Geochemistry and crystallochemistry of oceanic hydrothermal manganese oxyhydroxides showing Mn-Cu association
Hydrothermal iron and manganese oxides have been found in association with sulfides dredged on the E.P.R. near 7°N. The Mn phase, mainly a 10-7 Å phyllomanganate, presents a very important enrichment in Cu (up to 30% as weight of oxide). The Fe phase, mainly hydro-goethite, has a very low content of metals of economic interest. Also Mn-Cu oxide particles have been collected in sediment traps near the hydrothermal vents at 13°N. Using the Mn oxide samples of 7°N, Cu shows two simultaneous oxidation states: +I and +11 (ESCA and XAS edge measurements). Cu is adsorbed on the Mn(O,OH) 2 layers and partially belongs to the water layers (EXAFS results). This seems to be the first report of naturally occurring Cu-buserite in this environment. Lead isotope abundances, the presence of Cu(I), thermodynamic considerations on the stability of the Mn-Cu oxyhydroxides and unsuccessful attemps made with synthetic 10-7 Å phyllomanganates (buserite and birnessite types) at low temperature in order to stabilize Cu(I) and incorporate it in the interlamellar space of the manganate, lead us to accept a high temperature origin for the formation of the Mn-Cu oxyhydroxides. We present a transport model for Cu and Mn precipitation from oceanic hydrothermal fluid, to explain the formation of Mn-Cu oxyhydroxides.