The distinctiveness of carbonaceous debris in uranium reservoirs under arid sedimentary backgrounds and its implication for uranium mineralization: A case study of northern Ordos Basin

Abstract

In the northern Ordos Basin, numerous studies have revealed close associations between carbonaceous debris (i.e., CD) and uranium mineralization in the Zhiluo Formation (i.e., J₂z), which is greatly influenced by the humid sedimentary backgrounds. However, recent exploration has shown that the reservoirs in the Lower Cretaceous under arid sedimentary backgrounds are also rich in uranium resources, but CD within them is scarce and shows significant differences. The present study investigates the macerals, maturity and mineral composition of CD in the Huanhe Formation (i.e., K₁h) by mainly using optical microscope, photometer, scanning electron microscopy and energy dispersive spectroscopy. The results show that CD in K₁h is composed of vitrinite (mainly telinite) and inertinite (mainly fusinite), with inertinite content (i.e., IC) ranging from 0 to 19.81 %. Its vitrinite reflectance (i.e., VR) ranges from 0.415 % to 0.987 %. The plant cell cavities and fissures inside the CD are rich in uranium minerals, pyrite, and various other minerals. Compared with CD in J₂z (IC = 0 to 12 %, VR = 0.258 % to 0.514 %), most CD in K₁h is of significantly higher IC and VR. The analysis reveals that 96 % of CD in J₂z (VR < 0.5 %) originates from synsedimentary plant remains or peat debris and only 4 % (VR > 0.5 %) originates from ancient strata. By contrast, CD in K₁h can be classified into two genetic categories: endogenous supply from the basin (VR < 0.5 %) and exogenous supply from provenance (VR > 0.5 %), with the latter accounting for 62.5 %. The former originates from coal debris of Middle Jurassic or synsedimentary plant remains. It can be subdivided into two subcategories according to diverse coal debris or varying degrees of modification by fire and oxidation: exhibiting low IC (<12 %) and high IC (>12 %) respectively. The latter originates from coal debris or old CD transported from the provenance. It can also be subdivided into two subcategories based on the IC of the parent rocks. Importantly, the CD, although rare, exhibits close associations with pyrite and uranium minerals, suggesting its significant role as the key enrichment agent for uranium precipitation under arid sedimentary backgrounds.