Geochemical Characteristics and Chemical Electron Microprobe U-Pb-Th Dating of Pitchblende Mineralization from Gabal Gattar Younger Granite, North Eastern Desert, Egypt ()
1. Introduction
Gabal Gattar area is located in the north Eastern Desert, at a distance of about 70 km southwest of Hurghada city between latitudes 27˚02ꞌ00" - 27˚08ꞌ30"N and longitudes 33˚13ꞌ26" - 33˚25ꞌ47"E (Figure 1). Geomorphologically, The Qattarian batholith is nearly of oval shape, of 30 km long in N-S direction and about 20 km wide, covering an area of about 600 km2. Wadi Al Ghozah major fault of nearly N55˚E trend divides the batholith into northern and southern parts. The area is characterized by rough, steep slopped and ragged mountainous, where Gabal Gattar, (1963 m a.s.l.); Gabal Um Dissi (1556 m a.s.l.); Gabal Thelma (1733 m a.s.l); Gabal Abu El Hassan (1550 m a.s.l.); Gabal Abu El Hassan El Ahmar (1234 m a.s.l.)
and Gabal Abu Samyuok (1750 m a.s.l.) represented high peaks in the area (Figure 2).
The area was studied geologically, mineralogically and radiometrically by numerous authors e.g. Ghobrial and Lotfi [1], Moussa and Abu El Leil [2], Stern et al. [3], El Rakaiby and Shalaby [4], Willis et al. [5], Attawiya [6], Sayyah and Attawiya [7], Salman et al. [8], El Kammar et al. [9], El Shershaby [10], El Sayed et al. [11], Raslan [12], Wasfi et al. [13] and Abdel Warith et al. [14].
Few authors studied the isochron age of Gabal Gattar younger granite, e.g. Schurmann [15] determined an age of 484 Ma for Gabal Gattar younger granites (K/Ar method), Stern and Hedge [16] gave an age of 579 Ma for the granites of Gabal Gattar, (zircon analyses), Hashad [17]
Figure 2. TMlandsat image for Gabal Gattar area.
obtained an Rb-Sr age for Gattar granites lying within 450 - 675 Ma and Moussa [18] gave an age of 570 Ma for Gattar granites (Rb-Sr method). The lack and scarcity of the chronological data for uranium mineralization prevent the correlation for this mineralization with others in the different areas in the Eastern Desert.
This paper focuses on petrographical, mineralogical and electron microprobe investigations of the pitchblende from Gabal Gattar younger granite.
2. Geologic Setting
Geology of the concerned area is focused mainly on the younger granite of Gabal Gattar. Gabal Gattar represent the northern parts of a big pink granite batholith. This granite mass occurs as mountain terrain forming moderate to high relief hills, ridges and multi-peaks. It is composed mainly of younger granite forming marginal sharp intrusive contacts with the surrounding countryrocks which include; metavolcanics, diorite and Hammamat sediments (Figure 3, after Rose [19]). Younger granite range in colors from pale pink to pink and sometime show reddish pink color along fault planes and shear zones. They are massive, varying in grain size from coarse-grained pegmatitic texture to fine-grained, but medium-grained is the prevailing one and show few mafic minerals. Silicification, hematitization, Kaolinization, chloritization, fluoritzation and episyentization are the most common alteration features recognized along the major faults and shear zone dissected Gattar younger granites.
Pegmatites, quartz veins and aplites dykes are the most abundance encountered at the marginal parts of this granites. Dykes show variable distribution and composition in Gattar area. They represented by two main groups, the first group acidic dykes which comprising granite porphyry and granophyres dykes, while the second group represented by intermediate dykes which include basaltic and andesitic dykes. Gabal Gattar granite is strongly jointed and fractured. The most predominate strike direction of these joints and fractures are: NE-SW, NW-SE, ENE-WSW and NNW-SSE. Dykes are mainly extending in NE-Sw, ENE-WSW and E-W. Gabal Gattar area was traversed by several strike-slip faults trending in the NE-SW, N-S, NW-SE, E-W, ENE-WSW and NNW-SSE directions. According to the field relations, faults in Gabal Gattar area can be arranged chronologically, starting with the youngest as follow, N-S, NNW-SSE, NW-SE, NE-SW and E-W directions. Uranium mineralization in Gabal Gattar younger granite is structurally controlled vein-type (Salman et al. [8]). The vein-type U-mineralization in Central Eastern Desert is mostly controlled by fractures trending NNE-SSW, ENE-WSW and NW-SE (El Shazly et al. [20] and Bakhitand Kassas [21]).
3. Petrography of the Host Rock
Pitchblende mineralization occurs in the younger granite of Gabal Gattar as vein-type. These younger granites are pink to reddish in color, massive and show few mafic minerals. This granite displays in some areas hematitic alteration especially in the fault zones. Microscopic studies revealed that the rock is medium to coarse-grained and essentially composed of quartz, perthite, potash feldspars with subordinate amount of plagioclase, biotite and secondary muscovite. Accessory minerals are zircon, fluorite and some apatite. Quartz occurs as subhedral to anhedral megacrysts up to 3 × 2.5 mm and small crystals up to 0.1 × 0.2 mm. It is found in two generation, the older fills the interstices between the feldspar crystals, whereas the younger is graphically intergrowth with
perthite crystals (Figure 4A).
Perthite occurs as subhedral megacrysts up to 2.5 × 3.25 mm. Perthitic veinlets are the most predominate type of perthite in these rocks (Figure 4A). Potash feldspare are mainly perthite with few amount of microcline and orthoclase crystals. The orthoclase occurs as anhedral to subhedral crystals reaching 2.3 × 0.25 mm in size. Plagioclase represents the few constituent mineral in these rocks. It is represented by euhedral crystals up to 2.5 × 2.3 mm. These plagioclase crystals are oligoclase to albite in composition. Some of these plagioclase crystals show alteration to sericite in the periphery, while the core still clears preserving the twinning (Figure 4B). Biotite represents the chief mafic minerals. It occurs as subhedral to anhedral crystals. These biotite crystals show pleochroism from yellow to yellowish brown colour. Biotite crystals reach up to 1.1 × 0.4 mm, while the small crystals up to 0.4 × 0.25 mm. They are variably altered to chlorite (Figure 4C). Muscovite occurs as secondary mineral associated with biotite or as interstitial between quartz and feldspars. It is found as irregular medium flakes (Figures 4A & B). Zircon is found as euhedral prismatic crystals included with perthite and pitchblende mineralization (Figure 4D). Fluorite occurs as anhedral to subhedral crystals displaying distinct cleavage. It varies from colors from violet to light violet (Figure 4E).
4. Pitchblende Mineralogy
Mineralogical and petrographical features of pitchblende mineralization and accompanied accessory minerals were determined from thin section through optical observation in transmitted and reflected light using a scanning electron microscope (SEM) with a back-scattered electron (BSE) imaging (Figures 5-7).
Pitchblende mineralization occurs as veinlets and patches filling the fractures. It is amorphous in shape, dense and bluish gray to black incolor accompanied by significant content of zircon, fluorite and lead. Zircon occurs as euhedral prismatic crystals exhibiting its characteristic interference colors and show depleted content of radioelements. Fluorite is largely present and identified by its violet colors and distinct cleavage. Lead was identified by scanning electron microscope (SEM) and microprobe analyses (Figure 7).
5. Analytical Methods
The pitchblende mineralization in a polished thin section prepared for the conventionalelectron microprobe analyses using a CAMECA SX-100 electron microprobe at the Centre de Recherches Petrographiqueset Geochimiques, Nancy, France. The analysis was carried out underthe following instrument operating conditions; a 15 kV accelerating voltage and a beam current of 10 nA. Data of microprobe analyses for the pitchblende mineralizationis listed in Table 1.
6. Chemical Dating of Pitchblende
Chemical U-Pb-Th dating using the electron probe
A 
B
Figure 6. (A) EDX-ray pattern of pitchblende venilets. (B) Back-scattered electron imaging (BSE) of pitchblende venilets.
A 
B
Figure 7. (A) EDX-ray pattern of pitchblende patches. (B) Back-scattered electron imaging (BSE) of pitchblende patches.
microanalyzer (EPMA) has become increasingly popular for their it’s a relatively quick and low-cost method to obtain ages for detrital monazites and zircons using the electron microprobe analyses, but less accurate than iso- topes methods (e.g. Cocherie et al. [6]; Pyle et al. [22]). The principles of (EPMA) Th-U-Pb dating of monazite and zircon were first proposed by Suzuki and Adachi [23, 24], then Montel [25] systematically summarized the
Table 1. Electron microprobe analyses (wt%) of the pitchblende mineralization from Gabal Gattar younger granite.
Table 2. Summary of the CHIME dating method for pitchblende mineralization from Qattar younger granite.
method of EMPA monazite dating, and till now this method has been discussed in many articles published in China (e.g. Zhou et al. [26]; Zhang et al. [27,28]; Liu and Chen [29]; Liu et al. [30] & 2006; Dang et al. [31]).
CHIME (chemical Th-U-total Pbisochron method) dating method, which is based on precise electron microprobe analyses of Thand/or U-bearingminerals such as monazite, xenotime, zircon and polycrase (Suzuki and Adachi [22,23]). CHIME age calculation program is a computer program for the CHIME age calculation saves significantly the time taken to estimate the isochron age from a dataset of ThO2, UO2 and PbO or Th, U and Pb analyses of Thand/or U-bearing minerals.
CHIME age calculation for the Qattar pitchblende mineralization has been calculated using U, Th and Pb content measured with an electron microprobe analyses Table 2. The data was performed using CHIME age-computer program yielded ages between 543 to 657 Ma. This is the first time that a Pan-African age (543 - 657 Ma) is recorded for a U-mineralization in Gabal Gattar younger granite.
7. Results
A total of 43 spot on the pitchblende mineralization were analyzed using electron microprobe analyses. Electron scanning microscope (ESM) and electron probe microanalyses (EPMA) revealed that this pitchblende contains significant Zr content reach upto (66.80% ZrO2) which indicate that volcanic rocks were probably the source of this mineralization. Lead content of the pitchblende mineralization is high and with moderate volcanics (up to 7.71% PbO). In contrast, it is low in ThO2, Y2O3 and REE2O3. According to the calculation U-Pb chemical ages using U, Th and Pb content measured with an electron microprobe for this pitchblende yielded ages within 543 - 657 Maindicating a Pan-African age for this mineralization. This is the first time that a Pan-African age (543 to 657 Ma) is recorded for a U-mineralization in Gabal Gattar younger granite in the north Eastern Desert, Egypt.
8. Conclusion
Detailed petrographic observation, and electron scanning microscope and electron microscope analyses revealed that the pitchblende of Gabal Gattar younger granite was accompanied with high content of a number of accessory minerals such as zircon, lead and fluorite. In contrast, this pitchblende is low in ThO2, Y2O3 and REE2O3. High Zr and Pb content associated with pitchblende mineralization from Gattar granite indicates that the source of this mineralization derived from volcanic magma not from granitic magma. The results given in the present study, essentially by the chemical Th-U-total Pb dating, allow us to define a preliminary age lying between 543 and 657 Ma for pitchblende mineralization of Gabal Gattar younger granite. The age obtained for pitchblende mineralization indicates that the mineralization formed in the same age of Gabal Gattar intrusion. This is the first time that a PanAfrican age (543 - 657 Ma) is recorded for a U-mineralization in Gabal Gattar younger granite.
Acknowledgements
We are grateful to Prof. D. Michel Cuney and his assistance Prof. D. Marc Bround, Centre de Recherchessur la del’ Uranium, Nancy, France for all their help in microprobe analyses and interpreting the data.