Distribution and characteristics of kimberlite indicator minerals from the Aptian intermediate hosts of the Voronezh anteclise

Abstract

Introduction: Within the Voronezh anteclise (VA), numerous grains of kimberlite indicator minerals (KIMs) have been found in terrigenous deposits of different ages. The available KIM findings are distributed extremely unevenly, both in terms of area and the section. The deposits of the Aptian age in the territory of VA are the most promising for the study since they contain a record number of KIMs. Methodology: In 2012–2018, during further geological studies at a scale of 1:200,000, 32 heavy concentrate samples from the Aptian deposits were collected and examined. The typomorphic features of indicator minerals were studied using a binocular microscope. The chemical compositions of the KIMs were determined using a Jeol 6380 LV electron microscope equipped with an Inca-250 energy-dispersive quantitative analysis system (Voronezh State University). A facies map of the Aptian age was compiled using the data collected by the author during the further geological study of the territory of the VA and the data provided by his predecessors. Results and discussion: KIMs in the formations of the Aptian stage are dominated by chromespinelides. Their number in the Tulskoe, Dubovets, Koshkino, and Lebyazhie localities reaches several thousand signs per sample. The studied grains are characterised by wide variations in their chemical compositions and a significant number of high-chromium varieties. In the genetic diagrams by N. V. Sobolev and C. Fipke, chromespinelides are characterised by a kimberlite trend, while some of the composition points fall into the area of the diamond association. In the Lebyazhie locality, about 32% of the analysed grains in the diagram by N. V. Sobolev correspond to chromespinelides from inclusions in diamonds. Such a ratio is typical for highly kimberlite bodies with a high diamond yield. The sections contain grains of mantle chromespinelides represented by combinatorial crystals and octahedrons with vicinal faces. Conclusion: As a result, the formations of the Aptian stage of the Voronezh anteclise were studied. Kimberlite indicator minerals, chromespinelides, and small diamonds were isolated from the deposits of the Kriusha and Volchinsk formations (continental alluvial sands) and the shallow-marine formations of the Zusha stratum. The revealed KIMs indicate the presence of diamond-bearing bodies within the Voronezh anteclise which were located to the south-east of the localities containing indicator minerals.