COBALT 



147 



iron-nickel-copper sulfides containing cobalt are an 

 important type of hypogene deposit associated with 

 intrusive mafic igneous rocks. Large typical deposits 

 of this kind occur in the famous Sudbury district of 

 Ontario, where the ore has an average cobalt content 

 of 0.07 percent (Andrews, 1962, p. 93). The suite of 

 ore minerals consists of pyrrhotite, pentlandite, py- 

 rite, marcasite, magnetite, cobaltite, and gersdorf- 

 fite in pods, lenses, veins, stringers, and disseminated 

 grains in a layered sequence of norite, quartz dior- 

 ite, micropegmatite, and rocks of transitional compo- 

 sition and texture. The igneous host rocks of Precam- 

 brian age occur in an elliptical basin about 37 miles 

 long and 17 miles wide. The ore deposits are large: 

 A decade ago Andrews (1962, p. 93) reported proved 

 reserves of more than 250 million tons of ore. The 

 origin of these and similar deposits elsewhere in the 

 world is complex and hotly debated. Cornwall (1966, 

 p. 27-29) reviewed the hypotheses of origin ranging 

 from segregation by differentiation in situ to hydro- 

 thermal deposition, and even to impact by meteorite. 

 (See "Nickel.") More recently, Cheney and Lange 

 (1967) have offered a summary of a process of sul- 

 f urization as an explanation of the origin of the Sud- 

 bury-type deposits of nickel and cobalt. 



Deposits of the Sudbury type, however, are not 

 restricted to Canada. Indeed, similar deposits are 

 known in the United States, and some have been 

 known for a long time. The Gap nickel mine, Lan- 

 caster County, Pa., is in the base of a metamorphosed 

 gabbroic intrusive body. The ore is nickel-copper 

 sulfide minerals that contain 0.1-0.3 percent cobalt. 

 An estimated 1 million tons of ore remains at the old 

 mine. Large deposits of this type also occur near 

 Union, Maine, and in the keel of the Moxie Lake- 

 Moosehead Lake gabbro body (Young, 1968, p. 129, 

 134). The huge deposits of the Katahdin Iron Works 

 area are in gabbro and norite where the cobalt con- 

 tent of much mineralized rock is 0.05-0.1 percent. 

 Samples from several bodies of mafic rock in the 

 Calais area, Maine, indicate a cobalt content of 0.1 

 percent. 



Cobalt is associated with pyrrhotite deposits in 

 mafic rocks in Connecticut, at West Torrington and 

 in the Mount Prospect area near Litchfield (Camer- 

 on, 1943), and in Massachusetts, at Dracut (Dennen, 

 1943). At the Phillips mine and vicinity near Peek- 

 skill, N.Y., gabbro of Precambrian age has mineral- 

 ized rock that contains 0.003-0.24 percent cobalt.. 

 Other deposits of this type are reported in Washing- 

 ton, Oregon, and California. 



Huge concentrations of copper-nickel sulfides oc- 

 cur in the lower part of the Duluth Gabbro Complex 

 southeast of Ely, Minn. Samples from three drill 



cores contain 0.038 percent cobalt, 0.39 percent cop- 

 per, and 0.14 percent nickel (Grosh and others, 

 1955). It is now estimated that about 14 billion tons 

 of rock containing 0.25 percent total copper and 

 nickel occur in a southwest trend 35 miles long (P. 

 K. Sims, Director, Minnesota Geol. Survey, written 

 commun., Sept. 22, 1972) . No cobalt values are avail- 

 able for the large tonnage reported, but if the entire 

 deposit contains an average of 0.01 percent cobalt — 

 one-fourth of that reported in 1955 — then about 3 

 billion pounds of cobalt would be available. Current 

 knowledge of these deposits suggests that identified 

 resources of 1 billion pounds of cobalt are a conserva- 

 tive estimate. 



The mafic rocks of the Stillwater Complex in Mon- 

 tana contain some bodies of a few million tons rich 

 in nickeliferous sulfides (Cornwall, 1966, p. 45-46) 

 that also contain small amounts of cobalt. 



Other cobaltiferous ores in large deposits are as- 

 sociated with mafic igneous rocks and metamorphic 

 rocks in the Lynn Lake and Thompson-Moak Lake 

 area, Manitoba. The deposits in the Thompson-Moak 

 Lake area are of especial geologic interest because 

 the nickel and cobalt probably were derived from 

 peridotite and serpentine when they were metamor- 

 phosed at a temperature of at least 600 °C to rocks 

 of the garnet-amphibolite assemblage. Evidence sug- 

 gests that sulfur was available in the hydrothermal 

 solutions which deposited the mobilized nickel and 

 cobalt in sulfide minerals, as first suggested by 

 Michener (1957). If this process does indeed pro- 

 duce deposits of this type, peridotite bodies through- 

 out the world could have valuable nickel-cobalt 

 deposits if the bodies were sufficiently metamor- 

 phosed in an environment that had sulfur available. 

 (See "Nickel" ; also Cornwall, 1966, p. 20-23.) 



CONTACT METAMORPHIC DEPOSITS 



Deposits of magnetite, chalcopyrite, and cobalti- 

 ferous pyrite formed by contact metamorphism of 

 carbonate rock by sills and dikes of diabase comprise 

 an important type of cobalt deposit in the United 

 States. Typical deposits are near Cornwall, Lebanon 

 County, and Morgantown (the Grace mine), Berks 

 County, Pa. These and many other deposits occur in 

 the Triassic basin, an area about 75 miles long and 

 5-20 miles wide in southeastern Pennsylvania. At 

 Cornwall, a discordant saucer-shaped body of dia- 

 base of Triassic age has silicated and mineralized ad- 

 jacent carbonate rocks of Cambrian age. Recent 

 descriptions of these commercially valuable iron 

 deposits at Cornwall (Lapham, 1968) and Morgan- 

 town (Sims, 1968) indicate that the mine-run ore 

 contains 0.02-0.056 percent cobalt, most of which is 



