SEQUENTIAL LEACHING OF MARINE FERROMANGANESE PRECIPITATES - GENETIC-IMPLICATIONS

Sequential leaching experiments were carried out on twenty-one hydroge netic crust samples from different locations in the central Pacific an d the results are compared with four crust and nodule samples of diffe rent genetic origin. Fe-vernadite (delta-MnO2) is the most important m inor and trace metal-bearing mineral phase and shows high concentratio ns of Co, Ni, Cd, Tl, Pa, Zn, and Cu, which are present in seawater ma inly as hydrated and labile complexed cations. Elements forming carbon ate and hydroxide complexes and oxyanions in seawater like Pb, Mo, V, and minor proportions of Cu and Zn are bound in the FeOOH fraction. Th e Ca carbonate phase, and the residual fraction which consists of alum inosilicates and crystalline oxides, show only minor heavy metal assoc iations, e.g., Cu and Zn. Only a small part of the high Ti concentrati ons in hydrogenetic crusts is of detrital origin; Ti mainly forms a hy drogenetic phase, probably consisting of TiO22H(2)O intergrown with t he amorphous FeOOH phase. Analyses of different sample types and crust layers show that aging and diagenetic effects can change the phase as sociations of some elements. The most notable change was observed for Pb which, during phosphatization of crusts, is transferred from the Mn -Fe oxide fraction to the apatite fraction. A colloid-chemical model f or the hydrogenetic precipitation of ferromanganese crusts on seamount s is proposed. In the first stage, Mn2+-rich water from the oxygen min imum zone is mixed with oxygen-rich deep-water, and oxidized Mn(IV) an d other metals like Fe, Ti, Al, and Si form oxide and hydroxide colloi ds. These form mixed colloidal phases and scavenge trace metals by sor ptive processes which are dominated by coulombic and chemical interact ions between colloid surfaces and dissolved metal species. In stage tw o, the colloidal phases precipitate on the substrate rocks of the seam ounts as ferromanganese oxide encrustations, incorporating the sorbed heavy metals into the mineral phases.