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 620C. 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.
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