Journal of Economic Geology (Dec 2022)
Determination of physicochemical conditions of causative intrusion in the Masjeddaghi Cu-Au porphyry-epithermal deposit: constraints on chemical composition of biotite
Abstract
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. Introduction The Masjeddaghi porphyry-epithermal Cu-Au deposit is located in the margin of the Arasbaran Belt, in the Alborz-Azarbaijan structural zone in the NW of Iran. The host rock of mineralization is diorite porphyry, which has been intruded into the andesitic volcanic rocks. Paleocene andesitic and lavas crop out in the southeastern and eastern parts of the area. Paleocene andesitic rocks, including lavas, dikes, pyroclastic, and epiclastic rocks overlying on the Permian-Triassic rocks. The mineralized host rocks composed of the dioritic stocks that have formed the main mass of Masjeddaghi and is dated at 54 Ma (Hassanpour and Alirezaei, 2017). The Middle Eocene diorite porphyry includes granular to porphyry texture and with some dikes extending with east - west trend in the northern and eastern parts of the Masjeddaghi district. It is called the main Masjeddaghi Porphyry. This magmatic sequence and associated Cu-Mo-Au mineralization and alterations have been dated at about 54 Ma (Hassanpour and Alirezaei, 2017). The hydrothermal alteration types consist dominantly of potassic, phyllic, argillic, propylitic and locally of silicification around the veins (Ebrahimi et al., 2021; Hassanpour and Alirezaei, 2017; Ebrahimi et al., 2017). The chemical composition of biotite is sensitive to chemical and physical factors which are related to magmatic and hydrothermal activities such as halogens (e.g., F and Cl) and metals compositions, elemental distributions, water concentrations, temperature and pressure. Geological and geochemical features of porphyry-epithermal Cu-Au Masjeddaghi ore deposit have been studied in detail by Ebrahimi et al., 2021; Hassanpour and Alirezaei, 2017; Ebrahimi et al., 2017. In this research study we attempt to characterize the physicochemical attributes of causative magma in the Masjeddaghi porphyry ore deposit, by using biotite mineral chemistry. Material and methods All samples are from the least altered diorite intrusion and from potassic alteration zone were collected from the surface and drill cores (up to 700m) in the Masjeddaghi area. More than 150 thin and thin-polished sections were studied and subsequently, four samples were selected for EMPA (Electron Micro-Probe Analyses) analysis. Twenty-six points of biotite grains were selected and analyzed by using a Cameca SX-100 instrument in czechoslovakia (central European laboratory). The analyses were conducted with 15 kV accelerating voltage and 10 nA beam current. The results were processed by using the MICA+ software. (Yavuz, 2003). Results On the basis of petrographic studies, diorite porphyry consists of plagioclase, hornblende, biotite and quartz minerals. Biotite phynocrysts are within fine grained matrix and biotite chemistry can be used as an important indicator to evaluate magma crystallization condition (Selby and Nesbitt, 2000). Three kinds of biotites have been recognized in the samples: magmatic, re-equilibrated and hydrothermal type. On the basis of Mg, (Mn+Fe2+) vs. (AlVI+Fe3++Ti), Fe2+/(Fe2++Mg) vs. AlIV and (Mg-Li) vs. FeT + Mn + Ti-AlVI) diagrams all plotted in the phlogopite field. The samples in the MgO-10×(TiO2) -(FeO+MnO) diagram fall into the field of re-equilibrated primary biotite (Nachit et al., 2005). The temperature of biotites in diorite samples varies between 417-522 ºC (Bean, 1974) and 516-641 ºC (Henry et al., 2005). Discussion Biotites in the Masjeddaghi Cu-Au porphyries are mainly Plogopite and are dominantly from the samples taken from potassic alteration zone and situated in the re-equilibrated biotites field. The ore deposit tectonically is related to the calk-alkaline magma. Generally, biotites are calk-alkaline, with high content of MgO and reducedAl2O3 which refers to Al and Mg replacements in an ochtahedral setting. This ore deposit has a Magnesium rich calk-alkaline magma. The oxygen fugacity of biotites is in the range of hematite-magnetite (HM) and nickel-nickel oxide (NNO), which indicates high oxygen fugacity of the magma in this ore deposit. In the Masjeddaghi biotites, there is no linear/parallel trend between halogen fugacity and 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. Halogen ratios of the Masjeddaghi biotites with other porphyry deposits in the world show similarity with Bingham and Santa Rita ore deposits. Moreover, these biotites indicate the calk-alkaline signature and mantle source for the magma.
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