Egyptian Journal of Geology, v. 57, 2013, p. 287-300

 

GEOLOGY AND RADIOELEMENTS POTENTIALITY OF PHANEROZOIC TRACHYTES OF WADI AL-OWAYRISHAH AND ITS SURROUNDINGS, CENTRAL

EASTERN DESERT, EGYPT

HASSAN1, I. S., DARDIER2, A. M., ABDEL GHANI2, I. M., IBRAHIM3, S. K., EL-SAWEY2, E. H. AND DESSOUKY2, O. K.

1Science department, Faculty of basic education, Kuwait

2Nuclear Materials Authority, P.O. Box 530, El-Maadi, Cairo, Egypt 3Geology Department, Suez Canal University, Ismailia

ABSTRACT

Trachyte rocks of Wadi Al-Owayrishah and its surroundings crop out in the form of sheets and dykes. They are striking and dipping in a concordant manner with the primary and secondary structures of the enclosing host rocks. The trachyte rocks may be related to the rifting event that produced monogenetic volcanoes of basaltic rocks and/or alkaline felsic rocks. Radioactive minerals associated with trachytes are mainly represented by secondary minerals. Concentrations of radioactive minerals are essentially controlled by distribution of amygdales and phyenocrysts. The main sources of uranium anomalies in the study area are trachytes themselves, superheated solutions and hydrothermal solutions. The anomalies are controlled by low permeability of country rocks in contact (siltstones), as well as the presence of iron oxy-hydroxides alteration as assistance agent for uranium adsorption, fixation and accumulation. Secondary uranium minerals such as uranophane and uranium-bearing minerals such as samarskite, zircon and fluorite, as well as thorite and thorium-bearing minerals such as monazite, allanite and thorium-REEs silicate were identified using XRD and SEM techniques.

Keywords: W. Al-Owayrishah, Kab Al-Abyad, W. Al-Farkhah, Trachytes, Petrography, Radioelements potentiality, mineralization, CED, Egypt

INTRODUCTION

Wadi (W.) Al-Owayrishah and its surroundings are located in the central Eastern Desert of Egypt. The central Eastern Desert domain (CED) extends from Qena-Safaga road southwards to Idfu-Mersa Alam road (El Gaby et al., 1988) or to the line starting at Ras Benas on the east extending NW and turning around Hafafit dome SW to Kom Ombo on the west (Stern and Hedge, 1985). The alkaline volcanic rocks and related intrusives, including dyke swarms, ring dyke complexes and Tertiary basalts, (mainly Oligocene), belong to Post Pan-African (Phanerozoic) volcanics. During the Phanerozoic, continental intraplate volcanic activity in Egypt was intermittent and resulted in extrusion of volcanic rocks of wide compositional variation, size and mode of eruption. Geochronological studies on these Phanerozoic volcanics (Meneisy and Kreuzer, 1974; El Shazly, 1977; Hashad and Hassan, 1978; Hashad and El Reedy, 1979; Ressetar et al., 1981; Franz et al., 1987; Meneisy, 1990; Stairs et al., 1991; Ibrahim et al., 2001 and Saleh et al., 2004) revealed three phases of activity in Egypt. These phases are: Paleozoic (395 - 233 Ma.), Mesozoic (191-74 Ma.) and Tertiary (48-15 Ma.). Both the Paleozoic and Mesozoic volcanic rocks range in composition from olivine basalt to trachyte, whereas the Tertiary volcanics are largely of basaltic composition.

There are few detailed field investigations combined with radiometric and mineralogical studies carried out on the Late Carboniferous to Early Permian alkaline volcanism (trachytes) of W. Al-Owayrishah and

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its surroundings area. This paper introduces a new petrographical, radiometrical and mineralogical data on the trachytes of W. Al-Owayrishah, Kab Al-Abyad and W. Al-Farkhah. The aim of the present study is to outline the field concepts, petrographic, radiometric and mineralogical characteristics which affect the distribution of uranium occurrences in trachyte rocks.

FIELD OBSERVATIONS

Wadi (W.) Al-Owayrishah, Kab Al-Abyad (Fig.1) and W. Al-Farkhah (Fig. 2) trachytic sheets and dykes exposed in the central Eastern Desert, southwest Quseir city. Field and radiometric investigations of the studied areas were carried out by El-Ghawaby (1967), Salman (1968) EL-Kassas (1969) El-Manharawy (1972) Hashad and Mahfouz (1976) Hussien and EL-Kassas (1979) Bakhit et al. (1989) and Tawfik (2010).

Trachyte rocks (Late Carboniferous to Early Permian, El-Manharawy, 1972) represent the youngest basement rock type in the studied area. They generally crop out as sheets and dykes. The trachyte sheets are more abundant than the trachyte dykes. The recorded sheets and dykes usually comprise more or less parallel belts injected along faults, weak structural lines and parallel to the bedding planes of the molasses-type Hammamat sedimentary rocks (Fig. 3a). The trachyte bodies are striking and dipping in a concordant way with the primary and secondary structures of the enclosing host rocks. The trachyte rocks may be related to the rifting event that produced monogenetic volcanoes of basaltic rocks and/or alkaline felsic rocks.

The studied trachytes have significant variations in textures, grain sizes and types of alterations such as ferrugination, silicification, hematitization, kaolinization and patches of malachite-azurite associated with iron oxy-hydroxides along joint surfaces (Fig. 3b). At many locations within the study area the trachytic rocks, especially dykes, are porphyritic with large phenocrysts of K-feldspar. Fine-grained types are also found. Porphyritic texture is the predominant one and consists of relatively large, euhedral or subhedral phenocrysts dispersed in much finer grained or glassy groundmass. It is a characteristic of coherent lavas, syn-volcanic extrusions and clasts derived from them. It is one of the most important criteria for distinguishing coherent facies from pyroclastic, resedimented volcaniclastic and volcanogenic sedimentary deposits. The trachytic dykes have patterns of significantly increasing grain size consistent with their formation in a subvolcanic complex.

At W. Al-Owayrishah and Kab Al-Abyad area, the trachyte sheets are fine- to medium-grained, hard, massive and compact. They are of grey and pink colours but often pinkish grey due to the presence of hematite. Very often, these rocks are altered (mostly hematitized) especially along faults, fissures and contacts with the surrounding rocks (Fig. 3c). The most intensive alteration in the trachyte with Hammamat sedimentary rocks, especially siltstone, was noticed at the southern part of W. Al-Owayrishah. Several trenches were previously dug in this area to follow the upper contacts and alteration patches within trachyte bodies which show relatively high radioactivity levels.

At W. Al-Farkhah area, the trachyte rocks show low to moderate relief invading the tectonic mélange and Hammamat sedimentary rocks. They are hard, compact, massive, fine- to medium-grained usually porphyritic and show different colors as pink, pale pinkish grey, cream and reddish pink. Occasionally, trachytes show vesicular texture as a result of exsolved volatiles from lava. Vesicles are varying in their size, shape and abundance in the dykes reflecting the interplay of several controls, including original magma volatile content and viscosity, rates of decompression and diffusion, coalescence and interference of adjacent vesicles, and deformation during flowage.

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Fig. 1: Geologic map of W. Al-Owayrishah and Kab Al-Abyad area

Fig. 2: Geologic map of W. Al-Farkhah area

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Amygdales are the former vesicles that have been partially or completely infilled with secondary minerals. Amygdales founded in the porphyritic trachyte of W. Al-Farkhah are aligned within the trachytic-textured groundmass and filled with jasper, hematite and goethite (Fig. 3d). Some of these amygdales are easily isolated with weathering processes leaving vesicles stained with their relics. Most of these amygdales cause local ferrugination. It is worth mentioning that the highest radioactivity levels at W. Al-Farkhah area are restricted to an intensely hematitized shear zone with a thickness ranging from 40 to 70 cm and dip about 45˚ to the north. This zone includes rounded to elliptical metallic black minerals in the rock ranging in size from few millimeters up to 3 cm in diameter. Also, visible bright yellow–orange uranophane and bright yellowish green radioactive minerals are usually associating hematite and goethite.

Generally, along the contact zone between the trachyte and Hammamat sedimentary rocks, the trachyte becomes highly permeable, crumbly and contains vugs of few millimeters in diameter filled with goethite and radioactive minerals. The upper contact is generally more mineralized than the lower one, but the trachyte itself is much less mineralized than the contacts.

Fig. 3: Field photographs showing a) Trachyte sheets (T) parallel to the bedding and the foliation of Hammamat siltstones (H) and have the same dip, b) Close up view of trachyte showing malachite-azurite associated with iron oxy-hydroxides along joint surfaces, c) Close up view showing hematitized trachyte zone and d) Close up view of vesicular trachytes enclosing amygdales of different sizes filled with jasper and iron oxy-hydroxides (hematite and goethite); some of these amygdales were removed by weathering leaving vesicles stained with their relics.

a b

c d

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PETROGRAPHY

Trachytes are fine-grained with an aphanitic to porphyritic texture and holocrystalline except for small constituents of secondary chalcedony or cryptocrystalline quartz. The mineral assemblage consists of essential alkali feldspars which form around 80% to 90% of the whole rock with some mafic minerals especially hornblende and riebeckite and/or mica. Most of trachytes have little or no quartz but some quartz-trachytes contain quartz (from 5% up to 20%). Most samples show trachytic (Fig. 4a) and porphyritic textures (Fig. 4b), as well as common presence of vesicles and amygdales. The alkali feldspars are mainly represented by sanidine, minor orthoclase and albite. Sanidine is very common occurring in two generations, i.e. both as large euhedral phenocrysts and in smaller imperfect rods or laths forming a finely crystalline groundmass. Calcite and carbonate micro veinlets are also recognized especially at the contact when the rock is being ferruginated and contains uranium minerals that fill the amygdales and the cavities. Other constituents are the ferromagnesian minerals, mainly represented by chlorite, hornblende and riebeckite. They are generally obscured by intense hematitization.

Radioactive minerals associated with trachytes are mainly represented by secondary minerals. These secondary radioactive minerals are represented by autunite, beta-uranophane and uranophane always associated with iron-oxy hydroxides.

At W. Al-Owayrishah, autunite (Fig. 4c) is found in trachyte filling the cavities and is associated with iron-oxy hydroxides and carbonates. The same phenomena have been repeated at Kab Al-Abyad. The radioactive minerals are mainly represented by uranophane (Fig. 4d) and autunite (Fig. 4e). They are included in the K-feldspar crystals and fill the fractures and cavities in the porphyritc trachyte. The Radioactive minerals mostly associated with clay minerals and iron-oxy hydroxides. Uranophane and autunite are usually rimmed with wide paleochroic halos.

At W. Al-Farkhah area, uranium minerals associated with trachyte are not of the same obviousness of the above mentioned locations. Uranophane fills the amygdales of the trachyte and sometimes coats goethite (Fig. 4f).

Petrographic examination of the thin sections especially the altered ones which have noticeable mineralization, revealed a close relationship between the presence of phenocrysts ratio as well as the amygdales to the groundmass with the presence and distribution of radioactive minerals. Ratios of radioactive minerals are essentially controlled by distribution of amygdales and phyenocrysts rather than the groundmass. The highest radioactive ratio (2.8%) is recoded in W. Al-Owayrishah altered trachytes which possess the highest phenocrysts ratio (16.1%). Also, Al-Farkhah trachytes show high radioactive minerals ratio (2.6%) as they record the highest amygdales ratio (10.3%) (table 1 and Fig. 5).

Table 1: Ratios of radioactive minerals, amygdales, phenocrysts and groundmass of the studied altered mineralized trachytes.

Location Radioactive

minerals Amygdales Phenocrysts Groundmass

Kab Al-Abyad 0.8% 1.5% 10.3% 87.4% W. Al-Owayrishah 2.8% 3.9% 16.1% 77.2% W. Al-Farkhah 2.6% 10.3% 8% 79.1%

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Fig. 4: Photomicrographs showing a) general view of trachytic texture in trachyte, C.N., b) porphyritic texture in trachyte, C.N., c) autunite associated with carbonates and iron-oxy hydroxides in trachyte, W. Al-Owayrishah C.N., d) iron-oxy hydroxides associated with radioactive mineral (uranophane) with conspicuous pleochroic halos in trachyte, Kab Al-Abyad, C.N., e) autunite associated with carbonates in trachyte, Kab Al-Abyad, C.N. and f) goethite coated with uranophane in trachyte, W. Al-Farkhah, P.L.

Fig. 5: Pie-diagrams showing ratios of radioactive minerals, amygdales, phenocrysts and groundmass of the examined thin sections of the studied altered mineralized trachytes.

Kab Al-Abyad W. Al-Farkhah W. Al-Owayrishah

a b

c d

e f

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RADIOELEMENTS POTENTIALITY AND MINERALIZATION

The trachyte masses of Kab Al-Abyad show average eU (equivalent uranium) content of 21.9 ppm and eTh (equivalent thorium content) of 32.2 ppm. The maximum value of eU reaches 111.3 ppm along some previously dug trenches along the upper contact with the siltstones, especially those zones which show iron oxy-hydroxide alterations. The maximum value of eTh reaches 65.5 ppm. On the other hand, the minimum value of eU and eTh are 4.3 and 20.6 ppm respectively especially along unaltered zones.

Along W. Al-Owyrishah area, the average contents of eU and eTh are 21.2 ppm and 31.6 ppm respectively. The maximum value of eU is 209.9 ppm associated with maximum value of eTh reaches up to 87.1 ppm and that found on the dump around the main exploratory trench which is dug to follow the upper contact of the trachyte with Hammamat siltstones. The trachyte hand specimens of the anomalous spots show visible secondary uranium minerals associated with iron oxy-hydroxides. Violet fluorite dissemination and manganese dendrites are also present. The minimum values of eU and eTh are 4.1 ppm and 18.7 ppm respectively. The surface distribution of radioactivity eU (ppm) and eTh (ppm) of the studied areas are shown in figure (6).

Fig. 6: Surface radioelements distribution contour maps for the studied area, a) eU for W. Al-Owayrishah area, b) eTh for W. Al-Owayrishah area, c) eU for W. Al-Farkhah area and d) eTh for W. Al-Farkhah area.

a b

d c

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Trachytes of W. Al-Farkhah show thorium anomalies rather than uranium. The average eU is 24.4 ppm while that of eTh is 134.8 ppm. The maximum eTh content exceeds 260.0 ppm while that of eU reaches 45.5 ppm around some previously dug trenches following the contact with siltstones. The mineralized parts are associated with ferruginated shear zones in the amygdaloidal trachytes which are so clear on the walls of the main tunnels at W. Al-Farkhah area. Generally, Al-Farkhah area is characterized by Th-bearing minerals rather than uranium, suggesting that their origin is related to Th-rich hydrothermal fluids rather than meteoric water and/or superheated solutions.

The equation eU-(eTh/3.5) reflects the uranium mobilization. If the result of this equation equals zero, it indicates absence or at least very restricted uranium mobilization. When it is greater than zero it means that uranium was enriched (added to rock). The negative values mean uranium leaching out.

Along alteration zones, trachytes of Kab Al-Abyad and W. Al-Owayrishah (Figs. 7a and 7b), show eU-(eTh/3.5) values above zero and sometimes reach up to 200, especially at the contact with the ferruginated siltstones which are characterized by low permeability. The siltstones act as barrier preventing uranium from escaping during mobility processes. The presence of the iron oxy-hydroxides alteration, which is the main alteration process, couldn't be ignored. This may be due to the high ability of iron oxides to adsorb uranium from its bearing solutions (Hussein et al., 1965) and/or the prevalence of oxidation conditions and complexing ions that cause precipitation of uranium as complex uranyl ions (Cuney, 2003). Few samples show eU-eTh/3.5 values lower than zero, suggesting local U-leaching especially along kaolinized zones and/or near the contact with conglomerates. The leached uranium may represent the main source of uranium that added in ferruginated zones. Uranium is transported by meteoric water, superheated solutions and hydrothermal fluids as soluble carbonate and fluoride complexes as indicated from the occurrence of calcite veinlets cutting the host rock and secondary violet fluorite dissemination at most alteration zones.

Fig. 7: Uranium mobilization in the trachytes of a) Kab Al-Abyad, b) W. Al-Owayrishah and c) W. Al-Farkhah

(a) (b)

(c)

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Different situation has been found along W. Al-Farkhah. The radiometric measurements of the equivalent thorium increase in wide range if compared with those of the equivalent uranium. On mobility diagrams, all the measurements, mostly, lied under the zero line (Fig. 7c).

Generally, the main sources of uranium anomalies in the area are trachytes themselves, superheated solutions and hydrothermal solutions. Radioactivity of the trachyte rocks is not affected by their trends but is related to their mineralogical composition, their texture, type and degree of alteration and the type of country rocks.

Three main factors affect the existence of uranium anomalies and mineralization all over the study area, and they are: a) the trachytes (main source) must possess uranium concentrations exceeding the normal values, b) the country rocks in contact must be of low permeability such as siltstones to avoid uranium escaping during the mobility processes, and c) presence of iron oxy-hydroxides alteration, which is very important assistance agent for uranium adsorption, fixation and accumulation.

The mineralogical studies revealed the presence of uranophane encountered in Kab Al-Abyad, W. Al-Owayrishah, and W. Al-Farkhah, (Fig. 8) in the form of fracture-fillings and disseminated modes influenced by the existence of iron oxy-hydroxide. Samarskite is uranium-bearing mineral (Fig. 9), found partially stained with iron oxy-hydroxide, at W. Al-Owayrishah. The studied zircons that encountered in W. Al-Owayrishah trachyte are partially metamicted enclosing radioactive materials. The Environmental Scanning-Electron Microscope (ESEM) analyses of zircon crystals show association with uranophane (Fig. 10). Fluorite is found as disseminated grains associated with the radioactive spots that recorded in the studied trachyte rocks especially at Kab Al-Abyad and W. Al-Owayrishah (Fig. 11). Thorium anomalies of W. Al-Farkhah trachytes are restricted to thorite and Th-bearing minerals that encountered and associated with goethite, hematite, monazite and allanite as well as Th-rich REEs silicate (Fig. 12).

CONCLUSIONS

Late Carboniferous to Early Permian trachyte rocks of W. Al-Owayrishah, Kab Al-Abyad and W. Al-farkhah represent the youngest basement rock type in the studied areas that invade the oldest rock types which are represented by tectonic mélange, suture zone molasse-type Hammamat sedimentary rocks and collision-related granites. The trachyte bodies are striking and dipping in a concordant way with the primary and secondary structures of the enclosing host rocks and have several recognized alteration types such as silicification, hematitization, kaolinization and patches of malachite-azurite associated with iron oxy-hydroxides. The mineralization is recorded in the fractures and joints, mostly perpendicular to the contact.

Petrographic study proved strong relationships between the existence of radioactive minerals and the distribution of phenocrysts and amygdales rather than groundmass. The proposed model for these relationships show that the highest radioactive ratio (2.8%) is recoded in W. Al-Owayrishah altered trachytes which possess the highest phenocrysts ratio (16.1%). Also Al-Farkhah trachytes show high radioactive minerals ratio (2.6%) as they record the highest amygdales ratio (10.3%).

The high radioactive trachyte anomalies are related to the presence of some radioactive minerals mainly represented by uranophane and uranium-bearing minerals such as samarskite, zircon and fluorite, as well as thorite and thorium-bearing minerals such as monazite, allanite and thorium-REEs silicate.

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a)

Fig. 8: a) Stereo-photograph showing different uranophane grains at Kab Al-Abyad, W. Al-Owayrishah, and W. Al-Farkhah. b) EDX and BSE images of uranophane grains. c) X-ray diffractogram of uranophane and beta- uranophane

b)

c)

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Fig. 9: EDX and BSE images of samarskite in Fe-oxides

Fig. 10: EDX and BSE images of zircon associated with uranophane

Fig. 11: a) Stereo-photograph showing fluorite grains with violet to dark violet colors. b) X-ray diffractogram of fluorite grains, W. Al-Owayrishah trachytes

a)

b)

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Fig. 12: EDX and BSE images of a) monazite inclusions in Fe-oxides grains isolated from amygdales. b) allanite grain inclosing patches of thorite isolated from amygdales. c) Th-rich REEs silicate found in the amygdales, W. Al-Farkhah trachytes

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انيروزوى فى وادى العويرشة والمناطق المحيطه جيولوجية وامكانية تواجد العناصر المشعه فى صخور التراكيت لعصر الف مصر -وسط الصحراء الشرقيه، به

، ٢السيد حسن الصاوى، ٣ساميه كمال ابراهيم، ٢اسماعيل محمد عبد الغنى، ، ٢أحمد محمد دردير، ١إمبارك سيد حسن ٢اسامه خيرى عبده دسوقى

جامعة قناة - قسم الجيولوجيا ٣، القاهره - د النوويةهيئة الموا ٢، الكويت -كلية العلوم الأساسية - قسم العلوم ١ الأسماعيلية -السويس

الخلاصة

صفائح وسدود معظمها تميل تظهر صخور التراكيت الموجوده بوادى العويرشة ومناطق الدراسه المحيطه بها فى شكل صخور التراكيت وتكوينها بمنطقة الدراسه قد يكون له . المضيفةر بشكل متطابق فى أتجاه التراكيب الأوليه و الثانويه للصخو

. علاقه باحداث التصدع التى انتجت براكين احاديه النشأه التى ينشأ عنها صخور البازلت أو صخور قلويه حمضيه

واجدها على وقد اتضح أن المعادن المشعه المصاحبه للتراكيت هى معادن ثانويه ويعتمد توزيع نسب المعادن المشعه وتويرجع المصدر الأساسى لتواجد شاذات اليورانيوم فى منطقة . توزيع الامجدال والفينوكرست المتواجد فى صخور التراكيت

وقد اتضح من خلال الدراسه ان وجود هذه الشاذات . الدراسه هى صخور التراكيت نفسها والمحاليل الحاره والمحاليل الحرمائيه قوب صغيرة متلاحمه مع التراكيت بالأضافه الى تواجد اكاسيد الحديد التى تساهم فى أمتصاص مرتبط بوجود صخور ذات ث

.وتثبيت معادن اليورانيوم

تم التعرف من خلال الدراسه على معادن يورانيوم ثانويه من اليورانوفين ومعادن حامله لليورنيوم مثل السمارساكيت والزركون لمعادن الحامله للثوريوم والعناصر الأرضية النادرة مثل معادن المنوازيت والألنيت وذللك من والفلوريت وكذلك معدن الثوريت وا

.خلال استخدام تقنيات مثل حيود الأشعه السينيه والميكرسكوب الألكترونى