122 ECONOMIC GEOLOGY 



Bunker Hill Mine, The Federal Mining and Smelting Company, 

 and the Hercules Mine are some of the principle producers. 

 Ninety-nine per cent, of the lead and the same per cent, of the 

 silver in the Coeur d'Alene District comes from the Revette 

 quartzite and Burke sandstones, quartzites and shales (Figs. 79 

 and 80). 



According to F. L. Ransome, there are no sediments in the dis- 

 trict younger than the Algonkian, except the fluviatile deposits 

 some of which may be of Tertiary age. The post-Algonkian 

 intrusives are monzonite and syenite. The ore deposits of the 

 district are divided into three classes: (1) Lead-silver deposits; 

 (2) gold deposits, and (3) copper deposits. 



The lead-silver deposits occur in metasomatic fissure veins 

 formed largely by replacement along zones of fissuring or of com- 

 bined fissuring and faulting, and partly by the filling of open 

 spaces. The ore bodies are tabular and the mineralized fissures 

 have the characteristics of faults. They differ from the great 

 faults of the region in that the more important faults are not ore 

 bearing. Fissuring and cleavage are so closely related to each 

 other that the structure may be termed a shear zone. Ries con- 

 siders that the metamorphism was adequate to produce new 

 minerals. The most characteristic minerals are galenite and 

 siderite, the carbonate of iron. Sometimes the galenite replaces 

 the sericitic quartzite. Sometimes the quartzite is replaced by 

 siderite and in turn by galenite. 



The galenite was not all deposited during one period for some- 

 times the masses of coarsely crystallized galenite is traversed by 

 veinlets of a more compact variety of the same mineral. The 

 lead minerals found in the oxidized zone are cerussite and pyromor- 

 phite; the silver minerals, native silver and cerargyrite; the copper 

 minerals, azurite and malachite; and the hydrated oxide of iron, 

 limonite. 



Geological Horizon. Lead minerals are not confined to any 

 geological horizon. They occur widely distributed in the rocks 

 of all ages. They are, however, most abundant in the Cambrian, 

 Ordovician, Carboniferous, and Cretaceous ages. 



Extraction of the Metal. (1) The Reduction Process. Lead 

 oxides are readily reduced to the metallic state by carbon accord- 

 ing to the equation PbO+C = Pb+CO. 



(2) The Roast-reaction Process. The ore is crushed and intro- 

 duced into a reverboratory furnace in small quantities. The ore 



