Quaternary uranium mineralization in the Qaidam Basin, northern Tibetan Plateau: Insights from petrographic and C-O isotopic evidences
Aiertiken Abudukeyumu, Hao Song, Guoxiang Chi, Qi Li, Chengjiang Zhang
Abstract
There are several sandstone-type uraniferous basins around Tibetan Plateau. The Qaidam Basin is the largest sedimentary basin in the internal Tibetan Plateau, hosting important oil and coal resources, and has good potential for sandstone-hosted uranium resources. In this study, we examine the petrographic and mineralogical characteristics of a uranium mineralization interval at the Neogene-Quaternary transition (NQT) in the western Qaidam Basin using optical microscopy and electron probe microanalyzer (EPMA), including back scattered electron (BSE) imaging and energy-dispersive spectroscopy (EDS). The sedimentary lithofacies features and C-O stable isotopes of carbonate cement from two boreholes were analyzed, especially for the uraniferous interval. Whole-rock analyses of ores and barren rocks yielded highly variable U concentrations ranging from 1.5 to 2548 ppm. The main uranium minerals are coffinite and uraninite. The co-occurrence of uranium minerals with calcite indicates that uranium mineralization and calcite cementation were coeval. Coffinite mainly occurs at the interface between calcite cement and other minerals. The carbonates have δ13CV-PDB ranging from −29.67 to +3.93‰, suggesting involvement of organic carbon during uranium mineralization. This is also supported by the general trend of decreasing δ13CV-PDB and increasing U content from the conglomerate/coarse sandstone to the mudstone samples, and the overlap of δ13CV-PDB values between the calcite cement and organic matter in sedimentary rocks. The organic carbon may have been derived from hydrocarbons sourced from the deeper part of the basin and/or biogenic gases at shallow depths. The unconformity at the NQT may represent a favorable interval where oxidizing fluids carrying uranium met with reducing fluids carrying hydrocarbons and resulted in uranium mineralization. Thus, the NQT may serve as a guide for uranium exploration in this basin.