In Paleozoic rocks of Southeastern Bosnia (SEB) there are numerous, but very small vein and replacement barite ore deposits containing up to 10% Pb-Zn-Fe-Cu sulphides. Their geochemical characteristics are compared with large barite monosulphide (Hg-tetrahedrite) ore deposits in Mid-Bosnian Schist Mountains (MBSM). The 87Sr/86Sr isotope ratios in the barites of both area are very similar (0.710972 and 0.714170 in SEB, 0.711764 and 0.712548 in MBSM) and indicate epigenetic hydrothermal origin of barite ore deposits. To the same conclusion point elevated Sr content in barites of both area (0.48 to 2.83% in SEB, 1.44% in MBSM) and the δ13C and δ18O values in neominerals of barite ore deposits which are shifted toward lower values relative to typical values occurring in Devonian host rock. The δ18O values in barites from SEB (+14.2‰ to +15.6‰) are remarkably lower than those from MBSM (+15.8‰ to +22.4‰) what can be explained by lower temperature and lower salinity of mineralisation fluids in barite ore deposits of SEB. The δ34S values in barites of SEB are positive (+11.6‰ to 17.7‰) and enriched in heavy sulphur isotope in comparison with sulphides of SEB (-0.41 to +4.26‰), whereas those in tetrahedrites are negative (-4.95 in SEB, -5.50 to -15.40‰ in MBSM) indicating two remarkably different sulphur sources and time of formation.Barite ore deposits of both area, SEB and MBSM, have been genetically linked to fluids originated in the processes of Late Variscan S-type of magmatism and metamorphism of Upper Proterozoic and Lower Paleozoic rock complexes. However later, heating processes in Post Variscan/Eoalpine time affected significant parts of barite ore deposits in MBSM. They caused fluidization of sulphides in barite deposits, ascending of subcrustal deep-seated fluids enriched in H2S and mercury (± fluorine), which by passing through Upper Proterozoic and Caledonian ore deposits amalgamated their Au and Ag content and led to the formation of a new mineral, Hg-tetrahedrite rich in Au (10-50 g/t) and Ag (1000 do 3000g/t).