LEAD 



317 



high-grade deposits, particularly those that pro- 

 duced lead as a coproduct with silver and other 

 metals. These advances include the development of 

 high-volume mining and milling techniques that 

 have allowed the profitable exploitation of large 

 low-grade disseminated ore bodies; the successful 

 application of new techniques of continuous smelt- 

 ing of intimately mixed zinc and lead ores; the 

 recovery of byproduct lead concentrate from the 

 milling of massive sulfide ores in Sweden, Japan, 

 and elsewhere; and, most importantly, the contin- 

 ued discovery and development of conventional types 

 of ore bodies. Certain trends, however, tend to indi- 

 cate a softening demand for lead in the future, 

 particularly in the United States and Western 

 Europe. These trends include severe limitations on 

 the permissible levels of sulfur dioxide, lead oxide, 

 and other emissions from smelters; significant re- 

 ductions in the lead content of high-octane gasoline ; 

 the virtual elimination of lead oxides in paint; and 

 the substitution of other substances for the lead 

 used in cable sheathing, type metal, and other ap- 

 plications. Whether or not increased consumption 

 of lead in some uses (such as batteries) will offset 

 declining consumption in other uses (such as tetra- 

 ethyl lead gasoline additive) over the near term is 

 difficult to predict. It seems certain, however, that 

 world production and consumption of newly mined 

 lead will increase over the long term, but eventually 

 must decline as the primary sources approach ex- 

 haustion and the recycling of scrap lead increases. 



GEOLOGIC ENVIRONMENT 



GEOCHEMISTRY 



Lead is a chemical element of atomic number 82. 

 The atomic weight varies because of variations in 

 isotope composition which are due to unequal in- 

 corporation of radiogenic lead, the product of 

 atomic decomposition of both uranium and thorium. 

 Common lead of isotopic composition Pb^°* 1.5 per- 

 cent, Pb^<"* 23.6 percent, Pb^" 22.6 percent, and 

 Pb=°8 52.3 percent has an atomic weight of 207.2. 

 Lead occurs in nature rarely as the metallic element 

 and in the quadrivalent state but predominantly in 

 the divalent state with an ionic radius of 1.26 A 

 for sixfold coordination. Commonly it diadochically 

 replaces Ba+= (1.44 A), Sr+^ (1.21 A), and K+^ 

 (1.46 A) in the lattice of certain silicates, phos- 

 phates, and other minerals. 



Lead is the 34th most abundant element in the 

 lithosphere, averaging about 15 ppm (parts per 

 million). In crustal abundance, it ranks not only 

 below copper and zinc but also below such less com- 



mon and even rare elements as yttrium, neodymium, 

 lanthanum, and possibly gaUium (Mason, 1958, p. 

 44). In igneous rocks it ranges in concentration 

 from approximately 5 ppm in gabbro and related 

 rocks to 10 ppm in oceanic crust and approximately 

 20 ppm in granite. Typical concentrations in sedi- 

 mentary rocks range from an average of 7 ppm in 

 sandstone and 9 ppm in carbonate rocks to 20 ppm 

 in shales ; deep-sea clays reportedly average 80 ppm 

 (Turekian and Wedepohl, 1961, table 2). Inasmuch 

 as lead ores of minimum minable grade average 

 about 4 percent, they represent a concentration fac- 

 tor of 2,600 times the average crustal composition, 

 or about 2,000 times the average concentration in 

 granite. 



Lead appears to be chiefly siderophile, according 

 to Rankama and Sahama (1950, p. 729-730), al- 

 though it may also be chalcophile ; in the lithosphere 

 it shows a rather pronounced affinity for sulfur and 

 also for oxygen. Lead is essentially absent as an 

 early magmatic sulfide and is particularly enriched 

 in pneumatolytic and hydrothermal solutions ge- 

 netically associated with quartz-bearing igneous 

 rocks. It is the major constituent of more than 200 

 known minerals. 



In primary ore deposits, lead occurs predominantly 

 as the sulfide. During weathering, this sulfide is slow- 

 ly oxidized to lead sulfate, a process that is facili- 

 tated by the concurrent oxidation of the commonly 

 associated pyrite to ferric sulfate, which acts as an 

 oxidizing agent. The lead sulfate, which is only mod- 

 erately soluble, and therefore relatively stable, is in 

 turn converted by the action of carbon dioxide or 

 soluble bicarbonates to the even more stable lead 

 carbonate, which is the most chemically resistant 

 lead compound in the zone of oxidation. Other rarer 

 compounds that may be formed in the oxidation 

 zone include arsenates, chlorides, vanadates, phos- 

 phates, chromates, and molybdates. In time even the 

 stable lead carbonate is physically disaggregated or 

 is taken into solution as the moderately soluble lead 

 bicarbonate and finds its way to the sea. 



ORE MINERALOGY 



In the wide variety of lead-producing deposits 

 throughout the world, the overwhelmingly dominant 

 ore mineral is galena (PbS) . This metallic, steel-gray 

 mineral with perfect cubic cleavage is one of the 

 most widely distributed and easily recognized ore 

 minerals. Very commonly it contains silver, usually 

 as occluded blebs of argentite (AgoS), argentiferous 

 tetrahedrite (3[Cu-Ag]2S-Sb2S3), and similar min- 

 erals ; it becomes silver ore when the value of silver 

 exceeds that of the contained lead. Near the surface, 



