DIAMOND in SOUTH AFRICA
Dry Diggings (suite)
The different kinds of rocks constituting the material, which fills the pipes, are separated by no trace of bedding planes, but masses of rock, slightly different in character have been recently observed to be separated from each other in quite another way.
Vertical, or nearly vertical crevices, not more than three-eighths of an inch across and filled with a mineral substance resembling talc in character, penetrate the material down to the lowest depth to which the mines hive been worked. These divide the whole contents of each pipe into a number of vertical or nearly vertical columns, each differing slightly from the others in composition, but showing no difference in its own mass.
These small variations in the material filling the pipes, as well as its character as a whole, do not depend in any way upon the nature of the various rocks of the surrounding reef. It was formerly contended that the character of the reef had a more or less marked influence on the richness in diamonds of the material filling the pipes; thus it was feared that when the base of the black shale in the Kimberley mine had been reached, the yield of diamonds would cease, since the formation of diamonds was supposed to have been dependent on some way on the presence of carbon in these shales. In consequence of this belief, the value of mining claims for a time fell; but the yield of diamonds at lower levels, where the pipe is surrounded by melaphyre, turned out to be just as good as at the higher levels in the shale.
Between the material filling the pipes and the enclosing rocks or reef there is always a sharp line of demarcation and never a gradual transition. Usually the two sets of rocks are in immediate contact, but not infrequently they are separated by a space, sometimes of considerable width, into which project beautiful crystals of calcite. Other secondary minerals are also found in the numerous crevices, in addition to the vertical cracks, the rock is penetrated.
The actual diamond-bearing rock itself, which fills the pipes, must now be considered. In the upper portion of the pipes it consists of a light yellow, soft, sandy or friable material known to the diamond miners as "yellow ground" or "yellow stuff". This upper portion, which has a thickness of from 50 to 60 feet, has now, in the Kimberley mine, been completely removed in the course of mining operations, in the other mines, however, a little still remains to be seen. The rock at a greater depth has the character of a volcanic tuff or breecia, it is of a green or bluish-green colour, and is known as the "blue ground" or "blue stuff". Throughout the whole depth to which it has been worked it shows no deviation from these characters.
The passage of the "yellow ground" into the "blue ground" is as a rule abrupt, and the line of division is never quite horizontal, but inclined from 5º to 15º to the horizon. Sometimes there is an intermediate reddish layer, known as, "rusty ground" which passes upwards into the "yellow ground" and below into the "blue ground. " Neither the "yellow ground" nor the "rusty ground" is anything more than the weathered upper portions of the "blue ground"; the latter originally filled the pipes up to the surface, but the portion exposed to atmospheric influences became altered and transformed into what is now known as "yellow ground. " Similarly the "rusty ground" is a layer in which the alteration has not proceeded as far as in the "yellow ground"; the uppermost layer, therefore, of " blue ground" marks the level below which the weathering process has not yet commenced. In the early history of the mines, this change in the colour of the diamantiferous material also had the effect of diminishing the value of claims, since it was feared that the "blue ground" might be deficient in yield. Experience of course showed that these apprehensions were groundless, for the rock at greater depths proved as rich, if not richer, than the upper levels.
The "blue ground," which thus fills the pipes, and from which the uppermost "yellow ground " has been derived, has the appearance of dried mud, and consists of a green, or dark bluish-green ground-mass, which gives its colour to the whole rock. It binds together numerous fragments, larger or smaller in size, and with sharp, or in some cases rounded corners, of a green or bluish-black serpentine rock. The actual material of the mud-like ground-mass, and of the blocks which it cements together, is identical, the one being in a finely divided condition, and the other in compact masses. These are the chief constituents of the "blue ground", but it contains also numerous mineral grains as well as fragments of foreign rocks in large numbers.
Although the groundmass is not very hard, it has a certain amount of toughness, which renders it difficult to work with a pickaxe; it readily yields, however, to the chisel. It can be scratched with the fingernail and is somewhat greasy to the touch. The qualitative chemical composition of the "blue ground" is almost identical throughout the whole mass, but certain differences in the quantitative composition of different portions are detected in analysis. All analyses which have been made of this material record the presence of silica and magnesia in varying amounts, some ferrous oxide, usually only a little lime, some water and carbonic acid, and little or no alumina. The material is thus essentially a mixture of hydrated magnesium silicate and calcium carbonate.
The following is a quantitative analysis of a specimen of "blue ground" from the Kimberley mine given by Professor Maskelyne and Dr. Flight:
The carbon dioxide (carbonic acid) is present in nearly sufficient amount to combine with the whole of the lime to form calcium carbonate; deducting this, the remainder, consisting of hydrated magnesium silicate, with some of the magnesium replaced by ferrous oxide, has approximately the composition of the mineral serpentine. It has thus become customary to speak of the whole rock as a serpentine breccia, and this term, or that of volcanic tuff or agglomerate, will be used in referring to the diamantiferous material.
Rafal Swiecki, geological engineer email contact
This document is in the public domain.