The black shale-hosted Mn-carbonate deposits of Urkut (Bakony Mts., Hungary) are Toarcian in age (Falciferum Ammonite zone). Subsequent to burial, during the Late Cimmerian orogeny erosion nearly uncovered the bedded Mn-carbonate ores where they were oxidized, thereby forming laminated noduliferous Mn-oxide ores with a clay mineral-bearing ferruginous crust. The clay mineral-bearing samples of the primary Mn-carbonate deposit yielded scattered K/Ar apparent ages of 293 to 110 Ma, indicating that associated detrital minerals partly retained radiogenic ⁴⁰Ar during transportation, deposition, and diagenetic changes. In contrast, more than half of the Mn-oxide ores gave K/Ar ages of 105 to 91 Ma. The other Mn-oxides yield ages older than 105 Ma. Selective dissolution of cryptomelane demonstrates that older K/Ar ages of Mn-oxides are caused mostly by incorporated clastic minerals. It has been shown by comparing the Ar retention of Mn-oxide ores and the clay mineral rich host rock that the uniform K/Ar ages of the Mn-oxide ores correspond to the real ages of oxidation, which occurred during the Austrian and Subhercynian phases of the Alpine orogeny.