Geochemistry of ferromanganese crusts and nodules from the South Tasman Rise, southeast of Australia

Eighteen samples of ferromanganese oxide crusts up to 15 cm thick, and nodules, have been dredged or cored from elevated parts of the seafloor south of Tasmania. Ferromanganese deposits overlie a variety of basement rocks. Nodules are dominantly ellipsoidal and mononucleate, and range up to 13 cm in diameter. Surface textures for both crusts and nodules range from smooth to rough, gritty or botryoidal. Typically internally laminated, the ferromanganese deposits are dominantly ferruginous vernadite, while quartz, plagioclase, fluorapatite, phillipsite, goethite, and, in five samples todorokite, are minor phases. Both crusts and nodules show enrichment of Fe, Co and Pb and depletion of Mn, Ni, Cu and Zn, to be expected with a vernadite-dominated mineralogy. Cobalt and Pb content increase with decreasing water depth, with highest values in the range 1–2 km. The mineralogy, chemistry and physical appearance of the deposits indicate a hydrogenetic origin, probably in an environment swept by strong, erosive, and oxygenated bottom currents. The samples analyzed are similar in composition to seamount and ridge crusts and nodules from elsewhere in the Southwest Pacific. They are markedly different to those described from the North Pacific and, in particular, are generally much lower in Co. One Co-rich crust from a seamount on the South Tasman Rise (water depth 1700 m) does not fit in the distribution of Co in ferromanganese deposits of the southern Pacific, and may reflect a local zone of oxygen depletion associated with the subtropical convergence. Its significant enrichment in Co (0.8–1.0%) suggests a detailed survey may be warranted to fully assess the economic potential of this material.