Mineralium Deposita (2003) 38: 953–967
DOI 10.1007/s00126-002-0342-z
A RT I C L E
Klaus Germann Æ Volker Lüders Æ David A. Banks
Klaus Simon Æ Jochen Hoefs
Late Hercynian polymetallic vein-type base-metal mineralization
in the Iberian Pyrite Belt: fluid-inclusion and stable-isotope
geochemistry (S–O–H–Cl)
Received: 16 January 2002 / Accepted: 16 November 2002 / Published online: 6 February 2003
Springer-Verlag 2003
Abstract Late Variscan vein-type mineralization in the
Iberian Pyrite Belt, related to the rejuvenation of preexisting fractures during late Variscan extensional tectonism, comprises pyrite–chalcopyrite, quartz–galena–
sphalerite, quartz–stibnite–arsenopyrite, quartz–pyrite,
quartz–cassiterite–scheelite, fluorite–galena–sphalerite–
chalcopyrite, and quartz–manganese oxide mineral
assemblages. Studies of fluid inclusions in quartz,
stibnite, and barite as well as the sulfur isotopic compositions of stibnite, galena, and barite from three
occurrences in the central part of the Iberian Pyrite Belt
reveal compelling evidence for there having been different sources of sulfur and depositional conditions.
Quartz–stibnite mineralization formed at temperatures
of about 200 C from fluids which had undergone twophase separation during ascent. Antimony and sulfide
are most probably derived by alteration of a deeper
lying, volcanic-hosted massive sulfide mineralization, as
indicated by d34S signatures from )1.45 to )2.74&. Subcritical phase separation of the fluid caused extreme
fractionation of chlorine isotopes (d37Cl between )1.8
and 3.2&), which correlates with a fractionation of the
Cl/Br ratios. The source of another high-salinity fluid
trapped in inclusions in late-stage quartz from
Editorial handling: V. Bouchot
K. Germann (&)
Institut für Angewandte Geowissenschaften – Fachgebiet
Lagerstättenforschung, TU-Berlin, Ernst-Reuter-Platz 1,
10587 Berlin, Germany
E-mail: [email protected]
V. Lüders
GeoForschungsZentrum Potsdam,
Telegrafenberg, P.B. 4.3, 14473 Potsdam, Germany
D.A. Banks
School of Earth Sciences,
University of Leeds, Leeds, LS2 9JT, UK
K. Simon Æ J. Hoefs
Göttinger Zentrum Geowissenschaften, Abt. Geochemie,
Universität Göttingen, Goldschmidtstr. 1,
37077 Göttingen, Germany
quartz–stibnite veins remains unclear. By contrast,
quartz–galena veins derived sulfide (and metals?) by alteration of a sedimentary source, most likely shalehosted massive sulfides. The d34S values in galena from
the two study sites vary between )15.42 and )19.04&.
Barite which is associated with galena has significantly
different d34S values ()0.2 to 6.44&) and is assumed to
have formed by mixing of the ascending fluids with
meteoric water.
Keywords Pyrite Belt Æ Vein mineralization Æ Fluid
inclusions Æ Infrared microscopy Æ Chlorine isotopes
Introduction
Regarding their general stratigraphic and structural situation, the vein-type mineralizations of the Iberian
Pyrite Belt (IPB) studied here fit into the common metallogenetic evolution pattern of the European Hercynides. Late and post-Hercynian vein deposits of Zn–Pb,
barite and fluorite are widespread in the central and
western regions of the European Hercynian orogen (see,
for example, Tischendorf and Schwab 1989; Möller and
Lüders 1993). Their ages extend from Late Hercynian to
Mesozoic times, and underlying sedimentary rocks are
thought to be the fluid sources, rather than the granitic
intrusions. Structurally controlled antimony mineralizations are another typical feature of the Hercynian
orogenic belt in Europe (see Wagner and Cook (2000)
for NW Germany, Munoz et al. (1992) for the French
Paleozoic basement, and Gumiel and Arribas (1987) for
the Iberian Peninsula). These antimony deposits can be
considered to be a common and widespread mineralization style, all across the European Hercynides
(Wagner and Cook 2000), and can be regarded as a kind
of metallogenic marker characterizing the late Hercynian extensional tectonic evolution (Munoz et al. 1992).
The IPB is part of the South Portuguese Zone
(Fig. 1), located at the southern edge of the Iberian
954
Fig. 1 Location of deposits in the Spanish part of the Iberian Pyrite
Belt (modified after Leistel et al. 1998)
Hercynian belt and the European Hercynian orogen. It
extends for some 250 km with widths of 25–70 km, and
is one of the largest provinces of volcanic- and sedimenthosted massive sulfide deposits in the world, containing
more than 1,700 Mt sulfide ore (Leistel et al. 1998).
Mining activity dates back to the Chalcolithic era but at
present only five of the more than 80 mines which operated during the last hundred years are still active,
namely, Sotiel-Migollas, Aznalcóllar-Los Frailes, Rio
Tinto, Tharsis, and Neves-Corvo (Sáez et al. 1999).
Neves-Corvo is currently the only active mine in the
Portuguese part of the belt and is an important source of
copper and tin.
The massive sulfide deposits and their host rocks
formed under an epicontinental extensional regime
during the Late Devonian–Early Carboniferous, thus
corresponding to a regionally distributed metalliferous
‘‘peak’’ around 350 Ma within the Western Hercynides
(Lescuyer et al. 1998). Mineralization is hosted by a
strongly folded series of rocks composed of felsic and
mafic volcaniites, black shales, siltstones, and cherts of
the Volcano-Sedimentary Complex (VSC) which is intercalated between the Late Devonian Phyllite–Quarzite
Group (PQ) and the Lower Carboniferous Culm Group,
the latter consisting of flysch sediments (Schermerhorn
1971). During the main orogenic phase (middle
Westfalian), rocks and sulfide ores underwent multiphase deformation and low-grade regional metamorphism (Schermerhorn 1971; Silva et al. 1990; Munhá
1990, Onézime et al. 2002).
At the northeastern boundary of the IPB, a plutonic
complex (Sierra Norte Batholith) is situated, the age of
which is still under discussion. Some authors suggest
that the largest part of the plutonic complex is pre-tectonic and related to the first volcanic episode of the VSC
(e.g., Schütz et al. 1987; Thiéblemont et al. 1995). Others
consider the batholith’s emplacement to be of Late
Hercynian age and post-date the deformation of the IPB
(e.g., De la Rosa et al. 1993; Quesada 1998).
Besides the massive sulfide deposits, the IPB hosts
several other types of mineralization, namely, stratiform
manganese ores and manganese deposits and vein-type
base-metal. The latter comprise mineral associations
such as pyrite–chalcopyrite, quartz–galena–sphalerite,
quartz–stibnite–arsenopyrite, quartz–pyrite, quartz–
cassiterite–scheelite, and fluorite–galena–sphalerite–
chalcopyrite.
Structurally
controlled
vein-type
mineralization is known from all three main lithostratigraphic units, i.e., the pre-volcanic PQ, the VSC, and the
post-volcanic lower Carboniferous Culm Group.
For some of the PQ-hosted, sulfide-bearing quartz veins
in the Valverde area (Fig. 1), Bonijoly et al. (1994)
proved that the vein structures formed prior to the main
955
phase of deformation, and that their mineralogical
and geochemical composition is similar to that of the
massive sulfides. These oldest veins are thus interpreted
to be the roots of massive sulfide bodies, representing
paleofaults which controlled the distribution of the
stratiform sulfide deposits in the volcano-sedimentary
basin.
The predominant type of disconformable mineralizations in the VSC is represented by stockworks located
beneath the stratiform massive sulfides and which acted
as feeder veins. The major structures of these stockworks, as demonstrated by the Rio Tinto feeder zone,
are arranged in a regular pattern which can be interpreted as the expression of tectonic activity contemporaneous with emplacement of the orebodies (Bonijoly
et al. 1994).
The uppermost VSC sequence and the base of the
overlying turbidite sediments of the Culm Group host a
third type of vein deposit which postdates Hercynian
transpressional deformation. As Routhier et al. (1980)
showed, these types of post-massive sulfide and postfolding veins are restricted to a section of the sedimentary column which corresponds to an important sedimentary discontinuity—the Upper Viséan transgression.
Their mineralogy and geochemistry is clearly different
from that of the massive sulfides and their stockworks
(Leistel et al. 1998). Besides manganiferous quartz veins,
base-metal-bearing quartz veins mainly occur.
With the exception of some Cu-rich stockwork ores,
vein-type mineralization has been mined intensively at
only a few places due to its low economic importance,
and is less well studied, in contrast to the massive sulfide
deposits. Wipfler and Sedler (1995) suggested that the
youngest type of vein mineralization, the post-massive
sulfide manganiferous and sulfide-bearing quartz veins,
was derived from underlying stratiform manganese and
massive sulfide deposits, respectively. Quartz-hosted
fluid inclusions indicate decreasing temperatures for
fluids in repeatedly opened vein systems. Our study aims
to characterize the origin of the ore-forming fluids and
the P–T–X conditions during the formation of some of
the post-volcanic and post-folding vein-type mineralization, using fluid-inclusion and stable-isotope data of
some previously mined vein deposits in the Spanish part
of the IPB (Fig. 1).
Regional geological setting
From the long list of late vein-type occurrences and
small mines compiled by Pinedo Vara (1963) or mentioned in other regional publications, three localities in
the center of the Spanish part of the Pyrite Belt were
selected, namely, Mina Nerón, Mina Aurora and Mina
Silillas (Fig. 1 and Table 1). Geological fieldwork and
sampling at these sites was done by Sedler (1989) and
Wipfler (1989) who also presented data on the geochemistry and mineral parageneses and some fluid-inclusion data from quartz.
The Mina Silillas orebody is located in the northern
flank of the Valverde del Camino-Calañas anticline
where it follows a northwest-dipping fault zone, N36E
in direction, between Culm Group shales and finegrained sedimentary rocks of the uppermost VSC
sequence (Sedler 1989). The following primary and secondary mineral parageneses were described from Silillas
(Sedler 1989):
1.
2.
3.
4.
Barite, galena, quartz;
Quartz, galena, chalcopyrite, fahlore;
Malachite, cuprite, native copper;
Quartz, manganese oxides.
The stibnite veins of Mina Nerón, often taking the
form of quartz–stibnite banded ores (Fig. 2), are located
about 10 km west of Mina Silillas. They are hosted by
graywackes and shales of the Culm Group and are situated in the northern limb of the Puebla de Guzmán anticline. They occupy a north-dipping fault zone N105E
in direction, which was developed more or less parallel to
the prevailing direction of the cleavage and thrust planes
(Wipfler 1989), and show evidence of repeated opening.
The early mineral assemblage is chalcopyrite, pyrite,
fahlore and frequently idiomorphic quartz, and a ‘‘main
ore’’ stage contains stibnite, milky quartz, and subordinate arsenopyrite (Wipfler 1989). The hydrothermal activity ended with the formation of clear quartz which
occurs as euhedral crystals in fissures or replaces older
milky quartz (Fig. 2a). The Mina Aurora area is situated
at the eastern margin of the Puebla de Guzmán anticline,
within basal Culm sediments in which mineralized fracture zones are developed with N60E (according to Pin-
Table 1 Location and properties of the vein-hosted mineralizations at Mina Nerón, Mina Silillas and Mina Aurora, Huelva province,
Spain
Location
Primary mineralization
Geochemi calcharacter
Gangue
Host rock
Thickness of veins
Direction of veins
Mina Nerón
Mina Silillas
Mina Autora
2.5 km east of Montes de San Benito,
Huelva province
Stibnite, arsenopyrite, chalcopyrite,
pyrite, tetraedrite, tennantite, freibergite
Sb, As; (Cu)
Quartz; dolomite, ankerite
Graywacke of the Culm Group
0.05–3 m
N95E
6 km west of Calañas,
Huelva province
Galena, chalcopyrite, fahlore
2–3 km southeast of Alosno,
Huelva province
Galena, sphalerite
Pb, Cu, Ba; (As,Sb)
Quartz, barite
Shale; fault VS/Culm Group
max. 4 m
N36E
Pb, Zn; (Ba)
Quartz,barite
Fault VS/Culm Group
1m
N100E, N60E
956
deformation, including folding and thrusting. Thus,
these undeformed mineralized veins are most probably
related to the rejuvenation and repeated reopening of
pre-existing fractures during late Variscan extensional
tectonics.
Both the Puebla de Guzmán and the CalañasValverde del Camino anticlines host some prominent
massive sulfide deposits. The giant Tharsis deposit,
with reserves of about 115 Mt (Sáez et al. 1999), is
located some 10 km southwest of Nerón and about
7 km north of Aurora, and the Sotiel-Migollas Group
of massive sulfide deposits (reserves of 117 Mt) lies
another 10 km SE of the Silillas. No published information is available on possible indications of massive
sulfide bodies buried below the Culm sediments in the
Nerón-Silillas-Aurora area, whereas east of Valverde
the giant Masa Valverde deposit (120 Mt) has been
discovered quite recently under Culm sediment cover
(Toscano et al. 1993).
Analytical methods
Fig. 2 a Banded quartz–stibnite ore from Mina Nerón (sample
10000 in Tables 2 and 3) b Photomicrograph of halite-bearing fluid
inclusions in a clear zone of late-stage chevron quartz from Mina
Nerón (sample 16041 in Tables 2 and 3). c Infrared photomicrograph of a primary two-phase fluid inclusion in stibnite (sample
16039 in Tables 2 and 3) orientated parallel to {010}
edo Vara 1963) and N100E (Routhier et al. 1980) strike.
The mineral paragenesis comprises quartz, barite, galena
and sphalerite.
In terms of their structural and lithostratigraphic
position, all three vein deposits occur at or near to the
flanks of anticlines and are hosted either by the Basal
Shaly Formation (Moreno and Sequeiros 1989; Moreno
1993) of the Culm Group, which overlies the third and
uppermost volcanic sequence of the VS Complex, or by
the first turbidite facies of the Culm Group.
Wipfler and Sedler (1995) have demonstrated that
the strike direction of undeformed quartz veins is approximately parallel to the main foliation in the study
area. Structural analysis of the central part of the Pyrite Belt by Routhier et al. (1980) indicated a time sequence of fractures and related veins from N90/100E
to N40/60E and finally N140/150E. In addition, a
close temporal and spatial relationship between the
observed Variscan folding phases and their subsequent
relaxation periods, represented by fractures, was assumed by Routhier et al. (1980). However, neither the
Pb-Cu-barite veins nor the quartz-Sb veins which, according to the Routhier scheme, would belong to the
oldest fracture generation, are affected by polyphase
Fluid inclusions were studied in quartz and barite in transmitted
light, and in stibnite in near-infrared light using a U.S.G.S. heatingfreezing system. The thickness of doubly polished wafers for microthermometric studies was between 200 and 250 lm for quartz
and barite, and for stibnite between 100 and 120 lm. For calibration, synthetic standards and natural inclusions were used.
Hydrogen isotope ratios on bulk fluid inclusions in quartz were
measured by mechanical crushing of about 5 g of quartz grains of 1
to 5 mm in size, according to the method described in detail by
Simon (2001). The released water was trapped, reduced to H2 by
zinc (Bloomington, Indiana University, USA), and measured on a
Finnigan MAT 251 gas source mass spectrometer. The absolute
error of determination is about 5 to 10&.
Oxygen isotope ratios were measured by the laser ablation
continuous flow technique described in Fiebig et al. (1999) and
Wiechert et al. (2002). A 300-lm spot size was ablated by a 193-nm
ArF Excimer laser (Lambda Physik), with an average energy of
20 J/cm2 on the sample surface. The resulting oxygen was analyzed
by a Finnigan Delta plus mass spectrometer.
Sulfur isotope analyses of sulfides were performed by preparing
sulfur dioxide through combustion at 1,000 C with V2O5. Sulfur
isotopic analyses of sulfates were carried out on H2S prepared by
reaction at 350 C with Kiba solution. H2S is precipitated as CdS,
converted to Ag2S, and oxidized with V2O5 at 1,000 C to produce
SO2 which was used for the mass spectrometer measurements.
Sulfur isotope ratios are reported as d34S relative to the Cañon
Diablo Troilite (CDT).
Chemical analysis of the fluid inclusions was carried out at the
University of Leeds using the bulk crush-leach method as detailed in
Banks et al. (2000a). Quartz, barite and stibnite samples were crushed
to 1–2 mm grain size and the quartz samples cleaned in aqua-regia,
followed by nitric acid. All samples were then boiled several times in
18.2 MW water and dried prior to analysis. Approximately 0.5 to 1 g
of material was crushed, transferred to a suitable container and leached with 18.2 MW water for anion and Na, K and Li analysis, or
with acidified LaCl3 solution for the analysis of other cations. Anions
were determined by ion chromatography, Na, K and Li by flame
emission spectroscopy, and other cations by inductively coupled
plasma emission spectroscopy. Replicate analyses show the precision
to be on average 5% RSD for the analysis of these samples. The 35Cl
and 37Cl isotopes were determined by thermal ionization mass
spectrometry on aliquots of the water-extracted salts, following the
method of Banks et al. (2000b).
957
Results
Fluid-inclusion petrography
Milky gangue quartz samples from quartz–stibnite
banded ores from Mina Nerón and from the studied
occurrences contain numerous fluid inclusions, but their
sizes generally do not exceed 2 lm. Due to the high
number of fluid inclusions in milky quartz, a clear
classification, i.e., primary vs. secondary origin of the
inclusions, is impossible. The shapes of the inclusions
are irregular or rounded and they either consist of a
liquid phase and a vapor bubble or, in the case of Mina
Nerón, they are often vapor-rich. Only a few combined
microthermometric data of Tmice and Th were obtained
from such samples (Fig. 3 and Table 2). Fluid inclusions in younger quartz crystals, in small vugs within
milky quartz, are considerably larger (up to 15 lm) and
show rounded to elongated forms, consisting of a liquid
phase and a vapor bubble at room temperature. Some
well-crystallized, late-stage quartz samples from Mina
Nerón, with chevron structures, contain two-phase fluid
inclusions in the milky growth zones and polyphase
inclusions, i.e., two-phase fluid inclusions with cubes of
halite crystals and/or other daughter minerals of unknown composition, in their clear zones (Fig. 2b). The
inclusions are preferentially orientated in milky growth
zones or arranged parallel to crystal planes in clear
zones and, therefore, they appear to be of primary origin (Roedder 1984). They are highly variable in size
(20–100 lm) and shape (irregular and rounded–elongated forms). Barite samples from Mina Aurora always
contain monophase aqueous fluid inclusions. They
generally have a rounded–elongated form and sizes between 10 and 30 lm.
FTIR spectroscopic investigations of stibnite samples
from Mina Nerón indicate a transmittance of 15–25% to
near-infrared radiation in the studied wavelength range
between 880 and 1,300 nm. Under the IR microscope,
Fig. 3 Tmice vs. Th diagram of
fluid inclusions in stibnite,
quartz, and barite from veintype mineralization in the
Iberian Pyrite Belt. Note: barite
contains only monophase
aqueous inclusions, probably
due to low formation
temperatures
Table 2 Summary of microthermometric data derived from studies of fluid inclusions in quartz, stibnite, and barite (m massive quartz, x
crystallized quartz)
Locality Sample no. Material Te (n)
Nerón
Aurora
Silillas
10000
16039
10000x
10000m
16041x
16041m
16039m
1032
1030
1031
11113
Stibnite
Quartz
Barite
Quartz
Quartz
)20 (1)
)19 (1)
)52 to )32 (3)
–
)62 to )44 (14)
–
)20.8 to )14 (5)
)27 to )21 (4)
)30 to 24.3 (13)
–
–
Tmice (n)
Mean Tmclath/hydr. (n)
Th (n)
)3.3 to )0.9 (6)
)3.1 to )1 (25)
)21.4 to )15.1
)3.6 to )0.8 (17)
)49.3 to )19.7 (27)
)3.2 to )1.1 (10)
)7.3 to )0.6 (15)
)9 to 0.6 (10)
)16 to 13.9 (13)
)1.3 to )1 (7)
)23.1 to )7.8 (7)
)1.9
)2.5
)17.1
)2.4
)29.2
)1.9
)3.7
)5.5
)14.6
)1.2
)16.4
188
153
135
141
125
133
142
–
–
198
101
10.5 (1)
11.3 to 14.7 (6)
–
–
0.1 to 16.3 (4)
–
)2.1 to )1.5 (5)
–
–
–
–
to
to
to
to
to
to
to
Mean Tmhalite
213
200
138
217
152
192
189
(5)
(19)
(3)
(17)
(27)
(10)
(15)
to 245 (7)
to 163 (7)
199
178
137
176
142
165
165
–
–
233
119
–
–
–
–
166 to 176 (7)
–
–
–
–
–
–
958
the samples mostly show striation in the direction of
growth. Primary two-phase fluid inclusions are elongated (Fig. 2c), and orientated parallel to crystallographic planes, {110} and/or {010}, as observed in
stibnite from other occurrences (Lüders 1996; Bailly et al.
2000). The size of primary fluid inclusions in stibnite
varies between 30 and 70 lm. Secondary fluid inclusions
of smaller size (<25 lm) occasionally occur along trails
crosscutting the samples in various directions.
Microthermometry
The results of fluid-inclusion studies in quartz, stibnite
and barite are shown in Fig. 3 and Table 2. From the Th
vs. Tm diagram (Fig. 3), two groups of fluid inclusions
can be distinguished: (1) low-salinity fluid inclusions
with homogenization temperatures between 150 and
245 C, and (2) high-salinity fluid inclusions with
homogenization temperatures between 100 and 150 C.
Some fluid inclusions in quartz and monophase inclusions in barite from Mina Aurora vary in salinity
between that of these two groups.
All fluid inclusions in stibnite from Mina Nerón have
low salinities, i.e., 1.6–4.9 wt% equivalent NaCl. The
first melting of ice was observed at about )20 C, close
to the eutectic of the pure H2O–NaCl system. Additionally, the melting of clathrate between 10 and 14 C
in a few inclusions indicates traces of a dissolved gaseous
component in the fluid. Final homogenization temperatures were not obtained from these gas-bearing inclusions, due to their decrepitation prior to final
homogenization. The homogenization temperatures of
aqueous two-phase inclusions in stibnite were generally
observed between 150 and 200 C.
Fluid inclusions in milky gangue quartz from Mina
Nerón, which is associated with stibnite (Fig. 2a), have
similar salinities and homogenization temperatures as
the fluid inclusions in stibnite (Fig. 3). Melting of
clathrate was not observed in low-salinity inclusions in
milky quartz. Fluid inclusions in euhedral late-stage
quartz crystals with chevron structures (quartz II in
Fig. 3) from vugs contain low-salinity fluid inclusions in
milky growth zones whereas clear zones contain highsalinity or even halite-bearing fluid inclusions (Fig. 2b).
The high-salinity fluid inclusions show ice melting temperatures as low as )27 C. In some of these inclusions,
melting of hydrate was observed between )2 and +4 C
(metastable?). The homogenization of the vapor phase in
halite-bearing inclusions occurs at temperatures of
about 150 C, always prior to the homogenization of the
daughter mineral (170 to 175 C). From the final melting
temperatures of halite daughter crystals, a salinity of up
to about 30.5 wt% equiv. NaCl can be inferred for these
inclusions (Bodnar 1994). Although this salinity estimate
is only valid for inclusions where Th vapor and Tm
halite occur at the same temperature, the error range
from an underestimate of salinity will be less than
1.3 wt% for the measured difference between vapor
homogenization temperatures and halite dissolution
temperatures (Bodnar and Vityk 1994).
Fluid inclusions in quartz from Mina Silillas have
moderate to high salinity but lower homogenization
temperatures compared with the fluid inclusions from
Mina Nerón, whereas fluid inclusions in quartz from
Mina Aurora have slightly lower salinities but the
highest homogenization temperatures of the three occurrences (Fig. 3). Monophase aqueous inclusions in
barite from Mina Aurora have ice melting temperatures
varying between )16 and )1.5 C. The absence of a
vapor bubble in fluid inclusions in barite may be indicative of a low formation temperature of <80 C.
Crush-leach analyses
The results of the crush-leach analysis are presented in
Table 3. The data show the fluids to be dominated by
Na and Cl with significant but variable amounts of Ca
and K, and lesser amounts of Li, Mg and Br. Fluorine
was not detected and, although SO4 was present, there
was a strong possibility that the analyses were influenced
by oxidation of sulfides during crushing and leaching,
and so the data have been omitted.
There is a large variation in the Cl/Br ratio of the
fluid inclusions (Fig. 4). Quartz and stibnite (paired
samples) have similar Cl/Br and Na/Br ratios, but within
the whole sample suite the fluid inclusions have a large
range of values. This is despite the samples having essentially the same gross salinity. There is a correlation
with salinity, as can be seen from the bulk analyses of
quartz sample 16041 (*), where selective cutting and
analyzing parts of this sample which contains different
salinity inclusions (a=high salinity+halite, b=high
salinity, c=dominantly low-salinity fluids) was carried
out. Halite-bearing inclusions are the most Br-enriched
and the lowest-salinity fluid inclusions (salinity equivalent to that of the quartz–stibnite mineralization) the
least. However, the range of Cl/Br and Na/Br ratios, in
the whole sample suite, is much larger than that covered
by these three different salinity fluids in sample 16041.
Quartz and barite from Mina Aurora have different Cl/
Br and Na/Br ratios, the fluid inclusions in quartz being
more Br-rich, and are within the range of values from
Mina Nerón. The quartz from Mina Silillas has Cl/Br
and Na/Br ratios which are the same as those of the
halite-bearing fluid from Mina Nerón.
Cation ratios, Na/K, Na/Li and Na/Ca also show
considerable variation (Figs. 5 and 6). Low-salinity fluid
inclusions from the quartz–stibnite mineralization of
Mina Nerón have lower Na/K and Na/Li ratios compared to the high-salinity fluids, and are especially distinct in terms of their Na/K ratios. The fluid inclusions
in quartz and barite from Mina Aurora have the same
Na/K ratios but markedly different Na/Li ratios, with
fluids in barite being depleted in Li relative to the other
samples. There is no correlation of Cl/Br with Na/K
(Fig. 7) or Na/Li. However, fluid inclusions in quartz
959
Table 3 Crush-leach analyses of fluid inclusions hosted by the different vein minerals with the d37Cl, dD and d34S of the fluids and minerals
(anion and cation data reported in ppb (as analyzed); n.a. not analyzed, cont. analyses contaminated by minerals not removed prior to
crushing)
Locality Sample no. Mineral Na
Nerón
1023
1025
16039
16040
10000m
10000x
10000m
10000x
16039m
16039x
16040m
16041m
16041x
16041a
16041b
16041c
Aurora 1027
1031
1027
1030
1031
1026
1027
1030
1032
Silillas 11112
11113
11109
11112
K
Ca Li
Mg Ba Sr Fe
Mn Zn
F
Cl
Br
SO4 d34S
d37Cl FIs. dD FIs.
(&)CDT (&)SMOC (&)SMOW
Stibnite
1,031
2,656
1,763
1,978
Quartz 4,974
9,548
8,360
12,584
2,468
135
338
212
102
405
187
577
542
225
20,658
4,243
1,821
6,095
Quartz 1,870
416
Galena
488 8,723 48 367 63 153 405 65
92
13.7
<20
8
253
16.7
175
6
36
21
983
2,100
1,157
1,764
524
4.5
13.1
7.4
7.1
49.2
33.2
41.7
50.5
29.7
57
105
83
126
79
28
35
27
42
16
31
72
58
95
30
99
117
cont.
387
cont.
Barite
Quartz
17,010 1,608
9
369 <20
380 <20
32
9
Galena
Fig. 4 Fluid-inclusion compositions compared to the trend shown
by evaporating seawater. Sample marked * is the bulk analysis of
fluid inclusions in sample 16041 which contains halite daughter
crystals. Samples marked a–c represent halite-bearing, high-salinity
and low-salinity portions of sample 16041
from Mina Nerón show a good correlation of Cl/Br and
Na/Ca, with the most Br-rich fluids having the lowest
Na/Ca ratio.
The d37Cl values of fluid inclusions in quartz (Fig. 8)
cover a large range from )1.8 to +3.2& (SMOC). The
low-salinity fluids associated with the mineralization have
<20
338
655
<20
<20
<20
<20
<20
<20
1,995
4,900
3,494
3,909
8,925
17,429
15,371
24,029
5,122
5
18.2
23
23.1
47
127.2
33.6
61.2
17.2
n.a.
n.a.
n.a.
n.a.
956
333
cont.
915
cont.
126 <20
<20
<20
<20
<20
<20
51,264
9,385
2,855
13,730
3,859
865
639.4
131.6
27.9
89.9
24.4
4.6
cont.
115
142
173
n.a.
n.a.
5
127
16
59
3 cont.
4
113
11 cont.
)2.03
)2.29
)1.45
)1.82
)2.67
)2.74
)24.7
0.85
3.2
)1.77
)0.91
)0.26
)8.9
)9.9
)14.7
)23.7
)17.2
)19.04
)19.63
)17.63
6.44
)0.20
<20 34,097 99.2 n.a.
6.35
4.08
<20 1,125 14.9 n.a.
<20 1,161 14.4 n.a.
)15.47
)15.42
Fig. 5 Comparison of the fluid-inclusion temperatures using the
crush-leach analyses and equations for the Na/K and Na/Li
geothermometers derived by Verma and Santoyo (1997). Samples
designated by *, a, and c belong to the different portions of sample
16041 as identified in Fig. 4. Sample b contained too little K for
analysis and is not shown
an excellent correlation with Cl/Br (r2=0.91). The halitebearing fluid is quite distinct from these others, having a
much lower Cl/Br ratio and a d37Cl value which is within
the range expected from evaporation of seawater. Saline
brines trapped in quartz from other Spanish Pb–Zn
960
Fig. 6 The Na/Ca vs. Cl/Br ratios of quartz-hosted fluid inclusions
from Mina Nerón show a good correlation which may indicate
either fluid mixing or immiscibility. Only the bulk analysis of
sample 16041 is shown, plotting at one extreme. However, because
of its d37Cl value (Fig. 8), the highest-salinity inclusions found in
this sample could not be involved, either through mechanical
mixing or by dilution with other fluids, in producing the observed
correlation of the other samples
Fig. 7 Na/K vs. Cl/Br ratios of the fluid inclusions relative to the
trend expected for evaporating seawater. Samples designated by *,
a, and c belong to the different portions of sample 16041 as
identified in Fig. 4. Sample b contained too little K for analysis and
is not shown. The variation in Cl/Br at constant Na/K for some
quartz and stibnite samples from Mina Nerón is due to fluid
immiscibility. Other samples which contain variable proportions of
the late high-salinity fluid have quite different ratios
mineralization, in the Maestrat Basin, have quite different
distributions of Cl/Br ratios and d37Cl values, and show
no correlation between the chlorine isotopic composition
and the Cl/Br ratios (Banks 2001).
Origin of the fluids
Determination of halogen concentrations, especially Cl
and Br, and their ratios in fluid inclusions can be used to
Fig. 8 Cl/Br vs. d37Cl. The d37Cl values of fluid inclusions in
massive gangue quartz from Mina Nerón plot on a trend line
typical for fluids which underwent two-phase separation, as
observed for seafloor mineralization (Lüders et al. 2002). Samples
with vapor-rich inclusions have negative d37Cl values whereas the
residual fluid becomes more enriched in 37Cl. Evaporation alone
would only result in d37Cl fractionation of ±0.5& (shown by the
two dashed lines). The bulk analysis of sample 16041 from Mina
Nerón (identified by *), which is dominated by high-salinity, halitebearing fluid inclusions in late-stage quartz, had a different origin
to the other fluids or different formation conditions. For
comparison, high-salinity fluid inclusions in quartz from Spanish
Pb–Zn brines in the Maestrat Basin (Banks 2001) differ clearly
from the data in this study. The error bars represent 2r errors
provide information on the origin of the salinity. In the
absence of evaporite minerals, Cl and Br behave conservatively during fluid flow because they are not significantly involved in fluid–rock interactions (Banks et al.
1991; Bohlke and Irwin 1992). In addition, they can be
used to determine if fluid mixing or evaporation during
fluid flow and mineral deposition has occurred. The Cl/
Br and Na/Br ratios of fluid inclusions in different
minerals from the studied deposits are shown in Fig. 4
relative to the seawater evaporation line based on data
compiled by Fontes and Mattray (1993). The dashed line
in the Na/Br versus Cl/Br plot represents Na to Cl ratios
of 1, and any removal or addition of halite produces a
data array parallel to this line.
High-salinity and halite-bearing inclusions from
Mina Silillas and Mina Nerón (late fluids in vug quartz)
plot close to the seawater evaporation line, with Cl/Br
and Na/Br ratios which would be generated when
almost all the halite had been precipitated from seawater. The lower-salinity fluids, associated with the
quartz–stibnite mineralization and quartz from Mina
Aurora, also plot close to or on the seawater evaporation line, albeit with higher Cl/Br and Na/Br ratios.
However, the salinity of these fluid inclusions is too low
for them to represent only evaporated seawater, and
they would have to have been diluted by a low-salinity
fluid. Fluids which plot on the seawater evaporation line
at these points should have approximately 35 wt% total
dissolved solids, whereas the salinity of these fluids is less
961
than 5 wt%. To maintain these Cl/Br and Na/Br ratios,
the diluting fluid must have had a very low salinity.
Quartz samples and the barite from Mina Aurora have
Cl/Br and Na/Br ratios which are close to seawater
values, and their salinity would be consistent with such
an origin. Other samples have significantly higher Cl/Br
and Na/Br ratios which would be consistent with a
contribution from the dissolution of halite in the fluid.
However, an origin for the fluids which involves seawater evaporation, halite dissolution and dilution by a
low-salinity fluid is not compatible with the Cl isotope
data (Fig. 8). d37Cl values should be within the range of
+0.3 to )0.5& if the fluids were derived by evaporation
of seawater or dissolution of halite (Eggenkamp et al.
1995). However, the values of the fluids are between )1.8
and +3.2& and show an excellent correlation with the
Cl/Br ratios. The linear regression through the data in
Fig. 8 passes within error of each sample and of seawater. The late quartz, which contains the highest-salinity halite-bearing fluid inclusions, does not plot close
to the other samples. This has the lowest Cl/Br ratio and
a d37Cl value both of which are consistent with a seawater evaporation origin for the fluid. The same origin is
likely for the quartz-hosted fluid inclusions from Mina
Silillas.
Fluid–rock interactions
Unlike the halogens, the cation content of the fluid is
more likely to reflect some degree of re-equilibration
with rocks along the flow path and at the site of mineralization, and ultimately may bear no resemblance to
its original composition. An assessment of the closeness
to equilibration can be made by comparing temperatures derived from mineral geothermometers and
fluid-inclusion temperatures. The fluid-inclusion
homogenization temperatures are the minimum temperatures of the fluid and corrections for pressure have
to be applied, which are often difficult to determine
exactly. The temperatures obtained from mineral geothermometers are only valid if the fluid has reached
equilibrium with the appropriate mineral assemblage
and, using only one geothermometer, this is difficult to
assess. In this study (Fig. 5) we have used the Na/K and
Na/Li geothermometers of Fournier (1979) and Fouillac
and Michard (1981), which have improved equations
determined by Verma and Santoyo (1997) to estimate
the temperature of the different fluids. None of the
samples plot on the combined geothermometer line, and
the Na/K and Na/Li geothermometers give markedly
different temperatures.
The majority of quartz and stibnite from Mina
Nerón, and of quartz and barite from Mina Aurora
give temperatures of between 250 and ca. 180 C using
the Na/K geothermometer. Halite-bearing fluid inclusions from Mina Nerón have much larger Na/K ratios
which are outside the validity of the geothermometer.
Temperatures obtained with the Na/Li geothermometer
are much higher, between 300 and 450 C but most are
close to 350 C. Fluid inclusions in quartz and stibnite
at Mina Nerón have homogenization temperatures of
about 185 C, and the late-stage high-salinity fluids
have halite dissolution temperatures of about 170 C.
At Mina Aurora, homogenization temperatures of ca.
240 C were recorded for fluid inclusions in quartz.
The two mineral geothermometers give different temperatures, and this may be due to the fluids not
achieving equilibrium with the appropriate mineral
assemblages. However, it is also possible that both are
correct but reflect the temperature at different stages of
the fluid’s history. The temperatures obtained from
Na/K ratios are almost identical to the fluid-inclusion
temperatures, and we suggest they may be reflecting
equilibration of the fluid at the site of mineral deposition. Sodium and K are more readily re-equilibrated
than Li which is essentially conservative once in solution (Fontes and Matray 1993). Therefore, the temperatures obtained from the Na/Li ratios may be
reflecting the maximum temperature reached by the
fluid prior to its ascent. We believe that the initial
temperature of the ore-forming fluids was approximately 300 C, and that the Na/K geothermometer
probably indicates the true temperature of the mineral
formation whereas the Na/Li geothermometer more
likely reflects the initial temperature of the hydrothermal fluids. Consequently, if the mineralization occurred
in a near-surface environment (<500 m), as indicated
by the stratigraphic position of the deposits, it is very
likely that the ascending fluids locally underwent partial two-phase separation.
We suggested above that the highest-salinity fluid
inclusions were derived from seawater which evaporated
past the point of halite precipitation. If this were the
case, then the fluid should contain little Ca and large
amounts of Mg. However, we observe the opposite
(Table 3), and this is common for most high-salinity,
low-temperature brines where dolomitization controls
the Ca and Mg contents of the fluids. In Fig. 8 there is a
good correlation between the highest-salinity fluid
(marked with *) and the lower-salinity mineralizing
fluids. We assume that in these fluids the variation in Cl/
Br ratios was due to phase separation. It is possible the
correlation of Cl/Br and Na/Ca was caused by this
process, but we are unaware of data on the behavior of
Ca in these circumstances. However, in Fig. 7 there is
not a similar correlation of Na/K and Cl/Br for the
fluids we suggest have undergone phase separation. It is
not possible that the correlation is caused by the presence of small amounts of the late highest-salinity fluids
(as secondary inclusions) which were mixed with the
low-salinity fluids during crushing, based on the evidence from the Cl isotopes. If the highest-salinity fluids
at Mina Nerón represent evaporated seawater, they
should plot on the seawater evaporation trend line in
Fig. 7. They clearly do not, as they have ca. 7 to 9 times
less K than expected. This could be explained if, for
example, illite was produced from the fluid.
962
Fig. 9 Cl/Br vs. dD diagram
Deuterium and oxygen isotopes
The dD values of fluid inclusions hosted in quartz and
stibnite from Mina Nerón, obtained by mechanical extraction, fall within a narrow range of )24.7 to )8.9&
and are not correlated with the Cl/Br ratios (Fig. 9).
Considering a 2 sigma uncertainty of 5 to 10& (Simon
2001), all samples could have formed from a single fluid
with a mean dD composition of about 15&.
A profile through a well-crystallized quartz sample
from Mina Nerón indicates a correlation between the
salinity of fluid inclusions, in distinct growth zones, and
the d18O isotopic composition (Fig. 10). The lowest d18O
values of 19.3 to 20.7& were measured in growth zones
containing low-salinity fluid inclusions which have a Cl/
Br ratio of 153. By contrast, growth zones containing
high-salinity fluid inclusions without a halite daughter
mineral have a lower Cl/Br ratio of 103 and d18O values
between 20.8 and 21.6&. In the uppermost zone of the
quartz crystal where high-salinity fluid inclusions predominantly contain halite daughter crystals, a further
increase of the d18O values up to 23.4& occurs, accompanied by a decrease of the Cl/Br ratio to 72.
Sulfur isotopes in galena and stibnite
Stibnite samples from Mina Nerón have d34S values
from )1.45 to )2.74&, and clearly differ from the d34S
values of galena samples from Mina Silillas and Mina
Aurora which have considerably lower d34S values
(Table 3). Galena from Mina Silillas has d34S values of
about )15.5&, and galena from Mina Aurora between
)17.6 and )19.6&, the latter being lower than the
reported d34S values of )15 to +10& from sulfides of
the massive sulfide deposits in the IPB (Sáez et al. 1999).
For example, at the Tharsis deposit, which is the only
Fig. 10 d18O distribution in a chevron quartz of the late
mineralization stage from Mina Nerón (sample 16041x in Tables 2
and 3)
known massive sulfide deposit in the IPB with negative
sulfur isotopic signatures, the d34S values are between
)10.7 and +1.3& (Tornos et al. 1998). At some massive sulfide deposits, the d34S values of barite lie between
+15 and +20&, which leads to the assumption that
sulfate was derived from seawater. By contrast, sulfur
isotopes in barite from Mina Aurora have d34S values
between )0.2 and +6.4&, indicating a different source
of sulfate.
Origin of sulfur
The differences in sulfur isotopic compositions indicate
that stibnite, galena, and barite either derived their sulfur from distinctly different sources, or formed under
quite different physico-chemical conditions. Sulfur isotopic fractionation in hydrothermal ore deposits is a
function of temperature, pH, ƒO2, isotopic composition
of the ore-forming fluid, and/or dissolved element species in the fluid (Ohmoto and Rye 1979). Assuming a
homogeneous sulfur source for all three deposits, significant changes in ƒO2 and pH would have been required to change the d34S values of sulfides and barite
during ore deposition. Ohmoto (1972) has demonstrated
that for a fluid with d34SSS=0& and T=250 C, the
d34S values for sphalerite could vary between +5.8 and
)24&, and for coexisting barite between 0 and +24.2&,
within geologically reasonable limits for ƒO2 and pH. At
low ƒO2 and pH values, the d34S values of sulfides will be
similar to d34SSS whereas at high ƒO2 values, the d34S
values of minerals can differ significantly from d34SSS.
963
However, at high ƒO2 values the proportion of aqueous
sulfate in the fluid will increase significantly with respect
to H2S in the fluid, and considerable amounts of sulfates
should precipitate. Small changes in ƒO2 and pH would
cause large changes in the d34S values of either sulfate or
sulfide (Ohmoto and Lasaga 1982). Assuming a common sulfur source for all three deposits, the variations of
the d34S values can be explained by changes in ƒO2 and
pH. The variation of the large negative d34S values of
galena (Table 2) from Mina Aurora and Mina Silillas
can be attributed to a significant increase in ƒO2, considering an initial d34SSS=0& for the parental oreforming fluid (Ohmoto 1972). The small negative d34S
values of stibnites from Mina Nerón would have required only a slight increase in ƒO2 or a decrease in pH.
The positive d34S values of barite samples from Mina
Aurora (Table 3) could also have resulted from significant changes in ƒO2. However, the data from quartzhosted fluid inclusions (associated with galena) and
massive barite from Mina Aurora (Fig. 3) do not support the contemporaneous formation of both minerals
from the same fluid.
Alternatively, the observed variation in the sulfur
isotopic compositions of galena, stibnite, and barite
could indicate different sources of sulfur. The large
negative d34S values of the galena samples from Mina
Aurora and Mina Silillas (Table 3) are comparable to
d34S values of sediment-hosted massive sulfide deposits
(Sáez et al. 1999) and sedimentary sulfides in the Iberian
Pyrite Belt (Routhier et al. 1980). Therefore, deeply
circulating fluids could have derived sulfur as well as
metals by alteration of sedimentary rocks and/or preexisting sediment-hosted massive sulfide deposits. Marcoux et al. (1994) and Wipfler and Sedler (1995) have
shown that galena samples from late vein-type mineralization have a more radiogenic Pb isotopic composition than the massive sulfide mineralization. At Mina
Aurora, barite seems to have precipitated at a late stage
of mineralization. The variations in sulfur isotopic
compositions between )0.2 and 6.4& can be explained
by mixing of a cold SO24 -dominant fluid, probably residual evaporated seawater, with variable amounts of
dissolved biogenic sulfide with ascending, hydrothermal
Ba-bearing fluids.
The small negative d34S values of the stibnite samples
from Mina Nerón are similar to those of volcanic-hosted
massive sulfide deposits in the IPB (Routhier et al. 1980;
Sáez et al. 1999). These deposits are characterized by
anomalously high concentrations of mobile elements
such as As, Sb Tl, and F (Möller et al. 1983) which are
assumed to be secondary dispersion haloes from underlying orebodies resulting from the action of diagenetic or metamorphic fluids. Therefore, it seems
plausible that the stibnite mineralization at Mina Nerón
was deposited from fluids which altered the volcanichosted massive sulfides and derived metals and sulfur
from deeper orebodies, under the assumption that only
small isotopic fractionation occurred from the leaching
stage to the deposition stage.
Metallogenic model
The data obtained from studies of sulfur isotopes and
fluid inclusions in quartz, stibnite and barite show there
to be different sources of metals and sulfur, and different
depositional conditions for some vein-type mineralizations in the IPB. Vein-type deposits in the study area are
situated along E-W-striking (normal) fault zones which
were active during the transition from Hercynian uplift
to extensional tectonism. Therefore, the vein mineralization seems to be related to a late stage of metamorphism (Routhier et al. 1980). Extensional tectonics
allowed fluids from deeper reservoirs to ascend along
faults to higher crustal levels, accompanied by fluctuations in pressure and temperature (Cathelineau et al.
2000). However, it should be noted that rocks of the
uppermost VS Complex and of the Culm sequence host
all the studied vein-type deposits. The thickness of flysch
sediments in the eastern part of the IPB (in the Huelva
province) certainly does not exceed some 500 m, according to Schermerhorn and Stanton (1969). Therefore,
it can be considered that mineral deposition occurred
close to the paleosurface, in the upper 1 km of the crust
where the hydrothermal fluids were likely to have been
hydrostatically pressured (Hedenquist and Henley 1985).
At Mina Nerón, banded quartz–stibnite ore (Fig. 2a)
was deposited from low- to moderate-salinity fluids
probably containing small proportions of dissolved
gases. Similar fluids are present in late metamorphicstage quartz from dissolution vugs and small veinlets
within the massive sulfide orebodies, where it is associated with polymetallic sulfide assemblages, and/or late
chlorite and carbonates (Cathelineau et al. 2001). The
precipitation of milky gangue quartz seems to have occurred at temperatures similar to that of stibnite. A
similar low-salinity fluid precipitated gangue quartz at
Mina Aurora but at slightly higher temperatures
(Fig. 3). By contrast, well-crystallized quartz with
chevron structures from Mina Nerón contains low-salinity fluid inclusions with Th values generally above
150 C in the milky zones, and high-salinity fluid inclusions with lower Th values in the clear zones. Fluid
inclusions with halite daughter crystals can be found in
the outermost parts of the quartz crystals (Fig. 2b). Such
a significant variation in salinity of the fluid inclusions
within a single deposit can either be produced by continuous, non-adiabatic boiling in the vein structure, thus
indicating near-surface depositional conditions, or it is a
result of fluid mixing. In the latter case, a high-salinity
fluid (>30 wt% NaCl equiv.) must have been the
dominant fluid which mixed with the metal-bearing lowsalinity fluid. Both scenarios are discussed in the light of
the results of fluid-inclusion studies.
Fluid mixing model
A simple mixing model requires at least two fluids, with
different compositions, from different sources to explain
964
the observed variations in temperature, salinity, and
isotopic compositions. It is likely that the ore-forming
fluids ascended from depth due to the opening of the
shear zones during late stages of metamorphism. In the
case of fluid mixing this hot, metal-bearing fluid must
have mixed in differing proportions with a colder (descending?), highly saline brine. The source of this brine
in the study area is unclear. However, the Cl/Br ratios of
the high-salinity inclusions exclude the dissolution of
evaporites and strongly indicate that the fluid was a residual brine derived from seawater evaporation past the
point of halite precipitation (Fig. 4). If these brine-type
fluids were derived from evaporated seawater, from a
near-surface reservoir such as a sabkha, one would expect that such a brine was enriched in heavy isotopes D
and 18O, as illustrated by water from the Red Sea (Craig
1966). For oxygen such a trend can be observed by the
clear zones in quartz hosting highly saline fluid inclusions with heavier d18O values, compared to milky zones
which host fluid inclusions with considerably lower salinity (Fig. 10) whereas no positive dD values were
measured (Fig. 9 and Table 3). If we assume that quartz
precipitated under a hydrostatic pressure regime at a
depth of about 500 m, a pressure correction for the
mean Th values of about 180 C for the low-salinity
fluid inclusions would be negligible. The mean d18O
value of quartz in the growth zone hosting this type of
inclusions is about 20&, and the d18O value of water in
equilibrium with the quartz at 180 C would be about
7.0& (Zheng 1993). By contrast, clear zones in the
studied chevron quartz sample, hosting the high-salinity
fluid inclusions which were trapped at minimum temperatures of about 175 C (as indicated by halite dissolution after homogenization of the vapor phase), are
associated with a fluid which had an initial d18O value of
about 10&. The fractionation between quartz and pure
water is 13& at 170 C, and 13.8& at 180 C (Zheng
1993). High concentrations of salts such as NaCl and/or
KCl would lower the fractionation to about 0.5&
(Horita et al. 1993). If high amounts of CaCl2 are contained in the fluid, the fractionation between quartz and
the fluid can be lowered to about 2& (Horita et al.
1993). However, the initial d18O value of the brine can be
assumed to be between 8.5 and 10&. Thus, the initial
d18O values as well as the dD values of both fluids are
typical for formation waters or metamorphic fluids, and
therefore have probably been modified by water–rock
interactions in the sub-surface from the typical values
expected of sabkha-type fluids. A mixing model where a
hot, low-salinity fluid mixes with considerable amounts
of a cold, near-surface brine would not account for the
variations in the d18O of the quartz and temperatures of
175 C indicated by the high-salinity fluid inclusions. If
we assume that the temperature of the high-salinity fluid
was less than 50 C, the fluctuation in temperature must
be considerably higher than observed. Such a mixing
model would only account for the observed variations in
salinity and temperatures (indicated by the presence of
monophase, aqueous inclusions) of fluid inclusions
hosted in barite from the Mina Aurora. Furthermore,
the strong fractionation of the d37Cl values cannot be
explained by evaporation and mixing alone. Solomon
et al. (2002) suggested that brine pools developed by
exsolution of supercritical fluids from magmas during
the main period of volcanism in the Spanish Pyrite Belt.
The origin of highly saline fluids in such brine pools at
depth is reasonable. However, in our case, these brines
must have formed by seawater evaporation rather than
magmatic fluids. In the latter case, the Cl/Br ratios of the
brines should be considerably higher than observed
(Bohlke and Irwin 1992), and the chlorine isotopic
composition should show a magmatic signature (Banks
et al. 2000b).
Phase-separation model
Under a hydrostatic pressure regime, an ascending fluid
with a salinity of about 3 wt% NaCl equiv. and an initial temperature of about 300 C (mean temperature
derived from Na/Li and Na/K geothermometers) would
start boiling at a depth of about 900 m (Cunningham
1978). In any case, if the pressure at the site of mineralization was less than 90 bar, as indicated by the
stratigraphic position of the deposits (<500 m), the
hydrothermal fluid responsible for the vein-type deposits
must have undergone phase separation into a low-salinity vapor and a saline brine (Fig. 11). In the temperature range under consideration for the ore-forming
fluid (250±50 C), any dD variation will be less obvious
and fractionation scatters around 0±10& (Driesner and
Seward 2000). If some equilibration with the country
rock and boiling processes had occurred, a variation of
10& of the dD values is possible.
A phase-separation model would also account for
both the observed variations in the Cl/Br ratios and the
fractionation of the chlorine isotopes. With the
Fig. 11 Temperature–pressure–depth diagram showing the twophase boundary for a H2O–NaCl solution of seawater salinity
(3.2 wt% NaCl equiv.; modified after Cunningham 1978; Bischoff
and Pitzner 1985). The shaded field marks the range of trapping
temperature fluid inclusions in quartz and stibnite from Mina
Nerón. High-salinity fluids are probably the product of extreme
boiling (for details see text)
965
exception of the sample which contains halite daughter
crystals in the fluid inclusions, all the other four quartz
samples from quartz–stibnite ores from Mina Nerón
have a large variation in chlorine isotopes which correlates with a large variation in Cl/Br ratios (Fig. 8). A
similar correlation was first observed for sphaleritehosted fluid inclusions from modern seafloor mineralization, where fluid-inclusion microthermometric data
have revealed compelling evidence for two-phase separation (Lüders et al. 2002). However, negative d37Cl
values were not measured in fluid inclusions from seafloor mineralization, due to the escape of significant
amounts of vapor from the systems. At the Mina Nerón
site, it seems that considerable amounts of vapor (and
condensed vapor?) are trapped as fluid inclusions in vein
quartz, as indicated by the presence of numerous, small
vapor-rich inclusions besides aqueous two-phase inclusions. These samples have negative d37Cl values and
larger Cl/Br ratios when compared with the aqueous
two-phase fluid inclusions (Fig. 8). In this scenario, the
most saline fluids should have the most positive d37Cl
values. This is not the case at the Mina Nerón where the
late-stage fluid trapped in clear zones of chevron quartz
has the lowest Cl/Br ratio and a d37Cl value close to 0&.
This fluid clearly originated from a different source or
was produced by a different process, and indicates that
at least two different fluid sources were involved during
the formation of late-stage quartz II at the Mina Nerón
site. The halogen data are most consistent with its origin
being that of a highly evaporated seawater, but there is
still the question of how the fluid became so hot. As
mentioned above, a simple mixing model where a cold,
near-surface brine mixed with an ascending hydrothermal fluid should have produced a larger temperature
decrease than observed. Therefore, the mixed brine appears to have had a temperature similar to that of the
ascending fluid. This brine may represent a formation
water heated by the escape of steam from the upflowing
fluid. The large bromine enrichment in the fluid may
have been caused by halite precipitation. However, we
can exclude the possibility that this saline brine was
derived from the same ascending hydrothermal fluid
which underwent extreme boiling at higher levels close
to the surface. If this had occurred, the d37Cl values
would be expected to be even more positive than those
we recorded in other samples, which is not the case.
Therefore, these high-salinity fluids did not play an important role in the formation of quartz–stibnite mineralization, but seem to have had a greater role in the
formation of galena and quartz mineralization at the
Mina Silillas site.
Conclusions
Detailed studies of sulfur isotopic compositions of sulfides and fluid inclusions hosted in various minerals
from late Hercynian vein-type mineralization in the
central part of the IPB have revealed evidence for
different sources of sulfur (and metals) as well as different depositional conditions.
There is strong evidence that quartz–stibnite mineralization is the product of a non-adiabatic phase-separation (boiling) process under a hydrostatic pressure
regime (<100 bar). Phase separation caused pronounced fractionation of chlorine isotopes which correlate with the fractionation of Cl/Br ratios. The
chlorine isotope data clearly differ from other Spanish
Pb–Zn mineralization which is assumed to have deposited from brines similar to those in this study.
The sources of antimony and sulfide are assumed to
come from a hidden, underlying volcanic-hosted massive
sulfide orebody. High-salinity fluids which are involved
in the formation of late-stage quartz, and the Pb–Zn
mineralization at other localities, are not derived from
dissolution of evaporites. They most closely resemble
formation waters which originated as residual brines.
The sulfur isotopic compositions of galena samples
from two occurrences indicate a sedimentary source of
sulfide, as it is also proposed for the metals from the Pb
isotopic signatures in galena (Marcoux et al. 1994). It is
also likely that boiling occurred during the formation of
quartz–galena mineralization. Barite is thought to have
formed by mixing of the ascending fluids with meteoric
water.
From detailed studies of vein-type deposits in the
Iberian Pyrite Belt, it may be possible to trace deeperlying massive sulfide orebodies. Although there are different sources for the metals and sulfur and different
depositional conditions for the polymetallic ores, their
origin is apparently linked to the older massive sulfide
deposits.
Acknowledgements We are particularly indebted to I.K. Sedler and
E.L. Wipfler for providing sample material from the studied vein
mineralizations. We are grateful to M. Cathelineau and another
anonymous Mineralium Deposita referee for their constructive
reviews of the manuscript and useful comments.
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