Manganese deposits of the Devonian Magnitogorsk palaeovolcanic belt (Southern Urals, Russia)

Recent studies provide an overview on hydrothermal–volcaniclastic–sedimentary manganese deposits from the Magnitigorsk palaeovolcanic belt in Southern Urals, Russia (~ 100 deposits). This report presents data on the structures of ore bodies, chemical composition, and mineralogy of manganese ores. These deposits are divided into two types with different ore-bearing band structures, chemical and mineral ore compositions, and ore component accumulation conditions. Manganese ores of the type 1 deposit are associated with jasperites. Ores from type 2 deposits are localised within thin-banded jaspers. Type 1 deposits are thought to form during hydrothermal activity in areas where hydrothermal fluids leak into the seafloor. Type 2 deposits are formed within a closed basin as hydrothermal activity decreases away from hydrothermal vents, and typically contain a greater volcaniclastic contribution. Manganese ores comprised more than 70 mineral types and form three groups of mineral assemblages: (a) minerals formed during low grade regional metamorphism (T ≈ 200–260 °C, P = 2–3 kbar) that compose the main part of ore bodies, (b) mineral assemblages in segregatic and metasomatic veins filling late stage tectonic fractures, and (c) an assemblage of subsurface supergene minerals. Most of these minerals (which total approximately 38 minerals) are metamorphic silicates. The formation of metamorphic mineral assemblages is controlled by the presence of organic material (OM) in primary sediments. The conditions under which type 1 deposits formed favour the accumulation of OM. The metamorphism of these sediments is responsible for the formation of oxide–carbonate–silicate (“reduced”) ores, which consist of Mn-silicates and carbonates (such as tephroite, rhodonite, caryopilite, and rhodochrosite), andradite, quartz, hematite, and hausmannite. Insignificant preserved quantities of OM were found in type 2 deposits. During the metamorphic alteration of these sediments, oxide-silicate (“oxidised”) ores were formed with the dominance of braunite.