Composition and genesis of the clays associated with quartz and sand deposits at Paganico (Grosseto, Central Italy)

The clays associated with quartz sands at Paganico (Grosseto, Central Italy) are interesting both for their genesis and their potential uses. The Paganico deposits, ranging in age from Pliocene to Quaternary, mostly consist of very pure quartz sands derived from the erosion of Verrucano metasandstones sedimented in small lacustrine basins. Locally a clayey facies, made up of a soapy and white clay, is associated with the coarser detrital facies.The clayey beds are 2–3 meters thick and of limited extent, but their components may be recovered in great quantities from setting ponds, where they accumulate as tailings of quartz sand washing processes. The clay consists of kaolinite (up to 30%) and illite (10–70%), with subordinate amounts of alunite (5–10%), the remaining being mostly quartz. The genesis of these clays is complex. All the Paganico quartz sand and clay deposits are detrital in origin, being formed by Verrucano debris sedimentation in lacustrine basins. In some cases the deposition has been followed by gley-forming-like processes, involving seeping of organic-rich waters inducing leaching and partial removal of iron. In these cases partially iron-depleted, mottled quartz sand bodies were formed. The associated clays, when present, have not been affected by noticeable modifications, being mostly illitic in composition. In other quarries evidences of hydrothermal actions are present; extensive silica precipitation close to observed or inferred faults, abundance of alunite, complete leaching of iron, presence of nearby stibine mineralizations point to a hydrothermal fluid action. Kaolinite and illite are major constituents of these clays. The presence of alunite hinders the use of clays in the ceramic industry