The implications of reworking on the mineralogy and chemistry of lower carboniferous K-bentonites

Potassium-bentonites have been found in the Courceyan Lower Limestone Shales near Burrington Combe and Oakhill, Somerset, consisting of thin, greenish yellow, plastic clays interbedded within a mudrock and limestone sequence. Mineralogically, the clay fraction is composed of virtually monomineralic interstratified illite-smectite containing 7-10% smectite layers. The clay fraction of the surrounding mudrocks, however, consists of an illite-chlorite dominated assemblage. Their mineral composition, trace element content, and the relative abundance of zircon crystals suggest an origin from burial of montmorillonite originally formed from volcanic ash. The presence of anomalously high trace element contents with both euhedral and rounded zircon grains in the Oakhill K-bentonites suggests a secondary or reworked origin for these samples. In contrast, the presence of a non-anomalous trace element content and large (>100 mu m) euhedral zircon grains suggests that the Burrington K-bentonite is primary in origin. Modelling of whole-rock rare-earth element (REE) patterns shows that the Oakhill REE pattern can be derived from the Burrington pattern by the addition of small contributions from zircon and monazite, two major heavy minerals present. These K-bentonites probably represent the oldest Carboniferous K-bentonites so far recorded in the British Isles.