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Thus we find the oxides of titanium and of iron present in great abundance, though even here that of the different varieties of quartz exceeds the amount. The material of these plateau-deposits is known as "gurgulho", it occurs usually in horizontal beds, and is built up of coarse blocks of the surrounding rocks, with a more or less red clayey earth. In this material the diamond and its associated minerals are embedded; so indiscriminately, however, is ever thin colored by the red clayey earth that it is impossible to distinguish one mineral from another until the material has been washed. In some deposits a natural process of washing has removed the earthy material, and here the diamonds and their associated minerals are from the first distinctly seen. The rock and mineral fragments in the "gurgulho" are very slightly, if at all, rounded, and the diamonds themselves still preserve their perfectly sharp edges and corners, and the original natural characters of their faces.

The proportion of diamonds and their associated minerals present in a given weight of the material is rather smaller in "gurgulho" than in other deposits, but the average size of the stones is greater. The distribution of the diamonds is sometimes very irregular, large numbers, aggregating in weight up to 1,700 and 2,000 carats, being found in a single small nest, while few, if any, in a considerable area of the surrounding "gurgulho."

Under the diamond-bearing "gurgulho," and resting immediately upon the solid rock, there is usually a layer of clay, in which also a few diamonds are to be found. Above the "gurgulho," just as in the valley deposits, is a layer of red clay of varying thickness, from which diamonds are absent. When this layer is absent, as it sometimes is, the "gurgulho" forms the actual surface of the ground and is covered by vegetation. It is said that the observation of diamonds attached to the roots of plants, scratched up to the surface by fowls, or picked up by children at play, has led to the discovery of rich deposits.

Plateau deposits at other places, such as, for example, Sao Joao da Chapada, on the plateau of Diamantina and about twenty miles west of this town, differ very widely from those just considered. The mines here were situated on the watershed between the Rio Jequetinhonha and the Rio das Veihas, and on the prolongation of the straight line which connects the important deposits of Boa Vista, on the hills near Curralinho with those of La Sopa. These deposits were discovered in 1833. Extensive workings were carried on for a long period, but were finally discontinued owing to the exhaustion of the deposit. In spite of this the place remains extremely important from the scientific point of view, for here may be gathered data which afford material help in solving the problem as to the nature of the original mother-rock of diamond in this region.

The diamond occurs here in variously colored clays, which lie in a trench 40 meters deep, 60 to 80 meters wide, and 500 meters long, somewhat resembling a deep railway cutting. These clays are distinctly bedded, being inclined 50º to the east, and regularly and conformably interbedded with them are beds of itacolumite inclined at the same angle. All the strata, the clays as well as the itacolumite, are penetrated by numerous small veins filled for the greater part with quartz (rock-crystal), rutile, and hematite.

The yield of diamonds from these clays was very variable, but on the whole the deposit was considered poor. Tschudi, who visited the place in 1860, reported that in his presence forty-four carats were obtained in two hours, while on another occasion only ten small stones were found in twelve tons of material. The associated minerals are the same as elsewhere, the three just mentioned being especially abundant. It is a noteworthy fact, that where the associated minerals occurred in abundance there diamonds were plentiful, but where, on the other hand, the minerals were present to only a sparing extent, diamonds also were hard to find.

The minerals associated with the diamond are present at this locality in less proportion than in the ordinary "cascalho" or "gurgulho," and the same is true also of the diamond itself. As has already been mentioned, the minerals, which occur most frequently, are quartz, hematite, and rutile, other oxides of iron and of titanium, tourmaline, etc. All are found, like the diamond itself, in perfectly sharp crystals. Even the softest of the minerals found here have preserved intact the sharpness and angularity of their edges and corners. None show any indication of having been transported by running water.

These circumstances have led those who have personally investigated the deposit, namely, Orville A. Derby and Gorceix, to the conclusion that here, in these beds, the diamond is seen in its original home, and that here, in the quartz-veins by which the rocks are penetrated, it slowly took on the shape and form in which we now know it. Although no diamond has ever been found actually in a quartz-vein, yet the minerals associated with it occur in such situations with great frequency, and their constant association with the diamond, not only here but at all other localities of Minas Geraes, seems to point to a common origin for both. The fact that the diamond itself has never been found in a quartz-vein may perhaps be explained by the extreme rarity of its occurrence as compared with that of other minerals. The clays in which the precious stone lies, are decomposition products of the rocks which were originally penetrated by the quartz-veins, and which, like the surrounding schistose rocks, have been reduced to their present state of disintegration by exposure to the action of weathering agencies.

The deposit at Cocaes, near Ouro Preto, appears to be very similar to that at Sao Joao, which we have been considering. The diamonds here occur at a height of 1,100 feet above sea level, on a plateau of itacolumite overlying mica-schist and beneath this granite-gneiss. The minerals here associated with diamond are quartz, ilmenite, anatase, rutile magnetite hematite, martite, tourmaline monazite kyanite, fibrolite and gold. Of these minerals the first three in the list predominate, and quartz only occurs in rounded fragments.

The occurrence at Grao Mogol, in the district of Minas Novas, is of a different type again. This town is situated in the extreme north of the State of Minas Geraes, on the left or northern bank of the Rio Jequetinhonha and about 190 miles northeast of Diamantina. As well as in the normal "gurgulho", diamonds are here found in a solid, compact, conglomeratic sandstone containing much green mica, especially along the planes of bedding. According to some accounts this is to he regarded as a single, isolated, sandstone block of enormous size others, however, attribute to the diamond-bearing rock an extension of 300 to 400 meters. This deposit was discovered in 1833 and was worked in the thirties and forties, fragments of the sandstone being detached by means of blasting powder. All the fragments of sandstone with embedded crystals of diamond, which are sometimes, though rarely, to he seen in mineralogical collections, have come from this locality. Such specimens are not, however, in all cases genuine, for the crystals of diamond have sometimes been artificially set in the rock.

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