Morphology of Central Pacific Seamounts: Implications for Mn-Crust Mining

Abstract

ABSTRACT Mining of cobalt-rich manganese crusts in the upper slopes and summits of seamounts, is likely to take place in the Central Pacific Basin. The distribution, abundance, and mineralogy of these deposits are influenced by their geologic setting. The mineral deposits are generally thickest on the most stable (least mobile) substrates. But they will be most readily mined on friable substrates (such as volcanic breccias). Characteristics conducive to mining include: minimum slope angles, relative absence of bathymetric obstacles (benches, fault scarps, and volcanic features), absence of poorly consolidated sediments which minimize manganese crust growth, and presence of amenable substrates. On the basis of these criteria large portions of the Line Islands chain are targets for marine mining. Many of the seamounts within the Line Island group, however, are characterized by rejuvenated volcanism, which has resulted in very rugged summits and a thick mantling of the seamount with volcanoclastic rocks. In order to successfully mine crusts from the Line Islands seamounts, site specific surveys will be needed to identify sites with minimal bathymetric obstacles. Small, very mobile mining devices would be appropriate for this rugged terrain. INTRODUCTION Mining of cobalt-rich manganese crusts on the upper slopes and summits of seamounts is likely to take place in the Central Pacific Basin because (1) the crusts are richer in metal content and more extensively distributed than previously recognized, (2) many crust deposits are situated within Exclusive Economic Zones of island nations seeking economic development, (3) the deposits occur at relatively shallow depths compared to manganese nodules and (4) these deposits provide alternate sources of strategic minerals and rare earth elements. Manganese crust resource studies have been carried out in several seamount chains in the Central Pacific Basin by Japanese, French, and German scientists. The proposed Sale of Marine Mineral leases in the Hawaiian Archipelago and Johnston Island Exclusive Economic Zones raises the possibility of manganese crust mining on seamounts in the Hawaiian and Line Islands seamount chains. Since the distribution, abundance and mineralogy of the manganese crusts are strongly influenced by their geologic setting, it is important that we gain a clearer understanding of the geologic setting in which manganese crusts are formed. Geological and geophysical studies of Central Pacific seamounts of the Hawaiian, Line, Gilbert and Phoenix Islands (Fig. 1) indicate that the distribution of bathymetric features, seamount shapes, and substrates vary significantly from one province to another. These reflect differences in geologic history of the seamount provinces. In this paper, seamount characteristics are compared for the Line Islands (Island nation of Kiribati) and Hawaiian seamount chains. Examination of seamounts in the Hawaiian chain show limited resources, because of the youth of the seamount chain. Resource potential is greater in the Line Islands seamount chain, in large part because the crusts are much thicker in that seamount chain. The Line Islands chain however is characterized by rough topography, which would make mining using conventional dredge-type apparatus difficult.