Typomorphic features of garnet xenocrysts from the crater and diatreme parts of the high diamondiferous V. Grib kimberlite pipe (Arkhangelsk diamondiferous province): application in prospecting and exploration for diamonds in the north of the East European platform

Abstract

Research subject. Morphology of garnet xenocryst grains from the crater and diatreme parts of the highly diamondiferous V. Grib kimberlite pipe of the Arkhangelsk diamondiferous province. Aim. To reconstruct the main stages of endogenous and exogenous morphogenesis of garnet xenocrysts from the crater and diatreme parts of the kimberlite pipe based on a detailed study of their morphological features. To demonstrate the applicability of the data obtained in the interpretation of the morphology of grains of kimberlite indicator minerals from modern alluvial sediments during prospecting and exploration for diamonds in the north of the East European Platform. Materials and methods. A detailed study of the morphology of 655 garnet grains was carried using the methods of optical and electron-scanning microscopy. Garnet grains were selected from the heavy mineral concentrate of crushed rocks, representing the crater (sandstones) and diatreme (kimberlite) parts of the V. Grib pipe. All visually diagnosed garnet grains were handpicked selected from the rocks of the crater part, extracted from the core of one borehole and representing an interval from 70 to 174 m from the surface. For comparison, out of 1100 grains of garnet xenocrysts, selected by random sampling from the concentrate of minerals of the heavy fraction of kimberlite of the diatreme part, 180 garnet grains were selected, representing four-color groups: purple, red, red-orange and orange. Results. The majority of garnets from the crater (96%) and diatreme (89%) parts of the V. Grib kimberlite pipe were shown to exhibit the “primary” magmatic morphology. Nevertheless, the garnet grains with well-developed pyramidal shingle-like (3% in the crater and 11% in the diatreme) and drop-shaped (1% in the crater) reliefs have been identified. These types of garnets were formed under the influence of postmagmatic processes. For the first time, in the deep diatreme part of the pipe (more than 200 m from the surface), composed of dense kimberlite, garnets with a well-developed pyramidal shingle-like relief were found, which had been previously interpreted for the East European Platform only as components of the secondary deposits or the upper horizons of the crater parts of the pipes. Conclusions. Garnets with a well-developed pyramidal shingle-like relief, found in modern alluvial sediments, can be interpreted as from a secondary deposit only if they show signs of mechanical abrasion; the absence of traces of mechanical abrasion on such grains does not allow us to exclude the proximity of the primary source. The presence of drop-shaped relief garnets in the crater of the V. Grib pipe (150-168 m from the surface) also calls into question the unambiguous interpretation of such grains as a sign of secondary deposits: only the maximum degree of chemical abrasion of garnets with the formation of cuboids grains can indicate their redeposited nature. The results confirm the need to improve the method of prospecting for diamond deposits according to the typomorphic features of kimberlite indicator minerals recovered from the modern sediments, sampled in the north of the East European Platform.