Ferromanganese Crusts Research Articles

The structure and crystal chemistry of vernadite in ferromanganese crusts.

The structure and crystal chemistry of vernadite in ferromanganese crusts from the Magellan Seamount in the north-west Pacific Ocean have been investigated using synchrotron X-ray diffraction (XRD), X-ray pair distribution function (PDF) and high-resolution transmission electron microscopy (TEM). XRD patterns of vernadite mainly show two strong diffraction peaks at 2.42-2.43 Å and 1.41 Å without or with a broad (001) diffraction peak, indicating thin layer nanophases along the c-direction. TEM images show flat and curved sheet-like nanocrystals with (001) layer thickness of ∼7.2 Å and ∼9.6 Å, and their interstratified structure. PDF patterns of the vernadite are similar to those from synthetic δ-MnO2 and defective birnessite, suggesting a phyllomanganate framework. Combined XRD/PDF patterns suggest that vernadite in the outer part is associated with a higher density interlayer species at triple-edge sharing sites. The proportion of the 10 Å phase increases from the outer (young) part to the inner (old) part of the Mn crusts due to aging and sorption of Mn, Co and Ni from ambient seawater. This study suggests that this combined method of synchrotron radiation XRD/PDF and high-resolution TEM is a powerful tool to determine atomic structures of poorly crystallized nano-minerals. The mixture model of vernadite structure will help to understand the partitioning and distribution of trace elements in the ferromanganese crusts.

New Data on Ferromanganese Crusts of Mendeleev Rise, Arctic Ocean

This paper considers the compositions of ferromanganese crusts and nodules (FMCNs) growing on bedrock of Mendeleev Rise, collected in 2016 using a research submarine. The concentrations of Fe, Mn, Co, Ni, Zn, Cu, Cr, and Pb are measured in the collected FMCNs. Analysis of the variations in the contents of ore elements in the studied FMCNs shows that these changes are determined by the following factors: (1) the relative heights of submarine relief (higher relief is characterized by higher concentrations of Co and Ni); (2) substrate character (if it is a tuff, then its alteration provides additional Mn and Zn enrichment of FMCNs); (3) participation of bottom ooze (in the case of extensive contact of substrate fragments with ooze under the influence of pore waters, FMCNs become enriched in elements such as Mn, Co, and Ni); (4) chemical properties of elements (Mn hydroxides concentrate Co and Ni, whereas Fe hydroxides host Cu and Zn).

Characterization and Mapping of a Deep-Sea Sponge Ground on the Tropic Seamount (Northeast Tropical Atlantic): Implications for Spatial Management in the High Seas

Ferromanganese crusts occurring on seamounts are a potential resource for rare earth elements that are critical for low-carbon technologies. Seamounts, however, host vulnerable marine ecosystems (VMEs), which means that spatial management is needed to address potential conflicts between mineral extraction and the conservation of deep-sea biodiversity. Exploration of the Tropic Seamount, located in an Area Beyond National Jurisdiction (ABNJ) in the subtropical North Atlantic, revealed large amounts of rare earth elements, as well as numerous VMEs, including high-density octocoral gardens, Solenosmilia variabilis patch reefs, xenophyophores, crinoid fields and deep-sea sponge grounds. This study focuses on the extensive monospecific grounds of the hexactinellid sponge Poliopogon amadou (Thomson, 1878). Deep-sea sponge grounds provide structurally complex habitat, augmenting local biodiversity. To understand the potential extent of these sponge grounds and inform spatial management, we produced the first ensemble species distribution model and local habitat suitability maps for P. amadou in the Atlantic employing Maximum Entropy (Maxent), General Additive Models (GAMs), and Random Forest (RF). The main factors driving the distribution of the sponge were depth and maximum current speed. The sponge grounds occurred in a marked bathymetric belt (2,500 – 3,000 m) within the upper North Atlantic Deep Water mass (2.5∘C, 34.7 psu, O2 6.7–7 mg ml-1), with a preference for areas bathed by moderately strong currents (0.2 – 0.4 ms-1). GAMs, Maxent and RF showed similar performance in terms of evaluation statistics but a different prediction, with RF showing the highest differences. This algorithm only retained depth and maximum currents whereas GAM and Maxent included bathymetric position index, slope, aspect and backscatter. In these latter two models, P. amadou showed a preference for high backscatter values and areas slightly elevated, flat or with gentle slopes and with a NE orientation. The lack of significant differences in model performance permitted to merge all predictions using an ensemble model approach. Our results contribute toward understanding the environmental drivers and biogeography of the species in the Atlantic. Furthermore, we present a case toward designating the Tropic Seamount as an Ecologically or Biologically Significant marine Area (EBSA) as a contribution to address biodiversity conservation in ABNJs.

Enrichment mechanisms of antimony and arsenic in marine ferromanganese oxides: Insights from the structural similarity

Marine ferromanganese crusts and nodules as potential mineral deposits have received increasing attention. However, much less knowledge is available concerning the incorporation and enrichment mechanisms for antimony (Sb) and arsenic (As) in marine ferromanganese oxides. In this study, the surface complexations of Sb(V) and As(V) on synthetic ferrihydrite and Mn oxides (δ-MnO2 and birnessite) were investigated by a combination of adsorption experiments, extended X-ray absorption fine structure (EXAFS) analyses, and quantum chemical calculations. The speciation, distribution, and local structure of Sb and As in different types of natural marine ferromanganese oxides were determined by X-ray absorption near edge structure (XANES) and EXAFS analyses to reveal the enrichment mechanisms for the two elements in ferromanganese oxides at the molecular level. To the best of our knowledge, the Sb EXAFS analyses for different types of marine ferromanganese oxides are herein reported for the first time. Results showed that Sb(V) is preferentially adsorbed on Mn oxides through energetically favorable bidentate–mononuclear complexation because of the structural similarity between the octahedron SbV(OH)6− and MnO6 unit, although bidentate–binuclear (corner-sharing) and bidentate–mononuclear (edge-sharing) complexes can be formed on ferrihydrite and Mn oxides for the adsorption of Sb(V). By contrast, tetrahedral AsVO43− is mostly adsorbed on ferrihydrite and Mn oxides with the formation of bidentate–binuclear complexes. In natural marine ferromanganese oxides, Sb and As can be retained by Fe and Mn (oxyhydr)oxide components, and the disparate distribution of the two elements to Mn oxides may largely depend on the Mn/Fe ratio and constituent minerals. The larger enrichment factor of Sb than that of As in marine ferromanganese oxides may result from their preferential attachment modes onto the Fe and Mn phases and different inhibition effects from coexisting anions in seawater. Compared with As, a part of Sb may be strongly associated with the lateral sites in Mn oxides via the formation of bidentate edge-sharing complexes, with which anions such as sulfate in seawater do not significantly compete. The findings from this study provide the molecular-scale insights into the enrichment processes and mechanisms of Sb and As in marine ferromanganese oxides. Our study also helps elucidate the incorporation mechanisms and geochemical behaviors of other oxyanions in marine and surface environments.

Archaeal community structure in sediments from a seamount in the Mariana Volcanic Arc

Seamounts are subsurface mountains in the ocean. Examination of the abundance and distribution of Archaea in seamount ecosystems may provide a better understanding of their ecological functions. Most studies of marine archaeal assemblages in seamount area have focused on hydrothermal vents or ferromanganese crusts. We investigated the archaeal communities from a seamount of the Mariana Volcanic Arc, in the tropical western Pacific Ocean by using high-throughput sequencing. Thaumarchaeota was dominant in the sediments of all sample stations. Community diversity and species richness were greatest at stations near the top of the seamount, and lowest at the deepest station. One sample station on the steep southeast slope that faced the Yap-Mariana trench had a unique composition of Archaea. In summary, depth has an important influence on archaeal community structure, and the geographic properties and sediment characteristics may explain the unique distribution patterns of Archaea in this seamount. This study provides a foundation for future research on Archaea in seamounts.

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.

Mineralogy of Cobalt-Rich Ferromanganese Crusts from the Perth Abyssal Plain (E Indian Ocean)

Mineralogy of phosphatized and zeolitized hydrogenous cobalt-rich ferromanganese crusts from Dirck Hartog Ridge (DHR), the Perth Abyssal Plain (PAP), formed on an altered basaltic substrate, is described. Detail studies of crusts were conducted using optical transmitted light microscopy, X-ray Powder Diffraction (XRD) and Energy Dispersive X-ray Fluorescence (EDXRF), Differential Thermal Analysis (DTA) and Electron Probe Microanalysis (EPMA). The major Fe-Mn mineral phases that form DHR crusts are low-crystalline vernadite, asbolane and a feroxyhyte-ferrihydrite mixture. Accessory minerals are Ca-hydroxyapatite, zeolites (Na-phillipsite, chabazite, heulandite-clinoptilolite), glauconite and several clay minerals (Fe-smectite, nontronite, celadonite) are identified in the basalt-crust border zone. The highest Ni, Cu and Co contents are observed in asbolane and Mn-(Fe) vernadite. There is significant enrichment of Ti in feroxyhyte−ferrihydrite and vernadite. The highest rare earth element (REE) content is measured in the phosphate minerals, less in phyllosilicates and Na-phillipsite. The geochemical composition of minerals in the DHR crusts supports the formation of crusts by initial alteration, phosphatization and zeolitization of the substrate basalts followed by oscillatory Fe-Mn oxyhydroxides precipitation of hydrogenous vernadite (oxic conditions) and diagenous asbolane (suboxic conditions).

Electron microscopy study on the formation of ferromanganese crusts, western Pacific Magellan Seamounts

Variations in mineralogy and chemical composition, layer structures, redox states of Fe and Mn, and microbial diversity are closely linked to the biogeochemical process when a ferromanganese crust layer forms. A sample collected from the Magellan Seamount (OSM11), in the western Pacific, was characterized in five well-defined layers, top to bottom (L1–5). Fe-vernadite occurs in all layers, compared to detritus quartz, feldspar, goethite, and hematite in L1 and L3, and carbonate fluorapatite (CFA) in L4–5. The relatively high concentrations of Ca and P in L4–5, and Fe, Co, and Si in L1 and L3 correspond to the mineralogical variations in the crust layer. Disappearance of elongated voids along the convex growth line is likely due to void filling precipitation of CFA in L4–5, resulting in the void reduction (31.6 to 6.0%). The oxidation states of Fe in Fe-vernadite measured by electron energy loss spectroscopy (EELS) ranges from 36 to 63% of Fe3+/Fetot, and a layer where CFA appeared (L4) contains a more reductive form of Fe (Fe3+/Fetot = 36–48%). Presence of Fe- (coxC) and Mn-oxidizing gene (cumA), particularly displaying a strong PCR band of coxC in L2–3, indicates a dominant oxidizing condition. Direct evidence of microbial activity in Fe - and Mn-oxide precipitation with various redox conditions was identified in the focused ion beam-sectioned microfossil. Particularly, a spectrum image displaying a more reducing form of Fe around the voids previously occupied by microorganisms, strongly supports that microorganisms play an important role in the redox reaction in the growth of a crust.

Барий в железомарганцевых образованиях Японского моря: особенности выделения и взаимоотношение с основными рудными фазами

Барий в железомарганцевых образованиях Японского моря: особенности выделения и взаимоотношение с основными рудными фазами

Chemical Composition and Genesis of Ferromanganese Crusts from the Sonne Ridge (Kuril Basin, Sea of Okhotsk)

Abstract —The paper presents results of a comprehensive study of ferromanganese crusts from a submarine volcano on the Sonne Ridge, which was discovered during the 178th cruise of the RV Sonne (2004) investigating the northern slope of the Kuril basin in the framework of the KOMEX program. Results of study of the structures, textures, and mineral and elemental compositions of the crusts suggest their hydrogenous origin. The contents of major, trace, and rare-earth elements and Y (REY) in four mineral phases of ferromanganese crusts of the Sea of Okhotsk have been determined by phase analysis for the first time. The REY patterns of the manganese and ferrous in-situ phases confirms the hydrogenous genesis and indicates no hydrothermal influence.

Potential Mitigation and Restoration Actions in Ecosystems Impacted by Seabed Mining

Mining impacts will affect local populations to different degrees. Impacts range from removal of habitats and possible energy sources to pollution and smaller-scale alterations in local habitats that, depending on the degree of disturbance, can lead to extinction of local communities. While there is a shortage or even lack of studies investigating impacts that resemble those caused by actual mining activity, the information available on the potential long-lasting impacts of seabed mining emphasise the need for effective environmental management plans. These plans should include efforts to mitigate deep-sea mining impact such as avoidance, minimisation and potentially restoration actions, to maintain or encourage reinstatement of a resilient ecosystem. A wide range of mitigation and restoration actions for deep-sea ecosystems at risk were addressed. From an ecological point of view, the designation of set-aside areas (refuges) is of utmost importance as it appears to be the most comprehensive and precautionary approach, both for well-known and lesser studied areas. Other actions range from the deployment of artificial substrates to enhance faunal colonisation and survival to habitat recreation, artificial eutrophication, but also spatial and temporal management of mining operations, as well as optimising mining machine construction to minimise plume size on the sea floor, toxicity of the return plume and sediment compression. No single action will suffice to allow an ecosystem to recover, instead combined mitigation/restoration actions need to be considered, which will depend on the specific characteristics of the different mining habitats and the resources hosted (polymetallic sulphides, polymetallic nodules and cobalt-rich ferromanganese crusts). However, there is a lack of practical experience regarding mitigation and restoration actions following mining impacts, which severely hamper their predictability and estimation of their possible effect and success. We propose an extensive list of actions that could be considered as recommendations for best environmental practice. The list is not restricted and, depending on the characteristics of the site, additional actions can be considered. For all actions presented here, further research is necessary to fully encompass their potential and contribution to possible mitigation or restoration of the ecosystem.

拓洋第3海山マンガンクラスト表面の主成分・微量元素から探る海水, 堆積物-クラストの相互作用

拓洋第3海山マンガンクラスト表面の主成分・微量元素から探る海水, 堆積物-クラストの相互作用

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.

Variability in Contents of the Major Ore Components of Ferromanganese Formations in the Lakes–Seas–Oceans Series

Based on statistical methods, the available literature data were processed to analyze regularities in the distribution of five ore elements (Fe, Mn, Cu, Co, Ni) in ferromanganese nodules and crusts in an idealized lakes–seas–oceans profile. Application of the cluster analysis confirmed the regularities revealed by calculations of the mean contents of these elements and unraveled some additional features of their distribution.

Mineral Phase-Element Associations Based on Sequential Leaching of Ferromanganese Crusts, Amerasia Basin Arctic Ocean

Ferromanganese (FeMn) crusts from Mendeleev Ridge, Chukchi Borderland, and Alpha Ridge, in the Amerasia Basin, Arctic Ocean, are similar based on morphology and chemical composition. The crusts are characterized by a two- to four-layered stratigraphy. The chemical composition of the Arctic crusts differs significantly from hydrogenetic crusts from elsewhere of global ocean by high mean Fe/Mn ratios, high As, Li, V, Sc, and Th concentrations, and high detrital contents. Here, we present element distributions through crust stratigraphic sections and element phase association using several complementary techniques such as SEM-EDS, LA-ICP-MS, and sequential leaching, a widely employed method of element phase association that dissolves mineral phases of different stability step-by-step: Exchangeable cations and Ca carbonates, Mn-oxides, Fe-hydroxides, and residual fraction. Sequential leaching shows that the Arctic crusts have higher contents of most elements characteristic of the aluminosilicate phase than do Pacific crusts. Elements have similar distributions between the hydrogenetic Mn and Fe phases in all the Arctic and Pacific crusts. The main host phases for the elements enriched in the Arctic crusts over Pacific crusts (Li, As, Th, and V) are the Mn-phase for Li and Fe-phase for As, Th, and V; those elements also have higher contents in the residual aluminosilicate phase. Thus, higher concentrations of Li, As, Th, and V likely occur in the dissolved and particulate phases in bottom waters where the Arctic crusts grow, which has been shown to be true for Sc, also highly enriched in the crusts. The phase distributions of elements within the crust layers is mostly consistent among the Arctic crusts, being somewhat different in element concentrations in the residual phase.

Nickel isotopic variation in black shales from Bohemia, China, Canada, and Finland: a reconnaissance study

We present δ60Ni values for black shales, determined by double-spike MC-ICP-MS. The samples comprise Paleoproterozoic Talvivaara Ni–Zn–Co–Cu black shales from Finland, Neoproterozoic black shales from the Tepla-Barrandian Unit, Czech Republic, Early Cambrian Ni–Mo-rich black shales from the Yangtze Craton, and Devonian Ni–Zn–PGE black shales from Yukon, Canada. In addition, the sample set includes a black smoker sample from the Logatchev hydrothermal field, Mid-Atlantic Ridge. The δ60Ni values vary from − 0.84 ± 0.09 to + 0.62 ± 0.04‰ (2SD) with a median of − 0.10‰ (n = 28). Ni isotopic compositions were predominantly lighter than those of abiotic terrestrial and extraterrestrial samples (0.15 and 0.27‰), mantle (0.23‰, Gall et al. 2017), present-day seawater (1.44‰, Cameron and Vance 2014), dissolved Ni from riverine input (0.84‰, Cameron and Vance 2014), ferromanganese crusts (0.9–2.5‰, Gall et al. 2013), Devonian/Mississippian organic-rich marine sediments, lower Jurassic organic-rich marine sediments (0.2–2.5‰, average 0.92‰, n = 18, Porter et al. 2014), and euxinic sediments of the Black Sea (0.14–0.51‰, Vance et al. 2016). However, the range of δ60Ni values in our black shale samples was close to that of the weathering products of mafic/ultramafic rocks (ore and soil samples) ranging from − 0.60 to + 0.30‰ (Ratie et al. 2015; Spivak-Birndorf et al. 2018), Ni-sulfide ores hosted by Archean komatiites from Australia and Canada (− 0.10 to − 1.03‰, average − 0.70‰, n = 8, Gueguen et al. 2013), and Archean Ni-rich magmatic sulfides from Zimbabwe (− 0.28 to − 0.47‰, n = 6, Hoffman et al. 2014). Based on our observations and considering the extremely low contribution of direct biological uptake of Ni, and a dominant Ni residence in early Fe/Ni-sulfides, we suggest that our mostly light Ni isotopic compositions in metal-rich black shales result from sulfidization of organic matter and Ni removal into sulfides.

Ferromanganese Crusts in the Central Basin, Sea of Japan

The paper presents a comparative analysis of ferromanganese crusts and concretions (FMC) recovered during the dredging of 14 seamounts in the Central Basin, Sea of Japan. The major rock-forming elements in FMC are Mn, Fe, and Si. In terms of the Mn content, the studied 53 samples are divided into four groups: (1) less than 10% (given than concentrations of 2–8% are lacking); (2) 10‒25%; (3) 25‒42%; and (4) 42‒63%. The (Mn + Fe)/Si ratio increases from group 1 to group 4, and average value in them is 1.6, 2.5, 6.7, and 70.7, respectively. Taking Fe/Si and Mn/Si values into consideration, concretions of these groups belong to the following varieties: (1) ferrosiliceous; (2) mangano-ferrosiliceous; (3) siliceous-ferromanganese, and (4) manganiferous. The highest concentration of nonferrous metals is observed in FMC of groups 2 and 3. Their concentration is slightly lower in group 4 and very low in group 1. The internal structure of FMC in these groups is variable, suggesting their different formation settings. Crusts of group 1 were formed during the precipitation of Mn from a hydrothermal plume on the older ferrosiliceous crusts. Crusts of groups 2 and 3 were likely formed by the diffuse percolation of Mn-bearing hydrothermal solutions along fractures and weakened zones in volcanic rocks, with their subsequent cementation by manganiferous hydroxides from sedimentary or volcaniclastic deposits on seamounts. Crusts of group 4 were formed at sites of the hydrothermal solution discharge on the seafloor. FMC of different groups are recovered during the dredging of most volcanic seamounts in the Central Basin (Sea of Japan). Since the dredging is accomplished at a depth interval of a few hundreds of meters, the detection of concretions of a certain type is governed by the distance to the nearest hydrothermal source.

Operational Improvement of Hemire ROV for Deep-sea Survey and Application to Exploration of Ferromanganese Crusts of Western Pacific Seamount

Operational Improvement of Hemire ROV for Deep-sea Survey and Application to Exploration of Ferromanganese Crusts of Western Pacific Seamount

Geochemistry of cobalt-rich ferromanganese crusts from the Perth Abyssal Plain (E Indian Ocean)

Geochemistry of cobalt-rich ferromanganese crusts from the as-yet-unexplored Dirck Hartog Ridge (DHR), a linear bathymetric feature of poorly understood origin, located in the centre of the Perth Abyssal Plain (PAP), is described for the first time, based on 3 samples analyzed with XRF, ICP-MS and EPMA. Mean bulk concentrations of Fe and Mn were 20.8 and 18.4 wt%, respectively; individual crust layers, however, proved chemically variable. The mean concentration of metals of the highest economic potential (Cu + Ni + Co) was 0.69 wt%. The mean total rare earth elements (REE) and yttrium (ΣREY) contents in the samples (1908 mg/kg) proved moderate. The crusts showed strong positive Ce anomalies, negative Y anomalies and a low YSN/HoSN ratio (mean 0.56). Observations on the texture and chemical composition of the crust samples studied allowed us to distinguish two major and two subtypes of representative colloform structures built of laminae. The Type I colloform structures showed the Mn/Fe ratio to range from 0.86 to 5.69, the ranges in Type II colloform structures being 0.49–2.47 (Subtype II-1) and 6.91–221.42 (Subtype II-2). The Subtype II-2 colloform structures were enriched in Ni (up to 7.31 wt%). The Co and Cu concentrations showed a high variability, 0.03–1.64 and 0.03–0.22 wt%, respectively. According to the current genetic models and discrimination diagrams, the crusts studied were classified as hydrogenetic, although individual laminae and groups of laminae could be regarded as diagenetic or hydrothermal.

Assessment of the Mineral Resource Potential of Atlantic Ferromanganese Crusts Based on Their Growth History, Microstructure, and Texture

The decarbonisation of our energy supply is reliant on new technologies that are raw material intensive and will require a significant increase in the production of metals to sustain them. Ferromanganese (FeMn) crusts are seafloor precipitates, enriched in metals such as cobalt and tellurium, both of which have a predicted future demand above current production rates. In this study, we investigate the texture and composition of FeMn crusts on Tropic Seamount, a typical Atlantic guyot off the coast of western Africa, as a basis for assessing the future mineral resource potential of Atlantic Seamounts. The majority of the summit is flat and covered by FeMn crusts with average thicknesses of 3–4 cm. The crusts are characterized by two dominant textures consisting of either massive pillared growth or more chaotic, cuspate sections of FeMn oxides, with an increased proportion of detrital and organic material. The Fe, Mn, and Co contents in the FeMn oxide layers are not affected by texture. However, detrital material and bioclasts can form about 50% of cuspate areas, and the dilution effect of this entrained material considerably reduces the Fe, Mn, and Co concentrations if the bulk samples are analyzed. Whilst Tropic Seamount meets many of the prerequisites for a crust mining area, the thickness of the crusts and their average metal composition means extraction is unlikely to be viable in the near future. The ability to exploit more difficult terrains or multiple, closely spaced edifices would make economic feasibility more likely.