Rehabilitation of the Ecological Functions of the Senegal River Delta

This report is an initial effort to evaluate the options for rehabilitating the Ndiaël. It is the result of a desk study carried out in November 2000 and of two month-long field mission held in May and August of 2001. The findings will serve in preparation for the Environmental Management Component of the Long Term Water Sector Project, which also includes a management plan for the Lac de Guiers and the rehabilitation of the Baie de Hann (Dakar). The work was researched in the context of two other World Bank projects: the Coastal and Marine Biodiversity Management GEF project and the Senegal River Basin GEF project

Oscilações climáticas Plio-Pleistocênicas e sua influência na prospecção de ouro na área do garimpo do Manelão (PA)

In the Manelão deposit the gold is associated with quartz veins hosted in the São Manoel metavolcano-sedimentary sequence. At period Plio- Pleistocene, under a humid climate, an autochthonous and immature lateritic cover was developed on the São Manoel sequence. This lateritic cover contains gold particules of high purity suggesting lixiviation or remobilization and redeposition processes of Au-Ag in a lateritic enviroment. During Pleistocene the climate changed to arid or semi-arid favouring the erosion of the lateritic profile through colluvial processes associated with periodical floods. This colluvial deposit cover lateritic profile destroying a possible geochemical disperson of pattern of the supergenic gold and harming the geochemical exploration surface. At the end of Pleistocene and beginning of Holocene the humid climate condition returned and associated with the intemperic processes form stone line and latosols. The lateritic and colluvial cover were the source area for the current alluvial material of the São Manoel river, in which the gold occurs free in the lower sedimentary strata and forms concentrations around 10 g/ton.No garimpo do Manelão, o ouro está associado a veios de quartzo encaixados na seqüência metavulcano-sedimentar São Manoel. No período Plio-Pleistoceno, sob um clima úmido, desenvolveu-se sobre a seqüência São Manoel uma cobertura laterítica autóctone e imatura. Essa cobertura residual contém partículas de ouro com elevada pureza, sugerindo processos de lixiviação da liga Au-Ag ou de remobilização e redeposição do Au em ambiente laterítico. Durante o Pleistoceno, o clima tornou-se árido a semi-árido, favorecendo a erosão parcial da cobertura laterítica, através dos processos coluviais associados a enxurradas periódicas. Esse depósito coluvial recobre o perfíl laterítico, destruindo um possível padrão de dispersão geoquímica do ouro supergênico e prejudicando a prospecção geoquímica de superfície. No final do Pleistoceno e início do Holoceno, o clima úmido retorna, juntamente com os processos de intemperismo, formando stone line e latossolos. As coberturas laterítica e coluvial serviram de área-fonte para os aluviões atuais a subatuais do igarapé São Manoel, onde o ouro ocorre livre nos estratos sedimentares mais basais, formando concentrações da ordem de 10 g/ton

Bauxites and laterites of the Serra de Trucara (State of Para, Brazil). Geological survey and bauxitic evolution. / Bauxites et latérites de la Serra de Trucara (Etat du Para, Brésil). Etude géologique, évolution bauxitique

The Serra de Trucara, to the west of the town of Tucurui, consists of two geological units of the Tucurui Group : basalts with minor interbedded petites (Caraipé Formation) and graywackes (Morrote Formation). Only on the Caraipé Formation, which is the most important unit of the Serra de Trucara, was developed a thick mantle of weathered and decayed ferroaluminous rocks, indurated on the top. The typical lateritic profile includes, from bottom to top, a lithomarge up to 10 m thick, a hardened aluminous level (2 to 4 m) capped by a ferroaluminous crust (2 to 5 m). These formations were affected by erosive episodes resulting in the formation of colluvial deposits which, in turn, underwent new pedogenetic transformations. Based on the initial analyses made of these formations, the following evolution can be suggested : (1) formation of a lateritic crust by alteration of basalts and petites ; (2) accumulation of alumina near the base of the ferrallite ; (3) formation and deposition of colluvium ; (4) deposition of the upper clayey level ; (5) incision by the most recent hydrographie system, associated with new phases of colluvium formation and aluminization.Bauxites et latérites de la Serra de Trucara (Etat du Para, Brésil). Etude géologique, évolution bauxitique La Serra de Trucara, à l'Ouest de la ville de Tucurui, est constituée par des basaltes à intercalations de pélites (Formation Caraipé) et des grauwackes (Formation Morrote) datés du Protérozoïque moyen à supérieur. Seule la Formation Caraipé, qui forme l'ossature de la Serra de Trucara, porte un épais manteau d'altérites ferroalumineuses cuirassées à leur partie supérieure. Le profil latéritique type comprend, de bas en haut, une lithomarge qui peut atteindre 10 m d'épaisseur et passe à un niveau alumineux induré (2 à 4 m) surmonté par une cuirasse ferroalumineuse (2 à 5 m). Ces formations ont été affectées par des épisodes érosifs qui se sont traduits par la formation de produits colluviaux à leur tour affectés par de nouvelles transformations pédogénétiques. Les premières études concernant ces formations permettent d'envisager l'évolution polyphasée suivante : (1) formation d'une croûte latéritique par altération des basaltes et des pélites intercalées ; (2) accumulation de l'alumine vers la base de la ferralite ; (3) formation et mise en place des dépôts colluviaux ; (4) mise en place du niveau argileux supérieur ; (5) incision par le réseau hydrographique plus récent, associé à de nouvelles phases de colluvionnements et d'aluminisation.Hieronymus Bernard, Kotschoubey Basile, Truckenbrodt Werner, Boulègue Jacques. Bauxites and laterites of the Serra de Trucara (State of Para, Brazil). Geological survey and bauxitic evolution. / Bauxites et latérites de la Serra de Trucara (Etat du Para, Brésil). Etude géologique, évolution bauxitique. In: Sciences Géologiques. Bulletin, tome 42, n°1, 1989. Altérations météoriques : comportement des oxyhydroxydes Fe, Al, Mn. pp. 3-14

Petrographic and geochemical study of the ironstones from the base of the Pimenteiras Formation, Western border of the Paraíba Basin, Tocantins

Ironstones that occur at the base of the Devonian Pimenteiras Formation (northwestern border of the Parnaíba basin) were investigated along the Xambioá-Vanderlândia and Colinas do Tocantins-Couto Magalhães sections. The ironstones consist of discontinuous decimeter-thick layers intercalated in sandstones and shales that underlie rocks of the Neoproterozoic Araguaia belt. The Xambioá-Vanderlândia ironstones are texturally and chemically quite distinct from those of the Colinas do Tocantins-Couto Magalhães area. Besides the presence of oolites, the former shows smaller amounts of terrigenous material (especially quartz) and higher iron oxide-hydroxides proportions. They also are more enriched in V, Sr, Zr and ΣREE, and poorer in Al2O3 and Rb. The North American Shale Composite (NASC) normalized REE distribution pattern is also different, especially concerning the IREEN values, which, being higher in the oolithic ironstones, display convex curves and, being lower in the non-oolithic ironstones, display concave ones. In the field, spatial relationships could not be defined between both ironstone types. It is then suggested that they represent different lithological facies of the same iron formation. Probably, the deposition of the non-oolithic facies occurred in deeper and quieter waters away from the continental border, where larger amounts of detrital sediments were discharged, whereas the deposition of the oolithic facies took place in shallower and more agitated waters, with less supply of terrigenous material. Iron was largely derived from the erosion of continental areas where reduced environments favored its mobilization and transport by rivers as suspended particles, colloids, and organic complexes.Ironstones, que ocorrem na base da Formação Pimenteiras (Devoniano), na borda noroeste da Bacia do Parnaíba, foram investigados ao longo dos perfis Xambioá-Vanderlândia e Colinas do Tocantins-Couto Magalhães. Esses ironstones formam camadas de espessura decimétrica, descontínuas e intercaladas em arenitos e siltitos, que jazem sobre rochas do cinturão Araguaia. Além da textura oolítica, os ironstones de Xambioá-Vanderlândia diferem dos de Colinas do Tocantins-Couto Magalhães por conterem menores quantidades de material terrígeno, notadamente quartzo, e maiores proporções de oxi-hidróxidos de Fe. São ainda mais enriquecidos em V, Sr, Zr e ΣETR e mais empobrecidos em Al2O3 e Rb. Diferem também no padrão de distribuição dos ETR normalizados ao North American Shale Composite (NASC), especialmente com relação aos valores de (ETRI)N, os quais, mais altos nos ironstones oolíticos e mais baixos nos não oolíticos, geram curvas convexas e côncavas, respectivamente. No campo, não foram estabelecidas as relações espaciais entre as duas variedades de ironstones, porém sugere-se que elas representem diferentes fácies da mesma formação ferrífera. Possivelmente, a deposição da fácies não oolítica ocorreu mais afastadamente da borda continental, em ambiente de águas mais profundas e calmas, onde foram descarregadas maiores quantidades de sedimentos detríticos; a deposição da fácies oolítica transcorreu em águas mais rasas e agitadas, com menor suprimento de material terrígeno. O transporte do Fe poderia ter resultado, em grande parte, da erosão fluvial de áreas continentais marcadas por ambientes redutores, o que teria favorecido a solubilidade daquele metal na forma de complexos orgânicos