mineralogy of polymetallic mineralized pegmatite of ras

123Mineralogy of polymetallic mineralized pegmatiteJournal of Mineralogical and Petrological Sciences, Volume 105, page 123─134, 2010

doi:10.2465/jmps.090201M.F. Raslan, [email protected] Corresponding author

Mineralogy of polymetallic mineralized pegmatite of Ras Baroud granite, Central Eastern Desert, Egypt

Mohamed F. Raslan*, Hassan E. El-Shall**, Sayed A. Omar* and Ahmed M. Daher*

*Nuclear Materials Authority P.O. Box, 530, El Maadi, Cairo, Egypt **University of Florida, Gainesville, FL, 32611, USA

An economically important rare-metal mineralization is recorded in the pegmatite bodies of Gabal Ras Baroud younger granitic pluton, Central Eastern Desert of Egypt. These pegmatite bodies are of variable size and are compositionally zoned. Radiometric measurements of some anomalous pegmatite samples show that their equivalent uranium (eU) content is 219-328 ppm, whereas their equivalent thorium (eTh) content is 783-1101 ppm. On the other hand, the analysis of several separated mineral grains of some pegmatite samples using a scanning electron microscope and X-ray diffraction revealed the presence of several economic minerals. These minerals include zircon, thorite, phlogopite mica, and columbite, in addition to the samarskite-Y mineral. Tho-rite was found as numerous inclusions of variable size and pattern in zircon. Electron microprobe analysis con-firmed the presence of samarskite-Y whose composition corresponds to the empirical formula [(Y0.49, REE0.41, Th0.06, Si0.05, Ca0.03, U0.02, Fe0.01, Zr0.00)Σ1.05(Nb0.75, Ta0.17, Ti0.01)Σ0.94O4]. Accordingly, the mineralized Ras Baroud pegmatite can be considered as a promising target ore for its rare-metal mineralization that includes mainly Nb, Ta, Y, U, and REE together with Zr and Th.

Keywords: Samarskite-Y, Zircon and thorite inclusions, Ras Baroud granitic pegmatite, Eastern Desert, Egypt

INTRODUCTION

Several studies (e.g., Akaad and Noweir, 1980; Hassan and Hashad, 1990) have broadly classified Egyptian gran-itoids into two main groups: older syn- to late-tectonic granite referred to as grey granites (850-650 Ma, subduc-tion-related, I-type granitoids ranging in composition from trondhjemite to granodiorite) and younger post-tec-tonic granites referred to as pink granite (600-480 Ma, ranging in composition from biotite monzogranite to proper granites). Hussein et al. (1982) added a third group of alkaline granites that was previously identified as younger granites. The younger granites (second group) fall within Bowden’s (1985) petrotectonic-time associa-tions of the Pan-African orogeny (i.e., typically equiva-lent to the A2-subtype of Eby, 1992). Rare-metal mineral-ization is particularly and genetically associated with post-orogenic, geochemically distinctive varieties of the second group of granitic rocks. Rare-metal mineralization can be attributed either to magmatic or post-magmatic metasomatic processes (Schwartz, 1992; Abdalla et al., 1998).

According to Cerny’s (1990) pegmatite classifica-tion, the rare-earth element (REE) subclass is character-ized by the niobium-yttrium-fluorine (NYF) and zirconi-um-niobium-fluorine (ZNF) family signatures. NYF pegmatites are distinguished by the signature Y, Nb > Ta, HREE, U, Th, and F, whereas ZNF pegmatites are distin-guished by the signature Zr, Nb >> Ta, Y, Th, P, and F. From the viewpoint of exploration, post-orogenic, A2-

type granites are the most favorable sites for the localiza-tion of rare-metal pegmatitic mineralization of NYF af-finity. These granites are characterized by mineralogical and geochemical signatures, i.e., they are transolvus, alka-line, and metaluminous to mildy peraluminous with an-nite-siderophyllite mica as a sole mafic mineral (Abdalla and El Afandy, 2003).

Several studies worldwide have revealed the pres-ence of granite-pegmatite-hosted rare-metal mineraliza-tions including Nb-Ta oxides and zircon (e.g., Matsubara et al., 1995; Hanson et al., 1998; Erict, 2005; William et al., 2006; Pal et al., 2007).

The study area is located in the Central Eastern Des-ert of Egypt directly to the north of the Qena-Safaga as-phaltic road and it covers an area of approximately 70 km2 (Fig. 1A). The area of Ras Baroud is actually sur-

124 M.F. Raslan, H.E. El-Shall, S.A. Omar and A.M. Daher

rounded by both younger and older granitic masses and its geology has been studied by several authors (e.g., Abu El Ela, 1979; Kaoud, 1982; Hassan and Hashad, 1990; Omar, 1995; Zalata et al., 1996).

Several Nb-Ta occurrences have been recorded in different localities of the Eastern Desert, namely, El Naga, Abu Khurg, Abu Dabbab, Noweibi, and Abu Rushied (Hussein, 1990). However, these mineralizations are mainly restricted to the granite pegmatite bodies associat-ed with the younger granite that are widely distributed in the Eastern Desert (Omer, 1995; Ibrahim et al., 1996; Ab-

dalla et al., 1998; Ibrahim, 1999; Attawiya et al., 2000; Ammar, 2001; Abdalla and El Afandy, 2003; Abd El Wa-hed et al., 2005; Abd El Wahed et al., 2006; Abdel Warith et al., 2007). Relevant literatures indicate that Nb-Ta min-eralization in Egypt has a direct connection with albite granites in the Eastern Desert (Sabet and Tsogoev, 1973). Such a type of granite is commonly termed “apogranite” and it is believed to be a special type of metasomatic granitoid (Beus, 1982).

Nb-Ta mineralization has been recorded in the peg-matite bodies of Gebel Ras Baroud granite as well as in

Figure 1. (A) Geologic map of Gabal Ras Baroud, Eastern Desert, Egypt (after Omar, 1995). (B) Pegmatitic pockets in Ras Baroud granite. (C) Pegmatitic bodies in Wadi El Ba-roud area. (D) Sketches showing the studied pegmatites in Ras Ba-roud granitic mass.

125Mineralogy of polymetallic mineralized pegmatite

the granite itself and in the surrounding wadi stream sedi-ments (Kaoud, 1982; Mahdy et al., 1991).

Sayyah et al. (1993) have studied various clusters of distinguishable megascopic crystals of columbite-tantalite and alvarolite (mangano-tantalite, Mn Ta2O6) minerals scattered within the pegmatitic bodies of Ras Baroud granite. Raslan (2009) identified samarskite-Y, columbite, and zircon from the stream sediments surrounding the Ras Baroud younger granite pluton. However, a systematic and detailed mineralogical study of the different mineral-izations associated with Ras Baroud pegmatite has not yet been performed. Accordingly, the aim of the present paper is to identify the mineralogical and chemical characteris-tics of the radioactive as well as the economic heavy min-erals of Ras Baroud pegmatites.

GEOLOGIC SETTING

The younger granites occur as isolated plutons of high re-lief intruding the older granitoids. The younger granites are well represented in the area by Gabal Ras Baroud and Gabal Abu Hawis where they occur as small isolated plu-tons of circular to semicircular outline with respective outcrop areas of 40 km2 and 8 km2. It is interesting in this regard to mention that Abu Hawis granite is almost de-void of pegmatitic bodies.

Gabal Ras Baroud younger granitic pluton occupies the northern part of Wadi El Baroud and the granite is me-dium to coarse-grained, and pink to reddish or pink to whitish in color. The pluton is strongly jointed and faulted and is intruded by numerous dikes without definite direc-tions.

In Ras Baroud granitic mass, radioactive anomalies are hosted in the pegmatitic bodies. Pegmatites enclosing the radioactive anomalies are recorded at the marginal zone of the Ras Baroud granitic mass were they arrange roughly parallel to its periphery (Fig. 1A). These pegma-tites are of variable size ranging between 0.4 m to 30 m in width and 0.5 m to 50 m in length, and they are composi-tionally zoned. The pegmatite pockets are subrounded to elliptical in shape and they exhibit sharp contacts with the granite host. All pegmatite bodies are formed in an outer zone of blocky feldspars with subordinate randomly dis-tributed mica pockets and an inner core of massive quartz that also occurs as disconnected bodies between the feld-spars (Figs. 1B-1D). The feldspar zones enclose several clusters of mica pockets and contain highly radioactive, visible black minerals as well as violet fluorite. The micas are mostly lepidolite with subordinate amounts of musco-vite, and the width of the aggregated mica never exceeds 50 cm. The investigated pegmatites exhibit variations in the intensity of the pinkish color and granularity. Accord-

ing to Suror et al. (2004), two pegmatite lithotypes are distinguished at the Ras Baroud area, namely, the graphic and perthitic pegmatite, based not only on the size and textural bases but also on the type and composition of the feldspar. In these pegmatite bodies, radioactive anomalies occur in the form of small localized disconnected zones and they are associated with disseminated violet fluorite. From the paragenetic point of view, zones in pegmatites develop inward from the wall within a restricted magmat-ic hydrothermal system.

A black visible mineralization occurring as long pris-matic crystals having lengths of up to 10 cm is dispersed in the feldspar zone and less commonly at its contact with the quartz core. These black megascopic crystals have been identified as Nb-Ta oxides. The chemistry of these columbite crystals exhibits a compositional range from mangano- to ferrocolumbite at the cores and rims, respec-tively. The mineral chemistry of Ras Baroud columbite suggests that the Nb ore resulted from a fractionated F- and Li-rich fluid in connection with topaz-lepidolite gran-ite. The cryptically zoned columbite crystals with Fe-rich rims suggest remarkable reducing conditions during the late stages of columbite crystallization (Suror et al., 2004). The anomalous value of Nb and Ta elements may be attributed to their existence as disseminated Nb-Ta minerals or incorporated in the lattice of micas or other minerals.

According to Omar (1995), the modes of occurrence of Nb-Ta mineralization in Ras Baroud area can be ex-pressed as follows:(a) Distinguishable clusters of fan-shaped megascopic,

long, prismatic crystals of columbite-tantalite miner-als confined in the feldspar zone of the zoned pegma-tites.

(b) Associated with radioactive anomalies where the min-eralization occurs as invisible disseminations in the feldspar zones of pegmatite bodies and disseminations in the micas, fluorite, specularite, and other minerals in both the younger granite and the pegmatites.

(c) Distinct concentrations in the alluvial overburden sedi-ments formed through the destruction and weathering processes of the pegmatite bodies.

SAMPLING AND ANALYTICAL TECHNIQUES

Four mineralized grab samples were collected from Ras Baroud pegmatite bodies representing the highest values of anomalous field radioactivity. These samples were pre-pared for gamma-ray spectrometric analysis in order to determine their uranium and thorium contents. In addi-tion, a large bulk composite sample representing the dif-ferent mineralized zones of Ras Baroud pegmatite bodies


and weighing approximately 70 kg was collected for min-eralogical investigation. The sample was properly crush-ed, ground, and sieved before subjecting the liberated size fractions to heavy-mineral separation using bromoform (specific gravity = 2.85 gm/cm3). From the obtained heavy fractions, pure mineral grains were manually picked and investigated under a binocular microscope. Some of the picked mineral grains were subjected to X-ray diffraction analysis using a Phillips X-ray diffrac-tometer (Model PW-105018) and an environmental scan-ning electron microscope (ESEM). This instrument in-cludes a Philips XL 30 energy-dispersive spectrometer (EDS) unit. The applied analytical conditions were an ac-celerating voltage of 30 kV with a beam diameter of 1-2 µm for a counting time of 60-120 s and a minimum de-tectable weight concentration ranging from 0.1 wt% to 1 wt%. All these analyses were carried out at the laborato-ries of the Egyptian Nuclear Materials Authority (NMA).

In addition, thin, polished sections of some mineral grain varieties were prepared and analyzed using a JEOL 6335F field-emission scanning electron microscope. This instrument is fitted with an Oxford Energy Dispersive X-ray Spectrometer (EDS) for elemental analysis of mi-cro areas, a backscattered electron detector that allows compositional analysis, and a cathode luminescence de-tector that can image complex characteristic-visible spec-tra for detailed molecular structure information. The ap-plied analytical conditions were an accelerating voltage of 0.5-30 kV, 1.5 nm (at 15 kV)/5.0 nm (at 1.0 kV). The im-aging modes are secondary electron imaging (SEI) and backscatter electron imaging (BSI). This instrument can be remotely operated, and it can be used for X-ray micro-analysis of small areas, lines scans of relative concentra-tions for multiple elements, and X-ray maps of relative concentrations of multiple elements.

Finally, thin, polished sections of some mineral grains were also analyzed using a JEOL SUPERPROBE 733 with an accelerating voltage of 15 kV and a beam size of approximately 1 µm. The used standards included niobium metal (Nb), tantalum metal (Ta), biotite (Ti-Fe-Si), uranium metal (U), monazite (Th), fluorite (Ca), and cubic zirconia (Zr-Y).

RESULTS AND DISCUSSIONS

The systematic and detailed mineralogical examination of the heavy minerals obtained from the bulk composite sample of Ras Baroud pegmatite revealed the presence of several economic minerals. Thus, in addition to the Nb-

Ta minerals (columbite and samarskite-Y), a Th-U min-eral (thorite or uranothorite) was discovered in close asso-ciation with zircon. A phlogopite mica species associated

with hematite was also identified. Microscopic examina-tion of the heavy fractions of the four size classes (−0.800 + 0.600 mm), (−0.600 + 0.400 mm), (−0.400 + 0.200 mm), and (−0.200 + 0.072 mm) revealed that the content of the accessory minerals in the bulk composite sample of the studied Ras Baroud pegmatite amounts to approxi-mately 2.5 vol%. Phlogopite mica represents approxi-mately 50 vol% of the obtained heavy fractions and zir-con and samarskite account for the rest. The contents of heavy and accessory minerals have been determined us-ing the counting technique. These data indicate that mica is the predominant mineral followed by zircon and samar-skite in all size fractions (−0.800-0.072 mm). In addition to these minerals, some scattered columbite grains occur in much lower amounts.

Table 1 lists the results of the gamma-ray spectro-metric analysis for U, Th, K, and Ra concentrations in the radioactive samples collected from the highly anomalous radioactive zones of Ras Baroud pegmatites. The uranium content (eU) was found to range between 219 ppm and 328 ppm with an average of 263 ppm whereas the thori-um content (eTh) ranges between 783 ppm and 1101 ppm with an average of 938.5 ppm.

Zircon-thorite association

Under a binocular microscope, zircon occurs as pale to dark brown massive compact grains that are generally translucent to opaque (Fig. 2A). The surface of the zircon grains is generally ill-defined, rough, and dull. It is opti-cally anomalous, possessing a zonal structure with almost translucent to isotropic zones. Morphologically, the crys-tals are typically short to equidimensional, with a length/width ratio of 1:1, and they tend to exhibit square to trap-ezoid, rhombic, or hexagonal cross sections. Almost all the investigated zircon crystals are metamictized toward the core and characteristically contain several black inclu-sions of thorite. Accordingly, the data obtained for some of the separated zircon grains by XRD analysis confirm that all the analyzed grains are almost completely com-posed of zircon and thorite minerals (Table 2).

*R-1, 2, 3, and 4 refer to grab samples collected from Ras Baroud radioactive pegmatite pockets.

Table 1. Radioelement measurements of the studied Ras Baroud radioactive pegmatite


In addition, several zircon crystals were subjected to semiquantitative analyses using ESEM and a field-emis-sion scanning electron microscope. Thus, while the ESEM microphotograph (Fig. 2B) reflects the morphological fea-tures of the investigated zircon as well as its uranothorite inclusions, the EDAX analysis (Figs. 2C and 2D) con-firms the semiquantitative chemical composition of zircon and the thorite inclusion, respectively. However, the latter tends to be uranothorite species due to the presence of a remarkable amount of uranium (23.7%) together with Th (46.7%) and Si (11.3%).

The uranothorite inclusions are present in variable

sizes ranging from 1 µm to 30 µm. They occur either as numerous minute inclusions in a zonal distribution pat-tern, especially in the outer zone of the zircon grain (Figs. 2E-2H), or as randomly distributed inclusions of varying sizes (Figs. 3A-3C). The scanline elemental analyses along the zircon crystal revealed the heterogeneous distri-bution of Zr, Hf, and Si within the crystal. On the other hand, it is quite clear that uranium and thorium contents increase in the bright parts of thorite inclusions (Fig. 3D).

The obtained SEM data show the semiquantitative chemical composition of zircon (Fig. 3E) and thorite (Fig. 3F). It is actually noteworthy that Ali et al. (2005) and

Figure 2. (A) Massive brown zircon grains containing black thorite in-clusions separated from Ras Baroud pegmatite. Binocular microscope. (B) Backscattered electron image of zircon (note the bright thorite inclu-sions). (C) and (D) EDX analyses of zircon and thorite of Ras Baroud pegmatite, respectively. (E), (F), and (G) Zircon crystal in polarized light, corresponding BEI image, and enlarged area within that crys-tal showing the pattern of thorite distribution near the edge of the zir-con crystal. (H) BEI image of Ras Baroud zircon showing distribution pattern of thorite inclusions in zir-con.


Abdel Warith et al. (2007) reported the presence of thorite inclusions in rare-metal mineralization and accessory heavy minerals (zircon, samarskite, and spessartine gar-net) that are separated from some Egyptian granitic rocks and their associated pegmatites.

Phlogopite

Light- to dark-brown mica was identified using both XRD and ESEM analyses. The obtained XRD data for picked mica flakes revealed that the studied mica is main-ly stained with hematite (Table 3). The obtained ESEM data (Figs. 4A and 4B) reflect the morphological feature of mica and its semiquantitative chemical composition, respectively.

Table 2. XRD pattern of thorite inclusions in zircon grains separat-ed from RasBaroud pegmatite pockets

Figure 3. (A) Backscattered electron image of enlarged area in a zircon crystal separated from the Ras Ba-roud pegmatite (Fig. 2H), showing the bright thorite inclusions of vari-able size. (B) and (C) BEI photomi-crograph showing the random dis-tribution of thorite in zircon and the corresponding enlarged area. (D) Scanline showing elemental analy-ses (wt%) of zircon in Figure 2F. (E) and (F) EDX analyses of zircon and thorite, respectively.


Columbite

Angular to subangular black massive columbite grains were actually detected in the studied bulk sample of Ras Baroud pegmatite. Most columbite grains characteristical-ly contain surface cavities that are rich in iron (Raslan, 2005). Various ESEM data of the separated columbite grains are presented in Figures. 4C-4F. The obtained re-sults confirm the semiquantitative chemical composition of the separated columbite grains where the major ele-ments include Nb (67.2%), Fe (16.8%), Ta (5.3%), and Y (4.3%) with U (1.0%), Gd (2.3%), and Yb (1.9%) as trace elements.

Samarskite (Y)

Samarskite is a group of Nb-Ta mineral varieties that oc-cur in pegmatite granites and have the general formula Am Bn O2 (m+n), where A represents Fe2+, Ca, REE, Y, U, and Th whereas B represents Nb, Ta, and Ti. According to Hanson et al. (1999), the complete metamict state, altera-tion, and the broad variation of cations in the A-site of these mineral varieties render their crystal structure a problematic case. Therefore, these authors have proposed a nomenclature for the samarskite group of minerals based on their classification into three species. Thus, if REE + Y are dominant, the name samarskite-(REE + Y) should be used with the dominant of these cations as a suffix. If U + Th are dominant, the mineral should be named ishikawaite whereas if Ca is the dominant cation,

the mineral should be named calciosamarskite. Hanson et. al. (1999) have also reported that ishikawaite and calcio-samarskite are depleted in the light rare-earth elements (LREE) and enriched in the heavy rare-earth elements (HREE) together with Y. Recently, samarskite-(Yb) has been identified as a new species of the samarskite group (William et al., 2006), i.e., a Yb-dominant analog of sa-marskite-Y. On the other hand, samarskite-Y has also been described as a mineral with Y + REE dominant at the A-site (Nickel and Mandarino, 1987). Finally, it has to be mentioned that Warner and Ewing (1993) have pro-posed that samarskite should be formulated as ABO4. It is interesting to mention that ishikawaite with an average assay of approximately 50% Nb2O5 and 26% UO2 has been identified for the first time in Egypt in the mineral-ized Abu Rushied gneissose granite (Raslan, 2008).

In the present study, the samarskite-Y variety has been well identified using both microscopic investigation as well as proper analysis by ESEM, field-emission scan-ning electron microscope, as well as by electron micro-probe analyses. ESEM data of the studied samarskite grains (Figs. 4G and 4H) show that the mineral is en-riched in Nb (42.5%) and Y (20.7%) whereas the assay of the other REEs is much lower; namely, Ce (1.4%), Nd (2.9%), Sm (1.9%), Gd (1.8%), and Yb (4.5%). On the other hand, both uranium and thorium are commonly present and their assay attains 2.70% and 5.52%, respec-tively. Under a binocular microscope, the defined samar-skite crystals are generally massive with a granular form and having a characteristic pendent vitreous or resinous luster. In addition, the investigated samarskite grains are generally translucent, compact, metamict, and hard. Un-der a polarizing microscope, the samarskite grains are mainly velvet-yellow brown to bloody red in color (Figs. 5A and 5B). They occur as translucent grains that are 0.1-0.8 mm in size. Separated samarskite grains are pres-ent as short prismatic to tabular crystals, exhibiting vitre-ous luster and conchoidal fractures. Samarskite crystals commonly exhibit partial to complete alteration effects along micro cracks and grain peripheries as well as the metamictization effect. Altered varieties are darker brownish to opaque in thin sections and whitish grey in reflected light. The metamictization effects include the development of sinuous sineresis cracks associated with the expansion isotropism of the crystal. Although most crystals are granular, some crystals are rod-like in shape (Fig. 5B).

On the other hand, several samarskite crystals have also been subjected to semiquantitative analyses using a field-emission scanning electron microscope and the ob-tained SEM data (Figs. 5C and 5D) show that both Nb and Y are the essential components. Other elements pres-

Table 3. XRD pattern of phlogopite mica stained with hematite (Ras Baroud pegmatite pockets)


ent in small to minor amounts include U, Th, and Nd. The distribution of uranium and thorium within the crystals is actually heterogeneous and their contents increase with Y (Fig. 5E).

The chemical composition of the studied samarskite and the microprobe spots are shown in Figures. 5C, 5F, and 5G. The obtained microprobe analyses (Table 4) have resulted in the following averages in wt%: Nb2O5, 40.44; Ta2O5, 5.18; TiO2, 0.25; UO2, 2.19; ThO2, 7.21; Y2O3, 18.68; MnO, 0.05; ZrO2, 0.05; CaO, 0.55; FeO, 0.11, SiO2, 0.70; and a total REE of 24.78 with an average sum of 100.39 wt%. Table 4 shows the chemical empirical for-mula that is recalculated on the basis of 4 oxygens; viz, [(Y0.49, REE0.41, Th0.06, Si0.05, Ca0.03, U0.02, Fe0.01, Zr0.00)Σ1.05

(Nb0.75, Ta0.17, Ti 0.01)Σ0.94 O4]. In the meantime, the micro-probe analyses were plotted on the ternary diagram of Hanson et al., (1999) which shows the A-site occupancy of the samarskite group of minerals (Fig. 6). From the lat-ter, it was found that all the corresponding data points plot in the samarskite-Y field. Finally, it is interesting to note that some EMPA analyses of the defined samarskite crys-tals exhibit a significant enrichment in Ta content attain-ing up to 14.0% and 12.8%.

From the obtained data, it is quite clear that the stud-ied Nb-Ta mineral variety of Ras Baroud pegmatite gran-ite reflects the chemical composition of a Y- and REE-

rich samarskite species. The lines of evidence of the latter (samarskite-Y) can be summarized as follows:

Figure 4. (A) and (B) Backscattered electron image of phlogopite mica separated from Ras Baroud pegma-tite and corresponding EDX analy-ses. (C) and (D) BEI image of Ras Baroud columbite and its EDX analyses. (E) and (F) Photomicro-graph showing a columbite grain separated from Ras Baroud pegma-tite and the corresponding elemen-tal analyses. (G) and (H) Backscat-tered electron image of a samarskite grain and its EDX analyses.


1) The obtained EMPA data revealed that Nb2O5 is domi-nant in the investigated mineral where it ranges from 31.76% to 43.93% with an average of 40.2%. The av-erage of Ta2O5 and TiO2 attains 5.43%, which is much lower than Nb2O5. The samarskite minerals group comprises only those species in which the Nb content in the B-site is higher than that of Ta and Ti (Hanson et al., 1999).

2) The studied mineral actually falls within the composi-tional limits of both samarskite-Y and ishikawaite. Both samarskite-Y and ishikawaite have a dominant Nb in the B-site and the distinction between either va-

riety must be based on the content of B-site occupan-cy.

3) In the studied Ras Baroud samarskite species, the Y content ranges from 17.6% to 19.9% with an average of 18.7% and the ΣREEs ranges from 23.0% to 27.3% with an average of 24.8%. In other words, the average Y + REEs would attain up to 43.4%. Samarskite-Y has been described as the mineral species in which Y + REE are the dominant components at the A-site (Nick-el and Mandarino, 1987).

In short, it can be concluded that the investigated sa-marskite species separated from Ras Baroud radioactive

Figure 5. (A) and (B) Ras Baroud sa-marskite crystals of various habits (in crossed nicols, C.N.). (C), (D), and (E) BEI of a samarskite crystal with microprobe spots, EDX spectrum of the samarskite and its elemental scanline, respectively. (F) and (G) BEI and secondary electron images of the studied samarskite crystals showing the spots of microprobe analyses within crystals.


pegmatite is characterized by the dominance of Y + REE in the A-site whereas Nb in the B-site is higher than both Ta + Ti. Therefore, the defined samarskite species in the present work belongs to the compositional limits of the samarskite-Y mineral species, as specified in the litera-ture.

CONCLUSIONS

Radioelement measurements for anomalous pegmatite as-sociated with the Ras Baroud younger granite pluton re-vealed an equivalent uranium content (eU) of 219-328 ppm and an equivalent thorium (eTh) of 783-1101 ppm. ESEM analyses confirm the presence of a samarskite-Y mineral species whose microprobe analyses revealed the empirical formula [(Y0.49, REE0.41, Th0.06, Si0.05, Ca0.03, U0.02, Fe0.01, Zr0.00)Σ1.05(Nb0.75, Ta0.17, Ti0.01)Σ0.94O4]. In addition, both the SEM and the XRD analyses revealed the pres-ence of zircon, thorite, and phlogopite mica. Thorite was found as numerous inclusions of variable size and pattern embedded in the zircon mineral grains. The mineralized Ras Baroud pegmatite granite is actually considered to be a promising ore material for its rare-metal mineralization that includes mainly Nb, Ta, Y, U, and REE together with Zr and Th.

ACKNOWLEDGMENTS

The field-emission scanning electron microscope and mi-croprobe analyses were respectively carried out during the leave of the first author on a post doctoral fellowship in

FeO●, Total iron.

Table 4. Selected EMPA analyses of samarskite-Y from Ras Baroud pegmatite

Figure 6. Ternary diagram showing A-site occupancy of samar-skite-group minerals after Hanson et al. (1999). Ras Baroud sa-marskite-Y is represented by the closed squares.


the Particle Engineering Research Center (PERC) and Major Analytical Instrumentation Center (MAIC), Uni-versity of Florida, USA. The authors sincerely thank Prof. Dr. N.T. El Hazek, Egyptian Nuclear Materials Authority, for his support, review of the manuscript, and fruitful dis-cussions.

SUPPLEMENTARY MATERIALS

Color version of Figures 1, 2, 3, and 5 is available online from http://www.jstage.jst.go.jp/browse/jmps.

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Manuscript received February 1, 2009Manuscript accepted October 28, 2009Published online March 12, 2010Mauscript handled by Koichiro Fujimoto

mineralogy of polymetallic mineralized pegmatite of ras

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