The mainland of Equatorial Guinea (Rio Muni) offers the wide variety of mineral potential that is expected on an Archaean cratonic setting with later Pan-African overprinting; with possibilities for gold, diamonds, columbo-tantalite, platinum-group elements, bauxite and base metals.
Rio Muni comprises the Archaean terranes of the Ntem Complex and the Monts de Cristal Massif of the northern Congo Craton, both of which were partly re-worked during the Paleoproterozoic Eburnian orogeny. They consist of largely granitic gneisses, charnockites, mafic intrusions and broad mylonitic shear zones (including an Eburnian terrane boundary), with subordinate amounts of banded ironstones, metasediments, and post-orogenic intrusions. Pan African transpressional structures are common in the west and are associated with granitic intrusions and pegmatite bodies, which also occur throughout the interior. Sub-greenschist shales, argillaceous dolomites, and quartzites occur in the southwest. Higher-grade sedimentary packages, also attributed to the Pan-African, are found along the northern border of the country where they are associated with major strike-slip and thrust faults and post-tectonic granitic intrusions.
The coastal strip of Rio Muni comprises Cretaceous sands, shales, and carbonates with basal conglomerates, all deposited during the rifting phase of Atlantic opening. Trans-Atlantic fracture zones link to major onshore lineaments, at least one of which shows evidence of Cenozoic rifting (the Benito Rift).