Possible sources of the alluvial diamonds Udzha basin, northern Anabar region near kimberlite Tomtor field, Yakutia.

The NE of the Siberian platform in Udzha and Anabar river locate the richest alluvial placers of diamonds Since the discovery of placers > 700 were found, but no industrial bodies. A study of kimberlite magmatism has established that there are kimberlites of three ages on the territory of the North-East of the Siberian platform – middle Paleozoic (single), lower Triassic (few) and Jurassic-Cretaceous (prevailing). The latter are almost non-diamond-bearing.

The nearest kimberlite fields of Kuranakh and Tomtor are poor in diamonds. Some placers in the basin of the Udzha river, the right tributary of the Anabar, contain Cr-rich (≤14 wt.% Cr2O3) sub-calcic pyrope garnet associated with diamond. Comparison of kimberlite indicator minerals (KIMs) from the basins of Udzha and Chemara (its right tributary) shows similarity and a large diversity of pyropes, mostly of lherzolitic type. Cr- diopsides found in the Devonian collector suggest a close kimberlite source.

Mainly eclogitic placer diamonds are abundant in the upper reaches of the Chimara river in the northeastern part of the region. They occur in Permian, Jurassic, and Neogene rocks and in Quaternary alluvium where they coexist with pyrope and ilmenite. The diamonds in this region have mostly eclogitic features (Shatsky et al., 2015).

Reconstructions using monomineral thermobarometry (Ashchepkov et al., 2010) for the sources of pyrope and diamond show that the areas of the Anabar and Udzha placers share the similarity in the structure of mantle roots since 7.5 GPa, with a convective branch at the base.

The P-Fe trend for the Jurassic is slightly inclined, which is typical of the Kuranakh field. For the Devonian kimberlites, non-inclined trends are typical. The subcontinental lithospheric mantle (SCLM) beneath the Udzha basin is rich in pyroxenitic garnets as typical for the Anabar region.

There are 3 intermediate collectors of pyropes and associated diamonds: Permian, Jurassic and Neogenic and alluvium.  A study of the chemistry and thermobarometry of the kimberlite indicator minerals show some variations which possibly indicate different kimberlite sources ( Fig.1).

The detailed trace element geochemistry of the KIM from Udzha and Chemyra rivers show high variations and systematic differences.

Fig.1. PTX diagrams for kimberlite indicator minerals (KIM) from three correctors in the Udzha basin.

Fig.2 TRE distributions for KIM from Udzha alluvium

Fig.3 TRE distributions for KIM from Udzha alluvium

Supported by  RBRF grant 19-05-00788.