©2014 Society of Economic Geologists, Inc. Economic Geology, v. 109, pp. 775–783 SCIENTIFIC COMMUNICATIONS ILMENITE AS A DIAMOND INDICATOR MINERAL IN THE SIBERIAN CRATON: A TOOL TO PREDICT DIAMOND POTENTIAL LAURA CARMODY, 1,† LAWRENCE A. TAYLOR, 1 KEVIN G. THAISEN, 1 NIKOLAY TYCHKOV, 2,4 ROBERT J. BODNAR, 3 NIKOLAY V. SOBOLEV, 2 LYUDMILA N. POKHILENKO, 2 AND NIKOLAY P. POKHILENKO 2 1 Planetary Geosciences Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA 2 V.S. Sobolev Institute of Geology and Mineralogy, Russian Academy of Science, Siberian Branch, Novosibirsk, Russia 3 Department of Geosciences, 4044 Derring Hall, Virginia Tech, Blacksburg, Virginia 24061, USA 4 Novosibirsk State University, 630090, Novosibirsk-90, 2 Pirogova Str., Russia 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, chro- mite, 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 dia- mond 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. Introduction Diamond formation is thought to occur most commonly within the upper mantle between 120 and 200 km (Gurney et al., 1993; Taylor and Anand, 2004; Shirey et al., 2013), but can extend into the transition zone and lower mantle (~660 km; Kaminsky et al., 2001, 2009). Diamond petrogenesis is intrin- sically linked to the migration of fluids (melts or volatile- bearing fluids), and it is possible to determine the chemistry of these fluids due to their encapsulation within diamonds. Such research has led to the proposal that diamond forma- tion involves carbonate-bearing, volatile-rich brines (Navon et al., 1988; Izraeli et al., 2001; Pal’yanov et al., 2005; Paly- anov et al., 2007; Kopylova et al., 2010). As a reduced-carbon phase, diamonds must be transported to the more oxidized surface environment rapidly to prevent both retrograde reac- tions to their low-pressure graphite structure and resorption of the diamond, resulting in a decrease in size (Gurney and Zweistra, 1995). This rapid transport mechanism is provided by the rapid ascent of volatile-rich kimberlite and lamproite magmas through thick, cratonic, continental interiors. These melts are capable of transporting xenocrystal diamonds and diamondiferous xenoliths to the surface (Gurney, 1989; Gur- ney et al., 1993). However, only about 10% of kimberlite pipes are diamond bearing (Spetsius and Taylor, 2008), and even the most diamond-rich kimberlites only contain ~0.000007% (~70 ppb) diamond. Thus, searching for diamonds to explore for diamondiferous kimberlites is not very effective and is equivalent to searching for the proverbial needle in a haystack. However, kimberlites contain a variety of mineral phases that serve to characterize and identify them, and these kimberlite indicator minerals represent a much more effective tool, com- pared to diamonds, to search for diamond deposits. Thus, it is highly desirable to improve and enhance kimberlite indicator mineral evaluation techniques in prospecting for diamonds in order to prioritize the economic potential of a kimberlite or lamproite. Major and minor minerals in rocks closely associated with kimberlites and diamonds are much more abundant than the diamonds in the rocks, and these more common mineral phases are commonly used in exploration. Minerals that can indicate the presence of a kimberlite can be extracted from stream and soil samples. Kimberlite indicator minerals form within the same mantle region as the diamonds and are sub- sequently transported to the surface by the kimberlitic mag- mas. The most common indicator minerals used in diamond exploration are garnet, chromite, olivine, clinopyroxene, and ilmenite (Sobolev et al., 1969, 1973, 1997; Mitchell, 1973; Dawson and Stephens, 1975; Sobolev, 1977; Griffin et al., 0361-0128/14/4209/775-9 775 Submitted: December 5, 2012 Accepted: June 10, 2013 † Corresponding author: e-mail, l.carmod1@utk.edu