Mantle conditions and kimberlite geochemical criteria controlling diamond survival in kimberlites

Diamonds are formed at high pressure, temperature and low oxygen fugacity conditions in the Earth's deep mantle. However, during their residence in the mantle and transport to the surface by kimberlite magma, they may be exposed to deformation or oxidising conditions outside of their stability field, causing diamond corrosion and resorption processes. The infiltration of silicate-rich melts and hydrothermal fluids are key processes that cause this corrosion (oxidation). Understanding the processes involved in diamond transport and preservation is important for diamond exploration and to assess the potential diamond grade for economic geology of kimberlite deposits. However, a thorough understanding of variations in diamond grade and survival or preservation conditions in the Lunda province (LP) has not yet been developed. Here a diamond preservation index model has been developed. Chemical analyses and petrography both support the view that the observed variations in diamond grade or abundance within Lunda Province (NE Angola) are associated with oxygen fugacity (fO2), oxidation state, viscosity of kimberlite magma, speed of kimberlite emplacement. Variations in temperature and pressure can impose conditions that are sufficiently oxidising for diamond resorption or to form CO2. Several kimberlite pipes were sampled and it is known that they transported different amounts of diamond to the Earth's surface. In this study, both mantle xenoliths and kimberlite rocks from five kimberlite pipes (Catoca, Camatxia, Camagico, Caixepa and Camutue) from the Lucapa graben (NE Angola) were investigated, with the highest diamond concentrations occurring in LP. Alteration characteristics of kimberlite indicator minerals (KIMs) from the sampled deposits were investigated by optical microscopy, SEM, electron probe microanalysis (EMPA), XRF, XRD and 57Fe Mossbauer spectroscopy. The results of petrography and geochemical studies of peridotite, eclogite and kimberlite rocks during this project have revealed that the sampled kimberlite diamond deposits have experienced several alteration processes and the analysed diamond indicator minerals (olivine, garnet, spinel, ilmenite and pyroxene) reflect these alteration / redox reactions. The hydrothermal-metasomatic processes are associated with factors that controlled kimberlite emplacement. Mossbauer spectroscopy and EMPA results reveal that in the diamondiferous pipes of Camatxia, Caixepa and Camutue, iron redox ratios in ilmenites are considerably lower, indicating lower ƒO2 and consequently better diamond preservation conditions than for Catoca diamonds. The observed low diamond quality from Catoca pipe and the higher measured Fe3+/ΣFe ratio of Catoca indicator minerals agrees with a model which supports the use of ilmenite redox ratios to measure diamond and estimate the resorption on diamond. Results from Mossbauer and EMPA suggest that even with higher Fe3+/ΣFe Fe3+, and a lack of the original Mg-rich ilmenite and complex ilmenite formation, the presence or mineralization of diamond within Lucapa graben kimberlites cannot be ruled out.