Hydrocarbon Fluid Geological Characteristics of the Xishanyao Formation and Its Uranium Metallogenic Significance, Northwest Junggar Basin

The nature,type,function and mechanism of hydrocarbon fluids in Xishanyao Formation in Northwest Junggar Basin have not been systematically studied,and the genesis of the greenish alteration sandstone and its correlation with the sandstone-hosted uranium mineralization is still not clear. Through a series of measurements (e. g. rock-mineral identification,whole-rock geochemical test,fluid inclusion and C-O isotopic analysis),the mineralogical,geochemical,and isotopic characteristics have been researched for the primary gray sandstones and the greenish alteration sandstones,respectively. Petrographic observations indicate that the greenish alteration sandstones widespread a strong clay alteration (mainly montmorillonitization and chloritization),pyritization and carbonatization,and has less kaolinite content and high reduction capacity compared with the gray sandstones. The carbonate cements occurred in the greenish alteration sandstones have a light C-O isotope signature characterized by negative δ¹³CV-PDB values ranging from -12.8‰ to -8.6‰,and δ¹⁸O_V-PDB values changing from -11.7‰ to -7.7‰. Furthermore,a large amount of methane and a little carbon dioxide have been identified within fluid inclusions. Therefore,the greenish alteration sandstones mostly experienced a reworking event by the deeply alkaline reductive hydrocarbon fluid,resulting in the concealment and protection of the paleo-oxidization sandstone belt and the early-formed uranium ore bodies,thus increasing the difficulty for prospecting. Besides,it produces the current discovered uranium ore bodies due to the interaction between the greenish sandstone and the modern uranium-bearing and oxygenated underground water.