Mueilha rare metals granite, Eastern Desert of Egypt: An example magmatic-hydrothermal system in the Arabian-Nubian Shield

5 years ago

Mueilha rare metals granite, Eastern Desert of Egypt: An example magmatic-hydrothermal system in the Arabian-Nubian Shield

The Mueilha granite pluton is one of the rare-metals bearing peraluminous granitic plutons in the Arabian-Nubian Shield. It represents the apical part of a highly evolved magma chamber emplaced at a shallow level subsequent to the post Pan-African orogeny. The pluton can be seen as a highly leucocratic medium-grained albite/oligoclase framework infilled with quartz, K-feldspar and muscovite that are variably overgrown by K-feldspar, muscovite, quartz and topaz megacrysts. The increasing number and size of the K-feldspar megacrysts at the expense of the whitened albite framework imparts variably red colour to the Mueilha granite. Contacts between the un-altered milky white and altered red granites are usually gradational, but may be locally sharp or may form narrow transition zones resulting from abrupt variations in texture and lithology. Textural relations also indicate an initial stage of hydrothermal albitization of magmatic plagioclase and crystallization of topaz megacrysts resulting from infiltration of Na- rich fluorine bearing fluids. A subsequent stage of metasomatic enrichment is characterized by extensive growth of large K-feldspar, quartz and muscovite megacrysts at the expense of the albite/oligoclase crystals as a result of infiltration by K-Si rich hydrous fluids. Post-magmatic infiltration of hydrous fluids along the fault planes is shown by the intense replacement of alkali feldspar megacrysts by quartz, development of myrmekitic intergrowth pockets along the K-feldspar megacrysts and sealing of the micro-fractures by cryptocrystalline mixtures of clay minerals, iron oxides, sericite and chlorite. Compositionally, the altered red granitic rocks are have SiO2, Fe2O3total, K2O/Na2O, Σ REE, Y, Th, U, Zr and Zn and lower Al2O3, Ga, Ta, Nb and Mo compared to the un-altered milky white granites. LILE and Sn do not show clear variation trends throughout the Mueilha granites pluton, suggesting their immobility during hydrothermal alteration. Microthermometric measurements indicate that the interactions with the hydrothermal fluids started at minimum temperature >400°C, most likely during the late-stage crystallization of the Mueilha granite and continued after their complete solidification (i.e. subsolidus conditions) at temperatures as low as 120°C. The high fertility of Mueilha granite is most plausibly the result of partial melting within the undepleted juvenile crust of the Arabian–Nubian Shield that has formed during the Pan-African orogeny.

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DOI: S0024493717303389