Thorium and rare earth sulfates – new minerals of the Bykogorsk deposit (features of internal structure, chemical composition, genesis)

Abstract

Introduction: the Bykogorskoye uranium deposit was mined using the underground (heap) leaching method. The technological features of mining and the long period of deposit development led to the formation of rich secondary sulfate mineralization in the mine's tunnel chambers. This article provides information on the unique uranium, thorium and rare earth fluorine-containing sulfates of the Bykogorskoye deposit.

Methodology and factual material: during the field work, adit No. 10 (horizon 495 m) was studied. Flaky crystals of hademite exhibiting weak UV reflexes were found. During a detailed study of the discovered hademite crystals on an optical microscope, numerous small (1−30 μm) dark and brown precipitates of various morphologies were recorded in them, representing individual mineral phases. The selected crystals were studied in detail on a Jeol-6380 LV scanning electron microscope (laboratory of the Voronezh State University Central Collective Research Institute). The chemical compositions of the minerals were determined on an INCA-250 energy-dispersive microanalyzer. Gamma spectrograms of radiogenic minerals were obtained using a 4x4 cm NaJ (Tl) single crystal with a Burle 2060 photomultiplier.

Results and discussion: the studied hademite crystals are not uniform in composition. Areas with a high average atomic number (Z_cp) were found - hademite (HD2) with an increased (up to 7 wt.% UO₃) concentration of uranium and areas with low Z_cp values. (HD1) not containing uranyl ions. A detailed study of the hademite surface using a scanning electron microscope revealed mineral inclusions with high Z_cp parameters. Local microprobe analysis established two original mineral phases - 1) fluoride aluminosulfates of rare earths (ST1) with an increased content of ThO₂ (up to 6 wt.%); 2) fluoride aluminosulfates of thorium (ST2) with a very high content of ThO₂ reaching 51 wt.%. The studied hademite is a young mineral in which radioactive decay products have not yet accumulated. The gamma spectrogram does not show any daughter products of Ra²²⁶ decay (Pb²¹⁴ and Bi²¹⁴).

Conclusion: the formation of fluorine-containing aluminosulfates is associated with underground (heap) leaching processes widely used at the Bykogorskoye deposit. There are two main variants of aluminosulfate crystallization: 1) rapid evaporation of sulfuric acid solutions on the walls of the workings. 2) chemical precipitation associated with an increase in the pH of the environment.