Petrochemical criteria of diamond potentials of Yakutian unaltered kimberlites and their petrological meaning

V. B. Vasilenko*, L. G. Kuznetsova*, A. V. Tolstov**, and A. Ya. Rotman**

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

**ALROSA Company, Mirnyi, Sakha Republic, Russia

 

Recently, we have reported a correlation between the contents of K₂O and diamonds in kimberlites of diamond fields of the Vilyui–Markha zone, Yakutian Diamondiferous Province [Vasilenko et al., 2010]. Two quasilinear regression zones have been found: for calcium-rich rocks (CaO > 10 wt%) and for kimberlites with lower calcium contents. Both altered and unaltered kimberlites were included.

Separate consideration of this phenomenon in unaltered rocks showed that the division of kimberlites into more and less rich in calcium was related to secondary metasomatism. Nevertheless, the correlation between diamond and K₂O contents still existed in the form of a single quasilinear sequence (Fig. 1).

Fig.1. Mean compositions of diamond fields.

 

The sequence is described by the empirical equation:

LgA = –1.42 + 1.86 K₂O (1),

where LgA is the logarithm of the mean diamond content in the pipe and K₂O is the mean content of K₂O.

 

The correlation coefficient between LgA and K₂O is 0.95 at r₀₁ = 0.80. It means that the correlation is linear and highly significant. To apply Eq. (1) to practical assessment of the mean diamond content in an unknown pipe from the mean K₂O content, one should make sure that the mean composition of the pipe with regard to certain oxides is in agreement with the compositions of pipes from which the equation was derived. Empirical distributions of frequencies of rock-forming oxide concentrations in Yakutian kimberlites, which can be used for this purpose, are described in [Vasilenko et al., 1997].

A total of 2281 analyses were done to calculate nine mean compositions; 253 analyses per one field on the average. Hence, the diamond potential of an unexplored field can be predicted with a sufficient probability only from a close number of analyses. Prediction from smaller numbers will be less reliable. The application of Eq. (1) can be considered by the example of the Zarya pipe. First, let us check that the contents of rock-forming oxides fall to the corresponding empirical distributions of oxides in reference pipes. This condition is met in our case. Substitute the mean K₂O content in Zarya to Eq. (1). It follows that the mean diamond content in the pipe is 0.17 ct/t, which is close to the true value.

It should be mentioned that the pipes studied form a succession of diamond content and alkalinity values. This observation can be explained by the suggestion that the parental kimberlite melts formed under gradually varying conditions. According to the petrochemical population model of Yakutian kimberlites proposed by V. B. Vasilenko and L. G. Kuznetsova (Vasilenko et al., 1997, 2010, 2002), kimberlite composition is determined by the depth of the parental melt zone. We mean by a population a set of chemical compositions of rocks formed under identical conditions. The deepest kimberlite populations have the lowest contents of TiO₂ and highest, of K₂O (Vasilenko et al., 2010). Correspondingly, the shallowest populations are rich in TiO₂ and poor in K₂O. Population compositions follow this trend from the deepest (population 1) to the shallowest (population 7).

A negative linear correlation was found between diamond contents in fields and TiO₂ contents in kimberlites (Fig. 2).

 

Fig. 2. Mean TiO₂ contents in populations. Numerals from 1 to7 are population numbers.

 

The similarity between the correlations LgA = f K₂O and LgA= f TiO₂ and their opposite directions suggests that the succession of deposits in Fig. 1 is related to the fact that these deposits are formed mainly by a discrete set of close populations, and certain populations are modal in particular deposits. For example, the Aikhal pipe is dominated by populations 1 and 2; Yubileinaya, by populations 3 and 4; and Sytykanskya, by 5 and 6.

The associations of certain populations with particular kimberlite deposits and the housing kimberlite fields reveal a succession of populations from deeper to shallow (see Table).

Table

Relative contents (%) of unaltered kimberlite populations in fields of the Vilyui–Markha zone, Yakutian Diamondiferous Province

Field	Number of analyses	Populations
		1	2	3	4	5	6	7
Mirnyi	283	50	30	6	6	5	3	-
Nakyn	249	46	37	17	-	-	-	-
Alakit–Markha	807	10	9	21	29	20	10	1
Daldyn	1099	-	4	46	37	9	3	1
Upper-Muna	353	-	-	16	49	31	2	2

 

From south to north, fields dominated by deep populations, such as Mirnyi and Nakyn, give way to sets without population 1 and even population 2 in the Upper-Muna field. In our opinion, this observation points to lower pressures in magmatic chambers or shallower chamber locations. It is worth noting that the trends of population spectra in kimberlite fields match the trends of decreasing depths of the lithosphere bottom (Vasilenko et al., 2000). From this match we infer that the sites of parental kimberlite melt formation are confined to the lowermost lithosphere. Thus, kimberlite fields should not necessarily meet the common requirement of heterochronism. It is reasonable to suggest that certain parts of a mantle plume induced selective lithosphere melting in various areas of the region, and different kimberlite parental melt compositions arose depending on the depths of these areas. The relief of the lithosphere bottom and its depth are the principal factors determining the composition and diamond potential of kimberlites in various areas of the province.

 

V. B. Vasilenko, N. N. Zinchuk, and L. G. Kuznetsova, Petrochemical models of diamond deposits in Yakutia / Nauka, Novosibirsk, 1997 [in Russian].

V. B. Vasilenko, N. N. Zinchuk, and L. G. Kuznetsova, Geodynamic control of locations of kimberlite fields in the central and northern regions of the Yakutian Diamondiferous Province, petrochemical view / Vestnik VGU, Seriya Geologiya, 2000, No. 3, pp. 37–55.

V. B. Vasilenko, A. V. Tolstov, L. G. Kuznetsova, and V. A. Minin, Petrochemical criteria for assessment of the diamond potential of Yakutian kimberlite deposits / Geokhimiya, 2010, No. 4, pp. 366–376.

Vasilenko V.B., Zinchuk N.N., Krasavchikov O.V., Kuznetsova L.G., Khlestov V.V., and Volkova N.I. Diamond potential estimation based on kimberlite major element chemistry //Journal of Geochemical Exploration, 2002. V. 76, N. 2. P. 93-112.