Explore the vast range of titles available.

The southern part of the West African Craton includes the Baoulé-Mossi Domain, the world’s premier Paleoproterozoic gold province (~10,000 metric ton gold endowment). Structural, metamorphic, and geochronological data suggest gold mineralisation occurred during three episodes that span much of the Eoeburnean and Eburnean orogenic cycles. Eoeburnean orogenic and rare skarn-hosted gold deposits formed between ca. 2200 and 2135 Ma during repeated episodes of volcanism, plutonism, and shortening, which thickened the Paleoproterozoic crust. Early Eburnean orogenic and placer gold deposits formed between ca. 2110 and 2095 Ma during inversion, metamorphism, and subsequent oblique shortening of intra-orogenic basins filled after ca. 2135 Ma. This episode of mineralisation terminated when the Baoulé-Mossi Domain docked with the Archean Kénéma-Man Domain at ca. 2095 Ma. Late Eburnean orogenic and less common intrusion-related gold deposits formed between ca. 2095 and 2060 Ma during strike-slip to oblique-slip tectonics, post-collisional high-K plutonism and crustal reworking across the western and southern Baoulé-Mossi Domain. Eoeburnean gold deposits include ca. 10 % of the gold endowment of the Baoulé-Mossi Domain, whereas the Early Eburnean and Late Eburnean deposits include ca. 50–70% and 20–40%, respectively. Here, we highlight the favourable confluence of accretion-collision tectonics, involving juvenile crust formation as well as protracted magmatic, metamorphic, and deformation histories that resulted in diachronous gold events spread over at least 100 myr throughout the Baoulé-Mossi Domain.

The Tabakoto gold deposit is part of the highly endowed west-Malian gold belt, which hosts several world-class deposits. Located in the Paleoproterozoic Kédougou-Kéniéba Inlier (KKI) of the West African Craton (WAC), the deposit is contained in Birimian metasedimentary rocks of the Kofi series that are intruded by magmatic dikes. The Tabakoto deposit is characterized by multiple overlapping stages of magmatic dike intrusions and hydrothermal alteration during distinct regional-scale deformation events. The metasedimentary rocks are characterized by a S0/1 foliation delineated by the alternation of metagreywacke and meta-argillite. This foliation is affected by upright folds with N-S trending axial planes marked by a S2 schistosity and cross-cut by conjugate steep-dipping dextral NE-SW and sinistral NW-SE trending faults. These structures record regional-scale E-W shortening, first associated with crustal thickening and then with N-S stretching, evolving from ductile to brittle deformation. The gold-bearing V2 quartz-pyrite veins developed in dikes and metasedimentary units, are parallel to the S2 schistosity of the DT2 deformation event. These veins contained reduced phases such as pyrrhotite, loellingite and scheelite as well as bismuth. The second gold-bearing V3a quartz-carbonate veins are located in the NE-SW and NW-SE trending faults of the DT3 deformation event. The latter veins are marked by sodic alteration followed by carbonate (i.e., dolomite-ankerite ± calcite ± siderite) and phyllic (i.e., chlorite-muscovite-sericite) alterations. In the quartz-pyrite veins, gold occurs as inclusions in pyrite, arsenopyrite, and pyrrhotite, whereas in quartz-carbonate shear veins, gold is present in microfractures cross-cutting pyrite and arsenopyrite and at the contact between grains of pyrite and arsenopyrite, and of arsenopyrite and pyrrhotite. The litho-structural and mineralogical features of the Tabakoto deposit portray a polyphase mineralization with (i) an intrusion-hosted orogenic gold system, where the metabaslatic to metarhyolitic dikes acted as suitable emplacement for fluids flow and (ii) a hydrothermal fluid circulation under ductile to brittle conditions.

The NNW-NNE Libiri doleritic dyke swarm of the Liptako region of West Niger is one of multiple mafic dyke swarms that crosscut the Birimian formations of the Léo-Man shield in the West African Craton. This NNWNNE trending swarm consists of two different groups based on mineralogy and geochemical composition. Both groups have a matching assembly of minerals (ferro-edenite, pyroxene, plagioclase, biotite, apatite, chlorite, grains with granophyric texture and titanomagnetite-ilmenite). In detail Group I dykes have a distinct intergranular texture and some particular minerals (bytownite, pigeonite, clinoenstatite and sanidine) that differ from those in Group II dykes which have a partially preserved ophitic texture and less calcic plagioclases (albite to labradorite). Variation in Mg# and differentiation index (DI) indicate that Group I (Mg# = 55–65, DI = 16.26–20.19) is more primitive in composition than Group II (Mg# = 30–35, DI = 22.82–50.00). REE and other incompatible element values are elevated in Group II compared to Group I, and the REE slopes are parallel one to each other, which suggests that the two groups are co-genetic and that Group II units are more evolved magmas from Group I. The presence of negative Nb and Ta values normalized to primitive mantle in both groups and the La n /Yb n (4.73–6.21) ratios suggests that the magma was formed by partial melting of hydrated garnet lherzolite from the lithospheric mantle, modified by interaction with previously metasomatized lithosphere in subduction setting. Alteration minerals such as ferro-edenite, biotite, epidote and chlorite on augite crystal margins indicate later deuteric alteration or low grade late magmatic metamorphism. Petrography and chemistry of a sample SD (Samira Dyke, a sample from the Libiri pit in the Samira gold mine, which was previously dated at 1791 ± 3 Ma and belong to Group II dykes) suggests the entire swarm of NNW-SSE to NNE-SSW trending dykes belong to the Libiri swarm of a plume-generated Large Igneous Province that extends into formerly adjacent Sarmatia (Baltica) and Amazonia. L’essaim de dykes doléritiques de Libiri NNW-NNE de la région du Liptako, à l’ouest du Niger, est l’un des multiples essaims de dykes mafiques qui recoupent les formations birimiennes du bouclier de Léo-Man, dans le Craton Ouest-Africain. Cet essaim d’orientation NNO-NNE, se compose de deux groupes différents sur la base de la minéralogie et de la composition géochimique. Les deux groupes présentent un assemblage correspondant de minéraux (ferro-édénite, pyroxène, plagioclase, biotite, apatite, chlorite, grains avec une texture granophyrique et titanomagnétite-ilménite). En détail, les dykes du groupe I présentent une texture intergranulaire distincte et certains minéraux particuliers (bytownite, pigeonite, clinoenstatite et sanidine) qui diffèrent de ceux des dykes du groupe II qui présentent une texture ophitique partiellement préservée et des plagioclases moins calciques (albite à labradorite). La variation du Mg# et de l’indice de différenciation (DI) indique que le groupe I (Mg# = 55–65, DI = 16,26–20,19) est plus primitif dans sa composition que le groupe II (Mg# = 30–35, DI = 22,82–50,00). Les valeurs des terres rares et d’autres éléments incompatibles sont plus élevés dans le groupe II que dans le groupe I, et les pentes des spectres de terres rares sont parallèles entre elles, ce qui suggère que les deux groupes sont co-génétiques et que les unités du groupe II sont des magmas plus évolués que ceux du groupe I. La présence des valeurs négatives de Nb et de Ta normalisés par rapport au manteau primitif dans les deux groupes et les rapports La n /Yb n (4,73–6,21) suggèrent que le magma a été formé par la fusion partielle d’une lherzolite à grenat hydratée du manteau lithosphérique, modifiée par l’interaction avec la lithosphère précédemment métasomatisée dans le contexte de subduction. Les minéraux d’altération tels que la ferro-édénite et la chlorite sur les marges des cristaux d’augite indiquent une altération deutérique ultérieure ou un métamorphisme tardi-magmatique de faible grade. La chimie de l’échantillon SD (Samira Dyke, un échantillon de la fosse de Libiri dans la mine de Samia, a été précédemment daté à 1791 ± 3 Ma et apparient aux dykes du groupe II) suggère que l’ensemble de l’essaim de dykes orientés NNO-SSE à NNE-SSO appartient à l’essaim Libiri d’une grande province ignée générée par le panache qui s’étend jusqu’à la Sarmatie (Baltica) et l’Amazonie, autrefois adjacentes.

The Sassandra-Cavally (SASCA) domain (SW Côte d'Ivoire) marks the transition between the Archean Kenema-Man craton and the Paleoproterozoic (Rhyacian) Baoule-Mossi domain. It is characterized by the tectonic juxtaposition of granulite-facies and amphibolite-facies rocks. Migmatitic grey gneisses, garnet-cordierite-sillimanite migmatitic paragneisses and garnet-staurolite-bearing micaschists reached peak pressure conditions ranging from ∼ 6.6 kbar at 620 °C to ∼ 10 kbar at 820 °C. These conditions are associated with the first recorded deformation D1 and correspond to a Barrovian geothermal gradient of ∼ 25 °C/km. Subsequent exhumation, associated with a second deformation D2, was marked by decompression followed by cooling along apparent geothermal gradients of ∼ 40 °C/km. A D3 deformation phase is marked by folding and local transposition of the regional S1/S2 foliation into E-W trending shear zones. LA-ICP-MS U-Pb dating of monazite, which displays complex internal structures, reveals four age groups correlated to textural position of monazite grains and analytical points: (1) rare relictual zones yield dates at the Archean-Paleoproterozoic transition (ca. 2400-2600 Ma); (2) a cluster of dates centered at ca. 2037 Ma on grains aligned along the S2 foliation of the migmatitic grey gneiss, attributed to D2; (3) a cluster of dates centered at ca. 2000 Ma, and (4) dates spreading from ca. 1978 to 1913 Ma, documented for the first time in the West African Craton monazites. The ages of the latter two groups are similar to the ones identified in the Guiana Shield, and could be attributed to a disturbance by fluids, to a periodic opening of U-Pb system or to an episodic crystallization of monazite during slow cooling lasting several tens of Myrs. These data allow to propose a model for the tectonic evolution of the SASCA domain at the contact between the Rhyacian Baoule-Mossi domain and the Archean Kenema-Man nucleus whereby crustal thickening is achieved by crustal-scale folding and is followed by and concomitant with lateral flow of the thickened partially molten crust accommodated by regional transcurrent shear zones. This combination of crustal thickening controlled by tectonic forces and gravity-driven flow leads to the juxtaposition of granulite- and amphibolite-facies rocks.

Since the rise of the gold price in 2000, artisanal and small-scale gold mining (ASGM) is a growing economic activity in developing countries. It represents a source of income for several millions of people in West Africa. Exploitation techniques have evolved from traditional gold panning to mechanization and use of chemical products that are harmful for the environment. Government strategies to control and regulate this activity are impeded by the difficulties to collect spatial information, due to the remote location and the mobile and informal natural of ASGM. Here we present and discuss the value of remote sensing techniques to complement the knowledge on artisanal mining impacts, including for detection of illegal sites, the evaluation of the degradation of soils and waters, the deforestation and the monitoring of expansion of ASGM with time. However, these techniques are blind regarding gender issues, labor relations, mobility, migration, and insecurity and need to be considered with knowledges from other disciplines. Remote sensing is also instilled with various powers accruing to those enabled to produce and interpret these data. Remote sensing should be therefore used in a reflexive manner that accounts for the social, ethical and political implications of ASGM governance informed by space observations.Article HighlightsArtisanal and small-scale gold mining is an important economic activity in developing countries with gold resourcesObservations from space can be used to detect mining sites and evaluate their impact on the environmentsRemote sensing should be used in a reflexive and critical manner that accounts for the social, ethical and political implications of ASGM governance informed by space observations

Artisanal and small‐scale gold mining (ASGM) represents a significant economic activity for communities in developing countries. In southeastern Senegal, this activity has increased in recent years and has become the main source of income for the local population. However, it is also associated with negative environmental, social, and health impacts. Considering the recent development of ASGM in Senegal and the difficulties of the government in monitoring and regulating this activity, this article proposes a method for detecting and mapping ASGM sites in Senegal using Sentinel 2 data and the Google Earth Engine. Two artisanal mining sites in Senegal are selected to test this approach. Detection and mapping are achieved following a processing pipeline. Principal component analysis (PCA) is applied to determine the optimal period of the year for mapping. Separability and threshold (SEaTH) is used to determine the optimal bands or spectral indices to discriminate ASGM from other land use. Finally, automatic classification and mapping of the scenes are achieved with support vector machine (SVM) classifier. The results are then validated based on field observations. The PCA and examination of spectral signatures as a function of time indicate that the best period for discriminate ASGM sites against other types of land use is the end of dry season, when vegetation is minimal. The classification results are presented as a map with different categories of land use. This method could be applied to future Sentinel scenes to monitor the evolution of mining sites and may also be extrapolated to other relevant areas in the Sahel. Plain Language Summary In recent years, artisanal and small‐scale gold mining (ASGM) has become a significant source of income for communities in southeastern Senegal. However, this activity is also associated with negative environmental and social impacts. The lack of information regarding the constantly evolving geographical distribution and extent of the mining sites is an obstacle for the regulations of this activity by the government. To fill this gap, we have elaborated and tested a method using publicly available multispectral remote sensing data (Sentinel 2) and a cloud computing service (Google Earth Engine). The objective maps of environmental impacts produced by this method shall lead to a better knowledge of the areas affected by artisanal mining activities in the Sahelian region. This low‐cost approach, using public data and computing resources, is designed to become a decision support tool for governments of developing countries. Key Points artisanal and small‐scale gold mining Sentinel 2 Google Earth Engine

Artisanal and small-scale gold mining (ASGM) represents a significant economic activity for communities in developing countries. In south-eastern Senegal, this activity has increased in recent years and has become the main source of income for the local population. However, it is also associated with negative environmental and social impacts. Considering the recent development of ASGM in Senegal, and the difficulties of the government in monitoring and regulating this activity, this article proposes a method for detecting and mapping ASGM sites in Senegal using Sentinel 2 data and the Google Earth Engine. Two artisanal mining site in Eastern Senegal are selected to develop and test this approach. Detection and mapping are achieved using Principle Component Analysis (PCA), Separability and Threshold (SEaTH) and Support Vector Machine classifier (SVM). The results are validated by ground-truth observations. The PCA indicates that the best period for identifying artisanal mining sites against other types of land-use is the end of dry season, when vegetation is minimal. This result is confirmed by examining the spectral evolution over time of different types of land-use. Input variables for SVM classification are defined by the SEaTH. The classification results are presented as a map with 5 color-coded categories of land-use. The method can be used to map the evolution of mining sites as a function of time using future Sentinel acquisitions. This approach may also be extrapolated to other areas in the Sahel where authorities are also confronted with the difficult regulation of artisanal gold mining activities in remote areas.