The morphology and nature of primary diamondiferous occurrences

Abstract

Diamondiferous volcanic rocks, principally kimberlites and lamproites, have erupted since before 1.7 Ga, often repeatedly in the same cratonised environment. The diamond bearing volcano, whether kimberlite or lamproite, is of variable size and structure (and hence variable diamond reserves) largely dependent on groundwater availability at the time of eruption. Examples are reviewed from recently discovered pyroclastic cones or aprons (Saskatchewan), craters (Tanzania, Zaire) and diatremes (Western Australia, southern Africa). Hypabyssal intrusions are also prospective either for their intrinsic diamond content or because of their contiguous relationship to the larger pipes. Other volcanics (nephelinites, monchiquites, carbonatites) which have been reported to be associated with sporadic diamonds, mostly in Africa, are also mentioned as well as those with accidentally included diamonds (Russia, S.E. Australia) Diamonds in ultramafic massifs (obducted ophiolites) are divided into depleted peridotite (Tibet, Type A) and garnet pyroxenite (Beni Bousera and Ronda, Type B). The former contain very sparse diamonds, which are associated with high Cr chromite but no other “kimberlite diamond indicators”. The latter contain graphitised diamonds which are locally abundant. Precambrian metadunite “pipes” have yielded diamonds in Burkina Faso and possibly Surinam. Locally abundant microdiamonds, with a high proportion of cubes, occur in a wide variety of medium grade metamorphic rocks in Kazakhstan. Parallels are drawn with similar occurrences in western Norway, Dabie Shan, eastern China and northern Australia. The presence of calc silicate host rocks (Dabie Shan) is linked with compositionally similar diamond inclusions in alluvials in SE Australia. Meteoritic (impact) diamonds, including polycrystalline “yakutite”, contain the high pressure polymorph, lonsdaleite (Popagai, Siberia). A coarser variety, carbonado, is abundant in Brazil, but may not be meteoritic. It is associated with, and contains inclusions of, the gorceixite-florencite group of low temperature minerals. These are also found in alluvials with sparse diamond thought to have been derived from ophiolitic ultramafic massifs. It is concluded that whilst kimberlite and lamproite pipes will remain targets into the next century, other potential targets include interstratified kimberlite pyroclastics, obducted alpine peridotites and sheared metamorphic terrain of the Kazakhstan type.