The formation of giant deposits in the collisional environment usually undergoes multiple stages of magmatism, but the genetic relationship between different stages of magmas and the formation mechanism of ore-forming porphyry magma has not been effectively restricted. The giant Qulong porphyry copper deposit, located in the hinterland of Qinghai-Tibet Plateau, has several stages of Miocene magmas, including pre-ore granodiorite (also known as RMCL pluton), ore-forming granite porphyry (P and X porphyry), and post-ore high-Mg diorite porphyry (HMD), and diorite enclaves are found in the granodiorite. Such abundant magmatic sequence provides a good opportunity for the systematic study of the formation mechanism of ore-forming magma of porphyry deposits in collisional environment. This paper systematically analyzed the zircon trace elemental and Hf-O isotopic compositions of various types of magmatic rocks in Qulong and estimates the redox state of Miocene magmas in the Qulong mining area, which constrain the origin and evolution of the magma, especially the formation process of ore-forming porphyry.The results show that the zircons in the RMCL pluton and the diorite enclaves before mineralization have high Ce/Ce^* ratios (average values of 111 and 117, respectively) and similar Hf-O isotopic compositions, with the value of εHf(t) of +7~+10 and +7~+9 respectively, and the value of δ¹⁸O of +5.6‰~+7.1‰ and +4.7‰~+7.0‰ respectively. Whereas zircons from the ore-forming P porphyry have similar δ¹⁸O values (+4.6‰~+6.4‰), but the Ce/Ce^* ratios (29~405, with an average of 149) and the values of εHf(t) (+5~+10) change variably. Zircons from the post-ore high-Mg diorite porphyry (HMD) have similar δ¹⁸O values (+5.2‰~+6.3‰) with the P porphyry, but lower εHf(t) values (+3~+7) and changeable Ce/Ce^* ratios (34~252, with an average of 159).
The positive εHf(t) value and moderate-high δ¹⁸O value of the RMCL pluton and diorite enclaves indicate that they were originated from the juvenile lower crust. The low εHf(t) and δ¹⁸O values of the HMD show that it was formed from the evolved mantle with a strongly altered Hf isotopic composition. The zircon Hf isotopic composition of the P porphyry, varies between the RMCL pluton and the HMD, showing that it was originated from magma mixing between the crust-derived magma (represented by RMCL pluton) and the mantle-derived magma (represented by HMD). Among the Qulong Miocene magmatic rocks, the zircon Ce/Ce^* ratio from the HMD is the highest, indicating that the magma represented by the diorite porphyry also has the highest oxygen fugacity. Therefore, the addition of mantle-derived material during the formation of the P porphyry can provide water and raise the oxygen fugacity of the magma, which promotes the decomposition of saturated sulfides. The whole process is one of the keys to the mineralization of the P porphyry.