Chapter 46 Diamonds and Associated Heavy Minerals in Kimberlite: A Review of Key Concepts and Applications

Abstract

Kimberlite, a variety of ultramafic volcanic and sub-volcanic rock, is the dominant source of diamonds worldwide. It is widely accepted that the majority of diamonds are not formed within the kimberlite and much evidence points towards an ancient origin for most diamond in the deep lithospheric keels of Archaean cratons. The kimberlites, therefore, are transporting agents that “sample” deep, occasionally diamond-bearing, mantle material and rapidly convey it to surface. The dominant source rocks for diamond are highly depleted peridotite (harzburgite or dunite) and high-pressure eclogite. These are minor components of the mantle lithosphere, which is dominated by less-depleted peridotite (primarily lherzolite) and lesser amounts of non-diamondiferous eclogite. While other diamond sources are known, these rarely contribute significantly to diamond populations in kimberlite. Despite the limited range of source lithologies, diamond population characteristics in any given kimberlite are typically highly complex and indicative of several distinct populations, most likely formed in discrete events occurring at different times, ranging from the Archaean to the age of kimberlite emplacement. In addition to rare diamond, disaggregation of mantle rocks sampled by kimberlite yields relatively large quantities of other mantle minerals, commonly referred to as kimberlitic indicator minerals. From an exploration point of view, the most important indicator minerals are garnet, chromite, ilmenite, Cr-diopside and olivine. Several of these minerals display diagnostic visual and compositional characteristics, making them ideal pathfinders for kimberlite. The more chemically resistant minerals (garnet, ilmenite and chromite) are particularly useful due to their greater ability to survive weathering in the surface environment. Thus, sampling of surface materials to recover kimberlitic indicator minerals and tracing these back to their source is a key component of most diamond exploration programs. Studies of diamond inclusions and diamond-bearing xenoliths permit geochemical characterisation of diamond source materials and have led to major advances in the understanding of the relationship between diamond and its host rock in the mantle.