Method of complex analyses of the compositions of kimberlite indicator minerals to assess the presence of large diamonds

For industrial deposits that contain particularly large and expensive diamonds, a diagram of the compositions of KIM (kimberlite indicator minerals) for Cr, Al, Fe, Mg, Mn is proposed. The proximity or rather convergence of the compositions of KIM is also complemented by a high correlation of the frequency of occurrence of their cluster groups (Ivanov, 2017), such as pyropes, chromites, picroilmenites and pyroxenes. The increase in oxygen fugacity  in  KIM is correlating with Fe and Ti , this fact can also explain the intensity of metasomatism of kimberlites, shown in the proposed graph by an increase in ilmenite content. This type of diagram was proposed by Mitchell (1986) for chromite compositions, which the authors supplemented with compositions of  picroilmenites, pyropes and pyroxenes. The manganese  concentration is shown by the size of the figurative point  – a bubble diagram. For better perception, the drawing is supplemented with different colors of the composition groups of  KIM. Below are diagrams for two industrial deposits that contain expensive and large diamonds. The diagrams show the approximate regions of diamond-bearing associations for chromite and pyrope compositions with red asterisks. The blue lines show two main trends in chromite compositions, the horizontal one is picrate trend and the vertical one is kimberlite. For picroilmenite compositions, the diagram shows two main trend lines: the red line for paramagnetic compositions and the black line for ferrimagnetic compositions. For pyrope compositions, trend lines are shown in red for a number of cluster groups G10 by (Dawson, Stephens, 1975) and for groups G11 – in black. The diamond content in kimberlites of the Aykhal pipe is several times higher than in the Komsomolskaya pipe, but the cost of diamond crystals from the latter is several times more expensive than in the Aykhal pipe. A distinctive feature of the compositions of the Komsomolskaya pipe KIM, as well as the above-proposed kimberlite pipes (Griba and Karowe ) – is the presence of diamond-bearing websterite parageneses of chromites and pyropes, which corresponding in the diagram, to areas with elevated Cr, Fe  values . The kimberlite of the Aykhal pipe is characterized by a higher degree of metasomatism, which is recorded in the diagram by the trend of more ferruginous picroilmenite compositions, which affects the quality of its diamonds.

CONCLUSIONS. The proposed method is based on a comprehensive assessment of KIM compositions. The diagram allows to assess the presence of expensive and large diamonds in kimberlite pipes at the  exploration  stage, as well as to reconstruct the composition of deep mantle rocks, based on  the KIM graphically presenting analyses of their compositions for five elements.

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2. Mitchell R.H. Kimberlites: mineralogy, geochemistry and petrology. New York, Plenum Press. 1986. 442 P.

3. Dawson J.B., Stephens W.E. Statistical classification of garnets from kimberlites and xenoliths. J. Geol. 1975. 83, 589-607.