Fluid inclusions and sulfur stable isotopes of the Sarab 3 iron ore deposit (the Shahrak mining area - north Bijar)

Cenozoic magmatism in Central Iran has caused formation of contact metmorphed rocks especially skarns (Calagari and Hosseinzadeh, 2006, Karimzadeh Somarain and Moayyed, 2002). The skarns consist of valuable ore deposits. The Shahrak mining area is located on the border of Central Iran and Sanandaj-Sirjan zone (SSZ). This mining area includes 9 iron ore deposits. The Sarab 3 iron ore deposit is located to the south of them. The volcanic rocks of the study area include dacite, andesite, rhyolite and andesitic basalt has occurred during the Eocene period. The intrusive rocks of the study area include post early Miocene diorite- granodiorite, diorite and granite. The iron mineralization stage has formed in limestone-dolomite contact with intrusive igneous rocks (diorite- granodiorite and diorite) as a skarn deposit. The main ore of the Sarab 3 iron ore deposit is the magnetite and hematite. Limonite and goethite, pyrite, pyrrhotite and chalcopyrite can also be seen. The ore deposit geometry is characterized by massive to lens-like shape.

In addition to study of drilling cores, 70 samples were taken from them and the mine pit of the Sarab 3 iron ore deposit in order to study thin sections, thin-polish section, fluids inclusion and sulfur stable isotope. Finally, 20 samples were selected and studied at the Bu Ali Sina University of Hamedan. Fluid inclusion studies were performed on six doubly polished thin sections. These samples were taken from calcite in magnetite hosts. Measuring the temperature parameters was carried out at the mineralogical laboratory of Iran Mineral Processing Research Center to assist the Stage: THMS600 with Linkam model on ZEISS microscope. The temperature range is -196 to +600°C. The machine also has two controllers, heating (TP94) and cooling (LNP), a nitrogen tank (for the nitrogen pump for freezing) and a water tank (for cooling the device in high temperature). Calibration of Stage in heating has a precision of ± 0.6°C which was carried out with cesium nitrate with a melting point of 414 °C and freezing was carried out at a precision of ± 0.2 ° with a standard N-hexane material with a melting point of -94.3°C. Five sulfide samples were selected from an open pit of the Sarab 3 iron ore deposit and the isotopic ratio of their sulfur was measured at the Isotope Lab of the University of Queens, Canada.

During the retrograde mineralization stage in the Sarab 3 iron ore deposit, the effects of the remaining fluid on the skarn rocks and adjacent hornfels result in release of calcium from the skarn and then transport of volatile matter into it. At this stage, the fluid is barren and it has a lower temperature and salinity than its original state. As a result of retrograde reactions, the replacement of high calcium calc-silicate minerals with a series of lower calcium minerals occurs. Also, some amounts of dissolved calcium are combined with carbonate ions in the fluid and thus calcite is formed in the faults and microfractures. Study of the fluid inclusions in the Sarab 3 iron ore deposit shows that the manufacturer fluids have been related to the retrograde phase and have lower salinity and temperature. The study of sulfur stable isotopes in the Sarab 3 iron ore deposit shows that sulfur may have been derived from one of these two sources: It has been created directly from the magmatic differentiation fluid or by the dissolution of previously sulfide igneous sources. The values of δ34S of mineralization fluids have been calculated from the Pyrite-H2S separation factor (Ohmoto and Rey, 1979), assuming that H2S is the most important sulfur compound in the mineralization fluid. Considering the amount of δ34S in the Sarab 3 iron ore deposit (3 to 3.6 permil) it can be stated that all of them can be attributed to hydrothermal fluids with magmatic sources. Also the amount of δ34S of H2S in the fluid equilibrated with sulfides of the Sarab 3 iron ore deposit was close to zero (0.8-1.9‰).

Due to the emplacement of intrusive bodies in the limestone-dolomite of the Qom formation with Oligo-Miocene age, the skarn mineralization has occurred in the Sarab 3 Iron ore deposit. The study of sulfur stable isotopes on pyrite in magnetite ore, has shown the source of mineralization fluids to be derived from magma. Skarns have been formed in several stages the last of which is the retrograde stage. Retrograde fluids have been overprinted on ore and affected it. Calcite veins and sulfides have been formed in the retrograde stage in the Sarab 3 iron ore deposit. In this study it was found that the temperature of fluid in the Sarab 3 iron ore deposit was about 115-324ºC and the salinity was about 0.4-35 wt.% NaCl.