An active Au-Ag producing mine, Mastra gold deposit occurs in a fault zone dipping 65-80 degrees towards 20-40 degrees in andesitic volcanics and volcanoclastics of Eocene age. Hydrothermal alteration is represented mainly by propilitic alteration, argillic alteration and silicification. The main ore minerals are pyrite, chalcopyrite, galena, sphalerite, tetrahedrite, silver and gold with minor secondary digenite, covellite, chalcocite, hematite and limonite. Quartz, barite, adularia, calcite, cerussite, gypsum, sericite and clay minerals are the gangue minerals. Quartz veins display massive, comb, crustiform, coquade and brecciated textures, typical for epithermal conditions. Homogenisation temperatures measured in quartz and barite are in the ranges of 113 to 331 degrees C and 150 to 390 degrees C, respectively. Sulphur isotope values obtained from pyrite, chalcopyrite, galena and sphalerite range from -6,2 parts per thousand to -1,7 parts per thousand. Oxygen isotope values from quartz in mineralised veins and hydrogen isotope values from fluid inclusions in these quartz vary from +8,1 parts per thousand to +12,4 parts per thousand and from -56,2 to -96,4 parts per thousand, respectively. The sulphur and oxygen isotope thermometries yielded temperatures in the ranges of 145-400 degrees C and 310-405 degrees C, respectively for the ore formation. These results, coupled with mineralogical and textural data, indicate that the Mastra deposit is a low sulphidation type epithermal system.