Features of the formation of placer gold in the Agenda river section (Bauntovsky Evenki district of the Republic of Buryatia)

Abstract

Introduction: It is a common view that the most important factor for the formation of placer gold deposits is the destruction of mineralized bedrocks and the transfer of the goldbearing material followed by its sedimentation. Recently, several studies have appeared focusing on “covert” nanosized gold. Following changes in geological, geochemical, and geomorphic conditions and factors, nanosized gold tends to self-assemble into macroscopic particles and then into kernels, flakes, intergrowths, and even nuggets [1‒5]. It is important to investigate such processes in order to complement or reconsider the existing models of formation of placer gold deposits.

The geological structure of the region: From the point of view of orography, the Agenda licensed area is located in the North-West part of the Vitim Plateau. The territory has a complicated landscape and is located in the junction between the fold rim of southern Siberian platform, the Aldanian bend, and the Zabaykalsky region of Late Paleozoic-Mesozoic activation.

The purpose of the study was to investigate the goldbearing potential of the Agenda river valley and to determine and assess potential sources of gold. While doing this, it was important to consider various viewpoints with regard to the formation of placer gold deposits in the studied region and the general mechanism of the formation of placer gold deposits with indefinite primary sources.

Methodology: In our study, we performed a mineralogical analysis of the free gold content in the heavy fraction under binoculars and studied the chemical composition of gold by means of scanning electron microscopy and X-ray spectral microanalysis using a JEOL-6380 scanning electron microscope and a JNCA-250 microprobe.

Results: The appearance of the analyzed gold particles and their chemical composition indicate that all the gold in the Agenda river is a stable solid solution of gold and silver in a ratio from 85:15 to 95:5. Chemical zoning and the presence of impurities of other chemical elements indicating growth or self-purification of gold were not registered. The geomorphology and chemical (mineral) composition of the surface of the gold grains indicate that the material has been in the hypergenesis zone long enough to form the placer deposit.

Conclusion: The source of gold in the studied region were poor veins, ore mineralization clusters, and localised areas of quartz veining, which could provide for the actual sources of placers. Apparently, the source is largely contributed to by nanosized gold, which under hypergenesis and low hydrodynamics is capable of self-assembly. Due to the dynamic environment and increased mobility of gold nanoparticles [4], most of the gold easily migrated down the river to the terminal basin. The study presents a new approach to the transition from a valent chemical element (usually nanosized) to a visible gold particle (zero-valent gold) in the vicinity of a definite source and in a stable environment. It can help to elaborate the existing models of formation and adjust the extraction techniques used in such deposits.