Conditions for uranium biomineralization during the formation of the Zoovch Ovoo roll-front-type uranium deposit in East Gobi Basin, Mongolia

• Published in: Ore geology reviews 2021-11, Vol.138, p.104351, Article 104351
• Main Authors: Rallakis, Dimitrios, Michels, Raymond, Cathelineau, Michel, Parize, Olivier, Brouand, Marc
• Format: Article
• Language: English
• Subjects: Applied geology; Bioreduction; Earth Sciences; East Gobi Basin; Mongolia; Organic matter; Roll-front; Sciences of the Universe; Sulfur isotopes; Uranium
• ISSN: 0169-1368; 1872-7360
• DOI: 10.1016/j.oregeorev.2021.104351

[Display omitted] •Uranium occurs at Zoovch Ovoo very intimately associated with organic matter.•(Bio)-oxidation of pyrite liberates intermediate valence sulfur species.•U reduction through biomineralization by bacteria and fungi.•Continuous bacterial activity would increase the uranium concentration.•Significant reducing agents for uranium are micro-organisms and sulfides. The Zoovch Ovoo uranium roll-front-type deposit is hosted in the Sainshand Formation, a Late Cretaceous siliciclastic reservoir, which constitutes the upper part of the post-rift infilling of the Mesozoic East Gobi Basin in SE Mongolia. The Sainshand Formation consists of unconsolidated medium-grained sand, silt and clay intervals deposited in fluvial-lacustrine settings. The uranium deposit is confined within a 60–80 m thick siliciclastic sequence inside aquifer-driven systems. The overall system experienced shallow burial and was never subjected to temperatures higher than 40 °C. This study proposes a comprehensive metallogenic model for this uranium deposit. Sedimentological and mineralogical observations from drill core samples to the microscopic scale (optical and Scanning Electron Microscopy) together with in situ geochemistry of late-formed phases (Laser Ablation–Inductively Coupled Plasma Mass Spectrometry, Electron Probe Microanalysis, Fourier Transform–Infrared Spectroscopy) were considered for the reconstruction of the main stages of U trapping. In the mineralized zone, the uranium ore is expressed as Ca–enriched uraninite (UO2) and less commonly as Ca–enriched phospho-coffinite (U, P)SiO4. Trapping mechanisms include i) complexation (i.e. uranyl-carboxyl complexes), ii) adsorption on organic or clay particles) and iii) reduction by pyrite and by bacterial activity to amorphous uraninite. In all cases, the organic matter plays either the role of trap for uranium or nutrient for bacteria that can trap uranium through their metabolism. The shallow burial diagenesis conditions do not allow direct reduction of U(VI) by organic carbon. The δ34S values of the iron disulfide are very diverse, fluctuating in extreme cases between −50 to + 50‰, with an average δ34S value for framboidal pyrite at 2‰, and −20‰ for euhedral pyrite. The positive and negative values reflect close versus open fractionation systems, while bacterial sulphate reduction (BSR) is active during the whole diagenetic history of the deposit as an essential source of reduced sulfur. Therefore, using detrital organic