Geochemistry of apatite and zircon and their constraints on the origin of Mesozoic intrusion-related Cu-Pb-Zn deposits in the Nanling region, South China

In addition to its world-renowned W-Sn deposits, the Nanling region of South China also hosts several Cu-Pb-Zn polymetallic deposits that are genetically linked to the Mesozoic granodioritic plutons. In this study, we conducted in situ trace elemental and isotopic analyses on zircon and apatite along with whole-rock geochemical analysis to constrain the Cu-Pb-Zn ore formation. The granodiorites from the Baoshan, Tongshanling, and Shuikoushan deposits yielded zircon U-Pb ages of ca. 160 Ma, broadly coeval to the Cu-Pb-Zn mineralization. The average oxygen fugacity (fO2) for the granodiorites is ΔFMQ (logfO2 value relative to the fayalite-magnetite-quartz oxygen buffer) +0.74 (Baoshan), ΔFMQ +0.07 (Tongshanling), and ΔFMQ +1.67 (Shuikoushan), with the corresponding apatite Cl content of 0.36−0.46 wt%, 0.21−0.36 wt%, and 0.29−0.98 wt%, respectively. These fO2 and Cl values are higher than typical granite-related W-Sn mineralization (ΔFMQ −1.17, Cl = 0.01−0.48 wt%), and may have facilitated the Cu-Pb-Zn mineralization. Zircon Ce/Sm and Eu/Eu* ratios for the granodiorites, respectively, are 2.16−9.30 and 0.36−0.60 (Baoshan), 2.6−7.96 and 0.16−0.57 (Tongshanling), and 5.92−12.13 and 0.44−0.69 (Shuikoushan). This suggests that the Baoshan and Shuikoushan granodiorites may have undergone stronger amphibole fractionation than the Tongshanling granodiorite. This, in turn, implies that the granodiorites from Baoshan and Shuikoushan had higher water content than those from Tongshanling. The inferred “wetter” granodioritic magma at Baoshan and Shuikoushan may account for their larger deposit size than that of Tongshanling. Zircon grains of the granodiorites from Baoshan, Tongshanling, and Shuikoushan have εHf(t) values of −15.5 to −8.2, −16.9 to −8.2, and −10.6 to −7.4, while the apatite grains have εNd(t) values of −9.2 to −6.7, −8.8 to −6.0, and −8.7 to −4.2, respectively. The Nd-Hf isotopes are decoupled in the Baoshan and Tongshanling granodiorites, deviating from the terrestrial array. Thus, the Baoshan, Tongshanling, and Shuikoushan granodiorites were primarily derived from partial melting of the lower continental crust, while some melts from the subduction-metasomatized mantle were likely involved in the Baoshan and Tongshanling granodiorite formation. This may have led to the higher Cu/(Pb + Zn) ratio in the Baoshan and Tongshanling deposits. The formation of granodiorite-related Cu-Pb-Zn deposits in the Nanling region was likely associated with the Paleo-Pacific plate subduction and slab rollback in an extensional setting.