Platinum Ores

natural mineral formations containing platinum group metals (platinum, palladium, iridium, rhodium, osmium, and ruthenium) in economic concentrations. Significant accumulations of platinum ores in the form of deposits are encountered very rarely. Deposits of platinum ores are either primary or placer and are composed of platinum or complex ores (many primary deposits of copper and copper-nickel sulfide ores and placer deposits of gold with platinum or with iridosmine).

Platinum group metals are unevenly distributed within deposits of platinum ores. Their concentrations range from 2–5 g to several kilograms per ton in primary platinum ores, from tenths to hundredths (sometimes thousandths) of grams per ton in primary complex ores, and from tens of milligrams to hundreds of grams per cu m in placer deposits. The main form in which platinum metals are found in ore is platinum minerals, about 90 of which are known. The most frequently encountered are polyxene, ferroplatinum, platiniridium, nevyanskite, sisers-kite (a variety of iridosmine), zviagintsevite, paolovite (Pd₃Pb₂), froodite, sobolevskite, plumbopalladinite, and sperrylite. The dispersed form of platinum group metals in platinum ores found as infinitesimally small impurities enclosed in the crystal lattice of ore and rock-forming minerals is of lesser importance.

Primary deposits of platinum ores are platiniferous complex sulfide and platinum chromite ores in a variety of forms with massive and disseminated structure. These ore bodies are closely related genetically and spatially to intrusions of basic and ultrabasic rocks and are predominantly magmatic in origin. Primary deposits of platinum ores are encountered in platform and folded regions and are always centered on large fractures in the earth’s crust. The formation of these deposits took place at various depths, from 0.5–1 to 3–5 km, and in various geological epochs, from the Precambrian to the Mesozoic.

Complex deposits of copper-nickel sulfide platinum ores occupy a leading position among the natural sources of platinum metals. The area of these deposits is up to dozens of square kilometers, and the commercial ore zones are many dozens of meters deep. Their platinum mineralization is associated with bodies of solid and disseminated copper-nickel sulfide ores of complexly differentiated intrusions of gabbro-diabases (deposits of the Noril’sk Ore Region in the USSR and Insizva in the Republic of South Africa), stratiform intrusions of gabbro-norites with hyperbasites (deposits of the Merensky horizon in the Bushveld complex in the Republic of South Africa and the Monchegorsk deposit in the USSR), and stratified bodies of norites and granodiorites (the Sudbury copper-nickel deposits in Canada).

The main ore minerals of platinum ores are pyrrhotite, chal-copyrite, pentlandite, and cubanite. The main platinum group metals in copper-nickel ores are platinum and palladium, which occurs in greater quantities (Pd:Pt ≧ 3:1). The contents in ores of the remaining platinum group metals (rhodium, iridium, ruthenium, and osmium) are tens and hundreds of times less than those of palladium and platinum. In copper-nickel sulfide ores there are many minerals of platinum metals, mainly intermetallic compounds of palladium and platinum with bismuth, tin, tellurium, arsenic, lead, and antimony; solid solutions of tin and lead in palladium and platinum and of iron in platinum; and arsenides and sulfides of palladium and platinum.

Placer deposits of platinum ores are mainly Mesozoic and Cenozoic eluvial-alluvial and alluvial placers of platinum and iridosmine. Commercial deposits are exposed on the earth’s surface (open deposits) or covered by a sedimentary stratum 10–30 in thick (closed deposits). The largest such deposits extend dozens of kilometers and are hundreds of meters wide; the width of the productive metalliferous beds reaches several meters. These deposits formed as a result of weathering and decomposition of platiniferous clinopyroxene-dunite and serpentine-saxonite bodies. Commercial deposits are known both on platforms (the Siberian and African platforms) and in eugeosynclines in the Urals, Colombia (Chocó region), and Alaska (Good News Bay). Minerals of platinum group metals in placers are often found in concretions with one another, as well as with ohromites, olivines, and serpentines.

Platinum ores are extracted by opencut and underground mining methods. Most of the placer deposits and some primary deposits are mined by the opencut method. Dredges and hydromechanical devices are commonly used in opencut mining of placer deposits. Underground mining is the major means for developing primary deposits and sometimes for removing rich closed placers.

A slime of “raw” platinum, which is a platinum concentrate containing 70–90 percent minerals of platinum group metals, with the remainder consisting of chromites, forsterites, and serpentines, is produced by wet concentration of metalliferous sands and chromite ores. Such platinum concentrates are then refined. Complex sulfide platinum ores are concentrated by flotation, with subsequent multioperation pyrometallurgical, electrochemical, and chemical treatment.

The main countries producing platinum ore are the USSR, the Republic of South Africa, and Canada. The world reserves of platinum group metals (excluding the USSR) were estimated at about 7,000 tons in 1972 (6,200 tons in South Africa, about 500 in Canada, 155 in Colombia, and 93 in the USA). In 1972 the production of platinum group metals (in tons) was 45.2 in South Africa, 12.4 in Canada, 0.8 in Colombia, and 0.5 in the USA (total world production was 59 tons). The major industrial deposits of platinum group metals are the Merensky horizon deposits in the Bushveld complex in South Africa, the deposits in Sudbury (Ontario) and Thompson-Wabowden (Manitoba) in Canada, the placer deposits along the Chocó River in Colombia, and placers (in Alaska) and sulfide deposits of copper in the USA.