Kimberlites and Lamproites: Primary Sources of Diamond

INTRODUCTION A variety of mantle-derived igneous rocks comprise the primary sources of diamond, with the principal hosts being kimberlite and lamproite. Primary diamonds or graphite pseudomorphs after diamond are also known to occur in some lamprophyres (Jaques, Kerr et al., 1989), alkali basalts and alpine type peridotites (Kaminskii, 1984). Significant quantities of diamond have not yet been found in these rocks. Secondary diamond deposits are formed from these primary source rocks by weathering and transpartation. These deposits are commonly very rich in high-quality diamonds. Examples include those of the Ural Mountains (USSR), the marine deposits of Namibia and the alluvial deposits of West Africa, Brazil and Venezuela. The identity of the rocks which were parental to these types of deposits is not always evident from the mineralogy of the detrital phases present. The nature and origins of secondary diamond deposits are not discussed further in this work. Currentiy, diamonds are extracted from both kimberlites and lamproites and most exploration activity for diamond is directed toward the discovery of further exploitable deposits in these rocks. For these purposes, it is important to be able to determine rapidly the correct identity of a potentially diamondiferous rock, as exploration and assessment techniques for kimberlites and lamproites are different. Determining the correct identity of such rocks in some instances is not trivial or easy, as many rocks belonging to different petrological clans are petrographically similar. Identification of a rock as kimberlite or lamproite does not guarantee that that it will contain economic amounts of diamond. There are two reasons for this observation. Firstly, it is now accepted that diamond is a xenocryst in both rock types. Secondly, magmatic processes may act to resorb and eliminate any entrained diamonds. Thus, a given barren intrusion may never have contained diainonds, due to failure to incorporate xenocrysts or any originally present may have been completely resorbed during emplacement and cooling of the magma. It is particularly important to realize that kimberlites and lamproites are merely vehicles which transport diamondfrom the uppermantle to the crust. Detailed discussion and description of current hypotheses regarding the origin of diamond are beyond the scope of this work. However, some understanding of diamond genesis is essential to appreciate the distribution of diamonds in the primary source rocks.