326 



UNITED STATES MINERAL RESOURCES 



specific districts ; the common practice of maintain- 

 ing secrecy about the development of new ore re- 

 serves or the loss of established reserves; and 

 political and economic factors leading to a paucity 

 of data from Communist nations such as China and 

 the U.S.S.R. These factors may result in a margin 

 of error of as much as 25 percent for some countries, 

 but it should be noted that virtually all past esti- 

 mates have erred in being considerably smaller than 

 subsequent estimates based on more complete data 

 and that most of the larger reserves are contained 

 in deposits that are either well explored by drilling 

 or extensively developed in operating mines. 



In estimating lead reserves, price-cost and other 

 economic factors should be considered together with 

 technological advances in mining, milling, and smelt- 

 ing practices. The data of table 65 include all low- 

 grade ores that can be mined at the currently pre- 

 vailing relatively high price of lead. Decreasing 

 prices and increasing costs will diminish ore re- 

 serves, and some of these ore reserves may be 

 irretrievably lost if the higher grade parts of the 

 deposits are mined, thereby eliminating the possi- 

 bility of blending ores to produce acceptable mill 

 feeds. Conversely, a price increase, or more ex- 

 pectably, technological advances that substantially 

 reduce mining and milling costs or permit the utili- 

 zation of complex ores,- can abruptly increase appar- 

 ent ore reserves by the inclusion of large tonnages 

 of lead-bearing materials formerly considered to 

 be unprofitable. In this category, in addition to 

 conventional ores of subeconomic grade, are large 

 volumes of lead-bearing slags, mill tailings, and 

 mine wastes that are not considered in this report. 



CONDITIONAL RESOURCES 



Lead deposits of subeconomic grade have been 

 recognized and explored to various degrees through- 

 out the world, both on land and in the ocean basins. 

 These deposits include not only conventional ore 

 bodies that are similar to productive deposits ex- 

 cept for a lower grade, but also unconventional de- 

 posits that previously have not been mined. The 

 total quantity of lead in these presumed resources 

 is not accurately known, but estimates of their 

 contained metal — disregarding the potentially large 

 problems inherent in achieving reasonable mining 

 and metallurgical recoveries — may range as high as 

 1.5 billion tons of lead. 



The predominant resource of low-grade lead- 

 bearing material in this category is the manganese 

 nodules that occur in considerable volume on parts 

 of the ocean floor and on the beds of some fresh- 



water lakes. Although these nodules are considered 

 to be of potential value chiefly as a source of man- 

 ganese, nickel, copper, and cobalt, they contain an 

 average of about 0.10 percent of lead, which may 

 be recovered in some of the hydrometallurgical 

 processes now being investigated. Mero (1965) esti- 

 mates 1.656 trillion tons of nodules for the Pacific 

 Basin alone. These nodules contain an average of 

 0.09 percent of lead, approximately 1.3 billion tons. 

 Over large areas where the nodules are particu- 

 larly rich in nickel, copper, and cobalt, and where 

 they presumably would be mined first, the average 

 lead content is 0.18-0.30 percent and thus would be 

 an important early coproduct. Similar deposits oc- 

 cur in the Atlantic and other oceans and in parts of 

 Lake Michigan. Although mining and metallurgical 

 reduction techniques are currently under investiga- 

 tion, no attempts at commercial exploitation have 

 yet been made, and a question may be raised as to 

 the validity of considering the nodules a mineral 

 resource in the conventional sense at this time. 



An example of another large but more conven- 

 tional resource of subeconomic lead-bearing material 

 is the famous Kupferschiefer and adjacent beds of 

 the Zechstein and equivalent formations of Europe 

 (Deans, 1950). Although this formation is best 

 known and once was widely mined in East Germany 

 for copper, it is believed to contain far greater 

 quantities of lead and zinc, some of which was re- 

 covered as coproducts and byproducts. According 

 to generalized estimates by Richter-Bernburg 

 (1941), the portion of the Zechstein Formation 

 extending between Magdeburg and Richelsdorf in 

 East Germany alone may contain as much as 100- 

 150 million tons of lead and 200-250 million tons of 

 zinc as compared with only 50 million tons of copper. 

 However, mining is not feasible under current eco- 

 nomic conditions because of the limited thickness of 

 the ore-bearing stratum, the relatively low tenor of 

 the ore, the depth of large parts of the ore bed, 

 and other factors. Mineral-bearing parts of the 

 same stratigraphic unit also extend into West Ger- 

 many, Poland, where it is currently being mined 

 for copper, silver, and other metals, and northeast- 

 ern England; the total quantity of their contained 

 lead is not known but is probably much less than in 

 East Germany. 



Similar, but less well known, syngenetic deposits 

 are currently being studied in the Belt Supergroup 

 of northern Idaho and northwestern Montana. Pre- 

 liminary estimates based on initial, incomplete 

 surveys indicate a potential of 1 million tons or 

 more of contained lead. However, the mining of 

 adjacent and more valuable copper ore bodies may 



