The chemistry and mineralogy of ferromanganese encrustations on rocks from the Sierra Leone Rise, equatorial Mid-Atlantic Ridge and New England Seamount Chain

Abstract

Chemical and mineralogical compositions of ferromanganese oxide coatings on rocks dredged from the New England Seamounts, the Sierra Leone Rise and the Mid-Atlantic Ridge near the Equator have been determined in an investigation of regional differences in Atlantic ferromanganese deposits. Most encrustations are clearly of hydrogenous origin, consisting mainly of todorokite and δMnO 2, but several recovered from the equatorial fracture zones may be hydrothermal accumulations. Mn Fe ratios are similar to those observed in nodules and encrustations elsewhere in the Atlantic, generally falling below unity except for samples from the New England Seamounts. Cu (0.02-0.2%) is lower than in other Atlantic deposits. Ni contents vary from 0.064 to 0.478%, the latter value being found near the St. Paul's Fracture Zone. A large difference in Co is observed between the Northwest and Equatorial Atlantic (0.048-0.135%, New England Seamounts; 0.355-1.010%, near the Equator), ferromanganese coatings on phosphatized limestones from the Sierra Leone Rise being richest in Co. Differences in the chemistry of the water column and in growth rates of the ferromanganese coatings may be important in producing this regional contrast in composition. Fine-scale changes in element abundances within the encrustations indicate that the nature of the substrate has little influence on compositional variations.