Redox State of Parental Magmas of Cretaceous-Paleocene Gazu Porphyry Cu Deposit: Zircon Trace-Element Data

Abstract

The Gazu is the only Cretaceous-Paleocene Cu porphyry deposit in Iran and is located in the Tabas block with outcrops of scattered basement fragments attributed to the Neoproterozoic northern Gondwanan active margin continental arcs. The main question is why only a small, uneconomic porphyry deposit formed in this region? In this paper, we discuss this issue by calculating the oxygen fugacity of ore-bearing and barren intrusive rocks and zircon trace element geochemistry. The ore-bearing intrusions have both Cretaceous magmatic and Neoproterozoic inherited zircons. The Cretaceous-early Paleocene magmatic zircons have very low to moderate (10–385, mean 110) Ce4+/Ce3+ ratios. These ratios are lower (26-143, 68 on average) for inherited zircons. The oxygen fugacity (estimated logfO2) of Cretaceous Gazu porphyry magmas ranges from ΔFMQ - 1.4 (±1.3) to ΔFMQ + 2.3 (±1.3) (ΔFMQ ~ +1 on average). The logfO2 from Neoproterozoic inherited zircons are in the range of ΔFMQ - 1.2 (±1.3) to ΔFMQ + 1.6 (±1.3). A more positive range of Ce4+/Ce3+ and ΔFMQ values for Cretaceous zircons compared to those of inherited Neoproterozoic zircons indicates a more oxidized source magma during the crystallization of Cretaceous magmatic zircons. As the magma ascended through the reduced continental crust, AFC and mixing processes in the MASH zone reduced the logfO2 of the parental magma. Low-volume melting of the subcontinental crust, influenced by low-volume slab- metasomatized mantle magmas, led to the formation of a single, small porphyry copper deposit (Gazu).