Petrogenesis and metallogenic potential of the Jurassic porphyries in the northwest Zhejiang Province, South China: insights from SHRIMP zircon geochronology, geochemistry, and Sr–Nd–Hf isotopes of the Huangbaikeng ore-bearing porphyries

The northwest Zhejiang Province is a key domain for providing deep insight into the crust-mantle interaction and tectonic evolution of the South China block. In this paper, we collect geochemical, geochronological, and isotopic data of the Jurassic porphyries in this region, and investigated the Huangbaikeng ore-bearing porphyry in the Tongcun Mo-Cu deposit, using it as an example to uncover the porphyry petrogenesis and evaluate their metallogenic potential. Two varieties of the Huangbaikeng porphyry were distinguished: the medium-to coarse-grained type and medium-to fine-grained type. Zircon Sensitive High-Resolution Ion Microprobe U-Pb dating indicates that they were emplaced at 161.8 +/- 2.8 and 162.7 +/- 3.5 Ma, respectively, which are consistent with the molybdenite Re-Os ages of 163.9-161.8 Ma. The inherited zircons age spectrum significantly recorded a series of geological events, for example, assembly and breakup of the Columbia and Rodinia supercontinent, and the Triassic collision of Yangtze and North China blocks. Whole rock Sr-Nd and Jurassic zircon Hf isotopic data yield mostly negative epsilon(Hf)(t) values (0.5 to -8.4) and epsilon(Nd) (t) values (-0.79 to -4.82). Besides the Huangbaikeng porphyry, all the Jurassic porphyries in the northwest Zhejiang Province have a wide range of SiO2 contents (76.78-60.91 wt.%). They do not contain typical aluminous minerals (e.g. cordierite and garnet), and are mainly metaluminous to weakly peraluminous with high Na2O, low FeOT/MgO, and Zr + Nb + Ce + Y concentrations in composition. They thus fit the I-type granite definition. Some major and trace elements show strong correlations with SiO2, possibly indicating extensive fractional crystallization during their magma evolution. Tectonic discriminations imply that these plutons were likely formed in a volcanic arc regime possibly related to subduction of the Palaeo-Pacific plate. Sr-Nd-Hf isotopic data suggest a mixed source of the Mesoproterozoic crust and 30-50% mantle components. Compared with the adjacent Dexing Cu-bearing porphyies, which have more positive eHf(t) and eNd(t) values with more significant mantle components (55-70%), the Jurassic porphyries in the northwest Zhejiang Province probably lack metallogenic potential to form a giant porphyry copper deposit as Dexing.