Thick ferromanganese deposits from the Dampier Ridge and the Lord Howe Rise off eastern Australia

Chemical and mineralogical data are presented for four ferromanganese samples (two nodules and two crusts) from two stations of the West German vessel Sonne. Three samples came from a dredge on the flanks of the Dampier Ridge in water 2400- 2700 m deep. One came from a core on the Lord Howe Rise in water 1549 m deep. Thick ferromanganese deposits overlie a variety of substrates including granite, gabbro and feldspathic sandstone. The ferromanganese deposits, which are up to 20 cm thick, range from round mononucleate nodules with small volcanic nuclei, to polynucleate nodules, to nodules bound together as crusts, and to laminated crusts. Both nodules and crusts are hydrogenetic in origin and have low contents of Ni, Cu and Co, and low Mn:Fe ratios of 0.48-0.91. A comparison of these results with those from three deeper water stations of Galathea and Tangaroa indicates that Mn:Fe ratios, Ni% and Cu% increase markedly in deeper water, where Mn:Fe ratios exceed 2.5, and Ni+Cu+Co values exceed 1.25%. Any future search for nodules of economic significance should be concentrated in the even deeper water areas (>5000 m) east and southeast of Gascoyne Seamount.