Geochemical and mineralogical studies of zircon, apatite, and chlorite in the giant Dexing porphyry Cu-Mo-Au deposit, South China: Implications for mineralization and hydrothermal processes

The Tongchang granodiorite porphyry intrusion is located at the giant Dexing porphyry Cu-Mo-Au deposit in South China, and both mechanism of petrogenesis and mineralization remain controversial. In this study, results of zircon U-Pb dating of three granodiorite porphyries yield ages of 169.8 +/- 1.3 Ma, 167.1 +/- 1.1 Ma, and 166.2 +/- 1.3 Ma, respectively, indicating that their formation ages are contemporaneous. The zircon Hf isotopes of the three samples are consistent within the range of error (epsilon Hf(t) = 2.5-4.8), indicating that they share the same magma source. Values of Sr isotopes range from 0.70414 to 0.70759 (av. 0.70520) and 0.70530 to 0.70745 (av. 0.70624) for altered apatite and 0.70368 to 0.70493 (av. 0.70443) for fresh apatite. Major elements (F and Cl) of altered apatite (F = 1.51-2.44 wt%, Cl = 1.33-2.32 wt%) are significantly different from unaltered apatite (F = 2.54-4.62 wt%, Cl = 0.004-1.20 wt%). Trace element compositions, such as delta Eu, delta Ce, REE distribution patterns, and etc., between different apatites are also diverse. All these apatite geochemical data indicate that some apatite grains were modified by hydrothermal fluids, which were derived from the crust characterized by higher oxygen fugacity than the initial magma, indicating that they have undergone the decomposition of plagioclase. Chlorite geothermometers show that the formation temperature of chlorite from three granodiorite porphyry samples ranges from 221 to 269 degrees C (av. 240 degrees C), 236 to 254 degrees C (av. 246 degrees C), and 196 to 232 degrees C (av. 215 degrees C), respectively, indicating the temperature of hydrothermal alteration. The apatite and zircon data suggest that the Dexing porphyries were derived from partial melting of the subducted paleo-Pacific plate during the Jurassic rather than from the lower crust. Subducted oceanic crust with high oxygen fugacity and initial Cu content and Cl content are the key factors in forming the giant Dexing porphyry Cu-Mo-Au deposit.