40Ar-39Ar fast-neutron activation dating of biotite from the Weilasituo Cu-Zn polymetallic deposit in Inner Mongolia was conducted by microprobe. The average age 133.4±0.8 Ma is considered to be the time limit of the last or the late hydrothermal activity in the Weilasituo copper-zinc deposit. According to regional age relationship, the ore deposit seems to have been controlled by regional post-orogenic lithspheric thinning and strongly related to the magmatism of the late Yanshanian granite. The late regional Yanshanian granite might have provided the necessary heat and material resources for the formation of the Weilasituo deposit.