Discrimination of diamond resource and non-resource domains in the Victor North pyroclastic kimberlite, Canada

The Jurassic Victor kimberlite complex comprises several steep-sided pipes, including the Victor North pyroclastic kimberlite (VNPK) pipe which encompasses one of the main diamond resources in the mine plan. The VNPK pipe also contains some low-grade domains that appear to be non-economic. Distinguishing between the high- and low-grade units within this pipe is complicated by the gradational nature of the boundary.In this paper, detailed petrography and mineral chemistry analyses are used to establish reliable discriminators for the high- and low-grade kimberlite units. The high- and low-grade units are both spinel-bearing carbonate group-I kimberlites; dolomite is more abundant than calcite in all units. Based on textural relationships and composition, it is inferred that the carbonate is primary. Oxygen fugacity estimates on groundmass intergrowths of rutile and ilmenite indicate similar oxidation states for the magmas that produced the high- and low-grade kimberlite deposits. However, chromite core compositions within zoned groundmass spinels are markedly different between the high- and low-grade kimberlite units. In addition, compositions of groundmass ilmenite and dolomite show the same pattern as core compositions of chromite: the groundmass minerals in the low-grade unit have lower magnesium-numbers than corresponding phases in the high-grade unit. The chemical compositions of groundmass chromite and ilmenite suggest crystallisation with olivine, and thus record the oxygen fugacity associated with the early crystallisation history of the kimberlite magma.These observations suggest that the high- and low-grade units were formed from two distinct magma pulses that had a different origin in the asthenosphere and/or experienced different crystallisation histories. The similar oxygen fugacity estimates for the high- and low-grade kimberlite magmas indicate that differential resorption of diamond was not responsible for the grade difference. Therefore, it is most likely that the individual magma pulses entrained contrasting amounts of diamond during their ascent towards the surface, causing the observed grade differences in the VNPK.