Diamonds from low-grade kimberlite

Abstract

Based on exploration and sampling work on the Siberian platform (SP) the uneven diamond content of various diatremes was established. During the exploration apart from the pipes with high or medium content of a useful component also the diatremes with low to very low diamond content were discovered. To obtain representative data the diatremes of two kimberlite fields of the Central Siberian (CS) and six kimberlite fields of the Lena-Anabar (LA) diamondiferous sub-provinces (DSP) were studied. A comprehensive study of diamonds from low-grade pipes of the Alakit-Markhinsky and Daldynskoye kimberlite fields of CSDSP was carried out. 16 kimberlite bodies from six kimberlite fields from the LADSP were studied. Geology and mineral-petrographic composition of kimberlites, crystal-morphological and physical properties of diamonds from certain prospective areas were investigated. In addition to classifying diamonds into specific varieties according to Yu. L. Orlov (1984), for many crystals, we examined impurity centers, the ratio of the concentration of optically active defects, spectra of absorption of diamonds IR and UV for the visible regions. For many grains we determined the isotopic composition of carbon; carried out analyses using the electron paramagnetic resonance (EPR) method; measured the photoconductivity upon excitation with visible light; analyzed photoluminescence spectra; studied the internal structure and composition of the mineral inclusions. The obtained results were compared with similar materials for the known primary (and sometimes alluvial) deposits of the region. Based on the study of the typomorphic features of diamonds from low-grade kimberlite pipes from selected fields of the SP we conclude that they are characterized by a high content of typical rounded minerals of the Ural (Brazilian) type with shagreen and plastic deformation bands. This indicates a significant dissolution of the mineral in the kimberlite of such diatremes, which distinguishes them from similar bodies with high diamond content and makes it possible to predict the grade already at the initial stages of exploration work. Increased content of small semitransparent smoky-brown rounded diamonds with shagreen and plastic deformation bands for crystals from veins was noted, which is also typical for low-grade pipes.

Keywords

• kimberlite pipes
• typomorphism of diamonds
• low-grade
• siberian platform