Tailings affected by neutral mine drainage (NMD) have received comparatively little attention despite their potential to release trace elements, including rare earth elements (REE) and sulphate to downstream hydrological systems. This study presents a comprehensive geochemical characterisation of apatite iron ore (AIO) tailings from Kiirunavaara mine in northern Sweden, providing baseline geochemical and mineralogical data needed to evaluate the long-term stability of REE and other elements of potential concern in NMD conditions. Three vertical cores (KI_01, KI_02, and KI_03; up to 10 m deep) and groundwater samples were examined. The tailings were dominated by silicate minerals (ca. 74 wt%, albite, biotite, quartz, actinolite) with smaller amounts of carbonates (calcite, dolomite), sulphides (pyrite, chalcopyrite) and sulphates (gypsum, anhydrite). Hydraulic sorting during deposition created clear textural and compositional differences, with coarser material near the discharge points and finer material further away. The tailings contained average light REE and heavy REE content of 1100 ppm and 180 ppm, respectively, hosted in primarily unaltered apatite with minor monazite and allanite. This is supported by strong correlations between REE, P, F and As, and by intact grain boundaries in mineral analyses. The groundwater in the tailings was circumneutral to alkaline (pH 6.8–9.7) and mostly anoxic. Dissolved REE concentrations in groundwater were low, with ΣREE average value of 0.29 µg/L, showing that REE mobility is limited under current conditions. Sulphide minerals displayed minimal oxidation, reflected by low dissolved O2 and the absence of secondary iron phases, suggesting restricted oxygen ingress. In contrast, gypsum and anhydrite showed signs of dissolution, contributing to elevated sulphate and calcium in groundwater, especially during spring melt.