llmenite group minerals in the oldest Russian diamondiferous kimberlites of Kimozero, Karelia
Putintseva E. V., Spiridonov E.M.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2017. V. 146. N 3. P. 69-86
Morphology and variations in composition were studied for ilmenite group minerals from two hases of rocks of the oldest Russia’s Kimozero kimberlite occurrence. Mn-bearing picroilmenite and Fe-bearing geikielite from phenocrysts in kimberlites of both phases are similar in morphology and composition. Ilmenite in cementing mass of the II phase kimberlite is enriched in magnesium and forms fringes of small regular-shaped chromospinelide microphenocrysts, it is absent in the I phase kimberlites. Ilmenite, manganilmenite and Fe-bearing pyrophanite (with 22—24 wt % MnO) are widespread in the matrix of the II phase kimberlites, they are twice richer in Nb and significantly richer in Mn than ilmenite, up to manganilmenite, in the cementing mass of the I phase kimberlites. Ilmenite and manganilmenite of the I phase kimberlites are significantly richer in Zn (up to 1.5 wt % ZnO). Ilmenite of the II phase kimberlites contains up to 3 wt % Cr2O3. By the Nb content (up to 3.5 % Nb2O5) Kimozero kimberlite rocks do not differ from standard kimberlites of the world. The common peculiarity in composition of the ilmenite group minerals from Kimozero kimberlites is their significant enriching in manganese. It may be assumed that kimberlites in which all types of ilmenite group minerals, from megacrysts and impregnations down to small segregations in the cementing mass, are enriched in Mn, did appear with participation of carbonatite melts with elevated alkalinity.
Key words: kimberlite, ilmenite group minerals, picroilmenite, manganilmenite, manganese-bearing, Kimozero, Karelia.