Features of the weathering crust of the kimberlites of north-east Angola

Abstract

Introduction: The results of a comprehensive study of the geological structure and material composition of the weathering crust of the kimberlites in north-east Angola are presented. An important applied problem is the assessment of the degree of enrichment of the weathering crust by deep minerals for forecasting and searching for new diamond deposits of both primary and placer types. Methodology: The Catoca kimberlite pipe, characterized not only by an insignificant erosional section and preservation of the crater structure, but also by a wide petrographic spectrum of rocks and their intense post-magmatic and hypergenic variation, was chosen as a reference object. The collection of ore samples and flow samples from the upper horizons of the Catoca pipe and a number of other bodies of this field, as well as rocks containing kimberlites, were collected for the study of the geological structure and material composition of weathering crusts of weakly eroded Angola kimberlite pipes. Results and discussion: Two genetic types of rocks were distinguished in the Catoca kimberlite pipe: kim- berlites with their derivatives and a complex of volcanic-sedimentary formations filling the upper parts of the crater of diatreme. In general, the profile of the weathering crust is divided into two parts: lower and upper. The lower part (disintegration zone) is represented by slightly altered autolithic kimberlite breccias, fractured and partially ocherized. The upper, the most hypergene-altered zone, in turn, is divided into carbonate and non-carbonate parts. Conclusions: Two processes of changes in the kimberlite rocks of the region were revealed. The first process is the phased conversion of olivine to high-temperature, then low-temperature serpentine. The second process is the degradation of serpentine with the simultaneous emergence of a series of swellable minerals, starting with saponite through a series of intermediate metastable mixed-layer formations, out of which the ordered variety of lyzardite-saponite, identified for the first time, was relatively more stable compared to other ordered mixed-layer phases of the trioctahedral type. This finding emphasizes the specific conditions of crust formation of the weathering kimberlites of the African region.