Microprobe analyses of metacherts from the Tapolcsany Metachert Member of the Tapolcsany Formation and of limestones from the Lazberc Formation of the Uppony Mountains were completed to study Mn carbonate mineralization. Carbonate minerals in metachert samples include calcite, siderite and ankerite, which contain variable but low amounts of Mn, with a mean Mn content of 0.83 weight percent. In one sample (No. 5), the Mn content is high, 8.15 weight percent, with the host mineral being Mn-rich ankerite. Limestone samples are composed of calcite and dolomite. Iron and manganese contents are low, but somewhat higher in dolomite than in calcite. The environment of deposition of the Tapolcsany Metachert was euxine, with a high primary pyrite and organic matter content, which favored a high dissolved manganese content. These conditions however, did not characterize the sedimentary environment of the Lazberc Formation, which was more oxic. In chronological order the original mineral composition underwent different changes: 1) — primary carbonate minerals included mostly small calcite (and rhodochrosite?) grains; 2) — regional-dynamothermal metamorphism probably changed the primary assemblage. 3) — Fe-Mg metasomatism caused changes due its chemistry; small quantities of manganese were probably transported by the metasomatic fluids; 4) — surface weathering changed the original composition of the rocks; Fe and Mn was dissolved from the carbonates and Fe-Mn-oxide concentrations were formed in which the amount of Fe and Mn was much higher than in the pre-weathered rocks.