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The connection thus existing between the deposits of the plateau and those of the valley leads to the view that the mineral associations of diamond, whether they occur on the hills or in the valleys, are essentially the same over the whole of Minas Geraes. The material of these deposits consists mainly of grains and fragments of the surrounding rocks, from the weathering of which they have been derived, and includes besides the diamonds various minerals which may be in a fresh unaltered condition, or more or less weathered and decomposed. The mineral most frequently and abundantly present everywhere is quartz, of which the transparent and colorless varieties occur, as well as the compact varieties such as hornstone, jasper, etc. All three modifications of titanium dioxide, namely, rutile, anatase, and brookite are met with, the last being represented by the variety known as arkansite; crystals of anatase are sometimes completely altered to rutile, while preserving at the same time their own external form; these pseudomorphs are known in Brazil as "captives". Other minerals found in the deposits are oxides and hydroxides of iron, especially magnetite, ilmenite (titaniferous iron-ore), hematite, hematite having the external crystalline form of magnetite (the so-called martite), and limonite; also iron-pyrites, either unchanged or altered into brown hydroxide of iron (goethite), tourmaline, various kinds of garnet, fibrolite, lazulite, psiloinclane, talc, mica, yttrotantalite, xenotinie and monazite, kyanite, various complex hydrated phosphates (goyazite, etc.), diaspore, staurolite, sphene, and topaz, both white and blue but not yellow. In addition to diamonds, gold is frequently washed for, and is associated with platinum, the latter, however, not in sufficient quantity to be of any commercial importance. Some of the minerals mentioned above are distinguished in the district by local names; thus the black rounded pebbles of tourmaline are known as "feijas" (black beans), and the brown pebbles, consisting of a hydrated phosphate, or of titanium or zirconium oxides, are called "favas" (broad beans).

The minerals mentioned above are not of equally frequent occurrence; the most constant associates of diamond, after the different varieties of quartz, are the oxides of titanium (rutile, anatase, and brookite), hematite and martite, pebbles of black tourmaline, and specially xenotime and monazite. Even these, however, occur in varying abundance and frequency at different places; thus the same minerals do not occur associated together in the same way in every river, nor indeed in every part of the same river, this depending on the fact that the lighter minerals are transported at a greater rate than the heavier, and that some are more liable to be altered and reduced to powder than are others.

It should be mentioned here that while in the deposits of Salobro, in Bahia, corundum is found associated with diamond, it is entirely absent from the State of Minas Geraes.

Diamond-diggers are guided to a certain extent in their search for the precious stone by the presence or absence of the minerals usually associated with it, which they refer to as the formation. While by reason of their more sparing occurrence and small size diamonds may easily be overlooked, the associated minerals occur usually in larger and more conspicuous crystals and fragments, and are therefore more readily seen. Where the "formation" is absent a search for diamonds is useless, and never undertaken, since they are never found apart from their associates. It by no means follows, however, that diamonds are to be found wherever the "formation" exists; they may be altogether absent, or present in numbers insufficient to pay for the labor of working.

The different constituents of the "formation" are not regarded alike by the diamond-diggers. Those, to which a special importance is attached, as being certain indicators of the presence of diamonds, are tourmaline pebbles ("feijas"), the oxides of titanium (especially anatase, less so rutile and brookite), iron oxides (magnetite, ilmenite, hematite, and limonite), the phosphates ("favas"). Other minerals, such, for instance, as lazulite, are considered unimportant. Opinions on this matter naturally vary, and are to a certain extent arbitrary, but it may be taken as a safe rule that the presence of those minerals, which are most constantly associated with diamond, is an indication, which must not be disregarded.

We now pass to a more detailed consideration of the three classes of diamond-bearing deposits, namely those of the rivers, valleys, and plateaux, as they occur in the district of Diamantina and elsewhere in Minas Geraes.

The river-deposits are the richest of the three; they are found in the valleys below the existing high-water level, and at the present time are the only deposits of importance, not only in this district but also in the whole of Brazil, and this in spite of the fact that the average size of the stones so found is smaller than that of the diamonds of other deposits. In connection with the question of size, it is remarkable that stones found in the lower courses of a river are smaller than those in the upper part, and that eventually a point is reached in the river course at which the diamonds disappear altogether. This is strikingly shown in the Rio Jequetinhonha and other rivers, in which, 60 miles below Diamantina, only very small stones are to be found. In these rivers the material of the diamantiferous deposit is much rounded, more so than in others; the edges and corners of the diamonds are also considerably worn. The fact that the stones diminish in size the farther down the river they are found can easily be explained, when it is considered that material transported by water becomes more and more worn as the distance over which it travels increases, and, moreover, that the smaller the stones become, the more easily are they transported by water and the greater will be the distance they are carried from their original situation.

The diamantiferous debris which lies in the beds of the rivers and the bottom of the valleys consists mainly of rounded fragments of rocks and quartz brought down by the streams and rivers from their sources and upper reaches. This debris is usually mixed with clay to a greater or less extent, the resulting material in the state in which it is worked for diamonds being known to the diamond-diggers as cascalho. This is usually loose and incoherent, showing no signs of bedding; at times, however, it has a firmer consistency due to the presence of the clay. The upper portion of the mass is sometimes bound together, to a greater or less depth, by a ferruginous cementing material so as to form a conglomerate. This conglomerate, consisting largely of rounded quartz grains and pebbles, occurs in extensive beds or in isolated blocks known as "tapanhoacanga," or "canga," which may enclose crystals of diamond. Such a fragment of conglomerate, with a diamond embedded in it; similar specimens are often exhibited in collections as Examples of the occurrence of diamond in its mother-rock, a view that, as we have seen, is incorrect.


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