Geological Overview

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Chad is dominated by two main geological units, the Precambrian crystalline basement and the Palaeozoic to Quaternary sedimentary cover (see map). The basement rocks occur in several massifs representing the edges of three major cratons.

The majority of the sedimentary cover lies within a central depression. There are five main regions in Chad: the Tibesti massif is situated in the northwest, the Ouaddaï and Guera massif along the central eastern edge of the country, the lake Chad Basin in the centre and the Mayo Kebbi massif in the southwest.

 

Precambrian

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The two largest areas of basement rock are Tibesti in the north, and Ouaddaï which covers most of the eastern side of Chad extending from Ouaddaï down the Guera massif.

Other notable areas of basement rock are Balbokoum and Mayo Kebbi in the far south. Gneisses and granites are the main rock types found in at Guera and in the northern part of the Ouaddaï massif, while greenstones dominate Mayo Kebbi, Tibesti and the southern part of Ouaddaï.

The basement rocks have been structurally and regionally overprinted by the Panafrican orogenesis which occurred at the end of the Proterozoic.

There are two main Precambrian formations at Tibesti, separated by an unconformity. They have been intruded and weakly metamorphosed by post-tectonic granites.

The lower metamorphic assemblage outcrops in the north Tenere and Enneri Misky massif and comprises amphibolite, pyroxenites, quarzites marbles, gneisses and mica schists. The upper assemblage consists of a basal conglomerate overlain by thick layers of arkosic sandstone, pelitic schists and limestones. These are interlayered with rhyolites and mafic lava flows or stills.

Precambrian volcano-sedimentary rocks occur at Ouaddaï. They have been intruded by various calcalkaline and alkaline granitoid stocks as well as being separated by a large batholith comprising granitoids, migmatites and gneisses.

 

“The research programme provided an excellent indication for Chad’s mineral potential”

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To the south of the bacholith a large area of volcano-sedimentary rocks form the Ankarouba Series. To the north, lesser amounts form the Gaz Beida Group.

The Guera massif in the centre of the Lake Chad Basin is subdivided into three main massifs (Abou Telfan, Aboutouyour and Melfi) and numerous smaller massifs separated by dunes, the highest point is “MT du Guera” at 1.613m and the average elevations of the lowlands are 400-500m. The Guera massif, Mayo Kebbi and the Yade (Baibokoum) massif all have a similar geology to Ouadday except that the volcano sedimentary rocks form greenstone belt.

 

Palaeozoic TO RECENT

 

Palaeozoic to Quaternary sedimentary sequences cover almost 85% of Chad. There are two main sedimentary basins, the Erdis (to the northeast) and Lake Chad (in central and southern Chad).

The Lake Chad Basin is subdivided into several sub-basins: Kanem, Doba and Salamat. The Erdis Basin extends of Koufra in Libya, which evolved as a rift during the Panafrican Orogenesis.

Palaeozoic sediments consist of mainly continental material that was deposited in three main areas:

• Northern Chad, in the Ennedi massif (north of the Erdis Basin) and Borkou (in southern and western Tibesti);

• Northeastern Chad, where thick layers of Massif Sandstones extend westwards into Tibesti and   Ouaddaï;

• Erdis basin where continental material infilled the basin throughout the Palaeozoic except for a few minor interruptions caused by sea transgressions in the Carboniferous. The thickness of Palaeozoic sediments within the Erdis Basin totals 3.500m.

During the Upper Jurassic and beginning of the lower cretaceous, a number of large grabens were formed which generated the Lake Chad Basin. Initially sedimentation was predominately terrestrial and was delved from the extensive erosion of the bordering massifs under wet tropical climate conditions.

This style of deposition is characterized by a series of wet and dry periods. Deposition continued until the quaternary and resulted in very thick sedimentary sequences such as at Doba where 6.000-7.000m of terrestrial sediments were laid down.

 

Marine sediments were deposited during the Upper Cretaceous in the Erdis Basin and Tibesti area. In some areas this series, kwon as the lame formation, is over 200m in depth and included conglomerates, marine limestones, sandstones marls and argillites.

In Erdis, the marine sediments form a large plateau whit lateritic crust, ferruginous sandstones and argillite. In Tibesti they cover a much wider area and have a coarser character.

     The most important volcanic period in Chad began in the Early Cenozoic. The tick alkaline succession, which covers around 30,000km2 of the Tibesti massif, comprises rhyolite, trachyte, phonolite, ash stone, ignimbrite and basalt. These were emplaced by large shield volcanoes that were active from the end of the Cenozoic to the early Quaternary.

 

Exploration

 

Although numerous mineral occurrences have been reported since the early 150’s, it was not until the recent UNDP/DRGM geological and mining research programs that they were evaluated in any detail.

The research program, which considerably updated Chad’s mineral inventory, also identified new mineral deposits and has provided an excellent indication of Chad’s mineral potential.

It is important to note that chad is still very under explored compared whith other African countries. There has been non-systematic reconnaissance using modern exploration techniques no airborne geophysical data for example are available for 95% of the country. The only area covered by an aeromagnetic and radiometric survey is the Mayo Kebbi area.

The Tibesti massif is thought to be one of Chad’s most attractive areas for mineral development. It is known to host significant occurrences of tungsten, tin, niobium and tantalum.

 Preliminary exploration has also indicated that further studies should be made on the Tibestian volcano-sedimentary formations for gold, uranium, silver and base metals, as well as precious and sim-precious gemstones.

Gold exploration over the past few years has focussed on the Precambrian formation of Ouaddaï, Lake Fitri, in Guera and Mayo Kebbi.

The excellent gold potential in Mayo Kebbi has been kwon since the 1940’s, when the only recorded gold production came from two primary gold deposits owned by Compagnie Minère de l’Oubangui Oriental: one situated near Gamboke, operated between 1939-41 and produced 180kg.Au.

The other was a small operation near Leré, which produced 4.9Kg. In 1941. However, it was not until the end of the 1980’s that any notable exploration was conducted in the area under the DRGM/UNDP programme. Geochemical exploration survey of the Mayo Kebbi greenstone belts identified three distinct gold region. Lere, Mourbame and Pala- where quartz veins and silicified zones show visible gold and have returned grades up to 33g/t.

The Lere gold occurrences are located west of town of Lere. The gold is hosted in Precambrian amphibolite and green schists, and in local grantoid intrusions such as the Mayo Kebbi batholitch and the alkaline granite of Zabili.

 

The rocks are crosscut by mafic to ultramafic instructions and by microgranite dykes one strong.one strong geochemical anomaly was delineated at Teubara in the Zabili green schists.

Gold in the Mourbame area is also contained within a greenschist assemblage comprising meta-volcanic and meta-sedimentary formations.

The gold which ca be visible is found in quartz veins in shears along whit pyrite, arsenopyrtre, chalcopyrite and malachite, which are concentrated in the meta-sedimentary formation.

To date only trench samples have been taken. These have returned grades of 1-33g/t gold.

There are four main areas of gold mineralisation in the Pala region; Gamboke, Goueigoudoum, Massonebare and Mbbibou. These are all located in volcano-sedimentary formations. Whit abundant igneous intrusions such as serpentine, diorite and microgranites.

Mineralisation occurs in quartz veins, often as stockworks cutting the host rocks. The gold is sometimes visible and often associated with galena pyrite, chalcopyrite, arsenopyrite, coveilite and malachiste.   

Most of the areas have been investigated by trenching but only the Massonebare and Gouelgoudoum deposits have been drilled, revealing grades of 3.5-4g/t over 1-1.3m and 1.3-3.3g/t over 0.3-3. 25m.respectively.

One particularly rich alluvial deposit discovered in the Mayo Kebbi region was Mayo N’Dala, located north of Pala. The one grades 4g/m3 of gold with over 3t of gold metal resource.

Small-scale production from the deposit began in 1992 and a further four small-scale mining permits were issued to local groups to work this resource. In 1994, a 400g gold nugget was found in the Mayo N’Dala River.

The Ouaddaï region is also kwon to host gold which was first discovered in Goz Beida during 1963. In 1988 the UNDP/DRGM project underlook further exploration work in the form of geological mapping and regional geochemical surveys on the Ouadday massif.

The project identified some 40 gold anomalies around Goz, Belida, Ade Ardelik, Ehbara, Karoub and Koukou Angara. Further detailed analyses of these anomalies identified auriferous quartz veins and stockworks within volcano-sedimentary schist assemblages.

 

The most promising gold occurrences noted were:

 

• Am Ouchar: gold is found in a shallow dipping shear zone, thought to be a trust Channel and trench sample indicated 33g/t of gold. Typical intersections are 4.73g/t over 16m. 5.7g/t over 12m and 6.8g/t over 20m. The mineralisation can be traced at surface for over 100m along strike.

 

• Goz Belda: several anomalies were discovered, the most promising is associated with a granite/quartzite contact and is traceable at surface over a distance of 500m. Exploration trenches gave grades of 1.5g/t over 4m and 3g/t over 2.. Alluvial gold has also been found at Goz Beida, the primary source of which has not yet been identified.

 

• Echbara: five anomalies were identified including one that extends over 1.600m. The down-dip continuity of these anomalies has not yet been verified.

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• Ade Ardelik: five gold anomalies were defined, one of which was traced over 1.100m and graded 4.3g/t gold. Following the discovery of gold nuggets in alluvium in the Aozou area and Tibesti, and the favourable geological terrain. It is considered that there is a good potential of finding primary gold deposits in this

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• Base Metals

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          To date discoveries of base metals in Chad have been uneconomic. Copper sulphides, silver-bearing galean and traces of zinc were found in breccias in the Ofouni granites stock in Tibesti.

          The UNDP/DRGM programme also discovered copper sulphides and galena in the gold-bering veins of massone-bare and in poyeme. Malachite is reported in various locations south of Teubara and in hornblendes east of Mourbame.

          When taking into account the cooper mineralisation in neighbouring Niger, where mineralisation reported in extensions of geological formations found in Chad. It is thought that economic base metal deposits in Chad are undiscovered, rather than non-existent.

 

Uranium

RADIOACTIVE mineralisation, essentially uranium, bas be identified in both Tibesti and Mayo Kebbi, where they occur in veins associated whit alkaline granite and syenite stocks. sedimentary uranium in Cambro-Ordovician conglomerates and sandstones has also been reported at Bouboa, Ouadi Bakou near Fada and Tibesti