Chin.J.Geochem.(2011)30:453–478 DOI: 10.1007/s11631-011-0531-5
Mineralogy, geochemistry and age dating of shear zone-hosted Nb-Ta-, Zr-Hf-, Th-, U-bearing granitic rocks in the Ghadir and El-Sella areas, South Eastern Desert, Egypt Ali M.A.1* and Lentz D.R.2 1
Nuclear Materials Authority, 530.P.O. El Maadi, Cairo, Egypt
2
Depatment of geology,University of New Brunswick, NB, Canada
* Corresponding author, E-mail:
[email protected]
Received May 13, 2010; accepted June 10, 2010 © Science Press and Institute of Geochemistry, CAS and Springer-Verlag Berlin Heidelberg 2011 Abstract The rare metal minerals of mineralized altered granites within the Ghadir and El-Sella shear zones, are represented by betafite, U-minerals (uraninite and uranophane), zircon, monazite, xenotime, and rutile in the Ghadir shear zone. While they are columbite-tantalite minerals as ferrocolumbite, pyrochlore, and fergusonite, Th-minerals (cheralite, uranothorite, and huttonite monazite), Hf-zircon, monazite and xenotime in the El-Sella shear zone. Hf-zircon in the El-Sella and Ghadir shear zones (increasing from the core to the rim) contains high inclusions of U-Th, and REE minerals such as cheralite, uranothorite, huttonite monazite and xenotime especially in the El Sella shear zone. The rare-metal minerals, identified from peralkminous granites of the shear zones are associated with muscovite, quartz, chlorite, fluorite, magnetite, and biotite that are restricted to the two shear zones. Uraninite (low Th content) occurring in the Ghadir shear zone indicates the hydrothermal origin, but there are thorite, uranothorite, cheralite, and Hf-zircon in the El Sella shear zone, also indicating the hyrothermal proccess after magmatic origin. Compositional variations of Ta/(Ta+Nb) and Mn/(Mn+Fe) in columbite from 0.07–0.42 and 0.04–0.33, respectively, and Hf contents in zircon are so high as to be 12%, especially in the rim in the El Sella shear zone. This feature reflects the extreme degree of magmatic fractionation. Four samples from the altered granites of the Ghadir shear zone also are very low in TiO2 (0.04 wt%–0.17 wt%), Sr [(82–121)×10-6], and Ba [(36–380)×10-6], but high in Fe2O3T (0.46 wt%–0.68 wt%), CaO (0.64 wt%–1.23 wt%), alkalis (8.59 wt%–8.88 wt%), Rb [(11–203)×10-6], Zr [(98–121)×10-6], Nb [(9–276)×10-6], Ta [(2–139)×10-6], U [(14–63)×10-6], Th [(16–105)×10-6], Pb [(13–32)×10-6], Zn [(7–8)×10-6], Y [(15–138)×10-6], Hf [(3–9)×10-6], and ∑REE [(81–395)×10-6, especially LREE [(70–322)×10-6]. They are very high in Zr/Hf (15.07–85.96) and Nb/Ta (7.17–21.48), and low in Rb/Sr (2.56–3.36) and Th/U (0.096–3.36). Four samples of the altered granites from the El Sella shear zone are very low in TiO2 (0.23 wt%–0.38 wt%), Sr [(47–933)×10-6], and Ba [(82–175)×10-6] , with high Fe2O3T (1.96 wt%–2.87 wt%), CaO (0.43 wt%–0.6 wt%), alkalis (4.46 wt%–10.7 wt%), Rb [(109–313)×10-6], Zr [(178–1871)×10-6], Nb [(11–404)×10-6], U [(56–182)×10-6], Th [(7–188)×10-6], Ta [(0.5–57)×10-6], Pb [(12–28)×10-6], Zn [(1–13)×10-6], Y [(62–156)×10-6], Hf [(3–124)×10-6], and ∑REE [(101–184)×10-6], especially HREE [(7–139)×10-6]. This is consistent with the very fractionated, fluorine-bearing granitic rocks that were altered and sheared in the El Sella shear zone. Zr/Hf (14.23–39.79) and Nb/Ta (1.98–7.01) are very high, and Rb/Sr (0.14–1.7) and Th/U (0.25–2.5) are low in the Ghadir shear zone. Field evidence, textural relations, and the composition of ore minerals suggest that the main mineralizing event was magmatic (615+/-7 Ma, and 644+/-7 Ma CHIME monazite), especially in the El Sella shear zone, with later hydrothermal alteration and local remobilization of the high-field-strength elements. Key words uranium; thorium; zircon; columbite-tantalite; monazite; xenotime; two mica-granite; age dating; Ghadir shear zone; El Sella shear zone; South Eastern Desert, Egypt
1 Introduction Generally, granitic rocks are the favourable host www.gyig.ac.cn www.springerlink.com
rocks for U-mineralization in many parts of the world. Genetically, the economic uranium deposits associated with granitoid rocks are mostly located in anatectic
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melts or in strongly peraluminous two-mica granites (Cuney et al., 1984; Poty et al., 1986; Friedrich et al., 1987, 1989). In general, the granites are rich in incompatible elements (Rb, Nb, Sn, Th, and U) (Rameshbabu, 1999; Pal et al., 2001, 2007). The genetic relationship between granite intrusions and associated uranium mineralization has been discussed for a long time. Uranium existing in granites can be genetically divided into two categories; primary uranium and secondary uranium (Jiashu and Zehong, 1982). The former is fixed in rocks during magma crystallization, while the latter is precipitated in various geological events later from dissolved and transported uranium, which in turn comes from the primary uranium. The secondary uranium can be subdivided into three types: (1) absorbed uranium in altered minerals such as montmorillonite, chlorite and limonite; (2) interstitial uranium at the grain boundaries, formed as a result of hydrothermal solution migration along the interstices of minerals in granites; and (3) uranium in microfractures, the formation of this kind of uranium takes place during the circulation of hydrothermal solutions after the deformation of rocks. These three types of secondary uranium are regarded as the final preserved products through all geological processes such as metasomatism, hydrothermal alteration and weathering alteration. Egyptian granitic rocks of Pan-African age accounting for about 40% of the exposed Precambrian of the Eastern Desert and Sinai are subdivided into two distinct major groups, namely the Older and Younger granites. The Older granites have been referred to in the Egyptian literature as Grey granites (Hume, 1935; El-Ramly and Akaad, 1960), syn- to late-orogenic plutonites (El-Shazly, 1964), synorogenic granites (El-Gaby, 1975) and G1-granites (Hussein et al., 1982). They were emplaced around 930–850 Ma ago, and possibly extended to 711 Ma (El-Manharawy, 1977; Hashad, 1980; Dixon, 1981; Stern and Hedge, 1985; Hassan and Hashad, 1990). On the other hand, the younger granites (about 30% ) were previously mapped as Gattarian granites (Hume, 1935), red and pink granites (El-Ramly and Akaad, 1960), late to post-orogenic granites (El-Gaby, 1975) and G-II to G-III granites (Hussein et al., 1982). They were emplaced around 622–430 Ma ago (El-Manharawy, 1977; Hashad, 1980; Dixon, 1981; Stern and Hedge, 1985; Hassan and Hashad, 1990; Abdel Wahed et al., 2002; Moghazi et al., 2004; Moussa et al., 2008). Some of the younger granites are associated with a major part of the radioactive anomalies and host the most interesting uranium mineralization. Most Egyptian uranium occurrences in the younger granites of Gabal Um Ara (Ibrahim, 1986; Abdalla et al., 1994), Gabal Gattar (Roz, 1994), Gabal Missikat (Abu Dief,
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1985), Gabal Erediya (Hussein et al., 1986; Abu Dief, 1992; Osmond et al., 1999; Abdel Naby, 2008), and Gabal El Sella (Assaf et al., 1996; Ibrahim et al., 2005) belong to metaluminous to slightly peraluminous granites. Uranium mineralization in these granites belongs to the vein type except at Gabal Gattar and Gabal Um Ara, where uranium mineralization belongs to the fracture-filling type. Few peraluminous two-mica (muscovite-biotite) granites in Egypt are associated with visible U-mineralization (Ibrahium et al., 2005; Ibrahium and Ali, 2003). In general, the marginal parts of all these granitoid intrusions are more radioactive than their central parts due to the alteration of the marginal parts, which includes albitization (sodic metasomatism), microclinization (potassic metasomatism), ferrugination, silicification and kaolinitization, automatically giving a measure of its fertility. 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). Stern and Hedge (1985) and Sultan et al. (1988) suggested through their compilation of Landsat thematic mapper scenes of the Arabian-Nubian Shield (ANS) that the Najd system extends into the Egyptian Eastern Desert and dominates the structural pattern within its central part. 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 stress; however, both types of deformation led to antiformal structures on a regional scale.
2 Analytical methods Eight mineralized samples were studied in detail,including four samples from the Ghadir shear zone and four samples from the El Sella shear zone. The major-, trace-, and REE-element compositions were measured by pressed pellet XRF (Longerich, 1995) and ICP-MS (Inductively Coupled Plasma-Mass Spectrometry) techniques at the laboratories of Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada. Polished thin sections were studied under reflected and transmitted light in order to determine mineral associations and parageneses. Backscattered electron images were collected by scanning electron microscopy-energy dispersive spectrometry (BSE) (Model JEOL 6400 SEM) at the Microscopy and Microanalyses Facility, University of New Brunswick (UNB), Fredericton, New Brunswick, Canada. Mineral compositions were determined on the JEOL JXA-733 superprobe; operating conditions were 15 kV, with a beam current of 50 nA and peak counting time was 30 second for all elements. The standards used are jadeite,
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kaersutite, quartz, and apatite (for Na, Al, Si, P, and Ca, respectively ), SrTiO3 (for Ti), CaF2 (for F), Fe, Nb, Hf, Ta, Sn, Th, and U metals (for Fe, Nb, Hf, Ta, Sn, Th, and U, respectively), YAG (for Y), cubic zirconia (for Zr), La-, Ce-, Nd-, Sm-, Pr-, Er-, Gd-, Eu-, Tb-, Dy-, and Yb-Al; Si-bearing glass (for La-, Ce-, Nd-, Sm-, Pr-, Er-, Gd-, Eu-, Tb-, Dy-, and Yb-) and crocoite (for Pb).
3 Geology and petrography of the Ghadir Shear zone El-Sharkawy and El-Bayoumi (1979) classified the rocks that crop out along the eastern Ghadir as granites belonging to the older and younger (Gattarian) granites. Takla et al. (1992) classified the granitoids of Wadi Ghadir as (a) diorites and granodiorites that are similar in their characters to the older granites and (b) the younger granites that have characters similar to alkalic and other anorogenic granitic suits. The granitoid rocks in the Ghadir area have been subdivided according to their field relationships, petrography and geochemical characteristics into two types of magmatic granites by Ibrahim and Ali (2003). Granites of the first magmatic type include quartz diorite and gneissose granodiorite, which have a metaluminous character and were emplaced during pre-plate collision under high water-vapour pressure. The second magmatic type is represented by perthitic-leucogranite and muscovite-biotite granite and this type of granites was emplaced during syn-collision at moderate water-vapour pressure (Ibrahim and Ali, 2003). Older granitoids represent the oldest rock unit in the study area (Fig. 1). Muscovite-biotite granites intruded into the older granitoids and several xenoliths
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were taken from these rocks of different sizes and shapes. They are pink in color and characterized by exfoliation and cavernous weathering. All the rock units are dissected by numerous mafic (basalt and dolerite) and alkaline bostonite dykes, which mainly extend in the NW-SE, NE-SW and nearly E-W directions. These faults are mainly of the strike-slip type (sinistral) and vary in length from 1.5 to 4 km. The Ghadir shear zone (altered granite) is trending NE-SW and dissects the southeastern part of Gabal Ghadir two-mica granites and extends for 0.5 kilometer in length and several meters from about 0.5 to 5 meters in width (Figs. 2 and 3a). The two-mica granite becomes mylonitized and cataclased within the shear zone. Ferrugination, silicification and kaolinitization with few dark patches of manganese dendrites are the main rock alterations developed within the studied shear zone (Fig. 3b). Uranophane is present as needle-like aggregates associated with iron oxides filling fractures and varying in colour from pale yellow to orange yellow. Petrography, all constituents show cataclastic effects and have corroded edges. Quartz (30%–40%) sometimes shows clear signs of mylonitization and annealing (Fig. 3b). Brecciation took place prior to and/or is contemporaneous with hydrothermal solutions. Alkali feldspars are represented by the string-type perthite with subordinate microcline. They are penetrated by relatively fine-grained veinlets of quartz and are kaolinized along the cleavage planes and fractures. The cracks promoted the movement of late iron- rich hydrothermal solutions, which caused iron staining and gave the rock its red coloration. Cataclased albite (5%) is commonly altered to sericite and the abundance of sericite increases in the
Fig. 1. Geological map of the Ghadir area, South Eastern Desert, Egypt (modified after Takla et al., 1992).
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mineralized samples. Accessories are mainly metamectic zircon, monazite, xenotime, rutile, fluorite and opaques (Fig. 3c). Primary mineral of uraninite is observed in a cubic mineral in the studied sections either as a primary phase associated with iron oxides (Fig. 3d) or as a secondary phase (uranophane) resulting from the alteration of other primary minerals (uraninite?). The petrographic examination suggests that uranophane is present in the form of long acicular radiating aggregates associated with iron oxides and displays bright yellow and blue interference colours (Fig. 3d).
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the Ghadir shear zone and is formed due to reduction reaction. It is present as minute euhedral crystals associated with monazite, xenotime, and zircon and it ranges from 20 to 50 µm in size (Fig. 4a). Uraninite crystals are characterized as being much lighter in colour under scann electron microscope than the other uranium minerals. The EPMA analyses of these crystals revealed the chemical composition of uraninite. These results indicated that the major elements in uraninite are UO2 (77.66%), ThO2 (0.02%), and PbO (0.042%) within the elemental composition of phosphorous (P2O5=12.81), CaO (5.55) and K2O (1.46). Also, minor amounts of LREE and Y were reported as being substituted by monazite (Table 1). 4.1.2 Secondary U-minerals In the Ghadir shear zone and in the oxidation zone, U4+ is oxidized to uranyl ion U6+ which is soluble and mobile in solutions and easily combined with other cations such as Ca2+, and with anions such as silicates to form secondary uranium including uranophane. 4.1.2.1 Uranophane (CaO·2UO3·2SiO2·6H2O)
Fig. 2. Map of the shear zone in the Ghadir muscovite-biotite granite, South Eastern Desert, Egypt.
4 Mineralogy of the Ghadir shear zone 4.1 Uranium and thorium mineralization 4.1.1 Uraninite (UO2) Uraninite is a common accessory mineral occurring in pegmatites and peraluminous granites, and is probably the most important source of dissolved U in groundwaters emanting from weathered granite terrains (Frondel, 1958; Förster, 1999). Uraninite is isometric (fluorite structure type, Fm3m) with the nominal composition UO2+x(Z=4), however, pure UO2 is not known in nature, being alaways at least partly oxidized (xCa, U, Th) (Nb, Ta)O4] It occurs only in the muscovite-biotite granite of the El Sella shear zone and is founded as an alteration product phase, light gery in color and containing potential concentrations of Y, U, Th and Ca. The chemical analyses of SEM-BSE images and EPMA anaylses were obtained. The type of the columbite series is composed of Y, Nb, Ta and U (Fig. 7c, f and Table 2). 6.2 Accessory minerals Mineralogical examinations of the bulk accessory heavy minerals revealed the presence of zircon (Hf), xenotime, monazite, rutile and pyrite. The present study presents a brief description of accessory heavy minerals in the rare metal granites as follows: 6.2.1 Zircon (ZrSiO4) Hussein (1978) and Abdalla et al. (2008) were stated that the radioactive zircon was usually zoned. The radioactive zircon is also characterized by
metamictization. The explanation of the origin of “Metamict State” is that the internal order of an originally crystalline form has been destroyed by ∝-particle bombardment from radionuclides within the structure. Zircon may be partially or completely modified giving amorphous zircon with more isotropic character; such a mineral is called metamict zircon. Zircon crystals in the studied mineralized ganites are mainly characterized by high metamictization. It is known that zircon can incorporate uranium into its lattice and enclose radioactive materials as mineral inclusions. According to Knorring and Hornung (1961), Nb and Ta mineralizations are generally associated with Hf-zircon. Zircon has been found as the most abundant accessory mineral in the studied granites of the El Sella shear zones. In the study area, zircon occurs as euhedral prismatically zoned crystals with clusters of inclusions of radioactive minerals (Fig. 8a–f). They are mostly pale to light grey in colour in reflected light. In the studied younger granites, it is characterized by intense inclusions. They also possess inclusions of other zircon crystals rich in HREE and Hf (Fig. 8a–f). The EMPA analysis (Table 4) reflects the chemical composition of zircon. The core of zircon in the El Sella zone contains high inclusions of radioactive minerals but it has low Hf contents. Its rim has high Hf (12%) contents (Table 4). These inclusions are found in the zircon as inclusions (uranothorite, monazite, xenotime and cheralite).
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6.2.2 Monazite [(LREE, Th)PO4] It is the most common accessory mineral in many magmatic and metamorphic rocks, especially in the rocks which are characterized by mildly to strongly peraluminous compositions. Under binocular microscope, monazite crystals are mainly subhedral to anhedral, and are included in the columbite and zircon minerals (Fig. 8c, e). The studied monazite crystals have high thorium contents. The ESM-BSE images reflect the chemical composition of monazite which is composed of LREE and Th-U minerals (Fig. 8c, e). 6.2.3 Xenotime (YPO4) It is an accessory mineral in granites and occurs in zircon and columbite crystals. The studied xeotime crystals have high REE contents. The ESM images reflect the chemical composition of xenotime which is composed of Yand P with REE elements (Fig. 7b, e). 6.2.4 Pyrite (FeS2) Pyrite is the main sulphide encountered in the Table 4 Rock Sample SiO2 ZrO2 HfO2 P2O5 CaO FeO MnO TiO2 Y2O3 Ce2O3 Tb2O3 Dy2O3 Yb2O3 PbO ThO2 UO2 Total Si Zr Hf P Ca Fe Mn Ti Y Ce Tb Dy Yb Pb Th U Zr/Hf Th/U
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study area. Generally, pyrite is either disseminated in the El Sella shear zone or partially or entirely oxidized to oxy-hydroxides such as hematite and goethite. This process can be classified as pseudomorphic desulphitization under oxidizing conditions. Desulphitization of pyrite precursor creates voids that can be refilled by secondary materials enriched in U, REE and Ti. Pyrite occurs as well developed cubic octahedron crystals with a pale-brass yellow color and a metallic luster (Fig. 6d). It mainly contains Fe and S with Ni and Mo as impurities. This explains the reducing conditions during the late stage of columbite crystallization, which is also responsible for the formation of pyrite. 6.2.5 Rutile (Ti3O4) It is most common in igneous rocks, especially in the mineralized granites, but it is very important in sediments and their metamorphosed equivalents (Deer et al., 1966). In the mineralized shear zone of El Sella muscovite-biotite granite, magnetite occurs as circular crystals in the form of aggregates with monazite and zircon.
Microprobe analyses for zircon from the El-Sella shear zone, South Eastern Desert, Egypt
El Sella El Sella El Sella El Sella Zircon Zircon Zircon Zircon rim core innerrim core, no (Z2) (Z1) (Z3) inclusion (Z4) 33.09 31.55 30.57 30.62 59.01 56.27 49.27 55.28 3.79 8.94 11.68 5.36 0.691 0.2579 0.5833 1.0514 0.000 0.0000 0.4745 0.1360 0.396 0.3484 1.5240 1.1042 0.000 0.0000 0.0000 0.0000 0.000 0.0000 0.0000 0.0000 0.413 0.2268 0.5564 0.9803 0.084 0.0038 0.2610 0.0342 0.000 0.0000 0.0000 0.0000 0.081 0.0765 0.2185 0.2943 0.747 0.2270 0.5671 0.7910 0.031 0.0000 0.0000 0.0000 0.000 0.1871 0.2242 0.2845 0.746 1.0565 1.4468 1.7058 99.09 99.14 97.38 97.64 Structure formula on the basis of 4 oxygen atoms (apfu) 1.021 0.9738 0.9435 0.9451 0.886 0.8449 0.7398 0.8300 0.036 0.0846 0.1106 0.0507 0.017 0.0064 0.0146 0.0263 0.000 0.0000 0.0132 0.0038 0.010 0.0087 0.0381 0.0276 0.000 0.0000 0.0000 0.0000 0.000 0.0000 0.0000 0.0000 0.009 0.0047 0.0116 0.0204 0.002 0.0001 0.0054 0.0007 0.000 0.0000 0.0000 0.0000 0.002 0.0016 0.0046 0.0061 0.016 0.0047 0.0165 0.0165 0.032 0.0000 0.0000 0.000 0.000 0.0061 0.0073 0.0093 0.007 0.0106 0.0145 0.0171 15.57 6.29 4.22 10.31 0.000 0.18 0.16 0.17
El Sella Zircon rim, no inclusion (Z5) 33.86 65.53 1.4994 0.0974 0.0000 0.1426 0.0000 0.0000 0.0403 0.0000 0.0000 0.0608 0.0960 0.0029 0.0302 0.4414 101.78 1.0451 0.9839 0.0142 0.0024 0.0000 0.0036 0.0000 0.0000 0.0008 0.0000 0.0000 0.0013 0.0020 0.0001 0.0010 0.0044 43.70 0.07
El Sella
El Sella
El Sella
El Sella
Zircon (Z6) core
Zircon rim (Z7)
Zircon core (Z8)
Zircon rim (Z9)
32.90 62.07 2.9213 0.3298 0.0122 0.1624 0.0000 0.0000 0.1746 0.1070 0.0000 0.0843 0.4752 0.0507 0.0000 0.7286 100.02
31.33 64.55 5.88 1.1245 0.0308 0.5847 0.0000 0.0000 1.2342 0.1295 0.0000 0.1975 0.7794 0.0000 0.2919 0.9783 98.04
31.20 56.14 5.69 0.5784 0.1051 1.1832 0.0000 0.0000 0.3330 0.0000 0.0269 0.1085 0.6140 0.0057 0.0150 1.0388 97.04
31.20 59.35 2.3497 0.2149 0.3491 1.0539 0.0000 0.0000 0.2051 0.0076 0.0000 0.1780 0.6017 0.0000 0.1690 1.1082 96.99
1.0154 0.9320 0.0277 0.0082 0.0003 0.0041 0.0000 0.0000 0.0036 0.0022 0.0000 0.0018 0.0099 0.0011 0.0000 0.0073 21.25 0.0000
0.9669 0.8330 0.0557 0.0281 0.0009 0.0146 0.0000 0.0000 0.0257 0.0027 0.0000 0.0041 0.0162 0.0000 0.0096 0.0098 9.43 0.2984
0.9629 0.8429 0.0539 0.0145 0.0029 0.0296 0.0000 0.0000 0.0069 0.0000 0.0006 0.0023 0.0128 0.0001 0.0005 0.0104 9.87 0.38
0.9691 0.8911 0.0223 0.0054 0.0097 0.0263 0.0000 0.0000 0.0043 0.0002 0.0000 0.0037 0.0125 0.0000 0.0055 0.0111 25.26 0.15
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Fig. 7. SEM-BSE images: a. U-Th minerals as inclusions in zircon in the El Sella shear zone; b. uranothorite and xenotime as inclusion in zoned zircon (Hf); c. cheralite associated with pyrochlore and fergusonite in the El Sella shear zone; d. huttonit monazite and uranium columbite associated with Hf (zircon); e. (Hf) zircon enriched in uranothorite and xenotime associated with pyrochlore; f. fergusonite associated with pyrochlore with zircon.
7 Geochemistry of the mineralized shear zones The major, trace and REE elements for the eight mineralized samples were measured by XRF and ICP-MS at the Labratories of Department of Earth Sciences, Center for Earth Research, Memorial University of Newfoundland, St. John,s, Newfoundland, A1B3X5, Canada. The complete chemical analysis with some ratios of the two shear zones (Ghadir granite (4 samples) and El Sella granite (4 samples) are listed in Table 5. From the table it is seen that the alteration processes maintained serious enrichment in all major oxides, although at different magnitudes,
except for one alkali oxide, K2O, which is slightly depleted, especially in the El Sella shear zone. Silica and total iron oxides are the most enriched oxides in the alteration zone, especially in the Ghadir and El Sella shear zones, respectively. Al2O3 is enriched in another fresh granite, indicating alteration processes such as kaloinization. The fractionation of trace elements seems to be more prominent, especially for the REEs which exhibit very serious enrichment. Geochemically the mineralized Ghadir shear zone granites are characterized by their higher contents of some metals such as U, Th, Nb, Y, Rb, Cr, and Ga but depletion in Zr, and Hf. While in the El Sella shear zone granites are characterized by their higher
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contents of some metals such as U, Th, Nb, Pb, Y, Zr, Hf, Cr, V, Ni and Ga (Fig. 8). These metals are incorporated with some high REE in the secondary uranium-thorium minerals such as monazite, xenotime and zircon. This is confirmed by the higher values of U and Th contents ranging from 14×10-6 to 63×10-6 and 56×10-6 to 182×10-6, respectively in the Ghadir shear zone but ranging from 16×10-6 to 105×10-6 and 7×10-6 to 188×10-6, respectively in the El Sella shear zone (Table 5). The possible economic potential for these occurrences is justified by their enhanced average U contents in the Ghadir and El Sella shear zones, which are 33.5×10-6 and 123.2×10-6, respectively, higher than the regional Clark value recommended for silicic igneous rocks (about 4.7×10-6), with low Th/U ratio (
