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.

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.

Located in the South-West Pacific, at the northern extremity of the mostly submerged Zealandia continent, the New Caledonian Exclusive Economic Zone (EEZ) covers 1,470,000 km² and includes basins, ridges and seamounts where abundant ferromanganese crusts have been observed. Several investigations have been conducted since the 1970s on the nature and composition of ferromanganese crusts from New Caledonia’s seamounts and ridges, but none have covered the entire EEZ. We present data from 104 ferromanganese crusts collected in New Caledonia’s EEZ during twelve oceanographic cruises between 1974 and 2019. Samples were analysed for mineralogy, geochemical compositions, growth rates, and through a statistical approach using correlation coefficients and factor analysis. Crust thicknesses range from 1 mm to 115 mm, with growth rates between 0.45 mm/Ma and 102 mm/Ma. Based on textures, structures, discrimination plots, and growth rates, we distinguish a group of hydrogenetic crusts containing the highest mean contents of Co (0.42 wt%), Ni (0.31 wt%), and high contents of Mo, V, W, Pb, Zn, Nb, from a group of hydrothermal and/or diagenetic deposits showing high mean contents of Mn (38.17 wt%), Ba (0.56 wt%) and low contents of other trace metals. Several samples from this later group have exceptionally high content of Ni (0.7 wt%). The data shows that crusts from the southern part of the EEZ, notably seamounts of the Loyalty Ridge and the Lord Howe Rise, present high mineral potential for prospectivity owing to high contents of valuable metals, and constitute a great target for further investigation.

Eight ferromanganese crust samples spanning the complete depth range of Tropic Seamount in the northeast Atlantic were analyzed for Pb and Nd isotopes to reconstruct water mass origin and mixing over the last 75 Ma. Pb isotopes were determined by laser ablation multicollector inductively coupled plasma mass spectrometer (LA‐MC‐ICP‐MS), which enables the rapid production of large, high spatial resolution data sets. This makes it possible to precisely correlate stratigraphy between different samples, compare contemporaneous layers, and create a composite record given the abundance of hiatuses in crusts. Pb and Nd isotope data show the influence of various oceanic and continental end‐members in the northeast Atlantic Ocean. This reflects its evolution from a restricted, isolated basins in the Late Cretaceous with influxes from the Tethys Ocean, to an increasingly well‐mixed, large‐scale basin, with a dominant Southern Ocean signature until the Miocene. Less radiogenic Nd isotope signatures suggest Labrador Sea Water influenced the northeast Atlantic basin as early as 17–15 Ma, flowing through a northern route such as the Charlie‐Gibbs Fracture Zone. Pb and Nd isotopes highlight the increasing influence of Saharan eolian dust input about 7 Ma, imparting a less radiogenic excursion to the binary mixing between North Atlantic water masses and riverine discharge from West and Central Africa. This highlights the influence of eolian dust input on the open ocean Pb and Nd budget and supports an early stage of North African aridification in the Late Miocene. This signature is overprinted about 3 Ma to the present by a strong North Atlantic Deep Water signature following the onset of Northern Hemisphere glaciation.

Seafloor massive sulphide (SMS) deposits are potential sources of Cu, Zn, Au and Ag, however, how seafloor weathering affects the content of these metals in the deposits remains poorly understood. Key sulphide weathering products, i.e. Fe-oxyhydroxide and atacamite, can provide insight into metal mobilisation during and after oxidation of sulphides. Ten Fe-oxyhydroxide and three massive sulphide samples from the Semenov hydrothermal cluster (13°30′N, Mid-Atlantic Ridge) were studied; five Fe-oxyhydroxides and all sulphides were analysed by LA-ICP-MS with one sulphide sample analysed by LA-TOF-ICP-MS mapping. Additionally, a sequential leaching study comprising nine Fe-oxyhydroxide samples was performed. Copper is mainly sequestered into the sulphide weathering products atacamite (~ 60%) and Fe-oxyhydroxide (~ 30%). Initially, Fe‑oxyhydroxide precipitates as ferrihydrite, inheriting the metal content of its sulphide precursor (e.g., pyrite or chalcopyrite). Concurrently, Au and Ag are displaced from the sulphide and become concentrated along the oxidation front at the sulphide rims rather than being incorporated into the nascent Fe‑oxyhydroxide. As oxidation proceeds, further breakdown of sulphides such as chalcopyrite, releases Cu that initially adsorb onto existing Fe‑oxyhydroxide. When sufficient mobilised Cu reacts with oxygenated seawater, atacamite precipitates, sequestering the Cu in a stable mineral form, serving as an exploration indicator for Cu‑rich Fe‑oxyhydroxide deposits and underlying massive sulphide deposits. Complete sulphide oxidation leads to the loss of Au and Ag from the solid system into seawater or their migration downwards toward a redox boundary within the SMS mound. Crystallisation of ferrihydrite into more stable goethite liberates substantial amounts of Cu (92%) and Zn (57%). However, the metal loss caused by this process may be limited if released metals re-adsorbed onto adjacent Fe‑oxyhydroxide. These results reveal that Fe‑oxyhydroxides can act as effective long‑term traps for Cu and Zn, suggesting that older, fully or partially oxidised off-axis SMS deposits may retain considerable base metals, expanding exploration areas.