Fe-Mn Crusts Research Articles

Magnetite magnetofossils record biogeochemical remanent magnetization in hydrogenetic ferromanganese crusts

AbstractRecords of natural remanent magnetization (NRM) and geomagnetic polarity reversals preserved within ferromanganese (Fe-Mn) crusts, together with the application of fine-resolution magnetostratigraphic analysis, have been successfully demonstrated. However, because Fe-bearing precipitates or minerals are thought to be either oxides and/or hydroxides precipitated from ambient oxic seawater or detrital minerals, the magnetic properties of the ferromagnetic minerals and the genetic mechanisms remain controversial; moreover, the origin of the NRM is unclear. Here, we show that nanometer-scale magnetite crystals found in Fe-Mn crusts from the Pacific Ocean and South China Sea are magnetosome fossils based on their narrow size range, chain arrangement, chemical purity, and crystallographic perfection, as indicated by transmission electron microscopy. Furthermore, our new data from rock magnetic and electron paramagnetic resonance analyses, combined with a previously reported micro-magnetostratigraphic sequence, indicate that magnetotactic bacteria and their post-mortem remains contribute to a biogeochemical remanent magnetization of Fe-Mn crusts. In addition, the results provide evidence for a previously unappreciated pathway for the biogeochemical cycling of iron in the deep ocean.

The early stages of ferromanganese ore genesis on the guyots of the Magellan Seamounts (The Pacific Ocean)

Fe-Mn crusts play an important role in marine mineral deposit research because of their widespread occurrence and high concentrations of valuable and rare metals. Most Fe-Mn crust deposits occur on the tens of thousands of seamounts found in the ocean. Data on the structure, texture, composition, age, and deposit characteristics will help define which factors are key for the creation of mineral accumulation and which combination of factors leads to the formation of potentially economic concentrations of metals. In this paper, we address the structure and characteristics of the oldest Fe-Mn crust stratigraphic sections (Late Cretaceous and Paleocene) collected from the Magellan seamounts. A complete section of the crusts on the Magellan Seamounts includes four layers, each 2—4 cm thickness: the Late Paleocene (?) Early Eocene layer I 1, the Mid Late Eocene layer I—2, the Miocene layer II and the Quaternary layer III. In some cases, the main CMC section is underlain by relict layers. The chemical and mineral composition of the layers was determined both by X-ray diffraction and precision methods; concentrations of the main ore components and phosphorus were determined by the methods of classical chemistry. The age of 12 samples was determined, the mineral composition of four, the chemical composition of 22 samples. The results of the relict layers analysis allow to distinguish two groups of samples among them. Among the relict layers, two age ranges are established — the second half of Late Cretaceous (R1) and the first half of Paleocene (R2). High concentrations of barium, lithium, gallium, and zinc suggest that hydrothermal sources could be the source of the material. But not through direct delivery, but via the phase of transfer of sea bottom water. Thus, the analysis of lithological and geochemical parameters and fossil fauna of foraminifera in the relict layers of the Magellan Seamounts ore section indicates two stages of their formation: Late Campan Maastricht and Early Middle Paleocene. The discreteness of the formation of relict layers in time once again proves that the sharply changing environmental conditions controlled the growth of the CMC ore section.

Platinum enrichment and phase associations in marine ferromanganese crusts and nodules based on a multi-method approach

Marine ferromanganese crusts and nodules host high concentrations of many economically interesting metals such as platinum (Pt), with Pt concentrations in the range of 44 to 3207 ppb in 182 analyzed samples from various locations in the global ocean. Different mechanisms have been proposed to explain this strong enrichment in the MnFe oxide phases compared to seawater, including reduction of seawater Pt(II) to Pt(0) or oxidation to Pt(IV) and surface adsorption on the Fe-oxyhydroxide or Mn-oxide phases. To shed more light on the process of Pt enrichment in nodules and crusts, we applied a multi-method approach including bulk analyses, statistics (correlations and Q-mode factor analyses), sequential leaching, sorption experiments, and XANES and EXAFS measurements. Our analyses lend new support to the heterogeneous oxidation/sorption mechanism advanced by Maeno et al. (2016) to explain the initial Pt(II) enrichment on FeMn crusts, with Pt oxidation only taking place on the Mn-oxide phase but not on the Fe-oxyhydroxide phase of crusts. Nodules show lower concentrations and a lesser association of Pt with the Mn phase as the latter is partly of diagenetic origin while the origin of Pt is hydrogenetic. Our results also indicate that sorption of Pt2+(aq) to FeO(OH) is possible if Pt2+(aq) exists under conditions typical of nodule and crust growth in the oceans. Platinum concentrations are consistently highest in phosphatized FeMn crusts where they appear to be related to the occurrence of a 10 Å manganate, so they are likely to be the best exploration target for Pt globally in marine FeMn crusts. However, further work is needed to understand the geochemical reactions that dominate mass transfer of Pt under conditions under which crusts are phosphatized and to identify the exact mineral type which hosts the Pt enrichment.The complex Pt geochemistry in marine FeMn crusts and nodules due to the different phase associations and enrichment processes on the different carrier phases make a straightforward interpretation of Pt data in specific samples difficult. Our findings suggest that only spatially resolved, species-specific methods available via synchrotron-based X-ray and modern electron-probe micro-analyzers (in development) could provide the combination of spatial resolution and species-specificity to determine which of several proposed mechanisms are actually responsible for Pt enrichment in FeMn crusts.

Microbial metabolisms in an abyssal ferromanganese crust from the Takuyo-Daigo Seamount as revealed by metagenomics.

Rocky outcrops covered with thick Fe and Mn oxide coatings, which are known as ferromanganese (Fe-Mn) crusts, are commonly found on slopes of aged seamounts in bathyal and abyssal zones. Although the presence of diverse microorganisms on these Fe-Mn crusts has been reported, little is known about their metabolism. Here, we report the metabolic potential of the microbial community in an abyssal crust collected in the Takuyo-Daigo Seamount, in the north-western Pacific. We performed shotgun metagenomic sequencing of the Fe-Mn crust, and detected putative genes involved in dissolution and precipitation of Fe and Mn, nitrification, sulfur oxidation, carbon fixation, and decomposition of organics in the metagenome. In addition, four metagenome-assembled genomes (MAGs) of abundant members in the microbial community were recovered from the metagenome. The MAGs were affiliated with Thaumarchaeota, Alphaproteobacteria, and Gammaproteobacteria, and were distantly related to previously reported genomes/MAGs of cultured and uncultured species. Putative genes involved in the above reactions were also found in the crust MAGs. Our results suggest that crust microbial communities play a role in biogeochemical cycling of C, N, S, Fe, and Mn, and imply that they contribute to the growth of Fe-Mn crusts.

Fractionation of Zn isotopes in terrestrial ferromanganese crusts and implications for tracing isotopically-heterogeneous metal sources

Zinc liberated from geological sources during surface weathering is the main source of Zn to the surface environment, and is ultimately sequestered by sinks such as Mn oxides and oxyhydroxides. This behaviour has been applied to geochemical mineral exploration studies and terrestrial ferromanganese crusts have been shown to have anomalously-high Zn/Mn ratios around known Zn-Pb ore deposits. This poses the question: if Zn isotopic compositions of these Fe-Mn crusts can also discriminate the underlying anomalous metal sources from the surrounding non-mineralized lithologies. Isotopic fractionation during fractionation onto Mn oxides in well-studied marine Fe-Mn crusts has shown that isotopically-heavy Zn is preferentially-adsorbed in that environment, which has a comparatively isotopically-homogeneous Zn reservoir. However, isotopic fractionation behaviour in terrestrial Fe-Mn crusts that may have more heterogeneous Zn sources is unknown. Here, we present the first Zn and Pb isotope study of terrestrial Fe-Mn crusts to test if Zn isotopic compositions can be used to delineate isotopically-different lithological metal sources.Our data from Fe-Mn crusts that formed overlying isotopically-variable lithologies; hydrothermal polymetallic (Zn-Pb) deposits, and non-mineralized Archean rocks show that, while adsorption of Zn onto Mn oxyhydroxides in terrestrial Fe-Mn crusts generally favours isotopically-heavy Zn, the magnitudes of isotopic fractionation varies in Fe-Mn crusts overlying mineralized and non-mineralized areas. Fractionation factors (Δ66/64Zncrust – source up to 0.54‰) in crusts overlying non-mineralized lithological sources with a δ66Zn value of 0.30‰ are comparable with the predicted equilibrium fractionation onto birnessite. Conversely, magnitudes of Δ66/64Zncrust – source up to 0.37‰ in crusts overlying isotopically-light Zn-Pb mineralization, that has a mean δ66Zn value of -0.15‰, are much lighter than non-mineralized counterparts with similar Zn/Mn ratios. This difference may be explained by changes in hydrogeochemical complexation of Zn, with localized chloro-complexes that carry isotopically-light Zn from the isotopically-light mineralization.These findings demonstrate that different lithological Zn sources that are isotopically-variable give rise to isotopic heterogeneity in the surface Zn reservoir. This can be reflected by differing fractionation patterns following adsorption onto Mn oxyhydroxides in terrestrial Fe-Mn crusts that overly different Zn sources. Furthermore, Pb isotope compositions of the Fe-Mn crusts also closely reflect that of the underlying geology. This highlights that the use of Zn isotopes may be a valuable means of vectoring towards anomalous metal sources such as ore deposits.

Distribution of and hydrographic controls on ferromanganese crusts: Tropic Seamount, Atlantic

Hydrogenetic ferromanganese crusts are likely to be exploited as resources for critical metals in the near future, yet the processes controlling where and how they grow are poorly understood. Using detailed mapping of seafloor outcrop and well constrained hydrographic modelling alongside scanning electron microscope imagery of samples from the Tropic Seamount, a star-shaped guyot located in the Tropical East Atlantic, we investigate the relationship between currents, ferromanganese crustal texture and the locations and intensity of crustal erosion. Here, we report the distribution of FeMn crusts and explore factors controlling their growth and erosion. We find that just over 35% of the summit plateau of the guyot exposes some form of ferromanganese crust mineralisation, with the rest variably covered by plains of mobile sediment and slim cliff exposures of carbonate. The steep flanks of the guyot largely expose ferromanganese crust both in situ and as debris flows. The strongest currents are located on the upper flanks of the guyot, the central part of its eastern limb, and across the summit plateau. Three categories of surface morphologies are identified; from pristine botryoidal surfaces to flat areas that have been completely polished by the erosive action of currents and sediment. The relationship between the outcrop of crusts, their erosional states and the hydrographic current regime to which they are exposed is complicated. There is a general correlation between the degree of erosion and location across the seamount, with the least eroded being found on the flanks below 2000 m water depth and the most heavily eroded crusts largely restricted to the summit area. Furthermore, the pristine samples all reside in areas that rarely experience current magnitudes over 0.2 m/s, suggesting that above this the currents have the ability to erode ferromanganese crust. However, there is a strong overlap between the measured current magnitudes at the locations of partially and completely eroded crusts, as well as partial overlap with the current magnitudes measured at pristine crust locations. This complexity is likely due to the presence of cliffs and plateaus increasing current magnitudes and turbidity at a scale smaller than the model resolution.

Vanadium isotopic fractionation during the formation of marine ferromanganese crusts and nodules

Vanadium (V) isotopes can be significantly fractionated during the delivery, cycling, and burial to the ocean. The utilization of V isotopes might provide a useful geochemical tracer for the evolution of the oceans. This study provides the first detailed investigation of V isotopes in ferromanganese (Fe-Mn) crusts and nodules, which are widely distributed in the modern oxic ocean. Our results show a large variability of V isotope compositions in Fe-Mn crusts and nodules with a δ51V range from −0.89 to −1.65‰, much greater than current analytical uncertainty (±0.10‰, 2 SD). Therein, the most recent layers of hydrogenetic Fe-Mn crust and hydrogenous nodules have a narrower V isotope range (−0.89‰ to −1.25‰), with no correlation to their collection location or water depth. Thus, our results suggest a relatively homogeneous V isotope composition of marine hydrogenetic Fe-Mn crusts and nodules, with an average δ51V of −1.05 ± 0.16‰ (2 SD). The hydrogenetic Fe-Mn crusts and nodules are depleted in 51V compared with the recently reported seawater value (0.2 ± 0.15‰) by ∼1.2 ± 0.2‰ (2 SD). This can be explained by isotope fractionation during the adsorption of V onto Fe-Mn oxyhydroxides. The various δ51V in Fe-Mn nodules (−0.98‰ to −1.65‰) might be caused by diagenetic precipitation of V from pore fluids with lighter isotope composition compared to seawater, thus driving a negative V isotope shift recorded in Fe-Mn nodules with a diagenetic imprint. Additionally, a depth profiles across an Fe-Mn crusts reveals systematic changes in δ51V from −1.04 ± 0.13‰ (2 SD) in upper layers to 1.32 ± 0.06‰ (2 SD) in lower layers representing the relatively older deposition. The observed δ51V depth profiles might record the changes of V isotope composition of seawater. The proposed temporal variations of δ51V in seawater could be controlled by the isotope fractionation and fluxes of various V sources and sinks to the ocean that is likely related to the global redox state of the oceans. Another potential interpretation of this isotopic shift in the crusts could be related to the modification of the primary V isotope signature due to diagenetic remobilization and reorganization. However, there is no obvious evidence that definitively documents a diagenetic control or signal. This study highlights the burial of V with Fe-Mn oxyhydroxide as an important control on the V isotope composition of seawater, and the potential application of Fe-Mn crusts to track the temporal V isotope variations of seawater.

Nickel exchange between aqueous Ni(II) and deep-sea ferromanganese nodules and crusts

Deep-sea ferromanganese (Fe-Mn) nodules and crusts are rich in traditional and non-traditional metals with both current and emerging economic value. Mn(III,IV) oxides (e.g., phyllomanganates) are important host phases for these metals (e.g., Ni), which are structurally incorporated during nodule and Fe-Mn crust formation. Recrystallization of phyllomanganates can be catalyzed by aqueous Mn(II) (Mn(II)aq) during (bio)geochemical Mn redox cycling. The fate of structurally incorporated metals during such recrystallization of Mn(III,IV) oxides remains, however, poorly constrained. Here, we use a 62Ni isotope tracer to determine the exchangeability of dissolved Ni with structurally incorporated Ni in two deep-sea Fe-Mn nodules and one Fe-Mn crust. Ni exchange between solid and solution was investigated during reactions in 1 mM Mn(II)aq and in Mn(II)-free solutions under variable pH conditions (pH 5.5 and 7.5) over time. Sample characterization shows that all samples are of hydrogenetic or mixed hydrogenetic-diagenetic origin and Ni is preferentially associated with the phyllomanganates. Our Ni exchange experiments reveal that in some samples up to 25% of incorporated Ni is exchangeable with the fluid after 14 days. The prevalent reaction pathways exhibit pH-dependent behavior during phyllomanganate recrystallization and differ between sample types, with Mn(II)aq enhancing Ni exchange in the Fe-Mn crust-fluid system and Ni exchange being independent of Mn(II)aq concentrations in the Fe-Mn nodule-fluid systems. The exchangeability of structurally-incorporated Ni in Fe-Mn nodules and crusts indicates a labile behavior that potentially makes it available for biogeochemical processes in the marine environment.

Hydrogenetic, Diagenetic and Hydrothermal Processes Forming Ferromanganese Crusts in the Canary Island Seamounts and Their Influence in the Metal Recovery Rate with Hydrometallurgical Methods

Four pure hydrogenetic, mixed hydrogenetic-diagenetic and hydrogenetic-hydrothermal Fe-Mn Crusts from the Canary Islands Seamount Province have been studied by Micro X-Ray Diffraction, Raman and Fourier-transform infrared spectroscopy together with high resolution Electron Probe Micro Analyzer and Laser Ablation Inductively Coupled Plasma Mass Spectrometry in order to find the correlation of mineralogy and geochemistry with the three genetic processes and their influence in the metal recovery rate using an hydrometallurgical method. The main mineralogy and geochemistry affect the contents of the different critical metals, diagenetic influenced crusts show high Ni and Cu (up to 6 and 2 wt. %, respectively) (and less Co and REY) enriched in very bright laminae. Hydrogenetic crusts on the contrary show High Co and REY (up to 1 and 0.5 wt. %) with also high contents of Ni, Mo and V (average 2500, 600 and 1300 μg/g). Finally, the hydrothermal microlayers from crust 107-11H show their enrichment in Fe (up to 50 wt. %) and depletion in almost all the critical elements. One hydrometallurgical method has been used in Canary Islands Seamount Province crusts in order to quantify the recovery rate of valuable elements in all the studied crusts except the 107-11H, whose hydrothermal critical metals’ poor lamina were too thin to separate from the whole crust. Digestion treatment with hydrochloric acid and ethanol show a high recovery rate for Mn (between 75% and 81%) with respect to Fe (49% to 58%). The total recovery rate on valuable elements (Co, Ni, Cu, V, Mo and rare earth elements plus yttrium (REY)) for the studied crusts range between 67 and 92% with the best results for Co, Ni and V (up to 80%). The genetic process and the associated mineralogy seem to influence the recovery rate. Mixed diagenetic/hydrogenetic crust show the lower recovery rate for Mn (75%) and Ni (52.5%) both enriched in diagenetic minerals (respectively up to 40 wt. % and up to 6 wt. %). On the other hand, the presence of high contents of undigested Fe minerals (i.e., Mn-feroxyhyte) in hydrogenetic crusts give back low recovery rate for Co (63%) and Mo (42%). Finally, REY as by-product elements, are enriched in the hydrometallurgical solution with a recovery rate of 70–90% for all the studied crusts.

New data concerning the ferromanganese crusts of the Mendeleev rise (Arctic Ocean)

The results of study of Fe-Mn crusts from the Mendeleev Rise in the Arctic Ocean in 2016 sampled with manipulators from a submarine are presented. Concentrations of Fe, Mn, Co, Ni, Zn, Cu, Cr and Pb are measured in collected Fe-Mn crusts. The variations in the content of these elements are determined by the chemical properties of the elements, the height of the underwater topography, the composition of the underlying substrate and the presence of bottom silt.

The Geochemistry and Bioturbation of Clay Sediments Associated with Amalgamated Crusts at the Gagua Ridge

Based on the analysis of geochemical and mineralogical compositions, deep sea clay sediment characteristics and their material sources were examined in the eastern flank of the Gagua Ridge. The mineralogy mainly consists of detrital clay minerals, quartz, and authigenic phillipsite. There is scarce biogenic debris (siliceous or calcareous). The consolidated sediments are more enriched in Si, Al, K, Na, Li, Sc, Cr, Rb, and Cs than the associated crusts and nodules. The unmixed sediment samples were mainlycontributed by Asian eolian dust. The onset of the outer Fe-Mn crust growth nearly coincides with the Central Asia aridification event at ~3.5 Ma, which resulted in an abrupt increase in eolian flux of Asian dust. Intensified surface primary productivity is assumed to bring more metals to deep waters, and eventually facilitate the outer Fe-Mn crust formation. Authigenic phillipsite may come from the alteration of local basic volcanic glasses and cause excess Al, high Al/Ti, and low Si/Al ratios. However, phillipsites hardly affect the abundance of rare earth elements (REEs) and their patterns. In addition, the investigation of two kinds of burrows inside the consolidated sediments reveals that the inner nodules of the amalgamated crusts may remain on the oxic sediment surface, due to frequent benthic activities.

Improving confidence in ferromanganese crust age models: A composite geochemical approach

Accurate age models for marine ferromanganese (Fe-Mn) crusts are essential to understand paleoceanographic changes and variations in local environmental factors affecting crust growth rate and their lateral continuity. However, no absolute method exists for dating these deposits beyond the age of 10 Myr, which requires the combination of a number of approaches. Here, we present a composite age model for a 15 cm thick Fe-Mn crust sample obtained by unique core drilling using a remotely operated vehicle at a water depth of 1130 m, on the summit of Tropic Seamount, in the north-east Atlantic. The age model is based on cross-validation of laser-ablation U-Pb dating, Co-chronometry and Os isotopes. These enable robust calibration of the age-depth model using the Bayesian statistical modelling of Markov Chain Monte Carlo (MCMC) simulations. The results show that this Fe-Mn crust commenced growth in the Late Cretaceous between 73 and 77 Ma, and grew at a rate between 1 and 24 mm/Myr, averaging 4 mm/Myr. The phosphatised carbonate substrate, capping Tropic Seamount and underlying most of the Fe-Mn crusts, yields a U-Pb age of 84 ± 4 Myr, and provides the upper age limit for the model. Less radiogenic excursions of 188Os/187Os in the vertical profile through the crust permit the identification of key inflection points in the Os isotope seawater curve at the Eocene-Oligocene and Cretaceous-Paleogene transitions. Growth rates estimated from the empirical Co-chronometer are combined with the age envelope defined by the Os data and used to validate the MCMC simulations. The model identifies five hiatuses that occurred during the Pliocene (2.5 ± 1.9–5.3 ± 1.7 Ma), Early Miocene (16 ± 1–27 ± 2 Ma), Oligocene (29 ± 2–32 ± 1 Ma), Eocene (41 ± 2–52 ± 0.6 Ma), and the Late Paleocene (55 ± 1–59 ± 1.4 Ma). A major phosphatisation event affecting the Fe-Mn core can be dated to the Late Eocene (38 ± 1.2 Ma), which coincides with a recorded change in the global oceanic system, from warm and sluggish circulation to cold and vigorous thermohaline-driven meridional overturn at the onset of Antarctic glaciation.

Ferro-manganese nodules from the Kara Sea: Mineralogy, geochemistry and genesis

Sixteen ferromanganese nodules collected from a large area in the Kara Sea were studied by optical and scanning electron microscopy, X-ray powder diffraction, X-ray fluorescence analysis, inductively coupled plasma mass spectrometry, and electron microprobe analysis. The geological setting, chemical and mineralogical features suggest that Fe-Mn concretions of the Kara Sea mainly were formed by diagenetic process, while hydrogenetic process is secondary if present. The studied nodules are characterized by the alternation of Mn- and Fe-P-enriched laminae. The Fe-P-rich laminae are almost completely composed of an amorphous phase under X-ray. We consider that the phosphatization process did not occur during formation of the Fe-Mn crusts and nodules from the Kara Sea. Several Mn-bearing phases are present in the studied samples (buserite-1, birnessite and X-ray amorphous phase). All studied nodules have similar contents of the main chemical elements. In all cases, the samples are depleted in light REE, enriched in heavy REE, and have negative Ce and Y anomalies. Our data indicate that diagenetic crusts and nodules differ significantly in terms of chemical composition from other types of Fe-Mn concretions. Based on their Co, Ni, Cu, Ce, Zr and Mo contents, we could differentiate diagenetic concretions from hydrogenetic and hydrothermal (both high and low-temperature) concretions.

Distribution of Rare Earth Elements plus Yttrium among Major Mineral Phases of Marine Fe–Mn Crusts from the South China Sea and Western Pacific Ocean: A Comparative Study

Marine hydrogenetic Fe–Mn crusts on seamounts are known as potential mineral resources of rare earth elements plus yttrium (REY). In recent years, increasing numbers of deposits of Fe–Mn crusts and nodules were discovered in the South China Sea (SCS), yet the enrichment mechanism of REY is yet to be sufficiently addressed. In this study, hydrogenetic Fe–Mn crusts from the South China Sea (SCS) and the Western Pacific Ocean (WPO) were comparatively studied with mineralogy and geochemistry. In addition, we used an in situ REY distribution mapping method, implementing laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and a sequential leaching procedure to investigate the partitioning behavior of REY in the Fe–Mn crusts. The typical Fe–Mn crusts from SCS were mainly composed of quartz, calcite, vernadite (δ-MnO2), and amorphous Fe oxides/hydroxides (FeOOH). The Fe–Mn crusts from the Central SCS Basin and the WPO contained quartz, δ-MnO2, FeOOH, todorokite, and phillipsite. Furthermore, geochemical analysis indicated that the typical SCS crusts had a higher growth rate and lower REY concentrations. The LA-ICP-MS mapping results showed that the δ-MnO2 and FeOOH dominated the occurrence phases of REY in the SCS crusts. Four mineral phases (i.e., easily exchangeable and carbonate, Mn-oxide, amorphous FeOOH, and residual aluminosilicates) in these Fe–Mn crusts were separated by a sequential leaching procedure. In the SCS and WPO crusts, the majority of total REY (ΣREY) was distributed in the Mn-oxide and amorphous FeOOH phases. The post-Archean Australian shale-normalized REY patterns showed that light REY (LREY) and heavy REY (HREY) were preferentially adsorbed onto δ-MnO2 and FeOOH, respectively. It is noteworthy that ~27% of ΣREY was associated with the residual aluminosilicates phase of the WPO crusts. The La/Al ratios in the aluminosilicates phase of the typical SCS crusts were the values of the upper crust. We conclude that large amounts of terrigenous materials dilute the abundance of REY in the SCS crusts. In addition, the growth rates of Fe–Mn crusts have a negative correlation with the FeOOH-bound and aluminosilicate-bound REY. As a result of the fast growth rates, the SCS crusts contain relatively low concentrations of REY.

Petrological, geochemical, and hydrothermal characteristics of Ordovician cherts in the southeastern Tarim Basin, NW China, and constraints on the origin of cherts and Permian tectonic evolution

Cherts occur extensively as stratal wedges embedded in the Middle-Lower Ordovician carbonate successions in the circum-Tangguzibasi areas, south-east Tarim Basin, where thick (up to 2 km), dark grey well-stratified limestones deposited in shelf and platform environments are prominent. Based on detailed core and thin section observations, with petrographic, microthermometric, and geochemical data for the Middle-Lower Ordovician cherts in some key wells, two lithotypes of chert are identified across the marginal zone in the western Tangguzibasi areas: replacement and filled cherts. Replacement chert is characterized by granular microcrystalline quartz or radially fibrous chalcedony and commonly retains the pre-existing dolomite rhombs and elliptical ooids floating in these cherts. Filled chert, the most common type, is generally rimmed by fine-gained euhedral quartz that grades abruptly into vein-cavity-fill mega-quartz. Compositionally, these chert deposits are strongly enriched in Cr, Zn, Ba and U and generally show apparent right-convex REE patterns with low ∑REE values (avg. 4.24 ppm), weak negative Ce anomalies (Ce/Ce*: avg. 0.93) and low to intermediate positive Eu anomalies (Eu/Eu*: avg. 1.19), particularly the replacement cherts. Microthermometry of fluid inclusions from coarse quartz reveals high homogenization temperatures from 135 to 195 °C for trapped primary fluids with high salinities (11.0–18.9 wt%). These siliceous deposits also have low La/Ho and Th/U, relatively high Y/Ho and Ba/Sr, high Ce/Ce* (∼1.06), intermediate to high (Lu/La)N (0.5–1.0) and low (La/Ce)N (mostly 1.31–1.45). All these data suggest that the cherts formed under the strong influence of hydrothermal fluids originating from detritus-poor hydrogenetic Fe-Mn crusts at deep continental margins. Extensive silicification occurred after intense dolomitization and calcitization, while fracturing/faulting were greatly intensified in the host carbonates, induced by intense extension because of the uplift force of the magma cushion under oblique collision during the Early Permian. Most silica could have directly precipitated from hydrothermal fluids to chalcedony fibres around voids and vein cherts along fracture conduits. These petrological and geochemical data also provide a useful clue to better understand the extensive silica precipitation, intense tectono-hydrothermal activity and significant lithospheric fracturing events as a result of the closure of the South Tienshan and Palaeo-Tethys Oceans during the Early-Middle Permian.

Ferromanganese Crusts in the South Atlantic Ocean: Compositional Evolution and Specific Features of Ore Formation

Abstract—Layer-by-layer chemical and mineralogical analysis was carried out to consider the evolution of the Fe–Mn crusts in different tectonic settings in the South Atlantic Ocean. The ferromanganese crust in the rift valley of the Mid-Atlantic Ridge was formed owing to the hydrothermal–hydrogenetic supply. The formation of crust in the Transform Fracture zones of the Angola Basin was related to the halmyrolysis of magmatic rocks and subsequent hydrogenetic supply, which affected the proportions of goethite and vernadite, the Mn/Fe ratio, and Ce anomaly, as well as the content of some elements (Co, Cu, Al, REE). It is revealed that the Fe–Mn crusts taken in the abyssal part and on the seamount top in the Cape Basin are hydrogenetic in origin and consist of vernadite significantly enriched in Co, Ce, Tl, and Pt. The relationship of trace elements with manganese minerals is estimated by leaching of crusts by reagents, which either completely dissolve manganese oxyhydroxides (1 M NH2OH · HCl + 25%CH3COOH) or no (3.5 N H2SO4). It was shown that 100% Co and only 28% Ce are incorporated in the manganese minerals, while REE, Y, Sr, and U form a sorption complex of oxhyhydroxides. To estimate the chemical peculiarities of the hydrogenetic Fe–Mn crusts of the Atlantic Ocean, they were compared with the crusts of the Pacific Ocean. It is shown that the crusts of the Pacific Ocean have higher manganese and cobalt contents. The additional manganese could be derived from a hydrothermal source.

Helium and Argon Isotopes in the Fe-Mn Polymetallic Crusts and Nodules from the South China Sea: Constraints on Their Genetic Sources and Origins

In this study, the He and Ar isotope compositions were measured for the Fe-Mn polymetallic crusts and nodules from the South China Sea (SCS), using the high temperature bulk melting method and noble gases isotope mass spectrometry. The He and Ar of the SCS crusts/nodules exist mainly in the Fe-Mn mineral crystal lattice and terrigenous clastic mineral particles. The results show that the 3He concentrations and R/RA values of the SCS crusts are generally higher than those of the SCS nodules, while 4He and 40Ar concentrations of the SCS crusts are lower than those of the SCS nodules. Comparison with the Pacific crusts and nodules, the SCS Fe-Mn crusts/nodules have lower 3He concentrations and 3He/4He ratios (R/RA, 0.19 to 1.08) than those of the Pacific Fe-Mn crusts/nodules, while the 40Ar/36Ar ratios of the SCS samples are significantly higher than those of the Pacific counterparts. The relatively low 3He/4He ratios and high 40Ar concentrations in the SCS samples are likely caused by terrigenous detrital input with high radiogenic 4He and 40Ar contents. The SCS crusts and nodules have shorter growth periods, implying that in situ post-formation radiogenic 3He, 4He and 40Ar produced by decay of U, Th and K have no effect on their isotope compositions. Thus, the SCS crusts/nodules inherited the noble gases characteristics of their sources. Helium and Ar isotope compositions in the SCS Fe-Mn crusts and nodules reflect the product of an equilibrium mixture between air-saturated seawater and radiogenic components during their growth, while the partial 3He excess in some SCS samples may represent a little mantle-derived origin. The different He and Ar isotope compositions of the Fe-Mn crusts and nodules between the South China Sea and the Pacific Ocean are due to their different sources and genetic processes. The characteristics of He and Ar isotope compositions in the SCS polymetallic crusts and nodules are similar to the properties of hydrogenetic Fe-Mn oxide/hydroxide precipitates, which reflects mainly the product of an equilibrium mixture between air-saturated seawater and radiogenic components.

The Rare Earth Elements Potential of Greek Bauxite Active Mines in the Light of a Sustainable REE Demand

More recent data of Greek bauxites from the Parnassos-Ghiona active mines prove that rare earth elements (REEs) occur mainly in the form of authigenic/diagenetic LREE3+ carbonate and phosphate minerals (bastnasite/parisite group and florencite). Bulk geochemistry of representative samples, from underground mines and open pits, showed an increased content in LREE (ΣLREE—from La to Gd—varying between 106 and 913 ppm; avg. ΣLREE = 321 ppm; n = 17), and lower HREE (ΣHREE—from Tb to Lu including Y—varying between 45 and 179 ppm; avg. ΣHREE = 95; n = 17). The overall REE concentration (ΣREE + Y+Sc) varies from 192 to 1109 ppm (avg. 463 ppm; n = 17). The most abundant REE is Ce (min: 67 ppm; max: 655 ppm; avg. 193 ppm; n = 17), exhibiting in general a positive geochemical anomaly (avg. Ce/Ce* -CeA-: 2.6), identical to the case of marine Fe–Mn-crust and terrestrial desert vanish, implying also that Ce4+ may exist either in REE-oxides and/or epigenetically sorbed in Fe-oxides. On the other hand, Nd content, which is more interesting for the industry, is lower (avg. 41 ppm; n = 17). The concentration of REEs is much higher in Fe-rich (red) bauxite, compared to Fe-depleted (white) bauxite (avg. ΣREE + Y+Sc = 569 ppm and 268 ppm, respectively). The new data presented herein show a rather lower REE potential of Parnassos-Ghiona bauxites, compared to previous literature, but similar values compared to karst-type bauxites of the globe. Although their REE concentration is higher compared to that of various geochemical reference materials (i.e., positive REE geochemical anomalies in comparison with chondrites, UCC, PAAS, NASC, and ES), it is vitally lower compared to REE resources being mined, such as REE–Fe–Nb–Th deposits. A trend similar to REE geochemical trend also stands for most of the trace elements that are present in Greek bauxites—mainly HFSE—except for LILE. Besides, Greek bauxite metallurgical residue’s (red mud) REE content seems to be remarkably increased by almost two times compared to that of the Parnassos-Ghiona bauxite parent material. Scandium is another critical element. In the studied bauxites, it varies from 29 to 73 ppm (avg. 47 ppm; n = 17); it is typical for the Mediterranean and EU bauxites and laterites, but much lower compared to the exploitable Australian laterites. The Fe-rich samples contain higher concentrations of Sc compared to Fe-depleted (avg. 54 and 33 ppm, respectively). This means that common red Greek bauxite is rather more exploitable, with regard to Sc (and the rest REE), but not the white one (which is of high quality in terms of Al). Bulk geochemistry indicates that Sc is correlated to Fe but not to Zr, while microscale observations demonstrated the presence of fine-grained scandian-zircons. This is in line with a very recent study proving that Sc is mainly present in Fe-oxide minerals (mainly hematite and goethite) and zircons. Bulk geochemical Fe–Pb and Fe–As positive correlations are also verified among the associated trace elements. Finally, the investigation of the REE vertical distribution in a characteristic deposit of the B3 horizon (i.e., Pera Lakkos mine case study), showed that the REE concentration is increased in the Fe-rich domain (lying above the footwall limestone), as well as in the coal layer interstratified between the Fe-depleted domain and the hanging wall limestone. However, it is revealed that the Ce/Ce* (CeA) is increased in the coal layer and is raised to the uppermost Fe-depleted domain, but not the lowermost Fe-rich bauxite domain. This might be attributed to the Ce3+ ↔ Ce4+ and the LREE re-mobilization during the supergene/epigenetic processes.

Deep-sea mining: Interdisciplinary research on potential environmental, legal, economic, and societal implications.

Deep-sea mining refers to the retrieval of marine mineral resources such as Mn nodules, FeMn crusts, and seafloor massive sulfide deposits, which contain a variety of metals that serve as crucial raw materials for a range of applications, from electronic devices to renewable energy technologies to construction materials. With the intent of decreasing dependence on imports, supporting the economy, and potentially even overcoming the environmental problems related to conventional terrestrial mining, a number of public and private institutions have rediscovered their interest in exploring the prospects of deep-sea mining, which had been deemed economically and technically unfeasible in the early 1980s. To date, many national and international research projects are grappling to understand the economic environmental, social, and legal implications of potential commercial deep-sea mining operations: a challenging endeavor due to the complexity of direct impacts and spillover effects. In this paper, we present a comprehensive overview of the current state of knowledge in the aforementioned fields as well as a comparison of the impacts associated with conventional terrestrial mining. Furthermore, we identify knowledge gaps that should be urgently addressed to ensure that the world at large benefits from safe, efficient, and environmentally sound mining procedures. We conclude by highlighting the need for interdisciplinary research and international cooperation. Integr Environ Assess Manag 2018;14:672-691. © 2018 SETAC.

Marine ferromanganese oxide: A potentially important sink of light chromium isotopes?

Marine ferromanganese oxides are widely distributed in oxic marine sediments and are a sink for a range of metal elements derived from seawater. Their potential as a sink of chromium (Cr) isotopes has yet to be investigated and may have important implications for our understanding of the Cr-isotope system. In this study, we reported Cr-isotope data of modern oceanic Fe–Mn crusts for the first time. The δ53Cr values of the surface scrapings of the Fe–Mn crusts from the central North Pacific seamounts, range from −0.85 to −0.15‰, with an average of −0.42 ± 0.34‰ (2σ, n = 11), which are lower than published data from Pacific seawater (0.53–1.43‰ with an average of 0.79‰). These results reveal preferential removal of light Cr isotopes from seawater into Fe–Mn crusts, with an isotopic fractionation (△53Cr = δ53CrFe-Mn crust − δ53Crseawater) estimated from −2.3 to −0.7‰, averaging at ca. −1.2‰. We suggest that such significant negative isotopic fractionation may have been reached through adsorption of dissolved Cr (VI) oxyanions in the oxic water column onto the surface of Mn-oxide phases during the precipitation of Fe–Mn crusts. However, our modelling results imply that the deposition of Mn-oxide sediments would make insignificant contribution to elevation of the global seawater average δ53Cr value.