Carbonatites undergo various magmatic-hydrothermal processes during their evolution that are important for the enrichment of rare earth elements (REE). This geochemical, petrographic, and multi-isotope study on the Kangankunde carbonatite, the largest light REE resource in the Chilwa Alkaline Province in Malawi, clarifies the critical stages of REE mineralization in this deposit. The δ⁵⁶Fe values of most of the carbonatite lies within the magmatic field despite variations in the proportions of monazite, ankerite, and ferroan dolomite. Exsolution of a hydrothermal fluid from the carbonatite melts is evident based on the higher δ⁵⁶Fe of the fenites, as well as the textural and compositional zoning in monazite. Field and petrographic observations, combined with geochemical data (REE patterns, and Fe, C, and O isotopes), suggest that the key stage of REE mineralization in the Kangankunde carbonatite was the late magmatic stage with an influence of carbothermal fluids i.e. magmatic–hydrothermal stage, when large (~200 µm), well-developed monazite crystals grew. The C and O isotope compositions of the carbonatite suggest a post-magmatic alteration by hydrothermal fluids, probably after the main REE mineralization stage, as the alteration occurs throughout the carbonatite but particularly in the dark carbonatites.

This study presents a potential advancement for passive mine water treatment, transforming it from a purely remedial activity into a proactive carbon capture technology, by addressing the limitations of a conventional system in acid mine drainage (AMD) system. By introducing crushed basalt into an AMD system at the Shojin River, Japan, we utilize the acidic nature of the system to accelerate enhanced rock weathering (ERW). ERW is a carbon reduction process that utilizes crushed ultramafic rock powder to capture atmospheric carbon dioxide (CO2). This approach slightly raised the pH, thereby increasing the Fe colloid precipitation and stability for improved As sequestration, while the same weathering process simultaneously mineralizes atmospheric CO2 into stable bicarbonate alkalinity. In contrast, stable ferrihydrite colloids were observed at our circumneutral control site (Ainai Mine drainage), thereby emphasizing the pH-dependency of remediation in passive treatment systems. While most ERW research has focused on agricultural soils, this study explores a new frontier — evaluating ERW efficacy in mine-impacted environments. These settings provide naturally acidic and metal-rich conditions that can substantially accelerate silicate dissolution and CO2 drawdown. The insights gained here establish a foundation for expanding ERW applications to diverse geochemical fields such as mine tailings, industrial effluents, and water treatment systems.