Specific features of particular geological-genetic types of diamond-bearing magmatites

Globally recognized types of diamond-bearing magmatites can generally be classified into two groups, the mantle-derived and crustal ones. The first group comprises kimberlites, lamproites, kimberlite-lamproites, picritic monchiquites, lamprophyres, komatiites, and garnet pyroxenites. The crustal rocks are represented by impactmetamorphogenic types, or impactites. There is currently no consensus regarding the genetic type of diamonds found in crystalline rocks (of the Kokchetav type): some researchers assign them to the mantle group, while others argue for the crustal origin. Kimberlites, lamproites, and kimberlite-lamproites typically occur as diatremes and dikes, and less frequently as stockworks and sills. Other types of primary diamond deposits are hosted in crustal rocks that mainly occur as intrusive massifs (plutons). The original source of kimberlites and lamproites is represented by disintegrated mantle rocks (peridotites and eclogites), which fragments were brought into the Earth's crust by the magma. The lamproites differ from the kimberlites in their high content of Ti, K, P, and certain other elements. There are no significant differences between the diamonds found in these two types of magmatic rocks. Metamorphogenic diamonds from the crystalline complex are very small (rarely exceeding a fraction of a millimeter) and occur as crystals of cubic habit, skeletal forms, and their aggregates. They formed under crustal conditions at P-T parameters characteristic for stability of this mineral. A new type of primary diamond deposit, associated with impactites, formed as a result of an impact of cosmic bodies onto the Earth's surface. The diamond crystallization in these deposits resulted due to the solid-phase transition of graphite to diamond under high pressures generated during the impact. Diamonds from kimberlites and lamproites are valuable for both gem and technical applications; the technical significance is characteristic of other diamond types as well.