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

The studied mineralized lamprophyre dyke in Abu Rusheid area is trending NNW-SSE, and occurs withinAbu Rusheid mineralized shear zone, measuring 0.2 - 1.0 m in width and 0.5 - 1.0 km in length. It was emplacedparallel with the Abu Rusheid shear zone. The dyke is mainly composed of plagioclases, amphiboles, mica (musco-vite and biotite), relics of pyroxenes with K-feldspars and quartz derived from surrounding country rocks asphenocrysts 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 ironoxides. 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 solutionsand re-precipitatedas REE-minerals xenotime-(Y) and monazite-(Nd) around flourapatite, fluorite, zircon andrutile. The solid solutions between monazite-(Nd) and xenotime-(Y) were formed as a product precipitation fromhydrothermal solutions. Also, the apatite mineral in the lamprophyre dyke was subjected to the heating duringthe 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. Themonazite-(Nd) associated with fluorapatite in the studied dyke is poor in Th (0.02 ≤ Th ≤ 0.81 wt%), but usuallyrich in U (0.92 ≤ U ≤ 2.91 wt%), which indicates that monazite formed as a result of flourapatite metasomatism.