MINERALOGICAL AND GEOCHEMICAL CHARACTERISTICS OF EL-MISSIKAT FLUORITE MINERALIZATION, CENTRAL EASTERN DESERT, EGYPT: TRACE ELEMENTS AND FLUID INCLUSIONS CONSTRAINTS

Fluorite of different colours occurs as disseminations and veinlets in the host granitic rocks in El-
Missikat area, central Eastern Desert, Egypt. This paper addresses the mineralogical and geochemical
characteristics, including mineral chemistry, whole rock geochemical analyses of the granitic host rock.
Trace, REY (REE+Y) elements, fluid inclusions and LA-ICP-MS analyses for individual fluid inclusions
in fluorite were also carried out. Petrographically, the host granite represented by biotite granite. Mineral
chemistry data of primary phases are given. Plagioclase is albite (average 2.79 mol%). Biotite is of
primary magmatic origin and crystallized at 500 - 600oC. Geochemically, El-Missikat granite is
peraluminous A- type and was generated in post-collision environment (within plate).
The average ΣREE content of Type V (violet), G (green) and Type W (white) fluorite samples are
75.5, 200.9 and 203.2 ppm, respectively (i.e. increasing from violet fluorite to green and white fluorites.
These differences in ΣREE were possibly related to changes in pH condition and bulk chemical
composition of the fluids. Tb/Ca, Tb/La and Y/Ho ratios of the fluorite types indicate that they were
formed from the interaction between magmatic fluids and granitic wall-rocks. The fluorites show strongly
negative Eu and positive Y anomalies similar to REY pattern of the host granites, indicating that the
source of REE and trace elements of hydrothermal fluids is the host granite leached by fluids. The present
work revealed that the contents of Y and total REE contents are responsible for the appearance of green
and white colours. The Sr content is high in violet fluorite and may be responsible for violet colour.
The fluid inclusions (FI) of the fluorite have homogenisation temperatures ranging from 201oC to
296oC in Type A (high salinity FI) and 160oC to 165oC in Type B (low salinity FI). The melting
temperature of ice in fluorite indicates salinities up to 19.4 equiv. wt% Na CI (Type A) but Type B FI
have a low range of salinity (0.53 to 4.49 equiv. wt% Na Cl). The density of fluids is 0.9 to 1.0 g/cm3 in
Type A and 0.9 g/cm3 in Type B. From the LA-ICP-MS data for the individual FI of El-Missikat fluorite,
it is clear that the Type A FI contains very high concentrations of elements than that in Type B FI. The
abundance of the analyzed elements in the present FI is: Ca > Na > S > K > Sr > Y > Fe, Pb > Cu > U >
Cs, W, Te, Ag, As > Th > Au. The deposition mechanisms for fluorite may be fluid mixing, changes in
pressure and temperature and fluid-rock interaction.