Mineral chemistry of monazite-(Nd), xenotime-(Y), apatite, fluorite and zircon hosting in lamprophyre dyke in Abu Rusheid area, South Eastern Desert, Egypt

Mohamed A Ali

doi:10.5474/geologija.2012.007

Abstract

The studied mineralized lamprophyre dyke in Abu Rusheid area is trending NNW-SSE, and occurs within Abu Rusheid mineralized shear zone, measuring 0.2 - 1.0 m in width and 0.5 - 1.0 km in length. It was emplaced parallel with the Abu Rusheid shear zone. The dyke is mainly composed of plagioclases, amphiboles, mica (muscovite and biotite), relics of pyroxenes with K-feldspars and quartz derived from surrounding country rocks as phenocrysts embedded in fine-grained groundmass. The lamprophyre dyke hosts REE-minerals monazite-(Nd), xenotime-(Y), and REE-bearing minerals apatite, fluorite, zircon-(Hf), rutile with inclusions of xenotime and iron oxides. The emplacement of lamprophyre dyke caused heating in the mineralized shear zone of Abu Rusheid area. The lamprophyre dyke was subsequently affected by hydrothermal alterations (e.g. chlorite-carbonate, muscovitization, fluoritization). The REE were remobilized from the mineralized shear zones by hydrothermal solutions and re-precipitated as REE-minerals xenotime-(Y) and monazite-(Nd) around flourapatite, fluorite, zircon and rutile. The solid solutions between monazite-(Nd) and xenotime-(Y) were formed as a product precipitation from hydrothermal solutions. Also, the apatite mineral in the lamprophyre dyke was subjected to the heating during the emplacement, which lead to its alteration and breakdown with concominant precipitation of xenotime-(Y) and monazite-(Nd). The chemistry of monazite-(Nd) and xenotime-(Y) obtained by scanning electron microscopy (SEM), and electron probe microanalysis (EPMA), showed that these minerals are enriched in U and Th. The monazite-(Nd) associated with fluorapatite in the studied dyke is poor in Th (0.02 ≤ Th ≤ 0.81 wt%), but usually rich in U (0.92 ≤ U ≤ 2.91 wt%), which indicates that monazite formed as a result of flourapatite metasomatism.

Highlights

The Abu Rusheid area is one of the most important areas in the South Eastern Desert of Egypt, especially in rare-metal mineralization and is included in wadi Sikait and wadi El Gemal area
The present study shows the detailed mineral chemistry of REEminerals (monazite-(Nd), xenoti me-(Y)) and REE-bearing minerals within mineralized lamprophyre dyke, and discusses the genesis of these minerals
Four samples of the altered lamprophyre dyke were studied in detail using optical microscopy, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA)

Summary

Introduction

The Abu Rusheid area is one of the most important areas in the South Eastern Desert of Egypt, especially in rare-metal mineralization and is included in wadi Sikait and wadi El Gemal area. It lies about 97 km southwest of Marsa Alam town and is accessible from the Red Sea through wadi El-Gemal desert track (Fig. 1). The Abu Rusheid area lies to the NE of the major shear zone known as the Nugrus thrust fault (Greiling et al, 1988) or Nugrus strike-slip fault (Fritz et al, 2002). Shear zones are known to form important mechanical weaknesses that affect the geology of the continental lithosphere as a kinematic response to deformation (Butler et al, 1995). Greiling et al (1993) believe that shear zones in the Pan African basement of the Eastern Desert may be related to compressional, as well as extensio nal stresses; both types of deformation led to antiformal structures on a regional scale. Abdel-Monem & Hurely (1979) estimated the age date of psammitic gneisses by zircon detrital age, which ranges between 1120 Ma and possibly 2060 Ma suggesting that this basement may be the so called Nile Craton

Methods

Results

Discussion

Conclusion