Occurrence of uranium minerals in the Xiaomeigou Formation in northern Qaidam Basin, Northwest China

Abstract

The Lenghu sandstone type uranium deposit as industrial value in the northern Qaidam Basin, Northwest China. This study analyzed the LA-ICP-MS U-Pb dating of detrital zircons, mineralogical features, electron probe, backscatter electron imaging and track etch of the altered minerals from uranium-bearing strata in the Xiaomeigou Formation of Lower Jurassic on the basis of field investigation in order to trace the source, parent rock lithology, geochronological age, uranium minerals, compositional characteristics, occurrences of uranium mineralization.The U-Pb ages of detrital zircons from the ore-bearing member are ranging from 117 to 270 Ma and 370–480 Ma, respectively, indicating that the provenance and uranium source were mainly derived from the Permian-Triassic intermediate-acid igneous rocks in the eastern Altyn Tagh and western Saishiteng Mountains, a small amount of them are from the Ordovician-Silurian intermediate-acid igneous rocks in the Xiaosaishiteng Mountains. Two types of ores in Xiaomigou Formation in Lenghu region are mainly composed of graywacke and carbonaceous sandstone with main uranium minerals are coffinite, pitchblende and trace amounts of lead selenite. They are closely associated with clay minerals, organics and pyrite, which lie in the intergranular space of quartz, feldspar and other altered lithic mineral grains or in cleavage joints of altered clay minerals that occur in fine columnar, gel-like or irregular sawtooth shape. It is inferred that the Fe2+ in feldspar and biotite reacts with H2S or S2- bearing minerals participated in the formation of pyrite, where the U6+ was captured by late multiple fluid processes occurred reduction-precipitation-adsorption uranium mineralization in the reduction environment provided by Fe2+. Based on the analysis of the existence of high Y and low Y types of uranium minerals, it is inferred that uranium in uranium minerals is the residual of original uranium minerals derived from the pre-enrichment of uranium-bearing detrital particles in sedimentary and diagenetic stages, and at the later stage, the high radioactivity abnormal geological bodies migrated to the inner basin through the multi-stage reformation of uranium-containing and oxygen-containing fluids without Y or at very low contents.