Oxide minerals of Kaavi kimberlites (Finland)

The mineralogical and petrogeochemical features of the Neoproterozoic kimberlite rocks of the Lahtojoki and Niilonsuo pipes of the Kaavi cluster (Kaavi-Kuopio, Finland) have been studied, differences in their petrogeochemical composition, quantitative and chemical composition of oxide minerals of deep (mantle) and kimberlite genesis have been revealed. The kimberlites of the pipes are moderately titanic, but the TiO2 content in the kimberlites of Niilonsuo is higher (2.11 wt.%) than in the kimberlites from the breccia of the Lahtojoki pipe (1.07 wt.%). The kimberlites of the Niilonsuo pipe also differ in higher concentrations of Fe2 O3 , Ca, P, K, Rb, V, Nb, Ba, Th, U, Ta and REE. In the Lahtojoki kimberlite breccias the main TiO2 concentrator mineral is magnesian ilmenite (13,3—15,2 wt.% MgO; 0,5—4,4 wt.% Cr2 O3 ), (macrocrysts up to 4 mm); the fine-grained matrix of rocks contains small grains of rutile, chromespinelides, Mn-ilmenite and sometimes titanomagnetite. Macrocrystals of magnesian ilmenite have been not found in the kimberlites of the Niilonsuo pipe, perovskite acts as the main mineral of titanium, and chromespinelids and titanomagnetite are less common. Long-term crystallization of relatively large (up to 200 μm) perovskite grains proceeded according to estimates using an Nb-Fe-perovskite oxybarometer under a wide range of oxygen fugacity (fо2 ) of the kimberlite melt (NNO from -3,8 to 5,1). Chromespinelids from the groundmass of kimberlite pipe rocks differ in composition, but have the same specific zonality — enrichment of Al and Mg in the edge zones of crystals, which is possibly due to the dissolution of phlogopite phenocrysts in the rising kimberlite melt. In addition to oxide minerals, djerfisherite is widely distributed in the groundmass of kimberlites of the Niilonsuo pipe, the composition of which for the rocks of the body has been described for the first time. The combination of features of oxide mineralization indicates unfavorable conditions for the preservation of diamonds during their transportation by kimberlite melt.

Finland, Kaavi-Kuopio, kimberlites, mineralogy, perovskite, spinel, ilmenite, djerfisherite, rutile, groundmass

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