Ferromanganese Crusts near Johnston Island: Geochemistry, Stratigraphy and Economic Potential

Abstract

The 200-mi exclusive economic zone (EEZ) around Johnston Island, 700 mi west of Hawaii, contains economically attractive concentrations of cobalt, nickel, platinum, and rare earth elements contained in manganese crusts on the surface of seafloor plateau areas. Detailed mineralogy and geochemistry reveal these deposits to be largely hydrogenous in origin although also containing elemental additions due to biological, weathering, hydrothermal, detrital, and diagenetic processes. The major mineralogical phase is vernadite (delta manganese dioxide) laid down in strataform crustal deposits. The most valuable metal, cobalt, shows concentrations highest in the upper layers of the crusts and decreasing with stratigraphic depth. Upcoming submersible dives will fully document the significance of this deposit.