114 



UNITED STATES MINERAL RESOURCES 



more than 98 percent of the chromite resources of 

 the world. The Steelpoort Main Seam in the Bush- 

 veld Complex in South Africa, for example, contains 

 "reserves of merchantable ore conservatively esti- 

 mated at half a billion tons" (Cameron and Des- 

 borough, 1969, p. 24). The chromite in stratiform 

 deposits is predominantly of the high-iron type, but 

 the high-chromium ores of the Great Dyke in 

 Rhodesia are an outstanding exception. The regu- 

 larity of the layering facilitates mining, even of thin 

 seams, and is a reliable guide in prospecting. All the 

 chromite-bearing layered complexes are geologically 

 old, and in many the chromite layers have been dis- 

 membered by folding and faulting. Though mining 

 may be difficult and expensive in highly deformed 

 rocks, the layered structure usually can be used to 

 guide exploration for sizable blocks of ore that have 

 been separated. 



The chromite deposits in the Stillwater Complex 

 in Montana, which contains about 80 percent of the 

 chromium resources of the United States, are of the 

 layered high-iron type. They occur in an east-west 

 belt about 30 miles long by half a mile wide. Thirteen 

 chromite-rich zones are known ; they range in thick- 

 ness from 1 inch to more than 12 feet ; several can 

 be traced laterally more' than 15 miles ; and one can 

 be traced more than 25 miles. The layers dip from 

 about 50° to vertical and "extend downward to great 

 depths except where terminated by major faults" 

 (Jackson, 1963, p. 58). Two of the upper zones were 

 mined in the eastern part of the complex under U.S. 

 Government purchase contracts during World War 

 II and the Korean war. In the Benbow mine nearly 

 65,000 long tons of concentrates averaging 41.5 per- 

 cent CroOs was recovered from a highly faulted verti- 

 cal layer 4-5 feet thick (Materials Advisory Board, 

 1959, p. 42). In the Mountain View area about 

 830,000 long tons of concentrates averaging 38.5- 

 38.8 percent CraOs was recovered from two layered 

 zones which range in width from 2 to 16 feet and 

 from 3 to 8 feet, respectively (Price, 1963, p. 1) . 



Analogous to the Montana deposits are major de- 

 posits of the high-iron class in the layered Bird River 

 complex in southeastern Manitoba, Canada. The com- 

 plex has been folded to vertical and broken by numer- 

 ous faults, and the probable total length of the 

 chromite-bearing zone is 6-10 miles (Davies, 1958). 



Extensive layered chromite deposits in anorthosite 

 near Fiskenaesset in southwestern Greenland may 

 represent the largest potential resources of chrom- 

 ite in North America. The chromite occurs in layered 

 zones that average 1.5-10 feet in thickness, reach a 

 maximum of 65 feet, and consist of interlayered 



anorthosite and chromitite layers, usually y^-iy=i 

 inches thick. The chromite layers have a minimum 

 exposed length of 75 miles. The chromite mineral 

 averages about 33 percent CraOa, 0.9:1 in Cr:Fe 

 ratio, and contains 0.2-0.5 percent V2O5, averaging 

 about 0.3 percent. The average grade of rock in 

 minable widths probably ranges from 15 to 25 per- 

 cent CroOa. Although the rocks have been intricately 

 folded, layers are favorably situated for mining over 

 considerable areas. For example, it is estimated that 

 one layer 3-22 feet thick along a strike length of 

 4,600 feet contains a million tons of recoverable 

 chromite or 350,000 tons of CrjOa and 3,000 tons of 

 VaOg. The chromite is exposed in completely bare 

 outcrops near the coast; nearby fjords are deep, and 

 Fiskenaesset harbor is accessible the year round 

 (Ghisler and Windley, 1967). 



The tremendous resources of chromite in South 

 Africa and Rhodesia are in two great layered com- 

 plexes, the Bushveld and the Great Dyke, respective- 

 ly. The Bushveld Complex is a rudely basin-shaped 

 layered mass 26,000 square miles in extent. As many 

 as 29 chromite layers or groups of layers occur in 

 various segments of the chromite-bearing zone, 

 which is exposed over a distance of more than 70 

 miles along the eastern margin alone (Cameron and 

 Desborough, 1969). Although some of the layers are 

 less than 1 inch thick, several layers are of minable 

 thickness and quality. The relation of ore reserves to 

 geometry of deposits is illustrated by the Steelpoort 

 Main Seam, mentioned earlier. If the inclined seam 

 averages 40 inches in true thickness over a known 

 distance of 40 miles and if the ore has a specific 

 gravity of 4.2, each foot of ore below the outcrop 

 represents 100,000 tons. In view of the 40-mile 

 length, projecting only a mile downdip from the out- 

 crop seems minimal but yields "half a billion tons" 

 (Cameron and Desborough, 1969, p. 24). The total of 

 6.1 billion tons given for resources of the entire 

 Bushveld Complex (Republic of South Africa) in 

 table 24 is believed to be conservative. 



The Great Dyke in Rhodesia is a layered complex 

 332 miles long by 3-6 miles wide. In cross section the 

 Dyke is synclinal with dips of 20°-40° near the mar- 

 gins. Chromite layers occur over the entire length of 

 the Dyke, and individual layers have been traced con- 

 tinuously the entire width, though they vary in num- 

 ber from 6 to 12 in dilf erent segments (Worst, 1960) . 

 The layers range in thickness from 2 to 18 inches, 

 and nearly all the chromite is the high-chromium 

 variety. Only layers 6 inches or more thick are mined 

 below the outcrop; some of the thicker layers are 

 mined more than 1,000 feet down the slope. 



