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-
tourmaline.
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
downwards.
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
orogeny
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).
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