FEATURES OF THE CRUST OF WEATHERING ON THE KIMBERLITE PIPE OTHINGAU (SOUTHWEST ANGOLA)
Keywords:
kimberlite pipe, crust of weathering, perovskite, saponite, spinellids, ilmenite, geothermometer, Angola.Abstract
The results of petrographic, X-ray diffraction and microprobe studies of the rocks of the up-per horizons of the Othingau kimberlite pipe (Southwest Angola) are presented. The rocks of the crust of weathering are predominantly represented by altered massive kimberlite breccias of the brownish gray colour and porphyry structure. The matrix of the rock is composed of a small car-bonate-smectite aggregate, in which monoclinic pyroxenes, potassium feldspar, perovskite, il-menite, spinellids, magnetite, zircon, apatite have been diagnosed. There is a high content of perovskite, which indicates a specific mineral composition of the kimberlite pipe. Smectite is represented by a trioctahedral species – saponite, which is a more acidic mineral than serpentine, so a more acidic medium is needed for its formation. It is possible that postmagmatic solutions, when interacting with xenogenic material, were enriched with silica and, with their further expo-sure to olivine or its substitution products, saponite was formed. The chemical composition of the rock reflects the nature of superimposed mineral low-temperature transformations and the presence of xenogenic material. There is an increased con-tent of CaO due to the presence of calcite in cement and saturation of feldspars with fragments thereof. The presence of syngenetic intergrowths of ilmenite and magnetite, as well as grains with the structure of the solid solution magnetite–ilmenite decomposition, made it possible to calculate the temperature of mineral formation by the Donald Lindsley geothermometer, which is 484– 546 °C in the first case and 593–637 °C – in the second.
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