Summary of the Geology of Sierra Leone

The Republic of Sierra Leone is located on the West Coast of Africa, between latitudes 7° and 10° north and longitudes 10.5° and 13° west. The Republic of Guinea is to the north and northeast; Liberia is to the east and southeast, and the Atlantic Ocean on the west and south. It has 300 miles of coastline. From an approximate 70-mile coastal belt of low-lying land, the country rises to a mountain plateau near the eastern frontier rising 1219.2 m to 1828.8 m with a rich forest region. The Western Area encompasses the Freetown Peninsula, on which the capital and main commercial centre of Freetown stands; is 24 miles long and 10 miles wide. A mountainous peninsula, that rises in places to 92 m above sea level – one of the few parts of the West African Coast where there is high land so near the sea. Where the lush green forest spills down hillsides to meet the most beautiful white sandy beaches along the Atlantic Ocean.

This 27,925 square mile (73,326 sq. km) country has a population of approximately seven (7) million people. Sierra Leone is divided into four main Provinces, West, North, East and South. There are twelve Districts in the entire country. Most of the country is underlain by rocks of Precambrian age (Archaean and Proterozoic) with a coastal strip about 50 km in width comprising marine and estuarine sediments of Tertiary and Quaternary to Recent Age.

The Precambrian (mainly Archaean) outcrops over about 75% of the country and typically comprises granite-greenstone terrain. It represents parts of an ancient continental nuclei located on the edge of the West African Craton. Regional reconnaissance mapping indicates that the Archaean basement can be subdivided into infracrustal rocks (gneisses and granitoids); supracrustal rocks (containing greenstone belts); and basic and ultrabasic igneous intrusions. The infracrustal gneisses and granitoids were formed and reworked during two major orogenic cycles, an older Leonean episode (~2,950-3,200 Ma) and a younger Liberian episode (~2,700 Ma).

The Leonean orogenic episode commenced with the intrusion of a basic igneous suite (the Pre-Leonean amphibolites) and by the formation of a greenstone belt represented by the Loko Group which is now deeply eroded. The Loko Group comprises amphibolites, silimanite quartzites and ironstones. It appears to have formed on a gneiss/granitoid basement in which several granitoid bodies related to an earlier plutonic orogenic episode have been distinguished mainly in the northern part of the country. Only the main deformational phase of the Leonean orogenic episode which resulted in folds and fabrics trending east-west has been distinguished. Minor gold and cassiterite mineralization associated with portions of the Loko Group is probably related to a late Leonean granitization event which accompanied the formation of major shear zones in the craton. Other volcano-sedimentary sequences are preserved within the granites, gneisses and migmatites. Highly folded greenstone belts predominate in the north and central Sierra Leone.

In the southeast, the metamorphic facies increases, first with the Kambui Schists and finally with the Mano-Moa Granulites. Greenstone belts of the Kambui Supergroup are believed to have been deposited upon a post-Leonean basement and accompany by basic to ultrabasic intrusives. The Kambui Supergroup includes most of the schist belts exposed in the Sula Mountains and the Kangari, Kambui, Nimini and Gori Hills; the Marampa Group; and the two small greenstone belts of Serekolia and Sankarama in the northeast. These greenstone belts comprise a lower volcanic unit composed of ultrabasic lavas and basic lavas with pillow layers, overlain by a sedimentary unit comprising tuffs, pelitic and psammitic sediments, with conglomerate layers and ironstone bands. The greenstone belts are the principal hosts of the gold mineralisation of the country. Other associated mineral deposits include molybdenite, columbite-tantalite, chromite and possible nickel and copper.

The Marampa Group, bounded on its eastern margin by a tectonic contact, is important for its iron-ore deposits and forms the upper part of the Kambui Group. Late Liberian granitoids, marginal to, and within, the Kambui Supergroup are associated with important zones of shearing and deformation where gold, sulphide and molybdenite mineralization has been concentrated. The Rokel-Kasila Zone bounds the main part of the West African Craton on its west and south-western margin in Sierra Leone, and appears to form part of a north-south orogenic belt. Within this belt, the Marampa Group appears to represent some of the oldest rocks.

The Kasila Group is a distinctive group of mafic gneisses and granulites, trending NW–SE, flanking amphibolites emplaced along the western margin of the West African Craton. It comprises the main outcrop of crystalline schists, gneisses and granulites of the Liberian complex.  The unit is bounded on the east by intrusive granites whilst on the west; it is overlain by sediments, sands and clays of Pleistocene to recent age. The lithology of the Kasila Group is dominated by fine- to medium-grained basic granulites with minor horizons of quartz magnetite, quartz diopside, and silimanite rocks. The granulites are intruded by deformed gabbros, anorthosites, and ultramafics in which relict igneous textures have survived the Pan- African reworking in zones where shearing was less intense. The Kasila Group comprises of siliceous gneisses with lesser amounts of charnockites, garnet-hornblende gneisses, garnet-plagioclase gneisses and minor hornblendite and pyroxenite. Protoliths probably included basalt along with siliciclastic and chemical sedimentary rocks metamorphosed to granulite and almandine–amphibolite facies.

The western portion of the group is predominantly amphibolites, gneisses and metasedimentary migmatites, whereas the eastern portion is metabasic and subordinate metasedimentary granulites containing thin layers and lenses of banded iron formation, quartzite and leucogabbro. The Kasila Group, also considered to be part of the Kambui Supergroup, comprises a high-grade series of granulites, consisting of garnet, hypersthene and hornblende gneisses, quartzites and associated migmatites. When eroded, significant secondary deposition of titanium minerals have been formed from this unit as well as bauxitization which is as a result of weathering process.

A late Precambrian to Cambrian sedimentary and volcanic assemblage, the Rokel River Group, was deposited unconformably on a basement complex. Deposition was probably in a fault-bounded basin of the intracratonic type along the line of the Rokel-Kasila Group following the formation of the tectonic zone at the end of the Liberian or during the Eburnean Orogeny. The Rokel River Group and the Kasila Group to the west were deformed during the Rokelides orogenic episode (~550 Ma).  Deformation increased in intensity westwards.

The Saionya Scarp Group forms a small ingression into Sierra Leone in the northwest of the country, and is composed of horizontally-bedded arkoses, grits and shales with intruded dolerite sills. The group appears to belong to that part of the Gres Horizontaux of Guinea which has been classified as Ordovician, based on the discovery of the graptolites Monograptus riccartonensis and Monograptus priodon in shales near Telimele. In Sierra Leone, the Saionya Scarp Group rests on The Rokel River Group. Dolerite intrusions are common as dykes trending mainly east-west within the basement complex, and as extensive sills above the Rokel River Group.

The Freetown igneous complex forms an intrusive body on the coast, with arcuate outcrop concave towards the west. It is composed of a layered complex of gabbro, norite, troctolite and anorthosite. Platinum occurs in the gravels of many of the streams that cut the outcrops of anorthosite and anorthositic gabbro in the noritic gabbro complex of the Freetown Peninsula. The relation of this complex with the other units is obscured by the coastal veneer of Tertiary sediments of the Bullom Group which lies unconformably on the basement. Tertiary and more recent weathering has led to lateritization across a large part of Sierra Leone, affecting mainly the greenstone belts and the extensive dolerite intrusions.

Geological Map of Sierra Leone