Unique Rhenium Deposit in the Carboniferous Coal-bearing Sands of the Russian Plate: Communication 1. Geological Structure

Abstract

The Briketno-Zheltukhinskoe U–Mo–Re deposit is located in the southern sector of the Moscow lignite basin (central East European Platform). It is restricted to the paleochannel and deltaic coal-bearing sandy sediments of the Visean Bobrikov horizon lying on the Famennian limestone. They are overlain by the younger Neogene–Quaternary sands and clays. Tectonically, the deposit is ascribed to the southern shoulder of the Pachelma aulacogen crosscut by the SN-trending fault zone. The presence of a thick gray-colored unit of inequigranular moderately and poorly sorted sands containing the coalified plant detritus served as the main lithological-facies factor for the localization of U–Mo–Re mineralization. The Bobrikov sandy formation was accumulated in the fluvial, swampy, onshore deltaic, and shallow-sea lagoonal environments with warm humid climate. A combined syn-epigenetic model is proposed to explain the formation of the U–Mo–Re paleovalley-type deposit. Subeconomic stratabound U (±Mo, Re) mineralization associated with the carbonaceous and clayey horizons was accumulated during sedimentation and diagenesis. During epigenesis, the U–Mo–Re ores were deposited by oxidized groundwater owing to its lateral filtration through highly permeable sand horizons and metal accumulation at the reducing and sorption barriers. The general subhorizontal stratiform structure of the ore deposit is complicated by the subvertical morphological (swell of sand horizon plus strong pyritization) and concentration (Re + Mo column-like maximums) ore shoots. They occur in dome-like structures that disturb the horizontal structure of the host sedimentary unit. It is possible that these domes and ore shoots are the result of hydrothermal-hydraulic processes caused by the activity of deep crustal (likely, mantle as well with allowance for the Re mineralization) fluids.