G. Ruggieri1, M. Cathelineau2, M.C. Boiron2, Ch. Marignac3, G. Gianelli1
During the main hydrothermal activity of the field, some specific zones were dominated by one mineral assemblage which has been stable for a relatively long duration: the biotite (tourmaline, Ca plagioclase, pyrite) zone at great depth, and the chlorite zone at a shallower level. The chlorite-quartz -adularia (+/- epidote, +/- carbonate) association is the dominant mineral assemblage in the main reservoir at present. Quartz-chlorite veins are relatively common at shallow to intermediate depth (1000 to 2500 m b.g.l.). The occurrence of authigenic chlorite should reflect in part the recent evolution of the field and can be generally considered in equilibrium with the present day conditions. Studies have been carried out on core samples from four wells: Capannoli 2B (C2B, at 2585 m b.g.l.), Monteverdi 1 (MV1, at 2227 m b.g.l.) and Monteverdi 2A (MV2A 1871 m b. g. l.) and Sasso 22 ( 1480 and 1600 m b.g.l.). The physico-chemical conditions have been investigated on the basis of microthermometric and Raman spectrocopy data, and chlorite crystal-chemistry.
Microthermometric and Raman data
Two main types of fluid inclusions are observed: 1) two-phase (liquid-rich) inclusions, 2) two-phase (vapour-rich) or apparently one-phase (vapour) inclusions. Microthermometric and Raman analyses demonstrated that the compositions of both liquid-rich (Lw) and vapour-rich (Vw) inclusions are water rich but may contain in some instances a low density CO2 component (Lw(c) and Vw(c) inclusions). A large number of inclusions in the MV2A sample are primary as they occur along growth zone of quartz crystals, and in some cases, Lw and Vw inclusions appear to be trapped simultaneously.
- C2B: Lw inclusions (Te from -40 to -30oC) of this sample show final ice melting temperature (Tm ice) between -10.7 and -0.1oC which distribute among two groups showing: I) a Tm ice values from -0.1 to -4.0oC (mode at -1.6oC) , and II) a Tm ice ranging from -10.7 to -10.5oC. Homogenization temperature (Th) of Lw inclusions range from 328 to 367oC (mode at 344oC). Most of Lw inclusions show homogenization in the liquid phase while some inclusions exhibit a critical homogenization at 365 to 367oC. Th for Vw inclusions were observed between 309 and 367oC (mode at 345oC).
Few inclusions of group I (Lw(c)) also show the presence of clathrate during freezing runs. Clathrate melting temperature (Td cl) ranges from 5.4 to 5.8oC. Lw(c) inclusions display a volatile phase (around 4 mol. % in the global composition) containing 92.5 moles % of CO2, 4.7 moles % of CH4 and 2.8 moles % of N2. Raman analyses also demonstrated that some Lw inclusions which do not show a clathrate melting, contain also a low density volatile phase dominated by CO2, and CH4 (up to 6.7 moles % in the volatile phase).
- MV1: Lw inclusions ((Te around -45oC) show Tm ice ranging between -1.0 and -6.1oC (mode at -2.0oC). Some Vw inclusion show a Tm ice at 0.0oC indicating that the fluid is almost pure water. Lw inclusions homogenize to the liquid phase between 309 and 383oC, and distribute among two groups of inclusions: 352 to 383oC (mode at 368oC), and 309 to 320oC (mode at 315oC). The Th of Vw inclusions range between 326 and 389oC (mode at 351oC).
Other Vw(c) inclusions contain CO2 ( Th clathrate range: 0.9 to 3.9oC). Raman analyses show that the composition of the gas phase ranges between 88.9 and 94.4 moles % of CO2, 5.2 and 5.6 moles % of CH4, 0.0 and 5.9 moles % of N2. Volatiles represent around 8 to 9 moles % of the global composition. Traces of CO2 were also found in the vapour phase of a few Lw inclusions.
- MV2A: Lw(c) inclusions (NaCl dominated fluid) show a Tm ice in the temperature range -0.3 to -4.5oC (mode at -1.8oC) and Th from 254 to 343oC, with a mode at 302oC. Volatile phase composition obtained by Raman on Lw(c) inclusions is: 88.4/91.9 moles % of CO2, 8.1/11.56 moles % of CH4.
-Sasso 22: Lw inclusions are dominant and show Tm ice in the range of 0.0 to -4.9oC at 1480 m depth, and 0.0 to -0.5
P-T-X conditions of chlorite formation
Lw inclusions found in the quartz hosting chlorite are genetically reated to the formation of these minerals. The presence of coexisting liquid-rich (Lw) and vapour-rich (Vw) inclusions attests that boiling occurred in some instances during quartz-chlorite crystallisation. Lw and Vw inclusions do not systematically exhibit the same Th. However, at C2B, Vw inclusions with irregular shape show high Th (357/361oC) are associated to Lw inclusions with similar Th range (353/367oC).
Composition of the fluid: most samples show Lw inclusions with rather similar Te and Tm ice; this suggests that the composition of the geothermal fluids during quartz-chlorite crystallization was relatively homogeneous. As indicated by the Te values, the fluids contain divalent cation (Ca2+ and/or Mg2+) in addition to Na+ (possibly K+) and Cl-. The presence of Ca2+ could result from the interaction of the geothermal fluids with evaporitic layers containing anhydrite, dolomite and calcite (for example the Triassic formation of "Anidriti di Burano"). Maximum salinities calculated from Tm ice are comprised in the 0.2/9.3 wt% NaCl eq. range. This quite wide range of Tm ice may reflect two processes: 1) a salinity increase due to boiling; 2) a dilution of the fluids with nearly pure waters issued from the condensation of the aqueous vapours. The high salinity inclusions (C2B sample: 14.5 and 14.7 wt% NaCl eq.; MV1 sample: 8.1 and 9.3 wt% NaCl equiv.) can be interpreted as the results of sporadic inputs of saline fluids which may have strongly interacted with evaporitic layers. This type of fluid at Larderello were also reported by previous fluid inclusion studies (Cathelineau et al., 1989; Marignac et al., 1986; Valori et al., 1992).
Microthermometric and Raman analyses showed that some Vw and Lw inclusions contain moderate amounts of non condensable gases (mostly CO2 with small amounts of N2 and CH4). Lw fluids contain up to 3.5 moles % of CO2 while Vw inclusions trapped up to 9 moles % of CO2. They may represent the original geothermal fluid before boiling. During boiling the volatiles phases (CO2, CH4, N2) were separated from the original liquid and move to the vapour phases.
In the C2B sample Th of Vw inclusions showing irregular shape (357/361oC) are similar to the Th (353/367oC) of most of the associated Lw inclusions. In this case the Th of Vw and Lw inclusions correspond to the true temperature of the fluid. Using the curve of Haas (1971) neglecting the CO2 effect, the fluid pressure is estimated to be around 185 bar. The occurrence of Vw inclusions testifies boiling processes also during the formation of quartz-chlorite veins in MV1 and MV2A samples. For MV1 sample the Th (366/377oC) of Lw inclusions coeval with Vw inclusions can be confidently considered as the temperature of the boiling fluid. If we discard the effect necking-down and heterogeneous trapping, the Th ranges in the studied samples can be explained in two ways: the inclusions with lower Th could be formed actually at lower temperature than those trapped at boiling and/or they were trapped at higher pressure. Present-day temperature in the three wells at the depth of the sampling (341oC for C2B, 293oC for MV1 and 268oC for MV2A) are lower (C2B well) or distinctively lower (MV1 and MV2A wells) than the temperature estimated for the boiling fluids (353/367oC for C2B, 366-377oC for MV1 and 307-341oC for MV2A), clearly indicating that cooling could be a realistic process.
Fluids in the shallow part of the Larderello geothermal field have a low to moderate salinity (<10 wt% NaCl equiv.) suggesting a prevalently meteoric origin. Some of the inclusions trapped small amounts of CO2 (+/- CH4 and N2). More saline fluids were also found, and document more occasional fluid flows. A characteristics of all the samples is the evidence that boiling occurred sometimes during quartz-chlorite crystallization. Temperature and pressure of the vein formation were estimated to be in the 325 - 370oC range. Pressure have probably changed with time, and pressure drop may have promoted boiling process (case of C2B and MV1 samples). The comparison between present-day estimated temperature and data from inclusions indicate that a cooling process occurred at Monteverdi and Sasso 22 at the sampling depth. In the Capannoli 2B well in-hole temperature is comparable with the formation temperature indicating a long-lived thermal regime from the time of fluid inclusion trapping.
This work has been supported by the programme JOULE II (No JOU-CT93-0318).