Diamond potential of metamorphic rocks in the Kokchetav Massif, northern Kazakhstan

There are two known diamondiferous bodies in Kazakhstan, the Kumdy-Kol deposit and the Barchi-Kol occurrence. Both are located within the western part of a metamorphic belt that outcrops at the central part of the Kokchetav Massif. The metamorphic belt is interpreted as a mega-mélange that comprises structural units underlain by HP and UHP rocks. Diamondiferous varieties of the UHP rocks within the Kumdy-Kol deposit form a narrow (~1 km thick), NE-trending band; they comprise less than one percent of the total UHP rock volume. The economically important diamondiferous zone consists of variegated rocks, from silicate to essentially carbonate varieties with a wide spectrum of mineral assemblages. Garnet-biotite gneiss makes about 80 % of the diamondiferous zone. Certain regularity is discernable in spatial distribution of diamonds: linear diamond-rich zones alternate with barren ones. The diamond grade ranges from several carats per ton (cpt) to several hundreds cpt. Different morphological varieties are present among the diamonds: octahedra, cubes, cube-octahedron combination forms, skeletal and spheroid crystals. A wide range of δ¹³C (−8.9‰ through −27 ‰) and δ¹⁵N (+5.3 ‰ through +25 ‰) values have been measured. Diamonds from different rock types differ in their carbon isotopic pattern: diamonds from gneiss have ‘lighter’ isotopic compositions relative to those of pyroxene-carbonate and garnet-pyroxene rocks. Graphite, coesite, clinopyroxene, rutile, titanite, kyanite, K-feldspar, biotite, phengite, quartz, phlogopite, albite, apatite, chlorite and carbonates are minerals forming intergrowths with diamonds. The most frequent combination is graphite +diamond. We conclude that the model of crustal fluid-metasomatic formation of diamonds in metamorphic rocks reflects the observed regularities best. The morphology of diamondiferous bodies favors the notion of their formation from a carbon-bearing fluid. The micrometer size of the diamonds, along with isotopic signatures, hint at their formation under metastable crustal conditions. This model presumes that the formative process occurred at relatively low temperature and pressure within an open disequilibrium catalytic system. It does not exclude an UHP episode in the geological history of the metamorphic rocks.