Sequence and sedimentary characteristics of upper Cretaceous Sifangtai Formation in northern Songliao Baisn, northeast China: Implications for sandstone-type uranium mineralization

摘要

Sequence and sedimentary characteristics play an important role in the mineralization of sandstone-type uranium deposit. Two third-order sequences, including four system tracts namely lowstand system tract (LST), transgression system tract (TST), highstand system tract (HST) and regression system tract (RST), and two sedimentary facies, including braided-meandering river and meandering river are identified in the Upper Cretaceous Sifangtai Formation in northern Songliao Basin. Sandstones with better porosity and permeability formed in LST are good uranium reservoir, and the compacted fine-grained deposits in TST are good confining layers. They form a reservoir-confining assemblage that is beneficial to uranium mineralization. The uranium is mainly hosted in the braided-meandering river depositional system, the channel bar ore-hosting sandstones are connective and have good reservoir property, the maximum porosity and permeability are 42% and 1140 mD, respectively. Suitable thickness and sand percentage of sand bodies are beneficial to uranium enrichment, the industrial grade uranium deposits mainly occur in areas with sand percentage of 0.6-0.8 and sand body thickness of 8-12 m. Organic matter and sulfur act as adsorbents and reducing agents in the progress of uranium mineralization, braided-meandering river with strong hydrodynamic conditions bring a large amount of terrestrial carbon debris, which creates conditions for the formation and adsorption of sulfur and uranium. A sequence and sedimentation uranium metallogenic model of Sifangtai Formation has been established, large and high grade uranium deposits are formed in braided-meandering river sand bodies of LST, which have good reservoir properties, suitable thickness, sufficient organic matter and reducing sulfur content, and regional confining bed.