New samples of halloysite from the type-locality of Angleur were investigated. X-ray, chemical, optical, and thermal analyses of this halloysite show a strict relation of hydration with a continuous transition from halloysite-(10 Å) to halloysite-(7 Å). Oxydation of a sulfide orebody associated with the Streupas fault is taken as the main source for acids which have played a role in the karstification of Tournaisian dolomitic limestones and in the halloysite genesis. The source-rock for Al and Si is derived from leaching of illites from Namurian black shales.