Abstract: Gejiu is one of the largest polymetallic tin ore concentration areas in the world and is a granite-related magmatic-hydrothermal system. There are two main kinds of orebodies in the south area of the Gejiu deposit. One is stratoid orebody, and the other is contact orebody. The contact orebody and oxidized orebody, which closely occur in the depression of granite body, have not been given accurate ages. This paper presents new 40Ar/39Ar biotite age for the contact copper orebody and muscovite age for the oxidized orebody from the contact zone of Xinshan granite intrusion in the Gejiu tin-copper deposit so as to constrain the timing and duration of mineralization after the formation of Xinshan granite body. The biotite and muscovite yielded 40Ar/39Ar plateau ages of 82.47±0.49Ma and 76.17±0.42Ma, normal isochronal ages of 82.38±0.48Ma and 76.07±0.66Ma, and inverse isochronal ages of 82.38±0.49Ma and 76.07±0.73Ma, respectively. Combined with field geological contact relationships and the other geological dating results, the authors have reached the conclusions that the 40Ar/39Ar age of biotite is closely related to ore deposition, and constrains the formation of the contact copper orebody at 82.38±0.48Ma, and also records the rapid cooling process after the Xinshan granite formation. The 40Ar/39Ar age of 76.07±0.73Ma in muscovite indicates the age of the formation of oxidized orebodies, and also records the late stage of movements of the N-S trending faults in the east segment of the Gejiu deposit. These dating results are coincident with the data obtained from the Gejiu polymetallic tin-copper deposit, and hence the tin-copper deposits belong to the same structure-magma-fluid system.