LEAD 



319 



secondary ore minerals and have been mined as 

 lodes. 



The general grade of the Kupferschief er bed that 

 has been mined for its copper content is 2-3 percent 

 copper, 1.3 percent zinc, about 1.5 percent lead, and 

 approximately 4 ounces per short ton silver. Else- 

 where, the content of zinc and lead is higher, as at 

 Uftrungen, where the bed averages 2.5 percent lead 

 and 1.7 percent zinc (Deans, 1950, p. 343). Overall, 

 the Kupferschiefer may contain an average zinc and 

 lead concentration more than 10 times higher than 

 that of copper (Wedepohl, 1971, p. 269) . 



The genesis of the Kupferschiefer has been the 

 subject of much controversy. Most geologists who 

 have studied the deposits favor a syngenetic origin 

 because of the virtual restriction of the ore minerals 

 to a thin stratum that is rich in organic carbon, 

 the fine dissemination of the ore minerals, the ap- 

 parent bacterial origin of the contained sulfur, the 

 patterns of both vertical and regional zonation of the 

 ore metals, and the wide occurrence of ore minerals 

 in correlative units in western Poland, western Ger- 

 many, and northeast England. According to Richter- 

 Bemburg (1941), however, the metals occur in the 

 Kupferschiefer in such great volume that they could 

 not have been derived entirely from the denudation 

 of ore deposits in the surrounding landmasses during 

 the short time available for deposition of the thin ore 

 bed. He suggested that they were enriched over a 

 long period of time in deep-lying ground water, which 

 later rose to mingle with the early Zechstein sea. 

 Wedepohl (1971), in a more recent study, favored 

 the gradual deposition of metals in the anoxic bottom 

 waters of the Kupferschiefer sea as it transgressed 

 over a peneplane cut into red-bed country rocks. He 

 further stated that time factors and the occurrence 

 of the main ore-bearing zones at the margins of the 

 Kupferschiefer sea virtually rule out an origin from 

 submarine springs of cryptovolcanic affinity. 



Although other possibly syngenetic strata-bound 

 deposits of copper are not uncommon, particularly in 

 south-central Africa, lead-bearing deposits of un- 

 questioned syngenetic origin are rare. Strata-bound 

 deposits of lead minerals in the Precambrian Belt 

 Supergroup of northern Idaho and northwestern 

 Montana have recently been described by Clark 

 (1971). In this area, the somevt^hat better explored 

 strata-bound copper deposits — whose origin is still 

 in debate (see "Copper" chapter) — are locally over- 

 lain by zones of galena-impregnated rock that con- 

 tain as much as 6 percent of lead. Unlike the Kupfer- 

 schiefer, the host rocks are most commonly carbon- 

 ate-rich quartzite and siltite, and zinc minerals are 

 rare to absent. Deposits that are closely similar to 



the separated strata-bound copper and lead deposits 

 of the Belt Supergroup have been described at Dzhez- 

 kazgan in central Kazakhstan by Shcherba (1971, p. 

 173-174). It should also be noted that many strata- 

 bound deposits in Europe and northern Africa, con- 

 sidered by European geologists to be syngenetic, 

 have many of the characteristics of the stratiform 

 epigenetic deposits described in the following section. 

 (See Brown, 1970.) 



STRATA-BOUND DEPOSITS OF EPIGENETIC ORIGIN 



Ore deposits that are essentially restricted to one 

 or a few well-defined tabular stratigraphic units, 

 chiefly limestone or dolomite, but which are clearly 

 younger than the lithification of the host rocks and 

 are not obviously related to any igneous rocks, ac- 

 count for most of the lead that has been produced 

 in the world or is known as reserves (Bauchau, 1971, 

 p. 9). For some of these deposits, the widely used 

 term "strata-bound" may be misleading. As noted by 

 Brown (1970, p. 104), the ores occur mainly in cer- 

 tain preferred horizons or strata, and most are suf- 

 ficiently conformable to bedding to be described as 

 stratiform ; as the ore bodies commonly occur in 

 several formations of different ages, however, they 

 also may be closely related to cross-breaking frac- 

 tures, breccia pipes, and other tectonic features. 



In general, the most common host rocks of the 

 epigenetic stratiform deposits are shallow-water 

 marine carbonate rocks. In many areas they are most 

 heavily mineralized near the rims of the sedimentary 

 basins, adjacent to domelike positive areas, at facies 

 contacts within the basins, or near unconformities 

 within the basin strata. The minerals of the deposits 

 consist predominantly of galena, sphalerite, and py- 

 rite or marcasite. In some mines chalcopyrite, siegen- 

 ite ([Co,Ni]3Si), and other sulfides are present, and 

 nickel, cobalt, and copper, as well as cadmium, silver, 

 and germanium, are recovered as byproduct metals. 

 In many deposits, the ore consists predominantly of 

 either galena or sphalerite. In some deposits in the 

 Kentucky-Illinois, central Kentucky, and central 

 Tennessee districts, barite or fiuorite are major co- 

 product constituents, but elsewhere the gangue min- 

 erals commonly include only calcite, dolomite, and 

 jasperoid. 



The epigenetic stratiform deposits are typically 

 flat lying, although in central Kazakhstan and else- 

 where subsequent tectonic activity may have com- 

 pressed some of them into folds or tilted them steep- 

 ly within fault blocks. The ore bodies are character- 

 istically large and commonly are localized by strati- 

 graphic pinch-out zones, by masses of breccia of 

 diverse origin, by zones of minor faults and frac- 



