Overprinting of primary magmatic features by fluid-mediated processes in a F-bearing felsic rare-metal system: the Mueilha granite (Eastern Desert, Egypt)

• Granite evolution as a result of magmatic-hydrothermal and subsolidus processes. • The magmatic evolution in granitic systems is overprinted by fluid-mediated processes. • Fluids and F play a crucial role for rare metal redistribution. • F facilitates autometasomatism. • F promotes the formation of disseminated style Nb-Ta-REY-mineralization. Fluids are major fractionation agents in granitic systems because they partly control the behaviour and partitioning of elements, including rare metal, during the magmatic-hydrothermal transition and their subsequent redistribution during the later subsolidus stage. The exsolution of magmatic fluids from a volatile-saturated magma and their subsequent circulation commonly result in important textural and geochemical changes with primary magmatic features being entirely overprinted and earlier minerals chemically re-equilibrated. The changes documented herein serve as a basis for tracking the equilibration of a rare-metal granite with interacting fluids. The Mueilha F-Nb-Ta-REE-Y granitic system (Eastern Desert of Egypt) is composed of different facies such as the “red granite”, representing the main volume of the intrusion, and the “border facies”, occurring along the red granite south-western margin. Detailed mineralogical and textural studies of the latter facies document the pervasive and intense transformation of the original highly-evolved granite primarily via coupled dissolution-precipitation. Based on these observations, and complementary whole rock, and mineral major and trace element chemistry, a complex five-stage model of evolution from magmatic to metasomatic is proposed and discussed. As part of this evolution, the border facies represents residual melts extracted at a very late stage of crystallization of the red granite. This process led to a selective enrichment in Nb and Ta, and a corresponding depletion in Li, F, REEs, and Y within the border facies, highlighting the critical role of extraction timing in shaping the composition of residual melts. It is suggested that the weak peraluminous character of the intrusion combined with its F content, albeit low (<0.2 wt%), facilitated the observed autometasomatism and associated redistribution of rare metals and REE-Y within the intrusion to form magmatic and hydrothermal disseminated style mineralization that is typically encountered in rare-metal granite and pegmatite worldwide. We suggest that this study may serve as a useful reference point for investigating comparable settings elsewhere.