GEOCHEMICAL AND PETROGRAPHIC STUDIES OF TA MINERALIZATION IN THE NUWEIBI ALBITE GRANITE COMPLEX, EASTERN DESERT, EGYPT

The Nuweibi albite granite is one of 14 known Sn-Ta-Nb bearing granito ids in the Eastern Desert region of Egypt. The granite is a highly leu cocratic, albite-rich rock with accessory columbite-tantalite, cassite rite, microlite and ixiolite as well as topaz,garnet and white mica. A ges of 450-600 Ma were obtained from zircons by the Pb-207/Pb-206 evap oration method. Great uncertainty is caused by the small size and poor quality of the grains, but the precision is sufficient to indicate th at the granite is late- or postorogenic with respect to the Panafrican orogeny. The Nuweibi granite is divided into a western and an eastern part by a regional fault. Both parts of the granite are compositional ly similar but there are important differences and a clear composition al gap between them, so they are considered separate facies of an intr usive complex. The eastern part of the granite is more highly minerali zed, has higher modal albite contents and higher Ta/Nb ratios, both in the whole rock and in the ore minerals. It is suggested that the two parts of the granite evolved from a common source and were emplaced se quentially, the eastern part representing a later, more fractionated m agma. Textural evidence strongly suggests that the granite has a magma tic origin overall, but disturbance of geochemical trends at the whole -rock scale and at the scale of zoning profiles in individual grains o f columbite-tantalite indicate postmagmatic overprinting. By analogy w ith other Ta-bearing albite granites, the sodic bulk composition of th e Nuweibi granite can be explained by fluorine enrichment in the magma . Fluorine contents in the magma were high enough to stabilize topaz, and muscovites contain 2-4 wt.%. F. However, whole-rock F contents are low. We speculate that the low Ca, Al and P contents of the magma pre vented abundant F-bearing minerals to form. and led to loss of fluorin e to now-eroded roof rocks.