Pyrite Morphology Research Articles

Impacts of sulfate-driven anaerobic oxidation of methane on the morphology, sulfur isotope, and trace element content of authigenic pyrite in marine sediments of the northern South China Sea

The presence of sulfate-driven anaerobic oxidation of methane (SD-AOM) generally influences the precipitation of authigenic pyrite in seepage areas. However, the relatively weak comprehensive studies on the morphological and geochemical characteristics of pyrite limit the understanding of its formation and application to diagenetic processes. Here, we analyzed the pyrite texture, abundance, sulfur isotopic composition, degree of pyritization, and the trace element (TE) content of pyrite from two sites (A27 and SH1) in a seepage area of the South China Sea, so as to better constrain the impacts of SD-AOM on pyrite formation. Pore-water results revealed that present-day methane seepages occurred at both sites. Under normal sedimentary environments, pyrite resulting from organiclastic sulfate reduction (OSR) showed low abundances with negative δ34S. In contrast, high contents of 34S-enriched pyrite aggregates, long pyrite tubes and large framboid sizes typified by radial overgrowths were documented at the lower part of both core sediments, suggesting the significant impacts of SD-AOM. At the proposed sulfate-methane transition zone (SMTZ), pyrite is a mixing product of OSR and SD-AOM, but the latter enhances the morphologic and isotopic anomalies of pyrite. Besides, SD-AOM increased the degree of pyritization and accelerated the pyrite-iron accumulation in sediments. With pyritization proceeding, Mo, Ni, Mn, and Cr were gradually incorporated into the pyrite phase, presenting a relatively high affinity of these TEs for pyrite in the studied locations. However, the contents of TE in pyrite were positively correlated with its concentrations in the current ocean, from which we can get that TE in diagenetic pyrite was basically influenced by seawater chemistry. The comprehensive results reflect that the authigenic pyrite can act as an excellent geological archive of modern and ancient oceans. In particular, pyrite formed in seepage-impacted sediments allows identifying the methane seepage and characterizing the biogeochemical cycling of carbon, sulfur, and TEs.

Sedimentary pyrite in carbonaceous shales of the Mamfe Cretaceous basin, SW Cameroon: Morphologies, composition, pyrite framboid size frequency distribution, and formation pathways

Sedimentary pyrite morphologies, spatial distribution relative to their spectral compositions were studied in carbonaceous shales (Fc) of the Mamfe Cretaceous Basin (MCB) in SW Cameroon. Lithofacies analysis of Fc reveals laminated (Fcl), oolitic (Fco), marlstone (Fcm), algal mat (Fca) and pelletoidal (Fcp) pyritized microfacies. Results of petrographic, scanning electron microscope (SEM), backscattered electron imaging (BSEI) microscopic study coupled with energy dispersive atomic x-ray (EDAX) reveal micron-sized, densely packed framboidal and non-framboidal morphologies. Diameter measurement of 519 framboids exhibit a monomodal distribution with a narrow size-range of 0.50–14.0 μm and mean diameter range of 2.0–6.5 μm, accounting for 70.4–95.3% of all framboids measured. Non-framboidal morphologies include discrete euhedral, etched or zoned crystals, including core-sheath cell-like microstructures. Euhedral morphologies show a close spatial relationship with syngenetic mineral suite (calcite, microdolomite, chalcedony and evaporites). Meanwhile, degraded morphologies with moldic pores are mineralized in Zn and appear recrystallized, etched, in-filled by silica and clays or rimmed by ferroan carbonates and oxides. A three transitory pathways model suggests that morphological and compositional variations in pyrites are linked to authigenic precipitation, co-precipitation of Fe and Zn which depict sulphate reduction conditions under saline and carbonate lacustrine settings, followed by alteration processes. Pyritized core-sheath replicas in microstructures reminiscent of cells are indicative of a microbial paleoecosystem and could play an intrinsic role in biogeochemical recycling of S and Fe and the timing of fossilization which precedes the decay of microorganisms. Rimmed pyrite morphologies are potential indicators of local redox paleoenvironmental settings.

Genesis and Distribution of Pyrite in the Lacustrine Shale: Evidence from the Es3x Shale of the Eocene Shahejie Formation, Zhanhua Sag, East China.

Pyrite is a common mineral in sedimentary rocks and is widely distributed in a variety of different morphologies and sizes. Pyrite is also widely distributed in the Es3x shale of the Eocene Shahejie Formation in the Zhanhua Sag, Bohai Bay Basin. A combination of geochemical and petrographic studies has been applied to address the formation and distribution of pyrite in the Es3x shale. The methods include thin section analysis to identify the representative samples of the shale-containing pyrite, total organic carbon (TOC) content analysis, X-ray fluorescence, X-ray diffraction, electron probe micro-analysis, and field emission scanning electron microscopy (FE-SEM) coupled with the energy dispersive spectrometer, to observe the characteristics, morphology, and distribution of pyrite in the lacustrine shale. The content of pyrite in the Es3x shale ranges from 1.4 to 11.2% with an average content of 3.42%. The average contents of TOC and total organic sulfur (TS) are 3.48 and 2.53 wt %, respectively. Various types of pyrites are observed during the detailed FE-SEM investigations including pyrite framboids, euhedral pyrite, welded pyrite, pyrite microcrystals, and framework pyrite. Pyrite framboids are densely packed sphere-shaped masses of submicron-scale pyrite crystals with subordinate large-sized euhedral crystals of pyrite. Welded pyrite forms due to the overgrowth and alteration of pyrite crystals within the larger pyrite framboids. Pyrite microcrystals are the euhedral-shaped microcrystals of pyrite. The framework pyrite is also observed and is formed due to the pyritization of plant/algal tissues. Based on the growth mechanism, the pyrites can be divided into syngenetic pyrites, early diagenetic pyrites, and late diagenetic pyrites. The presence of pyrite, especially the abundance of pyrite framboids, suggests that the environment during the Es3x shale deposition in the lacustrine basin was anoxic. Their dominant smaller size suggests the presence of an euxinic water column, which is consistent with the lack of in-place biota and high TOC contents. This research work not only helps to understand the pyrite mineralization, role of organic matter, and reactive iron in pyrite formation in the shale but also helps to interpret the paleoredox conditions during the deposition of shale. This research work can also be helpful to other researchers who can apply these methods and conclusions to studying shale in other similar basins worldwide.

Relations between pyrite morphologies and uranium mineralization in the Shuanglong region, northern China

Pyrite is widely distributed in ore-hosting sandstones of the Zhiluo Formation in the Shuanglong uranium deposit in the southern Ordos Basin, northern China. Sandstones from the uranium-bearing stratum was collected to analyze morphologies of pyrite and its relations with uranium mineralization by means of scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS). The results show that colloidal and framboidal pyrite, and pentagonal dodecahedron microcrystalline widely occur in sandstones, and uranium minerals are unevenly distributed around pyrite. According to the occurrences of pyrite, morphologies of pyrite are orderly evolved from microcrystalline, and the formation of pyrite is grouped into four periods. Moreover, the pyrite might preferentially or simultaneously precipitate over uranium minerals. The smaller the diameter of pyrite, the more favorable for uranium enrichment. Additionally, variation trends of U contents are opposed to those of the Fe and S contents as well as the Fe/S atomic ratios. Moreover, the decrease rate of S contents is greater than that of Fe contents. It indicates that S and Fe from pyrite both play roles in uranium mineralization, and the S contribution dominates over the Fe contribution. Additionally, the aforementioned conclusion is also supported by the existences of Fe (III), S, SO32-, and SO42. Compared with carbonaceous debris (CD), pyrite might play a dominant role in uranium enrichment. During the process of enrichment and reduction of uranium by pyrite, CD might provide beneficial reducing environment for precipitation and preservation of pyrite and uranium minerals. The study could provide guide for understanding metallogenic mechanism to some extent.

Morphology of framboidal pyrite and its textural evolution: Evidence from the Logatchev area, Mid-Atlantic Ridge

The special and representative polymorphous pyrites of the Logatchev area, Mid-Atlantic Ridge, have a close evolution relationship. There are relatively complete pyrite intermediates undergoing a series of evolutions from disordered nano-micron crystals → framboids → colloidal and subhedral structures. This study focuses on a sulfide-bearing tuff sample of the Logatchev area and presents the detailed mineralogical, textural and geochemical results of a series of intermediate products, from pyrite framboids to subhedral crystals, using results from secondary ion mass spectrometry, field emission scanning electron microscopy, electron backscattered diffraction, and electron microprobes. Framboids packed with nanocrystals are significantly with negative δ34S (−28.3 to −17.4‰), probably formed via organoclastic sulfate reduction (OSR) in the seawater column or shallow marine sediment. And their associated microcrystal framboids have less-negative δ34S value (−22.3 to −8.8‰) and high Cu/Zn ratio, which are interpreted as framboids continuous growth by venting fluids with high temperature in an open system. During the diagenetic stage, the evolution of framboidal pyrites to subhedral crystals occurs mainly through two ways: cementation and rearrangement after growth. As an intermediate, the δ34S values of cemented framboids (−7.6 to +8.7‰) are close to the colloidal and subhedral structure pyrites (+9.1 to +11.2‰), reflecting the subsequent precipitation of pyrite and secondary overgrowth in a closed environment after burial. During the recrystallization process from nano-micron crystals to cemented framboids to colloidal or subhedral structures, the decreasing trend of SiO2 contents (come from the surrounding basement) and increasing Fe, S, Mo contents revealed the consequence of continuous expulsion of silicate in the intergranular pore space, which provides evidence for framboids aggregated and evolving to euhedral pyrite. Some intermediates from framboids to massive pyrites have a certain local ordering. Regulated by surface energy, grains in some framboidal pyrites are physically rotated and repositioned during the growth process, which show either 〈100〉 or 〈111〉 preferred orientations. The planes with higher energy are combined to form a regular arrangement and assembled into a polygonal frame until it evolves into a subhedral structure. The size uniformity of grains derived from framboidal pyrite is key to aggregation and self-organization. The research results are helpful for understanding the genesis mechanism of framboidal pyrite and its evolution process to a euhedral structure, which is of great significance to explore the structural growth of pyrite aggregates and the changes in the environmental response.

Formation of Pyrite Spherules From Mixtures of Biogenic FeS and Organic Compounds During Experimental Diagenesis

AbstractPyrite () is the most common iron sulfide on the Earth's surface and has widely been used as a paleo‐environmental proxy. Yet the information recorded by pyrite depends on whether it was formed through abiotic or biogenic routes. It is thus of importance to properly identify its origin. Here, we investigate the final morphology of pyrite produced upon a simulated diagenetic history from biogenic and abiotic iron sulfide/phosphate systems. Abiotic starting material obtained by chemical synthesis and biogenic starting material produced from pure culture of Desulfovibrio desulfuricans were submitted to increasing diagenetic conditions (75°C or 150°C from 1 to 10 days). Mineralogical products were characterized by X‐ray diffraction and electron microscopy. For both biogenic and abiotic starting materials, the final state was characterized by the association of pyrite and lipscombite (), the most stable phases in these conditions. Intermediate phases such as greigite for iron sulfides and beraunite/wolfeite for iron phosphates were present in the abiotic residue but were not detected in the biogenic residue. Distinct pyrite morphologies were observed depending on the presence of organic matter. Indeed, while abiotic starting material led to the formation of submicrometric single crystals of pyrite with euhedral shapes similar to the subunits of well crystallized framboids, biogenic starting material produced micrometric spherulitic clusters of pyrite resembling the so‐called pseudo‐framboids. Although further experiments are required to ensure that it can be used as biosignatures, such specific morphologies, likely related to the presence of organic matter, may help recognizing biogenic pyrite in the geological record.

Paleoredox conditions, hydrothermal history, and target vectoring in the Macmillan Pass base-metal district, Yukon, Canada: 2 – Pyrite paragenesis and mineral chemistry

ABSTRACTThe MacMillan Pass district in Yukon, Canada, hosts the Tom and Jason clastic sediment-hosted Zn-Pb-Ag-(Ba) deposits. Pyrite-bearing drill core samples were collected from seven drill holes that intersected sulfide mineralization and time-stratigraphically equivalent rocks at varied spatial distances extending up to 3 km away from the deposits to assess the relative timing of pyrite mineralization and the chemistry of pyrite paragenesis. There are four pyrite morphologies: framboids and polyframboids (Py1), subhedral to euhedral inclusion-free crystals (Py2a), silicate inclusion-bearing nodules with serrated edges (Py2b), and euhedral idiomorphic overgrowths on preexisting pyrite morphologies (Py3). These morphological varieties correspond in time from syngenetic to earliest diagenetic growth (Py1), early to late diagenetic growth (Py2a, Py2b), and metamorphic crystallization and/or recrystallization of previous textural varieties (Py3). A representative subset of pyrite grains was analyzed for trace element contents and distributions by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Analyses by LA-ICP-MS reveal that each textural variety of pyrite has a distinct trace element composition that also varies depending on stratigraphic unit. A suite of clastic sediment-hosted sulfide mineralization-related elements was incorporated into Py2 within sulfide mineralized units at greater abundances than that in unmineralized units (e.g., Zn, As, Pb, Tl, Bi). Lead abundances and Pb/Se and As/Mo values in pyrite are the most robust vectoring tools documented. The timing for clastic sediment-hosted Zn-Pb mineralization was syn and/or post late diagenesis (Py2b).A Ba-enriched horizon was identified in rocks and this is interpreted to be the distal time-stratigraphic equivalent unit to Zn-Pb mineralization. The Ba-enriched horizon contains Py2 with anomalous metal (Tl, Co, Mn, Cd, Zn, Sb) contents and abundant macroscopic baryte, and it is interpreted to represent the distal expression of sulfide mineralization-forming hydrothermal activity. Four genetic models for mineralization are reviewed; however, the only model that is consistent with our whole rock and pyrite geochemistry involves venting of buoyant hydrothermal fluid, mixing with ambient seawater, and remaining or sinking into unconsolidated sediments, with lateral migration up to 2–3 km from the vent source.

Pyrite Morphology and δ34S as Indicators of Deposition Environment in Organic-Rich Shales

This study is focused on the mineralogical, chemical, and isotopic characterization of pyrites from the rocks of the Bazhenov Formation (Upper Jurassic–Lower Cretaceous organic-rich shales, Western Siberia, Russia). Scanning electron microscopy (SEM) revealed pyrites of different morphologies: small and large framboids, small crystals, and large euhedral crystals; all morphotypes were usually combined into aggregates. Isotope ratio mass spectrometry (IRMS) and secondary ion mass spectrometry (SIMS) showed that small framboids and microcrystalline pyrite are isotopically light, with δ34SCDT varying from −55 to −20‰. Large framboids and euhedral crystals of pyrite are isotopically heavy with δ34SCDT up to +26‰. Both morphology and δ34S were suggested to be controlled by the redox conditions and sedimentation regime. The abundance of small framboids suggests that pyrite sedimentation occurred under anoxic conditions; the presence of the large framboids and euhedral crystals of pyrite suggest the accumulation of sediments occurred at suboxic conditions, possibly in the presence of oxygen.

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.

Effects of methane seepage activity on the morphology and geochemistry of authigenic pyrite

As an important product of methane seepage, pyrite contains abundant methane seepage information. It is right that the formation mechanisms of authigenic pyrite at methane seepage is still not so clear. We aim to reveal the importance of authigenic pyrite in methane seepage activity through different analytical methods. To this end, the morphological and geochemical characteristics of pyrite in a core obtained from the ‘Haima seep’ sedimentary area were analyzed. The results show that compared with pyrite formed under normal marine sedimentary environments, the pyrite content, degree of pyritization, and sulfur isotope value are higher in the sulfate-methane transition zone (SMTZ), and pyrite has larger mean diameters and standard deviations. These geochemical anomalies suggest four major changes in methane flux at site Q6, focusing anaerobic oxidation of methane (AOM) for extended periods of time in four subsequent layers termed paleo-SMTZ. Here, changes in methane flux to the SMTZ is mainly due to dissociation of subsurface methane hydrates. Therefore, this study can help us better understand the formation of pyrite under methane seepage activity and provide a scientific basis for identification of methane seepage activity.

Chemostratigraphy and pyrite morphology across the Wuchiapingian‐Changhsingian boundary in the Middle Yangtze Platform, South China

Separated mass extinction occurred at the beginning and the ending of the Late Permian, called end‐Guadalupian and end‐Permian mass extinctions, respectively. Palaeoenvironmental changes in the middle of the Late Permian could give more insights into the environmental causes for the Permian extinctions. In this contribution, we analysed carbonate carbon isotope and carbonate‐associated‐sulphate sulphur isotope compositions to constrain the carbon and sulphur cycles across the Wuchiapingian‐Changhsingian boundary (WCB) at Yanglinqiao in Zigui in the Middle Yangtze Platform. Pyrite morphology and pyrite framboids size distribution were used to analyse redox conditions variations across the WCB. Our results show that an episodic euxinia event occurred across the WCB. This anoxic/euxinic event across the WCB was a global event, and was mainly caused by the upwelling during the cooling event. Both inorganic‐ and organic‐carbon isotopic compositions show covariance of a negative shift across the WCB at Yanglinqiao and other places in South China. This carbon‐isotopic negative excursion across the WCB was a regional carbon cycle perturbation and may be linked to the light‐carbon input from the upwelling water mass. Although the seawater δ34S across the WCB recorded a negative excursion in two locations in South China, it reflects the riverine input of 32S‐enriched sulphate and represent a regional signal. These environmental anomalies such as oceanic anoxia/euxinia, global carbon cycle perturbation, cooling, end of the Emeishan large igneous province movement across the WCB may have delayed the recovery from the end‐Guadalupian mass extinction.

The mechanism of ultrasound oxidation effect on the pyrite for refractory gold ore pretreatment

The effect of ultrasonic cavitation on the oxidation of the crystal pyrite in deionized water has been discussed. The effect of solution alkalinity, ultrasonic power and temperature on the surface morphology of pyrite and the chemical forms of sulfur and iron was discussed. The surface of pyrite is severely corroded under ultrasound. The sulfur in the pyrite is oxidized to S2O32- and SO42-. Most of the iron is in the form of Fe2+, few of it is in the form of Fe3+. With the increase of alkalinity, ultrasonic power and temperature, the surface corrosion of pyrite becomes more and more obvious. With the increase of the NaOH concentration, the Fe(II) concentration in solution decreases and the deposition of iron oxides on the pyrite surface prevents pyrite oxidation. With the increase of the ultrasonic power, the Fe(II) concentration in solution increase. The effect of the NaOH concentration on the SO42- and S2O32- concentration indicates pyrite oxidation is in mild alkalinity under ultrasound. The concentration of SO42- and S2O32- increases along with increase of the ultrasonic power and decreases tremendously with increase of the temperature. The pyrite oxidation is initiated by hydroxyl radical and promoted by Fe3+.

Revisiting Ediacaran sulfur isotope chemostratigraphy with in situ nanoSIMS analysis of sedimentary pyrite

Abstract Reconstructions of ancient sulfur cycling and redox conditions commonly rely on sulfur isotope measurements of sedimentary rocks and minerals. Ediacaran strata (635–541 Ma) record a large range of values in bulk sulfur isotope difference (Δ34S) between carbonate-associated sulfate (δ34SCAS) and sedimentary pyrite (δ34Spy), which has been interpreted as evidence of marine sulfate reservoir size change in space and time. However, bulk δ34Spy measurements could be misleading because pyrite forms under syngenetic, diagenetic, and metamorphic conditions, which differentially affect its isotope signature. Fortunately, these processes also impart recognizable changes in pyrite morphology. To tease apart the complexity of Ediacaran bulk δ34Spy measurements, we used scanning electron microscopy and nanoscale secondary ion mass spectrometry to probe the morphology and geochemistry of sedimentary pyrite in an Ediacaran drill core of the South China block. Pyrite occurs as both framboidal and euhedral to subhedral crystals, which show largely distinct negative and positive δ34Spy values, respectively. Bulk δ34Spy measurements, therefore, reflect mixed signals derived from a combination of syndepositional and diagenetic processes. Whereas euhedral to subhedral crystals originated during diagenesis, the framboids likely formed in a euxinic seawater column or in shallow marine sediment. Although none of the forms of pyrite precisely record seawater chemistry, in situ framboid measurements may provide a more faithful record of the maximum isotope fractionation from seawater sulfate. Based on data from in situ measurements, the early Ediacaran ocean likely contained a larger seawater sulfate reservoir than suggested by bulk analyses.

Preservation of organic carbon in the Cretaceous Hue Shale on the North Slope of Alaska: Insights from pyrite morphology

The Cretaceous Hue Shale is a condensed mudstone section of the Brookian sequence with type II and type III kerogen, and it is one of the major source rocks that fill the world-class conventional oil fields on the North Slope, Alaska. The bottom section of the Hue Shale is referred to as Gamma Ray Zone (GRZ) or Highly Radioactive Zone (HRZ) and believed to be the most organic-rich section. The thermal maturity increases from immature to over oil window towards the southwest. Although, the research on the Hue Shale as an unconventional shale reservoir is still at an early stage, where exploration activities are very limited. The Hue Shale thickens to the east in general. However, regional variations of thickness and more importantly total organic carbon (TOC) have been reported.In this research, a suite of nineteen core samples of the Hue Shale are selected. These cores are collected from six wells covering a wide spectrum of thermal maturity with four wells in the National Petroleum Reserve Alaska and two wells in the state land. A combined geochemical and petrographic methodology is utilized to address the regional redox conditions and the preservation of organic carbon. TOC, Tmax, hydrogen index, and oxygen index are acquired through pyrolysis to characterize the abundance, thermal maturity, the origin, and the evolution of organic matter. Scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) is used to investigate the mineralogy and diagenesis of the Hue Shale. Pyrite crystal forms, and pyrite framboid size distributions are used as an indicator of the redox conditions during deposition. The results show substantial variance in oxygen levels in the water column. High abundance and small sizes of pyrite framboids, which indicates a depleted oxygen environment, show better preservation potential of organic matter.

Forms of Gold and Some Typomorphic Characteristics of Native Gold of the Pavlik Orogenic Deposit (Magadan Oblast)

Abstract—The ratios of vein and ore minerals bearing native gold, the shape and size native gold particles, their surface features, variations in chemical composition, and the presence and distribution of impurities and inclusions in them, allow sufficiently substantiated assumptions about ore-forming settings and composition, phase state, and certain other parameters of ore-forming fluids that precipitated native gold. If these data are insufficient for a full-fledged judgment on the ore formation processes, they can at least be used to curtail various genetic speculations. Certain typomorphic features of native gold were studied from one of the largest orogenic gold deposits in northeastern Russia. New data on the grain size distriubtion and chemical composition of native gold and gold contents in sulfides of the Pavlik deposit were obtained. Optical and SEM images were used to determine the ore texture, morphology, and internal structure of native gold and auriferous pyrite and arsenopyrite. ICP-MS, EDS, EDX were used to assess the composition of gold grains, EPMA and LA-ICP-MS were used to qualitatively and quantitatively characterize gold-bearing grains of pyrite and arsenopyrite. It was established that microscopic (finely dispersed (>0.0005–0.01), dusty (>0.01–0.05), extremely fine (>0.05–0.1)) (peak size at 0.075 mm), visible (very fine (>0.1 –0.25)), and fine (>0.25–1.00)) (size peaks at 0.25, 0.55 and 0.85 mm) native gold in association with sphalerite, pyrrhotite, pyrite, arsenopyrite, quartz, carbonates, albite, and sericite is widespread at the Pavlik deposit. Gold particles with an average size of 0.3 mm predominate by weight. Comparison of grain size distribution data with the technological properties of ores and the results of mining in the Omchak ore–placer district made it possible to establish that the share of dusty and fine-dispersed size classes accounts for at least 15–20% of the gold mass at the deposit. ICP-MS analysis of a bulk sample of a monofraction of native gold, which included native gold grains from different parts of the deposit, showed that the composition of the monofraction includes Fe, Hg, As, Zn, Cu, and Pb impurities. The average gold fineness was 806 with additional peaks of 775, 855, and 985 (EPMA determinations). The heterogeneity of native gold is due to the redistribution of silver during late deformation events and, to a lesser extent, removal of silver from gold in the ore oxidation zone. Inclusions in gold are represented by microvoids, disseminated fragments of host metasomatites, and crystals or crystal fragments of potassium feldspar, albite, ankerite–dolomite, and arsenopyrite. The low gold content in pyrite and arsenopyrite, with a high (up to 287 g/t) gold content in arsenic-bearing pyrite and ratios of molar amounts of gold and arsenic in arsenopyrite, may indicate that gold is bound in the crystal structure of sulfides. The relationship between native gold and vein and ore minerals, the features of native gold, and the regularities of gold distribution in pyrite and arsenopyrite do not contradict the hypothesis that native gold was deposited at a single mineralization stage in a different substrate: (1) in the intergranular space (quartz–carbonate metasomatites), (2) in the interstitial space (in arsenopyrite and carbonates), (3) in microfissures in the host minerals. Obviously, the deposition of native gold is an independent process occurring against a background of metamorphogenic (quartz–carbonate veins) and magmatic–metamorphogenic (disseminated pyrite and arsenopyrite) mineralization. Comparison of native gold from ore and placer objects of the Omchak ore–placer district has culminated in methods for using the typomorphic features of native gold to search for placer sources. It seems that various inclusions in native gold with a porous surface are promising for identifying old sources and reconstructing its crystallization conditions.

Characteristics and controlling factors of LORS from the Chang 7–3 section of the Triassic Yanchang Formation in the Ordos Basin

Lacustrine organic-rich shale (LORS) developed in the Chang 7–3 section of the Triassic Yanchang Formation in the Ordos Basin. LORS has been proven to be a source of crude oil and tight oil, as well as a source and reservoir for shale oil in the basin. A total of 35 LORS samples were collected from Well Z233 and analysed by mineralogy and element geochemistry to reveal the characteristics and controlling factors of LORS. The following results were obtained: ① In the oil shale, the organic matter is composed of type I and type II kerogen, with a predominance of type II. The organic matter is mature and reaches high levels of abundance. ②Through further analysis. The values of w(U)/w(Th), w(V)/w(Cr), w(Ni)/w(Co), the size and morphology of pyrite, indicates that the lake water of the LORS deposition period in the study area is anoxic or suboxic state. the TOC of the samples was found to have significant positive correlations with the w(Mo) and w(Babio) contents. Paleoproductivity can be inferred to be the key factor controlling organic matter enrichment in the LORS. ③Volcanic and hydrothermal activities promote the development of LORS, provide nutrients needed by organisms, promote the growth of organisms, and improve paleoproductivity. Hydrothermal activity leads to the formation of an anoxic environment at the bottom of the lake, and tuff will cover the sediments, which promotes LORS preservation. This study reveals in detail paleoproductivity is the main factor in the formation of LORS during the evolution of the basin. Volcanic and hydrothermal activities promoted the increase of paleoproductivity, and finally the oil shale layers with the richest organic matter have been formed. And thereby deepens our theoretical understanding of organic matter enrichment.

Hydrothermal Monodisperse Microspherulite Pyrite: Novel Synthesis Process and Electrochemical Study of Its Oxidation.

A simple one-step hydrothermal method was developed to synthesize pyrite (FeS2) sheet- and bulklike pyrite mineral. The Fe/S molar ratio determines the phase of FeS2, including pyrite and marcasite. The reaction temperature and time are key factors to regulate the structure, morphology, and size of pyrite. Scanning electron and transmission electron microscopy showed the formation of monodisperse microspherulite within 1 h reaction time, and the particles aggregated to large irregular polyhedron particles with increasing reaction time up to 4 h. Electrochemical oxidation tests demonstrated that their electrochemical activity significantly decreased with increasing synthesis time. At an elevated temperature of 200 °C, bulk pyrite was obtained after a 24 h reaction time, which could have promising applications in hydrothermal pyrite ore oxidation research.

The diagenetic origin and depositional history of the Cherry Valley Member, Middle Devonian Marcellus Formation

We report δ13C, δ18O, δ34S records coupled with elemental, pyrite morphology, and fluid inclusion data spanning the Cherry Valley Member in the Middle Devonian Marcellus shale from two cores located across a depth gradient in the Appalachian basin. The goal of the study was to evaluate the diagenetic and redox history of this carbonate-interbedded layer within a prominent shale gas system. Three lithological units were identified in the Cherry Valley Member: a nodular clay-rich marlstone lower unit, a fossiliferous limestone middle unit, and a calcareous mudstone upper unit. Low δ13C and δ18O values of the carbonate matrix display a mixing trend among marine carbonate, authigenic carbonate, and late stage diagenetic cementation. The petrology of barite nodules and associated δ34Sbarite values suggest that a shoaling sulfate-methane transition zone created by methane advection contributed to the nodule growth and pyritization. Along with other early and late diagenetic processes, this mechanism provided the main carbonate source for the lower unit due to elevated rates of aerobic methane oxidation. The δ34Spyrite values, U and Mo contents, and pyrite framboid size distributions together constrain the redox history of the Cherry Valley Member. An anoxic-euxinic marine environment rather than a fully oxygenated environment continued through the lower unit in areas with deeper paleo-bathymetry until it was interrupted by an episode of dysoxia during the deposition of the middle unit. Basin-scale anoxia was then renewed during a subsequent 3rd order sea level rise in the upper unit. The entire Cherry Valley Member was ultimately altered by warm burial fluid interaction through vertically distributed veins. The combined body of evidence indicates that the duration and spatial distribution of oxygenated marine conditions during the deposition of Cherry Valley Member within the Marcellus Formation may have been overestimated.

An experimental study of the morphological evolution of pyrite under hydrothermal conditions and its implications

A hydrothermal experiment study on the morphological evolution of pyrite has been conducted to simulate the pyrite behaviors during the gold precipitation conditions (180–220 °C, S/Fe molar ratio is 4/1 to 1/1). The experiment products mainly consist of pyrrhotite and pyrite through X-ray powder diffraction and transmission electron microscopy analysis. The scanning electron microscopy results show that the pyrite morphology is related to the temperature and S/Fe molar ratio. When S/Fe molar ratio drops from 4/1 to 1/1 at 200 °C, the pyrite morphologies change from a combination of {210} and {111} to a combination of {111} and {100} or a single form such as {100}. When S/Fe molar ratio is 3/1 and temperature drops from 220 °C to 180 °C, the pyrite morphologies vary from a combination of {210} and {111} to a combination of {111} and {100} or a single form such as {100}. The grain sizes of pyrite are positively correlated with the pyrite proportion in the products. The morphological evolution of pyrite under experimental conditions is similar to the hydrothermal gold deposits in the Jiaodong Peninsula, Shandong Province, China. The pyrite appears as {210} or a combination of two single forms under experimental conditions, consistent with the observations in gold-bearing hydrothermal mineralization. These results indicate that the pyrite morphologies could reflect the physicochemical conditions of the ore-bearing fluids, and provides a useful morphological typomorphism tool for gold exploration.

Variations in hydrocarbon generating potential of the Chang 7 shale: Evidence from pyrite morphology and sulfur isotope

Hydrocarbon generating potential in source rocks is influenced by various controlling factors. Bottom water redox condition during source rock deposition is an important one among these factors and can directly control the organic matter enrichments in source rocks. The study of pyrite morphology and sulfur isotopic compositions in pyrite provides useful information on depositional redox conditions in ancient marine and lacustrine systems. The results of our study in the Ordos Basin, North China show that pyrite morphology varies from framboid to euhedral crystals throughout the Chang 7 Shale. Further, the sulfur isotopic compositions of pyrite (δ34Spy) display a close linkage with the framboid size distributions of throughout the Chang 7 Shale deposition. The presence of relatively small pyrite size (average = 8.7 μm) and lighter sulfur isotopic compositions of pyrite (average = 4.9‰) in the Chang 73 suggests the bottom water redox conditions were anoxic to dysoxic. In contrast, pyrites in the Chang 72 and Chang 71 are relatively larger (average = 12.7 μm), and have heavier sulfur isotopic compositions (average = 7.1‰) as compared of those in the Chang 73 shales, suggesting a long-term oxic-dysoxic bottom water conditions. Previous paleontological and geochemical data suggest that marine incursion may have supplied ample sulfate into the basin water promoting bacterial sulfate reduction and led to the forming of anoxic-dysoxic lake bottom water redox conditions during Chang 73 shale deposition. In addition, the coupled organic matter enrichment in the Chang 73 and oxygen-depletion conditions is also reflected by the coincidence of higher total organic carbon content in the Chang 73 deposits. A decreasing marine influence with subsequent restriction of the lake basin led to distillation of sulfate, followed by a gradual return to oxic-dysoxic bottom waters during the deposition of the Chang 72 and Chang 71.