Mineral chemistry and isotopic composition of magmatic, re-equilibrated and hydrothermal biotites from Darreh-Zar porphyry copper deposit, Kerman (Southeast of Iran)

Abstract

Abstract The Darreh-Zar porphyry copper in the southeastern part of the Urumieh–Dokhtar magmatic belt developed as a consequence of hydrothermal alteration related to the Darreh-Zar granodioritc porphyry stock intruded into mafic volcanic rocks. Hydrothermal alteration and Cu–Mo mineralization are focused on the Darreh-Zar porphyry pluton belonging to Miocene time and the neighboring basaltic country rocks. A core zone of potassic alteration is enclosed by a peripheral zone of chlorite–sericite, sericitic, argillic and propylitic alteration. Judging by petrographic features as well as chemical composition, the biotites studied display variable size, color, and shape and occur as magmatic (M), re-equilibrated (R) and hydrothermal (H) types. The M and R-type biotites are found in the Darreh-Zar porphyry and the H-type formed during potassic alteration within the Darreh-Zar porphyry and the volcanic host rocks as well. The hydrothermal biotites are characterized by the highest amounts of Al 2 O 3 and K 2 O whereas the magmatic types are hosted by high level of TiO 2 (up to 4.38 wt.%). X Mg values vary between 0.31 and 0.61 which is increased from M to R-biotites then decreased toward low temperature potassic alteration zone. Fluorine and chlorine contents from biotite types also vary between 0.07 to 0.56 wt.% and 0.14 to 0.38 wt.% respectively. These values change to 2.94 to 2.15 wt.% and − 3.6 to − 4.4 wt.% for IV(F) and IV(Cl) respectively. In all types of biotites, positive correlation is observed between X Mg , Al 2 O 3 , Na 2 O + K 2 O + BaO and F but with FeO and Cl it is negative. The TiO 2 content clearly decreases from the M to R and H-type (Average: 3.98, 2.9 and 2.4 wt.% respectively). A direct relation may be noted between Mg# and oxygen fugacity. Log ( f H 2 O/ f HF) and log ( f H 2 O/ f HCl) values range from 6.24 to 5.46 and 4.49 to 4.12 wt.% respectively. Oxygen fugacity increases during formation of re-equilibrated biotites but it decreases in the course of potassic alteration. Crystallization temperatures range from 600° to 650 °C, 550 to 600 °C and 450 to 550 °C for magmatic, re-equilibrated and hydrothermal biotites respectively. The M-biotites from Darreh-Zar have the δD values of − 89 to − 101‰ and the calculated δD values of the aqueous fluids (− 66 to − 78‰) in equilibrium with the biotites are consistent with a large component of magmatic fluid which shows similarities with the biotite originated from the primary mantle source. The δ 18 O also varies between 7.4 to 8.2‰ for fluids in equilibrium with biotites. The obtained M-biotite data from Darreh-Zar porphyry indicates that there is no clear correlation between δD values, Fe and Mg contents but for δD values, Al and Fe 3 + /Fe 2 + some affinities were observed. The isotopic data indicate that M-biotites are formed at high temperature (620 ± 20 °C) from fluids with the highest δ 18 O H 2 O (7.8 ± 0.4‰) in comparison with the biotites from some other copper porphyries.