Ilmenite Group Minerals in the Russia’s Oldest Diamondiferous Kimberlites of Kimozero, Karelia

Abstract

The paper discusses the morphology and compositional variations of ilmenite group minerals from kimberlites of two phases at the Kimozero locality, the oldest in Russia. Phenocrysts of Mn-rich picroilmenite and Fe-rich geikielite in kimberlites of both phases are similar in morphology and composition. Ilmenite from cement in the second-phase kimberlites enriched in Mg and rimming small regularly shaped chrome spinel phenocrysts is not present in the first-phase kimberlites. Ilmenite, manganilmenite, and Fe-bearing pyrophanite (22–24 wt % MnO) abundant in the cement of the second-phase kimberlites are twice as rich in Nb and substantially richer in Mn than ilmenite up to manganilmenite from the cement of the first-phase kimberlites. Ilmenite and manganilmenite of the first-phase kimberlites is enriched in Zn (up to 1.5 wt % ZnO). Ilmenite from the second-phase kimberlites contains up to 3 wt % Cr₂O₃. In Nb concentration, kimberlitic rocks of the Kimozero are similar to those found in other parts of the world (up to 3.5 wt % Nb₂O₅). Significant Mn-enrichment of the ilmenite group minerals is a common feature of Kimozero kimberlitic rocks. It is suggested that kimberlites in which all ilmenite group minerals—from megacrysts and phenocrysts to small segregations in the cement—are enriched in Me, formed with the participation of carbonatite melts with increased alkalinity.