Siliceous minerals in kimberlites

Abstract

Introduction: Several thousand kimberlite bodies (pipes, dykes, and veins) of various shapes and sizes have been discovered in cratons all over the world. The structure of tubular (diatreme) structures consists of: a) a funnel or funnel-shaped expansion, which in case of non-eroded volcanic edifices is topped with a ring swell (crater zone); b) a vertical vent (diatreme zone); c) a feeder in the form of one or more dyke bodies (root zone). Each of these zones of the kimberlite structure is characterised by a particular morphology, textural and structural features, and material composition of rocks and the mechanism of formation, which allows considering each of the identified three zones of the kimberlite system as an independent geological feature. The concentration of secondary minerals (including the intensity of rock silification) is usually higher in the upper parts of the diatremes due to the influence of the host rock material and an increased degree of fracture density, as well as due to their reconversion under the influence of hypergene processes. Оbjects, research methodology and discussion of the results: Quartz and chalcedony are quite characteristic secondary minerals for kimberlite rocks in cratons. Their highest concentrations are usually associated with the upper parts of the diatremes affected to different extents by hypergene processes. There are both colourless quartzes and quartzes with various shades of brown, grey, and purple. The presence of amethyst-like precipitates among quartzes in the upper parts of kimberlite pipes allows attributing the majority of silica minerals present in such rocks to low-temperature formations, which has been confirmed by thermobarogeochemical studies of the crystals. The comparative study of quartz revealed that the most informative are its luminescent characteristics which show the features of microdefectiveness of these natural formations in terms of structure and impurities and, accordingly, the physicochemical parameters of quartz-generating media. Chalcedony is a fairly common mineral in kimberlites (especially in the upper parts of diatremes), which is often present in the lower parts of quartz crystalline druses and acts as a kind of substrate. Chalcedony often forms phantom crystals of other minerals. The crystallisation conditions for local nest-like quartz precipitates (including amethyst-like) were significantly influenced by the host rocks in the diatreme and the composition of fluids involved in the hydrothermal metasomatic transformation of kimberlites. Conclusion: The study revealed a large variety of quartzes in kimberlite rocks in the Siberian and other cratons all over the world. Particularly there were many concentrations of quite large druses of purple amethyst quartz which were found during the development of the upper horizons of the diatreme. The kimberlites of the lower horizons in the same pipes had fewer voids and almost no quartz mineralisation. This allows us to state that quartz druses (especially amethyst-like) were formed during hypergenesis due to silica mobilisation and deposition. The fact that crystalline druses of quartz with calcite inclusions predominate in the upper horizons of diatremes is of great practical importance since it emphasises the degree of denudation of the pipes, and during erosion and redeposition in sedimentation basins, specific mineral precipitates (especially their amethyst-like varieties) can be used to determine the provenance ar ea of magmatic material to conduct stratigraphic and paleogeographic reconstructions. Intensive hydrothermal-metasomatic processes in kimberlite diatremes were not enough to create large cavities with freely growing quartz crystals. The crystallisation conditions for local nest-like quartz precipitates (including amethyst-like crystals) were strongly influenced by the host rocks in the diatremes as well as the composition of the fluids involved in the formation of mineral associations which quartz grew on. The strong local influence of host rocks in diatremes in the absence of large cavities, low partial pressures of CO2 in combination with low crystallisation temperatures determined the specific features of microdefectiveness in terms of structure and impurities, which ultimately resulted in reduced values of the luminescent characteristics of quartzes from kimberlites (including amethyst-like) as compared to minerals from other sources.