Ultrathin gold in the sedimentary cover of the Voronezh anteclise

Abstract

Introduction: The previous works of the authors confirmed the presence of ultrathin gold (UTG) in the sedimentary cover of the Voronezh anteclise (VA). The new data studied in this work has been obtained over the past 15 years in the area of anteclise while performing GDP-200 (additional site exploration). Methodology: It was possible to identify the UTG particles with the size of a few dozen micrometers using MKTS and TSVK-100 concentration units. For the determination of the elemental composition a local X-ray spectral analysis (OXFORD INS system). Some of the samples were studied using Au-spectrometry in the laboratories of TSNIGRI, Voronezh State University, and IMGRE, and satisfactory correlation between these two methods was obtained. In total, over 600 samples were analysed, and 170 of them contained UTG. The identified gold grains were studied using microscopical methods as well as X-ray spectral analysis of the elementary composition. About 300 analyses were performed to determine the content of Au in the gold grains. Results: New locations of UTG were found on different stratigraphic levels. The increased content of Au was discovered in the upper part of the sedimentary cover. Joint presence of UTG and intermetallic compounds is typical for the discovered locations. Maximum concentrations of UTG were found within the sections with increased permeability in the areas of tectonic disturbances and on geochemical barriers represented by clay and ferritised rocks and phosphorites. Results and Discussion: The geochemical features of the gold grains are the almost constant presence of the platinum-group metals (PGMs), silver, and lead. A great quantity of volatile elements, mostly mercury, is also typical. Сu and Zn prevail in the intermetallic compounds without gold. The content of gold in phosphorites is uneven, reaching 0.4 g/t in some cases. Conclusions: The analysis of genetic features of the UTG of VA allows classifying it as being sedimentary exhalative, which is also known in other regions of Russia and around the world. The collected data on UTG, its relation to the endogenous processes, and confinedness to the areas of tectonic disturbances allowed designing a forecast map for the search of this metal within the area of VA.