Geological, geochemical and fluid inclusion investigations on the Duna Pb-Ba-(Ag) deposit, Central Alborz, North Central Iran

The Duna Pb-Ba-(Ag) mine is located ca. 155 km north of Tehran in the central Alborz structural zone, northern Iran. The ore mineralization occurs as stratabound, epigenetic, E-W and WSWENE trending veins and veinlets in fracture-controlled as well as massive and open-space filling textures within a Permian dolomitic limestone host rock. Field observations as well as mineralogical and petrographic studies show dolomitization, silicification and possibly haematisation in the host rock. In the mineralization zone, galena and barite are the main minerals, followed by pyrite, quartz, chalcopyrite, tetrahedrite, sphalerite, calcite and supergene minerals such as covellite, malachite, azurite, cerussite, anglesite, and Fe-oxides. The chemical analyses of the highgrade ore samples show an average grade of 18.66 wt. % for Pb, 19.99 wt. % for Ba, and 120 ppm for Ag together with substantial quantities of Zn (0.15 wt. %), As (690 ppm), Cu (0.86 wt. %), Sb (0.25 wt. %), and Sr (0.56 wt. %). The amount of silver in some samples from the tunnel and discordant layers is up to 7030 ppm. The positive Eu/Eu⃰ ratio and the weak negative Ce/Ce⃰ anomaly in the ore samples were most likely inherited from magmatic water. The presence of minerals such as pyrite and chalcopyrite together with the co-precipitation of sphalerite and chalcopyrite suggest a high-temperature for mineralizing fluids. The homogenization temperatures of fluid inclusions from barite in concordant layers span between 135 and 165 ºC with salinities between 18.54 and 23.65 wt. % NaCl equivalent, while the homogenization temperatures of fluid inclusions from barite of discordant layers span between 113 and 285 ºC with salinities between 7.34 and 23.65 wt. % NaCl equivalent. The structural, geological, geochemical, and mineralogical studies together with the paragenesis of the ore minerals and fluid inclusion data allow consideration of the Duna Pb-Ba-(Ag) mine as a two stage mineralization scenario; 1st stage /older/MVT-type (Early Cimmerian tectonic phase), and the second stage/younger/Irish-type (Laramide orogenic movements). The structural data, high temperature of the fluid inclusions, positive Eu/Eu⃰ ratio and high silver content, especially in the discordant layers, indicate the involvement of a magmatic water mixed with meteoric and connate fluids comparable to the Irishtype mineralization in the second stage, which formed along brecciated zones of the thrust faults. The second stage of mineralization was most likely influenced by the Akapol granitoid intrusive mass, which overprinted the 1st stage/older/MVT-type.