Arabian Nubian Shield Summary
Introduction
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).
Location
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:
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Saudi Arabia: In the Saudi Arabian ANS rocks, four gold mines were 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 resource 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 resource 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: It had gold resources 1.3 million ounces in 2007 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 resource 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 big gold mine is discovered with many prospective projects.
Sukari Gold Mine: The recent audit showed that the Sukari gold mine has 13.68 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 one gold mine is 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 are 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 is 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 produced over 1 million ounce of gold and the current audit with underground potential stands 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 found in western Ethiopia ANS and 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.
Western Ethiopia Summary
Tulu Kapi & Ankore Exploration Licence Area
Introduction
Nyota's wholly owned subsidiary, Nyota Minerals (Ethiopia) Limited (formerly Golden Prospect Mining Company), was granted an exploration licence over Tulu Kapi & Ankore area in May 2005. The Tulu Kapi & Ankore exploration licence areas were under licence to Tan Range Exploration Corporation (TREC), a Canadian based exploration company, from 1996 to 1998.
The presence of gold mineralization in the Tulu Kapi and Ankore area is known since the early 1930's. United Nation Development Program (UNDP) and Ministry of Mines were also conducted systematic exploration activities in the area in the early 1970's.
Since 2005, GPMC conducted systematic detail geological and structural mapping, geochemical sampling and four phases of exploration core and RC drilling activities in the Tulu Kapi & Ankore exploration licence area.
Location
The Tulu Kapi & Ankore exploration licence area is located in western Ethiopia, in Oromia regional state, Ghimbi zone and in the Genji wereda. The size of the licence area is currently 8.44 Km2 and is bounded by the following coordinates: 9°4'10" - 9°6'51"N and 35°32'32" - 35°34'45" E.
The exploration licence area is accessible in all seasons along three routes with 4WD vehicles: Ghimbi-Genji, Ghimbi-Keley and Ghimbi-Gulliso to Tulu-Kapi camp at the top of Tulu Kapi ridge.
History of Exploration Activities
The gold mineralization in the Tulu Kapi and Ankore licence area is known since Italian times. In early 1930's an Italian company conducted surface mining activities by hashing techniques in both Tulu Kapi and Ankore areas. They crushed the hard-rocks and dug the overburden first, then used hydraulic mining and panning activities to recover the gold. The mining activities were conducted in an area of around 500m by 500m. The Italians were also dug two drives following quartz veins reaching a distance of 50m into the Tulu Kapi ridge.
UNDP conducted detailed geological mapping, stream sediment sampling and geophysical survey in the Tulu Kapi and Ankore area, which were followed by three test bore-holes drilling activities.
TREC took an exploration license of a 20.3 Km2 area around Tulu Kapi & Ankore in 1996. They conducted geological mapping, grid soil sampling, Metallic-Ion mobilization (MMI) geochemical survey and IP-Resistivity geophysical survey, which were followed by five shallow bore-holes drilling program in 1998.
Since 2005, Nyota Ethiopia (then GPMC) conducted detailed exploration activities in the three detail areas in the exploration licence areas, namely, Tulu Kapi, Ankore and North of Tulu Kapi detail areas. The Tulu Kapi detail area is the one explored at an advanced stages of exploration activities including detail grid based geological and structural mapping, trenching, geophysical survey and drilling of both RC and diamond bore-holes.
The Ankore and North Tulu Kapi detail areas were explored by detailed exploration works of grid based geological and structural mapping, trenching and grid-soil sampling. But the results of the detail exploration works in the two detail areas didn't warrant to follow it by further drilling activities like the Tulu Kapi detail area.
Land
The surface topography in and around Tulu Kapi and Ankore exploration licence area is hilly and there are also deeply dissected valleys. The hilltops are subdued by recent erosion and are dome-like in shape. Maximum altitude varies between 1600 and 1770m above sea level.
According to an EMRDC (1984) report, the total annual precipitation of the surrounding area ranges from 1400 to 1550 mm, with a peak precipitation between May and October. The climate of the area is semi-arid with a daytime temperature ranges between 25 - 35°C in the dry season and a night time temperature falling to 11 - 16°C during the rainy season.
Most of the exploration licence area is covered by crop farming lands. Small river valleys are also covered by coffee and fruit plantations.
Geology
The Tulu Kapi and Ankore exploration licence area was mapped by UNDP in 1969 and TREC also mapped some of the detail areas. Nyota Ethiopia conducted a 1:5,000 scale geological mapping of the licence area in 2005/06 and also mapped the three detail areas at a scale of 1:2,000.
The geology of Ghimbi sheet in which the licence area is found comprises rocks that range in age from Precambrian to Tertiary (GSE, 2000). The Tulu Kapi & Ankore exploration licence is underlain by metasediments, which consists of metasandstone, phyllites and quartzites at the central par, followed by meta-intrusive units ranging from metagranites, metasyenites, metadiorite-granodiorites to metagabbro at the western and eastern parts of the licence area. There are talc schists, chlorite schists and mica schist derived from deformations of the two major groups of lithological units at the south-eastern part of the licence area.
There are four types of syenite units mapped at the Tulu Kapi detail area, namely, mafic-rich syenite (>25% mafic minerals), felsic syenite (<25% mafic minerals), porphyry syenite and alkali-feldspar syenite (>95% K-feldspar). There are also metadiorite unit at the south-eastern part and a number of basic dykes within the mafic rich and felsic syenite units. The gold in the detail area is specifically hosted in the mafic and felsic syenite units.
The Tulu Kapi detail area consists of minor thrusts trending along NW directions and NE-SW trending sigmoidal strike-sleep shear zones. In most places foliation strikes NNE-SSW and in some cases it strikes N-S and NNW-SSE. Rose diagram plot of quartz veins orientation data indicates three main strike directions: N30°W, N30°E and N65°E.
Mineralization
The Tulu Kapi and Ankore exploration licence area is granted for gold and base-metals exploration in the licence area. Gold is known to be mined from both Tulu Kapi and Ankore ridges starting from the Italian times in early 1930's.
The gold mineralization in the Tulu Kapi detail area is associated with sheared altered syenite unit trending along NW-SE directions and dipping shallowly towards south-west. The mineralogical studies showed that the paragenesis of the gold mineralization in Tulu Kapi area is by two stages of albitization processes followed by sulphide-carbonate-quartz and gold mineralized fluids along structures. The gold is always associated with sulphides, dominate by pyrites, galena, sphalerites, chalcopyrites and phyrotites, with an average of 3% sulphur.
Based on density (number of veins and veinlets per meter) of quartz veins and veinlets and types of sulphides association, there are three gold mineralized lodes are identified so far. Lode-1 being found at shallow depth with high density of quartz veins and veinlets and dominated by pyrites as disseminated and veinlets form. Lode-2 found at depth with moderate density of quartz-carbonate veins and veinlets and associated with base-metal sulphides of galena and sphalerite in addition to the pyrites. Lode-3 is the deepest lode with low density of quartz-carbonate veins and veinlets and associated with phyrotite in addition to galena, sphalerite and pyrites.
The mineralogical and metallurgical studies showed that the gold is free, deposited along fractures in sulphides and silicates and no metallurgical problem. The two metallurgical tests showed that the gold recovery is over 95% with gravity-flotation-leach circuit with cyanide in normal 24 to 36 hours.
Within the Tulu Kapi detail area, there are two potential areas for gold at north (UNDP drilled area) and south-west with similar geological settings. At UNDP site, Nyota drilled more than 13 RC holes and currently conducting core drilling activities to understand the structural control of the gold mineralization. The RC chip assays showed gold mineralization up to 5m at an average gold grade of 2 g/t, but the diamond hole intersections showed wider albitized zones.
At the south-west of Tulu Kapi main ridge, there was gold anomalous zones of TREC's grid soil samples and Nyota followed by both RC and diamond hold drilling activities. The vertical RC holes showed narrow albitized zones but the current diamond hole drilling activities are showing wider albitized zones in the area.