The Pipes or Craters
The five diamond mines or craters are all contained in a circle 3½ miles in diameter. They are irregularly shaped round or oval pipes, extending vertically downwards to an unknown depth, retaining about the same diameter throughout ([Fig. 3]). They are said to be volcanic necks, filled from below with a heterogeneous mixture of fragments of the surrounding rocks, and of older rocks such as granite, mingled and cemented with a bluish-coloured, hard clayey mass, in which famous blue clay the imbedded diamonds are hidden.
FIG. 3. KIMBERLEY MINE. THE “PIPE.”
FIG. 4. SECTION OF KIMBERLEY MINE.
To face p. 18.
The craters or mines are situate in depressions, which have no outlets for the water which falls upon the neighbouring hills. The watersheds of these hills drain into ponds, called pans or vleis. The water, which accumulates in these ponds during the rainy season, evaporates during the dry months, only one of them holding water throughout the dry season. The rocks which surround the craters are capped by red soil or calcareous tufa, and in places by both, the red soil covering the tufa.
The diamantiferous breccia filling the mines, usually called “blue ground,” is a collection of fragments of shale, various eruptive rocks, boulders, and crystals of many kinds of minerals. Indeed, a more heterogeneous mixture can hardly be found anywhere else on this globe. The ground mass is of a bluish green, soapy to the touch and friable, especially after exposure to the weather. Professor Maskelyne considers it to be a hydrated bronzite with a little serpentine.
The Kimberley mine is filled for the first 70 or 80 feet with what is called “yellow ground,” and below that with “blue ground” ([Fig. 4]). This superposed yellow on blue is common to all the mines. The blue is the unaltered ground, and owes its colour chiefly to the presence of lower oxides of iron. When atmospheric influences have access to the iron it is peroxidised and the ground assumes a yellow colour. The thickness of yellow earth in the mines is therefore a measure of the depth of penetration of air and moisture. The colour does not affect the yield of diamonds.
Besides diamonds, there have been detected more than eighty species of minerals in the blue ground, the more common being magnetite, ilmenite, garnet, bright green ferriferous enstatite (bronzite), a hornblendic mineral closely resembling smaragdite, calc-spar, vermiculite, diallage, jeffreysite, mica, kyanite, augite, peridot, eclogite, iron pyrites, wollastonite, vaalite, zircon, chrome iron, rutile, corundum, apatite, olivine, sahlite, chromite, pseudobrookite, perofskite, biotite, and quartz. The blue ground does not show any signs of passing through great heat, as the fragments in the breccia are not fused at the edges. The eruptive force was probably steam or water-gas, acting under great pressure, but at no high temperature. According to Mr. Dunn, in the Kimberley Mine, at a depth of 120 feet, several small fresh-water shells were discovered in what appeared to be undisturbed material.
A selection of thin sections of some of these rocks and minerals, mounted as microscopic objects and viewed by polarised light, are not only of interest to the geologist, but are objects of great beauty.
The appearance of shale and fragments of other rocks testify that the mélange has suffered no great heat in its present condition, and that it has been erupted from great depths by the agency of water vapour or some similar gas.
The rock outside the pipes and encasing them is called “reef.” Inside some of the mines occur large masses of “floating reef,” covering an area of several thousand square feet. In the De Beers Mine is what is called “the snake,” a dyke of igneous rock taking a serpentine course across the mine, and standing like a vein nearly vertical, varying in thickness from 2 to 7 feet. The main body of the blue ground is entirely analogous to the snake rock, naturally more decomposed, but in essential points the microscopic appearance of the blue ground and of the “snake” is in an extraordinary degree alike. Mr. Gardner Williams supposes that the “snake” is a younger eruptive formation coming from the same volcanic source as the blue ground. No diamonds have been found either in the “snake” or the floating reef. The ground, however, is generally richer in diamonds in the neighbourhood of the floating reef.
Before the discovery of the mines there was nothing in the superficial appearance of the ground to indicate the treasures below. Since the volcanic ducts were filled with the diamantiferous ground, denudation has planed the surface and the upper parts of the craters, and other ordinary signs of volcanic activity being smoothed away, the superficial and ubiquitous red sand covered the whole surface. The Kimberley Mine seems to have presented a slight elevation above the surrounding flat country, while the sites of other mines were level or even slightly depressed. The Wesselton Mine, within a mile of Dutoitspan, has only been discovered a few years. It showed a slight depression on the surface, which had been used as a shoot for dry rubbish. There are other diamantiferous pipes in the neighbourhood, but they are small and do not contain stones in payable quantities. More recently another diamantiferous pipe has been discovered about 40 miles off, near Klipdam, and is now worked as the Leicester Mine. Other hoards of diamonds may also be near; where there are no surface signs, and the pipe itself is hidden under 10 or 20 feet of recent deposits, it is impossible to prospect the entire country. Accident has hitherto been the chief factor in the discovery of diamond mines.
How the great pipes were originally formed is hard to say. They were certainly not burst through in the ordinary manner of volcanic eruption, since the surrounding and enclosing walls show no signs of igneous action, and are not shattered or broken up even when touching the “blue ground.” It is pretty certain these pipes were filled from below after they were pierced and the diamonds were formed at some previous time and mixed with a mud volcano, together with all kinds of debris eroded from the rocks through which it erupted. The direction of flow is seen in the upturned edges of some of the strata of shale in the walls, although I was unable to see any upturning in most parts of the walls of the De Beers Mine at great depths.