GEOLOGICAL AND RADIOACTIVITY STUDIES ON THE DYKES OF WADI EL AKHDAR AREA, SOUTHERN SINAI, EGYPT

The present work deals with the dykes of Wadi El Akhdar area in southern Sinai which is located
between lat. 28˚42′-28˚55′ N and long. 33˚40′-33˚53′ E and covers an area of about 700km2. The area
is characterized by basement rocks unconformably covered by Phanerozoic sedimentary rocks in the
northeastern portion. Field surveys indicated that the studied dykes traverse most rocks in Wadi El Akhdar
area except the quartz-syenite and Phanerozoic sedimentary rocks. The different rock units of the area
under consideration are traversed by various types of post-granitic dykes. Frequently, many of dykes
have curved outcrops, running in remarkable parallelism with each other and vary in inclination from
steep inclined to vertical, 0.5 to 20m in thickness and between 0.1 to 6km in length. Different types of
dykes are distinguished; acidic (granophyers, rhyolites, porphyritic rhyodacites and dacites), intermediate
(andesites) and basic (dolerites and basalts). The relative ages assigned from relationship between dykes
are given as follows: The acidic dykes were intruded first, followed eventually by two periods of a variety
of dykes. Photogeological studies of the dyke lineaments showed that the studied area is affected by the
following trends of dyke lineations; NNE, NE and NW directions. The N35˚W and N25˚E sets are the
most dominant dyke trends, while the less dominant trends are E-W, N15˚W and N56˚W. The trends ENE
and NW of dyke swarms are consistent with the main trends of dyke swarms in the Precambrian rocks
of Egypt. The regional trends of dyke swarms are strongly consistent with the regional trends of fractures
including faults and joints, i.e. dykes are structurally controlled. Radiometric measurements of dykes
indicated that, the acidic dykes are relatively more enriched in eU (av. =22.3 ppm) than other types of
dykes (av.=3.30 ppm) in intermediate dykes and (av.=2.10 ppm) in basic dykes. On the other hand, the
acidic dykes contain most eTh contents. Their average content of eTh is 25.70 ppm whereas it is 8.94 ppm
in intermediate dykes, and 4.10 ppm in basic ones. The eU contents in acidic dykes may be due to their
accessory minerals (i.e. apatite, zircon and sphene).