The Nuheting deposit,Erlian Basin,NE China: Synsedimentary to diagenetic uranium mineralization |
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| Institution: | 1. Université de Lorraine, GeoRessources-CNRS-CREGU, BP239, F54506 Vandoeuvre-lès-Nancy, France;2. East China Institute of technology, 56 Xuefu Road, Linchan, 344000 Fuzhou, Jiangxi, China;3. Geological Team No.208, BOG, CNNC, Baotou, Inner Mongolia, China;1. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China;2. Department of Geological Sciences, University of Manitoba, Winnipeg R3T 2N2, Canada;1. U.S. Geological Survey, Energy Resources Program, Denver Federal Center, Denver, CO 80225, USA;2. U.S. Geological Survey, Mineral Resources Program, W. 904 Riverside, Spokane, WA 99201, USA;3. U.S. Geological Survey, Mineral Resources Program, 954 National Center, Reston, VA 20192, USA;4. U.S. Geological Survey, Mineral Resources Program, Denver Federal Center, Denver, CO 80225, USA |
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| Abstract: | Sedimentary units deposited during the post-rift stage of the Erlian Basin located in northeast China present an alternation of sandstone and mudstone layers. This sedimentological architecture is at the origin of confined permeable reservoirs hosting sandstone-type uranium deposits. The study of the Nuheting deposit offered the opportunity to identify synsedimentary/early diagenetic uranium concentrations and diagenetic mineralization hosted in mudstone-dominated layers of the Erlian Formation, indicating that a stock of uranium was present in the basin prior to the genesis of sandstone-hosted uranium deposits. Therefore, this pre-existing stock may constitute a significant source of uranium for the formation of roll front deposits present in other parts of the Erlian Basin.Detailed petrographic and geochemical study of drill-core samples from the Nuheting deposit led to the characterization of different stages related to the formation of the uranium ore bodies and allowed to propose a new metallogenic model. Uranium mineralization of the Nuheting deposit is mainly hosted in dark gray silty mudstone of wetland depositional environment of the Late Cretaceous Erlian Formation. Petrographic observations and EMP analyses evidenced that a significant amount of uranium was associated with clay minerals (interstratified clays, smectite, chlorite, palygorskyte, illite and kaolinite), either adsorbed on mineral surfaces as U (VI) ions or reduced mainly as UO2 nano to microcrystals disseminated in the clayey matrix, which corresponds to synsedimentary/early diagenetic concentrations. Trace elements on pyrite analyzed by LA-ICPMS, petrographic observations and whole-rock geochemical data led to the characterization of a diagenetic uranium mineralization. High As (1–50 ppm), Mo (10–500 ppm) and Se concentrations in the whole rock and the incorporation of these elements in pyrite highlight reducing conditions within the host-rocks during the diagenesis of the Erlian Formation. During the early diagenetic stage, uranium was either desorbed from clay minerals and organic materials to be reduced or directly reduced and precipitated as P-rich coffinite and pitchblende on pyrite crystals. During the late diagenetic stage, uranium was redistributed in situ and locally deposited mainly as coffinite on pyrites. Finally, an epigenetic stage of cementation was identified with sulfate and carbonate minerals, which may enclose some uranium minerals. This epigenetic stage of fluid circulation may be responsible for a minor uranium remobilization. Therefore, the Nuheting deposit experienced three main stages: (i) a synsedimentary/early diagenetic uranium concentration and mineralization, (ii) a late diagenetic in situ uranium remobilization and deposition on pyrite and (iii) an epigenetic cementation. Rock-Eval pyrolysis indicates that the organic matter contained in host-rocks of the Nuheting deposit is of type IV, inherited from land plant, and do not contain free hydrocarbons (very low S1). Therefore, our results do not support that migrated hydrocarbons were involved as a reducing agent for uranium mineralization. |
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