Eburnean Orogeny Research Articles

A historical overview of Moroccan magmatic events along northwest edge of the West African Craton

Located along the northwestern edge of the West African Craton, Morocco exhibits a wide variety of magmatic events from Archean to Quaternary. The oldest magmatic rocks belong to the Archean Reguibat Shield outcrops in the Moroccan Sahara. Paleoproterozoic magmatism, known as the Anti-Atlas granitoids, is related to the Eburnean orogeny and initial cratonization of the WAC. Mesoproterozoic magmatism is represented by a small number of mafic dykes known henceforth as the Taghdout mafic volcanism. Massive Neoproterozoic magmatic activity, related to the Pan-African cycle, consists of rift-related Tonian magmatism associated with the Rodinia breakup, an Early Cryogenian convergent margin event (760–700 Ma), syn-collisional Bou-Azzer magmatism (680–640 Ma), followed by widespread Ediacaran magmatism (620–555 Ma). Each magmatic episode corresponded to a different geodynamic environment and produced different types of magma.Phanerozoic magmatism began with Early Cambrian basaltic (rift?) volcanism, which persisted during the Middle Cambrian, and into the Early Ordovician. This was succeeded by massive Late Devonian and Carboniferous, pre-Variscan tholeiitic and calc-alkaline (Central Morocco) volcanic flows in basins of the Moroccan Meseta. North of the Atlas Paleozoic Transform Zone, the Late Carboniferous Variscan event was accompanied by the emplacement of 330–300 Ma calc-alkaline granitoids in upper crustal shear zones. Post-Variscan alkaline magmatism was associated with the opening of the Permian basins.Mesozoic magmatism began with the huge volumes of magma emplaced around 200 Ma in the Central Atlantic Magmatic Province (CAMP) which was associated with the fragmentation of Pangea and the subsequent rifting of Central Atlantic. CAMP volcanism occurs in all structural domains of Morocco, from the Anti-Atlas to the External Rif domain with a peak activity around 199 Ma. A second Mesozoic magmatic event is represented by mafic lava flows and gabbroic intrusions in the Internal Maghrebian flysch nappes as well as in the external Mesorif. This event consists of Middle-Upper Jurassic MORB tholeiites emplaced during opening of the Alpine Tethys ocean. The Central High Atlas also records Early Cretaceous alpine Tethys magmatism associated with the aborted Atlas rift, or perhaps linked to plume activity on the edge of the WAC.Cenozoic magmatism is associated with Tertiary and Quaternary circum-Mediterranean alkaline provinces, and is characterized by an intermittent activity over 50 Ma from the Anti-Atlas to the Rif Mountain along a SW–NE volcanic lineament which underlines a thinned continental lithosphere.

A review of the Birimian Supergroup- and Tarkwaian Group-hosted gold deposits of Ghana

Ghana is the largest producer of gold in West Africa, a region with over 2,500 years of history with regards to gold production and trade. Modern exploration for and mining of gold in Ghana dates from 1874 with the establishment of the British Gold Coast Colony, which was followed in 1957 by the independence of Ghana and increased gold production since the early 1980s through Ghana's Economic Recovery Plan. At the time of writing, gold production (108.2 tonnes or 3.48 million ounces [Moz] in 2014) accounted for approximately one-third of Ghana's export revenues, with 36% of gold production coming from small-scale mining. The majority of the gold occurs in two styles of mineralisation, namely mesothermal quartz vein-hosted and associated gold in metavolcanics and metasediments and modified palaeoplacer gold in conglomerates. These styles of mineralisation occur in the Palaeoproterozoic Birimian Supergroup and Tarkwaian Group that make up Ghana's mainly southwest to northeast trending Birimian belts. Significant gold resources also occur as hydrothermal mineralisation in basement-type granitoids which show some geological association with the Birimian Supergroup-hosted mesothermal mineralisation. The majority of the gold mineralisation is believed to have formed between approximately 2.15 and 2.06 Ga during the Eburnean orogeny. The mesothermal quartz vein gold mineralisation is usually confined to tectonic corridors within the Birimian belts and is strongly associated with shear zones and fault systems. The quartz veins show multiple stages of formation and are steeply dipping, with the gold mineralisation occurring either as free gold within fractures in the veins or as invisible gold within disseminated sulphides in the host rocks surrounding the veins. The vein- and sulphide-hosted gold is strongly associated with deformational fabrics formed by the Eburnean extensional and compressional events, respectively, suggesting that disseminated sulphide mineralisation predates quartz vein-hosted mineralisation. The fluid from which the gold precipitated is believed to have been of metamorphic origin and carbon dioxide (CO 2 ) dominated, with lesser water (H 2 O) and nitrogen (N 2 ) and minor methane (CH 4 ). Gold precipitation was probably caused by decrease in pressure, temperature and CO 2 -H 2 O immiscibility, at depths of between 7 and 11 km. The palaeoplacer gold mineralisation shows some hydrothermal modification and occurs mostly within the conglomerates and to a lesser extent within the interbedded quartzite of the Banket Series of the Tarkwaian Group. The gold occurs as free gold in the matrix of the conglomerates and as intergrowths and overgrowths on and inclusions in, other heavy mineral grains. Based on strong evidence for palaeoplacer mineralisation, the gold was likely deposited with the conglomerates in alluvial fans and braided tributary channels. Because the Eburnean compressional event associated with the Birimian Supergroup-hosted mesothermal gold mineralisation also overprinted the gold-bearing Banket conglomerates, the former gold occurrences could not have acted as the source for the palaeoplacer gold. Hydrothermal gold mineralisation occurs in the Palaeoproterozoic belts and basin granitoids that intrude the Birimian belts, as well as in the sedimentary basins occurring between the belts. Gold mineralisation within the granitoids occurs as micro-inclusions in sulphides in small, steeply dipping stockworks and as sulphide disseminations concordant with regional faults and shears. A gold-bearing fluid similar to that for the Birimian Supergroup-hosted quartz vein gold mineralisation, but with a larger H2O component, is proposed to have formed the granitoid-hosted gold mineralisation.

K–Ar mineral ages and thermal history of magmatic and metamorphic Palaeoproterozoic units from the northern part of Kedougou Kenieba Inlier, West African Craton (Eastern Senegal)

AbstractThe northern part of the Kedougou Kenieba Inlier (Eastern Senegal) is composed of Palaeoproterozoic volcano‐plutonic and volcano‐sediment areas cross‐cut by differentiated plutonic bodies of gabbroic to granitic composition. They are emplaced in an arc‐type environment and accreted during the Eburnean Orogeny. We present here new K–Ar ages obtained on several plutons from different mineralogical phases, each characterized by a different closure temperature for the K–Ar system. Ages from hornblende, between 2013 ± 29 and 2141 ± 30 Ma, constrain the cooling history of the magma bodies through the 550 °C isotherm. They are slightly younger than U–Pb ages previously obtained from the same units, attesting of a rapid cooling, tectonically controlled, of about 7 °C/Myr. Ages obtained on biotite minerals range from 1955 ± 28 to 1893 ± 27 Ma, suggesting that from 550 to 325 °C, cooling rate decreased to about 3 °C/Myr. Significantly younger ages ranging from 1704 ± 24 to 1615 ± 23 Ma have been obtained on plagioclase and microcline. They can be interpreted in terms of either a slow cooling with a rate of only 1 °C/Myr, from about 1.9 to 1.6 Ga, or either reflecting the occurrence of a rather limited thermal event younger than 1.7 Ga. Since no age younger than 2.0 Ga has previously been obtained for plutonic rocks from the early Birimian in the West African Craton, ages of 1.7–1.6 Ga obtained here from feldspars might be the first evidence for a regional re‐heating event having occurred in the East–North Kedougou Kenieba inlier. Copyright © 2015 John Wiley & Sons, Ltd.

Geochronology and lithostratigraphy of the Siguiri district: Implications for gold mineralisation in the Siguiri Basin (Guinea, West Africa)

Geochronology and lithostratigraphic characterisation of the Siguiri district in the Palaeoproterozoic Birimian terrane of West Africa has provided a reappraisal of the Siguiri Basin stratigraphy. This new insight into the evolution of the central part of the basin further highlights first order controls on the location of the world class Siguiri orogenic gold district. Three metasedimentary formations occur in the Siguiri district: the Balato, Fatoya and Kintinian formations. These formations consist of fine-grained organic-rich shales, mudstone and siltstone/greywacke/limestone interbeds, decimetre- to metre-thick graded greywacke beds and debris flow deposits. The Balato Formation displays low-energy, hemipelagic marine or lake sediments, whereas the Fatoya Formation consists of distal turbidite deposits (thin-bedded turbidites and channel-fill deposits). Altogether they form a basal regressive sequence. The overlying Kintinian Formation is shale-rich, and contains a stack of polymict conglomeratic interbeds (up to 100m in thickness) around the base of the Formation in the Siguiri district. This conglomerate unit transects the whole Siguiri Basin and is interpreted to be the product of repeated subaqueous debris flow deposits and represent olistostromes (or mélanges of sedimentary origin) with various autochthonous and allochthonous clasts. The youngest detrital zircon age population for each of the Balato, Fatoya and Kintinian formations yields ages of 2113±10Ma, 2113±5Ma and 2124±7Ma respectively, thus constraining the maximum age of deposition for each of these formations. The minimum age of deposition of these sedimentary rocks is constrained by a crosscutting volcanic breccia dated at 2092±5Ma, and by the Maléa monzogranite intrusion dated at 2089±12Ma. Comparison of the sedimentary facies and geochronology of the Siguiri sedimentary rocks with other sedimentary basins in the West African Craton, suggests that the Kintinian Formation is part of the Lower Tarkwa Group, best described as “late orogenic basin” sediments. The Kintinian olistostromes are inferred to highlight the onset of renewed tectonic activity associated with the first phases of compression during the Eburnean Orogeny in the Siguiri Basin at ca. 2115Ma. The peculiar morphology of these olistostromes compared to the bulk of the Siguiri Basin sedimentary rocks, illustrates a major change in lithofacies. This change is interpreted to have developed along fundamental basement structures, controlling the early architecture of the Siguiri Basin as well as the location of world-class gold mineralisation in the Siguiri district. These fundamental structures are thought to have acted as first order fluid pathways during the E-W progressive compressional event responsible for orogenic gold mineralisation in the Siguiri district.

Gold mineralisation in the Essakane goldfield in Burkina Faso, West African craton

The West African craton is known for its structurally hosted Au deposits in Ghana, Burkina Faso, Côte d'Ivoire, Mali and Niger. The Essakane goldfield in northeast Burkina Faso has produced 1 606,000oz of gold since 2010 from the Essakane Main Zone. The Essakane goldfield is made up of several exploration and artisanal sites that include; Essakane Main Zone, Gossey, Falagountou, Sokadie, Tin Zoubratan, Essakane North and South, Korizéna, Bom Kodjélé, Tin Taradat, Tassiri, Gaigou, and Takabangou. Gold mineralisation in sheeted and stockworks quartz–carbonate and tourmaline veins occurs with pyrite, arsenopyrite, and traces of pyrrhotite, galena and hematite. It is hosted in sheared, folded and contact metamorphosed volcanic, volcanoclastic and sedimentary Birimian Supergroup sequences. The maximum age of gold mineralisation in the Essakane goldfield is syn-deformational and formed during the Eburnean Orogeny (D2) at 2130–1980Ma.

Lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton: Interplay of coeval contractional deformation and extensional gravitational collapse

We present new litho-structural and metamorphic maps of the Paleoproterozoic (2.25–2.00Ga) West African Craton in northern Ghana, based on the interpretation of field observations and airborne geophysical datasets. The study reveals contrasting metamorphic domains consisting of high-grade gneisses and low-grade volcano–sedimentary belts, separated by shear zones and assembled during the Paleoproterozoic Eburnean orogeny (2.15–2.07Ga). Supracrustal rocks were buried and metamorphosed at conditions corresponding to the amphibolite–granulite facies transition, during a (D1) deformation event, consistent with N–S horizontal shortening, which generated reverse shear zones. High and low metamorphic grade rocks were brought in contact along extensional shear zones formed during N–S extension (D2). These structures were overprinted by contractional deformation associated with E–W shortening and N to NNE stretching (D3), coeval with amphibolite-facies metamorphism. The late-stage tectonic evolution (D4–D7) is characterised by strain localisation in successive generations of narrow shear zones, and the re-activation of inherited structures in a dominantly transcurrent regime. U–Pb dating of zircon and monazite from magmatic and metamorphic rocks reveals that D1-D3 deformations form a continuous and overlapping time sequence between ca. 2140 and 2110Ma. The changes in deformation style from D1 to D3 are interpreted to reflect a shift from dominant horizontal tectonic forces to an interplay between tectonic and gravitational forces, which allowed for the exhumation of the lower crust in an anatectic migmatite dome. We suggest that doming is accommodated by lateral extensional sliding of the upper crust and amplified by coeval orthogonal shortening. The abrupt rotation of shortening directions points to a change in boundary conditions applied to the orogen. We hypothesise that it is due to the collision of northern Ghana with the Paleoproterozoic province in modern-day south-western Burkina Faso, which shows a contrasting geological history. The results reveal that the Eburnean orogeny in NW Ghana shared some thermo-mechanical similarities with modern orogenic belts. The findings bring new insight in Paleoprotezozoic plate tectonics, at the transition between archaic and modern geodynamics.

The Bagassi gold deposits on the eastern margin of the Houndé greenstone belt, Burkina Faso

The Bagassi gold deposits are situated on the West African craton and hosted in Palaeoproterozoic rocks of the Houndé greenstone belt, southwest Burkina Faso. High-grade gold mineralisation is hosted in quartz–gold±pyrite veins-lodes (V1A), in dilational zones and narrow shears in the Bagassi granitoid, and forms the majority of the resource–reserve portfolio in the Bagassi exploration permits, with gold grades of 18–21g/t. Shear hosted gold-pyrite mineralisation in quartz veins in dilational jogs (V1B) occurs along narrow discontinuous shear zones that trend north-northwest in Birimian-aged metabasaltic units, and forms a secondary gold resource. Gold mineralisation is restricted to formation in the late Eburnean Orogeny and formed during a change from east-west to transcurrent compression and shearing. The Bagassi deposits demonstrate that granitoids emplaced prior to onset of the Eburnean Orogeny represent viable gold mineralisation in host rocks that are increasingly seen to be associated with significant gold resources.

The Syama and Tabakoroni goldfields, Mali

ABSTRACT Gold deposits in the Syama and Tabakoroni goldfields in southern Mali occur along a north-northeast trending mineralised litho-structural corridor that trends for approximately 40 km. The deposits are interpreted to have formed during a craton-wide metallogenic event during the Eburnean orogeny. In the Syama goldfield, gold mineralisation in 9 deposits is hosted in the hanging-wall of the Syama-Bananso Shear Zone in basalt, greywacke, argillite, lamprophyre, and black shale. Gold is currently mined primarily from the oxidised-weathered zone of the ore bodies. In the Syama deposit, mineralisation hosted in altered basalt is associated with an intense ankerite–quartz–pyrite stockwork vein systems, whereas disseminated style mineralisation is also present in greywackes. In contrast, the Tellem deposit is hosted in quartz–porphyry rocks. In the Tabakoroni goldfield, gold mineralisation is hosted in quartz veins in tertiary splay shears of the Syama-Bananso Shear Zone. The Tabakoroni orebody is associated with quartz, carbonate and graphite (stylolite) veins, with pyrite and lesser amounts of arsenopyrite. There are four main styles of gold mineralisation including silica-sulphide lodes in carbonaceous fault zones, stylolitic quartz reefs in fault zones, quartz–Fe–carbonate–sulphide lodes in mafic volcanics, and quartz–sulphide stockwork veins in silicified sediments and porphyry dykes. The several deposit styles in the goldfield thus present a number of potential exploration targets spatially associated with the regional Syama-Bananso Shear Zone and generally classified as orogenic shear-hosted gold deposits.

Geological setting of the Wassa gold deposit, SW Ghana

Including past production, current indicated and inferred resources, Wassa is a 5 Moz poly-deformed early-orogenic gold deposit located on the eastern flank of the Ashanti Belt, in southwest Ghana. It is hosted by metamorphosed volcanic, intrusive and sedimentary rocks of the Sefwi Group (ca. 2260–2160 Ma). Early mineralization has an Eoeburnean age (2164 ± 22 Ma, Re–Os on pyrite) and is characterized by quartz veins, by a carbonate alteration of the host rocks, and by deformed gold-bearing pyrite. Remobilization of this gold occurred during the late stages of the Eburnean Orogeny (~ 2.1 Ga) and is associated with quartz-carbonate veins with visible gold and euhedral pyrites.

Provenance of the HP–HT subducted margin in the Variscan belt (Cabo Ortegal Complex, NW Iberian Massif)

AbstractThe Variscan Upper Allochthon is a continental‐affinity terrane that recorded a Cambrian–Ediacaran magmatic arc generation, a subsequent transition to a passive margin, and a collision‐related high‐P metamorphism during the Devonian–Carboniferous amalgamation of Pangea. The objective of this article is to decipher which continental margin subducted in the Devonian high‐P–high‐T (HP–HT) event. To do so, a provenance study is presented using combined U–Pb (n = 613) and Lu–Hf (n = 463) isotopic LA–ICP–MS zircon analyses and Sm–Nd whole–rock (n = 5) determinations. These analyses have been performed on five samples of the Banded Gneisses (Cabo Ortegal Complex, NW Iberia), which forms a part of the HP–HT bottom member of the Upper Allochthon. Palaeozoic–Neoproterozoic zircon ages (34.7%) have a maximum abundance at 522–512 Ma, peaks at 575, 561, 545 Ma and minor abundance peaks between 780 and 590 Ma, and show from their Lu–Hf compositions a volcanic arc mixing pattern. This arc was probably related to the Cadomian arc system. The Mesoproterozoic population is scarce and scattered (2.8%), and due to its Lu–Hf pattern, it is proposed that this population is also West Africa Craton derived. The Paleoproterozoic population (39.6%) is concentrated at 2.07 Ga and it is linked to the Eburnean Orogeny, where depleted mantle derived magmas intruded an Archean craton margin. This craton is represented by the Archean population (22.8%), which is grouped at 3.0, 2.68‐2.61 and 2.52‐2.48 Ga, and shows long‐term reworking processes and at least two juvenile magma intrusions. These data show that the Variscan Upper Allochthon has a West African provenance and therefore, it strongly suggests that the NW Iberian allochthonous complexes and their correlative European terranes are also West Africa derived. These results allow us to finally clarify that the first high‐P event, recorded during the eo‐Variscan amalgamation of Pangea, was attained by the subduction of the margin of Gondwana under Laurussia.

3-D numerical modelling of the influence of pre-existing faults and boundary conditions on the distribution of deformation: Example of North-Western Ghana

High-strain zones bound and separate the high-grade tectono-metamorphic terranes from low-grade greenstone belts in the North-Western parts of Ghana. These belts are bounded by granitoid domains characterized by two main episodic pulses of magmatic intrusive events, which occurred between 2213Ma and 2086Ma. High-strain zones are thought to play a significant role in creating fluid pathways, particularly for partially molten material from lower crustal sources to the upper crust. In this study, a three-dimensional thermo-mechanical model has been used to explore the evolution of high-strain zones and relief under compressional and simple shear boundary conditions. Different orientations of a system of branched strike-slip faults were tested. The effects of the frictional angle and density contrast on the evolution of relief were also compared in this study. The resulting model indicates domains of tensile vs. compressional strain as well as shear zones. This shows that the internal fault zones as well as the host rock in between the faults behave as relatively weaker domains than the external regions. Under both applied compressive and simple shear boundary conditions, these weakened domains constitute preferential zones of tensile and shear strain accommodations in the upper crust, which may favour infilling by deeper partially molten rocks. This processes is suggested by the authors as the most likely processes to have occurred in pre-existing branched shear zones systems in North-Western Ghana during the Eburnean orogeny (around 2.20–2.10Ga).The orientations of faults in these models play an important role in controlling the evolution of relief and localized deformation. In particular, greatest the largest relief is obtained when faults dip parallel to each other and when they are inclined at depth, as they thus facilitate strain rotation and material transfer from depth. The host rock density does not play a primary role in producing relief compared to variations in friction angle at crustal scale of our model. Relief increases by 200–300m when the host rock density is increased by 200kg/m3, whereas relief reduces by about 1200m when decreasing the host rock friction from φ=20° to 10°. This study suggests a model for interpreting the evolution and locus of exhumation of partially molten rocks in North-Western Ghana.

Geochemistry and geochronology of orthogneisses in Bode Saadu area, southwestern Nigeria and their implications for the Palaeoproterozoic evolution of the area

Bode Saadu area, southwestern Nigeria is underlain by metasedimentary and metaigneous rocks including orthogneisses which have been intruded by largely undeformed granitic rocks of probable Pan-African (ca. 600Ma) age. The metamorphic rocks have been subjected to several phases of both ductile and brittle deformation. Three suites of orthogneisses of granodioritic composition have been identified in the area, locally they contain xenoliths of metasediments. Geochemical analyses show that they are calc-alkaline, magnesian, metaluminous and possess the characteristics of granitoids emplaced in magmatic arcs.LA-ICP-MS in-situ dating of zircons in these orthogneisses has yielded upper Concordia intercept Palaeoproterozoic ages of 2236±29Ma, 2228±32Ma and 2179±28Ma. These indicate the times of crystallization of these rocks and attest to a long period of magmatism associated with the Eburnean orogeny in this part of Nigeria. The gneisses bear the imprints of several deformation events in the form of foliations, folds and shear zones. The geochronological data enable constraints to be placed on the timing of the development of some of these structures and also indicate that some of the metasedimentary units are at least of Palaeoproterozoic age.Similar Palaeoproterozoic orthogneisses have been documented in some other parts of southwestern Nigeria, in Central Hoggar area of the Tuareg Shield and in the Borborema Province of Brazil and support their correlation as part of West Gondwana.

Compilation of West African mineral deposits: Spatial distribution and mineral endowment

The West African Craton is highly endowed in minerals, and their spatial and temporal distribution varies from single to multi-phase mineralization events. They are broadly related to three major tectono-metallogenic elements and formed during distinct mineral epochs: (1) In both Archean Shields (Kénéma-Man and Reguibat) and Paleoproterozoic domains (Baoulé-Mossi, Eglab). These are characterized by giant iron ore deposits that formed between ca. 2.5–2.3Ga, nearly all gold, porphyry copper, lead–zinc and sedimentary manganese ore that developed between 2.2 and 2.1Ga, and primary diamonds that formed between two intervals at ca. 2.2–2.0Ga and in the Mesozoic. (2) Across Pan-African and Variscan belts. These are distinguished by major Precambrian IOCG's, copper–gold that formed at ca. 2.1Ga and approximately 680Ma, and Neoproterozoic sedimentary iron ore and phosphate deposits. (3) Within intracratonic and coastal basins. These include the development of Cenozoic lateritic bauxites over Mesozoic dolerites, Tertiary/Quaternary mineral sands deposits, oolitic iron ore and sedimentary phosphate deposits. Geological, spatial and temporal correlations using the multi-commodity West African Mineral Deposit Database highlight that gold and non-gold commodities formed in multiple phases. This commenced in the Liberian Orogeny (2.9–2.8Ga) with the enrichment of iron ore, nickel sulphides, diamonds and gold in the earth's crust. The pre-Eburnean or Tangaean–EoEburnean–Eburnean I Event yielded gold, and the major Eburnean Orogeny yielded gold, iron ore, manganese, diamonds, magmatic nickel sulphides, copper–gold, lead–zinc, and REE minerals. Throughout the Pan-African event sedimentary manganese deposits, lead–zinc, REE minerals, sedimentary phosphates, and again gold were formed. Primary diamonds and magmatic nickel sulphides are related to the break-up of Gondwana, followed by an intense lateritic weathering period that formed bauxite deposits along the craton margin.

The relative and absolute chronology of strato-tectonic events in the Gorom-Gorom granitoid terrane and Oudalan-Gorouol belt, northeast Burkina Faso

The integration of field mapping and analytical studies of magmatic units of the northeast of Burkina Faso has provided new information on the tectonic and magmatic evolution of the Oudalan-Gorouol belt and the Gorom-Gorom granitoid terrane. Structural, geochemical and geochronological analyses have helped to clarify the geological evolution of the region during the Tangaean Event (D1) and Eburnean Orogeny (D2) through to the Wabo Tampelse Event (D3). Further to this, zircon U–Pb geochronology data have demonstrated that the Oudalan-Gorouol belt and the Gorom-Gorom granitoid terrane represents some of the oldest outcropping geology in the Palaeoproterozoic Baoulé-Mossi domain recognised to date, with the oldest age at 2253±9Ma. The geochronology and geology suggests that the basement or a pre-Birimian crust to the Birimian Supergroup may be found in the northeast of Burkina Faso.The Eburnean Orogeny in the northeastern Burkina is preceded by the two phases of deformation (D1−x and D1), and two phases of magmatism. D1−x is associated with the emplacement of the Dori Batholith at the onset of D1 (2164–2141Ma). D1 ductile–brittle deformation formed F1 folds and discrete high-strained mylonite zones that deformed the Oudalan-Gorouol belt and the Gorom-Gorom granitoid terrane during a southwest-directed palaeo-principal transport direction. The pre-Birimian to Birimian supracrustal rocks and intrusions were regionally metamorphosed during D1 to greenschist to amphibolite facies.The Eburnean Orogeny (2130–1980Ma) is characterised by northwest–southeast shortening; it was followed by north–northwest to south–southeast shortening with development of sinistral strike-slip faults and shears. D2 brittle-(ductile) deformation is manifested by refolding of F1 by northeast-trending F2, and development of a pervasive northeast-trending S2 to S2-C foliation. Metamorphic grade attained greenschist facies during D2 with development of the mineral assemblage of quartz-chlorite–muscovite±actinolite.The Wabo Tampelse (D3) deformation event is brittle in character and does not significantly affect regional geological architecture in the study area.

Petrological and geochronological constraints on lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton

AbstractNew petrological and geochronological data are presented on high‐grade ortho‐ and paragneisses from northwestern Ghana, forming part of the Paleoproterozoic (2.25–2.00 Ga) West African Craton. The study area is located in the interference zone between N–S and NE–SW‐trending craton‐scale shear zones, formed during the Eburnean orogeny (2.15–2.00 Ga). High‐grade metamorphic domains are separated from low‐grade greenstone belts by high‐strain zones, including early thrusts, extensional detachments and late‐stage strike‐slip shear zones. Paragneisses sporadically preserve high‐pressure, low‐temperature (HP–LT) relicts, formed at the transition between the blueschist facies and the epidote–amphibolite sub‐facies (10.0–14.0 kbar, 520–600 °C), and represent a low (~15 °C km−1) apparent geothermal gradient. Migmatites record metamorphic conditions at the amphibolite–granulite facies transition. They reveal a clockwise pressure–temperature–time (P–T–t) path characterized by melting at pressures over 10.0 kbar, followed by decompression and heating to peak temperatures of 750 °C at 5.0–8.0 kbar, which fit a 30 °C km−1 apparent geotherm. A regional amphibolite facies metamorphic overprint is recorded by rocks that followed a clockwise P–T–t path, characterized by peak metamorphic conditions of 7.0–10.0 kbar at 550–680 °C, which match a 20–25 °C km−1 apparent geotherm. These P–T conditions were reached after prograde burial and heating for some rock units, and after decompression and heating for others. The timing of anatexis and of the amphibolite facies metamorphic overprint is constrained by in‐situ U–Pb dating of monazite crystallization at 2138 ± 7 and 2130 ± 7 Ma respectively. The new data set challenges the interpretation that metamorphic breaks in the West African Craton are due to diachronous Birimian ‘basins’ overlying a gneissic basement. It suggests that the lower crust was exhumed along reverse, normal and transcurrent shear zones and juxtaposed against shallow crustal slices during the Eburnean orogeny. The craton in NW Ghana is made of distinct fragments with contrasting tectono‐metamorphic histories. The range of metamorphic conditions and the sharp lateral metamorphic gradients are inconsistent with ‘hot orogeny’ models proposed for many Precambrian provinces. These findings shed new light on the geodynamic setting of craton assembly and stabilization in the Paleoproterozoic. It is suggested that the metamorphic record of the West African Craton is characteristic of Paleoproterozoic plate tectonics and illustrates a transition between Archean and Phanerozoic orogens.

Probing the edge of the West African Craton: A first seismic glimpse from Niger

AbstractConstraints on crustal and mantle structure of the Eastern part of the West African Craton have to date been scarce. Here we present results of P receiver function and SK(K)S wave splitting analyses of data recorded at International Monitoring System array TORD in SW Niger. Despite lacking in lateral coverage, our measurements sharply constrain crustal thickness (∼41 km), VP/VS ratio (1.69 ± 0.03), mantle transition zone (MTZ) thickness (∼247 km), and a midlithospheric discontinuity at ∼67 km depth. Splitting delay times are low with an average of 0.63 ± 0.01 s. Fast directions follow the regional surface geological trend with an average of 57 ± 1°. We suggest that splitting is due to fossil anisotropic fabrics in the crust and lithosphere, incurred during the Paleoproterozoic Eburnean Orogeny, with possible contributions from the later Pan‐African Orogeny and present‐day mantle flow. The MTZ appears to be unperturbed, despite the proximity of the sampled region to the deep cratonic root.

Thermo-mechanical modeling of lower crust exhumation—Constraints from the metamorphic record of the Palaeoproterozoic Eburnean orogeny, West African Craton

The aim of this paper is to explore the link between the metamorphic record of a Palaeoproterozoic orogeny and its thermo-mechanical evolution, in order to discuss the burial and exhumation processes of moderate- to high-pressure volcano-sedimentary rocks (6–8 to >10kbar) in the Birimian Province (2.2–2.0Ga) of the West African Craton. Metamorphic data collected in Burkina Faso, southwest Ghana and eastern Senegal suggests a Palaeoproterozoic Birimian crust dominated by moderate apparent geothermal gradients of 20–30°C/km (M2a), which fit a purely conductive thermal crustal regime. This produced greenschist- to amphibolite-facies metamorphic assemblages that developed during the Eburnean orogeny (∼2.1Ga), associated with regional shortening and granitoid intrusions. The M2a gradient is superimposed on an early thermal regime (M1: <10–15°C/km) that produced high-P greenschist- to blueschist-facies metamorphic assemblages. M1 most likely recorded an earlier thermal history in the volcano-sedimentary basins where the protolith of the metamorphic terrains formed. Whether M1 is related to distributed burial associated with sediment accumulation or early subduction-related tectonics remains an open question, not directly addressed here.Thermo-mechanical models were built based on the assumption of an initial crust made up of a succession of volcanic island arcs emplaced on top of a basement dominated by CaO-rich Tonalite–Trondhjemite–Granodiorite suites (TTGs), and tectonically paired with sedimentary basins. From the results of these thermo-mechanical models, we propose a mechanism of burial, heating and exhumation of meta-sediments through the Birimian crust controlled by simultaneous folding/shortening and gravitational instabilities in the juvenile crust. We postulated here the pre-existence of thick sedimentary basins (depth 15km) in the Birimian orogenic crust to model the late-stage exhumation of meta-sediments, collected in the thermal aureole of CaO-poor granitoids, and which record elevated metamorphic pressures (P>6–8kbar). At around 25km depth, TTGs reached appropriate PT conditions to start melting. The buoyant and low-viscosity partially molten material, comprising a fraction of CaO-poor melt and residual material, then ascended through the overlying sediments. It entrained upwards lower crustal material as well as the surrounding sediments. This scenario explains the important breaks in metamorphic conditions observed in the Birimian province between the thin slivers (<2km thick) of high-pressure rocks (P>8kbar) preserved in the thermal aureoles of granites and the adjacent synforms of greenstones, which consist of low-grade metasediments (P<6kbar; T<450°C). This scenario is in agreement with a significant recycling of the TTGs in the genesis of CaO-poor granitoid melts. We discuss the implications of our results for the tectono-metamorphic and mechanical evolution of Palaeoproterozoic orogenies.

Constraints on the timing of late-Eburnean metamorphism, gold mineralisation and regional exhumation at Damang mine, Ghana

The Damang gold deposit in southwest Ghana is unique among known deposits in Ghana, comprising gold mineralisation in two distinct styles. Early gold, hosted in a stratigraphically controlled, auriferous quartz-pebble conglomerate horizon, is overprinted by later gold mineralisation contained in a sub-horizontal fault-fracture quartz vein array. A multi-system geochronological study is used to constrain the timing of igneous activity, regional metamorphism, gold mineralisation and the thermal history at Damang. U/Pb analysis of zircons from Birimian volcaniclastic and intrusive rocks constrain volcanism and associated intrusive activity at 2178.0±9.3Ma and 2164.6±8.0Ma respectively, which is consistent with previous studies. The age of formation of staurolite-grade, amphibolite facies peak metamorphic mineral assemblages at 2005±26Ma is provided by U–Th–total Pb EPMA analysis of metamorphic monazite grains in the Tarkwa Phyllite. Measured 40Ar/39Ar biotite ages range between 1980±9Ma and 1898±11Ma. Argon diffusion modelling with the programme DIFFARG suggests that this age range could be achieved by a period of rapid cooling, at a rate of approximately 17°C/Ma, followed by a prolonged period of much slower cooling, at a rate of 0.15°C/Ma. The period of rapid cooling is interpreted to represent localised exhumation of the Damang host rocks during the latest stage of the Eburnean orogeny at the time of hydrothermal gold mineralisation. Given these age constraints, hydrothermal gold mineralisation is inferred to have occurred between approximately 2030Ma and 1980Ma. These ages constrain metamorphism, fluid flow and gold mineralisation at Damang and are the youngest currently recognised in the Birimian of SW Ghana.

Polycyclic Evolution of Paleoproterozo&amp;#239;c Rocks in the Southwestern Part of the Mako Group (Eastern Senegal, West Africa)

After an extensional stage giving rise to tholeiitic basalts and gabbros, the south western part of Kédougou Kéniéba Inlier (KKI) was affected by three phases of compressionnal deformation. The first phase (D1) was related to NE-SW stress that led to northeastward thrusting. Early granitoids would be implemented during this tectonic event. The second phase (D2) was due to East-West compressional stress that created combined strike sleep faults: NNW-SSE and NNE-SSW respectively sinistral and dextral. During this phase, some volcanic rocks and gabbros were mylonitized, hydrothermalized and transformed into quartzites. These quartzites were considered as sedimentary rocks in previous studies. Granitic plutons and andesites took place during this tectonic phase. D3 deformation occurred towards the end of the Eburnean orogeny. It would be held in two episodes. In the first episode, WNW-ESE stresses created conjugate shears: sinistral NNW-SSE and dextral NE-SW. In the second episode, stresses moved toward the NE-SW direction and made the resulting brittle structures arranged in NS dextral corridors.

Structural setting of gold deposits in the Oudalan-Gorouol volcano-sedimentary belt east of the Markoye Shear Zone, West African Craton

The Oudalan-Gorouol volcano-sedimentary belt (OGB) of Burkina Faso and Niger hosts meta-volcanic and metasedimentary sequences of the Birimian Supergroup that were folded and deformed during emplacement of the Dori Batholith (D1−x), the Tangaean Event (D1) and the Eburnean Orogeny (D2).The emplacement of the Dori Batholith accompanied aureole deformation (D1−x) and the development of proto-mylonite, migmatite, gneiss and schist on the northern margin of the batholith. Contact metamorphic grade reached granulite facies with partial melting of the supracrustal sequences. Emplacement of the Dori Batholith was succeeded by emplacement of monzonite dykes and sills through the OGB.The Tangaean Event (D1) accompanied formation of (a) the Saoga Branch of the Markoye Shear Zone (MSZ), (b) the Mukosi and Billiata mylonite zones that are hosted in the MSZ, (c) the Afu Branch of the Kargouna Shear Zone Complex (KSZC), and (d) northwest-trending thrust-folds (F1) that crosscut the OGB and coalesce with the MSZ. Metamorphic grade attained amphibolite facies in mylonite or proto-mylonite zones in the Saoga and Afu branches. D1 was succeeded by emplacement of alkali-granite plutons of the Dolbel Batholith.The Eburnean Orogeny, D2, accompanied formation of (a) the Korizéna Branch of the MSZ, (b) the Waho Branch of the KSZC, and (c) northeast–trending shear-faults that crosscut the OGB. D2 is manifested by refolding of F1 by northeast-trending F2, and development of a pervasive northeast-trending S2 to S2-C. Metamorphic grade attained greenschist facies during D2 with development of the mineral assemblage quartz–chlorite–muscovite±actinolite. D2 was succeeded by emplacement of northwest-trending gabbro and dolerite dykes.The OGB hosts structurally-controlled gold deposits that are sited along five metallogenic corridors and include the Essakane, Tin–Fal, Bom Kodjelé, Kossa and Tassiri Trends. Gold mineralisation is preferentially located where northeast-trending faults and shears crosscut northwest-trending thrust-folds, or where northwest-trending thrust-folds coalesce with north–northeast trending shears. An intimate relationship thus exists between D1 and D2 structures and gold mineralisation in the OGB. Gold in sheeted-stockwork veins is hosted in competent rocks units including conglomerate beds, greywacke, quartzite, monzonite dykes, pyroxenite–gabbro sills and D1 buck quartz veins. Gold in fine veinlets may also be hosted in massive shale units.