Abstract:

This study presents major and trace element data of the uranium-bearing Lower Cretaceous Luohe Formation (Fm.) sandstones in Zhenyuan area (southwestern margin of the Ordos Basin), and the metallogenic conditions and regularity.The results show that the uranium ores in the Luohe Formation is mainly uraninite (UO₂), and the uranium-bearing sandstone may have sourced from the quartzite in ancient sedimentary terranes or craton/recycled orogenic belts.These sandstones have the same REE patterns with LREE-enrichments, HREE depletions and negative Eu anomalies, suggesting that the wallrocks have a uniform detrital source, sedimentary environment and tectonic setting.The V/Cr, Ni/Co, U/Th and V/(V+Ni) features of the uranium-bearing sandstone indicate that the local uranium mineralization was mainly developed in a weakly-oxidizing to weakly-reducing transitional environment.The δEu and δCe values increase with the U content, reflecting that the U enrichment and mineralization have experienced the transformation from oxidizing to reducing environment, and the sand body colors have changed from light red to grayish white.The contents of Th, Sc, Co, Mo in the uranium-bearing sandstones are positively correlated with that of U, and thus the former four elements can be used as fertility indicators for sandstone-type uranium deposits in the Luohe Formation.Formation of sandstone-type uranium deposits in the study area is closely related to the reduction caused by the upward seeping of oil and gas from depth.The incursion of oil and gas fluids likely increases the reducing capacity of the sand body, which promotes the uranium deposition and concentration from the oxidizing uranium-bearing fluid in the redox transition zone.This eventually formed the widespread uranium anomalies in the Luohe Formation.

Key words: sandstone-type uranium deposit, redox reaction, element geochemistry, Zhenyuan area, southwestern Ordos Basin