Rare-earth element (REE) behaviour in the alteration facies of the active magmatic–hydrothermal...

Ž .Journal of Volcanology and Geothermal Research 88 1999 325–342

ž /Rare-earth element REE behaviour in the alteration facies of thežactive magmatic–hydrothermal system of Vulcano Aeolian

/Islands, Italy

Paolo Fulignati ), Anna Gioncada, Alessandro SbranaDipartimento di Scienze della Terra, UniÕersita degli Studi di Pisa, Õia S. Maria, 53, 56126 Pisa, Italy

Received 29 July 1998; accepted 6 December 1998

Abstract

Ž .This work reports a study on the rare-earth element REE behaviour in the high-sulfidation-type superficial alterationand in the phyllic and propylitic alteration in the subsurface of the island of Vulcano. The results of this study demonstratenotable differences in the REE behaviour in the different alteration facies. In silicic alteration, all REE are equally stronglydepleted; in advanced argillic alteration, LREE are quite immobile whereas HREE are strongly depleted; in intermediateargillic, phyllic and propylitic alteration, REE patterns are almost unchanged compared with their fresh rock equivalents.These features indicate that the behaviour of REE in hydrothermal alteration facies at La Fossa is mainly controlled by pH,

Ž 2y y.availability of complexing ions in the fluid mainly SO and, in the silicic facies, F and the presence of secondary4

minerals able to host REE in their structures. The origin of the acid fluids is related to the hypogenic introduction ofprevalently magmatic acid gases produced by the degassing of the shallow magmatic system of La Fossa volcano. In theactive high-temperature fumarolic field of La Fossa, REE behaviour illustrates the transition from a dry vapor system,corresponding to the high-temperature fumaroles, to a wet condensing system in the surrounding area. The results of thisresearch, carried out on an active high-sulfidation system, provide a useful tool for the study of high-sulfidation epithermalore deposits and particularly help to understand the alteration processes during the main stage of hypogene wall rockalteration. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Rare-earth elements; Magmatic–hydrothermal system; High-sulfidation; Hydrothermal alteration; Vulcano

1. Introduction

Ž .In the past, rare-earth elements REE were oftenconsidered to be immobile, unaffected by the leach-ing and geochemical redistribution processes that

) Corresponding author. Fax: q39-050-500675; E-mail:[email protected]

characterize hydrothermal alteration. In contrast,more recent studies on the REE contents of hy-

Ždrothermal fluids Michard and Albarede, 1986;.Michard, 1989; Lewis et al., 1997 and in hydrother-

mally altered rocks in epithermal and porphyry cop-Žper ore deposits Alderton et al., 1980; Taylor and

Fryer, 1980, 1982, 1983; Palacios et al., 1986; Lot-.termoser, 1990; Hopf, 1993; Arribas et al., 1995 ,

have shown that, under particular conditions, REEcan be mobilized by hydrothermal alteration. REE

0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0377-0273 98 00117-6

( )P. Fulignati et al.rJournal of Volcanology and Geothermal Research 88 1999 325–342326

Ž . Ž .Fig. 1. a Sketch map of the northern part of Vulcano island modified after Fulignati et al., 1998 ; the areal distribution of theŽ .hydrothermal alteration facies is shown. Light gray areas: silicic alteration; dark gray areas: advanced argillic alunitic alteration; black

Ž . Ž . Ž .area: intermediate argillic alteration; ruled areas: fossil hydrothermal alteration advanced argillic type . 1 crater rim. 2 Piano caldera rim.Ž . Ž . Ž .3 La Fossa caldera border fault. 4 location of geothermal wells. b Schematic cross-section of the La Fossa and Vulcanello

Ž .magmatic–hydrothermal system, modified after Fulignati et al. 1998 .

( )P. Fulignati et al.rJournal of Volcanology and Geothermal Research 88 1999 325–342 327

mobility is favoured by: low pH, high waterrrockŽ 2y yratios and abundant complexing ions CO , F ,3

y 3y 2y.Cl , PO , SO in the hydrothermal solutions4 4Ž .Michard, 1989; Lottermoser, 1990, 1992 . In partic-ular, REE seem to be preferentially complexed by

y 2y ŽCl and SO in acid conditions Wood, 1990;4.Haas et al., 1995 .

In this work we present the behaviour of the REEduring hydrothermal alteration on the surface and inthe subsurface of the island of Vulcano. In particular,we focus our attention on the alteration facies relatedto the magmatic–hydrothermal system of La Fossa,

Žthe active volcanic centre of the island Fulignati et.al., 1998 . The intent is to improve our understand-

ing of the physical–chemical conditions that charac-terize the active hydrothermal alteration environmentof La Fossa. Mineralogical and geochemical studieshave identified at La Fossa a high-sulfidation-type

Ž .hydrothermal system Fulignati et al., 1998 , accord-Ž .ing to the definition proposed by Hedenquist 1987 .

Active hydrothermal systems with similar character-istics have been recognized in other island arc volca-

Ž .noes Hedenquist et al., 1993, 1994 and in activeŽacid crater lakes Christenson and Wood, 1993;

.Rowe, 1994 .

2. Geological background

Vulcano is part of the Aeolian volcanic island arcŽ . ŽFig. 1 and lies within a graben structure Barberi et

.al., 1994 linked to a NNW–SSE dextral strike slipfault, the Tindari–Letojanni discontinuity, which bi-sects the arc. The rocks of Vulcano belong to theshoshonitic association and range from basalts to

Ž .rhyolites Keller, 1980 . The active volcanic centreof the island is La Fossa, which is located inside thevolcano-tectonic structure known as the La Fossa

Ž .caldera Fig. 1 . The volcanic activity of La FossaŽhas developed in the last 6000 years Frazzetta et al.,

.1984 and ranges in composition from latitic toŽ .rhyolitic Fig. 2; Table 1 . In historical times vol-

canic activity occurred north of the La Fossa cone,generating the Vulcanello volcanic complex. Thecomposition of Vulcanello products ranges from

Ž .shoshonitic to trachytic Fig. 2 .Since the last eruption occurred in 1888-1990, La

Fossa has shown fumarolic activity, particularly in-

Ž .Fig. 2. A TAS Total Alkali vs. Silica, Le Bas et al., 1986Žclassification diagram of Vulcano rocks data from Del Moro et

.al., 1998 . B: basalt; TB: trachybasalt; SH: shoshonite; LT: latite;TR: trachyte; RY: rhyolite.

tense in the 1920s and from 1978 to the presentŽ .Barberi et al., 1991 . Vulcanello showed fumarolicactivity up until about 30 years ago. At present, at La

ŽFossa, a high-temperature fumarolic field 1008C-T.-6008C has developed inside and on the northern

Žpart of the crater; lower temperatures T around.1008C characterize the fumarolic activity in the

areas surrounding the crater. Low-temperature fu-Ž .marolic emissions TF1008C are also present out

of the La Fossa cone, in the Baia di Levante area.The main constituent of the La Fossa fumarolic

Ž .fluids is water 80 000–97 000 mmolrmol , followedŽ .by CO 18 000–176 000 mmolrmol , S species2

ŽSO , 800–11 000 mmolrmol, and H S, 100–11 7002 2. Ž . Žmmolrmol , HCl 370–9300 mmolrmol , HF 80–

. Ž .2800 mmolrmol and N 200–3500 mmolrmol2Ždata from Chiodini et al., 1993, 1995, 1996, col-

.lected in the period 1979–1995 . Acid species areŽ .much lower H S, 300–3300 mmolrmol or not2

Ž .detectable SO , HCl, HF in the Baia di Levante2Žfumaroles, dominated by H O and CO Chiodini et2 2

.al., 1995, 1996 . The redox state of high-temperatureŽ .fumaroles T)300–3508C is dominated by the

Ž .SO rH S gas buffer Chiodini et al., 1993 .2 2

2.1. The magmatic–hydrothermal system of La Fossa

The magmatic system of the La Fossa volcano isshallow and fed by volatile-rich basaltic magmas.

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Table 1Chemical analyses of the hydrothermally altered rocks of Vulcano island in the different facies

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Sulfur is reported as a major element or as a trace element in dependence of the software used to recalculate the XRF analysis.Ž .The high L.O.I. values are influenced by the abundance of S, Cl and F in the altered rocks, as reported in Fulignati et al. 1998 .

Total Fe as Fe O .2 3

b.d.l.sbelow detection limit.n.d.snot determined.

Ž .Representative analyses of unaltered La Fossa volcanic products are also reported data from Del Moro et al., 1998 except Cl, S and F: this work; REE analyses by INAA .


Melt inclusion studies have shown that strong de-gassing of H O, Cl and S occurs during the differen-2

Žtiation of these magmas Clocchiatti et al., 1994;.Gioncada et al., 1998 . The introduction of magmatic

fluids into the conduit system causes high-tempera-ture recrystallisation and metasomatism of the vol-canic and sub-volcanic rocks. Lateral to the volcanicconduits, the magmatic fluids undergo neutralizationby mixing with groundwater and water–rock interac-

Ž .tion primary neutralization , forming neutral pH,low permeability hydrothermal zones. This is shownby the phyllic and propylitic neutral paragenesisŽ .Fig. 1 found in the subsurface thanks to geothermal

Ž .boreholes Fulignati et al., 1998 . During their as-cent, magmatic acid gases, such as HCl, HF andSO , may condense in groundwater, forming acid2

solutions that react with rocks to produce surficialhydrothermal alteration. The disproportionation of

Ž .SO in H O at relatively low temperatures -3008C2 2Ž . Ž .forms H SO according to reactions 1 and 2 :2 4

4SO q4H Os3H SO qH S 1Ž .2 2 2 4 2

3SO q2H Os2H SO qS 2Ž .2 2 2 4

Ž .Sakai and Matsubaya, 1977; Giggenbach, 1987 .Silicic, advanced argillic and intermediate argillic

Ž .alteration facies develop Fig. 1 , reflecting the pro-gressive neutralization of extremely acid fluidsŽ .Fulignati et al., 1998 . We believe that the silicicalteration at Vulcanello probably originated undersimilar chemical–physical conditions.

2.2. Hydrothermal alteration facies

Silicic alteration was mainly found in the craterarea of La Fossa in association with the activefumarolic field, and in the inner part of the altered

Ž .zone of Vulcanello Fig. 1 . In this facies, the pri-mary rock undergoes a complete leaching by ex-

Žtremely aggressive fluids with pH-2 Fulignati et.al., 1998; Fulignati and Sbrana, 1998 . These fluids

produce a porous residual rock made up of 90–95%SiO . The alteration assemblage is dominated by2

amorphous silica. Other phases are: realgar, barite,native sulfur and, in correspondence with the highest

Žtemperature fumaroles T)2508C, Chiodini et al.,.1996 , cristobalite and incrustations of halide and

Ž .sulfide sublimates Fulignati and Sbrana, 1998 .

The silicic alteration zones are surrounded byextensive areas characterized by pervasive alteration

Ž .of the advanced argillic alunitic type, which oftencompletely obliterates the original rock. The miner-alogical paragenesis is dominated by abundant alu-nite. Some alunitized zones are covered by a hardcrust of gypsum and jarosite probably of supergeneorigin.

In a small zone located on the western side of theLa Fossa crater, the advanced argillic facies gradesinto an intermediate argillic alteration facies. Alsohere the style of alteration is pervasive and obliter-ates the original lithologies. The intermediate argillicfacies is characterized by the presence of halloysiteand smectites and by the almost complete absence ofalunite, reflecting a pH increase in the hydrothermal

Žfluids and lower sulfate ion activity Fulignati and.Sbrana, 1998 .

Advanced argillic alteration was also found atŽFaraglione and in two areas Punta Luccia and Monte

.Saraceno corresponding to old eruptive conduitsŽwithout active fumarolic activity fossil hydrother-

. Ž .mal alteration zones Fig. 1 .A high-sulfidation-type hydrothermal system has

been active at La Fossa at least since the VI century,as demonstrated by the presence of the same alter-ation facies found in the lithics of hydrothermallyaltered rocks occurring in the Breccia di Commenda

Ž .deposits Fulignati et al., 1998 .The stratigraphic sequence and hydrothermal al-

teration paragenesis of the wells drilled from the1950s to the 1980s have been studied by Faraone et

Ž . Ž .al. 1986 , Cavarretta et al. 1988 , Gioncada et al.Ž . Ž .1995 and Fulignati et al. 1996 . The study of thesewells has revealed that hydrothermal fluid circulationdeveloped at depth, lateral to the conduit system ofLa Fossa with subsequent development of phyllicŽmixed layer illitersmectite and chloritersmectite

. Žparagenesis and propylitic chlorite, corrensite, adu-.laria and epidote paragenesis alteration facies.

3. Analytical methods

Major elements, As and Cl were analysed byŽ .X-ray fluorescence XRF using a Philips PW 1480

apparatus, following the procedure proposed by


Ž .Franzini et al. 1975 , based on the overall correctionof the matrix effects. Rare-earth elements were anal-ysed by inductively coupled plasma mass-spectrome-

Ž .try ICP-MS on approximately 50 mg of whole rockpowder with a VG w , Elemental Plasma Quad 2 Plus.Details of the operating parameters and analytical

Ž .procedure are given in D’Orazio 1995 . Fluorinewas determined using a fluorine-selective electrode.

The study of REE behaviour in the hydrothermalalteration paragenesis was carried out on rocks af-fected by the three surficial alteration facies previ-

Žously described silicic, advanced argillic and inter-.mediate argillic . Altered rocks were also sampled

close to the high-temperature fumarole vents. Tem-peratures were taken at the sampling point, about 40cm below the surface, using a thermocouple appara-tus suitable for acid fluids. Cutting samples from thegeothermal borehole ‘Vulcano Porto 1’ from 615 m,

Ž . Ž .735 m phyllic facies and 925 m propylitic faciesof depth were also analysed.

4. Analytical results

REE patterns of the hydrothermally altered rocks,Žnormalized to the chondrite values Mc Donough

ŽFig. 3. Chondrite-normalized REE patterns of samples from the silicic alteration facies rocks compared to unaltered equivalent rocks gray. Ž . Ž . Ž .areas . Chondrite-normalizing values from Mc Donough and Sun 1995 . a rhyolitic and trachytic rocks; b latitic rocks.


Ž . Ž .Fig. 4. Representative isocon diagrams left side and percentages of gains and losses right side focused on REE behaviour in VulcanoŽ . Ž . Ž .surficial hydrothermal facies; a silicic alteration; b advanced argillic alteration; c intermediate argillic alteration. In isocon diagrams the

Ž . Ž .component abundances of fresh rocks C are plotted on the horizontal scale and those of their altered equivalents C are plotted on theŽo. Ža.Ž .vertical scale. The isocon position was determined considering the variation of density d . Percentages of gain and losses plotted on the

Ž . Ž A O .Ž A O . Ž .histograms were calculated using the isocon equation: DC rC y100s r rr C rC y1 where DC rC =100 is the percentagei i i i i i

variation of the ith element in the altered rock compared with the fresh rock, r A and r O are density of the altered and fresh rocks,respectively. Values of q4 to q1sa gain of more than 100%, 50 to 100%, 25 to 50% and 10 to 25%, respectively. 0s immobile elementsŽ .variation in the range q10 and y10% . Values of y1 to y4s losses of 10 to 25%, 25 to 50%, 50 to 90%, more than 90%, respectively.


.and Sun, 1995 , were compared with REE patternsof fresh rocks considered to be equivalent on thebasis of the fact that they belong to the same erup-

Ž .tive unit. In addition, we compared the LarSm ,cnŽ . Ž .LarYb and TbrYb ratios between the alteredcn cn

and fresh rocks with the aim of deducing the possi-ble fractionation processes which the REE may haveundergone, during hydrothermal alteration. REE val-ues relative to the unaltered rocks were taken from

Ž .Del Moro et al. 1998 and are reported in Table 1.With the aim of evaluating the absolute mobility of

the REE, we took into account the variations of massinduced by alteration. For this reason, the data were

Žprocessed using the isocon diagram method Grant,.1986 , which represents a graphical solution of the

Ž .Gresens 1967 equation, considering the variationsin density between altered and unaltered rocks.

In the silicic alteration facies both LREE andHREE are strongly depleted compared to the unal-

Ž .tered rocks Figs. 3 and 4a . The extent of theimpoverishment is variable and seems to be related

Ž .to the temperature at the sampling point Fig. 3 . The

Ž .Fig. 5. High-temperature fumarolic field of La Fossa from Bukumirovic et al., 1997, modified and areal distribution of the hydrothermalalteration facies in the La Fossa area. Selected samples collected at fumaroles are shown.


Fig. 6. Chondrite-normalized REE patterns of samples from the high-temperature fumarolic field shown in Fig. 5; for each sample theground temperature is reported.

Ž . Ž . Ž . Ž .Fig. 7. Variations in LarSm , LarYb , TbrYb ratios a, b, c , illustrating the REE fractionation in the hydrothermally alteredcn cn cnŽ . Ž .rocks compared to unaltered equivalent rocks. The EurEu) ratio d shows the variation of the Eu anomaly. Ruled blocks refer tocn

rhyolitic and trachytic rocks, gray blocks to latitic rocks.


Fig. 8. Chondrite-normalized REE patterns of samples from theadvanced argillic alteration facies rocks compared to unaltered

Ž .equivalent rocks gray areas . Chondrite-normalizing values fromŽ . Ž .Mc Donough and Sun 1995 . a alteration of rhyolitic and

Ž . Ž .trachytic rocks; b alteration of latitic rocks; c an example ofREE behaviour in advanced argillic alteration facies is shown by

Ž . Ž .F0 unaltered and F1 altered samples, collected from the samedeposit.

less depleted REE patterns mostly correspond to thesamples collected from fumaroles characterized by

Ž .high temperatures about 220–2308C . With the aimof better understanding this process, we studied REEbehaviour in samples collected in correspondencewith and moving away from the highest-temperaturefumaroles on the northern edge of the La Fossa

Ž .crater Fig. 5 . These fumaroles reach temperaturesin excess of 4008C. As illustrated in Fig. 6, the REEpatterns show almost no impoverishment, comparedto their fresh rock equivalents, corresponding with

Ž .the highest-temperature fumaroles CR-13, CR-17 ,but a slight impoverishment away from the fu-

Ž .maroles CR-14, CR-15, CR-4 .Some samples from the silicic alteration facies, if

compared with the fresh rock equivalents, show aweak HREE fractionation with respect to the LREE,

Ž . Ž .as shown by the LarYb ratio Fig. 7b . In addi-cnŽ . Ž . Ž .tion, LarSm and TbrYb ratios Fig. 7a,ccn cn

Fig. 9. Chondrite-normalized REE patterns of samples from theŽ . Ž .intermediate argillic a and phyllic–propylitic b alteration facies

Ž .rocks compared to unaltered equivalent rocks gray areas . Chon-Ž .drite-normalizing values from Mc Donough and Sun 1995 .


also reveal a weak fractionation inside the LREE andŽ .HREE. In all the studied samples, the EurEu) iscn

Ž .in the range calculated for the fresh rocks Fig. 7d .REE patterns in the advanced argillic facies show

a quite constant immobility or slight enrichment ofŽ .the LREE Fig. 4b and Fig. 8a,b . By contrast, the

HREE are strongly impoverished. This behaviour isŽ .well represented for example by the F0 unaltered

Ž .and F1 samples altered , which were collected in theŽ .Faraglione area about 1 m from each other Fig. 8c .

This evidence is well illustrated by means of theŽ . Ž . Ž . ŽLarYb , LarSm and TbrYb ratios Fig.cn cn cn. Ž .7 . The LarYb ratio, in particular, points out thecn

strong fractionation undergone by the HREE withŽ . Ž .respect to the LREE. The LarSm and TbrYbcn cn

ratios show that also the LREE and the HREErespectively are fractionated with respect to the freshrock equivalents. The europium anomaly seems to be

Ž .unaffected by advanced argillic alteration Fig. 7d .REE patterns in the intermediate argillic facies are

Žsimilar to the fresh rock equivalents Fig. 4c and Fig.. Ž .9a . This conclusion is confirmed by the LarYb ,cn

Ž . Ž .LarSm and TbrYb ratios which are quitecn cn

constant and similar to the unaltered rock values.Some samples show a slight enrichment in bothLREE and HREE.

The REE patterns of the hydrothermally alteredrocks from the geothermal borehole ‘Vulcano Porto

Ž .1’ phyllic and propylitic alteration do not divergeŽ .from the patterns of the fresh rocks Fig. 7b .

Ž . Ž .LarSm and LarYb ratios are slightly lowercn cn

compared with the unaltered rock equivalents. Thisprobably reveals a weak fractionation among the

LREE and between the LREE and HREE, respec-tively.

5. Discussion

5.1. Silicic alteration

The strong impoverishment in all REE, in theŽ .silicic facies Figs. 3 and 4a , derives from the

complete breakdown of the primary minerals and theglasses in the original rock, mainly caused by the

Žvery low pH conditions pH-2 as inferred by.Fulignati et al., 1998 , with subsequent REE removal

by the altering fluids. The REE depletion is favouredby the absence, in this alteration facies, of secondaryminerals which can host the REE in their structuresŽ .Table 2 .

As regards the REE speciation in the fluids whichproduced the silicic alteration, given the relatively

Ž .low temperature -3008C , low pH, oxidizing con-ditions and the abundance of sulfur, we consider thatsulfate ion can be the most important complexingagent in the fluids, according to the results of WoodŽ . Ž .1990 and Lewis et al. 1998 . Nevertheless, wecannot exclude a contribution of the Cly in complex-

Žing REE Wood, 1990; Lottermoser, 1990, 1992;.Haas et al., 1995 , as suggested by the high HCl

Ž .content in fumarole gases Chiodini et al., 1993 .REE patterns in the silicic facies do not show anyfractionation compared to the unaltered rocks. This

q Žagrees with the stability constants for LnSO Lns4

Table 2Summarizing table of the REE behaviour in the hydrothermal alteration facies of Vulcano. Temperatures of phyllic and propylitic facies are

Ž .taken from Gioncada et al. 1995

Alteration facies Silicic Advanced argillic Intermediate argillic Phyllic Propylitic

pH -2 2-X-4 )4 Near neutral Near neutralTemperature -2508C F1008C F1008C 1508–2008C 2358CLREE Strongly depleted Nearly immobile or Nearly immobile Immobile Immobile

La and Ce enrichedHREE Strongly depleted Strongly depleted Nearly immobile Immobile ImmobileEurEu) Unmodified Unmodified Unmodified Unmodified Unmodified

2y y y 2y 2yDominating complexing agent SO )F , Cl SO SO – –4 4 4

REE bearing mineral phase none Alunite–natroalunite Smectites, halloysite goethite – –


.lanthanides , showing that the LREE and HREE areŽequally complexed by the sulfate ion Wood, 1990;

.Haas et al., 1995 . In some cases, a slight fractiona-tion of HREE with respect to LREE is present. Thisevidence may suggest that also the Fy ion, which

Ž .preferentially complexes HREE Lewis et al., 1997 ,has an important role in leaching REE in the silicicalteration. We may presume that the fluids responsi-ble for the silicic alteration at La Fossa have signifi-cant contents of fluorine, on the basis of the highfluorine contents in the altered rocks in correspon-

Ž .dence of the fumarolic vents Table 1 and of theHClrHF values in the fumarolic fluids between 0.8

Žand 13.7 in the period 1988–1995 Chiodini et al.,.1993, 1996 .

We believe that the sorption effects cannot ex-plain the behaviour of REE in the silicic facies, sincesorption should result in a fractionation of LREEwith respect to HREE because the strength of sorp-

Žtion decreases with increasing ionic radius Bau and.Moller, 1992 . This is in agreement with the results

Ž .of Haas et al. 1995 , showing that REE have astrong tendency to form complexes in aqueous solu-tions in the presence of significant amounts of com-plexing ions.

5.1.1. High-temperature fumarolesIn the area of the La Fossa crater the silicic facies

corresponds to a thermally anomalous wide area. Thehighest temperatures measured in this work corre-

Ž .spond to the fumarolic vents 250–4138C . Tempera-tures radially decrease away from the vents to around1008C, measured on a wide, nearly circular areacentered on the main fumarolic field, having a radiusof about 150 m.

The strong leaching process that characterizes thesilicic alteration leaves a rock composed almost en-tirely of residual silica, irrespective of its primarycomposition. The SiO content can therefore be used2

as a diagnostic parameter of the extent of develop-ment of the silicic alteration, since the altered rockshave SiO contents higher than the unaltered La2

Fossa volcanics. If we compare the SiO content to2Ž .other major and trace elements Fig. 10a,b,c , a good

inverse linear relation can be observed: with theapparent increase of silica the other elements de-crease. The variability in the degree of depletion ofREE depends therefore on the extent of the leaching

Ž .Fig. 10. Al O wt.%, K O wt.% and La ppm vs. SiO wt.% of2 3 2 2

samples collected inside the areas interested by silicic alteration.The gray areas show the composition of unaltered rocks. Theinsets show the variations of the same elements in the samples

Ž .from the high-temperature fumarolic field see Fig. 5 .

process at the expense of the original rocks. Themost depleted patterns belong to the rocks character-

Žized by a ‘mature’ silicic alteration SiO )95%,2.Al O -1% . The fact that the silicic alteration is2 3

more intensely developed going away from the high-Ž .est-temperature fumaroles Figs. 5 and 6 reflects a

progressive increase in the aggressiveness of thefluids generating this type of alteration. This evi-


dence is in agreement with the fact that the hightemperature fumaroles represent a gas and dry vapoursystem, hotter than 4008C where the redox state ofthe fumarolic fluid is dominated by the SO –H S2 2

Ž .gas buffer Chiodini et al., 1993 . These conditionsprevent the disproportionation of SO , thus consider-2

ably decreasing the amount of H SO available. In2 4Ž .addition, at high temperatures higher than 2508C

the coefficient of dissociation of acids is noticeablyŽ .lower Ruaya and Seward, 1987 , thus reducing the

aggressiveness of HCl and H SO . Away from the2 4

fumarolic vents the temperature decreases rapidlyŽ .Fig. 6 . In a few meters, a transition to a wet systemoccurs, produced by the condensation of fumarolicfluids, the dissociation coefficient increases and thedisproportionation reactions can produce H SO . All2 4

these factors together favour a lowering of pH of thefluids and the consequent leaching of the rocks.Major element variations confirm this picture: sam-ples near to the high temperature zones are almostunaltered compared to their fresh rock equivalents.Going away from the fumarolic vents there is aprogressive depletion in all elements and a corre-sponding increase in SiO , reaching the composition2

Ž .of the typical silicic alteration facies Fig. 10 .

5.2. AdÕanced argillic alteration

In the advanced argillic alteration, as describedbefore, REE fractionation is much more evident thanin the silicic alteration, as highlighted in Fig. 7a,b,cand Fig. 8. Similar REE behaviour was found also

Ž .by Arribas et al. 1995 in the advanced argillicŽparagenesis of the epithermal system Au– Cu–Te–

.Sn high-sulfidation-type of Rodalquilar in Spain.The fluids that produced the advanced argillic alter-ation facies, which occurs as an outward zonation

Ž .from the silicic facies Fig. 1 , derive from thepartial neutralization and dilution of fluids involvedin the silicic alteration, carried out by shallow watersŽ .Fulignati et al., 1998 . The presence of alunite as acharacteristic mineral phase in this hydrothermal fa-cies indicates a high activity of the sulfate ion in the

Ž .hydrothermal solutions Knight, 1977 . However, thesulfate ion abundance in solution does not explainthe observed strong fractionation of the HREE withrespect to the LREE. In fact the scarce dependence

of the sulfate complex on the REE3q ionic radiusallows us to infer a very minor or no REE fractiona-

Ž .tion Wood, 1990 . We believe that the relevant lossof HREE can be produced by the acid alterationŽ .although pH is above 2 undergone by the rocks,resulting in the breakdown of the primary mineralsand glass and leaching of REE by hydrothermalsolutions. On the other hand, the substantial LREEimmobility in the advanced argillic alteration canprobably be ascribed to the entrance of these ele-

Ž .ments into the alunite lattice Table 2 . LREE can, infact, take the place of potassium in the large radius Asite, considering the general formula of the alunite–

Ž . Ž .jarosite mineral group AB XO OH . Further-3 4 2 6

more, a lanthanum- and cerium-bearing phase of thisŽ .mineral group florencite has been reported to occur

Žin high-sulfidation epithermal systems Arribas et al.,.1995 . The importance of the secondary mineral

assemblage on the REE behaviour in the hydrother-Ž .mal paragenesis was also suggested by Hopf 1993

in a study on the REE patterns in hydrothermallyaltered rocks from New Zealand geothermal systems.

5.3. Intermediate argillic alteration

REE immobility in the intermediate argillic alter-ation confirms the close relationship between REEmobility and the pH of the hydrothermal solutionsŽMichard and Albarede, 1986; Michard, 1989; Lewis

.et al., 1997 . This hydrothermal paragenesis reflectsless acid pH conditions and a lower activity ofSO2y, Cly and Fy ions with respect to the silicic4

and advanced argillic alteration facies. On the basisof the spatial relationships between the areas charac-terized by advanced argillic and by intermediateargillic alteration, we can infer that the intermediateargillic alteration is produced by magmatic–hydro-thermal fluids which were aggressive in origin, butwhich have been partially neutralized by the water–rock interaction processes which generated the silicicand advanced argillic alteration and possibly by mix-ing with meteoric fluids. The slight increase in REEshown by rocks altered to the intermediate argillicfacies could be attributed to the partial destabiliza-

Žtion due to a decrease in the concentration of com-.plexing ions in solution and to the increase in pH of

the ionic complexes transporting the REE in solu-


tion, resulting in the deposition of REE in this facies.Since the intermediate argillic facies is characterized

Ž .at Vulcano by halloysite kaolin group and by smec-tites and goethite, we suggest that REE could befixed in the structure of these minerals. In order to

Žverify this hypothesis, the clay-size fraction -2.mm of the altered rock was analysed. The results

showed slight enrichment in REE, in particularŽLREE, compared with the bulk rock sample Fig.

.11 . The ability of phyllosilicates and goethite to beenriched in REE by sorption is reported in experi-

Žmental studies on kaolinite Laufer et al., 1984; Aja,. Ž . Ž1998 , on montmorillonite smectite group Bruque

.et al., 1980; Miller et al., 1982 and on goethiteŽ .Koeppenkastrop and De Carlo, 1992 . In particular,in goethite an enrichment in LREE compared toHREE has been observed, similar to the enrichmentshown by the Vulcano sample. The low activity of

Ž y y 2y.complexing ions Cl , F and SO in the fluids4

producing the intermediate argillic alteration mayfavour sorption effects of REE elements onto clayminerals and goethite.

5.4. Phyllic and propylitic alteration

The REE patterns of phyllic and propylitic alteredrocks from the ‘Vulcano Porto 1’ well do not showrelevant variations with respect to unaltered rocks, as

Ž .reported also in Arribas et al. 1995 for similaralteration facies. The immobility of REE in the

neutral pH conditions characterizing the phyllic andpropylitic facies underlines the dependency of REEmobility on the pH of alteration. In addition, REEimmobility suggests low waterrrock ratios in thesealteration facies.

5.5. Eu behaÕiour

The observation that the REE patterns of allalteration facies at Vulcano do not show relevant Eufractionation with respect to the patterns of unaltered

Ž .rocks Fig. 7d may suggest that Eu is in the formEu3q, reflecting a temperature of alteration lowerthan 2508C. In fact, at temperatures higher than

2q Ž2508C, Eu should predominate in solution Sver-.jensky, 1984 ; therefore, Eu should be fractionated

with respect to the other REE3q, as a consequence ofthe fact that the change Eu3q™Eu2q is coupled byan increase in the ionic radius, allowing the ionEu2q to have a higher strength of sorption and toform hydroxide complexes more stable than Eu3q

Ž .Bau, 1991 . Also, the low pH values and the ele-vated oxygen fugacity characterizing the high-sulfi-

Ž .dation leaching environments Hedenquist, 1995should favour the stability of the trivalent Eu in

Ž .solution Sverjensky, 1984; Bau, 1991 .A positive Eu anomaly in the low pH, sulfate-rich

geothermal fluids of Yellowstone is reported byŽ .Lewis et al. 1997 . These authors suggest that the

positive Eu anomaly in the fluids derives from the

Ž .Fig. 11. Enrichment factors of the REE in the clay-size -2 mm fraction of a sample characterized by intermediate argillic alteration,compared to the bulk altered rock.


Ž .preferential breakdown of Eu-rich phases sanidinepresent in the host rock, together with the fact thatEu is not fixed in any secondary minerals. Althoughthe latitic to rhyolitic volcanics of La Fossa arecharacterized by the presence of plagioclase and

Žalkali–feldspar as phenocrysts with maximum phe-.nocryst abundance of around 20% in volume and in

the groundmass, presenting a strong positive EuŽ .anomaly Del Moro et al., 1998 , Eu is not fraction-

ated in the altered rocks. This agrees with the obser-vation that the alteration, rather than selectively af-fecting feldspars, is intensely pervasive in all thedifferent facies. To verify if a selective alteration onfeldspars could have an influence on the Eu anomalyin the altered rocks, we subtracted from the composi-tion of an unaltered latitic rock an amount of plagio-clase corresponding to the plagioclase abundance.We calculated that the Eu fractionation would resultin an increase in the Eu anomaly of the altered rockwith respect to the unaltered one of 15% at most. Inthe case of rhyolitic products, the lower phenocrystabundance would result in a still more limited effecton the Eu anomaly.

6. Conclusions

The study of REE behaviour in the alterationfacies of the active magmatic–hydrothermal systemidentified on Vulcano island, showed the strong REEmobility in acidic and extremely acidic conditions.The maximum REE impoverishment occurs with thesilicic alteration, where all REE are equally depletedwithout significant fractionation. By contrast, in theadvanced argillic alteration facies, LREE are immo-bile and HREE are strongly depleted. REE mobilityis almost absent in the intermediate argillic, phyllicand propylitic facies.

The strong REE depletion in the silicic and ad-vanced argillic facies is related to low pH and abun-dance of mainly SO2y ion in solution. The scarce4

REE mobility in the intermediate argillic, phyllic andpropylitic facies may be due to the higher pH condi-tions and, in the case of phyllic and propylitic facies,also to the lower waterrrock ratio characteristic ofthese alteration typologies.

The weak HREE fractionation in the silicic faciesmay indicate that Fy plays an important role in REE

complexing also in acid conditions. The absence ofREE-bearing minerals in this facies means that thesecondary mineralogy cannot influence REE be-haviour here.

The relevant HREE fractionation with respect tothe LREE in the advanced argillic facies is mainlydue to the presence of secondary minerals which canfix the LREE into their structures, in our case alu-nite. In turn, the presence of alunite depends on pHand sulfate ion activity.

The slight REE enrichment shown in the interme-diate argillic facies could be due to the destabiliza-tion of the ionic complexes which carry the REE in

Žsolution, caused by chemical–physical variations pHincrease, temperature and abundance of complexing

.ions decrease . The REE released from hydrothermalŽsolutions are fixed in phyllosilicates kaolinite, smec-

.tites, goethite abundant in this alteration facies. Ourresults confirm the importance of the secondary min-eralogy in the control of the REE behaviour in thehydrothermal paragenesis.

In the high-temperature fumarolic field located onthe northern rim of the La Fossa crater, the REEbehaviour is linked to the transition from a drysystem, in accordance with the high-temperature fu-marolic vents, to a surrounding wet system, origi-nated by the condensation of the fumarolic fluids.The natural fumarolic system can therefore be de-scribed as a high-temperature conduit dominated bydry vapor and gases and controlled by the SO –H S2 2

gas buffer, surrounded by lower temperature con-densing zones where rocks are almost saturated inacid solutions. In accordance with the fumarolicvents, REE patterns are almost unaltered with respectto the fresh rocks. This is due to the high tempera-ture that prevents SO disproportionation processes2

and reduces the dissociation constants of the acidspecies, thus reducing their aggressiveness. In addi-tion, the fumarolic fluids, due to the high tempera-tures, consist of dry vapour, thus preventingwaterrrock interaction processes. A few meters awayfrom the fumarolic vents, with decreasing tempera-ture, the fumarolic fluids condense and their acidityincreases, therefore allowing the progressive REEmobilization in the altered rocks.

This study shows how REE behaviour in thehydrothermal alteration paragenesis gives importantinformation concerning the chemical–physical condi-


Ž .tions pH, T , fO in which leaching processes2

occurred. The results of this study, carried out on anactive high-sulfidation hydrothermal system, can alsorepresent a useful tool to study high-sulfidation ep-ithermal ore deposits, because they help to under-stand the surficial alteration mechanisms which de-velop in these systems during the phase defined by

Ž .Arribas et al. 1995 as ‘main stage of hypogenewall-rock alteration’.

Acknowledgements

The authors wish to thank Dr. Massimo D’Oraziofor help in ICP-MS analyses, Massimo Menichini forXRF analyses and Dr. M. Bertoli for fluorine deter-mination. We are in debt also with Roberto Cioni

Žand Massimo Guidi Istituto di Geochimica e.Geocronologia Isotopica, C.N.R. Pisa for helpful

comments on an early version of the manuscript.This paper has also greatly benefited from the criti-cal reviews of Pat Browne and Anita Grunder. Thisresearch was supported financially by a CNR-GNVgrant to A. Sbrana.

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