Mined sub-aerially stored kimberlite provided a natural laboratory in which to examine the potential for carbon sequestration in ultramafic materials. A 15 cm hand sample of ~50-year-old ‘cemented’ coarse residue deposit (CRD) collected from a cemented surface layer in the Cullinan Diamond Mine tailings in Gauteng, South Africa, demonstrated the encouraging effects of weathering on mineral carbonation of kimberlite. The examination of petrographic sections using light microscopy, X-ray fluorescence microscopy (XFM) and backscatter electron – energy dispersive spectroscopy demonstrated that weathering produced extensive, secondary Ca/Mg carbonates that acted as an inter-granular cement, increasing the competency of the CRD, i.e., producing a hand sample. Nearly every grain in the sample, including primary, un-weathered angular carbonate clasts were coated in secondary, μm- to mm-scale carbonate layers, which are interpreted as secondary materials. DNA analysis of an internal, aseptic sample of secondary carbonate revealed that the weathered kimberlite hosts a diverse micro- biome consistent with soils, metal cycling and hydrocarbon degradation that was found within the secondary carbonate, interpreted as a biomaterial. The formation of secondary carbonate demonstrates that ‘waste kimberlite’ from diamond mining can serve as a resource for carbon sequestration.

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