Rare-metal pegmatiles from the southeastern desert, egypt: Geology, geochemistry, and petrogenesis

The pegmatite province of the Southeastern Desert (SED) is parl ofa district that extends from eastern Egypt to the Red Sea (and covers an area of approximatly 120 km'). Rare-iretal pegmatites within the SED are divided into three major types: (1) unzoned, Sn-mincralized bodies: (2) zoned Li-, Nb-, Ta-, and Be-bearing bodies; and (3) peginatites containing colored, gem-quality tourmaline. Rb-Sr isotope data reflect a crustal origin for the rare-metal peginatites and indicate that the original SED magma was generated during the regional metainorphic peak predating intrusion of posttectonic leucogranites. These bodies developed an early border zone consisting of coarse to very coarse-museovite + qUartZ + alkali feldspar, followed by an intermediate zone of dominantly quartz + feldspar + muscovite rock. Garnet, tourmaline, beryl, galena, pyrite, aroblygonite, apatite, and monazite are rare accessories in both zoned and unzoned peginatites. Cassiterite occurs in replacement zones and along fractures in albile + quartz + muscovite-rich portions. The highest concentrations of coarse-grained cassiLerite occur in irregular greisenized zones that consist dominantly of micaceous aggregates of green Li-rieli muscovite, quartz, and albi[e. Different metasoniatic post-solidification alterations include sodie and postassic ineLasoniatislo, greisenization and tourmalinization. The pegmatite-generating granites are metalummous, showing a differentiation trend from coarsegrained, unfractionated, intermediate plagioclase-rich granite toward a highly fractionated, fine- to medium-grained, locally albite-rich rock. Economically important ore minerals were introduced by rare inetal-bearing fluids-either primarily or during the breakdown of the primary mineral assemblages. These include niobimntantalum oxides, Sn-oxides (eassiterite), Li-sihr_ ates (petalite, spodumene, euctyptite, and pollucite), Li-phospha(es (amblygonile, niontebrasite, and lithopilite), and minor REE minerals (Hfzircon, monazite, xenotime, thorian, loparite, and ytti-io-fluorite). Pollucite is typically associated with spodumene, petalite, amblygointe, quartz, and feldspar. Primary pollucite'llas Si Al ratios of 2.53 to 2.65 and CRK (Cs + Rb + lQCs + Rb + K + Na + Ca + Li) of 79.5 to 82.2. Thorian loparite is essentially a member of' the loparite (NaLREETi,0,)-lueshite (NaNb0,)-ThTi,0,-ThNb,,0l2 quaternary system with low or negligible contents of otber end-members. The mineral compositionally evolves from inobian loparite to mobian thorian and thorian; loparite gave rise to ceriobetafite and belyankiniLe with high ThO, content. Thorian loparite is metamict or partly metainict, and upon heating regains a structure close to that of'synthetie loparite, NaLaTiA.