Framboidal Pyrite Research Articles

Geological factors controlling variations in the mineralogical and elemental compositions of Late Permian coals from the Zhijin-Nayong Coalfield, western Guizhou, China

Determination of the geochemical and mineralogical characteristics of coal can provide useful information on the geological setting and evolution of coal basins and assist in identifying factors that control the distribution of the minerals and elements of interest. The Late Permian coals of southwestern China have received much attention because some of these coals are usually considered to be potential sources of some critical metals and have high concentrations of potentially toxic elements.Mineralogical and geochemical analyses were carried-out on the Late Permian coals from the Kulishu and Sijichun mines of the Zhijin-Nayong Coalfield, Western Guizhou, China. The diagram of Al2O3/TiO2-Nb/Yb shows that the sediment source of the Kulishu and Sijichun coals is dominantly the Emeishan magmatic province with a smaller contribution from a Neoproterozoic metamorphic-granite complex, both of which are the two major components of the Kangdian Upland. The sediment source of the roof and floor rocks is dominantly basaltic volcanics in the Emeishan large igneous province (ELIP), while the partings were probably derived from eruptions of the Emeishan felsic-alkaline magmas and a mixture of ashes derived from a distal volcanic arc. Highly elevated total sulfur contents (dominated by pyritic and organic sulfur), abundant framboidal pyrite, as well as disseminated fine-grained pyrite provide evidence for input of seawater during the peat and coal formation. Paragonite mineralization with enrichment of Li, Nb (Ta), Zr (Hf), REY and Pb in the Sijichun coals and partings of the Kulishu coal are attributed to the intrusion of alkaline magma. The pyrite, quartz, albite, anatase and gypsum occurring as veins-, cleat-fillings and separate well-crystallized aggregates of epigenetic origin in the studied coals are indicative of the influence of hydrothermal fluids from a magmatic source.

Enrichment and distribution of elements in the middle Miocene coal seams in the Orhaneli coalfield (NW Turkey)

This study focuses on ascertaining controlling factors and differences on mineralogical and elemental contents for reconstructing palaeoenvironmental conditions of both seams applying standard coal characteristics, mineralogical, coal-petrologic, and elemental data of the Orhaneli coalfield. The studied coalfield, one of the important coalfields in NW Turkey, includes two workable coal seams, the lower with 0.5–2 m and the upper with 5–6 m in thickness, which are separated by 2–5 m thick, grayish-green limnic claystone. The floor rocks of the both seams are limnic claystones, whereas the roof rock of the upper seam is the tuffite layers-bearing lacustrine marls. Matrix and xylite-rich lithotypes are common in the upper seam, while alternations of xylite-clayey bands (detro-xylite and xylo-detritic) are common in the lower seam. The lower seam is also characterized with the presence of altered tuff layers, while freshwater gastropod shell-bearing bands were identified from the upper seam. Even though the banding character and maceral composition of the coals were invariant between the seams, there are distinct differences in mineralogy and elemental composition, that can be related to the nature of the clastic sediment influx and contemporary volcanic inputs into freshwater forested mires.The lower seam is characterized by high ash yields (%44.2–64.3, d) and relatively high concentrations of aluminosilicate-affiliated elements and REY (rare earth elements and Y). In contrast, the upper seam has relatively low ash yield, and is slightly enriched B concentration. The B enrichment in the upper seam is presumably controlled by the presence of clay mineral (e.g., illite) and clastic volcanogenic accessory minerals (e.g., apatite). Arsenic is the only enriched element in both seams due to development of anoxic conditions. Although no traceable As was detected from syngenetic framboidal pyrite grains, the contemporary volcanic inputs (e.g., air-fall ash/tephra) along with leached surface waters into palaeomires might be source for As, while the development of anoxic conditions palaeomires could cause formation of syngenetic pyrite grains. Thus, the As concentrations of the studied samples are elevated. Nevertheless, the difference on ash yields and mineralogical compositions between studied seams might be related with different clastic influx ratios and contemporary volcanic inputs, in turn, aluminosilicate affiliated elements display different concentration coefficients (CC) values in the studied seams, and high B concentrations that are not expectable for a limnic coal Besides, enrichment of Cr, V and Ni in the upper seam are controlled by clastic V-bearing chromite grains from Orhaneli chromite deposit in the adjacent area. Furthermore, the contemporary volcanic inputs into the palaeomire of the lower seam, and their alteration under anoxic conditions caused REY enrichment and the formation of kaolinite and smectite matrices of clay aggregates in coal and altered tuff layers in this seam.

Depositional Conditions of Cretaceous Ironstones Deposit in the Chulym-Yenisey Basin (Western Siberia)

This study reconstructs the depositional conditions of ironstones within the Chulym-Yenisey basin and assesses the iron source. The detrital minerals of the studied deposits include quartz and feldspar. The authigenic minerals are goethite, siderite, aragonite, dolomite, calcite, apatite, barite, and pyrite. The clay components include minerals of the chlorite group (possible chamosite), nontronite, kaolinite, illite, and beidellite. Local bacterial sulfate reduction led to the formation of pyrite framboids in siltstone layers. The subsequent diagenetic iron reduction promoted the formation of chamosite from siderite. The goethite precipitation occurred in an oxidic aqueous environment. The Cretaceous continental sediments of the Ilek and Kia Formations of the Chulym-Yenisei depression consist of fine- and medium-grained, cross-stratified, poorly sorted litho-feldspatho-quartzose sandstones of fluvial channel origin alternating with bluish-gray siltstones and ironstones of floodplain–lacustrine–bog origin. Thin layers of iron-bearing rocks within siltstones formed in meromictic waters. The changes in geochemical proxies demonstrate fluctuations of paleoenvironmental conditions within the Cretaceous sequence. Siltstones and sandstones formed under humid and arid conditions, respectively. The primary iron source for sediments of the Chulym-Yenisey depression was determined as volcanogenic and igneous rocks of the Altai-Sayan mountainous region.

Deformation-enhanced diagenesis and bacterial proliferation in the Nankai accretionary prism

Abstract. Understanding diagenetic reactions in accreted sediments is critical for establishing the balance of fluid sources and sinks in accretionary prisms, which is in turn important for assessing the fluid pressure field and the ability for faults to host seismic slip. For this reason, we studied diagenetic reactions in deformation bands (shear zones and veins) within deep mud sediments from the Nankai accretionary prism (SW Japan) drilled at site C0001 during IODP Expedition 315, by means of microscopic observation, X-ray diffraction, and major- and trace-element analyses. Deformation bands are not only more compacted than the host sediment but are also enriched in framboidal pyrite, as observed under microscopy and confirmed by chalcophile-element enrichments (Fe, S, Cu, As, Sb, Pb). In tandem, one shear zone sample displays a destabilization of smectite or illite–smectite mixed layers and a slight crystallization of illite relative to its sediment matrix, and another sample shows correlated increases in B and Li in shear zones and veins compared to the host sediment, both effects suggesting a transformation of smectite into illite in deformation bands. The two diagenetic reactions of sulfide precipitation and smectite-to-illite transformation are explained by a combined action of sulfate-reducing and methanogen bacteria, which strongly suggests an increased activity of anaerobic microbial communities localized in deformation bands. This local bacterial proliferation was possibly enhanced by the liberation of hydrogen from strained phyllosilicates. We suggest that the proliferation of anoxic bacteria, boosted by deformation, may contribute to the pore water freshening observed at depth in accretionary prisms. Deformation-enhanced metabolic reactions may also explain the illitization observed in major faults of accretionary prisms. Care is therefore needed before interpreting illitization, and other diagenetic reactions as well, as evidence of shear heating, as these might be biogenic instead of thermogenic.

Biogeochemical properties and potential risk of shallow arsenic-rich sediment layers to groundwater quality in Western Bangladesh.

The arsenic-contaminated groundwater has attracted attention in much south and southeast Asian deltas, however, mainly on the deep aquifers. Here, arsenic (As) concentration and its fractionation of the sediment cores in a shallow aquifer in Bangladesh were investigated using ICP-MS, FE-EPMA, XRD and 14C-AMS chronology techniques. The results of the present study indicated that the peak concentrations of As (54.7-79.1µg/g) were in peat layers (at a depth of 7.5-8.0m). Several types of iron (oxyhidr)oxides and framboidal pyrite, which contain As also, were found in the peat samples. The high concentrations of As were in an exchangeable form, As-bearing iron crystalline and As-bearing organic materials. We revealed that the As-rich peat layers were formed from 3170 to 3901cal yrs before, due to the sea level decrease in this area. The 16S rRNA gene-based phylogenetic analysis revealed that the bacterial strains in the As-rich peats were mainly affiliated with genera Acinetobacter, Enterobacter, Escherichia, Bacillus, Clostridiaceae and Acinetobacter. The geo-accumulation index (Igeo) and ecological risk index assessment were calculated for the sediments, which shows that As-rich sediment layers were in range of moderately to heavily contaminated and considerable classes, respectively. Under the permanent saturated condition, the As-rich peat layers should be considered as an important potential driver of the groundwater As in this area.

Petrographic characteristics of the Ordovician and Silurian deposits in the Baltic Basin (N Poland) and their relevance for unconventional hydrocarbon accumulations

The Ordovician and Silurian deposits in the Baltic Basin are represented by siltstones, mudstones, limestones, marls, sandstones and, most commonly, claystones with a markedly shaley structure. Claystones that are particularly rich in organic matter are considered as potential reservoirs for shale gas accumulations. Samples from 11 boreholes, including core samples of the Piaśnica, Słuchowo, Kopalino, Sasino, Prabuty, Jantar, Pelplin and locally Pasłęk, Kociewie and Puck formations, were analysed by optical microscopy, cathodoluminescence (CL), scanning electron microscopy (SEM) and energy-dispersive spectrometry and X-ray diffraction analyses (XRD). The Piaśnica, Sasino, Jantar, Pasłęk and Pelplin formations are characterized locally by a dominance of black bituminous claystones with a high content of organic matter, and a generally consistent clay mineral composition. Individual samples only show variations in silt fraction and carbonate content. Very important is the negligible proportion of swelling minerals in the clay fraction; the percentage of smectite in the mixed-layered illite/smectite minerals does not exceed 15%. It is important for the rocks to have adequate [SiO2 >10%; Quartz (Q) + Feldspars (Fs) + Carbonates >40%] brittleness. The content of quartz, feldspars and carbonates (here >40%) suggests that the claystones are susceptible to hydraulic fracturing. The claystones show micropores between detrital grains, between the flakes of clay minerals, within pyrite framboids, and organic matter and secondary micropores within grains.

Environmental changes in the Middle Triassic lacustrine basin (Ordos, North China): Implication for biotic recovery of freshwater ecosystem following the Permian-Triassic mass extinction

Increasing evidence shows that the recovery of marine ecosystems after the Permian/Triassic mass extinction (PTME) was coupled with environmental amelioration. Recently, abundant insects and complete trophic structures were unraveled from the Ladinian (late Middle Triassic) in the Ordos basin, central China, deciphering recovery of lacustrine ecosystem from the PTME. Coeval environmental settings, however, remain unclear. In the Ordos basin, the Ladinian succession is dominated by black organic-matter-rich shales of the Chang 7 Member of the Yanchang Formation, and their depositional environments and palaeo-productivity are reconstructed based on major and trace element concentrations, pyrite framboid size and morphology, and total organic carbon (TOC) contents. High values of TOC and phosphorous (P) concentrations, as well as more enrichment of trace-metal elements such as Copper (Cu) and zinc (Zn) all indicate a rather high primary productivity. Such a high productivity during the deposition of the Chang 7 3 sub-member is also reinforced by and the presence of abundant organic-matter-enrich laminae in black shales and the diverse chrysophytes, as well as intensive occurrence of tuff layers and possible hydrothermal fluids. Analyses of framboidal pyrite size and morphology as well as C org /P ratio and trace-metal elements such as molybdenum (Mo) and uranium (U), indicate the dysoxic to oxic conditions in water columns of the Ordos lake during the late Middle Triassic. The Ladinian-Carnian cooling likely promoted the occurrence of dissolved oxygens in the lake. As a result, although the massive black shales of the Chang 7 3 sub-member were deposited and suggest a rather high primary productivity, they also indicate slightly oxygen-poor to oxygenated conditions of water columns, which are inductive for lacustrine biota to inhabit, and also offer hospitable habitats for freshwater lake ecosystems to recover from the PTME. • A rather high primary productivity occurred in a Ladinian freshwater lake. • Slightly oxygen-poor to oxygenated water conditions developed in a Ladinian lake. • Ordos lake offered hospitable habitats for freshwater ecosystems to recover.

Fluid migration along faults and gypsum vein formation during basin inversion: An example in the East Walanae fault zone of the Sengkang Basin, South Sulawesi, Indonesia

In this study, field, petrographic, and sulfur isotope investigations were conducted for gypsum veins that were formed in the sedimentary rocks of the Walanae Formation in the Sengkang basin, South Sulawesi, Indonesia. Four types of gypsum veins developed in the study area: (1) veins sub-parallel or parallel to bedding, (2) veins perpendicular to bedding, (3) vein networks that fill in complex fractures, and (4) veins that surround carbonate concretions. Their internal structures suggest that fibrous gypsum in these veins grew antitaxially from both sides of the median zone toward the vein walls. Judging from the structural relationships of the orientation of veins with the Sengkang anticline, these gypsum veins were most likely formed as a result of hydraulic fractures or layer-parallel compression within a high fluid pressure environment triggered by the activation of the East Walanae fault under the E-W lateral compression during the Pleistocene basin inversion. The relatively wide range of sulfur isotope values indicates that sulfur in gypsum veins was delivered by the fluids that reacted with framboidal pyrites in the sedimentary rocks.

Mineralogy and Geochemistry of Seabed Sediments of the Chiloé–Taitao Area, Southern Chile, and Implications for Ore Deposits

The oceanic seabed contains a variety of mineral resources related exclusively to submarine environments. Limited information has been documented for the seabed offshore of Chile, which is particularly interesting due to its geodynamic context and large area. Mineralogical and geochemical analyses of 16 sites within the Chiloé–Taitao area, from 83 to 3388 m in depth, were carried out. The most abundant minerals are quartz, feldspars, pyroxenes, amphiboles, epidote, and biotite, with lower quantities of zircon, white mica, olivine, pyrite, magnetite, ilmenite, and hematite. Framboidal pyrites are mainly present at >900 m depth, and could be associated with methane hydrates, which have been reported in the area and its surroundings. Tenorite, sphalerite, tennantite, cordierite, birnessite, and tellurobismuthite were revealed by XRD analysis at low concentrations but at many sites. Birnessite, a Fe–Mn nodule-forming mineral, was widely detected, and Pearson correlations showed elemental associations related to the presence of Mn oxides. Samples did not evidence Fe–Mn nodules, probably due to the redox and depth conditions. Nonetheless, it is probable that to the west, polymetallic nodules are present in deeper zones. In the southern part of the area, reflective grains were identified, with up to 58.3 wt.% Cu; these grains might be derived from the continent or formed by in situ diagenetic precipitation.

Examination of multiple sources of selenium release from coal wastes and strategies for remediation

Selenium (Se) has been mobilised by leaching from coal and associated waste rock exposed by mining activities in Western Canada, with deleterious impact on aquatic wildlife. Waste rock characterisation indicates that up to 7% of the Se, as Se(IV), may be associated with organic matter, with ≈9%, as Se(0), associated with euhedral pyrite. Small 1−2 µm mineral particles with average Se concentration of 1.0 ± 0.4 wt% account for the remaining Se with the largest components likely to be associated with Fe oxide/hydroxide/carbonate as Se(0) and framboidal pyrite as Se(IV) and Se(0). No evidence was found for the presence of Se(−I), Se(−II) or Se(VI). In the first 8 weeks of leaching Se release was not correlated to the addition of aqueous silicate, added to aid pyrite passivation, but was reduced by approximately one third when the waste was treated with manure. This suggests the primary initial source of leached Se was not pyrite. Added organic C results in increased microbial numbers, particularly aerobic microbes, and promotes the formation of extensive coating of extracellular polymeric substances resulting in depletion of O2 at particle surfaces, reducing oxidation of Se(IV) and therefore reducing the leach rate of Se. Subsequent to 8 weeks of leaching the rates of release of Se from the treated wastes were similar regardless of treatment strategy but were reduced as compared to the untreated waste rock, suggestive of partial framboidal pyrite geochemical and microbial passivation. Se leaching was not correlated to S leaching, but the source(s) of the leached S was not known as approximately half of the S within the waste rock was non-sulfidic. These results indicate that utilisation of local organic carbon-containing wastes for coverage of coal waste rock may be a cost-effective strategy to reduce Se leaching to acceptable rates of release regardless of whether the Se is associated with framboidal pyrite or organics.

Characteristics and evolution of pore types in marine carbonate mudrocks, selective early to late Jurassic succession, central Saudi Arabia

This study's main goal was to recognize pore types, sizes, and abundance within three carbonate mudrocks units (parts of Upper Marrat, Tuwaiq Mountain Limestone, and Hanifa formations) located in Central Saudi Arabia. The methods used for this study were petrographic thin section analysis, helium porosity measurements, and X-ray diffraction. Backscattered scanning electron microscopy with energy dispersive scanning, micro-computed tomographic scanning, Total Organic Carbon “TOC” analysis, nuclear magnetic resonance, N2 adsorption analyses were used for qualitative and quantitative pore type characterization. Three carbonate mudrocks lithofacies were specified in Hanifa Formation Hawtah Member: LFH1, LFH2, and LFH3. Two lithofacies were defined in the Tuwaiq Mountain Limestone mudstone: LFT1 and LFT2. Two mudstone lithofacies were identified in the Upper Marrat Formation: LFM1 and LFM2. Intra-skeletal, intraparticle, and secondary OM-hosted pores are found in the Hanifa mudstone lithofacies. Intra-skeletal, small scale interparticle (pyrite framboids), and OM-hosted pores were predominant in Tuwaiq Mountain Limestone mudstone lithofacies. Small scale interparticle pores (euhedral calcite) and secondary OM-hosted pores were found in both mudstone lithofacies of the Upper Marrat Formation. All pore types were linked to corresponding diagenetic processes and burial events. NMR-relaxation time curves and pore size distribution curves revealed that LFH1 and LFH3 lithofacies has higher pore abundance than in the LFH2 lithofacies. LFT1 and LFT2 lithofacies have similar bimodal relaxation time distribution with slightly higher abundance observed in the LFT1 lithofacies. LFM1 and LFM2 lithofacies have similar bimodal relaxation time distribution with slightly higher abundance observed in the LFM1 lithofacies. These results can be used as input parameters to generate fluid flow simulation to determine these carbonate-rich mudrock reservoirs' potentiality.

Marine depositional signatures of the Aptian Oceanic Anoxic Events in the Eastern Tethys, Lower Indus Basin, Pakistan

The depositional signatures of the Cretaceous OAE1a and OAE1b intervals in the Parh Formation, Lower Indus Basin, Pakistan, Eastern Tethys, have been studied to better constrain the Aptian–Albian ocean–climate system. Based on the microfacies attributes, an outer ramp to deep basin is suggested for the deposition of Parh Formation. The quasi-periodic occurrences of radiolarian-rich microfacies and the cyclicity between the outer ramp and deep basin settings throughout the section suggest eutrophication in this part of the Eastern Tethys influenced by Milankovitch cycles. The clastic influx during OAE1a and OAE1b intervals indicate the role of continental runoff in the eutrophication of the basin. The overall negative covariance between radiolarian-rich microfacies and clastic input suggests that upwelling was a major driver of nutrient enrichment rather than continental runoff. The organic enrichment along with pyrite framboids and lack of burrows suggest that dysoxic–anoxic bottom water conditions were common throughout the Aptian and not just in the OAE1a and OAE1b intervals. The poor ventilation of the sea floor was a result of restricted circulation and may have triggered sustained anoxia during Aptian in this part of the Eastern Tethys. Bottom-water currents recharged coarser clastics to the deeper part of the basin during OAE1a and OAE1b intervals. However, the effects of deep ventilated oxic water during the events were reduced by persistent ocean anoxia. Furthermore, the dominance of carbonate sediments throughout the Aptian–lower Albian succession suggests relatively alkaline oceans in this part of Eastern Tethys. A pronounced sea-level rise, as suggested elsewhere was not observed during OAE1a and OAE1b intervals in this part of the Tethys. KEY POINTS The Aptian–Albian hemipelagic sequence of the Parh Formation, Lower Indus Basin, Pakistan, Eastern Tethys has been deposited in an outer ramp to deep basin environment. The quasi-periodic occurrences of radiolarian-rich microfacies and cyclic alternation of depositional environments suggest eutrophication of the basin influenced by Milankovitch cycles. Organic enrichment, pyrite framboids and lack of burrows suggest dysoxic–anoxic bottom-water conditions throughout the Aptian to early Albian.

Transformation of Fe-bearing minerals from Dongsheng sandstone-type uranium deposit, Ordos Basin, north-central China: Implications for ore genesis

Abstract Iron-bearing mineral assemblages and their distribution patterns directly reflect the redox environment in sediments, which plays a decisive role in the migration and precipitation of U. The Dongsheng sandstone-type U deposit hosted in fluvial and/or deltaic sandstones of the lower member of the Middle Jurassic Zhiluo Formation in the northeastern Ordos Basin has experienced multiple fluid events that impacted the redox conditions. Highly enriched in barren gray sandstones, pre-ore U (Umean = 12.05 ppm) associated with Fe-Ti oxides, clay minerals, and organic matter is likely one of the key sources of U for the mineralization. Different contents of Fe-bearing minerals, including biotite, Fe-Ti oxides, pyrite, hematite, goethite, and chlorite that were formed or altered under different redox conditions, resulted in sandstone units with distinct colors. The red sandstone is hematite-rich, indicating a highly oxidizing environment. The green sandstone is chlorite-rich and formed because of reducing hydrocarbon-rich fluids that overprinted the hematite-rich sandstone. The barren and mineralized gray sandstones consist of pyrite (with a higher content in mineralized sandstones), Fe-Ti oxides, and carbonaceous debris, which are indicators of a reducing environment. Based on the paragenetic relationship and sulfur isotopic compositions of ore-stage pyrite, bacterial sulfate reduction was responsible for the formation of framboidal pyrite (δ34S = –31.2 to –3.8‰), and the sulfur of this pyrite mainly came from the oxidation of pre-ore pyrite (δ34S = –19.1 to +20.3‰). Euhedral and cement pyrite overprinting framboids were produced via Ostwald ripening with δ34S values ranging from –56.9 to –34.3‰, lower than any values of framboidal pyrite. Therefore, these mineralogical and geochemical characteristics of the Dongsheng deposit suggest U mineralization involves both biogenic and abiogenic redox processes.

Control of variability of primary grain assemblages on the stratigraphic differences in diagenetic processes and products in organic-rich sediments

Diagenesis exerts a key control on the evolution of mineralogical systems in shales, and by extension, on the development of rock properties amenable to unconventional reservoir prospectivity. To develop an understanding of the control of primary grain assemblage on the diagenetic pathways and products in shales, integrated high-resolution petrographic and multiple geochemical studies were conducted on samples from a vertical transect of oil-gas mature (vitrinite reflectance ranging from 0.98 to 2.72%) Whitehill Formation, a shale unit identified as the main potential gas reservoir in South Africa. Results show that the formation comprises three intervals with contrasting primary grain assemblages, including organic matter content, maceral type, and detrital mineral grains, as well as the submillimeter-scale arrangement of these constituents. The lower interval occurs as parallel-laminated black shale dominated by components derived from primary production, such as Tasmanites cysts, colonial algae, radiolarian tests, and amorphous organic matter with up to 5.78% TOC, and depleted in siliciclastic detritus. The mid and upper intervals contain biologically produced components (up to 16.57% TOC) and high quantities of siliciclastic mud dominated by clay minerals and micas, and are organized into thick discontinuous gray laminae. Petrographic data suggest that the high organic content supported a variety of bacterial metabolic pathways, which caused the precipitation of mineral cements in the sediment pore spaces prior to compaction. However, the early diagenetic reactions and products in these intervals display systematic variations that reflect heterogeneity in their primary grain assemblages. The presence of framboidal pyrite throughout the lower interval suggests sulfate reduction as the predominant metabolic pathway. In absence of significant volumes of siliciclastics, the acidity associated with this process was poorly buffered, and the pore waters were reducing. These conditions led to the dissolution of opaline radiolarian tests, which recrystallized within Tasmanites cysts and interstitial pores to quartz. On the contrary, carbon isotopes indicate that organic oxidation in the mid and upper sections was mainly promoted by methanogenic archaea and that the associated acidity was effectively buffered by the detrital aluminosilicates. The resulting bicarbonate alkalinities allowed for the formation of interstitial carbonate cements (dolomite, calcite). Aluminum released during the acid-consumption reactions recrystallized within sheltered and interstitial pores to chlorite. Considering that many other shale successions exhibit similar stratigraphic variability of primary grain assemblages as the Whitehill Formation, this type of study will improve the characterization of shale mineralogical systems and bulk properties amenable to sustainable unconventional exploration.

The Source of Organic Matter and Its Role in Producing Reduced Sulfur for the Giant Sediment-Hosted Jinding Zinc-Lead Deposit, Lanping Basin, Yunnan, Southwest China

Abstract The Jinding deposit, located in the northern part of Lanping basin in southwest China, is the second largest Zn-Pb deposit in China and the third largest Mississippi Valley-type deposit identified globally. The deposit consists of several large tabular orebodies within the Jinding dome. Two stages of sulfide mineralization (sphalerite, galena, and pyrite) are identified, which are mainly hosted in the siliciclastic strata of Early Cretaceous and Paleocene age. The early sulfide minerals are mostly fine grained (<100 μm) and disseminated in the host rocks, whereas the late minerals are ty pically coarse grained (up to 1 mm in diameter) and colloform. It is estimated that about 3.17 × 106 m3 of reduced sulfur (H2S) was involved in the sulfide mineralization of the Jinding deposit, although its origin remains equivocal. Here, we investigate the biomarker signatures of organic matter and the mechanism of generation of the H2S. The organic matter in the Jinding deposit occurs mainly as petroleum filling fractures and cavities in the wall rocks and solid bitumen intergrown with sulfides or calcite. Abundant solid bitumen is also found on the surfaces of the carbonate rocks in the Sanhedong Formation as well as in the rock fractures associated with framboidal pyrite. The petrographic characteristics and maturity-related biomarker parameters show that the solid bitumen in the ores has higher thermal maturity than that in the Sanhedong Formation, suggesting that it was generated at different temperatures in the two settings. The source-related parameters suggest that the solid bitumen in the ores and Sanhedong Formation probably both originated in a mixed marine shale and carbonate environment and that the source rocks for the bitumen precursor were late Triassic marine strata. The δ34S values, ranging from –30 to –10‰ for the fine-grained and disseminated sulfide minerals and from –24.50 to –16.27‰ for the solid bitumen in the early (main) mineralization stage, suggest that H2S was generated by microbial sulfate reduction. We propose that this occurred in the Triassic strata prior to or during migration of hydrocarbons to the Jinding dome to form a H2S-enriched paleo-oil reservoir. This hypothesis is supported by the similarity of the δ34S values (–27.62 to –17.38‰) of solid bitumen in the Sanhedong Formation (the source rocks) to that of bitumen in the ores. The late-ore sulfide, however, displays significantly higher δ34S values, ranging from –8 to 0‰. We propose that the H2S of this stage was mainly generated by thermochemical sulfate reduction as a result of the interaction between hydrocarbons, sulfate, and hydrothermal fluid. The hydrocarbons were oxidized into bitumen that has δ34S values from –7.38 to –4.61‰.

Depositional environment of Middle Triassic organic‐rich shales in the Ordos Basin, Northwest China

The Middle Triassic Tongchuan Formation in the Ordos Basin of northwestern China is a typical lacustrine deposit. A major component of this formation is thick layers of organic‐rich shales that are probably a potential hydrocarbon source and preserve the earliest known Mesozoic‐type lacustrine ecosystem. The exact depositional environment of the shales in the Tongchuan Formation, however, remains unknown. To address this question, we carried out high‐resolution carbon (δ13Corg) and sulphur (δ34Spy) isotope analysis as well as undertook total organic carbon (TOC)/pyrite contents and pyrite morphology investigation, and framboidal pyrite size measurements in shales of the Bawangzhuang section of the southern Ordos Basin. Remarkably high TOC (23 ± 9%) and pyrite (7 ± 3%) contents were obtained from the shales, which indicate a large amount of organic carbon and pyrite burial during shale deposition. Framboids are the dominant pyrite morphology in the pyrite crystals and show large and variable mean diameters (7.0 ± 1.7 μm to 14.3 ± 6.8 μm) across the section, indicating oxic–dysoxic bottom water during shale deposition. δ13Corg and δ34Spy revealed narrow and less variable values, ranging from −31.8‰ to −28.1‰, and −4.1‰ to 4.9‰, respectively. The δ13Corg values suggest balanced and consistent carbon cycles. Integrated with pyrite content and morphological patterns, consistent δ34Spy values probably demonstrate a relatively open environment for the formation of sedimentary pyrite, and thus a shallow chemocline that was quite close to the water‐to‐sediment interface during shale deposition. Overall, the organic‐rich shales of the Tongchuan Formation were probably deposited under oxic–dysoxic bottom‐water conditions. Shallow chemocline depth combined with moderately high sedimentation rate and high primary productivity may have played crucial roles in the deposition and formation of the organic‐rich shales in the Tongchuan Formation. The shallow chemocline also facilitates the fossil preservation in a lacustrine environment.

Conditions for uranium biomineralization during the formation of the Zoovch Ovoo roll-front-type uranium deposit in East Gobi Basin, Mongolia

The Zoovch Ovoo uranium roll-front-type deposit is hosted in the Sainshand Formation, a Late Cretaceous siliciclastic reservoir, which constitutes the upper part of the post-rift infilling of the Mesozoic East Gobi Basin in SE Mongolia. The Sainshand Formation consists of unconsolidated medium-grained sand, silt and clay intervals deposited in fluvial-lacustrine settings. The uranium deposit is confined within a 60–80 m thick siliciclastic sequence inside aquifer-driven systems. The overall system experienced shallow burial and was never subjected to temperatures higher than 40 °C. This study proposes a comprehensive metallogenic model for this uranium deposit. Sedimentological and mineralogical observations from drill core samples to the microscopic scale (optical and Scanning Electron Microscopy) together with in situ geochemistry of late-formed phases (Laser Ablation–Inductively Coupled Plasma Mass Spectrometry, Electron Probe Microanalysis, Fourier Transform–Infrared Spectroscopy) were considered for the reconstruction of the main stages of U trapping.In the mineralized zone, the uranium ore is expressed as Ca–enriched uraninite (UO2) and less commonly as Ca–enriched phospho-coffinite (U, P)SiO4. Trapping mechanisms include i) complexation (i.e. uranyl-carboxyl complexes), ii) adsorption on organic or clay particles) and iii) reduction by pyrite and by bacterial activity to amorphous uraninite. In all cases, the organic matter plays either the role of trap for uranium or nutrient for bacteria that can trap uranium through their metabolism. The shallow burial diagenesis conditions do not allow direct reduction of U(VI) by organic carbon. The δ34S values of the iron disulfide are very diverse, fluctuating in extreme cases between −50 to + 50‰, with an average δ34S value for framboidal pyrite at 2‰, and −20‰ for euhedral pyrite. The positive and negative values reflect close versus open fractionation systems, while bacterial sulphate reduction (BSR) is active during the whole diagenetic history of the deposit as an essential source of reduced sulfur. Therefore, using detrital organic matter as a carbon source, microorganisms play a significant role in uranium trapping, either as a direct reducing agent for uranium or pyrite formation, which will trap uranium through redox driven epigenetic processes.

Microscopic and NanoSIMS characterization of black shale-hosted pre-kinematic pyrites: Possible gold source of the orogenic gold deposits in the Truchas Syncline (Variscan Iberian Massif)

The Truchas Syncline contains orogenic gold deposits (OGDs), hosted mainly by the Armorican Quartzite Fm. In order to look for the source of gold in these deposits, pre-kinematic sulphides from black shales from the overlying Luarca Fm. were studied using both optical and scanning electron (SEM) microscopy, as well as nanoscale secondary ion mass spectrometry (NanoSIMS). NanoSIMS images allowed the observation of relevant structures (pyrite with high As content nodules) hidden to SEM.Microscopy and 36S, 75As32S, 75As, 75As34S, and 197Au mappings, as well as δ34S analysis (‰ Vienna-Canyon Diablo Troilite) allowed 4 types of pyrite to be characterized. According to their sequence of deposition, the pyrite types identified are the following: (1) framboid nuclei (FmPy), with δ34S between +4.1‰ and +57.5‰; (2) overgrowths (OgPy), with presence of Au, As and δ34S of approximately +20.5‰; (3) pyrite with high As content (AsPy), replacing previous pyrites, with δ34S of approximately +23‰, nodular shapes and non-detected Au; (4) subhedral pyrite (ShPy), with δ34S of approximately +19.7‰ and Au and As distribution showing (a) nuclei, (b) rims with Au and As and (c) structures interpreted as replacements of previous pyrite. Luarca Fm. pyrite framboids reach 75 µm in size, larger than framboids described in most previous NanoSIMS work, with microcrystals sufficiently large to be analyzed without contamination from edge effects. The FmPy formed in early diagenetic conditions by dissimilatory reduction of marine sulphate plus Rayleigh distillation or anaerobic oxidation of methane processes. The first FmPy nuclei formed in the periphery of framboids, and then started to form in the central region until completion of the framboidal structure. Growth of OgPy, AsPy and ShPy corresponded to more advanced diagenetic conditions, although the last stage of ShPy growth followed an aggregation model, in which ShPy metamorphic pyrites grew over ShPy diagenetic pyrites.The sulphur isotopic signature of the four types of pyrite is in good agreement with a source from Ordovician marine sulphate. A hypothesis stating that the source of Au in OGDs in the district could be in Luarca Fm. is supported by the results of the present research. Firstly, a source of Au in the district could have been the replacement of previous pyrites by AsPy and the release of Au to the system, instead of the framboidal to euhedral pyrite recrystallization process observed in other gold deposits. Secondly, by the correspondence between δ34S isotopic signature of the pyrites from Luarca Fm. and those from sulphides in OGDs. However, these results do not rule out other possible sources.

Geochemical and sedimentary facies study – Implication for driving mechanisms of organic matter enrichment in the lower Silurian fine-grained mudstones in the Baltic Basin (W Lithuania)

More than 100 m-long core sections from Western Lithuania (Baltic Basin), presenting an almost continuous lower Silurian fine-grained siliciclastic organic matter-rich sedimentary record from Aeronian to Homerian, were investigated to evaluate the main forces governing the organic matter enrichment. The methods applied include organic and inorganic geochemistry, organic petrology, sedimentological and petrographic studies along with framboidal pyrite analysis, as well as analysis of stable nitrogen and organic carbon isotopes. The petrographic observations showed distinct mudstone diversity reflecting a variety of paleoenvironmental depositional conditions evolving against a backdrop of post-Ordovician climate, thus, sea level variations, and progressing development of foreland basin throughout the early Silurian. Reflectance measurements on vitrinite-like particles, supported by molecular organic geochemical and Rock-Eval data prove that kerogen from the studied section reached the oil window maturity stage. Rock-Eval pyrolysis, organic geochemical and petrographical as well as δ13Corg data clearly confirm the marine origin of the organic matter. Various geochemical proxies and sedimentary features indicate that in Aeronian a period of more humid climate favored enhanced nutrient input, which in turn intensified organic matter productivity resulting in large amounts of organic matter accumulation. Redox proxies (TOC/TS ratio, trace elements enrichment, molecular organic geochemical indices, framboidal pyrite analysis, TOC/P ratio) demonstrate a rather broad spectrum of bottom water oxygenation levels in the lower Silurian sediments accumulated under oxic - dysoxic – anoxic conditions interrupted by several episodes of euxinia. A highly condensed section of Aeronian black-grey mudstones manifests a very good organic matter preservation under sediment-starved and oxygen-depleted settings. Here, the oxygen-depleted water bottom conditions are well evidenced by low values of δ15N and δ13Corg pointing to the presence of diazotrophic N-fixing and chemoautotrophic organisms. The Sheinwoodian-Homerian section of monotonous grey calcareous mudstones evenly and moderately enriched in organic matter reflects the most steady environmental settings developed under more arid climate in comparison to Llandovery. Throughout the early Silurian sedimentation rates were overall low, although probably enhanced in Wenlock, and played a role in organic matter preservation. It was a critical factor during sedimentation of grey-greenish mudstone of the Telychian characterized by comparatively the lowest organic matter preservation efficiency. Variations in the intensity of the pycnocline, thus organic matter preservation, were probably controlled by climate-triggered sea level changes, although tectonically-driven paleoenvironmantal effects, bathymetry, and, at some point, intensified paleoproductivity inducing oxygen-restricted conditions, were likely contributing. Although MoEF/UEF ratios mainly indicate fully open marine settings during the entire early Silurian, Mo–TOC covariation clearly displays strict basinal restriction in the Aeronian. The profile of δ13Corg demonstrates six carbon isotope excursions along the lower Silurian section. The identified Late Aeronian and Early Sheinwoodian Carbon Isotope Excursions are linked to global sea-level changes, thus, changes in oceanic water circulation pattern. A high percentage of tiny pyrite framboids within the interval of Early Sheinwoodian CIE could reflect an increased pyrite burial under highly reducing conditions and might be associated with an Oceanic Anoxic Event. The three positive δ13Corg excursions within the Telychian seem to be related to an early diagenetic partial oxidation of organic matter.

Genetic model for Jurassic shale-hosted Zn-Pb deposits of the Arak Mining District, Malayer-Esfahan metallogenic belt: Insight from sedimentological, textural, and stable isotope characteristics

The genetic model, including conditions under which mineralization formed, and relative timing of mineralization are critical questions for SEDEX (or shale-hosted massive sulfide, SHMS) deposits. There is increasing awareness that sub-seafloor replacement is an important process in the formation of some SEDEX deposits. We have studied the Hossein-Abad and Western Haft-Savaran Zn-Pb SEDEX deposits located in the Arak basin of the Malayer-Esfahan Metallogenic Belt, Iran, to address these questions of genesis. This metallotect formed in a back-arc paleotectonic setting as a result of the subduction of the Neo-Tethys oceanic plate beneath the Sanandaj-Sirjan Zone. The rocks that host the mineralization are Jurassic organic matter-bearing, fine-grained sandstones, siltstones, and shales. Asymmetric lenticular bedding, unidirectional flow (based on oblique silt lamination direction relative to horizontal bedding), graded bedding, and clay-rich interbeds indicate sediments were deposited from turbidity currents in a low-energy basin environment.There are three ore facies in the Hossein-Abad and Western Haft-Savaran Zn-Pb deposits: 1) bedded ore; 2) massive ore; 3) feeder zone. Bedded ore contains pyrite framboids and polyframboidal clusters. The size range of the pyrite framboids (3 to 6 µm in diameter) indicates they formed in the water column and not in the subsurface. Characteristic structures in bedded ore are: 1) sulfide-bearing silt injections into clay-filled burrows, 2) injection of sulfide-bearing silt into flame structures of claystone laminae, and 3) organic matter in claystone oriented obliquely relative to bedding. These structures are the result of seismic deformation induced by synsedimentary earthquakes, whereby sulfides that formed in permeable unconsolidated sediment were injected into the organic matter-bearing claystone unit.The δ18O and δ13C values of siderite, calcite and dolomite in veins from the feeder zone and massive ore range from 12.2 to 23.8‰ and −16.7 to 1.7‰, respectively. These values indicate that formational water, seawater and organic matter oxidation-decomposition all played a role in hydrothermal carbonate formation. Melting and homogenization temperatures for CO2 for CO2-bearing fluid inclusions range from −57.5 to −60 °C and 6.6 to 29.5 °C, respectively, and indicate the presence of < 15 mol percent CH4. The CO2 homogenization temperature range suggests the CO2-rich phase is a CO2-CH4 mixture. The CH4 and CO2 in the H2S-bearing fluid were likely generated by biodegradation and oxidation of organic matter via BSR and methanogenesis.The sulfur isotope compositions of pyrite, galena, sphalerite and chalcopyrite from the feeder zone and the massive ores range from δ34S −4.3 to + 7.2‰ and display equilibrium fractionations, indicating that the sulfur originated from two processes: (a) bacterial reduction of seawater sulfate (BSR) (distal from mineralization site), and (b) thermochemical reduction of seawater sulfate (TSR) (in both massive ore and feeder zone). Sulfur isotope geothermometric calculations (Δgalena-sphalerite) for two samples give temperatures in the range 209–224 °C for the massive ore facies. Such high temperatures negate a contribution of sulfur from BSR. However, BSR likely occurred in sediments distal to the feeder zone, where seawater sulfate was bacterially reduced to H2S, and this H2S migrated to the mineralization site during diagenesis. Collectively, the stable isotope data indicate mineralization formed in response to mixing occurred between ascending hot metalliferous hydrothermal fluid that rose up the fault and fracture network, with H2S-bearing fluid and sulfate-bearing percolating seawater, triggering sulfide deposition.