LEAD 



323 



Africa (Tsumeb Corp. Ltd., 1961), Turlan, U.S.S.R. 

 (Knyazev, 1954), and many others. Deposits of 

 doubtful replacement origin include the Broken Hill, 

 Mount Isa, and Hilton ore deposits in Australia, the 

 Sullivan ore body in Canada, and one or more of the 

 massive sulfide deposits in the metamorphic ter- 

 ranes of Scandinavia. 



The environments of deposition of hydrothermal 

 replacement deposits range from high pressures and 

 essentially magmatic temperatures to lovs^ tempera- 

 tures and a few atmospheres of pressure. Studies 

 of fluid inclusions, isotopic ratios, and the stability 

 fields of minerals and mineral associations indicate 

 that the highly productive limestone replacement ore 

 bodies were deposited from brines of moderate 

 density at temperatures ranging from 175 °C to 

 500 °C and at pressures of 140 to 400 atmospheres. 

 Commonly the terminal phases of ore deposition 

 show relatively low temperatures. Prospecting for 

 these ore bodies in areas where they are entirely 

 concealed is often expensive, frustrating, and un- 

 rewarding. 



Tabular or sheetlike masses of ores that occupy 

 a fracture or sets of fractures are perhaps the most 

 widespread and best known type of ore deposit. In 

 past centuries they have been the principal source 

 of lead ores, and today they are the chief type of 

 deposit being mined in several highly productive 

 mining districts, including the famous Coeur d'Alene 

 district of Idaho. Veins are found in all types of 

 rocks, assuming all possible attitudes and directions. 

 The simplest veins are those in which the ore and 

 gangue minerals partly or wholly occupy the origi- 

 nal open spaces along a fracture. Filled veins com- 

 pletely occupy a fracture, commonly leaving no 

 residual masses of country rock; many of them are 

 banded, and some show evidence of rebrecciation 

 and renewal of ore deposition. Replacement veins, 

 which generally occur in limestone and other reac- 

 tive rocks, so extensively replace their walls that 

 the character of the localizing fracture is commonly 

 obliterated and the veins become indistinguishable 

 from massive replacement deposits. In most dis- 

 tricts, veins occur in groups or systems of sub- 

 parallel trend, in which individual veins branch and 

 divide, commonly joining with adjacent veins by 

 means of diagonal structures. They range in width 

 from narrow seams to exceptional deposits that are 

 50 feet or more wide. Networks of narrow, short 

 veins, or close-spaced "horsetail" vein systems 

 locally are mined as stockworks or compound lodes. 

 Persistent veins follow faults of moderate to large 



displacement; in the Harz Mountains, Germany, 

 some of the lead-bearing veins are traceable for 12 

 miles, and in the Silverton area, Colorado, single 

 ore-bearing fissures are 5 miles or more in length 

 (Lindgren, 1933, p. 164). The vertical extent of 

 most veins is generally less than their length. Some 

 disappear within a few tens of feet below the sur- 

 face ; others have been followed downdip for as much 

 as 10,000-11,000 feet. Because of greatly increased 

 operating costs at depth and the relatively small 

 volume of ore in most ore shoots, only those base- 

 metal veins with a high proportion of gold or silver 

 can be mined below 2,000-3,000 feet. 



Few veins contain ore of minable grade for their 

 entire length or depth. The ore minerals are con- 

 centrated in ore shoots, which may be enclosed in 

 halos of low-grade material or separated by 

 stretches of barren gangue minerals. The shoots 

 have a wide range of size, shape, and continuity, 

 ranging from small bunches or kidneys to great 

 vertical or horizontal sheets, or to narrow vertical 

 pipes. They may occur at places where there is a 

 change in the strike or dip of the vein or in the 

 character of the wallrocks, or at vein intersections. 

 Recognition of such habits is useful in exploring for 

 new ore shoots, although more subtle factors re- 

 lated to changes in the temperature, pressure, and 

 chemistry may have been predominant in the pre- 

 cipitation of the ore metals. In the Coeur d'Alene 

 district of Idaho, the highly productive Star-Morn- 

 ing ore shoot has a dip length of 6,700 feet, a strike 

 length of about 4,000 feet, and a maximum width of 

 about 50 feet (Hobbs and Fryklund, 1968, p. 1426). 

 In contrast, the lead ore shoots in the North Pen- 

 nine district of England, which are localized at the 

 traces of relatively thin beds of hard limestone and 

 sandstone along the vein, are as much as 3,000 feet 

 in length but extend only 65 feet from top to base 

 although they are locally connected with other over- 

 lying and underlying horizontal ribbon ore shoots 

 by steep pipe-shaped ore shoots as much as 585 feet 

 long (Dunham, 1949, p. 77-78). 



In mineralogy and grade, the lead-producing ore 

 shoots in veins are similar to the replacement and 

 other type of ore bodies. The dominant minerals are 

 galena and sphalerite and pyrite. In some deposits, 

 argentiferous tetrahedrite, chalcopyrite, silver-lead 

 sulfosalts, and, rarely, cobalt, nickel, and uranium 

 minerals are also important. Gangue minerals com- 

 monly include quartz, siderite, calcite, barite, and 

 fluorite. The grade of ore ranges from a few per- 

 cent to as much as 50 percent or more combined 

 lead and zinc. As indicated, many lead-producing 

 veins are mined chiefly for their content of silver 



