Regional Geological Setting
Most of West Africa is underlain by the West African craton, an ancient, stable block of earth's crust. The oldest portion, which underlies Sierra Leone, Liberia and parts of Guinea and Cote d'Ivoire, is an Archean (>2,500 m.y.) core, which is surrounded by younger Paleoproterozoic (2,200-2,000 m.y.) sedimentary and volcanic rocks that form regularly spaced belts of alternating rock type. Rocks of this age in West Africa are known as Birimian.
Both volcanic belt rocks and sedimentary basin rocks are intruded by "belt and basin" type granitic plutons, dikes and stocks, respectively. All of these rocks display multiple episodes of tectonic deformation and are weakly to moderately metamorphosed. Gold mineralization is prolific in quartz-carbonate veins along faults and shear zones resulting from these tectonic episodes, particularly along the margins of belts such as the Ashanti and Sefwi Belts.
Each of the volcanic belts is partially overlain by younger sedimentary rocks consisting of quartzite, shale and quartz pebble conglomerate. Collectively, these rocks are known in West Africa as Tarkwaian after the town of Tarkwa, in southern Ghana, where they host modified paleoplacer gold deposits. This style of gold mineralization is strikingly similar to the gold deposits of the Witwatersrand in South Africa. Several multi-million ounce gold deposits (Tarkwa, Teberebie, Iduapriem), are hosted by Tarkwaian sedimentary rocks which also host structurally controlled vein mineralization.
The Asankrangwa Belt is a 5-10 km wide swath of northeast-striking, steeply northwest dipping shear zones and minor intrusions that host gold-bearing quartz-carbonate vein systems. These shear zones are similar to the ones that juxtapose Birimian basin and volcanic rocks and perhaps are the structurally higher continuation of equivalent shear zones in the Kumasi Basin. Accordingly, these shear zones host gold mineralization in a manner similar to that in gold deposits along the margins of the Ashanti and Sefwi Belts. Soil geochemical anomalies trend the same orientation as these shears. This is an emerging belt of mineralization, which is rapidly gaining recognition as more gold is found.
The Kibi Belt is the eastern-most Birimian volcanic belt in Ghana, and the belt least explored by modern exploration techniques. It is similar to the Ashanti Belt in composition and architecture, including the presence of Tarkwaian sedimentary rocks. Gold occurs in veins along shear zones near the margins of the belt (historic Kibi Mine). Abu's Asafo concession straddles the southwest margin of this belt and covers belt-bounding shear zones.
The Ashanti Belt of Ghana has been a world-famous gold producer for over 100 years. The premier gold deposit along this belt is Obuasi, which began production in 1898. Since then, underground mining operations have mined vein material along ~7 km of strike length and to a depth of 1.6 km. This world class mine has total contained gold in excess of 60M oz consisting of past production and current resources. Similar vein deposits such as Prestea and Konongo have famed and robust production histories. In recent years, many new multi-million ounce deposits have been discovered along and adjacent to the Ashanti Belt, helping to keep alive the world-class status of this belt.
The Ashanti Belt, along with other Birimian volcanic belts of West Africa, is overlain by Tarkwaian sedimentary rocks. In the southern portion of the Ashanti Belt, these rocks host modified paleoplacer deposits similar to those of the Witwatersrand, South Africa. Several multi-million ounce deposits have been mined or are currently being mined from these rocks. New discoveries have found that some Tarkwaian rocks also host vein mineralization as at Dunkwa.