Picroilmenite from Kimberlite Pipes of Central Yakutia

Picroilmenite is one of the most important indicator minerals of kimberlite rocks, which can be used in solving petrological problems and in the search for diamond deposits. The present study shows the results of studying picroilmenite grains from the Manchary and Aprelskaya pipes within the Khompu-May kimberlite field (Central Yakutia). The rocks composing the pipes are represented by porphyritic kimberlite and kimberlite breccia, between which there are gradual transitions. Rocks forming the upper pipe horizons are highly carbonatized and supergenetically altered. Porphyritic segregations are represented by carbonatized serpentine pseudomorphs from macro-, megacrysts and olivine phenocrysts. Pyrope, picroilmenite mega-, macrocrysts and chromospinellide macrocrysts are found in both pipes. Most weakly altered parts of mesostasis are microgranular and formed mostly by phlogopite, with xenomorphic segregations of calcite and serpentine. Picroilmenite in both kimberlite bodies occurs as irregular and rounded macrorysts ranging from 0.7 to 10 mm and megacrysts ranging from 10 to 25 mm. Micrograins of this mineral were not diagnosed in the mesostasis. Individual grains of picroilmenite from the Manchary pipe are surrounded by a polymineral rim composed of either ferrospinel and magnetite, or perovskite and magnetite. High-and low-chromium varieties which correspond to two parageneses are identified among the picroilmenite grains from the Manchary pipe. Crystallization trend of high-chromium ilmenites from the Manchary pipe is clearly seen in the diagram in the coordinates Fe₂O₃-FeTiO₃-MgTiO₃ and associated with the presence of Cr-rich phlogopite from lherzolites xenoliths. Picroilmenite grains from the Aprelskaya kimberlite pipe are more magnesian in comparison with similar grains from the Manchary pipe. Picroilmenite from both pipes in the coordinates Fe₂O₃-FeTiO₃-MgTiO₃ is characterized by a magmatic kimberlite trend of the mineral composition evolution. The distribution of mineral composition points from the studied pipes in the diagram in the coordinates MgO - Cr₂O₃ has form of the "Haggerty parabola" (Haggerty, 1975) - typical for picroilmenites from kimberlites of industrial diamond-bearing middle Paleozoic pipes of Yakutia (Aikhal, Mir, Udachnaya). In general, picroilmenite of Central Yakutia pipes differs from picroilmenite of the Aikhal, Mir and Udachnaya pipes by the presence of the parabola right branch in the Haggerty diagram and an indistinct left branch. The Aikhal, Mir, and Udachnaya pipes are characterized by a clear demonstration of the left branch and a weak right. At the same time, the composition points of the high-chromium picroilmenite variety from the Manchary pipe in the Haggerty diagram coincide with the high-chromium picroilmenite from the Grib kimberlite pipe (Arkhangelsk diamondiferous province). Thus, the study showed the genetic polygeny of picroilmenite from the Manchary and Aprelskaya kimberlite pipes, and also the correlation with mineralogical diamond potential of both pipes traced by comparison with the known industrial ilmenite diamondiferous pipes of Yakutia and Arkhangelsk region.