The metallogeny of Ghana

The supracrustal and intrusive rocks of Ghana make up part of the West-African craton of Early to Late Proterozoic (Birimian) ages and located mainly at the western part of the country, associated with mineralisations constituting a major metallogenetic epoch.

Gold is economically the most important metallic mineral, followed by manganese and bauxite, and a number of deposits are being exploited. Iron ore and columbite-tantalite possess mineral potentials in the country. Also known to occur, but having lower economic potential are copper, lead, zinc, chromium and nickel.

Among the non-metallic mineral occurrences, diamond has the greatest potential and is being mined at several sites.

Metallogenetic epochs

(I) Early to Mid Proterozoic Epoch

The Early to Mid Proterozoic period comprises the most significant metallogenic epoch in the country. Gold is predominantly mineralised in the Early to Mid Proterozoic rocks and occurs in association with arsenic, silver and antimony. The deposits occur mostly at the transition zones of the volcanic belt and sedimentary basin of the Birimian. There is a spatial relationship between gold and manganese occurrences on a regional scale. It has been established that gold mineralisation in Birimian rocks in Ghana is strikingly concentrated in a narrow corridor along volcanic belt/sedimentary basin boundaries. This phenomenon is ascribed to the presence of chemical sediments as well as widespread and intensive tectonisation in the belt/basin transition zones. This class of gold deposits comprises quartz vein-type and sulphide-type.

Another class of economically prominent gold ore deposit is represented by the Au-bearing quartz-pebble conglomerates of the Tarkwaian Group, which also hosts diamonds.

(II) Mid to Late Proterozoic Epoch

The Pan African terrain of West Africa contains less economic deposits than the older Birimian and Archaean rocks of the craton. There seems to be two main factors responsible for this. First, the proportion of mafic to ultramafic rocks among the supracrustals is relatively small. Secondly, the poly-cyclic nature of the basement has resulted in large tracts becoming barren of useful minerals.

(III) Tertiary to Quaternary Epoch

Residuals (gold, manganese, bauxite) and placers (gold, diamond) deposits are often due to Tertiary to Quaternary weathering and erosion under tropical conditions.

Gold: Obtained from weathering of Birimian and Tarkwaian rocks

Diamond: Obtained through weathering and erosion of Birimian (Birim diamondiferous area) and Tarkwaian (Bonsa Diamondiferous area)

Manganese: Mn02 as a residual product from the weathering of Birimian ocks (MnC03)

Bauxite: As a residual product from the weathering of Birimian (Kibi, Nyinahin deposits) and Voltaian (Ejuanema deposits).

Iron: From the weathering of Birimian (Oppon Mansi deposits) as well as Voltaian (Shieni deposits).

Metallogenetic provinces

Western half of Ghana constitutes a metallogenetic province in which gold, manganese, iron ore and diamonds occur. In particular, the transitional zones between Birimian Volcanic belts and sedimentary basins are noted for gold mineralisations.