NICKEL 



439 



is not a true mineral but rather a mixture of nickel 

 serpentine, nickel talc, and possibly other silicates. 



Besides the nickel mnierals given in table 86, 

 nickel occurs in minor amounts in pyrrhotite 

 (Fei_^S) and pyrite (FeSa), particularly where 

 these sulfides are associated with mafic or ultramafic 

 rocks. In most of these occurrences the total amount 

 of included nickel is less than 1 percent. Nickel may 

 replace iron to a considerable extent in these min- 

 erals, but substantial amounts may also be present 

 as intergrowths of pentlandite or other nickel sul- 

 fides. Native terrestrial iron also contains nickel, 

 ranging in amounts from 2 to 75 percent, but such 

 native nickel-iron is rare; it occurs in basalts and 

 carbonaceous sediments and with limonite and or- 

 ganic matter in petrified wood. The most important 

 occurrence of native terrestrial iron is on Disko 

 Island, Greenland, where it occurs in basalt as par- 

 ticles and, in places, as masses as large as 20 tons. 



TYPES OF DEPOSITS 



Sulfide deposits are by far the most important 

 present source of nickel, in regard to both quantity 

 of nickel and number of deposits. In 1964 two sul- 

 fide districts in Canada were producing about 80 

 percent of the free world total ; one of these, the 

 Sudbury district, has produced more than half the 

 total world supply since about 1905. In recent years, 

 however, major deposits have been discovered in 

 other parts of the world, particularly in Australia 

 and South Africa, as well as two large but very low 

 grade deposits in the United States. 



Another type of deposit, in which sulfides (to- 

 gether with arsenides of nickel, cobalt, and copper) 

 occur in hydrothermal veins, has been found in both 

 Canada and the United States, but these deposits 

 are small and unimportant as sources of nickel. 



Important nickel deposits are found in the later- 

 itic mantles formed by the weathering of peridotite, 

 include dunite, pyroxenite, and serpentinite, mostly 

 in tropical and subtropical climates. The weathering 

 of fresh peridotite has yielded nickel-silicate later- 

 ites in which the hydrosilicate garnierite is the 

 principal nickel mineral. These deposits contain 

 more than 1.5 percent nickel. Large deposits of this 

 type occur in New Caledonia, and medium to large 

 deposits occur in Indonesia, Venezuela, Brazil, 

 United States (Oregon), and elsewhere. The weath- 

 ering of serpentinite (serpentinized peridotite) has 

 also formed nickeliferous iron laterite that averages 

 0.9-1.4 percent nickel. Large deposits of this type 

 occur in Cuba and the Philippines, and medium to 

 large deposits occur in Indonesia, the U.S.S.R., the 



Western United States, Guatemala, and other locali- 

 ties. 



Recent oceanographic research has revealed the 

 presence of vast deposits of nickel-bearing manga- 

 nese-oxide nodules on deep ocean floors, particularly 

 in the Pacific Ocean. The nodules contain 0.1-1.1 

 percent nickel as well as somewhat lesser amounts 

 of copper and cobalt. 



Most of the nickel deposits of the world that were 

 known as of 1950 were shown by the U.S. Bureau 

 of Mines (1952, fig. V-1), but since then, major de- 

 posits have been discovered in the United States, 

 Canada, Australia, Guatemala, Colombia, and South 

 Africa. 



SULFIDE DEPOSITS 



The nickel-bearing sulfide deposits typically con- 

 sist predominantly of pyrrhotite (Fei_^S) and 

 associated pentlandite [(Fe,Ni)9S8] and chalco- 

 pyrite (CuFeSa). Many of the deposits contain 

 minor but recoverable amounts of platinum metals, 

 cobalt, and selenium. These deposits occur in or near 

 peridotite or norite intrusions and are generally 

 considered to be genetically related to them. The 

 sulfides occur as disseminations, massive bodies, or 

 veins and stringers in the igneous rocks. Some of 

 the masive ores contain fragments of the host rock. 

 The individual ore bodies are normally elongate, 

 lenticular, or sheetlike and may extend for hundreds 

 or thousands of feet. 



Many deposits occur at or near the base of peri- 

 dotite or norite intrusives and are particularly well 

 formed in local hollows or reentrants along the 

 basal contacts. Magmatic segregation is considered 

 by many geologists to have been the mechanism by 

 which these deposits were formed ; immiscible liquid 

 sulfide droplets were segregated from the parent 

 mafic or ultramafic magma at an early stage of 

 crystallization, settling downward and coalescing to 

 form a sulfide zone at the base of the intrusive. The 

 evidence in favor of this mechanism seems to be 

 strongest in many small deposits associated with 

 relatively small intrusives. For some larger depos- 

 its, although the mode of occurrence is similar, the 

 origin is disputed; typical of these are the large 

 deposits in the Sudbury district, Ontario, Canada, 

 which occur along the base of a basin-shaped or 

 funnellike norite intrusive. Hawley (1962) has re- 

 cently summarized the case for origin of these de- 

 posits by magmatic segregation. Others (for exam- 

 ple, Yates, 1948) support a theory of epigenetic 

 origin — introduction of the sulfides into the norite 

 contact zone by hydrothermal solutions that mi- 

 grated from depth. Different theories of origin have 



