Chemical characteristics and composition of hydrothermal biotite from the Dalli porphyry copper prospect, Arak, central province of Iran

Abstract

The Dalli porphyry copper deposit is hosted by the Miocene–Pliocene subvolcanic plutons with chemical composition from diorite to granodiorite that intruded into the andesitic and dacitic volcanic rocks and variety of sedimentary sequences within the Urumieh–Dokhtar Magmatic Arc. Three main hydrothermal alteration zones including potassic, phyllic and propylitic types have been described in the volcano-plutonic rocks. Early hydrothermal alteration started with potassic style in the central part of system produced a secondary biotite–K-feldspar–magnetite assemblage and accompanies to chalcopyrite and pyrite mineralization. This paper summarizes the detailed biotite mineral chemistry from the potassic and phyllic alteration zones. The FeO, TiO2, MnO, K2O, and Na2O (wt.%) concentrations of biotite from the phyllic alteration zone are lower than biotite from the potassic alteration zone. The F and Cl (wt.%) 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 positively correlated with the fluorine/chlorine intercept values [IV (F/Cl)]. Biotite geothermometry for the potassic and phyllic alteration zones yield a range from 402° to 450°C and 280° to 343°C, respectively at Dalli porphyry copper deposit. The scatter in log (XF/XOH) ratios vs. XMg and XFe plots also reflects the evidence of 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 fluids those associated with the phyllic alteration zone at Dalli porphyry copper deposit. The relation between log (fH2O/fHCl) and log (fH2O/fHF) fugacity ratios indicates that biotite from the Dalli volcano-plutonic rocks is distinctly similar to biotite from the porphyry copper deposit at Bingham.