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Abstract
Gabal (G.) Milaha area represents a part of the north Eastern Desert of Egypt; it covers about 620 km2 of crystalline basement rocks. It is bounded by latitudes 27° 27` and 27° 39` N and longitudes 32° 55` and 33° 12` E. According to the field studies, the rock types, of Precambrian age, cropping out in the area are classified into: (1) metavolcanics (oldest), (2) metagabbros, (3) older granitoids, (4) Dokhan volcanics, (5) Hammamat sedimentary rocks, (6) younger granites, (7) ring complex, and (8) Dykes and veins (youngest).
The metavolcanics (basic, intermediate and acidic rocks) and related metapyroclastics represent the oldest rocks in the study area. The basic metavolcanics mainly consist of metabasalts, the intermediate metavolcanics are composed of meta-andesites while the acidic varieties are very rare and are mainly represented by metadacites. They are intruded by metagabbros, older granitoids and younger granites that took xenoliths of different shapes and sizes from them. The contact between the metavolcanics and the intrusive rocks is sharp, while the contact between the Dokhan volcanics and metavolcanics is not clear due to the similarity of colour between them. Sometimes, the metavolcanics appear as roof pendant over the younger granites which send several offshoots through them.
The metagabbros are exposed as small isolated and scattered hills at W. Ladid al Ji’dan and W. Milaha ath- Thimilah. Along the shear zones, they are subjected to hydrothermal solutions that are manifested by silicification, carbonitization, and hematitization. These rocks intrude the metavolcanics and took xenoliths of different shapes and sizes from them. They are intruded by the older granitoids and younger granites, which send several offshoots into them.
The older granitoids in the study area are mainly ranging from tonalite to granodiorite. They are characterized by low to moderate relief, exfoliation and bouldery weathering with characteristic monumental shapes. In some parts, These rocks are carbonitized and hematitized, especially in the highly fractured zones due to the effect of hydrothermal solutions. Sometimes quartz veins and veinlets fill these fractures. The older granitoids intruded the metavolcanics and display sharp intrusive contacts. On the other hand, the older granitoids are extruded by the Dokhan volcanics and intruded by the younger granites, which took xenoliths of different shapes and sizes from them.
The Dokhan volcanics extrude the metavolcanics, metagabbros and older granitoids. They are intruded by the younger granites which took xenoliths of various shapes and sizes from them. The Dokhan volcanics in the study area are represented by a successive sequence of lava flows ranging in composition from intermediate to acidic varieties with their related pyroclastics. The amygdaloidal texture indicates that lava was rich in gases and volatiles which escaped near the surface during the semi-consolidation state. The lava flows are affected by low grade metamorphism and alteration processes. The main alteration processes are chloritization, sericitization and silicification. The microfractures of these rocks are sometimes filled with quartz and feldspar veinlets.
The Hammamat sedimentary rocks of the study area are exposed as scattered masses of low to moderate topography. They are mainly represented by conglomerates, greywackes and siltstones. These rocks are slightly metamorphosed and have different colours of light grey, greenish grey, deep green and black. The Hammamat sedimentary rocks are intruded by the younger granites and locally intercalated with Dokhan volcanics.
The younger granites are medium to coarse grained, greyish pink to red in colour and form a number of moderate to relatively elevated outcrops. They intrude the metavolcanics, metagabbros, older granitoids, Dokhan volcanics and Hammamat sedimentary rocks and send several offshoots through some of them especially, metavolcanics and metagabbros. The contacts between these younger granites and the older rocks are sharp intrusive and usually dip toward the latter. The main features of these younger granites are exfoliation, cavernous and bouldery weathering. These rocks are altered in some parts due to secondary processes especially along the fault zones and contacts; the most common alteration features are chloritization, kaolinitization and epidotization. The studied younger granites are massive. Their peripheries become fine grained along their contact with the older rocks and become darker in colour due to the assimilation of the enclosed xenoliths. The microfractures along these younger granites are sometimes filled with quartz and feldspar veinlets.
The ring complex is made up of three rock types arranged as an outer rim, an inner core and capped volcanic rocks. The outer rim is formed of nepheline syenite. It is undeformed, massive, white and shows heterogeneity in grain size and texture. The nepheline syenite is partially altered to kaolinite and shows cavernous weathering especially in the altered pegmatitic syenite where the large crystals are easily weathered. The inner core lies at the central mass of the main ring complex and built up of quartz syenite of pink colour. It is medium and coarse grained with locally intensive spheroidal weathering. The quartz syenite intrudes the nepheline syenite rocks and took xenoliths from them. The volcanic rocks occur as a cap on the top of the complex and are enclosed as xenoliths within the complex. They are dominantly trachytic in composition and include both flows and agglomerates.
Numerous acidic, intermediate and basic dykes of variable mineralogical composition, length and thickness are recorded traversing the mapped area. These post-granitic dykes represent the last igneous manifestation in the area. The majority of them show a uniform thickness ranging from 0.5 to 5 m, however, few dykes attain a variable thickness along their strike. They are fairly straight indicating that they have been apparently emplaced along sharply defined fractures. However, some dykes show local sinuous behavior and gentle curvature. They have a tendency to occur in parallel swarms. According to the field relationships, the acidic dykes are the oldest while the basic dykes are the youngest.
The acidic dykes are the most abundant dykes in the studied area. They comprise felsites, rhyolites, dacite and aplites. Aplites (micro-granites) are encountered as dykes or sheets cutting through all types of country rocks. The acidic dykes mostly strike in NNW-SSE and N-S directions with vertical or steep dips to the ENE, WSW, E and/or W
directions respectively. The acidic dykes are usually of light yellow, pink and red colours.
The intermediate dykes are dark grey to green in colour, hard, massive, fine to medium grained and porphyritic in texture. They include andesite and andesite porphyry. The basic dykes are less common in the studied area. They are mainly represented by fine grained, dark grey to black basalts to dolerites.
Quartz veins are less common in the studies area and generally occur filling joints, fractures and fault zones. Quartz veins mostly strike in the NNE-SSW direction with vertical to steep dips. The quartz veins are generally of milky–white colour, sometimes they are stained with iron oxides.
Pegmatites are not commonly encountered in the studied area, they are found as small pockets, lenses or pegmatitic veins along some fault zones. They are associated with the younger granites and exhibit sharp contact with them. The pegmatite pockets are of variable size ranging from 0.5 to 1.5 m in width and from 1 to 2.5 m in length. They are mainly composed of intergrowth of milky quartz and reddish pink K-feldspar with or without mica, which is mainly represented by muscovite and biotite. One of these pegmatite bodies record high radioactivity levels (70-190 ppm), so that it is considered as anomalous pegmatite. This anomalous pegmatite is found as a lens of 2.0 x 8.5 m in dimensions at the peripheral part of the younger granites of W. Ladid al- Ji’dan. It is considered as the most radioactive rocks in the area.
The studied area displays primary and secondary structures. Primary structures comprise layering, volcanic flows, vesicular tops of lavas, volcanic bombs, bedding, lamination and graded bedding. The secondary structures are represented by folds, foliation, joints and faults.
In the studied area, the majority of joints are tension joints. These tension joints may be still empty or filled with quartz, feldspars, epidote, iron oxides, manganese oxides and aplite. The total number of the measured joints on the various rock exposures in G. Milaha area reaches 1300 joints. The poles of these joints are plotted on an equal area stereographic projection. The main joint sets could be distinguished into:
1- The first predominant joint set is striking N-S and steeply to very steeply dipping to both the E and W directions, the latter is greater in number.
2- The second dominant joint set is striking ENE-WSW and steeply dipping to both NNW and SSE directions, the former is greater in number.
3- The third abundant joint set is striking NNE-SSW and dipping to both ESE and WNW directions. It is noticed that the majority of these joints are steeply dipping to the WNW direction.
In the study area, faults are traced according to several field observations as: displacement of veins, dykes and contact lines, mylonitization, and slickensided surfaces along fault planes. The fault zones are marked by fault breccia and alteration features, especially silicification, hematitization, kaolinitization and chloritization. Based on length and number proportions of the measured faults revealed that the most predominant fault trends are N-S, NE-SW, NNE-SSW, ENE-WSW and NNW-SSE directions.
Geochemically, the metabasalts of the studied area originated from a low-K tholeiitic magma in an immature island arc, the meta-andesites and metadacites originated from a medium-K calc-alkaline magma in an immature island arc. The studied metagabbros are sub-alkaline (slightly tholeiitic) of island arc tectonic setting environment.
The intermediate Dokhan volcanics are considered as medium-K rocks originated from calc-alkaline magma in island arc to active continental margin environment. The acidic Dokhan volcanics are considered as medium to high-K rocks originated from calc-alkaline magma in island arc to active continental margin environment.
The sources of the studied Hammamat rocks are felsic and intermediate igneous rocks (volcanic and plutonic) of continental arc tectonic environment.
The geochemical studies revealed that the studied older granitoids were derived from upper mantle and could be considered as volcanic arc granitoids or continental arc granitoids. On the other hand, the younger granites were considered as late-differentiate granites emplaced at the end of the Pan-African orogenic activity in Egypt. They were derived from sialic crust and originated under low pressure extensional conditions. They could be considered as late orogenic granites. Generally, both the older granitoids and the younger granites are intruded due to orogenic magmatism. The studied older granitoids are considered as I-type granitoids, originated from metaluminous calc- alkaline magma. They are emplaced at a moderate depth in the earth crust at water pressure more than 3.5 kb. They have been derived from
the mantle (M-type) and crystallized at temperature ranging from 950 to 1100o C. On the other hand, the younger granites are considered as I-type granites, originated from peraluminous calc-alkaline magma. They are intruded at shallow depth in the earth crust at water pressure between 1 and 3 kb. They have been derived from the crustal sialic materials (C-type) and crystallized at a temperature ranging from 800 to 850oC.
The studied ring complex are originated from alkaline magma in within plate environment.
The area of Gabal Milaha was radiometrically surveyed by using portable gamma-ray scintillometer detector (Gamma-gun), model FD-3013, which measure the total gamma activity in terms of parts per million (ppm). During the survey, almost all lithologic types exposed in the studied area were more or less radiometrically surveyed. Particular attention was paid to all structural features such as contacts, joints, fault planes and other fractures.
The field background radioactivity levels for G. Milaha area show very wide range due to lithologic variation, the anomalous pegmatites show the highest radioactivity level. The radiometric data, of G. Milaha area, expressed in ppm are grouped in nine zones:-
1- Basic and intermediate metavolcanics and metagabbros are mainly represented by the range of radioactivity less than 3.0 ppm as well as the acidic metavolcanics.
2- Intermediate Dokhan volcanics are represented by the zone of radioactivity varying from 2.0 to 4.0 ppm.
3- Older granitoids, acidic Dokhan volcanics and Hammamat sedimentary rocks are represented by the range of radioactivity between 2.0 and 6.0 ppm.
4- Syenogranites are represented by the range of radioactivity varying from 6.0 to 16.0 ppm.
5- Nepheline syenites (outer rim of W. Dib ring complex) and trachytes are represented by the range of radioactivity between 10.0 and 20.0 ppm.
6- Quartz syenites are represented by the range of radioactivity between 10.0 and 30.0 ppm.
7- Alkali feldspar granites are represented by the range of radioactivity between 20.0 and 30.0 pmm.
8- Normal pegmatites are represented by the range of radioactivity which varies from 25.0 to 50.0 ppm.
9- Anomalous pegmatites are represented by higher range of radioactivity between 70.0 and 190.0 ppm.
In the present work, the field background radioactivity level of the younger granites and pegmatites is much higher than the surrounding rocks. The uranium contents in the studied older granitoids range from 3.1 to 4.0 ppm with an average of 3.5 ppm. The Th content varies from 10.0 to 11.6 ppm with an average of 10.7 ppm. The Th/U ratio values range from 2.87 to 3.25 with an average of 3.1. On the other hand, the uranium content in the fresh younger granites ranges from 6.3 to 13.4 ppm with an average of 10.08 ppm and the Th content varies from 12.1 to 25.2 ppm with an average of 19.34 ppm. The Th / U ratio is ranging from 1.86 to 1.98 with an average of 1.92.
The studied younger granites are considered as uraniferous granites, which are characterized by the following items:
1- They are affected by tectonics, which caused fracturing, faulting and shearing; these weak zones are very suitable channels for penetrating hypogene and supergene fluids.
2- Petrographically, it is represented as two mica-two feldspar granites.
3- The very high contents of alkali feldspars.
4- The presence of dispersed fluorite in the studied samples.
5- High silica content (more than 73 %).
6- Zr/Sr ratios are greater than 1.65.
7- K/Rb ratios are more than 125.
8- High values of D.I. (more than 88)
9- K2O/Na2O ratios are more than 1.
10- Fe2O3/FeO ratios are more than 1.
11- L.O.I is always less than 1%.
12- They are considered as S-type peraluminous granites.
13- They are considered as post orogenic granites.
14- Uranium contents are more than twice the Clarke value (4 ppm).
15- Th/U ratios are less than 3.
Pegmatites show higher uranium contents relative to both types of the studied younger granites. The uranium contents in the studied normal pegmatites range from 30.6 to 40.2 ppm with an average of 35.7 ppm while the Th content ranges between 31.1 and 41.1 ppm with an average of 36.3 ppm. The uranium content in the anomalous pegmatites varies from 81.4 to 99.2 ppm with an average of 90.7 ppm, while the Th content varies from 65.2 to 78.3 ppm with an average of 71.7 ppm. Generally, the average of U and Th in the studied normal and anomalous pegmatites is higher than U and Th of world uraniferous pegmatites and Egyptian uraniferous pegmatite.
It could be concluded that the high uranium content in younger granites is related to the presence of zircon, but in pegmatites the high uranium content is related to the presence of columbite, pyrochlore and zircon as well as iron oxy hydroxides.
It is important to mention that during this study the high uranium content in both younger granites and pegmatites could be considered as source rock for uranium. So, this fact is rather important in planning for more detailed uranium exploration in similar rock types in other areas.