116 



UNITED STATES MINERAL RESOURCES 



GENESIS OF PRIMARY CHROMITE DEPOSITS 



All chromite of economic interest is believed to 

 have been formed by crystallization from fluid mag- 

 ma essentially simultaneously with the associated 

 silicates. Chromite and other minerals that crystal- 

 lize early are denser than the magma, and the crys- 

 tals, therefore, settle as they form and collect in 

 layers on the iloor of the magma chamber. In most 

 stratiform complexes the layering and many delicate 

 primary igneous and sedimentary features (Jackson, 

 1961) are preserved intact despite metamorphism 

 and deformation. Many podiform deposits also pre- 

 serve settled textures, some identical with those in 

 the stratiform deposits, but also some that are 

 unique (Thayer, 1964, 1969) . The similarities of the 

 podiform and stratiform deposits indicate a common 

 origin by crystal settling, but differences between 

 them are attributed to crystallization of podiform 

 deposits in the upper mantle, and of stratiform de- 

 posits in the crust where they are now. Intrusion of 

 peridotite and chromite from the upper mantle into 

 the upper crust by solid flowage at high temperature, 

 probably over distances of many miles, is believed to 

 have fragmented the original layering and imposed 

 metamorphic features on the podiform deposits and 

 their host rocks (Thayer, 1969). Some features of 

 chromite deposits that were interpreted 30-40 years 

 ago as evidence of hydrothermal origin are now at- 

 tributed to alteration of primary magmatic minerals. 



SECONDARY DEPOSITS AND ASSOCIATED METALS 



Erosion of chromite-bearing rocks may form sec- 

 ondary deposits of two kinds, black sands and la- 

 teritic soils. Because chromite is heavy and resists 

 weathering, it may accumulate as grains in black 

 sands or soil to form placers. Under favorable tropi- 

 cal or subtropical conditions, solution of magnesium 

 silicates from ultramafic rocks by ground water 

 forms lateritic soils which contain as much as 50 per- 

 cent iron, 2-4 percent CrjOs, 2-2.5 percent nickel, 

 about a tenth as much cobalt as nickel, and some 

 aluminum (Gross, 1970a, p. 27). Such ferruginous 

 laterites are now being worked for nickel and cobalt 

 in several countries and for iron ore in a few places, 

 but economic recovery of the chromium has not been 

 achieved. Chromite has been recovered from black 

 sands in the United States and Japan on a relatively 

 small scale, and from alluvial soil in Rhodesia by 

 flotation on a large scale. 



No other minerals are mined with primary chrom- 

 ite, although platinum occurs with layered chromite 

 deposits in some complexes in the Ural Mountains. 

 Some gold and platinum have been mined from 

 chromite-bearing beach sands along the Oregon 



coast (Griggs, 1945), but no other minerals have 

 been recovered during operations for chromite 

 (Thayer and Ramp, 1969). 



The second largest known chromium resource in 

 the United States is in black-sand deposits in raised 

 beaches along the southwestern coast of Oregon 

 (Griggs, 1945). The black sand forms lenses and lay- 

 ers that range in thickness from a few inches to 42 

 feet and in width from a few tens of feet to more 

 than 1,000 feet, and are as much as a mile long. Depth 

 of overburden is as much as 75 feet of sand, clay, and 

 gravel. The chromite is associated with ilmenite, 

 rutile, magnetite, and heavy silicates, all of which 

 are hard to separate because of similar physical 

 properties. Zircon in the deposits is of definite eco- 

 nomic interest. The explored deposits that contain 5 

 percent or more CrjOa approximate 2 million long 

 tons. The concentrates would average about 40 per- 

 cent CraOs, with a Cr:Fe ratio of 1.6:1. 



RESOURCES 



IDENTIFIED RESOURCES 



A comparison of world production (fig. 13) and 

 world resources of various categories (table 24) 

 shows that chromite resources are ample for the 

 foreseeable future. The only minable reserves in the 

 Western Hemisphere, however, are in Brazil and 

 Cuba. The figures for high-chromium chromite in the 

 United States are postulated entirely on intensive 

 exploration for extensions of known podiform ore 

 bodies or new ore bodies in known districts under 

 the stimulus of very high prices. Since 1918, mining 

 of podiform deposits has been on a precarious basis ; 

 rarely have significant reserves been blocked out 

 ahead of mining. The figures for high-iron presumed 

 resources are based on an estimate of tonnages down 

 to the level of the Stillwater River valley in the Still- 

 water Complex (Jackson, 1968) and in the Oregon 

 black sands (Griggs, 1945). The Canadian and Green- 

 land deposits are lower in Cr203 content and higher 

 in iron than most Stillwater chromite. Western Hem- 

 isphere reserves (excluding the U.S. national stock- 

 pile) and presumed resources exclusive of Greenland 

 total about 25 million tons, possibly a 12- to 15-year 

 supply for the countries in the region. 



Most of the Eastern Hemisphere resources is in 

 the Bushveld and Great Dyke complexes in southern 

 Africa, where labor is cheap and thin seams can be 

 mined. The reserves of 1.05 billion tons for the Bush- 

 veld Complex are regarded as assured, although they 

 have not actually been blocked out (Cameron and 

 Desborough, 1969). The estimate of Rhodesian re- 

 serves is based mainly on layers of minable thickness 



