Cesano 1 well, Cesano geothermal field, Metropolitan City of Rome Capital, Lazio, Italyi

Regional Level Types
Cesano 1 well	Borehole
Cesano geothermal field	- not defined -
Metropolitan City of Rome Capital	Metropolitan City
Lazio	Administrative Region
Italy	- not defined -

Cesano 1 geothermal well, drilled in 1975 by the AGIP-ENEL joint venture (ENEL, the National Electrical Energy Agency, as operator) to a depth of 1435 m, is located 2.3 km north of the village of Cesano, right on the edge of the Baccano caldera, east of Bracciano Lake.

A volcanic sequence (mainly phreatomagmatic pyroclastics with few interbedded lava flows) extends to 1070 m, and is followed by a flyschoid allochtonous complex (shales, marls, sandstones, and limestones; Middle Cretaceous to Miocene in age) down to 1390 m. Marls and marly limestones with cherts and flints, the so-called 'Carbonatic Basement', conclude the drilled sequence.

Hydrothermal minerals have been found in all the horizons: the most frequent are sulphates (such as gypsum, anhydrite, aphthitalite, görgeyite, glauberite, etc.) and calcite; occasionally ankerite, dolomite, K-feldspar, magnesian calcite, sulphides, and zeolites have been detected.

The volcanic sequence includes two formations. The first (0 to 350 m) consists essentially of pyroclastic deposits (tuffs, scoriae, lapilli, etc.), In the upper part of the formation, down to 130 m, lacustrine layers were found, intercalated with pyroclastics, while in the lower part (130 to 350 m), the formation is constituted by pyroclastics only. The second volcanic formation is represented by a polygenic explosion breccia, consisting of various volcanic materials mixed with sharp-edged fragments of sedimentary rocks. The latter are generally carbonate fragments turned up from the wall rocks. The volcanic elements, instead, are probably attributable to leucitic lavas; but their precise determination is prevented by the deep hydrothermal alteration of the rock, whose products (especially chlorite, epidote, sericite, chalcedony, calcite, and pyrite) form a sort of groundmass wrapping up the carbonate fragments. This type of alteration is properly defined by the term propylitisation. The hydrothermal process occurred along the walls of a diatreme, as indicated by the predominance in the breccia of carbonate components. There are only a few vertical variations along the diatreme breccia, represented by local variations in granulometry, in abundance of carbonate fragments and sometimes in concentrations of chalcedony. Zeolite mineralisations appear inside the volcanics, between 400 and 900 m. Generally the zeolites are present in geodes, cavities or have an interstitial emplacement.

Sulfate mineralisations are found in the more surficial parts of the well down to a depth of 1400 m and are thus far from evaporitic levels; they are prevalently made up of gypsum in the most shallow parts of the well (first hundreds of metres) and anhydrite in the deeper levels. They occur in a network of fractures, suggesting the existence of a deep self-sealing process in the sedimentary units. In the Cesano 1 well, the pyroclastic unit contains veins and veinlets of alkaline sulphates (görgeyite, aphthitalite, etc.), indicating a circulation of sulphate-rich fluids subsequent to the last tensional phase of Baccano tectonics. These veins, coated with calcite crystals, have a width which in some cases may exceed 1 cm (especially in the more superficial levels) and can be found even at considerable depth, where calcite generally prevails. The common characteristic is that they always cut any preexisting deposit. Mineralisations with K-feldspar (adularia), associated with sulfides and carbonates (ankerite, dolomite, and calcite), are found at the transition between the impermeable flysch and the marls or marly limestones of the 'Scaglia' formation. These mineralisations are in turn cut by even younger carbonate and sulfate veins.

Cesano No. 1 well is the type locality for cesanite, found in a vein occurring within the marls of the allochthonous complex. The vein, which yielded cesanite, görgeyite, and pyrite, is at about 45° from the vertical defined by the walls of the core; the stabilised temperature value at this depth is 190°C. The vein is white, with sugary texture and grain size ranging from 0.2 to 1.5 mm. Individual crystals, mostly subhedral and loosely packed, in general do not show preferred orientation but, rather, a kind of decussate texture with rational grain boundaries. Occasionally small rugs have been observed, where individual crystals show development of the upper terminal faces. Some rugs are coated by a thin crust of brownish-grey fine-grained gypsum. Close to the walls of the vein, cesanite crystals are smaller, and are intergrown with görgeyite.

Also at Cesano No. 1 well, similarly to the Russian type locality, kalistrontite has been discovered in a drill core: the mineral, which appears as colourless crystals with perfect cleavage, is here associated with calcite, anhydrite, and celestine inside a calcareous rock.

Other Regions, Features and Areas containing this locality

References