In-situ U–Pb SIMS dating and trace element (EMPA) composition of zircon from a granodiorite porphyry in the Wushan copper deposit, China

Abstract

Zircons from a granodiorite porphyry at the Wushan copper deposit in the Lower Changjiang Metallogenic Belt, east central China, were dated using a Cameca IMS 1270 secondary ion mass spectrometer (SIMS); their chemical compositions and Hf isotopes were analyzed using a JEOL JX A8800 electron microprobe (EMPA) and a Neptune LA-MC-ICP-MS, respectively. The U–Pb dating of zircon reveals two age groups for the granodiorite porphyry; i.e. 144.6 ± 3.9 Ma and 121.0 ± 2.5 Ma. Zircons of the two age populations display distinct chemical compositional characteristics with respect to UO2/HfO2 ratios, and show a negative correlation of (UO2 + Y2O3 + ThO2) and HfO2. The older age group of ∼145 Ma, defined by the majority of the zircons, is interpreted as the magmatic emplacement age, whereas the younger age group of ∼121 Ma is interpreted as rejuvenation due to subsequent thermal or hydrothermal events. Other interpretations such as presence of two distinct magmatic crystallization stages or inheritance of the older zircons are also discussed in the paper. The ∼145 Ma magmatism at Wushan is consistent with the ages of most of the granitods in the Lower Changjiang Metallogenic Belt. The Hf isotope composition of zircons (ɛHf = −2.1 and −7.0; TDM Hf ages = 0.87 and 1.05 Ga), together with other available geochemical and Sr–Nd isotope data suggest that the granodioritic magma was derived from mixing of juvenile mantle with older crustal material. Due to the close spatial and temporal relationship of the granodiorite porphyry and the skarn mineralization at Wushan, the magamatic hydrothermal mineralization likely took place between 145 Ma and 121 Ma, likely around 121 Ma.