Fluid–rock interactions and the role of late Hercynian aplite intrusion in the genesis of the...

Fluid–rock interactions and the role of late Hercynian aplite

intrusion in the genesis of the Castromil gold deposit,

northern Portugal

J. Vallancea,b,*, M. Cathelineaua, M.C. Boirona, S. Fourcadec,T.J. Shepherdd, J. Nadend

aUMR G2R-CNRS 7566, BP 23, 54501 Vandoeuvre-les-Nancy Cedex, FrancebSection des Sciences de la Terre, Departement de Mineralogie, Rue des Maraıchers 13, CH1205 Geneva, Switzerland

cGeosciences Rennes, Campus de Beaulieu, 35042 Rennes Cedex, FrancedBritish Geological Survey, Nicker Hill, Keyworth, Nottingham NG 12 5 GG, UK

Received 3 December 2001; received in revised form 28 June 2002

Abstract

Castromil (northern Portugal) is one of several important orogenic gold deposits located within the ‘‘Central Iberian’’

geotectonic zone of northwest Iberia. The deposit occurs at the margin of a Variscan, syn- to late-D3 biotite granite, and is

spatially associated with a small tourmaline aplite body that intrudes the granite at its contact with a secondary anticline of

Palaeozoic arenaceous and argillaceous metasediments of the Valongo Belt. Identification of the ore fluids and their pathways,

and the reconstruction of the P–T–X conditions during mineralisation were obtained by combining the geometric

characteristics of veins and microstructures together with a detailed study of the inclusion fluids.

Several stages of fluid percolation following contact metamorphism can be recognised. At each stage, the contact zone,

characterised by intrusive aplites, related faults and fractures, appears to have focused the hydrothermal flow and acted as a

structural conduit for deeper-sourced hydrothermal fluids. The earliest fluid stage (Stage I) is characterised by aqueous-carbonic

fluids dominated by CO2 and CH4 that were probably generated by high-temperature fluid–rock interaction (400–500 jC) withgraphitic schists interbedded with the metasediments. These fluids were responsible for significant alteration (greisenisation) of

the aplite and its host granite, and the formation of silicified, flat lying structures that can be traced along the strike length of the

deposit. At temperatures between 400 and 500 jC, fluid pressure ranges from 230 to 300 MPa, which is equivalent to a depth of

10F 1.5 km. The second stage of mineralisation (Stage II: As-ore stage) is also characterised by aqueous-carbonic fluids and

represents the main phase of quartz–arsenopyrite–pyrite deposition.

The third stage of mineralisation (Stage III: Au-ore stage) was accompanied by intense microfracturing of the preexisting

quartz veins and the preferential deposition of gold along microfractures in the sulphides. The introduction of gold corresponds

to the percolation and mixing of two distinctive aqueous fluids of contrasting salinity at relatively low temperatures (150–275

jC). Based on compositional and temperature data, it is suggested that during the main phase of uplift, shallow waters

penetrated deep into the basement, allowing gold to be leached from potential source rocks (most probably the Palaeozoic

metasediments) and deposited in structural and geochemical traps formed during earlier stages of the hydrothermal system.

0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0009 -2541 (02 )00278 -4

* Corresponding author. Section des Sciences de la Terre, Departement de Mineralogie, Rue des Maraıchers 13, CH1205 Geneva,

Switzerland.

E-mail address: [email protected] (J. Vallance).

www.elsevier.com/locate/chemgeo

Chemical Geology 194 (2003) 201–224

The decrease in pressure during the As-ore stage corresponds to a significant tectonic uplift (around 5–6 km), and probably

marks the transition from lithostatic to hydrostatic pressure conditions. Furthermore, if uplift had already been initiated during

aplite emplacement, the prevailing sub-isothermal high-temperature conditions provide an explanation for the presence of

decrepitated aqueous-carbonic inclusions in metamorphic quartz lenses and veins in the surrounding metasediments.

To conclude, localised heat flows linked to late Hercynian magmatism at deeper structural levels appears to be the main

cause of fluid circulation at Castromil. Evidence suggests that contact zones related to faulting along a secondary anticline of the

Valongo Belt controlled both aplite intrusion and subsequent long-lived hydrothermal fluid circulation. The proposed genetic

model differs from orogenic gold deposit models in emphasising the role of late stage aqueous fluids in the development of

economic grade (10–15 g/t) gold ores.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Fluid inclusions; Gold mineralisation; P–T conditions; Graphitic schists; Aplite; Portugal

1. Introduction

In northwest Iberia, gold deposits occur both in

granites and Palaeozoic metasedimentary rocks. Good

examples of granite-hosted deposits are Penedono and

Grovelas in Portugal and Corcoesto, Tomino and Pino

in Spain (Barakat, 1995; Cathelineau et al., 1993;

Essarraj, 1992; Gonzales Clavijo et al., 1994). Meta-

sediment-hosted deposits are typified by those of the

Valongo and Vila Pouca de Aguiar areas of Portugal

(Couto, 1993; Doria, 1999; Doria et al., 1999; Noro-

nha and Ramos, 1993) (see Fig. 1).

Most of the deposits show remarkable similarities

in paragenesis and ore fluid compositions (Catheli-

neau et al., 1993; Noronha et al., 2000). However, the

rheology of the host rock plays a key role in the

formation of fluid conduits and on the geometry of

these deposits, especially for the early quartz struc-

tures and their subsequent behaviour during deforma-

tion. Host rocks are also possible source reservoirs for

the elements and fluids. The understanding of the

interactions between successive fluids and the host

rocks is essential to reconstruct (i) the evolution of the

P–T–X conditions, and (ii) the transport and deposi-

tion of elements responsible for ore genesis.

Temporal relationships between magmas and gold

mineralisation are often ambiguous particularly

because no precise radiometric ages are available.

However, the existence of gold mineralisation cross-

cutting a Stephanian C coal basin (Alto do Sobrido,

Valongo district) shows that gold deposition occurs

during the latest stage of the Variscan orogeny. The

role of late magmatism is also of great importance, as

magmas are generally considered as a fertile source for

the ore fluids and metals (Lang and Baker, 2001;

Spycher and Reed, 1989; Simmons and Browne,

2000). For the Portuguese deposits, Neiva and Neiva

(1990) and Neiva (1992) suggested a genetic relation-

ship between hydrothermal fluid activity and late-stage

fractional crystallisation of granites. In contrast, Noro-

nha et al. (2000) advocates that the granites are not the

source of the fluids and act only as a heat engine.

The Castromil gold deposit occurs at the margin of

a syn- to late-D3 biotite granite and is spatially

associated to an hectometric tourmaline-bearing aplite

stock which intrudes the granite near its contact with

the Palaeozoic metasedimentary series (Fig. 2). These

metasediments include graphitic schists. The deposit

provides a good example of a close spatial relation-

ship between granite and Au-mineralisation.

The present study concerned the identification of

the percolating ore fluids, their pathways, and the

reconstruction of the P–T–X evolution during ore

formation, in order to decipher the role of granites in

the formation of the Au-ores. A complete character-

isation of the palaeofluid pathways, at each given

stage, has been obtained by combining the geometric

and chronological characteristics of the veins and

microstructures together with a detailed description

of hydrothermal alteration, ore mineralogy and fluid

chemistry.

2. Geological setting

The northwestern part of Portugal is an important

Variscan metallogenic province, particularly for gold,

which has been mined since Roman and pre-Roman

J. Vallance et al. / Chemical Geology 194 (2003) 201–224202

times. Although there are no mines currently in

production, the most important mines were Jales and

Tres Minas in the Vila Pouca de Aguiar area of

northern Portugal (Fig. 1).

The Castromil deposit was also mined during

Roman times and traces of old workings are still

preserved and reflect its overall elongated shape

(approximately 200 m wide, 2000 m length) with

the main axis parallel to the granite margin

(N140jE). The River Sousa divides the deposit into

two parts: Covas de Castromil to the north and Serra

de Quinta to the south (Fig. 3).

The deposit occurs at the contact between the

Castelo de Paiva granite and its contact metamorphic

aureole of Silurian rocks (Fig. 2). The Palaeozoic

metasedimentary sequence of the Valongo region

belongs to the Central Iberian Zone (CIZ), which

represents the autochthonous domain of the north-

western part of the Iberian Peninsula.

The main phases of deformation observed in this

region (denoted D1 and D3) define the main regional

structures (Noronha et al., 1981). During D1, the

Valongo anticline, striking N140jE with an axial

plane dipping 60j to the northeast was formed (Fig.

2). The Valongo anticline is asymmetric, the eastern

flank dips 35j to the east and the western flank is

subvertical (Dias and Ribeiro, 1991). The core of this

anticline comprises pre-Ordovician schists and grey-

wackes of uncertain age (Precambrian to Cambrian).

The sequence of Palaeozoic metasediments consists

of, from bottom to top, Ordovician quartzites and

schists, followed by Silurian quartzites, greywackes,

Fig. 1. Regional geological map showing the location of main gold occurrences within the principal geotectonic zones of northwest Iberia. Ca:

Castromil; Pe: Penedono; VPA: Vila Pouca de Aguiar; Pi: Pino, Fr: Franca; Ch: Chaves; Gr: Grovelas; To: Tomino, Co: Corcoesto (after

Cathelineau et al., 1993).

J. Vallance et al. / Chemical Geology 194 (2003) 201–224 203

lydites, black schists and graphitic schists (Romano

and Diggens, 1973). Devonian arenites and schists,

and Carboniferous rocks (Stephanian B–C coal basin)

outcrop in the western part of the structure. The

Valongo anticline is affected by a D3 brittle ductile

shear zone that strikes parallel to the axial plane.

After D3, the syn- to late-D3 Castelo de Paiva

biotite granite was intruded along the eastern boundary

of the anticline (Fig. 2). Near its contact, the granite

and metasedimentary rocks are crosscut by late aplite

and tourmaline aplite stocks striking N040–050jEand N130–140jE (Fig. 2).

3. Structure and paragenesis

3.1. Structural control

The Castromil gold deposit is separated from the

andalusite schists of the contact aureole (contact

metamorphism of the late D3 granite) to the west by

a late N130jE striking and 70jN dipping fault. The

contact zone can be interpreted as having formed

along the eastern flank of a secondary anticline of

the Valongo Belt. New structural observations made

during this study clearly show that the dip of the

Fig. 2. Geological map of the Valongo area showing the location of Castromil and other gold occurrences (modified from Carta Geologica de

Portugal, Folha 1; Pereira and Ribeiro, 1992).


Fig. 3. Geological map (A) and cross section (B) of Castromil showing the distribution of sample locations referred to in the text.


schistosity changes from 70j to the west to 70j to eastat about 200 m from the contact zone (Fig. 3).

The fault exhibits early reverse movements. Roof-

pendants of metasedimentary rocks are observed at

the margin of the granite and display similar foliation

and dips to the adjacent schists. To the north, the

granite displays complex inter-fingering relationships

with the metasediments. The aplite is emplaced pri-

marily along the contact zone between the granite and

the metasediments.

Brittle deformation related to fault movement is

responsible for the formation of open structures within

the main fault (stockwork), and on both sides (F 300

m) of the contact in the enclosing granite and meta-

sedimentary rocks. Thus, a specific fracture geometry

is observed in the these formations: low-angle struc-

tures oriented N050–080jE and dipping 30–50j to

the northwest, and associated sets of fractures dipping

50–70j, forming horse tail features.

Maximum hydrothermal alteration is observed

along the main fault (contact zone) in the silicified

breccia and the stockwork zone, which probably

formed in relation with fluid overpressuring. Greise-

nisation is also developed on low-angle structures (a

few tens of meters long, 15–30 cm thick) which

crosscut the intrusive bodies (Fig. 4A). The hydro-

thermal alteration envelope (greisenisation) in the

granitoids defines the maximum extent of the deposit

to the east, whilst the metasediments represents the

western boundary of the ore zone (Fig. 3). Thus,

although subhorizontal quartz–muscovite veins are

locally found in the greywackes, the maximum devel-

opment of alteration is confined to the granitoids.

3.2. Mineralogy

The main zone of hydrothermal mineralisation is

characterised by four main stages.

Fig. 4. Aspects of the mineralisation: (A), typical 20-cm-wide, silicified structure (subvertical outcrop); (B), Q1 quartz stockwork developed in

greisenised aplite; (C), Q3 quartz veinlet crosscutting a Q1 quartz–arsenopyrite (FeAsS)–phengites (Ph) assemblage; (D), gold (Au) in

microfracture sealed by Q3 quartz crosscutting early arsenopyrite (FeAsS).


3.2.1. Stage I: quartz stockwork and related greisens

The stockwork comprises a network of quartz-

filled fractures and microfractures. Quartz (Q1) is

milky and widely developed in the aplite and centi-

metric quartz veins of the silicified structures (Fig.

4B). Plagioclase and K-feldspar in the granite are

replaced by phengites (phengite 1), and the magmatic

quartz grains are partially recrystallised to clear quartz

along their boundaries. Mineralised samples are all

located in the hydrothermally altered zone (eastern

side composed of greisenised granite and aplite),

especially in the main stockwork, but also along the

low-angle structures.

3.2.2. Stage II: As-stage

Re-opening of the preexisting fractures lead to the

re-sealing of the structures and pores by a new

generation of quartz (Q2) in the stockworks and

silicified structures. It is accompanied by pyrite,

arsenopyrite and phengite (phengite 1). Locally, large

quantities of sulphides are developed (more than 1 m

thick bodies of massive pyrite–arsenopyrite without

quartz are locally observed).

3.2.3. Stage III: Au-stage

Gold, in association with bismuth minerals, galena

and minor phengites (phengite 3), is linked to a third

stage of quartz deposition (Q3) that forms millimetric

veinlets crosscutting and/or brecciating all former

structures. Gold is observed as electrum in Q3 frac-

tures (breccia cement) affecting pyrite and arsenopyr-

ite formed during the preceding stage (Fig. 4C and D).

Quartz Q1 represents the main infilling of the frac-

tures. By comparison, quartz Q2 and Q3 are insignif-

icant, implying a reduced supply of silica to the

system.

3.2.4. Stage IV: post-ore stage

Late normal movements, attesting to a late exten-

sional event, reflect the long-lived tectonic activity of

the fault, as shown by the crystallisation of euhedral

quartz combs (geodic quartz Q4) on both sides of the

opened structures. This event appears to be unrelated

to the main hydrothermal system, and can be consid-

ered as a post-ore stage.

In the metasediments, several metamorphic quartz

veins were sampled: (i) 10- to 20-cm-long and 5-cm-

thick quartz lenses concordant with the foliation, and

(ii) 2- to 10-cm-thick (red to purple coloured) quartz

veins that crosscut the foliation. These veins show a

complex history, with reverse and normal movements.

Graphite is present in the metamorphic quartz (Qm),

giving it a greyish aspect under the microscope.

4. Methods

Fluid evolution has been studied by looking at

relationships between fluid inclusions, their host min-

eral, the geometry of the host microstructures and the

location of ore minerals, in quartz veins and in quartz

grains from the host rocks. Geometry of the palae-

ofluid channelways has been investigated through the

characterisation of fluid inclusions with respect to the

deformation chronology, following the procedure of

Boiron et al. (1992).

Sampling along profiles was carried out across the

mineralised zones. For each sample, horizontal and

vertical sections were prepared for macroscopic obser-

vation of the textures. From these, oriented blocks

were then chosen for the preparation of oriented thin

sections and wafers. The geometry and chronology of

fluid migration was investigated by combining macro-

scopic observations of oriented horizontal and vertical

planes and transmitted light microscopy observations

on oriented thin sections. Quantification of the abun-

dance and the geometric characteristics of each

marker were carried out using a systematic and

statistical analysis of the microstructural markers in

horizontal and oriented planes. Systematic measure-

ment of the microfissures was carried out in each

section using an interactive videographic analyzer

adapted to such studies (Nogueira and Noronha,

1995). Results are given in the form of rose diagrams,

which take into account either the cumulative number

or the cumulative length of cracks in a given direction.

Frequencies have been distinguished for each angular

section from 0 to 180j.Typology and petrography of the fluid inclusions

was carried out on orientated wafers from representa-

tive samples of the host rocks and quartz veins. Fluid

inclusions have been related to the different stages of

fluid percolation on the basis of the relative chronol-

ogy of mineral assemblages and microstructures.

Notation of the fluid inclusion types follows nomen-

clature previously published (Boiron et al., 1992),


which takes into account the nature of the dominant

chemical phases and phase changes observed. It is

based on the total homogenisation Th (L–V to the

vapour noted V, L–V to the liquid noted L) and the

quantity of C–H–O–(N–S) species detectable by

Raman spectroscopy (subscript c, when C–H–O–S

species are the only components and water is not

visible; c-w, when water and homogenisation of

carbonic phase are observable; w-c, when both water,

CO2, CH4 and N2 species are present but are detected

only by clathrate melting and Raman spectroscopy;

and w, when C–H–O– (N–S) species are not

detected by any methods).

Microthermometric characterisation of the fluids

was performed on wafers using a Chaix-Meca heat-

ing–freezing stage (Poty et al., 1976) and a Fluid Inc.

stage. Molar fractions of CO2, CH4, N2 and H2S were

determined in individual fluid inclusions using a

DILOR-LABRAM Raman spectrometer at CREGU,

Nancy. Molar fraction of NaCl, molar volume, bulk

composition and P–T parameters were determined by

combining results from microthermometry and

Raman analysis. Bulk composition and molar volume

were computed from the P–V–T–X properties of

individual inclusions in the C–O–H–S system

(Dubessy, 1984; Dubessy et al., 1989; Thiery et al.,

1994, Bakker, 1997). The P–T properties were mod-

elled using the V–X data and the equation of state

from Bowers and Helgeson (1983) revised by Bakker

(1999) for aqueous-carbonic fluid inclusions. For

aqueous fluids, data from Zhang and Frantz (1987)

for the H2O–NaCl system have been used.

5. Fluid characterisation

5.1. Microthermometry and Raman spectroscopy

5.1.1. The aqueous-carbonic fluids

These occur in all types of quartz, as fluid inclu-

sion planes (FIP) in the host rocks, and as primary

inclusions in quartz Q1 and Q2 (Table 1). They are of

moderate salinity (1–10 wt.% eq. NaCl) but variable

in composition. Minimum trapping temperatures

range from 300 to 400 jC.

5.1.1.1. Metamorphic quartz lenses. Aqueous-car-

bonic fluid inclusions (Tables 2 and 3) are the

predominant inclusions in the metamorphic quartz

lenses and veins (Qm) veins, and in quartz grains

from the metasediments (greywackes). These fluid

inclusions generally contain graphite inclusions

where the lenses are spatially associated with graph-

itic schists. They show also abundant decrepitation

textures (highly irregular, dendritic textures (Fig.

5A)) that indicate re-equilibration processes (Vityk

and Bodnar, 1995). Decrepitation may also be a

consequence of granite and aplite intrusion since

these inclusions are observed in the contact aureole.

Such fluid inclusions show strong volatile/water

ratio variability; the volatile phase representing

30–100% of the fluid inclusion volume. The melt-

ing temperature of CO2 (Tm CO2) ranges from

� 63.8 to � 58.2 jC with two modes around

� 61.5 and � 58.5 jC. The homogenisation of

the volatile phase (Th CO2) occurs to the liquid,

Table 1

Summary of the relationships between deformation, mineralisation, quartz types and fluid stages at Castromil

Tectonic/deformation/magmatism Quartz-type alteration Ore minerals Fluids

Retrograde

metamorphism

Qm Aqueous-carbonic

fluids

Granite intrusion and contact

metamorphism

Aplite intrusion Greisenisation Aqueous-carbonic

fluids

Stockwork formation Recrystallisation of

magmatic quartz

Phengite 1

Q1–phengite 2 Aqueous-carbonic

Brecciation Q2 FeS2–FeAsS fluids

Q3–phengite 3 Au–PbS–Bi minerals Aqueous fluids

Microfissuration/recrystallisation Q4 Aqueous fluids


Table 2

Summary of fluid inclusion types and associated mineral assemblages from the Castromil deposit with indication of the microthermometric data

Stage Fluid inclusions Microthermometric data

Habitus Type Tm CO2 (jC) Th CO2 (jC) Tm ice (jC) Tm cl (jC) Th (jC)

Quartz veins and lenses (Qm)

in the metasedimentary rocks

Secondary Vc-w, Lc � 63.8/� 58.2

(� 61.5 and � 58.5)

� 17.9/25.3 L,

C or V

7.8/14 (10.5) 366/380

Greisen stage Secondary in host

granite and aplite

L or Vc-w � 63.5/� 57.2

(� 59)

13.5/29 L,

C or V

4.8/9.3 (7) 322/405

(370)

As-ore stage

(arsenopyrite, pyrite)

Secondary in host

granite, primary and

secondary in Q1

primary in Q2

L or Vc-w, Lc, Vc � 68.7/� 57.3

(� 68 and � 58)

1/28 L,

C or V (19)

5/11.4 (9.5) 284/365

(340)

Au-ore stage Secondary in host

granite, primary in Q3

Lw � 8.2/� 0.1 100/300

(210)

Post-ore stage Primary in Q4, secondary

in host granite

Lw � 11.5/� 7.2 175/250

(170)

For each microthermometric parameter, ranges and mode (in parenthesis) are given. Tm CO2: melting temperature of solid CO2. Th CO2: homogenisation temperature of CO2.

Tm ice: melting temperature of ice. Tm cl: melting temperature of clathrate. Th: homogenisation temperature. Homogenisation mode: L—liquid; C—critical; V—vapour;

D—decrepitation. All values are in jC. Nomenclature for fluid inclusions is explained in the text.

J.Valla

nce

etal./Chem

icalGeology194(2003)201–224

209

Table 3

Chemical compositions obtained by Raman microprobe spectrometry of selected fluid inclusions and corresponding microthermometric data,

from the Castromil deposit

Stage Type of Inclusion Microthermometry Raman data Bulk composition

occurrence Tm CO2 Th CO2 Mode Tm cl Th Mode CO2 CH4 N2 H2O CO2 CH4 N2 NaCl

Vc-w Quartz veins 4-1 � 61.9 9.2 L 11.4 350* V 80.7 17.3 2.0 58.1 33.2 6.8 0.8 1.1

and lenses in 4-3 � 61.0 9.4 L 10.7 366 V 80.6 17.2 2.2 58.3 32.9 6.7 0.9 1.2

the meta- 4-5 � 63.8 n.o. n.o. 10.4 350* V 85.9 2.6 11.5 87.4 10.9 0.2 1.1 0.4

sedimentary 39-2 � 58.7 22.9 L 8.6 380* V 96.4 2.0 1.6 14.0 82.8 1.7 1.3 0.2

rocks (Qm) 39-3 � 58.8 22.4 L 9.0 380 V 92.4 5.3 2.3 62.9 33.7 1.8 0.8 0.8

39-16 � 58.2 21.2 L 7.8 380* V 95.0 2.0 3.0 44.2 52.6 1.1 1.6 0.5

Lc 39-5 � 61.4 � 16.9 L n.o. n.o. n.o. 75.6 1 23.4 0.0 75.6 1 23.4 0.0

39-6 � 60.8 � 9 L n.o. n.o. n.o. 73.6 1 25.4 0.0 73.6 1 25.4 0.0

39-19 � 60.6 � 10.4 L n.o. n.o. n.o. 74.2 1 24.8 0.0 74.2 1 24.8 0.0

Greisen stage FIP in granite 25-2 � 58.4 25.0 L 6.4 320* L 95.5 3.0 1.5 82.7 14.6 0.4 0.2 2.1

(Lc-w, Vc-w) 25-3 � 58.3 25.0 L 6.6 320* L 94.8 3.2 2.0 80.8 16.4 0.5 0.3 2.0

25-12 � 58.3 24.5 L 7.1 350 L 94.9 3.3 1.8 81.1 16.4 0.5 0.3 1.8

FIP in 11-2-1 � 57.4 25.5 V 6.8 322 L 95.7 1.8 2.5 89.0 8.7 0.1 0.2 2.0

greisenised 11-2-9 � 58.2 26.2 V 5.3 330* L 98.8 0.6 0.6 84.0 13.2 0.1 0.1 2.6

granite 11-2-8 � 58.3 28.8 V 4.8 382 C 97.6 0.7 1.7 84.5 12.3 0.1 0.2 2.9

FIP in aplite 26-15 � 57.6 24.5 C 7.6 306 L 94.8 1.4 3.8 88.8 9.2 0.1 0.3 1.6

26-18 � 57.2 27.5 L 5.5 400 L 97.4 1.6 1.0 74.5 22.7 0.3 0.2 2.3

26-12 � 57.4 28.0 L 5.5 405 L 97.1 1.6 1.3 75.1 22.0 0.3 0.3 2.3

26-2-2 � 57.8 13.5 L 5.8 305* V 96.7 1.5 1.8 39.0 57.8 0.9 1.1 1.2

26-2-3 � 57.5 27.0 L 7.1 305* V 96.3 1.7 2.0 34.8 62.0 1.1 1.3 0.8

As-ore stage FIP in granite 27-6 � 61.2 3.6 V 5.0 374* L 75.0 21.1 3.9 88.5 6.2 1.1 0.2 4.0

(Lc-w, Vc-w) 27-4-2 � 59.2 12.4 C 9.3 255* L 84.7 11.5 3.8 81.7 14.7 1.7 0.6 1.3

27-2 � 63.5 1.0 V 7.6 300* V 71.9 23.8 4.3 83.9 9.8 2.6 0.5 3.3

27-9 � 60.0 2.6 V 5.5 300* V 73.8 21.7 4.5 70.7 19.9 5.3 1.1 3.0

47-2 � 68.7 n.o. n.o. 10.5 335 V 49.3 42.0 8.7 87.1 6.8 4.4 0.9 0.8

47-3 � 67.7 n.o. n.o. 9.6 336* V 46.1 46.0 7.9 87.6 6.0 4.5 0.8 1.1

47-6 � 66.8 n.o. n.o. 8.8 336* V 40.0 50.0 10.0 89.3 4.3 3.7 0.8 1.9

FIP in aplite 24b-9 � 59.8 16.7 V 8.4 365 L 86.7 7.7 5.6 90.9 7.6 0.4 0.3 0.8

24b-13 � 60.7 9.3 V 9.0 365 L 75.6 15.8 8.6 85.0 10.6 1.8 1.0 1.6

10-1-5 � 59.4 2.9 V 9.4 362 L 76.9 21.7 1.4 94.5 4.7 0.5 0.1 0.2

10-1-1 � 59.7 3.3 V 10.8 331 L 79.5 16.2 4.3 93.8 5.2 0.6 0.2 0.2

20-1-4 � 57.7 17.5 V 9.1 336* L 83.0 12.1 4.9 90.7 7.8 0.8 0.3 0.4

20-1-3 � 60.7 17.5 V 8.1 379 L 85.3 12.4 2.3 89.7 8.0 0.8 0.2 1.3

26-2-1 � 57.5 26.0 V 7.1 305* V 95.8 1.8 2.4 27.6 68.8 1.3 1.7 0.6

20-8 � 58.2 n.o. n.o. 7.3 373 C 90.9 7.0 2.1 83.2 13.8 0.9 0.3 1.9

20-2-3 � 57.7 26.0 V n.o. n.o. n.o. 94.6 4.0 1.4 80.1 17.3 0.6 0.2 1.8

FIP in Q1 21-1-1 � 58.7 18.8 C 9.0 343 L 87.5 11.1 1.4 91.5 7.1 0.6 0.1 0.7

21-1-2 � 58.4 20.0 C 8.6 353 L 88.4 10.5 1.0 91.0 7.3 0.6 0.1 1.0

21-1-3 � 58.6 19.5 C 8.6 357 C 86.8 12.5 0.7 88.3 9.6 1.0 0.1 1.0

21-1-5 � 58.4 19.0 V 9.4 275* L 81.1 18.2 0.7 89.4 9.3 0.8 0.1 0.4

21-1-6 � 59.0 13.0 V 9.6 355 C 83.3 16.0 0.7 90.8 7.7 1.2 0.05 0.25

21-1-8 � 58.2 19.5 C 9.0 345 L 88.4 10.2 1.3 90.9 7.7 0.6 0.1 0.7

32c-3 � 58.2 19.7 V 9.4 312 L 88.2 8.7 3.1 89.3 9.0 0.6 0.2 1.1

32c-7 � 57.8 18.0 V 8.8 321 L 92.2 5.3 2.5 89 9.3 0.4 0.2 1.1

32c-5 � 57.5 18.5 V 9.8 284 L 90.9 7.0 2.1 89.5 9.6 0.6 0.1 0.2

21-4 � 58.5 19.5 V 8.6 360 C 88.3 10.5 1.2 88.8 8.8 0.8 0.1 1.5

21-7 � 59.0 14.7 V 9.4 343 L 84.7 14.1 1.2 88.3 8.9 1.1 0.1 1.6

21-2-4 � 58.4 22.5 L 6.8 345 V 93.1 5.5 1.4 43.7 51.3 3.0 0.7 1.3

21-2-5 � 58.5 22.5 L 6.6 341 V 92.2 6.3 1.4 42.5 51.8 3.5 0.8 1.4

Lc, Vc FIP in aplite 20-2-2 � 57.7 26.0 L n.o. n.o. n.o. 95.7 2.7 1.6 0.0 95.7 2.7 1.6 0.0

20-2-4 � 57.5 26.0 V n.o. n.o. n.o. 95.7 1.6 2.7 0.0 95.7 1.6 2.7 0.0


critical or vapour phase in the range � 17.9 to + 25.3

jC. The melting temperature of clathrate (Tm cl) is

observed between + 7.8 and + 14 jC with a mode

around + 10.5 jC. Total homogenisation (Th) ranges

from 366 to 380 jC.

Raman analyses show that CO2 is the main com-

ponent of the volatile phase and is always higher than

70 mol% (Table 3; Fig. 6). The CH4 content ranges

from 2 to 17.3 mol%. Nitrogen is always detected and

ranges from 1.6 to 25.4 mol%. Bulk composition is

Table 3 (continued)

Stage Type of Inclusion Microthermometry Raman data Bulk composition

occurrence Tm CO2 Th CO2 Mode Tm cl Th Mode CO2 CH4 N2 H2O CO2 CH4 N2 NaCl

Lc, Vc FIP in aplite 20-2-5 � 58.3 23.5 L n.o. n.o. n.o. 95.3 1.7 3.0 0.0 95.3 1.7 3.0 0.0

Lc FIP in Q1 21-2-1 � 59.3 22.0 L n.o. n.o. n.o. 93.7 4.5 1.8 0.0 93.7 4.5 1.8 0.0

21-2-3 � 58.8 20.0 L n.o. n.o. n.o. 90.2 8.5 1.3 0.0 90.2 8.5 1.3 0.0

Compositions are given in mol%. Tm CO2: melting temperature of solid CO2. Th CO2: homogenisation temperature of CO2. Tm ice: melting

temperature of ice. Tm cl: melting temperature of clathrate. Th: homogenisation temperature. Homogenisation mode: L—liquid; C—critical;

V—vapour. Values in italic and asterisk indicate decrepitation temperature. All values are in jC. n.o.: non-observed. Nomenclature for fluid

inclusions is explained in the text.

Fig. 5. Principal characteristics of the main generations of fluid inclusions: (A), re-equilibrated aqueous-carbonic inclusions in pre-ore quartz

(Qm); (B), aqueous-carbonic inclusion showing three phases at room temperature; (C), aqueous-carbonic and carbonic inclusions; (D), aqueous

fluid inclusion planes (FIP) network in pre-ore Qm quartz.


characterised by a H2O content in the range of 0 to 87

mol%, a CO2 content between 11 and 83 mol%, a

CH4 content of 0.2 to 6.8 mol% and N2 from 0.8 to 25

mol%. NaCl is in the range 0.2 to 1.2 mol%.

5.1.1.2. Greisens. The aqueous-carbonic fluid inclu-

sions (Tables 2 and 3) occur as fluid inclusion planes

(FIP) in the magmatic quartz of the granitoids or as

primary inclusions in quartz Q1 from the greisen

zone. The volatile phase represents 30–60% of the

inclusion volume (Fig. 5B). Tm CO2 ranges from

� 63.5 to � 57.2 jC with a mode around � 59 jC.Th CO2 (to the liquid or vapour phase) is from + 13.5

to + 29 jC, Tm cl is from + 4.8 to + 9.3 jC with a

Fig. 6. Volatile geochemistry of the aqueous-carbonic inclusions. (A) CO2–CH4–N2 ternary plot; (B) CO2 content versus density of the volatile

phase; (C) H2O content versus CO2/CH4 ratio; (D) H2O content versus minimum trapping temperature (Th).


mode at + 7 jC, Th is from 322 to 405 jC, with a

mode around 370 jC.The volatile phase is dominated by CO2 (95 to 99

mol%), with variable amounts of CH4 (0.6–3.3

mol%) and N2 (0.6–3.8 mol%) (Table 3; Fig. 6).

Bulk compositions show a H2O content ranging from

34.8 to 89 mol%, CO2 content from 8.7 to 62 mol%, a

CH4 content of 0.1 to 1.1 mol% and N2 content

ranging from 0.1 to 1.3 mol%. NaCl is in the range

0.8 to 2.9 mol%.

5.1.1.3. As-ore stage. Aqueous-carbonic inclusions

occur as primary fluid inclusions in the Q2 quartz, and

as FIP in the former quartz (Tables 2 and 3). Fluid

inclusions show two, more rarely three, phases at

room temperature (liquid H2O, vapour CO2F liquid

CO2). The volatile phase represents 30 to 100% of

the inclusion volume (Fig. 5C). Tm CO2 ranges from

� 68.7 to � 57.3 jC with two modes around � 68 jCand � 58 jC. Th CO2 (liquid, vapour or critical) is

from + 1 to + 28 jC, Tm cl is from + 5 to + 11.4 jCwith a mode at + 9.5 jC, Th is from 284 to 373 jC,with a mode around 340 jC.

The composition of the volatile phase is highly

variable with CO2 content ranging from 40 to 96

mol%, variable amounts of CH4 (1.5–50 mol%) and

N2 (0.7–10 mol%) (Table 3). Traces of H2S (up to 0.1

mol%) have been detected in some fluid inclusions.

Bulk composition is characterised by a H2O content

ranging from 27.6 to 94.5 mol%, CO2 content

between 4.3 and 68.8 mol%, a CH4 content of 0.4

to 5.3 mol%, and N2 contents ranging from 0.1 to 1.7

mol%. NaCl is in the range 0.2 to 4 mol%.

5.1.2. Bulk chemistry of aqueous-carbonic fluids

The aqueous-carbonic fluids from metamorphic

quartz lenses are heterogeneous populations charac-

terised by a volatile phase dominated by CO2, and

highly variable methane and N2 contents (Fig. 6A and

C) and bulk densities. They result probably from

series of trapping events under different conditions.

The density of the volatile phase is however relatively

high and ranges from 0.5 to 0.8 g�cm� 3, except for

one inclusion which shows a low density of 0.23

g�cm� 3 (Fig. 6B). Some inclusions in Qm, from a

quartz vein crosscutting greywakes, are characterised

by important amounts of N2 (up to 25 mol%) and a

low content of CH4 ( < 3 mol%). On other hand, lower

density aqueous-carbonic inclusions from a quartz

veinlet crosscutting graphitic schists show inclusions

with significant CH4 and N2 contents ranging from 21

to 37 mol% and 2 to 17 mol%, respectively, plus

graphite inclusions. Such gas contents demonstrate

that graphitic lithologies produced significant

amounts of CH4 during thermal events.

The aqueous-carbonic fluids from the greisen

zones are characterised by high CO2 (up to 95

mol%) and CH4 and N2 contents, which are signifi-

cant but lower than in the metasediments. The density

of the volatile phase shows two groups of inclusions

(Fig. 6B): (i) those displaying the highest density

(0.6–0.8 g�cm� 3), which are considered as fluid

inclusions trapped at relatively high pressure, and

(ii) fluids having the lowest density (0.25–0.4

g�cm� 3), which are indicative of a pressure decrease.

The latter show a slight enrichment in CH4 and N2. It

is interesting to note that these two groups of fluids

display important variations in their CO2/CH4 and

CO2/N2 ratios. The highest CO2/CH4 ratios corre-

spond to the greisenised zone where fluid circulation

has been more intense.

The aqueous-carbonic fluids from the As-ore stage

are characterised by high CH4 contents (up to 50%),

although in most cases, CO2 remains the dominant

species. Secondary Vc-w inclusions in quartz from the

granite display a strong enrichment in CH4 (40–50

mol%) and a noticeable N2 content (8–10 mol%).

These fluids show an evolution of the volatile phase

and their content from those with relatively high

values (around 0.70 g�cm� 3) and a high CO2 content

to those which are enriched in CH4 and displaying

low density (0.11 g�cm� 3) (Fig. 6B). Considering the

H2O content and the CO2/CH4 ratio (Fig. 6C), two

end-members can be identified. Fluids without water

show the highest variation in the CO2/CH4 content

(10 to 60). Unmixing process could probably explain

such variations in their composition, without signifi-

cant variation in minimal trapping temperatures.

The significant CH4 content in fluids percolating

the granitoids can be explained by the presence of

graphitic schists roof-pendants such as it has been

observed near the border of the granitic massif. Thus,

the presence of metasediments rich in graphite plays a

major role in the chemistry of the aqueous-carbonic

fluids. Fluids associated with hydrothermal circulation

(As-ore stage) contain higher amounts of water than


the previous ones. However, this group of fluid dis-

plays noticeable variations in their H2O content and in

their bulk density.

5.2. The aqueous fluids

Aqueous fluid inclusions (Au-ore stage) are

observed in FIP (Fig. 5D) in all types of quartz

except Q4, and as rare primary inclusions in Q3

(Table 1). Melting temperature of ice (Tm ice) is in

the range of � 8.2 to � 0.1 jC. Th (to the liquid

phase) is observed between 100 and 300 jC (Table

2).

Aqueous fluid inclusions related to the post-ore

stage occur as FIP in metamorphic quartz lenses. A

solid remaining present at high temperature (up to

300 jC) is currently observed in these inclusions,

and considering its rhombohedral shape is assumed

to be a carbonate. Thus, these fluids are probably the

last to circulate in the study area. Tm ice ranges

from � 11.5 to � 7.2 jC. Minimal trapping temper-

atures (Th) are in the range 175–250 jC (Table 2).

Primary inclusions in quartz comb Q4 are low-

salinity fluids and characterised by low Th below

200 jC (Fig. 7).

The Th–salinity diagram (Fig. 7) shows that Au-

ore stage aqueous fluids display a large range of

salinity from 0 to 12 wt.% eq. NaCl with fluid salinity

decreasing with decreasing Th. The dilution process is

well defined in FIPs oriented N030–060jE and

N130–140jE, and probably indicates mixing between

two end-members, a saline (10–12 wt.% eq. NaCl)

and a dilute fluid.

6. Geometry of the fluid pathways and

microfracturing

Systematic measurements of FIP direction were

performed on oriented samples in order to reconstruct

the fluid pathways. Three main domains are distin-

guished: the granitic domain to the east, the mineral-

ised zone and the metamorphic host rocks in the

southwestern part of the study area (Fig. 8). FIPs

show three principal directions: N030–060jE,N170–010jE and N130–140jE set (Figs. 8 and 9).

However, their relative abundance varies from domain

to domain. The N030–060jE direction is typical of

the mineralised zone (Figs. 8 and 9). The N170–

010jE direction is well represented in all three

Fig. 7. Homogenisation temperature (Th) versus salinity for the Castromil fluids showing the mixing of saline fluids and cooler dilute fluids

during the ore stage. (FI—fluid inclusion; shaded area corresponds to area of aqueous-carbonic fluids).


domains. The N130–140jE is minor but better devel-

oped in the mineralised zone and the metamorphic

rocks.

The abundance of aqueous-carbonic fluid inclusions

related to the greisen and As-ore stages is important in

the mineralised zone and, to a lesser degree, in the host

metamorphic rocks. In contrast, they are rare in the

granite domain attesting a preferential percolation of

these fluids along the granite boundary.

Petrographic examination of FIP content does not

show, however, any preferential directions for a cer-

tain type of fluid, implying that the same direction of

microfracture was used at several stages of fluid

percolation, The N030–060jE direction, typical of

the mineralised area, corresponds to the orientation

of the silicified structures (Fig. 4A) formed during

the earlier stage (Stage I), showing that the FIP

directions reflect the macroscopic fracture network

at the deposit scale. Macroscopic fractures initiated

during an earlier percolation event could strongly

influence the orientation of the associated FIP.

Consequently, FIP directions do not reflect directly

Fig. 8. Orientation of fluid inclusion planes (FIP) in the metasedimentary rocks, mineralised area and host granite.


the palaeostress field but suggest a N170–010jEstress field involving the reworking of the N030–

060jE and N130–140jE directions as conjugate

systems.

In the metamorphic rocks, all the fluid types are

present, but only aqueous fluid inclusions are

observed in well-defined FIPs oriented N170–

010jE, N130–140jE and locally N100jE (sample

Fig. 9. Rose diagram showing the preferred orientation of fluid inclusion planes (FIP) in the main cross section through the Castromil deposit

along the line of the railway cutting (see Fig. 8).


Cas 37) (Fig. 8). In the mineralised area, greisen fluids

are observed in the N030–060jE and N130–140jEdirections (Fig. 9). As-ore fluids (Stage II) are present

in N170–010jE, N030–060jE and, to a lesser

degree, in N130–140jE direction. Aqueous fluids

(Au-ore stage) are observed in N170–010jE and

N030–060jE directions. In the granite, the major

direction of fluid percolation is N170–010jE but

locally N030–060jE.

7. P–T reconstruction

Representative isochores were calculated for each

type of fluid by combining results from microther-

mometry and Raman analysis. The derived chrono-

logical reconstruction is shown on a pressure versus

temperature plot (Fig. 10). Maximum estimates to be

considered (Fig. 10) are (i) peak metamorphic con-

ditions (450 jC and 400 MPa; Guedes, 2001), and (ii)

the high temperature– low pressure metamorphism

associated with the emplacement of late to post-

tectonic granites (300–350 MPa and 500–550 jC;Noronha et al., 2000).

7.1. Greisen stage

The highest densities observed are those of the

aqueous-carbonic fluids trapped in FIPs in magmatic

quartz of the aplite. Microthermometric measurements

yield minimal trapping temperature around 400 jCand minimal trapping pressure in the range 210–240

MPa. Considering the isochores and a trapping in the

range 400–500 jC, a pressure of 230–300 MPa is

inferred (i.e. slightly below contact metamorphism

conditions). These high fluid pressures imply a trap-

Fig. 10. Pressure– temperature reconstruction of conditions prevailing during the pre-ore and main stages of mineralisation. P–T reconstruction

is based on representative isochores, petrographic, microthermometric and Raman data. Indicates that the upper boundary for temperatures is

unknown and not constrained by mineral assemblages.


ping under lithostatic conditions, corresponding to a

depth of around 10F 1.5 km.

7.2. As-ore stage

Fluids displaying the highest pressures (150–220

MPa) for temperatures in the range 400–500 jC are

those containing the lowest water contents. These

fluids, observed in Q1 quartz, probably record litho-

static pressures after the sealing of the fracture system

related to the stockwork formation. Some inclusions

show much lower volatile phase densities and lower

trapping pressures (100–200 MPa) for temperature of

400–500 jC (Fig. 10). These estimates attest to a

pressure decrease during uplift. Therefore, between

Stages I and II, there is a pressure drop of >150 MPa

but a cooling of < 50 jC. Considering the structural

level during the Stage I (10F 1.5 km), the pressure

drop cannot be explained simply by a change from

lithostatic to hydrostatic regime but implies an uplift

of the basement from 10F 1.5 to 4–5.5 km, with a

change from lithostatic to hydrostatic conditions.

Even for this case, the thermal gradients are very high

(70 jC/km) and indicate a very rapid uplift of the

basement and/or the presence of magma intrusions at

shallow levels acting as strong heat engines. The

aplite body may represent an early pre-hydrothermal

witness of this late magmatism.

7.3. Consequence of pressure drop

Unmixing of the aqueous-carbonic fluids in

response to a pressure drop is only partially recorded.

Minimum trapping temperatures for inclusions having

the lowest density are in the range 285–365 jC. Ifone considers the intersection of isochores and corre-

sponding isopleths, the minimum trapping temper-

ature is less than 370 jC for pressures higher than

50 MPa (Fig. 10). During stockwork formation,

certain fluid inclusion associations can be attributed

to unmixing. Thus, in the same quartz (Q1), inclusions

with no visible water (carbonic liquids, Lc) and water-

rich aqueous-carbonic vapours (Vc-w) display similar

volatile composition (inclusion 21-2-1 to 21-2-5,

Table 3). The occurrence of carbonic liquids followed

by carbonic vapours (Vc), apparently free of water,

can be interpreted as the result of unmixing at

decreasing pressure down to 40–50 MPa. The latter

estimates imply a hydrostatic regime at such temper-

atures to be compatible with a realistic geothermal

gradient (hydrostatic gradient, 70 jC/km).

7.4. Fluid mixing during Au-ore stage

Aqueous fluids associated with Q3 quartz and gold

have minimum trapping temperatures in the range

100–270 jC. Pressures could be similar or lower

than those of the preceding stage, in the absence of

a burial event (Fig. 10). Trapping temperatures are

slightly above Th (150–280 jC), thus confirming the

cooling and dilution process described above, and

differ by about 200 jC from the preceding stage

without any apparent change in pressure (i.e. at a

similar depth).

8. Origin of fluids: isotope geochemistry of quartz

The 18O/16O ratios were measured on separated

quartz chips (down to ca. 1.5 mg) extracted from fluid

inclusion wafers. The analytical procedure follows the

conventional fluorination method of Clayton and

Mayeda (1963) and is given, with related uncertain-

ties, in Essarraj et al. (2001).

A summary of the d18O data (with respect to

SMOW) is presented in Table 4. The metamorphic

quartz (samples CAS 37, CAS 22 and CAS 4) shows a

d18O value ranging from 15.3xto 17.1x. Q1, Q2,

Q3 quartzes display similar ranges of d18O values:

14.1–15.1x, 14.45xand 14.2–14.8x, respec-

tively. Late geodic Q4 quartz shows d18O values

ranging from 15.5xto 11.6x.

From the preceding section, we know that the

likely deposition temperatures for the successive

quartz generations are the following: metamorphic

Qm, 475F 25 jC; milky Q1 and Q2, 425F 25 jC;Q3, 225F 25 jC; and comb quartz Q4, 190F 10 or

140F 40 jC depending of the sample. Using the

quartz–H2O fractionation curve of Zheng (1993),

the d18O values of the corresponding hydrothermal

fluids were estimated, with the following results:

(i) The metamorphic quartz was deposited from a

CO2-bearing fluid with a d18O water component

of between 11.6xand 14.6x. These values

fix the composition of the metamorphic fluids in


the Castromil area. The upper end of the range

suggests isotopic equilibration with a crustal

reservoir containing metasediments.

(ii) The milky Q1 and Q2 quartzes crystallised from a

CO2-bearing fluid with a d18O water component

ranging from 9.6xto 11.4x. Likewise, these

values indicate a fluid that has equilibrated with

basement lithologies such as granitoids (e.g.

Kerrich, 1987).

(iii) Interpretation of the Q3 quartz data is less simple.

These values may have been inherited from older

quartz that did not re-equilibrate with fluids

active during gold deposition (Stage III). Mor-

phology and FI data, however, argue to the

contrary. Q3 quartz was mostly neoformed during

this stage. In this context, Q3 quartz was

deposited from an aqueous fluid with a d18Ocomposition of between 3.2xand 5.8x,

which records a significant depletion in d18O.As discussed in Boiron et al. (2002), the

magnitude of such depletion cannot be attributed

to CO2–H2O unmixing, which is neither identi-

fied as a major process in the Castromil example

nor in most other Variscan gold deposits. In

theory, metamorphic fluids that have equilibrated

with the same type of basement at lower temper-

atures could account for these values. Never-

theless, we favour an interpretation calling for the

input of an isotopically lighter fluid in the system,

since the fluid inclusion data document a process

of mixing between relatively saline fluids and a

dilute end-member. The shift towards values of

4.5F 1.5 could be compared to that already

inferred for gold stage fluids in other deposits

from the Hercynian belt (Laurieras, for instance,

Essarraj et al., 2001).

(iv) Late comb quartz Q4. One sample, from a

N170F 20jW joint (CAS 2) in the metamorphic

host rocks, crystallised from dilute aqueous

fluids at a temperature of around 180–200 jCfrom a fluid with a d18O composition of between

� 1.8xand � 0.4x(i.e. with a significant

component of surface-derived waters that had

more or less isotopically exchanged with the

basement). Two other quartz samples from along

the main N115F 55jNE trending fault (comb

quartz with epithermal-like textures devoid of

any inclusions suitable for study) were probably

deposited at low temperature, from a fluid with a

d18O composition of between � 9.5xand

2.5x. This is interpreted as a surface-derived

fluid (‘‘meteoric water’’) that had undergone

only limited interaction with basement litholo-

gies.

9. Model for the deposit formation and conclusion

9.1. Comparison with other deposits in the Valongo

Belt

A comparison with metasediment-hosted gold

deposits in the Valongo area (Ribeiro da Igreja, Alto

do Sobrido and Banjas; Couto, 1993) provides a

further insight into the influence of the host rocks

and P–T conditions in the formation of the gold

deposits. These deposits occur (i) in high-angle frac-

tures distributed subparallel to the axial plane of

Table 4

Summary of O stable isotope data versus SMOW for the different

quartz generation of the Castromil deposit

Quartz

type

Sample

reference

d18Oquartz

(x)

Estimated

crystallisation

temperature

d18O fluid (x)

Qm CAS 37

chip a

16.73 475F 25 jC 13 to 13.7

Qm CAS 37

chip b

17.10 475F 25 jC 13.4 to 14.6

Qm CAS 22 15.65 475F 25 jC 12 to 12.6

Qm CAS 4 15.26 475F 25 jC 11.6 to 12.2

Q1 CAS 30 15.13 425F 25 jC 10.6 to 11.4

Q1 CAS 26 14.08 425F 25 jC 9.6 to 10.5

Q1 CAS 32 14.39 425F 25 jC 9.9 to 10.7

Q2 CAS 33 14.45 425F 25 jC 9.9 to 10.8

Q3 CAS 32c

chip a

14.78 225F 25 jC 3.1 to 5.8

Q3 CAS 32c

chip b

14.82 225F 25 jC 3.2 to 5.8

Q3 CAS 32 d 14.23 225F 25 jC 2.6 to 5.3

Q4 CAS 19 11.23 190F 10 jC � 1.8 to � 0.4

Q4 CAS 2

chip a

14.8 140F 40 jC � 6.3 to 1.8

Q4 CAS 2

chip b

15.54 140F 40 jC � 5.6 to 2.5

Q4 CAS 7 11.58 140F 40 jC � 9.5 to � 1.4

d18O value for fluids are calculated using estimated crystallisation

temperature of quartz from fluid inclusion studies and the quartz–

H2O fractionation curve from Zheng (1993).


secondary antiforms, and along specific lithologies

(now occurring as clay-rich layers in between quartz-

ite formations) crosscut by these fractures, and (ii)

along subvertical faults, striking N045F 15jE (e.g.

subperpendicular to the main axis of the Valongo

anticline). The occurrence of ores in specific litholo-

gies was interpreted by Combes et al. (1992) as the

result of the remobilisation during deformation and

prograde low-grade regional metamorphism of ‘‘tur-

bidite-hosted’’ gold from saddle reefs systems. This

model is in contradiction with the available P–T data

and structural observations where the gold is seen to

be associated with microstructures that clearly post-

date the deformation and metamorphism. Thus, as at

Castromil, Couto (1993) and Couto et al. (1990) show

that at Ribeiro da Igreja and Montalto, the early ore

fluids, which are high-temperature aqueous-carbonic

fluids, and which predate the introduction of Sb–Au,

also postdate the peak of metamorphism and defor-

mation.

In Sb-rich deposits such as Alto do Sobrido, quartz

containing Sb-sulphosalts, stibnite and gold formed

from low-salinity aqueous fluids with moderate Th

(150–200 jC).

9.2. Metallogenic model for Castromil and Valongo

Belt deposits

The aplites that are emplaced in a N130–150jEdirection at the contact between metasedimentary

series and the granite, and in a N050jE direction,

defined the main zone for the different stages of fluid

percolation (Fig. 11). Aqueous-carbonic fluids

enriched in CH4 by interaction with black schists are

responsible for greisenisation, formation of the silici-

fied structure and subsequent precipitation of quartz

and sulphides (pyrite and arsenopyrite). At this stage

(Stage III), a significant uplift (around 5–6 km)

occurs, accompanied by localised fluid unmixing.

During the latter phase of uplift, a reactivation of

the main fault zone creates intense microfracturing of

the quartz structures and permits the circulation of

aqueous fluids. This important phase of fluid circu-

lation were facilitated by decompression of the sur-

rounding rocks as evidenced by the presence of

aqueous fluids in microfractures outside the main

channel zone. In addition, the rheological contrast

between the quartz veins and the host rocks explains

the preferential formation of gold ores within the early

(Q1 and Q2) quartz structures. The mixing of aqueous

fluids in the highly microfissured quartz structures

leads to the deposition of gold. The abundant associ-

ation of gold and Q3 in late microfractures affecting

arsenopyrite and pyrite shows that electrochemical

interaction between early sulphides and gold com-

plexes acted as an additional process for gold depo-

sition (Moller and Kersten, 1994).

According to Boiron et al., 2003, most of the fluids

associated with gold deposits in western Europe are

waters that have equilibrated at decreasing P and T

within the upper crust. The main driving forces behind

their migration are major discontinuities acting as

drainage zones (Fig. 11), the emplacement of syn-

tectonic and then post-tectonic plutons (aplite stocks

at Castromil for example), uplift and general decom-

pression of the rock units. The final stage is charac-

terised by a progressive dilution of the crustal fluids

by solutions penetrating the basement from the sur-

face. The penetration of waters during the main uplift

stage of the basement may allow gold to be dissolved

and transported from the metamorphic series and/or

pre-concentrations (as it was observed in the Vila

Pouca de Aguiar area, Noronha et al., 2000), and re-

deposited in structural and geochemical traps formed

during earlier stages of quartz deposition.

The deposition of gold at Castromil is similar to

that observed in other deposits in the Valongo region

(Couto, 1993) and in northern Portugal (Noronha et

al., 2000), and is characterised by two main stages:

(i) An early stage with pyrite and arsenopyrite

deposition associated with aqueous-carbonic fluid

resulting from water– rock interactions with

graphite-bearing schists.

(ii) A later stage with gold and bismuth precipitation

in fractures crosscutting earlier sulphides in

response to the mixing of hot (300 jC) and

relatively saline (10–12 wt.% eq. NaCl) aqueous

fluids with a dilute (0.5 wt.% eq. NaCl) and cold

(150 jC) aqueous fluid.

Marked similarities are found between the study

area and other European deposits, previously

described in the literature (French Massif Central,

Galicia, Spain, Portugal and Bohemian massif) with

respect to (i) the timing of gold introduction and the


sequence of mineral assemblages; (ii) the nature of the

fluids and their geochemical features; (iii) the origin

of the two main fluid types: fluids equilibrated with

the host metamorphic rocks and meteoric, the latter

fluid type having undergone rather intense interaction

with the host rocks during their migration through the

microfractured basement (Vallance, 2001; Vallance et

al., 2001).

Fig. 11. Diagramatic representation of the genetic model for the Castromil and Valongo gold deposits. (A) Intrusion of the syn to late biotite

granite controlled by secondary anticlines and contact metamorphism; (B) aplite intrusion along the granite–metasediment contact and

greisenisation; (C) formation of the Q1 stockwork and gold deposition by fluid mixing; (D) general model at the Valongo anticline scale

including data from Couto (1993). 1: black schists; 2: aplite; 3: contact metamorphic aureole; 4: greisen-type alteration; 5: quartz stockwork.


However, the Castromil deposit shows certain

differences, which are summarised in Fig. 11:

(i) The volumetric importance of aqueous-carbonic

fluids in a well defined zone. This major

percolation event implies strong hydrothermal

alteration, extensive greisenisation and an impor-

tant interaction with the graphitic schists. The

quantitative importance of aqueous-carbonic flu-

ids has not been reported for other gold deposits

in northern Portugal (and especially the meta-

sedimentary hosted gold deposit of the Valongo

anticline). However, the style of alteration style

has much in common with Sn–W deposits.

(ii) The absence of a Sb stage (rare sulphosalts). This

is probably due to the granite being relatively

depleted in Sb compared to the metasedimentary

rocks of the Valongo region (Castro Reis et al.,

1997), or to the complete absence of this late

stage.

(iii) A dramatic decrease in pressure with minimal

change in temperature. The very low pressure

(around or < l50 MPa) at high temperature (370–

400 jC) implies high heat flows which are

thought to be related to the presence of a

magmatic intrusion at shallow depth acting as a

strong heat engine and remobilizing gold from

the metasedimentary rocks. Aplite bodies, which

are spatially associated with the two deposits,

may be a distal expression of this late magma-

tism.

(iv) Gold deposition is linked to the mixing between

two aqueous fluids of relatively contrasted

salinity. This observation is at variance with

conventional mesothermal models where gold is

considered to be deposited from aqueous-car-

bonic fluids (Groves et al., 1998; McCuaig and

Kerrich, 1998; Mikucki, 1998). At Castromil,

aqueous-carbonic fluid circulation is related to an

earlier stage pre-ore stage that formed the main

gold trap (i.e. quartz veins with sulphides, mainly

arsenopyrite and pyrite).

(v) The inferred presence of a granite in the core of

the anticline based on the observation of apatite,

scheelite and wolframite in the earliest quartz

veins (Couto,1993). P–T reconstruction also

supports the presence of a granite at depth (Fig.

11). Indeed, the relatively low pressure and high

temperature (up to 400 jC for 80 MPa) imply a

geothermal gradient of around 60 jC/km, which is

quite common at the exocontact zone of magmatic

intrusions. However, no evidence of magmatic

fluids were found (such as saline brines or Ca-, K-,

Li-rich, high-temperature fluids). The granite

acted therefore solely as an heat engine, enhancing

hydrothermal circulation but not contributing

directly to the metals or fluid fluxes.

Acknowledgements

This work was facilitated by the EC funded

CRAFT Project ‘New technologies for the exploration

of high grade oreshoots’ (contract no. BRST CT98

5397). Dr. Carlos Nascimento (Connary Minerals) is

especially acknowledged for his technical and scien-

tific assistance whilst researching the Castromil

deposit, and for having permitted release of the

present data. The Instituto Geologico e Mineiro (IGM)

is acknowledged for its assistance during field work.

The authors wish thank R. Moritz, S. Hagemann and

F. Noronha for comments that have helped to improve

the manuscript. This work has been completed in

2001–2002 with the help and within the framework

of GDR TRANSMET, especially stable isotope

analyses. [RR]

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