Yanshanian Magmatism And Au Mineralization In The Qiyugou Au Orefield Xiong'ershan

There are abundance of vein-type Au deposits in the southern North China Craton,and most of them emplaced at Early Cretaceous.Whether they are orogenic Au deposits or intrusion-related Au deposits is debated for a long time.The Qiyugou Au orefield contains Au resources of more than 66 t in this region.Three types of Au,including granite-hosted Au,breccia pipe-hosted,and fault-hosted Au(vein-type Au),and abundant granite were emplaced in this orefield.Thus,the Qiyugou Au orefield is potential to check the relationship between magmatism and Au mineralization.In this study,we present detailed investigation of intrusions and mineralization,and the new findings and conclusions can summarized as follows:1.We obtained SIMS zircon U-Pb ages of 158.6±1.1-157.3±1.1 Ma,132.3 ±1.3-130.8± 1.9Ma,and 128.9± 0.9-127.4 ± 0.4Ma for quartz porphyry,hornblende monzogranite,and hornblende monzogranite porphyry,respectively.These dating results are corresponding to the crosscutting relationships.Evidence from the major and trace elements,Sr-Nd-Pb-Hf-O isotopic compositions suggests that the hornblende monzogranite was partial melting from the enriched mantle metasomatized by subducted South China Block in Trassic.The processes are triggered by roll-back of westward subduction of Paleo-pacific plate and asthenosphere upwelling.2.Zircon xenocrysts form the hornblende monzograntie generally show positive?Hf(t)and low ?18O,which suggests that the melt from eastward flat subducted Paleoproterozoic oceanic crust underplated in the lower crust3.Five molybdenite form stage I of granite-hosted Au yield a weight average Re-Os age of 132.9± 1.5 Ma,which is consistent with the zircon U-Pb ages of the hornblende monzogranite within errors.According to field investigation and previous studies,we suggest that the mineralization ages of breccia pipe-hosted Au and fracture-hosted Au are close to the mineralization age of granite-hosted Au.One sericite yields a weighted plateau 40Ar-39Ar age of 125.9 ± 1.4 Ma,which is caused by thermal resetting of the emplacement of hornblende monzogranite porphyry4.Away from the hornblende monzogranite,metal zonation of three types of Au shows Au-Mo,Au-Cu and Au-Ag-Pb.Breccia pipe-hosted Au,fracture-hosted Au,and granite-hosted Au have telluride and Bi-sulfosalt minerals and hematite.The S isotopic compositions of the breccia pipe-hosted Au and fracture-hosted Au are similar to the granite-hosted Au.The Pb isotopic compositions of the three types of Au are consistent with that of the hornblende monzogranite.All of above evidence combined with previous fluid inclusion and H-O-He-Ar studies imply that the three types of Au mineralization are genetically related to hornblende monzogranite and constitute an intrusion-related Au system.