Contribution à la reconnaissance de la structure de la zone nord côtière du bassin d'Essaouira par méthodes de sondages électriques et de tomographie électrique

International audienceLes ressources en eau du bassin d’Essaouira sont en cours de destockage notamment celles desaquifères superficiels. L’utilisation des eaux des aquifères profonds nécessite une connaissanceapprofondie des caractéristiques géo-électriques des formations perméables. Pour cela nous avonsutilisé la prospection électrique pour comprendre la stratigraphie et la structure géologique du bassin.Le programme d’acquisition des données a comporté la mise en oeuvre de 29 sondages électriques et 4sections de résistivité. L’acquisition des données a été suivie par l’établissement de courbes et decoupes géo-électriques sur lesquelles sont indiqués les couches avec leurs résistivités, épaisseurs etprofondeurs

Gravimetric evidences of active faults and underground structure of the Cheliff seismogenic basin (Algeria)

International audienceThe Cheliff basin (ex El Asnam) is known as one of the most seismic active zone in Algeria and the West Mediterranean region. We can cite the El Asnam earthquake which occurred in 10.10.01980 with magnitude of 7.3. It was generated by a thrust fault with NE–SW sinistral component. Until now, there is a little information about existence of deep active faults, which generate this strong activity. The gravity field is an important resource of information on crustal structure. The aim of this work is giving a reliable geometry of the major faults relative to the kinematics of this region.The results obtained from various filtered maps (derivatives, upward continuation) of the gravity data, were used to generate a structural map of the studied area. Whilst the continuous wavelet transform method can help in automatic detection of elongated structures in 3-D, to estimate their strike direction, shape and depth. It gives a 3-D image or a model of the region and confirms the existence of several faults, localized or inferred, from former geological studies

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

International audienceThis 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 to understand 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

Aeromagnetic modeling of Precambrian subsurface structures of the Tasiast area, NW Mauritania : Mining consequence

International audienceThe NW sector of Mauritania represents the Archean province of the Reguibat Ridge. It is mainlycomposed of the typical Archaean cratonic association of granitoid gneisses, greenstones belts andgranites. The greenstone belts contain high mineral potential, mainly gold. This is particularly thecase of the Chami Greenstone Belt, which hosts the large Tasiast gold mine whose gold depositsare hosted along a NS oriented shear zonewithasurfaceof70x15km2.Themineral-bearing bodies of Tasiastare presented in both forms of (i) veins of quartz-carbonatewhitefeldspar-pyrrhotite-pyrite, vein lets of quartz-carbonated containing gold, and (ii) adjacentgold disseminated and hosted within rocks from greens chistto amphibolite facies, banded ironformations (BIF) withmagnetite-quartzite, and adjacent volcano-clasticrocks. The whole of the semineralizations follow a series of reactivated thrust faults, includingTasiast and Piment fracturezones (Heronetal.,2016). Atthenorthernendofthe Chami Greenstone Belt, Piment mineralizationsare hosted within BIF, felsicvolcanic rocks, and associated clastic rocks, whilein its western branch,they are hosted within mafic rocks (Sedore and Masterman, 2012; Heron et al., 2016). The clayalteration and the high salinity of fluid inclusions indicate possible orogenetic gold deposits datedbetween 1.85 Ga and 1. 5 Ga(Higashihara et al.,2004; Marutani et al.,2005).Thanks to the available aeromagnetic data recorded at 100 m elevation northern Chamiarea,suitable computations on the anomalyma preduced to the pole were used to propose a subsurfacestructural model. In fact, up ward continuations, apparent magnetic susceptibility, directionalderivatives, analyticalsignal, 3D Euler deconvolution and spectral analyses were investigated tobuild this model (Thompson, 1982; Blakely and Simpson, 1986; Reid et al., 1990; Cowan andCowan, 1993; Blakely, 1996; Mushayandebvu et al., 2001; Roest et al., 1992). They helped decipherthe main evidenced structural features withthemain NNE-SSW, NW-SE and~N120° directions,identified on most all computed maps combined within situ measurements. They probablycorrespond to mafic dykes, fault sands hears rooted up to 5 km depth, particularly to the North,SW and NE, and southwards and west wards by spectral analyses and 3D Euler deconvolution,respectively (Fig. 1). The base mentis deeper to wards the West and South and out crops to wardsthe Neinagreement with the geological observations. In addition, the high values of magneticsusceptibility (χm) measured in situ on BIF (25.7-35.10-3SI) southof Akjoujt could beat the origin ofmetallotects sources associated with magnetic minerals for gold bearing and base metalsexploration

Aeromagnetic modeling of Precambrian subsurface structures of the Tasiast area, NW Mauritania : Mining consequence

International audienceThe NW sector of Mauritania represents the Archean province of the Reguibat Ridge. It is mainlycomposed of the typical Archaean cratonic association of granitoid gneisses, greenstones belts andgranites. The greenstone belts contain high mineral potential, mainly gold. This is particularly thecase of the Chami Greenstone Belt, which hosts the large Tasiast gold mine whose gold depositsare hosted along a NS oriented shear zonewithasurfaceof70x15km2.Themineral-bearing bodies of Tasiastare presented in both forms of (i) veins of quartz-carbonatewhitefeldspar-pyrrhotite-pyrite, vein lets of quartz-carbonated containing gold, and (ii) adjacentgold disseminated and hosted within rocks from greens chistto amphibolite facies, banded ironformations (BIF) withmagnetite-quartzite, and adjacent volcano-clasticrocks. The whole of the semineralizations follow a series of reactivated thrust faults, includingTasiast and Piment fracturezones (Heronetal.,2016). Atthenorthernendofthe Chami Greenstone Belt, Piment mineralizationsare hosted within BIF, felsicvolcanic rocks, and associated clastic rocks, whilein its western branch,they are hosted within mafic rocks (Sedore and Masterman, 2012; Heron et al., 2016). The clayalteration and the high salinity of fluid inclusions indicate possible orogenetic gold deposits datedbetween 1.85 Ga and 1. 5 Ga(Higashihara et al.,2004; Marutani et al.,2005).Thanks to the available aeromagnetic data recorded at 100 m elevation northern Chamiarea,suitable computations on the anomalyma preduced to the pole were used to propose a subsurfacestructural model. In fact, up ward continuations, apparent magnetic susceptibility, directionalderivatives, analyticalsignal, 3D Euler deconvolution and spectral analyses were investigated tobuild this model (Thompson, 1982; Blakely and Simpson, 1986; Reid et al., 1990; Cowan andCowan, 1993; Blakely, 1996; Mushayandebvu et al., 2001; Roest et al., 1992). They helped decipherthe main evidenced structural features withthemain NNE-SSW, NW-SE and~N120° directions,identified on most all computed maps combined within situ measurements. They probablycorrespond to mafic dykes, fault sands hears rooted up to 5 km depth, particularly to the North,SW and NE, and southwards and west wards by spectral analyses and 3D Euler deconvolution,respectively (Fig. 1). The base mentis deeper to wards the West and South and out crops to wardsthe Neinagreement with the geological observations. In addition, the high values of magneticsusceptibility (χm) measured in situ on BIF (25.7-35.10-3SI) southof Akjoujt could beat the origin ofmetallotects sources associated with magnetic minerals for gold bearing and base metalsexploration

Methodology-selection by Fuzzy Analytic Hierarchy Process for Studying Net Pays

International audienceNet pay intervals are conventionally determined, applying cutoff values on geological well-logs. Recently developed methodologies utilize more complicated algorithms: Bayesian classifier, artificial neural network and Dempster-Shafer theory. The outputs of these methodologies are not completely compatible. It rises this question that which method should be recommended in each situation: (i) industrial use, (ii) research goal and (iii) general situations. Fuzzy analytical hierarchy process is used here to compare effectiveness of the four net pay determination methods in each three situations. Six criteria were defined: precision, generalization ability, fuzziness, simplicity of methodological concepts, user-friendly and speed of the algorithm. For the precision and generalization ability, mean squared error of training and generalization data are used, respectively. Mean squared error and speed (inverse of time) of the algorithm are continues variables, and provide quantitative comparison. While qualitative comparison is done for the criteria of simplicity and user-friendly. For the criterion of the fuzziness, a ranking, i.e. categorical variable, is used based on the number of classes that the classifier could provide. The comparison is done based on the results of net pay determination (the four methods) on sandy Burgan and carbonated Mishrif reservoirs, Iranian offshore oilfields. The results show that from viewpoint of general situation, Bayesian and Dempster-Shafer-based methods are the best. Artificial neural network and Dempster-Shafer are the most suitable methods for industrial mode, and Bayesian and artificial neural network are the best methods for research applications. Finally, cutoff methodology is never prioritized

Aeromagnetic modeling of Precambrian subsurface structures of the Tasiast area, NW Mauritania : Mining consequence

International audienceThe NW sector of Mauritania represents the Archean province of the Reguibat Ridge. It is mainlycomposed of the typical Archaean cratonic association of granitoid gneisses, greenstones belts andgranites. The greenstone belts contain high mineral potential, mainly gold. This is particularly thecase of the Chami Greenstone Belt, which hosts the large Tasiast gold mine whose gold depositsare hosted along a NS oriented shear zonewithasurfaceof70x15km2.Themineral-bearing bodies of Tasiastare presented in both forms of (i) veins of quartz-carbonatewhitefeldspar-pyrrhotite-pyrite, vein lets of quartz-carbonated containing gold, and (ii) adjacentgold disseminated and hosted within rocks from greens chistto amphibolite facies, banded ironformations (BIF) withmagnetite-quartzite, and adjacent volcano-clasticrocks. The whole of the semineralizations follow a series of reactivated thrust faults, includingTasiast and Piment fracturezones (Heronetal.,2016). Atthenorthernendofthe Chami Greenstone Belt, Piment mineralizationsare hosted within BIF, felsicvolcanic rocks, and associated clastic rocks, whilein its western branch,they are hosted within mafic rocks (Sedore and Masterman, 2012; Heron et al., 2016). The clayalteration and the high salinity of fluid inclusions indicate possible orogenetic gold deposits datedbetween 1.85 Ga and 1. 5 Ga(Higashihara et al.,2004; Marutani et al.,2005).Thanks to the available aeromagnetic data recorded at 100 m elevation northern Chamiarea,suitable computations on the anomalyma preduced to the pole were used to propose a subsurfacestructural model. In fact, up ward continuations, apparent magnetic susceptibility, directionalderivatives, analyticalsignal, 3D Euler deconvolution and spectral analyses were investigated tobuild this model (Thompson, 1982; Blakely and Simpson, 1986; Reid et al., 1990; Cowan andCowan, 1993; Blakely, 1996; Mushayandebvu et al., 2001; Roest et al., 1992). They helped decipherthe main evidenced structural features withthemain NNE-SSW, NW-SE and~N120° directions,identified on most all computed maps combined within situ measurements. They probablycorrespond to mafic dykes, fault sands hears rooted up to 5 km depth, particularly to the North,SW and NE, and southwards and west wards by spectral analyses and 3D Euler deconvolution,respectively (Fig. 1). The base mentis deeper to wards the West and South and out crops to wardsthe Neinagreement with the geological observations. In addition, the high values of magneticsusceptibility (χm) measured in situ on BIF (25.7-35.10-3SI) southof Akjoujt could beat the origin ofmetallotects sources associated with magnetic minerals for gold bearing and base metalsexploration

Syn-graben hidden folded structuring revealed by advanced gravity analysis: Kalaa Khesba case study (Northwestern Tunisia)

International audienc