Аннотация:Diamonds and accompanying minerals in the north
Yakutia placers, Russia
Posukhova T*, Sokolova M
Lomonosov Moscow State University *tposukhova@mail.ru
Nowadays we have different hypotheses about formation of the diamondiferous
sediments at the North of the Siberia platform [1]: this is tuffisitic rock - unusual new
hosts of the diamonds; this is a square intermediate collector; this is a halo 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 [2]. 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. 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. These 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.
[1] Garanin V.K., Posukhova T.V. and Degtyareva A.V. (1994). Morphological
features of the placer diamonds from alluvial deposit of the Anabar river // Bulletin of
the Russian Mineralogical society, N1, pp. 71-80.
[2] Kudryavtseva G.P., Garanin V.K. and Posukhova T.V. (1996). Morphology of
diamonds from Russian deposits: Arkhangelsk Province, North Yakutia, Timan, Ural //
IMA-abstract, Toronto.