LEAD 



321 



fluids that percolated through the older Paleozoic 

 strata and later were mixed with a minor proportion 

 of younger lead derived from the host rocks or the 

 prevailing oceans at the date of deposition (Brown, 

 1970, p. 109). The specific origin of the metals in 

 these deposits and their environment of deposition 

 are the subject of much vigorous debate among Euro- 

 pean geologists, some of whom favor a syngenetic or 

 a meteoric origin for the deposits. 



Strata-bound or stratiform deposits that closely 

 resemble the Mississippi Valley- and Alpine-type de- 

 posits but contain lead whose apparent isotopic age 

 is the same as the apparent age of ore deposition, 

 constitute the third class of strata-bound epigenetic 

 deposits. These ore bodies are genetically similar to 

 the vein and replacement deposits described in the 

 following sections. An example of such a deposit is 

 Pine Point in Canada, which contains lead from a 

 homogeneous source with model age of about 250- 

 275 million years, appreciably younger than the 

 Devonian host rocks (Gumming and Robertson, 

 1969) . Additional examples may be found among the 

 Appalachian Valley lead and zinc deposits of east 

 Tennessee, Virginia, and Pennsylvania and in other 

 stratiform deposits in other parts of the world. 



VOLCANO-SEDIMENTARY DEPOSITS 



The second most productive deposits of lead and 

 zinc are lenticular bodies of massive sulfides that are 

 conformable or semiconformable with interstratified 

 volcanic, volcano-sedimentary, and sedimentary 

 rocks. These deposits range from the relatively little 

 deformed Kuroko deposits of Miocene age in Japan 

 to the recrystallized massive deposits of Paleozoic 

 and Precambrian ages in metamorphic rocks in other 

 parts of the world. Some of these latter deposits form 

 the largest individual concentrations of metal sul- 

 fides yet discovered. 



The unmetamorphosed volcano-sedimentary, or 

 Kuroko, deposits of Japan are massive pods and 

 lenses of various sizes composed of intimate mix- 

 tures of sphalerite, galena, some chalcopyrite, acces- 

 sory sulfosalt minerals, barite, and gypsum (Kino- 

 shita, 1931; Horikoshi, 1969). Byproducts of gold, 

 silver, tin, tungsten, arsenic, barite, and other ele- 

 ments are recovered locally. The deposits occur chief- 

 ly in shale, sandstone, green tuff, and other clastic 

 volcano-sedimentary rocks of Miocene age. Many 

 show a zonal arrangement with lead- and zinc-rich 

 ore (kuroko) in the center, chalcopyrite- and pyrite- 

 rich ore (oko) forming an intermediate zone, and 

 silica-rich pyrite ore (kieko) forming an irregular 

 outer zone next to gypsum-rich altered country rock 

 (Japan Geol. Survey, 1960, p. 141). The deposits are 



essentially parallel to the stratification of the enclos- 

 ing volcano-sedimentary rocks, but many are obvi- 

 ously replacement deposits and are surrounded by 

 envelopes of altered rocks. One of the larger ore 

 bodies, the Matsumine, is more than 1,000 meters 

 long, 500 meters wide, and 30 meters thick and 

 originally contained 30,000,000 metric tons of ore. 

 The grade of this ore body is estimated to be 2-2.5 

 percent copper, 1 percent lead, 3.5 percent zinc, and 

 17-24 percent sulfur (Maruyama and Sato, 1970, p. 

 177). Other minable Kuroko deposits range down- 

 ward in size to the Suehiro ore body, 50 meters long, 

 40 meters wide, and 30 meters thick, which contains 

 about 40,000 metric tons of ore that averages 4.7 per- 

 cent copper, 1.7 percent lead, 3.8 percent zinc, and 

 20.2 percent sulfur (Maruyama and Sato, 1970, p. 

 177). Some parts of many Kuroko ore bodies, how- 

 ever, consist essentially of massive galena and mar- 

 matitic sphalerite. 



Geologic and mineralogic relations indicate that 

 the Kuroko ore bodies were formed by submarine 

 exhalative processes during the geosynclinal accumu- 

 lation of their country rocks. They are most com- 

 monly found as replacements of water-laid tuffs and 

 similar rocks beneath caprocks of lava on the flanks 

 of domes that are cored by rhyolite intrusions. Some* 

 of the deposits merge downward with veins and 

 stockworks that may represent the conduits for the 

 relatively low temperature mineralizing solutions 

 that formed the massive ore bodies. 



Deposits somewhat similar to the Kuroko deposits 

 have been described at Atasu and Achisai in Kazakh- 

 stan by Shcherba (1971, p. 172-174). 



Massive sulfide deposits in metamorphic rocks 

 have many features in common with the Kuroko de- 

 posits including large size, common association with 

 volcanic rocks, strata-bound or stratiform character, 

 simple mineralogy with abundant iron sulfide, and an 

 absence of obvious structural localization. Some of 

 them differ from the Kuroko deposits in containing 

 somewhat higher concentrations of lead and zinc, 

 and in being somewhat coarser textured; both fea- 

 tures may be attributable to recrystallization during 

 metamorphism. It is possible that many of these 

 deposits are large replacement bodies that were em- 

 placed long after lithification, uplift, and even meta- 

 morphism of the country rock. However, a sub- 

 marine-exhalative origin is suggested by their 

 apparent concordant nature, their independence from 

 faults and other structural features, and the common 

 absence of nearby intrusive bodies. 



As a group, the massive sulfide deposits in meta- 

 morphic rocks range in size from small pods contain- 

 ing only a few tons or less to enormous bodies con- 



