Provenance of Late Mesozoic Strata and Tectonic Implications for the Southwestern Ordos Basin,North China:Evidence from Detrital Zircon U-Pb Geochronology and Hf Isotopes

In order to determine the provenance and variation characteristics of sandstone-type uranium deposits located in the southwest Ordos Basin,U-Pb geochronology and Hf isotope analyses were conducted on detrital zircons from the Late Mesozoic strata of the SD01 well in the Zhenyuan area.The detrital zircon U-Pb ages of four samples exhibited four main peaks at 250–330,420–500,1720–2000,and 2340–2580 Ma,with a small number of zircons dated at 770–1100 Ma.The detrital zircon age spectrum and further restriction provided by the in-situ Hf isotopic data suggest that the provenance of each stratum was mainly derived from the crystalline basement rock series(Khondalites,intermediate-acidic intrusive rocks,and metamorphic rocks)of the Alxa Block to the northwest and the Yinshan Block to the north,with minor amounts of Caledonian magmatic rocks and Jingning Period rocks from the western part of the northern Qilian orogenic belt to the west and the northern Qinling orogenic belt to the south.The provenance of the sandbody has not changed significantly and is of the Middle Jurassic–Early Cretaceous.The clear variations in the zircon ages of the samples from the Zhiluo and Anding formations were likely influenced by climate change during the Middle–Late Jurassic.The Triassic zircon age(<250 Ma)first appeared in Early Cretaceous strata,suggesting that tectonic activity was relatively strong in the northern Qinling orogenic belt during the Late Jurassic and produced extensive outcrops of Indo-Chinese granite,which were a source of basin sediments.

Characteristics of Uranium Mineralization in Red Clastic Formations in the Southwestern Margin of the Ordos Basin

In the southwestern margin of the Ordos Basin,uranium mineralization is primarily hosted by predominantly oxidative red clastic formations in the Lower Cretaceous.The main target layers for uranium exploration are the Madongshan and Liwaxia formations of the Liupanshan Group,followed by the Jingchuan Formation of the Zhidan Group.The host rocks(medium-fine feldspar quartz sandstone),which are bleached to a light grayish white color,contain a minor organic matter component and pyrite.Uranium mineralization changes from surficial infiltration or phreatic oxidation in the upper part to interlayer oxidation in the lower part.Uranium ore bodies are mostly lenticular or tabular in shape,locally shaped like crescent rolls.Individual ore bodies are typically small and shallow.Uranium predominantly manifests as pitchblende and coffinite.Coffinite is usually short and columnar or granular in habit,whereas pitchblende occurs as an irregular colloidal covering on the surface or in fissures of ferric oxide,silicate,clay or carbonate.Secondary uranium minerals are torbernite,uranophane,and uranopilite.Minerals associated with uranium are mainly pyrite,chalcopyrite and,to a minor extent,arsenopyrite and fluorite.The associated elements are Mo,V,Se,Co,Ni,and Mn,the host sandstone being high in Cu and Ba.Overall,the red clastic formations in the southwestern margin of the Ordos Basin are characterized by’five multiples but one low’which means multiple target layers,multiple stages of mineralization,multiple ore body shapes,multiple kinds of uranium minerals,multiple associated elements,but low organic matter.This implies an overall complex uranium metallogenic environment and mineralization process.It is recommended that future uranium exploration should take into consideration regional metallogenic conditions and mineralization features,with target layers in the wide-smooth synclinal slope being focused on.Most uranium deposits are small to medium in size,and the main type of uranium mineralization can vary by target layer.