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Dry Diggings

The nature of these deposits was not at first known, and they were supposed to be similar to the river deposit and to consist nearly of superficial layers of alluvium. It was soon recognised, however, that this was by 110 means the case, and that the deposits were absolutely unique in character. The geographical position of these deposits has been already described; they are situated on a high plateau, far removed from any watercourses and formed of rocks belonging to the Karoo formation. This formation, which has a total thickness of about 10,000 feet, consists of sandstones and shales with numerous intruded dykes and bosses of igneous rocks, variously referred to, according to the form of the mass and the character of the rock itself, as trap, dolerite, melaphyre, basalt, diabase, etc. The age of the sedimentary rocks is not exactly known as yet, but in any case they are later than the Carboniferous, the lower beds probably corresponding with the Permian, and the upper beds with the Trias of Europe. In this upper and younger part occur the deposits of diamonds in Griqualand West, which we have now to consider.

The account, which follows deals mainly with the half-dozen mines having the richest yield, and especially with the four best-known Kimberley mines, others being passed over as insignificant or not completely examined. The main features of all are identical, and as the individual deposits differ only in unessential points, it is unnecessary to consider each one singly m ally great detail.

The diamond-bearing material is contained in pipes or funnel-shaped depressions which penetrate the Upper Karoo beds in a vertical direction to an unknown depth. The outline of a cross-section of one of these depressions may be circular, elliptical, kidney-shaped, or more or less irregular. The rock, which fills these pipes differs entirely from the surrounding beds of the Karoo formation, the so-called "reef," and is sharply separated from them. The occurrence of diamonds is confined exclusively to the material filling the pipes; nowhere in the surrounding reef of sandstone and shale, or elsewhere in the Karoo beds, has a single stone been found, although enormous quantities of these rocks have been removed in the course of the mining operations.

The upper extremities of the pipes are elevated above the surface to the height of a few yards each, thus forming a small kopje; in the case of the Wesselton mine, however, there was a slight depression. The pipes vary in diameter from 20 to 750 yards, the usual diameter being from 200 to 300 yards. In 1892 the diamond-bearing material had been excavated in the Kimberley mine, which is the deepest of all, to a depth of 1261 feet, and, as in the other mines, with no sign of exhaustion; the rock is therefore continued to an unknown depth.

The cross-sections of different pipes taken at the earth's surface differ widely both in shape and area, as will be seen from the following data: Du Toit's Pan (Dutoitspan), 192,000 square yards in area, of a flat horse-shoe shape, 750 yards long and 200 yards broad Bultfontein, 118,000 square yards in area, almost circular in outline with a diameter of 363 yards; De Beer's (De Beers), 66,000 square yards in area, elliptical in shape, measuring 320 yards from East to West and 210 from North to South; Kimberley, 49,000 square yards in area, oval in shape, 290 yards long and 220 yards broad, with a small projection measuring 37 yards towards the East. The size of the pipe of the Jagersfontein mine is not exactly known, its cross-section is between 100,000 and 110,000 square yards; exact details respecting the Koffyfontein mine are also wanting, but in any case it is smaller than the mine last mentioned. A peculiar feature of the Kimberley mine is the gradual contraction of the pipe in sectional area as greater depths are reached; thus at a depth of about 300 feet the two diameters are reduced to 260 and 160 yards respectively, and the contraction is continued as still lower depths are reached. A section of the Kimberley mine is given below.

The rooks composing the reef are on the whole much the same everywhere; still in the various mines certain differences do exist.

The neighbourhood for a considerable distance round Kimberley is covered with a layer of red clay 1 to 5 feet thick; underlying this is a bed from 5 to 20 feet thick of calcareous tufa, also of wide distribution. This tufa is of recent origin and has no genetic connection with either the reef or the diamond bearing pipes, since it covers both indiscriminately, and to a certain extent penetrates cracks and crevices in them. Beneath this tufa lie the rocks of the Karoo formation, which constitute the reef.

The uppermost part of the reef in the Kimberley mine consists of a series of bedded shales, 40 to 50 feet thick, greenish-grey in the upper part and yellowish or greyish in the lower; they are of varying hardness, and at different levels in the mine are interbedded with a fine-grained to compact olivine-basalt. Beneath these pale shales are about 270 feet of black bituminous shales, very similar in character to the shales of the English coal-measures; certain of these beds are impregnated with iron-pyrites, and they often contain nodules of clay-iron-stone, small bands of calcite, and thin layers of coal, while interbedded with them near their base is a sheet of basalt one foot thick. Beneath the black shales is a hard grey or green amygdaloidal diabase (melaphyre), the base of which is not exposed in the open workings, but is seen in the underground shafts at a depth of 440 feet below the upper surface of the mass.

Beneath this igneous rock the shafts penetrate a bed of quartzite of about the same thickness, and under this again black shales, both of which are penetrated in places by dykes of eruptive rock (dolerite). The deepest shaft of the mine has not yet penetrated to the base of the black shales, so that the total thickness of these beds is unknown. Probably at still greater depths, as yet untouched by mining operations, there are deep-seated rocks, such as granite, gneiss, or olivine-rocks, but this question will be discussed later.

In the De Beer's mine, a sheet of basalt 47 to 61 feet in thickness is met with in the upper part of the reef, otherwise the beds are the same as in the Kimberley mine. A similar sheet of basalt is present in Du Toit's Pan mine, but is absent from the Bultfontein mine. The walls of the pipe consist here, as far as they have been laid bare, only of shales, which are much displaced, sometimes having an inclination of at least 15º to the horizon; this is also the case to a certain extent in the De Beer's mine, while in other places the beds are horizontal. In the Du Toit's Pan and Bultfontein mines the shales have not yet been penetrated to their base, and their thickness appears to be greater here than in the Kimberley and De Beer's mines, which lie a little further to the north.

The material filling the pipes, like that of the surrounding rock, is essentially the same in every mine, and in every part of each mine, but in all mines the upper portions of the pipes to a fairly considerable depth have suffered the effects of weathering. Observable differences do exist, however, and an experienced miner can sometimes recognise not only from which mine, but also from what part of a particular mine, any given specimen of material has been taken. These small differences are usually connected with variations in colour, hardness, and composition the nature of the enclosed minerals and fragments of foreign rock, etc. and are, as a rule, unimportant.

Diamond Geology [ 1  India  3  4  5  6  7  8  Brazil  10  11  12  13  14  15  16  17  18  19  20  Borneo  22   South Africa  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  Venezuela, Guyana  42  Australia  44  Argyle  Congo  46  47  48  49  50  51  52  53  54  55  Angola  57  58  59  Guinea  ]

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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011