Sulfide Zone Research Articles

Supergene formation of sulfur-rich, tochilinite-bearing serpentinites in the Oman ophiolite

Mass transfer processes between fluids and ultramafic rocks produce subsurface environments encompassing a wide range of redox conditions. A notable locality where an extensive range of redox conditions is observed in one location is Hole BA1B, a ∼ 400 m borehole drilled by the Oman Drilling Project. A sulfur-enriched serpentinite zone, containing up to 0.6 wt% S, occurs between shallow oxidized serpentinites (<30 m) and deep partially serpentinized harzburgite (>150 m). All three alteration zones are predominantly composed of serpentine. However, microanalysis of samples from the sulfur-enriched zone shows that mesh textures after olivine are composed of serpentine, brucite, and tochilinite mixtures, yielding optically black thin-section samples that characterize this sulfidic zone. It is proposed that sulfur accumulates in this zone via a process similar to those found in supergene ore deposits. Reaction-path models show that at shallow conditions open to atmospheric input, sulfur is mobilized via oxidative weathering of serpentinized dunite and harzburgite. Sulfate-bearing fluids percolate deeper and react with host rocks in a system closed to atmospheric input. As fluids become more reduced, dissolved sulfate is precipitated as sulfide minerals yielding rocks with ∼0.4 wt% S, like those observed in Hole BA1B. Despite enrichment of S in the sulfidic zone in Hole BA1B, Ni and Co contents are uniform throughout all three layers in the borehole. This is consistent with model results which show that Ni (and, by analogy, Co) is less mobile than S, and can be hosted in serpentine and NiFe alloys in addition to sulfides. The sulfur enrichment process may occur abiotically. However, sulfide enrichment via microbial reduction of sulfate and other sulfur species can also facilitate the formation of the sulfidic zone. Bioenergetic calculations show that abundant energy is available for sulfur reducing microbes, consistent with previous work demonstrating the presence of active, sulfate-reducing microorganisms in Hole BA1B and other nearby boreholes. This suggests that the observed sulfur enrichment is an ongoing process. Overall, this work shows that variable redox conditions are attained as fluids percolate and react with serpentinized ultramafic rocks at variable extents of interaction between aquifer fluids, host ultramafic rocks, and the atmosphere.

Survival strategies of aerobic methanotrophs under hypoxia in methanogenic lake sediments

BackgroundMicrobial methane oxidation, methanotrophy, plays a crucial role in mitigating the release of the potent greenhouse gas methane from aquatic systems. While aerobic methanotrophy is a well-established process in oxygen-rich environments, emerging evidence suggests their activity in hypoxic conditions. However, the adaptability of these methanotrophs to such environments has remained poorly understood. Here, we explored the genetic adaptability of aerobic methanotrophs to hypoxia in the methanogenic sediments of Lake Kinneret (LK). These LK methanogenic sediments, situated below the oxidic and sulfidic zones, were previously characterized by methane oxidation coupled with iron reduction via the involvement of aerobic methanotrophs.ResultsIn order to explore the adaptation of the methanotrophs to hypoxia, we conducted two experiments using LK sediments as inoculum: (i) an aerobic "classical" methanotrophic enrichment with ambient air employing DNA stable isotope probing (DNA-SIP) and (ii) hypoxic methanotrophic enrichment with repeated spiking of 1% oxygen. Analysis of 16S rRNA gene amplicons revealed the enrichment of Methylococcales methanotrophs, being up to a third of the enriched community. Methylobacter, Methylogaea, and Methylomonas were prominent in the aerobic experiment, while hypoxic conditions enriched primarily Methylomonas. Using metagenomics sequencing of DNA extracted from these experiments, we curated five Methylococcales metagenome-assembled genomes (MAGs) and evaluated the genetic basis for their survival in hypoxic environments. A comparative analysis with an additional 62 Methylococcales genomes from various environments highlighted several core genetic adaptations to hypoxia found in most examined Methylococcales genomes, including high-affinity cytochrome oxidases, oxygen-binding proteins, fermentation-based methane oxidation, motility, and glycogen use. We also found that some Methylococcales, including LK Methylococcales, may denitrify, while metals and humic substances may also serve as electron acceptors alternative to oxygen. Outer membrane multi-heme cytochromes and riboflavin were identified as potential mediators for the utilization of metals and humic material. These diverse mechanisms suggest the ability of methanotrophs to thrive in ecological niches previously thought inhospitable for their growth.ConclusionsOur study sheds light on the ability of enriched Methylococcales methanotrophs from methanogenic LK sediments to survive under hypoxia. Genomic analysis revealed a spectrum of genetic capabilities, potentially enabling these methanotrophs to function. The identified mechanisms, such as those enabling the use of alternative electron acceptors, expand our understanding of methanotroph resilience in diverse ecological settings. These findings contribute to the broader knowledge of microbial methane oxidation and have implications for understanding and potential contribution methanotrophs may have in mitigating methane emissions in various environmental conditions.

Influence of oxidation–reduction potential and pH on polysulfide concentrations and chain lengths in the biological desulfurization process under haloalkaline conditions

Biological desulfurization under haloalkaline conditions has been applied worldwide to remove hydrogen sulfide (H2S) from sour gas steams. The process relies on sulfide-oxidizing bacteria (SOB) to oxidize H2S to elemental sulfur (S8), which can then be recovered and reused. Recently, a dual-reactor biological desulfurization system was implemented where an anaerobic (sulfidic) bioreactor was incorporated as an addition to a micro-oxic bioreactor, allowing for higher S8 selectivity by limiting by-product formation. The highly sulfidic bioreactor environment enabled the SOB to remove (poly)sulfides (Sx2−) in the absence of oxygen, with Sx2− speculated as a main substrate in the removal pathway, thus making it vital to understand its role in the process. The SOB are influenced by the oxidation–reduction potential (ORP) set-point of the micro-oxic bioreactor as it is used to control the product of oxidation (S8 vs. SO42−), while the uptake of Sx2− by SOB has been qualitatively linked to pH. Therefore, to quantify these effects, this work determined the concentration and speciation of Sx2− in the biological desulfurization process under various pH values and ORP set-points. The total Sx2− concentrations in the sulfidic zone increased at elevated pH (8.9) compared to low pH (< 8.0), with on average 3.3 ± 1.0 mM-S more Sx2−. Chain lengths varied, with S72− only doubling in concentration while S52− increased 9 fold, which is in contrast with observations from abiotic systems. Changes to the ORP set-point of the micro-oxic reactor did not produce substantial changes in Sx2− concentration in the sulfidic zone. This illustrates that the reduction degree of the SOB in the micro-oxic bioreactor does not enhance their ability to interact with Sx2− in the sulfidic bioreactor. This increased understanding of how both pH and ORP affect changes in Sx2− concentration and chain length can lead to improved efficiency and design of the dual-reactor biological desulfurization process.

Barite-bearing “septarian concretions” (upper Cretaceous – Paleocene) from the Biban area (north Algeria)

The "septarian barite concretions" have garnered significant interest worldwide. This study presents the first detailed investigation of these sedimentary structures in Algeria, with a primary focus on elucidating their genesis through petrography, SEM-EDS, major and trace elements (including REEs), and sulfur isotope analysis. The sampled concretions occur in Upper Cretaceous-Paleocene marl layers from the Biban area, primarily in two main sectors (Koudiat Djebassa and Sidi Ziane), situated within the Southern Tellian Atlas in Northern Algeria (Maghrebides). While these barite concretions exhibit diverse morphologies, our analysis focuses specifically on oblate ellipsoidal concretions. The flattened shape of these concretions may stem from compressed echinoid tests or small, oblate septarian cracked concretions representing the flattened cores. Notably, these concretions feature an outer cortex of radial fibrous barite crystals, measuring centimeters in thickness. Petrographic examination coupled with SEM-EDS analysis suggests that concretion growth occurred from porewaters in unconsolidated, soft sediments at shallow burial depths beneath the sediment-water interface, manifesting in three growth episodes: (i) an initial stage characterized by primarily thin lamellar barite crystals intertwining to form rosette-like structures in concretion cores, (ii) a later stage marked by radial-fibrous barite crystal growth in concretion margins, and (iii) the filling of septarian cracks by barite. The presence of terrigenous sediment, particularly abundant in rosette-like barite crystals, indicates growth within highly porous sediment. Conversely, the radial fibrous outer cortex is related to an effective displacive growth mode. The δ34S values of the barite concretions (ranging from 16 ‰ to 24.4 ‰), consistent with seawater values, which document that seawater was the primary sulfur source. Furthermore, REE geochemistry further supports the seawater origin of diagenetic fluids rather than a hydrothermal one. Concretions are suggested to have formed in organic-rich, fine-grained sediments at shallow depth below the sea floor, in sedimentation halts, and as a result of microbial degradation of organic matter. Barium was supplied to porewaters within the methanogenic zone, precipitating as barite when, migrating upwards, reached the boundary with the overlying sulfidic zone.

An integrated geophysical approach for gold from Wayand‐Nilambur granulite terrain, Kerala (India)

The Wayand‐Nilambur granulite terrain in Kerala (India) is well‐known for vein‐type gold mineralization within the quartz veins. Based on the previous geological, petro‐physical and geophysical characteristics of Wayand‐Nilambur gold deposit, the integrated geophysical survey was carried out to delineate the favourable prospecting zones. In the present study, detailed ground geophysical surveys, that is, magnetic, electrical resistivity/chargeability surveys were carried out in an area of 2 km2 in Kattikallu and Kalkulam blocks to delineate the ore deposits in terms of depths and extensions through the structural shear zone, locate the anomalous sources and the geometry of the Au‐rich sulphide zone and its depth continuity in the subsurface. These surveys brought out prominent resistivity and chargeability zones over known magnetite‐quartz veins that are associated with low‐grade sulphide bands at the central part of the Kattikallu block. The prominent potential zone is characterized by strong bipolar magnetic anomalies over the quartz veins. Based on the 2D inversion of resistivity data, the resistivity low zone of order 80–600 Ω m and chargeability of 21–25 mV/V are observed at a depth range of 5–20 m. In Kalkulam block, the magnetic survey has also brought out high intensity anomaly zones over quartz veins, the same quartz veins are mapped by high chargeability of 10–35 mV/V and low resistivity of 185–400 Ω m. The dipole‐dipole configuration produces two parameters, that is, resistivity and chargeability, these methods distinguish the anomalies along the two selected profiles in the study area. An attempt was made for combining the resistivity and chargeability values to identify the anomalous zone boundaries. The results of inversion indicated that the conductive bodies are located at the subsurface, with depths ranging from 5 to 25 m. Based on this integrated geophysical study, we suggested two borehole sites for further geo‐scientific studies in view of mineralization.

Trace and minor elements in the ore minerals: An assessment of geochemical distribution in overprinting epithermal on porphyry mineralization at Chodarchay, NW Iran

The Chodarchay overprinting epithermal on porphyry mineralization occurs in the Tarom-Hashtjin subzone of the Alborz magmatic arc in northwestern Iran. Rock units include intrusions (phases I and II) and volcano-volcaniclastic rocks. The alteration types consist of potassic, phyllic, argillic, and propylitic. The porphyry type is mostly accompanied by potassic alteration. The Chodarchay Fault implies two extensional and compressional mechanisms. The porphyry type is associated with the extensional and the epithermal type with the compressional stage of the fault. Mineralization occurred mainly inside the Eocene volcanic and volcano-sedimentary rocks; some ores are associated with the Late Eocene quartz monzonite. The mineralization is divided into three zones from depth to surface: 1) Hypogene sulfide zone (deep); 2) Enriched supergene sulfide zone (near-surface); 3) Oxidized and leached zone (surface). The major sulfide minerals consist of chalcopyrite, pyrite, sphalerite, and galena. Based on the results of the new analyses, chalcopyrite hosts Ag as a solid-solution and contains Ag-rich Bi-sulfosalt micro-inclusions. Non-constant Cd/Zn ratio in the chalcopyrite can indicate varying physiochemical conditions during its crystallization. Low As contents in the pyrite caused relatively clean pyrite with very few elements. The Ni concentrations in the pyrite imply that it was sourced from mantle mafic–ultramafic rocks. Sphalerite is the only Au-host mineral among the sulfide minerals as a nano-mineral phase. Solid-solution Ag exists in the sphalerite. The sphalerite Fe/Zn ratios show that it was precipitated between 220 and 280 °C. Silver and Bi have the highest measured concentrations among all analyzed elements in the galena, mainly partitioning within the galena, but Hg and Cd are abundant in the sphalerite. Bornite is one of the major carriers of Ag and Bi. Silver and Bi exhibit considerable variations in the Cu-(Fe) sulfides. A new occurrence of sulfosalt minerals was identified at Chodarchay (N = 14). The discovered Bi-sulfosalts include cuprobismuthite group minerals (cuprobismuthite, hodrushite, and paderaite), aikinite-bismuthinite series (aikinite and friedrichite), emplectite, makovickyite, galenobismutite, gustavite, heyrovskyite, neyite, cupropavanite; and As-Sb-bearing Cu sulfosalts (watanabeite and colusite). This number of sulfosalts had not previously been reported from any of the Tarom or Alborz deposits or mines. The Au-Ag occurrence was reported from the neyite, emplectite, and gustavite. The Ag-rich minerals crystallize from fractionated fluids that cooled during rising. Silver-carrier sulfosalts may have been exsolution from pre-existing Ag-bearing chalcopyrite in the cooling system. Most of the Ag-bearing sulfosalt minerals formed at the margins and fractures of the chalcopyrite, indicating that the cooling started from these points. Thus, Ag at Chodarchay is hosted inside Cu-(Fe) sulfides, the significant part of them being chalcopyrite and Ag-rich sulfosalt micro-inclusions inside the chalcopyrite. The fluid temperature for the Ag and chalcopyrite precipitation is from 200 to 350 °C. Bismuth assemblages precipitated at > 350 °C. The cuprobismuthite group crystallizes at temperatures > 300 °C. The supergene process and rare secondary enrichment of Ag and Au took place in the oxidation zone; this enrichment is dependent on the presence of manganese. Based on the fire-assay results, the concentration of gold is greater at shallower depths.

Assessment of the danger of the release of deep hydrogen sulfide from the Black Sea to the surface

The relevance of this study is due to the fact that the Black Sea is the largest body of water with a hydrogen sulfide zone. The proven reserves of hydrogen sulfide in the water column of the Black Sea amount to five billion tons. In terms of volume, this corresponds to 3.5 trillion cubic meters. At the same time, a mixture of hydrogen sulfide with air is explosive starting with a concentration of hydrogen sulfide in the air of 50 g/ m3. In addition, hydrogen sulfide is toxic, since it is a nerve gas, deadly already at 1 g/ m3. It is of interest to assess the possibility of the release of hydrogen sulfide waters of the sea to the surface with their subsequent degassing, as well as the consequences of increasing the concentration of hydrogen sulfide in surface waters and in the air for coastal areas. The distribution of hydrogen sulfide in the Black Sea is well studied. Especially a lot of measurement data was obtained for the top layer at a depth of 1000 m. Data on the concentration of hydrogen sulfide at depths from 1000 m to 2000 m is significantly less for technical reasons. The maximum concentration of hydrogen sulfide in the sea is reached at depths above 1500 m. A numerical assessment of the consequences of the release of deep waters to the surface has been obtained. It is shown that the concentration of hydrogen sulfide in the air will not exceed 1 g/m3, which is almost two orders of magnitude less than the explosive concentration. The balance estimates of oxygen and hydrogen sulfide fluxes in the Black Sea are considered in connection with forecasts of a rise in the boundary of the hydrogen sulfide zone. It is shown that the rise of the boundary is a consequence of the imbalance of these flows, however, the amount of net hydrogen sulfide production cannot be estimated accurately enough. With an accuracy of estimates of the fluxes themselves of 20-30%, the resulting increase in the amount of hydrogen sulfide per year is a statistically insignificant amount. The release of hydrogen sulfide to the surface in the foreseeable future is possible if the stratification of waters is disrupted by mechanical intervention.

Nature and Origin of a Massive Sulfide Occurrence in the Karrat Group: Evidence for Paleoproterozoic VMS Mineralization in Central West Greenland

Abstract Mafic volcanic rocks of the Kangilleq Formation of the Paleoproterozoic Karrat Group host volcanogenic massive sulfide (VMS) mineralization in the area of central Kangiusap Kuua, central West Greenland. The mafic volcanic rocks display evidence of subaqueous, effusive eruption and redeposition by mass debris flows generated along fault scarps on the sea floor. A zone of semiconformable quartz alteration and disconformable chlorite alteration within hydrothermal breccias and mafic tuff breccias near the top of the volcanic sequence is interpreted to reflect a synvolcanic hydrothermal system. Conformable, massive to semimassive, and discordant, stringer-style sulfide mineralization is hosted within the quartz- and chlorite-altered volcanic rocks. The massive to semimassive sulfide mineralization is ~10 m thick and crops out along strike for ~2,000 m. The stringer zone is ≤10 m thick with individual sulfide stringers ranging in width from 5 to 90 cm. All sulfide zones are dominated by coarse pyrrhotite and pyrite, with trace amounts of sphalerite and chalcopyrite. The pillow lavas are subalkaline with geochemical characteristics typical of modern transitional to tholeiitic mid-ocean ridge or back-arc basin basalt. Trace element and Nd isotope data suggest that these lavas erupted in an epicratonic, back-arc basin. Characteristics of the host rocks indicate a period of localized rifting, volcanism, and VMS formation during genesis of the Karrat Group, which is dominated by siliciclastic rocks.

Mineralogy and mineral chemistry of the ABM replacement-style volcanogenic massive sulfide deposit, Finlayson Lake district, Yukon, Canada

The ABM deposit is a bimodal-felsic, replacement-style volcanogenic massive sulfide deposit (VMS) that is hosted by back-arc affinity rocks of the Yukon–Tanana terrane in the Finlayson Lake VMS district, Yukon, Canada. Massive sulfide zones occur as stacked and stratabound lenses subparallel to the volcanic stratigraphy, surrounded by pervasive white mica and/or chlorite alteration. Remnant clasts of volcanic rocks and preserved bedding occur locally within the massive sulfide lenses and indicate that mineralization formed through subseafloor replacement of pre-existing strata. Three mineral assemblages occur at the ABM deposit: (1) a pyrite–chalcopyrite–magnetite–pyrrhotite assemblage that is associated with Cu–Bi–Se–Co-enrichment and occurs at the center of the massive sulfide lenses; (2) a pyrite–sphalerite assemblage, which occurs more commonly towards lens margins and is enriched in Zn–Pb–Ag–Au–Hg–As–Sb–Ba; and (3) a minor assemblage comprising chalcopyrite–pyrrhotite–pyrite stringers associated with pervasive chlorite alteration, which occurs mostly at the sulfide lens margins. Petrographic observations of preserved primary, zone refining, and metamorphic textures in combination with in situ geochemistry show that the pyrite–sphalerite assemblage formed at lower temperatures (< 270 °C) than the other two mineral assemblages (~ 270–350 °C), and that mineral chemistry in all mineral assemblages was affected by greenschist facies metamorphism, although the effects are limited to recrystallization, small-scale remobilization (< 1 m) and trace element redistribution.

The trace-element compositions of amphibole, magnetite and ilmenite as potential exploration guides to metamorphosed Proterozoic Cu–Zn±Pb±Au±Ag volcanogenic massive sulfide deposits in Colorado, USA

AbstractOrthoamphibole, clinoamphibole and magnetite are common minerals in altered rocks associated spatially with Palaeoproterozoic volcanogenic massive sulfide (VMS) deposits in Colorado, USA and metamorphosed to the amphibolite facies. These altered rocks are dominated by the assemblage orthoamphibole (anthophyllite/gedrite)–cordierite–magnetite±gahnite±sulfides. Magnetite also occurs in granitoids, banded iron formations, quartz garnetite, and in metallic mineralisation consisting of semi-massive pyrite, pyrrhotite, chalcopyrite, and sphalerite with subordinate galena, gahnite and magnetite; amphibole also occurs in amphibolite. The precursor to the anthophyllite/gedrite–cordierite assemblages was probably the assemblage quartz–chlorite formed from hydrothermal ore-bearing fluids (~250° to 400°C) associated with the formation of metallic minerals in the massive sulfide deposits.Element–element variation diagrams for amphibole, magnetite and ilmenite based on LA-ICP-MS data and Principal Component Analysis (PCA) for orthoamphiboles and magnetite show a broad range of compositions which are primarily dependent upon the nature of the host rock associated spatially with the deposits. Although discrimination plots of Al/(Zn+Ca) vs Cu/(Si+Ca) and Sn/Ga vs Al/Co for magnetite do not indicate a VMS origin, the concentration of Al+Mn together with Ti+V and Sn vs Ti support a hydrothermal rather than a magmatic origin for magnetite. Principal Component Analyses also show that magnetite and orthoamphibole in metamorphosed altered rocks and sulfide zones have distinctive eigenvalues that allow them to be used as prospective pathfinders for VMS deposits in Colorado. This, in conjunction with the contents of Zn and Al in magnetite, Zn and Pb in amphibole, ilmenite and magnetite, the Cu content of orthoamphibole and ilmenite, and possibly the Ga and Sn concentrations of magnetite constitute effective exploration vectors.

PLATINUM-PALLADIUM OCCURRENCE VASILINOVSKOE: A NEW TYPE OF NOBLE-METAL MINERALIZATION IN THE URALS

For the first time, a new noble-metal (Pt–Au–Pd) Vasilinovskoe ore occurrence discovered near the village of Kharp in the Yamalo-Nenets Autonomous Okrug is described. It is associated with amphibolized gabbroids and subordinate pyroxenites of the Kershor complex, dated mainly to the Late Ordovician. In these rocks, mineralization zones with an apparent thickness from 0.5 to 50 m (sulfides 3–5 vol.%, occasionally more) are developed. In areas with scattered or finely nested sulfide inclusions, feldspar-quartz, epidote and other veins are often present. According to assay data, in substantially sulfide 0.5–1 kg samples of these, in general, low–sulfide zones, the Pd content reaches 1.4 g/t, Au – 0.8 g/t, and Pt – 0.2 g/t. PGE minerals are represented by abundant secretions of micron–sized palladium tellurides – merenskite PdTe2, temagamite Pd3HgTe3, kotulskite PdTe, as well as other noble metal compounds – sadberite PdSb, arsenopalladinite Pd8(As,Sb)3 and others. In addition to these palladium minerals, the magnetite–chalcopyrite–pyrite association contains microinclusions of native silver, native bismuth and native tin. In the later polysulfide–feldspar–carbonate–quartz association, Au and Ag tellurides, native gold (including Hg-bearing), Se-containing argentite, greenockite are found. The formation of parageneses of precious metals is associated with late magmatic processes, as well as with redistribution by subsequent magmatogenic hydrothermal fluids, up to a temperature of ~250°C; pressure decreased from ~0.9–1.3 to ~0.4–0.5 kbar.

Shallow base metal exploration in northern New Brunswick, Canada

This research paper explores the effective utilization of 2-D Resistivity and Induced Polarization (IP) techniques for identifying sulfide deposits in complex geological settings. To enhance the accuracy of the findings, the geophysical data were complemented with geological information, petrophysical data, Digital Elevation Model (DEM), and Electromagnetic data Versatile Time Domain Electro-Magnetics (VTEM) and Total Magnetic Intensity (TMI) Reduced-To-Pole (RTP).The analysis of DEM, magnetic susceptibility, and magnetic intensity revealed a lack of correlation with the DEM, indicating that these parameters alone were insufficient to confirm the presence of sulfide deposits. Consequently, five regions exhibiting high to moderate conductivity were chosen for conducting six survey lines using the Wenner array with a 10 m electrode spacing. The objective was to map sulfide zones in Restigouche County, located in northeast New Brunswick.Geophysical measurements were acquired using a RESECS instrument, and the field data were processed using the Res2Dinv software after removing erroneous data points. Since the study area was characterized by thick overburdened glacial sediments and lacked surface exposure, information from numerous boreholes and core samples from the western part of the study area were utilized.The study achieved successful mapping of the shallow alteration zone containing pyrite mineralization in most of the survey lines. Specifically, resistivity responses below 50 Ω⋅m, coupled with high chargeability, likely corresponded to the alteration zone containing pyrite. Additionally, resistivity responses above 50 Ω⋅m, combined with chargeability and RTP readings, were indicative of the alteration zone with pyrite.

Redox gradient shapes the abundance and diversity of mercury-methylating microorganisms along the water column of the Black Sea.

In the global context of seawater deoxygenation triggered by climate change and anthropogenic activities, changes in redox gradients impacting biogeochemical transformations of pollutants, such as mercury, become more likely. Being the largest anoxic basin worldwide, with high concentrations of the potent neurotoxic methylmercury (MeHg), the Black Sea is an ideal natural laboratory to provide new insights about the link between dissolved oxygen concentration and hgcAB gene-carrying (hgc+) microorganisms involved in the formation of MeHg. We combined geochemical and microbial approaches to assess the effect of vertical redox gradients on abundance, diversity, and metabolic potential of hgc+ microorganisms in the Black Sea water column. The abundance of hgcA genes [congruently estimated by quantitative PCR (qPCR) and metagenomics] correlated with MeHg concentration, both maximal in the upper part of the anoxic water. Besides the predominant Desulfobacterales, hgc+ microorganisms belonged to a unique assemblage of diverse-previously underappreciated-anaerobic fermenters from Anaerolineales, Phycisphaerae (characteristic of the anoxic and sulfidic zone), Kiritimatiellales, and Bacteroidales (characteristic of the suboxic zone). The metabolic versatility of Desulfobacterota differed from strict sulfate reduction in the anoxic water to reduction of various electron acceptors in the suboxic water. Linking microbial activity and contaminant concentration in environmental studies is rare due to the complexity of biological pathways. In this study, we disentangle the role of oxygen in shaping the distribution of Hg-methylating microorganisms consistently with MeHg concentration, and we highlight their taxonomic and metabolic niche partitioning across redox gradients, improving the prediction of the response of marine communities to the expansion of oxygen-deficient zones. IMPORTANCE Methylmercury (MeHg) is a neurotoxin detected at high concentrations in certain marine ecosystems, posing a threat to human health. MeHg production is mainly mediated by hgcAB gene-carrying (hgc+) microorganisms. Oxygen is one of the main factors controlling Hg methylation; however, its effect on the diversity and ecology of hgc+ microorganisms remains unknown. Under the current context of seawater deoxygenation, mercury cycling is expected to be disturbed. Here, we show the strong effect of oxygen gradients on the distribution of potential Hg methylators. In addition, we show for the first time the significant contribution of a unique assemblage of potential fermenters from Anaerolineales, Phycisphaerae, and Kiritimatiellales to Hg methylation, stratified in different redox niches along the Black Sea gradient. Our results considerably expand the known taxonomic diversity and ecological niches prone to the formation of MeHg and contribute to better apprehend the consequences of oxygen depletion in seawater.

Different integrated mechanisms drove the two pulses of the Late Ordovician mass extinction

Across the Ordovician–Silurian transition, the first large-scale extinction event in geological history, the Late Ordovician mass extinction (LOME), occurred. The LOME was a double-pulse event, and the two extinction pulses occurred in the late Katian – early Hirnantian (LOME-1) and middle Hirnantian (LOME-2). At present, the comprehensive driving mechanism of the two extinction pulses is still controversial. In this study, two cores (JY87 and JY143) from the middle Yangtze shelf in South China were selected as the research objects. Combined with previous research results, four main stressors, namely, volcanism, ocean redox, climate and nutrient supply, were analyzed. On this basis, the two-pulse driving mechanism of the LOME was comprehensively studied. The comprehensive mechanisms driving the two extinction pulses were different. During the LOME-1, the intensity of volcanism was relatively weak. Volcanic silicate weathering, high nutrients and long-term accumulation of high organic carbon burial flux may have jointly caused cold conditions and led to the expansion of the anoxic sulfide zone and enhancement of sulfidation. The cold conditions had less effect on deep-water benthos, but enhanced euxinia led to their extinction. In contrast, shallow-water benthos were less affected by the enhanced euxinia, but the cold conditions led to their extinction. During the LOME-2, a sudden and brief increase in volcanism that was extremely intense occurred, bringing in abundant nutrients and SO2, resulting in more widespread and long-lasting euxinia. This occurrence led to the extinction of benthos that survived during the LOME-1.

Geophysical mapping of the occurrence of platinum group elements in the main sulphide zone of the Great Dyke in Zimbabwe

This work sought to establish the geophysical signature of platinum group elements in a mineralized sulphide host rock. It has been established that complex resistivity of rocks depends on frequency. The variation of resistivity magnitude and phase for the samples were taken for frequencies in the range 10−2 to 102 Hz. In the first part, the mathematical relationship between resistivity amplitude and phase with frequency was obtained based on the Cole-Cole model equation. A MATLAB Code based on the direct inversion of the apparent resistivity spectrum was used to extract the “Cole-Cole” parameters. In the second part an experimental procedure to obtain the data was designed and a discussion of the results obtained was made. Overally, the results confirm the model to be a good candidate for use in mapping the occurrence of platinum group elements in sulphide zones. Its frequency dependency parameter c is the critical anomaly detection parameter. Several efforts to solve this problem by other geophysical methods failed to yield the desired or useable results. The reason was the complex and inverse nature of the problem. The practical method chosen from a host of geophysical methods was complex resistivity measurements based on the “Cole-Cole model”. The knowledge gap being bridged in this study involves the use of this method to detect the base of the main sulphide zone.

Application of 2-D Electrical Resistivity Imaging, and Induced Polarization Methods for Delineating Gold Mineralisation at Felda Chiku 3, Kelantan, Malaysia

A geophysical survey has been carried out to assess the distribution of gold minerals at the Felda Chiku 3, Gua Musang district, Kelantan, Peninsular Malaysia. A 2-D geo-electrical resistivity imaging (ERI), combined with Induced Polarization (IP) method, and Oasis Montaj modelling were applied to delineate the potential conductive zones associated with sulphide mineralization. Most Peninsular Malaysia’s gold deposits occur in hydrothermal sulphides as discrete ore formations within the host rocks. A good correlation between the ERI and the IP profiles was observed during the interpretation of the model that successfully identifying the low resistive, and high chargeable sulphide zones, which correspond to the gold mineralization zones. The correlations could be linked to the conductive features at depth ranging from about 25 m - 135 m, trending along N-S directions. The study suggests further geo-electrical investigation to be carried out towards the south-western part of the area as more potential mineralized zones with N-S trends could be found in this region. Further studies would be able to give the extent of the gold deposits in Kelantan and by extension, allow for better informed mineral exploration and drilling operations to mine the gold in the region.

АНАЭРОКОНТУР ЧЕРНОГО МОРЯ

The work is devoted to the issue of the allocation of a new contour biotope in the Black Sea. Currently, in hydrobiology, it is customary to distinguish the following contour biotopes or contact zones of the sea: aerocontour, psammocontour, lithocontour, pelocontour, potamocontour (Zaitsev, 2012, 2015), which describe the ecological structure of any reservoir except for the Black Sea. This is due to the fact that most of the deep-water area of this water basin does not have a peloconture with higher forms of life due to hydrogen sulfide in its depths. This biotope exists is only in coastal shelf areas up to depths of 150–200m, where there is still oxygen. Until the end of the last century, it was believed that the Black Sea had only oxygen and hydrogen sulfide zones, with a mixing layer (or C-layer) between them. However, at the beginning of this century, a revolutionary discovery was made in hydrochemistry (Konovalov, Murray, 2001; Konovalov et al., 2005): it turned out that the C-layer does not exist and that the oxygen and hydrogen sulfide layers of water are separated by a special water mass where there is neither oxygen nor hydrogen sulfide. It turned out that this water mass (called the Suboxic layer) has a thickness of about 30-50 m, characteristic physicochemical properties and a certain history of origin (Konovalov et al., 2005). Like any water mass in the sea, it is a biotope that in hydrobiology does not yet have a name, is poorly described and its role in the ecosystem is insufficiently studied. However, there is already evidence that it plays an extremely important role in the ecosystem of the Black Sea because clusters of specific hydrobionts form on its borders and intensive chemosynthesis occurs in it, comparable in production with the photic zone (Polikarpov et al., 1990). This biotope is an obvious contour biotope, the opposite of the aeroconture, which is in contact with the oxygen environment. It is located on the border with anoxic layer and therefore the author called it the "anaerocontour" of the Black Sea. Based on the generalization of existing data and the results of his own research, the author of this work for the first time provides a generalized description of the structural and functional characteristics of this contour biotope and describes its role in the functioning of the ecosystem of the Black Sea. The results of this work are of fundamental interest for understanding biological processes in extreme conditions and their impact on long-period changes in the ecosystem of the Black Sea.

Mineralogical and textural evolution of the Alvo 118 copper-bearing gossan: Implications for supergene metallogenesis in Carajás Mineral Province, Brazil

The Carajás Mineral Province is home to one of the most extensive copper-gold belts globally, where hypogene orebodies were partially transformed into gossans. The Carajás Mountains host mature gossan profiles modified by lateritization. Conversely, the surrounding denuded landscape holds deep immature gossans characterized by generation mechanisms that remain poorly understood. Alvo 118 is a typical deposit from this area, with numerous drill cores available for evaluation. Mineralogical, textural, and mineral chemistry investigations showed that the hypogene mineralization, consisting of massive and disseminated chalcopyrite with minor nukundamite, was partially oxidized. The resulting gossan comprises a range of copper-bearing minerals, including malachite, pseudomalachite, cuprite, tenorite, native copper, ramsbeckite, chrysocolla, and libethenite, with relics of a secondary sulfide zone represented by chalcocite. The host rocks are relatively well-preserved from supergene alteration near the gossan due to the chalcopyrite's low acidifying potential and the deep occurrence of the gossan orebody, resulting in an immature profile. The gossan orebody is sectioned into four zones; the mineral succession of each zone indicates independent evolution, with a general transition from slightly acidic to alkaline conditions and increasing oxidation potential, with local oscillations related to water table fluctuations. This environment was established by the interaction of acid solutions derived from sulfide dissolution with gangue minerals (calcite and apatite) and secondarily with host rocks (chloritites and granodiorites), leading to system buffering. Ti, Mn, Zn, Ni, V, and Co were incorporated into the gossan mainly by goethite, while malachite contains Ba, Ca, Cr, Mn, and Zn. Gold and Pb were detected in goethite, cuprite, tenorite, and native copper. This set of mineralogical zones and mineral successions record an important and unique stage of the supergene evolution of the Carajás Mineral Province, with prominent copper-gold mineralization showing no effects from lateritization.

Increasing Wear Resistance of Low-Carbon Steel by Anodic Plasma Electrolytic Sulfiding

The paper considers the problem of increasing the wear resistance of steel products. For the first time, the technology of anodic plasma electrolytic sulfiding is proposed to increase the wear resistance of low-carbon steel. The composition, structure, and frictional properties of modified surface layers after sulfiding have been studied. The type and mechanism of wear are determined. The influence of the sliding speed of the sample over the counter body on the friction and wear resistance of the samples after processing is analyzed. The possibility of saturation of low-carbon steel with sulfur in an electrolyte with sulfur compounds is shown. The iron sulfide FeS in the surface layer is found. It has been established that the thickness of the sulfide zone and the relative amount of FeS in it have a positive effect on reducing the coefficient of friction and mass wear. The greatest decrease in the friction coefficient by 5.5 times and weight wear by 64 times occurs after sulfiding at 500 °C for 10 min. It was found that the mechanism of wear of sulfided samples is fatigue wear during dry friction and plastic contact.

Mercury, organic matter, iron, and sulfur co-cycling in a ferruginous meromictic lake

Mercury (Hg) speciation in natural waters is controlled by redox conditions and microbiological activity. Water columns of meromictic lakes have large and stable redox chemical and biological gradients that allow the investigation of many Hg chemical transformations. In this study, Hg speciation (elemental Hg = Hg0, methylated Hg = MeHg) and partitioning between truly dissolved (<3 kDa), colloidal (<0.45 μm and >3 kDa), and particulate (>0.45 μm) fractions, were determined throughout a high-resolution water column profile in the ferruginous meromictic Lake Pavin (Massif Central, France) in July 2018. Total Hg concentrations (THg) in water ranged between 0.4 and 8.8 pmol L−1. The particulate phase represented 10–70% of the THg, with a peak found in the mesolimnion associated with the particulate organic carbon maximum. In the mesolimnion, the colloidal fraction represented 12–68% of THg, and the highest value was found at the top of the sulfidic zone, whereas the truly dissolved Hg species (70 ± 9%) dominated in all the rest of the sulfidic zone. MeHg ranged from less than 10% of THg in the oxic mixolimnion to more than 90% in the anoxic monimolimnion. The Hg methylation was most active within the suboxic zone where iron and sulfate reduction are occurring. These results, associated with those of the partition of organic matter (OM), sulfur, and iron, in conjunction with thermodynamic calculations, allow us to present a conceptual scheme for the Hg cycle in the lake. Atmospheric Hg deposited onto surface waters of the lake is partially photo-reduced and returned to the air, another part is scavenged by biogenic particulate matter and conveyed at depth by settling organic material. Water stratification and redox changes create a sequence of reactions from oxic to ferruginous waters where Hg is successively (i) desorbed from particulate OM where mineralization occurs, (ii) adsorbed onto iron-oxy(hydr)oxides, (iii) desorbed where they dissolved, (iv) precipitate as HgS, (v) methylated, and (vi) reduced as Hg0 in the deepest part of the lake. In brief, the (micro)biological uptake, OM, iron and sulfur recycling, through associated microbial consortia, control the Hg cycling in the Pavin waters.