Economic Geology
Vol. 98, 2003, pp. 1397–1411
Origin of Ore-Forming Brines in Sediment-Hosted Zn-Pb Deposits of
the Basque-Cantabrian Basin, Northern Spain
FIDEL GRANDIA,† ÀNGELS CANALS,
Departament de Cristal lografia, Mineralogia i Dipòsits Minerals, Universitat de Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain
ESTEVE CARDELLACH,
Departament de Geologia, Universitat Autònoma de Barcelona, Edifici C, UAB, 08193 Bellaterra, Spain
DAVID A. BANKS,
School of Earth Sciences, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, United Kingdom
AND JOAQUIM
PERONA
Departament de Geologia, Universitat Autònoma de Barcelona, Edifici C, UAB, 08193 Bellaterra, Spain
Abstract
Fluid inclusion data (microthermometry and Na-K-Li-Cl-Br chemistry) from Mississippi Valley-type Zn-Pb
deposits in the Basque-Cantabrian basin, north Iberian Peninsula, indicate that fluid mixing occurred during
mineralization. Cl/Br ratios of the ore-forming brines suggest that the high salinity was primarily acquired by
evaporation of seawater. Only in deposits near salt domes (Orduña and Murgía diapirs) do the ore-forming
brines have halogen signatures indicative of halite dissolution.
Mixing of fluids frequently cannot be detected using only microthermometry or halogen data, although microthermometric data can indicate mixing where one of the end members is fresh water or a highly diluted
fluid. Combining the two allows both the recognition of mixing and an estimation of the relative proportion of
the different fluids involved. The effects of mixing on the salinity and halogen ratios of the resulting mixture
have been calculated using four types of fluids: (1) seawater, (2) evaporated seawater before the onset of halite
precipitation, (3) seawater evaporated past the point of halite precipitation, and (4) a halite-saturated brine at
25ºC derived from halite dissolution. The calculated Cl/Br-Na/Br mixing curves have been compared to data
from the Zn-Pb deposits of the Basque-Cantabrian basin and show that mixing between a residual brine and a
halite-dissolution brine can account for the deposits in the Western Biscay district (Txomin, Matienzo, Barambio). Most of the ore-forming brines from the studied deposits around the Orduña and Murgía salt domes (Altube, Monteleón, Jugo) originated by halite dissolution, although there is also significant contribution (20–50
wt %) from a highly evaporated brine.
Introduction
HALOGEN (Cl, Br, and to a lesser extent I) concentrations and
ratios have been used as tracers of fluid movement and indicators of the origin of salinity in fluids in sedimentary basins
(Carpenter, 1978; Land and Prezbindowski, 1981; Stoessell
and Moore, 1983; Walter et al., 1990; Banks and Yardley,
1992; Kesler et al., 1995, 1996). Their usefulness lies in the
fact that they behave conservatively in solution, as they do not
readily exchange with rocks during water-rock interactions.
Brines formed by dissolution of evaporites and by seawater
evaporation have very different Na-Cl-Br systematics (Fig. 1).
During seawater evaporation, Br is concentrated in the residual brine, as it cannot be accommodated in the halite crystal
lattice. Conversely, when halite is dissolved, the resulting
brine will have low Br concentration and the Cl/Br and Na/Br
ratios of the resulting fluids will be distinctly higher.
Halogen data from fluids related to Mississippi Valley-type
deposits worldwide are interpreted by using Cl/Br versus
Na/Br diagrams, and most indicate that basinal brines acquired their salinity by seawater evaporation (Kesler et al.,
1996; Viets et al., 1996; among others). Only in areas in the
† Corresponding
author: e-mail, [email protected]
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vicinity of salt diapirs is there evidence of salinity acquired by
dissolution of evaporites (Posey and Kyle, 1988). However,
Chi and Savard (1997) suggested that the brines derived from
seawater evaporation and halite dissolution are not mutually
exclusive and that mixing can occur in sedimentary basins.
According to these authors, data plotted on the seawater
evaporation trajectory in a Cl/Br versus Na/Br diagram can be
explained by mixing of evaporated seawater with a halite-dissolution fluid, and brines having Cl/Br and Na/Br ratios
higher than seawater could have a contribution from evaporated seawater. They concluded that Cl/Br versus Na/Br diagrams could not be used to distinguish between evaporated
seawater and halite-dissolution fluid.
The aim of this paper is to provide new insights concerning
the role of mixing in the interpretation of data in Na/Br versus Cl/Br diagrams. Microthermometric and halogen data obtained in sphalerite, dolomite, quartz, barite, and calcite from
Mississippi Valley-type deposits in the Basque-Cantabrian
basin, northern Iberian Peninsula, are presented and compared with several theoretical mixing scenarios. The results
extend the utility of the Na/Br versus Cl/Br diagrams for the
interpretation of fluid inclusion data, brine sources, and fluid
movement in sedimentary basins.
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GRANDIA ET AL.
26000
Basque-Cantabrian
Basin
22000
Halite
18000
1200
PYREN
EAN RIF
T
14000
600
NTIC
O
Seawater
Halite
precipitation
Madrid
SYSTE
M
Barcelona
Maestrat Basin
ATLA
(Cl/Br)
6000 10000 14000 18000 22000 26000
EM
ST
SY
molar
6000
800
FT
RI
CEA
N
1000
N
IA
ER
IB
10000
IBERIAN MASSIF
Seawater evaporation
trajectory
400
N
200
EM
ST
SY
FT
I
R
TIC
BE
TERTIARY BASINS
MESOZOIC RIFT SYSTEMS
HERCYNIAN BASEMENT
Mg-K salts precipitation
0
0
200
400
600
(Na/Br)
800
1000
1200
molar
FIG. 1. Cl/Br versus Na/Br ratios of halite, seawater, and residual fluids
precipitated from the progressive evaporation of seawater. Data for the evaporation seawater trajectory are taken from Fontes and Matray (1993). Data
for halite are from McCaffrey et al. (1987).
Zn-Pb Deposits in the Basque-Cantabrian Basin
(North Iberian Peninsula)
Geologic setting
Mesozoic basins in the north Iberian Peninsula contain ZnPb Mississippi Valley-type mineralization primarily in the
Basque-Cantabrian basin (Herrero, 1989; Velasco et al.,
1994), and in the Maestrat basin (Michel, 1974; Grandia et
al., 2003). These basins are associated with rift systems that
developed during the Mesozoic between the Iberian and European plates (Fig. 2; see reviews in Vergés and García-Senz,
2001; Salas et al., 2001). After the extensional stages, collision
of the two plates occurred during the Alpine orogeny in the
Tertiary.
The Mesozoic stratigraphic record in the Basque-Cantabrian
basin includes sedimentary rocks from the Triassic up to the
Upper Cretaceous. The Triassic series, deposited during the
first rifting stages, shows the characteristic germanic-type
succession, with red clastic sediments (Buntsandstein and
Middle Muschelkalk facies), platform carbonates (Lower and
Upper Muschelkalk facies), and evaporite-rich shale units
(Keuper facies). Jurassic rocks typically consist of platform
limestones and marly facies. The intense fault activity during
a second rifting event that began in the Late Jurassic led to
the deposition of a very thick succession of sedimentary rocks,
mainly during the Early Cretaceous. The initial sedimentation, from Kimmeridgian to Barremian, was clastic (Purbeck
and Weald facies: Rat, 1983). The Aptian to Albian period is
characterized by the deposition of the Urgonian facies (Rat,
1959), which typically consists of shallow-platform limestones
with abundant rudist-rich reefs. Their accumulated thickness
may be up to 4,000 m in the western and central parts of the
Basque-Cantabrian basin (García-Mondéjar, 1985). During
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FIG. 2. Schematic geological map of the Iberian Peninsula, showing the
location of the rifting systems during the Mesozoic. The boundary between
the Iberian and European plates is shown by a dashed line along the Pyrenean rift system.
this period, the basin was divided into several sub-basins. In
the central and eastern regions of the Basque-Cantabrian
basin, the Urgonian facies are essentially made up of terrigenous sediments (e.g., Bilbao Formation), whereas reefal carbonates are predominant in the west (e.g., Reocín and Ramales formations: García-Mondéjar et al., 1996). Sediments
directly overlying the Urgonian facies are commonly siliciclastic (Utrillas and Valmaseda formations) and were deposited in continental, delta, and talus environments. Upper
Cretaceous-Paleocene rocks range from continental clastics
to platform limestones to flysch-type marls.
Mineralization
Zn-Pb mineralization in the Basque-Cantabrian basin is
found in five districts (Fig. 3A; Velasco et al., 1994): (1) the
Santander district, which includes the largest epigenetic ZnPb deposit in the Iberian Peninsula (Reocín mine, ~60 Mt at
8% Zn: Abajo and Piret, 1989); (2) the Western Biscay district; (3) the Bilbao district, where the mineralization consists
mainly of iron carbonates; (4) the Guipúzcoa district, with
some important syngenetic deposits (e.g., Troya mine); and
(5) the Northern Biscay district. Although no absolute dating
of mineralization in these districts is available, it is believed
that most deposits could have been formed in a relatively
short period of time, between the Late Cretaceous and the
Tertiary.
This study is mainly concerned with mineralization in the
Santander and Western Biscay districts, where the main ZnPb epigenetic deposits occur. In the Santander district (Fig.
3B), mineralization is enclosed within dolomitized platform
limestones of Aptian age (Reocín Formation). Morphologically, sulfides are characteristically stratiform or irregular replacements and vug fillings (Barbanson, 1987; Velasco et al.,
1994). Sphalerite is usually the major sulfide, commonly precipitating as colloform and banded growths. Galena is commonly present as skeletal or dendritic growths, evidence of
rapid precipitation. Carbonate gangue is usually dolomite; at
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ORE-FORMING FLUIDS OF THE BASQUE-CANTABRIAN BASIN, SPAIN
Santander Western Biscay Cantabrian Sea
District
District
Northern
Santander
Bilbao Biscay
District District
A
B
Cantabrian Sea
5 km
San
Sebasti n
Bilbao
43°30'
N
San Vicente
Comillas
de la Barquera
liva
Mine
Novales
Tertiary
Orduña & Murgía
N salt domes
Udías
Guipúzcoa
District
Vitoria
Cabezón Reocín
de la Sal
Mesozoic
Torrelavega
La Florida
Triassic salt domes
Ebro
Basin
Paleozoic
43°15'
50 km
4°15'
C
4°00'
Paleozoic
Early Cretaceous
Main towns
Triassic
Late Cretaceous
Faults
Jurassic
Tertiary
Zn-Pb deposits
5 km
D
N
Amurri
43°16'
Ramales
de la
Victoria
Matienzo
Lanestosa
Orduña
Diapir
Ambasaguas
Matienzo
Treto
Txomin
Concha
Monteleón
43°14'
Paúl
La Antigua
3°30'
Early Cretaceous
(carbonate rocks)
3°25'
Early Cretaceous
(siliciclastic rocks)
Latest AlbianCenomanian
Murgí
3°20'
3º00'
Iturlum
Jugo
Beluntza
Murgía
Diapir
43°12'
Triassic
Jurassic
43º00
Altube
Orduñ
N
3 km
Barambio
Aperregui
Mina de Vila
2º50'
Triassic
(with embedded Jurassic blocks)
Aptian
Late Cretaceous
Albian- Early
Miocene
Cenomanian
Main towns
Faults
Zn-Pb deposits
42º55
Zn-Pb deposits
Faults
Main towns
FIG. 3. A. Geological map of the Basque-Cantabrian basin, with the main Zn-Pb districts (modified from García-Mondéjar et al., 1996). B. Geological map of the Santander district, showing the location of the major Zn-Pb deposits (after Instituto Geológico y Minero de España, 1970). C. Geological map of the Western Biscay district, showing the location of the
major Zn-Pb deposits (after Instituto Geológico y Minero de España, 1970). D. Geological map of the Orduña and Murgía
salt domes and the surrounding area (after Instituto Geológico y Minero de España, 1970). Peridiapiric Zn-Pb-Ba deposits
are indicated. Although the Barambio vein is close to these salt domes, its geological features are comparable to veins in the
Western Biscay district, and therefore, this deposit is considered to belong to this district (see text).
Reocín, several precipitation stages of this carbonate have
been recognized (Velasco et al., 2000). Marcasite is locally
very abundant in this deposit. Other similar but smaller deposits are found at the Novales, Udías, and La Florida mines.
Zn-Pb occurrences in the Western Biscay district (Fig. 3C)
are hosted by three distinct lithologic units: dolomitized Aptian-Albian platform carbonate rocks (Ramales Formation),
platform-talus transition rocks, and basinal marls (Herrero,
1989). Most mineralization consists of sulfide replacement
and dissemination, although some veins cementing northwest-southeast–trending fractures also occur. The most important deposits in this area are the Txomin and Matienzo
mines, with a total tonnage of about 3 Mt at 11 percent Zn.
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Mineralization in the Txomin mine consists of a stratiform
unit of up to 1.5 m in thickness, developed in the platformtalus transition facies. The hypogene assemblage consists of
sphalerite, galena, pyrite, dolomite, calcite, and quartz. The
Matienzo vein system is located 5 km to the east of the Txomin mine and fills a fracture zone in the talus facies. Vein
thicknesses can reach up to 3 m, and the veins are filled by
sphalerite, galena, fluorite, and calcite. Authigenic K feldspar
is found in country rocks near the veins. Based on isotopic
data and cathodoluminescence petrography, Simón and
Grandia (1998) showed that a clear genetic link between the
Txomin and Matienzo deposits could be established and concluded that the differences in morphology were caused by the
1399
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GRANDIA ET AL.
permeability of the particular enclosing rocks. Other mineralized fracture systems include the Treto vein, located near the
Matienzo and Txomin area, and the Barambio vein, located
close to salt domes of Triassic evaporites farther south (Fig.
3D). Some of these domes contain vein-type and stratabound Zn-Pb mineralization. The most significant deposits
are in the Orduña (Monteleón and Paúl) and Murgía (Altube,
Aperregui, and Jugo) diapirs, which are hosted by rocks of
Jurassic to Turonian age (Perona et al., 2002). The paragenesis consists of sphalerite, galena, pyrite, barite, dolomite, calcite, and bitumen.
Numerous Zn-Pb deposits are also found in Paleozoic rocks
at the margins of the Basque-Cantabrian basin. There is a
general consensus that many of the deposits are post-Paleozoic in age. Samples from the Áliva mine have been included
in this study for comparison. The Áliva mine (0.6 Mt, 13% Zn)
is located at the western margin of the Basque-Cantabrian
basin (the Picos de Europa region, Cantabrian zone, Hercynian Massif), near some of the deposits in the Santander district (Fig. 3A), and is hosted by dolomitized limestones of
Carboniferous age.
Fluid Inclusions
Previous data
Little data relating to fluid composition in deposits from
the Santander district exist. Moderate salinity (15–20 wt %
NaCl equiv) and low homogenization temperatures
(60º–80ºC) were obtained in fluid inclusions in dolomite from
Reocín (Bustillo and Ordóñez, 1995). Fluid inclusion studies
in most deposits of the Western Biscay district show broad
ranges in both the salinity and temperature of the oreforming brines. In the Txomin mine, a progressive dilution
through the paragenetic sequence is observed, ranging from
~17 wt percent NaCl equiv in sphalerite to less than 5 wt
percent NaCl equiv in late quartz (Ente Vasco de la Energía,
1997; Simón et al., 1999). This salinity drop does not
correspond to a temperature decrease, which ranges from
145º to 225ºC. Brines with similar salinities and temperatures
have been found in fluorite and sphalerite from the Matienzo
veins (~15–16 wt % NaCl equiv, 150º–170ºC; Velasco et al.,
1994; Simón et al., 1999). Secondary fluids with moderate
salinity have also been reported in Matienzo (up to 7 wt %
NaCl equiv; Herrero et al., 1988).
In the Áliva mine, the origin of the mineralization has been
related to migration of fluids with moderate salinity, from 6.4
to 23.6 wt percent NaCl equiv, and temperatures from 95º to
175ºC (Gómez-Fernández et al., 2000).
New data
In the present study, 49 samples of ore and gangue minerals from the deposits described above and from salt domes
were selected for fluid inclusion analysis (Table 1). Samples
were collected from surface outcrops, underground works,
and drill cores. Doubly polished sections of 14 samples were
studied for microthermometry, which was performed on a
Linkam THMS-600 heating-freezing stage, calibrated with distilled water and pure CO2 fluid inclusions. The reproducibility
of the measurements below 0ºC was ±0.2ºC and ±2ºC for the
homogenization temperatures. The relative concentrations of
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Na, K, Li, Cl, and Br in inclusion fluids were analyzed following the method of Banks and Yardley (1992). 1- to 2-mm
mineral grains were cleaned by boiling and washing three
times in milli-Q water. The dried samples (about 2 g) were
crushed in an agate mortar, and the powder was transferred
to a 7-ml Sterlin container. Approximately 6 ml of milli-Q
water was added to redissolve the dried salts. The samples
were filtered through 0.2-µm nylon filters prior to analysis for
Cl and Br by ion chromatography (Dionex 4500i). Na, K, and
Li were analyzed by flame emission spectrometry on the
same leach solutions. The analytical precision was usually better than 10 percent (2σ error) for the Cl and Br, about 10 percent (2σ) for the K and Na at low concentrations (less than
200–300 ppb), and about 5 percent (2σ) above these values.
Calcium was not determined because many of the samples
were carbonate (one fluorite), and because of the small
amount of sample material available for multi-acid leaching.
Aqueous, two-phase primary fluid inclusions in sphalerite
from the Reocín deposit (Fig. 4A) have similar homogenization temperatures (54º–110ºC) to those reported by Bustillo
and Ordóñez (1995). However, ice-melting temperatures, the
last phase to melt in most inclusions, range from –32.3º to
–28.6ºC, resulting in salinities from 25 to 26 wt percent NaCl
equiv, which are much higher than those reported by these
authors (Figs. 5A and 6A). Data from similar inclusions in
sphalerite from the Udías area indicate salinities of about 23
wt percent NaCl equiv. In both cases, selected samples contained a few secondary fluid inclusions that were not studied
due to their small size (less than 5 µm). Data from sphalerite
and quartz of the Barambio vein indicate that ore-forming
brines had salinities from 9.2 to 15.9 wt percent NaCl equiv,
with homogenization temperatures from 150º to 180ºC. Many
inclusions contained an opaque solid that likely derived from
maturation of an organic compound after trapping (Fig. 4B).
In such inclusions, homogenization temperatures were considered invalid.
In the Altube, Aperregui, and Jugo deposits (Murgía salt
dome), the salinity of primary inclusions in sphalerite ranges
from ~10 to more than 25 wt percent NaCl equiv, with temperatures between 150º and 250ºC (Figs. 5B and 6B). Barite
from the same area (Iturlum occurrence) hosts inclusions
with a lower salinity (as low as 5.4 wt % NaCl equiv; mean
value of 14.7 wt %). Similar salinities have been found in the
Orduña salt dome (Figs. 5C and 6C), as seen in fluid inclusions from Monteleón, with 15 to 25 wt percent NaCl equiv,
and from Paúl, with 12.7 to 25 wt percent NaCl equiv. All
studied fluid inclusions were two phase and liquid rich (Fig.
4C, D). In fluid inclusions from Altube, Aperregui, and Monteleón, CH4 clathrate was the last phase to melt (Fig. 4D),
whereas hydrohalite melts before ice in most other inclusions.
In sample 40.90 ALT-3 (sphalerite) from the Altube deposit,
the last phase to melt in all inclusions was hydrohalite (Tmh
from –19.7º to –17.6ºC), resulting in higher salinities on average (25 wt % NaCl equiv). As the salinity deduced from hydrohalite and ice melting is more than 15 wt percent NaCl
equiv and the content of volatile components is low (no separate phase is observed at room temperature), errors in the calculated salinity due to clathrate formation are insignificant.
In the Áliva mine, measurements in primary fluid inclusions (Fig. 4E) in sphalerite yielded salinities from 15 to 15.9
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ORE-FORMING FLUIDS OF THE BASQUE-CANTABRIAN BASIN, SPAIN
A
B
T=-113 ºC
Ice
T=+25 ºC
Bitumen ?
Vapor
Hydrohalite
50 µm
C
50 µm
Vapor
D
T=+25 ºC
T=+25 ºC
T=-28 ºC
CH4 clathrate
20 µm
E
20 µm
F
T=+25 ºC
180 µm
Vapor
T=+25 ºC
50 µm
FIG. 4. Fluid inclusions from the studied deposits in the Basque-Cantabrian basin. A. Primary, aqueous fluid inclusion in
sphalerite from the Reocín deposit (Santander district). The picture is taken at –113ºC, and a hexagonal crystal of hydrohalite, which is the last phase to melt in this inclusion, is clearly observed close to the vapor bubble. B. Aqueous, primary
fluid inclusion in sphalerite from the Barambio mine (Western Biscay district). Black shadow to the left-hand side of the inclusion is a solid of unknown composition, likely of organic origin. As most inclusions in the sample contained this solid phase,
it is not considered to be trapped during inclusion growth. C. Aqueous, two-phase fluid inclusion in sphalerite from the Paúl
deposit (Orduña salt dome). Fluid inclusions in this deposit record the lowest homogenization temperatures in the Triassic
salt domes of the Basque-Cantabrian basin. D. Primary fluid inclusion in sphalerite from the Aperregui deposit (Murgía salt
dome). This inclusion is two phase at room temperature (aqueous liquid and vapor bubble); at low temperatures, a CH4
clathrate is seen. Fluid inclusions with small amounts of CH4 have been observed in almost all deposits around the Triassic
salt domes in the Basque-Cantabrian basin. E. Large, primary fluid inclusion in sphalerite from the Áliva deposit (western
Paleozoic margin of the Basque-Cantabrian basin). F. Secondary fluid inclusions in fluorite from the Matienzo mine. Due to
their moderate salinity (up to 7 wt % NaCl equiv; Herrero et al., 1988), these inclusions may have significant influence on
the leachate composition during the crush-leach analysis of the sample.
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1401
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Sphalerite
Dolomite
Sphalerite
Dolomite
Sphalerite
Sphalerite
Sphalerite
Mineral
1402
198.30 ALT-2
35.20 ALT-3
36.70 ALT-3
40.90 ALT-3
40.90 ALT-3
111.80 ALT-5
126.00 ALT-5
144.00 ALT-6
198.50 ALT-6
209.00 ALT-6
206.40 ALT-7
233.10 ALT-7
233.20 ALT-7
233.20 ALT-7
307.50 ALT-7
312.75 ALT-7
DM-03-5
DM-03-5
DM-04-1
DM-02-1
DM-05-1
Murgía diapir
Salt domes
TX-1
TX-2
TX-3
TX-4
C-98-1
MAT-1
MAT-2
MAT-3
BA-1
BA-2
Sphalerite
Calcite
Sphalerite
Sphalerite
Calcite
Sphalerite
Calcite
Calcite
Sphalerite
Sphalerite
Sphalerite
Sphalerite
Sphalerite
Calcite
Sphalerite
Quartz
Sphalerite
Barite
Barite
Barite
Sphalerite
Sphalerite1
Calcite1
Dolomite1
Quartz1,2
Sphalerite
Sphalerite3
Fluorite2
Calcite
Sphalerite
Quartz
Western Biscay district
FL-1
FL-2
RE-1
RE-2
RE-3
NOV-1
UD-1
Santander district
Sample no.
Altube
Altube
Altube
Altube
Altube
Altube
Altube
Altube
Altube
Altube
Altube
Altube
Altube
Altube
Altube
Altube
Jugo
Jugo
Vila
Iturlum
Aperregui
Txomin
Txomin
Txomin
Txomin
Treto
Matienzo
Matienzo
Matienzo
Barambio
Barambio
La Florida
La Florida
Reocín
Reocín
Reocín
Novales
Udías
Locality
Sandstone (Albian)
Limestone (Albian)
Limestone (Albian)
Sandstone (Albian)
Sandstone (Albian)
Sandstone (Albian)
Sandstone (Albian)
Limestone (Albian)
Limestone (Albian)
Sandstone (Albian)
Sandstone (Albian)
Sandstone (Albian)
Sandstone (Albian)
Sandstone (Albian)
Limestone (Albian)
Sandstone (Albian)
Sandstone (Albian)
Sandstone (Albian)
Dolostone (Jurassic)
Dolostone (Jurassic)
Dolostone (Jurassic)
Dolostone (Aptian)
Dolostone (Aptian)
Dolostone (Aptian)
Dolostone (Aptian)
Marls (Aptian)
Marls (Aptian)
Marls (Aptian)
Marls (Aptian)
Sandstone (Albian)
Sandstone (Albian)
Dolostone (Aptian)
Dolostone (Aptian)
Dolostone (Aptian)
Dolostone (Aptian)
Dolostone (Aptian)
Dolostone (Aptian)
Dolostone (Aptian)
Host rock
Replacement
Replacement
Vein
Vein
Vein
Vein
Replacement
Replacement
Vein
Replacement
Replacement
Vein
Vein
Vein
Replacement
Replacement
Vein
Vein
Replacement
Replacement
Breccia cement
Replacement
Replacement
Replacement
Replacement
Vein
Vein
Vein
Vein
Vein
Vein
Replacement
Replacement
Replacement
Replacement
Replacement
Replacement
Replacement
Ore type
182
180
160
148
140
217
199
148
153
204
182
145
179
120
126
160
150
240
202
196
173
217
295
160
95
154
264
212
225
202
176
195
99
215
110
162
125
77
125
55
7
17
41
6
4
8
1
1
4
7
9
33
92
1
42
26
22.5 23.5
25.5
3
33
15
9.5
15.9 11
10
3
7
24
18
24.5 11
25
5.4
22
14.7 24 28
23.5 24.7 33
17.8 21
24 47
10.9 16.9 20.1 7
21
11.9 23.5
24.5 25 25.5 4
16.6 19.8 20.5 10
12.8
9.2
15
15.5
16 17.8
15.7 17.1 18.2 13
15
1
16.1
1
4.2 5.7 7.3 17
22
25
Min. Mean Max. n
Min. Mean Max.
n
wt % NaCl equiv
Thomogenization
Microthermometry
1,292
402
1,043
7,368
1,941
2,037
1,418
1,749
1,417
2,178
1,362
3,852
1,861
1,877
740
2,356
500
894
2,032
514
1,638
52
334
591
181
28
206
161
142
906
937
631
574
638
597
192
345
408
(Cl/Br)molar
TABLE 1. Fluid Inclusion Data for Samples from Zn-Pb Mineralization in the Basque-Cantabrian Basin
1,054
321
825
7,569
1,569
1,733
986
1,368
1,272
1,970
1,030
3,588
1,632
1,483
467
1,810
305
704
1,711
346
1,429
52
324
392
153
26
137
124
101
699
818
411
412
452
463
60
114
312
26
160
75
8
119
99
81
90
18
30
26
15
31
74
15
116
64
42
167
12
47
23
106
517
24
79
83
225
193
18
27
1,620
1,863
1,638
3,090
288
475
386
(Na/Br)molar (Na/Li)molar
Leachate data
26
79
10
12
79
63
23
68
17
13
12
10
20
54
0.6
29
9
27
56
7
16
20
156
143
1,173
10
38
71
26
38
67
50
9
7
7
(Na/K)molar
1402
GRANDIA ET AL.
0361-0128/98/000/000-00 $6.00
Monteleón
Monteleón
Monteleón
Monteleón
Monteleón
Paúl
Paúl
Paúl
La Antigua
Áliva
Áliva
Sphalerite
Sphalerite
Dolomite
Calcite
Sphalerite
Sphalerite
Dolomite
Sphalerite
Calcite
Sphalerite
Sphalerite
Ore type
Dolostone (Jurassic)
Replacement
Dolostone (Jurassic)
Replacement
Dolostone (Jurassic)
Replacement
Dolostone (Jurassic)
Replacement
Dolostone (Jurassic)
Replacement
Black shales (Turonian)
Vein
Black shales (Turonian)
Vein
Black shales (Turonian)
Dissemination
Limestone (Turonian)
Vein
Dolostone (Carboniferous) Replacement
Dolostone (Carboniferous) Replacement
Host rock
21
23
12.7 21.5
15.9 21.5
15
2
40
12
12
204
196
118
170
101
162
153
80
13
9
53
15.6 15.9 11
25
23
25
15
192
135
179
84
25
15.1 23.5
2
184
180
175
Min. Mean Max. n
n
wt % NaCl equiv
Min. Mean Max.
Thomogenization
Microthermometry
2,600
3,537
2,472
1,987
2,503
2,468
2,348
911
2,728
791
812
(Cl/Br)molar
2,241
2,939
2,059
1,389
2,374
2,146
1,891
673
2,332
613
725
2 Microthermometric
69
29
222
118
75
190
398
3
53
329
414
(Na/Br)molar (Na/Li)molar
Leachate data
data from Ente Vasco Energía (1997)
data from Simón et al. (1999); leachate data for samples TXO-4 and MAT-2 were obtained from the same specimens studied by Simón et al. (1999)
3 Microthermometric data (mean value) from Velasco et al. (1994)
Locality
Mineral
1 Microthermometric
DO-03-bloc
DO-03-4
DO-03-1
DO-03-bloc
DO-03-5
DO-04-1
DO-04-1
Paul n. Inf.
DO-02-7
ALI-1
ALI-2
Orduña diapir
Sample no.
TABLE 1. (Cont.)
68
13
120
77
79
87
133
1
127
46.1
62.9
(Na/K)molar
ORE-FORMING FLUIDS OF THE BASQUE-CANTABRIAN BASIN, SPAIN
18
60
14
60
60
1403
100
100
100
140
1403
20
16
A
14
12
N 10
Santander and Western
Biscay districts
RE-1
BA-1
UD-1
BA-2
ALI-1
TX-4
MAT-2
8
6
4
2
0
140
180
Th (°C)
B
N 8
6
180
Th (°C)
8
C
6
140
180
Th (°C)
220
220
220
260
12
209.00 ALT-6
10
233.10 ALT-7
312.75 ALT-7
260
260
300
16
Murgía salt dome
40.9 ALT-3 sph
DM-02-2
DM-03-5 sph
DM-05-1
4
2
0
300
10
Orduña salt dome
DO-03- 4
DO-03- 5
DO-04-1 sph
Paul n. Inf.
N
4
2
0
300
FIG. 5. Homogenization temperatures of primary fluid inclusions in the
studied samples. See Table 1 for location of the samples. Data from the Mississippi Valley-type deposits in Western Biscay and Santander districts (including the Áliva mine hosted by Paleozoic rocks) are shown in the histogram
(A). Histograms (B) and (C) show the data from the Murgía and Orduña salt
domes, respectively.
1404
GRANDIA ET AL.
wt percent NaCl equiv and homogenization temperatures
from 153º to 170ºC (Figs. 5A and 6A). These data are within the
range of values reported by Gómez-Fernández et al. (2000).
The broad ranges in both homogenization temperatures
and salinity in many deposits (Fig. 7) suggest that mixing of
fluids occurred during ore formation. Inclusion fluids in samples from the Western Biscay district are typically less saline
than those found in the deposits around salt domes, although
a continuous spectrum from high to low salinity is observed.
Data from the Reocín deposit does not plot in this trend because of the very low homogenization temperatures.
Leachate data show wide ranges in Cl/Br ratios (Table 1),
plotting above and below the seawater value (Cl/Brmolar ~660:
Fontes and Matray, 1993). Most leachates from the Matienzo
and Txomin mines (Western Biscay district) and one from the
Reocín mine (Santander district) have ratios consistent with
highly evaporated brines (past the point of halite precipitation, Cl/Brmolar ~200). The other samples from Reocín and
from other deposits in the Santander district have intermediate values between those of seawater and highly evaporated
brines. Cl/Br ratios slightly higher than that of seawater are
found in samples from Áliva (~800) and Barambio (~900). In
contrast, the samples from deposits close to Triassic salt
domes show a broad range of values, with ratios consistent
with evaporated, halite-saturated fluids to ratios associated
with halite dissolution. This variation is observed at a local
scale in the Altube mine (Murgía salt dome), where
(Cl/Br)molar values range from 402 to 7,368 in samples collected from the same drill core.
The Na/Br ratio in all samples is in accordance with the degree of evaporation or halite dissolution deduced from the
Cl/Br ratios (Fig. 8). Na/K and Na/Li ratios in samples from
deposits hosted by Cretaceous dolostones (Txomin and Santander district deposits) also reflect the degree of evaporation
indicated by the Cl/Br ratios (Na/Kmolar of 7–71; Fig. 9;
Na/Limolar of 23–3,090). In contrast, the samples from marlhosted veins in Western Biscay district (Matienzo and Treto)
typically have much higher Na/K ratios (as high as 1,173).
Moreover, many samples near salt domes have Na/K and
Na/Li ratios (Na/Kmolar of 0.6–133; Fig. 9; Na/Limolar of 3–398)
much lower than expected for a fluid dissolving halite
(Na/Kmolar > 300: Dean, 1978, Herut et al., 1998; Na/Limolar >
50,000: Pueyo, pers. commun., 2001).
As the “crush-leach” technique is a bulk sample analysis,
the presence of secondary fluid inclusions may influence the
ratios above reported. In those cases in which secondary fluid
inclusion salinity is known, the changes in the primary Cl/Br
ratios of the brines could be estimated in a Cl/Br versus Cl
plot. However, primary fluid inclusions predominate in the
samples analyzed in this study, and only in samples from the
Santander district and from the Matienzo vein may the influence of secondary fluids be significant (Fig. 4F).
30
A
25
20
N
Santander and Western
Biscay districts
BA-1
RE-1
BA-2
UD-1
ALI-1
TX-4
MAT-2
15
10
5
0
-32 -28
-24
-20
-16
Tmi
-12
-8
-4
0
35
30
B
Murgía salt dome
40.9 ALT-3 cc
40.9 ALT-3 sph
209.00 ALT-6
312.75 ALT-7
233.10 ALT-7
DM-02-1
DM-03-5 sph
DM-05-1
25
N
20
15
10
5
0
-32 -28
-24
-20 -16
Tmi
-12
-8
-4
0
16
14
C
Orduña salt dome
DO-03- 4
DO-03- 5
DO-04-1 sph
Paul n. Inf.
12
10
N
8
6
4
2
0
-32 -28
-24
-20
-16
-12
-8
-4
0
Tmi
Discussion: Cl vs. Cl/Br vs. Na/Br Diagrams
and Mixing Lines
FIG. 6. Ice-melting temperatures of fluid inclusions in the studied samples. See Table 1 for location of the samples. Data from the Mississippi Valley-type deposits in Western Biscay and Santander districts (including the
Áliva mine hosted by Paleozoic rocks) are shown in the histogram (A). Histograms (B) and (C) show the data from the Murgía and Orduña salt domes,
respectively.
0361-0128/98/000/000-00 $6.00
Theoretical cases
Mixing of fluids has been invoked in most Mississippi Valley-type deposits to explain mineral precipitation. The main
evidence for the mixing is based on microthermometric data
1404
1405
ORE-FORMING FLUIDS OF THE BASQUE-CANTABRIAN BASIN, SPAIN
300
260
T homogenization (ºC)
220
180
140
100
60
20
-34 -32 -30 -28 -26 -24 -22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2
0
Tmelting ice (ºC)
FIG. 7. Homogenization–versus–ice-melting temperature plot for fluid inclusions from some Zn-Pb deposits in the
Basque-Cantabrian basin and in its western Paleozoic margin. (●
●) = Santander district deposits, (●) = Áliva mine, () = Txomin mine, (
) = Matienzo-Treto veins, () = Barambio vein, () = Murgía salt dome deposits, ( ) = Orduña salt dome deposits. Note the broad ranges in both ice-melting and homogenization temperatures of fluid inclusions from the Murgía and
Orduña salt domes. Data in quartz from the Txomin mine are from Ente Vasco de la Energía (1997) and Simón et al. (1999).
Data in fluorite from the Matienzo mine is from Simón et al. (1999).
8000
1000
Seawater
evaporation
trajectory
800
7000
600
SW
400
6000
(Cl/Br) molar
200
0
5000
0
200
400
600
800
1000
(ºC)
4000
3000
2000
1000
0
0
1000
2000
3000
4000
5000
6000
7000
8000
(Na/Br) molar
FIG. 8. (Cl/Br)molar versus (Na/Br)molar plot of fluids from the Basque-Cantabrian basin. (●
●) = Santander district deposits,
(●) = Áliva mine, () = Txomin mine, (
) = Matienzo-Treto veins, () = Barambio vein, () = Murgía salt dome deposits,
( ) = Orduña salt dome deposits. Inclusion fluids in the deposits from the Western Biscay district typically have very low ratios, consistent with highly evaporated brines (past the point of halite precipitation, Cl/Brmolar ~200). Most samples from deposits in the Santander district have intermediate values between those of seawater and highly evaporated brines. Samples
from deposits close to Orduña and Murgía salt domes show a broad range of values, with ratios consistent with evaporated,
halite-saturated fluids to ratios associated with halite dissolution. Data for the evaporation seawater trajectory are taken from
Fontes and Matray (1993).
0361-0128/98/000/000-00 $6.00
1405
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GRANDIA ET AL.
8000
8000
7000
Mixing model
curve
7000
6000
5000
6000
Na loss?
(Cl/Br)molar
4000
3000
5000
2000
1000
4000
0
0.2
0.8
0
0.6
0.4
100 200 300 400 500 600 700 800
3000
2000
Mixing model curve
SW
1000
Seawater evaporation trajectory
0
0
50
100
150
200
(Na/K) molar
FIG. 9. (Cl/Br)molar versus (Na/K)molar plot of fluids from the Basque-Cantabrian basin. A model curve of the mixing trend
between a seven-times diluted, highly evaporated seawater and a halite-dissolution brine is shown. This mixing model is also
used in Figure 12. Weight fractions of the evaporated brine are indicated. See Figure 6 for legend and Table 2 for end-member compositions of the model curve. Data for the evaporation seawater trajectory are taken from Fontes and Matray (1993).
(see Wilkinson, 2001, and references therein). Halogen data
may also help to identify mixing processes in cases where the
end-member fluids acquired their salinity by different mechanisms (i.e., halite dissolution or seawater evaporation). However, Chi and Savard (1997) suggest that, in some cases, the
proportion of the end members in a mixture is difficult to
constrain solely on the basis of halogen ratios, and values of
Cl/Br lower than the seawater value may result from mixtures
with major contribution of a halite-dissolution brine.
In order to test the effect of mixing on the salinity and halogen ratios, four mixing scenarios have been evaluated using
four types of fluids (Table 2): (1) seawater, (2) an evaporated
seawater before the onset of halite precipitation, (3) a highly
evaporated seawater, and (4) a halite-saturated brine at 25ºC derived from halite dissolution. Fluids 3 and 4, diluted five times
by fresh water, have also been used in the modeling. These fluids have been selected because they are common in present-day
sedimentary basins and they are thought to be involved in ore
formation processes (e.g., Hanor, 1994). Cl/Br and cation (Na/K
and Na/Li) ratios of fluids 1, 2, and 3 have been taken from
Fontes and Matray (1993). For fluid 4, a (Cl/Br)molar ratio of
20,000 has been used, a value typical of a fluid dissolving halite.
TABLE 2. Element Concentrations and Ratios of the End-Member Fluids Used in the Mixing Models
Fluid
Cl (ppm)
Br (ppm)
Na (ppm)
Ca (ppm)
K (ppm)
Mg (ppm)
TDS (ppm)
(Cl/Br)molar
(Na/Br)molar
(Na/K)molar
1: Seawater
2: Evaporated
seawater at
halite saturation
3: Highly evaporated
seawater at
carnallite saturation
3: Highly evaporated seawater
at carnallite saturation
(5 times diluted)
4: Halitedissolution
brine
4: Halitedissolution brine
(5 times diluted)
19,354
67
10,763
410
399
1,291
32,284
145,847
480
78,987
373
2,990
11,130
239,807
229,887
5,570
6,151
45
649
82,113
324,415
45,977
1,114
1,230
9
130
16,423
64,883
159,457
18
103,543
350
350
350
264,063
31,891
4
20,709
70
70
70
52,814
650
558
46
684
572
45
93
4
16
93
4
16
19,938
19,983
502
19,938
19,983
502
Data for seawater (fluid 1) and for fluids derived from seawater evaporation (fluids 2 and 3 ) are from Fontes and Matray (1993); concentrations of fluid
2 correspond to a halite-saturated brine and of fluid 3 to a carnallite-saturated brine; fluid 4 concentrations correspond to a halite-saturated brine at 25 ºC
from halite dissolution
TDS = Total dissolved salts
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ORE-FORMING FLUIDS OF THE BASQUE-CANTABRIAN BASIN, SPAIN
Although (Cl/Br)molar ratios as low as 6,000 have been found in
the latest halites crystallizing during seawater evaporation (Fig.
1; McCaffrey et al., 1987), and recrystallization of halite may
generate Br-rich brines (Stoessell and Moore, 1983), relatively
low (Cl/Br)molar ratios (6,000–10,000) in the halite-dissolution
fluid do not significantly change the results of the modeling.
The curves are constructed from linear mixing equations, assuming a conservative behavior of each dissolved element (i.e.,
no mineral precipitation occurs). The curves are represented in
two types of diagrams: Cl/Br versus Na/Br ratios and Cl/Br ratios versus Cl concentrations (Figs. 10 and 11).
Case 1, mixing of fluid 1 with fluid 4: This model illustrates
the effects of adding increasing amounts of an ordinary seawater to a halite-dissolution brine, such as the brines associated with salt diapirs. In this model, all data plot above Cl/Br
and Na/Br seawater values, and small changes in the proportions of end-member fluids in the mixing produce large variations of these ratios. Since the end-member fluids have contrasting salinities, variations of the Cl/Br and Na/Br ratios are
coupled with large variations of salinity.
Case 2, mixing of fluid 3 with fluid 4: This model tests the
effects of adding a highly evaporated seawater to a halite-dissolution brine. Although the salinity of both fluids is different
(about 6 wt %), a contribution of fluid 3 greater than 10 wt
percent in the mixture is enough to reduce the Cl/Br and
Na/Br ratios to values below those of seawater, and in a Cl/Br
versus Na/Br diagram, the mixtures plot on the seawater
evaporation trajectory. Also, salinity changes rapidly as the
mixing proceeds, whereas Cl/Br and Na/Br ratios are almost
constant, resembling a simple dilution of fluid 3. Only accurate information about the slope of the Cl/Br versus [Cl]
curve for different mixture proportions indicates the involvement of a halite-dissolution brine (Fig. 11). For contributions
of fluid 3 lower than 10 wt percent, very small variations in
salinity are expected (less than 2 wt %) and, thus, mixing is
not detected by microthermometry. In this case, significant
variations of Cl/Br and Na/Br merely represent small changes
in the proportion of fluids in the mixture.
Case 2b, mixing of diluted fluid 3 with fluid 4: Highly evaporated brines may mix with low-salinity fluids (e.g., fresh
water) prior to the mixing with fluids dissolving evaporites,
reducing their salinity but not the Cl/Br ratios. If the salinity
of fluid 3 is, for instance, lowered by a factor of five, a proportion of fluid 4 greater than 65 wt percent in the mixture is
needed to produce Cl/Br ratios higher than the value for seawater. As in case 2, salinity changes are not detected by microthermometry at contributions of fluid 3 smaller than 10 wt
percent; conversely, contributions greater than 10 wt percent
result in significant changes in salinity, but the Cl/Br and
Na/Br ratios are almost constant, as would be predicted by dilution with fresh water. Again, by coupling microthermometric and halogen data, it is possible to detect the presence of an
evaporated brine with part of the salinity being related to
halite dissolution (Fig. 11).
8000
800
7000
600
400
0.8
200
0.4
molar
(Cl/Br)
0.6
5000
4000
Case 3b
0.2
6000
0.4
0.6
SW
0.4
Seawater evaporation
trajectory
Case 1
Case 2b
0.6
0.4
Case 2
1
0
0
200
400
600
800
Case 3
0.6
0.2
3000
2000
0.2
0.6
1000
0.8
0.4
0.2
0
0
1000
2000
3000
4000
(Na/Br)
5000
6000
7000
8000
molar
FIG. 10. (Cl/Br)molar versus (Na/Br)molar ratios of the mixtures in the theoretical mixing scenarios. The mixing is calculated
by addition of increasing amounts (in grams) of one brine into the other. The weight of each element in a certain mixture
will depend on the salinity of both fluids, the concentration of the element in the end members, and the degree of mixing.
The final Cl/Br and cation ratios are obtained by dividing the weight of both elements corresponding to the mixture. The
fluid fractions indicated in the plot are the weight fraction of one fluid in the mixture. Case 1 (●): in this model, increasing
amounts of an ordinary seawater are added to a halite-dissolution brine. Case 2 (●
●): this model tests the effects of adding a
highly evaporated seawater to a halite-dissolution brine. Case 2b (●): the same scenario as case 2, but the highly evaporated
brine is diluted by a factor of five prior to the mixing with a fluid dissolving evaporites. Case 3 (): an evaporated seawater
before the onset of the halite precipitation is added to a brine dissolving halite. Case 3b (): the same scenario as case 3, but
the fluid dissolving halite is diluted by a factor of five prior to the mixing with seawater.
0361-0128/98/000/000-00 $6.00
1407
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GRANDIA ET AL.
8000
7000
Case 1
Case 3b
0.4
(Cl/Br) molar
6000
5000
Case 2b
4000
0.6
0.2
3000
Case 3
2000
Seawater evaporation
trajectory
0.2
0.8
SW
1000
1
Case 4
0
0
0.2
0.2
1
0.4
0.8 0.4
50000
0.6
0.8
0.6
0.6
0.4
0.6
0.8
1
0.4
100000
150000
Case 2
Onset of halite precipitation
0.2 0.4
0.6 0.8 1
200000
250000
[Cl] ppm
FIG. 11. (Cl/Br)molar versus [Cl] of the mixtures in the theoretical mixing scenarios (see Fig. 10 for mixing calculations and
explanations of cases 1, 2, 2b, 3, and 3b). (●) = case 1, (●
●) = case 2, (●) = case 2b, () = case 3, ( ) = case 3b, () = case 4.
Case 4 illustrates the addition of seawater-evaporation fluids (before halite precipitation) to fresh water or seawater.
Case 3, mixing of fluid 2 with fluid 4: In this case, an evaporated seawater before the onset of the halite precipitation is
added to a brine dissolving halite. In a Cl/Br versus Na/Br plot,
the mixing of fluid 4 with an evaporated seawater is similar to
that in case 1, but the ratios decrease more steeply; for example, a contribution of 40 wt percent of fluid 2 results in a mixture with (Cl/Br)molar = 1,709, whereas in case 1, the ratio was
6,190. Since both fluids have a similar salinity, a very narrow
range of salinity in the mixture results and no significant variations are detected by microthermometric studies. Therefore,
halogen ratios provide clear evidence for mixing, but the relative proportions of the end-member fluids are only observed
by plotting the data in a Cl/Br versus [Cl] diagram.
Case 3b, mixing of fluid 2 with diluted fluid 4: Like case 2b,
dilution of a halite-dissolution brine by fresh water prior to
the mixing with fluid 2 reduces its salinity but does not
change the Cl/Br ratio. Here, an arbitrary dilution factor of
five has been chosen. As the diluted fluid 4 has a low salinity
(~50,000 ppm total dissolved salts), Cl/Br and Na/Br ratios
are mainly controlled by fluid 2. In mixtures with more than
80 wt percent of diluted fluid 4, small changes in the relative
proportions of fluids will produce broad ranges in both ratios.
In contrast, mixtures with less than 80 wt percent of fluid 4
will have similar Cl/Br and Na/Br ratios, and so, small changes
in such ratios could indicate major changes in the relative
proportions of the end members. In this case, large variations
in salinity are always observed unless the proportion of diluted fluid 4 is greater than 80 wt percent.
Case 4, mixing of fluid 1 with fluid 2: Addition of seawaterevaporation fluids (before halite precipitation) to fresh
water or seawater, which may occur in many evaporative environments, also does not change the Cl/Br and Na/Br ratios.
0361-0128/98/000/000-00 $6.00
Microthermometric data will reveal salinity variations, although they will be unable to distinguish the origin of the
end-member fluids.
Basque-Cantabrian basin data
Microthermometry suggests that the fluid mixing was common in deposits from the Basque-Cantabrian basin (Velasco
et al., 1994; Simón et al., 1999; and present work). Although
widespread Triassic evaporite (mainly gypsum and minor
halite) deposits occur in the Basque-Cantabrian basin, most
inclusion fluids from Mississippi Valley-type ores are consistent with evaporated seawater rather than halite-dissolution
fluids, according to the Cl/Br versus Na/Br plots (Fig. 8).
Only in the deposits near the salt domes do the ore-forming
fluids have a halogen signature that suggests the involvement
of brines from the dissolution of halite. However, the nature
of the end-member fluids and the relative proportions of end
members involved in mixing are not well constrained. Combining the salinity with the halogen ratios in a Cl/Br versus
[Cl] plot, the mixing between fluids of different origin and
salinity is better defined and the end-member fluids are better constrained (Fig. 12).
Data from deposits near salt domes define a high-Cl/Br,
high-salinity to low-Cl/Br, low-salinity trend from a mixture
where one of the end members is a brine generated by halite
dissolution (e.g., fluid 4) and the other is a highly evaporated
fluid that underwent dilution prior to mixing (fluid 3 diluted
about seven times). This trend defines a curve similar to that
explained for case 2b in Figure 11, where significant changes
in salinity (i.e., >5 wt % NaCl equiv) of the mixture are not
observed until the proportion of the low-salinity fluid reaches
~20 wt percent, whereas the Cl/Br ratio decreases by a factor
1408
ORE-FORMING FLUIDS OF THE BASQUE-CANTABRIAN BASIN, SPAIN
1409
8000
7000
(Cl/Br) molar
6000
5000
4000
3000
2000
0.2
SW
Onset of halite precipitation
1000
Seawater evaporation
trajectory
0.4
0.8
0
0.6
1
0
50000
100000
150000
200000
250000
[Cl] ppm
FIG. 12. (Cl/Br)molar versus [Cl] plot of fluids from the Basque-Cantabrian basin. Bars show the [Cl] range determined by
microthermometry. Symbols are averages: (●
●) = Santander district deposits, (●) = Áliva mine, () = Txomin mine, (
) = Matienzo-Treto veins, () = Barambio vein, () = Murgía salt dome deposits, ( ) = Orduña salt dome deposits. Solid line shows
the addition of a highly evaporated brine that underwent dilution by a factor of seven to a fluid-dissolving halite. This model
curve allows the explanation of the broad ranges of Cl/Br ratios in samples from an individual deposit (e.g., Altube deposits)
or from the same district (e.g., Murgía salt dome). The mixing model also shows a possible link between ore-forming brines
around Triassic diapirs and fluids in Mississippi Valley-type deposits in the Basque-Cantabrian basin. Dashed line represents
the addition of seawater or fresh water to a previous mixture formed by ~25 wt percent of highly evaporated brine and ~75
wt percent of a halite-dissolution brine. This sort of mixing allows the explanation of the data of Áliva and Barambio mines.
of 10. This mixing curve may explain the large changes in the
Cl/Br ratios observed in leachates from the Altube mine
(Murgía salt dome) in samples having a narrow range in mean
salinity values (within 5 wt % NaCl equiv). Mixtures dominated by the low-salinity fluid in the Altube mine are also suggested, as some samples have low Cl/Br ratios (as low as 402).
The broad ranges in salinity in some samples indicate different degrees of mixing and mineral precipitation.
Although the theoretical mixing lines explained above are
based on fluids with present-day Na-K-Cl-Br ratios, we believe that they can be used to interpret the fossil fluid data.
Differences in these ratios in Phanerozoic seawater (Lowenstein et al., 2001) are likely smaller than analytical errors of
crush-leach analyses.
From the mixing model curve (Fig. 12), it is deduced that
mineralization in most deposits around the Murgía and Orduña salt domes precipitated from mixtures dominated by
greater than 75 wt percent of fluids dissolving halite, with a
significant contribution of a diluted, evaporated brine. In this
model curve, the theoretical end member dissolving halite
had a salinity of around 26 wt percent NaCl equiv, in accordance with a halite-saturated fluid at 25ºC. Because homogenization temperatures measured in the deposits are up to
290ºC, and halite solubility increases with temperature, either
the end member did not reach halite saturation or it was
saturated and subsequently diluted prior to the mixing related
to mineralization. The low-salinity end member could be
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comparable to the fluids found in other areas of the Western
Biscay district, such as those in the late stages of mineralization in the Txomin mine, where highly evaporated brines had
been diluted (Simón et al., 1999)
The interpretation of the data in the Txomin-Matienzo area is
not straightforward, and no well-defined trends are observed.
The presence of secondary fluids with a moderate salinity (up to
7 wt % NaCl equiv; Herrero et al., 1988; Fig. 4F) could account
for the Cl/Br in some samples. It is clearly observed that the fluids were diluted prior to ore deposition (Fig. 12).
The leachates from the Barambio vein, despite having
Cl/Br ratios higher than seawater, do not fit well with mixing
curves involving a halite-dissolution brine. These halogen ratios and salinities could be explained by the presence of a
third fluid (e.g., seawater or fresh water) that mixed with a
previously formed mixture of a highly evaporated brine and a
halite-dissolution brine (dashed line in Fig. 12).
The scarcity of microthermometric data in most deposits
from the Santander district does not allow a mixing curve to
be defined. Also, the effect on the Cl/Br ratio by the secondary fluid inclusions cannot be assessed because their salinity is not known. However, fluid mixing could have occurred
as shown by the presence of samples with different Cl/Br ratios distributed along the seawater evaporation trajectory. In
this case, one of the end-member fluids could have been a
highly evaporated brine and the second fluid could have been
either an evaporated brine that was not halite saturated or a
1409
1410
GRANDIA ET AL.
diluted halite-dissolution brine. The Áliva data, with a behavior similar to Barambio, could also be related to a diluted
halite-dissolution brine.
As Na/K and Na/Li ratios in halite are much higher than in
seawater or in any residual fluid after seawater evaporation,
large variations in these ratios are expected when small proportions of a highly evaporated fluid are added to a brine dissolving halite. Consequently, mixing processes can also be
traced by using such ratios. However, unlike Cl/Br, the ratios
involving cations such as Na, K, and Li may be more sensitive
to fluid-rock interaction. This is evident in some samples from
deposits around Murgía and Orduña salt domes, which have
Na/K and Na/Li ratios orders of magnitude lower than those
expected based on the mixing scenarios discussed above (Fig.
9). The mixing model curve used in the plot of Cl/Br versus
[Cl] indicates that mixing alone cannot explain the Na/K ratios of samples from the salt domes (Fig. 9). Dissolution of Krich salts also cannot account for these low ratios, since they
are absent in the stratigraphy of the area (Serrano and
Martínez del Olmo, 1990). Therefore, other mechanisms
such as halite precipitation or interaction with siliciclastic
rocks (Chan et al., 2002) must be invoked. Perona et al.
(2002) reported albite precipitation in host rocks from the
Monteleón (Orduña diapir), Aperregui, and Jugo (Murgía diapir) deposits, suggesting that the low Na/K ratio could be
caused, at least in part, by precipitation of autigenic albite
from the ore-forming fluids. On the other hand, the fluids in
the Matienzo area show wide ranges in the Na/K ratio, with
all values higher than those for highly evaporated brines (up
to 1,173, beyond the scale of the Fig. 9). These values may be
explained by the loss of K by absorption and/or precipitation
of a K-rich phase in the sediments. It is interesting to notice
that autigenic K feldspar was recognized in country rocks
near the veins (Simón and Grandia, 1998). Samples from deposits hosted by carbonate rocks do not show major Na-K-Li
exchange, indicating either differences in the reactivity of the
rock and/or differences in the water/rock ratio.
Although the involvement of highly evaporated brines has
been accepted as an explanation for the Cl/Br ratios of most
Mississippi Valley-type, ore-forming brines (Wilkinson, 2001,
and references therein) with implications for fluid migration
paths (Leach et al., 2001), the proposed mixing models in this
study provide additional information, and possible alternative
explanations for the origin of the halogen ratios. Kesler et al.
(1995) noted similar Na-Cl-Br systematics of fluid inclusions
in galena from Mississippi Valley-type deposits of Viburnum
Trend that were interpreted to be the result of the presence
of fluids of different origin (seawater evaporation and halite
dissolution) in successive stages of the mineralizing process.
The Cl/Br and Na/Br ratios of fluids leached from these
galenas could also be explained by small changes in the
proportions of the end members in a mixture of halitedissolution brine with a highly evaporated seawater, similar to
case 2 in this study, as already pointed by Viets et al. (1996)
and Chi and Savard (1997). However, it is important to note
that only in a set of samples where both microthermometric
and halogen data are available can the proportions of the
respective end members in the mixtures be estimated. This
can help to identify the possible sources of the ligands for the
ore components and thereby influence exploration strategies.
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Conclusions
Microthermometric data can indicate mixing if one of the
end members is fresh water or highly diluted fluid, but they
cannot identify mixing between fluids with a high proportion
of brine. In contrast, halogen data can be used to identify mixing between brines, potentially allowing for the distinction of
solute sources from water sources. Combining these data can
provide a quantitative estimate of the relative proportions of
a variety of different fluids that may be involved in mixing.
Previous microthermometric studies of fluid inclusions in
ore and gangue minerals of Mississippi Valley-type deposits
from the Basque-Cantabrian basin determined that at least
two fluids were involved in the mineralization. One of them
was highly saline, ca. 25 wt percent NaCl equiv, and the other
had a lower but unknown salinity. In this study, the combination of microthermometric and halogen data has shown that
brines of different origin were involved in the mineralizing
processes: brines derived from seawater evaporation with
variable degrees of evaporation, brines dissolving halite from
salt domes, and low-salinity fluids (seawater or fresh water)
that acted as dilutants. Data from the Western Biscay district
(Txomin, Matienzo, and Barambio deposits) can be accounted for by mixing between a residual brine and a halitedissolution brine. On the other hand, most of the ore-forming
brines from the studied deposits around Orduña and Murgía
salt domes (Altube, Monteleón, Jugo) originated by halite dissolution, although a significant contribution (20–50 wt %)
from a highly evaporated brine was recognized.
Acknowledgments
This work is part of the Ph.D. thesis of the senior author at
the Universitat Autònoma de Barcelona. We thank Sarah
Gleeson and an anonymous reviewer for their constructive
comments. Also, we wish to thank Luís Muñoz and Álex
Franco (Ente Vasco de la Energía) for providing samples
from Mississippi Valley-type deposits in the BasqueCantabrian basin, and Joachim Beck (Outokumpu Minera
Española) and Juan García (Ente Vasco de la Energía) for the
use of geological data from the Murgía and Orduña salt
domes. This work has been financed through the DGESIC
PB98-0901 Project (Spanish Ministry of Education and Culture).
February 6, 2002; May 28, 2003
REFERENCES
Abajo, A., and Piret, C., 1989, Reocín mine: Mining Magazine, v. 160-8,
p. 102–116.
Banks, D.A., and Yardley, B.W.D., 1992, Crush-leach analysis of fluid inclusions in small natural and synthetic samples: Geochimica et Cosmochimica
Acta, v. 56, p. 245–248.
Barbanson, L., 1987, Les mineralizations Zn-Pb-Ba-Hg-Cu de socle et
couverture carbonates de la province de Santander (Nord de l’Espagne):
Unpublished Ph.D. thesis, Orléans, France, Université d’Orléans, 292 p.
Bustillo, M., and Ordóñez, S., 1995, Lower Cretaceous Pb-Zn ores of
Cantabria, northern Spain: New considerations based on petrological and
geochemical evidence: Transactions of Institution of Mining and Metallurgy, v. 104, p. B55–B65.
Carpenter, A.B., 1978, Origin of chemical evolution of brines in sedimentary
basins: Oklahoma Geological Survey Circular, v. 79, p. 60–77.
Chan, L-H. , Starinsky, A., and Katz, A., 2002, The behavior of lithium and
its isotopes in oilfield brines: Evidence form Heletz-Kokhav field, Israel:
Geochimica et Cosmochimica Acta, v. 66, no. 4, p. 615–623.
1410
ORE-FORMING FLUIDS OF THE BASQUE-CANTABRIAN BASIN, SPAIN
Chi, G., and Savard, M.M., 1997, Sources of basinal and Mississippi Valleytype mineralising brines: Mixing of evaporated seawater and halite-dissolution brine: Chemical Geology, v. 143, p. 121–125.
Dean, W.E., 1978, Trace and minor elements in evaporites, in Dean, W.E.,
and Schreiber, Ch., eds., Marine evaporites: Oklahoma, Society of Economic Paleontologists and Mineralogists Short Course 4, p. 86–104.
Ente Vasco de la Energía, 1997, Report on Mineralization at Txomin, western Bilbao syncline, Basque region, Spain: Unpublished report, Bilbao,
Spain, 74 p.
Fontes, J.Ch., and Matray, J.M., 1993, Geochemistry and origin of the formation brines from the Paris Basin, France: 1. Brines associated with Triassic salts: Chemical Geology, v. 109, p. 149–175.
García-Mondéjar, J., 1985, Aptian and Albian reefs [abs]: Institut d’Estudis
Ilerdencs, European Regional Meeting, 6th, Lleida, April 1985, Excursion
Guidebook no. 9, p. 329–352.
García-Mondéjar, J., Agirrezabala, L.M., Aranburu, A., Fernández-Mendiola, P.A., Gómez-Pérez, I., López-Horgue, M., and Rosales, I., 1996, Aptian-Albian tectonic pattern of the Basque-Cantabrian basin (northern
Spain): Geological Journal, v. 31, p. 13–45.
Gomez-Fernández, F., Both, R.A, Mangas, J., and Arribas, A., 2000, Metallogenesis of Zn-Pb carbonate-hosted mineralization in the Southeastern region of the Picos de Europa (Central Northern Spain) province: Geologic,
fluid inclusion, and stable isotope studies: ECONOMIC GEOLOGY, v. 95, p.
19–40.
Grandia, F., Cardellach, E., Canals, À., and Banks, D.A., 2003, Geochemistry
of the fluids related to epigenetic carbonate-hosted Zn-Pb deposits in the
Maestrat basin, Eastern Spain: Fluid inclusion and isotope (Cl, C, O, S, Sr)
evidence: ECONOMIC GEOLOGY, v. 98, p. 933–954.
Hanor, J.S., 1994, Origin of saline fluids in sedimentary basins: London Geological Society, Special Publication no. 78, p. 151–174.
Herrero, J.M., 1989, Las mineralizaciones de Zn, Pb, F en el sector
occidental de Vizcaya: Mineralogía, geoquímica, y metalogenia:
Unpublished Ph.D. thesis, Leioa, Spain, Universidad del País Vasco, 285 p.
Herrero, J.M., Pérez-Álvarez, M., Touray, J.C., and Velasco, F., 1988, Late
thermal events in the Mesozoic Basque basin: Evidence from secondary
fluid inclusions in Mississippi Valley-type fluorite occurrences: Bulletin de
Minéralogie, v. 111, p. 413–420.
Herut, B., Gavrieli, I., and Halicz, L., 1998, Coprecipitation of trace and
minor elements in modern authigenic halites from the hypersaline Dead
Sea brine: Geochimica et Cosmochimica Acta, v. 62, p. 1587–1598.
Instituto Geológico y Minero de España, 1970, Síntesis mapa geológico:
Madrid, hojas 4, 5, 11, 12, scale 1:200,000.
Kesler, S.E., Martini, A.M., Appold, M.S., Walter, L.M., Huston, T.J., and
Kyle, J.R, 1995, Na-Cl-Br systematics of mineralizing brines in Mississippi
Valley-type deposits: Geology, v. 23, p. 641–644.
Kesler, S.E., Martini, A.M., Appold, M.S., Walter, L.M., Huston, T.J., and
Furman, F.C., 1996, Na-Cl-Br systematics of fluid inclusions from Mississippi Valley-type deposits, Appalachian basin: Constraints on solute origin
and migration paths: Geochimica et Cosmochimica Acta, v. 60, p. 225–233.
Land, L.S., and Prezbindowski, D.R., 1981, The origin and evolution of
saline formation water, Lower Cretaceous carbonates, south-central Texas,
USA: Journal of Hydrology, v. 54, p. 51–74.
Leach, D.L., Bradley, D., Lewchuk, M.T., Symons, D.T.A., de Marsily, G.,
and Brannon, J., 2001, Mississippi Valley-type lead-zinc deposits through
geological time: Implications from recent age-dating research: Mineralium
Deposita, v. 36, p.711–740.
Lowenstein, T.K., Timofeeff, M.N., Brennan, S.T., Hardie, L.A., and
Demicco, R.V., 2001, Oscillations in Phanerozoic seawater chemistry: Evidence from fluid inclusions: Science, v. 294, p. 1086–1088.
0361-0128/98/000/000-00 $6.00
1411
McCaffrey, M.A., Lazar, B., and Holland, H.D., 1987, The evaporation path
of seawater and the coprecipitation of Br– and K+ with halite: Journal of
Sedimentary Petrology, v. 57, p. 928–937.
Michel, B., 1974, Contributions a l’étude des mineralizations plombozincifères dans le Crétacé Inferieur du Maestrazgo: Unpublished Ph.D.
thesis, Nancy, Frnace, Université de Nancy, 178 p.
Perona, J., Grandia, F., Canals, À., Cardellach, E., Maestro, E., and García,
J., 2002, Geología y mineralogía de los depósitos peridiapíricos de Zn-Pb de
Orduña y Murgía (Cuenca Vasco-Cantábrica, prov. Álava): Boletín de la
Sociedad Española de Mineralogía, v. 25A, 79–80.
Posey, H.H., and Kyle, J.R., 1988, Fluid-rock interactions in the salt dome
environment: An introduction and review: Chemical Geology, v. 74, p.
1–24.
Rat, P., 1959, Les pays Crétacés Basco-Cantabriques (Espagne): Unpublished
Ph.D. thesis, Dijon, France, Université de Dijon, 525 p.
––––1983, Les régions Basco-Cantabriques et Nord-Ibériques: Mémoires
Géologiques Université de Dijon, v. 9, p. 1–24.
Salas, R., Guimerà, J., Mas, R., Martín-Closas, C., Meléndez, A., and Alonso,
A., 2001, Evolution of the Mesozoic central Iberian rifting system and its
Cainozoic inversion (Iberian Chain): Publications Scientifiques du Múseum, París, v. 186, p. 145–185.
Serrano, A., and Martínez del Olmo, W., 1990, Tectónica salina en el dominio
Cántabro-Navarro: Evolución, edad, y origen de las estructuras salinas, in
Ortí, F., and Salvany, J.M., eds., Formaciones evaporíticas de la Cuenca del
Ebro y cadenas periféricas, y de la zona de Levante: Empresa Nacional de
Residuos Radioactivos Sociedad Anónima and Universitat de Barcelona, p.
39–53.
Simón, S., and Grandia, F., 1998, Estudio metalogenético de las dolomitizaciones del área Txomin-Ranero: Ente Vasco de la Energía internal
report, Bilbao, Spain, 61 p.
Simón, S., Canals, À., Grandia, F., and Cardellach, E., 1999, Estudio
isotópico y de inclusiones fluidas en depósitos de calcita y dolomita del
sector oeste del Anticlinal de Bilbao y su relación con las mineralizaciones
de Fe-Zn-Pb: Boletín de la Sociedad Española de Mineralogía, v. 22, p.
55–71.
Stoessell, R.K., and Moore, C.H., Jr., 1983, Chemical constraints and origins
of four groups of Gulf Coast reservoir fluids: American Association of Petroleum Geologists Bulletin, v. 67, p. 896–906.
Velasco, F., Herrero, J.M., Gil, P.P., Alvarez, L., and Yusta, I., 1994, Mississippi Valley-type, Sedex, and iron deposits in Lower Cretaceous rocks of
the Basque-Cantabrian basin, Northern Spain: Society for Geology Applied
to Mineral Deposits, Special Publication 10, p. 247–270.
Velasco, F., Alonso, J.A., Cueto, J., Herrero, J.M., Muñiz, F., Seebold, I., and
Yusta, I., 2000, Relación entre dolomitización y mineralización en el
yacimiento de Zn-Pb de Reocín, Cuenca Vasco-Cantábrica, España:
Cadernos del Laboratorio Xeológico de Laxe, v. 25, p. 135–137.
Vergés, J., and García-Senz, J., 2001, Mesozoic evolution and Cenozoic
inversion of the Pyrenean rift: Publications Scientifiques du Múseum,
París, v. 186, p. 187–212.
Viets, J.G., Hofstra, A.H., and Emsbo, P., 1996, Solute compositions of fluid
inclusions in sphalerite from North American and European Mississippi
Valley-type ore deposits: Ore fluids derived from evaporated seawater: Society of Economic Geologists, Special Publication 4, p. 465–482.
Walter, L.M., Stueber, A.M., and Huston, T.J., 1990, Br-Cl-Na systematics in
Illinois Basin fluids: Constraints on fluid origin and evolution: Geology, v.
18, p. 315–318.
Wilkinson, J.J., 2001, Fluid inclusions in hydrothemal ore deposits: Lithos, v.
55, p. 229–272.
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