Academic literature on the topic 'West African Craton (WAC) volcanism'

Relevance. The Paleoproterozoic region of the West African craton (WAC) includes numerous world-class gold deposits. These deposits are located in belts of volcanic and volcanic-sedimentary Birimian greenstone rocks. Gold mineralization of the orogenic type is spatially, chronologically, and genetically associated with the compression stage in the northwest and southeast. Several deposits are located in more or less unaltered and deformed zones west of Côte d’Ivoire (Guinea) and the Keniba–Kedugou Senegal-Mali salient. To date, the role of this type of rocks from the standpoint of the model of “local structural traps” for hydrothermal and (or) metamorphogenic gold-bearing fluids remains underestimated. The purpose of the research. The purpose and objectives of the research are to study and reveal the relationship of precious metal mineralization with regional and provincial geotectonic events, igneous and volcanic rock complexes of different ages, types of metamorphic and metasomatic transformation of ore-bearing rocks and types of ores within the West African Сraton. Particular attention is paid to the origin and location of gold mineralization in the carbonate formations of the lower (early) Birimian dating up to 2.12 billion years. Conclusions. The author demonstrates the differences between the Proterozoic gold mineralization of the preorogenic and, in fact, orogenic stages of the geological development of West Africa (Burkina Faso, Ghana) from modern regional-geological, geohistorical and metallogenic positions. The results of our own research have shown and proved the endogenous nature of the precious metal mineralization of the region. This is especially important for gold ores in sedimentary carbonate strata, which for a long time were considered syngenetic (placer).

AbstractThe Palaeoproterozoic Birimian Supergroup of the West African Craton (WAC) consists of volcanic belts composed predominantly of basaltic and andesitic rocks and intervening sedimentary basins composed predominantly of wackes and argillites. Mafic metavolcanic rocks and granitoid-hosted enclaves from the Palaeoproterozoic Lawra Belt of Ghana were analysed for geochemical and Sr–Nd isotopic data to constrain the geological evolution of the southeastern part of the WAC. The metavolcanic rocks display mainly tholeiitic signatures, whereas the enclaves show calc-alkaline signatures. The high SiO2 contents (48.6–68.9 wt%) of the enclaves are suggestive of their evolved character. The high Th/Yb values of the samples relative to that of the mantle array may indicate derivation of their respective magmas from subduction-modified source(s). The rocks show positive εNd values of +0.79 to +2.86 (metavolcanic rocks) and +0.79 to +1.82 (enclaves). These signatures and their Nd model ages (TDM2) of 2.31–2.47 Ga (metavolcanic rocks) and 2.39–2.47 Ga (enclaves) suggest they were probably derived from juvenile mantle-derived protoliths, with possible input of subducted pre-Birimian (Archean?) rocks in their source(s). Their positive Ba–Th and negative Nb–Ta, Zr–Hf and Ti anomalies may indicate their formation through subduction-related magmatism consistent with an arc setting. We propose that the metavolcanic rocks and enclaves from the Lawra Belt formed in a similar island-arc setting. We infer that the granitoids developed through variable degrees of mixing/mingling between basic magma and granitic melt during subduction, when blobs of basic to intermediate parental magma became trapped in the granitic magma to form the enclaves.

AbstractThis Special Publication combines results obtained by interdisciplinary groups from numerous academic 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 evolution 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. Mineralization is mainly controlled by tectonics within shear zones, orogenic belts, basins and faulting systems occurring 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 understanding 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.

This Special Publication combines results obtained by interdisciplinary groups from numerous academic 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 contribute to new knowledge in mineral research. They show that the geodynamic evolution 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. Various modelling techniques, when integrated, help in understanding the mechanisms of mineralization emplacement, some of which are still a matter of debate. The challenge for further studies is mitigation for sustainable development that can be appropriately used to minimize such damage.

Abstract New geodynamical data from West Africa bring consistent informations on the pre-Mesozoic reconstruction within a large area running from the western Sahara to the Colombian cordillera. These new data support a Neoproterozoic Ocean (WANO) between the Amazonian (AMC) and West African (WAC) cratons previously to the Iapetus and Rheic oceans. We delineate 31 blocs detached from the surrounding three continents: NAC (North American Craton), AMC and WAC. 7 came from the WAC margin, 7 from the NAC, 6 from the AMC and 11 from an intermediate volcano sedimentary domain (COB) built on a 1200–1000 Ma oceanic crust. These imbricated blocks formed a tight mosaic by the Hercynian/Alleghanian tectonic event which gave way to the Pangea super-continent. But, during the Atlantic Ocean opening these blocks began to move. They were separated by new oceanic basins. However, previously to the Pangea, blocks from the COB domain formed two sets of garlands located on the northwestern Gondwana margin. The northern one moved to the North until the Silurian to collide the NAC (Taconic tectonic event) meanwhile the southern one remains on the Gondwana margin. All together were gathered by the Carboniferous/Permian time. Then, the framework for the opening of the Atlantic Ocean was not totally disconnected from the “Variscan” collage and many variscan weakness zones were re-used as initial breaking zones. Beyond this tectonic impact, the pre-mesozoic assemblage allows us to compare this “Caribbean” island arc with another one: the Indonesian “Banda” arc. Thus, West Africa is a geological key area for correlations between the Caribbean, the Appalachian, the Brazilian “Nordeste” and the West European domains and for the understanding of the Atlantic Ocean opening process.

Abstract. New temperature measurements from eight boreholes in the West African Craton (WAC) reveal superficial perturbations down to 100 m below the alteration zone. These perturbations are both related to a recent increase in the surface air temperature (SAT) and to the site effects caused by fluid circulations and/or the lower conduction in the alterites. The ground surface temperature (GST), inverted from the boreholes temperatures, increased slowly in the past (~0.4 °C from 1700 to 1940) and then, more importantly, in recent years (~1.5 °C from 1940 to 2010). This recent trend is consistent with the increase of the SAT recorded at two nearby meteorological stations (Tambacounda and Kedougou), and more generally in the Sahel with a coeval rainfall decrease. Site effects are superimposed to the climatic effect and interpreted by advective (circulation of fluids) or conductive (lower conductivity of laterite and of high-porosity sand) perturbations. We used a 1-D finite differences thermal model and a Monte-Carlo procedure to find the best estimates of these site perturbations: all the eight borehole temperature logs can be interpreted with the same basal heat-flow and the same surface temperature history, but with some realistic changes of thermal conductivity and/or fluid velocity. The GST trend observed in Senegal can be confirmed by two previous borehole measurements made in 1983 in other locations of West Africa, the first one in an arid zone of northern Mali and the second one in a sub-humid zone in southern Mali. Finally, the background heat-flow is low (31±2 mW m−2), which makes this part of the WAC more similar with the observations in the southern part (33±8 mW m−2) rather than with those in the northern part and in the Pan-African domains where the surface heat-flow is 15–20 mW m−2 higher.

Abstract. New temperature measurements from eight boreholes in the West African Craton (WAC) reveal superficial perturbations down to 100 meters below the alteration zone. These perturbations are both related to a recent increase of the surface air temperature (SAT) and to the site effects caused by fluids circulations and/or the lower conduction in the alterites. The ground surface temperature (GST) inverted from the boreholes temperatures is stable in the past (1700–1940) and then dramatically increases in the most recent years (1.5 °C since 1950). This is consistent with the increase of the SAT recorded at two nearby meteorological stations (Tambacounda and Kedougou), and more generally in the Sahel with a coeval rainfall decrease. Site effects are superimposed to the climatic effect and interpreted by advective (circulation of fluids) or conductive (lower conductivity of laterite and of high-porosity sand) perturbations. We used a 1-D finite differences thermal model and a Monte-Carlo procedure to find the best estimates of these sites perturbations: all the eight boreholes temperatures logs can be interpreted with the same basal heat-flow and the same surface temperature history, but with some realistic changes of thermal conductivity and/or fluid velocity. The GST trend observed in Senegal can be confirmed by two previous boreholes measurements made in 1983 in other locations of West Africa, the first one in an arid zone of northern Mali and the second one in a subhumid zone in southern Mali. Finally, the background heat-flow is low (30 ± 1 m Wm−2), which makes this part of the WAC more similar with the observations in the southern part (33 ± 8 m Wm−2) rather than with those in the northern part and in the PanAfrican domains where the surface heat-flow is 15–20 m Wm−2 higher.

The Birimian of West African Craton (WAC) is known for its gold potential. Among Birimian structures, N-S and NE-SW trends have been defined as controlling gold mineralizations in Kedougou-Kenieba Inlier (KKI), whereas some of these structures are not gold-bearing. To determine structures related to gold mineralization, airborne electromagnetic data collected during the “Système Minier” of Mali have been processed and interpreted. Identified lineaments have been followed in the ground to establish their link with gold showings in the Kenieba area. Geophysical interpretations show trends similarity for different orders of conductivity. They allowed for characterizing resistance and conductive structures with prevalent N-S and NE-SW directionalities. Conductive structures are qualified as good conductors and are strongly related to known Artisanal Mining Sites (AMS). They coincide with tourmaline sandstones and quartz-albite veins, which are both often artisanally recognized as indicators of gold mineralization in Western Mali. Field observations show that resistance structures correspond to felsic rocks. These structures can bear gold only when silicified and they have spatial relations with Artisanal Mining Sites (AMS) within the Kenieba area. This study shows the efficiency of electromagnetic methods to characterize Birimian structures in relation to the gold mineralization in Kedougou-Kenieba Inlier (KKI).

AbstractDating the pre-Middle Ordovician metavolcanic rocks and metagranites of the Ollo de Sapo Domain has, historically, been difficult because of the small compositional variation, the effects of the Variscan orogeny and, as revealed in this paper, the unusually high fraction of inherited zircon components. The first reliable zircon data (U–Pb ion microprobe and Pb–Pb stepwise evaporation) indicate that the Ollo de Sapo volcanism spanned 495±5 Ma to 483±3 Ma, and was followed by the intrusion of high-level granites from 483±3 Ma to 474±4 Ma. In both metavolcanic rocks and metagranites, no less than 70–80% of zircon grains are either totally Precambrian or contain a Precambrian core overgrown by a Cambro-Ordovician rim. About 80–90% of inherited zircons are Early Ediacaran (602–614 Ma) and derived from calc-alkaline intermediate to felsic igneous rocks generated at the end of the Pan-African arc–continent collision. In the Villadepera region, located to the west, both the metagranites and metavolcanic rocks also contain Meso-Archaean zircons (3.0–3.2 Ga) which ultimately originated from the West African Craton. In the Hiendelaencina region, located to the east, both the metagranites and metavolcanic rocks lack Meso-Archaean zircons, but they have two different inherited zircon populations, one Cryogenian (650–700 Ma) and the other Tonian (850–900 Ma), which suggest older-than-Ediacaran additional island-arc components. The different proportion of source components and the marked variation of the 87Sr/86Srinit. suggest, at least tentatively, that the across-arc polarity of the remnants of the Pan-African arc of Iberia trended east–west (with respect to the current coordinates) during Cambro-Ordovician times, and that the passive margin was situated to the west.

SUMMARY Since the Mesozoic, central and eastern European tectonics have been dominated by the closure of the Tethyan Ocean as the African and European plates collided. In the Miocene, the edge of the East European Craton and Moesian Platform were reworked in collision during the Carpathian orogeny and lithospheric extension formed the Pannonian Basin. To investigate the mantle deformation signatures associated with this complex collisional-extensional system, we carry out SKS splitting analysis at 123 broad-band seismic stations in the region. We compare our measurements with estimates of lithospheric thickness and recent seismic tomography models to test for correlation with mantle heterogeneities. Reviewing splitting delay times in light of xenolith measurements of anisotropy yields estimates of anisotropic layer thickness. Fast polarization directions are mostly NW–SE oriented across the seismically slow West Carpathians and Pannonian Basin and are independent of geological boundaries, absolute plate motion direction or an expected palaeo-slab roll-back path. Instead, they are systematically orthogonal to maximum stress directions, implying that the indenting Adria Plate, the leading deformational force in Central Europe, reset the upper-mantle mineral fabric in the past 5 Ma beneath the Pannonian Basin, overprinting the anisotropic signature of earlier tectonic events. Towards the east, fast polarization directions are perpendicular to steep gradients of lithospheric thickness and align along the edges of fast seismic anomalies beneath the Precambrian-aged Moesian Platform in the South Carpathians and the East European Craton, supporting the idea that craton roots exert a strong influence on the surrounding mantle flow. Within the Moesian Platform, SKS measurements become more variable with Fresnel zone arguments indicating a shallow fossil lithospheric source of anisotropy likely caused by older tectonic deformation frozen in the Precambrian. In the Southeast Carpathian corner, in the Vrancea Seismic Zone, a lithospheric fragment that sinks into the mantle is sandwiched between two slow anomalies, but smaller SKS delay times reveal weaker anisotropy occurs mainly to the NW side, consistent with asymmetric upwelling adjacent to a slab, slower mantle velocities and recent volcanism.

2013/2014
La domanda alla base di questa ricerca è stata se il metodo della gravimetria satellitare possa essere utilizzato per seguire le unità geologiche anche in luoghi difficilmente accessibili. L’obiettivo di questa ricerca è di verificare se le missioni satellitari di nuova generazione permettano di identificare la più grande delle province ignee della Terra (Bertrand et al., 2013), nota come CAMP (Central Atlantic Magmatic Province) in Africa nord-occidentale. Oltre alle motivazioni scientifiche, una possibile applicazione è l’esplorazione di risorse minerarie e lo sfruttamento di energia geotermica. Tale provincia ignea è una LIP (Large Igneous Province) che si estende in Nord e Sud America, Atlantico, Europa ed Africa (istituita in Marzoli et al. 1999). Essa si è sviluppata a seguito della frammentazione del super-continente Pangea al limite Triassico-Giurassico, ca. 200 Ma fa. A causa probabilmente del riscaldamento globale del mantello e/o dalla convezione dello stesso innescata da dislivelli di blocchi litosferici, dai dicchi-sorgente si produssero i cosiddetti basalti da flusso e si verificò un intenso vulcanismo con imponenti colate laviche tali da suggerire a taluni ricercatori che gli elementi volatili presenti nel magma abbiano contribuito ad aumentare i gas serra con conseguenze nel clima globale e nelle estinzioni di massa. I depositi in esame sono costituiti da lave, tholeiti continentali, doleriti, basalti e gabbri. Ciò che rimane di questa attività vulcanica sono dicchi singoli o in sciami, batoliti, sill, colate laviche e plateau basaltici (nei fondali oceanici). L’Africa nord-occidentale è costituita principalmente da un cratone composto da rocce molto antiche dell’Archeano (3000-2500 Ma). Esso emerge a nord nello scudo Reguibat e, a sud, la dorsale dell’Uomo o del Leone (Lucazeau et al., 1991). Il cratone è circondato dalle zone di geosutura (greenstone e cinture mobili) associate al cosiddetto evento termo-tettonico Pan-Africano, verificatosi ca. 650 Ma fa con l’assemblaggio del continente africano da blocchi crostali più piccoli. Nelle Mauritanidi affiora il basamento ercinico (ca. 350 Ma) mentre negli Atlas e nelle Magrebidi prevalgono rocce più recenti connesse all’orogenesi alpina (0-150 Ma). Al centro del cratone si trova il bacino paleozoico Taoudenni che riempie una vasta area depressa. Tutto il territorio in esame è caratterizzato da una forte presenza di rocce metamorfiche dense e magmatiche di ogni età, con presenza di più di un “punto caldo” che potrebbe essere definito un terreno igneo (Bryan et al. (2008) con più LIP spesso sovrapposte o limitrofe. Mediamente, tutte queste rocce magmatiche e metamorfiche hanno una densità di 3000 kg/m3 (Kröner, 1977 ), maggiore di quella della crosta standard e dei sedimenti. Particolare attenzione è stata dedicata ad un lineamento tettonico noto come Pelusium Megashare System (PMS) che attraversa tutta l’Africa nord-occidentale (Neev et al., 1982) chiaramente visibile in tutte le immagini satellitari di Google Earth ma che è riportato solo in pochissime pubblicazioni. Per la prima volta in questa tesi si ipotizza un collegamento tra la CAMP e PMS. La gravimetria satellitare consente di rilevare variazioni di densità nella crosta terrestre. Ove vi sono rocce più dense, il segnale rilevato (detto anomalia gravimetrica) è positivo e viceversa. La gravimetria da satellite si è rivelata un valido strumento per identificare le aree con surplus di massa. La risposta all’interrogativo iniziale è dunque affermativa anche se, da quanto esposto, risulta difficile o impossibile associare ad un certo segnale positivo una data LIP. L’elaborazione dei segnali è avvenuta partendo dai dati del satellite GOCE (ultima generazione, a un’orbita di 250 km ma già ammarato) e GRACE (obsoleto ma tuttora in orbita a ca. 450 km). I dati utilizzati sono dei modelli del campo di gravità terrestre che contengono i coefficienti di Stokes per lo sviluppo in armoniche sferiche del potenziale. I modelli utilizzati sono l’EGM2008 (comprendente anche dati di terra, con risoluzione massima 10 km se sviluppato al massimo ordine di 2159) e GOCO TIM R4 (con una risoluzione massima di 80 km, la migliore mai ottenuta da dati satellitari globali). Il modello EGM2008 è stato sviluppato fino all’ordine e grado 720 per eliminare dati spuri (Pavlis, 2012) e, in tal modo, ha permesso di raggiungere una risoluzione di ca. 27 km se si considera metà lunghezza d’onda. Dopo il controllo della qualità dei dati, essi sono stati elaborati nel seguente modo, come esposto nei capitoli 2, 3 e 4. Ai dati grezzi sono state applicate tre riduzioni per sottrarre gli effetti di gravità indesiderati che mascherano il segnale cercato più debole. È stato sottratto l’effetto di gravità della topografia, dei sedimenti e dell’interfaccia crosta-mantello (ICM). Partendo dall’anomalia “in aria libera” (FA), è stata quindi ottenuta l’anomalia di Bouguer (BA) e la BA corretta per i sedimenti. Poi, calcolata la Moho (ICM) isostatica, si è prodotto il residuo isostatico corretto per i sedimenti. I campi elaborati sono la gravità gz (espressa in milli Gal, mGal) ed il gradiente Tzz (misurato in Eötvös, E). Sono state usate le risoluzioni di 0.5° e di 0.05°, computati ad una quota di 4000 m s.l. m perché maggiore del più altro rilievo montuoso dell’area. Dopo aver modellato dei casi a geometria semplice (cap. 6) si è passati alla modellizzazione di tre casi reali. I tre siti scelti per l’approfondimento sono: Tindouf (Algeria), Taoudenni (Mali), Timbuktu (Mali). Nel bacino di Tindouf un sill doleritico CAMP è annesso al suo probabile dicco-sorgente reso evidente dalla gravimetria che identifica bene anche una vicina miniera di Ferro. Nel bacino Taoudenni, le due anomalie principali suggeriscono la presenza di cumuliti magmatici spessi una dozzina di chilometri e connessi con la superficie attraverso dicchi obliqui. Il sito presso Timbuktu è trattato nel dettaglio perché al di sotto del vicino lago Faguibine è stata rivelata un’intrusione magmatica lunga ca. 250 km. In superficie vi sono evidenze di magmatismo (per es. fumarole) tali da preoccupare le popolazioni locali (El Abbass et al., 1993). Tra i risultati inaspettati, si ricorda il forte segnale gravimetrico generato dalle peridotiti in Marocco ed un’importante anomalia (80 mGal) nel Grand Erg Occidental (Algeria) al di sotto del Sahara che sembrerebbe essere causata da un corpo denso lungo ca. 600 km.
The question behind this research was whether the method of satellite gravimetry can be used to follow the geological units even in inaccessible places. The goal of this research is to verify if the new generation of satellite missions serve to identify the largest of the Earth's igneous provinces (Bertrand et al., 2013), known as CAMP (Central Atlantic Magmatic Province) in Northwest Africa . Besides the scientific reasons, a possible application is the exploration of mineral resources and the exploitation of geothermal energy. This is an igneous province (LIP Large Igneous Provinces) that extends throughout North and South America, the Atlantic Ocean, Europe and Africa (established in Marzoli et al. 1999). It developed as a result of the fragmentation of the super-continent Pangea at Triassic-Jurassic limit, ca. 200 My ago. Probably because of mantel global warming and/or its convection triggered by differences in thickness of lithospheric blocks, from source-dikes were produced the so-called continental flow basalts (CFB) and there was an intense volcanism with massive lava flows that this would suggest to certain researchers volatile elements present in the magma have contributed to increasing greenhouse gases with consequences in the global climate and mass extinctions. The deposits in question consist of lavas, tholeites continental dolerites, basalts and gabbros. What remains of this volcanic activity are individual dykes or in swarms, batholiths, sills, lava flows and basaltic plateau (in the ocean). The north-western Africa consists mainly of a craton made of most ancient rocks dell'Archean (3000-2500 Ma). It emerges in the shield Reguibat north and on the south, the Man or the Lion shield (Lucazeau et al., 1991). The craton is surrounded by areas of geosutura (greenstone belts and mobile belts) associated with the so-called Pan-African thermo-tectonic event, occurred ca. 650 Ma ago with the assembly of the African continent by smaller crustal blocks. Mauritanides emerges in the Hercynian basement (ca. 350 Ma) while in the Atlas and Magrebides prevail younger rocks (Alpine orogenesis, 0-150 Ma). At the center of the craton is the Paleozoic basin Taoudenni that fills a large area depressed. All the territory concerned is characterized by a strong presence of dense magmatic and metamorphic rocks of all ages, with the presence of more than a "hot spot" that could be called “igneous terrane” (Bryan et al. (2008) with more LIPs overlapping or adjacent. On average, these igneous and metamorphic rocks have a density of 3000 kg / m3 (Kröner, 1977), greater than that of the standard crust and the sediments. Particular attention was dedicated to a tectonic lineament known as Pelusium Megashare System (PMS) that runs through the north-western Africa (Neev et al., 1982) clearly visible in all the satellite images of Google Earth but is reported only in very few publications. For the first time this thesis suggests a link between CAMP and PMS. Satellite gravimetry can detect density variations in the Earth's crust. Where there are rocks denser, the detected signal (called gravity anomaly) is positive and vice versa. The gravimetry by satellite has proved a valuable tool to identify areas with surplus Mass. The initial response to the question is therefore affirmative although, from the above, it is difficult or impossible to associate a positive signal a date LIP. Signals processing occurred from the data of the GOCE satellite (last generation, in an orbit of 250 km, mission already finished) and GRACE (obsolete but still in orbit at ca. 450 km). The data used are the models of the Earth's gravity field containing the coefficients of Stokes for the development of potential in spherical harmonics. The models used are the EGM2008 (also including land data, with a maximum resolution 10 km if developed to the maximum order of 2159) and TIM GOCO R4 (with a maximum resolution of 80 km, the best ever obtained by global satellite data). The model EGM2008 has been developed up to the order and degree 720 to remove spurious data (Pavlis, 2012) and, thus, allowed to reach a resolution of ca. 27 km considering half wavelength. After quality control of the data, they were processed in the following flowchart, as discussed in Chapters 2, 3 and 4. Raw data were processed applying three reductions to subtract the effects of gravity that mask the signal. It was reduced by the effect of gravity of the topography, sediment and crust-mantle interface (CMI). Starting by the anomaly "free air" (FA), was thus obtained the Bouguer anomaly (BA) and BA correct for sediment. Then, once calculated the isostatic Moho (CMI), has produced the sediment-corrected-isostatic residual. The fields processed are gravity gz (in milli Gal, mGal) and gradient tzz (measured in Eötvös, E). It has been used the resolutions of 0.5 ° and 0.05 °, computed at an altitude of 4000 m a.s.l., higher of mountains in the area. After mng cases with simple geometry (ch. 6) we moved to the modeling of three real case histories. The three sites chosen for the study are: Tindouf (Algeria), Taoudenni (Mali), Timbuktu (Mali). In the basin of Tindouf a CAMP doleritic sill is attached to its likely source source-dyke evident by gravimetry that identifies well a nearby mine of Iron. In the Taoudenni basin, the two main anomalies suggest the presence of magmatic cumulites a dozen kilometers thick and connected with the surface through oblique dikes. The site at Timbuktu is discussed in detail because in the nearby lake Faguibine was revealed a magmatic intrusion long ca. 250 km. On the surface there is evidence of magmatism (e.g. Fumaroles) such that worry local populations (El Abbass et al., 1993) .Among the unexpected results, please note the strong signal generated by gravimetric peridotites in Morocco and a major anomaly (80 mGal) in the Grand Erg Occidental (Algeria) below the Sahara that would seem to be caused by a dense body ca. 600 km long.
XXVI Ciclo
1972