Potassium-argon isotopic age determinations from the metasomatic alteration of the Great Limestone, Northern Pennine Orefield

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SUMMARY K-Ar isotopic ages of illite-bearing iron carbonates associated with metasomatism of the Great Limestone are argued to relate to mineralisation which occurred between 252±5 and 240±6Ma. There is evidence that the ages of some of the illites investigated may have been partially reset by later hydrothermal events occurring between 240Ma and 160Ma. It is concluded that, after the iron-carbonate rocks were formed, certain vein systems were sealed from further fluid circulation.