Dr C . C. Rundle writes: In Special Publication No. 8 of the Geological Society, The Caledonides of the British Isles–reviewed, Drs Nutt, Ineson & Mitchell presented K-Ar data relevant to the age of mineralization at Parys Mountain, Anglesey, which, they suggest, indicates a period of mineralization at around 353 ± 7 Ma. It is the purpose of this note to demonstrate that their interpretation of the K-Ar data is erroneous and that there is no justification for suggesting this age for the mineralization.

(1) Nutt et al. presented K-Ar dates for ‘control’ samples of illite separated from ‘unaltered country rock well removed from any known mineralization’ which, to my mind, demonstrate admirably the unreliability of illite and illite–chlorite mixes as geochronometers. Four of these samples come from Ordovician country rocks which must be in excess of c. 421 Ma in age (see Gale et al. 1979) yet have given ages of 416, 409, 396 and 383 Ma, all with an analytical precision of ±4 Ma. It is also noteworthy that the sample with the youngest age is the only one with chlorite contamination. The authors comment that all the control samples are included in an isotopic date of 394±9 Ma. My interpretation would be that these dates are the result of variable argon loss, and that this type of material does not yield geologically meaningful ages.

(2) The errors in K-Ar dating of altered wallrock material are unlikely to be of a random nature. The problem is

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