COPPER 



173 



the U.S.S.R. Deposits of this type represent the 

 largest potential for discovery of new deposits. Not 

 only are many of them difficult to recognize in the 

 field, but the geologic controls of their origin are 

 only beginning to be understood. 



A genetic classification of sedimentary copper de- 

 posits should distinguish ores formed by chemical 

 or biological processes from those formed by me- 

 chanical processes; those deposited simultaneously 

 with the enclosing sediment from those formed by 

 later alteration; and those formed in a reducing 

 environment from those formed in an oxidizing 

 environment. However, these processes are not 

 mutally exclusive, and the criteria for distinguish- 

 ing them are frequently equivocal. Idealized descrip- 

 tions of different types of sedimentary copper de- 

 posits presented below, followed by descriptions of 

 several important deposits that may be combinations 

 of types. 



Type 1. — Rocks of intermediate Precambrian age 

 formed when atmospheric oxygen began to become 

 abundant and to mobilize copper that was previously 

 stable. These deposits may include copper reworked 

 or leached out of earlier disseminated deposits. The 

 deposits of the African copper belt and those in the 

 Belt Supergroup of western Montana may be of 

 this type. 



Type 2. — Syngenetic marine deposits formed on 

 the sea floor by precipitation or adsorption on or- 

 ganic matter. These deposits should show a system- 

 atic relation to lithologic facies and may be char- 

 acterized by lateral persistence within a bed or 

 facies. Mineral textures are typical of sedimentary 

 rocks and the deposits may contain other metals, 

 such as uranium, vanadium, and silver, that could 

 precipitate under similar conditions. The Kupfer- 

 schiefer and equivalent formations in Europe may 

 be of this type. 



Type 3. — Deposits formed by reaction of connate 

 brines or meteoric ground water with chemically 

 reactive strata such as black shale, sandstone con- 

 taining carbonaceous matter, or carbonate rock. As 

 discussed by Lindgren (1911), such deposits prob- 

 ably form during compaction and lithification and 

 are localized by variations in permeability, struc- 

 ture, and chemical reactivity of the host strata. The 

 deposits may be nonpersistent and highly variable 

 in size, shape, grade, and texture of the ore. Typical 

 ore is more coarse grained than syngenetic ore and 

 may include local replacement of original framework 

 grains. The associated metals are the same as found 

 in syngenetic deposits. Favorable host rocks include 

 nonmarine arkosic sandstones. The "red-bed" cop- 

 per deposits near Cuba, N. Max., are an example of 



this kind of deposit. The other types of deposits 

 have probably been modified to some degree by the 

 processes that form this type of deposit. 



THE KUPFERSCHIEFER 



The Permian Kupferschiefer of Mansfeld, in the 

 German Democratic Republic, is a thin-bedded bitu- 

 minous marine marl or shale about 50 centimeters 

 thick containing copper, lead, zinc, and silver in eco- 

 nomic concentrations and many other metals in 

 unusually large amounts (Richtsr-Bernburg, 1941 ; 

 Wedepohl, 1964, 1971). The equivalent Zechstein 

 shales of Lower Silesia in Poland contain economic 

 concentrations of copper in some areas and of lead 

 in other areas (Haranczyk, 1970). Many Europeans 

 regard the mineralization of the Kupferschiefer and 

 equivalent units as syngenetic because the minerali- 

 zation is confined to a thin stratigraphic unit and 

 its distribution is related to lithologic facies 

 (Richter-Bernburg, 1951). Most advocates of the 

 syngenetic hypothesis also suggest some extraordi- 

 nary way of concentrating greater than usual 

 amounts of copper from sea water. Submarine min- 

 eral springs and erosion of older copper ores are 

 among the sources suggested for the copper. More 

 recently, Brongersma-Sanders (1968) suggested that 

 heavy metals could be concentrated in black shales 

 and phosphorites by upwelling nutrient-rich waters. 

 Davidson (1964) rejected all the arguments for 

 unusual environments of deposition in favor of an 

 epigenetic episode of mineralization by saline ground 

 waters derived from the Zechstein salt deposits 

 overlying the Kupferschiefer, citing as an example 

 the metal-rich thermal ground waters encountered 

 in wells near the Salton Sea as potential mineraliz- 

 ing solutions. Wedepohl (1971, p. 272) suggested a 

 correlation between the local metal content of the 

 Kupferschiefer and the soluble trace metals in the 

 Lower Permian red sandstones over which the Kup- 

 ferschiefer sea transgressed. 



Much of the Kupferschiefer in West Germany is 

 probably of submarginal grade, but there may be 

 as much as several tens of millions of tons of rock 

 with an average grade of 0.3 percent copper. 



THE AFRICAN COPPER BELT 



Nearly one-fifth of the world's production of cop- 

 per comes from deposits in upper Precambrian sedi- 

 mentary rocks that extend for about 300 miles 

 across parts of Zambia and the Katanga province 

 of Zaire in Africa (U.S. Bur. Mines, 1970). These 

 deposits are usually called "strata-bound copper 

 deposits" because of some uncertainty as to their 

 origin. Individual beds containing disseminated cop- 



