Mineralizações magmatogénicas sulfuretadas de Ni-Cu no compartimento oeste da Sequência Gabróica Bandada de Beja
Magmatic, Ni-Cu sulphide mineralization in the western compartment
of the Beja Layered Gabbroic Sequence
A. P. Jesus(1), A. Mateus(1), J. Munhá(1), J.M.C. Branco(2), V.H. Araújo(2)
(1) Dep.
Geologia, CREMINER / Centro de Geologia, Fac. Ciências Univ. Lisboa, Ed. C6,
Piso 4, 1749-016 Lisboa
(2)
Rio Narcea Gold Mines SA, Rua Circular Poente, Bloco 36 R/c, Loja 1 – P.I.T.E.,
7000-171 Évora
ana.jesus@fc.ul.pt; amateus@fc.ul.pt; jmunha@fc.ul.pt; jmc.branco@rngm.pt; victor.araujo@rngm.pt
SUMÁRIO
No compartimento oeste da Sequência Gabróica Bandada de Beja (LGS), alguns cumulados piroxeníticos hospedam sulfuretos maciços intercumulus contendo até 0.89 wt% Ni e 0.93 wt% Cu. Os magmas que fraccionaram as
fácies gabróicas relacionadas com estas mineralizações têm potencial para segregar líquidos sulfuretados com
[Ni]5 wt%. Mas a possibilidade de gerar líquidos niquelíferos (máx. 12 wt% em 100% sulfuretos) durante os
estádios iniciais da evolução do LGS é alta, valorizando o potencial da faixa gabróica sob cobertura Cenozóica.
Palavras-chave: Complexo Ígneo de Beja, Minérios sulfuretados magmatogénicos de Ni-Cu
SUMMARY
In the western compartment of the Beja Layered Gabbroic Sequence (LGS), some pyroxene cumulates host intercummulus massive sulphides that show up to 0.89 wt% Ni and 0.96 wt% Cu. Magmas from which the gabbroic facies related to these ores were fractionated have potential to segregate sulphide melts containing up to 5
wt% Ni. But the possibility of generate Ni-rich melts (max.  12 wt% in 100% sulphide) during the early steps of
LGS evolution is high, thus turning in a very promising target the belt underneath the Cenozoic cover.
Key-words: Beja Igneous Complex, Magmatic Ni-Cu sulphide ores
1. Introduction
The metallogenic potential of the Beja Layered
Gabbroic Sequence (LGS) has been systematically
assessed since 1999, taken advantage of early exploration surveys carried out by the Serviço de Fomento
Mineiro [e.g. 1, 2]. Accordingly, the LGS western
compartment (from W Torrão to Beringel) emerged
as a very promising target and several studies were
performed in order to: (i) unravel the internal architecture of the igneous complex [3-5]; (ii) evaluate
the spatial distribution of oxide and sulphide enrichments (Fig. 1); and (iii) characterize the recognized oxide and sulphide accumulations. Available
data for the Torrão-Ferreira do Alentejo sector allow
grouping into five Series the outcropping LGS gab-
broic succession, which, from NE to SW, were labelled Soberanas I and II, and Odivelas I, II and III
[2-5]. Massive Fe-Ti-V oxide accumulations (type I
mineralization) are hosted in olivine leucogabbros
and cumulates of the Odivelas I lower section [2, 4,
6, 7]. Anastomosed veins of Po + Cpy  Py (type II
mineralization) are related to late metasomatic haloes and pegmatite dykes that affect gabbroic rocks
in the upper section of Odivelas III [2, 8]. In the
Ferreira do Alentejo-Beringel sector two other
rhythmic Series were put in evidence (Beringel I and
II – BRG I and II), the latter hosting intercummulus
massive Ni-Cu-Co sulphides (type III mineralization) [9].
The major goal of this work is to report and discuss
1
Fig.1: Spatial distribution (qualitative estimation) of
oxide and sulphide enrichments for the LGS
western sector. Grid coordinates according with
the international Gauss-ellipsoid, Lisbon datum. Details on geological features can be found in [5].
the geochemical features of type III mineralization
and its gabbroic envelope. Additionally several lines
of evidence will be examined in order to deduce
significant geochemical guides for Ni-Cu exploration in LGS.
2. Type III mineralization
The most important occurrence of this type so far
identified is located at the Figueirinha quarry, close
to the contact between BRG II and the Border
Group (see Fig. 1 and [5]). As reported in [9], the
intercummulus sulphides here observed are massive
and mostly composed of Pen + Po + Cpy + linnaeite
group minerals (Linn. s.l.) + Py. Pyroxenites hosting
intercummulus sulphides comprise mainly randomly
oriented, subhedral Cpx with typical adcumulate
texture. Tiny blebs of Po with Pen-Cpy exsolutions
occur as inclusions within coarser Cpx grains, forming graphic intergrowths. As sulphide modal proportion increases, Cpx grains are disrupted, displaying
abundant corrosion gulfs but only mild retrograding
effects. Rare Pl occurs interstitially or included
within the Cpx-sulphide framework being slightly
altered to a fine- grained mixture of chloriteprehnite. Minor magnetite and ilmenite coexist with
Pen-Po-Cpy blebs suggesting O2 conditions of late
stage (~620 ºC) crystallization close to the QFM
buffer.
Table 1 illustrates the most relevant (major and
trace) metal and sulphur contents for representative
samples of type III mineralization. The concentra-
tions obtained clearly reflect the chemical signature
showed by the (i) sulphide mineralogy (mainly S,
Fe, Ni, Cu, Co) and the (ii) accessory oxide assemblage (which incorporates minor amounts of Cr and
Mn, besides Fe, Ti and V) [9]. Trace amounts of
other base metals, like Pb and Zn, are also preferentially incorporated in sulphides (mainly in Cu-rich
phases and Po), their scarcity reflecting the ultramafic nature of the parental melt. The whole-rock
contents obtained for Sc, Se and Sr are, as expected,
quite low and sometimes just slightly above the
detection limit of the analytical method used; sulphides (chiefly Pen) can incorporate trace amounts
of these elements, namely of Se and Sr (as confirmed by means of Micro-PIXE analysis).
Tab.1: Summary of whole-rock chemical data for
mineralised samples (Figueirinha quarry)
S (%)
Ni (%)
Cu (%)
Co (ppm)
Cr (ppm)
Mn (ppm)
Ti (ppm)
V (ppm)
Pb (ppm)
Zn (ppm)
Sc (ppm)
Se (ppm)
Sr (ppm)
FG-6A
6.17
0.31
0.93
413
201
246
1898
85
4
32
16
16
7
FG-6B
8.22
0.89
0.71
1194
121
216
741
43
6
27
5
27
23
FG-6
8.23
0.46
0.96
531
171
197
1592
66
4
28
12
18
5
2
3. Geochemistry of the gabbroic envelope
A general overview concerning the gabbroic facies
succession in BRG II and their main mineralogical
features can be found in [5]. In order to fully characterize the chemical signature of these facies, thirty
samples were collected along four, sub-parallel profiles. The obtained results will not be completely
discussed here, but just those that help to unravel the
Ni-Cu ore-forming process.
A first inspection of Mg and Ni co-variation along
sampling profiles (Fig. 2) suggest that Ni is preferentially included in olivine. Both plots enable, additionally, an evaluation of the general BRG II evolution, being clear that rocks of the Basal Group fractionated from the most primitive magma. The trend
observed between rocks of the Basal Group and
troctolites also supports the hypothesis that they
represent products of the LGS primordial magmas. It
should be noted that whole-rock contents reflect
mostly the bulk result of accumulation processes,
therefore providing an image of the mineralogical
assemblage actually present. These plots also put in
evidence the rhythmic nature of the Series, confirming the inferences discussed in [5].
Fig. 3: (A) log Ni/Cu and (B) log Cu versus log S
contents for gabbroic rocks and mineralised samples
of BRG II.
Since there is a correlation between metal ratios
(Ni/Cu) and increasing sulphidation, it could be
suggested that most Ni and Cu are related to sulphides (Fig. 3A). The strong positive correlation
between Cu and S contents (Fig. 3B) clearly demonstrates that significant dissemination of Cpy occurs
in these rocks, as observed. Moreover, the log[Cu]–
log[S]
co-variance
suggests
that
both
(CliqS/CliqCu)initial and (Ds/liqS – Ds/liqCu) are roughly
constant, thus allowing to deduce a self-similar behaviour for the distribution. Note, however, that this
behaviour could be simply self-affine (statistically
self-similar) if the clustering/dispersion obtained for
(log[Cu], log[S]) values around (100, 1000) reflect
an intrinsic property of the Cu-S distribution that
implies rescaling by different factors (presently
unknown). Nonetheless, and independently of the
mathematical descriptor used to characterize the
contents distribution, it is clear that the log[Cu]log[S] co-variance reflects an important geochemical
feature that can be used as an exploration guide for
type III mineralization, i.e. it records appropriate
conditions for the segregation of sulphide melts.
Fig.2: (A) Mg and (B) Ni content variations for
BRG II Groups, excluding the mineralised samples.
4. Ni content variation in olivine
Using the approach reported in [10] and assuming
that Figueirinha sulphides were segregated before
magma reached Ni depletion due to extensive olivine fractionation, an estimation of the olivine Ni
content at the time of sulphide deposition can be
performed. The calculations make use of the average
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measured metal contents in Pen and Cpy and presume that all the analyzed Ni and Cu are incorporated in these sulphides, respectively; no correction
for the presence of olivine is needed because no
olivine is present in these samples. The only minerals that could induce some uncertainty in calculations are Pen or Linn. s.l.. The effect of Py in S estimation is for sure negligible given its relative scarcity; Linn. s.l. presents roughly the same (Ni,Co):Fe
ratio of Pen and if its development is ascribed to low
temperature sulphide transformation the bulk Ni:S
ratio should be kept essentially the same. The estimated Ni contents were subsequently plotted in a
diagram that relates the Ni content in 100% sulphide
with the Ni-content of olivine that would fractionate
from the same melt. The same procedure was further
applied to samples of type II mineralization and all
the LGS gabbroic Series [5], using in the latter the
maximum Ni-Ol content. Fig. 4 summarizes the
results obtained, and it is clear that the estimated Ni
concentration for sulphide melts lies above 4.7 wt%,
therefore comparable with values usually reported
for economic Ni-Cu ore-forming systems.
Fig.4: Empirical relation between Ni in Ol and Ni in
2500
2000
ppm Ni in Ol
Border G Troct
SB I
ODV I
ODV II
ODV III
BRG I
BRG II
Figueirinha
Ventoso
1500
1000
500
0
15
10
5
0
wt% Ni in 100% sulfide
sulphide melt that would segregate from a magma
[10].
5. Conclusions
Following [5], Border Group troctolites and their
equivalents in Soberanas I Series should represent
the LGS primordial U-type magmas. Additionally,
type III mineralization so far identified is confined
to the SW limit of LGS, quite near to the contact
between the Lower Group rocks of BRG II and the
Border Group. Considering these features together,
the possibility of having adequate conditions to
segregate Ni-rich sulphide melts (maximum estimate
12.1 wt % Ni in 100% sulphide) is high, the sulphide
accumulation occurring mostly at the transition from
the feeder zone (from SW) to the magma chamber.
This interpretation opens for the first time a sustained hypothesis that (primitive) LGS rocks underneath the Cenozoic sedimentary cover (between
Torrão and Ferreira do Alentejo villages) include NiCu(-PGE?) sulphide mineralization. Furthermore, it
fully justifies the exploration efforts recently per-
formed in BRG II, which have potential to segregate
sulphide melts containing up to 4.7 wt% Ni.
Acknowledgements
A. Mateus and J. Munhá appreciated the logistic
support of CREMINER and Centro de Geologia
(POCA-PETROLOG, UI : 263; POCTI / FEDER ) FCUL. A. Jesus acknowledges a PhD Grant
SFRH/BD/6355/2001.
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