Iron-rich metalliferous sediments on the East Pacific Rise: prototype of undifferentiated metalliferous sediments on divergent plate boundaries

Abstract

Sediments in a zone on the East Pacific Rise with an especially high spreading rate were studied chemically, mineralogically, and microscopically. They consist of a mixture of metalliferous sediments and plankton tests. The metalliferous sediments were formed by an acidic, hydrothermal leaching of tholeiitic basalt with seawater and subsequent precipitation in contact with cold near-bottom seawater. We assume the precipitation from hydrothermal solutions in this part of the East Pacific rise to be undifferentiated due to the high spreading rate and the resulting rapid flow on the water through the basalt. Thus, these metalliferous sediments are an initial stage type that have not undergone differentiation. Mn, Mo, La, Cu, V, Ni, Fe, Zn, Co, and Y, all of which are leachable in acidic, hydrothermal solutions, are enriched in the metalliferous sediments in comparison to the tholeiitic basalts. Zr, Al, and Ti, on the other hand, which under the same conditions are not easily leached, are reduced in their concentrations. All components of the metalliferous sediments precipitated as hydroxides or as adsorbed ions on the hydroxides of other elements. This is due to the oxidizing conditions in the near-bottom seawater. The sedimentation rate is high; the almost 3-m-long cores reach only to the Late Pleistocene. The only distinctly observable diagenetic process for this period of time is the formation of goethite from amorphous iron oxides. Only for Na, K, and Rb does it seem possible that a distinct enrichment in the sediments by adsorption from the seawater could have taken place. Ca, Sr, Pb, and perhaps Sc, are primarily bound to the planktonic carbonate part of the sediments.