The gold deposits of West Africa largely lie within the Proterozoic domain of the Man Shield, the southernmost subdivision of the West African (or Guinean) Craton. The Man shield, which underlies Niger, Ghana and Cote d’Ivoire, comprises early Proterozoic rocks, with Archaean rocks to the west.
Volcano- sedimentary belts and extensive felsic plutonic rocks that are similar to granite greenstone and meta-sedimentary belts of other Precambrian shields are present in both the Archaean and Early Proterozoic terrains. The early Proterozoic volcanic and sedimentary rocks of the Man shield are referred to as the Birimian Group and this group is broadly divided into phyllites, tuffs and greywackes of the Lower Birimian, and various basaltic to andesitic lavas and volcanoclastics of the Upper Birimian.
These subdivisions are largely believed to be coeval and have been deformed and regionally metamorphosed to grades ranging from lower greenschist to lower amphibolite facies. The Birimian Group has been intruded by two distinctive granitoid types.
The larger basin-type granitoids (and gneisses) are muscovite and/or biotite-rich, and are distinctly foliated and deformed, providing a pre-tectonic appearance. The smaller belt-type (arc related) granitoids (Dixcove Suite), on the other hand, are hornblende-rich, lack the characteristic foliation of the former, and are generally interpreted to be syn or post-tectonic. Despite their appearance, the belt-type granitoids are dated as being 60-90 million years older than the larger basin-type granitoids, however these dates are frequently inconsistent with field observations.
The younger Proterozoic Tarkwaian sediments, thought to unconformably overlie the Birimian Group, consist of a thick series of arenaceous and, to a lesser extent, argillaceous sediments believed to be derived from erosion of the Birimian. Economically important conglomerates and quartzites, termed the Banket Group, comprise the basal portion of the series.
The Tarkwaian Series is largely confined to elongate north northeast trending basins, believed to represent intra-cratonic rifts. The margins of these basins commonly coincide with major (frequently mineralised) structures representing the contact between Upper and Lower Birimian sequences.
It is conceivable that reactivation of the major structures during a period of crustal extension may have been responsible for rifting and subsequent preservation of the Tarkwaian Series. The Birimian belt-basin development was followed at around 2.1 billion years by the Eburnean tectono-thermal event, a single-stage progressive SE-NW compressional and regional metamorphic episode, expressed by foliation and shear development oriented between 025° and 050°. Metamorphic grade is now generally lower greenschist facies with higher grade facies locally developed proximal to intrusive bodies
The Birimian Group of the Man Shield constitutes one of the world’s important metallogenic provinces for gold. The spatial distribution of gold mineralisation appears to be governed by north to northeast trending belts of metavolcanic rocks, ranging from 15 to 40km in width, that are associated with the Upper Birimian. Almost without exception, the major gold deposits are located at or close to the margins of the metavolcanic belts, adjacent to the strongly deformed contacts between the Upper and Lower Birimian sequences.
Gold mineralisation in the majority of the Birimian of West Africa is found in three principal settings: The typical mesothermal lode gold deposits, typified by Obuasi and Bibiana (Ghana) is the most significant of these deposit styles and are generally closely related to major structures at the Upper and Lower Birimian contact. Deposits are of numerous styles, including quartz reefs hosted within sediments (frequently carbonaceous) and appear associated with major shear structures and subsidiary oblique faults.
Lower grade mineralisation may also be present as disseminations or associated with sheeted quartz veining within sediments, tuffs and basic dykes situated in close proximity to major structures.
The second style is the granitoid hosted sheeted vein swarms and stockwork zones. These deposits are typically lower grade than reef style mineralisation and appear to be confined to the smaller belt-type or Dixcove Suite granitoids and their regional equivalents.
Banket deposits represent the third significant style of mineralisation and are hosted by quartz pebble conglomerate located towards the base of the Tarkwaian Series and are typified by the Tarkwa Mine (Ghana). The gold mineralisation is interpreted to be of detrital origin, derived from erosion of the Birimian Series upon which the Banket Group lie. Epigenetic sheeted or stockwork quartz veining is, however, being increasingly recognised within lower portions of the Tarkwaian Series.