Contribution of deep organic fluids to uranium mineralization in Cretaceous sandstones of the southwestern Ordos Basin, China

Abstract

In recent years, significant advancements have been achieved in uranium (U) prospecting within the Lower Cretaceous strata of the southwestern Ordos Basin, China. U deposits have been unearthed in Zhenyuan and Pengyang County, predominantly found within the Cretaceous sandstones of the Luohe and Huanhe formations. These ore-bearing strata primarily originate from arid environments, characterized by a deficiency of organic matter (OM). This deposit type manifests within lenticular, gray sandstone interbedded within red sandstone. Its genesis markedly diverges from oxidation-reduction sandstone-type U deposits, yet systematic studies on the properties of ore-forming fluids, particularly their sources and contributions to U mineralization, remain lacking. Core observations were conducted, and representative samples were collected for laboratory analysis. This study discerns the characteristics of ore-forming fluids within the U-bearing sandstone and deliberates the source of these fluids and their impact on U mineralization. Results indicate an abundance of flow-patterned , OM-rich U ores, derived from the deep mudstone rocks of the Yanchang Formation. The ore-forming fluids, characterized as OM-rich and low-temperature (130 to 180°C) hydrothermal fluids, originate from fluid discharge within the source rocks. Subsequently, ore-forming materials dissolve within these OM-rich fluids and migrate towards the oxidized sandstone in shallower regions. Significant changes in temperature and oxygen fugacity lead to fluid differentiation or cracking, facilitating U precipitation into U ores. The characteristics of Cretaceous U mineralization in the study area are consistent with the exudative model of sandstone-type U metallogenesis.

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