Mineralogy, geochemistry and genesis of the polymetallic crusts and nodules from the South China Sea

Abstract

Recent marine mineral investigations in the South China Sea (SCS) have found increasingly significant abundances of Fe–Mn polymetallic crusts and nodules with variable substrates and nuclei. There is, however, no systematic and complete study of the SCS crusts and nodules. The lack of mineralogical and geochemical studies affects our explicit understanding of the mineralization characteristics and genesis of the crusts and nodules. In this study, we analyzed a set of SCS polymetallic crusts and nodules obtained from ten sites to determine their mineralogy, chemistry, and Fe and Os isotopes. The SCS crusts/nodules are composed predominantly of vernadite (δ–MnO2) and amorphous Fe oxides/hydroxides. The nodules also contain minor todorokite. TEM imaging and electron diffractions show that the Fe–Mn minerals are nano–sized minerals with very poor crystallinity. Nitrogen adsorption/desorption analysis indicates an average specific surface area of 257.128 m2/g for the SCS crusts and nodules, and main apertures of 2–10nm for their nanopores in the Fe–Mn minerals. It is noteworthy that the SCS crusts/nodules contain substantial amount of terrigenous detritus, suggesting that marginal sea–type Fe–Mn deposits are strongly influenced by terrigenous sedimentary input. Furthermore, geochemical analysis indicates that the SCS crusts/nodules have relatively lower Co (1482ppm), Cu (556.2ppm), Ni (3132ppm), Mo (276.2ppm), and Zn (598.3ppm) compared to open oceanic crusts/nodules, but relatively high concentrations of REY (1647ppm), PGE (234.0ppb), Ba (1056ppm), Pb (1878ppm), Sr (995.9ppm), Te (28.81ppm), and Tl (67.87ppm). Iron isotopic compositions suggest that the Fe sources of the SCS crusts/nodules may have been Fe dissolved in terrigenous (fluvial) inputs. Osmium isotopes indicate that the main ore–forming elements came directly from the ambient seawater, which was mainly influenced by terrigenous inputs, although there may have been mixing with minor mantle–derived signatures.Evidence from mineralogy, major element geochemistry, shale–normalized rare earth element (REE) patterns, and the Fe and Os isotopes suggest that the SCS crusts/nodules are mainly hydrogenetic, whilst the formation of some nodules may have been contributed by minor pore waters of their surrounding sediments. The SCS crusts/nodules had probably much higher growth rates (mean 19.20mm/Ma) than most hydrogenetic Fe–Mn deposits in other open oceans, which are typical of marginal sea Fe–Mn deposits that contain sufficient supply of ore–forming terrigenous detritus.Grades of some critical metals of economic interest (notably Co, Cu, Ni, Mo, and Zn) are slightly lower in the SCS crusts and nodules than in the crust and nodules from Pacific prime mineralization zones. Concentrations of Mn, REY, Pt, Mo, Co, Te, and Bi, however, show relatively high concentrations relative to the continental crust. The SCS crusts and nodules could thus represent a strategic REE, Co and Pt resource, and will likely catalyse additional investigation of seabed resources in the South China Sea.