Evaluation of crystallization conditions and porphyry Cu Mineralization potential of the Gheshlagh-Aftabrow pluton, central part of the Urumieh-Dokhtar magmatic belt, Iran

Abstract

Gheshlagh-Aftabrow granodiorite (GAG) is emplaced within Eocene volcanic rocks in the central part of the Urumieh-Dokhtar magmatic belt (UDMB) of Iran and comprises of host granodiorites and associated mafic microgranular enclaves (MMEs). The Al-in-hornblende barometry and the amphibole–plagioclase thermometer are applied to calculate pressure and temperature. The host granodiorite yields crystallization temperatures and pressures varying from 635 to 715 °C (with an average of 670 °C) and from 0.5 to 1.8 kbar (with an average of 1.3 kbar, equivalent to a depth of 4.5 km), respectively. The mafic enclave shows a slightly higher crystallization temperature (690–720 °C, with an average of 705 °C) and pressure (1.1–1.8 kbar, with an average of 1.4 kbar), equivalent to a depth of 5 km. The estimated ƒO2 contents by amphibole chemistry in the host rocks and their mafic enclaves range − 11.2 to − 13.3 and − 11.5 to − 12.8, respectively, indicating that these rocks probably crystallized at relatively oxidizing conditions above the nickel–nickel oxide (NNO) buffer, yielding an overall range between NNO + 1.1 and NNO + 2.6, that is consistent with the general presence of magnetite and corresponds to their calc-alkaline affinity and the formation of the rocks in the subduction environment. The granodiorites and associated MMEs yield broadly similar water contents of 2.5–3.7 wt% and 2.6–3.7 wt%, respectively. The calculated densities of the host granitic magmas vary from 2319 to 2353 kg/m3, which are different from the determined densities of the mafic magma from which the enclaves are derived (2500–2626 kg/m3). The minimum calculated solidification time for the Gheshlagh-Aftabrow intrusion is about 20 × 104 years, and for the enclaves, it is less than 0.2 to 14 h that could not be enough time for pervasive mixing. Most of the amphiboles have Mg# value between 0.5 and 0.7, that together with low difference in densities of the magmas forming host rocks and enclaves may indicate that mantle–crust mixing was significant in the GAG generation. However, some geochemical signatures together with high ƒO2 content in the GAG samples indicate that these rocks are essentially comparable to granitoids associated with porphyry copper deposits, but according to the field evidences, the lack of extensive alteration, whole-rock chemistry (e.g., low Sr/Y and high Zr/Hf), plagioclase geochemistry, low water contents, and mineralogical evidences (e.g., lack of the common alteration phases in porphyry systems), it can be concluded that the studied pluton is barren regarding porphyry Cu mineralization.