The exact timing of main stage mineralization in southwest England has never been certain. K-Ar ages for micas and feldspars from a metallized pegmatite are discordant but suggest formation at about 285 m.y. The K-Ar ages of micas from greisens are also discordant and generally younger than Rb-Sr ages (282 + or - 3 m.y.). K-Ar ages of eleven K-feldspars associated with the major polymetallic fissure mineralizations range from 212 + or - 4 to 244 + or - 4 m.y. and, excepting one sample, correlate inversely with triclinicity, which corresponds to that of intermediate to maximum microcline. It can be tentativeiy inferred from this that these feldspars are more than 260 m.y. old. This is confirmed by Rb-Sr ages for seven of the K-feldspars which have a mean of 270 m.y.The major mineralization appears to be closely associated temporally with porphyry dike injection rather than with emplacement of the Cornubian batholith. It can be inferred that argon loss from the feldspars was related to the processes causing structural readjustments, the major catalyst for which was probably heat from the granite and hydrothermal circulations persisting more than 60 m.y. after the granites were emplaced.