Unique Rhenium Deposit in the Carboniferous Coal-bearing Sands of the Russian Plate: Communication 2. Ore Mineralogy

Abstract: The paper reports geochemical and mineralogical data on ores of the Briketno-Zheltuhinskoe U–Mo–Re deposit, which is confined to the Visean paleochannel coal-bearing sandy sediments of the Moscow lignite basin. In addition to main ore elements (Re, Mo, U, and Se), the ores have elevated contents of the following metals (in order of decreasing abundance): Zr, Zn, Mn, Ni, As, Co, V, Pb, Y, Cu, and highly dispersed Tl and Ag. It was found that Re correlates positively with Mo, Ag, Zn, U, Se, and Co. The ores are incoherent and analytical scanning electron microscopy is the main method applied for their study. In order to clarify the speciation of U, Mo, and Re, sequential extraction experiments were performed. The main part of Re is associated with the organic and ion-exchange forms. Molybdenum is approximately equally distributed between the strongly bound (sulfide) and organic-related forms. Uranium mainly occurs in a weakly bound form and easily migrates with water. Authigenic minerals are mainly represented by pyrite, less common marcasite, and accessory sulfides (sphalerite, chalcopyrite, galena, etc.) and selenides (dzharkenite, clausthalite). Mo-minerals (containing up to 1.5 wt % Re) are represented by the tiny grains of molybdenite and jordisite. Uranium occurs as micron-size inclusions of uraninite (pitchblende) in colloform pyrite and is present as a macro-impurity in rhabdophane and zircon. The predominant part of U, Mo, and Re was found in adsorbed forms in carbonaceous, clayey, and strongly altered sulfide particles. Arkose and subordinate ash material together with heavy metals (Fe, Ni, Cu, Zn, Pb, U, Mo, Re), and also S and Se, were transported by river streams and accumulated in terrigenous sediments of shallow marine basin. These elements were adsorbed by plant detritus and, partly, by clay particles and iron oxyhydroxides/sulfides during diagenesis. The main economic U–Mo–Re ores were formed at the second, epigenetic stage, in the course of the lateral filtration of oxygenated groundwater through highly permeable sand horizons. This process was accompanied by the redeposition of U, Mo, Re, Se, Cu, Ni, and Zn from dispersed mineralization of the first stage at the reducing (plant detritus) and sorption (clay sediments) geochemical barriers. In this case, U, Mo, and Re were fixed in the mineral and, mainly, adsorbed forms. Significant contribution to the ore formation was likely provided by endogenous factors associated with activation of faults in the East European Platform (EEP) basement, epiplatform alkaline-basaltic magmatism, and the influx of deep fluids into the ore deposition zone. © 2020, Pleiades Publishing, Inc.