Mineral chemistry of hydrothermal biotite from the Kahang porphyry copper deposit (NE Isfahan), Central Province of Iran

• Biotite has been equilibrated with a heterogeneous fluid at Kahang.
• Biotite formed mainly in the early stages of hypogene potassic alteration.
• The biotites are Mg-rich, falling close to the phlogopite compositional field.
• The Fe–F avoidance mainly is consistent with biotites from the potassic zone.
• Hydrothermal fluids, for biotitization at Kahang, range in temperature about 333°C.

The Kahang porphyry Cu deposit, located northeast of Isfahan city in central of Iran, is associated with a composite Miocene stock and ranges in composition from diorite through granodiorite to quartz-monzonite. Field observations and petrographic studies show that the emplacement of the Kahang stock occurred in several pulses, each associated with its related hydrothermal activity. Early hydrothermal alteration started with a potassic style in the central part of the system and produced a secondary biotite–K-feldspar–magnetite assemblage accompanied by chalcopyrite and pyrite mineralization. Propylitic alteration that took place at the same time as the potassic alteration occurred in the peripheral portions of the stock. Subsequent phyllic alteration overprinted earlier potassic and propylitic alterations. Biotite grains from the potassic and phyllic zones show distinct chemical compositions. The FeO, TiO2, MnO, K2O, and Na2O concentrations in biotite from the phyllic alteration zone are lower than those from the potassic alteration zone. The F and Cl contents of biotite from the potassic alteration zone display relatively high positive correlation with the XMg. The fluorine intercept values [IV(F)] from the potassic and phyllic alteration zones are strongly correlated with the fluorine/chlorine intercept values [IV(F/Cl)]. Biotite geothermometry for the potassic and phyllic alteration zones, based on the biotite geothermometer of Beane (1974), yields a temperature range of 422° to 437 °C (mean = 430 °C) and 329° to 336 °C (mean = 333 °C), respectively. The position of data in log (XF/XOH) ratio vs. XMg and XFe diagram suggests that biotite formed under dissimilar composition and temperature conditions in the potassic and phyllic alteration zones. Calculated log fugacity ratios of (fH2O/fHF), (fH2O/fHCl), and (fHF/fHCl) show that hydrothermal fluids associated with the potassic alteration were distinctively different from those fluids associated with the phyllic alteration zone at Kahang porphyry Cu deposit. The results of this research indicate that the chemistry of biotite is related to the chemical composition of the magma and the prevailing physical conditions during crystallization.