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The expected growth of the global economy and the projected rise in world population call for a greatly increased supply of materials critical for implementing clean technologies, such as rare earth elements (REEs) and other rare metals. Because the demand for critical metals is increasing and land-based mineral deposits are being depleted, seafloor resources are seen as the next frontier for mineral exploration and extraction. Marine mineral deposits with a great resource potential for transition, rare, and critical metals include mainly deep-sea mineral deposits, such as polymetallic sulfides, polymetallic nodules, cobalt-rich crusts, phosphorites, and rare earth element-rich muds. Major areas with economic interest for seabed mineral exploration and mining are the following: nodules in the Penrhyn Basin-Cook Islands Exclusive Economic Zone (EEZ), the Clarion–Clipperton nodule Zone, Peru Basin nodules, and the Central Indian Ocean Basin; seafloor massive sulfide deposits in the exclusive economic zones of Papua New Guinea, Japan, and New Zealand as well as the Mid-Atlantic Ridge and the three Indian Ocean spreading ridges; cobalt-rich crusts in the Pacific Prime Crust Zone and the Canary Islands Seamounts and the Rio Grande Rise in the Atlantic Ocean; and the rare earth element-rich deep-sea muds around Minamitorishima Island in the equatorial North Pacific. In addition, zones for marine phosphorites exploration are located in Chatham Rise, offshore Baja California, and on the shelf off Namibia. Moreover, shallow-water resources, like placer deposits, represent another marine source for many critical minerals, metals, and gems. The main concerns of deep-sea mining are related to its environmental impacts. Ecological impacts of rare earth element mining on deep-sea ecosystems are still poorly evaluated. Furthermore, marine mining may cause conflicts with various stakeholders such as fisheries, communications cable owners, offshore wind farms, and tourism. The global ocean is an immense source of food, energy, raw materials, clean water, and ecosystem services and suffers seriously by multiple stressors from anthropogenic sources. The development of a blue economy strategy needs a better knowledge of the environmental impacts. By protecting vulnerable areas, applying new technologies for deep-sea mineral exploration and mining, marine spatial planning, and a regulatory framework for minerals extraction, we may achieve sustainable management and use of our oceans.

This study summarizes a compilation of studies and cartographical work on seabed mineral deposit types in pan-European seas developed under the GeoERA-MINDeSEA project. In total, 692 occurrences and 1194 individual mineral samples of volcanogenic massive sulfides and hydrothermal mineralization, ferromanganese crusts, phosphorites, marine placer deposits, polymetallic nodules, and their associated strategic and critical raw material (CRM) elements have been characterized. The GeoERA-MINDeSEA project has been built based on extensive studies carried out previously, which include geophysical surveys, sampling stations, underwater photography and remotely operated vehicle (ROV) surveys, and mineralogical, geochemical and isotopic studies. This study develops pan-European and national databases, and expands strategic and CRM knowledge through a compilation of mineral potential and metallogenic studies of CRM resources in European seas. For the first time, the GeoERA-MINDeSEA portal publishes harmonized marine mineral resource information, case studies and maps, and identifies potential areas for responsible resource exploration and extraction, strategic management, and marine spatial planning. This study also provides recommendations for future target areas, studies and standards to be used across Europe as part of this project.

Despite its status as a critical metal in the EU and the U.S., the geochemical behavior of gallium (Ga) in the Earth’s surface environment is not yet well constrained, which is in marked contrast to its geochemical partner aluminum (Al). Especially hydrogenetic ferromanganese crusts which may represent a potential future unconventional resource of critical metals, are surprisingly underexplored with regard to their Ga-Al systematics. This study presents the first data on the Ga-Al pair in hydrogenetic Fe-Mn crusts from various locations in the Pacific Ocean and from the Rio Grande Rise in the SW Atlantic. We also provide Ga-Al data for certified reference materials for Fe-Mn nodules NOD-P-1, NOD-A-1, and JMn-1. Our results also show that marine hydrogenetic Fe-Mn crusts cannot be considered an unconventional Ga resource, in marked contrast to several other critical metals. Aluminum and Ga concentrations in these precipitates are not controlled by detrital components. Both elements show a geochemical behavior comparable to that of the hydroxide-dominated elements Zr and Hf during scavenging from ambient seawater by hydrogenetic Fe-Mn crusts. The Ga/Al mass ratios of Atlantic (0.10–0.16 g/kg) and Pacific crusts (0.18–1.2 g/kg) are about one and a half orders of magnitude lower than those of respective ambient seawater and mimic the inter-oceanic fractionation of the Ga-Al pair between these two ocean basins. Without exception, the Atlantic Fe-Mn crusts show lower Ga/Al ratios than the Pacific crusts. The systematically lower Ga/Al ratios of the crusts relative to those of seawater reveal preferential scavenging of Al relative to Ga during the formation of Fe-Mn crusts. This is in line with previous studies that reported a higher particle reactivity of Al compared to Ga in the marine environment. However, this fractionation trend contradicts predictions based on a simplified model commonly used to approximate trace element scavenging by marine metal (hydr)oxides based on hydrolysis constants, which favors Ga enrichment relative to Al as the hydrolysis constant of Ga is higher than that of Al. The observed Ga-Al fractionation may be related to the preferential retention of Al compared to Ga on the surface of the Fe-Mn (oxyhydr)oxides because in contrast to Ga, desorption of Al is characterized by very slow reaction kinetics or is even irreversible. Alternatively, stronger organic complexation of dissolved Ga than Al in seawater (which is supported by the stability constants of various organic Ga and Al complexes) and the resulting preferential retention of Ga over Al in solution by dissolved organic ligands may be another viable mechanism to explain the observed Ga-Al fractionation between hydrogenetic Fe-Mn crusts and seawater. While more experimentally determined thermodynamic data are necessary, the rather consistent difference between hydrogenetic Fe-Mn crusts and ambient seawater supports an important role of organic ligands in the chemical speciation of Ga in seawater.

Metal sulfide minerals exist in several marine environments and are in thermodynamic disequilibrium with oxygenated seawater from the time of their formation. Oxidation is both ubiquitous and heter...

The older layers of thick ferromanganese (FeMn) crusts from the central Pacific Ocean have undergone diagenetic phosphatization, during which carbonate fluorapatite (CFA) filled fractures and pore ...

Deep-ocean polymetallic nodules form on or just below the vast, sediment-covered, abyssal plains of the global ocean. Polymetallic nodules primarily consist of precipitated iron oxyhydroxides and manganese oxides, onto which metals such as nickel, cobalt, copper, titanium and rare earth elements sorb. The enormous tonnage of nodules on the seabed, and the immense quantities of critical metals that they contain, have made them a target for future mining operations. Mining of polymetallic nodules has been spurred by the need for critical metals to support growing populations, urbanization, high-technology applications and the development of a green-energy economy. Nevertheless, an improved understanding of the affected ecosystems and their connectivity, as well as the environmental impacts of deep-ocean mining, is required before operations begin. Opportunities exist, however, to ensure that this new industry applies adaptive management to continually refine operations with the goal of environmental protection and invests in the development of green technologies for extractive metallurgy and mining. In this Review, we explore the chemical processes that control the concentration of critical metals in deep-ocean polymetallic nodules, discuss the mining and metallurgical techniques required, and highlight the opportunities and potential risks that are presented by this new industry. Deep-ocean polymetallic nodules contain an enormous tonnage of critical metals, which are vital natural resources for green-energy technologies and vehicles. This Review highlights the formation processes of these nodules and outlines the advantages and disadvantages for this developing industry moving forward.

Records 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.

International audience; Our understanding of the up to seven orders of magnitude partitioning of thallium (Tl) between seawater and ferromanganese (FeMn) deposits rests upon two foundations: (1) being able to quantify the Tl(I)/Tl(III) ratio that reflects the extent of the oxidative scavenging of Tl by vernadite (δ-MnO2), the principle manganate mineral in oxic and suboxic environments, and (2) being able to determine the sorption sites and bonding environments of the Tl(I) and Tl(III) complexes on vernadite. We investigated these foundations by determining the oxidation state and chemical form of Tl in FeMn crusts and nodules from the global oceans at a Tl concentration ranging from several hundreds ppm (mg/kg) down to the low ppm level. Seventeen hydrogenetic crusts and eleven nodules from the Pacific, Atlantic, and Indian oceans and Baltic Sea were characterized by chemical analysis, X-ray diffraction, Raman spectroscopy, Mn K-edge X-ray absorption near-edge structure (XANES) spectroscopy, and Tl L3-edge high energy-resolution XANES (HR-XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The Tl concentration increases linearly from 1.5 to 319 ppm with the Mn/Fe ratio in Fe-vernadite from hydrogenetic crusts, whereas the percentage of Tl(III) to total Tl varies between 62% and 100% independent of both the Mn/Fe and Mn(III)/Mn(IV) 2 ratios. The data, complemented by molecular modeling of the Tl(III) coordination and by XANES calculations, suggest that the enrichment of Tl in Fe-vernadite is driven by (1) the oxidative uptake of octahedrally coordinated Tl(III) above the vacant Mn(IV) sites and on the layer edges of the vernadite layers, and (2) the sorption of Tl(I) on the crystallographic site of Ba at the surface of the vernadite layers, which is analog to the surface site of K. Thus, Tl has a high affinity for vernadite regardless of its oxidation state, and the lack of correlation between Tl(III) and the Mn/Fe ratio in FeMn crusts is explained by the affinity of Tl(I) for the Ba site. The Tl concentration varies between 2 and 112 ppm in surface and buried nodules independent of the Mn/Fe ratio, and the percentage of Tl(III) varies between 0% and 100%. Nodules subjected to sediment diagenesis with replacement of layered vernadite by tunneled todorokite are depleted in Tl and have more reduced thallium. Knowledge of the complex interplay of mineralogy, surface chemical processes, and crystallographic siting are required to understand the variability of Tl concentrations, redox state, and acquisition processes by marine FeMn deposits.

Manganese nodules are a potential source of critical metals such as Cu, Ni, and Co and are widely distributed on the abyssal plains of the global oceans. A polymetallic nodule metallogenic belt with a heterogeneous and spatially clustered nodule distribution was recently discovered in NW Pacific inter-seamount basin (NPIB) areas. However, the geological processes that regulate the nodule occurrence in that region are unresolved. Here, we report on the characteristics of a high-density field of manganese nodules in the abyssal plain north of Suda Seamount. Ship-borne multibeam bathymetric data reveal a typical seamount sector-collapse topography characterized by radial lineaments of debris channels and ridges formed by rapid debris-avalanche flow. Backscatter data linked with underwater observation indicate that manganese nodules are more concentrated (50%–80% areal coverage) along the main body of the debris apron compared to adjacent neighboring areas (

Deep-water ferromanganese (Fe-Mn) nodules cumulate trace elements from the underlying sediment pore-water and seawater during the concentric mineral layers' growth process over millions of years. They can record long-term tectonic movements and paleoceanographic changes. To systematically ascertain paleoceanographic evolution of the deep South China Sea (SCS) since the last 3.2 Ma, we comprehensively investigated microstructure, mineralogy, geochemical element concentrations as well as Sr, Nd and Pb isotopic compositions in the Fe-Mn nodules nearby the Dongsha Islands in the northern SCS. Results indicate that these deposits were formed in a continental marginal environment by a mixture of diagenetic and hydrogenetic processes. Overall, the studied nodules can be divided into three stages with different microstructures and chemical compositions, which indicate different formation environments. Stage 1a is the innermost and oldest layer with a porous mottled texture, and has the lowest Mn/Fe ratio, Cu, Ni and Zn, which implies comparatively high oxidization conditions. In contrast, the Mn mineralogy of Stage 2 shows conditions similar to those of buserite formed under less oxidative conditions. Between Stage 1a and 2, Stage 1b represents a mixture of Stage 1a and 2 and has inhomogeneous growth structures. Based on the Nd and Sr isotopic composition of the detrital phases and Pb isotopes, three potential terrigenous source regions including South China, Luzon and Taiwan that discharge into the northern SCS from 3.2 Ma to 1.06 Ma. After 1.06 Ma, the terrigenous fraction in the Fe-Mn deposits on the northern slope of the SCS is a two end-members mixture of fluvial input from Taiwan Islands and eolian dusts from the Chinese loess. Our study indicates that SCS bottom water variation occurred at ~3.2 Ma, 2.1 Ma, and 1.06 Ma, respectively, which could be attributed to global cooling and the influence of Luzon arc-forearc uplift.

The Chiatura deposit is considered one of the world's largest deposits of metallurgical grade manganese, yet its geochemistry is poorly known, which is the focus of this paper. The Oligocene sedime...

Seamount phosphates are increasingly regarded as potential resources for rare earth elements (REE) and plus yttrium (REY). Carbonate fluorapatite (CFA) formed within seamount ferromanganese (Fe Mn) crusts is the most common seamount phosphate mineral. However, reports on the mineralogy and geochemistry of CFA are few and thus its origin and acquisition of trace elements are not well understood. In this study, we analyzed the major and trace elements of CFA in Fe Mn crusts collected from Western and Central Pacific seamounts to investigate the genesis of trace elements in the CFA. This is the first study to use in situ analytical techniques such as electron microprobe analyzer (EPMA) and laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) to analyze seamount CFA. We found that the CFA hosts abundant minor and trace elements and propose that ionic substitutions are responsible for the high contents of SO3, SiO2, REY, Sr, Na, Fe, and Mn in the CFA veins found in the Fe Mn crusts, i.e., SiO32− and SO42− substitute for PO42−, while REE3+, Y3+, Na+, Fe2+, and Mn2+ substitute for Ca2+. REE3+ substitutions for Ca2+ in the CFA are charge-compensated by Na+ substitution for Ca2+. Fourier transform infrared spectroscopy (FTIR) analysis shows that CO32– mainly substitutes for PO42− in the CFA crystal structure, and there is a minor substitution of PO42− by CO3F3−. Ocean water is the major source of the P and REY, which when precipitated as seamount CFA is characterized by high ∑REE (345 to 6016 ppm) and heavy-REE (HREE) enrichments. Monazite grains dispersed in the seamount CFA contribute trace amount of REY. These results shed light on the composition and element mobility of seamount CFA with economic potential, which also provides valuable insights into global ocean chemical cycling (e.g. REE).

The world increasing demand of electric vehicles (EVs) that use lithium-ion batteries (LIB), in which cobalt is one of the essential elements, focused the attention on its demand that is calculated will increase of 7-13% annually until 2030. The actual production of cobalt, usually extract as by-product of nickel and copper mine, is reduced to almost 20 countries between which the Democratic Republic of the Congo is the bigger producer with 55% of the world production. In Europe cobalt is produced only in Finland that actually provides 2.300 tonnes, the 2% of the world production. In this way several projects have been promoted by European Union, with the Raw Material Initiative, in order to find and evaluate the sustainable production of important materials in Europe.MINDeSEA[1] project is part of the GeoERA and represent the collaboration of 12 national geological institution partners, to characterize marine deposits and their contents in Critical Raw Materials (CRM) and to generate a comprehensive cartography and metallogenic models of them. The first preliminary map produced in 2019 represents the localization and evaluation of cobalt rich deposits in the oceans within the EEZ and ECS of the European countries. Cobalt deposits are represented essentially by hydrogenetic Fe-Mn crusts located essentially in the Macaronesian area of the north east Atlantic Ocean (in the Portugal and Spain), submarine plateaus, as the Galicia Bank (in the north west Spanish) and in the Arctic Ocean ridges (Norway and Iceland). The report differentiates between occurrences (0.05 wt. %), with the possibility of more than 200 Mt resources per potential deposit.Detailed mineralogical, geochemical and metallogenic studies are being developed in crusts from the Macaronesia. Fe-Mn crusts absorb dissolved elements in seawaters on the surface of the fresh precipitated oxy-hydroxides during their slow growth through millions of years. Several elements are concentrated in Fe-Mn crusts and between them cobalt is one of the most enriched trace metals (average 0.6 wt. %) accompanied by other strategic and critical metals such as nickel, copper, tellurium, molybdenum and rare earth elements plus yttrium (REY) (respectively 3000, 500, 150, 500 and 3500 µg/g). Micro Raman and micro X-Ray diffraction can be used to differentiate the mineralogy in laminae of less than 20 microns. On the other hand, electron probe micro-analyzer (EPMA) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), are useful in order to quantify contents of CRM in the different mineral phases. These are innovative techniques in order to identify critical-elements bearing minerals and thus choose the metallurgic method for a more efficient and sustainable extraction of the interesting elements.The evaluation of a seamount as a future mine site has to take into account all these mineralogical and chemical features as well as a proper knowledge of the seamount (morpho-structure, geology, oceanography, ecosystems) and the Fe-Mn crust thickness and extension [1] This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 731166

The oceans and seas cover more than 70% of the planet, representing a promising new frontier for mineral resources exploration, and an enormous challenge for science and technology. Communities are demanding actions to address global climate change, and the necessary high- and green-technologies required for a transition from a carbon-based to green-energy-based world. The global ocean is at the core of these issues. The seabed mineral resources host the largest reserves on Earth for some critical metals like cobalt, tellurium, manganese, and the rare earth elements, critical for Industry. But seabed geology and ecosystems are widely unexplored, and new geological and environmental studies are required to address the impacts of potential mining activities. In addition, a regulatory framework for minerals extraction and marine spatial planning are necessary for seabed mining sector development.The pan-European seas cover about 15 millions square kilometres in the Arctic and Atlantic oceans and the Mediterranean, Baltic, and Black seas, from shallow waters up to 6000 m water depth. Spanning a large diversity of environments and resource settings, including high and low temperature hydrothermal deposits, phosphorites, cobalt-rich ferromanganese crusts, and manganese nodules, deep-sea deposits are particularly attractive for their polymetallic nature with high contents of rare and critical metals. Moreover, shallow-water resources, like marine placer deposits, represent another source for many critical metals and gems. The GeoERA-MINDeSEA[1] project is compiling data and genetic models for all these deposit types based on extensive studies, carried out previously, which include geophysical surveys, dredging stations, underwater photography and ROV surveys, and mineralogical, geochemical, and isotopic studies.The preliminary MINDeSEA results show the potential of the pan-European seas for critical metals, and the enormous gaps of information covering vast marine sectors. More than 600 mineral occurrences are reported in the MINDeSEA database. Seamounts and banks in the Macaronesia sector (Portugal and Spain) and the Arctic ridges (Norway, Denmark, Iceland) show a high potential for Fe-Mn crusts, rich in energy-critical elements like Co but also Te, REEs, and Mn. Fe-Mn crusts are accompanied by phosphorites on the seafloor of continental shelves and slopes along the western continental margins. Seafloor polymetallic sulphides and metalliferous sediments precipitating from hot hydrothermal solutions and plumes are forming today in the Azores Islands (Portugal), the Arctic (Norway, Denmark) and, the Mediterranean volcanic arcs (Italy and Greece). They are among the most important marine resources for Cu, Zn, Ag, and Au. In addition, hydrothermal deposits may contain economic grades of Co, Sn, Ba, In, Bi, Te, Ga, and Ge. Placer deposits of chemically resistant and durable minerals have been discovered on shallow-water settings ( [1] This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 731166

The South China Sea (SCS) is connected to the West Pacific through a deep channel in the Luzon Strait. Thus the SCS deep water is sensitive to the evolution of Pacific Ocean circulation, which significantly influences the global climate system. Geochemical data (Pb isotope and redox-sensitive elements data) and magnetic data were determined for a Fe–Mn nodule obtained from Jiaolong seamount in the central SCS. These records reflect interactions between changes in ice sheets, deep Pacific circulation, and weathering inputs to the deep SCS during the Pliocene and Quaternary. Our results show that the SCS deep-water environment can be divided into three major Stages (Stages 1–3). Stage 1 (∼4.8–1.4 Ma) was characterized by a well-oxygenated Pacific Deep Water (PDW) and lower dust inputs; then moderate stable deep-water ventilation and greater inputs of Asian dust occurred during Stage 2 (1.4–0.9 Ma). During Stage 3 (

Hydrothermal iron (Fe)-rich sediments were recovered from the Tagoro underwater volcano (Central Atlantic) that formed during the 2011–2012 volcanic event. Cruises in 2012 and 2014 enabled the monitoring and sampling of the early-stage establishment of a hydrothermal system. Degassing vents produced acoustic flares imaged on echo-sounders in June 2012, four months after the eruption. A novel hydrothermal vent system was discovered and sampled in 2014 during a ROV dive. The system is characterized by hornito-like structures and chimneys showing active CO2 degassing and anomalous temperatures at 120–89 m water depth, and along the SE flank at 215-185 m water depth associated with secondary cones. Iron- and silica-rich gelatinous deposits pooled over and between basanite in the hornitos, brecciated lavas, and lapilli. The low temperature, shallow-water hydrothermal system was discovered by the venting of Fe-rich fluids that produced a seafloor draped by extensive Fe-flocculate deposits precipitated from the neutrally buoyant plumes located along the oxic/photic zone at 50-70 m water depths. The basanite is capped by mm- to cm-thick hydrothermally derived Fe-oxyhydroxide sediment, and contains micro-cracks and degasification vesicles filled by sulfides (mostly pyrite). Mineralogically, the Fe-oxyhydroxide sediment consists of proto-ferrihydrite and ferrihydrite with scarce pyrite at their base. The Fe-rich endmember contains low concentrations of most trace elements in comparison with hydrogenetic ferromanganese deposits, and the sediments show some dilution of the Fe oxyhydroxide by volcanic ash. The Fe-oxyhydroxide phase, with a mean particle size of 3–4 nm, low average La/Fe ratios of the mineralized deposits from the various sampling sites, and the positive Eu anomalies indicate rapid deposition of the Fe oxyhydroxide near the hydrothermal vents. Electron microprobe studies show the presence of various organomineral structures, mainly twisted stalks and sheaths covered by iron-silica deposits within the mineralized samples, reflecting microbial iron-oxidation from the hydrothermal fluids. Sequencing of 16 s rRNA genes also reveals the presence of other microorganisms involved in sulfur and methane cycles. Samples collected from hornito chimneys contain silicified microorganisms coated by Fe-rich precipitates. The rapid silicification may have been indirectly promoted by microorganisms acting as nucleation sites. We suggest that this type of hydrothermal deposit might be more frequent than presently reported to occur in submarine volcanoes. On a geological scale, these volcanic eruptions and low-temperature hydrothermal vents might contribute to increased dissolved metals in seawater, and generate considerable Fe-oxyhydroxide deposits as identified in older hot-spot seamounts.

Marine ferromanganese oxide crusts (Fe–Mn crusts) are potentially important metal resources formed on the seafloor by precipitation of dissolved and colloidal components from ambient seawater onto rocky surfaces. The unique properties and slow growth rates of the crusts promote adsorption of numerous elements from seawater: some, such as Te and Co, reach concentrations rarely encountered elsewhere in nature. Consequently, Fe–Mn crusts are potential sources of metals used in technologies considered essential for the transition to a low-carbon economy. However, the precise distributions and metal concentrations of Fe–Mn crusts at regional and local scales are poorly constrained because of the diversity of geological, oceanographic and chemical processes involved in their formation.

Minerals in stratigraphic sections are valuable tools for reconstructing past environmental conditions. Given the state of preservation of clay minerals, it is possible to determine under what conditions they formed, which provides clues about continental weathering (inherited minerals) and geochemical conditions in the water column or pore waters (neoformed or transformed) of the sedimentary environment. This study presents new mineralogical and chemical data from the Baskil section, a well-preserved middle Eocene Neo-Tethys sequence from eastern Turkey. Greater terrigenous input is marked by the increase of silicate minerals (e.g. phyllosilicates, quartz, and albite) in the section from 40.5 to 40 Ma, which diluted the marine carbonate content. This period is correlative with the global Middle Eocene Climatic Optimum (MECO) event. Authigenic palygorskite increased from the middle to the uppermost portion (~40 to 37.3 Ma) of the section, indicating dryer conditions in the continent and availability of Si and Mg in the water column and pore waters favorable for its formation. Additionally, we suggest a stratified water column with warmer and more saline conditions at greater depths (e.g. >200 m) after ~40 Ma favoring palygorskite and possibly authigenic dolomite precipitation. The mineralogical variations and element chemistry of rocks in the Baskil section reflect how detrital sources and weathering regimes changed in this area during the middle Eocene, and how these changes can be related to global, regional, and local processes.

The growth of ferromanganese (FeMn) crusts on soft substrates is uncommon. FeMn crusts generally accrete on hard-rock surfaces, where sedimentation rates are low and the rocks free of sediment cover. Here we use X-ray Powder Diffraction, Fourier Transform-Infra Red spectroscopy, Raman spectroscopy, X-ray Computed Micro-Tomography, Scanning Electron Microscopy and Particle Size Distribution analysis to investigate FeMn crusts associated with a porous, weakly consolidated, and bioturbated siltstone consisting mainly of authigenic palygorskite. FeMn crusts occur both on the surface of the siltstone and as FeMn lining and/or infilling of bioturbation burrows. Our results show that variations in the water redox conditions lead to a micrometric alternation of hydrogenetic vernadite and diagenetic todorokite and asbolane. These variations affected the oxidation state of Mn, which increased during the diagenetic formation of todorokite. The mineralogy of the FeMn oxides lining bioturbation traces is similar to that of the crusts growing on the siltstone, suggesting that they are genetically related and probably contemporaneous. Bioturbation burrows lined by FeMn oxides are unfilled (the siltstone porosity is 5–10%) or filled by bioclastic carbonate sediment. The soft siltstone possibly resulted from the alteration of volcaniclastic-pyroclastic sediments, forming authigenic palygorskite. Bioturbation of the sediment by burrowing organisms lead to seawater percolation, followed by hydrogenetic or diagenetic precipitation of FeMn linings under oxic/suboxic conditions within the burrows. These findings emphasize the importance of weakly-consolidated substrates with bioturbation cavities for the formation of marine polymetallic 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.

The sources of terrestrial material delivered to the California margin over the past 7 Myr were assessed using 187Os/188Os, Nd, and Pb isotopes in hydrogenetic ferromanganese crusts from three seamounts along the central and southern California margin. From 6.8 to 4.5 (± 0.5) Ma, all three isotope systems show more radiogenic values at Davidson Seamount, located near the base of the Monterey Canyon System, than in Fe-Mn crusts from the more remote Taney and Hoss seamounts. At the Taney seamounts, approximately 225 km farther offshore from Davidson Seamount, 187Os/188Os values, but not Pb and Nd isotope ratios, also deviate from the Cenozoic seawater curve towards more radiogenic values from 6.8 to 4.5 (± 0.5) Ma. However, none of the isotope systems in Fe-Mn crusts deviate from seawater at Hoss Seamount located approximately 450 km to the south. The regional gradients in isotope ratios indicate that substantial input of dissolved and particulate terrestrial material into the Monterey Canyon System is responsible for the local deviations in the seawater Nd, Pb, and Os isotope compositions from 6.8 to 4.5 (± 0.5) Ma. The isotope ratios recorded in Fe-Mn crusts are consistent with a southern Sierra Nevada or western Basin and Range provenance of the terrestrial material which was delivered by rivers to the canyon. The exhumation of the modern Monterey Canyon must have begun between 10 and 6.8 ± 0.5 Ma, as indicated by our data, the age of incised strata, and paleo-location of the Monterey Canyon relative to the paleo-coastline.

Little is known about marine mineral deposits in the Arctic Ocean, an ocean dominated by continental shelf and basins semi-closed to deep-water circulation. Here, we present data for ferromanganese crusts and nodules collected from the Amerasia Arctic Ocean in 2008, 2009, and 2012 (HLY0805, HLY0905, HLY1202). We determined mineral and chemical compositions of the crusts and nodules and the onset of their formation. Water column samples from the GEOTRACES program were analyzed for dissolved and particulate scandium concentrations, an element uniquely enriched in these deposits. The Arctic crusts and nodules are characterized by unique mineral and chemical compositions with atypically high growth rates, detrital contents, Fe/Mn ratios, and low Si/Al ratios, compared to deposits found elsewhere. High detritus reflects erosion of submarine outcrops and North America and Siberia cratons, transport by rivers and glaciers to the sea, and distribution by sea ice, brines, and currents. Uniquely high Fe/Mn ratios are attributed to expansive continental shelves, where diagenetic cycling releases Fe to bottom waters, and density flows transport shelf bottom water to the open Arctic Ocean. Low Mn contents reflect the lack of a mid-water oxygen minimum zone that would act as a reservoir for dissolved Mn. The potential host phases and sources for elements with uniquely high contents are discussed with an emphasis on scandium. Scandium sorption onto Fe oxyhydroxides and Sc-rich detritus account for atypically high scandium contents. The opening of Fram Strait in the Miocene and ventilation of the deep basins initiated Fe-Mn crust growth ∼15 Myr ago.

This study examines Fe-Mn crusts that form on seamounts along the California continental-margin (CCM), within the United States 200 nautical mile exclusive economic zone. The study area extends from approximately 30° to 38° North latitudes and from 117° to 126° West longitudes. The area of study is a tectonically active northeast Pacific plate boundary region and is also part of the North Pacific Subtropical Gyre with currents dominated by the California Current System. Upwelling of nutrient-rich water results in high primary productivity that produces a pronounced oxygen minimum zone. Hydrogenetic Fe-Mn crusts forming along the CCM show distinct chemical and mineral compositions compared to open-ocean crusts. On average, CCM crusts contain more Fe relative to Mn than open-ocean Pacific crusts. The continental shelf and slope release both Fe and Mn under low-oxygen conditions. Silica is also enriched relative to Al compared to open-ocean crusts. This is due to the North Pacific silica plume and enrichment of Si along the path of deep-water circulation, resulting in Si enrichment in bottom and intermediate waters of the eastern Pacific. The CCM Fe-Mn crusts have a higher percentage of birnessite than open-ocean crusts, reflecting lower dissolved seawater oxygen that results from the intense coastal upwelling and proximity to zones of continental slope pore-water anoxia. Carbonate fluorapatite (CFA) is not present and CCM crusts do not show evidence of phosphatization, even in the older sections. The mineralogy indicates a suboxic environment under which birnessite forms, but in which pH is not high enough to facilitate CFA deposition. Growth rates of CCM crusts generally increase with increasing water depth, likely due to deep-water Fe sources mobilized from reduced shelf and slope sediments. Many elements of economic interest including Mn, Co, Ni, Cu, W, and Te have slightly or significantly lower concentrations in CCM crusts relative to crusts from the Pacific Prime Crust Zone and other open-ocean basins. However, concentrations of total rare earth elements and yttrium average only slightly lower contents and in the future may be a strategic resource for the U.S.

The Urkut manganese deposit, one of the World's largest, is located in the central part of the Transdanubian Range, western Hungary. The deposit is interbedded with Mesozoic limemarlstone. The Fe-Mn-oxide indications of a feeder and mound zone embedded in limemarlstone at the footwall of the Mn-carbonate ore deposit were studied using 45 samples (Urkut Mine, Shaft III, deep level). Microstructural and textural (optical microscopy, SEM-EDS) observations, mineralogy (XRD-μXRD), and geochemistry (ICP, C and O by IR-MS) were used to characterize the host marlstone and the Fe-Mn oxides of the feeder and mound zone. High-resolution in situ and bulk organic matter analyses were performed for the first time using GC–MS, FTIR-ATR, and Raman spectroscopy. Stromatolite-like, filamentous and coccoid microstuctures built up of Fe-Mn-oxides (ferrihydrite, goethite, manganite, pyrolusite, hollandite, birnessite, hausmannite) and silica occur in the micritic marlstone host rock among common calcite biodebris (microfossils and Echinozoa fragments) and rare detrital clasts (quartz, feldspar). The clay minerals occur as greenish patches in the limemarlstone and show boring traces. The calcite matrix of the limemarlstone and idiomorphic dolomite are authigenic. δ 13 C PDB values of the carbonate in the host limemarlstone reflect greater organic matter contributions approaching the mineralized areas (0.64 to − 21.35‰). Temperature calculation based on δ 18 O SMOW values of the carbonate, assuming equilibrium conditions, show elevated temperatures toward the mineralized areas (9.93 to 29.87‰). In places, the Mn oxides appear with Fe oxides in laminated, micro-stromatolite-like structures. In these oxide zones, variable kinds of organic compounds occur as intercalated microlaminae identified by FTIR and Raman line-profile analyses as aromatic hydrocarbons. Results indicate that metal-bearing fluids infiltered the unconsolidated micritic limemarl. Fe-oxide enrichment occurred most probably through iron oxidizing microbes under suboxic, neutrophilic conditions, while Mn oxide formed most probably by active surface catalyses. At the sediment/water interface, Fe-Mn-oxide stromatolite mounds (chimneys) formed in rift zones from the discharge of fluids of elevated temperature. The host marl itself may have originated by microbially mediated reactions (clay minerals and calcite micrite).

Marine iron–manganese oxide coatings occur in many shallow and deep-water areas of the global ocean and can form in three ways: 1) Fe–Mn crusts can precipitate from seawater onto rocks on seamounts; 2) Fe–Mn nodules can form on the sediment surface around a nucleus by diagenetic processes in sediment pore water; 3) encrustations can precipitate from hydrothermal fluids. These oxide coatings have been growing for thousands to tens of millions of years. They represent a vast archive of how oceans have changed, including variations of climate, ocean currents, geological activity, erosion processes on land, and even anthropogenic impact. A growing toolbox of age-dating methods and element and isotopic signatures are being used to exploit these archives.

The spatial distribution of cobalt-rich crust thicknesses on seamounts is partly controlled by water depth and slope gradients. Conventional distance–direction-based variogram have not effectively expressed the spatial self-correlation or anisotropy of the thicknesses of cobalt-rich crusts. To estimate resources in cobalt-rich crusts on seamounts using geostatistics, we constructed a new variogram model to adapt to the spatial distribution of the thicknesses of the cobalt-rich crusts. In this model, we defined the data related to cobalt-rich crusts on seamounts as three-dimensional surface random variables, presented an experimental variogram process based on the distance–gradient or distance–“relative water depth,” and provided a theoretical variogram model that follows this process. This method was demonstrated by the spatial estimation of the thicknesses of cobalt-rich crusts on a seamount, and the results indicated that the new variogram model reflects the spatial self-correlation of the thicknesses of cobalt-rich crusts well. Substituted into the Kriging equation, the new variogram model successfully estimated the spatial thickness distribution of these cobalt-rich crusts.

Mineral formation in the modern oceans can take place over millions of years as a result precipitation from ambient ocean water, or orders of magnitude more rapidly from hydrothermal activity related to magmatic and tectonic processes. Here, we review associations between transform faults and related fracture zones and marine minerals. We define marine transform faults as strike-slip or oblique faults that accommodate lateral offsets along plate boundaries or shifting crustal blocks, and fracture zones as relicts of transform faulting extending beyond mid-ocean ridge offsets. We consider specifically the modern ocean and exclude regions where the transform or fracture has clearly not generated the mineral deposit, such as the Clarion-Clipperton fracture zone manganese nodule field. As a result, the summarized deposits are mainly hydrothermal in origin. Oceanic transform faulting has rarely been considered of interest for the mineralization and formation of ore deposits; however, there are locations in the modern oceans where transform faults and fracture zones are spatially related to mineral deposits. These occurrences suggest that transform faulting and fracture zones may be linked to mineralization at (A) intersections with other tectonic features, (B) where transform faults begin to resemble rifts through intra-transform crustal thinning, spreading, and the formation of pull-apart basins, and (C) as a result of serpentinization reactions due to exposure of deep-seated rocks by fracturing and faulting.

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 A 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 A and 7 A 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.

Recent geochemical data suggest the occurrence of an O2 overshoot during the mid-Paleoproterozoic (~2.3–2.0 Ga). This O2 overshoot appears to be consistent with carbon isotope records that suggest high burial rates of organic carbon during that period, the so-called Lomagundi Event. However, little is known about the changes in the ocean redox conditions associated with the O2 overshoot. To better understand the mid-Paleoproterozoic ocean chemistry, we investigated the microstructures, major and trace element concentrations, Re-Os and Mo (δ98/95Mo) isotopes, and total organic carbon contents of Mn-ore and phyllite samples from the Nsuta Mn deposit in the Birimian Supergroup of Ghana which were deposited during the O2 overshoot (at ~2.2 Ga). The Mn-ore samples contain early diagenetic rhodochrosite (Mn carbonate). The trace element compositions and Re-Os isotopes of the Mn-ore samples suggest that the rhodochrosite originated from primary manganese oxides (MnO2) deposited at ~2.2-Ga. The δ98/95Mo values of the least-altered Mn-ore samples range between −1.10‰ and −0.55‰ (relative to NIST3134), suggesting seawater δ98/95Mo values of 1.85 ± 0.18‰ (1SD) during the O2 overshoot. Such high seawater δ98/95Mo values can be best explained by enhanced removal of isotopically light Mo through adsorption onto Mn oxides. To form extensive Mn-oxide deposits, bottom seawater with O2 concentrations of > 10 μM would have expanded at ~2.2 Ga. The oxidizing conditions might have supported the emergence of stem group eukaryotes during the mid-Paleoproterozoic.

The Chiatura deposit is considered to be among the largest deposits of metallurgical grade manganese ore in the world, yet its geochemistry is poorly known, which is the focus of this paper. The Oligocene sedimentary Mn deposit is located in the Chiatura region of central Georgia and formed in a restricted arm of the Paratethys on stable crystalline basement during a regressive-transgressive cycle. An Oligocene basal conglomerate is overlain by sandstone that is in turn overlain by the ore deposits. The Chiatura sedimentary deposits form horizontal beds, 1.5 to 4 m thick, average 2 m, of which 1 to 1.5 m is high-grade ore, covering an area of some 150 km2. The Chiatura deposit contains three types of ore, primary oxide ores, carbonate ores, and oxidized carbonate ores and this paper focuses on the oxide ores. The common manganese minerals in the oxide ore are manganite, pyrolusite, braunite, and psilomelane, and less commonly bixbyite, rhodochrosite, and vernadite. Eighteen manganese-oxide and four wall-rock (low manganese content) samples were analyzed for geochemistry and found on average 36.6 wt% MnO, 2.45 wt% Fe2O3, 23.5 wt% SiO2, 5.88 wt% Al2O3, 3.84 wt% CaO for major oxide concentrations. The mean main trace element concentrations of the samples are 3944 ppm Ba, 946 ppm Sr, 511 ppm Ni, 150 ppm Zn, 94 ppm V, 84 ppm Cu, and 55 ppm Co. These element concentrations are generally low and indicate an enrichment assemblage of V, Ni, Co, Cu, Zr, As, Ba, Cd, Pb, and Zn. The total rare earth element (REE) concentrations of the deposits vary from 83 ppm to 521 ppm (mean 199 ppm). The Post-Archean Australian Shale (PAAS)-normalized REE concentrations have similar trends and show heavy REE (HREE) enrichments. All manganese-oxide samples show negative Ce (mean 0.58) and generally positive Eu (mean 1.20) anomalies, except for four samples. These geochemical data indicate that the Chiatura deposit precipitated rapidly in oxic seawater with a characteristic negative Ce anomaly. The carbonate and mixed ores compared to the oxide-hydroxide ores formed in a deeper-water environment of the shallow sea. Further, both Pb isotopic data and chemical discrimination diagrams show that the Chiatura deposit was affected by both hydrothermal activity located in deeper water, and by terrestrial input. The manganese metals in the oxide ores were syngenetically transported to the area of mineralization from deeper waters than the region where the carbonate ores formed.

Highlights • Geological characteristics of deep-sea minerals vary widely. • Deep-sea mineral occurrences differ in their resource potential. • Sizes of most favorable areas of formation influence exploration efforts. Abstract Marine minerals such as manganese nodules, Co-rich ferromanganese crusts, and seafloor massive sulfides are commonly seen as possible future resources that could potentially add to the global raw materials supply. At present, a proper assessment of these resources is not possible due to a severe lack of information regarding their size, distribution, and composition. It is clear, however, that manganese nodules and Co-rich ferromanganese crusts are a vast resource and mining them could have a profound impact on global metal markets, whereas the global resource potential of seafloor massive sulfides appears to be small. These deep-sea mineral commodities are formed by very different geological processes resulting in deposits with distinctly different characteristics. The geological boundary conditions also determine the size of any future mining operations and the area that will be affected by mining. Similarly, the sizes of the most favorable areas that need to be explored for a global resource assessment are also dependent on the geological environment. Size reaches 38 million km2 for manganese nodules, while those for Co-rich crusts (1.7 million km2) and massive sulfides (3.2 million km2) are much smaller. Moreover, different commodities are more abundant in some jurisdictions than in others. While only 19% of the favorable area for manganese nodules lies within the Exclusive Economic Zone of coastal states or is covered by proposals for the extension of the continental shelf, 42% of the favorable areas for massive sulfides and 54% for Co-rich crusts are located in EEZs.