LATE PRECAMBRIAN ACIDIC VOLCANICS SOUTH SINAI, EGYPT: IMPLICATIONS FOR GEOLOGY AND RADIOACTIVITY

Three extrusions of Late Precambrian acidic volcanic, south Sinai, Egypt are selected for studying
their geology and radioactivity. Field observations revealed that, these volcanic rocks are believed to
have erupted later than the Dokhan volcanics. These volcanic rocks are represented mainly by lava
flows of rhyoltic composition, lithic and crystal tuffs. They have many geochemical characteristics of
metaluminous to peraluminous A-type magma that were emplaced into within plate tectonic environment
and evolved through differentiation processes from mantle-derived basaltic magma with noticeable
crustal contribution. The fractionation of plagioclase in the early formed basaltic rocks leads to the
pronounced negative Eu anomaly displaced by these volcanics. These volcanics are considered as source
for uranium since they contain more than 8 ppm average uranium content. The average thorium and
uranium contents are 17ppm and 15 ppm for Ras Naqab volcanics, 16.7 ppm and 8.5 ppm for Iqna Sharay,a
while Umm Shouki volcanics have 17 ppm average Th and 8.6 ppm average U. Three modes of uranium
occurrence are inferred. The first assumes incorporation of U and Th within accessory minerals such as
zircon, monazite and uranothorite where thorium is three times more abundant than uranium. In the second
case, uranium is equal to one half of the thorium or slightly more which indicates that a process of uranium
enrichment (U-gain) may be occurred. In this case, uranium is adsorbed on ferroxides, altered minerals,
or along grain boundaries. When uranium is less than one quarter of thorium or completely disappear, a
process of uranium leaching (U-loss) may take place.
The U & Th-bearing rare earth mineral tornebohmite is classified as tornebohmite-Ce where its Ce
content is about 39.40 wt %. Tornebohmite may be formed by hydrothermal fluids penetrating the
studied volcanics. These fluids may be accompanied and contemporaneous with the syenogranite intrusion
which occurs at the southwestern part of Ras Naqab area.