Lower Carboniferous stratiform iron-manganese mineralizations (Rheinisches Schiefergebirge, Germany): products of submarine hydrothermal activity and diagenetic manganese redistribution

Abstract

Lower Carboniferous iron-manganese ores of volcanic-sedimentary origin occur in the eastern part of the Rheinisches Schiefergebirge (Germany). Major constituents are rhodochrosite, bementite, hematite, and quartz. The ores are only weakly metamorphic and exhibit excellently preserved primary textures and fossils. Metamorphic equivalents of these ores are the manganese deposits of the Iberian Pyrite Belt. Most strikingly, the iron-manganese ores show a clearly developed zonation. Positions of former hydrothermal vents are indicated by chert mounds which are intersected by vertical pipes of coarse-grained hematite. The chert mounds are covered and surrounded by beds of fine-grained hematite. Manganese ore forms the distal part of each deposit. The formation of the ores was controlled by complex interactions of hydrothermal activity, intrusive and extrusive volcanism, formation of intramagmatic sulphide ores as well as diagenetic redistribution of certain elements: SiO2, copper, nickel, and iron were mobilized due to hydrothermal alteration of basic subvolcanic intrusive rocks. Copper and nickel were mostly re-deposited as sulphides within the intrusives, whereas manganese, iron, and SiO2 were transported by hot convecting waters to the sea floor. Here, SiO2 and the dissolved metals were precipitated by mixing with oxygenated sea water. Subsequently, manganese was separated from iron by diagenetic reactions. Enrichments of cosmic spherules and conodonts indicate that this process required a long time.