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Clay Minerals; September 2007; v. 42; no. 3; p. 353-372; DOI: 10.1180/claymin.2007.042.3.08
© 2007 Mineralogical Society of Great Britain and Ireland
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Clay mineral reactions in an active geothermal area (Mt. Amiata, southern Tuscany, Italy)

S. BATTAGLIA1,*, F. GHERARDI1, G. GIANELLI1, L. LEONI2 and F. ORIGLIA1

1 Istituto di Geoscienze e Georisorse, Italian National Research Council, Via Moruzzi 1, 56124, Pisa, Italy, and 2 Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126, Pisa, Italy

* E-mail: battaglia{at}igg.cnr.it

(Received 25 June 2006; revised 20 June 2007)

This study characterizes the effects of fluid migration into a predominantly shale cover which seals the active geothermal system of Mt. Amiata (Tuscany, Italy). During Alpine orogenesis the shale unit was affected by regional metamorphism at the limit of the diagenesis-anchizone. Subsequently, the phyllosilicate clay minerals of the shales underwent significant alteration at diagenetic temperatures (175±25°C as determined by the geochemical model) by the pervasive circulation of fluids activated by the geothermal field. The overall mineralogical assemblages indicate that the main transformations consisted mostly of destabilization of illite and formation of kaolinite together with large amounts of I-S mixed layers, with higher smectite content and decreased Reichweite I-S ordering (from R3 to R1) with respect to the original, unaltered phases. Application of computer modelling indicates that the circulation of CO2-rich geothermal fluids into the shale unit was responsible for the observed phyllosilicate clay mineral transformations.

KEYWORDS: clay minerals, retrograde diagenesis, Mt. Amiata geothermal field, XRD, geochemical modelling






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