Evolution of fluids associated with metasedimentary sequences
from Chaves (North Portugal)
Alexandra Guedes a,*, Fernando Noronha a, Marie-Christine Boiron b, David A. Banks c
aGIMEF-Departamento de Geologia e Centro de Geologia, Faculdade de Ciencias, Prac�a Gomes Teixeira, 4099-002 Porto, PortugalbUMR 7566 G2R and CREGU, BP 23, 54501 Vandoeuvre les Nancy cedex, France
cSchool of Earth Sciences, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
Abstract
In order to identify and characterise fluids associated with metamorphic rocks from the Chaves region (North Portugal), fluid
inclusions were studied in quartz veinlets, concordant with the main foliation, in graphitic-rich and nongraphitic-rich lithologies
from areas with distinct metamorphic grade. The study indicates multiple fluid circulation events with a variety of compositions,
broadly within the C–H–O–N–salt system. Primary fluid inclusions in quartz contain low salinity aqueous–carbonic, H2O–
CH4–N2–NaCl fluids that were trapped near the peak of regional metamorphism, which occurred during or immediately after
D2. The calculated P–T conditions for the western area of Chaves (CW) is P= 300–350 MPa and Tf 500 jC, and for the
eastern area (CE), P= 200–250 MPa and T= 400–450 jC. A first generation of secondary fluid inclusions is restricted to
discrete cracks at the grain boundaries of quartz and consists of low salinity aqueous–carbonic, H2O–CO2–CH4–N2–NaCl
fluids. P–T conditions from the fluid inclusions indicate that they were trapped during a thermal event, probably related with
the emplacement of the two-mica granites.
A second generation of secondary inclusions occurs in intergranular fractures and is characterised by two types of aqueous
inclusions. One type is a low salinity, H2O–NaCl fluid and the second consists of a high salinity, H2O–NaCl–CaCl2 fluid.
These fluid inclusions are not related to the metamorphic process and have been trapped after D3 at relatively low P
(hydrostatic)–T conditions (P< 100 MPa and T < 300 jC).Both the early H2O–CH4–N2–NaCl fluids in quartz from the graphitic-rich lithologies and the later H2O–CO2–CH4–N2–
NaCl carbonic fluid in quartz from graphitic-rich and nongraphitic-rich lithologies seem to have a common origin and
evolution. They have low salinity, probably resulting from connate waters that were diluted by the water released from mineral
dehydration during metamorphism. Their main component is water, but the early H2O–CH4–N2–NaCl fluids are enriched in
CH4 due to interaction with the C-rich host rocks.
From the early H2O–CH4–N2–NaCl to the later aqueous–carbonic H2O–CO2–CH4–N2–NaCl fluids, there is an
enrichment in CO2 that is more significant for the fluids associated with nongraphitic-rich lithologies.
The aqueous–carbonic fluids, enriched in H2O and CH4, are primarily associated with graphitic-rich lithologies. However,
the aqueous–carbonic CO2-rich fluids were found in both graphitic and nongraphitic-rich units from both the CW and CE
studied areas, which are of medium and low metamorphic grade, respectively.
D 2002 Elsevier Science B.V. All rights reserved.
Keywords: Fluid inclusions; Graphitic lithologies; Metamorphism; Portugal
0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
PII: S0009 -2541 (02 )00120 -1
* Corresponding author. Tel.: +351-223-401477.
E-mail address: [email protected] (A. Guedes).
www.elsevier.com/locate/chemgeo
Chemical Geology 190 (2002) 273–289
1. Introduction
One of the major aims of the study of fluid
inclusions in metamorphic rocks is to provide
information about the composition of the fluid
phase during metamorphism and to estimate the
pressure and temperature conditions. However,
deformation following metamorphism frequently
causes decrepitation and necking down of the
primary inclusions, and most of the fluid inclusions
present are usually secondary. A compilation of the
work on fluid inclusion in metamorphic rocks is
summarised in Crawford (1981), Roedder (1984)
and Crawford and Hollister (1986). Most of the
objectives in these studies are, not only the char-
acterisation of the composition of the fluid phase
present, but also the estimation of the pressure and
temperature conditions of metamorphism. However,
a metamorphic rock may contain a wide range of
fluids of different composition and origin at differ-
ent times, and these may all become trapped in
different inclusions over the full P–T path taken by
the rock.
Nowadays, it is generally accepted that regional
metamorphism in conjunction with a high fluid flow
could lead to the formation of ore deposits.
Fluid systems associated with different Variscan
ore deposits in North Portugal, mainly Sn–Li ore
deposits associated with two-mica granites, W, Mo
and Bi deposits associated with biotite granites and
gold mineralizations, have shown the presence of
similar aqueous–carbonic fluids (Noronha, 1983;
Noronha et al., 1993, 1995, 2000; Doria, 1999; Lima,
2000) whose origin is still unclear. Previous studies of
fluid inclusions from metamorphic veins in the eastern
part of the Verin–Chaves–Vila Pouca de Aguiar–
Regua regional fault system, a region of low grade
regional metamorphism, indicate that again the chem-
istry of fluids is primarily controlled by the presence
of C-rich lithologies (Doria et al., 1993; Doria, 1999;
Guedes, 2001; Guedes and Noronha, 2000; Guedes et
al., 2001).
The purpose of this study is to describe the fluids
associated with the metasediments of the Chaves
region, to obtain geothermobarometric data from fluid
inclusions and also to determine the possible controls
and origin of the composition of the fluids related with
metamorphism. Therefore, fluid inclusions in quartz
segregation veinlets, concordant with the main folia-
tion in graphitic-rich and nongraphitic-rich lithologies
from areas with distinct metamorphic grade were
studied.
Investigations of fluid inclusions in metamorphic
rocks are commonly carried out on quartz segrega-
tions in which inclusions are often larger and more
abundant than in the enclosing rock matrix. In this
quartz, the inclusions could have the same composi-
tion and density as the inclusions trapped during
growth of the minerals in the host rock (Crawford,
1981).
The study of fluid inclusions in metamorphic
quartz from the western (medium grade of metamor-
phism) and eastern areas (low grade of metamor-
phism) of the region will allow a comparison
between the PTX characteristics of the two areas.
Fluid inclusions in quartz segregation veinlets hosted
by graphitic-rich and nongraphitic-rich lithologies in
both areas will determine the influence of the lithol-
ogy on the fluid composition.
The data obtained can be extended to provide a
clear understanding of similar fluids associated with
ore deposits.
2. Geological setting
The Chaves region is located close to the boundary
between two large tectonic units of the Iberian Her-
cynian belt, the Middle Galicia Tras-os-Montes Zone
to the north and the Centro Iberian Zone to the south,
and is traversed by an important NNE–SSW fault, the
Verin–Chaves–Vila Pouca de Aguiar–Regua fault
(VCVR) (Fig. 1).
The geology of the region is well known, from
several studies carried out in the region, namely the
study of metamorphism, lithogeochemistry and tec-
tonics (Noronha, 1983, 1992; Ribeiro, 1998; Ribeiro
et al., 1999; Noronha et al., 2000). This previous work
provides key data, which allows this study to be
placed in context.
The region comprises metasedimentary rocks,
which are of upper Ordovician to lower Devonian in
age. They are mostly composed of phyllites, quartz-
iferous or micaceous schists with interbedded black
schists and lydites (Noronha, 1992; Noronha and
Ribeiro, 1983).
A. Guedes et al. / Chemical Geology 190 (2002) 273–289274
At least three episodes of ductile deformation (D1,
D2 and D3) and a later, essentially, brittle deformation
phase (D4) affected the region (Noronha, 1992; Noro-
nha and Ribeiro, 1983; Ribeiro et al., 1999). D1
developed a well-marked schistosity (S1) striking
NW–SE. D2 implied an S2 sub-horizontal schistosity
and is related with the thrusts responsible for the
parautochthonous character of the metasedimentary
sequences. D3 deformation produced a subvertical
crenulation cleavage, striking N120jE. All the
sequences show a late- to post-D3 ductile–brittle
shear deformation. D4 is a brittle phase responsible
for the main fracture systems striking N10jW to
N20jE.In this region, metamorphism occurred prior to D3
(340 to 320 Ma), a key aspect of which is the different
metamorphic conditions, which vary across the
VCVR fault, with medium P and high T to the west
(Noronha, 1983), and lower P and T to the east
(Ribeiro, 1998; Ribeiro et al., 1999).
In the eastern part (CE), there are two structural
domains (structural domain of Tres Minas and struc-
tural domain of Carrazedo) which have different
regional isograds: a chlorite isograd (structural domain
of Tres Minas) with an association of quartz +white
mica + chloriteF biotiteF garnetF opaque miner-
alsF tourmalineF zirconF sphene and leucoxene to
the south and a biotite isograde (structural domain
of Carrazedo) with quartz + white mica + biotiteFplagioclaseFK-feldsparF chloriteF opaque miner-
alsF tourmalineF spheneF zirconF apatite to the
north. Maximum conditions of T= 350 to 450 jC and
P= 350 to 400 MPa were attained during metamor-
phism (Ribeiro, 1998; Ribeiro et al., 1999).
To the west of the VCVR fault (CW), there are
two regional metamorphic isograds, a biotite isograd
with an association of quartz +muscovite + biotiteFchloriteF garnetF plagioclase to the north and an
andalusite isograd with quartz +muscovite + biotiteFandalusiteF sillimaniteF stauroliteF plagioclaseFgarnet to the south (Noronha and Ribeiro, 1983). The
existence of relic staurolite in the andalusite crystals
indicates a thermal gradient, probably as a result of the
uplift during the orogen. The metamorphism in CW
Fig. 1. Location of the Chaves region in North Portugal. 1: Middle Galicia Tras-os-Montes Zone; 2: Centro Iberian Zone; CW: Chaves western
area; CE: Chaves eastern area; A: Quartzifereous schists and micaceous schists with interbedded black schists and lidites (*), the light gray color
correspond to the chlorite isograd and the dark gray to the andalusite isograd; B: Hercynian syntectonic two-mica granites; C: Hercynian post-
tectonic biotite granites; D: Fault; E: Thrust. Gray circles indicate the sample location.
A. Guedes et al. / Chemical Geology 190 (2002) 273–289 275
occurred during D2 at f 340 Ma and the maximum
P–T conditions correspond to the andalusite zone. A
temperature of 500 jC was reached and the estimated
pressure is 300 to 350 MPa.
The thermal peak (T= 500 to 550 jC and P= 250
to 350 MPa) is related to the emplacement of two-
mica granites during D3 (315 to 310 Ma), after which
there is evidence of retrograde metamorphism in the
two areas CE and CW (Ribeiro et al., 1999).
The present study was carried out on quartz vein-
lets interbedded with quartziferous schists, micaceous
schists, lydites and black shales generally within the
chlorite isograd at CE and within the andalusite iso-
grad at CW.
Different types of granites, all of Hercynian age,
occur in the region and are essentially divided into
peraluminous two-mica granites and biotite granites.
The first group of granites (observed close to the
sampling location) is dominantly syn D3 and the
second group can be syn D3, syn to late D3, or post
D3 (Noronha and Ribeiro, 1983; Ferreira et al., 1988;
Almeida, 1994; Martins, 1998).
3. Analytical methods
Prior to microthermometry, all inclusions were
optically studied in order to outline the general
characteristics of the fluid inclusion populations (pri-
mary, pseudosecondary or secondary) based on cri-
teria proposed by Roedder (1984).
Microthermometric characterisation of the fluid
inclusions was performed on doubly polished thick
sections ( < 200 Am) using a Chaixmeca (Poty et
al., 1976) and a Linkam THMSG 600 heating–
freezing stage (Shepherd, 1981). The stages were
calibrated with melting-point standards at T>25 jCand with natural and synthetic fluid inclusions at
T < 0 jC. The rate of heating was monitored in
order to obtain an accuracy of F 0.2 jC during
freezing, F 1 jC when heating over the 25 to 400
jC range and F 4 jC over the 400 to 600 jCrange. Salinity, expressed as wt.% eq. NaCl, was
calculated from microthermometric data using equa-
tions from Bodnar and Vityk (1994). In volatile-
bearing fluid inclusions, CO2 was identified by
melting of a solid phase below � 56.6 jC. The
volumetric fraction of the aqueous liquid (flw) have
been estimated at room temperature by reference to
the volumetric chart of Roedder (1984).
Molar fractions of CO2, CH4 and N2 were
determined in individual fluid inclusions by
micro-Raman analysis performed with a DILOR
Raman spectrometer at CREGU, Nancy. The pre-
cision of the Raman analyses of fluid inclusions is
better than 5% (RSD).
Fig. 2. (A) Quartz veinlets interbedded with black shales. (B) Quartz
veinlets interbedded with phyllites.
A. Guedes et al. / Chemical Geology 190 (2002) 273–289276
Bulk compositions were determined by combin-
ing the results of microthermometry, phase volume
ratios and Raman analyses using the computer
programs of Dubessy (1984), Thiery et al.
(1994a,b), Bakker (1997) for the C–O–H system,
the tables from Bodnar and Vityk (1994) for the
H2O–NaCl system and the tables of Oakes et al.
(1990) for the H2O–NaCl–CaCl2 system.
The ionic composition of the fluid inclusions
was determined by the crush–leach technique as
detailed in Banks and Yardley (1992). The anions
F, Cl, Br and SO4 were analysed by ion chroma-
tography on double-distilled water leaches using a
Dionex 45001 HPLC. The cations were not ana-
lysed due to the impurities in the quartz grains,
which would result in anomalous concentrations.
4. Results
4.1. Quartz types
Quartz segregation veinlets in different litholo-
gies (black shales, lydites, micaschists and phyllites)
with thicknesses varying from cm to mm were
studied (Fig. 2). Their concordance with S2 indi-
cates they were syn to late regional metamorphism,
which occurred during D2. Petrographic studies
were carried out to characterise the different quartz
generations present. Several types of metamorphic
quartz were recognised:
� a major filling of anhedral milky quartz (Q1A),
with a strained extinction and sub-granulation. This
Fig. 3. Quartz types. Q1A: grains of deformed milky quartz; Q1B: grains of clear quartz with a slight undulose extinction; Q2: grains of
recrystallized quartz resulting from quartz Q1A.
A. Guedes et al. / Chemical Geology 190 (2002) 273–289 277
type of quartz contains a great number of small
fluid inclusions (Fig. 3),� a clear sub-euhedral quartz grains with a slight
strained extinction (Q1B) (Fig. 3),� a recrystallised quartz (Q2) with a mosaic texture
resulting from the former milky quartz (Fig. 3).
This type of quartz occurs at the junctions of the
quartz grains of types Q1A and Q1B, showing
black dots, which correspond to decrepitated fluid
inclusions; in microcrystalline agglomerates and at
the border of the polygonal quartz Q1B.
The quartz Q1A and Q1B are the most common
in the studied segregation veinlets. These quartz are
frequently fractured, and following the classification
of Simmons and Richter (1976), the fractures are
essentially of two types: grain boundary cracks
(Fig. 4A) and intergranular cracks (Fig. 4B). These
microfractures are frequently healed by fluid inclu-
sions. We considered that the studied quartz are syn
to late relatively to the regional metamorphism. The
grain boundary cracks probably formed during syn-
tectonic recrystallization, due to the reduction in
permeability and consequently to the rise of fluid
pressure.
4.2. Fluid inclusion types
Petrography, microthermometry and Raman spec-
troscopy carried out on the fluid inclusions in quartz
reveal the presence of multiple fluid types with a
variety of compositions, broadly within the C–H–O–
N–salt system (Fig. 5). From the oldest to the young-
est, the following fluid inclusion (FI) generations have
been distinguished:
� primary fluid inclusions in Q1B containing low
salinity aqueous–carbonic, H2O–CH4–N2–NaCl
fluids,� secondary inclusions in discrete cracks, at the grain
boundaries of the quartz Q1A and Q1B, and
primary inclusions in Q2, containing low salinity
aqueous–carbonic, H2O–CO2–CH4–N2–NaCl
fluids,� secondary inclusions containing aqueous, low
salinity, H2O–NaCl and high salinity, H2O–
NaCl–CaCl2 fluids in intergranular fractures of
quartz Q1A, Q1B and Q2.
The results from microthermometric and micro-
Raman analysis are summarised in Tables 1 and 2,
respectively, and the fluid inclusions are described
using the classification scheme of Boiron et al.
(1992): L, for inclusions with total homogenisation
to liquid; V, for inclusions with total homogenisa-
tion to vapour; c indicates the presence of volatile
phase dominated by CO2; m indicates the presence
of a CH4 volatile phase; w indicates the presence of
an aqueous phase (water). The combinations result-
ing from the relative abundance of one phase to
another, i.e. for a fluid inclusion with total homog-
enisation to liquid and a volatile phase dominated
by CO2 and no visible or detected water are Lc, a
dominant CO2 volatile phase with some water are
Lc-w; a dominant aqueous phase with a CO2
volatile phase are Lw-c; Lw are inclusions with
only phase present is aqueous.Fig. 4. (A) Grain boundary cracks in a quartz grain. (B)
Intergranular cracks in quartz grains.
A. Guedes et al. / Chemical Geology 190 (2002) 273–289278
4.2.1. Microthermometric and Raman spectroscopy
data
Primary fluid inclusions in Q1B contain low sal-
inity aqueous–carbonic, H2O–CH4–N2–NaCl fluids.
These fluids are represented by isolated Lw-m two-
phase fluid inclusions in quartz Q1B from the graph-
ite-rich lithologies. Relevant phase transitions include
the melting of ice (Tm ice), the melting of clathrates
(TmCl) and the total homogenisation (Th). Measure-
ments of total homogenisation have been obtained
between 228 and 330 jC to the liquid phase (Fig. 6a).
The fluids trapped in these fluid inclusions have a
high water content (between 89.7 and 97.6 mol%),
low salinity (between 0.4 and 0.6 mol% NaCl) and a
volatile phase dominated by CH4 (75.9 to 100 mol%)
with variable amounts of N2. Their density is high
(0.6–0.9 g/cm3) (Tables 1 and 2; Fig. 7A and B).
Secondary inclusions in discrete cracks, at the grain
boundaries of the quartz Q1A and Q1B, and primary
inclusions in Q2, contain low salinity aqueous–
carbonic, H2O–CO2–CH4–N2–NaCl fluids. These
fluids are represented by groups or planes of se-
veral types of fluid inclusions: Lc, Vc, Lm, Vm,
Lc-w, Vc-w, Lm-w, Vm-w, Lw-c, Vw-m, Vw-c and
Lw-m in quartz Q1A, Q1B and Q2 from both
graphitic-rich and nongraphitic-rich lithologies
(Table 1). These fluid inclusions show many rele-
vant phase transitions including the homogenisation
of CH4 (ThCH4), the melting of CO2 (TmCO2), the
melting of ice (Tm ice), the melting of clathrates
(TmCl), the homogenisation of CO2 (ThCO2) and
the total homogenisation (Th). The total homoge-
nisation temperatures are scattered between 265 and
405 jC (Fig. 6a,b).
Fig. 5. Fluid types. A: Lw-m fluid inclusions containing low salinity aqueous–carbonic, H2O–CH4–N2–NaCl fluids; B: Lm inclusions in
discrete cracks, at the grain boundaries of the quartz containing low salinity aqueous–carbonic, H2O–CO2–CH4–N2–NaCl, fluids; C: Lw1
inclusions containing aqueous, low salinity, H2O–NaCl, fluids in intergranular fractures of quartz.
A. Guedes et al. / Chemical Geology 190 (2002) 273–289 279
Table 1
Microthermometric data
Location Petrography Microthermometry (jC)
Lithology Quartz
type
Fluid type Fluid
inclusion
type
Occurrence ThCH4 Mode TmCO2 Tm ice TmCl ThCO2 Mode Th
CW GL Q1B H2O–CH4–N2–NaCl Lw-m Isolated/group � 4.8/� 1.7 12/15.5 260/280
GL/NGL Q1A H2O–CO2–CH4–N2–NaCl Lc Group/FIP � 65/� 58.8 � 14.6/18 L
NGL Q1B Vc Group/FIP � 64.9/� 62.2 � 14.5/� 4 V
GL Q1B Lm Group/FIP � 97.7/� 92.7 L
GL/NGL Q1B Vm Group/FIP � 109/� 91.4 V
GL/NGL Q1A Lc-w Isolated/group � 65.1/� 58.2 � 4.5 10/11 � 26.4/16.6 to V
and 17.5/25.1 to L
V and L 280/342
NGL Q1A Vc-w FIP � 63.7/� 59.6 � 6 11.5 � 16.5/5.2 V 267/270
Q1B Isolated
GL Q1B Lm-w FIP � 93.4/� 73.2 V � 1 14.5/16.5 320/353
Q2 Isolated
GL/NGL Q1B Vm-w FIP � 99/� 88.4 V � 3.5 17 328
Q2 Isolated
GL/NGL Q1A Lw-c Group/FIP � 65/� 58.7 � 6/� 4 9.5/12.5 270/370
Q1B Isolated
Q2 Isolated/group
NGL Q1A Vw-c FIP � 64.8/� 59.6 � 5/� 4 8/12 324/326
Q1B Isolated/FIP
Q2 FIP
GL Q1B Lw-m Group/FIP � 3.5/� 0.9 9/16 270/390
Q2 Group
GL/NGL Q1A/
Q1B/Q2
H2O–NaCl Lw1 FIP � 1.9/� 0.2 152/246
GL/NGL Q1B H2O–NaCl –CaCl2 Lw2 FIP � 47.1/� 32.2 148/180
CE GL Q1A/Q1B H2O–CH4–N2–NaCl Lw-m Isolated/group � 3.1/� 1.2 10/15 228/330
NGL Q1A H2O–CO2–CH4–N2–NaCl Lc FIP � 59.9/� 59.1 6/11.6 L
NGL Q1A Lc-w FIP � 61.2/� 58.5 5.9/11.9 to L
and 9.5/22.3 to V
L and V 370/390
Q2 Group
NGL Q1B Lw-c Isolated/group/FIP � 6.8/� 2.3 7.5/11.9 265/405
Q1A Group/FIP
Q2 Group
NGL Q1B Vw-c Isolated/FIP � 60.5 � 6/� 3 9.8/11 375/390
Q1A Isolated/FIP
GL/NGL Q1A/Q1B H2O–NaCl Lw1 FIP � 2.8/� 0.4 138/275
GL: graphitic-rich lithologies; NGL nongraphitic-rich lithologies; ThCH4: homogenisation temperature of CH4; TmCO2: melting temperature of CO2; Tm ice: melting temperature of ice; TmCl: melting
temperature of clathrate; ThCO2: homogenisation temperature of CO2; Th: total homogenisation. L: liquid; V: vapour.
A.Guedes
etal./Chem
icalGeology190(2002)273–289
280
Table 2
Summary of Raman spectroscopy data with calculated bulk composition (in mol%) of representative fluid inclusions from each fluid type
Location Fluid type Fluid Volatile phase composition Bulk composition
inclusion typeCO2 CH4 N2 H2O CO2 CH4 N2 NaCl d
CW H2O–CH4–N2–NaCl Lw-m 0 75.9/95.2 4.8/24.1 93.7/97.6 0 1.6/4.3 0.1/1.4 0.6/0.7 0.74/0.90
H2O–CO2–CH4–N2–NaCl Lc
Vc
Lm
Vm
Lc-w 91.6 3 5.4 31.7 61.5 2 3.6 1.2 0.54
Vc-w 59.7 30.2 10.1 78.8 11.6 5.2 1.7 2.7 0.27
Lm-w 0/14.2 73.2/78.3 7.5/23.8 87.2/91.7 0/2.3 6/9.6 0.9/2.1 0/0.2 0.58/0.66
Vm-w 0/1.2 66/93.3 6.7/34 36.8/63.8 0/0.7 33.7/49.3 2.5/21.5 0 0.11/0.15
Lw-c 55.3/77.1 3.3/34.9 9.8/19.6 86.5/90.8 5.3/10.8 0.4/2.2 0.6/2.2 0.1/1.1 0.46/0.79
Vw-c 79.7 18.8 1.5 86.9 10.8 2 0.2 0.1 0.64
Lw-m 0 75.6 24.4 95 0 2.8 0.9 1.3 0.71
H2O–NaCl Lw1 99.0/99.9 0.1/1.0 0.80/0.90
H2O–NaCl–CaCl2 Lw2 93.8/94.5 5.5/6.2 1.1
CE H2O–CH4–N2–NaCl Lw-m 0 85.9/100.0 0/14.1 89.7/93.1 0 5.5/9.4 0/0.9 0.4/0.6 0.6
H2O–CO2–CH4–N2–NaCl Lc
Lc-w 83.8 6.4 9.8 13.7 72.4 5.5 8.4 0 0.75
Lw-c 68.4/77.5 14.9/23.4 6.3/15.9 89.9/95.1 3.5/7.4 0.5/1.1 0.2/1.2 0.4/0.7 0.60/0.74
Vw-c 68.2/75.3 18.1/30.4 1.4/6.6 90.3 7.2 1.1 0.4 1.0 0.7
H2O–NaCl Lw1 98.6/100.0 0/1.4 0.80/1.00
A.Guedes
etal./Chem
icalGeology190(2002)273–289
281
Fig. 6. (a) Th versus n diagram for the different fluids. (b) Th versus Tm ice diagram for aqueous–carbonic fluids. (c) Th versus Tm ice diagram
for aqueous fluids.
A. Guedes et al. / Chemical Geology 190 (2002) 273–289282
These fluids have a variable water content (13.7–
95.1 mol%) and a low salinity (0.4–2.7 mol% NaCl).
Some volatile phase analyses show variable CO2, CH4
and N2 contents (Fig. 7A,B). Their density is also
quite variable (between 0.1 and 0.8 g/cm3) (Tables 1
and 2; Fig. 7A and B).
A great number of these fluid inclusions are
healing grain boundary microcracks in quartz Q1A
and Q1B and show a wide range in the degree of
filling (flw between 0 and 0.8). The Lw-m (H2O-
rich) and Vm-w (volatile-rich) inclusions are exam-
ples of coeval fluid inclusions (Table 1). These may
have resulted from post trapping disturbances, such
as water leakage or partial contamination by late
aqueous liquids, from a simultaneous trapping of
aqueous liquids and carbonic vapours, or to suc-
cessive trapping of fluids that were more and more
enriched in water at different pressures (Cathelineau
et al., 1993).
The petrography, microthermometry and Raman
spectroscopy of FI in some samples reveal two types
of inclusions, which may represent two parts of an
immiscible H2O–CH4–N2–NaCl fluid. The two
inclusion types (Lw-m and Vm-w) are found healing
the same microfracture of the quartz Q1B (only with a
slight deformation). They contain CH4–N2 with very
little H2O and H2O–CH4–NaCl with very little N2.
The volatile components CH4 and N2 are partitioned
into the vapour-rich inclusions, whilst salt is parti-
tioned into the aqueous inclusions. This evidence and
the similarity to other studies (Cathelineau et al.,
1993; Hall et al., 1991) point to this fluid type having
resulted from an immiscibility process. However,
successive trapping of fluids at a varying pressure
that are more and more enriched in water cannot be
excluded (Pichavant et al., 1982).
Secondary inclusions containing aqueous, low sal-
inity, H2O–NaCl (Lw1) and high salinity, H2O–
NaCl–CaCl2 fluids (Lw2) are represented by fluid
inclusion planes (FIP) of Lw1 and Lw2 inclusions in
quartz Q1A, Q1B and Q2 from both graphitic and
nongraphitic lithologies. Lw1 inclusions with Tm ice
ranging from � 2.8 to � 0.2 jC homogenise to the
liquid phase between 138 and 275 jC. Lw2 inclusionswith eutectic temperatures between � 88 and � 75.8
jC and Tm ice from � 47.1 to � 32.2 jC homoge-
nise to the liquid phase between 148 and 180 jC (Fig.
6a and c).
The water content of H2O–NaCl fluids varies
between 98.6 and 100 mol%, with 0 to 1.4 mol%
NaCl. The density is between 0.8 and 1 g/cm3. The
H2O–NaCl–CaCl2 fluids have water contents of 93.8
to 94.5 mol% and 5.5 to 6.2 mol% CaCl2, the density
is f 1.1 g/cm3 (Table 2).
The histogram of Th (Fig. 6a) for the different
fluids shows that the H2O–CO2–CH4–N2–NaCl
fluids were trapped at higher minimum trapping
temperatures (265–405 jC; mean values around
350 jC) than the H2O–CH4–N2–NaCl fluids
(228–330 jC; mean values around 270 jC). The
aqueous fluids were trapped at lower temperatures
(138–275 jC) and from their ice melting temper-
Fig. 7. Ternary plot in the CO2–CH4–N2 diagram of the volatile phase and in the H2O–CO2–CH4 +N2 diagram of bulk composition for the
aqueous–carbonic fluids. GL: graphitic-rich lithologies; NGL: nongraphitic-rich lithologies.
A. Guedes et al. / Chemical Geology 190 (2002) 273–289 283
atures two groups: H2O–NaCl and H2O–NaCl–
CaCl2 can be distinguished (Fig. 6c).
From Table 2 and Fig. 7A and B, it can be seen
that the main component of both H2O–CH4–N2–
NaCl and H2O–CO2–CH4–N2–NaCl fluids is
water. However, other components such as CO2,
CH4, and N2 and NaCl are present. From Fig. 7A,
enrichment in CO2 from the early H2O–CH4–N2–
NaCl to the later aqueous–carbonic H2O–CO2–
CH4–N2–NaCl fluids is clear. This enrichment is
more prominent in the fluids associated with non-
graphitic lithologies.
The aqueous–carbonic fluids enriched in H2O and
CH4 are dominantly associated with graphitic-rich
lithologies (Fig. 7B). It is worth nothing that the
arrow in Fig. 7B corresponds to composition of fluid
inclusion from a sample collected near graphitic-rich
lithologies.
The aqueous–carbonic CO2-rich fluids were found
in both graphitic and nongraphitic lithologies from the
CE and CW areas, which are of low and medium
metamorphic grade, respectively.
4.2.2. Ionic data
A partial fluid composition was determined on
bulk samples from quartz grains with a single domi-
nant fluid inclusion population (samples containing
only one of the aqueous–carbonic fluids described in
the microthermometric analysis), and the recon-
structed fluid chemistry (Table 3) calculated using
the concentration of the major anions in the different
samples and the average salinity deduced from micro-
thermometry (Table 2).
The dominant anion in the fluids is Cl � with
significant amounts of SO42� . The Br content ranges
from 5 to 105 ppm with Br/Cl molar ratios between
0.0002 and 0.0053 for nongraphitic lithologies and
0.0004 to 0.0017 for graphitic ones.
For the graphitic lithologies, the Br/Cl ratios are
similar to the value of seawater (0.0015). The
average value (Br/Cl = 0.0010F 0.0005r) and asso-
ciated variability of the ratios is probably represen-
tative of the natural variability due to measurement
of a single fluid. It is unclear what this fluid is at
present but may represent seawater, which has
gained Cl � and H2O from the breakdown of
hydrous metamorphic minerals during metamor-
phism. Br/Cl ratios from the nongraphitic litholo-
gies are not tightly constrained and cover a large
range and can be significantly more Br-rich than
seawater. The range in values may indicate mixing
of two distinct fluids, one of which may be similar
to the fluid in graphitic lithologies, the other Br-
rich fluid from an unknown source. However, the
range of values could also have resulted from fluid
immiscibility, as this will fractionate Br� and Cl �
(Banks, 2001), Br� will tend to go to the vapour
Fig. 8. Br/Cl molar projection for aqueous –carbonic fluids
associated with graphitic-rich lithologies (GL) and nongraphitic-
rich lithologies (NGL). The dashed line corresponds to seawater Br/
Cl molar value.
Table 3
Fluid inclusion compositions reconstructed from crush– leach
analysis
Sample Lithology F � Cl � Br� SO42� Br/Cl
X3 GL 2534 9990 13 627 0.0006
X7 GL 2989 8764 8 3404 0.0004
X17 GL 40 6781 19 349 0.0012
X18 GL 283 21490 82 3624 0.0017
X19C NGL 0 28183 12 3805 0.0002
X22 GL 44 1591 5 422 0.0015
MA196AK NGL 21 1602 6 191 0.0017
X1 GL 0 8879 14 2269 0.0007
X2 GL 2226 6279 11 584 0.0008
MA122K NGL 0 9668 16 628 0.0008
MA125K NGL 0 7315 87 3396 0.0053
MA195K NGL 976 12978 105 2927 0.0036
Data in ppm. GL: graphitic-rich lithologies; NGL: nongraphitic-rich
lithologies.
A. Guedes et al. / Chemical Geology 190 (2002) 273–289284
phase and Cl � tends to remain in the liquid. So
the possibility that originally the same fluid existed
in both lithologies and that fluid immiscibility has
taken place to account for the variable Br/Cl ratios
cannot be excluded (Fig 8).
5. Fluid evolution
5.1. Chronology of entrapment
The trapping conditions of the inclusions were
determined using isochores constructed from the
molar volumes of the fluid inclusions, their composi-
tion and equations of state for the systems concerned.
The isochores for P–T interpretation were selected
from representative fluid inclusions for each type of
fluid.
The present study involves a variety of fluids.
Not all fluid inclusion types were found at all
sample localities, suggesting that the composition
was locally controlled. The textural aspects of
primary fluid inclusions within the quartz Q1A
and Q1B and their isochores indicate that these
were trapped during the maximum conditions of
regional metamorphism. It is also inferred by tex-
tural evidence that secondary fluid inclusions in
discrete healed fractures at quartz grain boundaries
of quartz Q1A and Q1B, and the primary fluid
inclusions in Q2 were the second oldest fluid
trapped. These inclusions have higher minimum
formation temperatures; however, the isochores
show a high variation in density and they could
have been trapped over a range of pressures.
The secondary fluid inclusions, located in healed
fractures which cross-cut the previous fluid inclusion
types, represent the last fluid to be trapped. The
mutual chronology of the two types of fluid inclusions
(H2O–NaCl, H2O–NaCl–CaCl2) is not clear. These
fluids are not related with the metamorphic process as
they were trapped in fractures that cross-cut the
metamorphic quartz.
5.2. P–T estimation
The metamorphic quartz Q1A and Q1B from
Chaves region seems to have precipitated in H2O–
CH4–N2–NaCl fluids (Lw-m fluid inclusions).
At the CW area, these fluid inclusions are charac-
terised by Th values around 270 jC. They have an
isochore which intercepts the metamorphic domain
assumed from mineral paragenesis (Tf 500 jC,P= 350–400 Mpa). This means that these fluids
may have been formed at maximum P–T conditions
of 350 MPa and 500 jC, respectively (Fig. 9A). The
minimum trapping conditions of H2O–CO2–CH4–
N2–NaCl fluids were 350 jC and 150 to 200 MPa.
These fluids were responsible for the recrystallization
of quartz and record maximum temperatures of 550
jC. This fluid type seems to have resulted from an
immiscibility process, which is also suggested by the
isochores a and aV, from coeval Lw-m and Vm-w
fluid inclusions which may represent the two parts of
an immiscible H2O–CH4–(N2–NaCl) fluid. The two
inclusion types contain CH4–N2 with very little H2O
and H2O–CH4–NaCl with very little N2. The more
volatile components CH4 and N2 are partitioned into
the vapour-rich inclusions, whilst salt is partitioned
into the aqueous inclusions. Density variations in
these inclusions suggest that immiscible fluids were
trapped over a range of pressures (50–300 MPa)
probably during the uplift associated with the Hercy-
nian orogen and the two-mica granites emplacement
(Fig. 9A).
In the CE area, the isochores and the Th recorded
for Lw-m inclusions (H2O–CH4–N2–NaCl fluids)
indicates minimum formation temperatures of around
280 jC, and indicate they may have been formed at
maximum P–T conditions of 250 MPa and 450 jC,respectively, once the isochores intercept the P–T
conditions assumed for low grade metamorphism
(Fig. 9B).
During the recrystallisation of quartz Q1, H2O–
CO2–CH4–N2–NaCl fluids were prevalent in the
system and minimum temperature (Th) and pressure
conditions of 375 jC and 200 MPa are indicated.
The mineral assemblage quartz +white mica + bioti-
te + andalusiteF chloriteF opaque minerals + tour-
malineF zirconF sphene and leucoxeneF apatite,
together with the fluid inclusion isochores, indicate
the maximum P–T conditions were similar to those
at the thermal peak of the metamorphism (150–200
MPa and 500–550 jC) and related to the intrusion
of two-mica syntectonic granites in the area.
Finally, aqueous H2O–NaCl and H2O–NaCl–
CaCl2 fluids flowed through the metamorphic se-
A. Guedes et al. / Chemical Geology 190 (2002) 273–289 285
quences of the region and were trapped in fluid
inclusion planes at minimum temperatures of 138
and 148 jC, respectively (Fig. 9A and B). P–T
conditions of trapping are difficult to constrain due
to the lack of mineralogical indicators at that stage.
However, they were probably trapped under hydro-
Fig. 9. (A) Isochores for fluid types observed at CW, and a proposed P–T path. a and aV are two end-members of immiscible fluids; bi: lower
limit of biotite stability. The stability fields of kyanite (ky), andalusite (and) and sillimanite (sil), and the low-pressure, high-temperature reaction
curves are in the KFASH-system, cld: chloritoid; st: staurolite; alm: almandine; kfs: K-feldspar (after Bucher and Frey, 1994); Th1: minimum
trapping temperature of H2O–CH4–N2–NaCl fluids; Th2: minimum trapping temperature of H2O–CO2–CH4–N2–NaCl fluids. (B) Isochores
for fluid types observed at CE, and a proposed P–T path; bi: lower limit of biotite stability. The stability fields of kyanite (ky), andalusite (and)
and sillimanite (sil) (after Bucher and Frey, 1994); Th1: minimum trapping temperature of H2O–CH4–N2–NaCl fluids; Th2: minimum trapping
temperature of H2O–CO2–CH4–N2–NaCl fluids.
A. Guedes et al. / Chemical Geology 190 (2002) 273–289286
static pressure conditions at shallow structural levels,
as they are similar to the aqueous fluids circulated
at shallow levels on a regional scale as described by
Doria (1999) and Noronha et al. (2000).
5.3. fO2 evolution
The oxygen fugacity of the fluids in the C–O–H
system was calculated using the molar fraction of the
volatile components obtained by Raman analysis of
fluid inclusions and the estimated temperature of their
trapping assuming they were in equilibrium. The fO2
of both aqueous–carbonic fluids is lower than the
Ni–NiO buffer, indicating a reducing metamorphic
environment, which is in agreement with the type of
host rock (black shales and lydites) hosting most of
the quartz veinlets. In Fig. 10, it can be seen there is a
increase in fO2 from the H2O–CH4–N2–NaCl fluids
(not plotted due to the absence of CO2) to the H2O–
CO2–CH4–N2–NaCl fluids (for minimum trapping
temperatures 350 jC). The fO2 is around 10� 31 in the
fluid inclusions from quartz associated with nongra-
phitic lithologies and around 10 � 32 in the fluid
inclusions of quartz associated with graphitic litholo-
gies.
The composition of the aqueous–carbonic fluids
suggests derivation from reduced lithologies at fO2
conditions below Ni–NiO probably from the devo-
latilization and thermal maturation of organic mat-
ter.
6. Conclusions
Fluid inclusions in metamorphic quartz veinlets
from the Chaves region contain evidence of multiple
fluid incursions of different fluids, broadly within the
C–H–O–N–salt system. Primary fluid inclusions
contain low salinity aqueous–carbonic, H2O–CH4–
N2–NaCl fluids. Secondary inclusions occur in dis-
crete cracks, at the grain boundaries of the quartz, and
contain low salinity aqueous–carbonic, H2O–CO2–
CH4–N2–NaCl fluids. Late low salinity, H2O–NaCl
and high salinity, H2O–NaCl–CaCl2 fluids are
present in intergranular fractures.
During metamorphism, H2O–CH4–N2–NaCl flu-
ids were trapped in the metamorphic quartz close to
the peak metamorphic conditions, which occurred
during or immediately after D2. The calculated P–T
conditions for the CW area are P= 300–350 MPa and
Tf 500 jC, and for the CE area, 250 MPa and 450
jC. The P–T conditions for the H2O–CO2–CH4–
N2–NaCl fluids indicate they were trapped between
lithostatic and hydrostatic pressures probably during
uplift associated with the Hercynian orogen and the
emplacement of two-mica syntectonic granites. There
is evidence to indicate some of these fluids resulted
from immiscibility. The youngest H2O–NaCl and
H2O–NaCl–CaCl2 fluids are the only ones not related
with metamorphism and appear to have been trapped
after D3 at relatively low P (hydrostatic)–T condi-
tions.
Both the early H2O–CH4–N2–NaCl fluids present
in quartz from the graphitic lithologies and the later
H2O–CO2–CH4–N2–NaCl fluids present in quartz
from graphitic and nongraphitic lithologies seem to
have a common origin and evolution. These fluids are
probably the products of mineral dehydration, as their
main component is water. However, the early H2O–
CH4–N2–NaCl fluids have volatile phase enriched in
CH4 due to their interaction with the C-rich host
rocks. Both aqueous–carbonic fluids have a low
salinity, probably as the result of connate waters being
diluted by the water released from mineral dehydra-
tion during metamorphism.
The fO2 obtained for the aqueous–carbonic fluids
is indicative of a reducing metamorphic environment,
which is in agreement with the type of host rock,
black shales and lydites of most of the quartz veinlets.
From the early H2O–CH4–N2–NaCl to the later
Fig. 10. T versus log fO2 diagram for aqueous–carbonic fluids. GL:
graphitic-rich lithologies; NGL: nongraphitic-rich lithologies.
A. Guedes et al. / Chemical Geology 190 (2002) 273–289 287
aqueous–carbonic H2O–CO2–CH4–N2–NaCl flu-
ids, there is an enrichment in CO2 which is more
significant for the fluids associated with nongraphitic-
rich lithologies. The aqueous–carbonic fluids rich in
H2O and CH4 are dominantly associated with graph-
itic-rich lithologies. However, aqueous–carbonic
CO2-rich fluids were found in both graphitic and
nongraphitic lithologies from CE and CW areas,
which were respectively of low and medium meta-
morphic grade.
Acknowledgements
This work has been supported by the project
PRAXIS 12/2.1/CTA/82/94 and by a grant of FCT/
PRAXIS XXI. The authors are gratefully acknowl-
edged to Jordi Bruno and two anonymous reviewers
for their help in improving the manuscript. [EO]
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