of Lower Cambrian black shales from the Lesser Himalaya, India. A comparison of Os isotope data among the Chinese Ni–Mo sulfide ores, the seafloor massive sulfide deposits and contemporaneous seawater suggests a possible similar hydrothermal forming mechanism for these deposits. The PGE concentrations, Pt anomaly (Pt/Pt ⁎), Pt/Pd, Ir/Pd, Au/Pd ratios of the Chinese Ni–Mo sulfide ores are similar to those metal-rich black shales of submarine volcanogenic hydrothermal origin from Czech Republic, Namibia and Canada, but are different from those data of the ocean floor Fe–Mn crusts that have scavenged their PGE and other metals directly from seawater. It is therefore suggested that the Ni–Mo sulfide ores from the Niutitang black shale sequence in South China had a submarine-hydrothermal origin. The anoxic environments and abundant organic matter in the ocean basin also played a key role for the extreme metal enrichments. The discharge of hydrothermal fluids into the Cambrian Ocean may have had a great effect on life during the Cambrian Explosion.

Biogenic origin of intergrown Mo-sulphide- and carbonaceous matter in Lower Cambrian black shales (Zunyi Formation, southern China)

The Lower Cambrian Zunyi Formation in southern China hosts complex intergrown phases of Mo-sulphides and anthracite-like carbonaceous matter and graphite (MoSC), pyrite and Ni-sulphides. These sulphides occur in a few centimeters thick polymetallic stratiform layer in black shales above phosphorites, cherts, dolomites and barite. The ore horizon contains 7 wt.% Mo, 2 wt.% Ni, 19 wt.% S and 13 wt.% C org and C tot (CO 2: 58 wt.%), 14 wt.% CaO, 1.2 wt.% As, 2000 ppm Se, 5400 ppm Zn, 450 ppm U and V, and up to 1 ppm platinum group elements. Three types of MoSC phases, always including framboidal pyrite, are distinguished: (1) rounded structures, few hundred microns large, (2) relict inclusions in diagenetic pyrite (3) grains interstitial to sulphurised and phosphatised organic matter. At the nanoscale, MoSC is represented by nanometric porous rounded structures of MoS 2 finely intergrown with carbonaceous matter of typical nano-“onion” structures. MoSC also contains nanocrysts of quartz, clays, Ca-sulphate, carbonate and Ni-sulphides. Micro-Raman spectroscopy shows that the carbon phase has a maturation grade of anthracite indicating temperatures below 300 °C. MoSC is mainly composed of 25 wt.% Mo, 23 wt.% S, 5.6 wt.% Fe, 2.5 wt.% Ni, 33 wt.% C, 1.5 wt.% O, 0.6 wt.% N and several hundreds of ppm of Cu, As, Se, Sb, Pd, Zn, Co and Sn. Electron energy loss spectra show that N is related to nanosized carbonaceous matter. The mean C/N ratio (∼ 57) of MoSC is similar to those observed in marine organic matter, and those reported from zooplankton at hydrothermal vents. The mineralized shales are crosscut by hydrothermal veins composed of carbonate or barite, always associated with quartz, apatite, and Fe and Zn sulphides, a paragenesis also observed as nanocrysts in MoSC. Although Mo originated from seawater, its extreme concentration is due to an enhanced growth of bacterial consortia in a diffuse hydrothermal vent system. These consortia may be dominated by nitrogen fixing and Mo dependant hyperthermophilic or mesophilic species containing nitrogenase, composed also of Mo, S, Fe. Organic matter decay led thus to the formation of MoSC and the included framboidal pyrite.

Origin of MoSC phases in Lower Cambrian black shales (Southern China)

Origin of MoSC phases in Lower Cambrian black shales (Southern China)

Re–Os dating of the Lower Cambrian black shales in Guizhou province of South China

Re–Os dating of the Lower Cambrian black shales in Guizhou province of South China

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.

Release of heavy metals during weathering of the Lower Cambrian Black Shales in western Hunan, China

Weathering of heavy metal enriched black shales may be one of the most important sources of environmental contamination in areas where black shales are distributed. Heavy metal release during weathering of the Lower Cambrian Black Shales (LCBS) in western Hunan, China, was investigated using traditional geochemical methods and the ICP-MS analytical technique. Concentrations of 16 heavy metals, 8 trace elements and P were measured for samples from selected weathering profiles at the Taiping vanadium ore mine (TP), the Matian phosphorous ore mine (MT), and Taojiang stone-coal mine (TJ). The results show that the bedrock at these three profiles is enriched with Sc, V, Cr, Co, Ni, Cu, Zn, Pb, Th, U, Mo, Cd, Sb, Tl, and P. Based on mass-balance calculation, the percentages of heavy metals released (in % loss) relative to immobile element Nb were estimated. The results show significant rates of release during weathering of: V, Cr, Co, Ni, Cu, Zn, U, Mo, Cd, Sn, Sb, and Tl for the TP profile; Sc, Cr, Mn, Co, Ni, Cu, Zn, Pb, Th, Cd, and Sn for the MT profile; and Sc, Mn, Co, Ni, Zn, Th, Cd, Sn, and Tl for the TJ profile. Among these heavy metals, Co, Ni, Zn, Cd, and Sn show very similar features of release from each of the three weathering profiles. The heavy metals released during weathering may affect the environment (especially topsoil and surface waters) and are possibly related to an observed high incidence of endemic diseases in the area.

Pb-Pb isotope dating of black shales from the Lower Cambrian Niutitang Formation, Guizhou Province, South China*

Abstract The Lower Cambrian black shales of the Niutitang Formation and their lateral equivalents occur widely on the Yangtze Platform. South China. In this study, we examine two sections of the Niutitang Formation in Guizhou Province. We employ a stepwise acid-leaching technique to obtain Pb-isotope compositions of the black shales. At the Zhongnan section, the black shales display large Pb isotope variations, with 206Pb/204Pb of 20.5396–33.5709, and 207Pb/204Pb of 15.8407–16.5782. In contrast, Pb isotope data show a relatively limited range in 206Pb/204Pb (18.2797–18.5972) and 207Pb/204Pb (15.6813–15.7159) at the Zhijin section. Data for all acid-leached black shale samples from the two sections yield a Pb-Pb isochron age of 531 ± 24 Ma. This age may represent the sedimentary age of the black shales. The initial Pb-isotope composition of the black shales fallsnear the upper-crust evolution curve in a Zartman Pb-Pb evolution diagram. Hence, it is likely that the Pb in the black shales may have been derived from an upper continental crustal source.

Re-Os Dating of Polymetallic Ni-Mo-PGE-Au Mineralization in Lower Cambrian Black Shales of South China and Its Geologic Significance

Re-Os Dating of Polymetallic Ni-Mo-PGE-Au Mineralization in Lower Cambrian Black Shales of South China and Its Geologic Significance

Re-Os Dating of Polymetallic Ni-Mo-PGE-Au Mineralization in Lower Cambrian Black Shales of South China and Its Geologic Significance

Black shales of the basal Lower Cambrian Niutitang Formation, southeast China, host a regionally distributed concordant, several centimeter-thick, sulfide layer which displays extreme metal enrichment, i.e., Mo-NiSe-Re-Os-As-Hg-Sb >1,000 times enriched and Ag-Au-Pt-Pd >100 times enriched over bulk continental crust. Mineable portions have about 5.5 wt percent Mo, 3.5 wt percent Ni, and 1 g/t PGE + Au. A six-point 187 Os/ 188 Os versus 187 Re/ 188 Os isochron on molybdenum-nickel ore samples defines an age of 541 ± 16 Ma (2σ) with an initial 187 Os/ 188 Os ratio of 0.78 ± 0.19. This age is in agreement with the depositional age of the black shale host; the initial ratio is close to present-day seawater. The sulfide layer/average seawater metal ratio is on the order of 10 6 to 10 8 , about 10 to 100 times higher than that for the black shale host and for average metalliferous black shale. Synsedimentary metal enrichment from seawater under anoxic (sulfate-reducing) conditions appears likely but requires an unusually low sedimentation rate and/or high replenishment rate of fresh seawater to the marine basin. The paleogeographic setting of the Lower Cambrian continental margin of the Yangtze craton indicates local basins controlled by synsedimentary rifting. Stagnant water episodically replenished by upwelling oxidized seawater is thought to be responsible for the formation of the polymetallic sulfide layer and of phosphorite, barite, and sapropelic “stone coal” (combustible black shale) beds.

Characteristics of sulfur species and their implications in Lower Cambrian black shales from southern margin of Yangtze Platform

There are different sulfur forms in the black shales from the Early Cambrian of the Yangtze platform. With its emphasis on pyrite and organosulfur, this paper discusses their distribution and formation. The research shows that sulfur phases take regular variations laterally as well as vertically in the research areas. In western researched profile with high terrigenous supply at the time it formed, there exists a larger amount of pyrite and less organosulfur, and pyrite amount declines while organosulfur content increases upwards along the profile. This black shale profile is characterized by relatively light sulfur isotope composition with evolution trend of becoming heavier both for pyrite and organosulfur from bottom to top along the profile. Opposite situation occurs in eastern profiles which were located farther away from terrigenous land. Here pyrite amount obviously decreases and organic matter has combined more sulfur, although these two kinds of sulfur species take similar trend in content variation along profiles to that for western profile. At the same time more 34 S is accumulated in sulfur species of black shale samples from eastern profile, and sulfur isotope composition gradually turns lighter from bottom to top. In combination with other information of iron, organic carbon contents and petrographic features, it can be established that sea-level change, supply of terrigenous matters, tectonic background and natures of paleoceanic chemistry have exerted great influence on the distribution of sulfur species in these black shales.

Facies and diagenesis of some upper proterozoic dolomites of South China

The Dengying Formation exposed at Sansha, Human Province, central South China, comprises unfossiliferous dolomitized platform carbonates and a solution-collapse breccia sequence. Microfacies analysis indicates a cyclic, increasingly restricted lagoonal to oolithic platform marginal facies. Sedimentation was repeatedly interrupted by periods of subaerial exposure as suggested by features of meteoric diagenesis and karstification. During a final regressive phase selective karst-dissolution of strata, interpreted as dolomite-evaporite interbeds, created a solution-residual/collapse breccia sequence. The Dengying Fm. is disconformably overlain by Lower Cambrian transgressive phosphatic conglomerates and fossiliferous black shales. These local facies data add another piece to the puzzle of Proterozoic/Cambrian Yangtze platform sedimentational history and demonstrate the applicability of microfacies analysis to uppermost Proterozoic strata.

Early diagenetic or sedimentary origins of fossiliferous Ni-Mo-Au-Pt deposits in lower cambrian black shales of China : R.M. Coveney, Jr., J.B. Murowchick (Department of Geosciences, University of Missouri, Kansas City, MO 64110, USA) and R.I. Grauch (MS 973, U.S. Geological Survey, Box 25046, Federal Center, Denver, CO 80225, USA)

Early diagenetic or sedimentary origins of fossiliferous Ni-Mo-Au-Pt deposits in lower cambrian black shales of China : R.M. Coveney, Jr., J.B. Murowchick (Department of Geosciences, University of Missouri, Kansas City, MO 64110, USA) and R.I. Grauch (MS 973, U.S. Geological Survey, Box 25046, Federal Center, Denver, CO 80225, USA)