Porretta quartz occurrences, Porretta Terme, Alto Reno Terme, Metropolitan City of Bologna, Emilia-Romagna, Italyi

Regional Level Types
Porretta quartz occurrences	Group of Occurrences
Porretta Terme	Municipality (Former)
Alto Reno Terme	Commune
Metropolitan City of Bologna	Metropolitan City
Emilia-Romagna	Region
Italy	Country

Quartz occurrences in the area around Porretta Terme in the so-called Porretta Sandstone (Arenarie di Porretta) or «Macigno di Porretta», presently referred to as Arenarie di Suviana, belonging to the terrigenous Miocene turbidite formations of the Tuscan Domain.

The occurrences are very similar to those of the Nervia Valley (Val Nervia) in Liguria.

The Porretta sandstone is a dark grey-coloured, medium-grained lithic arkose, with white mica (muscovite) lamellae oriented parallel to the stratification planes. From a petrographic point of view, these rocks mainly consist of quartz, feldspars and phyllosilicate grains with a minor amount of siliciclastic rocks fragments. A certain amount of extrabasinal carbonatic clasts is also present. The carbonatic cement generally tends to occlude the interstices among grains.

The sandstone was extracted from different quarries located south and south-west of the Porretta Terme village (e.g. Madonna del Ponte, della Costa, Puzzola, and San Rocco quarries); some of these quarries are no more visible since they have been hidden by the new buildings of the thermal resort. This sandstone has been widely employed up to the last century in building works realized in the Porretta area such as the Madonna del Ponte church and along the Reno valley up to Bologna.

Veins and clefts in sandstone, up to some decimetres in width, are characterised by the presence of calcite and quartz crystals. Quartz crystals typically developed skeletal growth forms and show inclusions of light yellow-grey clay. Crystals frequently contain also water and gaseous hydrocarbon inclusions (Gambari, 1868; Bombicci, 1869 and 1874; Chamberlin, 1908; Mullis, 1988). These crystals have smooth and lustrous, in part curved faces and very often they form aggregates of two or more crystals. Several large crystal aggregates, one of which weighs more than 14 kg, were found to the south of Porretta Terme during the excavation of the Porrettana Railway tunnels (Bombicci, 1869).

Quartz crystals are found everywhere in rock clefts of the Arenarie di Suviana (Porretta Sandstone) formation, in the area located between Porretta Terme to the north and the border with Tuscany to the south, the church of Madonna del Ponte (right side of Reno River, municipal territory of Castel di Casio) to the east and Corno alle Scale (upper Silla stream valley, municipal territory of Lizzano in Belvedere) to the west. This area includes the top localities of Madonna del Ponte quarry (Castel di Casio municipality), Della Costa quarry (former Porretta Terme municipality, now Alto Reno Terme), Porettana Railway tunnels and cuts between Porretta Terme and Pracchia (Pistoia Province, Tuscany), Rio Maggiore (former Porretta Terme and Granaglione municipalities, now Alto Reno Terme), Granaglione and Casa Calistri (former Granaglione municipality, now Alto Reno Terme).

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References