112 



UNITED STATES MINERAL RESOURCES 



During the last 5 years, the United States has con- 

 sumed about 7 milHon long tons of chromium ore, 

 equal to about 28 percent of total world production, 

 but it has mined none at all since 1962 (fig. 13). Our 

 presumed resources are nearly all low grade; al- 

 though theoretically they might last about 5 years 

 at our present rate of consumption, they would be 

 difficult to mine and expensive to use. Since 1962, our 

 entire supply of chromite has been imported from the 

 Eastern Hemisphere, and for the last 5 years about 

 45 percent of the chi'omium we have used in metal- 

 lurgy has been imported from the Soviet Union. 



EXPLOITATION 



The alltime production of chromite in the United 

 States is about 1,850,000 long tons; of this total 

 about 950,000 tons, all high-iron ore and concen- 

 trates, came from Montana. The remaining 900,000 

 tons came almost entirely from podif orm deposits in 

 California, Oregon, Pennsylvania, Maryland, and 

 Alaska. Since 1900, only in wartime has the United 

 States produced more than 3,500 tons of chromite a 

 year. During World War 1, the maximum domestic 

 production of 82,430 long tons equaled less than half 

 of consumption, and the gap between domestic pro- 

 duction and consumption has widened ever since 

 (fig. 13). Before 1940 all production was from podi- 

 form deposits, but since then, production of concen- 

 trates from the Stillwater Complex has predomi- 

 nated. Of 1,085,185 long tons produced during the 

 Korean war for Government purchase programs, 

 about 800,000 tons of Stillwater concentrates was 

 stockpiled because of its low grade (38.5 percent 

 CrsOa). Moreover, despite a base price of $115 a ton 

 for standard lumpy metallurgical ore during the 

 Korean war, the podiform deposits as a group yield- 

 ed only about 24,700 tons a year. From the period 

 1942-44 to the period 1952-57, an increase in price 

 from $87 to $115 in 1954 constant dollars — a price 

 increase of 32 percent — stimulated a rise in produc- 

 tion of usable ore from podiform deposits by only 10 

 percent (Materials Advisory Board, 1959, p. 44) . 



Because the podiform deposits are small and most 

 are in relatively inaccessible areas, no serious en- 

 vironmental problems are anticipated. Large-scale 

 mining and milling operations at the Stillwater Com- 

 plex, however, would entail large dumps and tailings 

 ponds along the sides of a scenic part of the Still- 

 water River valley. 



GEOLOGIC ENVIRONMENT 



Chromium is a rock-forming element that is close- 

 ly associated with magnesium and nickel in the ultra- 



mafic rocks peridotite, pyroxenite, and serpentine. 

 From a normal range of 1,100-3,400 ppm (parts per 

 million) and an average of about 1,800 ppm in ultra- 

 mafic rocks, the content of chromium drops to about 

 200 ppm in gabbro and basalt, and 5 ppm in granite. 

 Chromium is concentrated with the clay fraction of 

 sedimentary rocks, normally in amounts of less than 

 100 ppm ; but in phosphorites from Idaho, Wyoming, 

 and Utah, it averages about 1,000 ppm (Michael 

 Fleischer, written commun., 1972) . 



MINERALOGY OF CHROMITE AND CHROMITE ORES 



The mineral chromite is the only commercial 

 source of chromium and varies compositionally with- 

 in wide limits permitted by the formula (Mg,Fe- + ) 

 (Cr,Al,Fe^+)20... The Cr^O, content of the pure min- 

 eral chromite ranges from about 15 to 64 percent, 

 but directly usable ores, which are mixtures of 

 chromite and silicate minerals, range from about 33 

 to 55 percent CrjOs. Compositionally, three general 

 kinds of chromite ore are recognized: (1) high- 

 chromium, which contains 46 percent or more CraOs 

 and has a Cr:Fe ratio of 2:1 or more; (2) high-iron, 

 which contains 40-46 percent CrsOs and has a Cr:Fe 

 ratio of less than 2:1; and (3) high-aluminum, which 

 contains more than 20 percent AI2O3, and more than 

 60 percent AI2O3 and CraO,, combined. Although all 

 three kinds are interchanged or blended for some 

 uses, the high-chromium ores are used for metal- 

 lurgy, and the high-aluminum ores mostly for re- 

 fractories. Only high-iron ores are used for making 

 chemicals, and they are also of major importance for 

 making alloys and refractories. 



All chromite ores consist of mixtures of chromite 

 and associated minerals that are waste or gangue. 

 Massive lumpy ore that contains little gangue is pre- 

 ferred by industry for metallurgical and refractory 

 uses. Natural deposits, however, range from massive 

 or high grade to disseminated or low grade, and ore 

 commonly grades from one to the other. The low- 

 grade ores may be concentrated by simple washing 

 if they are friable or by grinding and separating the 

 chromite from gangue by various physical methods. 

 In general, milling of primary ores containing less 

 than 10 percent CraOs (20 percent chromite) has not 

 been successful. 



PRIMARY CHROMITE DEPOSITS 



Primary chromite deposits occur only in certain 

 kinds of ultramafic or closely related anorthositic 

 rocks (Thayer, 1971) and are of two major geologic 

 types: stratiform (layered) and pod-shaped (Jack- 

 son, 1961 ; Thayer, 1960, 1964) . 



