الفهرس 
geologic setting and petrography178Only 14 pages are availabe for public view
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Abstract
The present work deals with mineralogical and geochemical studies of the Lower Nubia sandstones and the volcanic rocks of Gabal El Ghorfa area, Wadi Natash, south Eastern Desert, Egypt. During the field trips, the author discovered visible mineralization by necked eyes in Lower Nubia sandstones (LNSS) for the first time. So, the mineralogy and geochemistry of the LNSS are studied in details to give a complete picture on Gabal El Ghorfa ring dyke.
The studied area covers about (130) km² of the exposed volcanic rocks at Gabal El Ghorfa area. It is delineated by longitudes 34° 07′ 03″ to 34° 17 31″ E, and latitudes 24° 26َ 30ً to 24° 32َ 20″ N. from the geological and petrographical studies, we concluded these main rock types : Alluvium (recent) ……………………… (Youngest)
Upper Nubia sandstones (UNSS)
Volcaniclastic sediments
Volcanic flows
Lower Nubia sandstones (LNSS)
Metamorphic rocks ………. ……………... (Oldest)…
1-Geologic setting:-
The Natash volcanic sequence is represented by three distinct flow units, separated by sequences of volcaniclastic sediments. Each of the three flow units shows a gradual change in composition upwards from alkali olivine basalt through trachybasalt, trachyandesite, to trachytes. The lowest flow unit rests on basal part of Nubia Sandstone, while the upper Nubia Sandston overlies the upper volcanic flows and volcaniclastic sediments. The volcaniclastic sediments comprise agglomerates, pumice, scoria and lithc tuffs and tuffs. The latter contain leaf imprints of Cenomanian age (Crawford et al., 1984), which are in good agreements with a 90Ma K-Ar age for the lavas determined by Ressetar et al., (1981).The volcanic sequences and their intercalated volcaniclastic sediments dip 5o westwards. Erosion has re-moved the upper flow and the upper volcaniclastic sequence from most locations in the east.
Two ring structures (Gaziret Khashm Natash and Western Ghorfa) and one ring dyke (Gabal El Ghorfa) have extruded through the three volcanic flows, particularly in the eastern area. The three rings range in composition from volcanic flows (represented by trachybasalt, trachyandesite, normal and alkaline trachyte) to volcaniclastic sediments (tuffs and agglomerates). They extruded into Lower Nubia sandstones (LNSS), which represented by conglomerate (at the base) followed by greywacke, quartz arenite and calcareous sandstone (at the top).
Gabal El Ghorfa forms a ring dyke with a diameter of 1.0 km and is situated in the southern part of Wadi Natash. Gabal El Ghorfa can be classified into:-a) lower Nubia sandstones and b) normal and alkaline trachyte, while Gaziret Khashm Natash forms a horse shoe-ring structure, 0.5 km in diameter and is dominated by a) lower Nubia sandstones, b) trachybasalt and trachyandesite and c) tuffs and agglomerates and Western Ghorfa constitutes a small ring structure consisting of a) lower Nubia sandstones and b) trachybasalt, trachyandesite and trachyte.
2-Petrographically:-
The volcanic rocks are characterized by very fine grain size and include; alkali olivine basalt. The rock is fine-grained holocrystalline, compact, and black to dark grey in color with aphyric to porphyritic texture. The essential minerals are olivine, plagioclase, pyroxene and K-feldspars. Epidote carbonate, iron oxides and clays are the secondary minerals. Trachybasalt is pale brown in color and composed mainly of sub-equal amounts of plagioclase (An50-55), alkali feldspars and olivine. Apatite and opaque are accessories. . The fine columnar laths of plagioclase are following the flow (trachytic) texture of alkali feldspars. Trachyandesite are composed of feldspars plagioclase (An40-43) and K-feldspars (orthoclase and sanadine) about (62-70 in vol. %), and hornblende (22-30 in vol. %). Accessory minerals are represented mainly by apatite and opaques. The dominant textures are trachytic and subtrachytic. Trachyte is characterized by buff to reddish brown in color according to the intensity of iron oxide stained alteration, mottled by pale green patches. Trachyte could be extinguished into two types according to the presences or absences of alkaline minerals; a) normal trachyte is composed mainly of sanadine (80-90 in vol. %), plagioclase (An14). Carbonate and quartz are secondary minerals. Autunite and opaques are accessory minerals, and b) alkaline trachyte composed of sanadine (75 in vol. %), alkaline pyroxenes (aegirine and aegirine augite) and minor amount of secondary quartz and carbonates. This type of trachyte is characterized by absences of plagioclase.
Pyroclastic rocks comprise; a) Trachytic lapilli tuffs, these rocks are white, grey, red and maroon in color. They are composed of secondary quartz, fine euhedral to sub-hedral crystals of sanidine, and mafic minerals as well as lithic fragments of trachyte, b) Pumice, the rock is characterized by cavernous appearance, have rough surface and the average of porosity about 70%; some vugs are occupied by carbonates. The rock is trachytic in composition, composed of K-feldspars (sanadine and anorthoclase) (50 in vol. %), plagioclase and opaque. carbonate and muscovite is secondary minerals and c) Scoria is deep black in color, porous, and basaltic in composition. The dominant texture is glomeroporphyritic texture. The main components of the rock are plagioclase (An55-68) represents about (30 in vol. %) of the rock, pyroxene, hornblende, actinolite and olivine.
The lower member of Nubia sandstones are well to badly sorted and contains amount of clay matrix <5%. They are white to pale brown in color and subdivided into greywacke, quartz arenite and calcareous sandstone. Greywacke grains are represented by angular, sub-angular to sub-rounded quartz, and sub-angular microcline feldspars. Accessories are zircon, monazite, epidote and rutile. Quartz arenite is composed mainly of monocrystalline mature, well sorted grains of quartz and microcline and perthite feldspars. Zircon, epidote and uranium minerals are the main accessories. Calcareous sandstone is black in color according dominance of both carbonates and iron oxide (about 40%of the rock) representing the matrix, while the grains are mainly quartz.
3-Mineralogical results revealed that:-
a) The volcanic rocks contain autunite and kasolite as uranium minerals, with association of brass alloy, wolframite, cerussite, and molybdite.The accessory minerals are represented by monazite, xenotime, allanite and plumbogummite.
b) The Lower Nubia sandstones contain metaheinrichite, autunite, uranophane and uranothorite as radioactive minerals, with association of gold, argentite, pyrite, chromite, galena, zincite, atacamite and barite. The main accessory minerals are zircon, monazite, allanite, columbite, yttrocolumbite, yttrotantalite, apatite, titanite, fluorite, rutile, spinel and garnet.
4-The geochemical studies revealed that:-
1) The volcanic rocks
Trachyte is higher in SiO2 and K2O (64.5% & 4.6% respectively) and lower in Al2O3, TiO2, Fe2O3, CaO and P2O5 (13.41%, 0.4%, 6%, 1.8% &0.3% respectively) than other volcanic rocks. Al2O3 and MnO content are the highest in trachybasalt (17%&0.4% respectively). Basalt rocks have the highest content of CaO, MgO and P2O5 (8.8%, 7.5 %&0.7% respectively), and have the lowest Na2O content (3.1%). TiO2, Fe2O3 and Na2O content (3.2%, 14% &11% respectively) are the highest.
The studied volcanic rocks having nearly the same values of agpaitic index (Av.=1 for trachyte, Av.=1 for trachyandesite), (Av.=0.6 for trachybasalt and Av.=0.5 for basalt).The highest Ba is recorded in trachytic volcanic rocks (Av.=1158 ppm), while basaltic volcanic rocks have the highest Sr, Cr and Ni (Av.=725, 229&134 ppm, respectively). The highest HFSE (Hf=17.7 ppm and Ta=8.5 ppm) are recorded at trachyte volcanic rocks. Among the studied volcanic rocks, there is a progressive increase in the Rb/Sr ratio with increasing differentiation from trachytic volcanic rocks (Av.=0.4),through trachyandesite (Av.=0.5) to trachybasalt (Av.=0.2).
The average ratios of Ba/Sr (5 for trachyte, 4.6 for trachyandesite, 4 for trachybasalt and 1 for pumice volcanic rocks) are considerably higher than the characteristic averages for calc-alkaline andesites (Ba/Sr=0.7) given by Taylor, (1969). Nearly all samples are generally not affected by alteration. Generally El Ghorfa volcanic rocks show high U (211-190 ppm) and Th(43-55 ppm) contents .
Geochemically, the investigated volcanic rocks were originated from an alkali magma rich in total alkalis, and the tectonic setting is continental basalt. The studied samples might be a result of a continuous magmatism and that they were derived from the same magma source by fractionation processes. The high TiO2magma is being the early fractionated, while the low TiO2 magma is produced from the more fractionated varieties. The studied volcanic rocks imply a significant degree of fractional crystallization along trend BC. This leads to the conclusion that the trachytic magma did not incorporate significant amount of crust that in turn does not affect the gross ultimate composition of the trachyte .The observed enrichment in the REE of the studied volcanic rocks is mainly due to increasing their LREE abundances.
The studied volcanic rocks have steep LREE, nearly flat HREE and a negative Eu anomaly. The studied volcanic rocks have nearly they differ in their ΣREE content and intensity of their –ve Eu anomalies. The –ve Eu anomaly is either due to the partitioning of Eu into feldspar during fractionation, which is an important process in developing alkalinity, or the presence of residual feldspar in the source (Singh et al., 2006). Another alternative explanation for the negative Eu anomaly is based on the high oxygen fugacity in the melt due to volatile saturation (Grenne and Roberts, 1998). The low Y and REE contents result from sub-solidus metasomatic effects which led to the mobilization of these elements in the form of carbonate complexes.
2) The Lower Nubia sandstones deposited in semi-arid to semi-humid climatic conditions. Sr and V show weakly positive correlation with SiO2(r=0.2 and 0.1) respectively, whereas large cation elements (Rb and Pb), show decreasing trend with silica(r=-0.3 and -0.2) respectively. This indicates that clastic detritus was associated with quartzose source component. In most mafic igneous rocks (Cr, Co, Ni, Ti and V) show similar chemical behavior which on a weathering regime gets fractionated. Sr content is relatively high where it enters the basin of deposition and is mainly associated with calcite.
The positive relationships between uranium and thorium chemically suggest strong U- enrichment. Moreover, Th vs. U/Th, show strongly negative correlation, confirming U- enrichment in the studied samples, high U (140-185ppm) contents and high Th(50-57ppm), once compared with the average concentration of the arenaceous sediments reported by IAEA(1979) (U=0.5-2ppm and Th= 2-6ppm). from the tectonic setting point of view, the lower Nubia sandstones display the passive margin environments. from the complete rare earth elements (REE) data,chondrite –normalized rare earth element of the lower Nubia sandstones are characterized by (1) enrichment in light rare earth element (LREE), (2) depletion in heavy rare earth element (HREE), extreme depletion of heavy REE relative to the light is most likely to indicate the presence of garnet in the source and (3) negative Eu- anomaly.
The removal of feldspars from a felsic melt by crystal fractionation or by partial melting of a rock in which feldspars is retained in the source will give rise to a(-ve) Eu anomaly in the melt. The principal carriers of REEs are the accessory minerals such as monazite, allanite and columbite. The REE is generally not lost during weathering. High enrichment of LREE in lower Nubian sandstone samples is controlled by the abundance of monazite mineral. The Eu anomaly seen in the LNSS samples may not be attributed to effects of weathering, and attributed to the presence of Eu depleted felsic rocks, such as granite in the source region (Gao and Wedepohl, 1995).
5- The radiometric measurements:-
Exhibit an expanded enrichment of Uch with respect to eU. The U-bearing solutions have been originated by leaching from normal trachyte or by ascending hydrothermal solutions (oxidizing environments) along channel ways. The secondary uranium minerals formed by the combination of the U-bearing solutions with other cations such as Ca, Pb, Ba and anions such as P2O5 and SiO2 (forming uranophane, kasolite, autunite, metaheinrichite).
It is worthy to state that this work reveals the presences of a group of newly recorded minerals such as uranium mineral (metaheinrichite, , with association of gold, argentite, zincite, atacamite and barite.