ABSTRACT

A black granule layer in the Precambrian West-Congolian Mountain Range has been studied. Situated in a calcareous and dolomitic unit of the "Schisto-Calcaire" group of the West-Congolian System, this layer, called the "Pseudo-Oolithe de Kisantu," is found in all this part of southwest Africa. It is composed of millimeter-sized spheroids or ovoids of pure talc in a siliceous matrix, with minor amounts of calcite, dolomite and sulfate. These oolites have a homogeneous chemical composition and do not contain organic matter. Their oxygen isotopic composition (+18 versus SMOW) is within the range of low-temperature clay minerals and outside the composition range of hydrothermal talcs. After comparison with other authigenic Mg-clays found n salt-bearing or evaporitic environments, the proposed origin for this oolitic talc layer is a two-step process: 1) direct precipitation of stevensite or sepiolite at low temperature in shallow marine, low salinity waters; 2) transformation of the stevensite (or sepiolite) into talc during diagenesis.