fluid inclusions in minerals from the geothermal fields ... - Science Direct

0375 - 6505/85 $3.00 + 0.00 Pergamon Press Ltd. ~ 1985 CNR.

Geothermics, Vol. 14, No. 1, pp. 59-72, 1985.

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FLUID INCLUSIONS IN MINERALS FROM THE GEOTHERMAL FIELDS OF TUSCANY, ITALY H. B E L K I N , * B. DE V I V O , t G. G I A N E L L I { and P. L A T T A N Z I § *U.S. Geological Survey, 959 National Center, Reston, VA 22092, U.S.A., tCNR, Centro Geocronologia e Geochimica, Citta Universitaria, Roma, Italy, {,CNR, Istituto Internazionale per le Ricerche Geotermiche, Via del Buongusto 1, Pisa, Italy, and §Dipartimento di Scienze della Terra, Universita di Firenze and CNR, Centro Mineralogia e Geochimica dei Sedimenti, Firenze, ltalv (Received ~Iarch 1984; accepted for publication Ju(v 1984)

AbslracI--A reconnaissance study on fluid inclusions from the geothermal fields of Tuscany indicates that the hydrothermal minerals were formed by fluids which were, at least in part, boiling. Four types of aqueous inclusions were recognized: (A) two-phase (liquid + vapor) liquid rich, (B) two-phase (vapor + liquid) vapor rich, (C) polyphase hypersaline liquid rich and (D) three phase--H20 liquid + CO2 liquid + CO2-rich vapor. Freezing and heating microthermometric determinations are reported for 230 inclusions from samples from six wells. It is suggested that boiling of an originally homogeneous, moderately saline, CO2-bearing liquid phase produced a residual hypersaline brine and a CO,-rich vapor phase. There are indications of a temperature decrease in the geothermal field of Larderello, especially in its peripheral zones. INTRODUCTION O n e o f the most i m p o r t a n t p r o b l e m s in the m o d e l i n g and u n d e r s t a n d i n g o f v a p o r - d o m i n a t e d g e o t h e r m a l systems is the c o m p o s i t i o n o f the g e o t h e r m a l fluid that p r o d u c e s the steam by boiling at d e p t h . W h i t e et al. (1971) have suggested that this boiling process p r o d u c e s a residual brine and a lowering o f the original water table. The fluid inclusions present in the minerals s a m p l e d d u r i n g drilling in g e o t h e r m a l areas are i m p o r t a n t as they could p r o v i d e samples o f this fluid a n d hence their s t u d y could place constraints on the n a t u r e and origin o f the g e o t h e r m a l fluid. The aim o f the present p a p e r is: (1) to ascertain whether the p h y s i c o - c h e m i c a l characteristics o f the fluids t r a p p e d within the m i n e r a l s from the g e o t h e r m a l fields o f T u s c a n y are c o m p a r a b l e with those presently o b s e r v e d in the reservoir and (2) to o b t a i n i n f o r m a t i o n on the origin and evolution o f the g e o t h e r m a l fluid.

GEOTHERMAL

FIELDS OF TUSCANY

The L a r d e r e l l o - T r a v a l e field is a w e l l - k n o w n e x a m p l e o f the relatively u n c o m m o n v a p o r d o m i n a t e d g e o t h e r m a l system. T h e geology o f the a r e a has been described in detail by Gianelli et al. (1978). Wells p r o d u c e s u p e r h e a t e d steam and have g e n e r a t e d electricity since 1904. Liquid water is present only in wells drilled in the p e r i p h e r a l zones o f the field (Celati et al., 1973). The D and '"O isotopic c o m p o s i t i o n o f the well fluids indicates a meteoric origin followed by i s o t o p e exchange with the reservoir rocks (Panichi et al., 1974). U p to now, a highly saline brine has nol been f o u n d , not even in the deepest wells. Saline waters with a b o u t 4000 p p m chloride are present in s o m e p e r i p h e r a l areas (Panichi et al., 1974). D o w n - h o l e fluid s a m p l i n g is difficult technically and is greatly h i n d e r e d by flashing o f the liquid phase present in small fractures o f the rocks (Truesdell and W h i t e , 1973). The g e o t h e r m a l fields o f the M o n t e A m i a t a a r e a ( B a g n o r e and P i a n c a s t a g n a i o ) lie on two s t r u c t u r a l highs, south a n d southeast o f the r h y o d a c i t e A m i a t a v o l c a n o ( C a l a m a i et al., 1970; 59

60

H. Belkin, B. De Vivo, G. Gianelli and P. Lattanzi

Bagnoli et al., 1980). The upper productive horizon is present at the top of the Mesozoic carbonate-anhydrite Burano Formation. Superheated steam is produced from this horizon. Other productive fractures, with a fluid, probably in the liquid phase, have recently been found in the low-grade metamorphic units underlying the Burano Formation. The fluid produced has a very high initial gas/steam ratio, and is characterized by high CO2 content (Cataldi, 1967; Atkinson et al., 1978; Bagnoli et al., 1980). The hydrothermal mineral assemblages at Larderello were studied in detail by Cavarretta et al. (1980, 1982, 1983). In the peripheral areas (field temperatures between 150 and 200°C), the most abundant minerals are quartz, calcite, chlorite and pyrite. In hotter and deeper parts of the field (temperatures in the range 200-300°C), where the main productive fractures are located, quartz, chlorite, calcite, K-spar and K-mica predominate, accompanied by wairakite, hematite, anhydrite and barite. Veins within the metamorphic units underlying the carbonate reservoir (temperatures 250-350°C) consist of K-spar, epidote, chlorite and sutphides, with minor sphene, prehnite, albite, clinopyroxene and amphibole. Figure 1 summarizes the hydrothermal mineral distribution with respect to temperature. 150

200

250

300

550

I

I

I

I

I

4 0 0 °C

0Z

Cc Ch Mu Do Wo gs r

Ep Sp Ac Wo Px Gr K-t PL An B(:] Hm Py

Cp Po SL Go Co Cu Fig. 1. H y d r o t h e r m a l mineral distribution with t e m p e r a t u r e in the Larderello field (after C a v a r r e n a et al., 1982). A b b r e v i a t i o n s : Q z = quartz, Cc = calcite, Ch = chlorite, M u = K-mica, D a = datolite, W a = w a i r a k i t e , Pr = prehnite, Ep = e p i d o t e - c l i n o z o i s i t e solid s o l u t i o n , Sp = sphene, A c = t r e m o l i t e - f e r r o t r e m o l i t e solid solution, W o = w o l l a s t o n i t e , Px = d i o p s i d e - h e d e n b e r g i t e solid s o l u t i o n , Gr = g r o s s u l a r - a n d r a d i t e solid s o l u t i o n , Kf = K-spar, PI = sodic p l a g i o c l a s e , A n = anhydrite, Ba = barite, H m = h e m a t i t e , Py = pyrite, P o = pyrrhotite, Cp = c h a l e o p y r i t e , S1 = sphalerite, Ga = galena, C o = cobaltite, Cu = cubanite.

Three lines of evidence suggest that the observed mineral assemblages are compatible with field temperatures: (1) The hydrothermal mineral assemblages generally do not show alteration, nor reaction toward retrograde lower temperature phases and thus they were mostly formed during the last mineral-forming event which affected the rocks saturated by the geothermal fluids. (2) The pyrrhotite-chalcopyrite pair, the cubanite-chalcopyrite-pyrite assemblage, and the transition laumontite-wairakite (Cavarretta et al., 1980) occur in these rocks. Based on

Fluid Inclusions in Minerals, Tuscany

61

experimental data reported by Barton and Skinner (1967), Liou (1970) and Sugaki et al. (1975), these assemblages are suggestive of temperatures compatible with present-day down-hole values. (3) The COa, O2 and $2 fugacities calculated from mineral equilibria that model the natural assemblages reproduce the partial pressure values of the same gases calculated at the reservoir temperatures from the gas composition (D'Amore and Nuti, 1977; Cavarretta et al., 1982; D'Amore and Gianelli, 1983). A few occurrences of relict minerals have also been reported in the Larderello - Travale field (Cavarretta et al., 1982, 1983). These phases can be related to a metamorphic-metasomatic event preceeding the crystallization of the hydrothermal mineral assemblages. The age and origin of this post-tectonic thermal event are still a matter of debate. The two prevalent hypotheses (Cavarretta et al., 1980) are: (1) Hercynian "Abukuma-type" metamorphism, or (2) Alpine metamorphism linked to the Neogene- Quaternary magmatism of Tuscany. The relics so far observed consist of: (1) wollastonite, andradite garnet and diopside-rich clinopyroxene and (2) tourmaline, biotite and plagioclase (An,7-60). They occur either in veins or in layers parallel to the main schistosity of the metamorphic units underneath Larderello. The hydrothermal minerals are less abundant and less well known at Piancastagnaio and Bagnore. Veins within the phyllitic rocks underlying the carbonate-anhydrite formation consist of quartz, chlorite, albite and pyrite. Epidote and quartz appear in the deepest parts of some wells. Measured in-hole temperatures range from 150 to 300°C. FLUID INCLUSIONS Studied samples Figure 2 shows the site of the Larderello geothermal field wells from which the core samples

o

1 2 3 4

t i g . 2. L o c a t i o n of ,,',ells and t e m p e r a t u r e d i s t r i b u t i o n at the top of the main reservoir in the k a r d e r e l l o field ( m o d i f i e d after Calore, 1979). LP = L a r d e r e l l o P r o f o n d o , VP ~ V a l P a v o n e 2 , $22 = S a s ~ o 2 2 , L = L a g o P u n t o n e 3 , SS = S e r r a z z a n o Sperimentale. {1) Neogene and flysch units ( " c o v e r " ) , (2) o p h i o l i l e and (3) Mesozoic C e n o z o i c units.

62

H. Belkin, B. De Vivo, G. Gianelli and P. Lattanzi

for fluid inclusion study have been taken. The temperatures at the top of the carbonate reservoir (after Calore, 1979 and Catore et al., 1979) are also reported. Core samples were kindly made available by ENEL (Italian National Electricity Board). Eight samples from the Larderello field have been examined: LP 789 from the well "Larderello Profondo"; VP 673 from "Val Pavone 2"; L 1448 from "Lago Puntone 3"; $22-1480, $22-2163, $22-2636 and $22-2767 from "Sasso 22," and SS 1824 from "Serrazzano Sperimentale." For comparison, one sample from Piancastagnaio (PC 1660, from the "Piancastagnaio 26" well) was also studied. The numbers in the name of the samples represent their depth (meters) below ground level. All the rock samples consist of various metamorphic units that underlie the Mesozoic carbonate reservoirs. Their petrography is described in detail below. L P 789. Fine-grained quartz-sericite schist. A felt of fine white mica (sericite) and some weltrounded detrital quartz clasts represent the original paragenesis of the rock. Tourmaline, zircon and rutile are accessories. Small veins are filled by authigenic calcite, pyrite and some hematite. Secondary quartz occurs in healed fractures in the quartz-rich portions of the sample. It can be assigned to the low-grade metamorphic terrigenous Verrucano Formation, intersected by the well between approximately 500 and 800 m. Small hydrothermal veins are widespread: authigenic minerals are, in order of decreasing abundance, calcite, sericite, K-spar, pyrite, epidote and quartz. Chlorite and hematite were observed in other portions of the same core. L 1448. Fine-grained graphitic schist crossed by small veins, with scattered ore minerals. The more abundant authigenic phases are K-spar, chlorite and epidote. Quartz occurs as veins in detrital minerals. Clinopyroxene occurs as small prismatic crystals often included in K-spar. Ore minerals, in order of abundance, are pyrite, pyrrhotite, chalcopyrite, sphalerite and galena. VP 673. Quartz-rich meta-arenite with thin veins of authigenic quartz. SS 1824. Quartzitic phyllite composed mainly of quartz, K-mica, chlorite, albite and ilmenite. K-spar and pyrite occur in veins. In other portions of the same core, epidote, quartz and pyrrhotite were observed. $22-1480. Metagreywacke with granolepidoblastic texture. Quartz, K-mica, chlorite, albite, ilmenite and sphene are the primary minerals. Epidote, K-spar, K-mica, calcite and quartz occur in veins. $22-2163. Metagreywacke similar to the above sample, with additional metamorphic epidote and minor biotite. Hydrothermal minerals are K-spar, epidote, chlorite and sphene. $22-2636. Fine-grained granolepidoblastic gneiss made up of quartz, muscovite, biotite, plagioclase (An,0-s0), and minor almandine garnet and hornblende. Zircon, apatite, sphene and ilmenite also occur as accessory minerals. Authigenic minerals include K-spar, epidote, chlorite, quartz, calcic amphibole, pyrrhotite and chalcopyrite. $22-2767. Fine- to medium-grained granolepidoblastic gneiss, with poorly defined schistosity. The primary minerals are quartz, biotite, K-mica, plagioclase-andesine, ilmenite, pyrrhotite and accessory tourmaline and zircon. Chlorite partly replaces biotite, and very finegrained patches of white mica are probable pseudomorphs after cordierite. K-spar, epidote, chlorite, albite, calcite, K-mica, quartz and sphene occur in the veins. The last two samples were taken from a fractured horizon intersected between 2400 and 3000 m, in which K-spar, epidote, chlorite and pyrite, with subordinate quartz, albite and sphene, are the main authigenic minerals (Bertini et al., 1980). PC 1660. Calcite-bearing quartzitic phyllite with abundant graphite. Zircon, tourmaline and apatite are accessories. Microfractures are filled by quartz and some chlorite. Classification and distribution o f fluid inclusions Fluid inclusions are very abundant in all samples. They mostly occur along planes of healed

Fluid Inclusions in Minerals, Tuscany

63

fractures (secondary inclusions) in detrital quartz crystals; primary inclusions in hydrothermal K-spar and quartz overgrowths were also observed. We have assumed that both primary inclusions in hydrothermal minerals and the secondary inclusions in detrital minerals were formed by the same fluids. Four types of inclusions were recognized (Fig. 3): (A) two-phase (liquid + vapor) liquid rich; they occasionally contain cubic crystals or anisotropic flakes which do not dissolve upon heating and probably represent accidental trapping; salinity ranges from very dilute to very high (22 wt% NaC1 eq.), (B) two-phase (vapor + liquid) vapor-rich, extremely dilute to moderate (10 wt% NaC1 eq.) salinity, (C) polyphase hypersaline (liquid + vapor + NaC1 + an elongate highly birefringent mineral: CaSO2) and (D) three phase (liquid brine + liquid CO2 + CO.,rich gas), very low salinity.

Fig. 3. (A) A typical group of type A inclusions. They are two-phase secondary low-salinity (2 wt% NaCI eq.) inclusions in quartz from $22-2636; they homogenize in the liquid phase at 340°C. (B) A vapor-dominated type B inclusion in quartz from $22-2636. The liquid phase (arrow) is of low salinity (0.5 wt% NaCI eq.). The inclusion homogenizes in the vapor phase at 339°C. (C) A three-phase hypersaline (28 wt% NaCI eq.) type C inclusion in quartz from VP 673. The cubic daughter crystal of halite (H) dissolves at about 215°C. Final homogenization occurs in the liquid phase at 242°C. (D) A typical three-phase CO2-rich type D inclusion in quartz from $22-2163. The CO2 vapor (V) and CO2 liquid (L) homogenize at 29.8°C in the vapor phase. A small amount of low-salinity aqueous fluid lies in the high-curvature "ends" of the inclusion (arrows). Total homogenization (COs + H20) occurs in the vapor phase at 322°C.

Type A and B inclusions are nearly ubiquitous. Type C occur occasionally in two shallow Larderello samples. Type D inclusions occur especially at Piancastagnaio, in agreement with the high CO~ content of present-day geothermal fluids; they also occur at Sasso 22 and, exceptionally, at Lago Puntone. Salinities range widely in each sample, except at Sasso 22, where a remarkably uniform moderate salinity (max. 3.0 wt% NaC1 eq.) is observed. Microthermometric determinations (Table 1 and Figs 4(a) and 4(b)) were carried out at the University of Florence and at the U.S. Geological Survey using two independently calibrated CHAIXMECA* heating and freezing stages. We estimate the temperature uncertainty in the range of T,,-ice to be -_+0.1°C and in the range of Th H20 V - L to be -_+2.0°C. Multiple doubly-polished plates were prepared from all the samples. The samples were heated only once to avoid spurious measurements that could be introduced by stretching or rupture of the inclusion walls. A particular caveat should be mentioned regarding the salinity. We express the salinity in terms of weight per cent NaC1 equivalent (Roedder, 1963). However, we know from T~ *Any use of trade names in this report is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey, the University of Florence or the CNR.

64

H. Belkin, B. De Vivo, G. Gianelli and P. Lattanzi

!

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65

Fluid Inclusions in Minerals, Tuscany []

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Fig. 4. Histograms of (A) homogenization temperatures (Th) and (B) salinities of fluid inclusions from Larderello and

Piancastagnaio.

measurements (see below) that at least Ca 2÷ and possibly Mg 2÷ and K ÷ occur in the inclusion fluids. Furthermore, Hedenquist and Henley (1984) have pointed out that CO2 as a solute can depress the freezing point ( m a x i m u m depression = - 1.48°C) and this effect can be mistaken or confused with salinity. Fluids with low CO2 pressures (between 0 and 10.4 bars pC02) freeze without clathrate formation. Errors can be significant especially at very low salinities. Thus, a

66

It. Belkin, B. De Vivo, G. Gianelli and P. Lattanzi

component of the freezing point depression in the inclusion fluids is probably due to dissolved CO2. We assume that type B, vapor-rich, inclusions are formed by boiling and represent the trapping of immiscible steam droplets. Boiling preferentially partitions salts into the liquid phase and volatiles (e.g. CO.,) into the vapor phase. It is reasonable to assume, therefore, that the major salinity error that results from CO~ solute freezing-point depression will be in type B Tm measurements. Type A and some type B inclusions homogenize in the liquid phase; most type B inclusions homogenize in the gas phase. Type B have, on average, homogenization temperatures (T~) distinctly higher than those of type A and C; this is due, at least in part, to heterogeneous trapping or widespread necking down, resulting in erroneously high T~ for type B. Necking down can produce both higher and lower T,,; however, modification to an original vapor-rich inclusion by necking-down processes would tend to produce a vapor-rich inclusion with a higher T~. In type C inclusions, NaCI was identified from the equant cubic habit of the crystals, and from their isotropy; upon heating, they usually dissolve at temperatures slightly below vapor/liquid homogenization. In type D inclusions, temperatures of the triple point are very close to that of pure CO2 ( - 56.6°C). CO2 homogenization occurs in the gas phase at Sasso 22 and Lago Puntone, and in the liquid phase at Piancastagnaio. Except for type C and D inclusions, salinity was estimated from the depression of the freezing point (i.e. temperature of melting of the last ice crystal on warming = T,3. Type A and B inclusions may well contain small amounts of CO2, which could cause slight errors in the salinity estimates due to clathrate formation (Collins, 1979, Hedenquist and Henley, 1984). An additional limitation on some of these measurements arises from poor optics, so that in many cases T~ had to be estimated on the basis of the last gas bubble movement. The salinity of type C inclusions was estimated from the relative volumes of the halite daughter crystal and inclusion liquid, and adding it to the saturated solution, assumed to be 23.3 wt% NaC1. In type D inclusions, the salinity of the aqueous phase was calculated from the decomposition temperature of the CO2 clathrate, using the equation given by Bozzo et al. (1973). Because of optical difficulties, this determination was only occasionally possible. Upon cooling, halite in type C inclusions sometimes reacted with the liquid to form hydrohalite (NaC1-2H~O), which was observed to decompose metastably on reheating (heating rate - - 0.5°C/rain) between +2.8 and +4.2°C. In some cases, it was possible to obtain reliable data on the temperature of the first appearance of melt in previously frozen inclusions (i.e. eutectic temperature, T,.). Type C inclusions show T~ in the range of - 39 to - 52°C. As the eutectic for the system CaCI~,N a C I - H 2 0 is at --52°C (Crawford, 1981), it is reasonable to assume that we are dealing with N a - C a - C I fluids, possibly with Mg as well. This assumption is strengthened by the widespread occurrence of Ca-bearing minerals in the hydrothermal assemblages. Since the inclusions contain NaC1 at room temperature, the fluid composition must lie on the NaC1 side of the phase diagram (Fig. 5). In the Na-rich C a - N a brine the first phase to disappear upon melting at Te is CaCI~. 6H20 (Crawford, 1981). As the composition of the fluid moves along the cotectic ice-NaC1-2H~O, melting of NaC1-2H20 and ice occur. After all the ice is melted, further warming causes the liquid composition to move across the NaCI-2H20 field to the peritectic between NaC1.2H20 and NaC1. Further warming causes NaC1.2H20 to melt incongruently (usually sluggishly) to form NaCI + liquid. When the last hydrohatite is gone, the system consists of just NaC1 and liquid, so that liquid composition leaves the reaction curve, and moves toward the NaCI corner of the diagram. The metastable disappearance of hydrohalite above + 0 . 1 ° C is a typical phenomenon. Some type A inclusions also show To in a

Fluid Inclusions in Minerals, Tuscany

52,0°C~ /'+

+

67

20.8°C ?aC'7",

CaC2l

50% N aCI

so%

t ig. 5. Phase boundaries and melting points of solid phases in the svstem NaCl

CaCI,

H,O. After Crawford (1981).

similar range. By contrast, all inclusions from Sasso 22 have T,. between - 2 1 . 8 and - 24.0°C, suggesting that the fluid trapped is a N a - ( K ? ) - d o m i n a t e d brine. Besides optical evidence in type D inclusions, the possible presence of gases under pressure in the inclusions was checked with a crushing stage (Roedder, 1970). Gas, probably CO2, was found to be present. Unfortunately, the poor optical quality and the small size of the inclusions prevented reliable estimates of the volumetric expansion of such gases upon pressure release. DISCUSSION

Temperature estimates Homogenization temperatures were assumed to be equal to trapping temperatures. Widespread coexistence of more saline liquid-rich and less saline gas-rich inclusions was taken as evidence of boiling. The only possible exception is $22-2767, where type B inclusions are quite rare, and when present are not obviously related to type A inclusions. The actual trapping temperature of type A inclusions in this sample might well be higher than Th. Figure 6 reports on the T+ axis the temperature values deduced from the present fluid inclusion study and from two calculation methods: (1) the octahedral vacancy of M g - F e 2+ chlorites of Larderello (Cavarretta et al., 1982) and (2) the K - s p a r - a l b i t e geothermometer (Cavarretta, Gianelli and Puxeddu, unpublished results). The former calculation method was successfully applied in the Salton Sea geothermal field by McDowell and Elders (1980). The latter calculation method is a low-temperature extrapolation of the a l b i t e - m i c r o c l i n e geothermometer (Bachinski and Muller, 1971), as calibrated for the Salton Sea geothermal system by McDowell and McCurry (1978). In the same Figure, on the 7", axis are reported down-hole or computed temperatures of the reservoir rocks (Calore, 1979; Cappetti et al., 1982). Reliable down-hole temperatures for the wells from which fluid inclusion samples were studied are relatively scarce; this fact somewhat hinders the appraisal of the comparison between the diverse temperature estimates. However, if we assume that a difference greater than 30°C between T~ and 77.,.in Fig. 6 represents an actual difference (higher or lower) between the present-day reservoir temperatures and the crystallization temperatures of the minerals, then we can suggest that the thermal regime at Larderello, and, to the extent of the few available data, at Piancastagnaio, did not dramatically change since the crystallization of the hydrothermal minerals. Either the temperature of the geothermal system has remained, at least in part, fairly constant over a long period, or the formation of hydrothermal minerals has been relatively recent. Significant cooling is evident only at Val Pavone, in the eastern peripheral zone of the field, and, to a lesser extent, at Serrazzano and Lago Puntone. The data for Sasso 22 deserve some detailed discussion. Both fluid inclusion Th and petrologic temperatures from

6~

H. Belkin, B. De Vivo, G. Gianelli and P. Lattanzi (range and average) (range bnd average)

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Fig. 6. T,. represents in-hole and calculated reservoir temperatures vs T, that are temperatures computed from the following geothermometric methods: (1) from fluid inclusions, (2) from octahedral vacancy of M g - Fe 2÷ chlorites and (3) from the K-spar - albite solvus. Symbols of the wells are organized as in text, e.g. $22-3279 = Sasso 22 well, 3279 m below ground level.

samples above 2500 m are consistently higher than present-day down-hole temperatures, and they appear to define a thermal gradient 20°C/kin lower than present (50°C/km; Fig. 7). This may testify to an actual change in the thermal regime at Sasso 22. However, it is also possible that the hydrothermal minerals were deposited by fluids hotter than the surrounding rocks: for instance, assuming ascent of periodically boiling fluids from a near-constant temperature

1000 -

I~

.ooo

~ i

k

,ooo-

o, 300

350

400

T (°C)

Fig. 7. Temperature/depth distribution in the Sasso 22 well. Circles = present-day in-hole values; triangles = average T~ of type A inclusions; squares = temperatures calculated with the K-spar/albite thermometer. Solid line = present thermal gradient, - 5 0 ° C / k m ; dashed line = hypothetical fossil gradient, - 3 0 ° C / k m . The fluid inclusion temperature of the sample at 2767 m depth was not taken into account in calculating the fossil gradient, because it may not represent actual trapping temperature (see text).

Fluid Inclusions in Minerals, Tuscany

69

reservoir located at a depth corresponding to present-day 3000 m. In this case, rock temperatures and the gradient defined by them, might well have been comparable to presentday conditions. We have no direct age determinations of the hydrothermal minerals at Larderello. The initiation of hydrothermal activity has been recently estimated to be 3 Myr by Del Moro et al. (1982) on the basis of blocking temperatures for K / A r and Rb/Sr ages of biotites from the Larderello basement rocks. These biotites were formed or re-equilibrated during a high temperature-low pressure metamorphism linked to the intrusion of Pliocene granitoids in southern Tuscany. This intrusion is considered to have been the origin for the convective hydrothermal circulation of fluids through rocks fractured by post-intrusion faulting. Del Moro et al. (1982) assume a blocking temperature of 4 0 0 - 4 5 0 ° C for the K/Ar and Rb/Sr systems in biotites, and, using the present temperature of the field, they calculate a cooling rate of about 40°C/Myr for the uppermost 2 - 3 kin, and of about 16°C/Myr for crustal sections below 3 kin. Part of the cooling is ascribed to uplift and erosion, but heat dissipation by convective circulation is considered the main cause of cooling of the upper parts of the geothermal system. The lower cooling rate at depth is related to the lower permeability of the basement rocks. Pressure and depth estimates The coexistence of vapor- and liquid-rich inclusions, suggesting boiling conditions of the fluids at the time of trapping, allows an estimate of the trapping pressure, which will correspond to the vapor pressure of a fluid. From the pressure, the depth below the paleo-water table can be calculated, assuming (Haas, 1971) that the system was open to the surface (that is, no restriction or throttling points were present in the channelways) and no gas bubbles were contained in the fluid column (because they would decrease the average density of the column). The calculated depths below the paleo-water table for the samples where type A and B inclusions coexist are reported in Table 2. Even considering possible errors in estimating the actual temperature and salinity of the fluids, it appears that most depth estimates are too low when compared to present depths. Most probably, two major effects, both involving the presence of CO~,, have influenced the temperature and pressure regime of the geothermal system. First, CO2 as a solute in the fluids may significantly lower the vapor pressure of the fluid (Takenouchi and Kennedy, 1964) and second, the presence of CO: bubbles may markedly lower the overall fluid density (Mahon et al., 1980). These effects result in a depth estimate that is too small. To develop a correct depth to boiling point curve for Larderello and associated fields the detailed CO2 systematics of the geothermal fluid must be known.

Table 2. (alculated pressures and depths of trapping for fluid inclusions from l.ardcrello, following Haas (1971) Sample

Y,, (%7)

Salinity (~t% N a ( I eq.)

Pressure (bar)

l)epdl (m)

VP 673 I.P 789 L 1448 SS 1824 $22-1480 $22-2163 $22-2636

256 249 330 287 308 324 325

10.8 4.(1 3.5 8.7 1.8 2.0 2.7

40 39 125 70 95 119 I 18

435 430 1600 800 1200 1540 1530

7",, and salinilies represent average xalucs for type A inclusions.

70

H. Belkin, B. D e Vivo, G. Gianelli and P. L a t t a n z i

No attempt was made at calculating pressure from the density of the CO2 + H20 type D inclusions, as suggested, for example by Touret (1977), because of the large deviations from ideality in H20 + CO2 mixtures below 300°C. Homogenization of CO~ in the liquid phase in type D inclusions at Piancastagnaio appears to indicate somewhat higher pressures than at Larderello, where homogenization occurs in the gas phase. N a t u r e a n d e v o l u t i o n o f the g e o t h e r m a l f l u i d Probably the most significant observation from the fluid inclusion study is the coexistence of liquid- and vapor-rich inclusions, with a wide range of salinity, together with hypersaline inclusions. This coexistence can be explained as the result of boiling and condensation processes. Boiling and condensation of an initially homogeneous, moderately saline, CO2bearing fluid (type A inclusions) would have produced vapor- and CO2-rich low-salinity fluids (type B and D inclusions) and a slightly cooler hypersaline brine (type C). Type C inclusions are the only, yet compelling, evidence that a hypersaline fluid was at times present in the geothermal field of Larderello. There are no indications regarding the fate of this residual brine. It could remain trapped in the pores and microfractures of the rocks (sealed by mineral crystallization) and, therefore, not circulate through the system any longer. Otherwise, following the model of White et al. (1971), it could concentrate under the receding deep water table. The low to moderate salinity Na + CI waters reported by Panichi et al. (1974) from nonproductive wells probably have compositions quite different from that of the deep geothermal fluid. From the recent San P o m p e o well, Batini et al. (1984) report the presence of a deep fluid characterized by temperatures higher than 400°C and pressures greater than 240 bars. Unfortunately it was not possible, for technical reasons, to obtain a fluid sample. If it exists in the liquid phase, then a salinity of at least 3 wt% NaC1 eq. must be assumed (Sourirajan and Kennedy, 1962).

CONCLUSIONS The hydrothermal minerals in the Larderello and, possibly, Piancastagnaio fields were formed by fluids which were, at least in part, boiling. The initial fluid (type A liquid-rich twophase inclusions) was homogeneous and moderately saline. Boiling and condensation produced low-salinity, vapor- and CO2-rich fluids, trapped as vapor- and CO2-rich inclusions, and a hypersaline brine (inclusions containing solid NaC1). The thermal regime in the geothermal fields since the deposition of the hydrothermal minerals can be reasonably modeled by a slow monotonic cooling, mostly due to convective heat dissipation, as suggested by Del Moro et al. (1982). In the peripheral zones of the field, this cooling may have been more pronounced. Acknowledgments--The research is part of a cooperation program between the Italian National Research Council (CNR) and the Italian National Electricity Board (ENEL). Financial support was provided by CNR. We acknowledge with gratitude the careful reviews of Daniel O. Hayba (USGS) and Richard W. Henley(Geothermal Research Centre, New Zealand). Grateful thanks are also extended to Edwin Roedder for his kind help and advice to one of the authors (De Vivo) during the latter's visit to the U.S. GeologicalSurvey, Reston, Virginia, U.S.A. on a grant from the Italian National Research Council.

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