Genetic Features of Variety III Cuboid Diamonds from Placers of the Northeastern Siberian Platform

This paper presents the results of a comprehensive study of a cuboid diamond of variety III according to the mineralogical classification of Y.L. Orlov, which was first discovered in Carnian (Upper Triassic) deposits of the Bulkur anticline in the northeastern Siberian platform. It is established that the crystal has a cubic shape with signs of intense dissolution and is characterized by a zonal–sectorial fibrous internal structure. The central area of the diamond is saturated with microinclusions. The studied cuboid diamond belongs to the IaAB type according to IR spectroscopy data. An accumulation of minerals, which is represented by chamosite (Fe-rich chlorite), quartz, and pyrite, as well as rare native metals (Fe, Cu, and Ag) and intermetallides (chromferide), is present on the diamond surface. The chemical composition and morphology of chamosite indicate its low-temperature hydrothermal–diagenetic origin (50–150 °C, pressure < 1 kbar) in the marine or lagoon sedimentary environment of the rift basin of the Siberian platform during the Triassic. The discovery of a diamond of variety III, characteristic of large industrial kimberlite pipes (Mir, Udachnaya, and Aikhal), in placers of the Leno-Anabar diamond-bearing subprovince indicates a possible unknown primary kimberlite source.