DIAMOND in SOUTH AFRICA
Dry Diggings (suite)
The blocks of foreign rock embedded in the breccia, which are often known as boulders, have usually perfectly sharp edges and corners, though occasionally these may be rounded. The size of these rock fragments varies from that of a small splinter to that of a block several thousand cubic yards in dimensions. In the pipe of De Beer's mine there is a block of olivine-basalt called "the island", which has a sectional area of 330 square yards, and has been traced to a depth of 237 yards. Large masses of similar rock occur commonly in all the mines, they are referred to as "floating reef," in contradistinction to the "main reef" which surrounds the pipe. This "floating reef" is more frequently met with in the upper than in the lower levels of the pipes. Smaller fragments of the same rock have, however, been met with at the greatest depths to which the mine shafts have been sunk, and here as elsewhere they form a large proportion of the material filling the pipes, through which they are distributed with the greatest irregularity.
Some of the rock fragments agree completely in character with the rocks of the main reef, frequently consisting of amygdaloidal basalt (melaphyre), shales, etc. In some places, the highly bituminous and carbonaceous shales are present in such large amounts that some-times the presence of the firedamp characteristic of coalmines has been observed. It has been asserted that diamonds occur only in those portions of the agglomerate in which bituminous shales are present in large amount, and it has been argued from this that the diamonds were actually formed from the carbonaceous matter present in these shales. There is, however, reason to believe, as will be shown below, that the diamonds were formed not in the pipes themselves, but at far greater depths in the interior of the earth from which they have been brought up by the action of volcanic forces.
Beside the blocks, which have evidently been detached from the reef surrounding the pipes, there are others which have not been found in situ in the neighbourhood, and which, therefore, must necessarily have been brought up from below. In the Kimberley mine at depths below 230 feet, there are found large blocks, several cubic yards in extent, of grey or greyish-white sandstone, the grains of which are bound together by a calcareo-argillaceous cement. They are of much the same character as the sandstone, which in other localities forms part of the Middle Karoo formation, and which from geological considerations must form part of the reef at the Kimberley diamond mines at a great depth below the surface. There also occur, though not so frequently, fragments of quartzite, mica-schist, talc-schist, eclogite, and granite. The last named rock is rarely found, and when met with is so decomposed as to be only doubtfully recognisable as granite. It was found in numerous large blocks, and in smaller fragments in the upper portion of a small mine known as Doyl's Rush about a mile from Kimberley. Such rocks crop out at the surface some distance north of the diamond-fields; it is therefore probable that their southern extension lies at a great depth below Kimberley, and forms the base of the reef. Rock fragments of materials not found in the reef enclosing the pipes, which in all probability have been brought up from below, are called "exotic fragments".
The minerals embedded in the agglomerate are usually distributed through it with some regularity, but very sparingly; they constitute only about 1/4000 of the total mass of the rock and are therefore rather inconspicuous. A complete collection can only be made from the residue left after the process of diamond washing
Among these minerals the most important, but not the most frequently occurring, is the diamond: it is found in crystals developed regularly on all sides, and also in fragments, such as would result front the breakage of larger crystals. It is remarkable, however, that different portions of the same crystal are never found lying close together. The edges and corners of the crystals are always perfectly sharp, not even the faintest trace of rounding can be detected, and so the stones of the dry diggings are easily distinguished from those of the river diggings. A more detailed description of the special characteristics of Cape diamonds will be given later; here we are concerned only with their mode of occurrence.
The diamond is a constituent part of the agglomerate in which it is embedded, and its mode of occurrence in no way differs from that of other minerals contained in the rock. Each crystal or fragment of a crystal occurs alone, firmly embedded in the agglomerate, from which it can be extracted only with difficulty; its surface is usually clean, but in some cases is coated with a layer of limonite (iron hydroxide) or with a calcareous film, both of which are easily removable. Until recently, no diamond had ever been observed attached to another mineral in such a way as to suggest that the two grew side by side at the same time. The discovery, however, of a diamond crystal attached in this way to a garnet shows that such a growth does take place, though rarely.
Diamonds are to be found at the surface, and downwards through the "yellow ground"; the "rusty ground", and the "blue ground", as far as the deepest mines have yet penetrated they do not occur, however, in equal number in all mines, nor in different portions of the same mine; numerical details will be given later. In the Kimberley mine, which is unique in this respect, the richness of the yield increases rapidly as lower levels are reached. The different columnar divisions of each pipe vary in the number of diamonds contained, some being so poor that the working of them is unprofitable, others on the contrary being just the reverse. The total number of diamonds contained in a given mass of any particular column is so constant that it is quite possible to calculate beforehand how many carats of stones a certain amount of "blue ground" will yield.
The presence of diamonds in the "blue ground" is of enormous economic importance regarded as a rock constituent; however, they are quite insignificant, being present in such small amount that, had they been less highly prized, and of less general interest, they would probably have been scarcely mentioned in a petrographical description of the rock. A striking illustration of their sparing occurrence is furnished by the fact that in the richest part of the richest mine, namely the Kimberley mine, they constitute only one part in two millions, or 0.00005 per cent of the "blue ground". In other mines the proportion is still lower, namely one part in forty millions, a yield which corresponds to five carats per cubic yard of rock, and which can be profitably worked. When the absolute amount of diamond present is so small, slight variations in this amount in different parts of the rock, though of great economic importance, are of little scientific significance.
In respect to the associated minerals of the diamond in the agglomerate, certain differences exist between the various mines and between different parts of the same mine. These minerals occur either in homogeneous grains of the same kind, or in small groups consisting of minerals of various kinds. Those of most frequent occurrence are red garnet, green enstatite, and vaalite (an altered mica), others are less widely distributed, and some indeed being regarded as great rarities. The most important of the minerals associated with the diamond in these deposits will be now considered in order.
Rafal Swiecki, geological engineer email contact
This document is in the public domain.