Characteristics of sandstone-type uranium mineralization in the Hangjinqi region of the northeastern Ordos Basin: Clues from clay mineral studies

Abstract

The uranium deposits in the northeastern Ordos Basin are closely related to clay minerals and coal seam both in space and genesis, and the generation of kaolinite in the Zhiluo Formation is controlled by the coal seam of the Yan'an Formation. Through a systematic analysis, the clay mineral types, assemblage characteristics, and the type of fluid interactions in the uranium bearing rock series were extrapolated by a statistical approach and the drawing of a plain plane figure. By doing this, the constraints of coal seam parameters on uranium mineralization have been identified. The results show that: the dominant clay mineral assemblage in the Zhiluo Formation sandstones is smectite(S) + illite/smectite mixed layer(I/S) + chlorite(C) + kaolinite(Kao), whereas I/S + C + Kao was found in the formations’s mudstones and Kao + I/S + S + C or Kao + I/S + C + S was found in the uranium bearing sandstones. The similarity of authigenic clay minerals in the sandstones and mudstones indicates that, they experienced the same pH and temperature conditions in the early diagenesis, while the difference reflects the change in fluid type and action intensity during the mineralization stage. Since the effect of acid fluid and alkaline fluid on the lower member of the Zhiluo Formation are stronger than that of the upper member, it is speculated that the acidic fluid mainly originated from the coal seam of the Yan'an Formation during diagenesis and the alkaline fluid comes from the deep strata after late Cretaceous era. The reducing fluid produced by the coal seam of the Yan'an Formation enhanced the transformation of clay minerals. Additionally, the parameters such as the thickness of the coal seam, the number of coal layers and the distance between the coal seam and Zhiluo Formation bottom are all interrelated with uranium mineralization. By systematically quantifying the following parameters and uranium mineralization information, it is speculated that, the conditions below were the most reducing medium for the targeted sand body layer, which serves as the most suitable conditions for the precipitation and enrichment of uranium ore: 1) where the shortest distance between coal seam of the Yan'an Formation and Zhiluo Formation bottom interface is less than or equal to 10 m, 2) where the accumulated thickness of the coal seam in the Yan'an Formation ranges between 5 and 10 m, and 3) where the number of coal seam is between 4 and 10 layers. Proposal of the optimum metallogenic parameters of the underlying coal seam is of great significance for enhancing prospecting conditions in coalfield exploration areas and also improving exploration efficiency in the future.