COPPER 



169 



t common minerals of the oxidized zone, and the 

 * bright colors of these minerals have been a guide to 

 prospectors for thousands of years. 



TYPES OF DEPOSITS 

 Most copper deposits fall into one of five major 

 types. (1) The most important of these are the 

 porphyry copper deposits and vein and replacement 

 deposits ; these account for almost two-thirds of the 

 world's copper resources and are discussed together 

 because of their common genetic association with 

 felsic intrusive rocks. (2) A fourth of the world's 

 identifiable copper resources is in sedimentary rocks, 

 and (3) about 5 percent is in volcanic rocks as 

 massive sulfide deposits. (4) Smaller portions of 

 world resources are in nickel-copper ores formed by 

 magmatic processes in mafic intrusives and (5) in 

 native copper ores of the Keweenaw type. These 

 types, as well as certain uncommon or unique types 

 of deposits, are discussed in order of their relative 

 importance as copper resources. 



PORPHYRY COPPER DEPOSITS AND RELATED TYPES 



Porphyry copper deposits and most of the im- 

 portant vein and replacement copper deposits are 

 genetically associated with intrusions of felsic ig- 

 neous rocks. The copper in these deposits is believed 

 by some geologists (for example, Fournier, 1967) 

 to be a primary constituent of the associated intru- 

 sion. A part of the copper is trapped in disseminated 

 grains by the rapid crystallization of the magma, 

 which in turn gives rise to the characteristic por- 

 phyritic texture of the intrusion. Another part of 

 the copper is mobilized by water and other volatiles 

 escaping from the hot congealed rock mass and is 

 deposited in fractures in the intrusion and its wall 

 rocks. A third part may escape completely from the 

 intrusion and form vein and replacement deposits 

 in nearby reactive host rocks. Thus porphyry copper 

 deposits commonly occupy the central part of large 

 or small mining districts containing vein and re- 

 placement deposits of copper, lead, zinc, silver, gold, 

 iron, and manganese. 



PORPHYRY COPPER DEPOSITS 



An excellent review of the geology of porphyry 

 copper deposits of the North American Southwest 

 with detailed descriptions of 15 individual deposits 

 was assembled by Titley and Hicks (1966). Suther- 

 land-Brown (1969) and Sutherland-Brown, Cathro, 

 Panteleyev, and Ney (1971) wrote excellent descrip- 

 tions of deposits in British Columbia. 



Porphyry copper deposits are defined herein as 

 deposits of disseminated copper sulfide that are in 

 or near a felsic intrusive body ; in most deposits, at 



least one facies of the intrusive is a porphyry. This 

 definition does not include size, grade, or amenabil- 

 ity to mass-mining methods because these factors 

 restrict consideration of the similar but subeco- 

 nomic occurrences that are so important to regional 

 metallogenic studies. Lowell and Guilbert (1970) 

 tabulated the geologic features of 27 major por- 

 phyry deposits, 18 of them in the North American 

 Southwest, and arrived at a typical or average de- 

 posit. This hypothetical ore body is oval and pipe- 

 like, it measures 3,500 by 6,000 feet in plan, and 

 it contains 150 million tons of ore averaging 0.8 

 percent copper and 0.15 percent molybdenum. Ore 

 below the zone of secondary enrichment contains 

 on the average only 0.45 percent copper. On the 

 average 70 percent of the ore body is in the intru- 

 sion, and 30 percent is in the country rock. Sulfide 

 minerals in descending order of abundance are 

 pyrite, chalcopyrite, molybdenite, and bornite. 

 Supergene sulfides may form a higher grade zone 

 of secondary enrichment. 



Porphyry copper deposits have petrologic asso- 

 ciations that are dependent on their tectonic en- 

 vironment. Deposits formed on thick continental 

 crust are usually associated with quartz monzonite, 

 as in Arizona (Lowell and Guilbert, 1970). Deposits 

 in island arcs are associated with quartz diorite, 

 diorite, or granodiorite, as in Bougainville (Mac- 

 namara, 1968), the Phihppines (Madamba, 1972; 

 Loudon, 1972), and Puerto Rico (Cox, 1971). Simi- 

 larly, British Columbia porphyry deposits of two 

 distinct ages and of distinct petrologic affinities 

 seem to be related to two types of crustal plate 

 motion (Hodder and Hollister, 1972). Thus, deposits 

 of Triassic and Jurassic age were formed during 

 plate convergence as the North American plate 

 moved westward over the Pacific plate; as the East 

 Pacific rise drifted under the American plate, de- 

 velopment of tensional structures in the continental 

 crust and a new regimen of magmatic activity gave 

 rise to the Cretaceous and Tertiary deposits. The 

 deposits formed in the earlier cycle in a thin or 

 poorly developed continental crust are associated 

 either with syenite, monzonite, or fennite as at Sti- 

 keen and Copper Mountain or with quartz diorite 

 and granodiorite as at Highland Valley. The later 

 deposits that formed in a thicker continental crust 

 are associated with quartz monzonite. 



Breccia pipes are present in 20 of the 27 deposits 

 considered by Lowell and Guilbert (1970), and wide 

 zones of closely and irregularly fractured rock 

 (crackle zones) surround almost all the deposits. 

 This last feature is a very useful one in the recogni- 

 tion of porphyry deposits in the field. 



