Ferromanganese Crusts in the South Atlantic Ocean: Compositional Evolution and Specific Features of Ore Formation

Resumo

Abstract—Layer-by-layer chemical and mineralogical analysis was carried out to consider the evolution of the Fe–Mn crusts in different tectonic settings in the South Atlantic Ocean. The ferromanganese crust in the rift valley of the Mid-Atlantic Ridge was formed owing to the hydrothermal–hydrogenetic supply. The formation of crust in the Transform Fracture zones of the Angola Basin was related to the halmyrolysis of magmatic rocks and subsequent hydrogenetic supply, which affected the proportions of goethite and vernadite, the Mn/Fe ratio, and Ce anomaly, as well as the content of some elements (Co, Cu, Al, REE). It is revealed that the Fe–Mn crusts taken in the abyssal part and on the seamount top in the Cape Basin are hydrogenetic in origin and consist of vernadite significantly enriched in Co, Ce, Tl, and Pt. The relationship of trace elements with manganese minerals is estimated by leaching of crusts by reagents, which either completely dissolve manganese oxyhydroxides (1 M NH₂OH · HCl + 25%CH₃COOH) or no (3.5 N H₂SO₄). It was shown that 100% Co and only 28% Ce are incorporated in the manganese minerals, while REE, Y, Sr, and U form a sorption complex of oxhyhydroxides. To estimate the chemical peculiarities of the hydrogenetic Fe–Mn crusts of the Atlantic Ocean, they were compared with the crusts of the Pacific Ocean. It is shown that the crusts of the Pacific Ocean have higher manganese and cobalt contents. The additional manganese could be derived from a hydrothermal source.