ИСТИНА

Nowadays we have different hypotheses about formation of the diamondiferous sediments at the North of the Siberia platform (Grakhanov et al., 2010): this is tuffisitic rocks – unusual new hosts of the diamonds; this is a square intermediate collectors; this is a halos of unknown kimberlitic bodies. Unusual diamond crystals of the “Ebalyakh types” were found in these rocks (V and VII types according to Yu. L. Orlov, 1985). We examined morphology and physical properties of diamonds from different placers of Northern Yakutia (Kudryavtseva et al., 1996). The proportions of external forms vary for diamond crystals from different sources. The rhombododecahedra dominate (35.5-70 %) and aggregates are abundant (25.5-35.5 %). Degree of resorption is high. Colorless crystals of the Ia type predominate. Their spectra have characteristic series of lines corresponded to the A and B nitrogen defects. Brownish color of crystals is connected with plastic deformation of crystals and existence of C-defects. We examined minerals accompanying diamonds from Triassic sediments of the river Bulkur at the mouth of the Lena River. They are characterized by high content of pyropes, very high content of diamonds, anomalous content of the anatase, absence of quartz in the light fraction. Garnet-chromite association with leucoxene and chlorite is typical for the heavy fraction. Minerals were investigated with different methods: IR-spectroscopy, SEM, X-ray structural and micro-beam analysis. Our investigation shows, that the rocks are composed of oval and clastic grains of quartz, rutile, feldspars, pyrite, chlorites (fig). These grains are cemented by cryptocrystalline aggregates of phyllosilicates (chlorite, montmorillonite, illite, and others). Whole grains have traces of mechanical wear, the remains of diatoms are revealed. According to investigating data we can conclude that studied sediments were formed as polymictic conglomerates and haven’t the volcanogenic materials. This complex rocks contain rounded and fragmented mineral grains, most of which bear the traces of mechanical processing which are typical for coastal marine sediments. The composition of the garnet was investigated and their distribution on different chemical-genetic groups was established. High quantity of garnets from ilmenite-bearing and eclogitic rocks was established. Such almandine garnets are not typical for lamproites and group II kimberlites, and they are not stable in the supergene processes. As a result we can conclude that the studied Carnian deposits are the collectors of xenogeneic kimberlitic minerals. The minerals with low chemical and mechanical stability were established in collector and this is not typical for ancient haloes with long supergene history. It is suggested that the Triassic placers could be formed by direct erosion of the close indigenous sources.