Abstract
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.
Highlights
Introduction iationsHydrogenetic ferromanganese (Fe-Mn) oxide deposits are known to be distributed widely in all oceans of the planet, the largest known fields being located in the PacificOcean [1]
We report mineralogical and geochemical studies of 104 selected samples using analytical methods such as X-ray powder diffraction (XRD), X-ray fluorescence (XRF) and inductivelycoupled plasma mass spectrometry (ICP-MS), and discuss the nature, age and mineral associations of those Fe-Mn deposits
Two types of samples can be distinguished from the macroscopic study: (1) Brown to black Fe-Mn encrustations which are referred here as Fe-Mn crusts, and (2) Grey to dark and rarely brownish grey Mn-rich (±Ca-Fe) samples
Summary
Introduction iationsHydrogenetic ferromanganese (Fe-Mn) oxide deposits are known to be distributed widely in all oceans of the planet, the largest known fields being located in the PacificOcean [1]. Ferromanganese crusts grow by precipitation of metals from ambient cold seawater and accumulation on the seafloor, forming layers of Mn oxides and Fe-oxyhydroxides. Their thickness ranges from less than a millimeter up to 25 cm [4]. The hydrogenetic accumulation of Mn oxides and Fe-oxyhydroxides requires stable conditions over long periods of time (million years) to form thick crusts [5]. Their distribution, textures and composition are impacted by several parameters, such as surface bioproductivity, depth of the Licensee MDPI, Basel, Switzerland
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