2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Diamond dissolution during postmagmatic kimberlite alteration

L.G. Kuznetsova and V.B. Vasilenko

Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
e-mail: titan@ uiggm.nsc.ru

In our previous study (Vasilenko et al., 2008), the stability of rock-forming, rare, and rare-earth elements during secondary kimberlite alteration was studied by the new method of the assessment of secondary kimberlite alteration from the content of secondary quartz (Q). Similarly, the hypothesis of diamond dissolution during postmagmatic alteration can be tested.

At the first glance, postmagmatic processes, which occur in kimberlites at low and medium temperatures, cannot influence the amount and appearance of diamonds. Nevertheless, geological descriptions of contacts between basaltoids and kimberlites record changes in diamond crystal morphology in the latter and a decrease in their sizes towards the contact with dolerites. Studies of the metasomatic alteration of kimberlites of pipe 4 (Krasnopresnenskaya) under the action of an intruding dolerite sill have shown that the zone of kimberlites experiencing metasomatism in dolerite endocontacts is 20 to 120 m thick (Shamshina et al., 1988). The same authors have detected traces of diamond dissolution in metasomatic kimberlites, and the amount of these traces increases towards the contact with dolerites. As suggested by Zinchuk (2000), kimberlite minerals (picroilmenite, pyrope, and chrome-spinellide) are unstable in metasomatism zones and they undergo profound physicochemical changes. Diamond is also unstable under these conditions. It undergoes catalytic oxidation (Khar'kiv et al., 1980). Therefore, it is reasonable to expect the presence of specific cavities on diamonds and an overall decrease in diamond content in metasomatism zones.

The hypothesis can be tested by comparison of the contents of diamonds with the amounts of secondary quarts in samples or groups of samples. The definition of a single kimberlite diamond potential test and its relation to petrochemical data should be clarified.

Diamond potentials were determined during prospect drilling in a core sample 4–8 m in length and 50 to 150 kg in weight. Several weights were taken from this bulk sample for petrochemical analysis. The mean composition of such a weight was correlated with diamond content. The increase in the contents of secondary quartz, produced by hydration of olivine and phlogopite, reflects the degree of kimberlite alteration. The content of secondary quartz (Q) is calculated as Q = SiO₂ – 0.81 MgO – 2.80 K₂O.

In the correlation of variations in diamond (D) and secondary quartz contents, primary data on sampling intervals were grouped into classes at intervals of 2.0% w/w Q. The mean diamond content was calculated for each class. The mean Q and A values in classes were correlated by constructing an empirical regression line. Regression sequences were obtained for various pipes (Table 1). Each of them points to a decrease in diamond content with increasing Q; that is, with increasing degree of kimberlite alteration. It is pertinent to ask where diamonds were removed to. Studies of granulometric distributions of diamonds in the samples showed that diamond grains dissolved during washing with postmagmatic solutions (Vasilenko et al., 2008). An example is Botuobinskaya pipe, where the decrease in diamond content correlates with a decrease in their mean weight (Table 1).

Table 1. Diamond potentials in rocks with different alteration degrees.

Variation of kimberlite diamond potentials with increasing Q in the pipes examined follows empirical regression lines with negative slopes (Fig. 1). These lines are specific for each pipe. Calculation of the pairwise regression coefficient between LgQ and A for all the 28 mean class contents revealed a significant correlation with r = –0.84 at r₀₁ = 0.51. The corresponding regression line is shown in Fig. 2. These regularities prove the hypothesis of the decrease in diamond content in kimberlites.

Fig. 1. Figurative points corresponding to the compositions of rocks in diamondiferous pipes: ●, Botuobinskaya; ♦, Aikhal; ■, Internatsional'naya; ▲, Udachnaya-West; ∆, Udachnaya-East.

Fig. 2. Variation in the mean relative diamond content in kimberlites of the pipes under study correlated with secondary kimberlite alteration.

Here are some possible practical applications of the detected regularities: (1) Before interpretation of crystallographic and geochemical features of diamonds in kimberlites, one should check whether the rock has experienced secondary alteration. The same is true for other mineralogical and petrochemical features (Table 2). (2) In the confirmation of geophysical anomalies by drilling, the core may contain up to 10% secondary quartz. As shown in Fig. 2, these kimberlites may lose up to 45% of the initial amount of diamonds during postmagmatic alteration. The practical use of this correlation demands consideration of the rock composition, because different compositions show different regression lines.

Table 2. Variation in the contents of rock-forming oxides in kimberlites of Aikhal pipe

References

Vasilenko V.B., Tolstov A.V., Minin V.A., and Kuznetsova L.G. Normative quartz as an indicator of the mass transfer intensity during the postmagmatic alteration of the Botuobinskaya pipe kimberlites (Yakutia) // Russian Geology and Geophysics. 2008, Vol. 49. No. 12. P.894-907.

Zinchuk N.N. Postmagmatic minerals of kimberlties. Moscow: Nedra-Biznes-Tsentr. 2000 [in Russian].

Khar'kiv A.D., Afanas'ev V.P., Kvasnitsa V.N., et al. Signs of catalytic oxidation at the high-temperature action of kimberlite melt on diamonds // Dokl. Akad. Nauk SSSR. 1980. Vol. 250. No. 4. P. 949–952.

Shamshina E.A., Kryuchkov A.I., Rogovoi V.V., et al. Mineralogic features of kimberlite rocks altered by the action of a trap sill // Topomineralogy and Typomorphism of Minerals. Collection of Papers. Yakutsk: Yakutsk Branch of the USSR Academy of Sciences. 1988. P. 47–55 [in Russian].