Abstract
Two weathering profiles developed over disseminated Cu mineralization hosted by granodiorites (porphyry type) and felsic volcanics respectively, in a savannah tropical environment (Burkina Faso) have been studied in detail. A mineralogical and geochemical study was carried out in order to determine the characteristics of both profiles and the behaviour of Cu in such deeply weathered environments. Our investigation was focused on the upper part of the weathering profile, respectively 4.0 and 10.5 m below the surface. The mineralogical study reveals that in the first case (profile A) the predominant clay minerals are smectites and kaolinite while in the second (profile B) a more kaolinitic composition is indicative of more severe leaching. In fact, field observations seem to demonstrate that the latter situation is more clearly related to an ancient lateritic-type weathering while the first one results from more recent processes. In both cases the Cu contents through the profiles are high (several thousands of ppm) and in good agreement with the grades obtained in depth, in the mineralized rock. Nevertheless, some leaching can be observed in the upper soil horizons, but the contents still remain highly anomalous, in the 1000 ppm range. It is shown that Cu is distributed in the main secondary minerals constituting the weathering products, whether they are silicates (smectites, phyllites, kaolinites) or oxides (goethite, hematite, Mn oxides). The main stable Cu-bearing mineral seems to be the kaolinite: indeed, smectites turn into kaolinite in the upper part of the profiles while goethite seems to be depleted in Cu under the same conditions. As concerns geochemical exploration, two observations can be noted. Firstly, Cu is very stable in such supergene environments, and secondly, the best size fraction in which to detect the Cu secondary dispersion haloes in soil or stream sediment samples is the <63 μ fraction, in terms of anomaly intensity or contrast.
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