Recrystallization of diamonds at the kimberlite serpentinization

Vasilenko V.B., Tolstov A.V., Kusnetsova L.G., Minin V.A.

V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia. tols61@mail.ru

 

Petrochemical features of Yakutian serpentinized kimberlites allow us to outline two tendencies that clearly evident in secondary alteration of rocks (Vasilenko et al., 2008, 2009: Tolstov et al., 2009). The first tendency is decrease of diamond grade of kimberlites accompanied by decrease in diamond crystal sizes and second is sharp increase of diamond grade of the rocks. The model of diamond grade increasing process proposed here are developed on the basis of study of  kimberlite sampling interval (appx. 10km of drill core). The comparing groups were divided in the rocks with different alteration degree of Botuobinskaya, Nurbinskaya and Mayskaya pipes using concentrations of secondary SiO₂ (Q = SiO₂ - 0.81 MgO – 2.8 K₂O) in different fractions.

Average diamond grain sizes of different size fractions in the primary and altered kimberlites are changes in opposite direction. In the fraction of -8+4 and -4+2mm of Botuobinskaya pipe diamonds the average size of crystals are decreases with increasing alteration degree but increases in all other size fractions.  In the Nurbinskaya pipe average diamond grain size is increases in the two coarsest fractions and decreases in finer size fractions and in the Mayaskaya pipe opposite direction of size changes are shown by fraction of -4+2 and -2+1mm.

The tendencies revealed in this study are fully identical to interrelation of coarse and fine crystal sizes in the metasomatic process. Korzhinskii (1955) show that grow of very coarse crystals are realized in non-favorable environment, particularly at the conditions of dissolution and mobility of mineral composition components that are could be realized at diffusional and infiltration metasomatic processes. The small size crystals will dissolve with decrease of component concentrations in the solutions but the coarsest and finest crystals will grow.

Comparison of regularity of crystals size changes in different size fractions with crystal grow behavior at the metasomatic process (Korzhinskij 1955) allow us to make conclusion that serpentinization of kimberlites led to recrystallization and size increase of diamonds and those altered diamonds must be different in morphology and other features from crystals originally present in the rocks. According to data of Zinchuk and Koptil (2003) amount of diamond crystal with signature of recryctallization in the Botyobinskaya pipe kimberlites are higher comparing to the other Yakutian diamond deposits, therefore this data supports our model.

 

References:

1.Vasilenko V.B., Tolstov A.V., Kusnetsova L.G., Minin V.A. Normative quartz as an indicator of mass-transport intensity at postmagmatic alteration of Botuobinskaya pipe kimberlites// Russian Geology and Geophysics. 2008, v 49, p 1189-1204.

2. Vasilenko V.B., Tolstov A.V., Kusnetsova L.G., Minin V.A. Chemical composition and diamond grade of Nyurbinskaya pipe kimberlites at secondary alteration process.// Geochemistry International. 2009, № 11, p 1-9.

3. Zinchuk N.N., Koptyl V.I. Typomorphism of Siberian platform diamonds. Moskow, “Nedra”. 2003, 603p.

4. Korzhinskii D.S. Essay of Metasomatic Processes.// Main Problems in Studying Magmatogenic Ore Deposits (AN SSSR, Moscow, 1955), pp. 334–456.

5. Tolstov A.V., Minin V.A., Vasilenko V.B., Kusnetsova L.G. A new body of highly diamondiferous kimberlites of the Nakyn field of Yakutian kimberlite province.// Russian Geology and Geophysics. 2009, v 50, p 227-240.