Research Article, Geoinfor Geostat An Overview Vol: 6 Issue: 2

Geochemical Characteristics and Radioactive Elements Estimation along Trenches of Um Ara area, South Eastern Desert, Egypt

Saleh GM^* and Kamar MS

Nuclear Materials Authority, El-Maadi, Cairo, Egypt

*Corresponding Author : Saleh GM
Professor, Nuclear Materials Authority, El- Maadi, Cairo, Egypt
Tel: +20 502262562
E-mail: drgehad_m@yahoo.com

Received: Februay 19, 2018 Accepted: March 28, 2018 Published: April 04, 2018

Citation: Saleh GM, Kamar MS (2018) Geochemical Characteristics and Radioactive Elements Estimation along Trenches of Um Ara area, South Eastern Desert, Egypt. Geoinfor Geostat: An Overview 6:2. doi: 10.4172/2327-4581.1000178

Abstract

Keywords: Radioactive minerals; Um Ara granites; Uranophane; Microclinization; Mining work; Egypt

Introduction

The Arabian-Nubian Shield (ANS) forms a suture between the East and West Gondwana at the northern end of the East African Orogen, which encompasses the ANS in the north and the Mozambique Belt in the south. The Arabian-Nubian Shield, covering large parts (~ 3×106 km²; Kusky et al. [1]) of NE Africa, Sinai and Arabia. It evolved during the Pan-African orogeny. It has been suggested that the evolution of the Arabian-Nubian Shield was neither entirely ensialic nor ensimatic, but it represents a complex amalgamation of arcs, oceanic plateaus and microcontinents assembled during Neoproterozoic closure of the Mozambique Ocean [2,3]. The evolution of the ANS encompasses four major tectono-magmatic episodes that occurred between 800 and 550 Ma [4]. These are: (1) rifting of the supercontinent Rodinia (~ 800 Ma; Kampunzu et al. [5].; Loizenbauer et al. [6].), (2) sea-floor spreading, arc and back arc basin formation, and accretion of the juvenile crust (870-690 Ma), (3) continental collision (750-650 Ma), and (4) east-west crustal shortening (640-550 Ma) when East and West Gondwana collided.

The Egyptian basement complex of the Eastern Desert (ED) and South Sinai constitutes its northwestern segment. The Arabian- Nubian Shield (ANS) is regarded the best preserved and largestexposed area of Neoproterozoic juvenile continental crust on the Earth [7-9]. The ANS juvenile crust has evolved during most of the Neoproterozoic time (900-600 Ma; [4]).

Granitoid rocks in the Eastern Desert of Egypt have long been classified as syn- to late-orogenic granitoids (Older granitoids) and post-orogenic to anorogenic granites (Younger granites) [10-12]. The older granitoids range from quartz diorite to rare true granite (predominantly muscovite trondhjemite, tonalite, and granodiorite). These are I-type, subduction-related plutons, with ages between 870 and 614 Ma using Rb-Sr and U-Pb methods [2,13,14]. Younger granites are mainly monzogranites, syenogranites, and alkali-feldspar granites, I-type (partly subduction-related) but mainly alkaline to peralkaline A-type plutons, formed between 610 and 550 Ma based on the Rb-Sr and U-Pb dating [15] and developed during extension.

The Egyptian granitoids are of I- and A- type affinity, and were generated either by partial melting of mafic lower crust or by fractionation of a mantle-derived magma [14,17-19]. The younger granitoids are widely distributed in the Nubian Shield of Egypt, constituting approximately 30 % of its plutonic proportion [20]. Recently, El Sayed [16] classified the younger granitoids into orogenic I-type arc-related, orogenic A-type arc-related, and anorogenic A-type rift-related granites. Most of the geological work on the younger granitoids in Egypt dealt with the arc-related types and little attention has been given to the post-orogenic and anorogenic granitoids [21,22].

Alkali-metasomatism refers to the geological behavior by which alkali (K or Na)-rich hydrothermal fluids interact with the country rocks and form the altered rocks mainly composed of albite or potassium feldspar and accompanied by some other altered minerals. It is impossible for potassium and sodium in the nature to occur simultaneously in the hydrothermal fluids, the main alteration could be observed in alkali-metasomatism is single albitization or single feldsparization, when the albitization dominates, it could be called as alkali-metasomatism; when the feldsparization dominates, it could be called as potassium-metasomatism. Alkali-metasomatic type uranium deposit is one of the main industrial uranium deposits, has been discovered around the world [23]. K-metasomatism is characterized by replacement of sodic and calcic phases by potassic phases. Sometimes replacement of K instead of Na and Ca may accompanied by silication and addition of metals and hydration.

The common Egyptian uranium occurrences are mainly: veintype (G. El-Missikat [24], G. Sella [25,26]), metasomatised granites (G Um Ara [27]), shear zones in calc-alkaline granite (G Gattar [28], two mica granites (G Nasb Aluba [29] and G Ribdab [30]) and shear zones hosted lamprophyre bearing-REEs and U in Abu Rusheid area [31]. Uranyl silicates are typical alteration phases of the primary uranium mineralization of the alteration zone of many uranium deposits. Uranyl silicate minerals are typical representatives of the long-term alteration products, indicating usually late stage, or the high-grade of weathering. Uranyl silicates, as uranophane and/or kasolite, are common constituents of massive aggregates of alteration phases, replacing primary uraninite in situ; aggregates of uranyl silicates occur often along with uranyl phosphates in the strongly oxidized portions of the primary mineralization [32].

Although previous investigations showed the presence of uraninite as a primary uranium mineral in the alkali-feldspar granites [33,27], more recent studies [35,36] reported only the occurrence of uranophane and β-uranophane as the main uranium minerals.

Abdel Naby et al. [37] suggested a two-stage metallogenetic model for the alteration processes and origin of U- and Th-bearing minerals in Um Ara area: 1) the first stage was dominated by hydrothermal alteration and accompanied by albitization, k-feldspathization, desilicification, chloritization, hematitization, silicification, argillization,fluoritization and corrosion of primary U-bearing minerals. Solid-solution between thorite and zircon occurred during this stage. The second stage occurred by mobilizing the early formed primary U-bearing minerals near-surface by the action of meteoric water. Uranium was transported as a calcium uranyl carbonate complexes. When these complexes lost their stabilities by precipitation of calcite, they decomposed in the presence of silica to form uranophane.

The main objectives of this study were to investigate and determine the geochemical alteration processes along trenches and drilled borehole at the northwestern part of Um Ara granitic pluton, which is totally composed of fine grained alkali feldspar granites rich by uranium and thorium mineralization.

Geologic Setting and Petrography

The google earth image covers a major part of Um Ara area (Figure 1) was used for general rock type identification and association. From the studies of the different geological features, rock types, tectonics and mineralization of Um Ara area (Figure 2) can be classified into Dokhan volcanic and post-collision granitoids (monzogranite and alkali feldspar granite), and associated with dykes and quartz veins. The geology of Um Ara area has been extensively studied by many authors [27,33,38,40,64].

Dokhan volcanics

The investigated Dokhan volcanic (DV) rocks range in colour from green, pink and brown to purple and greyish black. The Dokhan volcanics of Um Dubr, based on field relation, could be subdivided into the following two types: a) Lower Dokhan volcanics (LDV), they crop out in the form of three elongated parallel belts of NNW-SSE trend and are conformably overlain by rocks of the upper (acidic) subdivision [42]. They are intruded by younger granites, and are dissected by different dyke swarms. Andesite, quartz andesite, imperial porphyry and amygdaloidal andesite and pyroclastics are recognized. b) Upper Dokhan volcanics (UDV) occur in the form of two elongated belts of NNW-SSE and N-S trends [42]. These rocks are characterized by a moderate to high topography and more felsic ones. The rocks are commonly porphyritic, massive and white to pink in colour on fresh surfaces with euhedral sulphide disseminations. Thus the upper Dokhan volcanic and their equivalents in the Eastern Desert and Sinai may represent a good target for exploration for uranium in Egypt [42].

Monzogranites

The monzogranite (coarse grained granites) is an oval-shaped pluton trending N-S to NW-SE. It is coarse-grained, massive of pale pink colour and exhibits an equigranular texture. The rock consists essentially of k-feldspar, quartz, plagioclase and biotite. Accessory minerals are opaques, apatite, titanite and zircon, while muscovite and chlorite are secondary minerals. The K-Ar age for Um Ara monzogranite is 589Ma and for the other granites 556Ma [39]. K-feldspar is euhedral to subhedral crystals up to 2.5mm across, in some places altered to clay and sericite. The perthite and antiperthite textures were existed. The perthitic films are arranged subparallel to each other and parallel to the cleavage planes of k-feldspar host. Quartz is present as large subhedral as well as fine-grained crystals occupying interstices between other constituents. It is also enclosed by both microcline and plagioclase crystals forming graphic and myrmekitic textures. Plagioclase is oligoclase in composition (An_{12- 20}) occurs as euhedral to subhedral prisms up to 3.5mm. Biotite is the dominant ferromagnesian mineral in this rock type, sometimes interlayered with chlorite. It occurs as long flakes as well as very short laths. It may contain minute inclusions of titanite, apatite, quartz and zircon (Figure 3). Opaques are present as disseminated crystals of irregular outlines or as fine aggregates in association with biotite. Muscovite has been observed in a few samples as fine alteration aggregates.

Alkali-feldspar granites

It is subsolvus, fine- to medium-grained, equigranular of light yellowish-brown to pink colour. The alkali-feldspar granites (fine grained granites) send apophyses and off shoots (ranging in thickness from 1.5 cm to 250 m) into the northern Dokhan volcanics of Gabel Um Dubr [42]. Visible secondary uranium crystals are identified in hand specimens of the alkali-feldspar granite. They occur as stains along the fracture surfaces and as acicular crystals filling cavities. The later hydrothermal activities were manifested by hematitization, albitization, K-feldspathization, chloritization, silicification with partial kaolinization together with the frequent presence of manganese oxides, green and violet fluorite and uranium mineralization (uranophane) (Figure 4).

Detailed geological map was constructed by N-S traverses. The mapping was accompanied by a systematic sampling (16 samples in trenches) at 50-100 m intervals from Um Ara mining area (Figure 2). Sixteen trenches (T) have been digged (Figure 5) in the fine grained granites to explore and investigate the alteration processes and mineralization with depth. Field observations and microscopic examinations revealed the presence of three modes of occurrence for the rare metals mineralization associated with the Um Ara granite area. These modes are: (1) disseminated mineralization of columbite, Nb-Mn-Zn rich ilmenite, Hf-rich zircon, xenotime, thorite, monazite, (2) uranium crystals and most commonly as fracture-filling and (3) minor placer occurrences of columbite, zircon, xenotime and thorite [37]. Two modes of occurrences are exhibited by the uranophane in the Urn Ara granites; disseminated in the granites and fracturefillings in the amazonitized and albitized granites. The fracture-filling uranophane is closely associated with late stage Fe and Mn oxide alteration.

Petrographically, it consists mainly of quartz, perthitic microcline with orthoclase, plagioclase of oligoclase composition, biotite and muscovite. Accessories are fluorite, garnet, titanite, apatite, and opaques. Quartz occurs as unstrained, anhedral to subhedral grains up to 1.5mm across. Locally they form granophyric, graphic and worm-like myrmekitic intergrowths. K-feldspar is medium-grained up to 3.0 mm consisting of stringers and patches perthitic crosshatched microcline with minor orthoclase (Figure 6). Some orthoclase perthite crystals exhibit simple Carlsbad twinning. Plagioclase crystals are medium-grained and partly saussuritized. The exsolved fine-grained plagioclase is fresh relative to the host microcline and orthoclase.

Biotite crystals are subhedral, pale reddish-brown colour and slightly altered to chlorite. Muscovite occurs as long prisms and small laths, and as inclusions in plagioclase with preferred orientation. Corroded boundaries of muscovite are common as well as wavy extinction.

Fluorite crystals are intergranular to orthoclase perthite indicating their late crystallization. Euhedral crystals of garnet are present through some samples of the alkali feldspar granite. Opaque minerals occur as subhedral grains as well as irregular granules mostly enclosed by biotite. Titanite occurs either as well developed small crystals always associated with the biotite or as thin rims around titaniferous opaques. Apatite occurs as short prisms or minute needles or sixsided grains enclosed in plagioclase feldspar and biotite. The presence of leached biotite, hematite and fluorite clots in the alkali feldspar granite indicates the role of fluid- and fluorine-rich volatile activity.

Geochemistry

Analytical methods

Sixteen (16) and ten (10) samples were collected representing the altered granites and core samples respectively. The collected samples were chemically analyzed using different analytical procedures at the laboratories of the Nuclear Materials Authority of Egypt (NMA). Precision of the analytical data was monitored by international rock standards and is better than 3% for major oxides and 5% for trace elements. The REEs were analyzed using an ICP-AES spectrometer. Loss on ignition (LOI) was calculated by heating about 3g of a rock powder in a porcelain crucible at about 1000�?�?c for 4 hours. The results of the chemical analyses are given in Tables 1 to 4. All samples represent north Um Ara granitic rocks (alkali feldspar granites) and each sample is represented by the location (number of trench and its UTM) and chemical value (ppm).

Table 1: Chemical composition of major oxides (Wt%) and trace elements (ppm) analyses of uraniferous granitic rocks of the trenches samples, Um Ara area, SED, Egypt.

Table 2: REEs concentration in trenches at Um Ara area, SED, Egypt.

Table 3: Chemical composition of major oxides (Wt%) and trace elements (ppm) analyses of uraniferous granitic rocks of the core samples, Um Ara area, SED, Egypt.

Table 4: REEs concentration in core samples at Um Ara area, SED, Egypt.

Geochemical characteristics

There are a variation in trace elements and rare earth elements (REEs) concentrations in both trenches and core samples as follows: The average concentration of Cu, Zn, Zr, Rb, Y, Pb, Ga, Nb and all rare earth elements except Gd and Tm in core samples are higher than that in trenches, while the concentration of Ba, Sr, U, Th, Gd and Tm in trenches are higher than that in core samples (Figures 7,8).

According to Meyer et al. [43], Um Ara granites trenches and core samples are plotting on AKF ternary diagram (Figure 9), where A= Al₂O₃ - (Na₂O + K₂O), K= K₂O and F= FeO + MnO + MgO. It shows that only four samples of the altered granites from trenches fall alteration fields, two of them fall in sericite facies (due to sericitization processes) and the other two samples fall in propylitic field. All the core samples fall out of alteration fields.

The weathering trends are displayed on (Na₂O + CaO) - Al₂O₃ - K₂O triangular diagram (Figure 10) [44,45], Um Ara granites trenches and core samples show a relative enrichment in K₂O and Al₂O₃ compositions due to the relative alteration of feldspars to clay minerals.

The Na-K variations diagram after Cuney et al. [46] shows five trends; Na-metasomatism, K-metasomatism, silicification, desilicification and argillation (Figure 11). Most of Um Ara granites trenches and core samples lie in K-metasomatism trend.

The most prominent geochemical characteristics of Um Ara granites trenches and core samples include substantial variations in the concentrations of Na, Ca, Mg and Fe (Figure 12) [47]. Increasing alteration intensity is coupled with a gradual decrease in Na₂O and CaO, culminating in intensely altered samples having Na₂O+CaO values wt%. This trend is indicative of plagioclase destruction and formation of muscovite/sericitization.

The alteration box plot is a graphical representation that uses two alteration indices: the Ishikawa alteration index (AI) = 100 (K₂O + MgO)/ (K₂O + MgO + Na₂O + CaO) and the chlorite carbonate pyrite index (CCPI) = 100 (MgO + FeO) / (Mgo + FeO + Na₂O + K₂O (Figure 13) [48]. The Ishikawa (AI) index was defined by Ishikawa et al. [49] to quantify the intensity of sericite and chlorite alterations. The key reactions measured by the index involve the breakdown of sodic plagioclase and replacement by sericite and chlorite. Reactions that describe these alteration processes include:

3NaAlSi₃O₈+K⁺+ 2H⁺ = K Al₃ Si₃ O₈(OH)₂ + 6SiO₂ + 3Na+ (1)

Albite Sericite Quartz

2KAl₃Si₃O₁₀(OH)₂ +3H₂SiO₄+9 Fe⁺²+6 Mg⁺²+18H₂O=3Mg₂Fe₃Al₂ Si₃O₁₀(OH)₈+2K⁺+28H⁺

Sericite Chlorite

The first reaction is typical of sericite replacement of albite [50,51]. The second reaction is important close to massive sulphide mineralization where chlorite rich assemblages become dominant over sericite rich assemblages [52,53]. Reaction (1) involves a loss of Na₂O (and CaO) and gain of K₂O, whereas reaction (2) involves a loss of K₂O and gains in FeO and MgO, on the basis of constant Al₂O₃. The alteration box plot is a useful way of discriminating geochemical trends due to diagenetic alteration from those due to hydrothermal alteration directly related to sulphide ores (Figure 13). Um Ara granites trenches and core samples are restricted to the diagenetic alteration trend no.9: k-feldspar-albite, whereas k-feldspar replacing albite [54].

The concentrations of U and Th in Um Ara granites trenches and core samples more than the concentrations in upper continental crust is 3-4 ppm for U and 10-15 ppm for Th [55] and also are higher than the world average concentration of U (4ppm) and Th (18 ppm) in granites [56].

The most Th and U enriched rock samples have lower ratio of Th/U. Low Th/U ratio in samples may suggest fractionation of phases like Th-orthosilicate and zircon [57]. Um Ara granites trenches and core samples shows less Th/U ratio (Figure 14) than that of the crustal value [55]. All the granite samples from Um Ara granites trenches and core samples plot along the U and Th and away from the K which implies the deficiency of K and enrichment of U and Th (Figure 15).

There are two groups of trenches samples, the first one richer in Th than U, therefore they plotted between lines U/Th=0.5 and 1. The second group is richer in U than Th, therefore they plotted under the line of U/Th=2 (Figure 16). All the core samples are richer in U than Th, so they lie under the line U/Th=2 (Figure 16). All Um Ara granites trenches and core samples define hydrothermal U enrichment trend.

The lowest concentration of uranium and thorium in core samples is at 10m depth, while the highest values at 60m depth (Figure 17).

The Um Ara granites trenches and core samples have variable total REEs contents 117.41 to 308.28 and 256.51 to 397.22 ppm respectively (Table 2, Table 4). ΣLREEs in trenches and core samples range from 108.47 to 298.41 ppm and from 236.84 to 384.78 ppm respectively, whereas ΣHREEs range from 5.48 to 14.01 ppm and from 12.44 to 19.67 ppm respectively. The lowest concentration of total REEs in core samples is at 100m depth, while the highest values at 145m depth (Figure 18).

The Chondrite-normalized REEs patterns (Figure 19), of Um Ara granites trenches and core samples are characterized by strong (ΣLREEs)n enrichment and low (ΣHREEs)n content due to the presence of some accessory minerals like apatite, and zircon. The (Eu/ Eu*)_n in Um Ara granites trenches and core samples ranges from 0.19 to 1.06 and from 0.11 to 0.58 respectively suggests that plagioclase was an early fractionating phase.

The tetrad effect has been progressively recognized particularly for the granitic rocks which have undergone a high degree of fractional crystallization, hydrothermal alteration, and mineralization [58-62]. Masuda et al. [41] classified the tetrad effects into two different types, M and W-type (M-type in solid sample as residues and W-type in the interacting fluids as extract). The tetrad effect in the studied Um Ara granites trenches and core samples were calculated according to Irber equations [61]:

t₁=(Ce/Ce^t x Pr/Pr^t)^0.5 , t₃=(Tb/Tb^t x Dy/Dy^t)^0.5 and T_1,3= (t₁ xt₃)^0.5

When TE_1,3 values are higher than 1.1, the rock is considered to show the tetrad effect. The studied Um Ara granites trenches have TE_1,3 values range from 0.43 to 1.29, so they do not show tetrad effect in most samples (Table 2), whereas in core samples TE_1,3value ranges from 0.88 to 1.88 showing tetrad effect. The fractionation of elements with similar ionic radius and charge is regarded to be sensitive to change in the melt composition during magma differentiation [60,63].

The geochemical data suggest that the Um Ara granites trenches and core samples has evolved by means of fractional crystallization through removal of K-feldspar, biotite, plagioclase, together with some accessories such as apatite and Fe–Ti oxide phases. From the HREE pattern, the elements (Tb, Ho, Tm and Lu) form positive anomaly rather than (Dy, Er and Yb) in some samples of Um Ara granites trenches and core samples it may be suggested that fluorinecomplexes played some role during its evolution.

Mineralogical features

The heavy minerals were separated using heavy liquid (bromoform) separation technique, followed by magnetic fractionation using Frantz isodynamic separator. The heavy minerals were picked under a binocular microscope and identified by Environmental Scanning Electron Microscope (ESEM) techniques. All analyses have been done at Nuclear Materials Authority (NMA), Cairo, Egypt.

The altered granites contain uranium, thorium minerals, uranium bearing minerals and Nb-Ta mineral. The uranium mineral is uranophane (Figure 20) and thorium mineral is uranothorite. The uranium bearing minerals are fluorite, monazite, xenotime, allanite, zircon while Nb-Ta mineral is columbite (Figure 21).

Uranium and thorium estimation from trenches excavations

The estimation of uranium and thorium depend on the samples taken from the trenches and calculated as follows:

Total samples = Total collected samples.

Av. element content (ppm) = Total element content ÷ No. of selected samples.

Element grade (%) = Av. element content ÷ 10000.

Volume (m3) = Length × width × depth of selected area.

Element tonnage (t) = Volume × 2.60 (average density of granites)

Element metal (t) = Element tonnage × element grade.

The average of uranium and thorium mineralization grade and the total tonnage in all trenches are calculated as illustrated in table 5 and that of the total trenches are calculated in Table 6.

Table 5: Illustrating the tonnage (ton), element grade (%) and element metal (ton) for uranium and thorium mineralization in Um Ara granites trenches area, SED, Egypt.

Table 6: Illustrating the tonnage (ton), element grade (%) and element metal (ton) for uranium and thorium mineralization in the all Um Ara granites trenches, SED, Egypt.

Conclusion

The following are the main detailed geological, petrological and geochemical works, and previous results revealed the following characteristics for Um Ara area:

The basement rock units of Um Ara area is a part of the Arabian- Nubian Shield between 581400m E-584900m E and 2503000m N-2504600m N. They are represented by Dokhan volcanic and post-collision granitoids (monzogranite, and alkali feldspar granite), dykes and quartz veins. It could be concluded that the major NNWSSE, NNE-SSW and NE-SW trending fracture sets were acted as the principal channel ways for the mineralized solution while the re-deposition took place within almost all the minor fracture trends with no preferred orientation.

Um Ara altered granites and core samples are considered to be low grade uranium mineralization. This area seems to have been affected by many groundwater activities. The origin of the secondary uranium minerals is mainly related to alteration of primary uranium minerals by the action of oxidizing fluid (supergene alteration) mobilization of uranium and then re-deposition in other forms. Redistribution by circulating meteoric waters might have taken place. The rocks are characterized by containing of mineralization as uranium mineral (uranophane), thorium mineral (uranothorite) and others minerals such as (columbite, fluorite, monazite, xenotime, allanite and zircon).

Highest uranium concentrations, that are associated with altered trench granites and core samples rocks, show strongest broad uranium anomaly, with irregular shape. The radioactivity was measured systematically in the field over a grid with 0.5 meter spacing, constructed in the trenches. The rocks and their alteration products are the main constituents of these trenches. Hematization is mostly present as well as other different types of alterations such as sericitization, albitization, K-feldspathization, chloritization, silicification, argillization, fluoritization, kaolinitization and limonitization. These alterations are concentrated along the fractures of altered granite and are associated with mineralization.

Um Ara altered granites trenches are an abnormal radiation and have an extension. So, these anomalies are suitable to be a place for uranium extraction. The preliminary estimation of average element grade (%) of uranium and thorium mineralization in trenches is: U (0.0246%) and Th (0.01654%). The preliminary estimation of average element metal (by tons) of radioactive elements in trenches: U (2.38) and Th (1.6). Preliminary results show the area of interest for uranium associations need further investigations using state-of-the-art to discover economic deposits for development and utilization of the resource. The result is the continuous of its mining work besides leachability experiments.

Acknowledgment

The authors would like to thank the late Prof. Dr. Mostafa E. Darwish and Dr. Reda El Arafy who kindly made available facilities.

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