ZINC 



705 



Perhaps the most significant known occurrence of 

 this type is the famed Kupferschiefer, chiefly in 

 East Germany, but extending from England east- 

 ward to Poland. This is a bed of carbonaceous shale 

 generally less than 1 meter thick that contains 

 several percent combined copper, lead, and zinc as 

 finely disseminated sulfides. This mineralized bed is 

 more than 20,000 square kilometers in areal extent 

 and occurs in the basal part of the Permian Zech- 

 stein Formation of East Germany or its equivalents. 

 Copper is more abundant than zinc and lead in some 

 beds, and zinc and lead are more concentrated than 

 copper in others. Where the Kupferschiefer has 

 been mined for copper, the zinc content is less than 

 IV2 percent. Elsewhere, as at Uftrungen and near 

 Bottendorf, the zinc content is higher, being about 

 1.7 and 3.2 percent, respectively. 



Other zinc-bearing stratiform deposits seem to be 

 rare. Some investigators believe the deposits at 

 Tynagh, Eire, have a sedimentary origin, although 

 others cite contrary evidence. It is possible that 

 other stratiform deposits exist among oil shales or 

 other carbonaceous sediments elsewhere in the world. 

 As much as 1 percent Zn is reported in a variety of 

 black shales around the world (Tourtelot, 1970). 

 Another possibility of zinc occurrence is in evaporite 

 sequences where zinc might occur in minerals other 

 than sulfides. This suggestion is raised by the occur- 

 rence of several tenths of a percent zinc in samples 

 of Devonian and Triassic dolomites associated with 

 evaporite sequences in Alberta and elsewhere in 

 western Canada (Weber, 1964) and associated with 

 the modern evaporites of Lake Eyre in South Au- 

 stralia (Davidson, 1965). The sphalerite nodules in 

 the Middle Silurian dolomites of southern Ontario 

 (Sangster and Liberty, 1971) might also be classi- 

 fied as being of diagenetic origin in stratiform 

 deposits. 



DEPOSITS FORMED BY SUPERGENE ENRICHMENT 

 OR LATERIZATION 



Much has been written about the supergene en- 

 richment of silver-bearing base-metal deposits, 

 chiefly those occurring in the Western United States. 

 Such enrichment in the weathered parts of primary 

 ore deposits is generally not considered to be a 

 separate type of deposit but only a process by which 

 a lower grade primary sulfide ore or protore is 

 made exploitable by chemical reconcentration during 

 weathering. Most of the zinc ores mined until the 

 early part of the 20th century were from such oxi- 

 dized deposits (Heyl and Bozion, 1962). Most of 

 these deposits were clearly formed by decomposition 

 of sulfides in the bedrock during a weathering cycle, 



when the metals were redeposited nearby as carbon- 

 ate, silicate, or oxide minerals or as secondary sul- 

 fides if they were in a reducing environment. In 

 many places, as early mining exhausted these en- 

 riched supergene products, the nearly one-to-one 

 relation with primary sulfide occurrences in bedrock 

 was evident. In the Western States (Heyl, 1963, 

 1964) many secondary zinc deposits clearly were 

 formed by alteration of the primary sulfides vir- 

 tually in place or by direct replacement of limestone 

 by smithsonite or hemimorphite from zinc-bearing 

 solutions. However, in the Appalachian region, the 

 oxidized zinc deposits were formed as accumulations 

 of smithsonite and hemimorphite in saprolitic clay 

 pockets between pinnacles of the carbonate bedrock. 

 According to Heyl and Bozion (1962, p. A19-A21), 

 the zinc came from primary disseminations of spha- 

 lerite previously deposited in the unweathered car- 

 bonate host. These zinc-rich saprolites are analogous 

 to the terra rossa-type residual aluminous or ferru- 

 ginous laterite pockets formed by the deep weath- 

 ering of impure limestone. 



Until the advent of successful froth-flotation 

 processes shortly after the beginning of the 20th 

 century, sulfide ores containing sphalerite could not 

 be concentrated economically, and most zinc mined 

 in the world was obtained from the oxidized deposits. 

 Major districts producing such ores in the United 

 States included Friedensville, Austinville, Embree- 

 ville. Mascot, and Jefl'erson City in the Appalachian 

 region and Leadville, Tintic, Ophir, and Cerro Gordo 

 in the Western States. Much of the early production 

 in the districts of the Mississippi Valley region was 

 also from the enriched ores of the oxidized zone. 

 (See Heyl and Bozion, 1962.) Significant amounts 

 of this type of ore were also mined in Belgium, Ger- 

 many, and Poland, as well as in Africa. 



In contrast to the enrichment of zinc ores ob- 

 tained in temperate climates, where the zones of 

 oxidation are shallow, are the enriched oxidized ores 

 of zinc found in tropical-weathering regimes. Here, 

 the oxidized ores are perhaps best likened to later- 

 ites in that the process forming them, in effect, is 

 laterization. Perhaps one of the best documented 

 deposits that can be classified as a zinc-bearing 

 laterite is that at Vazante in Minas Gerais, Brazil. 

 This deposit is estimated to contain more than 5 

 million tons of zinc ; lead, copper, and vanadium may 

 also be recoverable. Carvalho and others (1962) have 

 described the Vazante deposit and the exploration 

 of it during the 1950's. Epigenetic zinc, lead, and 

 copper (?) sulfides in a lower Paleozoic rock were de- 

 composed by weathering, with the development of 

 a strong ferruginous gossan (chapeau de ferro). 



