DIAMONDS: CRUSTAL DISTRIBUTION AND FORMATION PROCESSES IN TIME AND SPACE, AND AN INTEGRATED DEPOSIT MODEL
Many kimberlites and lamproites are barren of diamonds. Where diamonds are present, concentrations are <3ppm, acquired mainly by random sampling of diamond source rocks in the sub-continental lithosphere. Diamond-formation processes in the lithosphere were episodic since at least 3.57 Ga, largely confined to cratons. All primary diamond deposits show evidence of two or more diamond generations. Earliest diamond-forming episodes at ~ 3.4 ± 0.2 Ga appear to have been a series of world-wide metasomatic events triggered by CO2 rich, probably subduction-related fluids that produced dominantly octahedral diamonds associated with garnet harzburgite. Further diamond populations have formed by various processes through time such as craton accretion, slab melting, magmatic modifications of the lithospheric mantle, obduction tectonics and metasomatic infiltration, all associated with subduction. In the process, the diamonds formed in association with metasomatised harzburgite were supplemented predominantly by metasomatic diamond growth in eclogite with occasional significant contributions from lherzolitic, websteritic and grospyditic sources. A minor additional contribution of diamonds, (e.g. in eclogitic majorite), has been noted to be derived from sublithospheric ultra-deep sources in a minority of diamondiferous diatremes and secondary deposits. The youngest diamond formation episodes are associated with metasomatic processes occurring shortly before kimberlite / lamproite emplacement in the crust, mainly as fibrous cubes and fibrous coats on pre-existing diamonds and are associated with CO2/Cl rich fluids.
A multistage model for the Kaapvaal craton that takes into account the tectonic history of the craton as well as the complexities observed within the diamond populations of its various primary deposits is presented. Although the details of this model are craton specific, the general features are applicable to other cratons.