Mineral phases on the surface of Variety VII diamond crystals from Carnian (Upper Triassic) deposits of the Bulkur anticline

Carnian deposits within the Bulkur anticline are characterized as "hurricane-like" diamondiferous. Among crystals typical for kimberlite pipes there are diamonds of V–VII varieties of unknown genesis. According to earlier researches a distinctive feature of this diamond variety is a light carbon isotope composition (δ1¹³C) ranging from –17 to –25‰ and a high nitrogen content of 1200–2000 ppm. The primary source of these diamonds has not yet been identified. In order to establish genetic relationship between mineral phases on diamond surface and host deposits, this paper presents the results of a comprehensive mineralogical study of variety VII diamonds (according to Yu.L. Orlov's classification) from Carnian (Upper Triassic) deposits of the Bulkur anticline. A set of analytical methods were used to identify chlorite (presumably chamosite), illite, and nontronite that fill cracks and form crusts on the surface of polycrystalline diamond aggregates. It was established that chamosite presented as ingrowths and foliate-flaky aggregates in cracks, while illite and nontronite form thin fine-flaky crust (less than 20 μ). This mineral assemblage is characteristic of low-temperature hydrothermal alteration and diagenesis of volcano-sedimentary strata in a marine environment. Raman spectra revealed apatite in a fragment of a crushed diamond polycrystal. Apatite formation is associated with hypergene alteration of primary host rocks prior to their transformation by hydrothermal processes in shallow marine environments. The results obtained complement the understanding of diamonds postgenetic changes from placer deposits in the north of the Siberian Platform and can be used to reconstruct conditions of formation of Carnian diamond-bearing deposits.

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