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 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 (musco vite 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, flu o ritization). The REE were remobilized from the mineralized shear zones by hydrothermal solutions and re-preci pitated 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.


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 area is located in the huge fault systems of Sikait-Nugrus environ. The age of the leucogranites in the Sikaite-Nugrus (Abu Rusheid granites) area is 610 ± 20 Ma determined by Rb-Sr me thod (Moghazi et al., 2004). 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). This shear zone separates high-temperature metamorphic rocks of the Hafafit complex in the SW from mainly low grade ophiolitic and arc volcanic assemblages to the NE (Bennett & Mosley, 1987). 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 extensional stresses; however, 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.
hassan (1964) studied geology and petrography of the radioactive minerals and rocks in wadi Sikait and wadi El Gemal area. Also, hassan (1973) and hilMy et al. (1990) studied geo logy, geochemistry and mineralization of radioactive columbite-bearing psammitic gneisses of wadi Abu Rusheid. el-geMMizi (1984), saleh (1997 and iBrahiM et al. (2004) studied the Abu Rusheid area and recorded several types of mi neralization, such as Ta-Nb minerals, zircon, thorite, and secondary uranium minerals. raslan (2005) identified columbite, Hf-rich zircon and dark Li-mica (zinnwaldite) in Abu Rusheid mi neralized gneiss. The latter author has further been able to identify mineral ishikawaite in Abu Rusheid minera-lized gneiss (raslan, 2008). dawood (2010) studied the mineral chemistry and genesis of uranyl minerals associated with psammitic gneisses of Abu Rusheid area, and conclu ded that the composition and genesis of uranyl mineralization provide additional information about the behavior of radionuclides in arid environments and at very oxidi zing conditions. ali et al. (2011) studied the mineralogy and geochemistry of Nb-, Ta-, Sn-, U-, Th-, and Zr-bearing granitic rocks in Abu Rusheid shear zones, and proved, on the basis of field evidence, textural relations, and compositions of the ore minerals, that the main mineralizing event was magmatic (629 ± 5 Ma, CHIME monazite), with later hydrothermal alteration and local remobilization of highfield-strength elements.
iBrahiM et al. (2007a) recorded the occurrence of REE up to (1.5 %) in the studied lamprophyre dykes. Also, iBrahiM et al. (2007b) studied the base-metal mineralization hosting uranium and geochemistry of the lamprophyres within the shear zones of Abu Rusheid area.
The present study shows the detailed mine ral chemistry of REEminerals (monazite-(Nd), xenotime-(Y)) and REE-bearing minerals (apatite, fluorite, and zircon) within mineralized lamprophyre dyke, and also discusses the genesis of these minerals.

Geological setting
The basement rocks sequence of the Precambrian rocks at Abu Rusheid area are arranged as follows: (1) ophiolitic mélange, consisting of ultramafic rocks and layered metagabbros with a metasedimentary matrix; (2) cataclastic rocks (peralkalic granitic gneisses) in the core of the granitic pluton, which are composed of protomylonites, mylonites, ultramylonites, and silicified ultramylonites, (3) mylonitic granites; and (4) post-granitic dykes and veins (iBrahiM et al., 2004). The rare-metal peralkalic granitic gneisses and cataclastic to mylonitic rocks (ali et al., 2011) are highly foliated and contain primary mineralization of uraninite, thorite and zircon minerals. These rocks are intruded by two mica (muscovitebiotite) granites. The cataclastic rocks are crosscut by NNW-SSE trending altered and mineralized shear zone with >1 km in length and 0.5 -5 m in width (Fig.1).
The lamprophyre dyke of 0.2 -0.5 m in width and > 1 km in length was emplaced along the altered and mineralized shear zone. Lamprophyre dyke is fine-to medium-grained, highly fractured parallel to the mineralized shear zone and characterized by dark grayish to black colors. Iron oxides, fluorite and carbonatite are present as  fillings in fracture planes. The studied dyke is completely altered and cross-cuts rare-metal peralkalic granitic gneisses and cataclastic mylonitic rocks in the Abu Rusheid mineralized shear zone in the direction of NNW-SSE (Figs. 2, 3b).
Petrographically, the altered lamprophyre dyke is mainly composed of plagioclases, amphiboles, relics of pyroxenes and secondary muscovite after biotite. The pyroxene is altered to amphiboles and iron oxides (Fig. 3c), while the biotite is altered to muscovite and iron oxides. K-feldspars and quartz are found in the lamprophyre dyke as phenocrysts embedded in the fine-grained groundmass. They were formed from xenoliths and relics of mineralized shear zone (granitic gneisses). Numerous apatite and zircon crystals, associated with muscovite and iron oxides were found. Apatite, fluorite, monazite, xenotime and zircon occur as accessory minerals ( Fig. 3d to f). Carbonate, chlorite, epidote, sericite, iron oxides and muscovite are secondary minerals.

REE-minerals and REE-bearing minerals
Due to their large ionic radii and charge, the REE, as well as U and Th, behave incompatibly during magmatic processes. The LREE tend to be concentrated in highly fractionated basic rocks, such as carbonatites (Förster, 2000), whereas HREE and especially Y tend be concentrated in fractionated acidic rocks, such as alkaline grani tes and pegmatites. Concerning the heavy REE (HREE), hanson (1978) invoked zircon as the major source of HREE in granitic rocks, while JeFFeries (1984) considered partitioning of HREE between zircon, xenotime, and apatite. Mineyev (1963) reported that the HREE can easily form sodium fluoro-complexes (e.g., Na(REE)F 4 ) under hydrothermal conditions. These complexes from preferably with HREE of smaller ionic radii (huMphris, 1984).
The crystal structures of many of REE-minerals are poorly known because the phases are metamict in nature (Th and U commonly sub-stitute for REE in minerals, as mentioned above). The smaller HREE, or Y-group, exhibits irregular coordination numbers with oxygen most commonly of 8, whereas the larger light REE (LREE), or Ce-group, exhibits larger coordination numbers, most commonly of 9 (e.g., Miyawaki & nakai, 1987). The LREE have relatively large ionic radii similar to those of Ca 2+ and Th 4+ . The trivalent LREE (Ce-group) are very similar crystalchemically to Ca 2+ , and they commonly substitute for Ca in rock-forming minerals. Substitution of a trivalent REE cation for divalent Ca is achieved by compensation of charge through a coupled substitution.
Zircon is an important member of REE-bearing minerals and a common accessory mineral in plutonic igneous rocks, especially those of granite group. It is generally present as small early formed crystals often enclosed in later minerals, but may form large well developed crystals in granites and pegmatites (deer et al., 1966). hussein (1978) and aBadalla et al. (2008) stated that the radioactive zircons are usually zoned. The radioactive zircon is also characterized by metamictization. The explanation for the origin of the "Metamict State " is that the internal order of the originally crystalline form has been destroyed by α-particles bombardment from radionuclides within the structure. Zircon may be partially or completely modified giving amorphous zircon with a more isotropic character. Such minerals are called metamict zircon.

Analytical methods
Four samples of the altered lamprophyre dyke were studied in detail using optical microscopy, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Polished thin sections were studied under reflected and transmitted light of an optical microscope in order to determine mineral associations and paragenesis. Backscattered electron images (BEI) were collected with the scanning electron microscope (JEOL 6400) at the Microscopy and Microanalyses Facility, University of New Brunswick (UNB), New Brunswick, Canada. Chemical compositions of minerals were determined on the JEOL JXA-733 Super probe at 15 kV, a beam current of 50 nA and peak counting times of 30 second for all elements. Standards used for the EPMA analysis were jadeite, kaersutite, quartz, and apatite for Na, Al, Si, and P, and Ca, respectively, SrTiO 3 for Ti, CaF 2 for F, pure metals were used for Fe, Nb, Hf, Ta, Sn, Th, and U, YAG for Y, cubic zirconia for Zr, (La, Ce, Nd, Sm, Pr, Er, Gd, Eu, Tb, Dy, and Yb)-bearing Al-Si glass for La, Ce, Nd, Sm, Pr, Er, Gd, Eu, Tb, Dy, and Yb and crocoite for Pb.

Mineralogical investigations
Results of detailed mineralogical examination of REE-minerals and REE-bearing minerals in altered lamprophyre dyke are briefly presented in the following sections.

Monazite-Nd ((Nd, Ce, La, Sm, Th)PO 4 )
Monazite-(Nd) occurs as euhedral to subhedral crystals in mineralized lamprophyre dyke in the Abu Rusheid area. The monazite crystals are generally light gray in color and range in size from 5 to 30 µm (Fig. 4a, c). Monazite was found associated with iron oxides and xenotime, rimming apatite and fluorite, as inclusions in the zircon, and in the fractures and fissure track in the dyke. Monazite in the studied lamprophyre dyke is frequently heterogeneous. This means that the same crystal of monazite consists of two distinct parts; which appear dark gray and light gray in SEM (BEI) images (Figs. 5c,d). Chemical composition of these distinct parts in monazite, obtained by the EPMA analysis, are given in Table 1 and shown in Figures 6 and 7. The results showed that contents of the major oxides in the darker part of monazite are: P 2 O 5 (22.99 to 26.11 wt%), Nd 2 O 3 (12.72 to 19.73 wt%), Ce 2 O 3 (2.31 to 16.37 wt%), La 2 O 3 (3.02 to 4.23 wt%), Pr 2 O 3 (5.15 to 6.30 wt%), Sm 2 O 3 (3.75 to 4.59 wt%), Gd 2 O 3 (3.82 to 4.75 wt%), Dy 2 O 3 (5.45 to 6.18 wt%), ThO 2 (0.02 to 0.21 wt%), UO 2 (0.71 to 1.18 wt%), while the lighter part consists of P 2 O 5 (20.40 to 24.26 wt%), Nd 2 O 3 (13.01 to 17.12 wt%), Ce 2 O 3 (10.18 to 16.42 wt%), La 2 O 3 (3.30 to 6.15 wt%), Pr 2 O 3 (3.46 to 4.68 wt%), Sm 2 O 3 (0.97 to 2.96 wt%), Gd 2 O 3 (2.94 to 4.01 wt%), Dy 2 O 3 (5.05 to 6.16 wt%), ThO 2 (0.21 to 0.81 wt%), UO 2 (0.92 to 2.91 wt%). It can be inferred from these results that the darker part of monazite-(Nd) is enriched in HREE and depleted in LREE, Th, and U; while the lighter part is depleted in HREE, Y and Ca, and enriched in Th and U. Monazite and xenotime in the Abu Rusheid lamprophyre dyke also occur in the mo-nazite-xenotime solid solution, which indicates presence of hydrothermal solutions enriched in REE, Y, P, U, F, and Zr. The emplacement of the lamprophyre dyke in the mineralized shear zone heated the wall rocks of the shear zone, which enriched in REE, Y, P, U, F, and Zr. These elements were remobilized by hydrothermal solutions and re-precipitated in the fractures and fissure tack in the lamprophyre dyke as REE-minerals, such as monazite-(Nd), xenotime-(Y) and REE-bearing minerals, such as apatite, fluorite, zircon, rutile with inclusions of xenotime and iron oxides.
Hydrothermal and igneous monazite can be distinguished by the ThO 2 content, which is generally < 1 wt% for hydrothermal monazite and 3 to > 5 wt% for igneous monazite (schandl & gorton, 2004). Also, monazites formed under high-grade metamorphic conditions show a wide range of ThO 2 concentrations and are strongly zoned with respect to Th. The studied monazite is relatively heterogeneous and strongly zoned with respect to Th. The Th content in the studied monazite is low and is consistent with its hydrothermal origin (Table 1).

Xenotime and monazite stability
The studied monazite is strongly zoned, as observed by SEM. According to heinrich et al. (1997), prograde zoning of metamorphic monazite and the partitioning of REE between monazite and xenotime are a function of temperature and pressure. It would appear that the U-Th partitioning between coexisting monazite and xenotime is also temperature and pressure depen dent and that crystals might be zoned in respect to their U/Th ratios. Xenotime can thus be used as a sensitive trace element geothermometer, both within monazite-xenotime pair (andrehs & heinrich, 1998). The data obtained by EPMA analyses of the stu died monazite-(Nd) and xenotime-(Y) were plotted on the CePO 4 -YPO 4 -ThSiO 4 ternary system at different temperatures (Fig. 8), accor ding to gratz & heinrich (1997) and seydoux-guillauMe et al. (2002). It can be inferred from the diagram that the studied xenotime from Abu Rusheid lamprophyre dyke formed at a tempera ture around 600 °C, while monazite formed at temperatures between 600 and 1100 °C (Fig. 8).

Apatite (Ca 10 (PO 4 ) 6 F 2 )
Apatite mainly occurs in the form of massive, euhedral to subhedral grains with size ranging from 10 to 40 µm (Figs. 4a, d, e, 5a). The apatite crystals are generally light gray to black in color under the transmitted light of an optical microscope (Figs. 3d -f). Chemical composition of apatite, obtained by EPMA, corresponded to fluorapatite (Table 3). The EPMA analyses (Table 3) also showed that the contents of major oxides and elements in the core of fluorapatite are: CaO (52.93 to 56.6 wt%), P 2 O 5 (39.78 to 42.12 wt%), and F (3.18 to Fig. 5. SEM-BSE images of a) apatite altered and breakdown to xenotime and monazite on the rim of apatite b) fluorite rimmed by monazite and xenotime on a rime, c) and d) heterogeneous monazite grain with darker and lighter parts, asso ciated with iron oxides. 4.15wt %), while in the rim of fluorapatite their values are: CaO (54.97 to 57.2 wt%), P 2 O 5 (38.66 to 42.39wt %), and F (3.55 to 3.7wt %). Minor amounts of LREE, Y, Fe, U, Sr and Si were also found as substituents in apatite, especially in the rim of apatite (Fig. 5b, Table 3). Uranium usually substitutes for Ca in apatite.
As pointed out by zhu & sverJensky (1991), the formation of apatite with a composition of environments is F-rich, even though the activity of Cl in the fluid is much higher than that of F. The formation of the studied apatite is similar. In some carbonatitic apatite (hogarth, 1989), F appears to overfill the site, especially when significant CO 3 2is present (piccoli & candela, 2002). However, carbonatitic apatite contains very high Sr (commonly > 2500 ppm) and low Y contents (commonly < 400 ppm) (Belousova et al., 2002). The EPMA data (Table 3) show that studied apatite from the Abu Rusheid lamprophyre dykes are similar to other Kiruna type apatite occurrences and would seem to exclude a carbonatitic affinity.

Rutile (TiO 2 )
Rutile was only found as a small crystal in the lamprophyre dyke in the Abu Rusheid area, which contains inclusions of xenotime. Generally, it is altered to xenotime in the fissure tracks of the rutile (Fig. 4c).

Iron oxides
Iron oxides are most common and very important in the mineralized lamprophyre dyke especially in the fractures and fissure tracks (Fig. 3f). They are associated with monazite, xenotime, apatite, fluorite, zircon, and rutile with inclusions of xenotime.

Discussion
The field observation shows that the mineralized lamprophyre dyke cuts the mineralized shear zone of Abu Rusheid area. The NNW-SSE faults along the shear zone itself acted as channel ways for the movement and focusing of hydrothermal fluids. Consequently, the lamprophyre dyke was subjected to the intense alteration processes (ferrugination, fluoritization, and carbonatization), brecciation and mineralization.
The mixing of volatile fluids with meteoric water and fluid-wall rock interactions caused changes in pH and oxygen activity and resulted in deposition of base metals. Precipitation of iron oxides probably decreased the pH of the solution thus giving rise to acidic fluids. The sudden      (alexandrov et al., 1985). The emplacement of the lamprophyre dykes in the host rocks was accompanied by high temperature and CO 2 and caused heating of the host rock. This lead to leaching of trace elements from minerals and their remobilization through the foliation, joints, fractures and shear zones, accompanied by hydrothermal solutions, rich in Cu, Y, Pb, W, V, Zn, and U. The rare-metals precipitated from saline and reduced fluids, originating from the two mica granites, in the form of sulfides due to cooling, fluid mixing and wall rock reactions (iBrahiM et al., 2007b). The lamprophyres, together with hydrothermal solutions and alterations, play an important role in the concentration and formation of the minerals in shear zones. The lamprophyres formed from CO 2 -rich magma. The CO 2 is cha racterized by its ability to fix all mineralization. So the lamprophyres are considered as a chemical trap for mineralization (iBrahiM et al., 2007b). Crystallization of fluorite, galena, and pyrite in the mineralized shear zone of Abu Rusheid area reflects the important role of F and S. Zr, Hf, Th, and Ti are typical high field strength elements (HFSE), which are gene rally conside red immobile during hydrothermal water-rock interaction. Experimental and natural evidence, however, have demonstrated that Zr, Ti, and Th may become mobile especially in high-tempera ture magmatic or hydrothermal environments contai ning strong complexing agents such as F, S, and others (keppler, 1993). The fact that F may play a prominent role in the hydrothermal mobilization of HFSE has been indicated for Zr, Th and REE (Moine & salvi, 1999). Also, fluoride complexes, e.g., (Zr, F) (REE, F 4 ) 3 , are known to be stable under hydrothermal conditions (Mineyev, 1963). The abundantly detected zircon and Th-bearing minerals, demonstrably of hydrothermal origin, can be attributed to the role of F-rich fluids. Thus, although Zr and Th are generally considered as highly immobile elements, the occurrence of zircon indicates that significant concentrations of Zr and Th can be tran sported via specified F-rich fluids.
Monazite associated with fluorapatite is poor in Th (0.00 ≤ ThO 2 ≤ 1.1 wt%), but usually rich in U (0.3 ≤ UO 2 ≤ 2.5 wt%), and the Th/U ratio is always lower than unity and ave-rages to 0.09. In this type of monazite, spatial relations and compositional features, suggest that the Th-poor, U-rich monazite-(Ce) formed as result of fluorapatite metasomatism (zieMann et al., 2005). The studied monazite-(Nd) in the Abu Rusheid dyke, which is associated with fluorapatite, is poor in Th (0.02 ≤ ThO 2 ≤ 0.81 wt%) and rich in U (0.92 ≤ UO 2 ≤ 2.91 wt%), with the Th/U ratio always lower than unity, averaging 0.16. This in dicates that monazite-(Nd) also formed as a result of flourapatite metasomatism.

The mineralized lamprophyre dyke in the
Abu Rusheid area, occurs in a great fracture systems trending in NNW-SSE direction. It cuts cross the cataclastic mlyonite and mineralized granitic gneisses. The rocks are intruded by a huge intrusion of muscovite-biotite granites. The dyke in the Abu Rusheid area is characterized by HREE-minerals, such as xenotime-(Y) and monazite-(Nd), and REEbearing minerals, such as apatite, fluorite, zircon, rutile with inclusions of xenotime and iron oxides. 2. The studied monazite-(Nd) in the lamprophyre dyke is frequently heterogeneous. There are two distinct parts in the same crystal. The part, which appears dark gray in BEI image, is enriched in HREE and depleted in LREE, Ca, Th and U, while the lighter gray part is depleted in HREE, Y and Ca and enriched in Th and U. The studied monazite is also characterized by low Th contents and shows strong zoning that reflects its hydrothermal origin. 3. The solid solutions between monazite and xenotime indicate that they formed at high temperatures from hydrothermal solutions. Composition of the studied monazite-(Nd) and xenotime-(Y), plotted on the CePO 4 -YPO 4 -ThSiO 4 ternary system indicated that the studied monazite and xenotime formed at a temperature around 600 °C. 4. The studied monazite-(Nd) in the Abu Rusheid dyke, which is associated with fluorapatite, is poor in Th (0.02 ≤ ThO 2 ≤ 0.81 wt%), but usually rich in U (0.92 ≤ UO 2 ≤ 2.91 wt%), and the Th/U ratio is always lower than unity and averages to 0.16. This indicates that the monazite formed as a result of flourapatite metasomatism. The event most likely responsible for metasomatism of the fluorapatite is documented by the associated monazite-(Nd). 5. Apatite was formed by magmatic fractionation and was affected by hydrothermal solutions enriched in REE, P and Y. 6. The emplacement of lamprophyre dykes within the mineralized shear zones was accompanied by high temperatures and CO 2 . Presence of hydrothermal solutions, heated by the lamprophyre dykes, caused leaching and remobilization of REE, P, Zr, F, Y, and Ti from minerals in the wall rocks, which were then re-precipitated as the REE-minerals monazite-(Nd) and xenotime-(Y) in the fractures and fissure tack in the apatite, fluorite, rutile, zircon and iron oxides. Fluorite and galena also indicate the displacement of REE, P and Y by hydrothermal solution and re-deposition in the fracture and fissure tack in the apatite, fluorite, rutile, zircon and iron oxide minerals. 7. The studied lamprophyre dyke in the Abu Rusheid area contains high concentrations of REE, P, U, F, Zr, Ti, and Fe, especially in the form of HREE minerals, such as xenotime-(Y), monazite-(Nd, and REE-bearing apatite, fluorite, zircon, and rutile with inclusions of xenotime. Accordingly, the mineralized lamprophyre dyke in the Abu Rusheid area could represent a very good source of REE-minerals, especially HREE.