关键词: 花岗斑岩, 银矿床, 地质年代学, 地球化学, Sr-Nd-Hf同位素, 白石坡, 大别山

Abstract: The Baishipo silver deposit is located in the northern margin of the Dabie Orogen, central China. Silver mineralization characterized by Ag-Pb-Zn quartz veins is spatially associated with the Baishipo granite porphyry. Elemental, whole-rock Sr-Nd and zircon U-Pb-Hf isotopic analyses have been carried out in this study, in order to understand the mineralizational setting. In situ zircon U-Pb dating yielded an Early Cretaceous crystallization age (142±2 Ma, MSDW=1.9) for the Baishipo granite porphyry. The granite porphyry is characterized by high concentrations of SiO2 and potassium, belongs to the high-K calc-alkaline series, and is generally strongly peraluminous. It is enriched in light rare earth elements (LREEs) and large ion lithophile elements, but depleted in heavy REEs and high field strength elements (Nb, Ta and Ti), and characterized by negative Eu anomalies. They have initial 87Sr/86Sr ratios ranging from 0.714762 to 0.715890 and negative εNd(t) values clustered of -13.8. In situ zircon Hf isotopic analyses show that the εHf(t) values of zircons from the granite porphyry vary from -16.6 to -13.5 with calculated two-stage Hf model ages of 1.81 to 1.98 Ga, suggesting a homogeneous crustal source without significant magma contamination or mixing. The geochemical and Sr-Nd-Hf isotopic signatures suggest that the Baishipo granite porphyry is a highly fractionated granite and is likely to be derived by partial melting of the ancient crust of the Yangtze Block. We propose that the Baishipo granite porphyry and related silver deposit formed in a thickened crust setting, before the Cretaceous tectonic collapse in the western Dabie Orogen.

Key words: granite porphyry, silver deposit, geochronology, geochemistry, Sr-Nd-Hf isotopes, Baishipo, Dabie Orogen