Geothermometry of skarn zones in the Khosrow Abad iron deposit, Northeast of Sonqor

Khosrow Abad iron skarn deposit is located in the northwest of the Sanandaj-Sirjan structural zone, in the western segment of the iron metallogenic zone (Sonqor series). Its geological units include skarnified meta basaltic andesite, crystallized limestone and quartz monzonite intrusion. The main minerals in the prograde and retrograde skarn zones are; garnet, clinopyroxene, epidote, albite, actinolite, chlorite, dolomite, calcite, quartz, serpentine, and talc. Geothermometry of skarn development was evaluated using skarn silicate minerals pair, verified by the microthermometry and calcite twining deformation. The estimated temperature from the two-feldspar geothermometry in the quartzmonzonitic intrusive body (endoskarn) is in range of 507º to 805ºC. The geothermometry of garnet-clinopyroxene pair minerals, indicate temperatures of 301º to 567ºC for prograde skarn zone. Based on chlorite and calcic amphibole geothermometry, the calculated temperatures are in range of 129º to 315ºC and 216º to 332ºC respectively. Geothermometry of the retrograde skarn zone is consistent with the microthermometry of fluid inclusions in calcite and deformation of calcite twining in crystallized limestone (170º to 300ºC) and skarn (127° to 142°C) unit. Andradite is stable at temperatures above 430 °C, even under high sulfidation conditions. At temperatures below 430 °C and relatively high fugacity of sulfur (more than 10-6), andradite altered to the complex of calcite, quartz and pyrite. With decreasing of fS2 (about 10-6), such fluids form magnetite complex. Therefore, it can be concluded that retrograde metasomatizing fluids in Khosrow Abad iron skarn deposit had a sulfur fugacity of 10-6.5 and a temperature of more than 430 °C.