Genesis of secondary Mn-oxide ores in the Úrkút deposit, Hungary

The Urkut Basin of western Hungary is well known for its early Jurassic Mn-carbonate deposit that is related genetically to Jurassic black shale formation. This carbonate deposit has undergone multistage oxidation during the Cretaceous that produced secondary Mn-oxides. Among the geochemical characteristics that distinguish the host sedimentary rocks and Mn-carbonates from the Mn-oxides is an extreme enrichment in Sr (up to 1%) without an observable independent Sr-phase. This enrichment indicates a special process of formation. The δ18O values of these Mn-oxide ores fall between 1.5 and 5.5%. (VSMOW) showing a slight bimodal distribution. This isotopic variability is interpreted as a product of superposing effects of the interaction of surficial and deep-seated magmatic fluids. The δ18OMnO2 values (from 1.5 to 5.5%.), the assumed δ18Owater values and formation temperatures fit well with the MnO2-water fractionation curve based on the data of Yeh et al. (1985) and Hoefs et al. (1987).