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Evidence is increasing that the primary mechanism of metallogenesis at oceanic spreading centers is the convective cooling of basaltic intrusives and extrusives by circulating seawater. This massive circulation of seawater leaches and alters the basalt in fractures extending below the seafloor, and the mobilized metals are deposited in two major reservoirs: (1) a subsurface sulfide reservoir with reducing, acidic, high-temperature (~300°C) conditions and (2) a surface hydroxide reservoir with oxidizing, alkaline, low-temperature (~2°C) conditions. Whereas the subsurface reservoir corresponds to a classical hydrothermal ore deposit, the surficial metalliferous sediments found along the crest of the East Pacific Rise (EPR) and Bauer basin resemble iron-rich laterite in both mode of formation and major element composition. Minor and trace element enrichments have various origins. B, V, Sc, Y, and the rare earth elements are adsorbed from seawater, whereas Cu, Ni, Zn, and Ba originate from both biogenic and hydrothermal sources. Hydrothermal sources appear to be the dominant cause of iron and manganese enrichment. Manganese accumulates at up to 27 times the normal authigenic rate near 20°S lat. on the EPR crest. Manganese accumulation rates range from normal values to six times the authigenic rate in the Bauer basin, suggesting that scattered areas of localized hydrothermal metallogenesis exist there. Bottom-current winnowing and transport of EPR sediments is an alternative explanation for Bauer basin metallogenesis. The naturally carbonate-free etal contents of the Bauer basin sediments are assessed for their resource potential.
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