Archaean gneisses and granite-greenstone terrains cover much of northeast DRC and extend into the Central African Republic (“CAR”), western Uganda and southern Sudan. Old basement gneisses, dated at about 3.5 Ga, are known as the Bomu (amphibolite-pyroxene gneisses and granites) and West Nile Complexes. Scattered greenstone belts known as the Ganguan and Kibalian Greenstone Belts have been dated at older than 2.9 Ga and 2.81 Ga respectively.
The Archaean Ganguan supracrustal series overlies the Bomu Complex, and includes quartzites, slates and metavolcanics (talc schists). It is considered to be part of the Kibalian-Ganguan greenstones. The Ganguan series is intruded by aplitic and quartz veins and by small doleritic massifs.
The Upper Congo Granite Greenstone (“UCGG”) association of north Congo belongs to the granite-greenstone belts of the northeastern DRC and CAR. In the northeast DRC, the greenstone belts are referred to as the Kibalian (Supergroup) of Archaean age. Greenstones form a number of zones of approximately 10 to 100 km² in area, composing metavolcanics and some metasediments. Granitoids form a significant part of the Precambrian rocks in northeastern DRC.
Some Upper Kibalian sediments have been identified with some andesitic volcanics, resting upon a Lower Kibalian volcanic granitoid association in DRC. The metavolcanics of the Lower Kibalian have been subdivided into ultramafic, mafic, intermediate and andesitic. The sediments of the upper Kibalian are pelites and banded iron formation (“BIF”, also referred to as itabirites ). The Lower Kibalian is intruded by 2.81 Ga old tonalites, whereas the Upper Kibalian is intruded by 2.46 Ga old granodiorites and granites that represent most of the volume of the UCGG belt.
The UCGG associations of the Archaean greenstone belts of the northern Congo craton have been classified according to their characteristics and to that of their basement as:
- Type A – UCGG association (about 95% of the gold output) comprises greenstones with abundant mafic-ultramafic volcanics and scarce sediments. Associated granitoids correspond to a typical tonalite-trondgheimite-granodiorite (“TTG”) suite. The tonalites of this UCGG association intruded 2.8 to 2.9 Ga ago
- Type B – UCGG association comprises mafic-intermediate volcanics and sediments (mainly BIFs). Associated granodiorites and granites (2.4 to 2.5 Ga), represent most of the volume of the entire greenstone belts, intruded this type B association and its basement.
The areas of the volcanic-granitoid Lower Kibalian display a synclinorial tectonic style, while the greenstones of the Upper Kibalian form belts less than 10 km wide, 30 to 60 km long, made up of units isoclinally folded along subvertical axial planes and horizontal axis. These units “float” within the granitoids.