The epigenetic alteration stable isotope characteristics of the Dongsheng uranium ore district and their geological implications

Some large to giant sandstone-type uranium deposits were discovered in Yimeng uplift area of the north Ordos basin, especially in Hanggin Banner-Dongsheng area, in recent decades, which suggests that this area is the most favorable potential area in search for sandstone-type uranium deposits. The main characteristics of these uranium deposits are under the control of green sandstone alteration zone in Zhiluo Formation, with some epigenetic alteration phenomena, such as the whitened sandstone, carbonation and calcite vein, and pyritization. In this paper, the authors studied and discussed systematically the formation process, genesis and the relationship with uranium mineralization by means of carbon, hydrogen, oxygen, sulfur stable isotope tracing. The microscopic observation shows that there are large particles of nodular iron pyrites in the uranium ore in association with uranium minerals, and both are products formed at the same time. The larger negative data of sulfur isotope in this type of pyritization shows that microbial effect on sulfur source quality is obvious. The sulfur isotope comparison with dispersed slack of Zhiluo Formation, coal seams of Yanan Formation, crude oil and coal-bearing Yanchang Formation and Yan'an Formation of the Upper Palaeozoic period shows that the sulfur is close to the Upper Palaeozoic coal organic sulfur. The hydrogen and oxygen isotopes of kaolinite from large white sandstone located at the top of Yan'an Formation show that the genesis was related to low temperature hydrothermal process. The carbon and oxygen isotope features of calcite from the ore-bearing bed indicate that the carbon sources were mainly provided by organic function, whereas the main carbon source was from gas source rocks of Upper Palaeozoic, with no relation to the Ordovician and Yanchang Formation oil and gas. The hydrogen and oxygen isotopic composition of the epigenetic fluid inclusions indicates that the ore-forming fluids were mainly composed of low temperature atmospheric precipitation, and formed the low temperature hydrothermal gaswater mixed solution together with gas of Upper Palaeozoic. This feature of fluid effect led to the formation of all kinds of epigenetic alteration in this area which was closely associated with uranium mineralization. In short, the kinds of epigenetic alteration of stable isotopes show that the Dongsheng uranium deposit is characterized by late epithermal solution and reformation of oil and gas. The results obtained by the authors provide evidence for the genesis of the Dongsheng uranium deposit.