Controls on kaolinite and dickite distribution, Highland Boundary Fault Zone, Scotland and Northern Ireland

Abstract

Kaolinite and dickite occur widely in central Scotland and Northern Ireland. Stable hydrogen and oxygen isotope compositions of both minerals are similar, suggesting that the formation of kaolinite occurred first at temperatures of <50°C from meteoric water, probably as a result of alteration of Lower Carboniferous volcanic rocks, and that dickitization followed locally as a result of local heating that accompanied the intrusion of dykes during Late Carboniferous–Permian times. This mechanism of dickite formation explains why the higher‐temperature polytype dickite occurs in a region from the Firth of Clyde to Perthshire, in association with dyke swarms, whilst kaolinite occurs elsewhere. The original kaolinite precipitation was focused along the high permeability fault zone now marked by the Highland Boundary Fault Zone and its presumed trace in Northern Ireland.