DIAMOND IN BRAZIL
This diamond-bearing sandstone has been, and is how by some geologists, considered to be itacolumite. Those who hold this view regard the sandstone or itacolumite as the original mother-rock of the diamonds now found in it, it being considered that they are as truly constituents of the rock as are the quartz-grains. Later and more detailed examination has, however, rendered it probable that this sandstone is not itacolumite but the more recent quartzite, such as we have seen to be superimposed unconformably in the Sierra do Espinhaco on the bed of itacolumite. This quartzite, although very similar in general appearance to itacolumite, is yet geologically quite distinct and probably of a much later date, having no doubt been formed of material derived from the weathering of the diamantiferous itacolumite. Which of these two views is the more correct has not yet been definitely decided. If the rock be really itacolumite, the origin of diamond at this place will differ from that at Sao Joao; if, on the other hand, it should be the later quartzite, which is more probably the case, then its occurrence here is in complete harmony with that at Sao Joao, for the later-formed quartzite must of necessity contain not only the constituents of itacolumite but also the minerals, including the diamond, which filled the veins by which it was penetrated, as is in fact the case. A comparison of the different diamantiferous deposits leads inevitably to the conclusion that each may be regarded as typical of some one stage in the development of a single process.
Thus at Sao Joao da Chapada, and elsewhere on the plateau, we see the diamond still at the place and in the rock in which it was originally formed, though the latter has been, in part at least, altered by weathering to a soft clayey mass. In such a case we may distinguish the deposit as original or primary.
Other plateau-deposits, where the rock fragments are but slightly, if at all, rounded, must have been laid down at an early period when the plateau had been eroded by water-courses to only a slight extent and before the present valleys came into existence. The diamonds and their associated minerals were indeed carried away by water from the disintegrated mother-rock, in which at Sao Joao they still remain, but they were re-deposited at spots not very far distant, as is proved by the fact that they are so little water-worn. The diamonds and other minerals were probably re-deposited on the floors of shallow lakes, a hypothesis that would account for the bedding of the material in which they occur. Such deposits are described as secondary or derived.
The slow but never ceasing erosion of the plateau by streams and rivers resulted in the course of ages in the formation of the valleys of the present day. Here, high up on the sides of the valley, the most ancient deposit marks the level of the original riverbed. The material for this deposit was derived partly from the primary or original deposit and partly from the secondary plateau-deposit. Some of the diamonds of the oldest valley-deposits have therefore changed their situation twice, and the material, having undergone a second transportation, is therefore more appreciably rounded. As the rivers carved out for themselves deeper and ever deeper channels, so fresh deposits were laid down, the material of each successively lower level being more and more water-worn, until the wearing down process culminates in the much rounded material of the present river-bed. The older valley-deposits are now to be found forming the terraces high up on the sides of the present valleys, while those in the present riverbed itself constitute the deposits described above as the river-deposits.
What has been said above with regard to the original mode of occurrence of diamond in Minas Geraes may be summarized as follows: The home of the diamond is located in those portions of the plateau-deposits in which the diamantiferous rivers take their origin. The diamonds gradually decrease in size and number and finally disappear further down the valleys. The rock, which is in situ in the neighborhood of the plateau-deposits, is everywhere itacolumite, interbedded with schists and covered by the younger quartzite. This itacolumite is evidently the source from which the diamonds and the material in which they are found have been derived. So much was established by L. von Eschwege as early as the beginning of the nineteenth century; this observer noticed that in the Diamantina district diamonds were found only in those rivers, such, for example, as the Rio Jequetinhonha, which flow down the western slopes of the Serra do Espinhaco which are formed of itacolumite, and that, on the other hand, no diamonds are found in those rivers, such, for example, as the Rio Doce and its tributaries, which rise on the eastern side of this range, these slopes being formed of gneiss, mica-schist, etc.
As has been already pointed out, those rivers which flow through strata composed of itacolumite are diamantiferous, while in those flowing through districts from which itacolumite is absent no diamonds are found.
The occurrence of the minerals associated with diamond, especially the most important of them, such as quartz (rock-crystal), oxides of iron and of titanium, tourmaline, etc. is also confined to the itacolumite; they do not, however, occur embedded in the rock but in the veins, consisting mainly of quartz, by which it and the interbedded schists are penetrated. The circumstance that the diamond is invariably associated with these minerals, and with them alone, points to the conclusion that it originated in the mineral-veins, as was first insisted upon by Gorceix. This conclusion receives additional support from the fact that Brazilian diamonds, instead of exhibiting a perfect and complete development on all sides such as is characteristic of embedded crystals, frequently show on one side an area by which they seem to have been attached during their growth and development, impressions of quartz-crystals being sometimes seen on such areas of attachment. Moreover, diamonds have been found enclosed in, or attached to, the surface of crystals of quartz, anatase, and hematite, and this could scarcely be explained except on the supposition that these minerals have all grown together at the same time and in the same vein. It has been stated by Gorceix that "a few diamonds have in places been met with actually in the mineral-veins themselves, and, though in small numbers, have been extracted; he compares such occurrences with that of the yellow topaz found near Ouro Preto in quartz-veins penetrating decomposed schists. In these districts, then, the diamond is a vein mineral, while in other localities it is an original constituent of the primitive crystalline rocks." Strange… A sedimentary paleoplacer, possibly subjected to a low-grade metamorphose, should be more appropriate choice for the origin of diamonds. In Guyana and southern Venezuela, diamonds are associated with the erosion of large paleoplacers (conglomerates) within a 1.7 billion years old sedimentary Roraima formation, later penetrated by sills and dikes of basic intrusives.
The precise method of winning diamonds adopted in Brazil depends in ore or less upon the nature of the deposit. A diamond-working was known in Brazil as a servico, those in a river-deposit being distinguished as "servicos do rio," while those in valley- and plateau-deposits are distinguished respectively as "servicos do campo" and "servicos da serra." The methods in each case have changed but little during the whole period the deposits have been worked; Negroes working formerly as slaves, but now as freemen perform the greater part of the labor.
Rafal Swiecki, geological engineer email contact
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