Jean-Claude Parisot

Analysis of poorly stratified lateritic terrains overlying a granitic bedrock in West Africa, using 2-D electrical resistivity tomography

Abstract Two-dimensional electrical resistivity tomography has been employed to investigate the subsurface structure of a thick lateritic weathering mantle overlying a granitic bedrock in southeastern Senegal. The resistivities were measured along two kilometric profiles insuring continuous coverage. Exploration pits exposed the different weathering layers, i.e., a saprolite, a mottled zone, a soft ferricrete and a ferricrete, whose respective thicknesses were used to constrain the measured apparent resistivity, despite their spatial variations. Colour-modulated pseudo-sections of apparent resistivity versus pseudo-depth including the groundsurface topography clearly show spatial variations in electrical properties of the weathering layers since their apparent resistivity changes faster than their respective thickness. The data from a cross-borehole survey along with estimates of resistivity for aquifers and granite were integrated into the pseudo-sections to provide more useful results about the real resistivity ranges of the weathering layers. The resulting geo-electrical images document the geometric relations between the different layer boundaries, in particular those of the aquifers with the bedrock and groundsurface topographies. The spatial relationships between the granitic bedrock and groundsurface topographies suggest that a large part of the actual lateritic weathering mantle is allochthonous. This also implies that the actual topography of the bedrock surface was mainly shaped by weathering processes while the hillslope geomorphic patterns result from erosion processes or lateritic weathering of reworked materials leading to ferricrete development according to the different landforms observed. It is suggested that climatic changes were implied in the landscape evolution of our study area.

Chemical and physical transfers in an ultramafic rock weathering profile: Part 1. Supergene dissolution of Pt-bearing chromite

Abstract Chemical weathering and supergene dissolution processes of Pt-bearing chromite have been studied in a lateritic weathering profile developed on ultramafic rocks in New Caledonia (southwest Pacific). The chemical distributions of alkaline earth, transition metals, and precious metals (including Pt and Pd) were determined in a weathering profile varying from bedrock at the base upward through coarse and fine saprolites, and capped by a mottled zone and a lateritic colluvial nodular horizon. Chemical analyses and mass-balance calculations suggest that progressive weathering of the parent rock is characterized by an enrichment of Fe, Co, and Mn, a segregation of Ni at the boundary between the bedrock and the coarse sapolite and in the lower part of the fine saprolite, and a depletion of Mg, Ca, Si, Al, and Cr. The higher concentration of transition metals at the interface between the coarse and fine saprolite is due to vertical transfer and precipitation at the base of the weathering profile. In such a lateritic environment, the Pt-bearing chromite grains are progressively dissolved and the Pt-group minerals (PGM) are released in the weathering mantle with a preferential depletion of Pd with regard to Pt.

Ferricrete genesis and supergene gold behaviour in Burkina Faso,West Africa

The lateritic weathering process of Precambrian albitite formations containing gold sulphide mineralization has been studied at Larafella, southern Burkina Faso. The regional landscape consists of three land surface units with respective altitudes of 300 m, 280 m and 265–250 m. Plateaus on a barren schist formation form the first two upper units; there are remnants of two old planation surfaces capped by ferricretes. The third unit is a large recent pediment covered by a soft ferricrete. Within the weathering profile of this lower surface, preservation of the parent rock structures, Au and As content, Au particle morphology and the Ag content indicate the inheritance of the parent mineralized rock. The preservation of unweathered sulphides within the soft ferricrete and their absence in the underlying saprolite indicate that the ferricrete formed on a stripped surface at the expense of a slightly weathered rock, and preceded the formation of the clay horizons and the deepening of the weathering profile. Some sulphide grains and Au particles affected by transportation marks are also present within the ferricrete of the upper units. Transported bauxitic and ferruginous debris of previous lateritic formations have also been described. The upper units are accumulation surfaces, each with 8–10 m of reworked material on the bedrock. The results provided here improve our knowledge of the geomorphic dynamics that can be useful for Au exploration in lateritic environments.

Gold: a useful tracer in sub-Sahelian laterites

Sequential and connected lateritic sequences from the Tertiary to present day can be observed in sub-Sahelian conditions in northern Burkina Faso (Michel, 1973; Grandin, 1 975 ). Here, complex iron auriferous crust systems have developed on the stable West African Craton from Proterozoic auriferous greenstone belts and volcanic tuff. In order to distinguish between chemical and physical processes which have led to the development of the lateritic systems, we propose to use visible gold particles as pathfinders. In this paper, we focused on the lowland lateritic system (profile of pit P4) located downslope to the middle land lateritic system, where gold is associated with hydrothermally altered quartz veins and pebbles (Fig. 1 ) .

Evolution des particules d'or en milieu latéritique soudano-sahélien : alluvions et colluvions dans l'environnement d'un gîte filonien, Piéla, Burkina Faso

On a barren bedrock, transported auriferous materials of colluvial and alluvial origin present differences in the distribution, composition and morphology of gold particles. Under Sudano-Sahelian conditions, transport processes shape evolved particles in short distances.