CHEMICAL NATURE OF Ag-BEARING TETRAHEDRITES FROM THE ENFERMARIAS
PROSPECT (MOURA, PORTUGAL) REVEALED BY EPMA AND MICRO-PIXE ANALYSIS
M. Barroso1, J. Figueiras2, L. C. Alves3, A. Mateus4
1
CREMINER, Faculdade de Ciências da Universidade de Lisboa, Portugal, migbarroso@hotmail.com
2
Dep. Geologia & CREMINER, Fac. Ciências da Universidade de Lisboa, Portugal, jmvf@fc.ul.pt
3
Dep. Física, Instituto Tecnológico e Nuclear, Portugal, lcalves@itn.pt
4
Dep. Geologia & CREMINER, Fac. Ciências da Universidade de Lisboa, Portugal, amateus@fc.ul.pt
Abstract
The prospect of Enfermarias is a superposition of mineralising events, with different metals deposited at
different times. Silver is most abundant in the late DFS mineralisation (where it occurs as sparse Ag-Au
particles), but is also concentrated in rims of td grains lately deposited in the much earlier MS mineralisation.
The geological setting, chemical composition, chemical zoning patterns and mineral associations of the td found
at Enfermarias have been thoroughly characterized, as part of an ongoing work aimed at the elucidation of the
complex history of this prospect.
Introduction
The Zn-Pb(-Ag-Sb-Au) prospect of Enfermarias, located in the Moura-Ficalho area of the
Magnetite-Zinc Belt (Ossa-Morena Zone), shows five different mineralisation types, two of them
containing accessory amounts of a late Ag-bearing tetrahedrite (Barroso, 2002). The mineralisation
termed MS is mainly composed of strongly fractured pyrite and of deformed sphalerite aggregates with
minor amounts of galena, chalcopyrite, magnetite, arsenopyrite and sparse Ag-bearing tetrahedrite in
strongly chloritised intermediate to mafic metavolcanics. The mineralisation labelled DFS mainly
consists of galena, sphalerite, pyrite, Ag-bearing tetrahedrite and, more rarely, gold-silver alloy
particles, commonly found as fine disseminations, that occasionally also fill small veinlets, in silicatebearing marbles or in strong metasomatised rocks (essentially made up of chlorite/serpentine,
actinolite/tremolite and talc) – for details see Barroso (2002) and Martins (2003). The main purpose of
this work is to characterise the chemical variability shown by the tetrahedrite (td) grains observed in
Enfermarias, as revealed by electron microprobe (EPMA) and proton-induced X-ray emission (microPIXE) analysis. According to the available data, td occurs always in close association to galena (gn –
in the MS mineralisation as fringes around this sulphide) and is the main Ag-bearing mineral in this
prospect, showing characteristically strong chemical heterogeneity, sometimes suggesting manifest
deviations from the ideal stoichiometry.
Materials and Methods
Representative samples were collected from an initial suite of drill-core specimens of borehole
SDM-3 whose mineralogy was previously examined through detailed optical microscopy. EPMA was
performed with a CAMECA-Camebax microprobe at IGM; the analytical conditions were 15 kV beam
acceleration voltage, 20 nA current and counting times of 20 s; a 2 m beam was used and element
calibration involved metallic (Ag, Cu, Cd, Bi) and mineral (sph, py, gn) standards. Micro-PIXE
measurements carried out at ITN used an Oxford Microbeams nuclear microprobe type set-up with a
2MeV proton beam (generated by a Van de Graaff accelerator) focused to 3 m and beam currents
close to 100 pA; particular conditions of X-ray accumulation, as well as procedures followed in data
acquisition, spectra fitting and mapping, are described in Alves et al. (2000). Whenever too strong
interference from Ca is present, the K line was used to collect data on Sb, together with an enhanced
beam current of 800 pA and a 2 mm thick Perspex filter to screen out low energy X-rays.
Results
The EPMA data available (28 analyses) show that the analysed td has variable but significant
contents of Ag, Zn, and Fe, besides minor amounts of As (Table 1). Its average composition is:
50
(Cu8.10Ag1.65Zn1.15Fe0.87Cd0.02)(Sb3.85As0.36)S12.59. The cationic proportions reveal always an excess of
(Sb+As+Bi), whereas (Cu+Ag) and S show a deficit when compared to the ideal formula. The sum of
trivalent ions (Sb+As+Bi) ranges between 4.08 and 4.30 atoms per half unit cell, and (Cu+Ag) falls in
the interval [9.58, 9.95], while S never reaches 13 atoms; the obtained negative correlation between
Ag and Cu contents is justified by their mutual replacement. Deficit in S could reflect volatilisation
during analysis or, instead, record vacancies in S equipositions. Complementarily or alternatively, the
very fact of (Sb+As+Bi) be in excess suggests
Table 1 – EPMA results for tetrahedrites of Enfermarias;
that they can partly behave as anions, thus
representative analyses of the two main groups (1 to 4 –
atomic proportions calculated on the basis of 16 cations)
occupying some of the positions ideally filled
and range shown by the analysed elements (N =28).
with S. Testing of these hypotheses needs the
use of analytical methods other than those here
Weight
1
2
3
4
described, and requires the prior separation of
%:
td grains, which is rather difficult to
S
24.66 22.73 24.27 20.42 20.42 – 24.79
accomplish due to their tiny dimensions and
Fe
3.30
2.86
0.48
2.22 0.48 – 3.46
As
1.18
2.23
2.37
0.99 0.74 – 2.37
mineralogical association.
Cu
35.86 26.72 35.34 18.32 18.32 – 35.96
According to EPMA data, two main td
Ag
2.83 14.11
1.41 25.20 1.41 – 25.20
Zn
3.83
4.50
7.38
4.68 3.68 – 7.38
groups can be recognised in Enfermarias, one
Cd
0.02
0.30
0.15
0.03 0.00 – 0.28
with an average Ag content of 0.5 atoms and
Sb
28.55 25.86 26.76 26.43 25.54 – 29.23
another characterised by higher values of this
Bi
0.11
0.00
0.00
0.00 0.00 – 0.20
Total 100.34 99.31 98.16 98.29
metal (usually, 2.0-2.5 atoms). Td of the first
Atoms:
group occurs in the DFS mineralisation, while
S
12.831 12.382 12.838 11.798
the Ag-enriched grains typically occur in the
Fe
0.986 0.894 0.146 0.736
As
0.263 0.520 0.536 0.245
MS type. For both td groups, the (Fe+Zn+Cd)
Cu
9.413 7.343 9.432 5.340
cations are scattered in the interval [1.95, 2.15],
Ag
0.438 2.284 0.222 4.327
Zn
0.977 1.202 1.914 1.326
showing a negative correlation with (Cu+Ag)
Cd
0.003 0.047 0.023 0.005
contents.
Sb
3.912 3.709 3.728 4.021
Micro-PIXE
point
analysis
and
Bi
0,009 0,000 0,000 0.000
Total 28.831 28.382 28.838 27.798
elemental mapping were performed in two
polished thin sections including td and gn
grains of the MS and DFS mineralisation types (samples E3-85 and E3-33, respectively). Results are
summarised in Table 2 and in Fig. 1; as an example, the elemental mapping obtained for E3-85 is
showed in Fig.2. The analytical results obtained with micro-PIXE need strong correction due to
interference from the surrounding minerals which are sampled because the beam penetration is larger
than the thickness of most td grains, and must therefore be considered semi-quantitative. However,
comparison of micro-PIXE and EPMA data sets show that both display similar variabilities, lending
confidence that, despite interference, the elemental maps obtained by micro-PIXE are a true image of
the elemental distributions in td. The latter provide indeed information that enable the understanding
of the heterogeneous chemical character recorded by td grains included in the MS mineralisation. In
fact, a clear chemical zoning is shown by these grains, typically with Ag-rich rims and Sb-Cu richer
cores sometimes with Zn-bearing domains (Fig. 2). Conversely, td from the DFS mineralisation do not
display evident chemical zoning, are significantly poorer in Ag and show higher As contents; Fe and
Zn values are also much more
uniformly distributed in these Table 2 – Micro-PIXE results for tetrahedrites (td) and galena (gn) in samples
E3-85 and E3-33. Representative concentration results are presented (td 1 refers
grains.
to the Ag-rich rims) as well as the concentration range found for the n number of
In
the
MS point analysis.
mineralisation,
gn
is
E3-85
E3-33
homogeneous and close to
ideal PbS, only including
td 1
td 2
gn
td
gn
Weight
n=6
n=3
n=4
%:
traces of Cu. The chemical
S
21.2 16.5-25.7
24.8 24.4-25.4
10.9
21.9 20.0-22.2
11.6
variability of this sulphide
Fe
3.51 2.70-5.36
2.97 2.97-3.31
0.07
2.11 2.02-2.49
0.44
increases, however, when it
As
0.38 0.08-0.94
1.4 1.2-1.4
3.33 2.40-3.57
Cu
19.26 16.27-29.95
28.8 28.1-28.8
0.10
32.9 32.5-33.0
3.6
occurs
in
the
DFS
Ag
22.6 14.9-26.2
10.6 9.9-10.6
7.8 7.03-8.55
2.3
mineralisation,
showing
Zn
3.5 3.1-5.6
4.3 4.3-4.9
5.6 5.1-5.6
0.5
meaningful contents of Ag,
Sb
29.5 20.3-33.5
27.1 27.1-27.8
26.3
7.5
Pb
89.0
73.6
Sb and Cu; no exsolutions or
Cr
0.03
51
inclusions of any type are observed at a
microscopic scale and the elemental maps
obtained with micro-PIXE do not support also
such a kind of features.
10
5
10
4
10
3
E-85
S
Ag
E-33
Cu
Sb
Counts
Fe
Zn
As
Mn
Discussion and conclusions
10
In the Enfermarias ore-forming system, td
deposition is a relatively late process, usually
10
post-dating gn, both in the MS and DFS
mineralisation types. EPMA and micro-PIXE
10
results show however that the chemical nature
300
400
500
600
700
800
900
C hannel
displayed by td and gn is different in each
mineralogical setting. When included in the
DFS mineralisation, td tends to be chemically Fig.1 – Micro-PIXE spectra obtained from point analysis on
tetrahedrite grains in E3-33 and E3-85 samples.
uniform, showing Ag/(Ag+Cu) and Zn/(Zn+Fe)
ratios of ca. 0.05 and ranging from 0.46 to 0.56, respectively; the adjoining gn deviates from ideal
composition, showing irregularly distributed minor amounts of Ag, Sb and Cu. The Ag/(Ag+Cu) and
Zn/(Zn+Fe) ratios displayed by td in the MS mineralisation are higher, usually varying between 0.200.27 and 0.53-0.64, respectively, although the former ratio may reach values as high as 0.45; the
coexistent gn exhibits typically an homogeneous chemical composition without meaningful traces of
other metals besides Cu. Another feature that characterises td in the MS mineralisation concerns the
development of chemical zoning, the rims being significantly Ag-enriched in relation to core domains.
No other type of chemical patterns nor specific coronas were found in these minerals that can be used
to infer the progress of particular Ag-Cu exchange reactions between td and gn (as those suggested by
Sack et al., 2002). The zoning of Ag in td included in the MS mineralisation suggests a late influx of
this metal coeval of the late stages of td growth. This agrees with the observation that the later DFS
mineralisation has much higher whole rock Ag concentrations and also bears Ag-Au alloys (Barroso,
2002). However, in seeming contradiction, td in DFS is always Ag-poor. This may be due to an age
difference between both td, to differential enhancing or inhibiting of td crystallisation and/or Ag
incorporation in both mineral environments, or even to two different peaks of Ag influx to the system
(as seems to be the case of Cu,
Si
S
Cu
Ag
for instance). Work in progress
will try to address these
4
problems.
2
1
0
6
7
2
5
10
8
1
9
3
Max.
K
Pb
Zn
Sb
Min.
100 µm
Fig. 2 – Elemental mapping of td, gn and phyllosilicate grains in sample E385. Circles in the Ag map mark point analysis locations.
Acknowledgments
The financial support was
provided by CREMINER. We
thank IGM for access to drillcores and non-published internal
reports. EPMA technical advice
from Dr. J. Farinha Ramos is
appreciated.
References
Alves, L.C., Breese, M.B.H., Alves, E., Paúl, A., da Silva, M.R., da Silva, M.F., Soares, J.C. (2000) – Micronscale analysis of SiC/SiCf composites using the new Lisbon Nuclear Microprobe. Nucl. Instr. Meth., B161163: 334-338.
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Enfermarias (Moura, Portugal). MSc Thesis, F. Ciências, U. Lisboa.
Martins, R. (2003) – Caracterização de processos metassomáticos correlativos da recristalização e (re-)
deposição de sulfuretos na Jazida de Enfermarias (Moura, Portugal). MSc Thesis, F. Ciências, U. Lisboa.
Patrick, R.A.D & Hall, A.J (1983) – Silver substitution into synthetic zinc, cadmium and iron tetrahedrites. Min.
Mag., 47: 441-451.
Sack, R.O., Kuehner, S.N., Hardy, L.S. (2002) – Retrograde Ag-enrichment in fahlores from the Coeur d’Alène
mining district, Idaho, USA. Min. Mag., 66: 215-229.
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