Revista Brasileira de Geofísica
Print version ISSN 0102-261X
PALMA, Jorge J. C. and PESSANHA, Ivo B. M.. Deep-seabed ferrromanganese deposits. Rev. Bras. Geof. [online]. 2000, vol.18, n.3, pp. 433-446. ISSN 0102-261X. http://dx.doi.org/10.1590/S0102-261X2000000300015.
Deep-seabed Fe-Mn deposits are very promising mineral resources due to their high contents of important metals as Ni, Co, Cu, Mn, Pt, Tl, Te, and other metals. Nevertheless, they show so variable distribution, morphology, and physical, chemical, and mineralogical proprieties as to impact future mining operation and metallurgical processing. Systematic investigation of the main areas of occurrence in the Pacific Ocean and Central Indian Ocean Basin over the past 30 years allows one to identify main controls of formation and growth of nodule fields and polymetallic encrustations. In the present work this knowledge is summarily reviewed. The basic condition to form Fe-Mn deep-seabed deposits is a combination of sources of metals and circulation of superficial waters to provide a due flux of Fe, Mn, as well as minor elements like Na, Ca, Sr, Ni, Cu, Co, and Mo. Hydrogenetic and/or diagenetic processes may form Fe-Mn deposits. Hydrogenetic precipitation of colloidal Fe/Co-rich oxy-hydroxides may accrete the upper side of nodules or pave basaltic outcrops swept by bottom currents over million years. Low latitudes and weak to moderate primary productivity zones favor biogenetic and diagenetic growth of high-grade Mn, Ni, and Cu nodules. Diagenetic growth occurs mostly on seafloor 4000 m to 5500 m deep, in or below the range of calcite compensation depth. Moderate pelagic sedimentation and locally rough topography favor fast growth of Mn/Ni/Cu-rich nodules. South Atlantic geological and oceanographic settings apparently meet requirements for hydrogenetic formation of Co-rich crusts rather than Mn-rich nodules but only systematic studies will allow properly assess of potential deep-seabed Fe-Mn deposits in this region.
Keywords : Marine Mineral Resources; Polymetallic nodules; Manganese nodules; Polymetallic crusts.