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Guinea's Main Geological Features


In terms of geology, Guinea is located on the south-west of the North-African tectonic plate. Basement structures are exposed in its southern framework (Leon-Liberia craton). Subhorizontal formations of platform mantle show on the day surface in the north-west. The north-east is marked by Lower Proterozoic igneous rocks of the Birim group.

The oldest rocks in the Pre-Cambrian basement are: Lower Archean gneiss, crystalline schist, quartzite, amphibolite and adergneiss of Kasil, Dabol and Simandu series. Metasomatic charnokite and gneissic granite make up an integral part of the Archean rock unit. Jaspilite is linked with large deposits of magnetite-hematite ore.

Lower Proterozoic igneous-sedimentary rocks of amphibolic and greenschist metamorphic facies are exposed among the rocks of crushed Archean basement. These Lower Proterozoic rock units include gneiss, green schist, amphibolite, quartzite of Mongo series (earlier - Kambui series of Archean age), as well as Early Proterozoic intrusions of basic and ultrabasic consistence, like amphibolite, gabbride, ultrabasite. They are forming superimposed geosynclinal structures.

Among the Early Proterozoic structures there are large solid masses of polygenic-metasomatic granitoids and minor intrusions of granites. The latter define the gold content of Guinea. In the western parts of the country, a consolidated Archean-Lower Proterozoic basement is covered by a submeridional Late Proterozoic Faleme-Rokell deflection. The deflection is composed of weakly metamorphized sandy-schistose and tuffaceous formations of the Segu-Madina-Kuta group. Related to the above deflection, there are late Proterozoic masses of polygenous-metasomatic granitoid rock, granite blocks and giant granites with muscovite. To the east of the Faleme-Rokell deflection, the deposits of the Segu-Madina-Kuta series possessing a platform nature are followed by a layer of the Upper-Proterozoic-Paleozoic platform mantle. Limestone deposits are Upper-Proterozoic as well, while sedimentary iron deposits lie in the Devonian layer.

Mesozoic intrusions are widely spread in Guinea. They were formed during the Mesozoic activation of the platform.

A considerable part of Guinea's territory is covered with Paleogene-Neogene weathering lateritic crusts. Connected with these are: iron deposits, kaolinitic clays and most abundant bauxite layers.

In Guinea's geologic profile important are Archean, Lower Proterozoic and Paleozoic deposits that boosted the development of the lateritic residuum and the products of their repricipitation. Cainozoic depositions were found also in coastal plains and river valleys.

Strongly metamorphized Archean layers, represented here by Kasil and Dabol series, are Guinea's oldest deposits. Soviet geologists divided the Kasil group into four suites (from below up): Kisikisin, Forekary, Maher, Kunsutin. The rocks are metamorphized in the amphibolite and granulitic facies, often diafluorated to green schist. Represented by various garnetiferous gneisses, crystalline schist, quartzite, amphibolite. The thickness of the series is between 2,400 and 3,400 meters. The composition of the Dabol series resembles that of Kasil. Its stands out, however, for the absence of garnetiferous rocks. The thickness is 3,500 to 4,000 meters.

Proterozoic deposits comprise two units: Lower Proterozoic unit, lying in the higher structural stage of the basement, and Upper Proterozoic unit of sedimentary and metamorphized layers of transitional type.

Lower Proterozoic deposits are represented by Mongo series, that used to be considered similar to Archean Kambui series, but is lying stratigraphically higher than Dabol group. Two structural-facies zones can be distinguished here: southern - eugeosynclinal, and northern - miogeosynclinal. Layers in the southern zone: down below are epidote-biotite, epidote-biotite-amphibolite gneisses, biotite-actinolite, actinolite and binary schists and adergneiss, as well as interbeds and lenses of magnetite and cummingtonite-magnetite quartzite; up above are epidote-biotite gneisses, adergneiss, ferrous quartz and others. Thickness - 4,500-5,000 meters. Layers in the northern zone: amphibole-biotite gneisses and crystalline schists, adergneiss. Thickness - 2,500-3,000 meters. By lithological composition, degree of metamorphism and geological history the layer resembles Lower Proterozoic Birrim series.

Upper Proterozoic deposits are divided into two series: Segu-Madina-Kuta and Yukunkun. The first one, Segu-Madina-Kuta, comprises a number of suites: Kolente, Rokell-River and Baniya on the south; Kanta, Dira, Pananpu, Bassari and Kuluntu on the north. Yukunkun series consists of the following suites: Mali, Lumbalumbita, Batapa, Bundu, Kubiya and Gaban, which seems to be identical to the Bundu suite.

Paleozoic deposits are composed of Ordovician, Silurian, Devonian sediments. They are lithologically similar: mudstone, siltstone, malmrock. Ordovician deposits contain, besides the three abovementioned, gritstone and pudding rock.

Intrusive deposits are made up of Archean, Early Proterozoic and Late Proterozoic polygenous-metasomatic and intrusive structures, as well as Mesozoic intrusions.

Archean intrusions and polygenous-metasomatic structures are developed throughout the metamorphic mass of basement and are represented by a rich and diverse rock series belonging to three tectonic-magmatic cycles. The first layer coincides with the original magmatism of the cycle, the following two, most probably, make up a comagmatic series of granite-gneiss masses. The following intrusive-metasomatic formations can be distinguished: amphibolites, pyroxenites, peridotites, hypersthenite; charnokites, anorthosites; plagiogranites, gneissic granites and granodiorites.

Early Proterozoic intrusions and polygenous-metasomatic formations are represented by pre-fold intrusions of main deposit and post-fold polygenous-metasomatic gneissic granite formations, together with intrusive granite masses that came later. It is amphibolites, gabbro-amphibolites, gabbros, gabbro-norites, gabbro-diorites; gneissic granites and gneissic granodiorites, granites, plagiogranites, monzonites.

Mesozoic intrusions are mostly intrusions of trappean formations (dikes, sills), or else masses of basic and ultrabasic rock units and circle intrusions of nephelinic syenite. Intrusions are represented by the following: dolerites, gabbro-dolerites, greenstones, dunite peridotites, pyroxenites, gabbro, gabbro-norites, norites; nephelinic syenites, microsyenits.




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