Microbial processes and the origin of the Úrkút manganese deposit, Hungary

The Transdanubian Range (Hungary, ALCAPA Unit) preserves a series of black shale-hosted Mn-carbonate deposits and cherty, Fe-rich Mn-oxide mineralized rocks associated with varicolored metalliferous claystones. Coccoid clumps (spherules), oval, tubular, and filamentous morphologies were observed by petrography, SEM-EDS, and TEM studies and are interpreted to be mineralized cellular materials. Local selective enrichment of bioessential elements (Mn, Fe, S, As, P, Mg, Ba, Sr, Co, Ce) occurs, and together with low δ13C values of the Mn carbonates also supports microbial mediated reactions. The results strongly suggest that the formation of Mn–Fe and Si minerals was associated with microbial metabolic processes. The role of aerobic chemolithoautotroph bacteria was essential in sequestering metal ions (Mn2 +, Fe2 +) from solution, which were deposited in the sediment pile and serve as a paleoenvironmental indicator of oxic conditions. These deposits are examples of, and therefore provide important criteria for identifying, non-sulphidic, oxic, microbial mineral processes. We provide a new genetic model for giant black shale-hosted Mn-carbonate deposits that involves episodic aerobic microbial processes.Although a part, if not the whole of the black shale-hosted Mn-carbonate deposits is of biogenetic-bacterial sedimentary origin, a hydrothermal/exhalative source of metals may have contributed to the formation of the deposits. An Fe–Mn-oxide chimney system is proposed to be a proximal facies to geofluid vents that occurred along fracture systems, which may have provided metals from deep-seated sources.