ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, Archaeometry and Cultural Heritage

Abstract. — The provenance determination of the ancient stone materials results the main insight archeometric application in the archaeological field, even if it can result difficult to reach when the studied material consists of white or coloured marbles. A petrographic application on grey marbles, using a scanning electron microscope equipped with a electronic microprobe system, is here reported. The marbles of Porta Praetoria (Aosta, Italy), manufactured by the Ancient Romans in the Republican Age, are studied. According to historical documents, two types of marble may have been used: the marble coming from Villeneuve or from Aymaville villages (Aosta Valley). Both marble varieties occur as lenses interbedded in Upper Piedmont Zone, an ophiolitic unit belonging to Penninic domain of the Western Alps. The sample from the Porta Praetoria is constituted by a grey marble, characterized by calcite and dolomite levels of different grain size, which reflect different grey tonality and confer to the rock a typical veined appearance (“bardiglio ” variety). Minor silicate minerals, concentrated in the dolomite layers, are also present. In particular, this marble is characterized by the contemporaneous presence of three dioctahedral micas, with the potassic mica enriched in the celadonitic end-member as crystallized at high pressure metamorphic conditions. In additions, phlogopite and Mg-chlorite are also present. Comparable paragenesis is shown by the Aymaville marble, whereas the marble from Villeneuve resulted devoid of phlogopite and Mg-chlorite. Therefore, on the basis of petrographic and microchemical features, it may be suggested that the grey marble used for the edification of the Porta Praetoria in Aosta comes from the village of Aymaville, located in the right side of the central Aosta Valley. Riassunto. — In questo lavoro viene presentato uno studio petrografico di una varietà di marmo di notevole significato archeometrico, impiegato per alcune importanti costruzioni storiche della città d