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

Abstract

Fluorite of different colours occurs as disseminations and veinlets in the host granitic rocks in ElMissikat 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 600C. Geochemically, El-Missikat granite is peraluminous Atype 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 201C to 296C in Type A (high salinity FI) and 160C to 165C 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/cm in Type A and 0.9 g/cm 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.