Silicates chemistry as indicators of physicochemical and geothermometry conditions on porphyry ore system: A case study of the Haftcheshmeh Cu–Mo deposit, NW Iran

Abstract

The Haftcheshmeh porphyry Cu–Mo deposit is located in the northwestern part of the Arasbaran Metallogenic Zone (AMZ), NW Iran. The Cu–Mo mineralization in this deposit is spatially and temporally related to late Oligocene gabbrodiorite and early Miocene granodiorite stocks. The chemistry of biotite, amphibole, and plagioclase in the ore–related granodiorite and gabbrodiorite intrusions was investigated to determine the physicochemical conditions of crystallization (temperature, pressure, oxygen fugacity, water activity, and fluorine–chlorine activity). Mg–biotite, Mg–hornblende, andesine/oligoclase and quartz are the main minerals in these rocks, associated with zircon, apatite, titanite, rutile and Fe–Ti oxides as accessory minerals; plus secondary biotite, K–feldspar, epidote, chlorite and muscovite as secondary minerals. The chemical compositions of the Mg–biotite and Mg–hornblende minerals reveal that the Oligo–Miocene magmas were most likely generated from mantle–derived oxidized I–type magma that was only weakly to moderately contaminate by crustal materials. The log(fH2O/fHF) and log(fH2O/fHCl) for the Mg–rich and relatively F–rich magmatic biotite indicate an oxidized, water– and halogen–rich melt. The halogen and water fugacities, show that the Mg–rich biotites crystallized under conditions that were similar in terms of pressure, temperature, flour, and chlorine content. Al–in–hornblende barometry and hornblende––plagioclase thermometry suggest that the Mg–hornblendes in the Haftcheshmeh granodiorite and gabbrodiorite intrusions crystallized under similar P–T conditions (from 0.11 to 0.96 kbar and 712 to 1060 °C, respectively). Oxygen fugacities obtained from the amphibole–only chemometer suggest that the gabbrodiorite and granodiorite intrusions have crystallized from oxidized (NNO + 1.82 and NNO + 2.92) magmas during amphibole crystallization. The presented method for fingerprinting magmas via trace element chemistry in biotite, amphibole and plagioclase by microprobe studies, demonstrate grouping of V, Cr, Ni and Nb concentrations in these silicates, despite the presence of large variation and zonation in other trace elements (Ba, Sr, Pb, Zn, P, Cl, F) that indicate complex petrogenetic processes. Our results show that trace elements in such silicates can be used to fingerprint of magma process.