Geochemical Zones Research Articles

Isotopic and Geochemical Zonality of the Paleocene Magmatism of the Asia–Pacific Transition Zone

Isotopic and Geochemical Zonality of the Paleocene Magmatism of the Asia–Pacific Transition Zone

Uncovering the hidden world of riverbed sediments: The role of sediment heterogeneity and cross-bar channel fills in the hydrogeochemical dynamics of the hyporheic zone

Groundwater-surface water interaction (hyporheic exchange) is critical in numerous hydrogeochemical processes; however, hyporheic exchange is difficult to characterize due to the various spatial (e.g., sedimentary architecture) and temporal (e.g., stage fluctuations) variables that influence it. This interdisciplinary study brings forth novel insights by integrating various methodologies including geophysical surveys, physical and chemical sediment characterization, and water chemistry analysis to explore the interplay of the numerous facets governing hyporheic zone processes within a compound bar deposit. The findings reveal distinct sedimentary facies and geochemical zones within the compound bar, driven by the sedimentary architecture. Cross-bar channel fills are identified as critical structures influencing hydrogeochemical dynamics, acting as baffles to groundwater flow and modulating nutrient transformations. Geophysical imaging and hydrogeochemical analyses highlight the complex interplay between sediment characteristics and subsurface hydraulic connectivity, emphasizing the role of sediment heterogeneity in controlling hyporheic exchange and solute mixing. The study concludes that sediment heterogeneity, particularly the presence of cross-bar channel fills, plays a pivotal role in the hydrogeochemical dynamics of the hyporheic zone. These structures significantly influence hyporheic flow paths, solute residence times, and nutrient cycling, underscoring the necessity to consider the fine-scale sedimentary architecture in models of hyporheic exchange. The findings contribute to a deeper understanding of riverine ecosystem processes, offering insights that can inform management strategies for water quality and ecological integrity.

Tourmaline composition probes serpentinite-derived fluid mobility in subduction zones

Serpentinite dehydration in subduction zones plays a pivotal role in geochemical cycling on Earth. A number of geochemical studies on arc magmas have elucidated the contributions of serpentinite-derived fluids to mantle sources. However, due to complex geological overprints during subduction zone processes, discerning serpentinite signatures in exposed metamorphic rocks within fossil subduction zones remains challenging. In this study we address these difficulties through in-situ investigations of tourmaline, the geochemistry of which reflects the host environment as well as potential fluid-induced processes. The presence of zonations in tourmaline makes it an excellent recorder of consecutive geological events. Integrated major and trace elements along with in-situ boron isotopes of tourmaline from the high-pressure Sopron area (Hungary) in the Eastern Alps were used to unravel fluid action sourced from serpentinite. Despite the presence of color zoning, tourmaline in the orthogneiss (Tur-G) has low XMg [Mg/(Mg + Fe)] of ca. 0.3–0.6 and δ11B values of around −11 ‰, along with variable trace element compositions. Petrological observations and geochemical analyses suggest that the inner domains of Tur-G are of igneous origin, while the outer rims are likely affected by subsequent metamorphic events. Tourmaline in metasomatized kyanite-quartzite (Tur-K) veins exhibits distinct geochemical zoning, and preserves metamorphic cores and fluid-induced rims. The inner domains of Tur-K display low XMg (<0.6), relatively high trace element concentrations and δ11B values of less than −10 ‰, whereas the overgrowths exhibit extremely high XMg values (>0.99), low trace element concentrations and high δ11B values reaching up to +21 ‰, clearly indicating the incorporation of serpentinite-derived Mg-11B-rich fluids. Through comparison with other metamorphic and metasomatic tourmalines in (ultra)high-pressure rocks globally, we establish that tourmaline with high XMg > 0.85 and δ11B values >0 ‰ may serve as an effective proxy for detecting serpentinite-derived fluids in subduction zones.

Microbial communities and functions are structured by vertical geochemical zones in a northern peatland

Northern peatlands are important carbon pools; however, differences in the structure and function of microbiomes inhabiting contrasting geochemical zones within these peatlands have rarely been emphasized. Using 16S rRNA gene sequencing, metagenomic profiling, and detailed geochemical analyses, we investigated the taxonomic composition and genetic potential across various geochemical zones of a typical northern peatland profile in the Changbai Mountains region (Northeastern China). Specifically, we focused on elucidating the turnover of organic carbon, sulfur (S), nitrogen (N), and methane (CH4). Three geochemical zones were identified and characterized according to porewater and solid-phase analyses: the redox interface (<10 cm), shallow peat (10–100 cm), and deep peat (>100 cm). The redox interface and upper shallow peat demonstrated a high availability of labile carbon, which decreased toward deeper peat. In deep peat, anaerobic respiration and methanogenesis were likely constrained by thermodynamics, rather than solely driven by available carbon, as the acetate concentrations reached 90 μmol·L−1. Both the microbial community composition and metabolic potentials were significantly different (p < 0.05) among the redox interface, shallow peat, and deep peat. The redox interface demonstrated a close interaction between N, S, and CH4 cycling, mainly driven by Thermodesulfovibrionia, Bradyrhizobium, and Syntrophorhabdia metagenome-assembled genomes (MAGs). The archaeal Bathyarchaeia were indicated to play a significant role in the organic carbon, N, and S cycling in shallow peat. Although constrained by anaerobic respiration and methanogenesis, deep peat exhibited a higher metabolic potential for organic carbon degradation, primarily mediated by Acidobacteriota. In terms of CH4 turnover, subsurface peat (10–20 cm) was a CH4 production hotspot, with a net turnover rate of ∼2.9 nmol·cm−3·d−1, while the acetoclastic, hydrogenotrophic, and methylotrophic methanogenic pathways all potentially contributed to CH4 production. The results of this study improve our understanding of biogeochemical cycles and CH4 turnover along peatland profiles.

“Hidden” mineralogical-geochemical zonality revealed by the laser microanalysis of sulfides at the Kyzyk-Chadr deposit, Republic of Tyva

A comprehensive study of the Kyzyk-Chadr porphyry copper deposit included determination of the composition and spatial distribution of trace elements in ore-forming sulfide minerals over the ore- hosting rock volume. The study was carried out using the mass spectrometric analysis with laser ablation of the analyzed material. Steady qualitative and quantitative elements of a “hidden” mineralogical and geochemical zonality have been identified, that are expressed in variations in the concentration of trace elements and their ratios in pyrites and chalcopyrites over the volume of the ore-bearing porphyry copper stockwork. The identified regularities correlate with the established centrifugal vector and concentric pattern of the mineralogical ore-metasomatic zonality of the ore deposit.

Zoneamento geoquímico vertical do depósito de Pb-Zn (Cu-Ag) Santa Maria, região de Caçapava do Sul, Rio Grande do Sul

The Pb-Zn (Cu-Ag) Santa Maria deposit, part of the Camaquã Mines, south-center of the Rio Grande do Sul state, corresponds to one of the largest base metal deposits hosted by the Neoproterozoic volcano-sedimentary successions of the Camaquã Basin. Because of their economic importance, Camaquã Mines already were the target of a serie of studies, with the consequent proposition of diverse metalogenetic models, which point to an epithermal or magmatic-hydrothermal origin. Hydrothermal deposits have a common geochemical zoning pattern of indicator elements which can be used as an effective prospective criterion. Several geochemical hydrothermal halos are identified in this paper that can potentially serve as a useful exploration guide in the Camaquã Mines region. For that, the vertical geochemical zoning of the Santa Maria deposit was investigated through petrographic and/or geochemical study of 562 samples from his 3 ore bodies. The results obtained here are preliminary (they need to be tested in soil geochemical profiles over Santa Maria ore bodies), but indicate that the multiplicative ratios (Ag × Cd)/(Pb × Zn) and (Ag × Cd)/(Cu × Zn) and the elemental ratios Ag/Zn and Ag/Cu can be useful in the search for new Pb-Zn deposits in the Camaquã Mines.

Chemostratigraphic correlation of Devonian-Carboniferous sedimentary sequences at Roboré SUB-BASIN, Bolivia

A chemostratigraphic analysis was performed on cutting samples from Devonian-Carboniferous intervals belonging to three wells located in the Roboré sub-Basin, east of Chaco Plain, southeast Bolivia.Using multivariate statistical analysis (Factor Analysis), we have established two main geochemical associations serving as control for the chemistry of the analyzed samples: detrital (clay and heavy minerals) and redox-organic matter association.In addition, different geochemical zones (chemozones or chemofacies) were established by the application of a hierarchical statistical analysis (constrained cluster analysis), both for the detrital and redox-organic matter associations. Furthermore, total correspondence of the established chemozones was observed and better definition of the predefined stratigraphic contacts was obtained after a chemostratigraphic correlation scheme was applied. Taking together our chemostratigraphic data, we have identified an interval that represents a possible marker of Lower Carboniferous glacial event in the Roboré sub-Basin. We have evaluated the general characteristics of the sources of sediment provenance along the studied sequences. In conclusion, we proposed a large active continental margin accretionary complex tectonic setting for Los Monos (Late Devonian) and Tupambi (Mississippian) formations in the Roboré sub-Basin (Chaco Plain).

Comprehensive molecular-isotopic characterization of archaeal lipids in the Black Sea water column and underlying sediments.

The Black Sea is a permanently anoxic, marine basin serving as model system for the deposition of organic-rich sediments in a highly stratified ocean. In such systems, archaeal lipids are widely used as paleoceanographic and biogeochemical proxies; however, the diverse planktonic and benthic sources as well as their potentially distinct diagenetic fate may complicate their application. To track the flux of archaeal lipids and to constrain their sources and turnover, we quantitatively examined the distributions and stable carbon isotopic compositions (δ13 C) of intact polar lipids (IPLs) and core lipids (CLs) from the upper oxic water column into the underlying sediments, reaching deposits from the last glacial. The distribution of IPLs responded more sensitively to the geochemical zonation than the CLs, with the latter being governed by the deposition from the chemocline. The isotopic composition of archaeal lipids indicates CLs and IPLs in the deep anoxic water column have negligible influence on the sedimentary pool. Archaeol substitutes tetraether lipids as the most abundant IPL in the deep anoxic water column and the lacustrine methanic zone. Its elevated IPL/CL ratios and negative δ13 C values indicate active methane metabolism. Sedimentary CL- and IPL-crenarchaeol were exclusively derived from the water column, as indicated by non-variable δ13 C values that are identical to those in the chemocline and by the low BIT (branched isoprenoid tetraether index). By contrast, insitu production accounts on average for 22% of the sedimentary IPL-GDGT-0 (glycerol dibiphytanyl glycerol tetraether) based on isotopic mass balance using the fermentation product lactate as an endmember for the dissolved substrate pool. Despite the structural similarity, glycosidic crenarchaeol appears to be more recalcitrant in comparison to its non-cycloalkylated counterpart GDGT-0, as indicated by its consistently higher IPL/CL ratio in sediments. The higher TEX86 , CCaT, and GDGT-2/-3 values in glacial sediments could plausibly result from selective turnover of archaeal lipids and/or an archaeal ecology shift during the transition from the glacial lacustrine to the Holocene marine setting. Our in-depth molecular-isotopic examination of archaeal core and intact polar lipids provided new constraints on the sources and fate of archaeal lipids and their applicability in paleoceanographic and biogeochemical studies.

THE SPATIAL DISTRIBUTION AND MOVEMENT OF SOLUBLE SALTS FROM ROMANIAN SOILS

The objective of this paper is to present a sketch map of the accumulation of soluble salts in soils from Romania, as a support for improving saline soils or preventing undesirable anthropogenic salinization phenomena.The map highlights the spatial distribution and movement of easily soluble salts in soil at the scale of the entire country, It is an overview on origin, sources and pathways of migration and accumulation of easily soluble salts in interdependence with natural factors and conditions.Taking into account the movement pathways of easily soluble salts in soil, two geochemical zones were separated on the map: the eluviation - transit zone and the transit - accumulation zone. In the last zone, areas where the accumulation of salts becomes dominant have been delimited, their transit being insignificant. The movement directions of easily soluble salts in soil are also highlighted on the map.From a naturalistic point of view, the map highlights the close connection between areas with accumulation of salts and areas where geological, geomorphological and hydrogeological conditions lead to a poor drainage. It also highlights the connection and importance of salt sources from geological deposits or salt masses in the hilly area in the formation, expansion and chemistry of saline soils in the plain or floodplain areas located in the neighbouring lowlands.

Genetic model of tabular orebody in Nalinggou uranium deposit, Ordos Basin: Constraint from clay minerals studies

Significant advancements have been achieved in the field of Uranium (U) exploration within the Ordos Basin. As a result, a sequence of U deposits characterized by tabular orebodies has been successfully identified. Extensive research has been conducted by scholars on the morphological characteristics, occurrence location, and control factors of the tabular orebody. However, a comprehensive understanding of its genesis and specific formation process remains elusive. Samples of sandstone from various geochemical zones in the Nalinggou U deposit were gathered for the purpose of microscopic observation, electron probe analysis, scanning electron microscopy, clay X-ray diffraction, and U elements analysis. Coffinite often occurs on the surface of particles and in association with other adsorbents such as organic matter, clay minerals, and titanium oxide, indicating strongly that sequential adsorption-reduction could be involved in the formation of U minerals. The clay minerals found in ore-bearing layers exhibit distinct characteristics. Specifically, there is a noticeable boundary between the content of kaolinite in the paleo-oxidation zone and the ore zone. This boundary aligns with the abrupt change in U content, indicating a consistent pattern. Moreover, the ore-bearing layer has undergone multiple stages of fluid transformation. These stages include the early U-bearing oxidizing meteoric precipitation and organic acidic fluids, as well as the late basin brine containing hydrocarbon. The formation of the tabular orebody can be attributed to the prolonged and stable interaction between weakly alkaline U-bearing oxidizing meteoric precipitation and the organic acidic fluid migrating towards the discharge area. This interaction leads to sequential adsorption-reduction processes, which occur due to the decrease in pH and the presence of reducing agents. These processes are responsible for the production of U mineralization on the interface where the two fluids interact. U metallogenic model of the tabular orebody in U deposits in the northern Ordos basin was established. This study presents an analysis of the metallogenic mechanism and evidence of fluids interaction interface for the tabular orebody. The findings of this study can be utilized as exploration guides for sandstone-type U deposits in the central-eastern basins of China

Compositional data analysis (CoDA) and geochemical signatures of the terminal complex aquifer in an arid zone (northeastern Algeria)

The Biskra region, one of the arid regions in North-East Algeria, has known remarkable agricultural development, necessitating additional mobilization of groundwater resources, which are almost the only source of water in the region. In the long term, this heavy mobilization may threaten the quantity and quality of groundwater. This study was carried out to gain a better understanding of groundwater quality in the Terminal Complex (TC) aquifer of the Biskra region, to determine the hydrogeochemical processes controlling water mineralization, and further to investigate the suitability of the groundwater for consumption. For this purpose, multivariate statistical analysis, based on compositional data analysis (CoDA), including robust PCA, cluster analysis (HCA), ilr-ion diagram, and correlation analysis as well as graphical approaches, was carried out on the hydrochemical results of forty-four (44) groundwater samples analysed by standard methods. The results showed that most groundwater parameters exceeded WHO-recommended safe limits for drinking water. Robust CoDA revealed three distinct processes controlling groundwater hydrochemistry: the first “carbonate link” [NO3−, HCO3−, pH] at the northern boundary, which was progressively replaced by the second link [Ca2+, Mg2+, SO42−], and the third “mineralization link” [Na+, Cl−, K+, EC, TDS], located on the Outaya, Biskra, Sidi Okba, and El-Houche axes. In addition, around 93% of samples were within the geochemical zone [SO42− +Cl− >CO32-- HCO3−], while only 7% fell within the [CO32− - HCO3−> SO42− + Cl−] zone, with the following chemical facies dominating: Ca–SO4 (52.27%), Ca–Mg–SO4 (22.72%), Na–Cl (20.45%), and Ca–HCO3 (4.56%). The modified water quality index (WQI) revealed five quality classes: excellent (9.10%), good (11.36%), poor (65.90%), very poor (11.36%), and unsuitable quality (2.28%), with an apparent deterioration in water quality from north to south, following the same direction of groundwater flow. Finally, the use of CoDA with isometric log-ratio plotting, for the first time in the study area, has enabled a more thorough and reliable assessment of the processes controlling groundwater quality, which is valuable for rationalizing pumping and can help decision-makers to plan current and future water resource management practices.

The Yubileinoe Porphyry Gold–Copper Deposit (Western Kazakhstan): Geological Position and Conditions of Formation

In the Uralian Fold Belt, there are quite numerous and well-studied porphyry copper (±Mo) deposits corresponding to the traditional “diorite” (most) or “monzonite” (Talitsa, Verkhneuralskoe) models. Along with them, there are relatively small but gold-rich massifs of porphyric granitoids, including the large Yubileinoe porphyry Au–Cu deposit, which is located at the southernmost extremity of the Urals. In this study, two main types of regional hydrothermal–metasomatic alteration were distinguished based on applying quantitative petrography and areal multielement geochemical studies in the scale of the ore district: (1) an earlier synvolcanic secondary alteration of volcanics, similar to those observed in volcanic massive sulfide-bearing fields (albitization, propylitic, and listvenitic alteration) and (2) a later plutonogenic alteration of the porphyry type. The plutonogenic hydrothermal–metasomatic (HM) complex is represented by K-feldspathization, hornfels and skarn alteration at the progressive phase, and propylitization, sericitization, and beresitization at the regressive phase. They are caused by hydrothermal alteration in the apical part of the stock, composed of the mineralizing Frasnian granite porphyry complex that hosts the Yubileinoe gold deposit. A lateral series of geochemical zonality (from the periphery of volcanotectonic structures to their center) has been established for the volcanogenic stage of hydrothermal activity: CrNiCo → PbZnCuCrNi → AuAg (CrNi) → BaAuAg. A large positive anomaly of the lithochalcophilic type was found for the HM plutonogenic complex in the ore field of the Yubileinoe deposit. The concentric zonality of this anomaly is characterized by the development of Ag, W, Sn, Pb, As, and Sb halos at its periphery, and Au, Cu, Bi and Mo at its focus (“core”). The stable and radiogenic isotope geochemical data for most of the porphyry copper deposits of the Urals indicate the predominant mantle source of their rocks and ore matter. Their paleotectonic position is reconstructed as a mature stage of intraoceanic island arcs. Unlike many other porphyry objects in the Urals, the totality of geochemical, isotope–geochemical and geological features of the Yubileinoe deposit indicate the predominantly crustal magma source. According to these features, this deposit is closer to Andean-type porphyry deposits, and its position can be reconstructed as an active margin of the Mugodzhar microcontinent, i.e., a suprasubduction regime, transitional from a mature island arc to the marginal continental one. According to the complex of features, this deposit in the Urals is a close analogue of the porphyry gold deposits of the Maricunga Belt in Chile. The magmatic complexes from the Silurian (Wenlock) to the Devonian (Frasnian) age, which are parental to the porphyry gold–copper systems of the Urals, correspond to the early phase of the Wilson cycle. This phase is the most ore-productive with the formation of giant Cr and Fe–Ti–V deposits associated with ultramafic–mafic complexes. It is likely that the differentiation of mafic magmas in the large-volume chambers occurring in the lower part of the lithosphere causes the appearance (as an extreme member) of diorite melts with a noticeable enrichment in gold and copper.

Genetic Mechanism of Tabular-Shaped Orebody of the Hailijin Sandstone-Type Uranium Deposit in the Songliao Basin: Constraints on the Clay Mineralogy of Ore-Bearing Sandstone

The Hailijin (HLJ) sandstone-type uranium deposit was newly discovered in the southwestern Songliao Basin in recent years. Different from the roll-front orebody of the sandstone-type uranium deposits with (phreatic oxidation) interlayer redox origin (or phreatic oxidation), the orebody of the HLJ uranium deposit is tabular-shaped and multi-stratiform. The kaolinite content in ore-controlling gray sandstones is significantly higher than that in oxidized sandstones, which have the highest kaolinite content in the less oxidized zone of sandstone-type uranium deposits in the basins of western China (such as Yili Basin and Turpan-Hami Basin). In order to identify the properties of ore-forming fluids and the genesis of the tabular-shaped orebody of the HLJ uranium deposit, trace element, scanning electron microscopy (SEM), X-ray diffraction (XRD), and uranium mineral electron probe (EPMA) analyses of different geochemical zone sandstones in ore-bearing strata were carried out. As a result, kaolinite, illite, and illite/smectite formation (I/S) appear to alternate with one another in ore-controlling gray sandstones, and the content of kaolinite is the highest in ores. SEM analysis also suggests that uranium minerals are commonly adsorbed on the surface of foliated and vermicular kaolinite or trapped within micropores of kaolinite. In this case, it is inferred that kaolinite in ore-controlling gray sandstones is of epigenetic origin, and the ore-bearing sandstones have undergone at least one transformation of acidic fluids. Combined with the regional paleoclimate, regional tectonics, and regional burial history, it is concluded that the acidic fluid originated from the uranium-rich source rocks of the Lower Cretaceous Jiufotang Formation, and the tabular-shaped orebody of the HLJ uranium deposit was formed by exudative metallogeny. When the uranium-rich acidic organic fluids exuded upward from deep levels along the faults to the target strata, the solubility of uranium and other polymetallic elements decreased because of the decrease in temperature and pressure, and uranium eventually precipitated and accumulated in sandstones with suitable permeability and porosity. However, it cannot be ruled out that the superimposition and transformation of uranium mineralization was caused by phreatic oxidation or local interlayer redox during the interval of exudative metallogeny.

Machine Learning (ML)-Based Copper Mineralization Prospectivity Mapping (MPM) Using Mining Geochemistry Method and Remote Sensing Satellite Data

The exploration of buried mineral deposits is required to generate innovative approaches and the integration of multi-source geoscientific datasets. Mining geochemistry methods have been generated based on the theory of multi-formational geochemical dispersion haloes. Satellite remote sensing data is a form of surficial geoscience datasets and can be considered as big data in terms of veracity and volume. The different alteration zones extracted using remote sensing methods have not been yet categorized based on the mineralogical and geochemical types (MGT) of anomalies and cannot discriminate blind mineralization (BM) from zone dispersed mineralization (ZDM). In this research, an innovative approach was developed to optimize remote sensing-based evidential variables using some constructed mining geochemistry models for a machine learning (ML)-based copper prospectivity mapping. Accordingly, several main steps were implemented and analyzed. Initially, the MGT model was executed by studying the distribution of indicator elements of lithogeochemical data extracted from 50 copper deposits from Commonwealth of Independent States (CIS) countries to identify the MGT of geochemical anomalies associated with copper mineralization. Then, the geochemical zonality model was constructed using the database of the porphyry copper deposits of Iran and Kazakhstan to evaluate the geochemical anomalies related to porphyry copper mineralization (e.g., the Saghari deposit located around the Chah-Musa deposit, Toroud-Chah Shirin belt, central north Iran). Subsequently, the results of mining geochemistry models were used to produce the geochemical evidential variable by vertical geochemical zonality (Vz) (Pb × Zn/Cu × Mo) and to optimize the remote sensing-based evidential variables. Finally, a random forest algorithm was applied to integrate the evidential variables for generating a provincial-scale prospectivity mapping of porphyry copper deposits in the Toroud-Chah Shirin belt. The results of this investigation substantiated that the machine learning (ML)-based integration of multi-source geoscientific datasets, such as mining geochemistry techniques and satellite remote sensing data, is an innovative and applicable approach for copper mineralization prospectivity mapping in metallogenic provinces.

Evidence for compositionally distinct upper mantle plumelets since the early history of the Tristan-Gough hotspot

Recent studies indicate that mantle plumes, which transfer material and heat from the earth’s interior to its surface, represent multifaceted upwellings. The Tristan-Gough hotspot track (South Atlantic), which formed above a mantle plume, documents spatial geochemical zonation in two distinct sub-tracks since ~70 Ma. The origin and the sudden appearance of two distinct geochemical flavors is enigmatic, but could provide insights into the structural evolution of mantle plumes. Sr–Nd–Pb–Hf isotope data from the Late Cretaceous Rio Grande Rise and adjacent Jean Charcot Seamount Chain (South American Plate), which represent the counterpart of the older Tristan-Gough volcanic track (African Plate), extends the bilateral-zonation to ~100 Ma. Our results support recent numerical models, demonstrating that mantle plumes can split into distinct upper mantle conduits, and provide evidence that these plumelets formed at the plume head-to-plume tail transition. We attribute the plume zonation to sampling the geochemically-graded margin of the African Large Low-Shear-Velocity Province.

Complex geochronological record of an emblematic Variscan eclogite (Haut‐Allier, French Massif Central)

AbstractTwo eclogite samples from the Haut‐Allier record a prograde evolution from ~20 kbar, 650°C to 750°C, 22–23 kbar followed by heating up to 850–875°C and partial melting. Incipient decompression in high‐pressure granulite facies conditions (19.5 kbar, 875°C) was followed by exhumation to high‐temperature amphibolite facies conditions (&lt;9 kbar, 750–850°C). Following a detailed geochemical, petrological, and geochronological investigation using trace‐element data and laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircon, apatite, and rutile, the eclogites reveal an Ordovician (c. 490 Ma) rifting event followed by Devonian (c. 370–360 Ma) subduction and Carboniferous (c. 350 Ma) exhumation in this part of the French Massif Central. The previously proposed Silurian age for the subduction, which strongly influenced many tectonic models, is definitively rejected. In the light of other geological data from the French Massif Central, including the lithological and geochemical zoning of calc‐alkaline Devonian volcanism, we propose a southward polarity of the subduction and question the very existence of the so‐called Massif Central Ocean. Furthermore, we infer that following subduction, the eclogites were relaminated to the upper plate and exhumed at the rear of the magmatic arc pointing to similarities with the geodynamics of the Bohemian Massif.The petrochronological record of zircon is particularly complex. Metamorphic zircon with clear eclogitic rare‐earth elements patterns (no Eu anomaly and flat heavy rare‐earth elements) and inclusions (garnet, rutile, and omphacite) shows concordant apparent ages that spread from c. 380 down to c. 310 Ma. This apparent age pattern strongly contrasts with the well‐defined age of apatite and rutile of c. 350 Ma. Apparent zircon ages younger than 350 Ma unequivocally testify that zircon can recrystallize outside the conditions of the eclogite facies, which resets the U–Pb while preserving an apparent eclogitic signature. Local fractures filled by analcite, thomsonite, plagioclase, and biotite testify to late interaction of the eclogites with alkaline fluids at relatively low temperatures. This interaction, possibly at c. 310 Ma or later, could lead to the recrystallization of zircon while leaving apatite unaffected.

The World’s Largest Bauxite-Bearing Province, Fouta Djallon–Mandingo (West Africa). Part 4: a Zoning Mechanism in Laterite Bauxite-Bearing Weathering Crust

The results of detailed exploration and year-round observations of the hydrogeological regime at large deposits, as well as special hydrochemical studies of various types of water and changes in the gas composition of the underground atmosphere in the lateritic weathering crust, of the very large bauxite-bearing province, Fouta Djallon–Mandingo, thanks to a reliable factual base, made it possible for the first time to draw a number of important conclusions about the conditions and features of zoning formation in the lateritic weathering profile. The weathering profile consists of two metasomatic columns spatially combined in plan, but vertically separated: an upper one, consisting of hydroxides and oxides of iron and aluminum, and a lower one, composed of clayey eluvium and products of hydration and decomposition of aluminosilicates—polymineral clays and kaolinite. In both columns, the processes of change begin with the same rainwater, but differ in the hydrodynamic parameters in each of the columns. Changes in the weathering profile occur in accordance with the main patterns of low-temperature infiltration metasomatism. The modern hydrogeological and gas regime in bauxite-bearing lateritic covers controls the mineralogical and geochemical zoning. Features of biological and soil processes and hydrodynamics and hydrochemistry in the weathering profile lead to a threshold nature of the change in physicochemical and biochemical conditions, providing the action of geochemical (oxidation, reduction, gley, adsorption) barriers, which is the main mechanism of zonal distribution of matter in lateritic weathering crusts.

THE WORLD’S LARGEST FOUTA DJALLON–MANDINGO BAUXITE PROVINCE (WEST AFRICA): PART 4. THE MECHANISM OF ZONING FORMATION IN LATERITE BOXITE-BEARING WEATHERING CRUSTS

Obtained as a result of detailed exploration and year-round observations of the hydrogeological regime at large deposits, as well as special hydrochemical studies of various types of water and changes in the gas composition of the underground atmosphere in the lateritic weathering crust of the largest bauxite-bearing province FDM, thanks to a reliable factual base, allowed us to draw a number of important conclusions on the conditions and features of the formation of zoning in the lateritic weathering profile for the first time. The weathering profile consists of two metasomatic columns spatially combined in plan, but vertically separated: the upper, lateritic cover, composed mainly of the products of the final hydrolysis of lateritic weathering, that is, hydroxides and oxides of iron and aluminum; the lower one, composed of clayey eluvium, products of hydration and decomposition of aluminosilicates - polymineral clays and kaolinite. In both columns, the processes of changing begin with the same rainwater, but differ in hydrodynamic parameters in each of the columns. Changes in the weathering profile occur in accordance with the main patterns of low-temperature infiltration metasomatism. The modern hydrogeological and gas regime in the bauxite-bearing lateritic covers controls the mineralogical and geochemical zoning. Peculiarities of biological and soil processes, hydrodynamics and hydrochemistry in the weathering profile lead to the threshold nature of the change in physicochemical and biochemical conditions, providing the action of geochemical (oxidative, reducing, gley, adsorption) barriers - the main mechanism of zonal distribution of matter in lateritic crusts.

Long-term warming modulates diversity, vertical structuring of microbial communities, and sulfate reduction in coastal Baltic Sea sediments.

Coastal waters such as those found in the Baltic Sea already suffer from anthropogenic related problems including increased algal blooming and hypoxia while ongoing and future climate change will likely worsen these effects. Microbial communities in sediments play a crucial role in the marine energy- and nutrient cycling, and how they are affected by climate change and shape the environment in the future is of great interest. The aims of this study were to investigate potential effects of prolonged warming on microbial community composition and nutrient cycling including sulfate reduction in surface (∼0.5 cm) to deeper sediments (∼ 24 cm). To investigate this, 16S rRNA gene amplicon sequencing was performed, and sulfate concentrations were measured and compared between sediments in a heated bay (which has been used as a cooling water outlet from a nearby nuclear power plant for approximately 50 years) and a nearby but unaffected control bay. The results showed variation in overall microbial diversity according to sediment depth and higher sulfate flux in the heated bay compared to the control bay. A difference in vertical community structure reflected increased relative abundances of sulfur oxidizing- and sulfate reducing bacteria along with a higher proportion of archaea, such as Bathyarchaeota, in the heated compared to the control bay. This was particularly evident closer to the sediment surface, indicating a compression of geochemical zones in the heated bay. These results corroborate findings in previous studies and additionally point to an amplified effect of prolonged warming deeper in the sediment, which could result in elevated concentrations of toxic compounds and greenhouse gases closer to the sediment surface.

Геохимические особенности и зональность залежи Филизчайского колчеданно-полиметаллического месторождения (южный склон Большого Кавказа)

The article discusses the geochemical features and zoning of the ore deposit of the Filizchay deposit on the southern slope of the Greater Caucasus. The spatial distribution of valuable (Cu, Zn, Pb) components, their sum, as well as the distribution of Ag, which have a high content for the Filizchay deposit, are considered. A field model was built on the basis of the Leapfrog GEO program. In order to quantify zoning, the index of geochemical zoning was calculated based on the ratios of the concentrations of elements accumulated in the upper part of the ore deposit (Zn, Pb, Ag) and elements accumulated in the lower part of the deposit (Cu, Co, Mn). It has been established that in the Azerbaijan part of the southern slope of the Greater Caucasus, as we move from the northwest to the southeast, there is a regional ore zonality, expressed in the change of chalcopyrite-pyrrhotite ores by sulfide-polymetallic ones, as a result of which there is a decrease in the concentration of copper and, accordingly, an increase in the amount of zinc and especially lead. According to the degree of intensity of manifestation of zinc, lead and silver mineralization in the deposit, two parts can be distinguished: the western one is richer, where almost all the maxima are concentrated, and the eastern one is poorer, which accounts for most of the minima. Additional exploration work is recommended in the eastern part. The distribution of elements in individual wells established a clear zoning in terms of the thick ness of the ore deposit, manifested in a regular change in the mineral and chemical composition of ores from a lying to a hanging side. This leads to the accumulation of copper, cobalt, tin and manganese in the foot, and the concentration of zinc, lead, cadmium, thallium, mercury, antimony, etc. in the hanging side of the ore deposit.