Giovanni Gianelli

Geological model of a young volcano-plutonic system: The geothermal region of Monte Amiata (Tuscany, Italy)

Abstract Geological, geophysical and petrologic data point to the presence of a granitic body below the geothermal region of Mt Amiata (central Italy). A broad area of about 900–1300 km 2 centered on Amiata volcano shows a regional uplift of the Pliocene beds to 950 m a.s.l. The uplift began during the lower Pliocene, with a regression of the Pliocene sea from an uplifted area centered in the volcano zone. The temperature distribution below the Piancastagnaio field shows an updoming of the isotherms, forming a thermal high, probably present since the earliest stages of interaction between geothermal fluids and country rocks. A re-evaluation of the petrologic data from the xenoliths included in the lava flows allows an estimate of the P-T conditions of the magma body; a minimum temperature of 575°C and pressures of 1550–2200 bars can be estimated for the confining rocks around the magma body. Magmatologic data show a temperature of 800–900°C and a P load - 1000 bar. Therefore the roof of the magma body should be present at about 6 km depth. Seismic reflection data reveal the continuous and widespread occurrence of a reflecting horizon ( K ) of all over the geothermal region. This horizon is present at a depth of 5–6 km. By analogy with Larderello, it is interpreted as a fracture interval filled with hot fluids, contact metamorphic minerals and hydrothermal minerals generated in the uppermost part of the granite and the basal levels of the wall rocks. By integrating geophysical and geological data, a two-dimensional gravimetric model of the volcano-plutonic system of Mt Amiata is proposed, with the following features: roof depth = 5–6 km, T = 820°C, d (magma) = 2.15 g cm −3 , d (wall rock) = 2.8 g cm −3 , shape of the intrusion is lens shaped or mushroom-like with possible thickening and roots just below Piancastagnaio. This model fits well with gravimetric data, which show a negative anomaly in correspondence with the uplifted area.

Evidence for Li-rich brines and early magmatic fluid-rock interactionin the Larderello geothermal system☆

Abstract The geochemical features of fluids accompanying the first stages of geothermal activity linked to magmatic intrusions have been documented for the Larderello geothermal system (Italy). Deep drilling has provided samples which preserve evidence of this early geothermal activity. Four wells (San Pompeo 2, Monteverdi 7, Sasso 22, and Serrazzano, VC 11) penetrated the deeper parts of the Larderello system, located in a metamorphic basement underlying the Tertiary nappe complex which constitutes the main aquifer at Larderello. The drill holes terminated close to the inferred roof of a granitic complex thought to be responsible for geothermal activity. Fluid inclusion data were obtained from recrystallized quartz lenses and quartz veins in samples displaying high temperature assemblages (plagioclase-actinolite-biotite-tourmaline; clinopyroxene ± andradite-wollastonite) and also from magmatic quartz in a leucogranite dike. The inclusions are mainly secondary in origin, oriented in fluid inclusion planes (FIP) related to hydrothermal circulation in the Larderello system. Several generations of high temperature fluids were trapped and include: 1. (1) H2OCO2 dominated vapors displaying variable but significant contents of CH4 and N2; 2. (2) aqueous vapors containing LiCl, with variable salinity; 3. (3) aqueous LiCl brine, often oversaturated with respect to halite at room temperature; 4. (4) complex brine, always oversaturated at room temperature with respect to two (halite and sylvite) or more (n ≤ 4) salts. The presence of LiCl was confirmed by identification of the salt hydrate (LiCl5H2O) at very low temperature using Raman spectroscopy. Bulk salinities could be roughly estimated at around 30 wt% eq. LiCl for the LiCl brine. Geometric and chronologic relationships between FIP reveal close relationships and mutual contamination between the H2OCO2 vapors and LiCl brine, indicating synchronism in their trapping. These fluids were generated and trapped at pressures of 100–130 MPa, nearly 23 MPa above the estimated present-day lithostatic pressure. This implies a denudation rate between 0.2 and 0.5 mrn· a−1 since the onset of hydrothermal activity, compatible with the setting of Larderello in a young (Tortonian) collision belt. Fluid inclusion trapping temperatures (425–650°C) show a monotonous increase towards the inferred granite, and are around 100–200°C higher than the highest present-day temperatures. The results are interpreted as recording the interaction between magmatic and contact metamorphic fluids in the early Larderello system. The H2OCO2 vapors resulted from the reheating of the basement metamorphic series (often C-rich) under relatively high temperatures during contact metamorphism. Lirich fluids expelled from an underlying Li-rich leucogranite migrated through the metamorphic series and the already cooled granite dikes and experienced local boiling. The fluid inclusion data demonstrate the involvement of magmatic fluids during the initial development of this high energy geothermal field.

Water-rock interaction processes in the Aluto-Langano geothermal field (Ethiopia)

Abstract Aluto-Langano is a water-dominated geothermal field in the Main Ethiopian rift system. The hydrothermal mineral assemblage is characterized by the presence of illite-montmorillonite ( T T T b ) range from 240°C to 350°C. The calculated CO 2 concentration in the liquid phase ranges from 0.07 to 0.66 mol. Thermodynamic properties and phase relations of calc-silicates show that the presence or absence of these minerals is mainly controlled by the P CO2 in the system, even where temperature has played an important role. Fluid inclusion data from two deep wells (LA-3 and LA-6) indicate an increase in temperature and CO 2 content of the geothermal fluid after the trapping of the inclusions. The wells produce an alkali-chloride-bicarbonate water. Enthalpy-chloride and Na/1000-K/100-√Mg diagrams indicate that none of the discharge waters can be taken as representative of the deep fluid in full equilibrium with the reservoir rock, and that important mixing processes characterize even the hottest zone of the system. These mixing processes could be responsible for the continuous dynamic evolution of the hydrothermal system, as evidenced by the authigenic mineralogy.

Hydrothermal alteration in the Aluto-Langano geothermal field, Ethiopia

The hydrothermal mineral assemblages found in eight wells (with a depth range of 1320–2500 m) of the active geothermal field of Aluto-Langano (Ethiopia) indicate a complex evolution of water-rock interaction processes. The zone of upflow is characterized by high temperatures (up to 335°C) and the presence of a propylitic alteration (epidote, calcite, quartz and chlorite, as major phases) coexisting with calcite and clay minerals. The zone of lateral outflow is characterized by mixing of deep and shallow waters and the occurrence of a calcite-clay alteration that overprints a previous propylitic assemblage. Clay minerals have a mushroom-shaped zonal distribution consistent with the present thermal structure of the field. Microprobe analyses have been carried out on chlorite and illite in order to apply several geothermometers. Most of the chlorite is iron-rich chlorite. It is found that the temperatures calculated from the chlorite geothermometer (159–292°C) after Cathelineau and Nieva [Contrib. Mineral. Petrol. 91, 235–244 (1985)] are in good agreement with in-hole measured temperatures (155–300°C). In the upflow zone, temperatures calculated from this geothermometer (217–292°C), together with fluid inclusion data of Valori et al. [Eur. J. Mineral. 4, 907–919 (1992)], and computed saturation indices of alteration minerals, indicate thermal stability or slight heating. On the other hand, evidence of a significant cooling process (up to 171°C) in the outflow zone is provided by the comparison between fluid inclusion homogenization temperature (240–326°C) and in-hole temperature (155–250°C). The apparent salinities (0.8–2.3 wt% NaCl eq.) of the fluid inclusions are generally higher than the salinity of the present reservoir fluid (0.29–0.36 wt% NaCl eq.). Clay minerals (illite, smectite, Ill/S mixed layers, vermiculite and chloritic intergrades) generally occur at temperatures consistent with their stability fields.

Evolution of the latera geothermal system II: metamorphic, hydrothermal mineral assemblages and fluid chemistry

Abstract The Latera field (Vulsini volcanic complex, Latium, Italy) is one of the geothermal areas of the peri-Tyrrhenian belt along which a regional, high thermal anomaly has been detected. So far nine deep wells have been drilled within the Latera caldera and four of them have been productive. The geothermal reservoir is located within the fractured carbonatic rocks of the Tuscan nappe; the overlying volcanic units, sealed by hydrothermal minerals (mainly calcite and anhydrite), act as an impervious cover. The fluid produced by the wells comes from a deep aquifer (about 1000–1500 m depth) which at present is not connected with the shallow aquifer in the volcanoclastic units. Fluid temperatures range between 200 and 230°C; in-hole temperatures as high as 343°C at 2775 m depth have been measured in dry wells. The study of the newly formed mineral assemblages from both volcanic and sedimentary units as sampled from the geothermal wells can be used to reconstruct the thermal evolution of the geothermal field. The intrusion of a syenitic melt, up to a depth of about 2000 m, dated 0.86 Ma, represents the major thermal event for the units in the area and is assumed to represent the first step in the geothermal evolution of the Latera system. The above mentioned newly formed mineral assemblages can be divided into three groups: (a) “contact-metasomatic”: calcite, anhydrite, diopsidic pyroxene, grossularitic garnet, phlogopite, wollastonite or monticellite; (b) “high-temperature hydrothermal”: calcite, anhydrite, K-feldspar, vesuvianite, melanitic garnet, tourmaline, amphibole, epidote, sulphides; (c) “low-temperature hydrothermal”: calcite, anhydrite, K-feldspar, clay minerals, sulphides. Group (a) minerals are now relics. Part of (b) and all of (c) group are still in equilibrium with the existing conditions in different parts of the geothermal system. Thermodynamic calculations on the observed mineral assemblages permitted estimates of the P, T conditions and gas fugacities.

Water-rock interaction and hydrothermal mineral equilibria in the Tendaho geothermal system

Abstract The Tendaho geothermal system occurs within a NW–SE-trending rift basin filled with Quaternary volcanics (mainly basalts) and fluvio-lacustrine sediments. Three deep (TD-1, TD-2 and TD-3) and one shallow (TD-4) geothermal wells have been drilled. The waters of productive wells TD-1, TD-2 and TD-4 are typical Na–Cl geothermal waters with reservoir temperature ranging between 220 and 270°C. Chemical analysis of core samples (altered basalts) shows increased Ca, Fe, Mg, Al content (owing to the dissolution of plagioclase and femics and the precipitation of wairakite, laumontite, epidote, garnet, calcite and clay minerals) and decreased Na, K, Si and Ti (owing mostly to the dissolution of glass matrix). Petrographic study of cuttings and core samples shows: (1) evidence of an early stage of calcite, zeolite (wairakite or laumontite) and quartz crystallization, while calcite underwent different stages of dissolution/precipitation, possibly due to abrupt changes in pH and CO 2 partial pressure; (2) that epidote, garnet, prehnite, pyroxene and amphibole crystallization occurred after wairakite or laumontite. Chlorite is the main layered silicate in the basaltic rocks in wells TD-1 and TD-2, temperatures beyond the stability of smectite and chlorite/smectite (C/S) interlayers must have been reached. The smectites and C/S interlayers in association with chlorite in well TD-3 indicate that this well has had a more complex thermal history, with variable temperatures. The smectites occur at temperatures above 120°C, which is considered to be the threshold for the transformation of smectite to illite. No evidence of disequilibrium conditions of smectites has been found at reservoir temperatures currently present at Tendaho. Fluid inclusion data indicate heating in the well TD-1, and thermal conditions similar to the ones measured in the deepest part of well TD-2, while the uppermost part of this latter has undergone cooling. Intense cooling has affected well TD-3, drilled far from the upflow zone of the field, probably in an area characterized by self-sealing, cooling and very low permeability.

Geothermal exploration in the Republic of Djibouti : thermal and geological data of the Hanlé and Asal areas

Abstract During geothermal exploration in the Republic of Djibouti in the period 1987–1989, two deep wells were drilled in the Hanle area and four in the Asal area. The two wells at Hanle encountered low temperatures (maximum recorded temperature was 124°C at 2020 m depth). Volcanic rocks of Miocene age were found at the bottom of one Hanle well. Minor recent crustal stretching may account for the absence of any significant thermal anomaly. The four wells at Asal met high temperatures (up to 358°C at 2095 m depth), and high-enthalpy fluid was produced from two of them. Drilling data and new structural studies contribute to the development of a model for the Asal Rift, which is characterized by very steep faults with no evidence of flattening at depth and by the migration of the spreading axis from southwest to northeast. The authigenic mineral assemblage studied in all the wells is in good agreement with the measured temperatures in five of the six wells.

Water–rock interaction in the active geothermal system of Pantelleria, Italy

Abstract The geothermal fluid sampled in a deep (990 m b.s.l.) well drilled into the peralkaline volcano of Pantelleria consists for the most part of seawater. The fluid is depleted in Ca and Mg and enriched in K, Rb, Cs and F due to interaction with rocks of trachytic composition. A forward geochemical model has been used to simulate the water–rock interaction processes occurring within the geothermal system in the southern part of the island. The simulation assumes different mixtures of seawater with groundwater and volcanic gas. The system model that emerges from the simulation is the following: the seawater flows through fractures and faults, reaching temperatures of over 300°C in the southern part of the island where an upflow zone of volcanic gas is present. Volcanic gases increase the amount of C and S in the system, lower the pH, and enhance the dissolution of trachyte. After reaction with trachyte the pH increases and the fluid reaches saturation conditions with respect to albite, quartz, saponite, K-feldspar and muscovite, in agreement with the natural hydrothermal mineral assemblages. The model confirms that the reservoir fluid is a mix between seawater and meteoric water, more saline fluids possibly existing in deeper levels. The fluid chemistry and the hydrothermal minerals are similar to those found in the Icelandic geothermal systems of Reykjanes and Svartsengi.

Hydrothermal metamorphism in the Larderello Geothermal Field

Abstract The various tectonic units underlying the Larderello — Travale geothermal region have undergone hydrothermal metamorphism. The hydrothermal mineral assemblages are generally consistent with the temperatures now measured in the wells, leading to the hypothesis that solid phases deposited from a liquid medium during a hot-water stage that preceded the vapour-dominated one.

Geochemistry and stratigraphic correlations ― application to the investigation of geothermal and mineral resources of Tuscany, Italy (Contribution to the knowledge of the ore deposits of Tuscany. II)

Abstract Major-element geochemistry (364 analyses) is used to obtain quantitative information on the initial mineralogical composition and the depositional environment of Paleozoic, more or less metamorphic, rocks from Tuscany and Elba. Furthermore, this method can be used to improve stratigraphic correlations by comparing the chemical compositions of the metamorphic rocks and their possible non-metamorphic counterparts of known stratigraphic position. The stratigraphic units were divided into two sets. The first includes the units of known stratigraphic or tectonic position, taken as reference groups: Permian Red Porphyries, Carboniferous rocks, Buti Group l.s., Porphyritic Schists, Porphyroids, Lower Phyllites and metabasites from the Apuan Alps and Elba. The second set comprises the units of uncertain position: metapelites and metapsammites from outcrops, mines and boreholes from the Boccheggiano—Niccioleta area, Calamita Schists and metabasites associated with these two units, Micaschists, Gneisses and associated amphibolites. The main results are: 1. (1) The Porphyroids and Porphyritic Schists differ strongly from the Permian Red Porphyries, thus confirming that they belong to two different volcanic episodes. 2. (2) All the reference units consist of shales, sandstones s.s. or graywackes, differing by their degree of maturity (increasing from the Lower Phyllites to the Buti Group and to the Carboniferous formations). 3. (3) The Carboniferous rocks, the Buti Group and the Lower Phyllites have distinct chemical compositions. 4. (4) The Boccheggiano Formation l.s. and the Calamita Schists are similar and include rocks chemically equivalent to the Lower Phyllites, the Buti Group and the Carboniferous formations. 5. (5) The Micaschists and Gneisses derive from shales and graywackes respectively, and chemically recall the Lower Phyllites. 6. (6) The metabasites from the Apuan Alps and from Niccioleta are “within-plate basalts”, whereas the amphibolites interlayered within the Micaschists and Gneisses seem “ocean-floor basalts”.