Sedimentary and ore-forming characteristics of uranium mineralization in the Shizigou formation from the Southwest Qaidam Basin, Northwest China

Abstract

The sandstone-type uranium deposits are the dominant source of uranium in China. The Qaidam Basin, the largest inland sedimentary basin from the Chinese northern Tibetan Plateau, is shaped by various tectonic processes from the surrounding East Kunlun, Altyn, and Qilian orogenic belts, and is characterized by uranium mineralization and enrichment of petroleum. The Shizigou Formation is identified as one of the U-bearing strata in the Qigequan area. We present a case study at the Qigequan area in the southwestern edge of the Qaidam Basin, to investigate the governing factors for anomalous uranium enrichment in this stratum. U-Pb dating of detrital zircon from sandstones of the Shizigou Formation reveales two age peaks at 450 Ma and 260 Ma, respectively. The zircon CL images suggest that the majority of zircon grains assessed are magmatic origin, though some show complex core-rim texture, irregular shape core with overgrowth domains, patched zoning and sector zoning, indicating a metamorphic origin. The bimodal age distribution, textures, and REE distribution of the selected detrital zircon grains all indicate that the sedimentary source of the Shizigou Formation originated from the Qimantagh area (East Kunlun). The paleoclimate proxies, namely the Sr/Cu, Sr/Ba, V/Sc, V/Cr, and Fe³⁺/Fe²⁺ values, combined with the chemical index of alteration (CIA) and index of compositional variation (ICV) reveal that the Shizigou Formation was formed in a relatively arid and oxidized environment that suffered a low degree of chemical weathering, which provides condition for the transportation of U-bearing ore-forming fluids. The escaping organic matters in the underlying strata act as reducing materials that control the precipitation of uranium-complexes in ore-forming fluids. On the other hand, the surrounding strata containing the source rocks and hydrocarbon make an effective barrier for the uranium orebodies to be preserved. The source of ore-forming materials, sedimentary environment and organic matters have made significant contributions to the uranium mineralization in the southwest Qaidam Basin.