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Mueilha rare-metal albite granite south Eastern Desert of Egypt: an example for successive fluid-rock reactions

Mohamed . A. Abu El-Rus 1  Mohamed A. Mohamed 1 A ndress Lindh 2,3

1Department of geology, Assiut University, Assiut, 71516, Egypt.

 2Department of Geology, Lund University, Sölvegatan 12, S-223 62 Lund, Sweden 3Present address: 14, Rue de la Garde, F-17560 Bourcefranc-le-Chapus, France.


Mueilha granite pluton occurs as an oval-shaped, small mass covering ~7.5Km2. The pluton is apparently heterogeneous and easily distinguished into the main mass characterized by milky white porphyritic quartz  syenites, surged along the vertical to subvertical  joints by more fractionated coarse-grained red alkali granites. The most extensive and best exposed body of red alkali granites occurs along the joints crossing-cuts the west and northern part of the pluton. The contact between red and milky white granites is usually gradual, locally it is distinctly sharp, or show a quick transition zones characterized by a considerable heterogeneity, as a result of the abrupt textural and lithological changes. These transition zones represent the replacement front and may reveal in short-interval how change of the original milky white granite to red alkali granites took place.  The textural relations, however,  revealed  that the transformation of the milky white granites to red granites  was due to pervasive K-Si rich-fluids that were working during the late stage of mamatic differentiation of the  Mueilha pluton  and continued after its consolidation.  The early alteration stage is distinguished by development of the sporadic subhedral to euhedral megacrysts of k-feldspars and quartz which usually encompass oriented fine albite inclusion arranged in a concentric form.  This stage of alteration is also characterized by grown of muscovite at the expense of the original biotite. The concentric zonation of crystal inclusions within megacrysts which mostly express straight outlines is a strict evidence of  growth these megacrysts in the presence melts. With increasing alteration, the number of K-feldspar and quartz megacrysts is increasing and the rocks becomes red alkali granite rocks.  Alkali granite rocks along some fault planes show cataclasis textures with replacement of k-feldspars megacrysts by quartz as well as filling the fractures and dislocations by cryptocrystalline intergrowths formed essentially of muscovite, iron oxides, quartz, carbonate and chlorite. Crystallization such intergrowths suggests a final low temperature alteration stage due to the activity of  Si-rich fluids after consolidation of the Mueilha granite pluton.  Chemical data show losses of Ca, Na, Fe, Mg and Ti and gains Si, K, HFES and HREE during metasomatism stages prior to the consolidation of the Mueilha granite pluton and  an increase in Si,  but no significant additions of alkalis, HREE during the metasomatism stages after  consolidation of the Mueilha granite pluton.