Laser-generated, step-heated 40 Ar- 39 Ar dates of single grains of fuchsite have been determined from the alteration footwall of the Archean Kidd Creek massive sulfide deposit. The grains are very small (50-200 mu m in diam) yet give age spectra showing excellent plateaus for 90 percent or more of their respective gas releases, with an average integrated age of 2618 + or - 8 Ma (20Sigma ). This result provides a date which is interpreted as relating to a K metasomatic alteration event at Kidd Creek; this is consistent with earlier suggestions, based on a Pb isochron and U-Pb dating, that some alteration was significantly younger than the felsic volcanic rocks (2717 + or - 2 Ma) hosting the massive sulfides. The 40 Ar- 39 Ar laser date may also provide the constraint for the fact that the sericite-fuchsite alteration postdated the 2.67 to 2.69 Ga regional metamorphism in the Abitibi belt. The age of 2618 + or - 8 Ma for K metasomatism is coincident with an age of 2617 + or - 8 Ma for fuchsite from the nearby Hollinger gold mine. Late processes dated by fuchsite and rutile from gold deposits of the Abitibi belt generally fall between 2.58 and 2.63 Ga. This range of ages between the various ore deposits suggests that hydrothermal activity may not have been synchronous at all deposits and/or some ore deposits were affected by more than one thermal episode.