Analysis on the Occurrence Characteristics of Uranium Minerals and Mineralization Conditions in the Yimin Formation of Kelulun Depression, Hailar Basin

The sandstone-type uranium deposit in the Kelulun Depression is the first industrial uranium deposit discovered in the Hailar Basin. In order to preliminarily identify the types, occurrence characteristics, and mineralization significance of uranium minerals in the study area, this paper conducts a systematic study of the ore-bearing sandstone in the target layer by using polarized light microscopy combined with electron probe analysis on the basis of a field investigation. The results show that uranium mineralization in the study area occurs at the front edge of the braided river delta of the Yimin Formation. The rock type of the ore-bearing target layer is mainly medium-fine-grained feldspathic litharenite, with the overall development post-generation alteration. Uranium minerals are mainly coffinite, followed by uranium-titanium oxide. Coffinite is mainly distributed in colloidal or stellate forms in or around pyrite, and a few are filled in the edges of debris particles or dissolution pits in irregular granular form. Uranium-titanium oxides are mostly distributed in irregular granular forms in the biotite cleavage cracks. The coffinite and uranium-titanium oxide, as the first discovered independent uranium minerals in the study area, fill the gap left by the absence of uranium minerals in the Hailar Basin. In terms of uranium mineralization conditions, the Permian granite and intermediate-acidic volcanic rocks in the west of the study area provide sufficient uranium sources for the sandstone-type uranium mineralization in the area. Under the action of reducing media such as oil and gas, organic carbon, and sulfides in the area, U⁶⁺ in the ore-forming fluid is reduced to U⁴⁺ and precipitated into mineralization in the form of coffinite and uranium-titanium oxide.