The World mid-Ocean Ridge System is a volcanically active area of the sea floor some tens of thousands of km in length, at which new ocean crust is being created. Associated with crustal formation is the leaching of newly solidified lavas by circulating sea water, leading to the formation of metal-rich hydrothermal solutions (Corliss 1971). The composition of these solutions depends on a number of factors including the ratio of water, available for leaching to rock being leached (Seyfried & Bischoff 1977), the composition of the rocks, the temperature of leaching, and the nature of the leaching and other chemical reactions taking place below the sea floor. Nevertheless, in many instances it is evident that hydrothermal solutions rich in Fe, Mn, Cu, Zn and S, and sometimes other elements, formed as a result of the sub sea-floor leaching, are circulating in the upper part of the oceanic crust under mid-ocean ridges, and periodically discharge onto the sea floor. The sequence of precipitates that occur when such hydrothermal solutions rise, cool, and mix with sea water is probably best exemplified in the Atlantis II Deep of the Red Sea, where sulphides, silicates and oxides, in that order, all form by fractional precipitation (Bischoff 1969). Nowhere else in the oceans has such a sequence of hydrothermal precipitates been found in one place, but parts of the sequence occur in a number of locations which, combined, allow us to view this mid-ocean ridge ore-forming process as a whole.

Metal-rich hydrothermal deposits found to

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