Structural and microstructural regularities of the distribution of diamond in metamorphic rocks of the Kumdy-Kol and Barchi-Kol deposits, Kokchetav Massif, northern Kazakhstan

The diamond content of metamorphic rocks of the Kumdy-Kol and Barchi-Kol deposits in the Kokchetav Massif, northern Kazakhstan, varies widely from less than one carat to several hundred carats per tonne (cpt). Diamond mineralization is neither controlled by a special rock type(s), nor is its spatial distribution random, as could be expected from the UHP nature of the parent process. Diamond mineralization occurs in linear zones coinciding with tectonic discontinuities and associated metasomatic rocks. The morphology of diamondiferous zones follows branching linear patterns of metasomatic zones developed along the fault systems. Results of a factor analysis of whole-rock chemical compositions including the abundance of diamond confirm a close relationship of diamond content with metasomatic rocks, and demonstrate that factors characterizing metasomatic processes also reflect the diamond content in rocks from both the Kumdy-Kol and the Barchi-Kol deposits. However, the fluids that produced metasomatic alterations were different at these two localities. In the case of the former, predominantly hydrous fluid carried a moderate amount of CO₂, whereas at Barchi-Kol, the fluid was rich in CO₂. Crystals of diamond occur in intergranular interstices and within the grains of the rock-forming minerals. Their distribution is not random but tends to be associated with fractures in rocks and rock-forming minerals, including secondary minerals. Diamond grains form chains and clusters along the fractures. Diamondiferous clusters are morphologically variable: five to ten microcrystals may form 2D patterns; 3D botryoidal aggregates are observed as well. These data confirm the hypothesis favoring a crustal fluid - metasomatic origin of diamond in the metamorphic sequence of the Kokchetav Massif.