The 40 Ar/ 39 Ar technique of K-Ar dating has been used to determine the age of hydrothermal sericite associated with mineralization in the Goonumbla porphyry copper-gold deposits, Lachlan fold belt, New South Wales, Australia. The 40 Ar/ 39 Ar age spectra from Goonumbla hydrothermal sericite indicate that vein and groundmass (potassic and phyllic) alteration were broadly contemporaneous at 439.2 + or - 1.2 Ma (1Sigma ). The 40 Ar/ 39 Ar dates obtained from hydrothermal alkali feldspar are considerably younger than those from coeval hydrothermal sericite. U-Pb dating of magmatic zircons by ion microprobe yields an age of 438.5 + or - 3.6 Ma (1Sigma ) for the Nelungaloo Volcanics which underlie the mineralized Goonumbla sequence, providing a maximum age for mineralization and suggesting that the two successions are largely coeval.The 40 Ar/ 39 Ar age spectra obtained in this investigation from fine-grained sericite illustrate the amenability of this hydrothermal phase to dating by the 40 Ar/ 39 Ar technique and suggest that there is considerable potential for application of this dating method to Phanerozoic ore deposits. The close agreement in ages between the 40 Ar/ 39 Ar and U-Pb results gives confidence in the application of both dating techniques to altered Paleozoic rocks.