Weathering and cementation of historic kimberlite residues from South Africa: Implications for residue stabilization and CO₂ sequestration

Enhanced weathering and carbon dioxide (CO₂) mineralization of ultramafic mine wastes, including kimberlite residues from diamond mining, provides secure storage of this greenhouse gas and may physically stabilize mine impoundments. Yet, the outcomes of these processes over extensive periods (i.e., decades) remain relatively unknown. This study examined coarse residues from historic impoundments at the Cullinan and Voorspoed diamond mines in South Africa that have weathered over 50 to >100 years to investigate weathering and cementation pathways. Cemented residues (n = 7) were mainly composed of lizardite (9.6-43.2 wt%), saponite (10.2-34.7 wt%), and augite (6.6-27.8 wt%), and had minor abundances of calcite (1.7-8.8 wt%). Electron microscopy and Raman spectroscopy revealed that three plausible pathways contributed to cementing residues: (1) secondary clay precipitation, (2) carbonate precipitation and recycling, and (3) particle entrainment and infilling. Quantitative mineralogical analyses of the cement (<63 μm) and clast (>63 μm) fractions showed that the abundances of most minerals were similar between fractions, indicating that infilling of pore spaces with fine-grained particles contributed substantially to residue cementation. Stable and radiogenic carbon isotope (¹³C, ¹⁴C) analyses of carbonates indicated limited incorporation of organic matter and atmospheric CO₂, an indication of surface weathering. Residue cementation led to some strength development (unconfined compressive strengths = 0.1-0.2 MPa), demonstrating the potential for mineral weathering to stabilize mine residues without chemical additives. Modifying residue management practices during the operational, closure, and reclamation phases at Cullinan, Voorspoed, and other mines, such as through residue co-disposal or implementing enhanced weathering practices, may improve residue stabilization and CO₂ sequestration.