Abstract: The construction of China’s underground laboratory for high-level radioactive waste disposal has revealed the F34-1 fracture zone within the spiral ramp, providing a unique opportunity to assess its barrier properties. This study combines field geological surveys with mineralogical and geochemical analyses to characterize the fracture zone and evaluate its sealing capacity. F34-1 strikes NNE (dip direction 115°, dip angle ~85°) and narrows with depth, exhibiting wedge-out geometry. Groundwater inflow decreases sharply downward, indicating negligible hydraulic connectivity with the regional system. Fracture infill is composed mainly of calcite, quartz, and clay minerals, with clays dominating shallow sections and calcite–quartz assemblages prevailing at depth. Geochemical results show Si–Al enrichment and low Fe–Ca contents, reflecting strong sorption and retardation potential for radionuclides. Trace element signatures indicate hydrothermal alteration and mineral precipitation in deeper zones, further reducing permeability. Isotopic dating (~257 Ma) suggests formation contemporaneous with host rock diagenesis, confirming long-term stability. These findings demonstrate that F34-1 possesses excellent sealing capacity and radionuclide retardation ability, providing key evidence for near-field barrier performance at the Beishan Underground Laboratory.