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Mineralization and sustainable development in the West African Craton: from field observations to modelling

Tahar Aïfa Université de Rennes, CNRS, Géosciences Rennes–UMR 6118, Bât. 15, Campus de Beaulieu, 35042 Rennes, France 0000-0002-1573-7668 [email protected]

Abstract: This Special Publication combines results obtained by interdisciplinary groups from numerous aca- demic institutions working on Paleoproterozoic formations to decipher the origins of the main mineralization resources in the West African Craton (WAC) and their impacts on African economic development. Structural, geophysical, sedimentological, stratigraphical, geochemical, petrophysical and mineralogical analyses have been used to highlight the complexities involved in mineralization emplacement and its origin and evolution within the WAC. Fourteen articles, mainly of basic research carried out in the WAC and surrounding areas, contribute to new knowledge in mineral research with updated references. They show that the geodynamic evo- lution of the WAC is complex from one area to another: it involves subduction, collision and obduction during several deformation phases ranging from Birimian (2.3–2.0 Ga) to Pan-African (650–450 Ma) events. Miner- alization is mainly controlled by tectonics within shear zones, orogenic belts, basins and faulting systems occur- ring in the various corridors. Mineralized fluid circulation is stressed and injected into appropriate formations and precipitate several types of well-documented ore deposits: porphyry, metal-bearing, volcanogenic massive sulfide, sedimentary exhalative and lateritic. Various modelling techniques, when integrated, help in under- standing the mechanisms of mineralization emplacement, some of which are still a matter of debate. Traditional and industrial exploitation of ore deposits, mainly gold, may inadvertently cause pollution to water tables and rivers, thus affecting the environment including watersheds. The challenge for further studies is mitigation for sustainable development that can be appropriately used to minimize such damage. The aim of this volume is thus to bring new insights to research activities on ore deposits within the WAC.

Several special publications have been devoted to the meetings were followed by a field trip to high- research on mineralization and ore deposits, some light outcrops that contain ore deposits. This volume of which focused on modelling and emplacement presents a series of articles dealing with ore deposit mechanisms of valuable minerals/ores within investigations in the WAC and surrounding areas. rocks. However, new specifications have made it This collection of 14 research papers arose as the possible to enforce limits on the impact of mining result of keynote presentations of guests and volun- on the environment and the health of civil popula- teers given mainly at the fourth meeting of the tions by the mining industry. Not only have they con- IGCP project 638 (2016–21, ‘Paleoproterozoic Biri- tributed through financial support to the economy of mian geology for sustainable development’). the developing countries but also in the mitigation A significant number of papers related to the and remediation of mines during and after their geology of the WAC have been published, as sum- exploitation and their impact on the environment, marized by Van Hinsbergen et al. (2011) in a previ- health, employment, etc. (e.g. Pokorny et al. 2019). ous special publication on the formations of Africa Many meetings resulted in proceedings or special (from 3.8 Ga to present). But papers focusing on publications on the geology and mineral deposits of WAC mining research are limited (e.g. Aïfa 2008, the WAC, such as the International Geoscience Pro- 2018; Saddiqi et al. 2018). gramme IGCP 485 (2003–07, ‘Cratons, metacratons In this special publication, the focus is on the and mobile belts: Keys from the West African craton WAC and its surrounding areas, dedicated mainly boundaries, Eburnian v. Pan-African signature, mag- to ore deposits from exploration using various tools matic, tectonic and metallogenic implications’), (geophysics, geochemistry, petrology, sedimentol- IGCP 502 (2004–08, ‘Global comparison of volca- ogy, etc.) to the implementation of modelling work nic-hosted massive sulfide districts: the controls on through the application of recent exploration tech- distribution and timing of VMS deposits’) and work- niques and ore identification (Fig. 1). shops including a meeting on the ‘Geology and met- An attempt to correlate the WAC with other ter- allogenesis of the Hoggar and Eglab massifs’ held in ranes/plates (Rio de La Plata, Baltica, etc.) since Tamanrasset (Algeria) on 1–3 March 2005. Most of the Archean regarding mineralization areas has also

From: Aïfa, T. (ed.) 2021. Mineralization and Sustainable Development in the West African Craton: From Field Observations to Modelling. Geological Society, London, Special Publications, 502,1–29. First published online May 14, 2021, https://doi.org/10.1144/SP502-2021-21 © 2021 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

2 T. Aïfa

Fig. 1. (a) Map of the West African Craton (WAC) and surrounding areas with locations of studies presented in this special volume shown by red boxes. The dashed line shows the approximate modern shield margin of the WAC. The ages of the main formations are given in alphabetical order. been made (e.g. Onstott et al. 1984; Franceschinis stratigraphy, metamorphic petrology, tectonics, et al. 2019). They indicate that most of the areas structural geology, geochemistry, geochronology, where mineralization occurs are linked or occupy metallogenesis, gravity and magnetism, seismic, geo- the boundary between terranes (large-scale shear graphical information systems (GIS), remote sensing, zones) (e.g. Diène et al. 2015; Masurel et al. 2017). etc (e.g. Chapman et al. 2002; Chorlton 2007; Mickus As noted in a simplified sketch (Fig. 2), at such 2008; Johnson et al. 2013; Yousefi and Nykänen boundaries, along with subducting plates or along 2017). This volume will be of interest to all those ridges, significant mineral deposits are often devel- involved in mining research or exploration concerned oped (Kious and Tilling 1996). Some studies have with the above investigative tools. Several special focused on the relationship with alteration/minerali- issues have been published in the past suggesting a zation within the WAC formations (e.g. Ben Aissi great interest in the timing of the formation of metal et al. 2005; Fon et al. 2012; Giorgis et al. 2014). ore deposits (e.g. Franklin 1996; Sangster 1996b; Ore deposit focused research activity is increasing Blundell et al. 2002; Ennih et al. 2004; Aïfa 2008, nowadays due to the recent demand for metallic ore in 2018; Ennih and Liégeois 2008; Belkasmi and different fields, mainly electronics, health, industry Ennih 2010; Driouch et al. 2010; Jessell and Liégeois and agriculture, and owing to rising prices of certain 2015; Jessell et al. 2016; Saddiqi et al. 2018). metals such as platinum group elements (PGEs) Ore deposits provide new evidence for magma- (e.g. Maier 2005; Augé et al. 2012). The use of mod- tism associated with cratons or metacratons and ern investigative tools in the field as well as in the lab- their boundaries where major fractures or shear oratory has greatly improved the understanding of the zones exist, through which fluid circulations and mechanisms of ore deposits and their interpretation, alterations may occur (e.g. Taylor et al. 2009; John- since the areas of research for such investigations son et al. 2013). Hydrothermal fluids can be the concern a field as wide as sedimentology, source of mineralization such as oxides, sulfides, Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

Introduction to volume SP502 3

Fig. 1. Continued. (b) Compiled map of ore deposits in and around the WAC. gold or PGE ore deposits (e.g. Milési et al. 2006; Jef- McQueen 2005), with their own emplacement mech- ferson and Delaney 2007; Tshibubudze and Hein anisms, are sub-divided into five main types: 2013). What controls the genesis of ore deposits in 1. Production of different types of deposits: por- ‘stable’ areas like the WAC and what are the geody- phyry, copper, sulfur, gold–silver by infills of namic and the metamorphic processes that lead to the veins, but also lead–zinc veins (Fig. 3a). Most of formation of ore deposits? Several issues have been Africa’s copper production comes from the Central addressed to help the reader understand the genesis African Copper Belt. However, the WAC copper of the ore deposits, their evolution and how they mines are located in its northwestern limit or in the may be extracted (see e.g. Cook and Marshall north Atlas range. Porphyritic copper deposits form 2004; McLaurin et al. 2005; Els and Eriksson beneath stratovolcanoes (Fig. 3a) where copper can 2006; Zaw et al. 2007; Chapman et al. 2009, 2017). be concentrated in hot, salty hydrothermal fluids in The papers presented in this volume include the molten igneous rock of the volcano. Copper set- results from large-scale field exploration and pre- tles as the molten rock cools and solidifies and/or is senting vast amounts of new data using techniques expelled with the hydrothermal fluids and deposit based on various processing methods. into adjacent rocks as the fluids cool and release Based on geodynamic models linked to the Afri- their metallic charge. An average porphyritic copper can cratonic lithosphere, one can distinguish areas deposit may contain 140 Mt of ore and consists of with different Archean lithosphere thicknesses, in 0.5% copper (Singer et al. 2002). particular within the WAC, revealed by seismic 2. Presence of metal-bearing: it requires mag- tomography (Celli et al. 2020). However, several matic intrusions such as the example of the Sampleu models of mineralization of ore deposits (e.g. mafic–ultramafic intrusion in western Côte d’Ivoire 4 Downloaded from http://sp.lyellcollection.org/ .Aïfa T. byguestonSeptember25,2021

Fig. 2. Sketch of plate tectonic framework in which several major types of mineral deposits are formed. The main types of ore deposits (1–5 as described in the text and Fig. 3) are shown. VMS, volcanogenic massive sulfide; MVT, Mississippi Valley type. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

Introduction to volume SP502 5

Fig. 3. The main models of mineralization through the cross-section. (a) Strato-volcano showing the relative locations of porphyry copper deposits, lead–zinc veins, gold–silver veins, and sulfur deposits. (b) Metal-bearing hydrothermalism showing extensional tectonics between plates beneath the ocean; the metallic hydrothermal fluids reacting with the seawater to form mineral deposits. (c) Volcanogenic massive sulfide (VMS) deposits. Hot hydrothermal fluids move along fractures in volcanic rocks towards the seabed and when these emerge and mix with cold ocean water, minerals of iron, copper, lead and zinc sulfide can form on the seabed. (d) Sediment-hosted (sedex) lead and zinc deposits. The fault is active during accumulation of sediments and mineral deposits and hydrothermal fluids move along the fault and fractures towards the seabed, minerals of iron, lead and zinc sulfide are deposited. (e) Soil profile in a laterite showing the progression of weathering effects on rock; aluminium and nickel are enriched by weathering.

(e.g. Gouedji et al. 2020)(Fig. 3b). Zinc and lead are floor or emplace at depth in shallow intrusions, pro- commonly found in ore deposits that form from viding the heat necessary to generate and mobilize hydrothermal fluids at low temperatures. Several hydrothermal solutions (Lydon 1988; Franklin 1993, types of deposits are recognized based on how the 1996; El Basbas et al. 2020). The oldest (Archean) hydrothermal fluids were generated and their move- VMS deposits have been reported to contain less ment, the distribution of minerals in the deposit, and lead than younger ones (e.g. Meso-Cenozoic). the geological context. Two deposit types formed on They can also contain recoverable amounts of cop- or near the ancient ocean floor where hydrothermal per, gold and silver. They are a common deposit fluids moved to the seabed along fractures and type but are currently not important for metal pro- vented to form hot springs (Fig. 3b). duction in the WAC (Belkabir et al. 2008; Marcoux 3. Volcanogenic massive sulfide (VMS) deposits: et al. 2008; Moreno et al. 2008; Outigua et al. 2020) classified as hydrothermal deposits, many of which (Fig. 3c). However, hydrothermal solutions mixed are associated with magmas generated at convergent with seawater and deposited amounts of iron, zinc plates boundaries such as porphyry copper (type 1) and lead on the mounds and layers of the seabed deposits. VMS deposits are associated with the erup- can thus form a VMS-type deposit (Fig. 3c). tion of volcanic rocks underwater. Indeed, deposits 4. Sedimentary exhalative (sedex) deposits: they form where the magma cools, either on the ocean concern the hydrothermal solutions, generated as Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

6 T. Aïfa water-rich sediments, buried deep in basins, then The sulfur-rich liquid, which is denser than the expelled when pressure is released during folding silicate-rich magma, will not mix again with the and faulting (Goodfellow et al. 1993; Lydon 1996; silicate-rich magma. Eventually, liquids rich in sul- Kolb and Petrov 2016)(Fig. 3d). Sedex lead and fur–iron nickel–copper separately accumulate and zinc deposits are a type of hydrothermal deposit solidify to form a variety of minerals that include found in sedimentary basins. These basins generally pentlandite (iron nickel sulfide), which is mined for form when the continents are rifted apart. Sedex its high nickel content (Naldrett 2004; Arndt et al. deposits are typically found in large basins filled 2005). Nickel laterite deposits are present throughout with more than 10 km of sediment. Silver, and less equatorial Africa, wherever tropical climatic condi- commonly copper, can be present in economical tions and nickel-rich rocks coexist. The most signifi- quantities in sedex deposits. Globally, sedex depos- cant nickel laterite deposits are located in Burundi its are younger than 1800 Ma. Examples of sedex and Côte d’Ivoire (Hammerbeck and Schurmann deposits in Africa relate to lead–zinc deposits that 1998). form from hydrothermal fluids associated with sedi- West African countries have experienced the mentary basins. However, in these deposits, hydro- highest levels of exploration activity for the past thermal fluids move through aquifers, mix with two decades, particularly in Mali, Ghana, Burkina other fluids, and deposit minerals in open cavities Faso and Côte d’Ivoire (Wilburn 2004). Unlike the within carbonate sedimentary rocks such as lime- other parts of Africa where lode/vein gold deposits stone. These ore bodies are referred to as Mississippi are associated mainly with rock units formed prior Valley type (MVT) (Sangster 1996a, b; Leach et al. to 2.5 Ga, the major source of gold in the WAC 2005; Haldar 2020). The movement of hydrothermal has been rock units formed between 2.5 and fluids in the sedimentary basins is driven by forces 1.6 Ga. These younger rock units and related struc- resulting from interactions at the plates boundaries. tures trend north to NE and underlie much of western These deposits form in relatively undeformed sedi- Ghana, Côte d’Ivoire and Burkina Faso, and extend mentary rocks that generally occur inland from the into the western part of Nigeria and southern Mali mountain belts formed at convergent plate margins (Fig. 1a). It is in these areas that many of the recent (Fig. 2). Known MVT deposits are younger than important gold discoveries have been made and that 500 Ma. Jurassic carbonate rocks of the High Atlas are most prospective for new discoveries. Among the Mountains in northeastern Morocco and in northern five ore deposit types – porphyry, metal-bearing, Algeria (Fig. 1b) host the largest known MVT depos- VMS, sedimentary exhalative and lateritic – many its in Africa (Bouabdellah et al. 1999). regions in the WAC have been subjected to deep 5. Soil profile and alteration: it is related to alter- weathering, and the potential for lateritic nickel ation soil which may have produced either iron deposits is being recognized, such as those already (Mauritania, Algeria, etc.) or bauxite (Burkina-Faso, known, for example in Côte d’Ivoire. The Bian- Côte d’Ivoire, etc.) (e.g. Ciampalini et al. 2013; kouma–Touba deposit in Côte d’Ivoire (Fig. 1b) Giorgis et al. 2014; Metelka et al. 2015, 2018). In has a resource of 169 Mt at 1.77% nickel and fact, bauxite, for example, is formed by weathering 0.08% cobalt, yet requires massive investment of aluminium-bearing rocks in high relief terrain since it is far from railway and harbour facilities. and good drainage in tropical climate (e.g. alternat- The primary nickel (sulfide) deposits in the WAC ing wet and dry seasons). A similar process can be are mostly associated within magnesium-rich intru- given for iron (Fig. 3e). Most nickel and cobalt sive rocks (ultramafic rocks). Whereas the diamond come from two general types of deposits: nickel lat- potential of the central part of the WAC, buried erite and igneous (magmatic) nickel–sulfide depos- beneath the sands of the Sahara, is still to be evalu- its. Nickel laterite (Fig. 3e) is formed by the ated. Recent exploration in Mauritania (Reguibat weathering of nickel-bearing rock in areas with a Shield) and Algeria (Yetti–Eglab Shield and Reg- tropical climate and good drainage (Freyssinet gane Basin) has found commercial-size diamonds et al. 2005). During this process, silica and other and G10 pyrope garnets (larger than 1 mm) and components are leached from the rocks, leaving a work continues to locate the kimberlite source soil enriched in nickel, cobalt and iron. The main rocks (Allek and Hamoudi 2008; Kahoui et al. nickel minerals are garnierite (hydrous nickel sili- 2008; Markwitz et al. 2016). cate) and nickeliferous limonite (hydrated nickel- bearing iron oxide). Igneous (magmatic) nickel–sul- fide deposits are associated with igneous rocks rich Minerals and mineralization in magnesium, nickel and chromium. These mafic and ultramafic rocks form as magmas cool and crys- The article of Ait Hmeid et al. (2020) concerns the tallize in layered igneous complexes. Then, a sepa- Ibourhardayn bentonite clay deposit of Beni Bou rate liquid, rich in sulfur, iron, nickel, copper and Ifrour within the Gourougou volcanic massif, in occasionally PGEs, can form within the magma. northeastern Morocco, in terms of mineralogical, Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

Introduction to volume SP502 7 chemical and geotechnical characteristics (e.g. (Leblanc 1981; Maacha et al. 1998; Maacha 2013). Al-Juboury 2018). This massif is mainly composed In this work, the authors try to demonstrate, after a of sedimentary, volcanic and granodioritic rocks review of general and local geology (Precambrian from the Mesozoic to the Neogene (El Rhazi and lithostratigraphy and geodynamics of the Anti-Atlas Hayashi 2002). The Ibourhardayn deposit results through Pan-African Orogeny) how mineralization from the weathering of volcanic ash and brecciated can be controlled by tectonics (faults, lithology) pyroclastic flows which have typical transport char- and constrained by mineral paragenesis and mineral acteristics (Ddani et al. 2005). It consists of two sig- chemistry. However, the lithostratigraphy of the Bou nificant bentonite layers: lower and upper layers Azzer inlier has been recently refined (Chèvremont resting on (1) Miocene marls covered by trachyande- et al. 2013; Blein et al. 2014). site tuffs, on top of which (2) a thin volcanic ash (cin- The 2500 m thick pile of ophiolitic sequence is eritic) facies occurs, respectively. They are hosted in composed of mafic and ultramafic rocks and a a sandy marl basement with biodetritic levels con- dyke complex associated with a volcanosedimentary taining algae and bryozoans. Fossil fauna and flora sequence, including pillow lavas and pyroclastic are typical of shallow marine waters for lagoon flows (Leblanc 1981). The ultramafic rocks under- lacustrine environments (Asebriy and Cherkaoui went serpentinization, including disseminated and 1995; Ddani et al. 2005). podiform chromites (Ikenne et al. 2005; Ahmed After a quick review of the bentonite deposits et al. 2009; Hodel et al. 2017). It occurred through over the world and detailed chemical description of (1) oceanic-like serpentinization at temperatures of the bentonites, the authors used some useful charac- 200–350°C of fresh serpentinites, followed by (2) a teristics for sustainability as good candidates for pol- hydrothermal event involving Cl-rich acidic fluids lutant adsorbents. Experimental analysis on the that significantly altered them. The calc-alkaline bentonite was performed to extract its physico- geochemistry of the mafic rocks and the chemistry chemical and mineralogical compositions and its of chromitic minerals are compatible with a supra- textural properties using X-ray diffraction, particle subduction arc setting (Bodinier et al. 1984; Ikenne size distribution, sedigraphy, bulk density, liquid et al. 2005; Ahmed et al. 2009). limit, plastic limit and shrinkage limit. A genetic model for the Bou Azzer deposits has The Ibourhardayn materials are of Ca-bentonite been proposed through the timing of mineralization type (Inglethorpe et al. 1993; Zeng et al. 2019). and the source(s) of metals for this odd mineralized The differences in the plasticity of the samples ana- system. It can be explained through two main argu- lysed are due to the variation in grain-size distribu- ments (1) occurrence of possible discrete events of tion and to mineralogical composition, that is polyphase mineralization/remobilization, and (2) samples with high clay content have a high plasticity, identification of the source(s) of metals and arsenic. whereas the silt fraction is characterized by relatively The following conclusions were thus drawn (Maacha low plasticity. They also show that this bentonite et al. 1998), complemented by stable isotope geo- represents a mixture of montmorillonite, anorthite, chemistry (Levresse 2001; Dolansky 2007; Maacha albite and orthoclase feldspar, with some calcite 2013): and impurities. Additionally, it has a very high water retention capacity and therefore a high swell- (1) a primary stage of Co–Ni arsenide–sulfarse- ing capacity, similar to some other bentonite out- nide mineralization, hosted within an east– crops (Ben Hamouda et al. 2017). west, ENE Pan-African faulting system (c. In their paper on the Bou Azzer inlier, Ikenne 615 Ma); et al. (2020) carried out a synthesis on the Co–Ni– (2) accompanied by extensional deformation at Cu deposits at Bou Azzer (Anti-Atlas, Morocco). the onset of basin opening in the early In this well-known mining area, mineralization, Ediacaran; associated with large bodies of serpentinite, results (3) reactivated during the Pan-African compres- from alteration of Neoproterozoic ophiolite sive tectonics starting from c. 615 Ma; sequences including peridotite protoliths through (4) a second mineralization stage (Maacha et al. Pan-African Orogeny (800–540 Ma). These ophio- 1998, 2015) during extensional tectonics at lites were subject to (1) a major tectonic lineament, the Ediacaran–Cambrian transition; the Anti-Atlas Major Fault (AAMF; Leblanc 1981; (5) extensional tectonics would have favoured El Boukhari et al. 1992; Saquaque et al. 1992) and upward movement of serpentinite domes (2) tectono-magmatic events of Pan-African, Hercy- through the reactivation of the mineralized nian and Alpine ages, at the origin of the polyphase faulting system; history of the Anti-Atlas (Choubert 1952; Leblanc (6) high salinity basinal brines with high chloride 1981; Hassenforder 1987). They induced progres- content (Leblanc and Lbouabi 1988; En-Naciri sive enrichment in Co and Ni mineralization with 1995; En-Naciri et al. 1995; Lebedev et al. the primary mineralization and its remobilization 1999; Essaraj et al. 2005; Dolansky 2007), Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

8 T. Aïfa

leaching and mobilizing of metals (e.g. Co, et al. 2008), but a feeder or stockwork sulfide zone Ni), possibly from serpentinites. The mixing has not been identified. Cu-rich mineralization is of such a fluid with a serpentinite-derived composed of numerous structurally controlled fluid may have led to As and S (Maacha veins whose morphology suits that of a lode rather 1994; Maacha et al. 1998; Maacha 2013). It than that of a stratiform orebody. In these Cu- and is also suggested that black shale of the Imiter Zn-rich orebodies pyrrhotite, however coarser within Group could be the source of all metals and As Cu-rich lenses, is deformed and recrystallized. The (Bouabdellah et al. 2016). authors showed that it is crosscut and replaced by (7) The sequence of mineral precipitation with pyrite infilling veins, where pyrite also crystallizes decreasing temperatures and salinities within along the S1 cleavage and in pressure shadows the advancing paragenetic sequence could around bioclasts in the disseminated ore. In terms also have been driven by the redox-dependent of hydrothemal fluids, lead isotopic signature indi- solubility of Co and Ni (Scholten et al. 2018). cates two types of fluids involved in the Draa Sfar Considering the existence of brannerite and the deposit: a less c. 18.30 (more c. 19.01) radiogenic negative Eu anomalies in carbonates associ- fluid (206Pb/204Pb) that formed the Zn-rich (Cu-rich) ated with Co–Ni arsenide–sulfarsenide ores, a lens, respectively (Marcoux et al. 2008). From initial reducing environment for mineral precipitation Zn-rich to subsequent more Cu-rich orebodies, has been proposed (Oberthür et al. 2009). hydrothermal fluid circulation may either have (8) The sulfur isotopic composition of sulfides changed to reflect a different source rock or (pyrite, chalcopyrite, sphalerite), precipitated increased in temperature to dissolve and carry cop- after arsenides and sulfarsenides, is centred per. In addition, the Zn-rich lens is enriched in silver, on c. 3‰ with outliers down to − 32.2‰ lead and arsenic, whereas in the Cu-rich lens, higher (Maacha et al. 1998; Levresse 2001; Dolansky concentrations of cobalt and gold can be observed. 2007). While most of the values could be com- The Cu-rich mineralized bodies can be inter- patible with the leached H2S derived from preted as being derived from syn- to post-tectonic magmatic sulfides, the outliers could be mobilization of dispersed metals in the host rocks, explained by H2S being supplied and derived probably interacting with metamorphic remobiliza- from the leaching of sulfides of biogenic ori- tion of pretectonic, massive and disseminated miner- gin, that is a source of sulfur prevailing in the alization. Finally, Sidi M’Barek ore deposits can be organic-rich shales recommended as metallic considered as remobilized, pre-tectonic Zn–Pb sys- sources (Bouabdellah et al. 2016). tems that have been partially overprinted and further remobilized by syntectonic Cu–Au systems. Outigua et al. (2020) did an interesting piece of The paper of Hallarou et al. (2020a) deals with research work on the Sidi M’Barek (Draa Sfar) mas- the Liptako (Niger) locality where the Kourki Paleo- sive sulfide deposits in central Jebilet (north of Mar- proterozoic porphyry Cu–Mo deposits are revisited. rakech, Morocco). It has been extensively studied A petrological analysis was performed on various using various techniques, from geophysical explora- facies namely (1) granitoids, (2) gabbros, clastic sed- tion to geochemistry and petrology (Moreno et al. iments, schists, (3) gneiss panels included within 2008; Jaffal et al. 2010; Rziki et al. 2012; Bouabdel- granitoids, and (4) an NW–SE-trending dolerite lah et al. 2016; Essaifi et al. 2019; Soulaimani et al. dyke crosscutting both previous facies. Based on 2020). Indeed, Cu-rich lenses, in which gold is being field observations and thin sections, the authors recovered profitably as a by-product, are the contin- show that epidote occurs as veins/veinlets within uation of the Zn-rich lens representing the northern mineralized zones whereas molybdenite is in the prolongation of the Draa Sfar VMS deposit. The pre- form of aggregates of elongated crystals, dissemi- vious studies on the Draa Sfar deposit were concen- nated randomly in the gangue. Molybdenite is also trated on the Zn-rich lens (Hibti 2001; Ben Aissi found in the form of nuggets in the veins of grey et al. 2005; Moreno et al. 2008; Rziki 2012). At quartz, but sometimes disseminated in granodiorites. Sidi M’Barek, this Zn-rich lens, NNE-trending, is Most of the Cu–Mo mineralization can be found composed of massive sulfides forming a 400 m within hydrothermal breccias, filled with quartz long and a thickness of c. 5 m, dismembered by veins and disseminated in the porphyry diorite and cross-faults into several smaller lenses. The Cu-rich the quartz–diorite stockworks. The mineralogical massive sulfides, 150 m long and 10 m thick, form assemblage is marked by the predominance of pyrite, several north–south to NNE subvertical small lenses. molybdenite, chalcopyrite and quartz. Networks They host massive pyrrhotite containing dissemina- resembling stockwork veinlets rich in quartz and car- tions of chalcopyrite or banded ore with alternation bonate are also abundant. Most of them are mineral- of chalcopyrite and pyrite- and/or pyrrhotite-rich ized, with chalcopyrite and a little molybdenite. bands. This deposit was classified as a VMS deposit Some gold values that were found have been discov- (Belkabir et al. 2008; Marcoux et al. 2008; Moreno ered in the granodiorites of Kourki. Mo–Cu + Au Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

Introduction to volume SP502 9 metal assemblage and the mineralization style The Central African Fold Belt (CAFB) resulted (hydrothermal, stockworks, breccias) are character- from the convergence of three main cratonic blocs: istic of an arc-type porphyry deposit. the São Francisco–Congo Craton, the WAC and The porphyry Cu–Mo deposits are characterized the Saharan Metacraton Bloc. It was strongly by the significant development of alteration zones affected by tectonics during the Pan-African Orog- that affect the entire mineralized zone. They also eny (600–500 Ma), leading to a complex metamor- affect the host of porphyry intrusions. Generally, phism with various grades of metamorphic rocks, four types of alteration are found in porphyry Cu– including orthogneisses, amphibolites, micaschists, Mo deposits: potassic, phyllitic, argillic and propy- mica-bearing quartzites, hornfels, quartzite veins litic (Wilkin and Bornhorst 1992). Field and micro- and mylonitic zones (Soba et al. 1991; Ngako scopic observations indicate: (1) sericitization, (2) et al. 2003, 2008; Toteu et al. 2006; Ganwa et al. epidotization and (3) propylitic types of hydrother- 2008, 2011; Njonfang et al. 2008; Kankeu et al. mal alterations. 2009, 2012). The northeastern part of the CAFB is A geochemical study using major and trace characterized by an important mineralization poten- elements including REE is also presented on Monzo- tial (Freyssinet et al. 1989; Ntep 1993, 1994; Ntep diorite quarzitic, granodiorite and microgranodiorite. et al. 2001; Suh et al. 2006; Toteu et al. 2006; Fon The main results show that Kourki rocks constitute a et al. 2012; Omang et al. 2014, 2015; Vishiti et al. calc-alkaline lineage, corresponding to granodiorites 2015, 2017; Ebot et al. 2016), resulting from hydro- and quartzitic monzodiorites. Rare earth element thermalism (Holler and Ghandi 1995; Li 2001). On (REE) analysis shows, for all the granitoids, a strong this part of the mobile belt, south of the Adamaoua homogeneity as in the case of the major elements, Plateau (central Cameroon), the authors chose with a Mo grade ranging between 1.03 and seven soil profiles that developed on four types of 302.94 ppm with the highest content within monzo- rocks in the Meïganga area. They present some min- diorite. Meanwhile the Zr grade, ranging between eralization indices to trace the pathway of minerals, 74.40 and 210.67 ppm, unusual in felsic rocks their chemical element mobilization and evolution (here monzodiorite), may reach 207.27 ppm, corrob- to further evaluate the occurrence of residual orating the occurrence of chromite. ore deposits. Finally, a genetic model was proposed for the A digital elevation model (DEM), covering an Kourki granitoids, probably derived from partial area of c. 12 245 km2, shows three morphological melting of a metasomatized garnet-bearing amphi- landscapes: upper, intermediate and lower, with alti- bole rock with a crustal component, emplaced at c. tudes between 1200 and 1500 m above sea-level 2.137 Ga (Léger et al. 1992). Based on the isotopic (asl), 900 and 1200 m asl, and 600 and 900 m asl, analysis (Hallarou et al. 2020b), the host rock pyrites respectively. of the Cu–Mo mineralization have been dated Based on the weathering patterns at Meïganga, between 2110 + 51 and 2158 + 51 Ma by the the mineral association reveals high contents (up to Re–Os isotopic system, consistent with the gran- 22.3%) and the recurrence of kaolinite as the main itoids age. This occurred during the D1 tectonic secondary mineral. The dominant rock weathering phase upstream of the Eburnean Orogeny, hence process – strong hydrolysis, during which part of creating a juvenile Paleoproterozoic crust. Such the silica, but almost all of the alkali and alkaline magmatic accretionary system related to the earth elements are leached, causing in-situ crystalli- emplacement of tholeiitic plumes in an ocean zation of kaolinite – induces shallow weathered basin, formed plateaus (Soumaila et al. 2008). The soil profiles (,4 m thick) (Nahon 1991; Shoji oceanic crust subducted beneath the continental et al. 1993; Dubroeucq et al. 1998). In order to assess crust, accompanied by calc-alkaline volcanism. the weathering trend and intensity at Meïganga, The island arcs induced from the intra-oceanic sub- valuable weathering parameters (chemical index of duction accreted on the Archean formations. The alteration, CIA; weathering index of Parker, WIP; Kourki porphyry Cu–Mo deposit which represents total reserve in bases, TRB; silica exportation ratio, the result of episodic geological events related to Ki), SiO2–Al2O3–Fe2O3 ternary and laterites classi- magma cooling, thus formed in an island arc-type fication diagrams were used by the authors. They subduction context. demonstrated that in the soil profiles, the TRB and Tchaptchet et al. (2020) worked on the morpho- WIP parameters globally decrease upward (TRB, logical, mineral and geochemical characterization of 416 to 11 mg kg−1; WIP, 56 to 4%), except in the soil soil profiles to check, through trends and degrees of profile on quartzite veins (TRB, 2 to 100 mg kg−1; intensity, the role of tropical weathering as a tool for WIP, 0 to 29%), which may be attributed to contam- mineralizations (lithogenic concentrations of useful ination by alkali and alkaline earth elements dur- elements and occurrence of residual ore deposits). ing lateral migrations since located in lowlands In fact, intensity of weathering depends strongly on (c. 930 m high). While the CIA (65–92%) and Ki climate and tectonic activity, among other factors. (2–9) values in almost all profiles indicate intense Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

10 T. Aïfa rock weathering, which can result from oscillating Ouattara et al. 2018a, b), Agbahou (Houssou 2013; periods of hot and humid tropical climate with Houssou et al. 2017), Dougbafla-Bandama (Ouattara .1500 mm of annual rainfall that affected the Ada- et al. 2017) and Hiré (Gnanzou 2006). The first maoua Plateau between Eocene and Holocene, Bonikro pit, located in the southern part of the maintaining the subsoil well-drained in permanent deposit where the shear zone can be seen (Ouattara humid conditions favourable to intense leaching. et al. 2018a), the higher-grade ores were found The authors concluded that kaolinization remains around NNE quartz veins. In the second and main the major soil-forming process in this area, regard- pit, the shear zone appears in the central part. The less of the rock. authors try to understand the influence of the Micaschists and quartzite veins are believed to Bonikro shear zone (BSZ) on the mineralization, host minerals such as graphite (C), gold (Au), zinc and hence on gold deposits, combining both field- (Zn), copper (Cu), lead (Pb), barium (Ba), tungsten work and laboratory analysis, focusing on (W), arsenic (As), uranium (U) and diamond (C) mineralized veins. (Ntep 1993; Vishiti et al. 2015, 2017; Ebot et al. Bonikro open-pit benches were used to investi- 2016). The granites also contain gold (Au), sapphire gate (micro)structural observations and core analy- (Al2O3), monazite ((Ce, La, Nd, Th)PO4), silver ses (polarizing microscope¸ scanning electron (Ag), molybdenum (Mo), lead (Pb), tin (Sn), manga- microscope) for comparison. Fluorescent minerals nese (Mn), arsenic (As) and uranium (U) (Ntep 1993, between scheelite-bearing and nonscheelite-bearing 1994). The exploitable U concentrations (1500– within the veins were separated through a Minera- 2000 ppm) in the soil profiles on quartzite veins light lamp UVG-47. and micaschists are above the exploitable limit The distribution of gold in the Bonikro deposit (1000 ppm) and thus can be compared to the residual occurs along the BSZ, its related structures and ore deposits of U at Meïganga (Omang et al. 2014, sheared rocks (e.g. faults, veins and schistosities). 2015). Major elements that can be considered as Gold is hosted exclusively in the north–south- indicators of mineralization are: Fe2O3 for Mo; trending BSZ corridor, dominated by quartz and Al2O3 for Nb, Mo, W, Nb and U; and TiO2 for Nb, pyrite, where contact is established between both pri- V and Cr. mary lithologies, that is the eastern volcanics and the Gold is found mostly in greenstone belts, espe- western volcaniclastics and sediments. S2 (pluton– cially in the interfaces of the Birimian volcanic and basalt contact) is a key indicator of mineralization, volcanoclastics (Burkina Faso, Côte d’Ivoire, recognizable by its abundant pyrite, and indicating Ghana, Mali, Senegal, etc.) (e.g. Davis et al. 1994; that the gold enrichment is post-magmatic and syn- Béziat et al. 2008; Ballo et al. 2016; Dabo et al. to post-tectonic. In fact, the intensity of shearing in 2016; Augustin and Gaboury 2017), but also in the the basalt controls ore grade increase, since calcite contacts between both these formations, affected is replaced successively by dolomite and ankerite by shear zones (Milési et al. 1989; Houssou 2013; in the areas where pyrite and pyrrhotite are abundant. Ouattara 2015; Ouattara et al. 2015; Houssou et al. Hence, higher-grade ore is found in intensely 2017; Ouattara et al. 2018a). The paper of Ouattara sheared zones. Sheeted, planar and transversal et al. (2020) concerns gold mineralization in the veins appear to control most of the gold deposits. Oumé–Fettèkro Greenstone Belt, one of the largest The sheeted veins, 0–45° N oriented, are the oldest Precambrian belts (300 km long and 40-50 km at Bonikro. They cut potassic feldspar megacrystals, wide) of Côte d’Ivoire. This greenstone belt, com- showing that they were definitely formed after the posed of NNE–SSW schist and quartzite, sandstone magmatic events. These veins are the main hosts and conglomerates, is affected by different injections for scheelite and most of the visible gold. The planar of meta-basites and acidites (Yacé 1982; Lemoine veins, 9–18°N oriented, are of secondary type, com- 1988; Mortimer 1990; Daouda 1998; Houssou prising quartz associated with powellite and frac- 2013, Ouattara 2015; Ouattara et al. 2015; Houssou tured pyrite. Finally, transversal veins, 120–150°N et al. 2017). oriented, are the youngest vein type. They consist The authors concentrated on the Bonikro deposit of larger quartz veins cutting the two previously min- to try to establish a relationship with the shear zones. eralized types of veins and are composed of milky This is a particular area where sheared granodiorite quartz, calcite, albite, biotite and sulfides (molybde- hosts gold within areas of faulting and ‘boudinaged’. nite up to 5%). The molybdenite is sheared and The Fettèkro is one of the numerous greenstone belts unevenly distributed around broken pyrites in the NE–ENE trending along with their shear zones, con- zones where the transversal veins cut the planar or sidered as the most productive gold belt in Côte sheeted veins. d’Ivoire. In its southern part, namely the Oumé–Fet- In northern Bonikro, mineralization is correlated tèkro Greenstone Belt, extensive exploration with the three domains of the BSZ: the first controls resulted in the discovery of four gold deposits: the contact between sediments and granodiorite, Bonikro (15.9 kt at 1.8 g t−1)(Ouattara 2015; north–west oriented; the second, north–south Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

Introduction to volume SP502 11 oriented, within the granodiorite; and the third, this zone, some of the dykes intruding the Early Pro- north–east trending, represents the interface between terozoic and the Archean basement were probably pluton and basalt. These changes in orientation of the emplaced during an extensional event predating the granodiorite and the contacts between intrusives and opening of the Pharusian Ocean (Condie 1992; primary lithologies have influenced the fluid circula- Lefort et al. 1994; dos Santos et al. 2008). Several tion pathways and thus gold deposits. Strain slips are magmatic episodes are evidenced (Sabaté and also present in the northern part of the pit; they Lameyre 1973; Kahoui et al. 1996; Drareni et al. formed from the stress that resulted from the intru- 1995; Kahoui and Mahdjoub 2004; Peucat et al. sive setting and the effects of fractures. 2005; Merabet et al. 2014, 2016, 2020). The structural history of the Bonikro gold deposit The dyke package, which was feeding the post- can be summarized as follows: (1) regional folding tectonic Eglab volcanics, gives a preliminary age of event that set up the north–east-trending structures; 1948 + 9 Ma on baddeleyite using the 207Pb/ (2) regional shear zone, i.e. the BSZ, redirected the 206Pb dating. This result broadly fits with the previous primary faults, and (3) occurrence of secondary Rb/Sr dating of the post-tectonic volcanics at 1.99 + structures with dextral thrust veins that reactivated 0.03 Ga (Sabaté 1978, 1979). Dating carried out on shear faults and fractures, and controlled mineraliz- zircons of host rocks (quartz–dioritic gneisses) were ing fluid flows. also made and fits with previous results (Mahdjoub et al. 1994; ages in the range of 2.21–2.18 Ga; Peucat et al. 2005). Taking account of dating limitations at Geodynamics and emplacement older ages, an error of c. 5–10 Ma will not bring mechanisms major distortions in the APWP path. A rough com- parison of the current palaeomagnetic data with The research work of Aïfa and Merabet (2020) was those previously determined (e.g. Sabaté and undertaken in the Yetti–Eglab terranes on north– Lomax 1975; Aïfa et al. 1993) is necessary. Because south-oriented Proterozoic intrusions to try (1) to no contact test was generally available for these sam- compute new palaeopoles and build an apparent pled dykes, a detailed rock magnetic study was per- polar wander path (APWP) for the WAC and (2) to formed for the identification and stability of the better constrain the WAC’s geodynamic evolution. magnetic carriers (e.g. Aïfa 1993). The main mag- In fact, a palaeolatitude v. age diagram in the light netic mineral is multidomain stable magnetite and/ of other rare palaeopoles available in close by ter- or titanomagnetite, however hematite, pyrrhotite ranes, blocks or (micro)plates, together with previ- and goethite may also occur. ous geological and radiochronological studies, In terms of palaeomagnetic directions, several suggest that this craton could have participated suc- palaeopoles were computed, considering palaeolati- cessively in the assembly of the Columbia and Rodi- tudes v. ages, to build suitable interpretations in nia supercontinents (Aïfa et al. 2001; Lefort and Aïfa terms of geodynamics: 2001, 2002; Lefort et al. 2004; Kouyaté et al. 2013). The Reguibat uplift is an area of 1500 × (1) The Yetti–Eglab zones, the deeper basement of 400 km2, covered, in its northern and southern which is made from Archean Chegga gneisses parts, by the Paleozoic Tindouf and Taoudeni basins. (Drareni et al. 1995), were probably separated It is composed of the Yetti and the Eglab zones, by an ocean because they follow different almost undisturbed since 2.1 Ga, and not affected paths between 2.1 and 2.08 Ga. However, a by the Pan-African Orogeny (600 Ma) that borders c. 2.73 Ga oceanic relic, composed of garnet– it. In the Eglab area eastward, subtle extensional tec- hornblende banded grey gneisses, is inter- tonics can be recognized, highlighted by a transgres- preted as Archean oceanic crust in the Eglab sive stromatolite-bearing sedimentary cover: the massif (Peucat et al. 2005). It could be associ- Hank Formation (c. 875–890 Ma, Clauer and Bon- ated with rifting, crustal breakup and oceaniza- homme 1971; Bertrand-Sarfati 1972; Bertrand- tion of the 3.0–3.6 Ga old craton during the Sarfati and Moussine-Pouchkine 1992; Aïfa et al. Liberian events (Peucat et al. 2005; Taylor 1993; Tewari and Seckbach 2011). This formation et al. 2019). deposited before the opening of the Pharusian (2) A possible existence of oceanic crust sup- Ocean (Caby and Andreopoulos-Renaud 1989; ported by a calc-alkaline volcanic arc at c. Helmstaedt and Scott 1992 : Condie 1992, 2003; 2.03 Ga (Sabaté 1978). Given the K2O/SiO2 Condie et al. 2000)atc. 800 Ma. Some east–west- geochemical gradient, subduction of this tilted blocks supported by listric normal faults can crust was dipping eastwards (Lefort and Aïfa be observed in the field, separated by large north– 2002; Peucat et al. 2005). Some magnetiza- south-oriented doleritic dyke swarms and represent tions possibly predate the collision between the westernmost remnants of the Pharusian continen- the two terranes that generated the Aftout gran- tal margin and foreland. In the easternmost part of ites (1.92 Ga) (Peucat et al. 2005), slightly Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

12 T. Aïfa

predating the amalgamation of cratons to form 563 + 8/–3 Ma, respectively. They are high-K the Columbia Supercontinent (Rogers and calc-alkaline and weakly hyperaluminous with Santosh 2002a, b, 2003, 2004). Such crustal ferroan affinity (Kwékam et al. 2015). According thickening could be indicative of a diamondif- to similar magnetite granites within both Dschang erous area (Kahoui et al. 2008, 2012). This and Nkambe regions, they could belong to a single accretion may have developed whilst the Paleoproterozoic domain, rich in iron mineralization. WAC was drifting close to the northern low In fact, BG and MG Nd isotopes display ɛNd from − latitudes. A very large volume of granites 16 to − 18 and from − 15 to − 17 with Nd TDM (the Aftout granites) witness the existence of indicating Paleoproterozoic ages in the ranges an oceanic closure at this time. The main 2112–1923 Ma and 1875–1713 Ma, respectively. crustal break (the Yetti-Eglab ‘suture zone’) They are of a potential metallogeny source of Cu, known in the field between these two domains Pb, Zn and Mo metals, typically post-collisional, (Sabaté 1978) probably represents a microplate with a subduction signature probably being inherited boundary. from their protoliths. The authors also show that the (3) The palaeopoles, dated around 1.8 Ga (Rio de inherited zircon U–Pb age of these granites are at the la Plata Craton (RPC), Halls et al. 2001) sug- origin of remobilization of an old Paleoproterozoic gest, because there is no Precambrian orogeny crust similar to the BIF of the Nyong series of the between the WAC and the RPC, no collision, Congo Craton. The geochemical results indicate a although they were located at approximately crustal growth during the late Pan-African Orogeny. the same palaeolatitude. They correspond Dschang granites are typically derived from partial either to the collapse of the Birimian and con- melting of older rock in the crust (Roberts and Clem- temporaneous orogens or to the very beginning ens 1993; Barbarin 1999; Frost et al. 2001). In terms of the fragmentation of Columbia. of petrogenesis, the Dschang MG granites are char- (4) The palaeopole dated at 1.5 Ga (Sabaté and acteristics of upper crustal granites (Kemp and Haw- Lomax 1975) obtained on the younger dyke kesworth 2003). generation may correspond with a period of The age of Dschang granites (578 + 6/–11 and extension, probably contemporaneous with 563 + 8/–3 Ma) is consistent with their emplace- the collapse of the Birimian Orogen. It may ment after the end of arc-related magmatism and correspond to the beginning or the continua- subsequent collision between the Congo–São Fran- tion of the fragmentation of Columbia. cisco Craton and the Sahara Metacraton during the (5) The palaeopole obtained from the transgres- lateral wrench movements along the N70°E Central sive Hank Formation shows that the WAC Cameroon sinistral shear zone (CCSZ) (Ngako was, at c. 900 Ma, located at low and warm lat- et al. 2008). Such emplacement age coincides with itudes (31°) since the stromatolites were able to the metamorphism and D4 deformation event along develop on a continental margin. Compared the CCSZ (Bouyo Houketchang et al. 2009; with the data compiled by Onstott and Dorbor Tchaptchet Tchato et al. 2009; Kwékam et al. (1987), nearly the same path for the WAC from 2013; Tcheumenak Kouémo et al. 2014). These 2.2 to 1.9 Ga is established. results revealed, through injection of mantle melts within the upper continental crust, a remobilization The similarities in the pole position for Baltica and of the older crust, here Paleoproterozoic in age. for the WAC at 0.95–0.89 Ga (Aïfa et al. 1993), It would have triggered the dispersion of ante- 1.5 Ga (Pesonen et al. 1991) and 1.79 Ga suggest Panafrican mineralizations. that the WAC and Baltica have been drifting at sim- Amadou et al. (2020) studied the Proterozoic ilar latitudes for a very long period of time. However, deposits of the Firgoun region within the Gourma they have probably never been attached to one and Taoudeni basins (west Niger) through field another before their amalgamation to the Rodinia observations: sedimentological and structural data Supercontinent at c. 1 Ga, since the Grenvillian analysis. The Gourma Basin, considered as a sedi- Orogen that affected Baltica is absent in western mentary prism with terrigenous and carbonate infills, Africa. The lack of deformation in the c. 1Ga is marked by a high subsidence rate with a thick sedi- Hank cover of the Reguibat uplift suggests that the mentary pile (.8000 m) on top of a major unconfor- WAC never collided completely with Rodinia mity on Paleoproterozoic terrains, southeastern edge (Lefort et al. 2004). of the WAC. The Gourma Basin recorded two stages The manuscript of Kwékam et al. (2020) deals of evolution with an increase in the Pan-African with I-type granites of Dschang (SW Cameroon). deformation rate eastwards: (1) an aulacogenous Using laser ablation U–Th–Pb geochronology, stage (Moussine-Pouchkine and Bertrand-Sarfati whole rock geochemical, Rb–Sr and Sm–Nd isotopic 1978), linked to the opening of the Neoproterozoic analyses, these biotite (BG) and magnetite (MG) Ocean (870–800 Ma) (Black et al. 1979); (2) a granites were dated at c. 578 + 7/–11 Ma and c. compressive tectonic event, associated with the Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

Introduction to volume SP502 13 development of thrust nappes (folds and tangential structures exhibiting syn-sedimentary normal structures) (Caby 1989). faults, with a stretching direction trending The Firgoun area deposits were dated from Meso- 140° N, possibly linked to the Neoproterozoic to Neo-Proterozoic (Miningou et al. 2010, 2017; Ocean opening northeastwards; Konaté et al. 2018). They include two lithostrati- (2) a first NW–SE compressive episode D2a, graphic units: (1) the basal Firgoun Sandstone For- marked by isopachous folds in the basal depos- mation, unconformably overlying the Birimian its, has affected most of the Firgoun area basement and (2) the uppermost Béli-Garous Forma- deposits. According to their constant layer tion corresponding to a sedimentary sequence com- thickness, the isopachous folds are post- prising glacial and non-glacial lithofacies. The depositional deformation structures. In the authors described accurately these outcrops in the uppermost deposits, the folding related to the field and through microscopic analysis. They show D2a episode was marked by anisopachous that Firgoun Sandstone Formation presents symmet- open folds, exhibiting layer thickness varia- ric ripples associated with oblique bedding, asym- tions, thus syn-depositional. metric current ripples, flat-crested sinuous or (3) A second NE–SW compressive episode D2b, undulatory ripples, herring bone structures and also marked by brittle structures, such as HCS, suggesting a shallow marine environment thrusts and reverse faults, crosscutting all the with fluvial episodes. While Béli-Garous Formation previous structures. The intragranular fractures consists of (1) several sedimentary structures (sym- in cataclastically deformed porous sandstones metric ripples, herring bone structures and hum- are linked to the compressive deformation epi- mocky cross-stratification) characteristic of shallow sode D2b. marine context, (2) diamictite interbeds–carbon- ates–silexites and other glacial features (stepped The single U–Pb geochronological age recorded in fractures and cryoturbations) suggesting glacial to the Firgoun lowermost sediments is c. 1800 Ma marine environment. According to their characteris- (Konaté et al. 2018). The uppermost siliciclastic sed- tics, the lowermost sedimentation of Firgoun Sand- iments of the Firgoun region could be Late Cryoge- stone Formation consists of fluvial to marine nian (630–610 Ma), since they exhibit some environments that evolved vertically from marine lithological similarities to the Late Cryogenian sedi- to glacial environments. ments of the Taoudeni, Gourma and Volta Basins. The Firgoun area deposits also show brittle and Finally, the authors agree that the compressive defor- ductile deformations in the field, mainly fractures, mation phases recorded in the Firgoun area are Pan- and folds and schistosity, respectively. The brittle African in age. Such tectonic events, particularly deformation has affected all the basal deposits induc- those of the Pan-African Orogeny, may have had a ing normal faulting acting during sedimentation direct influence on the distribution and uprising of (sandstones displaying secondary fractures, oriented the continental blocks and consequently on the ori- 40–60° N, with quartz infills) and post-depositional gin of the major Neoproterozoic climatic changes. thrusts, oriented 60–80° N, associated with westerly For the Gourma aulacogen, a possible relationship verging nappes. The ductile deformation exhibits between Neoproterozoic sedimentation and the isopachous folds (concentric folds, with axial planes, breakup of the Rodinia Supercontinent has also oriented 30–50° N and dipping 40–50° NE), some- been proposed (Michard et al. 2008). times accompanied by microfractures of variable The Neoproterozoic to Cambrian deposits within direction (55° N, 140–150° N). The Béli-Garous the Taoudeni Basin and its surroundings, including Formation, overlying the Firgoun Sandstone Forma- glacial deposits of West Africa, could be partly con- tion, records brittle deformation, characterized by trolled by the Pan-African events, consistent with the intragranular fractures (infilled with manganese or sedimentary record of the Pan-African events in the ferromanganese oxides) in cataclastically deformed Firgoun region. porous sandstone. However, in lithofacies Fr6, two The paper of Baraou and Konaté (2020) deals categories of deformation structures have been with new radiometric dating from south Maradi for- observed. These are anisopachous folds for which mations, southern Niger. They formed during the the axial plane mean orientations are 30–50° N, Pan-African Orogeny (between 750 and 450 Ma, 45–40° NE with plunging axes 10–20° NE. On a Ajibade and Wright 1989) as a result of the regional scale, they form a network of elongated par- convergence-collision of the WAC, the Congo Cra- allel hills, oriented 45° N. ton, the Saharan Metacraton and the São Francisco In terms of deformation phases, three phases Craton to amalgamate in West Gondwana (Black were recorded: and Liégeois 1993; Abdelsalam et al. 2002; Caby 2003; Liégeois et al. 2003; PRDSM 2005). How- (1) an extensional deformation stage D1, marked ever, the age spread of the Pan-African Orogeny dur- in the basal deposits by brittle deformation ing the convergence of East and West Gondwana is a Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

14 T. Aïfa matter of debate. Some authors have suggested ages in the Nigerian Pan-African Province (Dada et al. ranging from 750 to 450 Ma (Ogezi 1977; Van Bree- 1993; Ferré et al. 2002), and in northern Benin men et al. 1977; Ajibade and Wright 1989), while (Adissin Glodji 2012). others have argued that the Trans-Saharan Belt was Zircons dating from the schist belt rocks of Mar- formed between 750 and 500 Ma (Caby 1989; Cas- aka by U–Pb method gave an age of 617.9 + 2.8 Ma taing et al. 1993; Stern 1994, Villeneuve and Cornée (MSWD = 2.3). U–Pb zircons can be interpreted as 1994; Trompette 1997; Abdelsalam et al. 2002), a detrital age for the volcanic material in the sedi- whereas McCurry (1976), based on K–Ar dating of ments. In addition, this volcanic material was inter- mica in metasediments and volcanics, proposed preted as the schist protolith, and is from arc-type ages varying from 600 to 450 Ma. The southern Mar- volcanism. Radiometric dating of feldspars of the adi Province forms an alternatively juxtaposed Rourouka porphyroid granite by the K–Ar method, domain (synclinal form) of four bands with a NE– on whole rock, gave an age of 508 + 10 Ma, SW trend of schist belts: the Maraka Schist Belt which is similar to the metamorphic age of the Mar- (MSB); the Eastern Goumata Schist Belt (GSB); aka schist (505 + 15 Ma) using the K–Ar method the Garin Liman Schist Belt (GLSB); and the Maid- on whole rock. As this porphyritic granite is unde- aparo Schist Belt (MpSB). They are composed of formed, the obtained age could correspond to cool- gneiss, mylonitic granitic gneiss, quartzites and post- ing of the granite intrusion during the Early tectonic granites including pegmatites dykes Paleozoic (Cambrian). (Mignon 1970; PRDSM 2005; Baraou et al. 2018). Garin Wali and Fiawa mylonitic gneisses are As no zircon was found in the collected samples affected by north–south to 20° N mylonitic foliation of mylonitic gneiss and porphyroid granites, the K– with a sinistral S–C fabric. Radiometric dating of Ar method was used. Scanning electron microscope feldspars from these gneisses gave K–Ar ages of (SEM) images of schist and cathodoluminescence 491 + 10 Ma and 475 + 10 Ma, respectively. They (CL) images of porphyritic gneiss exhibit zircon correspond to an Early Paleozoic (Ordovician) reacti- grains for laser ablation inductively coupled plasma vation and are compatible with previous ages on the mass spectrometry (LA-ICP-MS). They were used to porphyroid granite of Rourouka ((510 + 20 Ma). determine the age of magmatism and/or metamor- Finally, the south Maradi Pan-African formation phism of different petrographical facies, through ages range from 638.3 + 3.0 Ma (Neoproterozoic) K–Ar and U–Pb methods on zircon. to 475 + 10 Ma (Ordovician), revealing the polycy- Dan Issa migmatite gneiss is characterized by a clic character of the Pan-African events. The U–Pb wide variety of petrographical facies, including method yielded older ages from migmatite gneiss palaeosomes of porphyritic gneiss and granitic leu- (638.3 + 3.0 Ma) and schist (617.9 + 2.8 Ma), cosome. Microscopic analysis of the palaeosomal while the younger ages are obtained by the K–Ar component shows mineral assemblages comprising method from mylonitic gneiss (491 + 10 and 475 quartz, orthoclase, microcline, sodium plagioclase + 10 Ma) and granite (508 + 10 Ma). In migmatite and biotite. The CL image from the palaeosome of gneiss, the U–Pb ages (638.3 + 3.0 and 589.6 + migmatite gneiss shows zircon grains. The core of 1.9 Ma) obtained on zircons do not allow attributing some zircons has strong luminescence (inherited a Paleoproterozoic age to their protolith. Such a nuclei) and some have low luminescence (altered lack of Paleoproterozoic rocks could be linked to nuclei). The Th/U ratio varies from low (0.191– significant remobilization during the Neoproterozoic 0.394) to high (0.493–0.808) values, revealing crys- and Early Paleozoic in the West Gondwana tallized (inherited) and recrystallized (altered) zir- amalgamation. cons, respectively. According to Williams and In the paper of Gouedji et al. (2020), the mafic– Claesson (1987) the Th/U ratios of metamorphic ultramafic Samapleu intrusion, west of Côte d’Ivoire, (magmatic) zircons are often low (high). The concor- is investigated to characterize its petrology, geo- dia diagram for all zircons analysed indicates two chemistry and metamorphism. Base metal deposits weighted averages for the mean U–Pb ages: one (Ni, Cu and Co), PGEs and chromite are associated with an average of 638.3 + 3.0 Ma (mean square with this intrusion. In fact, it is a feeder dyke of the weighted deviation (MSWD) = 5.4, high average) Yacouba layered complex composed of three assem- which could represent zircon crystallization (igneous blages (Ouattara 1998; Gouedji et al. 2014; Hamilton crystallization) and the other with an average of et al. 2014). The Yacouba layered complex forms 589.6 + 1.9 Ma (MSWD = 0.39, low average), dismembered and stretched assemblages that extend could correspond to the age of the zircon overgrowth for more than 25 km in a corridor from Yepleu- (remobilized system) during a Pan-African Bounta to Samapleu-Yorodougou villages. It con- high-grade metamorphism. sists of layered assemblages composed of mafic– Similar Ar–Ar ages of 565 + 5 Ma on horn- ultramafic sequences with petrographic markers of blende, and U–Pb ages of 589 + 11 and 605 + high/ultrahigh-temperature metamorphism and coe- 10 Ma on zircon were obtained on this type of gneiss val deformation (Gouedji et al. 2014, 2018), and Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

Introduction to volume SP502 15 locally magnetite-rich anorthosite units. Its geody- and associated intrusive rocks, located in the eastern namic context (2.09 Ga) will be better understood in margin of the Banfora Belt (BB), SW Burkina Faso, the frame of its geochemical and metamorphic charac- west of the Birimian Houndé and Boromo belts teristics, and the relationship with the granulite gneiss within the Léo-Man Shield (WAC). Its size is host (pressure–temperature (P–T ) conditions) during 160 km long and 35 km wide, trending NNE– emplacement and recrystallization within the Man SSW, continues in Côte d’Ivoire, where it is called Archean domain at 2.8 Ga. A more than 40 m thick the Katiola–Marabadiassa Greenstone Belt and Ban- layer of laterite alteration covers the Samapleu intru- dama Volcanosedimentary Belt (Ouattara 1998). sion, so drilling crosscuts the ultramafic, maficand The geological environment is dominated by the hybrid (remains of the granulite host) (|Gouedji sedimentary and volcanosedimentary rocks that et al. 2014, 2018) units. Combined with the mineral- have undergone greenschist facies metamorphism ogy and geochemistry, the P–T–time path of metamor- (Baratoux et al. 2011; Metelka et al. 2011; Ilboudo phism through pseudosections of diagnostic et al. 2017). Several magmatic episodes with assemblages of these units has been investigated. ma fic, intermediate and felsic volcanic rocks crop- The authors describe in detail the ultramafic, ping out on the eastern side of the belt, along a mafic and hybrid units which are composed of (1) north–south trend, could make the BB a target for peridotite, chromitite, spinel websterite, olivine and mineral exploration. They are intruded by gabbro, plagioclase, (2) gabbro–norite, norite and anortho- diorite and granite bodies. The Banfora, Houndé site, and (3) grey lithofacies with a mineralogical and Boromo belts exhibit similar volcanic units, in composition varying from noritic to anorthositic particular the eastern margin of the BB and the west- pole, respectively. The samples from the hybrid lith- ern margin of the Houndé Belt (Baratoux et al. 2011; ofacies, with very low TiO2,Na2O, MnO and K2O Metelka et al. 2011). As there are few geochemical contents, were used to quantify the P–T conditions studies, the authors targeted magmatic rocks from of the Samapleu intrusion emplacement. Their both mafic and felsic composition. They therefore wide variability in the geochemistry of major oxides focused on: (1) the geochemical nature of the mag- reflects the possible mingling of mafic–ultramafic matic environment (self-consistency of data, mag- intrusion and host rock. The phase relation model- matic series, geotectonic settings) supported by ling suggests that the hybrid lithofacies at the contact petrographic descriptions of the different facies; (2) of the Samapleu intrusion preserves evidence of a geodynamic model, based on the correlation with ultrahigh-temperature (≥1000°C) and pressure (c. the other belts; and (3) assessment of the BB’s poten- 9 kbar) conditions. It is followed by near isobaric tial in terms of mineral resources. cooling down to 750–800°C at 7–8 kbar, suggesting Studies based on radiometric and electromagnetic that these metamorphic conditions and the develop- data supported the hypothesis that volcanic rocks ment of the hydrid lithofacies are coeval with the (basalt, andesite, rhyolite) (,10%) estimated to be Samapleu intrusion emplacement. In addition, the 2–4 km thick are intercalated with volcanosediments high MgO (10–13%) levels recorded indicate that in the eastern part while the remaining part is com- Samapleu magmas were relatively hot (Precambrian posed only of volcanosediments (c. 90%) (Baratoux thermal gradient of c. 50°C/km; Chardon 1997), et al. 2011; Metelka et al. 2011). They are cross-cut implying an emplacement depth of the intrusion of by a syn-kinematic elongated granite emplaced 22 km (lower crust) with a crystallization tempera- throughout a major shear fault, called the Greenville ture of 1100–1200°C. An Eburnean age (2.09 Ga, Fault in the central part. The eastern and western sides U–Pb age obtained on rutile) was obtained on hybrid are separated by syn-kinematic two-mica granite and lithofacies, which would be the age of metamor- granodiorite bodies along a NNE–SSW trend, coin- phism that produced these formations (Gouedji ciding with the Greenville–Ferkés-Sédougou–Bobo et al. 2014). Dioulasso Shear Zone (GFBSZ), a first-order crustal Chondrite-normalized REE patterns of the Sama- scale shear zone (Lemoine et al. 1990; Baratoux et al. pleu rocks are generally flat (REE concentrations 2011). The GFBSZ extends from Liberia southwards, between one and ten times the chondrite values), passing through Côte d’Ivoire, and to the BB (Bur- slightly enriched in light REEs and relatively cons- kina Faso) northwards. tant in heavy REEs. Finally, the signatures of trace The granitoids intruding the belt in the Katiola– elements and REEs show that they are affected by Marabadiassa area, gave radiometric ages ranging crustal contamination with a small assimilation between 2123 + 3 and 2097 + 3 Ma (zircon Pb– proportion of the granulitic continental crust during Pb; Doumbia et al. 1998). Detrital zircons from the its emplacement. The magma could have been sedimentary rocks of the Bandama Basin were emplaced by underplating and feeding from a mantle dated (Pb–Pb) at c. 2133 and 2107 + 7 Ma. Based plume at the base of the continental crust. on the zircon Pb–Pb ages of pre- and post-basin gran- The paper of Ilboudo et al. (2020) aims to con- itoid intrusions, their emplacement age was esti- tribute to the understanding of the Banfora volcanic mated at between 2108 + 12 and 2097 + 3 Ma. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

16 T. Aïfa

In addition to these syn-kinematic granites, the BB a target for gold and base metals. Generally, border is cross-cut by granodioritic, dioritic (+ quartz- granites are of metallogenic interest because they bearing) and tonalite plutons that are responsible constitute a better receptacle for gold-bearing hydro- for local contact metamorphism. thermal fluid. The authors suggest that the main tar- The authors also performed rock classifications gets, located on the borders of the BB, may contain using thin sections on the sampled outcropping orogenic gold; massive sulfide and porphyry-type rocks (granitoids, mafic and felsic volcanics and deposits as well as VMS (exhalite on top of mafic- associated intrusives). They were subjected to induc- felsic volcanic activity) are documented in the Biri- tive coupled plasma-atomic emission spectrometry. mian system of Burkina Faso (Franklin 1996; It was found that they are mainly composed of basalt Gaboury and Pearson 2008; Hein 2016). Further, and andesite with associated mafic and felsic intru- the BB should not be excluded from exploration sives, altered or under greenschist or amphibolite companies (Ilboudo et al. 2017) for alluvial diamond facies metamorphism, suggesting an igneous proto- (Castaing et al. 2003). lith (I-type). The two-mica granite, accommodated by the GFBSZ, is sourced from sedimentary proto- lith (S-type). Bulk chemical (major, traces and Geological, geochemical and geophysical REEs) composition of the volcanic and intrusive modelling rocks confirmed the petrologic results. These mafic volcanic and plutonic rocks are calc-alkaline, tholei- In the paper of Aïfa and Lo (2020), investigations of itic or komatiitic. Precambrian subsurface structures were addressed The main results were interpreted to indicate the through aeromagnetic data on the gold-bearing nearby Houndé Belt, that shows a genetic link with Tasiast area (NW Mauritania). In fact, the inferred the Banfora, is an ancient oceanic floor at least structures, composed of dykes and shears, are ori- 8 km thick made up of tholeiitic basalts (Baratoux ented mainly NNE–SSW, NW–SE and c. 120° N et al. 2011; Augustin and Gaboury 2017, 2019). at subsurface (Ba et al. 2020). The gold deposits This floor formed above a deep mantle plume that are hosted along a north–south-oriented dextral pre-enriched basalts in gold (Weber et al. 2012). shear zone with an area of 70 × 15 km2. Mineraliza- Its evolution led to the formation of an island arc dur- tion follows a series of reactivated thrust faults at the ing the compressive D1 deformation phase, with a northern end of the Chami Greenstone Belt, includ- hypothetical subduction zone eastwards. During ing Tasiast and Piment fracture zones. They host a D1, calc-alkaline magmatism developed (andesite) large proportion of the gold-bearing deposit and with tonalite–trondhjemite–granodiorite (TTG) are composed of banded iron formations (BIFs), emplacement and sedimentary basins were formed felsic volcanic rocks and associated clastic and in possible syn-deformational settings during the mafic rocks. Tasiast–Tijirit Terrane comprises three late Eburnean, for example the Tarkwaian (Ghana). lithostratigraphic units: (1) TTG-type granitic gneiss Gold mineralization appeared later during transpres- rocks assembled into the Ctel Ogmâne Complex; sional phases (c. 2150 Ma). The BB, like the Bor- (2) greenstone belt lithologies assembled into the omo, probably has a similar geological history to Lebzenia Group; and (c) late granitoid intrusions the Houndé Belt and a genetic model similar for belonging to the Tasiast Suite. The Tasiast ore bodies the overall belt in western Burkina Faso is supported. are presented as layered quartz–carbonate–albite– However, this interpretation is not supported by geo- pyrrhotite–pyrite veins, carbonate quartz veinlets chemical data as far as the BB is concerned. Rocks, containing gold, and adjacent gold disseminated ranging from tholeiitic (high-Mg + Fe) basalts, with and hosted within higher greenschist to amphibolite komatiitic affinities (pyroxenolite) to dacite–rhyo- facies rocks, ribbons or BIF with magnetite- lite, are exposed eastwards, with an evolution from quartzite, and adjacent volcano-clastic rocks. All of mid-ocean ridge basalt (MORB) to arc-type. Eco- these mineralization events follow a series of reacti- nomically, both I- and S-type could be productive vations of reverse faults, including the fracture zones for mineral resources. Although a limited number of Tasiast and Piment. The clay alteration and the of selected basalts at Banfora did not allow an overall high salinity of fluid inclusions suggest possible oro- assessment of their potential compared with the min- genetic gold deposits dated within the range 1.85– eralized basalts of the Mana world-class gold deposit 1.5 Ga (Higashihara et al. 2004; Marutani et al. (Houndé Belt), the geological setting shows slight 2005). similarities with the Houndé Belt although dacite The geophysical processing of aeromagnetic data and rhyolite could constitute volcanic centres at Ban- (3D Euler deconvolution, spectral analysis, etc.) fora. Finally, based on the old and recent data avail- using several filters provided a clear 3D subsurface able, the authors suggest an eight-part revision of the image at depths up to 5 km. A structural map was lithostratigraphy chronology (e.g. Ilboudo et al. derived from this investigation and corroborates 2019). Rhyolites host-disseminated sulfides and are the outcropping structures, either for the in-situ Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

Introduction to volume SP502 17 magnetic susceptibilities or for the induced magnetic be highlighted SW and NE of the study area, whereas data. On the reduced to pole (RTP) magnetic map the basement topography is shallower in its centre one can notice magnetic anomalies presenting and northwestwards. In addition, the massive bodies extreme values concentrated much more on small on the map could be granitoids, some of which crop surfaces around structures, for example the Tasiat out at different locations (Gunn et al. 1997). area, hardly visible on the total magnetic intensity Nyame et al. (2020) present a paper related to (TMI) map. The computed upward continuation Birimian Mn mineralization within the Kibi- from 500 to 2000 m with an interval of 500 m on Winneba Metavolcanic Belt, in the Mankwadzi area, the RTP map reveals the progressive evolution of west of Accra. Birimian terrains throughout are the various structures (amplitude attenuation till dominantly stratiform within host metavolcanic- disappearance of some of the anomalies) and their metasedimentary sequences such as greenstone, phyl- location at large depths. The strong magnetic suscep- lite and schist (quartz schist, quartz-sericite schist, tibilities measured in situ on BIF (25.7–35 × etc.). This feature (or at least relicts thereof) seems − 10 3 SI) correspond to NE–SW linear structures, to be prevalently displayed irrespective of the post- mainly west of Akjoujt. In addition, some NNW– formation modification that may have affected the SSE to north–south structures also exhibit high rocks. In general, the stratiform or stratabound nature apparent magnetic bulk susceptibility values. To means that they are mostly conformable with the characterize the azimuthal intensity, directional enclosing rocks (Hein and Tshibubudze 2016), except derivative maps were computed with respect to X where post-depositional effects may have locally and Y, deciphering the weak (strong) azimuth been tectonically displaced and/or have altered the structures, respectively. original Mn-bearing horizons. Analytic signal (AS) and first derivative maps The study took into account several items and show similarities highlighted mainly within the was conducted through: (1) field observations, (2) main trending directions. But the AS map is more sampling for petrological analysis with SEM exami- significant when dealing with same geometry bodies, nation of thin sections and (3) ground geophysical for example four main linear and massive bodies can surveys (magnetic and resistivity) along with profiles be distinguished. In addition, upward continuation within 250 × 300 m2 rectangle of the targeted area. and 3D Euler Deconvolution Solutions (EDS) The Mn-bearing formation, NNE trending, outcrops maps for different elevations show that NNE–SSW over a 200 m long and 20–30 m wide area. In the linear structures disappear at 2000 m and 1000 m, field, it is associated with and often grades into horn- respectively : either the signal-to-noise ratio is weak blende schist to the NW and amphibolite to the SE of or no magnetic anomalies are detectable at 2000 m Mankwadzi. It is dominantly composed of fine to high. medium grains of quartz, some of which are occa- The results of the power spectral analysis (Okubo sionally recrystallized and stretched (foliated or et al. 1985; Tanaka et al. 1999) indicate depths.4km deformed) and mica. Dark bands, however, consist (deep sources) to the SW, and north and NE. They fol- mostly of opaque Mn (?) oxide minerals that can low the same behaviour – depths,120 m (shallow be identified under a microscope. In some hand sam- sources) – within the same areas, while 3D-EDS ples, veins and veinlets of similar opaque phases are maps show that the major sources (.0.5 km) are also observed. But in many outcrops and hand sam- mainly in the south and the west. Such differences ples, the host hornblende schist is macroscopically can be explained by materials with various magnetic massive and sometimes strongly foliated. The main susceptibility values at depth, corroborating field minerals examined under the microscope include observations where the basement crops out to the quartz, feldspar, biotite and hornblende with occa- NE and is deeper westwards and southwards (coastal sional grains of pyrite and arsenopyrite. SEM images basin). Hence, the magnetic sources are much deeper of Mn thin sections corroborate the distinctly banded southwestwards, and from 3000 to 5000 m very few to massive features observed both in hand samples magnetic sources can be distinguished. and under the microscope. Individual bands often In conclusion, the authors state that the maps of show variations in width and minerals density: the various magnetic gradients computed correspond garnet-rich bands alternate with garnet-poor layers to lengthening of dykes/abnormal contacts. The BIF in which sub-euhedral quartz or other silicate miner- west of Akjoujt may present metallotects for gold- als may be present, with contacts in between fairly bearing exploration and base metals deposited in sharp. Mn-garnet usually occurs either in a matrix paragenetic balance. A structural map is proposed of dark, amorphous silica (chert) or in the garnetifer- showing the main linear trending structures that ous bands. While the massive varieties of Mn rock would correspond to Mesoarchaean and Paleoproter- often do not have visible banded areas and the garnet ozoic dykes, shears or faulting systems roughly ori- crystals of varying size appear to be cemented by ented 120° N, and north–south to NNE–SSW, silica (chert) and are distributed throughout the respectively. However, deeper basement areas can rock. The garnet exhibits, through semi-quantitative Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

18 T. Aïfa

SEM analyses in banded and massive Mn rock, surrounding areas for localizing mineralization by greater variability in Mn, Si, Al and Fe contents. means of indirect or direct methods using geophysi- Regarding geophysical exploration through RTP cal and geochemical/geological field sampling for data, obtained from the transformation of the TMI, processing and laboratory analyses. The most signif- the locations of observed Mn floats appear to fall icant results concern the detailed analyses of miner- on the flanks of the transformed maxima, suggesting alization to extract information linked to their that the inferred maximum RTP intensities may rep- genesis and emplacement mechanisms in various resent subsurface Mn ore in the area. A comparison geodynamic contexts. This mainly concerns the por- of the apparent resistivity map with the analytic sig- phyry, VMS, sedex and lateritic sedimentary types. nal map suggests that the western anomalous zone Mineral deposits do not occur randomly; their distri- depicted from the magnetic survey is also character- bution is intimately linked to their geological history. ized by relatively low apparent resistivity values, The more we know about the geology of each area of suggesting that the zone is conductive. It strongly the WAC and the processes that control the regional correlates, thus, with the Mn occurrence reported and local distribution of mineral deposits, for exam- in this area. Additionally, the Mn outcrop in the ple faulting systems, shears, stress regimes, host rock Mankwadzi area exhibits a width and NE-trending types, etc., the better we can know where new depos- extents enclosing hornblende schist, supported its will be found and what they will contain. In fact, by interpretation from ground geophysical data. such investigations could not be possible without the They reveal a NE-trending, highly magnetic but appropriate and cutting-edge tools on increasingly low resistivity main western zone with source depths precise and targeted data (ICP-MS, Raman spectro- .7 m roughly coinciding with the Mn occurrence. scopy, high resolution geophysics and radiometric The shallow (,2.5 m), less persistent zone to the techniques, etc.) east, characterized by relatively high resistivity, The perspectives of knowledge of ore deposits appears to coincide with Mn floats in a dominantly could be targeted with artificial intelligence (e.g. lateritized zone. The Mn minerals, often associated fuzzy logic, neural networks, etc.) for a better with magnetic signatures, show paramagnetic and approach to analysing mineralization mechanisms diamagnetic characteristics based on the ore oxida- (porphyry, metal-metalliferous, VMS, sedimentary tion state, thereby inducing positive and negative and lateritic exhalative). This would help localizing magnetic susceptibility values, respectively (Kro- them through physical and chemical models, often páceǩ et al. 1975; Walid et al. 2013). used for analyses, but also for forecasting and Manganese deposits in the Birimian of Ghana computing reserves. (e.g. Nsuta) or Burkina Faso (e.g. Tambao), exhibit A particular challenge for West Africa is the several supergene-derived Mn oxide minerals such exploration of the hidden parts of the Precambrian as cryptomelane, pyrolusite, psilomelane, nsutite WAC where they are covered by younger rocks, and lithiophorite may be present in weathered sand and vegetation. The application of airborne lateritic or saprolitic horizons. They are overlying geophysical surveys (e.g. magnetic and radiometric) primary, mainly Mn, carbonate. The Mn oxide min- will be particularly important for exploration in these erals of the study area seem to be the products of areas. supergene alteration of primary Mn minerals in an Younger sedimentary basins are also known to underlying (protore) Mn. host a variety of mineral deposits rich in zinc, lead, phosphorous, aluminium, nickel and titanium, as well as various industrial mineral products. How- Conclusions ever, these sedimentary basins are still largely under explored. The prospects for natural mineral resources in West Africa are high and are produced there from some of the largest and richest mineral deposits world- Acknowledgements I am grateful to all the geoscien- wide. The presence of such ore/mineral deposits tists who accepted to peer review the proposed articles to suggests that others are likely to exist because the produce this special publication dedicated to Prof. Th. mineralization processes on explored formations Lasm. Their helpful suggestions and criticisms of the orig- commonly form clusters of mineral deposits. Min- inal manuscripts greatly improved the quality of the papers eral deposits are mainly associated with large-scale within this volume. I warmly thank all the referees, some of geological processes that occur in time and space. whom peer reviewed several articles, especially Richard fl England (University of Leicester, UK), Goodluck Anudu The magmas and hydrothermal uids, fractures and (University of Aberdeen, UK), Martin Lompo (University faults, mountain ranges and sedimentary basins are Ouagadougou, Burkina Faso), Sankar Bose (University of necessary for localizing mineral deposits. Kalkota, India), Fei Liu (Chinese Academy of Geological In this volume, several new approaches were pre- Sciences, Beijing, China), Simon Mitchell (University of sented in various parts of the WAC and its the West Indies, Kingston, Jamaica), Luca Barale (Institute Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

Introduction to volume SP502 19 of Geosciences and Earth Resources, Torino, Italy), Qiang 34, 119–136, https://doi.org/10.1016/S0899-5362 Yu (Chang’an University, China), Dick Tommy Claeson (02)00013-1 (Sveriges Geologiska Undersökning, Uppsala, Sweden), Adissin Glodji, C.L. 2012. La zone de cisaillement de Kandi Huan Cui (Vrije Universiteit Brussels, Belgium), Henri et le magmatisme associé dans la région de Savalou- Masquelin (Universidad de la República, Montevideo, Uru- Dassa (Bénin): étude structurale, pétrologique et géo- guay), Salah A. Al-Khirbash (Sultan Qaboos University, chronologique. PhD thesis, Université de Lyon. Muscat, Oman), Israa S. Abu-Mahfouz (University of Ahmed, A.H., Arai, S., Abdel-Aziz, Y.M., Ikenne, M. and Oxford, UK), Mohammad Ismaiel (University of Hydera- Rahimi, A. 2009. Platinum-group elements distribution bad, India), Chokri Jallouli (King Saud University, Riyadh, and spinel composition in podiform chromitites and Saudi Arabia), Dean Bullen (University of Portsmouth, associated rocks from the upper mantle section of the UK), Lauren Hoyer (University of KwaZulu-Natal, South Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Africa), Mohhamad Mehdi Farahpoor (Lorestan Univer- Morocco. Journal of African Earth Sciences, 55, sity, Iran), Sean C. Johnson (University College, Dublin, 92–104, https://doi.org/10.1016/j.jafrearsci.2009.02. Ireland), Abdelhak Boutaleb (FSTGAT-USTHB, Algiers, 005 Algeria), Martin Stephen Griffin (Amec Foster Wheeler, Aïfa, T. 1993. Different styles of remagnetization in Devo- Ashford, UK), Yacouba Coulibaly (Felix Houphouet- nian sediments from the north western Sahara (Algeria). Boigny University of Cocody-Abidjan, Côte d’Ivoire), Geophysical Journal International, 115, 529–537, Saeed Alirezaei (Shahid Beheshti University, Iran), Athok- https://doi.org/10.1111/j.1365-246X.1993.tb01204.x pam K Singh (Wadia Institute of Himalayan Geology, Aïfa, T. 2008. Investigations of ore deposits within the Dehra Dun, India), Ali Ismail Aljubory (Mosul University, West African Craton and Surrounding Areas. Journal Iraq), Jinsen Zhang (Hebei University of Engineering, of African Earth Sciences, 50,55–272, https://doi. China), Bernard Bingen (Geological Survey of Norway, org/10.1016/j.jafrearsci.2007.10.007 Trondheim, Norway), Bernhard Schulz (Technische Uni- Aïfa, T. 2018. Paleoproterozoic Birimian geology for versitat Bergakademie Freiberg, Germany), Jean-Clair sustainable development. Journal of African Earth Sci- Duchesne (Université de Liège, Belgium), Stefan ences, 148,1–108, https://doi.org/10.1016/j.jafrear G. Mueller (University of Western Australia, Perth, Austra- sci.2018.07.012 lia), Kaushik Das (Hiroshima University, Japan), Francesco Aïfa, T. and Lo, K. 2020. Aeromagnetic modelling of Arboit (University of Dublin Trinity College, Ireland), Precambrian subsurface structures of the Tasiast area, Ismail Omer Yilmaz (Middle East Technical University, NW Mauritania. Geological Society, London, Special Ankara, Turkey), Amira El Tohamy (Nuclear Materials Publications, 502, https://doi.org/10.1144/SP502- Authority, Cairo, Egypt), Mohammad Ebrahimi (Univer- 2019-101 sity of Zanjan, Iran), Chandan Kumar Boraiaha (Central Aïfa, T. and Merabet, N.E. 2020. Rock magnetic study on University of Kerala, India). the Yetti-Eglab Intrusions, Sahara: contribution to the I would also like to express my special thanks to Randell West African Craton geology. Geological Society, Lon- A. Stephenson (University of Aberdeen, UK), as editor of don, Special Publications, 502, https://doi.org/10. the GSL special publications, for his kind help during the 1144/SP502-2019-198 procedures of peer-review and to the GSL editorial staff, Aïfa, T., Lefort, J.P., Ouddane, M. and Calza, F. 1993. Mise Bethan Phillips, Maggie Simmons, Tamzin Anderson and en évidence d’antiformes générées en régime extensif Rachael Kriefman, for their support from handling queries sur la marge orientale du craton Ouest-Africain (région to production of this volume. Finally, I especially thank des Eglab): arguments paléomagnétiques et gravimétri- Moussa Dabo (UCAD, Dakar, Senegal) for his involve- ques. Bulletin du Service Géologique de l’Algérie, 4, ment and contribution to IGCP 638 (UNESCO-IGCP 121–136. program). Aïfa, T., Lefort, J.P. and Drareni, A. 2001. 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