Abstract
Abstract This paper considers the occurrence of Mn-oxide-rich hydrothermal deposits in the modern oceans and presents a case history of Jurassic Mn-cherts from a Tethyan area in Southern Greece. Manganese-oxide hydrothermal sediments are known from a wide variety of oceanic settings, including both on and off the axis of spreading ridges, in back-arc basins, arc-related areas, rifts and in small ocean basins. These Mn-rich sediments differ markedly from the chemical composition of better known metalliferous deposits associated with high-temperature black smokers (i.e. Fe-rich ochres and Fe-Mn-rich umbers). The ancient example discussed in detail here is that of Late Jurassic Mn-rich cherts spatially associated with basic-intermediate composition volcanics within sutured oceanic units in the Peloponnese, S Greece. The Mn enrichment there is interpreted as related to hydrothermal activity associated with Late Jurassic off-axis volcanism, rather than to a spreading ridge source. The Mn is strongly fractionated from Fe and is concentrated in thin beds and laminations, interpreted as of primary depositional origin. The silica is assumed to be mainly biogenic, as reflected in an association with red radiolarian cherts. The source hydrothermal activity is likely to have been of low-temperature type. Similar Mn-oxide deposits occur elsewhere in the Mesozoic Tethyan area, including N Greece, SW Turkey, Cyprus and Oman, and in other orogenic belts (e.g. in California). There are also metamorphosed counterparts of Mn-rich hydrothermal deposits that include metalliferous pelites and cherts; e.g. Cyclades, S Greece and Franciscan Complex, California. The record of deep-sea hydrothermal deposits exposed on land following tectonic emplacement is biased towards off-axis Mn-deposits within abyssal sediments, since axial spreading ridges (including massive sulphides, umbers and ochres) are rarely preserved, owing to subduction.
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