Optimization of the use of diamond indicator minerals in diamond exploration in kimberlites

Abstract

Ti-rich minerals occur in xenoliths of metasomatized mantle peridotites, MARID and PIC. Xenocrysts produced by disaggregation during the intrusion have equivalent compositions. Early crystallization in the first intrusive stage in kimberlites and related rocks produces olivine phenocrysts that may have trapped inclusions of co-crystallizing rutile, Mg-rich ilmenite to geikielite and chromite. Geikielite may replace Ti-bearing minerals of the xenocrystic suite, producing a suite of magnesian ilmenites of intermediate compositions that follow an Mg enrichment trend depending on the grade of interaction xenocrysts/magma. Phlogopite microphenocrysts can start to crystallize in this stage. A second magmatic stage produces saturation in pyrophanite, that can precipitate or react with all the above Ti-rich minerals, producing crystallization of a suite of intermediate manganoan ilmenites whose compositions depend on that of the replaced Ti-mineral and the grade of interaction of the magma with the above crystals. Lately in this stage, qandilite-ulvöspinel-magnetite start to crystallize, together with perovskite, along with phlogopite microphenocrysts. Finally, the crystallization of ulvöspinel-magnetite crystals may be produced in disequilibria, inducing the development of atoll textures. Groundmass phlogopite crystallize in the late magmagtic stage. Monticellite can be formed in this stage. Interstitial glass can be produced at the end of this stage. Subsolidus hydrothermal processes are widespread in most of the kimberlites, with replacement of the early minerals by hydrous and carbonic fluids of kimberlitic provenance. Olivines and glass are replaced by serpentines and carbonates. A sequence of ilmenites (geikielite followed by pyrophanite) can precipitate directly or replace the above Ti-bearing minerals. Spinels and perovskites may also be replaced by Ti hydrogarnets and late perovskite or kassite accompanied by aeschynite. The composition of the replacing ilmenite depends on that of the replaced mineral. Hence, these ilmenites can retain Nb, Cr, Zr when replacing rutile or perovskite, chromite or crichtonite. Therefore, the trace composition of ilmenite cannot be used to extract petrogenetic information. Mg- or Mn-ilmenites cannot be used as DIM because they are very late minerals formed during the intrusion. When plotting the composition of these minerals in the existing IUGS classification diagrams there is an extensive overlap among kimberlites and related rocks. Thus the existing classification diagrams are not useful and we propose some amendments to the existing classification.