Geology and minerogenetic condition of the Lenghu sandstone-type uranium deposit in the northern margin of Qaidam Basin

The Lenghu sandstone-type uranium deposit is the newly discovered industrial deposit in the northern margin of Qaidam basin. Based on the field geological survey, the ore-bearing rocks in the Dameigou Formation in this area were studied by means of polarizing microscope and electron probe analysis to further clarify the characteristics of uranium source, petrology and mineralogy of Lenghu sandstone-hosted uranium deposit. Results show that the ore-bearing rocks in the Dameigou Formation are mainly (silty) sandy mudstone and thin layer coal and fine-grained quartz greywacke, accompanied with a series of epigenetic alteration phenomena. The Lower Proterozoic Dakendaban Group and Hercynian granite in the northeastern part of the study area provide abundant uranium sources for uranium mineralization. The uranium minerals in this deposit are mainly pitchblende and minor adsorption state of uranium. In general, the pitchblende occurs in irregular granular, stellate, "beadlike, linear" and powdery forms at the edge of pyrite, in the crack or on the contact between pyrite and calcite. The adsorbed uranium mainly occurs in the carbon chips and coal lines. The pitchblende deposit is the first exposed and discovered independent uranium mineral in the northern margin of Qaidam basin, which generally fills the blank of no independent uranium mineral exposed in this area. The presence of calcite in ore-bearing layer indicates that the uranium mineralization fluid is rich in CO₂, H₂O etc. volatile and mineralizer. In addition, many exposed reducing medium (oil and gas, carbon and pyrite, etc.) provides reductant required for REDOX reaction of this type of deposit, which eventually reduces U⁶⁺ to U⁴⁺ and results in precipitation of pitchblende.