Heavy fraction minerals from altered kimberlites of the Сatoca field (Angola)

Abstract

Introduction: The article presents the results of a comprehensive study of the features of the composition and distribution of the heavy fraction from the kimberlites of the Catoca field (north-eastern Angola). The reference object of this study is the kimberlite pipe. In the weakly eroded Catoca kimberlite pipe with a preserved crater structure, a wide petrographic spectrum of rocks was determined. The rocks were often intensively altered as a result of hypergene processes with the almost complete destruction of relict minerals and shading of the primary structural and textural features. Methodology: In order to study the features of the distribution of the heavy fraction in the sections of kimberlite rocks and the concentration rate of deep minerals in the weathering zone, we analysed a representative amount of data on the recovery of core samples. We studied the features of the heavy fraction distribution in more than fifty sections of wells, which exposed the Catoca field kimberlites. For analysis, we used the results of recovery of core samples at the concentration plant, for comparison, we also used the data of studies of crushed bulk samples and lump ore samples. Results and discussion: Based on the complex of features, we found rocks of three facies: hypabyssal (subvolcanic), vent (or diatreme), and crater. Each of them is characterised by its own ratio of different types of magmatic (hypabyssal and vent facies) and volcanogenic-sedimentary (crater facies) formations. Conclusions: The noted features of the composition of the rocks filling the diatremes of the described region led to certain changes in the content and depth distribution of the heavy fraction. The most important of the fraction in terms of prospecting are the kimberlite indicator minerals (KIMs). Garnets (mainly pyrope), picroilmenite, and clinopyroxenes are the most preserved minerals in the studied kimberlite rocks. The highest concentrations of heavy minerals (including KIMs) are found in the vent facies. In general, this parameter is typical for the diatremes of the Catoca field. This highlights the possible high perspective of the use of the blacksand mineralogical method of prospecting for kimberlite pipes in this zone and other territories similar in geological structure.