Abstract：The Gejiu polymetallic tin ore district in Yunnan Province is the largest tin deposit in the world, and the Kafang copper (tin) deposit is a component part of this ore district. The stratoid ore body of the Kafang copper (tin) deposit, closely related to meta-basalt, is characterized by enriched copper. The basalt has been altered and the alteration intensity is related to the copper mineralization. This paper presents a new 40Ar-39Ar phlogopite age for the meta-basalt in the Kafang tin-copper deposit in order to constrain the timing and duration of the alteration of basalt and the mineralization. Phlogopite yielded 40Ar-39Ar plateau age of 85.49±0.63 Ma, isochronal age of 85.62±0.42 Ma, and inverse isochronal age of 85.59±0.81 Ma. These ages are substantially consistent with each other, pointing to late Cretaceous. These results are coincident with the data obtained from the Gejiu polymetallic tin-copper deposit.