A K–Ar mean age of 49.8 ± 0.7 m.y. was determined for four biotite concentrates from granite porphyry on the Mt. Haskin Mo and Mt. Reed Mo–W properties east of Cassiar, British Columbia.Biotite K–Ar ages of 71.7 ± 2.6 m.y. and 68.3 ± 2.7 m.y. obtained from a young phase of the Cassiar intrusions, occurring along the eastern margin of the Cassiar Batholith, place an upper limit on the age of the molybdenum mineralization on the Cassiar Molybdenum property. A 62.0 ± 2.2 m.y. age, determined for a biotite concentrate from the coarse alaskite phase of the Mt. Leonard Boss, dates the molybdenum mineralization on the Adanac property.These ages suggest that the early Tertiary metallogenic epoch, documented in central British Columbia and southeastern Alaska, can be extended through northern British Columbia.