Mineralogy and chemical composition of ferromanganese crusts from the Cruzeiro do Sul Lineament - Rio Grande Rise, South Atlantic

Samples dredged from the Cruzeiro do Sul Lineament exhibit ferromanganese crusts precipitated above phosphate-rich substrates (phosphorites, phosphate-impregnated volcanic rocks and older phosphatized crusts). Co-chronology of crusts, paleontological data 87Sr/86Sr ratios substrate indicate that last phosphatization event in region happened between 17.6 14.6 Ma ago. When ceased, oxic conditions were resumed hydrogenetic crust precipitation started at a maximum rate 1.6–3.0 mm/Ma, forming layers Fe-vernadite phosphorites. These differ composition formed prior to phosphatization, which impregnated by carbonate fluorapatite recrystallized 10 Å Mn-phase, expense lower crystallinity Mn-phase. Mn/Fe non-phosphatized vary 1.05 1.41. The as well metal content (Co = 0.65–1.04 wt%, Zn 0.04–0.06 Cu 0.02–0.06 wt% Ni 0.29–0.48 wt%) are similar described other parts world.