Extreme fractionation and magmatic–hydrothermal transition in the formation of the Abu Dabbab rare-metal granite, Eastern Desert, Egypt

Abstract

The Abu Dabbab rare-metal granite in the Central Eastern Desert of Egypt is a peraluminous alkali-feldspar leucogranite stock with minimummelt composition of albite-rich haplogranite and is characterized by economic resources of columbite group minerals (CGM) and cassiterite. The bulk-rock trace element composition is characterized by very low Sr, Ba, REE+Y, Zr, Ti, Ni, Cr concentrations, elevated Li and F, and very high Sn, Ta and Ga. The non-chondritic, low Y/Ho ratios and extremely low Nb/Ta and Zr/Hf ratios of the Abu Dabbab granite are consistent with highly evolved peraluminous granites. The chondrite-normalized REE patterns with significant tetrad effects (TE1,3 ~ 1.6–2.6) and strongly negative Eu anomalies are indicative of the magmatic to hydrothermal transition. Heterogeneous 147Sm/144Nd (0.20411–0.51923) and 143Nd/144Nd (0.512968–0.514056) values of the bulk-rock reflect a disturbed Sm-Nd isotope system. Nevertheless, the initial εNd values calculated for a wide range of Neoproterozoic ages (500–700 Ma) are consistently positive, suggesting a juvenile source.Laser ablation ICP-MS analyses of the CGM, cassiterite, wolframite and Mn-ilmenite show elevated rare metal concentrations and complex REE tetrad patterns, reflecting fluid-melt interactions. The metasomatic fluids associated with the Abu Dabbab rare-metal granite were primarily low-salinity (<6 wt.% NaCl eq.), high-density (~0.8–0.9 g/cm3), CO2-rich aqueous fluids with up to ~50 mol% CO2 (+CH4). The minimum entrapment conditions of carbonic-aqueous inclusions are calculated as 2–3 kbar and 350–360°C, whereas aqueous inclusions in the granite and crosscutting quartz veins most likely correspond to lower-pressure conditions and fluid phase separation by decompression. The extreme level of fractionation and complex tetrad effects of the bulk-rock and ore minerals coupled with criteria indicating the magmatic-hydrothermal transition are in accord with regional seismic tomography and heat flow data. A much larger, unexposed parental granite system, from which the exposed Abu Dabbab rare-metal tip has fractionated, must be present at depth. The interaction between extreme fractionation and fluid-melt interaction at shallow crustal levels gave rise to economic-grade Ta-Nb-Sn±W mineralization in the granite body and associated quartz veins.