We present evidence that precipitation of primary Mn-oxide minerals in the Vani volcanic hosted hybrid epithermal-VMS-type Mn-oxide and barite deposit was in part biogenically mediated. Manganese-oxides pseudomorphically replace small (1-5 mu m) spherical cell-like structures, and branching filamentous constructions (< 60 mu m long) probably representing manganese oxidizing bacteria. In addition, silicified consortia of spherical (5-10 mu m), filamentous, sheathed, septate and spiral (similar to 50-200 mu m) fossilized bacteria, proposed to represent photosynthetic thermophilic cyanobacteria, were found in quartz paragenetically related to the Mn ore. Fluid inclusions indicate formation temperatures around 100 degrees C. XRD and EMP analyses suggest X-ray-amorphous hollandite-group like Mn-oxide phases, and poorly crystalline todorokite and vernadite. These findings suggest a biological link between bacterial and mineralization processes.