« ПредыдущаяПродолжить »
rocks passed through in the different Shafts
Kimberley De Beers Bultfontein Du Toits par
By F. P. MENNELL, F.G.S.
The last few years have inaugurated a new era in the history of the South African Diamond deposits, inasmuch as rich bodies of diamantiferous ground have been located at great distances from localities which have previously been worked successfully. The group, of which the Premier mine is the best known member, is the most striking example of this extension of area, but the centre of gravity of the diamond-mining industry seems to be gradually shifting northward, and the opening up of the interesting deposit of the Somabula Forest, so far North of any other known occurrence, foreshadows the development of an important branch of the industry in the Cinderella of the South African provinces, as Rhodesia has been not inaptly termed.
No detailed or authentic description of the Somabula field has so far appeared. The writer briefly referred to the occurrence of a remarkably gravelly deposit West of Gwelo in his “ Geology of Southern Rhodesia,” and ascribed its formation to the Tertiary period, a course which appears fully justified by more recent and detailed investigation. He had already seen diamonds and other gems from the locality, but had not been made aware of their
Last year he made an examination of the ground on behalf of the South African Option Syndicate, who hold a large area on the field, and who have just erected plant for producing diamonds on a large scale. The reports of their preliminary operations will have shewn that a rich deposit of good quality stones has been opened up. A large quantity were disposed of at a price which works out at £3 175. per carat, and a smaller parcel, sold more recently, fetched 26 per carat.
The following notes, for permission to publish which I am indebted to the Syndicate, are intended to give some idea of the geological and mineralogical features of the field, of which I hope a much more detailed account will be given at an early date.
The diamond area may be described as a tongue of the Somabula Forest, stretching along the central plateau of Rhodesia from the Uvungu River for about seven miles in the diretion of Gwelo. The beds of which it consists are undoubtedly younger than the Forest Sandstones, as shewn by their numerous pebbles of agates derived from the lavas interbedded with those rocks, but they are probably not very different in age, and may perhaps be regarded as the uppermost portion of the Forest Sandstone series. They directly overlie the granite of the watershed, on to the apex of which they extend, but further down the Uvungu River the ordinary Forest Sandstones are met with. The general sequence appears to be :
(5) Surface rainwash, etc., chiefly redistributed
sav io ft. Somabula
(4) Red and white sands
(3) Gravel, with partings of clay, etc. Beds
(2) White micaceous sand
resting unconformably upon (1) Granite.
sav 40 ft. say 40 ft.
The top of the upper sandy beds is not seen, but the thickness given is probably well within the mark. The levels of the granite bedrock also vary considerably, and the beds themselves tend to assume a lenticular shape, so that it is impossible to give more than a rough idea of their proportionate development.
The UPPER SANDS, which have been entirely removed by denudation from some of the ridges, and from all the lower ground, are sometimes clayey and stained red by iron oxides, and sometimes fine and white. In a shaft at one spot on the slope of a ridge an actual thickness of 16 feet was passed through before reaching the underlying gravel. It may be stated, however, that so heavy an overburden is met with on few parts of the diamondiferous area.
The Gravel itself is composed of beautifully-rounded pebbles in a matrix of sandy clay, sometimes ferruginous. There are some concretionary masses of iron, cemented sandstone, and the gravel is converted in places into a hard conglomerate by infiltrated iron oxides or more rarely by silica. The pebbles are mostly of quartz, frequently rock crystal, but they also include jaspery banded-ironstone, chert, agate, hard sandstone or quartzite, and occasional large and small pieces of silicified wood, as well as fragments of granite and chloritic schist. Large boulders are comparatively rare. The silicified wood, though distributed about in all sorts of positions, may possibly have formed in situ, the granite and schist last mentioned are the only other constituents of the deposit that are not well rounded. The presence of the agate, as already mentioned, shews the deposit to be newer than the lavas of the Forest Sandstone series. In one shaft, 25 feet of gravel had been passed through at the time of my visit, without any indications of approaching the base, and more recent work has shewn that my estimate of 40 feet is probably a moderate one for the maximum thickness.
The Lower SANDY BEDS have obviously derived most of their materials from the underlying granite. They are micaceous throughout, the upper and lower parts South of the Railway being clayey with a bed of clean sand in between. The base shews fragments of decomposed granite, and it is difficult to fix the point where the bed rock really begins.
It is clear that in these sands and gravels we are not dealing with the insignificant accumulations of the present-day river system. This is no less evident from the character and distribution of the deposits than from their position on the crest of what is now the main watershed of the country. Their extent is quite in keeping with a lacustrine origin ; on the other hand, they correspond closely with the alluvial deposits of rivers which have eroded their valleys practically to the lowest possible level, and have for long been chiefly occupied in widening them and spreading the materials furnished by the process evenly over their flood plains. The Somabula beds may therefore be set down, provisionally, as due to the action of an important Tertiary River or river system, probably a feeder of the
great lake which must once have filled the adjacent portion of the Zambezi basin, and draining an area chiefly occupied by granite and the Archaean banded ironstone.
The gravels of the Somabula are interesting from their unique lithological character, as far as Rhodesia is concerned, but their chief interest naturally arises from their being the source of various gem stones, particularly the diamond. The diamonds themselves are peculiar, as almost invariably of a green shade in the rough: this is, however, entirely lost in cutting. They occur in very good crystals, principally octahedra, spinel twins of two octahedra, twinned tetrahedra, twinned hexatetrahedra, dodecahedra, etc. Etched triangles are characteristic of the tetrahedral faces. Worn stones are almost entirely absent. The mineralogical associates of the precious stone are not precisely similar to those of the Kimberley diggings or of the more recent Transvaal discoveries, but they nevertheless present a general resemblance to those of the localities named. Garnets are often common, but are not of the blood-red Kimberley variety. Ilmenite is uncommon. Both magnetite and haematite, of which grains are numerous, are evidently derived from the Banded Ironstone, while the source of the zircon and of the mica (muscovite) is equally clearly the granite : these minerals have no necessary connection with the original matrix of the diamond. This is also probably the case with the beryls which
The typical minerals are (besides the diamond) enstatite staurolite, chrysoberyl, kyanite, and sapphire. Enstatite is the commonest of the minerals popularly grouped together under the name of olivine at Kimberley, but that found here is a brownish variety. Staurolite is an abundant constituent of the sorted material from the puddling machines. Some of the grains are fairly clear, and might almost be taken for garnets on account of their red-brown colour.
Chrysoberyl is quite abundant for so rare a stone. The prevailing variety is yellow, but the opalescent (precious) catse ye,” and the form known as alexandrite,'' which is green by day and red by candlelight, also occurs. This mineral would seem to be usually a product of contact metamorphism : at the same time, although it is not strictly analogous in a chemical sense, its similarity of composition and isomorphism with olivine are to be noted. As a gem, the ordinary variety is actually called chrysolite," one of the names properly applied to olivine. Kyanite and sapphire are unequivocal contact minerals, so are rutile and tourmaline, whose occurrence may also be noted. The presence of the first-named is interesting; owing to its softness, which makes its survival rather remarkable. Of the sapphires, both the blue and colourless varieties occur, while true rubies and oriental amethysts are also found, though they are distinctly rare, even for such scarce stones. Another stone whose occurrence may be noted is the so-called "Somabula Blue”; this is harder and heavier than common beryl, and is biaxial with a wide ophiaxial angle, cleavage flakes shewing the bisectrix normal to them. seems unquestionably, therefore, a variety of topaz: when cut it is one of the most beautiful gems imaginable.
With regard to the origin of the gems, the mere richness of the deposit is sufficient to indicate a source for the diamond, although many of the constituents of the gravel itself have obviously travelled far. Despite theories to the contrary, it seems evident from the evidence obtained in New South Wales, as well as in this country, that what is commonly called “blue ground is in all cases the original source of the diamond, and the occurrence of enstatite, as well as the presence of garnets, points to the same origin in the case of the Somabula field. I am aware that Professor Gregory, without making an inspection of the ground, has pronounced the opinion that the diamond comes from pegmatite veins, but such an idea is so completely at variance with the local conditions, and with all that we know of diamond occurrences, that it scarcely merits discussion. I have little doubt that it will not be long before the pipe which produced the diamonds is discovered, and that it will present, apart from slight local peculiarities, all the usual features of the South African mines already known.