الفهرس 
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Abstract
The Precambrian rocks of Egypt (l00.000 Km2
) outcrop over
an extensive area in the Eastern Desert, south western part of the
Western Desert and south Sinai. The Precambrian belt represents the
NE part of the Arabian Nubian shield which is exposed in both sides
of the Red Sea, and belonging to the Pan-African organic belt.
The Egyptian Precambrian belt of the Eastern Desert IS
composed of an upper-Proterozoic volcano- sedimentary complex
containing scattered ultramafic bodies and intruded by syn-to late -
tectonic plutonites (mainly tonalite to granodiorite) with extensive
senes of calc-alkaline volcanic rocks. This sequence IS
unconformably overlain by molasse sediments of the· Hammamat
group. The sequences are cut by alkaline to sub-alkaline, late - to
post tectonic granites.
In the central and southern Eastern Desert , the deformed and
largely mylonitized rocks of the infrastructure crop out in gneiss
domes disposed along the axes of genetic lines trending NNW-SSE
which also functioned as magnetic area (EI Gaby, 1983 ) .
Remobilize infrastructure rocks are well displayed at Wadi
Feiran, SW Sinai ( EI Gaby & Ahmed 1980), along the Qena Safaga
road (Sabet et. a!., 1972 , Akaad et a!., 1973).
Wadi Beida area lies in the southern part of the Eastern Desert
about 30Km west of Shalatein town between latitudes 22° 54’ _23°
04’ 30” N and longitudes 35° 10’ - 35° 22’ 30” E and covers an area
Cfiapter 0/1
The present thesis involves detailed field, geological,
geochemical, mineralogical and structural studies for the rock units
at Wadi Beida area. These rock units comprise dismembered
ophiolities, arc volcanic assemblage, Arc-grantitoid and younger
gabbros.
The ophiolites occupy and extend in NW-SE direction in the
northeastern part of the mapped area. They embrace ultramafic
rocks, metagabbros, pillow lavas and cherts. These rocks are set, in
places, inside a schistose matrix of highly sheared ultramafic rocks,
foliated metasiltstone and metamudstone. Elsewhere, this matrix
encloses small bands of quartz carbonates. All these components
show tectonic contact in-between and alltogether were tectonically
transported and intermingled with the metavolcanics. At Bir Beida,
the contact of ophiolites against the metavolcanics is characterized
by strong shearing in NW-SE direction. Veinlets of talc, chromite
lenses and magnesite bands are aligned along this contact.
,
The Ultramafic rocks consist of intensively sepentinized
hurzbergite, lherzolite and subordinate dunite. They are represented
by main outcrop on the right side at the entrance of Wadi Beida and
by unmappable slices east and southeast of G. Beida. These rocks
are brown, dark grey to black and have a general NW -SE elongation.
In localities, the ultramafics contain veinlets and pockets (5-11 em
thick) of chrysotile indicating the tectonic effect. Petrographically,
the ultramafic rocks are represented by serpentinites (with relics of
hurzbergite, lherzolite and dunite). The serpentinites are composed
of antigorite, low contents of chrysotile, lizardite and ankerite
together with opaques (about 15-20% of the whole rock). The
opaques are mainly iron oxides and chromite with traces of sulfides.
The iron oxides are represented by magnetite that is densely altered
Cfiapter’VI Summary and Condusions
into goethite. The chromite occurs as disseminated crystals and is
partly altered into magnetite.
The metagabbros are exposed in the east of the mapped area
and cover an area of about 60 km2
• They form low to moderate relief
and have, along their eastern and western sides, tectonic contacts
against the ultramafics and their highly sheared derivatives
respectively. In places, these rocks exhibit layering , and rosette
feature. The layering is mostly disturbed and is not continued for
long distances. The layers are generally striking NW-SE and dipping
at moderate and steep angles towards plagiogranites. The latter
forms white bodies in the rocks. The metagabbros are coarse-grained
but they are fine-grained in the uppermost parts.
Petrographically, the metagabbros are mainly composed of
plagioclase, actinolitic hornblende and pyroxenes. Opaques and
sphene are the main accessory minerals. Biotite, chlorite, epidote
and calcite are the secondary minerals. Quartz is found in the
strongly sheared varieties and is mainly of secondary origin.
Plagioclase (An 36-44) reaches to 60% of the rock. It shows
lamellar and composite twinning and zoning. In samples, the core of
plagioclae crystals are charged with epidote while the peripheries are
clear indicating high and low calcic compositions respectively.
Occasionally, these crystals are obscured by dense kaolinitization
and sericitization. Actinolitic hornblende is partly or completely
chloritized and still shows relics of pyroxenes. These relics include
augite, enstatite and sometimes diopside; being altered to chlorite
and epidote.
Opaques (2-10%) are dominated by ilmenite (70 % of the
opaques), magnetite (30 % of the opaques) and traces of pyrite.
Ilmenite is occasionally homogeneous and shows two types of
alterations; high temperature alteration to sphene and/or hematiterutile
graphic intergrowths and low temperature alteration to
goethite. Magnetite is Ti-poor variety. Hematite-ilmenite exsolution
intergowth (metamorphic origin; Takla et al., 1981) is widespread in
most samples. Magnetite external granules in ilmenite are rarely
encountered.
The pillow lavas are encountered near Bir Beida and trending
NNW-SSE .The pillows are generally dark green and exhibit finegrained
basaltic composition with amygdales filled with epidote,
carbonates and iron oxides. The individual pillows are circular or
oval-shaped with massive cores and a zone of vesicles near the
periphery. The pillows range in size from 2S cm to 1 m and show
general dipping toward the NE.
Cherts occur as dark grey to grayish, yellowish and greenish
gray bands. They are tectonically interleaved with the ultramafic
rocks and pillowed basalt. The thickness of the chert bands vary
from S to 30 ern. Petraographically, the pillow lavas are composed of
plagioclase laths, tremolite, epidote and clorite together with few
amounts of sphene, quartz, calcite and opaques. The opaques
constitute about S% of the rock and are dominated by magnetite
which is partly martitized. They are characterized by schistose and
amygdaloidal textures. Some amygdales are filled with calcite,
epidote and chlorite.