Sequential deuteric and hydrothermal alterations in the Late Neoproterozoic Um Naggat rare metal-bearing granite, Central Eastern Desert, Egypt

The Late Neoproterozoic rare metal-bearing granite of Um Naggat pluton records deuteric and magmatichydrothermal fluid activities that culminated in overlapping sequential metasomatic alterations. The pluton is petrographically discriminated into subsolvus biotite, hypersolvus alkali-feldspar, and subsolvus albite granites. The residual fluid interacted and re-equilibrated with the mica and feldspars. Two phases of albitization, greisenization, chloritization and hematitization, and one phase of silicification, as well as microclinization are recognized. Compared to the alkali-feldspar and albite granites, the microclinization is restricted to the biotite granite, whereas greisenization and albitization increase in the former two granites and reaches their climax in the albite granite. Repeated contrasting acid-and alkaline-based alterations emphasize the fluctuation of the acidic-basic behavior of the fluid as a consequence of dominantly hydrogen-and alkali-exchange reactions.