Geology of Ghana

Last updated
Location of the Jubilee Field offshore Ghana Jubilee field 002.svg
Location of the Jubilee Field offshore Ghana

The geology of Ghana is primarily very ancient crystalline basement rock, volcanic belts and sedimentary basins, affected by periods of igneous activity and two major orogeny mountain building events. Aside from modern sediments and some rocks formed within the past 541 million years of the Phanerozoic Eon, along the coast, many of the rocks in Ghana formed close to one billion years ago or older leading to five different types of gold deposit formation, which gave the region its former name Gold Coast.

Contents

Stratigraphy, Tectonics & Geologic History

Ancient rocks from the Paleoproterozoic, Neoproterozoic and perhaps early Paleozoic cover almost all of Ghana, except near the coast where Paleozoic, Mesozoic and Cenozoic rocks are common.

Main Precambrian rock units include the metamorphosed and folded Dahomeyan, Birimian, Tarkwaian System, Togo Series, and the Buem Formation. Paleozoic rocks include the Voltaian System, and the Sekondi-Accraian Formations. [1]

Paleoproterozoic (2.5-1.6 billion years ago)

Paleoproterozoic rock units belonging to Birimian Supergroup, common across West Africa, dominate northwest and southwest Ghana. The Birimian Supergroup has two units, one a succession of sedimentary rocks including phyllite, tuff and greywacke overlain by conglomerate, sandstone and shale and the other a volcanic tholeiitic magma series. Originally, geologists separated the units into the Lower Birimian and the Upper Birimian, but subsequent research found that the volcanic rocks form several hundred kilometer belts within the larger supergroup, with low-grade metamorphism of the tholeiitic lava rocks.

The volcanic belts from north to south are the Lawra Belt, Bole-Navrongo Belt, Sefwi Belt, Asankrangwa Belt, Ashanti Belt and Kibi-Winiba Belt. Except for the Lawra Belt, they all trend northeast-southwest. Between the different belts are basins filled with dacite, argillite, volcaniclastic and granitoid rocks with isocline folding. At the transition between the basins and the volcanic belts are small outcrops of chert, carbonates and manganese-rich sediments, which are inferred to be the exhalative remains of the eruptions that formed the volcanic belts. [2]

The volcanic and sedimentary rocks were folded during the Eburnean orogeny 2.2 to 2.0 billion years ago and intruded by granitoids. The orogeny also uplifted and eroded the rocks, filling a large graben with a folded molasse series of new sedimentary rocks, known as the Tarkwaian Group. Around the same time, the Birimian Supergroup was intruded again, this time by Cape Coast type batholith granites and granodiorite, along with the Winneba type which has an Archean sial origin from the upper most layer of the mantle. Ghana also has a few other types of granites from this time period, including the Dixcove type, which formed an unfoliated intrusion in the volcanic belt and the rare Bongo potassic granitoids, which formed after the Eburnean orogeny. [3]

Neoproterozoic (1 billion-539 million years ago)

In the Neoproterozoic, Ghana was affected by the Pan-African orogeny. Today, the Pan-African mobile belt terrane spans eastern and southeastern Ghana, with several different units. The Dahomeyan System comprises both mafic and felsic gneiss while the Togo Series includes quartzite, shale and small amounts of serpentinite. The Buem Group is a mix of sediments and igneous rocks, including shale, sandstone, basalt, trachyte and volcanoclastic rocks.

The Volta Basin formed during the Neoproterozoic and includes the one kilometer thick sandstones and claystones of the Bambouaka Supergroup, which emplaced between 1.1 billion and 700 million years ago. In the Neoproterozoic, the Snowball Earth event led to widespread glaciation in what is now Africa. Glacial erosion created an unconformity between the Bambouaka Supergroup and the Pendjari Supergroup (also known as the Oti Supergroup), which in some cases is directly atop more ancient crystalline basement rock. The 2.5 kilometer thick, 600 to 700 million year old Pendjari Supergroup has tillite and dolomitic limestone, rich in barite and silixite at its base and siltstone and argillite in its upper layers. The third major unit in the Volta Basin is the Tamale Supergroup, with a thickness of 500 meters, claystone and siltstone at its base, giving way to coarse sandstone. Geologists debate the age of the Tamale Supergroup which may have formed in the Late Neoproterozoic or possibly in the Cambrian of the early Paleozoic era. [4]

Phanerozoic (539 million years ago-present)

The Phanerozoic, the current eon in which multicellular life became commonplace is mainly recorded along the coast of Ghana. The Accraian Series deposited in the Early Devonian and Middle Devonian while the Sekondian Series formed between the Middle Devonian and the Early Cretaceous, overlapping with the Late Jurassic to Early Cretaceous Amisian Group. The Late Cretaceous Apollonian Group is among the most recent rock units. The past 66 million years of the Cenozoic are mainly recorded with marine, lagoonal and riverine sedimentary rocks as well as with unconsolidated sediments that cover most of the country and form soils. [4]

Hydrogeology

The three major units of the Volta Basin are important aquifers in eastern Ghana with very low, if any, intergranular permeability. The middle and upper Pendjari Supergroup and Obosum Group sandstones, mudstones and siltstones typically have low mineralization, but contain some fluoride, while the deeper Bambouaka Supergroup rocks have generally high sulfate, iron and manganese concentrations and in some places fluoride and high salinity. Although generally impermeable, fractured rock aquifers in basement rocks cover most of southern and western Ghana with slight acidity, low salinity, low hardness and low iodine, but high fluoride. In some places in southwest Ghana, groundwater from weathered basement rock contains high levels of arsenic associated with gold in mineralized veins.

Groundwater recharge is poorly studied in Ghana except for 15 monitored wells in the northeast. [5]

Natural resource geology

Geologic map of the Tarkwa gold district in Ghana showing significant folding and faulting USGS geologic map Ghana.png
Geologic map of the Tarkwa gold district in Ghana showing significant folding and faulting

Gold plays an important role in the economy of Ghana, with up to 1500 tons of gold produced throughout its history. Ghana has five major types of gold deposits. Native gold in steeply dipping quartz veins in shear zones at the margins of Birimian basins, arsenopyrite in sulfur bodies, disseminated mineralization in basin granitoid rocks and alluvial placer gold in river deposits in gravel are major sources of gold. In some cases, ancient placer gold has mineralized and reconstituted along with other minerals such magnetite and hematite in the quartz-pebble conglomerates of the Tarkwaian Group.

From the late 15th century until the mid 19th century, two-thirds of Africa's gold production was estimated to have originated from the Gold Coast. Annual production in the early 1980s was 12,000-15,000 kg. The major primary gold lodes are found in the shear zone between the Lower Birimian phyllites and Upper Birimian greenstones, and consist of quartz veins and lenticular reefs. The gold is usually accompanied with arsenopyrite. Mines along the west side of the Tarkwa syncline include the Obuasi Gold Mine, and those located at Prestea and Konongo. Sedimentary gold is found in the Banket conglomerate near the base of the Tarkwaian. This includes the Iduapriem Gold Mine, Teberebie Mine, and those located at Tarkwa, and Abosso, located along the eastern margin of the Tarkwa syncline. Accompanying minerals include rutile, zircon, and detrital hematite, all within gravel horizons. Placer gold is mainly dredge mined along the Ofin River at Dunkwa-on-Offin. Additionally, eluvium, beach sands, terraces, and Pleistocene stream sediments contain alluvial gold. [6] [7]

Significant diamond deposits are found in river gravels in the central and eastern part of Ghana, although the country does not have kimberlite pipes where diamonds formed in the geologic past. Awaso, in southwest Ghana hosts a bauxite mine run by the Ghana Bauxite Company. The bauxite formed from the weathering of Paleoproterozoic phyllite. Nsuta in the Sekyere Central District of the Ashanti Region has a manganese oxide and carbonate mine. The country also has small, poorly researched deposits of asbestos, chromite, andalusite, mica, barite, cassiterite, columbite, monazite, beryl, spodumene and molybdenite. It also has nepheline syenite and on-shore alluvial deposits of rutile and ilmenite. Ghana has widespread quarrying of sand, gravel, kaolin and other clay deposits for road and building construction as well as brick production. [8]

See also

Related Research Articles

The West African Craton (WAC) is one of the five cratons of the Precambrian basement rock of Africa that make up the African Plate, the others being the Kalahari craton, Congo craton, Saharan Metacraton and Tanzania Craton. Cratons themselves are tectonically inactive, but can occur near active margins, with the WAC extending across 14 countries in Western Africa, coming together in the late Precambrian and early Palaeozoic eras to form the African continent. It consists of two Archean centers juxtaposed against multiple Paleoproterozoic domains made of greenstone belts, sedimentary basins, regional granitoid-tonalite-trondhjemite-granodiorite (TTG) plutons, and large shear zones. The craton is overlain by Neoproterozoic and younger sedimentary basins. The boundaries of the WAC are predominantly defined by a combination of geophysics and surface geology, with additional constraints by the geochemistry of the region. At one time, volcanic action around the rim of the craton may have contributed to a major global warming event.

Birimian

The Birimian rocks are major sources of gold and diamonds that extend through Ghana, Côte d'Ivoire, Guinea, Mali and Burkina Faso. They are named after the Birim River, one of the main tributaries of the Pra River in Ghana and the country's most important diamond-producing area. Ghana and Mali are the second and third largest producers of gold in Africa, respectively.

Wernecke Mountains Group

The Wernecke Mountains are located in the East-central Yukon Territory of Canada. They have provided important wildlife habitat for animals such as grizzly bears and caribou as well as trapping and hunting land for the indigenous people of the Yukon for many centuries. They are the northernmost expression of the North American Cordillera, a chain of mountains stretching from Alaska to southern Mexico. This area has a northern climate characterized by short summers and long winters, with average temperatures from −26 °C (−15 °F) in the winter to 22 °C (72 °F) in the summer While essentially unpopulated, this area is well known for its rich mineral deposits, including gold, silver and copper.

Geology of Germany Overview of the geology of Germany

The geology of Germany is heavily influenced by several phases of orogeny in the Paleozoic and the Cenozoic, by sedimentation in shelf seas and epicontinental seas and on plains in the Permian and Mesozoic as well as by the Quaternary glaciations.

Gabon is situated at the northwestern margin of the Congo Craton—a region of stable, ancient crust—and preserves very ancient rock units across 75% of the country, with overlying sedimentary units from the Cretaceous and other more recent periods.

Geology of the Republic of the Congo

The geology of the Republic of the Congo, also known as Congo-Brazzaville, to differentiate from the Democratic Republic of the Congo, formerly Zaire, includes extensive igneous and metamorphic basement rock, some up to two billion years old and sedimentary rocks formed within the past 250 million years. Much of the country's geology is hidden by sediments formed in the past 2.5 million years of the Quaternary.

Geology of the Democratic Republic of the Congo

The geology of the Democratic Republic of the Congo is extremely old, on the order of several billion years for many rocks. The country spans the Congo Craton: a stable section of ancient continental crust, deformed and influenced by several different mountain building orogeny events, sedimentation, volcanism and the geologically recent effects of the East Africa Rift System in the east. The country's complicated tectonic past have yielded large deposits of gold, diamonds, coltan and other valuable minerals.

Geology of Ivory Coast

The geology of Ivory Coast is almost entirely extremely ancient metamorphic and igneous crystalline basement rock between 2.1 and more than 3.5 billion years old, comprising part of the stable continental crust of the West African Craton. Near the surface, these ancient rocks have weathered into sediments and soils 20 to 45 meters thick on average, which holds much of Ivory Coast's groundwater. More recent sedimentary rocks are found along the coast. The country has extensive mineral resources such as gold, diamonds, nickel and bauxite as well as offshore oil and gas.

The geology of Malawi formed on extremely ancient crystalline basement rock, which was metamorphosed and intruded by igneous rocks during several orogeny mountain building events in the past one billion years. The rocks of the Karoo Supergroup and newer sedimentary units deposited across much of Malawi in the last 251 million years, in connection with a large rift basin on the supercontinent Gondwana and the more recent rifting that has created the East African Rift, which holds Lake Malawi. The country has extensive mineral reserves, many of them poorly understand or not exploited, including coal, vermiculite, rare earth elements and bauxite.

The geology of Mauritania is built on more than two billion year old Archean crystalline basement rock in the Reguibat Shield of the West African Craton, a section of ancient and stable continental crust. Mobile belts and the large Taoudeni Basin formed and filled with sediments in the connection with the Pan-African orogeny mountain building event 600 million years ago and a subsequent orogeny created the Mauritanide Belt. In the last 251 million years, Mauritania has accumulated additional sedimentary rocks during periods of marine transgression and sea level retreat. The arid country is 50% covered in sand dunes and has extensive mineral resources, although iron plays the most important role in the economy.

The geology of Mozambique is primarily extremely old Precambrian metamorphic and igneous crystalline basement rock, formed in the Archean and Proterozoic, in some cases more than two billion years ago. Mozambique contains greenstone belts and spans the Zimbabwe Craton, a section of ancient stable crust. The region was impacted by major tectonic events, such as the mountain building Irumide orogeny, Pan-African orogeny and the Snowball Earth glaciation. Large basins that formed in the last half-billion years have filled with extensive continental and marine sedimentary rocks, including rocks of the extensive Karoo Supergroup which exist across Southern Africa. In some cases these units are capped by volcanic rocks. As a result of its complex and ancient geology, Mozambique has deposits of iron, coal, gold, mineral sands, bauxite, copper and other natural resources.

The geology of Niger comprises very ancient igneous and metamorphic crystalline basement rocks in the west, more than 2.2 billion years old formed in the late Archean and Proterozoic eons of the Precambrian. The Volta Basin, Air Massif and the Iullemeden Basin began to form in the Neoproterozoic and Paleozoic, along with numerous ring complexes, as the region experienced events such as glaciation and the Pan-African orogeny. Today, Niger has extensive mineral resources due to complex mineralization and laterite weathering including uranium, molybdenum, iron, coal, silver, nickel, cobalt and other resources.

Geology of Tanzania

The geology of Tanzania began to form in the Precambrian, in the Archean and Proterozoic eons, in some cases more than 2.5 billion years ago. Igneous and metamorphic crystalline basement rock forms the Archean Tanzania Craton, which is surrounded by the Proterozoic Ubendian belt, Mozambique Belt and Karagwe-Ankole Belt. The region experienced downwarping of the crust during the Paleozoic and Mesozoic, as the massive Karoo Supergroup deposited. Within the past 100 million years, Tanzania has experienced marine sedimentary rock deposition along the coast and rift formation inland, which has produced large rift lakes. Tanzania has extensive, but poorly explored and exploited natural resources, including coal, gold, diamonds, graphite and clays.

The geology of Morocco formed beginning up to two billion years ago, in the Paleoproterozoic and potentially even earlier. It was affected by the Pan-African orogeny, although the later Hercynian orogeny produced fewer changes and left the Maseta Domain, a large area of remnant Paleozoic massifs. During the Paleozoic, extensive sedimentary deposits preserved marine fossils. Throughout the Mesozoic, the rifting apart of Pangaea to form the Atlantic Ocean created basins and fault blocks, which were blanketed in terrestrial and marine sediments—particularly as a major marine transgression flooded much of the region. In the Cenozoic, a microcontinent covered in sedimentary rocks from the Triassic and Cretaceous collided with northern Morocco, forming the Rif region. Morocco has extensive phosphate and salt reserves, as well as resources such as lead, zinc, copper and silver.

Geology of Senegal

The geology of Senegal formed beginning more than two billion years ago. The Archean greenschist Birimian rocks common throughout West Africa are the oldest in the country, intruded by Proterozoic granites. Basins formed in the interior during the Paleozoic and filled with sedimentary rocks, including tillite from a glaciation. With the rifting apart of the supercontinent Pangaea in the Mesozoic, the large Senegal Basin filled with thick sequences of marine and terrestrial sediments. Sea levels declined in the Eocene forming large phosphate deposits. Senegal is blanketed in thick layers of terrestrial sediments formed in the Quaternary. The country has extensive natural resources, including gold, diamonds, and iron.

Geology of Sudan

The geology of Sudan formed primarily in the Precambrian, as igneous and metamorphic crystalline basement rock. Ancient terranes and inliers were intruded with granites, granitoids as well as volcanic rocks. Units of all types were deformed, reactivated, intruded and metamorphosed during the Proterozoic Pan-African orogeny. Dramatic sheet flow erosion prevented almost any sedimentary rocks from forming during the Paleozoic and Mesozoic. From the Mesozoic into the Cenozoic the formation of the Red Sea depression and complex faulting led to massive sediment deposition in some locations and regional volcanism. Sudan has petroleum, chromite, salt, gold, limestone and other natural resources.

The geology of Nigeria formed beginning in the Archean and Proterozoic eons of the Precambrian. The country forms the Nigerian Province and more than half of its surface is igneous and metamorphic crystalline basement rock from the Precambrian. Between 2.9 billion and 500 million years ago, Nigeria was affected by three major orogeny mountain-building events and related igneous intrusions. Following the Pan-African orogeny, in the Cambrian at the time that multi-cellular life proliferated, Nigeria began to experience regional sedimentation and witnessed new igneous intrusions. By the Cretaceous period of the late Mesozoic, massive sedimentation was underway in different basins, due to a large marine transgression. By the Eocene, in the Cenozoic, the region returned to terrestrial conditions.

Geology of Sweden

The geology of Sweden is the regional study of rocks, minerals, tectonics, natural resources and groundwater in the country. The oldest rocks in Sweden date to more than 2.5 billion years ago in the Precambrian. Complex orogeny mountain building events and other tectonic occurrences built up extensive metamorphic crystalline basement rock that often contains valuable metal deposits throughout much of the country. Metamorphism continued into the Paleozoic after the Snowball Earth glaciation as the continent Baltica collided with an island arc and then the continent Laurentia. Sedimentary rocks are most common in southern Sweden with thick sequences from the last 250 million years underlying Malmö and older marine sedimentary rocks forming the surface of Gotland.

The geology of Nunavut began to form nearly three billion years ago in the Archean and the territory preserves some of the world's oldest rock units.

The geology of South Korea includes rocks dating to the Archean and two large massifs of metamorphic rock as the crystalline basement, overlain by thick sedimentary sequences, younger metamorphic rocks and volcanic deposits. Although extent is small, Geology is diverse, and There are diverse rocks which is formed during long-time from Precambrian to Cenozoic Era in Korea Peninsula.

References

  1. Kesse, G.O. (1984). Foster, R.P. (ed.). The occurrence of gold in Ghana, in Gold '82: The Geology, Geochemistry and Genesis of Gold Deposits. Rotterdam: Geological Society of Zimbabwe, A.A. Balkema. pp. 648–650. ISBN   906191504X.
  2. Schlüter, Thomas (2008). Geological Atlas of Africa. Springer. pp. 116–117.
  3. Eisenlohr & Hirdes (1992). "The structural development of the early Proterozoic Birimian and tarkwaian rocks of southwest Ghana, West Africa". Journal of African Earth Sciences (and the Middle East). Journal of African Earth Sciences. 14 (3): 313–325. doi:10.1016/0899-5362(92)90035-B.
  4. 1 2 Schlüter 2008, pp. 116–117.
  5. "Hydrogeology of Ghana". British Geological Survey.
  6. Wright, J.B.; Hastings, D.A.; Jones, W.B.; Williams, H.R. (1985). Wright, J.B. (ed.). Geology and Mineral Resources of West Africa. London: George Allen & UNWIN. pp. 45–47. ISBN   9780045560011.
  7. Taylor, Ryan; Anderson, Eric (2018). Quartz-Pebble-Conglomerate Gold Deposits, Chapter P of Mineral Deposit Models for Resource Assessment, USGS Scientific Investigations Report 2010-5070-P (PDF). Reston: US Dept. of the Interior, USGS. p. 9.
  8. Schlüter 2008, pp. 118–119.