Carbonatitic metasomatism in ultramafic rocks at the Blue Hill, Namibia

Carbonatite metasomatism at crustal pressures can significantly modify igneous and sedimentary rocks and understanding these processes is critical for interpreting mantle-derived magmas and their associated carbonate-bearing fluids. The Blue Hill complex in Namibia, located within the carbonatitic Gross Brukkaros volcanic field, represents an ideal natural laboratory to study the interaction between ultramafic lamprophyres (damtjernites) and carbonatitic fluids. In this setting, the damtjernites occur in direct spatial association with numerous carbonatite dykes, enabling detailed investigation of alteration processes. Evidence from olivine, spinel and whole-rock chemistry supports a mantle-derived UML parental melt that experienced post-emplacement CO₂ degassing, contributing to monticellite formation and possibly the adjacent fenitization of the country rock. Three distinct alteration types are identified: textures A (serpentine-magnetite-phlogopite-chlorite) and B (pectolite–apatite–hydrogarnet–calcite), which occur concentrically within the main complex, whereas C (phlogopite-serpentine-apatite-hydrogarnet-calcite) forms crosscutting veins. This study aims to explore to what extent the alteration of the damtjernite reflects carbonatitic activity within the Gibeon province. Texture A records mainly deuteric (late-magmatic) alteration with minor meteoric input, while texture C shows a clear carbonatitic signature with Ba-Al-rich phlogopite, Mg-calcite and abundant apatite. Texture B remains more complex and might represent either a separate CO₂-rich fluid, possibly related to texture C, or the evolved residue of the fluid responsible for texture A. Geochronological data is in accordance with a multi-stage history of magmatism and fluid interaction, with emplacement of the Blue Hill at 84 ± 4 Ma and later resetting at 60 ± 14 Ma. Notably, the first reported age for the nearby Gross Brukkaros carbonatite (66 ± 8 Ma) appears to be younger than the Blue Hill emplacement. These constrains provide one of the few documented examples of carbonatitic liquids modifying pre-existing ultramafic rocks, expanding our understanding of metasomatism in carbonatite-bearing provinces. • Alteration of ultramafic rocks by carbonatitic activity. • Three alteration textures reflect separate melt/fluid-related processes. • Texture C records carbonatitic metasomatism with Ba-phlogopite and Mg-calcite. • Geochronology confirms multi-stage overprinting.