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]].