Immiscibilty between silicate magma and aqueous fluids in Egyptian rare-metal granites: melt and fluid inclusions study

Abstract

Rare-metal granites of Nuweibi and Abu Dabbab, central Eastern Desert of Egypt, have mineralogical and geo- chemical specialization. These granites are acidic, slightly peraluminous to metaaluminous, Li-F-Na-rich, and Sn-Nb- Ta-mineralized. Snowball textures, homogenous distribution of rock-forming accessory minerals, disseminated mineraliza- tion, and melt inclusions in quartz phenocrysts are typical features indicative of their petrographic specialization. Geochemical characterizations are consistent with low-P-rare metal granite derived from highly evolved I-type magma in the late stage of crystallization. Melt and fluid inclusions were studied in granites, mineralized veins, and greisen. The study revealed that at least two stages of liquid immiscibility played an important role in the evolution of magma-hydrothermal transition as well as mineral deposition. The early stage is melt/fluid case. This stage is represented by the coexistence of type-B melt and aqueous-CO2 inclusions in association with topaz, columbite-tantalite, as well as cassiterite mineral inclu- sions. This stage seems to have taken place at the late mag- matic stage at temperatures between 450 °C and 550 °C. The late magmatic to early hydrothermal stage is represented by vapor-rich H2 Oa nd CO2 inclusions, sometimes with small crystallized silicic melt in greisen and the outer margins of the mineralized veins. These inclusions are associated with beryl, topaz, and cassiterite mineralization and probably trapped at 400 °C. The last stage of immiscibility is fluid-fluid and represented by the coexisting H2O-rich and CO2-rich inclu- sions. Cassiterite, wolframite ± chalcopyrite, and fluorite are the main mineral assemblage in this stage. The trapping tem- perature was estimatedbetween 200 °C and 350 °C. The latest phase of fluid is low-saline, low-temperature (100-180 °C), and liquid-rich aqueous fluid.