Modern submarine hydrothermal mineralization: examples from Santorini and the Red Sea

Summary Submarine hydrothermal mineralization is known to occur on parts of the World Mid-Ocean Ridge System and in association with island arc volcanism. Two areas of submarine hydrothermal activity have been examined in detail — one in the Atlantis II Deep of the Red Sea and the other off the volcano of Santorini in the Cyclades Volcanic Arc, eastern Mediterranean. In each case hydrothermal solutions are mixing with sea water, resulting in the fractionation and precipitation of a variety of metal-bearing phases as the physico-chemical properties of the solution change. Sulphides occur locally as a minor component of the newly precipitated hydrothermal sediments off Santorini, which are principally composed of iron oxides. Iron precipitates close to the hydrothermal outlets and decreases in concentration away from the volcanic source. By contrast, manganese is low in the sediments richest in iron, but increases in concentration more than twenty-fold away from the hydrothermal outlets and iron-rich zone. The distribution of certain minor elements in the sediments follows that of Mn (e.g. Zn) and Fe (e.g. Mo), indicating that scavenging may be a control on their distribution. Early-formed precipitates from the Atlantis II Deep brines also include sulphides, principally of Cu, Zn and Fe. Subsequent precipitates include iron silicates, after which the remaining iron is oxidized and precipitated as ferric oxide and hydroxide. Finally, Mn2+ is oxidized to Mn4+ and can scavenge considerable quantities of trace metals out of solution on precipitation. Manganese precipitated in the waters over the Deep is probably redissolved on falling back into the brine. By contrast, manganese and those minor elements that escape from the Deep can precipitate up to 10 km from it, giving rise to well-developed geochemical dispersion haloes. The selective fractionation and precipitation of hydrothermally introduced metals on mixing with sea water in these two environments results in a geochemical zonation away from the metal source in each case. The principal fractionation, other than the early precipitation of metal sulphides, is between Fe and Mn, but some other metals also show distinctive concentration gradients. These observations may be of great potential value in geochemical exploration for submarine hydrothermal orebodies, as the extent of the dispersion of the most mobile elements can be many times the extent of the potential orebody itself. The significance of these observations in regard to metallogenesis on mid-ocean ridges in general has been discussed elsewhere. Metalliferous sediments found to date on the World Mid-Ocean Ridge System are of much lower grade than those in the Red Sea, but these may, in part, represent the analogues of the widely dispersed Fe and Mn oxides around the Atlantis II Deep. On this basis, Cronan concluded that higher-grade deposits, including metal-rich sulphides, might occur within the upper part of the oceanic crust and within the deep fracture zones that cut mid-ocean ridges in all the oceans. Furthermore, studies on the geochemical relationship between metalliferous sediments and ferromanganese oxides on mid-ocean ridges suggest that the two are genetically related in that they receive their metals from the same sources, but in different proportions and at different rates. Ancient volcanogenic orebodies sometimes show features similar to those described here, indicating that in the modern environment we may be observing a not uncommon volcanic ore-forming process in operation.