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Defining, exploring, imaging and assessing reservoirs for potential heat exchange - Potsdam, Germany, Workshop1
Defining, exploring, imaging and assessing reservoirs for potential heat exchange - Potsdam, Germany, Workshop1
6-8 November 2006 GeoForschungsZentrum Potsdam (GFZ)
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Fluid evolution and water-rock interaction in the Larderello geothermal field: evidence from mineralogical, petrographic and fluid inclusions studies
Hydrothermal and contact metamorphic minerals, filling veins or replacing previous 
phases, are often found in core-samples and cuttings from geothermal wells. These 
minerals result from fluid-rock interaction processes. The past fluid circulation 
in geothermal systems are also recorded by fluid inclusions trapped in minerals. 
Mineralogical, petrographic and fluid inclusions studies, therefore, can provide 
useful data on the physical-chemical evolution of the fluid migrating in geothermal 
systems. A good example of the information which can be obtained using these 
studies is represented by the reconstruction of the fluid history in the Larderello 
geothermal field. The Larderello geothermal field is a long-living systems 
characterized by different phases of evolution and a complex hydrothermal activity. 
The examination of cuttings and core-samples showed that contact-metamorphic rocks, 
developed during the early stage of the fluid circulation and related with the 
intrusion of the granites of 3.8–1.0 Ma age, occur in the deep part (2.5–4.5 km) of 
the field. Whereas, a more recent hydrothermal activity was responsible for both 
the widespread propylitic and sericitic alterations found in veins at different 
depths and for the replacement of the deep contact-metamorphic mineral assemblage 
and the granite alteration. The contact-metamorphic assemblage consist of post-
tectonic biotite, cordierite, andalusite, tourmaline, plagioclase, corundum, K-
feldspar etc.. These minerals commonly crystallize in Paleozoic metapelites and 
gneisses. Whereas, amphibolite and carbonatic rocks are transformed in mafic 
hornfelses (consisting of hornblende, plagioclase, biotite, quartz and Fe-Ti-
oxides) and carbonatic hornfelses (consisting of dolomite, calcite, phlogopite, 
wollastonite, diopside, andraditic garnet or olivine), respectively. In some 
places, contact metamorphic rocks are affected by a retrograde metamorphism before 
the hydrothermal alteration. Epidote, serpentine minerals, actinolite and a Mg-Fe 
chlorite are formed during these metamorphic phase. Fluid inclusion data indicate 
that the fluids present during these stage were Li-Na-rich high-salinity fluids, 
and aqueous-carbonic fluids with varying proportions of H2O and CO2 that formed 
during the contact metamorphism. These fluids were trapped at 425-690 °C, under a 
lithostatic pressure regime of 90-130 MPa or between lithostatic and hydrostatic 
conditions. These pressure conditions are in agreement with the present-day depths 
of the contact metamorphic minerals assuming an uplift rate of 0.2 mm/years in the 
last 4 Ma. The Na-Li-rich fluids were probably exsolved from granites, whereas 
aqueous-carbonic fluids are interpreted as metamorphic fluids produced by heating 
(de-hydration reactions) of Paleozoic rocks during contact metamorphism. The 
carbonic phase of aqueous-carbonic fluids may have originated from high-temperature 
graphite–water interaction in the metamorphic basement (often C-rich), and/or from 
decarbonation reactions. Fluid inclusion temperatures agree with the temperature 
estimated by contact metamorphic minerals, for example corundum in equilibrium 
texture with K-feldspar, observed in some core-samples, indicates temperature of 
about 620C. The hydrothermal stage at Larderello produced in the reservoir rocks 
basically two types of late hydrothermal alteration: a propylitic-type alteration, 
characterised by epidote, chlorite, quartz, calcite, K-feldspar, titanite, 
actinolite, anhydrite, albite and pyrite in variable proportions; and sericitic-
type, with K-mica, chlorite and quartz in different proportions. The propylitic-
type alteration is probably related to the circulation of nearly neutral-pH 
solution, whereas the sericitic-type alteration, which has been found in cordierite-
bearing contact metamorphic rocks and granite, formed from a solution with a pH 
below neutrality. The hydrothermal phases occur as fracture filling, hydraulic 
breccias cement and, more commonly, as filling of secondary porosity originated by 
the dissolution of previous minerals. Fluid inclusion studies indicate that fluids 
with different compositions were present during this hydrothermal activity: aqueous 
liquids with low-to-moderate salinity of meteoric-derivation, relatively high-
salinity waters formed during boiling processes or as consequence of evaporite-
fluid interaction, low-density vapours derived from boiling, and nearly pure H2O 
resulting from condensation of vapours. All these fluids were trapped at 
temperatures varying from 150 to 400 °C, under hydrostatic pressures (<35 MPa). The 
final evolution of the hydrothermal system resulted in the development of the 
present-day vapour-dominated conditions. Present-day temperatures of about 200–400 °
C are consistent with the stability of the hydrothermal minerals.
Id: 16
Place: GeoForschungsZentrum Potsdam (GFZ)
14473 Potsdam, Telegrafenberg
Starting date:
06-Nov-2006   15:10
Duration: 03'
Contribution type: Poster
Primary Authors: Dr. RUGGIERI, Giovanni (CNR-Istituto di Geoscienze e Georisorse)
Co-Authors: Dr. GIOVANNI, Gianelli (CNR-Istituto di Geoscienze e Georisorse)
Dr. GIOLITO, Chiara (CNR-Istituto di Geoscienze e Georisorse)
Presenters: Dr. RUGGIERI, Giovanni
Material: slides Slides
poster Poster

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