Petrographic and isotopic studies of the altered acid volcanics of the Tolfa-Cerite area, Italy: the genesis of the clays

Alunite-kaolinite deposits occur in argillized zones in acid volcanic rocks at Tolfa. Over seventy samples were examined by one or more of the following methods: thin section, DTA, X-ray, IR, TG, SEM, chemical, and hydrogen and oxygen isotope analysis. Kaolinite and dickite, of high structural order, are the dominant clays with micamontmorillonite and halloysite subordinate. In many samples kaolinite and dickite coexist; their relative amounts are variable but the dickite content tends to increase with depth. SEM studies show that dickite crystals—up to 30 µm—are bigger than kaolinite. The D/H and18O/16O ratios of six clays, a whole rock, two biotites from fresh host rock, two chalcedony veins and three local meteoric waters indicate that the clays formed in a meteoric-hydrothermal environment of acid hot spring type at about 80°C. The fresh volcanic host rocks are strongly enriched in18O relative to ‘normal’ igneous rocks due to derivation from or exchange with18O-rich sedimentary rocks at depth prior to hydrothermal and solfataric activity. In the absence of present-day geothermal activity the life of this hot spring system was less than 4 m.y.