COPPER 



179 



sea water results in deposition of the overlying 

 bedded siliceous ironstones. 



The source of metals for these deposits is the 

 same magma that gives rise to the surrounding vol- 

 canic rocks. Different lava types give rise to de- 

 posits of different mineralogy. For example, pyrite- 

 pyrrhotite-chalcopyrite deposits are commonly 

 associated with mafic volcanics, and sphalerite- and 

 galena-bearing deposits tend to be found with silicic 

 volcanic rocks (Anderson, 1969). 



Copper-bearing massive sulfide deposits contain 

 as much as 10 percent copper, with a mean in com- 

 mercial ores of about 2.5 percent, and they range in 

 size from a few hundred thousand tons to more than 

 50 million tons. The wide range in production grade 

 of copper is due in part to the variable content of 

 zinc, lead, silver, and gold in these ores as well as 

 the economics of recovery of sulfur and iron from 

 them. 



COPPER IN NICKEL ORES 



Many of the commercial nickel deposits in mafic 

 rocks throughout the world have copper associated 

 with them, and this "byproduct copper" is an im- 

 portant contribution to the supply, especially in 

 Canada and the U.S.S.R. These deposits are gen- 

 erally interpreted to have been formed by a mag- 

 matic-segregation process as a part of the intrusion 

 of the bodies of mafic rock. 



The chalcopyrite-pentlandite ores related to mafic 

 intrusives range in composition from mostly copper 

 sulfides to mostly nickel sulfides, depending on the 

 composition of the primary magma; the proportion 

 of nickel increases roughly with maficity (T. P. 

 Thayer, oral commun., 1972). In certain cumulate 

 mafic bodies this may result in high-nickel sulfide 

 at the base and more cupreous sulfide in higher 

 strata (Thayer, 1971, p. 242). In these ores the 

 sulfide has generally separated as an immiscible 

 liquid, and the richest concentrations in these sulfur- 

 rich magmas formed when the sulfide separated be- 

 fore crystallization had gone very far. Many papers 

 in Wilson (1969) deal with the origin of ores of 

 this type. 



The most significant producer of copper from 

 nickel-bearing mafic rocks is the Sudbury district 

 of Ontario. In this area of complex geology, the 

 ores, some stratiform and some crosscutting, are 

 polygenic in relation to their enclosing rocks but 

 were probably all derived initially as products of 

 crystallization and the separation of a sulfide-rich 

 dense liquid. Variation in the Ni:Cu ratio in differ- 

 ent ore bodies makes it possible for a mining com- 

 pany to adapt the ratio in the ore produced to 



metal-market conditions. 



Besides the Sudbury district, nickel sulfide de- 

 posits containing appreciable quantities of copper 

 are found as segregations in mafic intrusive bodies 

 in Maine and New Brunswick, in Quebec, Manitoba, 

 Saskatchewan, and particularly in the coastal crys- 

 talline belt of British Columbia and southeast 

 Alaska. The copper-nickel ores of Norway, Sweden, 

 Finland, and the Kola Peninsula, U.S.S.R., are 

 mostly polygenic and complex. The nickel-copper- 

 cobalt ores of Noril'sk in north-central Siberia are 

 also an important source of copper for the U.S.S.R. 

 Nickel ores in Precambrian ultramafic and mafic 

 rocks in the newly discovered Kambalda district in 

 Western Australia contain 0.25-0.57 percent copper. 



The deposits in Minnesota are in the Precam- 

 brian Duluth Gabbro Complex in the northern part 

 of the State. Disseminated copper and nickel sul- 

 fides form discontinuous lenses and layers in the 

 basal part of the gabbro complex. Recent studies 

 show that although these deposits are not minable 

 under present conditions, they represent a very im- 

 portant future copper resource (P. K. Sims, oral 

 commun., 1972). 



NATIVE COPPER DEPOSITS OF THE KEWEENAW TYPE 



On the Keweenaw Peninsula, Mich., deposits of 

 native copper occur in amygdaloidal flow tops and 

 conglomerate beds in the Portage Lake Volcanics. 

 These strata-bound and in part conglomerate-bed- 

 controlled ores are mostly interpreted to be epige- 

 netic in origin and may have affinities to some 

 strata-bound ores in sedimentary rocks. (Bateman 

 (1950, p. 498) likened them to the sandstone ores 

 of Corocoro, Bolivia.) The proximity in both space 

 and time of ores in the Portage Lake rocks to the 

 mineralized Nonesuch Shale, 350-6,000 feet (110- 

 1,850 m) above, also suggests this affinity. 



Opposing this reasoning are the widespread minor 

 copper occurrences in basaltic rocks all over the 

 world, suggesting that the copper was originally 

 syngenetic but has been redistributed and concen- 

 trated to minable grades by later processes (Corn- 

 wall, 1956, p. 620). 



More than 5 million tons of copper metal has 

 been produced on the Keweenaw Peninsula since 

 mining began in 1845, and although there is little 

 activity in the district now, large ore reserves 

 remain. 



Most of the copper is the native metal and was 

 introduced into vesicles, fractures, and interstitial 

 pore spaces after deformation of the rocks. More- 

 over, the copper deposits fit into a pattern of low- 

 grade regional metamorphic zoning of silicate min- 



