Zircon U-Pb geochronology, Hf isotopes and geochemistry of intrusive rocks in the Gazu copper deposit, Iran: Petrogenesis and geological implications

- Three stages of intrusive magmatism (68-60 Ma) occurred in the Gazu copper deposit.
- Hornblende granite and hornblende quartz monzonite porphyry (65.9-65.1 Ma) are the main ore bearing rocks.
- Hf isotope data show magma derivation from mantle with interaction with Cadomian crust.
- The Gazu intrusive rocks formed in a subduction regime in the north of CIM along a NE-SW trend.

The Gazu Cu deposit is located at the boundary between the Lut and Tabas Blocks in Central Iranian Microcontinent. The intrusions in the Gazu area are characterized by SiO2 contents ranging from 52.6 to 66.4 wt.%, K2O from 1.98 to 3.64 wt.%, and Na2O from 2.5 to 5.2 wt.%. The intrusions have features typical of high-K calc-alkaline rocks, are metaluminous to weakly peraluminous, and formed in a volcanic arc setting. Zircon U-Pb age data for hornblende monzonite porphyry, hornblende monzodiorite porphyry, pyroxene monzonite, hornblende granite, hornblende quartz monzonite porphyry, and hornblende diorite porphyry are 68.1 ± 1.7 Ma, 67.74 ± 0.72 Ma, 67.41 ± 0.94 Ma, 65.9 ± 3.7 Ma, 65.1 ± 1.2 Ma, and 60.2 ± 1.2 Ma, respectively. Copper mineralization is related to the hornblende granite and hornblende quartz monzonite porphyry. Field observations and the U-Pb dating indicated three stages for intrusive rock emplacement: pre-mineralization intrusions (hornblende monzonite porphyry, hornblende monzodiorite porphyry, and pyroxene monzonite), syn-mineralization intrusions (hornblende granite and hornblende quartz monzonite porphyry), and post-mineralization intrusions (hornblende diorite porphyry). Mantle-normalized, trace-element spider diagrams display enrichment in large ion lithophile elements, such as Rb, Ba, K, and Cs, and depletion in high field strength elements, e.g., Nb, Ti, Zr, Y, and heavy rare earth elements, with light rare earth element enrichment ((La/Yb)N = 6.4-13.4) and a weak negative Eu anomaly (Eu/Eu* = 0.87-1.01). Zircon Hf isotope data indicate that the Gazu intrusive rocks are characterized by low positive εHf(t) values, with an average of 4.23, and show young Hf model ages (789-188 Ma), indicating derivation from juvenile crust. Relatively low positive εHf(t) values, the wide range for εHf(t), and the presence of zircon xenocrysts can be explained by involvement of a mantle component that mixed with older continental crust (Cadomian).