Occurrence of Ishikawaite (Uranium-Rich Samarskite) in the Mineralized Abu Rushied Gneiss, Southeastern Desert, Egypt

Transcription

1 International Geology Review, Vol. 50, 2008, p DOI: / Copyright 2008 by Bellwether Publishing, Ltd. All rights reserved. Occurrence of Ishikawaite (Uranium-Rich Samarskite) in the Mineralized Abu Rushied Gneiss, Southeastern Desert, Egypt MOHAMED FAHMY RASLAN 1 Nuclear Materials Authority. P.O. Box, 530, El Maadi, Cairo, Egypt Abstract Ishikawaite, with an average assay of about 50% Nb 2 and 26% UO 2 has been identified for the first time in Egypt in the mineralized Abu Rushied gneissose granite. The mineral is associated with columbite, Hf-rich zircon, and dark Li-mica mineral (zinnwaldite). The mineralogy and geochemistry of the studied ishikawaite were determined using microscopic investigation as well as quantitative analysis by both field emission scanning electron microscope and electron microprobe analyses. Analytical results indicate a structural formula of (U, Fe, Y, Ca) (Nb, Ta)O 4 for the ishikawaite, with U ranging from 0.12 to 0.61 per formula unit. Introduction THE ABU RUSHIED gneissose granite is situated some 90 km southwest of Mrsa Alam on the Red Sea Coast and belongs to the Precambrian basement of the Southeastern Desert of Egypt (Fig. 1). The mineralized Abu Rushied gneissose granite is considered promising for its rare-metal mineralization that includes mainly Nb, Ta, U, Th, and REE together with Zr and Hf. The studied mineralization, which is restricted to a psammitic gneissose type, has been attributed to metasomatic Nb-Ta mineralization (Abdel Aziz et al., 1967; Hassan, 1973). The origin of the psammitic gneiss host rock is indeed controversial; several authors considered it as a metamorphosed sedimentary unit of quartzofeldspathic composition (Hassan, 1964; Abdell Monem and Hurley, 1979; El Gemmizi, 1984; El-Ramly et al., 1984; Eid, 1986 and Saleh, 1998). However, Ibrahim et al, (2000) considered it as a highly mylonitic gneissose granitic rock, ranging in composition from the granodiorites to adamellites. Mineralogically, about 35 minerals of Nb and Ta are known, the most important of which are those having the structure A m B n O 2 (m+n) of Nb, Ta, Ti, and the REE. The latter can be categorized into the following three series: 1. The pyrochlore-microlite series [(Na, Ca) 2 (Nb, Ta) 2 O 6 (O, OH, F)], characterized by the presence of Na and Ca. Varieties with U, Th, Ti, Ce, Y, and other REE have been described, and the presence of 1 raslangaines@hotmail.com these substitutents is the rule rather than the exception. Fergusonite is a mineral variety in which the REEs wholly replace (Na and Ca), resulting in the formula Y (Nb, Ta) O The columbite-tantalite series [(Fe, Mn) (Nb, Ta) 2 O 6 ]. 3. The euxenite-polycrase series (Ca, REE, U, Th) (Nb, Ta, Ti) 2 (O, OH) 6 includes the varieties samarskite (Y, Ce, U, Fe 2+ ) 3 (Nb, Ta, Ti) 5 O 6, with even more substitutions and betafite (Y, U, Ce) 2 (Ti, Nb, Ta)O 6 OH. These minerals are typically metamict and isotropic and are common in granite pegmatites associated with zircon, monazite, xenotime, and allanite. It is interesting in this regard to mention that the author in a previous study of the Abu Rushied mineralized gneiss (Raslan, 2005) identified columbite, Hf-rich zircon, and dark Li-mica (zinnwaldite). In the present work, the author has further been able to identify ishikawaite for the first time in Egypt. This mineral is a variety of samarskite that represents a group of mineral varieties having the general formula (A 3+ B 5+ O 4 ) where A represents Fe 2+, Ca, REE, Y, U, and Th, while B represents Nb, Ta, and Ti. According to Hanson et al. (1999), the complete metamict alteration and the broad variation of cations in A-sites of these mineral varieties render their crystal structures a problematic case. The present paper is thus concerned with characterization of the newly identified mineral species ishikawaite of Abu Rushied mineralized gneissose rocks /08/1037/ $

2 OCCURRENCE OF ISHIKAWAITE 1133 FIG. 1. Geologic map of the Abu Rushied Sikeit area, Southeastern Desert of Egypt (after Ibrahim et al. 2000). Methodology To verify the objectives of this study, a representative sample of the mineralized Abu Rushied gneissose granite was collected (Fig. 1). The sample was crushed, ground, and sieved before the liberated size fractions were subjected to heavy mineral separation using Bromoform (sp. gr. = 2.85 gm/cm3). From the obtained heavy fractions, pure mineral grains were hand-picked and investigated under the binocular microscope. In addition, the studied samples were analyzed using the field emission scanning electron microscope (JEOL 6335F). This instrument is fitted with an Oxford energy dispersive X-ray spectrometer (EDS) for elemental analysis of micro areas, a backscattered electron detector that allows compositional analysis, and a cathode luminescence detector that can image complex, characteristic visible spectra to obtain detailed information on molecular structure. The applied analytical conditions involved 0.5 to 30 accelerating voltage, 1.5 nm (at 15 KV)/5.0 nm (at 1.0 KV). Magnification ranged from 10 to 500,000 with a digital image up to pixels and pixels for image display. Imaging modes are secondary electron imaging (SEI) and backscattered electron imaging (BSI). This instrument can operate in remote locations and can provide X-ray microanalysis of small areas, line scans of relative concentrations for multiple elements, and X-ray maps of relative concentrations for multiple elements. Finally, thin-polished sections of ishikawaite were prepared and analyzed using a JEOL Superprobe 733 with an accelerating voltage of 15 Kv and

3 1134 MOHAMED FAHMY RASLAN FIG. 2. Black massive crystals of U-rich samarskite variety from Abu Rushed radioactive gneiss, binocular microscope. a beam size of approximately 1 micron. The crystals used for the elemental analysis involved TAP (thallium acid phthalate), PET (pentaerythritol), and LIF (lithium fluoride) and the samples were carbon coated (less than 200 angstroms in thickness). The analyzed elements were loaded into the quantitative program (PRZ) and oxygen was made by difference (weight percentage is calculated by subtracting the measured weight). The used standards included niobium (Nb), tantalum (Ta), biotite (Ti-Fe-Si), uranium metal (U), monazite (Th), fluorite (Ca), and cubic zirconia (Zr-Y). Mineralogical and Geochemical Investigation Microscopic examination Under the binocular microscope, the investigated ishikawaite crystals were found to be distributed in almost all size fractions ( to mm), with a tendency to increase with decreasing grain size. The crystals of this samarskite variety were found to be liberated in the coarse size fractions (over mm). It should be noted that soft flakes of dark mica are the predominant mineral (20% by weight of the original sample) in the studied radioactive gneiss, together with other hard accessory heavy minerals (zircon and columbite). Therefore, the variation in hardness and habits between the accessory heavy minerals causes the investigated mineral to be liberated by detachment rather than size reduction (Raslan, 2005), so that ishikawaite occurs as liberated coarse crystals. Ishikawaite in the studied mineralized gneiss occurs as a heavy accessory mineral occurring generally as black translucent massive grains of anhedral to subhedral and granular form. These grains are generally characterized by a dark brown streak and by a resinous to vitreous luster. As illustrated in Figure 2, the ishikawaite grains are usually hard, compact, and metamict. Scanning electron microscope study Several ishikawaite crystals were subjected to semiquantitative analyses using a scanning electron microscope. The obtained SEM data (Figs. 3 and 4) show Nb, U, and Fe as the essential components. Other elements present in small to minor amounts include Ta, Th, Y, and Si. The distribution of uranium and thorium within the crystals is actually heterogeneous, and their contents increase in the bright parts within the crystal. Scan-line elemental analyses along the crystals revealed the presence of silica impurities in most crystals. The contents of Ta and Y generally increase or decrease together, especially in the darker portions of the investigated crystals. On the other hand, the elemental scan map for the U-rich variety of the studied samarskite variety reflects the chemical composition of the

4 OCCURRENCE OF ISHIKAWAITE 1135 FIG. 3. SEM backscattered image of Abu Rushied ishikawaite crystal showing the analyzed small areas within the crystal and line scan together with the corresponding EDX and elemental scan map.

5 1136 MOHAMED FAHMY RASLAN FIG. 4. SEM backscattered image of Abu Rushied ishikawaite crystal showing the line scan within the crystal and corresponding elemental scan map.

6 OCCURRENCE OF ISHIKAWAITE 1137 TABLE 1. Selected EMPA Analyses of Abu Rushied Ishikawaite Oxides Abu Rushied ishikawaite Average 1 4 Ishikawa 3 Kunar 4 TiO Nb SiO b.d.l. 2 b.d.l. Ta MnO UO ZrO b.d.l. CaO ThO FeO Y 2 O ΣREE Total FeO = total iron. 2 b.d.l. = below detection limit. 3 Average of eight microprobe analyses of the Ishikawa sample (Hanson et al., 1999). 4 Kunar uranium-rich samarskite (Hanson et al., 1999). FIG. 5. Backscattered electron image of U-rich samarskite variety (ishikawaite), showing the distribution of EMPA analyses within the crystal. investigated mineral and its silicate impurities. Additionally, silicate impurities were found to be generally free from Nb, Ta, and Fe, whereas U and Th are present in small amounts. Electron microprobe analyses The obtained microprobe analyses (Table 1 and Fig. 5) gave as average in wt%: Nb , Ta , TiO , UO , ThO , Y 2 O ,

7 1138 MOHAMED FAHMY RASLAN TABLE 2. Chemical Formulae Based on 4 Oxygens Sample Ti Nb Si Ta Mn U Zr Ca Th Fe Y ΣREE Sum A Sum B MnO 0.63, ZrO , CaO 0.57, FeO 12.51, SiO , and total REE of 0.10, with average sum equal to wt%. The uranium percentage increases in the outer margins of the crystal (bright zones) and decreases in the core, whereas Nb is enriched in the core (darker zones) and decreases in the rims (Table 1 and Fig. 5). Table 2 shows the chemical and empirical formulae that are recalculated on the basis of 4 oxygens. In the meantime, the four microprobe analyses were plotted on the ternary diagram of Hanson et al. (1999), which shows the A-site occupancy of samarskite-group minerals (Fig. 6). The latter shows that all the data points plot in the ishikawaite field. From the analytical data it is quite clear that the studied mineral reflects the chemical composition of a U-rich samarskite variety in the Abu Rushied mineralized gneiss, which is ishikawaite as indicated by the following evidence: 1. Both samarskite-y and ishikawaite have a dominant Nb in the B-site and the distinction between either variety must be based on the content of B-site occupancy. The obtained EMPA data revealed that Nb 2 is the dominant in the investigated mineral; in wt% it ranges from to with an average of 49.49%. Thus, the studied mineral falls actually within the compositional limits of both samarskite-y and ishikawaite. FIG. 6. Ternary diagram showing A-site occupancy of samarskite-group minerals after Hanson et al, (1999). Abu Rushied ishikawaite is represented by the closed circles. 2. The samarskite group of minerals must comprise only those that have Nb>Ta and Ti in the B-site (Hanson et al., 1999), and the studied mineral contains an average Ta + Ti = 3.69% < Nb= 49.49%. 3. Samarskite-Y has been described as a mineral with Y + REE dominant at the A-site (Nickel and Mandarino, 1987). According to Fleischer and Mandarino (1995), the currently accepted formula of the ishikawaite species is [(U, Fe, Y, Ca) (Nb, Ta) O 4 ] and that ishikawaite was first described as a uranium-rich, REE-poor mineral by Kimura (1922). Also, Cerny and Erict (1989) have described ishikawaite as a probable uranium-rich variety of samarskite. 4. The investigated mineral is actually rich in both uranium and thorium, where the former ranges from to 51.28% with an average of 26.37%, whereas the latter varies from 0.75 to 8.12% with an average of 3.85%. 5. Hanson et al (1999) have proposed a nomenclature for the samarskite group of minerals. They thus classified this group of minerals into three species. If REE+Y is dominant, the name samarskite-(ree+y) should be used with the dominant of these cations as a suffix. If U +Th is dominant, the mineral is properly named ishikawaite, whereas if Ca is dominant, the mineral should be named calciosamarskite. They also reported that ishikawaite and calciosamarskite are depleted in light rareearth elements (LREE) and enriched in the heavy rare-earth element (HREE) Y. The studied Abu Rushied samarskite species contains a Y content

8 OCCURRENCE OF ISHIKAWAITE 1139 ranging from 0.04 to 1.87% with an average of 0.84%, which reflects the enrichment of HREE. 6. The investigated samarskite variety separated from the Abu Rushied radioactive gneiss is characterized by dominant U + Th, Nb> Ta + Ti and relatively rich in Y. 7. In summary, the studied mineral most probably falls within the compositional limits of other ishikawaites cited in the previous literature. Conclusions The uranium-rich samarskite variety (ishikawaite) of the structural formula [(U, Fe, Y, Ca) (Nb, Ta) O 4 ] was recorded for first time in the radioactive gneiss of Wadi Abu Rushied area, Southeastern Desert of Egypt. It is associated with columbite, Hf-rich zircon, and dark Li-mica (zinnwaldite) as indicated from the separated heavy mineral fractions. Ishikawaite is found as massive grains with anhedral, subhedral to granular forms and occurs generally as black translucent crystals with a dark brown streak and resinous to vitreous luster. SEM data proved the presence of silica impurities in most of the studied crystals. The obtained data fall within the compositional limits of other ishikawaites cited in the published literature. It can thus be concluded that Abu Rushied mineralized gneiss should be considered as a potential source for several economic metal values such as Nb, Ta, Zr, Hf, U, Th, Li, and REE. Acknowledgments All the preformed analyses (SEM and EMPA) were carried out during the author s fellowship in the Material Science and Engineering Research Center, Major Analytical Instrumentation Center (MAIC) and Particle Engineering Research Center (PERC), respectively, University of Florida. The author sincerely thanks Prof. Dr. Hassan El-Shall, Professor of Material Science and Engineering, University of Florida, for his interest as well as for providing EMPA analyses. The author sincerely thanks Prof. Dr. N. T. El Hazek, Egyptian Nuclear Materials Authority, for his interest, reading the manuscript, and fruitful discussions. The author wishes to offer his thanks to Prof. Dr. M. E. Ibrahim, Head of the Research Sector, and Egyptian Nuclear Materials Authority for his interest as well as for providing the working sample. REFERENCES Abdel El Aziz, A. A, Bugrov, V., Hassan M. A., et al., 1967, Preliminary report on complex expedition No. 6 (Geological, geophysical and geochemical survey of apogranites and rocks which are bound with them in the erea of Wadi Nugrus. Abu Rushied, Zabara and Hangaliya): Cairo, Egypt: Geological Survey of Egypt, unpubl. report. Abdel Monem, A. A., and Hurley, P. M., 1979, U-Pb dating of zircons from psammitic gneisses, Wadi Abu Rusheid Wadi Sikeit area: Egyptian Institute of Applied Geology, Jeddah, Bulletin, v. 3, p Cerny, P., and Ercit, T. S., 1989, Mineralogy of niobium and tantalum: Crystal chemical relationships, paragenetic aspects, and their economic implications, in (Moller, P., Cerny, P., and Saupe, F., eds., Lanthanides, tantalum, and niobium: Berlin, Germany, Springer- Verlag, p Eid, A. S., 1986, Mineralogy and geochemistry of some mineralized rocks in Wadi El- Gemal, Eastern Desert, Egypt: Unpubl. Ph.D. thesis, Ain Shams University, 165 p. El Gemmizi, M. A., 1984, On the occurrence and genesis of mud zircon in the radioactive psammitic gneiss of Wadi Nugrus, Eastern Desert, Egypt: Journal of the University of Kuwait, Science, v. 11, p El-Ramly, M. F. Greiling, R., Kroner, A., and Rashwan, A. A., 1984, On the tectonic evolution of the Wadi Hafafit area and environs, Eastern Desert of Egypt: Faculty of Earth Sciences, King Abdulaziz University, Jeddah, Bulletin, v. 6, p Fleischer, M., and Mandarino, J. A., 1995, Glossary of mineral species Tucson, AZ: The Mineralogical Record, Hassan, M. A., 1964, Mineralogical and petrographical studies of radioactive minerals and rocks in Wadi- Sikait Wadi El Gemal area, Eastern Desert, Egypt. Unpubl. M.Sc. thesis, Faculty of Science, University of Cairo. Hassan, M. A., 1973, Geology and geochemistry of radioactive columbite-bearing psammatic gneiss of W. Abu Rushied, South Eastern Desert, Egypt: Annals of the Geological Survey of Egypt, v. III, p Hanson, S. L., Simons, W. B., Falster, A. U., Foord, E. E., and Lichte, F. E., 1999, Proposed nomenclature for samarskite-group minerals: New data on ishikawaite and calciosamarskite: Mineralogical Magazine, v. 63, p Ibrahim. M. E., Assaf. S. H., and Saleh. G. M., 2000, Geochemical alteration and spectrometric analyses in Abu Rusheid altered uraniferous gneissose granites, South Eastern Desert, Egypt: Chemie der Erde, v. 60, p Kimura, K., 1922, Ishikawaite: A new mineral from Ishikawa, Iwaki: Journal of the Geological Society of Tokyo, v. 29, p (in Japanese).

9 1140 MOHAMED FAHMY RASLAN Nickel, E. H., and Mandarino, J. A., 1987, Procedures involving the IMA Commission on New Minerals and Mineral Names, and guidelines on mineral nomenclature: Canadian Mineralogist, v. 25, p Raslan, M. F., 2005, Mineralogy and physical upgrading of Abu Rusheid radioactive gneiss, South Eastern Desert, Egypt, in The 9th International Mining, Petroleum, and Metallurgical Engineering Conference, Faculty of Engineering Cairo University, Min. 27. Saleh, G. M., 1998, The potentiality of uranium occurrences in Wadi-Nugrus area, South Eastern Desert: Unpubl. Ph.D. thesis, Faculty of Science, Mansoura University, 171 p.