Characteristics of Altered Ilmenite in Uranium-Bearing Sandstone and Its Relationship with Uranium Minerals in the Northeastern Ordos Basin

Abstract

In recent years, the close relationship between uranium and Ti-Fe oxides in the sandstone-type uranium deposits has been extensively recognized. However, the altered characteristics of ilmenite and its relationship with uranium enrichment still remain unclear. With this paper based on heavy-mineral sorting of uranium ore selected from the Tarangaole-Nalinggou deposit in the northeastern Ordos Basin, electron probe, backscattering image, energy spectrum and scanning electron microscopy were systematically performed. The ilmenite in the sandstone can be divided into four groups, including unaltered, weakly altered, moderately altered, and strongly altered ilmenite. The alteration of ilmenite in uranium ores is notably more intense than that of the surrounding rocks. In addition, weakly, moderately, and strongly altered ilmenite associated with uranium minerals in uranium ores demonstrate that the more intensity ilmenite altered, the closer its relationship with uranium minerals is. The ilmenite has likely been somewhat altered before mineralization, and the alteration intensifies by later exposure related to an oxygen-containing fluid. The alteration mechanism comprises a process of competitive diffusion between Fe²⁺ and O²⁻ ions. In the early stage, Fe ions was mainly diffused on the particle surface. Subsequently, diffusion of O ions into the particles began to be dominate. Most of the leached iron is stripped or carried away by fluid. In an alkaline and reductive environment, the remaining iron is reduced to form the surrounding pyrite, and TiO₂ in a form of titanium sol recrystallizes (i.e., anatase). Backscattering images show that uranium and altered ilmenite are close in space. Coffinite is often distributed along the edges of altered ilmenite as burrs in shape. Colloidal or knitted coffinite associated with anatase is formed in the voids of altered ilmenite. The chemical composition of altered ilmenite varies considerably from the core to edge, and the mineral assemblage sequence is from girdle with ilmenite, to leucosphenite, to anatase, and to coffinite. There is no brannerite that is symbiotic with altered ilmenite. It is considered to be a uranium-containing titanium mineral aggregate caused by the reduction and adsorption of uranium. As the altered product of ilmenite, TiO₂ is an aggregation agent, increasing the concentration of uranium by adsorption. Together with Fe²⁺ and S₂ ⁻ in secondary pyrite, this aggregate creates a uranium-rich environment in the microzone for the formation of coffinite. Therefore, the alteration of ilmenite plays a geochemical role in the processes of sedimentary, diagenesis and mineralization, in which Fe is removed, Ti is enriched, and U is adsorbed and reduced.

Keywords

Ordos Basin / Tarangaole-Nalinggou uranium deposit / altered ilmenite / uranium occurrence / enrichment of uranium / ore deposit geology

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