Rhodochrosite concretions with fish and plant fossils occur in the Úrkút manganese carbonate deposit (Jurassic) and were investigated for mineralogy and geochemistry. These concretions are mainly composed of rhodochrosite and Mn-bearing calcite, but X-ray diffraction and electron microprobe analysis showed also the presence of Ca-rich kutnohorite. Cathodoluminescence microscopy revealed kutnohorite as luminescent mineral as infiltration and veinlets suggesting early diagenetic origin. Chrysotile needles were also detected in one of the kutnohorite veinlets by transmission electron microscopy. Stable C and O isotope results showed low-T very early diagenetic, bacterially mediated decomposition of organic matter and formation of Mn-carbonate in the ore section, which effect was the strongest around the organic remnants. Compared to the polarized light microscopy, cathodoluminescence microscopy was able to give visible picture about the texture of the very fine-grained Mn-carbonate concretions; therefore it was proved to be a useful method in study of diagenetic processes in manganese deposits.