The area emerged from the waves as early as the Carboniferous period; consequently the accumulation of platinum, and in some localities of gold, in the surface-deposits, were not swept away. They were concentrated later on in the alluvia—perhaps at the time of the most intense glaciation, probably in the Pleistocene.
Throughout the Urals, the primary source of the platinum is associated with the eruptive basic rocks, among which the platiniferous and auriferous dunite forms three great masses. The principal outcrops of platiniferous dunite and platinum-bearing alluvia are connected with the second of the four parallel bands mentioned above. Towards the south, the band becomes discontinuous, and finally dies out altogether. Here a few outcrops of platiniferous olivine-rock contain a small percentage of platinum, with osmiridium and other members of the group associated with it.
The two principal platinum-producing districts are in the central Urals, and are: (1) The Shuvaloff Estates, Isov district, on the River Iss, near Goroblagodat. (2) The Demidoff Estates, Nizhni-Tagilsk district, on the Martian River in the South Verkhotur district. Prior to 1879 the latter field furnished the larger part of the platinum, but since then the former has been the most productive, and now supplies about 80 per cent. of the total output. The whole of the platinum is derived from gravel deposits, which are usually auriferous, and associated with dunite.
The Isov district.—Platinum is concentrated in the channels of the Rivers Iss, Veeya and Tura. In the north of this region it is obtained from the Sosnovki, Kytlymi, and Mala Kosva Rivers; further north again, platinum occurs with gold in the Vagran River, and in the system comprising the Rivers Lobva, Niasma, Lialia, Aktai, Emerlo and Talits, the gold here being predominant. Other sources of supply of platinum and gold in this district are on the Ivdevl River. In the south of the area, platinum deposits are worked on the tributaries of the Tagil, Salda, Imiaun and Tura Rivers[[15]] p. 921.
The Nizhni-Tagilsk district.—The richest placers occur in the valleys of the Visim, Martian, Sisim, Chaush and Cherna Rivers. Further south, platinum is found with gold in the gravels of the Nevian, Verkhne-Iset, Bilenibaev, Alapaev, Sysert, Kyshtym and Mias areas, and also in the Tanalyk, Sakmar and Urtazym Rivers.
Other localities of smaller importance are the Nikolae Pavdinsk and Rastes districts in the northern Urals, and the Systersk mining district.
The placers are derived from country rock, made up of serpentine gabbro, diallage and olivenite, the principal associated minerals being quartz, zircon, ilmenite, chromite, magnetite, spinel, native gold and palladium. Gold is present in the concentrate in very variable quantities, and sometimes contains silver, but the latter generally occurs in combination with palladium. The crude platinum usually includes some iridium, rhodium, ruthenium and iron.
The basic igneous rocks, from which the platinum is derived, are exposed in the form of discontinuous elliptical outcrops near the summits of the Urals, particularly on the western side of the mountains. These outcrops attain larger dimensions in the northern and central Urals than further south. The process of concentration of platinum in the gravels has clearly extended over a very long period of time, and it is probable that the richer gravels have been reconcentrated, perhaps several times[[52]] p. 299. Platinum nuggets are rarely found, but three of large size were discovered in the Nizhni-Tagilsk district, their weights respectively being 25½ lb., 21 lb., and 11½ lb.
Duparc, in a brief description of the geology of the deposits, states that they are essentially of magmatic origin. The structure of the rock is in the nature of concentric bandings; the felspathic rocks at the outer edge gradually grade into the intermediate stage of pyroxenes, until the central dunite is reached, composed of olivine and chromite. The richness of the gravels is in proportion to the size of the dunite deposits, and to the extent of erosion of these rocks. According to Duparc, platinum ore derived from a pyroxenite source usually contains high percentages of platinum and palladium, but low percentages of osmium and iron. The same writer estimated in 1916 that the reserves were sufficient for about twelve years, provided that the same methods of working and rate of extraction were employed during that period[[53]].
The amount of the platinum in the wash is very variable. The average yield was formerly over ½ oz. per c. yd., but latterly, owing to the gradual exhaustion of the richer deposits, the average returns have not exceeded from 2 to 3 dwt. per c. yd. The crude metal assays about 83 per cent. platinum, from 5 to 7 per cent. osmium and iridium, with small amounts of ruthenium, palladium, and gold, and about 10 per cent. impurities, mostly iron and copper[[54]]. The thickness of the pay gravels varies from 3 to 6 ft., and about 4 ft. may be taken as the average width, the overburden running from a few to over 60 ft. The extent of the productive area is about 170 sq. miles, and from 15,000 to 20,000 miners were employed before the war[[4]] p. 558. It has been noticed that the platinum deposits are characterized by their uniformity, of course not being so sinuous in direction as gold deposits.