Origin of Early Cretaceous A-type granite and related Sn mineralization in the Sanjiaowo deposit, eastern Guangdong, SE China and its tectonic implication

Abstract

The recently discovered Sanjiaowo Sn deposit is located in eastern Guangdong, SE China. The Sn mineralization, hosted in granite porphyry and the Lower Jurassic Jinji Formation low-grade metamorphic wall rocks, is considered to be granite-related. Zircon U–Pb dating of the granite porphyry yielded concordant ages of 140.5 ± 1.4 Ma, which is consistent with the cassiterite U–Pb age of 139.8 ± 5.0 Ma, indicating a temporal link between the emplacement of granitic plutons and Sn mineralization. The granite porphyry samples are weakly peraluminous and show an A-type geochemical signature with high Na2O + K2O (7.19–8.51 wt.%), REE (261–308 ppm), Zr + Nb + Ce + Y (353–476 ppm) contents, FeOT/MgO (0.93–0.99), and Ga/Al (2.65–3.42) ratios. They display bulk rock initial 87Sr/86Sr of 0.7110 to 0.7165, εNd(t) of −5.90 to −4.04, and zircon εHf(t) values from −8.36 to −4.16 with Mesoproterozoic TDM2 ages for both Nd and Hf isotopes. Detailed elemental and isotopic data suggest that the Sanjiaowo Granite porphyry belongs to the A2-subtype and were formed by partial melting of a crustal source of Mesoproterozoic overall residence age with minor input of mantle materials.The low calculated zircon Ce4+/Ce3+ (6.5–81.2) and EuN/EuN∗ (0.05–0.24) ratios demonstrates that the Sanjiaowo granitic magma had a low oxygen fugacity, which may have facilitated the Sn enrichment and played a key role in the Sn mineralization. Lead isotopes indicate that there is no difference for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios between the granite porphyry and the sulfide minerals from the orebodies. Moreover, the sulfur isotopic compositions for the sulfides (pyrite and pyrrhotite: δ34S = −1.6–1.0‰, average = −0.32) is similar to magmatic δ34S, but different from the δ34S in the Jinji Formation strata. Consequently, these data favor of a magmatic-hydrothermal origin for the Sn mineralization in the Sanjiaowo deposit. Integrating new and published data on the tectonic evolution, we suggest that the granite porphyry and associated Sn mineralization in the Sanjiaowo deposit formed in a back-arc extensional environment at ∼141 Ma, related to the rollback of the paleo-Pacific Plate.