The Arabian-Nubian Shield

The Arabian-Nubian Shield

The Arabian-Nubian Shield (ANS) is an exposure of Precambrian crystalline rocks on the flanks of the Red Sea. The crystalline rocks are mostly Neoproterozoic in age. The ANS extends from Jordan and southern Israel in the north to Eritrea and Ethiopia in the south and from Egypt in the west to Saudi Arabia and Oman

in the east . The ANS consists of gneisses, granitoids, various (meta)volcanic and (meta)sedimentary rocks. Many authors interpret the early evolution of the ANS as the accretion of island arcs and of oceanic terranes (e.g. Vail 1985; Stoeser and Camp 1985; Harris et al., 1990; Samson and Patchet 1991; Abdelsalam and Stern 1996; Johnson and Kattan, 2001).

The Afif and Ar Rayn Terranes in Saudi Arabia, both with a continental signature have also been included in the Arabian-Nubian Shield (Stoeser and Camp, 1985; Abdelsalam and Stern,

1996). Occasionally attention has been given to features that are generally associated with extension, such as dykes and sedimentary basins (e.g. Schürmann, 1966; Grothaus et al., 1979;

Stern et al., 1984; Husseini 1989; Rice et al., 1991; Greiling et al., 1994; Blasband et al., 2000).

Arabian Nubian shielf 

Geology of Arabian Nubian shielf 

The ANS contains many remnants of oceanic crust, in the form of ophiolites. Typical ophiolite sequences are found in the Eastern Desert, Egypt, in Sudan and in western Saudi Arabia. Locally, complete ophiolitic sequences can be observed including peridotites, gabbros,sheeted dykes, pillow lavas and sedimentary rocks that reflect a deep-sea environment.In many cases, the ophiolites have been dismembered and are now found in tectonic

mélanges. The ophiolites were dated at approximately 870-740 Ma

Area	Lithology	Age (Ma)
Egypt Wadi Ghadir,	Serpentinites, layered     gabbros, sheeted dykes,pillow lavas, black shales.	746±19 (Pb-Pb)
Egypt         QiftQuseir,	The Eastern Desert Ophiolitic Melange       Group/Abu Ziran Supergroup: Dunites,peridotites,     layered gabbros, sheeted dykes, pillow lavas and deep sea   sediments (red pelites).	ca. 800
Sudan Onib and Gerf,	Ultramafic cumulates,    interlayered gabbros, sheeted dykes and pillow basalt.	ca. 840-
Halaban    Ophiolite, Al Amar Suture,	Ultramafics,gabbros, cherts Gabbros indicate the ophiolite were former in back-arc basin	Ca. 700

                              

Geochemistry of a number of mafic schistose units throughout the ANS indicates a MORB provenance (Bentor 1985; El Gaby et al., 1984; El Din et al., 1991; Rashwan 1991). Some interpret the ophiolites to have been formed in back-arc basins and others believe that they were formed at mid oceanic spreading ridges (Bentor 1985; El Gaby et al., 1984; El Din et al., 1991; Rashwan 1991; Pallister et al., 1988). The ophiolites are thought to have been formed in the Mozambique Ocean that was formed upon rifting of Rodinia (Abdelsalam and Stern, 1996; Stern, 1994).

 Island-arc remnants

Typical island-arc related rocks are found throughout the ANS. Tonalites, gabbros, basalts, andesites and metavolcanics with a calc-alkaline island-arc geochemistry are common in the Eastern Desert and the Sinai, Egypt (Bentor 1985; El Gaby et al., 1984; El Din et al., 1991; Rashwan, 1991). Gabbro-diorite suites are typically observed in plutonic complexes in ancient island-arcs as the Umm Naggat Complex in the Eastern Desert of Egypt (Mohamed and Hassanen, 1996). Many amphibolites throughout the Eastern Desert, Egypt have island-arc protoliths (Bentor 1985; El Din 1993). The formation of the island-arc rocks has been dated at ca. 900-700 Ma. The oldest island-arc remnants in Saudi Arabia (900-850 Ma) consist of tholeiitic andesites (Jackson 1986; Brown et al., 1989) and are thought to represent young immature island arcs (Jackson 1986). Thickening and melting of the immature tholeiitic crust caused the formation of more mature island arcs with rocks of calc-alkaline character. Low- to high K tonalites, trondhjemites and andesites were formed during this phase and have been dated at 825-730 Ma (Schmidt et al., 1980; Jackson 1986; Brown et al., 1989).  Many of the island-arc related rocks are also thought to have been formed in the Mozambique Ocean, as the ophiolites (Abdelsalam and Stern 1996).

Formation of Arabian-Nubian Shield 

The Arabian-Nubian Shield is the northern half of a great collision zone called the East African Orogen. This collision zone formed near the end of Neoproterozoic time when East and West Gondwana collided to form the supercontinent Gondwana. The most intense part of the collision occurred in what is today southern Africa, where older crust in Tanzania. Mozambique, and Madagascar was remobilized to form the Mozambique Belt. This great collision was responsible for the Pan-African orogeny near the end of Neoproterozoic time. The crust of the Mozambique Belt is quite different from that of the Arabian-Nubian Shield, which is predominantly 'juvenile' crust, that is, crust that formed from partial melting of Earth's mantle, although much older Archean and Paleoproterozoic crustal materials is exposed west of the Nile in Egypt, in the SE part of the shield in Arabia, in eastern Ethiopia, and in Yemen.

The ANS took about 300 million years to form. The oldest rocks associated with the formative cycle of the ANS crust formed by coalescence of Island arcs and back-arc basins and perhaps oceanic plateaus. The oldest rocks associated with this cycle are about 870 million years old and are found in eastern Sudan and SE Arabia. Some of the oldest rocks are ophiolites, which testify that formation of ANS continental crust began with formation of oceanic crust by seafloor spreading, followed by the development of subduction zones and Island arcs. The various island arcs collided and these tectonic terranes sutured together during the time period 780 to 620 Ma to form an increasingly broad and thick nucleus of juvenile continental crust. This thickening resulted in the formation of several suture zones, marked by obduction of ophiolites and intense deformation. Crustal thickening was also accompanied by melting and magmatic fractionation of mafic magmas that ponded deep in the crust. These melts rose upwards to be emplaced as granitic plutons. Magmatism during this episode is characterized by tholeiites and calc-alkaline suites.

The welt of juvenile ANS crust was trapped between great tracts of converging continental crust. A protracted episode of continental collision started at about 610 Ma ago and continued for about 50 million years. Collision was more intense in the south, in the Mozambique Belt, but it also strongly affected the ANS. N-S oriented upright folds and shear zones deformed the arc terranes and sutures in the southern ANS, forming elongate structures such as the Hamisana Shear Zone in NE Sudan. Farther north and east, the ANS was affected by the formation of the great NW-SE trending Najd system of strike-slip faults. The composition of igneous rocks became distinctively more evolved as the collision continued and the crust continued to thicken. Deep erosion, possibly by a continental ice sheet, happened during this time. All tectonic and magmatic activity ended by the time the Cambrian sandstones were deposited, about 530 million years ago.

A number of features have been ascribed to late stage extensional tectonics including a widespread NE-SW trending dyke swarm, NE-SW trending normal faults and NW-SE trending sedimentary basins filled with post-orogenic molasse deposits 

Gold mines in The Arabian-Nubian Shield

The ANS was the site of some of man's earliest geologic efforts, by 

the Egyptians to extract gold from the rocks of Egypt and NE Sudan. This was the most easily worked of all metals and does not tarnish. All of the gold deposits in Egypt and northern Sudan were found and exploited by Egyptians, but new gold discoveries have been found in Sudan, Eritrea, and Saudi Arabia. Pharonic Egyptians also quarried granite near Aswan and floated this down the Nile to be used as facing for the pyramids. The earliest preserved geologic map was made in 1150 BCE to show the location of gold deposits in Eastern Egypt; it is known as the Turin papyrus. The Greek name for Aswan, Syene; is the type locality for the igneous rock syenite. The Romans followed this tradition and had many quarries especially in the northern part of the Eastern Desert of Egypt where porphyry and granite were mined and shaped for shipment. Precious and industrial metals, including gold, silver, copper, zinc, tin, and lead, have been mined in Saudi Arabia for at least 5,000 years. The most productive mine in Saudi Arabia, Mahd adh Dhahab ("Cradle of Gold"), has been periodically exploited for its mineral wealth for hundreds or even thousands of years and is reputed to be the original source of King Solomon's gold, although this may be more of a legend as there is no historical evidence that it ever occurred. Today, mining at Mahd adh Dhahab is conducted by the Saudi Arabian Mining Company, Ma'aden. Deposits of iron, tungsten, mineral sands, copper and phosphates have been found in many locations. Mining in the Eastern Desert of Egypt and Sudan is limited due to shortage of water and infrastructure. One option would be to bring water from the Nile by pipeline.

 The Precambrian shield of Ethiopia occupies a position of particular interest, lying at the interface between the predominantly gneissic terrain of the Mozambique belt to the south in East Africa and the pan-African Juvenile Arabian-Nubian Shield belts of Sudan, Egypt and Saudi Arabia to the north.

In western Ethiopia,is thought to exist both the juvenile belts of the Arabian-Nubian Shield (ANS) and the reworked older crust of the Mozambique belt. But the petrological and age dating studies showed that both the high grade gneisses of Geba and Baro domains and the low-grade Birbir domain rocks are part of the ANS.

The recent studies also showed that the ANS rocks goes down all the way to the southern Ethiopia and all the rocks around Lege-Dembi and Moyale areas showed ages ranging from 900 to 450 million years. Zircon age dating on the syenite of Tulu-Kapi and Genji Monzo-Granite showed that the magma formations had older crustal contamination of 1.5 billion years old, while the emplacement of the intrusions dated at around 650 to 700 million years).

The Arabian Shield is mainly found in Saudi Arabia, with small window of exposures at the southern part of Yemen. The southern wing of the ANS, the Nubian Shield is found at the eastern and southern parts of Egypt along the Red Sea, northern and western parts of Sudan, in Eritrea, and at four windows of exposures in Ethiopia.

The ANS rocks are exposed in the northern, western, southern and eastern Ethiopia. Smaller windows of exposures of ANS are also known to occur in .

Somalia and northern part of Kenya

Mineralization/Discovery

Compared with the Mozambique belt rocks, the ANS was thought to have narrow structures and dry for mineral deposit discoveries in older days. But the recent gold discoveries in Saudi Arabia, Egypt, Sudan, Eritrea and Ethiopia within the ANS rocks, demonstrated that the ANS to have bigger structures to host world class deposits like that of Sukari in Egypt with 13.68 million ounces of gold.

The recent discoveries in the different countries of the ANS rocks are given below with the host rocks and associated mineralisations:

Saudi Arabia: In the Saudi Arabian ANS rocks, four gold mines have been discovered so far with many prospective projects, namely, Mahd Ad Dahab Mine, AL Hajar Mine, Bulgah Mine and Sukhaybarat Mine:

Mahd Ad Dahab Mine: In 2007 it had gold resources of 1.7 million ounces and hosted in mafic to felsic volcano-sedimentary sequences. The ore body comprises vein complexes and the mineralization is associated with quartz, pyrite, chalcopyrites, sphalerite, galena and silver.

Al Hajar Mine: In 2007 it had gold resources of 0.4 million ounces and hosted in felsic to mafic volcanic rocks. The main mineralization is associated to vein and stock-works with base-metal sulphides.

Bulgah Mine: In 2007 it had gold resources of 1.3 million ounces and hosted along N-S trending shear zone within an intrusion. The ore-body is found along quartz filled fractures and associate with sulphides of arsenopyrites, pyrite, chalcopyrite and sphalerite.

Sukhaybarat Mine: In 2007 it had gold resources of 0.68 million ounces and hosted in diorite and sedimentary formations. The ore body is hosted along shear zones filled with quartz, arsenopyrites, pyrite, chalcopyrite and galena.

Egypt: In Egypt, so far only one large gold mine has been discovered with many prospective projects:

Sukari Gold Mine: The recent audit showed that the Sukari gold mine has 13.08 million ounces of gold as global resources. The Sukari gold is hosted by a large sheeted vein-type and brittle-ductile shear zone hosted gold deposit developed in a granitoid intrusive complex. Gold mineralization is hosted exclusively by a granitoid body of granodiorite-tonalite composition referred to as the Sukari Porphyry. The ore body is associated with quartz veins and silicified zones with sulphides of pyrite and galena.

Sudan: In Sudan, so far only one gold mine has been discovered over big Volcanogenic Massive Sulphide (VMS) deposit as gossans, with many prospective grounds in the ANS:

Hassai Gold Mine: Since 1992, a private company called La Mancha Resources and Sudan government Joint-Venture Company mined 2 million ounces of gold from oxide caps of shallow VMS deposits within 30 kilometre radius. Six of the pits showed bigger VMS deposits and an estimated 50 million tons of VMS mainly copper at 1.3% and zinc are delineated.

Eritrea: In Eritrea, two gold mines have been discovered so far, one in gossan over VMS deposit at Bisha and the other in sheared metasediment-intrusive contacts in Zara:

Bisha Gold Deposit: Over 1 million ounce of gold is estimated in the gossan over the big 20 million ton VMS deposit in Bisha. Nevsun Resources PLC, the company which owns Bisha went in to production of the gold deposit in late 2010. The VMS showed up to 1.5% Cu, Zn, Ag and Co.

Zara Gold Deposit: Zara gold deposit is discovered by an Australian Company at the north-western part of Eritrea and the latest audit showed over 950,000 ounces of global gold. The gold mineralization is hosted along shear zones with metasediment and granitoid contacts. The gold mineralization is associated with sub-vertical dipping quartz vein with small sulphides.

Ethiopia: In Ethiopia, there is only one gold mine running, Lege-Dembi in southern Ethiopia and one new gold deposit has been discovered, Tulu-Kapi in western Ethiopia. But there are several advanced exploration projects in different parts of the country within the ANS:

Lege Dembi Gold Mine: Lege-Dembi gold mine was discovered in early 1990′s and was run by the state owned gold mine till 1998. Since its privatization it has produced over 1 million ounce of gold. The current audit with underground potential stands at an estimated 3 million ounces of global gold. The ore-body is hosted along shear zone at the contact of metasediment and amphibolite unit, and associated with sulphides of galena and chalcopyrite.


Tulu Kapi Gold Deposit: The Tulu Kapi gold deposit is located in western Ethiopia ANS and was discovered in early 2008. The gold mineralization is hosted in sheared albitized syenite unit and associated with sulphides of pyrite, galena, sphalerite, chalcopyrite and phyrotite.