The Kyzyl-Tash manganese deposit (South Urals, Russia): The Devonian prototype of a low-temperature hydrothermal mound of the modern ocean

The paper presents a detailed description of the geological structure and c omposition of the Kyzyl-Tash manganese deposit, which is located within the Irendyk structural-lithological zone of the Magnitogorsk-Mugodzhary paleov olcanic belt. Manganese ores are spatially and genetically related to a lar ge silicite body occurring within the tuff sequence of the Ulutau Formation (D(2)ef-D(3)fr(1)). The main of the silicite body is composed of hydrother mal hematite-quartz rocks (jasperites). The deposit flanks include also bed ded jaspers. Mineralization is localized in the topmost part of the silicit e body. Ore bodies are characterized by a lenticular form and confined to d epressions in the paleohydrothermal mound-on the southern and northern slop es, as well as in its central part, between two summits. Two genetic types are recognized amid manganese ores: (1) carbonate-silicat e ores formed in the course of metamorphic transformation of Mn-bearing roc ks; and (2) oxide ores representing a product of near-surface oxidation of Mn minerals. Hypergene ores show a limited distribution. The main type of M n ores of the deposit is represented by carbonate-silicate ores. In mineral composition, there are several ore varieties: (1) caryopilite-carbonate-te phroite, (2) rhodonite, (3) calcite-rhodonits-quartz, (4) andradite-rhodoni te, (5) rhodonite-hematite-andradite, (6) caryopilite, (7) rhodonite-grossu lar-caryriopilite, (8) caryopilite-rhodonite-tephroite, and (9) epidote-rho donite. The ore-bearing silicite body was formed at the sea bottom in a discharge a rea of low-temperature fluids genetically related to submarine volcanism. M ost of silica and iron accumulated directly near the hydrotherms, thus, for ming ferruginous-siliceous mud. Mn precipitated in the upper zones of the f luid flow, where the latter was at a maximum diluted by seawater. Suspended Mn minerals accumulated there in the topographic depressions. Mn precipita tion occurred mainly in the carbonate (rhodochrosite, manganocalcite) and, probably, partly silicate (smectite, neotockite) forms. Subsequent metamorp hic transformations of ore-bearing formations occurred under conditions of the prehnite-pumpelliite facies (T = 250-300 degrees C, P approximate to 0. 5-1.0 kbar, X-CO2 < 1%) and resulted in the formation of Mn silicates.