Fe-rich and Na-rich megacryst clinopyroxene and garnet from the Luxinga kimberlite cluster, Lunda Sul, Angola

Abstract

The Luxinga kimberlite cluster forms part of a kimberlite field partially hosted by the Alto Cuilo exploration project in Lunda Sul province, northeast Angola. Compositional analysis of kimberlite indicator minerals recovered from drill core representing 18 of the Luxinga-area kimberlites has highlighted the presence of a megacryst mineral suite with significantly fractionated compositions, i.e. poorer in Mg/(Mg+Fe) than commonly described in existing literature. Garnet megacrysts are relatively common at Luxinga, clinopyroxene megacrysts are rare, and low-Cr ilmenite megacrysts are recorded present, but are not quantified in this work. The garnet and clinopyroxene megacrysts show elevated Na 2O content and Na/Ti ratios across a range of Mg/(Mg+Fe), and specifically at low Mg/(Mg+Fe) of ~ 0.58 and ~ 0.81 respectively. The unusual high-Fe, high-Na megacryst garnet compositions overlap with sodic garnet compositions typically considered characteristic of diamond-facies eclogite, websterite or pyroxenite, resulting in high “eclogitic” diamond potential assessments for several kimberlites in the Luxinga cluster. However, our exploration work has demonstrated low, or no diamond content in kimberlites from Luxinga. We have drawn on data for many different low-Cr garnet types at Luxinga and from a global database to design simple screens that isolate unusual Fe- and Na-rich garnet compositions. We document that they are related by fractionation to common garnet megacrysts (G1 garnets) and hence institute a “reclassified megacryst garnet” (G1R garnet) as an additional category to a recently-published garnet classification scheme. G1R garnets constitute 31% of the analysed megacryst population at Luxinga and would clearly impact the diamond potential perceived for the kimberlites. Our global database of garnet compositions suggests that such Fe-rich and Na-rich megacrysts are comparatively rare, however, and general implementation of the new G1R classification by diamond explorers is not necessarily indicated.