Thermobarometry and chemistry of garnet and pyroxene minerals of Oshvand skarn deposit, Nahavand, West Iran

The Oshvand Skarn deposit is located about 10 km east of Nahavand city in the Sanandaj-Sirjan structural zone. In this area, garnet (grossular-andradite) is the most abundant mineral in the skarn zone and their compositions are in the range of Adr(67.2 - 45.9%) Gro(50.1 - 31.8%) Sps(1.8 - 0.3%) and Prp(2.5 - 0.6%). EPMA data indicate a decrease in andradite and increase in the amounts of grossular and pyralspite group from the core to the margin of the garnet minerals. Increasing the substitution of Al by Fe+ 3 from the core to the rim of the garnet crystals indicates an increaseing FO2 during the growth of the garnet crystals. Pyroxene is the second most abundant mineral in the skarn zone and has a more homogeneous composition than garnet. The main pyroxenes of this deposit are in the range of hedenbergite to diopside with the combination of Fs(41.2 - 30.6%) En(26.6 - 9.5%) and Wo(49.3 - 42.8%) which has an average ratio of Fe/(Fe + Mg) in the range of 0.81 to 0.53. Thermobarometry studies indicate the formation of the progressive stage of skarnification at a pressure of 1 to 2 kbar and a temperature less than 600°C, depth of formation is about 5 km and  FO2 is about -16 to -27. The EPMA results of garnets and pyroxenes of skarn zones indicate that their composition is located at the range of Fe-Cu-Au type skarns.