Abstract
The Rio Grande Rise (RGR) is a large elevation in the Atlantic Ocean and known to host potential mineral resources of ferromanganese crusts (Fe–Mn), but no investigation into their general characteristics have been made in detail. Here, we investigate the chemical and mineralogical composition, growth rates and ages of initiation, and phosphatization of relatively shallow-water (650–825 m) Fe–Mn crusts dredged from the summit of RGR by using computed tomography, X-ray diffraction, 87Sr/86Sr ratios, U–Th isotopes, and various analytical techniques to determine their chemical composition. Fe–Mn crusts from RGR have two distinct generations. The older one has an estimated age of initiation around 48–55 Ma and was extensively affected by post-depositional processes under suboxic conditions resulting in phosphatization during the Miocene (from 20 to 6.8 Ma). As a result, the older generation shows characteristics of diagenetic Fe–Mn deposits, such as low Fe/Mn ratios (mean 0.52), high Mn, Ni, and Li contents and the presence of a 10 Å phyllomanganate, combined with the highest P content among crusts (up to 7.7 wt %). The younger generation is typical of hydrogenetic crusts formed under oxic conditions, with a mean Fe/Mn ratio of 0.75 and mean Co content of 0.66 wt %, and has the highest mean contents of Bi, Nb, Ni, Te, Rh, Ru, and Pt among crusts formed elsewhere. The regeneration of nutrients from local biological productivity in the water column is the main source of metals to crusts, providing mainly metals that regenerate rapidly in the water column and are made available at relatively shallow water depths (Ni, As, V, and Cd), at the expense of metals of slower regeneration (Si and Cu). Additionally, important contributions of nutrients may derive from various water masses, especially the South Atlantic Mode Water and Antarctic Intermediate Water (AAIW). Bulk Fe–Mn crusts from the summit of RGR plateau are generally depleted in metals considered of greatest economic interest in crusts like Co, REE, Mo, Te, and Zr, but are the most enriched in the critical metals Ni and Li compared to other crusts. Further investigations are warranted on Fe–Mn crusts from deeper-water depths along the RGR plateau and surrounding areas, which would less likely be affected by phosphatization.
Highlights
Ferromanganese (Fe–Mn) crusts are ubiquitous chemical sedimentary rocks occurring on oceanic elevations formed by the precipitation of Fe oxyhydroxide and Mn oxide directly from seawater onto a hard surface [1]
Carbonate Fluorapatite (CFA) is the main phosphate mineral that impregnates Fe–Mn crusts during phosphatization events, and it has white to off-white colors, which accounts for the whitish haze in phosphatized Fe–Mn crusts
The mineralogical terminologies of marine Mn oxides are not always clear and often divergent, the compilation of [35] was used here because it offers a practical guide based on systematic descriptions of crystallographic features based on synchrotron studies
Summary
Ferromanganese (Fe–Mn) crusts are ubiquitous chemical sedimentary rocks occurring on oceanic elevations (ridges, seamounts, and guyots) formed by the precipitation of Fe oxyhydroxide and Mn oxide directly from seawater onto a hard surface [1]. This process is called hydrogenesis and is remarkably. Co, Te, Bi, and many heavy rare earth elements (REE) plus yttrium (HREY) are more enriched in Fe–Mn crusts compared to most land-based mineral deposits. For these reasons, interest in mining Fe–Mn deposits from the ocean floor has intensified
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