Sequential leaching of marine ferromanganese precipitates: Genetic implications

Sequential leaching experiments were carried out on twenty-one hydrogenetic crust samples from different locations in the central Pacific and the results are compared with four crust and nodule samples of different genetic origin. Fe-vernadite (delta-MnO2) is the most important minor and trace metal-bearing mineral phase and shows high concentrations of Co, Ni, Cd, Tl, Pa, Zn, and Cu, which are present in seawater mainly as hydrated and labile complexed cations. Elements forming carbonate and hydroxide complexes and oxyanions in seawater like Pb, Mo, V, and minor proportions of Cu and Zn are bound in the FeOOH fraction. The Ca carbonate phase, and the residual fraction which consists of aluminosilicates and crystalline oxides, show only minor heavy metal associations, e.g., Cu and Zn. Only a small part of the high Ti concentrations in hydrogenetic crusts is of detrital origin; Ti mainly forms a hydrogenetic phase, probably consisting of TiO2*2H(2)O intergrown with the amorphous FeOOH phase. Analyses of different sample types and crust layers show that aging and diagenetic effects can change the phase associations 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 for the hydrogenetic precipitation of ferromanganese crusts on seamounts is proposed. In the first stage, Mn2+-rich water from the oxygen minimum zone is mixed with oxygen-rich deep-water, and oxidized Mn(IV) and other metals like Fe, Ti, Al, and Si form oxide and hydroxide colloids. These form mixed colloidal phases and scavenge trace metals by sorptive processes which are dominated by coulombic and chemical interactions between colloid surfaces and dissolved metal species. In stage two, the colloidal phases precipitate on the substrate rocks of the seamounts as ferromanganese oxide encrustations, incorporating the sorbed heavy metals into the mineral phases.