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Exploring High Temperature reservoirs: new challenges for geothermal energy - Volterra, Italy, Workshop2
Exploring High Temperature reservoirs: new challenges for geothermal energy - Volterra, Italy, Workshop2
1-4 April 2007 Volterra, Tuscany, Italy
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Present-day and paleo-geothermal fields related to granites: origin and composition of related fluids
Paleo-hydrothermal systems related to granite 
intrusions and deep migmatisation which characterize 
the late stage of collision belt, the exhumation and the 
evolution of metamorphic core complex involve a series 
of fluid types including in particular pseudo-
metamorphic fluids deeply equilibrated with 
metamorphic rocks, and meteoric waters, sometimes 
having preserved pristine isotopic features, indicating 
short time penetration and insignificant interaction with 
rocks. The latter experienced quick heating and 
convection in the shallow crust, and are responsible 
subsequently for the enhanced cooling of the crust 
especially along discontinuities. These features display 
striking similarities with active geothermal systems such 
as the Larderello system where most fluid types, 
already identified in paleo-hydrothermal systems and 
their surroundings, are found and considered to have 
been migrated and partially mix within a short period of 
time and a few kilometres of the shallow crust. 

The initiation of the active metamorphism around the 
inferred deep seated intrusion (Larderello): lessons 
from paleofluids
Several zones can be distinguished in the Larderello 
geothermal field (southern Tuscany, Italy) based on the 
nature of the fluid, mineralogical assemblages and the 
fluid flow as a function of depth. The deepest levels of 
field are characterized by mineralogical assemblages 
produced by currently active thermal metamorphism 
that is thought to have started a few million years ago, 
e.g. during or later than the main intrusions that are 
dated from 1.6 to 3.8 Ma. At depths between 2 and 4 
km below ground level (b.g.l.), the rocks are 
characterized by the assemblage quartz-biotite-
A detailed study of fluid inclusions has been carried out 
in order to investigate the P-T conditions and the 
nature of the fluids circulating in this deep zone. A 
series of representative deep samples from 8 wells 
(Bruciano, Lumiera, Badia, Canneto, Sperimentale, 
Padule 2, Monteverdi 3 and 5) that contain the quartz-
biotite assemblage have been selected from the 
available cores, and compared with the results from 
previous studies (Monteverdi 7, San Pompeo 2, Sasso 
22, Serrazzano VC 11). The fluid inclusions were 
studied to enable a reconstruction of the fluid 
composition, determination of the CO2/CH4/H2O ratios 
and fO2  (Boiron et al. 2007).
The dominant fluids belong to the C-H-O-N system and 
occur as CO2-CH4-N2-H2O vapors with CO2 as the 
dominant gas. The CO2/CH4 ratio and the water 
content, which are not affected by immiscibility or mixing 
processes, are compatible with values expected from 
calculations of water-graphite equilibrium. A thermal 
metamorphic origin is suggested for the genesis of the 
aqueous-carbonic fluid, especially the volatiles (CO2-
CH4). Such fluids may be considered as the dominant 
type, compared to other fluid types in the deeper 
levels, especially within the K-horizon. The water 
component of the aqueous-carbonic vapours has a 
rather low salinity (< 5 wt.% eq. NaCl) and may 
correspond either to water produced by dehydration 
reactions or to waters that have extensively 
equilibrated with the metamorphic host-rocks. Two 
other fluid sources may also contribute to the variety of 
fluids found in the biotite zone: i) the deep Li-rich brines 
considered as having a magmatic origin (Cathelineau et 
al., 1994), ii) the brines resulting from water-halite 
interactions within the evaporites, which penetrated 
The carbonic vapours were generated and trapped 
under lithostatic conditions of 100 to 120 MPa and high 
temperatures between 600 and 420°C, depending on 
the depth and the distance to the granitic intrusives. 
These fluids may be considered as representative of 
the dominant fluid at depth within the pressurised 
zones that have been inferred by geophysical 
investigations. After cooling, and in many cases 
decompression, aqueous fluid inclusions were trapped 
at pressures fluctuating between lithostatic and 
hydrostatic. The estimated pressure corresponds to 
depths between 2 and 4 km, which is in agreement 
with the transition from lithostatic to hydrostatic 
conditions. The end of the proposed fluid path 
corresponds to an evolution towards the present-day 
temperature and hydrostatic pressure conditions (Valori 
et al., 1992, Ruggieri et al.,1999).

Metamorphic core complex and exhumation of collision 
belts:  initiation of convection cells, granite intrusions, 
and hydrothermal systems at the end of the Hercynian 
In the Limousin area of the French Massif central (FMC), 
several hydrothermal systems were active during the 
late Carboniferous and were responsible for rare metal 
(Sn, W, Ta) and precious metal (Au) deposition in 
various settings. The space and time relationships 
between granitic magmatism, thermal and tectonic 
events and fluid circulation at the crustal scale have 
been studied in detail, and show many similarities with 
present day active systems such as that of the 
Larderello area. A detailed geochemical study of fluids 
from representative quartz-sealed faults, especially 
hosting late Hercynian gold concentrations, shows that 
fluids percolating the mineralised faults had two main 
distinct reservoirs: one was a quite shallow and the 
other rather deep-seated. Both fluids have lost a great 
part of their original geochemical signature through 
interactions with host metamorphic formations. Early 
fluids, present during the primary sealing of the faults 
by quartz, are considered to have effectively 
equilibrated with the metamorphic pile and then 
predominantly flowed upwards along the faults. They 
are characterised by CH4/CO2/H2O ratios rather typical 
of fluids equilibrated with graphite, and moderate to 
medium chlorinities with a high Br/Cl ratio (Boiron et al., 
2003). The quartz-wolframite or quartz-cassiterite 
mineralized systems display similar metamorphic fluids.  
Gold appears however late (later micro-fractures and 
associated with Pb-Bi-Sb sulfosalts and sulphides) and 
related to fluids experiencing mixing (salinities 
decreasing to very low values indicating their 
progressive dilution by waters of more surficial origin in 
the fault system). Low salinity fluids are then 
extensively found in the granites, as shown by the 
systematic study of fluid inclusion planes in several 
zones of the St Sylvestre granite (El Jarray et al., 1993, 
Andre et al., 2000). Circulating at lower pressures, but 
still rather high temperatures, the episyenite fluids 
(responsible for the total dissolution of the granite 
quartz grains), and the Sn-W depositing fluids at Vaulry 
in the Li-rich Blond leucogranite (Vallance et al. 2001) 
are considered as linked to the last generation of 
granite intrusions (305-310 Ma) or to concealed 
intrusions. Renewal of hydrothermal activity resulted in 
the percolation of heated meteoric water (very low 
salinity fluids), at a depth of 3.5 km, the waters having 
the features of high relief (more than 6000 m altitude) 
fluids, on the basis of isotopic studies. This late 
evolution depicts the deep penetration of meteoric 
waters in the shallow crust and their convection around 
hot spots linked probably to concealed deep-seated 
intrusions (still not necessarily identified or dated at 
that period), a striking similarity with the Larderello 
geothermal system.

Acknowledgements: This synthetic overview was 
possible thanks to long lived collaboration with CNR-
Pisa (G. Ruggieri, G. Gianelli) for the Tuscan geothermal 
area, and collaborators from G2R (A-S Andre, O. 
Vanderhaeghe, J. Vallance), CRPG (C. Marignac) in 
Nancy, Geosciences Rennes (S. Fourcade) and 
University of Leeds (D. A. Banks).


M.C., CUNEY M., LEROY J.,(1999) Percolation de fluides 
tardi-hercyniens dans le granite de Saint Sylvestre (NW 
Massif Central Français) : données des inclusions 
fluides sur un profil Razès-St Pardoux. C.R. Acad. Sci. 
Paris, 329, 23-30.

S., VALLANCE J., (2003) Mixing of metamorphic and 
surficial fluids during the uplift of the Hercynian upper 
crust: consequences for gold deposition. Chemical 
Geology, 194, 119-141.

contact metamorphism and CO2-CH4 fluid production in 
the Larderello geothermal field (Italy): the fluid inclusion 
data. Chemical Geology, 237, 303-328.

G., PUXEDDU M., (1994) Evidence of Li-rich brines and 
early magmatic water-rock interaction in a geothermal 
field : The fluid inclusion data from the Larderello 
geothermal field. Geochimica Cosmochimica Acta, 58, 

Percolation microfissurale de vapeurs aqueuses dans le 
granite de Peny (Massif de Saint Sylvestre, Massif 
Central) : relation avec la dissolution du quartz. C.R. 
Acad. Sci., Paris, t.318, 1095-1102.

C., (1999) Boiling and fluid mixing in the chlorite zone of 
the Larderello geothermal system, Chemical Geology, 
154, 237-256

M.C., FOURCADE S., (2001) Microfracturing and fluid 
mixing in granites : W-Sn ore deposition at Vaulry (NW 
french Massif Central).Tectonophysiscs, 336, 43-61.

fluid migration in a deep part of the Larderello field : a 
fluid inclusion study of the granite sill from well 
Monteverdi 7.J. Volcano. Geotherm. Res., 51, 115-131
Id: 54
Place: Volterra, Tuscany, Italy
Campus SIAF, SP del Monte Volterrano
Localita' Il Cipresso
Volterra, Italy
Starting date:
02-Apr-2007   14:00
Duration: 30'
Primary Authors: Dr. CATHELINEAU, Michel (G2R° and CREGU, Nancy Université, CNRS, France)
Co-Authors: Dr. BOIRON, Marie-Christine (G2R° and CREGU, Nancy Université, CNRS, France)
Presenters: CATHELINEAU, Michel
Material: slides Slides

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