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 lowgrade
 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.