Fig. 22.—Sources of crude platinum produced from 1909 to 1917.
Russia.
—Russian placer deposits have supplied approximately 95 per cent. of the platinum in the world. The principal placer deposits rich in platinum are in the central Urals, in the Perm government, near Nishni-Tagilsk, Nishni-Turinsk, and Verkhoturshi. The richer deposits are on the eastern slope of the mountains, principally on the Iss and Veeya tributaries of the Tura River of the Obi drainage. Several important placers are found on the west slope of the mountains on the headwaters of the Chusovaia and Kama rivers of the Volga drainage. Near Nishni-Tagilsk platiniferous placers occur both on the Taguil, a tributary of the Obi, and on the Martian, a headwater stream of the Volga drainage. In these placers platinum is the predominant metal, but gold is also found. The greater part of the output in the past has been by hand-washing, but of recent years much of the ground has been reworked by dredges, though it is still safe to say that over 75 per cent. of the output is from hand labor. It is hardly practicable in a report of this sort to discuss in detail the individual deposits, even were the information at hand. The best information available can be summarized as follows: most of the platinum has undoubtedly originated from the disintegration of dunitic, pyroxenitic, or peridotitic rocks. The period of weathering has been very long and there have been many changes in the drainage systems, which have now reached maturity. The stream grades are low; the inter-stream relief is relatively slight. The platinum has probably been reconcentrated many times and is at present won principally from present valley gravels, though the pay channels do not always follow the present river channels. Some bench-ground representing old river channels is worked, particularly in the Nishni-Tagilsk region. The pay gravels ordinarily rest on bed-rock, though concentration on clay beds forming false bed-rock is fairly common. The pay dirt ranges from a few inches to as much as 6 feet in thickness. It has few large boulders, but has considerable clay in many places. The overburden ranges from 2 to 16 feet in thickness, averaging 8 to 10 feet. It consists of a thick basal portion of practically barren gravel and sand, or clay and sand lenses interlayered with gravel, which is overlain by 2 to 3 feet of clay sand and vegetable matter similar to the “muck” of the Alaskan mines.
In shallow ground, up to 3 feet deep, mining is carried on by open-cut methods; in the deeper ground shafts and drifting have been employed. For the deposits in river channels crude hand dredges were and still are used for raising the gravels. About 35 more or less modern dredges were engaged in platinum mining prior to the war. Clay is so generally found in the gravels that specially designed machines have been used to save the platinum and the newer dredges have special devices to cope with this problem.
Colombia.
—The platinum district of Colombia covers the upper waters of the Atrato and San Juan rivers in the Choco district of northwestern Colombia. Platinum is known as far north as Bete, on the Atrato, and in many of the tributaries of the San Juan which enter from the east. Most of the platinum exported from Colombia has come from the Condoto River, a headwater stream of the San Juan. It is stated that in this stream the platinum constitutes about 75 per cent. of the precious metal in the gravel. On the Atrato the platinum content is lower, ranging from 5 to 15 per cent. Platinum and osmiridium also occur in the streams south of the San Juan drainage, which enter directly into the Pacific, though little authentic data concerning them are available.
According to Dr. Tulio Ospina,[189] Director of the School of Mines, Medellin, Colombia, platinum is found widespread in conglomerates which cover an extensive area in the Atrato and San Juan basins. The metal has been reconcentrated in the present stream channels from which the major output of platinum comes. According to the statements of various observers, there are places on the interstream areas in which platinum has been concentrated. These areas are, from all accounts, old stream channels. The primary platinum deposits evidently are to be looked for on the west slope of the western ridge of the Andes, though no literature which gives detailed information on the geology of this range has been found. Peridotite, dunite, and other basic igneous rocks are represented in the platiniferous gravels.
[189] Proceedings Second Pan-American Scientific Congress, vol. 8, 1917.
There is almost no information upon which to base an estimate as to the possible reserves of platinum in Colombia. Little has been published on the subject that gives good data on the geology of the country, but from all accounts it seems safe to assume that Colombia holds much promise and should be more carefully prospected. Over 90 per cent. of the platinum mined in Colombia is won by natives, mostly women, who raise the gravel in calabaches and wash the gravels in bateas. The dry season is utilized, for then the low places in the river are more exposed. There is one dredge in operation in Colombia at present, though other dredging operations have been tried, which, for various causes, were not successful.