Specific composition of picroilmenites from kimberlites of the Yakutian diamondiferous province

Alymova N.V., Kostrovitsky S.I., Yakovlev D.A.

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

The prospecting targeted at discovering new kimberlite bodies is given much attention in terms of examination of indicator minerals of diamonds, like picroilmenite, garnet and spinel. Picroilmenite is referred to the most important common minerals of kimberlite rocks applied for prospecting purposes, therefore it is critical to uncover the differences in the composition of ilmenite from different fields of the Yakutian province. Sobolev N.V. pointed to the necessity to use picroilmenite to locate kimberlite fields (1980). However, the efforts of the other researchers (Iljupin et al., 1992; Genshaft et al., 1983; Garanin et al., 1984) to define specifics of composition of ilmenite from different fields did not succeed. It is probably because the authors made efforts to compare statistic parameters of composition distribution. The conclusion on reduction of total magnesium and chromium contents of ilmenite in the northern fields of the province against the southern ones (Tabunov, 1979) is not verified. In our data, ilmenite are characterized by a wide variation of compositions within entire Yakutian province and. The compositions do not vary much, but the Maly Botuoba field, where kimberlites contain a widespread ferro-magnetic variety of the mineral. It is more promising and informative to consider the trends of ilmenite composition variability.

The compositional features of minerals nearly from all kimberlite bodies and secondary haloes within the field were first compared at the Daldyn field (Alymova et al., 2004). Available data indicate that: (1) The average composition of ilmenite from heavy fraction of kimberlites in the pipe is invariable value not depending on sampling site, texture-structural type of kimberlites, phase of intrusion; it can be correctly defined only due to a representative number of analyses; (2) The kimberlite pipes of one cluster within one field show similar trends of mineral composition (within coordinates TiO₂, FeOtot, MgO and Cr₂O₃). The mineralogical passportization of available pipe in the Daldynskoe field essentially extended prospecting capacities to discover new kimberlite pipes.

More detailed investigations were performed to define the compositional features of picroilmenite in the other southern diamondiferous fields (Kostrovitskiy, 2006). Recently, the Amakin Party of the ALROSA JSC have made up the passports of the main kimberlite bodies from the northern poorly studied kimberlite fields of the Yakutian province.

Each of studied kimberlite bodies of the Yakutian province contains picroilmenite with distinct individual characteristics of crystallization trends (Fig. 1a, b). It is presumed that the source the melt for picroilmenite from kimberlites of the Yakutian province was the same that is indicated by affinity of their averaged compositions. But crystallization of ilmenite in different fields evidently proceeded under varying specific conditions causing appearance of different crystallization trends.

Fig. 1 a. Correlation plot (MgO-Cr₂O₃) for compositions of picroilmenite from kimberlites of the Yakutian province.

Fig. 1 b. Correlation plot (MgO-Al₂O₃) for compositions of picroilmenite from kimberlites of the Yakutian province.

This study was financially supported by RFBR integration projects N 23.1 and 72.

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