Petrological and Geochemical Research on the Telaaobao Sandstone–Hosted Uranium Deposit in the North-Western Ordos Basin, China: Implications of Multiple Sources

Abstract

The newly discovered Telaaobao uranium deposit, hosted within Lower Cretaceous sandstones in the north-western Ordos Basin, represents a significant new stratigraphic target for uranium exploration. This study offers detailed petrological and geochemical analyses of different types of rocks from the Huanhe Formation, aiming to investigate the factors controlling uranium mineralisation in this deposit. The Huanhe Formation is generally characterised by low to moderate concentrations of total rare earth element (∑REE) (70–213 ppm) and Y (7.5–30.1 ppm), right-inclined REE patterns and slightly negative Eu anomalies, which resemble those of the upper continental crust (UCC). The trace element characteristics further reveal that the sedimentary sources have an average composition typical of intermediate-felsic rocks, with the addition of some old sediments. The U content in sediments is mostly higher than that of the average UCC, even in non-mineralised rocks from both the oxidised (2.5–58.8 ppm) and reduced zones (2.0–49.6 ppm), indicating that the provenance rocks and the sedimentary strata can serve as enriched uranium sources for the deposit. The Lower Cretaceous strata in the north-west margin of the Ordos Basin, which were formed in an arid depositional environment and lacked organic matter and other reducing agents, restricted the formation of uranium deposits. Nevertheless, influenced by multiple stages of tectonic activity, the release of deep-seated reducing fluids, such as oil and gas, greatly enhanced the reducing capacity of the sandstone, creating an extensive green reduction zone. The current development of uranium ore bodies is mainly controlled by the oxidation–reduction transition zone. Additionally, high U (591–4080 ppm) enrichment has also been found in the oxidised zone, where mineralised rocks display distinct petrological and geochemical characteristics from those in the transition zone. These rocks exhibit anomalous increases in REEs (∑REE = 168–722 ppm), with bell or flat shape REE patterns and high Y concentration (147–866 ppm). These features resemble the uranium minerals that are typically formed in deep-originated hydrothermal fluids, especially in unconformity-related uranium deposits. In this case, the hydrothermal fluids may have also introduced uranium into the Telaaobao deposit as a potential source.