The Pantelleria caldera geothermal system: Data from the hydrothermal minerals

ELSEVIER

Journal of Volcanology

and Geothermal

Research 75 ( 1997) 25 l-270

The Pantelleria caldera geothermal system: Data from the hydrothermal minerals Paolo Fulignati, Giuseppe Malf’itano, Alessandro Sbrana

*

Dipartimento di Scienze della Terra, Universid di Piss, Via S. Maria, 53, 56126 Pisa, Italy Received 8 September

1996; accepted

15 September

1996

Abstract This paper proposes, on the basis of petrographic and mineralogic data on cutting and cores from two deep wells (“Pantelleria 1” and “Pantelleria 2”), the first model of the active hydrothermal system of the island of Pantelleria. Phyllosilicates were studied in detail because they are considered key minerals in the identification of hydrothermal processes. The results of these studies emphasize differences between the intracaldera and pericaldera areas of the island. Within the 45 ka caldera there is a high-temperature (240-260°C at 600-800 m depth) active hydrothermal system with five zones of characteristic alteration minerals with increasing depth. Rocks are unaltered to a depth of 200 m, contain smectite and mixed-layer chlorite-smectite (C/S) between 200 and 380 m, chlorite, illite, chalcedony and quartz from 380 to 500 m, albite, adularia and saponite from 500 to 680 m, mixed-layer biotite-vermiculite from 680 m to the depth drilled (1100). Outside the caldera, but near the rim, a low-temperature and low-permeability (< 140°C) hydrothermal system is characterized by smectite, dolomite and ankerite at depths of 390 to 650 m, chlorite and calcite at 650-900 m, and mixed layers of chlorite-smectite, illite-smectite and iron carbonates (ankerite, siderite) from 900 m of the well at 1203 m. The superimposition of hydrothermal mineral assemblages is evidence for cooling in the hydrothermal system both inside and outside the caldera. We propose that a high-temperature hydrothermal system developed inside the caldera. In an early stage in the area surrounding the subvolcanic body, biotite isograd is reached and an alkali-metasomatism zone develops inside the

body itself. This phase may also account for the development of a chlorite-albite-adularia zone extending to 400 m. A cooling phase (nearly 50°C) followed, resulting in the substitution of biotite by mixed-layer biotite-vermiculite and by the crystallization of Fe-rich saponite instead of chlorite, within the currently active reservoir. A cooling phase has also been identified in the well outside the caldera. Keywords: hydrothermal

alteration;

phyllosilicates;

saporite; chlorite; mixed layers; geothermal

1. Introduction The island of Pantelleria is a Pleistocene-Recent volcanic complex in the continental rift of the Sicily

* Corresponding [email protected].

author.

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0377-0273/97/$17.00 Copyright PII SO377-0273(96)00066-2

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system; Pantelleria

channel. The subaerial portion of the island consists of the eruption of pantellerite and trachyte lava-flows and domes, and ignimbrites from caldera-forming pyroclastic eruptions between 300 ka and 4 ka. The chemical composition of the erupted magmas is typical of a continental rift environment, and varies from alkalic basalts, dominant in the northern part of the island (Mahood and Hildreth, 1986; Civetta et al.,

0 1997 Elsevier Science B.V. All rights reserved.

252

P. Fulignati et al./ Journal of Volcanology and Geothemtal Research 75 (19971251-270

19881, to pantellerites and trachytes forming the central-southern part of the island. The latter is dominated by a polyphased nested caldera (Mahood and Hildreth, 1986). This structure probably formed during collapse following two major explosive eruptions in two main phases, the oldest occurring around 114 ka, and the youngest at around 45 ka (Mahood and Hildreth, 1986). The caldera of Pantelleria has an uplifted central part, because of resurgence inside the caldera (volcano-tectonic horst of Montagna Grande) in recent times, between 18 and 4 ka (Mahood and Hildreth, 1983, 1986; Civetta et al., 1988). A geological sketch map of Pantelleria is shown in Fig. 1. Steaming ground, mofettes and hot springs CT,,,

0

1

2

3h-n

98°C) are found throughout the island, for example, in the Fossa de1 Russo, Montagna Grande, and Bagno dell’Acqua areas, and particularly in the intra- and pericaldera zone. This is evidence of a still active hydrothermal field. In the period 1990-94, two deep exploration wells were drilled (Fig. 1) as part of a geothermal research project involving geophysical, thermal, geochemical and geological surveys. The boreholes showed the existence of a high-enthalpy geothermal field in the central part of the island, in the youngest caldera structure (Chierici et al., 1995a,b). The aim of this paper is to propose a model of the hydrothermal system, located in the subsurface of the island, based on petrographic and mineralogical data.

P. Limani

Fig. 1. Strnctural sketch map of Pantelleria island (from Chierici et al., 1995a,b, modified). 1 = tectonic lineaments; 2 = main intracaldera volcanic vents (trapdoor-hingeline volcanic vents of Mahood and Hildreth, 1986); 3 = Montagna Grande volcano-tectonic horst border faults; 4 = La Vecchia caldera rim (114 ka); 5 = Cinque Denti caldera (Green Tuff caldera) rim (45 ka); 6 = thermal springs and steaming ground areas; 7 = PpTl and PPT2 geothermal wells.

P. Fulignati et al. / Journal of Volcanology and Geothemud Research 75 (1997) 251-270

2. Materials and methods This work was developed using cuttings and cores coming from the deep wells Pantelleria 1 and 2. Petrographic study in thin section was followed by diffractometric studies for the identification of the phyllosilicates and by SEM-EDS microanalysis. X-ray diffraction: Clay size (< 2 pm> fractions from powdered cuttings were prepared by standard sedimentation techniques for X-ray diffraction (XRD). Seven analyses of oriented samples were performed after the following treatments: (1) air drying; (2-3) saturation with ions K+ and Mg2+; (4) glycolation using ethylene glycol in a desiccator at 60°C; and (5-6-7) heating at 100, 300, and 550°C. X-ray diffraction patterns were obtained using a Philips diffractometer PW 1710 at 36 kV and 24 mA using CuK (Y . SEM-EDS: Polished thin sections were studied using a SEM-EDS PV 9900 at 20 kV beam current and about 10 nA. Quantitative analyses were obtained using a Philips HAX software using ZAF correction.

3. Petrography and authigenic mineral distribu-

tion 3.1. Well Pantelleria I The deep well Pantelleria 1, located in the southem sector of the Pantelleria caldera (Fig. 11, cuts through 700 m of ignimbrites and lavas which show a trachytic-comenditic chemical composition; below 700 m the well is in a subvolcanic intrusive body, which is a peralkaline-syenite with comenditic affinity (Fig. 2). Petrographic study allowed the identification of five different mineral alteration zones: Zone (1) (O-200 m) unaltered Zone (2) (200-380 m) smectite and mixed layers of chlorite-smectite (C/S) Zone (3) (380-500 m) chlorite, illite, quartz Zone (4) (500-680 m> albite, adularia, saponite Zone (5) (680-I 100 m> mixed layers of biotitevermiculite 3.1.1. Zone 1 (O-200 m) In this interval the host rocks are represented by unaltered porphyritic trachytic-comenditic lavas,

253

which can be correlated with the Monte Gibele lavas filling the Cinque Denti caldera. Below 150 m there are low-crystallinity smectites as an alteration of the glassy groundmass of host rocks and also opal filling the cavities of the lavas. 3.1.2. Zone 2 (200-380 ml In this interval the primary rocks are trachytic comenditic lava flows and the ignimbritic unit of the Green Tuff, Fig. 2, with pantelleritic composition at the base and comenditic at the top. Smectites and mixed layers of C/S develop as alteration products at the expense of the glassy groundmass and of the clinopyroxene and anortoclase phenocrysts. In addition, pyrite is well developed and from 247 m downwards calcite occurs. 3.1.3. Zone 3 (380-500 rn) The primary lithology is represented by a thick comenditic ignimbritic unit (Fig. 2). Hydrothermal alteration is widespread. The secondary minerals characterizing this interval are chlorite (restricted to 400-460 m>, present both in veins and as an alteration of phenocrysts, illite, present in the whole interval, and adularia, appearing below 460 m. A strong silicization of the ignimbrite is present in this interval, chalcedony is present up to 450-470 m and quartz develops below this depth with pervasive style. Both chlorite and mixed layers occur in association with calcite and sulphides in replacing the anorthoclase, with only sulphides if alteration replaces aegirine-augite. Sulphides are abundant, and below 465 m, in association with pyrite, sphalerite was found as small anhedral aggregates surrounded by euhedral crystals of pyrite. Sulphides seem to replace primary oxides of the ilmenite-magnetite group. Illite is abundant in the interval 385-435 m as replacement of anorthoclase. A quartz-adularia assemblage was found in microfractures below 470 m. This zone could represent the cover of the geothermal field developed by self-sealing processes. 3.1.4. Zone 4 (500-680 rn) Below 500 m, the host rock consists entirely of lavas, which appear strongly brecciated (core of 601-611 m) between 600 and 650 m. In this zone there is the disappearance of chlorite and mixed layers of C/S and the appearance of saponite as

P. Fulignati et al. /Journal

254

of Volcanology and Geothermal Research 75 (1997) 251-270

Measured Temperatures

-t

600 -

I I I t

600 900 lOOO-

I

llOOw

Fig. 2. Distribution

of authigenic

w

ignimbrites

minerals,

hydrothermal

laws

q

subintrusive

zones and stabilized temperatures

I

I

body

(from Chierici et al., 1995a,b)

in Pantelteria 1

well.

dominant phyllosilicate. The saponite-sulphides assemblage substitutes microphenocrysts and phenocrysts of aegirine-augite in part or completely. When the alteration involves anorthoclase, the previous assemblages are associated with calcite. The anorthoclase is less stable in this zone than at shallower levels; in some crystals there is total albitization. The quartz-adularia assemblage appears more developed than in the previous zone associated with sphene. Pyrite is the most abundant sulphide; pyrrotite and sphalerite occur in smaller quantities. 3.1.5. Zone 5 (680-1100 rn) In this zone there is a transition from trachyticcomenditic lavas to a sub-volcanic unit having a mineral paragenesis consisting of anorthoclase, sanidine, aegirine-augite, fayalitic olivine in reaction to clinopyroxene and arfvedsonite phenochrysts. The

groundmass is formed of feldspars, aenigmatite, arfvedsonite, aegirine-augite and quartz; the chemical composition of this rock is similar to the trachytes of the post Green Ignimbrite Monte Gibele volcanic center (SiO, 65%, Ce 154 ppm, La 86 ppm, Sr87/Sr86 0.70345 + 2). Hydrothermal alteration reaches a depth of 1000 m and decreases downwards; it is scarce in the last 50 m probably due to the reduction in permeability of the subvolcanic body. This zone is characterized by the appearance of mixed-layer biotite-vermiculite. This mineral is found, sometimes associated with sulphides and sphene, as irregular aggregates overlying anorthoclase and more rarely aegirineaugite. Crystals of epidote are rarely present as an alteration of anorthoclase. This mineral always shows evidence of instability (resorption) represented by calcitic reaction rims. The presence of fractures and

P. Fufignati et al. / Journal of Volcanology and Geothemtd Research 75 (1997) 251-270

veinlets filled by a quartz-adularia assemblage is widespread to 850 m. Below this depth the crystals of anorthoclase are influenced by a strong process of hydrothermal albitization. The presence of high-temperature hydrothermal minerals like biotite and the appearance of ‘chess board’ and ‘veinlet’ structures, caused by the base exchange reaction of Na for K in the crystals of anorthoclase (albitization), are characteristic of alkali metasomatism zones (Pirajno, 1992). 3.2. Well Pantelleria 2 The second well is located just north of the caldera rim (Fig. 3). In the first 180 m of this well

25.5

we can find an alternation of pantelleritic ignimbritic units with interbedded pantelleritic pumiceous layers. Below 180 m there is a totally different sequence made up of alkali-basaltic scoriaceous lavas and hyaloclastitic lavas intersected by doleritic dikes in the lower part (Fig. 3). The hydrothermal mineral association in the Pantelleria 2 drillhole is different from the other well, reflecting both a lower thermal grade and different host-rock lithology. Four alteration zones have been distinguished based on optical observation: Zone 1 (O-390 m) unaltered rock Zone 2 (390-650 m) smectite, dolomite, ankerite association m

hkawred Temperalures a,

b

Lilhology

I 600

I

I

t 900

Fig. 3. Distribution well.

of authigenic

minerals,

hydrothermal

zones and stabilized temperatures

(from Chierici et al., 1995a,b) in Pantelleria

2

256

P. Fulignati et al. /Journal

of Volcanology and Geothermal Research 75 (1997) 251-270

Zone 3 (650-900 m> chlorite, calcite association Zone 4 (900-1203 m> mixed layers, iron carbonates (ankerite, syderite) association 3.2.1. Zone I (O-390 ml The host rock here is mostly pantelleritic and trachytic ignimbrite, and in this zone hydrothermal alteration is almost completely missing. At about 270 m we observe some microcrysts of plagioclase in basaltic lavas partially replaced by smectites and carbonates. Chalcedony is present inside cavities and microfractures.

lithologies, the secondary mineral association is: carbonates + mordenite + corrensite + swelling chlorite. Among the carbonates there is the association: calcite + siderite + breunnerite + ankerite; illite is present only at 900 m. In doleritic levels carbonates and a smectite similar to zone 2 develop. In the last 50 m, in the doleritic dikes rectorite, a regular mixed-layer illite-smectite phyllosilicate appears; this mineral substitutes clinopyroxene crystals. Quartz is abundant throughout this interval.

4. Chemistry of the hydrothermal minerals and 3.2.2. Zone 2 (390-650 rn) The beginning of this zone coincides with the transition from scoriacious alkali-basaltic lavas to lavas with interbeds of hyaloclastites. Generally, hydrothermal alteration becomes more pervasive in this interval, particularly below 550 m. As regards secondary mineral association, carbonates occur as a substitution for the glassy groundmass and carbonates plus smectites as a substitution for clinopyroxenes. There is noteworthy deposition of mordenite, a hydrothermal mineral of the sodic zeolites group. In this case, the mordenite is often associated with carbonates. Near 600 m, fibrous aggregates of illite replace plagioclase crystals. From 640 m on, quartz occurs. 3.2.3. Zone 3 (650-900 ml The host rock is represented by basaltic hyaloclastites with minor interbeds of lavas and some doleritic dikes. Hydrothermal alteration is very strong and is characterized by well crystallized chlorite, present at 710-750 m, associated with calcite and corrensite. Corrensite, associated with mordenite, replaces plagioclase microcrystals and hyaloclastite clasts. Quartz and albite microveins occur as secondary minerals in this zone. 3.2.4. Zone 4 (900-1203 ml The host rock seems to be like the previous zone except for an increase in subvolcanic lithologies (Fig. 3). This zone is characterized by the presence of ‘swelling’ chlorite and the lack of chlorite as a secondary mineral in the hyaloclastites. Corrensite is observed as far as a depth of 1000 m. In these

XRD data The study of the minerals was carried out using EDS microanalysis. The phyllosilicates group was the object of particular study also by X-ray powder diffractometry (XRD). The following minerals were identified: feldspars, quartz, sulphides, carbonates, sphene, barite, apatite and phyllosilicates. 4.1. Pantelleria I drillhole 4.1.1. Feldspar Hydrothermal feldspars identified here are represented by albite and adularia. These phases were found from 462 m to the bottom (Table 1). Both albite and adularia in this well show a stoichiometric composition which does not change with increasing depth. 4.1.2. Carbonates Calcite is the only carbonate identified and its chemical composition is quite homogeneous (Table 2); Ca is always more than 90 mol% with rare substitution of Mn, Fe. 4.1.3. Sulphides The sulphides present are pyrite and sphalerite; the first is more abundant and sometimes it shows substitutions of Zn for Fe. At 462 and 600 m sphalerite was found; sphalerite analysis at 462 m indicates little FeS (4 mol%), Ni and Mn. The amount of FeS in the analysis of sphalerite at 600 m is less than at 462 m, in fact, it remains under 1.5 mol% and there are no traces of Mn and Ni.

P. Fulignati et al. / Journal of Volcanology and Geothermal Research 75 (1997) 251-270

Table 1 Average structural

formulas

of adularia and albite Albite

Adularia Depth (m): No. of deter-m.:

462

528

1

Na,O AW, SiO,

257

600 1

6

685 1

462 1

600 2

765 6

1035 3

0.00 18.46 64.13 16.28

0.52 18.24 64.12 16.53

CaO TiO,

0.00 0.35

0.09 0.00

MnO Fe@,

0.00 0.23

0.00 0.23

0.00 1.15

0.00 0.00

0.12 1.87

0.00 0.18

0.00 0.21

0.00 0.05

0.00 0.32

Sum

99.45

99.73

100.08

99.59

99.62

98.78

99.99

99.95

100.00

Si Al

2.98 1.01

2.98 1.00

2.97 1.01

2.97 1.01

2.98 0.96

2.97

3.00

2.99

2.99

Ti Mn Fe3+

0.01 0.00 0.01

0.00 0.00 0.01

0.00 0.00 0.04

0.00 0.00 0.00

0.00 0.00 0.07

1.01 0.00

1.00 0.00

1.02 0.00

0.99 0.00

0.01

0.01

0.00

0.01

Na K Ca

0.00 0.97 0.00

0.00 0.98 0.00

0.03 0.94 0.00

0.05 0.97 0.01

0.00 0.93 0.01

1.02 0.01 0.00

0.99 0.00 0.00

0.98 0.01 0.01

1.01 0.01 0.00

K2O

Structural

formulas

Table 2 Average structural

have been calculated

formulas

0.35 18.52 64.15 15.91 0.00 0.00

1035 1

0.52 18.51 64.02 16.36 0.18 0.00

of sphene

Na,O MgO

610 5

685 3

CaO TiO, Fe0

0.00 0.00 5.54 32.35 0.05 28.49 27.20 5.49

1.69 0.47 1.14 31.42 0.24 24.99 28.93 6.45

Sum

99.12

95.33

Si

1.07

1.10

Mg Al Ti Fe

0.00 0.22 0.68 0.15

0.02 0.05 0.76 0.19

Na K Ca

0.00 0.00 1.01

0.11 0.01 0.93

A1203

SiO, K2O

Structural gens.

formulas

have been calculated

11.93 19.40 67.06 0.12 0.09 0.00

11.70 19.37 68.54 0.07 0.10 0.00

11.53 19.73 68.29 0.20 0.15 0.00

11.92 19.18 68.39 0.12 0.07 0.00

on the basis of 8 oxygens

Sphene Depth (m): No. of determinations:

17.56 64.29 15.63 0.15 0.00

on the basis of 5 oxy-

4.1.4. Sphene Sphene analysis indicates some substitution of Al and Fe3+ for Ti and, more rarely, Na for Ca (Table 2). The abundance of sphene seems correlated with the high-Ti content of Pantelleria volcanics. 4.1.5. Other secondary minerals Besides the previously mentioned minerals, other neogenic phases were identified: such as a subordinate amounts of stoichiometric barite and rare-earth rich apatite at 600 m. 4.2. Clay minerals Five minerals of the phyllosilicates group were identified: saponite, chlorite, mixed-layer chloritesmectite, illite, mixed-layer biotite-vermiculite. The XRD data are reported in Table 3. To calculate structural formulas, 22 Ox was used for saponite, illite and mixed-layer biotite-vermiculite; 28 Ox for chlorite and mixed-layer C/S. 4.2.1. Saponite Saponite was found from 500 m tn; this mineral has a characteristic d,, of 14.8 A after MgCl,

258

P. Fulignati et al. / Journal of Volcanology and Geothermal Research 75 11997) 2.51-270

Table 3 XRD data Depth

d,,

(ml

WA

320 401 500 609 670 800 905 1000 1100

14.71 14.55 15.10 14.81 14.90 14.79 14.76 14.78 13.88

(A, Glicole etil.

10.17 10.18

17.11 16.96 17.02 17.09 17.14 16.98 16.99 16.96 16.69

7.18 7.16 7.33

13.23

KC1 (550°C)

15.00-14.7-10.10 10.00

iden. phases

12.30 14.30-13.72 13.94

7.18 7.12

13.17

9.94 9.97 9.76 9.86 10.02 9.91 9.99 9.97 10.09

12.71

Sm, Ch, Sm, Sm Sm Sm Sm Sm. Sm

C/S C/S, Ill, Sm C/S, Ill

Hyd

Table 4 Average

structural

formulas

of saponites

Saponites Depth cm): No.of determ.:

462 8

500 6

610 21

SiO, TiO, Al,% Fe0 * MnO Mg“ CaO Na,O K,O

45.67 0.00 8.05 33.35 0.19 9.64 2.46 0.00 0.37

49.84 0.00 6.89 25.62 0.00 15.09 2.14 0.00 0.37

49.27

Total

99.73

Si Al(W)

715 6

835 13

905 16

970 5

1035 I

7.51 25.59 0.59 14.09 2.13 0.00 0.24

45.29 0.00 8.30 34.74 0.33 8.08 2.27 0.00 0.67

44.40 0.15 8.85 33.52 0.41 8.44 2.25 0.00 1.94

43.73 0.14 8.48 35.89 0.21 8.48 2.50 0.00 1.59

45.22 0.13 8.63 31.64 0.61 9.61 2.18

45.71 0.00 8.68 29.24 0.37 11.93 1.88

0.00 1.94

0.00 I .88

99.95

99.48

99.68

99.96

99.78

99.96

99.69

6.66 1.34

6.93 1.07

6.89 1.11

6.66 1.34

6.54 1.46

6.53 1.47

6.60 1.40

6.59 1.41

Al(W) Ti Fe Mn Mg

0.04 0.00 4.07 0.02 2.09

0.06 0.00 2.98 0.00 3.13

0.14 O.Oll 2.99 0.07 2.94

0.10 0.00 4.28 0.04 1.77

0.08 0.02 4.13 0.05

1.85

0.02 0.02 4.48 0.03 1.61

0.08 0.01 3.86 0.07 2.09

0.06 0.00 3.53 0.04 2.56

Total

6.22

6.17

6.14

6.19

6.13

6.16

6.11

6.19

Ca Na K

0.38 0.00 0.07

0.32 0.00 0.06

0.32 0.00 0.04

0.36 0.00 0.13

0.36 0.00 0.36

0.40 0.00 0.30

0.34 0.00 0.36

0.29 0.00 0.35

Total

0.45

0.38

0.36

0.49

0.72

0.70

0.70

0.64

14.67

14.55

14.50

14.68

14.85

14.86

14.81

14.83

0.66

0.49

0.50

0.7

0.69

0.74

0.65

0.58

Total cat. Fe/Fe

+ Me

Structural

formulas

have been calculated

0.00

on the basis of 22 oxygens;

1

total iron has been considered

as ferrous.

P. Fulignati et al. / Journal of Volcanology and Geothermal Research 75 (1997) 251-270

Al),O,,(OH), formula unit) were found. According to the classification of Suquet and PCzerat (19881, based on layer charge values (saponites X < 0.5 and vermiculites X > 0.7; in the interval between 0.5-0.7 it is not possible to distinguish saponites and vermiculites), a transition from saponites (600 m) to vermiculites (835-905 m) was observed. Intermediate terms occur from 905 m and at the bottom of the well. However, XRD study never identified vermiculite; therefore, if we consider that we are not sure of the difference between high-charge smectites and low-charge vermiculites based on chemical data, we can consider all these phyllosilicates as high-charge trioctahedral smectites. As regards the saponites of this well, the Fe/Fe + Mg ratio is another parameter which changes with depth. This ratio is equal to 0.48-0.52 at 600 m, increases to 0.74-0.76 at 905 m and then it decreases to 0.58-0.59 at 1035 m. Below 835 m, saponite has a small amount of Ti. At 528 m and up to 350 m, there was also identification of dioctahedral smectites, probably interstratified with saponites (Fig. 4).

treatment. This expands to 17 & with ethylene glycol solv$ion and collapses at 13 A and between 9.8 and 10 A after treatment with KC1 and after heating at 55O”C, respectively. Analysed saponites (see Table 4) are Fe-rich and poor in Al like those described in the groundmass of rhyolitic tuffs at Oya, Japan (Kohyama et al., 1973); these were defined by this author as Fe-saponites. K, Ca and, in a smaller quantity Mg, are the interlayer cations. Below 650 m smectites show a progressive increase of interlayer cations. In particular, there is an increase of K+(X) and a noteworthy substitution of A13+ for Si4+ in the tetrahedral sites. Such a substitution is supposed: K+(X)

+ A13+(IV) * Si4+(IV)

The increase of A13+(IV) and K+ seems to be related to a rise in temperature. We think this substitution was not produced by a variation in rock chemistry because chemical analyses showed quite a homogeneous composition throughout the well. The increase of cations in (X) sites with increasing depth (Table 4) shows an increase of layer charge: We pass from a charge of 0.43-0.46 at 600 m to 0.70-0.73 between 835 and 905 m. At the bottom of the well intermediate layer charges such as 0.60-0.65 (these values are calculated on the basis of the Mg,(Si,

21-

259

4.2.2. Chlorite Chlorite as a discrete phase was oaly identified at 401 m. Chlorite has d,, of 14.5 A after MgCl, treatment. This value undergoes very small variations both after ethylene glycol solvation and after

q chlorite (401m) layers c/S (401m) 0 mixed layers US (462m) 0 sapcmite (500.1000m) n smectite (528m)

l mixed

20~

chlorite

16-

151 0

I 1

r 2

I 3

Al128

4

5

6

7

Ox

Fig. 4. PPTl well. Sum of the major non-interlayer cations (Si + Al + Fe + Mg) versus Al. All analyses recalculated on a diagram proposed by Schiffman and Fridleifsson (1991).

on 28 Ox basis. Based

P. Fulignati et al. / Journal of Volcanology and Geothermal Research 75 (1997) 251-270

260

heating at 550°C (14.3 A). The limited variation in d,, after ethylene glycol treatment probably represents a combination of discrete chlorite and chloriterich mixed-layer C/S (Schiffman and Fridleifsson, 1991; Purvis, 1990). EDS analyses of chlorite are shown in Table 5. Considering the classification of Bayliss (Bayliss, 1975) these chlorites are defined as Fe-clinochlore or Mg-chamosites. Chemical analyses show Fe/Fe + Mg and Al/Al + Fe + Mg relations of 0.69 and 0.3 l-0.33, respectively. The chlorites analyzed show octahedral vacancy of 0.3-0.5 mo1/28 Ox. This

Table 5 Average structural

formulas

of chlorites,

mixed layers C/S,

phengites

4.2.3. Mixed-layer chlorite-smectite Mixed-layer C/S were identified between 320 and 500 m. XRD study of this interlayered C/S showed wider and less defined peaks than discrete chlorite; this means these are random interlayers. The d,, is 14.5 A after MgCl, treatment, which

and mixed layers biotite-vermiculite

Chlorites

Mixed-layer

Depth (m): No. of determ:.

407 3

407 6

462 7

407 2

1035 1

835 3

970 5

1035 3

SiO 2 TiO,

34.39 0.00 17.86 36.44 2.27 7.89 0.61 0.00 0.47

37.71 0.06 12.80 36.93 0.86 10.29 1.04 0.00 0.25

57.88 0.00 18.07 12.40 0.00 2.32 0.20 0.00 8.95

60.75 0.00 14.36 9.33 0.23 3.90 0.21 0.00 10.36

43.39

K2O

32.13 0.00 18.25 37.88 1.89 9.24 0.55 0.00 0.00

0.53 8.94 1.20 0.00 5.25

42.93 0.22 10.32 27.69 0.34 11.55 1.11 0.00 5.70

44.47 0.31 10.62 27.89 0.33 10.73 1.05 0.00 4.55

Total

99.94

99.93

99.94

99.82

99.14

99.96

99.86

99.95

Si Al(W)

6.24 1.I6

6.62 1.38

7.21 0.79

7.60 0.40

7.98 0.02

6.49 1.51

6.30 1.70

6.44 1.56

AI Ti Fe Mn

2.42 0.00 6.15 0.31 2.67

2.67 0.00 5.87 0.37 2.26

2.10 0.01 5.91 0.14 2.93

2.40 0.00 1.36 0.00 0.45

2.20 0.00 0.03 0.76

0.00 0.02 4.03 0.07 1.99

0.08 0.02 3.40 0.04 2.53

0.25 0.03 3.38 0.04 2.32

11.55

11.17

11.09

4.21

4.01

6.11

6.07

6.02

Ca Na K

0.11 0.00 0.00

0.13 0.00 0.11

0.21 0.00 0.06

0.03 0.00 1.50

0.03 0.00

1.73

0.19 0.00 1.00

0.17 0.00 1.07

0.16 0.00 0.84

Total

0.11

0.24

0.27

1.53

1.76

1.19

1.24

1.oo

11.66

19.41

19.29

13.74

13.77

15.30

15.31

15.02

0.70

0.72

0.67

0.75

0.57

0.67

0.57

0.59

A’203

Fe0 * MnO MgO CaO Na,O

Mg Total

Total cat. Fe/Fe

+ Mg

C/S

vacancy in the octahedral sites represents a common characteristic of chlorites of hydrothermal origin (McDowell and Elders, 1980; Bettison and Schiffman, 1988; Cathelineau, 1988; Schiffman and Fridleifsson, 1991; Shau and Peacor, 1992).

Mixed-layer

Phengites

Structural formulas have been calculated on the basis of 22 oxygens for phengites chlorites and mixed layers C/S; total iron has been considered as ferrous.

1.02

0.22 8.22 32.21

biotite-vermiculite

and mixed layers biotite-vermiculite,

and 28 oxygens for

P. Fulignati et al. / Journal of Volcanology and Geothermal Research 75 (1997) 2.51-270

expands to 15 A after ethylene glycol solvation, and ultimately collapses to 13.7 A after heating at 550°C with an asymmetrical peak. This asymmetry indicates the presence of smectite in the mixed-layer C/S (Bettison-Varga et al., 1991). The percentage of chlorite in the mixed-layer C/S was derived approximately by using the diagram of Fig. 5. In this case, a dcm, of 15 A represents 80% chlorite. Table 5 shows EDS analyses of these interlayers. It is possible to distinguish the mixed-layer C/S from chlorite (Fig. 6) due to their greater number of interlayer cations (Na, K, Ca > 0.10 cations/28 Ox) and to a greater amount of Si(IV) cations (> 6.25 cations/28 Ox) (Bet&on and Schiffman, 1988). The diagram of Fig. 4 represents compositional differences between smectite, chlorite and their interlayer terms. Along the straight-line between saponite and chlorite there are terms that are more stable with increasing temperature. Along the saponite-beidellite straight-line, terms are stable with decreasing temperature (Schiffman and Fridleifsson, 1991). The spots corresponding to phyllosilicate analyses at 407 m fall near the saponite-chlorite straight-line and they are very near the chlorite composition. Analyses of interlayers at 462 m fall in the middle of the saponite-chlorite straight-line, showing a higher amount of saponite than chlorite analyzed at 407 m. At 500, 528 and 600 m our analyses indicate almost discrete saponites; the EDS data therefore confirm the results of the XRD study. In this well we note quite a progressive transition from chlorite to saponite with depth, contrary to other geothermal wells stud-

%

chlorite

Fig. 5. Peak migration curves for ethylene glycol solvated, randomly interstratified 001 chlorite/O01 smectite (after Reynolds, 1980).

261

Fig. 6. PPTl well. Sum of interlayer cations (Na + Ca + K) versus Si content of smectite, saponite, C/S and chlorite. All analyses recalculated on a 28 Ox basis.

ied (McDowell and Elders, 1980; Cathelineau et al., 1985; Schiffman and Fridleifsson, 1991). The Fe/(Fe + Mg) ratio varies from 0.48 to 0.75 and does not seem correlable with saponite-chlorite transition. In fact, some saponites have the same Fe/Fe + Mg value as chlorite and mixed-layer C/S. 4.2.4. Illite Illite was found between 400 and 500 m and at a depth of 1000 m. XRD analyses give a d,, of 10.1-10.2 A in the sample treated with MgCl,; after K+ treatment and heatirtg at 550°C d,, collapses to approximately 9.8-10 A. This is perhaps caused by the presence of ‘smectite’ layers inside the illite structure. EDS analyses on illites show some substitutions of Fe*+ and Mg*’ for A13+ in the octahedral sites, so we can consider these micas as phengites. The amount of K is always less than 2 (22 Ox). At 407 and 1035 m illites were found with K of 1.5 and 1.73 cations, respectively. The filling of interlayers of illites and particularly the amount of K, generally present a good positive correlation with the temperature of crystallization (Cathelineau, 1988). This agrees with the thermometric log of the well. 4.2.5. Biotite-vermiculite Mixed-layer biotite-vermiculite was identified below 685 m. The X-ray investigation shows a d,, of 13.3 A in the sample saturated with Mg*+. It does not change after ethylene glycol treatment, while it collapses to 12.7 A after K’ treatment and heating at 550°C.

P. Fulignati et al./Joumal

262

of Volcanology and Geothermal Research 75 (1997) 251-270

interval. The analyses show little substitution of Ca for Na (K is always absent) and Fe3+ for A13+ (Table 6).

.

0.0 0.0

1.0

0.5

1.5

2.0

K

4.3.3. Dolomite Dolomite occurs only at a depth of 500 m; the analyses show a stoichiometric composition with small amounts of Fe per Mg.

W

‘1 “.

I .

0.0

1.0

0.5

1.5

2.0

K Fig. 7. PPTl well. Variation diagrams of Ca and Fe + Mg versus K for biotite-vermiculite mixed layers.

EDS study revealed the presence of Ca (Table 5) in the structure of this interlayer. Ca shows a negative linear correlation with K (Fig. 7a), whereas (Fe + Mg) has an almost constant value with varying K (Fig. 7b). For this reason we may affirm that Ca is the dominant interlayer cation in the vermiculite layers, while Mg occurs in small quantities (McDowell and Elders, 1980). As regards biotite, the K leaching and Ca enrichment trend is usually correlated with an increase of vermiculite alteration (Craw, 1981; Craw et al., 1982; Moon et al., 1994). Therefore, these investigated phyllosilicates can be considered as biotites partially transformed into vermiculites. A similar case was observed for mixed-layer biotite-vermiculite in a Salton Sea geothermal well (McDowell and Elders, 1980). The Mg/(Mg + Fe) value generally increases with depth. 4.3. Borehole Pantelleria

4.3.2. Calcite In the Pantelleria 2 well, calcite occurs at depths between 600 and 1100 m. Particularly between 700 and 900 m calcite is the only carbonate which was found. The calcite analyzed shows little substitution of Fe, Mg and Mn for Ca.

2

4.3.1. Albite

Albite is the only hydrothermal feldspar identified in this well and it was found only in the 750-900 m

4.3.4, Siderite Siderite is rarely found as a pure phase. The most common cations which substitute Fe’+ are Mn’+ and Mg2+ and there is a totally solid solution between siderite and rhodocrosite and between siderite and magnesite. In this well pistomesite is found (30% < MgCO, < 50%) from 400 m of depth except in the interval of 750-900 m. Sideroplesite (5% < MgCO, < 30%) is quantitatively subordinate to the Table 6 Average structural

formulas of albite and mordenite Mordenite

Albite

Depth (m): No. of determinations:

1100 6

750 2

900 3

Na,O MgO Al,% SiO, K,O CaO Fe&+

5.65 0.26 13.03 78.77 0.80 1.15 0.08

10.81 0 20.19 68.47 0 0.32 0.16

10.94 0 22.64 64.74 0 0.16 0.14

Sum

99.74

99.95

98.62

Si Mg Al Fe3+

40.16 0.20 7.83 0.04

2.98 0 1.04 0

2.87 0 1.18 0

5.58 0.52 0.63

0.91 0 0.01

0.94 0 0.01

Na K Ca

Structural formulas have been calculated on the basis of 8 oxygens for albite and 96 oxygens for mordenite.

263

P. Futignafi et af. / Joumat of VoJcanology and Geothermal Research 75 f1997)251-270 Table 7 Average structural

of calcite, dolomite,

breunnerite,

Calcite

siderite and ankerite Dolomite

Breunnerite

Siderite

Ankerite

Depth (m): No. of determ.:

600 3

750 3

900 3

1100 2

500 3

600 4

500 4

1100 6

1200 1

500 4

1100 7

1200 1

MO CaO MnO Fe0

1.49 94.01 1.63 2.85

0.19 97.84 1.13 0.71

0.81 97.3 1.02 0.84

1.21 92.9 2.54 3.32

42.79 55.4 0 1.77

45.79 15.3 0 37.75

24.27 4.22 2.26 68.39

19 7.38 4.33 69.27

26.35 3.98 0 69.66

29.87 48.52 1.22 20.37

28.75 5 1.23 0.83 18.53

28.63 46.27 1.16 23.92

Sum

99.98

99.87

99.97

99.97

99.96

98.84

99.14

99.98

99.99

99.98

99.34

99.98

0

1

1

1

1

2

1

1

1

1

2

2

2

Mg Ca Mn Fe

0.02 0.94 0.01 0.02

0 0.98 0.01 0.01

0.01 0.97 0.01 0.01

0.02 0.94 0.02 0.03

1.02 0.95 0 0.02

0.59 0.14 0 0.27

0.36 0.05 0.02 0.57

0.29 0.08 0.04 0.59

0.39 0.04 0

0.78 0.91 0.02 0.3

0.75 0.96 0.01 0.27

0.75 0.88 0.02 0.35

pistomesite and was identified 1100 m (Table 7).

only at a depth of

4.3.5. Magnesite Magnesite forms a completely solid solution with siderite while the substitutions of Mn and Ca for Mg are limited. Magnesite-rich Fe is defined as breunnerite and it can contain Fe between 5 and 50 mol% (Deer et al., 1967). Breunnerite was found at 600 and 1100 m depth. The amount of FeCO, in these analyses varies from 17 to 35 mol% without any relation with depth. 4.3.6. Ankerite In the Pantelleria 2 well we found ankerite in the 500-600 m and 1085-1200 m intervals. The composition of this mineral results uniform, showing limited substitution of Mn and Mg (max. 2 mol%). 4.3.7. Mordenite This silica-rich zeolite is abundant between 1000 and 1115 m. The X-ray investigations reveal a characteristic reflection (020) at 9.07-9.08 A. The chemical composition of mordenite from the EDS analyses shows a high Na content (average Na/K = 9.71). The content in tetrahedral atoms, which can be expressed by the (Si/Si + Al) ratio, varies from 0.83 to 0.84, in agreement with the range (0.80-0.85) known for this zeolite (Passaglia, 1975).

0.57

4.3.8. Sulphides and other minerals Sulphides are not abundant in this well and the only phases identified are: pyrite, in the 600-1200 m interval and chalcopyrite, analyzed only at 750 m depth. Other hydrothermal minerals were identified in small quantities: anhydrite at 750 m and apatite.

4.4. Clay minerals In the Pantelleria 2 well, five types of phillosilicates were identified using X-ray investigation: smectite, corrensite, rectorite, chlorite and ‘swelling’ chlorite. X-ray analysis data are shown in Table 8. Structural formulas are calculated on the basis of 22 Ox for smectite and rectorite; 28 Ox for chlorite and 25 Ox for corrensite.

4.4. I. Smectite Smectite was identified throughout the well except at 700-750 m. In the samples of the fir$ 650 m, XRD analysis reveals a d,, at 15.0-15.3 A after MgCl, treatment, while below 700 m smectites have a d,, between 14.6 and 15 A. The d,, expands to 17 A after ethylene glycol solvation for the samples from the first 600 m. This expansion is smaller (d,, = 16.7 A) between 650 and 1000 m. Incomplete expansion of smectite is probably caused by some percentage

P. Fulignati et al. / Journal of Volcanology and Geothermal Research 75 (1997) 251-270

264 Table 8 XRD data Depth

d,,

(ml

M&l,

305 385 601 650 700 800 900 1005 1115 1203

29.87 29.62 29.70 29.32 24.94

(A, Glicole etil. 15.36 15.54 14.90 15.29 14.56 14.86 14.60 15.00 15.11 14.79

12.32

7.11 7.18 7.17 1.20 7.21 7.17

31.19 30.97 30.44 31.75 26.79

17.04 17.17 17.76 16.78 16.83 16.53 16.77 16.74 17.12 17.06

KC1 (550°C)

Iden. phases

13.51 9.61 15.04 14.74

13.48

7.14 7.16 7.18 7.27 7.23 7.19

22.96

13.99 13.75 13.84 13.97 14.12

23.26 23.28 22.26

11.94 12.01 11.90 12.20 10.98

9.96 9.75 9.78 9.93 9.79 9.69 9.72 9.68 9.73 10.26

Sm Sm Sm, Ill(?) Sm Ch, Co, Sm Ch, Co, Sm SCh, Co, Sm SCh, Co, Sm SCh, Sm Rect, Sm

dioctahedral and a &octahedral smectite. In fact, octahedral cations vary from 4.5 to 5.1 per formula unit. These phyllosilicates represent dioctahedral smectite interlayers (Fig. 8).

of chlorite within the mineral structure. At the bottom d,, corresponds to 17 A. For eve? sample, the basal peak collapses to about 9.8 A after KC1 treatment and heating at 550°C. Both dioctahedral(1 lo-1203 m) and trioctahedral (900 m) smectites were identified. Trioctahedral smectites are characterized by an Al amount higher than saponite (Fig. 81, due to the presence of limited chlorite interlayers in their structures. The Fe/(Fe + Mg) ratio is very low, from 0.34 to 0.38. At 460, 600, 1100 m we found smectites with a number of cations in the octahedral site intermediate between a

4.4.2. Chlorite Chlorite was found0 between 700-800 m and it has a d,, of 14.5 fi after MgCl, treatment; it expands to 14.7- 15 A after ethylene glycol treatment. This limited expansion means that in the chlorite there are minimal amounts of expandable phyllosilicates such as smectites (Purvis, 1990; Schiffman and Fridleifsson, 1991). In the sample treated

21 Al chlorite(750m)

,

l corrensib (750.900m)

20-

154. 0

D 0

seponite (900,1lOOm)

chlorite

smecGte (460,600,1100,1200m)

1.I. 1

2

I' 3

0x4

5

6

Al128 Fig. 8. PPT2 well. Sum of the major non-interlayer cations (Si + Al + Fe + Mg) versus Al. All analyses Based on a diagram proposed by Schiffman and Fridleifsson (1991).

recalculated

on a 28 Ox basis.

265

P. Fulignatiet al./ Journal of Volcanologyand Geothemal Research 75 (1997) 251-270

mulas of these chlorites have Si > 6.25 cations/28 Ox, which represents the highest value for a discrete chlorite (Bettison and Schiffman, 1988). The Fe/(Fe + Mg) ratio is between 0.46 and 0.50.

with KCl, d,, collapses to 13.8 A after heating at 550°C. By means of the diagram in Fig. 5, we can estimate a quantity of chlorite from 80 to 95%. EDS analyses are shown in Table 9. The chlorites analysed can be classified as Mg-Si-chamosite, following the classification of Bayliss (1975). As seen for the Pantelleria 1 chlorites, these chlorites also have some octahedral vacancy of 0.50-0.35 cations. As regards the chlorites of the Pantelleria 2 well, we can not exclude, however, that the octahedral vacancy in their structure could be correlated with the presence of some ‘smectite’ layers (Schiffman and Fridleifsson, 1991). In fact, structural forTable 9 Average structural

formulas

of chlorites,

corrensites,

saponites,

4.4.3. ‘Swelling ’ chlorite This particular type of chlorite typically expands after ethylene glycol solvation but does not collapse after heating to 550°C (Bain and Russel, 1981; Wilson, 1987). By X-ray diffraction the presence of ‘swelling’ chlorite between 900 and 1115 m was discovered. In the sample: treated with MgCl, d,, corresponds to 14.6-15.1 A. It expands to 16.7-17.1

smectites and rector&es

Chlorites

Corrensites

Rectorites

Saponites

Smectites

Depth (m): No. of determ.:

750 2

750 3

900 2

1100 2

1200 7

900 7

460 2

600 4

1100 5

1200 3

SiO, TiO, Fe0 * MnO MgO CaO Na,O K,O

34.78 0.00 18.25 28.89 0.15 17.49 0.36 0.00 0.00

37.29 0.00 18.04 27.60 0.21 15.59 0.74 0.00 0.29

38.82 0.00 16.17 14.42 0.00 19.53 0.79 0.16 0.00

36.79 0.00 18.33 26.89 0.00 16.87 0.70 0.23 0.04

56.31 0.00 32.11 1.69 0.00 0.95 1.16 0.15 6.90

50.44 0.00 11.68 17.83 0.00 17.22 1.84 0.37 0.42

60.99 1.14 16.93 12.50 0.00 4.94 1.87 0.00 1.Ol

62.91 0.03 17.14 10.23 0.00 6.51 2.19 0.39 0.47

62.50 0.00 19.98 6.72 0.03 5.75 1.45 0.77 1.86

67.92 0.00 22.04 4.87 0.00 3.21 0.45 0.00 1.19

Total

99.77

99.76

99.89

99.85

99.27

99.80

99.38

99.87

99.06

99.68

Si Al(IV)

6.39 1.61

6.06 1.94

6.19 1.81

5.95 2.05

6.96 1.04

6.74 1.26

7.71 0.29

7.80 0.20

7.72 0.28

8.00 0.00

Al(W) Ti Fe Mn Mg

2.34 0.00 4.44 0.00 4.79

1.51 0.00 3.75 0.03 3.77

1.23 0.00 3.26 0.00 4.64

1.44 0.00 3.63 0.00 4.06

3.64 0.00 0.17 0.00 0.17

0.58 0.00 1.99 0.00 3.43

2.23 0.11 1.32 0.00 0.93

2.30 0.00 1.06 0.00 1.20

2.63 0.00 0.69 0.00 1.06

3.09 0.00 0.48 0.00 0.57

Total

11.57

9.06

9.13

9.13

3.98

6.00

4.59

4.56

4.38

4.14

Ca Na K

0.07 0.00 0.00

0.13 0.00 0.06

0.13 0.05 0.00

0.12 0.07 0.01

0.15 0.04 1.09

0.26 0.10 0.07

0.25 0.00 0.16

0.29 0.09 0.07

0.19 0.18 0.29

0.06 0.00 0.18

Total

0.07

0.19

0.18

0.20

1.28

0.43

0.41

0.45

0.66

0.24

Total cat.

19.64

17.25

17.45

17.35

13.26

14.43

13.00

13.01

13.04

12.38

Fe/Fe

0.48

0.50

0.41

0.47

0.50

0.37

0.59

0.47

0.39

0.46

A1203

+ Mg

Structural formulas have been calculated on the basis of 22 oxygens oxygens for chlorites; total iron has been considered as ferrous.

for smectites,

saponites and rectorites;

25 oxygens for corrensites;

28

P. Fulignati et al. / Journal of Volcanology and Geothermal Research 75 (1997) 251-270

266

A after ethylene glycol treatment and ultimately it presents limited contraction to 13.9 A. 4.4.4. Corrensite Corrensite is defined as a 1: 1 regular interstratification of trioctahedral chlorite with trioctahedral smectite or trioctahedral vermiculite (Bailey, 1981). Distribution of corrensite in the Pantelleria 2 well is limited to the interval of 700-1005 m. XRD analyses show jhat this phyllosilicate has the d,, of 29.32-29.87 A in the sample treated with MgCl,; it expands to 30.44-3 1.75 after ethylene glycol solvation and collapses to about 23 A after KC1 treatment and heating at 550°C. The peak of this last d,, is generally not clear; we note, however, the appearance of a clear peak at 12 A, which should be the (002) reflection (Blatter et al., 1973; Wilson, 1987; Shau et al., 1990). Corrensite data are reported in Table 9. Fig. 9 illustrates some differences of composition between smectite, saponite, chlorite and corrensite. Data reported in this diagram do not show 0.8 0.7 0.6 fO.5 9

0.4

a! 0.3 0.2 0.1 0.0

0.7

0.6

0.5

0.9

0.8

1 .o

Sii Si+AI 0.8 0.7

4.4.5. Rectorite Rectorite consists of a regular 1: 1 illite-smectite interstratification (Wilson, 1987). It was found only at the bottom0 of the well (1203 m). Rectorite has d,, of 24.9 A after MgCl, treatment. The dO02ff 12.3 A is also very0 clear. d,, expands to 26.8 A, while d,, to 13.5 A after ethyltne glycol solvation. Finally, d,, collapses to 22.3 A and do,, to 11 A after KC1 treatment and heating to 550°C. The appearance of rectorite in the last 50 m of the Pantelleria 2 drillhole is simultaneous with the disappearance of mixed-layer C/S. The structural formula calculated by EDS analyses indicates, for this mineral, a relatively uniform composition (Table 91. A representative structural formula calculated on the basis of rectorite analyses is the following:

chlorite . corrensite 0 saponite

q

(W

0.6 z” 0.5

This mineral is K-Al rich and Na-Mg poor. It can be considered, therefore, as a regular interstratification between K-mica layers and beidellite layers (Kawano and Tomita, 19911.

i IA 0.4 z

a regular smectite-chlorite transition with increasing depth, as has been found in other geothermal wells (McDowell and Elders, 1980; Cathelineau et al., 1985; Schiffman and Fridleifsson, 1991). At 1100 m, for example, corrensite, swelling chlorite and dioctahedral smectites co-exist. This transition is probably hidden by retrohydrothermalism which influenced this zone. Moreover, regarding differences of composition between saponite, chlorite and corrensite, we note a general increase of the Fe/(Fe + Mg) ratio with a decrease of the Si/(Si + Al) ratio and the sum of interlayer cations, passing from saponite to corrensite to chlorite (Fig. 9a-b). These phyllosilicates show an analogous trend in the analyses of hydrothermal oceanic association of the Costa Rica rift and offshore Taiwan (Shau et al., 1990; Shau and Peacor, 1992).

0.3 0.2 0.1 0.0 ^, U.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0:s

5. Chlorite geothermometer

Ca+K Fig. 9. PPT2 well. Saponite-corrensite-chlorite compositional variations in Fe/Fe + Mg versus Si/Si + Al and Ca + K diagrams.

Chlorite composition was used to estimate the temperature of crystallization by means of a geother-

P. Fulignati et al./ Journal of Volcanology and Geothemud Research 75 (1997) D-270

mometer proposed by Cathelineau (1988). The correlation between temperature and number of Al(IV) cations is the following: T (“C) = -61.92 + 321.98(Aliv) based on a structural formula such as: (Fe, Mg),(Si,

Al),O,,(OH),. Thisgeothermometer was applied to the chlorites of 407 m for Pantelleria 1 and 750 m for Pantelleria 2. The following values of temperature were derived: 210-242°C in the Pantelleria 1 well and 183-213°C in the Pantelleria 2 well. In both cases, these values are greater than those measured by direct thermometric tests reported in Figs. 2 and 3. If this geothermometer is considered to be valid, then in the past the temperature in both the wells must have been greater than now.

6. Discussion The geothermal wells drilled on Pantelleria island, which led to the discovery of a geothermal field on the island, give information about the hydrothermal system present in the subsurface of the volcanic complex. The structural setting of Pantelleria can be divided into two distinct main sectors, the central south, dominated by a resurgent active nested caldera, the northwest dominated by basaltic eruptions (Mahood and Hildreth, 1986; Civetta et al., 1988). The results of these boreholes emphasized the differences between the two areas, showing the presence of a high-temperature active hydrothermal system within the caldera and of a low-temperature, low-permeability hydrothermal system which has developed just outside of the caldera. In the southern sector of the Cinque Denti caldera (PPTl drillhole) there is a very high temperature anomaly. Temperatures measured were 240°C at 600 m and 270°C at 1100 m. On the northern caldera edge, the temperature anomaly is much smaller. 90°C was measured at 600 m and 125°C at 950 m (Figs. 2 and 3).

267

These rocks develop between 200 and 500 m, on previous ignimbritic lithologies. The reservoir is a water-dominated type, with temperatures near boiling point (Chierici et al., 1995a,b). It develops in trachitic-comenditic lavas which are very fractured and hydrothermalized. The reservoir, within trachitic lavas crossed by dikes, is characterized by an association with albite, adularia, calcite, quartz, sphene, sulphides. Fe-saponite is the stable phyllosilicate in this permeable zone. The simultaneous crystallization of adularia and calcite may suggest that there are boiling fluids, according to the measured temperatures (Fig. 2) in agreement with observations made regarding other geothermal systems (Browne and Ellis, 1970; Browne, 1978). The permeable zone extends for several hundred meters within a microsyenitic body strongly hydrothermalized to 1000 m. In this lower part of the reservoir we can observe a change of composition of saponite; in fact there is an increase of the layer charge with depth and temperature. This should suggest a probable transition from saponite to vermiculite, defining a characteristic hydrothermal zone. Vermiculite has been found in layers of transition from chlorite zones to biotite zones in other hydrothermal systems (Salton Sea, McDowell and Elders, 1980) and in metamorphic settings (Velde, 1978). The equilibrium temperatures indicated by these authors completely agree with the measured temperature of the PPTl well for the stability zone of these phyllosilicates (250-270°C). The coexistence of mixed-layer biotite-vermiculite and high-charge saponite (vermiculite) below 680 m confirms the presence of a well defined hydrothermal zone. In a previous paper De Vivo et al. (1992), studying intrusive and subvolcanic xenoliths, suggest the lack of well developed hydrothermal systems in the subsurface of the island. The results of our study partially contrast these conclusions. A hydrothermal system develops above a microsyenitic subvolcanic body. Moreover, the outer portion of the intrusion is affected by hydrothermal circulation.

6.1. Intracaldera area 6.2. Pericaldera area The upper part of the geothermal field identified is composed of rocks self-sealed by silica and clay minerals, which represent the cover of the field.

The lithologic profile of the pericalder hydrothermal system is very different from that seen in the

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previous area, showing that the drilling crosses a sector of the volcanic complex uninfluenced by volcano-tectonic collapses. The PPT2 well, in the northwest area of Pantelleria, shows a low-temperature hydrothermal alteration ( < 2OO”C),similar to low-temperature areas of other hydrothermal systems (Browne, 19781, developed within basaltic-hyaloclastitic lithologic sequences. The PPT2 well was drilled in a sequence self-sealed by pervasive argillization of hyaloclastitic units and lavas and by deposition of carbonates. This is confirmed by the stabilized thermic profile of the survey. The monotony of the above-mentioned hydrothermal system, Fig. 3, is interrupted at about 700 m by the appearance of a chlorite, corrensite, albite and calcite association in the doleritic dikes. This mineral association indicates the development of hydrothermal reactions with temperatures higher than the low-temperature association of the rest of the well. In particular, crystallization of calcite instead of Fe-carbonates is correlated to a higher temperature of deposition (Browne and Ellis, 1970; McDowell and Elders, 1980). This restricted hydrothermal zone could have been created by the intrusion of the doleritic dikes mentioned above. The prevalence of stable Fe-carbonates in the cooling zones of the geothermal systems (Reyes et al., 1993; Simmons and Christenson, 1994) suggests that this pericaldera area represents a flow area of cold fluids at the interface with the intracaldera hydrothermal plume.

7. Hypothesis of evolution of the hydrothermal system The discussion of the previous paragraph concerns the mineral association in equilibrium with present temperatures referred to the cover and reservoir units of the intracaldera geothermal field. Some considerations can be made about the evolution of temperatures and about the changes in permeability of the hydrothermal system suggested by the superimposition of contrasting mineral associations. The hypothesis concerning the history of the hydrothermal system is the following. There is a phase where, near the subvolcanic body, biotite isograd is reached and a probable alkali-metasomatism zone (trioctahedral micas and chess-board albitization) de-

velops, which influences the subvolcanic body itself. The occurrence of alkali-metasomatism processes in the subsoil of the island is also suggested by the presence of high-T brines in fluid inclusions in granitoid rocks studied by De Vivo et al. (1992). This initial phase could be correlated with the development of a chlorite-albite-adularia zone extending to 400 m. After this phase, a diffused cooling phase (nearly 50°C) of the hydrothermal system follows. This is confirmed by the substitution of biotite by mixed-layer biotite-vermiculite and by the crystallization of Fe-rich saponite instead of chlorite, within the currently active reservoir (500-750 m). Saponite is found as typical phyllosilicate of a high-T reservoir also in other geothermal fields (Miles, Greece and Chipilapa, El Salvador); it is considered as a characteristic metastable phase of a newly-formed geothermal reservoir where high fracture permeability and boiling fluid are present (Papapanagiotou et al., 1995; Patrier et al., 1996). The zone from 400 to 500 m, sealed by high-T phases and silica mineral deposition, did not undergo a re-equilibrium of temperatures of around 160°C to present temperatures probably because of the almost total impermeability of the rocks. Chlorite-saponite transition (Fig. 4) occurs through a zone of deposition of mixed-layer C/S at 465 m, which should represent the transition to the currently permeable zone. Retrohydrothermalism is also observed in the PPT2 well. This is evident from the coexistence of low-T dioctahedral smectites and a corrensite-rectorite-swelling chlorite association. Such retrohydrothermalism phenomena have been observed in several geothermal systems (Hulen and Neilson, 1986; Flexser, 1991) and they can be simply justified by the short geological life of the hydrothermal systems. From the point of view of connection between volcanological evolution of the island and hydrothermal system evolution, the first phase could date back to the 44-37 ka period, in which the isostatic readjustment of the volcanic system after the caldera forming Green Tuff eruption occurred. The intrusion in the bottom of PPTl could go back to that age, if we consider that the sub-volcanic rocks have the same composition of the rocks erupted in that period, as reported in the paragraph on petrography. In this case, the cooling phase could be corre-

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lated to the volcano-tectonic rising of the Montagna Grande horst between 18 and 3 ka and to subsequent strong fracturing of the central area of the caldera, which could have played the role of an infiltration area for cool fluids. The possibility that the early stage of hydrothermalism could be correlated to the recent phase of activity (18-3 ka) is, in our opinion, remote. This is because there is an excessively short time interval for a phase of increasing hydrotbermalism to be followed by a phase of retrohydrothermalism. Moreover, the composition of the sub-volcanic body does not match the composition of the younger erupted products, which are pantelleritic in composition.

Acknowledgements The authors are indebted to Gail Mahood and to Roberto Scandone for the review of the manuscript that greatly improved its quality. We thank Agnese Bilancieri for the drawings. The authors gratefully acknowledge CESEN Spa and the Ente Minerario Sicilian0 for the permission to study the samples and to publish the data collected in the framework of a Research Contract between Pisa University and CESEN Spa. In particular, we thank R. Nannini and R. Chierici for the friendly collaboration and discussions during the drilling of the boreholes.

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