Typomorphism of Placer Gold and Prerequisites for the Discovery of Gold Ore Occurrences in the Southern Part of the Lena-Anabar Trough (North-East of The Siberian Platform)

Abstract

The article examines the results of exploratory work carried out in the southern part of the Lena-Anabar trough in the north-eastern part of the Siberian platform. Alluvial deposits of the Polovinnaya river, which belongs to the Uele-Udzhinsky placer cluster, were sampled for predictive assessment of commercial prospects for placer occurrences of gold. The mineralogical-geochemical features of the placer gold were studied in order to identify potential primary sources. It is proved that the Mesozoic and Cenozoic deposits are the main sources of gold. Its typomorphic features are small size (-0.5 mm), toroidal and flake shape, shagreen surface with casts of pressing of the minerals, and high fineness (950-999 %). The internal structures of gold particles are represented by thick high-grade shells, translation lines and high-grade intergranular veinlets. Along with this, the discovery of the autochtonous gold of ore habit in the studied gold area suggested the presence of an additional primary source. Very small size (-0.2 mm) and angular-lumpy shapes of the individuals, almost unrounded surface and low fineness are typical features of slightly altered gold. The discovery of small (3-4mm) fragments of hydrothermal-metasomatic rocks in the alluvium of the Polovinnaya river confirms the above assumption. The epigenetic mineral composition of these fragments is represented by pyrite, potassium feldspar, fluorapatite, and quartz. Micro-X-ray spectral analysis determined the smallest (first microns) mineral phases of native gold, argentite, galena, sphalerite and greenockite in them. It is worth noting that the described fragments are quite fragile, and they are destroyed with a slight impact with a steel needle. The fragility of these fragments indicates the close proximity of the rewashed ore occurrences. In addition, increased gold content in small classes of heavy concentrate material of small-volume samples was determined according to atomic absorption analysis. All this together is a prerequisite for the discovery of ore occurrences with finely-dispersed and fine gold. It is assumed that the ore occurrences can be localized in the Anabar-Eekit fault zone, which extends parallel to the Pronchishchev ridge, through the Anabar-Olenek trough, to the Sololiy uplift of the Olenek arch. In the area of the Olenek arch, this zone is associated with superimposed gold-bearing low-temperature hydrothermal mineralization, expressed by K-feldspathization and sulfidization of the Permian deposits.