SOUTH AMERICA
Brazil
José Vieira do Couto, in 1801, first pointed out that platinum occurs in the sands of the Lages River, near Conceiçao, Minas Geraes. According to E. Hussak[[78]], platinum in that region occurs only in the alluvium of rivers having their rise on the eastern slope of the Serra do Espinhaço. The platinum is accompanied by black pebbles of quartz-tourmaline rocks, magnetite, hematite, rutile, octahedrite, xenotime, monazite, senaite, pseudomorphs of rutile after octahedrite (captivos), zircon and gold, which is sometimes of a copper-red colour (palladium-gold). Palladic gold was formerly called ouro branco (white gold), by the miners.
At Condado, further north on the eastern slope of the same Serra, platinum also occurs with very similar associations. The platinum from both localities occurs in bunchy, mammillated and globular forms, concave within, with thin walls, having a radiated fibrous structure under the microscope. It is frequently in thin foliated crusts, having the characteristic structure of hematite. It was accurately described by Wollaston in 1805 and 1809. Hussak thinks the primary formation was an olivine rock, or gabbro, while the platinum is secondary, having been most probably re-deposited from solutions resulting from the decomposition of platiniferous pyrites or of sperrylite (PtAs2), minerals derived, possibly, from the neighbouring schistose quartzite, or from the overlying conglomeratic quartzite.
The platinum of Condado, although comparable, as regards density, to the Russian platinum, is very rich in palladium, is non-magnetic and contains practically no iron. An analysis by G. Florence gave the following percentages: insoluble residue, 0·92; platinum, 73·99; iridium, 0·08; palladium, 21·77; iron, 0·10 (= 96·86), undetermined (rhodium and osmium), 3·14.
In the Rio Abaeté, Minas Geraes, platinum—very different in appearance and chemical composition from that of the Serra do Espinhaço—occurs in placer deposits, associated with gold, diamonds and the following minerals: rolled pieces of a hydro-phosphate of barium and aluminium (gorceixite = “marumbé” of miners), garnet, almandite, pyrope, ashy-blue oxide of titanium (bagageira—regarded as a good indicator for diamonds), magnetite, chromite and calcium-titanate (perovskite). Pyroxene-olivine rock, a typical picrite-porphyry, rich in perovskite, and granular magnetite rocks, rich in titanium, have been observed by Oliveira in the vicinity. Hence it is highly probable that the platinum, as in the Urals, came from olivine rocks. The platinum occurs in thin laminæ, strongly rolled, and, rarely, in cubical crystals with the edges visibly rounded. It is strongly magnetic and contains no palladium. Minute crystals of osmiridium may occur with those of platinum, and in the platinum particles are found regular inclusions of osmiridium, as at Nizhne Turinsk, in the Urals, the platinum of which locality it resembles in chemical composition, magnetic properties and crystalline structure. The following analysis shows the percentage and composition of a general sample: insoluble residue, 7·57; iron, 9·62; palladium, trace; copper, trace; platinum metals, 82·81.
The auriferous alluvial of the Cuyabá and Coxim rivers in the southern part of the State of Matto-Grosso, also contain some platinum. According to Luiz Caetano Ferraz[[79]], platinum occurs in the River Coxipó-Mirim, where golddredging is carried on, combined with palladium, iron, osmium and iridium in small spherical grains, flattened on one side, of a brilliant white colour and strongly magnetic. It is found in alluvial deposits, associated with various kinds of quartz and oxides of iron, marcasite, arsenopyrite, rutile, anatase, almandine, garnet, black tourmaline, monazite, staurolite, white topaz, sphene, cassiterite, wolfram, graphite, galena and native silver.
In the State of Bahia, platinum has been found in Ituassú, Feira de S. Anna and Serra do Assuruá, and it is said to occur at Sâo Bartholomeu, and in the Serras do Pitango and Macahubes[[80]].
Platinum also occurs in Brazil as rare disseminations in the gold-bearing jacutinga, intercalated in the itabirites (e.g. at Gongo Socco Mine, long since abandoned). The jacutinga occurs as narrow bands and nuclei in the itabirites, containing a high percentage of gold, with much talc, clay and pulverulent pyrolusite. As accessory minerals zircon, rutile, cassiterite and tourmaline occur. Hussak thinks that the gold-bearing jacutinga has been derived from altered pegmatite veins.
From analyses made by Johnson (1833–41) on the Gongo Socco bullion, it would appear that the percentages of silver and platinum decreased while those of copper and palladium increased with depth (Henwood). The percentages of palladium varied from 3·89 to 4·80, and that of platinum from 0·04 to 0·12.
At Candonga, gold occurs in an eruptive rock rich in magnetite enclosed in itabirite, and is probably of contact-metamorphic origin. The gold occurs in grains of high standard, and with it are found fine indented scales of palladic gold, of a bright copper-red colour.
At Itabira do Matto Dentro gold occurs in jacutinga, lying between a micaceous iron schist, rich in quartz, and an enormous solid bed of itabirite. The palladium-gold may be copper-red, dark-brown or silver-white in colour. Native platinum also occurs with the gold.
Grains of platinum have also been found in the most northerly of the auriferous lenticular masses, which occur near the Bruscus River, near Pernambuco, in Cambrian crystalline schists. The matrix is a coarse white quartz containing small quantities of the arsenides and sulphides of iron, and the sulphides of copper, lead and zinc.
Platinum, although widely distributed in Brazil, occurs in such small quantities that so far there has been no production; but in the near future richer and more extensive deposits may be discovered, or it may be found practicable to win the metal from those already known, as an important by-product.
Palladium-gold, or porpezite, is a natural alloy of palladium and gold, and may contain up to 10 per cent. of the former metal. It is found in Brazil, in gold-washings, and also in the gold-bearing jacutinga reefs at Gongo Socco, Candonga and Itabira do Matto Dentro. In 1870 Henwood showed that the palladic gold from Gongo Socco contained, to a moderate depth from surface, from 0·04 to 0·12 per cent. of platinum. (Palladium-gold has also been reported from gold-washings in the Caucasus, near Batoum.) Ruer concludes, from an examination of the freezing-point curves of artificial alloys of gold and palladium, that these alloys form a continuous series of mixed crystals, and that there is no indication of chemical combinations[[81]].
Colombia
This republic is the second largest producer of platinum in the world, and in pre-war years supplied about 5 per cent. of the world’s total output. Owing to the decline of the Russian supply, and the increased demand for the metal, the industry has in recent years received a considerable stimulus, and in 1916 Colombia’s production rose to approximately one-third of the Russian output.
Platinum was first introduced into Europe from Colombia in 1735, although the metal was known in America for some time previously[[48]] p. 608. In 1810 the value of platinum stood at $5 to $6 (U.S.) per oz., and in 1823 the price had further dropped to from $3 to $4 per oz. As a result, platinum was rejected as waste in the operation of refining gold by the “dry-blowing” system. Later, when platinum became valuable, much of this dumped metal was recovered, notably in Quibdo, the capital of the Chocó district, where much gold-refining was carried on.
The following table gives the outputs of crude platinum from Colombia, in recent years, principally produced in the Chocó district:
| Year. | Oz. (troy). |
|---|---|
| 1911 | 12,000 |
| 1912 | 12,000 |
| 1913 | 15,000 |
| 1914 | 17,500 |
| 1915 | 18,000 |
| 1916 | 25,000 |
| 1917 | 32,000 |
| 1918 | 35,000 (estimated) |
The larger portion of the production is shipped to the United States, the exports to that country for the years 1910–18 being as follows:
| Year. | Oz. (troy). |
|---|---|
| 1910 | 1,600 |
| 1911 | 5,503 |
| 1912 | 6,627 |
| 1913 | 10,461 |
| 1914 | 12,387 |
| 1915 | 13,121 |
| 1916 | 25,588 |
| 1917 | 21,278 |
| 1918 | 27,030 |
The crude platinum is estimated at 84 to 85 per cent. fine. The price in 1917 fluctuated from £16 to £20 10s. per oz., the average for the year being nearer the latter amount.
During 1918 the United States Government fixed the price at $105 per oz., which stimulated production.
The deposits are alluvial, and consist of re-concentrates of older gravels. The principal source of supply is at the head of the San Juan River, which enters the Pacific Ocean north of Buenaventura, the richest deposits occurring in the Condoto, Opagado and Tamanal Rivers, tributaries of the San Juan[[82]]. Platinum is also obtained in the Upper Atrato River, which flows northward to the Caribbean Sea. It is nearly always found associated with gold. In the gravels of the San Juan River the two metals are present in about equal proportions, and in those of the Atrato the ratio is approximately 85 of gold to 15 of platinum.
The area including the watersheds of the San Juan and Upper Atrato Rivers is known as the Chocó district. T. Ospina, Director of the Colombian School of Mines[[83]], estimates that in the area are 5,000 sq. miles of gold and platinum deposits, the Mira River forming the southern boundary of the area. In 1916 he estimated that there were in it 68,000,000 c. yd. of actually profitable gravel, with a reserve of 336,000,000 yd. of possibly profitable ground. Platinum has also been recovered in much smaller amounts on the Micay River in the Barbacoas district, near the frontier of Ecuador. The stream beds in which platinum occurs are those in which Tertiary conglomerates have become eroded; the river gravels about the areas underlain by that formation are barren. The conglomerates are composed of rounded boulders of basic rocks, such as diabase, melaphyre, peridotite and dunite[[48]] p. 620.
At Novita Vieja, in the centre of the Chocó district, a bed of conglomerate 6 to 12 ft. thick has been laid bare, over an area 2½ miles long and ½ mile wide, through the sluicing away of the overlying sands. It contains 0·5 oz. of gold, and 0·5 oz. of platinum, per ton[[84]]. According to Castillo, the parent rock is a typical gabbro, pyroxene predominating over the felspar[[85]] p. 826.
Platinum has been found in the Chocó district in serpentine rock. Granite also occurs in the same district, traversed by quartz lodes containing palladium, iridium, osmium and rhodium.
Colombian crude platinum contains from 80 to 85 per cent. platinum, the remaining 15 to 20 per cent. consisting chiefly of iridium and osmium. The sands in which it occurs are described as brown in colour, and carrying, besides platinum and gold, the heavy minerals chromite, magnetite and ilmenite [[85]] p. 384.
In the past, mining operations have been very irregularly carried on, by primitive methods of working, but dredges are now being employed in increasing numbers. In 1915 a dredge was operated on the Condoto River, in the province of Chocó, by the Anglo-Colombian Development Company, and showed good results. Native methods of working are very simple. The alluvial gravels derived from dried-up beds of ancient rivers are hand-washed with the use of the batea or dish. Where the metal occurs in the bed of a river it is obtained by diving girls, who work down to the platiniferous gravels, removing the gravel in small dishes, the men being employed in washing the gravels on the river banks[[86]]. About 90 per cent. of the total output is recovered by these primitive methods.
A large portion of the industry is in the hands of two companies, one of which is the South American Gold and Platinum Company, of New York, a Lewisohn company, which has absorbed the interests of the above-mentioned Anglo-Colombian Development Company, Ltd., the Gold Fields American Development Company, Ltd., and Johnson, Matthey & Co., Ltd., of London. The second company—the British Platinum and Gold Corporation, Ltd.—has recently amalgamated with the Paris (Transvaal) Gold Mines, Ltd., taking in the latter’s interests on the Opogodo and other places.
The question of transporting platinum concentrate to the coast is not a matter of much difficulty, as the Atrato River is navigable as far as Quibdo, and the San Juan can be ascended by vessels of moderate draught for over 140 miles inland.
An estimate of average working costs appears to be 6d. per c. yd. for dredging, and 3d. for hydraulicking[[87]]. In 1917 new platiniferous deposits were discovered in the Caceres district, between the Cauca and Nechi Rivers, in the department of Antioquia. The mineralized area extends along the Caceri River, a distance of 14¼ miles, the width at the north end being 1¼ miles, and 300 ft. at the southern extremity [[10]] p. 545.
Ecuador
Platinum occurs, in association with the gold obtained from steam gravels, in the area covered by the Rivers Bogota, Cachabi, Uimbi, Santiago and Cayapas, but it has not so far been found in sufficient quantities to be of economic importance.
Dredging has been employed in mining these deposits, but does not appear to have been a success, and operations are now largely confined to native washings[[88]].
French Guiana
Platiniferous gold-bearing sands are found in the Aporuague River, the metal, according to an analysis by A. Danmer, having the following composition in percentages: platinum, 41·96; gold, 18·18; silver, 18·39; copper, 20·56[[89]].