The Meandu Creek Bentonite, Queensland

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Abstract
Remnant ash particles, vitric lapilli and pumice fragments established that the economic clay deposit at Meandu Creek was formed by devitrification and alteration of pyroclastic material. The assemblage of trace elements niobium, zirconium and yttrium suggested that the parent ash was more akin in composition to alkali‐acid volcanic rocks than to the basaltic lavas that blanketed the surrounding highlands. Variations in composition, pre‐depositional and diagenetic history of the parent ash were responsible for subdivision of the deposit into three horizons characterised by different clay minerals. Hydrated halloysite, allophane and gibbsite in the basal horizon were formed by subaerial weathering of the ash, and were subsequently deposited and preserved in an aqueous environment. Montmorillonite and α‐cristobalite in the central horizon were produced by alteration of the ash in place under conditions of impeded drainage. Kaolinite in the upper horizon formed by the weathering of the montmorillonite in a freely leached environment.

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