Origin and evolution of ore-forming fluids in the magnetite±apatite Lake Siah deposit (Bafq): Evidence of fluid inclusions and oxygen stable isotope

The Lake Siah magnetite ± apatite deposit is situated in the northeastern of Bafq and Central Iran tectonic zone. The host rock of deposit is composed from lower Cambrian volcano-sedimentary sequence that has exposed as caldera complex. The iron mineralization is as massive ore and includes magnetite and hematite which form with apatite, quartz and calcite gangue minerals. Based on fluid inclusions petrography in quartz and apatite, at least five types of fluid inclusions are recognized including, multi-phase solid bearing (L+V+S1+S2), three phase solid bearing (L+V+S), liquid-rich two phase (L+V), vapor-rich two phase (V+L) and monophase vapor (V(. The three phase fluid inclusions are the most abundant of fluid inclusions type and indicate homogenization temperature between 123 to 530°C (avg. 344°C) and salinity content 30 to 58 wt% NaCl equiv. (avg. 40.60 wt% NaCl equiv.). Three phase fluid inclusions in the apatite are characterized by homogenization temperature between 175 to 420°C (avg. 385°C) and salinity between 34 to 53 wt% NaCl equiv. (avg. 41.58 wt% NaCl equiv.). Calculated pressure on the basis of microthermometric measurements on fluid inclusions in the H2O-NaCl system is calculated between 10 to 500 bar which is corresponding with depth between 0.5 to 1.5 km. The values δ18O of fluid in equilibrium with quartz is 7.5 to 11 ‰ which indicated magmatic waters origin. According to wide range of fluid inclusion homogenization temperature and salinity accompanied with positive correlation between salinity and homogenization temperature of fluid inclusions, mixing of hot and high salinity fluid with cold and low salinity water (similar to meteoric waters) is main factor for occurrence of the Lake Siah deposit.