Geological Map and data - Geolog by wulinqing


									Geological Map of Chad
                     Micaschists and quartzites
                     Lower Tibesti Series
                     (micaschists, gneiss
                     quartzites, amphiblites,
                     dolerites and rhyolites)
                     Upper Tibesti Series
                     (schsts, meta-sedimentary
                     rocks and rhyolites)

                     Lower Grit Series
                     Silurian Middle Grit Series
                     Lower Devonian Upper Grit Series
                     Upper Devonian grits
                     and conglomerates
                     Carboniferous Marine Series
                     (grits, sandstones and limestones)

                     Lame Series (conglomerates,
                     sandstones, limestones
                     and basalts)
                     Nubie Series (grits,
                     conglomerates and siltstones)

                    Eocene Marine Series
                    (limestones, calcareous
                    sandstones and siltstones)
                    Terrestrial sediments
                    (iron stones, grits
                    laterites and siltstones)
                    Cenozoic volcanics

                    Fluvio-lacustrine deposits,
                    dunes and laterite

      Back Data     0               250 km
Africa Mining 2000

     Extracts from Mining Journal Chad Supplement September 1995

Economic Geology and Recent Exploration               programme. Geochemical exploration surveys of
Although numerous mineral occurrences have            the Mayo Kebbi greenstone belts, identified three
been reported since the early 1950s, it was not       distinct gold regions, Lere, Mourbame and Pala,
until the recent UNDP/DRGM geological and             where quartz veins and silicified zones show
mining research programmes that they were             visible gold and have returned grades up to 33 g/t.
evaluated in any detail.          The research
programmes, which considerably updated Chad's         The Lere gold occurrences are located west of
mineral inventory, also identified new mineral        the town of Lere. The gold is hosted in
deposits and have provided an excellent               Precambrian amphibolites and greenschists, and in
indication of Chad's mineral potential.               local granitoid intrusions such as the Mayo Kebbi
                                                      batholith and the alkaline granite of Zabili. The
It is important to note that Chad is still very       rocks are crosscut by mafic to ultramafic
under-explored compared to other African              instrusions and by microgranite dykes. One
countries.    There has been no systematic            strong geochemical anomaly was delineated at
reconnaissance using modern exploration               Teubara in the Zabili greenschists.
techniques, no airborne geophysical data for
example are available for 95% of the country.         Gold in the Mourbame area is also contained
The only area covered by an aeromagnetic and          within a greenschist assemblage, comprising meta-
radiometric survey is the Mayo Kebbi area.            volcanic and meta-sedimentary formations. The
                                                      gold, which can be visible, is found in quartz veins
The Tibesti massif is thought to be one of Chad's     in shears, along with pyrite, arsenopyrite,
most attractive areas for mineral development. It     chalcopyrite    and     malachite,     which     are
is known to host significant occurrences of           concentrated      in     the      meta-sedimentary
tungsten, tin, niobium and tantalum. Preliminary      formations. To date only trench samples have
exploration has also indicated that further studies   been taken. These have returned grades of 1.2-
should be made on the Tibestian volano-               33 g/t gold.
sedimentary formations for gold, silver, base
metals, as well as precious and semi-precious         There are four main areas of gold mineralisation
gemstones associated with the granitic stocks.        in the Pala region; Gamboke, Goueigoudoum,
                                                      Massonebare and Mbibou. These are all located
Gold                                                  in volcano-sedimentary formations, with abundant
Gold exploration over the past few years has          igneous intrusions such as serpentine, diorite and
focused on the Precambrian formations of              microgranites. Mineralisation occurs in quartz
Ouadday, Lake Fitri in Guera and Mayo Kebbi.          veins, often as stockworks cutting the host rocks.
                                                      The gold is sometimes visible and is often
The excellent gold potential in Mayo Kebbi has        associated with galena, pyrite, chalcopyrite,
been known since the 1940s, when the only             arsenopyrite, covellite and malachite. Most of the
recorded gold production came from two                areas have been investigated by trenching, but
primary gold deposits owned by Compagnie              only the Massonebare and Gouelgoudoum
Minière de l'Oubangui Oriental: one, situated         deposits have been drilled, revealing grades of
near Gamboke, operated between 1939-41 and            3.5-4.6 g/t Au over 1-1.3 m and 1.3-3.3 g/t over
produced 180 kg Au. The other was a small             0.3-3.25 m respectively.
operation near Lere, which produced 4.9 kg in
1941. However, it was not until the end of the        One particularly rich alluvial deposit discovered in
1980s that any notable exploration was                the Mayo Kebbi region was Mayo N'Dala, located
conducted in the area under the DRGM/UNDP             north of Pala. The ore grades 4 g/m3 of gold

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Africa Mining 2000

with over 3 t of gold metal resource. Small-scale      Base Metals
production of the deposit began in 1992 and a          To date, discoveries of base metals in Chad have
further four small-scale mining permits were           been uneconomic. Copper sulphides, silver-
issued to local groups to work this resource. In       bearing galena and traces of zinc were found in
1994, a 400 g gold nugget was found in the Mayo        breccias in the Ofouni granitic stock in Tibesti.
N'Dala River.                                          The UNDP/DRGM programme also discovered
                                                       copper sulphides and galena in the gold-bearing
The Ouadday region is also known to host gold,         veins of Massonebare and in Poyeme. Malachite is
which was first discovered in Goz Beida in 1963.       reported in various locations south of Teubara
In 1988 the UNDP/DRGM project undertook                and in hornblendes east of Mourbame.
further exploration work in the form of
geological mapping and regional geochemical            When taking into account the copper
surveys on the Ouadday massif. The project             mineralisation in neighbouring Niger, where
identified some 40 gold anomalies around Goz           mineralisation is reported in extensions of
Beida, Ade Ardelik, Echbara, Karoub and Koukou         geological formations found in Chad, it is thought
Angarana. Further detailed analyses of these           that economic base metal deposits in Chad are
anomalies identified auriferous quartz veins and       undiscovered, rather than non-existent.
stockworks within volcano-sedimentary schist
assemblages.       The most promising gold             Other Metals
occurrences noted were:                                There is an excellent potential for the discovery
                                                       of a wide range of other metals in Chad including
•   - Am Ouchar - gold is found in a shallow           tin and tungsten, iron ore, bauxite and various
    dipping shear zone, thought to be a thrust.        minor metals.
    Channel and trench samples indicated 33 g/t
    of gold. Typical intersections are 4.73 g/t        Tin and tungsten mineralisation has been
    over 16 m, 5.7 g/t over 12 m and 6.8 g/t over      recorded in association with post-tectonic granite
    20 m. The mineralisation can be traced at          stocks in the Yedri massif in Tibesti. The greisen
    surface for over 100 m along strike.               mineralisation    contains     large    cassiterite
•   - Goz Beida - several anomalies were               accumulations that are often capped with a dome
    discovered, the most promising is associated       of quartz veins and stockworks, which also host
    with a granite/quartzite contact and is            wolframite, cassiterite and copper.       Gigantic
    traceable at surface over a distance of 500 m.     wolfram crystals weighing several hundred
    Exploration trenches gave grades of 1.5 g/t        kilograms have been found in the Yedri massif.
    over 4 m and 3 g/t over 2 m. Alluvial gold has     During the 1930s, 22 t of wolframite was
    also been found at Goz Beida, the primary          collected from the surface of the massif. Similar
    source has not yet been identified.                mineralisation has been reported at other Tibesti
•   Echbara - five anomalies were identified,          massifs.
    including one that extends over 1,600 m. The
    down-dip continuity of these anomalies has         Some iron deposits have been worked on a small-
    not yet been verified.                             scale in the past. Iron can be found in the
•   Ade Ardelik - five gold anomalies were             Precambrian ferruginous quartzites of Hadjer
    defined, one of which was traced over 1,100        Hadid (near Gourgoundji) and Ouadday; in
    m and graded 4.3 g/t gold.                         haematite schists of the Koukou Angarama region
                                                       in Ouadday; in oolitic iron formations near Tile
Following the discovery of gold nuggets in             Nougar (in the far south of Guera); and in laterite
alluvium in the Aozou area and Tibesti, and the        crusts in various areas. No reserve estimations
favourable geological terrain, it is considered that   have been made for these deposits.
there is a good potential of finding primary gold
deposits in this area.                                 The Koro bauxite deposit, located south of
                                                       Guidari in southern Chad, comprises several

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Africa Mining 2000

small plateaus topped by ferruginous crusts that     been unsuccessful. It is thought that some alluvial
trapped an oolitic bauxite layer up to 10 m thick.   diamonds on the C.A.R. border could derive
The layer grades 57.3% Al2O3 and reserves have       from the neighbouring country. However, where
been estimated at around 7 Mt. Other bauxite         the hydrographic network is poorly developed
crusts were formed above the basalts and             and is located too far from the border, for
sandstones of Massalit, above the northern end of    example at Guera and Ouadday, the primary
the Ouadday batholith and above terrestrial          sources, it is believed, must be within Chad.
sediments in the Ennedi region.
                                                     Oil And Gas
The post-tectonic granitic stocks of Tibesti also    Exploration for oil and gas began in the 1960s in
host niobium, tantalum and beryllium. Columbo-       the Chad Basin, with the first oil discovery being
tantalite and beryl are commonly associated with     made in 1974 by Conoco. Fifteen years later the
cassiterite, although in Chad they occur mainly in   Sedigui deposit, in the Kanem Basin, was
pegmatites in the Yedri massif and the Orda          discovered by a consortium controlled by Esso,
Oudengui massif (southeast of Yedri). Large          with Shell and Elf as partners.
pegmatite veins have been recorded, such as at
Yedri, where seven 4 km long veins were found        The Sedigui deposit has estimated reserves in the
with columbo-tantalite content grading 47.5% Ta      order of 150 Mbbl and is currently being
and 19.5% Nb. Columbo-tantalite has also been        developed by the Société d'Etudé et d'Exploition
found in alluvial deposits in Tibesti (grading 145   de la Raffinerie du Chad (SEERAT). Sedigui is
g/m3) and in Ouadday (200-840 g/m3).                 expected to be on stream by 2005.             The
                                                     government holds a 51% interest in SEERAT,
Various titanium deposits have been reported,        along with Esso 18.4%, Shell 18.4% and Elf 12.3%.
including an alluvial ilmenite deposit, grading 50   Crude oil will be exported through a new 1,200
kg/m3, at Guera massif and an alluvial rutile        km pipeline which crosses Cameroon to the
deposit downstream from Bousso. Manganese            newly constructed terminal at the Atlantic port of
mineralisation has recently been discovered in the   Kribi.
Goz Beida area and comprises a 0.2-0.3 m thick
manganese hydroxide layer, with a strike length      Extensive exploration has also been conducted in
of 50 m.                                             the Doba and Doseo Basins in southern Chad
                                                     where reserves were first discovered in 1989 by
Diamonds                                             Conoco. The Esso, Shell and Elf consortium
The main areas of alluvial diamond production in     identified oil deposits in the Lower and Upper
Chad are in Abeche, Biltine, Am Zoer and Adre        Cretaceous detrital sediments of the Doba Basin
in Ouadday, and at Melfi-Bitkine in Guera. Local     and its eastern extension, the Doseo Basin.
prospectors have also discovered alluvial            Preliminary geophysics and drilling tests have
diamonds in many regions, including Melfi, central   indicated reserves amounting to 500 Mt of oil and
Ouadday and the Lim River in Baibokoum. The          510,000 million m3 of natural gas. Further
latter region, which is located in the far           evaluation is under way in order to define the
southwest of Chad, has similar geology to that of    deposits characteristics.
the small-scale diamond mining areas across the
border in the Central African Republic and           The Erdis Basin was explored for hydrocarbons
Cameroon. This supports the belief that further      by the Petropar/SNPA consortium during 1961-
finds will be made.                                  65, but the results have never been released. As
                                                     the Libyan extension of the Erdis Basin has
In 1993 the UNDP/DRGM project conducted an           producing oil wells, the potential for discovering
exploration programme over 2,500 km2 in the far      new oil deposits within the Chad sector is
south of Chad looking for diamonds and their         deemed very likely.
indicator minerals, such as ilmenite, pyrope and
chrome diopside. To date the programme has

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Evaporites                                           uranium, in Cambro-Ordovician conglomerates
Chad has large reserves of a number of               and sandstones, has also been reported at
evaporites, the two most common and extensive        Bouboa, Ouadi Bakou near Fada and Tibesti.
being trona and salt. Evaporites are renewable
with continued evaporation.        It has been       Industrial And Construction Minerals
estimated that for every cubic metre of water        The UNDP/DRGM project identified a number of
evaporated around 250,000 t of salt is produced.     promising marble deposits in Mayo Kebbi, Guera
                                                     and Ouadday.           High-quality marble was
There are two main regions where large               discovered at Zoubou in Guera, with a range of
quantities of trona can be found. The area           colours from pure white to grey. At Zafai, Rey
around Borkou, Ennedi and Tibesti hosts trona        and Modo near Goz Beida, large reserves of
both in salt ponds, such as at Ounianga Kabir,       marble have been found totalling 3.5 million m3.
Ounianga Seghir and Ouadi Doum, and on the           These deposits are regarded as economically
surface of the sand where direct precipitation has   viable both in terms of quantity and quality. A
occurred. This can be seen to the south of Faya      number of granites and igneous rocks occur in
and in the Tibesti calderas. The second region,      Chad that are suitable for ornamental stones,
to the northeast of Lake Chad, hosts trona in        such as the granites southeast of Lake Trene; the
dune valleys on the lakeshore. These trona           Sodjembaye syenite north of Pala; and several
deposits are around 25 km wide and extend from       granite intrusions in the Guera massif.
the Niger border to Doum-Doum.
                                                     Although sand and gravel deposits are
Trona mining has always been on a small-scale        widespread, the main production comes from
and is centred on the Liwa-Kaya region.              quarries near N'Djaména. The main aggregate
Production is estimated at 12,000 t/y, of which      quarry is Dandi, south of Lake Chad. Clay is also
70% is traded with neighbouring countries, mainly    mined on a small-scale for the local manufacture
Nigeria. Current renewable trona reserves are        of bricks.
estimated at 650,000 t/y.
                                                     Large reserves of limestone are found in Mayo
Numerous salt lakes can be found in Chad. The        Kebbi, Guera, Ouadday, Ennedi, Tibesti and
largest concentration of these salinas is in the     Tenere. Preliminary studies on a deposit near
Borkou and Ennedi region, stretching from Faya       Baore in Mayo Kebbi have indicated its suitability
to the north of Fada and covering 60,000 km2.        to supply a local Portland cement plant, with
There are 15 large salinas that are well known for   production from the deposit totalling 50,000 t/y.
their salt content. These cover an area of about     High-grade limestone is occasionally extracted for
20 km2 and comprise 1-4 km long basins with          the production of lime at Tagobo Faulbe in Mayo
widths of up to 2 km and grades of 22-91% NaCl.      Kebbi, which has estimated reserves of 90,000 t.
The Demi, Oueta, N'Teiguei, Bedo and Orori
salinas have extremely high NaCl grades.             Other significant minerals known to exist in Chad
Several gypsum occurrences have been reported
in late Cenozoic formations in the Bahrel-Ghazal     •   large quantities of graphite, found mainly in
area, north of N'Djaména. However, little                layers of Precambrian graphitic schists in
information is known on the exact extent of              Guera, Ouadday and Baib-okoum;
these deposits.                                      •   significant kaolinite deposits at Abou-Dela,
                                                         southeast of Mongo, with an estimated
Uranium                                                  525,000 t of reserves;
Radioactive mineralisation, essentially uranium,     •   good quality diatomite resources totalling
has been identified in both Tibesti and Mayo             3,000 Mt that extend over 115,000 km2
Kebbi, where they occur in veins associated with         between Lake Chad and Fada.
alkaline granite and syenite stocks. Sedimentary

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