Specific composition of minerals from groundmass of kimberlites of the diamondiferous deposits of Yakutia

D.А. Yakovlev,S.I. Kostrovitskiy, L.F.Suvorova

Institute of Geochemistry, SB Russian Academy of Sciences, Irkutsk, Russia

The Yakutian kimberlite province, occurring in the north-east of the Siberian platform, includes five diamondiferous fields clustered in the center of the Siberian craton. The study defined the distribution of groundmass minerals in rock, the ratios and variations of chemical composition of groundmass minerals depending on the texture-structural and petrochemical features of kimberlites from the pipes of the Daldyn, Alakit-Markhin, Maly Botuoba and Verkhne-Munsky field (Yakovlev et al. 2009). The petrochemical types of kimberlites have been identified from the contents of principal oxides for the Yakutian diamondiferous province (Kostrovitskiy et al., 2004). All diamondiferous fields, but Nakyn, contain two petrochemical types of kimberlites: (1) magnesium low-Ti, low-K (Aikhal and International pipes) and (2) magnesium-ferriferous high-Ti, low-K (Udachnaya, Zarnitsa, Aikhal, Yubileynaya and Zapolyarnaya pipes). The kimberlites are commonly serpentinized (serpentine from 20 to 90%) and contain from 15 to 50% of carbonate in the groundmass, producing small xenomorphic grains, in places associated with serpentine it is involved in the composition of pseudomorphoses after olivine. The freshest ones are kimberlites of Udachnaya pipe containing from 20 to 65% of olivine in the groundmass. It is noteworthy, that the same samples show higher content of carbonate (20-50%). The exception is an essentially silicate, serpentine-bearing (75-80%), kimberlite breccia from Udachnaya-Zapadnaya pipe. An increased content of olivine in the brecciated porphyry kimberlite deep horizons of International pipe (olivine 20-35%), and most pipes of the Verkhne-Munsky field. Fresh olivine in the kimberlite groundmass of the Yubileynaya and Aikhal pipes was preserved as rare relics. In the kimberlites of the Zarnitsa pipe olivine is completely serpentinized (serpentine 65-70%). A slight positive correlation was revealed between magnesium and NiO contents (0.2-0.4 mass %) in fresh olivine of в Udachnaya-Vostochnaya pipe. In most zonal olivine phenocrysts the ferrum content increases from the center to the edge. The olivine with an inverse zonation occurs more rarely (the ferrum content decreases from the center to the edge).

The groundmass spinel occurs as grains, sized from 5 to 30 µm, idiomorphic zonal grains, sized from 5 to 150 µm, differently corroded on the edges. The homogeneous grains are basically titanium magnetite, much more rarely magnesium chromite or ulvospinel. They are different from the Udachnaya-Zapadnaya pipe and other explored pipes. Higher spinel abundances in the groundmass are defined in kimberlites of Udachnaya-Vostochnaya, Yubileynaya, Aikhal and Zarnitsa pipes. The groundmass of kimberlites of magnesium-ferriferous petrochemical type (in Udachnaya, Zarnitsa, Zapolyarnaya and Novinka pipes) there is a combined occurrence of spinel and perovskite: diverse aggregates of titanium magnetite with perovskite, their joint growth in the rims of zonal chromites, magnetite rims around perovskite, that might indicate their simultaneous formation. By the diagram in figure 1, practically all zonal spinel of the groundmass lies in the field of chromite-ulvospinel-magnetite trend 1 (after Mitchell, 1986). Mostly oxidized is spinel from the Zarnitsa pipe and International pipe with higher chromite (from 44 to 52% mass % Cr₂O₃). Noteworthy, there is no ulvospinel in kimberlites of Zarnitsa pipe. The porphyry kimberlites of Yubileinaya and Aikhal pipes are devoid of titanium magnetite, but contain higher ulvospinel than in the other pipes.

Fig. 1. Diagram Fe²⁺/(Fe²⁺+Mg)- Fe³⁺/(Fe³⁺+Al+Cr) for groundmass spinel.

Perovskite is common for porphyry kimberlite groundmass. The kimberlites of the Verkhne-Munsky field are show dominance of perovskite over picroilmenite. In kimberlites of International pipe perovskite was found only in the sample of porphyry kimberlite with fresh olivine, whereas in the Aikhal pipe perovskite was not discovered. Perovskite is zonal, has rims composed of titanium magnetite, rarely Mn-ilmenite. As a rule, the CaO content increases from the center to the edges, while the contents of REE decreases.

Phlogopite is present in the groundmass of all investigated kimberlites. In places the phlogopite crystals are split by a thin system of fractures, deformed and display zonation. The higher content of phlogopite in the groundmass occurs in porphyry varieties of kimberlites from pipes International, Yubileiny and Aikhal. In porphyry kimberlites with fresh olivine from Udachnaya-Vostochnaya phlogopite is much more infrequent than in kimberlite breccias of Udacnnaya-Zapadnaya pipe. The phlogopite groundmass from Zarnitsa pipe contains more titanium (2-4 mass % TiO₂) against phlogopite from Yubileynaya, Aikhal, Deimos and International pipes (about 2 mass % TiO₂). Phlogopite in porphyry kimberlites of many pipes are distinguished by the zonation: from the center to the marginal parts the BaO content increases from 1 to 8 mass %, and K₂O content decreases from 10 to 4 mass %. Phlogopite of groundmass is subdivided into two groups considering TiO₂ content (fig. 2): low-titanium less than 2 mass % TiO₂ (Aikhal, International and Novinka pipes) and low-Ti from 2 to 4 mass % TiO₂ (Udachnaya, Zarnitsa and Yubileynaya pipes), basically corresponding to the magnesian and magnesian-ferriferous types of kimberlite.

Fig. 2. Diagram Fe²⁺/(Fe²⁺+Mg)- Fe³⁺/(Fe³⁺+Al+Cr) for groundmass phlogopite.

Apatite was discovered in the groundmass of kimberlites of all texture-structural types of Aikhal, International and Novinka pipes referred to the magnesium type. In the groundmass of Yubileynaya, Zarnitsa and Udachnaya pipes apatite is very rare. It is remarkable that samples with picroilmenite never contain apatite in the groundmass. The minimum contents of CaO in apatite are measured in porphyry kimberlite, maximum in more altered and carbonated breccias. The content of SrO (1.5-2.2 mass %) and F (2-4.5 mass %) in apatite from the Verkhne-Munsky kimberlites (Novinka pipe) is much higher than in the other kimberlites. In the Aikhal pipe kimberlites the maximum REE contents (0.8-0.9 mass % CeO) in apatite are defined in porphyry kimberlites, minimum ones are determined in carbonated kimberlite breccias.

References:

Kostrovitsky S.I., Morikyo T., Serov I.V., Rotman A. Ya. On kimberlite origin // Doklady RAS, 2004, v. 399, № 2, p. 236-240.

Yakovlev D.A., Kostrovitsky S.I., Alymova N.V. Mineral composition features of kimberlites from the Upper Muna field (Yakutia) //«News of Dept. Earth Sci. RANS. Geology, prospecting and exploration of ore deposits », 2009, N. 1 (34), с. 111-119.

Mitchell, R. H. (1986). Kimberlites: Mineralogy, Geochemistry and Petrology. NewYork: Plenum, 442 pp.