Report of a combined OGM and Marine, Mineral Deposits and Volcanic Studies Group Meeting held at Burlington House, 21–22 November 1978. The meeting was organized by Dr D. S. Cronan.

Metalliferous sediments associated with active spreading centres can be divided into 3 types: (1) sulphide deposits associated with silicates and oxides, as in the Red Sea; (2) sharply fractionated silicate and oxide deposits of very localized extent, as on some mid-ocean ridges in the open ocean such as in the FAMOUS area; and (3) widely dispersed predominantly oxidic deposits of iron and manganese which comprise the bulk of metalliferous sediments on mid-ocean ridges. These different deposits could all be formed as a result of fractional precipitation of metals from hydrothermal solutions derived by seawater leaching hot newly formed oceanic crust. Sulphide deposits would represent the early formed precipitates, silicates those precipitated next, and oxides those precipitated last. The widely dispersed predominantly oxide bearing metalliferous sediments common on mid-ocean ridges have been formed as a result of iron and manganese bearing solutions representing the residual liquids of sub sea-floor hydrothermal fractionation processes discharging from hydrothermal vents, together with some particulate iron and manganese oxides, becoming well mixed in the bottom waters, and precipating their constituents relatively uniformly over a wide area. That such a process is not confined just to the present time is evinced by the widespread occurrence of metalliferous sediments directly overlying oceanic basement found in many Deep Sea Drilling Project cores on the flanks of mid-ocean ridges.

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