The Bikkulovskoe manganese deposit (South Urals): Geological setting, composition of metalliferous rocks, and formation model

The results of investigation of the Bikkulovskoe manganese deposit confined to volcanosedimentary piles of the Magnitogorsk paleovolcanic belt are presented. The paper characterizes the geological setting of the deposit and mineral-chemical compositions of ores and enclosing rocks (volcanomictic sandstones; ferruginous, manganiferous, and ferruginous-siliceous tuffites; and jasperites). Analysis of the data obtained made it possible to identify four sequential stages of deposit formation: (1) accumulation and diagenesis of ore-bearing sequences (D 2–3 ); (2) burial and low-grade ( T = 200–250°C, P = 2 to 3 kbar) regional metamorphism of rocks (D 2–3 -C 1 ); (3) tectonic deformations of volcanosedimentary piles (C 2 -P); and (4) hypergenesis and partial denudation of rocks (MZ-CZ). According to the model proposed for the accumulation of ore-bearing rocks, the productive member was formed in a zone of hydrothermal solution outflow to the seafloor surface. Discharge of solutions and precipitation of Fe and Si began below the seafloor surface (rather than above the surface) in the near-bottom sequence of volcanomictic sediments. Upon reaching the seafloor, the impoverished solutions mixed with seawater and gave up metals completely: Fe and Mn were transferred to sediments to make up the ore-bearing bed. Thus, zonal sediments with ferruginous tuffites at the base and manganese ores at the top were formed.