Geologic, mineralogic and geochemical properties of Mastra (Gümüşhane) Au-Ag deposit

Mastra Au-Ag deposit is situated near Demirkaynak village, five km?s to the northwest of Gümüşhane. In this area, which is covered mainly by andesitic volcanics and volcanoclastic rocks of Eocene age, the ore zone is composed of quartz veins within a fault zone striking N50-700W and dipping 65-800NE. Hydrothermal alteration is represented mainly by chloritisation, carbonatisation, epidotisation, sericitisation, silicification and clay alteration. These alteration minerals are found in different combinations, and form, from the outer zones to iner zones of the deposit, a) propilitic alteration zone, b) argillic-sericitic alteration zone, and c) silicification zone. The propilitic zone varies in composition which is made up mainly of chlorite and clay minerals, and carbonates to a lesser extent at the outer zones, and carbonates and clay minerals, and chlorite to a lesser extent at the inner zones. Native Au, Ag, pyrite, chalcopyrite, sphalerite, sulphosalts, galena, digenite and covellite/chalcosite are the main ore minerals, and quartz, barite, adularia, calcite, serusite, gypsum, hematite, limonite, sericite, and clay minerals are the gangue minerals. various types of crystallisation, growth and replacement textures are present in the quartz veins. Among such textural features are massive-, comb-, crustiform-, cocard-, brecciated-, breccia-fill-, mosaic-, flamboyant-, ghost-sphere-, saccaroidal-, and lattice textures. Of these, brecciated, cocard, comb and void-filling textures are the indication of epithermal conditions. ? 34S values from pyrite, sphalerite, galena and chalcopyrite range from -1,7? to ?6,2?. These values are indicative of magmatic origin. ? 18O of hidrotermal fluid, which are in equilibrium with quartz, are between +4,5?-+6,2?. ? 18O of hidrotermal fluid which are in equilibrium with clay are between +5,8?-+9,8?. It was seen that ore deposit occurs in a wide temperature interval of 113-390oC. Occurance temperatures in ? 34S geotermometer were determined as T1:147±15oC, T2: average 287±21oC and T3: average 365±28oC while in ? 18O geothermometry it was average 348,5oC. In the results of alteration, ore microscopy, fluid inclusion and, isotope analysis it was determined that Mastra Au-Ag deposit is an adularia-sericite type epithermal system.