Al-Azhar Bull. Sci. Vol. 21, No. 2 (Dec.): PP. 19-43, 2010

TECTONIC FRAMEWORK OF EAST GABAL NUQRA AREA,

SOUTH EASTERN DESERT, EGYPT

MOSTAFA, M.S., IBRAHIM, I. H. AND HASSAAN, A. H.

Nuclear Material Authority, Cairo, Egypt.

Abstract

A singular complex-shaped, circular structure of crescent-shape (centered on

24o22

’N and 33

o45

’30

’’E), about 10 km in diameter, are located to the east of Gabal

Nuqra at the extreme western part of Natash volcanic in the south of Eastern Desert,

Egypt. This structure is a clear incomplete ring dyke complex that produced by

volcano-plutonic systems comprising a suite of alkaline granite intrusions and

volcanic ring dykes (dacite porphery, alkaline rhyolite and granophyre) extruded by a

recent trachy-basltic flow. These rocks cut into the local geology of the Nubian

Sandstone, which defines the rim of the structure that emplaced through out three

magmatic events, the 1st (granites) associated with development of the Nubian

sandstone basin, the 2nd

(volcanic ring dykes) is post to its formation (Natash

volcanics) whereas the 3rd

(trachy-basltic flow) is related to the Red Sea rifting.

The detailed field studies show that east Gabal Nuqra area has been affect by

at least three tectonic events where the reactivation interaction of the preexisting NW-

SE with the E-W to ENE-WSW basement fabrics as normal configure the Nubian

sandstone basin while the granitic intrusions are mainly controlled by the NW-SE

structural trend whereas their reactivation localize the emplacement of the ring dykes

and their interaction with NE-SW ones configure the trachy-basltic flow.

Hydrothermal activities enriched in REEs, Zr, Nb, Ba, U and Th that could be

late to post 2nd

magmatic event have been clearly recorded at the ring dykes complex

at east Gabal Nuqra area that confirmed by the presence of hydrothermal zircon,

REE-silicate, cerrusite (Pb-carbonate), kasolite and barite among the whole ring

structure. These rocks show eU-contents ranging from 2 ppm to 45 ppm and eTh-

contents ranging between 5 ppm and 98 ppm. The all area is affected by highly

ferrugination and silicification.

Key words: Gabal Nuqra, ring dykes, fault systems, tectonic setting & Nubian

sandstone basin.

Introduction

Over, 50 discrete ring complexes (mostly per-alkaline granites) had been

emplaced throughout the Arabian Shield (Stoesser and Elliott 1980) and more than

130 ring complexes in Egypt, Sudan and Ethiopia with their greatest concentration

between Lat. 18˚ to 25˚ N in the Eastern Desert and Nile Valley. Razvalyayev and

20

Shakhov (1978) showed that all of the ring complexes of the Arabian Nubian Shield

are located along the lines of rift valley fractures. As in West African reactivation of

pre-existing fault pattern and transcurrent faults in particular appear to control the

location of the ring complexes. In Egypt, the intrusions alignment corresponds to

onshore extension of transform faults, pre-date ocean-floor spreading (Serenesists et

al. 1979). Without doubt the 180 Ma of the Mesozoic era was the time of the greatest

and most widespread of all the ring complex developments in Africa. The Eastern

Desert (ED) of Egypt, contains a number of ring complexes and alkaline plugs, which

range in age from Pan-African late-orogenic (Wadi Dib, 554 ± 15 Ma) to the

anorogenic complexes between 404 ± 8 Ma (Zargat Naam) and 351 ± 7 Ma (Tarbtie

North) described elsewhere (Serencsits et al. 1979), to the younger group of E1

Khafa (92 ± 5 Ma) and Abu Khruq (89 ± 3 Ma). The latter complex is a 6 x 5 km

oval intrusion of nepheline syenite and aegirine granite ringdykes. Early

geochronological K- Ar investigations (Higazy and E1-Ramly 1960) provided

isotopic ages of 55 Ma and 40 Ma for these rocks, which compare with the Uweinat

activity (Vail 1989).

Fig. (1): Landsat TM ratio image, using 5,3,2 bands in R,G,B, showing a well

defined rock types.

21

El-Ramly et al., (1971) classified the alkaline rocks in the south ED into four

main groups; a) complete ring complexes, b) incomplete ring complexes (crescent-

shaped intrusions), c) lava fields with agglomerate plugs (domes) and ring dykes and

d) isometric mass of alkaline granites and syenites. Hashad and El-Reedy (1979)

recorded three phases of this igneous activity forming anorogenic alkaline rocks by

using Rb/Sr and K/Ar dates into the 250–200 Ma, the 150–125 Ma and the 110–70

Ma. About sixteen ring complexes have been identified in the ED intrude the

Proterozoic gneisses, metasediments, island arc volcanics and older granitoids ((El-

Ramly and Hussein 1982). They ranges in age from 554 Ma (W. Dib) to 89 Ma

(Abu Khruq), ranging from Cambrian to Cretaceous age (Lutz, 1979).

The present work aims to study the structure framework, rock types,

mineralization and radioactivity as well as reconstructing the tectonic setting

corresponding to this new ring dyke complex that located to the east G. Nuqra

between Long. 33o40´30˝ – 34

o49´30˝E & Lat. 24

o20´ – 24

o24´30˝N (Fig.1).

Geologic Setting

The ring structure of east G. Nuqra is a clear incomplete ring dyke complex

concave to the north-east with a maximum elevation point reaching about 200 m

above wadi level with diameter exceeds 10 km located at the extreme weastern part of

Natsh volcanic and east of G. Nuqra. The petrographic analysis, IV, shows that the

rock types of the area comprise a suite of plutonic granite intrusions and volcanic ring

dykes extruded by a recent trachy-basltic flow that cut all into the local geology of the

Nubian Sandstone, which defines the rim of the structure and overlies unconformably

the metavolcanics (Fig.2). The trachy-basalts flow is exposed at the eastern part of

ring dyke complex enclosing parts of the Nubian sandstone. The visible thickness of

these volcanic rocks ranges from 100 to 150 m. Although, both these ring dykes and

the trachy-basltic flows are extruded in the Nubian Sandstone, no observable field

relations have been recorded between them.

The main masses of ring dyke complex east of G. Nuqra are composed mainly

of greenish, coarse to very coarse grained quartz-bearing alkaline granite grading

north-westwards and north-eastwards into alkali-feldspar granites. Generally the

alkaline granite includes quartz veinlets and pegmatoidal patches. The main mass as a

whole is cut by numerous ring dykes includes dacite, granophyre and alkaline rhyolite

dykes.

The contacts between the ring dyke complex and the Nubian sandstone are

almost sharp (Fig. 3a), with inclined bedding (Fig. 3b) near the contacts to nearly

vertical and distinct structure contacts (Fig. 3c) and significant by a thick zone of

brecciation. Northwards, along the contacts with the alkaline granite, the Nubian

sandstone is thermally affected as indicating the altered and ferruginated NW-SE and

ENE-WSW fractures that sometimes filled with quartz and jasper vein (Fig. 3d).

22

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23

Fig. (3) Field Photographs showing the sharp contact between Nubian sandstone and

granite, (a); the structure contact between granite and Nubian Sandstone, (b); the

subvertical and inclined bedding of Nubian sandstone near the contact, (c); the alteration

of Nubian sandstone and a fracture filling with jasper vein, north G. Nuqra area, (d); a

close up view of the contact between alkaline granite and dacite porphery, (e) and a rhyolite dyke invaded between granite and Nubian sandstone, (f), G. Nuqra area.

A series of thick sub-latitudinal and radial dyke systems are the most

predominant in east of G. Nuqra area and composed of dacite porphery (Fig. 3e),

granophyre and alkaline rhyolite (Fig. 3f). They are exposed in the center to western

parts of the main mass of the ring dyke complex. These dykes took crescent shape and

extend to north-westward with dip steeply towards the centre of the complex. (60°-

70°). Further to the north, some sub-latitudinal dykes of brecciated aegirine granites

are encountered.

24

Structural Analysis And Tectonic Setting

The structural features prevailing East G. Nuqra area are characterized by

NW-SE, E-W to ENE-WSW, NNW-SSE to N-S and NE-SW main orientations. East

G. Nuqra ring structure and its hosted Nubian series are elongated in the NW-SE

structural trend and are delimited by E-W ones, whereas the trachy-basaltic flows are

localized at their intersection with the NE-SW trend (Fig. 2). The detailed field study

has been carried out among about 31 sites of measurements, distributed along East G.

Nuqra ring structure and its hosted Nubian series (Fig.2). Each site contains several

tens of fault kinematics defined by: (1) the coherence and the abundance of fault

populations and (2) by the quality of the outcrop. For each site an attention was paid

to cross-cut relationships; superposition of movement marks (slickenside lineations)

recorded on single fault plane and the unconformity contact surfaces as a good

chronological surfaces that separate between rock units of different ages which give

the possibility to express the tectonic events in terms of age (Fig.4).

Fig. (4): Examples of the field relative chronological criteria used in dating the tectonic

events recorded at East G. Nuqra area. (a) Example of absolute chronological criteria,

NW-SE structural contact between the Permo-Cretaceous Nubian series and the intruded

ring complex, (b) Example of relative chronological criteria recorded along ring structure,

where the NW-SE dextral strike-slip fault;1 has been reactivated as normal;2 then cross-cut by NE-SW trending.

The analysis of about 1110 fault slip data sets (875 normal faults, 192 strike-

slip faults & 43 reverse faults) as well as 89 joint sets using the direct inversion

method of Angelier (1990), revealed a well determination of the paleostress

orientations (Fig. 5). The geometry of normal fault populations show multidirectional

orientation dominated by E-W, N-S, NW-SE and NE-SW main trend clusters

(Fig.5a.1), whereas the strike-slip faults exhibit incomplete systems of E-W, NW-SE

& NNW-SSE trending dextral faults accompanied by NW-SE & NNW-SSE oriented

sinistral faults with NE-SW to ENE-WSW minor ones (Fig.5b.1). The strikes of the

reverse fault populations that restricted only to the Nubian series along bedding planes

are arranged around the NE-SW direction (Fig.5c.1).

25

The calculated paleostress tensors characterize the tectonic regimes in term of

mean stress axes, i.e. the directions of extension, 3 and/or that of compression, 1. A

total of 94 paleostress tensors have been calculated where 72 tensors characterizing

extensional deformation, 17 tensors exhibit strike-slip regime within extensional

context and 5 tensors corresponding to compression (Fig.5).

i - Extensional paleostress tensors:

Among the Nubian formations (Late Cretaceous) as well as the ring structure,

the measured normal faults show one broad main NW-SE trend, recorded with N-S to

NNE-SSW, ENE-WSW to E-W and NE-SW less dominant trends. These normal

faults are characterized by moderate to high dip angle that ranges between 60° and

80° while the dip direction is almost perpendicular to their azimuths. The slickenside

lineations of these faults show two geometrical categories dip-slip and oblique-slip as

more than 43% of the rake readings are higher than 70°, and about 55% are ranging

between 45° and 60°. These later ones may be interpret either as reactivation of the

pre-existing fault planes or as going under later on deformation phases.

The calculated paleostress tensors exhibit multi-directional pattern with 3

main extension (3) trends: NE-SW, NW-SE, NNE-SSW with ENE-WSW and

WNW-ESE minor ones. This multi-directional extension pattern indicates that, the

Nubian sandstone rocks have been subjected to successive extension phases.

- The NE-SW extension corresponds to the NW-SE to WNW-ESE trending normal

fault populations. In general, the calculated stress tensors are dominated by N40° to

N55° and N220° to N235° trending 3. The field investigation showed that some of

the NW-SE trending normal faults recorded tilted geometry (sites 19, 23; Fig.2),

while they were reactivated in others (site 21, Fig.2). This may indicate that this

extensional trend was initiated before the intrusion of the granite and volcanic ring

dykes suite and then rejuvenated after the emplacement took place. It is believed that

the fault slips correspond to N40° to N55° and N220° to N235° trending 3 (NE-SW

extension) are the main tectonic factor controlling the deposition in the Nubian basin.

26

Fig. (5): Summary of fault slip data analyses showing the geometrical frequency

distribution in roses of strikes, (1); dip angles, (2); pitch angles, (3) and the calculated

stress tensors, measured for:(a) normal fault systems,(b) strike-slip fault systems and (c) reverse fault systems at East G. Nuqra area.

27

- The NW-SE trending extension shows N 120° to N150° and N130° & N330° mean

orientation of 3 (sites 11,24,25 Fig.2). These calculated tensors correspond to NNE-

SSW and NE-SW trending normal faults. The field investigation of these faults

showed some of the NW-SE trending normal fault population are synsedimentary

ones (sites 11,24,41; Fig.2).

- The ENE-WSW to E-W trending extensional tensors are corresponding to the

NNW-SSE and/or NNE-SSW trending normal faults (sites 19,20; Fig.2). These

tensors are dominated by N75° to N115° and N255° to N295° mean trends of 3.

- The NNW-SSE to N-S trending tensors are corresponding to the ENE-WSW and/or

E-W trending normal faults (sites 22,25; Fig.2). They are dominated by N155° to

N175° and N335° to N355° mean trends of 3. The fault slips correspond to the

NNW-SSE to N-S extension are believed to be the bounder limits of the Nubian basin

as faulted bounded basin trending NW-SE and block faulting the E-W old one.

Reverse paleostress compressional tensors:

In the Cenomanian and Cenomanian-Turonian Nubian series (Fig.6a) the

reverse faults exhibit branched main tend around NE-SW direction with less dominant

NNW-SSE and WNW-ESE to E-W oriented ones (Figs. 5c1 & 6d). These reverse

faults are recorded along the Nubian bedding planes where they show low to

moderate dip angles range between 10° and 25°, while the angle of the slickenside

lineations are between 70°-80° (Fig. 5c1-c3). The calculated compresional paleostress

tensors correspond to these reverse fault populations revealed two WNW-ESE and

NNW-SSE trending compressions, 1 (Fig.5c4).

Strike-slip paleostress tensors:

The conjugated strike-slip fault systems exhibit two main clusters of azimuth,

the 1st cluster consists of ENE-WSW to E-W trending right lateral and WNW-ESE to

NW-SE trending left lateral strike-slip fault populations whereas the 2nd

is dominated

by NW-SE trending right lateral and NNW-SSE trending left lateral (more than 85 %

of the measurements; Fig.5b). These faults are of high dip angle (Fig.5b2), whereas

the slickenside lineations show 25°-40° main rake clusters (Fig.5b3). The calculated

compresional tensors corresponds to these strike-slip fault systems exhibit two main

clusters of 1 that oriented around the NW-SE and NE-SW associated with NW-SE

as more predominant trend (Fig.5b4).

28

Fig. (6): Photographs demonstrate the examples of the Nubian tectonics recorded at East

G. Nuqra area where, (a)= the structural contact between Nubian series and the ring

structure, (b)= the Cretaceous part of the Nubian series, (c)= paleostress analysis for the E-

W normal fault system and (d)= orientation and paleostress analysis for the intra-bedding

reverse fault system.

- The NW-SE trending major compression tensors corresponding to the ENE-WSW to

E-W trending right lateral and WNW-ESE to NW-SE trending left lateral strike-slip

fault system (Fig.7a&b) are dominated by N140° to N145° peak of 1 (Fig.5b4).

- The NE-SW to ENE-WSW trending compression tensors corresponding to NW-SE

trending right lateral and NNW-SSE trending left lateral strike-slip fault system are

dominated by N20° to N55° and N200° to N235° mean trends of 1 with N30° to

N35° trending peak of 1 associated with N 120° to N150° and N130° & N330° mean

orientation of 3 within an extension context (Fig.7d). The field observations recorded

the oblique geometry for some of the NW-SE trending right lateral and NNW-SSE

trending left lateral strike-slip system (sites 22,31 Fig. 7) indicating later reactivation

within extension context.

29

Synthesis:

The chronology obtained from the field relations are dominated by NE-SW

extensional trend. This extensional trend could be considered as syn-depositional

event that reactivated the preexisting NW-SE trending basement fabrics as dip-slip

normal faults as well as the E-W to ENE-WSW fabrics as oblique slip one delimiting

the NW-SE subsidence faulted bounded Nubian basin during Late Cretaceous period.

This extension was followed by successive extension trends caused the reactivation of

the pre-existing normal faults and dominated by ENE-WSW to E-W, NNE-SSW and

NW-SE trending extensions respectively. The NW-SE main compression event could

be attributed as an echo of the Senonian tectonic phase of inversion responsible for

Late Cretaceous folding in northern Egypt.

Fig. (7): Examples of fault tectonics affecting the ring complex at East G. Nuqra area,

(a&b) = fault kinematics and paleostress analysis of the main NW-SE strike-slip system,

(c)= paleostress analysis for the N-S normal fault system displace left laterally by NNW-

SSE strike-slip system and (d)= orientation and paleostress analysis for the NE-SW strike-

slip fault system.

30

Most of the preexisting normal fault trends were reactivated as oblique strike-

slip one to accommodate this compression. After this compressional episode, the

extension again predominated during the Paleogene Neogene period through a

predominant E-W and N-S multidirectional extension during the Paleocene.

Moreover, the NE-SW normal faults, NW-SE extension, have been found cutting

through all rock types among the study area. Red-staining of rocks due to fluid–rock

interaction during hydrothermal circulation in NW-SE and NE-SW fractures is a

common feature in the Nubian sandstone sequences. The advent of the Neogene

period was marked by intense tectonic activities, which had a great effect on the

present day morphology of Egypt. During the Late Oligocene time, the Gulf of Suez-

Red Sea rifting started as a result of a NE-SW extension. Subsequently, the dominated

NE-SW extension was associated with a strike-slip regime characterizing by NE-SW

extension and NW-SE compression. The two systems, strike-slip and distensif, pass to

each other by the permutation of 1 and 2 within a global extensional context

(Mostafa et al. 2004). This deformation phase could be explained so that the opening

of the Red Sea resulted in shearing along the Dead Sea fault zone during the Middle

Miocene.

Petrography

Petrographically, the incomplete ring complex of G. Nuqra is composed of

alkaline granites, ring dykes (dacite porphery, granophyre and alkaline rhyolite)

extruded by trachy-basaltic flows (youngest), whereas the whole structure is intruded

and extruded through the Nubian sandstones (oldest). Brief descriptions of

petrographic characteristics of these rocks from the oldest to the youngest are given

herein after.

The Nubian sandstones are moderate sorted, pale to dark brown in color and

composed mainly of quartz arenite (Fig. 8a). They are composed of sand size grains

of quartz (80-90 %), feldspars (5-15 %) and opaques, carbonates, chlorites, rutile,

epidote and zircon as accessories (5%). Quartz is present as subangular to sub

rounded grains and sometime stained by iron oxides. Feldspars comprise plagioclase

and perthite. The plagioclase occurs as sericitized subhedral to anhedral grains.

Perthite occurs as subhedral grains, sometimes kaolinitized.

Alkaline granites are medium- to coarse-grained, pinkish grey to yellowish

pink in color and composed mainly of K-feldspar (orthoclase perthites and perthites),

quartz, plagioclase and few alkali pyroxene (aegirine-augite). Sericite, kaolinite, and

epidote are secondary minerals. Potash feldspars are represented by orthoclase

perthite and perthite (63.4 – 70.5 Vol.%) are commonly subhedral to tabular laths (up

to 1.4 x 2.3 mm in dimension) and exhibit cloudy color due to slight alteration. The

31

orthoclase perthite form anhedral to subhedral crystals (up to 1.8 x 2.2 mm) and show

simple twinning. Perthite is subhedral crystals (up to 1.5 x 2.5 mm in dimension) and

characterized by patchy, flame and string types. Quartz is fine- to medium-grained

crystals (up to 2.5 mm across) and shows undulose extinction. They are intergrowth

with feldspars forming skeletal shape (Fig. 8b) and graphic textures (Fig. 8c). This

intergrown feldspar represents a later generation corresponding to the simultaneous

crystallization of feldspar and quartz interstitial. Plagioclases (An8-12) (0.2 – 6.1 Vol.

%) are mainly albite crystals grow at the peripheries of perthites and sometimes its

borders are corroded by quartz.

Table (1): Model analysis for alkaline granites, G. Nuqra area.

S. No. K-feld Plag. Qz Aeg. Op Acc. A P Q

1 70.5 0.6 21.4 4.6 3.3 0.6 77 0 23

2 69.7 0.4 22.6 3.9 3.0 0.5 75.5 0 23.5

3 70.1 0.2 22.1 3.7 3.5 0.4 76 0 24

4 67.1 5.6 22.5 2.1 2.1 0.6 70 6 24

5 65.5 5.1 24.1 2.6 2.2 0.5 69 5.5 25.5

6 63.4 6.1 24.3 3.4 2.4 0.4 67.5 6.5 26

S. No. = serial number, K-feld. = potash feldspar including perthite, Plag. = plagioclase, Qz. =

quartz, Aeg. = aegerine, Op. = opaques and Acc. = accessories. A = potash feldspar content, P = plagioclase content, Q = quartz content.(Note: All values are in volume percent).

Aegirine-augite occurs mostly as later products of crystallization of alkaline

magmas. They constitute (2.1 - 4.6 Vol. %) of the rocks and shows green color with

pleochroic from yellow green to green color. They are represented by euhedral to

subhedral crystals (up to 0.3 x 1.5 mm in dimension) of varies outlines which occur

between quartz and alkali feldspar. The iron oxides are exsolved from alkali pyroxene

during the alteration processes and arranged along the cleavage planes of mafic minerals

(Fig. 8c) while others crystals from feldspar are dissolved and filled with opaques and

secondary minerals (Fig. 9e) reflecting the effect of metasomatism.

Opaques occur as fine irregular grains. Hydrothermal zircon (Fig. 8d) is rare

and occurs as rounded and cracked crystals. Kasolite is euhedral to subhedral of dark

brown. Apatite occurs as inclusion of fine prisms in aegirine-augite.

Dacite porphery is fine-grained, hard, massive, reddish brown to greyish brown in

color and exhibit porphyritic textures. They are composed of potash feldspar,

plagioclase, aegerine-augite and few biotite. Opaques, quartz and apatite are

accessories. Kaolinite, sericite and carbonate are alteration products of feldspars. The

feldspar laths show preferred parallel orientation giving sub-trachytic and porphyritic

textures (Fig. 8e).

32

Fig. (8): Photomicrographs showing Qz arenite composition of Nubian sandstone,(a);

Graphic texture, alkaline granite (b) radiated acicular crystal as pseudomorph pyroxene,

alkaline granite, (c); Secondary zircon (Zr) along cracks and quartz boundaries, alkaline

granite,(d), Porphyretic texyure, dacite porphery, (e) micrographic texture, granophyre

dyke,(f); six sided of developed augite, alkaline rhyolite,(g) and olivine associated with subtrachytic texture, trachy-basalt, (h).

33

Potash feldspar is usually perthite, varies in size from tabular phenocrysts

(1.2×2.3 mm) to microlites (0.1×0.3 mm). Plagioclase occurs either as phenocrysts

(0.8×1.1 mm) or as fine groundmass. Biotite is fairly common, presents as subhedral

tablets in groundmass. It occurs as brown to deep brown in color, as well as, nearly

opaques. Quartz occurs with fewer amounts as subhedral and anhedral crystals that

form the groundmass with feldspar and iron oxides.

Granophyre is fine- to medium-grained massive rocks of pinkish grey color.

They are composed essentially of potash feldspar, quartz, plagioclase, and aegirine-

augite with sphene, apatite and zircon as accessories. Orthoclase perthite may grade

into orthoclase with spotted perthite and sometimes replaced by sodic plagioclase.

Plagioclase occurs as individual lath forming and showing polysynthetic twinning.

Quartz are intergrowth with feldspars and forming micrographic and granopheric

(Fig. 8f) textures. Aegirine-augite occurs as irregular outlines replaced by iron oxides

and biotite especially along cleavages. Opaques and zircon are associated with

ferromagnesian minerals.

Alkaline rhyolite is fine-grained of yellow to yellowish red colors. The

phenocrysts are mainly of partially albitized orthoclase, augite and albite embedded

in a fine-grained groundmass of orthoclase, quartz and augite. The rhyolites show

porphyritic and spherulitic textures. Augite is occurred as well developed crystal with

six sided (Fig. 8g) and showing second order interference colors.

Trachy-basaltic is greenish-black colour, fine grained, characterized by

subtrachytic texture (Fig. 8h) and composed mainly of plagioclase, olivine and

opaques. The phenocrysts are represented by olivine and its pseudomorphs (chlorite

and opaque minerals) and plagioclases. The groundmass is highly ferruginated and

chloritised. Apatite is accessory and calcite amygdules are recorded.

Ground Gamma-Ray Radioactivity and Mineralogical Studies

Uranium and thorium distribution maps (Aero Service Division, Western

Geophysical Company of America, 1984) show U- and Th-anomalies along the

eastern G. Nuqra (Fig. 9). The ground measured radioactivity (Table 2) in the study

area shows eU and eTh concentrations.

The eU contents ranging between 4 and 45 ppm and while the eTh-contents

ranging between 9 and 98 ppm in granite. Dacite porphery eU contents ranging

between 3 and 18 ppm and while the eTh-contents ranging between 5 and 36 ppm.

Granophyre eU contents ranging between 2 and 13 ppm and while the eTh-contents

ranging between 8 and 26 ppm . alkaline rhyolite eU contents ranging between 3 and

34

11 ppm and while the eTh-contents ranging between 8 and 23 ppm. The granites

show relatively higher radioactivity level than ring dykes. Accordingly, the eastern

and southern parts of the ring dykes show higher radioactivity level due to the

outcropping granites, with almost no radioactive or economic minerals records. The

kasolite, zircon and apatite are the most common recorded accessory minerals.

Fig. (9): Uranium and thorium distribution maps of the studied area, after Aero Service Division, Western Geophysical Company of America, 1984

35

Cerrusite (PbCO3) (Fig. 11a) is associated barite mineral in hydrothermal

environment. Cerrusite is easy soluble mineral within ascent hydrothermal solutions

due to elevated temperatures causing decomposition reactions of minerals then

transport by hydrothermal solutions and form a metal-bearing complex.

Ni-silicate (Fig. 11b) in the studied ring dyke complex is suspected to have

been derived from the nearby peridotites or serpentinized peridotites body during

hydrothermal alteration and circulation. The analyzed sample data show 73.6% NiO.

Zircon (ZrSiO4) in the studied ring dyke (Figs. 11c&d ) is mainly hydrothermal

type and different from magmatic in both textural and compositional characteristics.

Hoskin and Schaltegger (2003) conclude that the hydrothermal zircons may be zoned

or unzoned; spongy in texture; anhedral or faceted in morphology; and either high or

low in common-Pb. In addition, light rare earth element (LREE) abundances may

differ from associated magmatic zircons (e.g., Hoskin, 2005; Pettke et al., 2005). The

studied zircon is mainly hydrothermal origin and associated REEs, Pb and Ba. These

so-called hydrothermal zircons are believed to precipitate from aqueous fluids, in

most cases at relatively low temperatures, rather than from magmas. The analyzed

sample data show 53.9 % ZrO2.

Barite (BaSO4) in the study area is euhedral crystals (Figs. 11e&f) and fracture

filling (Fig.3d). The origin of barite mineral requires two discrete sources, one for

barium and another for sulfate. Jewell (2000) suggests that barite can form in three

ways: (1) diagenetic replacement, (2) hydrothermal exhalation, or (3) biological

precipitation. The first two mechanisms are most important, particularly in the

Archean. However, the most common origin for barite in the Archean appears to be

hydrothermal exhalation. Barite precipitates when a reduced fluid carrying Ba2+ mixes ‏

with a fluid carrying SO2-

4 . The analyzed sample data show 57.6% BaO.

36

Table (2): eU (ppm), eTh (ppm) and K (%) contents, eU/eTh and eTh/eU

ratios of different rocks, G. Nuqra area.

Rock Types Radiometric measurements

eU (ppm) eTh (ppm) K (%) eTh/eU

Trchy-basalts

(n=8)

Min. 03 1 0.8 3.3

Max. 1.2 3 2.6 2.5

Average 0.8 2 1.2 2.5

Granites

(n=42)

Min. 4 9 1.75 2.25

Max. 45 98 4.29 2.18

Average 23 76 3.45 3.3

Rin

g D

yk

es

Dacite porphery

(n=16)

Min. 3 5 0.39 1.67

Max. 18 36 3.44 2.0

Average 9 21 1.71 2.34

Granophyre

(n=18)

Min. 2 8 0.19 4.0

Max. 13 26 4.4 2.0

Average 6 15 2.6 2.5

Alkaline

rhyolite

(n=18)

Min. 3 8 2.08 2.67

Max. 11 23 4.16 2.1

Average 5 13 3.37 2.6

REEs-silicate is irregular shape (Fig. 11g) enriched by REEs (Ce, La, Pr, Nd,

Sm and Gd, in decreasing order) It contains 25.7% Ce2O3 and 16.3% La2O3, Pr2O3

11.9% , Nd2O3 11.2 %.

Kasolite [Pb(UO2)SiO4.2H2O] is yellowish brown to dark brown color of

secondary uranium minerals. It shows radiated or fan likes shape (Fig. 11h). Dawood

et al., (2010) stated that fluoride and carbonate complexes played a significant role in

the formation of kasolite whereas high temperature hydrothermal solutions reacted

with pre-existing uranium-bearing metamictized accessory minerals such as

pyrochlore, U-rich thorite and/or zircon to form uranous fluoride complexes. These

complexes are predominant in reducing environment and at pH 4 and finally formed

kasolite.

37

Fig. (10): Semi-quantitative analyses using the EDAX –Scanning Electron Microscope.a)

Cerrusite (Pb-carbonate), , b) Ni-silicate, c) Zircon, d) Zircon corroded by Fe-Ti-silicate, e)

euhedral barite, f) Barite filling fractures, g) REEs-silicate and h) Kasolite.

38

Sequence Formation Of G. Nuqra Ring Dyke Complex

The NW-SE fault separating the basement rocks from the Nubian sandstone

seems to have played an important role in the formation of the ring dyke complex. It

may explain the appearance of alkaline granites and alkali-feldspar granites of the

main mass to be intruded along an incomplete ring faults. This event was followed by

the intrusion of sub-latitudinal and radial dacite porphyry, granophyre and alkaline

rhyolite, dykes. Being confined to the zones of tectonic activity, these dykes are

slightly brecciated. The complex is cut by sub-latitudinal and radial faults along

which it is displaced. Finally small trachy-basalt lava flows are extruded in Nubian

sandstone without any field relations between these flow and ring complex dykes. The

detailed field studies show that east Gabal Nuqra area has been affect by at least four

tectonic events where the reactivation interaction of the preexisting NW-SE with the

E-W to ENE-WSW basement fabrics as normal configure the Nubian sandstone basin

while the granitic intrusions are mainly controlled by the NW-SE structural trend

whereas their reactivation localize the emplacement of the ring dykes and their

interaction with NE-SW ones configure the trachy-basltic flow.

Discussion And Conclusion

The initial phase of the post-Hercynian break-up of the Pangaea super

continent spanned Late Permian to Middle Jurassic times and culminated in the

development of a new divergent/transform plate boundary between Gondwana and

Laurussia (Guiraud and Bellion, 1996). The post-Hercynian and most likely Lower

Cretaceous Nubian Series as host of the East G. Nuqra structure, together with the

trachy-basaltic flows of probably Oligo-Miocene age that intercepts the structure,

constrain the formation age of the East G. Nuqra structure to'syn-or post-Nubian'

(probably post-Lower Cretaceous) and pre- Oligo-Miocene. This stage includes

Mesozoic rifting (extension) events, Senonian phase of tectonic inversion

(compression) and Paleogene-Neogene extensions (Gulf of Suez - Red Sea rifting).

The integrated use of geological, geophysical, and geochemical data from North

African passive margin onshore and offshore samples indicate a crustal thinning

induced from the Tethyan rifting that responsible for the subsequent evolution of the

39

North African passive margin during the Late Cretaceous (Kort et al., 2009). This

thinned crust was an area of mantle upwelling that favored the increase of isotherms,

the uprise of magma, and the circulation of hydrothermal fluids along the inherited

preexisting deep seated NW-SE, ENE-WSW & NE-SW faults, which are reactivated

during the different extension events. Thus, Early Cretaceous rifting might have been

associated with magmatic activity that could have affected the development of the

basin of the Nubian Series.

The most types of alterations which have been distinguished and recorded in

the study areas are mainly silicification and ferrugination, while kaolinization, and

chloritization are less common. Silicification is the most common types of alteration

in G. Nuqra area. The common style of silicification is the formation of close-spaced

fractures in a network, or "stockworks", which are filled with quartz. The stockworks

are sometimes present in the wallrock of Nubian sandstone along the margins of ring

dyke complex (Fig. 3d). Ferrugination causes increasing in total FeO, (reaching up to

13% at the expense of all other oxides) and indicates presence of alkaline

hydrothermal solution having temperature range between 300°C and 350°C

(Helgeson, 1974). In G. Nuqra ring complex, some aegerine-augite flakes are altered

to chlorite (dark green) with releasing iron oxides (deep red) along the rims forming

radial shape (Fig. 9d).

All mineralization is localized within N25°W & N75°W trending faults which

crosscut the gently dipping volcanics at steep angles. Some quartz veins dip generally

to southwest, but those in the northern portion of the district often dip to the northeast.

In the study area, the age of mineralization post dating the deposition of the Nubian

series and the eruption of ring dyke complex. So, this structure is a clear incomplete

ring dyke complex that produced by volcano-plutonic systems comprising a suite of

alkaline granite intrusions and volcanic ring dykes (dacite porphery, granophyre and

alkaline rhyalite) extruded by a recent trachy-basltic flow. These rocks cut into the

local geology of the Nubian sandstone, which defines the rim of the structure that

emplaced through out three magmatic events, the 1-(granites) associated with

development of the Nubian sandstone basin, the 2nd

(volcanic ring dykes) is post to its

formation (Natash volcanics) whereas the 3-(trachy-basltic flow) is related to the Red

Sea rifting.

40

Paragenetic studies suggest that hydrothermal solution rich by mineralization

was emplaced during a single event requiring a plumbing system that enables fluids to

rise upward, perhaps along faults in the Nubian series, and then migrate laterally

along permeable horizons. Dykes were emplaced at depths in excess of one kilometer

while mineralization is formed near the surface. The mineralization studies in the area

yield the following conclusions: (1) mineralization is fracture-controlled and

magmatic-hydrothermal in origin rather than syngenetic or diagenetic; (2) the SO2"4

indicates an external magmatic-hydrothermal source of sulphur; (3) Ni-silicate

indicates that metals were derived from the nearby peridotites during hydrothermal

alteration and circulation. (4) the mineral occurrences are proposed to be of post

Cretaceous age as the Nubian sandstone gave an age of 104±7 Ma, using Rb/Sr

isochron by Hashad and El Reedy (1979).

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