Natural Amalgam of Gold in Placers of the Ebelyakh Area -Indicator of Tectono-Fluid Activation (North-Eastern Siberian Platform)

Abstract

The results of studying the typomorphic features of placer gold in the unique diamond-bearing placers of the Ebelakh river basin (Anabar diamond-bearing region) are given. Detailed study of the morphology, micro-relief of the surface and internal structure of the gold and photographing was performed using a scanning electron microscope JEOLJSM-6480LV. The gold fineness was determined using the Camebax-Micro microanalyzer and the JEOL OXFORD INCA-sight energy spectrometer. The internal structure of native gold was studied by etching gold in mounted thin sections using a proven method, using a reagent: HCl + HNO3 + FeCl3 × 6H2O + CrO3 + thiourea + water. It is determined that, modern placer occurrences were formed mainly due to the redeposition of fine high-grade gold from ancient gold-bearing intermediate sources. At the same time, in the placer of the Kamenisty creek a natural amalgam of gold is discovered, which is an aggregate consisting of several cemented rounded gold particles of small size (-0.25 mm) of various morphology. The cement is superimposed medium-grade gold with Hg content of up to 13.3%. Its internal structure is porous, and zoning is noted due to the uneven distribution of Hg. The surface of mercury gold is characterized by a brain-like microsculpture. Along with this, in some placers of the studied area, it was possible to find small (>0.2 mm) gold particles with a specific surface with the thinnest (first microns) fringe of mercury (ртутистого) gold. Obviously, these are single grains of the disintegrated aggregates of auro amalgam. We also found gold particles in an aluminosilicate film of variable Al, Si, K, Fe composition, and the smallest (1-2 microns) mineral phases of gold, sphalerite, galena, zincite and zinc-containing iron are identified on this film. It is assumed that natural amalgamation of placer gold occurred as a result of migration of mercury-containing gold-bearing hydrothermas on fault zones during tectonomagmatic activation. Based on the above-mentioned, it is assumed that there are adjacent sources of supply - low-temperature hydrothermal ore occurrences confined to the faults and formed as a result of tectonomagmatic activation processes. This proposition is confirmed by the identification of hydrothermal-metasomatic formations of K-spar-quartz-pyrite composition with disseminated gold-sulfide mineralization on the studied territory. The similarity of geological conditions of localization and the material composition of apocarbonate hydrothermal-metasomatic formations of the Ebelyakh area with the gold-bearing rocks of the Central Aldan ore district allow to estimate positively the prospects for their further study.