Framboidal Pyrite Research Articles

Pyrites in a salt marsh-ria system: Quantification, morphology, and mobilization

Galician Rias are among the most productive ecosystems in the world. Consequently, the soils of their salt marshes and sediments of the intertidal flats show high organic matter contents, reactive Fe, and sulfate, which promote pyrite synthesis and accumulation, using sulfate for organic matter decomposition. This work studies the morphological variability and concentration of pyrites (individual crystals and framboids) in different geochemical environments found in the Ria de Ortigueira (salt marsh soils and bottom sediments in the inner, middle, and outer section), addressing their dynamics in the marsh-ria system in relation to the hydrodynamic characteristics defined by tides and river discharges. Framboidal pyrites were the dominant morphology in marsh soils and sediments in the middle and inner sections of the Ria, while isolated crystals dominated its outer section. The results showed that lower marsh soils (colonized by Spartina) are the most favorable environment for pyrite synthesis, showing high pyritic Fe concentrations and high degrees of pyritization, largely exceeding the values observed in sediments from Galician Rias and from most sedimentary environments worldwide. However, the amount of framboidal pyrites present in the lower marsh (SPE: 4–5 × 104 framboids) was clearly lower than in bottom sediments of the inner and middle part of the Ria de Ortigueira (∼2–7 × 106 framboids), mainly due to the fact that pyrites were found to form large framboids in lower salt marsh soils. Thus, the amount of framboidal pyrites does not seem to be a good indicator of redox conditions in modern marine sediments. Pyrite crystals found in the sediments of the Ria showed poorly defined vertices and facets, indicating their degradation and suggesting that a significant amount of the pyrites found in the middle and inner sections derive from marsh collapse. Finally, the output of framboidal pyrites towards the outer Ria de Ortigueira reflects the low intensity of residual flows in this Ria. Therefore, the pyrites observed in the outer section consisted only of isolated crystals, presumably formed in situ under low sulfate-reducing activity conditions.

Geochemistry and textural evolution of As-Tl-Sb-Hg-rich pyrite from a sediment-hosted As-Sb-Tl-Pb ± Hg ± Au mineralization in Janjevo, Kosovo

The sediment-hosted As-Sb-Tl-Pb ± Hg ± Au Janjevo occurrence is located in the southern part of the Kizhnica-Hajvalia-Badovc ore field, in the Trepça (Trepča) Mineral Belt (TMB) in Kosovo. The As-Sb-Tl-Pb ± Hg ± Au mineralization is hosted by Upper Triassic marbles and occurs in the form of quartz-stibnite veins and dolomitized irregular pockets associated with jasperoid rocks. The main sulfide most widespread in the mineralization studied is pyrite which is the main carrier of thallium. A wide range of techniques was used to characterize the geochemistry and textural evolution of pyrite/marcasite from Janjevo: polarized reflected-light microscopy, electron microprobe (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), X-ray powder diffraction (XRPD), Mössbauer spectroscopy, and isotopic studies. The study of 42 thin and polished sections resulted in the identification of 4 generations of pyrite/marcasite. The first generation of pyrite related to the pre-ore stage occurs as framboidal pyrite with different degrees of evolution (Py1a), as well as recrystallized pyrite/marcasite (Py1b). Au-enriched pyrite (Py2) occurs as elongated fibrous aggregates and is genetically associated with the formation of stibnite veins (Sb stage). As-Tl-Sb-Hg pyrite (Py3) forms irregular colloform aggregates which fill the stylolites and also overgrow older generations of pyrite. The youngest generation is represented by marcasite (Py4), which forms euhedral crystals. Co/Ni ratio < 1 in all pyrite/marcasite generations implies a sedimentary fingerprint. On the other hand, δ34S isotopic studies confirm a magmatic sulfur source for As-Tl-Sb-Hg pyrite (Py3), as well as co-occurring stibnite and realgar. The most important minor and trace elements in pyrite/marcasite are As (up to 14.1 %), Tl (up to 3.94 %), Sb (up to 3.73 %), and Hg (up to 0.55 %), which are mainly hosted by Py3. In addition to arsenic, thallium, antimony, and mercury, other generations of pyrite show enrichment in Ni, Co, and Cu (framboidal pyrite Py1), Au, and Se (Au-enriched pyrite Py2), and Se (marcasite Py4). The highest gold content of up to 6.08 ppm was recorded at Py2. Studies on Py3 geochemistry confirm the presence of 2Fe2+ ↔ Tl+ + Sb3+ heterovalent substitution in pyrite. As-Tl-Sb-Hg pyrite (Py3) is the main thallium host mineral in the mineralization studied, with significantly less thallium present in the Pb-Sb ± Tl ± As sulfosalts. In addition, the high arsenic content of Py3 is due to the presence of arsenic in the form of As1− and As0 related to amorphous arsenic-rich nanoparticles. The reported Py3 is the best phenomenon of the crystallization of colloform As-Tl-Sb-Hg-rich pyrite under hydrothermal conditions. Pyrite-rich sediment-hosted As-Sb-Tl-Pb ± Hg ± Au mineralization from Janjevo is associated with the distal manifestation of a concealed porphyry system.

Pyrite Dissolution in the Cretaceous Yogou Formation of the Niger (Chad) Basin: Implications for Basin Evolution under a Rift Tectonic Setting.

This is the first study of pyrite minerals in the entire West and Central African Rift System (WCARS). Several polished organic-rich core samples from the Cretaceous Yogou Formation of the Niger (Chad) Basin located in the WCARS were investigated for their pyrite content using FE-SEM and SEM-EDS imaging techniques. An attempt was made to classify the types and provenance of the pyrites and to highlight the control of rift fractures on the oxidation and dissolution of pyrites in the region. Three major types of pyrites are present in the studied formation, including euhedral pyrite (EPy) crystals, pyrite framboids (FPy), and sunflower pyrites (SPy). A statistical analysis of 307 FPy shows that the framboids are diagenetically formed with an average diameter of 6.61 μm. SPy is present in a relatively low amount compared to framboids. The pyrites underwent a variety of diagenetic modifications, from mechanical compaction to oxidation, dissolution, and recrystallization. Unoxidized pyrites primarily contain Fe, S, and C, but oxidized pyrites also contain O, Al, and Si. There is a strong correlation between the fractures and the spatial distribution of the physicochemical alteration of the pyrite in the study. Dissolution in relatively deep-buried samples occurs mainly along fracture planes. The fractures provide a pathway for oxidants and other metal elements to reach the pyrites. The pattern of pyrite dissolution reflects the timing of fracture formation and fracture activities as a purveyor or drainage for fluids in the organic-rich samples investigated. The pyrites are associated intimately with organic matter (OM); thus, the relationship between the fracture and the pyrites' transformation is significant in the assessment of organic matter preservation at deep-burial depth.

Petrographic and geochemical characteristics of oncolites in the Ediacaran Dengying Formation of the Upper Yangtze Area, China: Implications for their Paleo-environment

The Ediacaran Period represents a critical transition in the Earth's evolution, which could be potentially revealed by analyzing the oncolites formed during this period. This study investigates the genesis of oncolites in the Ediacaran Dengying Formation in the Yangba profile, Nanjiang County, Sichuan Basin. Moreover, efforts are made to analyze the effects of terrigenous input from perspectives of rare earth elements, probe into the variation of redox conditions via the microscopic analysis of trace elements, and exemplify the coupling relationship between the oxygen isotope, strontium isotope, paleosalinity (indicated by Z values), and sea level variation. The oncolites in the Yangba profile are generally featured by thin skins and dispersal grains and are highly differentiated in size and shape. A large number of microbial laminated fabrics and framboidal pyrite are developed in the dark laminae, and the former is yellow-green under the fluorescence microscope. According to the 13C-enrichment and petrographic analyses, oncolites and organic matter should be autochthonously formed by geochemical processes linked to microbial mats. Three cycles of sedimentary microfacies formed in subtidal low-energy zone and subtidal high-energy zone, and ten upward-coarsening cycles of oncolites are identified vertically, which, together with the vertical variations of δ18O and 87Sr/86Sr, indicate that the sea level has experienced six stages of variations during the deposition of the 2nd Member of the Ediacaran Dengying Formation (Deng-II Member): namely, transgression, lowstand fluctuating regression, transgression, continuous regression, transgression, and again lowstand fluctuating regression. Negative Ce anomalies (Ce/Ce*) and low V/Cr and low V/(V + Ni) ratios indicate the presence of molecular O2 in the water column, whereas characteristic Ni/Co, U/Th, and framboidal pyrite suggest the contemporaneous presence of anoxic water. The oncolites are suggested to be rarely impacted by terrigenous input, as is evident by the Er/Nd ratio of < 0.1, the average Y/Ho ratio of 35, the average La/Yb ratio of 36, and the Z value of > 130. To conclude, Upper Yangtze Deng-II Member oncolites are exemplified to be of microbial origins; the marine redox structure is featured by a temporarily stratified, oxygenated water body and a less oxygenated deeper water body during the Ediacaran, and frequent change of seawater energy, which are consistent with the Ediacaran seawater properties.

A multi-proxy record of palaeoenvironmental and palaeoclimatic conditions during Plio-Pleistocene peat accumulation in the eastern flank of the Isparta Angle: A case study from the Şarkikaraağaç coalfield (Isparta, SW Central Anatolia)

The Şarkikaraağaç coalfield is located in the north-western part of the Lake Beyşehir Basin, which is the most significant graben area in the eastern flank of Isparta Angle, and hosts a 300-Mt coal resource. This study focuses on the first palaeoenvironmental and palaeoclimatic reconstruction of Pliocene and early Pleistocene coal-bearing sequences cored in the two coal exploration wells (SK-1 and SK-2) using coal petrography, mineralogy, faunal (ostracod and mollusk), and floral (palynology and diatom) data from four coal seams (from bottom to top: B, A, X-1, and X-0) with the variable total thickness (1 to 7 m). According to the palynological data, warm and humid climate conditions prevailed during the early Pliocene, and the precursor peats of seams A and B were mainly accumulated under limno-telmatic conditions, with high contributions of herbaceous peat-forming plants. Nevertheless, the co-occurrence of syngenetic carbonate minerals and framboidal pyrites along with calcareous fossil and diatom remains implies neutral to weakly alkaline conditions within palaeomires of these seams. Furthermore, ostracod and gastropod fauna from these seams might imply nutrient-rich shallow water conditions and spring support (e.g., karstic aquifer) into palaeomires. Thus, algal activity within the palaeomires was presumably high, and freshwater algae and diatoms were commonly identified in these seams.With the development of increased uplift ratio of central Taurides and climatic changes towards to late stages of Pliocene and particularly early Pleistocene, the common peat-forming plants within palaeomires and vegetation in the surrounding areas were changed. The increased precipitation caused an elevation of water levels in the study area; hence, the precursor peats of the seam X-1 were accumulated under wet forest mire conditions. This increase also explains the existence of ostracod and mollusk fauna related to river and spring support underlying sequences of the seam X-1. With the ceasing of accumulation of peat of the seam X-1, the climate became drier, and very shallow water conditions have been common. The development of spring support and relatively high precipitation in a short period of time allowed for final peat accumulation (seam X-0) during the early Pleistocene. Nevertheless, this period was followed by the development of relatively colder conditions in the study area, and cold small water conditions were developed during the end of the early Pleistocene. Furthermore, microthermic vegetation was common in the vicinity of the palaeomires during the early Pleistocene. Overall, the peat accumulation and water level of lakes in the study area seem to be controlled by climatic oscillations and uplift of margins of the Lake Beyşehir Basin during the Pliocene to early Pleistocene.

Dynamic ocean redox conditions during the end-Triassic mass extinction: Evidence from pyrite framboids

The end-Triassic (∼201 Mya) records one of the five largest mass extinction events of the Phanerozoic. Extinction losses were coincident with large igneous province volcanism in the form of the Central Atlantic Magmatic Province (CAMP) and major carbon isotope excursions (CIEs), suggesting a link between these phenomena. Marine anoxia has been implicated as a causal factor in the crisis, but there remains some uncertainty regarding the role of marine redox changes in marine extinction phases because both intensity and duration of marine anoxia are poorly constrained. We employ high resolution pyrite framboid size-frequency analysis at two Triassic-Jurassic (Tr-J) boundary sections: Kuhjoch in Austria (the Tr-J Global Boundary Stratotype Section and Point; GSSP) and St. Audrie's Bay in England (former GSSP candidate) in order to further evaluate the role of marine anoxia in the end-Triassic mass extinction (ETME). The St. Audrie's Bay section records predominantly anoxic conditions punctuated by weakly oxygenated (dysoxic) conditions through the Tr-J transition, even during shallow-water intervals. Kuhjoch experienced both anoxic and dysoxic conditions during the ETME but became better oxygenated near the Tr-J boundary. Marine anoxia is therefore implicated in the extinction at both locations. A similar redox history is known from the Central European Basin, Western Tethys and Panthalassa, where marine anoxia developed in the lead up to the ETME prior to reoxygenation around the Tr-J boundary. • High-resolution pyrite framboids size distribution from two European Triassic-Jurassic boundary sections. • Global dynamic ocean redox evolution through the Triassic-Jurassic boundary is reconstructed. • Oxygen restriction occurred in extremely shallow waters and was likely one of the key causes of the crisis.

Genesis of the Loma Galena Pb-Ag Deposit, Navidad District, Patagonia, Argentina: A Unique Epithermal System Capped by an Anoxic Lake

Abstract Loma Galena (978,852 t Pb, 206 Moz Ag) is one of eight epithermal deposits in the world-class Navidad Pb + Ag ± (Zn, Cu) district located in the Cañadón Asfalto continental foreland basin, northern Patagonia, Argentina. This basin formed during the Jurassic in an extensional tectonic regime during the breakup of Gondwana. Host rocks comprise major listric faulted and tilted blocks of K-rich andesite to dacite lava flows (173.9–170.8 Ma; U-Pb ages for zircon) unconformably overlain by mudstone interbedded with stromatolitic and pisolitic limestones, sandstone, coal, and an Sr-rich evaporite layer deposited in a lacustrine environment. The mineralization occurs as disseminations in the organic-rich sedimentary rocks, in veins and hydrothermal breccia dikes in the hanging walls and footwalls of NW- and NE-striking normal faults, in volcanic autobreccias, and in a phreatic breccia at the contact of volcanic and sedimentary rocks. The earliest hydrothermal minerals consist of veins of colloform, crustiform, and cockade calcite 1 (δ13Cfluid –4.7 to 0.8‰; δ18Ofluid 4.8–11.6‰) and siderite. The precipitating fluids were likely basement-exchanged basinal brines having salinities of 9.5 to 16.4 wt % NaCl equiv and temperatures of 154.7° to 212°C. The interaction of these fluids with the host volcanic rocks formed calcite, albite, adularia, and celadonite-glauconite-group minerals followed by chlorite and siderite as fO2 decreased. Fluids intermittently boiled, as evidenced by bladed (platy) texture in calcite 1. Subsequent mineralizing stages contributed to the metal endowment of Loma Galena. The abundance of organic-rich mudstone and δ34S from –15.4 to 12.9‰ for sulfides suggests that the bottom waters of the lake were anoxic and the loci of microbial sulfate reduction (evaporites have δ34S 35‰). Mixing of upflowing metal-rich basinal fluids carrying some S from depth with this H2S-rich connate water efficiently precipitated Ag-bearing framboidal pyrite, colloform pyrite-marcasite, chalcopyrite, bornite, tennantite-tetrahedrite, sphalerite, and galena as veins, breccias, and disseminations in host rocks. The highest grade and tonnage of the ores are found in autobreccias at the junction of the uppermost lava flow and in the overlying mudstone, where the addition of a strong microbial signature is recorded in sulfides. This event also led to partial dissolution of magmatic and hydrothermal feldspar and calcite 1 in the altered volcanic rocks. Mineralization was followed by hydrothermal brecciation and successive precipitation of chalcedony (δ18Ofluid 2.6–4.8‰), barite (δ34S 15.7–22‰; 160.9°–183.8°C; 7.7–9.7 wt % NaCl equiv), calcite 2 (δ18Ofluid –10.2 to –3.7‰, 58°–95°C; 1.9–7.0 wt % NaCl equiv), strontianite, and quartz in brecciated veins and breccias; kaolinite (δ18Ofluid 2–6.2‰), illite-smectite, smectite, and carbonates with minor chalcedony and barite in the volcanic rocks; and calcite, chalcedony, and barite in the sedimentary rocks. A trend of decreasing salinity with decreasing temperature and lowering δ18O of the fluids with time suggests dilution of the basinal fluids by mixing with Jurassic meteoric water (δ18O −9 to −5.2‰). Loma Galena is a unique example of a polymetallic epithermal system formed in a sublacustrine anoxic environment that promoted the efficient deposition and preservation of Ag-bearing sulfides, thereby contributing to the large size and relatively high grade of the deposit.

Pushing the limits: Resolving paleoseawater signatures in nanoscale fluid inclusions by atom probe tomography

New insight into the geochemistry of ancient environments can be gained through structural and chemical analyses of nanometer-scale features within minerals. Here, we present recent developments using atom probe tomography (APT) enabling direct visualization of nanoscale fluid inclusions trapped within pyrite (FeS2) and thereby chemical characterization of remnant seawater. Pyrite framboids (spherical clusters of nanocrystals) were sampled from the Middle Devonian Leicester Pyrite Member (New York). Scanning transmission electron microscopy shows low density features distributed within the pyrite (<4 nm in size). Three-dimensional chemical visualization and analysis by APT reveals these to be nanoscale fluid inclusions preserving the elemental signatures of the water column in which the framboids formed, as alkalis consistent with seawater including Na, K, Mg, and Ca are identified. Further, within the inclusions Mg/Ca is measured to be ∼0.6±0.2 – consistent with conditions for calcite-dominated seawater existing in the Middle Devonian. The results and approach developed in this study show the potential to reconstruct paleoenvironmental conditions from coupled elemental and structural analyses of nanoscale fluid inclusions.

Evolution of organic matter quantity and quality in a warm, hypersaline, alkaline lake: The example of the Miocene Nördlinger Ries impact crater, Germany

The Nördlinger Ries (NR) impact crater in southern Germany contains Miocene lacustrine fine-grained post-impact sediments, which were deposited under saline, alkaline, and hydrologically closed aquatic conditions. To obtain information on the organic matter quantity, quality and environmental evolution, samples from one central (NR 1973) and one more marginal (Enkingen SUBO-18) well were analyzed by pyrolytic, organic geochemical and micro-petrographic techniques. Deposits of the marginal drilling can be linked to the thicker stratigraphic units of the well from the lake center based on total organic carbon (TOC), hydrogen index (HI) and lithology. Central deposits contain mainly hydrogen-rich type I kerogen with a mean HI of 447 mg HC/g TOC, whereas the marginal sequence contains type II-III kerogen with an average HI of 206 mg HC/g TOC. In the center, high total sulfur (TS) over TOC ratios of 1.5 on average, and δ13C values of biomarkers in combination with low α-/total MTTC (α-/total methyl-trimethyltridecyl chromans ∼ &amp;lt;0.6) and Pr/Ph ratios &amp;lt;0.2 suggest the prevalence of hypersaline water. Salinity only decreased during deposition of the uppermost Miocene units, when the then shallow alkaline lake turned, at least temporarily, into acidic-neutral conditions during the time of peat deposition. A similar trend, but at overall lower salinities is recorded for the marginal site. Water stratification in the center is stronger than in the marginal domain based on salinity variation. Anoxic bottom water prevailed in the whole lake, as recorded by exceptionally low Pr/Ph ratios (∼&amp;lt;0.2), the occurrence of des-A-lupane and small pyrite framboids (&amp;lt;5 µm) except for the youngest layers of the Miocene lake. Autochthonous, halophilic red algae/plankton prevailed as reflected by abundant C27steranes, while terrestrial material and aerobic bacteria significantly contributed to the OM only in the latest stages of lake as evident from abundant vitrinite/inertinite particles, high values of diterpenes/C27sterane (up to 255) as well as low values of C27-29regular steranes/C29-32hopanes (&amp;lt;0.5). The same trend, but less well resolved, can be seen at the margin of the paleo-lake.

Marine redox variation and hydrographic restriction in the early Cambrian Nanhua Basin, South China

The early Cambrian Nanhua Basin is an important archive of information related to contemporaneous marine redox evolution and its relationship to the radiation of early animals. However, the nature of watermass conditions in the Nanhua Basin remains uncertain owing to conflicting redox signals. Here, we generated a redox proxy dataset comprising Fe speciation, redox-sensitive element (RSE), pyrite framboid size distributions, and molybdenum isotopic data (δ98Mo), in combination with proxies for hydrographic restriction (i.e., Co × Mn, Cd/Mo, and Mo/TOC), in order to evaluate environmental conditions in the basinal Yuanjia section. We then compared these data with previous results for outer-shelf (Jinsha, Zhongnancun, and Yangjiaping), slope (Daotuo, Songtao, and Longbizui), and basinal facies (Silikou) to develop an integrated picture of environmental (especially redox) changes in the early Cambrian Nanhua Basin. At Yuanjia, the lower member (LM, 0–28 m) is characterized by weakly euxinic conditions and an intense Fe-Mn shuttle, and the upper member (UM, 28–65 m) by moderately euxinic conditions and a weak Fe-Mn shuttle. Our integrated analysis demonstrates co-existence of oxic surface waters, euxinic mid-depth waters, and ferruginous deep waters in the Nanhua Basin, with euxinic conditions prevailing in shelf-slope facies of the Yangtze Block and basinal facies of the Cathaysia Block but declining in intensity from the LM to the UM. The Nanhua Basin changed from being moderately restricted with strong upwelling during the LM to relatively more restricted with weak upwelling during the UM. We infer that redox fluctuations in the early Cambrian Nanhua Basin were linked to hydrographic restriction, although global factors such as low seawater sulfate concentrations may have played a role also. Our study highlights the need for caution in assuming that redox variation in the early Cambrian Nanhua Basin was representative of contemporaneous global-ocean redox states.

Intensive ocean anoxia and large δ13Ccarb perturbations during the Carnian Humid Episode (Late Triassic) in Southwest China

The Carnian Humid Episode (CHE) represents a dramatic dry to wet climate transition in the Late Triassic. Manifestations of this climate shift and its associated biological and environmental responses are not fully understood. Here, we carried out carbonate carbon isotope, trace metal, and pyrite framboid analyses at Wolonggang in southwest China to trace palaeoenvironmental changes during this critical interval. The CHE at Wolonggang is marked by the development of fine laminated carbonaceous siltstones and black shales overlying the intensely bioturbated Zhuganpo limestone deposited in the latest Julian 1. This lithological change is accompanied by two negative δ 13 C carb excursions, a strong enrichment of redox-sensitive trace metals U and Mo, and the development of abundant small pyrite framboids with an average diameter < 5 μm. The sedimentary and geochemical evidence collectively suggest a sharp decrease in carbonate production and a major carbon cycle disturbance, coinciding with the onset of intense ocean anoxia and the mass mortality of burrowing faunas. Unlike other areas in the western Tethys, the aftermath of the CHE at Wolonggang is characterized by long-lasting ocean anoxia, with weak carbonate production persisting at least to the late Tuvalian. This is evidenced by the deposition of finely laminated silty carbonates, marls, and carbonaceous siltstones with rare benthic fossils in the Wayao Formation. Our study suggests that anoxic-euxinic environments widely developed along the southwest margins of the Yangtze Platform and in the Nanpanjiang Basin, probably contributing to the loss of marine faunas during the CHE. • Sedimentary and bio-chemostratigraphic investigations were carried out in a key Carnian section in SW China. • Carbon cycle instability was seen during the Carnian Humid Episode. • Anoxia was basin-wide and long-lasting during the CHE and its aftermath. • Warming and anoxia played key roles in the loss of marine faunas during Carnian.

Do bacterial viruses affect framboid-like mineral formation?

Abstract. Framboidal pyrite has been a matter of interest of many studies due to its abundance in a wide range of environments and being a marker of redox conditions. However, the clear origin of framboidal pyrite remains unresolved. Our studies are preliminary laboratory investigations on the influence of the shape and physicochemical properties of bacteriophages on the synthesis of framboid-like structures. This paper discusses the possible role of bacteriophages (bacterial viruses) in the precipitation of sulfide minerals (FeS and CuS) and their impact on the formation of framboid-like structures. Here, two bacteriophages (Escherichia phage P1 and Pseudomonas phage Φ6), which differ significantly in shape and physicochemical properties, were used. Our observations suggest that viruses may bind ions from the solution. Moreover, we showed that bacteriophages P1 can lead to the formation of finer mineral particles of FeS and CuS, whereas the framboid-like structures were found only in experiments with precipitation of FeS. However, the lipid-enveloped Pseudomonas phage Φ6 did not cause the formation of similar structures. It is assumed that Escherichia phage P1 can promote the formation of FeS-based framboid-like or spherical structures. The proposed four-step conceptualized mechanism facilitating the framboid-like structure synthesis via viruses is as follows: (i) binding of ions by capsids, (ii) bacteriophages behaving like a crystallization surface, (iii) destabilization of the colloid (ζ potential ± 0), and (iv) formation of fine agglomerates and subsequent formation of small crystallites. Further studies are required to find all factors that may be affected by bacteriophages during sulfide precipitation. In addition, it is important to consider viruses present in sedimentation environments, despite possible difficulties in laboratory culturing. The consideration of such viruses may make laboratory testing more valid in terms of sedimentation environments.

Formation and preservation of Eocene lacustrine microbialites in the western Qaidam Basin (northeastern Qinghai-Tibetan Plateau, China): Petrological, mineralogical, and geochemical constraints

Studying the lacustrine microbialites in the Qaidam Basin may provide clues to the occurrence and mineralization processes of microbial communities in extremely harsh terrestrial environments. In the Eocene lacustrine systems of the Qaidam Basin, thick thrombolites (~0.4 to 1 m thick) generally formed in near-shore settings with rare to plentiful terrigenous admixtures and are characterized by four different clotted structures (i.e., maze-like, cortoid-like, tightly bound, and aggregated). In contrast, the small-scale stromatolitic, thrombolitic, and composite clotted-laminated microbialites (cm level) exhibit domical, columnar, and stratiform shapes, likely indicating that they developed in very shallow settings (e.g., a lagoon). The early lithification of the outer crusts and the internally laminated and clotted textures facilitated the preservation of the microbialites. The laminated and clotted textures of the Eocene microbialites analyzed in this study were composed of crypto- to micro-crystalline (mimetic) dolomite, which likely formed during the early mineralization process. This was likely caused by the strong evaporative lacustrine conditions on the Qinghai-Tibetan Plateau during the Eocene-Oligocene climate transition. The microbial mineralization process in dolomite-supersaturated pore fluids may also have contributed to early dolomitization. In contrast, the medium to coarsely crystalline components of the microbialites are composed of calcite minerals, implying that these components experienced cementation/recrystallization during diagenesis. In addition, large quantities of authigenic sulfate (celestite-barite) and sulfide (framboidal pyrite) minerals are exclusively distributed within the mimetically-dolomitized, laminated and clotted microbial structures. It is inferred that the saline lake conditions and the mineral dissolution (both carbonate and terrigenous particles) contributed to the SrSO4- and BaSO4-supersaturated fluid conditions and enabled the crystallization of celestite-barite. The development of Eocene lacustrine microbialites on the northeastern Qinghai-Tibetan Plateau provides a case for comparison with widely-distributed records of lake microbial carbonates, and the microstructural characteristics and analytical tools used in this study may offer a new perspective to explore the complex mineralization processes of microbialites.

Organoclastic sulfate reduction in deep-buried sediments: Evidence from authigenic carbonates of the Gulf of Mexico

The fate of organic matter in deep-buried marine sediments determines the formation of hydrocarbon reservoirs, shapes the deep biosphere, and affects the global carbon cycle. Several geochemical processes such as silicate weathering, iron and manganese (hydr)oxide reduction, and sulfate reduction influence the remineralization of organic matter in buried sediments. However, little information on deep-seated organoclastic sulfate reduction is available and the involved geochemical processes are largely unknown. Here, authigenic carbonates recovered from Ship Shoal Block 296 (SS296) of the Gulf of Mexico, all dominated by low-magnesium calcite, were investigated to constrain organoclastic sulfate reduction in deep-buried sediments. The SS296 carbonates had been previously suggested to be of a deep origin and transported upward by salt diapirism. The presence of cone-in-cone textures in samples 3110R1 and 3110R2 as well as vitrinite reflectance values of 0.48% to 0.58% indicate carbonate formation at a relatively deep burial. Likewise, clumped isotope thermometry yielded formation temperatures from 45 °C to 53 °C for sample 3110R1, 60 °C for sample 3110R2, and 11 °C to 37 °C for sample 3110R3. In spite of formation at greater depth, the temperatures of samples 3110R1 and 3110R2 are still within the temperature range of microbial sulfate reduction, reflecting formation depths between 1 km and 2.7 km. The presence of framboidal pyrite and its relatively low δ34S values (−27.8‰ to +11.5‰) agree with the occurrence of microbial sulfate reduction during carbonate precipitation. The carbon isotopic composition of the carbonates (δ13C: −16.6‰ to −8.9‰) and enclosed organic matter (δ13Corg: −26.8‰ to −20.5‰) suggests that organic matter is the dominant carbon source of carbonate minerals. This study confirms that microbial sulfate reduction occurs in deep-buried sediments, driving the degradation of organic matter. Our findings suggest that organoclastic sulfate reduction might be common in deep-seated sedimentary environments, controlling the fate of organic matter in deep subsurface environments and affecting the global carbon cycle.

Petrological, mineralogical, and geochemical compositions of coal in the Ombilin Basin, West Sumatra, Indonesia

Coal deposits have received considerable attention in recent years owing to their by-product waste being a promising alternative source of critical elements. The Ombilin Basin is well known as one of the largest coal-producing basins in Indonesia. This study investigated the petrological, mineralogical, and geochemical compositions of four Eocene coal seams in Ombilin Basin basement rock, and their relevance regarding critical element enrichment. Samples of coal and basement rock from the Ombilin Basin were analyzed using thin- and polished-section scanning electron microscope energy-dispersive X-ray spectroscopy, inductively coupled plasma atomic emission spectroscopy, and inductively coupled plasma mass spectrometry. Coals from the Ombilin Basin are high-volatile bituminous C coals with mean random vitrinite reflectance values of 0.58%–0.66%. Vitrinite content is the most dominant maceral (61.06 vol% mmf), followed by liptinite (24.95 vol% mmf), and inertinite (13.75 vol% mmf). Minerals observed in the coal samples comprised pyrite, quartz, and clay. Pyrite was present as discrete crystals, massive crystals, framboidal pyrite, and cell-filling on other macerals. Both syngenetic and epigenetic pyrite was found in the Ombilin coal. Based on the petrological and mineralogical analyses, basement rock in the Ombilin Basin can be divided into four groups: granodiorite, andesite-basaltic, marble, and meta-sediment. In comparison with World Hard Coal and common Chinese coals, some layers/partings in the coal seams in the Ombilin Basin are enriched in U (37.6 ppm), V (175 ppm), and total rare earth elements and yttrium (REY; 126.15 ppm). REY-bearing minerals in the Ombilin coal samples were attributed to clay and sulfide minerals, while others were found as dispersed and scattered, indicating that REY are organically bound. Inorganic materials in Ombilin coal deposits have mainly intermediate-felsic provenance, are derived from Cretaceous granitoid rocks and andesite intrusions, and have been subjected to freshwater conditions.

Multimodal study of the impact of stimulation pH on shale pore structure, with an emphasis on organics behavior in alkaline environments

The tight nature of shale formations calls for hydraulic fracturing techniques able of altering the pore architecture to facilitate the production of hydrocarbon resources. The composition, pH, salinity, density and chemistry of Hydraulic Fracturing Fluids (HFF) vary substantially depending on reservoir characteristics. pH is among the most important stimulation fluid properties, ranging from very acidic, for carbonate reservoirs, to basic, in the case of clay-rich formations. pH regulates what geochemical reactions take place between the stimulation fluid and shale, as well as pore architecture alteration. The dissolution of carbonates and pyrite, and the precipitation of common minerals such as barite, gypsum and iron oxides in acidic environments have been extensively documented. In contrast, alkali stimulation environments and the role of organic components have received less attention. This research provides insight into the role of both minerals and organic components during alkaline stimulation, and the resulting pore architecture alterations. A set of reactive experiments are performed using three different shales, with varying organic matter (OM) content, at different pH (3, 6, 8, 10, 12). The analysis of pore architecture alteration induced by the reaction was performed via nitrogen (N2) gas adsorption and Time-Domain Nuclear Magnetic Resonance (TD-NMR). Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) was used to measure the composition of the stimulation fluid post-reaction. The system with a larger impact on the pore architecture (pH 12) underwent a more extensive analysis through the application of Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) and x-ray Diffraction (XRD). Results show that mineral dissolution contributed to the creation of secondary porosity, mainly through dissolution of clays and silicates. We show that the abundance and distribution of organic matter both play a significant role in changes in pore architecture. Most samples showed an increase in inter-organic porosity, likely due to the organic acids within kerogen reacting with the stimulation fluid. Samples with a larger amount and more widespread distribution of organic matter show the most significant alterations. Finally, we also highlight the dissolution process of iron-rich minerals, including framboidal pyrite, as another important source of secondary porosity.

Complex fluid source of the multistage pyrite-bearing Huilvshan gold deposit (west Junggar, NW China): Insight from pyrite texture, sulfur isotope and trace element compositions

Hydrothermal gold deposits commonly form during protracted multistage ore-forming processes, however, discriminating among different ore-forming fluids and tracing their origin to unique or different repositories is a challenging task. The Huilvshan gold deposit in West Junggar (Xinjiang province, NW China), was affected by three hydrothermal stages and thus provides an opportunity to examine complex ore-forming processes. The deposit consists of gold-bearing quartz-sulfide veins and disseminated sulfides, hosted within Early Carboniferous basalts and tuffs. Three stages of hydrothermal pyrite (Py1, Py2, and Py3) were identified. Gold is only present in Py2 where it occurs as native gold inclusion and in invisible form. Anhedral Py1, disseminated in the altered basalt, is characterized by higher Ni (13.7 to 635 ppm) concentrations than Py2 and Py3. Euhedral-subhedral Py2 is the richest in Au (1.46 to 25.7 ppm) among three stages of pyrite. Py3 is subdivided into grains with zonal texture (Py3a) and irregular grains (Py3b). Both sub-types have high Sb (56.8 to 1599 ppm) and Tl (0.02 to 68.7 ppm) concentrations. In situ δ34S values of Py1 (-6.0 to 2.7 ‰) are similar to those of Py2 (-5.9 to 3.9 ‰), whereas Py3 has extremely negative δ34S values (-44.3 to-18.0 ‰).Ar-Ar ages of hydrothermal muscovite coexisting with Py2 indicate that the Huilvshan gold deposit formed at ∼300 Ma. The sulfur isotopic systematics of Py1 and Py2, close spatial and temporal relationship between the Huilvshan gold deposit and adjacent felsic intrusions, and paucity of regional metamorphic rocks in the region all suggest that hydrothermal fluids in stage I (Py1) and II (Py2) were derived from magmatism. On the other hand, stage III pyrite and associated sulfides more likely precipitated from a hydrothermal fluid that circulated in tuffs, based on the extremely negative δ34S values of Py3, similar to those measured from framboidal pyrite in the host tuffs.Therefore, textural, trace elemental and sulfur isotope data from pyrite suggest different sources for the hydrothermal fluids in the Huilvshan gold deposit, and that magmatic activity may have contributed the most to the main ore-forming stage. The genesis of this Huilvshan gold deposit could provide significant insights into the origin of other hydrothermal gold deposits that show multiple sulfide generations.

Metallogeny of Manto-type stratabound Cu-(Ag) mineralization in Iran: Relationship with NeoTethyan evolution and implications for future exploration

Stratabound or “Manto-type/volcanic red-bed” Cu-(Ag) deposits occur along the Urumieh-Dokhtar magmatic arc (UDMA), Sanandaj-Sirjan Zone (SSZ), Alborz Magmatic Assemblage (AMA), Sabzevar Zone, and Lut Block structural zones of Iran, and are hosted by Cretaceous and Eocene volcanic and volcano-sedimentary rocks. The most important deposits of this type are East Narbaghi, Khankishi, Kahak, Veshnaveh, and Koshkouieh in the UDMA; Keshtmahaki in the SSZ; Mari, Qeblebolagh, and Yamaghan in the AMA; Abbasabad, Dochileh, Gol-Cheshmeh, Zangaloo, Abri, Rahbari, and Cheshmeh-Marzieh in the Sabzevar Zone; and Vorezg in the Lut Block. Back-arc extensional basins are the principal tectonic setting for the Manto-type deposits of Iran, that developed during the subduction of the NeoTethyan crust beneath the Iranian micro-continental plates. The stratigraphic sequences of these deposits are commonly formed from volcanics, volcaniclastics, and sedimentary rocks, which are similar to other volcanic-hosted stratabound copper districts around the world. Iranian Manto-type deposits are stratigraphically controlled in which Cu-(Ag) mineralization occurs as vein-veinlets, disseminated grains, and filling vesicles. Copper mineralization is commonly closely related to framboidal pyrite and pyrobitumen. The mineralogy of the Cu-(Ag) mineralization in all Manto-type deposits of Iran is simple and consists of chalcocite, bornite, native copper, chalcopyrite, acanthite, and minor secondary digenite, covellite, cuprite, tenorite, chrysocolla, anilite, malachite, azurite, and Ag-bearing clausthalite as the main economic minerals, accompanied by pyrite, chlorite, calcite, quartz, and zeolite. Copper sulfides occur mainly as a replacement of diagenetic pyrite, which in turn, replaced pyrobitumen. Iranian Manto-type deposits are interpreted to be the result of mineralizing hydrothermal fluids derived from footwall volcanic rocks that migrated upwards, driven by the late diagenetic processes. The UDMA and Sabzevar Zone are the most favorable metallogenic provinces in Iran for Manto-type Cu-(Ag) exploration since these two zones host the largest copper deposits of this type, including Abbasabad, Koshkouieh, East Narbaghi, and Abri deposits. Consequently, reconstruction of orebearing basins, such as presented, is not only critical to understanding the genesis of ancient deposits and their tectonic setting, but also for guiding exploration in deposit-proximal areas.

Abundant Fe(III) Oxide‐Bound Arsenic and Depleted Mn Oxides Facilitate Arsenic Enrichment in Groundwater From a Sand‐Gravel Confined Aquifer

AbstractEnrichment mechanisms of high‐arsenic (&gt;10 μg/L) groundwater in confined aquifers with sand‐gravel sediments were rarely documented. Both groundwater and sediment were sampled to investigate mechanisms of groundwater arsenic mobility in sand‐gravel confined aquifers from the Songnen Basin, China. The results show that, although high arsenic groundwater was present in both shallow sandy (&lt;60 m) and deep sand‐gravel aquifers (60–100 m), relatively higher arsenic groundwater was hosted in the deep confined aquifers. The significantly higher proportion of biologically degradable organic carbon (BDOC) in deep confined groundwater than those in shallow unconfined ones (p = 0.03) suggests that higher arsenic concentrations were associated with higher DOM biodegradability. High‐arsenic groundwater was the result of Fe(III) oxide reduction facilitated by BDOC degradation. Arsenic‐rich pyrite framboids were observed in shallow aquifers, which would lower groundwater arsenic concentrations. Sand‐gravel sediments in deep aquifers had remarkably higher contents of extracted Fe(III) oxides and the bound arsenic, HCl‐extractable Fe(II) minerals, and adsorbed arsenic, but considerably lower contents of Mn oxides and Mn(II) minerals than the shallow sandy sediments. Siderite, which is not as efficient as pyrite in sequestering arsenic, was the dominant Fe(II) mineral in the deep aquifer. In the deep aquifer, Mn oxide reduction was almost completed and Fe(III) oxide reduction was predominated, which resulted in high concentrations of groundwater arsenic due to high arsenic/iron molar ratio in the Fe(III) oxides. This study proposed that geogenic high‐arsenic groundwater can occur in sand‐gravel aquifers being high in Fe(III) oxide‐bound arsenic but poor in Mn oxides under reducing conditions.

Catastrophic event sequences across the Permian-Triassic boundary in the ocean and on land

This review forms the preface of a special issue dealing with Environmental stresses and biotic responses during the Palaeozoic-Mesozoic transition, and develops an improved understanding of the sequence of catastrophe events associated with the Permian-Triassic (P–Tr) transition in particular. The 16 papers that comprise this special issue mostly focus on the Paleo-Tethys and Panthalassia regions, and enable us to recognize seven distinct phases of biotic and environmental evolution that may be precisely correlated between marine and terrestrial environments. Phase 1 represents the normal marine environments and biota that existed at 252.104 Ma, prior to the onset of major environmental perturbation. Phase 2 is marked by the onset of a negative carbon (δ13Ccarb) isotopic excursion, and coincides with a significant decline in radiolarian diversity, the collapse of communities in shallow metazoan reefs and deep-water settings, and the loss of rainforests on land (including the disappearance of Gigantopteris flora). Phase 3 is characterized by a pulse of biodiversification on the eve of the end-Permian extinction (EPE). Phase 4 comprises the EPE event itself, dated to 251.941 Ma, and coincides with an extreme episode of widespread anoxia, biodiversity loss, community structural collapse, and volatile volcanic eruptions. Phase 5 is marked by the renewed onset of oxic conditions in all oceanic environments except those in microbialite-rich shallow platform settings where abundant pyrite framboids signal dysoxia. Phase 6 includes the earliest Triassic extinction (ETE) event, dated to 251.880 Ma, and associated with euxinic to anoxic conditions and ocean acidification. An extended phase of deoxygenation and acidification returned to the marine settings during Phase 7, possibly driving a calcium crisis. The terrestrial extinction event (Phase 2) probably commenced c. 60 kyrs (in South China) or 370 kyrs (in Sydney basin) before the marine P–Tr extinction, the latter consisting of two distinct events (Phases 4 and 6), spaced c. 60 kyr apart, each being marked by both a carbon isotope excursion and ecologic collapse. The environmental changes associated with these three extinction phases in the ocean and on land can be attributed to the effects of volcanic emissions (e.g., CO2, SO2, halogens and metals) during the eruption of the Siberian Traps Large Igneous Province and convergent margin volcanism related to Tethyan tectonics.