Depositional architecture and post-depositional alteration of the Toutunhe Formation (J2t) in the Louzhuangzi area, Southern Junggar Basin: Implications for uranium mineralization

In recent years, economically significant sandstone-hosted uranium mineralization has been identified in the Louzhuangzi area along the southern margin of the Junggar Basin. However, the controls on uranium enrichment and their links to depositional architecture and post-depositional fluid processes remain insufficiently constrained. This study integrates field geological investigations, drill-core lithofacies logging, and systematic sampling with petrographic and micro-analytical techniques, including optical microscopy, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). The objective is to elucidate the depositional characteristics, alteration processes, and uranium occurrence mechanisms of the ore-bearing sandstones within the Toutunhe Formation (J2t). Results show that the upper member (J2t2) represents meandering-river deposits, whereas economically significant uranium mineralization is hosted in the lower member (J2t1), characterized by braided-river sandstone units with high permeability. The sandstones of the Toutunhe Formation (J2t) exhibit intense oxidation by surficial fluids, overprinted by post-mineralization hydrothermal alteration and sulfide-forming alteration associated with reducing fluids. Uranium is closely associated with pyrite, organic matter, and clay minerals. Uranium minerals are dominated by coffinite and pitchblende (~68%), with UO₂ contents of 52.89-86.07%. Minor Ti-bearing uranium phases (~16%), interpreted as possible brannerite, contain 38.01-41.46% UO₂ and 31.67-36.09% TiO₂, while nanoscale uranium minerals (~16%) show UO₂ contents of 9.15-60.28%. These results indicate that uranium mineralization was controlled by the coupling of braided-channel architecture and multi-stage fluid processes. Uranium was initially precipitated from oxidized fluids and subsequently modified and preserved by later thermal and reducing fluids, highlighting the importance of multi-fluid interactions in sandstone-hosted uranium systems.