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ABSTRACT: Oceanic Ferromanganese Geochemistry

Sergei I. Andreev

The diversity of ferromanganese oxide deposits is due to variable manganese and iron hydroxide contents on one hand, and that of the transition metals nickel, copper, and cobalt as major ore components on the other. Manganese nodules used to be the assessment criterion; however, more information indicated that they do not determine the ratios and contents of ore components, and, therefore, are not a universal index of geochemical types. Geochemical classification of the oceanic ferromanganese deposits might be made based on cobalt and nickel+copper contents. Using a new method of geochemical classification, we managed to recognize from the ferromanganese series six geochemical types of nodules and crusts. They frequently differ in cobalt and nickel+copper concentrations: larion-Clipperton type, rich in nickel and copper (summing over 1.7%); Peruvian type, rich in nickel (>=1.0%) and twice depleted in copper; Central Pacific type showing both low cobalt (^sim0.3%) and nickel+copper (0.9-1.7%); South Pacific type with persistent high cobalt concentrations (0.3-0.4%) and low nickel+copper (<0.9%); Hawaiian type showing high cobalt (0.4% and more), low nickel (0.4-0.6%), and drastically low copper (<0.2%). The sixth is the hydrothermal geochemical type showing extraordinarily low transitional metals (<0.10%) at quite variable manganese and iron concentrations. Each of the geochemical types of nodules and crusts is formed under predominantly one of four mechanisms: hydrogenetic sedimentary, diagenetic, or hydrothermal. The types occur on the seafl or, being in strict subordination to vertical zonality with respect to carbonate compensation depth (CCD). Clarion-Clipperton type is immediately below CCD; Peruvian type is colinear and mainly beneath CCD; Central Pacific type is immediately above; South Pacific type is 500 to 600 meters beneath the CCD; and Hawaiian type is much higher than CCD at the oxygen minimum depth. Hydrothermal crusts are formed within a wide depth range, regardless of CCD.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990