Fe-Mn oxide indications in the feeder and mound zone of the Jurassic Mn-carbonate ore deposit, Úrkút, Hungary

Abstract The Urkut manganese deposit, one of the World's largest, is located in the central part of the Transdanubian Range, western Hungary. The deposit is interbedded with Mesozoic limemarlstone. The Fe-Mn-oxide indications of a feeder and mound zone embedded in limemarlstone at the footwall of the Mn-carbonate ore deposit were studied using 45 samples (Urkut Mine, Shaft III, deep level). Microstructural and textural (optical microscopy, SEM-EDS) observations, mineralogy (XRD-μXRD), and geochemistry (ICP, C and O by IR-MS) were used to characterize the host marlstone and the Fe-Mn oxides of the feeder and mound zone. High-resolution in situ and bulk organic matter analyses were performed for the first time using GC–MS, FTIR-ATR, and Raman spectroscopy. Stromatolite-like, filamentous and coccoid microstuctures built up of Fe-Mn-oxides (ferrihydrite, goethite, manganite, pyrolusite, hollandite, birnessite, hausmannite) and silica occur in the micritic marlstone host rock among common calcite biodebris (microfossils and Echinozoa fragments) and rare detrital clasts (quartz, feldspar). The clay minerals occur as greenish patches in the limemarlstone and show boring traces. The calcite matrix of the limemarlstone and idiomorphic dolomite are authigenic. δ 13 C PDB values of the carbonate in the host limemarlstone reflect greater organic matter contributions approaching the mineralized areas (0.64 to − 21.35‰). Temperature calculation based on δ 18 O SMOW values of the carbonate, assuming equilibrium conditions, show elevated temperatures toward the mineralized areas (9.93 to 29.87‰). In places, the Mn oxides appear with Fe oxides in laminated, micro-stromatolite-like structures. In these oxide zones, variable kinds of organic compounds occur as intercalated microlaminae identified by FTIR and Raman line-profile analyses as aromatic hydrocarbons. Results indicate that metal-bearing fluids infiltered the unconsolidated micritic limemarl. Fe-oxide enrichment occurred most probably through iron oxidizing microbes under suboxic, neutrophilic conditions, while Mn oxide formed most probably by active surface catalyses. At the sediment/water interface, Fe-Mn-oxide stromatolite mounds (chimneys) formed in rift zones from the discharge of fluids of elevated temperature. The host marl itself may have originated by microbially mediated reactions (clay minerals and calcite micrite).