Provenance Analysis Of The Middle Jurassic Zhiluo Formation And Uranium Metallogenic Model Of Dongsheng Sandstone-type Uranium Deposit In Northeastern Ordos Basin

Ordos basin is one of the most important energy base in western North Chinacraton, which absorbs many researchers`attention and interesting. A large number ofscholar has analyzed the tectonic and evolution, sedimentary cycle, sedimentary faciesof the basin and uranium deposit`s characteristics and uranium metallogenic model ofDongsheng uranium deposit. In spite of these positive results, we still know littleabout the features of the ore-bearing sedimentary formation——Zhiluo formation andare insensible of water-rock reaction process and genetic mechanism of uraniummineral in the mineralization. In this background, we do the following works in twoaspects:（1） detailed petrological and geochemical studies were carried out on the sandstonesof Middle Jurassic Zhiluo Formation from8drilling cores in the northeastern OrdosBasin, in order to determine their provenance and tectonic setting. The samples arecoarse greywacke with moderate to high SiO₂content （69.31%⁷7.06%） andAl₂O₃/SiO₂ratio（0.15⁰.25）, a low content of Fe₂O₃, MgO and TiO₂, which impliesthat these sandstones are in felsic，lacking of mafic materials. The rocks are enrichedin large ion lithophile elements （LILEs, such as K, Rb, Sr and Ba）, and depleted inhigh field strength elements （HFSEs, such as Nb, Ta, Ti and P）. These featuresindicate that the source region have affinity of subduction-related tectonic setting.Similarly, other discrimination diagrams, such as （TFe₂O₃+MgO）-TiO₂,（TFe₂O₃+MgO）-Al₂O₃/SiO₂, Th-Sc-Zr/10, Th-Co-Zr/10, further suggest that theZhiluo Formation sandstones were deposited as detritus from a continental island arcor active continental margin. Together with other characteristics, such as LREEenrichment, relatively flat HREE patterns, without or slightly Eu anomalies, flatpatterns with positive Eu anomalies in the PAAS-normalized rare element plots,allowed us to conclude that the rocks are strongely related to the phanerozoicgranitoids from the northern margin of North China craton.（2） We analysis38sandstones from3different geochemical district and divide theminto4groups in consideration of uranium content and REE pattern: group I, group Ⅱ,group Ⅲ-1and group Ⅲ-2.Group Ⅰ and group Ⅱ have a similar flat REE patterns, with Eu positive anomalies and slight HREE deficit and group Ⅱ have a much higheramount of REE. Group Ⅲ have very different REE patterns. The Ⅲ-1show a LREEdeficit and flat HREE enrichment patterns, but the Ⅲ-2display a MREE enrichmentbut both LREE and HREE deficit patterns. Combining results of slice method, SEM,electron microscope, and electronic probe, we infer humus play an important role inenrichment of uranium and MREE, and conclude dissociation of plagioclases controlsthe process of uranium minerals, HREE and calcite`s deposit.