WADI LADID AL-JI,DAN ALKALI FELDSPAR GRANITES AND ASSOCIATED PEGMATITES, NORTH EASTERN DESERT, EGYPT: GEOLOGY, MINERALOGY AND RADIOACTIVITY

This article deals with the geology, structure, mineralogy and radioactivity of the alkali feldspar granites
at wadi Ladid al-Ji,dan, north Eastern Desert of Egypt. Geologically, the area consists mainly of alkali
feldspar granites. These granites are medium to coarse grained, pink to red in colour and form numbers
of moderate to relatively high elevated outcrops. These rocks are altered in some parts due to secondary
processes especially along the fault zones and contacts.
The area displays primary and secondary structures. Primary structures comprise layering, volcanic
flows, vesicular tops of lavas and volcanic bombs. The secondary structures are represented by exfoliation,
joints and faults. The granites are dissected by several sets of joints predominating in the N-S direction,
dipping 840-880 mainly to west; E-W direction, dipping 70o-75o mainly to south and NW-SE direction,
dipping 80o-82o to SW direction. The majority of joints are tension ones. These tension joints are commonly
filled with quartz, feldspars, epidote, iron oxides, manganese oxides and aplite. The studied area is dissected
by major regional faults. Along the fault zones; the granites are marked by fault breccia and alteration
features, especially silicification, hematitization, kaolinitization and chloritization. The area is cutting by
faults trending mainly in N-S, NE-SW and NNW-SSE directions.
The average uranium and thorium contents of the alkali feldspar granites are 12 and 23 ppm and reaches
up to 36 and 37 ppm in the normal pegmatites. In the anomalous pegmatites the maximum values recorded
are 91 and 72 ppm. Generally, the averages of uranium and thorium contents in the studied pegmatites
are higher than that of the world and Egyptian uraniferous pegmatites. The high uranium content in alkali
feldspar granites is related to the presence of some accessory minerals such as zircon, fluorite and apatite
but the high uranium content in the pegmatites is related to the presence of columbite, pyrochlore, zircon,
fluorite as well as iron oxy hydroxides.