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Remobilisation of base metals and gold by Variscan
metamorphic fluids in the south Iberian pyrite belt:
evidence from the Tharsis VMS deposit
Christian Marignaca,*, Bocar Diaganab, Michel Cathelineaub,Marie-Christine Boironb, David Banksc, Serge Fourcaded,
Jean Vallanceb
aCRPG-CNRS, BP20, 54501 Vandoeuvre-les-Nancy Cedex, FrancebCREGU and UMR G2R 7566, BP 23, 54 501Vandoeuvre-les-Nancy Cedex, France
cSchool of Earth Sciences, Leeds University, Woodhouse Lane, Leeds, UKdGeosciences Rennes, Campus de Beaulieu, 35042 Rennes Cedex, France
Received 14 February 2002; received in revised form 25 June 2002
Abstract
The Tharsis massive sulphide deposit, one of the major VMS-type deposits in the Iberian pyrite belt (IPB) was severely
deformed by the Variscan tectono-thermal events. The question of whether or not these events affected the metal distribution in
the deposit has been addressed by simultaneously studying the mineral parageneses (Tharsis stockwork) and the fluid
circulation (at local and regional scales). The results are:
(1) The early paragenesis in the stockwork (Q1 quartz–pyrite–chlorite–phengiteF cobaltiteF ankerite) was strongly
overprinted by a late post-kinematic mineral deposition, including new quartz veins (Q3 quartz) and base metal sulphides
(chalcopyrite, sphalerite, Bi and Te minerals, pyrite and galena) and gold.
(2) At a regional scale, fluids accompanying the peak metamorphism conditions (ca. 300 MPa, ca. 300 jC) were of C–O–H–
N–NaCl type, CO2-dominated with CH4 and N2, and are considered to be ‘‘metamorphic’’ on the basis of microthermometry
and geochemistry. The late- to post-kinematic evolution (‘‘retrograde’’ stage) was characterised by a pressure drop, down to
40 MPa (lithostatic to hydrostatic transition), and a heat input leading to temperatures z 430 jC, then decreasing to
temperature around 170 jC. Fluids of the ‘‘retrograde’’ type exhibit both dilution of the C–O–H–N–NaCl fluid by a low
salinity ‘‘meteoric’’ water and progressive loss of volatile components.
(3) Fluids of the retrograde type pervasively percolated through the Tharsis stockwork and were responsible for the strong
mineral overprint on the early (deformed) paragenesis. All the measurable fluid inclusions (f.i.) record these late fluids.
There are primary fluid inclusions in Q1, but they are systematically imploded due to the external overpressure generated
by the Variscan tectonic events. Although base metal distribution in the stockwork is basically the result of the
0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
PII: S0009 -2541 (02 )00275 -9
* Corresponding author.
E-mail address: [email protected] (C. Marignac).
www.elsevier.com/locate/chemgeo
Chemical Geology 194 (2003) 143–165
‘‘retrograde’’ fluid circulation, it remains unclear whether these metals were newly introduced into the stockwork from
elsewhere or simply redistributed from the existing primary assemblages in the massive sulphide bodies.
D 2002 Elsevier Science B.V. All rights reserved.
Keywords: VMS; Iberian pyrite belt; Gold; Fluid inclusions; Metamorphic fluids
1. Introduction
The Iberian pyrite belt (IPB) is the largest prov-
ince of volcanogenic massive sulphide (VMS) depos-
its in the world. A great deal of structural,
petrological and geochemical data has been published
and were recently summarised (Leistel et al., 1998a;
Saez et al., 1999; and references therein). The depos-
its were formed during the early Carboniferous
(Tournaisian) in a transtensional environment charac-
terised by abundant bimodal volcanism and coeval
sedimentation of black shales (the VS Group) in
several E–W trending troughs. These troughs were
developed through the dislocation of a former plat-
form characterised by the late Devonian shales and
quartzites of the PQ Group. Starting from the Visean,
the IPB was incorporated in a south-verging fold-
and-thrust belt with progressive evolution of the
deformation from North to South (Ribeiro and Silva,
1983; Ribeiro et al., 1990; Quesada, 1998; Onezime
et al., 2001) and coeval sedimentation of syntectonic
flyschs (the ‘‘Culm’’). The VMS deposits suffered
both ductile deformation and thermal overprinting
(e.g., Tornos et al., 1998), under the weak metamor-
phic conditions of the prehnite-pumpellyite facies
(Munha, 1990).
Several fluid inclusion studies have been recently
undertaken in the huge VMS deposits of South Spain,
in an attempt to reconstruct ore-forming conditions
(e.g., Almodovar et al., 1998; Nehlig et al., 1998).
However, the possibility that metamorphic fluids could
have been trapped within the deposits, and possibly
overprinted the earlier, ore-depositing fluids in some
instances, was not considered in these works or was
disregarded (Nehlig et al., 1998). In addition, there are
until now no data on the metamorphic fluids at the
regional scale in the whole IPB. Nevertheless, it has
been shown, in the Neves-Corvo deposit in Portugal
(Moura et al., 1997a,b) that metamorphic fluids had
indeed percolated through the deposit and were res-
ponsible for significant mass transfer, involving ore-
forming elements.
The present study addresses the question of compar-
ing fluid circulating at the regional scale with the fluids
involved in the genesis and subsequent modifications
of the Iberian VMS deposits. For this purpose, the
Tharsis area was chosen because: (i) the Tharsis deposit
is well known through recent publications (Tornos et
al., 1998) and several exploration drill cores were
available (courtesy of SEIEMSA); and (ii) good out-
crops with suitable lithologies are present in the deposit
area. At Tharsis, the study was focused on the stock-
work, since it is currently considered that primary gold
is mainly concentrated in the stockworks of the Iberian
VMS deposits (Leistel et al., 1998b).
The study was conducted on a multi-disciplinary
basis and encompassed ore petrography and fluid
inclusion study (microthermometry and Raman micro-
spectrometry), bulk analysis of the fluids (crush-leach
experiments) and stable isotope (d18O) analysis.
2. Geological setting of the Tharsis deposit
With 120 Mt ore, the Tharsis deposit is one of the
largest VMS ore bodies in the IPB. The deposit is
representative of those Iberian VMS deposits that are
associated more closely with black shales than with
penecontemporaneous acid volcanism (Saez et al.,
1999). It is located in the so-called ‘‘Puebla de Guz-
man anticlinorium’’, in fact an antiformal stack of
nappes (Fig. 1) resulting from the inversion of an
early Carboniferous synvolcanic extensional basin
(Quesada, 1998; Tornos et al.,1998). At the mine
scale, three subunits of the Puebla de Guzman nappe
are stacked onto shales and quartzites of the late
Devonian PQ Group (Fig. 2). These units are dis-
membered parts of the regional Volcano-Sedimentary
(VS) Group of Tournaisian age. From bottom to top:
(i) the lower unit (LU), comprising massive sulphides,
C. Marignac et al. / Chemical Geology 194 (2003) 143–165144
Fig. 1. Geological sketch of the Tharsis area, simplified from the IGME (1982) map and adapted from Quesada (1998) for the nappe structure.
C.Marig
nacet
al./Chem
icalGeology194(2003)143–165
145
shales and a stockwork; (ii) the intermediate unit,
(IU), mainly massive spilites (a former sill, with
hydrothermal ankerite-rich breccias); and (iii) the
upper unit of silicified rhyolites. The LU displays a
complex internal structure (Fig. 2), with two massive
sulphide bodies (Filon Norte and San Guillermo) and
an intercalated sheet of shales, containing the pyrite–
chlorite–quartz stockwork (interpreted as the feeder
of the San Guillermo body). It is likely that the Filon
Norte and San Guillermo bodies were initially part of
a single VMS deposit (Tornos et al., 1998).
The massive sulphides are mainly pyrite, with
subordinated sphalerite, chalcopyrite and galena, and
a host of minor sulphides (including Bi minerals) and
sulphosalts (mainly tetraedrite). Arsenic minerals are
rare, mostly expressed in the stockwork, with the
specific presence of cobaltite (thought to be associated
with gold; Leistel et al., 1998b). The massive ores are
mainly of the banded types, including carbonated ores
(with siderite) that are specific to Tharsis. The deposit
is renowned for its numerous ‘‘synsedimentary’’ tex-
tures involving the banded ores (Tornos et al., 1998).
Synkinematic hydrothermalism is present, mainly
in the PQ group (pyrite and chlorite veinlets and
disseminations) (Tornos et al., 1998). However,
although the massive sulphides and the stockwork
evidently suffered strong synfolial deformation, and
the ore minerals display strong evidence of recrystal-
lisation, it is claimed that there was no significant syn-
or post-kinematic ore deposition (Tornos et al., 1998).
Fig. 2. Cross-section of the Tharsis open pit (adapted from Tornos et al., 1998; SEIEMSA, unpublished documents).
C. Marignac et al. / Chemical Geology 194 (2003) 143–165146
3. Paragenetic succession in Tharsis stockwork
The Variscan tectono-thermal events are well
expressed at the sample scale. In good agreement
with the results of structural analysis at the regional
(Onezime et al., 2001) and local (Tornos et al., 1998)
scale, we find consistently two schistosities (often at
right angles) in the studied samples. It is therefore
easy to partition the observed mineral associations
into specific assemblages by considering the timing of
crystallisation relatively to the Variscan deformation,
thus defining three main stages, ante-, syn- and post-
kinematic, respectively (Fig. 3). This is particularly
evident in the stockwork samples where deformation
features are by far more conspicuous than in massive
ore.
(a) The early stage comprises all the ante-kine-
matic minerals and is thought to represent the initial
(syn-VMS) paragenesis in the stockwork. These min-
erals occur as networks of veins and veinlets, with
three types: (i) barren quartz veins (Q1); (ii) frequent
pyrite veinlets with chloritised walls overprinting type
(i); disseminated pyrite within chlorites is also found;
and (iii) rare quartz (Q1)F ankeriteF phengite veins
and veinlets, with sulphides, that are associated to
carbonated wall-rocks. A special mention must be
made for Co-bearing minerals (cobaltite CoAsS,
glaucodot [Co, Fe]AsS) occurring either as isolated
crystals associated with pyrite in the type (iii) veinlets
(Fig. 4a), or as independent veinlets in chloritised
black shales, and being clearly ante-kinematic in both
cases. This early cobalt-bearing paragenesis was first
described by Marcoux et al. (1996) and is thought to
be closely related with gold occurrence in the shale-
hosted deposits of the Tharsis type (Leistel et al.,
1998b).
(b) The synkinematic stage is principally marked by
recrystallisation and neocrystallisation in ‘‘pressure
shadows’’ and/or ‘‘pull apart’’ structures around early
sulphides at the microscopic scale: mainly, quartz
fibres (Q2), and subordinated chlorite (Fig. 4a,b). Mass
transfer remained of limited importance, although a
few synkinematic structures may be found at the out-
crop scale outside the stockwork: mainly tension
gashes filled with Q2 within the massive sulphide ore
(Filon Norte). A synkinematic Q2 stockwork (includ-
ing hydraulic fractures) is locally known at the footwall
of the IU (Fig. 5), but there is no evidence for the
superimposition by a significant Q2 stockwork of the
early stockwork underlying the massive sulphides.
(c) The late- to post-kinematic stage is characterised
in the mineralised stockwork by the inception of
significant fluid circulation, as evidenced by the devel-
opment of new mineralised veins and veinlets and the
crystallisation of a polymetallic sulphide assemblage.
There is a distinct late-kinematic stage, with for-
mation of a network of quartz (Q3), phengite and
chlorite veinlets, that clearly overprint synkinematic
assemblages, but are deformed by some late incre-
ments of the Variscan deformation. In particular, the
Q3 quartz systematically exhibits undulose extinction
typical of a slight plastic deformation (Fig. 4c). Also,
vermicular phengites and/or chlorites (Fig. 4d), that
crosscut plastic deformation features in the Q1 quartz,
are currently kinked, or even foliated in some instan-
ces. In addition, the deformed Q1 quartz or the Q2
fibres are often locally partially recrystallised into
patches of a Q2–3 quartz exhibiting the same defor-
mation features as the Q3 quartz and therefore con-
sidered to be coeval. Arsenopyrite is rare and clearly
crystallised during this late-kinematic stage.
The post-kinematic stage itself is typified by (i) the
crystallisation of a hyaline undeformed Q4 quartz in
small dissolution vugs, together with late chlorite
(undeformed radiated booklets) and calcite; and (ii)
the deposition of polymetallic sulphides in micro-
cracks (particularly, within early pyrite) and dissolu-
tion patches. That the sulphides were coeval with Q4
is demonstrated by their filling of vugs rimmed by Q4
crystals. The main minerals are sphalerite, chalcopyr-
ite (with typical ‘‘chalcopyrite disease’’ features in
sphalerite), pyrite and galena, crystallising in this
order. A few undetermined Bi and Te minerals (likely,
joseite) were found in this study, generally coeval
with chalcopyrite. According to the observations of
Leistel et al. (1998b), gold was deposited together
with this late paragenesis containing chalcopyrite and
Bi–Te minerals.
Thus, to the contrary of earlier conclusions (Tornos
et al., 1998), it seems that a significant hydrothermal
event occurred within the Tharsis stockwork at the end
of the Variscan tectono-thermal event. This hydrother-
mal event was indeed responsible for the late poly-
metallic gold-bearing assemblage that was formerly
considered to represent a late influx of high-temper-
ature (up to 400 jC) fluids at the end of the massive
C. Marignac et al. / Chemical Geology 194 (2003) 143–165 147
Fig. 3. Paragenetic succession in the Tharsis stockwork.
C. Marignac et al. / Chemical Geology 194 (2003) 143–165148
Fig. 4. Details of the paragenetic relationships in the Tharsis stockwork: (a) two generations of Q2 quartz fibres around cobaltite (THA 12–
111.4); (b) same around pyrite (THA 12–111.4); (c) late-kinematic Q3 quartz veinlet with chlorite (chlorite 3) selvages (THA 15–30.20); (d)
late-kinematic chlorite 3 vermicules growing onto grain boundaries of deformed Q3 quartz (THA 15–30.20); (e) late sphalerite overprinting S2
schistosity; note that sphalerite overgrew phengite booklets (THA 12–143.85); (f) late Bi–Te mineral (joseite?) in microcracks in earlier (late
kinematic) arsenopyrite (MIS).
C. Marignac et al. / Chemical Geology 194 (2003) 143–165 149
sulphide deposition (Marcoux et al., 1996; Leistel et al.,
1998b). It may be speculated that the late-kinematic
inception of hydrothermal circulation was related to the
brittle–ductile transition allowing fluid ingress in the
stockworks through the still active thrust planes. Evi-
dencemay be found in the THA 15/120 sample, located
close to the basal contact of the LU (Fig. 1), where late
Q3-chlorite hydrothermal veining is conspicuous.
4. Selection of samples for fluid inclusion and
geochemical studies
For the fluid inclusion and geochemical studies, we
selected representative samples from the Tharsis open
pit and from quartz veins in regional settings.
– At Tharsis (Fig. 2), the samples were taken in the
massive sulphide and the host formations sampled in
the open pit and from one drill core (THA 15) carried
out by SEIEMSA:
(i) In the stockwork: underlying the San Guillermo
body (THA 15–39 and THA 15–105.5) and close
to the phyllonite at the basis of the LU (THA 15–
120).
(ii) In the main open pit (Filon Norte), mainly Q2
quartz: N80jE25jN tension gashes in the massive
pyrite (samples TH1FT, with late sulphides:
sphalerite, chalcopyrite; and THA-1FT3); a
N85jE20jN tension gash in the spilites of the
IU (sample THA-3D); a folded quartz vein (likely,
refolded early tension gash) in the shales from the
IU (sample THA-3C); a tension gash with fibrous
Q2 quartz in the ignimbrite of the UU (THA 5). In
addition, Q4 quartz and an associated dolomite
were sampled from late quartz–chlorite–dolo-
mite veins in the spilite of the IU (sample TH4).
The Tharsis mine samples allowed the study of
fluid inclusions in the Q1, Q2–3, Q3 and Q4 quartz
generations (the Q2 fibres being usually devoid of
fluid inclusions). In the TH1FT and THA-3C samples,
the quartz veins are locally recrystallised into a clear
quartz (cQ), particularly at the quartz grain bounda-
ries; this cQ quartz is likely coeval with Q3. In
TH1FT, a chalcopyrite veinlet overprinted cQ and
was associated with euhedral overgrowths of a hyaline
quartz (hQ) (likely coeval with Q4) on cQ.
Fig. 5. Synkinematic hydraulic fracturing at the footwall of the
intermediate unit (sample THA 3B).
Fig. 6. Typology of fluid inclusions: (a) imploded early fluid inclusions in the Q0 quartz from Virgen de la Pena, (b) stripe of recrystallised clear
cQ quartz with Lc –w inclusions from Valverde, (c) rim of clear cQ quartz with a few secondary (FIP) Lc –w inclusions from Valverde, (d) a
sequence of clear cQ quartz (with Lc –w inclusions) and vuggy hyaline hQ quartz (with rare Lw inclusions in healed planes) from SP 110, (e) a
typical Lc –w inclusion from El Morante, (f) healed planes of Lw inclusions in Q4 associated with sphalerite (Sph) from THA 15–105.5.
C. Marignac et al. / Chemical Geology 194 (2003) 143–165150
– For an overview of the fluids percolating at a
regional scale, two sets of samples were selected:
(i) From the close vicinity of the Tharsis deposit, in
rocks from the same major tectonic unit that the
deposit (the Puebla de Guzman nappe) or from
the underlying Almendro nappe (Fig. 1):
– Quartzites from the PQ Group of the Virgen
de la Pena subunit in the Puebla de Guzman
nappe. The sample was taken at the top of the
Virgen de la Pena hill, in huge quartzite
masses containing a stockwork of ante-kine-
matic (folded) subvertical quartz veins with
the N30j, N150j and N170jE directions
(sample Vir Pe). At the microscopic scale,
the early Q0 quartz exhibits evidence of
plastic deformation, overprinted by a clear
quartz (cQ) in healed fractures and at
recrystallised grain boundaries. A late hyaline
more or less vuggy quartz (hQ) occurs in
recrystallisation/dissolution patches.
– Ignimbrites from the VS Group, at Villanue-
va de las Castilleros, in the Almendro nappe,
close to the thrust contact with the under-
lying Culm (Fig. 1). The sample comes from
a set of ante-kinematic quartz veins (sample
SP 110). At the microscopic scale, the early,
plastically deformed, Q0 quartz is over-
printed by a series of tension veins with
palissadic quartz (likely coeval with Q2 at
Tharsis), that exhibits late recrystallisation
features, with a clear quartz (cQ) at quartz
grain boundaries and eventually a vuggy
hyaline quartz (hQ) (Fig. 6d), that are likely
to be coeval with similar features in the Vir
Pe, THA-1FT and THA-3C samples.
(ii) From a late south verging thrust within the
Valverde nappe (Fig. 1), a probable equivalent of
the Puebla de Guzman nappe. These shales of
the PQ Group are upthrusted onto shales of the
VS Group. In the PQ shales, the S1 foliation is
folded (F2 folds, Onezime et al., 2001) and
contains, about 30 cm above the main thrust, a
series of synfolial quartz lenses (10 cm� a few
cm) paralleling the thrust plane. These are
deemed to be coeval with the thrusting and are
therefore considered to be representative of Q2
(sample VA).
(iii) From a Mn-bearing chert lens of the VS Group, in
the Berrocal nappe (Fig. 1), a probable equivalent
of the Paymogo nappe that overlies the Puebla de
Guzman nappe. The sample was taken at the top
of the El Morante hill, from a set of quartz-filled
tension gashes (Q2) inside the chert lens (El
Morante sample).
In both the VA and El Morante samples, the Q2
quartz displays the features of late recrystallisation
that were described above, with development of clear
quartz (cQ) along grain boundaries.
5. Fluid inclusion study
5.1. Methods
Petrography of fluid inclusions has been carried out
on 300-Am thick double-polished wafers. Microther-
mometric characterisation of the fluids was performed
using a Chaix-meca heating-freezing stage (Poty et al.,
1976) at low temperature (V 30 jC) and a Fluid Inc.
Stage for measurements at higher temperatures. The
accuracy was of F 0.1 jC at low temperature and of
F 2 jC at high temperature. Salinity, expressed in
equivalent wt.% NaCl, and fluid density of gas-free
fluid inclusions, are determined from melting temper-
atures of ice using the regression of Bodnar (1993). In
volatile-bearing fluid inclusions, CO2 was identified
by melting of a solid at TV� 56.6 jC. The volumetric
fractions of the volatile phase(s) (/vol) was estimated
at room temperature by reference to the volumetric
chart of Roedder (1984). Molar fractions of CO2, CH4,
N2 and H2S were determined in individual fluid
inclusions using a DILOR LABRAM Raman spec-
trometer following the procedure given by Dubessy
(1984). Molar fraction of NaCl, molar volume, bulk
composition and P–T parameters were determined by
combining results from microthermometry and Raman
analysis using the computer code by Bakker (1997).
The P–V–T properties of volatile-bearing inclu-
sions were modelled in the system H2O–CO2–CH4
using the V–X data and the equation of state of
Kerrich and Jacobs (1981) and Jacobs and Kerrich
(1981). For the purely aqueous fluids, the data from
Zhang and Frantz (1987) for the H2O–NaCl system
have been used.
C. Marignac et al. / Chemical Geology 194 (2003) 143–165152
5.2. Typology and relationships
Nomenclature follows that of Boiron et al. (1992).
Briefly, the fluid inclusions (f.i.) are named first on
the basis of the mode of their final homogenisation as
either liquids (noted L) or vapour (noted V); then
according to the nature of their components, noted by
a subscript, c for CO2-dominated volatile species, w
for water; for instance two-phase aqueous f.i. homog-
enising in the liquid phase are noted Lw. A specific
case must be made for mixed inclusions. For
instance, three-phase f.i. at room temperature that
finally homogenise in the liquid phase are noted as
Lc–w, whereas those inclusions homogenising to the
liquid that remain apparently two-phase even at low
temperature, but display the presence of volatile
species (more commonly as clathrates) are noted
Lw–c. In the case of the volatiles being only detect-
able by Raman microspectroscopy, the notation
becomes Lw–(c). Similar notations hold for V-type
inclusions.
5.2.1. Early fluid inclusions
In the Tharsis stockwork, there is evidence of some
primary f.i., in early Q1 with preserved growth zones.
However, these inclusions display features of post-
trapping modifications typical of implosion processes
according to the experimental data of Vityk and
Bodnar (1995), in particular, they frequently exhibit
stellar or annular morphologies. It is likely that these
inclusions imploded during the Variscan tectono-
metamorphic events. Unfortunately, as a result, these
primary inclusions have become unsuitable for micro-
thermometric measurements. Consequently, any P–
T–X information relevant to the deposition of the
massive sulphides is lost.
In the Vir Pe sample, numerous similarly imploded
inclusions are also observed (Fig. 6a), in healed
planes with secondary inclusions (FIP). Since the
host quartz veins are apparently ante-kinematic, it
may be speculated that they were coeval with the
primary f.i. in the Q1 quartz from the Tharsis stock-
work, i.e., that the early Q0 quartz stockwork in the
PQ quartzites could have resulted from the same
hydrothermal event that was responsible for the
VMS formation.
5.2.2. Volatile-rich inclusions
Lc–w inclusions were systematically found in the
clear quartz (cQ) from the Vir Pe, VA and El Morante
samples (Fig. 6b–e). There they mainly occur in
healed microfractures (‘‘fluid inclusion planes’’: FIP;
Roedder, 1984; Cathelineau et al., 1994), but are
locally present as isolated clusters. Vc–w inclusions
are also found, equally in FIP, but seem to be
restricted to cQ quartz in sample Vir Pe. Significantly,
no volatile-rich inclusions were found in the samples
from the Tharsis stockwork.
5.2.3. Aqueous-dominated inclusions
Lw type f.i. are found in all samples and all types of
quartz, generally in planar arrays cross-cutting grain
boundaries or as clusters in hyaline hQ quartz and in
Q4. Lw–c and Lw–(c) inclusions are also observed in
Table 1
Summary of the microthermometric data for the different types of fluid inclusions
Fluid inclusion type Location Tm CO2 Th CO2 Tm cl Tm ice Th
Lc–w Virgen de la Pena � 56.8/� 58.2 25/29 (L) 7.5/8.5 � 3.2/� 5.4 265/300
Valverde (VA) � 56.6/� 58.2 24/29 (L/V) 6.5/7.7 � 5/� 3 220/250
El Morante � 57.6/� 57.2 20/24 (V) 8/12 � 4.2/� 5.7 200/280
Lw– c Tharsis 3.3/3.6 � 3
Lw– (c) Tharsis � 1/� 3.9 140/360
Vw– c Tharsis 4.4/8.2 � 2/� 2.6 250/380
Vw– (c) Tharsis � 2.2/� 2.8 340/380
Villanueva � 3.4/� 3.5 330/420
Lw Virgen de la Pena � 3.2/� 4.3 150/250
Tharsis � 1/� 5.9 130/240
Tm CO2: melting temperature of CO2; Tm ice: melting temperature of ice; Tm cl: melting temperature of clathrate; Th CO2: homogenization
temperature of CO2, Th: total homogenization temperature. L: liquid, V: Vapor. All temperatures are given in jC.
C. Marignac et al. / Chemical Geology 194 (2003) 143–165 153
Table 2
Chemical composition of typical fluid inclusions from the studied areas obtained by Raman microprobe analysis and corresponding
microthermometric data
Sample IF Type Microthermometry Raman data Bulk Composition
numberTm
CO2
Th
CO2
Mode Tm
ice
Tm cl Th Mode CO2 CH4 N2 H2S H2O CO2 CH4 N2 NaCl
Tharsis
TH1/1ft 11 Vw– c no no – � 2.4 5.2 285 V 74.3 25.7 nd nd
TH1/1ft b Vw– c no no – � 2.5a 5.2 273 V 82.6 17.4 nd nd 92.1 6.5 1 nd 0.4
TH1/1ft 12 Lw– (c) no no – � 2.3 193 L 76.6 23.4 nd nd
TH1/1ft 13 Lw– (c) no no – � 2.3 146 L 83.3 16.7 nd nd
THA 15/39 1 Lw– c no no – � 3a 3.3 90.5 5.6 3.9 nd 95.7 3.4 0.1 0.1 0.7
THA 15/39 2 Lw– c no no – � 3a 3.6 84.4 6.4 9.2 nd 96 3.1 0.1 0.1 0.7
THA 15/39 12b Lw– (c) no no – � 2.6 276.5 L 95.1 3.2 1.7 nd
THA 15/39 11 Lw– (c) no no – � 5.8 176.2 L 94.3 5.7 nd nd
THA 15/120 17 Lw– (c) no no – � 1.8 221.5 L 24.5 18.5 57 nd
THA 15/120 24 Lw– (c) no no – � 1.6 198.2 L 16.3 18.2 65.5 nd
THA-3C 1 Lw– (c) no no – � 2.6 254.8 L 87 4 9 nd
THA-3C 12 Lw– (c) no no – � 2.6 219.6 L 88.4 2.3 9.3 nd
THA-3C 9 Lw– (c) no no – � 2.8 220.2 L 93.8 6.2 nd nd
THA-3C 13 Lw– (c) no no – � 2.8 282.3 L 86 5.6 8.4 nd
THA-3C 1v Lw– (c) no no � 3.6 245 L 81.6 5.2 13.2 nd
THA-3C 4v Lw– (c) no no � 3.3 d 65.3 10.7 24 nd
THA-3C 3 Vw– (c) no no – � 2.6 345.2 V 92.8 3.1 4.1 nd
THA-3C 22 Vw– c no no – � 2.6 8.2 250.3 V 73.1 6.2 20.7 nd 82.3 13.4 1 3.2 0.1
THA-3C 7 Vw– c no no – � 2.4 4.9 380 V 95.0 nd 5.0 nd 89.5 9.8 nd 0.4 0.3
THS 20 HS1 Vw– c no no – � 2 4.4 254.3 V 91.2 8.8 nd nd 90.3 8.9 0.7 nd 0.1
THS 20 HS2 Lw– (c) no no – � 1.8 167.6 L 71.5 28.5 nd nd
THS 20 HS13 Lw– (c) no no – � 1.7 152.5 L 78.4 21.6 nd nd
THS 20 HS3 Lw– (c) no no – � 1.2 174.3 L nd 100.0 nd nd
Villanueva
SP 110 1 Vw– (c) no no – � 5.5 8.2 375.7 V 85.8 3.5 10.7 nd 92.1 6 0.2 0.4 1.3
SP 110 2 Vw– (c) no no – � 4.9 8.5 380.2 V 87.5 5 7.5 nd 92.4 6 0.2 0.3 1.1
SP 110 3 Vw– (c) no no – � 5.2 8.5 375.6 V 88.3 5.2 6.5 nd 91.1 7.3 0.3 0.4 1
SP 110 1v Vw– (c) no no no 449 V 85.9 11.1 3 nd
SP 110 7v Vw– (c) no no no 405 V 81.6 5 13.4 nd
SP 110 2v Lw– (c) no no � 3.5 331 L 69 5.7 25.3 nd
Virgen de la Pena
Vir Pe -Sp2 a Lc –w � 58.1 22.7 L � 4.3 7.7 302.2 L 96.2 1.6 2.2 nd 84.6 13 0.2 0.2 2
Vir Pe -Sp2 b Lc–w � 57.2 22.8 L � 4.4 7.6 285.5 L 93.1 nd 6.9 nd 77.2 19.6 nd 1.3 1.9
Vir Pe -Sp2 c Lc –w � 57.2 24 L � 5.1 7.7 285.2 L 96.8 1.5 1.7 nd 80.1 17.7 0.2 0.3 1.8
Vir Pe -Sp2 h Lc–w � 57.2 22.8 L � 4.8 7.7 285.4 L 98.4 1 0.6 nd 73 25.2 0.2 0.1 1.5
Vir Pe -Sp2 s Lc –w � 57.3 24 L � 4.2 8.4 285.1 L 98.3 1.1 0.6 nd 83.9 13.7 0.1 0.1 2.1
Vir Pe -Sp2 q Lc–w � 56.8 24 L � 4.7 8.4 283.5 L 97.9 1.5 0.6 nd 86.3 11.4 0.1 0.1 2.1
Vir Pe -Sp2 1 Lc–w � 57.2 26.3 V – 8.5 286.5 L 98.4 1.6 nd nd 87 9.8 0.1 0 3.1
Vir Pe -Sp2 11 Lc–w � 57.2 no – � 4.1 8.4 283.9 L 96.9 nd 3.1 nd 94.7 4.2 nd 0.05 1.05
Valverde
VA 4 Lc–w � 57.6 25.3 L � 4.4 6.5 250.5 V 99.3 0.7 nd < 0.1 92.3 5.45 0.05 nd 2.2
VA 10 Lc–w � 57.8 24.3 L � 5.2 6.5a 230.5 L 99.2 0.7 nd 0.1 83.6 14.3 0.1 nd 2.0
VA 8 Lc–w � 57.2 25.2 L � 3.9 6.5a 230.4 L 99.5 0.4 nd 0.1 83.7 14.2 0.1 nd 2.0
C. Marignac et al. / Chemical Geology 194 (2003) 143–165154
healed fractures, mainly in Q2–3, Q3 or cQ quartz.
They are absent from the Q4 or hQ quartz, where only
the Lw inclusions may be found (Fig. 6f) (they are
sometimes also present as isolated clusters). Thus, the
Lw–c or Lw–(c) inclusions appear more akin to the
volatile-rich inclusions, whereas the Lw inclusions are
seemingly coeval with Q4 and therefore with the post-
kinematic stage of fluid circulation and ore deposition
(polymetallic sulphides and gold).
In the SP 110 sample, some patches of clear cQ
quartz exhibit healed fractures with vapour-rich
inclusions, containing a low-density volatile phase
when examined by Raman microspectrometry, and
therefore representative of the Vw–(c) class of inclu-
sions.
5.3. Microthermometry and Raman analysis
A summary of the main f.i. types and microther-
mometric measurements is given in Table 1. For all
the measured inclusions, the first melting temperature
was found to be close to � 21 jC, indicating that the
fluid salinity may be modelled by considering only
the NaCl content in all cases.
5.3.1. Volatile-rich inclusions
5.3.1.1. Lc–w inclusions. Lc–w f.i. have melting
temperatures of CO2 (Tm CO2) ranging from
� 56.6 to � 58.2 jC and homogenisation temper-
atures of CO2 (Th CO2) to the liquid or to the vapour
phase in the range of 20–29 jC (Table 1). Tm CO2
values are significantly lower than the melting point
of pure CO2 and indicate the presence of volatile
compounds other than CO2. The melting temperature
of ice (Tm ice) ranges from � 3.2 to � 5.7 jC with
a mode around � 4.5 jC. The melting temperature of
clathrate (Tm cl) is in the range of 6.5–12 jC with a
mode around 8 jC. The total homogenization tem-
perature (Th to the liquid phase) ranges from 200 to
300 jC.
5.3.2. Aqueous-dominated inclusions
5.3.2.1. Volatile-bearing aqueous inclusions. Most
volatile-bearing aqueous inclusions in the Tharsis
samples are aqueous two-phase inclusions homoge-
nising in the liquid or the vapour state (/vap from 0.1
to 0.8). They are usually small (8–13 Am). Clath-
rates are observed in a few cases (rare Lw–c inclu-
sions, see Table 1; more frequent Vw–c inclusions).
However, in many inclusions with apparently no
clathrate formation upon freezing, Raman micro-
spectrometry revealed the presence of low-density
volatile components (CO2[N2–CH4) (Table 2).
These are either liquid- or vapour-rich, although
most vapour inclusions appear to be pure water.
Vw–c fluid inclusions show Tm cl in the range of
4.4–8.2 jC. Tm ice ranges from � 2 to � 2.6 jC.The total homogenisation temperatures (Th to the
vapour phase) are rather scattered and range from
250 to 380 jC.
Table 2 (continued )
Sample IF Type Microthermometry Raman data Bulk Composition
numberTm
CO2
Th
CO2
Mode Tm
ice
Tm cl Th Mode CO2 CH4 N2 H2S H2O CO2 CH4 N2 NaCl
El Morante
El Morante 8 Lc –w � 56.7 22.5 V � 5.3 9.5 230.5 L 99.2 0.8 nd nd 94.3 5.3 0.02 nd 0.4
El Morante 5 Lc –w � 57.2 24 V � 4.3 8 250 L 99.7 0.3 nd nd 95.3 5.4 0 nd 1.3
El Morante 17 Lc –w � 57.5 20.5 V � 5.5 10a 240.3 L 99.2 0.5 0.1 nd 94.8 5.1 0.02 nd 0.1
El Morante 7 Lc –w � 56.7 24 V � 5.4 9 254.6 L 100 nd nd 0.1 94.1 5.3 0 nd 0.6
El Morante 14 Lc –w � 57.2 22 V � 5.5 9.5 265 L 100 nd nd nd 94.6 5.1 nd nd 0.3
El Morante 15 Lc –w � 57.5 24 V � 5.3 9.5 250.6 L 99 0.6 0.4 nd 92.4 7.1 0.03 0.01 0.4
El Morante 21 Lc –w � 57.2 20 V � 4.9 8.5 200.6 L 100 nd nd nd 92.8 6.3 nd nd 0.9
El Morante 20 Lc –w � 57.5 22.5 V � 5.6 10a 250.6 L 99.2 0.6 0.1 0.1 93 6.9 0.02 nd 0.1
Compositions are given in mol%. Abbreviations are the same as in Table 1. nd: not determined. no: not observed. Inclusion types are defined in
the text.a Indicates that temperatures have been estimated.
C. Marignac et al. / Chemical Geology 194 (2003) 143–165 155
5.3.2.2. Vw– (c) and Lw– (c) inclusions. Both types of
inclusions have similar melting temperatures of ice
(Tm ice comprised between � 1 and � 3.9 jC), butdiffer significantly when total homogenisation temper-
atures are considered. The Vw–(c) inclusions of the SP
110 (Villanueva) sample homogenise at rather constant
high temperatures (Th from 330 to 420 jC), whereasall the Lw–(c) (including those from SP 110) display
lower temperatures, scattered from 340 to 380 jC.
5.3.2.3. Lw inclusions. Tm ice varies from � 1
to� 5.9 jC, corresponding to low to moderate salin-
ity, between 2 and 9 wt.% eq. NaCl. The homoge-
nisation temperatures (Th) are largely variable, from
150 to 250 jC. Part of this variation may be ascribed,
in some cases, to a pronounced necking-down effect.
The highest Th are found in Q1, Q2–3, Q3 and cQ
quartz and span the same Th interval as the associated
Lw–(c) inclusions.
6. Fluid geochemistry
6.1. Crush leach data on bulk quartz
Small quartz grains have been analysed by the
crush-leach method (Banks and Yardley, 1992).
Reconstructed compositions (in mmol/kg solution)
are given in Table 3. Na/K (molar ratios) are between
15 and 80, Na/Li in the range of 50–300, Cl/SO4
f 120 to 560. The Na/Li and Na/K ratios are con-
sistent with those expected at temperatures from 150
to 250 jC (Fig. 7A), using the Fournier (1979) and
Fouillac and Michard (1981) empirical geothermom-
eters for geothermal systems, as revised by Verma and
Santoyo (1997). The halogens, Cl, Br and to a lesser degree I, can be used to identify the sources of fluid
(Bohlke and Irwin, 1992), because they are conserva-
tive in solution and are relatively unaffected by fluid–
rock interactions (Banks et al., 1991). With log (Br/
Cl) between � 3 and � 2.1 and log (I/Cl) in the range
of � 3.5 to � 2.8, the Tharsis fluids differ signifi-
cantly from both seawater and granite-related fluids
(Fig. 7B). They are similar to evolved basinal fluids.
6.2. Stable isotope (18O) geochemistry
The same samples that were used for crush-leach
experiments have been chosen for isotopic analysis.
Table 3
Reconstructed composition (in mmol/kg solution) of fluid inclu-
sions from crush-leach analyses
Sample Na K Li F Cl Br SO4 I
THA-1FT 370 13 3 11 691 2.4 6 0.23
THA-1FT3 379 13 2 3 468 0.5 2 0.72
THA 4 Quartz 332 18 2 6 381 0.8 3 nd
THA 4 Dolomite 254 5 1 0.2 388 0.1 1 0.14
THA 5 375 29 2 2 386 0.4 2 nd
THA 15–30.2 1008 79 4 1 1335 7.1 5 0.86
THA 15–120 485 6 9 2 569 3.8 1 0.26
nd: not determined.
Fig. 7. Ionic ratios (mol) of fluids determined from crush-leach
analyses. (A) Na/K–Na/Li diagram. Full line temperature estimation
deduced from the geothermometric cation relationships (Verma and
Santoyo, 1997). (B) log (Br/Cl)– log (I/Cl) diagram with reference
fluid compositions from literature. (1) Seawater (Fontes and Matray,
1993), (2) bulk earth (Bohlke and Irwin, 1992), (3) Au-bearing
quartz veins from NW Iberia (Boiron et al., 1996), (4) oil field
(Carpenter et al., 1974; Kharaka et al., 1987), (5) Canadian shield, (6)
Baltic shield (Frape and Fritz, 1987), (7) SW England fluids from
granites (Bottrell and Yardley, 1988).
C. Marignac et al. / Chemical Geology 194 (2003) 143–165156
The analytical procedure follows the conventional
fluorination method of Clayton and Mayeda (1963).
Two types of samples were analysed: (i) bulk quartz
samples (ca. 7 mg); (ii) chips of quartz (down to ca. 1
mg) extracted from the fluid inclusion wafers. These
chips correspond exactly to the material on which FI
studies were carried out. The uncertainties (estimated
from duplicates performed on different Ni fluorination
tubes) are generally on the order of 0.1–0.2xfor the
bulk samples. Duplicates could not be performed on
the chip samples but it is known from similar studies
that the uncertainties may be higher (Essarraj et al.,
2001). In this paper, we consider a conservative value
of 0.3xfor the uncertainties associated with the
quartz chips extracted from fluid inclusion wafers. A
summary of the data (d18O vs. SMOW) is presented in
Table 4.
The results suggest a rather clear-cut distinction
between the Q3 quartz on one hand (d18O in the
narrow 13.0–13.9xrange) and regional quartz
(mainly Q2 type) on the other hand (d18O in the
narrow 17.7–19.5xrange). Other samples from the
Tharsis mine are either consistent with the regional
range (THA-1FT–THA 4) or display intermediate
isotopic compositions (d18O spread from 14.6xto
16.5x). When the nature of quartz in the Tharsis
mine is considered, it may be concluded that Q2
quartz (or, more precisely, Q2-dominated, since minor
fractions of Q3 or cQ quartz may be present in the
bulk samples) everywhere display high d18O values,
comprised between 15.8xand 19.5x, whereas the
Q3 quartz are characterised by distinctly lower values
(13.0–13.9x) and the Q4 quartz stands in a some-
what intermediate range (14.6–17.4x).
These results are rather different from those ob-
tained for the Rio Tinto quartz stockwork by Halsall
and Sawkins (1989), with a range of d18O values
between 8.2xand 13.2x. The reasons for this
discrepancy may stem from the differences between
the analysed quartz: at Rio Tinto, mainly Q1 quartz
was sampled. On the other hand, the values found
by Tornos et al. (1998) for syntectonic quartz within
the phyllonite zones (17.4x) and for ‘‘quartz
vesicles’’ within the vulcanites of the LU (17.8–
18.5x) are consistent with our findings. In addition,
Tornos et al. (1998) reported d18O values for anker-
ites of the Tharsis mine: 15.0xfor a tension gash
within the phyllonites (likely coeval with Q2) and
18.2xto 20.9xfor veins in the spilites from the
LU.
7. Fluid evolution and P–T–t reconstruction
7.1. Volatile-bearing fluids
7.1.1. Bulk compositions and chemical evolution
All the volatile-bearing fluids in the Tharsis area
appear to be rich in CO2. Most of them are also rich
in N2 and poor in CH4, defining a ‘‘main trend’’
including both ‘‘regional’’ and ‘‘Tharsis mine’’ fluids
(Fig. 8a). However, some fluids trapped in Lw–(c),
Vw–c and Vw–(c) inclusions from the Tharsis mine are
on the contrary enriched in CH4 and poor in N2.
When bulk compositions are considered (Fig. 8b), it
is seen that ‘‘regional’’ fluids display a ‘‘dilution
trend’’ from a volatile-rich end-member (z 30
mol%) towards a water-dominated end-member, with
the ‘‘Tharsis mine’’ fluids being similar to the latter.
7.1.2. P–T–t path
Peak metamorphic conditions are apparently re-
corded by those VA and Vir Pe Lc–w inclusions
Table 4
Stable isotope data for the quartz from the Tharsis area
Sample number Type of
sample
Quartz
type
d18O quartz
xSMOW
Tharsis
THA-1FT bulk Q2 18.3F 0.1
THA-1FT3 bulk Q2 16.55F 0.2
THA-3C wafer Q2 17.3F 0.3a
THA 4 bulk Q4 17.40F 0.15
THA 5 bulk Q2 15.75F 0.2
THA 15–30.20 wafer Q3 13.0F 0.3a
THA 15–105.5 wafer Q4 14.6F 0.3a
THA 15–120 bulk Q3 13.5F 0.15
THA 15–120 wafer Q3 13.9F 0.3a
THA 15–120 wafer Q3 13.3F 0.3a
Virgen de la Pena bulk Q2 18.4F 0.1
SP2 wafer Q2 17.3F 0.3a
Villanueva, SP 110 bulk Q2 17.0F 0.3a
Valverde wafer Q2 19.5F 0.3a
El Morante bulk Q2 17.7F 0.3a
The different types of quartz are described in the text.
The ‘‘bulk’’ samples are from the same samples that were used for
the crush-leach analyses (see Table 3).a Estimated uncertainties for nonduplicated analyses (see text).
C. Marignac et al. / Chemical Geology 194 (2003) 143–165 157
trapped in cQ quartz that exhibit the highest minimal
trapping pressures (Fig. 9). This may be checked by
comparing with the regional petrographic studies.
From metamorphic mineral associations, oxygen iso-
tope data and epidote compositions from buffered
assemblages, Munha (1983, 1990) concluded that
the limiting conditions for the IPB were 290 jC and
180 MPa. However, the characteristic assemblage is
prehnite + chlorite + albiteF pumpellyite, which is in
fact bracketed by 210 jCV TV 290 jC and 200
MPaVPV 300 MPa (Bucher and Frey, 1994). As
may be seen in Fig. 9, these pressure estimates are
consistent with the range of minimal trapping pres-
sures displayed by the VA and Vir Pe fluid inclusions
and, consequently, we consider that P was lower than
300 MPa in the Tharsis area (Fig. 9). However,
whereas the VA isochores are also consistent with
the regional temperature estimations, the temperature
at Virgen de la Pena could have been slightly higher,
up to 320–330 jC (Fig. 9). This may be interpreted as
resulting from heat advection by the circulating fluids
and would therefore point to the CO2-rich fluids being
generated at a greater depth in the Variscan crust of
the IPB.
– The ‘‘retrograde’’ path should not have been
associated with a significant temperature drop, as
indicated by the Lc–w and Vc–w inclusions with the
lowest trapping pressures (Fig. 9). On the other hand,
decompression could have been significant, down to
ca. 100 MPa. These findings are corroborated by the
El Morante isochores (Fig. 9).
A further refinement of the late stages of the
‘‘retrograde’’ path is possible by considering the
aqueous-dominated volatile-bearing fluids. Most sig-
nificant are the Vw– (c) inclusions in the SP 110
sample, with high Th in the 405–420 jC range. These
temperatures are likely to be close to the true trapping
temperatures, since biotite was never a stable phase in
the mineral assemblages. Consequently, these low-
density volatile-bearing fluids circulated at pressures
around 40 MPa (Fig. 9). Such a low pressure is likely
to be hydrostatic in nature (thus implying a depth of
ca. 4 km). On the other hand, there is neither textural
nor microthermometric evidence of a phase separation
Fig. 8. Composition of the volatile-bearing fluid inclusions from regional and Tharsis samples: (a) composition of the volatile phase in the
CO2–CH4–N2 diagram; (b) bulk compositions in the H2O–CO2– (CH4 +N2) diagram.
C. Marignac et al. / Chemical Geology 194 (2003) 143–165158
process for the origin of ‘‘vapour’’ or ‘‘liquid’’ inclu-
sions. Therefore, the Lw–c and Lw–(c) inclusions are
best interpreted as recording a (more or less) isobaric
condensation trend, down to ca. 270 jC, as seen in
Fig. 9.
Thus, the end of the ‘‘retrograde’’ path in the Tharsis
area was indeed characterised by both a significant heat
input at the origin of a (re)-heating of the circulating
fluids, and a transition from lithostatic to hydrostatic
conditions. A detailed appraisal of the time relation-
ships between the decompression and the heat input is
not possible with the presented data—there is a lack of
f.i. recording the transition. The heat input may as well
have been progressive (path A in Fig. 9) or ‘‘instanta-
neous’’ (path B).
7.2. Aqueous fluids
A Th–Tm ice plot was drawn for all the aqueous-
dominated fluid inclusions from the Tharsis mine,
including the Lw, Lw–c, Lw– (c), Vw–c and Vw– (c)
types (Fig. 10).
In Fig. 10, a clear-cut distinction appears between
two groups of fluid inclusions, according to their
setting. The first group (group 1—white and grey
symbols in Fig. 10) comprises the inclusions that are
restricted to quartz generations earlier than Q4, all
being found in Q3. The second group (group 2—black
symbols in Fig. 10) consists of inclusions that are
present either in all generations of quartz or only in
Q4. For any given salinity, inclusions of the first
Fig. 9. Reconstruction of the Variscan P–T– t path for the fluids in the Tharsis area. For a given set of fluid inclusions, the shaded area
represents the permitted trapping conditions. The limiting conditions are either a maximum pressure of ca. 300 MPa (corresponding to the
estimated maximum pressure of the regional metamorphism, see text) or a maximum temperature of ca. 430 jC as fixed by the absence of biotite
in the studied area. The black field corresponds to the conditions of peak regional metamorphism. The arrows labelled A or B are for the
possible ‘‘retrograde’’ paths in the Tharsis area (see text).
C. Marignac et al. / Chemical Geology 194 (2003) 143–165 159
group consistently exhibit higher Th than inclusions
of the second group. This is true whatever the content
of the fluid inclusions, either pure water or containing
a minor volatile component. It may be considered that
the first group is representative of the fluids coeval
with Q3, whereas the second group is representative
of the fluids coeval with Q4. A similar distinction may
be observed for the regional aqueous-dominated
inclusions, with a ‘‘high temperature’’ group restricted
to cQ and a ‘‘low temperature’’ group characteristic of
hQ.
– As seen in Fig. 10, the group 2 inclusions
exhibits a clear trend of varying salinity (from 9 to
1.7 wt.% eq. NaCl) with fluctuating Th (between 150
and 230 jC, when the necking-down effects are
smoothed by removing the extremal Th values). This
trend may be interpreted as resulting from the mixing
of a ‘‘hot’’ saline fluid (z 9 wt.% eq. NaCl, z 230
jC) with a ‘‘cold’’ low-salinity fluid (V 1.7 wt.% eq.
NaCl, V 150 jC). The mixing may have occurred
under fluctuating pressures, as recorded by the fluc-
tuating Th at a given salinity. A range of Th up to 60
jC for a given salinity (Fig. 10) is consistent with the
pressures fluctuating above a hydrostatic pressure of
ca. 40 MPa, up to the corresponding lithostatic value
of 110 MPa (Fig. 9). The 40 MPa value is assumed
taking into account the pressure estimates of the
preceding section.
– The group 1 inclusions typically correspond to the
end of the ‘‘retrograde’’ path discussed in the preceding
section; therefore, the large range of Th displayed by
these inclusions really records the overall temperature
decrease from the ‘‘peak’’ 400–430 jC discussed
above. Nonetheless, when restricted ranges of Th are
considered, a significant dispersion of the salinity is
observed (Fig. 10). Furthermore, trends similar to the
group 2 trend may be observed: for instance, for the
inclusion in the sample THA 15–120 (Fig. 10). There-
fore, it is possible to interpret the dispersion pattern of
the group 1 inclusions as recording a continuous
Fig. 10. Tm ice–Th plot for aqueous-dominated fluid inclusions from the Tharsis mine.
C. Marignac et al. / Chemical Geology 194 (2003) 143–165160
process of fluid mixing at regionally decreasing Th,
according to the following scheme: (i) at each time (i.e.,
for each small given range of Th) a ‘‘hot’’ saline fluid
mixes with a ‘‘cold’’ low-salinity fluid; and (ii) the bulk
temperatures of the two corresponding reservoirs
decrease simultaneously with time.
7.3. Conclusion: fluids at the ‘‘retrograde’’ stage
Combining the interpretations of the two preceding
sections, it may be concluded that, in the Tharsis area,
at the end of the Variscan thermal event, the fluids
circulated at decreasing pressures but under temper-
atures that were increased up to 400–430 jC before
their progressive lowering. These fluids evolved from
early rather saline volatile-bearing fluids (CO2-domi-
nated) towards complex fluids exhibiting simultane-
ously (i) the mixing of the volatile-bearing fluids with
a low-salinity purely aqueous end-member; and (ii) a
progressive loss of the volatile content in the volatile-
bearing fluids.
8. Discussion
8.1. Source of fluids
The two types of fluids involved in the ‘‘retro-
grade’’ stage of fluid circulation at the regional scale
(volatile-rich and purely aqueous) may be readily
interpreted. The volatile-rich fluids are characterised
by a low, but significant, content in CH4 and N2
together with CO2, and exhibit CO2 to CH4 ratios that
are consistent with equilibrium with graphite in the
source region (at temperatures near 400 jC: Dubessy,1984; Dubessy et al., 1989). They are therefore likely
of ‘‘metamorphic’’ derivation: either, true devolatili-
sation fluids or fluids of any pristine source that
equilibrated with graphite (‘‘pseudo-metamorphic’’
fluids). In any case, it is worth underlining the
abundance of black shales in the IPB (Saez et al.,
1999) that are an accessible source of organic-derived
carbon. The low-salinity, purely aqueous end-member
is likely a fluid rooted in a shallow-level reservoir, i.e.,
a more or less evolved ‘‘meteoric’’ fluid. These
interpretations are consistent with the fact that the
volatile-bearing fluids are constantly hotter than the
purely aqueous fluids throughout the mixing process
that characterises the ‘‘retrograde’’ stage, meaning that
the former are deeper fluids than the latter.
The results of the geochemical studies lend some
support to these contentions.
– Calculation of the d18O of the waters associated
with the quartz (d18Ow), using the fractionation coef-
ficients given by Zheng (1993), leads to the following
results: for the Q2 stage, for a temperature of 300 jC,the d18Ow are in the range of 8.9–12.6x; for the Q3
stage, the d18Ow are either in the range of 9.7–8.8x,
if the maximum temperature of 420 jC is considered,
or in the range of 4.0–4.9xif the low temperature of
250 jC is taken into account; and for the Q4 stage, at
temperatures of 200 jC, the d18Ow are in the range of
3.0–5.8x. A global trend of lowering d18Ow with
time is apparent, which is entirely consistent with the
mixing model presented in the previous section. It is
worth noticing that the d18O values reported for the
syntectonic ankerites by Tornos et al. (1998) are
equally consistent with our findings, since, using the
fractionation coefficients of O’Neil et al. (1969), the
d18Ow calculated at 300 jC for these ankerites is in
the range of 9.9–11.3x.
– The ion ratios obtained by crush-leach analyses
of the brine +meteoric fluid mixtures are obviously
dominated by the saline end-member (dilution by
meteoric water reduces salinity but is not expected
to change cation ratios). Therefore, the cation signa-
ture of the regional and Tharsis fluids, as presented
above, reflects an origin as a deep-seated ‘‘basinal’’
fluid. The ‘‘basinal’’ nature of the deep fluids suggests
that they may represent evolved connate waters remo-
bilised by the Variscan thermal events, thus explaining
their rather high salinity.
8.2. Relevance for the VMS deposits of the IPB
The ‘‘retrograde’’ fluids were responsible for sig-
nificant mineral genesis and ore (re)-deposition in the
Tharsis deposit and, particularly, within the early
deformed stockwork:
– The volatile-bearing fluids that spanned the Q2–
Q3 (or cQ) stages of quartz deposition, both at the
regional and mine scales, were mainly related to
the formation of Q3 quartz and associated silicates
(phengite and/or chlorite) at Tharsis, marking the
late-kinematic influx of externally derived fluids
C. Marignac et al. / Chemical Geology 194 (2003) 143–165 161
into the mineralised structures (see above). How-
ever, late-kinematic sulphides (arsenopyrite and
sphalerite) were also probably deposited by the
same fluids.
– The aqueous-dominated fluids that circulated
during the deposition of Q4 (clusters of primary
f.i.) or thereafter (FIP) are therefore most likely
responsible for the deposition of polymetallic
sulphides and gold, since metal deposition and Q4
were coeval.
These results may be contrasted with the conclu-
sions of Leistel et al. (1998b). These authors, while
clearly identifying the chalcopyrite + Bi-minerals +
gold assemblage as a late paragenesis mainly formed
in the stockwork just beneath the massive sulphide ore
body, were of the opinion that it was a late high
temperature pulse of the syn-VMS (Tournaisian)
Variscan hydrothermalism. In reality, as far as the
stockwork is concerned, we show that: (i) the late
polymetallic stage at Tharsis was basically a Variscan
post-kinematic feature; and (ii) the temperatures
spanned a ca. 400–170 jC interval, with chalcopyrite
and Bi-minerals possibly deposited in the higher range
of temperatures (300–400 jC), whereas galena (and
gold?) could have been deposited at lower temper-
atures (170–250 jC).These conclusions, drawn for the Tharsis deposit,
are likely to be of a general validity for the VMS of
the whole IPB, because:
(i) The sequence of fluid circulation and ore (re)-
deposition that we have found at Tharsis and in
the surroundings is basically the same as was
described for the Neves-Corvo deposit by Moura
et al. (1997a,b).
(ii) The fluids found at Tharsis in the Q3 and Q4
quartz are very similar to those described by
previous workers in the Aznalcollar (Almodovar
et al., 1998), Rio Tinto (Nehlig et al., 1998) and
several northern (Sanchez-Espana et al., 2000)
deposits and deemed to be representative of the
deposition conditions for the VMS deposits. It
seems therefore that these studies were in reality
concerned with the post-kinematic stage of ore
deposition. This is logical, since, given the
generally accepted P–T conditions of ore depo-
sition in the Kuroko-type VMS deposits (V 350
jC, V 30 MPa, e.g., Franklin, 1993), and the
peak conditions of the Variscan tectono-thermal
event (similar temperatures, but up to 300 MPa),
implosion of primary inclusions trapped in the
early quartz was unescapable. Indeed, as stated in
a previous section, primary imploded inclusions
are effectively observed at Tharsis; they were also
described at La Zarza and Rio Tinto (Diagana,
2001) and at Neves-Corvo (Moura et al.,
1997a,b), and are likely to be discovered in the
other deposits of the IPB.
8.3. Cause of the ‘‘retrograde’’ heat input
The significant ‘‘retrograde’’ heat input found in
the Tharsis area is worth further discussion. Consid-
ering the high temperature (400–420 jC at least)
reached by the fluids in the very upper continental
crust (estimated depth of ca. 4 km), we are evidently
dealing with the inception of a high-enthalpy geo-
thermal system, and the source of heat must therefore
be sought in some kind of magmatic activity, most
likely a shallow-level granite intrusion. There is a
plutonic complex (Sierra Norte batholith) to the north-
east of the IPB. Made up of tonalites and granites,
with subordinated gabbros and diorites involved in
magma mixing and mingling, this batholith was
considered either as late Variscan (e.g., Schermerhorn,
1987) or as coeval with the bimodal volcanism of the
VS Group in the IPB (Schutz et al., 1987; Thieble-
mont et al., 1994; Stein et al., 1996). Most recently,
structural investigation of the Sierra Norte batholith
lead Onezime et al. (2001) to conclude that the
intrusion was late kinematic. Such a timing is con-
sistent with the timing of heat input at Tharsis, and it
is therefore likely that other plutonic complexes
underlie the IPB and were responsible for the late-
kinematic (‘‘retrograde’’) reheating that we observe at
the regional scale.
It is interesting to note that a comparable situation
was found for the Hajjar VMS deposit in Morocco,
which is very similar to the VMS deposits in the IPB
(Hibti and Marignac, 2001). There, synsedimentary
hydrothermal activity occurred during the late Visean,
and the tectono-thermal Variscan event is supposed to
be of Westphalian age. Nevertheless, primary syn-
VMS f.i. are imploded at Hajjar like at Tharsis, and
late- to post-kinematic fluids record both a pressure
C. Marignac et al. / Chemical Geology 194 (2003) 143–165162
decrease (from peak conditions at 240 MPa) and a late
heat in pulse (from 360 to more than 430 jC, biotitebeing a post-kinematic mineral at Hajjar) (Essarraj et
al., 1999).
9. Conclusions
(1) At Tharsis, there is no evidence of fluids related to
the deposition of the massive sulphides within the
available transparent minerals displaying fluid
inclusions. Deformation is such that early fluid
inclusions in the earliest quartz (Q1) have im-
ploded. Fluids trapped in recrystallised ores as
primary inclusions in newly formed quartz or in
healed fractures are post-deformational and are
therefore synchronous or posterior to the peak of
metamorphism. Fluid production began under P–
T conditions close to those at the peak meta-
morphism, but fluid movements within the
Tharsis orebody and stockwork seem to have
been favoured by decompression after a pressure
drop linked either to the basement uplift and/or
tectonic events.
(2) Peak metamorphic conditions were close to 300
MPa and 300 jC. A strong pressure decrease is
then documented by the aqueous–carbonic fluids,
and can be attributed to the sudden uplift of the
area. Coeval with the transition from lithostatic to
hydrostatic pressure, at a depth of ca. 4 km, a
strong reheating of the circulating fluids (up to at
least 430 jC) is documented (‘‘retrograde’’
stage).
(3) Fluids circulating at decreasing temperatures and
fluctuating pressures during the ‘‘retrograde’’
stage were aqueous–carbonic fluids (similar to
those of the earlier stages, but with a progressive
loss in the volatile-component) that mixed with a
low-salinity purely aqueous fluid. Microthermo-
metric and geochemical evidence indicate that the
volatile-bearing fluids are of metamorphic (or
‘‘pseudo-metamorphic’’) derivation, whereas the
low-salinity water is likely to be meteoric in
origin.
(4) ‘‘Retrograde’’ fluids have played a significant role
in the textural and mineralogical changes that
affected the ores and the stockwork of the Tharsis
massive sulphide deposit. Microtextures indicate
clearly the recrystallisation or the crystallisation
of newly formed quartz and sulphides (arsenopyr-
ite, chalcopyrite, Bi–Te–sulphides, sphalerite,
pyrite, galena, native gold). In particular, the
primary paragenesis of the Tharsis stockwork was
mainly quartz–pyrite–chlorite–phengite (and,
locally, ankerite and cobaltite). Base metals (and
gold) were quantitatively introduced in the stock-
work during the ‘‘retrograde’’ stage of the
Variscan tectono-thermal event. It remains open
as to whether these metals were newly introduced
into the stockwork from elsewhere or were simply
remobilised from the existing primary assemb-
lages in the massive sulphide bodies.
(5) Fluid inclusion studies in other deposits of the IPB
have reported results very similar to those found in
the present study. Excepted for the Neves-Corvo
deposit, for which the reported fluids and relation-
ships with Variscan events are the same as for
Tharsis (Moura et al., 1997a,b), the authors of
these earlier studies claimed to have studied the
pristine fluids associated with the massive sulphide
deposition (Almodovar et al., 1998; Nehlig et al.,
1998; Sanchez-Espana et al., 2000). However,
considering that (i) the synhydrothermal fluid
inclusions are unlikely to escape implosion when
submitted to the peak metamorphic conditions;
and (ii) that the observed inclusions yield basically
the same microthermometric data that in the
Tharsis and Neves-Corvo deposits; it seems likely
that these earlier studies equally dealt with the
‘‘retrograde’’ stage of fluid circulation in the IPB.
Acknowledgements
This work was supported by the GDR ‘‘Metal-
logenie no. 514’’, BRGM, La Source; and SEIEMSA.
J.-P. Milesi, Y. Deschamps and M. Joubert are thanked
for their technical and scientific assistance during the
visit of the Tharsis open pit, and sampling of the
SEIEMSA loggings. The company ‘‘Minas de Tharsis
S.A.’’ is thanked for having permitted sampling in the
open pit. E. Marcoux provided representative thin and
polished sections of the early stockwork ores and J.
Onezime provided the Villanova del Castillo sample.
F. Martineau is thanked for help in the O isotope
measurements. L. Diamond, J. Munha and F. Noronha
C. Marignac et al. / Chemical Geology 194 (2003) 143–165 163
carefully reviewed the manuscript and greatly helped
to its improvement. [RR]
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