The ratio of the two isotopes 39K and 41K has been measured close to the sharp contact between a granite and an amphibolite in the Spekboom area. Within the amphibolite a zone of about 3 cm shows abnormally high values of this ratio (up to 14.65). The value for the amphibolite further from the contact is close to 14.22. This behaviour is shown to be the same as that reported for an unaltered shale xenolith at Sea Point. It is further shown that the variation in isotope ratio can be qualitatively represented by assuming homogeneous diffusion of potassium from the granite into the aniphibolite.