Late Mesozoic continental basin "Red and Black beds" coupling formation constraints on the sandstone uranium mineralization in northern China

Based on the 973 program and the northern sandstone type uranium survey program, this study tried to explain the "red and black" sandstone constraint on uranium mineralization. On the basis of summarizing temporal and spatial relationship between red layers and black layers of global sandstone uranium deposits and the sieving of more than 100, 000 meters core drilling data of the northern continental basin, the authors selected Junggar, Ordos and Songliao basin as typical research objects. Through compilation of drilling chart of the basin, investigating well-connecting section of typical mining area and geochemical testing of key strata, the authors made a vertical and horizontal comprehensive analysis and comparison of "red and black" sandstone and uranium-bearing strata, and the results show that the formation ages of uranium-bearing strata gradually change from Middle Jurassic to Upper Cretaceous from west to east in China. There were at least 6 large-scale oxygen-rich red sedimentary events in Late Mesozoic period, which are respectively Ⅰ. Middle Jurassic-late Jurassic early stage (Bathonian-Oxfordian), Ⅱ. Early-middle stage of the Early Cretaceous (Berriasian-Barremian), Ⅲ. Middle stage of Early Cretaceous (Barremian), Ⅳ. Late stage of middle Cretaceous (Cenomanian), Ⅴ. Middle stage of late Cretaceous (Coniacian), and Ⅵ. Late stage of late Cretaceous (Campanian). The red layer and the black layer below constituting "red-black color structure" in Ⅰ, Ⅴ and Ⅵ stage are 3 important uraniumbearing strata in China. Geochemical and fossil data of the red and black layers in the typical basins show that the content of B, Sr and Cu and the ratios of Fe²⁺/Fe³⁺, B/Ga, Sr/Cu and FeO/MnO are obviously different between them. The content of Fe₂O₃ in the red layer is obviously higher, and the ratio of U and U/Th in the transition zone is obviously higher than other sides. Combined with development of strawberry pyrite in black layer and carbonate rocks in red layer, the authors hold that the red layer is a relatively strong oxidizing environment and the black layer is a relatively reducing environment, which respectively provide oxidationreduction conditions for the mineralization of sandstone uranium ore. The spectrum of uranium deposits is related genetically with red and black layer, industrial orebodies occur in tabular form in gray and grayish green sandstone. The red and black layer with the thickness of more than 500m is unfavorable for mineralization. The black and red layer formed in late Mesozoic continental basin has vertical zoning to represent the depositional environment change from oxidizing to reducing environment which provides "obstacle" and "field" for mineralization respectively. In conclusion, the authors put forward the "double color structure" metallogenic prospecting model for sandstone uranium deposit, in which the upper layer serves as the red oxide barrier, whereas the lower layer serves as black reducing barrier, and the sandstone uranium deposit (ore spot) transition zone is characterized by grayish green, gray sand enrichment and mineralization. The understanding obtained by the authors is significant not only for improving the metallogenic environment knowledge, metallogenic regularity and metallogenic model of sandstone uranium deposit but also for guiding the investigation of sandstone uranium deposits.