MINERALOGICAL AND FLUID INCLUSIONS EVIDENCE FOR THE GENESIS OF UMM ADDEBBAA-UMM KABU BERYL BELT, SOUTH EASTERN DESERT, EGYPT

Beryl mineralization in quartz veins and pegmatites, are common deposits of tectonic–hydrothermal
and/or igneous origin. The beryl-specialized granites association at Umm Addebaa–Umm Kabu belt is
manifested in the field by the development of a system of beryl-bearing pegmatitic pods and quartz veins.
The emplacement of these syn-tectonic pegmatitic leucogranites from which K- and Be-rich fluid phases
were derived, are confined to the shear zones, as well as a broad zone of alkali metasomatism.
Microthermometic studies of primary fluid inclusions within beryl growth zones are consistent with beryl
precipitation from H2O-CO2 ± CH4 bearing saline brines. The estimated fluid composition is approximately
0.88 mol% H2O, 0.017 mol% CO2 ± 0.001 mol% CH4 and 0.10 mol% NaCl (2- 11 wt.% NaCl eq.). Fluid
inclusion results are consistent with that mineralization in pegmatites and quartz veins that are formed
by two genetic stages. The first stage is characterized by temperature of formation in the range of 216.4
– 378.3 °C, with corresponding pressures along fluid inclusion isochore paths ranging from 1.04 to 2.25
bar. The second stage is of aqueous fluid represented with low temperature (177-255°C) and pressure # 1
bar, but high saline (16-22 wt.% NaCl eq.) which might explain mixing of the early carbonaceous fluid
with late meteoric water accompanied with pressure release. Thus, it can be inferred that the Be-bearing
solutions were moderately saline, but CO2 (and possible CH4)-rich fluid implies that Be was most probably
complexed by carbonate (+CH4) - chloride base.
The different paragenetic types of emerald and beryl associated with granitoid rocks indicates that
the chemistry of the Be-bearing fluids (rather than that of the bulk rock), and syn-tectonic intrusions of
leucogranites and pegmatites (Be deriving sources) along major ductile shear zones are the important
factors controlling the crystallization of beryl.