Chemistry of magmatic and alteration minerals in the Chahfruzeh porphyry copper deposit, south Iran: implications for the evolution of the magmas and physicochemical conditions of the ore fuids

Abstract

The Chahfruzeh deposit is a newly discovered porphyry-style copper deposit in the southern part of the Cenozoic Urumieh-Dokhtar magmatic arc of Iran. Mineralization is associated with a Miocene quartz-diorite to quartz-monzodiorite porphyritic intrusion(Chahfruzeh porphyry) intruded into older basaltic and andesitic lava fows and pyroclastic rocks. Alteration assemblages andalteration zoning, typical of porphyry copper deposits, are well developed. Mineralization occurs in quartz-sulfde stockworks and assulfde disseminations in the porphyritic intrusion and the immediate wall rocks. Pyrite and chalcopyrite are the main hypogene sulfdeminerals; bornite and molybdenite are rare. Representative magmatic and alteration minerals, including plagioclase, amphibole, biotite,sericite, and chlorite, are analyzed, and the data are used to constrain the crystallization conditions of the magmas and the nature andevolution of the hydrothermal fuids. Te fuorine chlorine fugacity in the magma during crystallization of the Chahfruzeh porphyry,represented by log ( fH 2O)/( fHF)log ( fH 2O)/( fHCl) and determined from the chemical composition of magmatic biotite, rangesbetween 5.23 and 6.80 and between 5.05 and 5.13, respectively. A comparison to several other intrusions associated with porphyry-style mineralization suggests that the Chahfruzeh intrusion crystallized at relatively high fH 2O/ fHCl ratios. Te intercept value (F/Cl) of the magmatic biotite in the Chahfruzeh porphyry ranges from 5.5 to 7.02. Te calculated F/Cl intercept values for the biotiteare consistent with those reported from many other porphyry copper systems. Te secondary reequilibrated biotite in the intrusion isdistinguished from the primary magmatic biotite by a slightly higher Mg component (XMg = 0.53 0.68 compared to 0.62 0.66). Tis canbe explained by the consumption of Fe to form pyrite and chalcopyrite. Chlorite occurs as an alteration product replacing magmaticbiotite and hornblende, as well as hydrothermal biotite. Chlorite geothermometry indicates a narrow range between 212 and 246 °Cfor the formation of chlorites from various alteration zones, implying that the whole system equilibrated with a common fuid at lowtemperatures.