Ein geologisches Modell des magmatisch-hydrothermalen Systems von Vulcano, Aeolische Inseln, Italien

Mineralogy and Petrology (1998) 62:195-222 Mineralogy

Pe o logy © Springer-Verlag 1998 Printed in Austria

Geologic model of the magmatic- hydrothermal system of Vulcano (Aeolian Islands, Italy)

P. Fulignati, A. Gioncada, and A. Sbrana

Dipartimento di Scienze della Terra, Universit~ di Pisa, Pisa, Italy

With 7 Figures

Received April 21, 1997; revised version accepted October 27, 1997

Summary

This paper presents a model of the active magmatic-hydrothermal (high-sulfidation) system of La Fossa volcano, based on mineralogical and geochemical studies of hydrothermal alteration on the surface and in the subsoil (geothermal wells and lithic clasts from explosive eruptions).

The main engine of this system is represented by the shallow magmatic feeding system of La Fossa, which produces substantial degassing of volatiles (H20, S, C1).

The introduction of magmatic fluids into the conduit system causes high temperature recrystallisation and metasomatism of the volcanic and sub-volcanic rocks. Lateraly to the volcanic conduits, the magmatic fluids undergo a primary neutralization, forming neutral low permeability hydrothermal zones. During their rise to the surface, the magmatic vapours may condense in groundwater, forming acid solutions that react with rocks to form superficial hydrotherrnal alteration. Silicic, advanced argillic and intermediate argillic alteration facies develop. This reflects the progressive neutralisation of extremely acid fluids. High contents of trace elements, like T1 and Bi, supporting evidence for magmatic fluid transport, were found close to the high temperature fumaroles (up to 500 °C) in the silicic alteration zone of La Fossa.

Zusammenfassung

Ein geologisches Modell des magmatisch-hydrothermalen Systems yon Aeolische Inseln, Italien

Vulcano,

Diese Arbeit stellt ein Modell ftir das aktive magmatisch-hydrothermale (high- sulfidation) System des La Fossa Vulkans vor. Dieses beruht auf mineralogischen und

196 R Fulignati et at.

geochemischen Studien der hydrothermalen Umwandlung an der Oberfl/iche und im Untergrund (geothermale Bohrungen und lithische Klasten von explosiven Eruptionen). Als Energiequelle fungiert das seichte magmatische Zufuhrsystem yon La Fossa, das signifikante Entgasung volatiler Phasen (H20, S, HC1) mit sich bringt. Das Eindringen magmatischer Fluide in die Zufuhrkan~ile vemrsacht Rekristallisation und Metasoma- tose der vulkanischen und subvulkanischen Gesteine bei hohen Temperaturen. In lateralen Bereichen der vulkanischen Zufuhrkaniile effahren die magmatischen Fluide eine primfire Neutralisation, wobei neutrale hydrothermale Zonen niedriger Permea- bilit/it entstehen. W~ihrend des Aufstiegs an die Oberfl/iche k6nnen die magmatischen Fluide im Grundwasser kondensiert werden, wobei sie saure L6sungen bilden, die wiederum mit den Gesteinen reagieren und zu Hydrothermalalteration fiihren. Dabei entstehen silizische, fortgeschrittene argillische und intermedi~ire agillische Umwand- lungsfazies. Dies entspricht der zunehmenden Neutralisation extrem saurer Fluide. Hohe Gehalte an Spurenelementen, wie T1 und Bi k6nnen als zus~itzliche Hinweise ftir magmatischen Fluidtransport gesehen werden; sie treten in der Niihe der Hoch- Temperatur-Fumarolen (bis 500 °C) in der silizischen Alterationszone yon La Fossa auf.

1. Introduction

Vulcano is part of the Aeolian volcanic island arc (Fig. 1) and is within a graben structure (Barberi et al., 1994) linked to a NNW-SSE dextral strike slip fault, the Tindari-Letojanni discontinuity, which dissects the arc. The rocks of Vulcano belong to the shoshonitic association and range from basalts to rhyolites (Keller, 1980). The active volcanic centre of the island is La Fossa, which is located inside the "La Fossa caldera", a volcano-tectonic pull-apart-like structure (Ventura, 1994; Mazzuoli et al., 1995). The last eruptions occurred in 1888-90; since then La Fossa has shown fumarolic activity with two main periods of unrest, in the 1920's and from 1978 to the present (Barberi et al., 1991). The La Fossa magmatic feeding system has been identified, on the basis of petrological and melt inclusions studies, as a two-stage magmatic system (Clocchiatti et al., 1994; Gioncada et al., 1997). The first stage is a relatively deep (P equil.>_700 bar, depth around 3000 m) basaltic to shoshonitic-latitic reservoir, hosted in siliceous metamorphic units in the crust. The second stage is a shallower magmatic reservoir (P equil .~300-600 bar) with differentiated magmas, of limited volume and again hosted in siliceous metamorphic units. Data on pre-eruptive volatile content and the differentiation history of the Vulcano magmas indicate that most of the degassing of volatile species (H20, S, C1) occurs at the transition from basalts to shoshonites/latites (first stage and transition to the second stage, Gioncada et al., 1997).

The eruptive complex of Vulcano is influenced by extensive areas of fumarolic emissions and hydrothermal alteration, mainly concentrated on the La Fossa cone and at the Baia di Levante beach (Fig. 1). The development of hydrothermal circulation in the subsoil of the island was revealed by the boreholes drilled in the area around the La Fossa cone during the geothermal exploration carried out on the island in the 1950's and in the 1980's by Agip-Enel-EMS (Agip S.p.A., 1987). In addition, hydrothermal fluids have influenced the dynamics of some rather distinctive explosive eruptions of La Fossa in various periods of the volcano's activity (Barberi et al., 1988; Capaccioni et al., 1995; Dellino and La Volpe, 1997). The recent phase of unrest at La Fossa (Barberi et al., 1991) makes the definition of

Geologic model of the magmatic-hydrothermal system of Vulcano 197

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a model for this magmatic-hydrothermal system essential for the evaluation of volcanic hazard in the area.

This paper presents new mineralogical and geochemical data regarding superficial hydrothermal alteration and the lithic clasts of altered rocks ejected in the "breccia di Commenda" and 1888-90 eruptions, whose dynamics is characterized by the involvement of hydrothermal fluids. In this work these new data are integrated with the previous knowledge on the hydrothermal circulation and the

198 R Fulignati et al.

magmatic system with the aim of proposing a geological scheme of the magmatic- hydrothermal system of the La Fossa volcano. This study of an active system can give valuable informations about analogous fossil hydrothermal systems and epithermal ore deposits, and might be useful for the understanding of their genesis.

The mineralogical and geochemical evidence presented in this paper indicates, together with the presence and composition of high-temperature fumaroles (Chiodini et al., 1993, 1995), that the Vulcano hydrothermal system represents an example of a high-sulfidation environment, similarly to other magmatic- hydrothermal systems associated with active island arc volcanoes characterized by shallow degassing magma chambers (Hedenquist et al., 1993; Christenson and Wood, 1993; Hedenquist et al., 1994a). This type of alteration is characteristic for the category of epithermal ore deposits defined as "high-sulfidation type" (Hedenquist, 1987). It is the product of very acidic conditions that occur within a sulfate-rich hydrothermal fluid formed by the absorption of magmatic vapour in shallow groundwater. The disproportionation of SO2 in H20 to form H2SO4+H2S (Sakai and Matsubaya, 1977) and the significant amount of HC1 and HF are responsible for the acidic conditions.

2. The subsoil hydrothermal system

2.1 Previous studies on subsoil hydrothermal alteration from geothermal wells

The study of the geothermal wells VU2bis, Isola di Vulcano 1/ld and Vulcano Porto 1/ld, drilled inside the La Fossa volcano-tectonic structure, give some interesting indications on the type of deep hydrothermal alteration in the area of La Fossa. The stratigraphic sequence and hydrothermal alteration paragenesis of the wells have been studied by Faraone et al. (1986), Cavarretta et al. (1988), Gioncada et al. (1995) and Fulignati et al. (1996), and are sketched in Fig. 2. The location of the wells is shown in Fig. 1.

In the wells Vulcano Porto 1 and ld the lavas and pyroclastic units present an advanced-argillic-type alteration which reaches a depth of 600m. It is characterised by natroalunite and silica in the surface portion and by pyrophyllite and alunite at depth (Fulignati et al., 1996). Below 600m, a neutral alteration develops, with mixed layer illite-smectite and chlorite-smectite minerals, changing downwards into a facies with chlorite, epidote and corrensite, at the bottom of the well. Anhydrite, calcite and pyrite are widespread throughout the well. In particular, anhydrite is abundant and deposited in veins and may play an important role in the self-sealing processes.

In the Isola di Vulcano 1 well neutral parageneses are observed throughout the well. They are characterized by a downward transition from smectites to corrensite, to chlorite, quartz and epidote. Also this well shows widespread distribution of anhydrite and pyrite to a depth of about 1360 m. The underlying intrusive body is scarcely affected by hydrothermal alteration.

There is a completely different situation in the VU2bis well, where the hydrothermal zoning is extremely compressed. Alunitisation influences only the first 20 meters of the well, followed by a few tens of meters of kaolinite, smectite and chalcedony alteration and then an illite-smectite alteration. Below 200 meters

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Table I. Selected EDS analyses of secondau clinopyroxenes from lithic clasts of the Breccia di Commenda and 1888-90 eruptions. Analyses are normalized to 100 due to the EDAX software used. Analyses of salite, Fe-salite and Fe-hedenbergite are recalculated following Papike (1987), aegirine-augite and Ca-aegirine following Morimoto (1988)

salite Fe-salite Fe-hedenbergite aegirine-augite Ca-aegirine

SiO2 54.17 51.96 50.09 53.78 53.50 TiO2 - 0.30 0.23 0.23 0,28 A1203 1.01 0.64 0.93 0.36 0.47 FeO 6.44 15.69 25.81 23.51 30.10 MnO - - 0.28 1.73 0.79 MgO 15.04 9.52 2.09 4.82 0.11 CaO 22.48 20.57 17.01 6.89 2.84 Na20 0.87 1.32 3.45 8.69 11.90 K20 . . . . . Cr203 - - O. 1 1 - -

T o t . 100.00 100.00 100.00 100.00 100.00

Si 1.99 1.98 1.98 2.00 2.00 Ti 0.00 0.01 0.01 0.01 0.01 A1 IV 0.01 0.02 0.02 0.00 0.00 A1 VI 0.03 0.01 0.02 0.03 0.03 Fe 3+ 0.04 0.09 0.25 0.58 0.81 Fe 2+ 0.16 0.41 0.60 0.16 0.14 Mn 0.00 0.00 0.01 0,05 0.03 Mg 0.82 0.54 0.12 0.27 0.01 Ca 0.88 0.84 0.72 0.28 0.11 Na 0.06 0.10 0.26 0.63 0.87 K 0.00 0.00 0.00 0.00 0.00 Cr 0.00 0.00 0.00 0,00 0.00

Wo 46.41 44.65 42.45 En 43.22 28.76 7.27 Fs 10.37 26.59 50.28

Jd 2.96 3.35 Ae 61.26 83.66 Q 35.77 12.99

Total Fe as FeO

of depth the alteration is composed of biotite, phlogopite and talc. At the well bottom, between 233 and 236m, there is a pervasive alteration with Fe-rich clinopyroxene also found in veins, andradite garnet and biotite, superimposed on a no-longer-recognisable protolith. The temperatures indicated by the secondary mineral assemblages are very high, > 4 0 0 ° C for the cpx-garnet association and 300-400 °C for the overlying phlogopite-talc facies. The mineralogy, texture and structure indicate that these rocks were subjected to circulation of fluids of a probably magmatic-hydrothermal origin. The mineralogical zoning described above and the evidence of high-temperature secondary facies can be explained


considering that well VU2bis was positioned in close proximity to the eruptive vent of the Faraglione cinder cone (Fulignati et al., 1996).

2.2 Data from the lithic clasts ejected in the hydrothermal-magmatic eruptions

In the products of the Breccia of Commenda and of the 1888-90 eruption, the following alteration classes can be distinguished in the lithic fraction:

a) silicified lavas and tufts; b) alunite ± gypsum altered lavas and tufts; c) phlogopite and talc altered lavas and subvolcanic rocks; d) salitic to Fe-hedenbergitic clinopyroxene (Fig. 2, Table 1) and garnet-bearing

rocks; e) recrystallised rhyolitic rocks, banded, with a Fe-hedenbergitic to Ca-aegirine

(Fig. 2, Table 1) clinopyroxene widespread in a quartz-feldspathic groundmass, showing lithophysae with quartz, tridymite and hematite crystallised in the vapour phase.

3. Superficial hydrothermal alteration

3.1 Types of hydrothermal alteration

XRD and EDS studies on samples of superficial altered rocks show three types of hydrothermal alteration. 1) "Silicic" facies. This type of alteration is characterised by the complete

destruction of the original rock. Both petrographic analyses and XRD spectra show the compete destruction of the primary mineral phases. The product of this alteration is a porous rock consisting exclusively of residual amorphous silica associated with native sulfur, realgar, occasionally barite and, in correspondence with the high-temperature fumaroles, also with cristobalite, halides (salammoniac, halite, etc.), sulfides of Pb-Bi and rare native elements (Table 2) as evidenced also by Garavelli et al. (1997).

2) "Advanced argillic" facies. This type of alteration is characterised by widespread replacement of the original rocks by minerals of the alunite group (natroalunite, alunite and jarosite) with great prevalence of alunite (Table 2). In the areas influenced by this alteration there is often deposition of gypsum as surface concretions.

3) "Intermediate argillic" facies. The alteration mineral typical of this facies is halloysite, sometimes associated with minor hematite and smectites (Table 2).

3.2 Areal distribution

3.2.1 Areas of active hydrothermal alteration

In the area of the La Fossa caldera there is abundant evidence for hydrothermal activity originating from the active circulation of fumarolic fluids. The La Fossa active cone contains the most developed hydrothermal alteration areas. These are followed by the less extensive zones of the Faraglione cinder cone and Vulcanello

202 E Fulignati et al.

Table 2. Secondary minerals identified in the superficial alteration facies of Vulcano through XRD and SEM-EDS investigations

Alteration Secondary minerals identified facies

XRD SEM-EDS

Intermediate halloysite [A12Si205 (OH)4] goethite argillic smectite

hematite

Advanced alunite [KA13 (SO4)2(OH)6 ] alunite goethite argillic natroalunite [NaKA13(SO4)2(OH)6] natroalunite

jarosite [KFe3 (804)2(OH)6] jarosite gypsum gypsum amorphous silica amorphous silica mizzonite [(Na,K)Ca(Si,A1)6OIzC1] native sulfur native sulfur halite wilco xite [MgAI(SO4)2E 18H20] barite

Silicic amorphous silica realgar

native sulfur amorphous silica salammoniac [(NH)4C1] native sulfur cristobalite realgar

goethite barite gold tellurium Pb-Bi sulfides

center (Fig. 1). On the cone of La Fossa, silicic alteration prevalently extends across a semi-circular area which occupies the northern part of the 1888-90 crater. The altered rocks in this facies are yellowish grey with an extensive surface induration and are constituted by silica and sulfur. This alteration facies is well- developed and encloses the area of the highest temperature fumaroles of the crater zone and coincides perfectly with the area of greatest thermal anomaly on the volcano as detected by thermal infrared data (Mazzarini et al., in prep.). The pyroclastic deposits from the 1888-90 eruption have been intensively altered, showing that a complete leaching of the rocks occurred in a very short time. In the crater area, minor silicic zones are present at the base of the southern edge of the crater. These zones cover an area of tens of square meters (Fig. 1). Outside the crater zone this type of alteration was identified in two very limited outcrops (a few square meters), corresponding with fumaroles south of the crater border. The silicic alteration affects rocks varying in composition from rhyolites to latites.

The zone described above is almost completely surrounded by alteration facies of the advanced argillic type. This facies is found on the crater edges associated with older successions and on the northern side of the volcanic cone, affecting the area of the Forgia crater (Fig. 1), conditioned by N-S fractures.


The same type of alteration is present in the area of the Faraglione cinder cone where it is widespread around the vent, decreasing in intensity outwards. The altered area extends northwards along the Spiaggia di Levante where it reaches the isthmus and the lava platform of Vulcanello (Fig. 1).

The intermediate argillic facies is a zone of no more than a few tens of square meters, which is exclusively found in the internal western zone of the crater as a transition from the advanced argillic. The advanced argillic and intermediate argillic alteration mainly affect latitic-trachytic products.

A similar hydrothermal system has developed at Vulcanello with small silicic spots surrounded by a wider sulfidised area of advanced argillic facies, developed on shoshonitic lavas.

3.2.2 Areas affected by fossil hydrothermal alteration

Areas of fossil hydrothermal alteration were identified within the Primordial Vulcano in the area bordering the La Fossa volcano-tectonic structure, near the Punta Luccia and Monte Saraceno volcanic vents (Fig. 1).

The hydrothermally altered rocks appear in layers with an irregular color gradation from white to red, from incoherent to semicoherent. The original lavas have been pervasively altered to advanced argillic associations, dominated by sulfate (mainly alunite) deposition. Massive deposition of gypsum and jarosite is superimposed on the alunite; textural evidence suggests a supergene origin for gypsum and jarosite due to circulation of surface rainwater fluids. In the area of Monte Saraceno, the bydrothermal pulse is marked also by a small phreatic crater.

Based on field relations between altered and fresh rocks it can be inferred that the two hydrothermal pulses were caused by two distinct periods of magmatic activity. The older one (Punta Luccia) occurred about 484-6.5 Ka (De Astis et al., 1989; Voltaggio et al., 1997) while the younger (Monte Saraceno) is older than 25 Ka.

3.3 Chemical composition of the hydrothermally altered rocks

Major and a wide range of trace elements, involving REE, T1 and Bi, were determined on samples of the three different superficial alteration facies and on samples showing phyllic/propylitic alteration of cuttings from the "Vulcano Porto l " geothermal well (Table 3). The altered rocks analysed (in particular those belonging to the silicic and advanced argillic parageneses) are modified with respect to their initial composition to such an extent that they contain high amounts of S, C1, and F (several wt%). In addition, the LOI (Loss On Ignition) is high in the rocks showing superficial hydrothermal alteration. During LOI determination (calcination at 850 °C) volatile elements tend to escape in vapour form. The LOI derived therefore is not given only by molecular water loss in the hydrated minerals, but is also strongly influenced by the presence of S, C1 and F in the altered rocks. A confirmation of this comes from the fact that LOI values and the amounts of S, C1 and F present in the altered rocks are positively correlated. For this reason it was decided to recalculate the sum of major elements on a LOI free basis.

Table 3, Chemical composition of representative samples of the alteration facies of Vulcano

silicic

Sample no. 258 268

advanced argillic intermediate argillic phyllic/propylitic

F1 C5 102 103 Vp1-735 Vpl-920

SiO2(wt%) 89,94 94.53 41.35 49.51 50.11 53.27 61.26 60.22 A1203 3.90 2.23 18.69 25.08 34.06 28 ,74 15.90 14.57 Fe203 0.38 0.16 7.38 0.53 11.68 11.60 7.19 5.55 MgO 0.12 0.08 0.00 0,00 1.12 2,76 3.83 6.39 CaO 0.13 0.12 3.85 0,26 0.29 0.80 5.05 6,88 NaaO 0.63 0.64 2,47 0.52 0.10 0.24 2.77 2.62 K20 0,56 0,70 4.23 6.05 0.15 0.51 2,97 2.84 TiO2 0.08 0.17 0.51 0.82 1.11 0.91 0,58 0.48 P205 0.02 0,02 0.51 0,83 0.48 0.46 0.31 0,30 MnO 0.02 0.01 0.01 0.00 0.08 0.22 0.13 0.14 SO3 4.22 1.34 21.00 16.40 0.81 0.50 n.d. n.d. Sum 100.00 100.00 100.00 100.00 100.00 100.00 99.99 99.99 L.O.I. 47.92 4.01 25,47 24.43 13.12 12.28 n,d. n.d, Ba(ppm) 28 166 594 886 871 522 627 680 Ni b.d.1. 8 7 2 20 24 23 16 Cr 1 3 52 64 55 483 69 53 V 1 2 234 186 267 224 195 130 Co b.d.1, b.d.1. 9 b.d.1. 39 42 27 19 Cu 6 6 266 15 854 777 125 57 Pb 35 5 6 32 41 43 30 20 Zn 11 6 15 6 143 137 90 76 As 767 1997 13 7 7 11 b.d.1 6 C1 177456 4407 366 99 299 72 n,d. 171 F 137735 20647 9386 2961 2770 3011 n.d. n.d. Nb 8 31 11 23 41 40 20 13 Zr 28 266 48 173 435 400 162 103 Y 3.0 8.2 6.0 7.9 62 54 34 22 Sr 11 36 771 1304 721 582 1059 805 Rb 456 95 191 23 10 26 179 83 La 6.5 9,4 43 135 101 104 41 39 Ce 11.3 18,5 81 224 186 197 77 72 Pr 1.16 2.16 9.9 19.7 19.8 19.9 9,2 8.3 Nd 3.6 7.6 37 56 69 71 34 30 Sm 0.59 1.69 5.1 7.7 12.4 12.9 6.6 5.9 Eu 0.02 0.06 0.84 0.55 2.00 2.03 1.31 1,03 Gd 0.40 1.03 2.0 2.9 8.2 7.9 5.1 4.3 Tb 0.07 0.23 0.31 0.51 1.45 1.43 0.77 0.69 Dy 0.47 1.45 1.43 2.06 8,6 8.3 4.34 3.81 Ho 0.10 0,33 0,27 0,38 1.87 1.63 0.84 0.75 Er 0.31 0.96 0.62 0.79 5.7 4.8 2.3 2,0 Tm 0.05 0.16 0.08 0.11 0.83 0,74 0.39 0,34 Yb 0.33 1.07 .051 0.66 6.1 5.1 2.3 2,1 Lu 0.05 0.17 0.07 0.10 1.03 0.87 0.33 0.3 Hf 1.6 7.8 2.5 6.8 8.4 8.6 3.5 3.2 Ta 2.70 3.6 0.73 2,05 2.57 2.50 0.97 1.02 Mo 4.4 4.1 0.74 9,3 1.91 2,22 1.92 4.03 T1 124 0.4 24.7 0,59 0.16 0,22 n.d. n.d. Th 6.3 29.1 17.8 44 50 52 16.4 17.6 U 1.84 10.7 5.0 2.9 11,4 13.2 5.0 5.4 Sc 4.9 1,9 13 18 22 20 18 14 Cs 2.15 5.1 3.11 2.21 1.00 1.25 2,22 1.35

Total Fe as Fe203, b.d.l, below detection limit

R Fulignati et al.: Geologic model of magmatic-hydrothermal system 205

Geochemical data were used to draw isocon diagrams, and typical isocons for each superficial alteration facies are shown in Fig. 3; chondrite normalized REE patterns of altered rocks are compared to the unaltered rocks considered to be equivalent in Fig. 4.

3.3.1 The isocon method

The variations in chemical composition of hydrothermally altered rocks with respect to fresh rocks can be represented using the isocon diagram method proposed by Grant (1986). This is a method of graphical resolution of Gresen's equation (1967). The construction of the diagram consists in tracing the straight isocon (a straight line that joins the points of equal geochemical concentration) fitting one or a series of elements considered immobile, such as A1203, TiO2 and Zr (Grant, 1986; Eaton and Setterfield, 1993). Alternatively, volume can be considered constant, taking as an isocon a straight line of equation

C A = (p° / f¢)C° (1)

where C A = concentration of an immobile element in the altered rock, C°-= concentration of an immobile element in the fresh rock, pa = density of the altered rock, pO = density of the fresh rock. In our case we adopted the latter method (constant volume) for construction of the isocon diagrams, because no element could be considered immobile due to the particular conditions of acid leaching found. The geometry of the diagram (Fig. 3) is such that the elements below the straight isocone are depleted whereas those above are enriched by the hydrothermal alteration (Grant, 1986). The enrichments or depletions of the various elements can be estimated by using the equation

(~XCi/Ci) × 100 -= (pa/p°)(C~/C°) - 1 (2)

where (ACi/Ci) x 100 is the percentage variation of the it%lement in the altered rock compared with the fresh rock (Fig. 3). If the assumption of constant volume is not correct and leaching results in a volume decrease, there would be an overestimation of calculated depletions and an underestimation of enrichments. The assumption of constant volume is supported by the observation that the leaching process is accompanied by an increase in the porosity of the rocks.

The analogous fresh rocks were selected on the basis of our knowledge of the composition of the units affected by hydrothermal alteration and on petrographic observation.

3.3.2 Silicic facies

The samples studied come from two areas: the inner fumarolic area and the southern external edge of the La Fossa crater and from Vulcanello. At the La Fossa crater, apart from the emergence of the hottest fumaroles (T > 540 °C), there is widespread emission of gases and mean temperatures measured at ground level during samplings were around 100°C.

The rocks in this area are essentially composed of silica and sulfur. They show (Table 3) SiO2 concentrations varying from 79 to 95 wt% and SO3 concentrations

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6"

4"

2"

0

"S "~Sc ~ t C e

# Cr A 1 2 0 3 ~ 8 ' ~

Sine ~D,¢Th eLOI F ~ ' / • Old

e o 2 # 7 • Zr La • zZLa"

Ti

/ " • MnO "Vb/ v s . • sio2 ":rB~ ~ S " • sio2 "srB~

~'~b .M~o •Cs *Cl #Cao

0 2

Rb, Na20 * K20

4 6 8 10 12 14 16 18 20

C(o)

4

2 I 1 I I I ° l i t . ]

Ii I -3 I ,

-4

(c)

Fig. 3. Isocon diagrams of the three different alteration styles identified in the superficial hydrothermalised zones of Vulcano and histograms showing gains and losses of selected elements during hydrothermal alteration as calculated from isocon diagram method, a silicic, b advanced argillic, e intermediate argillic. Values of +4 to + 1 = a gain of more than 100 percent, 50 to 100 percent, 25 to 50 percent and 10 to 25 percent respectively. 0 = immobile elements (variation in the range +10 and - 1 0 percent). Values of - 1 to - 4 = losses to I0 to 25 percent, 25 to 50 percent, 50 to 90 percent, more than 90 percent respectively

E Fulignati et al.: Geologic model of magmatic-hydrothermal system 207

1000 - 1000~

10 10

1 1

0.1 ~ t r ~ ~ ~ ~ ~ ~ ~ ~ t ~ t r 0.1 La Pr Pm Eu Tb Ho Tm Lu La Pr Pm Eu Tb Ho Trn Lu

(a) (b)

10

1

0.1

1000 -_--

100

1000 -

100

10

Ce Nd Sm GdrDDy Er Yb

t'a Pr Pm E~J Tb Ho Tm I'u 0.1

Ce Nd Sm Gd Dy Er Yb r P r ~ i t i { i

I~a Pr Pm Eu % Ho Tm Lu

(c) (d)

Fig. 4. Range of chondrite-normalized REE patterns of hydrothermally altered rocks (ruled areas) compared with unaltered equivalents (grey areas). REE values of unaltered rocks from Gioncada et al. (1997). Chondrite normalizing values from Mc Donough and Sun (1995). a silicic facies; b advanced argillic; c intermediate argillic; d phyllic and propylitic (VP1 well)

ranging from 1.3 to 16 wt.% A1203 varies between 1 and 3,9 wt.% and all the other elements have concentrations of less than 1 wt.%.

The isocon diagrams of the samples from the silicic alteration zone (Fig. 3a) confirm a general depletion in all the elements except for S, C1, F, LOI and in some cases Ba, Ni and Cr, of which there appears to be marked enrichment. The higher contents of C1 and F are found in the samples of altered rock taken closer to the fumarolic emissions.

The described general depletion of most elements produces very similar chemical patterns in most samples irrespective of their initial composition. REE patterns (Fig. 4a) show this clearly: all REEs are strongly depleted compared to the unaltered rocks and a negative Eu anomaly is present in all samples. The altered rocks always show an evident negative Eu anomaly (Fig. 4a), while the fresh rocks can be characterised by a strong Eu anomaly (rhyolites) or by a very weak Eu anomaly (latites).


Even though rocks characterised by silicic alteration have 79-95 wt.% SiO2, the isocone diagrams show that they are poor in silica (Fig. 3a). This indicates that they are residual rocks and the apparent enrichment in SiO2 is a relative enrichment, when compared to other elements that have been leached even more completely. In fact, these rocks are impoverished also in elements such as A1, Ti, Zr and Hf, which are generally considered immobile in many hydrothermal environments (Finlow-Bates and Stumpfl, 1981; Grant, 1986; Eaton and Setterfield, 1993).

The isocon diagram also allows calculations of mass variation, assuming the rock volume to be constant (Grant, 1986). We calculated a mass loss of 30-35% for the silicic facies.

3.3.3 Advanced argillic facies

Advanced argillic facies rocks are also impoverished in most elements (Fig. 3b) with respect to the original rock. Sulfur enrichment is marked, reaching a content of about 35 wt.% SO3. In this alteration facies the abundance of sulfur is accounted for mainly by the presence of alunite. In fact, this mineral in many cases is the only phase present. Magnesium is strongly impoverished whilst calcium shows a different behaviour in function of the presence (Ca enrichment) or absence (Ca impoverishment) of gypsum (Fig. 5), which, based on field and textural evidence, is of supergene origin. It is interesting to observe chemical variations of samples in Fig. 5. The analyses of the samples with advanced argillic alteration fall along a straight line joining the vertex of the alkaline earth elements with the side of K20+Na20-AI203+FeaO3, corresponding to the composition of samples where alunite is the dominating alteration mineral. The Ca content is therefore strictly controlled by the gypsum/alunite relationship present in the mineral paragenesis.

As far as the alkali are concerned, there is a pronounced Na impoverishment, whereas K is depleted to a lesser extent than Na because it is fixed by alunite. A1203 remains practically immobile.

The advanced argillic alteration presents HREE depleted and LREE immobile or weakly enriched (Fig. 4b). Decreases in the mass of the altered rocks were estimated at between 27 and 34%.

3.3.4 Intermediate argillic facies

This alteration facies is characterised by a weak increase in A1203 and Fe203 and a strong depletion in the alkaline and alkaline earth elements. This reflects an alteration mineralogy made up mainly of halloysite, a phyllosilicate composed of Si and A1, whose structure does not host large cations like Ca, Na and K (Fig. 3c, 5). In the "intermediate argillic" facies the REE pattern is very similar to that of the fresh rocks with a general enrichment of all the REE (Fig. 4c). In general, compared with the alteration facies previously described, it can be noted that various elements usually considered immobile, such as A1, Zr, Ce and Ti, plot close to the straight isocone line (Fig. 3c).

Geologic model of the magmatic-hydrothermal system of Vu]cano

A1203+Fe203 sio + + sio

[ ]

supergene gypsum

CaO+MgO ~- % Na20+K20

[] ©

209

Si02

• silicic alteration, La Fossa crater

• silicic alteration, Vulcanello

© advanced argillic alteration, Faraglione

[ ] fossil advanced argillic alteration

O advanced argillic alteration, La Fossa crater

A advanced argillic alteration, Vulcanello

+ intermediate argillic "alteration

fresh rocks ~%iii

Fig. 5. Triangular composite diagram showing the variation of SiO2 - A1203 + Fe203- Na20 + K 2 0 - CaO + MgO in the hydrothermal alteration facies with respect to the equivalent fresh rocks. This diagram has been obtained by the combination of the following four simple triangular diagrams: SIO2-A1203 +Fe203 -CaO+MgO, A1203 +Fe203 -Na20 +KzO-CaO+MgO, SiO2-A1203+FezO3-SiOz, Na20+K20-CaO+MgO-SiO2


Table 4. Results of sulfur isotope determination of representative samples of the superficial alteration facies of Vulcano

Sample no. Description 34S/32S

258 Ag2S - 2 . 2 258 bis Sulfur - 3 . 6 263 Sulfur - 2 . 8 102 Ag2S 5.0 FG23 Ag2S 8.8 P 1 Ag2S -9 .7 C 5 Ag2S - 2 . 2 L 1 Ag2S 4.4 S 2 Ag2S 9.8 VL 6 Ag2S 5.7 F 1 Ag2S - 1.4

Ca~

t~

22 20- - - - 182 16 14- 12 10" 8 6 4 2 0

- 2 -4 [] -6 -8

-10 i] -12

0 5

~ " " ~ " ~ s e a water S04--

[] •

1 I I i i

10 15 20 25 30 S % w t .

/ /

2

%

IOO

[] 834S silicic

• 834S advanced argillic

@ 834S intermediate argillic

- - After Cortecci et al. (1992) (pyrite and anhydrite found in "Vulcano Porto 1" geothermal well)

Fig. 6. ~348 values of the three different alteration facies rocks identified in the superficial hydrothermalised zones of Vulcano


Sulfur enrichment is limited compared with the silicic and advanced argillic facies. In this alteration facies the calculated decrease in mass is also high (around 38 wt.%).

3.4 Sulfur isotope composition of superficial hydrothermalized rocks

Eleven samples from different alteration facies were analysed for sulfur isotopes (Table 4). The results are shown in Fig. 6. The samples from the La Fossa silicic facies show slightly negative values ( -3 .6 to - 2.2); they are in the range of the crater fumaroles (Cortecci et al., 1992). The samples containing sulfate (alunite) as the main alteration mineral (ex. FG 23 in Table 4) generally show high ~34S.

4. Discussion

4.1 The alteration facies: genesis and spatial distribution

4.1.1 Superficial hydrothermal alteration

The chemical composition of the altered rocks shows the occurrence of a very efficient acid leaching process, particularly evident in the silicic and advanced argillic alteration which provokes a depletion of the rocks in most elements. In contrast, the ubiquitous enrichment in S, accompanied, close to the high temperature fumarolic vents, by high contents of C1, F, T1, Pb, As and Bi, derives from fumarolic fluid supply (Table 3).

In the silicic alteration zone the leaching process produced a rock composed almost completely of residual silica, after the mobilization of all other elements, also those usually regarded as immobile. The marked depleion in A1203 is particularly significant because this element tends to be mobilized under pH conditions < 2 (Stoffregen, 1987). The notable depletion in all REE shown by the silicic alteration (Fig. 4a) is indicative of low pH and abundant chloride, fluoride and sulfate complexing ions in the hydrothermal fluids (Michard, 1989; Wood, 1990; Lottermoser, 1992). The REE patterns in the silicic altered rocks are similar to each other, always showing an evident negative Eu anomaly (Fig. 4a). The original rock types may have different REE patterns (rhyolites have negative Eu anomaly while latites have weak or no Eu anomaly). The occurrence of a depletion in Eu may result from the dissolution of Eu-rich mineral phases by hydrothermal fluids (Lewis et al., 1997). Alternatively, hydrothermal fluids may preferentially leach europium relative to the other REE at temperatures above 250 °C (Sverjensky, 1984; Bau, 1991) or in an extreme reducing environment. These conditions favour the stability of the more soluble Eu 2+ with respect to the trivalent state. For the silicic alteration facies identified at Vulcano the first hypothesis is more suitable, given the superficial, low-temperature (outside the fumarolic vents, ground temperatures, measured during sampling are about 100 °C) and pH< 2 oxidizing environment. The total dissolution of plagioclase and K-feldspar, which are Eu- rich minerals abundant in the primary paragenesis of Vulcano rocks (Gioncada et al., 1997), should be responsible for the negative Eu anomaly in silicic altered rocks. The Eu anomaly can be furthermore enhanced by the high concentration of


Cl- in this type of hydrothermal solution because this ion represents a good complexing agent for Eu (Michard, 1989; Lottermoser, 1992).

Regarding the advanced argillic facies, the isocon diagrams (Fig. 3b) show a general depletion in most elements, indicating the efficiency of acid leaching processes. It must be noted that aluminium remains practically immobile, or at least undergoes a much less accentuated depletion compared to that in the silicic alteration. This different behaviour implies that pH never was below 2, thus preventing leaching of the A1 from the original rock (Stoffregen, 1987). The observed HREE fractionation (Fig. 4b) should be due to low pH, together with the abundance of sulfate-complexing ions in the hydrothermal fluids. The LREE remain practically immobile as they can be fixed in the molecular structure of the minerals of the alunite group: the LREE can, in fact, substitute K in the large radius site of A, considering the general formula of this family of minerals: AB3(XO4).

In the intermediate argillic facies the fact that A1203, Zr, Hf and Ti are not depleted suggests that the rocks influenced by this alteration facies have undergone less severe leaching, which has not affected the elements characterised by limited mobility in a hydrothermal environment. This fact can mainly be attributed to the lower acidity (4<pH<6) of the hydrothermal solutions that generate the intermediate argillic alteration. The slight REE enrichment observed (Fig. 4c) can be attributed to the instability of the complexing ions due to the partial neutralisation of the fluids. This process provokes the deposition of the REE leached in the preceding facies which are fixed in the clay minerals that characterise the intermediate argillic alteration (Fulignati, unpublished data). On the other hand, experimental studies have documented the capability of kaolinite and smectite group minerals in retaining REE (Bruque et al., 1980; Miller et al., 1982; Laufer et al., 1984). The increase in other elements (for instance Cr, Co, Ni), observed in the intermediate argillic alteration (Fig. 3c), many derive from a similar process of mobilization by acid leaching during the silicic and advanced argillic alteration and subsequent deposition following the neutralization of the fluids.

The acid conditions evidenced by the above discussed leaching processes are due to the presence of acid species such as SO2, HC1 and HF in the fumarolic fluids. SO2 disproportions below 400°C in oxidized shallow groundwaters following the reaction proposed by Sakai and Matsubaya (1977): 4SOz+4H20=3H2SO4+HaS. Evidence of SO2 disproportionation producing isotopically light sulfide and heavy sulfate come from the high (~348 shown by samples of alunitized rocks in the advanced argillic facies (Fig. 6). Sulfur isotope data (~534S in the range - 1 0 to +10, Table 4) indicate a prevalently magmatic origin of sulfur with minor processes of fluid-rock interaction, according to the interpretation proposed by Cortecci et al. (1992). The presence of a magmatic component in the fumarolic fluids is also supported by the measured contents of elements such as T1 (Table 3) and Bi (Fulignati and Sbrana, in prep.), commonly considered of magmatic origin (Symonds et al., 1987), in colxespondence of the high temperature fumarolic vents. Other geochemical evidence is based on stable and radiogenic isotope data (Carapezza et al., 1981; Bolognesi and D'Amore, 1993; Le Cloarec et al., 1994; Chiodini et al., 1995).


The above discussion indicates that surface hydrothermal alteration at Vulcano represents a "high sulfidation" environment, in accordance with the definitions proposed by Hedenquist (1987) to characterize a type of epithermal ore deposits whose genesis is linked to the hypogenic introduction of magmatic gases. The silicic alteration found at Vulcano presents many elements in common (chemical composition, porosity, geochemical environment) with the "vuggy silica" alteration described in high sulfidation epithermal ore deposits (Stoffregen, 1987; Hedenquist et al., 1994b; Arribas et al., 1995). Even if chemical composition consists in both cases of 90-95 wt.% of SiO2 (on an anhydrous basis), "vuggy silica" alteration consists predominantly of quartz, while silicic alteration at Vulcano is characterized by amorphous silica and, where rocks are lithified, opal. Considering the close relationship between high-sulfidation epithermal deposits and active volcanic-hydrothermal systems characterised by high temperature and acid fumarolic fluids (Hedenquist and Lowenstern, 1994), it may be proposed that the silicic alteration found at Vulcano represents the early stage of the "vuggy silica" alteration. The amorphous residual silica should recrystallize over time to give quartz (Hedenquist et al., 1994b). In addition, a strong analogy exists between the distribution of alteration facies in the superficial hydrothermal system at Vulcano and "high sulfidation" epithermal systems. A sequence of alteration zones has in fact been recognised extending outward from the volcanic conduits of La Fossa and Vulcanello (Fig. 1, 7), in correspondence of which there is or there was in the recent past (at Vulcanello) a preferential introduction of hypogenic acid fluids. The transition from silicic to advanced argillic to, where present, intermediate argillic alteration is indicative of decreasing acidity and corresponds to the neutralisation of the acid solutions by reaction with the rocks.

4.1.2 The subsoil hydrothermal alteration

Lithic clasts showing silicic and advanced argillic alteration represent a relevant fraction of the deposits of "Breccia di Commenda" and, to a lesser extent, of the 1888-90 explosive eruptions. The analogy of these secondary parageneses to those described in the superficial alteration suggests that these lithics are representative of the upper part of the conduit and the surface vent zone (Fig. 7).

Lithics from explosive eruptions also reveal the presence of a high temperature alteration paragenesis at depth. The paragenesis characterised by aegirine clinopyroxene (a mineral phase absent in the primary paragenesis of the rocks of Vulcano, Fig. 2, Table 1) were probably formed by metasomatic processes caused by fluids of a prevalently magmatic origin being injected into the conduit system of the volcano. These fluids consisting of vapor phases and chloride-rich brines, are responsible for high temperature metasomatic exchange reactions between silicates and chlorides, producing Na-silicates and acidic HCl-rich fluids, as hypothesised by Shinohara (1992). It must be noted that NaC1 of marine origin could also be involved in this kind of reaction, as proposed by Chiodini et al. (1993). The presence of garnet and salitic to Fe-hedenbergitic clinopyroxene (Fig. 2) can be interpreted as the product of alteration by magmatic-hydrothermal fluids, which have undergone dilution by groundwaters and/or exchange reactions with host rocks. As a consequence they are not able to produce alkaline metasomatism

214 P. Fulignati et al.

@

@

r~

~ c e ~

°


(Fig. 7). This alteration facies may also be interpreted as a thermo-rnetamorphic facies ("skarn") generated by the rise of magmatic fluids or even magma itself in zones where there has previously been deposition of hydrothermal calcic minerals like anhydrite, which is widespread in the subsoil secondary paragenesis, as revealed by geothermal drillholes. Situations of this type have been reported in other volcanic systems similar to Vulcano, such as Ruapehu and White Island (Wood, 1994; Wood and Browne, 1996). In the general picture of the geometry of the magmatic-hydrothermal system of the La Fossa volcano we consider that the phlogopite- and talc-bearing facies found in the lithic clasts represents the upward transition of the thermometamorphic and metasomatic facies, which should be confined to deeper zones bordering the feeding systems (Fig. 7).

The secondary mineralogical assemblages characteristic of the altered lithic clasts are closely similar to those shown by well VU2bis drilled in correspondence with the feeding vent of the Faraglione cinder cone (Fig. 2). It is therefore probable that such zoning of alteration is common for the vent systems of Vulcano (Fig. 7).

Phyllic and propylitic alteration characterises the deeper part of the Vulcano Porto 1 and Isola di Vulcano 1 deep wells (Fig. 1, 2). The REE distribution patterns relative to the propylitic and phyllic facies identified in the Vulcano Porto 1 borehole (Fig. 4d) do not show any significant variation with respect to the unaltered rocks. The observation that REE are immobile in the near-neutral pH propylitic and phyllic alteration facies seems to confirm the existence of a strong relation between REE mobility and pH of hydrothermal fluids. In addition, the immobility of REE in these parageneses supports a low water/rock ratio according to Michard (1989) and Bau (1991). Phyllic and propylitic alteration are not represented in the lithic clasts. This evidence may suggest that these types of alteration develop in the subsoil only laterally to the conduit zones (Fig. 7). The results of the geothermal boreholes indicate low permeability for these zones.

4.2 Assessment of a model for the magmatic-hydrothermal system of La Fossa

The data collected so far allow the construction of a geologic model of the magmatic-hydrothermal system of the La Fossa volcano. The heat engine of the hydrothermal system is represented by the La Fossa magmatic feeding system. A loss of volatiles (H20, S, C1) is documented to occur during evolution based on melt inclusion studies (Clocchiatti et al., 1994; Gioncada et al., 1997). Therefore, an exsolved fluid phase mainly consisting of H20, chlorides and sulfur is released from the magma. At the conditions of pressure, around or lower than 1 kbar (Gioncada et al., 1997), hypothesised for the La Fossa magmatic system, the exsolved fluid is probably partitioned into a hypersaline brine into which the chlorides and most of the metals are seperated (Hemley et al., 1992; Shinohara, 1994) and a vapor phase that can still contain a certain amount of salts (mainly chlorides) which allow to maintain a good capacity to mobilise metallic elements (Hedenquist et al., 1994a; Hedenquist and Lowestern, 1994). The brine, which is denser than the vapor phase, is confined at depth and, although we have no direct evidence such as hypersaline fluid inclusions, we consider that it is involved in the metasomatic processes (sodium contribution) evident in the aegirinic clinopyrox-


ene-bearing lithic clasts. The dilution of the brines by groundwater could have originated the Fe-hedenbergitic clinopyroxene, garnet and biotite parageneses.

The vapor phase, rich in gases such as SO2, H2S, HC1 and probably HF, during its rise can partially condense in relatively superficial waters of meteoric or marine origin, forming even more reactive solutions. Initially, the fluids produce a phlogopite and talc (T~350 °C) paragenesis. Following this, with a decrease in temperature, the fluids become ever more oxidising and acid, the SO2 disproportions producing H2SO4 and H2S, and there is formation of alteration parageneses of the advanced argillic type (lithics with alunite alteration paragenesis widespread in the deposits). Finally, nearer the surface, when the temperature drops to below 250°-300 °C, the acids H2SO4, HC1 and HF in the solutions increase their dissociation (Ruaya and Seward, 1987), lowering the pH to below 2 and generating a silicic alteration facies.

Laterally, out of the conduit zone, the acid solutions produced by hypogenic introduction of magmatic gases undergo neutralisation by mixing with groundwater and reaction with the host rock with a low water/rock ratio (primary neutralisation). Primary neutralisation evidence may be represented by the neutral paragenesis (phyllic and propylitic) found in the wells Vulcano Porto 1 and Isola di Vulcano 1 (Fig. 2). At the surface the extremely acid fluids are progressively neutralised moving away from the high temperature fumarolic chimneys.

In a passive degassing system like La Fossa, the vapour coming from the fumaroles, due to the gradual re-equilibration at lower pressure conditions during its rise, has a low chloride content and consequently a low capacity to transport metallic elements (Hedenquist et al., 1993, 1994a). Nevertheless, high contents of TI and Bi and the discovery of Au and Te grains characterize the samples close to the high-temperature fumarolic vents in the silicic facies, as is reported for many passively degassing volcanoes (Hedenquist, 1995 and reference therein). Sulfur, abundant in the fumarolic fluids of La Fossa, may partly contribute to the transport of metallic elements.

4.3 Remarks on the volume of magma degassing from the VI century to the present

The ~534S values indicate that most of the S fixed in hydrothermally altered rocks is of magmatic origin. The data collected on the hydrothermalised rocks, together with knowledge of the pre-emptive volatile content during the differentiation processes of the magmas of Vulcano, allow some considerations as to the volume of magma necessary to produce the sulfur both released into the atmosphere as SO2 from the fumarolic system and contained within the hydrothermalised rocks.

The geological data indicate that the high sulfidation hydrothermal system of La Fossa has been active at least since the VI century, with an areal distribution similar to the present one. The altered rocks of La Fossa contain on average 12wt.% SO3. From the field geometry of the distribution of hydrothermal alteration (Fig. 1, Fig. 7), about 3 million tonnes of sulfur is fixed in the rocks in secondary minerals.

A significant amount of sulfur is dispersed in the atmosphere by the fumarolic plume. On the basis of the data reported by Allard et al. (1992) and Bruno et al.

Geologic model of the magmatic-hydro thermal system of Vulcano 217

(1994), a mean release of SO2 of 15 tonnes per day can be hypothesized. If we assume a similar emission rate in the past, an amount of sulfur comparable to that fixed in the altered rocks has been released into the atmosphere in the last 1400 years.

Most of the sulfur degassing occurs at the transition from basalts, containing 2300ppm of sulfur, to shoshonites/latites generated by the differentiation of basalts, which have an average content of 500 ppm of S (pre-emptive content from melt inclusions, Clocchiatti et al., 1994; Gioncada et al., 1997). Sulfur is lost as a consequence of the decrease in solubility both during the magmatic differentiation and the decrease in pressure and is mobilised by the water-rich fluid. The Vulcano basaltic melts are believed to be undersaturated with respect to a S-bearing phase (Metrich and Clocchiatti, 1996), so it is unlikely that S is fractionated by immiscibile sulfide globules. A release of about 1800 ppm of S can be inferred at the basalt/shoshonites transition. From these data a volume of basaltic magma involved in degassing from the VI century to the present can be calculated to be around 2 km 3. If in the process of degassing an intermediate magma were involved, the volumes required would be three times greater, given the lower S content measured for the shoshonitic and latitic magmas. At La Fossa there is a lack of evidence, particularly geophysical, for the presence of a magmatic chamber of great dimensions (Ferrucci et al., 199l). Therefore we believe that the sulfur is released from a differentiating basaltic magma in a relatively deep reservoir.

5. Summary and conclusions

The eruptive complex of Vulcano is characterised by the marked development of a high-sulfidation-type hydrothermal system (Fig. 7), closely linked to the degassing of a shallow-sited, low-volume magma reservoir.

The hypogenic introduction of magmatic fluids (brine +vapour phases) into the conduit system of the volcano causes high temperature recrystallisation and the metasomatism of the volcanic and sub-volcanic rocks occupying the deeper parts of the structure. The hypersaline brines remain confined at depth whilst the vapour phases tend to rise to the surface, feeding the fumarolic system and partly entering hydrothermal systems lateral to the volcanic vents, where they undergo neutralisation due to water-rock interactions with a low water/rock ratio and mixing with marine and/or meteoric fluids.

During their rise to the surface some of the magmatic fluids partially condense in groundwater, acquiring even more acid conditions and giving rise to a sequence of shallow and superficial alteration facies (silicic, advanced argillic and intermediate argillic). This corresponds to the progressive neutralisation of the acid fluids via water-rock interaction and mixing with shallow groundwaters.

The silicic alteration derives from extremely acid (pH < 2) fluids that have provoked an almost complete leaching of the original volcanic products, leaving a rock mainly composed of residual silica. We propose that this alteration facies can represent the early stage of the "vuggy silica" alteration characteristic of high- sulfidation epithermal ore deposits.

The advanced argillic alteration is characterised by a complete substitution of the rock by sulfates of the alunite group. Circulation is dominated by fluids with a


higher pH (4.5 > pH > 2) and temperatures lower than those influencing the silicic alteration.

The intermediate argillic alteration is scarcely developed. The depositional conditions of the hydrothermal minerals indicate a medium acid pH (6 > pH > 4).

The presence of elements like T1, Te, Bi, Au in correspondence with the high temperature fumaroles reinforces the evidence that La Fossa of Vulcano is an active magmatic system subject to important degassing processes and that it contributes with volatiles and metals to the upper hydrothermal systern.

We attempted an estimation of the volume of magma required for the production of S involved in the fumarolic system and fixed in the hydrothermal alteration paragenesis from the VI century to the present. Based on data on the pre- emptive content of S in Vulcano magmas and on the existing constraints on the magmatic system of La Fossa, roughly 2km 3 of basaltic magma have been involved in degassing in the last 1400 years.

The close relationships between shallow-level magmatism and related hydrothermal systems are evident at Vulcano, where the active La Fossa magma reservoirs contribute heat, volatile species and metals to the shallow high- sulfidation hydrothermal system, developing in correspondence with the volcanic conduits (Fig. 7). The preferential rise of deep fluids through volcanic conduits is controlled by the presence of the active magma chamber of the volcano, which favours permeability due to volcano-tectonic fracturation and determines high porosity due to the acid leaching process which affects at least the upper part of the conduit system. Inside the La Fossa active volcano-tectonic area several high- sulfidation systems were recognised, both fossil (Pt. Luccia, Mr. Saraceno) and active (La Fossa, Vulcanello and Faraglione). These represent the superficial evidence for the existence of several shallow magmatic sub-systems that supply magmatic volatiles to the related hydrothemal systems.

Acknowledgements

This paper has greatly benefited from critical comments, suggestions and review by J. B. Lowenstern and H. E. Belkin. We appreciate very much the editorial handling of Prof. B. De Vivo. We thank M. D'Orazio for help in ICP-MS analyses, M. Menichini for XRF analyses and M. Bertoli for fluorine determination. This research was supported by a CNR-GNV grant to A. Sbrana.

Appendix 1: Analytical methods

Major elements, including SO3, and trace elements (Ba, Ni, Cr, V, Co, Cu, Zn, Zr, Nb, Sr, Rb, As, and C1) were analysed by X-ray fluorescence (XRF) using a Philips PW 1480 spectrometer, following the procedure of Franzini et al. (1975). REE elements, Hf, Ta, W, T1, Pb, Y, Mo, Sc, Cs, U and Th were analysed by inductively coupled plasma mass-spectrometry (ICP-MS) with a VG ® Elemental Plasma Quad 2 Plus. Operating parameters and sample preparation were those described by D'Orazio (1995). The determination of fluorine was done through fusion of the samples with sodium carbonate flux and subsequent measurement of the fluorine ions extracted by a fluorine-selective electrode, following the method outlined in D ' Orazio and Marianelli (1991).


Microanalysis of secondary minerals was carried out using a Philips XL 30 electronic scanning microscope (SEM) equipped with a EDAX DX4, using ZAF correction method. Operating conditions were 20kV voltage and about 10nA beam current. Standardisation was carried out before each analytical session using natural silicates and glasses.

XRD analyses were done on powdered samples, with a Philips diffractometer PW1710 at 36kV and 2 4 m A using a CU-Ko~ X-ray tube.

The above mentioned analyses were carried out at the Dipartimento di Scienze della Terra, University of Pisa. The determination of ~534S was done by Krueger Enterprises USA, on native sulfur and Ag2S; sulfur ions were extracted from the samples using the Kiba reagent (Kiba et al., 1955) and then precipitated as Ag2S.

Note added in proof

In this paper, we observed that all samples in the silicic facies show a significant negative Eu anomaly. We suggested that a negative Eu anomaly could also occur in altered rocks originally showing a nearly fiat REE pattern, as a consequence of a selective dissolution of Eu-rich primary minerals. Further REE data, based on a wider sampling of rocks affected by silicic alteration and having different primary lithologies, evidenced instead that the Eu anomaly of the hydrothermally altered rocks very nearly corresponds to that of the unaltered equivalent rocks. In the light of the new results, we conclude that the Eu anomaly is not affected by this type of alteration processes. These topics are discussed in a specific paper on REE behaviour in hydrothermally altered rocks of Vulcano (Fulignati et al., in prep.).

References

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Authors' address: P. Fulignati, A. Gioncada, and A. Sbrana, Dipartimento di Scienze delia Terra, Universit~ di Pisa, Via S. Maria 53, 1-56 126 Pisa, Italy

Documents

Ein geologisches Modell des magmatisch-hydrothermalen Systems von Vulcano, Aeolische Inseln, Italien