Giovanni Ruggieri

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.

Boiling and fluid mixing in the chlorite zone of the Larderello geothermal system

Abstract The geochemical features of the geothermal fluids produced within the boiling zone in the relatively shallow parts of the Larderello geothermal system (Italy) have been documented as a result of deep drilling which provided samples from 1480 to 2500 m depth. Four wells (Monteverdi 1, Monteverdi 2A, Sasso 22 and Capannoli 2B) have been sampled in the intermediate parts of the Larderello aquifer located in a metamorphic basement underlying the Tertiary nappe complex which constitutes the shallow aquifer at Larderello. Fluid inclusions from recrystallized quartz lenses and quartz veins in samples displaying a predominant quartz–chlorite–(epidote–adularia) paragenesis have been studied by microthermometry and Raman spectroscopy. The inclusions are primary and pseudo-secondary in origin when formed in authigenic quartz, or of secondary origin, when located in fluid inclusion planes related to microfracturing of metamorphic quartz lenses. Several generations of fluids are present and include: H 2 O–(CO 2 )-dominated vapours and liquids, and a series of aqueous liquids, for the most part of relatively low salinity. The T m -ice of both early and late inclusions are mostly between 0.0 and −4.5°C, indicating that the salinities of the hydrothermal fluid were very low to moderate. However, rare fluid inclusions with lower T m -ice (from −4.9 to −25.0°C) were also observed. These inclusions may record the occasional input of saline fluids, which may be derived from the interaction of the hydrothermal waters with the evaporites present in the shallow part of the Larderello field. At Capannoli 2B, earliest H 2 O–(CO 2 ) liquids were trapped under minimal pressures of 610–645 bars, which bracket the estimated present-day lithostatic pressure (640 bars). In all other samples, the main stage of quartz–chlorite crystallization occurs under boiling conditions attested by the presence of liquid and vapour-rich inclusions, that, in some instances, can be texturally interpreted to be coeval. Their trapping conditions (350–375°C, 160–215 bars) are higher than the present day temperatures at the same depth. Later fluid inclusions attest to a significant cooling of the fluids down to temperatures similar to the present-day temperatures. During this time, pressures were close to hydrostatic conditions. Most fluid inclusions were trapped within the liquid field, this indicating that a significant pressure drop has since affected the main aquifers or fractured zones which are, at present, under vaporstatic conditions.

Pressure fluctuation during uplift of the Northern Apennines (Italy): a fluid inclusions study

Abstract P–T conditions existing after the main syn-collision tectonic phase in the western part of the Northern Apenninic chain (Italy), e.g. in the Tuscan Nappe outcropping in the La Spezia area, were estimated on the basis of a detailed microstructural, structural, petrographic and fluid inclusion study of quartz of syn-tectonic (D2) veins developed in the Tertiary flysch at the top of the Tuscan Nappe (Macigno formation). Three main fluid events have been distinguished as follows. ⋅ During retrograde metamorphism (D1 to D2 phase), fluids in equilibrium with turbidites from the Tuscan Nappe were H2O–CH4 mixtures issued from water-organic matter interactions in temperatures conditions that may have reached at least 260 °C or more (280 °C) depending on the considered depth estimates and maximum pressures around 210–250 MPa. ⋅ Evidence of strong fluid pressure fluctuation between lithostatic and hydrostatic within the metamorphic formations (up to 100–150 MPa), possibly linked to fault-valve activity at the beginning of the uplift, triggered phase separation of the water–methane fluids and production of methane-rich and water-rich fluids; fluctuations in pressure during these events played a crucial role in quartz crystallization especially in extensional fissures formed perpendicular to the D2 folds axial foliation. ⋅ Changes in the fluid regime and sources with time are evidenced by the input of brines, which mix to distinct degrees and are trapped in healing microfissures during retrograde fluid evolution. Such mixing processes are an indication of the connection between separate fluid reservoirs with different temperature conditions. Consequently, mass and heat transfer have to be taken into account, as the downward percolation of cooler fluids probably accelerated the rate of cooling of the exhumed formations. These processes are certainly common to most orogenic terrains and can be quantitatively studied through P–V–T–X reconstruction using fluid inclusion data on the drainage zones where the mixing processes occurred. This work confirms that fluid inclusion studies can provide accurate quantitative estimates of fluid pressure variations during the evolution of orogenic terrains and could, therefore, constitute a useful tool in tectonic interpretations at the light of the models developed for pressure variations in the upper crust.

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.

Multi-stage fluid circulation in a hydraulic fracture breccia of the Larderello geothermal field (Italy)

Abstract The deep well MV5A, drilled in the western part of the Larderello geothermal field, crossed a 20-cm-thick hydraulic fracture breccia unit at a depth of 1090 m below ground level (b.g.l.). This breccia occurs in a fine-grained Triassic metasandstone and consists of angular to subangular clasts of up to some centimeters in size. Pervasive alteration has affected the breccia clasts and wall rock around the breccia, with the formation of Mg–Fe chlorite. After such alteration, hydrothermal circulation caused the precipitation of two generations of calcite cement. Then, ankerite partially replaced these two calcite generations. Ankerite also precipitated in late veinlets with chlorite. Late hydrothermal activity led to the crystallization of albite, quartz and finally, anhydrite. The calcite contains vapor-rich inclusions and two populations of liquid-rich (L1 and L2) inclusions. L1 inclusions are characterized by homogenization temperatures between 304 and 361°C and salinities from 7.4 to 11.6 wt.% NaCl equivalent; L2 inclusions revealed homogenization temperatures in the range of 189–245°C and salinities from 2.6 to 6.3 wt.% NaCl equivalent. The fluids contained in L2 inclusions were probably trapped coevally with some vapor-rich inclusions under boiling conditions after the L1 inclusions formed. Some of the abundant vapor-rich inclusions in calcite may also represent early, low-temperature inclusions affected by decrepitation and/or stretching and/or leaking during L1 trapping. The liquid-rich (L) inclusions trapped at later stages in ankerite, albite and anhydrite display, respectively, homogenization temperature ranges of 189–198°C, 132–145°C, and 139–171°C, and salinities ranging from 1.6 to 1.7 wt.% NaCl equivalent, 1.4 to 2.1 wt.% NaCl equivalent and 3.7 to 6.2 wt.% NaCl equivalent. The inclusions studied record the evolution, over time, of the fluids flowing in the breccia level: L1 inclusions capture high-temperature fluid (about 300 to 350°C) of high salinity (around 10 wt.% NaCl equivalent) at above-hydrostatic pressures (up to about 150 bar). The L2 inclusions in calcite and liquid-rich inclusions in ankerite and albite represent subsequent hydrothermal fluid evolution toward lower temperatures (about 250 to 130°C), pressures (45 to a few bar) and salinities (6.3 to 1.4 wt.% NaCl equivalent). During this stage, boiling processes and infiltration of meteoric waters probably occurred. Finally, moderately saline fluids (around 5 wt.% NaCl equivalent) at a temperature (about 160°C) close to that of present-day in-hole measurements was trapped in the anhydrite inclusions. The liquids trapped in liquid-rich inclusions circulated at 41,000 years (maximum age of calcite) or later. This age represents an upper limit for the development of vapor-dominated condition, in this part of the geothermal system. The fluids circulating at the breccia level were probably meteoric and/or connate waters. These fluids may have interacted with the anhydrite and carbonate bearing formations present in the Larderello area. The occurrence of the hot and saline fluids, trapped in L1 inclusions at above-hydrostatic pressure, suggests that similar fluids but with higher pressure (≥167 bar) and temperature (≥360°C) may have been responsible for rock fracturing.

Isotopic and fluid inclusion study of hydrothermal and metamorphic carbonates in the Larderello geothermal field and surrounding areas, Italy

Abstract Fluid inclusions have been studied on six calcite veins from the shallow part (480 to 1515 m below ground level) of the Larderello geothermal field and outcropping in peripheral zones of the geothermal area. Oxygen and carbon isotopic analyses have been carried out on these carbonate veins, as well as on the dolostone layers found inside the Paleozoic metamorphic units of the deep part of the field (from 1939 to 3177 m below ground level). Fluid inclusion observations suggest that boiling processes probably occurred during calcite precipitation in most of the veins. The fluids that formed or interacted with the calcite veins below the uppermost reservoir (made up of Mesozoic marine carbonates), and with the calcite hydrothermal veins of Sassa, were characterised by an apparent salinity from 1.3 to 5.3 wt.% NaCl eq. and a homogenisation temperature from 137 to 245°C. The fluid inclusions related to the calcite veins hosted above the uppermost reservoir show a wide range of apparent salinity (from 1.7 to 22.2 wt.% NaCl eq.) and homogenisation temperatures from 224 to 296°C. Apparent salinity/homogenisation temperature covariations of the latter veins are interpreted as being the result of a mixing process between a low-temperature, high-salinity fluid and a higher-temperature, moderate-salinity fluid. The oxygen isotopic compositions of the calcite veins ( δ 18 O from 10.34 to 11.45‰) located below the Mesozoic carbonates and in the outcrops ( δ 18 O from 9.42 to 17.07‰) indicate that the vapour in equilibrium with these veins was isotopically similar to the present-day discharge steam. The aqueous fluids in equilibrium with these veins could be meteoric water that interacted with the Mesozoic carbonates of the upper reservoir. The δ 13 C values of the CO 2 produced at Larderello and the constant concentration of this gas over time are, however, indicative of a deep source inside the reservoir that is probably related to the decarbonation reaction within the metamorphic units that form the present-day deep reservoir. Fluid inclusion salinities (up to 22.2 wt.% NaCl eq.) and isotopic results ( δ 18 O from 13.43 to 21.99‰, δ 13 C between −1.26 and −0.18‰) on the calcite veins hosted above the uppermost reservoir suggest that the water circulating in these veins has strongly interacted with Mesozoic carbonates or Neogene sediments containing evaporite layers. The isotopic values ( δ 18 O from 14.09 to 19.91‰, δ 13 C from −4.09 to 1.90‰) of dolomite samples present in the Paleozoic metamorphic rocks indicate a reaction with fluid of variable temperatures under different water/rock ratios. The isotopic composition of one sample reveals equilibrium with present-day discharge fluids. This fact aside, the remaining data indicate that the Paleozoic dolomitic layers do not seem to contribute significantly to the production of CO 2 .

Metallogeny, exploitation and environmental impact of the Mt. Amiata mercury ore district (Southern Tuscany, Italy)

The Mt. Amiata mining district (Southern Tuscany, Italy) is a world class Hg district, with a cumulate production of more than 100,000 tonnes of Hg, mostly occurring between 1870 and 1980. The Hg mineralization at Mt. Amiata is younger than 0.3 Ma, and is directly related to shallow hydrothermal systems similar to presentday geothermal fields of the region. There is likely a continuum of Hg deposition to present day, because Hg emission from geothermal power plants is on-going. In this sense, the Mt. Amiata deposits present some analogies with hot-spring type deposits of western USA, although an ore deposit model for the district has not been established. Specifically, the source of Hg remains highly speculative. The mineralizing hydrothermal fluids are of low temperature, and of essentially meteoric origin. Recent results by our research group indicate that, 30 years after mine closure, the environmental effects of Hg contamination related to mining are still recorded by the ecosystem, namely on waterways of the Paglia and Tiber River basins. In particular, the close spatial connection between the town of Abbadia San Salvatore, the Hg mine within its immediate neighborhood, and the drainage catchment of the Paglia River has an influence also on Hg speciation, transported mainly in the particulate form by the river system. The extent of Hg contamination has been identified at least 100 km from Abbadia San Salvatore along the Paglia-Tiber River system. Estimated annual Hg mass loads transported by the Paglia River to the Tiber River were about 11 kg yr-1. However, there is evidence that flood events may enhance Hg mobilization in the Paglia River basin, increasing Hg concentrations in stream sediment. The high methyl-Hg/Hg ratio in water in this area is an additional factor of great concern due to the potential harmful effects on human and wildlife health. Results of our studies indicate that the Mt. Amiata region is at present a source of Hg of remarkable environmental concern at the local, regional (Tiber River), and Mediterranean scales. Ongoing studies are aimed to a more detailed quantification of the Hg mass load input to the Mediterranean Sea, and to unravel the processes concerning Hg transport and fluid dynamics.

An overview on the characteristics of geothermal carbonate reservoirs in southern Tuscany

This paper focuses on brittle deformation and fluid-rock interaction, for enhancing permeability in carbonate geothermal reservoir. The relationships between fractures and fluid flow at different structural levels within a geothermal circuit are described through examples from exhumed geothermal systems cropping out in southern Tuscany, with emphasis on the carbonate reservoirs, located within the late Triassic evaporite level and/or at the base of the Tuscan Nappe. The description is based on the fact that geothermal fluids are mainly made up of meteoric water channelled to depth through structural conduits, affecting regionally hot rocks. In this pathway, the meteoric water is transformed in geothermal fluid, becoming chemically aggressive, thus favouring leaching of hosting rocks, and enhancing and maintaining permeability. The fluid-rock interaction is promoted by existing fractures and/or by unhomogeneities in the rock-textures, as it is the case of the Miocene cataclasite located within the late Triassic evaporite. Travertine deposits can occur if fluids reach the surface after having circulated in carbonate reservoirs. Since permeability is controlled by fluid-rock geochemistry and by the possibility to have fluids continuously renewed, we conclude that the fluid-rock interaction and high temperature of hosting rocks make the geothermal issue a specific case of study and therefore the conclusion on oil reservoirs formation cannot be completely transferred to geothermal exploitation issue.