The Geology Of Eritrea
The geological set up of Eritrea is made up of Precambrian basement rocks that are overlain un-conform- ably by predominantly Mesozoic sedimentary rocks and Tertiary to Quaternary volcanic and sedimen- tary rocks.
Precambrian Basement Rocks
Basement rocks in Eritrea cover more than 60% of the surface of the country. The basement rocks of Eritrea are part of the Arabian Nubian Shield (ANS) which are exposed in north east Africa (Egypt, Sudan, Eritrea, and Ethiopia) and in Saudi Arabia, northern and northwest- ern parts of Yemen and part of the western Middle East.
The shield is believed to repre- sent a mega suture between East and West Gondwana. Archean and Paleoproterozoic continental crust rocks make the older components of the shield and occupy a very small part of the basement rocks and the major part of the shield consists of Neoproterozoic (c. 870- 670 Ma) continental-marginal and juvenile Intra-oceanic magmatic- arc rocks.
In Eritrea, the basement rocks are not well studied despite their high mineral potential. The two geological maps so far completed cover the western part of the coun- try (Geology of Gash River Area) and the southern part of the coun- try (Geology of Mai Dima/Kohain Area). Compilation of geologic maps at 1:250,000 scale has recent- ly been completed for four map sheets. Other study, based largely on satellite image interpretation aided by limited ground controls, suggest that the rocks can be sub- divided into four tectonic blocks or segments, separated by tectonic boundaries.
Three of these blocks, the west- ern, central and eastern segments, underlie northern and central Eri- trea, whilst the fourth, the Danakil segment, occurs in the southeastern part of the country.
The western segment, the Barka Terrain is exposed in the north- western part of the country and underlies the Barka lowlands.
It is made up of amphibolite, amphibolitefacies pelitic schist containing kyanite and staurolite,quartzite and marble.
The central segment, referred to as the Hagar Terrain, extends from the Barka River up to the Adobha Abi valley in the east, and com- prises several large elliptical bod- ies of various tectonic units that are dominantly composed of oceanic and accretionary wedge materials. Occasionally, layered sequences of chloritic schists are seen, inter- layered with epidotic and chloritic metaba-salts, occasional thin and discontinuous marbles, and man- ganiferous and ferruginous cherts.
The Hagar Terrain displays an east verging thrust contact with the adjacent segment to the east. The Hagar Terrain is known to be prospective for chromite, platinum group elements, nickel, gold and copper mineralization.
The eastern segment, the Nakfa Terrain, is bounded by the Adobha Abi valley in the west and by the Red Sea escarpment to the east.
It is made up of calc-alkaline volcanic and volcanoclastic rocks conformably overlain by a metasedimentary sequence of chlo- rite schists, grits and polymict con- glomerates with occasional pelitic sericite schists and carbonates.
The metavolcanic rocks are in- truded by variably deformed plu- tonic to hypabyssal calc-alkaline bodies. The sequence is cut in places by post-kinematic granites and gabbros and is also transect- ed by several narrow shear zones sub- parallel to the regional strike. The Nakfa Terrain is considered to represent a relict island arc assem- blage. Several VMS (Volcanogenic Massive Sulphide) base metal oc- currences and gold showings are associated with this tectonic unit.
The southern segment, The Danakil Terrain, is composed of metamorphic rocks which may be grouped into three formations:-
(1) migmatitic hornblende biotite gneisses;
(2) a phyllitic formation consisting of schists, conglomerat- ic phyllites, crystalline limestones, and graphitic schists; and
(3) post- tectonic granitoids.
Recently compiled geologic map of the whole country at 1:1,000,000 scale has divided the Precambrian basement rocks into 7 domains, 5 groups and 3 formations. It is rath- er a more detailed presentation of the rock units of the country.
The lower Mesozoic sediments are represented by the Merbet (Adigrat) Sandstone which out- crops in the southern part of the country and in the Danakil area, and is commonly intercalated with siltstones and haematitic layers. It lies unconformably over thin lay- ers of conglomeratic sandstones which, in places, appear to have the characteristics of a glacial deposit.
Overlying the sandstone is the Jurassic Adailo (Antalo) Lime- stone. This unit is exposed over a large area in the Danakil and is made up of limestones that are compact, partly shelly, fossilifer- ous and layered.
Alternations of quartzitic layers are present in the lower part, whilst towards the upper part the se- quence becomes mainly gypsifer- ous to dolomitic. The Upper Sand
stone forms pockets of sandstones that have been preserved from ero- sion. Commonly this sandstone is medium to coarse grained, light coloured, and dominantly quartz- itic but partly conglomeratic.
Tertiary Volcanic and Sediments
The Tertiary volcanic can be di- vided into three units:
(1) the pla- teau-forming Tertiary basalts that are predominantly olivine basalts with intercalations of intermedi- ate lavas and tuffs;
(2) the lower Afar stratified basalts composed of basaltic lava flows and tuffs that are usually found intercalated with sediments of and Danakil for- mation; and
(3) the Afar Basalts composed of recent lava flows and volcanic cones, with minor acid to intermediate volcanic, mainly tra- chytes, rhyolites and ignimbrites. The Tertiary basalts are currently actively exploited for aggregates.
The Tertiary Sediments
The Tertiary sediments lie along the Rift escarpment and in central Afar. Three sedimentary formations have been identified: the Danakil, Dogali and Desset Formations.
The Danakil and Dogali Forma
tions are of late Tertiary age and are composed mainly of limestone intercalated with conglomeratic sandstones and siltstones. They are overlain by calcareous sands with coral reefs, partly consisting of pebbles of volcanic origin, and gravels with sand, silt and clay ho- rizons. The Desset Formation com- prises sandstones, clays and fine beds of anhydrite and halite un- conformably overlying the Dogali formation in the northern part of the coast, while the Red Series containing coarse clastic fresh wa- ter sediments occupies the southern part of the coast.
A thick evaporitic formation of bedded halite, gypsum, anhydrite, potassium and magnesium salts, with shell material fills the basin in the Danakil Depression. Deposits of sheet flood terraces, silt, sand and gravel are present in some lo- cations occasionally covered by windblown sands. Basaltic flows and related spatter cones represent Quaternary volcanic activity in the Danakil region.
(Source: Eritrea Mining Journal, October 2012)