Origin and development of Skarn-Forming fluids from the Band-e-Narges Skarn Iron ore, Central Iran

The Band-e-Narges magnetite deposit is located in central part of Urumieh–Dokhtar Magmatic Arc (UDMA). Wide I-type calk-alkaline and alkalin magmatic activity in the Koh-e- Latif region has been reported due to Eocene intrusive processes in UDMA. The iron ores are hosted by Cretaceous limestone intruded by granite and granodiorite units. Genetic model of this deposit was determined using petrological, stable isotope, fluid inclusion and mineralographical data. Five stages of paragenesis were observed in terms of mineralization in this area: prograde stage, retrograde stage, sulfide-quartz stage, carbonate stage and oxidation stage. According to mineralogy and geochemistry studies, formation of the skarn has resulted from a hydrothermal fluid changing carbonate units to hydrosilicate minerals. The ore minerals showed magnetite features with slight chalcopyrite and pyrite. The δ34S values ranged from +3.31 to +6.29 for the early retrograde stage pyrite and from +5.51 to +7.1 for that of late retrograde stage pyrite + anhydrite pairs. All the δ34S values of pyrite and anhydrite + pyrite were positive with a magmatic sulfur origin in these deposits. Fluid inclusions were observed according to petrographic and microthermometric inclusions within garnet, quartz, and calcite minerals at various stages. Due to high temperature (414 -448 ºC ) and middle salinity (up to 13.186 wt% NaCl) of fluid inclusions in prograde skarn-stage (garnet), the fluid inclusions showed a composition related to magmatic fluids following reaction with calcareous wall rock and fluid inclusions were trapped at pressures of 400 -500 bars, corresponding to depths of 1.5 -2 km in prograde stage. Fluid inclusions in quartz had moderate temperatures (152-303 ºC) and low salinity (7.9-11.3 wt% NaCl) indicating quartz-sulfide stage and late retrograde stage. The presence of fluid inclusions with moderate homogenization temperature (303 ºC) suggested that reboiling has occurred under hydrostatic pressure of 150-250 bars, equivalent to a depth of 1 -1.5 km in the late retrograde skarn and quartz-sulfide stages. Fluid inclusions in calcite had moderate temperatures (160 -287ºC) and low –to- high salinities (0.406-23wt% NaCl). A greater number of the fluid inclusions in the Band-e-Narges deposit had salinity (0.4-23.74 wt% NaCl) and homogenization temperatures (152-448 ºC) showing them as a moderate-high temperature and low–to-high salinity type of deposit. A decline in temperature and variation in salinity documented for the Band-e-Narges deposit would cause a notable decrease in Fe solubility and ore precipitation. Fluid compositions indicated that ore-forming fluid had a high fO2 value and rich Fe concentration in the early stage, while having relatively lower fO2 value and poor Fe concentration in the retrograde and sulfide stages. The data obtained from geology, mineralogy, geochemistry, salinities, and homogenization temperatures of the fluid inclusion populations at the Band-e-Narges iron deposit followed a model of boiling as a result of decrease in pressure, mixing, and cooling.