Geothermobarometry of Fe-Ti hosted gabbroid rocks in the Dar Gaz district (Kahnouj Ophiolitic Complex)

The Kahnouj Fe-Ti ore district is located 25 km southeast of Kahnouj city associated with the large gabbro intrusion of the Kahnouj ophiolitic complex. This ophiolite is one of the largest ophiolite assemblages of Iran (SE Iran), and part of neo-tethyan ophiolites (Kananian et al., 2001; Ghasemi Siani et al., 2021b). The Dar Gaz district is located in the middle part of Kahnouj ophiolitic complex and it is classified as the main ortomagmatic Fe-Ti ore mineralization. Although the geothermobarometric of iron-titanium oxide minerals in the Dar Gaz district has been studied by Karimi Shahraki et al. (2019), the geothermometry of silicate minerals (especially ferromagnesian) in the Dar Gaz gabrroic rocks has not been performed. Therefore, the main aim of this study is to determine the crystallization temperature and replacement of gabbroic rocks hosting Fe-Ti mineralization of the Dar Gaz district, using geothermometry of ferromagnesian silicate mineral.

A total of 100 thin-polish sections from different parts of the mining area were prepared and studied at the Iran Mineral Processing Research Center (IMPRC) and the Kharazmi University of Tehran with a Zeiss Axioplan 2 microscope. In order to achieve the temperature conditions of gabbroic rocks formation, 64 points (20 points of olivine, 13 points of clinopyroxene, 3 points of orthopyroxene, 14 points of plagioclase and 14 points of amphibole) from ferrogabbro to coarse-grained pyroxene-hornblende gabbro, 42 points (11 points of olivine, 10 points of clinopyroxene, 1 point of orthopyroxene, 8 points of plagioclase and 12 points of amphibole) from pyroxene-hornblende to fine-grained olivine gabbro, 30 points (12 points of clinopyroxene, 10 points of plagioclase, 8 points of amphibole) from fine-grained hornblende gabbro and 20 points (3 points of clinopyroxene, 11 points of plagioclase and 5 points of amphibole) from the diabasic dike of the Dar Gaz district were analyzed using CAMECA SX 100 electron microscopy (EPMA) with 20 kV and 20 nA conditions in the IMPRC.

The mafic rocks of the Dar Gaz district include ferrogabbro to coarse-grained pyroxene-hornblende gabbro, fine-grained pyroxene-hornblende gabbro, hornblende gabbro and diabasic dikes. Ferrogabbro to coarse-grained pyroxene-hornblende gabbro is one of the most important host rocks for Fe-Ti mineralization in the district.According to the thermo-barometers, the formation temperature and pressure of gabbroic rocks in the Dar Gaz district are in the range of 750 to 1258°C and a pressure of 2.5 and 6 kbars (clinopyroxene and amphibole barometers), and dibasic dikes are in the range of 700 to 1145°C and a pressure of 2.5 and 6 kbars were obtained. The highest crystallization temperature related to fine-grained pyroxen-hornblende gabbro unit (754 to 1258 °C) is the base of the sequence.The ascending of asthenosphere in the back-arc tectonic settings are from a magmatic chamber with a depth of about 15.34 to 21.20 km, and a pressure of about 4 to 8 kbars upwards. The average geometric results of pyroxene-ilmenite mineral pair geothermometry and pyroxene geothermometer of these rocks, their equilibrium temperature was determined between 901 to 1228°C, which is close to the magmatic temperatures.With comparison of temperature (700 to 1258°C), pressure (4 to 8 kbars) and oxygen fugacity (-19.25 to -25.25 bars) obtained for gabbroid rocks hosting Fe-Ti oxide mineralization with the temperatures obtained from ilmenite and titanomagnetite by Karimi Shahraki et al. (2019), it can be concluded that oxide mineralization is classified as orthomagmatic and occurs during the replacement, cooling and fraction of basic magma and formation of gabbroid intrusion associated with fractional crystallization.

Thermometry of pyroxenes at 2.5 kbars pressure indicates a temperature of 750 to 1258 °C for gabbroid bodies and 700 to 1145 °C for diabaic dikes. Thermometry of plagioclase and hornblende-plagioclase at 6 kbars pressure for coarse-grained ferrogabbro, fine-grained pyroxene-hornblende gabbro, hornblende gabbro and diabasic dikes are 868, 884, 776 and 784 °C, respectively. Amphibole thermometers at 6 kbar pressure for coarse-grained ferrogabbro, fine-grained pyroxene-hornblende gabbro, hornblende gabbro and diabasic dikes are 911, 948, 937 and 946°C, respectively. Comparison of temperature, pressure and high oxygen fugacity values obtained for gabbroic rocks and ilmenite and titanium magnetite ores of the Dar Gaz district, indicating oxidation conditions associated with fractional crystallization is the main factor for control of orthomagmatic mineralization in the back-arc environment.