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Cesano geothermal field, Bracciano Lake, Rome Province, Latium, Italy

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Location is approximate, estimate based on other nearby localities.
Latitude & Longitude (WGS84): 42° North , 12° East (est.)
Margin of Error:~3km
Name(s) in local language(s):Campo geotermico di Cesano, Lago di Bracciano, Roma, Lazio, Italia

The Cesano geothermal field is located in the eastern sector of the Sabatini Mts., north of Rome and est of Bracciano Lake. As elsewhere in the Sabatini Mts. the geothermal reservoir is composed of the Mesozoic formations (mainly carbonate) of the Umbria-Marche series.

In the period 1975-1983, a prospecting programme undertaken by ENEL (the National Electrical Energy Agency) in search of high enthalpy geothermal fluids brought to the drilling of twelve wells, reaching depths of 960 to 3200 m, in the area to the north of Cesano, inside and outside the Baccano caldera, namely:
• Cesano 1 - year: 1975; depth: 1435 m; coordinates: 42°06’00”N, 12°21’18”E
• Cesano 2 - year: 1975; depth: 2097 m; coordinates: 42°06’32”N, 12°21’00”E
• Cesano 3 - year: 1976; depth: 2061 m; coordinates: 42°05’48”N, 12°21’11”E
• Cesano 4 - year: 1976; depth: 3128 m; coordinates: 42°04’25”N, 12°21’33”E
• RC 1 (RC 001) - year: 1977; depth: 3048 m; coordinates: 42°06’39”N, 12°22’42”E
• Cesano 5 - year: 1978; depth: 1851 m; coordinates: 42°04’25”N, 12°21’22”E
• Cesano 6 - year: 1978; depth: 3219 m; coordinates: 42°07’47”N, 12°21’59”E
• Cesano 7 - year: 1980; depth: 2035 m; coordinates: 42°08’17”N, 12°19’31”E
• Cesano 8 - year: 1980; depth: 960 m; coordinates: 42°07’00”N, 12°21’08”E
• Cesano 13 - year: 1982; depth: 2054 m; coordinates: 42°07’57”N, 12°20’42”E
• Cesano 14 - year: 1982; depth: 2673 m; coordinates: 42°07’59”N, 12°19’48”E
• Cesano 18 - year: 1983; depth: 3002 m; coordinates: 42°04’24”N, 12°19’28”E

Two wells, situated in the Baccano caldera, encounterd brine reservoirs with salinity over 350 g/l rich in sodium and potassium chlorides and sulfates slightly overpressurised (150 bar); the temperatures are 210°C and 150°C respectively. Other two wells, external to the caldera, crossed fluids with different conditions, respectively, temperature of 145 and 220°C and salinity of 60-70 g/l. Because the liquid phase is extremely saline and the gas phase high in H2S, the wells are not economically exploitable for the generation of electric power as demonstrated in the early '80s by experimentation with a helical screw expander pilot plant. Exploitation of the dissolved salts in the fluids is instead possible.

The above area underwent an intense sequence of tensional tectonic events which produced a large sedimentary basin of Upper Miocene to Pleistocene age. During the Pleistocene and Holocene an intense, widespread sequence of volcanic events took place, burying the older structures to depths of as much as 1 km over an area of 5000 km 2. Volcanic activity in the Sabatini Mts. system has always been of a fissural nature, with no volcanic centres. In correspondence with volcanic chimneys, local residual high permeability reservoirs are present; self-sealing phenomena, subsequent to the hydromagmatic activity, and hot hydrothermal circulations, occurred for a long time in the area, closing in all the directions these residual permeable zones and increasing the salinity of the fluids. Fluid circulation, which is mainly extinct, seems to belong to a closed system. Two types of circulation have been recognised: the first of these is connected to the stratigraphic discontinuities (especially at the contact between the allochthonous flysch complex and the Meso-Cenozoic basal carbonate sequence) and is mainly characterised by K feldspar and sulfide-sulfate mineralisations; the second circulation type is tied to the fracture systems and is particularly observable in the pyroclastic cover where it is represented in the form of sulphate- and carbonate-rich dykes. This system seems to have undergone a mixing with surface waters.

The drilled wells first penetrated volcanic sequences, mainly made up of pyroclastic products of the most recent activity (Quaternary tuff and breccia of phreatomagmatic activity; Quaternary lava interbedded with tephra); at greater depths they penetrated sedimentary sequences (Upper Miocene - Quaternary clay, sand and conglomerates of post-orogenic sequence; Lower Cretaceous - Lower Miocene allochthonous flysch complex with shale, marl, sandstone and limestone; Middle Cretaceous - Oligocene marl and marly limestone with chert; Middle Lias - Middle Cretaceous limestone and cherty limestone; Lower Lias massive and grey limestone; Hettangian - Rhaetian dolomitic limestone; Hettangian - Rhaetian limestone with anhydrite; traces of main overthrusts), revealing present day fluids or evidence of their past circulation, indicating temperatures of more than 190°C. The stratigraphic and mineralogical analyses revealed important differences in the Trias sedimentary rocks, which are mainly carbonatic in the western sector of the area and anhydritic in the eastern sector: the two facies at present seem to be tectonically brought closer to one another.

According to Funiciello et al. (1979), who studied the mineralogy of wells Cesano 1 - Cesano 6 and RC 1, the following considerations can be drawn from analytic data:
- in well Cesano 1 zeolite mineralisations appear inside the volcanics, between 400 and 900 m, which encountered an active circulation of hot temperature fluids. Generally the zeolites are present in geodes, cavities or have an interstitial emplacement;
- sulfate mineralisations appear in all the drilled wells. These sulphates are generally found in the more surficial parts of the wells, 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 wells (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 most of the wells. In well Cesano 1 the pyroclastic unit contains veins and veinlets of alkaline sulfates (görgeyite, aphthitalite, glazerite), indicating a circulation of sulfate-rich fluids subsequent to the last tensional phase of Baccano tectonics. These veins are coated with calcite crystals, have a width which in some cases may exceed 1 cm (especially in the more superficial levels of Cesano 1) and can be found even at considerable depth, where calcite generally prevails. The common characteristic is that they always cut any preexisting deposit. In any case, they represent the last circulation in this unit;
- mineralizations with K feldspar (adularia), associated in many cases 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' in wells Cesano 1, Cesano 2, and Cesano 3, as well as near pyroclastic necks (Cesano 3). These mineralisations are in turn cut by even younger carbonate and sulfate veins. In some instances, near clayey and carbonate alterations of the Mesozoic units of the 'Scaglia', K feldspar-enriched levels can alternate with levels totally free of K feldspar. This is the effect of the in situ recrystallisation of clayey minerals and of their probable removal by an increase in the temperature and pressure of the system. Similar K feldspar concentrations were also noted inside the carbonate ejecta contained among the phreatomagmatic products, especially in the southernmost parts of the caldera. Based on these observations, it can be deduced that the genesis of this mineral is linked to thermal events which certainly precede fluid circulation: various processes, e.g. degassing and cooling, led to deposit of the solid phase causing self-sealing phenomena within the country rocks, especially in the central part of the Baccano caldera;
- epidote mineralisations are present only in the pyroclastites of well Cesano 1. Other rare euhaedral crystals of epidote are present in the deeper parts of well Cesano 4, apparently on the walls of the small marcasite veins found at the transition between carbonate units of the Lower Lias and dolomitic units of the Rhaetian.
- sulphide mineralisations were recognized in many wells. Those of Cesano 1 and, above all, Cesano 5 are particularly significant. In subsequent wells, the presence of pyrite together with K feldspar is the only evidence of a recently interrupted hydrothermal circulation. The widespread occurrence of realgar at depths below 1500 m in all the wells showing some evidence of circulation is also significant. In the deeper part of well RC 1, the well most enriched in H2S, sulfides and sulfosalts (chalcocite, boulangerite) were also observed as isolated specimens;
- feldspathoid mineralisations are present only in well Cesano 4 at a depth of approximately 2900 m, at the passage between the Lower Lias and the Rhaetian. Here sodalite and nosean were found associated with leucite, K feldspar, haüyne and phlogopite, related to a dyke emplacement. These types of association are in agreement with participation of various salts from the evaporitic levels in the mineralised system;
- phlogopite and pyrite mineralisations were recognised only in the deeper parts of well Cesano 4 (in the Upper Trias dolomites and dolomitized limestones) at the margins of a dyke of trachytic composition.

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27 valid minerals. 1 (TL) - type locality of valid minerals.

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Baldi, P., Cameli, G.M., Locardi, E., Moutón, J., Scandellari, F. (1976): Geology and geophysics of the Cesano geothermal field. Proceedings, Second United Nations Symposium on the Development and Use of Geothermal Resources, San Francisco, California, USA, 20-29 May 1975, Vol. 1, 871-881.

Calamai, A., Cataldi, R., Dall’Aglio, M., Ferrara, G.C. (1976): Preliminary report on the Cesano hot brine deposit (northern Latium, Italy). Proceedings, Second United Nations Symposium on the Development and Use of Geothermal Resources, San Francisco, California, USA, 20-29 May 1975, Vol. 1, 305-313.

Funiciello, R., Mariotti, G., Parotto, M. , Preite-Martinez, M., Tecce, F., Toneatti, R., Turi, B. (1979): Geology, mineralogy and stable isotope geochemistry of the Cesano geothermal field (Sabatini Mts. volcanic system, northern Latium, Italy). Geothermics, 8, 1, 55-73.

Buonasorte, G., Cameli, G.M., Fiordelisi, A, Parotto, M., Perticone, I. (1995): Results of geothermal exploration in central Italy (Latium-Campania). Proceedings of the World Geothermal Congress, Florence, Italy, 18–31 May 1995, Vol. 2, 1293–1298.

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