Geochemical Constraints on Fertile and Infertile Miocene Magmatic Suite in Dalli Area, Iran and Its Insights for Metallogeny

The Urumieh-Dokhtar magmatic arc (UDMA), hosting three giant and six large Miocene porphyry copper deposits, is the most important post collisional porphyry belt in the Tethyan domain. In the UDMA belt, there are also abundant coeval barren porphyry intrusions, but the factors controlling fertility of Miocene porphyry intrusions in the belt remains enigmatic. The Dalli deposit, located in the central segment of UDMA, contains both fertile and barren Miocene intrusions. In this study, we report zircon petrography, geochronology, trace element geochemistry and Hf isotopic compositions for the barren diorite and mineralization-related quartz diorite porphyry at Dalli. The results show that the diorite has inherited zircon cores overgrown by new zircons, whereas quartz diorite only has one type of zircon. The diorite yields a zircon U -Pb age of 17.4±0.3 Ma, which is slightly older than the quartz diorite porphyry (zircon U-Pb:15.6±0.1 Ma), and the inherited zircon cores yielded as zircon U-Pb of 172-920 Ma. Both the diorite and quartz diorite porphyry show slightly positive ε_Hf(t) values, which are respectively +2-+4 and +2-+5.Miocene zircons from both diorite and quartz diorite porphyry are characterized by obviously positive Ce anomalies and slightly depleted Eu anomalies. However, the inherited zircon cores exhibit more depleted Eu anomalies, suggesting low oxygen fugacity. These results suggest that the barren diorite possibly interacted with ancient crust, particularly with reduced material, which subsequently decreased magma oxygen fugacity whereas the fertile quartz diorite porphyry emplaced without such material exchange. On the basis of above observations, we propose magma process may be the first order of key factors controlling the fertility of intrusions in the central segment of the UDMA.