Geology of Ghana

Geolological Map of Ghana

 

About two-thirds of Ghana is dominated by Paleoproterozoic Birimian rocks consisting of five evenly spaced volcanic belts trending northeast-southwest. The intervening basins between the volcanic belts are filled by sediments. The remaining one-third is made up of post-Birimian rocks.

The supra-crustal rocks are highly deformed. However, the sedimentary rocks are particularly characterised by extensive folding.

The lavas are mainly of basaltic composition, though andesitic, dacitic and rhyolitic rocks are also present. Some pattern of facies distribution is shown by the Birimian sedimentary basins from the margins towards the basin centres.

The transition zone between the volcanic belts and the sedimentary basins is marked by a chemical facies, which has of late been found to be the site of much of gold mineralization in Ghana.

Previously it was thought that the Birimian sedimentary rocks were older than the volcanic rocks. However, recent studies indicate that the volcanic belts and sedimentary basins are lateral facies equivalents.

Both the volcanic belts and sedimentary basins are intruded by three types of granitoids differing in age, mineralogy and chemistry, namely:

 

• Granitoids in the sedimentary basins - the Cape Coast type - are dominated by two-mica granites
• Granitoids associated with the volcanic belts - the Dixcove-type - are dominated by hornblende-bearing granites
• The late K-rich granitoids (post-Tarkwaian) comprising the Bongo, Tongo and Banso granitoids

Each of the Birimian volcanic belts contains meta-sedimentary rocks - the Tarkwaian Group – that lie unconformably over the Birimian. The Tarkwaian consists entirely of conglomerates, sandstones, phyllites and slates derived from the Birimian country rocks.

The Dahomeyan System occupies the southeastern corner of Ghana and occurs as four alternate belts of acid and basic gneisses, trending south-southwest to north-northeast direction. A greater part of the System forms a monotonous low-lying plain broken by isolated inselbergs and ridges of mainly ultramafic intrusives and hills forming outliers of Togo rocks. The areas occupied by the basic gneisses are flat (Accra Plains), while the areas occupied by the acid gneisses give rise to gently undulating topography.

The Togo Series are made up of meta-sediments and Buem Formation dominated by meta-sediments and metavolcanics, support a range of hills lying in the eastern corner of the country.

Undeformed, flat-lying Neoproterozoic clastic sedimentary rocks forming the Voltaian System cover the eastern third of the country.

A strip of Paleo-Cretaceous to Tertiary sediments occurs along the coast. These consist of the Accraian Series (in Accra area), the Sekondian Group (Elmina, Komenda and Sekondi areas), Apollonian Formation (Tano and Keta areas), and the Amisian Formation (the Saltpond area).