Roles of dispersed organic matters in sandstone-type uranium mineralization: A review of geological and geochemical processes

Abstract

Among uranium deposits in sedimentary basins, sandstone-type uranium deposits are epigenetic. The development scales of sandstone-type uranium deposits are controlled by tectonic events, provenance, sand bodies, uranium sources, and reducing media, including that reducing media control the scales of redox transition zone. Dispersed organic matter (DOM) within sandstones, e.g., carbonaceous debris (CD), is one of the important reducing media. The characteristics of organic petrology and element of CD are similar in those of coals, being grouped into type III kerogen. However, the origin of CD within sandstones is unclear. Uranium occurs in uranium-bearing matter and uranium mineral, and coexists with CD in space. Adsorption, complexation, and reduction of organic matter are responsible for the mobility and precipitation of uranium in sediments. The adsorption and complexation capacity of DOM is the most intensive at the stage of immaturity to low maturity, and decreases with the increasing coalification. The reduction capacity is linked to organic matter maturity. Besides, porous macerals could provide the channels for uranium-bearing fluid migration and the space for uranium preconcentration. Under certain physical and chemical conditions, uranium-bearing fluid could permeate into macerals, and react, i.e., trapping followed by reduction. Nevertheless, the coordination characteristics of uranium within uranium-bearing DOM should be further investigated. Uranium enrichment concurrently alters organic matter, causing an increase in organic matter maturity and anisotropy through the effects of radiogenic heat or α-irradiation-induced crosslinking. However, α-irradiation-induced crosslinking should be the primary way. Besides, contributions of CD to uranium mineralization should be reckoned.