Fe-Mn Deposits Research Articles

Mineral formation and element migration in fractures and pores of hydrogenetic ferromanganese nodules

Ferromanganese (FeMn) nodules composed of nano-sized FeMn (oxyhydr)oxides are significant mineral resources rich in multiple critical metals (e.g., Co, Ni, Cu, and REY). The ubiquitous high porosity, large pore size, and pore connectivity in hydrogenetic nodules enable the constant influx of seawater and/or porewater driving widespread diagenetic processes (i.e., mineral dissolution and recrystallization) in the abundant fractures and pores. These diagenetic processes are critical to understanding the migration, partitioning, and distribution of the critical metals, but remain unclear yet. In this study, high-resolution in-situ analyses were conducted to investigate the mineralogical and geochemical characteristics of diagenetic precipitates in fractures and pores of a hydrogenetic FeMn nodule (MP5D33) from the central Pacific. Both oxic and suboxic processes were found in the fracture. The oxidized diagenetic products exhibit geochemical and mineralogical characteristics similar to thoese of hydrogenetic precipitates (e.g., vernadite), while the suboxic diagenetic processes primarily produced large-size 10 Å phyllomanganate. The diagenetic conditions within this studied fracture gradually shifted from oxic to suboxic, causing changes in the contents of multiple critical metals. The minerals in pores are mainly composed of 10 Å phyllomanganate that crystallized forms through heterogeneous nucleation on small-size vernadite in a suboxic environment. Additionally, minor todorokite likely transformed from 10 Å phyllomanganate undergoing long-term diagenesis. These findings shed some insights into the diagenetic process in the interior fractures and/or pores of FeMn nodules and enhance the understanding of the migration of critical elements associated with FeMn (oxyhydr)oxides during diagenesis in FeMn deposits.

Magnetization of Ferromanganese Crusts: Geochemical and Magnetic Insights From Rio Grande Rise and Tropic Seamount

AbstractFerromanganese (FeMn) crusts are Fe and Mn oxides that typically form on deep‐sea elevations by deposition of colloids from seawater. These mineral deposits are considered a source of critical metals and rare earth elements. Besides their potential economic value, FeMn crusts are extremely relevant in ocean science, since their very slow growth rates result in long‐term paleoenvironmental and paleoceanographic records. In this study, we applied geochemical, mineralogical, and magnetic analyses to unravel paleoenvironmental changes at two locations on opposite sides of the Atlantic Ocean, the Rio Grande Rise in the SW Atlantic and the Tropic Seamount in the NE Atlantic. Our results show that the occurrence of amorphous (non‐crystalline) Fe oxyhydroxides and the absence of Fe oxides in hydrogenetic, non‐phosphatized FeMn crusts prevented the development of primary remanent magnetization. In contrast, phosphatized FeMn crusts may have contained a remanent magnetic signal. Phosphatization resulted from increased primary productivity and occurred at different stages during the growth of the FeMn crusts, leading to suboxic conditions and partial dissolution of pre‐existing, remanence‐carrying magnetic minerals. Carbonate Fluorapatite (CFA) accumulation in the phosphatized layers of FeMn crusts replaced Fe and Mn, decreasing their magnetic content. Thus, magnetic variations do not reflect a primary magnetization but rather result from geochemical alterations. The loss of primary magnetization may hamper the use of FeMn deposits for magnetostratigraphic purposes.

Igelströmite, Fe3+(Sb3+Pb2+)O4, and manganoschafarzikite, Mn2+Sb3+2O4, two new members of the newly established minium group, from the Långban Mn–Fe deposit, Värmland, Sweden

Abstract. The two new minerals igelströmite, Fe3+(Sb3+Pb2+)O4, and manganoschafarzikite, Mn2+Sb23+O4, are found in the Långban Fe–Mn deposit, in open fractures in a fine-grained hematite ore, with minor amounts of aegirine, a serpentine-group mineral, fluorcalcioroméite, baryte, nadorite, mimetite and other late-stage minerals. Igelströmite is named after the Swedish geologist–mineralogist Lars Johan Igelström (1822–1897). Mohs hardness = 3–4 and Dcalc= 6.33(1) and 5.37(2) g cm−3 for igelströmite and manganoschafarzikite, respectively. Cleavage is distinct on {110}. Both minerals are brittle, with an uneven to conchoidal fracture. The chemical formulae obtained from microprobe data are (Fe0.593+Mn0.292+As0.063+Fe0.062+)Σ=1.00(Sb1.243+Pb0.652+As0.113+)Σ=2.00O4 and (Mn0.642+Fe0.252+Mg0.08)Σ=0.97(Sb1.973+As0.033+Pb0.012+)Σ=2.01O4. The crystal structures for igelströmite and manganoschafarzikite have been refined in space group P42/mbc from single-crystal X-ray diffraction data to R1 = 3.73 % and 1.51 %, respectively, giving the following sets of unit-cell parameters: a= 8.4856(2), 8.65159(8) Å; c= 6.0450(3), 5.97175(9); and V= 435.27(3), 446.986(11) Å3 for Z = 4. Both minerals are isostructural with minium, Pb4+Pb22+O4, where Pb4+O6 forms distorted octahedra, which connect via trans-edges to form rutile-like ribbons along c. The Pb2+ atoms appear in trigonal, flattened PbO3 pyramids, which are linked via corners to form zigzag (PbO2)n chains. The minium group, of general formula MX2O4(X= As3+, Sb3+, Pb2+), presently consists of the minerals minium, trippkeite, schafarzikite, igelströmite and manganoschafarzikite. For future new members, it is recommended to consider the X cation content for the root name and add prefixes to indicate the dominant metal at the M position.

New insight into genesis of the Maojun laterite Fe–Mn deposit in the Lanshan area, Hunan Province, South China: Evidence from detailed mineralogical and geochemical studies

Laterite Fe–Mn deposits are widespread in South China, with the majority of Fe–Mn ore being present in residual quaternary clay beds. However, detailed geological data on the lateritization of low-iron-content carbonate rocks are rare. In this study, we present new results on the mineralogy and geochemistry, as well as a genetic model, of the Maojun laterite Fe–Mn deposit in the Lanshan area, Hunan Province, South China. The profile sequence of laterite consists of an eroded bedrock horizon at its base, an intermediate black–brown clay layer containing earthy Fe–Mn ore, and a reddish-brown clay layer with nodular ferromanganese ore in contact with reddish-brown or yellowish-brown clay on top. Field evidence and chemical analysis indicated that during lateritization, the Mn-Fe-containing carbonate rocks of the Huanggongtang (D2h) and Shetianqiao formations (D3s) saw a more significant removal of mobile elements (Mg, Ca, and Na) whilst insoluble elements (Fe, Mn, Si, Al, Pb, and Zn) exhibited persistent enrichment in situ. Discriminative diagrams of Fe–Mn mineralization, as well as the assemblage-related enrichment of Y, U, Mo, Pb, and Zn and depletion of high-field-strength elements Zr, Nb, and Th, imply that subsequent hydrothermal circulation overprinted on the previously formed hydrogenous deposit. Mineralogical studies conducted using XRD, EPMA, HR-TEM, and TIMA indicated the predominance of iron and manganese oxides; hematite, goethite, limonite, and hollandite were identified as major oxide phases and cryptomelane, pyrolusite, and coronadite were present in minute quantities. Similar minerals constitute the upper ferromanganese nodule horizons, although they possess distinct textures and concentrations. The mineralogy, geochemical associations, and Ti mass balance show a continuous vertical evolution from top to bottom in the lateritic profile. Ferromanganese concretions from the drenching zone with poorly crystallized Al–goethite, Al–hematite, limonite, Fe–kaolinite, Fe–Mn oxides and hollandite predominate in shallow parts, and microcrystalline hematite, goethite and hollandite were found in deeper layers. Mn2+ can be rapidly oxidized and precipitated on the surface of hematite and limonite as high-valence-state (Mn4+) manganese oxides and binds strongly with mobile elements (Ba, K, Pb, Zn, Ca, and Ni). Petrographical, mineralogical, and geochemical studies show that three stages comprised the formation of the Maojun laterite Fe–Mn deposit.

Magmatic–Hydrothermal Origin of Fe-Mn Deposits in the Lesser Khingan Range (Russian Far East): Petrographic, Mineralogical and Geochemical Evidence

Iron and iron–manganese deposits form three closely spaced clusters within the Lesser Khingan Range of the Russian Far East. Fe-Mn mineralization is hosted in Vendian–Cambrian carbonates and composed of magnetite, hematite, braunite, haussmanite, rhodochrosite and pyrolusite. The iron–manganese ores are closely associated with explosive intermediate–felsic breccias, magnetite-rich lavas, dolerites and mineralized lithocrystalloclastic tuffs. Magmatic rocks display both concordant and discordant relationships with Fe-Mn mineralization and contain abundant xenoliths of host carbonates. Both magmatic rocks (with the exception of Nb-enriched dolerites) and Fe-Mn ores are characterized by variable enrichments in large-ion lithophile and light rare earth elements and strong depletions in high-field strength elements compatible with the broad subduction setting for explosive volcanism and associated hydrothermal Fe-Mn ore mineralization. Nd-Sr isotope systematics suggest contamination by both ancient and juvenile continental crust and the involvement of recycled pelagic sediment in the formation of Fe-Mn deposits in the Lesser Khingan Range of the Russian Far East.

The role of depositional environment and chemical composition on the triple oxygen isotope ratios of ferromanganese precipitates and their endmember components

Ferromanganese precipitates - crusts, nodules, and hydrothermal deposits – typically exhibit negative Δ'17O values. These deviations are thought to reflect partial incorporation of dissolved atmospheric O2, a component with strongly negative Δ'17O values, seawater oxygen, and deep sea O2 into Mn-oxide minerals, in combination with the other major mineralogical endmember components (Fe-oxyhydroxide and silicate minerals), and kinetic processes such as oxidation at mineral surfaces and biological respiration. Our understanding of the triple oxygen isotope systematics of these materials are limited, however; in the current body of literature, triple oxygen isotope data has only been generated for marine hydrogenetic (i.e., metals accrete directly from seawater) Fe-Mn crusts and Mn nodules. This study evaluates the underlying chemical and environmental factors contributing to the bulk triple oxygen isotopic composition of Fe-Mn precipitates of different genetic types (i.e., hydrothermal, hydrogenetic, mixed-type) from a variety of formation environments (e.g., volcanic arc, pull-apart basin, hotspot, freshwater lake). To improve our understanding of the triple oxygen isotope systematics of natural Fe-Mn precipitates and better constrain the endmember components, we isolate and measure close approximations of the Mn-oxide, Fe-oxyhydroxide, and silicate fractions. Hydrothermal Fe-Mn deposits show greater variation in δ'18O and Δ'17O values (6.0 to 29.6 ‰ and −0.286 to 0.007 ‰, respectively) than hydrogenetic Fe-Mn crusts (5.1 to 11.9 ‰ and −0.200 to −0.086 ‰, respectively). Lacustrine Mn nodule δ'18O and Δ'17O values (5.1 to 8.8 ‰ and −0.230 to −0.090 ‰, respectively) are similar to marine hydrogenetic Fe-Mn crusts, primarily reflecting tropospheric O2. The triple oxygen isotope ratios of nearly pure Mn-oxide samples cluster into distinct groups based on oxygen source, and mass-balance mixing models suggest a significant proportion of incorporated oxygen was photosynthetic in origin. The layered Fe-Mn precipitates in this study show irregular to rhythmic patterns of δ'18O and Δ'17O values depending on formation environment that mostly do not correlate with Mn, Fe, and Si contents. Shale-normalized rare earth element and yttrium (REYSN) patterns and anomalies (CeSN/CeSN*, EuSN/EuSN*, and YSN/HoSN), which can provide insight on formation conditions (e.g., redox, oxygen fugacity, and REY speciation of fluids), are compared with the triple oxygen isotope data. The REYSN anomalies increase from the bottom to the top of the profile of the layered marine sample, suggesting conditions became more oxic over time. Although the triple oxygen isotope ratios correlate with the Eu anomaly in most Fe-Mn precipitates, hydrothermal input can limit their utility as indicators of oxygen fugacity. The triple oxygen isotope ratios of Fe-Mn precipitates reflect complex processes and mixing of multiple sources. Constraining these oxygen sources will assist future studies in interpreting paleoenvironmental conditions (e.g., the saturation state and triple oxygen isotope ratios of dissolved O2 in bottom water) at the time of Fe-Mn precipitate formation.

Magmatic Native Gold: Composition, Texture, Genesis, and Evolution in the Earth’s Crust

Abstract —Here we report results of microforms’ studies of native gold and its alloys in igneous rocks, modified to varying degrees by secondary processes. We discuss the composition and occurrence of both the deep-seated magmatic gold-bearing alloys and the products of their transformation under conditions of the upper Earth’s crust. Gold-bearing Kamchatka adakites and ankaramites, Ildeus massif mafic-ultramafic intrusions and adakites from the Stanovoy fold system as well as dacites from the Bolivian Andes were formed during melting of either the suprasubduction mantle wedge or the subducted oceanic crust. In depleted peridotites from the Avachinsky Volcano in Kamchatka as well as suprasubduction ophiolites from Polar Urals, Eastern Sayan and the Western Mediterranean Betic–Rifean belt, the gold-bearing mantle was hybridized by subduction-related melts and high-temperature fluids. Volcanic rocks associated with the Lesser Khingan Fe–Mn deposits and Zolotaya Gora Au deposit in Southern Urals as well as Taragai ultramafic rocks in the South Khingan Range display subduction-related geochemical characteristics. Gold-bearing trachytes in the Virginian Appalachians (USA) represent felsic differentiates of mafic intraplate magmas. We propose that one of the principal forms of gold transport into the upper crustal environments is represented by Cu–Ag–Au alloys, which precipitated from mantle-derived silicate melt enriched in chalcophile and siderophile elements. Such Cu–Ag–Au alloy-rich magmatic rocks can either constitute primary sources of precious metals in the mantle-crust system or serve as geochemical precursors to the formation of native gold assemblages in epithermal and mesothermal ore deposits. Presence of magmatic gold particles in subduction-related igneous rocks and mantle restites hybridized by subduction-derived melts and high-temperature fluids suggest the existence of gold-rich horizons in the Earth’s mantle at depths comparable to typical depths of generation of primary convergent zone and some within-plate magmas.

МИНЕРАЛОГИЯ ЭЛЕМЕНТОВ ПЛАТИНОВОЙ ГРУППЫ В ЭКСПЛОЗИВНЫХ БРЕКЧИЯХ МЕСТОРОЖДЕНИЯ ПОПЕРЕЧНОЕ (МАЛЫЙ ХИНГАН, РОССИЯ)

The results of detailed study of the platinum-group minerals (PGM) from explosive breccias of the Poperechnoe Fe-Mn deposit (the Lesser Khingan Range, Russian Far East) are presented. Native PGMs are dominated by isoferroplatinum; rutheniridosmine, native iridium, platinum, and osmium are less com-mon. Micron-sized segregations of laurite, bowieite, сuproiridsite, cuprorhodsite, hollingworthite, as well as new mineral phases of (Ir,Rh,Os)7 (S,As)13 , (Rh,Ir,Ru) 7(S,As) 13, and Pd 3(Sb,As) were recognized as micro-inclusions in isoferroplatinum and on the surface of its grains. It is shown that the studied PGMs are derived from rocks of ultramafic formations: (1) vein pyroxenites, harzburgites, and dunites of metamorphic and cumulative complexes of the most depleted peridotite varieties of the suprasubduction wedge of island-arc ophiolites and (2) vein pyroxenites of cumulative high-pressure ultramafic complexes of the basement of the ensialic island arc and products of evolution of suprasubduction mantle melts. In terms of PGE contents, the compositions of isoferroplatinum from explosive breccias are divided into four groups: group I: isoferroplatinum of fluid-metamorphogenic genesis from harzburgite, group II: isoferroplatinum of fluid-metamorphogenic genesis from dunites and magmatogenic-fluid-metasomatic genesis from vein pyrox-enites; group III: isoferroplatinum of magmatogenic genesis from chromitites of dunites of the cumulative complex; and group IV: isoferroplatinum with an elevated Pd content, which is likely derived from the melt formed by explosive breccias. Three scenarios of PGM occurrence in the fluid-saturated andesite-dacite melt of explosive breccias at the Poperechnoe deposit are discussed: (1) directly from early ultramafic com-plexes in suprasubduction settings; (2) from the reservoir of ancient platinum placer deposits; and (3) from a mantle wedge above the Mesozoic subduction zone during the generation of initial island-arc melts. The first two petrogenetic models suggest «rejuvenation» of the 190Pt-4He age of isoferroplatinum grains as a result of the thermal effect of the andesite-dacite melt to the age of 125 ± 21 Ma. The third geodynamic scenario suggests that the Lower Cretaceous age of isoferroplatinum marks the processes of metamorphogenic-metasomatic transformation of the dunite-harzburgite mantle wedge and probably corresponds to the age of the subduction magmatism at the Poperechnoe Deposit in particular and within the Lesser Khingan terrane as a whole. PGM associations in andesite-dacite breccias of the Poperechnoe Deposit are a new type of potentially commercial noble-metal mineralization in the Russian Far East, while explosive breccias (fluidoliths) can be used as an exploration guide to discover PGE lode and placer deposits of volcanogenic-explosive genesis in the Russian Federation.

Fast-growing Arctic Fe–Mn deposits from the Kara Sea as the refuges for cosmopolitan marine microorganisms

The impact of biomineralization and redox processes on the formation and growth of ferromanganese deposits in the World Ocean remains understudied. This problem is particularly relevant for the Arctic marine environment where sharp seasonal variations of temperature, redox conditions, and organic matter inflow significantly impact the biogenic and abiotic pathways of ferromanganese deposits formation. The microbial communities of the fast-growing Arctic Fe–Mn deposits have not been reported so far. Here, we describe the microbial diversity, structure and chemical composition of nodules, crust and their underlying sediments collected from three different sites of the Kara Sea. Scanning electron microscopy revealed a high abundance of microfossils and biofilm-like structures within the nodules. Phylogenetic profiling together with redundancy and correlation analyses revealed a positive selection for putative metal-reducers (Thermodesulfobacteriota), iron oxidizers (Hyphomicrobiaceae and Scalinduaceae), and Fe-scavenging Nitrosopumilaceae or Magnetospiraceae in the microenvironments of the Fe–Mn deposits from their surrounding benthic microbial populations. We hypothesize that in the Kara Sea, the nodules provide unique redox-stable microniches for cosmopolitan benthic marine metal-cycling microorganisms in an unsteady environment, thus focusing the overall geochemical activity of nodule-associated microbial communities and accelerating processes of ferromanganese deposits formation to uniquely high rates.

Spatial patterns of microbial diversity in Fe-Mn deposits and associated sediments in the Atlantic and Pacific oceans.

Mining of deep-sea Fe-Mn deposits will remove crusts and nodules from the seafloor. The growth of these minerals takes millions of years, yet little is known about their microbiome. Besides being key elements of the biogeochemical cycles and essential links of food and energy to deep-sea, microbes have been identified to affect manganese oxide formation. In this study, we determined the composition and diversity of Bacteria and Archaea in deep-sea Fe-Mn crusts, nodules, and associated sediments from two areas in the Atlantic Ocean, the Tropic Seamount and the Rio Grande Rise. Samples were collected using ROV and dredge in 2016 and 2018 oceanographic campaigns, and the 16S rRNA gene was sequenced using Illumina platform. Additionally, we compared our results with microbiome data of Fe-Mn crusts, nodules, and sediments from Clarion-Clipperton Zone and Takuyo-Daigo Seamount in the Pacific Ocean. We found that Atlantic seamounts harbor an unusual and unknown Fe-Mn deposit microbiome with lower diversity and richness compared to Pacific areas. Crusts and nodules from Atlantic seamounts have unique taxa (Alteromonadales, Nitrospira, and Magnetospiraceae) and a higher abundance of potential metal-cycling bacteria, such as Betaproteobacteriales and Pseudomonadales. The microbial beta-diversity from Atlantic seamounts was clearly grouped into microhabitats according to sediments, crusts, nodules, and geochemistry. Despite the time scale of million years for these deposits to grow, a combination of environmental settings played a significant role in shaping the microbiome of crusts and nodules. Our results suggest that microbes of Fe-Mn deposits are key in biogeochemical reactions in deep-sea ecosystems. These findings demonstrate the importance of microbial community analysis in environmental baseline studies for areas within the potential of deep-sea mining.

Rare earth elements and origin of Fe[sbnd]Mn ores from the Lower Carboniferous Um Bogma Formation, Southwest Sinai, Egypt: A mixed source of metals and multiple formation processes

The economic FeMn ores of the Lower Carboniferous Um Bogma Formation, Southwest Sinai are an important mineral resource, and are also of interest because of their significant enrichment in Cu and U compared to other Phanerozoic Mn deposits. However, their origin remains controversial. Genetic theories range from sedimentary, to hydrothermal, to karst and to laterization origins. This study integrates geological, sedimentological, mineralogical, and geochemical data, particularly rare earth elements, to elucidate the origin of these deposits. Field relations showed the occurrence of FeMn deposits as lenses of Mn-rich material surrounded by Fe-rich clastic deposits, which then are overlain by dolostones, which, in the subsurface, are known to also contain MnFe lenses. The occurrence of the largest of these deposits at a major transgressive surface is consistent with syndepositional Mn mineralization, but not of ironstones. Geochemical proxies, based on modern hydrothermal, diagenetic and hydrogeneous Mn deposits, are mixed: some suggest a hydrogenous and some a hydrothermal source of Fe and Mn. The sweater (hydrogenous) source of Mn and Fe is indicated by the plotting of these ores in the hydrogenous field of the SiAl discrimination plot, by high ΣREE (42–327 ppm), low La/Ce ratios (0.261.5), relatively high Y/Ho ratios for Mn but not for Fe (averages of 35 and 28), and positive YSN anomalies (1–1.8). The ores plot in the sweater fields of the (La/Sm)SN-(La/Yb)SN and (Sm/Yb)SN-Y/Ho diagrams, which also supports this interpretation. However, the hydrothermal input to the source of these ores cannot be ignored and is indicated by high Ba, Cu, Zn, and by high LREE/HREE ratio as well as the plotting of the FeMn samples in the hydrothermal field of the Fe-Mn-(Ni + Co + Cu)*10, (Co + Ni)-(As+Cu + Mo + Pb + V + Zn), (Co + Ni + Cu)-Co/Zn, CeSN/CeSN*-YSN/HoSN and CeSN/CeSN*-NdSN diagrams. In addition, lower Er/Nd ratios than that of seawater and similarity of REE parameters such as ΣREY, Nd and CeSN/CeSN* between the diagenetic FeMn nodules and the FeMn ores of Um Bogma Formation suggest a post-depositional overprint on the REEs geochemistry of these ores. We propose, as most consistent with the evidence from tectonic subsidence history, sequence stratigraphy, and our geochemical-mineralogical data combined with that of others, an initial synsedimentary deposition of Mn, derived from a distally-located back-arc spreading center where modified seawater at relatively low temperature was the hydrothermal fluid released into the surrounding seawater. This plume of Mn-rich water, which can be far-travelled, deposited Mn on the unconformity surface during a rapid transgression. This accumulation of Mn was subsequently modified and significantly upgraded during karstification of the host dolostones soon after deposition of the dolostones. It is likely that the Fe was introduced at this stage via low-oxygen ground water flow along with As and possibly Ba. Finally, much later, there was a flow of warm, saline oxidizing water through the dolomitic aquifer that precipitated Cu, U and Zn in the weathered material accumulated in the karst cavities.

Benthic megafauna habitats, community structure and environmental drivers at Rio Grande Rise (SW Atlantic)

The Rio Grande Rise (RGR) is a large and geomorphologically complex feature located in the Southwest Atlantic, with great commercial and scientific interest due to its potential for mining rare earth elements that are critical for low-carbon technologies. Brazilian interest in this area led to the submission of a petition to the UN Commission on the Limits of the Continental Shelf in 2018 to include RGR on the limits of its continental shelf beyond 200 nautical miles. However, mining activities are potentially harmful to deep-sea ecosystems and will likely cause some extent of biodiversity loss. Thus, baseline and continuous environmental studies in the RGR are important to address potential conflicts between mineral extraction and the conservation of deep-sea biodiversity. The RGR is characterized by a series of summit plateaus of ∼600 m deep divided NE-SW by a rift valley, up to 2000 m deep. In 2018, the plateaus and rift of a small area in RGR (30°35′S – 31°03′S, 35°36′W – 36°16′W) were explored through 13 dives of the robotic underwater vehicle (RUV) HyBIS. Videos were analyzed for the description of structuring factors (topography and habitat types) and to record benthic megafauna occurrences. Video transects revealed highly heterogeneous and rapidly changing habitats. Eleven habitats, five in the rift and six in the plateaus are proposed based on geomorphology, slope, and substrate textures. We recorded 17,008 megabenthic organisms classified in 83 morphotypes and six different phyla, from which Porifera (42.7%) and Cnidaria (41.5%) were the most representative. Samples were characterized by a high dominance and the dissimilarities result chiefly from differences in abundance scores. PERMANOVA tests indicated that Habitat and Region variables were the most important to explain structure within the community data, followed by depth and slope. The rift floor exhibited a low abundance of megabenthic epifauna, except in a sinkhole in the northern part of the rift. The lower and upper rift wall were characterized by different communities delimited by the transition between the Antarctic Intermediate Water and the Upper Circumpolar Deep Water. The habitats formed by Fe–Mn deposits were dominated by distinct communities, which were rarely observed elsewhere. Additionally, we found variations in community structure at regional scales (20–30 km), with distinct communities on each side of the rift and at the southwest of the study area. Our results contribute toward understanding the diversity, biogeography, and environmental drivers of the RGR. Fauna distribution is patchy, linked to habitats with potential mining resources, and dominated by Vulnerable Marine Ecosystems (VMEs) indicator taxa. Extensive community analysis should occur at a given site prior to consideration for the exploitation of natural resources.

Abundance and microbial diversity from surface to deep water layers over the Rio Grande Rise, South Atlantic

Marine microbes control the flux of matter and energy essential for life in the oceans. Until now, the distribution and diversity of planktonic microorganisms above Fe-Mn crusts have received relatively little attention. Future deep-sea mining is predicted to affect microbial diversity. Here, we studied the ecology of picoplankton among pelagic zones of a Fe-Mn deposit region, at Rio Grande Rise, Southwestern Atlantic Ocean. We investigated microbial community composition using high-throughput sequencing of 16S rRNA genes and their abundance estimated by flow cytometry.The picoplankton populations were more abundant in epi- and mesopelagic waters, corresponding to the Tropical and South Atlantic Central Water masses. Bacterial groups related to heterotrophy, such as Oceanospirillales (Gammapoteobacteria), SAR11 (Alphapoteobacteria), Flavobacteriales (Bacteroida), and Rhodobacterales (Alphapoteobacteria), were the main representatives of the pelagic microbial community. Additionally, we detected abundant assemblages belonging to acetate-oxidizing manganese reducers, i.e., Alteromonas. No differences were observed in microbial community diversity among pelagic zones and water masses. These results provide the first insights into the picoplankton abundance, taxonomy and diversity, and ecological processes in the Rio Grande Rise of the Atlantic Ocean. This may also support draft regulations for deep-sea mining in the region.

Characteristics and possible formation process of ferromanganese beachrock in an intertidal zone of East China Sea influenced by buried ancient woods

The ferromanganese (FeMn) deposits could be found in many shallow water and deep-ocean places. Lots of studies have been performed to explore the occurrence, formation and economic value of FeMn nodules or crusts on the deep ocean floor. However, the occurrence and formation of FeMn beachrocks at the coastal zones has rarely been reported. In the present study, we collected two types of dendritic FeMn beachrock samples from an intertidal zone of Zhoushan Archipelago, East China Sea, where was significantly amended by an ancient wood layer formed during 5900–5600 cal. yr B.P. Then the microscale analysis including electron microprobe analyzer (EMPA), X-ray powder diffraction (XRD), and scanning electron microscopy-energy dispersive X-ray spectrometer (SEM-EDS) were performed to study the microscale characteristics on these FeMn beachrocks. Combined with our previous studies, a possible formation process was proposed: (1) Buried ancient woods produced enough organic acids to accelerate the weathering of nearby bedrock via chemical or biological reactions; (2) The seepage water could be regarded as a bridge to connect the ancients woods and the intertidal zones, and large amounts of dissolved Fe2+ and Mn2+ can be transported to the sandy beach and intertidal zones; (3) Precipitation of insoluble FeMn oxides occurred with the involvement of microbial processes and direct inorganic chemical sorption under mixing of seepage water and seawater; (4) After that, these FeMn oxides would experience nucleation, crystallization growth, and then diagenetic process to form the FeMn beachrocks. The present study spotted a light on the microscale features, formation process and biogeochemical cycling of nearshore/coastal diagenetic Fe-Mn deposits.

The Shamsabad Fe-Mn deposit, Markazi province, Iran: LA-ICP-MS and sulfur isotopic geochemistry

To determine the origin of the Shamsabad Fe-Mn deposit, LA-ICP-MS geochemistry in pyrolusite and the sulfur isotopes in galena were studied. The ore is lenticular and is hosted in Lower Cretaceous limestones. Hematite, iron hydroxides and pyrolusite are the ore minerals, accompanied by subordinate todorokite, coronadite, calcite, quartz, pyrite, galena, rhodochrosite and malachite. Field and petrographic evidence, including lamination and colloform textures of the ore and the presence of conformable limestone lenses within the ore, suggest a synsedimentary hydrothermal origin to the mineralization. In addition, the enrichment of As, Ba, Cu, Pb and Zn and depletion of Co and Ti in the pyrolusite is similar to hydrothermal Fe-Mn deposits. The LREE/HREE ratio, low ΣREE values and negative Ce anomalies indicate that hydrothermal fluids played an important role in mineralization. The Mn/Fe, Co/Ni, Co/Zn, La/Ce, LaN/NdN, DyN/YbN, and Y/Ho ratios, as well as all discriminative element diagrams for trace and major elements, are consistent with hydrothermal deposits. The δ34S values of the galena samples indicate that the sulfur originated from magmatic fluids, probably partially mixed with Lower Cretaceous seawater. According to the proposed hydrothermal-sedimentary genetic model, the hydrothermal solutions leaked into the sedimentary basin primarily through possible deep-seated faults in the bedrock. Hydrothermal fluids were enriched in ore-forming materials through leaching from the rocks with which they were in contact. The leakage of the metal-bearing hydrothermal solutions into the sedimentary basin and mixing with seawater altered the Eh-pH conditions, resulting in the deposition of ore materials in the Shamsabad deposit.

Formation of Fe-Mn coatings on foraminifera from the ultraslow-spreading southwest Indian Ridge: Implications for hydrothermal and diagenetic overprints in sediments

Detailed fine-scale morphological, mineralogical, and geochemical analyses were conducted on the Si-(Fe) hydroxide formations, planktonic foraminifera (PF) and the Fe-Mn coatings within a typical sediment sample collected from near the newly-discovered active hydrothermal vent field at the ultraslow-spreading Southwest Indian Ridge (SWIR). The analyses were conducted with a combination of different techniques, including Laser Scanning Confocal Microscope (LSCM), Laser Raman, high-resolution XRD, SEM-EDS, C-O isotope measurement, EMPA, and in-situ LA-ICP-MS. The mixed-type hydrothermal-hydrogenetic Si-(Fe) hydroxide formations (commonly occur as cavity-fillings inside the PF chambers) contain a considerable amount of organic matter, and are mainly composed of nontronite, goethite, tridymite, and opal-A with the bulk compositions of 8.13–40.91 wt% SiO2, 6.85–54.50 wt% FeO, and 0.43–23.99 wt% Al2O3. Such high Al content can be mainly attributed to the precipitation and accumulation of hydrothermal-derived Al-rich materials, and the Si-Fe-Al assemblage reveals that it is mainly sourced from plagioclase dissolution/alteration during subseafloor fluid-rock interaction. Meanwhile, the relative enrichments of high field strength elements (HFSEs), heavy rare earth elements (HREE), and Cr-Ni-Sc also imply that these Si-(Fe) hydroxide formations have another source from pyroxene alteration. In addition, their distinct microtextures (filamentous, orbicular, or beaded-like) are indicative of bacterial-related precipitation. The hydrogenetic Fe-Mn coatings have a layered texture and exhibit no apparent fluorescence, and are mainly composed of vernadite and goethite with the bulk compositions of 23.98–30.06 wt% FeO and 8.55– 21.52 wt% MnO. Their average Fe/Mn ratios, growth rates, and Zn-As contents are slightly higher than those of the open-ocean SWIR Fe-Mn deposits, which suggests an additional Fe input from submarine hydrothermal activity. Moreover, the altered PF shell is mainly composed of calcite and have three color types (white/yellowish/yellow). The contents of Si, Al, Fe, Cr, Ni, Cu, Zn, Ba, HFSEs, and REE increase as the yellowish color deepens, which indicates the diagenetic contamination from basalt alteration. Consequently, a new diagenetic Fe-carbonate phase (ankerite) was formed in the inner layer of the altered PF shells, and close correlations were also found between their C-O isotope ratios. Both the δ13C and δ18O values tend to drop from white- to yellow-type PF shells, accompanied with the decreasing contents of Ca, Na, Mg, and Sr. All these parameters and signatures would help to unravel the hydrothermal influence and diagenetic overprinting, which could cause confusion when using foraminifera shell geochemistry (e.g., Sr/Ca and Mg/Ca ratios) as a marine tracer in paleoclimate reconstruction.

Assessment of metal pollution and subsequent ecological risk in the coastal zone of the Olkhon Island, Lake Baikal, Russia

Olkhon Island is the largest island in Lake Baikal and a part of Baikal National Park, Russia. The first objective of this study is to establish relationships between the particle size of accumulating sediments and their elemental composition, as well as the concentrations of heavy metals (Hg, Cd, As, Pb, Cr, Co, Ni, Cu, and Zn). The second goal is to completely assess the contamination level and to identify the possible sources of heavy metals using geochemical indices, including enrichment (EF) and contamination (Cf) factors, contamination degree (Cd), geoaccumulation index (Igeo), and pollution load index (PLI). The results obtained are summarized as follows. Heavy metal pollution in the coastal zone of Olkhon Island ranged from moderate to significant levels for Hg, As, Cd, Pb, and Cu. The EF and Igeo indices showed that Hg, Cd, Pb, and Cu sources were more likely to be anthropogenic, whereas the As, Cr, Co, Ni, and Zn sources were similar to crustal sources. Thus, Hg, Cd, and Pb are the main pollutants in the study area and pose high ecological risks. Pearson correlation analysis indicated high positive correlations between Pb and Hg (0.741), As and Cd (0.730), and Cd and Pb (0.803), and strong positive correlations among Cr, Co, Ni, Cu, Zn and Fe. This can reflect the same source and migration pathway, either crustal or anthropogenic. However, it does not indicate that Cr, Co, Ni, Cu, and Zn have anthropogenic origins because these metals are linked with FeMn deposits. These findings could contribute to a more effective investigation of relationships between heavy metals and their sources. We emphasize that Hg, Cd, and Pb could rise to dangerous levels. These reliable results allow us to use our study as a model for studies relating to heavy metal contamination in different areas.

A magnetic approach to unravelling the paleoenvironmental significance of nanometer-sized Fe hydroxide in NW Pacific ferromanganese deposits

Ferromanganese nodules and crusts (Fe-Mn deposits) are being widely explored for their significant economic potential and paleoenvironmentally significant archives. Fe-Mn deposits contain abundant Fe-bearing minerals including detrital minerals, biogenic Fe-bearing components, but predominantly amorphous Fe hydroxides (AFH). Particularly, the hydrogenetic Fe that is formed in bottom water should be closely related with oceanic environmental and Fe-cycling processes. However, it remains challenging to characterize and quantify the x-ray amorphous AFH component in Fe-Mn deposits. To resolve this problem, we systematically investigated thermally treated hydrogenetic Fe-Mn deposits sampled from the northwestern Pacific Ocean to unravel the AFH component. Our results show that the nanometer-sized AFHs can be transformed into strongly magnetic nanometer-sized (approximately 10-20 nm) magnetite upon heating above 500°C, which can be feasibly quantified by systematic rock magnetic analyses. Using this novel approach, several Fe-Mn deposits at different water depths from the western Pacific Ocean are investigated. Our results indicate that the abundance of AFH increase at a water depth of ∼5000 m, which can be ascribed to bottom-current stratification. The magnetic approach to indirectly quantify the AFH component in Fe-Mn deposits has a great potential in exploring oceanic paleoenvironment significance.

Age of Ore-Bearing Rocks at Devonian Fe–Mn Deposits, Central Kazakhstan

Age of Ore-Bearing Rocks at Devonian Fe–Mn Deposits, Central Kazakhstan

Geochemical and mineralogical composition of ferromanganese precipitates from the southern Mariana arc: Evaluation, formation, and implications

The Mariana intraoceanic volcanic arc system in the western Pacific Ocean hosts abundant ferromanganese (Fe-Mn) precipitates. A suite (n = 22) of Fe-Mn precipitates were collected from the southern portion of the arc and their mineralogies and chemical compositions were determined. These results were used to decipher their genetic assemblage, assess their potential as a source of trace metals, and place them into context with respect to Fe-Mn precipitates sampled at higher latitudes in the Mariana arc and from other locations, globally. Minerals identified include vernadite, birnessite, 10 Å manganate, manganite, hematite, goethite, maghemite, calcite, rhodochrosite, quartz, phillipsite, various feldspar, pyroxene, and clay minerals. Element discrimination diagrams indicate that the samples are predominantly of hydrothermal and hydrogenetic-hydrothermal (i.e., mixed) origin, with most reflecting some influence of both. Rare earth element and Y (REY) profiles are distinguished by negative Ce and Y anomalies and positive Eu anomalies. Together, the samples form a continuum from the hydrogenetic to hydrothermal endmembers. Samples with the largest hydrogenetic component are friable with branching oxide/oxyhydroxide growth structures, contain mostly vernadite, and have the greatest concentrations of most metals (including the REY elements). Hydrothermal input produces denser, cemented deposits that contain more 10 Å and 7 Å manganate minerals and lower minor-metal contents. Calculated growth rates range from 6 mm to >190 m Ma−1. Average metal contents of Fe-Mn precipitates from the southern Mariana arc are low relative to hydrogenetic Fe-Mn crusts and hydrothermal Fe-Mn deposits from the northern Mariana arc and elsewhere, globally, and are therefore unlikely to be viable exploration targets.