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Sm/Nd and platinum-group element geochemistry of a late-Precambrian
Alaskan-type complex from the Eastern Desert of Egypt
H. M. Helmy
Geology Department, Faculty of Science, Minia University, Egypt
[email protected]
A. H. Ahmed
Central Metallurgical Research and Development Institute, Cairo
[email protected]
H. Kagami
Graduate School of Science and Technology Niigata University
S. Arai
Kanazawa University, Kanazawa, Japan
ABSTRACT. Abu Hamamid intrusion is a late-Precambrian zoned mafic-ultramafic intrusion located along
major fracture zone in the Eastern Desert of Egypt. It is made up of a peridotite core, enveloped by wehrlite,
hornblende-clinopyroxenite and hornblende gabbro, at the margin. The different petrographic units are
cumulates formed by fractional crystallization processes. The mafic minerals are more magnesian in the core
rocks. No chromitite or Cu-Ni-sulfide ores or Pt-Fe alloys were found at the present level of erosion. All
lithologic units are depleted in platinum group elements (< 25 ppb total PGE). The rocks show fractionated
chondrite-normalized PGE pattern. The Sm-Nd data yield model ages (TDM) of 750 to 796 Ma. Geochemical
and Sm/Nd isotope data suggest that the intrusion was formed from fairly primitive magma that had
experienced sulfide segregation prior to the development of the rocks.
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INTRODUCTION
The Eastern Desert of Egypt is covered by a PanAfrican (650-550 Ma) basement complex
representing a part of the Arabo-Nubian Shield. This
part is dissected by deep seated fault zones trending
ENE and extend for more than 100 km. Many zoned
mafic-ultramafic complexes intrude along these fault
zones and have many features characteristic of
Alaskan-type complexes (Helmy and El Mahallawi,
2003). Unusually, some of these complexes e.g.
Gabbro Akarem (Helmy and Mogessie, 2001) and
Genina Gharbia (Helmy, 2004) host sub-economic
Cu-Ni-PGE mineralization.
Abu Hamamid mafic-ultramafic complex has
geological and petrological characteristics typical of
Alaskan-type complexes. In common with other
zoned complexes in the Eastern Desert and
worldwide, Abu Hamamid do not host sulfide or
chromitite mineralization. In this contribution we
study the Sm/Nd isotope, rare-earth element (REE)
and platinum-group element (PGE) geochemistry of
this complex to estimate its age and to understand its
magmatic history. The genetic relationship between
the Abu Hamamid intrusion and the surrounding
island-arc rocks and other mineralized zoned
complexes is discussed on the light of the
geochemical and isotopic study.
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GEOLOGICAL SETTING
Abu Hamamid Alaskan-type complex is located
100 km to the west of the Red Sea coast at the
intersection of two major faults affecting low-grade
volcanic rocks (Shadli Metavolcanics: 710 Ma, Stern
et al., 1991). This area is a part of the Proterozoic
Shield
comprising
metasedimentary
and,
metavolcanic rocks, granitoids and ultramafic rocks.
The AH complex is an elliptical body 1.5 km
long and 500 m maximum width (Fig. 1). The
geologic contacts with the surrounding volcanic
rocks are hidden below a thick valley sediments.
This intrusion comprises a peridotite core enveloped
by wehrlite, hornblende clinopyroxenite and
hornblende gabbro, at the margin. The peridotite
core bodies occur as small rounded outcrops aligned
in a ENE direction parallel to the direction of
elongation of the intrusion. Blocks of the hornblende
gabbro are commonly observed in the wehrlite. The
contacts between the rock types are gradational over
a short distance.
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4.1
Figure 1. Geologic map of the Abu Hamamid
intrusion
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MINERALOGY
The ultramafic units of AH complex have cumulate
textures and the observed crystallization sequence is:
olivine (+cotectic spinel)- clinopyroxene (Cpx)hornblende and plagioclase.
Olivine is present as a major constituent in
peridote (60 % modal) and wehrlite (up to 30 %),
but less abundant in the hornblende clinopyroxenite
(< 10 %). Spinel is more abundant in the peridotite
while ilmenite is more common in the hornblende
gabbro. Augite and hornblende exist in all rock
units, hornblende being more abundant than augite
in peridotite. Detailed microscopic investigations did
not locate any platinum-group minerals.
Mafic minerals from the core of the intrusion are
highly magnesian, a consistent decrease in the Mg#
of olivine (from 82 to 73), Cpx (from 87 to 73) and
hornblende (from 84 to 64) is observed from core to
margin rocks. Two types of spinel are found; AlMg-rich and Fe-rich. The Al-Mg-rich spinel has a
wide-range of Al2O3 (16-32 wt.%), Cr2O3 (18 - 31
wt.%) and MgO (4-8 wt.%). A wide range of
Fe3+/Fe2+ (0.3 - 0.7) and Cr# (Cr/Cr+Al, 0.2 - 0.7)
ratios is revealed. Cr-magnetite contains Cr2O3
within the range 4.7 - 19 wt.%, whereas the Al2O3
and MgO contents vary from 0.3 to 4.4 wt.% and
from 0.6 to 1.9 wt.%, respectively. The Fe3+/Fe2+
and Cr# ratios in the Cr-magnetite are within the
range 1.4 - 1.9 and 0.4 - 0.9, respectively. The MnO
contents in both types of spinel are similar, normally
< 0.8 wt.%. Detailed mineralogy of the intrusion will
be published elsewhere (Helmy and Farahat, in
prep.)
GEOCHEMISTRY
Major elements
Hornblende gabbro samples are clearly
distinguished from the ultramafic units by their high
Al2O3 (average 23 wt.%) and Na2O (average 0.9
wt.%) and low MgO (average 7.7 wt.%) contents.
Relatively low Cr contents are found in the
hornblende gabbro (350 ppm).
The wehrlite rock samples show a narrow range
of chemical compositions (average 10.0 wt.% MgO
and 6.0 wt.% FeO). Al2O3 (18.0 – 19.5 wt.%), CaO
(11.0-16.0 wt.%) and total alkali (Na2O+K2O) (1.01.5 wt.%) contents are higher than those in peridotite
samples. Low Cr contents are found in wehrlite (500
ppm).
The peridotite samples contain 17-24 wt.% MgO
and 10.2-12.9 wt% total FeO. The relatively high
CaO (average 9.2 wt.%) and Al2O3 (average 9.2
wt.%) contents are related to the abundance of
amphiboles, augite and plagiocalse. Relatively low
Cr contents are found in peridotite (2250 ppm).
4.2 Rare-earth elements
The chondrite-normalized REE patterns of the
AH rocks are shown in Figure 2. The AH rocks have
depleted LREE patterns (La/Yb)n = 0.5-0.9). The
slight LREE depletion relative to HREE indicate that
the AH rocks represent residue after different
degrees of melt extraction. Small positive Eu
anomaly is characteristic of the various rock units.
The Eu-anomaly indicates the role of plagioclase
fractionation.
Figure 2. Chondrite-normalized REE pattern
of Abu Hamamid rock units
4.3 Platinum group elements
Ten samples were analysed for platinum group
elements and base metals. The Ir and Os contents in
wehrlite and hornblende gabbro are close 1 ppb,
only one hornblende gabbro sample (AH1) contain
3ppb Ru. The Rh contents are close to or below the
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detection limit (I ppb) in all rock units. Low Pd and
Pt contents are recorded in all rock units (less than 9
and 7 ppb, respectively). A narrow range of
variation in Pt and Pd contents characterizes Abu
Hamamid rocks (3 – 7 and 2 - 9 ppb, respectively).
The Pt:Pd ratio in samples in which both elements
were detected varies from 3:1 to 1:1, only one
hornblende clinopyroexnite sample (AH28) contain
lower Pt than Pd (Pt:Pd = 1:2).
The sample in which all PGE were detected is a
Cu-bearing (84 ppm) and Cr-rich (2300 ppm)
peridotite. The total IPGE (Os + Ru + Ir) content in
this sample is 14 ppb while total PPGE is 9 ppb.
The IPGE/PPGE ratio in this sample is >1 which is
not similar to other lithologic units. Chondritenormalized PGE pattern of Abu Hamamid rocks is
illustrated in Figure 3. Fractionated PGE pattern is
shown by wehrlite, hornblende clinopyroxenite and
hornblende gabbro.
Figure 3. Chondrite-normalized patterns of
different lithologic units, Abu Hamamid.
Chondrite (C1) value from Naldrett and Duke
(1980).
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NEODYMIUM ISOTOPIC COMPOSITION
Neodymium isotopic data for three samples of
peridotite, wehrlite and hornblende gabbro are
plotted on Figure 4. Epsilon-Nd at 773 Ma ranges
from +6.9 to + 7.7. These data are similar to ca. 710
Ma island arc rocks hosting the AH intrusion (Shadli
Volcanics, Stern et al., 1991). The depleted mantle
of Nelson and Depaolo (1985) predicts that the
depleted mantle had an Epsilon-Nd of + 7.7 at
770Ma, the data for AH are very close to this value.
The Epsilon Nd for AH falls in the range for
depleted mantle about 770 Ma ago.
Figure 4. Sm – Nd isochron diagram for Abu
Hamamid rocks
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DISCUSSION
Abu Hamamid complex show geological and
petrological characteristics typical of Alaskan-type
complexes as given by Irvine (1974). The only
difference, is that this complex do not host
chromitite mineralization, at least at the present level
of erosion. Also, this complex is different from other
concentrically zoned complexes from the Eastern
Desert of Egypt where some of these complexes
contain sub-economic Cu-Ni-PGE mineralization
(e.g. Gabbro Akarem; Helmy and Mogessie, 2001,
Genina Gharbia; Helmy 2004). Relative to Gabbro
Akarem (Mg# 66 of parent melt) and Genina
Gharbia (Mg# 61), Abu Hamamid is made of more
evolved magma (Mg# 59, Helmy and Farahat, in
prep.) of depleted mantle source. Gabbro Akarem
and Genina Gharbia are older in age (973 Ma, 945
Ma TDM, respectively, Helmy, un-published data).
Although depleted in PGE, AH Hamamid show
a chondrite-normalized PGE pattern similar to
Alaskan-type complexes (e.g. Alto Condoto
Complex, Tistl, 1994) and other concentrically
zoned complexes from the Eastern Desert. It is
widely accepted that PGE are commonly hosted in
platinum group minerals (PGM) or incorporated in
the structure of some sulfides. Chromite-rich rocks
of zoned ultramafic complexes contain the highest
PGE contents (Crocket, 1981). This is supported by
our geochemical data where the highest PGE content
is recorded in the sample with the highest Cr (2300
ppm) contents. The various lithologic units at Abu
Hamamid contain low Cr contents which should be a
primary magmatic feature
Magma generation is probably the stage at which
the low PGE content is established. Either partial
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melting or the nature of source region are possible
fundamental causes of PGE depletion. Low degrees
of partial melting (< 10 %) would produce melts
depleted in PGE. Although the low Cr contents may
support low degrees of partial melting as a cause of
PGE depletion, the very low sulfide content is
problematic. Low degrees of partial melting will
produce a melt rich in sulfides than that of Abu
Hamamid. The rocks could have been formed after
early sulfide segregation process. Sulfide-rich rocks
could be met with at lower stratigraphic level of the
intrusion. The other alternative, the source region
was depleted in PGE, may be supported by the
Sm/Nd isotope data and other trace- and REE
geochemistry. A complex scenario of partial melting
and pre-melting processes in the mantle is likely.
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REFERENCES
Crocket, J.H. 1981, Geochemistry of the platinum
group elements: CIM Special Volume 23, p. 391402.
Helmy, H.M. 2004. Cu-Ni-PGE mineralization in
the Genina Gharbia mafic-ultramafic intrusion,
Eastern Desert, Egypt. Canadian Mineralogist,
42, 351-370.
Helmy, H.M. & El-Mahallawi, M.M. 2003.
Gabbro Akarem mafic-ultramafic complex,
Eastern Desert, Egypt: a Late Precambrian
analogue of Alaskan-type complexes.
Mineralogy Petrology, 77, 85-108
Helmy, H.M. & Mogessie, A. 2001. Gabbro
Akarem, Eastern Desert, Egypt: Cu-Ni-PGE
mineralization in a concentrically zoned
mafic-ultramafic complex. Mineralium
Deposita ,36, 58-71
Irvine, T.N. 1974. Petrology of the Duke Island
ultramafic complex, southeastern Alaska.
Geological Society America Mem, 138, 240p.
Naldrett, A.J. & Duke, J.M. 1980. Platinum metals
in magmatic sulfide ores. Science, 208, 14171424.
Nelson, B.K. & DePaolo, D.J. 1985. Rapid
production of continental crust 1.7-1.9 b.y. ago:
Nd and Sr isotopic evidence from the basement of
the North American mid-continent. Geological
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Stern, R.J., Kröner A. & Rashwan A.A. 1991. A late
Precambrian (~710 Ma) high volcanicity rift in
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Rundschau, 80, 155-170
Tistl, M. 1994. Geochemistry of platinum-group
elements of the zoned ultramafic Alto Condoto
complex, Northwest Colombia. Economic
Geology, 89, 158-167.
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