Basal metalliferous sediments from sites 77B, 80 and 81 of the Deep Sea Drilling Project represent mixtures of pelagic clay, biogenic ooze, and a metalliferous component of hydrothermal origin. The metalliferous end-member of the sediments displays a strong inverse relationship (r = -0.88) between Mg and Mn. Mg is most likely tied up in an X-ray amorphous Mg-silicate ("sepiolite"), whereas Mn occurs almost exclusively in an oxide phase. Precipitation of the Mg-rich phase is favored by high flow rates and limited mixing of the hydrothermal end-member (source of silica) with seawater (source of Mg). Under those conditions much of the hydrothermal Mn2+, with its slow oxidation kinetics, may escape to the free water column. In contrast, in highly-diluted hydrothermal fluids, which provide a source solution for Mn-rich sediments, dissolved silica is diluted below saturation with respect to "sepiolite". The separation of the Mn and Mg phases may be further compounded by hydraulic fractionation. ?? 1981.