Practically all of the platinum metals produced in the world to date have been derived from placer deposits, though a little platinum and palladium have been obtained as a by-product from the electrolytic refining of copper and nickel matte.
The platinum placers of the world are so definitely related to intrusive basic igneous rocks, including pyroxenites, peridotites and dunites, that there is practically no question of the origin of the metal. In fact, in British Columbia, Spain, and Russia platinum has been found in place in these types of rocks, and it is reported that owing to the great value of platinum at present, a project is on foot to crush and wash certain of the bodies of dunite in Russia.
Platinum is not found in all placers derived from basic rocks, but so far as known it is rarely found in placers derived from other types of rocks. Prospecting for platinum placers therefore resolves itself first into a search for deposits of gravel derived in a large part from pyroxenite, peridotite, and dunite. Chromite is a characteristic heavy constituent of platinum sands and in some deposits can be recovered as a by-product in mining. Olivine is also present in considerable amounts. Magnetite and ilmenite are ordinarily present in the concentrates from platinum placers, but are more characteristic of placers in which gold is the most valuable constituent.
A large amount of prospecting has been done in Russia and Spain, based on the theory of origin outlined above. Much of this prospecting has been successful in locating platiniferous placers, but many of these recent discoveries do not seem to be of commercial worth. It is not possible to foretell whether placers derived from basic igneous rocks will be productive. The most that can be told is that certain placers derived from these rocks hold the greatest promise for the searcher for platinum. Platinum is such a rare metal and is found in such small quantities in its mother rock that it is necessary to have certain physiographic conditions present to predicate commercial deposits. Most important of these conditions is extremely prolonged or very rapid weathering of the primary deposits. In the Russian field, rock-weathering has been in progress for great geologic time; on the other hand, in Colombia, South America, the period of weathering seems to have been relatively short, but so rapid that the same result has been obtained.
Ordinarily, platinum is not found in commercial quantities in gravels that have not been reconcentrated, and the richer deposits of the world seem to be the results of repeated reconcentrations of platinum-bearing material. Crude or placer platinum is not pure metal, but contains, besides other metals of the platinum group, more or less iron, nickel, and copper. Russian crude platinum is ordinarily sold on the assumption that it contains 83 per cent. platinum metals; Colombian crude, 85 per cent. platinum metals. Some placer platinum, so-called, carries a large proportion of osmiridium. Thus the Oregon and California crude platinum carries from 25 to 45 per cent. iridium, and Tasmanian platinum is really nearly pure osmiridium.
The following analyses of Russian,[183] Colombian,[184] and American[184] platinum serve to illustrate the wide divergence of metal content of crude platinum.
Table 73.—Analyses of Crude Platinum from Various Parts of the World
| North America[183] | South America[183] | ||||||
|---|---|---|---|---|---|---|---|
| California | California | Oregon | British Columbia | Colombia | Colombia | Colombia | |
| Pt | 85.50 | 63.30 | 51.45 | 72.07 | 86.20 | 86.16 | 80.00 |
| Ir | 1.05 | 0.70 | 0.40 | 1.14 | 0.85 | 1.09 | 1.55 |
| Rh | 1.00 | 1.80 | 0.65 | 2.57 | 1.40 | 2.16 | 2.50 |
| Pd | 0.60 | 0.10 | 0.15 | 0.19 | 0.50 | 0.35 | 1.00 |
| Os | ... | ... | ... | ... | ... | 0.97 | |
| Io3[186] | 1.10 | 22.55 | 27.30 | 10.51 | 0.95 | 1.19 | 1.40 |
| Au | 0.80 | 0.30 | 0.85 | ... | 1.00 | ... | 1.50 |
| Fe | 6.75 | 6.40 | 4.30 | 8.59 | 7.80 | 8.03 | 7.20 |
| Cu | 1.40 | 4.25 | 2.15 | 3.39 | 0.60 | 0.40 | 0.65 |
| Sand | 2.95 | ... | 3.00 | 1.69 | 0.40 | ... | 4.35 |
| 101.15 | 99.40 | 100.25 | 100.15 | 100.25 | 100.35 | 100.15 | |
| Oceania[185] | Russia[187] | |||||||
|---|---|---|---|---|---|---|---|---|
| Borneo | N. S. W. | Australia Tasmania? | Taguil | Iso | Kamenonchy | Koswinsky | Kanjakowaky | |
| Pt | 82.60 | 75.90 | 61.40 | 76.16 | 80.10 | 82.46 | 83.50 | 60.39 |
| Ir | 0.66 | 1.30 | 1.10 | 2.68 | 1.38 | 1.79 | 2.74 | 6.90 |
| Rh | ... | 1.30 | 1.85 | 0.54 | 0.30 | 0.69 | 0.62 | 0.80 |
| Pd | ... | Trace | 1.80 | 0.27 | 0.30 | 0.18 | 0.28 | 0.19 |
| Os | ||||||||
| Io3[186] | 3.80 | 9.30 | 26.00 | 1.50 | 4.47 | 4.99 | 0.79 | 20.21 |
| Au | 0.20 | ... | 1.20 | ... | 0.09 | 0.27 | 0.07 | |
| Fe | 10.67 | 10.15 | 4.55 | 14.72 | 7.68 | 9.49 | 11.05 | 11.16 |
| Cu | 0.13 | 0.41 | 1.10 | 3.39 | 0.63 | 0.54 | 1.14 | 0.49 |
| Sand | ... | 1.22 | 1.20 | |||||
| 98.06 | 99.58 | 100.20 | 99.26 | 94.95 | 100.41 | 100.19 | 100.04 | |
[183] Duparc, Louis: “Le Platine et les gîtes platiniferes de l’Oural”: Soc. des Eng. Civ. de France. Bull. Jan.-Mar., 1916.