Miocene fossil hydrothermal system associated with a volcanic complex in the Andes of central Chile

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Journal of Volcanology and Geothermal Research 138 (2004) 139–161

ne fossil hydrothermal system associated with a volcanic

complex in the Andes of central Chile

cisco Fuentesa,*, Luis Aguirrea, Mario Vergaraa, Leticia Valdebenitoa,

Eugenia Fonsecab

aDepartamento de Geologıa, Universidad de Chile, Casilla 13518, Correo 21, Santiago, ChilebServicio Nacional de Geologıa y Minerıa, Casilla 10465, Santiago, Chile

Received 18 September 2003; accepted 2 July 2004

eposits in the Andes of central Chile have been affected by very low-grade burial metamorphism. At about 338Se Chacabuco area, approximately 53 km north of Santiago, two Oligocene and Miocene volcanic units form a ca.

rock pile. The Miocene unit corresponds to a volcanic complex composed of two eroded stratovolcanoes.

neral assemblages in both units were studied petrographically and using X-ray diffraction and electron

alyses. Most of the igneous minerals are wholly or partially preserved, and the ubiquitous secondary minerals are

afic phyllosilicates. The alteration pattern observed is characterized by a lateral zonation in secondary mineralogy

eral increase in temperature but not to stratigraphic depth. The following three zones were established, mainly

distribution of zeolites: zone I comprises heulandite, thomsonite, mesolite, stilbite and tri-smectite; zone II

ntite, yugawaralite, prehnite, epidote and chlorite; and zone III comprises wairakite, epidote, chlorite, diopside,

nite. For each zone, the following temperature ranges were estimated: zone I, 100–180 8C; zone II, 180–270 8C;245–310 8C. The alteration episode was characterized by a high Pfluid/Ptotal ratio (ca. 1.0), although slightly

h geothermal gradient of ca. 160 8C km�1 and fluid pressures below 500 bars. Although temperature was the

n the mineral zonation, several interrelated parameters, mainly fluid composition, porosity and permeability, were

. Hot, near neutral to slightly alkaline pH, alkali chloride hydrothermal fluids with very low dissolved CO2

ited the secondary minerals. The alteration pattern is the result of depositing fluids in outflow regions from a

ystem developed inside a volcanic complex during the Miocene. The hydrothermal system has been eroded to a

th of 1.7 km.

er B.V. All rights reserved.

s; Chile; hydrothermal system; zeolites; mafic phyllosilicates; outflows

ee front matter D 2004 Elsevier B.V. All rights reserved.

olgeores.2004.07.001

ding author. Tel.: +56 2 6784124; fax: +56 2 6963050.

ess: [email protected] (F. Fuentes).

www.elsevier.com/locate/jvolgeores

1. Introduction

The Andes of central Chile comprise three mor-

phostructural units, which from west to east are: the

Coastal Cordillera, the Central Depression, also called

Central Valley, and the Principal Cordillera (Fig. 1a).

The Central Depression and Principal Cordillera

consist largely of a several kilometer thick series of

upper Cretaceous to Cenozoic volcanic continental

rocks erupted from central volcanoes and deposited in

subsident basins, possibly grabens, after the closure of

an early Cretaceous extensive intra-arc basin (Vergara

and Drake, 1979; Aberg et al., 1984; Thiele et al.,

1991; Charrier et al., 1996). All these deposits have

been affected by low- to very low-grade metamor-

phism characterized by an increase in grade with

stratigraphic depth. This is thought to be a conse-

quence of burial metamorphism ranging from zeolite

to prehnite-pumpellyite facies without a change in

texture and structure (Levi et al., 1989). However,

some Eocene–Oligocene and Miocene units from the

Central Depression and the Principal Cordillera of

central Chile display geothermal-type metamorphic

patterns which interrupt the regional metamorphic

pattern and reflect variations along and across the

Andes in tectonic setting and thermal gradient (Levi et

al., 1989; Thiele et al., 1991; Vergara et al., 1993).

Between 328S and 358S, the Eocene–Oligocene unit isknown as the Abanico Formation and the Miocene

unit, the Farellones Formation (Vergara and Drake,

1979; Aguirre, 1985; Vergara et al., 1988; Rivano et

al., 1990; Charrier et al., 1996). Levi et al. (1989)

Fig. 1. (a) Location map of the studied area showing the three morphostructural units of central Chile. Abbreviations as follows: CC=Coastal

Cordillera, CD=Central Depression, PC=Principal Cordillera. (b) Geologic map of the Cuesta de Chacabuco area showing the location of the

analyzed samples (slightly modified from Fuentes et al., 2002). 1=Algarrobo unit, 2=Chacabuco unit, 3=Lo Valle Formation, 4=subvolcanic

stocks, 5=fault, 6=strike and dip of strata, 7=subhorizontal strata, 8=road, 9=cross–section of Fig. 3, 10=location of stratigraphic columns of Fig.

2, 11=sample location. Letters a–f are the stratigraphic columns of Fig. 2.

F. Fuentes et al. / Journal of Volcanology and Geothermal Research 138 (2004) 139–161140

assigned the metamorphism of the Abanico Formation

to the zeolite and prehnite-pumpellyite facies and that

of the Farellones Formation to the mordenite and

laumontite subfacies of the zeolite facies.

The sector studied here is located at the northern

boundary of the Central Depression, in the Cuesta de

Chacabuco area (32856VS–32859VS and 70843VW–

70835VW), ca. 53 km north of Santiago (Fig. 1a). In

this area, a series of volcanic and volcaniclastic

stratified rocks crops out and is intruded by various

types of subvolcanic rocks, the whole conforming a

mountainous chain almost perpendicular to the

Principal Cordillera. The regional alteration in this

area is characterized by zeolite facies assemblages

corresponding to the mordenite subfacies (Levi et al.,

1989). Recent studies in the area have clarified the

relationship of these deposits to units present in

central Chile. In particular, Fuentes et al. (2002) have

correlated the rocks at the Cuesta de Chacabuco area

with the Abanico and Farellones formations.

The strata of the Cuesta de Chacabuco area have

been examined with the aim of studying their

metamorphic characteristics in order to relate them to

the regional alteration pattern proposed by Levi et al.

(1989). This paper intends to demonstrate: (1) that the

alteration pattern observed in the Cuesta de Chacabuco

area reveals a high palaeothermal gradient, constitut-

ing an exception to the bnormalQ burial metamorphic

pattern; and (2) that this alteration process relates to a

Miocene hydrothermal system developed inside a

volcanic complex. As a result, this work shows, for

the first time, a geothermal-type alteration directly

related to a volcanic complex developed during the

Miocene in the Andes of central Chile.

2. Geological setting

The rocks exposed at the Cuesta de Chacabuco area

are continental volcanic rocks devoid of chronologi-

cally significant fossils. Their ages have been deduced

based on regional lithologic correlations and have been

grouped in different units by previous authors (Aguirre,

1960; Padilla, 1981; Rivano et al., 1993, among

others). As a result, a long-standing controversy has

arisen among Chilean geologist concerning this issue.

Here, we have adopted the framework recently

proposed by Fuentes et al. (2000, 2002) for this area

based on field observations, and geochronological and

geochemical studies. Based on these results, the

volcanic strata can be grouped into three units (Fig.

1b), which, with decreasing age, are: the upper Creta-

ceous ignimbrite-dominated Lo Valle Formation to the

west, the Oligocene Chacabuco unit composed of lavas

and continental sedimentary rocks to the centre, and the

lower Miocene Algarrobo unit corresponding to an

eroded volcanic complex, to the northeast.

The Lo Valle Formation (Thomas, 1958) consists

of subhorizontal silicic pyroclastic flows and ash-fall

deposits with intercalations of lavas and sedimentary

rocks. K/Ar ages of 70.5F2.5 and 64.6F0.5 Ma

(plagioclase separate and whole rock, respectively;

Drake et al., 1976) for ignimbrites located about 15

km SW of the Chacabuco tunnel (Fig. 1b) and three40Ar/39Ar plateau ages of 71.9F1.4, 72.4F1.4 and

71.4F1.4 Ma (plagioclase separates; Gana and Wall,

1997) for silicic pyroclastic rocks immediately south

of the studied area indicate a late Cretaceous age for

this formation.

The Chacabuco unit (first defined by Fuentes et al.,

2000, 2002) is made up of mafic to intermediate lavas

with intercalations of fluvial and lacustrine sedimen-

tary and volcaniclastic rocks and ash-fall deposits. It is

separated from the Lo Valle Formation by a NNE-

trending normal fault, the Infiernillo fault (Fig. 1b). The

lava flows and volcaniclastic strata of the Chacabuco

unit strike about 108E and are inclined 108 to 208 to theeast. Their total thickness in the area can be estimated to

be at least 500 m. The best exposures of this unit are

found at the southern entrance to the Chacabuco tunnel

where several reddish brown flows of basalts and

basaltic andesites crop out. These flows are 5–20 m

thick, partly brecciated and have amygdaloidal tops.

They are porphyritic with intersertal to intergranular

groundmass and contain phenocrysts of plagioclase

(An91Ab9Or0 to An50Ab50Or0), clinopyroxene (En50Fs16Wo34 to En47Fs7Wo46) and olivine (Fo74 to Fo80);

clinopyroxene also occurs as crystal aggregates up to 5

mm. A series of grayish red and grayish green ash-fall

deposits, claystones, matrix-supported volcaniclastic

sandstones with normal and reverse graded bedding,

and clast-supported conglomerates with up to 15 cm

diameter clasts, constitutes the lower part of the unit,

exposed next to the Infiernillo fault. This sequence was

deposited in a lacustrine to fluvial environment.

Fuentes et al. (2002) reported two 40Ar/39Ar ages close

F. Fuentes et al. / Journal of Volcanology and Geothermal Research 138 (2004) 139–161 141

to 29Ma for whole rock samples of tholeiitic lava flows

of the Chacabuco unit and interpreted a plateau age of

28.8F0.3 Ma as the best estimate for their age.

The Algarrobo unit (first defined by Fuentes et al.,

2000, 2002) is composed of two eroded structures

showing morphological, lithological and structural

features typical of stratovolcanoes. One is located

close to the Cobre de Chacabuco hill and the other near

the Algarrobo hill (Fig. 1b). The stratovolcano close to

the Algarrobo hill is deeply eroded and dissected,

specially on its western flank, in part as a result of

NNE and NNW fault systems. The erosion reveals the

internal structure of the volcano with its swarm of

radially distributed feeder dikes and subvolcanic

intrusions near the top. The deposits of the Algarrobo

unit, ca. 800 m thick, unconformably overly the

Chacabuco unit and consist of intermediate to mafic

lavas, pyroclastic flows, nuees ardentes, lahars, tuffs

and continental sedimentary rocks, mainly sandstones.

The volcanic rocks from this unit have a pronounced

calc-alkaline character (Fuentes et al., 2000). Andesitic

compositions predominate among the lavas and

pyroclastic deposits, with basaltic andesites occurring

subordinately. The lava flows are 10–30 m thick and

are commonly autobrecciated. Their texture is gen-

erally porphyritic with intersertal groundmass and

phenocrysts of plagioclase (An87Ab13Or0 to An30Ab67Or3), amphibole (magnesiohastingsite and par-

gasite) up to 1 cm long, and minor orthopyroxene

(En73Fs24Wo3 to En71Fs25Wo4) and clinopyroxene

(En43Fs20Wo37 to En43Fs10Wo47). The pyroclastic

flows are 30–200 m thick and locally grade into lahars

(Aguirre, 1960). Their matrix is characterized by

amphibole crystals up to 1 cm length, similar to those

found in the lavas, and carries fragments of amphibole-

bearing andesites with sizes between b1 cm and 2 m.

The swarm of dikes and subvolcanic intrusions that cut

all the deposits consist of andesites and dacites with

large amphibole phenocrysts. Their geochemistry,

mineralogy and age are similar to those of the andesitic

lava and pyroclastic flows. Two plateau ages of

19.6F0.3 and 18.6F0.4 Ma, obtained on amphibole

crystals from basaltic andesite lava and andesitic dike,

respectively, have been reported for the Algarrobo unit

(Fuentes et al., 2002).

Two subvolcanic stocks with an irregular to

ellipsoidal shape in aerial view and major and minor

lengths of 2.0–2.5 and 0.7–1.4 km, respectively, crop

out close to the two eroded stratovolcanoes (Fig. 1b).

These stocks, intermediate in composition, have a

porphyritic texture with intersertal to intergranular

groundmass and phenocrysts of plagioclase and

amphibole varying from almost pristine to wholly

altered. The stock close to the Cobre de Chacabuco

hill is a diorite porphyry with plagioclase and

clinopyroxene in its groundmass and a grain size

larger than the one close to the Algarrobo hill. Whole

rock K/Ar dating of a sample from the stock near the

Algarrobo hill yielded an age of 18.4F2.9 Ma

(Rivano et al., 1993).

3. Analytical methods

The alteration assemblages were studied through

petrography, X-ray diffraction (XRD) and electron

microprobe (EPMA). The petrographic studies were

carried out on 150 thin sections of rock samples.

Thirty-three of these samples, representatives of the

observed alteration, were selected for XRD and

EPMA analyses. The locations of these samples are

shown on Fig. 1b and in the columns of Fig. 2.

For XRD, three types of powders were prepared:

(1) of amygdales, (2) of veins and (3) of whole rock.

XRD analyses were performed at the Servicio

Nacional de Geologıa y Minerıa (SERNAGEOMIN),

Chile, using a Philips 1130-90 diffractometer with

CuKa radiation at 40 kV and 20 nA, Ni filter and a

divergence slit of 18.Secondary minerals were analyzed on polished thin

sections using a CAMECA SX100 electron microp-

robe at the University of Montpellier II, France.

Measurement conditions were a 15-kV accelerating

voltage, 10-nA sample current and 6–20-s counting

times depending on the analyzed element. A defo-

cused beam of 5–10 Am was used for zeolites to

prevent a rapid destruction of the mineral under

electron bombardment, whereas a beam of approx-

imately 1 Am was used for other secondary minerals.

4. Secondary mineral assemblages

Field examination of the rocks in the Cuesta de

Chacabuco area showed that alteration assemblages

have developed in three settings, namely within the


body of the rock, as amygdale infillings and as cross-

cutting veins. The first setting includes two alteration

domains: phenocrysts and groundmass/matrix. On the

other hand, as concluded in this study, a SW–NE to

E–W zonation in the secondary mineralogy is present

at the Cuesta de Chacabuco area with three zonal

divisions: zones I, II and III (Fig. 3). The distribution

of the secondary phases in these different zones is also

shown in Fig. 3. The following description of the

secondary mineral assemblages is made separately for

subvolcanic intrusives and for stratified rocks. Silicic

pyroclastic flows and ash-fall deposits of the Lo Valle

Formation have glassy matrix altered to smectite,

which occurs mainly as vermicular shapes, and to

hematite. The presence of cristobalite and illite was

also recognized in these rocks.

The compositional classification of the zeolites

used here is that from Gottardi and Galli (1985) based

on pure end members. The lava flows of the

Chacabuco unit present amygdales filled with Ca-

(epistilbite, chabazite), Na-(natrolite), CaNa-(thom-

sonite, mesolite) and CaNaK-(heulandite) zeolites

associated with calcite and smectite. A sandstone

bed intercalated in these lavas has clinoptilolite (NaK-

zeolite) in the matrix (sample FF12 in Fig. 2b),

whereas the matrix of a sedimentary breccia (sample

FF20 in Fig. 2c) contains chabazite (Ca-zeolite). Lava

flows of the Chacabuco unit at the westernmost sector

of the area studied, present analcime, stilbite, mor-

denite, tridymite and cristobalite. The rocks of the

Algarrobo unit can be divided into two types in

accordance with their epidote content. Tuffs and lava

flows without or with scarce epidote have amygdales

filled by Ca-(laumontite) and rare CaNaK-(heulan-

dite) (Fig. 4c) zeolites in association with smectite,

chlorite, prehnite, calcite and quartz. The laumonti-

te+heulandite association is found at the La Nipa

Grande hill (Figs. 3b and 4d), whereas the associa-

tions with chlorite+prehnite F epidote are found in

the zone between Quebrada de Los Mayas and Loma

Blanca (Fig. 3b). At the Loma Blanca zone, lavas with

ubiquitous epidote are present, accompanied by

chlorite, prehnite, calcite and quartz.

Vesicles in the subvolcanic rocks are filled with

zeolites, smectite, calcite, celadonite and quartz.

Amygdales in intrusive rocks that cut the Lo Valle

Formation are composed of Ca-(epistilbite), CaNa-

(levyne), NaCaK-(mordenite) and Na-(analcime) zeo-

lites in association with smectite, celadonite and

quartz (Fig. 4a). Intrusives cutting the Algarrobo unit

have amygdales consisting of Ca-(laumontite, chaba-

zite, yugawaralite) and NaCaK-(mordenite) zeolites

together with calcite, smectite and quartz.

Veins cutting the volcanic, volcaniclastic and

subvolcanic rocks are not wider than 12 cm and

bifurcate or form anastomosing systems. Country

rocks adjacent to these veins are generally pervasively

altered. Veins with Na-(barrerite), Ca-(stellerite) and

CaNa-(stilbite) zeolites accompanied by quartz cut the

Fig. 2. Stratigraphic columns showing sample location and

lithology. Location of these columns is indicated in Fig. 1.

1=Basaltic andesite and basalt flows, 2=sandstones, 3=conglom-

erates, 4=breccias, 5=fossil wood, 6=fossil leaves, 7=basaltic

stocks, 8=tuffs, 9=andesite flows, 10=flow-breccias, 11=sills and

dikes, 12=silicic ignimbrites, 13=unconformity.


Fig. 3. (a) Schematic SW–NE cross-section in the Cuesta de Chacabuco area showing alteration zones, volcanic units, lithology, secondary

mineral distribution, and ranges of mafic phyllosilicate non-interlayered cation (NIC) and chlorite layers (x%) contents. Strike of cross-section is

indicated in Fig. 1. 1=Tuff, 2=ignimbrite, 3=conglomerate, 4=sandstone, 5=claystone, 6=lava flow, 7=breccia, 8=flow-breccia, 9=sill and dike

with amphibole, 10=basaltic intrusive, 11=subvolcanic stock, 12=fault, 13=unconformity. (b) Schematic map showing the three zonal divisions

of secondary mineralogy in the Cuesta de Chacabuco area. The zones are identified by areas in white and shades of gray as in part (a). Symbols

as in Fig. 1.


lava flows of the Chacabuco unit. Lavas and tuffs of

the Algarrobo unit are cut by veins filled with Ca-

(stellerite), CaNaK-(heulandite) and CaNa-(mesolite)

zeolites and quartz from La Nipa Grande hill up to

Quebrada de Los Mayas zone (Fig. 3b). Veins cutting

the lavas of the Algarrobo unit between Quebrada de

Los Mayas and Loma Blanca zone (Fig. 3b) are

composed of Ca-(laumontite) zeolite (Fig. 4e), smec-

tite, calcite and quartz. In the Loma Blanca zone,

lavas cut by veins containing Ca-(laumontite, wair-

akite) zeolites (Fig. 4f), chlorite, scarce smectite and

quartz are found.

Veins cutting the amphibole-bearing subvolcanic

rocks, which in turn intersect the Chacabuco unit, are

filled by a Na-(barrerite) zeolite (Fig. 4b), calcite and

quartz. Veins cutting similar subvolcanic rocks, which

in turn cut the Algarrobo unit, have Ca-(stellerite,

laumontite), CaNa-(mesolite), NaCaK-(mordenite)

Fig. 4. X-ray diffractograms of samples (a) FF1, (b) FF14, (c) FF27, (d) FF31, (e) FF56 and (f) FF62. Abbreviations of minerals of Kretz (1983):

Mnt=montmorillonite, Anl=analcime, Ab=albite, Hul=heulandite, Lmt=laumontite, Qtz=quartz. Other abbreviations of minerals as follows:

Epist=epistilbite, Lev=levyne, Barr=barrerite, Wk=wairakite.


and Na-(barrerite) zeolites, associated with smectite

and quartz.

5. Chemical composition of the secondary minerals

5.1. Zeolites

Zeolites are found in amygdales and veins (or

veinlets), as replacement of the groundmass/matrix,

and forming partial or total replacements of primary

plagioclase phenocrysts. Because of the many types of

zeolites occurring in the rocks of the study area,

numerous shapes and habits were found. Thomsonite,

occurring at the westernmost sector of zone I (Fig. 3),

forms clear radiating fibers in the amygdales (Fig. 5a)

where it sometimes crystallized together with meso-

lite. Mesolite occurs as dirty fibro-radial assemblages

in amygdales or veins (Fig. 5e). Chabazite is present

in amygdales and has various morphologies, including

clear massive aggregates of very low birrefringence,

and radiating twinned crystals in association with

laumontite (Fig. 5d). Laumontite, one of the most

common zeolites identified, is found in amygdales

and veins, mainly as clear tabular forms often with

cleavage (Fig. 5c). Twinned laumontite, together with

twinned chabazite, was also seen in amygdales (Fig.

5d). Other zeolites form single crystals in veins

(stellerite and mesolite, Fig. 5e and f) or massive

aggregates in amphibole crystals (yugawaralite, Fig.

5b).

Zeolite microprobe analyses were chosen on the

basis that the value of their chemical balance-error

function, as defined by Passaglia (1970) (E%=

100[(Al+Fe3+)�(Na+K)�2(Mg+Ca)]/[(Na+K)+

2(Mg+Ca)]), is low (b10%). Representative analyses

of zeolites are given in Table 1 following the

nomenclature recommended by the International Min-

eralogical Association, IMA (Coombs et al., 1997).

According to these analyses, four compositional

groups can be distinguished (Fig. 6a): (1) Ca-zeolites;

(2) CaNaK-zeolites with predominant Ca; (3) CaNa-

zeolites devoid of or poor in potassium; and (4) CaK-

zeolites. The CaNa- and CaK-zeolites are only found in

zone I. The CaNaK-zeolites belong to zones I and III,

and the Ca-zeolites are found in all three zones. In Fig.

6b, samples from group (1) plot as chabazite, laumon-

tite and yugawaralite, those from group (2) as chabazite

and laumontite, those from group (3) as thomsonite and

mesolite, and those from group (4) as chabazite.

Mesolite from group (3) has low (Ca+Mg)/(Na+K)

and Si/Al ratios of ca. 1.3 and 1.4, respectively,

displaying low variability (Fig. 7 and Table 2). The

average Si/(Si+Al) ratio of 0.6 is the same that the

value given by Gottardi and Galli (1985) and the M/

(M+D) (M=monovalent-cations and D=divalent-cati-

ons) ratios of these zeolites are close to the range

given by these same authors. Thomsonite also from

the group (3) has a (Ca+Mg)/(Na+K) ratio of ca. 2.2

and a Si/Al ratio of about 1.2 (Fig. 7). Ca/Na ratios for

mesolite and thomsonite range from 1.1 to 1.5 and

from 1.9 to 2.6, respectively (Table 2).

Analyses of chabazite from group (4) show

(Ca+Mg)/(Na+K) ratios ranging from 1.1 to 2.1 and

slightly variable Si/Al ratios of about 2.3 (Fig. 7 and

Table 2). Ca/Na and Ca/K ratios range from 7.9 to

50.7 and from 1.2 to 2.2, respectively. The ranges of

(Ca+Mg)/(Na+K) and Si/Al ratios for chabazite from

group (2) are 3.2–4.0 and 2.1–2.4, respectively,

whereas for chabazite from group (1) they are of

7.9–15.1 and 2.1–2.5, respectively. The range of the

Ca/Na and Ca/K ratios of chabazite is 9.7–109.7 and

7.7–65.0 for group (1) and 3.3–6.9 and 9.3–70.4 for

group (2) (Table 2). The large variation in these ratios

for chabazite from the three groups is in accordance

with the large compositional variations for this

mineral reported by Gottardi and Galli (1985).

Laumontite from group (2) shows low and

slightly variable (Ca+Mg)/(Na+K) ratios ranging

from 3.1 to 4.2 and a constant Si/Al ratio of 2.2,

whereas laumontite from group (1) displays a higher

and more variable (Ca+Mg)/(Na+K) ratio of between

6.2 and 71.7, and a Si/Al ratio in the range 1.9–2.2

(Fig. 7 and Table 2). The two analyses of laumontite

with the highest (Ca+Mg)/(Na+K) ratios proceed

from the same sample and plot close to the Ca+Mg

apex in Fig. 6a. Yugawaralite from group (1) has a

chemical composition very near to the stoichiometric

formula Ca2Al4Si12O32d 8H2O (see Table 1), show-

ing consequently the highest Si/Al ratio of all

analyzed zeolites (Fig. 7 and Table 2). Nevertheless,

as indicated by Gottardi and Galli (1985), Na is

present in the analyses and is of about 0.2 atoms per

formula unit (pfu). The (Ca+Mg)/(Na+K) ratio for

the yugawaralite studied here ranges from 8.3 to 9.3

(Table 2).


5.2. Mafic phyllosilicates

They are present as patches in primary plagioclase,

as partial or total replacements of olivine, amphibole,

clinopyroxene, orthopyroxene, as replacement of the

groundmass/matrix, and filling amygdales and vein-

lets. Representative microprobe analyses of the mafic

phyllosilicates, recalculated on the basis of a chlorite

formula with 28 oxygens, are given in Table 3 and

plotted in Fig. 8a in terms of non-interlayer cations

(NIC=Si+Ti+Altotal+Fe+Mg+Mn) versus Altotal con-

tent. In this figure, the spread in NIC values indicates

Fig. 5. Microphotographs showing the distribution of zeolites in the analyzed samples. (a) Thomsonite as clear radiating fibers in amygdale

together with a clinopyroxene phenocryst. (b) Yugawaralite as massive aggregates in an amphibole crystal. (c) Laumontite with cleavage (right

side of microphotograph) in an amygdale associated with prismatic epidote (center of microphotograph) and chlorite (left side of

microphotograph). (d) Twinned laumontite (tabular crystals in the left side of amygdale) together with twinned chabazite in an amygdale. (e)

Mesolite as fibro-radial assemblages together with massive stellerite in a vein. (f) Single crystals of stellerite in a vein.


a trend in mafic phyllosilicate composition from

trioctahedral smectite (saponite), with no mixed

layers, towards an end-member chlorite (clinochlore).

Recalculation of chlorite layers in these minerals

using the method of Wise (in Bettison and Schiffman,

1988) gives values up to ca. 96%.

The trend in NIC values shows a broad correlation

with the mineral zones. Mafic phyllosilicates from

zone I have low NIC of 16.4–17.8 pfu, close to the

ideal total of 17.9 for pure saponite, and interlayer

cation (IC=Ca+Na+K) totals between 0.30 and 0.92

(Fig. 8a and b). Their chlorite layers range between

0% and 21%. However, analyses of a sample of this

same zone have NIC values of 18.7–19.2 and IC

contents of 0.32–0.41. In zone II, the mafic phyllo-

silicates have high NIC totals (18.2–19.8) close to the

ideal 20 for a tri-chlorite (clinochlore) and IC totals

between 0.44 and 0.05 (Fig. 8a and b). However, one

sample having the lowest porosity in zone II also has

the lowest NIC value and the highest IC content. With

the exception of this sample, the NIC and IC totals for

zone II are 18.9–19.8 and 0.43–0.05, respectively.

Recalculation of the analyses from zone II, without

the lowest-porosity sample, gives a range of 70–91%

chlorite layers for the phyllosilicates. Analyses of

zone III show NIC and IC totals of 19.3–19.9 and

0.33–0.03, respectively (Fig. 8a and b). The chlorite

layers of mafic phyllosilicates of this zone range from

76% to 96%.

On the other hand, three analyses of mafic

phyllosilicates from zone I roughly define a trend

from tri-smectite (saponite) towards di-smectite (bei-

dellite) (Fig. 8a), which have IC values greater than

0.86 (Fig. 8b). This trend has been also reported from

the shallowest part of the Nesjavellir geothermal field

in Iceland (Schiffman and Fridleifsson, 1991), where

phyllosilicates with similar compositions associated

with heulandite occur.

Table 1

Representative results of microprobe analyses of zeolites from the Cuesta de Chacabuco area

Zone I I I I II I II III III II II

Rock type BA BA BA B D B T A A D D

Zeolite type Tmp Mes Cbz Cbz Cbz Lmt Lmt Lmt Lmt Yu Yu

Sample FF9 FF9 FF9 FF82 FF58 FF82 FF49 FF115 FF115 FF58 FF58

SiO2 38.85 45.43 49.66 55.42 53.97 53.24 53.36 54.80 53.45 61.52 61.70

Al2O3 29.59 26.24 19.51 20.33 18.70 20.48 21.93 21.21 20.38 17.75 17.64

Fe2O3 0.03 0.01 0.02 0.02 0.03 0.00 0.05 0.07 0.22 0.17 0.13

MnO 0.01 0.00 0.00 0.00 0.00 0.02 0.00 0.01 0.02 0.01 0.01

MgO 0.00 0.00 0.05 0.30 0.02 0.00 0.00 0.00 0.03 0.01 0.01

CaO 12.52 9.73 7.63 8.17 8.88 9.96 11.86 10.91 10.46 9.11 9.25

Na2O 3.58 4.06 0.47 0.09 0.71 1.03 0.02 0.01 0.05 0.55 0.50

K2O 0.00 0.05 3.74 3.30 0.80 0.41 0.10 1.33 1.35 0.09 0.08

Total 84.58 85.52 81.09 87.62 83.10 85.14 87.32 88.34 85.96 89.21 89.33

No. of oxygens 80 80 24 24 24 48 48 48 48 32 32

Si 21.150 23.967 8.223 8.407 8.531 16.525 16.173 16.464 16.515 11.930 11.948

Al 18.986 16.312 3.806 3.634 3.484 7.491 7.835 7.511 7.420 4.057 4.026

Fe3+ 0.014 0.004 0.002 0.002 0.004 0.000 0.010 0.016 0.051 0.025 0.019

Mn 0.002 0.000 0.000 0.000 0.000 0.006 0.000 0.003 0.004 0.001 0.002

Mg 0.000 0.000 0.013 0.067 0.005 0.000 0.000 0.000 0.014 0.002 0.004

Ca 7.305 5.498 1.354 1.328 1.504 3.311 3.851 3.512 3.464 1.893 1.919

Na 3.779 4.156 0.152 0.026 0.218 0.622 0.014 0.004 0.031 0.206 0.188

K 0.000 0.033 0.790 0.638 0.161 0.164 0.040 0.510 0.533 0.023 0.019

Ti 0.000 0.000 0.001 0.001 0.003 0.001 0.004 0.001 0.000 0.003 0.003

Cr 0.001 0.000 0.000 0.002 0.002 0.000 0.000 0.005 0.000 0.000 0.000

Si/Al 1.114 1.469 2.161 2.313 2.449 2.206 2.064 2.192 2.226 2.941 2.968

E% 3.32 7.45 3.64 5.27 2.69 1.11 1.15 �0.16 �0.66 1.53 �0.18

Symbols: BA=basaltic andesite, B=basalt, D=dike, T=tuff, A=andesite. Abbreviations of minerals of Kretz (1983): Tmp=thomsonite,

Cbz=chabazite, Lmt=laumontite. Other abbreviations of minerals as follows: Mes=mesolite, Yu=yugawaralite. E%=charge balance (Passaglia,

1970).


Mafic phyllosilicates with more than 75% chlorite

layers can be considered almost pure chlorites and

have Mg/(Mg+Fe) values ranging from 0.46 to 0.69.

Their compositions correspond to pycnochlorites and

diabantites (Hey, 1954). Chlorites of zone III are

mainly pycnochlorites with Mg/(Mg+Fe) ratios

between 0.46 and 0.53, whereas chlorites of zone II

range from pycnochlorites to diabantites and have

Mg/(Mg+Fe) ratios between 0.62 and 0.69. Never-

theless, two chlorite analyses of zone III are diaban-

tites with high Mg/(Mg+Fe) values of about 0.65.

Two chlorite analyses of zone I correspond to

diabantites with Mg/(Mg+Fe) values of 0.59 and

0.64. In this study, therefore, the chlorite compositions

become pycnochlorites and are more iron-rich from

zones I and II to III. No correlation exists between the

Mg/(Mg+Fe) of chlorite and the Mg/(Mg+Fe) of the

whole rock (see Table 3), an opposite tendency to that

reported by other authors (see Beiersdorfer and Day,

1995 and references therein).

A Mg/(Mg+Fe) versus AlIV diagram for mafic

phyllosilicates (Fig. 8c) clearly shows the difference

between the zones. Phyllosilicates of zones I and II

have similar Mg/(Mg+Fe) ratios, with a larger

Fig. 6. (a) Projection of the zeolite compositions in a (Ca+Mg)–Na–

K diagram. Numbers indicate compositional groups, as follows:

1=Ca-zeolites, 2=CaNaK-zeolites, 3=CaNa-zeolites and 4=CaK-

zeolites. (b) 10d (SiO2/Al2O3)�(CaO+MgO)�(Na2O+K2O) dia-

gram used for the chemical classification of zeolites. Fields of

zeolite compositions were outlined using the authors’ data base

which consists of a compilation of numerous works published in

international journals (e.g., Iijima, 1978; Viereck et al., 1982; Zeng

and Liou, 1982; Gottardi and Galli, 1985; Cho et al., 1986, 1987;

Aguirre and Atherton, 1987; Deer et al., 1992; Vergara et al., 1993;

Cocheme et al., 1996; De’Gennaro and Langella, 1996; Ibrahim and

Hall, 1996; Di Renzo and Gabelica, 1997; Aguirre et al., 2000;

Vattuone et al., 2001). Abbreviations of minerals of Kretz (1983):

Hul=heulandite, Lmt=laumontite, Stb=stilbite, Cbz=chabazite,

Anl=analcime, Ntr=natrolite, Tmp=thomsonite. Other abbreviations

of minerals as follows: Mor=mordenite, Cl=clinoptilolite, Barr=bar-

rerite, St=stellerite, Yu=yugawaralite, Epist=epistilbite, F=faujasite,

O=offretite, Wk=wairakite, Lev=levyne, Ph=phillipsite, Go=gob-

binsite, Ga=garronite, Sc=scolecite, Gi=gismondine, Mes=mesolite,

Am=amicite. For both diagrams the symbols are as follows: open

triangles=analyses from zone I, crosses=analyses from zone II, full

circles=analyses from zone III.

Fig. 7. Plot of Si/Al against (Ca+Mg)/(Na+K) for zeolite compo-

sitions. Fields in white and gray represent different zeolites.

Abbreviations and symbols as in Fig. 6.


dispersion for zone I, but different AlIV values if the

lowest-porosity sample of zone II is excluded. The

phyllosilicates of zone III have Mg/(Mg+Fe) ratios

lower than those of zones I and II, and display the

highest AlIV values of all the zones. The Mg/(Mg+Fe)

values of all the mafic phyllosilicates are also higher

than the Mg/(Mg+Fe) values of the whole rock (see

Table 3).

5.3. Epidotes and prehnites

Epidote, associated with chlorite, appears as a

partial or total replacement of amphibole, clinopyrox-

ene and orthopyroxene, as patches in primary

plagioclase, as single crystals and as patches in the

groundmass, and filling amygdales and veinlets

together with prehniteFquartz, laumontite and chlor-

iteFcalcite. Representative microprobe analyses of

epidote from zones II and III are given in Table 4. The

mole fraction of octahedral Fe3+, expressed as XFe3+

[=Fe3+/(Fe3++Al)], ranges from 0.23 to 0.38 in zone II

and from 0.21 to 0.32 in zone III.

As in all hydrothermal occurrences, prehnite is not

abundant in the studied area (Wheeler et al., 2001).

Prehnite is found in amygdales alone or in association

with: (1) chlorite, (2) epidoteFquartz, (3) laumonti-

te+chloriteFquartz, (4) epidote+chloriteFcalcite, (5)

chabazite+chloriteFcalcite and (6) yugawaralite+epi-

dote+chlorite. It is also present as patches in primary

plagioclase together with epidote. Microprobe analy-

ses of prehnites from zone II, where it is more

common this mineral, are given in Table 5. The XFe3+

ratio ranges from 0.09 to 0.24, whereas the ferric iron

partitioning between epidote and prehnite in this

study, where associated in the same samples, is

characterized by XFe3+ in epidote (0.29–0.38)NXFe3+

in prehnite (0.15–0.22).

5.4. Other alteration minerals

Titanite is present as small rhomboidal crystals

associated with chlorite and calcite in pseudomorphs

after clinopyroxene, and as small grains together with

diopside, chlorite and quartz in pseudomorphs after

amphibole. This mineral has been found only in zone

III. Microprobe analyses (Table 6) show chemical

differences between both types of titanite. Titanite

replacing clinopyroxene has Al2O3 and Fe2O3 con-

tents in the ranges 2.7–3.2% and 4.4–4.6%, respec-

tively, whereas titanite-substituting amphibole has

lower Al2O3 and Fe2O3 contents of 1.0% and 2.0%,

respectively. The Ti/(Ti+Al) ratio is ca. 0.87 in the

first type and 0.96 in the second. However, consid-

ering the scarcity of data no conclusions can be

derived from these chemical differences.

Adularia is rare and has been found in zone I as small

patches at the core of primary plagioclase phenocrysts,

in association with smectite. Microprobe analyses show

uniform compositions close to pure K-feldspar (An0.4–

0.8Ab0.9–1.4Or98.7–97.8, Table 6) with minor Na and

insignificant Ca and Fe contents, all features typical of

low-temperature K-feldspar (Deer et al., 1992).

Diopside is a very rare phase and has been only

found in a hydrothermal breccia in direct contact with

the subvolcanic stock close to the Cobre de Chacabuco

hill, i.e., in zone III. It is present there as: (1) small pale

green crystals replacing the matrix, associated with

epidote, chlorite, biotite and quartz; (2) replacing

amphibole crystals in association with titanite, chlorite

and quartz; and (3) filling veinlets together with

chlorite. The quadrilateral components, expressed by

the Ca+Mg+Fe2+ values according to Morimoto et al.

(1988), are of 1.9, whereas the non-quadrilateral,

expressed by the twod Na contents, are insignificant

(=0.1, Table 6). The Mg/(Mg+Fe2++Mn) contents are

Table 2

Chemical ratios for zeolites from the Cuesta de Chacabuco area separated in compositional groups

Compositional group (1) Ca (1) Ca (1) Ca (2) CaNaK (2) CaNaK (3) CaNa (3) CaNa (4) CaK

Zeolite type Cbz Lmt Yu Cbz Lmt Mes Tmp Cbz

(Ca+Mg)/(Na+K) 7.9–15.1 6.2–71.7 8.3–9.3 3.2–4.0 3.1–4.2 1.1–1.5 1.9–2.5 1.1–2.1

Si/Al 2.1–2.5 1.9–2.2 2.9–3.0 2.1–2.4 2.2 1.3–1.5 1.1–1.2 2.1–2.4

Ca/Na 9.7–109.7 9.4–861.3 9.2–10.2 3.3–6.9 3.5–5.3 1.1–1.5 1.9–2.6 7.9–50.7

Ca/K 7.7–65.0 6.5–95.8 83.2–99.6 9.3–70.4 20.1–28.2 57.2–166.7 N940 1.2–2.2

Compositional groups are described in text. Abbreviations of minerals of Kretz (1983): Tmp=thomsonite, Cbz=chabazite, Lmt=laumontite.

Other abbreviations of minerals as follows: Mes=mesolite, Yu=yugawaralite.


ca. 0.8 and thus the compositions of these clinopyrox-

enes are very similar to those found in geothermal

systems (e.g., Bird et al., 1984).

6. Discussion

6.1. Alteration pattern

The secondary mineralogy shows a SE–NW to E–W

zonation towards the Cobre de Chacabuco hill which is

unrelated to stratigraphic depth. This zonation is mainly

marked by a successive change in the zeolites from a

heulandite+thomsonite+mesoliteFstilbite-bearing zone I,

through a laumontiteFyugawaralite-bearing zone II, and

to a wairakite-bearing zone III (Fig. 3).

This zonation is also shown by other alteration

minerals. Mafic phyllosilicates present a gradual

change in chemical composition between zones I

and III. NIC and AlIV values and the percentage of

chlorite layers increase whereas IC totals and Mg/

(Mg+Fe) ratios decrease from zone I to zone III (Figs.

3 and 8). Tri-smectites (saponite) and minor di-

smectites therefore dominate in zone I, pycnochlorites

to diabantites prevail in zone II and pycnochlorites

dominate in zone III. On the other hand, the

abundance of epidote increases from zones II to III,

being absent in zone I but ubiquitous in zone III. In

addition, diopside and biotite that are alteration

minerals formed at higher temperature appear only

in zone III.

The progressive changes shown here result from

the increase in temperature from zone I to zone III.

This leads to the following important observations:

(1) the Mg/(Mg+Fe) ratio of chlorite decreases with

temperature (a relationship reported in other studies;

e.g., Beiersdorfer and Day, 1995 and references

therein); (2) the Al/(Al+Si) ratio of chlorite

increases slightly with temperature, as has been

shown by other studies (see Beiersdorfer and Day,

1995 and references therein); and (3) the similar

XFe3+ values for epidotes from zones II and III

probably indicate that no relationship exists between

XFe3+ and temperature, as also shown by other

studies (see Beiersdorfer and Day, 1995 and

references therein).

The rocks of the Cuesta de Chacabuco area

display similar alteration mineral assemblages in

amygdale and vein (and/or veinlets) infillings

within each zone. Both alteration domains contain

Ca-, CaNa-, CaNaK-, NaCaK- and Na-zeolites in

association with other alteration minerals. This

suggests that the alteration assemblages in the

Fig. 8. (a) Plot of non-interlayer cation totals (NIC=Si+Ti+Altotal+-

Fe+Mg+Mn) against Altotal in mafic phyllosilicates. All cation

values are calculated based on a chlorite formula with 28 oxygens.

(b) Plot of interlayer cation totals (IC=Ca+Na+K) against Si in

mafic phyllosilicates. All cation values are calculated based on a

chlorite formula with 28 oxygens. (c) Plot of AlIV against Mg/

(Mg+Fe). All values were recalculated on a variable oxygen basis

between smectite (22 oxygens) and chlorite (28 oxygens), according

to the percentage of chlorite layers measured by recalculation of

microprobe analyses (after Hillier in Schmidt and Robinson, 1997).

Same symbols as in Fig. 6.


amygdales and veins formed during the same

alteration episode. In addition, there is no contrast

between the alteration assemblages present in the

subvolcanic and the volcanic and volcaniclastic

rocks within each zone. We therefore conclude that

all the alteration assemblages found in the rocks of

the Cuesta de Chacabuco area have been produced

by a single alteration event.

Table 3

Representative results of microprobe analyses of mafic phyllosilicates from the Cuesta de Chacabuco area

Zone I I I I I II II II III III III III III

Rock type BA BA B B B T D D BA BA BA H H

Sample FF9 FF19 FF82 FF82 FF106 FF49 FF58 FF58 FF60 FF60 FF60 FF131 FF131

SiO2 49.79 46.81 33.48 33.02 48.84 32.59 31.06 29.23 27.63 29.28 26.69 27.30 27.39

TiO2 0.09 0.04 0.02 0.00 0.05 0.00 0.01 0.01 0.03 0.00 0.00 0.03 0.02

Al2O3 14.73 6.99 15.13 13.72 4.60 17.18 19.02 19.07 17.88 16.83 19.26 20.16 19.34

FeO 3.56 16.38 21.27 18.70 9.90 16.92 17.31 20.24 26.64 24.42 26.19 26.13 26.88

MnO 0.15 0.04 0.31 0.31 0.03 0.31 0.66 0.77 1.31 0.95 1.33 0.47 0.40

MgO 11.67 14.31 16.98 18.62 20.36 21.00 15.63 19.01 13.72 15.19 13.94 14.02 14.37

CaO 2.13 2.51 1.39 1.23 1.15 0.55 1.72 0.10 0.46 0.22 0.11 0.17 0.22

Na2O 0.19 0.64 0.12 0.06 0.13 0.09 0.07 0.03 0.09 0.08 0.01 0.01 0.03

K2O 1.80 0.44 0.08 0.08 0.26 0.03 0.03 0.05 0.15 0.15 0.01 0.00 0.00

Total 84.10 88.15 88.78 85.73 85.32 88.66 85.50 88.51 87.91 87.11 87.53 88.29 88.65

No. of oxygens 28 28 28 28 28 28 28 28 28 28 28 28 28

Si 9.303 9.060 6.756 6.830 9.362 6.429 6.394 5.919 5.887 6.184 5.689 5.721 5.744

Ti 0.013 0.005 0.003 0.000 0.007 0.000 0.001 0.001 0.005 0.000 0.000 0.004 0.003

AlIV 0.000 0.000 1.244 1.170 0.000 1.571 1.606 2.081 2.113 1.816 2.311 2.279 2.256

AlVI 3.243 1.593 2.355 2.174 1.040 2.421 3.008 2.470 2.376 2.372 2.526 2.699 2.525

Fe2+ 0.556 2.651 3.589 3.235 1.587 2.790 2.981 3.428 4.747 4.313 4.668 4.579 4.716

Mn 0.023 0.006 0.053 0.054 0.005 0.051 0.115 0.132 0.235 0.170 0.239 0.084 0.071

Mg 3.250 4.128 5.109 5.740 5.818 6.175 4.798 5.738 4.358 4.782 4.429 4.379 4.492

NIC 16.388 17.444 19.108 19.204 17.818 19.438 18.902 19.769 19.721 19.636 19.863 19.745 19.807

Ca 0.425 0.521 0.300 0.272 0.235 0.116 0.379 0.023 0.104 0.050 0.026 0.039 0.048

Na 0.069 0.239 0.047 0.022 0.049 0.034 0.026 0.012 0.036 0.032 0.005 0.005 0.014

K 0.429 0.110 0.020 0.020 0.064 0.007 0.008 0.013 0.040 0.039 0.002 0.001 0.001

IC 0.923 0.869 0.367 0.315 0.349 0.157 0.413 0.048 0.181 0.121 0.033 0.044 0.063

Mg/(Mg+Fe)mp 0.85 0.61 0.59 0.64 0.79 0.69 0.62 0.63 0.48 0.53 0.49 0.49 0.49

Mg/(Mg+Fe)wr 0.67 0.44 0.65 0.65 0.63 – 0.56 0.56 0.42 0.42 0.42 – –

x% 0.0 20.5 76.5 78.6 14.3 79.4 70.1 89.7 95.7 86.3 94.5 88.9 93.5

No. of oxygens* 22.0 23.2 26.6 26.7 22.858 26.8 26.2 27.4 27.7 27.2 27.7 27.3 27.6

Si 7.310 7.516 6.415 6.517 7.643 6.145 5.985 5.788 5.833 6.001 5.622 5.585 5.664

Ti 0.010 0.004 0.003 0.000 0.006 0.000 0.001 0.001 0.005 0.000 0.000 0.004 0.003

AlIV 0.690 0.484 1.585 1.483 0.357 1.855 2.015 2.212 2.167 1.999 2.378 2.415 2.336

AlVI 1.858 0.837 1.833 1.707 0.492 1.961 2.303 2.240 2.281 2.066 2.402 2.444 2.378

Fe2+ 0.437 2.200 3.408 3.087 1.295 2.667 2.790 3.352 4.703 4.186 4.614 4.470 4.650

Mn 0.018 0.005 0.050 0.052 0.004 0.049 0.107 0.129 0.233 0.165 0.237 0.082 0.070

Mg 2.554 3.424 4.851 5.477 4.749 5.902 4.490 5.611 4.318 4.641 4.377 4.275 4.430

NIC 12.877 14.471 18.146 18.323 14.546 18.579 17.691 19.333 19.540 19.058 19.629 19.275 19.531

Ca 0.334 0.432 0.285 0.260 0.192 0.111 0.354 0.022 0.103 0.049 0.026 0.038 0.048

Na 0.054 0.198 0.045 0.021 0.040 0.033 0.024 0.012 0.036 0.031 0.005 0.005 0.014

K 0.337 0.091 0.019 0.019 0.053 0.006 0.008 0.013 0.040 0.038 0.002 0.001 0.001

IC 0.725 0.721 0.349 0.300 0.285 0.150 0.386 0.047 0.179 0.117 0.033 0.043 0.062

T (8C) – – 138 126 – 191 197 273 278 231 310 305 301

Symbols: BA=basaltic andesite, T=tuff, D=dike, B=basalt, H=hydrothermal breccia. NIC=non-interlayer cation total, IC=interlayer cation total.

x%=percentage of chlorite layers, T (8C)= temperature according to the Cathelineau (1988) geothermometer. No. of oxygens*=formulae based

on variable number of oxygens related to the percentage of chlorite (28 oxygens) to smectite (22 oxygens) layers. Mg/(Mg+Fe)mp and Mg/

(Mg+Fe)wr are Mg/(Mg+Fe) ratios of mafic phyllosilicate and whole rock, respectively.


The alteration pattern observed at the Cuesta de

Chacabuco area contrasts with the bnormalQ regional

alteration pattern established for this zone of central Chile,

which is related to stratigraphic depth and is lower in

grade (mordenite subfacies of the zeolite facies according

to Levi et al., 1989). Moreover, a typical feature in

geothermal systems is the great number of mineral phases

expressed by the coexistence of more than one zeolite

species (Fepidote) in the same sample (Boles, 1977), as

observed in the studied area. Such a large number of

mineral phases is rare in burial metamorphosed regions

(Boles, 1977). The established pattern therefore corre-

sponds to that of a geothermal field and is similar to other

patterns of this type previously reported for Cenozoic

deposits in central Chile (Thiele et al., 1991; Vergara et

al., 1993). Padilla and Vergara (1985) already reported

alteration of the geothermal field type for the intrusive

rocks of the Cuesta de Chacabuco area. However, they

did not observe the zonation established in this study and

their conclusions are not in agreement with the alteration

model presented here.

6.2. Temperature, pressure and geothermal gradient

estimates

As stated above, the lateral zonation found in this

study is similar to that present in geothermal systems

where temperature is the principal control on changes

in secondary mineral assemblages. Estimates of the

temperature in each of the three zones can be therefore

made by comparison with formation temperatures of

secondary minerals measured in active geothermal

systems together with experimental data on mineral

stability. Fig. 9 shows the stability relations of some

Ca-zeolites (Liou et al., 1991) and the petrogenetic grid

in the NCMASH (Na2O–CaO–MgO–Al2O3–SiO2–

H2O) system with average activity values (Frey et al.,

1991). Temperatures below approximately 150 8C are

compatible with the presence of heulandite and

thomsonite in zone I, on the basis of observations

made in the Icelandic Nesjavellir geothermal system

(Schiffman and Fridleifsson, 1991) and experimental

work (Liou et al., 1991, Fig. 9). For this same zone I,

Table 4

Representative results of microprobe analyses of epidotes from the Cuesta de Chacabuco area

Zone II II II II III III III III

Rock type T T D D BA A A A

Sample FF49 FF49 FF58 FF58 FF60 FF115 FF115 FF131

SiO2 36.46 37.02 36.64 37.56 37.53 37.15 37.57 37.11

TiO2 0.05 0.03 0.18 0.03 0.00 0.02 0.05 0.07

Al2O3 21.11 22.40 19.51 25.09 23.23 21.80 25.46 21.94

Fe2O3 15.66 14.58 18.45 11.50 13.90 16.14 10.41 16.02

MnO 0.09 0.11 0.10 0.96 0.39 0.05 0.36 0.39

MgO 0.01 0.00 0.02 0.00 0.00 0.00 0.00 0.04

CaO 22.83 22.83 22.75 22.65 22.80 23.01 23.14 22.38

Na2O 0.02 0.02 0.00 0.03 0.00 0.02 0.00 0.00

K2O 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00

Total 96.23 96.98 97.65 97.84 97.86 98.19 97.01 97.95

Cations per 12.5 oxygens

Si 2.988 2.992 2.987 2.979 2.996 2.981 2.990 2.983

AlIV 0.012 0.008 0.013 0.021 0.004 0.019 0.010 0.017

Ti 0.003 0.002 0.011 0.002 0.000 0.001 0.003 0.004

AlVI 2.027 2.125 1.862 2.324 2.182 2.043 2.378 2.061

Fe3+ 0.964 0.885 1.130 0.685 0.834 0.973 0.622 0.967

Mn 0.006 0.007 0.007 0.064 0.026 0.003 0.024 0.026

Mg 0.001 0.000 0.001 0.000 0.000 0.000 0.000 0.003

Ca 2.788 2.751 2.765 2.678 2.714 2.752 2.745 2.682

Na 0.002 0.001 0.000 0.003 0.000 0.002 0.000 0.000

K 0.000 0.001 0.000 0.000 0.001 0.000 0.002 0.001

Total 8.791 8.772 8.776 8.757 8.757 8.775 8.775 8.743

XFe3+ 0.32 0.29 0.38 0.23 0.28 0.32 0.21 0.32

Symbols: T=tuff, D=dike, BA=basaltic andesite, A=andesite. XFe3+=Fe3+/(Fe3++Al).


the comparison with secondary minerals in Icelandic

geothermal systems indicates that: (1) mesolite is stable

at temperatures below 100 8C (Kristmannsdottir and

Tomasson, 1978); (2) heulandite, stilbite and analcime

may also form at temperatures below 100 8C or can be

precipitated from fluids at temperatures greater than

100 8C (Kristmannsdottir and Tomasson, 1978); and

(3) tri-smectite is stable at temperatures below 150–175

8C (Kristmannsdottir, 1979; Schiffman and Fridleifs-

son, 1991). According to Liou et al. (1991), stilbite is

stable below 150 8C, whereas Frey et al. (1991)

indicated an upper temperature limit of about 180 8Cfor this zeolite (Fig. 9). However, the main difference

between the P–T diagrams of Liou et al. (1991) and

Frey et al. (1991) is the stability field of heulandite.

According to Frey et al. (1991), heulandite is restricted

to low pressures (b1800 bars) and to temperatures

below about 230 8C, and Cho et al. (1987) and Liou etal. (1991) indicated that formation of heulandite would

require a minimum pressure of 600 bars. However,

Liou et al. (1991) have shown that Na-bearing

heulandite can even form at pressures b600 bars.

Concerning zone II, the predominance of laumon-

tite indicates temperatures either above ca. 150 8C on

the basis of experimental work (Liou et al., 1991) and

observations in Iceland (Kristmannsdottir, 1979), or in

the range of 220–260 8C in accordance with Schiff-

man and Fridleifsson (1991) based on observations

made in the Nesjavellir geothermal field. The petro-

genetic grid of Frey et al. (1991) shows that

laumontite is stable in the range of approximately

180–260 8C and Liou et al. (1991) determined the

upper temperature limit of laumontite to be about 260

8C below 1000 bars (Fig. 9). The presence of

yugawaralite in zone II implies a very low pressure

and a very restricted P–T stability field (Zeng and

Liou, 1982). In Fig. 9, this field is restricted to a

pressure below 500 bar and a temperature range of

about 190–230 8C. Furthermore, in geothermal

systems with Pfluid/Ptotal ratio close to 0.3, yugawar-

alite occurs only at depths less than 500 m (Zeng and

Liou, 1982). The presence of prehnite in this zone

indicates temperatures between 200 and 280 8Caccording to Frey et al. (1991) or in the range 240–

300 8C on the basis of observations of Philippine

geothermal systems (Reyes, 1990), whereas the

appearance of epidote suggests temperatures higher

than 250 8C (Bird et al., 1984; Liou et al., 1985) or

200 8C (Cho et al., 1986; Reyes, 1990). Finally, the

occurrence of chlorite with nearly ideal NIC totals to

those of clinochlore indicates temperatures N245–265

8C based on Icelandic data (Schiffman and Fridleifs-

son, 1991).

In zone III, the appearance of wairakite imposes

strict limitations of formation. Wairakite is one of the

most common Ca-zeolites in geothermal systems,

where it coexists with epidote, just as it is found in

this study. In fact, wairakite is largely confined to

geothermal systems (Bird et al., 1984). There it occurs

at temperatures greater than 200–300 8C (Bird et al.,

1984; Reyes, 1990). Experimental work by Liou et al.

(1991) has shown that wairakite is stable between

approximately 210–370 8C and, according to Frey et

al. (1991), its stability field corresponds to temper-

atures of 220–370 8C and pressures below 1600 bar

(Fig. 9). Diopside as present in zone III, is found in

modern geothermal systems at temperatures above

300 8C (Bird et al., 1984). The presence of biotite

Table 5

Representative results of microprobe analyses of prehnites from the

Cuesta de Chacabuco area

Zone II II II II II II II

Rock type T T T D D D D

Sample FF49 FF49 FF49 FF58 FF58 FF58 FF58

SiO2 41.39 41.98 42.03 42.25 42.40 43.06 42.34

TiO2 0.04 0.05 0.02 0.07 0.06 0.10 0.01

Al2O3 17.76 19.45 20.66 20.35 20.01 22.20 18.34

Fe2O3 8.92 6.93 5.04 5.96 6.41 3.46 8.20

MnO 0.03 0.03 0.05 0.09 0.08 0.08 0.07

MgO 0.08 0.00 0.00 0.00 0.00 0.00 0.05

CaO 25.46 25.80 26.02 26.05 25.99 26.11 25.58

Na2O 0.09 0.04 0.05 0.03 0.06 0.09 0.07

K2O 0.00 0.01 0.01 0.00 0.00 0.00 0.00

Total 93.77 94.29 93.88 94.80 95.01 95.09 94.66

Cations per 11 oxygens

Si 3.000 3.000 2.997 2.993 3.000 3.007 3.025

AlIV 0.000 0.000 0.003 0.007 0.000 0.000 0.000

Ti 0.002 0.003 0.001 0.003 0.003 0.005 0.000

AlVI 1.516 1.638 1.733 1.691 1.669 1.827 1.544

Fe3+ 0.486 0.372 0.270 0.317 0.341 0.181 0.440

Mn 0.002 0.002 0.003 0.005 0.005 0.005 0.004

Mg 0.009 0.000 0.000 0.000 0.000 0.000 0.005

Ca 1.976 1.975 1.988 1.977 1.971 1.953 1.958

Na 0.013 0.006 0.007 0.004 0.008 0.012 0.010

K 0.000 0.000 0.001 0.000 0.000 0.000 0.000

Total 7.004 6.996 7.003 6.999 6.996 6.990 6.988

XFe3+ 0.24 0.19 0.14 0.16 0.17 0.09 0.22

Symbols: T=tuff, D=dike. XFe3+=Fe3+/(Fe3++Al).


indicates temperatures greater than 280 8C (Reyes,

1990). The abundance of chlorite also indicates

temperatures above 245–265 8C (Schiffman and

Fridleifsson, 1991).

The preceding discussion permits an estimate of

temperatures for each zone to be made: (1) ca. 100–

180 8C for zone I; (2) in the order of 180–270 8C for

zone II; and (3) approximately between 245 and 310

8C for zone III. Although reservations exist about the

validity of the geothermometers of AlIV in chlorite

(Bevins et al., 1991; De Caritat et al., 1993), the

values obtained by means of the calibration of

Cathelineau (1988), which is based on data from the

Los Azufres geothermal system, are in agreement with

these estimates. Temperatures derived from this geo-

thermometer and calculated for mafic phyllosilicates

with more than 75% chlorite layers are: (1) ca. 126–

138 8C for a sample from zone I; (2) 191–273 8C for

zone II; and (3) 131–310 8C for zone III. This last

temperature range is lower than the previous estimated

for zone III. However, there are only two values (131

and 180 8C) that do not correspond with the estimates;

these were obtained from a hydrothermal breccia in

direct contact with the subvolcanic stock close to the

Cobre de Chacabuco hill. These two analyses come

from chlorite in a veinlet in this sample and because

their alteration microdomain is different to others in

zone III, we believe that these two temperatures

reflect metastable equilibrium and early stages of the

alteration episode or a late stage of the alteration.

Table 6

Microprobe analyses of diopside, titanite and adularia

Mineral Diopside Titanite Adularia

Zone III III III III III I I

Rock type H H BA BA H BA BA

Sample FF131 FF131 FF60 FF60 FF131 FF9 FF9

SiO2 52.13 52.45 31.05 30.89 30.12 62.18 63.20

TiO2 0.31 0.39 31.73 31.43 39.31 0.00 0.02

Al2O3 1.29 1.39 2.68 3.17 1.02 18.29 18.60

Cr2O3 0.01 0.02 0.00 0.00 0.02 0.00 0.01

FeO 7.87 7.37 – – – 0.08 0.11

Fe2O3 – – 4.38 4.63 2.02 – –

MnO 0.25 0.27 0.04 0.02 0.01 0.00 0.02

MgO 13.69 14.01 0.01 0.15 0.02 0.00 0.01

CaO 23.04 23.10 26.95 27.02 27.73 0.08 0.17

Na2O 0.38 0.38 0.05 0.06 0.00 0.10 0.15

K2O – – 0.02 0.03 0.01 16.51 16.57

Total 98.96 99.36 96.90 97.40 100.27 97.24 98.85

No. of oxygens 6 6 20 20 20 8 8

Al 0.057 0.061 0.426 0.503 0.157 1.030 1.030

Si 1.957 1.958 4.197 4.156 3.937 2.970 2.968

Ti 0.009 0.011 3.225 3.180 3.864 0.000 0.001

Fe3+ 0.039 0.028 0.401 0.422 0.179 – –

Cr 0.000 0.000 0.000 0.000 0.002 0.000 0.000

Fe2+ 0.209 0.202 – – – 0.003 0.004

Mn 0.008 0.008 0.004 0.003 0.001 0.000 0.001

Mg 0.766 0.779 0.002 0.030 0.003 0.000 0.000

Ca 0.927 0.924 3.902 3.896 3.883 0.004 0.009

Na 0.027 0.027 0.013 0.016 0.001 0.009 0.014

K 0.001 0.000 0.003 0.005 0.002 1.006 0.993

Ni 0.001 0.001 – – – – –

Total 4.001 4.001 12.173 12.211 12.031 5.022 5.019

En 39.31 40.13 Al 10.51 12.25 3.74 An 0.39 0.84

Fs 13.12 12.31 Ti 79.58 77.47 91.99 Ab 0.88 1.38

Wo 47.57 47.56 Fe3+ 9.90 10.29 4.27 Or 98.73 97.78

Symbols: BA=basaltic andesite, H=hydrothermal breccia.


Excluding these two values, the range obtained for

zone III is 231–310 8C, in agreement with the other

estimates.

According to Liou et al. (1991), yugawaralite may

be stable in hydrothermal systems with relatively high

geothermal gradients and high Pfluid/Ptotal ratios, where

a possible zeolite zonation with the increasing of

temperature would be: mordenite, laumontite, yuga-

waralite and wairakite, as found in this study. These

same authors indicated that, if the geothermal gradient

and Pfluid/Ptotal ratio decrease, then yugawaralite is no

longer stable and the zonation would be: mordenite,

laumontite and wairakite. Because yugawaralite, in

contrast to laumontite, is not widespread in zone II, we

believe that local geochemical and hydrologic con-

ditions fluctuated during the alteration episode, slightly

changing the Pfluid/Ptotal ratio and/or the geothermal

gradient. However, the Pfluid/Ptotal ratio would have

been high and close to 1 during almost all the duration

of the alteration episode. In addition, the presence of

wairakite in the assemblages would indicate a high

geothermal gradient of about 70–80 8C km�1 (e.g.,

Kristmannsdottir, 1985; Liou et al., 1987, 1991; Frey et

al., 1991). Likewise, the great number of mineral

phases expressed by the coexistence of laumontite and

epidote, or laumontite and other zeolite species

indicates high gradients (Boles, 1977).

As previously shown, the overall temperature at the

Cuesta de Chacabuco area ranged from a minimum of

ca. 100 8C to a maximum of approximately 310 8C.On the other hand, the Chacabuco and Algarrobo

units in the studied area are approximately 500 and

800 m thick, respectively. The temperature variation

therefore occurred through a ca. 1300-m-thick rock

pile and consequently the geothermal gradient should

have been of about 160 8C km�1 (see Fig. 9). This

high gradient is in agreement with the range 150–175

8C km�1 estimated by Aguirre et al. (2000) for the

metamorphism of the Valle Nevado stratified

sequence belonging to the Farellones Formation and

located 70 km to the southeast of the area studied in

this work. Furthermore, these authors suggested that

this gradient was related to volcanic centres that

generated the middle and uppermost levels of the

Farellones Formation and which were associated with

localised areas of geothermal activity. This interpre-

tation is similar to the one postulated here.

6.3. Chemical and morphological controls

In addition to temperature and pressure, the

formation of secondary minerals in hydrothermal

systems is dependent on fluid composition, initial

composition of the whole rock (generally important at

temperatures lower than 200 8C) and permeability

(Browne, 1978). Based on observations in geothermal

systems (e.g., Reyes, 1990; Wheeler et al., 2001), all

secondary minerals found in the studied area indicate

hot, near neutral to slightly alkaline pH, alkali

chloride hydrothermal fluids. Among these minerals,

prehnite is the one best allowing the characterization

of these fluids. As suggested by Wheeler et al. (2001),

numerous observations in geothermal systems, includ-

ing fluid inclusion studies, show that the formation of

prehnite (or, indeed, any calc-silicate mineral) is

independent of the salinity of the fluids. Besides

temperature, the most important factor controlling

prehnite formation is the dissolved CO2 content in

Fig. 9. P–T diagram showing the stability relations of secondary

minerals. Solid lines are experimentally determined relations

between Ca-zeolites (Liou et al., 1991), in the presence of excess

quartz (Qtz) and fluid (F). Broken lines limit the fields for stilbite

(Stb), laumontite (Lmt), heulandite (Hul) and wairakite (Wk) in the

petrogenetic grid for the system NCMASH (Na2O–CaO–MgO–

Al2O3–SiO2–H2O) with average activity values (Frey et al., 1991).

The gray area at the left side of the diagram is the stability field for

prehnite and the one at the right side the stability field for epidote.

The darker gray overlapping zone of these two areas is the stabilility

field for prehnite+epidote. The dotted arrow shows the deduced

geothermal gradient for the Cuesta de Chacabuco area. Other

abbreviations are: Anl=analcime, Ab=albite, Yu=yugawaralite,

An=anorthite.


fluids, which must be low (Wheeler et al., 2001).

Prehnite stability depends more severely on this factor

than it is the case for wairakite, laumontite or epidote.

An activity of dissolved CO2 greater than ca. 0.01 mol

would prevent the formation of prehnite, even though

the temperatures are above 250 8C (Wheeler et al.,

2001). On the other hand, the great diversity of zeolite

species found here could indicate variable composi-

tions of the fluids, specially in their Na/Ca ratios,

permitting the formation of Na- and Ca-zeolites. This

variability in fluid chemistry is reflected also by the

presence of barrerite, a Na-zeolite which is extremely

uncommon and is known to occur only in four

localities in the world (see Vattuone et al., 2001 and

references therein).

The initial whole rock composition appears to be of

lesser importance as shown by a lack of correlation

between the Mg/(Mg+Fe) ratio of chlorite and rocks

(see Table 3) and by the occurrence of Na-zeolites in

rocks with relatively low Na/Ca ratios. Nevertheless,

this parameter must have influenced the formation of

the low-Si-bearing Ca zeolites present in zone I. In

this zone, many silica-deficient lava flows belonging

to the Chacabuco unit contain natrolite, chabazite,

thomsonite and mesolite, rather than of heulandite or

stilbite. In zones II and III, however, the whole rock

composition had little influence and its role in the

alteration was probably characterized by a reshuffling

of the elements into secondary minerals. For instance,

the ubiquitous albitization of primary plagioclase

released the Ca necessary to form the calc-silicates

found in the studied area.

The influence of thermal factors in the zonation of

the secondary mineralogy is occasionally opposed to

morphological parameters such as porosity and per-

meability (e.g., Levi et al., 1982; Schmidt and

Robinson, 1997). Samples with low porosity and

permeability are therefore almost unaltered, independ-

ent of the zone in which they occur, or contain

alteration assemblages of temperatures lower than that

expected for the zone. For instance, the lowest porosity

sample (FF52) analyzed has mafic phyllosilicates with

38% chlorite layers, a value much lower than the range

70–91% established for zone II. The opposite occurs

with samples of high porosity and permeability, as

tuffs, volcanic breccias and amygdaloidal lava flow

tops, where the characteristic assemblages of each

zone are found. Sample FF82, which has increased its

permeability due to the cross cutting fault, is a special

case (Fig. 1). This sample belongs to zone I but

contains laumontite and 58–79% chlorite layers.

Consequently, it is suggested that hotter depositing

fluids, originating near the stock close to the Cobre de

Chacabuco hill, were channelled by this fault. This

structure is not unique in the studied area but numerous

other faults occur which provide the major sources of

permeability (Figs. 1 and 3). Most of them are normal

faults and their presence is defined by sheared rock,

numerous veins and intense rock alteration.

6.4. Alteration model

Wheeler et al. (2001) suggested that the type of

fluids which formed the secondary minerals, charac-

terized by a near neutral to slightly alkaline pH and a

very low dissolved CO2 content, may occur deep in

the upflow regions of some geothermal systems but

this case is relatively rare. More probably, these fluids

occur in outflow regions moving laterally away or

downslope from upflow zones. In these regions, the

liquid is degassed and consequently has lost almost all

its CO2, which will also cause that the residual liquid

to become slightly more alkaline. Usually, geothermal

systems in steep terrains associated with composite

andesite volcanic centers have outflow regions

(Hochstein and Browne, 1999; Wheeler et al., 2001).

We therefore propose an alteration model correspond-

ing to the liquid dominated, high-temperature hydro-

thermal system of Hochstein and Browne (1999). In

this model, the reservoir temperature is high (N225

8C) and the permeabilities of rocks in the reservoir

and recharge areas are high and moderate, respec-

tively. The heat source of this system was probably

the diorite porphyry stock close to the Cobre de

Chacabuco hill. This stock was associated with the

Miocene volcanic complex, which built up the

Algarrobo unit. According to this model, the lateral

zonation observed in the alteration mineralogy in the

studied area was the result of fluids that flowed

laterally, cooling as they moved away from the

volcanic center and the associated stock, close to the

Cobre de Chacabuco hill, where the upflow zone was

probably located (Fig. 10). These outflows could have

extended in the area for more than 10 km as reported

by Hochstein and Browne (1999) for hydrothermal

systems in Java. These outflowing fluids could also


have been channelled by faults, where an intense

alteration occurred. The temperature estimates for the

easternmost sector of zone II and for the whole of

zone III are in better agreement with the presence of

the upflow region in these areas. A swarm of alunite-

bearing dikes close to the Cobre de Chacabuco hill,

reported by Aguirre (1960), suggests that low pH

fluids were present in the upflow region, consistent

with the alteration model presented here. These low

pH fluids moved downslope from upflow zones and

mixed with shallow groundwater to produce near

neutral to slightly alkaline pH fluids. However,

because this process of neutralization is slow and

fluid composition must dramatically change within a

relatively short distance, the presence of alunite is

better explained by minor ascending acid condensed

steams. The amount of material eroded from this

hydrothermal system can also be approximated from

the previously obtained pressure estimates. If

Pfluid=Ptotal, in all microdomains with the possible

exception of veins and veinlets, and assuming a baric

gradient of 300 bars km�1, then pressures below 500

bars (given by yugawaralite) indicate that no more

than 1.7 km of erosion has occurred.

7. Conclusions

The alteration pattern observed at the Cuesta de

Chacabuco area is characterized by a SE–NW to E–

W lateral zonation in secondary mineralogy. Three

zones have been distinguished: zone I comprises

heulandite, thomsonite, mesolite, stilbite and tri-

smectite; zone II contains laumontite, yugawaralite,

prehnite, epidote and chlorite; and zone III comprises

wairakite, epidote, chlorite, diopside, biotite and

titanite. This zonation is related to a lateral increase

in temperature from zones I to III but not to

stratigraphic depth. This alteration pattern constitutes

an exception to the bnormalQ burial metamorphic

pattern established for central Chile. The temperature

in the rock pile ranged between about 100 and 310

8C, and the geothermal gradient during this alteration

episode was high and estimated at ca. 160 8C km�1.

Although temperature was the main control on the

mineral zonation, several interrelated parameters,

mainly fluid composition, porosity and permeability,

were also important.

The lateral zonation in secondary mineralogy and

temperature was the result of hot outflowing, deposit-

ing fluids with very low dissolved CO2 content,

originating from the upper part of a reservoir located

close to a Miocene volcanic center near the Cobre de

Chacabuco hill.

In the studied area, the Chacabuco and Algarrobo

units can be correlated to the Abanico and Farellones

formations in central Chile, respectively. According to

the classical model proposed to account for the

development of the very low grade metamorphism in

the Cenozoic deposits of the Andes of central Chile

Fig. 10. Alteration model for the Cuesta de Chacabuco area based on the liquid dominated, high-temperature hydrothermal system of Hochstein

and Browne (1999). The light gray area in the outflow region is the inferred zone of mineral deposition. The dotted heavy line is the present

surface.


(Levi et al., 1989), the Abanico Formation would have

undergone a burial metamorphic episode previous to

the unconformable deposition of the Farellones For-

mation. A new and separate burial metamorphic

episode would then have affected the Farellones

Formation but not the Abanico Formation. This

episodic metamorphic model was based on the

assumption that mineralogical breaks existing at the

unconformity between these formations are breaks in

metamorphic grade (Levi et al., 1989). As shown in

this study, no mineralogical break exists between these

formations. Rather they show a clear continuum in

secondary mineralogy. Moreover, there is no evidence

of a metamorphic event previous to the alteration

episode studied here. Only a single Miocene thermal

event affected both formations therefore, a similar

conclusion to that of Bevins et al. (2003).

Acknowledgements

This research was supported by the FONDECYT

Project 1990050 and a CNRS/CONICYT 2000 Project.

The electron microprobe analyses benefited greatly

from the assistance and valuable experience of Dr. A.

Demant. We thank Division Andina (CODELCO) for

access to their brelaveductoQ road. Critical reviews andconstructive comments by Dr. P.R.L. Browne, Prof.

G.P. Glasby and Dr. S. Schmidt were very helpful in

improving the manuscript.

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Miocene fossil hydrothermal system associated with a volcanic complex in the Andes of central Chile

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