Regional Geology NW Africa
Arabian Nubian Shield Summary
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, where all Nyota Exploration Licences are found, 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 (Tesfaye et al., 2000).
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.
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:
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.
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.
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.
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.
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.