The study of the basal horizons of the sedimentary cover of the Voronezh anteclise: a path to forecasting its primary diamond potential

Abstract

Introduction: The forecasts concerning the diamond potential of ancient cratons are dominated by the concept that kimberlite diatremes tend to be found in the areas where the most powerful Archaean (older than 2.5 billion years) crust had developed. The ore body of Voronezh diamonds, similar to other diamond-bearing provinces, should be localised within these regions. Small diamonds within the Voronezh anteclise have only been found at different depths in rocks of basal horizons of the sedimentary cover. Diamonds can be accompanied by kimberlite indicator minerals, the key ones of which include the following: pyrope, picroilmenite, chrome-spinelide, and chrome-diopside. In order to reach the ore bodies of diamonds, both the study of accessories and a facies analysis of the basal horizons are necessary. Methodology: In 2004–2006, in various parts of the anteclise, over 50 reference samples 0.25–10 m3 in volume were taken from Paleozoic, Mesozoic, and Cenozoic aleuro-sandy rocks. They were processed in the Nedra Scientific Production Enterprise (Simferopol), where kimberlite indicator minerals were isolated from them. The typomorphic and geochemical features of the isolated kimberlite indicator minerals were studied using a binocular microscope and an electronic microscope with a Camebax 50-SX X-ray microprobe analyser (Moscow State University). The geological survey (GDP-200) conducted in 2003–2017 included over 250 steam sediment samples from Devonian, Carboniferous, Jurassic, Cretaceous, Paleogene, and Neogene deposits. The initial volume of the bulk sample was 20 litres (0.02 m3). The samples were washed on a Siberian tray in the following three stages: elutriating of samples, washing off light particles, and refining the concentrate. The mineralogical analysis was preceded by screening for particle size classes: +1.0 mm, –1.0 + 0.25 mm, –0.25 + 0.1 mm, and –0.1 mm; the division of the material in a heavy liquid (bromoform) with a density of 2.9 g/cm3; and the fractionation of the heavy fraction of samples into magnetic, electromagnetic, and non-magnetic components. The chemical composition of mineral grains was studied using a Jeol 6380 LV electron microscope with an Inca-250 energy dispersive quantitative analysis system (Voronezh State University) and a JSM 5300 electron microscope with a Link:ISIS spectrometer (Central Research Institute of Geological Prospecting for Base and Precious Metals). Over 1,000 tests were performed. Geological, facies, and paleogeographic maps were created. Their analysis revealed the relationships between the distributions of kimberlite indicator minerals with the facies features characteristic of basal horizons deposits. Results and discussion: 683 small diamonds and a large number of their accessory minerals were found in the studied samples. Accessory minerals are quantitatively dominated by pyropes, especially moderately chromic differences characteristic of lherzolites. Grains were found with compositions identical to garnets from diamond-bearing harzburgite-dunites, as well as those characteristic of inclusions in diamonds. Ilmenites with wide variations in chemical composition, including picroilmenites, are also widely represented in the studied samples. Compared to garnets and ilmenites, chrome-spinelides are rarer, however, even among them there are differences with the composition corresponding to a potential diamond-bearing coesite depth subfacies. Judging by their typomorphic features and composition, diamonds and their accessory minerals in the intermediate hosts of the Voronezh anteclise come from primary diamond-bearing rocks, which should be considered as a direct prospecting indicator for forecasts. Conclusions: The analysis of paleogeographic maps of a new generation (scales of 1:200,000 – 1:500,000) and the sampling data (50 samples weighing 0.5–20 tons each) support the hypothesis of the local origin of diamonds and the localization of their parent rocks in the sedimentary cover of the Voronezh anteclise. Paleogeographic studies, an indispensable component of exploration activities, and kimberlite indicator minerals data allowed to outline two areas with assumed productive diatremes.