GEOCHEMISTRY AND MINERALOGY OF SEAFLOOR HYDROTHERMAL AND HYDROGENETIC MN OXIDE DEPOSITS FROM THE MANUS BASIN AND BISMARCK ARCHIPELAGO REGION OF THE SOUTHWEST PACIFIC-OCEAN

Petrographic, chemical and mineralogical data are presented for ferromanganese deposits from the Manus Basin-Bismarck Archipelago region of the southwest Pacific. The samples were dredged from water depths of between 800 and 2700 m, range up to 3 cm in thickness, and ocur on a variety of substrates including basalt and partially lithified sediment. The physical appearance, bulk composition and mineralogy of the majority of these samples suggest a hydrogenetic origin. However, one sample from northwest of Manus Island and two from inside the active spreading ridge of the Manus Basin show extreme fractionation of Fe and Mn, depletion of minor trace metals and radiometric isotope compositions indicative of a hydrothermal origin, the first report of such material from this region. The Co content in the deposits is low compared to crusts from other parts of the Pacific Basin, and thus it would appear that the resource potential for Co in the Manus Basin-Bismarck Archipelago region may be limited. The presence of Mn-rich crusts of hydrothermal origin does, however, suggest that further sampling in selected areas for potentially economic reserves of polymetallic sulfides may be warranted.