evolution of fluids associated with metasedimentary sequences from

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.


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.


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.


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.


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

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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

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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.


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.


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.


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-


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.


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.


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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