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60-535: The craton appears to have formed when three Archean cratons fused: Leo-Man-Ghana, Taoudeni and Reguibat. The first two docked around 2.1 Ga (billion years ago), and the Reguibat Craton docked with the craton around 2 Ga. The roots of the combined craton extend to a depth of over 300 km (190 mi) in the sub-continental lithospheric mantle. The WAC stretches from the Little Atlas mountains of Morocco to

120-525: A greenhouse effect on massive scale, melting the ice and releasing CO 2 into the atmosphere. The climate quickly switched to one much warmer than today, resulting in the Cambrian explosion of life forms. During its wanderings, at different times covered by ice sheets, forests, marshes or arid desert, the surface of the West African Craton has been heavily eroded by ice, water and wind. In most places

180-457: A water world : there was continental crust , but much of it was under an ocean deeper than today's oceans. Except for some rare relict crystals , today's oldest continental crust dates back to the Archean. Much of the geological detail of the Archean has been destroyed by subsequent activity. The Earth's atmosphere was also vastly different in composition from today's: the prebiotic atmosphere

240-801: A diameter greater than 10 kilometers (6 mi) every 15 million years. This is about the size of the Chicxulub impactor. These impacts would have been an important oxygen sink and would have caused drastic fluctuations of atmospheric oxygen levels. The Archean atmosphere is thought to have almost completely lacked free oxygen ; oxygen levels were less than 0.001% of their present atmospheric level, with some analyses suggesting they were as low as 0.00001% of modern levels. However, transient episodes of heightened oxygen concentrations are known from this eon around 2,980–2,960 Ma, 2,700 Ma, and 2,501 Ma. The pulses of increased oxygenation at 2,700 and 2,501 Ma have both been considered by some as potential start points of

300-413: A feature in later, more oxic oceans. Despite the lack of free oxygen, the rate of organic carbon burial appears to have been roughly the same as in the present. Due to extremely low oxygen levels, sulphate was rare in the Archean ocean, and sulphides were produced primarily through reduction of organically sourced sulphite or through mineralisation of compounds containing reduced sulphur. The Archean ocean

360-408: A network of radiation damaged areas. Fission tracks and micro-cracks within the crystal will further extend this radiation damage network. These fission tracks act as conduits deep within the crystal, providing a method of transport to facilitate the leaching of lead isotopes from the zircon crystal. Under conditions where no lead loss or gain from the outside environment has occurred, the age of

420-698: A permanent chemical change in the ocean and the atmosphere after the Archean. The word Archean is derived from the Greek word arkhē ( αρχή ), meaning 'beginning, origin'. The Pre-Cambrian had been believed to be without life (azoic); however, fossils were found in deposits that were judged to belong to the Azoic age . Before the Hadean Eon was recognized, the Archean spanned Earth's early history from its formation about 4,540 million years ago until 2,500 million years ago. Instead of being based on stratigraphy ,

480-518: A primary diamond source with gold and copper deposits occurring in a sub-domain of the shield ( Paleoproterozoic Yetti). It experienced at least one major cooling event during the Mesozoic and three minor cooling events from the late Cretaceous to present day. Low temperature thermochronology data indicates that the western side of the shield has an igneous differentiation post- Triassic thermal history, largely controlled by vertical movements of

540-428: A result, newly-formed zircon crystals will contain no lead, meaning that any lead found in the mineral is radiogenic . Since the exact rate at which uranium decays into lead is known, the current ratio of lead to uranium in a sample of the mineral can be used to reliably determine its age. The method relies on two separate decay chains , the uranium series from U to Pb, with a half-life of 4.47 billion years and

600-449: A series of zircon samples has lost different amounts of lead, the samples generate a discordant line. The upper intercept of the concordia and the discordia line will reflect the original age of formation, while the lower intercept will reflect the age of the event that led to open system behavior and therefore the lead loss; although there has been some disagreement regarding the meaning of the lower intercept ages. Undamaged zircon retains

660-504: A significant amount of material to qualify as a resource-rich country. The mining of the WAC can establish a strong economy and government development support, helping the alleviation of poverty. Archean The Archean Eon ( IPA : / ɑːr ˈ k iː ə n / ar- KEE -ən , also spelled Archaean or Archæan ), in older sources sometimes called the Archaeozoic , is the second of

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720-525: Is a lack of extensive geological evidence for specific continents. One hypothesis is that rocks that are now in India, western Australia, and southern Africa formed a continent called Ur as of 3,100 Ma. Another hypothesis, which conflicts with the first, is that rocks from western Australia and southern Africa were assembled in a continent called Vaalbara as far back as 3,600 Ma. Archean rock makes up only about 8% of Earth's present-day continental crust;

780-617: Is difficult to reconstruct the early wanderings of the West African Craton, but around 1.13–1.071 billion years ago it seems to have been one of the cratons that came together to form Rodinia , a supercontinent . At that time, the Congo Craton lay to the west of the Amazonian Craton , and the West African Craton lay to the south where both were rotated about 180° and retain this relative configuration. Around 750 million years ago Rodinia rifted apart into three continents: Proto-Laurasia ,

840-525: Is evidenced by certain highly deformed gneisses produced by metamorphism of sedimentary protoliths . The moderate temperatures may reflect the presence of greater amounts of greenhouse gases than later in the Earth's history. Extensive abiotic denitrification took place on the Archean Earth, pumping the greenhouse gas nitrous oxide into the atmosphere. Alternatively, Earth's albedo may have been lower at

900-422: Is one of the oldest and most refined of the radiometric dating schemes. It can be used to date rocks that formed and crystallised from about 1 million years to over 4.5 billion years ago with routine precisions in the 0.1–1 percent range. The method is usually applied to zircon . This mineral incorporates uranium and thorium atoms into its crystal structure , but strongly rejects lead when forming. As

960-881: Is termed the lead–lead dating method. Clair Cameron Patterson , an American geochemist who pioneered studies of uranium–lead radiometric dating methods, used it to obtain one of the earliest estimates of the age of the Earth in 1956 to be 4.550Gy ± 70My; a figure that has remained largely unchallenged since. Although zircon (ZrSiO 4 ) is most commonly used, other minerals such as monazite (see: monazite geochronology ), titanite , and baddeleyite can also be used. Where crystals such as zircon with uranium and thorium inclusions cannot be obtained, uranium–lead dating techniques have also been applied to other minerals such as calcite / aragonite and other carbonate minerals . These types of minerals often produce lower-precision ages than igneous and metamorphic minerals traditionally used for age dating, but are more commonly available in

1020-732: The Cenozoic Era from 66 million to about 1.8 million years ago, the mountain chains that today compose the Atlas Mountains were lifted up as the European and African plates collided at the southern end of the Iberian Peninsula . Erosion has reduced the Little Atlas range so that it is today lower than the High Atlas range to the north. South of the mountains, the West African Craton is relatively flat, mostly desert or dry savanna apart from

1080-556: The Congo craton and Proto-Gondwana . The West African Craton may then have combined with other cratons to form Pannotia , a hypothetical supercontinent that existed from the Pan-African orogeny about 600 million years ago to the end of the Precambrian about 539 million years ago. Later it became part of Gondwana , and later still part of Pangaea , the supercontinent that existed during

1140-492: The Eburnean orogeny . The poor exposure of the greenstone belt in the area limits interpretations of a broad structural context. The limited greenstone belt exposure indicates a major deformation event associated with granitoid intrusions . The shield hosts world-class gold deposits, important iron ore concentrations, and the mineralization of aluminum ore, lead-zinc, manganese , phosphate , and uranium . A majority of

1200-607: The Great Oxygenation Event , which most scholars consider to have begun in the Palaeoproterozoic ( c.  2.4 Ga ). Furthermore, oases of relatively high oxygen levels existed in some nearshore shallow marine settings by the Mesoarchean. The ocean was broadly reducing and lacked any persistent redoxcline , a water layer between oxygenated and anoxic layers with a strong redox gradient, which would become

1260-622: The Gulf of Guinea , and is bounded by mobile belts of much younger rocks to the north, east and west. The oldest rocks were metamorphosed 2.9 to 2.5 billion years ago. In the Sahara it is mostly covered by more recent sediments from the Phanerozoic Eon. Further south, younger volcanic and sedimentary rocks outcrop in Ghana , Ivory Coast , and Sierra Leone , surrounded by even younger sediments laid down in

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1320-534: The Paleozoic and Mesozoic eras about between 335 and 175 million years ago, before North and South America separated from Eurasia and Africa and the continents started to drift towards current configurations. Proponents of the Snowball Earth theory claim that sometime before 650 million years ago the Earth was going through an extremely cold period. The oceans were frozen to great depths, and snow covering reflected

1380-449: The Pan-African and/or the Variscan orogenies. The WAC underlies the modern countries of Morocco , Algeria , Mauritania , Senegal , The Gambia , Guinea Bissau , Guinea , Mali , Burkina Faso , Sierra Leone , Liberia , Ivory Coast , Ghana , Togo and Benin . The metamorphic record of the craton is characteristic of Paleoproterozoic plate tectonics. A definitive evolution of

1440-500: The Precambrian . The WAC is made of two distinct regions north and south of each other; the Reguibat shield and the Man shield respectively. Both of these regions are mainly made of rocks that are either Archean or Paleoproterozoic in age, with western Archean nuclei, and rock types are separated by major shear zones. The fold belts surrounding the WAC were folded and metamorphosed during

1500-460: The actinium series from U to Pb, with a half-life of 710 million years. Uranium decays to lead via a series of alpha and beta decays, in which U and its daughter nuclides undergo a total of eight alpha and six beta decays, whereas U and its daughters only experience seven alpha and four beta decays. The existence of two 'parallel' uranium–lead decay routes ( U to Pb and U to Pb) leads to multiple feasible dating techniques within

1560-472: The geodynamic processes controlling the craton stabilization and maturation after the Archean - Proterozoic transition. Limited geochronological data indicate a prolonged period of metamorphic overprint, lasting approximately 70-million-years, with the support of Sm-Nd garnet-whole-rock isochron age data and U-Pb and Pb-Pb crystallization ages of zircon, monazite, and titanite. Overprinting relationships in

1620-472: The lithosphere , consisting of the crust and the rigid uppermost part of the mantle , solidified. The lithosphere rides on the asthenosphere , which is also solid but can flow like a liquid on geological time scales. The lithosphere is broken up into tectonic plates , which slowly move in relation to one another at speeds of 50–100 mm annually, colliding, combining into continents , splitting and drifting apart to form new continental configurations. It

1680-511: The Archean and become common late in the Archean. Cyanobacteria were instrumental in creating free oxygen in the atmosphere. Further evidence for early life is found in 3.47 billion-year-old baryte , in the Warrawoona Group of Western Australia. This mineral shows sulfur fractionation of as much as 21.1%, which is evidence of sulfate-reducing bacteria that metabolize sulfur-32 more readily than sulfur-34. Evidence of life in

1740-459: The Archean began, the Earth's heat flow was nearly three times as high as it is today, and it was still twice the current level at the transition from the Archean to the Proterozoic (2,500  Ma ). The extra heat was partly remnant heat from planetary accretion , from the formation of the metallic core , and partly arose from the decay of radioactive elements. As a result, the Earth's mantle

1800-522: The Archean without leaving any. Fossil steranes , indicative of eukaryotes, have been reported from Archean strata but were shown to derive from contamination with younger organic matter. No fossil evidence has been discovered for ultramicroscopic intracellular replicators such as viruses . Fossilized microbes from terrestrial microbial mats show that life was already established on land 3.22 billion years ago. Uranium%E2%80%93lead dating Uranium–lead dating , abbreviated U–Pb dating ,

1860-569: The Archean, the conditions necessary to sustain life could not have occurred until the Archean Eon. Life in the Archean was limited to simple single-celled organisms (lacking nuclei), called prokaryotes . In addition to the domain Bacteria , microfossils of the domain Archaea have also been identified. There are no known eukaryotic fossils from the earliest Archean, though they might have evolved during

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1920-464: The Late Hadean is more controversial. In 2015, biogenic carbon was detected in zircons dated to 4.1 billion years ago, but this evidence is preliminary and needs validation. Earth was very hostile to life before 4,300 to 4,200 Ma, and the conclusion is that before the Archean Eon, life as we know it would have been challenged by these environmental conditions. While life could have arisen before

1980-529: The Mineral Act, organized mining began in 1927 with significant mineral discovery in the 1930s and 40s resulting in medium to moderately sized mines; iron ore, two extensive alluvial gold field deposits, placer gold, alluvial diamonds, and chromite deposits. Quarrying and mining provides a livelihood for greater than 250 thousand people in Sierra Leone alone, employing approximately 15% of its population, producing

2040-435: The area has not been determined as a result of conflicting interpretation of the relationship between low-grade greenstone belts and high-grade gneissic terranes dominated by TTG suites. The three major, widely accepted tectonothermal events for the WAC; the 3.5 to 2.9 Ga Pre-Leonean and Leonean Orogeny, the 2.9 to 2.8 Ga Liberian Orogeny, and the 2.15-1.8 Ga Eburnean Orogeny . A definitive answer has strong implications on

2100-431: The area indicate copper mineralization is associated with the first deformation event in the WAC, with gold mineralization occurring during subsequent deformation events via reactivation of magmatic and hydrothermal fluids. The metamorphic rock of the WAC include, but are not limited to, high-grade amphibolite facies amphibolite , gneiss , paragneiss schist , calc-silicate rock , and migmatites . Portions of

2160-511: The areas near the Atlantic or Gulf of Guinea. However, below the surface there are ancient sedimentary basins such as the Taoudeni basin that may contain large reserves of oil and gas. The northern Reguibat shield covers parts of Algeria , Western Sahara , Morocco and Mauritania . It contains exposed Paleoproterozoic units to the east, and Archean to the west, including kimberlites making it

2220-442: The assembly and destruction of one and perhaps several supercontinents . Evidence from banded iron formations, chert beds, chemical sediments and pillow basalts demonstrates that liquid water was prevalent and deep oceanic basins already existed. Asteroid impacts were frequent in the early Archean. Evidence from spherule layers suggests that impacts continued into the later Archean, at an average rate of about one impactor with

2280-549: The beginning and end of the Archean Eon are defined chronometrically . The eon's lower boundary or starting point of 4,031±3 million years ago is officially recognized by the International Commission on Stratigraphy , which is the age of the oldest known intact rock formations on Earth. Evidence of rocks from the preceding Hadean Eon are therefore restricted by definition to non-rock and non-terrestrial sources such as individual mineral grains and lunar samples. When

2340-464: The craton dates back to the early 1960s which used quartz vein debris as a gold indicator. Placer gold is primarily exploited by the artisanal miners, with other deposit types being mined by open-pit or by excavation mining ( surface mining or sub-surface mining ). In 1985 the State of Burkina Faso created an official structure, known as Le Projet Orpaillage, for the management of gold mining and buying in

2400-401: The crust via burial and exhumation processes. The southern Man shield covers the countries Ivory Coast , Mali , Burkina Faso , Ghana , Sierra Leone , Liberia , and Guinea . In this shield, a large portion of the WAC consists of Paleoproterozoic Birimian continent. The Birimian units were initially produced in an immature volcanic arc setting, which was later metamorphosed during

2460-553: The crystalline remnants of the surviving Archean crust. These include great melt sheets and voluminous plutonic masses of granite , diorite , layered intrusions , anorthosites and monzonites known as sanukitoids . Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes , mudstones , volcanic sediments, and banded iron formations . Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite . Carbonate rocks are rare, indicating that

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2520-466: The deep oceans of the Archean probably covered the continents entirely. Only at the end of the Archean did the continents likely emerge from the ocean. The emergence of continents towards the end of the Archaean initiated continental weathering that left its mark on the oxygen isotope record by enriching seawater with isotopically light oxygen. Due to recycling and metamorphosis of the Archean crust, there

2580-506: The four geologic eons of Earth 's history , preceded by the Hadean Eon and followed by the Proterozoic . The Archean represents the time period from 4,031 to 2,500 Mya (million years ago). The Late Heavy Bombardment is hypothesized to overlap with the beginning of the Archean. The Huronian glaciation occurred at the end of the eon. The Earth during the Archean was mostly

2640-444: The geologic record. During the alpha decay steps, the zircon crystal experiences radiation damage, associated with each alpha decay. This damage is most concentrated around the parent isotope (U and Th), expelling the daughter isotope (Pb) from its original position in the zircon lattice. In areas with a high concentration of the parent isotope, damage to the crystal lattice is quite extensive, and will often interconnect to form

2700-494: The gold deposit formations occurred during the Eburnean orogeny , but a number of the remaining gold deposits formed before this orogeny during a period of oceanic arc- back-arc basin formation and erosion during the Neoproterozoic and Cretaceous . Within the West African Craton, there is a large amount of mining activity covering resources such as gold, copper, cobalt, silver, tin, and zinc. Artisanal mining activity in

2760-475: The heat from the sun through cloudless skies. Only simple forms of life could survive in locations such as deep oceanic hydrothermal vents . At the end of this period the edges of the West African craton became highly active, throwing up a ring of volcanoes . The thermal activity was caused by excessive mantle heat that had built up below the craton, insulated by the lithosphere . The volcanic eruptions created

2820-580: The lead generated by radioactive decay of uranium and thorium up to very high temperatures (about 900 °C), though accumulated radiation damage within zones of very high uranium can lower this temperature substantially. Zircon is very chemically inert and resistant to mechanical weathering – a mixed blessing for geochronologists, as zones or even whole crystals can survive melting of their parent rock with their original uranium–lead age intact. Thus, zircon crystals with prolonged and complicated histories can contain zones of dramatically different ages (usually with

2880-526: The neighboring island arcs and deposited in a forearc basin. Greenstone belts, which include both types of metamorphosed rock, represent sutures between the protocontinents. Plate tectonics likely started vigorously in the Hadean , but slowed down in the Archean. The slowing of plate tectonics was probably due to an increase in the viscosity of the mantle due to outgassing of its water. Plate tectonics likely produced large amounts of continental crust, but

2940-462: The notation can be ignored.) These are said to yield concordant ages ( t from each equation 1 and 2). It is these concordant ages, plotted over a series of time intervals, that result in the concordant line. Loss (leakage) of lead from the sample will result in a discrepancy in the ages determined by each decay scheme. This effect is referred to as discordance and is demonstrated in Figure ;1. If

3000-424: The oceans were more acidic, due to dissolved carbon dioxide , than during the Proterozoic. Greenstone belts are typical Archean formations, consisting of alternating units of metamorphosed mafic igneous and sedimentary rocks, including Archean felsic volcanic rocks . The metamorphosed igneous rocks were derived from volcanic island arcs , while the metamorphosed sediments represent deep-sea sediments eroded from

3060-510: The original rocks are buried far below more recent volcanic and sedimentary deposits. The visible features are usually of comparatively recent origin. The Little Atlas range formed about 300 million years ago when Euramerica and Gondwana ground against one another during the Alleghenian orogeny , a process that also formed the Appalachians in present-day North America . More recently, in

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3120-449: The overall U–Pb system. The term U–Pb dating normally implies the coupled use of both decay schemes in the 'concordia diagram' (see below). However, use of a single decay scheme (usually U to Pb) leads to the U–Pb isochron dating method, analogous to the rubidium–strontium dating method. Finally, ages can also be determined from the U–Pb system by analysis of Pb isotope ratios alone. This

3180-514: The region have also been metamorphosed to the greenschist facies , generally termed a greenstone belt in an Archean terrane. The timing of the facies is constrained by the in-situ U-Pb dating , with garnet composition providing constraints for prograde evolution at the blueschist - amphibolite facies transition. The tectonic environment is constrained by a combination of geophysics , surface geology, geochemistry , and metallogenesis. The Earth formed about 4.54 billion years ago. As it cooled,

3240-665: The region. Metallurgical studies on the gold panning rejects were funded by the United Nations Development Programme (UNDP) and the Bureau de Recherches Géologiques et Minières (BUMIGEB). The treatment of mine tailings collected from artisanal working gold was the principle operation conducted by the Compagnie d'Exploitation des Mines d'Or du Burkina (CEMOB), operating a heap leach plant which processed approximately 500 tonnes of gold panning rejects per day. Following

3300-684: The rest of the Archean continents have been recycled. By the Neoarchean , plate tectonic activity may have been similar to that of the modern Earth, although there was a significantly greater occurrence of slab detachment resulting from a hotter mantle, rheologically weaker plates, and increased tensile stresses on subducting plates due to their crustal material metamorphosing from basalt into eclogite as they sank. There are well-preserved sedimentary basins , and evidence of volcanic arcs , intracontinental rifts , continent-continent collisions and widespread globe-spanning orogenic events suggesting

3360-837: The time, due to less land area and cloud cover. The processes that gave rise to life on Earth are not completely understood, but there is substantial evidence that life came into existence either near the end of the Hadean Eon or early in the Archean Eon. The earliest evidence for life on Earth is graphite of biogenic origin found in 3.7 billion–year-old metasedimentary rocks discovered in Western Greenland . The earliest identifiable fossils consist of stromatolites , which are microbial mats formed in shallow water by cyanobacteria . The earliest stromatolites are found in 3.48 billion-year-old sandstone discovered in Western Australia . Stromatolites are found throughout

3420-451: The zircon can be calculated by assuming exponential decay of uranium. That is where This gives which can be written as The more commonly used decay chains of Uranium and Lead gives the following equations: (The notation Pb ∗ {\displaystyle {\text{Pb}}^{*}} , sometimes used in this context, refers to radiogenic lead. For zircon, the original lead content can be assumed to be zero, and

3480-410: Was a reducing atmosphere rich in methane and lacking free oxygen . The earliest known life , mostly represented by shallow-water microbial mats called stromatolites , started in the Archean and remained simple prokaryotes ( archaea and bacteria ) throughout the eon. The earliest photosynthetic processes, especially those by early cyanobacteria , appeared in the mid/late Archean and led to

3540-557: Was enriched in heavier oxygen isotopes relative to the modern ocean, though δ18O values decreased to levels comparable to those of modern oceans over the course of the later part of the eon as a result of increased continental weathering. Astronomers think that the Sun had about 75–80 percent of its present luminosity, yet temperatures on Earth appear to have been near modern levels only 500 million years after Earth's formation (the faint young Sun paradox ). The presence of liquid water

3600-786: Was significantly hotter than today. Although a few mineral grains have survived from the Hadean , the oldest rock formations exposed on the surface of the Earth are Archean. Archean rocks are found in Greenland , Siberia , the Canadian Shield , Montana , Wyoming (exposed parts of the Wyoming Craton ), Minnesota (Minnesota River Valley), the Baltic Shield , the Rhodope Massif , Scotland , India , Brazil , western Australia , and southern Africa . Granitic rocks predominate throughout

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