The Sanjiang metallogenic belt is one of the most important polymetallic ore-forming belt in China. The Duocai ore concentration area possesses various types of Cu polymetallic deposits in its middle and north part,and its mineral resource potential is huge; nevertheless, the ambiguous mineralization age seriously restricts the understanding of the ore deposits and regional mineralization regularity. In view of such a situation,the authors selected the Chayong copper polymetallic deposit in this belt and conducted a complete systematic research on its mineralization chronology based on detailed field exploration and microscopic mineralogical identification so as to fill the research blank of the ore-forming age in this area. The results indicate that there are two kinds of orebodies in the Chayong copper polymetallic deposit, mainly Cu-Mo orebodies, which were developed in the metamorphic siltstone and mainly occurred at the quartz and sulfide stage. The biotite in hornfels shows that the points of Mg-(Al^Ⅵ+Fe³⁺+Ti)-(Fe²⁺+Mn) in diagram fall within the range of magnesium biotite, the value of Al₂O₃/TiO₂ is between 6.359 and 10.087 and the ratio of Fe²⁺/(Fe²⁺+Mg) is comparatively uniform. Based on the above data, the authors hold that it is hydrothermal biotite unmodified by late fluids. The authors confirmed the closure age of hydrothermal biotite by the ⁴⁰Ar-³⁹Ar plateau age of 129.59 ±1.58 Ma. In addition, molybdenites which penetrate into chalcopyrite have the rhenium content from 0.864×10^-6 to 9.624×10^-6. These values indicate that the metallogenic material of molybdenite was probably derived from the crust. The Re-Os isochron age of molybdenite is 124.75±0.86 Ma. In conclusion, the age of hydrothermal biotite is similar to that of molybdenite, and it is held that both of them should belong to the same period of magmatic hydrothermal event as shown by their closure age. This study accurately determined the metallogenic age of a typical magmatic hydrothermal Cu polymetallic metallogenic deposit in the middle and northern part of the Sanjiang metallogenic belt, revealing that there was a mineralization event during the Early Cretaceous, and these data indicate a new direction for studying regional metallogenic regularity and for prospecting work in the Sanjiang metallogenic belt.