FERROMANGANESE CRUST AND NODULE DEPOSITS FROM THE CONTINENTAL-MARGIN SOUTH AND WEST OF TASMANIA

West of Tasmania and on the South Tasman Rise, from 41 degrees S to 50 degrees S, rocky outcrops in water depths of 1500-4500 m are commonly coated with ferromanganese crusts up to 20 cm thick. These crusts are unusually thick as compared to those in most parts of the world ocean . Manganese nodules of similar chemical composition are often associat ed with the crusts. The results from 43 samples analysed here build on previous work on a few samples that showed most crusts and nodule dep osits to be of low grade. Both types of deposits had been shown previo usly to be dominantly ferruginous vernadite and X-ray amorphous iron o xyhydroxide, and of hydrogenetic origin. The Tasmanian nodules are exc eptionally large in places and roughly spherical (up to 12 cm in diame ter). Unusually, many irregular nodules are cored by granite or other basement rocks. The nodules, excluding nuclei, average 12.1% Mn, 16.1% Fe, 0.14% Cu, 0.29% Ni and 0.20% Co. The three valuable metals (Cu, N i, Co) total 0.63% on average. These contents are not commercially int eresting as compared to those of the deep-ocean nodules from the centr al Indian Ocean and the northeast Pacific Ocean (high Ni and Cu), and the Cook Islands Exclusive Economic Zone (high Co). The crusts average 16.2% Mn, 17.2% Fe, 0.08% Cu, 0.32% Ni and 0.34% Co, the latter three valuable metals together averaging 0.74%, grades that are not of comm ercial interest. Calculated growth rates apparently vary from about 0. 5 mm/10(6) y above 2000 m water depth to about 20 mm/10(6) y below 300 0 m. Six bulk analyses of crusts taken at three stations within the lo wer part of the oxygen minimum zone (1500-2000 m water depth) average 0.79% cobalt, contents comparable to those in high-grade crusts from t he Marshall Islands, indicating that these shallow-water crusts have l ong-term economic potential. But the shallow-water crusts sampled are only 2-4 cm thick (approximately the cut-off grade for economic deposi ts). Further seabed sampling in the oxygen-minimum zone might reveal t hicker high-grade crusts.