COPPER 



177 



kilometers. Most of the deposits are in Quaternary 

 gravels, but some are in older rocks. 



MOBILIZED DEPOSITS OF PROBABLY SEDIMENTARY ORIGIN 



Two major deposits, Aitik in northern Sweden 

 and Mount Isa in Queensland, Australia, contain 

 copper ores that were probably sedimentary in ori- 

 gin but have been modified by metamorphism. The 

 ores of the old White Pine mine, Michigan, were 

 deposited by hydrothermal solutions, but the source 

 of the metal was probably the nearby strata-bound 

 copper present in the Nonesuch Shale, the ore of 

 the present mine. Many other deposits may also fall 

 into this general category, and classification is 

 harder as modification has increased. 



The large low-grade copper deposit of Aitik, near 

 Gallivare, Sweden, is in Precambrian gneisses, 

 schists, and quartzites (Zweifel, 1972). The copper 

 is in chalcopyrite and occurs with pyrite, magnetite, 

 and pyrrhotite, mainly as disseminations and 

 stringers. There was some hydrothermal alteration 

 along with the mobilization of the sulfides. 



At Mount Isa, lead-zinc-silver ores form masses 

 that are conformable with the bedding of the Pre- 

 cambrian Mount Isa Shale, but the generally asso- 

 ciated copper ores are in shears, particularly where 

 silica and dolomite have been introduced. Thus, the 

 lead-zinc-silver ores are reasonably interpreted as 

 syngenetic, but the origin of the copper sulfide ores 

 is less certain. 



ENVIRONMENTS OF DEPOSITION 



An understanding of the origin and environment 

 of deposition of sedimentary copper deposits has 

 practical application in the search for new or hid- 

 den ore deposits. A search conducted in depositional 

 environments comparable to those of known copper 

 deposits should have a far greater chance for success 

 than a random search. Environments worth consid- 

 eration include depositional environments where 

 syngenetic concentrations are optimum, as well as 

 postdepositional environments where diagenetic or 

 epigenetic concentration seem most likely, as fol- 

 lows: 



1. Deep marine basin sediments associated with 



midocean ridges or continental plate bound- 

 aries, such as the recent sediments along the 

 crest of the East Pacific rise and sediments 

 in association with the hot brine pools of the 

 Red Sea. 



2. Continental-margin marine black shales associ- 



ated with phosphorites, such as the Meade 

 Peak Phosphatic Shale Member of the Phos- 



phoria Formation of Permian age in western 

 Wyoming. 



3. Shallow marine or estuarine shale associated 



with evaporites or nonmarine deposits, such 

 as the Flowerpot Shale of Permian age in 

 Oklahoma and the Kupferschiefer of Per- 

 mian age in Germany and the equivalent 

 Zechstein beds in Poland. 



4. Sandstone, shale, and carbonate rocks exposed 



to migrating saline ground waters, such as 

 alkaline carbonated waters or chloride brines 

 derived from subsurface solution of evapor- 

 ites. Examples might be the "Speckled Sand- 

 stone" of probable Early Permian age in the 

 Salt Range of Pakistan, the Flowerpot Shale 

 of Permian age in Oklahoma, the Santa Rosa 

 Sandstone of Triassic age in New Mexico, 

 and the San Andres Limestone of Permian 

 age in New Mexico. 



5. First-cycle sedimentary rocks in a restricted 



or closed basin, such as the Agua Zarca 

 Sandstone Member of the Chinle Formation 

 of Triassic age along the west fiank of the 

 Nacimiento Range, N. Mex. 



6. Pyritic shale associated with submarine vol- 



canism, as perhaps the Nonesuch Shale of 

 Precambrian Y age in Michigan and Wis- 

 consin. 



7. Pyroclastic or porous carbonate-bearing rocks 



exposed to ground water from weathering 

 copper-sulfide deposits, like the Apache Leap 

 Tuff of Miocene age at the Inspiration mine, 

 Arizona (Throop and Busek, 1971). 



8. Stratified rocks of mostly Precambrian Y age 



(800-1,600 million years), especially if they 

 are first-cycle sediments deposited in shallow- 

 water or restricted basin environments. Ex- 

 amples are the Revett Formation of the Belt 

 Supergroup in northwestern Montana, and 

 the Roan Formation of the Katanga Super- 

 group in the African copper belt. 



9. Basal transgressive marine rocks directly over- 



lying deeply weathered crystalline rocks or 

 first-cycle arkosic clastic strata, such as the 

 Kupferschiefer of Permian age in central 

 Europe and the Roan Formation of Precam- 

 brian age in the African copper belt. 



10. Porous strata updip and marginal to deeply 



buried volcanic rocks of basaltic to interme- 

 diate composition, as the amygdaloidal flow 

 tops and conglomerate beds of the Portage 

 Lake Volcanics and perhaps also the None- 

 such Shale, both of Precambrian Y age in 

 the Upper Peninsula of Michigan. 



