Early Diagenetic Processes in the Formation of Carbonate-Hosted Mn Ore Deposit (Lower Jurassic, Tatra Mountains) as Indicated from Its Carbon Isotopic Record

The Lower Jurassic Huciska Limestone Formation (lower Sub-Tatric nappe) in the Western Tatra Mountains locally hosts lens-like bodies of Mn-ore deposit (the Banie Ore Bed). The ore is composed of crinoidal sediment containing both Mn-oxides (bixbyite, hausmannite, todorokite, birnessite) and Mn-carbonates (manganic calcite, kutnahorite, calcic rhodochrosite, rhodochrosite). This composition reflects deposition of Mn oxide-rich sediment as a result of episodes of downslope gravitational transport in a marginal part of the Krizna basin, and early diagenetic formation of Mn-carbonates resulting from bacterial reduction of manganese in a nearsurface environment. The diagenetic reduction of manganese brought about important decrease of d(13)C values (-1.2‰ to +0.5‰) of carbonate carbon in the Mn-ore deposit, which stands in a marked contrast to positive carbon values (+2.0‰ to +3,6‰) of the besting carbonate facies in the Huciska Limestone Formation. Enhanced deposition and diagenesis of manganese in the Huciska Limestone Formation is a result of elevated Mn content in the Toarcian Tethys, reflecting both increased Mn supply associated with initial stage of ocean floor spreading and development of widespread oxygen-minimum zones in basinal settings.