Silurian Black Shales Research Articles

Phenyl derivatives of polycyclic aromatic compounds as indicators of hydrothermal activity in the Silurian black siliceous shales of the Bardzkie Mountains, Poland

High relative concentrations of phenyl (Ph) derivatives of polycyclic aromatic compounds (PACs) were found in samples from the Silurian Żdanów section (Bardzkie Mountains, Poland). These sedimentary rocks were characterized by the occurrence of all previously described Ph-PAC groups, including: phenylnaphthalenes, phenylphenanthrenes, terphenyls, phenyldibenzofurans, phenyldibenzothiophenes, and quaterphenyls, among which the phenylnaphthalenes and phenylphenanthrenes were most abundant. In all samples the thermally more stable isomers clearly dominated, while those with ortho- or α-substitution were present only as traces or absent. This suggests a high maturity of the investigated section, which was also confirmed by other molecular and petrographic thermal maturity parameters. Considering the highest abundances of Ph-PACs occur in the older part of the section, we suggest that the major factor controlling OM maturation was ascending hydrothermal fluids. Moreover, the significant peri-condensed aromatic compounds detected in the samples may have formed by dehydrocyclization of phenyl- and naphthyl-PAC precursors. If the formation of the PhPACs is strictly connected with alteration by ascending solutions, the interpretation of the inorganic geochemical results should be expanded to consider the impact of hydrothermal fluid migration.

Shale Gas Potential of the Major Marine Shale Formations in the Upper Yangtze Platform, South China, Part III: Mineralogical, Lithofacial, Petrophysical, and Rock Mechanical Properties

The marine black shale formations on the Upper Yangtze Platform, South China, are currently exploration targets for shale gas. Here, we report on the mineralogy, lithofacies, petrophysics, and rock mechanics of samples collected from the Ediacaran (Upper Sinian), Lower Cambrian, and Lower Silurian black shale intervals. All three formations are composed of high proportion of quartz, low content of clay, and rare or nonexistent content of carbonates. The Ediacaran and Lower Cambrian shales deposited in restricted deep water marine platform to marine basin environments are characterized by a higher quartz content and lower clay content than the Lower Silurian shales that were deposited in a more restricted marine basin environment. The carbonate content varies from 0 to over 50%, with the higher values measured in the Lower Silurian samples. These stratigraphic units were formed during bottom water anoxic conditions; therefore, they were rarely influenced by bioturbation. Lithologically, laminated and nonlaminated siliceous mudstones predominate, with minor contributions of other lithotypes. Pores generally have diameters in the nanometer (nm) to micrometer (μm) range, and numerous pores occur in organic matter. Most of the measured samples have porosities less than 4%, although a few samples show porosity in excess of 10%. Pores with radii less than 50 nm contribute significantly to total pore volume and total porosity. Permeability is extremely low, and helium permeability coefficients (Klinkenberg corrected permeability coefficient) are less than 20.2 nD (nano-Darcy, ∼2 × 10–20 m2). The rock mechanical properties of the samples are characterized by high brittle behavior, which coincides with their high compressive and tensile strengths and elastic properties. The Lower Cambrian shale is generally more brittle than the Lower Silurian shales, which possess a relatively higher content of clay minerals. The rock mechanical properties of the measured samples, however, depend on the overall mineral compositions and physical properties.

Accumulation Conditions and Gas Content Characteristics of the Lower Silurian in Northwestern Hunan, China

The Lower Silurian marine shale is widely distributed in Northwestern Hunan and features in a large thickness of dark shale showed at outcrops. However, the accumulation conditions and gas content is unknown. The sedimentary facies, thickness and distribution, organic matter types and content, maturity, reservoir properties and gas content of the Lower Silurian black shale in Northwestern Hunan were investigated by field observation, sampling and experimental analysis. The results show that the black shales sedimentary environment is deep water continental shelf, with featured in abundant fossil. The burial depth of the Lower Silurian black shale is 0-3000 m, and its thickness is 10-40m while the average TOC is 1.0% and average Ro is 2.9%. For the disadvantaged sedimentary facies and shallow depth, the maximum gas content of the Lower Silurian black shale from well site desorption test is 0.59m3/t only, but the result of isothermal adsorption simulate test show that the Lower Silurian black shale have a good adsorption capacity, and can generate a large number of shale gas in Northwestern Hunan where better deposition conditions and suitable depth exist in.

A preliminary study on the pore characterization of Lower Silurian black shales in the Chuandong Thrust Fold Belt, southwestern China using low pressure N2 adsorption and FE-SEM methods

Although Lower Silurian black shales within and around the Sichuan Basin have received renewed attention during the past few years because of their potential prospectivity for shale gas, studies associated with shale gas reservoir evaluation are still rare. In this study, eight black shale core samples were collected from a well drilled recently in the Chuandong Thrust Fold Belt, southwestern China, and their geochemistry and pore structures were investigated using low pressure N2 adsorption analysis and field emission scanning electron microscopy (FE-SEM) observations. The results show that the black shales have total organic carbon (TOC) values ranging from 1.01% to 3.98% and their equivalent vitrinite reflectance values are in the range of 2.84%–3.05%. Both mineral matrix and organic matter pores are well developed with pore sizes ranging from several to several hundred nanometers. The total porosity for the eight samples ranges from 2.60% to 4.74% and the percentages of organic matter pores are estimated to be in the range of 31%–62%. The total surface area ranges from 5.06 m2/g to 19.32 m2/g and the micropore (<2 nm) surface area estimated by the t-plot method ranges from 3.13 m2/g to 9.27 m2/g. The TOC values have positive relationships with the total porosity, total surface area and the micropore (<2 nm) volume and surface area, indicating TOC may be an effective parameter for shale gas reservoir evaluation in the studied area.

The late Aeronian graptolitesedgwickiiEvent, associated positive carbon isotope excursion and facies changes in the Prague Synform (Barrandian area, Bohemia)

AbstractStudy of the lower Silurian black shale succession of the Prague Synform has enabled detailed insight into graptolite faunal dynamics and diversity trends from the mid-Aeronian diversity maximum through to the late Aeronian crisis. Graptolite diversity decreased from 33 taxa in theLituigraptus convolutusBiozone to 17 taxa in the upper part of theStimulograptus sedgwickiiBiozone and newly erectedLituigraptus rastrumBiozone. The graptolite assemblages of the latter biozones exhibit low species richness along with high dominance. Many graptolite species that became extinct in the early part of thesedgwickiiZone were promptly replaced by new forms. In the later part of thesedgwickiiZone, however, replacement of extinct species by new forms considerably decelerated. The increased rate of graptolite extinction recorded in theconvolutus–sedgwickiibiozone boundary beds coincided with subtle changes in black shale lithologies and a positive shift in δ13Corg(of 2.2 ‰) compared to baseline values. Sea-level drawdown can be inferred from siltstones and/or temporary nondeposition in the middlesedgwickiiZone. This level also sees total organic carbon (TOC) fluctuations and a strong positive δ13Corgexcursion with a peak shift of at least 7 ‰. ThesedgwickiiEvent exhibits substantial reorganization of the graptolite fauna, its taxonomic impoverishment and concomitant increase in species dominance rather than a sudden collapse of the pre-extinction assemblage. Associated changes in lithology, TOC and the pronounced δ13Corgexcursion suggest a relatively extended and probably multi-phase period of stressed conditions that affected the pelagic realm inhabited by graptolites in the course of the late Aeronian interval.

Supergene processes and uranium ore formation in the Ronneburg ore field, Germany

The Ordovician-Lower Carboniferous sequence of slightly metamorphosed gray carbonate-terrigenous rocks contains the Silurian black cherty shales enriched in carbon (6–9%), pyrite (6–7%), and uranium (∼30 ppm). The uranium ore is localized at the pinch-out of areal and linear zones of the Early Permian supergene (exogenic) oxidation of rocks expressed in reddening (hematitization). U, As, Sb, Cu, Ni, Mo, and Ag have been removed from the oxidized black shales and concentrated in the cementation zone in form of pitchblende and sulfides in wall-rock disseminations and veinlets largely hosted in carbonate-bearing rocks. In the Late Permian, during deposition of the upper Rotliegende and Zechstein, the fractures in the basement were filled with carbonates and sulfates; uranium was partly redeposited along with enrichment in Pb and Zn. Mesozoic and Cenozoic supergene processes altered uranium ore insignificantly.

Paleoenvironment of Lower Silurian Black Shale and its Significance to the Potential of Shale Gas, Southeast of Chongqing, China

Black shale is studied because of its economic value, paleoclimatic and paleogeographic significance. Due intensive development southeast of Chongqing, Longmaxi black shale, characterized by thickness in excess of 100 m, is becoming a location of concentrated research interest for shale gas exploration in China. The paleoenvironment, mineral composition, elements and reservoir characteristics of black shale of the Lower Silurian Longmaxi formation southeast of Chongqing were obtained by cores, outcrops, slices analysis, X-ray diffraction and X-ray fluorescence spectrum. The results show that Longmaxi black shale, including turbidite and suspended sediment, is deposited primarily in deep water. The sum of SiO2, Al2O3 and Fe2O3 content ranges from 73.82% to 90.54% by weight; quartz, feldspar and clay minerals are the dominant minerals in the rocks, ranging from 56% to 100%. Trace element contents of Mo (7.71 μg/g) and U (5.97 μg/g) are higher than that of earth crust and mudstone, the indexes of the anoxic reducing environment in the Longmaxi stage in the study area. The relative sea level of the Lower Silurian Longmaxi period, which correlates to total organic carbon content, experienced two rise-and-fall cycles; meanwhile, the reducibility of the ancient sea waters experienced a similar period of rise-and-fall cycles. Total organic carbon content is quite higher near the maximum flooding surface in study area. After mineral analysis, the reservoir property was found to be similar to the Ohio shale that provided the first commercial gas production in the United States, thereby inspiring and instructing shale gas exploration and exploitation southeast of Chongqing.

Black shales in Ukraine – A review

As a result of a long-lasting and complex geological history, organic-matter-rich fine-grained rocks (black shales) with widely varying ages can be found on Ukrainian territory. Several of them are proven hydrocarbon source rocks and may hold a significant shale gas potential.Thick Silurian black shales accumulated along the western margin of the East European Craton in a foreland-type basin. By analogy with coeval organic-matter-rich rocks in Poland, high TOC contents and gas window maturity can be expected. However, to date information on organic richness is largely missing and maturity patterns remain to be refined.Visean black shales with TOC contents as high as 8% and a Type III-II kerogen accumulated along the axis of the Dniepr-Donets rift basin (DDB). They are the likely source for conventional oil and gas. Oil-prone Serpukhovian black shales accumulated in the shallow northwestern part of the DDB. Similar black shales probably may be present in the Lviv-Volyn Basin (western Ukraine).Middle Jurassic black shales up to 500 m thick occur beneath the Carpathian Foredeep. They are the likely source for some heavy oil deposits. TOC contents up to 12% (Type II) have been recorded, but additional investigations are needed to study the vertical and lateral variability of organic matter richness and maturity.Lower Cretaceous black shales with a Type III(-II) kerogen (TOC > 2%) are widespread at the base of the Carpathian flysch nappes, but Oligocene black shales (Menilite Fm.) rich in organic matter (4–8% TOC) and containing a Type II kerogen are the main source rock for oil in the Carpathians. Their thermal maturity increases from the external to the internal nappes.Oligocene black shales are also present in Crimea (Maykop Fm.). These rocks typically contain high TOC contents, but data from Ukraine are missing.

Evidences for secondary cracking of oil in South Pars field, Persian Gulf, Iran

Condensates and natural gases in South Pars, world's largest gas field in Persian Gulf were studied for their geochemical characteristics and possibility of secondary cracking as a major gas producing mechanism. Carbon isotopic analysis of gas components proposes thermogenic origin for South Pars gas field. However, δ 13C values of Methane and Ethane are slightly depleted in comparison with generated gas from primary thermogenic gas whereas gases from secondary cracking of oils are systematically depleted in 13C in Methane and Ethane compared to gases from primary cracking. Gas composition of Head-Space gas samples were plotted in Ln(C 1/C 2) vs. Ln(C 2/C 3) diagram that all the data points indicate a noticeable shift toward trend of secondary cracking, indicating frequency of higher components i.e. C 2+. Silurian black shales are recognized as most important source rock for the South Pars field. Biomarker analysis of hydrocarbon liquid samples indicates Pristane to Phytane ratio is > 1 that could confirm mentioned shaly source rock which is deposited in suboxic to oxic marine environment. Considering of δ 13C of Silurian shales the “δ 13C Methane-δ 13C Source” was calculated and plotted against C 1/ΣC 1–5 indicating effect of secondary cracking of oil as producing mechanism for South Pars gas field. Moreover, pyro-bitumen as product of the secondary cracking was found filling secondary porosities in bottom part of the reservoir.

Coexistence of pyrophyllite, I-S, R1 and NH4+-rich illite in Silurian black shales (Sierra de Albarracín, NE Spain): metamorphic vs. hydrothermal origin

AbstractA set of Silurian black shales from Sierra de Albarracín (NE Spain) corresponding to two different sections was studied to determine the relative influence of diagenesis, igneous activity, and regional tectonics on the clay-mineral genesis. The coexistence of pyrophyllite, I-S interstratifications (R1), ammonium-rich illite, potassium illite, kaolin, and chlorite is not the result of prograde evolution during diagenesis – very low-grade metamorphism. Three different stages may be inferred: (1) sedimentation of black shales (Aeronian, Lower Silurian, to basal Ludfordian, Upper Silurian) and the subsequent diagenetic process producing the coexistence of quartz, illite, kaolinite, organic matter, etc.; (2) intrusion of andesitic sills producing hydrothermal alteration and crystallization of pyrophyllite, ammonium-rich illites, smectite, I-S R1 phases and jarosite; and (3) and folding of shales and sills and development of penetrative schistosity during the late Variscan leading to illite and paragonite recrystallization reaching the anchizone grade.

Changes in productivity and redox conditions in the Panthalassic Ocean during the latest Permian

The Gujo-Hachiman section in central Japan provides a rare window into environmental conditions within the Panthalassic Ocean, which encompassed more than half the Earth’s surface at 251 Ma. The section is characterized by a sharp transition from green-gray organicpoor cherts to black siliceous shales in the uppermost Permian. Normalization to the clay fraction demonstrates that apparent increases in the concentrations of organic matter and trace metals above this transition were due primarily to the loss of a diluent biogenic (radiolarian) silica fl ux and only secondarily to a small shift toward more reducing bottom waters. In the black shale, pyrite abundance increases by a factor of ~30× and is dominated by framboidal grains of probable syngenetic origin. These observations suggest that the expansion of lowoxygen conditions within the Panthalassic Ocean was focused within the oxygen-minimum zone rather than at the seafl oor. Such a pattern implies that (1) changes in nutrient fl uxes and primary productivity rates, rather than stagnation of oceanic circulation, were a key factor infl uencing oceanic redox conditions around the Permian-Triassic boundary, and (2) large regions of the Panthalassic Ocean underwent only limited redox changes, providing potential refugia for marine taxa that survived into the Triassic.

Identification of early Llandovery (Silurian) anoxic palaeo-depressions at the western margin of the Murzuq Basin (southwest Libya), based on gamma-ray spectrometry in surface exposures

ABSTRACT Following the melting of the Gondwanan icecap and the resulting postglacial sea-level rise, organic-rich shales were deposited in shelfal palaeo-depressions across North Africa and Arabia during the latest Ordovician to earliest Silurian. The unit is absent on palaeohighs that were flooded only later when the anoxic event had already ended. The regional distribution of the Silurian black shale is now well-known for the subsurface of the central parts of the Murzuq Basin, in Libya, where many exploration wells have been drilled and where the shale represents the main hydrocarbon source rock. On well logs, the Silurian black shale is easily recognisable due to increased uranium concentrations and, therefore, elevated gamma-ray values. The uranium in the shales “precipitated” under oxygen-reduced conditions and generally a linear relationship between uranium and organic content is developed. The distribution of the Silurian organic-rich shales in the outcrop belts surrounding the Murzuq Basin has been long unknown because Saharan surface weathering has commonly destroyed the organic matter and black colour of the shales, making it complicated to identify the previously organic-rich unit in the field. In an attempt to distinguish (previously) organic-rich from organically lean shales at outcrop, seven sections that straddle the Ordovician-Silurian boundary were measured by portable gamma-ray spectrometer along the outcrops of the western margin of the Murzuq Basin. It was found that the uranium content of the shales remained largely unaltered by the weathering processes and could therefore be used as a valid proxy parameter to distinguish between pre-weathering organically rich and lean shales. It is now possible to identify and map-out the thickness and approximate organic richness of the black shale using measurement of uranium radiation. Five of the newly measured sections are characterised by uranium-enriched intervals, representing areas of earliest Silurian palaeo-depressions. Major uranium peaks are absent in the spectral gamma-ray curves of two other sections, which are interpreted to mark earliest Silurian palaeo-highs. The new data on the distribution of Silurian black shales from the outcrop belt was integrated with subsurface data from the Murzuq Basin. The resulting map of the distribution of black shales may help with predictions of the occurrence of this unit in less well-explored areas of the basin. Graptolite biostratigraphic data suggests that the anoxic event centred on the middle Rhuddanian, with more oxygenated conditions and onset of deposition of organically leaner shales having commenced sometime during the late Rhuddanian. The presence of anoxic palaeo-depressions during the earliest Silurian within the Ghat outcrop belt indicates that the Tihemboka High at the western margin of the Murzuq Basin could not have been a positive structure during this time.

THE COMPOSITION OF Co-Ni-Fe SULFARSENIDES, DIARSENIDES AND TRIARSENIDES FROM THE SAN JUAN DE PLAN DEPOSIT, CENTRAL PYRENEES, SPAIN

The San Juan de Plan deposit, in the central Pyrenees of Spain, is situated in an ankeritic horizon in Silurian black shales. The main association consists of pyrite, sulfarsenides, diarsenides, and Co triarsenides (skutterudite), as well as a late stage consisting of cobaltite, Fe, Cu and Bi sulfides and native Bi. The sulfarsenides are arsenopyrite, alloclasite, members of the gersdorffite‐ cobaltite solid-solution series, arsenic-rich gersdorffite and cobaltite, whereas the diarsenides range from rammelsbergite to safflorite. Rammelsbergite is characterized by a composition within the range (Ni0.71‐0.97Co0.02‐0.29Fe0.00‐0.02)As1.75‐2.01S0.00‐0.26, exceeding the theoretical value (As1.93S0.07) given by Yund (1962). Two generations of the gersdorffite‐cobaltite solid-solution series have been found on the basis of textural features and chemical composition; the first, (Co0.10‐0.77Ni0.07‐0.80Fe0.03‐0.58)As0.95‐ 1.27S0.75‐1.06, shows a positive correlation between S and Fe contents, and between Co and As contents, whereas the second one, (Co0.02‐0.91Ni0.02‐0.96Fe0.00‐0.28)As0.99‐1.38 S0.67‐1.01, exhibits a considerable compositional field in the Fe-poor region of the system CoAsS‐NiAsS‐FeAsS , providing evidence of extensive substitution of Co by Ni, with a positive correlation between As and Ni contents. Arsenic-rich gersdorffite displays a broad range of As and S mutual substitution, (Ni0.65‐0.98Co0.02‐0.27Fe0.00‐0.06)As1.37‐ 1.83S0.22‐0.64, exceeding the experimental value of NiAs1.77S0.23 reported by Yund (1962). The presence of alloclasite and the extent of the solid-solution fields of the cobaltite‐gersdorffite series suggest that the main assemblage of minerals at the San Juan de Plan deposit formed at temperatures as high as 600°C. Preliminary geothermometric data obtained on graphite support such a high temperature.

FIRST DISCOVERY OF MIDDLE TOURNAISIAN CONODONTS IN THE GRIOTTE-TYPE NODULAR PELAGIC LIMESTONES, ISTANBUL AREA, NW TURKEY

The uppermost layers of the Ayineburnu member of the Buyukada Formation in the Istanbul-Gebze area are represented by Griotte-type pelagic nodular limestones. They yielded conodont elements (Bispathodus stabilis and Siphonodella lobata) that can iden- tify the interval from the upper part of the sandbergi Zone through the isosticha- Upper crenulata Zone - middle Tournaisian - Early Carbonif- erous. This is the first middle Tournaisian conodont record published from an autochthonous succession in the Istanbul Terrane, NW Tur- key. This level characterizes the very last carbonate deposition and is followed by lydites and black siliceous shales and finally by the accu- mulation of flysch-type deposits during the late Tournaisian and early Visean, that mark the onset of Variscan events to the north of the Is- tanbul Terrane. A correlation with the Lower Carboniferous succes- sions in the easterly-located Zonguldak area suggests that Istanbul and Zonguldak terranes were in quite different palaeogeographic positions during this time interval.

Geology and Metallogenic Evolution of the PolymetallicDeposits of the Alcudia Valley Mineral Field, Eastern Sierra Morena, Spain

The Alcudia Valley is intensely mineralized, with a large number of Pb-Zn-Ag-Cu deposits hosted by Neoproterozoic and Paleozoic sedimentary rocks. Five distinct types of deposits have been recognized. The most significant mineralizing events were related to the two major phases of deformation (HD1, HD2) and granite emplacement during the Hercynian orogeny. Stable isotope and lithogeochemical data demonstrate that the metals, as well as sulfur and carbon, were sourced from the local sedimentary rocks. Type A deposits are minor occurrences of strata-bound mineralization with Zn > Pb, occurring as disseminations and small veins in Late Ordovician limestone. A syndiagenetic origin is suggested by the limited stratigraphic distribution of the mineralization and anomalous base metal contents of Late Ordovician-Early Silurian black shales overlying the mineralized limestone. Minor remobilization of disseminated sulfides into joints took place during late diagenesis or early Hercynian deformation. Types B, C, and D are syntectonic deposits with Zn > Pb. Type B deposits are strongly deformed veins in HD1 fractures in Ordovician rocks. A process involving local mobilization of metals by a surface-derived fluid is indicated by depleted metal contents of host rocks adjacent to the veins, the similarity of δ 34S values of ore sulfides with disseminated pyrite in the host rocks, and the δ 18OH2O values. Type C deposits exhibit a greater variation in morphology and degree of deformation compared to other types and are found in both HD1 and HD2 fractures in Late Ordovician and Silurian rocks. The stratigraphic distribution and δ 34S values of sulfides are similar to type A deposits, suggesting that localized hydrothermal systems either remobilized type A mineralization or derived metals and sulfur from the same source. Type D deposits occur in HD2 shear fractures in Neoproterozoic rocks. The δ 13C values of carbonate gangue and the similarity of δ 34S values of ore sulfides and disseminated pyrite in the host rocks indicate that black shales of the Neoproterozoic sequence are the major source of sulfur and carbon. Mixing of a metamorphic fluid with a surface-derived fluid is suggested by fluid inclusion and oxygen isotope data. Type E deposits, the most abundant and economically most important type, are post-tectonic Pb > Zn veins occupying HD2 fractures and are widely distributed through the district in rocks ranging from Neoproterozoic to Late Ordovician. Mineral assemblages and Ag contents of the veins show a zonal relationship with respect to monzogranite outcrops, indicating that late Hercynian magmatism was the source of heat driving fluid migration through the fracture system. Fluid inclusion and oxygen isotope data are typical of basinal brines. Reaction of the fluid with black shales of the Neoproterozoic sequence is suggested by δ 34S values of sulfides and δ 13C values of carbonates of the main stages of the paragenetic sequence. The final paragenetic stage consists of barite, calcite, and pyrite, which may have been deposited during a later hydrothermal event, probably in the Early Triassic.

First finding of Late Silurian conodonts from the "Orthoceras Limestones", Camdag Area (NW Turkey): prelilninary constraints for the palaeogeography

The Paleozoic succession of the Camdag area is located between the Paleozoic rocks of the Istanbul and Zonguldak terranes in NW Turkey and has a key location for the paleogeographic position of these terranes. The Silurian deposits in this area includes from bottom to top: the Black Shale Member (gray to greenish gray, well cleaved shales with minor black siltstone and limy shale interlayers), the Shale-Limestone Member (black shales with limestone and dolomitic limestone interlayers), and the Shale-Si Itstone Member (alternation of black shales, light gray quartz-rich siltstones with few limestone lenses) that is concordantly followed by Lower Devonian deposits. The "Orthoceras Limestones" of the midc\le member yielded a poorly preserved conodont fauna indicating a Pridoli age. By this, the Paleozoic of the Camdag is considered to be the eastern continuation of the Istanbul Terrane. Moreover, this new finding of Late Silurian black shale - "Orthoceras Limestone" assemblage in NW Turkey, which is a wide spread paleogeographic marker in the Camic Alps and elsewhere in Europe may help to correlations with coeval and analogous facies of other paleobiogeographic areas in the peri-Gondwanan realm.

Sulphur isotope composition of Silurian shale-hosted PGE-Ag-Au-Zn-Cu mineralisations of the Prades Mountains (Catalonia, Spain)

Several Silurian metamorphosed shale-hosted sulphide occurrences have been studied in the Prades Mountains, southern part of the Coastal Catalonian Ranges. Most of the sulphides are found as stratiform or shale-disseminated occurrences. Pyrrhotite is the most abundant sulphide mineral. Chalcopyrite and arsenopyrite are less common. Gold, Pd-bearing lollingite (partly replaced by arsenopyrite), sperrylite, hessite, clausthalite, altaite, galena, sphalerite, molybdenite, scheelite, and V-Cr oxides and silicates are minor components. Sulphur isotopic analyses were made on three outcrops (Roca de Ponent, Coma Fosca and Sant Miquel) and in the Silurian black shales of the Sant Bernat series, in order to determine the origin of the sulphur which formed these deposits. The Coma Fosca and Sant Miquel outcrops yield a narrow range of δ34S values (–11.3 to –4.6‰), whereas the Roca de Ponent and Sant Bernat series have a wider range (–19.9 to –7.6‰ in pyrrhotite, and up to +36‰ in pyrite from the top of the Sant Bernat series). In the Roca de Ponent outcrop the δ34S values decrease with increasing stratigraphic height. All the deposits have a mean δ34S composition close to –9‰. Annealing of pyrrhotite took place during the Hercynian metamorphism. Later, during the retrograde metamorphism, pyrite replaced pyrrhotite, and arsenopyrite replaced lollingite. Large-scale compositional inhomogeneities survived metamorphism (from pyroxene to amphibolite facies). A hydrothermal source of S has been postulated for the Coma Fosca and Sant Miquel outcrops. Sulphur in these deposits is from leaching of sulphides from the underlying rocks. In the Roca de Ponent outcrop and in the Sant Bernat series, sulphur was derived from a hydrothermal source and from biogenic reduction of seawater sulphate. The contribution of the latter source increased with time. Biogenic reduction of seawater sulphide in a closed or semi-closed system for sulphate is considered responsible of the high δ34S values of pyrite from the uppermost part of the Sant Bernat series. Based on textural and isotopic evidence, we propose an exhalative origin for the precious metals, cogenetic with the stratiform sulphides.

SILURIAN — LOWER DEVONIAN BLACK SHALES IN MOROCCO: WHICH ARE THE ORGANICALLY RICHEST HORIZONS?

Lowermost Silurian (Rhuddanian, lower Llandovery) black shales represent the main Palaeozoic petroleum source rock throughout North Africa and Arabia. The unit also occurs in parts of Morocco with recorded TOC values of up to 10.5%. However, in contrast to many other North African and Arabian countries, the Silurian‐Lower Devonian shale‐dominated succession in Morocco also contains a number of other horizons with elevated organic contents. In order to evaluate the organic richness and better understand the depositional mechanisms of this shale succession in Morocco, samples were collected from petroleum exploration wells, from the spoil heaps of shallow water wells and from outcrops, and were subsequently analysed. Graptolite biostratigraphy provided a high‐resolution correlation framework. The data was integrated with that from published and unpublished studies, and the results may help in improved predictions of the source quality of Silurian—Lower Devonian strata in Morocco.An Aeronian (middle Llandovery) shale sample with a high organic content (4.35% TOC) was recovered from the NE margin of the Tadla Basin, central Morocco. Based on gamma‐ray data from the subsurface, however, this horizon appears to be laterally discontinuous within the basin. Secondly, late Telychian‐Wenlock shales from the eastern Atlas Mountains were found to have TOC values of around 2.5%, and may be correlated with age‐equivalent organic‐rich strata in the Ghadames Basin (eastern Algeria, western Libya, southern Tunisia) and Iraq. Late Silurian shale‐limestone alternations in Morocco apparently do not contain major amounts of organic mattel; although comparable deposits in parts of western Algeria are believed to be organic rich. Early Devonian graptolitic black shales from the Tadla Basin and its margins contain high amounts of organic matter (around 5% TOC); however; the lateral continuity of this unit is at present unclear.We propose that high primary productivity during the Silurian‐Early Devonian provided the basis for the formation of these organic‐rich shales in Morocco. Sea level changes may have been an important additional factor. For example, the Rhuddanian organic‐rich shales were deposited during the initial stages of a transgression, when circulation was still restricted due to the pronounced pre‐Silurian relief. At other times during the Silurian‐Early Devonian in Morocco, deposits with elevated organic content seem to have been formed during periods of high sea level, which may have been associated with high primary productivity and/or a rise in the oxygen minimum zone onto the shelf.

Characteristics and origin of the Middle Proterozoic Dongshuichang chambersite deposit, Jixian, Tianjin, China

The Jixian-type boron–manganese (B–Mn) deposits include both chambersite and rhodochrosite deposits, from which Dongshuichang is economically the best chambersite deposit. Although the ore bodies of the region are relatively small, they contain a rare B–Mn ore that is scientifically significant. Chambersite (Mn 3B 7O 13Cl) is a rare mineral, discovered as a mineral in 1957 and as an ore in 1971 at the Dongshuichang deposit near Jixian, Tianjin City. The deposit forms a monocline with a southwest dip and covers an area of 2 km 2. In the mine area, the Gaoyuzhuang Formation, the upper member of the Middle Proterozoic Changcheng System, consists mainly of stromatolitic and muddy dolostone with B–Mn-bearing siliceous shale and dark-gray dolostone. Individual ore lenses range in length from 1 to 4 m with a mean thickness of 0.25 m. The ores can be divided into high-grade and low-grade, the former is composed of spindle and granular chambersite aggregates and the latter of spheroidal chambersite in dark-gray micritic dolostone. The center of spheroids is commonly composed of micritic and columnar chambersite, dolomite, clay minerals, and organic matter, around which columnar chambersite grew, forming a sunflower shape. The organic-carbon content in high-grade ores is 0.63%; in low-grade ores, 0.83%; and in overlying black siliceous shale, 3.75%. Total rare earth element (REE) contents of two chambersite ore samples are 29 and 35 ppm, whereas that of rhodochrosite ore is 22 ppm. The chondrite-normalized REE patterns of chambersite and rhodochrosite ores are similar, with small negative Ce and Eu anomalies and a small positive Gd anomaly. The δ 13 C PDB values of two rhodochrosite ore samples are −4.3‰ and −7.3‰, indicating that the carbon source of Mn-carbonate minerals was mainly seawater bicarbonate with a small contribution from the degradation of organic carbon. The source of the Dongshuichang ore components was mainly submarine volcanic eruptions and a small amount from the weathering of pre-Proterozoic boron deposits. The chambersite precipitated under conditions of high salinity, high B, Mn, and Mg concentrations, and weak alkalinity. Low-oxygen conditions might be favorable for precipitation of chambersite.

A diverse metaphyte assemblage from the Neoproterozoic black shales of South China

A fossil assemblage of diverse megascopic benthic algae is well preserved on bedding planes in black siliceous shales of the Neoproterozoic Lantian Formation (upper Sinian) at Lantian village, Xiunng county, Anhui Province, South China. Based on morphological differences, 12 taxa are recognized. Most specimens show both a holdfast and a vegetative thallus, some with dichotomous branching, which reveals that they are multicellular benthic algae. Each type presents a certain morphological appearance, e.g., spheroidal - Chuaria sp., filamentous - Doushantuophyton rigidulum, fan-like - Flabellophyton lantianensis. Four to six different benthic forms and more than 15 individuals are well preserved on the same bed with an area less than 150 cm2. These confirm that a great morphological evolutionary radiation of seaweeds took place no later than the Lantian epoch, with an absolute age around 570 Ma. Growth cycles in Flabellophyton strigata n.sp. reveal that periodical events, such as seasonal variations, occurred in that epoch. A benthic assemblage consisting of three differentiated macroscopic remains is similar to the Miaohe Biota reported from the Doushantuo Formation of the Yangtze Gorges 1200 km away, allowing biostratigraphic correlation of both of these Neoproterozoic Formations.