The diamond-bearing blue-ground and allied rocks of South Africa

Abstract

Mr du Toit said that scores of volcanic necks or pipes filled with the peculiar bluish ultrabasic breccia known as blue-ground or kimberlite pierced the strata in South Africa, and were accompanied by numerous dykes of the same or of an allied material. The blue-ground contained a great variety of minerals, chief among which were olivine, enstatite, diopside, garnet, mica, and ilmenite, and the groundmass was of a serpentinous nature. The rock contained abundant inclusions of the strata pierced by the pipes, together with fragments of rocks brought up from great depths. These consisted of ultrabasic holocrystalline rocks, rich in magnesium silicates, along with eclogites (garnet-diopside rocks), and granulites of various types. These inclusions had, by some geologists, been regarded as segregations from the kimberlite magma, but appeared more probably to be portions of a deep-seated formation penetrated by the pipes—the denser and less silicious portion of the crust of the earth, in fact. The eclogites graded into the garnetiferous granulites, and seemed to have but little affinity to the ultrabasic rocks and the kimberlite. The blue-ground appeared to be a hybrid rock, the matrix of which was closely allied to, if not identical with, melilite-basalt, a rock which had been found in pipes in Cape Colony along with silicious breccias. The diamond had by some been considered to have originated in the blue-ground itself, but the occurrence of diamonds in the garnets of eclogite indicated a more probable source of the gems. The absence or scarcity of diamonds in all but a very small percentage of the pipes was also in favour of this view. The secondary changes in the kimberlite had led to the production of serpentine, with consequent increase in the bulk of the rock. This had caused a swelling of the material, and the strata surrounding the pipes had been tilted upwards at the contacts, whereas in other types of volcanoes, both ancient and modern, the reverse was usually the case. The age of the pipes was probably late Cretaceous.