DIAMOND IN BRAZIL
To the west of the Diamantina district is the Rio Abacte, a tributary on the left bank of the Rio de Sao Francisco; this river is fed by the Rio Fulda and Rio Werra, and on the left bank by its tributary the Rio Andrada. Into the Rio de Sao Francisco flow also the Rio Indaia, the Bambuy, the Barraehudo, as well as the Paricatu, with its tributaries Santo Antonio, d'Almas, (le Somno, de Catinga, de Prata, and others. The diamonds of this region were discovered by unlicensed searchers (garimpeiros) in 1785, who worked at first without a concession; in the Rio Aback they found one of the largest of Brazilian diamonds, which weighed 138 1/2 carats. Although in 1791 there were as many as 1,200 licensed workers at the place, the deposits seem to have been very quickly worked out subsequent to the year 1795, and in 1807 work here practically ceased. This district embraces a stretch of country on the eastern slope of the Serra da Mata da Corda, about 300 miles long, and it is here that the rivers mentioned above have their sources. On the western side of the same range, still in Minas Geraes, but near the border of the State of Goyaz, is the district of Bagagem, having about the same length as the district of the Rio Abacte, the two districts having together a width of about 250 miles. The whole area embraced by these districts, though it has been only partially explored, has yielded a large number of diamonds, many of which are of considerable size. These include a stone of 120 1/8 carats, and also the famous " Star of the South, the largest of Brazilian diamonds, which was found in 1853, and in its rough condition weighed 254 1/4 carats.
A new diamantiferous deposit, which, however, does not appear to be very rich, has been discovered and worked in the district at Agua Suja, about twelve miles south of Bagagem. The diamonds are here associated with blocks of rock, identical with that which occurs in situ not far away, together with much magnetite and also ilmenite, decomposed perofskite, pyrope, and rutile. Some of these minerals, more especially the perofskite and pyrope, have not hitherto been found associated with diamond at any other Brazilian locality. This association of minerals recalls the mineral constituents of the "blue ground of Kimberley in South Africa, which will be described later, as will also the various minerals hitherto found in Brazil associated with diamond.
One other diamantiferous district in the State of Minas Geraes remains to be mentioned, namely, that of Grao Mogol (Grao Mogor), which is situated about 180 miles north of Diamantina, in a mountain range to the northwest of, and on the left side of, the Rio Jequetinhonha. Although this district was first searched in 1813 diamonds were not found here until 1827 it is remarkable as being the only locality at which diamonds occur in the solid sandstone, which, at one time, was thought to be their original mother-rock. Though the yield from this district is now small, it was formerly rather considerable, 2000 people being employed in the industry in 1839.
The geological relations of the diamantiferous districts of the State of Minas Geraes, especially that of Diamantina, have been frequently investigated, and, at least in the case of the latter, are fairly well understood, though many doubtful points still await elucidation. The investigations which have been made are due, at the beginning of the nineteenth century, to L. von Eschwege; a little later to Spix and Martins; in the fifties, first to Heusser and Claraz and then to Claussen and Helmreichen; and in later times to various geologists resident in Brazil, namely, Gorceix, De Bovet, Orville A. Derby, and others.
We learn from their observations that the principal rock in the Serra do Espinhao is usually thinly laminated sandstone or quartzite, the lamina bearing numerous scales of pale green mica on their surface. Some of the thin laminae or slabs are so peculiarly constituted that they can be bent without being broken, such specimens being hence described as flexible sandstone. The increased size of the quartz grains and pebbles renders the rocks in places more coarse-grained in character, so that it resembles a conglomerate rather than sandstone. This laminated sandstone, which is of great geological antiquity, is usually regarded as a sedimentary rock rather than as belonging to the crystalline schists; it is very abundant in the Serra Itacolumi, in the southern part of Minas Geraes, and has thus come to be known as itacolumite. Interbedded with it are clay-slates, and various schists such as mica-schist, hornblende-schist, hematite- schist, etc. The rock is penetrated for short distances by veins, which usually contain crystals of quartz. The beds of itacolumite and associated rocks, together with the underlying gneiss, mica-schist, and hornblende-schist, are usually inclined at a steep angle.
On the mountaintops the itacolumite is overlain unconformablv by a younger quartzite, the bedding of which is less steeply inclined than is that of the underlying rock. It is very similar in appearance to the itacolumite, and in places it merges into a conglomerate just as the itacolumite does. From the fact that irregular and angular projections of the lower rock are covered by the younger quartzite, it is evident that the two rocks are perfectly distinct, and probably belongs to very different periods.
At some places, and conspicuously so in the basin of the Rio de San Francisco, the beds of itacolumite are associated with slates and limestones, in which fossils of Silurian and Devonian age are found. These slates and limestones have no direct bearing on the occurrence of diamonds, since, as we shall see later, the itacolumite must be regarded as the diamond-bearing rock; they may, however, serve to determine the geological age, at present unknown, of the itacolumite when its relation to these rocks has been made out, which has not yet been accomplished.
We have already seen that the mode of occurrence of the diamond differs in each of these districts. Three kinds of diamantiferous deposits are distinguished according to their situation, whether on the plateau or in the valley, and, in the latter case, whether above or below the present high water level. These are known respectively as river-deposits, valley-deposits, or plateau-deposits, according as they are found in the existing watercourses within the present limits of high water, on the sides of the valley above high-water level, or covering more or less large areas on the summits of plateaux.
Both the river-deposits and the valley-deposits are without exception constituted of sands, the plateau-deposits also having in part a similar constitution; these sands, or alluvial deposits, consist of debris which has been transported by water, and which contains more or less rounded rock-fragments, among which the diamonds occur singly and isolated. The amount of rounding, which the rock-fragments have undergone, may be regarded as indicating the distance to which they have been transported from their original situation. In places, however, the rock-fragments of the plateau-deposits show no trace of having been water-worn; when this is the case, such deposits have undoubtedly been formed on the spot they now occupy, and consist usually of much weathered rock-masses, as will be shown in a special description of certain plateau-deposits.
An attentive consideration of the distribution of the three classes of deposits leads to the recognition of a certain connection between them. The various diamond-bearing districts of the plateau are at the same time the collecting grounds of the diamantiferous streams and rivers; it is a natural conclusion then that the stones now found in the sands of the river valleys have been carried there, together with sand, gravel, and other debris, from their original situation on the plateau by these same rivers and streams. This is especially the case in the neighborhood of the town of Diamantina, which stands on a plateau, the surface beds of which consist of diamond-bearing rock. The rivers which have their sources in these rocks are in their lower courses rich in diamonds, whereas in other rivers, such, for example, as the Rio Doce and its tributaries, which rise among rocks from which diamonds are absent, no diamonds are to be found.
Rafal Swiecki, geological engineer email contact
This document is in the public domain.