Late Cretaceous-Cenozoic tectonic-sedimentary evolution and U-enrichment in the southern Songliao Basin

Abstract

Late Cretaceous-Cenozoic tectonic-sedimentary evolution is fundamental to U (Uranium) migration and U-enrichment in the southern Songliao Basin, northeast China. To establish the genetic connection between tectonic-sedimentary evolution and U-enrichment, this paper presents a mineralogical study, apatite fission-track (AFT) analysis, and SEM-CL analysis of quartz grains. Two stages of mineralization were proposed, including the U pre-enrichment stage during the Late Cretaceous, and the ore-forming stage during the Cenozoic. Based on the density plots of the AFT ages, paleoclimate studies, and SEM-CL analysis of quartz grains, it is inferred that the U pre-enrichment was closely related to the rapid uplift of the Great Xing'an Range and the Zhangguangcai Range, and the O2-rich atmosphere during 97–90 Ma in the southern Songliao Basin. The O2-rich atmosphere increased U mobility and led to the formation of red beds during the Late Cretaceous, which widely developed U pre-enrichment in the basins of Northeast China. The mineralogical study reveals that the Qianjiadian uranium deposit developed in two stages. The diagenetic process experienced the transition from early alkaline to late acidic conditions, to promote the Stage I mineralization (U pre-enrichment). The dominant occurrence spaces for U were the dissolution pores of quartz, tightly packed with calcite cement. U-enrichment during alkaline ambient conditions was mainly via adsorption. With the increase of the buried depth, the early alkaline diagenesis returned to acidic conditions due to the abundant organic material. U(VI) was reduced to pitchblende due to the reducing capacity of the organic matter (OM). Stage II (ore-forming stage) mineralization was strictly related to the hydrocarbon re-concentration. According to the thermal history study of the CC1 and Q2 boreholes, the southern Songliao Basin experienced four episodes of rapid uplift during the Cenozoic at 83–80 Ma, 68–52 Ma, 33–23 Ma, and −8 Ma, respectively. The Cenozoic uplifts led to hydrocarbon migration at 65 Ma, 23 Ma, and present. The re-concentration of hydrocarbon prompted the U mineralization in the Qianjiadian uranium deposit in the southern Songliao Basin during the stable period of tectonic movements. The reducing conditions caused by hydrocarbons promoted U mineralization. The dominant spaces of coffinite were intergranular dissolved pores, formed during the transformation from the feldspar to the kaolinite under acidic conditions. The post-rift phase and structural inversion phase directly controlled the U pre-enrichment stage and the ore-forming stage, respectively. Overall, the tectonic-sedimentary evolution played an important role in controlling the U leaching from granites, the dominant occurrence spaces, and U recycling.