538 



UNITED STATES MINERAL RESOURCES 



by Indians in Colombia before the arrival of the 

 Spanish. Although Colombian placers were first de- 

 scribed by Scalig-er (1557, p. 323-324), mining of 

 them did not begin until 1778, soon after the first 

 chemical work on platinum was done. Until 1823, 

 Colombia was the only source of platinum metals, but 

 soon after the 1822 discovery of platinum in placers 

 in the Ural Mountains, Russia became the world's 

 greatest producer, which it has remained except 

 during 1936-61. 



Platinum placers were mined as early as 1885 in 

 Canada, but lode deposits were not mined for plati- 

 num until 1919, when platinum metals began to be 

 recovered from the nickel-copper deposits of the Sud- 

 bury district. By 1936, Canada had become the prin- 

 cipal producer. In 1923 platinum lodes were dis- 

 covered in Transvaal, Republic of South Africa, and 

 in the following year in the Bushveld Complex. By 

 1956 and 1957, output from these lodes outstripped 

 Canadian production. Russia ranked third for a long 

 time, but as the Uralian placers became depleted, 

 lodes were found in Siberia that by 1961 again raised 

 Russian output to first rank, with the Bushveld and 

 Transvaal second and Canada third. In 1929, plati- 

 num placers were discovered in Alaska, and since 

 1934, Alaska has ranked fifth in world production, 

 and Colombia fourth. The Witwatersrand district, 

 South Africa, a producer of osmiridium, ranks sixth. 



Through most of the first quarter of the 20th 

 Century, placer deposits furnished the world's sup- 

 ply. The discovery of lodes in South Africa, Canada, 

 and Siberia so altered the situation that in 1971 

 these three countries produced more than 98 percent 

 of the world's output. Production records have not 

 been accurately kept, but production has amounted 

 to about 63,685,000 ounces through 1971. 



The geologic occurrence of platinum-group metals 

 as lodes or placers dictates that copper, nickel, gold, 

 silver, and chromium will be either coproducts or by- 

 products in the recovery of the platinum metals and 

 that platinum will be largely a byproduct. For ex- 

 ample, about 1 ounce of platinum-group metals is 

 recovered for each 35 tons of copper produced (Age- 

 ton and Ryan, 1970) from domestic ores. Because a 

 larger part of the domestic production is a byproduct 

 of copper mining, environmental efi'ects or problems 

 mainly are those directly related to the mining 

 and refining of copper. Indeed, a suggested use of 

 platinum catalysts is as a potential pollution control 

 of nitrogen oxide fumes. 



GEOLOGIC ENVIRONMENT 



GEOCHEMISTRY 



The crustal abundance of the platinum-group 



metals (Wright and Fleischer, 1965) is poorly 

 known, owing to the low concentration of these ele- 

 ments and inaccurate analytical techniques. Mason 

 (1958) estimated crustal abundances as follows: Pt, 

 5 ppb ; Pd, 10 ppb ; and Ir, Os, Rh, and Ru, 1 ppb each. 

 Useful equivalents are : 1 ppb (part per billion) = 

 0.001 ppm (part per million) = 0.000032 oz per ton 

 (troy ounce per ton) . 



The complex, unique, but poorly known chemical 

 nature of the platinum-group elements in geologic 

 environments prohibits accurate generalizations re- 

 garding their distribution and geochemical cycle. 

 The highest concentrations of the platinum elements 

 are generally in igneous rocks that formed at high 

 temperatures, an indication that the platinum metals 

 are indigenous to the earth's mantle. Certain condi- 

 tions of high temperature and (or) high sulfur par- 

 tial pressure may result in mobilization and concen- 

 tration of the platinum-group elements. Because of 

 their high specific gravity, the platinum metals are 

 concentrated by surficial or hydrologic processes to 

 form placer deposits. 



MAJOR MINERALS OF THE PLATINUM ELEMENTS 



The platinum metals are major constituents of 

 several metallic alloys and a number of minerals with 

 anions of sulfur, arsenic, antimony, bismuth, and 

 tellurium. As Wright and Fleischer (1965) listed 

 most of the known platinum minerals and references 

 to their mineralogy, this report considers only those 

 in domestic occurrences (table 110). With the excep- 

 tion of occurrences in the Stillwater Complex, Mon- 

 tana, virtually all the known platinum-group min- 

 erals in the United States come from placers, in 

 which the material ranges from microscopic-size 

 grains to nuggets several centimeters in the largest 

 dimension. Mertie (1940, p. 72) observed that most 

 nuggets and grains are composed of two or more 

 minerals intergrown to form the grains, and recent 

 microscopic and electron microprobe studies have 

 validated this observation. 



CLASSIFICATION OF DEPOSITS 



The major workable deposits of platinum metals 

 occur with nickel-copper and copper lodes associated 

 with mafic and ultramafic rocks and as platinum al- 

 loys in placers. Mafic and ultramafic rocks form at 

 least three distinctive types of igneous complexes 

 (Jackson and Thayer, 1972) : (1) Stratiform— 

 fioored, tabular complexes consisting of vertical 

 repetitions of well-layered rock sequences with cumu- 

 lus textures, that are intrusive into Precambrian 

 shields or into basaltic terranes of any age; (2) 

 Concentric — roughly cylindrical, zoned complexes 



