GENESIS AND RESOURCE POTENTIAL OF METAL-RICH FERROMANGANESE CRUSTS

Metal-rich ferromanganese-oxide crusts occur throughout the global ocean on seamounts, ridges, and plateaus where currents have swept the rocks clean of sediments for millions of years. Crusts precipitate from cold ambient ocean water onto rock substrates forming pavements up to 250 mm thick. Crusts form at water depths of about 400-4000 m, with the thickest and most Co-rich crusts occurring at depths of 800-2500 m. Gravity processes, sediment cover, submerged and emergent reefs, and currents control the distribution and thickness of crusts. Crusts are composed of vernadite and X-ray amorphous iron oxyhydroxide, with minor amounts of quartz and feldspar in most crusts and moderate amounts of carbonate fluorapatite in thick crusts. The physical properties of crusts, such as high mean porosity (60%), extremely high mean specific-surface area (300 m2/g), and their incredibly slow rates of growth (1-5 mm/Ma) allow for the adsorption of large quantities of economically important metals from ocean water onto the crust surfaces. Elements that commonly adsorb on vernadite are Co, Ni, Zn, and Tl, and on iron oxyhydroxide are Cu, Pb, Ti, Mo, As, V, W, Zr, Bi, and Te. Partitioning of elements between the Mn and Fe phases occurs because of the sorption of cationic ocean-water species onto the negatively charged surfaces of Mn colloids and sorption of anionic and neutrally charged ocean-water species onto the slightly positively charged surfaces of Fe colloids. Many of the most highly enriched metals undergo an oxidation reaction after adsorption onto the Mn (Co, Ce, and probably Tl) and Fe (likely Te and Pt) oxide surfaces. Crusts have been considered an important potential resource for Co, Ni, and Cu. However, it has recently been determined that crusts contain other metals that may offer significant additional incentive for recovery. For example, Ti has the highest value after Co, Ce has a greater value than Ni, Zr is equivalent to Ni, and Te has nearly twice the value of Cu. This analysis assumes that economic extractive metallurgy can be developed for each of those metals. Traditional economic evaluation of crust and nodule mining for Cu, Ni, and Co indicates that crust mining would be competitive with nodule mining only at times when the price of Co is high. However, adding Ti, rare-earth elements, Zr, Te, and possibly other metals to the economic models could change this outlook.