Argillization processes at the El Berrocal analogue granitic system (Spain): mineralogy, isotopic...

Argillization processes at the El Berrocal analogue granitic system

(Spain): mineralogy, isotopic study and implications for the

performance assessment of radwaste geological disposal

L. Perez del Villar a,1, E. Reyesb,*, A. Delgadob, R. Nunezb,M. Pelayoa, J.S. Cozar a

aCentro de Investigaciones Energeticas, Medio Ambientales y Tecnologicas (CIEMAT), IDAE,

C.H.E., Avda. Complutense 22, 28040 Madrid, SpainbDepartamento Ciencias de la Tierra y Quımica Ambiental. Estacion Experimental del Zaidın (CSIC),

Profesor Albareda 1, 18008 Granada, Spain

Received 17 April 2001; accepted 27 September 2002

Abstract

The El Berrocal granite/U-bearing quartz vein (UQV) system has been studied as a natural analogue of a high-level

radioactive waste repository. The main objective was to understand the geochemical behaviour of natural nuclides under

different physicochemical conditions. Within this framework, the argillization processes related to fracturing and formation of

the uranium–quartz vein were studied from a mineralogical and isotopic standpoint in order to establish their temperatures of

formation and thus complete the geothermal history of the system. For this purpose, d18O values were determined for pure

mineral from the unaltered granite and quartz from the uranium–quartz vein, as well as for mixture samples from the

hydrothermally altered granite (sericitised granite) and clayey samples from fracture fillings, including the clayey walls of the

uranium–quartz vein. The isotopic signature of quartz from the uranium–quartz vein and the monophasic nature of its fluid

inclusions led us to conclude that the isotopic signature of water in equilibrium with quartz was approximately in the range

from � 8.3xto � 5.7xV-SMOV, its temperature of formation being around 85–120 jC. The d18O values of pure sericite

from the hydrothermally altered granite, calculated by means of the oxygen fraction molar method, indicate that its

temperature of formation, in equilibrium with the aforementioned waters, is also in the range from 70 jC to approximately

120 jC. Clays from fracture fillings and clayey walls of the uranium–quartz vein are usually mixtures, in different

proportions, of illite, approximately formed between 70 and 125 jC; two generations of kaolinite formed at approximately

90–130 jC and at around 25 jC, respectively; smectite, formed at V 25 jC; and occasionally palygorskite, formed either

between 30 and 45 jC or 19 and 32 jC, depending on the fractionation equation used. These data suggest that sericite from

the hydrothermally altered granite, quartz from the uranium–quartz vein, illite and the first generation of kaolinite from the

fracture fillings resulted from the same hydrothermal process affecting the El Berrocal granite in relation to fracturing. Under

certain physicochemical conditions (Tc 100 jC, pHc 8 and log [H4SiO4] between � 4 and � 3), illite and kaolinite can be

paragenetic. As a result of weathering processes, smectite was formed from hydrothermal illite and inherited albite under

0009-2541/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

PII: S0009 -2541 (02 )00323 -6

* Corresponding author. Fax: +34-958-129600.

E-mail addresses: [email protected] (L. Perez del Villar), [email protected] (E. Reyes).1 Fax: +34-913-466542.

www.elsevier.com/locate/chemgeo

Chemical Geology 193 (2003) 273–293

alkaline weathering, while the second generation of kaolinite was formed from smectite, under acid conditions and close to

the sulphide-rich uranium–quartz vein. Palygorskite is an occasional mineral formed probably either during the thermal tail

of the above-described hydrothermal process or during weathering processes. In both cases, palygorskite must have formed

from alkaline Si–Mg-rich solutions. Finally, these data and processes are discussed in terms of natural analogue processes,

drawing some implications for the performance assessment of a deep geological radwaste repository (DGRR).

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Clays; Quartz; Fracture fillings; Granite; Oxygen isotopes; Natural analogue

1. Introduction

Over the last 20 years, the international scientific

community has paid particular attention to studying

natural analogues to gain information on the condi-

tions of mobilisation and retention of natural nuclides

and other analogue elements in geological formations,

especially igneous, clayey and salt formations (Chap-

man and Smellie, 1986; Chapman, 1994; Choppin and

Wong, 1996; Smellie et al., 1997; Hidaka and Hol-

liger, 1998; Miller et al., 2000). Granite formations

have been deemed highly favourable for geological

disposal of radioactive wastes (Chapman and Smellie,

1986; Parneix, 1992). In this context, the mineralog-

ical and geochemical analysis of the secondary min-

eral phases from granite formations, particularly of

clay minerals, is informative for charting the history

of the alteration processes of granite rocks and for

determining the role of these minerals in the retention

of natural radionuclides.

The El Berrocal granite/U-bearing quartz vein

(UQV) system has been studied as a natural analogue

of a deep geological radwaste repository (DGRR), the

main objective being to understand the distant past,

recent past and present behaviour of natural nuclides

under real conditions, as well as to interpret it in terms

of rock–water interaction (Rivas et al., 1997). In this

context, the low-temperature hydrothermal alteration

and weathering events in the El Berrocal system have

been comprehensively studied, since they caused the

most important mobilisation, migration and retention

processes that affected the natural radionuclides in the

system (Perez del Villar et al., 1994, 1995, 1996a,b,

1997; Reyes et al., 1998).

The aim of this work is to study the mineralogical

and isotopic composition of sericite from the hydro-

thermally altered granite, of quartz from the UQV and

the clay minerals from its clayey walls and from the

fracture fillings of the system, in order to:

Establish the approximate temperature of the

hydrothermal processes that affected the El Berro-

cal granite located close to the UQV, and of this

quartz vein itself.

Differentiate the argillization processes in fracture

fillings, according to the formation temperature of

the different clay minerals present in these

fractures.

All these data have been interpreted taking into

account the geological and geochemical background

of the system, emphasizing the relationships between

alteration and migration/retention processes affecting

natural radionuclides. Some implications for the per-

formance assessment of a DGRR in granite forma-

tions have been stated based on the geological

analogies and analogue processes observed at the El

Berrocal natural system.

2. Geological and geochemical background

The El Berrocal system is located some 90 km

southwest of Madrid, near the small town of Nom-

bela, in the province of Toledo (Spain). The system

takes its name from the El Berrocal granite pluton,

which is located in the central part of the Centro-

Iberian Zone, within the Spanish Hercynian Belt

(Julivert et al., 1972). The main granite facies of the

El Berrocal pluton, the so-called El Berrocal facies, is

the host-rock of a U sulphide-bearing quartz vein,

mined in the 1960s and known as ‘‘El Berrocal U

mine’’ (Arribas, 1965) (Fig. 1). Together, the host-

rock and the UQV, make up the so-called El Berrocal

L. Perez del Villar et al. / Chemical Geology 193 (2003) 273–293274

system. The geological materials that form the El

Berrocal system are: (i) the El Berrocal facies or

reference granite, (ii) the hydrothermally altered gran-

ite, (iii) the weathered hydrothermally altered granite,

(iv) the weathered reference granite, (v) the UQV,

including its clayey walls, and (vi) the fracture fillings

(Fig. 2).

The reference granite was classified mineralogi-

cally as a weakly altered alkaline-feldspar granite with

two micas, muscovite being dominant over biotite.

Notable among the accessory minerals are ilmenite,

zircon, monazite, xenotime, apatite, uraninite, cassi-

terite and primary sulphides. Muscovite II, fluorite,

sericite, chlorite, rutile, anatase, K-feldspar and albite

II comprise the main secondary minerals, formed

probably during the deuteric and/or early postmag-

matic hydrothermal processes that affected the pluton.

The geochemical features indicate highly evolved

hypocalcic granite, enriched in silica, phosphorus

and alumina. This granite is fertile in U and belongs

Fig.1. Geographical location and geological scheme of the El Berrocal pluton: (a) biotitic granite of San Vicente type; (b) two mica porphyritic

granite (El Berrocal facies); (c) fine-grained leucogranitic facies; (d) Almorox-Navamorcuende aplitic dyke; (e) uranium quartz vein (UQV); (f)

undifferentiated Tertiary sediments (sands, clays and conglomerates).

L. Perez del Villar et al. / Chemical Geology 193 (2003) 273–293 275

to the ilmenite granite series or to the S-type granites

(Perez del Villar et al., 1994).

The Rb–Sr isotopic data, as well as the structural

and textural relationships among the El Berrocal geo-

logical materials and the mineral paragenesis indicate

that the El Berrocal pluton intruded approximately

297F 1 Ma ago, with an initial 87Sr/86Sr ratio of

0.7175F 0.0029 (Perez del Villar et al., 1996a,b). This

strontium ratio is typical of melted granite derived

from the continental crust (Faure and Powell, 1972).

Furthermore, the El Berrocal facies was affected by an

initial set of hydrothermal events at high temperature

Fig. 2. N–S cross-section of the El Berrocal system showing the uranium mine gallery, and sampling points on boreholes.


(T>350 jC) defined as deuteric and/or early postmag-

matic alteration processes, which transformed the

original facies into the so-called reference granite.

These processes are weak but pervasive since they

affect the whole granite mass. The internal isochrone

determined from apatite, biotite, albite, K-feldspar and

bulk-rock sample of the El Berrocal facies suggests

that the bulk rock remained as a closed system for the

Rb–Sr pair during late alteration processes, and that

the minerals were closed for Sr isotopes around

289F 1 my ago (Perez del Villar et al., 1996a,b).

Consequently, the difference between the intrusion

age and the age estimated from the internal isochrone

(9 my) represents the time during which the first set of

hydrothermal alteration processes took place (Reyes at

al., 1998).

The second hydrothermal event recorded in the

system is related to N80E and N110E fracture fami-

lies. This process transforms the reference granite

close to the fractures into the so-called hydrothermally

altered granite. The alteration is evidenced by the

generalised sericitisation of feldspars, mainly albite,

the presence of secondary ankerite and calcite, which

are scattered in the rock, and the yellow-green colour

of the rock (Perez del Villar et al., 1994). Based only

on isotopic data, the minimum formation temperature

of secondary carbonates is between 61 and 72 jC for

ankerite, and between 35 and 52 jC for calcite (Reyes

et al., 1998). In both carbonates, the influence of

edaphic carbon is evident, being stronger in the latter

than in the former (Reyes et al., 1998).

In relation to this process, sulphide-bearing quartz

veins and their clayey walls were formed, filling the

aforementioned fractures. The U mineralisation took

place after the fracturing of these veins, mainly in

those oriented N110E. The uraniferous paragenesis is

formed by pitchblende, pyrite, barite and carbonates,

the latter being detected by the presence of dissolution

moulds filled with Fe (Mn) oxyhydroxides (Reyes et

al., 1998). Fracture fillings, composed mainly of clay

minerals, Mn-rich calcite and minor secondary quartz,

result primarily from the interaction between fracture

gouges and hydrothermal solutions.

The intensity and depth of the weathering effects

on the El Berrocal system are related to the degree of

fracturing and hydrothermal alteration of the granite,

as well as the mineralogical composition of the hydro-

thermally altered granite, UQV, its clayey walls, and

fracture fillings (see Fig. 2). Geothermometric studies

of Mn-rich calcites, based on d18O values, indicate at

least two generations of carbonates, the first formed

under hydrothermal conditions, at a temperature range

of between 25 and < 100 jC, and the second at

ambient temperature ( < 25 jC) (Reyes et al., 1998).

Almost all the samples analysed were mixtures of the

two types of carbonates. The isotopic signature of

carbon in both generations of carbonates also indi-

cates the influence of edaphic carbon (Reyes et al.,

1998).

As a function of the physicochemical conditions

under which the alteration processes took place in the

system, these processes produced mobilisation, migra-

tion and retention, mainly by precipitation, of U (IV)

and Th (IV), as analogues for Pu (IV) and Np (IV); U

(VI), for Np (V and VI); light rare-earth elements

(LREE), for Am, Cm and Np (III) (Chapman and

Smellie, 1986) and Zr as a fission product (Perez del

Villar et al., 1994, 1995; Reyes et al., 1998).

During the first set of deuteric and/or hydrothermal

events, uraninite, monazite, xenotime and apatite in

the granite were partially destabilised by F-rich fluids.

Thus, when uraninite crystals were surrounded by

fluorite they were totally or partially pseudomorphised

by complex U silicophosphates. Under the same

circumstances, monazite was totally or partially de-

pleted in REE, xenotime showed U and REE poor al-

tered zones and apatite was partially dissolved. The

presence of U silicophosphates coating chloritized

biotite and filling cleavage planes of distorted musco-

vite and microfissures in quartz implies the migration

of U away from uraninite, as well as its retention in

the rock matrix, respectively. On the contrary, Th from

monazite remains immobile, and no secondary REE-

bearing minerals have been found. These textural

features suggest that F is responsible for the alteration

of U, Th and REE-bearing minerals, as well as for the

transport of U and REE, probably as fluoride com-

plexes. This hypothesis was confirmed by the pres-

ence of secondary fluorite and U silicate compounds

filling dissolution voids in albite (Perez del Villar et

al., 1995).

The second hydrothermal process, responsible for

the hydrothermally altered granite and quartz veins,

mobilised and also precipitated U, Th, REE and Zr.

The mineralogical evidence for these processes are: (i)

the appearance of U silicophosphates lining and filling


microfissures of secondary pyrite; (ii) precipitation of

botryoidal mineraloids with a variable chemical com-

position from U>Th to Th>U silicophosphates; (iii)

presence of Th and Zr silicates filling microfissures in

quartz; (iv) neoformation of florencite and (v) the

general increase of U, Th, Ce, Y, and Zr contents in

the hydrothermally altered granite. In this case, the

chemical agent that mobilised U, Th, REE and Zr is

not as clearly defined as in the former case. However,

the presence of secondary carbonates in this hydro-

thermally altered granite implies that carbonate sol-

utions were at work in the system (Perez del Villar et

al., 1995).

In relation to this second hydrothermal event, U

mineralised the main quartz vein. Its mineral para-

genesis (pitchblende, carbonates, pyrite and minor

barite) suggests that U from uraninite scattered in

the granite was oxidised, mobilised and transported,

as uranyl–carbonate complexes, downwards through

the fractures. These solutions, reheated at depth by

tectonic events, moved upwards, probably by con-

vective flow. After destabilisation of uranyl–carbo-

nate complexes, caused by rock–water interaction

processes and a drop in temperature and pressure,

uranyl cations were probably reduced by H2S, pre-

cipitating pitchblende, carbonates and pyrite. The

coexistence of pyrite and barite suggests that the

precipitation occurred between � 200 and � 300 mV

(Krivovichev, 1979). The same process also affected

the hydrothermally altered granite close to the UQV

and, to a less extent, the intragranitic fractures.

The effects of weathering on the uranium minerals

of each part of the system may be summarised as

follows:

On the uraninite from the reference granite, the

effects vary depending on the nature and fracturing

degree of the host minerals. When uraninite is

included in feldspars, the former is usually dissolved

and only the outlines of the crystals remain, with

residual Th–Ca silicophosphates inside. Uranium

from uraninite, forming U silicophosphates, usually

fills adjacent microfissures or is adsorbed, togeth-

er with P, onto Fe oxyhydroxides.

On the hydrothermally altered granite, weathering

mainly redistributed the U that forms large autunite

crystals and/or is adsorbed onto secondary sericite

and Fe oxyhydroxides.

On the U ore body, the effects of weathering are

represented by strong oxidation of pitchblende and

sulphides, dissolution of carbonates and neoforma-

tion of secondary U minerals such as uranotyle,

autunite, in secular equilibrium, torbernite and ura-

nocircite. Furthermore, U is also adsorbed onto Fe

oxyhydroxides.

On fracture-filling materials, similar weathering

effects have been found. Uranium forms uranyl

silicates and phosphates, such as uranotyle, autunite

and phosphuranylite, and occasionally it is ad-

sorbed onto Fe oxyhydroxides. Currently, uranyl–

carbonate complexes are the predominant uranium

species in oxidising groundwaters of the system

(Perez del Villar et al., 1995; Reyes et al., 1998).

3. Sample collection and experimental methods

3.1. Sampling

Twelve samples from the reference granite (M)

were taken along the core of borehole 16, between

6.27 and 590.15 m in depth (Table 1, see Fig. 2) for

the separation of pure quartz, albite and muscovite, as

well as mixtures of chlorite and biotite, since both

phyllosilicates were closely associated in the samples.

Ten samples from the hydrothermally altered granite

(DTG) were taken along borehole S-13, between

84.93 and 100.65 m in depth, on both sides of the

UQV (Table 1, see Fig. 2). For reference granite, the

average sample size was cylinder-shaped, 20 cm long

and 8.6 cm in diameter, while for hydrothermally

altered granite, the average size was cylinder-shaped,

1 cm long and 8.6 cm in diameter.

Nine samples from the weathered hydrothermally

altered granite (LAR), six samples from the UQV

(DQ) and four from its clayey walls (DAR) were

taken from the U mine gallery (Table 1, Fig. 3).

Fracture fillings (RF) were sampled on the hole-cores

of S-13, S-14, S-15, S-16 and S-17 (Table 1, see

Fig. 2).

3.2. Analytical methods

For mineral separation from samples of the refer-

ence granite, 125–60 Am fractions were subjected to

electromagnetic methods and dense liquids. The pow-


der X-ray diffraction (XRD) method was used for

checking the purity of minerals. Mineralogical com-

position of the hydrothermally altered granite was

determined by combining optical microscopy with

the powder XRD method, by using a plane-rotating

sample holder, and by scanning as well as transmi-

ssion electron microscopy (SEM and TEM) coupled

to an energy dispersive X-ray analysis system (EDX).

The mineralogical composition of < 2 Am fractions

from this type of granite was also determined.

For preparation and study of clayey materials from

the walls of the UQV and fracture fillings, the fol-

lowing method was used. After being dried at 30 jC,the samples were smoothly ground in a porcelain mor-

tar to obtain a concentrate of the neoformed mine-

rals and another of the inherited minerals, under the

assumption that the former were preferentially con-

centrated in the soft fraction, while the second were

concentrated in the hard one. A 150-mesh (104 Am)

sieve was used to separate the two fractions. Only

soft fractions ( < 104 Am) were studied, after groun-

ding and sieving to a grain size of < 60 Am. Then,

the carbonates were eliminated to clean the silicate

fraction. An aliquot of the < 60 Am fraction was used

to determine the mineralogical composition, and ano-

ther to separate the < 2 Am fraction, according to the

Stoke’s law.

The < 60 and < 2 Am fractions were characterised

by XRD, combining the powder and oriented ag-

gregate methods. The former was applied to both

fractions, while the second, with ethylene glycol,

sulphoxide dimethyl and thermal treatments, was

applied only to the < 2 Am fractions. For semi-

quantification of the different clay minerals in the

samples, the Bradley and Grim (1961) method was

used. The percentage of apatite was calculated by

stoichiometry, under the assumption that the P2O5/

CaO ratio is 0.73 in the apatite of the reference granite

(Perez del Villar et al., 1992). The clay minerals in the

< 2 Am fractions were more accurately identified using

TEM+EDX, thus facilitating the calculation of their

respective structural formulae.

For isotopic measurements, 12 samples from the

reference granite were used for determining 43 d18Ovalues on pure quartz (12), albite (10), muscovite (12)

and biotite–chlorite mixtures (9). Similarly, 10 ser-

Table 1

Sampling from the El Berrocal site for mineralogical and isotopic purposes

Reference granite Hydrothermally altered granite U mine Fracture filling

(S-16)

Samples

Depths

(m)

(S-13)

Samples

Depths

(m)

gallerySamples Depths

(m)

M-2 6.27–6.45 DTG.10554 84.93 LAR-1 Hydrothermally RF.13-6 87.30

M-3 8.59–8.79 DTG.10555 85.40 LAR-2 altered granite, RF.13-7 87.86

M-4 13.53–13.73 DTG.10556 86.10 LAR-3 affected by RF.13-14 108.02

M-5 17.62–17.82 DTG.10557 86.24 LAR-4 weathering RF.13-17 113.07

M-6 21.60–21.90 DTG.10558 87.47 LAR-5 RF.14-10 106.85

M-10 48.70–40.90 DTG.10559 88.96 LAR-6 RF.15-17 129.50

M-16 115.00–115.19 DTG.10560 90.15 LAR-7 RF.15-18 132.29

M-26 185.09–185.20 DTG.10561 91.60 LAR-8 RF.15-29 143.72

M-39 300.16–300.36 DTG.10562 96.30 LAR-9 RF.16-54 220.31

M-49 399.90–400.10 DTG.10563 100.65 DQ-1 Uranium RF.16-55 221.31

M-59 500.34–500.54 DQ-2 quartz vein RF.16-8 238.16

M-69 589.95–590.15 DQ-3 RF.16-10 413.58

DQ-4 RF.16-21 471.04

DQ-5 RF.16-23 473.15

DQ-6 RF.16-40 569.15

DAR-1 Clayey Walls RF.16-45 596.65

DAR-2 RF.17-12 70.85

DAR-3 RF.17-15 76.83

DAR-4 RF.17-27 156.05

Types of samples and location.


icite-rich samples from the hydrothermally altered

granite, 9 samples from the weathered hydrothermally

altered granite, 6 quartz samples from the UQV, 4

clayey samples from its walls, and 19 samples of

clayey mixtures from fracture fillings, more or less

enriched in illite, smectite, kaolinite or palygorskite,

were also analysed. The d18O values of all the

samples were determined using a Finnigan-MAT

Fig. 3. Bird’s eye and side views of the inner part of uranium mine gallery (from 36 m to the end) showing fractures, quartz veins, including the

UQV and its clayey walls, and sampling points on weathered hydrothermally altered granite (LAR series), quartz veins (DQ series) and clayey

walls (DAR series).

Table 2

Oxygen isotope composition of pure minerals from the reference

granite (borehole S-16) (d values as xvs. V-SMOW)

Samples Q-d18O Ab-d18O Mus-d18O (B +Chl)-d18O

M.2 11.4 – 8.4 10.3

M.3 11.6 9.7 9.0 9.2*

M.4 11.7 8.9 9.9 9.0

M.5 12.0 7.9 9.5 –

M.6 12.4 9.7 7.9 5.6

M.10 11.5 11.1 8.7 –

M.16 11.5 9.1 10.1 –

M.26 11.3 10.8 8.0 4.6

M.39 11.4 9.5 8.3 8.0

M.49 11.6 – 9.2 5.2

M.59 11.0 10.0 8.4 7.1*

M.68 10.8 8.1 8.0 7.1**

Q: quartz; Ab: albite; Mus: muscovite; B: biotite; Chl: chlorite;

* 100% biotite; ** 100% chlorite.

Table 3

Mineralogical composition and d18OT of samples from the hydro-

thermally altered granite intersected by borehole S-13 (d values as

xvs. V-SMOW)

Samples Seric Q Ab K-Fd Chl Cc d18OT

DTG.10554 67 24 9 0 0 0 12.2

DTG.10555 62 31 7 0 0 0 8.1

DTG.10556 81 16 3 0 0 0 8.5

DTG.10557 65 31 4 0 0 0 7.5

DTG.10558 34 31 23 10 2 0 10.4

DTG.10559 75 15 0 10 0 0 12.8

DTG.10560 39 26 0 8 0 27 12.4

DTG.10561 34 22 11 8 0 25 12.9

DTG.10562 40 9 20 12 7 12 13.5

DTG.10563 52 7 15 7 19 0 13.0

Seric: sericite; Q: quartz; Ab: albite; K-Fd: K-feldspar; Chl: chlorite;

Cc: calcite; d18OT: isotopic composition of the bulk sample (x).


251 mass spectrometer, according to the ClF3 techni-

que (Borthwick and Harmon, 1982), later modified by

Vennemann and Smith (1990). These isotopic values

are reported using the d notation in units per mil (x)

relative to V-SMOW, d18O values being reproducible

to F 0.2. Using this procedure, the d18O values of

NBS-30 (biotite) and NBS-28 (quartz) are + 5.1xand

+ 9.7x, respectively.

For geothermometric purposes, the following frac-

tionation equations were used:

1000lna ¼ 3:38 ð106 T�2Þ � 3:40 ð1Þ

for quartz (Clayton et al., 1972);

1000lna ¼ 2:43 ð106 T�2Þ � 4:80 ð2Þ

for illite and sericite (Eslinger and Savin, 1973);

1000lna ¼ 2:5 ð106 T�2Þ � 2:9 ð3Þ

for kaolinite (Land and Dutton, 1978); and

1000lna ¼ 2:6 ð106 T�2Þ � 4:28 ð4Þ

for smectite (Savin and Lee, 1988). In addition, two

approaches were tested to calculate the temperature of

palygorskite formation. The first, by using the equa-

tion for smectite, since experimental fractionation

equation for palygorskite is not yet available in the

specialised literature, and smectite and palygorskite

are chemically similar (Torres-Ruiz et al., 1994). The

second, by applying the equation proposed by Gauth-

ier-Lafaye et al. (1993).

However, two main problems arise in determining

the temperature of the formation of these minerals.

Firstly, all the samples are mixtures either of different

clay minerals or clay minerals and other granite-

forming silicates, mainly quartz and albite. Secondly,

no data could be gathered on isotopic signatures of

water in equilibrium with these minerals. However,

fluid inclusion studies performed on quartz samples

Table 4

Mineralogical composition of granite, quartz and clayey samples taken from the El Berrocal uranium mine gallery

Min Hydrothermally altered granite affected by weathering Quartz from UQV Clayey walls

(%) LAR-

1

LAR-

2

LAR-

3

LAR-

4

LAR-

5

LAR-

6

LAR-

7

LAR-

8

LAR-

9

DQ-

1

DQ-

2

DQ-

3

DQ-

4

DQ-

5

DQ-

6

DAR-

1

DAR-

2

DAR-

3

DAR-

4

Q 55 34 24 30 25 30 23 27 41 100 100 100 100 100 100 22 25 50 46

Ab 1 traces 7 4 3 – 3 4 4 – – – – – – 3 – 3 4

K-Fd – 7 15 8 9 8 7 8 – – – – – – – – – – –

Ap traces traces traces traces traces traces traces traces traces – – – – – – traces traces 5 traces

T.Ph. 43 59 54 58 63 62 66 60 54 – – – – – – 74 74 44 50

Q: quartz; Ab: albite; K-Fd: K-feldspar; Ap: apatite (determined by stoichiometry); T.Ph: total phyllosilicates; – : undetectable.

Table 5

Mineralogical composition and isotopic signature of the < 2 Am fractions from samples taken in the El Berrocal uranium mine gallery

Min Hydrothermally altered granite affected by weathering Quartz from UQV Clayey walls

(%) LAR-

1

LAR-

2

LAR-

3

LAR-

4

LAR-

5

LAR-

6

LAR-

7

LAR-

8

LAR-

9

DQ-

1

DQ-

2

DQ-

3

DQ-

4

DQ-

5

DQ-

6

DAR-

1

DAR-

2

DAR-

3

DAR-

4

Q 19 18 17 18 17 14 18 14 12 100 100 100 100 100 100 9 16 13 11

Ab – – – – – – – – – – – – – – – – – – –

K-Fd – 14 23 10 11 12 16 5 17 – – – – – – – – – 18

Ap – – – – – – – – – – – – – – – – – 7 –

Ill 50 31 21 35 32 32 31 43 45 – – – – – 64 27 48 42

Sm 30 35 38 36 40 42 34 30 26 – – – – – – 26 48 32 28

K – 2 1 – – – – – – – – – – – – – 9 – –

d18O 11.2 11.9 13.6 13.7 13.6 14.7 14.4 12.9 8.6 14.2 14.4 15.3 12.7 14.2 12.6 9.7 14.3 10.2 10.1

Q: quartz; Ab: albite; K-Fd: K-feldspar; Ap: apatite (determined by stoichiometry); Ill: illite; Sm: smectite; K: kaolinite; – : undetectable; d18O:in xV-SMOV.


from the UQV have shown that primary fluid inclu-

sions are monophasic and that temperatures lower

than 100 jC can therefore be inferred. Furthermore,

the average d18O value (� 7.6x) of the present day

meteoric waters at the El Berrocal site (Perez del

Villar et al., 1997) was also considered for geother-

mometric purposes.

Under these restraints, an approximate d18O value

for water in equilibrium with quartz from the UQV

was firstly established. For sericite from the hydro-

thermally altered granite, an approximate theoretical

d18O value was calculated taking into account: the

semi-quantitative mineralogical composition of the

samples; the molar fraction of oxygen in each of the

quantified minerals; the structural formula of sericite;

and the isotopic signatures of bulk sericite-enriched

samples, pure quartz and albite from the reference

granite (Lawrence and Taylor, 1971; Ruiz-Cruz and

Reyes, 1998). The theoretical d18O value for pure

illite was calculated from d18O signatures of several

< 2 Am binary mixtures formed by illite + smectite

from fracture filling samples (RF series). For this,

the regression line between d18O values of the binary

mixtures and the illite/smectite ratios were calculated.

The d18O value thus determined was also assigned to

illite from the clayey walls of UQV. The isotopic

signature of pure smectite was measured on only one

sample from fracture fillings, formed by only smectite

(RF series). This value was also assigned to smectite

from the clayey walls of the UQV. The theoretical

Table 6

Chemical composition and structural formula of sericite– illite s.

str., with < 5% of expanding layers

Samples DAR-3 DAR-4 DAR-3 LAR-1 DAR-3 DAR-3

Oxides (%) 1 2 3 4 5 6

SiO2 50.63 48.90 49.62 49.15 51.03 51.29

Al2O3 32.92 32.53 36.83 38.23 36.54 36.54

Cr2O3 – – – – – –

Fe2O3 4.62 6.87 2.53 2.03 1.14 2.08

MgO – – – – – –

CaO – – – – 0.96 –

K2O 11.83 11.71 11.02 10.59 10.32 10.09

Total 100 100 100 100 99.99 100

Si4 + 6.467 6.308 6.265 6.180 6.395 6.413

Al3 + 1.533 1.692 1.735 1.820 1.605 1.587

Total 8.000 8.000 8.000 8.000 8.000 8.000

Al3 + 3.424 3.254 3.747 3.850 3.792 3.797

Fe3 + 0.444 0.667 0.240 0.190 0.110 0.200

Cr3 + – – – – – –

Mg2 + – – – – – –

Total 3.868 3.921 3.987 4.040 3.902 3.997

K+ 1.927 1.927 1.775 1.700 1.651 1.610

Ca2 + – – – – – –

Mg2 + – – – – – –

Total 1.927 1.927 1.775 1.700 1.781 1.610

D � 1.533 � 1.692 � 1.735 � 1.820 � 1.605 � 1.587

w � 0.396 � 0.237 � 0.039 0.120 � 0.294 � 0.009

= 1.929 1.929 1.774 1.700 1.899 1.596

Si/Al 1.304 1.275 1.142 1.089 1.184 1.191

% 79.47 87.71 97.80 100 84.52 99.43

D: tetrahedral charge; w: octahedral charge; =: interlayer charge;

%: of tetrahedral charge.

Table 7

Chemical composition and structural formulae of beidellitic

smectite

Samples DAR-2 DAR-2 DAR-2

Oxides (%) 39 40 41

SiO2 55.92 61.53 58.91

Al2O3 33.57 29.24 36.74

Cr2O3 1.10 – –

Fe2O3 3.18 2.86 1.87

MgO 3.32 3.40 –

CaO 1.55 0.64 1.41

K2O 1.38 1.25 1.07

Total 100.02 100.02 100.02

Si4 + 6.700 7.290 6.928

Al3 + 1.300 0.710 1.072

Total 8.000 8.000 8.000

Al3 + 3.440 3.370 4.020

Fe3 + 0.290 0.260 0.165

Cr3 + 0.100 – –

Mg2 + 0.170? 0.370? –

Total 4.000 4.000 4.185

K+ 0.210 0.190 0.161

Ca2 + 0.200 0.220 0.177

Mg2 + 0.420? 0.23? –

Total 0.830 0.64 0.338

D � 1.300 � 0.710 1.072

w � 0.170 � 0.37 0.555

= 1.450 ? 1.090 0.517

Si/Al 1.413 1.786 1.360

% 88.43 65.74 100

D: tetrahedral charge; w : octahedral charge; =: interlayer charge;

%: of tetrahedral charge.


d18O values of pure kaolinite and palygorskite were

calculated from samples enriched in these clay min-

erals (DAR and RF series), by using the d18O values

of their respective mixtures together with those meas-

ured and calculated for pure minerals present in the

mixtures. In these cases, the same method as for

estimating of the isotopic signature for pure sericite

was also applied.

4. Results

4.1. d18O values of pure quartz, albite, muscovite and

chlorite–biotite mixtures from the reference granite

The isotopic signatures (d18O values) of these

minerals (Table 2) were measured for correction

purposes. Both quartz and muscovite gave constant

values, while the d18O values for albite (An3) showed

greater variability. These variations were even more

accentuated in biotite–chlorite mixtures due to the

nature of the samples. However, two values for almost

pure biotite and one value for almost pure chlorite are

also recorded.

4.2. Mineralogy and d18O values of the hydro-

thermally altered granite

The mineralogical composition and d18O values of

the bulk samples from the hydrothermally altered

granite intersected by borehole S-13 are listed in Table

3. These data show that the main phyllosilicate in the

samples is sericite, while chlorite is also present in

three samples. The d18O values of the bulk samples

ranged between 7.5xand 13.5xV-SMOV. For iso-

topic purposes, it is noticeable that in the four first

samples (DTG.10554 to DTG.10557) K-feldspar,

chlorite and calcite were absent. Consequently, these

samples were used only for determining the theoret-

ical d18O value for pure sericite.

Table 8

Chemical composition and structural formula of kaolinite

Samples DAR-2 DAR-2 DAR-2 DAR-2 DAR-2 DAR-2

Oxides

(%)

42 43 44 45 46 47

SiO2 54.08 52.83 52.22 53.69 54.93 52.98

Al2O3 43.40 46.29 45.40 46.31 44.56 45.79

Fe2O3 1.46 – 1.18 – 0.51 0.83

MgO – – – – – –

CaO – – 0.67 – – 0.40

K2O 1.07 0.89 0.52 – – –

Total

Si4 + 4.045 4.060 4.000 4.040 4.060 4.013

Al3 + – – 0.09 – – –

Total 4.045 4.060 4.090 4.040 4.060 4.013

Al3 + 3.82 3.82 3.910 3.980 3.890 3.940

Fe3 + 0.08 – 0.070 – 0.030 0.47

Mg2 + – – – – – –

Total 3.900 3.930 3.980 3.980 3.920 3.987

K+ 0.10 0.08 0.05 – – –

Ca2 + – – 0.053 – – –

Mg2 + – – – – – –

Total 0.10 0.08 0.156 – – 0.064

D 0.180 0.240 0.27 0.16 0.24 0.050

w � 0.300 � 0.210 � 0.060 � 0.060 � 0.240 � 0.039

= 0.120 0.030 0.15 0.10 0 0.013

Si/Al 1.06 1.06 1.00 1.01 1.04 1.018

D: tetrahedral charge; w: octahedral charge; =: Interlayer charge. 42:analysed point.

Table 9

Mineralogical composition and oxygen isotope composition of soft

fractions from fracture fillings intersected by boreholes S-13, S-14,

S-15, S-16 and S-17

Samples Q K-Fd Ab Chl Ill Sm K Pgk d18

OV-SMOW

RF.13-6 2 – 3 – 61 33 1 – 9.5

RF.13-7 – – – – 82 17 1 – 8.2

RF.13-14 5 6 4 4 11 70 – – 13.2

RF.13-17 – – – – 1 51 48 – 11.7

RF.14-10 – – – – 19 66 15 – 12.8

RF.15-17 – – – – 46 54 – – 8.4

RF.15-18 9 – 4 – 87 – – – 6.3

RF.15-29 – – – – 79 21 – – 10.3

RF.16-54 – – – – 13 30 57 – 10.4

RF.16-55 – 5 – – 9 40 46 – 11.6

RF.16-8 – – – – 73 27 – – 10.0

RF.16-10 8 – – 89 3 – – – 7.9

RF.16-21 – – – – – 100 – – 16.5

RF.16-23 3 3 3 – 3 88 – – 15.1

RF.16-40 – – – – 10 90 – – 16.8

RF.16-45 – – – 68 27 5 – – 9.4

RF.17-12 1 – – – 7 21 – 71 15.1

RF.17-15 5 5 3 – 8 79 – – 15.4

RF.17-27 1 5 3 – 11 78 2 – 14.4

Q: quartz; K-Fd: K-feldspar; Ab: albite; Chl: chlorite; Ill: illite; Sm:

smectite; K: kaolinite; Pgk: palygorskite; d18OV-SMOW: x.


4.3. Mineralogy and d18O values of the weathered

hydrothermally altered granite, quartz from the UQV

and its clayey walls

The mineralogical composition of the bulk sam-

ples from these materials (Table 4) shows that phyl-

losilicates are generally the main components in the

samples, except in those from the UQV. The minera-

logical composition of their corresponding < 2 Amfractions (Table 5) shows that the main clay minerals

in the samples, except for the quartz ones, are illite

and smectite, the former predominating over the

latter. Only three samples (LAR-2, LAR-3 and

DAR-2) contain minor kaolinite. The d18O values

for hydrothermal pure quartz ranged between 12.6xand 15.3xV-SMOV, while for clay mixtures the

range was between 8.6xand 14.7xV-SMOV (see

Table 5).

The structural formulae of the main clay minerals

in the samples (Tables 6–8) correspond to dioctahe-

dral clay micas (sericite/hydromuscovite and illite),

aluminian dioctahedral smectite, specifically beidel-

lite, and kaolinite, respectively.

4.4. Mineralogy and d18O values of fracture fillings

intersected by boreholes drilled at the site

Mineralogy and oxygen isotopic signatures were

determined only in the < 2 Am fractions of these

materials. The results (Table 9) indicate that, in

general, the main clay minerals in the samples are

illite and smectite, while kaolinite and chlorite are

occasionally present. Only one sample (RF.17-12)

contains a significant amount of palygorskite and

another (RF.16-21) consists exclusively of smectite.

Chlorite is inherited from the granite; illite comes

from the alteration of albite and, to a less extent, of

K-feldspar and muscovite (Fig. 4a); and smectite,

with a web-like pore-lining morphology (Fig. 4b),

mainly from the alteration of albite (Fig. 4c). Kao-

linite is idiomorphic, with a face-to-face stack-type

texture (Fig. 4d), and palygorskite shows its typical

fibrous habit, but coated by Fe oxyhydroxides (Fig.

4e). The d18O value of pure smectite sample was

16.5xV-SMOW, while the d18O values of the re-

maining mixtures varied between 6.3xand 16.8xV-SMOW.

5. Estimation of isotopic signatures for pure

minerals and geothermometry

5.1. Isotopic signature of water in equilibrium with

hydrothermal quartz

As mentioned in Section 3, primary fluid inclu-

sions in quartz from the UQV were monophasic, and

temperatures lower than 100 jC can therefore be

inferred. On the basis of the highest d18O values of

these samples (15.3x) and a temperature of approx-

imately 100 jC (or lower), the isotopic signature of

water in equilibrium with quartz could be more

negative than � 5.7x. Similarly, given that the range

of isotope values for quartz was between 12.6xand 15.3x, the d18O range for water in equilibrium

was between � 8.3xand � 5.7x. However, though

this range can be considered quite likely, d18O va-

lues more negative than � 8.3xare possible if the

temperature of formation of quartz was lower than

100 jC.

5.2. Theoretical d18O values for sericite from the

hydrothermally altered granite

Three of the ten samples analysed (DTG.10555 to

DTG.10557, see Table 3) were selected to estimate

the theoretical d18O value of pure sericite, since they

were ternary mixtures of mainly sericite, magmatic

quartz and minor albite, with similar d18OT values.

Sample DTG.10554, with a similar mineralogical

composition, had an extremely high d18OT value

and was therefore rejected. For calculation, the

fraction molar method was used, taking into account

the structural formula of silica, albite (An3) and

Fig. 4. Secondary electron images showing: (a) the transformation of muscovite (1) into illite (2); (b) the web-like pore-lining texture of

smectite; (c) smectite from the alteration of albite. The chemical composition in points 1, 2, 3, 4, 5 and 6 corroborates the gradual albite–

smectite transformation; (d) the first generation of kaolinite (1), with its typical face-to-face stacked texture; (e) fibrous palygorskite coated by

Fe oxyhydroxides with some U and P adsorbed.


sericite, and applying the following general equa-

tion:

Xad18Oa þ Xbd

18Ob þ Xcd18Oc þ . . .Xnd

18On

¼ d18OT ð5Þ

Abbreviated as:

Xc

n¼a

Xnd18On ¼ d18OT ðn ¼ a . . . cÞ

where: a, b, c,. . .n =minerals in the sample or

mixture; d18On = isotopic signature of each pure

mineral in the sample or mixture, known from

pure minerals experimentally measured; Xn = num-

ber of mols of oxygen in each mineral, referred as

a fraction of the total mols of oxygen in the

sample or mixture.

Xc

n¼a

Xn ¼ 1

d18OT = d18O experimentally measured in the total

sample or mixture.

For the particular case of the hydrothermally

altered granite, a, b and c are sericite, magmatic quartz

and albite (An3), respectively. For quartz and albite,

the average d18O values were used (Table 2). The

mineralogy and d18OT of the samples used are shown

in Table 3.

For sericite, the structural formula used was (Si6.387Al1.613)(Al3.339Fe0.555)K1.927O20(OH)4, which repre-

sents the average formula of the first two formulae

(Eqs. (1) and (2)) listed in Table 6 since they are the

most representative for this phyllosilicate. Thus, the

theoretical d18O value for pure sericite ranges between

5.2xand 7.8xV-SMOV.

For calculating an approximate temperature of

formation of sericite, the range of the d18O values

(� 8.3xand � 5.7x) for waters in equilibrium with

hydrothermal quartz from the UQV was used since it

was assumed that both UQV and hydrothermally

altered granite resulted from the same geological

process. According to these parameters, the minimum

temperature varies from 70 to 117 jC (Fig. 5a).

5.3. Theoretical d18O values for smectite, illite,

kaolinite and palygorskite from the clayey walls of

UQV, weathered hydrothermally altered granite and

fracture fillings

The wide range of isotopic values found in these

samples is attributable to the fact that all the < 2 Amfractions were mixtures of different minerals that in

turn were impossible to separate physically (see

(Tables 4, 5 and 9)). This circumstance enabled the

calculation of only an approximate d18O value for

each pure clay mineral. However, sample RF.16-21

consists exclusively of smectite with an d18O value of

16.5x, which agrees with the theoretical d18O value

for a smectite in equilibrium with the current meteoric

waters at El Berrocal (d18O =� 7.6x), at a near-

surface temperature.

For estimating the theoretical d18O value of illite

from the clay mixtures, an initial approach was used

following a method similar to that of Whitney and

Northrop (1988). Thus, from the correlation between

the illite/smectite ratios and the experimental d18Ovalues determined from samples RF.13-7, RF.15-17,

RF.15-29, RF.16-8, RF.16-21 and RF.16-40, which

were binary mixture of illite and smectite (see Table

9), a theoretical d18O value ( + 6.7x) for illite was

estimated (Fig. 6). A second approach was used to

calculate the d18O value of illite, using the oxygen

fraction molar method (Eq. (5)) on samples formed

mainly of smectite and illite, with minor quartz or

quartz and albite (samples LAR-1; DAR-1, RF.13-6

and RF.15-18). For calculation, the d18O values used

were 16.5xfor smectite and 11.5xand 9.5xas

average values for magmatic quartz and albite, respec-

tively (see Table 2). With this second approach and

data, the d18O value for illite ranged between 4.6xand 8.0x.

The temperature of illite formation is difficult to

estimate since no data are available on the isotopic

signature of water in equilibrium with illite. However,

from the structural relationships among UQV, its

clayey walls and the hydrothermally altered granite,

it can also be assumed that these materials are formed

as a result of the same hydrothermal process. Con-

sequently, the isotopic range for water in equilibrium

with quartz from the UQV (� 8.3xand � 5.7x)

and the equation for illite–water fractionation (Eq. (2))

can be used to estimate the illite formation temper-


Fig. 5. (a) Plot of the Eslinger and Savin (1973) equation, from which the temperature range for sericite from the hydrothermally altered granite

has been estimated. (b) Idem for illite from fracture fillings and clayey walls of the UQV. (c) Plot of the Land and Dutton (1978) equation on

which the temperature of formation for the second generation of kaolinite has been calculated. (d) Idem for the first generation of kaolinite. (e)

Plot of the Savin and Lee (1988) equation for smectite–water fractionation, applied to palygorskite. (f) Plot of the Gauthier-Lafaye et al. (1993)

equation for palygorskite–water fractionation, also applied to the El Berrocal palygorskite.


ature, this is ranging from 69 to approximately 124 jC(Fig. 5b).

Prior to the estimation of the isotopic signature and

thermometry of the remaining clay minerals in the

samples, kaolinite and palygorskite, it is appropriate

to take into account the following assumption. Weath-

ering under acidic and intensive cation leaching con-

ditions or low-temperature hydrothermal processes

(Raymahashay, 1968) are usually the main geological

mechanisms that originate kaolinite, if the country

rocks are suitable. On the contrary, palygorskite is

usually formed in very restricted geological environ-

ments, under alkaline conditions, with high activities

of silica and magnesium.

In the El Berrocal system, kaolinite occurs either in

the shallow and oxidised zone of the UQV-clayey

walls, where pyrite is almost totally oxidised and

sulphate acid percolating waters (pH 3–4) are present

today, or in deeper fractures, where reducing, neutral

or weakly alkaline conditions prevail.

On these bases, kaolinites from these two different

environments were considered separately to determine

their respective d18O values and formation temper-

atures. Sample DAR-2, formed by smectite, illite,

kaolinite and hydrothermal quartz, has been consid-

ered for shallow kaolinite, while samples RF.13-17,

RF.14-10 and RF.16-54, which are mixtures of illite,

smectite and kaolinite, have been retained for deeper

kaolinites.

From sample DAR-2, the d18O value estimated for

pure shallow kaolinite is 19.1xV-SMOW by apply-

ing Eq. (5) and using the d18O values for smectite

(16.5x), the average value for hydrothermal quartz

(13.9x) and the highest values for illite (8.0x). In

the same way, for pure deeper kaolinites the d18Ovalues vary between 2.6xand 8.3xV-SMOW. The

difference between the isotopic signatures of the shal-

low and deeper kaolinites seems to corroborate the

different geological environments in which they were

formed, as mentioned above. Consequently, for esti-

mating the formation temperature of shallow kaolinite,

it seems reasonable to assume that it was formed in

equilibrium with the present-day meteoric waters, with

a weighted average d18O value of � 7.6xV-SMOW.

Thus, the maximum formation temperature estimated

for shallow kaolinite was 26 jC (Fig. 5c).

On the other hand, as kaolinite can also be formed

in hydrothermal systems under conditions similar to

Fig. 6. Correlation between the illite/smectite ratios and the experimental d18O values obtained from the illite/smectite samples from which a

theoretical d18O value for pure illite has been estimated.


those that form illite (Raymahashay, 1968), deeper

kaolinites can also be equilibrated with waters in

equilibrium with both hydrothermal quartz and illite,

provided that the three minerals resulted from the

same hydrothermal process. Thus, the temperature of

formation of deeper kaolinites ranged from 88 to

133 jC (Fig. 5d). A similar temperature range re-

sulted when present-day meteoric waters were used

for this purpose since both hydrothermal and mete-

oric waters at the site have very similar isotopic

signatures.

The presence of different generations of kaolinites

(specifically dickites), including one of Quaternary

age (dickite from sample DAR-2), has also been

confirmed in the El Berrocal system, by using Fourier

transform infrared spectroscopy (FTIR) (Allard et al.,

submitted for publication).

Regarding palygorskite, sample RF.17-12, formed

essentially by this mineral and smectite together with

minor illite and quartz, was considered only for

isotopic and geothermometric purposes. For pure

palygorskite the d18O value varied from 15.5xto

15.8xV-SMOW, and the estimated temperature

ranged between 31 and 45 jC, when the assumption

proposed by Torres-Ruiz et al. (1994) was applied

(Fig. 5e). However, this temperature range was

approximately 10 jC lower (19–32 jC) when the

equation proposed by Gauthier-Lafaye et al. (1993)

was used (Fig. 5f). In any case, both temperature

ranges agree with those estimated for low-temperature

carbonates from the hydrothermally altered granite

(52–35 jC) and for low-temperature carbonates from

fracture fillings, some of them formed at ambient

temperature (V 25 jC) (Reyes et al., 1998). Finally,

palygorskite as well as carbonates need similar phys-

icochemical conditions for precipitation.

6. Discussion and conclusions

6.1. Argillization processes

The approximate isotopic signature (d18O) esti-

mated for the hydrothermal waters at the El Berrocal

site is very similar to that for the present-day meteoric

waters, despite boiling and water–rock interactions

that could affect the hydrothermal ones. In this sense,

Reyes et al. (1998), on the basis of the isotopic

signatures (d18O and d13C) of carbonates, suggestedthat the hydrothermal water at the El Berrocal site had

a meteoric origin, being reheated in depth and moving

upwards by convective flow.

The d18O range estimated for sericite from the

hydrothermally altered granite and the intensity of

the sericitisation process suggests that this mineral

was formed at around 100 jC, with a high water/

rock ratio, as corresponds to a highly fractured

geological environment. Illite from fracture fillings

and clayey walls of the UQV was probably the result

of the same hydrothermal process, since its forma-

tion temperature is also around 100 jC. Both pro-

cesses—sericitisation of reference granite and illiti-

sation of fault gouges—were probably controlled by

the following two well-known general chemical

reactions:

K � feldspar þ bicarbonate� carbonate H2O

Zsericite or illiteþ SiO2 þ Kþ þ Carbonates ð6Þ

Albiteþ bicarbonate� carbonate H2Oþ Kþ

Zsericite or illiteþ SiO2 þ Naþ þ Carbonates ð7Þ

In the hydrothermally altered granite, these reac-

tions are supported by the presence of K-feldspar and

albite pseudomorphised by a mixture of sericite,

carbonates and microcrystalline quartz. Furthermore,

potassium released from reaction (6) was used in Eq.

(7) and sodium released from Eq. (7) probably

migrated outside the system. Neoformed sodium min-

erals have not been found in the samples. Similarly,

silica released from both reactions was probably spent

in the formation of quartz veins, particularly the UQV.

This quartz was also formed at approximately 100 jC.The presence of kaolinite formed at the same

temperature as illite suggests that both clay minerals

were also formed during the same alteration pro-

cesses, but only in those fractures where the phys-

icochemical conditions were suitable for its formation.

In this sense, the pH–log [H4SiO4] stability diagram

(Raymahashay, 1968) shows that the boundary

between the muscovite and kaolinite stability fields

at 95 jC is located at pH = 8 for a log [H4SiO4] range

between � 4 and � 3 (Fig. 7). Consequently, illite


and this first generation of kaolinite are assumed to be

paragenetic in those fractures where the pH ap-

proached 8. Above this pH value, only illite would

be formed.

These parameters agree with the presence in the

hydrothermally altered granite and fracture fillings of

carbonates formed below 100 jC and with some

influence of edaphic carbon (Reyes et al., 1998),

thereby corroborating the meteoric origin of the

hydrothermal waters circulating in the system.

According to the textures observed by SEM, smec-

tite comes mainly from the alteration of albite, prob-

ably as a result of the following general reaction:

Albiteþ bicarbonate� carbonate H2O

ZSiO2 þ Smectiteþ Carbonates ð8Þ

Illite can also be transformed into smectite by the

reaction:

Illiteþ SiO2 þ bicarbonate � carbonate H2O

ZSmectite þ Kþ þ Carbonates ð9Þ

Furthermore, geothermometric studies indicate that

smectite was formed at ambient temperature (e.g. in

relation to weathering processes) under alkaline con-

ditions. This agrees with the stability diagram at 25 jC

(see Fig. 7) and with the presence of carbonates formed

at environmental temperature, which also have a sub-

stantial influence of edaphic carbon (Reyes et al.,

1998).

Kaolinite formed at approximately 25 jC can be

explained as having formed under the acid conditions

caused by the oxidation of sulphides from the UQV.

Under these physicochemical conditions, mainly

smectite is transformed into kaolinite, according to

the following general reaction:

Smectiteþ acid H2O

ZKaoliniteþ Ca2þ þMg2þ þ Kþ þ Quartz ð10Þ

The fact that only one sample from the oxidised

zone of the clayey walls of the sulphide-bearing UQV

contains minor kaolinite (9%) is probably due to the

extremely low permeability of these clayey walls that

hindered the water–rock interactions between the

sulphate acid percolating waters and smectite.

For palygorskite, an occasional mineral in the

fracture filling materials, only one datum concerns

the temperature of its formation (31–45 or 19–32 jC)is available. This mineral was therefore probably

formed either during the thermal tail of the hydro-

thermal process described above and simultaneously

with the lower temperature carbonates or during

weathering processes. In both cases, palygorskite

must have formed from alkaline Si–Mg-rich solu-

tions. In any case, this fibrous clay mineral appears to

have formed specifically in those fractures of the

system where these physicochemical conditions

occurred.

6.2. Implications for the performance assessment

Some analogies between El Berrocal system and a

deep geological radwaste repository can be drawn:

The UQV contains pitchblende (UO2 + x), which is

analogous to the main component of the spent

nuclear fuel (source term).

The geometry of the UQV is very simple and it is

bordered on two clayey walls analogous to the

engineered clayey barrier envisioned to isolate the

spent nuclear fuel (near field).

Fig. 7. Stability fields of gibbsite, kaolinite, muscovite, K-feldspar

and montmorillonite in alkaline environment after Raymahashay

(1968).


The UQV is hosted in highly evolved S-type

granites (store rock or far field), which are well

represented in the European Hercynian Belt.

On the contrary, the UQV, highly fractured and

very permeable, is perturbed by mining activities and,

particularly, contains considerable amounts of sul-

phides and carbonates, which in principle are two

‘‘poisonous’’ minerals under oxidising conditions.

Their dissolution also facilitates the destabilisation

of pitchblende and encourages the migration of ura-

nium.

Finally, the granite country rock of the El Berrocal

UQV contains uraninite and is therefore the second

source of U in the system. This feature hampers the

delimitation of the dispersion haloes of U from the

UQV and from the granite itself. In addition, the El

Berrocal granite shows a very high fracture density,

though fractures are generally filled by clay minerals,

minor Fe oxyhydroxides and other minerals, mainly

phosphates, that participate in the retardation of U

(VI).

According to these geological and mineralogical

features, the El Berrocal system cannot be considered

a completely natural analogue of a deep geological

radwaste repository. However, in terms of the pro-

cesses affecting the system, both low-temperature

hydrothermal and weathering processes can be com-

pared to those expected in a nuclear repository after

burial, under extremely adverse circumstances.

The low-temperature hydrothermal process could

be similar to that expected in a radwaste repository

after burial, since a thermal peak related to radioactive

decay of fission products from the spent fuel is

envisaged (less than 100 jC in the ENRESA con-

ceptual model).

In the natural system under investigation, the low-

temperature hydrothermal process caused:

The accumulation in the UQVof U scattered in the

granite, as a result of a first downward step of U

mobilisation, under oxidising and weakly acidic or

alkaline conditions, and environmental tempera-

ture; and a second upward step of U (IV) preci-

pitation, under weakly alkaline and reducing con-

ditions at higher temperatures, but below 100 jC.The argillization of the fault gouges, including the

UQV walls, and the hydrothermal alteration of the

reference granite close to fractures, increasing their

clay contents and therefore their retention capa-

bilities for natural nuclides and other analogous

elements (Perez del Villar et al., 1995, 1997).

If a similar process occurs in a nuclear repository,

two possibilities can be considered. The first would be

that the oxidising meteoric waters percolate down to

the spent fuel, and the second that the meteoric waters

lose their oxidising capability (by rock–water inter-

action processes) before interaction with the spent

fuel. In the first case, the source term would be

destabilised, and U and other nuclides would be

dispersed as a function of their solubility in these

reheated waters. However, clays from an engineered

barrier, fracture fillings and country rock itself would

work against the migration of radionuclides as in the

El Berrocal natural system (Perez del Villar et al.,

1997). In the second case, the source term would

remain totally or partially unaltered. Only coffinitisa-

tion of the UO2 + x from the spent fuel could be

expected (Perez del Villar et al., 2002). Furthermore,

the hydrothermal system artificially created in the

nuclear repository would alter the granitic storage

rock, mainly through the fracture zones, increasing

its clay content and therefore its retention capability.

In addition, past and recent past weathering pro-

cesses were more intensive and deeper on the UQV

than in the hydrothermally altered and reference

granites due to their different secondary permeability.

The pH of these waters in the upper part of the UQV is

usually around 3–4, but close to 7 in the other

hydraulically active fractures. In both cases, waters

become weakly alkaline in depth (Perez del Villar et

al., 1995). Accordingly, the following analogies can

be established:

(a) In the UQV itself or source term, oxidation due

to weathering caused the total oxidation and

dissolution of pitchblende, sulphides (mainly

pyrite), and carbonates, as well as the precip-

itation of jarosite (pH 3–4) and Fe (Mn)

oxyhydroxides with minor U and P adsorbed

(Perez del Villar et al., 1995, 1997). Conse-

quently, this intense downward oxic and acid

alteration in the upper part of the UQV is of no

relevance in assessing repository behaviour. How-

ever, once the conditions of neutrality–alkalinity


and reducing potential are restored in depth, the

original mineral paragenesis [pitchblende, sul-

phides (mainly pyrite) and carbonates] becomes

stable, illustrating again the well-known stability

of natural UO2 + x under reducing conditions. In the

transition between oxidised and reduced zones, in

situ transformation of pitchblende into uranocircite

and autunite together with the precipitation of

torbernite occur, this zone being therefore relevant

as a geochemical discontinuity for natural nuclide

retardation.

(b) In the clayey walls or near field, with lower per-

meability than UQV itself, inherited albite and

particularly illite were transformed firstly into

smectite under weakly alkaline conditions increas-

ing therefore the buffer capability of this natural

clayey barrier. After this transformation, the

system became acid by the oxidation of sulphides

and minor smectite was occasionally transformed

into kaolinite, a clay mineral without any adequate

buffer and physicomechanical properties to work

as barrier against migration of U and other trace

analogue elements. Consequently, smectite–kao-

linite transformation must be avoided regardless

of the geological site selected for radwaste

disposal.

(c) In the shallow hydrothermally altered granite and

fracture fillings (far field), weathering also trans-

formed illite and residual albite into smectite,

generally increasing their retention and buffer

capabilities against the mobility of natural radio-

elements. Uranium is usually trapped by the

clayey minerals in the rock, Fe oxyhydroxides

and sometimes precipitated as autunite, as a result

of the presence of significant amounts of phos-

phorous in the system. This is a relevant anion-

forming element for the retention, by precipita-

tion, of the complex uranyl cation and other

analogous elements.

Acknowledgements

ENRESA, CIEMAT and EU provided the financial

supports for this work. Dr. N. Clauer and another

anonymous reviewer are also thanked for their useful

criticisms, comments and suggestions that improved

the manuscript. [PD]

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