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UNITED STATES MINERAL RESOURCES 



taining more than 100 million tons. The ore is 

 commonly fine grained, consisting of intimate ag- 

 gregates of pyrite or pyrrhotite, sphalerite, galena, 

 and chalcopyrite, with minor or subordinate amounts 

 of quartz, sericite, chlorite minerals, ankerite, and 

 other carbonate minerals. Host rocks include argil- 

 lite, metavolcanic rocks, schists of various mineral 

 associations, shale, and carbonate rocks. In folded 

 terranes, the ore bodies are typically, but not invari- 

 ably, found on the limbs of folds. In such widely 

 separated places as Bathurst, New Brunswick, 

 Jerome, Ariz., and Broken Hill, New South Wales, 

 the ore bodies are intimately associated with meta- 

 morphosed quartz porphyry. 



As typified by the Bathurst deposits, individual 

 ore lenses may be 3,000 feet or more long and as 

 much as 250 feet wide and may extend to depths 

 greater than 1,700 feet. Zones of en echelon ore 

 lenses in the Bathurst area are nearly a mile long 

 (Gates, 1971, p. 113). The grade of the larger con- 

 centrations of lead and zinc ores in the irregularly 

 zoned Brunswick and Heath Steele ore bodies is 

 close to 3-3.5 percent lead and 8.75 percent zinc; 

 copper, silver, and gold are also recovered. Addi- 

 tional examples of volcano-sedimentary deposits in 

 metamorphic rocks include numerous mines in Nor- 

 way (Yokes, 1962), Sweden (Kautsky, 1957), 

 U.S.S.R. (Smirnow, 1960), especially Leninogorsk 

 and adjacent areas (Nekhoroshev, 1941), and else- 

 where (Anderson, 1969). Other great massive ore 

 deposits of the world, such as Mount Isa and Broken 

 Hill in Australia, Rammelsberg and Meggen in West 

 Germany, Kidd Creek and Sullivan in Canada, and 

 Ducktown in the United States, may have originated 

 as volcano-sedimentary deposits, but conclusive evi- 

 dence of their origin has not been established or may 

 have been obliterated by metamorphism, recrystal- 

 lization, or subsequent tectonism. 



REPLACEMENT DEPOSITS 



The third most productive source of primary lead 

 is hydrothermal replacement ore deposits that are 

 commonly associated with postorogenic intrusions 

 of intermediate to acid composition. Most commonly 

 these deposits occur in such readily soluble rocks as 

 limestone and dolomite, but they also occur in 

 quartzite and shale and in igneous and metamorphic 

 rocks. They range in size from insignificant pods 

 to extensive masses like the Cerro de Pasco ore body 

 in Peru, which may have originally contained more 

 than 2.3 million tons of galena, 6 million tons of 

 sphalerite, and 100 million tons of pyrite, and sig- 

 nificant amounts of other hydrothermal minerals 

 (recalculated from Peterson, 1965, p. 446) . In most 



of the replacement deposits the dominant lead min- 

 eral is galena, which is associated with sphalerite, 

 chalcopyrite, and pyrite. In many deposits, silver, 

 arsenic, antimony, and cadmium are also abundant, 

 giving rise to a great variety of arsenides, anti- 

 monides, and sulfosalts. Oxidation, which locally 

 extends to considerable depth, results in a still 

 greater variety of minerals and commonly aids in 

 the enrichment of some lead ore by the solution and 

 migration of more soluble constituents. 



The form and size of replacement ore bodies are 

 largely determined by the chemical character of 

 their host rocks and the geologic structures that 

 localize the deposits. In less readily soluble igneous 

 and metamorphic rocks and in quartzite and other 

 refractory sedimentary rocks, replacement deposits 

 commonly assume, wholly or partly, the form of 

 such structural features as tabular fault and shear 

 zones, pipe-shaped masses of intrusion breccia, flat- 

 lying beds or layers of porous tuff, agglomerate, and 

 similar rocks, or podlike masses of breccia of diverse 

 origin. In the more soluble carbonate rocks, the re- 

 placement deposits similarly assume the general 

 character of the localizing structural or strati- 

 graphic feature, but they also typically extend ir- 

 regularly and unpredictably along a variety of minor 

 or obscure features, producing ore bodies that are 

 highly irregular and branching. In gently dipping, 

 relatively unfractured carbonate rocks, particularly 

 in central Mexico and central Colorado, the replace- 

 ment bodies form flat-lying tabular bodies and near- 

 horizontal pipelike bodies — both called mantos; 

 these bodies commonly extend outward along one 

 or more selected horizons from vertical columns or 

 chimneys of ore that are several hundred feet high, 

 which may or may not be localized by recognizable 

 fractures (Prescott, 1926). Most of the important 

 replacement deposits contain a few hundred thou- 

 sand to a few million tons of ore. Many of them are 

 small, but some exceed 100 million tons. The grade 

 is commonly of the order of 10-30 percent of lead 

 and zinc combined, 3-5 percent copper, 5-50 ounces 

 per ton silver, and a few hundredths of an ounce of 

 gold per ton. Some deposits show a pronounced 

 vertical or lateral zonation of the metals. 



Examples of some of the more important massive 

 replacement ore deposits include Tintic, Utah (Mor- 

 ris, 1968), Bingham, Utah (Eubright and Hart, 

 1968), Gilman, Colo. (Tweto and Lovering, 1947), 

 Leadville, Colo. (Emmons and others, 1927; Tweto, 

 1968), central Mexico (Prescott, 1926), central 

 Peru (Peterson, 1965), Sardinia, Italy (Novarese, 

 1950), Trep6a, Yugoslavia (Forgan, 1950), Carta- 

 gena, Spain (Pavilion, 1969), Tsumeb, South West 



