Characteristics and metallogenic significance of granite porphyry and pyroxene diorite in the Bianjiadayuan Pb-Zn-Ag polymetallic deposit, Inner Mongolia

Located in the southern section of the CAOB (Central Asian Orogenic Belt), the Bianjiadayuan Pb-Zn-Ag polymetallic deposit belongs to the Sn-Cu-Zn-Pb metallogenic belt of Da Hinggan Mountains. In this study, a series of analyses, such as LAICP-MS zircon U-Pb isotopic dating, major element and trace elements testing and electron microprobe analysis of albite, were performed for the granite porphyry and augite diorite. The results show that the age of granite porphyry and pyroxene diorite are ca. 138 Ma and ca.137 Ma respectively, indicating that the intrusive rocks are products of the magmatic activities in the Early Cretaceous. The pyroxene diorite belongs to high K calc-alkaline series and calc-alkaline series with SiO₂ (50.99%-52.89%), CaO (7.4%7-7.51%), MgO (3.64%-4.68%), and alkali (Na₂O+K₂O) 4.91%-5.36%. Granitic porphyry with miarolitic structure and microscopic identification shows that feldspar is all alkaline feldspar. Non-mineralized granite porphyry is characterized by high SiO₂ (50.99%-52.89%), alkali (Na₂O+K₂O=4.83%-9.42%), A/CNK (1.13-2.40), LREE enrichment, strong negative Eu anomalies (δEu=0.12-0.32), enrichment of LILE such as Rb, Th, U and K, depletion of HFSE such as Ta, Nb, P and Ti and transition elements such as Sr and Ba. According to the electron microprobe analyses, the An values of the albite in granite porphyry are by far lower than 10 (0.03-4.64). These features are similar to the features of typical highly evolved A₂ post-orogenic alkali feldspar granite pluton, suggesting that the magma was derived from the lithospheric mantle and formed in the tensional setting. Combined with the geological characteristics and previous research results, the authors hold hat the metallogenic geological body of the Bianjiadayuan area is the granite porphyry pluton, and there is still a great potential for mineralization in the deep part of western mining area.