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UNITED STATES MINERAL RESOURCES 



many of the galena ore bodies are altered to cerus- 

 site (PbCOa), anglesite (PbSOi), pyromorphite 

 (Pb4[PbCl] [POjJa) , and other minerals, but commer- 

 cial ores consisting chiefly of these secondary min- 

 erals now represent only a small fraction of the total 

 volume of lead ores currently being produced. A 

 great many other metallic lead minerals have been 

 recognized, including many silver-, copper-, bismuth-, 

 and antimony-bearing sulfosalts that are important 

 sources of silver and other metals. Only rarely are 

 these minerals found in concentrations large enough 

 to produce a significant quantity of lead. 



The primary metallic minerals most commonly as- 

 sociated vs^ith galena include pyrite (FeSs), sphaler- 

 ite (ZnS), chalcopyrite (CuFeS2), tetrahedrite 

 (SCuaS-SbaSs) or tennantite (SCusS-AsoSs), argen- 

 tite, boumonite (SPbS-CusS-SbaSj), and other sul- 

 fosalts, and, locally, marcasite (FeS2) and pyiThotite 

 (FerSs). Sphalerite, in particular, is such a common 

 associate of galena that sphalerite-free galena ore 

 bodies and galena-free sphalerite ore bodies are rare. 

 The primary gangue minerals found in lead deposits 

 include quartz in various forms ; calcite, dolomite, 

 and other carbonates; barite; and fluorite. Some of 

 these minerals, as well as many of the associated 

 minor metallic minerals, commonly are recovered as 

 coproducts and byproducts. 



TYPES OF DEPOSITS 



The ore bodies in which galena is a major or a re- 

 coverable minor constituent have been deposited in 

 a wide variety of physical and chemical environ- 

 ments. They are most effectively classified on the 

 basis of their geologic occurrence as: (1) strata- 

 bound deposits of syngenetic origin, (2) strata- 

 bound or stratiform deposits of epigenetic origin, (3) 

 volcano-sedimentary deposits and their metamorphic 

 equivalents, (4) replacement deposits, including man- 

 tos, (5) veins, and (6) contact pyrometasomatic 

 deposits in the aureoles of granitic plutons. As with 

 other types of classification systems, the above 

 scheme is not wholly satisfactory, inasmuch as one 

 type of deposit — such as a vein — may gradually as- 

 sume the characteristics of another type — such as a 

 replacement deposit. Also, there is much disagree- 

 ment among geologists and geochemists as to the 

 genesis of many ore bodies, particularly the strata- 

 bound deposits, which may have either a syngenetic 

 or an epigenetic origin. As pointed out by Brown 

 (1970, p. 108), it is remarkable that so many well- 

 qualified authorities can examine the same set of 

 facts about the strata-bound ore bodies and reach 

 such widely different conclusions as to their genesis. 

 In recent years, however, studies of fluid inclusions 



and of lead, sulfur, and oxygen isotopes and their 

 ratios in various deposits of unquestioned origin ap- 

 pear to have helped resolve questions of the origin of 

 some previously enigmatic ore bodies. (Note. — Us- 

 age of the terms "strata-bound" and "stratiform" in 

 this chapter is somewhat different from usage in 

 some other chapters.) 



STRATA-BOUND DEPOSITS OF SYNGENETIC ORIGIN 



The largest and most productive lead deposits are 

 those that are apparently restricted to one or more 

 sedimentary rock units, chiefly limestone, dolomite, 

 or shale. Many of these deposits underlie areas ex- 

 ceeding several square kilometers and are, in fact, 

 merely the richer parts of broadly mineralized ter- 

 ranes that extend for many hundreds or even thou- 

 sands of square kilometers. Detailed studies com- 

 monly reveal that these deposits may be further 

 divided into two general types: those in which the 

 ore minerals were deposited syngenetically with the 

 enclosing sediments or formed shortly afterward as 

 a result of diagenetic processes and those in which 

 the ore minerals chiefly occur in secondary features 

 and commonly are enclosed within aureoles of sec- 

 ondary dolomitization or some other form of post- 

 diagenetic alteration. Although deposits of the latter 

 type have many features in common with the re- 

 placement deposits, particularly the mantos, they 

 generally differ in the absence of any clear evidence 

 of their association with igneous rocks. These epi- 

 genetic deposits are discussed separately. 



The strata-bound ores of syngenetic origin are 

 typified by the "Kupferschiefer" bed, near the base 

 of the Zechstein Formation, which is currently being 

 mined in East Germany in the area of Mansfeld, 

 Richelsdorf, and Sangerhausen and in Poland in the 

 Subsudetic syncline. This layer of bituminous shale 

 is commonly 1 meter or less thick ; it locally overlies 

 a patchy marine conglomerate, the first stratum 

 deposited in the Upper Permian basin of central 

 Germany, and underlies a sequence containing lime- 

 stone, gypsum, halite, and sandstone. It is reportedly 

 mineralized over an area of 4,560 square kilometers 

 (Bauchau, 1971, p. 5). The ore minerals are finely 

 disseminated through the shale and consist pre- 

 dominantly of bomite, chalcocite, galena, sphalerite, 

 and tetrahedrite. Accessory elements include nickel, 

 cobalt, selenium, vanadium, molybdenum, and silver. 

 There are virtually no gangue minerals except for 

 wispy veinlets of barite and gypsum. Here and there 

 the Kupferschiefer is cut by small faults, and near 

 them the bed may be either enriched or impoverished 

 in the metals ; locally, some of these faults contain 



