Fuchsite and other Cr-rich in ultramafic enclaves mercury mining

Clay Minerals (2001) 36, 345–354
Fuchsite and other Cr- rich phyllosilicates
in ultramafic enclaves from the Almadén
mercury mining district, Spain
D. M O R A T A 1 , * , P. H I G U E R A S 2 , S. D O M Í N G U E Z- B E L L A 3 , J. P A R R A S 2 ,
F. V E L A S C O 4 A N D P. A P A R I C I O 5
1
Departamento de Geologõ´a, Facultad de Ciencias Fõ´sicas y Matemáticas, Universidad de Chile, Casilla 13518,
correo 21, Santiago de Chile, 2 Departamento de Ingenierõ´a Geológica y Minera, Universidad de Castilla-La Mancha,
13400 Almadén, Ciudad Real, Spain, 3 Departamento de Cristalografõ´a y Mineralogõ´a, Estratigrafõ´a, Geodinámica y
Petrologõ´a y Geoquõ´mica, Universidad de Cádiz, 11510 Puerto Real, Cádiz, Spain, 4 Departamento de Mineralogõ´a y
Petrologõ´a, Universidad del Paõ´s Vasco, Apdo. 644, 48080 Bilbao, Spain, and 5 Departamento de Cristalografõ´a y
Mineralogõ´a y Quõ´mica Agrõ´cola, Facultad de Quõ´mica, Universidad de Sevilla, Apdo. 553, 41041 Sevilla, Spain
(Received 26 January 2000; revised 29 October 2000)
A B S T R A C T : Fuchsite and other Cr-rich phyllosilicates , parageneti c with dolomite, are present in
some ultramafic enclaves from the ‘frailesca’ rock (a lapilli- to block-siz e pyroclasti c lithic-tuff), in
the Almadén mercury mining district, Spain. Analyses (EMPA and TEM) of fuchsite and Cr-chlorite
showed a relatively large range in levels of Cr2 O3. Petrographi c relationship s between these
phyllosilicate s and primary relics of Cr-spinel crystals, as well as their high Cr content, indicate that
these Cr-rich minerals originate d from primary chromian spinels through an early hydrotherma l
alteration stage. The hydrotherma l fluids accounting for this early alteration would be of relatively
high temperature , high aCO2 and aK , and variable aNa/K. In a later alteration stage, fuchsite was
partially or totally replaced by illite and Cr-illite, giving rise to an argillitic alteration.
KEYWORDS: fuchsite, Cr-chlorite, Cr-illite, Hg-mineralization, hydrothermal alteration, Almade´n, Spain.
Mercury mineralization in Central Spain is probably
related to a regional Silurian–Devonian intracontinental alkaline magmatism, present in the Almadén
mining district (Almadén syncline). From here,
>30% of the world mercury consumption is
supplied (Ortega & Hernández, 1992), making the
district possibly the largest geochemical Hg
anomaly on the planet. Two main hypotheses
have been proposed to explain the origin of this
high Hg anomaly: Saupé (1990) suggested the
leaching of Hg from Palaeozoic black shales by
hydrothermal fluids, whereas Ortega & Hernández
(1992) and Higueras (1995) suggested a genetic
relationship between the magmatism and the Hg
* E-mail: [email protected] l
mineralizations. In both hypotheses, hydrothermal
fluids are invoked as a means of transportation and
concentration of the Hg. Another important feature
of the mineralization is the relationship with
alteration processes which affected the alkaline
mafic igneous rocks in two different stages: a lateto post-magmatic event, and a Hercynian, orogenyrelated event, with different chemical and mineralogical characteristics (Hall et al., 1996, 1997).
In the Almadén Hg district, Cr-rich phyllosilicates (Cr-illite, fuchsite and Cr-rich chlorite) appear
to be directly associated with one of the alteration
processes involved in the Hg metallogeny. In
particular, the Cr-rich phyllosilicates are present in
ultramafic xenoliths from basaltic lavas and
pyroclasts throughout the mining district. In this
work we present the petrographic, geochemical and
structural characteristics of these minerals.
# 2001 The Mineralogical Society
346
D. Morata et al.
Hg deposits. However, the time and space
distribution of this breccia is larger than that of
the Hg deposits, and it can be found occasionally in
Ordovician series, and almost continuously in
Silurian and Devonian sequences all along the
syncline. During the Hercynian deformation, this
sequence was folded, metamorphosed (very low- to
low-grade) and intruded locally by acid plutonic
rocks.
Two main Hg-mineralization types can be
distinguished in the Almadén district (Hall et al.,
1996; Hernández et al., 1999): (1) Stratabound
mineralization, hosted in the basal Silurian
‘Criadero quartzite’ (Fig. 1b) and referred as
‘Almadén-type mineralization’. This mineralization
is associated directly with the Silurian–Devonian
alkaline mafic magmatism. The Almadén, El
Entredicho and Vieja Concepción deposits
(Fig. 1a) are included in this group. (2) Epigenetic
stockworks and vein mineralization, hosted in
various lithologies, and known as ‘Las Cuevassubtype’ (hosted in magmatic rocks) and ‘Pilar de
la Legua-subtype’ (hosted in quartzites) mineralizations. These epigenetic mineralizations were interpreted as tectono-metamorphic remobilization of the
older ‘Almadén-type mineralizations’ during the
Hercynian deformation, in relation to tectonics,
metamorphism and/or granitic intrusions (Jébrak &
GEOLOGICAL CONTEXT AND
PETROLOGICAL OUTLINE
The Almadén syncline, a Hercynian structure
located in the central part of the Iberian Peninsula
(Fig. 1a), is composed of early Ordovician to late
Devonian sediments (mainly shales, sandstones and
quartzites), with a few interbedded mafic igneous
rocks, resting unconformably on pre-Ordovician
shales and schists (Fig. 1b). The magmatic rocks
are largely alkali basalts (Higueras et al., 2000) and
comprise dykes, sills, flows and bedded pyroclastic
deposits, including lapilli-breccias, known locally as
‘frailesca’, that are well developed in most of the
a
Subvolcanic
tholeiites
Volcano-sedimentary
sucessions
Intermediate and
acidic rocks
Hesperian
Massif
b
Basalts
Upper Volcanic Group
Pyroclastic
(frailesca) rocks
DEVONIAN
The presence of Cr-rich phyllosilicates (fuchsite,
Cr-illite and/or chlorite) as secondary minerals in
mafic/ultramafic rocks and enclaves has been
reported from a number of alteration/mineralization
areas (Moritz & Croket, 1991; Christofides et al.,
1992; Jiang et al., 1992; Schandl & Wicks, 1993;
Craw & Angus, 1993; Arif et al., 1996). However,
the compositional constraints on these minerals in
relation to the alteration processes have never been
studied in great detail. In this paper, an hypothesis
for the relationship between these Cr-rich phyllosilicates and the two major alteration events defined
in the Almadén district is suggested.
Base quartzite
MADRID
LC
0
5
1 0 km
Almadén
Cor Gu
PL
Almadén-type mineralizations
Las Cuevas-type mineralizations
Pilar del Legua-type mineralizations
BTH
Almadenejos
NVC
Criadero quartzite
ORDOVICIAN
Devonian
Silurian
Ordovician
Precambrian
Las Cuevas
quartzite
SILURIAN
Ciudad Real
EE
Canteras quartzite
600
400
200
Armorican quartzite
0m
FIG. 1. (a) Geological map of the Almadén syncline in the Hesperian Massif, showing the location of the major
Hg deposits (after Higueras, 1995). BTH: Burcio-Tres Hermanas; Cor: Corchuelo; EE: El Entredicho; Gu:
Guadalperal ; LC: Las Cuevas; NVC: Nueva and Vieja Concepción; PL.: Pilar de la Legua. (b) Lithologica l
column of the Almadén syncline, showing the magmatic rock intercalate d. Modified after Higueras (1995).
Fuchsite in the Almadén mercury mining district, Spain
Hernández, 1995; Hall et al., 1997). The Las
Cuevas, El Nuevo Entredicho and La Nueva
Concepción mines, as well as the Corchuelo and
Guadalperal mineral showings (Fig. 1a ) are
included in this group.
Both mineralization types show genetic and/or
geometric relationships with the magmatic rocks of
the district and/or their alteration processes. In this
sense, a geometric and genetic relationship can be
established between the stratabound mineralizations
(‘Almadén-type’) and the pyroclastic rocks (the
‘frailesca’) of this mining district (Higueras, 1993;
1995). Moreover, a possible ‘reactivity’ relationship
was established between the ‘frailesca’ and the
epigenetic mineralization (Higueras et al., 1995,
1999).
Magmatic rock alteration and mineralization
In the magmatic rocks of the Almadén syncline
related to the Hg-mineralization, two alteration stages
were differentiated (Hall et al., 1997; Higueras et al.,
1999): a ‘regional’ stage, affecting all of the
magmatic rocks of the district, and a ‘local’ stage,
affecting only the magmatic rocks present in the
epigenetic Las Cuevas-type mineralization.
The ‘regional’ event gave rise to the pseudomorphic replacement of primary igneous minerals
(olivine, pyroxene, plagioclase) by chlorite + CaMg-Fe carbonates + silica, and could be interpreted
as a late-magmatic event (Higueras, 1993). In spite
of its large extent, it is possible to establish a
gradual distribution of its influence, with a stronger
expression in the stratabound Hg deposits, as
reflected by the zoning in the carbonate content
(Higueras, 1995).
The ‘local’ alteration stage is characterized by
the formation of pyrophyllite, kaolinite, illite and
illite-smectite (‘‘argillitic alteration’’, after Higueras
et al., 1995) and the partial to complete destruction
of the secondary minerals developed in the previous
regi onal al t erat ion st age. Thermodynami c
constraints, as well as preliminary fluid inclusion
data (Higueras et al. 1999) indicate low-salinity
fluids, temperature in the range 200 3008C and
pressure of >0.5 kbar. This mineral assemblage
developed zoned shells, some 10 20 m in
diameter, around the Las Cuevas-type mineralization, and can be interpreted as the reaction products
of the interaction between the high-S mineralizing
solutions and the host magmatic rocks (Higueras et
al., 1999).
347
DESCR IPTION OF T HE FRAILESCA
ROCK AND CHARACTERIZATION
O F T H E C R- R I C H
PHYLLOSILICATES
The frailesca rock is a lapilli- to block-size
pyroclastic lithic tuff, composed mainly of basaltic
fragments. Strongly hydrothermally altered ultramafic xenoliths, as well as various sedimentary
clasts are also present, all of them contained in an
argillaceous groundmass. The basaltic fragments
have relicts of olivine and pyroxene phenocrysts in
a glassy and highly-vesiculated matrix.
Spinel crystals, ranging from 100 500 mm in
size, appear mostly as vermicular grains included in
pyroxene crystals from the ultramafic xenoliths.
They are Cr-rich varieties and show compositional
variations, mainly in terms of Cr (=Cr/(Cr+Al3+ ))
and Mg (=Mg/(Mg+Fe2 + )) occupancies. These
variations can be established between samples from
different outcrops and even at a single-sample scale.
In some of the frailesca ultramafic xenoliths,
conspicuous pale-green mica, associated with CaFe-Mg carbonates and chlorite, is present as a
secondary phase (Fig. 2). This pale-green mica was
studied using X-ray diffraction (XRD), scanningelectron microscopy coupled with a microanalytical
system (SEM-EDX), electronic microprobe analysis
(EMPA) and transmission electron microscopy
(TEM).
The XRD study was carried out with Cu-Ka
radiation in a Siemens D5000 diffractometer at
40 kV, 30 mA, in the 2 3682y range. Counts were
FIG. 2. SEM image of the associatio n of rhombohedra l
carbonate s and small mica crystals. Scale bar = 10 mm.
348
D. Morata et al.
recorded at 0. 0282y intervals for 1. 0 s. A
y-compensating variable slit and sample spinning
were used. A profile-fitting peak-decomposition
program, part of MacDiff 4.1.2, by Petschick
(2000), was used on the ethylene glycol (EG)orientated aggregate patterns to determine the
precise position and intensities of the individual
peaks within broad diffraction bands. A Pearson VII
function was used, and the parameters obtained
were: the peak position in 82y; the height above the
baseline; the full width at half height; and the
mixing parameter for the function. The initial fit
results were iterated until the difference between
the experimental and decomposed pattern was <5%.
The smoother composite pattern produced by
summing these simulated peaks produces a trace
that closely approximates the average observed
XRD intensity. The residual obtained by subtracting
the measured from the calculated value exhibits
about the same variation due to noise and X-ray
counting statistics as the original recording (Fig. 3).
A complete listing of the peaks derived by fitting
the illustrated region and the entire backgroundsubtracted pattern is compiled in Table 1. The most
Ê region
significant reflections in the 9 11 A
Ê )
correspond to the 001 reflection of illite (10.01 A
Ê ,
and the 002 reflection of fuchsite at 9.92 A
according to Martõ´n Ramos & Rodrõ´guez Gallego
(1982) and Clifford et al. (1999). A low intensity
Ê ) was also
002 reflection of chlorite (7. 07 A
detected.
The EMPA analyses were performed using a
JEOL JXA-89 00 M from the Servicio de
Micros copõ´a electrónica Lui s Bru, of the
Universidad Complutense de Madrid, with analytical conditions of 15 kV and 20 nA, a beam
diameter of 1 mm, and with geostandards provided
by the Department of Petrology, Universidad
Complutense, Madrid, and the Smithsonian
Institute, Washington, USA. The EMPA data
(Table 2) from this pale green mica show highly
variable Cr2 O3 contents (11.67 0.25%) but almost
always >1%, which allow their classification as
fuchsites. Nevertheless, almost all Cr-rich micas
Residual
300
I
Smoothed
composite
200
Individual peaks
F
100
7
8
9
10
°2q Cu-Ka
FIG. 3. Example of the peak composition results for the <2 mm fraction of the EG-solvated sample. I: Illite;
F: Fuchsite.
349
Fuchsite in the Almadén mercury mining district, Spain
TABLE 1. XRD decompositio n results of the
< 2 m m frac tion o f th e et hy l en e g ly co l
(EG)-solvated pale green mica.
Intensity (counts )
dhkl
11.177
10.496
10.147
10.009
9.9202
9.762
9.5919
7.0772
5.0045
4.9331
5.049
3.3356
3.3049
24
38
35
188
112
57
15
21
56
15
9
61
27
from Las Cuevas mine have lower Cr2 O3 contents
(<2% and generally <1%), classifying them as Cr
illites (Fig. 4a). The variations in Cr content in
samples from the same area of mineralization (e.g.
El Entredicho mine) as well as between micas in
the same sample is remarkable. In general, fuchsites
show a good correlation between Cr and AlV I
(Fig. 4b), which indicates that Cr is present in the
mica structure through substitution of octahedral Al.
Other differences between Cr-rich micas in the
frailesca from these mines relate to the higher Na
content (>0.2 a.p.f.u. ) in micas from Las Cuevas
(Fig. 4c), contrasted with their lower K/(Na+K)
values (Fig. 4d). The higher Na values in fuchsites
from this mine with respect to those from El
Entredicho (characterized also by a reasonable
compositional homogeneity in the alkaline components) is also reflected in the chemical compositions
of other associated secondary minerals present in
both mineralizations. This feature must be related
Height
0.349
0.407
0.204
0.133
0.19
0.367
0.265
0.186
0.272
0.077
0.552
0.231
0.928
a
b
EE-11
8
Cr2O3
EE-11
VI
EE-12
EE-14
6
Al
5
EE-12
EE-14
LC-PL4
LC-PL4
4
4
2
Illites
0
40
50
3
SiO2
60
c
0
0.5
Cr
1
d K/(Na+K)
1
2
K
EE-11
1
EE-12
0.75
EE-11
EE-14
EE-12
LC-PL4
EE-14
LC-PL4
0
0
0.2
0.4
Na
0.5
0
0.5
Cr
1
FIG. 4. Compositiona l variations of the Cr-rich micas from the frailesca rocks of the Almadén district. The EE
and LC samples are from the El Entredicho and Las Cuevas mines, respectively. Contoured area represent s the
composition field of the Cr-rich micas from the Las Cuevas mine.
35.35
0.37
28.07
11.67
6.25
0.09
0.22
2.90
0.15
0.33
5.54
90.92
Wt.%
SiO2
TiO2
Al2 O3
Cr2 O3
FeO
MnO
NiO2
MgO
CaO
Na2 O
K2 O
Total
50.00
0.03
30.92
4.12
0.53
0.00
0.03
1.21
0.18
0.17
6.03
93.21
EE-11
2-11
49.58
0.05
29.90
4.06
0.49
0.03
0.02
1.21
0.17
0.24
6.31
92.05
EE-11
2-12
49.11
0.07
33.39
1.83
0.60
0.01
0.00
0.95
0.20
0.48
6.83
93.47
EE-11
2-21
Structural formula on the basis of 24 oxygens
Si
5.236
6.625
6.668
6.490
Al(IV )
2.764
1.375
1.332
1.510
Al(V I )
2.137
3.452
3.407
3.689
Cr
1.366
0.432
0.431
0.192
Ti
0.042
0.002
0.005
0.006
Mg
0.639
0.238
0.242
0.188
Ni
0.026
0.004
0.002
0.000
Fe2 +
0.775
0.058
0.055
0.067
Mn
0.011
0.000
0.004
0.001
Ca
0.024
0.025
0.024
0.028
Na
0.094
0.043
0.063
0.123
K
1.046
1.020
1.082
1.151
Scat.
14.159 13.274 13.314 13.445
EE-11
2-7
Sample
Point
6.573
1.427
3.395
0.382
0.009
0.279
0.000
0.069
0.000
0.037
0.061
1.227
13.460
49.25
0.09
30.66
3.62
0.61
0.00
0.00
1.40
0.26
0.24
7.21
93.34
EE-12
2-8
6.685
1.315
3.611
0.185
0.011
0.274
0.000
0.085
0.000
0.024
0.069
1.049
13.308
49.78
0.10
31.13
1.75
0.76
0.00
0.00
1.37
0.17
0.27
6.12
91.44
EE-12
4-24
6.486
1.514
3.699
0.026
0.048
0.340
0.003
0.102
0.007
0.031
0.058
1.122
13.436
49.01
0.48
33.43
0.25
0.93
0.06
0.02
1.72
0.22
0.23
6.65
93.00
EE-14
1-5
6.718
1.282
3.696
0.155
0.002
0.272
0.012
0.043
0.002
0.017
0.072
0.955
13.227
51.11
0.02
32.14
1.49
0.39
0.02
0.11
1.39
0.12
0.28
5.70
92.77
EE-14
2-8
6.905
1.911
3.826
0.631
0.048
1.109
0.016
0.623
0.011
0.087
0.152
1.340
13.845
51.82
0.48
34.54
5.54
5.38
0.09
0.15
5.34
0.61
0.60
7.44
95.04
6.089
1.095
3.004
0.020
0.000
0.188
0.000
0.042
0.000
0.013
0.043
0.871
13.190
43.69
0.00
28.71
0.19
0.38
0.00
0.00
0.95
0.09
0.17
5.09
88.19
Range
n = 31
6.563
1.437
3.803
0.120
0.006
0.080
0.000
0.024
0.006
0.014
0.303
1.093
13.449
49.50
0.06
33.54
1.14
0.22
0.05
0.00
0.41
0.10
1.18
6.46
92.66
6.296
1.704
3.922
0.037
0.003
0.038
0.000
0.017
0.000
0.022
0.146
1.517
13.701
47.45
0.03
35.98
0.35
0.16
0.00
0.00
0.19
0.16
0.57
8.96
93.84
LC-PL4-30a
4-50
4-51
6.538
1.462
3.814
0.111
0.004
0.069
0.002
0.018
0.005
0.011
0.412
1.052
13.497
50.70
0.04
34.72
1.08
0.17
0.05
0.02
0.36
0.08
1.65
6.40
95.25
4-48
6.905
3.206
3.826
0.136
0.006
0.123
0.002
0.029
0.006
0.022
0.412
1.685
13.701
51.82
0.06
35.98
1.33
0.27
0.05
0.02
0.64
0.16
1.65
8.96
95.86
4.764
1.095
1.770
0.037
0.000
0.038
0.000
0.011
0.000
0.006
0.146
1.391
13.430
43.69
0.00
33.00
0.35
0.10
0.00
0.00
0.19
0.04
0.57
6.40
92.66
Range
n = 10
TABLE 2. Selected EMPA analysis of fuchsite from the El Entredicho (samples EE-11, EE-12, EE-14) and Las Cuevas (sample LC-PL4-30a) mines. In the
‘range’ column, analyses for EE-11/2-7, which seem to correspond to mixtures of mica plus chromite, have been excluded. EE-11/2-12 can be considered as a
typical analysis of fuchsite, whereas EE-14/1-5 or LC-PL4-30a/4-51 should be typical analyses for Cr-illite.
350
D. Morata et al.
351
Fuchsite in the Almadén mercury mining district, Spain
with variations in the aN a/K of the fluids during the
alteration process. The Cr illites from Las Cuevas
mine are characterized by lower K and Cr.
A Cr-rich chlorite is also present, associated with
the fuchsite. This has also been reported in other
areas (Moritz & Crocket, 1991; Schandl & Wicks,
1993). EMPA analyses of Cr-chlorite (Table 3)
indicate strong chemical variations, mostly in terms
of Cr2 O3 (2.73 0.23%) and FeOt (30.85 22. 11%),
as well as the presence of interlayer cations (in
some analyses >0.1 a.p.f.u.). These Cr-rich chlorites
are characterized by higher Fe contents than those
Cr-chlorites normally associated with ultramafic
nodules (Newman & Brown, 1987; Deer et al.,
1992; Schandl & Wicks, 1993). As for the
associated fuchsites, these Cr-chlorites are characterized by a relatively broad compositional range.
All these facts could be interpreted as a consequence of a partial replacement of these Cr-rich
chlorites by normal chlorite.
A possible alternative explanation for the data
observed is the existence of a phase mixture
between fuchsite and Cr-rich chlorite, causing the
important compositional variations described.
However, as seen in Fig. 5, there is no evidence
of this kind of mixture, since intermediate
compositions are not detected.
G E N E S I S O F T H E C R- R I C H
PHYL LOSI LICATE S
and/or with vermicular morphologies in pyroxene
pseudomorphs), and the high Cr values measured in
some fuchsite crystals, could indicate that this mica
formed through the alteration of originally Cr-rich
phases (Cr-spinels) due to a high aCO2 (responsible
for the formation of the paragenetic carbonates) and
K+-rich hydrothermal fluids. In fact, a petrographic
relationship has been established between Cr-spinel
and fuchsite, dolomite and Cr-chlorite (Fig. 6). This
origin for the fuchsite, from originally Cr-rich
phases, could also explain the high Cr 2 O 3
(11.67%) and FeO (6.25%) values and the low
SiO2 (35.35%) content of some of the analysed
phyllosilicates (see Table 2). Moreover, it is
possible that some of the compositional anomalies
recorded in these Cr-rich micas are due to probe
analyses focusing on unaltered Cr-spinel relics and
neoformed fuchsite. In this sense, the high Cr2 O3
contents of the fuchsite analysed by Saupé (1990)
could be interpreted as a mixed analysis between
spinel and mica.
Compositional variations observed in the Cr and
Fe contents in fuchsite (Fig. 4a d, and Table 2) and
of Cr-rich chlorite (Table 3) could be explained by
an original heterogeneous distribution of these
elements in the igneous spinels (as previously
noted). On the other hand, the high Cr2O3 variations
in fuchsites could indicate a progressive destabilization of this mineral with the advance of the
argillization processes as it is reflected in the
subsequent neoformation of illite-type mica. This
The textural relations of the Cr-rich micas (as result
of the pseudomorphing of ultramafic-rock xenoliths,
Chl
K+Na
2
Dol
Fuch
Spl
1
Fuch
EE-11
EE-12
EE-14
LC-PL4
Cr-chlorites
0
0
0.1
Cr/Al VI
0.2
FIG. 5. Cr/AlVI vs. K+Na (a.p.f.u.) diagram for the
micas and chlorites.
FIG. 6. SEM image showing the partial replacement by
a Cr-spinel crystal (Spl) of fuchsite (Fuch), dolomite
(Dol) and Cr-chlorite (Chl). Qtz = quartz. Scale bar =
10 mm. All the phases were identified using SEM-EDX
analysis.
352
D. Morata et al.
TABLE 3. Selected EMPA analysis of Cr-chlorite from El Entredicho (samples EE-) and Las
Cuevas (sample LC-PL4-30a). Same analytica l condition s as for Table 2.
Sample
Point
3-41
LC-PL4-30a
3-42
3-47
EE-11
2-6
EE-11
2-17
EE-12
2-14
Wt.%
SiO2
TiO2
Al2 O3
Cr2 O3
FeO
MnO
NiO2
MgO
CaO
Na2 O
K2 O
Total
25.57
0.03
24.74
2.18
24.03
0.00
0.10
8.82
0.10
0.02
0.00
85.59
26.82
0.04
25.49
2.73
22.11
0.00
0.08
9.04
0.07
0.00
0.00
86.38
22.95
0.14
24.67
0.94
27.77
0.06
0.09
9.81
0.07
0.00
0.00
86.48
24.35
0.07
21.21
2.25
28.30
0.01
0.40
9.27
0.00
0.00
0.03
85.88
24.93
0.02
21.57
0.74
28.87
0.01
0.29
9.85
0.05
0.03
0.03
86.40
25.49
0.05
20.91
0.23
30.85
0.02
0.28
9.07
0.02
0.02
0.01
86.95
5.377
2.623
2.897
0.392
0.011
3.052
0.070
5.227
0.003
0.000
0.000
0.009
19.661
5.451
2.549
3.009
0.128
0.003
3.211
0.051
5.278
0.002
0.011
0.012
0.008
19.713
5.580
2.420
2.975
0.040
0.008
2.960
0.049
5.647
0.004
0.006
0.008
0.003
19.700
Structural formulae on the basis of 28 oxygens
Si
5.467
5.600
4.987
Al(iv )
2.533
2.400
3.013
Al(vi )
3.701
3.873
3.304
Cr
0.369
0.451
0.161
Ti
0.005
0.007
0.022
Mg
2.810
2.813
3.178
Ni
0.018
0.013
0.016
Fe2+
4.298
3.860
5.045
Mn
0.000
0.000
0.010
Ca
0.023
0.016
0.016
Na
0.007
0.000
0.000
K
0.000
0.000
0.000
Scat.
19.230
19.031
19.752
alteration sequence is supported by the 40Ar/39Ar
dating carried out on these fuchsite crystals (Hall et
al., 1997). The 40 Ar/39 Ar results indicate values
ranging from 365 to 427 Ma, with radiometric ages
varying between different samples and even within
the same sample. On the other hand, radiometric
40
Ar/39Ar analyses performed on illites formed from
the break-down of fuchsite gave a more precise age
of 361+2 Ma (Hall et al., 1997). The oldest
radiometric ages are related to the hydrothermal
alteration processes associated with the genesis of the
stratabound mineralizations (i.e. the ‘Almadén-type’),
whereas the younger ages are related to the argillitic
alteration and, consequently to the epigenetic-type
mineralizations in the Almadén Hg district.
d13 C isotopic data from carbonates associated
with the Hg mineralization in the Almadén district
(Eichmann et al., 1977; Rytuba et al., 1988)
suggested a mantle origin for the carbon. This
possible cause of the high aCO2 would agree with
an early hydrothermal alteration of the primary
igneous Cr-rich spinels. This early alteration stage
has a typical mantle signature.
CONCLUSIONS
The presence of fuchsite as well as minor Cr-rich
chlorite in the ultramafic-rock enclaves of the
‘frailesca’ rock from the mining district of the
Almadén area, can be interpreted as the result of an
early hydrothermal alteration process (~427 Ma)
acting on igneous Cr-rich spinels and related to the
formation of Hg deposits. The hydrothermal fluids
responsible for this early alteration would have
been characterized by a relatively high temperature,
high aCO2 and aK , and variable aN a/K , this last
parameter being higher in the case of the Las
Cuevas mine (where fuchsite has higher Na
Fuchsite in the Almadén mercury mining district, Spain
contents than those from the El Entredicho mine).
Finally, during a second alteration stage, the total
break-down of fuchsite into illite was only possible
in areas in which the hydrothermal fluids pervaded
completely. This second alteration stage would be
related to acid fluids and high aSiO2 and aS, at
temperatures lower than 2708C, giving rise to the
argillic alteration (Higueras et al., 1995, 1999),
which is characteristic of the epigenetic-type
mineralization. The partial break-down of fuchsite
into illite may account for the higher 40 Ar/39 Ar
radiometric ages obtained for the fuchsite. The
second alteration stage, characterized by the
formation of illite, must have occurred ~360 Ma
ago, and could be related to the Hercynian
metamorphic event, as proposed by Higueras et
al. (1995), or associated with the high geothermal
gradients related to the long-lasting magmatic
activity characteristic of the district (Higueras et
al., 1999).
Thus, the two alteration stages experienced by the
primary Cr-rich spinels agree with the two 40Ar/39Ar
ages obtained by Hall et al. (1997). According to our
genetic model, the origin of the Cr-rich phyllosilicates in the ultramafic enclaves from the ‘frailesca’
rock in the Almadén Hg district is related to an early
hydrothermal alteration stage (~427 Ma, 40Ar/39Ar
age) from primary Cr-rich spinels.
ACKNOWLEDGMENTS
This work was carried out with the financia l support of
the ‘Ayudas a la Investigaci ón (Financiación Interna )’
programme of the Universit y of Castilla-La Mancha.
Luis Puebla, Juan Luis Baldonedo and Alfredo
Fernández (Centro de Microscopõ´a electrónica Luis
Bru, Universidad Complutense de Madrid) provide d
assistance with the EMPA and TEM analytica l work.
The SEM-EDX study was carried out at the Universit y
of Cádiz. Detailed XRD study was performe d at the
Department of Geology and Geophysic s of the
Louisiana State University. Suggestions and improvements by Dr L. Aguirre (Universit y of Chile), Dr A.
Meunier (University of Poitiers, France) and Dr S.
Hillier (The Macaulay Land Use Research Institute ,
UK) are acknowledged. This work is a contributio n to
the IGCP-427 ‘‘Ore-forming processes in dynamic
magmatic systems’’.
REFERENCES
Arif M., Fallick A.E. & Moon C.J. (1996 ) The genesis of
emeralds and their host rocks from Swat, north-
353
western Pakistan: a stable-isotop e investigation.
Mineral. Deposita, 31 255 268.
Christofide s G., Thimiatis G., Koroneos A., Sklavounos
S. & Eleftheriadis G. (1992 ) Mineralog y and
chemistry of Cr-chlorites associate d with chromites
from Vavdos and Vasilika ophiolite complexes
(Chalkidiki , Macedonia, N. Greece). Chem. Erde,
54, 151 166.
Clifford T.N., Rex D.C., Green R., Le Roex A.P.,
Pienaar H.S. & Bühmann D. (1999) Chromian illiteankerite-quart z paragenese s from the Kintail district
of southern Ross-shire, Scotland. Mineral. Mag. 63,
37 52.
Craw D. & Angus P.V. (1993 ) Mafic/ultramafic clasts in
deformed biotite zone metaconglomerate , Macraes
mine, Haast Schist, New Zealand. New Zealand J.
Geol. Geophys. 36 395 398.
Deer W.A., Howie R.A. & Zussman J. (1992) An
Introduction to the R ock-fo rming Minerals.
Longman Scientific & Technical, London, 696 pp.
Eichmann R., Saupé F. & Schidlowski M. (1977)
Carbon and oxygen isotope studies in rocks of the
vicinity of the Almadén mercury deposit (provinc e
of Ciudad Real, Spain). Pp. 396 405 in: Time- and
Strata-boun d Ore Deposits (D.D. Klemm & H.D.
Schneider, editors ). Springer- Verlag, Berlin,
Heidelberg , New York.
Hall C.M., Higueras P., Kesler S.E., Lunar R., Dong H.
& Halliday A.N. (1996 ) Datación 39 Ar/40 Ar de
mineralizacione s de mercurio del Sinclinal de
Almadén. Geogaceta , 20, 483 486.
Hall C.M., Higueras P., Kesler S.E., Lunar R., Dong H.
& Halliday A.N. (1997) Dating of alteration episodes
related to mercury mineralization in the Almadén
district. Earth Planet. Sci. Lett. 148, 287 298.
Hernández A., Jébrak M., Higueras P., Oyarzun R.,
Morata D. & Munhá J. (1999) The Almadén mercury
mining district, Spain. Mineral. Deposita, 34,
539 548.
Higueras P. (1993) Alteration of basic igneous rocks
from the Almadén mercury mining district. Pp.
131 134 in: Current Research on Geology Applied
to Mineral Deposits (P. Fenoll, F. Gervilla & J.
Torres, editors ). University of Granada, Spain.
Higueras P. (1995) Procesos petrogenéticos y de
alteración de las rocas magmáticas asociada s a las
mineralizaciones de mercurio del distrit o de
Almadén. PhD thesis, Univ. Granada, Spain.
Higueras P., Parras J. & Sánchez C. (1995) Procesos de
alteración asociados a la removilizaci ón hidroterma l
de cinabrio en el yacimiento de ‘Las Cuevas’
(Almadén, Ciudad Real). Bol. Soc. Esp. Min. 18,
191 200.
Higueras P., Oyarzun R., Lunar R., Sierra J. & Parras J.
(1999) The Las Cuevas deposit, Almadén district
(Spain): Unusual case of deep-seate d advanced
argillic alteration related to mercury mineralization.
354
D. Morata et al.
Mineral. Deposita, 34, 211 214.
Higueras P., Oyarzun R., Munhá J. & Morata D. (2000)
The Almadén mercury metallogeni c cluster (Ciudad
Real, Spain): Alkaline magmatism leading to mineralization processes at an intraplate tectonic setting.
Rev. Soc. Geol. Espan~a, 13, 105 119.
Jébrak M. & Hernández A.M. (1995) Tectonic deposition of mercury in the Almadén district, Las Cuevas
deposit, Spain. Mineral. Deposita, 30, 413 423.
Jiang W.-T., Peacor D.R. & Slack J. F. (1992 )
Microstructures , mixed layering, and polymorphis m
of chlorite and retrograd e berthierin e in the Kidd
Creek massive sulfide deposit, Ontario. Clays Clay
Miner. 40, 501 514.
Martin Ramos J.D. & Rodrõ´guez Gallego M. (1982)
Chromium mica from Sierra Nevada, Spain.
Mineral. Mag. 46, 269 272.
Moritz R.P. & Crocket J.H. (1991) Hydrothermal wallrock alteration and formation of the gold-bearing
quartz-fuchsit e vein in the Dome mine, Timmins
area, Ontario, Canada. Econ. Geol. 86, 620 643.
Newman A.C.D. & Brown G. (1987) The chemical
constitutio n of clays. Pp. 1 128 in: Chemistry of
Clays and Clay Minerals (A.C.D. Newman, editor ).
Longman Scientific & Technical, London.
Ortega E. & Hernández A. (1992) The mercury deposits
of the Almadén syncline , Spain. Chron. Rech. Min.
506, 3 11.
Petschik R. (2000) MacDiff 4.1.2. Powder diffractio n
software. Available from the author at http://
www. geol. uni- erlangen. de/html/software/macdiff.html
Rytuba J.J., Rye R.O., Hernández A.M., Dean J.A. &
Arribas A. (Sr) (1988) Genesis of Almadén type
mercury deposits, Almadén, Spain. Int. Geol. Congr.
Washington , abstract.
Saupé F. (1990) Geology of the Almadén mercury
deposit, Province of Ciudad Real, Spain. Econ. Geol.
85, 482 510.
Schlandl E.S. & Wicks F.J. (1993) Carbonate and
associate d alteration in ultramafic and rhyolitic rocks
at the Hemingway property , Kidd Creek volcanic
complex, Timmins, Ontario. Econ. Geol. 88,
1615 1635.