Determination of physicochemical conditions of causative intrusion in the Masjeddaghi Cu-Au porphyry-epithermal deposit: constraints on chemical composition of biotite

The Masjeddaghi porphyry-epithermal Cu-Au deposit has located in the western part of the Alborz-Azarbaijan zone; in the south margin of Lesser Caucasus. The Eocene porphyritic quartz diorite intrusion has intruded into the andesite volcanic rocks and formed the main host rock of Cu-Au mineralization. Hydrothermal alteration types consisted dominantly of potassic, phyllic, argillic, and propylitic, and local silicification around the veins. Electron microprobe studies indicated that the Masjeddaghi biotites has been located in the phlogopite field and fall into the field of re-equilibrated primary biotite. Moreover, these biotites indicate the tectonomagmatic setting and magma characteristics related to calk-alkaline granitoids which were originated from mantle sources. The temperature of biotites from Masjeddaghi indicated a range between 417 ºC -641ºC. The conditions of oxygen fugacity in the magmatic biotites are in the range of hematite-magnetite (HM) and nickel-nickel oxide (NNO), which indicate high oxygen fugacity of the magma in this ore deposit. In the Masjeddaghi biotites, there is no linear/parallel trend between halogen fugacity,d log (ƒH2O/ƒHF), log (ƒH2O/ƒHCl) and log (ƒHF/ƒHCl) lines. Therefore, it is possible that biotites have not formed under the same conditions and were in equilibrium in a wide range of temperatures and compositions with hydrothermal fluids.