ILMENITE AS A DIAMOND INDICATOR MINERAL IN THE SIBERIAN CRATON: A TOOL TO PREDICT DIAMOND POTENTIAL

Abstract

Conventional diamond exploration seldom involves searching for diamonds in rock and soil samples; rather, it focuses on the search for “indicator minerals.” Kimberlite indicator minerals include garnet, olivine, chromite, pyroxene, and ilmenite, and these can be used to infer the presence of kimberlites and lamproites in the vicinity of where the samples were collected. Ilmenite has served as an effective indicator mineral for more than 40 years due to its resistance to chemical and physical weathering. As a result of its relatively high density compared to other indicator minerals, ilmenite grains often accumulate in placer deposits downstream from a kimberlite source. Although the ideal formula for ilmenite is FeTiO3, the crystal structure is also favorable to cation substitution owing to similarities in ionic radii and charge between Ti and Fe and other trace elements associated with its formation. We have investigated ilmenite trace element chemical signatures that can be related to the presence of diamond-bearing or diamond-free kimberlites. Our results suggest that the diamond potential of kimberlites is best reflected in the Zr/Nb ratio of ilmenite— these elements substitute for Ti in the ilmenite structure. An extensive compilation of compositions of ilmenite collected from heavy-mineral placers and from 14 kimberlites in northern Siberia (Yakutia) indicates that diamond pipes that have economically favorable diamond grades and abundances are associated with ilmenites having a Zr/Nb ratio of >0.37. Because of this, we suggest that ilmenite trace-element chemistry can be a useful tool to identify high-priority targets for diamond potential on the Siberian craton, and reconnaissance studies of other areas suggest that this relationship may be universally applicable.